The Effects And Mechanisms Of Microglial Proliferation And Activation Regulated By TREM2 In Alzheimer’s Disease

Alzheimer’s disease(AD),characterized by extracellular amyloid deposition and intracellular neurofibrillary tangles,is a progressive neurodegenerative disease.Its clinical manifestations are mainly cognitive dysfunction and memory loss,accompanied by a series of psychiatric symptoms and behavioral changes.At present,the etiology of AD has so far unknown.Although many factors involved in the pathogenesis of AD,Aβ theory is recognized as the main mechanism.The theory thinks that due to the metabolic abnormalities of brain,leading to brain related enzyme activity disorders,and then resulting in toxic effects of protein fragments.The toxic protein fragments in the brain cannot be cleared in time,making its deposition in the specific area of the brain and formation of amyloid piaques.The result is,eventually,leading to neuronal death,inducing Alzheimer’s disease.β-amyloid protein(Aβ),composed of 39-43 amino acids,is the main component of senile plaques.It is the product of amyloid precursor(APP)after proteolytic enzyme action,including Aβ40 and Aβ42 fragments.Which Aβ42 fragment content is less,but it is more likely to gather to form amyloid protein,and therefore has a stronger toxicity.Microglia is the resident immune cell in the brain.It is the first and most important immune defense in the central nervous system(CNS),and plays an important role in various neurological diseases and inflammatory responses.The activation of microglia in physiological state plays an important role in nervous system diseases.Recent genome-wide association studies have shown that a rare mutation of triggering receptor expressed on myeloid cell 2(TREM2)is considered to be a risk factor for the pathogenesis of AD.TREM2 is a surface receptor of microglia,a protein that is highly expressed by microglia and associated with microglia activation itself,involved in phagocytosis of Aβ,repair of damaged neurons and removal of cell debris.Studies have shown that TREM2 deficiency augment Aβ accumulation due to a dysfunctional response of microglia,which fail to cluster around Aβ plaques and become apoptotic.Previous studies had shown that NF-κB mediated the expression of TREM2 in many diseases.NF-κB is a key nuclear transcription factor,usually in the form of homologous or heterodimer inactive form exists in almost all types of cells in the cytoplasm,has a very important function.NF-κB as a signal transduction in the landscape of the hub,with a wide range of biological activity,involved in inflammation,cancer,immune,cell proliferation and apoptosis and other physiological and pathological process of gene regulation.Pyrrolidinedithiocarbamic acid(PDTC)is the inhibitor of NF-κB and it can inhibit the activation of NF-κB in many cell lines.Objective:In this study,we enhanced the expression of TREM2 and promoted the proliferation of microglia,and further examined the effect of TREM2 on the pathogenesis of AD model mice.Methods:1.Aβ1-42 injected into Kunming mice unilateral hippocampus to build a similar animal model of AD.The contralateral hippocampus was injected with different concentrations of PDTC.The mice were divided into blank group,Aβ group and PDTC group(6 mg / L,8 mg / L,600 mg / L,800 mg / L).Animals were sacrificed 7days after injection.2.The morphological changes of hippocampus were observed by HE staining.3.The expression of Aβ,TREM2,microglia,BDNF and DCX in hippocampus were detected by immunohistochemistry and western blot.4.Immunofluorescence technique was used to detect co-expression of Aβ and microglia,and co-expression of microglia and TREM2.Results:1.The results of HE staining showed that the hippocampus were significantly different injury in the mice which injected Aβ compared with the control.Compared with the Aβ group,the injury was decreased when treated with PDTC.2.The results of Aβ immunohistochemistry showed that there is not Aβ deposition in the hippocampus of the control group.Aβ deposition was found in the hippocampus of Aβ group.The Aβ deposition in the hippocampus of Aβ group was more than the mice treated with PDTC.The results were significantly different(P<0.01).3.The expression of TREM2 in the hippocampus of Aβ group were lower than that of the control group.The expression of TREM2 in the PDTC treated group was positively correlated with the PDTC dose.The results were significantly different(P<0.01).The results of microglia immunohistochemistry showed that the expression of microglia in the hippocampus of Aβ group and PDTC group was mainly distributed around Aβ.Compared with Aβ group,the expression of microglia in PDTC group were increased and there was a positive correlation with PDTC in the dose range.The results were significantly different(P<0.01).4.Immunofluorescence co-localization showed that there was a large number of activated microglia arounded with the Aβ deposition and activated microglia could infiltrate into Aβ deposition.TREM2 and microglia showed co-expression relationship.5.The expression of BDNF and DCX in Aβ group were decreased compared with that of blank group.The expression of BDNF and DCX in PDTC group were increased and the results were significantly different(P<0.01).And the expression of BDNF and DCX were dependent on PDTC dose in a certain concentration range.Conclusions:1.Treated with PDTC can regulate the expression of TREM2 in mice hippocampus.2.Microglia can be activated and maintained phenotype by increasing expression of TREM2,thereby removing Aβ and inhibiting the inflammatory response.3.Activated microglia may secrete neurotrophic factors associated with nerve regeneration,promote neuronal regeneration,and mitigate AD pathology.