Defining How Entamoeba histolytica Modulates Macrophage Functions
Date
2019-09-06
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Abstract
Entamoeba histolytica (Eh) colonizes the colonic mucus layer to establish asymptomatic infection and under certain circumstances not completely known, Eh breaches mucosal barriers to cause intestinal and extraintestinal amebiasis. Eh adherence and invasion to host cells triggers robust pro-inflammatory responses that are critical in disease pathogenesis. At present, it is unclear why there is an aggressive pro-inflammatory response that intensifies at the site of parasite invasion. In this study, I hypothesize that Eh contact with macrophages induces the release of the endogenous alarmin molecule HMGB1 to alert non-contacted cells of imminent Eh invasion to escalate innate host defenses. Preformed nuclear HMGB1 is a well-known alarmin protein that upon secretion plays critical roles in the pathogenesis of infectious inflammation. Eh in contact with macrophages induced rapid and early secretion of HMGB1. This was facilitated by Eh surface Gal-lectin mediated activation of PI3K and NF-κB signaling pathways and increased nuclear histone acetyltransferase activity that resulted in the acetylation of HMGB1 to cause active secretion without significant host cell death. In contrast to endotoxemia, Eh induced HMGB1 secretion was independent of caspase-1 mediated inflammasome activation and gasdermin D pores. Neutralization of HMGB1 in mice significantly inhibited Eh-induced pro-inflammatory responses in colonic loops, demonstrating a critical role for HMGB1 in shaping pro-inflammatory responses towards Eh. The second aim of this study was to investigate the role of autophagy associated protein ATG16L1 in response to Eh-induced pro-inflammatory responses. Autophagy maintains and regulates excessive inflammatory responses and tissue damage. Interestingly, Eh in contact with macrophages at the intercellular junction induced rapid degradation of ATG16L1 protein and dissociation of ATG5 from ATG12-ATG5 conjugates, which are essential components of immune regulatory autophagy processes. The cellular mechanism of ATG16L1 protein degradation was mediated by Eh induced activation of caspase-6. In vitro cleavage assays demonstrated that ATG16L1 protein was a potential proteolytic substrate for active caspase-6 and the presence of Eh increased ATG16L1 protein susceptibility towards degradation, which leads to dysregulation of immune regulatory processes with excessive inflammation. Taken together, these results revealed two distinct but novel interconnected outcomes following Eh interaction with macrophages at the intercellular junction 1), induction and secretion of the alarmin molecule HMGB1 to establish amplified pro-inflammatory responses and 2), caspase-6 activation that mediates proteolytic degradation of ATG16L1 to dysregulate immune responses to escalate the magnitude and duration of pro-inflammatory responses. Both these processes can play major roles in innate host defenses and/or disease pathogenesis in amebiasis.
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Keywords
HMGB1, alarmin, ATG16L1
Citation
Begum, S. (2019). Defining How Entamoeba histolytica Modulates Macrophage Functions (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca.