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Tutorial 2,Tutorial 2,Plasma MembranePlasma Membrane

IPAM Cells and Materials:IPAM Cells and Materials:

At the Interface between Mathematics, Biology and EngineeringAt the Interface between Mathematics, Biology and Engineering

Dr. Toshikazu HamasakiDr. Toshikazu Hamasaki

Dept. Bioengineering, UCLADept. Bioengineering, UCLA

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Plasma Membrane

Lipid Bi-layerCreates Hydrophobic BarrierHigher Cholesterol contents

(~20%) than Organelle membraneGlycolipid (external surface of

Plasma Membrane)Water : Poorly permeableO2, CO 2 : PermeableHydrophobic agents (drugs)Deter ent H dro hobic - H dro hilic

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Plasma Membrane: environmental boundary (barrier)Electrochemical gradient

Membrane potentialUnique intracellular environment

Stabilize pHHolds molecules inside

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Plasma Membrane: environmental boundary (barrier)

Ionic imbalance (particularly, Na + and K +) between inside and outside a cell,created by membrane ionic pumps, ion exchangers and channels, establishesresting membrane potential. This is used to drive other process (such as moleculeimport), as well as for information processing ( e.g. nerve cells).Activities of plasma membrane ionic pumps are energized by hydrolysis of ATP.

All the live cells establish and maintain the membrane potential.

-50 mV

0 mV

Na + Na +10 mM 145 mMK + K+

140 mM 4 mM

Calcium Calcium0.1 mM 2 mMCa 2+ Ca 2+

< 0.1 M 1 mMCl - Cl -3 mM 117 mM

ATP ATP1 mM < 0.1 M

Intracellular Extracellular(Cytoplasm) (Tissue fluid)

Membranepotential

Ion Channels

Exchangers

Pumps

Importers

Porins

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Plasma MembraneComponents of plasma membrane

LipidsPhospholipids

GlycolipidCholesterol

Proteins; transmembrane proteins, peripheral proteinsMany proteins are glycosylated

Membrane channels, pumps : Ion concentration gradient (in out)

Membrane potentialTransporters : transport molecule across plasma membrane, e.g. glucose transporterMembrane receptors : Information relay

(via particular signaling molecules, e.g . hormones, neurotransmitters)Communication between cells : Gap junctions, integrinsAdhesion molecules (Junctions); cell to extracellular matrix, cell to cellEndocytosis, Exocytosis : Intracellular membrane flow

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Phospholipid structure: e.g. Phosphatidylcholine

cis -doublebondkink

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More kinks

1. More difficult to pack phospholipid together membrane staysfluid at lower temp(Bacteria, yeast adjust the fatty acid composition according totemp, to maintain membrane fluidity)

2. The kinks shorten the length of hydrocarbon chains, so that themembrane is thinner.

Mobility of phospholipid

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Four major phospholipids found in mammalian plasma membrane

(PE) (PS) (PC)

There are many minor phospholipids exists, too.

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Cholesterol

Unique to plasma membraneStabilize membrane

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CholesterolUnique to plasma membrane

Stabilize membrane

ALSO: Precursor tosteroid hormones

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Glycolipids

Unique to plasma membrane(Extracellular side)

Neutral glycolipid (A)or,

negatively charged (B)

(due to sialic acid [NANA])

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Phospholipid arrangement in Plasma MembraneExt. Cellular Side:two unique lipids

Glycolipids (blue)Sphingomyelin (brown)

Cytospsmic Side:PS [negatively charged]PE

Cholesterol (not shown)

are found in both side.

(PE) (PS) (PC)

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Lipid Raft

Local area of amembrane wheresphingolipid,cholesterol andmembrane proteins areconcentrated.

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Plasma Membrane : Membrane ProteinsFunctional classification (1)

TransmembraneProteins

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Plasma Membrane : Membrane ProteinsFunctional classification (2)

Transmembrane Proteins

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Plasma Membrane : Membrane ProteinsFunctional classification (3)

Peripheral Proteins(only one side of the membrane)

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Association of membrane proteins withthe lipid bilayer (1)

Transmembrane Proteins

1. A single -helix2. Multiple -helices3. Rolled up -sheet

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Association of membrane proteins w/ the lipid bilayer (2)Peripheral Membrane Proteins

4. - helix (hydrophobic face)embedded in the lipid bilayer

5. Protein covalently attaches lipid chain fatty acid chain or prenyl group(cytoplasmic side)

6. Protein attaches phosphatidylinositol viaan oligosaccharide linker

7, 8. Noncovalent interaction betweenproteins

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helical transmembrane polypeptide chainMostly consists of hydrophobic

amino acids (yellow and green)

Seven trans-membrane helices

Hydropathy plotPrediction of transmembrane helix bysequence of amino acids

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Membrane receptor proteinsSignal transduction across the membrane

The signals are used to activate:

gene transcription(s) cell differentiationcell locomotionexocytosis / endocytosis

etc

Signal Transduction (an another tutorial section)

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These systems exist not only on plasma membrane, but also many organelle membranes.Overview of membrane transport proteins

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Na + /K +-ATPase (Na + /K +-pump)1 ATP used for exporting 3 Na + ions and importing 2 K + ions.Crucial for maintaining resting membrane potential.

Other pumps; e.g. Ca 2+ pump

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Transporters

ATP-independent systems(However, Na + gradient drives

these transporters;ATP-driven Na + /K +-pump

generates the gradient.)

Glucose transporter

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Many of the channels, pumps and transporters areinserted only on particular surface of the cell

Intestinalepithelial cell

Tight junction serves barriers between apical surface and baso-lateral surface of plasma membrane (many plasma membraneproteins, such as channels, pumps, receptors, are inserted only into

one or the other surface of the plasma membrane. These proteinscannot go (move) across the other side due to tight junction.

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Cellular JunctionsLocationFunction

Each junction consists of specific setof adhesion proteins

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Occluding JunctionsTight Junction

Separates two environments

Paracellular pathway:Passage through tight junction b/w cells

Small(er) moleculeswater, ions, etc

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Occluding JunctionsTight Junction

Separates two environments

External body vs

Extracellular fluid (body fluid)

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Tight JunctionTight Junction :

Separation of Apical vs basolateralplasma membrane (proteins)

Sealing strands: Plasma-membrane proteins(Occludin, Claudin)Visualized with freeze-fracture EM

Ca 2+-requirement

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Adherens Junctions(Zonula Adherens; Belt Desmosome)

Ca2+

-requirement

Cadherin

Anchoring Junctions:

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Desmosomes

Yellow marks: desmosomesKeratin filaments attach cytoplasmic

side of desmosomes

Epithelial Cells

Intercalated Discsin cardiac muscle

Maintain strong cell-cell adhesion

Anchoring Junctions:

h i i

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DesmosomesMaintain strong cell-cell adhesion

Anchoring Junctions:

F l Adh i

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Focal Adhesion

Integrin

Cytoskeletal fibers associated:Actin fibers

Focal adhesions Muscle (lateral) attachment

Intermediate filaments Hemidesmosomes

Cell-Matrix adhesion

F l Adh i

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Focal AdhesionCell-Matrix adhesion

CadherinIntegrin

Hemidesmosome (green)

I t i h ld b l l i

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Integrin holds basal lamina

Matrix-binding activity of Integrin is regulated by

signaling events (below)e.g. white blood cellsIntegrin (cluster) triggers intracellular signalling

Gap J nction

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Gap JunctionConnexin hexamer (per cell)

Connexon (hemichannel)Two connexons from adjacentcells to form intracellularchannels

Often found as patch (cluster)on a plasma membrane

Gap Junction

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Gap JunctionMolecules that pass through gap junctions dependent on type of connexons

Molecular size, charge, else?

Examples of GAP-junction-connection between cellsElectrical Synapses (Fishes, Insects etc)Cardiac muscle cells / smooth muscle cellsHepatocytesTracheal ciliated epithelial cells [IP 3]

Regulation of the channel (Open Close)

Summary of cell cell / cell matrix adhesions

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Summary of cell-cell / cell-matrix adhesions

Excitatory Properties of Plasma Membrane:

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Excitatory Properties of Plasma Membrane:Neuron

DendritesConducts impulses towardsthe cell body

Typically short, highly branched& unmyelinatedSurfaces specialized for contact

with other neurons(Post-synaptic terminals)

AxonsConduct impulses away from cell bodyLong, thin cylindrical process of cellArises at axon hillock

Impulses (Action potentials) arise fromthe initial segment (trigger zone)Side branches (collaterals) end in fine

processes called axon terminals

Synaptic end bulbs : containvesicles filled with neurotransmittersSynap ti c bou to ns

Resting Membrane Potential

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Nerve cells, (Neurons, Grial cells)All the other living cells

Potential energy difference at rest is (about) -70 mV(depending on the cell types)

Resting Membrane Potential

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Membrane channels (a partial list):

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Membrane channels (a partial list):Membrane conductance

Conductance:Each carried byspecific ion speciesNa +, K +, Ca 2+

Specific Ion Channels

Depolarization / Hyperpolarization

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Specific ion channels are openedaccording to the stimulus

Strength of the stimulus amount of change in membrane

potential(Not always; also effective range)

Local change in membranepotential spreads throughmembrane with decay

Depolarization / Hyperpolarizationof the Membrane Potential

Depolarization / Hyperpolarization

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Graded Potentials

Source of stimuliMechanical stimulation of membranes with mechanical gated

ion channels (pressure)Chemical stimulation of membranes

with ligand gated ion channels (neurotransmitter, hormone)

Graded/postsynaptic/receptor or generator potentialIons flow through ion channels and change membrane potential locallyAmount of change varies with strength of stimuli

Flow of current (ions) occurs only locallyDendrites and Cell bodies (usually not on axonal membrane)

Depolarization / Hyperpolarizationof the Membrane Potential

Action Potential

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Produced by voltage-gated ion channels

All or NoneVoltage threshold

Voltage-gated Na + Channel

Components of an Action Potential

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p

Voltage-gated Na + Channel:Deporalization activated

Voltage-gated K + Channel

Action Potential

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Action potentials start to arise only at trigger zone on axon hillock, inresponse to membrane depolarization due to graded potential(s)generated at dendrites / cell body

Action potential :bl f ll

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enables information processing all-or-none

The refractory period makes actionpotential to propagate unidirectional

Propagation of an action potential through an axon

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Timing delays with distance

Slow propagationHeight remains same

(red arrow)All or - None

p g p g

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Unmyelinated vs Myelinated axon

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A myelinated axon, myelin sheath [M] and aUnmyelinated axones [A] and a schwann cell [S] schwann cell [S]

Myelinated fibers: appear white jelly-rolllike wrappingsmade of lipoprotein = myelin

acts as electrical insulatorspeeds conduction of nerve impulses

Unmyelinated fibers: slow,small diameter fibers

only surrounded by neurilemmabut no myelin sheath wrapping

Saltatory action potential conduction :

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Saltatory action potential conduction :Speedup the action potential propagation