L7- Cell signalling- GPCR's Flashcards
What is the overview of cell signalling steps?
- Extracellular signal detected by receptor protein
- Signal transduction via enzymes
- Signal amplification (2nd messengers, phosphorylation cascade)
- Modification of effector proteins results in response
What are mediators that bind to extra/intracellular receptors?
- Peptides
- Amino acids
- Nucleotides
- Fatty acid metabolites
- Steroids
- Ions
How does intercellular communication occur?
- Contact dependent
- Paracrine signalling
- Synaptic signalling
- Endocrine signalling
How does contact dependent signalling occur?
Signalling cell binds to target cell via its receptor • Bi-directional • Membrane bound ligand receptor • Relatively slow • Development, immune responses
How does paracrine signalling occur?
Local signal which can be autocrine (cell produced substance which has an effect on the cell by which it is produced)
Restricted by diffusion (uptake, degradation, immobilisation)
How does synaptic signalling work?
- Specialised synapses
- Specificity through spatial precision (generic transmitters; GABA, Ach)
- Evoked by action potentials
- Rapid
How does endocrine signalling work?
- Hormones reach their target cells and organs via the bloodstream
- Slow signalling system
- Receptors are specific
How do the different signalling rates vary?
- Voltage dependent signals arr rapid
- Second messengers, modification of existing proteins is relatively fast
- Transcription dependent events are slow
What are cellular responses determined by?
- Receptor type
- Receptor coupling
- Effector molecules
What are the different types of cell surface receptors?
- Ligand gated ion channels (synaptic communication)- nicotine, acetylcholine neurotransmitters
- Voltage and/or Ca2+ mediated effects
What are the different ligand gated ion channels?
- NaChR (nicotinic acetylcholine), GluR, NMDA, AMPA are cation channels (Ca2+ and Na+)
- GABA and GLYR are anion (Cl-) channels
What are G protein coupled receptors?
- 7 transmembrane segments
- Extracellular ligand binding site
- Signals through trimeric GTP-binding proteins
- Largest receptor family
What is the order of events for the GPCR activation of G-protein?
- Ligand binding induces a conformational change of the receptor
- This change allows Gα to exchange GDP for GTP
- GTP binding activates both α and βγ and they dissociate
How does GTP binding act as a switch?
Receptor stimulation (GDP exchange for GTP) leads to activation of signalling cascade (signal transduction) by binding of the α subunit to target protein.
Gα protein is inactivated by GTP hydrolysis which allows the now inactive α subunit and βγ complex to re-associate.
What are the 6 families of G proteins?
- Gs- activate adenylyl cyclase
- Golf- activates adenylyl cyclase (olfactory neurons)
- Gi/o- inhibits adenylyl cyclase, complex and activates K+ channels
- Gt- activates cGMP phosphodiesterase
- Gq- activates phospholipase C-β
- G12/13- activates Rho GTPases
What is cAMP?
- Second messenger
- Produced by adenylyl cyclase
- Hydrolysed by cAMP phosphodiesterase to produce AMP
What are mediators acted upon by cAMP?
- Glucagon= glycogen+triglyceride breakdown
- Adrenaline= positive iono/chonotrope via beta adrenoceptors
- Vasopressin= water reabsorption
- PGE2= Pain, hypersensitivity
What does PKA do?
• Protein kinase A is activated by cAMP and it phosphorylates cytosolic targets like VGCC’s, phosphorylase kinase
How does gene regulation occur by cAMP?
Active PKA translocated to nucleus and activates CREB which activates target gene and causes gene transcription
Inactive CREB stops gene transcription
What are AKAPs?
A kinase anchoring protein
Keep signalling proteins near the site of action
How do Golf proteins work?
Activated Golf proteins stimulate AC which produces cAMP
cAMP stimulates cyclic nucleotide gated ion channels and depolarisation occurs, stimulating an action potential
What does the Gi protein do?
- Inhibits AC, reducing cAMP
* βγ complex can directly open K+ channels and inhibit VGCC’s
How does mammalian phototransduction work?
- In the dark, cGMP keeps CNG (cationic channels open)
- Rhodopsin is activated by absorbing proton (light)
- Gα transducin activates cGMP phosphodiesterase (PDE) which hydrolyses GMP
- Reduced cGMP causes closure of CNG channels and hyperpolarisation occurs (stops action potentials) - signal relayed to brain
What happens in darkness?
- High rate of mGluR6 activation which stimulates Gαo which inhibits TRPM1
- TRPM1 channels are associated with night blindness
What happens in llight?
- Reduced rate of mGluR6 activation
- Gαo inactivation and dissociation
- TRPM1 activation and propagation of action potential
What are novel direct actions of Gβγ?
Application of Gβγ inhibits TRPM3 activity directly
Inhibited by morphine
What does Gq do?
Activated by rhodopsin absorbing light
Acts by stimulating phospholipase C-β (PLC-β)
PLC-β hydrolyses PIP2 to DAG and IP3
Insect phototransduction in drosophla
What is PIP2?
Membrane lipid
Important for many membrane proteins
What is DAG and IP3?
DAG- membrane associated molecule, recruits PKC to membrane
IP3- cytosolic molecule- stimulates IP3R which causes influx of Ca2+ that activates PKC
How does GPCR desensitisation occur?
GPCR kinases phosphorylate the receptor after stimulation occurs
This stimulates the binding of arrestin protein
Association with clathrin coated pits occurs
Endocytosis occurs followed by dephosphorylation or recycling and breakdown
