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Keywords:

  • human milk oligosaccharides;
  • infant formula;
  • prebiotics

Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. PREBIOTIC EFFECTS
  5. ANTI-ADHESIVE EFFECTS
  6. GLYCOME-MODIFYING EFFECTS
  7. HUMAN MILK OLIGOSACCHARIDES AS SELECTIN-LIGAND ANALOGS
  8. CLINICAL IMPLICATIONS – NECROTIZING ENTEROCOLITIS
  9. SUPPLEMENTS FOR INFANT FORMULA
  10. BIOSYNTHESIS OF HUMAN MILK OLIGOSACCHARIDES IN THE HUMAN MAMMARY GLAND
  11. CONCLUSION
  12. Acknowledgments
  13. REFERENCES

Human milk oligosaccharides (HMO) are complex glycans that are present at very high concentrations in human milk but not in infant formula. The significant energy expended by mothers to make these complex glycans suggests they must be important. How do maternal HMOs benefit the breast-fed infant? How are HMOs synthesized in the human mammary gland? How can we provide formula-fed infants with HMOs or HMO-like glycans? This article reviews current knowledge and open questions on the biosynthesis and functions of HMOs.


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. PREBIOTIC EFFECTS
  5. ANTI-ADHESIVE EFFECTS
  6. GLYCOME-MODIFYING EFFECTS
  7. HUMAN MILK OLIGOSACCHARIDES AS SELECTIN-LIGAND ANALOGS
  8. CLINICAL IMPLICATIONS – NECROTIZING ENTEROCOLITIS
  9. SUPPLEMENTS FOR INFANT FORMULA
  10. BIOSYNTHESIS OF HUMAN MILK OLIGOSACCHARIDES IN THE HUMAN MAMMARY GLAND
  11. CONCLUSION
  12. Acknowledgments
  13. REFERENCES

Human milk is often referred to as the “gold standard” of nutrition for the first few months of human life. It contains all essential nutrients for the infant to grow and develop, but it also contains ingredients that may provide health benefits beyond traditional nutrients. Parts of these functional ingredients are the human milk oligosaccharides (HMOs). High amounts and structural diversity of these glycans are unique to humans. Only trace amounts of oligosaccharides are present in mature bovine milk and, as a consequence, in bovine milk-based infant formula. The potential health benefits of HMOs have been studied over the years with a special emphasis on prebiotic effects, but there might be several other mechanisms by which HMOs may benefit the breastfed infant.

One liter of mature human milk contains about 5–10 g of unbound oligosaccharides (in addition to lactose), which is similar to the amount of milk proteins and exceeds the amount of milk lipids. The building blocks of milk oligosaccharides are the five monosaccharides D-glucose (Glc3), D-galactose (Gal), N-acetylglucosamine (GlcNAc), L-Fucose (Fuc), and sialic acid (Sia; N-acetyl neuraminic acid [Neu5Ac] in humans and both Neu5Ac and N-glycolyl neuraminic acid [Neu5Gc] in most other species). Lactose (Galβ1-4Glc) forms the reducing end of milk oligosaccharides (Figure 1). Gal in lactose can be sialylated in α2-3 and/or α2-6 linkages to form 3′sialyllactose and 6′sialyllactose, respectively. Lactose can also be fucosylated in α1-2, α1-3 linkages to form 2′fucosyllactose and 3′fucosyllactose, respectively. These trisaccharides are called the short-chain milk oligosaccharides. To form more complex milk oligosaccharides, lactose is elongated with up to 15 N-acetyllactosamine repeat units (Galβ1-3/4GlcNAc). Lactose or the polylactosamine backbone can be sialylated in α2-3 and/or α2-6 linkages and/or fucosylated in α1-2, α1-3, and/or α1-4 linkages. Approximately 200 different complex oligosaccharides have been identified in human milk. In contrast, infant formula contains only trace amounts of oligosaccharides, which are also structurally less complex. Does the presence of HMOs in human milk benefit the breastfed infant? Does the absence of HMOs in infant formula represent a disadvantage for formula-fed infants? What are the effects of HMOs on breastfed infants, and what are the underlying mechanisms? Most of the effects depend on specific oligosaccharide structures. Do non-HMO glycans that are added to some infant formula have similar or different effects?

image

Figure 1. Structural composition of milk oligosaccharides. Milk oligosaccharides can be short-chain trisaccharides, e.g., sialyllactose or fucosyllactose, or complex high-molecular-weight glycans.

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PREBIOTIC EFFECTS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. PREBIOTIC EFFECTS
  5. ANTI-ADHESIVE EFFECTS
  6. GLYCOME-MODIFYING EFFECTS
  7. HUMAN MILK OLIGOSACCHARIDES AS SELECTIN-LIGAND ANALOGS
  8. CLINICAL IMPLICATIONS – NECROTIZING ENTEROCOLITIS
  9. SUPPLEMENTS FOR INFANT FORMULA
  10. BIOSYNTHESIS OF HUMAN MILK OLIGOSACCHARIDES IN THE HUMAN MAMMARY GLAND
  11. CONCLUSION
  12. Acknowledgments
  13. REFERENCES

Several different mechanisms have been described by which HMOs interfere with bacteria-host interactions and impact the composition of the infant's intestinal microbiota. Already in 1954, György et al.1 reported prebiotic effects of HMOs, showing that a mixture of HMOs promotes the growth of Bifidobacterium bifidum (Figure 2). LoCascio et al.2 reported recently that Bifidobacterium longum biovar infantis, an isolate from the infant gut, prefers certain short-chain HMOs over more complex, high-molecular-weight HMOs, correlating with the expression of bacterial enzymes capable of cleaving oligosaccharides. These results suggest that the prebiotic, bifidogenic effects of HMOs are structure-specific and may vary depending on the HMO composition in the milk of different individuals or change over the course of lactation.

image

Figure 2. Prebiotic effects of HMOs. This highly simplified scheme shows that desired (light) and undesired (dark) bacteria have different capabilities of metabolizing HMOs. In the presence of HMOs (right), the desired bacteria metabolize HMOs and thrive while undesired bacteria cannot metabolize HMOs. Metabolites from bacterial HMO degradation, e.g., short-chain fatty acids, create an environment that also benefits the growth of desired bacteria. In the absence of HMOs (left), both desired and undesired bacteria can grow.

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The prebiotic effects of HMOs are probably best known and most referenced. However, about 90% of all HMOs are found intact and not metabolized in the infant's feces. This suggests that HMOs have additional effects other than just serving as prebiotic “fuel” to establish and maintain a certain desired mircobiota composition. The following sections describe some of these postulated effects.

ANTI-ADHESIVE EFFECTS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. PREBIOTIC EFFECTS
  5. ANTI-ADHESIVE EFFECTS
  6. GLYCOME-MODIFYING EFFECTS
  7. HUMAN MILK OLIGOSACCHARIDES AS SELECTIN-LIGAND ANALOGS
  8. CLINICAL IMPLICATIONS – NECROTIZING ENTEROCOLITIS
  9. SUPPLEMENTS FOR INFANT FORMULA
  10. BIOSYNTHESIS OF HUMAN MILK OLIGOSACCHARIDES IN THE HUMAN MAMMARY GLAND
  11. CONCLUSION
  12. Acknowledgments
  13. REFERENCES

The virulence of most pathogenic microorganisms, e.g., Campylobacter jejuni, Escherichia coli, Vibrio cholera, and Shigella and Salmonella strains, often depends on their ability to adhere to the host's epithelial surface. Adhesion-related virulence factors are often lectins, glycan-binding proteins, which bind to oligosaccharides on the epithelial cell surface3 (Figure 3A). Some of these glycan-binding determinants are also part of HMOs, suggesting that HMOs serve as soluble ligand analogs, block pathogen adhesion, and protect the breastfed infant against infections and diarrhea4 (Figure 3B). For example, Campylobacter jejuni, one of the major causes of diarrhea worldwide,5 adheres to intestinal 2′- fucosyllactosamine,6 which is also present on HMOs. In fact, Ruiz-Palacios et al.6 showed that fucosylated HMOs inhibit Campylobacter binding to human intestinal mucosa ex vivo. The incidence of Campylobacter diarrhea in breastfed infants is inversely related to the amount of 2′-fucosyllactose in the mother's milk.7 Antimicrobial effects of HMOs have also been described for calcivirus diarrhea7 and infections with heat-stable enterotoxin of E. coli.8 Recent data from our lab in collaboration with Dr. Benhur Lee's team at the University of California Los Angeles and Dr. Carlito Lebrilla's team at the University of California Davis show that HMO might also reduce HIV-1 mother-to-child transmission.9

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Figure 3. Anti-adhesive and glycome-modifying effects of HMOs. Most bacteria (commensals and pathogens) express glycan-binding proteins (lectins), that bind to glycans on the host's epithelial cell surface (A), which is essential for bacteria to attach (a), and to proliferate and colonize the intestine (b). Some pathogens need to attach to the intestinal epithelial cell surface prior to invading the host (c). HMOs are structurally similar to the intestinal epithelial cell surface glycans. They can serve as bacterial lectin ligand analogs and block bacterial attachment (B). Human milk oligosaccharides (HMO) may also alter the intestinal epithelial glycosylation machinery and modify the cell-surface glycome (“glycocalyx”), which could impact bacterial attachment, proliferation, and colonization (C).

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According to the Joint United Nations Programme on HIV/AIDS an estimated 2.3 million children are infected with HIV-1 worldwide (UNAIDS/WHO AIDS Epidemic Update: December 2006), with more than 500,000 new infections in 2006 alone. Mother-to-child transmission accounts for more than 40% of all HIV-1 infections in children, with breast-feeding being the predominant route of postnatal transmission.10 Breastfed infants from HIV-1-infected mothers are constantly exposed to the virus over a period of many months. It is currently unknown why a vast majority of these infants does not become HIV-infected. Our data suggests that HMOs may be part of the protection mechanism.

HIV-1 entry across the infant's mucosal barrier is partially mediated through binding of the viral envelope glycoprotein gp120 to DC-SIGN (dentritic cell-specific ICAM3-grabbing non-integrin) on human dendritic cells (DC).11,12 DC-SIGN-bound ligands (or pathogens) are usually internalized into DC lysosomes, processed, and then presented to T cells, which triggers an immune response to fight the intruder. HIV-1, however, “hides” within the DC for several days and escapes the immune system before getting transferred to CD4+ T lymphocytes, where it multiplies and later causes disease. Other viruses, e.g., hepatitis C, ebola, dengue, or CMV, use a very a similar approach to escape the immune response, using DC-SIGN to gain access to the host.13

DC-SIGN is a carbohydrate binding protein of the C-type lectin family. It recognizes mannose-containing glycoconjugates, such as HIV-1-gp120, but has even higher binding affinities for Lewis blood group antigens (Lewis × [Galβ1-4{Fucα1-3}GlcNAc], Lewis y, Lewis a, and Lewis b).14 The Lewis antigens contain N-acetyllactosamine (Galβ1-3/4GlcNAc), which is fucosylated in the α1-2 position on Gal and/or α1-3/4 position on GlcNAc. Although monomeric Lewis epitopes bind to DC-SIGN,14 the presence of multivalent Lewis epitopes is required to compete with HIV-1-gp120 for DC-SIGN-binding.15

The presence of multiple Lewis blood group determinants as part of HMOs led us to hypothesize that HMOs compete with HIV-1-gp120 for binding to DC-SIGN and therefore contribute to the protective effects against HIV-1 mother-to-child transmission during breastfeeding. We have shown, using two independent in vitro assays, that physiological concentrations of HMOs significantly reduce HIV-1gp120-binding to DC-SIGN by more than 80%.9 Our lab now aims to identify the specific individual HMOs that interact with DC-SIGN. However, blocking DC-SIGN may be a two-edged sword. It may reduce the entrance of certain viruses such as HIV-1, but it may also reduce the ability of the infant's immune system to detect and fight other pathogens, potentially increasing the risk for infants to develop bacterial or viral gastroenteritis. Once individual HMOs have been identified that block HIV-1-gp120 binding to DC-SIGN, it will be important to use additional in vitro and in vivo models to assess whether these HMOs trigger adverse effects.

GLYCOME-MODIFYING EFFECTS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. PREBIOTIC EFFECTS
  5. ANTI-ADHESIVE EFFECTS
  6. GLYCOME-MODIFYING EFFECTS
  7. HUMAN MILK OLIGOSACCHARIDES AS SELECTIN-LIGAND ANALOGS
  8. CLINICAL IMPLICATIONS – NECROTIZING ENTEROCOLITIS
  9. SUPPLEMENTS FOR INFANT FORMULA
  10. BIOSYNTHESIS OF HUMAN MILK OLIGOSACCHARIDES IN THE HUMAN MAMMARY GLAND
  11. CONCLUSION
  12. Acknowledgments
  13. REFERENCES

HMOs have prebiotic and antiadhesive effects, which modulate bacteria-host interactions and the infant's intestinal microbiota composition. Now, new in vitro data16 suggest that HMOs may also have glycome-modifying effects, modulating the expression of intestinal epithelial cell surface glycans (“glycocalyx”), the attachment sites for most pathogens and commensal bacteria (Figure 3C).

Angeloni et al. (2005) showed for the very first time that Caco-2 cells change their cell surface glycan profile upon exposure to 3′sialyllactose, one of the major oligosaccharides in human milk.16 The group used plant lectins to detect changes of cell surface glycan epitopes. Upon exposure to 3′sialyllactose, cell surface expression of α2-3- and α2-6-linked sialic acid residues was significantly reduced. Fucose and galactose residues were also diminished. To evaluate the significance of these cell surface glycome changes in the context of bacteria-host interactions, Angeloni et al. assessed whether adhesion of enteropathogenic Escherichia coli (EPEC) is altered.16 EPEC adheres to epithelial cell surface glycans in the host's intestine. Indeed, 3′sialyllactose-induced changes in the epithelial cell surface glycome led to a 90% reduction in EPEC adhesion compared to control cells.16 These results suggest a novel mechanism by which milk oligosaccharides, such as 3′sialyllactose, regulate bacteria-host interactions (Figure 3C).

A reduction of cell surface glycan epitopes can be caused by either decreased production or increased degradation. Angeloni et al.16 showed that the gene expression levels of α2-3-sialyltransferases are reduced. Expression levels of ST3Gal I, II, and IV were reduced 2.5-, 2-, and 5-fold, respectively. ST3Gal III, V, and VI were not affected. These results suggest that exogenous 3′sialyllactose modifies the cell surface glycome profile by regulating the expression of genes that encode for enzymes involved in glycan assembly. Whether glycan-related genes other than ST3Gal are also differentially expressed upon exposure to 3′siallylactose is unknown. Whether HMOs other than 3′sialyllactose have similar, additive, synergistic, or adverse effects is also unknown.

The study by Angeloni et al.16 was the first to report that exogenous glycans regulate gene expression. These results were recently supported by Kuntz et al.,17 who showed that oligosaccharides isolated from human milk inhibit cell growth and induce differentiation and apoptosis in intestinal epithelial cell lines. How exogenous glycans such as 3′sialyllactose signal and regulate gene expression is unknown.

HUMAN MILK OLIGOSACCHARIDES AS SELECTIN-LIGAND ANALOGS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. PREBIOTIC EFFECTS
  5. ANTI-ADHESIVE EFFECTS
  6. GLYCOME-MODIFYING EFFECTS
  7. HUMAN MILK OLIGOSACCHARIDES AS SELECTIN-LIGAND ANALOGS
  8. CLINICAL IMPLICATIONS – NECROTIZING ENTEROCOLITIS
  9. SUPPLEMENTS FOR INFANT FORMULA
  10. BIOSYNTHESIS OF HUMAN MILK OLIGOSACCHARIDES IN THE HUMAN MAMMARY GLAND
  11. CONCLUSION
  12. Acknowledgments
  13. REFERENCES

The mechanisms described above mostly affect bacterial-host interactions in the infant's intestine, but HMOs may also impact the infant on a systemic level. HMOs are partially absorbed intact in the infant's intestine and appear in the urine of breastfed, but not formula-fed, infants.18,19 These observations indirectly prove the presence of HMOs in the systemic circulation, although the direct detection of HMOs in the infant's blood has not yet been reported. However, a blood concentration of 100–200 µg/mL can be calculated based on the HMO concentration in human milk, the daily intake, the infant's blood volume, and the amount excreted with the urine over time.

Given that HMOs reach the systemic circulation, they may alter protein-carbohydrate interactions not only on a local but also on a systemic level. Several human lectins show binding specificity for glycan epitopes that are part of HMOs. Siglecs bind different sialylated glycans; galectins bind specific terminal galactose, e.g., as part of lactosamine. Whether HMOs interfere with siglec- or galectin-mediated processes is currently unknown.

Selectins bind to specific fucosylated and sialylated oligosaccharides, e.g., sialyl Lewis × (sLex), on their respective glycoconjugate ligands. These glycan-binding determinants can be part of HMOs.20 Selectins are involved in cell-cell interactions in the immune system. P- and E-selectin mediate the initial step in the leukocyte adhesion cascade. Selectins expressed on activated endothelial cells bind to glycans on leukocytes, which leads to leukocyte deceleration (rolling). Subsequently, the leukocytes adhere tightly to the endothelial cells and then transmigrate into subendothelial regions at sites of inflammation. In addition to the leukocyte extravasation process, P-selectin is also involved in the formation of platelet-neutrophil complexes, a subpopulation of highly activated neutrophils primed for adhesion, phagocytosis, and enhanced production of reactive oxygen species. Both selectin-mediated cell-cell interactions stimulate or activate the immune system. HMOs resembled the physiological binding determinants of selectins and may compete with physiological ligands for selectin binding. In fact, in vitro and ex vivo assays show that leukocyte rolling and adhesion21 as well as platelet-neutrophil formation and activation22 are reduced in the presence of physiologically relevant concentrations of HMO. Most intriguing, the total fraction of sialylated HMO decreases leukocyte adhesion to a greater extent than the physiological binding determinant, sLex, suggesting that complex high molecular HMOs carry multiple binding sites, which have been reported to enhance selectin-ligand binding.

Similar to the data showing that HMOs reduce HIV-1-gp120 binding to DC-SIGN, a decrease in selectin-mediated cell-cell interactions has to be looked upon as a two-edged sword. Do HMOs compromise the infant's immune defense and harm the breastfed infant or do they keep an immature immune system in check and protect breastfed infants from overshooting immune responses? A “hyperactivated” immune response appears to play a major role in the pathogenesis of necrotizing enterocolitis (NEC), and breastfed infants are at lower risk of developing this devastating disorder. Do HMOs contribute to the protective effects of human milk? The following paragraph outlines NEC pathogenesis and points out several key events that could benefit from the presence of HMOs.

CLINICAL IMPLICATIONS – NECROTIZING ENTEROCOLITIS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. PREBIOTIC EFFECTS
  5. ANTI-ADHESIVE EFFECTS
  6. GLYCOME-MODIFYING EFFECTS
  7. HUMAN MILK OLIGOSACCHARIDES AS SELECTIN-LIGAND ANALOGS
  8. CLINICAL IMPLICATIONS – NECROTIZING ENTEROCOLITIS
  9. SUPPLEMENTS FOR INFANT FORMULA
  10. BIOSYNTHESIS OF HUMAN MILK OLIGOSACCHARIDES IN THE HUMAN MAMMARY GLAND
  11. CONCLUSION
  12. Acknowledgments
  13. REFERENCES

Necrotizing enterocolitis is the most frequent and often life-threatening disorder that affects the intestine of premature infants. More than 10% of premature infants with very-low-birth-weight develop NEC.23–25 Advances in neonatology have resulted in the improved survival of premature and low-birth-weight infants, resulting in a growing population of infants at risk for NEC. The mortality rate for NEC patients ranges from 10 to 50% and approaches 100% for patients with the most severe form of the disease.25 However, the exact etiology of the disease remains undefined. In fact, NEC may represent a syndrome, with common findings and a variety of etiologies. The primary insults leading to NEC could be perinatal hypoxia or a mild postnatal infection. These insults cause mild mucosal damage and impaired intestinal epithelial barrier function. Following (formula) feeding and the proliferation of the intestinal flora, an increased influx of bacteria and bacterial product (LPS) into the mucosa induces the endogenous production of inflammatory cytokines such as platelet-activating factor (PAF) and TNF-α. Both cytokines further increase the intestinal permeability, closing a vicious circle. PAF also synergizes with LPS and TNF-α, reaching a threshold necessary to trigger a cascade of inflammatory events, including mucosal neutrophil infiltration and activation, early key events in NEC pathogenesis. Eventually, vasoconstriction occurs and leads to ischemia and subsequent reperfusion. Reactive oxygen species (ROS) produced by activated neutrophils and intestinal epithelial xanthine oxidase may then cause severe tissue necrosis and breakdown of the intestinal barrier. Entry of large amounts of bacteria and LPS leads to sepsis, shock, and death.26Figure 4 shows a flow diagram of the proposed pathogenesis of NEC.

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Figure 4. Flow diagram of proposed NEC pathogenesis and potential benefits of HMOs. Adapted from Hsueh et al. (2002)26). Abbreviations: HMO, human milk oligosaccharides; NEC, necrotizing enterocolitis; PAF, platelet-activating factor; PNC, platelet-neutrophil complexes; ROS, reactive oxygen species.

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Numerous risk factors have been implicated in NEC pathogenesis, including prematurity, hypoxia, bacterial infection, and intestinal ischemia, but also formula feeding. In fact, several studies have shown that NEC incidence is sixfold higher in formula-fed infants than in breastfed infants.27–31 Why formula-fed infants are at higher risk for developing NEC can be partially explained by significant activity of the PAF degrading enzyme PAF-acetylhydrolase (PAF-AH), which is present in human milk but not in infant formula.32 In addition, long-chain polyunsaturated fatty acids (LC-PUFA) in human milk modulate inflammation. Supplementing infant formula with PAF-AH or LC-PUFA reduces NEC incidence in rats.33,34 But do these differences alone account for the sixfold lower NEC incidence in breastfed infants?

Infant formulas have greatly improved over the past decades. For example, formulas are now routinely supplemented with LC-PUFA, more closely related to the composition of human milk. However, despite these improvements, human milk is still advantageous and formula-feeding remains a risk factor.28,31 A recent study by Schanler et al.28 observed a lower incidence of NEC cases in premature infants who received at least 50 mL/kg/day of human milk. Sisk et al.31 reported in 2007 that enteral feeding containing at least 50% human milk was associated with a sixfold decrease in the risk of developing NEC. A similar sixfold risk difference between formula-fed and breastfed infants had already been reported almost two decades ago.27 The gap in the composition of formula and human milk gets narrower, but the difference in NEC risk still remains. Are HMOs part of the missing link?

As outlined in the different sections above and in the diagram of Figure 4, HMOs may interfere with key events in NEC pathogenesis. HMOs may be able to modify bacteria-host interactions and the intestinal microbiota composition through several different mechanisms, serving as prebiotics, or antiadhesives. They may also alter bacterial adhesion by modifying the intestinal epithelial cell surface glycome, which represents major attachment sites for pathogens and commensals alike. In vitro and ex vivo studies also suggest that HMOs reduce neutrophil infiltration and activation as well as ROS production, which are early key events in NEC pathogenesis.

To date, no intervention studies have been conducted in human preterm neonates that attempted to correlate the presence or absence of HMOs with NEC incidence. An elegant and powerful approach would be to provide a population of preterm neonates with HMO-supplemented formula and compare NEC incidence in this group with a group of preterm neonates that receive the same formula, but without HMOs, varying only one dietary parameter: the presence or absence of HMOs. Unfortunately, the limited availability of HMOs makes this study unfeasible.

SUPPLEMENTS FOR INFANT FORMULA

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. PREBIOTIC EFFECTS
  5. ANTI-ADHESIVE EFFECTS
  6. GLYCOME-MODIFYING EFFECTS
  7. HUMAN MILK OLIGOSACCHARIDES AS SELECTIN-LIGAND ANALOGS
  8. CLINICAL IMPLICATIONS – NECROTIZING ENTEROCOLITIS
  9. SUPPLEMENTS FOR INFANT FORMULA
  10. BIOSYNTHESIS OF HUMAN MILK OLIGOSACCHARIDES IN THE HUMAN MAMMARY GLAND
  11. CONCLUSION
  12. Acknowledgments
  13. REFERENCES

Since HMOs are mostly absent from infant formula, formula-fed infants may miss out on the potential benefits attributed to HMOs. To close this gap, several infant formula-producing companies searched for inexpensive alternatives and developed mixtures of galactooligosaccharides (GOS, Figure 5, left) and fructooligosaccharides (FOS, Figure 5, right) or inulin that mimic the prebiotic effects of human milk and promote a bacterial microflora that closely resembles that of breastfed infants.35,36 In addition to their prebiotic effects, GOS/FOS-supplemented formulas have also been reported to modulate the immune system in mice37 and to reduce the incidence of infectious episodes38 and atopic dermatitis in at-risk infants.39 GOS and FOS or inulin, however, are structurally very different from the oligosaccharides occurring naturally in human milk (HMO). Considering that most of the postulated effects of HMOs are highly structure-specific, infant formula oligosaccharides may have different effects than HMOs. To date, GOS, FOS, or inulin represent a reasonable and inexpensive way to supplement infant formula with prebiotic oligosaccharides, but extensive research is needed to clarify the specific effects of these “artificial” glycans and, more so, to understand the mechanisms by which HMOs benefit the breastfed infant. To provide formula-fed infants with the same benefits that breastfed infants receive with their mother's milk, HMO-like glycans may be needed as formula supplements.

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Figure 5. Structural composition of infant-formula oligosaccharides. In comparison to human milk oligosaccharides (HMO, Figure 1), oligosaccharides added to some infant formula are structurally different. Galactooligosaccharides (GOS) consist of one to seven galactose moieties with glucose at the non-reducing end. Fructooligosaccharide (FOS) is a β1-2 fructose polymer. Fructose is not an oligosaccharide that occurs naturally in human milk. Sialic acid and fucose, which appear to be important structural and functional components of human milk oligosaccharides, are absent in infant-formula oligosaccharides.

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BIOSYNTHESIS OF HUMAN MILK OLIGOSACCHARIDES IN THE HUMAN MAMMARY GLAND

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. PREBIOTIC EFFECTS
  5. ANTI-ADHESIVE EFFECTS
  6. GLYCOME-MODIFYING EFFECTS
  7. HUMAN MILK OLIGOSACCHARIDES AS SELECTIN-LIGAND ANALOGS
  8. CLINICAL IMPLICATIONS – NECROTIZING ENTEROCOLITIS
  9. SUPPLEMENTS FOR INFANT FORMULA
  10. BIOSYNTHESIS OF HUMAN MILK OLIGOSACCHARIDES IN THE HUMAN MAMMARY GLAND
  11. CONCLUSION
  12. Acknowledgments
  13. REFERENCES

Considering that the concentration of HMOs in mature human milk is similar to the concentration of total milk proteins and even exceeds the concentration of milk lipids, it is most intriguing that the biosynthetic pathway of this major human milk component is still unknown. HMOs carry lactose at the reducing end. Lactose is synthesized in the Golgi of mammary gland epithelial cells. In the presence of α-lactalbumin, which is specifically expressed during lactation, the substrate specificity of β1-4-galactosyltransferase shifts from N-acetylglucosamine to glucose (Glc), now linking galactose (Gal) to Glc to form lactose. The biosynthetic steps leading from lactose to HMO are currently unknown. The fucosylated and/or sialylated lactosamine backbone could be synthesized on glycoproteins or glycolipids and later transferred to lactose en bloc. The lactosamine backbone could also be synthesized onto lactose, one monosaccharide at a time. Understanding how HMOs are synthesized in the human mammary gland could guide us in generating HMOs as supplements for infant formula, providing formula-fed infants with the same benefits that breastfed infants receive with their mother's milk.

CONCLUSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. PREBIOTIC EFFECTS
  5. ANTI-ADHESIVE EFFECTS
  6. GLYCOME-MODIFYING EFFECTS
  7. HUMAN MILK OLIGOSACCHARIDES AS SELECTIN-LIGAND ANALOGS
  8. CLINICAL IMPLICATIONS – NECROTIZING ENTEROCOLITIS
  9. SUPPLEMENTS FOR INFANT FORMULA
  10. BIOSYNTHESIS OF HUMAN MILK OLIGOSACCHARIDES IN THE HUMAN MAMMARY GLAND
  11. CONCLUSION
  12. Acknowledgments
  13. REFERENCES

Human milk oligosaccharides are complex glycans that are highly abundant in human breast milk. Infant formula contains only trace amounts of these oligosaccharides. Whether the presence of HMOs in human milk provides a significant advantage for the breastfed infant is currently unknown. It is generally accepted that HMOs have prebiotic effects, selectively serving as a source of energy and nutrients for desired bacteria to colonize the infant intestine. Beyond these prebiotic effects, HMOs may very likely benefit the breastfed infant through multiple other mechanisms. HMOs have been shown to be antiadhesive, mimicking the attachment sites for certain pathogens and blocking their adhesion, colonization, and invasion. HMOs have been shown to reduce leukocyte adhesion, extravasation, and activation in in vitro and ex vivo models. Most recent data suggests that HMOs might also have intestinal epithelial cell surface glycome-modifying effects, changing the glycosylation machinery of intestinal epithelial cells, altering the expression profiles of pathogen attachment sites, and reducing infectious diseases. Whether these effects can be translated in vivo remains to be elucidated. More and more infant formula is now supplemented with non-HMO oligosaccharides, such as galactooligosaccharides (GOS) and fructooligosaccharides (FOS). These infant formula oligosaccharides are not naturally present in human milk and are structurally different from the oligosaccharides naturally occurring in human milk (HMOs). Most of the effects attributed to HMOs appear to be highly structure dependent. Therefore, infant-formula oligosaccharides may have different effects than HMOs. Extensive research is needed in this highly understudied, but very important, area of research to understand the functional benefits of HMOs.

REFERENCES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. PREBIOTIC EFFECTS
  5. ANTI-ADHESIVE EFFECTS
  6. GLYCOME-MODIFYING EFFECTS
  7. HUMAN MILK OLIGOSACCHARIDES AS SELECTIN-LIGAND ANALOGS
  8. CLINICAL IMPLICATIONS – NECROTIZING ENTEROCOLITIS
  9. SUPPLEMENTS FOR INFANT FORMULA
  10. BIOSYNTHESIS OF HUMAN MILK OLIGOSACCHARIDES IN THE HUMAN MAMMARY GLAND
  11. CONCLUSION
  12. Acknowledgments
  13. REFERENCES
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