The Effects And Mechanisms Of Glucocorticoids On Amyloid β-peptide Metabolism Of Astrocytes In Alzheimer's Disease

Alzheimer's disease (AD) is a chronic neurodegenerative disorder marked by a progressive loss of memory and cognitive function. The two hallmark neuropathological features are amyloidβ-peptide (Aβ) plaques and tau-laden neurofibrillary tangles. Although mutations in three different genes are known to underlie some cases of the rare, inheritable forms of the AD, the etiology of the more common sporadic cases remains unknown, and familial AD accounts for fewer than 10% of cases of AD and sporadic AD accounts for most case, it likely involves complex interactions between various genetic and environmental factors, such as a stressful lifestyle. Epidemiological evidence further supports a role for stress (such as a stressful lifestyle) as a risk factor for AD because elderly individuals prone to psychological distress are more likely to develop the disorder than age-matched, nonstressed individuals. Further, Aβlevels and amyloid plaque formation can be reduced by environmental enrichment, so it is important to investigate how environmental influences have an impact on and contribute to the pathogenesis of the disease.Neuroendocrine malfunctions may be involved in the disease process, particularly because it is established that stress hormones can negatively affect neuronal survival. Environmental factors such as stress activate receptors includingβ-adrenergic receptors and d-opioid receptor. Aβproduction can be reduced by activation of the muscarinic acetylcholine receptor or estrogen receptor, somatostatin,β-adrenergic receptor. Even some of them can ameliorate amyloid plaque pathology.Glucocorticoids (GC) are steroid hormones that readily cross the blood– brain barrier and bind to glucocorticoid receptors (GR). The glucocorticoid response to stressful stimuli is regulated by the hypothalamic–pituitary–adrenal (HPA) axis, which triggers the adrenal cortex to release GC (cortisol in primates, corticosterone in rodents). Activity of the receptor is crucial for many central nervous system (CNS) functions, including learning and memory. There is markedly elevated basal levels of circulating cortisol in AD, reflected by markedly elevated basal levels of circulating cortisol. Thus , GC as important neuroendocrine hormones in stress response have caused growing concern in the pathogenesis of AD.Aβis a major constituent of senile plaque and is derived from the sequential proteolytic cleavage of amyloid precursor protein (APP) byβ-secretase (β-site APP cleaving enzyme, BACE) andγ-secretase. Accumulation of the Aβin the brain can lead to the formation of amyloid deposits and is crucial for development of AD. The process that regulates the deposition of Aβin the brain is still under investigation. A better understanding of the mechanism leading to Aβproduction would facilitate the development of treatments for AD. Several glucocorticoid-responsive elements (GRE) within the APP and BACE promoter have been identified and these sites occur in a region of the promoter that positively influences transcription. So whether GC can increase the production of Aβvia influencing the expression of APP and BACE?An astrocytic gliosis and activation is always observed in brains of patients with AD. Although neurons are known to be the major source of Aβin AD, astrocytes, on the contrary, are known to be important for Aβclearance and degradation, for providing trophic support to neurons, and for forming a protective barrier between Aβdeposits and neurons. However, under certain conditions related to chronic stress, the role of astrocytes may not be beneficial. Studies have suggested that cultured astrocytes could generate modest amounts of Aβcompared with neurons. Recent work performed from transgenic mice (Tg2576) exhibiting amyloid plaques evidenced that astrocyte-derived Aβparticipate in plaque formation and maturation at later stages than neuronal Aβ. There is evidence demonstrating that astrocytes are an alternative source of BACE in animal models of chronic gliosis and in brains of AD patients and therefore may contribute to Aβplaque formation. And TGF-β1 has been confirmed to potentiate Aβproduction in human astrocytes and may enhance the formation of plaques burden in the brain of AD patients. These data led us to reconsider the participation of astrocytes in the amyloidogenic process. This would suggest that the mechanism for astrocytes play a role in the development of AD and that therapeutic strategies that target astrocyte activation in brain may be beneficial for the treatment of AD.The present study sought to determine whether GC modulate the hallmark neuropathological feature of AD-Aβformation and degradation through astrocyte and, if so, the underlying mechanism. The results will suggest a mechanism by which GC in stress affects AD neuropathology. The main contents and results of the research including:Firstly, we established the cell model of GC stimulate astrocytes. AdEASY adenovirus system was used to express APPSW mutant and wild type APP. Astrocytes infected with adenovirus were stimulated with dexamethasone or corticosterone (CORT). Unstimulated astrocytes secreted nearly no Aβ, but GC could trigger the Aβproduction in astrocytes, furthermore, the amounts of Aβwere correlated with the dose and time of treatment.Secondly, we determined the underlying mechanisms of how GC promote Aβsecretion in astrocytes. We investigated GC have a role in the biological effects through genomic effect or non-genomic effect. Results showed that bovine serum albumin-corticosterone (BSA-CORT) had no effects on Aβsecretion in astrocytes. Furthermore, the effect of GC occurs through activation of the GR, as an antagonist of this receptor type RU 38486 prevents GC mediated increases in Aβ, and the GR is widely known to mediate transcription during agonist binding, dimerization, and relocation to the nucleus. So it suggested that GC increased Aβsecretion in astrocytes via GR-mediated genomic effect.Thirdly, we investigated the levels of the BACE and its substrate APP in astrocytes after GC administration using real-time RT-PCR and Western blotting. Both the APP and BACE genes contain GC-response binding elements, making GC directly increase transcription of the APP and BACE genes, leading to the increased Aβproduction observed in astrocytes. Increases in APP and BACE proteins lead to increased processing of APP to C99 by BACE, which is consequently cleaved by theγ-secretase to release Aβ. Fourthly, we detected whether GC treatment of astrocytes in culture impeded degradation of Aβpeptides, in which in vivo application resulted in more compact Aβplaques containing more of the peptide. Cofocal microscope results demonstrated that GC reduced clearance and degradation of Aβin astrocytes, and this effect was mediated by GR.Fifthly, to study the link between elevated levels of stress hormones and AD genesis, we investigated the effects of GC on APP processing in astrocytes, as well as on the Aβburden in vivo. We found that GC increased the numbers of astrocytes both in cortex and hippocampus. And many GFAP+ astrocytes surrounded the Aβplaques. Consistent with stress being a risk factor for AD, we showed that administering GC to Tg-AD mice increased insoluble Aβload in both young and aged mice. Here we report the novel findings that levels of the BACE and its substrate APP in astrocytes are selectively increased after GC administration, resulting in increased production of Aβ.In conclusion, the present study indicates the contribution of GC to AD pathology–Aβplaque formation by increasing generation and reducing clearance of Aβin astrocytes. Our findings provide support for the hypothesis that elevated GC found in AD play a significant causal role in the development of the pathology and highlight a mechanism by which stress affects AD neuropathology and suggest that stress warrant additional consideration in the regimen of AD therapies.