Molecular characterization of pig UDP-N-acetylglucosamine-2-epimerase/N- acetylmannosamine kinase (Gne) gene: Effect of dietary sialic acid supplementation on gene expression in piglets

The UDP-N-acetylglucosamine-2-epimerase/N-acetylmannosamine kinase (Gne) gene encodes a key bifunctional enzyme important in initiating and regulating sialic acid (Sia) synthesis. Sia is commonly expressed as a terminal (non-reducing) sugar residue on oligosaccharide side chains on cell surface glycoconjugates where it mediates many carbohydrate receptor interactions, particularly in the neural and immune systems. The myriad of functions include nerve cell transmission, memory formation,cell-cell adhesion and communication, and lymphocyte extravasation. Sia is also a major component of human milk oligosaccharides where its level has been correlated with enhanced cognitive function and memory. There is no information, however, if early dietary Sia supplementation that increases the intracellular levels of Sia, may regulate Gne gene expression in developing piglets. To study this possible regulation, we cloned and sequenced a 4.3 kb of pig Gne cDNA (4326 nt), including 2169-nucleotides in the coding region. The Gne cDNA showed 91.9% and 98.9% identities to the human counterparts in the nucleotide and protein sequences, respectively. There are only eight amino acid differences between human and pig Gne, making the pig cDNA closer in homology to the human than the orthologs from mouse, rat, hamster, chicken, frog or zebrafish. The piglets Gne gene is highly expressed in the hippocampus, liver, and thymus followed by the pancreas, frontal cortex, lungs, kidneys, heart and spleen. The lowest level of expression was in skeletal muscle. Importantly, we have now shown that dietary Sia supplementation in developing piglets with a Sia-rich glycopeptide and active learning increased the Gne mRNA expression levels 2 to 3-fold in brain hippocampus and liver. These findings suggest that normal brain development and active learning increases the requirement for sialylated glycoconjugates, including neural cell adhesion molecules (N-CAM), and brain gangliosides. They further show that dietary supplements of Sia can modulate Gne gene expression, and that Sia is an essential nutrient during periods of rapid neural growth and brain development in piglets.