L10 Flashcards
what can regulate exocytosis?
Ca2+ (calcium)
- Ca2+ regulated exocytosis found in cells specialized for secreting products rapidly on demand
what diseases can be due to aberrent secretion?
- endocrine disorders - excess secretion of growth hormones
- type II diabetes
- immune disorders (high secretion of antibodies)
- mood disorders
why is controlling cell secretion important?
- important for maintaining cellular homoeostasis by adapting to changes in the local environment
what does Ca2+ have to bind to in order to trigger fusion?
synaptotagmin (Ca2+ sensor for transmitter release at central synapse)
-Ca2+ binds with phospholipids via the C2B domain of synaptotagmin to trigger fusion
(can bind to multiple Ca2+ ions)
what is the protein composition of synaptic vesicles + how was this determined?
- purified neurons(+their vesicles) from the brain and did mass spectrometry
- ~70 copies of synaptobrevin
- synaptotagmin (Ca2+ binding sensor)
- synaptophysin (interacts with synaptobrevin)
- glutamate transporters
- synapsin
- rab proteins
- V-ATPase
-syntaxin 1 and SNAP25 - contamination of PM
what does synapsin do?
tethers vesicles to actin cytoskeleton
what does V-ATPase do?
acidifies the inside of the vesicles
how many isoforms are there of synaptotagmin and how do they differ?
- 16 isoforms
- have different Ca2+ affinities
- have different membrane/vesicle localisations
describe the relationship of Ca+ conc/binding to sensor and level of release/exocytosis
- exponential release
- implies multiple ions are required to bind to synaptotagmin/sensor to get max/high release/exocytosis
Can Ca2+ bind with SNARE proteins?
Yes
- Ca2+ can bind with SNARE complex which is important for the regulation of docking
- binds with complexin which acts like a fusion clamp
What happens in KO synaptotagmin mouse?
- released synaptic transmission not at great as in WT
- evoked synchronised transmission is not depressed in KO
- small amount of vesicle fusion but is unsynchronised
- asynchronous release persists
- hypertonic sucrose induced released - so vesicles are still being made - just not fusing
- DOCKING DEFECT
WHAT DOES COMPLEXIN DO?
complexin clamps SNARES in a zigzag array
- this prevents premature membrane fusion but holds SNARES in place so that they are ready for rapid synchronous fusion/release/exocytosis
- when Ca2+ (bound to synaptotagmin)comes in it displaces complexin and allows SNARE to do final zip
what is synaptotagmin?
synaptotagmin is a major Ca2+ sensor in the membrane of presynaptic axon terminal
- it regulates NT release
- calcium binds to C2b domain which triggers fusion
- synaptotagmin+calcium can displace complexin from SNARE complex - which removes clamp and allows vesicle fusion to occur
What ensures that only the vesicles docked near voltage gated calcium channels (VGCCs) are released by an AP?
- low affinity of synaptotagmin 1,2,9
- low affinity synaptotagmin can only sense very high calcium concentrations which only occurs when the channel is open at the mouth of the channel
- (then calcium diffuses away rapdily
- therefore - only vesicles docked at mouth of VGCCs undergo fusion when there is calcium influx into the cell
what does fast Ca2+ dependent lipsome fusion require?
- the combined function of complexin II and synaptotagmin I
