The abnormal aggregation of α-synuclein in the substantia nigra is a crucial pathological feature of PD, and understanding the relationship between gut microbiota and α-synuclein is key to clarifying the pathogenesis of PD. In the early stages of PD, gastrointestinal dysfunctions such as constipation, dysphagia, salivation, delayed gastric emptying, and anorectal dysfunction occur, often preceding motor symptoms (Travagli et al., 2020; Talman and Safarpour, 2023). Researchers have proposed that intestinal lesions may precede brain lesion, introducing the concept of body priority (Nuzum et al., 2022). In fact, α-synuclein is not only found in the brain but also present in the peripheral nervous system. In Shannon et al. (2012a,b) first identified abnormal aggregation of α-synuclein in the colonic submucosal nerve fibers of patients with PD. Subsequent research confirmed, using different PD models, that abnormal aggregation of α-synuclein in Schwann cells in the intestine leads to gastrointestinal dysfunction and vagus inflammation via the toll-like receptor-2 (TLR2)/myeloid differentiation primary response gene 88 (MyD88)/nuclear factor kappa-B (NF-κB) signaling pathway (Cheng et al., 2023c; Jiang et al., 2023). The current hypothesis is that α-synuclein is transmitted to the central nervous system through the vagus nervous system in a prion-like manner (Jan et al., 2021).

Similar to prions, α-synuclein has the ability to escape from a cell and subsequently spread into neighboring cells. Both in vivo and in vitro experiments have confirmed that transplanted cells can detect host α-synuclein (Desplats et al., 2009; Hansen et al., 2011). For example, Holmqvist et al. (2014) administered various forms of recombinant α-synuclein as well as α-synuclein derived from human PD brain lysates directly into the intestinal wall of rats, detecting α-synuclein in the vagus nerve 48 h later, eventually reaching the dorsal motor nucleus of the vagus nerve in the brain stem. Another study by Kim et al. (2019) injected pathological α-synuclein prefabricated fibers into the duodenum and muscular layer of the pylorus, tracking the diffusion path of this pathological α-synuclein in the brain. It first appeared in the posterior cerebral tail, then in the basolateral amygdala, dorsal raphe nucleus, and finally in the substantia nigra (Kim et al., 2019). Related studies have shown that the inoculation of pathological α-Synuclein prefabricated fibers can directly affect the activation of microglia in the brain (Rey et al., 2016; Manfredsson et al., 2018; Uemura et al., 2019). These studies strongly suggest that enteric-derived α-synuclein can migrate to the brain through the vagus nerve. Abnormal aggregation of α-synuclein in the nigrostriatum can further lead to M1-like phenotype polarization of microglia, activation of nucleotide oligomerization domain (NOD)-like receptor heat protein domain protein 3, and impaired autophagy and phagocytosis of microglia (Wood, 2022; Lv et al., 2023).

α-synuclein can accumulate and migrate in the intestine, and intestinal microorganisms are also associated with its production. Amyloid fibrin (curli) is a major component of bacterial biofilms, enhancing bacterial adhesion, colonization, and invasion, thus increasing virulence. Curli is often produced by Salmonella spp and Escherichia coli. It is formed by seven subunits, with CsgA serving as the primary structural subunit, and α-synuclein structurally similar to curli (Sheng et al., 2020; Yan et al., 2020). In rodent models, transgenic mice overexpressing α-synuclein were colonized with curli-producing Escherichia coli via artificial gavage, resulting in significantly increased α-synuclein in the intestine and brain compared to controls (Sampson et al., 2020; Schmit et al., 2023). Wang C. et al. (2021) found through whole genome screening that the Escherichia coli gene CsgA is a crucial gene that exacerbates neuropathy in a PD model of C. elegans. However, the exact transition mechanism remains unclear, and whether other bacteria produce curli remains to be elucidated. Notably, targeted reduction of curli-producing bacteria could offer a breakthrough in developing new therapies.

SCFAs are a type of saturated fatty acids that contain fewer than six carbon atoms, including formic acid, acetic acid, propionic acid, butyric acid, and isobutyric acid. They are metabolic byproducts of gut microbiota, generated via the fermentation of polysaccharides, such as dietary fiber and resistant starch (Mirzaei et al., 2021). Importantly, 95% of SCFAs are transported and absorbed by colon epithelial cells, with only a small portion reaching peripheral tissues through systemic circulation. These SFCAs act as signaling molecules to regulate physiological functions or provide energy to target cells (Dalile et al., 2019). As signaling molecules, SCFAs mainly bind to G-protein coupled receptors (GPCRs) on cell membranes to regulate the host’s metabolic immune response, cell proliferation, and other physiological activities (He et al., 2020). Studies have shown that the fecal matter of patients with PD contains a higher abundance of certain SCFAs-producing bacteria (such as Prevotellaceae, Fusicatenibacter, Faecalibacterium, and Blautia) and the concentration of SCFAs decrease, correlating with the significant severity of clinical symptoms (Tan et al., 2021; Nishiwaki et al., 2022; Yang et al., 2022). This suggests that SCFAs may potentially play a significant role in the pathogenesis of PD.

Excessive activation of microglia in the substantia nigra and basal ganglia of patients with PD induces increased expression of inflammatory factors and macrophage infiltration, linking microglia activation and neuroinflammation to the pathogenesis of PD (Kwon and Koh, 2020). SCFAs, particularly butyric acid, seem to play a significant role in regulating the activation of microglia. Multiple studies have demonstrated that butyric acid has the potential to decrease the activation of astrocytes and microglia in the brains of mice with PD, inhibit the expression of inflammatory factors, upregulate brain- and glia-derived neurotrophic factors, and reduce dopaminergic neuron loss (Srivastav et al., 2019; Hou Y. et al., 2021; Guo et al., 2023; Ji et al., 2023). Moreover, the neuroprotective effect of butyric acid on PD may be mediated through the Janus kinase-2/signal transducer and activator of tran-ions-3 (JAK2/STAT3) and TLR4/MyD88/NF-kB signaling pathways (Guo et al., 2023; Ji et al., 2023).

The permeability of the BBB also influences PD progression. Reduced permeability allows inflammatory mediators and toxins to enter, contributing to neuroinflammation and loss of dopaminergic neurons. Butyric acid supplementation has been shown to significantly improve the levels of the tight junction proteins occludin and zonula occludens-1 in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced PD mouse models, maintaining the integrity of the BBB (Liu et al., 2017). SCFAs also play a direct role in regulating energy metabolism and insulin release through the stimulation of peptide YY (PYY) and glucagon-like peptide 1 (GLP-1) secretion in intestinal endocrine cells (Larraufie et al., 2018; May and den Hartigh, 2021). Sun et al. (2021) found that supplementation with Clostridium butyricum in MPTP-induced PD mice increased the number of SCFA-producing bacteria, promoted GLP-1 secretion by intestinal endocrine L cells, and activated the GLP-1 receptor in the brain, thereby improving dopaminergic neuron loss and neuroinflammation.

Additionally, studies have revealed a significant reduction in propionate levels in the feces of patients with PD. Supplementation with propionate in PD mice was found to improve motor symptoms, possibly by enhancing intestinal epithelial barrier function through the Automatischer Kassentresor (AKT) signaling pathway (Huang et al., 2021). Osteocalcin has been shown to increase Bacteroides levels in the gut microbiota of PD mice, boost fecal propionate levels, and alleviate motor deficits and the loss of dopaminergic neurons (Hou Y. F. et al., 2021). Therefore, the therapeutic effects of propionic acid in PD warrant further investigation.

Accumulated evidence suggests that SCFAs can benefit PD by alleviating BBB damage, inhibiting microglial activation, and improving neuroinflammation. However, conflicting results also exist: butyric acid has been found to promote the activation of microglia and astrocytes, leading to an upregulation of pro-inflammatory factors such as interleukin-6, interleukin-18, inducible nitric oxide synthase, and nitric oxide, thereby aggravating neuroinflammation (Qiao et al., 2020). These contrasting outcomes may result from differences in model types, treatment durations, dosages, and experimental protocols. Therefore, SCFAs have great potential as safe and effective targets for the treatment of PD.

One of the pathological characteristics of PD is the degeneration of dopaminergic neurons, and the most important current treatment is oral levodopa, which is absorbed from the intestinal tract to the systemic circulation, crosses the BBB, and is transformed into dopamine within the brain for therapeutic purposes (Lewitt, 2008). However, previous studies have found that Enterococcus faecalis (E. faecalis) in the intestine is also involved in the absorption and metabolism of levodopa (van Kessel et al., 2019; Wang Y. et al., 2021). The metabolism of levodopa and dopamine in patients with PD is correlated with the abundance of E. faecalis. The bacteria produce tyrosine decarboxylase, which has the ability to transform levodopa into dopamine, thereby weakening the efficacy of levodopa (van Kessel et al., 2019). Conversely, other studies have found that oral administration of berberine in PD model mice can increase the enzymatic activity of tyrosine hydroxylase, which acts as the rate-limiting enzyme in E. faecalis, thereby promoting the production of levodopa and alleviating PD symptoms (Wang Y. et al., 2021). This suggests that E. faecalis reduces the bioavailable levodopa, thereby limiting the amount of drug that reaches the brain and provides therapeutic benefit. but its biological activity on levodopa still requires further study.

Molecular hydrogen (H₂), a metabolite of the gut microbiota, is usually produced by Clostridium, anaerobic cocci, and Enterobacteriaceae (Smith et al., 2019). Recent studies have indicated that the quantity of H₂-producing bacteria in the intestines of patients with PD is significantly reduced, with H₂ content being 2.2 times lower than that in healthy controls (Suzuki et al., 2018). In fact, earlier studies found that drinking water with low concentrations of H₂ can mitigate the loss of dopaminergic neurons caused by MPTP, thus ameliorating symptoms in a mouse model of PD (Fujita et al., 2009). The underlying mechanism may be related to the antioxidant, anti-inflammatory and neuroprotective properties of H₂ (Ostojic, 2018).

Hydrogen sulfide (H₂S) is a gaseous neurotransmitter, and excessive H₂S produced by intestinal bacteria may induce PD. Sulfate-reducing bacteria are major producers of H₂S in the feces of healthy individuals (Dordevic et al., 2021). Importantly, bacterial species known to produce H₂S, such as Helicobacter pylori, Clostridium difficile, and Vibrio desulfuricum, are reportedly associated with the occurrence of PD (Huang et al., 2018; Kang et al., 2020; Murros et al., 2021; Nie et al., 2023). Excessive endogenous H₂S induces the release of cytochrome C (Cyt c) into the mitochondria of intestinal cells and increases cytoplasmic iron levels, thereby increasing reactive oxygen species (ROS) (Murros, 2022). The presence of α-synuclein, Cyt c and ROS together leads to the aggregation of α-synuclein in the intestine and its transmission to the brain, promoting the development of PD (Murros, 2022). However, exogenous inhalation of H₂S or injection of NaHS (an H₂S donor) has been found to have a beneficial effects on PD in both cell and animal experiments (Kida et al., 2011; Sarukhani et al., 2018; Tian et al., 2022; Hacioglu et al., 2023; Nagashima et al., 2023), possibly due to the upregulation of antioxidant protein-coding genes (Kida et al., 2011). Therefore, the role of H₂S in PD needs further study, as differences in H₂S concentration, the method of administration methods, and the target sites may lead to varying results. Moreover, studies suggest that low concentrations of H₂S inhalation are beneficial for stroke recovery, whereas high concentrations are beneficial for other conditions (Chan and Wong, 2017).

In conclusion, as depicted in Figure 2, the gut microbiota and its metabolites are involved in the pathogenesis of PD. However, the roles of some important metabolites in PD need further clarification through additional studies. Furthermore, bacterial metabolites such as bile acids (Zangerolamo et al., 2021), 5-hydroxytryptamine (Frouni et al., 2022) and γ-aminobutyric acid (Roberts et al., 2021) are also involved in the pathological progression of PD.

Sampson et al. (2016) initially established a Germ-free Thy1-αSyn mouse model, and subsequently introduced fecal samples from patients with PD versus healthy controls via gavage into these germ-free mice. Their findings revealed that the Thy1-αSyn mice receiving fecal transplants from patients with PD exhibited more pronounced motor function impairment compared to the group receiving fecal transplants from healthy controls. However, minimal changes were observed in body weight or gastrointestinal function (Sampson et al., 2016). Additionally, this study highlighted that SCFAs exacerbated α-synuclein aggregation in the brain and motor deficits in α-synuclein-overexpressing mice (Sampson et al., 2016). Notably, this study pioneered the use of FMT to explore the interplay between gut microbiota, its metabolites, and PD in a germ-free mouse model. Subsequent research further corroborated the direct therapeutic potential of FMT in PD mouse models (Sun et al., 2018; Zhao et al., 2021; Zhong et al., 2021; Zhang T. et al., 2022).

Sun et al. (2018) demonstrated that FMT could modulate the gut microbiota of MPTP-induced PD mice, leading to reduced SCFAs levels in feces, which in turn improved motor function while decreasing microglia and astrocytic activity in the substantia nigra. Moreover, FMT was found to decrease α-synuclein levels in the brains of PD mouse models (Zhong et al., 2021) and enhance gut and BBB permeability (Zhao et al., 2021). Mechanistic investigations into FMT therapy suggested that these effects may occur through inhibition of the TLR4/tumor necrosis factor-alpha (TNF-α) signaling pathway (Sun et al., 2018; Zhao et al., 2021; Zhong et al., 2021). Further clarification of the role of TLR4 in PD was provided by Perez-Pardo et al. (2019), where TLR4 knockout in PD mice reduced intestinal and brain inflammation, implicating TLR4 as a key receptor in PD-related neurodegeneration.

In these studies, donors were healthy mice of the same strain. To explore the therapeutic potential of human-derived gut microbiota in PD models, Xie et al. (2023) transplanted fecal samples from patients with PD and healthy individuals into MPTP-induced mice. Their results indicate that FMT from healthy human donors suppressed microgliosis and astrogliosis, restored nigra peristriatal cells, and preserved BBB integrity, possibly mediated via the AMP-activated protein kinase (AMPK)/superoxide dismutase 2 (SOD2) signaling pathway to mitigate mitochondrial damage (Xie et al., 2023). Yu J. et al. (2023) utilized bioinformatics analysis to identify the significance of nicotinamide mononucleotide adenylyltransferase 2 (NMNAT2) in PD pathogenesis induced by gut microbiota. Further investigation into the molecular mechanisms of FMT in PD treatment involved treating 6-OHDA-treated rats with FMT, resulting in upregulation of NMNAT2 expression. This upregulation, along with reduced glutathione (GSH) content, total SOD, and glutathione peroxidase (GSH-Px) activities, contributed to the mitigation of oxidative stress and amelioration of neurobehavioral defects in rats (Yu J. et al., 2023).

Regarding donor selection, most studies utilized fecal samples from young and healthy individuals, with minimal investigation into the fecal samples from elderly donors for FMT. Qiao et al. (2023) observed that while fecal transplantation from old and young mice did not alter the intestinal tract or neuroinflammation in PD mice, fecal samples from old mice improved motor disorders by enhancing neurogenesis in the hippocampus. Similar effects on the hippocampus of elderly mice were reported in prior studies (Rei et al., 2022), underscoring the importance of considering donor when performing FMT on PD patients.

In summary, these findings indicate that FMT holds promise in treating PD by mitigating inflammation via TLR4/TNF-α signaling pathway inhibition, improving mitochondrial damage through AMPK/SOD2 pathway activation, and reducing oxidative stress via NMNAT2 upregulation. Future research endeavors should further elucidate the therapeutic mechanisms of FMT in PD.

Although FMT has not been widely implemented in treating PD, several case reports and case-control studies suggest that FMT can normalize gut microbiota disturbances in patients with PD, leading to improvements in both motor and non-motor symptoms (Huang et al., 2019; Xue et al., 2020; Kuai et al., 2021; Segal et al., 2021). In Huang et al. (2019) conducted a pioneering study applying FMT to a patient with PD suffering from constipation. They inserted a transendoscopic enteral tube into the patient’s ileocecal area via colonoscopy, administering 200 ml of fecal bacterial solution daily for three days. During a three-month follow-up, the patient’s bowel movement time decreased from > 30 min to < 5 min. Remarkably, the patient’s resting tremor in both lower limbs nearly disappeared one week post-treatment, although symptoms in the right lower limb reappeared two months later without any adverse reactions (Huang et al., 2019). While this was merely a case report, it highlighted the potential of FMT in the management of PD.

In Xue et al. (2020) expanded on this research by applying FMT to a larger cohort of patients with PD. They conducted a self-controlled clinical study involving ten patients with PD who underwent FMT via colonoscopy and five patients with PD who underwent FMT via a nasojejunal tube. The results indicated improvements in motor function, anxiety, depression, and sleep symptoms among patients treated via colonoscopy, whereas those treated via the nasojejunal tube exhibited less significant improvements (Xue et al., 2020). This suggests that the route of FMT administration may impact its therapeutic efficacy, a hypothesis further supported by Gundacker et al. (2017), who proposed that bacterial loss during migration from the jejunum to the colon could explain the variance in effectiveness.

Cheng et al. (2023b) conducted an FMT trial involving 56 patients with mild-to-moderate PD, administering fecal bacterial capsules. Throughout the duration of the follow-up period, patients receiving FMT exhibited significant improvements in autonomic symptoms and gastrointestinal disorders compared to those in the placebo group, without experiencing any serious adverse reactions. This study represents the only clinical randomized controlled trial reported to date (Cheng et al., 2023b). Collectively, these studies underscore the potential of FMT for the treatment of PD.

Safety is a paramount consideration in FMT applications. Reported side effects include diarrhea, abdominal pain, venting, flatulence, nausea, and throat irritation, which are usually mild and transient (Xue et al., 2020; Kuai et al., 2021). However, one patient in Segal’s study experienced recurrent episodes of vasovagal presyncope 24 h post-FMT, lasting for 8 h—an adverse effect not previously documented in FMT treatments (Segal et al., 2021). The longest follow-up duration in current studies is one year, which is insufficient to conclusively determine the long-term therapeutic effectiveness of FMT in PD treatment (Xue et al., 2020).

In summary, FMT has demonstrated therapeutic potential for PD, but more extensive and rigorous prospective studies are necessary to confirm its efficacy, safety and sustainability.

Despite the promising results from animal and human studies on the treatment of PD with FMT, several key issues warrant deeper consideration, as they may significantly influence the efficacy of FMT.

First, the source of fecal donors is crucial for the treatment’s success. Donors can be either allogeneic or autologous. Allogeneic feces typically come from fecal banks or universal donors and undergo rigorous screening to ensure safety (Sorbara and Pamer, 2022). However, some pathogens may evade detection, posing potential risks post-transplantation. Therefore, autologous feces, stored before the onset of disease, might be a more suitable donor source (Sorbara and Pamer, 2022). Moreover, the age of allogeneic fecal donors is an important factor. Most current donors are healthy young adults, whose future risk of developing PD is unknown, which could inadvertently affect the progression of PD in recipients. Animal studies suggest that feces from elderly mice can enhance hippocampal neurogenesis and improve PD symptoms (Qiao et al., 2023), indicating that feces from healthy elderly donors might be more beneficial for FMT treatment.

The method of FMT administration is another critical consideration. Common methods include nasal jejunal tube transplantation, oral microflora capsule transplantation, and colonoscopy. Clinical studies have reported better outcomes with colonoscopic transplantation compared to nasojejunal tube transplantation (Xue et al., 2020). However, since PD patients are generally older and some may not tolerate colonoscopy, the balance between efficacy and safety is essential. Some studies also recommend using antibiotics like vancomycin to deplete the patient’s native gut microbiota, facilitating the colonization of donor microbiota (Contarino et al., 2023). However, the necessity of pre-antibiotic treatment remains undetermined, though animal experiments indicate that ceftriaxone might offer neuroprotective benefits in PD models (Zhou et al., 2021). More case-control studies are needed to evaluate the impact of antibiotic pretreatment on the efficacy of FMT.

Furthermore, there is no standardized FMT regimen for PD treatment. Variables such as the number of transplants and the interval between treatment cycles significantly influence outcomes. Most current cases report single infusions of bacterial fluid (Xue et al., 2020; Kuai et al., 2021; Segal et al., 2021), with one study administering daily infusions for three consecutive days (Huang et al., 2019). Symptom improvement, particularly for constipation, is more pronounced than motor symptom relief. Additionally, the longest follow-up duration in existing studies is one year (Xue et al., 2020), providing limited data on the long-term efficacy of FMT in PD treatment. Therefore, establishing a standard FMT regimen and determining the necessity of maintenance therapy are crucial areas for future research.

The ideal composition of the gut microbiota for PD treatment is still unclear. However, effective microbiota should possess anti-inflammatory properties, produce SCFAs, H₂, and H₂S, and should not interfere with anti-PD drug metabolism. Future developments in FMT may involve assembling well-characterized, functional bacterial strains into symbiotic microbiota to reestablish a healthy gut environment while avoiding potential pathogens (Sorbara and Pamer, 2022).