Hypoxia–ischemia plays a role in the physiological and pathological processes of various diseases and presents a common challenge for humans under extreme environmental conditions. Neurons are particularly sensitive to hypoxia–ischemia, and prolonged exposure may lead to irreversible brain damage. The primary mechanisms underlying this damage include energy depletion, mitochondrial dysfunction, oxidative stress, inflammation, and apoptosis. Mitochondria serve as primary organelles for adenosine triphosphate (ATP) production, and mitochondrial dysfunction plays a crucial role in mediating hypoxic pathophysiological processes. Hypoxic–ischemic preconditioning (H/IPC) is an endogenous cellular protective mechanism that reduces the damage caused by lethal hypoxic stressors. In this review, we summarize the potential role of H/IPC and its protective effects on mitochondrial quality control and function. This perspective offers a new approach for treating diseases caused by hypoxia–ischemia.

Background. It has been reported that ischemia and ischemic preconditioning (IPC) have different effects on the expression of tuberous sclerosis complex 1 (TSC1), which may contribute to the tolerance to ischemia/hypoxia with the increase of autophagy. The mechanisms of TSC1 differential expression are still unclear under ischemia/IPC conditions in hippocampal Cornu Ammon 1 (CA1) and Cornu Ammon 3 (CA3) area neuronal cells. While we have shown that 5‐Aza‐CdR, a DNA methyltransferase inhibitor, can upregulate TSC1 and increase hypoxic tolerance by autophagy in vivo and in vitro, in this study, we examined whether DNA methylation was involved in the differential expression of TSC1 in the CA1 and CA3 regions induced by hypoxic preconditioning (HPC). Methods. Level of rapamycin (mTOR) autophagy, a downstream molecular pathway of TSC1/TSC2 complex, was detected in HPC mouse hippocampal CA1 and CA3 areas as well as in the HPC model of mouse hippocampal HT22 cells. DNA methylation level of TSC1 promoter (‐720 bp~ ‐360 bp) was determined in CA1 and CA3 areas by bisulfite‐modified DNA sequencing (BMDS). At the same time, autophagy was detected in HT22 cells transfected with GFP‐LC3 plasmid. The role of TSC1 in neuroprotection was measured by cell viability and apoptosis, and the role of TSC1 in metabolism was checked by ATP assay and ROS assay in HT22 cells that overexpressed/knocked down TSC1. Results. HPC upregulated the expression of TSC1, downregulated the level of P‐mTOR (Ser2448) and P‐p70S6K (Thr389), and enhanced the activity of autophagy in both in vivo and in vitro. The increased expression of TSC1 in HPC may depend on its DNA hypomethylation in the promoter region in vivo. HPC also could reduce energy consumption in HT22 cells. Overexpression and knockdown of TSC1 can affect cell viability, cell apoptosis, and metabolism in HT22 cells exposed to hypoxia. Conclusion. TSC1 expression induced by HPC may relate to the downregulation of its DNA methylation level with the increase of autophagy and the decrease of energy demand.

Prior research has firmly established that the N-methyl-d-aspartate (NMDA) receptor subunit 2 B (GluN2B) and its phosphorylation contribute to ischemic/hypoxic brain injury. Hypoxic preconditioning (HPC) is an endogenous mechanism that protects the brain from both ischaemic and hypoxic damage. In this study, we explored the effects of HPC on GluN2B and its phosphorylation at two sites (tyrosine residues 1252 and 1336), catalysed by Fyn, in the hippocampus both in vivo and in vitro. Animal and cellular models of HPC were developed by subjecting mice and the mouse hippocampal neuronal cell line HT22 to repeated hypoxia. Levels of GluN2B and its phosphorylation at the tyrosine residues 1336 (pY1336 GluN2B) and 1252 (pY1252 GluN2B) were detected in HPC-treated hippocampi and HT22 cells using western blotting and immunofluorescence. The distributions of GluN2B, pY1336 GluN2B, and pY1252 GluN2B in the synaptic (TxP) and extrasynaptic components (TxS) were analysed by western blotting. Caspase-3 and spectrin, both markers of cellular injury, were further measured using western blotting. HPC downregulated GluN2B and pY1336 GluN2B levels in the hippocampus and HT22 cells. The changes in GluN2B and pY1336 GluN2B levels in the extrasynaptic components were similar to those in the hippocampus and HT22 cells, while the changes in the synaptic components showed the opposite trend which increased after HPC. The downregulation of GluN2B and pY1336 GluN2B may be associated with neuroprotection induced by HPC. Additionally, their localization at synaptic and extrasynaptic sites may play distinct roles in neuroprotection.

Hypoxic preconditioning (HPC) can increase the hypoxia tolerance of the mouse hippocampus both in vivo and in vitro by upregulating ATP levels, which may depend on mitochondrial homeostasis. The GSK-3β/β-catenin signaling pathway is involved in neuroprotection after brain ischemia. The aim of this study was to explore whether HPC can activate GSK-3β/β-catenin to improve the hypoxia tolerance of neuronal cells by enhancing mitochondrial homeostasis. In vitro and in vivo analyses revealed that HPC upregulates the activity of the GSK-3β/β-catenin pathway, maintaining mitochondrial morphological stability. Mechanistically, GSK-3β activity is negatively correlated with mitochondrial homeostasis. Pharmacological inhibition of GSK-3β reduced mitochondrial fission, whereas HPC suppressed GSK-3β expression to attenuate fission and preserve mitochondrial integrity. Conversely, GSK-3β overexpression abrogated HPC-mediated protection and exacerbated mitochondrial dysfunction. These findings elucidate a neuroprotective mechanism whereby HPC stabilizes mitochondrial dynamics via modulation of the GSK-3β/β-catenin pathway, providing novel experimental insights into HPC-mediated neuroprotection.