Neural agrin increases postsynaptic ACh receptor packing by elevating rapsyn protein at the mouse neuromuscular synapse.
Fluorescence resonance energy transfer (FRET) experiments at neuromuscular junctions in the mouse tibialis anterior muscle show that postsynaptic acetylcholine receptors (AChRs) become more tightly packed during the first month of postnatal development. Here, we report that the packing of AChRs into postsynaptic aggregates was reduced in 4-week postnatal mice that had reduced amounts of the AChR-associated protein, rapsyn, in the postsynaptic membrane (rapsyn(+/-) mice). We hypothesize that nerve-derived agrin increases postsynaptic expression and targeting of rapsyn, which then drives the developmental increase in AChR packing. Neural agrin treatment elevated the expression of rapsyn in C2 myotubes by a mechanism that involved slowing of rapsyn protein degradation. Similarly, exposure of synapses in postnatal muscle to exogenous agrin increased rapsyn protein levels and elevated the intensity of anti-rapsyn immunofluorescence, relative to AChR, in the postsynaptic membrane. This increase in the rapsyn-to-AChR immunofluorescence ratio was associated with tighter postsynaptic AChR packing and slowed AChR turnover. Acute blockade of synaptic AChRs with alpha-bungarotoxin lowered the rapsyn-to-AChR immunofluorescence ratio, suggesting that AChR signaling also helps regulate the assembly of extra rapsyn in the postsynaptic membrane. The results suggest that at the postnatal neuromuscular synapse agrin signaling elevates the expression and targeting of rapsyn to the postsynaptic membrane, thereby packing more AChRs into stable, functionally-important AChR aggregates.
School of Medical Sciences (Physiology), Bosch Institute, University of Sydney, Sydney, Australia.
Jennifer
J
Brockhausen
Rebecca N
RN
Cole
Othon L
OL
Gervásio
Shyuan T
ST
Ngo
Peter G
PG
Noakes
William D
WD
Phillips
D018171:Agrin / Q000378:metabolism* / Q000494:pharmacology; D000818:Animals; D002038:Bungarotoxins / Q000494:pharmacology; D002454:Cell Differentiation / Q000502:physiology; D002460:Cell Line; D005260:Female; D005455:Fluorescent Antibody Technique; D051379:Mice; D018345:Mice, Knockout; D009046:Motor Neurons / Q000378:metabolism; D018485:Muscle Fibers, Skeletal / Q000166:cytology / Q000378:metabolism; D009124:Muscle Proteins / Q000235:genetics / Q000378:metabolism*; D018482:Muscle, Skeletal / Q000254:growth & development / Q000294:innervation / Q000378:metabolism; D009469:Neuromuscular Junction / Q000187:drug effects / Q000254:growth & development / Q000378:metabolism*; D021381:Protein Transport / Q000502:physiology; D051381:Rats; D011940:Receptor Aggregation / Q000187:drug effects / Q000502:physiology*; D011950:Receptors, Cholinergic / Q000378:metabolism*; D013570:Synaptic Membranes / Q000187:drug effects / Q000378:metabolism; D009435:Synaptic Transmission / Q000502:physiology; D015854:Up-Regulation / Q000502:physiology*
D018171:Agrin; D002038:Bungarotoxins; D009124:Muscle Proteins; D011950:Receptors, Cholinergic; C051609:peripheral membrane protein 43K