In this review published in the Journal of Parkinson’s Disease scientists compare results of current research and provide recommendations to increase the comparability and utility of these studies with a view towards improving patient outcomes.
“Despite the large societal impact of PD, the underlying cause remains elusive and only symptomatic treatments exist,” explained Filip Scheperjans, MD, PhD, Department of Neurology, Helsinki University Hospital, Helsinki, Finland, lead author of the review and chief investigator of the study that first identified gut microbiome differences in PD.
“Since our study appeared in 2015, 15 human case-control studies with original data have found changes in the gut microbiome composition of PD patients, but their results vary.
The challenge is to identify whether these changes are actually relevant for PD patients and whether they play a role in the disease process.”
“As more studies investigate the gut microbiome composition in PD, it is important to compare the findings of these studies to get an overview of the changes present in the disease,” added the review’s first author, Jeffrey M. Boertien, MSc, Parkinson Expertise Center, Department of Neurology, University Medical Center Groningen, University of Groningen, The Netherlands.
“It is even more important to compare the methods used in the various studies. Especially as the studies report different and sometimes even contradictory results.”
This review systematically compares the methodologies and results of the currently available case-control fecal gut microbiome studies in PD, which encompass 16 studies, representing seven countries with study populations varying from 10-197 PD patients and 10-130 healthy controls.
These studies reported over 100 differentially abundant taxa covering all taxonomic levels, from phylum to genus or species, depending on methodology.
While several findings were replicated in various studies, such as an increase of Verrucomicrobiaceae and Akkermansia and a decrease of Prevotellaceae, the investigators also found numerous differences.
“There is currently no consensus on PD-specific changes in microbiome composition and their pathophysiological implications due to inconsistent results, differences in methodologies and unaddressed confounders,” observed Dr. Scheperjans.
Accordingly, procedures for collection, storage and shipment of the stool samples varied considerably; almost all studies used different DNA extraction kits; different DNA sequencing protocols were used; and different bioinformatics and statistical methods can further lead to different results.
In addition, the study populations differed considerably between studies in terms of age, percentage of females and Parkinson’s disease characteristics, such as disease duration and the clinical subtype.
The investigators recommend methods to increase the comparability and utility of both the currently available data as well as future microbiome studies in PD.
They propose the integration of the already available data to address various possible confounders.
They also propose that future gut microbiome studies should address questions that are relevant for patient care, for example, whether gut microbiome changes can distinguish PD from similar disorders such as multiple system atrophy.
“Specific changes might serve as a biomarker with which we can recognize PD or specific subtypes of PD.
Since gut complaints can occur very early in the disease process, this might help to identify patients in the early stages of the disease, possibly even before the appearance of motor symptoms such as tremor and rigidity,” commented Jeffrey Boertien.
“If gut microbiota play an important role in the disease process, this might lead to new treatment options for PD.”
Differences in study population, Parkinson’s disease characteristics and methodology make it difficult to compare gut microbiome studies in Parkinson’s disease. The image is credited to Academy of Finland.
“If we combine all data, it will be easier to distinguish changes that are associated with PD from noise,” added Dr. Scheperjans.
“However, further research is still required to increase our understanding of the possible role of gut microbiota in PD.
It is important to emphasize that no microbiota-based treatment for PD exists to date.
We advise PD patients not to start self-treatment with probiotics or undergo fecal microbiota transplantation without consulting with their doctors in order to avoid potential harm.”
PD is a slowly progressive disorder that affects movement, muscle control and balance. It is the second most common age-related neurodegenerative disorder affecting about 3% of the population by the age of 65 and up to 5% of individuals over 85 years of age.
During the 20th century, PD was thought to be primarily a brain disorder. However, it is often preceded by non-motor symptoms such as sleep disorder, depression and gastrointestinal symptoms, especially constipation.
The pathology present in the brains of PD patients, so-called Lewy bodies, can also be found in the nerve cells of the gut, leading to the hypothesis that PD might originate in the gut.
Parkinson’s disease (PD) has traditionally been characterized by motor impairment but is now considered a multisystemic disorder displaying a plethora of non-motor symptoms (Chaudhuri et al., 2006; Emamzadeh and Surguchov, 2018; Greenland et al., 2019).
For example, people with PD frequently report various gastrointestinal complaints including constipation and nausea, and have prolonged intestinal transit time, often years prior to their PD diagnosis (Martinez-Martin et al., 2011; Lin et al., 2014; Fasano et al., 2015; Adams-Carr et al., 2016). A key feature of the disease is the formation of insoluble alpha-synuclein (αSyn) aggregates within neurons (Goedert et al., 2013), contributing to the loss of dopaminergic neurons in the basal ganglia.
This Lewy body pathology also occurs more widely throughout the central and peripheral nervous systems, including the enteric nervous system (Beach et al., 2010).
The concept that PD is initiated following continuous gut aggravation has gathered significant momentum in recent years. Enteric αSyn is associated with greater intestinal permeability (Forsyth et al., 2011), and a positive relationship between inflammatory bowel diseases and future PD risk is evident in various populations (Lin et al., 2016; Peter et al., 2018; Weimers et al., 2018). Individuals with PD also exhibit an imbalanced gut microbiome (dysbiosis) and gastrointestinal inflammation (Keshavarzian et al., 2015; Hill-Burns et al., 2017; Heintz-Buschart et al., 2018).
Various studies report similar trends in the microbial composition of people with PD, where commensal bacteria (e.g., phylum Firmicutes) are reduced, while pathogenic gram-negative bacteria (Proteobacteria, Enterobacteriaceae, Escherichia sp.) and mucin-degrading Verrucomicrobiaceae are increased (Keshavarzian et al., 2015; Scheperjans et al., 2015; Unger et al., 2016; Hill-Burns et al., 2017; Li et al., 2017). Moreover, bacterial treatments in vitro and fecal microbial transplants in vivo also support the role of the gut microbiome in αSyn aggregation, gastrointestinal inflammation and motor symptom development (Sampson et al., 2016; Choi et al., 2018; Sun et al., 2018).
Gram-negative bacteria, elevated in people with PD, produce lipopolysaccharide (LPS), an endotoxin associated with intestinal inflammation (Guo et al., 2015; Nighot et al., 2017). Interestingly, the abundance of gram-negative Enterobacteriaceae is positively correlated with the degree of postural instability and gait difficulty in individuals with PD (Scheperjans et al., 2015).
In rodent models, LPS administration mirrors PD pathology. Direct stereotaxic injection of LPS into the substantia nigra causes microglial inflammation, oxidative stress, cellular apoptosis, reduced dopamine production and motor impairments (Sharma and Nehru, 2015). In the periphery, an intraperitoneal dose of LPS increased αSyn expression and intestinal permeability in the large intestine (Kelly et al., 2014), while chronic intranasal instillation resulted in progressive hypokinesia, selective dopaminergic neuronal loss and nigrostriatal αSyn aggregation (He et al., 2013).
Recently, intrarectal administration of Proteus mirabilis-derived LPS to mice was shown to reduce the tight junction cell marker occludin but increased tumor necrosis factor alpha levels, and caused toll-like receptor 4 overexpression in the colon 16 days after treatment (Choi et al., 2018).
These effects extended to the brain, with microglial activation throughout nigrostriatal regions and αSyn aggregation throughout central and enteric neurons, supporting evidence for environmentally-triggered gut-brain pathology in the context of PD (Choi et al., 2018).
With enteric levels of αSyn being associated with greater intestinal permeability and LPS translocation across the intestinal barrier in people with PD (Forsyth et al., 2011), there is the potential for gut microbiota to induce αSyn propagation along peripheral nerves toward the brainstem, and brain more widely.
As such, early gastrointestinal dysfunction in people with PD may be more than a prodromal symptom, but rather an early contributing factor for αSyn pathology in susceptible individuals. Therefore, the objective of this study was to investigate the gut microbiota in PD patients and αSyn over-expressing mice. Mice over-expressing human αSyn (C57BL-6N-Tg (Thy1-SNCA) 61Mjff/J) exhibit nigrostriatal pathology and dopamine depletion (Chesselet et al., 2012), and were chosen for this study as a progressive model of PD which allows for a clinically relevant exploration of the gut microbiome prior to the onset of motor impairments.
The identification of pro-inflammatory Gammaproteobacteria in PD patients but not the genetic rodent model led to the exploration of LPS as an environmental inflammatory trigger for PD in intestinal epithelial cells and αSyn over-expressing mice.
Diana Murray – IOS Press
The image is credited to Academy of Finland.
Original Research: Open access
“Increasing Comparability and Utility of Gut Microbiome Studies in Parkinson’s Disease: A Systematic Review”. Filip Scheperjans et al.
Journal of Parkinson’s Disease doi:10.3233/JPD-191711.