Recent clinical studies indicate that traumatic brain injury (TBI) produces chronic and progressive neurodegenerative changes leading to late neurological dysfunction but little is known about the mechanisms underlying such changes. Microglial-mediated neuroinflammation is an important secondary injury mechanism after TBI. In human studies, microglial activation has been found to persist for many years after the initial brain trauma, particularly after moderate-to-severe TBI. In the present study, adult C57Bl/6 mice were subjected to single moderate-level controlled cortical impact and were followed up by longitudinal T2-weighted magnetic resonance imaging in combination with stereological histological assessment of lesion volume expansion, neuronal loss and microglial activation for up to 1 year after TBI. Persistent microglial activation was observed in the injured cortex through 1 year post-injury, and was associated with progressive lesion expansion, hippocampal neurodegeneration, and loss of myelin. Notably, highly activated microglia that expressed major histocompatibility complex class II (CR3/43), CD68 and NADPH oxidase (NOX2) were detected at the margins of the expanding lesion at 1 year post-injury; biochemical markers of neuroinflammation and oxidative stress were significantly elevated at this time point. These data support emerging clinical TBI findings and provide a mechanistic link between TBI-induced chronic microglial activation and progressive neurodegeneration.

Traumatic brain injury (TBI) causes neuronal apoptosis, inflammation, and reactive astrogliosis, which contribute to secondary tissue loss, impaired regeneration, and associated functional disabilities. Here, we show that up-regulation of cell cycle components is associated with caspase-mediated neuronal apoptosis and glial proliferation after TBI in rats. In primary neuronal and astrocyte cultures, cell cycle inhibition (including the cyclin-dependent kinase inhibitors flavopiridol, roscovitine, and olomoucine) reduced upregulation of cell cycle proteins, limited neuronal cell death after etoposide-induced DNA damage, and attenuated astrocyte proliferation. After TBI in rats, flavopiridol reduced cyclin D1 expression in neurons and glia in ipsilateral cortex and hippocampus. Treatment also decreased neuronal cell death and lesion volume, reduced astroglial scar formation and microglial activation, and improved motor and cognitive recovery. The ability of cell cycle inhibition to decrease both neuronal cell death and reactive gliosis after experimental TBI suggests that this treatment approach may be useful clinically.flavopiridol ͉ glial scar ͉ neuron ͉ trauma

Apoptosis plays important roles in development, immunological competence, and homeostasis. It is characterized by marked changes in cellular morphology, including chromatin condensation, membrane blebbing, nuclear breakdown, and the appearance of membrane-associated apoptotic bodies, internucleosomal DNA fragmentation, as well as by cleavage of poly(ADP-ribose) polymerase (PARP).

Activated microglia and macrophages exert dual beneficial and detrimental roles after central nervous system injury, which are thought to be due to their polarization along a continuum from a classical pro-inflammatory M1-like state to an alternative anti-inflammatory M2-like state. The goal of the present study was to analyze the temporal dynamics of microglia/macrophage polarization within the lesion micro-environment following traumatic brain injury (TBI) using a moderate-level controlled cortical impact (CCI) model in mice. We performed a detailed phenotypic analysis of M1-and M2-like polarized microglia/macrophages, as well as nicotinamide adenine dinucleotide phosphate oxidase (NOX2) expression, through 7 days post-injury using real-time polymerase chain reaction (qPCR), flow cytometry and image analyses. We demonstrated that microglia/macrophages express both M1-and M2-like phenotypic markers early after TBI, but the transient up-regulation of the M2-like phenotype was replaced by a predominant M1-or mixed transitional (Mtran) phenotype that expressed high levels of NOX2 at 7 days post-injury. The shift towards the M1-like and Mtran phenotype was associated with increased cortical and hippocampal neurodegeneration. In a follow up study, we administered a selective NOX2 inhibitor, gp91ds-tat, to CCI mice starting at 24 h post-injury to investigate the relationship between NOX2 and M1-like/Mtran phenotypes. Delayed gp91ds-tat treatment altered M1-/M2-like balance in favor of the anti-inflammatory M2-like phenotype, and significantly reduced oxidative damage in neurons at 7 days post-injury. Therefore, our data suggest that despite M1-like and M2-like polarized microglia/macrophages being activated after TBI, the early M2-like response becomes dysfunctional over time, resulting in development of pathological M1-like and Mtran phenotypes driven by increased NOX2 activity.

Traumatic brain injury (TBI) causes chronic microglial activation that contributes to subsequent neurodegeneration, with clinical outcomes declining as a function of aging. Microglia/macrophages (MG/MΦ) have multiple phenotypes, including a classically activated, pro-inflammatory (M1) state that may contribute to neurotoxicity, and an alternatively activated (M2) state that may promote repair. In this study we used gene expression, immunohistochemical and stereological analyses to show that TBI in aged versus young mice caused larger lesions associated with an M1/M2 balance switch and increased numbers of reactive (bushy and hypertrophic) MG/MΦ in the cortex, hippocampus and thalamus. Ym1, an M2 phenotype marker, displayed heterogeneous expression after TBI with amoeboid-like Ym1-positive MG/MΦ at the contusion site and ramified Ym1-positive MG/MΦ at distant sites; this distribution was age related. Aged injured mice also showed increased MG/MΦ expression of MHC II and NADPH oxidase, and reduced antioxidant enzyme expression- which was associated with lesion size and neurodegeneration. Thus, altered relative M1/M2 activation and an NADPH oxidase-mediated shift in redox state may contribute to worse outcomes observed in older TBI animals by creating a more pro-inflammatory M1 MG/MΦ activation state.