*4.5. Amyotrophic Lateral Sclerosis*

Amyotrophic lateral sclerosis (ALS) is a rare motor neuron disease, strongly associated with mutations in *SOD1*, a ROS scavenging enzyme [325,326]. Characteristic features of mitochondrial dysfunction have been observed across ALS patients, and respiratory chain impairment has been highlighted as a common feature in the muscles of ALS patients, even prior to neuronal deficits being found [327–329]. This finding is consistent across both patient samples and experimental model systems and has highlighted mitochondrial dysfunction as a major pathological feature in ALS [330].

A small proportion of wildtype SOD1 is known to localise to the IMS under physiological conditions in both yeast and mammals [331,332]. Its antioxidant role in detoxifying ROS species produced by the ETC (mainly CI and CIII) is well established [332,333]. Disease associated variants of SOD1, however, have been shown to accumulate not only in the IMS but also within the matrix and the OMM, where it aggregates and interacts with OMM proteins (Table 2) [272,273]. This mislocalisation of SOD1 variants lead to excessive ROS production and subsequent mitochondrial dysfunction and toxic effects on the cells, which can be rescued by selective targeting of wildtype SOD1 to the IMS [271]. Evidence also shows alterations in activity of the respiratory complexes and in mitochondrial calcium buffering capacity associated with disease-causing SOD1 variants [317,334].

A proteomic screen of protein level changes in mitochondria from rat spinal cord of ALS-linked variant SOD1G93A showed vast changes in mitochondrial import and CI related proteins compared to SOD1WT mitochondria (Table 2) [274]. Levels of TOM subunits TOM20, TOM22, and TOM40 were increased in the affected mitochondria although, surprisingly, in vitro import assays highlighted a 30% reduction in protein import levels in these mitochondria compared to wildtype [274].

Furthermore, variants of mitochondrial IMS protein CHCHD10, which is crucial for cristae remodelling, have been linked to progression of ALS as well as frontotemporal dementia [275]. The native version of this protein is imported via the MIA pathway, where disulphide bonds are formed within the CHCHD of the protein [275]. A novel CHCHD10 variant, Q108P, discovered in a patient with rapidly progressing ALS, has been shown to almost completely abolish its import, resulting in reduced mitochondrial respiratory capacity, an effect that is rescued by overexpression of CHCHD4 (Table 2) [335]. Interestingly, the C9orf72 protein, which is often mutated in cases of ALS and frontotemporal dementia, has recently been shown to be an IMM protein vital for the assembly and stabilisation of CI, and its translocation occurs via the MIA pathway [336]. These studies demonstrate the importance of mitochondrial protein import and proper respiratory function in the prevention of motor neuron diseases such as ALS, highlighting import pathways as interesting potential targets for treatment.

#### **5. Repair Pathways**

In order to maintain the integrity and function of the mitochondria, a complex hierarchy of quality control mechanisms exists. This consists of repair mechanisms at the molecular, organelle, and cellular levels via a plethora of complex systems including mitochondrial chaperones and proteases, mitochondrial dynamics and distribution, mitochondrial-derived vesicles (MDVs), mitophagy, and apoptosis [337]. In addition to the emergence of links between mitochondrial import defects and neurodegenerative diseases, there is also evidence implicating stress response pathways in neurodegeneration. This evidence suggests that the pathways may have either a protective or exacerbating role in disease progression in different models. This section will discuss some of the stress response pathways that cells have developed in response to mitochondrial dysfunction for restoration of mitochondrial import function, respiratory capacity, and mitochondrial and cytosolic proteostasis.
