Studies of naturally occurring mutant mice, wld(s), showing delayed
Wallerian degeneration phenotype, suggest that axonal degeneration is an active process. We previously showed that increased
nicotinamide adenine dinucleotide (
NAD)-synthesizing activity by overexpression of
nicotinamide mononucleotide adenylyltransferase (NMNAT) is the essential component of the Wld(s)
protein, the expression of which is responsible for the delayed
Wallerian degeneration phenotype in wld(s) mice. Indeed, NMNAT overexpression in cultured neurons provides robust protection to neurites, as well. To examine the effect of NMNAT overexpression in vivo and to analyze the mechanism that causes axonal protection, we generated transgenic mice (Tg) overexpressing NMNAT1 (nuclear
isoform), NMNAT3 (mitochondrial
isoform), or the Wld(s)
protein bearing a W258A mutation, which disrupts
NAD-synthesizing activity of the Wld(s)
protein.
Wallerian degeneration delay in NMNAT3-Tg was similar to that in wld(s) mice, whereas axonal protection in NMNAT1-Tg or Wld(s)(W258A)-Tg was not detectable. Detailed analysis of subcellular localization of the overexpressed
proteins revealed that the axonal protection phenotype was correlated with localization of NMNAT enzymatic activity to mitochondrial matrix. Furthermore, we found that isolated mitochondria from mice showing axonal protection expressed unchanged levels of respiratory chain components, but were capable of increased
ATP production. These results suggest that axonal protection by NMNAT expression in neurons is provided by modifying mitochondrial function. Alteration of mitochondrial function may constitute a novel tool for axonal protection, as well as a possible treatment of diseases involving axonopathy.