The Triple Alliance: Microbiome, Mitochondria, and Metabolites in the Context of Age-Related Cognitive Decline and Alzheimer’s Disease

Abstract

Alzheimer’s disease (AD) is a progressive, age-related neurodegenerative disorder that affects a large proportion of the older population. It currently lacks effective treatments, placing a heavy burden on patients, families, health care systems, and society. This is mainly due to our limited comprehension of the pathophysiology of AD progression, as well as the lack of effective drug targets and intervention timing to address the underlying pathology. AD is a multifactorial condition, and emerging evidence suggests that abnormalities in the gut microbiota play a significant role as environmental and multifaceted contributors to AD, although the exact mechanisms are yet to be fully explored. Changes in the composition of microbiota influence host neuronal health through their metabolites. These metabolites regulate intestinal epithelia, blood-brain barrier permeability, and neuroinflammation by affecting mitochondrial function. The decline in the proportion of beneficial microbes and their essential metabolites during aging and AD is directly linked to poor mitochondrial function, although the specific mechanisms remain unclear. In this review, we discuss recent developments in understanding the impact of the microbiome and its metabolites on various cell types, their influence on the integrity of the gut and blood-brain barriers, systemic and brain inflammation, and cell-specific effects in AD pathology. This information is expected to pave the way for a new understanding of the interactions between microbiota and mitochondria in AD, providing a foundation for the development of novel treatments for AD.

Alzheimer’s disease (AD) is the most prevalent neurodegenerative condition that causes dementia in older persons (65 years or older). More than 27 million people worldwide are affected by AD (1), and there are an estimated 6.2 million older adults living with AD-related dementias (ADRD) in the United States alone. By 2030, there are projected to be 75 million individuals diagnosed with AD worldwide, and by 2050, there will be 131.5 million (2). ADRD has a huge emotional and financial burden on patients, their families, and the health care. Currently, no disease-modifying medications are available, and the most commonly prescribed FDA-approved medications are donepezil (an acetylcholine esterase inhibitor) and memantine (an N-methyl-D-aspartate antagonist), both of which are only useful for treating a limited number of symptoms (3) instead of AD pathology. For nearly 30 years, amyloid-targeting therapies have focused on developing promising medications, but most clinical trials targeting the reduction of amyloids and/or tau have failed in phase 3 (4,5). Aducanumab (Aduhelm) is a recently FDA-approved drug for the treatment of AD that reduces brain amyloids, but its efficacy is limited, and its long-term safety remains elusive. This is because we lack a full understanding of AD pathophysiology as well as effective targets for the development of treatments. Therefore, there is a dire need to develop a better understanding of AD pathology and establish effective drug targets to develop new classes of drugs to combat the increasing prevalence of ADRD.

There are several theories about the pathophysiology of AD, most of which emphasize the functions of amyloid-beta (Aβ) and tau protein (6), blood-brain barrier (BBB) disruption, and neuroinflammation (7). The etiology of AD, at least in part, is associated with the inability to clear Aβ from the brain (8), but in many cases, is also linked with higher Aβ formation. Aβ is comprised 36–46 amino acids (AAs) and has neurotoxic properties, thus contributing to AD pathology (9). Furthermore, the accumulation of tau protein is a common manifestation of AD pathology (10). As a structural protein, tau forms microtubules to assist in the transport of materials within neurons. Despite its positive functions, tau build-ups disrupt its role and thus contribute to the progression of AD pathology. BBB are barriers to protect neurons from factors and toxins present in the blood circulation, thus critical to maintaining proper neuronal functions (11). However, BBB breakdown in terms of increased permeability is another common manifestation of the AD brain, which allows the leakage of unwanted proinflammatory substances from blood circulation to the brain and causes neuroinflammation (12). In addition, neuronal death either due to Aβ/tau accumulation or BBB breakdown further exacerbates neuroinflammation, which significantly contributes to the neurodegenerative and cognitive decline pathology of ADRD (13). However, the factors and mechanisms that contribute to BBB breakdown, neuroinflammation, and Aβ/tau accumulation remain elusive.

Emerging evidence indicates that, abnormalities in trillions of microbes living in the gut (called the gut microbiome) are linked with increased AD pathology of BBB breakdown, neuroinflammation, and Aβ/tau accumulations (14). The gut microbiome regulates the function of the CNS via the gut–brain axis and operates as a “second brain” to play a role in AD (15). Gut microbes produce diverse metabolites that interact with and influence host cells in both beneficial and detrimental ways. These metabolites not only influence cellular functions in gut cells but many of these metabolites also cross the blood circulation and influence various organs and their cellular functions (16). In the gut, connections between the microbiota and host immune system result in the generation of proinflammatory mediators, such as cytokines and chemokines, as well as antibodies involved in brain immunity (17). Metabolites created by the gut microbiota control the development and activation of microglia and astrocytes, which govern key neurobiological functions, including BBB permeability, neuronal development, and brain immune activation (18). However, the mechanisms by which microbial metabolites affect various cells in the BBB, microglia, neurons, and other cells in the brain are not fully understood.

A growing body of evidence has demonstrated that systemic mitochondrial dysfunction, evident in circulating blood cells, is associated with dementia and the pathophysiology of AD (19). Moreover, noncellular factors present in circulation mediate some of the key bioenergetic differences associated with AD (20). Mitochondrial dysfunction may be a link to the potential mechanisms by which the microbiota and its metabolites can affect brain and AD pathology. It is evident that mitochondrial dysfunctions significantly contribute to the pathology of AD (21). The disruption of BBB is linked with mitochondrial dysfunction, which is caused by a variety of mechanisms such as oxidative stress and neuroinflammation (22). Mitochondrial impairment and inflammatory processes can activate BBB endothelial cells, causing structural changes in capillaries, increasing BBB permeability, and disrupting transporter function (23). However, we do not understand the integrative relationship of microbiota and its metabolites affecting mitochondrial function in different cell types such as endothelial cells of BBB, microglia, astrocytes, and neurons that are significantly impacted in AD pathology. Here, we review the literature establishing the relationship of microbiota and its metabolites with mitochondria in the context of AD development via affecting BBB, microglia, and other cell types in the brain. We also discuss how these strategies can be exploited to develop preventive and treatment strategies for AD.

Microbiota and Its Metabolites in AD

Microbiota Changes Linked to AD Pathology: Machinery

The microbiota has an impact on human health and contributes to several diseases, including AD. Naturally occurring AD may be better seen as a systemic issue that affects the entire human superorganism, including microbiota that lives inside humans, rather than in certain regions of the brain or the body (24). Numerous studies on humans have revealed that when compared to healthy controls, those with clinically diagnosed AD, dementia, or moderate cognitive impairment (MCI) have different gut microbiomes from those without these conditions (25). This is consistent with the concept that gut microbiota may play a role in the pathology of AD. Additionally, patients with MCI and AD dementia showed dysbiosis, as measured by microbiome diversity, such as α- or β-diversity (25,26). An interesting finding implies that the gut of older adults with AD has a significantly distinct microbiota composition (25). For instance, in patients with dementia (33 AD and 32 MCI), there was a significant decline in Firmicutes, with an increased abundance of Proteobacteria, Gammaproteobacteria, and Enterobacteria compared to 32 cognitively healthy controls (25). A similar decrease in Firmicutes and Bifidobacteria was noted by Vogt et al. in the fecal samples of patients with AD, but this loss was offset by an overabundance of Bacteroidetes (27). In addition, Zhuang et al., discovered that the gut microbiota composition differed between 43 AD patients and age-matched controls: enhanced Bacteroidetes and lowered Actinobacteria at the phyla were accompanied by increased Ruminococcaceae, Enterococcaceae, and Lactobacillaceae, as well as decreased Lanchnospiraceae and Bacteroidaceae (25,28). Recent studies have also demonstrated that changes in the gut microbiota have direct and causal relationship with AD. For example, Sun et al. found that fecal microbiota transplantation (FMT) of a healthy mouse to AD mice shows significant improvements in AD pathology (29). A clinical trial of FMT has also been proposed to ameliorate AD pathology in human subjects (30). Moreover, in phase III, a double-blind, placebo-controlled trial with 818 mild to moderate AD patients, it was discovered in recent clinical research that altering the gut microbiome with GV-971, a sodium oligomannate, improved cognitive performance in as little as 4 weeks, possibly by reducing neuroinflammation. This may be because dementia or a cognitively impaired state can change food habits and behaviors (31). These studies imply that alterations in gut microbiome composition may be less relevant to AD pathology but reflect secondary phenomena in which microbial metabolites can contribute to AD pathology. Herein the next section describes the link between microbial metabolites with cognitive decline and AD.

Microbial Metabolites Linked With Cognitive Decline and AD Pathology: Products of Machinery

The modulation of the microbiota is associated with both the pathogenesis of AD and its treatment. Fecal microbiome transplantation (FMT) has received more attention in current research toward the connection between microbiota and AD pathology, although the specific reason for this interaction is still unknown. The microbial metabolites are functional units of the microbiota that exhibit the biological effects of the microbiota’s composition and metabolic activities on host health, according to a growing body of evidence (32,33). Their metabolites also have beneficial and detrimental effects on host health, just like good and bad bacteria abundances do. There is association between cognitive impairment in AD and several well-known microbial metabolites, including short-chain fatty acids (SCFAs), tryptophan, kynurenine, urolithin, and trimethylamine N-oxide (TMAO) and amyloid (34). The common microbiota metabolites affecting cognitive function and AD have been depicted in Table 1.

Table 1.

Effect of Common Gut Microbiota and Its Metabolites On Cognitive Decline and AD

Gut Microbiota	Metabolites	Effect on Cognitive Decline and AD	References
Lactobacillus, Bifidobacterium	Gamma-aminobutyric acid (GABA), serotonin (5-HT)	The predominant inhibitory neurotransmitter, regulates mood, emotions, and cognitive functions.	(35)
Streptococcus, Lactobacillus, Streptococcus, Bifidobacterium, Lactococcus, Enterococcus	Dopamine	Dopamine regulates cognitive processes like learning and memory, as well as mental and motor actions.	(36)
Lactobacillus, Bacillus, Lactococcus, Streptococcus	Acetylcholine	Acetylcholine maintains cognitive flexibility, social interaction, and emotional personality.	(36,37)
Lactobacillus, Bifidobacterium, Propionibacterium, Clostridium, Roseburia	Short-chain fatty acids (SCFA)	Reduce blood-brain barrier (BBB) permeability and increase neurotransmitter production and release.	(36,38)

Short-Chain Fatty Acids (SCFAs)

In the cecum and proximal colon, the gut microbiota ferments nondigestible food fibers such as dietary fiber and complex carbohydrates to generate primary metabolites known as SCFAs (ie, acetate, propionate, and butyrate), which have a significant effect on host health by directly affecting host cells (39). Although, the precise mechanisms of microbiome-derived SCFAs are not fully understood, they are primarily known to act on host cells through a few mechanisms: (i) by activating free fatty acid receptors 2 and 3 (FFAR2/3); (ii) by entering host cells through monocarboxyl transporter 1/2 (MCT1/2), where they are involved in mitochondrial metabolism or inhibit histone deacetylase (HDAC) activity (40). However, the precise mechanisms and impact of SCFAs in AD pathology still remain poorly understood.

Multiple emerging evidence has demonstrated a link between SCFAs and AD (28). Studies, have shown that the diversity of the gut microbiota decreases in AD animal models and patients, leading to changes in the amount and concentration of SCFAs (41,42). The levels of SCFAs and the associated SCFA-producing microbiota in AD reduce over time or age, which supported the findings of subsequent studies that SCFAs and AD were closely related (43). For example, when compared to 3- and 6-month-old 3xTg-AD mice, the concentrations of SCFAs were significantly lower in 11-month-old 3xTg-AD mice (44). Additionally, in APP/PS1 transgenic mice, both butyric acid and isobutyric acid concentrations in the brain and feces decreased. Moreover, the concentrations of butyric acid in brain tissue were found to be positively linked with those in the feces (45). Further, it was found that the changed gut flora in Drosophila models of AD caused acetate to be present at lower concentrations (46). Propionic, butyric, and isobutyric acid concentrations are often found to be lower in AD mouse models than in wild-type mice due to the reduced abundance of acetobacter and lactobacillus (47). However, we do not yet understand why the abundance of butyrate-producing bacteria in the gut of AD patients and older adults decreases, and further studies are needed to investigate this phenomenon.

Furthermore, individuals with MCI had reduced levels of SCFAs in their stools compared to healthy controls (45). Microbiome diversity was found to decrease in the AD mouse model, including a decline in the abundance of butyrate makers (45,48). Butyrate treatment reduces the progression of AD pathology in the brain and prevents cognitive decline in the AD mouse model (49). Clinical investigations have proposed that giving SCFA mixture or just boosting dietary fibers could alleviate AD pathology and the microbial hypometabolism linked to neuronal dysfunction in AD (50).

Although, the precise mechanism of SCFAs in AD is not known, emerging literature shows multiple and variable pieces of evidence. For example, Lee et al. showed that butyrate restored the radiation-induced decrease of p-CREB and BDNF expression (51). Govindarajan et al. observed that butyrate treatment increased the expression of memory-consolidation genes like MYST4, Marcksl1, GluR1, SNAP25, and SHANK3 in an AD mouse model (52). Butyrate also enhanced synaptic plasticity in 8-week-old 5xFAD mice by boosting synapse-associated proteins and encouraging depotentiation and long-term potentiation, probably by activating the PI3K/AKT/CREB/BDNF signaling cascade (53,54). Butyrate also promotes the expression of genes involved in proliferation, apoptosis, and neurogenesis, which can aid in brain regeneration (55).

The pathogenic effects of Aβ can be modified in various ways by SCFAs, most probably by reducing the formation of toxic soluble Aβ aggregates (49). Oral sodium butyrate treatment causes a dose-dependent decrease in Aβ levels in the brains of 5xFAD mice (49). However, the role of putative mechanisms like FFAR2/3 signaling, MCT1/2, mitochondrial metabolic fuel shunt, and HDAC inhibition in AD models is still understudied. Further studies are also required to establish the ideal SCFA ratio in the brain and peripheral that will support healthy neuronal activities as the brain ages. In addition, further insight is needed as, to how the microbiome of the gut can be manipulated to achieve this balance of SCFAs between the periphery and brain to protect against cognitive decline in older adults.

Bile Acids (BAs)

Bile acids (BAs) are the combination of host and microbiome-derived metabolites of the pool of cholesterol derivatives and are involved in several biological and metabolic functions (eg, cholesterol homeostasis, and lipid digestion) (56). Cholesterol, which is a key lipid in the makeup of the brain, is eliminated through the formation of BAs (57). The liver produces the 2 main BAs, cholic acid (CA) and chenodeoxycholic acid (CDCA), which are then released into the gallbladder, and then supplied into the intestine with bile juice to be broken down by gut bacteria (57). Numerous genes involved in cholesterol metabolisms, such as ABCA7, ABCG1, BIN1, and CLU are the top loci for AD vulnerability found in genome-wide association study (33). It is noteworthy that, the distinct BA profile observed during dysbiosis is associated with cognitive decline in AD patients (33). In which they found that blood levels of a primary BA, CA, were significantly lower in older persons with AD compared to cognitively healthy older adults (33). In contrast to the earlier, the levels of a secondary BA (produced by gut bacteria), deoxycholic acid (DCA), and its conjugated forms like glycine and taurine were significantly higher in cognitive declined older adults (33). These findings indicate a strong link between microbiota-induced changes in BA and its species, however, their causal relationship with AD is not known. Thus, these findings necessitate further research on gut dysbiosis and a potential function for the gut–liver–brain axis in the progression of AD.

Taurine

For secretion into bile, the primary BAs must be conjugated to taurine, and the secondary BAs must be conjugated to primary BAs (58). Interestingly, AD patients often have higher serum taurine levels than controls (58). With this intriguing finding, researchers have investigated the level of taurine transporters (LC6A6 and SLC36A1) in the brain and observed that both these transporters were more abundant in the brains of AD patients, particularly in the prefrontal cortex (58). LC6A6 and SLC36A1 act as taurine transporters and increase the supply of taurine across the BBB (59). According to previous reports, taurine supplementation in rats improve neuropsychiatric disorders by enhancing ionic homeostasis and mitochondrial activity (60). Furthermore, we have previously shown that a probiotic cocktail of human origin reduces increased taurine levels which in turn suppresses the inflammation and leaky gut caused in older mice by modifying the microbiota and tight junctions (61). Although, observations indicate opposite effects, one explanation is that increased levels of taurine and its transporter in AD may serve as a protective mechanism. These data also indicate that taurine can impact both gut and brain function by acting peripherally and centrally.

Amino Acids (AAs)

Tryptophan is an essential amino acid (AA) available in dairy products and fruit that is absorbed through the gut epithelial membrane and acts as a precursor for kynurenine and serotonin (62). Kynurenine acts as a precursor for a neuro-active glutaminergic compound (62). Some derivatives of kynurenine can cross the BBB (63). The plasma concentrations of several kynurenines have previously been demonstrated to be reduced in AD patients compared with controls, suggesting that dysregulation of central and peripheral kynurenine may have significant functional effects (64). Additionally, tryptophan levels in AD patients were often lower than those in older controls (65). Higher kynurenine/tryptophan in older controls was a direct reflection of patients’ increased tryptophan degradation (65). Kynurenine/tryptophan levels and the levels of soluble immunological markers such as neopterin, IL-2 receptor, and TNF were found to be correlated in AD patients (65). These findings indicate that the degradation of tryptophan contributes to AD pathogenesis.

Moreover, AAs homeostasis dysregulation is linked to play a key role in the etiology of AD (66). Changes in glutamate metabolism in the AD brain can alter γ-aminobutyric acid (GABA) availability and affect neuronal function (67). Indoxyl sulfate is a tryptophan-derived metabolite and is produced in large quantities and varies greatly between people (68). In addition, changes in the amino-acid metabolic pathways such as methionine, tryptophan, and purine have been found in older adults with cognitive decline and AD (69). Thus, neuronal function, and AD pathogenesis may be affected by these AA-derived metabolites, however, we still lack a full understanding of how they contribute to AD pathology, and their significance in preventing, delaying, and treating cognitive decline and AD.

Urolithin A (UA)

Urolithin A (UA) is produced by gut bacteria belonging to the genus Gordonibacter (G. pamelaeae and G. urolithinfaciens) and Ellagibacterisourolithinifaciens from ellagic acid (70). UA has shown an autophagy-inducing activity with potential neuroprotection (70). Long-term intermittent administration of UA prevents cognitive deficits in 3xTg-AD mice, and reduces the Aβ plaque accumulation in the hippocampus (70). Although, the precise mechanisms are still not fully known, it has been demonstrated that UA can stimulate mitophagy, which could help to reduce the amount of Aβ plaque (71). Additional investigation is required to confirm the mechanism of action of UA as well as its utility in ameliorating AD pathology.

Trimethylamine-N-Oxide (TMAO)

Gut-derived TMAO is generated by bacteria of the genera Anaerococcus, Clostridium, Escherichia, and Proteus (72). Trimethylamines (TMA), are metabolites created by gut microbial metabolism of dietary choline, lecithin, and carnitine (73). TMAO is produced in the liver from eggs, dairy products, and fish (73). AD pathogenesis has shown higher concentrations of TMAO (74). Additionally, a combined computational analysis found that TMAO was strongly related to clinical features of AD, such as disease vulnerability and cognitive decline, thus signifying the beginning of AD (75). Furthermore, a finding demonstrated age-related increased circulatory TMAO levels in both wild-type and APP/PS1 mice aggravated AD-like phenotype (76). Moreover, the reduction of TMAO, attenuated cognitive deficits by reducing long-term potentiation in APP/PS1 mice. Additionally, it is known that TMAO promotes Aβ accumulation and degenerative processes in AD by enhancing β-secretase activity (77). Although, the link between TMAO and AD pathology is becoming increasingly clear, more study is required to comprehend the pathways through which TMAO contributes in the pathogenesis of AD.

Vitamins

Vitamins are necessary micronutrients that people get through their diet; however, gut microbiota also synthesize them and supply them to the host (78). Escherichia coli, Klebsiella pneumoniae, and Propionibacterium produce vitamin K, as do E. coli and Bacillus subtilis for vitamin B2 (riboflavin), Bifidobacterium, Lactococcus lactis, and Streptococcus thermophilus for vitamin B9 (folic acid), and Lactobacillusreuteri and Propionibacteriumfreudenreichii for vitamin B12 (cobalamin) (78). In the brain, vitamin K takes part in the synthesis of sphingolipids, which has the potential to ameliorate oxidative stress and inflammation-induced psychomotor behavior and cognition deficits (79). Moreover, research has shown low levels of Vitamin K in the cerebrospinal fluid (CSF) of AD patients (80). Therefore, gut-derived vitamins are one of the implications observed in AD and need more attention for better correlation.

Overall, the earlier findings indicated that, gut microbiota and its different types of metabolites play an important role in the development and prevention of cognitive decline and AD, but how remains largely unknown. Multiple evidence indicates that systemic- and neuro-inflammation exacerbating cognitive decline and AD progression; however, the precise source of inflammation remains elusive. In the following section, we discussed the impact of gut microbiota on gut permeability (leaky gut), which is linked with AD pathology.

Leaky Gut Is an Understudied Source of Systemic Inflammation and Is Linked With AD

Strong data suggest that, the microbiota and its metabolites are linked with AD, however, the precise mechanisms as to how abnormalities in the gut affect brain function remain unknown. One of the emerging, but preliminary, mechanisms is increased gut permeability (“leaky gut”) which triggers an inflammatory response in the circulatory system and brain resulting in the exacerbation of AD pathology (81). The gut permeability is largely controlled by 2 types of barriers (i) a thick and sticky mucus layer, which forms a physical filter and separation system between microbes and host cells; and (ii) a network of tight proteins that holds epithelial cells tightly to maintain intact epithelial wall integrity (82). However, the breakdown or dampening of these barriers allows the nonspecific leakage of antigens and/or proinflammatory ingredients from the intestinal lumen into the blood, triggering local, and systemic inflammation (83). Abnormalities in the microbiota and its metabolites can dampen both mucus and tight junction barriers (made of zonulin, occludin, and other junction proteins), although there is evidence that beneficial microbiota metabolites also promote these barriers functions thus reducing leaky gut, and inflammation thereby ameliorating AD pathology (84).

The key sign of gut dysbiosis is an increase in the Firmicutes/Bacteroidetes ratio, which may contribute to the development of intestine amyloid precursor protein (APP) in the early stage of AD (85). A rise in Aβ levels in the CNS and decreased cognitive performance were linked in APP mice with altered gut microbiota composition (86). Additionally, AD rats have aberrant Aβ- and tau-protein accumulation in neurons and activated intestinal innate immunity preceding CNS neuroinflammation (87). Similar to this, in Tg2576 animals, intestinal dysbiosis permits Aβ deposition in the gut prior to cerebral Aβ depositions begin (87).

SCFA production by gut bacteria, or lack thereof, is a potential contributor. The SCFA production capacity of microbiota is reduced during gut dysbiosis related to AD pathology (32). SCFAs show effectiveness in preventing protein–protein interactions, that are required for Aβ assemblies (88). Further, the leaky gut microbiota induces immune activation through gut-metabolite-dependent pathways. For instance, endotoxemia and systemic inflammation are produced when LPS, a component of the gram-negative bacteria’s cell walls, leaks from the gut (89). Several preclinical investigations suggested that administering LPS increased gut permeability, which resulted in a leaky gut (90). Proinflammatory cytokines and LPS-containing blood later enter the brain via BBB disruption and allow other neurotoxins to cause neuroinflammation and brain damage (91). It has been demonstrated that, intraperitoneal injection of LPS in mice causes an increase in hippocampal Aβ levels and cognitive impairment (92). Li et al. observed a decline in cognitive performance in younger rats after transplanting the fecal microbiota of aged rats into adult rats (93). This suggested that cognitive impairment is caused by age-related changes in the gut flora (93). However, successful treatments for aging-associated microbiome dysbiosis to encourage mucus production and reduce leaky gut are unknown. This relationship has also been proven clinically, suggesting that FMT of older to adults increased gut permeability due to increased LPS production (93). In vitro investigation have shown that bacterial LPS promotes amyloid fibrillogenesis (94). Through the systemic circulation, inflammation can spread from the periphery to the brain. Increased levels of circulating BAs produced by bacteria may worsen BBB permeability by rupturing tight junctions, allowing BAs or peripheral cholesterol to enter the brain (95). Cellular cholesterol boosts APP inclusion (as it directly binds to APP) into the phospholipid monolayers, where Aβ is generated, thus, increasing Aβ synthesis in the brain (95). On the contrary, treatment with SCFAs significantly reversed the gut permeability. This provides direct evidence of the role of the microbiota in gut integrity. It is not surprising that abnormalities in the gut lead to harmful consequences in the brain. The most common consequence of this dysregulation is inflammation, which can propagate from peripheral to central. For example, in inflammatory bowel disease (IBD) pronounced levels of leaky gut are associated with higher cognitive decline (96). It is noteworthy, that physiological levels of SCFAs have been demonstrated to strengthen the intestinal barrier (97). In preclinical studies, butyrate has been shown to upregulate tight junction proteins (TJPs) like occludin and claudin, and to decrease bacterial translocation (98).

Therefore, both leaky gut and systemic inflammation are linked to age-related cognition declines, according to research by Wang et al. (99). In older mice, it has been shown that gut permeability increases with inflammation (61). In a study by Zhan et al., mice were first given a 14-day course of broad-spectrum antibiotics. The fecal bacteria were then transplanted into pseudo-germ-free mice from senescence-accelerated mouse resilient 1 (SAMR1) mice, and it was discovered that SAMR1 mice showed improvements in cognitive performance in the pseudo-germ-free mice that received the SAMR1 mice’s fecal bacteria transplants (100). In addition, Dodiya et al. showed that FMT from age-matched AD male mice was given to AD male mice that had received antibiotic treatment. intestinal microorganisms have been reinstated while Aβ pathology and microglial morphology were only partially restored (101). Recent clinical research demonstrates that leaky gut, a condition where microbial and food antigens can enter the circulation from the gut lumen, increases low-grade inflammation and that these situations are prevalent in older persons (102). Therefore, reducing inflammation and leaky gut may help slow cognitive decline in the aging brain. Additionally, a study has shown that the removal of C. difficile and fecal microbiota transplantation simultaneously improved gastrointestinal, cognitive, and mental issues in AD patients (103). In contrast to earlier findings, it is implicated that IBD has a more pronounced leaky gut, associated with higher AD (96). Moreover, serum levels of acute phase proteins (APPs) linked with accelerated cognitive impairment such as soluble CD14, lipopolysaccharide binding protein (LBP), and C-reactive protein (CRP) were elevated (104), Thus, the earlier evidence implicates a direct involvement of gut microbiota in the regulation on cognitive performance and AD pathology.

Thus, a leaky gut plays a significant role in the propagation of inflammation from the periphery to the brain through BBB in AD. This allows for the interaction of proinflammatory cytokines and endotoxin with BBB. These cytokines and endotoxins can change the structure of BBB by affecting its endothelial layer or pericytes, allowing the proinflammatory mediators into the brain. Therefore, in the following section, we have elaborated the findings that demonstrate whether and how microbiota-metabolites affect BBB, AD pathology, and cognitive decline.

Gut Microbiota-Derived Metabolites Affecting BBB and AD

The BBB serves as a dynamic interface between peripheral blood circulation and the brain that controls brain homeostasis and safeguards the CNS, and can react to various physiological and pathological conditions (105). Brain endothelial cells serve as the initial barrier in the BBB structure between blood vessels and brain parenchyma (106). TJs, which are tightly enclosed by pericytes, encompassed by the basal lamina, and continuous with astrocyte end feet and endothelial cells, connect endothelial cells (106) (see Figure 1). During aging, the blood vessel elasticity reduces, and the blood vessel wall thickens both of which contribute to the disruption of BBB and ultimately worsen neurological conditions in older adults (107). The BBB dysfunction (increased permeability) results in microglial activation and neuroinflammation due to an increased influx of blood’s proinflammatory ingredients that leak into the brain, exacerbating neurodegeneration, and cognitive impairment. The breakdown of the BBB promotes the accumulation of insoluble neurofibrillary tangles (NFT) of p-tau and Aβ (108). In addition to a breakdown of TJPs of BBB, the number and density of an important cell type, pericytes is reduced (109), further suggesting BBB cell damage contributes to AD pathology.

Figure 1.

(A) The diagram illustrates the neurovascular unit that constitutes the blood-brain barrier (BBB). It includes pericytes and astrocytes, which play crucial roles in the structure and function of the BBB. (B) The diagram depicts the main pathways through which chemicals and substances enter the central nervous system. (a) It highlights how tight connections between endothelial cells prevent the penetration of polar medicines and water-soluble chemicals. (b) The endothelium contains transporters or carriers for amino acids, nucleosides, and other chemicals. (c) Lipid-soluble drugs including nanoparticles can diffuse through endothelial membranes, providing a pathway for their traversal. (d) Plasma proteins, including albumin, have weak transportation; however, cationization can enhance their absorption through processes like endocytosis and transcytosis.

Recent research has revealed that the effects of microbiota and its metabolites are not limited to the gastrointestinal (GI) tract but also have an affect outside of the GI tract (110). There is emerging evidence showing that the microbiota can alter the functionality of the BBB in AD pathology such as antibiotic-induced gut microbiome alteration causing compromised BBB permeability in rhesus monkeys (111). However, it is still not entirely clear how gut bacteria affect BBB permeability. The differential release of substances by various bacterial strains can have a detrimental or positive effect on BBB (112). For instance, LPS is a cell wall ingredient of pathogenic gram-negative microbes with strong proinflammatory effects that can contribute to BBB disruption (112); while commensal or beneficial microbes promote the production of metabolites such as SCFAs, which protects or improve BBB (113). Propionate, an SCFA, protects BBB by inhibiting oxidative stress in a nuclear factor erythroid-derived 2-like 2-dependent mechanism (114). Similarly, after oral administration of butyrate or colonization with the butyrate-producing bacterium Clostridium tyrobutyricum, reduces BBB permeability in GF-mice by up-regulating TJPs (50). Further, the Bifidobacterium infantis can raise plasma levels of tryptophan (115) a building block for diverse microbial metabolites including kynurenic acid, which can cross BBB and shows neuroprotective effect in pathological condition (116) (see Table 2). Any alteration of the BBB can lead to inflammation in the neuronal tissue of the BBB, which comprises endothelial and glial cells including astrocytes and microglia. Therefore, it is important to explore the cell-specific effect of microbiota and its metabolites. Here in this section, we highlighted the research on the effects of selective gut microbiota and its metabolites on specific brain cells.

Table 2.

Effects of Gut Microbiota on Blood-Brain Barrier Permeability

Permeability Modulator (microbiota)	Method and Model	Effect on BBB	References
Clostridiumtyrobutyricum, Bacteroides thetaiotaomicron	Germ-free C57Bl/6 mice	Increase tight junction protein and reduces permeability	(117)
Clostridium butyricum	C57Bl/6 mice	Increase tight junction protein and reduces permeability
Chenodeoxycholic acid and deoxycholic acid	Sprague–Dawley rats	Decrease tight junction protein and increases permeability	(95)
Acetate/propionate/butyrate	Germ-free BDF1 mice	Microglia activation

Microbial Metabolites Affecting Brain Cell Types Related to Cognitive Function and AD

The breakdown of the BBB is well known to let proinflammatory mediators into the brain, but at the same time, these mediators also stimulate nearby glial cells and neurons. Even in terms of normal physiology, they protect the brain, as the brain’s macrophages, astrocytes, and microglia remove harmful substances. However, in pathological conditions, particularly in AD, astrocytes, and microglia are more intensely activated, which directly affects neurons and causes neuroinflammation (36,118) (see Figure 2). The relationship between the microbiota and BBB has previously been highlighted in this section, our attention has now shifted to how the microbiota can control neuronal functioning.

Figure 2.

The consequences of AD on the gut–brain axis can have several effects. The gut and the brain are connected through a complex and effective communication system. Here are some potential consequences of AD on the gut–brain axis: (1) intestinal epithelial dysfunction: in a healthy state, the intestinal epithelium maintains its integrity through tight junctions between cells. However, during gut dysbiosis (an imbalance in the gut microbiota), these tight junctions can become compromised, leading to increased intestinal permeability or “leaky gut.” The decrease in gut microbiota diversity during gut dysbiosis allows proinflammatory bacteria or their toxic byproducts to proliferate, triggering immune system activation and resulting in inflammation. (2) Systemic inflammation: with a leaky gut, bacteria, and their byproducts can enter the body, triggering the release of cytokines into the systemic circulation. These cytokines promote inflammation throughout the body, including the brain. (3) Impact on amyloid-beta (Aβ) and tau: certain bacteria can produce amyloids that may be taken up or leaked from the gut into circulation and reach the brain. The systemic inflammation and cytokine release caused by the leaky gut also promote the production of Aβ and tau in the brain. (4) BBB permeability: systemic inflammatory cytokines can disrupt the tight junctions in the blood-brain barriers (BBBs), leading to the leakage of Aβ originating from the gut into the brain, or vice versa. These events trigger neuroinflammatory responses in astrocytes and microglia, contributing to neurodegeneration and the worsening of AD pathology. (5) Mitochondrial dysfunction: mitochondrial dysfunction can occur at various levels, including intestinal epithelial cells, leading to reduced tight junctions, immune cells producing inflammatory cytokines, BBB cells developing leakiness, astrocytes, and microglia causing neuroinflammation, and neurons experiencing dysfunction or degeneration. Overall, this exacerbates the progression of AD (36).

Microbiota-Metabolites Affecting Microglia

Gut microbial metabolites affecting the BBB can also show their effect on the different cell types of the CNS, like microglia, astrocytes, and neurons (118). For example, gut microbiota changes the growth and activity of the microglia cells in brains (110), and dysregulated microglial activity has been linked with increased AD pathology (119). Given the crucial role of microglia in AD and Aβ pathology, the next focused on microglia as the likely mediators of the microbiota metabolites that affect AD or Aβ pathology (120). It is reported that, the SCFAs regulate microglial homeostasis and obstruct Aβ protein aggregation (121). SCFA supplementation restored the morphological phenotype of microglia in GF mice (122). Another study showed that microglia from GF APP/S1 mice treated with SCFA caused an increase in microglial recruitment to Aβ plaques and exhibited a more active microglial phenotype compared to control GF APP/S1 mice (110,120). Further, the study investigated that, tauroursodeoxycholic acid (TUDCA), a BA, attenuates APP expression and Aβ accumulation in APP/PS1 mice (123). In vivo and in vitro reports have demonstrated that microglial cells express the G protein-coupled bile acid receptor 1 and Takeda G protein-coupled receptor 5 (GPBAR1/TGR5). Microglia’s intracellular cAMP levels increased because of TUDCA binding to GPBAR1/TGR5, inducing anti-inflammatory signals while suppressing proinflammatory ones (124,125). Additionally, metabolomic profiling was used to evaluate the blood levels of selective BA metabolites from the AD neuroimaging initiative (126). They investigated the relationship between BAs and the Aβ and tau levels and suggested BAs may act as one of the neurodegeneration biomarkers for AD (126). Furthermore, the triple-transgenic (3xTg) AD model used was to establish the causal relationship between the kynurenine pathway and microglia in context to the Aβ and tau pathologies in AD (127). Here, tryptophan 2,3 dioxygenase (TDO) expression in animal models and AD patient brains has been measured (127). The excitotoxin quinolinic acid and TDO production were found to be significantly higher in AD patients (127). This is due to the fact that human microglia turn the neurotoxic l-tryptophan into quinolinic acid (128). These findings provide prove that the kynurenine pathway may be crucial to AD.

Even during gut dysbiosis, there is an increased levels of TMAO, which can lead to microglial activation and oxidative stress (129). The likely mechanism, however, is the release of IL-1β from activated microglia, which decreased the expression of tight junction proteins reduced the synthesis of sonic hormone from astrocytes, and consequently enhanced BBB permeability (130). Additionally, the release of proinflammatory cytokines from microglia promoted astrocytic activation, which then produced the C–C Motif Chemokine Ligand 2 (CCL2) and C–X-C motif chemokines ligand 2 (CXCL2; proinflammatory cytokines) that promote cell mobilization and aggravate BBB breakdown following neuroinflammation (130) (see Table 3).

Table 3.

Gut Microbiota–Metabolites and Their Effect on Microglia, Astrocytes, and Neurons

Metabolites	Cell Type	Effect	References
SCFAs BA TMAO AA	Microglia	Enhances recruitment of microglia against Aβ and tau protein. Promotes G protein-coupled bile acid receptor 1 and Takeda G protein-coupled receptor 5 on microglia. Promotes release of IL-1β from activated microglia, which decreased the expression of TJ proteins. Enhances tryptophan 2,3 dioxygenase	(120) (123) (129) (127)
SCFAs TMAO	Astrocytes	Anti-inflammatory effect. Promotes release of inflammatory mediators and apoptosis.	(44) (131)
SCFAs BA TMAO	Neurons	Increased AMPK dependent mitophagy and clearance of Aβ and tau protein. Stabilizes the mitochondrial membrane and inhibiting the release of cytochrome c and prevents apoptosis. Increased oxidative stress, mitochondrial dysfunction, and inhibition of mTOR signaling in the brain.	(88) (95) (131)

SCFAs: Short chain fatty acids, BA: Bile acid, TMAO: Trimethylamine N-oxide, AA: Amino acid, Aβ: Amyloid beta, TJ: Tight junction, AMPK: AMP-activated protein kinase, mTOR: Mammalian target of rapamycin.

Microbiota-Metabolites Affecting Astrocytes

Astrocytes play an important function in the maintenance of synaptic plasticity, nutrients, or metabolite supply to the brain (132). Numerous pieces of evidence imply that changes in gut microbiota cause astrocytes to become activated in AD (133). Recently, it was suggested that gut microbiota influences astrocyte quantity and function in the brain and is involved in the pathways of microbiota/gut–brain axis in AD (134). Diet controls the quantity and characteristics of astrocytes in the brain via gut microbiota, which is the physiologically and therapeutically significant evidence for a gut–brain axis (134). In addition, it found that mice fed with high-fat diet had an excess of Bacteroidetes and Firmicutes in their guts along with reduced astrocyte density in the hypothalamus (135).

Recent studies have shown that reactive astrocytes build up in the aged brain that can take a harmful state in response to immune-related stimuli (131). Equally significant is the finding that some gut microbiota chemicals, such as SCFAs, are crucial for astrocytic activation. Consuming bioactive food that has a healthy microbiota inhibits LPS’s ability to activate astrocytic and microglial cells of 3xTg-AD mice, which lowers Aβ aggregation and tau hyperphosphorylation (44). Moreover, it has been shown that TMAO crosses the BBB and can produce a detrimental effect on astrocytes and neurons (136). In this context, increased TMAO level has been found in the cerebral fluid of AD and other neurological illness patients (136). These results are important because they imply that plasma TMAO could be used as a substitute for brain TMAO, allowing for noninvasive research on these pathways in humans (131). Additionally, they have demonstrated that TMAO, the enzyme required to convert TMA to TMAO, increases in the brain with age in tandem with increases in plasma flavin monooxygenase 3 (FMO3) concentrations (131).

Together, these findings show that food influences the amount and characteristics of astrocytes in the brain via gut microbiota, which is physiologically and therapeutically significant evidence for a gut–brain axis. However, the precise mechanism is not yet known. Research examining the mechanistic relationship between microbiota–metabolites and astrocytes may therefore provide further proof of the gut–brain axis in AD (see Table 3).

Microbiota-Metabolites Affecting on Neurons

In AD mice, abnormal Aβ accumulation in myenteric neurons is observed prior to the start of CNS neuroinflammation (87). Similar to humans, Tg2576 animals experience gut dysbiosis and Aβ deposits in the gut before Aβ deposits start to appear in the brain (87). Through gut-metabolite-dependent pathways, the leaky gut microbiota causes immunological activation, and it is demonstrated that injection of LPS increased Aβ levels in hippocampus neurons and caused cognitive deficits in rodent models (92). Another potential mechanism is connected to the gut microbiota’s secretion of certain bacterial metabolites (88). Research has demonstrated that increased bacterially produced BAs increased gut and BBB permeability by disrupting TJPs and allowing peripheral cholesterol to enter the brain, causing Aβ synthesis and neuroinflammation in AD (95).

The mechanism by which these metabolites contribute to the pathophysiology of AD is not well understood. However, existing evidence suggested that a trigger point could be a mechanism relying on mitochondria (34). Neuro-inflammation encourages mitochondrial-dependent neuronal autophagy, which is associated with lower levels of p-tau in the brain and better cognitive function in tau-transgenic AD rats (137). In contrast to the earlier findings, supplementation with SCFAs inhibits the protein–protein interaction and prevents Aβ assemblies in the neurons and mitigates cognitive deficits in AD mice (88). Therefore, the gut microbiome has the potential to modulate neuronal function and cognitive performance. To further explain the mechanism by which the gut microbiota and its metabolite contribute to the pathophysiology of AD, we have focused on the interaction of the gut–microbiome–mitochondria–brain axis in the following section. Although studies may specifically demonstrate that the microbiota modulates the microglia, astrocytes, or other neuronal cells, it would be interesting to discuss changes in the microglia, astrocytes, and neurons in the brain at the same time.

Microbiota–Metabolite–Mitochondrial Interactions: A Mechanistic Link

Before discussing how microbial metabolites affect mitochondria and AD pathology, we will first discuss the common mitochondrial pathways and their connection with AD pathology (138). The tricarboxylic acid (TCA) cycle and the electron transport chain (ETC) are the most important metabolic pathways in mitochondria, which oxidize acetyl-CoA and provide electron carriers to generate cell energy as ATP (138). During the ETC, electrons, such as nicotinamide adenine dinucleotide (NAD)/NADH and flavin adenine dinucleotide (FAD)/FADH, are involved in passing through the ETC complexes, resulting in the production of ATP (139).

Abnormalities in the TCA cycle, either due to insufficient supply of acetyl-CoA or downregulation of enzymes participating in TCA, cause an increase in the production of ROS, which is common in the AD brain (140). Furthermore, the AMP-activated protein kinase (AMPK) plays a crucial role in mitochondrial functions and biogenesis. The activation of AMPK promotes mitochondrial biogenesis, contributing to normal neuronal and other cellular functions, while the downregulation of AMPK has been implicated with AD (141).

Swerdlow et al. proposed the mitochondrial cascade hypothesis, describing the role of mitochondria in the development of AD (142). According to this hypothesis, the basic rate of ROS production determines the rate at which acquired mitochondrial damage accumulates, and these are controlled by an individual’s inherited, genetically specified ETC. Increased ROS production in response to damaged proteins, lipids, RNA, and mitochondrial DNA (mtDNA) instigates the generation of Aβ and β-sheet proteins, promotes senescence, and triggers responses to form neurofibrillary tangles (143). However, how the microbiome and its metabolites interact with mitochondrial TCA/ETC, AMPKs, and the mitochondrial cascade remains poorly understood.

Microbiome-derived SCFAs such as acetate and butyrate, being members of the fatty acid family, are the main substrates for de novo lipid metabolism, converting SCFAs into acetyl-CoA (144). This is one of the ways through which SCFAs directly contribute to the increase in the availability of acetyl co-A (145). Furthermore, secondary bile acids also contribute to host energy production, reducing ROS by potentiating mitochondrial function in both the peripheral and central nervous systems (146). Interventions on microbiota have been shown to recover mitochondrial-dependent hippocampal apoptosis pathways in animal models of AD (147). SCFAs are known to directly activate the master sensor of energy stress, AMPK, by increasing the AMP/ATP ratio (148). Microbiota and probiotics also affect mitochondrial function via the AMPK-signaling and reduced tau-phosphorylation in AD by modulating this mechanism (149). On the contrary, certain microbiome metabolites like TMAO and LPS exhibit increased brain aging and cognitive impairment, most likely brought on by reduced mitochondrial functions and increased ROS in the brain (150). Therefore, microbiota–metabolites, including good and bad metabolites, directly modulate mitochondrial function in aging and AD (see Figure 3).

Figure 3.

The triple alliance of microbiome, metabolite, and mitochondria plays a significant role in AD pathology: (1) microbiome-metabolite impact on mitochondria: microbiome metabolites can affect mitochondria in both beneficial and detrimental ways. For instance, certain microbial metabolites like TMAO and LPS may cause abnormalities in the mitochondria, impair electron transport chain (ETC) function, and increase the accumulation of reactive oxygen species (ROS). On the other hand, metabolites like short-chain fatty acids (SCFAs) and bile acids (BA) can prevent ROS formation and improve ETC function. During aging, hereditary changes occur in ETC genes, leading to ROS generation. (2) Abnormalities in ETC genes and mtDNA alterations: mitochondrial dysfunction can originate from the downregulation of ETC-related genes due to mtDNA alterations. (3) Mitochondrial dysfunction: Underactivity of mitochondrial bioenergetics leads to mitochondrial dysfunction. (4) Development of AD pathology: mitochondrial dysfunction contributes to the development of AD by facilitating the formation of Aβ, removing programmed cell death of weakened cells (senescence), and promoting the formation of neurofibrillary tangles (142).

Many abnormalities in the CNS are linked to the peripheral system (151). Inflammation can propagate from the periphery to the CNS via different modes, including systemic circulation, infiltration process, and immune activation (152). Evidence suggests that the peripheral system’s mitochondria can also communicate with CNS in AD pathology (153). Mitochondria play a crucial role in the maturation and differentiation of neuronal tissue including neurons, astrocyte, and glial cells (154). Various reports implicate mitochondrial dysfunction in AD. However, the actual cause of mitochondrial dysfunction is still under study. Intriguingly, it has been demonstrated that the deficits in mitochondrial efficiency can be mediated by the abnormal supply of mitochondrial substrates such as acetyl-co-A (155). Although microbiota plays a key role in the regulation of mitochondrial function, there is limited gathered information on the cross-talk of microbiota-mitochondria in the pathophysiology of AD. We have elaborated on the role of microbiota in the regulation of BBB integrity followed by their effect on neuronal tissues such as astrocytes, microglia, and neuronal cells during AD.

There is growing evidence that mitochondria play a critical role in managing BBB permeability and the related neural function (156). By increasing BBB permeability or by harming the BBB, peripheral system mitochondrial dysfunction can exacerbate AD pathogenesis (157). Although, the exact mechanism is still being investigated, it is speculated that, dysfunctional mitochondria produce nitrous oxide (NO), which opens the BBB, allowing extracellular material to enter, and transmitting inflammation to the microglia (157). The activation of microglia is significantly regulated by mitochondria. In the mouse models, microglial cells are stimulated to generate NO, an inflammatory mediator, by extracellular CytC, which is typically released by dying cells (158).

Over 50% of microglia had decreased mitophagy that was linked to an increase in the proportion of damaged mitochondria in the animal models of AD (71). Because mitochondrial damage-associated molecular patterns (DAMPs), which originate from cellular damage, are released into the cytoplasm when mitophagy is defective, this might cause inflammation (159). Additionally, it has been noted that Aβ hindered mitochondrial mitophagy and morphology and that improvements in mitochondrial mitophagy increased microglia’s capacity to phagocytose cells and lowered proinflammatory cytokines like IL-1β in AD mouse model (108). Intriguingly, changes in mitochondrial morphology have also been observed in the postmortem human AD hippocampus, a sign of accumulated mitochondrial damage (160). Therefore, restoration of mitochondrial mitophagy can be one of therapeutic strategies for AD. Recently, it was revealed that TREM2 contributes to the maintenance of the metabolic performance of microglia in the 5xFAD mice (160). TREM2−/− microglia from 5xFAD animals had reduced mitochondria and a lower ATP level (160). Cyclo-creatine, an analog of creatine that may supply ATP, was given to TREM2−/− animals to increase the availability of ATP. This enhanced microglial metabolism, microglial responsiveness to Aβ, and controlled mitophagy (160). Mitophagy and clearance of Aβ depend on mitochondrial health. It has been proposed that the mitochondrial biogenesis system controls Aβ and tau protein clearance through a variety of molecular signaling pathways (161). Initially, AMPK downregulates the expression of APP-cleaving enzyme protein, which reduces the formation of Aβ and promotes its clearance through mitophagy or autophagy and prevents the development of AD (162).

Additionally, the SCFA such as butyrate plays a critical role in controlling mitochondrial function in different cell types (163). The improvement of mitochondrial function, partially through an increase in oxidative phosphorylation, activates the melatonergic pathway (164). Such effects appear to be caused by an augmentation in the activity of the pyruvate dehydrogenase complex, which regulates the TCA cycle, and oxidative phosphorylation (165).

Mitochondria also play a central role in the function of astrocytes and neurons. To protect neuronal cells, it is believed that astrocytes discard excess produced glutamate by converting it to glutamine using glutamine synthetase and the mitochondria’s TCA cycle (166). This is important because the glycolytic enzymes of mitochondria mostly co-localized with GLT-1 (166), which is derived from L-tryptophan by gut microbiota such as Lactobacillus and Clostridium XI (167).

Furthermore, it is feasible to selectively damage the glial cells’ OXPHOS system using glia-specific mitochondrial gliotoxin, which also causes metabolic stress and inhibits neuronal synaptic transmission (166). As a result, the astrocyte’s unique metabolism meets the energetic needs of its astrocytic functions, indicating that the regulation of neuronal activity has a spatial and functional relationship with astrocytic metabolism.

Astrocytes in the AD brain have reportedly been subjected to oxidative stress, which causes DNA damage and functional impairment (168). Additionally, AD exposure to astrocytes can result in mitochondrial fragmentation and depolarization, which increases the production of ROS and impairs metabolism (168). Inferring the vulnerability of astrocyte mitochondria in AD, there is a decline in astrocyte mitochondrial function during AD. Because astrocyte mitochondria play a crucial role in determining how well they operate, it is important to note that SCFAs generated from the gut microbiome, including butyrate, may have considerable effects on astrocytes through the regulation of their mitochondria (169).

Microbiota-derived metabolites also have negative effects on the brain, and a recent study found that TMAO is detectable in cerebrospinal fluid (CSF), indicating that it enters the CNS and may thus be important for normal neurological function or problems (136). In fact, mice given dietary TMAO exhibit increased brain aging and cognitive impairment, which is most likely brought on by increased oxidative stress, mitochondrial dysfunction, and suppression of mTOR signaling in the brain (150). Overall, mitochondrial involvement led to attenuation of neurodegeneration and associated abnormalities, such as tightening of BBB, maturation of microglia, and clearance of Aβ and tau. Thus, both microbiota and mitochondria share interlinked mechanisms associated with AD.

Therapeutic Approach: Mitochondrial Targeting in AD

In recent studies, antioxidants like vitamins C and E have been demonstrated to reduce Aβ pathology and improve cognitive deficits in AD transgenic mice (170). Antioxidants that target mitochondria, such as MitoQ, MitoVitE, MitoPBN, glutathione choline esters, and N-acetyl-L-cysteine, have made substantial advances in the previous decade (171). Furthermore, supplementation of prebiotics modulates the gut–microbiome–brain axis and hence is reported to improve cognitive function by modulating the AMPK pathway in rodents (172). Therefore, activating AMPK may be a viable target for improving the disrupted brain energy metabolism associated with the development of AD (172). AMPK is reported to mitigate amyloid genesis by modulating neuronal cholesterol and sphingomyelin levels in the lipid rafts (173). Studies have reported that, activation of AMPK facilitates autophagy and promotes lysosomal degradation of Aβ and clearance of tau protein (173). This finding is consonant with the report that improvement in mitochondrial function potentiates mitochondrial-dependent proteasomal autophagy and the clearance of Aβ and tau protein in AD. Therefore, the gut microbiome has the potential to modulate cognitive function via mitochondrial-dependent mechanism in a preclinical and clinical setup. Thus, mitochondrial-targeted antioxidants could be interesting candidates for treating the aged and Alzheimer’s patients. Furthermore, mitochondrial-dysfunction-targeting medication that can directly modulate AMPK could be a possible powerful intervention treatment in AD. As a result, AMPK could be a new target for treating future risk factors in AD.

Summary and Future Direction

In this review, we elaborated on the role of microbiota metabolites in the regulation of age-related cognitive decline and AD. The information suggests that a leaky gut leads to systemic inflammation, which propagates upward to the brain through the BBB. Microbiota bidirectionally regulates BBB integrity and associated inflammation in neuronal tissues, including astrocytes and microglia. Existing reports demonstrate that microbiota metabolites, such as SCFAs, mitigate inflammation from the systemic to the brain, ultimately ameliorating AD pathology. The most common mechanism involved is mitochondrial AMPK-dependent mitophagy and clearance of Aβ and tau. The probable mechanism involves microbiota metabolites acting as a direct source of mitochondria. Furthermore, harmful microbiota metabolites like TMAO aggravate AD pathology by reducing mitochondrial function. Despite the diversity in AD, a common theme emerges: a synergy between mitochondria and microbiota early in life is disrupted at some point and later in life transformed into its opposite. Therefore, a microbiota-derived pre, post, or synbiotic that improves mitochondrial function could be a potential therapeutic approach for AD. Another approach could involve the colonization of beneficial microbiota species in the CNS or specific brain regions, reducing ROS and strengthening mitochondria. However, more research is required to elaborate on the gut–microbiome–mitochondria–brain axis as a possible mechanism involved in the pathophysiology of AD.

Funding

There was no specific grant was provided for this review article from public, private, or nonprofit funding organizations; however, Dr. Yadav’s lab would like to acknowledge and thanks for funding support from National Institutes of Health, National of Institute of Aging (R56AG069676, R56AG064075, RF1AG071762, R21AG072379, U01AG076928, R21DE032197), the Department of Defense (W81XWH-18-PRARP AZ180098), and the Ed and Ethel Moore Alzheimer’s Disease Research Program of the Florida Department of Health (22A17). Additional resources were provided by the University of South Florida (USF) Center for Microbiome Research, Institute of Microbiomes, Center for Excellence in Aging and Brain Repair, Department of Neurosurgery and Brain Repair, USF Morsani College of Medicine.

Conflict of Interest

Dr. Yadav is Co-Founder and Chief Scientific Officer of the Postbiotics Inc, but his role has not conflict with work presented in this manuscript. Other authors have no conflict of interest to report.