Delayed CO₂ postconditioning promotes neurological recovery after cryogenic traumatic brain injury by downregulating IRF7 expression

Aims: Few treatments are available in the subacute phase of traumatic brain injury (TBI) except rehabilitation training. We previously reported that transient CO₂ inhalation applied within minutes after reperfusion has neuroprotective effects against cerebral ischemia/reperfusion injury. In this study, it was hypothesized that delayed CO₂ postconditioning (DCPC) starting at the subacute phase may promote neurological recovery of TBI.

Methods: Using a cryogenic TBI (cTBI) model, mice received DCPC daily by inhaling 5%/10%/20% CO₂ for various time-courses (one/two/three cycles of 10-min inhalation/10-min break) at Days 3-7, 3-14 or 7-18 after cTBI. Beam walking and gait tests were used to assess the effect of DCPC. Lesion size, expression of GAP-43 and synaptophysin, amoeboid microglia number and glia scar area were detected. Transcriptome and recombinant interferon regulatory factor 7 (Irf7) adeno-associated virus were applied to investigate the molecular mechanisms.

Results: DCPC significantly promoted recovery of motor function in a concentration and time-course dependent manner with a wide therapeutic time window of at least 7 days after cTBI. The beneficial effects of DCPC were blocked by intracerebroventricular injection of NaHCO₃ . DCPC also increased puncta density of GAP-43 and synaptophysin, and reduced amoeboid microglia number and glial scar formation in the cortex surrounding the lesion. Transcriptome analysis showed many inflammation-related genes and pathways were altered by DCPC, and Irf7 was a hub gene, while overexpression of IRF7 blocked the motor function improvement of DCPC.

Conclusions: We first showed that DCPC promoted functional recovery and brain tissue repair, which opens a new therapeutic time window of postconditioning for TBI. Inhibition of IRF7 is a key molecular mechanism for the beneficial effects of DCPC, and IRF7 may be a potential therapeutic target for rehabilitation after TBI.