Gut microbiota metabolites: potential therapeutic targets for Alzheimer’s disease?

Sphingolipids are ubiquitously present in eukaryotes such as plants and animals. With advancements in chemical and lipid technologies, it has been discovered that a minority of bacteria possess enzymatic capabilities to synthesize sphingolipids (Bai et al., 2023). Bacterial phyla known to produce sphingolipids include Bacteroidetes, α-Proteobacteria, and δ-Proteobacteria (Brown et al., 2023). Within the gut microbiota, Bacteroidetes is currently the only known symbiotic bacteria capable of synthesizing sphingolipids. These bacteria use serine-palmitoyl transferase to catalyze the de novo synthesis of sphingolipids (Le et al., 2022; Heaver et al., 2018) and play a crucial role in inositol sphingolipid synthesis (Heaver et al., 2022). Especially in the absence of dietary intake, gut bacteria serve as a source of endogenous sphingolipids. Johnson’s research supported this notion, showing that in mice on a sphingolipid-deficient diet, intestinal epithelial cells absorb long-chain sphingoid bases (Sa(d18:0) and Sa(d17:0)) from the lumen and metabolize them via de novo synthesis pathways (Johnson et al., 2020), as described in Figure 3. Bacterial and dietary sphingolipids interact to maintain stable sphingolipid levels in the host (Rohrhofer et al., 2021), further highlighting the potential role of bacterial sphingolipids in the gut-brain axis.

Sphingolipids, which include ceramide, sphingosine-1-phosphate (S1P), and sphingosine, are bioactive substances commonly found in cell membranes. In particular, they are abundant in central nervous system tissues (van Echten-Deckert and Walter, 2012). Studies have shown that sphingolipids play a central role in cell signaling and regulation of homeostasis, and are crucial in processes like mitochondrial apoptosis, autophagy, immune regulation, and oxidative stress (Brown et al., 2023). These functions position sphingolipids as key players in the pathogenesis of various diseases, with AD drawing significant attention. In patients with AD, cerebrospinal fluid and blood samples have shown that blood sphingolipid concentrations are associated with brain atrophy and cognitive decline (Varma et al., 2018). A longitudinal cohort study also found increased expression of sphingolipid S1P 18:1 (Chua et al., 2020), which may induce the upregulation of astrocytes and various inflammatory mediators, suggesting that sphingolipids may be early biomarkers for AD, as shown in Figure 3. Moreover, experimental results from animal models further support this conclusion (Crivelli et al., 2021). As summarized by Luo (Luo et al., 2024), ceramide contributes to the stability of BACE1 and promotes excessive production of Aβ (Parveen et al., 2019). Additionally, neutral sphingomyelinase inhibitors can reduce the release of ceramide vesicles, inhibit Aβ aggregation, and improve cognitive function (Dinkins et al., 2017). Interestingly, acidic sphingomyelinase ceramide inhibitors can also reduce the release of ceramide and pro-inflammatory factors from microglia (Crivelli et al., 2023). Additionally, in Tg2576 AD model mice, decreasing the accumulation of ganglioside GM3 has been shown to lower soluble Aβ42 levels and amyloid plaque burden, thereby stabilizing remote memory (Dodge et al., 2022). Sulfatide deficiency can activate microglia, increase the expression of AD risk genes such as Apoe, Trem2, Cd33, and Mmp12, and exacerbate cognitive impairment in mouse models. These findings are consistent with recent research indicating that impaired sphingolipid metabolism is associated with the development of AD (Qiu et al., 2021). However, given that sphingolipids are considered early biomarkers of AD, further investigation is required to elucidate the relationship and underlying mechanisms between sphingolipids and AD. This research may offer potential strategies to delay the onset and progression of the disease.

Zhong and colleagues discovered that the activation of S1P transport protein is related to microglial inflammation and NF-κB pathway activation (Zhong et al., 2019). Additionally, the binding of S1P to S1PR1 mediates the secretion of pro-inflammatory cytokines and chemokines, increasing central nervous system inflammation (O’Sullivan et al., 2018). Kong’s research further found that ceramide promotes the binding of tubulin to voltage-dependent anion channel 1, further blocking mitochondrial energy metabolism and increasing the risk of AD (Kong et al., 2018). Additionally, the metabolic regulation of ceramides is closely linked to autophagy, mTORC1 activity, and Aβ secretion, further exacerbating the progression of AD pathology (Ordóñez-Gutiérrez et al., 2018). However, most research has focused on exogenous dietary sphingolipids, while the importance of endogenous sphingolipids for gut health has been largely overlooked. Interestingly, Brown and colleagues discovered that a deficiency in endogenous sphingolipids derived from Bacteroides can lead to intestinal mucosal damage, intestinal inflammation, and alterations in the host ceramide pool (Brown EM. et al., 2019). Multiple reports have indicated that Bacteroides sphingolipids influence the gut-brain axis through immune regulation, lipid metabolism, and other pathways. For example, inositol derived from Bacteroidetes may affect the secretion of host antimicrobial peptides, thereby participating in host immune defense (Heaver et al., 2022). Additionally, gut bacteria, as a source of endogenous sphingolipids, can increase the host’s available sphingolipid pool, thereby influencing host sphingolipid metabolism (Johnson et al., 2020). Further studies reveal that Bacteroides-derived sphingolipids are associated with MCP-1 secretion and macrophage release (Brown EM. et al., 2019). Moreover, Bacteroides fragilis can alter the homeostasis of host iNKT cells, thereby inhibiting the proliferation of iNKT cells in the colon and protecting the integrity of intestinal cells and mucosa (An et al., 2014). These findings reveal the multiple mechanisms by which gut microbiota-derived sphingolipids function within the gut-brain axis, suggesting that endogenous sphingolipids may hold potential value in the prevention and treatment of neurodegenerative diseases (Le et al., 2022; Wei et al., 2009).

Interestingly, the study found that hawthorn flavonoids increase levels of sphingolipids and phosphatidylcholine, which can reverse gut microbiota dysbiosis, inhibit Aβ accumulation, and suppress abnormal microglial activation in the hippocampus (Zhang J. et al., 2022). In addition, glycerophospho dihydroceramide derived from Porphyromonas gingivalis has been found to promote Aβ formation and Tau hyperphosphorylation in vitro. This process is also accompanied by an increase in senescence-associated secretory phenotype factors, and downregulates Sirt-1 expression. These may be potential mechanisms for the development of AD (Yamada et al., 2020). Targeted metabolomics studies have further confirmed that disruptions in sphingolipid metabolism are closely associated with the biological changes observed in the preclinical and prodromal stages of AD, particularly in relation to Tau phosphorylation, Aβ metabolism, and apoptosis (Varma et al., 2018). These data suggest that bacterial sphingolipids can regulate host sphingolipid synthesis and immune homeostasis, indirectly influencing the onset of AD, as illustrated in Figure 3. However, current research on gut microbiota-derived sphingolipids in AD patients is not yet comprehensive. It is anticipated that this field will soon emerge as a novel target for AD treatment.

As the role of bacterial sphingolipids in the pathogenesis of AD is gradually being revealed, phospholipids, another critical class of lipids, are also recognized for their significant role in the gut-brain axis and neurodegenerative disorders. Phospholipids, as major components of cell membranes, maintain membrane stability and fluidity, and provide an optimal environment for protein interactions and transport (Bohdanowicz and Grinstein, 2013). The brain, being the organ with the highest lipid content, exhibits significant alterations in lipid composition in AD (Svennerholm, 1968). High-performance liquid chromatography analysis revealed a reduction of approximately 20% in total phospholipids in the frontal cortex and hippocampus of AD patients, when normalized to DNA content (Guan et al., 1999). This disruption in phospholipid levels may be closely related to the pathogenesis of AD.

As early as 2001, Folch extracts from autopsy materials revealed significantly reduced levels of phosphatidylethanolamine (PE) and phosphatidylinositol in the brains of AD patients (Pettegrew et al., 2001). Changes in these phospholipids may influence the progression of AD through the gut-brain axis. Phospholipids synthesized by gut microbiota, particularly PE, could exacerbate this pathological process by disrupting the host’s phospholipid metabolism (Brown et al., 2023). For instance, researchers have screened and identified an immunologically active molecule produced by Akkermansia muciniphila: a diacyl phosphatidylethanolamine (a15:0-i15:0 PE) with two branched chains. This molecule can activate the immune system, induce the release of certain inflammatory factors, compromise the intestinal mucosal barrier, and trigger various immune diseases, including AD (Li Y. et al., 2022).

Despite this, the pathways through which phospholipids interact between the host and symbionts remain largely unknown. On the other hand, studies indicated that hypoxic conditions lead to increased intestinal angiogenin-4 secretion, resulting in a significant increase in Desulfovibrio abundance in mice intestines. Desulfovibrio species produce PE and phosphatidylcholine (Li Y. et al., 2022). These two phospholipids are major components of brain phospholipids (Blusztajn and Slack, 2023). The excessive breakdown of phosphatidylcholine and PE is considered one of the most prominent metabolic defects in AD. Their reduced concentrations are crucial factors in mitochondrial dysfunction, Aβ accumulation, and memory deficits (Zappelli et al., 2022). Additionally, recent studies have revealed the significant role of α-glycerophosphoethanolamine in protecting aging hippocampal neurons. It protects astrocytes by increasing glucose uptake, reducing total and oligomeric α-synuclein, and decreasing total tau accumulation (Daniele et al., 2020). Multi-omics analyses reveal that disruptions in glycerophospholipid metabolism lead to alterations in gut microbiota, potentially serving as a primary mechanism for heightened microglial activation and neuroinflammatory responses in APP/PS1 mice. This pathway is critically implicated in the pathogenesis and progression of AD (Qian et al., 2023). Reports indicate that phosphatidylcholine rich in DHA and EPA improves memory and cognitive function in APP/PS1 mice by inhibiting Aβ production, reducing oxidative stress, and decreasing apoptosis (Che et al., 2018). In summary, disruptions in phospholipid metabolism, particularly involving PE and phosphatidylcholine, may contribute to the onset and progression of AD through various mechanisms. Future research should explore these metabolic pathways to develop more effective AD treatment strategies.

Building on the relationship between phospholipid metabolism and AD, microbiota-derived LPS are also key factors influencing the progression of AD. LPS is currently the only glycolipid metabolite produced by the microbiome identified (Pahil et al., 2024). LPS is a potent endotoxin primarily found in Gram-negative bacteria, such as Bacteroides fragilis, Escherichia coli, and Shigella flexneri (Günther et al., 2020; Khalid et al., 2019). Zhao et al., (2017) first reported the presence of bacterial LPS in the hippocampus and temporal lobe cortex of AD patients, with levels in the hippocampus of late-stage AD patients being 26 times higher than in age-matched controls. This finding provides important insights into the relationship between LPS and the pathogenesis of AD. Similar results were observed in AD model mice further support this perspective. Interestingly, intermittent feeding with high doses of LPS can also cause learning and memory impairments and hippocampal neuronal damage in mice (Engler-Chiurazzi et al., 2023). These results indicated that the high expression of LPS in the blood and brain induced by gut infections may be a high-risk factor for the onset and progression of AD.

Further studies have shown that LPS affects various AD-related pathological products, including Aβ homeostasis, tau pathology, neuroinflammation, and neurodegeneration (Molinaro et al., 2020; Kim et al., 2021). Mice administered LPS exhibit a decline in memory performance, which may be related to increased expression of glial fibrillary acidic protein and p-Tau protein in the hippocampal CA3 region, along with elevated levels of pro-inflammatory factors and oxidative responses (Azzam et al., 2023). More importantly, studies on PgLPS transgenic mice have further elucidated the relationship between LPS and the AD phenotype. The study found that PgLPS transgenic mice induce AD phenotypes, including increased expression of pro-inflammatory factors, Tau hyperphosphorylation (Jiang M. et al., 2021), enhanced microglia-mediated neuroinflammation, and intracellular Aβ accumulation in neurons (Wu et al., 2017). These results reveal the multifaceted role of LPS in exacerbating AD pathology.

Furthermore, LPS-induced innate immune responses and neuroinflammation are crucial in AD pathogenesis. Luo’s research found that LPS is associated with microglial activation and neuroinflammation (Luo et al., 2024). LPS regulates the inflammatory response of microglia by modulating the expression of Toll-like receptor 4 and triggering receptor expressed on myeloid cells 2, thereby affecting Aβ clearance (Zhu et al., 2022; Zhou et al., 2019). At the same time, a non-targeted metabolomics study found that LPS can regulate energy metabolism in the hippocampus of APP/PS1 mice, promote pro-inflammatory activation of microglia (Agostini et al., 2020). Importantly, TLR4 can induce “Aβ/LPS tolerance,” increasing M1 polarization of microglia, which leads to reduced Aβ clearance and the expression of AD risk genes (Yang et al., 2021). Additionally, building on Luo’s research, it has been found that LPS can activate the chemotactic response of astrocytes, leading to increased inflammatory infiltration by neutrophils and T cells (Lopez-Rodriguez et al., 2021; Hennessy et al., 2015). In addition to inducing microglial and astrocyte activation, which increases AD risk, LPS can cross the gut barrier into the bloodstream, disrupt the BBB, and increase the release of pro-inflammatory factors (Brown and Heneka, 2024). This process suggests that LPS may drive the progression of AD by promoting inflammation and dysregulation of the immune system.

Notably, LPS can also directly induce apoptosis and necrosis in neural cells and neurons (Seim et al., 2019). Research indicated that colonization by Aeromonas and elevated LPS levels cause hippocampal neuronal apoptosis and spatial memory impairments in mice (Wang X. et al., 2023). Further studies revealed that exposure to LPS significantly increased the protein and mRNA expression levels of G protein-coupled receptor 17 (GPR17) in the hippocampus. GPR17 may contribute to neuronal damage and cognitive impairments by regulating the NF-κB p65/CREB/BDNF(cAMP response element-binding protein/brain-derived neurotrophic factor) signaling pathway, thus promoting the expression of oxidative stress and inflammatory factors (Liang et al., 2023). Additionally, the impact of LPS on myelination should not be overlooked.The overall level of myelination in the cortex and hippocampus of APP/PS1 mice, as well as postmortem AD tissue, is reduced. LPS can also affect the remyelination of axons (Regen et al., 2011; Dean et al., 2017), and age-dependent structural defects in myelin directly and indirectly promote the formation of Aβ plaques (Depp et al., 2023). As discussed by Luo and colleagues, LPS may become a new target for future AD research (Luo et al., 2024).

As another class of lipids produced by gut microbiota metabolism, SCFAs have a significant impact on the central nervous system, playing an essential role in maintaining the integrity of the BBB and regulating neuroinflammation (Hoyles et al., 2018; Liu J. et al., 2020). SCFAs, such as acetate, propionate, butyrate, and valerate, are lipids produced by the gut microbiome, such as Bacteroides, Firmicutes, and Akkermansia muciniphila, through the fermentation of dietary fiber (Lynch and Pedersen, 2016; Dalile et al., 2019). These SCFAs serve not only as an energy source for intestinal epithelial cells but also influence systemic organs, including the liver and brain, through the bloodstream (Qian et al., 2022; Zhou Y. et al., 2023). Oldendorf and coworkers found in 1973 that acetate, propionate, and butyrate, which are SCFAs, are able to pass through the BBB in rats after being injected with labeled SCFAs (Oldendorf, 1973). This finding further reveals the potential role of SCFAs in the pathogenesis of AD. SCFAs are indispensable for establishing a normal BBB and maintaining its integrity. Therefore, it is crucial to further explore the role of SCFAs in the BBB integrity.

Research has found that germ-free mice exhibit increased BBB permeability; however, gut microbes producing SCFAs reduce this permeability by upregulating the expression of tight junction proteins, indicating the role of SCFAs in maintaining cerebral homeostasis (Hoyles et al., 2018; Xie et al., 2023a). Further research found that propionate (at physiological concentration of 1 μM) inhibits pathways related to nonspecific microbial infections through a CD14-dependent mechanism, reduces LRP-1 expression, and protects the BBB against oxidative stress via the Nuclear factor erythroid 2-related factor 2 signaling pathway (Hoyles et al., 2018). The reduction in LRP1 levels can significantly inhibit the uptake and spread of Tau protein in neurons, helping to slow AD progression (Rauch et al., 2020). Conversely, broad-spectrum antibiotic treatment and germ-free mice both exhibit significantly increased barrier permeability and disrupted tight junctions between endothelial cells. Increasing butyrate intake can reverse this barrier disruption (Wen et al., 2020). Evidence suggests that histone acetylation is a key epigenetic mechanism in AD development (Yang et al., 2017). SCFAs may maintain BBB integrity through pathways mediated by GPR and the inhibition of histone deacetylases (Fock and Parnova, 2023; Matt et al., 2018).

Current research suggest that the primary pathogenic mechanisms of AD are Aβ deposition and abnormal phosphorylation of Tau protein, leading to neuroinflammation and neurodegenerative changes. In the feces of patients with mild cognitive impairment, propionate and butyrate are negatively correlated with Aβ42 (Nagpal et al., 2019). Clinical studies have shown that Aβ levels in AD patients are positively correlated with serum acetate and pentanoate levels, and negatively correlated with butyrate levels (Marizzoni et al., 2020). This suggests that SCFAs play a critical role in neuroprotection and Aβ metabolism, potentially providing protective effects by interfering with the assembly and conversion mechanisms of Aβ peptides (Ho et al., 2018). Subsequent studies have found that sodium butyrate can upregulate the PI3K/AKT/CREB/BDNF signaling pathway, inhibit the overactivation of microglia and Aβ deposition, and promote long-term potentiation and synaptic plasticity (Jiang Y. et al., 2021). Interestingly, other studies have shown that SCFAs can upregulate apolipoprotein E, increasing microglial activation and Aβ plaque recruitment (Colombo et al., 2021). These differing results may be due to variations in SCFAs concentrations and metabolite imbalances, leading to different outcomes in each study.

In neuroinflammation, microglia and astrocytes, as the brain’s innate immune cell populations, play a crucial role (Kwon and Koh, 2020). Recent studies indicated that SCFAs modulate the maturation of microglia and synaptic plasticity, thereby affecting learning, memory, and behavior. Erny et al. reported that mice lacking receptors for SCFAs exhibited microglial defects under germ-free conditions (Erny et al., 2021). Previous reports have found that acetate exerts anti-inflammatory effects by upregulating GPR41 levels in BV2 cells and inhibiting NF-κB (Luo et al., 2024; Liu J. et al., 2020). Importantly, gut-derived SCFAs, such as acetate, can regulate microglial functions in the 5xFAD AD mouse model, including mitochondrial metabolism and phagocytic function (Erny et al., 2021). Butyrate can also inhibit the secretion of pro-inflammatory cytokines by microglia, ameliorating related neuroinflammation (Matt et al., 2018). Astrocytes also play a crucial role in the central nervous system, regulating neurotransmitter secretion, synaptic function, and BBB integrity, thus maintaining central nervous system homeostasis.

A potential mechanism by which the MGBA affects behavior and cognition is through myelination in the prefrontal cortex (Ntranos and Casaccia, 2018). The PFC integrates inputs from multiple brain regions, processing complex decision-making and memory retrieval. SCFAs can alleviate behavioral and cognitive impairments by improving myelin abnormalities. For example, antibiotic treatment in neonatal mice can lead to oligodendrocyte damage and alterations in PFC myelin phospholipids, accompanied by behavioral deficits and gut dysbiosis, which can be prevented by SCFAs supplementation (Keogh et al., 2021). Butyrate can directly promote the differentiation and maturation of oligodendrocytes, inhibit demyelination, and enhance remyelination (Chen et al., 2019). In summary, SCFAs regulate brain function through various mechanisms, including maintaining BBB integrity, modulating glial cell activation, inhibiting demyelination, inhibiting Aβ deposition, and enhancing synaptic plasticity, as described in Table 1. Although the specific mechanisms of SCFAs remain controversial, their role as key mediators in the MGBA is widely recognized and is expected to be further elucidated in future research.

GABA, the central nervous system’s predominant inhibitory neurotransmitter (Lee et al., 2019), has the excitatory neurotransmitter glutamate as its metabolic precursor. It has been reported that an imbalance in GABA metabolism can lead to cognitive and memory impairments, and is associated with anxiety, depression, and AD (Govindpani et al., 2020). Autopsy findings reveal a downregulation of GABA signaling components in the middle temporal gyrus of AD patients, accompanied by reduced GABA levels in the cerebrospinal fluid (Carello-Collar et al., 2023). Numerous animal studies have found that GABA is primarily produced by intestinal microorganisms such as Lacticigenium, Bacteroides, Bifidobacterium, and Escherichia coli. This finding has been corroborated by human fecal transcriptome analysis (Chang et al., 2020; Strandwitz et al., 2019). Additionally, studies have found that germ-free mice have lower GABA levels in their colon (Eicher and Mohajeri, 2022). These findings support the view that gut microbiota can regulate host GABA levels. However, the connection and mechanism of action between gut GABA and the brain still need to be completed.

Although dietary glutamate cannot cross the BBB, GABA produced by bacterial metabolism can enter the brain and regulate neuronal and synaptic changes (Zhang H. et al., 2022). A recent human study found that fecal microbiome transplantation alters plasma GABA levels (Fu et al., 2023). Moreover, Bifidobacterium dentium was found to secrete GABA and acetate in vitro experiments (Engevik et al., 2021), leading to increased levels of GABA and tyrosine in mouse feces and brain (Luck et al., 2021). Thus, the gut microbiome may indirectly regulate GABA signaling to enhance brain function and behavior. In both the presynaptic and postsynaptic stages of brain transmission, GABA binds to particular transmembrane receptors on the cell membrane. Furthermore, it is involved in the proliferation of precursor neurons, the formation of synapses, and the suppression of inflammation (Tang X. et al., 2021). Similarly, the reduction of GABA_B receptors on glial cells can induce the upregulation of Aβ1-40 in transgenic mice (Osse et al., 2023). Animal research indicated that GABA can regulate IL-15 expression in the hippocampus, potentially impairing the new object recognition ability in mice (Di Castro et al., 2024). Additionally, research indicates that astrocytes play a critical role in the GABA metabolic cycle. Insufficient astrocyte metabolism leads to reduced glutamine synthesis, which in turn inhibits neuronal GABA synthesis, resulting in an imbalance between synaptic excitation and inhibition in the AD brain (Andersen et al., 2021).

Bi et al. (2020) suggested that dysfunction in the GABAergic system leads to neuronal excitatory/inhibitory imbalance, which in turn promotes the spread of Aβ and Tau pathology, exacerbating cognitive impairment in AD patients. Thus, abnormalities in the GABAergic system might represent a common target in the various aberrant signaling pathways of AD. Increasing research have revealed the critical role of GABA activity in cognitive functions such as learning and motor coordination (Wen et al., 2022b). Studies indicated that GABA activation promotes neuronal differentiation (Tozuka et al., 2005). However, other research showed that inhibiting GABA signaling can alleviate cognitive impairment in AD mice (Hardy et al., 1987). In the brains of late-stage AD patients, although the glutamate and GABA receptor subunits are relatively unaffected, these findings suggest that the brain continues to attempt to maintain the balance of excitatory and inhibitory tensions even in the final stages of the disease (Carello-Collar et al., 2023). Consequently, a new concept has emerged: the balance between inhibitory and excitatory expression of GABA may be crucial in preventing the onset of AD (Bi et al., 2020).

Unfortunately, fluctuations in GABA levels in the AD brain have been primarily confirmed through animal and post-mortem studies. Consistent observations include increased GABA levels in the hippocampus and astrocytes of AD model mice, along with an increase in amyloid protein deposition (Jo et al., 2014; Rice et al., 2019). Therefore, further exploration of the GABAergic system may become a key target for restoring excitatory/inhibitory balance and cognitive function in AD patients.

Gut microbes synthesize 90% of the body’s 5-HT (Boadle-Biber, 1993), for instance, Lacticigenium and Bifidobacterium (Tian et al., 2022), using tryptophan from dietary proteins within the gastrointestinal epithelial cells (Martin et al., 2017). Importantly, 5-HT cannot cross the BBB (Pagire et al., 2022), preventing it from directly influencing brain 5-HT levels. Intriguingly, research by Wang indicated that Clostridium butyricum can alleviate learning and memory impairments induced by Di-(2-ethylhexyl) Phthalate, attributed to the restoration of gut microbiome, suppression of inflammatory factors, and increased 5-HT secretion (Wang et al., 2023d). Consequently, the role of gut microbiota and 5-HT in regulating central nervous system function is gaining increasing attention.

The study found that TPH2 KO CUMS mice (chronic mild stress with serotonin inhibition), there was a significant decrease in extracellular serotonin levels, accompanied by gut microbiome dysbiosis and aggravated cognitive dysfunction. Feeding with Lacticigenium E001-B-8 significantly improved the cognitive behavior of these mice (Ma et al., 2023). Moreover, selective serotonin reuptake inhibitors (SSRIs), a class of antidepressants, increase extracellular serotonin levels by inhibiting the reuptake of the neurotransmitter serotonin by nerve synapses (Michely et al., 2020). Studies have found that citalopram improves cognition in APP mice by regulating mitochondrial biogenesis and autophagy. Additionally, it has been shown to increase dendritic spine density and synaptic activity in these mice (Reddy et al., 2021). Cellular experiments show similar results. Additionally, in vitro cell experiments have shown that citalopram reduces p-Tau and serotonin protein expression, improves mitochondrial respiration, and increases cell survival rates (Sawant et al., 2024). However, recent clinicopathological studies show that SSRIs accelerate cognitive decline by increasing Tau tangles (Sood et al., 2023). Additionally, there is evidence that increased serotonin synthesis is associated with temporal lobe atrophy (Markova et al., 2023). These findings highlight the crucial role of the gut microbiome in the central nervous system’s 5-HT signaling pathway.

Although the role of gut microbiota-derived 5-HT in the central nervous system is gaining increasing attention, the mechanisms underlying their connection remain unclear. Tryptophan, the precursor of 5-HT, influences the brain through the kynurenine and indole pathways (Cervenka et al., 2017). Additionally, 5-HT released from enterochromaffin cells in the gut signals to intestinal neurons via the Trapa1 pathway (Ye et al., 2021), enhancing vagal sensory neuron activity. Further research has found that gastric distension stimulate enterochromaffin cells to release 5-HT, which then activates 5-HT3 receptors on the gastric vagal nerve. It ultimately activates neurons in the paraventricular and supraoptic nuclie by increasing c-fos expression in the tractus solitarius and area postrema (Leon et al., 2021; Mazda et al., 2004). Importantly, the coordination of 5-HT receptor function is crucial for cognition and memory in AD patients. Abnormal serotonergic signaling has been observed in the hippocampus of AD model mice, further supporting the significant role of 5-HT in AD (Wang et al., 2023e).

More importantly, activation of 5-HT7R increases Tau phosphorylation and aggregation via CDK5, leading to neuronal death, long-term potentiation deficits, and memory impairment in transgenic mice (Labus et al., 2021; Ackmann et al., 2024). Wu also confirmed similar results, showing that the downregulation of 5-HT1AR and 5-HT2AR can promote neuronal resistance to Aβ neurotoxicity and improve cognitive deficits (Wu et al., 2024). Indeed, variations in the gut microbiome can influence the levels of tryptophan and serotonin, which in turn may influence the synthesis of crucial brain neurotransmitters (Doifode et al., 2021; Cryan et al., 2019). These findings collectively support the significant role of the interaction between 5-HT signaling and gut microbiota in the pathogenesis of AD. However, the lack of precise 5-HT detection methods has significantly slowed the progress in research on 5-HT signaling (Wan et al., 2021).

Acetylcholine, a common excitatory neurotransmitter, is prevalent in both the peripheral and central nervous system (Picciotto et al., 2012). In recent years, research have revealed a connection between gut microbiota and the central nervous system, particularly noting that various gut microbes release acetylcholine, such as Lactiplantibacillus plantarum, Escherichia coli, Bacillus subtilis, and Staphylococcus aureus (Horiuchi et al., 2003). Since acetylcholine cannot directly cross the BBB (Amenta et al., 2001), choline from the peripheral nervous system crosses the BBB with the help of carrier proteins and is then synthesized into acetylcholine in the central nervous system by choline acetyltransferase (Inazu, 2019). Studies have found reduced acetylcholine transferase activity across all cortical regions in AD patients (Rossor et al., 1982), suggesting that acetylcholine levels in the memory-related prefrontal cortex may be a potential factor influencing the development of AD (Sun Q. et al., 2022). Further research indicates that impairments in cholinergic neurotransmission are closely linked to the pathological process of AD. For example, disruptions in cortical cholinergic neurotransmission can affect the expression and processing of amyloid precursor protein amyloid precursor protein (APP), leading to the deposition of Aβ amyloid plaques, which in turn further disrupt cholinergic transmission (Schliebs, 2005). This series of studies was confirmed in another animal experiment. In AD model mice, obstruction of pyramidal neuron pathways in the prefrontal cortex and cholinergic neuron pathways in the basal forebrain is observed, which is associated with cognitive decline (Zhong et al., 2022). Clinical studies indicated that the basal forebrain cholinergic system can predict Aβ deposition in AD (Berry and Harrison, 2023), which positively correlates with the extent of hippocampal atrophy (Teipel et al., 2023).

Like acetylcholine, dopamine plays various roles in the central nervous system, but its mechanisms of action in AD may differ. Dopamine is a crucial neurotransmitter in the vertebrate system and a catecholamine that affect both the central and peripheral nervous systems (Miri et al., 2023). Interestingly, more than half of dopamine is produced in the gut, with Bacillus, Bacteroides, Brevilactibacter, and Bifidobacterium being the major contributors to its production and bioavailability (Eisenhofer et al., 1997). Dopamine metabolized by the microbiome is transported to the brain across the BBB, where it negatively regulates the NLRP3 inflammasome via dopamine receptors expressed in microglia and astrocytes, thereby influencing neuroinflammatory responses (Possemato et al., 2023). The dopamine D4 receptor modulates the transport of α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptor in the GABAergic interneurons of the prefrontal cortex through a unique signaling mechanism, regulating the strength of excitatory synapses in neurons and affecting cognition and emotion (Penn et al., 2017; Valles-Colomer et al., 2019). In AD model rats, degeneration of midbrain limbic dopamine neurons impairs the function of parvalbumin interneurons (La Barbera et al., 2022), leading to early hippocampal hyperexcitability and epilepsy-like activity (Spoleti et al., 2024), which contrasts with the role of acetylcholine in AD. Additionally, Calabresi et al. highlight that dopamine not only affects mood and cognition but also regulates behavior (Korn et al., 2021), learning, and memory (Calabresi et al., 2007).

Similar to dopamine, norepinephrine is another catecholamine produced by gut bacteria (Dicks, 2022). Although present in small amounts, it significantly affects human cognition and behavior and is an important neurotransmitter (Borodovitsyna et al., 2017). While primarily synthesized in the adrenal medulla, norepinephrine is also produced by gut bacteria such as Escherichia coli, Proteus vulgaris, and Bacillus subtilis (Ojeda et al., 2021). Reports indicated that norepinephrine can modulate synaptic plasticity (Fukabori et al., 2020), regulate microglial and astrocytic activation, and increase the production of BDNF, thereby providing neuroprotection (Giustino et al., 2020). These functions are crucial for maintaining brain health, particularly in the context of neurodegenerative diseases like AD. Postmortem cerebrospinal fluid samples from AD patients show reduced levels of norepinephrine and its metabolites (Kaddurah-Daouk et al., 2011), which may serve as evidence of norepinephrine’s involvement in the pathogenesis of AD. Studies in APP/PS1 transgenic mice showed that neuroinflammation and microglial activation are significantly associated with degeneration in the locus coeruleus-norepinephrine system (Cao et al., 2021). Additionally, deficiencies in this system are associated with Aβ deposition (JaMalatt and Tagliati, 2022), hyperphosphorylation of Tau protein (Chalermpalanupap et al., 2018), and spatial memory deficits (Betts et al., 2019; Lin et al., 2024). Locus coeruleus-norepinephrine may play a crucial role in the pathology of AD by reducing inflammatory cytokine release and enhancing amyloid clearance through the regulation of microglial migration and phagocytic functions (Flores-Aguilar et al., 2022; Li J. et al., 2022).

The gut microbiome also produce various other neurotransmitters, including histamine (produced by Enterococci) (De Palma et al., 2022) and tyramine (produced by Enterococcus) (O’Donnell et al., 2020). These neurotransmitters are hypothesized to regulate Aβ aggregation, Tau protein accumulation, and synaptic remodeling via the gut-brain axis, affecting brain physiology (Berry and Harrison, 2023; Fukabori et al., 2020; JaMalatt and Tagliati, 2022),. However, their functions have not been fully elucidated and represent a potential direction for future research. As our understanding of these complex interactions deepens, we hope to develop new therapeutic strategies to more effectively address AD, a global health challenge.