Impaired mitochondrial dynamics Mitochondria are dynamic organelles that undergo fusion and
fission events and directed transport. Several lines of research indicate that impaired mitochondrial dynamics may contribute to the aetiology of ALS.
2.7.1. Aberrant mitochondrial fission and fusion Mitochondria form a dynamic network that evolves according
to the energetic requirements of the cell. An individual mitochondrion can fuse to or fission from the mitochondrial network in response to various cues [144]. Fusion allows mitochondria to share mitochondrial metabolites, DNA and proteins, and allows for dissipation of small changes in MMP. Fission facilitates mitochondrial motility and allows the isolation of damaged parts of the network prior to disposal by mitophagy. In healthy cells, mitochondrial network morphology is governed by the interplay between Dynamin-related protein 1 (Drp1) and Fission 1 (Fis1) that promote fission, and Mfn1, Mfn2 and Optic atrophy 1 (Opa1) that promote fusion [reviewed in Ref. [145]] (Fig. 4). Evidence points towards imbalances in these two opposing pathways in ALS. In SOD1 G93A expressing SH-SY5Y and NSC-34 cell models, a
decrease in Opa1 and increase in Drp1 levels resulting in a fragmented mitochondrial network were reported [146]. In vivo, the levels of Mfn1 and Opa1 progressively decreased while the levels of activated Drp1 phosphorylated at Ser616 and Fis1 remained stable in the spinal cord of SOD1 G93A transgenic mice [147] (Fig. 4). In contrast, in SOD1 G93A mouse muscle fibres there was no reported change in Mfn1, Mfn2 or Drp1 levels [148]. Overexpression of wild type TDP-43 and to a greater extend ALS
mutant TDP-43 Q331K and M337V resulted in reduced mitochondrial length in primary motor neurons which was sensitive to Mfn2 levels [23]. Transgenic mice expressing wild type TDP-43 exhibited increased levels of Fis1 and activated phospho-Ser616 Drp1 and a reduction in Mfn1, which correlated with aberrant mitochondrial morphology and clustering [149]. In contrast, the same group reported no changes in the levels of phospho-Ser616 Drp1, Fis1 or Mfn1 in mutant TDP-43 M337V transgenic mice, despite a similar mitochondrial phenotype [150]. Possibly different pathways lead to fragmentation of the mitochondrial network in wild type and mutant TDP-43 transgenic mice, or, alternatively, external factors linked to disease such as changes in apoptotic factors result in mitochondrial fragmentation. Nevertheless, analysis of TDP-43 A382T patient fibroblasts showed that Fis1 levels were significantly increased compared to control cells [31] (Fig. 4). Counterintuitively, mitochondrial network fragmentation in C9ALS/FTD patient fibroblasts was associated with an increase in Mfn1 levels [31]. Possibly Mfn1 expression is increased to compensate for dysregulation of other fusion and fission factors. ER–mitochondria contact sites have been shown to regu-
late mitochondrial morphology. Mitochondrial fission occurs at ER–mitochondria contact sites where ER tubules wrap around and constrict mitochondria to aid division [151]. As discussed above reduced ER–mitochondria contacts are a common obser-
vation in various ALS models. Consistent with reduced fission, decreased ER–mitochondria contacts in motor neurons of SIGMAR1 knockout correlated with elongated mitochondria compared to controls [124] whereas, in VAPB P56S overexpressing cells increased ER–mitochondria contacts correlated with fragmented mitochondria [118] (Fig. 4). However, in SOD1, TDP-43 and FUS ALS models reduced ER–mitochondria contacts appear to correlate with fragmented mitochondria [23,26,51,115,152,153]. Therefore, there is no straightforward correlation between ALS-associated disruption of ER–mitochondria contacts and changes in mitochondrial network morphology. The relevance of exacerbated mitochondrial fission in the aeti-
ology of ALS is not clear. Perhaps, smaller mitochondria, which tend to be less energetically favourable and more prone to accumulating ROS induced damage if they cannot fuse back to the network [154], may exacerbate mitochondrial damage. Thus, the fragmented mitochondrial network observed may correlate directly with the reduced MMP, ATP levels and increased ROS reported in ALS. Alternatively, mitochondrial fragmentation may merely reflect mitophagy, a physiological quality control response to mitochondrial damage (see below). Indeed, it is well established that one of the first steps of mitophagy is to isolate damaged mitochondria from the mitochondrial network [reviewed in Ref. [155]]. However, as disease progresses and mitochondrial damage accumulates, mitophagy may become unable to cope and as a consequence mitochondrial fragmentation could become selfpropagating. In any case, evidence suggests that mitochondrial fragmentation participates in ALS. Inhibition of mitochondrial fission by overexpression of dominant negative Drp1 K38A rescued SOD1 G93A induced mitochondrial fragmentation and trafficking defects and increased motor neuron viability [156]. Similarly, promotion of mitochondrial fusion by co-expressing Mfn2 in wild type or TDP-43 M333V expressing primary motor neurons rescued mitochondrial fragmentation [23]. Whether rescuing mitochondrial network morphology is sufficient to improve neuron survival
in vivo remains unclear.