Traumatic brain injury (TBI) is a major cause of death and disability and is experienced by nearly 3 million people annually as a result of falls, vehicular accidents, or from being struck by or against an object. While TBIs can range in severity, the majority of injuries are considered to be mild. However, TBI of any severity has the potential to have long-lasting neurological effects, including headaches, cognitive/memory impairments, mood dysfunction, and fatigue as a result of neural damage and neuroinflammation. Here, we modified a projectile concussive impact (PCI) model of TBI to deliver a closed-head impact with variable severity dependent on the material of the ball-bearing projectile. Adult male Sprague Dawley rats were evaluated for neurobehavioral, neuroinflammatory, and neural damage endpoints both acutely and longer-term (up to 72 h) post-TBI following impact with either an aluminum or stainless-steel projectile. Animals that received TBI using the stainless-steel projectile exhibited outcomes strongly correlated to moderate-severe TBI, such as prolonged unconsciousness, impaired neurobehavior, increased risk for hematoma and death, as well as significant neuronal degeneration and neuroinflammation throughout the cortex, hippocampus, thalamus, and cerebellum. In contrast, rats that received TBI with the aluminum projectile exhibited characteristics more congruous with mild TBI, such as a trend for longer periods of unconsciousness in the absence of neurobehavioral deficits, a lack of neurodegeneration, and mild neuroinflammation. Moreover, alignment of cytokine mRNA expression from the cortex of these rats with a computational model of neuron–glia interaction found that the moderate-severe TBI produced by the stainless-steel projectile strongly associated with the neuroinflammatory state, while the mild TBI existed in a state between normal and inflammatory neuron–glia interactions. Thus, these modified PCI protocols are capable of producing TBIs that model the clinical and experimental manifestations associated with both moderate-severe and mild TBI producing relevant models for the evaluation of the potential underlying roles of neuroinflammation and other chronic pathophysiology in the long-term outcomes associated with TBI.

Aberrant inflammatory signaling between neuronal and glial cells can develop into a persistent sickness behavior-related disorders, negatively impacting learning, memory, and neurogenesis. While there is an abundance of literature describing these interactions, there still lacks a comprehensive mathematical model describing the complex feed-forward and feedback mechanisms of neural-glial interaction. Here we compile molecular and cellular signaling information from various studies and reviews in the literature to create a logically-consistent, theoretical model of neural-glial interaction in the brain to explore the role of neuron-glia homeostatic regulation in the perpetuation of neuroinflammation. Logic rules are applied to this connectivity diagram to predict the system's homeostatic behavior. We validate our model predicted homeostatic profiles against RNAseq gene expression profiles in a mouse model of stress primed neuroinflammation. A meta-analysis was used to calculate the significance of similarity between the inflammatory profiles of mice exposed to diisopropyl fluorophostphate (DFP) [with and without prior priming by the glucocorticoid stress hormone corticosterone (CORT)], with the equilibrium states predicted by the model, and to provide estimates of the degree of the neuroinflammatory response. Beyond normal homeostatic regulation, our model predicts an alternate self-perpetuating condition consistent with chronic neuroinflammation. RNAseq gene expression profiles from the cortex of mice exposed to DFP and CORT+DFP align with this predicted state of neuroinflammation, whereas the alignment to CORT alone was negligible. Simulations of putative treatment strategies post-exposure were shown to be theoretically capable of returning the system to a state of typically healthy regulation with broad-acting anti-inflammatory agents showing the highest probability of success. The results support a role for the brain's own homeostatic drive in perpetuating the chronic neuroinflammation associated with exposure to the organophosphate DFP, with and without CORT priming. The deviation of illness profiles from exact model predictions suggests the presence of additional factors or of lasting changes to the brain's regulatory circuitry specific to each exposure.

BACKGROUND: Gulf War illness (GWI) is an archetypal, medically unexplained, chronic condition characterised by persistent sickness behaviour and neuroimmune and neuroinflammatory components. An estimated 25-32% of the over 900,000 veterans of the 1991 Gulf War fulfil the requirements of a GWI diagnosis. It has been hypothesised that the high physical and psychological stress of combat may have increased vulnerability to irreversible acetylcholinesterase (AChE) inhibitors leading to a priming of the neuroimmune system. A number of studies have linked high levels of psychophysiological stress and toxicant exposures to epigenetic modifications that regulate gene expression. Recent research in a mouse model of GWI has shown that pre-exposure with the stress hormone corticosterone (CORT) causes an increase in expression of specific chemokines and cytokines in response to diisopropyl fluorophosphate (DFP), a sarin surrogate and irreversible AChE inhibitor. METHODS: C57BL/6J mice were exposed to CORT for 4 days, and exposed to DFP on day 5, before sacrifice 6 h later. The transcriptome was examined using RNA-seq, and the epigenome was examined using reduced representation bisulfite sequencing and H3K27ac ChIP-seq. RESULTS: We show transcriptional, histone modification (H3K27ac) and DNA methylation changes in genes related to the immune and neuronal system, potentially relevant to neuroinflammatory and cognitive symptoms of GWI. Further evidence suggests altered proportions of myelinating oligodendrocytes in the frontal cortex, perhaps connected to white matter deficits seen in GWI sufferers. CONCLUSIONS: Our findings may reflect the early changes which occurred in GWI veterans, and we observe alterations in several pathways altered in GWI sufferers. These close links to changes seen in veterans with GWI indicates that this model reflects the environmental exposures related to GWI and may provide a model for biomarker development and testing future treatments.

Gulf War Illness (GWI) is a chronic multi-symptom illness that has affected veterans of the 1991 Persian Gulf War for over two decades. Recently, research into GWI has greatly expanded, including investigations into potential initiating stimuli and conditions, current pathobiology, and promising treatments for this population of ill veterans. As the field of GWI research grows, it is important for researchers to further characterize and expand upon prior findings in order to bring the field closer to a comprehensive understanding of GWI and to develop therapies that treat the illness itself, not just its symptoms. | While the cause (s) of GWI remain largely unknown, most research supports a role for chemical exposures in theater (White et al., 2015) that initiate a protracted, largely neuroimmune-based disorder. Furthermore, the observation that a subset of veterans developed GWI, while nearly all soldiers were likely exposed to some combination of toxicants in theater, strongly supports the hypothesis that veterans with GWI may harbor some specific genetic-susceptibility. In the most recent publication from the Georgopoulos group, James et al. (2017) expand upon several of their previous studies verifying the protective role of HLA alleles related to brain function (Georgopoulos et al., 2015, James et al., 2016) in the observed subcortical brain atrophy associated with GWI (Christova et al., 2017). Here, James et al.’s (2017) evaluation of subcortical brain volumes in Gulf War veterans supported the suspected protective effect of the HLA class II allele DRB1*13:02 by finding a significantly higher subcortical volume in carriers of this allele. A hypothesis is presented that GWI is the result of persistent antigenicity resulting from the presentation of “novel” brain antigens following toxicant exposure. For example, exposure to acetylcholinesterase (AChE) inhibiting organophosphate compounds is well-studied as a potential initiator for GWI. In these models, there is the potential for either irreversibly phosphorylated, or “aged,” AChE or, as recently presented by Locker et al. (2017), the persistent “organophosphorylation” of neuroimmune-related targets to serve as a persistent antigen. Additionally, the presence of “auto-antibodies” against several neural proteins in the sera of veterans with GWI (Abou-Donia et al., 2017) suggests that changes in the brain following in-theater exposures has resulted in the release of brain antigens that have stimulated an immune response. Moreover, the extreme stress experienced in theater, which has been demonstrated to enhance the acute neuroinflammatory response to chemical toxicants (O'Callaghan et al., 2015, Locker et al., 2017), has the potential to perpetuate a persistent immune hypersensitivity to any potential GWI-related antigen (see Dhabhar and McEwen, 1996). The work presented in James et al. (2017) links these GWI studies to a potential mechanism involving genetic susceptibility based upon deficient antigen presentation facilitated by HLA class II allele genotype.