Essential role for complex N-glycans in forming an organized layer of bronchial epithelium

Mice lacking the complex subset of N-glycans due to inactivation of the Mgatl gene die at mid-gestation, making it difficult to identify specific biological functions for this class of cell surface carbohydrates. To circumvent this embryonic lethality and to uncover tissue-specific functions for complex N-glycans, WW6 embryonic stem cells with inactivated Mgatl alleles were tracked in chimeric embryos. The Mgatl gene encodes N- acetylglucosaminyltransferase 1 (GIcNAc-TI; EC 2.4.1.101), the transferase that initiates the synthesis of complex N-glycans. WW6 cells carry an inert β-globin transgene that allows their identification in chimeras by DNA-DNA in situ hybridization. Independent Mgatl-/- and Mgatl+/- mutant WW6 isolates contributed like parent WW6 cells to the tissues of embryonic day (E) 10.5 to EI6.5 chimeras. However, a cell type-specific difference was observed in lung. Homozygous null Mgatl-/- WW6 cells did not contribute to the epithelial layer in more than 99% bronchi. This deficiency was corrected by transfection of a Mgatl transgene. Interestingly, heterozygous Mgatl+/- WW6 cells were also deficient in populating the layer of bronchial epithelium. Furthermore, examination of lung bud in E9.5 Mgatl-/- mutant embryos showed complete absence of an organized epithelial cell layer in the bronchus. Thus, complex N-glycans are required to form a mnrphologically recognizable bronchial epithelium, revealing an in vivo, cell typespecific function for this class of N-glycans.