Exploring new pathways of neurodegeneration in ALS: the role of mitochondria quality control

Neuronal cells are highly dependent on mitochondria, and mitochondrial dysfunction is associated with neurodegenerative diseases. As perturbed mitochondrial function renders neurons extremely sensitive to a wide variety of insults, such as oxidative stress and bioenergetic defects, mitochondrial defects can profoundly affect neuronal fate. Several studies have linked ALS with mitochondrial dysfunction, stemming from observations of mitochondrial abnormalities, both in patients and in cellular and mouse models of familial forms of ALS. Mitochondrial changes have been thoroughly investigated in mutants of superoxide dismutase 1 (SOD1), one of the most common causes of familial ALS, for which excellent cellular and animal models are available, but recently evidence is emerging also in other forms of ALS, both familial and sporadic. Mitochondrial defects in ALS involve many critical physiopathological processes, from defective bioenergetics to abnormal calcium homeostasis, altered morphology and impaired trafficking. In this review, we summarize established evidence of mitochondrial dysfunction in ALS, especially in SOD1 mutant models of familial ALS. The main focus of the review is on defective mitochondrial quality control (MQC) in ALS. MQC operates at multiple levels to clear damaged proteins through proteostasis and to eliminate irreparably damaged organelles through mitophagy. However, since ALS motor neurons progressively accumulate damaged mitochondria, it is plausible that the MQC is ineffective or overwhelmed by excessive workload imposed by the chronic and extensive mitochondrial damage. This article is part of a Special Issue entitled ALS complex pathogenesis.