Complex asparagine-linked oligosaccharides are required for morphogenic events during post-implantation development.

Abstract

Complex asparagine (N)‐linked oligosaccharides appear late in phylogeny and are highly regulated in vertebrates. Variations in these structures are found on the majority of cell‐surface and secreted proteins. Complex N‐linked oligosaccharide biosynthesis is initiated in the Golgi apparatus by the action of Mgat‐1‐encoded UDP‐N‐acetylglucosamine:alpha‐3‐D‐ mannoside beta‐1,2‐N‐acetylglucosaminyltransferase I (GlcNAc‐TI). To determine if these structures govern ontogenic processes in mammals, mouse embryos were generated that lacked a functional Mgat‐1 gene. Inactivation of both Mgat‐1 alleles produced deficiencies in GlcNAc‐TI activity and complex N‐linked oligosaccharides. Embryonic lethality occurred by day 10.5, thus establishing that complex N‐linked oligosaccharides are required during post‐implantation development. Remarkably, embryonic development proceeded into day 9 with the differentiation of multiple cell types. Complex N‐linked oligosaccharides are important for morphogenic processes as neural tube formation, vascularization and the determination of left‐right body plan asymmetry were impaired in the absence of a functional Mgat‐1 gene.

Respiratory Distress and Neonatal Lethality in Mice Lacking Golgi α1,2-Mannosidase IB Involved in N-Glycan Maturation

“…The present phenotype is very different from those caused by genetic ablation of other enzymes acting in early stages of the N-glycan processing pathway. Systemic deletion of N-acetylglucosaminyltransferase I results in depletion of complex and hybrid N-glycan structures and is embryonic lethal at E9.5 due to multiple organ abnormalities, including the absence of an organized layer of bronchial epithelial cells in the lung (2,3,37). It was further demonstrated that homozygous null N-acetylglucosaminyltransferase I ES cells are unable to populate the bronchial epithelium (37).…”

“…They convert high mannose N-glycans to Man 5 GlcNAc 2 (reviewed in Ref. 1), thereby providing the substrate for maturation to hybrid and complex N-glycans that are absolutely required for normal mouse embryogenesis (2,3). These enzymes belong to glycosylhydrolase family 47 based on amino acid sequence similarity, and their enzymatic specificity further distinguishes them into two groups.…”

“…These animal models have provided insights into critical functions mediated by N-glycans that cannot be observed in cultured cells. This was first noted by the ability to grow cells lacking N-acetylglucosaminyltransferase I in culture (26), whereas gene ablation is embryonic lethal in the mouse (2,3).…”

Abnormalities of developmental cell death in Dad1‐deficient mice

“…Impairment of the hybrid and complex classes of N-glycans by gene targeting of the Mgat1 locus leads to embryonic lethality at about E10.5 (Ioffe & Stanley 1994;Metzler et al 1994). Mgat1 de®ciency leads to growth retardation, neural tube abnormalities, impairment of the vascular system, in addition to loss of N-acetylglucosaminyltransferase I activity.…”

Retinoic acid administration is associated with changes in the extracellular matrix and cardiac mesenchyme within the endocardial cushion

“…The disturbance in the asymmetry of these proteins resulted in randomization of early heart looping and concomitant cardiac defects (Smith et al, 1997). The randomization of left-right orientation such as reversals and isomerism in the heart and the viscera are also seen in genetic mutants (Seo et al, 1992;Metzler et al, 1994). Therefore, the observed effect of RA upon the superior and the inferior cushion in this research may be due to the inherent biological asymmetry between the left and right sides of the developing embryo.…”