N-glycan branching requirement in neuronal and postnatal viability.

The structural variations among extracellular N-glycans reflect the activity of glycosyltransferases and glycosidases that operate in the Golgi apparatus. More than other types of vertebrate glycans, N-glycans are highly branched oligosaccharides with multiple antennae linked to an underlying mannose core structure. The branching patterns of N-glycans consist of three types, termed high-mannose, hybrid, and complex. Though most extracellular mammalian N-glycans are of the complex type, some cells variably express hybrid and high-mannose forms. Nevertheless, a requirement for hybrid and complex N-glycan branching exists in embryonic development and postnatal function among mice and humans inheriting defective Mgat1 or Mgat2 alleles. The resulting defects in formation N-glycan branching patterns cause multiple abnormalities, including neurologic defects, and have inferred the presence of distinct functions for hybrid and complex N-glycan branches among different cell lineages. We have further explored N-glycan structure-function relationships in vivo by using Cre-loxP conditional mutagenesis to abolish hybrid and complex N-glycan branching specifically among neuronal cells. Our findings show that hybrid N-glycan branching is an essential posttranslational modification among neurons. Loss of Mgat1 resulted in a unique pattern of neuronal glycoprotein deficiency concurrent with caspase 3 activation and apoptosis. Such animals exhibited severe locomotor deficits, tremors, paralysis, and early postnatal death. Unexpectedly, neuronal Mgat2 deletion resulting in the loss of complex but not hybrid N-glycan branching was well tolerated without phenotypic markers of neuronal or locomotor dysfunction. Structural features associated with hybrid N-glycan branching comprise a requisite posttranslational modification to neuronal glycoproteins that permits normal cellular function and viability.