Proteomic Study Of Human Hippocampal Subregions Reveals The Key Role Of Astrocytes In The Pathogenesis Of Alzheimer’s Disease

BackgroundAlzheimer’s disease(AD),the leading cause of dementia,is one of the most common progressive neurodegenerative diseases featured by progressive dementia,language disorders and memory loss.The neuropathological manifestations of AD are extracellular plaques formed by β-Amyloid(Aβ),intracellular Neurofibrillary tangles(NFTs)consist of hyperphosphorylated tau protein,loss of neuron,activation of glial cells and loss of synaptic components.Moreover,the pathogenesis of AD has not been elucidated.The effects of drug therapy are also limited.Currently,the main drugs approved by FDA for the treatment of AD are Acetylcholinesterase(AchE)inhibitors and NMDA receptor(Nmethyl-D-asparticacid receptor)antagonists.AchE inhibitors include Tacrine,Donepezil,Galanthamine and Rivastigmine,which increase AchE levels and synaptic transmission.NMDA receptor antagonists,represented by Memantine,regulate glutamate activity.But these drugs can only reduce the symptoms of AD or delay the progression of the disease,rather than cure it or reverse the course.None of the recent promising drugs targeting Aβand tau have shown significant results in Phase Ⅲ trials.Even Aducanumab,the most recent drug that was approved by FDA for the treantment of AD in 2021,is also controversial because the clearance of Aβ isn’t mean an improvement in cognition.Moreover,studies of the mechanism of AD have mainly used mouse models,which can’t realistically simulate the disease in humans.The hippocampus is of great importance in learning and memory,and it is also one of the regions where AD occurs earliest and worst,the other is Entorhinal cortex(EC).Signal circuits between Medial entorhinal cortex(MEC)and hippocampus exert an essential role in spatial cognition.The hippocampus can be classified into Dentate gyrus(DG)and Cornuammonis(CA),among which CA can be divided into four histological subfields(CA1-CA4).Moreover,there is heterogeneity among these hippocampal subfields.Previous studies have made it clear that CA1 subfield has been seriously influenced in AD,however,the pathology in CA2-CA4 subfields and DG in AD has not been clarified clearly.In recent years,proteomics has often been used to study the pathological mechanism of AD.Proteomics is a science that studies the composition,function,distribution and interaction of proteins,which are the ultimate executors of life activities.Proteomic analysis is also a powerful tool for understanding changes in biological networks,pathways,and cell types in human tissues.By using this technique,many hypotheses about the pathological mechanism of AD can be explored,as well as the changes of proteomics in various regions of the human brain.There have been many studies using proteomics to analyze different brain regions,but proteomic studies related to the hippocampus were reported about a decade ago,and because of the technical limitations during that period,the number of proteins which were identified was limited.ObjectiveThe purpose of this study is to explore the changes of protein profiles in different subfields of the hippocampus of AD patients by using quantitative proteomics technology,and correlate it with the patients’ pathological scores and cognitive scores before death,so as to reveal the potential pathogenesis of AD.MethodsIn this study,44 cases of brain tissue collected from National Developmental and Functional Human Brain Tissue Resource Bank(Institute of Basic Medicine,Chinese Academy of Medical Sciences)were divided into three groups according to the“ABC”dementia score and ECog cognitive score of each case:Normal group(22 donors),Pre-AD group(9 donors)and AD group(13 donors).The four hippocampal subfields(CA1-CA4)and the EC region were analyzed by proteomics,and bioinformatics was applied to analyze the results.Immunohistochemistry(IHC)and Immunofluorescence(IF)were utilized to further confirm the previous results.ResultsOur results show that:(1)There were significant differences in the protein expression of AD in different brain subfields.(2)The number of S100A10 positive astrocytes increased significantly in all subfields and EC regions of the hippocampus,and participated in the phagocytosis of neuron debris.(3)Oligodendrocyte and myelin sheath density decreased in a region-specific manner in Pre-AD,which only happens in the EC region,but not in the CA1-CA4 subfields.ConclusionThis study suggests that we should carefully compare different subfields when studying different brain regions in order to minimize the bias caused by tissue selection,and also reveals the regional response of AD in the brain.It also reveals that there was myelin sheath loss in the brain of AD patients,which might be related to cognitive impairment and pathological changes of AD.It also revealed that the proliferation of S100A10 positive astrocytes in the hippocampus is closely related to AD and phagocytic cell debris after apoptosis of neurons to play a neuroprotective role,highlighting the role of glia cells in AD.