A rat model of slow Wallerian degeneration (Wld^S) with improved preservation of neuromuscular synapses

Abstract

The slow Wallerian degeneration phenotype, Wld(S), which delays Wallerian degeneration and axon pathology for several weeks, has so far been studied only in mice. A rat model would have several advantages. First, rats model some human disorders better than mice. Second, the larger body size of rats facilitates more complex surgical manipulations. Third, rats provide a greater yield of tissue for primary culture and biochemical investigations. We generated transgenic Wld(S) rats expressing the Ube4b/Nmnat1 chimeric gene in the central and peripheral nervous system. As in Wld(S) mice, their axons survive up to 3 weeks after transection and remain functional for at least 1 week. Protection of axotomized nerve terminals is stronger than in mice, particularly in one line, where 95-100% of neuromuscular junctions remained intact and functional after 5 days. Furthermore, the loss of synaptic phenotype with age was much less in rats than in mice. Thus, the slow Wallerian degeneration phenotype can be transferred to another mammalian species and synapses may be more effectively preserved after axotomy in species with longer axons.

Non-Nuclear WldSDetermines Its Neuroprotective Efficacy for Axons and SynapsesIn Vivo

“…In addition to these insights into the protective mechanism, the increased efficacy will improve assessment of which neurodegenerative disorders involve Wallerian-like degeneration and points to more optimal therapeutic strategies based around Wld S . Wld S protection has been shown to be strongly dosedependent by the weaker phenotype of C57BL/Wld S heterozygotes in both injury and disease (Perry et al, 1992;Mack et al, 2001;Samsam et al, 2003;Mi et al, 2005) and by strong correlation between expression level and phenotype strength in Wld S transgenic lines (Mack et al, 2001;Adalbert et al, 2005). Surprisingly, we found that reduced nuclear targeting of Wld S without altering total expression level strengthens the protective phenotype, rather than weakening it as a nuclear action would predict.…”

“…Similar redistribution with reductions in nuclear targeting was seen in DRG neurons (supplemental Fig. 1 We and others have previously shown that delay of Wallerian degeneration and related axon pathologies by Wld S is strongly dose dependent (Perry et al, 1990;Mack et al, 2001;Samsam et al, 2003;Adalbert et al, 2005). In particular, the strength of axon protection in transgenic mice correlates closely with Wld S protein level (Mack et al, 2001) (and unpublished observations) (note: the lower expressing lines are no longer available for comparison).…”

“…After testing in vitro we coupled the ⌬NLS R213A,R215A Wld S cDNA to a ␤-actin promoter, a system that consistently confers a Wld S phenotype in mice and rats using native Wld S (Mack et al, 2001;Adalbert et al, 2005). Lines were established from seven of the eight founders (lines 1-6, 8) by breeding to YFP-H mice (Feng et al, 2000) for convenient assay of axon degeneration (Beirowski et al, 2004).…”

Differential protection of neuromuscular sensory and motor axons and their endings in Wld mutant mice

“…However, not all models of neuro-degeneration are protected by coexpression of Wld S (Vande Velde et al, 2004;Fischer et al, 2005;Kariya et al, 2009). The reasons for failure of neuroprotection in these instance are unclear but since motor nerve terminals in Wld S mice and their transgenic equivalents lose their capacity to inhibit degeneration of motor nerve terminals as the mice age (Gillingwater et al, 2002;Adalbert et al,2005), mouse models with a late onset of disease are less likely to benefit or be protected from co-expression of the protective gene.…”

“…However, not all models of neuro-degeneration are protected by coexpression of Wld S (Vande Velde et al, 2004;Fischer et al, 2005;Kariya et al, 2009). The reasons for failure of neuroprotection in these instance are unclear but since motor nerve terminals in Wld S mice and their transgenic equivalents lose their capacity to inhibit degeneration of motor nerve terminals as the mice age (Gillingwater et al, 2002;Adalbert et al,2005), mouse models with a late onset of disease are less likely to benefit or be protected from co-expression of the protective gene.Interestingly, in the present study, protection of sen-sory axons and their endings showed a much weaker dependence on mouse age. The age-dependence of the protective phenotype is also much less in transgenic mice in which Wld S protein (or Nmnat-1 activity) was targeted to axons, even at very low expression levels Babetto et al, 2010).…”