Lecture 4 - Ion channels Flashcards
Describe the pathway from stimulus to effector
stimulus –> receptor –> control centre –> effector
What is an ion channel?
A channel is a transmembrane protein that transports molecules from one side of the membrane to the other. They are specific - Na+, K+ or Cl- ions, open or gated.
Name 3 types of channels
- Open/leakage channels
- Voltage gated channels
- Ligand gated channels
What are 3 essential functions of ion channels?
- Transport ions across membrane (e.g. secretion/absorption of fluids)
- Regulate membrane potentials (e.g. nerve & muscle cells for high-speed communication)
- Ca2+ influx into the cytoplasm (e.g. secretion & muscle contraction)
What is an alpha helices?
a right hand-helix conformation
What are subunits?
Single protein that forms with others to form protein complex
What is a transmembrane domain (TM)?
protein that spans the width of the membrane from the extracellular to intracellular sides usually a helical shape
What is a P-loop or pore?
Pocket where ion will bind
Describe the main structural features of all ion channels
- Transmembrane proteins made up of 2+ a-helices that cross the lipid bilayer
- Made up of 2-6 subunits which usually surround the ‘pore’
Ion channels are classified into subgroups based on:
- gating mechanisms
- ion selectively of the pore (defined by physical size of ‘filter’ and amino acids lining the pore)
Describe the evolution of ion channels
- 400 genes in humans code for membrane channels, knowledge of structure reveals evolutionary relationships
- the pH-regulated K+ channel KcsA from the bacterium Streptomyces lividans serves as a model for all channels
Describe the structures of simple ion (K+) channels
- TM helicase structures form a p-loop (pore) - highly selective (size & charge)
- on cytoplasmic side, TMs are more tightly packed creating a ‘gate’.
Membrane potential, mechanical stress and ligand control the gate.
What are the 2 main functions of voltage-gated ion channels?
- Na+ & K+ create action potentials in excitable cells
- Ca2+ transported into cytoplasm where 2nd messenger elicits a cellular response
How are voltage-gated ion channels different from simple ion channels?
- additional helices S1 and S4 form a separate ‘voltage-sensing domain’ lateral to the subunits
- large polypeptides that extend into the cytoplasm
- plugging mechanism
What are transient receptor potential (TRP) channels?
Share common structural features with voltage-gated channels, but evolved to sense chemicals and physical stimuli. Special receptors that only open in extreme temperatures (emergency response)
Describe the structure of ligand-gated ion channels
Similar in structure to voltage-gated but controlled by the binding of a ligand - e.g. cyclic nucleotide-binding domain, Calmodulin bound to N-terminal
What are gated channels controlled by?
intracellular or extracellular ligands
What do Na+/K+ selective channels control?
membrane excitability - depolarise cells
What do channels with added permeability to calcium do?
directly regulate activity of calcium sensitive proteins
What do Cl- selective channels control?
control membrane excitability - reduce resistance/hyperpolarise cells, reduce action potential firing
Describe the make up of a cys-loop type receptor with pentametric assembly - e.g. nicotinic acetylcholine receptor (nAchR)
In muscle composed of 5 subunits. Each has 4 TM’s (M1,M2,M3,M4) with large external facing N domain and intracellular loop between M3 and M4. M2 lines the pore.
Explain how multiple subunits exist within any one ligand-gated ion channel family
- different subunit combinations make up receptors in different parts of the brain
- complexity provides diversity and opportunity for drug targeting
- e.g. nAcha4 is involved in reward pathways and nicotine addiction
Explain how nicotinic receptors can be targeted for nicotine addiction
- Neuronal nAchRs - a2-10 & B2-4 - each has different affinity (depending on composition and location)
- a4B2 are expressed in cortex & hippocampus and have high affinity to agonists nicotine and vareniciline
- chronic exposure leads to receptor upregulation
- genetic studies shown specific polymorphisms in subunit genes (CHRNA4 - a4) and (CHRNA6 - a6) are linked to tobacco dependence
- Rare variants have been shown to be protective against nicotine dependence
How does a mutation in mAchR cause autosomal dominant nocturnal frontal lobe epilepsy?
- Mutations in the M2 region of the human a4 neuronal nicotinic subunit causes ADNFLE - 9 mutations have been identified to date
- use-dependent potentiation and delay in rising phase caused by a slow unblocked of closed receptors
- enhanced receptor function = increased nicotinic-mediated transmitter released = ADNFLE seizures
Describe features of glutamate receptors - tetrameric assembly
- glutamate is main neurotrasmitter in the brain
- tetramer with similar structure to KcssA except pore is inverted
- forms as dimer of dimers. ligand binding site is in a cleft that ‘closes’ when occupied
- vital to every aspect of brain function and dysfunction contributes to human diseases
Describe the diversity of glutamate receptors
Multiple genes, alternative splicing and RNA editing contribute to diversity (pharmacology, permeability and function) of glutamate receptors
AMPA receptors mediate fast excitatory synaptic transmission in the central nervous system
NMDA (N-methyl-D-aspartate receptor) - involved in learning and memory - slower than other isoforms
Kainate - similar to AMPA, but lesser role at synapses, linked to Schizophrenia, depression and Huntington’s
Describe the functional consequences of RNA processing in AMPA receptor subunits
RNA splicing
- each subunit exists as 2 splicing isoforms - flip and flop. Flop is faster than flip. Alternative splicing of 2 exons in the primary transcript = 2 protein isoforms with different domain in the extracellular loop.
- They have different kinetic properties
- FLOP = FASTER desensitization rate and reduced current responses to glutamate than flip
RNA editing
- The GluA2 Q/R site is located in the M2 of the subunit, inside the channel pore. CAG (glutamine) codon to a CCG (arginine) codon.
- The Effect of GluA2 Q/R editing on channel Ca2+ permeability. Mutant mice lacking enzyme responsible for RNA editing prone to seizures and early death
What are examples of dysfunction of glutamate receptors?
NMDA - the NMDA receptor is thought to be very important for controlling synaptic plasticity and mediating learning and memory functions.
Excess stimulation of NMDA in stroke leads to neuronal death
Explain how dysfunction of RNA modification can lead to pathological conditions
- e.g. downregulation of GluA2 Q/R editing in the motor neurons of ALS patients = increase in Ca2+ permeable AMPA receptors causes damage due to glutamate excitotoxicity
- downregulation of the editing enzyme ADAR2 not a mutation in the editing site.
- In glioblastoma, decreased ADAR2 activity correlated with increased malignancy increase in Ca2+ = Akt pathway promoting proliferation and tumourigenesis. This was reversed when GluA2 Q/R was edited.
- Potential target for therapeutic applications
Describe the features of P2X receptors - trimeric assembly
Adenosine triphosphate (ATP) - gated ion channel
- 3 subunits with 2 TM helices
- Large extracellular domain
- 3 ATP molecules needed to open channel
- Widely expressed
- P2X (1-7) subtypes of subunits
What does diversity of subunits leads to?
Potential for drug treatments
What is the extracellular ligand for P2X/Trimeric receptors?
ATP
What are examples of diseases and physiological conditions that can arise from problems at P2X/Trimeric receptors?
P2X(2) - hearing loss
P2X(4) - pain
P2X(7) - inflammation, neurodegenerative disease
What is the extracellular ligand at glutamate/tetrameric receptors?
Glutamate
What are examples of diseases and physiological conditions that can arise from problems at glutamate/tetrameric receptors?
Excess NMDA in stroke = neuron death
What is the extracellular ligand at Cys-loop/Pentameric receptors?
Nicotinic acetylcholine, GABA, seratonin, Glycine
What are examples of diseases and physiological conditions that can arise from problems at cys-loop/pentameric receptors?
Epilepsy
