Featured Articles

Cell migration: Cadherin glycans branch out

© Nevit Dilmen, C-C 3.0

Cadherins mediate cell–cell adhesion by forming homotypic contacts between cells in response to calcium binding. N-cadherin, implicated in tumor invasiveness and metastatic potential, is N-glycosylated, and the glycans are known to affect the stability of cell–cell contacts. The branched glycan structure β-1,6 N-acetylglucosamine (β-1,6 GlcNAc) is associated with the malignant transformation of cells, and its presence on N-cadherin regulates cell adhesion by an unknown mechanism. Now, Hua-Bei Guo et al. have identified the glycosylation sites of human N-cadherin, and examined how β-1,6 GlcNAc glycans affect adhesion. They report in the Journal of Biological Chemistry that the inhibition of glycosylation at three sites, or the loss of glycan branching, leads to increased cell–cell contacts and reduced cell migration through increased cis-dimerization of N-cadherin.

The transfer of β-1,6 branched GlcNAc onto N-glycans is catalyzed by β-1,6 N-acetylglucosaminyltransferase V (GnT-V). In earlier work, increased expression of GnT-V in fibroblast cells was shown to increase N-linked β-1,6 branching on N-cadherin, and to decrease homophilic cell–cell adhesion mediated by the receptor². In this study, the authors confirmed that the opposite is also true; knocking down GnT-V expression using siRNA caused decreased levels of β-1,6 branching on N-cadherin, and an increase in homotypic cell–cell aggregation. The latter, calcium-dependent effect was disrupted by an antibody directed against N-cadherin, but not one against integrin β1. Cell adhesion-mediated intracellular signaling was enhanced, and the rate of cell migration decreased, by GnT-V knockdown.

To characterize N-cadherin glycans, the authors generated mutants lacking each of eight potential N-glycosylation sites. Three sites were found to be occupied, and mutation of all three sites together resulted in a significant increase in cell–cell adhesion. A similar effect was produced by treatment with inhibitors of either N-glycosylation or β-1,6 branching. Combined mutagenesis and lectin-binding experiments showed that all three N-cadherin sites carry mature, branched glycans.

The cytoplasmic domain of cadherins binds to catenins, proteins that link cadherins to the actin cytoskeleton. Glycosylation of E-cadherin is thought to affect both its turnover rate at the cell surface and the stability of its complexes formed with catenins. Guo et al. in this study found no influence of glycosylation on the cell-surface expression of N-cadherin, and no difference in the formation of complexes with catenins. By contrast, chemical cross-linking experiments showed that cis-dimerization of N-cadherin is increased by reduced expression of branched glycans. This dimerization is thought to yield the functional unit for trans N-cadherin binding. Thus, loss of branched glycans can reduce migration by promoting N-cadherin dimerization and the formation of cell–cell contacts.

This study adds to the evidence that N-glycosylation of cadherins plays a prominent role in regulating calcium-dependent cell adhesion. Cadherin glycan alterations can alter cell behavior through different mechanisms, but decreased dimerization of N-cadherin appears to be one mechanism by which upregulated GnT-V activity can produce a migratory, invasive phenotype in cancer cells.

Author: Emma Leah