Peripheral clearance of brain-derived Aβ in Alzheimer's disease: pathophysiology and therapeutic perspectives

Alzheimer’s disease(AD)is the most common type of dementia,and no disease-modifying treatments are available to halt or slow its progression.Amyloid-beta(Aβ)is suggested to play a pivotal role in the pathogenesis of AD,and clearance of Aβfrom the brain becomes a main therapeutic strategy for AD.Recent studies found that Aβclearance in the periphery contributes substantially to reducing Aβaccumulation in the brain.Therefore,understanding the mechanism of how Aβis cleared in the periphery is important for the development of effective therapies for AD.In this review,we summarized recent findings on the mechanisms of Aβefflux from the brain to the periphery and discuss where and how the brain-derived Aβis cleared in the periphery.Based on these findings,we propose future strategies to enhance peripheral Aβclearance for the prevention and treatment of AD.This review provides a novel perspective to understand the pathogenesis of AD and develop interventions for this disease from a systemic approach.

Biofluid Biomarkers of Alzheimer's Disease:Progress,Problems,and Perspectives

Since the establishment of the biomarker-based A-T-N(Amyloid/Tau/Neurodegeneration)framework in Alzheimer's disease(AD),the diagnosis of AD has become more precise,and cerebrospinal fluid tests and positron emission tomography examinations based on this framework have become widely accepted.However,the A-T-N framework does not encompass the whole spectrum of AD pathologies,and problems with invasiveness and high cost limit the application of the above diagnostic methods aimed at the central nervous system.Therefore,we suggest the addition of an“X”to the A-T-N framework and a focus on peripheral biomarkers in the diagnosis of AD.In this review,we retrospectively describe the recent progress in biomarkers based on the A-T-N-X framework,analyze the problems,and present our perspectives on the diagnosis of AD.

Association of Polygenic Risk Score with Age at Onset and Cerebrospinal Fluid Biomarkers of Alzheimer’s Disease in a Chinese Cohort

To evaluate whether the polygenic profile modifies the development of sporadic Alzheimer’s disease(sAD)and pathological biomarkers in cerebrospinal fluid(CSF),462 sAD patients and 463 age-matched cognitively normal(CN)controls were genotyped for 35 singlenucleotide polymorphisms(SNPs)that are significantly associated with sAD.Then,the alleles found to be associated with sAD were used to build polygenic risk score(PRS)models to represent the genetic risk.Receiver operating characteristic(ROC)analyses and the Cox proportional hazards model were used to evaluate the predictive value of PRS for the sAD risk and age at onset.We measured the CSF levels of Aβ42,Aβ42/Aβ40,total tau(T-tau),and phosphorylated tau(P-tau)in a subgroup(60 sAD and 200 CN participants),and analyzed their relationships with the PRSs.We found that 14 SNPs,including SNPs in the APOE,BIN1,CD33,EPHA1,SORL1,and TOMM40 genes,were associated with sAD risk in our cohort.The PRS models built with these SNPs showed potential for discriminating sAD patients from CN controls,and were able to predict the incidence rate of sAD and age at onset.Furthermore,the PRSs were correlated with the CSF levels of Aβ42,Aβ42/Aβ40,T-tau,and P-tau.Our study suggests that PRS models hold promise for assessing the genetic risk and development of AD.As genetic risk profiles vary among populations,large-scale genome-wide sequencing studies are urgently needed to identify the genetic risk loci of sAD in Chinese populations to build accurate PRS models for clinical practice.

Improving Blood Monocyte Energy Metabolism Enhances Its Ability to Phagocytose Amyloid-βand Prevents Alzheimer’s Disease-Type Pathology and Cognitive Deficits

Deficiencies in the clearance of peripheral amyloidβ(Aβ)play a crucial role in the progression of Alzheimer’s disease(AD).Previous studies have shown that the ability of blood monocytes to phagocytose Aβis decreased in AD.However,the exact mechanism of Aβclearance dysfunction in AD monocytes remains unclear.In the present study,we found that blood monocytes in AD mice exhibited decreases in energy metabolism,which was accompanied by cellular senescence,a senescence-associated secretory phenotype,and dysfunctional phagocytosis of Aβ.Improving energy metabolism rejuvenated monocytes and enhanced their ability to phagocytose Aβin vivo and in vitro.Moreover,enhancing blood monocyte Aβphagocytosis by improving energy metabolism alleviated brain Aβdeposition and neuroinflammation and eventually improved cognitive function in AD mice.This study reveals a new mechanism of impaired Aβphagocytosis in monocytes and provides evidence that restoring their energy metabolism may be a novel therapeutic strategy for AD.

Polysaccharide Krestin Prevents Alzheimer’s Disease-type Pathology and Cognitive Deficits by Enhancing Monocyte Amyloid-βProcessing

Deficits in the clearance of amyloidβprotein(Aβ)by the peripheral system play a critical role in the pathogenesis of sporadic Alzheimer’s disease(AD).Impaired uptake of Aβby dysfunctional monocytes is deemed to be one of the major mechanisms underlying deficient peripheral Aβclearance in AD.In the current study,flow cytometry and biochemical and behavioral techniques were applied to investigate the effects of polysaccharide krestin(PSK)on AD-related pathology in vitro and in vivo.We found that PSK,widely used in therapy for various cancers,has the potential to enhance Aβuptake and intracellular processing by human monocytes in vitro.After administration of PSK by intraperitoneal injection,APP/PS1 mice performed better in behavioral tests,along with reduced Aβdeposition,neuroinflammation,neuronal loss,and tau hyperphosphorylation.These results suggest that PSK holds promise as a preventive agent for AD by strengthening the Aβclearance by blood monocytes and alleviating AD-like pathology.

Chronic hypoperfusion due to intracranial large artery stenosis is not associated with cerebralβ-amyloid deposition and brain atrophy

Background:Insufficient cerebral perfusion is suggested to play a role in the development of Alzheimer disease(AD).However,there is a lack of direct evidence indicating whether hypoperfusion causes or aggravates AD pathology.We investigated the effect of chronic cerebral hypoperfusion on AD-related pathology in humans.Methods:We enrolled a group of cognitively normal patients(median age:64 years)with unilateral chronic cerebral hypoperfusion.Regions of interest with the most pronounced hypoperfusion changes were chosen in the hypoperfused region and were then mirrored in the contralateral hemisphere to create a control region with normal perfusion.11C-Pittsburgh compound-positron emission tomography standard uptake ratios and brain atrophy indices were calculated from the computed tomography images of each patient.Results:The median age of the 10 participants,consisting of 4 males and 6 females,was 64 years(47-76 years).We found that there were no differences in standard uptake ratios of the cortex(volume of interest[VOI]:P=0.721,region of interest[ROI]:P=0.241)and grey/white ratio(VOI:P=0.333,ROI:P=0.445)and brain atrophy indices(Bicaudate,Bifrontal,Evans,Cella,Cella media,and Ventricular index,P>0.05)between the hypoperfused regions and contralateral normally perfused regions in patients with unilateral chronic cerebral hypoperfusion.Conclusion:Our findings suggest that chronic hypoperfusion due to large vessel stenosis may not directly induce cerebralβ-amyloid deposition and neurodegeneration in humans.

Naturally-Occurring Antibodies Against Bim are Decreased in Alzheimer’s Disease and Attenuate AD-type Pathology in a Mouse Model

Increased neuronal apoptosis is an important pathological feature of Alzheimer’s disease(AD).The Bcl-2-interacting mediator of cell death(Bim)mediates amyloid-beta(Aβ)-induced neuronal apoptosis.Naturally-occurring antibodies against Bim(NAbs-Bim)exist in human blood,with their levels and functions unknown in AD.In this study,we found that circulating NAbs-Bim were decreased in AD patients.Plasma levels of NAbs-Bim were negatively associated with brain amyloid burden and positively associated with cognitive functions.Furthermore,NAbs-Bim purified from intravenous immunoglobulin rescued the behavioral deficits and ameliorated Aβdeposition,tau hyperphosphorylation,microgliosis,and neuronal apoptosis in APP/PS1 mice.In vitro investigations demonstrated that NAbs-Bim were neuroprotective against AD through neutralizing Bim-directed neuronal apoptosis and the amyloidogenic processing of amyloid precursor protein.These findings indicate that the decrease of NAbs-Bim might contribute to the pathogenesis of AD and immunotherapies targeting Bim hold promise for the treatment of AD.

Is Alzheimer’s Disease Transmissible in Humans?

Alzheimer’s disease(AD),the most common type of dementia,has been identified as a protein misfolding disease with the accumulation of abnormally folded amyloid-β(Aβ)protein and hyperphosphorylated tau protein in the brain[1,2].Aβ is known to play a crucial role in the pathogenesis of AD.Deposition of Aβ protein in the brain parenchyma causing senile plaques is the obligatory event in AD pathogenesis.Aβ protein is also deposited in the media and adventitia of small and midsized cerebral arteries leading to cerebral amyloid angiopathy(CAA),which is also present in the large majority of AD patents.

Combining Multiple Factors to Predict Alzheimer’s Disease

Alzheimer's disease(AD)is the most common type of dementia that imposes a high burden on caregivers and has substantial economic cost for society.Although there are no drugs available to reverse the progression of AD,a large number of studies have shown that intervention for the risk factors for AD or administration of disease-modifying therapy at the preclinical/prodromal stage yields better outcomes.Therefore,early diagnosis or accurate prediction of AD in healthy populations and preclinical AD is an important prerequisite for effective intervention.