Serum Protein N-Glycans in Colostrum and Mature Milk of Chinese Mothers

O-linked glycosylations in human milk casein and major whey proteins during lactation

As glycosylations are difficult to analyze, their roles and effects are poorly understood. Glycosylations in human milk (HM) differ across lactation. Glycosylations can be involved in antimicrobial activities and may serve as food for beneficial microorganisms. This study aimed to identify and analyze O-linked glycans in HM by high-throughput mass spectrometry. 184 longitudinal HM samples from 66 donors from day 3 and months 1, 2, and 3 postpartum were subjected to a post-translational modification specific enrichment-based strategy using TiO2 and ZrO2 beads for O-linked glycopeptide enrichment. β-CN was found to be a major O-linked glycoprotein, additionally, αS1-CN, κ-CN, lactotransferrin, and albumin also contained O-linked glycans. As glycosyltransferases and glycosidases are involved in assembling the glycans including O-linked glycosylations, these were further investigated. Some glycosyltransferases and glycosidases were found to be significantly decreasing through lactation, including two O-linked glycan initiator enzymes (GLNT1 and GLNT2). Despite their decrease, the overall level of O-linked glycans remained stable in HM over lactation. Three different motifs for O-linked glycosylation were enriched in HM proteins: Gly-Xxx-Xxx-Gly-Ser/Thr, Arg-Ser/Thr and Lys-Ser/Thr. Further O-linked glycan motifs on β-CN were observed to differ between intact proteins and endogenous peptides in HM.

Online LC-ESI-MS/MS comparative analysis of N/O-glycopatterns in human colostrum from different ethnic groups in Northwest China

Human milk oligosaccharides, including free oligosaccharides and glycoconjugates, exert a key role in neonatal health and development. Changes in free oligosaccharides of milk from different ethnic groups have been documented. In this study, human milk was collected from Han, Hui, and Tibetan populations in northwest China, and differences in N/O-glycome among these three ethnic groups were systematically compared using online high-performance liquid chromatography-tandem mass spectrometry. Among the 63 detected N-glycans, 35 showed significant differences between the three ethnic groups (p < 0.05). Among the 70 detected O-glycans, four neutral O-glycans and six acidic O-glycans exhibited significant differences among the three ethnic groups (p < 0.05), with six acidic O-glycans reported for the first time. Overall, the extent of milk N/O-glycosylation was higher in the Han population than in the Hui or Tibetan groups. This trend was particularly pronounced for the main sialylated N/O-glycans. Except for sulfated O-glycans, which were higher in the milk from Tibetan mothers, the other types of N/O-glycans were present in similar proportions across all ethnic groups. Understanding the composition of N/O-glycans in human milk can help research on the structure-function relationship of glycans.

Label-Free Liquid Chromatography-Mass Spectrometry Quantitation of Relative N- and O-Glycan Concentrations in Human Milk in Japan

Human milk is abundant in carbohydrates and includes human milk oligosaccharides (HMOs) and N/O-glycans conjugated to proteins. HMO compositions and concentrations vary in individuals according to the maternal secretor status based on the fucosyltransferase 2 genotype; however, the profile of N/O-glycans remains uninvestigated because of the analytical complexity. Herein, we applied a label-free chromatography-mass spectrometry (LC-MS) technique to elucidate the variation in the composition and concentration of N/O-glycans in human milk. We used label-free LC-MS to relatively quantify 16 N-glycans and 12 O-glycans in 200 samples of Japanese human milk (1-2 months postpartum) and applied high performance anion exchange chromatography with pulsed amperometric detection to absolutely quantify the concentrations of 11 representative HMOs. Cluster analysis of the quantitative data revealed that O-glycans and several HMOs were classified according to the presence or absence of fucose linked to galactose while N-glycans were classified into a different group from O-glycans and HMOs. O-glycans and HMOs with fucose linked to galactose were more abundant in human milk from secretor mothers than from nonsecretor mothers. Thus, secretor status influenced the composition and concentration of HMOs and O-glycans but not those of N-glycans in human milk.

Online PGC-LC-ESI-MS/MS comparative analysis of variations in human milk O-glycopatterns from different secretor status

O-glycome is one of the important components of glycoconjugates in human milk which is speculated to provide protective features similar to those observed in free oligosaccharides. The effects of maternal secretor status on free oligosaccharides and N-glycome in milk have been well researched and documented. Currently, milk O-glycome of secretors (Se+) and nonsecretors (Se-) was investigated through reductive β-elimination combined with porous graphitized carbon-liquid chromatography-electrospray ionization-tandem mass spectrometry. A total of 70 presumptive O-glycan structures were identified, of which 25 O-glycans (including 14 sulfated O-glycans) were reported for the first time. Notably, 23 O-glycans exhibited significant differences between Se+ and Se- samples (p < 0.05). Compared to Se- group, the O-glycans of the Se+ group was two times more abundant in the total glycosylation, sialylation, fucosylation, and sulfation (p < 0.01). In conclusion, approximately one-third of the milk O-glycosylation was influenced by maternal FUT2-related secretor status. Our data will lay a foundation for the study of O-glycans structure-function relationship.

Human milk whey glycoprotein N-glycans varied greatly among different maternal secretor status

Human milk glycans are complex carbohydrates, which play a pivotal role in infant health and neonatal development. Maternal secretor status is known to affect free oligosaccharides in milk. Here, the milk N-glycome of secretor (Se+) and nonsecretor (Se-) individuals was qualitatively and quantitatively analyzed by hydrophilic interaction chromatography-electrospray ionization-tandem mass spectrometry. The total glycosylation, fucosylation, and sialylation of N-glycans was three times higher in the Se+ group compared to the Se- group (p < 0.001) per equal volume of milk. Importantly, 52 out of 63 N-glycans-including the eight most abundant ones-differed greatly between Se+ and Se- individuals (p < 0.05). Moreover, nine N-glycans (H5N3F1, H6N3, H3N5F1, H5N5F1, H5N5F1S1, H5N4F3S1, H6N4F2S1, H6N5F4S1, and H8N7S1) were >10 times more abundant in Se+ milk than in Se- milk. These findings lay a glycomics-basis for designing personalized nutrition supplements for infants.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2019-2020

This review is the tenth update of the original article published in 1999 on the application of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2020. Also included are papers that describe methods appropriate to analysis by MALDI, such as sample preparation techniques, even though the ionization method is not MALDI. The review is basically divided into three sections: (1) general aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, fragmentation, quantification and the use of arrays. (2) Applications to various structural types such as oligo- and polysaccharides, glycoproteins, glycolipids, glycosides and biopharmaceuticals, and (3) other areas such as medicine, industrial processes and glycan synthesis where MALDI is extensively used. Much of the material relating to applications is presented in tabular form. The reported work shows increasing use of incorporation of new techniques such as ion mobility and the enormous impact that MALDI imaging is having. MALDI, although invented nearly 40 years ago is still an ideal technique for carbohydrate analysis and advancements in the technique and range of applications show little sign of diminishing.

Variations in Oligosaccharides and N/O-Glycans in Human Milk through the Eight-Month Lactation Period

Oligosaccharides and N/O-glycans are abundant in human milk and have numerous biological functions (for instance sialylated glycans provide sialic acid for the growth of infant brains), but their variation trends during lactation need further exploration. Qualitative and quantitative analyses of oligosaccharides and N/O-glycans in human milk at different lactation stages (from 7 days to 8 months) were performed using UHPLC-ESI-MS/MS. Thirty-four oligosaccharides, twenty-three N-glycans, and six O-glycans were identified. Oligosaccharides showed the highest abundance in human colostrum and decreased with the progression of lactation, and the abundance of N/O-glycans fluctuated as lactation progressed, while a high abundance of sialylated oligosaccharides and sialylated N/O-glycans was observed in human colostrum. These findings provide evidence for breastfeeding support and contribute to the development of infant formula supplemented with human milk glycans.

Preterm birth and human milk proteome: are we ready for individualized fortification?

PURPOSE OF REVIEW

Preterm birth is one of the most pressing clinical problems in obstetrics and neonatology worldwide. One of the most sophisticated components of human milk is the proteome and a better understanding of it can lead to precision guides for feeding preterm infants. In this review, we will examine recent research focused on the human milk proteome and individualized protein fortification of human milk.

RECENT FINDINGS

In both preterm and term birth, the protein content in mother's own milk dropped rapidly in the early postnatal period. Preterm milk had a higher protein content and contained different protein and endogenous peptide compositions compared with term milk. The peptides in gastrointestinal fluids of preterm infants still need further investigation. Individualized fortification is more superior to standard fortification, but it only focuses on the total protein amount. There is no data concerning the composition and posttranslational modifications of proteins and endogenous peptides with fortification and their longterm effects.

SUMMARY

Comprehensive identification and characterization of the human milk proteome have led to the targeted breast milk fortification theory of preterm infant feeding and has also been enriched by clinical trials. However, to achieve fortification of key proteins and/or endogenous peptides, as standard clinical practice, requires additional studies. Future research should explore the long-term effect of protein fortification and pay more attention to quality rather than quantity in relation to infant body composition and growth outcomes.

Glycoproteomics Analysis Reveals Differential Expression of Site-Specific Glycosylation in Human Milk Whey during Lactation

Protein N-glycosylation in human milk whey plays a substantial role in infant health during postnatal development. Changes in site-specific glycans in milk whey reflect the needs of infants under different circumstances. However, the conventional glycoproteomics analysis of milk whey cannot reveal the changes in site-specific glycans because the attached glycans are typically enzymatically removed from the glycoproteins prior to analysis. In this study, N-glycoproteomics analysis of milk whey was performed without removing the attached glycans, and 330 and 327 intact glycopeptides were identified in colostrum and mature milk whey, respectively. Label-free quantification of site-specific glycans was achieved by analyzing the identified intact glycopeptides, which revealed 9 significantly upregulated site-specific glycans on 6 glycosites and 11 significantly downregulated site-specific glycans on 8 glycosites. Some interesting change trends in N-glycans attached to specific glycosites in human milk whey were observed. Bisecting GlcNAc was found attached to 11 glycosites on 8 glycoproteins in colostrum and mature milk. The dynamic changes in site-specific glycans revealed in this study provide insights into the role of protein N-glycosylation during infant development.

Human milk proteome: what's new?

PURPOSE OF REVIEW

The proteome is one of the most complicated and multifunctional components in human milk. Recently, numerous novel characteristics of the human milk proteome have been discovered, which are described and critically examined in this review.

RECENT FINDINGS

Recent human milk proteomics studies have focused on how external factors like geography and environment, or maternal and infant's factors affect the milk proteins, endogenous peptides, their posttransitional modifications (PTMs) and infant utilization. Most of these studies have shown that major protein and endogenous peptide profiles are similar for healthy women and infants. The human milk proteome has been expanded by providing novel insights into PTMs like glycosylation and phosphorylation, and how the proteins and peptides are digested and utilized by the infant. All human milk proteomics studies are subject to conditions in which the samples were collected, handled and stored.

SUMMARY

Significant technological advancements in mass spectrometry have considerably enabled a deeper and more comprehensive identification and characterization of the expanded human milk proteome. However, data concerning human milk from mothers with infections or illnesses and mothers nursing more vulnerable infants are still limited and the roles of the components of the human milk proteome have not yet been sufficiently elucidated.

Potential Applications of Endo-β-N-Acetylglucosaminidases From Bifidobacterium longum Subspecies infantis in Designing Value-Added, Next-Generation Infant Formulas

Human milk is the optimal source of infant nutrition. Among many other health benefits, human milk can stimulate the development of a Bifidobacterium-rich microbiome through human milk oligosaccharides (HMOs). In recent years, the development of novel formulas has placed particular focus on incorporating some of the beneficial functional properties of human milk. These include adding specific glycans aimed to selectively stimulate the growth of Bifidobacterium. However, the bifidogenicity of human milk remains unparalleled. Dietary N-glycans are carbohydrate structures conjugated to a wide variety of glycoproteins. These glycans have a remarkable structural similarity to HMOs and, when released, show a strong bifidogenic effect. This review discusses the biocatalytic potential of the endo-β-N-acetylglucosaminidase enzyme (EndoBI-1) from Bifidobacterium longum subspecies infantis (B. infantis), in releasing N-glycans inherently present in infant formula as means to increase the bifidogenicity of infant formula. Finally, the potential implications for protein deglycosylation with EndoBI-1 in the development of value added, next-generation formulas are discussed from a technical perspective.

Alteration of the goat milk glycoproteins N/O-glycome at different lactation stages

Goat milk oligosaccharides represent an unexplored multi-functional ingredient for the dairy industry. Here, we qualitatively and quantitatively compared the N/O-glycome at different lactation stages via online hydrophilic interaction chromatography-tandem mass spectrometry. Complex N-glycans and high mannose N-glycans constituted 82.1% and 17.9% of the glycan pool, respectively. N-glycans with isomers containing non-bisected antenna complex structures accounted for 30.8%. N-glycans modified with Neu5Ac, Neu5Gc and fucosylated were 3.7%, 5.3% and 35.3%. The triantennary trifucosylated complex N-glycan (H5N5F3) was reported for the first time. A comparison between colostrum and mature milk revealed a 1.20-fold decrease in total N-glycans and 1.66-fold decrease in fucosylation with ongoing lactation, echoing the trend in human milk. Similarly, Neu5Ac- and Neu5Gc-modified sialylation decreased by 1.69 and 3.62 times, respectively. In the O-glycome, 46.2% of structures were O-linked core 1, 23.1% were O-linked core 2, 7.7% were O-linked core 3 and core 4. As lactation progressed, overall O-glycans content decreased by 1.26-fold. Unlike human milk, Neu5Ac- and Neu5Gc-modified sialylation increased by 4.4 and 2 times, respectively. These findings will facilitate research on the structure-function relationship of goat milk oligosaccharides and the development of formula food targeting different age groups.