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Signal Transduction 2
BL4010
12.02.05
Signalling vocabulary
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Signal/stimulus
Effector
Receptor
Messenger
Ligand
Cascade
Heterotrimeric G Proteins
A model for their activity
• Binding of hormone, etc., to receptor
protein in the membrane triggers
dissociation of GDP and binding of
GTP to -subunit of G protein
• G-GTP complex dissociates from G
and migrates to effector sites,
activating or inhibiting
• But it is now clear that G also
functions as a signalling device
GTPase activating proteins (GAPs)
MAP Kinase Cascade
MAP Kinase Cascade
When signal molecule misbehave
Phospholipases Release Second
Messengers
• Inositol phospholipids yield IP3 and DAG
• PLC is activated by 7-TMS receptors and G
proteins
• PLC is activated by receptor tyrosine kinases (via
phosphorylation)
• Note PI metabolic pathways and the role of lithium
Phospholipase targets
Phosphotidyl inositols as secondary
messengers
Many different activators of phospholipase
Phospholipase C isozymes
• src-homology domains (SH)
– SH2 mediates interactions with phosphotyrosinated
proteins
– SH3 interacts with cytoskeletal proteins
Other Lipids as Messengers
Recent findings - lots more to come
• More recently than for PI, other phospholipids have
been found to produce second messengers!
• Phosphatidyl choline can produce prostaglandins,
diacylglycerol and/or phosphatidic acid
• Sphingomyelin and glycosphingolipids also produce
signals such as ceramide, a trigger of apoptosis programmed cell death
Ca2+ as a Second Messenger
Several sources of Ca2+ in cells!
• [Ca2+] in cells is normally very low: < 1M
• Calcium can enter cell from outside or from ER and
calciosomes (plants store Ca2+ in oxalate crystals)
• CICR - Calcium-Induced Calcium Release
– see animation
Calcium Oscillations!
M. Berridge's model of Ca2+ signals
• Ca2+ was once thought to merely rise in cells to signal
and drop when the signal was over
• Berridge's work demonstrates that Ca2+ levels
oscillate in cells!
• The purpose may be to protect cell components that
are sensitive to high calcium, or perhaps to create
waves of Ca2+ in the cell
Patch clamp
Ca2+-Binding Proteins
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Mediators of Ca2+effects in cells
Many cellular proteins modulate Ca2+ effects
3 main types: protein kinase Cs, Ca2+-modulated
proteins and annexins
Kretsinger characterized the structure of
parvalbumin, prototype of Ca2+-modulated proteins
"EF hand" proteins bind BAA helices
Calmodulin
Protein Modules in Signal Transduction
• Signal transduction in cell occurs via proteinprotein and protein-lipid interactions based on
protein modules
• Most signaling proteins consist of two or more
modules
• This permits assembly of functional signaling
complexes
Transduction of two second messenger
signals
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PKC is activated by DAG and Ca2+
Most PKC isozymes have several domains, including ATPbinding domain, substrate-binding domain, Ca-binding
domain and a phorbol ester-binding domain
Phorbol esters are apparent analogues of DAG
Cellular phosphatases dephosphorylate target proteins
Read about okadaic acid
Localization of Signaling Proteins
• Adaptor proteins provide docking sites for
signaling modules at the membrane
• Typical case: IRS-1 (Insulin Receptor
Substrate-1)
– N-terminal PH domain
– PTB domain
– 18 potential tyrosine phosphorylation sites
– PH and PTB direct IRS-1 to receptor
tyrosine kinase - signaling events follow!
Lipids Rafts
• first hypothesized in 1988
• nice review: Cary, L. A. & Cooper, J. A. (2000)
Molecular switches in lipid rafts. Nature. 404, 945-947
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Moffett, S., Brown, D. A. & Linder, M. E. (2000) Lipid-dependent
targeting of G proteins into rafts. J. Biol.Chem. 275, 2191-2198.
• Many actin binding proteins are known to bind to
polyphosphoinositides and to be regulated by them
• Activation of receptor causes reorganization of the rafts
Simons, K. et al. J. Clin.
Invest. 2002;110:597-603
J. Fantini, N. Garmy, R.
Mahfoud and N. Yahi
Lipid rafts: structure,
function and role in
HIV, Alzheimer’s and
prion diseases
Expert Reviews in
Molecular Medicine: 20
December 2002
Cells of Nervous Systems
Neurons and Neuroglia (Glial Cells)
• Neurons contain processes,
including an axon and dendrites
• Axon is covered with myelin
sheath and cellular sheath,
except at nodes of Ranvier
• The axon ends in synaptic
termini, aka synaptic knobs or
synaptic bulbs
• Three kinds of neurons: sensory
neurons, motor neurons and
interneurons
Ion Gradients
The source of electrical potentials in neurons
• Nerve impulses consist of electrical signals that are
transient changes in the electrical potential differences
(voltages) across neuron membrane
• Difference between Nernst potential and actual potential
represents a thermodynamic push
The Action Potential
series of changes in
potential that
constitute a nerve
impulse
• Small depolarization
(from -60 to -40 mV)
opens voltage-gated
ion channels - Na flows
in
• Potential rises to +30
mV, Na channels close,
K channels open. K
streams out, lowering
potential
Action potential
• Action potentials flow along the axon to the
synapse
• Number and frequency important, not
intensity
Voltage-Gated Na, K Channels
Clustered in Nodes of Ranvier
• These channels are voltage-sensitive - voltage changes
cause conformational changes and gating
Some 7000 sodium ions pass
through each channel during
the brief period (about 1
millisecond) that it remains
open.
Streptomyces
voltage gated K+
channel
Communication at the Synapse
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A crucial feature of neurotransmission
Ratio of synapses to neurons in human forebrain is 40,000
to 1!
Chemical synapses are different from electrical
Neurotransmitters facilitate cell-cell communication at the
synapse
Note families of neurotransmitters in Table 34.6
The Cholinergic Synapse
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A model for many others
Synaptic vesicles in synaptic knobs contain acetylcholine
(10,000 molecules per vesicle)
Arriving action potential depolarizes membrane, opening Ca
channels and causing vesicles to fuse with plasma membrane
Acetylcholine spills into cleft, migrates to adjacent cells and
binds to receptors
Toxin effects: botulism toxin inhibits Ac-choline release, black
widow's latrotoxin protein overstimulates
Two Classes of Ac-Ch Receptor
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Nicotinic and muscarinic
As always, toxic agents have helped to identify and purify
hard-to-find biomolecules
Nicotinic Ac-Ch receptors are voltage-gated ion channels
Muscarinic Ac-Ch receptors are transmembrane proteins
that interact with G proteins
Acetylcholinesterase degrades Ac-Ch in cleft
Transport proteins and V-type H+-ATPases return Ac-Ch to
vesicles - called reuptake
Other Neurotransmitters
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Excitatory and inhibitory
Glutamate is good example: nerve impulse triggers Cadependent exocytosis of glutamate
Glutamate is either returned to neuron, or carried into glial
cells, converted to Gln and taken back to the neuron from
which it was released
See 4 types of glutamate receptors in Fig. 34.68
NMDA receptor is best understood for now
Note phencyclidine (angel dust) story
GABA and Glycine
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Inhibitory Neurotransmitters
Inhibitory neurotransmitters diminish the actions of
activating neurotransmitters
See Figure 34.70 for glutamate degradation
Excitatory glutamate is broken down to inhibitory GABA,
which is broken down to non-signals
GABA & glycine receptors are chloride channels
Glycine receptor is site of action of strychnine
Catecholamine Neurotransmitters
• Epinephrine, norepinephrine, dopamine and L-dopa
are all neurotransmitters
• Synthesized from tyrosine - see Fig. 34.72
• Excessive production of dopamine (DA) or
hypersensitivity of DA receptors produces psychotic
symptoms and schizophrenia
• Lowered production and/or loss of DA neurons are
factors in Parkinsonism
Neurological Disorders
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Depression, Parkinsonism, etc.
Defects in catecholamine processing are responsible for
many neurological disorders
Norepinephrine and dopamine systems are keys
Breakdown of NE and DA by catechol-O-methyl
transferase and monoamine oxidase
Reuptake by specific transport proteins
MAO inhibitors are antidepressants
Tricyclics - antidepressants that block reuptake
Cocaine blocks reuptake, prolongs effects of DA
Peptide Neurotransmitters
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Lots more to learn here!
Likely to be many peptide NTs
Concentrations are low; purification is hard
Roles are complex
Endorphins and enkephalins are natural opioids
Endothelins affect smooth muscle contraction,
vasoconstriction, mitogenesis, tissue changes
Vasoactive intestinal peptide stimulates AC (to make
cAMP) via G proteins, and its effects are synergistic with
those of other neurotransmitters
Sensory Transduction
• Similarities between sight, taste, smell
• Specialized sensory cells translate stimulus into electrical
signals in adjacent neurons
• Vision is the paradigm system
• Absorption of light quanta by rhodopsin isomerizes retinal
(11-cis to all-trans)
• Light is absorbed by rhodopsin in the outer segments of
rod and cone cells
Recap: Signaling Pathways from
Membrane to the Nucleus
• The complete path from membrane to nucleus
is understood for a few cases
• Signaling pathways are redundant
• Signaling pathways converge and diverge
• This is possible with several signaling modules
on a signaling protein
Module Interactions Rule!
• The interplay of multiple modules on many
signaling proteins permits a dazzling array of
signaling interactions
• We can barely conceive of the probable extent
of this complexity
• For example, it is estimated that there are
more than 1000 protein kinases in the typical
animal cell - all signals!