NOX2 drives M1-like microglial/macrophage activation and neurodegeneration following experimental traumatic brain injury

Abstract

Following traumatic brain injury (TBI), activation of microglia and peripherally derived inflammatory macrophages occurs in association with tissue damage. This neuroinflammatory response may have beneficial or detrimental effects on neuronal survival, depending on the functional polarization of these cells along a continuum from M1-like to M2-like activation states. The mechanisms that regulate M1-like and M2-like activation after TBI are not well understood, but appear in part to reflect the redox state of the lesion microenvironment. NADPH oxidase (NOX2) is a critical enzyme system that generates reactive oxygen species in microglia/macrophages. After TBI, NOX2 is strongly up-regulated in M1-like, but not in M2-like polarized cells. Therefore, we hypothesized that NOX2 drives M1-like neuroinflammation and contributes to neurodegeneration and loss of neurological function after TBI. In the present studies we inhibited NOX2 activity using NOX2-knockout mice or the selective peptide inhibitor gp91ds-tat. We show that NOX2 is highly up-regulated in infiltrating macrophages after injury, and that NOX2 deficiency reduces markers of M1-like activation, limits tissue loss and neurodegeneration, and improves motor recovery after moderate-level control cortical injury (CCI). NOX2 deficiency also promotes M2-like activation after CCI, through increased IL-4Rα signaling in infiltrating macrophages, suggesting that NOX2 acts as a critical switch between M1- and M2-like activation states after TBI. Administration of gp91ds-tat to wild-type CCI mice starting at 24 hours post-injury reduces deficits in cognitive function and increased M2-like activation in the hippocampus. Collectively, our data indicate that increased NOX2 activity after TBI drives M1-like activation that contributes to inflammatory-mediated neurodegeneration, and that inhibiting this pathway provides neuroprotection, in part by altering M1-/M2-like balance towards the M2-like neuroinflammatory response.

NOX2 deficiency alters macrophage phenotype through an IL-10/STAT3 dependent mechanism: implications for traumatic brain injury

“…Given that NOX2 −/− TBI mice exhibit enhanced anti-inflammatory activation that is associated with improved outcomes following TBI [24], we set out to investigate mechanisms of NOX2-dependent macrophage phenotype switching using the LPS/IL-4 stimulation model. WT and NOX2 −/− BMDMs were treated with LPS/IL-4 for 24 h, and pro and anti-inflammatory activation markers were assessed.…”

“…NOX2 is highly expressed in macrophage/microglia up to 12 months after moderate-to-severe TBI [21], and inhibition of NOX2 is neuroprotective and reduces inflammatory-mediated neuronal cell death after TBI [23–26]. We recently reported that NOX2 inhibition also enhances anti-inflammatory activation of macrophage/microglia following TBI [24]. Thus, NOX2 may act as a critical switch between pro- and anti-inflammatory phenotypes after TBI.…”

“…There was also a significant increase in anti-inflammatory activation markers in NOX2 −/− BMDMs. These findings are significant because there are increased numbers of anti-inflammatory microglia/macrophages in the injured cortex of NOX2 −/− TBI mice [24]. Of note, it was not possible to promote this NOX2-dependent phenotype shift when BMDMs were treated with only pro-inflammatory stimuli (LPS or IFNγ alone; data not shown) or anti-inflammatory stimuli (IL-4 alone; data not shown), highlighting the importance of mixed environmental stimuli (combined LPS/IL-4) for NOX2-dependent macrophage phenotype responses.…”

“…Notably, NOX2 inhibition reduces markers of pro-inflammatory activation, limits tissue loss and neurodegeneration, and improves neurological recovery in TBI mice [12, 23–26]. NOX2 inhibition also results in enhanced expression of anti-inflammatory markers in macrophage/microglia [24, 27], and infiltrating macrophages are major contributors to the anti-inflammatory environment in the TBI brain in NOX2 knockout mice [24]. Based on accumulating pre-clinical evidence supporting NOX2-mediated neuroinflammation as an important therapeutic target for TBI, the goal of the current study was to elucidate the cell signaling mechanisms that regulate NOX2-dependent phenotypic changes in macrophages/microglia in the brain following TBI.…”

Exosomes from MiR-30d-5p-ADSCs Reverse Acute Ischemic Stroke-Induced, Autophagy-Mediated Brain Injury by Promoting M2 Microglial/Macrophage Polarization

“…The functions of microglia become more significant in response to a CNS insult such as ischemia. Increasing evidence suggests that an increase in the brain M2/M1 ratio and increases of M2 microglial proteins have anti-inflammatory properties and provide a neuroprotective effect [36,37]. IL-4 and IL-10 are markers secreted by M2 microglial cells, while TNF-α, IL-6 and iNOS are markers secreted by M1 microglial cells [38][39][40][41].…”