Breast milk oligosaccharides: structure-function relationships in the neonate

Abstract

In addition to providing complete postnatal nutrition, breast milk is a complex biofluid that delivers bioactive components for the growth and development of the intestinal and immune systems. Lactation is a unique opportunity to understand the role of diet in shaping the intestinal environment including the infant microbiome. Of considerable interest is the diversity and abundance of milk glycans that are energetically costly for the mammary gland to produce yet indigestible by infants. Milk glycans comprise free oligosaccharides, glycoproteins, glycopeptides, and glycolipids. Emerging technological advances are enabling more comprehensive, sensitive, and rapid analyses of these different classes of milk glycans. Understanding the impact of inter- and intraindividual glycan diversity on function is an important step toward interventions aimed at improving health and preventing disease. This review discusses the state of technology for glycan analysis and how specific structure-function knowledge is enhancing our understanding of early nutrition in the neonate.

Keywords: bifidobacteria; breast milk glycans; glycomics; immunity; intestinal barrier function.

Figure 1

An example of the structural diversity of human milk oligosaccharides (HMOs). (a) With few exceptions, all HMO structures consist of a lactose core linked to lacto-N-biose or to N-acetyllactosamine with n = 0–15 units. (b) Lactose can be fucosylated or sialylated by different linkages. (c) Lactose can be elongated enzymatically in repeats of lacto-N-biose (type I) or N-acetyllactosamine (type II). (d) Elongated type I or II chains can be fucosylated in different linkages to form structures that phenotypically describe secretor status and the Lewis blood group. (e) The elongated core HMO structures can be sialylated by α2-3 or α2-6 linkages at the terminal positions forming structural isomers. Abbreviations: 2′-FL, 2′-fucosyllactose; 3FL, 3-fucosyllactose; 3′-SL, 3′ sialyllactose; 6′-SL, 6′-sialyllactose; LNFP I, II, III, V, lacto-N-fucopentaose I, II, III, V; LNH, lacto-N-hexaose; LNnT, lacto-N-neotetraose; LNT, lacto-N-tetraose; LST a, b, c, sialyl-lacto-N-tetraoses a–c. Adapted from Reference 15 with permission.

Figure 2

Infant gut-associated bifidobacteria cleave a diverse range of specific linkages within human milk glycans using a variety of glycosyl hydrolases. Legend at bottom indicates the monosaccharide composition and corresponding potential glycolytic enzymes in bifidobacteria that cleave the specific linkages. The figure depicts the structure of HMO, a complex N-glycan, three different cores found in human O-linked glycans, and the glycolipid structure of ganglioside GD3. Adapted from Reference 51 with permission.

Figure 3

Possible strategies for human milk oligosaccharide (HMO) consumption in Bifidobacterium bifidum, B. infantis, B. breve, and B. longum. Dashed lines in the HMO panel represent potential linkages. Abbreviations: GNB, galacto-N-biose; LNB, lacto-N-biose. Adapted from Reference 48 with permission.

Figure 4

The variation of human milk oligosaccharide (HMO) fucose composition is dependent on maternal genetics that dictate the activities of distinct fucosyltransferases phenotypically described as secretor status and Lewis blood group. Abbreviations: FL, fucosyllactose; LNH, lacto-N-hexaose; LNnT, lacto-N-neotetraose; LNFP I, II, III, V, lacto-N-fucopentaose I, II, III, V; LNT, lacto-N-tetraose; LST a–c, sialyl-lacto-N-tetraoses a–c. Adapted from Reference 17 with permission.

Figure 1

(a) Individual human milk oligosaccharide (HMO) abundance as the normalized percent contribution of each isomeric oligosaccharide species in breast milk. The shaded box indicates the five most abundant oligosaccharide species in the total pool of HMOs analyzed, representing 70% of the overall detectable HMO pool. The arrow shows a particular abundance completely consumed by Bifidobacterium longum subsp. infantis (B. infantis). (b) Nano electrospray ionization Fourier transform ion cyclotron resonance (ESI-FT-ICR) (+) mass spectrometric analysis of B. infantis grown on media initially supplemented with 2.0% (w/v) HMO. Data represent the percent difference of HMO species abundance in the media before and at the end of fermentation, corresponding to 0 h and 94 h. Measurements were conducted in triplicates of individual biological and technical replicates. (c) Growth curves of B. infantis, B. breve, and B. longum on a semisynthetic de Man, Rogosa, and Sharpe (MRS) medium supplemented with 2% (w/v) HMO. Growth was measured as optical density (OD) of the media at 600 nm. Error bars are standard deviations of the mean for each available time point. Abbreviation: m/z, ratio of mass to charge. Adapted from Reference 82 with permission.

Figure 6

N-Glycosylation of milk proteins is altered in gestational diabetes mellitus (GDM). Milk proteins from mothers with and without GDM were compared using orthogonal signal corrected partial least-squares discriminant analysis. The maximum difference between milk glycans from women with and without GDM was captured in the first dimension or latent variable (LV[1]) of the model. Positive values indicate higher glycosylation in GDM compared to controls; conversely, negative values indicate lower glycosylation in GDM compared to controls. LF 5411 is an N-glycan that contains 5 hexoses, 4 N-acetyl-hexosamines, 1 fucose, and 1 sialic acid. Abbreviations: CH, complex hybrid; F, fucose; FS, fucose and sialic acid; HM, high mannose; LF, lactoferrin; S, sialic acid; sIgA, secretory immunoglobulin A. Adapted from Reference 122 with permission.