Mitochondria are present as tubular organelles in neuronal projections. Here, we report that mitochondria undergo profound fission in response to nitric oxide (NO) in cortical neurons of primary cultures. Mitochondrial fission by NO occurs long before neurite injury and neuronal cell death. Furthermore, fission is accompanied by ultrastructural damage of mitochondria, autophagy, ATP decline and generation of free radicals. Fission is occasionally asymmetric and can be reversible. Strikingly, mitochondrial fission is also an early event in ischemic stroke in vivo. Mitofusin 1 (Mfn1) or dominant-negative Dynamin related protein 1 (Drp1 K38A ) inhibits mitochondrial fission induced by NO, rotenone and Amyloid-b peptide. Conversely, overexpression of Drp1 or Fis1 elicits fission and increases neuronal loss. Importantly, NO-induced neuronal cell death was mitigated by Mfn1 and Drp1 K38A . Thus, persistent mitochondrial fission may play a causal role in NO-mediated neurotoxicity.

Huntington disease (HD) is an inherited and incurable neurodegenerative disorder caused by an abnormal polyglutamine (polyQ) expansion in huntingtin (HTT). PolyQ length determines disease onset and severity with a longer expansion causing earlier onset. The mechanisms of mutant HTT-mediated neurotoxicity remain unclear; however, mitochondrial dysfunction is a key event in HD pathogenesis1,2. Here we tested whether mutant HTT impairs the mitochondrial fission/fusion balance and thereby causes neuronal injury. We show that mutant HTT triggers mitochondrial fragmentation in neurons and fibroblasts of HD individuals in vitro and HD mice in vivo before the presence of neurological deficits and HTT aggregates. Interestingly, mutant HTT abnormally interacts with the mitochondrial fission GTPase dynamin-related protein 1 (DRP1) in HD mice and individuals which in turn stimulates its enzymatic activity. Importantly, mutant HTT-mediated mitochondrial fragmentation, defects in anterograde and retrograde mitochondrial transport, and neuronal cell death are all rescued by reducing DRP1 GTPase activity with the dominant-negative DRP1K38A mutant. Thus, DRP1 might represent a new therapeutic target to combat neurodegeneration in HD.

Fast axonal transport (FAT) requires consistent energy over long distances to fuel the molecular motors that transport vesicles. We demonstrate that glycolysis provides ATP for the FAT of vesicles. Although inhibiting ATP production from mitochondria did not affect vesicles motility, pharmacological or genetic inhibition of the glycolytic enzyme GAPDH reduced transport in cultured neurons and in Drosophila larvae. GAPDH localizes on vesicles via a huntingtin-dependent mechanism and is transported on fast-moving vesicles within axons. Purified motile vesicles showed GAPDH enzymatic activity and produced ATP. Finally, we show that vesicular GAPDH is necessary and sufficient to provide on-board energy for fast vesicular transport. Although detaching GAPDH from vesicles reduced transport, targeting GAPDH to vesicles was sufficient to promote FAT in GAPDH deficient neurons. This specifically localized glycolytic machinery may supply constant energy, independent of mitochondria, for the processive movement of vesicles over long distances in axons.