week 3 dr zain , cell signalling 2014,5.pptx

8/10/2019 WEEK 3 Dr Zain , cell signalling 2014,5.pptx

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Cellularresponse

GrowthDifferentiationGrowth factorsDNA replication

Deathapotosis

Degradationoxidation of fattyacids

Releaseneurotransmitters

hormonestorage

glucose glycogen


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Requirement of Biosignaling

Requires a receptor to detect signals;

The receptor must link to or generate anintracellular response;

Such linking molecules are known as secondmessengers

This transduction system must meet four specificcriteria.


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Criterion 1: specificity High specificity only the target cell is influenced;

Receptor binding site ligand (signal molecule)complementary and non-covalent interaction follows the law of mass action


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r er on : amp ca on

often short-lived& low concentration

A single receptor binding event may elicit responses in multiple enzyme


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Criterion 3: Desensitization

feedbackcontrol

the aim of biosignaling is to produce a rapid and majorcellular response to a transient signal.


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Subcellular mediators of cell membrane receptor activation

1. cAMP2. Inositol

trisphosphate (IP3)

3. Diacylglycerol (DAG)

4. Ca++ Adenylylcyclase

cAMP

PKA

PLC-

IP3

Ca 2+

Second messengers

PKC

DAG


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G protein

G refers to the fact that protein binds

Guanine nucleotides GDP, GTP) ;

G proteins are integral membrane protein,hetertrimers );

G proteins have similar and subunits, butdiffer in the type of -subunits;

When G-protein is activated, the subunitdissociates to interact with an enzymes thatgenerate second messengers e.g. cAMP).


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Various types of G-protein families -subunits

G protein Signal Effected enzyme EffectGs epinephrine adenylyl cyclase stimulatoryglucagon

Gi catecholamines adenylyl cyclase inhibitory

Gq acetylcholine phospholipase C stimulatorycatecholamines

Gt photons cGMP stimulatoryphosphodiesterase


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An extracellular signal such as epinephrine or

PGE1 can have quite different effects on

different tissues or cell types.Depend on:

The type of receptor:-1. The type of G protein with which the

receptor is coupled;

2. The set of PKA target enzymes in the cell .
http://e/START.EXE


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http://e/START.EXE


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1) When all subunits are associated together, the G protein is in aninactivate state and in this state the subunit preferentially bindsGDP;

2) Once epinephrine or other cAMP-linked hormones bind to thereceptor, the hormone-receptor complex interacts with the Gprotein to bring about its activation;

3) Following interaction of the hormone-receptor complex with the

G protein, GTP displaces GDP;4) Binding of GTP produces a conformational change in the G

protein that causes the subunit to dissociate.5) The subunit then interacts with adenylyl cylcase;

6) The subunit is inactivated when the bound GTP is hydrolyzedto become GDP, which is catalyzed by a GTPase activity that ispart of the subunit.


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Self-Inactivation of Gs


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cAMP is synthesizedfrom ATP via adenylate

cyclase, and degraded bycyclic AMPphosphodiesteraseinhibited by caffeine);

Once cAMP issynthesized, it mustultimately activate orinhibit enzymes involvedin fuel metabolism,which is accomplishedby the phosphorylationof these regulated

enzymes in a reactioncatal zed b PKA.

cAMP acts as a second messenger


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Cyclic AMP as aSecond Messenger

In many cases, a G protein is activatedwhich then activates an enzyme,adenylyl cyclase which converts ATP to

cyclic AMP (cAMP).

cAMP then serves as a second messenger

which activates another enzyme in the

cell, often a protein kinase (an enzyme

that phophorylates a protein, activating

it).

cAMP initiates a chain of events (the

signal transduction pathway) that

results in some specific response of the

cell to the first messenger (hormone).


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Activation of Glycogen Breakdownby Epinephrine

1. Epinephrine (adrenaline) triggers a largeincrease in the rate of glycogen breakdown intoglucose-1-P units that then feed into glycolysis.

2. Binding of one epinephrine molecule activatesabout 100 G protein molecules which thenactivate 100 adenylyl cyclase molecules.

3. Each adenylyl cyclase generates about 100cAMP producing 10,000 cAMP molecules eachof which can activate an inactive protein kinase

A.


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Inositol Triphosphate 2 nd Messenger


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Two second messengers: IP 3 and diacylglycerol


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1) IP 3 opens channels to release calcium ions fromintracellular stores

1) IP3 is able to increase [Ca 2+] by associating with the

IP 3 channel or IP 3 receptor;2) At least three molecules of IP 3 must bind to sites on

the cytosolic side of the membrane protein to openthe channel and release Ca 2+ (allosteric interaction).

3) Increase [Ca 2+] activates PKC4) IP 3 is a short-lived messenger (less than a few

seconds)5) 2. Diacylglycerol activates protein kinase C (PKC)6) PKC phosphorylates Ser or Thr residues of specifictarget proteins, changing their catalytic activities;1) Isozymes of PKC: target protein specific and tissue-

specific.


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IP 3


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DAG 2 nd Messenger


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Calcium as a second Messenger

activates the regulatory protein Calmodulin,

which stimulates many enzymes and

transporters

Calmodulin:integral subunit of Ca 2+/calmodulin-dependent

protein kinase

(CaM kinase)

regulatory subunit of phosphorylase b kinase of

muscle


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Smooth Muscle Contraction: Mechanism


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Smooth Muscle Contraction: Mechanism


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These receptors, are coupled to a Gs-protein , whichstimulates the formation of cAMP.

Although increased cAMP enhances cardiac myocytecontraction , Activation of 2 -adrenoceptors in the lungscauses bronchodilation and in vascular smooth muscle anincrease in cAMP leads to smooth muscle relaxation. The reason for this is that cAMP inhibits myosin light chainkinase that is responsible for phosphorylating smooth

muscle myosin.

Therefore, increases in intracellular cAMP caused by 2-agonists inhibits myosin light chain kinase thereby producing

less contractile force (i.e., promoting relaxation).

Mechanism of 2-agonists promoting relaxation ).
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IP 3- Coupled Signal Transduction The IP 3 pathway is linked to activation of 1-adrenoceptors ,angiotensin II (AII) receptors, and endothelin-1 (ET-1) receptors and therefore is stimulated by alpha-agonists ,angiotensin II and endothelin-1 .

These receptors are coupled to a phospholipase C (PL-C)-coupled Gq-protein , which when activated, stimulates theformation ofinositol triphosphate ( IP 3 ) fromphosphatidylinositol biphosphate (PIP 2 ).

Increased IP 3 stimulates Ca ++ release by the sarcoplasmicreticulum in the heart, thereby increasing inotropy as one ofits actions
http://www.cvphysiology.com/Blood%20Pressure/BP010.htmhttp://www.cvphysiology.com/Blood%20Pressure/BP010.htmhttp://www.cvphysiology.com/Blood%20Pressure/BP010.htmhttp://www.cvphysiology.com/Blood%20Pressure/BP010.htmhttp://www.cvphysiology.com/Blood%20Pressure/BP015.htmhttp://www.cvphysiology.com/Blood%20Flow/BF012.htmhttp://www.cvphysiology.com/Blood%20Flow/BF012.htmhttp://www.cvpharmacology.com/vasoconstrictor/alpha-agonist.htmhttp://www.cvpharmacology.com/vasoconstrictor/alpha-agonist.htmhttp://www.cvphysiology.com/Blood%20Pressure/BP015.htmhttp://www.cvphysiology.com/Blood%20Flow/BF012.htmhttp://www.cvphysiology.com/Cardiac%20Function/CF010.htmhttp://www.cvphysiology.com/Cardiac%20Function/CF010.htmhttp://www.cvphysiology.com/Blood%20Flow/BF012.htmhttp://www.cvphysiology.com/Blood%20Flow/BF012.htmhttp://www.cvphysiology.com/Blood%20Flow/BF012.htmhttp://www.cvphysiology.com/Blood%20Pressure/BP015.htmhttp://www.cvphysiology.com/Blood%20Pressure/BP015.htmhttp://www.cvphysiology.com/Blood%20Pressure/BP015.htmhttp://www.cvpharmacology.com/vasoconstrictor/alpha-agonist.htmhttp://www.cvpharmacology.com/vasoconstrictor/alpha-agonist.htmhttp://www.cvpharmacology.com/vasoconstrictor/alpha-agonist.htmhttp://www.cvphysiology.com/Blood%20Flow/BF012.htmhttp://www.cvphysiology.com/Blood%20Flow/BF012.htmhttp://www.cvphysiology.com/Blood%20Flow/BF012.htmhttp://www.cvphysiology.com/Blood%20Flow/BF012.htmhttp://www.cvphysiology.com/Blood%20Flow/BF012.htmhttp://www.cvphysiology.com/Blood%20Pressure/BP015.htmhttp://www.cvphysiology.com/Blood%20Pressure/BP010.htmhttp://www.cvphysiology.com/Blood%20Pressure/BP010.htmhttp://www.cvphysiology.com/Blood%20Pressure/BP010.htmhttp://www.cvphysiology.com/Blood%20Pressure/BP010.htm


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Gs Protein and Gi Protein Coupled Signal Transduction


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Gs-Protein and Gi-Protein Coupled Signal TransductionG-proteins are linked to an enzyme,adenylyl cyclase, that dephosphorylates

ATP toform cyclic AMP (cAMP).Gs- protein (stimulatory G-protein) activation(e.g., via-adrenoceptors) increases cAMP by activatingadenylyl cyclase. cAMPthen activates PK-A (cAMP stimulatedprotein kinase) and causes increasedcellular influx of Ca++by phosphorylation and activation of L-type calciumchannels, and enhanced release of Ca++ by thesarcoplasmic reticulum in the heart.These and other intracellular events increase inotropy (muscle contractility),chronotropy (heart rate), dromotropy (velocity of electrical conduction) andlusitropy (relaxation rate).

Activation ofGi-proteins (inhibitory G-protein), for example by adenosineand muscarinic agonists binding to their receptors, decreases cAMP (throughadenylyl cyclase inactivation), inactivates PK-A, decreases Ca++ entry into the celland release by the sacroplasmic reticulum, and increasesoutward,hyperpolarizing K +currents . Activation of the Gi-protein pathway therefore

enhancesrepolarization.Gi-protein activation produces effects that are opposite to those elicited by Gs-protein activation; however, Gi-protein effects are primarily found in theSAnode and AV node, and lead to a decrease in sinus rate and AV nodal conductionvelocity, respectively, with minimal effects on muscle contractility. In contrast, Gs-

protein strongly stimulates muscle contraction in addition to having nodal effects.
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I o n - C

h a n n e l

R e c e p

t o r s


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Note steps

involved:1. Ligand

Reception2. Receptor

Dimerizati

on3. Catalysis(Phosphor ylization)

4. Subsequent Protein

Activation5. Further

Transduction

6. Response


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teroid hormones


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Steroid hormones bind tointracellular receptors

Steroidhormone

The steroid-receptor

complex binds to DNA,turning specific genes onor off

TARGETCELL Receptor

protein

1

2

3NUCLEUS

DNA

Hormone-receptorcomplex

4

mRNATranscription

Newprotein

Cellular response:activation of a geneand synthesis ofnew protein

In this example, a newprotein is synthesized

teroid hormones

steroid receptor


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Regulation of transcription by steroid hormones

steroid receptor


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I n t r a c e

l l u l a r R e c e p

t i o n


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week 3 dr zain , cell signalling 2014,5.pptx

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