NEUROMUSCULAR DEVELOPMENT IN THE AVIAN PARALYTIC MUTANT CROOKED NECK DWARF (CN CN) - FURTHER EVIDENCE FOR THE ROLE OF NEUROMUSCULAR ACTIVITY IN MOTONEURON SURVIVAL/

Neuromuscular transmission and muscle activity during early stages of embryonic development are known to influence the differentiation and s urvival of motoneurons and to affect interactions with their muscle ta rgets. We have examined neuromuscular development in an avian genetic mutant, crooked neck dwarf (cn/cn), in which a major phenotype is the chronic absence of the spontaneous, neurally mediated movements (motil ity) that are characteristic of avian and other vertebrate embryos and fetuses. The primary genetic defect in cn/cn embryos responsible for the absence of motility appears to be the lack of excitation-contracti on coupling. Although motility in mutant embryos is absent from the on set of activity on embryonic days (E) 3-4, muscle differentiation appe ars histologically normal up to about E8. After E8, however, previousl y separate muscles fuse or coalesce secondarily, and myotubes exhibit a progressive series of histological and ultrastructural degenerative changes, including disarrayed myofibrils, dilated sarcoplasmic vesicle s, nuclear membrane blebbing, mitochondrial swelling, nuclear inclusio ns, and absence of junctional end feet. Mutant muscle cells do not dev elop beyond the myotube stage, and by E18-E20 most muscles have almost completely degenerated. Prior to their breakdown and degeneration, mu tant muscles are innervated and synaptic contacts are established. In fact, quantitative analysis indicates that, prior to the onset of musc le degeneration, mutant muscles are hyperinnervated. There is increase d branching of motoneuron axons and an increased number of synaptic co ntacts in the mutant muscle on E8. Naturally occurring cell death of l imb-innervating motoneurons is also significantly reduced in cn/cn emb ryos. Mutant embryos have 30-40% more motoneurons in the brachial and lumbar spinal cord by the end of the normal period of cell death. Elec trophysiological recordings (electromyographic and direct records form muscle nerves) failed to detect any differences in the activity of co ntrol vs. mutant embryos despite the absence of muscular contractile a ctivity in the mutant embryos. The cu-ryanodine receptor that is genet ically abnormal in homozygote cn/cn embryos is not normally expressed in the spinal cord. Taken together, these data argue against the possi bility that the mutant phenotype described here is caused by the pertu rbation of a central nervous system (CNS)-expressed alpha-ryanodine re ceptor. The hyperinnervation of skeletal muscle and the reduction of m otoneuron death that are observed in cn/cn embryos also occur in genet ically paralyzed mouse embryos and in pharmacologically paralyzed avia n and rat embryos. Because a primary common feature in all three of th ese models is the absence of muscle activity, it seems likely that the peripheral excitation of muscle by motoneurons during normal developm ent is a major factor in regulating retrograde muscle-derived (or musc le-associated) signals that control motoneuron differentiation and sur vival. (C) 1997 Wiley-Liss, Inc.