| Objective:Along with the society developed and life expectancy extended,the proportion of the elderly is increasing.It is of great significance to understand the biological process of aging,to improve the quality of life in order to achieve a healthy aging,and to prevent the occurrence of aging related diseases.It is not only a major scientific problem in the field of life science,but also a social problem with important strategic significance.Aging refers to the process of decreasing individual survival and overall function with age,which is an inevitable stage in the normal life process.Although there are various changes and manifestations of aging process,cognitive decline is the most important feature of the aging process,and its mechanism is complex and not clear.There are 100 trillion microorganisms living in the human intestinal tract,which are called gut microbiome.Changes in gut microbiome have been found to be associated with many diseases,including cardiovascular disease,irritable bowel syndrome(IBS),inflammatory bowel disease,obesity,and type 2 diabetes.From the population study,it was found that the gut microbiome of the elderly was significantly changed,which was the important mechanism and sign of the aging process.The changed gut microbiome could affect the health of the elderly.More and more evidence showed that gut microbiome participated in the regulation of brain function and behavior.Changes in gut microbiome may be involved in neurodegenerative diseases such as AD and PD,and psychiatric disorders such as ASD and depression.Some reports also showed the effects of gut microbiome on learning and memory,including: GF(Germ Free)mice showed defects in non-spatial memory and working memory compared with SPF(Specified Pathogen Free)mice;The combination of antibiotics resulted in impaired novel object recognition memory of SPF mice;Infected with C.rodentium to change the gut microbiome led to the memory impairment induced by stress,and probiotic intervention can improve learning and memory function and HPA axis function.However,all of these studies were conducted in special adult animal models,such as GF mice,pathogenic bacteria infection,probiotics or antibiotics.Up to date,there is no research to systematically observe the effect of gut microbiome on cognitive decline in the normal aging animal model.In this study,the cognitive behavior and gut microbiome were measured in normal aging rats.In order to clarify the effect of gut microbiome on the cognitive decline in brain aging,the fecal microbiome transplantation technique was used to transplant the gut microbiome from the aged rats to young rats.To observe whether the changed gut microbiome can cause the change of cognitive behavior,brain structure and function.To explore how gut microbiome affect the cognitive function in brain aging,oxidative stress in the brain were detected in aged rats and the young rats receiving the fecal microbiome transplantation.The study aimed to clarify the role of gut microbiome in cognitive decline in normal brain aging,which may provide a new evidence for the occurrence and possible mechanism of cognitive decline.Methods:(1)To compare cognitive behavior of aged rats with young rats by delayed matched to position task based on operant conditioning;To compare gut microbiome of aged rats with young rats by16 S rRNA gene sequencing;(2)To determine the effect of gut microbiome on cognitive decline in normal aging,Young rats were transplanted with gut microbiome of aged rats by fecal microbiota transplantation(FMT).To observe the cognitive behavior by operating conditioning and Y maze;To observe different brain regions and function connection by fMRI;To observe synapse and dendritic spines by electron microscopy and Golgi staining;To observe the expression of BDNF,NMDAR1 GR and Synaptophysin by western blot and IHC;(3)Detected oxidative stress in brain after FMT,to explore if gut microbiome have the role through oxidative stress mechanism.Results:(1)With aging,the cognitive function declined in the aged rats.In the delayed matched to position task(DMTP)based on operational conditioning,when the delay interval was longer(18 s,24 s),the correct lever rate of aged rats was significantly lower than that of young rats;In Y maze,the number of exploring new arm was reduced and the time was shortened in aged rats.The synaptic structure changed,the number of dendritic spines decreased,and the expression level of NMDAR1,BDNF and synaptophysin decreased.(2)Gut microbiome in aged rats changed.Analysis based on OTU and microbial groups showed that the gut microbiome of aged rat was significantly different from that of young rat,including decrease of the bacteria diversity and the change of the dominant bacteria.The Shannon index of aged rats decreased,indicating that the diversity of gut microbiome in aged rats decreased.Bacteroidetes,Firmicutes,Actinobacteria and Proteobacteria were the dominant bacteria in rats.The level of Bacteroidetes,Firmicutes and Actinobacteria were different between aged and young rats.At the level of the genus,Bulleidia?Collinsella?Lactobacillus?Treponema?Allobaculum?Ruminococcus?Parabacteroides increased,while Prevotella?Bacteroides?Sutterella?Phascolarctobacterium?Bilophila decreased.(3)Fecal microbiome transplantation(FMT)changed gut microbiome in rats.The gut microbiome of aged rats was transplanted to young rats.The gut microbiome of young rats have the characteristics of "aged donor rats",including the decrease of Shannon index and the decrease of diversity in FMT group.At the phylum level,Bacteroidetes decreased and Firmicutes increased in FMT group.At the genus level,Prevotella,Bacteroide,Parabacteroides,Corynebacterium,Catenibacterium and paludibacter decreased and Adlercreutia increased.(4)FMT changed the cognitive function of young rats.In DMTP task,when the delay interval was 18 s or 24 s,the correct lever rate of the FMT rats decreased.In Y maze,the number of exploring new arm was reduced and the time was shortened in FMT group.At the same time,FMT caused the functional connection between the hippocampus and the other brain regions changed in FMT group.FMT caused the change of synaptic structure and dendrite spines,and the decrease of important signal molecules such as BDNF,NR1 and synaptophysin,indicating that gut microbiome plays a role in the cognitive decline in normal brain.(5)The important mechanism that gut microbiome caused the cognitive decline in aging may be oxidative stress.Compared with young rats,the stress reaction product MDA increased,and the antioxidant SOD activity declined in hippocampus and the mPFC in aged rats.And the MDA content increased and the activity of SOD decreased in the hippocampus and mPFC in the young rats with the "elderly" gut microbiome.Conclusions:(1)With aging,the cognition was impaired,the synaptic structure changed and important signal molecules NMDAR1,BDNF and synaptophysin decreased in aged rats,and the gut microbiome was also changed in aged rats;(2)Gut microbiome may play an important role in cognitive decline in normal brain aging;(3)Gut microbiome may play a role in cognitive decline in normal brain aging by affecting oxidative stress. |
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