GUT DYSBIOSIS AND AGE-RELATED NEUROLOGICAL DISEASES; AN INNOVATIVE APPROACH FOR THERAPEUTIC INTERVENTIONS

Abstract

The gut microbiota is a complex ecosystem of bacteria, fungi, and viruses that acts as a critical regulator in microbial, metabolic, and immune responses in the host organism. Imbalances in the gut microbiota, termed “dysbiosis”, often induce aberrant immune responses, which in turn disrupt the local and systemic homeostasis of the host. Emerging evidence has highlighted the importance of gut microbiota in intestinal diseases, and more recently, in age-related central nervous systems diseases, e.g., stroke and Alzheimer’s disease. It is now generally recognized that gut microbiota significantly influences host behaviors and modulates the interaction between microbiota, gut, and brain, via the “microbiota-gut-brain axis”. Several approaches have been utilized to reduce age-related dysbiosis in experimental models and in clinical studies. These include strategies to manipulate the microbiome via fecal microbiota transplantation, administration of prebiotics and probiotics, and dietary interventions. In this review, we explore both clinical and pre-clinical therapies for treating age-related dysbiosis.

INTRODUCTION

The microbiota represents a dynamic environment inhabited by a wide array of microorganisms encompassing bacteria, fungi, archaea, protozoans, and even viruses present ubiquitously on host tissues; their collective genomes are known as the microbiome (1). They reside both inside and outside of the host including on the skin, and in the mouth, pharynx, respiratory tract, urogenital tract, and gastrointestinal (GI) tract, and play critical roles in modulating host homeostasis (2). New molecular techniques have allowed for the rapid identification of microbiome components by sequencing of the 16S ribosomal RNA (rRNA)-encoding gene to identifying specific bacteria (3–5). Initiatives like the Human Microbiome Project (HMP), developed by the National Institutes of Health (NIH) are using genome sequencing to create a reference database of the healthy human microbiota composition (NIH HMP 2012) (6).

These microorganisms regulate many factors of host physiology, such as immunogenic and hormonal properties. They are profoundly attuned to the dynamic host mucosal environment. Changes to this environment, known as “dysbiosis”, especially within the gastrointestinal tract, have been associated with a vast array of neurological diseases such as in stroke and Alzheimer’s (AD), through the MGB axis (bottom-up signaling) (7–10). Studies have implicated this bottom-up signaling in the migration of pro-inflammatory mediators from the gut to the brain via several routes including circulating blood, the immune system, microbial metabolites, the vagus nerve, and the enteric nervous system (11). Over the past decade, numerous studies have shown that the microbiota within the gut plays an integral part in regulating brain development, function, and behavior. Furthermore, changes to the immune milieu orchestrating the bi-directional communication across the MGB axis is well documented, however, how this interaction influences the exacerbation of the disease, and its contribution to aging is still an emerging concept. Over 100 trillion microorganisms have been identified in the human GI tract encoding over three million genes, compared with the 23,000 genes within the human genome (12, 13). The microbiota produces thousands of diverse metabolites that play crucial functional roles in the host (12–14). Currently, more than 50 bacteria phyla have been detected in the GI tract of humans translating to a diversity of over 36,000 species of microorganisms (15).

Predominantly, the spike in the microbial load within the GI tract occurs during birth. The intestines of neonates are mostly sterile or have only trace levels of microbes (16). Although there are studies that suggest a placental microbiome may exist, which could influence the microbial flora within neonates, these studies are limited (17–20). During birth, as neonates pass through the birth canal, they are exposed to the microbial population of the mother’s vaginal canal. The principal genus are Lactobacillus spp. and Bifidobacterium spp. as indicated by the distinct metabolites present within the gut lumen (16, 20, 21). Interestingly, many population-based studies have shown that infants who are delivered by cesarean have a significantly reduced bacterial load in the gut at one month of age relative to neonates that were born by traditional vaginal delivery, suggesting that the host biome is programmed early in life (22).

The microbiome composition, at a genetic level, changes through the lifespan due to multiple external and physiological factors. During the first 3 years of life, the microbiome fluctuates dramatically until it reaches a more stable and unique signature at a genetic level resembling an adult microbiota, which can be relatively stable for decades (23–25). Under normal physiological conditions, the microbiome modulates the development of the immune system and immunogenic potential within the host (26–30). In particular, the intestinal microbiota regulates the differentiation and development of tissue-resident immune cells within the central nervous system, a hallmark classification for the microglial cell lineage (31). In addition, it provides a stable source for several important nutrients and vitamins from well-established commensal bacteria, to facilitate proper metabolic homeostasis and function (26–30). Microbial diversity diminishes with aging, and elderly populations have differences in gut microbe composition compared to that of young adults (32, 33). However, understanding if the gut microbiota can itself influence aging is challenging, as the biome varies greatly between individuals and over time. Age-related changes in host physiology may shape the dysbiosis seen with aging, so it remains unknown if microbes themselves can drive the aging phenotype. Interestingly, work in simple model species, like the fruit fly, Drosophila melanogaster, have shown that changes in microbial composition precede intestinal and host aging, and manipulation of the microbiota can extend lifespan, suggesting that microbes can accelerate the aging process (34, 35). Age- or disease-related changes in microbiota composition can lead to a disruption in gut homeostasis in terms of integrity and overall physiology. This phenomenon is commonly referred to as dysbiosis, the term signifying the shift in the microbiota that is associated with a pathological state (36). Gut dysbiosis has been identified as a critical trigger for chronic low-grade inflammation, obesity, diabetes, cardiovascular disease, behavioral disorders, and neurodegenerative disease (37).

In this review, we will highlight the potential of 1) neurological disorders, e.g., stroke, AD, and 2) aging, to cause dysbiosis. Understanding the underlying mechanisms that lead to dysbiosis is imperative for the development of potential therapeutic interventions.

MICROBIOME: AN EMERGING KEY REGULATOR IN THE GUT-BRAIN AXIS

The microbes within the gut play a crucial role in human health. Biosynthesis of vitamins and amino acids using products of food breakdown/fermentation by microorganisms has been widely described (38). Moreover, it has been shown the microorganisms in the GI tract can provide a biochemical pathway for the metabolism of non-digestible carbohydrates, including more abundant polysaccharides (starches, cellulose, hemicellulose, pectins, and gums), oligosaccharides, non-absorbable sugars, and alcohols from the diet (39). The dietary consumption of these non-digestible molecules provides an avenue for the host to gain essential vitamins and minerals required for proper physiological function. Importantly, the gut microbiome also supports host protection. Commensal bacteria prevent pathogen invasion and growth by their crosstalk with the host intestinal epithelium and regulate protective mechanisms including secretory immunoglobulin A and mucous production for barrier protection (40). Beneficial bacteria in the gut help the host avoid colonization by pathogens through many competitive processes: nutrient metabolism, pH modification, antimicrobial peptide secretion, and effects on cell signaling pathways (41).

Recent studies demonstrate that the gut microbiome has an important role in connecting the brain and distant organs such as the gut, often referred to as the gut-brain axis or microbiota-gut-brain (MGB) axis (42). The MGB axis acts as a communication portal that integrates neural, hormonal, and immunological signaling in the host where microbial metabolites including short-chain fatty acids (SCFAs) and neurotransmitters play a key role inmodulating signaling cascades (7, 42, 43). The bidirectional communication within the brain and the gut is very intricate and dynamic allowing the central nervous system to regulate proximal immune responses in the gut to maintain host homeostasis. Numerous studies have highlighted the complexity of this communicative interaction and have shed light on how neurological disorders have detrimental effects on gut morphology and integrity. Very recent findings highlight that the gut microbiota could be a therapeutic target in many neurological diseases such as stroke (8, 9).

Various studies have shown that stroke, AD, and even aging itself can lead to loss of gut barrier integrity (i.e., “leaky gut”), changes in gut microbiota composition, and altered microbial metabolite secretion of SCFAs and neurotransmitters that govern the pathogenic processes of the disease. As an example, in AD, the environment within the gut promotes the colonization of certain bacteria and fungi that secrete amyloid proteins, leading to an increase in amyloid proteins within the CNS and systemically (44–53). This continuous supply of amyloid shifts the dynamic equilibrium of Aβ production and clearance leading to its accumulation in the brain. Stroke leads to dysbiosis and activates inflammatory cascades within the gut, leading to exacerbated brain inflammation and poorer recovery after an ischemic event (7, (54–56). Stroke is responsible for 1 out of every 20 deaths in the United States (57, 58). A major contributor to mortality is post-stroke infection or sepsis. We investigated the effects of age and the role of bacterial translocation from the gut in young (2-3 months) and aged (18-20 months) C57BL/6 male mice following transient middle cerebral artery occlusion or sham surgery. Gut permeability was examined, and peripheral organs were assessed for the presence of gut-derived bacteria following stroke. We found that while stroke-induced gut permeability and bacterial translocation in both young and aged mice, only young mice were able to resolve the infection. Bacterial species seeding peripheral organs also differed between young (Escherichia) and aged (Enterobacter) mice. (59) Consequently, aged mice developed a septic response marked by persistent and exacerbated hypothermia, weight loss, and immune dysfunction compared to young mice following stroke. This suggests that bacteria that are present in aged mice, and the age-related dysbiosis well documented by others, can influence disease outcomes. Factors like aging, diet, environment, and genetics all have a crucial role in the promotion of gut dysbiosis (60) and increase both the risk of developing neurological disorders and impair recovery (61–67).

FACTORS THAT INFLUENCE THE MICROBIOME

The microbiome is a dynamic environment that coordinates with host physiology and is highly fine-tuned to protect the host against luminal antigens (5). In addition, the overall microbial population can change within the lifespan of the individual as well as with dietary and disease-related physiological alternations. However, based on certain factors such as genetics, certain individuals have a heightened predisposition or genetic susceptibility toward gut dysbiosis and neurological disorders (68–70). Several factors are involved in this alteration and with temporal changes in the microbiome, including host genetics, diet, medications (e.g., antibiotics), and aging (Fig. 1). The etiology of these factors are complicated and are often challenging in regards to identification of causative, rather than correlative variables. Do host changes (i.e. increased inflammation, immune/cellular senescence, gut motility/barrier impairment) drive age related dysbiosis, or does the dysbiosis change the host? Many of these questions remain to be answered but emerging data from germ free mice or mice “rejuvenated” by young fecal transplants suggest that the microbial composition is a major contributor to changes in gut physiology, integrity, and overall host health with aging

Fig 1.

Factors that can influence gut microbiota composition and cause gut dysbiosis

a. Genetics

Large-scale population-based studies have shown that genetic interactions among the host and microbiota play an essential role in shaping the human microbiome composition. Predominantly, the genetic component is linked to the diversification of the microbiota during the progression of colonization within a wide range of gene transduction processes (71). Spontaneous mutations or lateral gene transfers within species of microbiota promote niche variations and maintain microbial diversity. Over the past decade, multiple studies have incorporated the use of integrated gene catalogs to incorporate and examine genetic sequencing data for most microbes (72, 73). Expanding the range of integrated gene analysis within the microbiome gene catalog will certainly aid in facilitating the quantitative characterization of metagenomics, metatranscriptomics, and metaproteomic on a genetic level to fully comprehend disease-specific differences within human pathological states (74).

Multiple studies have shown that host genetics have the potential to influence alpha-diversity or the degree of diversity within taxa (75, 76). Genome-wide association studies (GWAS) have made promising strides in predicting genetic variants and the association of population-based microbial phenotypes (77). However, modeling microbiome profiling datasets as heritable traits is still challenging due to the lack of a stable reference catalog. Additionally, sample sizes are too small under the criteria of Genome-wide association studies (GWAS) to accurately predict or validate significance individually, and comparative meta-analysis from cross-references are very distinct in their analytical methods of microbial profiling.

Modeling the transgenerational genetic composition of the microbiome as a diagnostic screen is crucial to highlight the heritability of microbe-induced diseases. Many studies have tried to elucidate how environmental factors, interactions among microbes, and host genetic differences shape the microbiome. It has been assumed that differing genetic backgrounds in the host contribute to differences in both the vulnerability to pathogens and the capacity to associate with symbionts. Several studies have shown that genetic differences between hosts influence the microbiome composition (78–80), while other studies have determined that the host plays a minor role, and it is mostly the environment that shapes the biome (81). Currently, it is difficult to separate genetic and environmental factors. Indeed, it is challenging to identify host effects as often environmental factors are not controlled for or taken into consideration in the course of analysis (82, 83).

b. Diet

Dietary intake is an essential factor that induces microbial shifts in the host to maintain immunogenic tolerance. The consumption of food is a major source of essential precursors for metabolite production required for homeostasis. Multiple studies have shown that dietary factors modulate the gut microbiota and have an impact on both the dynamic ecosystem, microbe-microbe interactions, and the overall microbiome profile (84, 85). Classification of certain diets (e.g., high fat, high carbohydrate intake, or high fiber) have the potential to facilitate the colonization of certain bacterial phyla, including Firmicutes, Actinobacteria, and Deltaproteobacteria (fat) and Prevotella (carbohydrate, fiber) (86–88). High protein-based diets, such as meats, and synthetic-based proteins, have been correlated to the growth of bile tolerant microbes, which predominantly produce metabolites such as essential/non-essential amino acids required for maintaining proper gut health and harmful byproduct removal (89). On the other hand, high-fat diets (i.e., saturated fats) results in chronic regional gut inflammation due to increases in specific genus, i.e., Clostridium, and ultimately contribute to dysbiosis (90, 91).

Bacteroides are commonly found in diets dominated by abundant animal proteins, amino acids, and saturated fats like those consumed by US populations, and are potentially harmful to overall gut health (4, 92). Higher consumption of animal proteins has been linked to increased plasma concentrations of trimethylamine N-oxide (TMAO), a biologically active molecule produced by the microbiota from TMA. Elevated TMAO levels are associated with an increased risk of atherosclerosis and other chronic vascular diseases (93, 94). Studies have shown that alterations in dietary habits (vegan/vegetarian versus omnivore/carnivore) and changes in systemic TMAO concentrations correlate with changes in specific bacterial taxa (95).

All these findings indicate that diet may play a critical role in shaping the gut microbial signature in the elderly population (96).

c. Antibiotics

The discovery and use of antibiotics have helped treat many different types of diseases and infections over the past century. These drugs are given to patients to eradicate pathogenic bacteria, but some antibiotics can also kill mutualistic, commensal microbiota, and lead to long-term changes in the biome. Prolonged use of such antibodies has shown to have profound negative effects upon normal physiology and the composition of the biome (97). Multiple studies have shown that administration of antibiotics promotes gut dysbiosis (98), contributes to several gut-related diseases, and interestingly can affect behavioral outcomes (i.e., increased prevalence of depression and autism) (99).

In addition, extensive use of antibiotics gives rise to colonized bacteria that become resistant due to natural selection. For instance, in a longitudinal-based clinical study, healthy volunteers treated with 150 mg of clindamycin reported an upregulation of antibiotic resistance genes among Bacteroides that persisted six months to 2 years after completion of treatment (100). Further, a dramatic loss in gut microbial diversity as well as in the representation of specific taxa was evident. As not all antibiotics target the same bacteria, it is common to use a combination of different antibiotics, e.g., ampicillin, gentamycin, metronidazole, neomycin, and vancomycin, to widen the antimicrobial spectra for the treatment of severe bacterial infection, thus altering the microbiome. Currently, the usage of probiotics and fecal microbiota transplantation (FMT) are emerging therapies that can be a potential intervention to address the detrimental effects of antibiotic-induced gut dysbiosis (8).

d. Aging

Aging itself is not considered a disease, but is a critical risk factor for a wide array of neurological disorders. Aging comes with an increased risk of infection and chronic low-grade inflammation known as “inflamm-aging”. This phenotype is mainly due to the physiological changes that result in frailty and immunosenescence, which leads to increased infections in the elderly population (101). Changes in microbiota diversity have been observed over the lifespan of humans and during the progression of aging (102) (Fig. 2). Many studies (Table I) have shown dramatic changes within the microbiome composition and gut structure in the elderly population (103).

Fig 2.

Changes in gut microbiota with aging. Examples of the dominant bacterial species found human intestinal tract during different stages of human development from birth. As aging occurs, there is a decrease in microbial diversity and significant shifts in the microbiome that can lead to gut dysbiosis.

Table I.

CHANGES IN THE MICROBIOME IN AGING

Disease	Study Title	Reference
Aging	A Human-Origin Probiotic Cocktail Ameliorates Aging-Related Leaky Gut And Inflammation Via Modulating The Microbiota/Taurine/Tight Junction Axis	(104)
Aging	A Longitudinal Study Of The Development Of The Saliva Microbiome In Infants 2 Days To 5 Years Compared To The Microbiome In Adolescents	(142)
Aging	Adolescence And Aging: Impact Of Adolescence Inflammatory Stress And Microbiota Alterations On Brain Development, Aging, And Neurodegeneration	(143)
Aging	Age-Related Alterations In Human Gut Cd4 T Cell Phenotype, T Helper Cell Frequencies, And Functional Responses To Enteric Bacteria	(144)
Aging	Age-Related Compositional Changes And Correlations Of Gut Microbiome, Serum Metabolome, And Immune Factor In Rats	(145)
Aging	Age-Related Differences In The Gut Microbiome Of Rhesus Macaques	(146)
Aging	Age-Related Variation Of Bacterial And Fungal Communities In Different Body Habitats Across The Young, Elderly, And Centenarians In Sardinia	(147)
Aging	Calorie Restriction Slows Age-Related Microbiota Changes In An Alzheimer’s Disease Model In Female Mice	(148)
Aging	Dietary Interventions In Mild Cognitive Impairment And Dementia	(149)
Aging	Dietary Restriction With And Without Caloric Restriction For Healthy Aging	(150)
Aging	Effect Of Short Chain Fatty Acids On Age-Related Disorders	(151)
Aging	Effects Of The Gut Microbiome On Aged-Related Cognitive Decline And Inflammation	(152)
Aging	Gastro-Intestinal And Oral Microbiome Signatures Associated With Healthy Aging	(153)
Aging	Gut Microbial, Inflammatory And Metabolic Signatures In Older People With Physical Frailty And Sarcopenia: Results From The Biosphere Study	(154)
Aging	Gut Microbiota And Aging-A Focus On Centenarians	(155)
Aging	Gut Microbiota And Microbiota-Related Metabolites As Possible Biomarkers Of Cognitive Aging	(156)
Aging	Gut Microbiota Combined With Metabolomics Reveals The Metabolic Profile Of The Normal Aging Process And The Anti-Aging Effect Of Fufang Zhenshu Tiaozhi(FTZ) In Mice	(157)
Aging	Gut Microbiota In Common Elderly Diseases Affecting Activities Of Daily Living	(158)
Aging	Ketogenic Diets Alter The Gut Microbiome Resulting In Decreased Intestinal Th17 Cells	(159)
Aging	Lactobacillus Fermentum JX306 Restrain D-Galactose-Induced Oxidative Stress Of Mice Through Its Antioxidant Activity	(160)
Aging	Lactobacillus Sp. Improved Microbiota And Metabolite Profiles Of Aging Rats	(161)
Aging	Microbial Stimulation Reverses The Age-Related Decline In M Cells In Aged Mice	(162)
Aging	Modifying Progression Of Aging And Reducing The Risk Of Neurodegenerative Diseases By Probiotics And Symbiotic	(163)
Aging	Probiotics And Prebiotics As A Therapeutic Strategy To Improve Memory In A Model Of Middle-Aged Rats	(164)
Aging	The Gut Microbiome And Frailty	(165)
Aging	The Interaction Between Gut Microbiota And Age-Related Changes In Immune Function And Inflammation	(166)
Aging	The Microbiome And Aging	(167)
Aging	The Molecular Mechanisms Of Probiotic Strains In Improving Aging Bone And Muscle Of D-Galactose-Induced Aging Rats	(168)
Aging	The Role Of The Microbiota In Sedentary Lifestyle Disorders And Ageing: Lessons From The Animal Kingdom	(169)
Aging	Transplant Of Microbiota From Long-Living People To Mice Reduces Aging-Related Indices And Transfers Beneficial Bacteria	(170)

Gut epithelial layers are maintained by goblet cell-producing mucins and tight junctions (TJ), which form a paracellular barrier to regulate not only the flux of ions and solutes but also commensal flora. Studies have shown that aging leads to a breakdown of the epithelial barriers of the GI tract, which is prone to bacterial challenges such as infection, facilitating bacterial translocation and increasing infection rates and overall inflammation in the elderly (104) (Fig. 3). Loss of gut integrity induces pathological activation of the immune system, and “primes” a pro-inflammatory environment in the host (105). This is characterized by higher levels of inflammatory markers such as IL-6 and IL-S in the blood that have been linked to enhanced senescence (33).

Fig 3.

(A) Disruption in gut epithelial structure may allow the translocation of commensal bacteria and toxic signals into the host which cause both inflammation and infection. (B) Age-associated impairment of the histological architecture and the mucin production in the colon. Colonic tissues were collected from young (2-3 months) and aged (18-20 months) mice and stained with Alcian-blue and periodic acid-Schiff staining to examine the global structure and mucins.

A population-based study evaluated the microbiome diversity of individuals over 100 years of age (“the centenarians”) and compared this to 70-year-olds and young adults. As expected, centenarians showed a different pattern in their microbiome diversity compared with 70-year-olds and young adults, with a decrease in Firmicutes and enrichment in facultative anaerobes (33). A later study analyzed the bacterial metagenome in young (mean age: 38), adult (mean age: 66.4), and centenarian (mean age: 100.7) and demonstrated that an increase in genes related to tryptophan metabolism pathways correlated with a significant reduction in tryptophan levels in the serum in the centenarians, which was linked to cognitive deficits and dementia (106–108). Further analysis showed a reduction in genes involved in the production of SCFAs in centenarians and its correlation with overall health status. Butyrate is a key SCFA associated with improved intestinal structure and gut health. Butyrate has anti-inflammatory properties and suppresses nuclear factor-kappa B, signal transducer and activator of transcription 1 activation, and induces T_reg cell differentiation within the gut lamina propria (109). Higher levels of SCFA butyrate-producing bacteria were correlated with reduced inflammation and longevity in centenarians (33). These findings were supported by the publication of the ELDERMET project, which determined the composition of the intestinal microbiome of 500 elderly individuals (110).

GUT MICROBIOTA AS A THERAPEUTIC TARGET FOR AGING AND AGE-RELATED NEUROLOGICAL DISORDERS

Several studies have shown that the GI track and resident microbiota are susceptible to the progressive functional decline associated with aging and with neurological diseases such as stroke (Table II) and AD (Table III). Many of the pathological processes include the breakdown of the gut epithelial barrier, loss of enteric neurons, and altered mucosal immune function resulting in excessive production of pro-inflammatory cytokines. Concomitant with changes in GI physiology, aging alone was associated with significant shifts in the makeup of the gut microbiota including altered microbial richness and diversity and a decrease in bacteria with anti-inflammatory properties (111). Through this communicative route or through the microbiota-gut-brain axis, the gut microbiota can regulate brain function of the host and even cognition and memory.

Table II.

CHANGES IN THE MICROBIOME IN STROKE

Disease	Study Title	Reference
Stroke	A Gut Feeling About Stroke Reveals Gut-Brain Axis’ Active Role In Homeostasis And Dysbiosis	(171)
Stroke	Age-Related Changes In The Gut Microbiota Influence Systemic Inflammation And Stroke Outcome	(9)
Stroke	Distinct Commensal Bacterial Signature In The Gut Is Associated With Acute And Long-Term Protection From Ischemic Stroke	(172)
Stroke	Enteral Eco-Immuno-Nutrition Reduced Enteral Permeability And Serum Ghrelin Activity In Severe Cerebral Stroke Patients With Lung Infection	(173)
Stroke	Examining The Role Of The Microbiota-Gut-Brain Axis In Stroke	(7)
Stroke	Gut Dysbiosis Is Associated With Metabolism And Systemic Inflammation In Patients With Ischemic Stroke	(174)
Stroke	Gut Microbiota And Stroke	(175)
Stroke	Imbalance In The Force: The Dark Side Of The Microbiota On Stroke Risk And Progression	(176)
Stroke	Stroke Dysbiosis Index (Sdi) In Gut Microbiome Are Associated With Brain Injury And Prognosis Of Stroke	(177)
Stroke	The Association Of Post-Stroke Cognitive Impairment And Gut Microbiota And Its Corresponding Metabolites	(178)
Stroke	The Gut Microbiome Primes A Cerebro-Protective Immune Response After Stroke	(179)
Stroke	Microbiota Dysbiosis Controls the Neuro-inflammatory Response after Stroke.	(180)
Stroke	Age-related changes in the gut microbiota influence systemic inflammation and stroke outcome.	(9)
Stroke	Gut dysbiosis is associated with metabolism and systemic Inflammation in patients with ischemic stroke.	(174)

Table III.

CHANGES IN THE MICROBIOME IN ALZHEIMER’S DISEASE

Disease	Study Title	Reference
Alzheimer’s	Antibiotics, Gut Microbiota, And Alzheimer’s Disease	(181)
Alzheimer’s	Gut Microbiome Alterations In Alzheimer’s Disease	(182)
Alzheimer’s	Gut Microbiota And Pro/Prebiotics In Alzheimer’s Disease	(183)
Alzheimer’s	Gut Microbiota is Altered In Patients With Alzheimer’s Disease	(184)
Alzheimer’s	Metagenome Analysis Of Bodily Microbiota In A Mouse Model Of Alzheimer Disease Using Bacteria-Derived Membrane Vesicles In Blood	(185)
Alzheimer’s	Mild Cognitive Impairment Has Similar Alterations As Alzheimer’s Disease In Gut Microbiota	(186)
Alzheimer’s	Modifying Progression Of Aging And Reducing The Risk Of Neurodegenerative Diseases By Probiotics And Symbiotic	(163)
Alzheimer’s	Probiotics For Dementia: A Systematic Review And Meta-Analysis Of Randomized Controlled Trials	(187)
Alzheimer’s	Relationship Between Dementia And Gut Microbiome-Associated Metabolites: A Cross-Sectional Study In Japan	(188)
Alzheimer’s	The Gut Microbiome As A Therapeutic Target For Cognitive Impairment	(189)
Alzheimer’s	Time To Test Antibacterial Therapy In Alzheimer’s Disease	(190)

A few recent studies have attempted to manipulate the gut microbiome to eliminate this as a source of systemic inflammation in age-related neurological disorders. The crux of this emerging research is engaging the idea of the bidirectional communication between the brain and the gut. The gut or gut microbiota can communicate with the brain via the production of neurotransmitters, SCFAs, and other metabolites (7). These biochemical signals create a response in the brain, such as mood changes, behavioral changes, and changes in cognition. Different strategies have been tested in clinical trials to modify the microbiome, including fecal microbiota transplantation (FMT), administration of probiotics or antibiotics, or dietary interventions, which are non-invasive but may re-shape the host immune systems (Table IV). We summarize pre-clinical findings demonstrating the potential of manipulation of gut microbiota for therapeutic use in neurological disorders.

Table IV.

Interventional microbiome-based therapeutic approaches in clinical trials for Aging, Stroke, and Alzheimer’s. Additional Information on Clinical trials can be found at the website of NIH clinical trials (https://clinicaltrials.gov/).

Condition	Study Title	Drug	Intervention	Trial ID
Stroke	Cognition and Gut Microbiome Associated Study of Shanghai People With Acute Ischemic Stroke	Probiotics	Diet Supplement	NCT03812445
Stroke	Gut Microbiota in Acute Stroke Patients	Fecal Sample Collection	Observational	NCT03934021
Stroke	The Effectiveness of Oral Health Promotion on Pneumonia Complicating Stroke		Behavioral	NCT04095780
Condition	Study Title	Drug	Intervention	Trial ID
Alzheimer’s Disease	Taxonomic and Functional Composition of the Intestinal Microbiome: a Predictor of Rapid Cognitive Decline in Patients With Alzheimer’s Disease	Fecal Sample Collection	Observational	NCT03487380
Alzheimer’s Disease	The PREVENTION Trial: Precision Recommendations to Optimize Neurocognition (PREVENTION)		Behavioral	NCT04082611
Alzheimer’s Disease	Study: Diet and Exercise Study to Improve Brain Blood Flow: Blood Flow Improvement Trial	Diet and Exercise	Behavioral	NCT03117829
Alzheimer’s Disease	Oral Fecal Microbiota Transplant Feasibility Study in Alzheimer’s Disease	Fecal Microbiota Transplant	Diet Supplement	NCT03998423
Condition	Study Title	Drug	Intervention	Trial ID
Aging, and Cognitive Decline	The Impact of Cranberries On the Microbiome and the Brain in Healthy Ageing sTudy (COMBAT)	Freeze-Dried Cranberry	Dietary Supplement	NCT03679533
Aging	Acarbose Anti-aging Effects in Geriatric Subjects (Substudy B & C)	Acarbose	n α-Glucosidase Inhibitor	NCT02865499
Aging	Study of Acarbose in Longevity	Acarbose	Drug	NCT02953093
Aging	Age-associated Arterial Dysfunction, Western Diet, and Aerobic Exercise: Role of the Gut Microbiome	Western Diet	Diet	NCT03334201
Aging	Resistant Starch and Nonstarch Polysaccharide (Dietary Fibre) Intake and the Colonic Microbiome in Older People	Dietary Fiber/Resistant Starch	Diet Supplement	NCT02384174
Aging	Accelerated Genital Tract Aging in HIV: Estradiol Clinical Trial	Estradiol Vaginal Insert	Drug	NCT04079218
Aging	Synbiotic Use Effect Gut Bacteria and the Immune Response in Older People	Synbiotic	Dietary Supplement	NCT01226212
Aging	Low Sugar Protein Pacing, Intermittent Fasting Diet in Men and Women	intermittent fasting; Heart Healthy Diet	Diet	NCT04327141
Microbiome, Aging, Immune, and Inflammation	The RAMP Study-Rejuvenation of the Aging Microbiota With Prebiotics	Prebiotic	Dietary Supplement	NCT03690999
Microbiota, and Aging	Impact of Okara and Bio-okara Food Product on Gut and Glycaemic Health in Middle-aged and Older Adults in Singapore	Okara/Bio-Okara	Dietary Supplement	NCT03978104
Aging	Probiotic on Psychological and Cognitive Effects	Probiotic	Dietary Supplement	NCT03080818
Aging	Nutritional Strategies for Metabolic Health in Aging	Supplements	Behavioral	NCT04282603
Aging	Investigating the Effects of Daily Consumption of Blueberry (Poly)Phenols on Vascular Function and Cognitive Performance	Wild-Blueberry Powder	Dietary Supplement	NCT04084457
Aging	Effects of Aronia Berries on Vascular Endothelial Function and the Gut Microbiota in Middle-Aged/Older Adults	Aronia	Dietary Supplement	NCT03824041
Aging, and Cognitive Decline	Dietary Reduction of AGEs to Prevent Cognitive Decline in Elderly Diabetics	Low AGEs Diet	Behavioral	NCT02739971

A. Fecal Microbiota Transplantation (FMT) in Aging and Age-related Neurological Disorders (Table V).

Table V.

FECAL TRANSFER AS THERAPEUTIC APPROACH FOR AGING, STROKE, AND AD.

Disease	Study title	Treatment	Reference
Aging	Microbial Stimulation Reverses The Age-Related Decline In M Cells In Aged Mice	FMT	(162)
Aging	Heterochronic Fecal Transplantation Boosts Gut Germinal Centers In Aged Mice	FMT	(191)
Aging	Transplant Of Microbiota From Long-Living People To Mice Reduces Aging-Related Indices And Transfers Beneficial Bacteria	FMT	(170)
Stroke	Age-Related Changes In The Gut Microbiota Influence Systemic Inflammation And Stroke Outcome.	FMT	(9)
Stroke	Commensal Microbiota Affects Ischemic Stroke Outcome By Regulating Intestinal Γδ T Cells	FMT	(192)
Stroke	Higher Risk Of Stroke Is Correlated With Increased Opportunistic Pathogen Load And Reduced Levels Of Butyrate-Producing Bacteria In The Gut	FMT	(193)
Stroke	Microbiota Dysbiosis Controls The Neuro-lnflammatory Response After Stroke	FMT	(180)
Stroke	Transplantation Of Fecal Microbiota Rich In Short-Chain Fatty Acids And Butyric Acid Treat Cerebral Ischemic Stroke By Regulating Gut Microbiota	FMT	(194)
Alzheimer’s	Altered Microbiomes Distinguish Alzheimer’s Disease From Amnestic Mild Cognitive Impairment And Health In A Chinese Cohort	FMT	(195)
Alzheimer’s	Fecal Metabolite Of A Gnotobiotic Mouse Transplanted With Gut Microbiota From A Patient With Alzheimer’s Disease	FMT	(196)
Alzheimer’s	Fecal Microbiota Transplantation Alleviated Alzheimer’s Disease-Like Pathogenesis In APP/PS1 Transgenic Mice	FMT	(117)
Alzheimer’s	Sex-Specific Effects Of Microbiome Perturbations On Cerebral Aβ Amyloidosis And Microglia Phenotypes	FMT	(197)
Alzheimer’s	Transfer Of A Healthy Microbiota Reduces Amyloid And Tau Pathology In An Alzheimer’s Disease Animal Model	FMT	(116)
Alzheimer’s	Transplant Of Microbiota From Long-Living People To Mice Reduces Aging-Related Indices And Transfers Beneficial Bacteria	FMT	(170)
Alzheimer’s	Transplantation Of Fecal Microbiota Rich In Short-Chain Fatty Acids And Butyric Acid Treat Cerebral Ischemic Stroke By Regulating Gut Microbiota	FMT	(194)

1. Aging

Studies using germ-free (GF) mice, which are devoid of all microbiota, have shown an increase in intestinal permeability and disturbed gut morphology as a consequence of aging. FMT using aged microbiome (17 months) transplanted into young GF mice (12-14 weeks) increased inflammation, and similarly, young mice given aged microbiota had features consistent with “inflamm-aging”. This was reflected by a reduction of the beneficial bacterial genus Akkermansia and increased gut permeability. This implies that the aged microbiome itself has a detrimental effect on inducing inflammation and dysfunction in the gut barrier (105). Understanding the role of dysbiosis with aging and age-related neurological disorders suggests there is potential to alter the gut microbiota to attenuate inflamm-aging. This becomes even more significant when the wide variety of diseases that primarily affect the elderly population are considered.

2. Alzheimer’s Disease (AD)

AD is the most common cause of dementia. AD is one of the most prevalent diseases of the elderly (112). It affects 16% of people between 65-74 years, 45% of people between 75-84 years, and 36% of people older than 85 years (113). The link between the gut microbiota and AD appears to be due in part to microbiome induced neuro-inflammation (114). Multiple studies have hypothesized that the onset of amyloid plaques is associated with the dysbiosis within the gut and is regulated via the bidirectional communication of the gut-brain axis (115, 116). In this regard, we recently found that gut inflammation and microbial dysbiosis precedes brain amyloid-beta accumulation using Tg2576 mice, a mouse model of AD (115). Further, researchers have investigated the concept of fecal microbial transplantation (FMT) to alleviate cognitive decline, enhance gut immune responses and promote anti-inflammatory cascades in a mouse model of AD (116). Using ADLP^APT (AD-like pathology with amyloid and neurofibrillary tangles) transgenic mice (116), investigators found inflammation and microbial dysbiosis in the gut, compared with wild-type (WT) mice. Interestingly, ADLP^APT mice given a fecal transplant from WT donor mice had reduced amyloid and Tau pathology in the brain, enhanced cognition, and reduced gut and systemic inflammation. Using a different model of AD, APPswe/PSldE9 transgenic mice, FMT from healthy WT mice improved behavioral outcomes, decreased Aβ40 and Aβ42 levels and phosphorylation of Tau protein in the brain with reversed gut microbiota profiles in the recipient AD mice (117). Despite using three distinct animal models of AD, which could potentially have different gut microbiota profiles, manipulating the biome comprehensively could serve as an intervention to alleviate the effects of AD.

3. Stroke

Stroke is a major cause of mortality and morbidity in the United States (55). One of the major contributors to mortality is post-stroke infection. Recent studies demonstrated that stroke increases gut permeability and systemic bacterial translocation leading to lung infection, i.e., pneumonia in stroke patients (118). Brain injury also induces infection in a mouse model of ischemic stroke (54). Stroke-induced gut permeability and bacterial translocation were seen in both young and aged mice, but only young mice were able to resolve infection (54). The bacterial species colonizing peripheral organs also differed between young (Escherichia) and aged (Enterobacter) mice. Interestingly both are gut-originated bacteria (119), suggesting, as in humans, many stroke related infections arise from the gut.

Our recent study further emphasizes the translational potential of manipulating the microbiome (8). Using post-stroke FMT from young mice (young FMT), we reconstituted the biome of recipient aged mice (18-20 months). Compared with aged FMT as a control, young FMT had higher SCFA levels, improved behavioral outcomes, and enhanced gut integrity. In addition, mice with young FMT had 1) increased T_reg cells in both small intestines and the brain, 2) decreased microglial activation, and 3) decreased γν T cells in the brain, which secrete pro-inflammatory IL-17. We observed the same beneficial effect of young biome when we reconstituted the gut of aged mice with young biome before stroke (9). A summary of recent reviews and selected studies examining the microbiome in stroke can be found in Table II.

B. Probiotics and Prebiotics in Age-related Neurological Disorders (Table VI)

Table VI.

PROBIOTICS AS THERAPEUTIC APPROACH FOR AGING, STROKE, AND AD.

Disease	Study title	Treatment	Reference
Aging	Lactobacillus Sp. Improved Microbiota And Metabolite Profiles Of Aging Rats	Probiotics	(161)
Aging	The Molecular Mechanisms Of Probiotic Strains In Improving Aging Bone And Muscle Of D-Galactose-Induced Aging Rats	Probiotics	(168)
Aging	Lactobacillus Fermentum JX306 Restrain D-Galactose-Induced Oxidative Stress Of Mice Through Its Antioxidant Activity	Probiotics	(160)
Aging	A Human-Origin Probiotic Cocktail Ameliorates Aging-Related Leaky Gut And Inflammation Via Modulating The Microbiota/Taurine/Tight Junction Axis	Probiotics	(104)
Aging	Probiotics And Prebiotics As A Therapeutic Strategy To Improve Memory In A Model Of Middle-Aged Rats	Probiotics	(164)
Aging	Longevity In Mice Is Promoted By Probiotic-Induced Suppression Of Colonic Senescence Dependent On Upregulation Of Gut Bacterial Polyamine Production	Probiotics	(198)
Stroke	Association Of Gut Microbiota-Dependent Metabolite Trimethylamine N-Oxide With First Ischemic Stroke	Probiotics	(199)
Stroke	Clostridium Butyricum Pretreatment Attenuates Cerebral Ischemia/Reperfusion Injury In Mice Via Anti-Oxidation And Anti-Apoptosis	Probiotics	(141)
Stroke	Dysbiosis Of Gut Microbiota And Short-Chain Fatty Acids In Acute Ischemic Stroke And The Subsequent Risk For Poor Functional Outcomes	Probiotics	(200)
Stroke	Effects Of The Oral Ingestion Of Probiotics On Brain Damage In A Transient Model Of Focal Cerebral Ischemia In Mice	Probiotics	(140)
Stroke	Gut Microbiota-Derived Short-Chain Fatty Acids Promote Post-Stroke Recovery In Aged Mice	Probiotics	(8)
Stroke	Short-Chain Fatty Acids Improve Post-Stroke Recovery Via Immunological Mechanisms	Probiotics	(201)
Alzheimer’s	Effects Of Probiotic Supplementation On Short Chain Fatty Acids In The Appnl-G-F Mouse Model Of Alzheimer’s Disease	Probiotics	(202)
Alzheimer’s	Probiotics Ameliorate Intestinal Pathophysiology In A Mouse Model Of Alzheimer’s Disease	Probiotics	(203)
Alzheimer’s	Probiotics Ameliorate Intestinal Pathophysiology In A Mouse Model Of Alzheimer’s Disease.	Probiotics	(203)
Alzheimer’s	Probiotics Modulate The Microbiota-Gut-Brain Axis And Improve Memory Deficits In Aged SAMP8 Mice	Probiotics	(204)
Alzheimer’s	Short-Chain Fatty Acids: Microbial Metabolites That Alleviate Stress-Induced Brain-Gut Axis Alterations	Probiotics	(205)

Probiotics are defined as live and beneficial bacteria (120). The most common probiotics used today are species of Lactobacillus and Bifidobacterium (121). The prebiotic is typically made up of carbohydrates such as galacto-oligosaccharides, fructo-oligosaccharides, and inulin to complement and foster the growth of the probiotic bacteria; usually, both agents are taken in combination for an ideal re-constitution (122). Studies have already shown that the use of this approach in elderly individuals leads to a decrease in circulating pro-inflammatory cytokines such as interleukin (IL)-6, IL-1β, and tumor necrosis factor-α (TNF- α) (122). An investigative review by Sharma et al. has recently compiled studies that used probiotics as therapeutic agents for the treatment of cellular senescence (123). They propose that probiotic supplementation can help improve age-related gut dysbiosis, suppress age-associated inflammatory stress, and possibly enhance lifespan as seen in model organisms (124). The use of probiotics and prebiotics have been shown to be helpful to treat metabolic conditions (125–135). A new perspective within this therapeutic intervention is the incorporation of probiotics to treat the cognitive dysfunction that is linked with these diseases as well (124, 136). However, most of these studies are still in the pre-clinical phase as the effect of metabolic disease on cognitive decline is still inconclusive in human studies (137).

1. Probiotics in AD

More recently, research has examined the effects of probiotics on age-induced cognitive decline and other neurodegenerative diseases of aging via the bidirectional communication of the MBA axis (138). Administration of a probiotic to AD model mice improved glucose uptake in the brain by restoring expression levels of glucose transporter (GLUT) 3 and GLUT1, key glucose transporters (139). A reduction in the expression of these glucose transporters is seen in AD patients and is believed to contribute to the cognitive decline seen in the disease phenotype. With the daily administration of a probiotic, an improvement in cognition in treated mice compared to that of non-treated mice was seen. Based on these findings, although the cognitive symptoms of AD may not develop until later in disease progression, the use of probiotics could be both a preventative and a symptomatic therapy for the cognitive dysfunction seen with AD and possibly other neurodegenerative diseases as well (139).

2. Probiotics and prebiotics in stroke

Studies examining the use of probiotics to treat stroke are limited. One study found that oral ingestion of Bifidobacterium breve, Lactobacillus casei, Lactobacillus bulgaricus, and Lactobacillus acidophilus reduced ischemic injury after an experimental stroke in mice (140), but confirmation of changes in the microbial levels was not performed, and no behavioral benefits were seen. Administration of Clostridium butyricum by oral gavage two weeks prior to bilateral common carotid artery occlusion (BCCAO) reduced brain injury, but again, changes in the microbiome were not assessed directly. Levels of SCFAs (e.g., acetate, butyrate, and propionate) in young biome are higher than aged biome (8). For more refined and targeted microbial therapy, we selected bacterial strains that produce SCFAs (SCFA-producers) based on microbial and metagenomic sequencing. Transplanting selected SCFA-producers (Bifidobacterium longum, Clostridium symbiosum, Faecalibacterium prausnitzii, and Lactobacillus fermentum) improved stroke outcomes in aged stroke mice by decreasing brain IL-17 levels secreted by γν T cells (141).

C. Dietary interventions (Table VII)

Table VII.

DIATERY INTERVENTIONS AS THERAPEUTIC APPROACH FOR AGING, STROKE, AND AD.

Disease	Study Title	Treatment	Reference
Aging	Ketogenic Diets Alter The Gut Microbiome Resulting In Decreased Intestinal Th17 Cells	Dietary Intervention	(159)
Aging	Gut Microbiota Combined With Metabolomics Reveals The Metabolic Profile Of The Normal Aging Process And The Anti-Aging Effect Of Fufang Zhenshu Tiaozhi(FTZ) In Mice	Dietary Intervention	(157)
Stroke	Dietary Habits In Patients With Ischemic Stroke: A Case-Control Study	Dietary Intervention	(206)
Alzheimer’s	Bioactive Food Abates Metabolic And Synaptic Alterations By Modulation Of Gut Microbiota In A Mouse Model Of Alzheimer’s Disease.	Dietary Intervention	(207)
Alzheimer’s	Fructo-Oligo-Saccharides Ameliorating Cognitive Deficits And Neurodegeneration In APP/PS1 Transgenic Mice Through Modulating Gut Microbiota	Dietary Intervention	(208)
Alzheimer’s	Modified Mediterranean-Ketogenic Diet Modulates Gut Microbiome And Short-Chain Fatty Acids In Association With Alzheimer’s Disease Markers In Subjects With Mild Cognitive Impairment	Dietary Intervention	(209)
Alzheimer’s	Modified Mediterranean-Ketogenic Diet Modulates Gut Microbiome And Short-Chain Fatty Acids In Association With Alzheimer’s Disease Markers In Subjects With Mild Cognitive Impairment	Dietary Intervention	(209)

It is well known that a change in diet can influence the microbiota composition in the gut. Instead of adding beneficial bacteria to the gut, changes in diet aim to promote the growth of beneficial bacteria already present in the gut. A seminal study done by Claesson et al. showed that dietary changes in older individuals living in nursing facilities vs. communityd-welling elders had a significant effect on microbiota diversity (32). Previous data show that microbiome divergence is greater among older people compared to younger adults. Claesson hypothesized that the elderly had greater variance environmentally, and devised this study to specifically look at how much the environment affects diet and the aging microbiome. The variability in microbiota in the elderly, their diets, and their overall health was examined in community-dwelling groups and groups that were transitioning into a long-term nursing facility. The community-dwelling group that had a more balanced and diverse diet had a more diverse microbiome whereas the nursing facility group that had a more fixed diet that was higher in glucose, glycine, and lipids had a less diverse microbiome. The nursing facility group also had increased levels of circulating cytokines, were more frail and depressed, and had poorer overall nutrition. Given their results, they concluded that although other factors contributed to the decline in the health of these individuals, there was an association of diet with microbiota composition. Therefore, supporting dietary interventions could be a strategy to modulate the microbiota and, as a result, promote healthier aging. That said, changes in diet would be most effective as a preventative approach or used in conjunction with probiotics and prebiotics to essentially revert the aged dysbiotic microbiome to a ‘younger’ healthier composition.

CONCLUSION

It is becoming apparent that the gut microbiota play a pivotal role both in health and in age related diseases. In this review, we highlighted some of the ways that alterations of the gut microbiota could be both detrimental and beneficial in terms of aging and neurological diseases. Currently research using microbiota-based therapies for age-related diseases is in its infancy, but the research in this area has been growing exponentially over the past decade. The concept of manipulating the host gut microbiome, using FMT, dietary interventions, or pro/prebiotics as a therapeutic approach to address neurological diseases is gaining traction. As we begin to gain a better understanding of host-microbiota interactions, there are opportunities to produce a better approach to treat these diseases with more refined and targeted approaches.

Abbreviations:

AD

Alzheimer’s disease

CNS

central nervous system

FMT

fecal microbiota transplantation

GF

germ-free

GI

gastrointestinal

GLUT

glucose transporter

HMP

human genome project

IFN

interferon

IL

interleukin

MGB

microbiota-gut-brain

PAMP

pathogen-associated molecular pattern

PRR

pattern-recognition receptor

SCFA

short-chain fatty acid

TJ

tight junction

TLR

toll-like receptor

TMAO

trimethylamine N-oxide

TNF-α

tumor necrosis factor-α

T_reg

regulatory T cell