Freeze-fracture observations of the lactating rat mammary gland. Membrane events during milk fat secretion

Perilipin-2 promotes lipid droplet-plasma membrane interactions that facilitate apocrine lipid secretion in secretory epithelial cells of the mouse mammary gland

Secretory epithelial cells (sMEC) in mammary glands of lactating animals secrete lipids by a novel apocrine mechanism in which cytoplasmic lipid droplets (LD) contact and are enveloped by elements of the apical plasma membrane (APM) before being released into the lumen of the gland as membrane bound structures. The molecular properties of LD-APM contacts and the mechanisms regulating LD membrane envelopment and secretion are not fully understood. Perilipin-2 (Plin2) is a constitutive LD protein that has been proposed to tether LD to the APM through formation of a complex with the transmembrane protein, butyrophilin1a1 (BTN) and the redox enzyme, xanthine oxidoreductase (XOR). Using mice lacking Plin2 and physiological inhibition of apocrine lipid secretion, we demonstrate that LD-APM contact and envelopment are mechanistically distinct steps that they are differentially regulated by Plin2 and independent of LD secretion. We find that Plin2 is not required for formation of LD-APM contacts. However, it increases the percentage of LD that contact the APM and mediates enlargement of the LD-APM contact zone as LD undergo membrane envelopment. The effects of Plin2 LD-APM interactions are associated with increased abundances of BTN, XOR and Cidea, which are implicated as mediators of LD-APM contact formation, on membranes surrounding secreted LD, and with promotion of glycocalyx remodeling at LD-APM contact sites. We propose that Plin2 does not directly mediate contact between LD and the APM but acts by enhancing molecular interactions that stabilize LD-APM contacts and govern membrane envelopment of LD during apocrine lipid secretion. Plin2 does not appear to significantly affect the lipid content of milk in fully lactating animals, but it does increase lipid secretion at the onset of lactation in primaparous dams, which suggest a role in facilitating apocrine lipid secretion in sMEC during their initial transition to a secretory phenotype.

Symposium review: Intravital imaging of the lactating mammary gland in live mice reveals novel aspects of milk-lipid secretion

Milk fat comprises membrane-coated droplets of neutral lipid, which constitute the predominant source of lipids for survival of the suckling neonate. From the perspective of the dairy industry, they are the basis for the manufacture of butter and essential ingredients in the production of cheese, yogurt, and specialty dairy produce. To provide mechanistic insight into the assembly and secretion of lipid droplets during lactation, we developed novel intravital imaging techniques using transgenic mice, which express fluorescently tagged marker proteins. The number 4 mammary glands were surgically prepared under a deep plane of anesthesia and the exposed glands positioned as a skin flap with intact vascular supply on the stage of a laser-scanning confocal microscope. Lipid droplets were stained by prior exposure of the glands to hydrophobic fluorescent BODIPY (boron-dipyrromethene) dyes and their formation and secretion monitored by time-lapse subcellular microscopy over periods of 1 to 2 h. Droplets were transported to the cell apex by directed (superdiffusive) motion at relatively slow and intermittent rates (0-2 µm/min). Regardless of size, droplets grew by numerous fusion events during transport and as they were budding from the cell enveloped by apical membranes. Surprisingly, droplet secretion was not constitutive but required an injection of oxytocin to induce contraction of the myoepithelium with subsequent release of droplets into luminal spaces. These novel results are discussed in the context of the current paradigm for milk fat synthesis and secretion and as a template for future innovations in the dairy industry.

Compartmentalization of proteins in lipid droplet biogenesis

Our existing understanding of the structure, protein organization and biogenesis of the lipid droplet has relied heavily on microscopical techniques that lack resolution and the ability to preserve native cellular and protein composition. The electron microscopic technique of freeze-fracture replica immunogold labeling (FRIL) overcomes these problems, and is currently providing new perspectives in the field. Because of the property of frozen lipids to deflect the fracture plane, en face views of the lipid droplet and its component layers are revealed for high resolution visualization. By means of immunogold labeling, proteins involved in the accretion and mobilization of lipids, notably the PAT family proteins, can be localized at and in the droplet. Application of this approach demonstrates that, contrary to prevailing wisdom, the PAT family proteins are not invariably restricted to the surface of the lipid droplet but can occur throughout the core. The notion that lipid droplet biogenesis involves neutral lipid accumulation within the ER membrane bilayer followed by budding off, enclosed by a protein-containing phospholipid monolayer, is not substantiated. Instead, lipid droplets appear to develop externally to both ER membranes at specialized sites in which the ER enwraps the droplet, and the facing leaflets of the ER membrane and droplet surface are enriched in adipophilin. PAT family proteins are not, as often stated, specific to the lipid droplet, but are widely present in the plasma membrane where, under conditions of lipid loading, they adopt a similar configuration to that of specialized sites in the ER. FRIL has further provided new insights into the mechanism of secretion of a special type of lipid droplet, the milk fat globule. These examples highlight the contribution of the FRIL technique to critical appraisal and development of concepts in the lipid droplet field.

Butyrophilin controls milk fat globule secretion

The molecular mechanism underlying milk fat globule secretion in mammary epithelial cells ostensibly involves the formation of complexes between plasma membrane butyrophilin and cytosolic xanthine oxidoreductase. These complexes bind adipophilin in the phospholipid monolayer of milk secretory granules, the precursors of milk fat globules, enveloping the nascent fat globules in a layer of plasma membrane and pinching them off the cell. However, using freeze-fracture immunocytochemistry, we find these proteins in locations other than those previously inferred. Significantly, butyrophilin in the residual plasma membrane of the fat globule envelope is concentrated in a network of ridges that are tightly apposed to the monolayer derived from the secretory granule, and the ridges coincide with butyrophilin labeling in the globule monolayer. Therefore, we propose that milk fat globule secretion is controlled by interactions between plasma membrane butyrophilin and butyrophilin in the secretory granule phospholipid monolayer rather than binding of butyrophilin-xanthine oxidoreductase complexes to secretory granule adipophilin.

Structure and expression of goat GLYCAM1 gene: lactogenic-dependent expression in ruminant mammary gland and interspecies conservation of the proximal promoter

A macroarray approach used to list genes differentially expressed in goat mammary gland (gestation vs. lactation), other than milk protein genes, allowed us to detect the Glycosylation-dependent Cell Adhesion Molecule 1 (GLYCAM1) gene. GLYCAM1, a member of the glycoprotein mucin family, is a component of the milk fat globule membrane (MFGM). Its complete cDNA and gene sequences were determined and it was mapped by fluorescent in situ hybridization (FISH) on goat and cattle chromosome 5 (CHI5q21 and BTA5q21), and on sheep chromosome 3 (OAR3q21). Northern blot analyses confirmed its differential expression during the development and differentiation of the mammary gland of ruminants with a significantly higher mRNA amount during lactation than during pregnancy. An experimental in vivo induction model for lactation, developed by Kann et al., showed that the expression of GLYCAM1 is hormonally regulated in the mammary gland of ewes. Interspecies comparison of the gene promoter revealed the evolutionary conservation of a short proximal nucleotide sequence encompassing several transcription factor binding sites that could mediate the above-mentioned hormonal regulation.

Isolation of adipophilin and butyrophilin from bovine milk and characterization of a cDNA encoding adipophilin

The milk fat globule membrane-associated proteins adipophilin (alias adipocyte differentiation-related protein) and butyrophilin were purified from bovine milk by reverse-phase chromatography. The nucleotide sequence of bovine adipophilin was obtained via peptide mapping and sequencing of a mammary gland cDNA clone, which comprises 1841 nucleotides and has an open reading frame of 450 amino acids. By peptide mapping, 19% of the amino acid sequence was confirmed. The obtained amino acid sequence has 87 and 80% identical residues with human and mouse adipophilin, respectively. Alignment with the proteins perilipin and TIP47 revealed two highly conserved segments, which may assemble into amphipathic alpha-helices.

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.

Butyrophilin is expressed in mammary epithelial cells from a single-sized messenger RNA as a type I membrane glycoprotein

We investigated the expression of butyrophilin in eukaryotic cells with a view to determining the number of mRNA species, the incorporation of the peptide chain into microsomes, and the topology of the processed protein in biological membranes. Butyrophilin is synthesized from a single sized mRNA in both bovine and murine lactating mammary tissue and associates with microsomal membranes with a type I topology (Nexo.Ccyto) via a single hydrophobic anchor in the middle of the sequence. Several isoelectric variants of the protein were detected in cellular membranes from lactating bovine mammary tissue and in the milk-fat-globule membrane. We found no evidence for soluble forms of butyrophilin in postmicrosomal supernatants. The 66-kDa protein appears to be subjected to limited proteolysis, giving rise to a 62-kDa fragment lacking the C terminus and to other more minor fragments of lower Mr in the milk-fat-globule membrane. Antipeptide antibodies to epitopes within the N- and C-terminal domains were used to show that butyrophilin retains a type I topology in plasma membranes when expressed in insect cells from a baculovirus vector, and in secreted milk-fat globules. These data do not agree with previous suggestions that butyrophilin may exist in cytoplasmic soluble forms, or be reorganized in the plane of the lipid bilayer during secretion in lipid droplets from mammary cells. The results are discussed with reference to the role butyrophilin may play as the principal scaffold for the assembly of a complex with xanthine oxidase and other proteins that functions in the budding and release of milk-fat globules from the apical surface during lactation.

A review of the molecular and cellular biology of butyrophilin, the major protein of bovine milk fat globule membrane

The molecular and cellular biology of the milk protein butyrophilin is reviewed. Butyrophilin constitutes more than 40% by weight of the total protein associated with the fat globule membrane of bovine milk. Closely related proteins are abundant in the fat globule membranes of many other species. Butyrophilin is synthesized as a peptide of 526 amino acids with an amino-terminal hydrophobic signal sequence of 26 amino acids, which is cleaved before secretion in association with the fat globule membrane. Hydropathy analysis and in vitro translation of butyrophilin mRNA indicate that the protein associates with membranes in a type I orientation via a single stretch of 27 hydrophobic amino acids in the approximate middle of the sequence. Evidence that butyrophilin is incorporated into fat globule membrane as a transmembrane protein and as a cytoplasmically oriented peripheral component is discussed. The carboxy-terminal sequence of butyrophilin is significantly homologous to two other proteins: ret finger protein and the 52-kDa nuclear antigen A of Sjögren's syndrome. Expression of bovine butyrophilin mRNA correlates with the onset of milk fat secretion toward the end of pregnancy and is maintained throughout lactation. The possible function of butyrophilin in the secretion of milk lipid droplets is discussed.

An electron-microscope and freeze-fracture study of the egg cortex of Brachydanio rerio

We have examined the cortex of the teleost (Brachydanio rerio) egg before and during exocytosis of cortical granules by scanning, transmission, and freeze-fracture electron microscopy. In the unactivated egg, the P-face of the plasma membrane exhibits a random distribution of intramembranous particles, showing a density of 959/micron2 and an average diameter of 8 nm. Particles over P- and E-faces of the membranes of cortical granules are substantially larger and display a significantly lower density. An anastomosing cortical endoplasmic reticulum forms close associations with both the plasma membrane of the egg and the membranes of cortical granules. Exocytosis begins with cortical granules pushing up beneath the plasma membrane to form dome-shaped swellings, coupled with an apparent clearing of particles from the site of contact between the apposed membranes. A depression in the particle-free plasma membrane appears to mark sites of fusion and pore formation between cortical granules and plasma membranes. Profiles of exocytotic vesicles undergo a predictable sequence of morphological change, but maintain their identity in the egg surface during this transformation. Coated vesicles form at sites of cortical granule breakdown. Differences in particle density between cortical granules and egg plasma membranes persist during transformation of the exocytotic profiles. This suggests that constituents of the 2 membrane domains remain segregated and do not intermix rapidly, lending support to the view that the process of membrane retrieval is selective (i.e., cortical granule membrane is removed).

Development of intracellular lipid deposits in the lipid-laden cells of atherosclerotic lesions. A cytochemical and ultrastructural study

In atherosclerotic lesions of rabbits fed a cholesterol-rich diet, the lipid deposits of foam cells derived from monocytes, smooth muscle and endothelial cells were studied by physical, cytochemical and ultrastructural methods. Beginning with the third week of diet, the lipid material that could be visualized at the light microscope level by Oil red O and Nile red staining was progressively accumulated in the intimal cells of the atherosclerotic lesions. In the early stages of foam cell formation, the deposits occurred especially as intracytoplasmic non-membrane bound lipid inclusions (lipid droplets). In polarizing microscopy these appeared as a mixture of iso-, and anisotropic material. The latter were birefringent and showed an axial symmetry with a black cross image, suggesting that the lipids were in a liquid crystalline state. In chemically-fixed specimens, the content of lipid inclusions was preserved in various degrees. In freeze-fractured preparations they displayed a layered onion-like arrangement with smooth cleavage faces surrounding an amorphous core. Upon incubation with filipin, that specifically binds to 3 beta-hydroxysterols, the peripheral layers of the inclusions were labeled, revealing the existence of unesterified cholesterol. In the advanced stages of foam cell formation, lipids were additionally accumulated in the lysosomal compartment as polymorphic multilamellar structures concentrically arranged, with cleavage faces devoid of intralamellar particles. The presence of acid phosphatase showed that these features were modified lysosomes and were tentatively named lysosomal lipid bodies. In the latest stages examined cholesterol crystals developed within lysosomal lipid bodies usually enclosed in multilamellar structures. This lipid coat may represent the place of crystal formation and presumably acts as barrier for the turnover of the crystalline cholesterol, thus impeding plaque regression.

Monoclonal antibodies prepared against PAS-I butyrophilin and GP-55 from guinea-pig milk-fat-globule membrane bind specifically to the apical pole of secretory-epithelial cells in lactating mammary tissue

Monoclonal antibodies to the three major glycoproteins of guinea-pig milk-fat-globule membrane were isolated. The specificity of these antibodies was determined by solid-phase immunoassays and by immunoblotting and autoradiographic techniques after one- and two-dimensional gel electrophoresis. The antibodies bound to PAS-I, a sialoglycoprotein of Mr greater than or equal to 200 000 and the glycoproteins butyrophilin and GP-55, of Mr 63 000 and 55 000, respectively. Immunolocalization studies showed that all three proteins were highly concentrated in the apical pole of secretory-epithelial cells in mammary tissue during lactation. PAS-I, butyrophilin or GP-55, were not detected in either the basal cytoplasm of mammary epithelial cells or in myoepithelial cells, capillary endothelial cells or other cells found in the mammary gland. These proteins were either present in small amounts or were absent from mammary tissue taken in late pregnancy. The monoclonal antibodies characterized in this study will therefore be useful as probes for studies of the biogenesis of apical membrane proteins in mammary epithelial cells during lactation.

Glycosyl transferases in mouse and human milk fat globule membranes

Membranes isolated from mouse and human milk fat globules were found to contain the enzymes responsible for the synthesis of dolichol monophosphate mannose and dolichol monophosphate glucose as well as those involved in the transference of the glycosyl residues from the two dolichol derivatives to dolichol diphosphate oligosaccharides. The levels of most of the enzymes were comparable to those found in mouse mammary gland microsomes. The presence of enzymes involved in protein glycosylation via dolichol derivatives in the milk fat globule membrane provides evidence in favor of an outward flow of membrane components from the rough endoplasmic reticulum, where these enzymes are active in vivo, towards the cell surface.

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.

Membrane associations between subsurface cisternae of endoplasmic reticulum and the plasma membrane of rat Sertoli cells

Close membrane associations between the endoplasmic reticulum and the plasma membrane (ER-PM) occur in specialized regions of the rat Sertoli cell cytoplasm. They are characterized, in freeze fracture replicas, as mesa-like modifications of E membrane fracture faces or as corresponding discoid depressions on P membrane fracture faces. When these structures lie along transitional regions in the membrane fracture plane, they are seen to be complementary, and the space between them to be greatly reduced. These specialized close membrane associations may represent adhesive sites between the endoplasmic reticulum and the plasma membrane. However, their resemblance to vascular endothelial fenestrae which are known to be sites of increased membrane permeability may suggest other functional roles.

Freeze-fracture study of rat ventral prostate: secretory mechanisms in the epithelial cell

Membrane events during apocrine and merocrine secretion of rat ventral prostate epithelial cells were analyzed by freeze-fracture. Early morphological manifestation of both secretory modes involves focal clearance of microvilli characterized by their progressive inclination and attenuation over the apical membrane. Merocrine secretion includes invasion of the apical microfilament web by the secretory vesicle and clearance of membrane particles at the site of its interaction with the apical plasmalemma. During apocrine secretion, a portion of the plasma membrane cleared of microvilli projects into the lumen. Growth of this projection is accompanied by a progressive, but partial, clearing of membrane particles and results in the formation of a large bleb containing dilated endoplasmic reticulum cisternae. Completion of the process involves "degeneration" of the bleb and its release by constriction of a neck, and possibly, fusion of vesicular or tubular structures. Swelling and blebbing of microvilli are shown to be preparation artifacts.

Carotene in bovine milk fat globules: observations on origin and high content in tissue mitochondria

The location and origin of carotenoids in bovine milk fat globules was investigated using spectral absorption of lipid solutions at 461 nm to quantitate carotene. Release of membrane from globules as a result of churning to butter or by freezing and thawing of the globules yielded membrane preparations which were devoid of carotene. Globule cores from these procedures exhibited carotene concentrations comparable to those in total milk lipids. Fractionation of lactating bovine tissue and analysis of lipid extracts revealed that the intracellular fat droplets have carotene concentrations approximating those of secreted globules. However, intracellular membranes of the tissue, particularly the mitochondria, are much richer in carotenoids than formative or secreted fat globules. The evidence indicates that bovine milk fat globules acquire carotene during their formation in the cell, but that some minor fraction of the total carotene may be extracted from the enveloping secretory membrane. Mean carotene values (microgram/g of lipid) for fractions from three samples of lactating tissue were: whole tissue 47, mitochondria 461, microsomes 69, cytosol 67, fat droplets 8, milk 9. One tissue analysis indicated that Golgi membranes contain somewhat more carotene than do microsomes.

Membrane fusion during secretion: cortical granule exocytosis in sex urchin eggs as studied by quick-freezing and freeze-fracture

Exocytosis of cortical granules was observed in sea urchin eggs, either quick-frozen or chemically fixed after exposure to sperm. Fertilization produced a wave of exocytosis that began within 20 s and swept across the egg surface in the following 30 s. The front of this wave was marked by fusion of single granules at well-separated sites. Toward the rear of the wave, granule fusion became so abundant that the egg surface left with confluent patches of granule membrane. The resulting redundancy of the egg surface was accommodated by elaboration of characteristic branching microvilli, and by an intense burst of coated vesicle formation at approximately 2 min after insemination. Freeze-fracture replicas of eggs fixed with glutaraldehyde and soaked in glycerol before freezing displayed forms of granule membrane interaction with the plasma membrane which looked like what other investigators have considered to be intermediates in exocytosis. These were small disks of membrane contact or membrane fusion, which often occurred in multiple sites on one granule and also between adjacent granules. However, such membrane interactions were never found in eggs that were quick-frozen fixation, or in eggs fixed and frozen without exposure to glycerol. Glycerination of fixed material appeared to be the important variable; more concentrated glycerol produced a greater abundance of such "intermediates." Thus, these structures may be artifacts produced by dehydrating chemically fixed membranes, and may not be directly relevant to the mechanism by which membranes naturally fuse.

Membrane events involved in myoblast fusion

Myoblast fusion has been studied in cultures of chick embryonic muscle utilizing ultrastructural techniques. The multinucleated muscle cells (myotubes) are generated by the fusion of two plasma membranes from adjacent cells, apparently by forming a single bilayer that is particle-free in freeze-fracture replicas. This single bilayer subsequently collapses, and cytoplasmic continuity is established between the cells. The fusion between the two plasma membranes appears to take place primarily within particle-free domains (probably phospholipid enriched), and cytoplasmic unilamellar, particle-free vesicles are occasionally associated with these regions. These vesicles structurally resemble phospholipid vesicles (liposomes). They are present in normal myoblasts, but they are absent in certain fusion-arrested myoblast popluations, such as those treated with either 5-bromo-deoxyuridine (BUdR), cycloheximide (CHX), or pospholipase C (PLC). The unilamellar, particle-free vesicles are present in close proximity to the plasma membranes, and physical contact is observed frequently between the vesicle membrane and the plasma membrane. The regions of vesicle membrane-plasma membrane interaction are characteristically free of intramembrane particles. A model for myoblast fusion is presented that is based onan interpretation of these observations. This model suggests that the cytoplasmic vesicles initiate the generation of particle-depleted membrane domains, both being essential components in the fusion process.

Experimental studies of apocrine secretion in the dorsal prostate epithelium of the rat

Rat dorsal prostate epithelium was studied in intact adult animals, in animals castrated for three days and in rats after inhibition of prolactin secretion. Thin sections, electron-microscopic autoradiographs and freeze-fracture replicas were used to analyze the process of apocrine secretion in this gland. The rough endoplasmic reticulum and the Golgi apparatus of the secretory cells are well developed, but secretory granules are absent. The only sign indicating release of secretory material is the appearance of blebs originating from the apical plasma membrane. Freeze-fracture replicas of the apical plasma membrane reveal that the blebs develop randomly from the bases of microvilli-like protrusions. In vitro pulse labeling of the proteins using 3H-leucine resulted in a labeling of the apical blebs. A post-castration period of three days was sufficient to reduce drastically the number and size of the apical blebs conhree weeks by application of lisuride, a synthetic ergot alkaloid, also induced regressive changes in the secretory cells. The apical blebs were still present, but they were shrunken and their content appeared condensed. These experimental conditions proved that the apical blebs are closely related to the functional activity of the cells and are interpreted as true apocrine secretion in the rat dorsal epithelium.