Age‐Dependent Synapse Withdrawal at Axotomised Neuromuscular Junctions in Wld^s Mutant and Ube4b/Nmnat Transgenic Mice

Abstract

Axons in WldS 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 Wlds 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 Wlds 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 ∼30 days. Regenerated synapses in mature Wlds 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.

NAD and axon degeneration: From the Wlds gene to neurochemistry

“…92 Another study showed that in Wlds mice over seven months of age, axotomy-induced synapse withdrawal from motor endplates is no longer protected, and reverts to a wild-type pattern. 93,94 These findings indicate that the molecular pathways involved in the NAD + /Wlds mediated neural protection might be regulated by age-related changes. The age factor is probably working downstream of the Wlds gene.…”

Targeting NMNAT1 to Axons and Synapses Transforms Its Neuroprotective PotencyIn Vivo

“…The weaker Wld S protection at NMJs, compared to the axonal trunk (Gillingwater et al, 2002), could be explained by the longer distance that Wld S needs to travel to reach the nerve endings and by dilution into the extensive terminal axonal branches. The age dependence of synapse withdrawal could be due to progressive decrement of axonal transport efficiency correlating with aging.…”

“…Because Wld S protective efficacy is dose dependent (Mack et al, 2001), axon protection would not be expected at such low levels, but low doses of axonally targeted NMNAT1 delay Wallerian degeneration for even 5 weeks. Moreover, neither a heterozygous dose of Wld S protein in young mice nor a homozygous dose in older mice is sufficient to delay NMJ dener- vation (Gillingwater et al, 2002;Wong et al, 2009), whereas these very low doses of axonally targeted NMNAT1 preserve axotomized NMJs for 6 d and retain this ability even in older mice. Even though Ax-NMNAT1 has substantially higher potency than Wld S , important similarities indicate that its mechanism of protection is the same as the Wld S mechanism.…”

“…Motor nerve terminal preservation decreases with age in homozygous spontaneous mutant and transgenic Wld S mice, and is absent in Wld S mice older than 3 months (Gillingwater et al, 2002). However, extranuclear Wld S can protect NMJs efficiently in aged mice .…”

Ageing and the brain

“…Anti-apoptotic interventions do not prevent axonal degeneration [59]. Dendrites and synapses also seem to contain compartmentalized selfdegenerative programmes, which may be important in synaptic plasticity as well as in ageing and neurodegeneration [60]. Some of these do appear to be related to apoptosis [57].…”