trafficking and signalling Flashcards
what is synaptic strength?
synapses are not static, they can get stronger or weaker over time (plasticity)
what are strong synapses?
- lots of receptors
- greater influx of ions through channels
- greater influence on membrane potential
- greater influence in likelihood of firing AP
what are weak synapses?
- fewer receptors
- lower total influex of ions through channels
- smaller influence on membrane potential
- smaller influence on likelihood of firing action potential
how do we control synapse function?
regulating the number of neurotransmitter receptors at the synapse
how do we regulate the number of transmitter receptors?
protein synthesis, protein trafficking and degradation
what is protein trafficking?
proteins are sorted and targeted to different parts of the cell
creation and trafficking of plasma-membrane receptors
- membrane receptors are first synthesised in the RER
- ribosomes on RER synthesis the membrane protein and thread it into the surface membrane of the ER
- receptors than trafficked from ER to the golgi apparatus
- trafficked to surface
this is coded for by amino acid sequencing which contains all the signals needed for trageting
various cytosilic proteins interact eith protein sequences on receptors to control traffic
protein synthesis in the ER
transmembrane receptors have short hydrophobic AA sequences (signal peptide) that stay in the plane of the plasma membrane during synthesis
signal peptide inserted into translocation channel into ER
when transmembrane domains enters the translocation channel it will open and lets the protein out into the ER
signal peptidase cleaves off signal peptide
what can regulate protein synthesis in the ER?
miRNA
steps of the secretory pathway
- mRNA of transmembrane receptors contain signal sequence which targets it for translation in RER
- protein synthesised by ribosome and sits on RER membrane, nascent protein establishes its transmembrane topology
- transport vesicle buds off ER and carries receptor to golgi
- in receptors are sorted into further transport vesicles for delivery to plasma membrane
important fact to remember about transmembrane receptors:
they must always be embedded in a membrane, even when moving around the cell
this is why they needed to be shuttled betweeen compartments via vesicles
and this makes it so we have efficient systems for vescles for membrane compartments and fusing
issue with fusion of membranes
energentically unfavourable
why is fusing membranes energentically unfavourable?
have to expose hydrophilic heads and hydrophonic tails that are arranged in the bilayer
how do we make membrane fusion happen?
exocytosis of vesicle and membrane
vSNARES and tSNAREs
on vesicles and target membranes we have SNARE proteins
if vSNARE and tSNARE are complementary they can wind around each other and pull membrane close together
this generates enough force to fuse the membranes
how do you separate SNARE complex?
NSF (ATPase enzyme)
role of NSF
NSF uses energy from ATP to reset SNARE system to separate proteins
steps of exocytosis and membrane fusion
- receptors in vesicle membrane contains
SNARE proteins - SNAREs in vesicle membrane bind tightly to SNAREs in target membrane
- SNARE complex pulls membranes close together, so they fuse
- the vesicle membrane is incorporated into the target membrane
- hence receptors from the vesicle are transferred to the
target membrane
how do you create a vesicle on a membrane?
endocytosis
steps of endocytosis and vesicle creation
- receptors interact with clathrin (protein that polymerises when reaches certain conc) coat via AP2
- various proteins including clathrin cause the membrane to bend into circular shape (vesicle)
- dynamin pinches off the forming vesicles
- hence receptors are isolated in the vesicle and are uncoated
- receptors then pass through various compartments called the endosomes which sort them and send them to specific places in the cell
what happens to receptors following endocytosis?
- receptors are internalised by endocytosis on a vesicle
- this can fuse with an early endosome (sorting station)
- receptors may be sent back to the plasma membrane via recycling endosomes
OR
sent to late endosomes for further sorting, usually to lysosomes, where the receptors are degraded
what are lysosomes?
acidic organelles packed with proteases to degraded membrane receptors/vesicles
synaptic plasticity and AMPA receptors
long-term potentiation=synapse gets stronger which is primarily mediated by increased exocytosis of AMPA receptors=more receptors so more sensitive
long-term depression=synapse gets weaker=increasing rate of endocytosis=removed receptors from surface for degradation
protein important for endocytosis
AP2
protein important for exocytosis
NSF
protein important for endosomal transport
myosin V
protein important for vesicular delivery
GRIP1 and KIF1A
3 examples of receptor interacting proteins
scaffold proteins
trafficking proteins
kinase anchoring proteins
AMPA scaffolding proteins
PSD-95
GABAa scaffolding proteins
gephyrin
AMPA trafficking proteins
AP2, NSF, PICK1
GABAa trafficking proteins
AP2, NSF, GABARAP
AMPA kinase anchoring proteins
AKAP150/79
GABAa kinase anchoring proteins
AKAP150/79 and RACK1
what are kinase anchoring proteins?
bind to receptor and kinase (protein that phosphorylates other proteins) which allows rapid phosphorlyation to change activity of receptor
GABAaR trafficking
- receptors clustered in surface by binding to gephyrin
- receptors on surface are phosphorylated by intracellular loop which keeps them bound to gephyrin
- receptors can be dephosphorylated by phosphatase
- AP2 (endosystic) can bind to dephosphorlyated receptors and forms coat of clatharin, dyanmin pinch etc
- this shows how we can regulate receptors on surface under different conditions
what is HAP1?
involved in cellular issues during huntingtons diease
- normally functions in recycling of GABAR in endosomal system back to plasma membrane
- in huntingtons the HAP1 protein doesnt work which leads to reduced recyling of GABAR
- this leads to weaker inhibitory synapses
AMPAR and NMDAR trafficking
- NMDAR=calcium
- calcium activates PKC, CAMKII, calcineurin and PICK1
- depending on pattern of activation, kinases can promoted LTP by inserting AMPAR receptors of LTD by removing AMPAR receptors through interactions of endocytic/exocytic proteins
