The Therapeutic Effects And Mechanisms Of Huc-mscs On Aging-related Cognitive Decline

Aging is a comprehensive phenomenon with multi-organ participated,characterizing by a progressive loss of physiological integrity,leading to impaired function and increased vulnerability to death.The brain is one of the most susceptible to aging [1,2].The brain aging includes physiological and pathological changes,the cognitive decline caused by the former is defined as cognitive aging [3,4],which is not considered as a disease,and the latter is associated with the neurodegenerative diseases,including Alzheimer’s disease(AD).The initial manifestations of aging brain is learning and memory or other brain dysfunction which is not match with age and educational level,and progressively developing to dementia such as learning and memory disorders,executive dysfunction and behavioral changes [5].Cognitive aging is authenticated that large neurons appear to shrink,few are lost[3,6],but its neurobiological substrates for function are decreased in dendritic spine density,synapses post synaptic density(PSD),reduced in neuroplasticity and neurogenic potential [7-11].Alzheimer’s disease(AD)is a progressive neurodegenerative disease,characterized by progressive loss memory,learning ability and daily living activity.The two major pathological features of AD are Aβ deposition(SP)and neurofibrillary tangles(NFTs)entangled by Tau proteins.In addition,large numbers of neuronal are lost in the brain AD as the other neurodegenerative disease [12].According to the data released by the Alzheimer’s Disease International(ADI)in 2016,the incidence of aging-related cognitive dysfunction increases with the average life expectancy of humans [13].The cognitive decline with aging affects daily life of older adults and their families,and brings significant social pressure.How to maintain cognitive integrity and prevent further deterioration of cognition have emerged as a leading public health concern with the increasing aging population,but there is still lack of specific interventions.At present,healthy lifestyle,such as physical exercise,cognitive stimulation,avoiding excessive exposure to neurotoxins and so on,may be beneficial for aging-related cognitive decline [14],but it is not enough or specific to prevent it.So in fact,new strategies of specific interventions for aging-related cognitive decline need arising pressingly.It has gradually been recognized that the aging systemic milieu is negative to the stable neural structure and function and maintaining normal neurogenesis,reflecting in impaired spatial learning and memory,decreased synaptic plasticity and so on [15,16].A series of particularly appealing studies have shown that young blood can activate the endogenous stem cells in various tissues such as,skeletal muscle,liver,spinal cord and brain [17,18] in aged animals by circulating “youthful” factors.This indicated that the aging brain could be systematically regulated by the circulating “youthful” factors.Recently,the transplantation of specific tissue-derived mesenchymal stem cells(MSCs)has been shown to be effective in the repairment or regeneration of several tissues,such as bone,heart and lung [19-21].It is generally accepted that the efficacy of MSCs is based on the secretion of a wide range of substances,including growth factors,cytokines,chemokines and metabolites which are very important to regulate multiple physiological functions of the organism [22,23].Human umbilical cord mesenchymal stem cells(hUC-MSCs)are closer to the fetal phase,easier to collection,and have higher proliferation and faster self-renewal ability compared with MSCs from other sources such as bone marrow,adipose tissue[24].These make it a more ideal seed cell for clinical cell therapy[25].On this basis,we propose that hUC-MSCs,as “young” stem cell,could play an important role on intervening aging-related cognitive decline,through systemic regulation by its large number of multiple functional cell regulatory factors.To explore the therapeutic effects and mechanisms of hUC-MSCs on aging-related cognitive decline,D-galactose-induced aging mice was choosed as the cognitive aging mice model and SAMP8(Senescence-accelerated mouse/prone 8)was choosed as a model for the research of Alzheimer’s disease.It has shown that injection of high doses of D-galactose by long-term subcutaneous to mice and other rodents [26],can cause metabolic disorders,accelerating its organs degenerative changes,especially brain.The changes is similar to the natural aging,reflecting the reduction in cell transcription [27] and hippocampus neurogenesis disorder [28-30].Therefore,the D-galactose-induced aging mice model was choosed for the study of cognitive aging.In addition,many characteristics of AD are found in SAMP8(Senescence-accelerated mouse/prone 8)mice,such as behavioral disorders,memory loss,amyloid deposition and tau phosphorylation,which rapidly evolved with early significant learning and memory impairment with aging [31-33].As a natural screening model of disease,its development and growth is more in line with the natural state of progress from brain aging to Alzheimer’s disease.In this research,SAMP8 was choosed as an AD mice model for the study of Alzheimer’s disease.As its large diameter and strong adhesion,MSCs are prone to be entrapped in the lungs,potentially increasing the risk for iatrogenic atelectasis and lethal pulmonary thromboembolism by intravascular transplantation [34,35].Previous researches have shown that intraperitoneal administration of MSCs to mice can confer significant lifespan and healthspan extension [36],alleviate neuropathology and symptoms associated with globoid cell leukodystrophy [37].It suggested the intraperitoneal injection as one of the optimal candidate transplantations,which is much safer and can realize the systematical regulation of hUC-MSCs with its secrecting youthful factors.As we known,the hippocampus in humans and mice was significantly impaired during aging,mainly due to down-regulation of neurogenesis and synaptic plasticity-related genes,decreasing in neonatal neurons and dendritic spine density,leading to the corresponding reduction in synaptic plasticity and cognitive function [38,39].Therefore,the above indicators are used to evaluate of the effects of hUC-MSCs in mice models.In addition,the hippocampus is the main area to regulate situational memory and spatial memory [40].In the cognitive evaluation,the related behavioral experiments are used to evaluate spatial learning and memory ability and situational memory,such as Morris water maze,Y maze,new object recognition,shuttle boxes and conditional fear experiments,etc.Firstly,in the study of cognitive aging mice,after modeling,a series of brain aging characteristics were showing: impaired in spatial learning and memory;reduction in synaptic plasticity and spine in CA1(Cornu ammonis 1)pyramidal neuron;the ultrastructure of the dendrites in the CA1 and DG(Dentate gyrus)regions showed that the PSD-95 of the excitatory synapses became thinner,the microfilaments of the neurons were disordered,the myelin edema was decomposed,mitochondrial structure damage;expression of PSD-95 decreased in hippocampus.After hUC-MSCs transplantation,the above characteristics of aging brain were significantly recoved or even revised.At the same time,hUC-MSCs promoted the neurogenesis in hippocampus by activating the endogenous neural stem/progenitor cells in the mice model.Further study showed that transplanted hUC-MSCs could exert their intervention by activating the ERK-CREB pathway in the hippocampus of mice model.Then,we found that hUC-MSCs could improve the cognitive ability of AD mice model,including ability to adapt to the environment,explore vitality,conditional memory ability,object and spatial recognition.The expression of AD-related proteins,including P-tau(S396),GSK-3β,BACE1 and APP,was down-regulated in the hippocampus and cortex in SAMP8 treated with hUC-MSCs,and the neural stem cells expression in cortical and hippocampus was up-regulated.Furthermore,the key factor HGF was screened by high throughput screening,bioinformatics analysis and multiple AD cell models.Next,the therapeutic effects of HGF were confirmed on SAMP8.What’s more,it confirmed that hUC-MSCs could activate the AKT pathway through HGF/c-Met system to further inhibit the activity of GSK-3β,thereby down-regulating the phosphorylation of Tau protein and finally repairing damaged nerve cells in AD cell model.In summary,the secretions from hUC-MSCs play important roles in the recovery of cognitive aging.Moreover,it demonstrates that the beneficial effects secreted from hUC-MSCs could promote synaptic plasticity,and recover the pathologic features and cognitive function on AD mice model,and HGF plays a key regulatory role in it.Collectively,our study provides a promising strategy to prevent ageng-related cognitive decline by hUC-MSCs.