Neuromuscular Junction Flashcards
why do we care about the NMJ?
it’s easy to study - very large, and simple while being accessible on peripheral tissues
what 3 cell types are involved in the NMJ?
- motoneuron
- muscle cell
- Schwann cell
basal lamina
aka synaptic basement membrane
covers the muscle fiber and Schawnn cells
- has a different composition in the NMJ synapse than extrasynaptically
explain AP signal passage through the NMJ
- nerve AP reaches presynaptic terminal and opens voltage gated Ca2+ channels
- Ca2+ entry triggers ACh vesicle fusion to the presynaptic membrane and ACh release
- ACh diffuses across synapse through basement membrane
- ACh bind to AChR which opens ligand-gated ion channels (Na+ moves in, K+ moves out)
- local postsynaptic membrane depolarization occurs (EPP)
- if threshold endplate voltage is reached, voltage gated Na+ channels open and muscle AP follows
how to pre and post synaptic membrane components align themselves?
- axon terminals on presynaptic membrane perfectly overlay AChRs on postsynaptic membrane
endplate potential (EPP)
local post-synaptic depolarization of the muscle fibre surrounding the AChRs
- only open AChRs
- are observed only when muscle AP is blocked (lab setting)
miniature endplate potential (mEPP)
electrical signal induced when a single ACh vesicle is released
explain electrical events that occur in neuromuscular transmission
- 1st have an EPP
- if EPP is large enough to reach AP threshold voltage, it triggers opening of voltage-gated Na+ channels
- in healthy individuals, EPP will surpass AP threshold every time
safety factor
EPP amplitude-AP theshold
- ensures EPP reliably activates myofiber
ie. some leeway to be sure we reach threshold
Novichok
a Russian poison synthesized artificial version neurotoxin
- targets ACh esterase (breaks down ACh) and blocks enzyme from working
- was deployed in the UK by spies, targets were hospitalized but survived, though the toxin can have long term effects, one bystander was killed
choline acetyl transferase (ChAT)
an enzyme that catalyzes ACh synthesis in the intracellular space
- takes choline + acetyl-CoA to make ACh
- is highly expressed in MNs so is often used as a marker in immunofluorescence
where do the components of ACh comes from for its synthesis
Acetyl CoA comes from mitochondria
choline comes from extracellular fluid
vesicular ACh transporter (VAChT)
packs ACh into secretory vesicles
immediately releasable pool of ACh
where is it found
- 2-10% of ACh vesicles come from here
- localized at active zones of presynaptic membrane
- is first to be released upon arrival of AP and Ca2+ influx
active zones
discrete spots of ACh vesicles
- when Ca2+ enters the cell, vesicles fuse to the presynaptic membrane
reserve pool of ACh
cycles into active zones to replete stores when depleted
acetylcholine esterase (AChE)
breaks down ACh into choline and acetate
- choline gets cycled back
- found in basement membrane of primary and secondary junctional folds
cytomatrix of active zones (CAZ)
dense collections of proteins that make up active zones
where are AChRs found?
in peaks of secondary junctional folds
synapsin
an intermembrane protein that helps vesicle move from reserve pool to the immediately releasable pool
- works when immediately releasable pool is depleted
synaptobrevin
found in presynaptic membrane
- works with SNARE complex to be responsible for nucleation and zippering
zippering vs nucleation
both require interaction/weaving of synaptobrevin and SNARE protein complex
- nucleation keeps ACh vesicles from leaving the active zone
- zippering brings vesicle into contact with presynaptic membrane, preparing it to be fused to it
SNARE protein complex
proteins expressed all over the body that are important for docking vesicles at the membrane
synaptogamin
- activated by Ca2+ to caused synaptogamin-mediated fusion of ACh vesicle and presynaptic membrane to allow for exocytosis
positive signals at the NMJ/muscle fiber
- are released from nerve terminal and result in triggering gene expression of synaptic proteins at the NMJ
- supersede negative signals in local regions
- descend onto sub-synaptic nuclei which are responsible for generating the proteins involved at the NMJ
negative signals at the NMJ/muscle fiber
- elicit electronegative pathways when released which turn on proteins which inhibit expression of NMJ proteins like AChRs
- suppress protein expression everywhere in the muscle fiber and prevent formation of other NMJs
if positive and negative signals have such opposing functions, how do things get done?
they are balanced, positive overrule in local regions where the signals are at the NMJ
what positive signals are there?
- agrin
- neuregulin
agrin mechanism
- is released by MN terminal and bind to its receptor Lrp4-MuSK complex
- MuSK recruits Dok-7 to stabilize the AChR cluster and rapsyn anchors AChRs to the postsynaptic membrane
neuregulin mechanism
NGR-1 protein is released by MN terminal which bind to Erbβ receptors on motor endplate
- Erbβ and MuSK stimulate MAPK pathway that converges on subsynaptic myonuclei
- when Erbβ binds to its receptor, it activates GABP transcription factor which dimerizes and binds to N Box synapse specific gene promoters
- GABP is only expressed in sub-synaptic nuclei
overall negative signal mechanisms
global AChR transcription is inhibited via:
- myogenin
- calcium sensitive proteins CaMKII and PKC
myogenin and negative signalling
when the AP goes down the muscle fiber it releases Ca2+ which inhibits myogenin
- myogenin usually activates AChR gene expression by binding to E box promoter region of AChRs
calcium sensitive proteins CaMKII and PKC involvement in negative signalling
- both proteins are activated by Ca2+
- signalling from CaMKII and PKC result in transcriptional repression of myogenin (suppress its promoter)
can mice survive without any of the following: MuSK receptor Lrp4 Dok-7 agrin
nope. they die prematurely bc of respiratory failure
