Plasminogen activation system in human milk

Fingerprinting of Proteases, Protease Inhibitors and Indigenous Peptides in Human Milk

The presence of proteases and their resulting level of activity on human milk (HM) proteins may aid in the generation of indigenous peptides as part of a pre-digestion process, of which some have potential bioactivity for the infant. The present study investigated the relative abundance of indigenous peptides and their cleavage products in relation to the abundance of observed proteases and protease inhibitors. The proteomes and peptidomes in twelve HM samples, representing six donors at lactation months 1 and 3, were profiled. In the proteome, 39 proteases and 29 protease inhibitors were identified in 2/3 of the samples. Cathepsin D was found to be present in higher abundance in the proteome compared with plasmin, while peptides originating from plasmin cleavage were more abundant than peptides from cathepsin D cleavage. As both proteases are present as a system of pro- and active- forms, their activation indexes were calculated. Plasmin was more active in lactation month 3 than month 1, which correlated with the total relative abundance of the cleavage product ascribed to plasmin. By searching the identified indigenous peptides in the milk bioactive peptide database, 283 peptides were ascribed to 10 groups of bioactivities. Antimicrobial peptides were significantly more abundant in month 1 than month 3; this group comprised 103 peptides, originating from the β-CN C-terminal region.

Ecologies, synergies, and biological systems shaping human milk composition-a report from "Breastmilk Ecology: Genesis of Infant Nutrition (BEGIN)" Working Group 2

Human milk is universally recognized as the preferred food for infants during the first 6 mo of life because it provides not only essential and conditionally essential nutrients in necessary amounts but also other biologically active components that are instrumental in protecting, communicating important information to support, and promoting optimal development and growth in infants. Despite decades of research, however, the multifaceted impacts of human milk consumption on infant health are far from understood on a biological or physiological basis. Reasons for this lack of comprehensive knowledge of human milk functions are numerous, including the fact that milk components tend to be studied in isolation, although there is reason to believe that they interact. In addition, milk composition can vary greatly within an individual as well as within and among populations. The objective of this working group within the Breastmilk Ecology: Genesis of Infant Nutrition (BEGIN) Project was to provide an overview of human milk composition, factors impacting its variation, and how its components may function to coordinately nourish, protect, and communicate complex information to the recipient infant. Moreover, we discuss the ways whereby milk components might interact such that the benefits of an intact milk matrix are greater than the sum of its parts. We then apply several examples to illustrate how milk is better thought of as a biological system rather than a more simplistic "mixture" of independent components to synergistically support optimal infant health.

Premature delivery impacts the concentration of plasminogen activators and a plasminogen activator inhibitor and the plasmin activity in human milk

BACKGROUND AND AIMS

Plasmin in human milk partially hydrolyzes milk proteins within the mammary gland and may enhance the hydrolysis of milk proteins within the infant's stomach. This study examined the effects of extremely preterm (EP)-, very preterm (VP)-, and term-delivery on plasmin activity and the concentrations of plasminogen activators [urokinase-type plasminogen activator (uPA) and tissue-type plasminogen activator (tPA)], plasminogen activator inhibitor type 1 (PAI-1) and the complexes of PAI-1/uPA and PAI-1/tPA in human milk.

MATERIALS AND METHODS

Human milk samples were collected from mothers who delivered extremely preterm infants [24-27 weeks gestational age (GA), n = 20], very preterm infants (28-32 weeks GA, n = 12), and term infants (38-39 weeks GA, n = 8) during 2-72 days postnatally. Plasmin activity was determined using fluorometric substrate assay, whereas concentrations of uPA, tPA, PAI-1, the PAI-1/uPA complex and the PAI-1/tPA complex were quantified by ELISA.

RESULTS

Plasmin activity, uPA and tPA were detected in all human milk samples, PAI-1 and the PAI-1/uPA complex were present in 42.5 and 32.5% of milk samples, respectively, and the PAI-1/tPA complex was not detected. Plasmin activity was correlated negatively with postnatal age and postmenstrual age (PMA) in the VP group and positively with postnatal age in the term group. uPA and tPA concentrations decreased with increasing postnatal age in both EP and VP groups but did not correlate in the term group. uPA concentration was correlated positively with GA in the VP group and tended to be elevated with increasing GA in the combined three groups. In contrast, tPA concentrations were correlated negatively with GA and PMA in the combined three groups (P < 0.008) and with PMA in the EP and VP groups. PAI-1 concentration tended to be correlated positively with postnatal age in the combined three groups. No correlation was detected with the PAI-1/uPA complex.

CONCLUSION

Premature delivery impacted the plasmin activity and the concentrations of uPA, tPA, and PAI-1 in human milk. Whether these changes in milk plasminogen activators and inhibitors have a role in balancing the proteolytic digestion of premature infants remains to be investigated.

Personalized Profiling Reveals Donor- and Lactation-Specific Trends in the Human Milk Proteome and Peptidome

BACKGROUND

Human milk is the most genuine form of personalized nutrition, whereby its nutritional and bioactive constituents support the changing needs of the growing infant. Personalized proteome profiling strategies may provide insights into maternal-infant relationships. Proteins and endogenous peptides in human milk play an important role as nutrients for growth and have distinct functionality such as immune defense. Comprehensive monitoring of all of the human milk proteinaceous components, including endogenous peptides, is required to fully understand the changing role of the human milk proteome throughout lactation.

OBJECTIVE

We aimed to investigate the personalized nature of the human milk proteome and peptidome for individual mother-infant dyads.

METHODS

Two individual healthy milk donors, aged 29 and 32 y and both of a normal BMI, were longitudinally observed over weeks 1, 2, 3, 4, 6, 8, 10, 12, and 16 postpartum. Milk collection was standardized. Comprehensive variations in the human milk proteinaceous components were assessed using quantitative LC-MS/MS methods.

RESULTS

We longitudinally profiled the concentrations of >1300 milk proteins and 2000 endogenous milk peptides spanning 16 wk of lactation for 2 individual donors. We observed many gradual and alike changes in both donors related to temporal effects, for instance early lactation was marked by high concentrations of proteins and peptides involved in lactose synthesis and immune development. Uniquely, in 1 of the 2 donors, we observed a substantial anomaly in the milk composition, exclusively at week 6, likely indicating a response to inflammation and/or infection.

CONCLUSIONS

Here, we provide a resource for characterizing the lactational changes in the human milk proteome, encompassing thousands of proteins and endogenous peptides. Further, we demonstrate the feasibility and benefit of personalized profiling to monitor the influence of milk on the development of the newborn, as well as the health status of each individual mother-infant pair.

Protein levels and protease activity in milk from mothers of pre-term infants: A prospective longitudinal study of human milk macronutrient composition

BACKGROUND AND AIMS

The composition and enzymology of human milk changes throughout the lactation period, and differ for mothers who give birth prematurely compared to those who deliver at full-term. Understanding the composition of milk from mothers of very low birth weight premature infants is of great significance, and the objective of this study was to evaluate the composition, protein profile and plasmin activity of milk from mothers who delivered infants at different gestational ages.

METHODS

Samples of human milk were donated by women (n = 74) in the Cork, Ireland, area who gave birth to full-term (>37 weeks gestation, FT), pre-term (32-37 weeks, PT) and very pre-term (≤32 weeks, VPT) infants. FT milk was collected at 1, 3, 6 and 10 weeks post-partum (PP), while PT and VPT milk was collected weekly until the FT due date of the infant and subsequently followed the FT protocol.

RESULTS

Gestational age did not significantly affect lactose or fat content or total energy content of milk. However, protein content, and levels of some individual proteins, were significantly affected by both gestational age at birth and duration of lactation, with significantly higher protein levels in PT or VPT milk samples at 0-7 days and 1-2 months, respectively. Plasmin activity was significantly higher in VPT milk, indicating differences in proteolytic processing in milk.

CONCLUSION

Compositional differences between the milk of mothers of term and pre-term infants were greatest in terms of the protein profile, which showed both qualitative and quantitative differences, as well as difference in proteolytic activity.

Bioactive peptides derived from human milk proteins: an update

PURPOSE OF REVIEW

Human milk contains a variety of bioactive proteins, and some of the bioactivities are exerted only after proteins are digested in the gastrointestinal tract. This review aims to overview recent studies on bioactive peptides in human milk and gastric digesta of breast-fed infants.

RECENT FINDINGS

Milk protein-derived peptides are endogenously present in human milk, and some of them have been reported to be bioactive peptides, such as a homologue of caseinophosphopeptide, an antimicrobial peptide, and an immunomodulatory peptide. A larger number of peptides are identified in gastric aspirates from breast-fed infants, and bioactive peptides such as angiotensin I-converting enzyme-inhibitory peptides, an antioxidative peptide, opioid agonist peptides are only found in the digesta of human milk but not in human milk per se. Casein is the greatest source of released bioactive peptides.

SUMMARY

Technological advances have considerably increased our capability to identify and characterize peptides derived from human milk proteins. However, their physiological significance and the roles of these bioactive peptides in growth and development of breast-fed infants have not yet been sufficiently elucidated, and further in-vivo experiments and clinical trials are warranted.

Milk Peptides Survive In Vivo Gastrointestinal Digestion and Are Excreted in the Stool of Infants

BACKGROUND

Human milk peptides released by gastrointestinal proteases have been identified with bioactivities that can benefit the infant but must first reach their respective sites of activity. Peptides in the stool either survived to or were released inside the intestinal tract, and thus had the opportunity to exert bioactivity there. However, it is unknown whether any milk peptides, bioactive or not, can survive in the stool of infants.

OBJECTIVE

The aim of this study was primarily to identify milk peptides in infant stool samples and secondarily test the hypotheses that the milk peptide profiles of stools are different between preterm infants at different days of life and between preterm and term infants.

METHODS

Infant stool samples were collected from 16 preterm infants (<34 weeks gestational age) at 8 or 9 and 21 or 22 days of life (DOL), and from 10 term infants (>34 weeks gestational age) at 8 or 9 DOL. Milk peptides were isolated from the stool samples and identified using tandem MS. The peptide counts and abundances were compared between infant groups.

RESULTS

In total, 118 exclusively milk-derived peptides from the caseins and α-lactalbumin were present in the stool samples, including some peptides with known or potential bioactivity. The remaining 8014 identified peptides could be derived either from milk or endogenous proteins. Although many individual milk peptides were significantly different between preterm infants at 8/9 and 21/22 DOL and between preterm and term infants, total peptide abundance and count were similar for all 3 groups.

CONCLUSIONS

This is the first study to confirm the survival of milk peptides in the stool of infants. Some of the peptides had potential bioactivities that could influence infant gut development. These results are important to understand the physiological relevance of human milk peptides to the infant.

The Functional Power of the Human Milk Proteome

Human milk is the most complete and ideal form of nutrition for the developing infant. The composition of human milk consistently changes throughout lactation to meet the changing functional needs of the infant. The human milk proteome is an essential milk component consisting of proteins, including enzymes/proteases, glycoproteins, and endogenous peptides. These compounds may contribute to the healthy development in a synergistic way by affecting growth, maturation of the immune system, from innate to adaptive immunity, and the gut. A comprehensive overview of the human milk proteome, covering all of its components, is lacking, even though numerous analyses of human milk proteins have been reported. Such data could substantially aid in our understanding of the functionality of each constituent of the proteome. This review will highlight each of the aforementioned components of human milk and emphasize the functionality of the proteome throughout lactation, including nutrient delivery and enhanced bioavailability of nutrients for growth, cognitive development, immune defense, and gut maturation.

The plasminogen receptor, Plg-RKT, is essential for mammary lobuloalveolar development and lactation

Essentials Plg-RKT-/- female mice give birth, but no offspring of Plg-RKT-/- female mice survive to weaning. Causal mechanisms of potential lactational failure in Plg-RKT-/- mice are unknown. Plg-RKT regulates extracellular matrix remodeling, cell proliferation, apoptosis, fibrin surveillance. Plg-RKT is essential for lactogenesis and mammary lobuloalveolar development.

SUMMARY

Background Lactational competence requires plasminogen, the zymogen of the serine protease, plasmin. Plg-RKT is a unique transmembrane plasminogen receptor that promotes plasminogen activation to plasmin on cell surfaces. Plg-RKT-/- mice are viable, but no offspring of Plg-RKT-/- female mice survive to weaning. Objectives We investigated potential lactational failure in Plg-RKT-/- mice and addressed causal mechanisms. Methods Fibrin accumulation, macrophage infiltration, processing of extracellular matrix components, effects of genetic deletion of fibrinogen, expression of fibrosis genes, and proliferation and apoptosis of epithelial cells were examined in lactating mammary glands of Plg-RKT-/- and Plg-RKT+/+ mice. Results Milk was not present in the stomachs of offspring of Plg-RKT-/- female mice and the pups were rescued by foster mothers. Although the mammary ductal tree developed normally in Plg-RKT-/- glands, lobuloalveolar development was blocked by a hypertrophic fibrotic stroma and infiltrating macrophages were present. A massive accumulation of fibrin was also present in Plg-RKT-/- alveoli and ducts. Although this accumulation was decreased when Plg-RKT-/- mice were made genetically heterozygous for fibrinogen, defects in lobuloalveolar development were not rescued by fibrinogen heterozygosity. Transcriptional profiling revealed that EGF was downregulated 12-fold in Plg-RKT-/- glands. Furthermore, proliferation of epithelial cells was not detectable. In addition, the pro-survival protein, Mcl-1, was markedly downregulated and apoptosis was observed in Plg-RKT-/- but not Plg-RKT+/+ glands. Conclusions Plg-RKT is essential for lactogenesis and functions to maintain the appropriate stromal extracellular matrix environment, regulate epithelial cell proliferation and apoptosis, and, by regulating fibrinolysis, preserve alveolar and ductal patency.

Analysis of Milk from Mothers Who Delivered Prematurely Reveals Few Changes in Proteases and Protease Inhibitors across Gestational Age at Birth and Infant Postnatal Age

Background: Peptidomics research has demonstrated that protease activity is higher in breast milk from preterm-delivering mothers than from term-delivering mothers. However, to our knowledge, the effect of the degree of prematurity and postnatal age on proteases and protease inhibitors in human milk remains unknown.Objective: We aimed to determine the change of proteases and protease inhibitors in milk from mothers who delivered prematurely across gestational age (GA) and postnatal age.Methods: Milk samples were collected from 18 mothers aged 26-40 y who delivered preterm infants and who lacked mastitis. For analysis, samples were separated into 2 groups: 9 from early GA (EGA) (24-26 wk GA)-delivering mothers and 9 from late GA (LGA) (27-32 wk GA)-delivering mothers. Within the 9 samples in each group, the collection time ranged from postnatal days 2 to 47. The activity and predicted activity of proteases in preterm milk were determined with the use of fluorometric and spectrophotometric assays and peptidomics, respectively. Protease and protease inhibitor concentrations were determined with the use of ELISA. Linear mixed models were applied to compare enzymes across GA and postnatal age.Results: Carboxypeptidase B2, kallikrein, plasmin, elastase, thrombin, and cytosol aminopeptidase were present and active in the milk of preterm-delivering mothers. Most milk protease and antiprotease concentrations did not change with GA or postnatal age. However, the concentration and activity of kallikrein, the most abundant and active protease in preterm milk, increased by 25.4 ng · mL-1 · d-1 and 0.454 μg · mL-1 · d-1 postnatally, respectively, in EGA milk samples while remaining stable in LGA milk samples.Conclusions: This research demonstrates that proteases are active in human milk and begin to degrade milk protein within the mammary gland before consumption by infants. Proteases and protease inhibitors in milk from mothers of premature infants mostly did not vary substantially across GA and postnatal age.

Proteolytic Systems in Milk: Perspectives on the Evolutionary Function within the Mammary Gland and the Infant

Milk contains elements of numerous proteolytic systems (zymogens, active proteases, protease inhibitors and protease activators) produced in part from blood, in part by mammary epithelial cells and in part by immune cell secretion. Researchers have examined milk proteases for decades, as they can cause major defects in milk quality and cheese production. Most previous research has examined these proteases with the aim to eliminate or control their actions. However, our recent peptidomics research demonstrates that these milk proteases produce specific peptides in healthy milk and continue to function within the infant's gastrointestinal tract. These findings suggest that milk proteases have an evolutionary function in aiding the infant's digestion or releasing functional peptides. In other words, the mother provides the infant with not only dietary proteins but also the means to digest them. However, proteolysis in the milk is controlled by a balance of protease inhibitors and protease activators so that only a small portion of milk proteins are digested within the mammary gland. This regulation presents a question: If proteolysis is beneficial to the infant, what benefits are gained by preventing complete proteolysis through the presence of protease inhibitors? In addition to summarizing what is known about milk proteolytic systems, we explore possible evolutionary explanations for this proteolytic balance.

Breastmilk-Saliva Interactions Boost Innate Immunity by Regulating the Oral Microbiome in Early Infancy

INTRODUCTION

Xanthine oxidase (XO) is distributed in mammals largely in the liver and small intestine, but also is highly active in milk where it generates hydrogen peroxide (H2O2). Adult human saliva is low in hypoxanthine and xanthine, the substrates of XO, and high in the lactoperoxidase substrate thiocyanate, but saliva of neonates has not been examined.

RESULTS

Median concentrations of hypoxanthine and xanthine in neonatal saliva (27 and 19 μM respectively) were ten-fold higher than in adult saliva (2.1 and 1.7 μM). Fresh breastmilk contained 27.3 ± 12.2 μM H2O2 but mixing baby saliva with breastmilk additionally generated >40 μM H2O2, sufficient to inhibit growth of the opportunistic pathogens Staphylococcus aureus and Salmonella spp. Oral peroxidase activity in neonatal saliva was variable but low (median 7 U/L, range 2-449) compared to adults (620 U/L, 48-1348), while peroxidase substrate thiocyanate in neonatal saliva was surprisingly high. Baby but not adult saliva also contained nucleosides and nucleobases that encouraged growth of the commensal bacteria Lactobacillus, but inhibited opportunistic pathogens; these nucleosides/bases may also promote growth of immature gut cells. Transition from neonatal to adult saliva pattern occurred during the weaning period. A survey of saliva from domesticated mammals revealed wide variation in nucleoside/base patterns.

DISCUSSION AND CONCLUSION

During breast-feeding, baby saliva reacts with breastmilk to produce reactive oxygen species, while simultaneously providing growth-promoting nucleotide precursors. Milk thus plays more than a simply nutritional role in mammals, interacting with infant saliva to produce a potent combination of stimulatory and inhibitory metabolites that regulate early oral-and hence gut-microbiota. Consequently, milk-saliva mixing appears to represent unique biochemical synergism which boosts early innate immunity.

Endogenous human milk peptide release is greater after preterm birth than term birth

BACKGROUND

Hundreds of naturally occurring milk peptides are present in term human milk. Preterm milk is produced before complete maturation of the mammary gland, which could change milk synthesis and secretion processes within the mammary gland, leading to differences in protein expression and enzymatic activity, thereby resulting in an altered peptide profile.

OBJECTIVE

This study examined differences in peptides present between milk from women delivering at term and women delivering prematurely.

METHODS

Nano-LC tandem mass spectrometry was employed to identify naturally occurring peptides and compare their abundances between term and preterm human milk samples at multiple time points over lactation. Term milk samples were collected from 8 mothers and preterm milk was collected from 14 mothers. The 28 preterm and 32 term human milk samples were divided into 4 groups based on day of collection (<14, 14-28, 29-41, and 42-58 d).

RESULTS

Preterm milk peptide counts, ion abundance, and concentration were significantly higher in preterm milk than term milk. Bioinformatic analysis of the cleavage sites for peptides identified suggested that plasmin was more active in preterm milk than term milk and that cytosol aminopeptidase and carboxypeptidase B2 likely contribute to extensive milk protein breakdown. Many identified milk peptides in both term and preterm milk overlapped with known functional peptides, including antihypertensive, antimicrobial, and immunomodulatory peptides.

CONCLUSION

The high protein degradation by endogenous proteases in preterm milk might attenuate problems because of the preterm infant's immature digestive system. This trial was registered at clinicaltrials.gov as NCT01817127.

Predicting the important enzymes in human breast milk digestion

Human milk is known to contain several proteases, but little is known about whether these enzymes are active, which proteins they cleave, and their relative contribution to milk protein digestion in vivo. This study analyzed the mass spectrometry-identified protein fragments found in pooled human milk by comparing their cleavage sites with the enzyme specificity patterns of an array of enzymes. The results indicate that several enzymes are actively taking part in the digestion of human milk proteins within the mammary gland, including plasmin and/or trypsin, elastase, cathepsin D, pepsin, chymotrypsin, a glutamyl endopeptidase-like enzyme, and proline endopeptidase. Two proteins were most affected by enzyme hydrolysis: β-casein and polymeric immunoglobulin receptor. In contrast, other highly abundant milk proteins such as α-lactalbumin and lactoferrin appear to have undergone no proteolytic cleavage. A peptide sequence containing a known antimicrobial peptide is released in breast milk by elastase and cathepsin D.

Extensive in vivo human milk peptidomics reveals specific proteolysis yielding protective antimicrobial peptides

Milk is traditionally considered an ideal source of the basic elemental nutrients required by infants. More detailed examination is revealing that milk represents a more functional ensemble of components with benefits to both infants and mothers. A comprehensive peptidomics method was developed and used to analyze human milk yielding an extensive array of protein products present in the fluid. Over 300 milk peptides were identified originating from major and many minor protein components of milk. As expected, the majority of peptides derived from β-casein, however no peptide fragments from the major milk proteins lactoferrin, α-lactalbumin, and secretory immunoglobulin A were identified. Proteolysis in the mammary gland is selective-released peptides were drawn only from specific proteins and typically from only select parts of the parent sequence. A large number of the peptides showed significant sequence overlap with peptides with known antimicrobial or immunomodulatory functions. Antibacterial assays showed the milk peptide mixtures inhibited the growth of Escherichia coli and Staphylococcus aureus . The predigestion of milk proteins and the consequent release of antibacterial peptides may provide a selective advantage through evolution by protecting both the mother's mammary gland and her nursing offspring from infection.

Tissue-type plasminogen activator and plasminogen embedded in casein rule its degradation under physiological situations: manipulation with casein hydrolysate

The aims of this study were to test the assumption that tissue-type plasminogen activator (t-PA) and plasminogen (PG) are closely associated with the casein micelle and form a functional complex that rules casein degradation. This assumption was essentially verified for bovine milk under conditions wherein the plasmin system was activated by treatment with casein hydrolysate. It was also shown that urokinase-type PA (u-PA), the second type of plasminogen activator present in milk, was not involved in casein degradation. In agreement with previous studies, we show that treatment with casein hydrolysate precipitously reduced mammary secretion, disrupted the tight junction integrity (increase in Na+ and decrease in K+ concentrations), induced hydrolysis of casein, and activated various elements of the innate and acquired immune system. In the present study, we have identified t-PA as the principal PA, which is responsible for the conversion of PG to plasmin. It was found that t-PA and plasminogen are present in freshly secreted milk (less than 10 min from its secretion), suggesting that they are secreted as a complex by the mammary gland epithelial cells. Further research is needed to provide the direct evidence to verify this concept.

Osteopontin is cleaved at multiple sites close to its integrin-binding motifs in milk and is a novel substrate for plasmin and cathepsin D

Osteopontin (OPN) is a highly modified integrin-binding protein present in most tissues and body fluids where it has been implicated in numerous biological processes. A significant regulation of OPN function is mediated through phosphorylation and proteolytic processing. Proteolytic cleavage by thrombin and matrix metalloproteinases close to the integrin-binding Arg-Gly-Asp sequence modulates the function of OPN and its integrin binding properties. In this study, seven N-terminal OPN fragments originating from proteolytic cleavage have been characterized from human milk. Identification of the cleavage sites revealed that all fragments contained the Arg-Gly-Asp(145) sequence and were generated by cleavage of the Leu(151)-Arg(152), Arg(152)-Ser(153), Ser(153)-Lys(154), Lys(154)-Ser(155), Ser(155)-Lys(156), Lys(156)-Lys(157), or Phe(158)-Arg(159) peptide bonds. Six cleavages cannot be ascribed to thrombin or matrix metalloproteinase activity, whereas the cleavage at Arg(152)-Ser(153) matches thrombin specificity for OPN. The principal protease in milk, plasmin, hydrolyzed the same peptide bond as thrombin, but its main cleavage site was identified to be Lys(154)-Ser(155). Another endogenous milk protease, cathepsin D, cleaved the Leu(151)-Arg(152) bond. OPN fragments corresponding to plasmin activity were also identified in urine showing that plasmin cleavage of OPN is not restricted to milk. Plasmin, but not cathepsin D, cleavage of OPN increased cell adhesion mediated by the alpha(V)beta(3)- or alpha(5)beta(1)-integrins. Similar cellular adhesion was mediated by plasmin and thrombin-cleaved OPN showing that plasmin can be a potent regulator of OPN activity. These data show that OPN is highly susceptible to cleavage near its integrin-binding motifs, and the protein is a novel substrate for plasmin and cathepsin D.

Characterization of carbohydrate structures of bovine MUC15 and distribution of the mucin in bovine milk

The present work reports the characterization of carbohydrate structures and the distribution of the newly identified mucin MUC15, a highly glycosylated protein associated with the bovine milk fat globule membrane (MFGM). Distribution of MUC15 was investigated in various fractions of bovine milk by densitometric scanning of Western blots. In raw milk, MUC15 was shown to constitute 0.08% (wt) of the protein and approximately 1.5% (wt) of the MFGM-associated proteins. Surprisingly, this study showed that in addition to the fat-containing fractions, such as MFGM and buttermilk, MUC15 was present in nonfat-containing fractions as well, such as skim milk and whey. Compositional and structural studies of the carbohydrates of bovine milk MUC15 showed that the glycans are composed of fucose, galactose, mannose, N-acetylgalactosamine, N-acetylglycosamine, and sialic acid. The carbohydrate was shown to constitute 65% of the total molecular weight, and the molar ratios of the individual sugars to protein of the O-linked glycans were determined. The glycan structures of MUC15 were further studied by enzymatic deglycosylation experiments using different endo- and exoglycosidases as well as a panel of lectins. The N-linked glycans were shown to contain mainly hybrid-type N-glycans. In addition, the N-glycans were shown to be sialylated and contain terminal poly-lactosamine structures. The O-linked glycans were found to constitute some unsubstituted Core-1 structures and a substantial number of sialylated Core-1 O-linked glycans. By comparing the results of peanut agglutinin lectin binding, enzymatic deglycosylation, and monosaccharide composition analysis, we concluded that bovine MUC15 also contains more complex O-glycans containing high amounts N-acetylglucosamine residues. Furthermore, a small subset of the O-linked glycans is decorated with lactosamine on their terminal ends.

Lack of plasminogen leads to milk stasis and premature mammary gland involution during lactation

The extracellular serine protease, plasmin, is activated from its precursor, plasminogen (Plg), by the urokinase-type and tissue-type Plg activators (uPA and tPA respectively). One of the main plasmin substrates, fibrin, is formed from fibrinogen via thrombin activity. We have previously shown that mice deficient for Plg are strikingly less able to support a litter during lactation compared to wild type mice. Here we suggest a mechanism responsible for this lactation defect. Reduced epithelial content and increased apoptosis are observed in Plg-deficient mammary glands at lactation day 7. Immunofluorescence analysis reveals the presence of fibrin(ogen) in the stroma surrounding mammary alveoli and adipocytes and identifies fibrin(ogen) as a component of breast milk in both wild type and Plg-deficient mice. Furthermore, a large accumulation of fibrin(ogen) together with apoptotic epithelial cells is observed in the lactating mammary alveoli and ducts of some Plg-deficient mice. This suggests that fibrin plays a key role in the malfunction of mammary glands in the absence of Plg, possibly through blockade of mammary ducts inducing milk stasis, inhibiting milk expulsion and thereby inducing premature apoptosis and involution.

Human colostrum: Identification of minor proteins in the aqueous phase by proteomics

Human colostrum is an important source of protective, nutritional and developmental factors for the newborn. We have investigated the low abundance proteins in the aqueous phase of human colostrum, after depletion of the major proteins secretory IgA, lactoferrin, alpha-lactalbumin and HSA by immunoabsorption, using 2-D LC and gel-based proteomic methods. One hundred and fifty-one proteins were identified, 83 of which have not been previously reported in human colostrum, or milk. This is the first comprehensive proteomic analysis of human colostrum produced during the first 48 h of lactation.

Human milk as a carrier of biochemical messages

Not only does breast milk provides an ideal nutrient composition for the newborn, but it also contains a variety of substances that may actively influence growth and development of the infant and stimulate neonatal protection against gastrointestinal diseases. Hormones, growth factors, cytokines and even whole cells are present in breast milk and act to establish biochemical and immunological communication between mother and child. In addition, milk nutrients such as nucleotides, glutamine and lactoferrin have been shown to influence gastrointestinal development and host defense. The unique properties of milk as a mediator of biochemical messages will be presented and the clinical significance of breastfeeding in the prevention of neonatal gastrointestinal diseases will be discussed.