The classical symptoms of PD are characterized by progressive motor dysfunction, including rest tremor, bradykinesia, rigidity, and postural instability, which is noticeable when there is already 60% dopaminergic neuronal loss in the substantia nigra.
In addition to motor symptoms, non-motor manifestations such as rapid eye movement sleep behavior disorder (RBD), gastrointestinal dysfunction, olfactory disruption, neuropsychiatric symptoms, and sensory dysfunction also exist in patients with PD and compromise their quality of life.5 Of note, these non-motor PD symptoms can occur years before the onset of classical motor symptoms.6 Thus, they are therefore now considered prodromal clinical markers before the onset of the classical motor manifestations, according to the International Movement Disorders Society.7 Among these prodromal symptoms, constipation is the most prevalent and earliest pre-motor feature and can precede motor symptoms by decades.
The pathological hallmark of PD is progressive dopaminergic neuronal degeneration and intraneuronal accumulations of misfolded α-synuclein (Lewy bodies) in the substantia nigra.3 The pathological α-synuclein can transmit from cell to cell in a prion-like fashion to promote the neurodegenerative process of this disease.9,10 Recent postmortem evidence indicates that Lewy body pathology is first detectable outside the brain, starting from neurons in the gut enteric nervous system (ENS) and olfactory bulbs.11 These neuropathology observations are consistent with the findings that non-motor symptoms of PD, especially constipation, can precede the onset of motor symptoms.8 Further in vivo PD animal model studies demonstrated that pathological forms of α-synuclein, after injection into the intestinal wall, can be transported from the gut to the brain via the vagus nerve, reaching the dorsal motor nucleus of the vagus nerve in the brainstem.12, 13, 14 However, although this pathology finding has given rise to the idea that PD pathology originates in the peripheral ENS and then invades the brain via retrograde axonal transport through the vagus nerve, a small fraction of patients do not show pathology in the ENS. In about one-third of patients, the PD neuropathology begins in the brain itself and then travels downward.15 Hence, it is hypothesized that PD can be divided into a gut-first (body first) and a brain-first subtype.15 The former is tightly associated with chronic constipation and RBD during the prodromal phase and the latter is most often sparing of gastrointestinal symptoms and is RBD-negative during the prodromal phase. These findings reinforce the concept that PD is a heterogeneous disorder with diverse initial triggers and propagation trajectories of α-synuclein, suggesting that tailored disease-modifying therapy is needed for patients with different subtypes.
The various recently observed gut microenvironmental changes in the early stages of the disease may play a vital role in PD, especially those with the body-first subtype of the disease. Patients whose disease begins in the gut may benefit most from interventions that target the gut. In this review, we summarize recent evidence for altered gut microenvironments contributing to PD through the gut–brain axis. Furthermore, there is a plethora of evidence, including our previous study, for altered gut microbiota in patients with PD compared with unaffected controls.
There is much evidence that the vagus nerve transports α-synuclein from the gut to the brain.12 Pathological α-synuclein fibrils injected into the duodenum can move from the muscular layer of the duodenum to the brain and then from neuron to neuron across the synapses through the vagus nerve in a PD animal model.12 Furthermore, recent evidence indicates that protein nucleation and aggregation may be influenced by E. coli's secretion of curli, which induces neuronal deposition of α-synuclein in the ENS.29,31 The abundance of E. coli at the colonic mucosa correlates with enteric α-synuclein deposition in PD patients.
A plethora of studies have investigated changes in the gut microbiota in patients with PD compared with healthy controls using either 16 S rRNA gene amplicon surveys or shotgun metagenomic sequencing analysis.35, 36, 37, 38, 39, 40, 41, 42 Meta-analyses have shown that the relative abundance of various phyla of anti-inflammatory and short-chain fatty acid (SCFA)-producing bacteria, including Blautia, Coprococcus, Roseburia, Lachnospira, Fusicatenibacter, and Faecalibacterium, are reduced in PD patients compared with controls. In contrast, the amounts of Lactobacillus, Bifidobacterium, and Akkermansia are higher in PD patients than in unaffected participants from different ethnicities.38, 39, 40, 41 Notably, opportunistic pathogens and pro-inflammatory bacteria at the phylum level, including Corynebacterium, Porphyromonas, Alistipes, Bacteroides, Escherichia, and Megasphaera, are also enriched in PD.40,43 In metagenomic research, gene markers from the gut microbiome were found to accurately discriminate PD patients from healthy controls, with most of the identified markers belonging to Bacteroides and Escherichia species.44 Although PD medications affect the structure of the gut microbiota, these changes are detectable in drug-naïve early-stage PD patients.36,43,45
Changes in the gut microbiota also correlate with disease progression in PD. A decrease in the SCFA-producing microbiota and increase in pro-inflammatory bacteria correlate with motor and cognitive severity in patients with PD.38,40,46 Compared with transplantation of fecal gut microbiota from healthy donors, such transplantation from PD patients leads to worsened motor symptoms in a transgenic rodent model of PD.47 A 3-year longitudinal follow-up study of PD patients revealed that a reduced amount of Roseburia species predicted faster progression of both motor and non-motor symptoms of PD.48 A lower abundance of SCFA-producing bacteria, including Fusicatenibacter and Faecalibacterium, correlates with elevated fecal inflammatory calprotectin levels in PD patients.49 Furthermore, enrichment in Bacteroides and Bifidobacterium has been linked to elevated expression of systemic and fecal inflammatory markers IFN-γ, TNF-α, and neutrophil gelatinase-associated lipocalin in patients with PD.42,50 The increased levels of Lactobacillaceae and Bifidobacteriaceae in PD patients require further investigation, as they are usually recognized as probiotics for improving constipation.51 Therefore, distinct gut microbiota species promote enteric α-synuclein aggregation or gut inflammation to facilitate the occurrence and progression of PD.
The composition of gut microbiota also influences the pharmaceutical treatment responses in PD patients. The growing literature has shown the role of the gut microbiome in the pharmacokinetics of prescription drugs and the effects that the drugs can have in turn on the composition of the gut microbiome,52,53 indicating a potential interaction between PD medications and the microbiome. Prior studies have shown that anti-PD medications, including catechol-O-methyl transferase (COMT) inhibitors and anticholinergics, have gastrointestinal side effects, which may be related to the changes of the gut microbiome.54,55 Furthermore, bacterial tyrosine decarboxylase, which can convert levodopa to dopamine, could limit its bioavailability and may contribute to the interindividual responses to levodopa treatment among patients with PD.56,57 Enterococcus faecalis was found to be the dominant microorganism responsible for levodopa decarboxylation and restrict levels of levodopa in the treatment of PD.57 Dopamine produced by gut bacterial metabolism of levodopa decarboxylation can also impair intestinal motility, which could provide an explanation for bacterial overgrowth in the small intestine associated with motor fluctuation in PD.58 These observations lend support to the notion that the composition of the gut microbiome may affect the treatment efficacy and potential side effects of levodopa treatment in patients with PD.
Srivastav et al. treated animals with an oral probiotic mixture containing Lactobacillus rhamnosus GG, Bifidobacterium animalis lactis, and Lactobacillus acidophilus for 30 days, after which the mice were given MPTP injections.62 The results showed that mice receiving this probiotic mixture reduced dopaminergic neurodegeneration by upregulating neurotrophic factors and increasing striatal neuronal responses to dopamine.62 Using the same toxin-induced PD model, Liao et al. fed mice with Lacobacillus plantarum PS128 for 28 days, then gave them MPTP injections for 4 days. The results showed that feeding with Lacobacillus plantarum PS128 mitigated neuronal degeneration, attenuated oxidative stress and neuroinflammation, and rescued the locomotor defects of MPTP-injected PD mice.63 Sun et al. treated MPTP-injected mice with the probiotic Clostridium butyricum for 4 weeks and demonstrated improved motor deficits, attenuated dopaminergic neuron loss, improved synaptic dysfunction, and reduced microglia activation in the treated mice.64 This beneficial effect was associated with increased colonic levels of glucagon-like peptide-1 (GLP-1), colonic G protein–coupled receptors GPR41/43, and other components of the cerebral GLP-1 receptor pathway.57 Of note, the incretin hormone GLP-1 was recently found to regulate neurogenesis and synaptic plasticity,65 and GLP-1 agonists may be neuroprotective in PD pathogenesis.66 Similarly, a 3-week treatment with Lactobacillus fermentum U-21 reduced nigra dopaminergic cell loss in a paraquat-toxin model of PD.67
Several studies have demonstrated the potential benefits of probiotic supplementation in patients with PD. A total of seven clinical trials were identified, which were all randomized clinical trials68, 69, 70, 71, 72, 73, 74 (Table 1). Consumption of fermented milk containing multiple probiotic strains can improve constipation in patients with PD.62 Similar beneficial effects in improving bowel movements were also noted in PD patients in most of the probiotic clinical trials.51,73 Evidence seems to demonstrate that probiotic intake can improve bowel movement and reduce gastrointestinal symptoms. There are two studies measuring non-gastrointestinal symptoms of PD as one of the primary outcomes of probiotic clinical trials.71,74 Probiotic supplementation containing Bifidobacterium bifidum, L. acidophilus, L. fermentum and Lactobacillus reuteri for a period of 12 weeks has been observed to improve some symptoms in PD patients as measured by total MDS-UPDRS scores.71
One recent open-label clinical trial with 87 participants showed improvement of non-motor symptom scores, reduced fecal inflammatory marker of calprotectin and increased fecal butyrate in patients with PD who received prebiotic supplement with resistant starch compared to those without prebiotic intervention.78 Several in vivo studies have shown that lower abundance of SCFAs butyrate-producing bacteria could be corrected by the administration of prebiotic fibers, which in turn reduce the gut inflammatory processes, improve gut barrier function, and peristalsis.79,80 SCFAs have a key role in modulating the cross-talks in the gut–brain axis through modulating the gut barrier and blood–brain barrier integrity, inflammatory processes, inhibition of histone deacetylase to promote neuronal survival.81 Of note, an in vivo study using a transgenic a-synuclein–expressing mouse model demonstrated that a germ-free environment eliminates PD phenotypes but that oral feeding with SCFAs remerged the disease-related neuropathology by microglial activation.82 As the effects of SCFAs may depend on the concentration and the different subtypes, the effect of SCFAs in the PD process requires more studies.
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