Wld ^s requires Nmnatl enzymatic activity and N16- VCP interactions to suppress Wallerian degeneration

Slow Wallerian degeneration (Wld ^s) encodes a chimeric Ube4b/nicotinamide mononucleotide adenylyl transferase 1 (Nmnat1) fusion protein that potently suppresses Wallerian degeneration, but the mechanistic action of Wld ^s remains controversial. In this study, we characterize Wld ^s-mediated axon protection in vivo using Drosophila melanogaster. We show that Nmnatl can protect severed axons from autodestruction but at levels significantly lower than Wld ^s, and enzymedead versions of Nmnatl and Wld ^s exhibit severely reduced axon-protective function. Interestingly, a 1 6-amino acid N-terminal domain of Wld ^s3p (termed N1 6) accounts for the differences in axon-sparing activity between Wld ^s and Nmnat1, and N16-dependent enhancement of Nmnat1 -protective activity in Wld ^s requires the N1 6- binding protein valosin-containing protein (VCP)/TER94. Thus, Wld ^s-mediated suppression of Wallerian degeneration results from VCP-N16 interactions and Nmnat1 activity converging in vivo. Surprisingly, mouse Nmnat3, a mitochondrial Nmnat enzyme that localizes to the cytoplasm in Drosophila cells, protects severed axons at levels indistinguishable from Wld ^s. Thus, nuclear Nmnat activity does not appear to be essential for Wld ^s-like axon protection.