O-Glycosylation regulates polarized secretion by modulating Tango1 stability

Abstract

Polarized secretion is crucial in many tissues. The conserved protein modification, O-glycosylation, plays a role in regulating secretion. However, the mechanisms by which this occurs are unknown. Here, we demonstrate that an O-glycosyltransferase functions as a novel regulator of secretion and secretory vesicle formation in vivo by glycosylating the essential Golgi/endoplasmic reticulum protein, Tango1 (Transport and Golgi organization 1), and conferring protection from furin-mediated proteolysis. Loss of the O-glycosyltransferase PGANT4 resulted in Tango1 cleavage, loss of secretory granules, and disrupted apical secretion. The secretory defects seen upon loss of pgant4 could be rescued either by overexpression of Tango1 or by knockdown of a specific furin (Dfur2) in vivo. Our studies elucidate a novel regulatory mechanism whereby secretion is influenced by the yin/yang of O-glycosylation and proteolytic cleavage. Moreover, our data have broader implications for the potential treatment of diseases resulting from the loss of O-glycosylation by modulating the activity of specific proteases.R egulation of secretory vesicle formation and polarized secretion in vivo is crucial to ensure the proper deposition of signaling molecules, morphogens, and matrix components that mediate growth and differentiation. Polarized secretion is also required in many differentiated tissues, such as the digestive tract, where secreted components along the apical surface form the mucous membrane that confers protection from mechanical and microbial insults (1) and provides immunoregulatory signals (2). Indeed, disruptions in the secreted mucous membrane are associated with diseases of the digestive tract, such as colitis and colon cancer (3-6).Recent studies aimed at identifying the factors that influence secretion have elucidated novel proteins that function in unique aspects of secretion, including the enzymes responsible for the addition of sugars to mucins and other proteins (mucin-type O-glycosylation) (7). O-glycosylation is an essential, evolutionarily conserved protein modification (8, 9) that has direct medical relevance, as aberrations are responsible for the human diseases familial tumoral calcinosis (10, 11) and Tn syndrome (12). Loss of this protein modification affected constitutive secretion and Golgi apparatus structure in Drosophila cell culture (7, 13) and secretion in the developing respiratory system in vivo (14). Additionally, loss of O-glycosylation also disrupted secretion of an extracellular matrix protein (Tiggrin) in the developing wing, resulting in aberrant basement membrane formation and disrupted integrin-mediated cell adhesion (15). Mammalian studies have confirmed the effects of O-glycosylation on secretion, as loss of a glycosyltransferase (ppGalNAcT-1) disrupted secretion of laminin and collagen during mammalian organogenesis, altering the composition of the basement membrane and disrupting proper FGF signaling and organ growth (16). Although these studies all point to a role for O-glycosylation in se...

Two matrix metalloproteinase classes reciprocally regulate synaptogenesis

“…At the Drosophila NMJ, a similar reciprocal suppression interaction between pgant glycosyltransferases involved in O-linked glycosylation regulates synaptogenesis via integrin-tenascin transsynaptic signaling (Dani et al, 2014). A recent study reported that pgant activity protects substrates from Furin-mediated proteolysis, which is a protease responsible for processing or activating Drosophila Mmp1 and Mmp2 (Zhang et al, 2014). Thus, Mmp proteolytic and glycan mechanisms could converge within the NMJ synaptomatrix to regulate trans-synaptic signaling.…”

Characterizing the O-glycosylation landscape of human plasma, platelets, and endothelial cells

“…Similarly, proximity to endoplasmic reticulum/Golgi trafficking signals is notable because O-glycosylation has been demonstrated to regulate secretion, suggesting a function in vesicular transport. [69][70][71] Given that vesicular trafficking signals remain poorly annotated, it will be of interest to further investigate the potential role of O-glycosylation in protein trafficking experimentally.…”

Pleiotropic effects of O-glycosylation in colon cancer

“…GalNAc-T6 is one of the 20 family members in mammals that are responsible for the initiation of O-linked glycosylation (2). Members of this family are essential in Drosophila and are involved in various cellular and developmental processes (6,7). Aberrant GALNT6 expression has previously been observed in several cancer types, but the biological role of GALNT6 in vivo is still unknown.…”

“…These cells (also known as SimpleCells or LS174T SC ) were compared with a version with GALNT6 ablated (LS174T SC ⌬T6) to identify the proteins that are specifically glycosylated by GalNAc-T6. Interestingly, they found a small number of proteins that appear to be GalNAc-T6 -specific targets, including MIA3 (also known as Tango1), which had previously been associated with colon cancer development (8) and is essential for secretory apparatus structure and secretion (7,9).…”