Traumatic brain injury (TBI) is a major risk factor for the development of multiple neurodegenerative diseases. With respect to the increasing prevalence of TBI, new therapeutic strategies are urgently needed that will prevent secondary damage to primarily unaffected tissue. Consistently, neuroinflammation has been implicated as a key mediator of secondary damage following the initial mechanical insult. Following injury, there is uncertainty regarding the role that accumulating CCR2 ϩ macrophages play in the injury-induced neuroinflammatory sequelae and cognitive dysfunction. Using CX3CR1 GFP/ϩ CCR2 RFP/ϩ reporter mice, we show that TBI initiated a temporally restricted accumulation of peripherally derived CCR2 ϩ macrophages, which were concentrated in the hippocampal formation, a region necessary for learning and memory. Multivariate analysis delineated CCR2 ϩ macrophages' neuroinflammatory response while identifying a novel therapeutic treatment window. As a proof of concept, targeting CCR2 ϩ macrophages with CCX872, a novel Phase I CCR2 selective antagonist, significantly reduced TBI-induced inflammatory macrophage accumulation. Concomitantly, there was a significant reduction in multiple proinflammatory and neurotoxic mediators with this treatment paradigm. Importantly, CCR2 antagonism resulted in a sparing of TBI-induced hippocampal-dependent cognitive dysfunction and reduced proinflammatory activation profile 1 month after injury. Thus, therapeutically targeting the CCR2 ϩ subset of monocytes/macrophages may provide a new avenue of clinical intervention following TBI.

Traumatic brain injury (TBI) is a leading cause of long-term neurological disability, yet the mechanisms underlying the chronic cognitive deficits associated with TBI remain unknown. Consequently, there are no effective treatments for patients suffering from the long-lasting symptoms of TBI. Here, we show that TBI persistently activates the integrated stress response (ISR), a universal intracellular signaling pathway that responds to a variety of cellular conditions and regulates protein translation via phosphorylation of the translation initiation factor eIF2α. Treatment with ISRIB, a potent drug-like small-molecule inhibitor of the ISR, reversed the hippocampaldependent cognitive deficits induced by TBI in two different injury mouse models-focal contusion and diffuse concussive injury. Surprisingly, ISRIB corrected TBI-induced memory deficits when administered weeks after the initial injury and maintained cognitive improvement after treatment was terminated. At the physiological level, TBI suppressed long-term potentiation in the hippocampus, which was fully restored with ISRIB treatment. Our results indicate that ISR inhibition at time points late after injury can reverse memory deficits associated with TBI. As such, pharmacological inhibition of the ISR emerges as a promising avenue to combat head traumainduced chronic cognitive deficits.brain trauma | memory deficits | translational control | eIF2α | hippocampus T raumatic brain injury (TBI) represents a major mental health problem (1-4). Even a mild TBI can elicit cognitive deficits, including permanent memory dysfunction (2, 4). Moreover, TBI is one of the most predictive environmental risk factors for the development of Alzheimer's disease and other forms of dementia (5-9). Current treatments have focused primarily on reducing the risk of TBI incidence, immediate neurosurgical intervention, or broad behavioral rehabilitation (10-13). Despite posing a huge societal problem, there are currently no pharmacological treatment options for patients that suffer from TBIinduced cognitive deficits.The integrated stress response (ISR) is an evolutionarily conserved pathway that controls cellular homeostasis and function (14). The central ISR regulatory step is the phosphorylation of the α-subunit of the eukaryotic translation initiation factor 2 (eIF2α) by a family of four eIF2α kinases (15, 16). Phosphorylation of eIF2α leads to inhibition of general protein synthesis, but also, to the translational up-regulation of a select subset of mRNAs (17, 18). In the brain, phosphorylation of eIF2α regulates the formation of long-term memory (19)(20)(21). Briefly, animals with reduced phosphorylation of eIF2α show enhanced long-term memory storage (19,(22)(23)(24), and increased phosphorylation of eIF2α in the brain prevents the formation of long-term memory (19,24,25).Similar to other chronic cognitive disorders (21, 26), TBI leads to a persistent activation of the ISR. TBI induces eIF2α phosphorylation even in brain regions that are distal to the injury site (27, 28). How...

BackgroundChronic neuroinflammation is a hallmark of several neurological disorders associated with cognitive loss. Activated microglia and secreted factors such as tumor necrosis factor (TNF)-α are key mediators of neuroinflammation and may contribute to neuronal dysfunction. Our study was aimed to evaluate the therapeutic potential of a novel analog of thalidomide, 3,6'-dithiothalidomide (DT), an agent with anti-TNF-α activity, in a model of chronic neuroinflammation.MethodsLipopolysaccharide or artificial cerebrospinal fluid was infused into the fourth ventricle of three-month-old rats for 28 days. Starting on day 29, animals received daily intraperitoneal injections of DT (56 mg/kg/day) or vehicle for 14 days. Thereafter, cognitive function was assessed by novel object recognition, novel place recognition and Morris water maze, and animals were euthanized 25 min following water maze probe test evaluation.ResultsChronic LPS-infusion was characterized by increased gene expression of the proinflammatory cytokines TNF-α and IL-1β in the hippocampus. Treatment with DT normalized TNF-α levels back to control levels but not IL-1β. Treatment with DT attenuated the expression of TLR2, TLR4, IRAK1 and Hmgb1, all genes involved in the TLR-mediated signaling pathway associated with classical microglia activation. However DT did not impact the numbers of MHC Class II immunoreactive cells. Chronic neuroinflammation impaired novel place recognition, spatial learning and memory function; but it did not impact novel object recognition. Importantly, treatment with DT restored cognitive function in LPS-infused animals and normalized the fraction of hippocampal neurons expressing the plasticity-related immediate-early gene Arc.ConclusionOur data demonstrate that the TNF-α synthesis inhibitor DT can significantly reverse hippocampus-dependent cognitive deficits induced by chronic neuroinflammation. These results suggest that TNF-α is a critical mediator of chronic neuroinflammation-induced neuronal dysfunction and cognitive impairment and targeting its synthesis could provide an effective therapeutic approach to several human neurodegenerative diseases.