Gut dysfunction is commonly observed in Parkinson's disease (PD) and gastrointestinal symptoms have been reported to precede neurological motor symptoms. This issue led to hypothesize that intestine is a putative site of origin of PD pathology, with the subsequent spread toward the central nervous system (CNS), via gut-brain connections.
Many studies have been devoted to the comprehension of possible triggering conditions able to affect the enteric nervous system (ENS), located in the gut submucosal and myenteric plexi, causing the spread up of such neuropathological features to the CNS. Different studies pointed out to the role of intestinal inflammation and the related impaired mucosa permeability (leaky gut) as key factor in the generation of neuropathological features in the enteric neurons, like alpha synuclein (a-syn) mysfolding and Leucine-Rich-Repeated-Kinase-2 (LRRK-2) protein expression. In both humans and animal models, the disruption of the normal microbiota, by antibiotic overexposure, has been described as one of the most important leaky gut-referred conditions.
The present research aims at investigating the effect of leaky gut syndrome induced by administration of broad spectrum antibiotics (ampicillin, vancomycin, neomycin, and metronidazole) and an antifungal (amphotericin B) in mice, and to evaluate: (1) the immunological activation of the ENS/EGC following dysbiosis, and relative alpha-syn mysfolding and LRRK-2 increased protein expression in the gut submucosal/myenteric plexi; (2) if ENS-induced activation by dysbiosis is associated with an ascending neuropathological signaling to the CNS; (3) the pathway by which dysbiotic-activation of ENS alteration propagates from the periphery to the brain, and (4) to correlate these events with a parkinsonism- related neuropathology in mice.
When PD is diagnosed, a typical set of movement -related symptoms has already appeared, following loss of striatal dopamine induced by the neurodegeneration of dopaminergic neurons in the midbrain. Therapy is based with dopamine-replacement , but, no available treatments are capable to arrest, or slow, the progressive nature of Parkinson's neurodegeneration. The possibility to both diagnose and pharmacologically interfere as early is possible with PD could maximize the efficacy of therapy and minimize its long-term failure. The hypothesis of a gut-brain axis in the development of PD in humans, has been postulated on the basis of Braak's theory [Braak et al., Neurobiol Aging. 2003 Mar-Apr; 24(2):197-211; Hawkes et al., Ann N Y Acad Sci. 2009 Jul; 1170:615-22] whereas misfolded alpha-synuclein is early detected in the ENS then propagate, as a prion-like disease to the CNS by vagal afferent way. Primitive changes of alpha-synuclein, as well as other PD-related proteins such as LRRK-2, are in fact commonly observed in chronic inflammatory gut disorders, and connection of three events are observed: 1. Changes of microbiota; 2 leaky gut induction and 3. Alteration of ENS. The exact mechanism(s) leading to ENS activation are largely obscure, as well as the mechanism(s) inducing ENS protein expression alteration. Moreover the exact process by which the ENS neurodegenerative process propagate to CNS is not clear. In this interplay, a very important role is hypothesized to be played by EGC, that are physiologically involved in the maintenance of gut homeostasis by their ability to reinforce the epithelial barrier function [Bassotti et al., Lab Invest 2007;87:628-32] and, more recently, they have been identified to regulate intestinal inflammatory responses [Barajon et al., J Histochem Cytochem 2009;57:1013-23] and to mediate host-pathogen interactions [Turco et al., Gut 2014;63:105-15]. EGC may thus behave as an¿entrance door¿ through which harmful stimuli from the intestinal lumen, prime the enteric nervous system, predisposing it to the above described alterations. Such perpetuation in the time, could be responsible for a primitive onset of PD-related neuropathological features in the gut, evolving to florid PD in the striatum and nigra, with TH neurons loss, according Braak's hypothesis. This project aims at light up, for the first time, at EGC as key cell type in mediating ENS neurodegeneration following dysbiotic-induced leaky gut condition induced by chronic antibiotic exposure. As known, antibiotic misuse due to self-medication, or unconsciousness their assumptions from foods deriving from animals raised with extensive antibiotic treatments is major health emergency in Western countries. While the risks associated with bacterial resistance and the lower effectiveness of chemotherapy due to incorrect use of antibiotics are clear, the possibility that such drugs may predispose in a subtle way subjects to a PD related neuropathic framework are completely obscure . Yet because antibiotics in chronic can cause sequelae of events such as: microbial alterations due to dysliosis, leaky gut and ENS alteration, with chronic enteropathies (constipation and diarrhea), our experimental model may, in a way, represent a new model of Braak's hypothesis, just through the fundamental involvement of EGCs in PD etiology. Depending upon the obtained results, our study will provide evidence about a possible role exerted by EGC in ENS-mediated alteration of alpha-synuclein mysfolding and Lewi¿s body formation, as well as, LRRK-2 expression in the frame of glia-mediated neuroinflammation occurring in leaky gut condition caused by dysbiosis. Humanization procedure, with fecal transplantation of PD-derived microbiota will also give us a corroboration of our experimental conditions, identifying a possible new key to view at PD-related etiology, according Braak¿s hypothesis. As a whole, the data that will be obtained from our research will not only allow us to confirm an enteric route in PD pathophysiology, but also a new therapeutic target for treating the disease in humans represented by EGCs pharmacological modulation. To date, in fact, although we know about the appearance of Parkinson's neuropathological alterations in ENS preceding the disease at the CNS level, we completely miss how this phenomenon happens. In this context, a priming of EGCs operated by dysbiosis induced in our experimental conditions will give us a significant confirmation of the role played by these cells in a more in-depth identification of PD's origin.