Alterations to Mucus Production and Secretion during Giardia Infection Involve Giardia Protease Activity and PAR2 Activation
Authors: Fekete Elena Rae , Amat Christina , Allain Thibault , Mihara Koichiro , Hollenberg Morley , Chadee Kris , Buret Andre 
Published: April 2020

Background

The protozoan parasite Giardia duodenalis has been shown to disrupt the intestinal mucus barrier via unknown mechanisms, potentially disrupting host mucosal immunity and contributing to development of post‐infectious disorders. Protease‐activated receptor 2 (PAR2) has been identified as a potential target for secreted Giardia cysteine proteases, and may play a role in disruption of mucus production and secretion in the intestines.

Methods

The human colonic epithelial cell line LS174T was infected with Giardia trophozoites (isolates NF, WB, S2, and GSM). Prior to infection, trophozoites were treated with E64, a broad‐spectrum cysteine protease inhibitor, and LS174T were treated with a pepducin PAR2 antagonist, BAPTA, a calcium chelator, or U0126, an ERK1/2 inhibitor. MUC2 mucin gene expression was assessed using quantitative PCR (qPCR), and intracellular mucin content was quantified by staining with fluorescein‐coupled wheat germ agglutinin (WGA) and imaging using confocal microscopy.

Chinese hamster ovary cells transfected with nano‐luciferase tagged PAR2 were incubated with Giardia trophozoites. Release of enzymes due to cleavage at the receptor N‐terminus by Giardia resulted in a luminescent signal proportional to the number of cleaved receptors.

Wild type (WT) and PAR2 deficient (PAR2 −/−) mice were infected with Giardia trophozoites. The colonic mucus layer was stained using WGA and qPCR was performed for Muc2 and Muc5ac in the small and large intestines.

Results

Giardia NF, S2, and WB, but not GSM, upregulate MUC2 expression in LS174T cells. This increase was attenuated by inhibition of Giardia cysteine proteases and by antagonism of PAR2 or inhibition of calcium release or ERK1/2 signaling in LS174T cells. Preliminary results suggest that WGA staining is decreased upon exposure of LS174T to Giardia NF trophozoites.

Giardia trophozoites cleaved PAR2 at the N‐terminus, suggesting they are capable of activating the receptor. The amount of cleavage was isolate‐dependent, reflecting differences in protease activity, and cleavage was significantly reduced by E64 treatment for all isolates.

Giardia infected WT mice show increased Muc2 and Muc5ac expression in the colon, and increased Muc5ac but decreased Muc2 expression in the jejunum. These changes were not seen in PAR2−/− mice. Both WT infected and PAR2−/− non‐infected mice showed thinning of the colonic mucus layer compared to WT controls. There was some recovery in thickness in PAR2−/− infected mice.

Conclusions

Giardia‐induced alterations to mucin gene expression in vitro and in vivo are dependent on PAR2 signaling. Giardia cysteine proteases cleave and activate PAR2, leading to both calcium release and MAPK pathway activation, which in turn contributes to altered gene transcription. Further, Giardia induces altered mucus secretion in vitro, which may contribute to thinning of the colonic mucus gel in vivo. Further study is required to determine the role of PAR2 in mucus secretion during Giardia infection.

Support or Funding Information

Crohn’s Colitis Canada