Lecture 8 Ligand gated ion channels Flashcards
Why do we need signalling systems
Recognise, transfer, amplify signals. Modulate (turn on and off) effector systems. Adapt and respond to changes in the environment
How do we respond specifically to stimuli
By selective expression of receptors and molecules involved in signal transduction
What can we not predict
Receptor or signalling transduction mechanism from the stimulus
Receptors detect
Chemical and physical stimuli
Most receptors are located where?
embedded in PM to detect EC signals
Physical interaction of receptor and stimuli provides
energy to change the structure of the receptor and so initiate signalling
IC Receptors detect
small membrane permeable stimuli e.g. gases and lipids
How many families of receptors in the mammalian genome
25
Members of each receptor family share
1 or more structurally homologous domains e.g. Ligand BD or signal transducing domains
Ion channels can generate…
chemical signals which are changes in the ion composition of cells
Ion channels cause
change in potential across the plasma membrane
How many human genes code for membrane channels
400
Name the 2 key features of ion channels
- ion selectivity of pore defined by physical size of filter and amino acids lining the pore
- Gating mechanism - voltage or ligand
Evolution of ion channels - 2 examples
- Duplication of a 2 TMD gave rise to a large family of channels with 2-24 TMD
- +ve charges in aa in TM4 gave rise to voltage sensitivity
Ion channel structure
- subunits come together with a pore in the middle
- P loops between 2 TMs creating highly selective filter
- on cytoplasmic side, TMs closely packed to form a gate that blocks ion passage
Example of a K ion channel and it’s structure
2 TMD tetrameric homomeric K+ need to dehydrate and pass single file Flow is down an electrochemical gradient
Example of a VG ion channel
Kv1
Structure of Kv1
4 subunits
6TMD
conserved filter and pore region
voltage sensing S4 domain adjacent to pore lining helices of 4 alpha subunits
Inactivation peptide in alpha - swings to block pore if -ve potential
Cytoplasmic beta subunits for extra regulation
What happens to the inactivation peptide of Kv1 when the membrane is depolarised
S4 senses this, pulls on S5 + S6 opening the gate
Define ligand gated ion channels
Channels gated by an IC generated or EC chemical stimuli
Example of ligand gated ion channel
Cyclic nucleotide gated
How many type of cyclic nucleotide gated ion channels
2
one by cGMP and one by cAMP
Similarities of cyclic nucleotide gated ion channels to Kv1
- tetramer
- 6TM
- S5/6 alpha helical domains
- central pore with p loop selectivity filter
Differences of cyclic nucleotide gated ion channels to Kv1
Added regulatory domain in the IC N/C terminus
Cyclic nucleotide binding domain IC - C terminus opens pore allowing permeability of Na+/Ca2+
these ions are bigger and so the channel is less selective
How many ligands must bind to cyclic nucleotide gated ion channels to activate them
3/4 sites occupies to open channel and cause transformational change and energy to be transduced
What is the conc/response curve of cyclic nucleotide gated ion channels
Sharp
Result of calcium binding to cyclic nucleotide gated ion channels
bind to N term associated with calmodulin and provides negative feedback
What is P2XR gated by and how many bind
3 ATP extracellularly
Structure of P2XR
Trimeric
2 TMD
Homomeric mostly - can be heteromeric
What are ionotropic receptors gated by
EC ligands
Selectivity examples of ionotropic receptors x 3
- Na+ / K+ selective channels control membrane excitability – depolarize cells
- Channels which added permeability to calcium also directly regulate activity of calcium sensitive proteins
- Cl- selective channels control membrane excitability – hyperpolarize cells
Name of pentameric channels
Cys loop
Examples of cys-loop channels
nAChR
GABAAR
5HT3
Location of N/C terminus in Cys loop channels
EC, 4TMD
Name and examples of tetrameric channels
Ionotropic glutamate R i.e. NMDA
Location of N/C terminus in tetrameric channels
N term EC, C term IC, 3TMD
Trimeric channel example
P2X e.g. P2XR
Glutamate receptor structure
Tetrameric
Form as dimers of dimers
Ligand BS in cleft near PM that closes when occupied
What happens to glutamate receptor cleft when glutamate binds
Cleft closes like a clamshell producing tension on TMD pulling the channel open
Glutamate receptors are selective for which ions
Na+
K+
How were glutamate receptors formed
From the fusion of S5-p-S6 and bacterial amino acid binding protein
What led to the emergence of different glutamate receptors
Multiple genes
Alternate splicing
RNA editing
3 Examples of Glutamate receptors
NMDA
AMPA
Kainate
NMDA is permeable to what
Ca2+, Na+
Excess stimulation of NMDA leads to what
Excitotoxicity
Increasing glutamate, increases Ca2+ –> stroke/neuron death
What activates NMDA
glycine
glutamate
These need a depolarisation of +30mV to dislodge Mg2= to allow calcium and sodium in
Speed of EPSP of NMDA
slow
Speed of EPSP for AMPA and KAINATE
fast
Which neurones are AMPA and KAINATE present on
AMPA - post synaptic
Kainate - pre and post as inhibit pre synaptic neurons
Ampa and kainite allow which ions through the channels
Na+, K+ for depolarisation
Example of Cys-loop R and its structure/location
nAChR pentameric alpha2, beta, gamma, E Muscles - has large external N term IC loop between M3 and M4 TM = M1-M4
How many and what does Ach bind to. Result of this
2 ACh bind to alpha subunits pocket
causes rotation or twisting motion of M2 subunits opening the pore = permeable to hydrated cations
How many subunits in any one ligand-gated ion channel family?
Multiple
Example of a nAChR and what it is involved in
nAChR alpha 4
One of many types in the brain
Involved in nicotine addiction and reward pathways - releases dopamine when nicotine binds
How does nAChR of muscle differ to the brain
Needs to be near to calcium calcium release machinery
Autoimmune loss of nAChR leads to
Myasthenia gravis
Epliepsy
Mutation leading to epilepsy in nAChR
Gain of function increasing excitability –> seizures
In CHRNA4 encoding alpha 4 in brain
= ADNFLE - autosomal dominant nocturnal frontal love epilepsy
How many mutations in ion channels found leading to epilepsy
9
Which receptor regulates LTP required for memory formation
Glutamate receptor
How do VG and LG ion channels work together
LG starts process
VG are the amplification system to generate an AP that spreads and controls contraction or release of NT
How do glutamate receptors control memory formation
- before LTP - probability of vesicle release is low and post synaptic responsiveness is low
- With LTP induction, rapid stimulation –> NMDA open –> Ca2+ in –> binds to calmodulin –> stimulates CAM Kinase II –>AMPA phosphorylates
- Probability of step 1 is now high
Explain contraction of muscles
Ap fires, Ca2+ open, Ach vesicles dock, fusion w/membrane, Ach release, activates nAChR, Na+ moves into post synaptic cell, depolarisation
Is GABA excitatory or inhibitory
Inhibitory NT of vertebrates
Two forms of GABA R
GABAAR - ionotropic/LG
GABABR - GPCR/metabotropic
Pathway of GABA R
GABA binds to EC, opens Cl- selective pore, drive rev pot Cl- = -75mV inhibiting firing of new potentials
