Gut-brain Interactions in Parkinson’s Disease

Introduction

Parkinson’s disease (PD) is a neurodegenerative disorder characterized by the progressive cell death of dopaminergic neurons and the cytoplasmic aggregation of the protein alpha-synuclein. Gut microbiota has been found to play an important role in the pathology, influencing both the accumulation of Alpha Syn and the activation of microglia responsible for the inflammatory response to neuronal damage. Gastrointestinal tract (GIT) disorders account for one of the most common non-motor symptoms with 50-80% of PD patients displaying such dysfunctions. This alteration of bowel function, mainly in the form of constipation, can precede the prototypical motor symptoms of Parkinson’s disease by over a decade. There is a marked difference in the composition of gut microbial populations in PD patients versus healthy controls with the relative abundance of specific bacterial genera being sufficient to distinguish between the different forms of PD. The initiating factors of Parkinson’s disease are currently unknown. However, environmental factors influencing primarily through the gut are likely to play a key role, most probably against a background of genetic vulnerability. This essay will discuss how gut-brain interactions influence Parkinson’s disease by describing the proposed mechanisms by which the microbiome impacts its pathology. It will also explore the value of microbiota as biomarkers for early detection of the disease and the potential for therapeutics to relieve non-motor symptoms.

Pathophysiology of Parkinson’s disease

Parkinson’s disease impacts 1-3% of the population over the age of 65 and is a type of amyloidosis featuring the cytoplasmic accumulation of fibrils of α-synuclein. Despite the characterization of the disease relating to the residence of these neuropathological hallmarks in the dopaminergic neurons of the substantia nigra and striatum, there is evidence for αSyn accumulating in the gut and propagating via the vagus nerve to the brain. For example, vagotomised individuals who have had part of their vagus nerve removed are at a decreased risk of Parkinson’s disease. Neurons in the enteric nervous system and olfactory bulbs were also found to contain aggregated αSynuclein in early-stage PD patients. The vagal nerve is a crucial mediator of communication between gut microbiota and the brain and relates to the pathway by which the microbiome would exert its effects on the pathology of Parkinson’s disease. This ENS affection to the CNS is consistent with the clinical observation that GIT dysfunctions sometimes precede the motor symptoms of Parkinson’s disease by years. However, the diagnostic value of having these αSyn positive aggregates present in the ENS is uncertain due to their presence in the healthy colon mucosa. There is debate over whether PD does truly begin in the gut, however, the agreement is made over the potential role of the gut microbiome in Parkinson’s disease pathology.

αSynuclein derived from the exosomes of neurons in PD patients triggers the activation of microglia. These activated microglia are important in normal biological function and normally respond to neuronal damage by removing the damaged cells by phagocytic function. However, the chronic production of inflammatory products such as cytokines, reactive oxygen intermediates, proteinases, and complement proteins by microglia characterize the slow destructive process of Parkinson’s disease. There are several microbial molecules that mimic host structures and promote an immune response during the development of PD.

The resultant loss of dopaminergic neurons leads to the cardinal motor symptoms of Parkinson’s disease patients: rigidity, slowness of movement (termed bradykinesia), and a tremor. These symptoms progressively worsen as the disease advances.

Taxonomic structure of the microbiome in Parkinson’s disease

The human body possesses not solely its own genome but also harbors the genetic information of all the microorganisms living in and with it, the so-called microbiome. Current available studies of the colon, oral and nasal microbiome have shown results regarding PD-specific alterations to bacteria, most convincingly those of the GI tract. These studies are mainly based on the technique of 16S rRNA gene amplification. The technique allows the diversity of this single ribosomal gene to be surveyed, allowing analysis of the genetic diversity of microbial communities. However, it may be biased due to the unequal expression of the gene between species leading to a skewed perception of abundance. Deeper insight into the community biodiversity and function of the microbiome can be gained through metagenomic sequencing, a process which allows genes of thousands of organisms to be sampled in parallel. There are also drawbacks to this technique relating to its inability to provide deep enough analysis to detect rare species. Accumulatively these techniques have shown bacteria associated with an altered intestinal barrier or immune function to be either significantly over or underrepresented in PD patients. In particular, Akkermansia & Lactobacillus increase in relative abundance, whilst Facalibacterium and Prevotella genus decrease. It has not yet been established whether there is a temporal or causal relationship between the gut microbiota and the core features of the disease. However, this dysbiosis allows PD cases to be distinguished from healthy individuals at a very early stage. A comparable microbial shift is found in REM sleep behavior disorder which is a risk factor for the later development of synucleinopathies including Parkinson’s disease. And also, is sufficient to distinguish between different forms of PD. Patients with the tremor-dominant phenotype of Parkinson’s disease display significantly lower Enterobacteriaceae than those with more severe postural and gait instability. High Enterobacteriaceae abundance is also correlated to a more severe PD phenotype. This accumulation of evidence indicates that alterations in the human intestinal microbiome represent a risk factor for Parkinson’s disease and are correlated with disease progression.

Microbiome composition as a diagnostic marker for Parkinson’s disease

Following these discoveries, the possibility of using modifications in the microbiome as a marker for early diagnosis has been stated. However, confounding variables that can affect the microbiome were likely responsible for the results of early studies, as has been noted and subsequently controlled for. These include the DNA isolation methods and experimental cohorts being recruited from PD patients with variations in diet, age, the severity of the stage of disease, and current medication. Therefore, it is important to interpret the data of these studies with caution as reproducibility and consistency can be below.

For example, a 16S rRNA amplicon sequencing study conducted in 2015 by Scheperjans and his team found the prevotella enterotype to be underrepresented in PD patients. The link to PD pathogenesis was attributed to decreased mucin synthesis associated with increased gut permeability when prevotella levels are low. This could lead to local and systemic expression of bacterial endotoxin, an environmental trigger of PD, and increased alpha Syn expression in the colon. However, no PD association to prevotellacea family was reported in the 2017 study by Hill-Burns and her colleagues involving 16S rRNA sequencing with a larger sample size (197 compared to 72) and statistical analysis included to control potential confounders. Therefore, following suggestions such as those of the Scheperjans study to investigate the effects of supplementing neuroactive sort chain fatty acids and the vitamins thiamine and folate to account for this deficit in prevotella and act as a potential therapeutic in PD patients could be regarded as premature. Other papers have also highlighted the reported inconsistencies in the influence of the prevotella genus and have stated that the change in abundance may only be present in the early stages of Parkinson’s disease. Attempts have now been made to standardize protocols in microbial studies to allow easier comparison of results.

Proposed functional mechanisms of the microbiome’s impact on Parkinson’s Disease pathology

Under normal conditions, Akkermansia muciniphila exerts beneficial effects on the mucosal layer of the intestine and improves the barrier function of the gut epithelium. In PD patients the relative abundance of this species is increased from the normal equilibrium. This may seem counter-intuitive regarding its link to the pathogenesis of Parkinson’s disease; however, the mucus is utilized as its energy source and degradation of the barrier could be caused if bacterial abundance is too high. Without sufficient barrier function, microbial antigens of opportunistic pathogens may activate immune cells and thus have inflammatory potential. A schematic representation of the process by which microbiota may increase the permeability of the blood-gut and blood-brain barrier is found in figure 1. The diagram is not specific to Akkermansia though does highlight the general process by which a route of transmission between the gut and brain could be formed following microbial dysbiosis and formation of fibrillar αSynuclein induced. Products of the microbiota such as LPS and metabolites would then be able to gain access to the CNS via the compromised barriers with detrimental effects.

Increased Lactobacillacae is associated with decreased levels of ghrelin, a hormone known to regulate nigrostriatal dopamine function and possibly restrict neurodegeneration in PD. Lactobacilli are also thought to modulate the activity of neurons of the enteric nervous system which as discussed previously is thought to be implicated in Parkinson’s disease. This modulation may affect cellular α-synuclein secretion, also a hallmark of PD. Currently, these links are still conceptual with insufficient data to support them conclusively. However, Lactobacillacae levels are thought to increase in line with Parkinson’s disease progression which does indicate this family of bacteria is important to the pathology in some form.

There are reduced levels of Lachnaspiraceae bacteria reported in Parkinson’s disease patients. These bacteria produce short-chain fatty acids (SCFAs) which are microbial metabolites that play a role in PD. Depletion of SCFAs is an attractive hypothesis implicated in the pathogenesis of Parkinson’s disease and could potentially explain inflammation and microglial activation in the brain plus the gastrointestinal features of the disease. The molecules are also featured in the schematic of fig 1.

There is currently little evidence of one underlying bacterial change having a significant impact on Parkinson’s disease risk, it is more likely that changes in the complex equilibrium of the entire microbiome will be correlated to PD. However, certain bacteria are capable of mimicking host molecules which may initiate the same destructive immune response found in Parkinson’s disease. The unclassified bacteria operational taxonomic unit 469 expresses an endonuclease with a domain like αSyn. Despite the extremely low abundance of such bacteria, it is possible that as a consequence of specific interactions with the host, the pathophysiology of Parkinson’s disease can be significantly modified.

Conclusion

The link between the gut and Parkinson’s disease is well established. However, the characterization of the mechanisms by which the microbiome elucidates its effects is yet to be determined in full. Analysis of the bacterial composition of the colon may be predictive for PD allowing discrimination between its forms, but only once studies have determined the relevant biomarkers through analysis at the highest possible resolution with confounding variables adequately controlled for in reproducible studies. Beyond this, a functional link between dysbiosis of particular genera comprising the microbiome and the pathophysiological markers of PD requires more evidence. Once this has been established, it would be feasible to develop Parkinson’s disease therapeutics targeting aspects of the microbiome.

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