Selective mitochondrial depletion, apoptosis resistance, and increased mitophagy in human Charcot-Marie-Tooth 2A motor neurons.

Charcot-Marie-Tooth 2A (CMT2A) is an inherited peripheral neuropathy caused by mutations in MFN2, which encodes a mitochondrial membrane protein involved in mitochondrial network homeostasis. Because MFN2 is expressed ubiquitously, the reason for selective motor neuron (MN) involvement in CMT2A is unclear. To address this question, we generated MNs from induced pluripotent stem cells (iPSCs) obtained from the patients with CMT2A as an in vitro disease model. CMT2A iPSC-derived MNs (CMT2A-MNs) exhibited a global reduction in mitochondrial content and altered mitochondrial positioning without significant differences in survival and axon elongation. RNA sequencing profiles and protein studies of key components of the apoptotic executioner program (i.e. p53, BAX, caspase 8, cleaved caspase 3, and the anti-apoptotic marker Bcl2) demonstrated that CMT2A-MNs are more resistant to apoptosis than wild-type MNs. Exploring the balance between mitochondrial biogenesis and the regulation of autophagy-lysosome transcription, we observed an increased autophagic flux in CMT2A-MNs that was associated with increased expression of PINK1, PARK2, BNIP3, and a splice variant of BECN1 that was recently demonstrated to be a trigger for mitochondrial autophagic removal. Taken together, these data suggest that the striking reduction in mitochondria in MNs expressing mutant MFN2 is not the result of impaired biogenesis, but more likely the consequence of enhanced mitophagy. Thus, these pathways represent possible novel molecular therapeutic targets for the development of an effective cure for this disease.

MFN2-related neuropathies: Clinical features, molecular pathogenesis and therapeutic perspectives.

Mitofusin 2 (MFN2) is a GTPase dynamin-like protein of the outer mitochondrial membrane, encoded in the nuclear genome by the MFN2 gene located on the short (p) arm of chromosome 1. MFN2 protein is involved in several intracellular pathways, but is mainly involved in a network that has an essential role in several mitochondrial functions, including fusion, axonal transport, interorganellar communication and mitophagy. Mutations in the gene encoding MFN2 are associated with Charcot-Marie-Tooth disease type 2A (CMT2A), a neurological disorder characterized by a wide clinical phenotype that involves the central and peripheral nervous system. Here, we present the clinical, genetic and neuropathological features of human diseases associated with MFN2 mutations. We also report proposed pathogenic mechanisms through which MFN2 mutations likely contribute to the development of neurodegeneration. MFN2-related disorders may occur more frequently than previously considered, and they may represent a paradigm for the study of the defective mitochondrial dynamics that seem to play a significant role in the molecular and cellular pathogenesis of common neurodegenerative diseases; thus they may also lead to the identification of related therapeutic targets.

iPSC-Based Models to Unravel Key Pathogenetic Processes Underlying Motor Neuron Disease Development.

Motor neuron diseases (MNDs) are neuromuscular disorders affecting rather exclusively upper motor neurons (UMNs) and/or lower motor neurons (LMNs). The clinical phenotype is characterized by muscular weakness and atrophy leading to paralysis and almost invariably death due to respiratory failure. Adult MNDs include sporadic and familial amyotrophic lateral sclerosis (sALS-fALS), while the most common infantile MND is represented by spinal muscular atrophy (SMA). No effective treatment is ccurrently available for MNDs, as for the vast majority of neurodegenerative disorders, and cures are limited to supportive care and symptom relief. The lack of a deep understanding of MND pathogenesis accounts for the difficulties in finding a cure, together with the scarcity of reliable in vitro models. Recent progresses in stem cell field, in particular in the generation of induced Pluripotent Stem Cells (iPSCs) has made possible for the first time obtaining substantial amounts of human cells to recapitulate in vitro some of the key pathogenetic processes underlying MNDs. In the present review, recently published studies involving the use of iPSCs to unravel aspects of ALS and SMA pathogenesis are discussed with an overview of their implications in the process of finding a cure for these still orphan disorders.