Tau inhibits mitochondrial calcium efflux and makes neurons vulnerable to calcium-induced cell death

Aggregation or phosphorylation of the microtubule-associated protein tau is the pathological hallmark in a number of diseases termed tauopathies, which include the most common neurodegenerative disorder, Alzheimer’s disease; or frontotemporal dementia, linked to mutations in the gene MAPT encoding tau. Although misfolded tau has strong familial and histopathological (as in intracellular tangles) association with neurodegenerative disorders, the cellular mechanism of tau-induced pathology remains to be controversial. Here we studied the effect of tau on the cytosolic and mitochondrial calcium homeostasis using primary cortical cultures treated with the protein and iPSC-derived neurons bearing the 10 + 16 MAPT mutation linked to frontotemporal dementia. We found that incubation of the primary cortical co-cultures of neurons and astrocytes with tau induced spontaneous Ca²⁺ oscillations in the neurons, which were also observed in iPSC-neurons with the 10 + 16 MAPT mutation. Importantly, tau inhibited mitochondrial calcium efflux via the mitochondrial Na⁺/Ca²⁺ exchanger (NCLX) in both neurons and astrocytes. This inhibition led to mitochondrial depolarisation in response to physiological and pathological calcium stimuli and made these cells vulnerable to calcium-induced caspase 3 activation and cell death. Thus, inhibition of the mitochondrial NCLX in neurons with misfolded or mutated tau can be involved in the mechanism of neurodegeneration.