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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
2
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
3
• 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
4
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
6
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
7
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
8
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 anadditional 100 dowstream target enzymesEach of the 10,000 downstream targetsactivates 100 control factors
so rapidly have
1,000,000 active control fac
9
Receptors relay signals via intracellular SIGNALING CASCADESCASCADES
Push doorbell
Ring bell
Enzymaticactivation
ofmore
ENZYMES
10
TrimericG-protein-linked
Cell-surface receptors -large &/or hydrophilic ligands
ion-channel-linked
enzyme-linked (tyrosine kinase)
11
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
12
Review:Remember the Na+/K+ ATPase (Na+/K+ pump)?
[Na+] inside ~10mM; outside ~150mM[K+] inside ~100mM; outside ~5mMcell has membrane potential ~ -60mV
-60mVK+
A-
Na+
Cl-
--
- --
-+ +
++
++
13
Gated ion channelsspecifically let ions through membrane“keys”: small molecules (ligand-gated) or change in membrane potential (voltage-gated)
+ + +++ + ++ ++ + ++ + +
-- - - - - - - - - - - - - -
-60 mV inside
14
Acetylcholine:common neurotransmitter
opens ligand-gated Na+ channels on muscle celland some nerve cells
15
Gated ion channelsspecifically let ions through membrane“keys”: small molecules (ligand-gated)
+ + +++ + ++ ++ + + + +
-- - - - - - - - - - - - - -+
+
-60 mV inside
16
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
++++
+++
++++
+
17
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
++++
+++
++++
+
+++
+++
18
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
+++
+
+++
+ ++ +
+
+++
+++ ---+
19
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++
+
+
++
++ ++ +
+++++++ ---
++
--
20
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+
21
Action potential:
nerve impulse; rapid, self-propagating electrical signal
Musclecell
22
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
23
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
24
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.
25
TrimericG-protein-linked
Cell-surface receptors -large &/or hydrophilic ligands
ion-channel-linked
enzyme-linked (tyrosine kinase)
26
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
27
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
28
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
29
•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
30
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
31Like Fig 11-9
GG--proteinprotein--GTP activation ofGTP activation of
Effector Enzyme adenylyladenylyl cyclasecyclaseproduces the 2nd messenger cAMPcAMP
ActivatedG-protein
32
cAMPcAMP
activatestarget enzyme
Protein Protein KinaseKinase A (PKAA (PKA)
phosphorylatestarget proteins
InactivePKA
ActiveActivePKAPKA
33
inactive
+ P active
PhosphorylasePhosphorylase kinasekinase
Protein Protein KinaseKinase AAPhosphorylates downstream target enzymes
Breaks downStarch
Into Glucose 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
35
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??
36
How to shut it off?
AutoShut-off
G-protein α-subunit is on a timer
InherentGTPase activity
No ligand
37
How to shut it off?
cAMP-phosphodiesterase
rapidly cleavescAMP
(so short lived) 38
How do you turn it off?
kinases – phosphatases
DiametricallyOpposed…
Remember: whether you activeor inactivate by adding P
depends on the specific protein
39
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, death40
TWO subclasses of trimeric G-protein-activated signal transduction pathways:
A. target protein adenylate cyclasecAMP-> PKA
B. target protein phospholipase C
41
target effector enzyme is Phospholipase C
PLC cleaves a membranephospholipid (Phoshatidyl inositol) to
two 2nd Messengers:
Inositol-1,4,5-Trisphosphate(InsP3) &
Diacylglycerol (DAG)42
InsP3WaterSoluble
DAGLipid
Soluble
PIP2
43
DAG ActivatesProteinKinase C
(StartsCascade)
InsP3Ligandfor ER
ligand-gatedCa++
channels↑ Ca++ levels 44
Protein Kinase C phosphorylatestarget proteins (ser & thr)
cell growthregulation of ion channelscytoskeletonincreases cell pH Protein secretion
Ca++Binds & activates calmodulin
Calmodulin-binding proteins activated(kinases & phosphatases)
Response:
45
Figure 11.12
321
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.
5
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
46
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