The role of nerve- versus muscle-derived factors in mammalian neuromuscular junction formation.
Neuromuscular junctions (NMJs) normally form in the central region of developing muscle. In this process, agrin released from motor neurons has been considered to initiate the formation of synaptic acetylcholine receptor (AChR) clusters (neurocentric model). However, in muscle developing in the absence of nerves and thus of agrin, AChR clusters still form in the muscle center. This raises the possibility that the region of NMJ formation is determined by muscle-derived cues that spatially restrict the nerve to form synapses from aneural AChR clusters, e.g., by patterned expression of the agrin receptor MuSK (muscle-specific kinase) (myocentric model). Here we examine at initial stages of synaptogenesis whether the responsiveness of myotubes to agrin is spatially restricted, whether the regions of NMJ formation in wild-type muscle and of aneural AChR cluster formation in agrin-deficient animals correlate, and whether AChR cluster growth depends on the presence of agrin. We show that primary myotubes form AChR clusters in response to exogenous agrin in their central region only, a pattern that can spatially restrict NMJ formation. However, the nerve also makes synapses in regions in which aneural AChR clusters do not form, and agrin promotes synaptic cluster growth from the first stages of neuromuscular contact formation. These data indicate that aneural AChR clusters per se are not required for NMJ formation. A model is proposed that explains either the neurocentric or the myocentric mode of NMJ formation depending on a balance between the levels of MuSK expression and the availability of nerve-released agrin.
Lin
Shuo
S
Biozentrum, University of Basel, CH-4056 Basel, Switzerland.
Landmann
Lukas
L
Ruegg
Markus A
MA
Brenner
Hans Rudolf
HR
D018171:Agrin / Q000172:deficiency; D000818:Animals; D003964:Diaphragm / Q000166:cytology / Q000196:embryology; D004622:Embryo, Mammalian; D005260:Female; D018507:Gene Expression Regulation, Developmental / Q000502:physiology; D018398:Homeodomain Proteins / Q000235:genetics; D021621:Imaging, Three-Dimensional; D008297:Male; D051379:Mice; D008822:Mice, Transgenic; D008954:Models, Biological; D024510:Muscle Development; D018485:Muscle Fibers, Skeletal / Q000378:metabolism; D009124:Muscle Proteins / Q000235:genetics / Q000378:metabolism*; D009469:Neuromuscular Junction / Q000378:metabolism*; D009924:Organ Culture Techniques; D011940:Receptor Aggregation / Q000502:physiology; D020794:Receptor Protein-Tyrosine Kinases / Q000172:deficiency; D011950:Receptors, Cholinergic / Q000502:physiology*; D014157:Transcription Factors / Q000235:genetics
D018171:Agrin; D018398:Homeodomain Proteins; D009124:Muscle Proteins; D011950:Receptors, Cholinergic; D014157:Transcription Factors; C084059:Hb9 protein, mouse; C511695:MuSK protein, mouse; D020794:Receptor Protein-Tyrosine Kinases