Mitochondrial impairment and synaptic dysfunction are associated with neurological defects in iPSCs-derived cortical neurons of MERRF patients

Abstract

Background Myoclonic epilepsy with ragged-red fibers (MERRF) syndrome is a rare inherited mitochondrial disease mainly caused by the m.8344A > G mutation in mitochondrial tRNALys gene, and usually manifested as complex neurological disorders and muscle weakness. Currently, the pathogenic mechanism of this disease has not yet been resolved, and there is no effective therapy for MERRF syndrome. In this study, MERRF patients-derived iPSCs were used to model patient-specific neurons for investigation of the pathogenic mechanism of neurological disorders in mitochondrial disease. Methods MERRF patient-derived iPSCs were differentiated into excitatory glutamatergic neurons to unravel the effects of the m.8344A > G mutation on mitochondrial bioenergetic function, neural-lineage differentiation and neuronal function. By the well-established differentiation protocol and electrophysiological activity assay platform, we examined the pathophysiological behaviors in cortical neurons of MERRF patients. Results We have successfully established the iPSCs-derived neural progenitor cells and cortical-like neurons of patients with MERRF syndrome that retained the heteroplasmy of the m.8344A > G mutation from the patients’ skin fibroblasts and exhibited the phenotype of the mitochondrial disease. MERRF neural cells harboring the m.8344A > G mutation exhibited impaired mitochondrial bioenergetic function, elevated ROS levels and imbalanced expression of antioxidant enzymes. Our findings indicate that neural immaturity and synaptic protein loss led to the impairment of neuronal activity and plasticity in MERRF neurons harboring the m.8344A > G mutation. By electrophysiological recordings, we monitored the in vivo neuronal behaviors of MERRF neurons and found that neurons harboring a high level of the m.8344A > G mutation exhibited impairment of the spontaneous and evoked potential-stimulated neuronal activities. Conclusions We demonstrated for the first time the link of mitochondrial impairment and synaptic dysfunction to neurological defects through impeding synaptic plasticity in excitatory neurons derived from iPSCs of MERRF patients harboring the m.8344A > G mutation. This study has provided new insight into the pathogenic mechanism of the tRNALys gene mutation of mtDNA, which is useful for the development of a patient-specific iPSCs platform for disease modeling and screening of new drugs to treat patients with MERRF syndrome.

Primary mitochondrial diseases: The intertwined pathophysiology of bioenergetic dysregulation, oxidative stress and neuroinflammation

“…The latter findings are in line with the study by Sharma et al., who showed that the increased NADH/NAD + ratio was the major determinant of the plasma metabolic signature of patients with MELAS and non‐MELAS m.3243A>G mutation variant 46 . It is noteworthy that the intensity of oxidative imbalance can be associated with the level of mutation load among MELAS fibroblasts, MELAS‐induced neurons, induced pluripotent stem cells (iPSCs)‐derived MERRF‐induced neural stem cells (iNSCs) and MERRF iPSC‐derived cortical neurons as evidenced by other studies 50,52,53 . In contrast, NARP patients had decreased levels of ROS and extremely low levels of catalase (CAT) protein; however, mitochondrial superoxide level was increased together with SOD2, GPX1 and GPX2 enzymes 54 .…”

“…46 It is noteworthy that the intensity of oxidative imbalance can be associated with the level of mutation load among MELAS fibroblasts, MELAS-induced neurons, induced pluripotent stem cells (iPSCs)-derived MERRF-induced neural stem cells (iNSCs) and MERRF iPSC-derived cortical neurons as evidenced by other studies. 50,52,53 In contrast, NARP patients had decreased levels of ROS and extremely low levels of catalase (CAT) protein; however, mitochondrial superoxide level was increased together with SOD2, GPX1 and GPX2 enzymes. 54 Another study involving cybrid fibroblasts derived from NARP patients with m.9032T>C mutation, and LHON-like patients m.9029A>G defect also reported increased ROS production together with upregulated mRNA level of mitochondrial SOD.…”

Clinical Approaches for Mitochondrial Diseases

“…Myoclonic epilepsy with ragged red fiber (MERRF) is a multi-system disease characterized by progressive myoclonus and seizures [96]. One of the most common mutation among the patients with different types of mtDNA mutations is m.A8344G, which affects the mitochondrial tRNA lysine [97][98][99][100][101]. Another prominent mutation in the same gene is m.G8363A, along with the m.A3243G, m.G3255A, and m.T3291C mutations, which have also been reported to cause MERRF with a defects in tRNA leucine [57][58][59]99,102,103].…”

“…Another prominent mutation in the same gene is m.G8363A, along with the m.A3243G, m.G3255A, and m.T3291C mutations, which have also been reported to cause MERRF with a defects in tRNA leucine [57][58][59]99,102,103]. Mutations in tRNA-coding genes induce the global impairment of mtDNA-encoded proteins rather than affecting certain complexes or pathways [101,104,105]. This can result in the overall dysfunction of mitochondria.…”