Cisd2 ensures adequate ER-mitochondrial coupling, thereby critically supporting mitochondrial function in neurons

Loss of CISD2, an iron-sulfur cluster transfer protein, results in type 2 Wolfram syndrome (WFS2), a disorder associated with severe impacts on pancreatic beta cell and neuronal functions. CISD2 has been implicated in Ca2+ signaling but the molecular basis and cellular consequences remain poorly understood. In this work, we demonstrate that Cisd2 intersects with intracellular Ca2+ dynamics at different levels, including as an interactor of IP3Rs and as a protein contributing to ER-mitochondrial tethering. As such, loss of CISD2 in HeLa cells results in reduced ER-mitochondrial Ca2+ transfer without majorly impact cytosolic Ca2+ signaling. In these cells, CISD2 deficiency promotes autophagic flux, yet has minimal impact mitochondrial function. However, studying the impact of CISD2 deficiency in iPSC-derived cortical neurons, relevant for WFS2, revealed a severe loss of glutamate-evoked Ca2+ responses in cytosol and mitochondria and loss of ER-mitochondrial contact. Correlating with the profound changes in cellular Ca2+ handling, mitochondrial function (oxygen consumption rate, ATP production, mitochondrial potential maintenance) was severely declined, while autophagic flux was increased. Overall, these deficiencies further impact the resilience of CISD2-deficient cortical neurons against cell stress as CISD2-KO neurons were highly susceptible to staurosporine, a cell death inducer. Overall, this work is one of the first to decipher the impact of CISD2 on ER-mitochondrial Ca2+ handling in disease-relevant cell models, thereby revealing a unique dependence of neurons on CISD2 for their mitochondrial health and cell stress resilience.