Cell Communication

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Cell Communication

• Signaling molecules
• Cell surface receptors

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Cell Communication
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Cell Communication An Overview

• Cells communicate with one another through
• Direct channels of communication
• Specific contact between cells
• Intercellular chemical messengers

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Receptor animation

• Cell surface receptors

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Cell Communication

• To survive, cells must
• Communicate with their neighbors
• Monitor environmental conditions
• Respond appropriately

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Cell Signaling
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Apoptosis

Fig. 7-1, p. 140
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Signals relayed between cells

• Direct intercellular signaling
• Cell junctions allow signaling molecules to pass
  from one cell to another
• Contact-dependent signaling
• Some molecules are bound to the surface of cells
  and serve as signals to cell coming in contact
  with them
• Autocrine signaling
• Cells secrete signaling molecules that bind to
  their own cell surface or neighboring cells of
  the same type

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Signals relayed between cells

• Paracrine signaling
• Signal does not affect cell secreting the signal
  but does influence cells in close proximity
  (synaptic signaling)
• Endocrine signaling
• Signals (hormones) travel long distances and are
  usually longer lasting

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Cell Signaling
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Signaling Molecules

• Small molecules
• Peptides
• Proteins
• LIGANDS

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Receptor affinity

• High affinity
• Low concentration of ligand most receptors are
  occupied
• Low affinity
• High concentration of ligand for most rectors to
  be occupied

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Receptor affinity

• Dissociation constant Kd
• Measures the affinity of the receptor-ligand
  complex
• The concentration of ligand at which half the
  receptors are occupied

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Example

• Erythroid progenitor cell 1000 surface receptors
  for erythropoietin (Epo)
• Only 100 receptors need to bind Epo to induce
  cell division
• Max cellular response less than Kd

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Vasoconstriction occurs when epinephrine
(adrenaline) binds to the a-adrenergic receptor
on vascular smooth muscle cells. One approach to
treating high blood pressures is to administer
competitive inhibitors that bind to the
a-adrenergic receptor. The Kd for binding of
epinephrine to this receptor is 0.6 mM. Which of
the following compounds might be good candidate
drugs for high blood pressure? Kd for binding to
the a-adrenergic receptor are shown.

1. Compound A Kd 1pM
2. Compound B Kd 0.6 mM
3. Compound C Kd 60 mM

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Intercellular Chemical Messengers

• Controlling cell
• Releases signal molecule that causes response of
  target cells
• Target cell processes signal in 3 steps
• Reception, transduction, response
• Signal transduction
• Series of events from reception to response

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3 stages of cell signaling

• Receptor activation
• Signaling molecule binds to receptor
• Signal transduction
• Activated receptor stimulates sequence of
  changes- signal transduction pathway
• Cellular response
• Several different responses
• Alter activity of 1 or more enzymes
• Alter structural protein function
• Change gene expression transcription factor

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Signal Transduction

Fig. 7-2, p. 142
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Amazing cells

• animation

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Which of the following best describes a signal
transduction pathway?

1. Binding of a signal molecule to a cell protein
2. Catalysis mediated by an enzyme
3. Series of changes in a series of molecules
   resulting in a response

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a. Reception by a cell-surface receptor
Polar (hydrophilic) signal molecule
Receptor embedded in plasma membrane
Activation
Target cell
Plasma membrane
Polar signal molecules cannot pass through the
plasma membrane. In this case they bind to a
receptor on the surface.
Fig. 7-3a, p. 142
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b. Reception by a receptor within cell
Nonpolar (hydrophobic) signal molecule
Activation
Receptor within cell
Nonpolar signal molecules pass through the plasma
membrane and bind to their receptors in the cell.
Fig. 7-3b, p. 142
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Intracellular receptors

• Some receptors are inside the cell
• Estrogen example
• Passes through membrane and binds to receptor in
  nucleus
• Dimer of estrogenreceptor complexes binds to DNA
• Transcription factors regulate transcription of
  specific genes

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Cell Communication Systems with Surface Receptors

• Peptide hormones and neurotransmitters
• Primary extracellular signal molecules recognized
  by surface receptors in animals
• Surface receptors
• Integral membrane glycoproteins
• Signaling molecule
• Bound by a surface receptor
• Triggers response pathways within the cell

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Surface Receptors

• Cell communication systems based on surface
  receptors have 3 components
• (1) Extracellular signal molecules
• (2) Surface receptors that receive signals
• (3) Internal response pathways triggered when
  receptors bind a signal

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Peptide Hormones

• Peptide hormones
• Small proteins
• Growth factors
• Special class of peptide hormones
• Affect cell growth, division, differentiation

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Neurotransmitters

• Neurotransmitters include
• Small peptides
• Individual amino acids or their derivatives
• Chemical substances

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Surface Receptors

• Surface receptors
• Integral membrane proteins
• Extend entirely through the plasma membrane
• Binding of a signal molecule
• Induces molecular change in the receptor that
  activates its cytoplasmic end

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Ligand

• Signaling molecule
• Binds noncovalently to receptor with high degree
  of specificity
• Binding and release between receptor and ligand
  relatively rapid
• Ligands alter receptor structure- conformational
  change
• Once a ligand is released, the receptor is no
  longer activated

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Response of Surface Receptor

Fig. 7-4, p. 143
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Cellular Response Pathways

• Cellular response pathways
• Operate by activating protein kinases
• Protein kinases add phosphate groups
• Stimulate or inhibit activities of target
  proteins, producing cellular response

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Cellular Response Pathways

• Protein phosphatases
• Reverse response
• Remove phosphate groups from target proteins
• Receptors are removed by endocytosis
• When signal transduction is finished

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Phosphorylation

Fig. 7-5, p. 144
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Amplification

• Each step of a response pathway catalyzed by an
  enzyme is amplified
• Each enzyme activates hundreds or thousands of
  proteins that enter next step in pathway
• Amplification
• Allows full cellular response when few signal
  molecules bind to receptors

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Amplification

Fig. 7-6, p. 145
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Which of the following steps in an intracellular
signaling pathway amplifies the signal?

1. Synthesis of a secondary messenger
2. Activation of a protein kinase
3. Binding of ligand to receptor
4. 1 2

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In reactions mediated by protein kinases, what
does phosphorylation of successive proteins do
to drive the reaction?

1. Make functional ATP
2. Change a protein from its inactive to active form
3. Change a protein from its active to inactive form

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Which of the following is an example of signal
amplification?

1. catalysis of many cAMP molecules by several
   simultaneously binding signal molecules
2. activation of 100 molecules by a single signal
   binding event
3. activation of a specific gene by a growth factor

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Cell surface receptors

• Enzyme-linked receptors
• Found in all living species
• Extracellular domain binds signal
• Causes intracellular domain to become functional
  catalyst
• Most are protein kinases

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Receptor Tyrosine Kinases

• Receptor tyrosine kinases bind signal molecule
• Protein kinase site becomes active
• Adds phosphate groups to tyrosines in the
  receptor itself, and to target proteins
• Phosphate groups added to cytoplasmic end of
  receptor are recognition sites for proteins
  activated by binding to the receptor

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Protein Kinase Activity

Fig. 7-7, p. 146
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G-ProteinCoupled Receptors

• G proteins Key molecular switches in
  second-messenger pathways
• Two major G-proteincoupled receptor response
  pathways involve different second messengers

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G-Protein-Coupled Receptors

• G-protein-coupled receptors activate pathways
• Binding of the extracellular signal molecule
  (first messenger) activates a site on the
  cytoplasmic end of the receptor

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G-protein-coupled receptors

• Signals binding to cell surface are first
  messenger
• Many signal transduction pathways lead to
  production of second messengers
• Relay signals inside cells
• Examples
• cAMP
• Ca2
• Diacylglycerol and inositol triphosphate

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G-Protein-Coupled Receptors

Fig. 7-8, p. 147
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Now what?

• How does binding a signaling molecule induce a
  cellular response?

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G-protein-linked-receptors

• 7-pass transmembrane receptor
• G protein

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G protein GTP binding protein

• G proteins are trimeric 3 subunits

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Inactive State
Receptor binds ligand G-protein associates with
receptor
GTP is exchanged for GDP - ? subunit and ??
subunit activated
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The G-protein ?-subunit and ?? subunits are
activated

• What next?

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The active subunits interact with target proteins
in the membrane

• What are some target proteins?

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G-Protein Activation

• Activated receptor turns on a G protein, which
  acts as a molecular switch
• G protein
• Active when bound to GTP
• Inactive when bound to GDP

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Active G Protein

• Active G protein
• Switches on the effector of the pathway (enzyme
  that generates second messengers)
• Second messengers
• Small internal signal molecules
• Activate the protein kinases of the pathway

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Response Pathways

Fig. 7-9, p. 147
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Second Messengers cAMP

• 1st of two major pathways triggered by
  G-protein-coupled receptors
• Effector (adenylyl cyclase) generates cAMP as
  second messenger
• cAMP activates specific protein kinases

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cAMP Receptor-Response Pathways

Fig. 7-10, p. 148
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cAMP

Fig. 7-11, p. 148
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Phospho-diesterase
Adenylyl cyclase
Pyrophosphate
ATP
cAMP (second messenger)
AMP
Fig. 7-11, p. 148
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• One effect of cAMP is to activate protein kinase
  A (PKA)
• Activated catalytic PKA subunits phosphorylates
  specific cellular proteins
• When signaling molecules no longer produced,
  eventually effects of PKA reversed

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cAMP has 2 advantages

• Signal amplification
• Binding of signal to single receptor can cause
  the synthesis of many cAMP that activate PKA,
  each PKA can phosphorylate many proteins
• Speed
• In one experiment a substantial amount of cAMP
  was made within 20 seconds after addition of
  signal

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Now what?

• How does binding a signaling molecule induce a
  cellular response?

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Membrane-bound Enzymes
Second messengers
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Adenylate cyclase
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Adenylate Cyclase
?
?cAMP
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Adenylylcyclase
Always on so cAMP is quickly broken down
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cAMP activates cAMP-dependent protein kinase
(A-kinase)
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A-kinase phophorylates serine/threonines of
selected proteins

• Regulates the activity of the target protein

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Activated A-kinase can modulate gene regulation
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Example of cell regulation by an increase in cAMP
levels

• Fight or flight response- muscle cells

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Fight or flight response

• When an animal is frightened or stressed the
  adrenal gland releases epinephrine into the
  bloodstream

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Epinephrine example

• Fight-or-flight hormone
• Different effects throughout body
• Stimulates heart muscle cells to beat faster
• Caffeine inhibits phosphodiesterase
• Enzyme removes cAMP once a signaling molecule is
  no longer present
• Inhibition causes cAMP to persist for longer so
  heart beats faster

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?-adrenergic receptors

• Circulating epinephrine binds ?-adrenergic
  receptors on muscle and liver cells
• Liberates glucose and fatty acids
• animation

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A-kinase phosphoryates an enzyme to break
down glycogen to release glucose
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ß- adrenergic receptors

• ß- adrenergic receptors are GPCR
• Different types are coupled to different G
  proteins
• Gs (stimulatory) G proteins activate adenylyl
  clyclase
• Gi (inhibitory) G proteins inhibit adenylyl
  cyclase (a1 and a2)

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Phophoprotein phosphatase (PP)
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Pathway Controls

• cAMP pathways are balanced by reactions that
  eliminate second messengers
• Stopped by protein phosphatases that continually
  remove phosphate groups from target proteins
• Stopped by endocytosis of receptors and their
  bound extracellular signals

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Sos - Guanine nucleotide exchange factor - Ras-GEF
Grb2- SH2 adaptor protein
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1. Receptor binds ligand
2. Tyrosines phosphorylated
4. Sos exchanges GTP for GDP
3. Grb2/Sos bind pY
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Activated Ras recruits Raf to the plasma
membrane - Raf - protein kinase that initiates
the MAP kinase cascade
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Ras and Ga (trimeric G proteins)

• Similar structure and function and ubiquity in
  eukaryotic cells suggest a single type of GTPase
  originated early in evolution
• Gene encoding this ancestral protein duplicated
  and evolved gt 100 different intracellular switch
  proteins

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Active GTP

• Ras-GTP active conformation

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Ras-mitogen-activated protein kinase (MAPK)

• Similar to cAMP signaling cascade - both provide
  pathways by which extracellular signals can
  influence gene expression

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Cascade of Protein Kinases

• Active Ras activates Raf (ser/thr kinase)
• Raf activates MEK
• MEK activates MAPK
• MAPK activates other proteins (transcription
  factors)

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Gene Regulation Ras

• Some pathways in gene regulation link certain
  receptor tyrosine kinases to a specific G protein
  (Ras)
• When the receptor binds a signal molecule, it
  phosphorylates itself
• Adapter proteins then bind, bridging to and
  activating Ras

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Activated Ras

• Activated Ras turns on the MAP kinase cascade
• Last MAP kinase in cascade phosphorylates target
  proteins in the nucleus
• Activates them to turn on specific genes
• Many of these genes control cell division

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Mutations

• Mutated systems can turn on the pathways
  permanently, contributing to progression of some
  forms of cancer

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The importance of Ras

• Early 1980s, several human tumors were found to
  contain a mutant of Ras
• Ras mutations are found in over 30 of all human
  tumors
• Mutation in the Ras gene that lead to tumor
  formation prevent the protein from hydrolzying
  the bound GTP back to GDP
• Ras always on

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The importance of Ras

• Ras is a small G protein held to the inner
  surface of the plasma membrane by a lipid group
  that is embedded in the inner leaflet of the
  bilayer
• Ras is a single subunit G protein
• Cycles between an active GTP-bound form and an
  inactive GDP-bound form
• Activates a kinase cascade (MAP Kinase)

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Gene Regulation

Fig. 7-13, p. 151
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Gene Activation Steroid Hormone Receptors

Fig. 7-14, p. 152
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Cell Response

• Cell response to a steroid hormone
• Depends on whether it has an internal receptor
  for the hormone
• Type of response within the cell
• Depends on the genes that are recognized and
  turned on by an activated receptor

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Cross-Talk

• Cell signaling pathways communicate with one
  another to integrate responses to cellular
  signals
• May result in a complex network of interactions
  between cell communication pathways

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Cross-Talk

Fig. 7-15, p. 153
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Core signaling Pathways
Many target proteins