In G93A mSOD1 mice [75], degeneration of the anterior

horn neurones was noted early on in the disease process [110]. Ultrastructural studies showed membrane bound vacuoles originating from the degenerating mitochondria, via distension of the outer mitochondrial membrane, expansion of the IMS, preceding disintegration of the IMM [56]. The notion of a causal role of this mitochondrial dysmorphology in the pathogenesis of ALS has arisen, due to the observations that these defects occur at a presymptomatic stage in G37R and G93A mSOD1 mice [56]. Furthermore, at the onset of disease symptoms, the dominant pathological event in the ventral horn is a rapid increase in the number of vacuolated mitochondria, this website correlating with decline in muscle strength and preceding motor neuronal cell death [56,74,111,112]. It is postulated that this death is due to apoptosis, with the relative density of cytochrome c immunoreactivity noticeably reduced in the swollen mitochondria, suggestive of its pro-apoptotic release into the cytosol [56]. However, over-expression of wild-type SOD1 may also lead to vacuolation of mitochondria [113], and as mitochondrial vacuolation is not seen in all mSOD1 mouse models, it is important to consider whether more subtle disruption of mitochondrial morphology occurs. The initial cause of this mitochondrial

dysmorphology is unclear, although mSOD1 has been implicated in the process, with vacuolation of mitochondria correlating with accumulation of mSOD1 in the mitochondrial IMS of transgenic see more mice [113]. Furthermore, mSOD1 Mannose-binding protein-associated serine protease has been found to be present in only mildly swollen mitochondria, suggesting that the translocation of mSOD1 into the IMS may trigger vacuolation

of the mitochondria, possibly via dysfunctional interaction with mitochondrial chaperones, eliciting structural damage [56,114]. A fragmented network of motor neuronal mitochondria in the anterior horn of SALS patients is suggestive of defective fusion, or an increase in the levels of fission [49]. This is supported by investigation of cultured motor neurones derived from G93A mSOD1 transgenic mice; mitochondria were found to have a lower aspect ratio, suggestive of ‘rounding up’ of individual mitochondria [115]. Furthermore, investigation of a mSOD1 expressing NSC-34 cell line revealed fragmentation of the mitochondrial network alongside remodelling of the mitochondrial cristae [12,116]. Recent analysis of mitochondrial morphology in differentiated NSC-34 cells transfected with IMS-targeted mSOD1 revealed a significant decrease in mitochondrial length, indicative of fragmentation of the mitochondrial network in the presence of mSOD1 [109]. Thus, loss of mitochondrial fusion or an increase in mitochondrial fission may be a component of the pathogenic process in ALS.