Traumatic brain injury (TBI) is a major cause of mortality and morbidity worldwide. Cerebral edema, a life-threatening medical complication, contributes to elevated intracranial pressure (ICP) and a poor clinical prognosis after TBI. Unfortunately, treatment options to reduce post-traumatic edema remain suboptimal, due in part, to a dearth of viable therapeutic targets. Herein, we tested the hypothesis that cerebral innate immune responses contribute to edema development after TBI. Our results demonstrate that high-mobility group box protein 1 (HMGB1) was released from necrotic neurons via a NR2B-mediated mechanism. HMGB1 was clinically associated with elevated ICP in patients and functionally promoted cerebral edema after TBI in mice. The detrimental effects of HMGB1 were mediated, at least in part, via activation of microglial toll-like receptor-4 (TLR4) and the subsequent expression of the astrocytic water channel, aquaporin-4 (AQP4). Genetic or pharmacological (VGX-1027) TLR4 inhibition attenuated the neuroinflammatory response and limited post-traumatic edema with a delayed, clinically implementable therapeutic window. Human and rodent tissue culture studies further defined the cellular mechanisms demonstrating neuronal HMGB1 initiates the microglial release of interleukin-6 (IL-6) in a TLR4 dependent mechanism. In turn, microglial IL-6 increased the astrocytic expression of AQP4. Taken together, these data implicate microglia as key mediators of post-traumatic brain edema and suggest HMGB1-TLR4 signaling promotes neurovascular dysfunction after TBI.

BackgroundOxidative stress is known to play an important role in the pathology of traumatic brain injury. Mitochondria are thought to be the major source of the damaging reactive oxygen species (ROS) following TBI. However, recent work has revealed that the membrane, via the enzyme NADPH oxidase can also generate the superoxide radical (O2 −), and thereby potentially contribute to the oxidative stress following TBI. The current study thus addressed the potential role of NADPH oxidase in TBI.Methodology/Principal FindingsThe results revealed that NADPH oxidase activity in the cerebral cortex and hippocampal CA1 region increases rapidly following controlled cortical impact in male mice, with an early peak at 1 h, followed by a secondary peak from 24–96 h after TBI. In situ localization using oxidized hydroethidine and the neuronal marker, NeuN, revealed that the O2 − induction occurred in neurons at 1 h after TBI. Pre- or post-treatment with the NADPH oxidase inhibitor, apocynin markedly inhibited microglial activation and oxidative stress damage. Apocynin also attenuated TBI-induction of the Alzheimer's disease proteins β-amyloid and amyloid precursor protein. Finally, both pre- and post-treatment of apocynin was also shown to induce significant neuroprotection against TBI. In addition, a NOX2-specific inhibitor, gp91ds-tat was also shown to exert neuroprotection against TBI.Conclusions/SignificanceAs a whole, the study demonstrates that NADPH oxidase activity and superoxide production exhibit a biphasic elevation in the hippocampus and cortex following TBI, which contributes significantly to the pathology of TBI via mediation of oxidative stress damage, microglial activation, and AD protein induction in the brain following TBI.

Traumatic brain injury (TBI) is a leading cause of death and disability in the United States. Current medical therapies exhibit limited efficacy in reducing neurological injury and the prognosis for patients remains poor. While most research is focused on the direct protection of neuronal cells, non-neuronal cells, such as astrocytes, may exert an active role in the pathogenesis of TBI. Astrocytes, the predominant cell type in the human brain, are traditionally associated with providing only structural support within the CNS. However, recent work suggests astrocytes may regulate brain homeostasis and limit brain injury. In contrast, reactive astrocytes may also contribute to increased neuroinflammation, the development of cerebral edema, and elevated intracranial pressure, suggesting possible roles in exacerbating secondary brain injury following neurotrauma. The multiple, opposing roles for astrocytes following neurotrauma may have important implications for the design of directed therapeutics to limit neurological injury. As such, a primary focus of this review is to summarize the emerging evidence suggesting reactive astrocytes influence the response of the brain to TBI.

J. Neurochem. (2010) 113, 637–648. Abstract Traumatic brain injury is a devastating neurological injury associated with significant morbidity and mortality. Medical therapies to limit cerebral edema, a cause of increased intracranial hypertension and poor clinical outcome, are largely ineffective, emphasizing the need for novel therapeutic approaches. In the present study, pre‐treatment with curcumin (75, 150 mg/kg) or 30 min post‐treatment with 300 mg/kg significantly reduced brain water content and improved neurological outcome following a moderate controlled cortical impact in mice. The protective effect of curcumin was associated with a significant attenuation in the acute pericontusional expression of interleukin‐1β, a pro‐inflammatory cytokine, after injury. Curcumin also reversed the induction of aquaporin‐4, an astrocytic water channel implicated in the development of cellular edema following head trauma. Notably, curcumin blocked IL‐1β‐induced aquaporin‐4 expression in cultured astrocytes, an effect mediated, at least in part, by reduced activation of the p50 and p65 subunits of nuclear factor κB. Consistent with this notion, curcumin preferentially attenuated phosphorylated p65 immunoreactivity in pericontusional astrocytes and decreased the expression of glial fibrillary acidic protein, a reactive astrocyte marker. As a whole, these data suggest clinically achievable concentrations of curcumin reduce glial activation and cerebral edema following neurotrauma, a finding which warrants further investigation.

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Cerebral vasospasm is a major cause of death and disability after subarachnoid hemorrhage (SAH); however, clinical therapies to limit the development of cerebral vasospasm are lacking. Although the causative factors underlying the development of cerebral vasospasm are poorly understood, oxidative stress contributes to disease progression. In the present study, curcumin (150 or 300 mg/kg) protected against the development of cerebral vasospasm and limited secondary cerebral infarction after SAH in mice. The protective effect of curcumin was associated with a significant attenuation of inflammatory gene expression and lipid peroxidation within the cerebral cortex and the middle cerebral artery. Despite the ability of curcumin to limit the development of cerebral vasospasm and secondary infarction, behavioral outcome was not improved, indicating a dissociation between cerebral vasospasm and neurologic outcome. Together, these data indicate a novel role for curcumin as a possible adjunct therapy after SAH, both to prevent the development of cerebral vasospasm and to reduce oxidative brain injury after secondary infarction.

Objective Traumatic brain injury (TBI) induces significant neurological damage, including deficits in learning and memory which contribute to a poor clinical prognosis. Treatment options to limit cognitive decline and promote neurological recovery are lacking, in part, due to a poor understanding of the secondary/delayed processes which contribute to brain injury. In the present study, we characterized the temporal and spatial changes in the expression of PSD-95, a key scaffolding protein implicated in excitatory synaptic signaling, following controlled cortical impact in mice. Neurological injury, as assessed by the open field activity test and the novel object recognition test, were compared with changes in PSD-95 expression. Methods Adult male CD-1 mice were subjected to controlled cortical impact to simulate a moderate traumatic brain injury in humans. The spatial and temporal expression of PSD-95 was analyzed in the cerebral cortex and hippocampus at various time points following injury. Neurological assessments were performed to compare changes in PSD-95 with cognitive deficits. Results A significant decrease in PSD-95 expression was observed in the ipsilateral hippocampus beginning at day 7 post-injury. The loss of PSD-95 corresponded with a concomitant reduction in immunoreactivity for NeuN, a neuronal-specific marker. Aside from the contused cortex, significant loss of PSD-95 immunoreactivity was not observed in the cerebral cortex. The delayed loss of hippocampal PSD-95 directly correlated with the onset of behavioral deficits, suggesting a possible causative role for PSD-95 in behavioral abnormalities following a head trauma. Conclusion Delayed loss of hippocampal synapses was observed following head trauma in mice. These data may suggest a cellular mechanism to explain the delayed learning and memory deficits in humans and provide a potential framework for further testing to implicate PSD-95 as a clinically-relevant therapeutic target.

Directed targeting of the Akt and NFkappaB signaling pathways may be a useful adjunct in the clinical management of pituitary tumors. Further elucidation of this pathway may yield novel information regarding the behavior of pituitary tumors in humans.