ABSTRACT
Axon degeneration is an early event in many neurodegenerative disorders. In some, the mechanism is related to injury-induced Wallerian degeneration, a proactive death program that can be strongly delayed by the neuroprotective slow Wallerian degeneration protein (Wld(S)) protein. Thus, it is important to understand the Wallerian degeneration mechanism and how Wld(S) blocks it. Wld(S) location is influenced by binding to valosin-containing protein (VCP), an essential protein for many cellular processes including membrane fusion and endoplasmic reticulum-associated degradation. In mice, the N-terminal 16 amino acids (N16), which mediate VCP binding, are essential for Wld(S) to protect axons, a role which another VCP binding sequence can substitute. In Drosophila, the Wld(S) phenotype is weakened by a similar N-terminal truncation and by knocking down the VCP homologue ter94. Neither null nor floxed VCP mice are viable so it is difficult to confirm the requirement for VCP binding in mammals in vivo. However, the hypothesis can be tested further by introducing a Wld(S) missense mutation, altering its affinity for VCP but minimizing the risk of disturbing other aspects of its structure or function. We introduced the R10A mutation, which weakens VCP binding in vitro, and expressed it in transgenic mice. R10AWld(S) fails to co-immunoprecipitate VCP from mouse brain, and only occasionally and faintly accumulates in nuclear foci for which VCP binding is necessary but not sufficient. Surprisingly however, axon protection remains robust and indistinguishable from that in spontaneous Wld(S) mice. We suggest that either N16 has an additional, VCP-independent function in mammals, or that the phenotype requires only weak VCP binding which may be driven forwards in vivo by the high VCP concentration.
