Cell Communication

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Cell Communication
Single cell Multicellular organism
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Budding yeast cells responding to mating factor
One haploid cell Another haploid cell Sexual
mating (requires many downstream
signal pathways to be activated)
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GENERAL PRINCIPLES OF CELL COMMUNICATION
Extracellular signal molecules bind to specific
receptors
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Extracellular signal molecules can act over
either short or long distance
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Autocrine signaling can coordinate decision by
groups of identical cells
Community effect in early development In tumor
biology---cancer cells stimulate their own
proliferation
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Gap junctions allow signaling information to be
shared by neighboring cells
Ca2, cAMP etc. but not for proteins or nucleic
acids Intracellular electrodes, small
water-soluble dyes Connexin 43 deficiency ---
abnormal heart development
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Each cell is programmed to respond to specific
combinations of extracellular signal molecules
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Different cells can respond differently to the
same extracellular signal molecules
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The concentration of a molecule can be adjusted
quickly only if the lifetime of the molecule is
short
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Nitric oxide gas signals by binding directly to
an enzyme inside the target cell
Nitroglycerine --- angina Viagra --- PDE
inhibitor CO
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Nuclear receptors are ligand-activated gene
regulatory proteins
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Ligand-binding domain
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The three largest classes of cell-surface
receptor proteins are ion-channel-linked,
G-proteins-linked, and enzyme-linked receptors
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Most activated cell-surface receptors relay
signals via small molecules and a network of
intracellular signaling proteins
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Some intracellular signaling proteins act as
molecular switches
Monomeric GTPase Trimeric GTPase
2 of human genes
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Signal integration by protein phosphorylation
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Intracellular signaling complexes enhance the
speed, efficiency, and specificity of the
response
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Complex forms transiently
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Interactions between intracellular signaling
proteins are mediated by modular binding domains
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www.cellsignal.com
PDZ Domain
  
Domain binding and function PDZ domains bind to
the C-terminal 45 residues of their target
proteins, frequently transmembrane receptors or
ion channels. These interactions can be of high
affinity (nM Kd). The consensus binding sequence
contains a hydrophobic residue, commonly Val or
Ile, at the very C-terminus. Residues at the 2
and 3 positions are important in determining
specificity. PDZ domains can also heterodimerize
with PDZ domains of different proteins,
potentially regulating intracellular signaling.
In addition to engaging in protein-protein
interactions, several PDZ domains including those
of syntenin, CASK, Tiam1 and FAP are capable of
binding to the phosphoinositide PIP2. PIP2-PDZ
domain binding is thought to control the
association of PDZ domain-containing proteins
with the plasma membrane. Structure Reference
Doyle, D.A. et al. (1996) Cell 85(7), 10671076.
The third PDZ domain from PSD-95.

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Lipid raft
Enriched in cholesterol and glycolipids
c-Src tyrosine kinase
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Cells can respond abruptly to a gradually
increasing concentration of an extracellular
signal
effector/target 116
Chicken oviduct cells Stimulated by estradiol
maximal activation
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One type of signaling mechanism expected to show
a steep thresholdlike response
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A cell can remember the effect of some signals
Signals trigger muscle cell determination
Autophosphorylation of Ca2/CaM-kinase II
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Cells can adjust their sensitivity to a signal
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SIGNALING THROUGH G-PROTEIN-LINKED CELL-SURFACE
RECEPTORS
1. The largest family of cell-surface
receptors 2. 5 of the C. elegans genes 3. Signal
molecules hormones, neurotransmitters and
local medicators 4. Rhodopsin-light receptor 5.
Genome sequencing --- vast numbers of new
family members 6. Major targets for drug discovery
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Trimeric G proteins disassemble to relay signals
from G-protein-linked receptors
Transducin-G protein in visual transduction
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The disassembly of a activated G-protein into
two signaling components
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The switching off of the G-protein a subunit by
the hydrolysis of its bound GTP
RGS proteins --- regulators of G protein
signaling, act as a subunit-specific GTPase
activating proteins (GAPs) 25 RGS proteins in
the human genome
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Some G-proteins signal by regulating the
production of cyclic AMP
gt10-6 M
5 X 10-8 M
Nerve cell culture, preloaded with a fluorescent
protein that changes its fluorescence when it
binds to cAMP.
(Science 260222-226, 1993)
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cAMP-dependent protein kinase (PKA) mediate most
of the effects of cyclic AMP
Role of cAMP, PKA in glycogen metabolism
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How gene transcription is activated by a rise in
cAMP concentration
(CRE, cAMP response element)
Role of protein phosphatases?
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Some G-proteins activate the inositol
phospholipid signaling pathway by activating
phospholipase C-b
(lt1 of total phospholipids)
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The two branches of the inositol phospholipid
pathway
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Ca2 functions as a ubiquitous intracellular
messenger
Ca2 signaling in fertilization of starfish,
detected by Ca2-sensitive fluorescence dye
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The main ways eucaryotic cells maintain a very
low concentration of free Ca2 in their cytosol
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The frequency of Ca2 oscillations influences a
cells response
In a liver cell
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Ca2/calmodulin-dependent protein kinases
(CaM-kinases) mediate many of the actions of
Ca2 in animal cells
A peptide derived from CaM-Kinase II
The structure of Ca2/calmodulin
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The activation of CaM-kinases II
2 of total mass in some brain regions,
especially in synapses

• It can function as a molecular memory device ---
• Learning defect (where things are in space) in
  mutant mice that
• lack the brain-specific subunit of
  CaM-kinase II
• (2) Same defect also observed in mutant mice that
  
• have their CaM-kinase II mutated at the
  autophosphorylation site

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CaM-kinases II as a frequency decoder of Ca2
oscillations
What a nice experiment it is!
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Smell and vision depend on G-protein-linked
receptors that regulate cyclic-nucleotide-gated
ion channels
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Cyclic GMP
A rod photoreceptor cell
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The response of a rod photoreceptor cell to light
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Extracellular signals are greatly amplified by
the use of small intracellular mediators and
enzymatic cascades
Amplification in the light-induced catalytic
cascade in vertebrate rods
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G-protein-linked receptors desensitization
depends on receptor phosphorylation
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SIGNALING THROUGH ENZYME-LINKED CELL-SURFACE
RECEPTORS
Six classes 1. Receptor tyrosine kinases 2.
Tyrosine kinase-associated receptors 3.
Receptorlike tyrosine phosphatases 4. Receptor
serine/threonine kinases 5. Receptor guanylyl
cyclases 6. Histidine-kinase-associated receptors
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Activated tyrosine kinases phosphorylate
themselves
angiogenesis cell/axon migration
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Three ways in which signaling proteins can
cross-link receptor chains
Monomeric vs. dimeric ligand
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Inhibition of signaling through normal receptor
tyrosine kinases by an excess of mutant receptors
As a tool for determining normal function of
receptor
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Phosphorylated tyrosine serves as docking sites
for proteins with SH2 domains
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The binding of SH2-containing intracellular
signaling proteins to an activated PDGF receptor
determine the binding specificity
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Ras is activated by a guanine nucleotide exchange
factor
GEF guanine nucleotide exchange factor GAP
GTPase-activating protein
In cells GTP gt GDP 10 fold
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The activation of Ras by an activated receptor
tyrosine kinase
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The MAP-kinase serine/threonine phosphorylation
pathway activated by Ras
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The organization of MAP-kinase pathway by
scaffold proteins in budding yeast
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PI 3-kinase produces inositol phospholipid
docking sites in the plasma membrane
Cell division vs. cell growth PI 3 kinase is one
of the major cell growth signaling transduces
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The recruitment of signaling proteins with PH
domains to the plasma membrane during B cell
activation
SH2 domain
Mutation of BTK leads to severely deficiency in
Ab production
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The PI 3-kinase/protein kinase B signaling
pathway can stimulate cells to survive and grow
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Brief summarization
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Signal proteins of the TGF-b superfamily act
through receptor serine/threonine kinases and
Smads
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Kinase catalytic domain 250 amino acids
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SIGNALING PATHWAY THAT DEPEND ON REGULATED
PROTEOLYSIS

• Notch
• Wnt
• Hedgehog
• NF-kB

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The receptor protein Notch is activated by
cleavage
In Drosophila, mutation in Delta leads to produce
a huge excess of neurons at the expense of
epidermal cells
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The processing and activation of Notch by
proteolytic cleavage
Inhibit neural differentiation
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Wnt proteins bind to Frizzled receptors and
inhibit the degradation of b-catenin
(c-Myc protein)
(APC, adenomatous polyposis coli, a tumor
suppressor )
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Multiple stressful and proinflammatory stimuli
act through an NF-kB-dependent signaling pathway
inflammation development cancer