The intestinal luminal sources of α-synuclein: a gastroenterologist perspective

Gut-brain axis and environmental factors in Parkinson's disease: bidirectional link between disease onset and progression

Parkinson's disease has long been considered a disorder that primarily affects the brain, as it is defined by the dopaminergic neurodegeneration in the substantia nigra and the brain accumulation of Lewy bodies containing α-synuclein protein. In recent decades, however, accumulating research has revealed that Parkinson's disease also involves the gut and uncovered an intimate and important bidirectional link between the brain and the gut, called the "gut-brain axis." Numerous clinical studies demonstrate that gut dysfunction frequently precedes motor symptoms in Parkinson's disease patients, with findings including impaired intestinal permeability, heightened inflammation, and distinct gut microbiome profiles and metabolites. Furthermore, α-synuclein deposition has been consistently observed in the gut of Parkinson's disease patients, suggesting a potential role in disease initiation. Importantly, individuals with vagotomy have a reduced Parkinson's disease risk. From these observations, researchers have hypothesized that α-synuclein accumulation may initiate in the gut and subsequently propagate to the central dopaminergic neurons through the gut-brain axis, leading to Parkinson's disease. This review comprehensively examines the gut's involvement in Parkinson's disease, focusing on the concept of a gut-origin for the disease. We also examine the interplay between altered gut-related factors and the accumulation of pathological α-synuclein in the gut of Parkinson's disease patients. Given the accessibility of the gut to both dietary and pharmacological interventions, targeting gut-localized α-synuclein represents a promising avenue for developing effective Parkinson's disease therapies.

Addition of α-synuclein aggregates to the intestinal environment recapitulates Parkinsonian symptoms in model systems

The gut-brain axis plays a vital role in Parkinson's disease (PD). The mechanisms of gut-brain transmission mainly focus on α-synuclein deposition, intestinal inflammation and microbiota function. A few studies have shown the trigger of PD pathology in the gut. α-Synuclein is highly conserved in food products, which was able to form β-folded aggregates and to infect the intestinal mucosa. In this study we investigated whether α-synuclein-preformed fibril (PFF) exposure could modulate the intestinal environment and induce rodent models replicating PD pathology. We first showed that PFF could be internalized into co-cultured Caco-2/HT29/Raji b cells in vitro. Furthermore, we demonstrated that PFF perfusion caused the intestinal inflammation and activation of enteric glial cells in an ex vivo intestinal organ culture and in an in vivo intestinal mouse coloclysis model. Moreover, we found that PFF exposure through regular coloclysis induced PD pathology in wild-type (WT) and A53T α-synuclein transgenic mice with various phenotypes. Particularly in A53T mice, PFF induced significant behavioral disorders, intestinal inflammation, α-synuclein deposition, microbiota dysbiosis, glial activation as well as degeneration of dopaminergic neurons in the substantia nigra. In WT mice, however, the PFF induced only mild behavioral abnormalities, intestinal inflammation, α-synuclein deposition, and glial activation, without significant changes in microbiota and dopaminergic neurons. Our results reveal the possibility of α-synuclein aggregates binding to the intestinal mucosa and modeling PD in mice. This study may shed light on the investigation and early intervention of the gut-origin hypothesis in neurodegenerative diseases.

The role of the microbiota-gut-brain axis and intestinal microbiome dysregulation in Parkinson's disease

Parkinson's disease (PD) is a complex progressive neurodegenerative disease associated with aging. Its main pathological feature is the degeneration and loss of dopaminergic neurons related to the misfolding and aggregation of α-synuclein. The pathogenesis of PD has not yet been fully elucidated, and its occurrence and development process are closely related to the microbiota-gut-brain axis. Dysregulation of intestinal microbiota may promote the damage of the intestinal epithelial barrier, intestinal inflammation, and the upward diffusion of phosphorylated α-synuclein from the enteric nervous system (ENS) to the brain in susceptible individuals and further lead to gastrointestinal dysfunction, neuroinflammation, and neurodegeneration of the central nervous system (CNS) through the disordered microbiota-gut-brain axis. The present review aimed to summarize recent advancements in studies focusing on the role of the microbiota-gut-brain axis in the pathogenesis of PD, especially the mechanism of intestinal microbiome dysregulation, intestinal inflammation, and gastrointestinal dysfunction in PD. Maintaining or restoring homeostasis in the gut microenvironment by targeting the gut microbiome may provide future direction for the development of new biomarkers for early diagnosis of PD and therapeutic strategies to slow disease progression.

The Role of Exposomes in the Pathophysiology of Autoimmune Diseases I: Toxic Chemicals and Food

Autoimmune diseases affect 5–9% of the world’s population. It is now known that genetics play a relatively small part in the pathophysiology of autoimmune disorders in general, and that environmental factors have a greater role. In this review, we examine the role of the exposome, an individual’s lifetime exposure to external and internal factors, in the pathophysiology of autoimmune diseases. The most common of these environmental factors are toxic chemicals, food/diet, and infections. Toxic chemicals are in our food, drink, common products, the air, and even the land we walk on. Toxic chemicals can directly damage self-tissue and cause the release of autoantigens, or can bind to human tissue antigens and form neoantigens, which can provoke autoimmune response leading to autoimmunity. Other types of autoimmune responses can also be induced by toxic chemicals through various effects at the cellular and biochemical levels. The food we eat every day commonly has colorants, preservatives, or packaging-related chemical contamination. The food itself may be antigenic for susceptible individuals. The most common mechanism for food-related autoimmunity is molecular mimicry, in which the food’s molecular structure bears a similarity with the structure of one or more self-tissues. The solution is to detect the trigger, remove it from the environment or diet, then repair the damage to the individual’s body and health.

Cross-Reactivity and Sequence Homology Between Alpha-Synuclein and Food Products: A Step Further for Parkinson's Disease Synucleinopathy

INTRODUCTION

Parkinson's disease is characterized by non-motor/motor dysfunction midbrain neuronal death and α-synuclein deposits. The accepted hypothesis is that unknown environmental factors induce α-synuclein accumulation in the brain via the enteric nervous system.

MATERIAL AND METHODS

Monoclonal antibodies made against recombinant α-synuclein protein or α-synuclein epitope 118-123 were applied to the antigens of 180 frequently consumed food products. The specificity of those antibody-antigen reactions was confirmed by serial dilution and inhibition studies. The Basic Local Alignment Search Tool sequence matching program was used for sequence homologies.

RESULTS

While the antibody made against recombinant α-synuclein reacted significantly with 86/180 specific food antigens, the antibody made against α-synuclein epitope 118-123 reacted with only 32/180 tested food antigens. The food proteins with the greatest number of peptides that matched with α-synuclein were yeast, soybean, latex hevein, wheat germ agglutinin, potato, peanut, bean agglutinin, pea lectin, shrimp, bromelain, and lentil lectin. Conclusions: The cross-reactivity and sequence homology between α-synuclein and frequently consumed foods, reinforces the autoimmune aspect of Parkinson's disease. It is hypothesized that luminal food peptides that share cross-reactive epitopes with human α-synuclein and have molecular similarity with brain antigens are involved in the synucleinopathy. The findings deserve further confirmation by extensive research.