Glycoconjugates in human and transgenic animal milk

Human milk samples contain a particularly rich collection of oligosaccharides compared with other milk samples. The synthesis of these molecules should depend on the expression of glycosyltransferases and the presence of sugar nucleotides in lactating mammary glands. We set out to produce transgenic animals expressing glycosyltransferases during lactation with the purpose of exploring the following issues: a) Is it possible to synthesize human milk oligosaccharides in lactating mammary glands of nonhuman animals?, b) Is it possible to express during lactation homologous, tissue specific glycosyltransferases that are not normally expressed in lactating mammary tissue?, and c) What is the effect of expressing a human glycosyltransferase in different animal species? Simultaneously, we embarked on a research program to study short-chain neutral human milk oligosaccharides--no larger than hexasaccharides--to understand the natural variation of milk sugars and glycoproteins. The reagents and methods developed to study human milk oligosaccharides and glycoproteins were also applied to the study of milk from transgenic animals. Our results indicate that mice predictably express transgene-encoded glycosyltransferases and their secondary gene products, oligosaccharides and remodeled glycoproteins. This was true even when the transgene encoded a homologous galactosyltransferase. Also, it was possible to synthesize fucosylated glycoconjugates in mouse milk using two different fucosyltransferases, thus demonstrating that is feasible to emulate the synthesis occurring in the human lactating mammary gland. Experiments with transgenic rabbits yielded different phenotypes, some of them unexpected. Taken together, our results answer the questions stated above but open even more intriguing areas of inquiry.