NAD+ and axon degeneration revisited: Nmnat1 cannot substitute for WldS to delay Wallerian degeneration

Abstract

The slow Wallerian degeneration protein (Wld S ), a fusion protein incorporating full-length nicotinamide mononucleotide adenylyltransferase 1 (Nmnat1), delays axon degeneration caused by injury, toxins and genetic mutation. Nmnat1 overexpression is reported to protect axons in vitro, but its effect in vivo and its potency remain unclear. We generated Nmnat1-overexpressing transgenic mice whose Nmnat activities closely match that of Wld S mice. Nmnat1 overexpression in five lines of transgenic mice failed to delay Wallerian degeneration in transected sciatic nerves in contrast to Wld S mice where nearly all axons were protected. Transected neurites in Nmnat1 transgenic dorsal root ganglion explant cultures also degenerated rapidly. The delay in vincristine-induced neurite degeneration following lentiviral overexpression of Nmnat1 was significantly less potent than for Wld S , and lentiviral overexpressed enzyme-dead Wld S still displayed residual neurite protection. Thus, Nmnat1 is significantly weaker than Wld S at protecting axons against traumatic or toxic injury in vitro, and has no detectable effect in vivo. The full protective effect of Wld S requires more N-terminal sequences of the protein.

Nicotinamide mononucleotide adenylyl transferase 1 protects against acute neurodegeneration in developing CNS by inhibiting excitotoxic-necrotic cell death

“…The Wlds mutant mouse that overexpresses a slow Wallerian degeneration protein (WldS), a fusion protein composed of the N-terminal 70 amino acids of ubiquitination factor E4 linked to full-length Nmnat1 exhibits slow axonal Wallerian degeneration in response to nerve injury in the peripheral nervous system (PNS). This WldS fusion protein protects axons from degeneration initiated by a variety of insults both in vitro and in vivo (15)(16)(17). Further, it has been shown in multiple axonal injury paradigms that expression of Nmnat1 robustly protects axons in vitro and in vivo in the PNS (18)(19)(20).…”

NAD synthase NMNAT acts as a chaperone to protect against neurodegeneration

“…Conflicting results exist in mammalian systems as to whether Nmnat1 exerts protective effects [13][14][15] and whether NAD is required 4,14,15 . Drosophila contains only one NMNAT gene, whose overexpression delays axonal degeneration 2 .…”

WldS, Nmnats and axon degeneration-progress in the past two decades

“…Initially, there was some controversy regarding whether individual fragments of the Wld S gene (UFD2 or Nmnat1), or the entire chimeric gene, are responsible for the phenotypes (Araki et al, 2004;Wang et al, 2005;Zhai et al, 2006;Conforti et al, 2007). In cultured neurons, we and others found that while inhibiting the ubiquitin proteosome system activity can slow down Wallerian degeneration (Zhai et al, 2003;MacInnis and Campenot, 2005), over-expressing Nmnat1 alone could mimic the protective effects of Wld S (Araki et al, 2004;Wang et al, 2005;Sasaki et al, 2006).…”

“…However, a study reported that over-expression of Nmnat1 in transgenic mice model failed to protect Wallerian degeneration of sciatic nerves in vivo (Conforti et al, 2007). The exact mechanisms for these different results are unclear.…”

“…For example, while endogenous Nmnat1 is a nuclear protein (Emanuelli et al, 2003), some of the over-expressed Wld S /Nmnat1 protein can be detected in the axons of cultured neurons Zhai et al, 2006). Perhaps the Nmnat1 expressed in transgenic mice generated by Conforti (Conforti et al, 2007) did not reach a high level and still primarily localized in the nucleus; therefore, insufficient amounts of Nmnat1 were available in the axons for the protective effects. In addition, although many reports suggest that Wld S is localized to the nucleus of neurons in the Wld S mice (Coleman, 2005), all of these studies relied on an antibody prepared against an 18-amino acid peptide in Wld S (between UFD2 and Nmnat1) (Conforti et al, 2000;Fang et al, 2005).…”