Proton extrusion during oxidative burst in microglia exacerbates pathological acidosis following traumatic brain injury

Abstract

Acidosis is among the least studied secondary injury mechanisms associated with neurotrauma. Acute decreases in brain pH correlate with poor long‐term outcome in patients with traumatic brain injury (TBI), however, the temporal dynamics and underlying mechanisms are unclear. As key drivers of neuroinflammation, we hypothesized that microglia directly regulate acidosis after TBI, and thereby, worsen neurological outcomes. Using a controlled cortical impact model in adult male mice we demonstrate that intracellular pH in microglia and extracellular pH surrounding the lesion site are significantly reduced for weeks after injury. Microglia proliferation and production of reactive oxygen species (ROS) were also increased during the first week, mirroring the increase in extracellular ROS levels seen around the lesion site. Microglia depletion by a colony stimulating factor 1 receptor (CSF1R) inhibitor, PLX5622, markedly decreased extracellular acidosis, ROS production, and inflammation in the brain after injury. Mechanistically, we identified that the voltage‐gated proton channel Hv1 promotes oxidative burst activity and acid extrusion in microglia. Compared to wildtype controls, microglia lacking Hv1 showed reduced ability to generate ROS and extrude protons. Importantly, Hv1‐deficient mice exhibited reduced pathological acidosis and inflammation after TBI, leading to long‐term neuroprotection and functional recovery. Our data therefore establish the microglial Hv1 proton channel as an important link that integrates inflammation and acidosis within the injury microenvironment during head injury.

Functions and Mechanisms of the Voltage-Gated Proton Channel Hv1 in Brain and Spinal Cord Injury

“…It is possible that the age-dependent protection seen in older KO mice is due to decreased transmigration and reactivity of neutrophils in the ischemic brain, rather than microglia-driven. Nevertheless, neuroprotection in young KO mice has been documented in models of traumatic CNS injury, hypoperfusion-induced white matter injury, and multiple sclerosis (Li et al, 2021;Ritzel et al, 2021). Moreover, our group (Ritzel et al, 2021) showed that protection persisted for weeks and months following TBI.…”

“…Nevertheless, neuroprotection in young KO mice has been documented in models of traumatic CNS injury, hypoperfusion-induced white matter injury, and multiple sclerosis (Li et al, 2021;Ritzel et al, 2021). Moreover, our group (Ritzel et al, 2021) showed that protection persisted for weeks and months following TBI. These findings indicate there could be important interactions between age, Hv1, and the onset, duration, and type of injury.…”

“…In a mouse moderate/severe brain injury using a controlled cortical impact (CCI) model of experimental TBI, Hv1 mRNA and protein expression were remarkably increased at 24 h post-injury and remained elevated for up to 4 weeks in the ipsilateral cortex and hippocampus (Ritzel et al, 2021). qPCR analysis revealed a virtual absence of Hv1 mRNA expression in the microglia-depleted brain (Ritzel et al, 2021), further confirming that expression of this channel is largely restricted to microglia within the CNS. Dysregulation of Hv1 expression in the injured spinal cord after moderate/severe contusion in mice was also recently reported (Li et al, 2021).…”

“…Although it is well known that Hv1 regulates intracellular pH, and aids in compensation for NOX-dependent generation of ROS, a direct connection between acidosis, ROS, and inflammation has not been established until recently. Work from our group has demonstrated that Hv1 is a key driver of tissue acidosis, oxidative stress, and neuroinflammation in models of traumatic brain injury and spinal cord injury (Li et al, 2021;Ritzel et al, 2021; see Figure 2).…”

“…The underlying mechanisms of pathological acidosis in CNS injury are not well understood, however, growing evidence suggests a positive association with inflammation. In a well-characterized CCI mouse model of experimental TBI, we investigated the cellular and molecular mechanisms of brain acidosis (Ritzel et al, 2021). We demonstrated that TBI causes intracellular and extracellular acidosis that persist for weeks after injury.…”