Vesicular transmitter release! Flashcards
What are the properties of vesicular transmitter release?
Rapid transduction of electrical stimulus into vescicular release - less than 1ms.
Very high spatial and temporal resolution - pin-point signalling.
Release of small packets/quanta of NT.
Triggered by Ca2+ mediated fusion of vesicles.
What is quantal release?
NTs are released in integer multiples of a packet (quantums) that correspond to contents of an individual synaptic vessicle.
End Plate Potential is due to summation of many cholinergic vesicles released at the same time.
Katz showed each synaptic cholinergic vesicle produced a potential of approx. 0.5mV.
What are the differences between SSVs and LDCVs?
Small Synaptic Vesicles - around 50nm in diameter.
Store small molecule NTs. Stay in the synaptic terminal, recylced and refilled.
SSVs remain docked in the active zone - others held in the reserve pool.
Large Dense Core Vesicles (LDCVs):
Store Neuropeptides. LCDVs are filled in the soma and trafficked to the nerve terminal, so can be found distributed throughout the neurone. Much larger - around 250nm in diameter.
What are examples of NTs being released at different rates?
Retinal + inner ear synapses are sensory so are designed to respond very quickly. can release 1000s of vesicles per ms
Compared to Fast cerebellar connections - release just 3 vesicles per ms.
What can control the different rates of synaptic vesicle release?
Efficiency and rate of replenishment - VMAT for NAergic synapses..
No. of veiscles in release ready state docked at active zone.
Response mechanism to Ca2+ trigger.
What are the stages of NT release?
Loaded SVs are target to the active zone.
1.) Docking - vesicular membrane tightly associates with PM, unless kept in reserve pool.
2.) Priming - Docked SSVs are primed to produce competent, readily release pool of vesicles in active zone.
3.) Fusion - In response to Ca2+ trigger above threshold, active fusion of vesicular membrane with PM to release vesicular contents.
4.) Recycling - SSV membranes are reculed by endocytosis and refilled.
How is fusion halted?
Prior to the formation of the fusion pore, there is an intermediate Hemifusion state = Where synaptic vesicle is primed and readily releasable upon Ca2+ trigger.
Hemifsuion state is stabilised by Complexins - which bind to SNAREs in 1:1 stoichiometry ie. 1 complex per SNARE.
Complexin arrests fusion in hemifused state where the cytoplasmic-facing leaflets are fused.
How does synaptotagmin work?
Synaptotagmin acts as the trigger for fast vesicular fusion.
Synaptotagmins are in association with complexins - which stabilise hemifused state.
Upon Ca2+ influx, Ca2+ is detected by synaptotagmin and displaces complexins from SNAREs.
= Allowing for full fusion.
Synaptotagmin is an integral VESICULAR membrane protein which has 2 C2 domains and 2 Ca2+ binding domains…
What are the different modes of endocytosis of vesicles?
Ultrafast endocytosis - with rapid recycling.
Kiss and Run = partial fusion with open pore but without full exocytosis.
Clathrin-mediated endocytosis - where synaptic vesicle is endocytosed in clathrin-coat.
How are SSVs held at the reserve pool?
SSVs in reserve pool are associated with cytoskeleton,
Synapsin is in unphosphorylated state - phosphorylation of synapsin causes recruitment to the active zone.
What is the role of RAB3?
RAB3 is GTPase protein on vesicular membrane surface.
GTP-bound RAB3 binds PM protein RIM - forming a scaffold complex with Mun13, Rabphilin and Ca2+ channels.
= Complex holds vesicle in close proximity to Ca2+ channels
Formation of scaffold is coordinated by lipid binding C2 domain of RIM.
Why are lipid binding C2 domains so common?
Lipid binding C2 domains are used to localise a protein to plasma membrane/phospholipid membranes.
Common signature of protein that moves from cyotosol to PM.
What is NSF?
NEM-sensitive factor (NSF).
NSF has ATPase activity…
NSF in complex with Soluble NSF attachment proteins (SNAPs) are rquired for ER-Golgi and Intra-golgi membrane transport.
Despite their role in ER-Golgi and intra-golgi membrane transport, in terms of vesicular transport - the functional role of NSF and SNAPs is in regulating SNARE dissassembly!
What are SNAREs?
Soluble NSF attachment protein RECEPTORS (SNAREs):
SNAREs are membrane anchored and involved in all membrane fusion secretory events.
SNAREs form a helical coiled-coil complex together = form a 4 helical bundle.
tSNAREs are on target MEMBRANE:
Syntaxin1 and SNAP-25.
Synaptobrevin2 = vSNARE = on VESICLE.
Together, Form a complex of four parallel helices:
A helix contributed each by Syntaxin 1 and Synaptobrevin 2, with 2 helices contributed by SNAP-25.
How does botox work?
BOTOX prevents ACh vesicular release by preventing formation of 4 helical bundle with Synaptobrevin 2, Syntaxin 1 and SNAP-25.
