Mitochondria are highly dynamic organelles that constantly fuse, divide, and move, and their function is regulated and maintained by their morphologic changes. Mitochondrial disease (MD) comprises a group of disorders involving mitochondrial dysfunction. However, it is not clear whether changes in mitochondrial morphology are related to MD. In this study, we examined mitochondrial morphology in fibroblasts from patients with MD (mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS) and Leigh syndrome). We observed that MD fibroblasts exhibited significant mitochondrial fragmentation by upregulation of Drp1, which is responsible for mitochondrial fission. Interestingly, the inhibition of mitochondrial fragmentation by Drp1 knockdown enhanced cellular toxicity and led to cell death in MD fibroblasts. These results suggest that mitochondrial fission plays a critical role in the attenuation of mitochondrial damage in MD fibroblasts.Biomolecules 2020, 10, 450 2 of 13 respiratory chain complexes [7]. However, in multiple cases, the exact genetic cause of Leigh syndrome remains unknown [8]. Currently, there is no cure or effective treatment for MD, but recent research has shown the potential benefits of some approaches, at least in preclinical in vivo models [9][10][11]. Many researchers assume that both bulk and mitochondrial autophagy play protective roles in MD because the accumulation of damaged mitochondria and other toxic aggregates causes deterioration of the pathophysiology of the cell [12]. Preclinical studies suggest that a potential therapeutic target of disease progression is the mammalian target of rapamycin (mTOR) pathway, a biological route fundamental for regulating cell metabolism and physiology [13]. In mice models of Leigh syndrome, treatment with rapamycin, an mTOR inhibitor, extended lifespan and reduced disease progression and severity [14]. Civiletto et al. provided evidence that rapamycin induces improvements in mitochondrial function and ultrastructure, indicating powerful clearance of dysfunctional organelles via activation of autophagic flux in skeletal muscle.Mitochondria are highly dynamic organelles that continuously fuse, divide, and move, and mitochondrial function is controlled and maintained by these morphologic changes. Mitochondrial fission is specifically mediated by dynamin-related guanosine triphosphatase (GTPase) protein 1 (Drp1); in addition, dynamin-related GTPases mitofusin (Mfn) and optic atrophy 1 (OPA1) are associated with the outer and inner mitochondrial membranes, respectively, and mediate fusion of these membranes [15][16][17][18][19]. The most direct consequence of mitochondrial division and fusion is the change in size of the mitochondria [20][21][22]. Mitochondrial fission via Drp1 has been thought to regulate mitophagy by dividing mitochondria into fragments suitable for autophagosome engulfment [23][24][25] and/or separating damaged subdomains of mitochondria for elimination [26]. Mitochondrial fusion enables efficient mixing of m...