CISD2 ensures adequate ER-mitochondrial coupling, critically supporting mitochondrial function in neurons

Loss of Cisd2, an iron-sulfur cluster transfer protein, results in type 2 Wolfram syndrome (WS2), a disorder associated with severe impacts on pancreatic β cell and neuronal functions. Cisd2 has been implicated in regulating intracellular Ca²⁺ signaling. However, the molecular basis and cellular consequences remain poorly understood. In this work, we demonstrate that Cisd2 intersects with intracellular Ca²⁺ dynamics at different levels, by interacting with the inositol-1,4,5-trisphosphate receptors and as a regulator of ER-mitochondria tethering. As such, loss of Cisd2 in HeLa cells results in reduced ER-mitochondrial Ca²⁺ transfer while only modestly impacting cytosolic Ca²⁺ signaling. In HeLa cells, Cisd2 deficiency promotes autophagic flux, yet has minimal impact on mitochondrial function. However, studying the impact of Cisd2 deficiency in human induced pluripotent stem cell -derived cortical neurons revealed a severe loss of glutamate-evoked Ca²⁺ responses in cytosol and associated uptake in mitochondria due to loss of ER-mitochondria contact sites. Correlating with the profound changes in cellular Ca²⁺ handling, mitochondrial function (oxygen consumption rate, ATP production, mitochondrial potential maintenance) declined severely, while autophagic flux was increased. Overall, these deficiencies further impact the resilience of Cisd2-deficient cortical neurons to cell stress as Cisd2-KO neurons were highly sensitive to staurosporine, an inducer of apoptosis. Overall, this work is one of the first to decipher the impact of Cisd2 on ER-mitochondria Ca²⁺ handling in a WS2 disease-relevant cell models, thereby revealing a unique dependence of neurons on Cisd2 for their mitochondrial health and cell stress resilience.