Calcium channels Flashcards
what is the concentration of calcium inside and outside the nerve terminal?
why is calcium homeostasis important?
internal ~1mM
external ~ 100nM
if levels of calcium are elevated for prolonged periods
- > can be damaging for synapse
- > evolves a calcium dependent form of neurotoxicity
what allows Ca2+ in?
what removes Ca2+ from the cell?
calcium channels
non-selective cation channels
- both can be ligand or voltage gated
calcium pump
- transports protons in
calcium exchanger
- transports Na+ in
what are the 2 important buffering organelles?
ER
- Ca2+ can go in and form stores
Mitochondria in synapse
what are genetically encoded calcium indicators called?
what are they?
how does it work?
Cameleons
GFP fused to calmodulin and M13 peptide of MLCK
Ca2+ binding to calmodulin
-> induces a conformational change
= alters emission properties
what are the features of a SNARE complex at 200nm from the Ca2+ channel?
1) lower concentration ~ 5-10uM
2) slower rises and falls
3) at equilibrium with movie buffers
- > so calcium can be mopped up
4) strongly dependent on buffers
5) calcium concentration is determined by mean activity of several neighbouring channels
what are the features of a SNARE complex at 20nm from the Ca2+ channel?
what are these known as?
1) higher conc ~ 100uM
2) quicker rises and falls
3) is not at equilibrium with mobile buffers
- > can overcome buffering activity as the Ca2+ conc is so high
4) almost independent of Ca-buffers
5) calcium conc determined by the local channel
Microdomains
what are micro domains critical to?
the fine spacial and temporal control needed to maintain fast excitatory neurotransmitter transmission
describe calcium-dependent NT release
e.g. glutamate
- action potential in bouton generated by Na+ channels opening
- depolarisation opens Ca2+ channels
- Ca2+ elevation occurs in micro domain
- Ca2+ binds to synaptotagmin causing opening of fusion pore
- glutamate passes through fusion pore and diffuses in the cleft
- opening of AMPA channel generates EPSC
how do action potentials trigger calcium influx?
what happens then?
when action potential reaches threshold
- > triggers depolarisation
- > inward Ca2+ current occurs
triggers NT release
-> EPSP
what is homosynaptic plasticity?
what are the 3 types?
the amount of NT released with each action potential is not constant
- but is a factor of the quanta
facilitation
potentiation
depression
what is facilitation?
what is potentiation?
what is depression?
progressive increase in release due to residual calcium at release site during a train of APs (short lasting)
increase following repetitive stimulation
termed PTP, long lasting, involves mitochondrial calcium buffering and release
progressive decrease in release during a train of action potentials due to decline in readily releasable vesicle pool
what are the features of voltage gated calcium channels?
mediate calcium influx in response to membrane depolarisation
couple electrical activity to neurophysiological events
e.g. NT release
members of a gene superfamily of transmembrane ion channels
e.g. voltage-gated sodium channels
complex proteins of 4-5 distinct subunits encoded by multiple genes
describe the structure of the voltage-gated calcium channel
alpha 1
= where most fundamental properties of the channel occur
auxiliary subunits
= important for trafficking, folding + overall function
describe the structure of the alpha subunit
10 genes
contains:
conduction pore
voltage sensing + gating
2nd messenger + toxin regulatory sites
4 homologous domains (I-IV), 6 membrane segments in each (S1-S6)
what do the homologous domains and membrane segments in the alpha subunit do?
3 amino acid changes in I, III, IV convert Na+ to Ca2+
S4 voltage sensor
S5-S6 pore loop determines ion conductance
what is the nomenclature based on sequence similarity in a1 subunits?
Cav1.1-4
Cav2.1-3
Cav3.1-3
what is the basic subdivision of voltage gated calcium channels?
High voltage activated
(HVA)
(-40 - -10mV)
Low voltage activated
(LVA)
(-60 - -70mV)
What are the 4 types of channel within HVA?
L-type
= ‘L’ong lasting, slowly inactivating currents
N-type
= ‘N’euronal, slowly inactivating
P/Q type
= ‘P’urkinjie,
P slowly inactivating, Q variable
R-type (IVA)
= R (follows Q, Resistant)
- slow/medium inactivation
what are LVA channels?
T-type
= ‘T’ransient,
rapidly inactivating
which types of channel are involved in NT release?
which types are involved in Ca2+ spikes?
which types isn’t involved in the regulation of NT release?
where are they found?
P/Q
N
P/Q
R
L
regulate post-synaptic signalling events
give examples of toxins that voltage gated calcium channels show differential sensitivity to
Omega agatoxins
Omega conotoxins
SNX482
what are the intracellular regulatory domains of the a1 subunit?
SNAREs
Phosphorylation site for PKC
CaMKII
- phosphorylates C terminus of channel in response to Ca2+ rises
G protein
- beta gamma subunit associates with channel
what are the features of the beta subunit?
encoded by 4 genes
facilitate correct folding + promote exit from ER
enhance Cav1 and Cav2 receptor trafficking
phosphorylation of b2 subunits via PI3K/Akt reduces channel degradation
what are the features of the a2-delta subunit?
4 genes encode the subunit
-> then cleaved post translationally into 2 subunits
enhance channel expression + trafficking
site of binding for anti-epileptic drugs
what are the effects of a naturally occurring a2d2 knockout?
‘ducky’
reduced calcium current in Purkinje neurones
what is heterosynaptic plasticity?
modulation of presynaptic NT release by signalling molecules coming in from neighbouring synapses
what are the important components in heterosynaptic plasticity?
Autoreceptors
- all GPCRs (Gi)
e. g. GABA B, cb1
receptors on presynaptic terminal
detect transmitter to regulate further transmitter release
Retrograde messengers
= diffusible second messengers produced post-synaptically that acts pre-synaptically to control further NT release
what are endocannabinoids?
endogenous ligands of cannabinoid receptors (CB1 and CB2
describe lipid derived regulatory signalling e.g. endocannabinoids
- glutamate released
- activates post synaptic receptors e.g. AMPA + mGluR1
- stimulates PLC
- acts on PIP2
- activates DAG
- DAG metabolised by DAG lipase into 2-AG
- 2-AG is lipid derived so can get out across the membrane and act back on nerve terminal receptors
describe gas regulatory signalling e.g. nitric oxide
what is this an example of?
- glutamte activates NDMA receptor
- triggers calcium influx
- drives enzymatic conversion of L-arginine -> NO via nNOS
- NO diffuses out and acts on nerve terminal
- guanylyl cyclase senses NO and produces second messenger molecule cGMP
- regulates further transmitter release
facilitation
= more transmitter coming out
what is an example of peptide regulatory signalling?
opioids
describe conventional neurotransmitter regulatory signalling
e.g. GABA
where can the transmitter come from?
- GABA stimulates post synaptic receptors
- GABA signals back onto GABA B receptors
- activates Gi
- beta gamma subunit controls transmitter release
- regulates further transmitter release
can come from other neighbouring synapses
describe growth factor regulatory signalling e.g. neurotrophins
BDNF (brain derived neurotrophic factor)
acts through receptor TrkB
-> exerts control over transmitter release
