Gut microbiota and its metabolites in Alzheimer’s disease: from pathogenesis to treatment

Gut bacterial microbiota dysbiosis and AD

Increasing evidence suggests a correlation between gut microbiota dysbiosis and AD. The gut microbiota composition of AD patients differs from that of healthy people, specifically regarding the microbial diversity and proportions of specific bacterial species (Hung et al., 2022; Wasén et al., 2022). A clinical study reported that AD patients had higher abundances of Deferribacteres, Bacteroidetes, and Phascolarctobacterium than the healthy population. However, the AD patients had lower abundances of Clostridiaceae (1.53% vs. 3.89%, P = 0.002), Lachnospiraceae (14.26% vs. 19.29%, P = 0.006), and Ruminococcaceae (8.08% vs. 14.25%, P = 0.019) (Liu et al., 2019). Various studies reported that the interaction between microbiota and the host immune system becomes imbalanced due to aging and disease. This imbalance increases proinflammatory bacteria and decreases anti-inflammatory bacteria (Pellegrini et al., 2023). This pattern is similar to opportunistic species in other neurologic disorders and various inflammatory bowel diseases (Cammann et al., 2023), supporting the hypothesis of immune inflammation in AD. Vogt et al. (2017) reported decreased microbial diversity in the gut microbiota of AD patients. Certain major bacterial phyla, such as Firmicutes and Clostridium, were the most affected. AD featured a decreasing trend in Actinobacteria and an increasing trend in Bacteroidetes, while Ruminococcus abundance was downregulated and Escherichia/Shigella abundance was upregulated (Cattaneo et al., 2017). Gut microbiota pathogenicity increases with life progression and the decline of various host functions, resulting in differential expression of microbial characteristics among different AD patients (de Cena et al., 2021; Martino et al., 2022) (Table 1). Furthermore, the microbial composition of AD patients exhibits regional heterogeneity. The highest proportion was found in a human gut bacterial sequence (ADAS) related to AD in Chinese and United States populations (Paley, 2019). However, there was no definitive evidence to establish a causative connection. Fecal microbiota transplantation (FMT) experiments in rodent models suggested a potential causal relationship between AD and gut microbiota balance and dysbiosis (Wang et al., 2021a; Nandwana & Debbarma, 2021). Gut microbiota variations trigger AD onset, while AD pathological changes disrupt the ecosystem by altering the gut environment. Therefore, AD and the gut environment interact closely and are mutually influential (Nguyen, Cho & Lee, 2023; Liu et al., 2020; Leblhuber et al., 2021; Simão et al., 2023).

The underlying pathological mechanisms of AD

Although overall changes in the gut microbiota of AD patients have been suggested, the specific pathogenic agents should be identified using further investigations. Experimental studies suggested that the aggregation of misfolded proteins, such as amyloid-beta and tau, and increased bacterial amyloid, such as curli fibers, cause neurodegenerative changes, significantly contributing to AD development (Chen et al., 2016; Pisa et al., 2015; Pluta et al., 2020). Experimental studies indicated that the human metabolites ‘canine uracil’ and myopeptide elevated DNJ-12 and DNJ-19J protein levels in an ‘AD rodent model’. These increased protein levels triggered the cytoplasmic unfolded protein response to eliminate A β42 aggregates and their associated toxicity. However, the ‘rodent model’ demonstrated that the genus Prevotella directly enhanced protein stability (Joshi et al., 2021; Walker et al., 2021). Furthermore, it was suggested that gut microbiota changes in AD mice are accompanied by corresponding changes to the V3 and V4 regions of the 16S ribosomal RNA genes (Shen, Liu & Ji, 2017).

Numerous human studies have demonstrated that bacteria, such as Helicobacter pylori, Borrelia burgdorferi, and Chlamydia pneumoniae, increase AD susceptibility by promoting tau protein hyperphosphorylation and elevating proinflammatory bacteria (Escherichia/Shigella) and decreasing anti-inflammatory gut microbiota (Ruminococcus) (Murray, Kemp & Nguyen, 2022; Cryan et al., 2019). Several experiments on AD mice reported increased abundance of Escherichia coli-Shigella and Desulfovibrio and an according increase in the abundance of Enterobacteriaceae, Pseudomonas, Clostridium, Prevotella, and Clostridium cluster IV RNA, contributing to amyloid-like protein deposition and microglial cell accumulation in the brain, triggering inflammatory responses and participating in AD pathogenesis (Chen et al., 2020; Borsom et al., 2023). Additionally, Xi et al. (2021) found an increased abundance of Pseudomonas and Faecalibacterium and a correlation between the higher abundance of the family Lachnospiraceae (Fusicatenibacter, Blautia, and Dorea) and more severe AD symptoms through non-targeted analysis of gut microbiota in AD patients’ fecal samples. Furthermore, a study using metabolic methods reported increased Candida and Proteobacteria in an AD mouse model. The results suggested a connection between gut microbiota imbalance and AD occurrence and progression (Favero et al., 2022).

Fujii et al. (2019) reported that transplanting fecal samples from AD patients into germ-free mice and subsequently administering this strain orally impaired the cognitive abilities of the mice. However, these specific pathogenic factors might interact with other contributing factors within the complex gut microbiota ecosystem rather than being limited to singular infectious elements. Therefore, examining the interplay and potential mechanisms of various microorganisms, including bacteria and fungi, and their respective roles in AD from different perspectives is crucial. The interactions between microbial populations might exhibit diverse complexities. Current research suggests that certain common bacteria and fungi influence key microbial communities within the body by forming symbiotic relationships or producing metabolites. A deeper understanding of the higher-order interactions that produce novel effectors in the microbial–host interaction might enable a more comprehensive understanding of the mechanisms underlying AD (Hu, Wang & Jin, 2016).

Gut non-bacterial microbiota dysbiosis and AD

AD pathogenesis extends beyond gut bacterial dysbiosis and involves non-bacterial microbiota. Bacteriophages contain randomly uniquely encoded peptides/proteins that can influence gut microbiota genotypic and phenotypic diversity, affecting microbial community functionality and contributing to AD development (Shkoporov, Turkington & Hill, 2022; Attai et al., 2022; Zhang et al., 2022b). Additionally, gut fungal ecosystem disruption was reported in AD patients, and the fungal–bacterial network function appeared to be associated with AD pathology (Castillo-Álvarez & Marzo-Sola, 2022). Previous studies have demonstrated the involvement of C17 fungal isolates (belonging to the genus Candida) in AD pathophysiology (Cruz-Miranda et al., 2020). The interactions between bacteria and non-bacterial microbiota in the gut are highly complex. Thus, studying only the bacterial characteristics without considering the role of other non-bacterial microbiota would distort the fungal–bacterial network function and limit research direction development, hindering understanding of the mechanisms underlying AD.

Interaction between gut microbiota and Western medicine treatment for AD

Currently, drug intervention remains the main approach for treating AD. Medications significantly affect the gut microbiota composition and function, which should not be overlooked. Drugs directly or indirectly influence different gut functions, thus directly or indirectly regulating bacterial metabolism and the ability to modulate bacterial activity and efficacy. The drugs used to treat AD, such as acetylcholinesterase inhibitors and NMDA receptor antagonists (memantine), affect the gut microbiota composition and microbial diversity. It has been suggested that memantine promotes the capture and bactericidal activity of neutrophils induced by pathogenic E. coli in the host gut (Peng et al., 2020; Xiao et al., 2022). Most studies indicated that AD drugs mainly promote gut microbiota and immune-inflammatory functions of derived metabolites, regulating inflammatory responses, stimulating the vagus nerve, and binding to various receptors through the brain-gut axis, thus improving cognitive function.

Interaction between gut microbiota and traditional Chinese medicine treatment for AD

Traditional Chinese medicine is increasingly applied in clinical practice. Traditional Chinese medicine increases gut microbiota species diversity and community diversity and regulates the gut microbiota structure (Table 2). Schisandra chinensis improved gut microbiota dysbiosis in AD rats, promoted SCFA production, regulated the inflammatory metabolic pathways, promoted the growth of beneficial bacteria, and decreased the relative abundance of pathogenic bacteria (Zhao et al., 2016; Wang et al., 2021b; Guo et al., 2020a; Fu, 2023). Furthermore, the relative abundance of OTU345, OTU380 (Lachnospiraceae), and OTU264 (Bacteroidales_S24-7_group) increased significantly in AD patients treated with a compound herbal medicine based on Xiaoyao powder, while the abundance of functional pathways, such as tryptophan metabolism, arginine and proline metabolism, and niacin metabolism, decreased (Weijie, Xingfei & He, 2021). Lachnospiraceae has a strong ability to produce anti-inflammatory butyrate; therefore, its increased abundance plays an important synergistic role in AD treatment (Minter et al., 2017). Additionally, the water extract of Gastrodia elata exerted a good reverse recovery effect on gut microbiota dysbiosis caused by D-galactose and aluminum chloride and increased the abundance of beneficial bacteria, such as Lactobacillus johnsonii, L. murinus, and L. reuteri (Wenbin, 2019). These studies demonstrated that traditional Chinese medicine modulates gut microbiota function and composition, benefiting AD treatment.

Interaction between gut microbiota and external therapies of traditional Chinese medicine (acupuncture) for AD

Traditional Chinese medicine offers diverse disease treatments. In addition to herbal interventions, numerous external therapies can improve AD clinical manifestations. Research has indicated that acupuncture can balance gut microbiota quantity and composition to suppress peripheral and central nervous system inflammation reactions (Jiang et al., 2021; He et al., 2021). Experimental studies have demonstrated that electroacupuncture increased the abundance of delta-Proteobacteria and epsilon-Proteobacteria in the intestine and improved learning and memory abilities in AD mice (Jiang et al., 2021).

New evidence from animal (Jiang et al., 2021; Yu et al., 2020) and human (Wang et al., 2020) studies supports acupuncture as a candidate method for treating AD. A randomized controlled clinical trial demonstrated that patients in the acupuncture group exhibited increased gut microbiota diversity compared to the non-acupuncture group, and their AD symptoms were improved through brain-gut axis regulation (Kong et al., 2023; Bao et al., 2023). Increased gut microbial diversity and beneficial bacteria and their derived metabolites stimulate the vagus nerve in the gut and enhance the immune response, thus improving brain function. Despite clinical support and ongoing research, the available data and studies in this area and sample sizes are limited. The increased gut microbiota diversity and abundance due to acupuncture have been confirmed, but the specific microbial abundances or the implications of their increase have not been clearly elucidated. Therefore, increased research on the effects of acupuncture and other traditional Chinese medicine external therapies on the gut microbiota is necessary. Thus, external therapies of traditional Chinese medicine, such as acupuncture, are potential alternative approaches for treating AD (Table 3).

Dietary interventions

Diet directly influences gut microbiota aggregation and genomic composition, which is a critical factor (Liu et al., 2020). Extensive research has summarized the connections between dietary patterns, gut microbiota, and AD, identifying them as effective candidates for preventing and impeding AD mechanisms and progression. The Mediterranean diet (MD) is characterized by high consumption of fruits, vegetables, legumes, and grains, as well as anti-inflammatory properties, and has long been considered a healthy dietary pattern (Liu et al., 2020; Long-Smith et al., 2020). A systematic review reported that adherence to the MD can reduce the risk of developing AD and help maintain beneficial gut bacteria, demonstrating its preventive role in AD (Solch et al., 2022). A recent meta-analysis of prospective data focusing on older adults reported significant improvements in AD symptoms, specifically in overall cognition and episodic memory, under MD interventions (Loughrey et al., 2017). The MD is associated with gut microbiota diversity and gut inflammation. A positive correlation was indicated between the MD and increased numbers of beneficial microbial species, such as certain Bacteroides species and their anti-inflammatory SCFA metabolites. This alleviation of gut inflammation contributed to reduced neuroinflammation and improved neurocognitive function (McGrattan et al., 2019). Taylor et al. (2017) observed the relationship between carbohydrate intake and AD, demonstrating a positive correlation between carbohydrate intake and increased cerebral amyloid-beta protein load in 26% of participants. Multiple studies suggested dietary recommendations for individuals with AD, including increasing folate, vitamin B6, and vitamin B12 intake while reducing saturated fat, excess dietary fat, and SFAs (Stefaniak et al., 2022; Zhu et al., 2023). Thus, dietary therapy can be a potential adjunctive treatment for AD. However, significant individual variation means that personalized diets should be tailored to maximize therapeutic efficacy.

FMT

FMT is a clinically valuable approach that involves transferring gut microbiota from a healthy donor to restore a dysbiotic gut microbiota in patients, aiming to normalize its composition (Wang et al., 2019a; Grabrucker et al., 2023). Various clinical trials have reported that FMT improved clinical manifestations in AD patients, including cognitive impairments. AD-like phenotypes, such as Iba-1 activation, amyloid-beta protein deposition, and spatial memory decline, were observed in 8-month-old endogenous melatonin reduced (EMR) mice, while FMT increased gut permeability, suppressed systemic inflammation, and alleviated AD-related phenotypes in the mice (Zhang et al., 2022a). Altered gut microbiota abundance is at the core of FMT. FMT increased the Bacteroides species and restored Bacteroidetes abundance in AD patients (Bäuerl et al., 2018; He et al., 2018). Additionally, a human case study involving an AD patient receiving FMT from a healthy 47-year-old woman reported significant improvements in cognitive function as measured by mini-mental state examination (MMSE) and clinical dementia assessment scores (Park et al., 2021). Despite demonstrating efficacy and safety in mice, challenges remain in the clinical application of FMT, including acceptance between recipients and donors, individual immune response mechanism variations, and the influence of lifestyle factors, such as diet, on gut microbiota composition. Future research should address these issues (Table 4).

Probiotics

Probiotics are supplements of live microorganisms that can promote a balanced gut microbiota in the host, resulting in positive effects (Fuller, 1989; Faulin & Estadella, 2023). Several probiotic strains exerted positive therapeutic effects in AD, with a focus on the genera Bifidobacterium and Lactobacillus. In mouse models, specific strains, such as Bifidobacterium longum A1 and L. helveticus Shirota, effectively counteracted AD (Kobayashi et al., 2017). Furthermore, a randomized controlled trial (RCT) demonstrated that increasing intake of L. acidophilus, L. helveticus, Bifidobacterium, and fermented milk products altered gut microbiota diversity and significantly improved MMSE scores (Leblhuber et al., 2018). Probiotics suppress inflammatory responses. Experimental studies demonstrated that probiotics are important in regulating the immune responses of Th1, Th2, Th17, Treg, natural killer (NK), and B cells (Dargahi et al., 2019). Additionally, some studies demonstrated that probiotics reduced mucosal inflammation by modulating cytokine networks and macrophage tissue and regulating local immune responses (Sichetti et al., 2018).

The oxidative stress response is an important mechanism by which probiotics participate in AD pathological and physiological reactions. Recent research demonstrated that the SLAB51 formulation significantly reduced oxidative stress levels in transgenic AD mice by inducing SIRT-1-dependent mechanisms. In that experiment, researchers used a SIRT-related gene to modulate the response of AD mice to SLAB51 and SLAB50 (Bonfili et al., 2018). Furthermore, integrating L. acidophilus, L. fermentum, L. plantarum, and Bacillus subtilis successfully improved the oxidative stress performance in rats injected with Aβ1-42 (Athari Nik Azm et al., 2018). However, other studies indicated that probiotic intake did not significantly improve AD symptoms. An RCT reported that the intake of two different probiotic capsule formulations did not significantly affect cognitive function or related inflammatory factors in AD patients (Leblhuber et al., 2019). Thus, while some evidence suggests the potential benefits of probiotic intake in improving cognitive function, much data is produced in laboratory settings, and confirmation is needed to establish their clinical usefulness and robustness.

Prebiotics

Prebiotics are sources of nutrition for gut microbiota that can promote beneficial bacterial activity, exerting positive effects on host organs (Ansari et al., 2020). Clinical research supports prebiotic intervention in AD. Fructooligosaccharides (FOS) and mannanoligosaccharides (MOS) are the most common prebiotics used to treat AD, along with compounds from yacon, inulin, and compounds derived from vegetables, herbs, and plants. Additionally, the recently discovered xylooligosaccharides (XOS), which are similar to oligosaccharides, have a wide range of sources and are the most abundant biopolymers in the plant kingdom. XOS exerted significant effects in altering gut microbiota composition and improving AD symptoms (Peterson, 2020; Liu et al., 2021; Arora, Green & Prakash, 2020; Ávila et al., 2020).

Multiple experimental studies indicated that prebiotics achieve their therapeutic effects in AD through various mechanisms. For example, Liu et al. (2021) reported that an 8-week intervention with MOS in 5xFAD AD mice improved cognitive function and spatial memory and reduced anxiety and compulsive behavior. In-depth analysis of the corresponding mechanisms revealed that MOS prevented intestinal barrier integrity damage and LPS leakage, rebuilt the gut microbiota composition, and significantly reduced inflammation by enhancing SCFA formation, achieving the intervention goals (Liu et al., 2021). Furthermore, oral administration of oligosaccharides derived from Marinda officinalis reduced Aβ accumulation in the brains of AD mice, reconstructed oxidative-reductive homeostasis, and reduced overall inflammation, improving memory and learning capabilities (Xu et al., 2020; Chen et al., 2017). Thus, the diverse prebiotics and the mechanisms they use render them a promising therapeutic approach worthy of further development and research.

Synbiotics

Synbiotics combine prebiotics and probiotics, providing substrates for beneficial bacterial proliferation. The synergistic effect of both components contributed to the development of synbiotics. A synbiotic supplement consisting of triptolide-rich plant prebiotic Trifolium pratense leaf extract (TFLA) and probiotics demonstrated stronger survival capacity in transgenic humanized Drosophila melanogaster with BACE1-APP-induced AD phenotypes compared to TFLA or probiotic intervention alone (Westfall, Lomis & Prakash, 2019). Another synbiotic formulation composed of Acetobacter, Delbrueckii, L. fermentum, Leuconostoc, Bifidobacterium, Sporolactobacillus, L. helveticus, Kluyveromyces, and Kluyveromyces marxianus in 4% kefir grain synthesis improved memory, visual-spatial, executive, and linguistic abilities in test subjects and decreased the expression of proinflammatory cytokines (IL-8, IL-12, and TNF-α) while increasing the overall anti/proinflammatory cytokine ratio (Arora, Green & Prakash, 2020; Ton et al., 2020). In summary, synbiotics are a powerful biological tool for treating AD, and the proper selection of probiotic strains and prebiotics lays a solid foundation for formulating synbiotic combinations that confer physiological benefits to the host.

Unresolved Issues

Although current research and the literature demonstrate the intimate connection between AD and gut microbiota ecosystem stability, many unresolved questions remain in the literature:

(1) How does microbial diversity change at different AD development stages?

(2) How do the brain-gut mechanisms and intestinal mucosal immune responses participate in AD physiological and pathological processes?

(3) Which external factors have the greatest effect on AD progression?

Additionally, intestinal mucosal resistance to external microbiota is individual-specific, so the development of microbiota-targeted therapies should consider the differences between recipients and donors, drug administration timing, and the short- and long-term risks after administration in the complex intestinal environment. Traditional Chinese medicine interventions have yielded promising results in AD diagnosis and treatment, but whether traditional Chinese medicine interventions and microbiota-based therapies differ requires further investigation. The relationship between the microbiota and the host is intricate; therefore, future developments should focus on the relationship. The field continues to face significant challenges, and it is hoped that more research will be conducted and new technologies developed to address these challenges and provide clinicians with more standardized, safe, and effective treatment options.