Effects of corticotrophin-releasing factor receptor 1 antagonists on amyloid-β and behavior in Tg2576 mice

Connecting the emotional-cognitive puzzle: The role of tyrosine kinase B (TrkB) receptor isoform imbalance in age-related emotional and cognitive impairments

Age-related cognitive and affective disorders pose significant public health challenges. Notably, emotional and cognitive symptoms co-occur across multiple age-associated conditions like normal aging, Alzheimer's disease (AD), and mood disorders such as depression and anxiety. While the intricate interplay underlying this relationship remains poorly understood, this article highlights the possibility that an imbalance between full-length (TrkB.FL) and truncated (TrkB.T1) isoforms of tyrosine kinase receptor TrkB in the neurotrophic system may significantly affect age-associated emotional and cognitive functions, by altering brain-derived neurotrophic factor (BDNF) signaling, integral to neuronal health, cognitive functions and mood regulation. While the contribution of this imbalance to pathogenesis awaits full elucidation, this review evaluates its potential mediating role, linking emotional and cognitive decline across age-related disorders The interplay between TrkB.T1 and TrkB.FL isoforms may be considered as a pivotal shared regulator underlying this complex relationship. The current review aims to synthesize current knowledge on TrkB isoform imbalance, specifically its contribution to age-related cognitive decline and mood disorders. By examining shared pathogenic pathways between aging, cognitive decline, and mood disorders through the lens of TrkB signaling, this review uncovers potential therapeutic targets not previously considered, offering a fresh perspective on combating age-related mental health issues as well as cognitive deficits.

Experimental laboratory models as tools for understanding modifiable dementia risk

Experimental laboratory research has an important role to play in dementia prevention. Mechanisms underlying modifiable risk factors for dementia are promising targets for dementia prevention but are difficult to investigate in human populations due to technological constraints and confounds. Therefore, controlled laboratory experiments in models such as transgenic rodents, invertebrates and in vitro cultured cells are increasingly used to investigate dementia risk factors and test strategies which target them to prevent dementia. This review provides an overview of experimental research into 15 established and putative modifiable dementia risk factors: less early-life education, hearing loss, depression, social isolation, life stress, hypertension, obesity, diabetes, physical inactivity, heavy alcohol use, smoking, air pollution, anesthetic exposure, traumatic brain injury, and disordered sleep. It explores how experimental models have been, and can be, used to address questions about modifiable dementia risk and prevention that cannot readily be addressed in human studies. HIGHLIGHTS: Modifiable dementia risk factors are promising targets for dementia prevention. Interrogation of mechanisms underlying dementia risk is difficult in human populations. Studies using diverse experimental models are revealing modifiable dementia risk mechanisms. We review experimental research into 15 modifiable dementia risk factors. Laboratory science can contribute uniquely to dementia prevention.

Anorexigenic neuropeptides as anti-obesity and neuroprotective agents: exploring the neuroprotective effects of anorexigenic neuropeptides

Since 1975, the incidence of obesity has increased to epidemic proportions, and the number of patients with obesity has quadrupled. Obesity is a major risk factor for developing other serious diseases, such as type 2 diabetes mellitus, hypertension, and cardiovascular diseases. Recent epidemiologic studies have defined obesity as a risk factor for the development of neurodegenerative diseases, such as Alzheimer's disease (AD) and other types of dementia. Despite all these serious comorbidities associated with obesity, there is still a lack of effective antiobesity treatment. Promising candidates for the treatment of obesity are anorexigenic neuropeptides, which are peptides produced by neurons in brain areas implicated in food intake regulation, such as the hypothalamus or the brainstem. These peptides efficiently reduce food intake and body weight. Moreover, because of the proven interconnection between obesity and the risk of developing AD, the potential neuroprotective effects of these two agents in animal models of neurodegeneration have been examined. The objective of this review was to explore anorexigenic neuropeptides produced and acting within the brain, emphasizing their potential not only for the treatment of obesity but also for the treatment of neurodegenerative disorders.

Converged avenues: depression and Alzheimer's disease- shared pathophysiology and novel therapeutics

Depression, a highly prevalent disorder affecting over 280 million people worldwide, is comorbid with many neurological disorders, particularly Alzheimer's disease (AD). Depression and AD share overlapping pathophysiology, and the search for accountable biological substrates made it an essential and intriguing field of research. The paper outlines the neurobiological pathways coinciding with depression and AD, including neurotrophin signalling, the hypothalamic-pituitary-adrenal axis (HPA), cellular apoptosis, neuroinflammation, and other aetiological factors. Understanding overlapping pathways is crucial in identifying common pathophysiological substrates that can be targeted for effective management of disease state. Antidepressants, particularly monoaminergic drugs (first-line therapy), are shown to have modest or no clinical benefits. Regardless of the ineffectiveness of conventional antidepressants, these drugs remain the mainstay for treating depressive symptoms in AD. To overcome the ineffectiveness of traditional pharmacological agents in treating comorbid conditions, a novel therapeutic class has been discussed in the paper. This includes neurotransmitter modulators, glutamatergic system modulators, mitochondrial modulators, antioxidant agents, HPA axis targeted therapy, inflammatory system targeted therapy, neurogenesis targeted therapy, repurposed anti-diabetic agents, and others. The primary clinical challenge is the development of therapeutic agents and the effective diagnosis of the comorbid condition for which no specific diagnosable scale is present. Hence, introducing Artificial Intelligence (AI) into the healthcare system is revolutionary. AI implemented with interdisciplinary strategies (neuroimaging, EEG, molecular biomarkers) bound to have accurate clinical interpretation of symptoms. Moreover, AI has the potential to forecast neurodegenerative and psychiatric illness much in advance before visible/observable clinical symptoms get precipitated.

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.

The effects of social environment on AD-related pathology in hAPP-J20 mice and tau-P301L mice

In humans, social factors (e.g., loneliness) have been linked to the risk of developing Alzheimer's Disease (AD). To date, AD pathology is primarily characterized by amyloid-β plaques and tau tangles. We aimed to assess the effect of single- and group-housing on AD-related pathology in a mouse model for amyloid pathology (J20, and WT controls) and a mouse model for tau pathology (P301L) with and without seeding of synthetic human tau fragments (K18). Female mice were either single housed (SH) or group housed (GH) from the age of 6-7 weeks onwards. In 12-week-old P301L mice, tau pathology was induced through seeding by injecting K18 into the dorsal hippocampus (P301LK18), while control mice received a PBS injection (P301LPBS). P301L mice were sacrificed at 4 months of age and J20 mice at 10 months of age. In all mice brain pathology was histologically assessed by examining microglia, the CA1 pyramidal cell layer and specific AD pathology: analysis of plaques in J20 mice and tau hyperphosphorylation in P301L mice. Contrary to our expectation, SH-J20 mice interestingly displayed fewer plaques in the hippocampus compared to GH-J20 mice. However, housing did not affect tau hyperphosphorylation at Ser202/Thr205 of P301L mice, nor neuronal cell death in the CA1 region in any of the mice. The number of microglia was increased by the J20 genotype, and their activation (based on cell body to cell size ratio) in the CA1 was affected by genotype and housing condition (interaction effect). Single housing of P301L mice was linked to the development of stereotypic behavior (i.e. somersaulting and circling behavior). In P301LK18 mice, an increased number of microglia were observed, among which were rod microglia. Taken together, our findings point to a significant effect of social housing conditions on amyloid plaques and microglia in J20 mice and on the development of stereotypic behavior in P301L mice, indicating that the social environment can modulate AD-related pathology.

Links between COVID-19 and Parkinson's disease/Alzheimer's disease: reciprocal impacts, medical care strategies and underlying mechanisms

The impact of coronavirus disease 2019 (COVID-19) pandemic on patients with neurodegenerative diseases and the specific neurological manifestations of COVID-19 have aroused great interest. However, there are still many issues of concern to be clarified. Therefore, we review the current literature on the complex relationship between COVID-19 and neurodegenerative diseases with an emphasis on Parkinson's disease (PD) and Alzheimer's disease (AD). We summarize the impact of COVID-19 infection on symptom severity, disease progression, and mortality rate of PD and AD, and discuss whether COVID-19 infection could trigger PD and AD. In addition, the susceptibility to and the prognosis of COVID-19 in PD patients and AD patients are also included. In order to achieve better management of PD and AD patients, modifications of care strategies, specific drug therapies, and vaccines during the pandemic are also listed. At last, mechanisms underlying the link of COVID-19 with PD and AD are reviewed.

CRHR1 antagonist alleviates LPS-induced depression-like behaviour in mice

BACKGROUND

Maladaptation of the HPA (hypothalamic-pituitary-adrenal) axis plays an important role in depression-like behaviour, but the specific molecular mechanisms are unknown. Here, we determined the roles of CRHR1 (corticotrophin releasing hormone receptor 1) and nectin3 in LPS (lipopolysaccharide)-induced depression-like behaviour in mice.

METHODS

C57BL/6 male mice were intraperitoneally injected with LPS (0.83 g/kg), and the open field, novelty-suppressed feeding, forced swimming, and tail suspension tests were performed after intraperitoneal injections of saline or antalarmin (20 mg/kg). The hippocampal mRNA levels of CRHR1 and nectin3 were determined by quantitative reverse transcription-PCR. The hippocampal protein levels of CRHR1, nectin3, and calbindin were measured by western blotting. The CORT (corticosterone) levels in the blood were measured by ELISA kits.

RESULTS

Antalarmin alleviated LPS-induced depression-like behaviour in male mice. Furthermore, antalarmin significantly inhibited changes in CRHR1, nectin3 and calbindin levels in the hippocampus and reduced the increase in CORT levels in LPS-treated mice.

CONCLUSION

CRHR1antagonist showed antidepressant effects in LPS-induced depressive mice, and CRHR1/nectin3 signalling may play a crucial role in this process.

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.

Sex differences in CRF1, CRF, and CRFBP expression in C57BL/6J mouse brain across the lifespan and in response to acute stress

Signaling pathways mediated by corticotropin-releasing factor and its receptor 1 (CRF1) play a central role in stress responses. Dysfunction of the CRF system has been associated with neuropsychiatric disorders. However, dynamic changes in the CRF system during brain development and aging are not well investigated. In this study, we characterized CRF1, CRF, and corticotropin-releasing factor binding protein (CRFBP) expression in different brain regions in both male and female C57BL/6J mice from 1 to 18 months of age under basal conditions as well as after an acute 2-hr-restraint stress. We found that CRF and CRF1 levels tended to increase in the hippocampus and hypothalamus, and to decrease in the prefrontal cortex with aging, especially at 18 months of age, whereas CRFBP expression followed an opposite direction in these brain areas. We also observed area-specific sex differences in the expression of these three proteins. For example, CRF expression was lower in females than in males in all the brain regions examined except the prefrontal cortex. After acute stress, CRF and CRF1 were up-regulated at 1, 6, and 12 months of age, and down-regulated at 18 months of age. Females showed more robust changes compared to males of the same age. CRFBP expression either decreased or remained unchanged in most of the brain areas following acute stress. Our findings suggest that brain CRF1, CRF, and CRFBP expression changes dynamically across the lifespan and under stress condition in a sex- and regional-specific manner. Sex differences in the CRF system in response to stress may contribute to the etiology of stress-related neuropsychiatric disorders.

Anxiety and Alzheimer's disease: Behavioral analysis and neural basis in rodent models of Alzheimer's-related neuropathology

Alzheimer's disease (AD) pathology is commonly associated with cognitive decline but is also composed of neuropsychiatric symptoms including psychological distress and alterations in mood, including anxiety and depression. Emotional dysfunction in AD is frequently modeled using tests of anxiety-like behavior in transgenic rodents. These tests often include the elevated plus-maze, light/dark test and open field test. In this review, we describe prototypical behavioral paradigms used to examine emotional dysfunction in transgenic models of AD, specifically anxiety-like behavior. Next, we summarize the results of studies examining anxiety-like behavior in transgenic rodents, noting that the behavioral outcomes using these paradigms have produced inconsistent results. We suggest that future research will benefit from using a battery of tests to examine emotional behavior in transgenic AD models. We conclude by discussing putative, overlapping neurobiological mechanisms underlying AD-related neuropathology, stress and anxiety-like behavior reported in AD models.

HIV-1 Tat Protein Promotes Neuroendocrine Dysfunction Concurrent with the Potentiation of Oxycodone's Psychomotor Effects in Female Mice

Human immunodeficiency virus (HIV) is associated with neuroendocrine dysfunction which may contribute to co-morbid stress-sensitive disorders. The hypothalamic-pituitary-adrenal (HPA) or -gonadal (HPG) axes are perturbed in up to 50% of HIV patients. The mechanisms are not known, but we have found the HIV-1 trans-activator of transcription (Tat) protein to recapitulate the clinical phenotype in male mice. We hypothesized that HPA and/or HPG dysregulation contributes to Tat-mediated interactions with oxycodone, an opioid often prescribed to HIV patients, in females. Female mice that conditionally-expressed the Tat_1-86 protein [Tat(+) mice] or their counterparts that did not [Tat(-) control mice] were exposed to forced swim stress (or not) and behaviorally-assessed for motor and anxiety-like behavior. Some mice had glucocorticoid receptors (GR) or corticotropin-releasing factor receptors (CRF-R) pharmacologically inhibited. Some mice were ovariectomized (OVX). As seen previously in males, Tat elevated basal corticosterone levels and potentiated oxycodone's psychomotor activity in females. Unlike males, females did not demonstrate adrenal insufficiency and oxycodone potentiation was not regulated by GRs or CRF-Rs. Rather OVX attenuated Tat/oxycodone interactions. Either Tat or oxycodone increased anxiety-like behavior and their combination increased hypothalamic allopregnanolone. OVX increased basal hypothalamic allopregnanolone and obviated Tat or oxycodone-mediated fluctuations. Together, these data provide further evidence for Tat-mediated dysregulation of the HPA axis and reveal the importance of HPG axis regulation in females. HPA/HPG disruption may contribute vulnerability to affective and substance use disorders.

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.

Identification of vascular dementia and Alzheimer's disease hub genes expressed in the frontal lobe and temporal cortex by weighted co-expression network analysis and construction of a protein-protein interaction

Background: It is difficult to distinguish cognitive decline due to AD from that sustained by cerebrovascular disease in view of the great overlap. It is uncertain in the molecular biological pathway behind AD and VaD.Objective: Our study aimed to explore the hub molecules and their associations with each other to identify potential biomarkers and therapeutic targets for the AD and VaD.Methods: We screened the differentially expressed genes of AD and VaD, used weighted gene co-expression network analysis and then constructed a VaD-AD-specific protein-protein interaction network with functional annotation to their related metabolic pathways. Finally, we performed a ROC curve analysis of hub proteins to get an idea about their diagnostic value.Results: In the frontal lobe and temporal cortex, hub genes were identified. With regard to VaD, there were only three hub genes which encoded the neuropeptides, SST, NMU and TAC1. The AUC of these genes were 0.804, 0.768 and 0.779, respectively. One signature was established for these three hub genes with AUC of 0.990. For the identification of AD and VaD, all hub genes were receptors. These genes included SH3GL2, PROK2, TAC3, HTR2A, MET, TF, PTH2R CNR1, CHRM4, PTPN3 and CRH. The AUC of these genes were 0.853, 0.859, 0.796, 0.775, 0.706, 0.677, 0.696, 0.668 and 0.652, respectively. The other signature was built for eleven hub genes with AUC of 0.990.Conclusion: In the frontal lobe and temporal cortex regions, hub genes are used as diagnostic markers, which may provide insight into personalized potential biomarkers and therapeutic targets for patients with VaD and AD.

HIV-1 Tat Dysregulates the Hypothalamic-Pituitary-Adrenal Stress Axis and Potentiates Oxycodone-Mediated Psychomotor and Anxiety-Like Behavior of Male Mice

Human immunodeficiency virus (HIV) is associated with co-morbid affective and stress-sensitive neuropsychiatric disorders that may be related to dysfunction of the hypothalamic-pituitary-adrenal (HPA) stress axis. The HPA axis is perturbed in up to 46% of HIV patients, but the mechanisms are not known. The neurotoxic HIV-1 regulatory protein, trans-activator of transcription (Tat), may contribute. We hypothesized that HPA dysregulation may contribute to Tat-mediated interactions with oxycodone, a clinically-used opioid often prescribed to HIV patients. In transgenic male mice, Tat expression produced significantly higher basal corticosterone levels with adrenal insufficiency in response to a natural stressor or pharmacological blockade of HPA feedback, recapitulating the clinical phenotype. On acute exposure, HIV-1 Tat interacted with oxycodone to potentiate psychomotor and anxiety like-behavior in an open field and light-dark transition tasks, whereas repeated exposure sensitized stress-related psychomotor behavior and the HPA stress response. Pharmacological blockade of glucocorticoid receptors (GR) partially-restored the stress response and decreased oxycodone-mediated psychomotor behavior in Tat-expressing mice, implicating GR in these effects. Blocking corticotrophin-releasing factor (CRF) receptors reduced anxiety-like behavior in mice that were exposed to oxycodone. Together, these effects support the notion that Tat exposure can dysregulate the HPA axis, potentially raising vulnerability to stress-related substance use and affective disorders.

Impact of Social Isolation on the Behavioral, Functional Profiles, and Hippocampal Atrophy Asymmetry in Dementia in Times of Coronavirus Pandemic (COVID-19): A Translational Neuroscience Approach

The impact of COVID-19 on the elderly is devastating, and nursing homes are struggling to provide the best care to the most fragile. The urgency and severity of the pandemic forces the use of segregation in restricted areas and confinement in individual rooms as desperate strategies to avoid the spread of disease and the worst-case scenario of becoming a deadly trap. The conceptualization of the post-COVID-19 era implies strong efforts to redesign all living conditions, care/rehabilitation interventions, and management of loneliness forced by social distance measures. Recently, a study of gender differences in COVID-19 found that men are more likely to suffer more severe effects of the disease and are over twice as likely to die. It is well-known that dementia is associated with increased mortality, and males have worse survival and deranged neuro-immuno-endocrine systems than females. The present study examines the impact of long-term isolation in male 3xTg-AD mice modeling advanced stages of Alzheimer's disease (AD) and as compared to age-matched counterparts with normal aging. We used a battery of ethological and unconditioned tests resembling several areas in nursing homes. The main findings refer to an exacerbated (two-fold increase) hyperactivity and emergence of bizarre behaviors in isolated 3xTg-AD mice, worrisome results since agitation is a challenge in the clinical management of dementia and an important cause of caregiver burden. This increase was consistently shown in gross (activity in most of the tests) and fine (thermoregulatory nesting) motor functions. Isolated animals also exhibited re-structured anxiety-like patterns and coping-with-stress strategies. Bodyweight and kidney weight loss were found in AD-phenotypes and increased by isolation. Spleen weight loss was isolation dependent. Hippocampal tau pathology was not modified, but asymmetric atrophy of the hippocampus, recently described in human patients with dementia and modeled here for the first time in an animal model of AD, was found to increase with isolation. Overall, the results show awareness of the impact of isolation in elderly patients with dementia, offering some guidance from translational neuroscience in these times of coronavirus and post-COVID-19 pandemic. They also highlight the relevance of personalized-based interventions tailored to the heterogeneous and complex clinical profile of the individuals with dementia and to consider the implications on caregiver burden.

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.

Neuroimmune nexus of depression and dementia: Shared mechanisms and therapeutic targets

Dysfunctional immune activity is a physiological component of both Alzheimer's disease (AD) and major depressive disorder (MDD). The extent to which altered immune activity influences the development of their respective cognitive symptoms and neuropathologies remains under investigation. It is evident, however, that immune activity affects neuronal function and circuit integrity. In both disorders, alterations are present in similar immune networks and neuroendocrine signalling pathways, immune responses persist in overlapping neuroanatomical locations, and morphological and structural irregularities are noted in similar domains. Epidemiological studies have also linked the two disorders, and their genetic and environmental risk factors intersect along immune-activating pathways and can be synonymous with one another. While each of these disorders individually contains a large degree of heterogeneity, their shared immunological components may link distinct phenotypes within each disorder. This review will therefore highlight the shared immune pathways of AD and MDD, their overlapping neuroanatomical features, and previously applied, as well as novel, approaches to pharmacologically manipulate immune pathways, in each neurological condition. LINKED ARTICLES: This article is part of a themed section on Therapeutics for Dementia and Alzheimer's Disease: New Directions for Precision Medicine. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.18/issuetoc.

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.

Neuronal calcineurin transcriptional targets parallel changes observed in Alzheimer disease brain

Synaptic dysfunction and loss are core pathological features in Alzheimer disease (AD). In the vicinity of amyloid-β plaques in animal models, synaptic toxicity occurs and is associated with chronic activation of the phosphatase calcineurin (CN). Indeed, pharmacological inhibition of CN blocks amyloid-β synaptotoxicity. We therefore hypothesized that CN-mediated transcriptional changes may contribute to AD neuropathology and tested this by examining the impact of CN over-expression on neuronal gene expression in vivo. We found dramatic transcriptional down-regulation, especially of synaptic mRNAs, in neurons chronically exposed to CN activation. Importantly, the transcriptional profile parallels the changes in human AD tissue. Bioinformatics analyses suggest that both nuclear factor of activated T cells and numerous microRNAs may all be impacted by CN, and parallel findings are observed in AD. These data and analyses support the hypothesis that at least part of the synaptic failure characterizing AD may result from aberrant CN activation leading to down-regulation of synaptic genes, potentially via activation of specific transcription factors and expression of repressive microRNAs.

OPEN PRACTICES

Open Science: This manuscript was awarded with the Open Materials Badge. For more information see: https://cos.io/our-services/open-science-badges/ Read the Editorial Highlight for this article on page 8.

Corticotrophin releasing factor receptor 1 antagonists prevent chronic stress-induced behavioral changes and synapse loss in aged rats

Mounting evidence suggests that chronic stress can alter brain structure and function and promote the development of neuropsychiatric disorders, such as depression and Alzheimer's disease. Although the results of several studies have indicated that aged brains are more vulnerable to chronic stress, it remains unknown whether antagonists of a key stress regulator, the corticotrophin releasing factor receptor 1 (CRF1), can prevent stress-induced anxiety and memory deficits in animal models. In this study, we evaluated the potential benefits of two CRF1 antagonists, R121919 and antalarmin, for preventing stress-induced anxiety-related behavioral and memory deficits and neurodegeneration in aged rats. We stressed rats using isolation-restraint for 3 months starting from the 18 months of age. Subsets of animals were administrated either R121919 or antalarmin through food chow for 3 months, followed by a series of behavioral, biochemical and morphological analyses. We found that stressed aged rats displayed body weight losses and increased corticosterone levels, as well as anxiety-related behaviors and memory deficits. Additionally, chronic stress induced a loss of cortical dendritic spines and synapses. However, R121919 and antalarmin both prevented stress-induced behavioral changes including anxiety-related behaviors and memory deficits and prevented synapse loss, perhaps through reversing HPA axis dysfunction. These results suggest that CRF1 antagonists may hold promise as a potential therapy for preventing stress-induced anxiety and memory deficits in aged individuals.

Sex differences in chronic stress responses and Alzheimer's disease

Clinical studies indicate that Alzheimer's disease (AD) disproportionately affects women in both disease prevalence and severity, but the mechanisms underlying this sex divergence are unknown. Though some have suggested this difference in risk is a reflection of known differences in longevity between men and women, mounting clinical and preclinical evidence supports women also having intrinsic susceptibilities towards the disease. While a number of potential risk factors have been hypothesized to affect these differences in risks, none have been definitively verified. In this review, we discuss a novel hypothesis whereby women's susceptibility to chronic stress also mediates increased risk for AD. As stress is a risk factor for AD, and women are twice as likely to develop mood disorders where stress is a major etiology, it is possible that sex dimorphisms in stress responses contribute to the increase in women with AD. In line with this, sex divergence in biochemical responses to stress have been noted along the hypothalamic-pituitary-adrenal (HPA) axis and among known molecular effectors of AD, with crosstalk between these processes also being likely. In addition, activation of the cortical corticotrophin-releasing factor receptor 1 (CRF1) signaling pathway leads to distinct female-biased increases in molecules associated with AD pathogenesis. Therefore, the different biochemical responses to stress between women and men may represent an intrinsic, sex-dependent risk factor for AD.

Anxiety-like behavior as an early endophenotype in the TgF344-AD rat model of Alzheimer's disease

Alzheimer's disease (AD) is characterized by progressive cognitive decline and the presence of aggregates of amyloid beta (plaques) and hyperphosphorylated tau (tangles). Early diagnosis through neuropsychological testing is difficult due to comorbidity of symptoms between AD and other types of dementia. As a result, there is a need to identify the range of behavioral phenotypes expressed in AD. In the present study, we utilized a transgenic rat (TgF344-AD) model that bears the mutated amyloid precursor protein as well as presenilin-1 genes, resulting in progressive plaque and tangle pathogenesis throughout the cortex. We tested young adult male and female TgF344-AD rats in a spatial memory task in the Morris water maze and for anxiety-like behavior in the elevated plus-maze. Results indicated that regardless of sex, TgF344-AD rats exhibited increased anxiety-like behavior in the elevated plus-maze, which occurred without significant deficits in the spatial memory. Together, these results indicate that enhanced anxiety-like behavior represents an early-stage behavioral marker in the TgF344-AD rat model.

Sex-biased cellular signaling: molecular basis for sex differences in neuropsychiatric diseases

The recognition that there are fundamental biological sex differences that extend beyond those that define sexual behavior and reproductive function has inspired the drive toward inclusion of both sexes in research design. This is supported by an underlying clinical rationale that studying both sexes is necessary to elucidate pathophysiology and develop treatments for the entire population. However, at a more basic level, sex differences, like genetic differences, can be exploited to better understand biology. Here, we discuss how sex differences at the molecular level of cell signaling and protein trafficking are amplified to create a state of vulnerability that under the right conditions can result in symptoms of neuropsychiatry disease. Although this dialogue focuses on the specific example of corticotropin-releasing factor, the potential for analogous sex differences in signaling and/or trafficking of receptors for other neuromodulators has broad biological and therapeutic implications.

Corticotropin-releasing factor overexpression gives rise to sex differences in Alzheimer's disease-related signaling

Several neuropsychiatric and neurodegenerative disorders share stress as a risk factor and are more prevalent in women than in men. Corticotropin-releasing factor (CRF) orchestrates the stress response, and excessive CRF is thought to contribute to the pathophysiology of these diseases. We previously found that the CRF₁ receptor (CRF₁) is sex biased whereby coupling to its GTP-binding protein, Gs, is greater in females, whereas β-arrestin-2 coupling is greater in males. This study used a phosphoproteomic approach in CRF-overexpressing (CRF-OE) mice to test the proof of principle that when CRF is in excess, sex-biased CRF₁ coupling translates into divergent cell signaling that is expressed as different brain phosphoprotein profiles. Cortical phosphopeptides that distinguished female and male CRF-OE mice were overrepresented in unique pathways that were consistent with Gs-dependent signaling in females and β-arrestin-2 signaling in males. Notably, phosphopeptides that were more abundant in female CRF-OE mice were overrepresented in an Alzheimer's disease (AD) pathway. Phosphoproteomic results were validated by demonstrating that CRF overexpression in females was associated with increased tau phosphorylation and, in a mouse model of AD pathology, phosphorylation of β-secretase, the enzyme involved in the formation of amyloid β. These females exhibited increased formation of amyloid β plaques and cognitive impairments relative to males. Collectively, the findings are consistent with a mechanism whereby the excess CRF that characterizes stress-related diseases initiates distinct cellular processes in male and female brains, as a result of sex-biased CRF₁ signaling. Promotion of AD-related signaling pathways through this mechanism may contribute to female vulnerability to AD.

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.

Sex-biased cellular signaling: molecular basis for sex differences in neuropsychiatric diseases

The recognition that there are fundamental biological sex differences that extend beyond those that define sexual behavior and reproductive function has inspired the drive toward inclusion of both sexes in research design. This is supported by an underlying clinical rationale that studying both sexes is necessary to elucidate pathophysiology and develop treatments for the entire population. However, at a more basic level, sex differences, like genetic differences, can be exploited to better understand biology. Here, we discuss how sex differences at the molecular level of cell signaling and protein trafficking are amplified to create a state of vulnerability that under the right conditions can result in symptoms of neuropsychiatry disease. Although this dialogue focuses on the specific example of corticotropin-releasing factor, the potential for analogous sex differences in signaling and/or trafficking of receptors for other neuromodulators has broad biological and therapeutic implications.

The CRF System as a Therapeutic Target for Neuropsychiatric Disorders

The major neuropsychiatric disorders are devastating illnesses that are only modestly responsive to treatment. Improving the treatment of these conditions will require innovative new strategies that depart from previously focused-on pharmacological mechanisms. Considerable preclinical and clinical data indicate corticotropin-releasing factor (CRF) signaling as a target for new psychotropic drug development. Here we review alterations in the CRF system reported in several psychiatric conditions. We also examine the preclinical work that has dissected the distinctive roles of CRF receptors in specific circuits relevant to these disorders. We further describe the clinical trials of CRF1 receptor antagonists that have been conducted. Although these clinical trials have thus far met with limited therapeutic success, the unfolding complexity of the CRF system promises many future directions for studying its role in the etiology and treatment of neuropsychiatric conditions.

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.

The stress response neuropeptide CRF increases amyloid-β production by regulating γ-secretase activity

The biological underpinnings linking stress to Alzheimer's disease (AD) risk are poorly understood. We investigated how corticotrophin releasing factor (CRF), a critical stress response mediator, influences amyloid-β (Aβ) production. In cells, CRF treatment increases Aβ production and triggers CRF receptor 1 (CRFR1) and γ-secretase internalization. Co-immunoprecipitation studies establish that γ-secretase associates with CRFR1; this is mediated by β-arrestin binding motifs. Additionally, CRFR1 and γ-secretase co-localize in lipid raft fractions, with increased γ-secretase accumulation upon CRF treatment. CRF treatment also increases γ-secretase activity in vitro, revealing a second, receptor-independent mechanism of action. CRF is the first endogenous neuropeptide that can be shown to directly modulate γ-secretase activity. Unexpectedly, CRFR1 antagonists also increased Aβ. These data collectively link CRF to increased Aβ through γ-secretase and provide mechanistic insight into how stress may increase AD risk. They also suggest that direct targeting of CRF might be necessary to effectively modulate this pathway for therapeutic benefit in AD, as CRFR1 antagonists increase Aβ and in some cases preferentially increase Aβ42 via complex effects on γ-secretase.