Gut microbiota dysbiosis in Parkinson disease: A systematic review and pooled analysis

The gut-brain axis in early Parkinson's disease: from prodrome to prevention

Parkinson's disease is the second most common neurodegenerative disorder and fastest growing neurological condition worldwide, yet its etiology and progression remain poorly understood. This disorder is characterized pathologically by the prion-like spread of misfolded neuronal alpha-synuclein proteins in specific brain regions leading to Lewy body formation, neurodegeneration, and progressive neurological impairment. It is unclear what triggers Parkinson's and where α-synuclein protein aggregation begins, although proposed induction sites include the olfactory bulb and dorsal motor nucleus of the vagus nerve. Within the last 20 years, there has been increasing evidence that Parkinson's could be triggered by early microbiome changes and α-synuclein accumulation in the gastrointestinal system. Gut microbiota dysbiosis that alters gastrointestinal motility, permeability, and inflammation could enable prion-like spread of α-synuclein from the gut-to-brain via the enteric nervous system. Individuals with isolated rapid eye movement sleep behavior disorder have a high likelihood of developing Parkinson's and might represent a prodromal 'gut-first' subtype of the condition. The gut-first model of Parkinson's offers novel gut-based therapeutic avenues, such as anti-, pre-, and pro-biotic preparations and fecal microbiota transplants. Crucially, gut-based interventions offer an avenue to treat Parkinson's at early prodromal stages with the aim of mitigating evolution to clinically recognizable Parkinson's disease characterized by motor impairment.

Gut microbiota regulation by Lactiplantibacillus plantarum SG5 enhances mitochondrial function in Parkinson's disease mice via the GLP-1/PGC-1α pathway

Motor dysfunction constitutes a prominent characteristic of Parkinson's disease (PD), a neurodegenerative disorder associated with compromised mitochondrial activity, perturbed gut microbial composition, and neuronal loss. In this study, we examined the regulatory mechanisms of Lactiplantibacillus plantarum SG5 (SG5) on mitochondrial function in PD mouse models, with a particular emphasis on its interaction with the GLP-1/PGC-1α pathway. Findings revealed that MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, MPTP) induced (male 6-8 weeks C57BL/6 mice) motor impairments and damage to dopaminergic (DA) neurons in PD mice, resulting in mitochondrial dysfunction, decreased mitochondrial biogenesis, disrupted dynamics, and autophagy, while promoting fission and apoptosis. Additionally, MPTP modified gut microbial diversity and community structure. Nevertheless, supplementation with SG5 alleviated motor deficits and DA neurons damage in PD mice, enhancing mitochondrial quality by elevating PGC-1α expression and restoring biogenesis, dynamics, and autophagy levels. Mechanistic investigations demonstrated that SG5 increased colonic GLP-1 expression, suggesting that GLP-1 might regulate mitochondrial function via the GLP-1R-mediated PGC-1α. Furthermore, SG5 counteracted MPTP-induced gut dysbiosis. Notably, both GLP-1R antagonists and PGC-1α inhibitors attenuated the protective effects of SG5 in PD mice. In conclusion, L. plantarum SG5 may enhance mitochondrial function in the substantia nigra (SN) of PD mice through the GLP-1/PGC-1α pathway, potentially delaying neurodegeneration. Its mechanism is closely related to the regulation of the gut microenvironment and GLP-1 levels, presenting novel microbiota-based therapeutic targets for PD.

James Parkinson and Other Historical Aspects of Parkinson's Disease

The history of Parkinson's disease is long and rich. Here, we discuss ancient remedies, James Parkinson's identification of the disease, Charcot's refinement of diagnostic criteria, and multiple other contributions to our current understanding. We also discuss the multiple treatment methods we have now and potential developments for the future.

A prebiotic dietary pilot intervention restores faecal metabolites and may be neuroprotective in Parkinson's Disease

Current treatment of Parkinson's Disease (PD) remains symptomatic, and disease-modifying approaches are urgently required. A promising approach is to modify intestinal microbiota and key metabolites of bacterial fermentation: short-chain fatty acids (SCFA), which are decreased in PD. A prospective, controlled pilot study (DRKS00034528) was conducted on 11 couples (PD patient plus healthy spouse as control (CO)). Participants followed a 4-week diet rich in dietary fibre, including intake of the prebiotic Lactulose. Gut metagenomes, faecal and urinary metabolites, and clinical characteristics were assessed. The dietary intervention significantly augmented faecal SCFA and increased Bifidobacteria spp., reducing PD-related gastrointestinal symptoms. The pre-existing bacterial dysbiosis in PD (depletion of Blautia, Dorea, Erysipelatoclostridium) persisted. Bacterial metabolite composition in faeces and urine positively changed with the intervention: Brain-relevant gut metabolic functions involved in neuroprotective and antioxidant pathways, including S-adenosyl methionine, glutathione, and inositol, improved in PD. These promising results warrant further investigation in larger cohorts.

Gut-brain axis and neuroplasticity in health and disease: a systematic review

The gut microbiota emerged as a potential modulator of brain connectivity in health and disease. This systematic review details current evidence on the gut-brain axis and its influence on brain connectivity. The initial set of studies included 532 papers, updated to January 2024. Studies were selected based on employed techniques. We excluded reviews, studies without connectivity focus, studies on non-human subjects. Forty-nine papers were selected. Employed techniques in healthy subjects included 15 functional magnetic resonance imaging studies (fMRI), 5 diffusion tensor imaging, (DTI) 1 electroencephalography (EEG), 6 structural magnetic resonance imaging, 2 magnetoencephalography, 1 spectroscopy, 2 arterial spin labeling (ASL); in patients 17 fMRI, 6 DTI, 2 EEG, 9 structural MRI, 1 transcranial magnetic stimulation, 1 spectroscopy, 2 R2*MRI. In healthy subjects, the gut microbiota was associated with connectivity of areas implied in cognition, memory, attention and emotions. Among the tested areas, amygdala and temporal cortex showed functional and structural differences based on bacteria abundance, as well as frontal and somatosensory cortices, especially in patients with inflammatory bowel syndrome. Several studies confirmed the connection between microbiota and brain functions in healthy subjects and patients affected by gastrointestinal to renal and psychiatric diseases.

Gut microbiota and Parkinson's disease

ABSTRACT

Emerging evidence suggests that dysbiosis of the gut microbiota is associated with the pathogenesis of Parkinson's disease (PD), a prevalent neurodegenerative disorder. The microbiota-gut-brain axis plays a crucial role in the development and progression of PD, and numerous studies have demonstrated the potential therapeutic benefits of modulations in the intestinal microbiota. This review provides insights into the characterization of the gut microbiota in patients with PD and highlights associations with clinical symptoms and underlying mechanisms. The discussion underscores the increased influence of the gut microbiota in the pathogenesis of PD. While the relationship is not fully elucidated, existing research demonstrates a strong correlation between changes in the composition of gut microbiota and disease development, and further investigation is warranted to explain the specific underlying mechanisms.

Outcomes of dietary interventions in the prevention and progression of Parkinson's disease: A literature review

Parkinson's disease (PD) is a progressive neurodegenerative disorder characterized by motor and non-motor symptoms, primarily due to the degeneration of dopaminergic neurons in the substantia nigra pars compacta (SNpc). Factors contributing to this neuronal degeneration include mitochondrial dysfunction, oxidative stress, and neuronal excitotoxicity. Despite extensive research, the exact etiology of PD remains unclear, with both genetic and environmental factors playing significant roles. Given the increasing prevalence of PD, particularly in aging populations, effective preventive and therapeutic strategies are urgently needed. Emerging research suggests that dietary interventions might offer promising approaches to managing PD progression. This literature review examines various dietary interventions that differ in their composition and mechanisms of action, including the Mediterranean, vegan, carnivore, paleo, and ketogenic diets, and their potential neuroprotective effects. By evaluating the current evidence, this review aims to identify dietary strategies that may improve the quality of life for individuals with PD. Additionally, it explores the underlying mechanisms through which diet may influence PD pathophysiology, thus providing insights into how nutritional modifications can be integrated into holistic management plans for the disease.

Fucoidan ameliorates rotenone-induced Parkinsonism in mice by regulating the microbiota-gut-brain axis

Microbiota-gut-brain axis, the bidirectional relationship between the gut microbiota and the brain, has been increasingly appreciated in the pathogenesis of Parkinson's disease (PD). Fucoidan, a sulphate-rich polysaccharide, has been shown to be neuroprotective by reducing oxidative stress in PD models. However, the role of microbiota-gut-brain axis in the neuroprotective activity of fucoidan has not been revealed. In this study, the therapeutic effects of fucoidan and involvement of microbiota-gut-brain axis in rotenone (ROT)-induced PD were investigated. The results showed that fucoidan gavage attenuated neuroinflammation, dopamine neuronal damage and motor dysfunction in ROT-induced PD mice. In addition, fucoidan treatment ameliorated gut dysfunction, intestinal inflammation and disruption of the intestinal barrier in PD mice. Fucoidan also affected the composition of gut microbiota in PD mice, indicated particularly by decreased abundance of Akkermansia muciniphila and Lactobacillus johnsonii and increased abundance of Lactobacillus murinus. Mechanistic studies showed that fecal microbiota transplantation (FMT) from the fucoidan-treated mice and probiotic Lactobacillus murinus supplement are as potent as fucoidan treatment in attenuating peripheral and central inflammation and ameliorating dopamine neuronal damage, which might be attributed to the downregulation of LPS/TLR4/NF-κB signaling pathway. Our study suggests that fucoidan might be potential candidates for the treatment of PD.

Breaking barriers in Parkinson's care: the multidisciplinary team approach

Parkinson's disease is a complex neurodegenerative disorder presenting a range of motor and non-motor symptoms that greatly impact both patients and caregivers. The diverse needs arising from these symptoms make a multidisciplinary team (MDT) approach crucial for effective management. This article explores the role and benefits of MDTs in Parkinson's care, highlighting how collaborative models improve clinical outcomes and quality of life. MDTs integrate neurologists, nurse specialists, therapists, and other professionals to deliver comprehensive, patient-centered care. The inclusion of patients and caregivers fosters shared decision-making, enhancing health outcomes. However, challenges like limited controlled trials, lack of comprehensive guidelines, and under-referral remain. Innovative models, such as telehealth and community-based care, offer promising solutions, especially in underserved regions. The article advocates for further research and standardized guidelines to optimize the MDT approach for Parkinson's disease.

Microbial biomarker discovery in Parkinson's disease through a network-based approach

Associations between the gut microbiota and Parkinson's disease (PD) have been widely investigated. However, the replicable biomarkers for PD diagnosis across multiple populations remain elusive. Herein, we performed a meta-analysis to investigate the pivotal role of the gut microbiome in PD and its potential diagnostic implications. Six 16S rRNA gene amplicon sequence datasets from five independent studies were integrated, encompassing 550 PD and 456 healthy control samples. The analysis revealed significant alterations in microbial composition and alpha and beta diversity, emphasizing altered gut microbiota in PD. Specific microbial taxa, including Faecalibacterium, Roseburia, and Coprococcus_2, known as butyrate producers, were notably diminished in PD, potentially contributing to intestinal inflammation. Conversely, genera such as Akkermansia and Bilophila exhibited increased relative abundances. A network-based algorithm called NetMoss was utilized to identify potential biomarkers of PD. Afterwards, a classification model incorporating 11 optimized genera demonstrated high performance. Further functional analyses indicated enrichment in pathways related to neurodegeneration and metabolic pathways. These findings illuminate the intricate relationship between the gut microbiota and PD, offering insights into potential therapeutic interventions and personalized diagnostic strategies.

Lactiplantibacillus plantarum SG5 inhibits neuroinflammation in MPTP-induced PD mice through GLP-1/PGC-1α pathway

Mounting evidence suggests that alterations in gut microbial composition play an active role in the pathogenesis of Parkinson's disease (PD). Probiotics are believed to modulate gut microbiota, potentially influencing PD development through the microbiota-gut-brain axis. However, the potential beneficial effects of Lactiplantibacillus plantarum SG5 (formerly known as Lactobacillus plantarum, abbreviated as L. plantarum) on PD and its underlying mechanisms remain unclear. In this study, we employed immunofluorescence, Western blotting, ELISA, and 16S rRNA gene sequencing to investigate the neuroprotective effects of L. plantarum SG5 against neuroinflammation in an MPTP-induced PD model and to explore the underlying mechanisms. Our results demonstrated that L. plantarum SG5 ameliorated MPTP-induced motor deficits, dopaminergic neuron loss, and elevated α-synuclein protein levels. Furthermore, SG5 inhibited MPTP-triggered overactivation of microglia and astrocytes in the substantia nigra (SN), attenuated disruption of both blood-brain and intestinal barriers, and suppressed the release of inflammatory factors in the colon and SN. Notably, SG5 modulated the composition and structure of the gut microbiota in mice. The MPTP-induced decrease in colonic GLP-1 secretion was reversed by SG5 treatment, accompanied by increased expression of GLP-1R and PGC-1α in the SN. Importantly, the GLP-1R antagonist Exendin 9-39 and PGC-1α inhibitor SR18292 attenuated the protective effects of SG5 in PD mice. In conclusion, we demonstrate a neuroprotective role of L. plantarum SG5 in the MPTP-induced PD mouse model, which likely involves modulation of the gut microbiota and, significantly, the GLP-1/PGC-1α signaling pathway.

Implications of Butyrate Signaling Pathways on the Motor Symptomatology of Parkinson's Disease and Neuroprotective Effects-Therapeutic Approaches: A Systematic Review

Parkinson's Disease (PD) is a prevalent neurodegenerative disorder characterized by motor and non-motor symptoms. Emerging evidence suggests that gut microbiota alterations, specifically involving short-chain fatty acids (SCFAs) like butyrate, may influence PD pathogenesis and symptomatology. This Systematic Review aims to synthesize current research on the role of butyrate in modulating motor symptoms and its neuroprotective effects in PD, providing insights into potential therapeutic approaches. A systematic literature search was conducted in April 2024 across databases, including ScienceDirect, Scopus, Wiley, and Web of Science, for studies published between 2000 and 2024. Keywords used were "neuroprotective effects AND butyrate AND (Parkinson disease OR motor symptoms)". Four authors independently screened titles, abstracts, and full texts, applying inclusion criteria focused on studies investigating butyrate regulation and PD motor symptoms. A total of 1377 articles were identified, with 40 selected for full-text review and 14 studies meeting the inclusion criteria. Data extraction was performed on the study population, PD models, methodology, intervention details, and outcomes. Quality assessment using the SYRCLE RoB tool highlighted variability in study quality, with some biases noted in allocation concealment and blinding. Findings indicate that butyrate regulation has a significant impact on improving motor symptoms and offers neuroprotective benefits in PD models. The therapeutic modulation of gut microbiota to enhance butyrate levels presents a promising strategy for PD symptom management.

Investigating the response of the butyrate production potential to major fibers in dietary intervention studies

Interventions involving dietary fibers are known to benefit host health. A leading contribution of gut microbiota is commonly recognized with production of short chain fatty acids (SCFA) suspected to play a key role. However, the detailed mechanisms are largely unknown, and apart from a well-described bifidogenic effect of some fibers, results for other bacterial taxa are often incongruent between studies. We performed pooled analyses of 16S rRNA gene data derived from intervention studies (n = 14) based on three fibers, namely, inulin-type fructans (ITF), resistant starch (RS), and arabinoxylan-oligosaccharides (AXOS), harmonizing the bioinformatics workflow to reveal taxa stimulated by those substrates, specifically focusing on the SCFA-production potential. The results showed an increased butyrate production potential after ITF (p < 0.05) and RS (p < 0.1) treatment via an increase in bacteria exhibiting the enzyme butyryl-CoA:acetate CoA-transferase (but) that was governed by Faecalibacterium, Anaerostipes (ITF) and Agathobacter (RS) respectively. AXOS did not promote an increase in butyrate producers, nor were pathways linked to propionate production stimulated by any intervention. A bifidogenic effect was observed for AXOS and ITF, which was only partly associated with the behavior of but-containing bacteria and largely represented a separate response. Low and high Ruminococcus abundances pre-intervention for ITF and RS, respectively, promoted an increase in but-containing taxa (p < 0.05) upon interventions, whereas initial Prevotella abundance was negatively associated with responses of butyrate producers for both fibers. Collectively, our data demonstrate targeted stimulation of specific taxa by individual fibers increasing the potential to synthesize butyrate, where gut microbiota composition pre-intervention strongly controlled outcomes.

The gut metabolome in a cohort of pregnant and lactating women from Antioquia-Colombia

Nutrition during the perinatal period is an essential component of health and one that can severely impact the correct development of a human being and its overall condition, in all the subsequent stages of life. The availability of several compounds, mainly macronutrients and micronutrients, plays a key role in the balanced nutrition of both mother and baby and is a process with direct relation to the gut microbiome. Thus, we hereby refer to the set of small molecules derived from gut microbiome metabolism as the gut metabolome. These continuous processes occurring in the gut of a gestating or lactating mother related to microbial communities and nutrients, can be revealed by metabolomics. In this study, we explore for the first time the gut metabolome of pregnant and lactating women, from our region of Antioquia-Colombia, applying untargeted metabolomics by LC-QTOF-MS, and molecular networking. Regarding the gut metabolome composition of the cohort, we found, key metabolites that can be used as biomarkers of microbiome function, overall metabolic health, dietary intake, pharmacology, and lifestyle. In our cohort, pregnant women evidenced a significantly higher abundance of prostaglandins, alkaloids, corticosteroids, organosilicons, and natural toxins, while in lactating women, lipids stand out. Our results suggest that unveiling the metabolic phenotype of the gut microbiome of an individual, by untargeted metabolomics, allows a broad visualization of the chemical space present in this important niche and enables the recognition of influential indicators of the host's health status and habits, especially of women during this significant perinatal period. This study constitutes the first evidence of the use of untargeted LC-QTOF-MS coupled with molecular networking analysis, of the gut microbiome in a Colombian cohort and establishes a methodology for finding relative abundances of key metabolites, with potential use in nutritional and physiological state assessments, for future personalized health and nutrition practices.

The Effect of Gut Microbiota-Targeted Interventions on Neuroinflammation and Motor Function in Parkinson's Disease Animal Models-A Systematic Review

Gut microbiome-targeted interventions such as fecal transplant, prebiotics, probiotics, synbiotics, and antibiotic gut depletion are speculated to be of potential use in delaying the onset and progression of Parkinson's disease by rebalancing the gut microbiome in the context of the gut-brain axis. Our study aims to organize recent findings regarding these interventions in Parkinson's disease animal models to identify how they affect neuroinflammation and motor outcomes. A systematic literature search was applied in PubMed, Web of Science, Embase, and SCOPUS for gut microbiome-targeted non-dietary interventions. Studies that investigated gut-targeted interventions by using in vivo murine PD models to follow dopaminergic cell loss, motor tests, and neuroinflammatory markers as outcomes were considered to be eligible. A total of 1335 studies were identified in the databases, out of which 29 were found to be eligible. A narrative systematization of the resulting data was performed, and the effect direction for the outcomes was represented. Quality assessment using the SYRCLE risk of bias tool was also performed. Out of the 29 eligible studies, we found that a significant majority report that the intervention reduced the dopaminergic cell loss (82.76%, 95% CI [64.23%, 94.15%]) produced by the induction of the disease model. Also, most studies reported a reduction in microglial (87.5%, 95% CI [61.65%, 98.45%]) and astrocytic activation (84,62%, 95% CI [54.55%, 98.08%]) caused by the induction of the disease model. These results were also mirrored in the majority (96.4% 95% CI [81.65%, 99.91%]) of the studies reporting an increase in performance in behavioral motor tests. A significant limitation of the study was that insufficient information was found in the studies to assess specific causes of the risk of bias. These results show that non-dietary gut microbiome-targeted interventions can improve neuroinflammatory and motor outcomes in acute Parkinson's disease animal models. Further studies are needed to clarify if these benefits transfer to the long-term pathogenesis of the disease, which is not yet fully understood. The study had no funding source, and the protocol was registered in the PROSPERO database with the ID number CRD42023461495.

Gut microbiome is not associated with mild cognitive impairment in Parkinson's disease

Gut microbiome differences between people with Parkinson's disease (PD) and control subjects without Parkinsonism are widely reported, but potential alterations related to PD with mild cognitive impairment (MCI) have yet to be comprehensively explored. We compared gut microbial features of PD with MCI (n = 58) to cognitively unimpaired PD (n = 60) and control subjects (n = 90) with normal cognition. Our results did not support a specific microbiome signature related to MCI in PD.

Gut Microbiome Changes in Patients With Idiopathic Normal Pressure Hydrocephalus

BACKGROUND

The gut microbiome is a complex system within the human gastrointestinal tract. The bacteria play a significant role in human health, and some can promote inflammation and pathologic processes through chemical interactions or metabolites. Gut microbiome dysbiosis has been linked to some neurological and other diseases. Here we aimed to examine microbiome differences between patients with a progressive neurological disorder, idiopathic normal pressure hydrocephalus (iNPH), compared with healthy controls (CO).

METHODS

We recruited 37 neurologically healthy CO and 10 patients with shunted iNPH. We evaluated these participants' cognition using the CERAD-NB test battery and CDR test, and collected a variety of information, including about dietary habits and health. We also collected fecal samples, which were subjected to 16S amplicon sequencing to analyze differences in gut microbiome composition.

RESULTS

We found that the iNPH group exhibited significantly different abundances of 10 bacterial genera compared with the CO group. The Escherichia/Shigella and Anaeromassilibacillus genera were most remarkably increased. Other increased genera were Butyrivibrio , Duncaniella , and an unidentified genus. The decreased genera were Agathobaculum , Paramuribaculum , Catenibacterium , and 2 unidentified genera.

CONCLUSIONS

Here we report the first identified microbiome differences in iNPH patients compared with healthy controls.

Dietary Interventions in Parkinson's Disease

Several dietary patterns and nutritional supplements have been linked to the development, progression, and symptomatic treatment of Parkinson's disease (PD). Most of the evidence, at this point, is preliminary and based largely on observational studies. Interventional studies are scarce, so the evidence on effectiveness remains inconclusive. Dietary interventions could, analogous to exercise, potentially have a beneficial effect on disease symptoms as well as on the progression of the disease and should therefore be researched in high quality studies. Further work is also needed to study whether dietary interventions, when applied to an at-risk population, have any potential to postpone the onset of manifest PD. In this paper, we summarize all ongoing clinical trials on dietary interventions in PD. We found 10 ongoing studies, all aimed at a different intervention. These studies are mostly exploratory in nature or represent phase I or phase II trials focusing on safety, biological responses, and symptomatic effects. Taken together, we conclude that research on dietary interventions in persons with PD is still in its early days. The results of the various ongoing trials are expected to generate new hypotheses and will help to shape the agenda for future research on this important topic.

The Dose-Response Effect of Fluoride Exposure on the Gut Microbiome and Its Functional Pathways in Rats

Metabolic activities within the gut microbiome are intimately linked to human health and disease, especially within the context of environmental exposure and its potential ramifications. Perturbations within this microbiome, termed "gut microbiome perturbations", have emerged as plausible intermediaries in the onset or exacerbation of diseases following environmental chemical exposures, with fluoride being a compound of particular concern. Despite the well-documented adverse impacts of excessive fluoride on various human physiological systems-ranging from skeletal to neurological-the nuanced dynamics between fluoride exposure, the gut microbiome, and the resulting dose-response relationship remains a scientific enigma. Leveraging the precision of 16S rRNA high-throughput sequencing, this study meticulously examines the ramifications of diverse fluoride concentrations on the gut microbiome's composition and functional capabilities within Wistar rats. Our findings indicate a profound shift in the intestinal microbial composition following fluoride exposure, marked by a dose-dependent modulation in the abundance of key genera, including Pelagibacterium, Bilophila, Turicibacter, and Roseburia. Moreover, discernible alterations were observed in critical functional and metabolic pathways of the microbiome, such as D-lyxose ketol-isomerase and DNA polymerase III subunit gamma/tau, underscoring the broad-reaching implications of fluoride exposure. Intriguingly, correlation analyses elucidated strong associations between specific bacterial co-abundance groups (CAGs) and these shifted metabolic pathways. In essence, fluoride exposure not only perturbs the compositional equilibrium of the gut microbiota but also instigates profound shifts in its metabolic landscape. These intricate alterations may provide a mechanistic foundation for understanding fluoride's potential toxicological effects mediated via gut microbiome modulation.

The link between the gut microbiome, inflammation, and Parkinson's disease

As our society ages, the growing number of people with Parkinson's disease (PD) puts tremendous pressure on our society. Currently, there is no effective treatment for PD, so there is an urgent need to find new treatment options. In recent years, increasing studies have shown a strong link between gut microbes and PD. In this review, recent advances in research on gut microbes in PD patients were summarized. Increased potential pro-inflammatory microbes and decreased potential anti-inflammatory microbes are prominent features of gut microbiota in PD patients. These changes may lead to an increase in pro-inflammatory substances (such as lipopolysaccharide and H2S) and a decrease in anti-inflammatory substances (such as short-chain fatty acids) to promote inflammation in the gut. This gut microbiota-mediated inflammation will lead to pathological α-synuclein accumulation in the gut, and the inflammation and α-synuclein can spread to the brain via the microbiota-gut-brain axis, thereby promoting neuroinflammation, apoptosis of dopaminergic neurons, and ultimately the development of PD. This review also showed that therapies based on gut microbiota may have a bright future for PD. However, more research and new approaches are still needed to clarify the causal relationship between gut microbes and PD and to determine whether therapies based on gut microbiota are effective in PD patients. KEY POINTS: • There is a strong association between gut microbes and PD. • Inflammation mediated by gut microbes may promote the development of PD. • Therapies based on the gut microbiome provide a promising strategy for PD prevention.

Gut Microbiota Composition in Patients with Neurodegenerative Disorders (Parkinson's and Alzheimer's) and Healthy Controls: A Systematic Review

This systematic review aims to provide a comprehensive understanding of the current literature regarding gut microbiota composition in patients with Parkinson's disease (PD) and Alzheimer's disease (AD) compared to healthy controls. To identify the relevant studies, a thorough search of PubMed, Medline, and Embase was conducted following the PRISMA guidelines. Out of 5627 articles, 73 studies were assessed for full-text eligibility, which led to the inclusion of 42 studies (26 PD and 16 AD studies). The risk of bias assessment showed a medium risk in 32 studies (20 PD studies and 12 AD studies), a low risk in 9 studies (5 PD studies and 4 AD studies), and 1 PD study with a high risk. Among the PD studies, 22 out of 26 studies reported a different gut microbiota composition between the PD cases and the healthy controls, and 15 out of 16 AD studies reported differences in gut microbiota composition between the AD cases and the healthy controls. The PD and AD studies consistently identified the phyla Bacteroidetes, Firmicutes, and Proteobacteria as prevalent in the gut microbiota in both the healthy groups and the case groups. Microbial dysbiosis was specifically characterized in the PD studies by a high abundance of Akkermansia, Verrucomicrobiaceae, Lachnospiraceae, and Ruminococcaceae in the cases and a high abundance of Blautia, Coprococcus, Prevotellaceae, and Roseburia in the controls. Similarly, Bacteroides and Acidobacteriota were abundant in the AD cases, and Acidaminococcaceae, Firmicutes, Lachnospiraceae, and Ruminiclostridium were abundant in the AD controls. The microbial signature assessment showed the association of several microbial taxa, including Akkermansia, Lachnospiraceae, Verrucomicrobiaceae, Bifidobacterium, Ruminococcacea, and Verrucomicrobia with PD and Ruminococcaceae, Bacteroides, and Actinobacteria with AD. The microbial diversity evaluations in the PD and AD studies indicated comparable alpha diversity in some groups and distinct gut microbiota composition in others, with consistent beta diversity differences between the cases and the controls across multiple studies. The bacterial signatures identified in this study that are associated with PD and AD may offer promising prospects for efficient management and treatment approaches.

[Relationships between microbiome and neurodegeneration]

BACKGROUND

Neurodegenerative diseases are often associated with changes in the (gut) microbiome.

OBJECTIVE

Based on studies in Parkinson's disease (PD) and Alzheimer's disease (AD), an overview of the current evidence of microbial changes and their possible role in the development of these diseases is given.

METHODS

Analysis, summary, and evaluation of the current literature on (gut) microbiome and neurodegeneration.

RESULTS

Numerous studies have shown dysbiotic changes in the gut microbiome of PD and AD patients compared to healthy individuals, some of which might occur even in the prodromal phase. Specifically, these patients show a reduction in bacteria involved in the synthesis of short-chain fatty acids. These microbial alterations have been associated with systemic inflammation and a compromised integrity of the intestinal barrier and blood-brain barrier. Bacterial molecules such as lipopolysaccharides may play an important role in these changes. Additionally, the bacterial protein curli, found on the surface of e.g., Escherichia coli, has been shown in vitro and in animal models to promote the misfolding of α-synuclein, thus suggesting a crucial pathomechanism. Moreover, certain oral bacteria appear to be more prevalent in AD patients and may contribute to the pathogenesis of AD.

CONCLUSION

Neurodegenerative diseases are associated with dysbiosis of the (gut) microbiome, which can have diverse systemic effects; however, it remains unclear whether this dysbiosis is a cause or a consequence of the diseases. Further investigation of this (prodromal) microbial imbalance could reveal new approaches for targeted therapeutic manipulation of the microbiome to modify and prevent these diseases.

Emerging role of the host microbiome in neuropsychiatric disorders: overview and future directions

The human body harbors a diverse ecosystem of microorganisms, including bacteria, viruses, and fungi, collectively known as the microbiota. Current research is increasingly focusing on the potential association between the microbiota and various neuropsychiatric disorders. The microbiota resides in various parts of the body, such as the oral cavity, nasal passages, lungs, gut, skin, bladder, and vagina. The gut microbiota in the gastrointestinal tract has received particular attention due to its high abundance and its potential role in psychiatric and neurodegenerative disorders. However, the microbiota presents in other body tissues, though less abundant, also plays crucial role in immune system and human homeostasis, thus influencing the development and progression of neuropsychiatric disorders. For example, oral microbiota imbalance and associated periodontitis might increase the risk for neuropsychiatric disorders. Additionally, studies using the postmortem brain samples have detected the widespread presence of oral bacteria in the brains of patients with Alzheimer's disease. This article provides an overview of the emerging role of the host microbiota in neuropsychiatric disorders and discusses future directions, such as underlying biological mechanisms, reliable biomarkers associated with the host microbiota, and microbiota-targeted interventions, for research in this field.

Conditioning of the immune system by the microbiome

The human intestinal microbiome has coevolved with its host to establish a stable homeostatic relationship with hallmark features of mutualistic symbioses, yet the mechanistic underpinnings of host-microbiome interactions are incompletely understood. Thus, it is an opportune time to conceive a common framework for microbiome-mediated regulation of immune function. We propose the term conditioned immunity to describe the multifaceted mechanisms by which the microbiome modulates immunity. In this regard, microbial colonization is a conditioning exposure that has durable effects on immune function through the action of secondary metabolites, foreign molecular patterns, and antigens. Here, we discuss how spatial niches impact host exposure to microbial products at the level of dose and timing, which elicit diverse conditioned responses.

Parkinson's Disease and Diabetes Mellitus: Synergistic Effects on Pathophysiology and GI Motility

PURPOSE OF REVIEW

Parkinson's disease and diabetes affect an increasing proportion of the aging global population. Both conditions extensively affect gastrointestinal (GI) motility with similar and differing clinical symptoms. Nonetheless, GI symptoms in Parkinson's disease and diabetes pose significant morbidity and impairment of quality of life. Their pathophysiology is poorly understood, and therefore, effective treatment options are lacking.

RECENT FINDINGS

Parkinson's disease patients have oropharyngeal dysphagia and constipation. They also have mild or absent upper GI symptoms associated with delayed gastric emptying, which is prevalent in 70% of patients. Delayed gastric emptying in Parkinson's disease leads to erratic medication absorption and fluctuating motor symptoms. Half of diabetics have upper GI symptoms, which correlate to gastric emptying and changes in brain activity of the insular cortex. The majority of diabetics also have constipation. Diabetics have an increased risk for developing Parkinson's disease and anti-diabetic medications are associated with risk reduction of developing Parkinson's disease. Hyperglycemia is associated with advanced glycated end products formation and acceleration of α-synuclein aggregation. GLP-1 receptor agonists have also demonstrated efficacy in improving motor symptoms and cognition in Parkinson's disease patients with diabetes. Parkinson's disease and diabetes are pan-enteric disorders with significant GI symptoms and impairment of gut motility. Both conditions have synergistic pathophysiologies that propagate neurodegenerative changes. Treatment options for GI symptoms in diabetic and Parkinson's disease patients are lacking. Anti-diabetic treatment improves motor symptoms in Parkinson's disease, however, its effect on GI symptoms is unclear.