N-glycosylation proteomic characterization and cross-species comparison of milk whey proteins from dairy animals

Identification of pasteurized mare milk and powder adulteration with bovine milk using quantitative proteomics and metabolomics approaches

Adulteration in dairy products presents food safety challenges, driven by economic factors. Processing may change specific biomarkers, thus affecting their effectiveness in detection. In this study, proteomics and metabolomics approaches were to investigate the detection of bovine milk (BM) constituents adulteration in pasteurized mare milk (PMM) and mare milk powder (MMP). Several bovine proteins and metabolites were identified, with their abundances in PMM and MMP increasing upon addition of BM. Proteins like osteopontin (OPN) and serotransferrin (TF) detected adulteration down to 1 % in PMM, whereas these proteins in MMP were utilized to identify 10 % adulteration. Biotin and N6-Me-adenosine were effective in detecting adulteration in PMM as low as 10 % and 1 % respectively, while in MMP, their detection limits extend down to 0.1 %. These findings offer insights for authenticating mare milk products and underscore the influence of processing methods on biomarker levels, stressing the need to consider these effects in milk product authentication.

Utilizing linkage-specific ethyl-esterification approach to perform in-depth analysis of sialylated N-glycans present on milk whey glycoproteins

Glycosylation of milk whey proteins, specifically the presence of sialic acid-containing glycan residues, causes functional changes in these proteins. This study aimed to analyze the N-glycome of milk whey glycoproteins from various milk sources using a linkage-specific ethyl esterification approach with MALDI-MS (matrix-assisted laser desorption/ionization-mass spectrometry). The results showed that the N-glycan profiles of bovine and buffalo whey mostly overlapped. Acetylated N-glycans were only detected in donkey milk whey at a rate of 16.06%. a2,6-linked N-Acetylneuraminic acid (a2,6-linked NeuAc, E) was found to be the predominant sialylation type in human milk whey (65.16%). The amount of a2,6-linked NeuAc in bovine, buffalo, goat, and donkey whey glycoproteomes was 42.33%, 44.16%, 39.00%, and 34.86%, respectively. The relative abundances of a2,6-linked N-Glycolylneuraminic acid (a2,6-linked NeuGc, Ge) in bovine, buffalo, goat, and donkey whey were 7.52%, 5.41%, 28.24%, and 17.31%, respectively. Goat whey exhibited the highest amount of a2,3-linked N-Glycolylneuraminic acid (a2,3-linked NeuGc, Gl, 8.62%), while bovine and donkey whey contained only 2.14% and 1.11%, respectively.

Quantitative phosphoproteome analysis reveals differential whey phosphoproteins of bovine milk during lactation

Whey proteins in bovine milk, as the most widely used nutritional components for infant formulae, have been paid more attention. However, the phosphorylation of proteins in bovine whey during lactation has not been thoroughly researched. In this study, a total of 185 phosphorylation sites on 72 phosphoproteins were identified in bovine whey during lactation. 45 differentially expressed whey phosphoproteins (DEWPPs) in colostrum and mature milk were focused on by bioinformatics approaches. Gene Ontology annotation indicated that blood coagulation, extractive space, and protein binding played a key role in bovine milk. The critical pathway of DEWPPs was related to the immune system according to KEGG analysis. Our study investigated the biological functions of whey proteins from a phosphorylation perspective for the first time. The results elucidate and increase our knowledge of differentially phosphorylation sites and phosphoproteins in bovine whey during lactation. Additionally, the data might offer fresh insight into the development of whey protein nutrition.

Organelles coordinate milk production and secretion during lactation: Insights into mammary pathologies

The mammary gland undergoes a spectacular series of changes during its development and maintains a remarkable capacity to remodel and regenerate during progression through the lactation cycle. This flexibility of the mammary gland requires coordination of multiple processes including cell proliferation, differentiation, regeneration, stress response, immune activity, and metabolic changes under the control of diverse cellular and hormonal signaling pathways. The lactating mammary epithelium orchestrates synthesis and apical secretion of macromolecules including milk lipids, milk proteins, and lactose as well as other minor nutrients that constitute milk. Knowledge about the subcellular compartmentalization of these metabolic and signaling events, as they relate to milk production and secretion during lactation, is expanding. Here we review how major organelles (endoplasmic reticulum, Golgi apparatus, mitochondrion, lysosome, and exosome) within mammary epithelial cells collaborate to initiate, mediate, and maintain lactation, and how study of these organelles provides insight into options to maintain mammary/breast health.

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.

Proteomic analysis of differentially expressed whey proteins in Guanzhong goat milk and Holstein cow milk by iTRAQ coupled with liquid chromatography-tandem mass spectrometry

Guanzhong goat and Holstein cow milk are the major milks supplied in China. Whey proteins play an important role in immune defense for newborn mammals. This study aimed to analyze the differentially expressed whey proteins of Guanzhong goat milk and Holstein cow milk by using isobaric tags for relative and absolute quantitation (iTRAQ)-based proteomics techniques. A total of 165 whey proteins were quantified, 114 of which differed significantly in abundance in goat and cow milks. According to the "up_keywords," in the online DAVID tool (https://david.ncifcrf.gov/home.jsp), 75% of these differentially expressed whey proteins were related to the category of "signal." Gene Ontology analyses classified these differentially expressed proteins into biological processes, cellular components, and molecular functions. The most common biological process was response to stress, the most common cellular component was related to extracellular region, and the most prevalent molecular function was binding. Kyoto Encyclopedia of Genes and Genomes pathway analyses showed that these proteins were mainly involved in the complement and coagulation cascade pathways. The results improve our understanding of the different biological properties of whey proteins in goat and cow milks.

Unique Microbial Catabolic Pathway for the Human Core N-Glycan Constituent Fucosyl-α-1,6-N-Acetylglucosamine-Asparagine

The gastrointestinal tract accommodates more than 10¹⁴ microorganisms that have an enormous impact on human health. The mechanisms enabling commensal bacteria and administered probiotics to colonize the gut remain largely unknown. The ability to utilize host-derived carbon and energy resources available at the mucosal surfaces may provide these bacteria with a competitive advantage in the gut. Here, we have identified in the commensal species Lactobacillus casei a novel metabolic pathway for the utilization of the glycoamino acid fucosyl-α-1,6-N-GlcNAc-Asn, which is present in the core-fucosylated N-glycoproteins from mammalians. These results give insight into the molecular interactions between the host and commensal/probiotic bacteria and may help to devise new strategies to restore gut microbiota homeostasis in diseases associated with dysbiotic microbiota.

The survival of commensal bacteria in the human gut partially depends on their ability to metabolize host-derived molecules. The use of the glycosidic moiety of N-glycoproteins by bacteria has been reported, but the role of N-glycopeptides or glycoamino acids as the substrates for bacterial growth has not been evaluated. We have identified in Lactobacillus casei strain BL23 a gene cluster (alf-2) involved in the catabolism of the glycoamino acid fucosyl-α-1,6-N-GlcNAc-Asn (6′FN-Asn), a constituent of the core-fucosylated structures of mammalian N-glycoproteins. The cluster consists of the genes alfHC, encoding a major facilitator superfamily (MFS) permease and the α-l-fucosidase AlfC, and the divergently oriented asdA (aspartate 4-decarboxylase), alfR2 (transcriptional regulator), pepV (peptidase), asnA2 (glycosyl-asparaginase), and sugK (sugar kinase) genes. Knockout mutants showed that alfH, alfC, asdA, asnA2, and sugK are necessary for efficient 6′FN-Asn utilization. The alf-2 genes are induced by 6′FN-Asn, but not by its glycan moiety, via the AlfR2 regulator. The constitutive expression of alf-2 genes in an alfR2 strain allowed the metabolism of a variety of 6′-fucosyl-glycans. However, GlcNAc-Asn did not support growth in this mutant background, indicating that the presence of a 6′-fucose moiety is crucial for substrate transport via AlfH. Within bacteria, 6′FN-Asn is defucosylated by AlfC, generating GlcNAc-Asn. This glycoamino acid is processed by the glycosylasparaginase AsnA2. GlcNAc-Asn hydrolysis generates aspartate and GlcNAc, which is used as a fermentable source by L.casei. These data establish the existence in a commensal bacterial species of an exclusive metabolic pathway likely to scavenge human milk and mucosal fucosylated N-glycopeptides in the gastrointestinal tract.

Characterization and comparison of milk fat globule membrane N-glycoproteomes from human and bovine colostrum and mature milk

Human and bovine milk fat globule membrane (MFGM) proteins have been identified and characterized; however, their glycosylation during lactation remains unclear. We adopted a glycoproteomics approach to profile and compare MFGM N-glycoproteomes in human and bovine milk during lactation. A total of 843, 718, 614, and 273 N-glycosite peptides corresponding to 465, 423, 334, and 176 glycoproteins were identified in human colostrum, human mature milk, bovine colostrum, and bovine mature milk, respectively. The biological functions of these MFGM N-glycoproteins were revealed through bioinformatics. Substantial differences were observed between human and bovine milk, and immune-related MFGM N-glycoproteins varied between colostrum and mature milk from both species. Our results expand current knowledge of MFGM N-glycoproteomes, and further demonstrate the complexity and biological functions of MFGM N-glycosylation. These data can provide references for the application of bovine MFGM N-glycoproteins in infant formula to resemble human milk and in functional foods.

Characterization and comparison of whey N-glycoproteomes from human and bovine colostrum and mature milk

Milk glycoproteins are crucial nutrients with a variety of functions. However, whey N-glycoproteomes in human and bovine milks have not been characterized during lactation. Herein, using lectin enrichment and liquid chromatography tandem mass spectrometry, 68, 58, 100, and 98 N-glycoproteins were identified in human colostrum and mature milk as well as bovine colostrum and mature milk whey. Gene Ontology and KEGG pathway analyses were used to elucidate the biological functions of whey N-glycoproteins in human and bovine colostrum and mature milks. Whey N-glycoproteomes differed dramatically between human and bovine milks and across lactation stages. The conserved and specific whey N-glycoproteins in all four sample types were also determined. Our results improve understanding of the properties and biological functions of whey N-glycoproteins in human and bovine milk and colostra, and provide insight into the potential application of some N-glycoproteins in infant formulae at different stages of development.

Quantitative N-glycoproteomics of milk fat globule membrane in human colostrum and mature milk reveals changes in protein glycosylation during lactation

The present study profiled the N-glycoproteome and quantified the changes of N-glycosylation site occupancy of MFGM proteins during lactation.

Comparative Analysis of Whey N-Glycoproteins in Human Colostrum and Mature Milk Using Quantitative Glycoproteomics

Glycosylation is a ubiquitous post-translational protein modification that plays a substantial role in various processes. However, whey glycoproteins in human milk have not been completely profiled. Herein, we used quantitative glycoproteomics to quantify whey N-glycosylation sites and their alteration in human milk during lactation; 110 N-glycosylation sites on 63 proteins and 91 N-glycosylation sites on 53 proteins were quantified in colostrum and mature milk whey, respectively. Among these, 68 glycosylation sites on 38 proteins were differentially expressed in human colostrum and mature milk whey. These differentially expressed N-glycoproteins were highly enriched in "localization", "extracellular region part", and "modified amino acid binding" according to gene ontology annotation and mainly involved in complement and coagulation cascades pathway. These results shed light on the glycosylation sites, composition and biological functions of whey N-glycoproteins in human colostrum and mature milk, and provide substantial insight into the role of protein glycosylation during infant development.