Subchronic glucocorticoid receptor inhibition rescues early episodic memory and synaptic plasticity deficits in a mouse model of Alzheimer's disease

Convergent effects of synthetic glucocorticoid dexamethasone and amyloid beta in human olfactory neurosphere-derived cells

Stressful life events and glucocorticoid (stress) hormones appear to increase the risk of Alzheimer's disease and hasten its progression, but the reasons for this remain unclear. One potential explanation is that when amyloid β (Aβ) pathology is accumulating in the preclinical disease stage, glucocorticoid receptor signalling during stressful events exacerbates cellular dysfunction caused by Aβ. Alternatively, Aβ may disrupt glucocorticoid receptor signalling. To explore these possibilities, we investigated whether the synthetic glucocorticoid dexamethasone and Aβ have overlapping effects on the cellular proteome and whether Aβ influences canonical glucocorticoid receptor function. Human olfactory neurosphere-derived (ONS) cells, collected from the olfactory mucosa of six adult donors, were treated with soluble Aβ40 or Aβ42 followed by dexamethasone. Proteins were quantified by mass spectrometry. After 32 h treatment, Aβ40 and Aβ42 both induced profound changes in innate immunity-related proteins. After 72 h, Aβ42 formed widespread aggregates and induced few proteomic changes, whereas Aβ40 remained soluble and altered expression of mitochondrial and innate immunity-related proteins. ONS cells revealed overlapping impacts of Aβ40 and dexamethasone, with 23 proteins altered by both treatments. For 16 proteins (including eight mitochondrial proteins) dexamethasone counteracted the effects of Aβ40. For example, caspase 4 and methylmalonate-semialdehyde dehydrogenase were increased by Aβ40 and decreased by dexamethasone. Consistent with this finding, Aβ40 increased, but dexamethasone decreased, ONS cell proliferation. For seven proteins, including superoxide dismutase [Mn] mitochondrial, dexamethasone exacerbated the effects of Aβ40. For some proteins, including complement C3, the effects of dexamethasone differed depending on whether Aβ40 was present or absent. Neither Aβ species influenced glucocorticoid receptor nuclear translocation. Overall, this study revealed that glucocorticoid receptor signalling modifies the intracellular effects of Aß40, counteracting some effects and exacerbating others. It suggests that cellular mechanisms through which glucocorticoid receptor signalling influences Alzheimer's disease risk/progression are complex and determined by the balance of beneficial and detrimental glucocorticoid effects.

Targeting the adenosine A2A receptor for neuroprotection and cognitive improvement in traumatic brain injury and Parkinson's disease

Adenosine exerts its dual functions of homeostasis and neuromodulation in the brain by acting at mainly 2 G-protein coupled receptors, called A1 and A2A receptors. The adenosine A2A receptor (A2AR) antagonists have been clinically pursued for the last 2 decades, leading to final approval of the istradefylline, an A2AR antagonist, for the treatment of OFF-Parkinson's disease (PD) patients. The approval paves the way to develop novel therapeutic methods for A2AR antagonists to address 2 major unmet medical needs in PD and traumatic brain injury (TBI), namely neuroprotection or improving cognition. In this review, we first consider the evidence for aberrantly increased adenosine signaling in PD and TBI and the sufficiency of the increased A2AR signaling to trigger neurotoxicity and cognitive impairment. We further discuss the increasing preclinical data on the reversal of cognitive deficits in PD and TBI by A2AR antagonists through control of degenerative proteins and synaptotoxicity, and on protection against TBI and PD pathologies by A2AR antagonists through control of neuroinflammation. Moreover, we provide the supporting evidence from multiple human prospective epidemiological studies which revealed an inverse relation between the consumption of caffeine and the risk of developing PD and cognitive decline in aging population and Alzheimer's disease patients. Collectively, the convergence of clinical, epidemiological and experimental evidence supports the validity of A2AR as a new therapeutic target and facilitates the design of A2AR antagonists in clinical trials for disease-modifying and cognitive benefit in PD and TBI patients.

40 Hz Electroacupuncture relieves the memory dysfunction of 5xFAD mice by regulating neuronal electrical activity

In this investigation, we probed the impacts of 40 Hz Electroacupuncture (EA) on the cognitive function and brain activity in 5xFAD mice. Three groups of mice were constituted: the Model group of 5xFAD mice, the Wild Type (WT) group of littermate controls, and the EA group of 5xFAD mice subjected to EA treatment. Behavioral tests were conducted to evaluate memory function and anxiety levels, while the presence of Aβ plaques were detected via immunostaining, and neuronal activity was measured using multichannel recordings. Our results indicated that EA therapy enhanced memory function and anxiety-like behavior in 5xFAD mice, as well as diminishing the abundance and dimensions of Aβ plaques in the hippocampus and mPFC regions. Notably, the suppression of astrocyte activation was observed, which was potentially associated with alterations in gamma oscillation. Furthermore, the synaptic transmission of neurons was amplified, suggesting a possible modulation in neural activity. These findings indicate that 40 Hz EA could influence cognitive performance and potentially affect neuronal activity in 5xFAD mice, while the direct connection between EA and neuronal electrical activity regulation requires further exploration. The potential frequency-specific effects of EA on protective mechanisms in the brain was not addressed in this study and thus presents a direction for future research.

Stress and the risk of Alzheimer dementia: Can deconstructed engrams be rebuilt?

The exact neuropathological mechanism by which the dementia process unfolds is under intense scrutiny. The disease affects about 38 million people worldwide, 70% of which are clinically diagnosed with Alzheimer's disease (AD). If the destruction of synapses essential for learning, planning and decision-making is part of the problem, must the restoration of previously lost synapses be part of the solution? It is plausible that neuronal capacity to restitute information corresponds with the adaptive capacity of its connectivity reserve. A challenge will be to promote the functional connectivity that can compensate for the lost one. This will require better clarification of the remodeling of functional connectivity during the progression of AD dementia and its reversal upon experimental treatment. A major difficulty is to promote the neural pathways that are atrophied in AD dementia while suppressing others that are bolstered. Therapeutic strategies should aim at scaling functional connectivity to a just balance between the atrophic and hypertrophic systems. However, the exact factors that can help reach this objective are still unclear. Similarities between the effects of chronic stress and some neuropathological mechanisms underlying AD dementia support the idea that common components deserve prime attention as therapeutic targets.

Updates on the Physiopathology of Group I Metabotropic Glutamate Receptors (mGluRI)-Dependent Long-Term Depression

Group I metabotropic glutamate receptors (mGluRI), including mGluR1 and mGluR5 subtypes, modulate essential brain functions by affecting neuronal excitability, intracellular calcium dynamics, protein synthesis, dendritic spine formation, and synaptic transmission and plasticity. Nowadays, it is well appreciated that the mGluRI-dependent long-term depression (LTD) of glutamatergic synaptic transmission (mGluRI-LTD) is a key mechanism by which mGluRI shapes connectivity in various cerebral circuitries, directing complex brain functions and behaviors, and that it is deranged in several neurological and psychiatric illnesses, including neurodevelopmental disorders, neurodegenerative diseases, and psychopathologies. Here, we will provide an updated overview of the physiopathology of mGluRI-LTD, by describing mechanisms of induction and regulation by endogenous mGluRI interactors, as well as functional physiological implications and pathological deviations.

Pre-symptomatic synaptic dysfunction and longitudinal decay of hippocampal synaptic function in APPPS1 mouse model of Alzheimer's disease is sex-independent

Alzheimer's disease (AD) is an incurable, age-related and progressive neurodegenerative disease characterized by cognitive impairments. Deficits in synaptic plasticity were reported in various models of AD-like pathology and are considered as an early contributing factor of cognitive impairment. However, the majority of previous studies were focused on overt, symptomatic stages of pathology and assessed long-term potentiation (LTP), whereas long-term depression (LTD) was much less investigated and the precise nature of its involvement remains poorly defined. To better understand the earliest synaptic dysfunctions along the pre-symptomatic stage of AD-like pathology, we performed a detailed analysis of underlying mechanisms and quantified basal synaptic activity, presynaptic release probability, and synaptic plasticity such as post-tetanic potentiation (PTP), as well as LTP and LTD. These parameters were studied in APPPS1 mouse model at two time points (early- and mid-) along the pre-symptomatic stage, which were compared with alterations monitored at two later time-points, i.e. the onset of cognitive deficits and the overt stage of full-blown pathology. Because sex is known to be an instrumental biological parameter in AD pathophysiology, all alterations were assessed in both males and females. Our data show that, as compared to wild-type (WT) littermates, initial neuronal hyperexcitability, seen at early pre-symptomatic stage shifts subsequently towards hypoexcitability at mid-pre-symptomatic stage and remains impaired at advanced stages. The pre-symptomatic changes also involve increased synaptic plasticity as assessed by paired-pulse facilitation (PPF), which returns to basal level at the onset of pathology and remains stable afterwards. Synaptic plasticity is impaired by mid-pre-symptomatic stage and manifests as lowered LTP and absence of LTD induction, the latter being reported here for the first time. Observed LTP and LTD impairments both persist in older APPPS1 mice. Remarkably, none of the observed differences was gender-dependent. Altogether, our data evidence that major impairments in basal synaptic efficacy and plasticity are detectable already during mid-pre-symptomatic stage of AD-like pathogenesis and likely involve hyperexcitability as the underlying mechanism. Our study also uncovers synaptic alterations that may become critical read-outs for testing the efficiency of novel, pre-symptomatic stage-targeted therapies for AD.

The Molecular Effects of Environmental Enrichment on Alzheimer's Disease

Environmental enrichment (EE) is an environmental paradigm encompassing sensory, cognitive, and physical stimulation at a heightened level. Previous studies have reported the beneficial effects of EE in the brain, particularly in the hippocampus. EE improves cognitive function as well as ameliorates depressive and anxiety-like behaviors, making it a potentially effective neuroprotective strategy against neurodegenerative diseases such as Alzheimer's disease (AD). Here, we summarize the current evidence for EE as a neuroprotective strategy as well as the potential molecular pathways that can explain the effects of EE from a biochemical perspective using animal models. The effectiveness of EE in enhancing brain activity against neurodegeneration is explored with a view to differences present in early and late life EE exposure, with its potential application in human being discussed. We discuss EE as one of the non pharmacological approaches in preventing or delaying the onset of AD for future research.

Alternative Pharmacological Strategies for the Treatment of Alzheimer's Disease: Focus on Neuromodulator Function

Alzheimer's disease (AD) is a neurodegenerative disorder, comprising 70% of dementia diagnoses worldwide and affecting 1 in 9 people over the age of 65. However, the majority of its treatments, which predominantly target the cholinergic system, remain insufficient at reversing pathology and act simply to slow the inevitable progression of the disease. The most recent neurotransmitter-targeting drug for AD was approved in 2003, strongly suggesting that targeting neurotransmitter systems alone is unlikely to be sufficient, and that research into alternate treatment avenues is urgently required. Neuromodulators are substances released by neurons which influence neurotransmitter release and signal transmission across synapses. Neuromodulators including neuropeptides, hormones, neurotrophins, ATP and metal ions display altered function in AD, which underlies aberrant neuronal activity and pathology. However, research into how the manipulation of neuromodulators may be useful in the treatment of AD is relatively understudied. Combining neuromodulator targeting with more novel methods of drug delivery, such as the use of multi-targeted directed ligands, combinatorial drugs and encapsulated nanoparticle delivery systems, may help to overcome limitations of conventional treatments. These include difficulty crossing the blood-brain-barrier and the exertion of effects on a single target only. This review aims to highlight the ways in which neuromodulator functions are altered in AD and investigate how future therapies targeting such substances, which act upstream to classical neurotransmitter systems, may be of potential therapeutic benefit in the sustained search for more effective treatments.

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.

Roles of traditional chinese medicine regulating neuroendocrinology on AD treatment

The incidence of sporadic Alzheimer's disease (AD) is increasing in recent years. Studies have shown that in addition to some genetic abnormalities, the majority of AD patients has a history of long-term exposure to risk factors. Neuroendocrine related risk factors have been proved to be strongly associated with AD. Long-term hormone disorder can have a direct detrimental effect on the brain by producing an AD-like pathology and result in cognitive decline by impairing neuronal metabolism, plasticity and survival. Traditional Chinese Medicine(TCM) may regulate the complex process of endocrine disorders, and improve metabolic abnormalities, as well as the resulting neuroinflammation and oxidative damage through a variety of pathways. TCM has unique therapeutic advantages in treating early intervention of AD-related neuroendocrine disorders and preventing cognitive decline. This paper reviewed the relationship between neuroendocrine and AD as well as the related TCM treatment and its mechanism. The advantages of TCM intervention on endocrine disorders and some pending problems was also discussed, and new insights for TCM treatment of dementia in the future was provided.

Complex Processes Underlying the Dynamic Changes of D-serine Levels in AD Brains

BACKGROUND

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by extracellular β-amyloid (Aβ) plaques and cognitive impairments. D-Serine, produced by the enzyme serine racemase (SR) in the brain, functions as an endogenous co-agonist at the glycine-binding site of N-methyl-D-aspartate receptor (NMDAR), has been implicated in the pathophysiological progression of AD.

OBJECTIVES

Evidence regarding the understanding of the role and dynamic modulation of D-serine during AD progression remains controversial. This literature review aims to offer novel research directions for studying the functions and metabolisms of D-serine in AD brains.

METHODS

We searched PubMed, using D-serine/SR and AD as keywords. Studies related to NMDAR dysfunction, neuronal excitotoxicity, D-serine dynamic changes and inflammatory response were included.

RESULTS

This review primarily discusses: (i) Aβ oligomers' role in NMDAR dysregulation, and the subsequent synaptic dysfunction and neuronal damage in AD, (ii) D-serine's role in NMDAR-elicited excitotoxicity, and (iii) the involvement of D-serine and SR in AD-related inflammatory pathological progression.

CONCLUSION

We also presented supposed metabolism and dynamic changes of D-serine during AD progression and hypothesized that: (i) the possible modulation of D-serine levels or SR expression as an effective method of alleviating neurotoxicity during AD pathophysiological progression, and (ii) the dynamic changes of D-serine levels in AD brains possibly resulting from complex processes.

Simultaneous Alteration of the Circadian Variation of Memory, Hippocampal Synaptic Plasticity, and Metabolism in a Triple Transgenic Mouse Model of Alzheimer’s Disease

Alzheimer’s disease (AD) is characterized by progressive memory deficits accompanied by synaptic and metabolic deficits, namely of mitochondrial function. AD patients also display a disrupted circadian pattern. Thus, we now compared memory performance, synaptic plasticity, and mitochondria function in 24-week-old non-transgenic (non-Tg) and triple transgenic male mice modeling AD (3xTg-AD) at Zeitgeber 04 (ZT-4, inactive phase) and ZT-16 (active phase). Using the Morris water maze test to minimize the influence of circadian-associated locomotor activity, we observed a circadian variation in hippocampus-dependent learning performance in non-Tg mice, which was impaired in 3xTg-AD mice. 3xTg-AD mice also displayed a lack of circadian variation of their performance in the reversal spatial learning task. Additionally, the amplitude of hippocampal long-term potentiation also exhibited a circadian profile in non-Tg mice, which was not observed in 3xTg-AD mice. Moreover, cerebral cortical synaptosomes of non-Tg mice also displayed a circadian variation of FCCP-stimulated oxygen consumption as well as in mitochondrial calcium retention that were blunted in 3xTg-AD mice. In sum, this multidimensional study shows that the ability to maintain a circadian oscillation in brain behavior, synaptic plasticity, and synaptic mitochondria function are simultaneously impaired in 3xTg-AD mice, highlighting the effects of circadian misalignment in AD.

Enduring glucocorticoid-evoked exacerbation of synaptic plasticity disruption in male rats modelling early Alzheimer's disease amyloidosis

Synaptic dysfunction is a likely proximate cause of subtle cognitive impairment in early Alzheimer's disease. Soluble oligomers are the most synaptotoxic forms of amyloid ß-protein (Aß) and mediate synaptic plasticity disruption in Alzheimer's disease amyloidosis. Because the presence and extent of cortisol excess in prodromal Alzheimer's disease predicts the onset of cognitive symptoms we hypothesised that corticosteroids would exacerbate the inhibition of hippocampal synaptic long-term potentiation in a rat model of Alzheimer's disease amyloidosis. In a longitudinal experimental design using freely behaving pre-plaque McGill-R-Thy1-APP male rats, three injections of corticosterone or the glucocorticoid methylprednisolone profoundly disrupted long-term potentiation induced by strong conditioning stimulation for at least 2 months. The same treatments had a transient or no detectible detrimental effect on synaptic plasticity in wild-type littermates. Moreover, corticosterone-mediated cognitive dysfunction, as assessed in a novel object recognition test, was more persistent in the transgenic animals. Evidence for the involvement of pro-inflammatory mechanisms was provided by the ability of the selective the NOD-leucine rich repeat and pyrin containing protein 3 (NLRP3) inflammasome inhibitor Mcc950 to reverse the synaptic plasticity deficit in corticosterone-treated transgenic animals. The marked prolongation of the synaptic plasticity disrupting effects of brief corticosteroid excess substantiates a causal role for hypothalamic-pituitary-adrenal axis dysregulation in early Alzheimer's disease.

Connecting Mind-Body Therapy-Mediated Effects to Pathological Features of Alzheimer's Disease

Alzheimer's disease (AD) is a complex, multifactorial neurodegenerative disorder that represents a major and increasing global health challenge. In most cases, the first clinical symptoms of AD are preceded by neuropathological changes in the brain that develop years to decades before their onset. Therefore, research in the last years has focused on this preclinical stage of AD trying to discover intervention strategies that might, if implemented effectively, delay or prevent disease progression. Among those strategies, mind-body therapies such as yoga and meditation have gained increasing interest as complementary alternative interventions. Several studies have reported a positive impact of yoga and meditation on brain health in both healthy older adults and dementia patients. However, the underlying neurobiological mechanisms contributing to these effects are currently not known in detail. More specifically, it is not known whether yogic interventions, directly or indirectly, can modulate risk factors or pathological mechanisms involved in the development of dementia. In this article, we first review the literature on the effects of yogic practices on outcomes such as cognitive functioning and neuropsychiatric symptoms in patients with mild cognitive impairment and dementia. Then, we analyze how yogic interventions affect different risk factors as well as aspects of AD pathophysiology based on observations of studies in healthy individuals or subjects with other conditions than dementia. Finally, we integrate this evidence and propose possible mechanisms that might explain the positive effects of yogic interventions in cognitively impaired individuals.

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.

Targeting Microglia-Synapse Interactions in Alzheimer's Disease

In this review, we focus on the emerging roles of microglia in the brain, with particular attention to synaptic plasticity in health and disease. We present evidence that ramified microglia, classically believed to be "resting" (i.e., inactive), are instead strongly implicated in dynamic and plastic processes. Indeed, there is an intimate relationship between microglia and neurons at synapses which modulates activity-dependent functional and structural plasticity through the release of cytokines and growth factors. These roles are indispensable to brain development and cognitive function. Therefore, approaches aimed at maintaining the ramified state of microglia might be critical to ensure normal synaptic plasticity and cognition. On the other hand, inflammatory signals associated with Alzheimer's disease are able to modify the ramified morphology of microglia, thus leading to synapse loss and dysfunction, as well as cognitive impairment. In this context, we highlight microglial TREM2 and CSF1R as emerging targets for disease-modifying therapy in Alzheimer's disease (AD) and other neurodegenerative disorders.

Developmental synaptic regulator, TWEAK/Fn14 signaling, is a determinant of synaptic function in models of stroke and neurodegeneration

Identifying molecular mediators of neural circuit development and/or function that contribute to circuit dysfunction when aberrantly reengaged in neurological disorders is of high importance. The role of the TWEAK/Fn14 pathway, which was recently reported to be a microglial/neuronal axis mediating synaptic refinement in experience-dependent visual development, has not been explored in synaptic function within the mature central nervous system. By combining electrophysiological and phosphoproteomic approaches, we show that TWEAK acutely dampens basal synaptic transmission and plasticity through neuronal Fn14 and impacts the phosphorylation state of pre- and postsynaptic proteins in adult mouse hippocampal slices. Importantly, this is relevant in two models featuring synaptic deficits. Blocking TWEAK/Fn14 signaling augments synaptic function in hippocampal slices from amyloid-beta-overexpressing mice. After stroke, genetic or pharmacological inhibition of TWEAK/Fn14 signaling augments basal synaptic transmission and normalizes plasticity. Our data support a glial/neuronal axis that critically modifies synaptic physiology and pathophysiology in different contexts in the mature brain and may be a therapeutic target for improving neurophysiological outcomes.

Navigating Alzheimer's Disease via Chronic Stress: The Role of Glucocorticoids

Alzheimer's disease (AD) is a chronic intensifying incurable progressive disease leading to neurological deterioration manifested as impairment of memory and executive brain functioning affecting the physical ability like intellectual brilliance, common sense in patients. The recent therapeutic approach in Alzheimer's disease is only the symptomatic relief further emerging the need for therapeutic strategies to be targeted in managing the underlying silent killing progression of dreaded pathology. Therefore, the current research direction is focused on identifying the molecular mechanisms leading to the evolution of the understanding of the neuropathology of Alzheimer's disease. The resultant saturation in the area of current targets (amyloid β, τ Protein, oxidative stress etc.) has led the scientific community to rethink of the mechanistic neurodegenerative pathways and reprogram the current research directions. Although, the role of stress has been recognized for many years and contributing to the development of cognitive impairment, the area of stress has got the much-needed impetus recently and is being recognized as a modifiable menace for AD. Stress is an unavoidable human experience that can be resolved and normalized but chronic activation of stress pathways unsettle the physiological status. Chronic stress mediated activation of neuroendocrine stimulation is generally linked to a high risk of developing AD. Chronic stress-driven physiological dysregulation and hypercortisolemia intermingle at the neuronal level and leads to functional (hypometabolism, excitotoxicity, inflammation) and anatomical remodeling of the brain architecture (senile plaques, τ tangles, hippocampal atrophy, retraction of spines) ending with severe cognitive deterioration. The present review is an effort to collect the most pertinent evidence that support chronic stress as a realistic and modifiable therapeutic earmark for AD and to advocate glucocorticoid receptors as therapeutic interventions.

Therapeutic implications of hypothalamic-pituitaryadrenal-axis modulation in Alzheimer's disease: A narrative review of pharmacological and lifestyle interventions

With disease-modifying treatments for Alzheimer's disease (AD) still elusive, the search for alternative intervention strategies has intensified. Growing evidence suggests that dysfunction in hypothalamic-pituitaryadrenal-axis (HPAA) activity may contribute to the development of AD pathology. The HPAA, may therefore offer a novel target for therapeutic action. This review summarises and critically evaluates animal and human studies investigating the effects of pharmacological and non-pharmacological intervention on HPAA modulation alongside cognitive performance. The interventions discussed include glucocorticoid receptor antagonists and 11β-hydroxysteroid dehydrogenase inhibitors as well as lifestyle treatments such as physical activity, diet, sleep and contemplative practices. Pharmacological HPAA modulators improve pathology and cognitive deficit in animal AD models, but human pharmacological trials are yet to provide definitive support for such benefits. Lifestyle interventions may offer promising strategies for HPAA modification and cognitive health, but several methodological caveats across these studies were identified. Directions for future research in AD studies are proposed.

Cortisol hypersecretion and the risk of Alzheimer's disease: A systematic review and meta-analysis

BACKGROUND

Morning cortisol levels have been reported to be elevated among patients with Alzheimer's disease (AD); yet no meta-analysis has been conducted to confirm the existence and magnitude of this association. It also remains unclear whether hypercortisolism is a risk factor for AD.

METHODS

PubMed, EMBASE, and PsycINFO were systematically searched for eligible studies. Cross-sectional data were pooled using random-effects meta-analyses; the differences in morning cortisol levels between patients and cognitively normal controls were quantified. Longitudinal studies were qualitatively synthesised due to methodological heterogeneity.

RESULTS

17,245 participants from 57 cross-sectional studies and 19 prospective cohort studies were included. Compared with cognitively normal controls, AD patients had moderately increased morning cortisol in blood (g = 0.422, P < 0.001; I² = 48.5 %), saliva (g = 0.540, P < 0.001; I² = 13.6 %), and cerebrospinal fluids (g = 0.565, P = 0.003; I² = 75.3 %). A moderate elevation of morning cortisol was also detected in cerebrospinal fluids from patients with mild cognitive impairment (MCI) versus controls (g = 0.309, P = 0.001; I² = 0.0 %). Cohort studies suggested that higher morning cortisol may accelerate cognitive decline in MCI or mild AD patients, but the results in cognitively healthy adults were inconsistent.

CONCLUSIONS

Morning cortisol was confirmed to be moderately elevated in AD patients and may have diagnostic and prognostic values for AD.

Air Pollution, Stress, and Allostatic Load: Linking Systemic and Central Nervous System Impacts

 Air pollution is a risk factor for cardiovascular and respiratory morbidity and mortality. A growing literature also links exposure to diverse air pollutants (e.g., nanoparticles, particulate matter, ozone, traffic-related air pollution) with brain health, including increased incidence of neurological and psychiatric disorders such as cognitive decline, dementia (including Alzheimer’s disease), anxiety, depression, and suicide. A critical gap in our understanding of adverse impacts of pollutants on the central nervous system (CNS) is the early initiating events triggered by pollutant inhalation that contribute to disease progression. Recent experimental evidence has shown that particulate matter and ozone, two common pollutants with differing characteristics and reactivity, can activate the hypothalamic-pituitary-adrenal (HPA) axis and release glucocorticoid stress hormones (cortisol in humans, corticosterone in rodents) as part of a neuroendocrine stress response. The brain is highly sensitive to stress: stress hormones affect cognition and mental health, and chronic stress can produce profound biochemical and structural changes in the brain. Chronic activation and/or dysfunction of the HPA axis also increases the burden on physiological stress response systems, conceptualized as allostatic load, and is a common pathway implicated in many diseases. The present paper provides an overview of how systemic stress-dependent biological responses common to particulate matter and ozone may provide insight into early CNS effects of pollutants, including links with oxidative, inflammatory, and metabolic processes. Evidence of pollutant effect modification by non-chemical stressors (e.g., socioeconomic position, psychosocial, noise), age (prenatal to elderly), and sex will also be reviewed in the context of susceptibility across the lifespan.

Age-related shift in LTD is dependent on neuronal adenosine A2A receptors interplay with mGluR5 and NMDA receptors

Synaptic dysfunction plays a central role in Alzheimer's disease (AD), since it drives the cognitive decline. An association between a polymorphism of the adenosine A2A receptor (A2AR) encoding gene-ADORA2A, and hippocampal volume in AD patients was recently described. In this study, we explore the synaptic function of A2AR in age-related conditions. We report, for the first time, a significant overexpression of A2AR in hippocampal neurons of aged humans, which is aggravated in AD patients. A similar profile of A2AR overexpression in rats was sufficient to drive age-like memory impairments in young animals and to uncover a hippocampal LTD-to-LTP shift. This was accompanied by increased NMDA receptor gating, dependent on mGluR5 and linked to enhanced Ca2+ influx. We confirmed the same plasticity shift in memory-impaired aged rats and APP/PS1 mice modeling AD, which was rescued upon A2AR blockade. This A2AR/mGluR5/NMDAR interaction might prove a suitable alternative for regulating aberrant mGluR5/NMDAR signaling in AD without disrupting their constitutive activity.

The medial prefrontal cortex - hippocampus circuit that integrates information of object, place and time to construct episodic memory in rodents: Behavioral, anatomical and neurochemical properties

Rats and mice have been demonstrated to show episodic-like memory, a prototype of episodic memory, as defined by an integrated memory of the experience of an object or event, in a particular place and time. Such memory can be assessed via the use of spontaneous object exploration paradigms, variably designed to measure memory for object, place, temporal order and object-location inter-relationships. We review the methodological properties of these tests, the neurobiology about time and discuss the evidence for the involvement of the medial prefrontal cortex (mPFC), entorhinal cortex (EC) and hippocampus, with respect to their anatomy, neurotransmitter systems and functional circuits. The systematic analysis suggests that a specific circuit between the mPFC, lateral EC and hippocampus encodes the information for event, place and time of occurrence into the complex episodic-like memory, as a top-down regulation from the mPFC onto the hippocampus. This circuit can be distinguished from the neuronal component memory systems for processing the individual information of object, time and place.

Passive immunotherapy for N-truncated tau ameliorates the cognitive deficits in two mouse Alzheimer's disease models

Clinical and neuropathological studies have shown that tau pathology better correlates with the severity of dementia than amyloid plaque burden, making tau an attractive target for the cure of Alzheimer's disease. We have explored whether passive immunization with the 12A12 monoclonal antibody (26-36aa of tau protein) could improve the Alzheimer's disease phenotype of two well-established mouse models, Tg2576 and 3xTg mice. 12A12 is a cleavage-specific monoclonal antibody which selectively binds the pathologically relevant neurotoxic NH226-230 fragment (i.e. NH2htau) of tau protein without cross-reacting with its full-length physiological form(s). We found out that intravenous administration of 12A12 monoclonal antibody into symptomatic (6 months old) animals: (i) reaches the hippocampus in its biologically active (antigen-binding competent) form and successfully neutralizes its target; (ii) reduces both pathological tau and amyloid precursor protein/amyloidβ metabolisms involved in early disease-associated synaptic deterioration; (iii) improves episodic-like type of learning/memory skills in hippocampal-based novel object recognition and object place recognition behavioural tasks; (iv) restores the specific up-regulation of the activity-regulated cytoskeleton-associated protein involved in consolidation of experience-dependent synaptic plasticity; (v) relieves the loss of dendritic spine connectivity in pyramidal hippocampal CA1 neurons; (vi) rescues the Alzheimer's disease-related electrophysiological deficits in hippocampal long-term potentiation at the CA3-CA1 synapses; and (vii) mitigates the neuroinflammatory response (reactive gliosis). These findings indicate that the 20-22 kDa NH2-terminal tau fragment is crucial target for Alzheimer's disease therapy and prospect immunotherapy with 12A12 monoclonal antibody as safe (normal tau-preserving), beneficial approach in contrasting the early Amyloidβ-dependent and independent neuropathological and cognitive alterations in affected subjects.

Insights into the Therapeutic Potential of Glucocorticoid Receptor Modulators for Neurodegenerative Diseases

Glucocorticoids are crucial for stress-coping, resilience, and adaptation. However, if the stress hormones become dysregulated, the vulnerability to stress-related diseases is enhanced. In this brief review, we discuss the role of glucocorticoids in the pathogenesis of neurodegenerative disorders in both human and animal models, and focus in particular on amyotrophic lateral sclerosis (ALS). For this purpose, we used the Wobbler animal model, which mimics much of the pathology of ALS including a dysfunctional hypothalamic–pituitary–adrenal axis. We discuss recent studies that demonstrated that the pathological cascade characteristic for motoneuron degeneration of ALS is mimicked in the genetically selected Wobbler mouse and can be attenuated by treatment with the selective glucocorticoid receptor antagonist (GRA) CORT113176. In long-term treatment (3 weeks) GRA attenuated progression of the behavioral, inflammatory, excitatory, and cell-death-signaling pathways while increasing the survival signal of serine–threonine kinase (pAkt). The action mechanism of the GRA may be either by interfering with GR deactivation or by restoring the balance between pro- and anti-inflammatory signaling pathways driven by the complementary mineralocorticoid receptor (MR)- and GR-mediated actions of corticosterone. Accordingly, GR antagonism may have clinical relevance for the treatment of neurodegenerative diseases.

Transient upregulation of translational efficiency in prodromal and early symptomatic Tg2576 mice contributes to Aβ pathology

TG2576 mice show highest levels of the full length mutant Swedish Human Amyloid Precursor Protein (APPKM670/671LN) during prodromal and early sympotomatic stages. Interestingly, this occurs in association with the unbalanced expression of two of its RNA Binding proteins (RBPs) opposite regulators, the Fragile-X Mental Retardation Protein (FMRP) and the heteronuclear Ribonucleoprotein C (hnRNP C). Whether an augmentation in overall translational efficiency also contributes to the elevation of APP levels at those early developmental stages is currently unknown. We investigated this possibility by performing a longitudinal polyribosome profiling analysis of APP mRNA and protein in total hippocampal extracts from Tg2576 mice. Results showed that protein polysomal signals were exclusively detected in pre-symptomatic (1 months) and early symptomatic (3 months) mutant mice. Differently, hAPP mRNA polysomal signals were detected at any age, but a peak of expression was found when mice were 3-month old. Consistent with an early but transient rise of translational efficiency, the phosphorylated form of the initial translation factor eIF2α (p-eIF2α) was reduced at pre-symptomatic and early symptomatic stages, whereas it was increased at the fully symptomatic stage. Pharmacological downregulation of overall translation in early symptomatic mutants was then found to reduce hippocampal levels of full length APP, Aβspecies, BACE1 and Caspase-3, to rescue predominant LTD at hippocampal synapses, to revert dendritic spine loss and memory alterations, and to reinstate memory-induced c-fosactivation. Altogether, our findings demonstrate that overall translation is upregulated in prodromal and early symptomatic Tg2576 mice, and that restoring proper translational control at the onset of AD-like symptoms blocks the emergence of the AD-like phenotype.

Stress and neurodegeneration

Neurodegenerative diseases are a great concern because of aging worldwide population. Despite substantial effort to advance our understanding of the etiology and potential treatment of neurodegeneration, there remains a paucity of information with respect to this complex disease process. Interestingly, stress has been implicated among the potential mechanisms implicated in neurodegenerative pathology. Given the increase in chronic stress in modern society, this premise warrants further investigation. The aim of this review is to evaluate the influence of stress on neurodegeneration, the effect of neurodegenerative diseases diagnosis on stress, and therapeutic strategies for neurodegenerative diseases with a special focus on stress reduction. Neurodegenerative disorders such as Alzheimer's, Parkinson's and Huntington's Disease showed an acceleration in disease progression and a worsening of symptoms under stress. Some therapies (e.g., yoga, meditation) focused on reducing stress showed beneficial effects against neurodegeneration. Nevertheless, more studies are necessary in order to completely understand the implications of stress in neurodegeneration and the usefulness of stress reduction in the treatment thereof.

Glucocorticoid receptors signaling impairment potentiates amyloid-β oligomers-induced pathology in an acute model of Alzheimer's disease

Dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis occurs early in Alzheimer's disease (AD), associated with elevated circulating glucocorticoids (GC) and glucocorticoid receptors (GR) signaling impairment. However, the precise role of GR in the pathophysiology of AD remains unclear. Using an acute model of AD induced by the intracerebroventricular injection of amyloid-β oligomers (oAβ), we analyzed cellular and behavioral hallmarks of AD, GR signaling pathways, processing of amyloid precursor protein, and enzymes involved in Tau phosphorylation. We focused on the prefrontal cortex (PFC), particularly rich in GR, early altered in AD and involved in HPA axis control and cognitive functions. We found that oAβ impaired cognitive and emotional behaviors, increased plasma GC levels, synaptic deficits, apoptosis and neuroinflammatory processes. Moreover, oAβ potentiated the amyloidogenic pathway and enzymes involved both in Tau hyperphosphorylation and GR activation. Treatment with a selective GR modulator (sGRm) normalized plasma GC levels and all behavioral and biochemical parameters analyzed. GR seems to occupy a central position in the pathophysiology of AD. Deregulation of the HPA axis and a feed-forward effect on PFC GR sensitivity could participate in the etiology of AD, in perturbing Aβ and Tau homeostasis. These results also reinforce the therapeutic potential of sGRm in AD.

Stress hormones as potential mediators of air pollutant effects on the brain: Rapid induction of glucocorticoid-responsive genes

Air pollution is associated with adverse effects on brain health including cognitive decline, dementia, anxiety, depression, and suicide. While toxicological studies have demonstrated the potential for repeated or chronic pollutant exposure to lead to disease states, characterisation of initial biological responses to exposure is needed to better understand underlying mechanisms. The brain is highly sensitive to glucocorticoids (primarily cortisol in humans, corticosterone in rodents), stress hormones that play important roles in cognition and mental health. We tested whether glucocorticoids could be implicated in central nervous system (CNS) effects of pollutant exposure by examining glucocorticoid-dependent signaling across brain regions after exposure to the common pollutant ozone. Male Fischer-344 rats were exposed for 4 h to air or 0.8 ppm ozone ± metyrapone (50 mg/kg), a drug that blocks corticosterone synthesis (n = 5/group). Key glucocorticoid-responsive genes (serum- and glucocorticoid-inducible kinase, SGK; glucocorticoid-inducible leucine zipper, GILZ), and a gene responsive to both glucocorticoids and oxidative stress (metallothionein (MT)-1), were increased by ozone in all brain regions (olfactory bulb, frontal lobe, cortex, midbrain, hippocampus, cerebellum, brainstem), correlating with plasma corticosterone levels. Metyrapone prevented the increase in SGK and GILZ, and reduced but did not eliminate the effect on MT-1, suggesting glucocorticoid-dependent and -independent regulation. Administering exogenous corticosterone (10 mg/kg) to air-exposed rats reproduced the ozone effects, confirming specificity. The results demonstrate that early pollutant effects include stress hormone-dependent signaling. As both ozone and particulate matter activate the hypothalamic-pituitary-adrenal axis, and elevated glucocorticoids are implicated in brain pathologies, stress hormones could contribute to CNS impacts of air pollutants.

Neurovascular glucocorticoid receptors and glucocorticoids: implications in health, neurological disorders and drug therapy

Glucocorticoid receptors (GRs) are ubiquitous transcription factors widely studied for their role in controlling events related to inflammation, stress and homeostasis. Recently, GRs have reemerged as crucial targets of investigation in neurological disorders, with a focus on pharmacological strategies to direct complex mechanistic GR regulation and improve therapy. In the brain, GRs control functions necessary for neurovascular integrity, including responses to stress, neurological changes mediated by the hypothalamic-pituitary-adrenal axis and brain-specific responses to corticosteroids. Therefore, this review will examine GR regulation at the neurovascular interface in normal and pathological conditions, pharmacological GR modulation and glucocorticoid insensitivity in neurological disorders.

Targeting Synaptic Plasticity in Experimental Models of Alzheimer's Disease

Long-term potentiation (LTP) and long-term depression (LTD) of hippocampal synaptic transmission represent the principal experimental models underlying learning and memory. Alterations of synaptic plasticity are observed in several neurodegenerative disorders, including Alzheimer’s disease (AD). Indeed, synaptic dysfunction is an early event in AD, making it an attractive therapeutic target for pharmaceutical intervention. To date, intensive investigations have characterized hippocampal synaptic transmission, LTP, and LTD in in vitro and in murine models of AD. In this review, we describe the synaptic alterations across the main AD models generated so far. We then examine the clinical perspective of LTP/LTD studies and discuss the limitations of non-clinical models and how to improve their predictive validity in the drug discovery process.

Role of Hypothalamic-Pituitary-Adrenal Axis, Hypothalamic-Pituitary-Gonadal Axis and Insulin Signaling in the Pathophysiology of Alzheimer's Disease

Alzheimer's disease (AD), the commonest progressive neurodegenerative disorder of the brain, is clinically characterized by the formation of extracellular amyloid plaques and intracellular neurofibrillary tangles. Recent studies suggest a relationship between the endocrinal dysregulation and the neuronal loss during the AD pathology. Dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis and hypothalamic-pituitary-gonadal (HPG) axis regulating circulating levels of glucocorticoid hormones has been implicated in the pathophysiology of AD. Likewise, dysregulated insulin signaling, impaired glucose uptake and insulin resistance are some of the prime factors in the onset/progression of AD. In this review, we have discussed the changes in HPA and HPG axes, implicated insulin resistance/signaling and glucose regulation during the onset/progression of AD. Therefore, simultaneous detection of these endocrinal markers in the early or presymptomatic stages may help in the early diagnosis of AD. This evidence for implicated endocrinal functions supports the fact that modulation of endocrinal pathways can be used as therapeutic targets for AD. Future studies need to determine how the induction or inhibition of endocrinal targets could be used for predictable neuroprotection in AD therapies.

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.

High Cortisol and the Risk of Dementia and Alzheimer's Disease: A Review of the Literature

Introduction: Cortisol effects on the brain are exerted through two distinct receptors, inducing complex and even opposite effects on the cerebral structures implicated in the various cognitive functions. High cortisol may also have deleterious effects on the brain structures and contribute to neurodegeneration, in particular Alzheimer’s disease (AD), via different mechanisms.

Objective: To examine the interrelationships between cortisol, cognitive impairment and AD.

Methods: Review of the literature.

Results: Clinical studies found that elevated cortisol was associated with poorer overall cognitive functioning, as well as with poorer episodic memory, executive functioning, language, spatial memory, processing speed, and social cognition; while in animals, glucocorticoid administration resulted in cognitive impairment and abnormal behavior. In cognitively healthy subjects, higher cortisol levels have been associated with an increased risk of cognitive decline and AD. Subjects with dementia and Mild Cognitive Impairment (MCI) due to AD have been found to have higher CSF cortisol levels than cognitively healthy controls. Elevated CSF cortisol may also be associated with a more rapid cognitive decline in MCI due to AD. Elevated cortisol levels have been also found in delirium. High cortisol may mediate the impact of stressful life events, high neuroticism, depression, sleep disturbances, as well as cardiovascular risk factors on cognitive performance, neurodegeneration, and cognitive decline. High cortisol may also exert neurotoxic effects on the hippocampus, and promote oxidative stress and amyloid β peptide toxicity. Further possible underlying mechanisms include the interactions of cortisol with inflammatory mediators, neurotransmitters, and growth factors.

Conclusion: Elevated cortisol levels may exert detrimental effects on cognition and contribute to AD pathology. Further studies are needed to investigate cortisol-reducing and glucocorticoidreceptor modulating interventions to prevent cognitive decline.

3,4-Dihydroxyphenylethanol Assuages Cognitive Impulsivity in Alzheimer's Disease by Attuning HPA-Axis via Differential Crosstalk of α7 nAChR with MicroRNA-124 and HDAC6

Cognitive impulsivity, a form of suboptimal cost-benefit decision making, is an illustrious attribute of an array of neurodegenerative diseases including Alzheimer's disease (AD). In this study, a delay discounting paradigm was used to assess the effect of 3,4-dihydroxyphenylethanol (DOPET) on cognitive impulsivity, in an oA42i (oligomeric amyloid β_1-42 plus ibotenic acid) induced AD mouse model, using a nonspatial T-maze task. The results depicted that oA42i administration elevated cognitive impulsivity, whereas DOPET treatment attenuated the impulsive behavior and matched the choice of the sham-operated controls. In addition, DOPET treatment has ameliorated the anxiety-like behavior in the oA42i-challenged mice. Probing the molecular signaling cascades underpinning these functional ramifications in the oA42i-challenged mice revealed reduced cholinergic (α7 nAChR; alpha 7 nicotinic acetylcholine receptor) function, dysregulated hypothalamic-pituitary-adrenal (HPA) axis (manifested by amplified glucocorticoid receptor expression and plasma corticosterone levels), and also aberrations in the neuroepigenetic (microRNA-124, HDAC6 (histone deacetylase 6), and HSP90 (heat-shock protein 90) expressions) as well as nucleocytoplasmic (importin-α1 expression and nuclear ultra-architecture) continuum. Nonetheless, DOPET administration ameliorated these perturbations and the observations were in line with that of the sham-operated mice. Further validation of the results with organotypic hippocampal slice cultures (OHSCs) confirmed the in vivo findings. We opine that HPA-axis attunement by DOPET might be orchestrated through the α7 nAChR-mediated pathway. Based on these outcomes, we posit that 3,4-dihydroxyphenylethanol might be a potential multimodal agent for the management of cognitive impulsivity and neuromolecular quagmire in AD.

Central Role of Glucocorticoid Receptors in Alzheimer's Disease and Depression

Alzheimer’s disease (AD) is the principal neurodegenerative pathology in the world displaying negative impacts on both the health and social ability of patients and inducing considerable economic costs. In the case of sporadic forms of AD (more than 95% of patients), even if mechanisms are unknown, some risk factors were identified. The principal risk is aging, but there is growing evidence that lifetime events like chronic stress or stress-related disorders may increase the probability to develop AD. This mini-review reinforces the rationale to consider major depressive disorder (MDD) as an important risk factor to develop AD and points the central role played by the hypothalamic-pituitary-adrenal (HPA) axis, glucocorticoids (GC) and their receptors (GR) in the etiology of MDD and AD. Several strategies directly targeting GR were tested to neutralize the HPA axis dysregulation and GC overproduction. Given the ubiquitous expression of GR, antagonists have many undesired side effects, limiting their therapeutic potential. However, a new class of molecules was developed, highly selective and acting as modulators. They present the advantage to selectively abrogate pathogenic GR-dependent processes, while retaining beneficial aspects of GR signaling. In fact, these “selective GR modulators” induce a receptor conformation that allows activation of only a subset of downstream signaling pathways, explaining their capacity to combine agonistic and antagonistic properties. Thus, targeting GR with selective modulators, alone or in association with current strategies, becomes particularly attractive and relevant to develop novel preventive and/or therapeutic strategies to tackle disorders associated with a dysregulation of the HPA axis.

Glucocorticoid receptor activation induces decrease of hippocampal astrocyte number in rats

RATIONALE

The decrease of astrocyte number and hypothalamic-pituitary-adrenal (HPA) axis overactivity are observed in individuals with major depressive disorder. Elevated levels of glucocorticoids induced by hyperactivation of the HPA axis may result in glucocorticoid receptor (GR) activation. However, it is unclear whether there is a direct link between GR activation and the decrease of astrocyte number.

METHODS

Animals were exposed to chronic unpredictable stress (CUS) for 28 days and treated with continuous subcutaneous injections of vehicle or corticosterone (CORT; 40 mg/kg/day) for 21 days. We then administered mifepristone on day 21 after CUS and on day 18 after the CORT treatment. We observed behavioral deficits in the sucrose preference test, open field test, and forced swim test. Protein expression was analyzed using immunofluorescence (IF) and western blot (WB).

RESULTS

Animals exposed to CUS exhibited behavioral deficits in tests measuring anhedonia, anxiety, and despair state. They also had decreases in glial fibrillary acidic protein (GFAP) expression and numbers of GFAP-positive cells in the hippocampus. The behavioral and cellular alterations induced by CUS were reversed by subchronic treatment with the GR antagonist mifepristone. We also found that the subcutaneous injection of glucocorticoids may induce depression-like behavior and reduce GFAP protein expression in rats, which was similarly reversed by mifepristone.

CONCLUSIONS

These findings provide experimental evidence that GR activation due to elevated CORT levels induces the decrease of hippocampal astrocyte number in rats.

Identification of an acute functional cross-talk between amyloid-β and glucocorticoid receptors at hippocampal excitatory synapses

Amyloid-β is a peptide released by synapses in physiological conditions and its pathological accumulation in brain structures necessary for memory processing represents a key toxic hallmark underlying Alzheimer's disease. The oligomeric form of Amyloid-β (Aβο) is now believed to represent the main Amyloid-β species affecting synapse function. Yet, the exact molecular mechanism by which Aβο modifies synapse function remains to be fully elucidated. There is accumulating evidence that glucocorticoid receptors (GRs) might participate in Aβο generation and activity in the brain. Here, we provide evidence for an acute functional cross-talk between Aβ and GRs at hippocampal excitatory synapses. Using live imaging and biochemical analysis of post-synaptic densities (PSD) in cultured hippocampal neurons, we show that synthetic Aβo (100 nM) increases GR levels in spines and PSD. Also, in these cultured neurons, blocking GRs with two different GR antagonists prevents Aβo-mediated PSD95 increase within the PSD. By analyzing long-term potentiation (LTP) and long-term depression (LTD) in ex vivo hippocampal slices after pharmacologically blocking GR, we also show that GR signaling is necessary for Aβo-mediated LTP impairment, but not Aβo-mediated LTD induction. The necessity of neuronal GRs for Aβo-mediated LTP was confirmed by genetically removing GRs in vivo from CA1 neurons using conditional GR mutant mice. These results indicate a tight functional interplay between GR and Aβ activities at excitatory synapses.

Obesity and episodic memory function

Obesity-related lifestyle factors, such as physical activity behavior and dietary intake, have been shown to be associated with episodic memory function. From animal work, there is considerable biological plausibility linking obesity with worse memory function. There are no published systematic reviews evaluating the effects of obesity on episodic memory function among humans, and examining whether physical activity and diet influences this obesity-memory link. Thus, the purpose of this systematic review was to evaluate the totality of research examining whether obesity is associated with episodic memory function, and whether physical activity and dietary behavior confounds this relationship. A review approach was employed, using PubMed, PsychInfo, and Sports Discus databases. Fourteen studies met our criteria. Among these 14 reviewed studies, eight were cross-sectional, four were prospective, and two employed a randomized controlled experimental design. Twelve of the 14 studies did not take into consideration dietary behavior in their analysis, and similarly, nine of the 14 studies did not take into consideration participant physical activity behavior. Among the 14 studies, ten found an inverse association of weight status on memory function, but for one of these studies, this association was attenuated after controlling for physical activity. Among the 14 evaluated studies, four did not find a direct effect of weight status on memory. Among the four null studies, one, however, found an indirect effect of BMI on episodic memory and another found a moderation effect of BMI and age on memory function. It appears that obesity may be associated with worse memory function, with the underlying mechanisms discussed herein. At this point, it is uncertain whether adiposity, itself, is influencing memory changes, or rather, whether adiposity-related lifestyle behaviors (e.g., physical inactivity and diet) are driving the obesity-memory relationship.

AD-Related N-Terminal Truncated Tau Is Sufficient to Recapitulate In Vivo the Early Perturbations of Human Neuropathology: Implications for Immunotherapy

The NH₂tau 26-44 aa (i.e., NH₂htau) is the minimal biologically active moiety of longer 20-22-kDa NH₂-truncated form of human tau-a neurotoxic fragment mapping between 26 and 230 amino acids of full-length protein (htau40)-which is detectable in presynaptic terminals and peripheral CSF from patients suffering from AD and other non-AD neurodegenerative diseases. Nevertheless, whether its exogenous administration in healthy nontransgenic mice is able to elicit a neuropathological phenotype resembling human tauopathies has not been yet investigated. We explored the in vivo effects evoked by subchronic intracerebroventricular (i.c.v.) infusion of NH₂htau or its reverse counterpart into two lines of young (2-month-old) wild-type mice (C57BL/6 and B6SJL). Six days after its accumulation into hippocampal parenchyma, significant impairment in memory/learning performance was detected in NH₂htau-treated group in association with reduced synaptic connectivity and neuroinflammatory response. Compromised short-term plasticity in paired-pulse facilitation paradigm (PPF) was detected in the CA3/CA1 synapses from NH₂htau-impaired animals along with downregulation in calcineurin (CaN)-stimulated pCREB/c-Fos pathway(s). Importantly, these behavioral, synaptotoxic, and neuropathological effects were independent from the genetic background, occurred prior to frank neuronal loss, and were specific because no alterations were detected in the control group infused with its reverse counterpart. Finally, a 2.0-kDa peptide which biochemically and immunologically resembles the injected NH₂htau was endogenously detected in vivo, being present in hippocampal synaptosomal preparations from AD subjects. Given that the identification of the neurotoxic tau species is mandatory to develop a more effective tau-based immunological approach, our evidence can have important translational implications for cure of human tauopathies.

Targeting glucocorticoid receptors prevents the effects of early life stress on amyloid pathology and cognitive performance in APP/PS1 mice

Exposure to chronic stress or elevated glucocorticoid hormone levels in adult life has been associated with cognitive deficits and an increased risk for Alzheimer's disease (AD). Since exposure to stress during early life enhances stress-responsiveness and lastingly affects cognition in adult life, we here investigated; (i) whether chronic early life stress (ELS) affects AD pathology and cognition in middle-aged APPswe/PS1dE9 mice, and (ii) whether it is still possible to rescue these late effects by briefly blocking glucocorticoid receptors (GRs) at a translationally relevant, middle age. Transgenic APPswe/PS1dE9 mice were subjected to ELS by housing dams and pups with limited nesting and bedding material from postnatal days 2-9 only. In 6- and 12-month-old offspring, this resulted in enhanced hippocampal amyloid-β (Aβ)-40 and -42 levels, and in reduced cognitive flexibility, that correlated well with the Aβ42 levels. In parallel, CORT levels and BACE1 levels were significantly elevated. Surprisingly, blocking GRs for only 3 days at 12 months of age reduced CORT levels, reduced hippocampal Aβ40 and -42, and β-site APP-cleaving enzyme 1 (BACE1) levels, and notably rescued the cognitive deficits in 12-month-old APPswe/PS1dE9 mice. These mouse data demonstrate that exposure to stress during the sensitive period early in life influences later amyloid pathology and cognition in genetically predisposed, mutant mice, and as such, may increase AD vulnerability. The fact that a short treatment with a GR antagonist at middle age lastingly reduced Aβ levels and rescued the cognitive deficits after ELS, highlights the therapeutic potential of this drug for reducing amyloid pathology.

Enhanced Hypothalamic NMDA Receptor Activity Contributes to Hyperactivity of HPA Axis in Chronic Stress in Male Rats

Chronic stress stimulates corticotrophin-releasing hormone (CRH)-expressing neurons in the paraventricular nucleus (PVN) of the hypothalamus and leads to hypothalamic-pituitary-adrenal (HPA) axis hyperactivity, but the mechanisms underlying this action are unknown. Because chronic stress enhances N-methyl-d-aspartate receptor (NMDAR) activity in various brain regions, we hypothesized that augmented NMDAR activity contributes to the hyperactivity of PVN-CRH neurons and the HPA axis in chronic stress. We performed whole-cell patch-clamp recordings on PVN-CRH neurons expressing CRH promoter-driven enhanced green fluorescent protein in brain slices from rats exposed to chronic unpredictable mild stress (CUMS) and unstressed rats. CUMS rats had significantly higher expression levels of the NMDAR subunits GluN1 in the PVN than unstressed rats. Furthermore, puff NMDA-elicited currents, evoked NMDAR currents, and the baseline frequency of the miniature excitatory postsynaptic currents (mEPSCs) in PVN-CRH neurons were significantly larger in CUMS rats than in unstressed rats. The NMDAR-specific antagonist 2-amino-5-phosphonopentanoic acid (AP5) significantly decreased the frequency of mEPSCs of PVN-CRH neurons in CUMS rats but did not change the frequency or amplitude of mEPSCs in unstressed rats. Bath application of AP5 normalized the elevated firing activity of PVN-CRH neurons in CUMS rats but not in unstressed rats. In addition, microinjection of the NMDAR antagonist memantine into the PVN normalized the elevated corticosterone (CORT) levels in CUMS rats to the levels in unstressed rats, but did not alter CORT levels in unstressed rats. Our findings suggest that synaptic NMDAR activity is enhanced in CUMS rats and contributes to the hyperactivity of PVN-CRN neurons and the HPA axis.

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.

Nuclear deterrents: Intrinsic regulators of IL-1β-induced effects on hippocampal neurogenesis

Hippocampal neurogenesis, the process by which new neurons are born and develop into the host circuitry, begins during embryonic development and persists throughout adulthood. Over the last decade considerable insights have been made into the role of hippocampal neurogenesis in cognitive function and the cellular mechanisms behind this process. Additionally, an increasing amount of evidence exists on the impact of environmental factors, such as stress and neuroinflammation on hippocampal neurogenesis and subsequent impairments in cognition. Elevated expression of the pro-inflammatory cytokine interleukin-1β (IL-1β) in the hippocampus is established as a significant contributor to the neuronal demise evident in many neurological and psychiatric disorders and is now known to negatively regulate hippocampal neurogenesis. In order to prevent the deleterious effects of IL-1β on neurogenesis it is necessary to identify signalling pathways and regulators of neurogenesis within neural progenitor cells that can interact with IL-1β. Nuclear receptors are ligand regulated transcription factors that are involved in modulating a large number of cellular processes including neurogenesis. In this review we focus on the signalling mechanisms of specific nuclear receptors involved in regulating neurogenesis (glucocorticoid receptors, peroxisome proliferator activated receptors, estrogen receptors, and nuclear receptor subfamily 2 group E member 1 (NR2E1 or TLX)). We propose that these nuclear receptors could be targeted to inhibit neuroinflammatory signalling pathways associated with IL-1β. We discuss their potential to be therapeutic targets for neuroinflammatory disorders affecting hippocampal neurogenesis and associated cognitive function.

Membrane-Associated Effects of Glucocorticoid on BACE1 Upregulation and Aβ Generation: Involvement of Lipid Raft-Mediated CREB Activation

Glucocorticoid has been widely accepted to induce Alzheimer's disease, but the nongenomic effect of glucocorticoid on amyloid β (Aβ) generation has yet to be studied. Here, we investigated the effect of the nongenomic pathway induced by glucocorticoid on amyloid precursor protein processing enzymes as well as Aβ production using male ICR mice and human neuroblastoma SK-N-MC cells. Mice groups exposed to restraint stress or intracerebroventricular injection of Aβ showed impaired cognition, decreased intracellular glucocorticoid receptor (GR) level, but elevated level of membrane GR (mGR). In this respect, we identified the mGR-dependent pathway evoked by glucocorticoid using impermeable cortisol conjugated to BSA (cortisol-BSA) on SK-N-MC cells. Cortisol-BSA augmented the expression of β-site amyloid precursor protein cleaving enzyme 1 (BACE1), the level of C-terminal fragment β of amyloid precursor protein (C99) and Aβ production, which were maintained even after blocking intracellular GR. We also found that cortisol-BSA enhanced the interaction between mGR and Gαs, which colocalized in the lipid raft. The subsequently activated CREB by cortisol-BSA bound to the CRE site of the BACE1 promoter increasing its expression, which was downregulated by inhibiting CBP. Consistently, blocking CBP attenuated cognitive impairment and Aβ production induced by corticosterone treatment or intracerebroventricular injection of Aβ more efficiently than inhibiting intracellular GR in mice. In conclusion, glucocorticoid couples mGR with Gαs and triggers cAMP-PKA-CREB axis dependent on the lipid raft to stimulate BACE1 upregulation and Aβ generation.SIGNIFICANCE STATEMENT Patients with Alzheimer's disease (AD) have been growing sharply and stress is considered as the major environment factor of AD. Glucocorticoid is the primarily responsive factor to stress and is widely known to induce AD. However, most AD patients usually have impaired genomic pathway of glucocorticoid due to intracellular glucocorticoid receptor deficiency. In this respect, the genomic mechanism of glucocorticoid faces difficulties in explaining the consistent amyloid β (Aβ) production. Therefore, it is necessary to investigate the novel pathway of glucocorticoid on Aβ generation to find a more selective therapeutic approach to AD patients. In this study, we revealed the importance of nongenomic pathway induced by glucocorticoid where membrane glucocorticoid receptor plays an important role in Aβ formation.

Physiological and pathophysiological control of synaptic GluN2B-NMDA receptors by the C-terminal domain of amyloid precursor protein

The amyloid precursor protein (APP) harbors physiological roles at synapses and is central to Alzheimer’s disease (AD) pathogenesis. Evidence suggests that APP intracellular domain (AICD) could regulate synapse function, but the underlying molecular mechanisms remain unknown. We addressed AICD actions at synapses, per se, combining in vivo AICD expression, ex vivo AICD delivery or APP knock-down by in utero electroporation of shRNAs with whole-cell electrophysiology. We report a critical physiological role of AICD in controlling GluN2B-containing NMDA receptors (NMDARs) at immature excitatory synapses, via a transcription-dependent mechanism. We further show that AICD increase in mature neurons, as reported in AD, alters synaptic NMDAR composition to an immature-like GluN2B-rich profile. This disrupts synaptic signal integration, via over-activation of SK channels, and synapse plasticity, phenotypes rescued by GluN2B antagonism. We provide a new physiological role for AICD, which becomes pathological upon AICD increase in mature neurons. Thus, AICD could contribute to AD synaptic failure.

DOI:http://dx.doi.org/10.7554/eLife.25659.001