The present study has demonstrated that there was a obvious alteration of gut microbiome composition in PD patients. The relative abundance of Rikenellaceae_RC9_gut_group, which was increased in both early and advanced PD patients, associated with both the motor symptoms and concentration of amino acids. Therefore, our results not only identify the fresh genera of gut microbiota associated with the severity of PD, but also may help us to explore the potential pathogenesis mediated by amino acids in PD.
The proposed microbiota-gut-brain axis suggests that there are some ways facilitating communication between the microbiota and the brain, which include the neuroendocrine, neuroimmune, the vagus nerve, the enteric nervous system and microbial metabolites . For example, Braak's staging thought that α -synuclein can be retrogradely transported from the gut to the brain via the vagus nerve . All these evidences show that PD may be the intestinal onset, which is regulated by intestinal microbiota. Therefore, it is necessary to conduct a comprehensive analysis of intestinal microbiota and identify microbial therapeutic targets for PD.
We performed 16S rRNA gene sequencing to investigate the intestinal microbiota composition of PD patients. our findings showed the β-diversity index (structure) of PD patients significantly different from that of healthy controls qualitatively, indicating that the intestinal microbiota composition altered in PD patients. At the phylum, genus and species levels, we observed significant differences of fecal microbiota in patients with PD compared with healthy controls in our study. We observed an increased abundance in Sphingomonas, in a correspondence with the results by Qian et al . Moreover, we also found that the abundance of Rikenellaceae_RC9_gut_group, known as the putative ‘pro-inflammatory' bacterial genus, was increased at the genus level. We speculate that the differences may be related to the different diet, geographical background and disease duration and other factors.
The targeted metabolomics method was applied to detect the change of amino acid content in feces. We found that there was a significant decrease in both absolute and relative concentrations for phenylalanine, tyrosine and isoleucine by quantitative analysis of amino acid concentrations in PD patient's fecal samples. However, the alteration of amino acid concentration in feces is inconsistent with that in the serum, which may be related to gastrointestinal symptoms in PD. PD patients are susceptible to delayed gastric emptying and slow digestion of the small intestine , and ultimately may lead to impaired absorption of amino acids.
Meanwhile, most intestinal microbes affect the host through their metabolites. For example, it has been reported that phenylalanine, tyrosine, leucine, isoleucine and valine were inversely correlated with Bacteroides that includes B. thetaiotaomicron, B. intestinalis, B. ovatus and B. uniformis . In addition, a study by Fan et al reported that the high concentration of dietary protein can increase the abundance of Rikenellaceae_RC9_gut_group . And then we found that Rikenellaceae_ RC9_gut_group in PD patients was negatively correlated with phenylalanine, tyrosine and isoleucine in feces, indicating that Rikenellaceae_ RC9_ gut_group may be a microorganism closely related to PD by affecting the metabolism of amino acids. However, Rikenellaceae_RC9_ gut_ group whether affects PD through phenylalanine, tyrosine and isoleucine remain to be experimentally verified.
It is worth noting that amino acids can participate in the pathogenesis of PD via a variety of mechanisms. BCAAs, especially leucine, can regulate the activity of mTORC1 to promote protein synthesis, and then accelerate the growth and proliferation of proinflammatory immune cells, such as Th1 and Th17 . Superoxide anion and nitric oxide (NO) free radicals are produced by cells of the immune system in inflammatory processes, which then form the nitrogen radical peroxynitrite (ONOO−) that leads to mitochondrial damage and dopaminergic neuron death. And it is reported that the proportion of Th1 cells in peripheral blood of patients with PD was increased . Aromatic amino acids (AAAs) are precursors of serotonin, dopamine and norepinephrine neurotransmitters. As previously described, serum tryptophan levels may be associated with depression and gastrointestinal regulation disorder . Taken together, various kinds of amino acids play different roles in PD, which may lead to different symptoms of PD patients.
There are still some limitations of the present study. Firstly, in order to reflect overall body metabolism, different biological samples (feces, blood, cerebrospinal fluid and et al.) from the same patients should be analyzed in the same study. Secondly, although it was proven that levodopa and large neutral amino acids (LNAAs) use the same transporter when crossing the blood-brain barrier , whether levodopa interferes with the plasma levels of BCAAs and AAAs in patients with PD is still unclear. Since all PD patients in our study were taking anti-parkinsonian medication, the medications were unlikely to be the main confounders. Finally, in order to obtain more detailed information about the microbiota, it is necessary to conduct metagenome and transcriptome analysis, which are conducive to the identification of BCAAs and AAAs specific strains.