2.3. Defective mitochondrial respiration and ATP production Reductions in cellular respiration and ATP production are well documented in ALS (Fig. 1). In post-mortem spinal cord of sporadic ALS patients the activity of all ETC complexes, complex I, II, III, and IV was found to be reduced [52,53]. In addition, the activity of complexes I and IV were reported to be impaired in skeletal muscle [54–56] while complex I activity and ATP levels were reduced in lymphocytes of sporadic ALS patients [57]. Counterintuitively, in fibroblasts obtained from skin biopsies of sporadic ALS patients the mitochondrial membrane potential (MMP) was increased compared to healthy controls. Possibly the increased MMP observed in fibroblasts reflects an attempt to rescue inefficient ATP synthesis or the metabolic differences between skin fibroblasts and central nervous system (CNS) cells [58]. Indeed, similar to fibroblasts obtained from sporadic patients, MMP was increased in C9orf72 patient fibroblasts [31] but was significantly decreased in iPSC-derived motor neurons reprogrammed from C9orf72 patient fibroblasts [32]. In SOD1 G93A transgenic mice impaired ATP synthesis and
depressed mitochondrial respiration rates in the brain and spinal cord were found well before disease onset and they persist throughout the course of the disease [39,45]. Associated with these changes, decreased complex I + III, II + III and IV activity was observed in the spinal cord of 17-week old symptomatic SOD1 G93A transgenic mice [45] and complex IV activity was reduced in forebrain mitochondria isolated from presymptomatic, symptomatic and end-stage SOD1 G93A transgenic mice [39,45]. Analysis of regional changes in ETC complex activities further showed a selective decrease of complex I and IV activity in the ventral horn of the spinal cord of these mice prior to the onset of disease which further declined as disease progressed [59]. In these cases, the reduced activity of complex IV may relate to impaired association of cytochrome c with the IMM [39,45]. A decrease in complex I activity was also confirmed in ex vivo motor cortex slices from SOD1 G93A mice [60] (Fig. 1). Analysis of mutant SOD1 ALS cell models recaptured the defects
found in SOD1 G93A mice. SOD1 I113T patient fibroblast lines showed reduced oxidative phosphorylation compared to nondisease controls [61]. Stable low level expression of SOD1 G93A or G37R in NSC-34 motor neuron-like cell lines reduced the activity of complex II and IV [62,63] accompanied by reduced MMP [64] while doxycycline-induced expression of SOD1 G93A, A4V, H46R, H80R, D90A, or D123H in the same cell line caused a decrease in complex I, II + III and IV activity which was accompanied by reduced cellular ATP levels [46]. In N2A neuroblastoma cells overexpression of SOD1 G37R resulted in reduced complex I activity which correlated with a drop in MMP and reduced levels of cytosolic ATP, but complex II, III and IV activities were unaffected [63]. In contrast to these studies, Bowling et al. found increased com-
plex I activity in post-mortem frontal cortex of familial ALS patients with an SOD1 A4V mutation [65] and the same group reported an increase in complex I activity in the forebrain of SOD1 G93A transgenic mice [66]. Furthermore, when they assessed mitochondrial respiration solely via complex II, the mitochondrial respiration rate was not different in SOD1 G93A transgenic mice compared to wild type SOD1 transgenic mice [67]. Similarly, others found no evidence for reduced mitochondrial respiration via complex I or II in presymptomatic SOD1 G93A transgenic mice [68]. It is not readily
apparent what accounts for the discrepancy between these studies, but these measurements are particularly sensitive to variations in experimental conditions and differences between the SOD1 G93A mouse strains cannot be excluded. Nevertheless, despite differences between studies and cell mod-
els, the overarching trend is tha