Dysfunctional ER-mitochondrion Crosstalk Leads to Subsequent ER Stress-induced Neuronal Apoptosis and Neurological Deficits in an Experimental Rodent Model of Severe Head Injury

Introduction: Previous studies have shown proper interorganelle communication between the endoplasmic reticulum (ER) and mitochondria (ER-mitochondrion crosstalk) is crucial for cellular homeostasis. Mitochondria-associated membranes (MAMs) provide an excellent platform and play an essential role in different signaling pathways to maintain cellular viability. However, the time course and potential pathological effects of this ER-mitochondrion physical and functional tethering in traumatic brain injury (TBI) remain unknown. We tested the hypothesis that dysfunctional ER-mitochondrion crosstalk at the acute phase of injury results in ER stress-induced neuronal apoptosis and neurological deficits in a mouse model of TBI. Methods: : Male C57BL/6 mice were subjected to severe TBI (sTBI) using a controlled cortical impact (CCI) device. Transmission electron microscopy, western blot, immunofluorescence staining, terminal deoxynucleotidyl transferase (TdT)-mediated dUTP nick end labeling (TUNEL), and MitoSOX assays were used to analyze posttraumatic ER-mitochondrion physical contact, Ca 2+ transfer from the ER to mitochondria, mitochondrial reactive oxygen species (ROS) production, the unfolded protein response (UPR), the neuroinflammatory response and ER stress-mediated apoptosis in the perilesional cortex. The brain water content (BWC) and Evans blue dye extravasation were used to assess the blood-brain barrier (BBB) integrity, and a modified neurological severity score (mNSS) to evaluate neurological function in mice. Results: : We showed sTBI led to significant MAM reorganization in the mouse cerebral cortex during the first 24 hr after injury. Enhanced ER-mitochondrion crosstalk peaked at 6 hr post injury and was significantly correlated with increased ER-mitochondrion Ca 2+ transfer, mitochondrial ROS overproduction, elevated ER stress and UPR levels, and augmented levels of proinflammatory cytokines. In vivo experimental downregulation of PACS2, a protein essential for ER-mitochondrion tethering, restored mitochondrial Ca 2+ homeostasis and alleviated mitochondrial oxidative stress by downregulating the IP 3 R 1 -GRP75-VDAC1 axis, inhibited ER stress and suppressed the inflammatory response through the PERK/eIF2α/ATF4/CHOP signaling pathway, blocked Caspase 12-dependent ER stress-mediated apoptosis, and reduced BBB permeability, resulting in significantly improved neurological function in mice subjected to sTBI. Conclusions: : These results indicate that dysfunctional ER-mitochondrion crosstalk at the acute stage of injury might be primarily involved in the neuronal apoptosis and neurological deficits following sTBI, and specific modulation of ER-mitochondrion crosstalk might be a novel promising therapeutic strategy for sTBI.