Home NEUR 310 Thursday, March 20, 2014

Thursday, March 20, 2014 CC-BY-NC
Transmitter Release

Maintainer: admin

1Transmitter and vesicle fusion

  • release is triggered by calcium and very fast, so there must be a calcium sensor linked to fusion
  • transmitter release is quantal : number of vesicles and number of transmitters
  • organization of active zone , where vesicles are close to clacium and positioned right across from the receptors
    • Bassoon and Piccolo : scaffolding proteins that might be important for this

  • fusion of synaptic vesicle:

    • related to other fusion like in ER, Golgi, TGN
    • differences: calcium sensitivity and specificity for active zone
    • require SNARES

  • spontaneous release

    • not evoked, just at some rate spontaenously fuse with the membrane
    • there seem to be two groups of synaptic vesicle: evoked release and ones for spontaneous release

  • can use a hypertonic shock (put in lots of sucrose) would cause all readily releasable vesicles to fuse

2SNARES

  • Types

    • VAMP (Synaptobrevin)
      • on vesicle, an R SNARE (with arginine)
    • Syntaxin
      • on acceptor membrane, a Q SNARE (with glutamine)
      • integral plasma membrane protein
      • can bind to synaptotagmin (a calcium sensor) on vesicle
    • SNAP25
      • on acceptor membrane, a Q SNARE (with glutamine)
      • attach to lipid on membrane (not integral membrane protein)

  • they form coil-coil complex

    • 1 VAMP + 1 Syntaxin + 2 SNAP25

  • tetanus and botulinim toxins: protease that cleave SNARE and thus block neurotransmitter release

3Other proteins

  • priming and priming of vesicles:
    • need to dock close to the calcium channels
      • synaptotagmin binding to SNAP 25 or syntaxin
      • tethering to calcium channels by Rim proteins
    • involves generating SNARE coiled-coil interaction

3.1Regulators of SNARE complex

  • Munc18 :

    • tighly associated with SNARE complex
    • binds to synatxin in a closed conformation to prevent a full coil-coil complex
    • also prevent unproductive syntaxin/SNAP25 heteromer (i.e. prevent them from coiling with each other when there's no need to.
    • this is only in synaptic vesicles

  • Munc13

    • discrupts Munc18
    • major point of regulation for vesicle release, such as in plasticity, for cells

  • complexin:

    • binds to late coiled-coil complex
    • synaptotagmin in presence of calcium, removes complexin binding to allow fusion

3.2Calcium sensing

  • Synaptotagmin

    • a synaptic vesicle protein with two C2 domain that binds calcium
    • knockout : changes in calcium dependence of synchronous release and no more fast calcium evoked release
      • slow calcium evoked release are intact
    • binds to 5 calcium and is fast
    • important when calcium level is high

  • "mysterious asynchronous sensor"

    • we don't know what it is, could be synaptotagmin 7
    • binds to 2 calcium and is slow
    • important when calcium level is low

3.3Recycling

  • NSF : an ATPase that breaks up SNARE complex after fusion so they can be reused
  • clatherin: clatherin-coated vesicles recycle the SNAREs
    • endophillin KO blocks almost all recycling. it has to do with forming clatherin-coated vesicles
  • dynamin: GTPase that buds/pinches off vesicles
    • Shibere: temperature-sensitive drosophila dynamin mutation

4Synaptic vesicles and dense core

  • synaptic vesicles do not bud off TGN ; probaby formed by endocytosis
  • neuropeptides from dense core vesicles are not released at active zones
    • outside of active zones, and diffuse to receptors
    • require high rates of stimulation and lower levels of calcium for release
      • not localized next to calcium channels, so levels need to increase throughout the entire presynaptic terminal before dense cores are released