Sex-Specific and Traumatic Brain Injury Effects on Dopamine Receptor Expression in the Hippocampus.

Traumatic brain injury (TBI) is a major health concern. Each year, over 50 million individuals worldwide suffer from TBI, and this leads to a number of acute and chronic health issues. These include affective and cognitive impairment, as well as an increased risk of alcohol and drug use. The dopaminergic system, a key component of reward circuitry, has been linked to alcohol and other substance use disorders, and previous research indicates that TBI can induce plasticity within this system. Understanding how TBI modifies the dopaminergic system may offer insights into the heightened substance use and reward-seeking behavior following TBI. The hippocampus, a critical component of the reward circuit, is responsible for encoding and integrating the spatial and salient aspects of rewarding stimuli. This study explored TBI-related changes in neuronal D2 receptor expression within the hippocampus, examining the hypothesis that sex differences exist in both baseline hippocampal D2 receptor expression and its response to TBI. Utilizing D2-expressing tdTomato transgenic male and female mice, we implemented either a sham injury or the lateral fluid percussion injury (FPI) model of TBI and subsequently performed a region-specific quantification of D2 expression in the hippocampus. The results show that male mice exhibit higher baseline hippocampal D2 expression compared to female mice. Additionally, there was a significant interaction effect between sex and injury on the expression of D2 in the hippocampus, particularly in regions of the dentate gyrus. Furthermore, TBI led to significant reductions in hippocampal D2 expression in male mice, while female mice remained mostly unaffected. These results suggest that hippocampal D2 expression varies between male and female mice, with the female dopaminergic system demonstrating less susceptibility to TBI-induced plasticity.

Thrombin, a Key Driver of Pathological Inflammation in the Brain.

Neurodegenerative diseases, including Alzheimer's disease (AD), are major contributors to death and disability worldwide. A multitude of evidence suggests that neuroinflammation is critical in neurodegenerative disease processes. Exploring the key mediators of neuroinflammation in AD, a prototypical neurodegenerative disease, could help identify pathologic inflammatory mediators and mechanisms in other neurodegenerative diseases. Elevated levels of the multifunctional inflammatory protein thrombin are commonly found in conditions that increase AD risk, including diabetes, atherosclerosis, and traumatic brain injury. Thrombin, a main driver of the coagulation cascade, has been identified as important to pathological events in AD and other neurodegenerative diseases. Furthermore, recent evidence suggests that coagulation cascade-associated proteins act as drivers of inflammation in the AD brain, and studies in both human populations and animal models support the view that abnormalities in thrombin generation promote AD pathology. Thrombin drives neuroinflammation through its pro-inflammatory activation of microglia, astrocytes, and endothelial cells. Due to the wide-ranging pro-inflammatory effects of thrombin in the brain, inhibiting thrombin could be an effective strategy for interrupting the inflammatory cascade which contributes to neurodegenerative disease progression and, as such, may be a potential therapeutic target for AD and other neurodegenerative diseases.

Sex Differences in Metabolic Indices and Chronic Neuroinflammation in Response to Prolonged High-Fat Diet in ApoE4 Knock-In Mice.

Late-onset Alzheimer's disease (LOAD) likely results from combinations of risk factors that include both genetic predisposition and modifiable lifestyle factors. The E4 allele of apolipoprotein E (ApoE) is the most significant genetic risk factor for LOAD. A Western-pattern diet (WD) has been shown to strongly increase the risk of cardiovascular disease and diabetes, conditions which have been strongly linked to an increased risk for developing AD. Little is known about how the WD may contribute to, or enhance, the increased risk presented by possession of the ApoE4 allele. To model this interaction over the course of a lifetime, we exposed male and female homozygote ApoE4 knock-in mice and wild-type controls to nine months of a high-fat WD or standard chow diet. At eleven months of age, the mice were tested for glucose tolerance and then for general activity and spatial learning and memory. Postmortem analysis of liver function and neuroinflammation in the brain was also assessed. Our results suggest that behavior impairments resulted from the convergence of interacting metabolic alterations, made worse in a male ApoE4 mice group who also showed liver dysfunction, leading to a higher level of inflammatory cytokines in the brain. Interestingly, female ApoE4 mice on a WD revealed impairments in spatial learning and memory without the observed liver dysfunction or increase in inflammatory markers in the brain. These results suggest multiple direct and indirect pathways through which ApoE and diet-related factors interact. The striking sex difference in markers of chronic neuroinflammation in male ApoE4 mice fed the high-fat WD suggests a specific mechanism of interaction conferring significant enhanced LOAD risk for humans with the ApoE4 allele, which may differ between sexes. Additionally, our results suggest researchers exercise caution when designing and interpreting results of experiments employing a WD, being careful not to assume a WD impacts both sexes by the same mechanisms.

Neurogenesis and chronic neurobehavioral outcomes are partially improved by vagus nerve stimulation in a mouse model of Gulf War illness.

Gulf War illness (GWI) is a chronic, multi-symptom disorder that has impacted approximately one third of Gulf War veterans. GWI and its symptoms have been linked to the exposure to neurological chemicals, including the anti-nerve gas drug pyridostigmine bromide (PB) and the insecticide permethrin (PER), among others. Mouse models utilizing these chemicals have reported symptomology analogous to human GWI. These changes include behavioral and cognitive impairment, neuroinflammation and hippocampal pathogenesis. Disease modifying interventions that target these pathological components are desperately needed. Vagus nerve stimulation (VNS) is FDA approved for drug-resistant epilepsy and depression. VNS has also been used off-label to target a myriad of symptoms, some of which are encompassed within the Kansas and CDC definitions of clinical GWI symptomology. A GWI animal model in which mice are exposed to a daily injection of PB and PER for 10 consecutive days has been shown to exhibit cognitive impairment and hippocampal pathology. The purpose of this study was to determine if 2-4 weeks of continuous vagus nerve stimulation initiated at 32 weeks after exposure to PB and PER would improve cognitive performance and hippocampal pathology. The results of the study revealed that exposure to PB and PER produces long-term cognitive deficits and reduced hippocampal neurogenesis. The results also showed that the VNS treatment was anxiolytic, improved some aspects of pattern separation deficits, and mitigated the reduced hippocampal neurogenesis. Thus, VNS improves outcomes in a mouse model of GWI and should be examined as a potential therapeutic strategy for mitigating some symptomology associated with GWI.

High Glucose and Hypoxia-Mediated Damage to Human Brain Microvessel Endothelial Cells Induces an Altered, Pro-Inflammatory Phenotype in BV-2 Microglia In Vitro.

Diabetes is strongly linked to the development of Alzheimer's disease (AD), though the mechanisms for this enhanced risk are unclear. Because vascular inflammation is a consistent feature of both diabetes and AD, the cerebral microcirculation could be a key target for the effects of diabetes in the brain. The goal of this study is to explore whether brain endothelial cells, injured by diabetes-related insults, glucose and hypoxia, can affect inflammatory and activation processes in microglia in vitro. Human brain microvascular endothelial cells (HBMVECs) were either treated with 5 mM glucose (control), 30 mM glucose (high glucose), exposed to hypoxia, or exposed to hypoxia plus high glucose. HBMVEC-conditioned medium was then used to treat BV-2 microglia. Alterations in microglia phenotype were assessed through measurement of nitric oxide (NO), cytokine production, microglial activation state markers, and microglial phagocytosis. HBMVECs were injured by exposure to glucose and/or hypoxia, as assessed by release of LDH, interleukin (IL)-1ß, and reactive oxygen species (ROS). HBMVECs injured by glucose and hypoxia induced increases in microglial production of NO, tumor necrosis factor-α (TNFα) and matrix metalloproteinase (MMP)-9. Injured HBMVECs significantly increased microglial expression of CD11c and CLEC7A, and decreased expression of the homeostatic marker P2RY12. Finally, bead uptake by BV-2 cells, an index of phagocytic ability, was elevated by conditioned media from injured HBMVECs. The demonstration that injury to brain endothelial cells by diabetic-associated insults, glucose and hypoxia, promotes microglial inflammation supports the idea that the cerebral microcirculation is a critical locus for the deleterious effects of diabetes in the AD brain.

Isoform-Specific Effects of Apolipoprotein E on Markers of Inflammation and Toxicity in Brain Glia and Neuronal Cells In Vitro.

Mutations to the cholesterol transport protein apolipoprotein E (ApoE) have been identified as a major risk factor for the development of sporadic or late-onset Alzheimer's disease (AD), with the e4 allele representing an increased risk and the rare e2 allele having a reduced risk compared to the primary e3 form. The reasons behind the change in risk are not entirely understood, though ApoE4 has been connected to inflammation and toxicity in both the brain and the periphery. The goal of this study was to better understand how the ApoE isoforms (ApoE2/3/4) confer differential AD-related risk by assessing cell-specific ApoE-related neuroinflammatory and neurotoxic effects. We compared the effects of ApoE isoforms in vitro on human astrocytes, a human immortalized microglia cell line (HMC3), and the human neuroblastoma cell line SH-SY5Y. Cells were treated for 24 h with or without recombinant ApoE2, ApoE3, or ApoE4 (20 nM) and inflammation and toxicity markers assessed. Our results indicated the expression of inflammatory cytokines IL-1ß, TNFα, and IL-6 in human astrocytes was increased in response to all ApoE isoforms, with ApoE4 evoking the highest level of cytokine expression. In response to ApoE2 or ApoE3, microglial cells showed reduced levels of microglial activation markers TREM2 and Clec7a, while ApoE4 induced increased levels of both markers. ApoE2 promoted neuron survival through increased BDNF release from astrocytes. In addition, ApoE2 promoted, while ApoE4 reduced, neuronal viability. Overall, these results suggest that ApoE4 acts on cells in the brain to promote inflammation and neuronal injury and that the deleterious effects of ApoE4 on these cells may, in part, contribute to its role as a risk factor for AD.

Thrombin Signaling Contributes to High Glucose-Induced Injury of Human Brain Microvascular Endothelial Cells.

BACKGROUND: Diabetes is one of the strongest disease-related risk factors for Alzheimer's disease (AD). In diabetics, hyperglycemia-induced microvascular complications are the major cause of end-organ injury, contributing to morbidity and mortality. Microvascular pathology is also an important and early feature of AD. The cerebral microvasculature may be a point of convergence of both diseases. Several lines of evidence also implicate thrombin in AD as well as in diabetes. OBJECTIVE: Our objective was to investigate the role of thrombin in glucose-induced brain microvascular endothelial injury. METHODS: Cultured Human brain microvascular endothelial cells (HBMVECs) were treated with 30 mM glucose±100 nM thrombin and±250 nM Dabigatran or inhibitors of PAR1, p38MAPK, MMP2, or MMP9. Cytotoxicity and thrombin activity assays on supernatants and western blotting for protein expression in lysates were performed. RESULTS: reatment of HBMVECs with 30 mM glucose increased thrombin activity and expression of inflammatory proteins TNFα, IL-6, and MMPs 2 and 9; this elevation was reduced by the thrombin inhibitor dabigatran. Direct treatment of brain endothelial cells with thrombin upregulated p38MAPK and CREB, and induced TNFα, IL6, MMP2, and MMP9 as well as oxidative stress proteins NOX4 and iNOS. Inhibition of thrombin, thrombin receptor PAR1 or p38MAPK decrease expression of inflammatory and oxidative stress proteins, implying that thrombin may play a central role in glucose-induced endothelial injury. CONCLUSION: Since preventing brain endothelial injury would preserve blood-brain barrier integrity, prevent neuroinflammation, and retain intact functioning of the neurovascular unit, inhibiting thrombin, or its downstream signaling effectors, could be a therapeutic strategy for mitigating diabetes-induced dementia.

Short-term treatment with dabigatran alters protein expression patterns in a late-stage tau-based Alzheimer's disease mouse model.

Proteins that regulate the coagulation cascade, including thrombin, are elevated in the brains of Alzheimer's disease (AD) patients. While studies using amyloid-based AD transgenic mouse models have implicated thrombin as a protein of interest, the role of thrombin in tau-based animal models has not been explored. The current study aims to determine how inhibiting thrombin could alter oxidative stress, inflammation, and AD-related proteins in a tau-based mouse model, the Tg4510. Aged Tg4510 mice were treated with the direct thrombin inhibitor dabigatran or vehicle for 7 days, brains collected, and western blot and data-independent proteomics using mass spectrometry with SWATH-MS acquisition performed to evaluate proteins related to oxidative stress, intracellular signaling, inflammation, and AD pathology. Dabigatran reduced iNOS, NOX4, and phosphorylation of tau (S396, S416). Additionally, dabigatran treatment increased expression of several signaling proteins related to cell survival and synaptic function. Increasing evidence supports a chronic procoagulant state in AD, highlighting a possible pathogenic role for thrombin. Our data demonstrate that inhibiting thrombin produces alterations in the expression of proteins involved in oxidative stress, inflammation, and AD-related pathology, suggesting that thrombin-mediated signaling affects multiple AD-related pathways providing a potential future therapeutic target.

Thrombin, a Mediator of Coagulation, Inflammation, and Neurotoxicity at the Neurovascular Interface: Implications for Alzheimer's Disease.

The societal burden of Alzheimer's disease (AD) is staggering, with current estimates suggesting that 50 million people world-wide have AD. Identification of new therapeutic targets is a critical barrier to the development of disease-modifying therapies. A large body of data implicates vascular pathology and cardiovascular risk factors in the development of AD, indicating that there are likely shared pathological mediators. Inflammation plays a role in both cardiovascular disease and AD, and recent evidence has implicated elements of the coagulation system in the regulation of inflammation. In particular, the multifunctional serine protease thrombin has been found to act as a mediator of vascular dysfunction and inflammation in both the periphery and the central nervous system. In the periphery, thrombin contributes to the development of cardiovascular disease, including atherosclerosis and diabetes, by inducing endothelial dysfunction and related inflammation. In the brain, thrombin has been found to act on endothelial cells of the blood brain barrier, microglia, astrocytes, and neurons in a manner that promotes vascular dysfunction, inflammation, and neurodegeneration. Thrombin is elevated in the AD brain, and thrombin signaling has been linked to both tau and amyloid beta, pathological hallmarks of the disease. In AD mouse models, inhibiting thrombin preserves cognition and endothelial function and reduces neuroinflammation. Evidence linking atrial fibrillation with AD and dementia indicates that anticoagulant therapy may reduce the risk of dementia, with targeting thrombin shown to be particularly effective. It is time for "outside-the-box" thinking about how vascular risk factors, such as atherosclerosis and diabetes, as well as the coagulation and inflammatory pathways interact to promote increased AD risk. In this review, we present evidence that thrombin is a convergence point for AD risk factors and as such that thrombin-based therapeutics could target multiple points of AD pathology, including neurodegeneration, vascular activation, and neuroinflammation. The urgent need for disease-modifying drugs in AD demands new thinking about disease pathogenesis and an exploration of novel drug targets, we propose that thrombin inhibition is an innovative tactic in the therapeutic battle against this devastating disease.

Inhibiting thrombin improves motor function and decreases oxidative stress in the LRRK2 transgenic Drosophila melanogaster model of Parkinson's disease.

Parkinson's disease (PD) is a complex neurodegenerative disease characterized by the presence of tremors, loss of dopaminergic neurons and accumulation of α-synuclein. While there is no single direct cause of PD, genetic mutations, exposure to pesticides, diet and traumatic brain injury have been identified as risk factors. Increasing evidence suggests that oxidative stress and neuroinflammation contribute to the pathogenesis of neuronal injury in neurodegenerative diseases such as PD and Alzheimer's disease (AD). We have previously documented that the multifunctional inflammatory mediator thrombin contributes to oxidative stress and neuroinflammation in AD. Here, for the first time, we explore the role of thrombin in a transgenic PD model, the LRRK2 mutant Drosophila melanogaster. Transgenic flies were treated with the direct thrombin inhibitor dabigatran for 7 days and locomotor activity and indices of oxidative stress evaluated. Our data show that dabigatran treatment significantly (p < 0.05) improved climbing activity, a measurement of locomotor ability, in male but had no effect on locomotor performance in female flies. Dabigatran treatment had no effect on tyrosine hydroxylase levels. Analysis of oxidative stress in male flies showed that dabigatran was able to significantly (p < 0.01) lower reactive oxygen species levels. Furthermore, Western blot analysis showed that the pro-oxidant proteins iNOS and NOX4 are elevated in LRRK2 male flies compared to wildtype and that treatment with dabigatran reduced expression of these proteins. Our results indicate that dabigatran treatment could improve motor function in PD by reducing oxidative stress. These data suggest that targeting thrombin may improve oxidative stress related pathologies in PD.