4 - Ligand gated ion channels

Question 1

What are the 3 essential functions of ion channels?

Answer 1

1. Transport ions across the membrane
   e.g. secretion/ absorption of fluids
2. Regulate membrane potentials
   e.g. nerve and muscle cells
3. Ca2+ influx into the cytoplasm
   e.g. secretion and muscle contraction

Question 2

What are the main structural features of all ion channels?

Answer 2

transmembrane proteins are made up of 2 or more alpha-helices that cross the lipid bilayers
made up of 2-6 subunits - usually surround pore

Question 3

How are ion channels classified into subgroups?

Answer 3

• Gating mechanism - voltage or ligand
• ion selectivity of the pore
  defined by the physical size of ‘filter’ and aa lining the pore

Question 4

what is an alpha helix?

Answer 4

a right hand helix conformation

Question 5

What is a beta sheet?

Answer 5

beta strands connected laterally by at least 2 or 3 backbone H bonds
form a sheet

Question 6

What is the P-loop?

Answer 6

the pocket where the ion will bind

Question 7

Structure of a simple ion (K+) channel?

Answer 7

TM helicase forms p-loop - highly selective
2 helices span memb - surround pore
in cytoplasm TMs packed - create gate
when ions flow in - TMs move - allow potassium to pass through

Question 8

What factors control the TM gate on the cytoplasmic side?

Answer 8

• membrane potential
• Mechanical stress
• ligands

Question 9

What are the 2 main functions of voltage-gated ion channels

Answer 9

• Na+ and K+ create action potentials in excitable cells
• Ca2+ transported into the cytoplasm where 2nd messengers elicit a cellular response

Question 10

How is the structure of a voltage gated channel different to a simple ion channel?

Answer 10

voltage gated channels have:
- additional helices S1 and S4 - form separate ‘voltage sensing domain’ lateral to subunits
- large polypeptides that extend to cytoplasm
- plugging mechanism

Question 11

What are transient receptor potential channels?

Answer 11

Share common structural feaures w/ voltage gated channels
but have evolved to sense chemicals and physical stimuli
e.g. hot/spicy food

Question 12

How are ligand binding ion channels different to voltage-gated?

Answer 12

are similar in structure but are controlled by the binding of a ligand

Question 13

What are examples of ligand gated ion channels?

Answer 13

• calmodulin bound to c-terminal
• cyclic nucleotide-binding domain

Question 14

What do Na+/ K+ selective channels control?

Answer 14

membrane excitability - they depolarise cells

Question 15

What do calcium-permeable channels control?

Answer 15

regulate activity of calcium-sensitive proteins

Question 16

What do chlorine selective channels control?

Answer 16

membrane excitability - reduce resistanc/ hyperpolarise cells
reduce action potential firing

Question 17

What is the structure of a Cys-loop type receptor?

Answer 17

pentameric assembly
e.g. nicotinic assembly - nicotinic acetylcholine receptor

Question 18

Neuronal nicotinic acetylcholine receptors and nicotine addiction:

Answer 18

neuronal nACHR exist as alpha 2-10 and beta 2-4
each have diff affinity depending on composition and location

Question 19

Which neuronal nACHRs have a high affinity to nicotine and varenicline?

Answer 19

Alpha4 beta 2
are expressed in cortex and hippocampus

Question 20

What does chronic exposure to nicotine lead to?

Answer 20

receptor upregulation
more receptors so more nicotine needed for same effect
specific polymorphisms in subunit genes CHRNA4 (alpha4) and CHRNA6 (alpha6) are linked to tobacco dependence

Question 21

What causes autosomal dominant nocturnal front lobe epilepsy?

Answer 21

mutation in nACHR in M2 region of alpha4 nicotinic subunit cause ADNFLE
enhanced receptor function = increased nicotinic - mediated transmitter release = ADNFLE seizures

Question 22

What is the main neurotransmitter in the brain?

Answer 22

glutamate

Question 23

What is the structure of a glutamate receptor?

Answer 23

tetramer - similar structure to KcsA
except pore is inverted
form as dimer/ dimers
ligand binding site in cleft ‘closes’ when occupied
vital to brain fuction

Question 24

What are the 3 main types of glutamate receptor?

Answer 24

AMPA
Kainate
NMDA

Question 25

What do AMPA receptors do?

Answer 25

mediate fast excitatory synaptic transmission in the CNS

Question 26

What do NMDA receptors do?

Answer 26

N-methyl-D-aspartate receptor -
involved in learning + memory
control synaptic plasticity
slower than other isoform

Question 27

What do kainate receptors do?

Answer 27

similar to ampa
but lesser role of synapses linked to schizophrenia, Depression + huntingtons

Question 28

What are the functional consequences of RNA splicing in AMPA receptor subunits?

Answer 28

alternative splicing = diff isoforms w/ diff functional properties
each subunit 2 isoforms - flip + flop
alternative splicing in 2 exons in primary transcript = 2 prot isoforms w/ diff domain in extracellular loop
different kinetic properties

Question 29

What are the functional consequences of RNA editing in AMPA receptor subunits?

Answer 29

can result in aa substitutions - affect receptor functions
GluA2 Q/R site located in M2 of subunit - inside channel pore
mutation leads to CAG (glutamine) codon becoming CGG (arganine) codon
alters permeability + conductance

Question 30

What is an example of dysfunction of glutamate NMDA receptors?

Answer 30

NMDA thought to be important for controlling synaptic plasticity + mediating learning and memory function
excess stimulation of NMDA - lead to neuron death

Question 31

Dysfunctions of RNA modification can lead to pathological conditions example:

Answer 31

Downregulation of GluA2 Q/R editing in motor neurons of ALS patients = increase in Ca2+ permeability receptors
causes damage due to glutamate excitotoxicity

Question 32

Structure of P2X receptors

Answer 32

trimeric assembly
e.g. Adenosine Triphosphate (ATP) gated ion channel
3 subunits w/ TM helicase
large extracellular domain
3 ATP molecules needed to open channel
widely expressed
P2X1-7 subtypes of subunits