Host glycosylation pathways and the unfolded protein response contribute to the infection byFrancisella

Abstract

Protein glycosylation processes play a crucial role in most physiological functions, including cell signalling, cellular differentiation and adhesion. We previously demonstrated that rapid deglycosylation of membrane proteins was specifically triggered after infection of human macrophages by the bacterial pathogen Francisella tularensis. Using a glycan processing gene microarray, we found here that Francisella infection modulated expression of numerous glycosidase and glycosyltransferase genes. Furthermore, analysis of cell extracts from infected macrophages by Lectin and Western blotting revealed an important increase of N- and O-protein glycosylation. We chose to focus in the present work on one of the O-glycosylated proteins identified by mass spectrometry, the multifunctional endoplasmic reticulum chaperone BiP (HSPA5/GRP78). We demonstrate that BiP expression is modulated upon Francisella infection and is required to support its intracellular multiplication. Moreover, we show that Francisella differentially modulates the BiP-dependent activation of three key proteins of the unfolded protein response (UPR), IRE1, PERK and ATF6. The effects exerted on human cells by Francisella may thus constitute a novel excample of UPR manipulation contributing to intracellular bacterial adaptation.

Role of Glycosylation/Deglycolysation Processes in Francisella tularensis Pathogenesis

“…Francisella infection modifies the unfolded protein response (UPR) (Barel et al, 2016) and manipulates autophagy (Miller and Celli, 2016). Both processes are involved in maintaining cellular homeostasis and helping destroy invading microorganisms.…”

“…Indeed, using a glycan processing gene microarray (Chacko et al, 2011), we observed significant changes in the level of glycosyltransferase and glycosidase gene expression profiles in human THP-1 monocytes, infected for 24 h with F. tularensis LVS (Barel et al, 2016). Expression of eight genes, encoding four glycosyltransferases and four glycosidases, was down-regulated upon infection.…”

Innate Immune Recognition: An Issue More Complex Than Expected

“…F. tularensis strain LVS has been shown to trigger rapid deglycosylation of host membrane proteins (Barel et al, 2012) due to the expression of enzymes producing N - and O -linked glycosylation (Barel et al, 2016). An important effect related to intracellular existence of Francisella is an increased expression of GRP78/BiP (Barel et al, 2016), which is an ER stress chaperone required for proper folding and assembly of newly synthetized proteins (Lee, 2005; Luo et al, 2006). In general, the expression of glycosylated GRP78/BiP is followed by activation of IRE1 (Barel et al, 2016) through a binding/release mechanism.…”

“…An important effect related to intracellular existence of Francisella is an increased expression of GRP78/BiP (Barel et al, 2016), which is an ER stress chaperone required for proper folding and assembly of newly synthetized proteins (Lee, 2005; Luo et al, 2006). In general, the expression of glycosylated GRP78/BiP is followed by activation of IRE1 (Barel et al, 2016) through a binding/release mechanism. IRE1 is an ER-transmembrane protein important for sensing and responding to misfolded protein-GRP78/BiP dissociation and is one of three recently known arms of UPR (Hetz and Papa, 2018).…”

“…Francisella , moreover, modulates the activation of the other two arms of UPR. While decreasing PERK (protein kinase RNA-like endoplasmic reticulum kinase) phosphorylation, this induces slight expression of the cleaved active form of ATF6 (Barel et al, 2016). The sequences of signaling events induced by Francisella inside the individual cell associated with so-called ER stress may be the cell-autonomous defense mechanisms designed to eliminate cell damage rather than to eliminate invading bacteria (Pillich et al, 2016).…”