Cell Communication
Single cell
Multicellular
organism
Budding yeast cells
responding to mating
factor
One haploid cell
Another haploid cell
Sexual mating
(requires many
downstream signal
pathways to be
activated)
GENERAL PRINCIPLES OF CELL COMMUNICATION
Extracellular signal molecules bind to specific receptors
Extracellular signal molecules can act over either short or long distance
Autocrine signaling can coordinate decision
by groups of identical cells
“Community effect” in early development
In tumor biology---cancer cells stimulate their own proliferation
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
Each cell is programmed to respond to specific combinations
of extracellular signal molecules
Different cells can respond differently to the
same extracellular signal molecules
The concentration of a molecule can be adjusted quickly
only if the lifetime of the molecule is short
Nitric oxide gas signals by binding directly to an enzyme
inside the target cell
Nitroglycerine --- angina
Viagra --- PDE inhibitor
CO
Nuclear receptors are ligand-activated gene regulatory proteins
Ligand-binding domain
The three largest classes of cell-surface receptor proteins are
ion-channel-linked, G-proteins-linked, and enzyme-linked receptors
Most activated cell-surface receptors relay signals via small
molecules and a network of intracellular signaling proteins
Some intracellular signaling proteins act as
molecular switches
2% of human genes
Monomeric GTPase
Trimeric GTPase
Signal integration by protein phosphorylation
Intracellular signaling complexes enhance the speed,
efficiency, and specificity of the response
Complex forms transiently
Interactions between intracellular signaling proteins are
mediated by modular binding domains
PDZ Domain
www.cellsignal.com
Domain binding and function: PDZ domains bind to the C-terminal 4–5 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 proteinprotein 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), 1067–1076.
The third PDZ domain
from PSD-95.
Binding Examples:
PDZ domain
proteins
Binding partners
domain
binding sites
Post-synaptic
Density Protein 95
(PSD-95)
NMDA receptor B via PDZ1
and PDZ2 of PSD-95
– IESDV-COOH
Post-synaptic
Density Protein 95
(PSD-95)
Kvl1.4 Shaker-type K+
channel via PDZ1 and
PDZ2 of PSD-95
– VETDV-COOH
Post-synaptic
Density Protein 95
(PSD-95)
Neural Nitric Oxide
Synthase (nNOS) via PDZ2
PDZ/PDZ
interaction
Lipid raft
Enriched in cholesterol
and glycolipids
c-Src tyrosine kinase
Cells can respond abruptly to a gradually increasing
concentration of an extracellular signal
Chicken oviduct cells
Stimulated by estradiol
effector/target : 1~16
maximal activation
One type of signaling mechanism expected to show
a steep thresholdlike response
A cell can remember the effect of some signals
Autophosphorylation of
Ca2+/CaM-kinase II
Signals trigger muscle cell determination
Cells can adjust their sensitivity to a signal
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
Trimeric G proteins disassemble to relay signals from
G-protein-linked receptors
Transducin-G protein in visual transduction
The disassembly of a activated G-protein into
two signaling components
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
Some G-proteins signal by regulating the production
of cyclic AMP
~5 X 10-8 M
>10-6 M
Nerve cell culture, preloaded with a fluorescent protein that changes
its fluorescence when it binds to cAMP.
(Science 260:222-226, 1993)
cAMP-dependent protein kinase (PKA) mediate most of
the effects of cyclic AMP
Role of cAMP, PKA in glycogen
metabolism
How gene transcription is activated by a rise
in cAMP concentration
(CRE, cAMP response element)
Role of protein phosphatases?
Some G-proteins activate the inositol phospholipid signaling
pathway by activating phospholipase C-b
(<1% of total phospholipids)
The two branches of the inositol phospholipid pathway
Ca2+ functions as a ubiquitous intracellular messenger
Ca2+ signaling in fertilization of starfish, detected by
Ca2+-sensitive fluorescence dye
The main ways eucaryotic cells maintain a very low
concentration of free Ca2+ in their cytosol
The frequency of Ca2+ oscillations influences
a cell’s response
In a liver cell
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
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 --(1) 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
CaM-kinases II as a frequency decoder of Ca2+ oscillations
CaM-kinase II is immobilized on a solid surface
+a brain protein phosphatase
+repetitive pulse of Ca2+/calmodulin at different frequency
Kinase activity assay
What a nice experiment it is!
Smell and vision depend on G-protein-linked receptors
that regulate cyclic-nucleotide-gated ion channels
Cyclic GMP
A rod photoreceptor cell
The response of a rod photoreceptor cell to light
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
G-protein-linked receptors desensitization depends on
receptor phosphorylation
SIGNALING THROUGH ENZYME-LINKED
CELL-SURFACE RECEPTORS
Six classes:
1.
2.
3.
4.
5.
6.
Receptor tyrosine kinases
Tyrosine kinase-associated receptors
Receptorlike tyrosine phosphatases
Receptor serine/threonine kinases
Receptor guanylyl cyclases
Histidine-kinase-associated receptors
Activated tyrosine kinases phosphorylate themselves
angiogenesis
cell/axon migration
Three ways in which signaling proteins can
cross-link receptor chains
Monomeric vs. dimeric ligand
Inhibition of signaling through normal receptor tyrosine
kinases by an excess of mutant receptors
As a tool for determining normal function of receptor
Phosphorylated tyrosine serves as docking sites
for proteins with SH2 domains
The binding of SH2-containing intracellular signaling
proteins to an activated PDGF receptor
determine the
binding specificity
Ras is activated by a guanine nucleotide exchange factor
GEF: guanine nucleotide exchange factor
GAP: GTPase-activating protein
In cells [GTP] > [GDP] ~10 fold
The activation of Ras by an activated receptor
tyrosine kinase
The MAP-kinase serine/threonine phosphorylation
pathway activated by Ras
The organization of MAP-kinase pathway
by scaffold proteins in budding yeast
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
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
The PI 3-kinase/protein kinase B signaling pathway can
stimulate cells to survive and grow
Brief summarization
Signal proteins of the TGF-b superfamily act through
receptor serine/threonine kinases and Smads
Kinase catalytic domain ~250 amino acids
SIGNALING PATHWAY THAT DEPEND ON
REGULATED PROTEOLYSIS
1. Notch
2. Wnt
3. Hedgehog
4. NF-kB
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
The processing and activation of Notch by
proteolytic cleavage
Inhibit neural differentiation
Wnt proteins bind to Frizzled receptors and
inhibit the degradation of b-catenin
(APC, adenomatous polyposis coli,
a tumor suppressor )
(c-Myc protein)
Multiple stressful and proinflammatory stimuli act through
an NF-kB-dependent signaling pathway
inflammation
development
cancer