Age-dependent synapse withdrawal at axotomised neuromuscular junctions in Wlds mutant and Ube4b/Nmnat transgenic mice

Axons in Wld(s) mutant mice are protected from Wallerian degeneration by overexpression of a chimeric Ube4b/Nmnat (Wld) gene. Expression of Wld protein was independent of age in these mice. However we identified two distinct neuromuscular synaptic responses to axotomy. In young adult Wld(s) mice, axotomy induced progressive, asynchronous synapse withdrawal from motor endplates, strongly resembling neonatal synapse elimination. Thus, five days after axotomy, 50-90% of endplates were still partially or fully occupied and expressed endplate potentials (EPPs). By 10 days, fewer than 20% of endplates still showed evidence of synaptic activity. Recordings from partially occupied junctions indicated a progressive decrease in quantal content in inverse proportion to endplate occupancy. In Wld(s) mice aged > 7 months, axons were still protected from axotomy but synapses degenerated rapidly, in wild-type fashion: within three days less than 5% of endplates contained vestiges of nerve terminals. The axotomy-induced synaptic withdrawal phenotype decayed with a time constant of similar to30 days. Regenerated synapses in mature Wld(s) mice recapitulated the juvenile phenotype. Within 4-6 days of axotomy 30-50% of regenerated nerve terminals still occupied motor endplates. Age-dependent synapse withdrawal was also seen in transgenic mice expressing the Wld gene. Co-expression of Wld protein and cyan fluorescent protein (CFP) in axons and neuromuscular synapses did not interfere with the protection from axotomy conferred by the Wld gene. Thus, Wld expression unmasks age-dependent, compartmentally organised programmes of synapse withdrawal and degeneration.