Anti-Tumor Necrosis Factor Therapy and Incidence of Parkinson Disease Among Patients With Inflammatory Bowel Disease

Altered Gut Microbiome and Intestinal Pathology in Parkinson's Disease

Parkinson's disease (PD) is a common neurodegenerative disorder arising from an interplay between genetic and environmental risk factors. Studies have suggested that the pathological hallmarks of intraneuronal α-synuclein aggregations may start from the olfactory bulb and the enteric nervous system of the gut and later propagate to the brain via the olfactory tract and the vagus nerve. This hypothesis correlates well with clinical symptoms, such as constipation, that may develop up to 20 years before the onset of PD motor symptoms. Recent interest in the gut-brain axis has led to vigorous research into the gastrointestinal pathology and gut microbiota changes in patients with PD. In this review, we provide current clinical and pathological evidence of gut involvement in PD by summarizing the changes in gut microbiota composition and gut inflammation associated with its pathogenesis.

Alpha-Synuclein Pathology and the Role of the Microbiota in Parkinson's Disease

There is a principle in science, known as Occam’s razor, that says the correct solution is usually the one with the simplest explanation. The microbiota-gut-brain axis, an interdependent series of communication loops between the enteric nervous system (ENS), the microbiota, the gut, and the brain, offers important insight into how changes in our gut affect distant organs like our brains. The inherent complexity of this axis with the crosstalk between the immune system, inflammatory states, and the thousands of bacteria, viral, and fungal species that together make up the microbiota make studying the interactions that govern this axis difficult and far from parsimonious. It is becoming increasingly clear that the microbiota is integral to this axis. Disruption of the healthy flora, a phenomenon collectively referred to as dysbiosis, has been implicated as a driver for several diseases such as irritable bowel syndrome, rheumatoid arthritis, obesity, diabetes, liver disease, and neurological disorders such as depression, anxiety, and Parkinson’s disease (PD). Teasing apart these complex interactions as they pertain to PD is critical for our understanding of this debilitating disease, but more importantly, for the development of future treatments. So far, treatments have been unable to stop this neurodegenerative disease, succeeding only in briefly dampening symptoms and buying patients time before the inevitable loss of function ensues. Given that the 10 years prognosis for death or life-limiting disability with someone diagnosed with PD is upwards of 80%, there is a desperate need for curative treatments that go beyond symptom management. If PD does begin in the periphery with bidirectional communication between the microbiota and the immune system, as recent literature suggests, there is an exciting possibility that progression could be stopped before it reaches the brain. This systematic review assesses the current literature surrounding the role of the microbiota in the pathogenesis of alpha-synucleinopathies and explores the hypothesis that alpha-synuclein folding is modulated by the microbiota. Furthermore, we discuss how changes in the gut environment can lead to pathology and outline the implications that advances in understanding the interactions between host and microbiota will have on future research and the development of potential biomarkers.

Is Parkinson's disease a chronic low-grade inflammatory bowel disease?

While the pathogenesis of Parkinson's disease is not fully understood, there is increasing evidence that inflammatory responses in the brain are implicated in both disease initiation and progression. The inflammatory process in Parkinson's disease is, however, not limited to the brain but also involves the gastrointestinal tract. High amounts of cytokines and inflammatory markers are found in the colon of Parkinson's disease patients and there is now strong epidemiological and genetical evidence linking Parkinson's disease to inflammatory bowel diseases. Recent findings obtained in both experimental inflammatory bowel diseases and Parkinson's disease further support a bidirectional link between gastrointestinal inflammation and brain neurodegeneration. Altogether, these observations suggest a role for gastrointestinal inflammation in the initiation and progression of Parkinson's disease.

Progress of Pharmacological Approaches in Parkinson's Disease

The progressive neurodegenerative process in Parkinson's disease (PD) is not restricted to dopaminergic midbrain neurons but involves the entire nervous system. In this review, we outline established treatment options at different disease stages and address new therapeutic approaches. These include, based on recent advances in the understanding of the pathophysiology of PD, genetic and disease-modifying approaches to reduce abnormal accumulation and aggregation of alpha-synuclein (aSYN), mitochondrial dysfunction, and dysfunction of lysosomal proteins. Moreover, we highlight clinical trials to reduce neuroinflammation and increase neurorestoration.

Peripheral-Central Neuroimmune Crosstalk in Parkinson's Disease: What Do Patients and Animal Models Tell Us?

The brain is no longer considered an immune privileged organ and neuroinflammation has long been associated with Parkinson's disease. Accumulating evidence demonstrates that innate and adaptive responses take place in the CNS. The extent to which peripheral immune alterations impacts on the CNS, or vice and versa, is, however, still a matter of debate. Gaining a better knowledge of the molecular and cellular immune dysfunctions present in these two compartments and clarifying their mutual interactions is a fundamental step in understanding and preventing Parkinson's disease (PD) pathogenesis. This review provides an overview of the current knowledge on inflammatory processes evidenced both in PD patients and in toxin-induced animal models of the disease. It discusses differences and similarities between human and animal studies in the context of neuroinflammation and immune responses and how they have guided therapeutic strategies to slow down disease progression. Future longitudinal studies are necessary and can help gain a better understanding on peripheral-central nervous system crosstalk to improve therapeutic strategies for PD.

Dissemination of brain inflammation in traumatic brain injury

Traumatic brain injury (TBI) is recognized as a global health problem due to its increasing occurrence, challenging treatment, and persistent impacts on brain pathophysiology. Neural cell death in patients with TBI swiftly causes inflammation in the injured brain areas, which is recognized as focal brain inflammation. Focal brain inflammation causes secondary brain injury by exacerbating brain edema and neuronal death, while also exerting divergent beneficial effects, such as sealing the damaged limitans and removing cellular debris. Recent evidence from patients with TBI and studies on animal models suggest that brain inflammation after TBI is not only restricted to the focal lesion but also disseminates to remote areas of the brain. The dissemination of inflammation has been detected within days after the primary injury and persists chronically. This state of inflammation may be related to remote complications of TBI in patients, such as hyperthermia and hypopituitarism, and may lead to progressive neurodegeneration, such as chronic traumatic encephalopathy. Future studies should focus on understanding the mechanisms that govern the initiation and propagation of brain inflammation after TBI and its impacts on post-trauma brain pathology.

T-Cell-Driven Inflammation as a Mediator of the Gut-Brain Axis Involved in Parkinson's Disease

Parkinson's disease (PD) is a neurodegenerative disorder affecting mainly the dopaminergic neurons of the nigrostriatal pathway, a neuronal circuit involved in the control of movements, thereby the main manifestations correspond to motor impairments. The major molecular hallmark of this disease corresponds to the presence of pathological protein inclusions called Lewy bodies in the midbrain of patients, which have been extensively associated with neurotoxic effects. Importantly, different research groups have demonstrated that CD4⁺ T-cells infiltrate into the substantia nigra of PD patients and animal models. Moreover, several studies have consistently demonstrated that T-cell deficiency results in a strong attenuation of dopaminergic neurodegeneration in animal models of PD, thus indicating a key role of adaptive immunity in the neurodegenerative process. Recent evidence has shown that CD4⁺ T-cell response involved in PD patients is directed to oxidised forms of α-synuclein, one of the main constituents of Lewy bodies. On the other hand, most PD patients present a number of non-motor manifestations. Among non-motor manifestations, gastrointestinal dysfunctions result especially important as potential early biomarkers of PD, since they are ubiquitously found among confirmed patients and occur much earlier than motor symptoms. These gastrointestinal dysfunctions include constipation and inflammation of the gut mucosa and the most distinctive pathologic features associated are the loss of neurons of the enteric nervous system and the generation of Lewy bodies in the gut. Moreover, emerging evidence has recently shown a pivotal role of gut microbiota in triggering the development of PD in genetically predisposed individuals. Of note, PD has been positively correlated with inflammatory bowel diseases, a group of disorders involving a T-cell driven inflammation of gut mucosa, which is strongly dependent in the composition of gut microbiota. Here we raised the hypothesis that T-cell driven inflammation, which mediates dopaminergic neurodegeneration in PD, is triggered in the gut mucosa. Accordingly, we discuss how structural components of commensal bacteria or how different mediators produced by gut-microbiota, including short-chain fatty acids and dopamine, may affect the behaviour of T-cells, triggering the development of T-cell responses against Lewy bodies, initially confined to the gut mucosa but later extended to the brain.

Acute inflammation down-regulates alpha-synuclein expression in enteric neurons

The protein alpha-synuclein whose expression is strongly implicated in Parkinson's disease (PD) is not only expressed in the CNS but also in the enteric nervous system (ENS). The growing body of evidence suggesting that gastrointestinal inflammation is involved in the development of PD led us to investigate the effects of inflammation on alpha-synuclein expression in primary culture of rat ENS and in mice with dextran sulfate sodium-induced colitis. Using western blot and qPCR, we found that both lipopolysaccharide and a combination of tumor necrosis factor-α and interleukin 1-β decreased the expression levels of alpha-synuclein in primary culture of rat ENS, an effect that was prevented in the presence of the p38 inhibitors SB203580 and BIRB 796. Lipopolysaccharide and tumor necrosis factor-α/interleukin 1-β had no effect on alpha-synuclein expression in primary culture of rat CNS and in human erythroid leukemia cells. In mice, acute but not chronic dextran sulfate sodium-induced colitis was associated with a decreased expression of colonic alpha-synuclein. As a whole, our findings indicate that acute inflammatory insults down-regulate alpha-synuclein expression in the ENS via a p38 pathway. They provide new insights into the widely discussed concepts of alpha-synuclein expression and aggregation in the ENS in PD and raise issues about the possible role of gastrointestinal inflammation in the development of PD. OPEN SCIENCE BADGES: This article has received a badge for *Open Materials* because it provided all relevant information to reproduce the study in the manuscript. The complete Open Science Disclosure form for this article can be found at the end of the article. More information about the Open Practices badges can be found at https://cos.io/our-services/open-science-badges/.

Do Th17 Lymphocytes and IL-17 Contribute to Parkinson's Disease? A Systematic Review of Available Evidence

Parkinson's disease (PD) is a neurodegenerative disease characterized by progressive loss of dopaminergic neurons, appearance of Lewy bodies and presence of neuroinflammation. No treatments currently exist to prevent PD or delay its progression, and dopaminergic substitution treatments just relieve the consequences of dopaminergic neuron loss. Increasing evidence points to peripheral T lymphocytes as key players in PD, and recently there has been growing interest into the specific role of T helper (Th) 17 lymphocytes. Th17 are a proinflammatory CD4+ T cell lineage named after interleukin (IL)-17, the main cytokine produced by these cells. Th17 are involved in immune-related disease such as psoriasis, rheumatoid arthritis and inflammatory bowel disease, and drugs targeting Th17/IL-17 are currently approved for clinical use in such disease. In the present paper, we first summarized current knowledge about contribution of the peripheral immune system in PD, as well as about the physiopharmacology of Th17 and IL-17 together with its therapeutic relevance. Thereafter, we systematically retrieved and evaluated published evidence about Th17 and IL-17 in PD, to help assessing Th17/IL-17-targeting drugs as potentially novel antiparkinson agents. Critical appraisal of the evidence did not allow to reach definite conclusions: both animal as well as clinical studies are limited, just a few provide mechanistic evidence and none of them investigates the eventual relationship between Th17/IL-17 and clinically relevant endpoints such as disease progression, disability scores, intensity of dopaminergic substitution treatment. Careful assessment of Th17 in PD is anyway a priority, as Th17/IL-17-targeting therapeutics might represent a straightforward opportunity for the unmet needs of PD patients.

Colonic inflammation affects myenteric alpha-synuclein in nonhuman primates

Background: Parkinson's disease (PD) patients frequently present gastrointestinal (GI) dysfunction that, in many cases, predates the onset of motor symptoms. In PD, the presynaptic protein alpha-synuclein (α-syn) undergoes pathological changes, including phosphorylation and aggregation leading to the formation of Lewy bodies, which can be found in neurons of the enteric nervous system (ENS). Inflammation has been proposed as a possible trigger of α-syn pathology. Interestingly, patients with inflammatory bowel disease and irritable bowel syndrome, conditions associated with GI inflammation, are at higher risk of developing PD. Captive common marmosets (Callithrix jacchus) develop colitis, providing a natural platform to assess the relationship between α-syn pathology and GI inflammation. Materials and Methods: Sections of proximal colon from marmosets with colitis (n=5; 5.3±2.3 years old; 4 male) and normal controls (n=5; 4.1±1.6 years old; 1 male) were immunostained against protein gene product 9.5 (PGP9.5), human leukocyte antigen DR (HLA-DR), cluster of differentiation 3 (CD3), cluster of differentiation 20 (CD20), glial fibrillary acidic protein (GFAP), 8-hydroxy-2'-deoxyguanosine (8-OHdG), α-syn, and serine 129 phosphorylated α-syn (p-α-syn). Immunoreactivity of each staining in the myenteric plexus was quantified using NIH ImageJ software. Results: Marmosets with colitis had significantly increased expression of inflammatory markers (HLA-DR, p<0.02; CD3, p<0.008), oxidative stress (8-OHdG, p<0.05), and p-α-syn (p<0.02) and decreased expression of α-syn (p<0.04) in the colonic myenteric ganglia compared to normal, healthy controls. Conclusion: Colonic inflammation is associated with changes in α-syn expression and phosphorylation in the myenteric plexus of common marmosets. Future evaluation of the vagus nerve and brain of animals with colitis will be key to assess the contribution of colitis-induced ENS α-syn pathology to PD-like pathology in the brain.

The Appendix in Parkinson's Disease: From Vestigial Remnant to Vital Organ?

Parkinson's disease (PD) has long been considered a brain disease, but studies now point to the gastrointestinal (GI) tract as a potential starting point for PD. In particular, the human vermiform appendix has been implicated in PD. The appendix is a tissue rich in immune cells, serving as part of the gut-associated lymphoid tissue and as a storehouse for the gut microbiome. The functions of the appendix converge with recent evidence demonstrating that gut inflammation and shifts in the microbiome are linked to PD. Some epidemiological studies have linked removal of the appendix to lowered PD risk, though there is controversy among these associations. What is apparent is that there is an abundance of aggregated forms of α-synuclein in the appendix relevant to PD pathology. α-Synuclein pathology is thought to propagate from gut to brain via the vagus nerve, which innervates GI tract locations, including the appendix. Remarkably, α-synuclein aggregates in the appendix occur not only in PD patients, but are also present in healthy individuals. This has led to the proposal that in the appendix α-synuclein aggregates are not unique to PD. Moreover, the molecular events leading to PD and the mechanisms by which α-synuclein aggregates transfers from gut to brain may be identifiable in the human appendix. The influence of the appendix on GI inflammation, autoimmunity, microbiome storage, and the lymphatic system may be yet unexplored mechanisms by which the appendix contributes to PD. Overall, the appendix represents a promising tissue site to advance our understanding of PD pathobiology.

Triggers, Facilitators, and Aggravators: Redefining Parkinson's Disease Pathogenesis

We hypothesize that Parkinson's disease (PD) pathogenesis can be divided into three temporal phases. During the first phase, 'triggers', such as viral infections or environmental toxins, spark the disease process in the brain and/or peripheral tissues. Triggers alone, however, may be insufficient, requiring 'facilitators' like peripheral inflammation for PD pathology to develop. Once the disease manifests, 'aggravators' spur further neurodegeneration and exacerbate symptoms. Aggravators are proposed to include impaired autophagy and cell-to-cell propagation of α-synuclein pathology. We believe clinical trials need to consider these three phases and target potential therapies at the appropriate stage of the disease process in order to be effective.

The risk of Parkinson's disease in inflammatory bowel disease: A systematic review and meta-analysis

BACKGROUND

Several studies have reported an increased prevalence of Parkinson disease (PD) amongst patients with inflammatory bowel disease (IBD) with conflicting results. We aimed to evaluate the risk of PD in the IBD population by conducting a meta-analysis (MA).

METHODS

A systematic review with MA of the existing literature was conducted. The main outcome of interest was the incidence of developing PD in patients previously diagnosed with IBD.

RESULTS

Four studies were included in this MA. The overall risk of PD in IBD was significantly higher than controls (RR 1.41, 95% c.i. 1.19-1.66). Crohn's disease had a 28% increased risk of PD and ulcerative colitis had a 30% increased risk of PD compared to controls (CD: RR 1.28, 95% c.i. 1.08-1.52, UC: RR 1.30, 95% c.i. 1.15-1.47).

CONCLUSION

The MA detected an increased risk of PD in the IBD population and CD/UC subgroup. These results merit further clinical validation in future studies.

Inflammatory Bowel Diseases and Parkinson's Disease

The etiology of Parkinson's disease (PD) is multifactorial, with genetics, aging, and environmental agents all a part of the PD pathogenesis. Widespread aggregation of the α-synuclein protein in the form of Lewy bodies and Lewy neurites, and degeneration of substantia nigra dopamine neurons are the pathological hallmarks of PD. Inflammatory responses manifested by glial reactions, T cell infiltration, and increased expression of inflammatory cytokines, as well as other toxic mediators derived from activated glial cells, are currently recognized as prominent features of PD. Experimental, clinical and epidemiological data suggest that intestinal inflammation contributes to the pathogenesis of PD, and the increasing number of studies suggests that the condition may start in the gastrointestinal system years before any motor symptoms develop. Patients with inflammatory bowel disease (IBD) have a higher risk of developing PD compared with non-IBD individuals. Gene association study has found a genetic link between IBD and PD, and an evidence from animal studies suggests that gut inflammation, similar to that observed in IBD, may induce loss of dopaminergic neurons. Based on preclinical models of PD, it is suggested that the enteric microbiome changes early in PD, and gut infections trigger α-synuclein release and aggregation. In this paper, the possible link between IBD and PD is reviewed based on the available literature. Given the potentially critical role of gastrointestinal pathology in PD pathogenesis, there is reason to suspect that IBD or its treatments may impact PD risk. Thus, clinicians should be aware of PD symptoms in IBD patients.

Can the gut be the missing piece in uncovering PD pathogenesis?

It is now well established that Parkinson's disease (PD) is not only a movement disorder of the CNS but also a gastrointestinal disorder affecting the enteric nervous system (ENS). The gut-brain axis is a bidirectional communication between the brain and the gastrointestinal tract, which comprises besides the CNS and the ENS, the intestinal epithelial barrier, the intestinal microbiota and the enteroendocrine systems. In this review, we present the clinical and pathological evidence suggesting that the gut-brain axis is dysfunctional in PD by discussing the possible role of gut microbiota, inflammation and permeability in the development of the disease.

The Dual Role of Hepcidin in Brain Iron Load and Inflammation

Hepcidin is the major regulator of systemic iron metabolism, while the role of this peptide in the brain has just recently been elucidated. Studies suggest a dual role of hepcidin in neuronal iron load and inflammation. This is important since neuronal iron load and inflammation are pathophysiological processes frequently associated with neurodegeneration. Furthermore, manipulation of hepcidin activity has recently been used to recover neuronal damage due to brain inflammation in animal models and cultured cells. Therefore, understanding the mechanistic insights of hepcidin action in the brain is important to uncover its role in treating neuronal damage in neurodegenerative diseases.

A punch in the gut - Intestinal inflammation links environmental factors to neurodegeneration in Parkinson's disease

Parkinson's disease (PD) is an etiologically heterogeneous disorder. Experimental, clinical and epidemiological data suggest that intestinal inflammation contributes to the pathogenesis of PD. This article reviews recent literature on gut microbiota and intestinal inflammation in PD. We propose that intestinal inflammation links environmental factors (e.g. an altered gut microbiota composition) to neurodegeneration in (genetically susceptible) PD patients. In addition, there is an epidemiological and genetic overlap between PD and inflammatory bowel disease. This overlap provides an opportunity to develop new treatment strategies for at least a subgroup of PD patients.

Therapeutic implications of how TNF links apolipoprotein E, phosphorylated tau, α-synuclein, amyloid-β and insulin resistance in neurodegenerative diseases

While cytokines such as TNF have long been recognized as essential to normal cerebral physiology, the implications of their chronic excessive production within the brain are now also increasingly appreciated. Syndromes as diverse as malaria and lead poisoning, as well as non‐infectious neurodegenerative diseases, illustrate this. These cytokines also orchestrate changes in tau, α‐synuclein, amyloid‐β levels and degree of insulin resistance in most neurodegenerative states. New data on the effects of salbutamol, an indirect anti‐TNF agent, on α‐synuclein and Parkinson's disease, APOE4 and tau add considerably to the rationale of the anti‐TNF approach to understanding, and treating, these diseases. Therapeutic advances being tested, and arguably useful for a number of the neurodegenerative diseases, include a reduction of excess cerebral TNF, whether directly, with a specific anti‐TNF biological agent such as etanercept via Batson's plexus, or indirectly via surgically implanting stem cells. Inhaled salbutamol also warrants investigating further across the neurodegenerative disease spectrum. It is now timely to integrate this range of new information across the neurodegenerative disease spectrum, rather than keep seeing it through the lens of individual disease states.