Transcript Introduction to Neurobiology Intracellular Signal Transduction

Intracellular Signal Transduction
Pathways and Cascades
Outline :
I.
II.
III.
IV.
V.
VI.
Review cell to cell communication
Ligands
Receptors
Activation of Intracellular signaling pathways and
production of second messengers
Effector Proteins
Mechanisms to terminate signal transduction
Main Concepts
1- Receptors and ligands mediate signal transduction
2-Signals can be amplified by the activation of multiple down stream pathways: ex. cAMP,
IP3, PIP3
3-Signal transduction results in a change in cellular function- ex. Ion channel function,
cytoskeletal organization (cell shape), or gene expression
4- Termination of signaling: desensitization, internalization, downregulation
Signal TransductionThe process of converting a signal
from outside the cell to a functional
change within the cell
Two Main Types of Cell-Cell
Communication
Direct: Gap
Junctions
Indirect: Receptors and Signal
Transduction
Defined by:
• Where the signal
originates
• Where the signal is
released (distance
travelled)
• Location of the target
cells
• What types of cells are
signaling
What is the Signal?
• Chemical communication
between cells
• Molecule (ligand) that will bind
to a receptor.
• Intended to be amplified to elicit
a response
Three classes of cell signaling molecules
Receptors propagate the SIGNAL
across the cell membrane
Intracellular Receptors:
Ligand must be
hydrophobic and able to
pass directly through
plasma membrane
Ex. estrogen
Cell Surface Receptors:
Ligands can be either hydrophilic or
hydrophobic
Three major classes defined by
mechanism used to transduce
ligand-binding into intracellular
signaling events.
Receptors are proteins that mediate a biological change following ligand binding.
Ligand + receptor
[Ligand-receptor complex]
Non-covalent Interactions (reversible)
Cell surface receptors are
membrane spanning proteins
that undergo allosteric changes in
response to ligand binding.
•
•
•
•
•
•
Ligand binding site(s) on the outside (extracellular domain) and sites for protein-protein
interactions on inside (cytoplasmic domain).
High affinity and selectivity for their ligands.
Function via signal transduction, though by very different molecular mechanisms.
Can regulate ion channels and therefore affect the membrane potential or excitability of neurons.
Have mechanisms for desensitization.
Are regulated by phosphorylation.
Receptor Properties
Reversibility
A ligand binds to its the receptor through non-covalent interactions
Affinity
How strongly a ligand binds to the receptor- Equilibrium dissociation constant = KD
Efficacy
How well an agonist can activate a receptor once it is bound- refers to
response
Specificity = Selectivity
How well a receptor can distinguish among different ligands
Location = Localization
Where is the receptor localized in the cell, on the plasma membrane or in
intracellular vesicles. Where in the plasma membrane-synaptic or extrasynaptic
G Protein Coupled Receptors: GPCRs
•
•
•
•
•
•
Largest family of cell surface receptors with >1000 genes (a lot of these are odorant
receptors)
Targets of the majority of therapeutic drugs (over 50% of all prescription
pharmaceuticals on the market)
Core structure: 7 transmembrane -helices (extracellular N-term, intracellular Cterm)
Respond to a massive number and variety of ligands
Divergent ligands –Photons-Retinal; small organic molecules; neurotransmitters and
neuromodulators; glycoproteins; hormones
GPCRs have mechanisms for signal amplification, can generate second messengers,
and regulate protein kinases and protein phosphorylation.
GPCRs (metabotropic receptors) compared to ligand
gated ion channels (LGICs; ionotropic receptors)
• Slow speed: (0.1 sec to min to hours)
• Signal transduction results in a change in cellular
function- including ion channel function, cytoskeletal
organization (cell shape) or gene expression.
• Mostly modulatory, rather than direct mediation of rapid
synaptic response. i.e. neuromodulation.
• Receptors can be presynaptic, postsynaptic, or on the cell
body
• Targets and effects
• Amplification
Why Target Molecules?
• Receptors propagate the signal
across the postsynaptic cell
membrane
• Intracellular signaling cascades
propagate and amplify the signal
that results in phosphorylation
(or dephosphorylation) of target
molecules
What Target Molecules?
• Kinases –
– Transfer phosphate groups from
ATP to substrates to activate
them (phosphorylate)
• Phosphatases –
– Remove inorganic phosphates
from activated molecules
(dephosphorylate)
• Kinase cascade
Effector
Second messenger
Later effector
EVERYBODY
STAND, PLEASE!!
What Responses?
• Transcriptional modification
• Activation (or inhibition) of
surface receptors
– Ion channels
– Ion pumps
– Neurotransmitter receptors
• Triggering of intracellular events
– Ca2+ release from intracellular
stores (ER)
Intracellular cascades
amplify the signal,
providing extremely
high sensitivity. The
activation of only a few
receptors can produce a
response
Amplification allows for
the induction of
responses in cells with
low density of receptors
or the induction of
responses at low
concentration of
signaling molecules
Channel or
transporter
altered [ion]
membrane
potential
Figure 15-1 Molecular Biology of the Cell (© Garland Science 2008)
Second messengers contribute to regulation of
cellular proteins by
phosphorylation/dephosphorylation
Examples of neuronal functions regulated by GPCRs
• Presynaptic receptors-inhibit (a few enhance) neurotransmitter
release
• Development-Neurite outgrowth—Go family, CB1 receptor
• Dendritic spine remodeling and postsynaptic organization
• Neuroprotection—A1 and A2 adenosine receptor, a2A-AR
• Plasticity –muscarinic receptors, learning & memory
• Behavior –dopamine, serotonin, opioid, adrenergic
This huge variety of functions suggests that there might be more than one type of
GPCR signaling cascade
Specific GPCR cascades
Receptors and heterotrimeric GPCRS
Mechanisms of
protein kinase
activation
One Major Target of cAMP:
cAMP Dependent Protein Kinase (PKA)
Consensus Sequence
RRXS/TX
Downstream Targets of PKA
PKA regulates CREB and Transcription
HAT
Regulation of
tyrosine hydroxylase
by protein
phosphorylation
How is GPCR signaling terminated?
RE
LE
GPCR regulation: desensitization and down-regulation
Main Concepts
1- Receptors and ligands mediate signal transduction
2-Signals can be amplified by the activation of multiple down stream
pathways: ex. cAMP
3-Signal transduction results in a change in cellular function- ex. Ion
channel function, cytoskeletal organization (cell shape) or gene
expression
4- Termination of signaling: desensitization, internalization,
downregulation