Axotomy-dependent and -independent synapse elimination in organ cultures of Wld(s) mutant mouse skeletal muscle

Progressive "dying back" neurodegenerative diseases are debilitating due to loss of connectivity after nerve terminal and axonal withdrawal, which impairs peripheral nerve function and leads ultimately to neuronal cell death. The mutant mouse (Wallerian degeneration slow; Wld(s)) provides an accessible model system to understand orthograde and retrograde degeneration, because in these mice axotomy induces slow, progressive withdrawal of nerve terminals from motor endplates. Axon degeneration itself is about 10 times slower than in wild-type mice. We describe an organ culture paradigm that permits direct observation of the progressive changes in morphology of neuromuscular junctions in Wld(s) mutant mice. Normal nerve terminal and motor endplate morphology were maintained at most Wld(s) neuromuscular junctions for up to 72 hr in vitro. At others, synaptic boutons were removed from postsynaptic junctional folds in piecemeal fashion, as observed in adults in vivo. By contrast, nerve terminals degenerated rapidly and synchronously in wild-type muscle cultures, resembling Wallerian degeneration in vivo. These observations confirm that in Wld(s) mice, axotomy triggers a mechanism of nerve-terminal withdrawal that seems qualitatively different from that in wild-type animals. The piecemeal dismantling of presynaptic terminals resembles that occurring during neonatal synapse elimination. Organ cultures of neonatal Wld(s) muscle maintained for 1-2 days in vitro also showed no evidence of synaptic terminal degeneration, but elimination of polyneuronal innervation progressed in vitro at approximately the same rate as in vivo. Taken together, the data suggest that both natural and axotomy-induced forms of synapse withdrawal may be accessible to continuous observation and analysis, in organ-cultures of Wld(S) mouse muscles. This offers several advantages over repeated visualization of synaptic remodeling that has thus far been possible only in vivo.