mtHsp70 overexpression enhances neuroprotection through ATF5-dependent UPRmt signaling after traumatic brain injury

Traumatic brain injury (TBI) induces secondary neuronal damage, in which mitochondrial dysfunction plays a central role. Mitochondrial heat shock protein 70 (mtHsp70) is a key mitochondrial chaperone involved in protein folding and proteostasis, yet its role in TBI pathology remains unclear. In the present study, we investigated the neuroprotective function of mtHsp70 and its underlying mechanisms using a controlled cortical impact (CCI) mouse model. We found that CCI selectively reduced mtHsp70 levels within mitochondria, accompanied by its cytoplasmic accumulation, while total cellular mtHsp70 expression remained unchanged. Stereotactic AAV-mediated overexpression of mtHsp70 in the cortex significantly reduced neuronal apoptosis, improved motor and cognitive behavioral outcomes, and increased neuronal survival following CCI. In vitro, mtHsp70 overexpression in HT22 cells attenuated H₂O₂-induced neuronal injury, improved mitochondrial respiration (OCR), and reduced mitochondrial protein aggregation. Mechanistically, mtHsp70 overexpression increased the expression of mitochondrial unfolded protein response (UPRmt)-related proteins, including HSP60 and Lonp1, and restored mitochondrial membrane potential. Importantly, ATF5 knockdown attenuated mtHsp70-induced upregulation of UPRmt-associated proteins and diminished mitochondrial respiratory improvement, suggesting that mtHsp70-mediated protection is dependent on ATF5-associated UPRmt signaling. Together, these findings indicate that mitochondrial mtHsp70 deficiency contributes to neuronal injury after CCI, whereas restoration of mtHsp70 improves mitochondrial proteostasis and neuronal survival. Targeting the mtHsp70-UPRmt pathway may represent a potential therapeutic strategy for TBI.