br Stress glucocorticoids as risk factor for Alzheimer
Stress/glucocorticoids as risk factor for Alzheimer’s Disease: preclinical studies A role for glucocorticoids in shaping the risk for AD is supported by animal studies using rats or mice injected with Aβ oligomers, tau protein or excitotoxins, or transgenic (Tg) mice carrying mutations associated with hFAD or tauopathies. The latter include Tg2576 mice expressing human APP with the Swedish mutation (KM670/671NL), mice carrying double mutations of APP and PSEN1, and 3xTgAD mice, carrying triple mutations of APP (Swedish mutations), PSEN1 (M146V), and MAPT (the gene encoding tau protein, P301L) [see 35 for a recent review on this topic]. In all these models, the endogenous glucocorticoid milieu was manipulated either by stress (most often by chronic application) or by corticosterone administration. Some studies have employed corticotrophin-releasing hormone (CRH) to activate the HPA axis, or CRH receptor antagonists to restrain HPA activation. In 1993, Elicia Elliot, at that time working in Robert Sapolsky’s Lab, demonstrated in rats that corticosterone administration exacerbated excitotoxic neuronal death caused by kainic Letrozole australia by eliciting antigenic modifications in tau protein resembling those present in neurofibrillary tangles found in the brain of AD patients . Few years later, it has been reported that rats receiving the synthetic glucocorticoid dexamethasone, showed an increase in amyloid ß precursor protein (AßPP) in several brain regions . These findings were confirmed in 3xTgAD mice, which represent a mixed model of AD and tauopathies (tau protein mutations have never been detected in AD). Combining in vitro and in vivo approaches, Green and coworkers, have investigated the effects of glucocorticoids (corticosterone and dexamethasone) on the molecular determinants of AD . Exposing mouse neuronal N2A cells to either corticosterone or dexamethasone, raised Aß1-42 formation by increasing the activity of the ß-APP cleaving enzyme BACE1. Increases in Aβ formation were also observed after subchronic (7 days) dexamethasone administration in 3xTgAD. In the same study, an age-dependent increase of serum corticosterone levels was demonstrated in 3xTgAD mice after 9 months of age. These studies paved the way to explore the effect of stress (of variable intensity and duration) on AD-vulnerable brain areas by analyzing pertinent pathological markers. Mice overexpressing human APP-CT100 containing the London mutation (V717I) were subjected to chronic restraint stress with the following characteristics: 8 months of duration, 6 h/day, and 4 days/week . In these mice, exposure to stress accelerated cognitive impairment as well as the density of vascular and extracellular deposits containing Aß peptide and carboxyl-terminal fragments of amyloid precursor protein (APP-CTFs) in the hippocampus. In the hippocampus and entorhinal and piriform cortices, stress caused neurodegeneration and tau phosphorylation and increased intraneuronal Aß and APP-CTFs immunoreactivity. Similar findings were obtained by Lee and co-workers, who exposed Tg2576 mice to a daily restraint stress of 2 h for 16 consecutive days . At the termination of stress procedure, increased Aß levels, plaque deposition, tau hyperphosphorylation, and neuritic atrophy were shown in the prefrontal cortex, hippocampus, parietal cortex, and piriform cortex. Of note, these changes were reversed by administration of the CRH receptor antagonist, NBI 27914, lending credit to the hypothesis that over-activation of the HPA axis is necessary for stress-induced AD-like pathogenesis. Tau hyperphosphorylation has a key pathogenic role in synaptic dysfunction and neuronal death associated with AD because phosphorylation detaches tau protein from microtubules resulting into cytoskeletal impairment and formation of neurofibrillary tangles , . A causal relationship between glucocorticoids and tau phosphorylation has been demonstrated in in vitro and in vivo studies. In PC12 cells engineered to express human tau, exposure to glucocorticoids markedly enhances tau phosphorylation at AD-related epitopes, and increases cell susceptibility to the neurotoxic action of Aβ⋅ Both effects were abolished by the GR antagonist, mifepristone. This study also explored the mechanism by which glucocorticoid treatment induces tau hyperphosphorylation, demonstrating the involvement of cyclin-dependent kinase 5 (CDK5) and glycogen synthase kinase-3 (GSK3) . The same research group performed in vivo studies in middle-aged rats, examining whether stress-induced glucocorticoid secretion could influence tau hyperphosphorylation . Male Wistar rats subjected to 1 month of chronic unpredictable stress showed tau hyperphosphorylation in the hippocampus and prefrontal cortex. Administration of dexamethasone to unstressed animals could also increase tau phosphorylation. Both stress and glucocorticoid administration increased levels of pERK1/2, pGSK3ß, p35 (an activator of CDK5), pJNK, and CaMKII in the prefrontal cortex and hippocampus. In line with biochemical data, both stress and dexamethasone administration impaired hippocampus- and prefrontal cortex-dependent memory, as assessed by the Morris water maze tests. In addition, neurotoxicity caused by intracerebral infusion of Aß was amplified by exogenous glucocorticoids. The Authors concluded that prolonged stress exposure via glucocorticoid hypersecretion could impact both the onset and progression of AD neuropathology. CRH plays a key role in both tonic and phasic (e.g. stress-induced) activation of the HPA axis. Carroll and colleagues focused on a possible role of CRH on AD neuropathology independently of its activity as main ACTH secretagogue . They used Tg2576 mice and PS19 mice expressing mutated (P301S) human tau. Two different stress protocols were applied for 1 month: (i) chronic restraint stress (CRS); and (ii) chronic unpredictable stress (CUS), the latter based on exposure to a number of stressors that varied in their sequence on a daily basis. In both transgenic lines, CRS, but not CUS, elicited an increase in Aß and hyperphosphorylated tau in the hippocampus and frontal cortex. CRS also induced a defect in hippocampal-dependent memory. Interestingly, implantation with a corticosterone-releasing pellet in PS19 mice did not mimic the effect of CRS in enhancing tau phosphorylation, whereas administration of a CRH receptor antagonist (NBI 27914) 15 min prior to daily stress application, prevented tau accumulation and memory impairment. These findings suggest that CRH acts centrally in causing AD-like neuropathology, an hypothesis supported by the evidence that transgenic mice overexpressing CRH showed increased levels of phosphorylated tau in the hippocampus. By using a phosphoproteomic approach, the same Authors found a gender-related difference in the vulnerability to CRF-induced AD neuropathology, with females being more vulnerable than males . This is in line with the greater prevalence of AD in women .