Study On The Mechanism Of Histone Deacetylase - 2 (HDAC2) In Chronic Stress - Induced Cognitive Impairment

BackgroundChronic stress has significant injury effects on cognition via diverse mechanisms including epigenetic regulation such as histone deacetylation. Chronic stress is also a risk factor of Alzheimer’s disease (AD) that with cognitive decline. Histone deacetylases family (Histone Deacetylases, HDACs) are crucial to catalyze histone deacetylations. In recent years, many specific HDAC members are found to participate in learning and memory such as HDAC2. HDAC2 belongs to the class I HDAC family, and participates in diseases with cognitive decline. Glucocorticoid receptor and HDAC2 both express in the in hippocampus, suggesting a possible functional link between them. Activated glucocorticoid receptor can combine with the promoter of Hdac2 and increase HDAC2 protein of hippocampus in a mouse model of Alzheimer’s disease. Glucocorticoid receptors are also activated under stress, indicating that HDAC2 may be involved in stress-induced cognitive dysfunction. In order to investigate whether HDAC2 is involved in chronic stress, experiments are carried out as followed.MethodsIn vitro experiments, the mouse derived neuroblastoma N2a cell line was used as the experiment material, corticosteroid as the drug. A treatment of corticosteroid at a dose of 2μM for 48 h was used to mimic the chronic stress intervention in vitro. After corticosteroid treatment, the cell morphology changed and the postsynaptic density marker PSD95 protein levels were evaluated to determine the dysfunction of N2a cells. The injury mechanisms including changes of HDAC2 protein, certain histones acetylation levels and protectable PI3K/AKT signal pathway phosphorylation levels were detected. After that, shRNA transfection was used to knockdown HDAC2 in N2a cells and corresponding improvement of the morphology and molecular markers were measured to evaluate the role of HDAC2 in corticosteroid induced N2a cell injury.In vivo experiments, both wild type C57BL/6J mice and APP/PS1 AD mouse were chosen as the experimental materials. The chronic restraint stress procedure was used to build up the chronic stress model. Related cognitive behavioral changes and HDAC2 protein levels were observed after stress of both mice. Synaptic plasticity markers such as PSD95 and GluR-1 levels, neurotrophic factors BDNF levels, early gene including Arc and Egr, as well as PI3K/AKT signaling pathway phosphorylation levels of C57 mice were evaluated before and after stress. Similar to the experiments in vitro, shRNA injection in hippocampus of C57 mice was done to knock down HDAC2, corresponding improvements of behavior tests and molecular markers were measured to evaluate the role of HDAC2 in the cognitive impairment induced by chronic restraint stress.ResultsIn vitro experiments, corticosteroid caused N2a cells injured after intervention:(1) the growth restrained, the average cell density decreased, the length of cell exogenous neurite structure significantly shortened; (2) the synaptic structure related marker PSD95 decreased significantly; (3) nervous protectoral PI3K/AKT phosphorylation (ser473) levels significantly reduced. At the meanwhile, HDAC2 protein levels rised significantly, and the histone H3K9 acetylation levels were down-regulated significantly. After HDAC2-shRNA transfection, both morphology and some molecular markers were partially improved.In vivo experiments, chronic stress can damage cognition in both mice and the hippocampus HDAC2 protein levels of both mice increased significantly. Synaptic plasticity related molecular PSD95 and GluR-1, neurotrophic factor BDNF, early gene Egr and AKT (ser473) phosphorylation levels were all decreased in C57 mice. Although hippocampus shRNA injection significantly down-regulated HDAC2 protein levels, limit improvement of behavior changes of C57 mice under chronic stress were found, while significant improvments were found after shRNA injection in mice without stress.ConclusionIn this study, HDAC2 are involved in the cognitive impairment caused by chronic stress and exert as a negative regulator but may be not the only one. The ability to resiste stress at different ages of life is not the same, the older, the more sensitive, showing that HDAC2 may be a cofactor for upregulate the sensitivity to stress. In some neurodegenerative diseases such as Alzheimer’s diseases, HDAC2 is significantly increased with limit specific mechanism. Chronic stress can increase HDAC2 protein levels may provide a new research direction for this.