Rescue of Peripheral and CNS Axon Defects in Mice Lacking NMNAT2

Abstract

NMNAT2 is an NADϩ -synthesizing enzyme with an essential axon maintenance role in primary culture neurons. We have generated an Nmnat2 gene trap mouse to examine the role of NMNAT2 in vivo. Homozygotes die perinatally with a severe peripheral nerve/axon defect and truncated axons in the optic nerve and other CNS regions. The cause appears to be limited axon extension, rather than dying-back degeneration of existing axons, which was previously proposed for the NMNAT2-deficient Blad mutant mouse. Neurite outgrowth in both PNS and CNS neuronal cultures consistently stalls at 1-2 mm, similar to the length of truncated axons in the embryos. Crucially, this suggests an essential role for NMNAT2 during axon growth. In addition, we show that the Wallerian degeneration slow protein (Wld S ), a more stable, aberrant NMNAT that can substitute the axon maintenance function of NMNAT2 in primary cultures, can also correct developmental defects associated with NMNAT2 deficiency. This is dose-dependent, with extension of life span to at least 3 months by homozygous levels of Wld S the most obvious manifestation. Finally, we propose that endogenous mechanisms also compensate for otherwise limiting levels of NMNAT2. This could explain our finding that conditional silencing of a single Nmnat2 allele triggers substantial degeneration of established neurites, whereas similar, or greater, reduction of NMNAT2 in constitutively depleted neurons is compatible with normal axon growth and survival. A requirement for NMNAT2 for both axon growth and maintenance suggests that reduced levels could impair axon regeneration as well as axon survival in aging and disease.

Axon Self-Destruction: New Links among SARM1, MAPKs, and NAD+ Metabolism

“…As described above, NMNAT2 is an endogenous axon survival factor that synthesizes NAD+ from NMN and ATP. Genetic knockout of NMNAT2 is embryonic lethal, and the embryos have dramatic defects in axon extension (Gilley et al, 2013, 2015). Remarkably, the embryonic lethality and axon defects in NMNAT2 knockout mice are rescued by loss of SARM1 as NMNAT2/SARM1 double knockout animals are healthy into adulthood (Gilley et al, 2015).…”

Deficiency of Nicotinamide Mononucleotide Adenylyltransferase 3 (Nmnat3) Causes Hemolytic Anemia by Altering the Glycolytic Flow in Mature Erythrocytes

“…Several articles have suggested the implication of Nmnat2 in axonal cell survival and protection (22,23). Nmnat1 and Nmnat2 also have essential roles in axonal growth and survival during embryogenesis as their deficiency in mice results in embryonic lethal (21,24,25).…”

Axon Degeneration: Too Much NMN Is Actually Bad?

“…Based on the NAD + depletion model, NMNAT2 loss is thought to trigger axon degeneration [13]. Indeed, depletion of NMNAT2 causes Wallerian degeneration in neurons that can be rescued by Wld S [13,14]. Consistent with the notion that it is the NMNAT activity of Wld S that is critical for its function, a cytoplasmic variant of NMNAT1 (cytNMNAT1) exhibits protection against Wallerian degeneration that is comparable to Wld S [15].…”

“…Mice lacking NMNAT2 exhibit severe axon growth defects and die perinatally [14], and these defects are rescued either by expressing Wld S or knocking out SARM1 [18]. This suggests that the observed defects in NMNAT2 knockout Axotomy results in the rapid degradation of NMNAT2, resulting in an increase in its substrate, nicotinamide adenine mononucleotide (NMN).…”