Impairment of ER-mitochondrial coupling provides neuroprotection in a model of oxytosis

The crosstalk between the endoplasmic reticulum (ER) and mitochondria facilitates calcium transfer between these organelles, thereby maintaining the driving force for calcium into the mitochondrial matrix to modulate mitochondrial respiration. Glucose-regulated protein 75 (GRP75/mortalin) physically links ER and mitochondria through facilitating the interaction between ER-bound inositol-1,4,5-triphosphate receptors (IP3R) and voltage-dependent anion channel 1 (VDAC1) on the outer mitochondrial membrane.
In our study, we investigated the role of ER-mitochondrial coupling in a model of glutamate toxicity in neuronal HT22 cells that is characterized by cell death and mitochondrial dysfunction. We confirm that GRP75 determines ER-mitochondrial contact formation as shown by an in situ proximity ligation assay. Using siRNA-mediated knockdown, CRISPR/Cas9-mediated knockout and pharmacological inhibition of GRP75, we show that relieving ER-mitochondrial crosstalk preserves mitochondrial integrity and thereby prevents cell death. In response to glutamate, we observe preservation of mitochondrial morphology, attenuation of mitochondrial ROS formation and reduction of both [Ca2+]c and [Ca2+]m levels. Thus, we provide for the first time evidence that disrupting ER-mitochondrial coupling by silencing GRP75 conferred protection against oxidative stress in neuronal HT22 cells by a mechanism involving the regulation of calcium homeostasis and redox balance.