Abstract
This study examines the genes coding for enzymes involved in bovine milk oligosaccharide metabolism by comparing the oligosaccharide profiles with the expressions of glycosylation-related genes. Fresh milk samples (n = 32) were collected from four Holstein and Jersey cows at days 1, 15, 90 and 250 of lactation and free milk oligosaccharide profiles were analyzed. RNA was extracted from milk somatic cells at days 15 and 250 of lactation (n = 12) and gene expression analysis was conducted by RNA-Sequencing. A list was created of 121 glycosylation-related genes involved in oligosaccharide metabolism pathways in bovine by analyzing the oligosaccharide profiles and performing an extensive literature search. No significant differences were observed in either oligosaccharide profiles or expressions of glycosylation-related genes between Holstein and Jersey cows. The highest concentrations of free oligosaccharides were observed in the colostrum samples and a sharp decrease was observed in the concentration of free oligosaccharides on day 15, followed by progressive decrease on days 90 and 250. Ninety-two glycosylation-related genes were expressed in milk somatic cells. Most of these genes exhibited higher expression in day 250 samples indicating increases in net glycosylation-related metabolism in spite of decreases in free milk oligosaccharides in late lactation milk. Even though fucosylated free oligosaccharides were not identified, gene expression indicated the likely presence of fucosylated oligosaccharides in bovine milk. Fucosidase genes were expressed in milk and a possible explanation for not detecting fucosylated free oligosaccharides is the degradation of large fucosylated free oligosaccharides by the fucosidases. Detailed characterization of enzymes encoded by the 92 glycosylation-related genes identified in this study will provide the basic knowledge for metabolic network analysis of oligosaccharides in mammalian milk. These candidate genes will guide the design of a targeted breeding strategy to optimize the content of beneficial oligosaccharides in bovine milk.
Highlights
Oligosaccharides are present in bovine milk as lactose-derived free forms and as glycoconjugates bound to proteins and lipids [1]
The gene expression profiles showed increased expression of most of the galactosyltrasnsferase genes in late lactation milk. This is the first published study on expression profiling of genes involved in milk oligosaccharide metabolism in any mammalian species
The study compared the annotation of bovine milk oligosaccharide profiles with the expression analysis of genes in milk oligosaccharide metabolism
Summary
Oligosaccharides are present in bovine milk as lactose-derived free forms and as glycoconjugates bound to proteins and lipids [1]. Recent studies on free milk oligosaccharides and glycoconjugates demonstrate both local and systemic beneficial effects to the suckling human neonate [2,3,4]. There is considerable evidence that breastfeeding provides long-term cognitive advantages to the infant, even through adulthood [10]. It has been shown in developing piglets that sialic acid supplementation of a milk formula acts as a conditional nutrient during periods of rapid brain growth [11]. It is thought that large amounts of sialylated oligosaccharides in human milk are responsible for the higher cognitive performance of breast-fed infants [11]
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