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External signal is received and convertedto another form to elicit a response

BCOR 011 Lecture 19 Oct 12, 2005I. Cell Communication – Signal Transduction

Chapter 11

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Lecture OutlineLecture Outline

1. Types of intercellular communication2. The primary receiver – Receptors

3. - the concept of AMPLIFICATION4. Types of receptors5. Ion Channels – Membrane depolarization6. Trimeric G-Protein coupled receptors

- the cAMP signal pathway- the phophatidyl inositol pathway, Ca++ release

7. Tyrosine Kinase – MAP Kinase Cascade8. Internal cytosolic receptor systems

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• Cells sense and respond to the environmentProkaryotes: chemicalsHumans:

light - rods & cones of the eyesound – hair cells of inner earchemicals in food – nose & tongue

• Cells communicate with each other

Direct contactChemical signals

External signals are converted to External signals are converted to Internal ResponsesInternal Responses

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General principles:

2. Signals have different chemical natures.

4. Cells respond to sets of signals.

3. The same signal can induce adifferent response in different cells.

5. Receptors relay signals viaintracellular signaling cascadescascades.

1. Signals act over different ranges.

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Endocrine

long distanceex. estrogen, epinephrine

Paracrinelocal ex. nitric oxide, histamines,prostaglandins

Neuronal/Synapticex. neurotransmitters

direct contactCell-cell recognitionex. delta/notch

Signals act over different ranges

Like Fig 11.4

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Cells detect signal & respond

Signal transduction: ability of cell to translatereceptor-ligand interaction into a change in behavioror gene expression

1º messenger

2º messengers

EffectorEnzymes Target

Enzymes

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EXTRACELLULARFLUID

Receptor

Signal molecule

Relay molecules in a signal transduction pathway

Plasma membraneCYTOPLASM

Activationof cellularresponse

Figure 11.5

Reception1 Transduction2 Response3

PrimaryPrimaryMessengerMessenger SecondarySecondary

MessengersMessengersTargetTargetEnzymesEnzymes

Cascade EffectCascade Effect

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Each protein in a signaling pathway–– AmplifiesAmplifies the signal by activating multiple

copies of the next component in the pathway

1 primary signal- activates an enzyme activity, processes 100 substrates per second

Primary enzyme activates 100 target enzymes

Each of the 100 enzymes activates an

additional 100 dowstream target enzymesEach of the 10,000 downstream targetsactivates 100 control factors

so rapidly have

1,000,000 active control fac

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Receptors relay signals via intracellular SIGNALING CASCADESCASCADES

Push doorbell

Ring bell

Enzymaticactivation

ofmore

ENZYMES

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TrimericG-protein-linked

Cell-surface receptors -large &/or hydrophilic ligands

ion-channel-linked

enzyme-linked (tyrosine kinase)

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Ion channel receptors

Cellularresponse

Gate open

Gate close

Ligand-gatedion channel receptor

Plasma Membrane

Signalmolecule(ligand)

Figure 11.7

Gate closed Ions

Examples:

Muscle Contraction

Nerve Cell communication

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Review:Remember the Na+/K+ ATPase (Na+/K+ pump)?

[Na+] inside ~10mM; outside ~150mM[K+] inside ~100mM; outside ~5mM

cell has membrane potential ~ -60mV

-60mVK+

A-

Na+

Cl-

--

- --

-+ +

++

++

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Gated ion channelsspecifically let ions through membrane“keys”: small molecules (ligand-gated) or change in membrane potential (voltage-gated)

+ + +++ + ++ ++ + ++ + +

-- - - - - - - - - - - - - -

-60 mV inside

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Acetylcholine:common neurotransmitter

opens ligand-gated Na+ channels on muscle celland some nerve cells

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Gated ion channelsspecifically let ions through membrane“keys”: small molecules (ligand-gated)

+ + +++ + ++ ++ + + + +

-- - - - - - - - - - - - - -+

+

-60 mV inside

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Influx of Na+ ions causes local, transient depolarization of membrane potential

nerve impulse (action potential)

Gated ion channelsspecifically let ions through membrane“keys”: small molecules (ligand-gated)

+ + +++ + ++ ++ + +

+

+ +

-- - - - - - -- - - -

+10 mV inside

++++

+++

++++

+

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Influx of Na+ ions causes local, transient depolarization of membrane potential

nerve impulse (action potential)

Gated ion channelsspecifically let ions through membrane“keys”: small molecules (ligand-gated) or change in membrane potential (voltage-gated)

+ + +++ + ++ ++ + +

+

+ +

-- - - - - - - -

+10 mV inside

++++

+++

++++

+

+++

+++

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Influx of Na+ ions causes local, transient depolarization of membrane potential

nerve impulse (action potential)

Gated ion channelsspecifically let ions through membrane“keys”: small molecules (ligand-gated) or change in membrane potential (voltage-gated)

+ + +++ + ++ ++ + +

+

+ +

-- - - - - - - -

+10 mV inside

+++

+

+++

+ ++ +

+

+++

+++ ---+

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Influx of Na+ ions causes local, transient depolarization of membrane potential

nerve impulse (action potential)

Gated ion channelsspecifically let ions through membrane“keys”: small molecules (ligand-gated) or change in membrane potential (voltage-gated)

+ + +++ + ++ ++ + +

+

+ +

-- - - - - - - -

+10 mV inside++

+

+

++

++ ++ +

+++++++ ---

++

--

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a. polarized

b. Action potential Initiated by LigandLigand--gated Nagated Na++

channels openingchannels openingLocal depolarization

Depolarization opensVoltageVoltage--gated Nagated Na++

channelschannelsre-polarization Na+channels close K+

channels openAction potentialPropagates to as moreVoltageVoltage--gated channelsgated channels open

Transmission of action potential

Na+

Na+

Na+

K+

K+

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Action potential:

nerve impulse; rapid, self-propagating electrical signal

Musclecell

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Signal transmitted to musclecell across a synapse

a. Depolarization opens voltage-gated Ca+2 channels

b. Ca+2 rushes in; Vesicles fuse with membrane

c. Neurotransmitter released; opens ligand-gated Na+ channels on muscle cell

Depolarizes muscle cell Signal: electrical to chemical to electrical

b.

c.

a.

Musclecell

Musclecell

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Depolarization of Muscle Cell Results in release of [Ca++]:

Ca++ from SERIn CytosolTriggers Activation of Myosin ATPaseTo “walk along” actin filaments – causing contraction

Typically in cytosol Ca++ is 10-7 M, maintained ultra-low by active transport “pumps” Ca++/ATPases “vacuums”

Ca++ Stored in SmoothEndoplasmicreticulum

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Acetylcholine degraded by acetylcholinesterase or removed by re-uptake & endocytosis

if not removed……

Turning off the synapse……..

Potent enzyme inhibitors

Post-synaptic membrane can’t repolarize

Paralysis, Tetany

a,b Pesticidesc Nerve gases

a.

b.

c.

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TrimericG-protein-linked

Cell-surface receptors -large &/or hydrophilic ligands

ion-channel-linked

enzyme-linked (tyrosine kinase)

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Trimeric G protein-linked receptors:largest family of cell-surface receptors

7-pass membrane receptor

activates G-protein

by GTP exchange

ligand

Ligand binding

G-proteinGTP

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G-protein-linkedReceptor Plasma Membrane

EnzymeG-protein(inactive)CYTOPLASM

Cellular response

Activated Effectorenzyme

ActivatedReceptor

Signal molecule Inctivateenzyme

Segment thatinteracts withG proteins

GDP

GDP

GTP

GTP

P i

Signal-binding site

Figure 11.7

GDP

TrimericTrimeric GG--proteinprotein--linked linked receptorsreceptors

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G-protein activation

“molecular switch”(b) Ligand binds

G-protein associates

(c) GDP-GTP exchange•α-Subunit dissociates

inactive

Active G-Protein-GTP-> allosteric modulator

of target effector enzymeactive

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•There are TWO broad subclasses oftrimeric G-protein-activated signal transduction pathways:

depends on their target effector enzymesA. A. adenylyladenylyl cyclasecyclaseB. B. phospholipasephospholipase CC

•All G-proteins – similar structure/activation

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An activated An activated GGαα--proteinprotein--GTPGTP–– Can trigger the formation of Can trigger the formation of cAMPcAMP, , which then acts as a second messenger which then acts as a second messenger in cellular pathwaysin cellular pathways

ATP

GTP

cAMP

Proteinkinase A

Cellular responses

G- protein- linkedreceptor

AdenylylcyclaseG protein

First messenger(signal moleculesuch as epinephrine)

Figure 11.10

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31Like Fig 11-9

GG--proteinprotein--GTP activation ofGTP activation of

Effector Enzyme adenylyladenylyl cyclasecyclaseproduces the 2nd messenger cAMPcAMP

ActivatedG-protein

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cAMPcAMP

activatestarget enzyme

Protein Protein KinaseKinase A (PKAA (PKA)

phosphorylatestarget proteins

InactivePKA

ActiveActivePKAPKA

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inactive

+ P active

PhosphorylasePhosphorylase kinasekinase

Protein Protein KinaseKinase AAPhosphorylates downstream target enzymes

Breaks downStarch

Into Glucose

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34Figure 11.13 Glucose-1-phosphate(108 molecules)

Glycogen

Active glycogen phosphorylase (106)Inactive glycogen phosphorylase

Active phosphorylase kinase (105)Inactive phosphorylase kinase

Inactive protein kinase A

Active protein kinase A (104)

ATP

Cyclic AMP (104)

Active adenylyl cyclase (102)Inactive adenylyl cyclase

Inactive G protein

Active G protein (102 molecules)

Binding of epinephrine to G-protein-linked receptor

(1 molecule)Transduction

Response

Reception

1

102

104

105

106

108

A SignalCascadeCascade

amplification amplification

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cAMP regulated pathways

Function target tissue signal

Glycogen breakdown muscle,liver epinephrineHeart rate cardiovascular epinephrineWater reabsorption kidney antidiuretic

hormone

What are targets for Protein Kinase A??

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How to shut it off?

AutoShut-off

G-protein α-subunit is on a timer

InherentGTPase activity

No ligand

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How to shut it off?

cAMP-phosphodiesterase

rapidly cleavescAMP

(so short lived)

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How do you turn it off?

kinases – phosphatases

DiametricallyOpposed…

Remember: whether you activeor inactivate by adding P

depends on the specific protein

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What if you can’t turn off cascade?

Vibrio cholera - causes cholera7 great pandemics, Ganges Valley, Bangladesh

Normal gut: H20, NaCl, NaHCO3 secretioncontrolled by hormones via Gs/cAMP signalpathways

V. cholera – secretes enterotoxin, chemically modifies αGs – no GTPase activity - stays ON

Severe watery diarrhea – dehydration, death

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TWO subclasses of trimeric G-protein-activated signal transduction pathways:

A. target protein adenylate cyclasecAMP-> PKA

B. target protein phospholipase C

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target effector enzyme is Phospholipase C

PLC cleaves a membranephospholipid (Phoshatidyl inositol) to

two 2nd Messengers:

Inositol-1,4,5-Trisphosphate(InsP3) &

Diacylglycerol (DAG)

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InsP3WaterSoluble

DAGLipid

Soluble

PIP2

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DAG ActivatesProteinKinase C

(StartsCascade)

InsP3Ligandfor ER

ligand-gatedCa++

channels↑ Ca++ levels

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Protein Kinase C phosphorylatestarget proteins (ser & thr)

cell growthregulation of ion channelscytoskeletonincreases cell pH Protein secretion

Ca++Binds & activates calmodulinCalmodulin-binding proteins activated(kinases & phosphatases)

Response:

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Figure 11.12

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IP3 quickly diffuses throughthe cytosol and binds to an IP3–gated calcium channel in the ERmembrane, causing it to open.

4 The calcium ionsactivate the nextprotein in one or moresignaling pathways.

6Calcium ions flow out ofthe ER (down their con-centration gradient), raisingthe Ca2+ level in the cytosol.

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DAG functions asa second messengerin other pathways.

Phospholipase C cleaves aplasma membrane phospholipidcalled PIP2 into DAG and IP3.

A signal molecule bindsto a receptor, leading toactivation of phospholipase C.

EXTRA-CELLULARFLUID

Signal molecule(first messenger)

G protein

G-protein-linkedreceptor

Variousproteinsactivated

Endoplasmicreticulum (ER)

Phospholipase CPIP2

IP3(second messenger)

DAG

Cellularresponse

GTP

Ca2+

(second messenger)

Ca2+

IP3-gatedcalcium channel

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SummarySummary- signaling is endocrine, paracrine, synaptic, or direct cell contact- signal transduction is mediated by receptor proteins- Receptors bind primary signal (ligand)- Some amplification event occurs- Example: ligand gated ion channel opens

influx of ions triggers change in activity (vesicle fusion in nerve end, contraction in muscle)

- Example: ligand binds to 7-pass membrane receptorcatalyzes GTP exchangeto Ga-subunit of trimeric G-proteinactive Ga-subunit-GTP is allosteric activator ofeffector enzymes:

- ADENYLATE CYCLASE: makes cyclic AMP- PHOSPHOLIPASE C: makes DAG and IP3

these second messengers activate target enzymesTrigger cascades cascades

- Must shut off cascade: removal of ligand, hydrolysis of GTP,phosphodiesterase, protein phosphatases, Ca++ ion pumps