In the present MR study, we found significant association of increased abundance of genera RuminococcaceaeUCG004 and higher risk of ALS. Besides, we found suggestive evidence of causal associations of Actinobacteria, Lactobacillaceae, Faecalibacterium, and Ruminiclostridium, Lachnoclostridium with AD, of Lentisphaerae, Lentisphaeria, Oxalobacteraceae, Victivallales, Bacillales, Eubacteriumhalliigroup, Anaerostipes, and Clostridiumsensustricto1 with PD, and of Lachnospira, Fusicatenibacter, Catenibacterium, and Ruminococcusgnavusgroup with ALS. What’s more, metabolites including amino acids, bile acids, amino acids, polyphenols produced by gut microbiota were also potentially related to the risks of neurodegenerative disorders, indicating their important roles in gut microbiota–brain axis.
A previous MR study have suggested that increase in Blautia and elevated γ-aminobutyric acid (GABA) were related to lower risk of AD[35]. However, our study failed to repeat these findings, nor Blautia or GABA including putrescine, glutamate, arginine or ornithin which produces GABA were found related to risk of AD, which is potentially due to lack of significance of results and scale of GWAS. Another MR study proved no causal association of trimethylamine N-oxide (TMAO) or its precursor with AD[36], which was consistent with our results. What’s more, our finding of Actinobacteria at family level as a risk factor of AD was opposite to previous studies [10], while the findings of relationships between Lactobacillaceae, Faecalibacterium with AD[11] were in accordance with the result of previous cross-sectional studies. Interestingly, genera Ruminiclostridium6 and Ruminiclostridium9 represent different effects on risk of AD in our analysis results, which remind us that inconsistencies in results of previous clinical studies were potentially due to insufficiently digging deeper into classification of genera level of gut microbiota. Besides, our study suggested that phenylacetate, which was a potential tracer of glibal metabolism was related to increased risk of AD[37]. In addition, mannitol, a microbial metabolite was found as protective factor of AD, which may provide new ideas for disease interventions.
What’s more, our study revealed suggestive causal effect of increased abundance of phylum Lentisphaerae, class Lentisphaeria, order Victivallales on protective effects of PD, however, no direct effect revealed after multivariable MR analysis, while no relevant result was reported in previous studies either, therefore, such results should be treated with caution. Other associations of Family Oxalobacteraceae, Order Bacillales, Eubacteriumhalliigroup, Anaerostipes and Clostridiumsensustrictol with risk of PD were in accordance with the result of previous cross-sectional studies[12, 13, 38]. In a previous clinical study, which compared the fecal microbiota of 25 ALS patients with 32 controls, significant higher abundance of uncultured Ruminococcaceae at genus level was observed in ALS patients[14]. However, our study found significant association between RuminococcaceaeUCG004 and higher risk of ALS, and suggestive association between Ruminococcusgnavusgroup and lower risk of ALS. Inconsistent results between these studies may likely be attributed to small study sample sizes of previous observational studies, sample heterogeneity, and different sequencing technologies. Therefore, a standardized classification system for gut microbiota at genus level or even more specific level is crucial to direct mechanism researches and provide more accurate clinical guidance.
Tryptophan is broken down by the microbiota into indole derivatives and also tryptamine and kynurenine metabolites, and those metabolites were considered important in gut-brain axis[39, 40]. Previous studies have revealed that glutamate signals are destroyed by serotonergic overdrive, and serotonergic dysfunction is associated with the development of motor and non-motor symptoms and complications in Parkinson's disease[41]. What’s more, kynurenine Pathway (KP) of tryptophan degradation is involved with several neuropathological features present in ALS including neuroinflammation, excitotoxicity, oxidative stress, immune system activation and dysregulation of energy metabolism[42], previous clinical studies have revealed that serum kynurenine in control were lower than that in ALS[43]. Our study proved that serotonin was protective factor of PD, while kynurenine was risk factor of ALS, and those molecules may become potential biomarkers to assess the progression of relative diseases. In addition, other amino acid such as glutamine and isoleucine were found causally associated with lower risk of AD and PD. Actually, up to 50% of all α-amino groups of glutamate and glutamine are derived from leucine. Leucine is a regulator of the mechanistic target of rapamycin (mTOR) complex 1 (mTORC1), which is critical on protein synthesis and degradation, autophagy as well as maintenance of glutamate homeostasis, and may have effects on the neuronal solute transport and the excitatory neurotransmitter function[44]. Moreover, in the glutamate-glutamine cycle, synaptically-released glutamate is rapidly transported into astrocytes, and glutamine is then released by astrocytes through SN-type glutamine transporters into the extracellular fluid. Aβ has been shown to reduce the surface expression of GLT-1and to impair astrocyte glutamate uptake[45, 46]. A recent study demonstrated that altered astrocyte glutamine synthesis directly impaired neuronal GABA synthesis in brain slices of the 5xFAD mouse model of AD[47], and our results provided clinical evidence to confirm that reduction of glutamine in peripheral blood was causally associated with occurrence of AD.
Bacterial metabolites produced from polyphenol precursors were also found at levels sufficient to exert biological effects enter circulation[48]. In vitro cultures have shown that polyphenol metabolites such as ferulic acid are able to exert protective effects on neuronal cultures and neurodegenerative models, mostly through a decrease in inflammatory responses[49, 50], however, in vivo evidence remains lacking. Our study suggested hippurate, belongs to the group of uremic toxins as a risk factor of ALS, which may indicate potential treatment of disease. Since those neurodegenerative diseases develop through a long prodromal phase, it is plausible that our findings may inform early interventions by targeting the microbiota via gut microbiota transplantation, psychobiotics, or antibiotics in the future.
Among the strengths of the study are the most comprehensive MR study on association of gut microbiota and metabolite traits with neurodegenerative diseases, and the largest sample size so far. However, our study still suffers from several limitations. Firstly, most of the results did not survive a strict FDR correction. However, MR was a hypothesis-driven approach, and it could be used to detect some causal relationships regardless of FDR adjusting when some biological evidence exists. Secondly, 16S rRNA gene sequencing describes gut microbiota from genus to phylum level only, and metagenomic and multiomic approaches may offer opportunities to target gut microbiota compositon at a more specific level, avoiding bias if species of more specific level associated with neurodegenerative diseases. Finally, gut microbiota is affected by several environmental factors including diet and lifestyle, whereas those confounders which were not available in present studies were hardly to be excluded.