Lactose and major milk proteins are present in secretory vesicle-rich fractions from lactating mammary gland

Hormonal regulation of mammary gland development and lactation

Lactation is critical to infant short-term and long-term health and protects mothers from breast cancer, ovarian cancer and type 2 diabetes mellitus. The mammary gland is a dynamic organ, regulated by the coordinated actions of reproductive and metabolic hormones. These hormones promote gland development from puberty onwards and induce the formation of a branched, epithelial, milk-secreting organ by the end of pregnancy. Progesterone withdrawal following placental delivery initiates lactation, which is maintained by increased pituitary secretion of prolactin and oxytocin, and stimulated by infant suckling. After weaning, local cytokine production and decreased prolactin secretion trigger large-scale mammary cell loss, leading to gland involution. Here, we review advances in the molecular endocrinology of mammary gland development and milk synthesis. We discuss the hormonal functions of the mammary gland, including parathyroid hormone-related peptide secretion that stimulates maternal calcium mobilization for milk synthesis. We also consider the hormonal composition of human milk and its associated effects on infant health and development. Finally, we highlight endocrine and metabolic diseases that cause lactation insufficiency, for example, monogenic disorders of prolactin and prolactin receptor mutations, maternal obesity and diabetes mellitus, interventions during labour and delivery, and exposure to endocrine-disrupting chemicals such as polyfluoroalkyl substances in consumer products and other oestrogenic compounds.

Lactose on the basolateral side of mammary epithelial cells inhibits milk production concomitantly with signal transducer and activator of transcription 5 inactivation

Mammary epithelial cells (MECs) are the only cells capable of synthesizing lactose. During lactation, alveolar MECs secrete lactose through the apical membrane into the alveolar lumen, whereas alveolar tight junctions (TJs) block the leakage of lactose into the basolateral sides of the MECs. However, lactose leaks from the alveolar lumen into the blood plasma in the mastitis and after weaning. This exposes the basolateral membrane of MECs to lactose. The relationship between lactose in blood plasma and milk production has been suggested. The present study determined whether lactose exposure on the basolateral membrane of mouse MECs adversely affects milk production in vitro. Restricted exposure to lactose on the basolateral side of the MECs was performed using a culture model, in which MECs on the cell culture insert exhibit milk production and less-permeable TJs. The results indicated that lactose exposure on the basolateral side inhibited casein and lipid production in the MECs. Interestingly, lactose exposure on the apical side did not show detectable effects on milk production in the MECs. Basolateral lactose exposure also caused the inactivation of STAT5, a primary transcriptional factor for milk production. Furthermore, p38 and JNK were activated by basolateral lactose exposure. The activation of p38 and JNK following anisomycin treatment reduced phosphorylated STAT5, and inhibitors of p38 blocked the reduction of phosphorylated STAT5 by basolateral lactose exposure. These findings suggest that lactose functions as a partial inhibitor for milk production but only when it directly makes contact with the basolateral membrane of MECs.

Colostrogenesis: Role and Mechanism of the Bovine Fc Receptor of the Neonate (FcRn)

Colostrogenesis is a separate and unique phase of mammary epithelial cell activity occurring in the weeks before parturition and rather abruptly ending after birth in the bovine. It has been the focus of research to define what controls this process and how it produces high concentrations of specific biologically active components important for the neonate. In this review we consider colostrum composition and focus upon components that appear in first milked colostrum in concentrations exceeding that in blood serum. The Fc Receptor of the Neonate (FcRn) is recognized as the major immunoglobulin G (IgG) and albumin binding protein that accounts for the proteins' long half-lives. We integrate the action of the pinocytotic (fluid phase) uptake of extracellular components and merge them with FcRn in sorting endosomes. We define and explore the means of binding, sorting, and the transcytotic delivery of IgG1 while recycling IgG2 and albumin. We consider the means of releasing the ligands from the receptor within the endosome and describe a new secretion mechanism of cargo release into colostrum without the appearance of FcRn itself in colostrum. We integrate the insulin-like growth factor family, some of which are highly concentrated bioactive components of colostrum, with the mechanisms related to FcRn endosome action. In addition to secretion, we highlight the recent findings of a role of the FcRn in phagocytosis and antigen presentation and relate its significant and abrupt change in cellular location after parturition to a role in the prevention and resistance to mastitis infections.

A Comparative Review of the Cell Biology, Biochemistry, and Genetics of Lactose Synthesis

Lactose is the primary carbohydrate in the milk of most mammals and is unique in that it is only synthesized by epithelial cells in the mammary glands. Lactose is also essential for the development and nutrition of infants. Across species, the concentration of lactose in milk holds a strong positive correlation with overall milk volume. Additionally, there is a range of examples where the onset of lactose synthesis as well as the content of lactose in milk varies between species and throughout a lactation. Despite this diversity, the precursors, genes, proteins and ions that regulate lactose synthesis have not received the depth of study they likely deserve relative to the significance of this simple and abundant molecule. Through this review, our objective is to highlight the requirements for lactose synthesis at the biochemical, cellular and temporal levels through a comparative approach. This overview also serves as the prelude to a companion review describing the dietary, hormonal, molecular, and genetic factors that regulate lactose synthesis.

Milk Processing Affects Structure, Bioavailability and Immunogenicity of β-lactoglobulin

Bovine milk is subjected to various processing steps to warrant constant quality and consumer safety. One of these steps is pasteurization, which involves the exposure of liquid milk to a high temperature for a limited amount of time. While such heating effectively ameliorates consumer safety concerns mediated by pathogenic bacteria, these conditions also have an impact on one of the main nutritional whey constituents of milk, the protein β-lactoglobulin. As a function of heating, β-lactoglobulin was shown to become increasingly prone to denaturation, aggregation, and lactose conjugation. This review discusses the implications of such heat-induced modifications on digestion and adsorption in the gastro-intestinal tract, and the responses these conformations elicit from the gastro-intestinal immune system.

DNA and RNA-sequence based GWAS highlights membrane-transport genes as key modulators of milk lactose content

Background

Lactose provides an easily-digested energy source for neonates, and is the primary carbohydrate in milk in most species. Bovine lactose is also a key component of many human food products. However, compared to analyses of other milk components, the genetic control of lactose has been little studied. Here we present the first GWAS focussed on analysis of milk lactose traits.

Results

Using a discovery population of 12,000 taurine dairy cattle, we detail 27 QTL for lactose concentration and yield, and subsequently validate the effects of 26 of these loci in a distinct population of 18,000 cows. We next present data implicating causative genes and variants for these QTL. Fine mapping of these regions using imputed, whole genome sequence-resolution genotypes reveals protein-coding candidate causative variants affecting the ABCG2, DGAT1, STAT5B, KCNH4, NPFFR2 and RNF214 genes. Eleven of the remaining QTL appear to be driven by regulatory effects, suggested by the presence of co-locating, co-segregating eQTL discovered using mammary RNA sequence data from a population of 357 lactating cows. Pathway analysis of genes representing all lactose-associated loci shows significant enrichment of genes located in the endoplasmic reticulum, with functions related to ion channel activity mediated through the LRRC8C, P2RX4, KCNJ2 and ANKH genes. A number of the validated QTL are also found to be associated with additional milk volume, fat and protein phenotypes.

Conclusions

Overall, these findings highlight novel candidate genes and variants involved in milk lactose regulation, whose impacts on membrane transport mechanisms reinforce the key osmo-regulatory roles of lactose in milk.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-017-4320-3) contains supplementary material, which is available to authorized users.

The intracellular source, composition and regulatory functions of nanosized vesicles from bovine milk-serum

A hypothesis that the source of milk-serum derived vesicles (MSDVs) is the Golgi apparatus (GA) was examined.

Human Milk Oligosaccharides (HMOS): Structure, Function, and Enzyme-Catalyzed Synthesis

The important roles played by human milk oligosaccharides (HMOS), the third major component of human milk, in the health of breast-fed infants have been increasingly recognized, as the structures of more than 100 different HMOS have now been elucidated. Despite the recognition of the various functions of HMOS as prebiotics, antiadhesive antimicrobials, and immunomodulators, the roles and the applications of individual HMOS species are less clear. This is mainly due to the limited accessibility to large amounts of individual HMOS in their pure forms. Current advances in the development of enzymatic, chemoenzymatic, whole-cell, and living-cell systems allow for the production of a growing number of HMOS in increasing amounts. This effort will greatly facilitate the elucidation of the important roles of HMOS and allow exploration into the applications of HMOS both as individual compounds and as mixtures of defined structures with desired functions. The structures, functions, and enzyme-catalyzed synthesis of HMOS are briefly surveyed to provide a general picture about the current progress on these aspects. Future efforts should be devoted to elucidating the structures of more complex HMOS, synthesizing more complex HMOS including those with branched structures, and developing HMOS-based or HMOS-inspired prebiotics, additives, and therapeutics.

Intracellular origin and secretion of milk fat globules

The cream or fat fraction of milk consists of fat droplets composed primarily of triacylglycerols that are surrounded by cellular membranes. In this review we discuss what is known about how these droplets are formed in and secreted by mammary epithelial cells during lactation. This secretion mechanism, which appears to be unique, is unlike the exocytotic mechanism used by other cell types to secrete lipids. Milk fat globules originate as small, triacylglycerol-rich, droplets that are formed on or in endoplasmic reticulum membranes. These droplets are released from endoplasmic reticulum into the cytosol as microlipid droplets coated by proteins and polar lipids. Microlipid droplets can fuse with each other to form larger cytoplasmic lipid droplets. Droplets of all sizes appear to be unidirectionally transported to apical cell regions by as yet unknown mechanisms that may involve cytoskeletal elements. These lipid droplets appear to be secreted from the cell in which they were formed by being progressively enveloped in differentiated regions of apical plasma membrane. While plasma membrane envelopment appears to be the primary mechanism by which lipid droplets are released from the cell, a mechanism involving exocytosis of lipid droplets from cytoplasmic vacuoles also has been described. As discussed herein, while we have a general overview of the steps leading to the fat globules of milk, virtually nothing is known about the molecular mechanisms involved in milk fat globule formation, intracellular transit, and secretion.

The lactose analog GalNAcbeta1-->4Glc is present in bovine colostrum. Enzymatic basis for its occurrence

We have isolated from bovine colostrum the lactose analog GalNAcbeta1-->4Glc. The enzymatic basis for its occurrence was studied by assaying the activities of GlcNAcbeta-R beta4-N-acetylgalactosaminyltransferase (beta4-GalNAcT) and GlcNAcbeta-R beta4-galactosyltransferase (beta4-GalT) in primary milk and several lactating bovine mammary gland fractions. As the beta4-GalNAcT, which appears to be tightly membrane bound, is induced by the milk protein alpha-lactalbumin (alpha-LA) to act on Glc, it is concluded that beta4-GalNAcT is responsible for the synthesis of GalNAcbeta1-->4Glc in the gland. The comparatively low level (15-20 mg/l) at which this disaccharide is produced may be due to the relatively poor interaction of beta4-GalNAcT with alpha-LA as well as to the fact that alpha-LA does not inhibit the action of the enzyme on N-acetylglucosaminides.

Secretion of prosaposin, a multifunctional protein, by breast cancer cells

Western blotting and immunodetection with three antibodies were used to probe conditioned media of breast cancer cells (MDA231, MDA435, MCF-7) for prosaposin, a lysosomal protein that occurs in milk. It was readily detected in media from these cells, and from that of an sv40-transformed mammary epithelial cell, HBL100, but not from medium of human neural tumor cells (SK-N-MC). In cultures of MCF-7 cells, the prosaposin pattern of secretion over time closely resembled that of procathepsin D, another lysosomal protein occurring in milk. Supplementing medium with 17beta-estradiol (0. 1-100 nM) dose dependently increased secretion of both proteins after 48 h without changes in cell viability. The influence of 17beta-estradiol on secretion could play a role in the trophic activity of prosaposin in cellular differentiation and cell death protection. In concert with other lysosomal proteins in the tumor environment, such as procathepsin D, prosaposin may be a factor in eliminating barriers to tumor metastasis by facilitating hydrolysis of membrane glycolipids. The number of milk proteins known to be secreted by breast cancer cells is growing. There is evidence that at least some of these may be secreted in an endocrine manner in the normal, non-lactating breast.

Production of low-lactose milk by ectopic expression of intestinal lactase in the mouse mammary gland

We have investigated, in mice, an in vivo method for producing low-lactose milk, based on the creation of transgenic animals carrying a hybrid gene in which the intestinal lactase-phlorizin hydrolase cDNA was placed under the control of the mammary-specific alpha-lactalbumin promoter. Transgenic females expressed lactase protein and activity during lactation at the apical side of mammary alveolar cells. Active lactase was also secreted into milk, anchored in the outer membrane of fat globules. Lactase synthesis in the mammary gland caused a significant decrease in milk lactose (50-85%) without obvious changes in fat and protein concentrations. Sucklings nourished with low-lactose milk developed normally. Hence, these data validate the use of transgenic animals expressing lactase in the mammary gland to produce low-lactose milk in vivo, and they demonstrate that the secretion of an intestinal digestive enzyme into milk can selectively modify its composition.

Beta1,4-galactosyltransferase and lactose biosynthesis: recruitment of a housekeeping gene from the nonmammalian vertebrate gene pool for a mammary gland specific function

Beta1,4-galactosyltransferase (beta4GalT-I) is a constitutively expressed trans-Golgi enzyme, widely distributed in vertebrates, which synthesizes the beta4-N-acetyllactosamine structure commonly found in glycoconjugates. In mammals beta4GalT-I has been recruited for a second biosynthetic function, the production of lactose; this function takes place exclusively in the lactating mammary gland. In preparation for lactose biosynthesis, beta4GalT-I enzyme levels are increased significantly. We show that mammals have evolved a two-step mechanism to achieve this increase. In step one there is a switch to the use of a second transcriptional start site, regulated by a stronger, mammary gland-restricted promoter. The transcript produced is distinguished from its housekeeping counterpart by the absence of approximately 180 nt of 5'-untranslated sequence. In step two, this truncated transcript is translated more efficiently, relative to the major transcript expressed in all other somatic tissues.

Origin and secretion of milk lipids

The cream fraction of milk comprises droplets of triacylglycerol coated with cellular membranes. In this review, we discuss how these droplets are formed and secreted from mammary epithelial cells during lactation. This secretory system is especially interesting because the assembled lipid droplets are secreted from the cytoplasm enveloped by cellular membranes. In other cells, such as hepatocytes and enterocytes, lipid is secreted by exocytosis from membrane-bounded compartments of the secretory pathway. Milk lipids originate as small droplets of triacylglycerol, synthesized in or on the surfaces of rough endoplasmic reticulum (ER)4 membranes. These droplets are released into the cytoplasm as microlipid droplets (MLDs) with a surface coat of protein and polar lipid. MLDs may fuse with each other to form larger cytoplasmic lipid droplets (CLDs). Droplets of varying size, are transported to the apical cytoplasm by unknown mechanisms and are secreted from the cell coated with an outer bilayer membrane. CLDs may increase in size in all regions of the cell, especially at the plasma membrane during secretion. Two possible mechanisms for lipid secretion have been proposed: an apical mechanism, in which lipid droplets are enveloped with apical plasma membrane, and a secretory-vesicle mechanism, in which fat droplets are surrounded by secretory vesicles in the cytoplasm and are released from the surface by exocytosis from intracytoplasmic vacuoles. A combination of both mechanisms may be possible. Following secretion, a fraction of the membrane surrounding the globules may be shed from the droplets and give rise to membrane fragments in the skim milk phase.

Production of cystic fibrosis transmembrane conductance regulator in the milk of transgenic mice

Here we describe the production of cystic fibrosis transmembrane conductance regulator (CFTR), the product of the gene associated with cystic fibrosis, in the milk of transgenic mice. Mammary specific expression was achieved by placing the CFTR cDNA under the control of the goat beta-casein gene promoter. By fractionation, CFTR was shown to be associated with the membranes that envelop milk fat globules as they are discharged from the apical surface of the mammary epithelia. Since milk fat globules may comprise up to 10% of whole milk, this represents a novel, inexpensive and efficient approach to produce CFTR and possibly other membrane-associated proteins. The availability of large quantities of CFTR could have important implications for the development of new therapies for cystic fibrosis.

Calcium and calmodulin-dependent phosphorylation of kappa-casein by a bovine mammary casein kinase

A calcium and calmodulin-dependent kappa-casein kinase activity has been described in the bovine mammary gland. This kinase required previously dephosphorylated kappa-casein for substrate, thus suggesting a physiological role for this enzyme. The kappa-casein kinase required magnesium and the presence of both calcium and calmodulin for full activity. Calmodulin concentrations of .32 microM achieved one-half maximal activation of this enzyme. The calcium and calmodulin-dependent kappa-casein kinase was found in preparations of mammary acini and could be localized in a membranous fraction by centrifugation. The particles containing this activity had a high density (1.309 g/cc) and cofractionated with caseins, suggesting this enzyme may be present in secretory granules.

Intramammary infusions of cytochalasin B and dimethyl sulfoxide do not suppress milk secretion in the goat

Cytochalasin B, an intracellular microfilament antagonist, was evaluated for its capacity to inhibit milk secretion in the goat. Intramammary infusions of the drug via teat canal in amounts to 6 mg had no effects on yields or fat and protein contents and minor, if any, effects on somatic cell counts of consecutive 12-h milkings. Our results suggest that the apical plasma membrane of lactating cells is impermeable to cytochalasin B and that the reduced secretion of lactose and casein caused by the drug in vitro may arise from its interference with glucose uptake at the base of cells. Dimethylsulfoxide, which we used (2 ml) in infusates to solubilize cytochalasin B, also was without effect on the foregoing lactation characteristics.

Internalization of ferritin-concanavalin A by the lactating mammary cell in vivo

Ferritin-concanavalin A (Fer-Con A) was used to label the apical plasma membrane of the lactating cell to determine whether membrane internalization takes place. Rat glands were infused in vivo via the teat with 0.2 mg of Fer-Con A in 0.2 ml tris buffer (pH 7.0) containing 0.1% trypan blue, the latter acting as a marker of the infusate. Tissues were obtained from separate animals 5, 10 and 60 min postinfusion. Fer-Con A was seen in alveolar lumina bound to the outer surfaces of apical plasma membrane, microvilli and milk fat globules. It was observed within lactating cells on the inner membrane surfaces of endocytotic vesicles, Golgi cisternae, and secretory vesicles containing casein micelles, and in multivesicular bodies and lysosomes. Internalization of the ferritin-lectin conjugate into casein-containing secretory vesicles was detectable in the 5-min postinfusion tissue. Lysosomes were the only structures in control tissue that contained particles bearing some resemblance to Fer-Con A. The data provide evidence that apical plasma membrane is internalized and distributed to a number of intracellular compartments.

Immunoreactive prolactin in subcellular fractions from bovine mammary tissue

Mammary tissue from lactating Holsteins at slaughter was homogenized and fractionated into subcellular compartments. Bovine prolactin was quantified in each subcellular fraction and in preslaughter milk by radioimmunoassay procedures validated for this purpose. Prolactin concentrations were ng prolactin/mg protein for all samples. Immunoreactive prolactin was in rough endoplasmic reticulum (24.8 ng/mg), Golgi apparatus (18.8 ng/mg), and secretory vesicle (10.2 ng/mg). All of these concentrations exceeded that in samples of milk (2.7 ng/mg). Nuclear fractions contained a material that reacted in the prolactin radioimmunoassay, but criteria for valid quantification could not be satisfied. These results confirm prolactin inside bovine mammary cells, and they are consistent with an intracellular pathway for transfer of prolactin from plasma into milk.

Antibodies to the major insoluble milk fat globule membrane-associated protein: specific location in apical regions of lactating epithelial cells

Milk lipid globules of various species are surrounded by a membrane structure that is separated from the triglyceride core of the globule by a densely staining fuzzy coat layer of 10- to 50-nm thickness. This internal coat structure remains attached to the membrane during isolation and extraction with low- and high-salt buffers, is insoluble in nondenaturing detergents, and is enriched in an acidic glycoprotein (butyrophilin) with an apparent Mr of 67,000. Guinea pig antibodies against this protein, which show cross-reaction with the corresponding protein in some (goat) but not other (human, rat) species, have been used for localization of butyrophilin on frozen sections of various tissues from cow by immunofluorescence and electron microscopy. Significant reaction is found only in milk-secreting epithelial cells and not in other cell types of mammary gland and various epithelial tissues. In milk-secreting cells, the staining is restricted to the apical cell surface, including budding milk lipid globules, and to the periphery of the milk lipid globules contained in the alveolar lumina. These findings indicate that butyrophilin, which is constitutively secreted by surface budding in coordination with milk lipid production, is located at the apical surface and is not detected at basolateral surfaces, in endoplasmic reticulum, and in Golgi apparatus. This protein structure represents an example of a cell type-specific cytoskeletal component in a cell apex. It is suggested that this antigen provides a specific marker for the apical surface of milk-secreting cells and that butyrophilin is involved in the vectorial discharge of milk lipid globules.