Mechanisms of Sperm Chemotaxis

ANRV336-PH70-05 ARI 10 January 2008 18:27

Mechanisms of SpermChemotaxisU. Benjamin Kaupp, Nachiket D. Kashikar,and Ingo WeyandForschungszentrum Julich, Institut fur Neurowissenschaften und Biophysik 1,D-52425 Julich, Germany; email: [email protected]

Annu. Rev. Physiol. 2008. 70:93–117

First published online as a Review in Advance onNovember 7, 2007

The Annual Review of Physiology is online athttp://physiol.annualreviews.org

This article’s doi:10.1146/annurev.physiol.70.113006.100654

Copyright c© 2008 by Annual Reviews.All rights reserved

0066-4278/08/0315-0093$20.00

Key Words

calcium, cyclic nucleotides, motility, receptor, signaling

AbstractSperm are attracted by chemical factors that are released by theegg—a process called chemotaxis. Most of our knowledge on spermchemotaxis originates from the study of marine invertebrates. In re-cent years, the main features of the chemotactic signaling pathwayand the swimming behavior evoked by chemoattractants have beenelucidated in sea urchins. In contrast, our understanding of mam-malian sperm chemotaxis is still rudimentary and subject to an ongo-ing debate. In this review, we raise new questions and discuss currentconcepts of sperm chemotaxis. Finally, we highlight commonalitiesand differences of sensory signaling in sperm, photoreceptors, andolfactory neurons.

93

Ann

u. R

ev. P

hysi

ol. 2

008.

70:9

3-11

7. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by U

nive

rsity

of

Mas

sach

uset

ts -

Am

hers

t on

04/1

2/13

. For

per

sona

l use

onl

y.


Chemotaxis: adirectional change ofcell movement eithertoward or away froma source of achemical attractantor repellent,respectively

Guanylyl cyclase(GC): an enzymethat catalyzes theformation of thesignaling moleculecGMP (3′,5′-cyclicguanosinemonophosphate)from the nucleosidetriphosphate GTP.Has both soluble(sGC) andmembrane-spanning(tmGC) forms

Resact: a smallpeptide derived fromeggs of the seaurchin Arbaciapunctulata. Evokessimilar biochemicalreactions as speractin sperm; however,only resact has beenestablished as achemoattractant

INTRODUCTION

The egg attracts sperm by releasing chemicalfactors. The chemoattractant gradient pro-vides cues that guide sperm to the egg—aprocess called chemotaxis. Given the key roleof chemotaxis in fertilization, it is surprisingthat until recently little was known about howsperm chemotaxis works. However, improvedmethods for optical and electrical recordingof sperm significantly advanced our under-standing of sperm physiology. In particular,the mechanism by which binding of chemoat-tractants is transduced into a cellular response,as well as the pattern of swimming behav-ior evoked by the chemoattractant, has beendescribed in sea urchin sperm. Much lessis known about chemotaxis of mammaliansperm. The chemical nature of the chemoat-tractants, their cognate receptors, and the un-derlying signaling pathways are subject to anongoing debate. It has become clear, however,that chemotactic signaling in marine inverte-brates and mammals may be radically differ-ent. To take this difference into account, thisreview is organized in two sections discussingchemotaxis in marine invertebrates andmammals.

CHEMOTAXIS IN MARINEINVERTEBRATES

Historical Perspective and Overview

Almost 100 years ago, Lillie at the Ma-rine Biological Laboratory in Woods Hole,Massachusetts, observed a hefty “aggluti-nation” reaction when he mixed sperm ofthe sea urchin Arbacia punctulata with egg-conditioned seawater (1). This was probablythe first observation of a chemotactic reactionovershadowed by other motility reactions (2)and marked the rise of sea urchin to becomethe most important model of sperm chemo-taxis. In the 1980s, Garbers and colleaguesidentified short peptides in the egg coat and areceptor-type guanylyl cyclase (GC) in spermas the peptide receptor (3–5). At that time,Vacquier and colleagues demonstrated that

resact, the peptide of A. punctulata, displayschemotactic activity and that Ca2+ ions arerequired for chemotaxis (6).

Brokaw and colleagues (7, 8) showed thatthe flagellar waveform of sperm whose mem-branes were rendered permeable with deter-gent depends on the concentration of Ca2+

ions ([Ca2+]) in the medium: At low [Ca2+],flagella beat more symmetrically, whereas athigh [Ca2+], the waveform becomes moreasymmetrical. Early studies by Miller &Brokaw (9) and Miller (10–12) revealed thatthe swimming trajectory of sperm from var-ious marine invertebrates in a gradient ofchemoattractant is characterized by the alter-nation of tight loops and wide circular arcs.Wide arcs are made in the direction of thesource, whereas tight loops occur when spermswim away from the source. Taken together,these observations inspired a plausible, yetincorrect, concept: Swimming up the gradi-ent keeps the intracellular Ca2+ concentra-tion ([Ca2+]i) low, the flagellar beat symmetri-cal, and the trajectory straight. If a sperm cellbears away and senses a decrease in chemoat-tractant, [Ca2+]i increases, the beat becomesasymmetrical, and the cell makes a correctingturn that brings it back on course.

Chemoattractants give rise to a host ofother cellular reactions, including changes inlevels of cyclic nucleotides, pHi, the phospho-rylation pattern of proteins, and membranevoltage. Sophisticated models of cell signalinghave been constructed to relate these eventsto changes in [Ca2+]i and to swimming behav-ior (13–18). It was not until recently that thesemodels were scrutinized through the use of ki-netic techniques able to resolve the sequenceof signaling events and the resulting naviga-tion of sperm in a chemical gradient. The newresults require substantial revisions of previ-ous signaling schemes.

Sperm of A. punctulata are exquisitely sen-sitive: They can react to the binding of a singlemolecule of chemoattractant and transducethis binding into an elementary voltage andCa2+ response (19, 20). Thus, Arbacia spermprovide an excellent eukaryotic cell system for

94 Kaupp · Kashikar ·Weyand

Ann

u. R

ev. P

hysi

ol. 2

008.

70:9

3-11

7. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by U

nive

rsity

of

Mas

sach

uset

ts -

Am

hers

t on

04/1

2/13

. For

per

sona

l use

onl

y.


the study of the cellular and physical prin-ciples governing single-molecule responses.The principles may be relevant for signalingin small cellular compartments such as synap-tic boutons, growth cones, or cilia.

The Signaling Pathwayand Its Molecules

Overview. The study of chemotactic sig-naling is most advanced in A. punctulatasperm (Figure 1). Resact, the chemoattrac-tant peptide, activates a GC on the flagel-lum and thereby stimulates the rapid synthe-sis of 3′,5′-cyclic guanosine monophosphate(cGMP) (19). cGMP opens K+-selectivecyclic nucleotide–gated (KCNG) channels toproduce a brief membrane hyperpolariza-tion (20). During hyperpolarization, voltage-dependent Ca2+ (Cav) channels recover frominactivation (20) and open on subsequent de-polarization produced by hyperpolarization-activated and cyclic nucleotide–gated (HCN)channels (21). The ensuing Ca2+ dynamicscontrol the flagellar beat and adjust the swim-ming trajectory of sperm in a gradient of thechemoattractant (22) (see Figures 1 and 2).In sperm of the sea urchin Strongylocentrotuspurpuratus, a similar cGMP-signaling path-way exists (23–26; for reviews see References27 and 28). Because no chemotactic activityhas been demonstrated for the S. purpuratuspeptide speract, the physiological significanceof the speract-induced responses is unclear.

Sperm of A. punctulata respond to a sin-gle molecule of chemoattractant and, at thesame time, respond to resact concentrationsover six orders of magnitude (19). In the fol-lowing subsection, we discuss the inventoryof molecules that endows this signaling path-way with both single-molecule sensitivity anda wide dynamic range.

The receptor. The chemoattractant recep-tor belongs to the family of receptor-type GCs(29). In fact, the GC gene from A. punctu-lata was the first member to be cloned (5).This family of receptors is characterized by

Cyclicnucleotide–gated(CNG) channels:nonselective cationchannels firstidentified in retinalphotoreceptors andolfactory sensoryneurons; CNGchannels are openedby the direct bindingof cAMP and cGMP

Hyperpolarization-activated and cyclicnucleotide–gated(HCN) channels:cation channels thatare activated bymembranehyperpolarizationand modulated bythe binding of cyclicnucleotides. UnlikeCNG channels, theyare weakly selectivefor K+ ions

Speract: a smallpeptide derived fromeggs of the sea urchinStrongylocentrotuspurpuratus. Evokessimilar biochemicalreactions as resact,although only resacthas been establishedas a chemoattractant

a single transmembrane span that divides themolecule into an extracellular domain (ECD)for ligand binding and an intracellular regionharboring a kinase-homology domain, a hinge(H) region, and a cyclase-homology domain,which carries the catalytic site. Surprisingly,the published sequence of A. punctulata GC islacking the conserved cyclase-homology do-main (5). Recently, Bungert and colleagues [S.Bungert, I. Weyand, A. Loogen, R. Seifert, W.Bonigk, A. Helbig, J. Enderlein, Q. Van, N.Kashikar, D. Hoppner-Heitmann, C. Klemm,E. Krause, E. Kremmer & U.B. Kaupp, un-published (hereafter referred to as “S. Bungertet al., unpublished”)] reexamined the primarystructure of this GC by cloning cDNA fromA. punctulata testis. The new amino-acid se-quence differs from the sequence of Singhet al. (5) by 203 residues in the C-terminalregion and harbors the prototypical motif ofthe catalytic site.

The GC is highly phosphorylated at restand becomes dephosphorylated on resactbinding (30, 31). The enzymatic activity ofthe dephosphorylated form is lower than thatof the phosphorylated form, suggesting thatthe receptor inactivates on dephosphorylation(32). Six phosphorylated serine residues wereidentified by mass spectrometry (S. Bungertet al., unpublished); five of these residues arelocated in the kinase-homology region. Themammalian isoforms GC-A and GC-B carrysix and five phosphorylated residues, respec-tively, in this domain (33, 34). Four of thesix phosphorylated residues undergo dephos-phorylation on stimulation by resact. Usingquenched-flow techniques, S. Bungert et al.(unpublished) demonstrated that in intactsperm the rates of inactivation and dephos-phorylation of GC are similar, arguing thatthe lifetime of the active receptor (∼300 ms)is controlled by dephosphorylation.

The lifetime of a single active receptoris expected to follow an exponential dis-tribution, and signals controlled by singlemolecules should be inherently variable. Inrod photoreceptors, the mechanisms un-derlying single-photon responses have been

www.annualreviews.org • Mechanisms of Sperm Chemotaxis 95

Ann

u. R

ev. P

hysi

ol. 2

008.

70:9

3-11

7. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by U

nive

rsity

of

Mas

sach

uset

ts -

Am

hers

t on

04/1

2/13

. For

per

sona

l use

onl

y.


Stimulation

–42 mV

Vo

ltag

e

Time

KCNG

HCN andLVA Cav

HVA Cav ?

0 1 20

10

20

30

40

50

60

70

80

90

5 10

Time (s)

pm

ol c

GM

P/1

08 c

ells

∆R

∆F

518

(mV

)

Time (s) Time (s)

(i) (ii) (iii)

0 5 10 15 20 25 300

5

10

15

20

25

300.04

0.02

–0.02

0

0.0 0.5 1.0 1.5 2.0

–0.04

b

c

GTP cGMP cAMP

GCOut

In

Resact

“Turn”

Flagellarasymmetry

K+

HC

N

KC

NG

LVA

Ca v

HV

A C

a v

[Ca2+] i [Ca2+]i

Ca2+Na+ Ca2+

?

a

Hyperpolarization Repolarization Depolarization

Figure 1Chemotactic signaling. (a) Model of chemotactic signaling events in Arbacia punctulata sperm. Binding ofresact to the receptor guanylyl cyclase (GC) activates cGMP synthesis. cGMP opens a K+-selectivecyclic nucleotide–gated (KCNG) channel, thereby causing hyperpolarization. On hyperpolarization,hyperpolarization-activated and cyclic nucleotide–gated (HCN) channels and low-voltage-activatedCa2+ channels (LVA Cav) allow the influx of Na+ and Ca2+, respectively. The opening ofhigh-voltage-activated Ca2+ channels (HVA Cav) may be involved in the sustained elevation of [Ca2+]i.By unknown mechanisms, Ca2+ ions interact with the motor proteins of the axoneme and cause a changein the flagellar beat and, finally, a change in the swimming trajectory. (b) Cellular responses onstimulation of sperm by resact. (i ) The time course of intracellular cGMP concentration evoked bydifferent resact concentrations (0.25 nM–250 nM, from bottom trace to top trace). (ii ) Voltage responsesafter stimulation with 125 pM, 25 pM, and 6.25 pM resact (from bottom trace to top trace). (iii) Ca2+signals of sperm stimulated with 0.25 pM–25 pM resact (from bottom trace to top trace). The black tracerepresents the Ca2+ level of unstimulated sperm. For details see References 19 and 20. (c) Distinct ionchannels contribute to the voltage response.


Ann

u. R

ev. P

hysi

ol. 2

008.

70:9

3-11

7. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by U

nive

rsity

of

Mas

sach

uset

ts -

Am

hers

t on

04/1

2/13

. For

per

sona

l use

onl

y.


carefully worked out. The absorption of a sin-gle photon by the visual pigment rhodopsinproduces uniform, highly reproducible lightresponses (35). Multiple phosphorylationsteps on rhodopsin confer reproducibilityof the single-photon response by tightlycontrolling rhodopsin’s active lifetime (36).Sperm chemotaxis relies on the precisetiming of Ca2+ transients that control theswimming trajectory. As in rods, a sequence ofphosphorylation/dephosphorylation stepsmay tightly control the lifetime of the activeGC as well and thereby generate uniformsingle-molecule responses. This mechanismmay improve the fidelity by which the timingand number of binding events are encoded.

A. punctulata sperm respond to femtomo-lar concentrations of resact (19). The exquisitesensitivity requires a high capture efficiency,i.e., a high density of chemoattractant recep-tors endowed with a high affinity. Previousstudies reported a density of ∼14,000 receptormolecules per sperm (37), whereas S. Bungertet al. (unpublished) observed a density that isorders of magnitude higher (∼1,000,000 GCmolecules per flagellum or ∼30,000 GCmolecules μm−2). Each GC molecule bindsa single resact molecule. Thus, sea urchinsperm developed a high receptor density tomaximize the probability of binding once achemoattractant molecule hits the flagellum.

Sperm can register chemoattractants fromfemtomolar to micromolar concentrationswithout becoming saturated. How do spermreconcile single-molecule sensitivity withsuch a wide dynamic range? The bindingisotherm is shallow and saturates at ∼1 μMresact (S. Bungert et al., unpublished). A fit ofthe Hill equation to the binding data yieldeda K1/2 in the subnanomolar range and a Hillcoefficient n of ∼0.45, suggesting that bind-ing is controlled by negative cooperativity andthat at least three ligand molecules bind tothe receptor. Negative cooperativity requiresseveral GC subunits to interact within anoligomeric complex. In fact, the GC in flag-ellar membranes exists as a homotrimer (S.Bungert et al., unpublished), consistent with

Cyclic nucleotidephosphodiesterases(PDEs): enzymesthat hydrolyzecAMP and cGMP to5′-AMP and5′-GMP,respectively. Inmammals, thesuperfamily of PDEscan be classified into11 families,PDE1–11

the prediction that at least three subunits par-ticipate in the allosteric interaction. However,other mechanisms may contribute to the ad-justment of binding affinity. Because of thehigh binding affinity and density, at picomo-lar to low nanomolar concentrations of re-sact, binding is far from chemical equilibrium;i.e., sperm register relative changes in con-centration rather than absolute values. Thus,sperm escape saturation even for high bindingaffinities.

The initial signal amplification is deter-mined by the number of cGMP moleculessynthesized per second by an active GCmolecule. Early accounts reported turnovernumbers of <0.1 cGMP molecules s−1 (31,37–39), far too low to permit rapid single-molecule responses. S. Bungert et al. (un-published) reexamined GC activity in intactmotile sperm, using quenched-flow tech-niques. The number of cGMP molecules syn-thesized within ≤100 ms after stimulation wasuncompromised by GC inactivation or cGMPhydrolysis by phosphodiesterase (PDE). Aturnover number of ∼5 cGMP molecules s−1

is in fair agreement with turnover numbersreported for other active receptor-type GCs(40). The extremely low synthesis rates re-ported by previous studies probably reflectthe residual activity of desensitized receptors.This result exemplifies the power of time-resolving techniques.

The K+-selective CNG channel. Owing totheir high sensitivity �F/mV, carbocyanineand oxonol dyes with distributed charge havebeen employed as fluorescent potentiomet-ric probes of electrical activity in sperm (27,41–43). These experiments provided the ini-tial evidence that egg-derived peptides elicithyperpolarization by opening a K+ channel.However, a definitive conclusion was ham-pered by several shortcomings. First, all butone study (42) used osmotically swollen spermor isolated flagella in unphysiological saline.The intracellular ion concentrations, in par-ticular [Ca2+]i, the ionic gradients, and themembrane voltage are undefined under these


Ann

u. R

ev. P

hysi

ol. 2

008.

70:9

3-11

7. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by U

nive

rsity

of

Mas

sach

uset

ts -

Am

hers

t on

04/1

2/13

. For

per

sona

l use

onl

y.


conditions and are likely to be different fromthose of intact cells. Second, owing to theirmechanism of voltage sensing, the responsetime of these dyes is inherently slow (1–20 s)and may misrepresent the true voltage re-sponse. In addition, these dyes also distributeacross membranes of intracellular organelles,are pH sensitive, disturb the resting voltage,Vrest, and bind to intracellular components(for a review see Reference 44). Recently,Strunker et al. (20) recorded electrical activ-ity from intact sperm, using the dye Di-8-ANEPPS, which does not suffer from thesedrawbacks. For short incubation times, thisdye stains only the outer leaflet of the plasmamembrane and does not reach intracellularorganelles. Moreover, the dye responds tochanges in Vm in less than 1 ms; finally, theelectrochromic mechanism of voltage sensingallows for dual-emission ratio measurementthat eliminates most of the fluorescence sig-nals unrelated to voltage.

The Vrest of sperm (∼−45 mV) is signif-icantly less negative than the Nernst poten-tial for K+ ions, suggesting that several ionchannels with different ion selectivity are in-volved (20, 42). Both resact and cGMP evoke

a brief hyperpolarizing pulse followed by amore persistent depolarization. The hyper-polarization is caused by the opening of aKCNG channel (20). The KCNG channelhas been cloned from A. punctulata (20; S.Bungert et al., unpublished) and S. purpura-tus (45) and functionally characterized in aheterologous expression system (S. Bungertet al., unpublished). The KCNG channel dis-plays a unique membrane topography. It iscomposed of four homologous repeats in asingle large polypeptide with a total Mw of∼250 kD. Each repeat embodies six trans-membrane segments, S1–S6, a hairpin poreregion between S5 and S6, a cyclic nucleotide–binding domain (CNBD), and a linker re-gion between S6 and the CNBD. The over-all structure of the KCNG channel is moreakin to those of Cav and voltage-dependentNa+ (Nav) channels, which form a pseu-dotetrameric channel from a single largepolypeptide, than to that of cyclic nucleotide–gated (CNG) channels, which form heterote-tramers from four smaller subunits. All fourpore motifs carry the GYGD signature se-quence of K+-selective channels (46). In theCNBD, amino-acid residues characteristic of

−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−→Figure 2Swimming behavior. (a) Chemotactic trails of sperm from marine invertebrates and urochordata. Thetrails toward the pipette containing the chemoattractant are characterized by alternating narrow loopsand wide circular arcs (modified after Reference 12). (b) Arbacia punctulata sperm swim on loops anddrifting circles in a resact gradient. Resact was released from caged resact by local UV irradiation (square).(c) Cyclic GMP (cGMP) evokes alternating periods of high and low curvature of the swimmingtrajectory; the arrow in the main graph indicates the release of cGMP from caged cGMP by UV flash.Numbers indicate peaks in the curvature and corresponding turns or bends in the trajectory. (Inset)Trajectory before (blue trace) and after (green trace) the release of cGMP (red ). (d ) Ca2+ fluctuationscontrol changes in curvature. Changes in intracellular Ca2+ concentration (as measured by fluorescence)(blue) and curvature (red ) on stimulation by cGMP (arrow). (e) Relative stimulus concentration ( green)and Ca2+ fluctuations (blue) of sperm navigating in a resact gradient. The resact gradient was establishedby a UV flash (arrow), resulting in a step in the stimulus function. As a result of the repetitive loop-likemovement (cf. panel b), the sperm cell is repetitively exposed to increasing and decreasing resactconcentrations, producing a periodic stimulus function. The Ca2+ spikes follow the periodic stimulusfunction with a delay (phase shift = 156◦). ( f ) Turn-and-run model of the motor response of sperm. Ina gradient of chemoattractant (blue background ), sperm measure relative changes in concentration duringthe sampling period ( yellow arrow). Sampling is followed by a delay phase when sperm continue to swimdown the gradient (green arrow) (� of approximately 160◦, corresponding to a delay of approximately300 ms). After the delay, a Ca2+ spike occurs, and the curvature increases, leading to a “turn” that isfollowed by a period of straight swimming (“run”) (red arrow). The next sampling period then begins. Fordetails see Reference 22.


Ann

u. R

ev. P

hysi

ol. 2

008.

70:9

3-11

7. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by U

nive

rsity

of

Mas

sach

uset

ts -

Am

hers

t on

04/1

2/13

. For

per

sona

l use

onl

y.


Cnidaria

Mollusca

Echinodermata

Urochordata

5 10 15 20 25Time (s)

Cu

rvat

ure

(μ

m–1

)

1

2 534

67

0.05

0.10

0

12

4

6

3

57

12

4

6

3

57

0 30 2 4 6 8 10

200

150

100

50

0

0.02

0.04

0.06

0.08

0.10

0 12

0.12

1.0

80

02 4 6 80 10

200

0.8

0.6

0.4

0.2

0

160

120

40

a

c

ef

d

b

Relative resact concentrationCa2+ fluctuations

Time (s)

Cu

rvat

ure

(μ

m–1

)

Flu

orescen

ce (ph

oto

ns)

Time (s)

No

rmal

ized

sti

mu

lus

Flu

orescen

ce (ph

oto

ns)

Φ


Ann

u. R

ev. P

hysi

ol. 2

008.

70:9

3-11

7. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by U

nive

rsity

of

Mas

sach

uset

ts -

Am

hers

t on

04/1

2/13

. For

per

sona

l use

onl

y.


cAMP- and cGMP-binding domains fromCNG and HCN channels are conserved.

The KCNG channel displays an exquisitecGMP sensitivity and selectivity. The het-erologously expressed KCNG channel be-comes activated by nanomolar concentrationsof cGMP (K1/2 ∼= 50 nM) in a noncooper-ative fashion. cAMP acts as partial agonistat approximately 1000-fold-higher concen-trations. Owing to the high cGMP sensitiv-ity, the KCNG channel opens in stimulatedsperm when only a few cGMP molecules aresynthesized (see below).

The HCN channel. Sea urchin sperm har-bor in their flagellar membrane two chan-nel isoforms that belong to the family ofHCN channels (21, 47). HCN channels be-come activated when the membrane is hyper-polarized (i.e., at a Vm more negative than∼−40 mV), and their open probability, Po, ata given voltage is enhanced by cAMP (for re-views see References 48–50). Although theirrelative permeability is 3–4-fold larger for K+

than for Na+ ions, under physiological con-ditions (high Na+ outside, high K+ inside),these channels give rise to a depolarizing in-ward current carried by Na+ ions.

HCN channels may serve multiple func-tions in sea urchin sperm. First, a small frac-tion of HCN channels are constitutively openindependently of voltage (21, 51). Owing tothis property and the weak K+ selectivity (re-versal voltage, Vrev, of ca. −25 to −35 mV),HCN channels may codetermine the Vrest.Second, HCN channels may contribute to re-polarizing the cell after the cGMP-inducedhyperpolarization and thereby initiate theopening of T-type Cav channels (see below).Third, the HCN channel from sea urchinis set apart from its mammalian cousins byunique properties (21). Sperm HCN chan-nels, after a hyperpolarizing voltage step, firstactivate and then inactivate. cAMP removesthe inactivation (21, 52) and consequently en-hances the Po profoundly. In contrast, mam-malian HCN channels do not inactivate andcAMP shifts the Po-Vm relation to more pos-

itive values of voltage without affecting themaximal current. Because of the exquisitecAMP sensitivity (K1/2 = 0.75 μM) and thelarge effect of cAMP on Po (21), the modu-lation of sperm HCN channels by cAMP ispredicted to have profound functional con-sequences. Finally, HCN channels are oftenreferred to as pacemakers because they con-trol rhythmic electrical activity in neurons andcardiac myocytes (48). Future studies need toexamine whether these channels are also im-portant for Ca2+ dynamics in sperm.

The Ca2+ entry channel(s). The Ca2+ re-sponse has been studied in cell suspensionsand single sperm. The macroscopic Ca2+ sig-nal displays two kinetic components referredto as the early and late kinetic phases (19). Thelate phase becomes prominent at resact con-centrations of >∼1 nM and persists for a coupleof minutes. When sperm are stimulated by astep increase of cGMP, the Ca2+ signals fadeaway in up to four distinct peaks or humps(19). The waveform may manifest cGMP-induced Ca2+ oscillations in single sperm,which wear away in population measure-ments. In fact, both resact and cGMP elicittrains of Ca2+ spikes in the flagellum (22).

Ca2+ channels open at or shortly after thepeak of the hyperpolarization (20). This re-sult supports a mechanism of recovery frominactivation. At Vrest of ca. −45 mV, Cav chan-nels are inactivated. During a hyperpolarizingvoltage pulse, Cav channels first recover frominactivation and then open. This mechanismhas two important ramifications. In the single-molecule regime, the locally generated hy-perpolarization passively spreads and affectsall Cav channels along the flagellum. As aconsequence, the information at the site ofligand binding gets lost, arguing for a tem-poral rather than a spatial mechanism of gra-dient sensing. Furthermore, this mechanismmay provide a second stage of amplification,which needs to be taken into account whenthe overall signal amplification is assessed.

The molecular identity of the Ca2+ entrychannels in invertebrate sperm is not certain.


Ann

u. R

ev. P

hysi

ol. 2

008.

70:9

3-11

7. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by U

nive

rsity

of

Mas

sach

uset

ts -

Am

hers

t on

04/1

2/13

. For

per

sona

l use

onl

y.


The most negative value of Vm at which Cav

channels open in sperm (ca. −70 mV) is diag-nostic of low-threshold or T-type Cav chan-nels (53). Alternatively, a small window cur-rent carried by L-type Cav channels (53) maybe enhanced on hyperpolarization. Fragmentsencoding various L- and T-type Cav chan-nels have been identified in testis cDNA ofS. purpuratus (54) and A. punctulata (20). Thefuture biochemical identification of the Ca2+

entry channel, its immunocytochemical local-ization, and its physiological characterizationwill further our understanding of Ca2+ signal-ing in sperm.

Open questions. Whereas the main featuresof the excitatory signaling pathway have beenunveiled with fair precision, we are still inthe dark as to the mechanisms that termi-nate signaling and adjust the cell’s sensitiv-ity. In particular, the enzymes that phospho-rylate and dephosphorylate the GC as wellas their mechanisms of regulation are notknown. Furthermore, the regulation of PDEactivity and its contribution to cGMP dy-namics are unclear. In photoreceptors and ol-factory neurons, Ca2+ influx provides neg-ative feedback that terminates the responseand initiates adaptation. The action of Ca2+

on cellular targets is either direct or con-ferred by small Ca2+-binding proteins (35).Whether any of these feedback mechanismsoperates in sperm needs to be addressed in thefuture.

Sperm, Photoreceptors,and Olfactory Sensory Neurons:An Illuminating Kinship

Sperm and photoreceptors register singlemolecules and photons, respectively. A com-parison of both cell types illustrates variationson a common signaling scheme that confersabsolute sensitivity (Figure 3).

The capture of chemoattractants—at leaston the timescale of the chemotaxis response—and photons is irreversible. Both sperm androds rely on relatively long-lived receptor in-

Odorant receptors(ORs): Gprotein–coupledseven-helix proteinsfirst identified in theolfactory epithelium.The large family ofORs contains ∼350different members inhuman and ∼1000 inmice

G proteins:proteins that bindGTP, which activatesthe protein. Theintrinsic GTPaseactivity converts theGTP to GDP, whichinactivates theprotein

Adenylyl cyclase(AC): an enzymethat catalyzes theformation of thesignaling moleculecAMP (3′,5′-cyclicadenosinemonophosphate)from the nucleosidetriphosphate ATP.Has both soluble(sAC) andmembrane-spanning(tmAC) forms

termediates whose lifetime is controlled bydephosphorylation and phosphorylation, re-spectively. In rods and possibly also sperm, se-quential inactivation steps tightly control thereceptor lifetime and thereby produce uni-form elementary responses (36). Both spermand rods respond with an elementary hyper-polarization of 1–2 mV. In sperm, the open-ing of KCNG channels leads to hyperpolar-ization. In rods, nonselective CNG channelsthat are open in the dark close on light stim-ulation. The initial amplification of the signalis approximately 1000-fold smaller in spermcompared with rods. In rods, a single pho-ton stimulates the hydrolysis of approximately10,000 cGMP molecules (55); because ofthe relatively high free cGMP concentration(2 μM), the fractional change, however, issmall. In contrast, in sperm a single chemoat-tractant molecule stimulates the synthesis ofapproximately 5 cGMP molecules. Althoughthe free cGMP concentration in the flagellumis not known, it must be in the low-nanomolarrange to keep the fraction of open KCNGchannels small. In the flagellum (volume≤1.6 fl), one cGMP molecule equals a con-centration of 1 nM. Thus, an increase in thecGMP concentration by a few nanomolar issufficient to open a small fraction of KCNGchannels.

Functional parallels also exist between sig-naling in sperm and that of olfactory sensoryneurons (OSNs) (Figure 3). The binding ofodorants activates a cAMP-signaling pathwayin OSNs. Owing to the relatively low bindingaffinity, the odorant dwell time is very brief,and therefore the probability is low that anodorant receptor (OR) activates G proteinsin the cAMP-signaling pathway (56). Becauseof the short lifetime determined by liganddissociation, there is no need for a phosphory-lation/dephosphorylation mechanism to inac-tivate short-lived receptor intermediates. Fur-thermore, adenylyl cyclases (ACs), like GCs,display a low rate of cAMP synthesis. Thus,the first stage of signal amplification is proba-bly as low as in sperm. Both sperm and OSNsillustrate that a low initial amplification does


Ann

u. R

ev. P

hysi

ol. 2

008.

70:9

3-11

7. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by U

nive

rsity

of

Mas

sach

uset

ts -

Am

hers

t on

04/1

2/13

. For

per

sona

l use

onl

y.


a

GTP cGMP

GCOut

In

Chemoattractant

K+

KC

NG

Ca v

Ca2+

∆Vm PM

GMP cGMP

Out

In

Photon Na+ / Ca2+

Na+ / Ca2+

CN

G

PMDisk membraneDisk membraneDisk membrane

ATP cAMP

Out

In

Ca2+

CN

G

Cl–

chan

nel

chan

nel

chan

nel

chan

nel

chan

nel

Cl–

PM

Rh

PDET

Odorant

OR

Golf

AC

Figure 3Similarities and differences in the signaling of sperm, photoreceptor cells, and olfactory sensory neurons(OSNs). (a) Simplified models of the signaling pathways in sperm from marine invertebrates (top),vertebrate rod photoreceptors (middle), and OSNs (bottom). AC, adenylyl cyclase; CNG, cyclic nucleotidegated; GC, guanylyl cyclase; Golf, olfactory-specific G protein; OR, odorant receptor; PDE,phosphodiesterase; Rh, rhodopsin; T, transducin; PM, plasma membrane. (b) Scheme of a sperm, a rodphotoreceptor, and an OSN. The table highlights similarities and differences between the signalingpathways. R, receptor; G, G protein; E, enzyme.

not preclude an overall high sensitivity. Thegeneration of the electrical responses in bothsperm and OSNs involves two types of ionchannels (Figure 3). In sperm, a small hyper-polarizing step produced by the opening of a

few KCNG channels affects all Cav channelsalong the flagellum. In OSNs, the initial Ca2+

influx through CNG channels opens Ca2+-dependent Cl− channels, thereby amplifyingthe depolarizing response (57).


Ann

u. R

ev. P

hysi

ol. 2

008.

70:9

3-11

7. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by U

nive

rsity

of

Mas

sach

uset

ts -

Am

hers

t on

04/1

2/13

. For

per

sona

l use

onl

y.


bSperm

Rodphotoreceptor

Olfactorysensory neuron

Receptor

Affinity

Enzyme cascade

Enzyme activity

1. Amplification(activated G proteinsper receptor)

2. Amplification(turnover number)

3. Amplification(activation ofadditional channels)

Long-lived

High

Single-stage(R/E)

Low (GC)

Absent

Low(~5 s–1)

Present

***

***

Irreversible absorption of photon.Maximal forskolin-activated turnover number.Cooperativity of CNG channel activation may be considered as a third amplification stage.

Short-lived

Low

Two-stage(R→G→E)

Low (AC)

Low(<1)

Low**(<120 s–1)

Present

Long-lived

High*

Two-stage(R→G→E)

High (PDE)

High (<500)

High(~4000 s–1)

Absent***

~ ~

~

50 µ

mCilium

Rudimentarycilium

Cilia

35 µ

m

10 µ

m

Swimming Behavior

Ca2+ signal and motor response. Spermpossess a flagellum of approximately 50 μm inlength. Motor proteins in the axoneme pro-duce bending waves that propel the spermforward. In three dimensions, sea urchin

Axoneme: a bundleof microtubules andassociated proteinsthat forms the coreof eukaryotic flagellaand cilia and isresponsible for theirmovements

sperm swim on helical trajectories (58). Whenconfined to a shallow observation cham-ber, sperm from many species swim in cir-cles at the water-glass interface (59–61). Ahydrodynamic model of sperm (62; J. Elgeti& G. Gompper, unpublished) provides a


Ann

u. R

ev. P

hysi

ol. 2

008.

70:9

3-11

7. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by U

nive

rsity

of

Mas

sach

uset

ts -

Am

hers

t on

04/1

2/13

. For

per

sona

l use

onl

y.


Caged molecules:light-sensitive,inactive derivativesof molecules that canbe converted by aflash of UV light tothe free, active form

rationale for this peculiar behavior. A key partof the model is a short proximal segment thatdetermines the position of the head relative tothe direction and beating plane of the flagel-lum. The curvature of this segment is denotedas bending parameter. When the head is dis-placed from the beating plane so as to producea chiral shape, sperm swim on helical paths.When sperm approach a surface, a combina-tion of the chiral shape, forward thrust, andhydrodynamic repulsion between the flagel-lum and the surface results in circular trajec-tories. The bending parameter determines thecurvature of the swimming trajectory. Thus,steering of sperm can be envisaged as a mod-ulation of the bending parameter, but otherasymmetries in the flagellar waveform mayalso contribute. It will be important to re-construct experimentally the flagellar beat inthree dimensions in freely swimming sperm.

The motor response, in the absence ofa chemoattractant gradient, was recorded inmotile sperm after the rapid release of cGMPfrom caged cGMP inside the cell by a briefflash of UV light (19). On stimulation, thewaveform of the flagellar beat changes, andsperm exit the circular swimming mode. Thenew trajectory is characterized by alternatingperiods of higher curvature (turns or bends)and lower curvature (run), resulting in a loop-ing path. After some time, sperm resume theircircular swimming mode. Turns and runs donot reflect steering down or up a chemical gra-dient, respectively, but rather constitute a sin-gle response unit. With a laser-stroboscopictechnique, Bohmer et al. (22) showed that thecurvature fluctuations are triggered by Ca2+

spikes in the flagellum (Figure 2d ). Simi-lar fluctuations of [Ca2+] and curvature wereevoked by resact in A. punctulata (22) and bysperact in S. purpuratus (23, 25).

The trajectory closely mirrors the changesin waveform of the flagellar beat. An asym-metric waveform is associated with turns orbends in the trajectory, whereas more sym-metric beating results in a straighter swim-ming path. The mechanisms underlying theCa2+ control of ciliary beating are ill-defined

(for a review see Reference 63). It is, however,noteworthy that the curvature begins to relaxwhile [Ca2+]i is still elevated (22, 24), indicat-ing some kind of adaptive mechanism.

Bohmer et al. (22) proposed a turn-and-run model that captures essential stages ofthe motor response (Figure 2f ). Sperm sam-ple the concentration field continuously. Thesampling time necessary to elicit a response isnot known but may be similar to the inactiva-tion time of the GC. Sampling is followed bya delay period. The delay time for the Ca2+

response is accounted for by the rate of hyper-polarization and the time it takes Cav chan-nels to recover from inactivation (20). Finally,sperm turn when [Ca+]i commences to rise;this turn is followed by a run period up thegradient. The delay of the motor response isslightly longer than that of the Ca2+ response(19), perhaps because of the time required toreach a [Ca2+]i threshold and to transduce arise of [Ca2+]i into activity of motor proteins.

Navigation strategy. Sperm from severalmarine invertebrates and urochordata swim ina gradient of the chemoattractant on trajec-tories characterized by repetitive loops (12)(Figure 2a). Each loop consists of a turnand a run segment. The entire trajectory canbe considered as a string of turn-and-runepisodes. The turns can be tight and the runslong; alternatively, the difference in curvaturebetween turn and run can be small, resultingin a pattern of a drifting circle. The trajec-tory of the center of the drifting circle can belinear or curved (19, 22). In a gradient, the pe-riodic changes in chemoattractant concentra-tion and Ca2+ fluctuations are synchronizedwith a phase shift of approximately half a cir-cle (∼150◦) (Figure 2e), suggesting that theperiodic stimulation, owing to circular swim-ming, is transduced into a periodic modula-tion of the flagellar waveform and, thereby,also of the curvature of the trajectory. Circlesare approximately 50–60 μm in diameter. Atan average speed of approximately 200 μm s−1,it takes sperm approximately 1 s to completeone revolution. Because the frequency of Ca2+


Ann

u. R

ev. P

hysi

ol. 2

008.

70:9

3-11

7. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by U

nive

rsity

of

Mas

sach

uset

ts -

Am

hers

t on

04/1

2/13

. For

per

sona

l use

onl

y.


spikes roughly matches the angular frequencyof circular swimming, a single Ca2+ spike isgenerated per revolution.

Recently, Friedrich & Julicher (64) de-veloped a theoretical description of spermchemotaxis. While swimming on circularpaths in a concentration gradient, sperm en-counter a periodic stimulation by the chemo-attractant. The signaling pathway transfersthe stimulus into a periodic modulation of thepath curvature. There exists a characteristicphase shift between the periodic stimulationand the periodic modulation of trajectory cur-vature, which depends on the dynamics of thesignaling system. The phase shift determinesthe direction of the drifting circular trajecto-ries. The model precisely recapitulates char-acteristic features of sperms’ swimming be-havior. For example, the trajectories of thedrifting circles can be straight, curved, or spi-ral shaped, depending on the phase shift. Inthree dimensions, the swimming path is a benthelix whose axis aligns with the gradient, amechanism that is based on the idea of helicalklinotaxis proposed by Crenshaw (58, 65, 66)and Crenshaw & Edelstein-Keshet (67).

Stimulation by a uniform concentrationfield of chemoattractant or by a step increaseof cGMP generates a series of Ca2+ spikesand several successive loops, indicating thatthe system itself can undergo oscillations forbrief periods of time (19, 22, 23). In a naturalhabitat, gradients are shaped by eddies, con-vection, and drifting of eggs rather than byhomogeneous diffusion of chemoattractant.As a result, sperm encounter discontinuousgradients or patches of concentration fields(plumes) rather than smooth, steady gradi-ents. Trains of loops may be a means by whichto navigate in irregular concentration fields.A step increase may signal that an egg is closeby. A series of leaps may be a fitting strategyeither for a successful finish or for jumping be-tween patches of chemoattractant. For such amultijump mechanism to work, the sense ofdirection must be preserved during looping,i.e., when the angular frequency and the spikefrequency match. Future work is necessary to

Fallopian tubes:two narrow tubes,also known asoviducts, leadingfrom the ovaries offemale mammals tothe uterus. Theampulla region of thefallopian tube is thesite of fertilization

Capacitation:poorly understoodpriming process thatsperm must undergoin the femalereproductive tractbefore they arecompetent forfertilization

examine whether sperm show directed move-ments by using such a multijump mode.

Other Invertebrates

Chemotactic signaling and behavior are bothconserved and diverse among invertebrates.In starfish, peptides also serve as chemoat-tractants (22, 68, 69), and a GC is the re-ceptor (68). Stimulation of Asterias amuren-sis sperm by asterosap, the chemoattractant,rapidly elevates cGMP level and causes Ca2+

entry. Furthermore, stimulation by the re-lease of cGMP from caged cGMP producesturn-and-run episodes similar to those ob-served in sea urchin sperm (22). Amino acids(l-tryptophan) and peptides have been iden-tified as chemoattractants in sperm of abaloneand molluscs, respectively (70, 71). In ascidiansperm, a sulfated steroid serves as a chemo-tactic factor (72). The underlying signalingpathways are not known, although releaseof Ca2+ from intracellular stores has beenproposed as a possibility for ascidian sperm(73). In the nematode Caenorhabditis elegans,amoeboid sperm crawl to the spermatheca, aconvoluted tube where fertilization happens.Polyunsaturated fatty acids (PUFAs) are prob-ably precursors of as yet unknown signalingmolecules that are released from the egg topromote sperm migration to the spermathecaand enable fertilization (74).

SPERM GUIDANCEIN MAMMALS

Historical Perspective and Overview

Chemotaxis of mammalian sperm is a rel-atively recent entry in the literature (for areview see Reference 75). It was commonlybelieved that the large number of sperm ejacu-lated into the female genital tract compete in arace for fertilization of the egg. However, onlya few sperm succeed in entering the fallopiantubes (76), and even fewer can fertilize theegg. Sperm undergo a maturation process,capacitation, which is required to penetratethe layer of cumulus cells surrounding the egg


Ann

u. R

ev. P

hysi

ol. 2

008.

70:9

3-11

7. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by U

nive

rsity

of

Mas

sach

uset

ts -

Am

hers

t on

04/1

2/13

. For

per

sona

l use

onl

y.


Hyperactivation:motility patterncharacterized byvigorous,whiplash-likeflagellar beating

and to undergo the acrosomal reaction, whichenables sperm to trespass the egg coat and tofuse with the egg membrane (for a review seeReference 77). Only a small fraction of sperm(∼10% in humans) at any given time adoptthe capacitated state (78, 79). This calls for ef-fective mechanisms of sperm guidance to theegg. Approximately 15 years ago, two labora-tories established that human follicular fluidcontains substances that cause sperm chemo-taxis in vitro (80–82). Thus the concept ofsperm chemotaxis in mammals found its roots(Figure 4).

Chemotaxis of mammalian sperm is prob-ably more complex than that of marine inver-tebrates. Sperm guidance may involve morethan one chemoattractant and also thermo-taxis (83). Both the oocyte and cumulus cellssecrete chemoattractants (84). The hydro-

dynamics encountered by sperm in the fal-lopian tube are different from that in theocean. To reach the egg, sperm have to“crawl” through a viscous environment onthe convoluted surface of epithelial cell lay-ers. Most importantly, two functional statesof sperm—capacitation and hyperactivation(for a review see Reference 85)—complicatethe study of chemotaxis considerably. Be-cause only the small fraction of capacitatedsperm are chemotactically responsive (78),the signal-to-noise ratio of behavioral assaysis low. Hyperactivation is characterized by avigorous whiplash-like beat of the flagellum.This swimming mode interrupts progressiveforward movement, and as a result, sperm be-come trapped in a small area, a process thatcould be mistaken as accumulation by truechemotaxis. Finally, many of the molecules

Ovary

Cumulus cell

Egg

Chemotaxis

Storagesite

Uterus (7–9 cm)105 sperm

Isthmus (2–3 cm)

Ampulla

(5–8 c

m)

Oviduct

Cervix (2–3 cm)106 sperm

104 sperm102 –103

sperm

Vagina (7–9 cm)107 sperm

FertilizationFertilizationsitesite

Fertilizationsite

Figure 4The mammalian female genital tract. The scheme, the dimensions of organs, and the sperm numbers arederived from studies in humans. Only a small fraction of the ejaculated sperm enter the storage site in theoviduct. Here, again, a small portion of the sperm population undergoes capacitation, which enablessperm to fertilize the egg at the fertilization site. The egg and the surrounding cumulus cells secretechemoattractants, which guide sperm to the egg-cumulus complex. Modified after Reference 75.


Ann

u. R

ev. P

hysi

ol. 2

008.

70:9

3-11

7. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by U

nive

rsity

of

Mas

sach

uset

ts -

Am

hers

t on

04/1

2/13

. For

per

sona

l use

onl

y.


involved in chemotaxis—like progesterone,cAMP, and Ca2+—have also been implicatedin capacitation, hyperactivation, and the acro-somal reaction. It is a formidable task to ex-perimentally dissect the different cellular re-sponses and the underlying pathways.

Chemoattractants and Receptors:One Too Many?

Given the complexities outlined above, itis not surprising that a multitude of sub-stances have been reported to cause chemo-taxis (for a review see Reference 75), andinvestigators have identified up to 32 recep-tors that—in principle—may serve as recep-tors for chemoattractants (for a review seeReference 86). Although the arduous journeyof sperm along the fallopian tube may proceedin several stages, each employing a differentchemoattractant, it is unlikely that more thana few different chemoattractants participate.We restrict the following discussion to thosechemoattractants that have garnered major at-tention in recent years.

Odorants. ORs are expressed ectopically ina variety of tissues other than the olfactory ep-ithelium (87). Transcripts of ORs were iden-tified in male germ cells (88), and an OR wasdetected on dog sperm (89). Subsequent re-ports revealed a repertoire of ORs in testisand sperm (90–94). In the nose, each OSNexpresses only one type of receptor (95, 96).It will be interesting to know whether individ-ual sperm display distinct profiles of receptorexpression. Spehr et al. (97, 98) identified ahitherto-undescribed receptor, hOR17-4 (orOR1D2), in both olfactory epithelium andsperm. A floral scent—bourgeonal—acts as anagonist of this receptor, and behavioral anal-ysis showed it to be a chemoattractant. Bour-geonal stimulates Ca2+ responses in a fractionof human sperm. An inhibitor of transmem-brane adenylyl cyclases (tmACs) blocked boththe Ca2+ signal and the chemotactic response.Mass spectrometry and immunohistochem-istry led to the identification of various iso-

Epididymis: inmammals, a narrow,tightly coiled tubeconnecting eachtesticle to its vasdeferens. Duringtheir transit throughthe epididymis,sperm undergomaturation processesnecessary to acquirethe ability to swimforward

forms of tmACs (99–101). In a similar vein,mOR23—a mouse OR—is expressed in roundspermatids (102). Lyral, another floral scent,is a ligand of mOR23, and stimulation with ei-ther lyral or membrane-permeable analogs ofcAMP causes an increase in [Ca2+]i in epididy-mal sperm. Behavioral experiments suggestedthat mouse sperm perform Ca2+-mediatedchemotaxis in a gradient of lyral. Given thelack of species specificity of chemoattractantsfor some mammals (103), it is surprising thatthe ORs from human and mouse sperm arenonorthologous proteins and that bourgeonaland lyral act species specifically. Several othercomponents of signaling pathways character-istic of sensory cells, including receptor pro-tein kinases, the G proteins Golf and gust-ducin, and the stop protein β-arrestin, havebeen localized to sperm (90, 101, 104, 105).Altogether, these studies suggest the follow-ing cAMP-signaling scheme of mammaliansperm chemotaxis:

Ligands → OR → Golf → tmAC

→ cAMP → [Ca2+]i → motor

response → chemotaxis.

As conspicuous as this signaling scheme is,we caution against a rash interpretation. Im-portant pieces of information are still miss-ing, and several inconsistencies require clar-ification. First, the genuine ligands of spermORs need to be identified. Second, in manystudies (see, however, References 89, 90, 104),ORs have been localized to progenitor germcells by sensitive molecular biological tech-niques. Because low levels of OR transcriptsmay originate from “illegitimate” transcrip-tion, it is advisable to establish the functionalexpression of OR proteins by biochemical orimmunocytochemical means. Third, the spe-cific components that constitute this cAMP-signaling pathway—G proteins, tmAC iso-forms, and Ca2+ entry channels—have notbeen unequivocally identified. Several or evenall nine isoforms of tmACs reportedly existin human sperm (99–101), yet other com-pelling findings argue that sperm harbor only


Ann

u. R

ev. P

hysi

ol. 2

008.

70:9

3-11

7. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by U

nive

rsity

of

Mas

sach

uset

ts -

Am

hers

t on

04/1

2/13

. For

per

sona

l use

onl

y.


the soluble bicarbonate-sensitive form of AC(sAC) (106–109) and lack Gs proteins that ac-tivate tmACs (106). Moreover, the expressionpattern of tmAC isoforms probed by immuno-cytochemistry (99–101) as well as the reportedand expected Mw are vastly different (99).Fourth, cAMP does not directly stimulateCa2+ entry but enhances Ca2+ signals evokedby depolarization (110). Moreover, mice thatlack genes for potential cAMP targets, e.g.,CNG channels, do not exhibit a fertiliza-tion phenotype (111, 112). Fifth, as Eisenbach& Giojalas (75) pointed out, the fraction ofhuman sperm responding chemotactically tobourgeonal (≥90%), the fraction of cells thatrespond with an elevation of [Ca2+]i (∼30%),and the fraction of cells that presumably arecapacitated (∼10%) do not match (97, 98). Fi-nally, the dependence of chemotaxis on bour-geonal concentration is not bell-shaped. Thiscriterion relies on the fact that, when saturatedwith chemoattractants, receptors cannot sig-nal any further change in concentration, andas a result the chemotaxis response drops (75).

Progesterone. Progesterone is secreted bycumulus cells in the female reproductivetract and is present at micromolar concen-trations in the direct vicinity of an ovu-lated egg (113). The radial distribution ofprogesterone-producing cumulus cells mayhelp to form a gradient from the centerto the periphery of the cumulus cell mass(114). Progesterone was long ago proposedas a chemoattractant (115–117). Initial stud-ies to establish progesterone as a chemoattrac-tant gave mixed results (for a discussion seeReference 118). Recently, using rigorous cri-teria, Teves et al. (114) showed that for humanand rabbit sperm progesterone is a chemoat-tractant at picomolar but not nanomolar con-centrations. Chemotaxis was observed in 8%of the sperm population, in agreement withthe fraction of capacitated sperm. Chemotac-tic activity of progesterone for both humanand rabbit sperm is not surprising becausechemotactic factors originating from the fol-

licular fluid of different species are identicalor closely related to each other (103).

The progesterone-signaling pathway me-diating the chemotactic response is unknown.A progesterone gradient in the nanomolar tomicromolar range generates a steady increasein [Ca2+]i superimposed by slow Ca2+ oscilla-tions in the neck region of sperm (119, 120).The Ca2+ oscillations alter the flagellar beat.The steady and dynamic components of theCa2+ signal seem to involve Ca2+ influx andrelease from intracellular stores, respectively(120). Whether Ca2+ dynamics control spermchemotaxis in a progesterone gradient is notknown (for a review see Reference 121). Thelowest progesterone concentration (∼1 nM)that elicits a noticeable Ca2+ response in mostsperm, and not only in the fraction of capac-itated sperm (119), is much higher than therange of concentrations at which chemotaxisoccurs (114).

Human sperm harbor both classic (ge-nomic) and unconventional (nongenomic)progesterone receptors (122). The rise of[Ca2+]i and the chemotactic response are be-lieved to be mediated via nongenomic pro-gesterone receptors on the plasma membrane(for a review see Reference 113). A bewilder-ing variety of candidate membrane receptorshave been described for the nongenomic ac-tion of progesterone (123). Moreover, in othertissues several ion channels are regulated bydirect binding of steroids (124, 125). Thus,ion channels as progesterone receptors alsoshould be considered in the case of sperm.

Other candidates. Allurin, a 21-kDa spermchemoattractant from eggs of the Africanclawed frog Xenopus laevis, shares homologywith the family of mammalian cysteine-richsecretory proteins (CRISP) (126, 127). X.laevis allurin has now been shown to bind tomouse sperm and to act as a mouse spermchemoattractant as well (D.E. Chandler, per-sonal communication).

Rantes (regulated upon activation normalT cell expressed and secreted) is a smallprotein of 68 amino acids. This chemokine is


Ann

u. R

ev. P

hysi

ol. 2

008.

70:9

3-11

7. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by U

nive

rsity

of

Mas

sach

uset

ts -

Am

hers

t on

04/1

2/13

. For

per

sona

l use

onl

y.


a chemoattractant for eosinophils, monocytes,and T lymphocytes (128, 129; for a review seeReference 130) as well as for human sperm(131). In somatic cells, various chemokinereceptors, e.g., CCR1, CCR3, CCR4, andCCR5, mediate the chemotactic activity ofRantes (132). The receptors CCR3 and CCR5have also been identified on human sperm(133). The signaling pathway is unknown.

Nitric oxide (NO) controls diverse physio-logical functions such as neuronal communi-cation, host defense, and vascular regulation.NO induces human sperm chemotaxis invitro (134). The proposed signaling path-way involves the activation of soluble GC,synthesis of cGMP, and subsequent activa-tion of cGMP-dependent protein kinases.In the female reproductive tract, cumuluscells produce significant amounts of NO(135). Further experiments are warranted totest the physiological significance of theseobservations.

Proteomics. Recently, the proteome of hu-man sperm has been analyzed (136). Surpris-ingly, many membrane receptors that havebeen reported to exist in sperm were not con-firmed by this analysis. For example, the OROR2AE1 and tmAC VIII have been identi-fied, but not OR1D2 or all other tmAC iso-forms. However, no peptides from the sperm-

specific CatSper ion channels (137–139) andNHE exchanger (140) that have been locatedunequivocally to sperm were detected. Thisresult underscores a caveat that some proteinsare missed in proteomic studies.

PERSPECTIVE

Although a coherent picture of chemotaxisin sea urchin sperm is emerging in quantita-tive detail, our understanding of mammaliansperm chemotaxis is still rudimentary, if notcontroversial. However, we anticipate that inthe years to come, the experimental strate-gies developed for sea urchin sperm will findtheir way into the study of mammalian sperm.Furthermore, the ability to electrophysiolog-ically record from mammalian sperm withthe patch-clamp technique (141) will greatlyadvance our understanding of sperm sig-naling. Finally, several sperm-specific iso-forms of important molecules—CatSper ionchannels (137–139), the Na+/H+ exchanger(140), sAC (107–109), and protein kinase A(142)—have been identified, and deletion ofthe genes encoding these molecules givesrise to characteristic fertilization or motil-ity phenotypes. Eventually, these studies willdelineate the signaling mechanisms for ca-pacitation, hyperactivation, chemotaxis, andthermotaxis.

SUMMARY POINTS

1. The study of chemotaxis is most advanced in sperm of the sea urchin Arbacia punctulata,in which the chemoattractant activates a cGMP-signaling pathway via a receptor-typeguanylyl cyclase.

2. Sperm of A. punctulata are exquisitely sensitive: They can detect a single moleculeof chemoattractant and transduce the binding into an elementary voltage and Ca2+

response.

3. The Ca2+ signals control the flagellar beat and adjust the swimming trajectory of thesperm in a gradient of the chemoattractant.

4. Functional parallels exist between signaling in sperm, photoreceptor cells, and olfac-tory sensory neurons.

5. Chemotaxis of mammalian sperm is still poorly understood and probably more com-plex than that of marine invertebrates.


Ann

u. R

ev. P

hysi

ol. 2

008.

70:9

3-11

7. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by U

nive

rsity

of

Mas

sach

uset

ts -

Am

hers

t on

04/1

2/13

. For

per

sona

l use

onl

y.


FUTURE ISSUES

1. Molecular identification and functional characterization of further components of thechemotactic signaling pathway in sea urchin sperm, such as the Ca2+ entry channel(s)and the phosphodiesterase(s), are necessary.

2. Sea urchin sperm respond to a single molecule of chemoattractant and, at the sametime, can respond to chemoattractant concentrations over six orders of magnitude.How does the sperm cell adjust its sensitivity?

3. A challenging task is to reconstruct experimentally the flagellar beat in three dimen-sions in freely swimming sperm.

4. For mammals, several candidates have been proposed as chemoattractants. Do mam-malian sperm respond to different chemoattractants during their journey through thefemale reproductive tract? If not, what is the single genuine chemoattractant? Howdo chemo- and thermotaxis each contribute to sperm guidance?

5. In mammalian sperm, very little is known about the chemotactic signaling pathway.Which intracellular messenger molecules are involved, and how do they regulate theswimming behavior?

6. Questions remain on the relationship between chemotaxis, hyperactivation, and ca-pacitation with respect to their individual physiological roles, timing, and molecularmechanisms.

DISCLOSURE STATEMENT

The authors are not aware of any biases that might be perceived as affecting the objectivity ofthis review.

ACKNOWLEDGMENTS

This work was supported by the Deutsche Forschungsgemeinschaft and the Fonds der Chemis-chen Industrie. We thank A. Eckert and H.D. Grammig for help in preparing the manuscriptand figures. We thank Dr. Larry Cohen (Yale University) for his reading of the manuscript.

LITERATURE CITED

1. Lillie FR. 1912. The production of sperm iso-agglutinine by ova. Science 360:527–302. Brokaw CJ. 1990. The sea urchin spermatozoon. BioEssays 12:449–523. Hansbrough JR, Garbers DL. 1981. Speract—purification and characterization of a

peptide associated with eggs that activates spermatozoa. J. Biol. Chem. 256:1447–524. Shimomura H, Dangott LJ, Garbers DL. 1986. Covalent coupling of a resact analogue

to guanylate cyclase. J. Biol. Chem. 261:15778–82

5. The firstmolecularidentification of amembrane-boundreceptor-typeguanylyl cyclase(see, however, thesubsection titledThe Receptor,above).

5. Singh S, Lowe DG, Thorpe DS, Rodriguez H, Kuang W-J, et al. 1988. Membraneguanylate cyclase is a cell-surface receptor with homology to protein kinases.Nature 334:708–12

6. The first reportof animal spermchemotaxis inresponse to adefinedegg-derivedmolecule.

6. Ward GE, Brokaw CJ, Garbers DL, Vacquier VD. 1985. Chemotaxis of Arbacia

punctulata spermatozoa to resact, a peptide from the egg jelly layer. J. Cell Biol.

101:2324–29


Ann

u. R

ev. P

hysi

ol. 2

008.

70:9

3-11

7. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by U

nive

rsity

of

Mas

sach

uset

ts -

Am

hers

t on

04/1

2/13

. For

per

sona

l use

onl

y.


7. Brokaw CJ, Josslin R, Bobrow L. 1974. Calcium ion regulation of flagellar beat symmetryin reactivated sea urchin spermatozoa. Biochem. Biophys. Res. Commun. 58:795–800

8. Brokaw CJ. 1979. Calcium-induced asymmetrical beating of triton-demembranated seaurchin sperm flagella. J. Cell Biol. 82:401–11

9. Miller RL, Brokaw CJ. 1970. Chemotactic turning behaviour of Tubularia spermatozoa.J. Exp. Biol. 52:699–706

10. Highlights thatsperm chemotaxisis widespread in theanimal kingdom.

10. Miller RL. 1975. Chemotaxis of the spermatozoa of Ciona intestinalis. Nature

254:244–4511. Miller RL. 1977. Chemotactic behavior of the sperm of chitons (Mollusca: Polypla-

cophora). J. Exp. Zool. 202:203–1112. Miller RL. 1985. Sperm chemo-orientation in the metazoa. In Biology of Fertilization,

Vol. 2: Biology of the Sperm, ed. CB Metz, A Monroy, pp. 275–337. New York: Academic13. Cook SP, Babcock DF. 1993. Selective modulation by cGMP of the K+ channel activated

by speract. J. Biol. Chem. 268:22402–714. Cook SP, Babcock DF. 1993. Activation of Ca2+ permeability by cAMP is coordinated

through the pHi increase induced by speract. J. Biol. Chem. 268:22408–1315. Cook SP, Brokaw CJ, Muller CH, Babcock DF. 1994. Sperm chemotaxis: Egg peptides

control cytosolic calcium to regulate flagellar responses. Dev. Biol. 165:10–1916. Darszon A, Labarca P, Nishigaki T, Espinosa F. 1999. Ion channels in sperm physiology.

Physiol. Rev. 79:481–51017. Darszon A, Beltran C, Felix R, Nishigaki T, Trevino CL. 2001. Ion transport in sperm

signaling. Dev. Biol. 240:1–1418. Shiba K, Ohmuro J, Mogami Y, Nishigaki T, Wood CD, et al. 2005. Sperm-activating

peptide induces asymmetric flagellar bending in sea urchin sperm. Zool. Sci. 22:293–99

19, 20. Evidencethat in sea urchinsperm even a singlemolecule ofchemoattractantcan initiate acGMP-signalingpathway: Elevationof cGMP leads to ahyperpolarizationcaused by theopening ofK+-selectivecGMP-gated ionchannels, followedby Ca2+ entry.

19. Kaupp UB, Solzin J, Hildebrand E, Brown JE, Helbig A, et al. 2003. The signalflow and motor response controling chemotaxis of sea urchin sperm. Nat. Cell

Biol. 5:109–1720. Strunker T, Weyand I, Bonigk W, Van Q, Loogen A, et al. 2006. A K+-selective

cGMP-gated ion channel controls chemosensation of sperm. Nat. Cell Biol.

8:1149–5421. Gauss R, Seifert R, Kaupp UB. 1998. Molecular identification of a hyperpolarization-

activated channel in sea urchin sperm. Nature 393:583–87

22. Shows thatCa2+ spikes in theflagellum controlchemotaxis ofinvertebrate sperm.

22. Bohmer M, Van Q, Weyand I, Hagen V, Beyermann M, et al. 2005. Ca2+ spikesin the flagellum control chemotactic behavior of sperm. EMBO J. 24:2741–52

23. Wood CD, Darszon A, Whitaker M. 2003. Speract induces calcium oscillations in thesperm tail. J. Cell Biol. 161:89–101

24. Wood CD, Nishigaki T, Furuta T, Baba SA, Darszon A. 2005. Real-time analysis of therole of Ca2+ in flagellar movement and motility in single sea urchin sperm. J. Cell Biol.169:725–31

25. Wood CD, Nishigaki T, Tatsu Y, Yumoto N, Baba SA, et al. 2007. Altering the speract-induced ion permeability changes that generate flagellar Ca2+ spikes regulates theirkinetics and sea urchin sperm motility. Dev. Biol. 306:525–37

26. Solzin J, Helbig A, Van Q, Brown JE, Hildebrand E, et al. 2004. Revisiting the role ofH+ in chemotactic signaling of sperm. J. Gen. Physiol. 124:115–24

27. Darszon A, Acevedo JJ, Galindo BE, Hernandez-Gonzalez EO, Nishigaki T, et al. 2006.Sperm channel diversity and functional multiplicity. Reproduction 131:977–88

28. Kaupp UB, Hildebrand E, Weyand I. 2006. Sperm chemotaxis in marine invertebrates—molecules and mechanisms. J. Cell. Physiol. 208:487–94


Ann

u. R

ev. P

hysi

ol. 2

008.

70:9

3-11

7. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by U

nive

rsity

of

Mas

sach

uset

ts -

Am

hers

t on

04/1

2/13

. For

per

sona

l use

onl

y.


29. Wedel BJ, Garbers DL. 2001. The guanylyl cyclase family at Y2K. Annu. Rev. Physiol.63:215–33

30. Ward GE, Vacquier VD. 1983. Dephosphorylation of a major sperm membrane proteinis induced by egg jelly during sea urchin fertilization. Proc. Natl. Acad. Sci. USA 80:5578–82

31. Suzuki N, Shimomura H, Radany EW, Ramarao CS, Ward GE, et al. 1984. A peptideassociated with eggs causes a mobility shift in a major plasma membrane protein ofspermatozoa. J. Biol. Chem. 259:14874–79

32. Ward GE, Moy GW, Vacquier VD. 1986. Dephosphorylation of sea urchin spermguanylate cyclase during fertilization. Adv. Exp. Med. Biol. 207:359–82

33. Potter LR, Hunter T. 1998. Identification and characterization of the major phospho-rylation sites of the B-type natriuretic peptide receptor. J. Biol. Chem. 273:15533–39

34. Potter LR, Hunter T. 1998. Phosphorylation of the kinase homology domain is essentialfor activation of the A-type natriuretic peptide receptor. Mol. Cell. Biol. 18:2164–72

35. Pugh EN Jr, Lamb TD. 2000. Phototransduction in vertebrate rods and cones: molecu-lar mechanisms of amplification, recovery and light adaptation. In Handbook of BiologicalPhysics, ed. DG Stavenga, WJ DeGrip, EN Pugh Jr, 3:183–255. North-Holland: ElsevierScience B.V.

36. Doan T, Mendez A, Detwiler PB, Chen J, Rieke F. 2006. Multiple phosphorylationsites confer reproducibility of the rod’s single-photon responses. Science 313:530–33

37. Shimomura H, Garbers DL. 1986. Differential effects of resact analogues on spermrespiration rates and cyclic nucleotide concentrations. Biochemistry 25:3405–10

38. Suzuki N, Yoshino K, Kurita M, Nomura K, Yamaguchi M. 1988. A novel group ofsperm activating peptides from the sea urchin Glyptocidaris crenularis. Comp. Biochem.Physiol. 90:305–11

39. Bentley JK, Tubb DJ, Garbers DL. 1986. Receptor-mediated activation of spermatozoanguanylate cyclase. J. Biol. Chem. 261:14859–62

40. Hwang J-Y, Lange C, Helten A, Hoppner-Heitmann D, Duda T, et al. 2003. Regulatorymodes of rod outer segment membrane guanylate cyclase differ in catalytic efficiencyand Ca2+-sensitivity. Eur. J. Biochem. 270:3814–21

41. Babcock DF, Bosma MM, Battaglia DE, Darszon A. 1992. Early persistent activationof sperm K+ channels by the egg peptide speract. Proc. Natl. Acad. Sci. USA 89:6001–5

42. Beltran C, Zapata O, Darszon A. 1996. Membrane potential regulates sea urchin spermadenylylcyclase. Biochemistry 35:7591–98

43. Galindo BE, Beltran C, Cragoe EJ Jr, Darszon A. 2000. Participation of a K+ channelmodulated directly by cGMP in the speract-induced signaling cascade of Strongylocen-trotus purpuratus sea urchin sperm. Dev. Biol. 221:285–94

44. Ebner TJ, Chen G. 1995. Use of voltage-sensitive dyes and optical recordings in thecentral nervous system. Progr. Neurobiol. 46:463–506

45. Galindo BE, de la Vega-Beltran JL, Labarca P, Vacquier VD, Darszon A. 2007. Sp-tetraKCNG: a novel cyclic nucleotide gated K+ channel. Biochem. Biophys. Res. Commun.354:668–75

46. Heginbotham L, Lu Z, Abramson T, MacKinnon R. 1994. Mutations in the K+ channelsignature sequence. Biophys. J. 66:1061–67

47. Galindo BE, Neill AT, Vacquier VD. 2005. A new hyperpolarization-activated, cyclicnucleotide-gated channel from sea urchin sperm flagella. Biochem. Biophys. Res. Commun.334:96–101

48. Pape H-C. 1996. Queer current and pacemaker: The hyperpolarization-activated cationcurrent in neurons. Annu. Rev. Physiol. 58:299–327


Ann

u. R

ev. P

hysi

ol. 2

008.

70:9

3-11

7. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by U

nive

rsity

of

Mas

sach

uset

ts -

Am

hers

t on

04/1

2/13

. For

per

sona

l use

onl

y.


49. Kaupp UB, Seifert R. 2001. Molecular diversity of pacemaker ion channels. Annu. Rev.Physiol. 63:235–57

50. Robinson RB, Siegelbaum SA. 2003. Hyperpolarization-activated cation currents: frommolecules to physiological function. Annu. Rev. Physiol. 65:453–80

51. Proenza C, Tran N, Angoli D, Zahynacz K, Balcar P, Accili EA. 2002. Different roles forthe cyclic nucleotide binding domain and amino terminus in assembly and expression ofhyperpolarization-activated, cyclic nucleotide-gated channels. J. Biol. Chem. 277:29634–42

52. Shin KS, Maertens C, Proenza C, Rothberg BS, Yellen G. 2004. Inactivation in HCNchannels results from reclosure of the activation gate: desensitization to voltage. Neuron41:737–44

53. Perez-Reyes E. 2003. Molecular physiology of low-voltage-activated T-type calciumchannels. Physiol. Rev. 83:117–61

54. Granados-Gonzalez G, Mendoza-Lujambio I, Rodriguez E, Galindo BE, Beltran C,Darszon A. 2005. Identification of voltage-dependent Ca2+ channels in sea urchin sperm.FEBS Lett. 579:6667–72

55. Rodieck RW. 1998. The First Steps in Seeing. Sunderland, MA: Sinauer

56. Evidence thatsignal amplificationin olfactorytransduction isfundamentallydifferent from thatof photo-transduction.

56. Bhandawat V, Reisert J, Yau K-W. 2005. Elementary response of olfactory recep-tor neurons to odorants. Science 308:1931–34

57. Lowe G, Gold GH. 1993. Nonlinear amplification by calcium-dependent chloride chan-nels in olfactory receptor cells. Nature 366:283–86

58. Crenshaw HC. 1996. A new look at locomotion in microorganisms: rotating and trans-lating. Am. Zool. 36:608–18

59. Rothschild L. 1963. Non-random distribution of bull spermatozoa in a drop of spermsuspension. Nature 198:1221–22

60. Woolley DM. 2003. Motility of spermatozoa at surfaces. Reproduction 126:259–7061. Cosson J, Huitorel P, Gagnon C. 2003. How spermatozoa come to be confined to

surfaces. Cell Motil. Cytoskelet. 54:56–6362. Elgeti J. 2006. Sperm and cilia dynamics. Thesis. Univ. Koln63. Salathe M. 2007. Regulation of mammalian ciliary beating. Annu. Rev. Physiol. 69:401–2264. Friedrich BM, Julicher F. 2007. Chemotaxis of sperm cells. Proc. Natl. Acad. Sci. USA

104:13256–6165. Crenshaw HC. 1993. Orientation by helical motion—I. Kinematics of the helical motion

of organisms with up to six degrees of freedom. Bull. Math. Biol. 55:197–21266. Crenshaw HC. 1993. Orientation by helical motion—III. Microorganisms can orient to

stimuli by changing the direction of their rotational velocity. Bull. Math. Biol. 55:231–5567. Crenshaw HC, Edelstein-Keshet L. 1993. Orientation by helical motion—II. Changing

the direction of the axis of motion. Bull. Math. Biol. 55:213–3068. Matsumoto M, Solzin J, Helbig A, Hagen V, Ueno S-I, et al. 2003. A sperm-activating

peptide controls a cGMP-signaling pathway in starfish sperm. Dev. Biol. 260:314–2469. Miller RL, Vogt R. 1996. An N-terminal partial sequence of the 13 kDa Pycnopodia

helianthoides sperm chemoattractant ‘startrak’ possesses sperm-attracting activity. J. Exp.Biol. 199:311–18

70. Riffell JA, Krug PJ, Zimmer RK. 2002. Fertilization in the sea: the chemical identity ofan abalone sperm attractant. J. Exp. Biol. 205:1439–50

71. Zatylny C, Marvin L, Gagnon J, Henry J. 2002. Fertilization in Sepia officinalis: the firstmollusk sperm-attracting peptide. Biochem. Biophys. Res. Commun. 296:1186–93

72. Yoshida M, Murata M, Inaba K, Morisawa M. 2002. A chemoattractant for ascidianspermatozoa is a sulfated steroid. Proc. Natl. Acad. Sci. USA 99:14831–36


Ann

u. R

ev. P

hysi

ol. 2

008.

70:9

3-11

7. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by U

nive

rsity

of

Mas

sach

uset

ts -

Am

hers

t on

04/1

2/13

. For

per

sona

l use

onl

y.


73. Yoshida M, Ishikawa M, Izumi H, De Santis R, Morisawa M. 2003. Store-operatedcalcium channel regulates the chemotactic behavior of ascidian sperm. Proc. Natl. Acad.Sci. USA 100:149–54

74. Kubagawa HM, Watts JL, Corrigan C, Edmonds JW, Sztul E, et al. 2006. Oocyte signalsderived from polyunsaturated fatty acids control sperm recruitment in vivo. Nat. CellBiol. 8:1144–48

75. Comprehensivereview of themechanisms thatguide mammaliansperm to the egg.

75. Eisenbach M, Giojalas LC. 2006. Sperm guidance in mammals—an unpaved roadto the egg. Nat. Rev. Mol. Cell Biol. 7:276–85

76. Williams M, Hill CJ, Scudamore I, Dunphy B, Cooke ID, Barratt CL. 1991. Spermnumbers and distribution within the human fallopian tube around ovulation. Hum.Reprod. 8:2019–26

77. Brewis IA, Moore HD, Fraser LR, Holt WV, Baldi E, et al. 2005. Molecular mechanismsduring sperm capacitation. Hum. Fertil. 8:253–61

78. Cohen-Dayag A, Tur-Kaspa I, Dor J, Mashiach S, Eisenbach M. 1995. Sperm capacita-tion in humans is transient and correlates with chemotactic responsiveness to follicularfactors. Proc. Natl. Acad. Sci. USA 92:11039–43

79. Jaiswal BS, Eisenbach M. 2002. Capacitation. In Fertilization, ed. DM Hardy, pp. 57–117. San Diego: Academic

80. Villanueva-Diaz C, Vadillo-Ortega F, Kably-Ambe A, de los Angeles Diaz-Perez M,Krivitzky SK. 1990. Evidence that human follicular fluid contains a chemoattractant forspermatozoa. Fertil. Steril. 54:1180–82

81. Shows thathuman spermaccumulate infollicular fluid andthat thisaccumulationcorrelates with theability of the egg tobe fertilized.

81. Ralt D, Goldenberg M, Fetterolf P, Thompson D, Dor J, et al. 1991. Spermattraction to a follicular factor(s) correlates with human egg fertilizability. Proc.

Natl. Acad. Sci. USA 88:2840–4482. Ralt D, Manor M, Cohen-Dayag A, Tur-Kaspa I, Ben-Shlomo I, et al. 1994. Chemotaxis

and chemokinesis of human spermatozoa to follicular factors. Biol. Reprod. 50:774–8583. Bahat A, Tur-Kaspa I, Gakamsky A, Giojalas LC, Breitbart H, Eisenbach M. 2003.

Thermotaxis of mammalian sperm cells: a potential navigation mechanism in the femalegenital tract. Nat. Med. 9:149–50

84. Sun F, Bahat A, Gakamsky A, Girsh E, Katz N, et al. 2005. Human sperm chemotaxis:Both the oocyte and its surrounding cumulus cells secrete sperm chemoattractants. Hum.Reprod. 20:761–67

85. Marquez B, Suarez SS. 2004. Different signaling pathways in bovine sperm regulatecapacitation and hyperactivation. Biol. Reprod. 70:1626–33

86. Naz RK, Sellamuthu R. 2006. Receptors in spermatozoa: Are they real? J. Androl.27:627–36

87. Feldmesser E, Olender T, Khen M, Yanai I, Ophir D, Lancet D. 2006. Widespreadectopic expression of olfactory receptor genes. BMC Genomics 7:121–38

88. The firstevidence thatmembers of theolfactory receptorgene family areexpressed in malegerm cells.

88. Parmentier M, Libert F, Schurmans S, Schiffmann S, Lefort A, et al. 1992. Ex-pression of members of the putative olfactory receptor gene family in mammaliangerm cells. Nature 355:453–55

89. Vanderhaeghen P, Schurmans S, Vassart G, Parmentier M. 1993. Olfactory receptorsare displayed on dog mature sperm cells. J. Cell Biol. 123:1441–52

90. Walensky LD, Roskams AJ, Lefkowitz RJ, Snyder SH, Ronnett GV. 1995. Odorantreceptors and desensitization proteins colocalize in mammalian sperm. Mol. Med. 1:130–41

91. Asai H, Kasai H, Matsuda Y, Yamazaki N, Nagawa F, et al. 1996. Genomic structure andtranscription of a murine odorant receptor gene: differential initiation of transcriptionin the olfactory and testicular cells. Biochem. Biophys. Res. Commun. 221:240–47


Ann

u. R

ev. P

hysi

ol. 2

008.

70:9

3-11

7. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by U

nive

rsity

of

Mas

sach

uset

ts -

Am

hers

t on

04/1

2/13

. For

per

sona

l use

onl

y.


92. Vanderhaeghen P, Schurmans S, Vassart G, Parmentier M. 1997. Specific repertoire ofolfactory receptor genes in the male germ cells of several mammalian species. Genomics39:239–46

93. Linardopoulou E, Mefford HC, Nguyen O, Friedman C, van den Engh G, et al. 2001.Transcriptional activity of multiple copies of a subtelomerically located olfactory recep-tor gene that is polymorphic in number and location. Hum. Mol. Genet. 10:2373–83

94. Fukuda N, Touhara K. 2006. Developmental expression patterns of testicular olfactoryreceptor genes during mouse spermatogenesis. Genes Cells 11:71–81

95. Vassar R, Ngai J, Axel R. 1993. Spatial segregation of odorant receptor expression inthe mammalian olfactory epithelium. Cell 74:309–18

96. Ressler KJ, Sullivan SL, Buck LB. 1994. A molecular dissection of spatial patterning inthe olfactory system. Curr. Opinion Neurobiol. 4:588–96

97. Spehr M, Gisselmann G, Poplawski A, Riffell JA, Wetzel CH, et al. 2003. Identificationof a testicular odorant receptor mediating human sperm chemotaxis. Science 299:2054–58

98. Spehr M, Schwane K, Heilmann S, Gisselmann G, Hummel T, Hatt H. 2004. Dualcapacity of a human olfactory receptor. Curr. Biol. 14:R832–33

99. Baxendale RW, Fraser LR. 2003. Evidence for multiple distinctly localized adenylylcyclase isoforms in mammalian spermatozoa. Mol. Reprod. Dev. 66:181–89

100. Wade MA, Roman SD, Jones RC, Aitken RJ. 2003. Adenylyl cyclase isoforms in rattestis and spermatozoa from the cauda epididymidis. Cell Tissue Res. 314:411–19

101. Spehr M, Schwane K, Riffell JA, Barbour J, Zimmer RK, et al. 2004. Particulate adenylatecyclase plays a key role in human sperm olfactory receptor-mediated chemotaxis. J. Biol.Chem. 279:40194–203

102. Fukuda N, Yomogida K, Okabe M, Touhara K. 2004. Functional characterization of amouse testicular olfactory receptor and its role in chemosensing and in regulation ofsperm motility. J. Cell Sci. 117:5835–45

103. Sun F, Giojalas LC, Rovasio RA, Tur-Kaspa I, Sanchez R, Eisenbach M. 2003. Lack ofspecies-specificity in mammalian sperm chemotaxis. Dev. Biol. 255:423–27

104. Neuhaus EM, Mashukova A, Barbour J, Wolters D, Hatt H. 2006. Novel function ofβ-arrestin2 in the nucleus of mature spermatozoa. J. Cell Sci. 119:3047–56

105. Fehr J, Meyer D, Widmayer P, Borth HC, Ackermann F, et al. 2007. Expression of theG-protein α-subunit gustducin in mammalian spermatozoa. J. Comp. Physiol. A 193:21–34

106. Hildebrandt JD, Codina J, Tash JS, Kirchick HJ, Lipschultz L, et al. 1985. Themembrane-bound spermatozoal adenylyl cyclase system does not share coupling char-acteristics with somatic cell adenylyl cyclases. Endocrinology 116:1357–66

107, 107a, 108.Evidence thatbicarbonatedirectly stimulatessAC and that malemice lacking sAC,the predominantsource of cAMP insperm, are infertile.

107. Chen Y, Cann MJ, Litvin TN, Iougenko V, Sinclair ML, et al. 2000. Soluble adeny-lyl cyclase as an evolutionarily conserved bicarbonate sensor. Science 289:625–28

107a. Esposito G, Jaiswal BS, Xie F, Krajnc-Franken MAM, Robben TJAA, et al. 2004.Mice deficient for soluble adenylyl cyclase are infertile because of a severe sperm-motility defect. Proc. Natl. Acad. Sci. USA 101:2993–98

108. Hess KC, Jones BH, Marquez B, Chen Y, Ord TS, et al. 2005. The “soluble”adenylyl cyclase in sperm mediates multiple signaling events required for fertil-ization. Dev. Cell 9:249–59

109. Xie F, Garcia MA, Carlson AE, Schuh SM, Babcock DF, et al. 2006. Soluble adenylylcyclase (sAC) is indispensable for sperm function and fertilization. Dev. Biol. 296:353–62

110. Schuh SM, Carlson AE, McKnight GS, Conti M, Hille B, Babcock DF. 2006. Signalingpathways for modulation of mouse sperm motility by adenosine and catecholamineagonists. Biol. Reprod. 74:492–500


Ann

u. R

ev. P

hysi

ol. 2

008.

70:9

3-11

7. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by U

nive

rsity

of

Mas

sach

uset

ts -

Am

hers

t on

04/1

2/13

. For

per

sona

l use

onl

y.


111. Biel M, Seeliger M, Pfeifer A, Kohler K, Gerstner A, et al. 1999. Selective loss of conefunction in mice lacking the cyclic nucleotide-gated channel CNG3. Proc. Natl. Acad.Sci. USA 96:7553–57

112. Brunet LJ, Gold GH, Ngai J. 1996. General anosmia caused by a targeted disruption ofthe mouse olfactory cyclic nucleotide-gated cation channel. Neuron 17:681–93

113. Correia JN, Conner SJ, Kirkman-Brown JC. 2007. Non-genomic steroid actions inhuman spermatozoa. “Persistent tickling from a laden environment”. Semin. Reprod.Med. 25:208–19

114. Teves ME, Barbano F, Guidobaldi HA, Sanchez R, Miska W, Giojalas LC. 2006. Pro-gesterone at the picomolar range is a chemoattractant for mammalian spermatozoa.Fertil. Steril. 86:745–49

115. Sliwa L. 1995. Effect of some sex steroid hormones on human spermatozoa migrationin vitro. Eur. J. Obstet. Gynecol. Reprod. Biol. 58:173–75

116. Villanueva-Diaz C, Arias-Martınez J, Bermejo-Martinez L, Vadillo-Ortega F. 1995.Progesterone induces human sperm chemotaxis. Fertil. Steril. 64:1183–88

117. Wang Y, Storeng R, Dale PO, Abyholm T, Tanbo T. 2001. Effects of follicular fluid andsteroid hormones on chemotaxis and motility of human spermatozoa in vitro. Gynecol.Endocrinol. 15:286–92

118. Jaiswal BS, Tur-Kaspa I, Dor J, Mashiach S, Eisenbach M. 1999. Human sperm chemo-taxis: Is progesterone a chemoattractant? Biol. Reprod. 60:1314–19

119. Harper CV, Kirkman-Brown JC, Barratt CLR, Publicover SJ. 2003. Encoding of pro-gesterone stimulus intensity by intracellular [Ca2+] ([Ca2+]i) in human spermatozoa.Biochem. J. 373:407–17

120. Bedu-Addo K, Barratt CLR, Kirkman-Brown JC, Publicover SJ. 2007. Patterns of[Ca2+]i mobilization and cell response in human spermatozoa exposed to progesterone.Dev. Biol. 302:324–32

121. Publicover S, Harper CV, Barratt C. 2007. [Ca2+]i signalling in sperm—making themost of what you’ve got. Nat. Cell Biol. 9:235–42

122. Losel R, Breiter S, Seyfert M, Wehling M, Falkenstein E. 2005. Classic and non-classicprogesterone receptors are both expressed in human spermatozoa. Horm. Metab. Res.37:10–14

123. Wehling M, Schultz A, Losel R. 2007. To be or not to be (a receptor). Steroids 72:107–10124. Herve JC. 2002. Non-genomic effects of steroid hormones on membrane channels. Mini

Rev. Med. Chem. 2:411–17125. Schlichter R, Keller AF, De Roo M, Breton JD, Inquimbert P, Poisbeau P. 2006. Fast

nongenomic effects of steroids on synaptic transmission and role of endogenous neuro-steroids in spinal pain pathways. J. Mol. Neurosci. 28:33–51

126. Olson JH, Xiang X, Ziegert T, Kittelson A, Rawls A, et al. 2001. Allurin, a 21-kDasperm chemoattractant from Xenopus egg jelly, is related to mammalian sperm-bindingproteins. Proc. Natl. Acad. Sci. USA 98:11205–10

127. Xiang X, Kittelson A, Olson J, Bieber A, Chandler D. 2005. Allurin, a 21 kD spermchemoattractant, is rapidly released from the outermost jelly layer of the Xenopus egg bydiffusion and medium convection. Mol. Reprod. Dev. 70:344–60

128. Schall TJ, Bacon K, Toy KJ, Goeddel DV. 1990. Selective attraction of monocytes andT lymphocytes of the memory phenotype by cytokine RANTES. Nature 347:669–71

129. Alam R, Stafford S, Forsythe P, Harrison R, Faubion D, et al. 1993. RANTES is achemotactic and activating factor for human eosinophils. J. Immunol. 150:3442–48

130. Krensky AM, Ahn YT. 2007. Mechanisms of disease: regulation of RANTES (CCL5)in renal disease. Nat. Clin. Pract. Nephrol. 3:164–70


Ann

u. R

ev. P

hysi

ol. 2

008.

70:9

3-11

7. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by U

nive

rsity

of

Mas

sach

uset

ts -

Am

hers

t on

04/1

2/13

. For

per

sona

l use

onl

y.


131. Isobe T, Minoura H, Tanaka K, Shibahara T, Hayashi N, Toyoda N. 2002. The effectof RANTES on human sperm chemotaxis. Hum. Reprod. 17:1441–46

132. Appay V, Rowland-Jones SL. 2001. RANTES: a versatile and controversial chemokine.Trends Immunol. 22:83–87

133. Muciaccia B, Padula F, Vicini E, Gandini L, Lenzi A, Stefanini M. 2005. Beta-chemokinereceptors 5 and 3 are expressed on the head region of human spermatozoon. FASEB J.19:2048–50

134. Miraglia E, Rullo ML, Bosia A, Massobrio M, Revelli A, Ghigo D. 2007. Stimulationof the nitric oxide/cyclic guanosine monophosphate signaling pathway elicits humansperm chemotaxis in vitro. Fertil. Steril. 87:1059–63

135. Lefievre L, Chen Y, Conner SJ, Scott JL, Publicover SJ, et al. 2007. Human spermatozoacontain multiple targets for protein S-nitrosylation: an alternative mechanism of themodulation of sperm function by nitric oxide? Proteomics 7:3066–84

136. Baker MA, Reeves G, Hetherington L, Muller J, Baur I, Aitken RJ. 2007. Identificationof gene products present in Triton X-100 soluble and insoluble fractions of humanspermatozoa lysates using LC-MS/MS analysis. Proteomics Clin. Appl. 1:524–32

137. Qi H, Moran MM, Navarro B, Chong JA, Krapivinsky G, et al. 2007. All four CatSper ionchannel proteins are required for male fertility and sperm cell hyperactivated motility.Proc. Natl. Acad. Sci. USA 104:1219–23

138, 139, 141.Identified CatSperproteins, a newfamily ofsperm-specificCa2+-permeableion channels invertebrates.

138. Ren D, Navarro B, Perez G, Jackson AC, Hsu S, Shi Q, Tilly JL, Clapham DE.2001. A sperm ion channel required for sperm motility and male fertility. Nature

413:603–9139. Quill TA, Ren D, Clapham DE, Garbers DL. 2001. A voltage-gated ion channel

expressed specifically in spermatozoa. Proc. Natl. Acad. Sci. USA 98:12527–31140. Wang D, King SM, Quill TA, Doolittle LK, Garbers DL. 2003. A new sperm-specific

Na+/H+ exchanger required for sperm motility and fertility. Nat. Cell Biol. 5:1117–22141. Kirichok Y, Navarro B, Clapham DE. 2006. Whole-cell patch-clamp measure-

ments of spermatozoa reveal an alkaline-activated Ca2+ channel. Nature 439:737–40

142. Nolan MA, Babcock DF, Wennemuth G, Brown W, Burton KA, McKnight GS. 2004.Sperm-specific protein kinase A catalytic subunit Cα2 orchestrates cAMP signaling formale fertility. Proc. Natl. Acad. Sci. USA 101:13483–88


Ann

u. R

ev. P

hysi

ol. 2

008.

70:9

3-11

7. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by U

nive

rsity

of

Mas

sach

uset

ts -

Am

hers

t on

04/1

2/13

. For

per

sona

l use

onl

y.

AR336-FM ARI 10 January 2008 19:46

Annual Review ofPhysiology

Volume 70, 2008Contents

FrontispieceJoseph F. Hoffman � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �xvi

PERSPECTIVES, David Julius, Editor

My Passion and Passages with Red Blood CellsJoseph F. Hoffman � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �1

CARDIOVASCULAR PHYSIOLOGY, Jeffrey Robbins, Section Editor

Calcium Cycling and Signaling in Cardiac MyocytesDonald M. Bers � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 23

Hypoxia-Induced Signaling in the Cardiovascular SystemM. Celeste Simon, Liping Liu, Bryan C. Barnhart, and Regina M. Young � � � � � � � � � � � 51

CELL PHYSIOLOGY, David E. Clapham, Associate and Section Editor

Bcl-2 Protein Family Members: Versatile Regulators of CalciumSignaling in Cell Survival and ApoptosisYiping Rong and Clark W. Distelhorst � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 73

Mechanisms of Sperm ChemotaxisU. Benjamin Kaupp, Nachiket D. Kashikar, and Ingo Weyand � � � � � � � � � � � � � � � � � � � � � � � � 93

ECOLOGICAL, EVOLUTIONARY, AND COMPARATIVEPHYSIOLOGY, Martin E. Feder, Section Editor

Advances in Biological Structure, Function, and Physiology UsingSynchrotron X-Ray ImagingMark W. Westneat, John J. Socha, and Wah-Keat Lee � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �119

Advances in Comparative Physiology from High-Speed Imagingof Animal and Fluid MotionGeorge V. Lauder and Peter G.A. Madden � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �143

vii

Ann

u. R

ev. P

hysi

ol. 2

008.

70:9

3-11

7. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by U

nive

rsity

of

Mas

sach

uset

ts -

Am

hers

t on

04/1

2/13

. For

per

sona

l use

onl

y.

http://arjournals.annualreviews.org/doi/abs/10.1146/annurev.physiol.70.113006.100627







http://arjournals.annualreviews.org/loi/physiol

http://arjournals.annualreviews.org/toc/physiol/70/1


ENDOCRINOLOGY, Holly A. Ingraham, Section Editor

Estrogen Signaling through the Transmembrane G Protein–CoupledReceptor GPR30Eric R. Prossnitz, Jeffrey B. Arterburn, Harriet O. Smith, Tudor I. Oprea,

Larry A. Sklar, and Helen J. Hathaway � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �165

Insulin-Like Signaling, Nutrient Homeostasis, and Life SpanAkiko Taguchi and Morris F. White � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �191

The Role of Kisspeptins and GPR54 in the NeuroendocrineRegulation of ReproductionSimina M. Popa, Donald K. Clifton, and Robert A. Steiner � � � � � � � � � � � � � � � � � � � � � � � � � � �213

GASTROINTESTINAL PHYSIOLOGY, James M. Anderson, Section Editor

Gastrointestinal Satiety SignalsOwais B. Chaudhri, Victoria Salem, Kevin G. Murphy, and Stephen R. Bloom � � � � �239

Mechanisms and Regulation of Epithelial Ca2+ Absorption in Healthand DiseaseYoshiro Suzuki, Christopher P. Landowski, and Matthias A. Hediger � � � � � � � � � � � � � � � �257

Polarized Calcium Signaling in Exocrine Gland CellsOle H. Petersen and Alexei V. Tepikin � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �273

RENAL AND ELECTROLYTE PHYSIOLOGY, Gerhard H. Giebisch,Section Editor

A Current View of the Mammalian AquaglyceroporinsAleksandra Rojek, Jeppe Praetorius, Jørgen Frøkiaer, Søren Nielsen,

and Robert A. Fenton � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �301

Molecular Physiology of the WNK KinasesKristopher T. Kahle, Aaron M. Ring, and Richard P. Lifton � � � � � � � � � � � � � � � � � � � � � � � � � �329

Physiological Regulation of Prostaglandins in the KidneyChuan-Ming Hao and Matthew D. Breyer � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �357

Regulation of Renal Function by the Gastrointestinal Tract: PotentialRole of Gut-Derived Peptides and HormonesA.R. Michell, E.S. Debnam, and R.J. Unwin � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �379

RESPIRATORY PHYSIOLOGY, Richard C. Boucher, Jr., Section Editor

Regulation of Airway Mucin Gene ExpressionPhilip Thai, Artem Loukoianov, Shinichiro Wachi, and Reen Wu � � � � � � � � � � � � � � � � � � � �405

Structure and Function of the Cell Surface (Tethered) MucinsChristine L. Hattrup and Sandra J. Gendler � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �431

viii Contents

Ann

u. R

ev. P

hysi

ol. 2

008.

70:9

3-11

7. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by U

nive

rsity

of

Mas

sach

uset

ts -

Am

hers

t on

04/1

2/13

. For

per

sona

l use

onl

y.














Structure and Function of the Polymeric Mucins in Airways MucusDavid J. Thornton, Karine Rousseau, and Michael A. McGuckin � � � � � � � � � � � � � � � � � � � �459

Regulated Airway Goblet Cell Mucin SecretionC. William Davis and Burton F. Dickey � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �487

SPECIAL TOPIC, OBESITY, Joel Elmquist and Jeffrey Flier, Special Topic Editors

The Integrative Role of CNS Fuel-Sensing Mechanisms in EnergyBalance and Glucose RegulationDarleen Sandoval, Daniela Cota, and Randy J. Seeley � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �513

Mechanisms of Leptin Action and Leptin ResistanceMartin G. Myers, Michael A. Cowley, and Heike Münzberg � � � � � � � � � � � � � � � � � � � � � � � � �537

Indexes

Cumulative Index of Contributing Authors, Volumes 66–70 � � � � � � � � � � � � � � � � � � � � � � � �557

Cumulative Index of Chapter Titles, Volumes 66–70 � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �560

Errata

An online log of corrections to Annual Review of Physiology articles may be found athttp://physiol.annualreviews.org/errata.shtml

Contents ix

Ann

u. R

ev. P

hysi

ol. 2

008.

70:9

3-11

7. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by U

nive

rsity

of

Mas

sach

uset

ts -

Am

hers

t on

04/1

2/13

. For

per

sona

l use

onl

y.





Date post:	08-Dec-2016
Category:	Documents
Upload:	ingo
View:	213 times
Download:	0 times

Mechanisms of Sperm Chemotaxis

Documents