Pathophysiology of mitochondrial spinocerebellar ataxias in iPSC-derived neuronal cells
Riikka Äänismaa1, Henriikka Vekuri1,, Simo Ojanen1, Meeri Mäkinen2, Tuula Manninen1, Susanna Narkilahti2, Anu Suomalainen1
1Molecular Neurology, Research Programs Unit, University of Helsinki, Helsinki, Finland; 2Neuro Group, BioMediTech, The Faculty of Medicine and Life Sciences, University of Tampere, Tampere, Finland
Mitochondrial dysfunction is a common cause of inherited spinocerebellar ataxias combined with sensory neuropathy and epilepsy. The most common examples of this disease group are autosomal recessively inherited infantile-onset spinocerebellar ataxia (IOSCA) and mitochondrial recessive ataxia syndrome (MIRAS), caused by defects of mtDNA replication, i.e. mutations in the helicase Twinkle and DNA polymerase gamma (POLG). Both IOSCA and MIRAS are characterized by CNS-specific mtDNA depletion and liver symptoms. All IOSCA patients and a subgroup of MIRAS patients develop severe, treatment-resistant epilepsy in adolescence, the underlying mechanism of which remains unknown.
We engineered induced pluripotent stem cell (iPSC) lines from IOSCA and MIRAS patients and from control individuals and differentiated them to neuronal cells; the differentiation capacity of all iPSC lines proved to be similar. We investigated their mitochondrial and molecular biological properties with live-imaging, qRT-PCR and immunostainings. The electrophysiologal properties of neuronal networks were studied with microelectrode array (MEA) platform, both at basal level over 4-5-week maturation period and upon pharmacological modulation.
We report here that reactive oxygen radical signaling is significantly modified in both IOSCA and MIRAS neurons in immature stage, accompanied by a progressive depletion of the respiratory chain complex I. A redox-active antioxidant n-acetyl-cysteine rescued the phenotype. The depletion of complex I coincided with abnormal electrophysiological activity in the patient neuronal networks with elevated spontaneous network activity, mimicking epileptic activity. We were able to reversibly rescue and stimulate the activity by metabolic and chemical interventions, indicating potential mechanisms underlying the epileptic activity.
Our data indicate that mitochondrial epilepsy, often manifesting as status epilepticus, can be modeled and tested in iPSC-differentiated neuronal cells derived from the human patients. Furthermore, our data support causal connection between oxygen radical signaling and complex I depletion in maturing neurons.