Research Highlights

Development: Sticky-sweet adhesions

Integrin-mediated cell adhesion and protein O-glycosylation have important developmental roles and are implicated in cancer progression. Reporting in the Journal of Biological Chemistry, Kelly Ten Hagen and colleagues make a connection between these two processes, presenting evidence that mucin-type O-glycosylation plays a critical role in integrin-mediated cell adhesion by influencing the makeup of the extracellular matrix (ECM).

Mucin-type O-glycosylation, the post-translational addition of the sugar GalNAc to proteins, is mediated by the conserved UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase family of glycosyltransferases (in mammals, ppGalNAcTs; in Drosophila, PGANTs). Four years ago, we highlighted how Ten Hagen and colleagues demonstrated the importance of a PGANT during development in Drosophila melanogaster; PGANT35A is involved in establishing basoapical cell polarity (O-glycosylation enzymes: Directing traffic during Drosophila embryogenesis). Later, the authors showed that another family member, PGANT3, glycosylates an ECM protein called Tiggrin. Flies with reduced levels of PGANT3 had less Tiggrin glycosylation and had blistered wings, an indication of defective adhesion between epithelial cell layers (Further reading). Whereas these findings identified PGANT3 as an important player in cell adhesion, its mechanistic role was still unclear.

In their new study, the authors used the wing-blistering phenotype to dissect the contribution of PGANT3 to integrin-mediated cell adhesion. By generating adult flies with novel point mutations in pgant3, they showed that loss of enzymatic activity in an otherwise stable PGANT3 protein is sufficient to abrogate Tiggrin glycosylation and cause wing blistering. PGANT3 appears to be specifically important for cell adhesion mediated by integrins, as a mild wing-blistering phenotype caused by partial loss-of-function mutations in an integrin-encoding gene was dramatically exacerbated by introduction of a single mutant pgant3 allele. Furthermore, integrin-dependent, but not integrin-independent, adhesion of cultured Drosophila cells was disrupted by RNAi-mediated knockdown of PGANT3 or Tiggrin.

The crux of the adhesion problem in pgant3 mutants appears to lie in faulty localization of Tiggrin, resulting from the loss of mucin-type O-glycosylation. During wing development, epithelial cell layers undergo several distinct adhesion and separation phases. Confocal imaging of normal wings showed that during adhesion phases, Tiggrin, O-glycans, and integrins localized to the basal interface between epithelial cell layers. In pgant3 mutants, however, Tiggrin and O-glycans were missing from this surface, although other membrane proteins localized normally. An interesting exception was an integrin that localized properly to the basal interface, but in a more diffuse pattern than normal, suggesting that integrin distribution may depend partly on Tiggrin localization.

A comparison of Tiggrin staining compared with that of a cell membrane marker confirmed that Tiggrin was no longer present on outer cell membranes during adhesion phases in pgant3 mutants, although it was expressed at normal levels as shown by western blot. Instead, Tiggrin accumulated in abnormal puncta inside the cell borders. Therefore, proper O-glycosylation by PGANT3 appears to be a required step for secretion of Tiggrin, without which integrin-mediated cell adhesion is compromised.

These findings highlight a new role for O-linked glycosylation in controlling the secretion and localization of an ECM component during eukaryotic development. Moreover, this work points to an intriguing link between O-glycosylation and integrin-mediated cell adhesion, two processes that have been strongly connected to cancer progression.

Related articles

von Elstermann, M. (2008) Glycosylation and development: A frog hops into the gap

von Elstermann, M. (2008) Protein O-glycosylation: Drosophila on acid

von Elstermann, M. (2006) O-glycosylation enzymes: Directing traffic during Drosophila embryogenesis

Author: Carole Weaver