The disulfide catalyst QSOX1 maintains the colon mucosal barrier by regulating Golgi glycosyltransferases

Abstract

Mucus is made of enormous mucin glycoproteins that polymerize by disulfide crosslinking in the Golgi apparatus. QSOX1 is a catalyst of disulfide bond formation localized to the Golgi. Both QSOX1 and mucins are highly expressed in goblet cells of mucosal tissues, leading to the hypothesis that QSOX1 catalyzes disulfide‐mediated mucin polymerization. We found that knockout mice lacking QSOX1 had impaired mucus barrier function due to production of defective mucus. However, an investigation on the molecular level revealed normal disulfide‐mediated polymerization of mucins and related glycoproteins. Instead, we detected a drastic decrease in sialic acid in the gut mucus glycome of the QSOX1 knockout mice, leading to the discovery that QSOX1 forms regulatory disulfides in Golgi glycosyltransferases. Sialylation defects in the colon are known to cause colitis in humans. Here we show that QSOX1 redox control of sialylation is essential for maintaining mucosal function.

Disulfide bond formation and redox regulation in the Golgi apparatus

“…produced in mice lacking QSOX1, such that these animals were hypersensitive to induced colitis [21]. As described below, no evidence was found that QSOX1 introduces disulfide bonds in vivo into the major glycoproteins that make up mucus.…”

“…In vitro, QSOX1 oxidizes an intestinal mucus-associated lectin protein, ZG16 [51], but it is not yet known whether ZG16 is a physiological substrate of QSOX1 in vivo or how the ZG16 disulfide might contribute to mucus properties [59]. In contrast, QSOX1 was found to introduce disulfides into certain Golgi glycosyltransferases in the intestinal epithelium, with clear physiological consequences [21]. Specifically, the sialyltransferases (STs) St6gal1 and St3gal1, enzymes that introduce sialic acid into glycoproteins and glycolipids, were excessively reduced, and thus presumably inactive [60][61][62], in QSOX1 knockout (KO) mice [21].…”

“…Specifically, the sialyltransferases (STs) St6gal1 and St3gal1, enzymes that introduce sialic acid into glycoproteins and glycolipids, were excessively reduced, and thus presumably inactive [60][61][62], in QSOX1 knockout (KO) mice [21]. Inactivity of the reduced STs was supported by the observation that colon mucosal tissue of QSOX1 KO mice had a much lower content of sialylated glycans than normal colon [21]. QSOX1 was seen to oxidize rapidly a regulatory disulfide bond adjacent to the active site of the St6gal1 human ortholog [21].…”

“…The product of goblet cell secretion, however, is mucus rather than ECM. Intestinal mucus was found to be unstable and poorly functional when produced in mice lacking QSOX1, such that these animals were hypersensitive to induced colitis [21]. As described below, no evidence was found that QSOX1 introduces disulfide bonds in vivo into the major glycoproteins that make up mucus.…”

“…Quiescin sulfhydryl oxidase 1 (QSOX1) and its less abundant paralog QSOX2 are the only sulfhydryl oxidases or oxidoreductases known to be specifically localized to the Golgi in animal cells [15,16]. This review will focus on QSOX1, which has been studied structurally [17][18][19], mechanistically [19,20], and functionally [21]. QSOX2, in contrast, is largely uncharacterized but, like QSOX1 [22][23][24][25], has been reported to be associated with cancer progression [26][27][28].…”