University of Zurich Zurich Open Repository and Archive Winterthurerstr. 190 CH-8057 Zurich http://www.zora.uzh.ch Year: 2007 Soluble erythropoietin receptor is present in the mouse brain and is required for the ventilatory acclimatization to hypoxia Soliz, J; Gassmann, M; Joseph, V Soliz, J; Gassmann, M; Joseph, V (2007). Soluble erythropoietin receptor is present in the mouse brain and is required for the ventilatory acclimatization to hypoxia. The Journal of Physiology, 583(1):329-336. Postprint available at: http://www.zora.uzh.ch Posted at the Zurich Open Repository and Archive, University of Zurich. http://www.zora.uzh.ch Originally published at: The Journal of Physiology 2007, 583(1):329-336.
Transcript
University of ZurichZurich Open Repository and Archive
Winterthurerstr. 190
CH-8057 Zurich
http://www.zora.uzh.ch
Year: 2007
Soluble erythropoietin receptor is present in the mouse brain andis required for the ventilatory acclimatization to hypoxia
Soliz, J; Gassmann, M; Joseph, V
Soliz, J; Gassmann, M; Joseph, V (2007). Soluble erythropoietin receptor is present in the mouse brain and isrequired for the ventilatory acclimatization to hypoxia. The Journal of Physiology, 583(1):329-336.Postprint available at:http://www.zora.uzh.ch
Posted at the Zurich Open Repository and Archive, University of Zurich.http://www.zora.uzh.ch
Originally published at:The Journal of Physiology 2007, 583(1):329-336.
Soliz, J; Gassmann, M; Joseph, V (2007). Soluble erythropoietin receptor is present in the mouse brain and isrequired for the ventilatory acclimatization to hypoxia. The Journal of Physiology, 583(1):329-336.Postprint available at:http://www.zora.uzh.ch
Posted at the Zurich Open Repository and Archive, University of Zurich.http://www.zora.uzh.ch
Originally published at:The Journal of Physiology 2007, 583(1):329-336.
Soluble erythropoietin receptor is present in the mouse brain andis required for the ventilatory acclimatization to hypoxia
Abstract
While erythropoietin (Epo) and its receptor (EpoR) have been widely investigated in brain, theexpression and function of the soluble Epo receptor (sEpoR) remain unknown. Here we demonstratethat sEpoR, a negative regulator of Epo's binding to the EpoR, is present in the mouse brain and isdown-regulated by 62% after exposure to normobaric chronic hypoxia (10% O2 for 3 days).Furthermore, while normoxic minute ventilation increased by 58% in control mice following hypoxicacclimatization, sEpoR infusion in brain during the hypoxic challenge efficiently reduced brain Epoconcentration and abolished the ventilatory acclimatization to hypoxia (VAH). These observationsimply that hypoxic downregulation of sEpoR is required for adequate ventilatory acclimatization tohypoxia, thereby underlying the function of Epo as a key factor regulating oxygen delivery not only byits classical activity on red blood cell production, but also by regulating ventilation.
J Physiol 583.1 (2007) pp 329–336 329
Soluble erythropoietin receptor is present in the mousebrain and is required for the ventilatory acclimatizationto hypoxia
Jorge Soliz1, Max Gassmann1∗and Vincent Joseph2∗
1Institute of Veterinary Physiology, Vetsuisse Faculty and Zurich Center for Integrative Human Physiology (ZIHP), University of Zurich,
Winterthurerstrasse 260, CH-8057 Zurich, Switzerland2Department of Pediatrics, Laval University, Center de Recherche (DO-711), Hopital St-Francois d’Assise, 10 rue de l’Espinay, Quebec (QC),
G1L 3L5, Canada
While erythropoietin (Epo) and its receptor (EpoR) have been widely investigated in brain,
the expression and function of the soluble Epo receptor (sEpoR) remain unknown. Here we
demonstrate that sEpoR, a negative regulator of Epo’s binding to the EpoR, is present in the
mouse brain and is down-regulated by 62% after exposure to normobaric chronic hypoxia (10%
O2 for 3 days). Furthermore, while normoxic minute ventilation increased by 58% in control
mice following hypoxic acclimatization, sEpoR infusion in brain during the hypoxic challenge
efficiently reduced brain Epo concentration and abolished the ventilatory acclimatization to
hypoxia (VAH). These observations imply that hypoxic downregulation of sEpoR is required for
adequate ventilatory acclimatization to hypoxia, thereby underlying the function of Epo as a key
factor regulating oxygen delivery not only by its classical activity on red blood cell production,
but also by regulating ventilation.
(Received 27 March 2007; accepted after revision 15 June 2007; first published online 21 June 2007)
Corresponding author J. Soliz: Institute of Veterinary Physiology, Vetsuisse Faculty, University of Zurich,
∗Both senior authors contributed equally to this work.
Erythropoietin (Epo) is a pleiotropic cytokine mostcommonly recognized as a factor that improves arterialoxygen carrying capacity (Gassmann et al. 2003; Sasaki,2003). Upon sustained hypoxaemic conditions Eposynthesis in the kidney is accelerated, resulting in increasedplasma Epo levels (Eckardt & Kurtz, 2005; Stockmann& Fandrey, 2006). Binding of Epo to its receptor(EpoR) on erythrocyte progenitor cells in the bonemarrow maintains their viability, promotes cell division,and increases haemoglobin synthesis culminating inincreased haematocrit levels (Fisher, 2003; Jelkmann,2005). While this endocrine loop is essential, it is notsufficient to ensure fast and adequate oxygen supply underconditions of limited oxygen availability such as in highaltitude or in patients suffering from impaired respiratoryfunctions. Sustained hypoxic exposure also affects therespiratory control network, leading to a progressive,species-dependent increase in minute ventilation overminutes, hours or even weeks. This process is definedas the ventilatory acclimatization to hypoxia (VAH) andpersists for a few days following return to normoxic
M. Gassmann and V. Joseph contributed equally to this work.
conditions. VAH mainly relies on increased sensitivityof peripheral chemoreceptors to hypoxia, associated withcentral facilitation of the ventilatory output (Forster et al.1981; Dempsey & Forster, 1982; Smith et al. 1986; Powellet al. 2000a; Prabhakar & Jacono, 2005). Here we show thatthe soluble form of EpoR (sEpoR) is involved in the VAH.
Endogenous expression of Epo has been reported in thenervous system (Digicaylioglu et al. 1995; Gassmann et al.2003) and transgenic mice overexpressing human Epo inneural cells have shown that brain-derived Epo facilitatesVAH (Soliz et al. 2005). Furthermore it was reported thatthe EpoR is present in respiratory areas of the brain-stem, such as NK-1R positive neurons in the pre-Botzingercomplex (proposed as the respiratory rhythm generator),and the nucleus tractus solitarii, which relays input fromperipheral chemoreceptors to the central respiratory areasto increase ventilation upon hypoxic exposure (Soliz et al.2005). In analogy to several other members of the cytokinesuperfamily type I transmembrane proteins, EpoR is alsosynthesized in a soluble form (sEpoR) that corresponds tothe extracellular domain of the complete receptor (Nagaoet al. 1992; Harris & Winkelmann, 1996; Westphal et al.2002). Synthesis of the sEpoR occurs by alternative splicingof EpoR mRNA. Following secretion into the extracellular
fluid, sEpoR binds Epo, thereby limiting its ability to bindEpoR (Kuramochi et al. 1990; Baynes et al. 1993). Whilethe presence of sEpoR has been reported in plasma andseveral tissues including liver, spleen, kidney, heart andbone marrow (Fujita et al. 1997), its cerebral expressionand function has not been investigated yet. In the presentwork we show that sEpoR is expressed in the mouse brain.Moreover, down-regulation of sEpoR upon exposure tochronic hypoxia reflects the key role of sEpoR to regulatethe VAH.
Methods
Measurement of ventilation
A total of 20 adult male C57/Bl6 mice (Charles RiverCanada) at the age of 3 months were housed for 1 weekat constant temperature (21 ± 1◦C) and light (from 07.00to 19.00 h). Brain tissue and blood were collected from 10mice to measure cerebral and plasma Epo before (n = 5)and after (n = 5) VAH, as described below. The other 10animals were used to measure ventilation in normoxia andacute hypoxia, before and after being surgically connectedto a cannula (see below) and exposed to 10% O2 for 3 days.A whole-body flow-though plethysmography (EMKATechnologies, France) was used to monitor ventilation asdescribed (Soliz et al. 2005). Briefly, mice were placedin a 600 ml chamber continuously supplied with air-flow at 0.7–0.8 l min−1 using flow restrictors. Ventilation(VE) was calculated as the product of tidal volume (V T)and respiratory frequency (f R) and normalized to 100 gof body weight (e.g. ml min−1 (100 g)−1). Ventilatorymeasurements were performed in normoxia (21% O2).Acute hypoxia was achieved by flushing air balanced inN2. The fraction of inspired O2 (FIO2
) in the chamber wasgradually decreased from 21% to 10% O2 during 15 min.Respiratory recordings at 10% O2 were performed for20 min. At the end of each experiment, body weight andbody temperature (rectal thermocouple, Physitemp, USA)were measured. In parallel to ventilation measurements,O2 consumption (VO2
, ml min−1 (100 g)−1) and CO2
production (VCO2, ml min−1 (100 g)−1) were determined
by O2 and CO2 analysers (AEI technologies). Ventilatorymeasurements were performed under similar conditionsbefore and after chronic hypoxic exposure. All animalprocedures have been approved by the ethics committee ofanimal care at Laval University and followed the guidelinesof the Canadian Council on Animal Care.
Intracerebroventricular infusions
For intracerebroventricular infusion, the animals weredeeply anaesthetized with a gas mixture of 4% halothane,70% N2 and O2, and maintained by reducing theinspired halothane concentration to 1–1.5%. Bodytemperature of the mice was maintained at 37◦C using
a temperature-controlled heating pad. They were thenstereotaxically implanted with a permanent stainless-steelcannula (Alzet, Durect Corp., Toronto, Canada) into theleft lateral ventricle of the brain at coordinates (Bregma:0; L: 1 mm, and DV: −3 mm) according to Paxinosand Watson (Paxinos & Watson 2000). Subsequently anosmotic minipump (Alzet pump model 1003D; 100 μl;flow rate 1.4 μl h−1; Durect Corp.) was connected to thecannula with medical grade vinyl tubing and placed in as.c. pocket in the dorsal region. Pumps were filled withsoluble EpoR solution (50 μg; R & D systems, Inc., USA)or vehicle (phosphate buffer). After surgery, mice wereallowed to recover in isolation and observed for at least 1 huntil complete recovery from anaesthesia.
Quantification of Epo, EpoR and sEpoR
Mice were anaesthetized by an intraperitoneal injectionof a mixture of ketamine (80 mg kg−1) and xylazine(10 mg kg−1). Blood samples were drawn by cardiacpuncture into heparinized Ependorff tubes and plasma wascollected after samples centrifugation at (18 000 g) Brainsamples were obtained after transcardial perfusion of micewith phosphate buffer (0.1 m, pH 7.4) and immediatelyfrozen in liquid nitrogen. For the extraction of totalprotein, brain tissues were complemented with lysisbuffer, homogenized and centrifuged, and supernatantswere harvested. Protein concentrations of both brain andplasma were determined by the Bradford protein assay(Bio-Rad, Hercules, CA, USA). Epo levels in brain andplasma were quantified using an 125I-Epo-based radio-immunoassay (RIA) (Amersham, Zurich, Switzerland),according to previously published protocols (Kilic et al.2005). The lower detection limit of our RIA was 4 U l−1,and the intrassay/interassay variances were < 2% and< 6%, respectively. For quantification of EpoR and sEpoRin brain samples, equal amounts of extracts were separatedby 10% SDS-PAGE and transferred onto a nitrocellulosemembrane. Transfer and equal loading of proteins wereconfirmed by Ponceau S staining. For immunoblotting,the membranes were incubated with an EpoR antibody(H-194, Santa Cruz Biotechnology Inc., Lab Force�G CH-4208, Nummingen, Switzerland; 1 : 200) andwith the appropriate horseradish peroxidase-conjugatedsecondary antibody. EpoR and sEpoR were detected at theexpected molecular weight (64 and 30 kDa, respectively;Nagao et al. 1992) by enhanced chemiluminescenceand quantified using a Gel Doc 2000 scanner withQuantity One software (Bio-Rad). The membrane wasthen stripped (10 ml stripping buffer complementedwith 70 μl β-mercaptoethanol) for 20 min and incubatedwith monoclonal anti-β-actin primary antibody (AC-15,Sigma, USA; 1 : 5000), followed by suitable secondaryantibody. The intensity of the Epo and sEpoR bands wasthen normalized to actin.
J Physiol 583.1 sEpoR in brain blocks the ventilatory acclimatization to hypoxia 331
Statistical analysis
Analysis was performed using the StatView software(Abacus Concepts, Berkeley, CA, USA). The reportedvalues are means ± s.d. For simple measurements, datawere analysed by one-way ANOVA. For hypoxic ventilationresponses, data were analysed by two-way ANOVA forrepeated measurements. Differences were consideredsignificant at P < 0.05.
Results
sEpoR is endogenously expressed in brain and isdown-regulated upon chronic exposure to hypoxia
We performed Western blot and RIA analysis on mousebrain extracts. Both EpoR and sEpoR were detectedunder normoxia and after 3 days of hypoxic exposure(10% O2; Fig. 1). While the levels of EpoR (Fig. 1A) andEpo (Fig. 1C) synthesized in the hypoxic brain were notsignificantly altered compared to the normoxic control,hypoxia reduced the expression of sEpoR in the brainby 62% (Fig. 1B). Exposure to chronic hypoxia increasedplasma Epo in mice (54%; Fig. 1D), confirming that theanimals were indeed hypoxaemic. These data demonstratethat sEpoR is endogenously expressed in the mousebrain and that its expression is down-regulated duringhypoxia.
Intracerebroventricular infusion of sEpoR blocks theventilatory acclimatization to hypoxia
Considering that mice achieve VAH within 3 days (Olson& Saunders, 1987; Malik et al. 2005), we hypothesizedthat the observed decrease in sEpoR expression in chronichypoxia enhances the ability of Epo to facilitate VAH.We tested this hypothesis by performing respiratorymeasurements in normoxia and also acute hypoxia (10%O2) before and after acclimatization to chronic hypoxia(10% O2 for 3 days; Fig. 2A). Minute ventilation (VE),respiratory frequency (f R) and tidal volume (V T), as well asmetabolic variables and rectal temperature were evaluatedin 10 adult male mice before acclimatization. During thefollowing 3 days mice received intracranial infusion ofsEpoR (0.7 μg of sEpoR per hour over 3 days, n = 5)and were compared to vehicle-treated controls (n = 5).During the treatment animals were exposed to normobaricchronic hypoxia (10% O2) and then returned to room airfor a second evaluation of ventilation under normoxicand acute hypoxic conditions. No differences werefound between vehicle and sEpoR treated groups beforeacclimatization (Fig. 2B–G, normoxia before treatment).However, while control mice showed a large increase innormoxic ventilation after chronic exposure to hypoxia,this ventilatory acclimatization was completely blocked
by sEpoR infusion (Fig. 2B–F). As metabolic rate wasalso increased following hypoxic exposure in controlmice (but not in sEpoR-treated animals), we correctedventilation to metabolic rate. We observed that chronichypoxia-mediated hyperventilation (higher VE
/VO2
–Fig. 2F) was blunted by the sEpoR infusion. This isconsistent with previous findings (Soliz et al. 2005)showing that Epo’s effect on the ventilatory response tohypoxia is neither due to altered body metabolism (Fig. 2E)nor to alterations in body temperature (Fig. 2G).
The acute hypoxic ventilatory response upon VAH isblunted in mice treated with sEpoR
As ventilatory acclimatization to hypoxia is also manifestedby an augmented response to subsequent acute hypo-xia (Powell et al. 2000b; Malik et al. 2005; Soliz et al.2005), acute hypoxic ventilation before and after theacclimatization period was compared (Fig. 3). In controlmice acute hypoxic ventilation was doubled whereas insEpoR treated mice it was completely abolished (Fig. 3A),due to increased tidal volume (V T; Fig. 3C) rather thanrespiratory frequency (f R; Fig. 3B) after acclimatization.These data demonstrate that sEpoR efficiently antagonizes
Figure 1. sEpoR is expressed in the mouse brain and isdown-regulated upon hypoxic exposureBrain tissue and plasma were collected from mice kept at normoxia(Nx; n = 5) and after 3 days of continuous exposure to hypoxia at10% O2 (Hx; n = 5). EpoR and sEpoR were detected in the normoxic(Nx) and hypoxic (Hx) brain (A and B). Note that Hx but not Nx reducedthe expression of sEpoR (B). The level of brain-derived Epo did notincrease after Hx (C), while plasma Epo was increased after hypoxicstimulus as expected (D). Western blot analysis is exemplified at thetop of A and B, including the negative control (no primary antibody).∗∗P < 0.01, ∗∗∗P < 0.001.
Epo’s ventilatory effect in brain. In agreement with themodel of VAH proposed by Powell (Fig. 3D), control micegradually increased minute ventilation from normoxiato acute hypoxia before acclimatization, and fromhypoxia before acclimatization to acute hypoxia afteracclimatization. In contrast, this effect was inhibited insEpoR-infused animals (Fig. 3E). A similar response wasobserved in the oxygen convection ratio (Fig. 3F). Onceagain, these alterations were not caused by differencesin body temperature after acclimatization (vehicle versussEpoR in Nx: 37.2 ± 0.4 to 37.7 ± 0.7◦C; in Hx: 35.9 ± 0.5to 36.3 ± 0.4◦C).
Figure 2. Intracerebroventricular infusion of sEpoR abolishes VAHA, schematic diagram showing the sequential steps of the experiment protocol. Normoxic (Nx) and acute hypoxic(acute Hx) minute ventilation was evaluated before and after VAH (shaded area) in mice receiving continuousintracerebroventricular infusion of sEpoR or vehicle. Before and after acclimatization, mice were kept at normoxicconditions and the minute ventilation was determined. After acclimatization, control mice showed a large increaseof normoxic ventilation (VE), respiratory frequency (fR), tidal volume (VT) and oxygen consumption (VO2 ). In contrast,this elevation in ventilation was abolished in the sEpoR group, showing that no acclimatization occurred (B, C, Dand E). Accordingly, the oxygen convection ratio (VE/VO2 ) increased progressively in the control mice group butremained unaltered in sEpoR animals (F). These differences were not caused by altered body temperature (G).∗∗∗P < 0.001.
Intracerebroventricular infusion of sEpoRdown-regulates the expression of cerebral Epoand EpoR
Following the ventilatory measurements described above,brain and plasma were collected to quantify Epo (byRIA) as well as EpoR and sEpoR (by Western blot).Treatment with sEpoR dramatically reduced the amountof detectable EpoR and Epo (Fig. 4A and C). The levelof sEpoR did not increase (Fig. 4B) probably due to therapid metabolism of exogenous sEpoR or because theresulting Epo–sEpoR complex was undetectable by either
J Physiol 583.1 sEpoR in brain blocks the ventilatory acclimatization to hypoxia 333
the Epo or EpoR antibody. Mice treated with sEpoR duringchronic hypoxic exposure had higher levels of plasmaEpo (Fig. 4D), a consistent sign of impaired VAH. Theseresults imply that reduced brain-derived Epo availability,as induced by sEpoR infusion during chronic hypoxia,abolishes the progressive augmentation of ventilation, thuscompromising the physiological acclimatization process tohypoxia.
Figure 3. Exogenous application of sEpoR abolishes subsequent augmentation of acute hypoxicventilation induced by VAHAcute hypoxic (acute Hx) ventilation was evaluated before and after VAH in mice infused with sEpoR or vehicle(A, B and C). In line with Powell (D), the VAH is observed in vehicle-infused mice as a supplemental increase ofrespiratory parameters measured in acute hypoxia after acclimatization (Hx after) compared to acute hypoxia before(Hx before) acclimatization (E). Soluble EpoR administration abolished this process (A, B, C and E). The oxygenconvection ratio (VE/VO2 ) follows a similar dynamic (F). The 15 min gradual reduction of F IO2 from normoxia toacute hypoxia is represented by the black triangles. ∗P < 0.05, ∗∗P < 0.01, ∗∗∗P < 0.001.
Discusion
Our results demonstrated for the first time thatendogenously synthesized Epo and sEpoR in the centralnervous system play a crucial role in the physiologicalprocess related to long-term oxygen homeostasis, therebyultimately contributing to the same goal as plasma Epo,i.e. increasing the overall capacity of oxygen delivery. Thisfinding is in agreement with previous data obtained in
Figure 4. Intracerebroventricular infusion of sEpoR decreasesthe levels of Epo and EpoR in the brain upon VAHBrain tissue and plasma were collected from mice infused with sEpoRor vehicle after VAH (3 days; 10% O2). Epo in brain and blood wasanalysed by RIA and EpoR variants were determined by Western blot.Western blot analysis is exemplified at the top of A and B, includingthe negative control (no primary antibody). Compared to the untreatedcontrol, the level of EpoR and Epo were decreased (A and C), whilesEpoR (B) did not alter in sEpoR-infused mice. Plasma Epo was higherin the sEpoR group compared to control (D). ∗P < 0.01, ∗∗P < 0.001.
genetically modified mice overexpressing Epo in the brain,in which the hypoxic ventilatory response persists aftercarotid body denervation (Soliz et al. 2005). Our dataprovide convincing evidence that the regulation of Epo andsEpoR in the central nervous system and at a systemic levelis finely tuned to establish a balance between ventilatoryand haematopoietic control of oxygen carrying capacity asshown by the proposed model for hypoxic acclimatizationpresented in Fig. 5. Acute hypoxia triggers during the
Figure 5. Model of ventilatory acclimatization tohypoxia (VAH) showing the contribution of cerebraland plasma EpoDuring the first minutes of hypoxia, carotid bodies sensethe drop of arterial oxygen pressure thus leading theshort-term response to hypoxia by promoting a fastincrease of ventilation. Persisting hypoxia activateslong-term regulation mechanisms. In mice, Epo synthesisin brain and kidney is initiated a few hours after hypoxicexposure. Plasma Epo augments the oxygen carryingcapacity (by gradual increase of the haematocrit) as wellas contributes to the regulation of ventilation (binding ofthe EpoR present in the carotid body glomus cells). Whilewe recently showed that catecholamines are involved incentral regulation of ventilation, other molecules such asnitric oxide and neuroglobin might be involved, too.
first minutes the response of the chemoreflex loop thatleads to an immediate increase in ventilation termedhypoxic ventilatory response (short-term regulation).If hypoxia persists, the organism enters a process ofacclimatization in which long-term regulatory responsesare initiated. Thereby, central and systemic availabilityof Epo is increased. In mice, hypoxic induction of EpomRNA expression in the kidneys is maximal after 2 h,while plasma Epo protein level peaks after 20 h, and despitecontinuous hypoxia both mRNA and protein are quicklylowered (Abbrecht & Littell, 1972; Chikuma et al. 2000).Most probably, the elevated Epo concentration in theblood immediately impacts the carotid bodies therebycontributing to the modulation of ventilation. Note that werecently showed a dense staining of EpoR in glomus cells(Soliz et al. 2005). At the same time, increased Epo plasmalevels progressively augments the number of circulatingred cells to augment the oxygen carrying capacity. Inbrain, the Epo mRNA reaches a peak expression after 4 hof hypoxia. Additionally, we observed that brain-derivedEpo protein levels peaks after 24 h (unpublished data)and then decreased to basal level after 72 h, this kineticbeing consistent with the HIF-1 expression in the hypo-xic brain (Stroka et al. 2001). However, during the decayof brain Epo, sEpoR expression is decreased to maintaincontinuously high Epo availability. The mechanism bywhich cerebral Epo controls VAH remains to be elucidated.We previously showed that Epo regulates breathing byaltering catecholaminergic metabolism in the brain-stem (Soliz et al. 2005). However we do not excludethe involvement of other factors, such as nitric oxide(Lipton et al. 2001) and neuroglobin (Hundahl et al.2005), both up-regulated in brain after hypoxic induction(Fig. 5).
On the other hand, although the mechanism involvedare not completely understood, it has been well establishedthat hypoxia causes changes in body metabolism (Gautier,
J Physiol 583.1 sEpoR in brain blocks the ventilatory acclimatization to hypoxia 335
1996). As such, acute hypoxia induces a decrease in oxygendemand and an increase of ventilation mediated by anaugmented chemosensitive drive (Kline et al. 1998; Klineet al. 2000; Kline et al. 2002). In parallel, VAH reestablishestissue oxygenation by offsetting minute ventilation to anew basal level. Furthermore, we and others reported inprevious studies that the metabolic rate is increased afterVAH (Malik et al. 2005; Soliz et al. 2005), but the identityof this event is unknown so far. At present, we cannotexplain why metabolic rate was not increased in sEpoRinfused mice. We speculate that the metabolic cost ofelevated minute ventilation, as seen in control mice, is notoccurring in sEpoR-treated animals due to their unalteredventilation. In line with this, we hypothesize that high Epoplasma levels reflect a hypoxaemia that in turn hindershigher metabolic rate.
Our data suggest that cerebral expression of Epo and/orsEpoR in brain is implicated in respiratory disordersoccurring at high altitude such as acute and chronicmountain sickness (Leon-Velarde et al. 1998; Josephet al. 2000; Joseph et al. 2002). Similarly, it is temptingto speculate that this system could also play a rolein the respiratory plasticity induced by exposure tointermittent hypoxia, either in adults or in newbornmammals. Therefore, expression and regulation ofcerebral Epo and sEpoR are of central importance in thephysiological response to hypoxia providing new insightsinto disease pathogenesis and to generate novel therapeuticapproaches.
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Acknowledgements:
The authors wish to thank Van Diep Doan and Beat Grenacher
for their superb assistance and Csilla Becskei, Edith Schneider,
Johannes Vogel and Michelle Scott for fruitful discussions. J.S and
M.G. are supported by grants of RoFAR and M.G. by the Swiss
National Science Foundation and the EU-project ‘Pulmotension’.
Animal experiments in Canada were supported by a NSERC
