Stress and Alzheimer's disease

Anxiety and Alzheimer's disease pathogenesis: focus on 5-HT and CRF systems in 3xTg-AD and TgF344-AD animal models

Dementia remains one of the leading causes of morbidity and mortality in older adults. Alzheimer's disease (AD) is the most common type of dementia, affecting over 55 million people worldwide. AD is characterized by distinct neurobiological changes, including amyloid-beta protein deposits and tau neurofibrillary tangles, which cause cognitive decline and subsequent behavioral changes, such as distress, insomnia, depression, and anxiety. Recent literature suggests a strong connection between stress systems and AD progression. This presents a promising direction for future AD research. In this review, two systems involved in regulating stress and AD pathogenesis will be highlighted: serotonin (5-HT) and corticotropin releasing factor (CRF). Throughout the review, we summarize critical findings in the field while discussing common limitations with two animal models (3xTg-AD and TgF344-AD), novel pharmacotherapies, and potential early-intervention treatment options. We conclude by highlighting promising future pharmacotherapies and translational animal models of AD and anxiety.

Co-administration of dl-3-n-butylphthalide and neprilysin is neuroprotective in Alzheimer disease associated with mild traumatic brain injury

dl-3-n-Butylphthalide is a potent synthetic Chinese celery extract that is highly efficient in inducing neuroprotection in concussive head injury (CHI), Parkinson's disease, Alzheimer's disease, stroke as well as depression, dementia, anxiety and other neurological diseases. Thus, there are reasons to believe that dl-3-n-butylphthalide could effectively prevent Alzheimer's disease brain pathology. Military personnel during combat operation or veterans are often the victims of brain injury that is a major risk factor for developing Alzheimer's disease in their later lives. In our laboratory we have shown that CHI exacerbates Alzheimer's disease brain pathology and reduces the amyloid beta peptide (AβP) inactivating enzyme neprilysin. We have used TiO2 nanowired-dl-3-n-butylphthalide in attenuating Parkinson's disease brain pathology exacerbated by CHI. Nanodelivery of dl-3-n-butylphthalide appears to be more potent as compared to the conventional delivery of the compound. Thus, it would be interesting to examine the effects of nanowired dl-3-n-butylphthalide together with nanowired delivery of neprilysin in Alzheimer's disease model on brain pathology. In this investigation we found that nanowired delivery of dl-3-n-butylphthalide together with nanowired neprilysin significantly attenuated brain pathology in Alzheimer's disease model with CHI, not reported earlier. The possible mechanism and clinical significance is discussed based on the current literature.

Exploring the bidirectional causal link between household income status and genetic susceptibility to neurological diseases: findings from a Mendelian randomization study

Objectives

Observational studies have revealed that socioeconomic status is associated with neurological disorders and aging. However, the potential causal effect between the two remains unclear. We therefore aimed to investigate the causal relationship between household income status and genetic susceptibility to neurological diseases using a bidirectional Mendelian randomization (MR) study.

Methods

An MR study was conducted on a large-sample cohort of the European population pulled from a publicly available genome-wide association study dataset, using a random-effects inverse-variance weighting model as the main standard. MR-Egger regression, weighted median, and maximum likelihood estimation were also performed concurrently as supplements. A sensitivity analysis, consisting of a heterogeneity test and horizontal pleiotropy test, was performed using Cochran's Q, MR-Egger intercept, and MR-PRESSO tests to ensure the reliability of the conclusion.

Results

The results suggested that higher household income tended to lower the risk of genetic susceptibility to Alzheimer's disease (odds ratio [OR]: 0.740, 95% confidence interval [CI] = 0.559-0.980, p-value = 0.036) and ischemic stroke (OR: 0.801, 95% CI = 0.662-0.968, p-value = 0.022). By contrast, higher household income tended to increase the risk of genetic susceptibility to Parkinson's disease (OR: 2.605, 95% CI = 1.413-4.802, p-value = 0.002). No associations were evident for intracranial hemorrhage (OR: 1.002, 95% CI = 0.607-1.653, p-value = 0.993), cerebral aneurysm (OR: 0.597, 95% CI = 0.243-1.465, p-value = 0.260), subarachnoid hemorrhage (OR: 1.474, 95% CI = 0.699-3.110, p-value = 0.308), or epilepsy (OR: 1.029, 95% CI = 0.662-1.600, p-value = 0.899). The reverse MR study suggested no reverse causal relationship between neurological disorders and household income status. A sensitivity analysis verified the reliability of the results.

Conclusion

Our results revealed that the populations with a superior household income exhibit an increased predisposition of genetic susceptibility to Parkinson's Disease, while demonstrating a potential decreased genetic susceptibility to ischemic stroke and Alzheimer's disease.

Life Experience Matters: Enrichment and Stress Can Influence the Likelihood of Developing Alzheimer's Disease via Gut Microbiome

Alzheimer's disease (AD) is a chronic neurodegenerative disease, characterized by the presence of β-amyloid (Aβ) plaques and neurofibrillary tangles (NFTs) formed from abnormally phosphorylated tau proteins (ptau). To date, there is no cure for AD. Earlier therapeutic efforts have focused on the clinical stages of AD. Despite paramount efforts and costs, pharmaceutical interventions including antibody therapies targeting Aβ have largely failed. This highlights the need to alternate treatment strategies and a shift of focus to early pre-clinical stages. Approximately 25-40% of AD cases can be attributed to environmental factors including chronic stress. Gut dysbiosis has been associated with stress and the pathogenesis of AD and can increase both Aβ and NFTs in animal models of the disease. Both stress and enrichment have been shown to alter AD progression and gut health. Targeting stress-induced gut dysbiosis through probiotic supplementation could provide a promising intervention to delay disease progression. In this review, we discuss the effects of stress, enrichment, and gut dysbiosis in AD models and the promising evidence from probiotic intervention studies.

Tarooneh extract relieves anxiety-like behaviors and cognitive deficits by inhibiting synaptic loss in the hippocampus and frontal cortex in rats subjected to chronic restraint stress

In traditional medicine, Tarooneh (a hardcover of the date palm; Phoenix dactylifera) has known as a sedative and relaxant medicine. In this study, we evaluated the protective effects of Tarooneh in the anxiety-like behavior, cognitive deficit, and neuronal damages in the CA1, CA3, and dentate gyrus (DG) regions of the hippocampus and frontal cortex neurons employing a rat model of chronic restraint stress. The animal received Tarooneh extract for 14 consecutive days in water, and chronic restraint stress was performed daily during this period. The results of the Barnes maze test showed that treatment with Tarooneh significantly improves spatial memory parameters such as latency time to find the target hole, number of errors, and distance traveling compared to the stress group. The EPM results showed that Tarooneh significantly increased the time spent in open arms and the percentage of entries into open arms and significantly decreased the frequency of head dipping behavior compared to animals in the stress group. Golgi-Cox staining indicates that loss of neural spine density in DG, CA1, CA3, and frontal cortex due to chronic restraint stress, was prevented with daily administration of Tarooneh. The results of cresyl-violet staining indicate that Tarooneh significantly increased the number of CV-positive neurons in the frontal cortex and CA1 region of the hippocampus compared to the stress group. Our results suggest that Tarooneh potentially prevented and improved effects in anxiety-like behavior, memory impairment, and synaptic plasticity loss in frontal and hippocampal neurons induced by chronic restraint stress. In conclusion, our results suggest that Tarooneh prevented and improved anxiety-like behavior, cognitive deficit, and neuronal damages in the CA1, CA3, and DG regions of the hippocampus and frontal cortex neurons induced by chronic restraint stress.

Understanding Alzheimer's disease in the context of aging: Findings from applications of stochastic process models to the Health and Retirement Study

There is growing literature on applications of biodemographic models, including stochastic process models (SPM), to studying regularities of age dynamics of biological variables in relation to aging and disease development. Alzheimer's disease (AD) is especially good candidate for SPM applications because age is a major risk factor for this heterogeneous complex trait. However, such applications are largely lacking. This paper starts filling this gap and applies SPM to data on onset of AD and longitudinal trajectories of body mass index (BMI) constructed from the Health and Retirement Study surveys and Medicare-linked data. We found that APOE e4 carriers are less robust to deviations of trajectories of BMI from the optimal levels compared to non-carriers. We also observed age-related decline in adaptive response (resilience) related to deviations of BMI from optimal levels as well as APOE- and age-dependence in other components related to variability of BMI around the mean allostatic values and accumulation of allostatic load. SPM applications thus allow revealing novel connections between age, genetic factors and longitudinal trajectories of risk factors in the context of AD and aging creating new opportunities for understanding AD development, forecasting trends in AD incidence and prevalence in populations, and studying disparities in those.

Understanding the relationships between physiological and psychosocial stress, cortisol and cognition

Stress is viewed as a state of real or perceived threat to homeostasis, the management of which involves the endocrine, nervous, and immune systems. These systems work independently and interactively as part of the stress response. The scientific stress literature, which spans both animal and human studies, contains heterogeneous findings about the effects of stress on the brain and the body. This review seeks to summarise and integrate literature on the relationships between these systems, examining particularly the roles of physiological and psychosocial stress, the stress hormone cortisol, as controlled by the hypothalamic-pituitary-adrenal (HPA) axis, and the effects of stress on cognitive functioning. Health conditions related to impaired HPA axis functioning and their associated neuropsychiatric symptoms will also be considered. Lastly, this review will provide suggestions of clinical applicability for endocrinologists who are uniquely placed to measure outcomes related to endocrine, nervous and immune system functioning and identify areas of intervention.

Early Chronic Stress Induced Changes within the Locus Coeruleus in Sporadic Alzheimer's Disease

Chronic exposure to stress throughout the lifespan has been the focus of many studies on Alzheimer's disease (AD) because of the similarities between the biological mechanisms involved in chronic stress and the pathophysiology of AD. In fact, the earliest abnormality associated with the disease is the presence of phosphorylated tau protein in locus coeruleus neurons, a brain structure highly responsive to stress and perceived threat. Here, we introduce allostatic load as a useful concept for understanding many of the complex, interacting neuropathological changes involved in the AD degenerative process. In response to chronic stress, aberrant tau proteins that begin to accumulate within the locus coeruleus decades prior to symptom onset appear to represent a primary pathological event in the AD cascade, triggering a wide range of interacting brain changes involving neuronal excitotoxicity, endocrine alterations, inflammation, oxidative stress, and amyloid plaque exacerbation. While it is acknowledged that stress will not necessarily be the major precipitating factor in all cases, early tau-induced changes within the locus coeruleus-norepinephrine pathway suggests that a therapeutic window might exist for preventative measures aimed at managing stress and restoring balance within the HPA axis.

Precision Medicine Approach to Alzheimer's Disease: Rationale and Implications

The neurodegenerative disease field has enjoyed extremely limited success in the development of effective therapeutics. One potential reason is the lack of disease models that yield accurate predictions and optimal therapeutic targets. Standard clinical trials have pre-determined a single treatment modality, which may be unrelated to the primary drivers of neurodegeneration. Recent proof-of-concept clinical trials using a precision medicine approach suggest a new model of Alzheimer's disease (AD) as a chronic innate encephalitis that creates a network insufficiency. Identifying and addressing the multiple potential contributors to cognitive decline for each patient may represent a more effective strategy. Here we review the rationale for a precision medicine approach in prevention and treatment of cognitive decline associated with AD. Results and implications from recent proof-of-concept clinical trials are presented. Randomized controlled trials, with much larger patient numbers, are likely to be significant to establishing precision medicine protocols as a standard of care for prevention and treatment of cognitive decline. Furthermore, combining this approach with the pharmaceutical approach offers the potential for enhanced outcomes. However, incorporating precision medicine approaches into everyday evaluation and care, as well as future clinical trials, would require fundamental changes in trial design, IRB considerations, funding considerations, laboratory evaluation, personalized treatment plans, treatment teams, and ultimately in reimbursement guidelines. Nonetheless, precision medicine approaches to AD, based on a novel model of AD pathophysiology, offer promise that has not been realized to date with monotherapeutic approaches.

Early Life Stress Enhances Cognitive Decline and Alters Synapse Function and Interneuron Numbers in Young Male APP/PS1 Mice

BACKGROUND

Exposure to stress early in life increases the susceptibility to Alzheimer's disease (AD) pathology in aged AD mouse models. So far, the underlying mechanisms have remained elusive.

OBJECTIVE

To investigate 1) effects of early life stress (ELS) on early functional signs that precede the advanced neuropathological changes, and 2) correlate synaptosomal protein content with cognition to identify neural correlates of AD.

METHODS

APPswe/PS1dE9 mice and littermates were subjected to ELS by housing dams and pups with limited bedding and nesting material from postnatal days 2-9. At 3 months of age, an age where no cognitive loss or amyloid-β (Aβ) pathology is typically reported in this model, we assessed hippocampal Aβ pathology, synaptic strength and synapse composition and interneuron populations. Moreover, cognitive flexibility was assessed and correlated with synaptosomal protein content.

RESULTS

While ELS did not affect Aβ pathology, it increased synaptic strength and decreased the number of calretinin+ interneurons in the hippocampal dentate gyrus. Both genotype and condition further affected the level of postsynaptic glutamatergic protein content. Finally, APP/PS1 mice were significantly impaired in cognitive flexibility at 3 months of age, and ELS exacerbated this impairment, but only at relatively high learning criteria.

CONCLUSIONS

ELS reduced cognitive flexibility in young APP/PS1 mice and altered markers for synapse and network function. These findings at an early disease stage provide novel insights in AD etiology and in how ELS could increase AD susceptibility.

Changes in personality traits in patients with Alzheimer's Disease

Changes in personality traits in patients with Alzheimer's disease (AD) are extremely common throughout the course of the pathology, and these behavioral changes present themselves as challenges in clinical management and as a significant cause of caregivers’ burden.

Chronic Stress Induces Hippocampal Mitochondrial Damage in APPPS1 Model Mice and Wildtype Littermates

BACKGROUND

Alzheimer's disease (AD) is a neurodegenerative disorder and the most common cause of dementia worldwide. Despite decades of investigation, the etiology of AD is not fully understood, although emerging evidence suggest that chronic environmental and psychological stress plays a role in the mechanisms and contributes to the risk of developing AD. Thus, dissecting the impact of stress on the brain could improve our understanding of the pathological mechanisms.

OBJECTIVE

We aimed to study the effect of chronic stress on the hippocampal proteome in male APPPS1 transgenic mice and wildtype (WT) littermates.

METHODS

APPPS1 and WT mice were subjected to 4 weeks of chronic stress followed by 3 weeks of continued diurnal disruption. Hippocampal tissue was used for proteomics analysis using label-free quantitative DIA based LC-MS/MS analysis.

RESULTS

We identified significantly up- and downregulated proteins in both APPPS1 and WT mice exposed to chronic stress compared to the control groups. Via interaction network mapping, significant proteins could be annotated to specific pathways of mitochondrial function (oxidative phosphorylation and TCA cycle), metabolic pathways, AD pathway and synaptic functions (long term potentiation). In WT mice, chronic stress showed the highest impact on complex I of the oxidative phosphorylation pathway, while in APPPS1 mice this pathway was compromised broadly by chronic stress.

CONCLUSION

Our data shows that chronic stress and amyloidosis additively contribute to mitochondrial damage in hippocampus. Although these results do not explain all effects of chronic stress in AD, they add to the scientific knowledge on the topic.

Childhood Stressors, Relationship Quality, and Cognitive Health in Later Life

OBJECTIVES

The rising prevalence of cognitive impairment, Alzheimer's disease, and related disorders signals the need for a better understanding of how social factors may affect cognitive health for millions of Americans. Drawing from cumulative inequality theory, we aim to understand the implications of a stressful childhood on social relationships and cognitive health in later life.

METHODS

This study utilizes longitudinal data (2006-2016) from the Health and Retirement Study to examine pathways, both direct and indirect through social relationships in adulthood, from childhood stressors to cognitive health trajectories over time.

RESULTS

Respondents reporting a greater number of stressors in childhood had worse cognitive health over time, but those negative effects were not as steep as time progressed. Early-life stressors are also associated with less social support and more social strain in adulthood which, in turn, are associated with initial cognitive health. Finally, pathway analyses confirm that childhood stressors are indirectly associated with initial cognitive health through social strain and social support.

DISCUSSION

Findings reveal that a stressful childhood creates chains of risks that have lifelong implications for cognitive health, both directly and indirectly by creating obstacles for developing healthy and supportive social relationships.

Health effects following exposure to dust from the World Trade Center disaster: An update

Exposure to dust, smoke, and fumes containing volatile chemicals and particulate matter (PM) from the World Trade Center (WTC) towers' collapse impacted thousands of citizens and first responders (FR; firefighters, medicals staff, police officers) of New York City. Surviving FR and recovery workers are increasingly prone to age-related diseases that their prior WTC dust exposures might expedite or make worse. This review provides an overview of published WTC studies concerning FR/recovery workers' exposure and causal mechanisms of age-related disease susceptibility, specifically those involving the cardiopulmonary and neurological systems. This review also highlights the recent findings of the major health effects of cardiovascular, pulmonary, and neurological health sequelae from WTC dust exposure. To better treat those that risked their lives during and after the disaster of September 11, 2001, the deleterious mechanisms that WTC dust exposure exerted and continue to exert on the heart, lungs, and brain of FR must be better understood.

Neurological Disorders in an Elderly Cohort Experienced Past Stressful Events: A Retrospective-prospective Study

BACKGROUND

Psychological stress may be a risk factor for dementia, but the association between exposure to stressful life events and the development of cognitive dysfunction has not been conclusively demonstrated. We hypothesize that if a stressful event has an impact on the subjects, its effects would be different in the three diseases.

OBJECTIVE

This study aims to assess the effects of stressful events in senior patients who later developed ischemic stroke, Alzheimer's, or Parkinson's disease.

MATERIAL AND METHODS

Together with demographic variables (age, sex, race, socioeconomic and cultural levels), five types of past stressful events, such as death or serious illness of close relatives, job dismissal, change of financial status, retirement, and change of residence, were recorded in 1024 patients with Alzheimer's disease, Parkinson's disease, and ischemic stroke. Time-todiagnosis (months from the event to the first symptoms: retrospective study) and evolution time (years of follow-up of each patient: prospective study) were recorded. The variance and nonparametric methods were analyzed to the variables time-to-diagnosis and evolution time to analyze differences between these diseases.

RESULTS

The demographic variables, such as age, sex, race, economic and cultural levels, were found to be statistically non-significant; differences in the economic level were significant (P<0.05). Significant differences (P<0.001) were found in the mean time-to-diagnosis between diseases (Alzheimer's disease>Parkinson's disease >Stroke), and minor differences (P<0.05) in evolution time.

CONCLUSION

Differences in time-to-diagnosis between the diseases indicate that the stressful effect of having experienced the death or serious illness of a close relative has an impact on their emergence. The measurement of time-to-diagnosis and evolution time proves useful in detecting differences between diseases.

Effect of Global Brain Ischemia on Amyloid Precursor Protein Metabolism and Expression of Amyloid-Degrading Enzymes in Rat Cortex: Role in Pathogenesis of Alzheimer's Disease

The incidence of Alzheimer's disease (AD) increases significantly following chronic stress and brain ischemia which, over the years, cause accumulation of toxic amyloid species and brain damage. The effects of global 15-min ischemia and 120-min reperfusion on the levels of expression of the amyloid precursor protein (APP) and its processing were investigated in the brain cortex (Cx) of male Wistar rats. Additionally, the levels of expression of the amyloid-degrading enzymes neprilysin (NEP), endothelin-converting enzyme-1 (ECE-1), and insulin-degrading enzyme (IDE), as well as of some markers of oxidative damage were assessed. It was shown that the APP mRNA and protein levels in the rat Cx were significantly increased after the ischemic insult. Protein levels of the soluble APP fragments, especially of sAPPβ produced by β-secretase, (BACE-1) and the levels of BACE-1 mRNA and protein expression itself were also increased after ischemia. The protein levels of APP and BACE-1 in the Cx returned to the control values after 120-min reperfusion. The levels of NEP and ECE-1 mRNA also decreased after ischemia, which correlated with the decreased protein levels of these enzymes. However, we have not observed any changes in the protein levels of insulin-degrading enzyme. Contents of the markers of oxidative damage (di-tyrosine and lysine conjugates with lipid peroxidation products) were also increased after ischemia. The obtained data suggest that ischemia shifts APP processing towards the amyloidogenic β-secretase pathway and accumulation of the neurotoxic Aβ peptide as well as triggers oxidative stress in the cells. These results are discussed in the context of the role of stress and ischemia in initiation and progression of AD.

Interplay between stress-related genes may influence Alzheimer's disease development: The results of genetic interaction analyses of human data

Emerging evidence from experimental and clinical research suggests that stress-related genes may play key roles in AD development. The fact that genome-wide association studies were not able to detect a contribution of such genes to AD indicates the possibility that these genes may influence AD non-linearly, through interactions of their products. In this paper, we selected two stress-related genes (GCN2/EIF2AK4 and APP) based on recent findings from experimental studies which suggest that the interplay between these genes might influence AD in humans. To test this hypothesis, we evaluated the effects of interactions between SNPs in these two genes on AD occurrence, using the Health and Retirement Study data on white indidividuals. We found several interacting SNP-pairs whose associations with AD remained statistically significant after correction for multiple testing. These findings emphasize the importance of nonlinear mechanisms of polygenic AD regulation that cannot be detected in traditional association studies. To estimate collective effects of multiple interacting SNP-pairs on AD, we constructed a new composite index, called Interaction Polygenic Risk Score, and showed that its association with AD is highly statistically significant. These results open a new avenue in the analyses of mechanisms of complex multigenic AD regulation.

The long-term consequences of trauma and posttraumatic stress disorder symptoms on later life cognitive function and dementia risk

Stress may be a risk factor for dementia, however it is unknown whether post-traumatic stress disorder (PTSD) symptoms are associated with incident dementia in community-dwelling older individuals. The aim was to determine whether lifetime major trauma with and without re-experiencing of PTSD symptoms is associated with later-life cognition and dementia risk. Participants were 1,700 community-dwelling older adults (65+) in the longitudinal ESPRIT study followed over 14 years. Lifetime major traumatic exposure and PTSD were assessed using Watson's PTSD Inventory. Cognitive tests assessed global cognition, visual memory, verbal fluency, psychomotor speed and executive function. Incident dementia was diagnosed according to DSM-IV criteria. Lifetime major trauma (versus no trauma) was associated with significantly increased executive function and increased global function in men, however women with lifetime trauma and re-experiencing symptoms had a significantly increased risk of low global cognition. Over 14 years, lifetime trauma without re-experiencing symptoms was associated with a significantly decreased risk of incident dementia, particularly for women. Lifetime major trauma without re-experiencing symptoms (but not with) may be protective for later life cognitive function. However, the mechanisms and moderating factors underlying these association requires further investigation.

Does sauna bathing protect against dementia?

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Frequent sauna bathing predicted decreased risk of dementia in a cohort from Finland.

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Result was independent of several dementia risk factors, and was not modified by sex.

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Findings support suggested benefits of sauna and passive body heating in the brain.

Repeated heat exposure like sauna bathing is suggested to beneficially affect against dementia development. The epidemiological evidence is, however, scarce. Therefore, we studied the association between heat exposure during sauna bathing (i.e., the frequency of sauna bathing, frequency of heat sessions, length of stay in heat, sauna temperature) and the subsequent risk of dementia. A prospective cohort study was conducted based on 13,994 men and women aged 30–69 and free from dementia diagnosis from the Finnish Mobile Clinic Follow-up Survey. During a follow-up of 39 years, a total of 1805 dementia patients were diagnosed. The sauna bathing data was gathered from a questionnaire. Analyses based on the Cox model included the sauna bathing variables and the potential confounding factors. Sauna bathing frequency was related to a reduced risk of dementia after adjustment for the potential sociodemographic, lifestyle, and metabolic risk factors of dementia considered. The hazard ratio of dementia between individuals sauna bathing 9–12 times per month in comparison with those not sauna bathing or sauna bathing less than four times per month was 0.47 (95% CI = 0.25–0.88) during the first 20 years of follow-up and 0.81 (95% CI = 0.69–0.97) during the whole follow-up. The results are in line with the hypothesis that sauna bathing provides protection against dementia. Further studies are required to verify the suggested benefits of sauna bathing.

Psychological Stress-Induced Immune Response and Risk of Alzheimer's Disease in Veterans from Operation Enduring Freedom and Operation Iraqi Freedom

PURPOSE

Psychological stress is a significant health problem in veterans and their family members. Traumatic brain injury (TBI) and stress lead to the onset, progression, and worsening of several inflammatory and neurodegenerative diseases in veterans and civilians. Alzheimer's disease (AD) is a progressive, irreversible neuroinflammatory disease that causes problems with memory, thinking, and behavior. TBIs and chronic psychological stress cause and accelerate the pathology of neuroinflammatory diseases such as AD. However, the precise molecular and cellular mechanisms governing neuroinflammation and neurodegeneration are currently unknown, especially in veterans. The purpose of this review article was to advance the hypothesis that stress and TBI-mediated immune response substantially contribute and accelerate the pathogenesis of AD in veterans and their close family members and civilians.

METHODS

The information in this article was collected and interpreted from published articles in PubMed between 1985 and 2020 using the key words stress, psychological stress, Afghanistan war, Operation Enduring Freedom (OEF), Iraq War, Operation Iraqi Freedom (OIF), Operation New Dawn (OND), traumatic brain injury, mast cell and stress, stress and neuroimmune response, stress and Alzheimer's disease, traumatic brain injury, and Alzheimer's disease.

FINDINGS

Chronic psychological stress and brain injury induce the generation and accumulation of beta-amyloid peptide, amyloid plaques, neurofibrillary tangles, and phosphorylation of tau in the brain, thereby contributing to AD pathogenesis. Active military personnel and veterans are under enormous psychological stress due to various war-related activities, including TBIs, disabilities, fear, new environmental conditions, lack of normal life activities, insufficient communications, explosions, military-related noise, and health hazards. Brain injury, stress, mast cell, and other immune cell activation can induce headache, migraine, dementia, and upregulate neuroinflammation and neurodegeneration in veterans of Operation Enduring Freedom, Operation Iraqi Freedom, and Operation New Dawn. TBIs, posttraumatic stress disorder, psychological stress, pain, glial activation, and dementia in active military personnel, veterans, or their family members can cause AD several years later in their lives. We suggest that there are increasing numbers of veterans with TBIs and stress and that these veterans may develop AD late in life if no appropriate therapeutic intervention is available.

IMPLICATIONS

Per these published reports, the fact that TBIs and psychological stress can accelerate the pathogenesis of AD should be recognized. Active military personnel, veterans, and their close family members should be evaluated regularly for stress symptoms to prevent the pathogenesis of neurodegenerative diseases, including AD.

Brain Injury-Mediated Neuroinflammatory Response and Alzheimer's Disease

Traumatic brain injury (TBI) is a major health problem in the United States, which affects about 1.7 million people each year. Glial cells, T-cells, and mast cells perform specific protective functions in different regions of the brain for the recovery of cognitive and motor functions after central nervous system (CNS) injuries including TBI. Chronic neuroinflammatory responses resulting in neuronal death and the accompanying stress following brain injury predisposes or accelerates the onset and progression of Alzheimer's disease (AD) in high-risk individuals. About 5.7 million Americans are currently living with AD. Immediately following brain injury, mast cells respond by releasing prestored and preactivated mediators and recruit immune cells to the CNS. Blood-brain barrier (BBB), tight junction and adherens junction proteins, neurovascular and gliovascular microstructural rearrangements, and dysfunction associated with increased trafficking of inflammatory mediators and inflammatory cells from the periphery across the BBB leads to increase in the chronic neuroinflammatory reactions following brain injury. In this review, we advance the hypothesis that neuroinflammatory responses resulting from mast cell activation along with the accompanying risk factors such as age, gender, food habits, emotional status, stress, allergic tendency, chronic inflammatory diseases, and certain drugs can accelerate brain injury-associated neuroinflammation, neurodegeneration, and AD pathogenesis.

Early-Onset Dementia in War Veterans: Brain Polypathology and Clinicopathologic Complexity

The neuropathology associated with cognitive decline in military personnel exposed to traumatic brain injury (TBI) and chronic stress is incompletely understood. Few studies have examined clinicopathologic correlations between phosphorylated-tau neurofibrillary tangles, β-amyloid neuritic plaques, neuroinflammation, or white matter (WM) lesions, and neuropsychiatric disorders in veterans. We describe clinicopathologic findings in 4 military veterans with early-onset dementia (EOD) who had varying histories of blunt- and blast-TBI, cognitive decline, behavioral abnormalities, post-traumatic stress disorder, suicidal ideation, and suicide. We found that pathologic lesions in these military-EOD cases could not be categorized as classic Alzheimer's disease (AD), chronic traumatic encephalopathy, traumatic axonal injury, or other well-characterized clinicopathologic entities. Rather, we observed a mixture of polypathology with unusual patterns compared with pathologies found in AD or other dementias. Also, ultrahigh resolution ex vivo MRI in 2 of these 4 brains revealed unusual patterns of periventricular WM injury. These findings suggest that military-EOD cases are associated with atypical combinations of brain lesions and distribution rarely seen in nonmilitary populations. Future prospective studies that acquire neuropsychiatric data before and after deployments, as well as genetic and environmental exposure data, are needed to further elucidate clinicopathologic correlations in military-EOD.