Calcium channels and signalling Flashcards
How is the concentrations of Ca2+ maintained across neuronal PM?
Extracellular Ca2+ conc = 1mM!!!!!
= very high.
Intracellular = 100nM…
= huge concentration difference
Maintained in order to prevent intracellular Ca2+ signalling events/onset of Ca2+ dependent processes.
Mitochondria take in Ca2+ too = important for synapses…
Ca2+ buffering with extracellular proteins… removal into organelles like ER and mitochondria.
Active pumping - Ca2+/H+ exchanger pump!
Ca2+/Na+ exchanger = NCX
How can Ca2+ be imaged?
Flura-2 is a Ca2+ chelator and fluoresces when exposed to UV light.
Or Radiometric imaging …
Or CAMELEONS - genetically encoded Ca2+ indicators.
Can actually use these methods to quantify free Ca2+ conc..
What are Calcium microdomains?
Calcium conc. is not uniform across whole cell.
Microdomains of Ca2+ are formed at sites where Ca2+ enters the cytoplasm, both at cell surface, or at internal stores…
Local rapid sparks of Ca2+ can combine to produce micromdomains to regulate specific cellular processes in different regions of the cell.
- particularly in neurones - important to process enormous amounts of input through miniaturisation of Ca2+ signalling systems..
e.g. synaptotagmin in close proximity to Ca2+ channel - in contact physcially with C-terminal domain of VG Ca2+ channels..
Immediate to point of entry are rapid rise of Ca2+ concentration, independent to mobile calcium buffers, whereby local concentration is determined by LOCAL channels!
How does Synaptotagmin interact with microdomains?
e.g. synaptotagmin in close proximity to Ca2+ channel - in contact physcially with C-terminal domain of VG Ca2+ channels..
- Ca2+ sensing to replace complexins to to allow hemi-fused state for full fusion pore opening and SSV release.
e.g glutamate
What are the different forms of homosynaptic plasticity?
Facilitation - short term progressive increase in release of NT quanta, due to residual Ca2+ at release sites during tetanic train of APs.
Potentiation - long-lasting increase in NT quanta release following tetanic stimulation - through mitochondrial buffering of Ca2+ to enhance conc. of readily-releasable Ca2+ upon stimulation. + Increased pool of readily realasable vesicles.
Depresion - Progressive decrease in release during tetanic train due to declined readily releasable pool of vesicles..
- Reduced no. of readily releasable pool of SSVs..
What are VGCCs?
Voltage-sensitive depolarisation dependent Ca2+ channels.
Complex of 4 to 5 distinct sub-units coded by multiple genes.
Belong to gene superfamily of TM ion channels including Na+ channels.
What is the general structure of VGCCs?
An a1 subunit plus auxiliary subunits.
alpha 1 subunit is 24 TM structure - composed of 4 homologous domains each with 6 TM segments…
Contain pore forming residues and voltage sensing regions in Alpha 1.
Containing Auxiliary Beta, gamma, delta and Alpha 2 domains…
What is the structure of the alpha 1 subunit?
4 homologous domains each containing 6 TM segments.
a1 subunit contains the conduction pore, voltage sensing, gating and majority of second messenger/toxin regulatory sites…
S4 of each of the 4 homologous domains contain voltage sensing residues.
S5-S6 pore loop of each of the 4 domains determines ion conductances.
Why are VGNA and VGCC so similar?
Belong to same gene superfamily of TM ion channels.
Just 3 AA changes to domains 1, III and IV, can convert absolutely selectivity of VGCC to switch conductance to Na+.
What determines the nomencalature of VGCC?
The diversity of alpha 1 subunits can change the nomenclature within the subfamily of Ca2+ channels..
Cav1.2 = alpha 1c.
cav2.1 = alpha 1a.
What are the subdivisions of VGCCs?
HVAs = high voltage activated caclium channels.
LVAs = low voltage activated calcium channels.
LVA= T-type - transient - in pacemaker potential.
HVAs are N, L, P/Q and R type VA Ca2+ channels.
L-type - long lasting.
N-type are NEURONAL - also slowly inactivating too.
P/Q - purkinje, slow inactivating.
R - resistant - medium inactivation… (Intermediate VA)..
What are the different subdivisions of VACCs responsible for?
L-type VA Ca2+ channels are mostly controlling signalling processes!!!
- mostly for neuropeptides!!, not NTs!!!
- can find in soma + axon, less so at synapse!
N, P/Q and R type HVA Ca2+ channels are responsible for controlling vesicular release of NT!!!!
- localised to synapse!
What are the different toxins for VACCs?
Omega agotoxins, omega conotoxins and SNX482 are all potent inhibitors of AP’s and NT release!
Particularly T-type LVAs.
How is the alpha 1 subunit regulated?
Alpha 1 subunit contains a Protein Kinase C phosphorylation site as well as G b/y subunit binding site in the S6-S1 linker.
= D2 autoregulators.. bind Gi/Go b/y subunit..
Contain CaM-KII binding sites… which phosphorylate A1 subunit.
Aswell a scalmodulin binding domain = CBD at the C-terminus…
AND
Synaptotagmin - synprint site for binding synaptotagmin!!!, Syntaxin !, SNAP25…!!!1
What is the effect of Calmodulin on VGCC?
Calmodulin Binding Domain = CBD is at the C-terminus of Alpha subunit.
Dual regulation on Channel.
CaV2.1 can be positively regulated during local Ca2+ increases e.g in microdomain.
but also negatively regulated during global CA2+ increases.
