Introduction: Clinical and experimental studies indicate that traumatic brain injury (TBI) can have profound pathophysiological effects on peripheral organs. We have previously reported widespread dysfunction of innate and adaptive immune responses after experimental TBI, however, the long-term effects of TBI on peripheral tissues are largely unknown. Moreover, our understanding of the molecular mechanisms underlying systemic immune impairment after TBI remains poor. Hv1, a highly conserved voltage-gated proton channel, is exclusively expressed in microglia of the central nervous system. We and others have shown that microglial Hv1 is a key driver of tissue acidosis, oxidative stress, and neuroinflammation in models of acute CNS injury. However, Hv1 is also widely expressed in peripheral immune cells. Ablation of Hv1 caused detrimental effects on neutrophil function and impaired mitochondrial respiration and glycolysis in peripheral tissues. The present study examined the pathogenesis and long-term impact of TBI on the peripheral organs in the presence and absence of Hv1 in mice using a controlled cortical impact (CCI) model.
Methods: Young adult (3-month-old) male Hv1 knock out (Hv1KO) mice and their wildtype (WT) littermates were subjected to moderate CCI surgery. Functional recovery was evaluated at 17-months post-injury by open field, Y-maze, novel object recognition (NOR), novelty-suppressed feeding (NSF), and social interaction (SI) test. At 18 months after completion of all behavioral tests, the ipsilateral cerebral cortex and the liver, lung, spleen, bone marrow were dissected and processed for flow cytometry and transcriptomic analysis using NanoString Neuropathology or Myeloid Innate Immunity Panels.
Results: In Hv1 KO mice, TBI resulted in significant higher mortality beginning at 13-months post-injury compared to WT animals. At 17-months post-injury, no significant differences were detected in spontaneous locomotor activities tested in the open field among the groups. In a battery of neurobehavioral tests, Hv1 KO/TBI mice displayed significant cognitive deficits as demonstrated by reduced % spontaneous alternation in Y maze test, reduced time with novel object in NOR test, and poor performance in NSF and SI tests. NanoString Neuropathology analysis demonstrated that Hv1KO/TBI mice had a pronounced dysregulation in genes related to Oxidative Stress, Cytokines, and Activated microglia in the ipsilateral cortex. Myeloid Innate Immunity Panels analysis showed impaired immune function in the liver, lung, and spleen tissues in the absence of Hv1.
Conclusions: Taken together, our data indicate an important role for Hv1 in regulating peripheral immune function-mediated functional damage after chronic TBI.
