(1)cell signaling
TRANSCRIPT
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"Receptor-ligand interactions
- cell signaling, adhesion,
motility, cell migration"
Patricia Zuk, PhD
Research Director
Regenerative Bioengineering and Repair(REBAR) Lab
Department of Surgery
David Geffen School of Medicine at UCLA
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No cell lives in isolation
survival depends on an elaborateintercellular communication network that
coordinates growth, differentiation andmetabolism
cells adjacent to one another frequentlycommunicate through cell-cell contact
other forms of communication coverlarger distances = extracellular signalingmolecules
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Extracellular Signalling
signaling molecules are released by signaling cells
the signal is called the ligand
the ligand binds to its specific receptoron a target cell
this ligand-receptor interaction induces a conformational orshape-change in the receptor
produces a specific response - called the cellular response
can include a vast array of compounds
e.g. small amino acid derivatives, small peptides,proteins
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Cell-to-cell communication by extracellular signaling usuallyinvolves six steps
(1) synthesis of the signaling molecule by the signaling cell
(2) release of the signaling molecule by the signaling cell
(3) transport of the signal to the target cell
(4) detection of the signal by a specific receptor protein receptor-ligand specificity
(5) a change in cellular metabolism, function, or development =cel lu lar respon se
triggered by the receptor-ligand complex specific to the ligand-receptor complex
(6) removal of the signal, which usually terminates the cellularresponse degredation of ligand
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Signaling molecules operate over various distances in animals
-extracellular signaling can occur over:
1. large distanc es or endocr ine sign al ing signaling molecules are called
hormones
- act on target cells distant from their site of synthesis
-usually carried through the bloodstream2. sho rt distances or paracr ine signal ing affects target cells within proximity
to the cell that synthesized the molecule
-usually mediated by neurotransmitters and some growth
factors
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Signaling molecules operate over various distances in animals
-extracellular signaling can occur over:
3. no d istance or autocr ine sign al ing the signal feeds-back and affects
itself
-action of many growth factors
-these compounds generally act on themselves toregulate proliferation
-seen frequently in tumor cells
-many compounds can act through two or even three types of cell signaling
e.g. growth factors (e.g. EGF) paracrine and autocrine and endocrine
-epinephrine endocrine and paracrine
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Circulating & Local Hormones
Circulat ing
hormones
act on distant targets travel in blood endocrine hormones
Local ho rmones
paracrine hormones &autocrine hormones
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Hormones
two types
lipid soluble water soluble
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Lipid-Soluble Hormones
-lipid-soluble hormones can easily
enter a cell by diffusing through the
plasma membrane
-PROBLEM: how do they travel in the
water-based blood??-SOLUTION: they are carried by
carrier-proteins
-these hormones then enter their
target cell where they result in a
specific cellular effect or response
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Water-soluble Hormones
-water soluble hormones can easily travel within the blood
-PROBLEM: how do they enter a cell and result in a cellular response??
-SOLUTION: binding to specific cell-surface receptors
-this binding activates the receptor and results in a series of cellular events called
the second messenger system
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Lipid-soluble Hormones
Steroids lipids derived from cholesterol in SER different functional groups attached tocore of structure provide uniqueness interact with specific intracellular
receptors (within the cell) to turnspecific genes on or off
effective for hours or days
Thyro id hormones tyrosine ring plus attached iodines arelipid-soluble
activate enzymes involved in thecatabolism of fats and glucose
help set our basal metabolic rate Retinoids
vitamin A derivatives have dramatic effects on proliferationand differentiation plus cellular death(i.e. apoptosis)
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Water-soluble Hormones
Am ino acid der ivat ives,
small pept ides andprotein hormones modified amino acids or
amino acids put together serotonin, melatonin,
histamine, epinephrine
larger peptide hormones insulin and glucagon
Eicosanoids
derived from arachidonicacid(fatty acid) prostaglandins or
leukotrienes
prostaglandins despite beinglipidphilic bind to cell
surface receptors
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Action of Lipid-Soluble Hormones
Hormone diffusesthrough phospholipid
bilayer & into cell
Binds to receptor
turning on/off specific
genes
New mRNA is formed
& directs synthesis ofnew proteins
New protein alters
cells activity
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Action of Water-Soluble Hormones
Can not diffuse through plasma
membrane Hormone receptors are integral
membrane proteins
act as first messenger Receptor protein activates G-protein in
membrane G-protein activates adenylate cyclase to
convert ATP to cAMP in the cytosol
Cyclic AMP is the 2nd messenger
Activates kinases in the cytosol to
speed up/slow down physiological
responses
Phosphodiesterase inactivates cAMP
quickly
Cell response is turned off unless
new hormone molecules arrive
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Cell-surface receptors belong to four major classes
GPCRsare involved in a range of signaling pathways, includinglight detection, odorant detection, and detection of certainhormones and neurotransmitters
Many different mammalian cell-surface receptors includingGPCRs are coupled to a trimeric signal-transducing G protein
made of an alpha, beta and gamm asubunit complex
Ligand binding activates the receptor, which activates the Gprotein, which activates an effector enzym eto generate an
in t racel lu lar second messenger e.g. adenyly l c yclaseconverts ATP to cAMP
depending on regulation at the effector enzyme this pathwaycan be either activated or inhibited
by the type of G protein activated by the hormone-receptorcomplex
Gsproteins result in stimulation of the effector enzyme Giproteins result in inhibition of the effector enzyme
adenyly l cy clase (AC)
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Four classes of cell-surface
receptors
-ligand binding changes the confirmation of the receptor so that specific ions flow
through it
-the resultant ion movement alters the electric potential across the plasmamembrane
-found in high numbers on neuronal plasma membranes
e.g. ligand-gated channels for sodium and potassium
-also found on the plasma membrane of muscle cells
-binding of acetylcholine results in ion movement and
eventual contraction of muscle
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-lack intrinsic catalytic activity
-binding of the ligand results in the formation of a receptor dimer (2 receptors)
-this dimer than activates a class of protein called tyros ine kinases
-this activation results in the phosphorylation of downstream targets by
these tyrosine kinases (stick phosphate groups onto tyrosines withinthe target protein)
-receptors for cytokines such as XXXX, interferons (XXXXXXX)
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-also called receptor tyros ine kinases OR ligand-tr iggered protein kinases
-similar to tyrosine-linked receptors - ligand binding results in formation of a dimer
-BUT: they differ from tyrosine-linked receptorsintr ins ic catalyt ic act iv i ty-means that ligand binding activates it and the activated receptor acts as a
kinase
-recognize soluble or membrane bound peptide/protein hormones that act as
growth factors
e.g. NGF, PDGF, insulin
-binding of the ligand stimulates the receptors tyrosine kinase activity,-results in phosphorylation of multiple amino acid residues within its target
such as serine and threonine residues
-this phosphorylation activates downstream targets
-its targets are generally other protein kinaseswhich phosphorylate their
own downstream targets (other kinases??)
Signal
transductionCascade
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Signal transduction cascades
-the successivephosphorylation/activation ofmultiple kinases results in acascade ofphosphorylation/activation
-this cascade is frequently called as ignal-t ransdu ct ion cascade
-this cascade eventually leads to aspecific cellular response
e.g. changes in cellphysiology and/or patterns ofgene expression
-RTK pathways are involved inregulation of cell proliferation anddifferentiation, promotion of cellsurvival, and
modulation of cellular metabolism
Signal
KINASE #2
KINASE #3
TARGET
EFFECT
KINASE #1
p
p
p
p
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Second messengers
produced by the activation of GPCRs and RTKs
Hormone stimulation of Gs protein-coupled receptors leads to activationof adenyly l cyc laseand synthesis of the second messenger cAMP most commonly studied second messenger cAMP does not function in signal pathways initiated by RTKs cAMP and other second messengers activate specific protein kinases (cAMP-
dependent protein kinases or PKAs)
cAMP has a wide variety of effects depending on the cell type and thedownstream PKAs and other kinases in adipocytes, increased cAMP activates a PKA that stimulates production of fatty
acids in ovarian cells another PKA will respond to cAMP by increase estrogen synthesis
second messenger systems allow for ampl i f icat ionof an extracellularsignal
one epinephine molecule can bind one GPCR this can result in the synthesis ofmultiple cAMP molecules which can go on to activate and amplified number ofPKAs
a blood level as low as 10 -10M epinephrine can raise blood glucose levels by 50%
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Second messengers
other second messengers include: IP3 and DAG breakdown products of phosphotidylinositol
(PI) produced upon activation of multiple hormone receptor types
(GPCRs and RTKs) calcium IP3 production results in the opening of calcium-channels on the plasma membrane of the ER release ofcalcium a rise in calcium in pancreatic beta cells triggers the exocytosis
of insulin a rise in intracellular calcium also triggers contraction of
muscle cells much study has been done on the binding of calcium to a
protein called calmodul inand the effect of this complex ongene expression
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MAP kinase pathways
best characterized signal transduction pathway activation of RTKs by growth factors, hormones etc.. result in activation of an adaptor protein called Ras GTPase ras induces a kinase signal cascade that starts with a kinase called rac
and culminates in activation of a MAP kinase (MAPK) in between are a series of kinases that are part of the cascade MAPK activation results in translocation into the nucleus and
phosphorylate many different proteins, including transcription factors thatregulate gene expression
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MAPK kinase paths
StressCytokines
StressHormonesCytokines
MitogensHormones
cdc42/rac
MEKKs
JNKK 1/2
JNK 1/2
elk
jun
ATFMAPKAP-2
HSP27
MEK 3/6
MEKKs
p38MAPK
ATF
elk
cdc42/rac then activates one of three MAPK paths:
Activation ofras/MEK/ERK path: proliferation &
differentiation
p38MAPK: stress response & apoptosis (MAPKAP-
2, HSP27)
Stress, cytokines, hormones & mitogens signal
through cdc42/rac
raf-1
MEK 1/2
ERK 1/2
elk
RSK
ras
grb2
sos
fos
JNK: stress response & proliferation (jun)
transcription
factors (nucleus)
MAPK kinase
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Common signaling pathways are initiated by differentreceptors in a class
The effects of activation of GPCRsand RTKs is more complicated thana simple step-by-step cascade
Stimulation of either GPCRs orRTKs often leads to production ofmultiple second messengers, andboth types of receptors promote orinhibit production of many of thesame second messengers
in addition, RTKs can promote asignal transduction cascade thateventually acts on the same targetas the GPCR
therefore the same cellular
response may be induced bymultiple signaling pathways bydistinct mechanisms
Interaction of different signalingpathways permits fine-tuning ofcellular activities
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Integrating Cells into Tissues:
Cell-Cell Adhesion and Communication
-a key event in the evolution of multicellularity is the ability for cells to adhere to one
another and be able to communicate with each other
-evolved a series ofcel l-adhesion moleculesorCAMsthat allow interaction with
each other and with the surrounding extracellular matrix (ECM)
-this results in coordinated functioning of tissues
-HOW??
-these interactions result in the activation of specific signal transduction
cascades eventually resulting in the desired cellular effect
-therefore the physical interaction of CAMs with the ECM can turn pathways on oroff cellular effect
e.g. cellular interactions with the adhesion protein b1-integrin can result in
activation of the MAPK cascade
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-various classes of CAMs found on cells
1. cadherins cell-cell adhesion
-calcium dependent
e.g, E-cadherin, P-cadherin
2. Ig superfamily of CAMs cell-cell adhesion
e.g. N-CAM, V-CAM
-calcium-independent
-some are found enriched on specific cell types N-CAM
3. Integri ns major cell-matrix adhesion molecule
e.g. a1 integrin, b1 integrin
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Cell-Cell adhesion: Cadherin-containing junctions
1. Adherens junction
2. Desmosome
-cadherins are a family of calcium-dependent
CAMs
-major molecules of cell-cell adhesion (homophilic)
-over 40 different are knowne.g. E or epithelial cadherin
N or neural cadherin
-tissues have specialized junctions made of cadherins
1. adher ens j un ct io n continuous band of
cadherins found in epithelial cells
-connects cells tightly and interact
with the actin portion of the cytoskeleton2. desm osomes also found in high #s in
epithelial tissues
-cells attach via cadherins connect
to protein plaques that attach to the
plasma membrane and to intermediate
filaments within the cytoskeleton
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Cell Matrix
three components to the ECM
insoluble col lagens structural framework of the ECM,provides strength and resiliency
proteoglycans cushions cells adhesive matr ix proteins bind these components to
receptors on the cell surface
different combinations result in unique ECM compositionswhich can directly affect the activity of the cells
the interaction of ECM components with specific CAMs (i.e.ECM receptors) can turn certain signaling paths ON or OFF
also the ECM is capable of sequestering growth factors andhormones by binding them and making them unavailable toturn cellular signaling on or off
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Cell-matrix interactions: integrins
integrins allow connection between the extracellular matrix with the cytoskeleton of the cell also can mediate cell-cell interactions (weak)
made of an alpha and a beta subunit 17 types ofa chains, 8 types ofb chains
these ab complexes act as receptors to specific matrix components e.g. a1b1 collagen and laminin a5 b1 - fibronectin
cells will express multiple integrin types
attachment can be regulated by up- or down-regulated expression of these integrins integrins can also form very specific adhesive junctions
foca l adhesions(FN to actin) complex of more than 20 proteins can increase and decrease based on physical stress
hemidesmosomes(intermediate filaments to collagens and laminins) epithelial cells
activation of these integrins can promote cell signaling
e.g. formation and activation of focal adhesions triggers a cascade initaited by aFocal Adhesion Kinase (FAK) modulates cell growth and motility
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Cell-Cell Communication: Gap Junctions
-almost all cells have specialized junctions
that allow the free passage of materials(e.g. signaling molecules) back and forth
-these junctions = gap junc t ions
-made of a channel protein called connexon
-connexons interact to form channels between
two cells
-one important compound small enough totraverse this junction is the second messenger
cAMP
-also allows passage of calcium ions and other
ions crucial to signaling (sodium, phosphate)
-these gaps are not free-swinging gates
-they actually open and close inresponse to ion concentrations
thereby restricting the flow of too
many ions
-they also exclude materials based
on size
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