N-linked polylactosamine glycan synthesis is regulated by co-expression of β3GnT2 and GCNT2

Poly-N-acetyllactosamine (PLN) is a unique glycan composed of repeating units of the common disaccharide (Galβ1,4-GlcNAcβ1,3)n . The expression of PLN on glycoprotein core structures minimally requires enzyme activities for β1,4-galactosyltransferase (β4GalT) and β1,3-N-acetylglucosminyltransferase (β3GnT). Because β4GalTs are ubiquitous in most cells, PLN expression is generally ascribed to the tissue-specific transcription of eight known β3GnT genes in mice. In the olfactory epithelium (OE), β3GnT2 regulates expression of extended PLN chains that are essential for axon guidance and neuronal survival. N-glycan branching and core composition, however, can also modulate the extent of PLN modification. Here, we show for the first time that the β1,6-branching glycosyltransferase GCNT2 (formerly known as IGnT) is expressed at high levels specifically in the OE and other sensory ganglia. Postnatally, GCNT2 is maintained in mature olfactory neurons that co-express β3GnT2 and PLN. This highly specific co-expression suggests that GCNT2 and β3GnT2 function cooperatively in PLN synthesis. In support of this, β3GnT2 and GCNT2 co-transfection in HEK293T cells results in high levels of PLN expression on the cell surface and on adenylyl cyclase 3, a major carrier of PLN glycans in the OE. These data clearly suggest that GCNT2 functions in vivo together with β3GnT2 to determine PLN levels in olfactory neurons by regulating β1,6-branches that promote PLN extension.