Maoyang Lu,Zhao Wang

MOE Key Laboratory of Protein Sciences,School of Pharmaceutical Sciences,Tsinghua University,Beijing 100084,PR China

ABSTRACT The human gut microbiota is a huge ecosystem that provides lots of functions for host development,immune system,and metabolism.Gut microbiota is linked to lots of diseases,including human metabolic diseases such as obesity,type 2 diabetes (T2D),irritable bowel syndrome,and cardiovascular disease(CVD).Few studies,however,have noted the relationship between aging and microbiota,the connection between aging and microbiota remain largely to be researched.In this review,recent research findings are summarized on the role of gut microbiota in aging processes with emphasis on therapeutic potential of microbiome-targeted interventions in anti-aging medicine.

Keywords:Gut microbiota Aging Fecal transplantation Aging-related diseases

1.Introduction

The human gut microbiota is a huge ecosystem that provides lots of functions for host development,immune system,and metabolism.Gut microbiota is linked to lots of diseases,including human metabolic diseases such as obesity,type 2 diabetes(T2D),irritable bowel syndrome,and cardiovascular disease(CVD).Few studies,however,have noted the molecular mechanisms,the connection between host and microbiome remain largely to be researched.Here we have reviewed how aging may affect gut microbiota in the complex interactions of diet,microbiota,and host metabolism and show the new theories about the signaling pathway of modulating gut microbiota.

Human intestinal tract has a large number of bacteria,these bacteria is related to the host immune system development,metabolic regulation,nutrient digestion and absorption [1].Because of the application of systems biology approaches and new computational tools,there has been increasing interest in exploring the gut microbiota and their combined genomes,the microbiome,as diagnostic and therapeutic targets for prolonging human lifespan and treating aging-related diseases[2].Through plenty of experiments,the molecular mechanism of gut-host interaction has been elucidated.

Microbiota is defined as the collection of microbial taxa in a given environment.The concept of microbiome is the collection of the genes/genomes encoded by the microbiota.Compositional and functional changes of the human gut microbiome have been linked to lots of chronic metabolic disease,like malnutrition [3]as well as obesity[4]and obesity-associated diseases such as cirrhosis[5].Then,alterations of the gut microbiome have also been linked to intestine-related diseases,including inflammatory bowel disease(IBD)[6],colorectal cancer[7],neuro-developmental disorders[8].Changes in lifestyle and diet have been argued to contribute to the shifting gut microbiota ecology.

Obesity,T2D,and IBD are characterized by reduced fecal microbial diversity and studies have shown that uses of dietary emulsifier alters the gut microbiota’s composition,which results in intestinal inflammation,devastation of gut barrier and development of the metabolic syndrome.These findings suggest that the modern lifestyle,especially diet,has the potential to affect the gut microbiota,which may contribute to disease development.Therefore,understanding factors which influence the gut microbiota might lead to the finding of new therapies for both metabolic and inflammatory diseases.

There are three major factors which influences the composition of gut microbiota,including host genetic background,diet and microbes.Human genetics might play a role in shaping the composition of the gut microbiota.For instance,people homozygous for loss-of-function alleles of the FUT2 gene have an altered microbiota.FUT2 encodes an enzyme which is required for the fucosylation of surface carbohydrates on intestine mucosal linings.Loss in FUT2 altered both the composition and the function of the gut microbiota.FUT2 gene has been linked to Crohn’s disease and IBD,suggesting that an altered gut microbiota may explain the association between FUT2 gene background and increased possibilities to Crohn’s disease.Besides FUT2 gene,FXR gene and NOD2 gene also plays a vital role in shaping gut microbiota.However,further investigations are needed to determine the extent to which host genotype impacts and shapes the microbiome,as studies in mice show that environmental factors like diets,might have nominating effects [9].Recently,there have been new reports on the relationship between host genetic background and gut microbiota,caspase recruitment domain family member 9(CARD9),a susceptibility gene for inflammatory bowel disease (IBD) that functions in the immune response against microorganisms,which promotes recovery from colitis by promoting interleukin(IL)-22 production,and Card9-/-mice are more susceptible to colitis.Card9 can change the composition of gut microbiota,when transplantation its feces to normal mice,which can lead to the occurrence of colitis in mice [10].This research proves that there is exactly relationship between host genetic background and composition/function of the gut microbiota,demonstrating the mechanism by researching the production of microbial metabolites.

Energy intake and the nutrients composition of the diet affect human health and impact the composition of the human gut microbiota;in particular,recent studies in mouse performed on different genetic background and different living environment shows that diet has a stronger effect than host genotype in determining gut microbial composition[11].The gut microbiota responds to dietary interventions very quickly and short-term consumption of diets composing of either animal or plant products can alter the overall structure of the gut microbiota [12].For example,people with proteins and fats as the main food,Bacteroides dominated the gut microbiota,while those with fiber and carbohydrate as the main foods,the gut microbiota dominated by the genus Prevotella[13].

The microbes,including pathogens and prebiotics,are an important key of the way of shaping gut microbiota.Recent research shows that the interaction between microorganisms and microorganisms also affects the gut microbiota.For example,Salmonella and Clostridium difficile,two antibiotic-associated pathogens,improve their numbers and activities after antibiotic treatment by utilizing microbiota-liberated mucosal carbohydrates such as sialic acid [14].Now,as a beneficial bacteria,probiotics should be mentioned.Probiotics is a living microorganisms that confer a health benefit on the host.Prebiotics is an activator of probiotics.Prebiotics is dietary ingredients that are fermented by specific gut microbes,which results in specific changes in the composition of the gastrointestinal tract microbiota,making benefits on host-health[15].

2.The gut microbiota’s changes during aging process

As mentioned above,the body’s aging process is accompanied by the occurrence and development ofinflammation,meanwhile the function of each organ has declined.Conversely,gut microbiota may have their own unique way of changing[16].At the same time,it could not be ignored that the elderly usually have a variety of comorbidities,changes in diet and exercise habits,and other changes associated with gut bacteria that affect the gut microbiota.Therefore,the question of how to interact with gut bacteria still deserves further exploration(Figs.1 and 2).

Compared to evidence related to aging and inflammation,less is known regarding associations between aging and the microbiome.In fact,in contrast to the thousands of peer-reviewed publications on aging and inflammation,a PubMed search for “aging and microbiome” yielded only 466 results and a search for “aging and dysbiosis” yielded a mere 34.Moreover,only a handful of studies to date have investigated the aging microbiome in humans.Still,at least two early studies in this area have documented that advanced age is associated with changes to both the composition and stability of gut microbiota[17].Biagi et al.reported that a group of centenarian from Northern Italy displayed low species diversity compared to younger adults(～30 years of age).They also noted specific changes within Firmicutes(one of the two dominant phyla commonly found in the gut)subgroups and enrichment of Proteobacteria-a group containing many opportunistic bacteria which can overtake commensal bacteria and induce pathology.These microbiome changes were also characterized by a loss of genes for short-chain fatty acid production and an overall decrease in the saccharolytic potential,while proteolytic functions were more abundant than in the intestinal metagenome of younger adults[18].Interestingly,these changes in bacterial content were also moderately associated with circulating plasma concentrations ofinflammatory cytokines interleukins six (IL-6) and eight (IL-8).Surprisingly,however,despite these interesting findings is happened among the centenarians,we did not find significant differences in microbiota composition between the younger adults and a group of older adults with an average age of 70 years.In contrast,a study of gut microbiora in Ireland found that core populations of people over the age of 65 did indeed change [19].These changes are mainly manifested in the substantial increase in the proportion ofBacteroidesspp.and Clostridium compared to younger individuals.Gut microbiota’s diversity of the elderly will decline,mainly in the diversity of related species,including Prevotella [20],which may lead to the instability of the composition of the entire microbial community.However,it would not be ignored that the difference in gut microbiota between the elderly is so great that the prediction of the phenotype is more difficult to carry out.A few key factors are the predominant predictors of gut microbiota in community-dwelling and long-term care residents,such as diet and the use of antibiotics.Differences in cohort studies in Italy and Ireland may be explained by diet differences.

Interestingly,the ELDERMET study reveals a possible link between gut microbiota and body vulnerability,a link that can be used to measure the difference between elderly and communitydwelling elderly living in long-term care and/or rehabilitation facilities[20].These differences are closely related to the degree ofinflammation,mainly in the changes of some systemic inflammatory factors,such as IL-6,IL-8,CRP,and TNF-α.The association with frailty in the cohort is recently demonstrated more formally along with concordant findings from 728 female twins enrolled in the Healthy Ageing Twin Study[21].These findings are similar to those from a prior small cohort of older adults from The Netherlands[20].Thus,available data suggests that the gut microbiome may play at least some role in the development of physical frailty among the elderly.

The composition of gut microbiota can change significantly with aging and aging-related diseases [22].Age-related changes in gut physiology,such as gastric motility disorders,achlorhydria,and degenerative changes in the enteric nervous system,have a significant impact on the composition and function of gut microbes[23].Long-term stimulation of the immune system can lead to decreased immune system function,leading to immunosenescence,which in turn causes the above age-related differences.Subsequent to this are many aging-related diseases,including gastrointestinal related(Clostridium difficile colitis) and other (cachexia,frailty,cancer)[24]diseases.Such inflammatory state might make the host more sensitive to gut bacteria.

Fig.1.Interaction between gut microbiota and aging.

Fig.2.The molecule signaling pathway between gut microbiota and aging.

The age-related changes in the gut microbiota composition include a decline in microbiota diversity,a decrease in saccharolytic bacteria and an increase in proteolytic bacteria,decreased abundance of core (dominant) species and increased abundance of subdominant species,an increase of certain Proteobacteria,a reduction of bifidobacterial,and also a decrease of the ratio of Firmicutes to Bacteroides (F/B) [25].Taking the number of bifidobacteria as an example,the number of bacteria will decrease from 90% of the total number of colonic microbial communities when as a baby to 5% as an adult.With the aging,this bacteria will decrease rapidly in centenarians[26].The pronounced changes in gut microbiota occur through the transition from adulthood to old age.Among the elderly,there is a decrease in the diversity of gut microbiota and a greater variation in the differences between individuals than young people,with reduced numbers of Bifidobacteria,Firmicutes,Faecalibacterium prausnitzii,Clostridium cluster XIV,Blautia coccoides-Eubacterium rectal and greater presence of Bacteroidetes and Enterobacteriaceae [27].The centenarian’s microbiota have shown to be less diverse than in adult persons,and have demonstrated decreased levels of Bifidobacterium,Bacteroides and Enterobacteriaceae,and increased Clostridium [28].Such aging-associated differences in gut microbiota can not necessarily be caused by aging; this change is mainly due to the decline in body health,including malnutrition and the use of antibiotics,etc.Therefore,recent studies have suggested that the loss of the core community ofintestinal gut microbiota is more related to aging-associated frailty than with chronological age.With the aging of the body,the body will have a series of aging-related diseases,such as chronic inflammation,neurodegeneration,cognitive decline,frailty,type 1 and type 2 diabetes.Age-related changes in gut microbes are a very important factor in these disease states.

3.The role of microbiota during aging process

The available data indicate that the composition of the intestinal flora can affect the rate of aging.The age-dependent relationship between host and gut microbiota is influenced by many factors,such as age-related lifestyle changes,inflammation,and the hobby[29].

Evidence suggests that changes in gut flora predict the lifespan of humans.Studies have shown that the composition of young people and 70-year-olds relatively similar,but the difference between centenarians is quite obvious.Among centenarians,the major changes in gut flora are characterized by the reconstruction of Firmicutes population and enrichment in Proteobacteria.This phenomenon is mainly associated with an increase in the inflammatory status of the centenarians,such as an increase in a range ofinflammatory markers.The changes in the centenarian microbiome are accompanied by a decrease in Faecalibacterium prausnitzii,which is reported to have strong anti-inflammatory activity.In addition,Eubacterium limosum is considered to be a landmark in predicting longevity because ofits substantial increase in the population oflongevity elders.In the recent analysis of the super-long-lived elderly,the levels of several probiotics have risen dramatically(e.g.,Akkermansia,Bifidobacterium,and Christensenellaceae).In addition,from the point of bacterial function,the main functional characteristics of the dominant microflora of centenarians are the decrease of short-chain fatty acid synthesis and the decrease of glycolysis ability.On the contrary,the proteolytic function is significantly stronger than young people [30].In this study,116 microorganisms were identified as markers oflongevity.

There are several clinical trials on the molecular mechanism of how intestinal flora affects the body’s health and the aging process.These clinical trials using probiotics and probiotics research,the results show that prebiotics and probiotics can significantly improve the health of the elderly.Scientists believe that bacteria can improve the chronic inflammation associated with aging process.This includes an increase in the synthesis ofinflammatory cytokines,IL-6,IL-8 and IL-10 in the elderly and thus activation oflymphocyte and natural killer cells.This indicates that the direct regulation of the structure of the gut microbiota can reduce the inflammatory response and improve the adaptive immune response to improve immunosenescence [31].This process also includes other mechanisms:for example:degradation of non-digestible carbohydrates,enhancement of antioxidant activity,production of vitamin B and conjugated linoleic acids,regulation of host fat deposition and metabolism,prevention ofinsulin resistance,and improved maintenance of mucosal barrier integrity and immune homeostasis[32].Interestingly,we carefully observe these processes that they can improve intestinal barrier function,increase the synthesis of short-chain fatty acids,and increase the synthesis of antimicrobial peptides for lipid metabolism and glucose homeostasis.Shockingly,this process can also simulate some changes in the calorie restriction process,raising the level of expression of some genes involved in xenobiotic metabolism[33].Moreover,gut microbiota could affect the host’s aging by modulating microRNAs expression[34].

Targets for anti-aging drugs are mainly Bifidobacteria and butyrate-producing colon bacteria,such as Faecalibacterium prausnitzii and Roseburia.The short-chain fatty acid butyrate is the product of bacterial metabolism in the colon.It is a source of colonic epithelial cells and helps to maintain the integrity of the intestinal barrier and improve anti-inflammatory and anti-cancer abilities.It can also exert its beneficial metabolic effects via prevention of metabolic endotoxemia,enhanced mitochondrial activity,and activation of gut gluconeogenesis [35].Moreover,butyrate has many other functions,such as preventing metabolic endotoxemia and enhancing mitochondrial activity,and it also regulates epigenetic processes by inhibiting histone deacetylase activity.Therefore,butyrate has strong therapeutic potential in age-related diseases.

Intestinal microorganisms can cause age-related inflammation and premature death in mice.Dysbiosis of gut microbes in older mice may result in intestinal leakage that releases the bacterial products which causes inflammation in the body,thereby impairing immune function and reducing lifespan.In fact,a similar phenomenon has been found in the human body.Often,older adults with high levels ofinflammatory factors in the body are more likely to have frailty,reduced self-care ability,and hospitalization.At the same time,they are also more vulnerable to infections,dementia and cardiovascular diseases.However,so far,it is unclear that how the composition of gut microbes,inflammation and deterioration of the health of the elderly are related.Aging-related inflammation leads to macrophage dysfunction and tissue damage.They feed a group of mice under germ-free conditions and compared it to that of a conventionally cultured control mouse.Compared with the conventional cultured mice,the germ-free mice showed no aging-related increase in inflammation and have longer average life expectancy.If mixed with older mice,more of germ-free mice have aging-related inflammation; these are not the effects when mixing with young mice.In normal humans and mice,the aging process is accompanied by an increase in the levels of proinflammatory cytokines such as tumor necrosis factor(TNF)and interleukin 6 (IL-6) in the blood and tissues.Among them,the increase of TNF will damage the function of macrophages,and increase the permeability of the intestinal wall.In contrast,GF mice do not show an increase in TNF levels with age.Consistent with this,TNF-deficient mice do not develop age-related inflammation.In normal mice,Humira,an anti-TNF monoclonal drug,can reduce the harmful changes in the intestinal microbiota due to aging process.This shows that the composition of the gut microbiota may vary depending on the host’s inflammatory status.In general,leakage ofintestinal flora toward the intestinal wall occurs throughout life.However,with age,this phenomenon induces an inflammatory response that can further cause an ecological imbalance in the gut microbiota.An imbalance in the gut microbiota increases the permeability of the gut,thereby increasing the leakage ofintestinal flora.This positive feedback process will increase with aging[36].

4.The gut microbiota and aging-related diseases

With aging process,the body will have a series of diseases,such as:Alzheimer’s disease (AD) and osteoporosis and so on.The gut microbiota will change during this diseases process.We will show how aging-related diseases interact with gut microbiota in this sector.

4.1.Alzheimer’s disease

At present,the mechanism of gut microbiota participating in AD has been tentatively explained.Gamma-aminobutyric acid(GABA)is the major inhibitory neurotransmitter in the human central nervous system.When patients have disorders of the gut microbiota,especially when the number of Bifidobacteria and Lactobacillus is reduced,GABA levels in the intestine will be affected,further leading to a decrease in GABA levels in the central nervous system.Glutamate is the major excitatory neurotransmitter in the human central nervous system.N-Methyl-D-aspartate glutamate receptor(NMDA)receptor is an important glutamine Acid receptors,mainly involved in the regulation of neuronal survival,dendrites and axonal structures,and synaptic plasticity [45].Neufeld et al.[46]reported that the expression of NMDA receptor NR2B mRNA in the hippocampus of germ-free mice is significantly down-regulated,indicating that gut microbiota is associated with the expression of NMDA receptors.The above study suggests that gut microbiota may participate in the pathological process of AD by affecting metabolism.

Amyloid is an aggregate ofinsoluble proteins formed by misfolding and plays an important role in the pathogenesis and progression of AD.A variety of gut bacteria,such as Bacillus,Staphylococcus aureus,Streptomyces andE.colican secrete amyloid protein or its degradation products,it can induce oxidative stress response,further activate microglia,release inflammatory factors,such as TNF-α,IL-1,IL-6,etc.As the aging process,the permeability of the intestinal epithelial layer and the blood-brain barrier increases,and these products can enter the brain through the blood-brain barrier;further induce the occurrence of AD.Clinical studies have reported [46],bifidobacteria,lactic acid bacteria can regulate the gut microbiota,improving the cognitive ability of AD patients.The mechanism may be related to the exogenous polyamines producing by gut microbiota.Polyamines not only inhibit the production ofinflammatory cytokines,but also have antioxidant effects.Intestinal microorganisms can regulate the metabolism of tryptophan,further affecting the content of 5-HT in vivo.The gut microbiota’s metabolites can promote the secretion of serotonin by enteroendocrine cells,thereby regulating the balance of AD neurotransmitter [47].In addition,probiotics can also promote the production of neurotrophic factors in the brain,reduce the activity ofinflammatory cytokines,thereby contributing to the prevention and early treatment of diseases associated with cognitive disorders.

In conclusion,the pathogenesis of AD is closely related to the imbalance of gut microflora.Transplantation of probiotics can regulate the gut microbiota which can activate the immune system,increase the release of neurotransmitters in the brain,and effectively improve AD gastrointestinal and neurological symptoms.Therefore,an in-depth study of the mechanism ofintestinal microbial involvement in AD will provide a new target for the treatment of AD.

4.2.Parkinson disease

In recent years,the role of gut microbiota in Parkinson’s disease has been gradually recognized.Helicobacter pylori (HP) infection is associated with the severity of activity symptoms in PD patients.The NAROZANSKA et al.[48]study suggest that HP infection is associated with fluctuations in PD motor symptoms,which attributes this correlation to HP’s effect on levodopa absorption.Another factor related to PD is small intestinal bacterial overgrowth (SIBO).Studies of Gabrielli et al.[49]have suggested that the incidence of SIBO is increased in patients with PD,and the incidence of SIBO in the PD group is 54%compared with 8%in the healthy group through the lactose breath test and glucose breath test.

At present,the interaction between anti-Parkinson’s disease drugs and gut microbes are not completely understood[50].Scheperjans et al.[51]found that taking Catechol-O-methyltransferase(COMT) inhibitor increased the abundance of Enterobacteriaceae and pointed out that COMT inhibitor is the only drug related to gut microbiota’s changes.Studies have shown that HP affects the absorption oflevodopa.We can eliminate HP by improving the absorption oflevodopa and improve the patient’s motor symptoms[48].

The mechanism of gut microbiota in Parkinson’s disease is about inflammation and genetic background.Gut microbiota can change intestinal barrier and intestinal mucosal permeability.This change can not only affect the gastrointestinal epithelial cells and immune system,but also has effect on ENS neurons and glial cells,and can be caused by pathogens and cytokines producing lipopolysaccharides to up-regulate the intestinal and systemic inflammatory responses[52].Intestinal bacteria enhance the inflammatory effect of α-syn by initiating a natural immune response and lead to misfolding of α-syn[53].

4.3.Osteoporosis

In recent years,research on gut microbiota and osteoporosis has become a hot topic in orthopedic research.Mc Cabe’s study[54]points out that gut microbiota can affect the bone metabolism through the release of small molecules such as estrogen,serotonin,immune regulation and affect absorption and metabolism of calcium and phosphorus.Sj?gren [55]finds that mice with a lack of gut microbiota have approximately 40%higher bone density in the distal femur than normal mice.In addition,the expression level of osteolytic cytokines such as IL-6 and TNF-α in bone of germ-free mice is significantly reduced.Therefore,the reduction of osteoclasts in germ-free mice may also be caused through regulation of the immune system.The number of osteoclasts in sterile mice is relatively low,and the formation of T cells and osteoclasts is significantly reduced,but the rate of bone formation is not reduced through culture of bone marrow mesenchymal stem cells(BMSCs).They also pointe out that the expression ofinflammatory factors in germ-free mice is relatively small,but there is no difference in serum calcium and serum phosphorus levels.In conclusion,the increase in bone mass in germ-free mice is due to the lack of bacterial as antigen to cause immune response.

Although animal models have confirmed that gut microbiota increases inflammation and accelerates postmenopausal bone resorption,no studies have yet confirmed the relationship between human gut microbiota and osteoporosis,suggesting that bone mass occurrence is the results of many factors.However,we have reasons to believe that disorder in the gut microbiota is one of the key factors leading to osteoporosis.

4.4.Obesity

The obesity problem has become a global public health problem that can lead to a series of metabolic syndromes such as T2D,CVD,and nonalcoholic fatty liver disease.Human genetics background,diet composition,and live environment are major risk factors for obesity[9].There are some ways to treat obesity-related diseases,including calorie restriction,physical activity,anti-obesity drugs,and bariatric surgery.Numerous evidences show that gut microbiota is another factor causing obesity.

Experiments have shown that germ-free mice is much more resistant to high-fat diet-induced obesity,and at the same time,fat storage increases after colonization of microbiota from conventional WT [56].Twins experiments illustrate the association between gut microbes and obesity.Transplantation of microbiota from both obese mice and human promoted higher fat accumulation in recipient germ-free mice compared with transplantation of microbiota from lean donors.After co-housing with lean mouse,the obese mouse will become thinner [57].This is also proved in the clinical case report,there is significant weight gain in a woman receiving fecal microbiota transplantation(FMT)treatment from a healthy overweight donor to treat C.difficile infection[58],suggesting that obesity may also be transmissible in humans.In summary,gut microbiota is a factor that promotes fat storage.

All of the existing methods to treat obesity can target for intestinal gut microbes,including bariatric surgery,drugs,caloric restriction.At present,from the most effective bariatric surgery Roux-en-Y gastric bypass(RYGB)point of view,it can promote an increase in the abundance of Enterobacteriaceae and all microbiota richness to change the structure ofintestinal microorganisms[59].The impact of surgery will last ten years.FMT of the surgically altered microbiota resulted in reduced fat-mass gain in recipient germ-free mice[60].The main reason is that the surgery will result in increased circulation of bile acids,and it will activate the FXR signal pathway.FXR signaling pathway is also an important factor to influence the obesity through modulating the gut microbiota composition.The gut microbiota promoted weight gain and hepatic steatosis in an FXR-dependent manner,and the bile acid profiles and composition of gut microbiota differed between Fxr-/-and WT mice.By transferring the gut microbiota from HFD-fed Fxr-/-and wild-type mice into GF mice,we showed that the obesity phenotype is transferable[61].Calorie restriction is another useful ways to treat with obesity and has been regarded as the only experimental regimen that can effectively lengthen lifespan in various animal models.A all life-long study shows that calorie restriction on both high-fat or low-fat diet significantly changes the overall structure of the gut microbiota.A life-cycle study shows that calorie restriction can significantly alter gut microbiota and increase strains that are positively correlated with longevity,for example,the genus Lactobacillus[62].

5.Regulation of gut microbiota and health

At present,there are several methods for prolonging life span:calorie restriction,drugs,food,and gene regulation.However,these methods are mainly based on the conclusions obtained from experimental animals and there is no real clinical trial.These methods of prolonging life will change the body’s health and regulate the gut micriobiota.So we will review the interaction between the gut microbiota and anti-aging methods.

5.1.Caloric restriction

In various experiments for promoting health and longevity,changes in gut microbiota have been reported.Alterations in the composition of the gut microbiota are revealed in various healthspan-promoting interventions.In a number of studies,an association has been found between the intestinal microbiota composition and weight loss caused by CR,a most reproducible lifeextending strategy now.The F/B ratio,in particular,is consistently found to be increased in obesity and reduced with weight lossproducing CR-based interventions.For example,in the study of the surgical and dietary weight loss therapy for obesity,the energyreabsorbing potential of the gut microbiota,indicated by the F/B ratio,is decreased by CR and increased following laparoscopic sleeve gastrectomy[63].Remarkably,the Firmicutes changes were accompanied by alterations in butyrate-producing bacterial species in both groups.The F/B ratio is also significantly decreased in obese individuals receiving a weight-loss dietary intervention [64].The weight gain-causing bacteria can,in turn,induce the expression of genes linked to carbohydrate and lipid metabolism thereby influencing dietary energy harvest.In animal models,the CR-induced life extension is accompanied by structural modulation of gut microbiota.For example,calorie restriction throughout the life process can significantly alter the gut microbiota structure in mice.Calorie restriction can significantly up-regulate strains that are positively correlated with longevity,e.g.,the genus Lactobacillus,and to down-regulate phylotypes negatively associating with lifespan[62].Such CR-induced changes were accompanied by significantly reduced levels of serum lipopolysaccharide-binding protein,suggesting that a structurally balanced architecture of gut microbiota may be established via CR.The authors suggest that CR can cause health benefits for the host through reduction of antigen load from the gut.Recently,it is confirmed that the gut microbiota will be predominant by the bacterium Lactobacillus only after two weeks of fasting,and the intestinal bacteria-derived antigen in the blood of the mice could be significantly reduced and the level of systemic inflammation could also be reduced.The researchers isolate the predominant strain of Lactobacillus lyrhei from the intestinal tract of fasting mice and found that this strain can reduce the production ofinflammatory factors in the Caco-2 cell model in vitro and extend the lifespan of nematodes.Not only that,in germ-free mice transplanted with old mouse’s gut microbiota,this strain of bacteria can significantly improve the intestinal barrier damage caused by oldflora,thereby reducing the intestinal bacteria-derived antigens in the blood and reducing the age-related systematic inflammation[65].

5.2.Genetic modulation

Nematodes and drosophila have become relatively good model organisms for studying aging due to their short life cycles The researchers identified 29 bacterial mutants inE.colithat could significantly extend the life of the host by 10%to 40%.One research is about nematodes.The results showed that about half of these 29 longevity-promoting bacteria are capable of drastically reducing the damage caused by tumor expansion and A-β protein deposition.This means that these bacterial mutants can not only prolong life but also significantly improve quality oflife.At the same time,these bacterial genes associated with the longevity of the host have been shown to be involved in a variety of signaling pathways that regulate aging,such as the insulin signaling pathway,the mTOR signaling pathway and the caloric restriction.This shows that bacteria can affect the lifespan of the host through a variety of molecular signaling pathway [66].Another research is about drosophila.The amount of bacteria in the intestine of Drosophila increases significantly with age,leading to an inflammatory state.This imbalance is driven by long-term activation of the stress response gene FOXO,which inhibits the activity of a class of molecules called PGRP-SCs(homologues of human PGLYRPs).PGRP-SCs regulate the body’s immune response to bacteria.Inhibition of PGRP-SC results in the deregulation of a signaling molecule,NFkB,which plays an important role in initiating an effective immune response to enteric bacteria.The resulting immune imbalance leads to the proliferation of bacteria,triggering an inflammatory response and generating free radicals.In the intestine,free radicals cause excessive proliferation of stem cells,leading to abnormal epithelial hyperplasia.When researchers increased PGRP-SC expression in intestinal epithelial cells,the bacterial balance is repaired and stem cell proliferation is limited.Simply enhancing the function of PGRP-SC is sufficient to prolong the lifespan of drosophila[67].

5.3.Food

Gut microbiota is the main participant in food digestion and is involved in the separation,synthesis,and absorption of major nutrients such as carbohydrates,fats,proteins,and vitamins.Both low-and high-sugar diets affect the structure of the gut microbiota.A low-sugar diet will alter the microbial structure of the intestine,reduce the number ofintestinal tumors in the mutant mice,and at the same time reduce the levels of butyrate-producing microorganisms[68],whereas a high-fat/high-sugar diet will alter the intestinal microflora structure of mice and promote mice developing obesity[69].In addition,the proportion of dietary fiber in the diet significantly affects the microbial structure of the intestine.A high-fiber diet increases the number of Bifidobacterium in the intestine and a low-fiber diet increases the number of Bacteroidese prevotella [70].The products of carbohydrate fermentation by intestinal microbes are mainly SCFAs,including acetic acid,butyric acid and propionic acid.Compared with people with protein and fat as their main dietary constituents,the content of short-chain fatty acids in intestinal microbial metabolites is relatively high among people who have long-term carbohydrates as their main diet.Gut microbiota participates in the metabolism of body proteins.On the one hand,it provides a nitrogen source for its own growth.It also helps the host to decompose and synthesize essential amino acids to meet physiological needs.Lactoferrin is a natural glycoprotein that is mainly found in breast milk and has a variety of biological functions.It can regulate the growth and reproduction ofiron-containing bacteria by recovering soluble iron from intestinal fluids.The sIgA in breast milk plays an important role in agglutinating pathogenic bacteria and neutralizing viral toxins,which can prevent neonatal intestinal infections[71].It has been reported that certain fruits can prolong life by regulating intestinal flora,such as pomegranate.When the gut microbiota transform the urolithin A in the pomegranate,they can prompt the body’s muscle cells to protect themselves from aging[72].

5.4.Drugs

At present,from the experiments of nematodes,drosophila and mice,metformin and rapamycin drugs can significantly prolong the life of the organism.Metformin is mainly used for the treatment of diabetes.In addition to diabetes,metformin has also begun to be used for diseases such as cancer,cardiovascular diseases,Alzheimer’s disease,obesity,retinopathy and other diabetic syndromes,nephropathy,etc.,and is known as the “magic drug.¨Metformin use significantly increased the ability to produce butyrate and propionate in the intestinal flora.Metformin significantly changes the bacterial composition of diabetic patients.Metformin will significantly increase the proportion of the bacterial genus Sutterella.Metformin also increased the catabolism of many amino acids including glycine and tryptophan.From a molecular mechanism perspective,metformin can regulate glucose synergistic protein-1(SGLT1)in the upper small intestine,activate SCLT1-dependent signaling pathways,and reduce glucose production [73].Surprisingly,as another anti-aging drug,prevents the expansion ofintestinal stem cells [74].More research is needed to focus on the results of anti-aging drugs in humans in the future.

6.How microbiota modulate lifespan and aging?

If you would like to check the relationship between gut microbiota and lifespan,examining axenic culture process is a good way.When having sterile culture,the life span ofC.eleganshave increasing two-fold [37].As we all know,Caloric restriction can prolong lifespan.Obviously,bacteria is food source,no one can exclude if dietary restriction is beneficial for extending lifespan.There are different researches checking how sterile culture influence drosophila life span and get different results.There are differences between sex and axenic culture,the results is that axenic culture can shorten males’ life span while not female [38].On the contrary,Ren,Tower and colleagues report that NO-bacteria have no influence on males’lifespan[39].Recently,Clark reported sterile culture can extend females’life span,while Petkau reported antibiotic which can break down the drosophila gut microbiota can also extend fly lifespan.Beside life span,intestinal aging has been delayed through antibiotic treatment.Clark attempted to transplant the stool homogenates of aged drosophila to young drosophila and the structure of the stool changes dramatically and the lifespan of drosophila decreased.Taken together,these experiments demonstrat that the gut flora is closely related to the lifespan and mortality of Drosophila[40].

In addition to aseptic culture,there are other cases to study the relationship between bacteria and lifespan.Regulation ofC.elegans E.coliin intestinal tract content can regulate nematode life span.For instant,Kanamycin administration can delay the proliferation ofE.coliin theC.elegansintestine and increase the lifespan ofC.elegans.However,some reports show that with the aging process,the accumulation of bacteria in different individuals is quite different.ManyC.elegansdo not accumulate bacteria [41].This shows that bacterial accumulation is not the cause of age-related disease.Therefore,differentC.elegansmay die for different reasons.All in all,although there are individual differences,it has been recognized that bacterial pathogenicity can promoteC.elegansdeath.

At present,the molecular mechanism of howC.elegans-E.colisystem affects the lifespan of the host through the intestinal flora has already begun to be studied.This model is relatively simple and allows for easy screening of E.coli mutants.Recent studies have identified certain mutant strains that extend lifespan and have discovered the molecular mechanisms how gut microbiota regulate the life span of host.E.colimutants prolong the lifespan of host by inhibiting the production of folic acid[41].A number of additional studies have demonstrated that diffusible molecules originating in bacteria can impactC.eleganslifespan [42].C.eleganslack the enzyme nitric oxide (NO)synthase.The source of NO is from bacteria.It has been reported in the literature that NO produced by bacteria can prolongC.eleganslifespan[43].Another mechanism is that small endogenous non-coding RNA expressed byE.coliplays an important role in regulating the lifespan ofC.elegans,and the main function is lncRNA DsrA [44].Therefore,bacterial-derived molecules can regulateC.eleganslifespan.

7.Conclusion and expectation

Over the past decades,researchers have established a link between the alteration of gut microbial composition and aging and aging-related disease.Aging affects the host health status by modulating the composition of the gut microbiota.It has been found that gut microbiota changes during aging period.In this review,recent research findings are summarized on the role of gut microbiota in aging processes with emphasis on therapeutic potential of microbiome-targeted interventions in anti-aging medicine.

In the future,new tools and new approaches are needed for further investigations to find how to prolong lifespan that means healthspan and treat aging-related diseases.

Acknowledgements

This work was financially supported by grants from the National Key R&D Program of China (2018YFD0400204),the Key International S&T Cooperation Program of China (2016YFE113700),the European Union’s Horizon 2020 Research and Innovation Program(633589) and the National Natural Science Foundation of China(81471396).