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

Abstract

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.

Keywords: Parkinson’s disease; appendix; gastrointestinal tract; vagus nerve; α-Synuclein.

Fig.1

The appendix as an initiation site for PD. Several possible mechanisms for the role of the appendix in PD are depicted. The appendix microbiota could influence PD by promoting cytokine release or by the release of endotoxins and/or bacterial metabolites (e.g., short-chain fatty acids). Disruption of the dual mucous layers in the appendix or microbial biofilm penetration of the inner mucous layer could result in host invasion of pathogenic microorganisms. Dysbiosis promotes a state of chronic tissue inflammation that results in the accumulation of α-syn pathology and aggregation-prone α-syn truncation products. α-Syn pathology can then propagate to the brain via the vagus nerve to initiate PD. Alternatively or in addition, α-syn pathology may accumulate in the brain via the circulatory system and a compromised BBB integrity or via a degenerating lymphatic system. Another mechanism could involve chronic tissue inflammation that upregulates α-syn, which in turn may act as an antigen to stimulate B cells, resulting in the production of autoreactive immunoglobulin and T cells. The activation of these peripheral immune cells might then lead to systemic autoimmunity toward α-syn. Both autoantibodies and self-reactive T cells might enter the CNS, resulting in neurotoxicity and PD.