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English

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Mitochondrial autoimmunity in Parkinson’s disease

Abstract

Parkinson's disease (PD) is a neurodegenerative disease caused by the destruction of dopamine neurons, yet, the underlying cause of this neuronal loss is still unknown. Parkinson's disease can be juvenile or early onset (before age 40) and sporadically (after the age of 60). PINK1 (PTEN-induced putative kinase 1) and Parkin are genes implicated in the juvenile form of PD that have also been shown to influence inflammatory responses and mitochondrial dysfunction. PINK1 and Parkin negatively regulate mitochondrial antigen presentation (MitAP). These proteins suppress the presentation of mitochondrial proteins on Major Histocompatibility Complex (MHC) Class I following stress. MitAP is dependent on the formation of mitochondrial vesicles (MDVs). These MDVs utilize traffic proteins such as Snx9, Rab9, and Rab7. Decreased levels of expression of any of these proteins inhibit MitAP. Mitochondrial instability also increases mitochondrial DNA leakage into the cytosol (through and unknown mechanism) resulting in activation of cytosolic DNA sensors like cGAS/STING. MitAP's implication in autoimmunity has been demonstrated in PINK1 KO mice infected with gram-negative intestinal bacteria resulting in auto-reactive T-cells specific to mitochondrial antigens and Parkinson's-like symptoms. In PD patients, auto-reactive T cells specific for alpha-synuclein, a protein whose dysfunction is associated with PD, have been found, suggesting that auto-reactive T-cells are present in PD. Thus, we hypothesized that PD patients would have increased levels of MitAP in antigen presenting cells (APCs) resulting in mitochondrial self-responsiveness as well as an inflammatory deregulation. This increased production of MDVs would result in an increase mtDNA leakage and ultimately be detected by the cGAS/STING pathway inducing the production of proinflammatory cytokines. Ultimately, these proinflammatory cytokines would induce the polarization of T-cells towards an autoimmune phenotype (Th17). Here, we show that APCs deficient in genes associated with MDVs biogenesis have reduced expression levels of pro-inflammatory cytokines after infection with Enteropathogenic Escherichia coli (EPEC) or the bacterial endotoxin LPS. We determine that in the absence of STING there is a strong inhibition of MitAP and cytokine expression in raw 264.7 macrophages after LPS treatment. Furthermore, we validate that in the absence of PINK1, LPS or EPEC treated BMDCs overproduce a set of pro-inflammatory cytokines (IL-6, IL-23 and IL-1β) that induce a biased polarization of T-cells to a Th17 phenotype. Finally, we observed mitochondrial antigen specific T-cells expressing IL-17 (a marker of Th17 and Tc17 lymphocytes, the types of lymphocytes that are strongly related to autoimmune diseases) in PD patients. This data highlights the importance of the MitAP pathway in the pathophysiology of Parkinson's disease through the regulation of autoimmunity and inflammation.

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