Increased tau phosphorylation and aggregation in the hippocampus of mice overexpressing corticotropin-releasing factor

Application of the occupational health risk assessment model in predicting the risk of noise-induced hearing loss in noise-polluted manufacturers in southern China

Objective. This study aimed to evaluate the correlation between occupational noise-induced hearing loss (ONIHL) and noise exposure in different positions within manufacturing facilities that primarily produce electronic devices, wooden furniture and hardware machinery in southern China and to investigate the practical application of the risk assessment model. Methods. Noise exposure data were collected from 115 workers in 10 manufacturing industries in southern China through on-site investigation. Subsequently, sample testing was conducted in accordance with Chinese standards. The 31 job positions investigated were classified into two similar exposure groups (SEGs). The occupational health risk assessment (OHRA) model was used to evaluate the risks of ONIHL in various job positions. Results. More than half of the positions in both groups had noise levels exceeding 85 dB. In both SEGs, male workers exhibited similar risk patterns of acquiring hearing impairment, with a more pronounced and earlier risk of hearing loss compared to female workers. Age was also found to be a risk factor for hearing impairments. Conclusion. The ORHA model demonstrated a positive correlation between noise exposure levels and NIHL. This model can help employers assess job management with noise exposure and take measures to implement an effective risk management system.

Stress induced exacerbation of Alzheimer's disease brain pathology is thwarted by co-administration of nanowired cerebrolysin and monoclonal amyloid beta peptide antibodies with serotonin 5-HT6 receptor antagonist SB-399885

Alzheimer's disease is one of the devastating neurodegenerative diseases affecting mankind worldwide with advancing age mainly above 65 years and above causing great misery of life. About more than 7 millions are affected with Alzheimer's disease in America in 2023 resulting in huge burden on health care system and care givers and support for the family. However, no suitable therapeutic measures are available at the moment to enhance quality of life to these patients. Development of Alzheimer's disease may reflect the stress burden of whole life inculcating the disease processes of these neurodegenerative disorders of the central nervous system. Thus, new strategies using nanodelivery of suitable drug therapy including antibodies are needed in exploring neuroprotection in Alzheimer's disease brain pathology. In this chapter role of stress in exacerbating Alzheimer's disease brain pathology is explored and treatment strategies are examined using nanotechnology based on our own investigation. Our observations clearly show that restraint stress significantly exacerbate Alzheimer's disease brain pathology and nanodelivery of a multimodal drug cerebrolysin together with monoclonal antibodies (mAb) to amyloid beta peptide (AβP) together with a serotonin 5-HT6 receptor antagonist SB399885 significantly thwarted Alzheimer's disease brain pathology exacerbated by restraint stress, not reported earlier. The possible mechanisms and future clinical significance is discussed.

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.

Animal Models of Cushing's Syndrome

Endogenous Cushing's syndrome is characterized by unique clinical features and comorbidities, and progress in the analysis of its genetic pathogenesis has been achieved. Moreover, prescribed glucocorticoids are also associated with exogenous Cushing's syndrome. Several animal models have been established to explore the pathophysiology and develop treatments for Cushing's syndrome. Here, we review recent studies reporting animal models of Cushing's syndrome with different features and complications induced by glucocorticoid excess. Exogenous corticosterone (CORT) administration in drinking water is widely utilized, and we found that CORT pellet implantation in mice successfully leads to a Cushing's phenotype. Corticotropin-releasing hormone overexpression mice and adrenal-specific Prkar1a-deficient mice have been developed, and AtT20 transplantation methods have been designed to examine the medical treatments for adrenocorticotropic hormone-producing pituitary neuroendocrine tumors. We also review recent advances in the molecular pathogenesis of glucocorticoid-induced complications using animal models.

Targeting the cannabinoid system to counteract the deleterious effects of stress in Alzheimer’s disease

Alzheimer’s disease is a progressive neurodegenerative disorder characterized histologically in postmortem human brains by the presence of dense protein accumulations known as amyloid plaques and tau tangles. Plaques and tangles develop over decades of aberrant protein processing, post-translational modification, and misfolding throughout an individual’s lifetime. We present a foundation of evidence from the literature that suggests chronic stress is associated with increased disease severity in Alzheimer’s patient populations. Taken together with preclinical evidence that chronic stress signaling can precipitate cellular distress, we argue that chronic psychological stress renders select circuits more vulnerable to amyloid- and tau- related abnormalities. We discuss the ongoing investigation of systemic and cellular processes that maintain the integrity of protein homeostasis in health and in degenerative conditions such as Alzheimer’s disease that have revealed multiple potential therapeutic avenues. For example, the endogenous cannabinoid system traverses the central and peripheral neural systems while simultaneously exerting anti-inflammatory influence over the immune response in the brain and throughout the body. Moreover, the cannabinoid system converges on several stress-integrative neuronal circuits and critical regions of the hypothalamic-pituitary-adrenal axis, with the capacity to dampen responses to psychological and cellular stress. Targeting the cannabinoid system by influencing endogenous processes or exogenously stimulating cannabinoid receptors with natural or synthetic cannabis compounds has been identified as a promising route for Alzheimer’s Disease intervention. We build on our foundational framework focusing on the significance of chronic psychological and cellular stress on the development of Alzheimer’s neuropathology by integrating literature on cannabinoid function and dysfunction within Alzheimer’s Disease and conclude with remarks on optimal strategies for treatment potential.

Differential effects of chronic stress on anxiety-like behavior and contextual fear conditioning in the TgF344-AD rat model of Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative brain disorder that leads to severe cognitive and functional impairments. Many AD patients also exhibit neuropsychiatric symptoms, such as anxiety and depression, prior to the clinical diagnosis of dementia. Chronic stress is associated with numerous adverse health consequences and disease states, and AD patients exhibit altered stress systems. Thus, stress may represent a causal link between neuropsychiatric symptoms and AD. To address this possibility, we examined the effects of chronic stress in the TgF344-AD rat model that co-expresses the mutant human amyloid precursor protein (APPsw) and presenilin 1 (PS1ΔE9) genes. Adult male transgenic (Tg+) and wild-type (WT) rats (6-7.5 months of age), with and without a history of chronic restraint stress, were tested for footshock-induced conditioned fear and for anxiety-like behavior in the elevated plus-maze. We found that non-stressed Tg+ rats showed increased anxiety-like behavior compared to non-stressed WT rats. In contrast, Tg+ and WT rats did not differ in levels of freezing immediately following footshock or during contextual re-exposure. Additionally, stressed Tg+ rats were not significantly different from stressed WT rats on any measures of anxiety or fear. Thus, while stress has been linked as a risk factor for AD-related pathology, it appears from the present findings that two weeks of daily restraint stress did not further enhance anxiety- or fear-like behaviors in TgF344-AD rats.

Chronic stress and Alzheimer's disease

Stress is a key factor in the development and progress of diseases. In neurodegenerative conditions, stress management can play an important role in maintaining the quality of life and the capacity to improve. Neurodegenerative diseases, including Alzheimer's disease, cause the motor and cognitive malfunctions that are spontaneously stressful and also can disturb the neural circuits that promote stress responses. The interruption of those circuits leads to aggressive and inappropriate behavior. In addition, stress contributes to illness and may exacerbate symptoms. In this review, we present stress-activated neural pathways involved in Alzheimer's disease from a clinical and experimental point of view, as well as supportive drugs and therapies.

Trait anxiety, a personality risk factor associated with Alzheimer's Disease

Alzheimer's disease (AD) is the most common neurodegenerative disease in elderly population and the leading cause of dementia worldwide. While senile plaques and neurofibrillary tangles have been proposed as the principal histopathologic hallmarks of AD, the exact etiology of this disease is still far from being clearly understood. AD has been recognized as pathological consequences of complex interactions among genetic, aging, medical, life style and psychosocial factors. Recently, the roles of neuroticism personality traits in AD incidence and progression have come into focus. More specifically, increasing evidence has further shown that the trait anxiety, one major component of neuroticism predicting the individual vulnerability in response to stress, is a risk factor for AD and may correlated with various AD pathologies. In this review, we summarized recent literature on the association of trait anxiety with AD. We also discussed the possible neuroendocrinological and neurochemical mechanisms of this association, which may provide clinical implications for AD diagnosis and therapy.

Vaccinium bracteatum Improves Spatial Learning and Memory by Regulating N-methyl-D-aspartate Receptors and Tau Phosphorylation in Chronic Restraint Stress-Induced Memory Impaired Mice

Vaccinium bracteatum Thunb. Leaves (VBL) are a component of traditional herbal medicines. However, molecular mechanisms of VBL in stress-related memory impairment are still unclear. This study aimed to investigate the spatial memory improvement effects of VBL in an animal model of chronic restraint stress (CRS) by using Y maze test and identified possible protective mechanisms against oxidative stress inducers (e.g., corticosterone and hydrogen peroxide [H2O2]) in SH-SY5Y neuronal cells. VBL showed neuroprotective effects via reduced release of lactate dehydrogenase (LDH) in corticosterone or H2O2-induced cell death that was mediated through the regulation of cleaved caspase-3 and Nrf2 pathways. Furthermore, CRS-exposed mice were orally administered VBL (10, 50, 100, and 200 mg/kg) daily for 21 days. CRS-exposed mice treated with VBL showed significantly increased spontaneous alternation in short-term memory (STM) and long-term memory (LTM) trials, and number of total arm entries in LTM trials as measured by the Y maze test. Moreover, VBL (50, 100, and 200 mg/kg) decreased acetylcholinesterase (AChE) activity in the hippocampus (HC, [Formula: see text] ¡ 0.01 and [Formula: see text] ¡ 0.001, respectively) and prefrontal cortex (PFC). CRS-exposed mice treated with VBL had dramatically decreased total Tau and Tau phosphorylation in the synapse of the HC and PFC which might be mediated by the regulation of CaMKII and GSK3[Formula: see text] phosphorylation. Additionally, VBL reduced CRS-induced upregulation of N-methyl-D-aspartate (NMDA) receptor subunits (NMDAR1, 2A, and 2B). Thus, VBL exerts spatial memory improvement by regulating CRS-induced NMDA receptor neurotoxicity and Tau hyperphosphorylation.

Reproductive status impact on tau phosphorylation induced by chronic stress

Sex and exposure to chronic stress have been identified as risk factors for developing Alzheimer's disease (AD). Although AD has been demonstrated to be more prevalent in females, sex is often overlooked in research studies, likely due to the complexity of the hormonal status. In female rats, the reproductive status can modulate the well-known increase in tau phosphorylation (pTau) caused by the exposure to acute physical and psychological stressors. To test the hypothesis that reproductive status can impact hippocampal pTau induced by chronic stress, cohorts of virgin, lactating (4–5 days pp), and post-maternal (1-month post-weaned) rats were subjected to a daily 30-min episode of restraint stress for 14 days and were sacrificed either 20 min or 24 h after their last stress/handling episode. Western blot analysis of two well-characterized AD-relevant pTau epitopes (AT8 and PHF-1) and upstream pTau mechanisms (e.g. GSK3β) analysis, showed that stressed post-maternal rats have increased pTau in comparison to stressed lactating rats 20 min after their last stress episode. Furthermore, an increase in pTau was also seen 24 h after the last stress episode in stressed post-maternal rats in comparison to their non-stressed controls in the detergent-soluble fraction. GSK3 analysis showed an increase in total levels of GSK3β in virgin rats and an increase of inactive levels of GSK3β in post-maternal rats, which suggests a different stress response in pTau after the rat has gone through the maternal experience. Interestingly, post-maternal rats also presented the more variability in their estrous cycles in response to stress. Besides no differences in pTau, non-stressed lactating rats showed an increase in inactive GSK3β 24 h after the last handling episode. Immunohistochemical detection of the PHF-1 epitope revealed increased pTau in the CA4/hilar subfield of the hippocampus of virgin and post-maternal rats exposed to chronic stress shortly after their last stress episode. Overall, lactating rats remained unresponsive to chronic restraint stress. These results suggest increased sensitivity of the virgin and post-maternal rats to hippocampal stress-induced pTau with chronic restraint stress compared to lactating rats. Because no differences were detected in response to stress by lactating rats and an exaggerated response was observed in post-maternal rats, current results support the hypothesis that lactation affects tau processing in the brain of the female.

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pTau increases in the hippocampus of stressed virgin and especially post-maternal rats but not in that of lactating dams.

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The hippocampal area CA4 of virgin and post-maternal rats is most affected by the chronic restraint stress.

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GSK3β overall levels and activity are modified by the reproductive condition and stress.

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Reproductive experience modifies pTau induced by chronic stress.

Corticotropin releasing factor-binding protein (CRF-BP) as a potential new therapeutic target in Alzheimer's disease and stress disorders

Alzheimer's disease is the most common cause of dementia and one of the most complex human neurodegenerative diseases. Numerous studies have demonstrated a critical role of the environment in the pathogenesis and pathophysiology of the disease, where daily life stress plays an important role. A lot of epigenetic studies have led to the conclusion that chronic stress and stress-related disorders play an important part in the onset of neurodegenerative disorders, and an enormous amount of research yielded valuable discoveries but has so far not led to the development of effective treatment strategies for Alzheimer's disease. Corticotropin-releasing factor (CRF) is one of the major hormones and at the same time a neuropeptide acting in stress response. Deregulation of protein levels of CRF is involved in the pathogenesis of Alzheimer's disease, but little is known about the precise roles of CRF and its binding protein, CRF-BP, in neurodegenerative diseases. In this review, we summarize the key evidence for and against the involvement of stress-associated modulation of the CRF system in the pathogenesis of Alzheimer's disease and discuss how recent findings could lead to new potential treatment possibilities in Alzheimer's disease by using CRF-BP as a therapeutic target.

Is AD a Stress-Related Disorder? Focus on the HPA Axis and Its Promising Therapeutic Targets

Alzheimer’s disease (AD) is a progressive neurodegenerative disorder that has important health and economic impacts in the elderly. Despite a better understanding of the molecular mechanisms leading to the appearance of major pathological hallmarks (senile plaques and neurofibrillary tangles), effective treatments are still lacking. Sporadic AD forms (98% of all cases) are multifactorial, and a panoply of risk factors have been identified. While the major risk factor is aging, growing evidence suggests that chronic stress or stress-related disorders increase the probability to develop AD. An early dysregulation of the hypothalamic-pituitary-adrenal axis (HPA axis or stress axis) has been observed in patients. The direct consequence of such perturbation is an oversecretion of glucocorticoids (GC) associated with an impairment of its receptors (glucocorticoid receptors, GR). These steroids hormones easily penetrate the brain and act in synergy with excitatory amino acids. An overexposure could be highly toxic in limbic structures (prefrontal cortex and hippocampus) and contribute in the cognitive decline occurring in AD. GC and GR dysregulations seem to be involved in lots of functions disturbed in AD and a vicious cycle appears, where AD induces HPA axis dysregulation, which in turn potentiates the pathology. This review article presents some preclinical and clinical studies focusing on the HPA axis hormones and their receptors to fight AD. Due to its primordial role in the maintenance of homeostasis, the HPA axis appears as a key-actor in the etiology of AD and a prime target to tackle AD by offering multiple angles of action.

Alzheimer's Disease and Its Potential Alternative Therapeutics

Alzheimer’s Disease (AD) is a chronic neurodegenerative disease that affects over 5 million individuals in the United States alone. Currently, there are only two kinds of pharmacological interventions available for symptomatic relief of AD; Acetyl Cholinesterase Inhibitors (AChEI) and N-methyl-D-aspartic Acid (NMDA) receptor antagonists and these drugs do not slow down or stop the progression of the disease. Several molecular targets have been implicated in the pathophysiology of AD, such as the tau (τ) protein, Amyloid-beta (Aβ), the Amyloid Precursor Protein (APP) and more and several responses have also been observed in the advancement of the disease, such as reduced neurogenesis, neuroinflammation, oxidative stress and iron overload. In this review, we discuss general features of AD and several small molecules across different experimental AD drug classes that have been studied for their effects in the context of the molecular targets and responses associated with the AD progression. These drugs include: Paroxetine, Desferrioxamine (DFO), N-acetylcysteine (NAC), Posiphen/-(−)Phenserine, JTR-009, Carvedilol, LY450139, Intravenous immunoglobulin G 10%, Indomethacin and Lithium Carbonate (Li₂CO₃).

Localization of amyloid beta peptides to locus coeruleus and medial prefrontal cortex in corticotropin releasing factor overexpressing male and female mice

A culmination of evidence from the literature points to the Locus Coeruleus (LC)-Norepinephrine system as an underappreciated and understudied area of research in the context of Alzheimer's Disease (AD). Stress is a risk factor for developing AD, and is supported by multiple clinical and preclinical studies demonstrating that amplification of the stress system disrupts cellular and molecular processes at the synapse, promoting the production and accumulation of the amyloid beta (Aβ42) peptide. Stress-induced activation of the LC is mediated by corticotropin releasing factor (CRF) and CRF receptors exhibit sex-biased stress signaling. Sex differences are evident in the neurochemical, morphological and molecular regulation of LC neurons by CRF, providing a compelling basis for the higher prevalence of stress-related disorders such as AD in females. In the present study, we examined the cellular substrates for interactions between Aβ and tyrosine hydroxylase a marker of noradrenergic somatodendritic processes in the LC, and Dopamine-β-Hydroxylase (DβH) in the infralimbic medial prefrontal cortex (ILmPFC) in mice conditionally overexpressing CRF in the forebrain (CRFOE) under a Doxycycline (DOX) regulated tetO promoter. CRFOE was sufficient to elicit a redistribution of Aβ peptides in the somatodendritic processes of the LC of male and female transgenic mice, without altering total Aβ42 protein expression levels. DOX treated groups exhibited lysosomal compartments with apparent lipofuscin and abnormal morphology, indicating potential dysfunction of these Aβ42-clearing compartments. In female DOX treated groups, swollen microvessels with lipid-laden vacuoles were also observed, a sign of blood-brain-barrier dysfunction. Finally, sex differences were observed in the prefrontal cortex, as females responded to DOX treatment with increased frequency of co-localization of Aβ42 and DβH in noradrenergic axon terminals compared to vehicle treated controls, while male groups showed no significant changes. We hypothesize that the observed sex differences in Aβ42 distribution in this model of CRF hypersignaling is based on increased responsivity of female rodent CRFR1 in the LC. Aβ42 production is enhanced during increased neuronal activation, therefore, the excitation of DOX treated female CRFOE LC neurons projecting to the mPFC may exhibit more frequent co-localization with Aβ due to increased neuronal activity of noradrenergic neurons.

Memantine Differentially Regulates Tau Phosphorylation Induced by Chronic Restraint Stress of Varying Duration in Mice

Exposure to chronic psychiatric stress has been linked to Alzheimer's disease-related tau hyperphosphorylation and abnormalities in glutamate neurotransmission. However, the pathological relationship between glutamatergic dysfunction and tau phosphorylation in the cerebral cortex under chronic psychiatric stress is not fully understood. The present study investigated the effects of memantine (MEM, 5 and 10 mg/kg), an uncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist, on chronic restraint stress- (CRS-) induced tau phosphorylation in mice. CRS administered for 16 or 28 consecutive days (1 h daily) induced significant tau phosphorylation in the brain. MEM treatment suppressed the elevation of phosphorylated tau (P-tau) levels induced by 16-day CRS in a dose-dependent manner. P-tau reduction was accompanied by the attenuation of the upregulation of GSK3β and CDK5 expression and the downregulation of PP2A activity induced by CRS. Additionally, MEM reduced CRS-induced upregulation of NMDA receptor subunit levels (GluN2A, GluN2B) in the frontal cortex. However, MEM markedly enhanced tau phosphorylation in the frontal cortex and other cerebral cortical regions following 28 days of CRS. The stimulatory effect of MEM on CRS-induced tau phosphorylation was correlated with increased activities of AKT, JNK, and GSK3β, inactivation of PP2A, and downregulation of Pin1 and HSP70. Moreover, MEM did not effectively reverse the NMDA receptor upregulation induced by 28-day CRS and even increased GluN2B subunit levels. In contrast to the duration-dependent effects of MEM on P-tau levels, MEM produced an anxiolytic effect in both regimens as revealed by elevated plus maze testing. However, MEM did not affect the body weight reduction induced by CRS. Thus, MEM exerts distinctive effects on CRS-induced tau phosphorylation, which might be related to the expression of GluN2B. The differential effects of MEM on P-tau levels have crucial implications for its clinical application.

Tauopathy and neurodegeneration: A role for stress

Neurodegenerative diseases are characterized by an irreversible and progressive loss of neuronal structure and function. While many alterations to normal cellular processes occur during neurodegeneration, a pathological accumulation of aggregated proteins constitutes a hallmark of several neurodegenerative disorders. Alzheimer's disease, specifically, is pathologically defined by the formation of amyloid plaques and tangles of hyperphosphorylated tau protein. Stress has emerged as an important factor in the development and progression of neurodegenerative diseases, including Alzheimer's. Very little is known, however, regarding the effects of stress on the mechanisms controlling abnormal protein aggregation and clearance. Chronic stress activates the hypothalamic-pituitary-adrenal (HPA) axis, causing an excessive secretion of glucocorticoids that are capable of impacting diverse physiological and cellular processes. The present review focuses on the influence of stress on a key feature of Alzheimer's disease pathology, emphasizing the relationship between tau phosphorylation and accumulation and its connection to HPA axis dysfunction.

Early Life Stress and Epigenetics in Late-onset Alzheimer's Dementia: A Systematic Review

Involvement of life stress in Late-Onset Alzheimer's Disease (LOAD) has been evinced in longitudinal cohort epidemiological studies, and endocrinologic evidence suggests involvements of catecholamine and corticosteroid systems in LOAD. Early Life Stress (ELS) rodent models have successfully demonstrated sequelae of maternal separation resulting in LOAD-analogous pathology, thereby supporting a role of insulin receptor signalling pertaining to GSK-3beta facilitated tau hyper-phosphorylation and amyloidogenic processing. Discussed are relevant ELS studies, and findings from three mitogen-activated protein kinase pathways (JNK/SAPK pathway, ERK pathway, p38/MAPK pathway) relevant for mediating environmental stresses. Further considered were the roles of autophagy impairment, neuroinflammation, and brain insulin resistance. For the meta-analytic evaluation, 224 candidate gene loci were extracted from reviews of animal studies of LOAD pathophysiological mechanisms, of which 60 had no positive results in human LOAD association studies. These loci were combined with 89 gene loci confirmed as LOAD risk genes in previous GWAS and WES. Of the 313 risk gene loci evaluated, there were 35 human reports on epigenomic modifications in terms of methylation or histone acetylation. 64 microRNA gene regulation mechanisms were published for the compiled loci. Genomic association studies support close relations of both noradrenergic and glucocorticoid systems with LOAD. For HPA involvement, a CRHR1 haplotype with MAPT was described, but further association of only HSD11B1 with LOAD found; however, association of FKBP1 and NC3R1 polymorphisms was documented in support of stress influence to LOAD. In the brain insulin system, IGF2R, INSR, INSRR, and plasticity regulator ARC, were associated with LOAD. Pertaining to compromised myelin stability in LOAD, relevant associations were found for BIN1, RELN, SORL1, SORCS1, CNP, MAG, and MOG. Regarding epigenetic modifications, both methylation variability and de-acetylation were reported for LOAD. The majority of up-to-date epigenomic findings include reported modifications in the well-known LOAD core pathology loci MAPT, BACE1, APP (with FOS, EGR1), PSEN1, PSEN2, and highlight a central role of BDNF. Pertaining to ELS, relevant loci are FKBP5, EGR1, GSK3B; critical roles of inflammation are indicated by CRP, TNFA, NFKB1 modifications; for cholesterol biosynthesis, DHCR24; for myelin stability BIN1, SORL1, CNP; pertaining to (epi)genetic mechanisms, hTERT, MBD2, DNMT1, MTHFR2. Findings on gene regulation were accumulated for BACE1, MAPK signalling, TLR4, BDNF, insulin signalling, with most reports for miR-132 and miR-27. Unclear in epigenomic studies remains the role of noradrenergic signalling, previously demonstrated by neuropathological findings of childhood nucleus caeruleus degeneration for LOAD tauopathy.

Chronic stress as a risk factor for Alzheimer's disease: Roles of microglia-mediated synaptic remodeling, inflammation, and oxidative stress

Microglia are the predominant immune cells of the central nervous system (CNS) that exert key physiological roles required for maintaining CNS homeostasis, notably in response to chronic stress, as well as mediating synaptic plasticity, learning and memory. The repeated exposure to stress confers a higher risk of developing neurodegenerative diseases including sporadic Alzheimer's disease (AD). While microglia have been causally linked to amyloid beta (Aβ) accumulation, tau pathology, neurodegeneration, and synaptic loss in AD, they were also attributed beneficial roles, notably in the phagocytic elimination of Aβ. In this review, we discuss the interactions between chronic stress and AD pathology, overview the roles played by microglia in AD, especially focusing on chronic stress as an environmental risk factor modulating their function, and present recently-described microglial phenotypes associated with neuroprotection in AD. These microglial phenotypes observed under both chronic stress and AD pathology may provide novel opportunities for the development of better-targeted therapeutic interventions.

The Corticotropin-Releasing Factor Family: Physiology of the Stress Response

The physiological stress response is responsible for the maintenance of homeostasis in the presence of real or perceived challenges. In this function, the brain activates adaptive responses that involve numerous neural circuits and effector molecules to adapt to the current and future demands. A maladaptive stress response has been linked to the etiology of a variety of disorders, such as anxiety and mood disorders, eating disorders, and the metabolic syndrome. The neuropeptide corticotropin-releasing factor (CRF) and its relatives, the urocortins 1–3, in concert with their receptors (CRFR1, CRFR2), have emerged as central components of the physiological stress response. This central peptidergic system impinges on a broad spectrum of physiological processes that are the basis for successful adaptation and concomitantly integrate autonomic, neuroendocrine, and behavioral stress responses. This review focuses on the physiology of CRF-related peptides and their cognate receptors with the aim of providing a comprehensive up-to-date overview of the field. We describe the major molecular features covering aspects of gene expression and regulation, structural properties, and molecular interactions, as well as mechanisms of signal transduction and their surveillance. In addition, we discuss the large body of published experimental studies focusing on state-of-the-art genetic approaches with high temporal and spatial precision, which collectively aimed to dissect the contribution of CRF-related ligands and receptors to different levels of the stress response. We discuss the controversies in the field and unravel knowledge gaps that might pave the way for future research directions and open up novel opportunities for therapeutic intervention.

Stress induced neural reorganization: A conceptual framework linking depression and Alzheimer's disease

Chronic stress is a risk factor for a number of physiological disorders including cardiovascular disease, obesity and gastrointestinal disorders, as well as psychiatric and neurodegenerative disorders. There are a number of underlying molecular and cellular mechanisms altered in the course of chronic stress, which may increase the vulnerability of individuals to develop psychiatric disorders such as depression, and neurodegenerative disorders such as Alzheimer's Disease (AD). This is evident in the influence of stress on large-scale brain networks, including the resting state Default Mode Network (DMN), the effects of stress on neuronal circuitry and architecture, and the cellular and molecular adaptations to stress, which may render individuals with stress related psychiatric disorders more vulnerable to neurodegenerative disease later in life. These alterations include decreased negative feedback inhibition of the hypothalamic pituitary axis (HPA) axis, decreased dendritic arborization and spine density in the prefrontal cortex (PFC) and hippocampus, and the release of proinflammatory cytokines, which may suppress neurogenesis and promote neuronal cell death. Each of these factors are thought to play a role in stress-related psychiatric disease as well as AD, and have been observed in clinical and post-mortem studies of individuals with depression and AD. The goal of the current review is to summarize clinical and preclinical evidence supporting a role for chronic stress as a putative link between neuropsychiatric and neurodegenerative disease. Moreover, we provide a rationale for the importance of taking a medical history of stress-related psychiatric diseases into consideration during clinical trial design, as they may play an important role in the etiology of AD in stratified patient populations.

Sexual dimorphism in predisposition to Alzheimer's disease

Clinical studies indicate that Alzheimer's disease (AD) disproportionately affects women in both disease prevalence and rate of symptom progression, but the mechanisms underlying this sexual divergence are unknown. Although some have suggested this difference in risk is a reflection of the known differences in longevity between men and women, mounting clinical and preclinical evidence supports women also having intrinsic susceptibilities toward the disease. Although a number of potential risk factors have been hypothesized to mediate these differences, none have been definitively verified. In this review, we first summarize the epidemiologic studies of prevalence and incidence of AD among the sexes. Next, we discuss the most likely risk factors to date that interact with biological sex, including (1) genetic factors, (2) sex hormones (3) deviations in brain structure, (4) inflammation and microglia, and (5) and psychosocial stress responses. Overall, though differences in life span are likely to account for part of the divide between the sexes in AD prevalence, the abundance of preclinical and clinical evidence presented here suggests an increase in intrinsic AD risk for women. Therefore, future studies focusing on the underlying biological mechanisms for this phenomenon are needed to better understand AD pathogenesis in both sexes, with the eventual goal of sex-specific prevention and treatment strategies.

A preclinical perspective on the enhanced vulnerability to Alzheimer's disease after early-life stress

Stress experienced early in life (ES), in the form of childhood maltreatment, maternal neglect or trauma, enhances the risk for cognitive decline in later life. Several epidemiological studies have now shown that environmental and adult life style factors influence AD incidence or age-of-onset and early-life environmental conditions have attracted attention in this respect. There is now emerging interest in understanding whether ES impacts the risk to develop age-related neurodegenerative disorders, and their severity, such as in Alzheimer's disease (AD), which is characterized by cognitive decline and extensive (hippocampal) neuropathology. While this might be relevant for the identification of individuals at risk and preventive strategies, this topic and its possible underlying mechanisms have been poorly studied to date. In this review, we discuss the role of ES in modulating AD risk and progression, primarily from a preclinical perspective. We focus on the possible involvement of stress-related, neuro-inflammatory and metabolic factors in mediating ES-induced effects on later neuropathology and the associated impairments in neuroplasticity. The available studies suggest that the age of onset and progression of AD-related neuropathology and cognitive decline can be affected by ES, and may aggravate the progression of AD neuropathology. These relevant changes in AD pathology after ES exposure in animal models call for future clinical studies to elucidate whether stress exposure during the early-life period in humans modulates later vulnerability for AD.

The relationship between stress and Alzheimer's disease

Stress is critically involved in the development and progression of disease. From the stress of undergoing treatments to facing your own mortality, the physiological processes that stress drives have a serious detrimental effect on the ability to heal, cope and maintain a positive quality of life. This is becoming increasingly clear in the case of neurodegenerative diseases. Neurodegenerative diseases involve the devastating loss of cognitive and motor function which is stressful in itself, but can also disrupt neural circuits that mediate stress responses. Disrupting these circuits produces aberrant emotional and aggressive behavior that causes long-term care to be especially difficult. In addition, added stress drives progression of the disease and can exacerbate symptoms. In this review, I describe how neural and endocrine pathways activated by stress interact with ongoing neurodegenerative disease from both a clinical and experimental perspective.

ATP-sensitive potassium-channel inhibitor glibenclamide attenuates HPA axis hyperactivity, depression- and anxiety-related symptoms in a rat model of Alzheimer's disease

Affective disorders including depression and anxiety are among the most prevalent behavioral abnormalities in patients with Alzheimer's disease (AD), which affect the quality of life and progression of the disease. Dysregulation of the hypothalamic-pituitary-adrenal-(HPA) axis has been reported in affective disorders and AD. Recent studies revealed that current antidepressant drugs are not completely effective for treating anxiety- and depression-related disorders in people with dementia. ATP-sensitive-potassium-(K_ATP) channels are well-known to be involved in AD pathophysiology, HPA axis function and the pathogenesis of depression and anxiety-related behaviors. Thus, targeting of K_ATP channel may be a potential therapeutic strategy in AD. Hence, we investigated the effects of intracerebroventricular injection of Aβ25-35 alone or in combination with glibenclamide, K_ATP channel inhibitor on depression- and anxiety-related behaviors as well as HPA axis response to stress in rats. To do this, non-Aβ25-35- and Aβ25-35-treated rats were orally treated with glibenclamide, then the behavioral consequences were assessed using sucrose preference, forced swim, light-dark box and plus maze tests. Stress-induced corticosterone levels following forced swim and plus maze tests were also evaluated as indicative of abnormal HPA-axis-function. Aβ25-35 induced HPA axis hyperreactivity and increased depression- and anxiety-related symptoms in rats. Our results showed that blockade of K_ATP channels with glibenclamide decreased depression- and anxiety-related behaviors by normalizing HPA axis activity in Aβ25-35-treated rats. This study provides additional evidence that Aβ administration can induce depression- and anxiety-like symptoms in rodents, and suggests that K_ATP channel inhibitors may be a plausible therapeutic strategy for treating affective disorders in AD patients.

Effects of chronic noise on the corticotropin-releasing factor system in the rat hippocampus: relevance to Alzheimer's disease-like tau hyperphosphorylation

Background

Chronic noise exposure has been associated with tau hyperphosphorylation and Alzheimer’s disease (AD)-like pathological changes, but the underlying mechanism is unknown. In this study, we explored the effects of long-term noise exposure on the corticotropin-releasing factor (CRF) system in the hippocampus and its role in noise-induced tau phosphorylation.

Methods

Sixty-four rats were randomly divided into the noise-exposed group and the control group, and rats in the exposure group were exposed to 95 dB SPL white noise for 30 consecutive days. The levels of CRF, CRFR1, CRFR2, and total tau and phosphorylated tau (p-tau) at Ser396 (S396) and Thr205 (T205) in the hippocampus were measured at different time points after the final noise exposure. The co-localized distribution of CRF and p-tau (T205) in the hippocampus was evaluated using double-labeling immunofluorescence.

Results

Long-term exposure to noise for 30 consecutive days significantly increased the expression of CRF and CRFR1 and their mRNAs levels in the hippocampus, which persisted for 7 days after final exposure. In contrast, CRFR2 was raised for 3–7 days following the last exposure. These alterations were also concomitant with the phosphorylation of tau at S396 and T205. Furthermore, there was co-localization of p-tau and CRF in hippocampal neurons.

Conclusion

Chronic noise leads to long-lasting increases in the hippocampal CRF system and the hyperphosphorylation of tau in the hippocampus. Our results also provide evidence for the involvement of the CRF system in noise-induced AD-like neurodegeneration.

Corticotropin-releasing factor receptor-1 modulates biomarkers of DNA oxidation in Alzheimer's disease mice

Increased production of hydroxyl radical is the main source of oxidative damage in mammalian DNA that accumulates in Alzheimer’s disease (AD). Reactive oxygen species (ROS) react with both nuclear DNA (nDNA) and mitochondrial DNA (mtDNA) to generate 8-hydroxy-2’-deoxyguanosine (8-OHdG), both of which can be measured in the urine. Knowledge of this pathway has positioned measurement of urine 8-OHdG as a reliable index of DNA oxidation and a potential biomarker target for tracking early cellular dysfunction in AD. Furthermore, epigenetic studies demonstrate decreased global DNA methylation levels (e.g. 5-methyl-2’-deoxycytidine, 5-mdC) in AD tissues. Moreover, stress hormones can activate neuronal oxidative stress which will stimulate the release of additional stress hormones and result in damages to hippocampal neurons in the AD brain. Our previous work suggests that treating AD transgenic mice the type-1 corticotropin-releasing factor receptor (CRFR1) antagonist, R121919, to reduce stress signaling, prevented onset of cognitive impairment, synaptic/dendritic loss and Aβ plaque accumulation. Therefore, to investigate whether levels of DNA oxidation can be impacted by the same therapeutic approach, urine levels of hydrogen peroxide, 8-OHdG, 5-mdC and total antioxidant capacity (TAC) were analyzed using an AD Tg mouse model. We found that Tg animals had an 80% increase in hydrogen peroxide levels compared to wild type (Wt) counterparts, an effect that could be dramatically reversed by the chronic administration with R121919. A significant decrease of 8-OHdG levels was observed in Tg mice treated with CRFR1 antagonist. Collectively our data suggest that the beneficial effects of CRFR1 antagonism seen in Tg mice may be mechanistically linked to the modulation of oxidative stress pathways.

Targeting psychologic stress signaling pathways in Alzheimer's disease

Alzheimer's Disease (AD) is the most prevalent progressive neurodegenerative disease; to date, no AD therapy has proven effective in delaying or preventing the disease course. In the search for novel therapeutic targets in AD, it has been shown that increased chronic psychologic stress is associated with AD risk. Subsequently, biologic pathways underlying psychologic stress have been identified and shown to be able to exacerbate AD relevant pathologies. In this review, we summarize the literature relevant to the association between psychologic stress and AD, focusing on studies investigating the effects of stress paradigms on transgenic mouse models of Amyloid-β (Aβ) and tau pathologies. In recent years, a substantial amount of research has been done investigating a key stress-response mediator, corticotropin-releasing hormone (CRH), and its interactions with AD relevant processes. We highlight attempts to target the CRH signaling pathway as a therapeutic intervention in these transgenic mouse models and discuss how targeting this pathway is a promising avenue for further investigation.

Distribution, physiology and pharmacology of relaxin-3/RXFP3 systems in brain

Relaxin-3 is a member of a superfamily of structurally-related peptides that includes relaxin and insulin-like peptide hormones. Soon after the discovery of the relaxin-3 gene, relaxin-3 was identified as an abundant neuropeptide in brain with a distinctive topographical distribution within a small number of GABAergic neuron populations that is well conserved across species. Relaxin-3 is thought to exert its biological actions through a single class-A GPCR - relaxin-family peptide receptor 3 (RXFP3). Class-A comprises GPCRs for relaxin-3 and insulin-like peptide-5 and other peptides such as orexin and the monoamine transmitters. The RXFP3 receptor is selectively activated by relaxin-3, whereas insulin-like peptide-5 is the cognate ligand for the related RXFP4 receptor. Anatomical and pharmacological evidence obtained over the last decade supports a function of relaxin-3/RXFP3 systems in modulating responses to stress, anxiety-related and motivated behaviours, circadian rhythms, and learning and memory. Electrophysiological studies have identified the ability of RXFP3 agonists to directly hyperpolarise thalamic neurons in vitro, but there are no reports of direct cell signalling effects in vivo. This article provides an overview of earlier studies and highlights more recent research that implicates relaxin-3/RXFP3 neural network signalling in the integration of arousal, motivation, emotion and related cognition, and that has begun to identify the associated neural substrates and mechanisms. Future research directions to better elucidate the connectivity and function of different relaxin-3 neuron populations and their RXFP3-positive target neurons in major experimental species and humans are also identified.

LINKED ARTICLES

This article is part of a themed section on Recent Progress in the Understanding of Relaxin Family Peptides and their Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.10/issuetoc.

Effects of chronic noise on mRNA and protein expression of CRF family molecules and its relationship with p-tau in the rat prefrontal cortex

Chronic noise exposure has been associated with Alzheimer's disease (AD)-like pathological changes, such as tau hyperphosphorylation and β-amyloid peptide accumulation in the prefrontal cortex (PFC). Corticotropin-releasing factor (CRF) is the central driving force in the stress response and a regulator of tau phosphorylation via binding to CRF receptors (CRFR). Little is known about the CRF system in relation to noise-induced AD-like changes in the PFC. The aim of this study was to explore the effects of chronic noise exposure on the CRF system in the PFC of rats and its relationship to tau phosphorylation. Male Wistar rats were randomly divided into control and noise exposure groups. The CRF system was evaluated following chronic noise exposure (95dB sound pressure level white noise, 4h/day×30days). Chronic noise significantly accelerated the progressive overproduction of corticosterone and upregulated CRF and CRFR1 mRNA and protein, both of which persisted 7-14days after noise exposure. In contrast, CRFR2 was elevated 3-7days following the last stimulus. Double-labeling immunofluorescence co-localized p-tau with CRF in PFC neurons. The results suggest that chronic noise exposure elevates the expression of the CRF system, which may contribute to AD-like changes.

Prediction of conversion from mild cognitive impairment to dementia with neuronally derived blood exosome protein profile

Introduction

Levels of Alzheimer's disease (AD)-related proteins in plasma neuronal derived exosomes (NDEs) were quantified to identify biomarkers for prediction and staging of mild cognitive impairment (MCI) and AD.

Methods

Plasma exosomes were extracted, precipitated, and enriched for neuronal source by anti-L1CAM antibody absorption. NDEs were characterized by size (Nanosight) and shape (TEM) and extracted NDE protein biomarkers were quantified by ELISAs. Plasma NDE cargo was injected into normal mice, and results were characterized by immunohistochemistry to determine pathogenic potential.

Results

Plasma NDE levels of P-T181-tau, P-S396-tau, and Aβ1–42 were significantly higher, whereas those of neurogranin (NRGN) and the repressor element 1-silencing transcription factor (REST) were significantly lower in AD and MCI converting to AD (ADC) patients compared to cognitively normal controls (CNC) subjects and stable MCI patients. Mice injected with plasma NDEs from ADC patients displayed increased P-tau (PHF-1 antibody)–positive cells in the CA1 region of the hippocampus compared to plasma NDEs from CNC and stable MCI patients.

Conclusions

Abnormal plasma NDE levels of P-tau, Aβ1–42, NRGN, and REST accurately predict conversion of MCI to AD dementia. Plasma NDEs from demented patients seeded tau aggregation and induced AD-like neuropathology in normal mouse CNS.

Corticotropin-releasing factor receptor-1 antagonism mitigates beta amyloid pathology and cognitive and synaptic deficits in a mouse model of Alzheimer's disease

INTRODUCTION

Stress and corticotropin-releasing factor (CRF) have been implicated as mechanistically involved in Alzheimer's disease (AD), but agents that impact CRF signaling have not been carefully tested for therapeutic efficacy or long-term safety in animal models.

METHODS

To test whether antagonism of the type-1 corticotropin-releasing factor receptor (CRFR1) could be used as a disease-modifying treatment for AD, we used a preclinical prevention paradigm and treated 30-day-old AD transgenic mice with the small-molecule, CRFR1-selective antagonist, R121919, for 5 months, and examined AD pathologic and behavioral end points.

RESULTS

R121919 significantly prevented the onset of cognitive impairment in female mice and reduced cellular and synaptic deficits and beta amyloid and C-terminal fragment-β levels in both genders. We observed no tolerability or toxicity issues in mice treated with R121919.

DISCUSSION

CRFR1 antagonism presents a viable disease-modifying therapy for AD, recommending its advancement to early-phase human safety trials.

Impact of CRFR1 Ablation on Amyloid-β Production and Accumulation in a Mouse Model of Alzheimer's Disease

Stress exposure and the corticotropin-releasing factor (CRF) system have been implicated as mechanistically involved in both Alzheimer's disease (AD) and associated rodent models. In particular, the major stress receptor, CRF receptor type 1 (CRFR1), modulates cellular activity in many AD-relevant brain areas, and has been demonstrated to impact both tau phosphorylation and amyloid-β (Aβ) pathways. The overarching goal of our laboratory is to develop and characterize agents that impact the CRF signaling system as disease-modifying treatments for AD. In the present study, we developed a novel transgenic mouse to determine whether partial or complete ablation of CRFR1 was feasible in an AD transgenic model and whether this type of treatment could impact Aβ pathology. Double transgenic AD mice (PSAPP) were crossed to mice null for CRFR1; resultant CRFR1 heterozygous (PSAPP-R1(+/-)) and homozygous (PSAPP-R1(-/-)) female offspring were used at 12 months of age to examine the impact of CRFR1 disruption on the severity of AD Aβ levels and pathology. We found that both PSAPP-R1(+/-) and PSAPP-R1(-/-) had significantly reduced Aβ burden in the hippocampus, insular, rhinal, and retrosplenial cortices. Accordingly, we observed dramatic reductions in Aβ peptides and AβPP-CTFs, providing support for a direct relationship between CRFR1 and Aβ production pathways. In summary, our results suggest that interference of CRFR1 in an AD model is tolerable and is efficacious in impacting Aβ neuropathology.

Functional Impact of Corticotropin-Releasing Factor Exposure on Tau Phosphorylation and Axon Transport

Stress exposure or increased levels of corticotropin-releasing factor (CRF) induce hippocampal tau phosphorylation (tau-P) in rodent models, a process that is dependent on the type-1 CRF receptor (CRFR1). Although these preclinical studies on stress-induced tau-P provide mechanistic insight for epidemiological work that identifies stress as a risk factor for Alzheimer’s disease (AD), the actual impact of stress-induced tau-P on neuronal function remains unclear. To determine the functional consequences of stress-induced tau-P, we developed a novel mouse neuronal cell culture system to explore the impact of acute (0.5hr) and chronic (2hr) CRF treatment on tau-P and integral cell processes such as axon transport. Consistent with in vivo reports, we found that chronic CRF treatment increased tau-P levels and caused globular accumulations of phosphorylated tau in dendritic and axonal processes. Furthermore, while both acute and chronic CRF treatment led to significant reduction in CREB activation and axon transport of brain-derived neurotrophic factor (BDNF), this was not the case with mitochondrial transport. Acute CRF treatment caused increased mitochondrial velocity and distance traveled in neurons, while chronic CRF treatment modestly decreased mitochondrial velocity and greatly increased distance traveled. These results suggest that transport of cellular energetics may take priority over growth factors during stress. Tau-P was required for these changes, as co-treatment of CRF with a GSK kinase inhibitor prevented CRF-induced tau-P and all axon transport changes. Collectively, our results provide mechanistic insight into the consequences of stress peptide-induced tau-P and provide an explanation for how chronic stress via CRF may lead to neuronal vulnerability in AD.

Corticotropin-Releasing Factor Receptor-1 Antagonism Reduces Oxidative Damage in an Alzheimer’s Disease Transgenic Mouse Model

Reports from Alzheimer’s disease (AD) biomarker work have shown a strong link between oxidative stress and AD neuropathology. The nonenzymatic antioxidant, glutathione (GSH), plays a crucial role in defense against reactive oxygen species and maintenance of GSH redox homeostasis. In particular, our previous studies on GSH redox imbalance have implicated oxidative stress induced by excessive reactive oxygen species as a major mediator of AD-like events, with the presence of S- glutathionylated proteins (Pr-SSG) appearing prior to overt AD neuropathology. Furthermore, evidence suggests that oxidative stress may be associated with dysfunction of the hypothalamic-pituitary-adrenal axis, leading to activation of inflammatory pathways and increased production of corticotropin-releasing factor (CRF). Therefore, to investigate whether oxidative insults can be attenuated by reduction of central CRF signaling, we administered the type-1 CRF receptor (CRFR1) selective antagonist, R121919, to AD-transgenic mice beginning in the preclinical/prepathologic period (30-day-old) for 150 days, a time point where behavioral impairments and pathologic progression should be measureable. Our results indicate that R121919 treatment can significantly reduce Pr-SSG levels and increase glutathione peroxide activity, suggesting that interference of CRFR1 signaling may be useful as a preventative therapy for combating oxidative stress in AD.

Aging as a Precipitating Factor in Chronic Restraint Stress-Induced Tau Aggregation Pathology, and the Protective Effects of Rosmarinic Acid

Stress is an important risk factor of Alzheimer's disease (AD). It has been evidenced that stress could induce tau phosphorylation and increase tau insolubility in brain; however, little is known about the interactional effect of stress with aging on tauopathy. Therefore, we explored the effects of aging on stress-induced tauopathy and the potential mechanism in mouse model of chronic restraint stress (CRS). Here we found that in general, the level of phosphorylated tau (P-tau) was higher in brain of middle-aged mice than that in adult mice under physiological conditions. CRS-induced tau phosphorylation and its insolubility were more prominent in middle-aged mice. The increase of AT8-labeled insoluble P-tau was dramatic in middle-aged mice, which was highly ubiquitinated but did not form PHF structures. The levels of chaperones were relatively lower in middle-aged mice brain; CRS further reduced the expression, especially for HDJ2/HSP40. CRS also suppressed the expression of Pin1, the peptidylprolyl cis/trans isomerase, in middle-aged mice but not in adult mice. Downregulation of HSP40 or Pin1 caused an increase of transfected extraneous tau in 293 cells. Rosmarinic acid (RA) could effectively suppress the elevation of P-tau and insoluble P-tau formation induced by CRS, and reversed the abnormal changes of chaperones and Pin1 particularly in middle-aged mice. Taken together, our findings provided evidence that aging could be a promoting factor in stress-induced tauopathy, which was relevant with malregulation of chaperones and Pin1, and RA might be a promising beneficial agent for stress-induced tauopathy.

Reproductive Stage and Modulation of Stress-Induced Tau Phosphorylation in Female Rats

Chronic stress is implicated as a risk factor for Alzheimer's disease (AD) and other neurodegenerative disorders. Although the specific mechanisms linking stress exposure and AD vulnerability have yet to be fully determined, our laboratory and others have shown that acute and repeated restraint stress in rodents leads to an increase in hippocampal tau phosphorylation (tau-P) and tau insolubility, a critical component of tau pathology in AD. Although tau phosphorylation induced by acute psychological stress is dependent on intact signaling through the type 1 corticotropin-releasing factor receptor, how sex steroids or other modulators contribute to this effect is unknown. A naturally occurring attenuation of the stress response is observed in female rats at the end of pregnancy and throughout lactation. To test the hypothesis that decreased sensitivity to stress during lactation modulates stress-induced tau-P, cohorts of virgin, lactating and weaned female rats were subjected to 30 min of restraint stress or no stress (control) and were killed 20 min or 24 h after the episode. Exposure to restraint stress induced a significant decrease in tau-P in the hippocampus of lactating rats killed 20 min after stress compared to lactating controls and virgins subjected to stress treatment. Lactating rats killed 24 hr after restraint stress exposure showed significant elevation in tau-P compared to lactating cohorts killed 20 min after stress. Levels of tau-P in these latter cohorts did not differ signficantly from control animals. Furthermore, glycogen synthase kinase (GSK)3-α levels were significantly decreased in stressed lactating animals at both timepoints. This suggests a steep, yet transient stress-induced dephosphorylation of tau, influenced by GSK3, in the hippocampus of lactating rats.

Tau Mislocation in Glucocorticoid-Triggered Hippocampal Pathology

The exposure to high glucocorticoids (GC) triggers neuronal atrophy and cognitive deficits, but the exact cellular mechanisms underlying the GC-associated dendritic remodeling and spine loss are still poorly understood. Previous studies have implicated sustained GC elevations in neurodegenerative mechanisms through GC-evoked hyperphosphorylation of the cytoskeletal protein Tau while Tau mislocation has recently been proposed as relevant in Alzheimer's disease (AD) pathology. In light of the dual cytoplasmic and synaptic role of Tau, this study monitored the impact of prolonged GC treatment on Tau intracellular localization and its phosphorylation status in different cellular compartments. We demonstrate, both by biochemical and ultrastructural analysis, that GC administration led to cytosolic and dendritic Tau accumulation in rat hippocampus, and triggered Tau hyperphosphorylation in epitopes related to its malfunction (Ser396/404) and cytoskeletal pathology (e.g., Thr231 and Ser262). In addition, we show, for the first time, that chronic GC administration also increased Tau levels in synaptic compartment; however, at the synapse, there was an increase in phosphorylation of Ser396/404, but a decrease of Thr231. These GC-triggered Tau changes were paralleled by reduced levels of synaptic scaffolding proteins such as PSD-95 and Shank proteins as well as reduced dendritic branching and spine loss. These in vivo findings add to our limited knowledge about the underlying mechanisms of GC-evoked synaptic atrophy and neuronal disconnection implicating Tau missorting in mechanism(s) of synaptic damage, beyond AD pathology.

Locus Coeruleus, norepinephrine and Aβ peptides in Alzheimer's disease

Monoaminergic brainstem systems have widespread projections that participate in many central processes and, when dysregulated, contribute to a plethora of neuropsychiatric and neurodegenerative disorders. Synapses are the foundation of these neuronal circuits, and their local dysfunction results in global aberrations leading to pathophysiological disease states. This review focuses on the locus coeruleus (LC) norepinephrine (NE) brainstem system and its underappreciated role in Alzheimer's disease (AD). Amyloid beta (Aβ), a peptide that accumulates aberrantly in AD has recently been implicated as a modulator of neuronal excitability at the synapse. Evidence is presented showing that disruption of the LC-NE system at a synaptic and circuit level during early stages of AD, due to conditions such as chronic stress, can potentially lead to amyloid accumulation and contribute to the progression of this neurodegenerative disorder. Additional factors that impact neurodegeneration include neuroinflammation, and network de-synchronization. Consequently, targeting the LC-NE system may have significant therapeutic potential for AD, as it may facilitate modulation of Aβ production, curtail neuroinflammation, and prevent sleep and behavioral disturbances that often lead to negative patient outcomes.