Toll-like receptors on enteric neurons: relevance for Parkinson's disease

Abstract

Parkinson’s disease is a neurodegenerative disease, which affected about 1.5-5 per 1,000 people in the Netherlands in 2011. The hallmark symptoms of Parkinson’s disease are deficiencies in motor functioning due to dopaminergic neuronal cell loss in the substantia nigra in the brain, and accumulation of the protein α-Synuclein. Non-motor symptoms like gastrointestinal problems have been found to precede the motor symptoms and diagnosis by years. This has led to the hypothesis that damage to neurons in the gastrointestinal tract occurs long before the substantia nigra are affected. This theory is known as the Braak’s hypothesis according to which unknown pathogens may enter the nervous system through both the nasal and intestinal mucosa resulting in prion-like spread of α-Synuclein to the brain, leading to neurodegeneration. Regarding the Braak’s hypothesis, the focus of study in this thesis was the immune signaling of enteric neurons in the context of Parkinson’s disease. A cell line of enteric neurons has been used as a model for the neurons in the gastrointestinal tract. The expression and function of a specific group of innate immune receptors, Toll-like receptors, which are important for the recognition of microbes such as bacteria, was studied on this cell line. The current scientific literature on Toll-like receptors on neurons in the context of stroke, Alzheimer’s disease and Parkinson’s disease is presented. The distinct importance of Toll-like receptor-2 and -4 in Parkinson’s disease is apparent, as well as the need to obtain more knowledge about all Toll-like receptors in Parkinson’s disease. In addition, a review of the current scientific literature on the Braak’s hypothesis is presented. The support for the hypothesis is weighed against the criticism, and it is concluded that the Braak’s hypothesis is most likely relevant for a large subset of patients suffering from Parkinson’s disease. The technical aspects of culturing the murine enteric neuronal cells are described. An effort was made to identify predictors for the unstable growth rate of the cells. Such predictors were not found, but the results were still useful to guide future research into these cell culturing techniques towards a better understanding of the growth rate. The expression of Toll-like receptors on the murine enteric neuronal cell line, and on primary enteric neurons of mice was investigated. Expression of Toll-like receptors-2, -3, -4 and -7 by the enteric neuronal cell line was demonstrated, as well as expression of TLR-2 and -4 on primary enteric neurons. α-Synucleine accumulation in the neuronal plexus of intestines from mice undergoing Parkinson’s disease was associated with TLR2 and -4 expression. Furthermore, the function of Toll-like receptors on the murine enteric neuronal cell line in the context of Parkinson’s disease was studied. In vitro exposure of enteric neurons to α-Synucleine in combination with a TLR4 ligand resulted in the expression and release of a proinflammatory neuropeptide and cytokines. In summary, the results presented in this thesis indicate that bacterial ligands in combination with α-Synuclein accumulation might be involved in the neuro-inflammatory response and associated intestinal problems of patients suffering from Parkinson’s disease.