Drosophila NMNAT Maintains Neural Integrity Independent of Its NAD Synthesis Activity

Abstract

Wallerian degeneration refers to a loss of the distal part of an axon after nerve injury. Wallerian degeneration slow (Wlds) mice overexpress a chimeric protein containing the NAD synthase NMNAT (nicotinamide mononucleotide adenylyltransferase 1) and exhibit a delay in axonal degeneration. Currently, conflicting evidence raises questions as to whether NMNAT is the protecting factor and whether its enzymatic activity is required for such a possible function. Importantly, the link between nmnat and axon degeneration is at present solely based on overexpression studies of enzymatically active protein. Here we use the visual system of Drosophila as a model system to address these issues. We have isolated the first nmnat mutations in a multicellular organism in a forward genetic screen for synapse malfunction in Drosophila. Loss of nmnat causes a rapid and severe neurodegeneration that can be attenuated by blocking neuronal activity. Furthermore, in vivo neuronal expression of mutated nmnat shows that enzymatically inactive NMNAT protein retains strong neuroprotective effects and rescues the degeneration phenotype caused by loss of nmnat. Our data indicate an NAD-independent requirement of NMNAT for maintaining neuronal integrity that can be exploited to protect neurons from neuronal activity-induced degeneration by overexpression of the protein.

NAD synthase NMNAT acts as a chaperone to protect against neurodegeneration

“…Drosophila contains only one NMNAT gene, whose overexpression delays axonal degeneration 2 . This study and our finding that NMNAT functions as a maintenance factor to protect against activity-induced neurodegeneration 1 suggest that NMNAT alone can protect against multiple neurodegenerative insults. Our recent finding that enzymatically inactive NMNAT retains neuroprotective capabilities also exposed a hitherto unknown molecular function 1 .…”

“…Overexpression of wild-type NMNAT or enzyme-inactive NMNAT (NMNAT-WR) 1 suppresses the degenerative phenotypes induced by overexpression of Drosophila ataxin-1 (dAtx-1). It also offers moderate protection against the severe phenotypes caused by overexpression of human ataxin-1 with an expanded (82) poly-glutamine tract (hAtx-1[82Q]) 16 (Fig.…”

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

“…Such protective effects could be mimicked by exogenously provided NAD or its biosynthetic precursors such as nicotinamide or nicotinamide riboside (Araki et al, 2004;Wang et al, 2005;Sasaki et al, 2006), suggesting that Wld S /Nmnat1 may act, at least partially, through NAD biosynthesis. Similarly, independent studies also showed that Nmnat1 could protect axon degeneration in Drosophila-based in vivo models Zhai et al, 2006).…”

“…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).…”

“…It is possible that the differences in both the expression levels and protein localizations of the over-expressed Nmnat1 vs. Wld S contribute to these discrepancies due to the nature of these gain-of-function studies. 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.…”