NMN Deamidase Delays Wallerian Degeneration and Rescues Axonal Defects Caused by NMNAT2 Deficiency In Vivo

Michele Di Stefano,Andrea Loreto,Giuseppe Orsomando,Valerio Mori,Federica Zamporlini,Richard P Hulse,Jamie Webster,Lucy F Donaldson,Martin Gering,Nadia Raffaelli,Michael P Coleman,Jonathan Gilley,Laura Conforti

doi:10.1016/j.cub.2017.01.070

Abstract

Axons require the axonal NAD-synthesizing enzyme NMNAT2 to survive. Injury or genetically induced depletion of NMNAT2 triggers axonal degeneration or defective axon growth. We have previously proposed that axonal NMNAT2 primarily promotes axon survival by maintaining low levels of its substrate NMN rather than generating NAD; however, this is still debated. NMN deamidase, a bacterial enzyme, shares NMN-consuming activity with NMNAT2, but not NAD-synthesizing activity, and it delays axon degeneration in primary neuronal cultures. Here we show that NMN deamidase can also delay axon degeneration in zebrafish larvae and intransgenic mice. Like overexpressed NMNATs, NMN deamidase reduces NMN accumulation in injured mouse sciatic nerves and preserves some axons for up to three weeks, even when expressed at a low level. Remarkably, NMN deamidase also rescues axonal outgrowth and perinatal lethality in a dose-dependent manner in mice lacking NMNAT2. These data further support a pro-degenerative effect of accumulating NMN in axons invivo. The NMN deamidase mouse will be an important tool to further probe the mechanisms underlying Wallerian degeneration and its prevention.

Highlights

Axon degeneration is a widely recognized hallmark of many neurodegenerative disorders and axonopathies, including peripheral neuropathies, Parkinson’s disease, multiple sclerosis, and others [1, 2]
Recent studies revealed a crucial role for the endogenous mammalian nicotinamide mononucleotide adenylyltransferase (NMNAT) isoform NMNAT2 in axon survival [4]
We previously found that the NAMPT inhibitor FK866 delays Wallerian degeneration in zebrafish larvae, likely through inhibition of NMN accumulation and a consequent rise in Ca2+ [11, 13]

Summary

Introduction

Axon degeneration is a widely recognized hallmark of many neurodegenerative disorders and axonopathies, including peripheral neuropathies, Parkinson’s disease, multiple sclerosis, and others [1, 2]. Any of the three natural NMNAT isoforms or the slow Wallerian degeneration protein (WLDS) (an aberrant fusion protein with NMNAT activity [6]) can robustly delay Wallerian degeneration when present at sufficient levels in axons [2]. This is likely achieved by maintaining axonal NMNAT enzymatic activity after loss of endogenous NMNAT2 through increased levels and/or greater relative stability of the introduced proteins [2, 4]. The findings that specific depletion of NMNAT2 in neuronal primary culture is sufficient to initiate WLDS-sensitive degeneration [4] and that mice lacking NMNAT2, which develop severe axonal defects and die at birth, can be rescued by WLDS expression [7] are consistent with this model

Results

Discussion

Conclusion