Effects Of High Cholesterol On Cognition Function In AD Rats Model And Its Mechanism

Alzheimer's disease is the most common form of dementia in later life, and a major cause of disability and death in the elderly. Recently study indicates vascular risk factors play important role in the pathogenesis and development of Alzheimer's disease. It is important to interfere in and reduce modifiable vascular risk factors to prevent and delay Alzheimer's disease.In all vascular risk factor, hypercholesteremia in midlife have the closest relation with AD in older age. There have been accumulating evidences pointing toward a potentially important link between high cholesterol, Aβ, and AD. Evidences from clinical and epidemiological studies suggest that high cholesterol might influence the development of AD. There are several investigations demonstrating that individuals with elevated serum cholesterol levels have increased susceptibility to AD. The prevalence of AD is higher in countries with high-fat and high-calorie diets and lower in those with low-in-fat diets. In addition, recent other studies have indicated that the prevalence of AD is reduced among people taking a class of cholesterol-lowering medicines. Experimental studies provided additional and important support for the role of cholesterol in AD. Several studies in vitro have reported that modifications of cholesterol content can influence the expression of the amyloid precursor protein (APP) and alter Aβproduction, and there had also circumstantial evidence that high cholesterol could change Aβdeposition. Cholesterol-enriched diet can destroy synaptic plasticity, but regular diet can reverse the impaiment of synaptic plasticity. Then, whether high cholesterol further aggravate AD patient's pathogenetic changes, and how to influence their cognition function are not fully understood.Inflammatory reaction in the central nervous system is an early pathological change in the brains of AD patients. In addition, there have been clear evidences of inflammatory process in the AD brain. Neuritic plaques activate microgial cell and astrocytes even before severe cognitive decline. A lot of active microgial cells and reactive astrocytes have the potential roles to injure neurons by the release of different cytotoxic factors including reactive oxygen species, nitric oxide, proinflammatory cytokines, proteolytic enzymes, complement factors, and excitatory amino acids et al. Evidences suggested that cholesterol-fed induced development of inflammation and AD-like pathological changes in rabbit brain. Another study shows high cholesterol induces the activation of both microglia and astrocyte in mice. High cholesterol induced expression increase of IL-6. Whether high cholesterol can increase overall inflammatory damage induced by Aβis unclear.Neurofibrillary tangles (NFTs) is a common feature of Alzheimer's disease (AD). Tau is a multifunctional microtubule-associated-protein, which can stable microtubule and promote assembly of microtubule. Hyperphosphorylated Tau is major component of neurofibillary tangles. Studies show Aβis trigger of pathogenesis and abnormal function of Tau, which results in functional disturbance and death of neuron. There are few studies of relation between high cholesterol and Tau hyperphosphorylation, effect of high cholesterol on Tau hyperphosphorylation induced by Aβhas not been reported.It is necessary to understand the mechanism of influence of high cholesterol on the pathogenesis and development of AD, and to supply experimental base for preventing and delaying AD by change of cholesterol content. Thus in the present study, there are main contents including effects of high cholesterol diet on cognitive impairment by Aβinjection in hippocampus of rats; By studies in vive and vitro, evaluate the effects of high cholesterol on neurons loss in hippocampus of AD rats; influences of high cholesterol on expression of IL-6, TNF-αand phosporylation of Tau induced by Aβ.Method:1. Effect of high cholesterol on Aβ-induced cognitive impairment and its'mechanism. SD rats (male, weight 250±20g,) were divided into four groups randomly: CH+ AD group,AD group, CH+Sal group, Sal group. Each group has eight rats. CH+AD group and CH+Sal group rats had eaten high cholesterol diet, AD group and Sal group rats had Standard rat diet for eight weeks, then aggregated beta-amyloid1-40 was injected into right hippocampus of CH+AD group and AD group to establish AD rat modles, Sal was injected into right hippocampus of CH+Sal and sal group rats. Morris water maze and shuttle box test were measured rat learning and memory function; The pathological changes were observed with HE staining; Nissle's staining was used to assessed loss of neuron; Aβ depositon was evaluated by congo red staining; GFAP, IL-6, TNF-αand (Pser202)Tau protein expression were analyzed by immunohistochemistry; IL-6 and TNF-αmRNA were further detected by in suit hybridization and RT-PCR.2. Effect of high cholesterol on Aβ-induced injury of neurons and activation of glial cell in vitro.The mixed hippocampal cells of 2d-old Sprague-Dawley rats were cultured. There were three groups: normal control group, Aβ(2μM) group, cholesterol (1mM)+Aβ(2μM) group. Lactic dehydrogenase (LDH) releasing assay was applied for measurement of the change of neuron cell. IL-6 and TNF-αcontents in culture supernatants were determined by ELISA and neurons were subjected to fluorescence staining.Result:1. Effect of high cholesterol on Aβ-induced cognitive impairment and its mechanism. CH+AD rats exhibited seriously learning and memory deficits in both Morris water maze and shuttle box. In Morris water maze test, mean value of escape latency of CH+Aβgroup was longer than the other groups, CH+AD group had shorter distance percentage of spending in the former platform quadrant and fewer frequency of crossing the former platform site than the SD+ Aβand other groups. In shuttle box, CH+AD group had fewer AAR, GAR and longer escape latency, comparing with AD and other group. It suggested high cholesterol aggravated cognitive impairment of AD rats.CH+AD group rats showed most significant defects of neurons in all groups. With the same food, Aβ-injected rats had greater neuronal loss than Sal rats. With the same injection, high cholesterol fed rat had more neurons loss than standard diet.IL-6 and TNF-αpositive cells of dentate guys and CA1 were more in CH+AD group rats than the other groups; There were more IL-6 and TNF-αpositive cell in cortex of high cholesterol diet group than standard diet group. In RT-PCR trail, expression of IL-6 mRNA and TNF-αmRNA is higher in CH+AD group rats than AD groups.High cholesterol induced increase of (Pser202)Tau positive cells in dentate guys and CA1, CH+AD rats had the most number of (Pser202)Tau positive cell.CH+AD rats had significantly more (Pser202)Tau positive cell than AD rats.2. Effect of high cholesterol on Aβ-induced injury of neurons and activation of glial cell in vitro. Percent of LDH release in hippocampal culture of Aβgroup is larger the control group. In all groups, CH+Aβhas the largest percent of LDH release, and larger than Aβgroup.In the normal control group, fluorescence stains of the neuron were even, neuronal body was full, nerve processes are long and formed a network. In the Aβgroup, the number of neurons decrease and nerve processes shorten, CH+Aβgroup has fewer neurons and shorter nerve process than Aβgroup.IL-6 and TNF-αcontents in culture supernatants of normal control group are low. IL-6 and TNF-αcontents of Aβgroup significantly increase, in all group, IL-6 and TNF-αcontents of CH+Aβgroup is highest, the differences in IL-6 and TNF-αcontents were significant between CH+Aβgroup and Aβgroup.Conclusion:1. High cholesterol may deteriorate cognitive impairment caused by Aβinjection into hippocampus of rats.2. High cholesterol increases the expression of tow important inflammatory cytokines IL-6 and TNF-α,and phosphorylation of (ser202)Tau in hippocampus of AD group,which causes more neurons defect and severer cognitive impairment.3. high cholesterol facilitates Aβ-induced injury of cultured neurons and activation of glial cells, which secrete IL-6 and TNF-α.Experiment in vitro further confirms that high cholesterol enhance injury of neurons and activation of glial cells induced by Aβ.