Neurotransmitter release

Neuronal chemical axo-dendritic communcaiton
1. NT release is a highly specialised form of membrane fusion
2. Very high spatial and temporal resolution
3. Involves the release of small packets or quanta of transmitter
4. Triggered by calcium-mediated fusion of vesicles
5. Very rapid transduction of an electrical stimulus into vesicular release - less than 1ms
Vesicular transmitter release
1. Small synaptic vesicles (SSVs)
  1. Small transmitters
  2. Diameter about 50nm
2. Large dense-core Vesicles (LDCVs)
  1. Peptides
  2. Diameter about 250nm
Structure and location of synaptic vesicles
1. Large concentration of protein on post synaptic side (about 1000 different proteins involved)
2. Reserve pool of vesicles pre-synaptically are bound to actin - released from actin upon stimulation
Action potential
1. 1. Action potential propagation in presynaptic neuron
2. 2. Ca2+ entry into synaptic knob
3. 3. Release of neurotransmitter by exocytosis
  1. 1. Docking - membrane of vesicles tightly associates with plasma membrane (otherwise vesicles are in a reserve pool)
  2. 2. Priming - creating of a competent readily releasable pool of vesicles
  3. 3. Fusion - active fusion of the vesicle membrane with the plasma membrane to release vesicular contents when the local calcium reaches a threshold level
  4. 4. Recycling - of vesicular membrane by endocytosis to form a new vesicle of transmitter by filling
4. 4. Binding of neurotransmitter to postsynaptic receptor
5. 5. Opening of specific ion channels in subsynaptic membrane
Vesicle fusion regulation
1. Vesicular proteins
  1. Synaptobrevin
    1. Important for controlling fusion
  2. Synaptotagmin
    1. Calcium trigger
    2. Drives membrane fusion
  3. Synapsin
    1. Important in holding vesicles in the reserve pool
  4. Rab's
    1. Important in fusion
2. Phosphorylation of synapsin 1 releases the vesicle form the reserve pool
  1. CamKII is activated and phosphorylates synapsin 1 causing it to release from actin
3. Rab 3 involved in docking and fusion
  1. GTP-bound Rab3 binds Rabphilin and the plasma membrane protein RIM
  2. GDP-vound Rab3 dissociates from vesicles
  3. C2 domain proteins are domains involved in calcium and lipid binding (common in proteins associated with the lipid membrane
4. Fusion requires N-ethlmaleimide (NEM) sensitive factors (NSF)
  1. Cell-free fusion assays of golgi derived vesicles - NEM sensitive protein fraction
  2. 76kDa homotrimer
  3. ATPase activity
  4. Required together with soluble NSF attachment proteins (SNAPs) for ER-golgi and intra-golgi membrane transport
Synaptic SNAREs
1. Soluble N-ethylmaleimide sensitive factor (NSF) attachment protein receptor
2. All membrane fusion secretory events involve the formation of a helical coiled coil complex assembled from membrane anchored proteins known as SNAREs
3. SNARE proteins responsible for synaptic vesicle fusion are:
  1. Syntaxin 1
    1. Presynaptic plasma membrane (integral) tSNARE
  2. SNAP 25
    1. Presynaptic plasma membrane (integral) - tSNARE
  3. Synaptobrevin 2
    1. Vesicular membrane (integral) - vSNARE
  4. Synaptic SNAREs form a complex of four parallel helices: Syntaxin and Synaptobrevin contribute one motif each and SNAP 25 two mofits
4. SNAREs squeeze target and vesicular membrane
5. Munc-13 and Munc-18 enable formation of the SNARE complex
  1. Muc-18
    1. Acts as a clasp
    2. Binds Syntaxin 1
    3. Designed to bind 4 helix bundles
6. SNARE complexes are 'clamped' by complexins
  1. Complexins bind SNARES with 1:1 stochiometry
  2. Complexins arrest fusion in an hemifusion state - inner but not outer leaflets fused
7. Synaptotagmin 1
  1. Can displace complexin from SNAREs in the presence of Ca2+
  2. Trigger for NT release
  3. Integral protein on vesicles with two C-terminal C2 domains (C2A/B)
  4. Calcium and phospholipid binding domains
  5. Acts to put a Ca2+ sensitive inhibitory constrain on exocytosis
  6. Acts upon the complexin-bound SNARE complex in response to Ca2+ and thus triggers fast diffusion

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Neurotransmitter release

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	Created by zambrella about 11 years ago