Milk lipid globules: control of their size distribution

Unveiling the applications of membrane proteins from oil bodies: leading the way in artificial oil body technology and other biotechnological advancements

Oil bodies (OBs) function as organelles that store lipids in plant seeds. An oil body (OB) is encased by a membrane composed of proteins (e.g., oleosins, caleosins, and steroleosins) and a phospholipid monolayer. The distinctive protein-phospholipid membrane architecture of OBs imparts exceptional stability even in extreme environments, thereby sparking increasing interest in their structure and properties. However, a comprehensive understanding of the structure-activity relationships determining the stability and properties of oil bodies requires a more profound exploration of the associated membrane proteins, an aspect that remains relatively unexplored. In this review, we aim to summarize and discuss the structural attributes, biological functions, and properties of OB membrane proteins. From a commercial perspective, an in-depth understanding of the structural and functional properties of OBs is important for the expansion of their applications by producing artificial oil bodies (AOB). Besides exploring their structural intricacies, we describe various methods that are used for purifying and isolating OB membrane proteins. These insights may provide a foundational framework for the practical utilization of OB membrane proteins in diverse applications within the realm of AOB technology, including biological and probiotic delivery, protein purification, enzyme immobilization, astringency detection, and antibody production.

Caleosin/peroxygenases: multifunctional proteins in plants

BACKGROUND

Caleosin/peroxygenases (CLO/PXGs) are a family of multifunctional proteins that are ubiquitous in land plants and are also found in some fungi and green algae. CLO/PXGs were initially described as a class of plant lipid-associated proteins with some similarities to the oleosins that stabilize lipid droplets (LDs) in storage tissues, such as seeds. However, we now know that CLO/PXGs have more complex structures, distributions and functions than oleosins. Structurally, CLO/PXGs share conserved domains that confer specific biochemical features, and they have diverse localizations and functions.

SCOPE

This review surveys the structural properties of CLO/PXGs and their biochemical roles. In addition to their highly conserved structures, CLO/PXGs have peroxygenase activities and are involved in several aspects of oxylipin metabolism in plants. The enzymatic activities and the spatiotemporal expression of CLO/PXGs are described and linked with their wider involvement in plant physiology. Plant CLO/PXGs have many roles in both biotic and abiotic stress responses in plants and in their responses to environmental toxins. Finally, some intriguing developments in the biotechnological uses of CLO/PXGs are addressed.

CONCLUSIONS

It is now two decades since CLO/PXGs were first recognized as a new class of lipid-associated proteins and only 15 years since their additional enzymatic functions as a new class of peroxygenases were discovered. There are many interesting research questions that remain to be addressed in future physiological studies of plant CLO/PXGs and in their recently discovered roles in the sequestration and, possibly, detoxification of a wide variety of lipidic xenobiotics that can challenge plant welfare.

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.

Chemistry of Human Breast Milk-A Comprehensive Review of the Composition and Role of Milk Metabolites in Child Development

Early nutrition has an enormous influence on a child's physiological function, immune system maturation, and cognitive development. Human breast milk (HBM) is recognized as the gold standard for human infant nutrition. According to a WHO report, breastfeeding is considered as an unequaled way of providing ideal food to the infant, which is required for his healthy growth and development. HBM contains various macronutrients (carbohydrates, proteins, lipids, and vitamins) as well as numerous bioactive compounds and interactive elements (growth factors, hormones, cytokines, chemokines, and antimicrobial compounds. The aim of this review is to summarize and discuss the current knowledge about metabolites, which are the least understood components of HBM, and their potential role in infant development. We focus on small metabolites (<1500 Da) and characterize the chemical structure and biological function of polar metabolites such as human milk oligosaccharides, nonprotein molecules containing nitrogen (creatine, amino acids, nucleotides, polyamines), and nonpolar lipids. We believe that this manuscript will provide a comprehensive insight into a HBM metabolite composition, chemical structure, and their role in a child's early life nutrition.

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.

Milk lipid globules and their surrounding membrane: a brief history and perspectives for future research

Most of the lipids in milk are triacylglycerols that occur in globules surrounded by a membrane derived from cellular membranes. This membrane, the milk-fat or milk-lipid globule membrane (MLGM), surrounds globules during the process of their secretion from the cell. The nature and cellular origin of the milk lipid globule membrane has been the subject of a considerable amount of research. Milk lipid globules originate as very small lipid droplets formed on or in the endoplasmic reticulum followed by release into the cytosol. These droplets consist of a triacylglycerol-rich core coated with a layer of proteins and polar lipids. How these droplets are formed, how they can grow in volume, how they move through the cell, and how they are secreted are questions that have been the basis for a number of investigations. While the general outlines of droplet formation, growth, movement, and secretion are known, virtually no molecular details of any of these processes have been elucidated. In this article I have presented a brief historical account of research on milk fat globules, their surrounding membrane, and on aspects of the intracellular origin, growth, and secretion of milk lipid globules. I have also attempted to call attention to those areas where further research is needed to gain a better understanding of the processes involved.

Caleosins: Ca2+-binding proteins associated with lipid bodies

We have previously identified a rice gene encoding a 27 kDa protein with a single Ca2+-binding EF-hand and a putative membrane anchor. We report here similar genes termed caleosins, CLO, in other plants and fungi; they comprise a multigene family of at least five members in Arabidopsis (AtClo1-5). Northern hybridization demonstrated that AtClo2-4 mRNAs levels were low in various tissues, while AtClo1 mRNA levels were high in developing embryos and mature seeds. Analysis of transgenic Arabidopsis plants expressing the GUS reporter under control of the AtClo1 promoter showed strong levels of expression in developing embryos and also in root tip cells. Antibodies raised against AtCLO1 were used to detect caleosin in cellular fractions of Arabidopsis and rapeseed. This indicated that caleosins are a novel class of lipid body proteins, which may also be associated with an ER subdomain.

Intracellular distribution and mobilization of unesterified cholesterol in adipocytes: triglyceride droplets are surrounded by cholesterol-rich ER-like surface layer structures

In addition to their central role in triglyceride storage, fat cells are a primary depot of unesterified cholesterol (FC) in the body. In comparison, peripheral cells contain very little FC. This difference in adipocytes versus peripheral tissues is inconsistent with the current theory of cholesterol homeostasis. Attempting to resolve this discrepancy, we examined intracellular storage sites of FC in murine 3T3-F442A adipocytes. Using the cholesterol-binding antibiotic, filipin, in combination with high resolution fluorescence microscopy, intense fluorescent staining characteristically decorated the periphery of triglyceride droplets (TGD) as well as the plasma membrane (PM) of fat cells. Filipin-staining was not visible inside the lipid droplets. Purification of TGD by subcellular fractionation demonstrated that the rise in total FC content of adipocytes upon differentiation was attributable to an increase in TGD-FC, which contributed up to one third of the total cellular FC. The protein component of purified TGD from cultured adipocytes as well as from murine adipocytes obtained from fresh tissues contained the lumenal endoplasmic reticulum (ER) immunoglobulin binding protein (BiP) and the integral ER membrane protein calnexin. Efflux experiments using the extracellular FC acceptors (β)-cyclodextrin or apolipoprotein A-I demonstrated that TGD-associated FC was releasable from TGD. Whereas FC efflux from adipocytes was unaffected in the presence of brefeldin A or monensin, the secretion of a control protein, lipoprotein lipase, was effectively reduced. In summary, our findings identify the TGD surface layer as primary intracellular storage site for FC within adipocytes. We suggest that the structural role of ER-resident proteins in this adipocyte TGD envelope has been previously neglected. Our findings support the suggestion that an ER-like structure, albeit of modified lipid composition, constitutes the lipid droplets' surface layer. Finally, the efflux process of FC from adipocytes upon extracellular stimulation with (beta)-cyclodextrin provides evidence for an energy-dependent intracellular trafficking route between the TGD-FC pool and the PM-FC sites which is distinct from the secretory pathway of proteins.

Mechanisms of lipid-body formation

Most organisms transport or store neutral lipids as lipid bodies - lipid droplets that usually are bounded by specific proteins and (phospho)lipid. Neutral-lipid bodies vary considerably in their morphology and are associated with an extremely diverse range of proteins. However, the mechanisms by which they are generated in plants, animals and microorganisms appear to share many common features: lipid bodies probably arise from microdomains of the endoplasmic reticulum (or the plasma membrane in prokaryotes) that contain lipid-biosynthesis enzymes, and their synthesis and size appear to be controlled by specific protein components.

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.

Regulation of milk lipid secretion and composition

Triacylglycerols make up 98% of the lipid content of milk, ranging in different species from 0 to 50% of the total milk volume. The fatty aid composition of the triacylglycerols depends on the species, the dietary fatty acid composition, and the carbohydrate-to-lipid ratio of the diet. The rate of lipid synthesis in the lactating mammary gland depends on the stage of mammary development and is decreased by fasting and starvation in ruminants and rodents but not in species that fast during lactation, such as seals and hibernating bears. Regulatory agents include insulin, prolactin, and non-esterified fatty acids. Dietary trans fatty acids may depress milk lipid synthesis under certain conditions. Evidence is presented that fatty acids may play a major regulatory role in acute changes in de novo mammary fatty acid synthesis, acting primarily on the activity of acetyl coenzyme A carboxylase.

Adipocyte differentiation-related protein is secreted into milk as a constituent of milk lipid globule membrane

Milk lipid globules from humans, cows and rats contained a protein identified as adipocyte differentiation-related protein (ADRP) associated with the globule surface membrane material. This protein, previously believed to be specific to adipocytes, was a major constituent of the globule surface and was present in a detergent-insoluble complex that contained stoichiometric amounts of butyrophilin and xanthine oxidase. Identification of ADRP was by sequence similarity of tryptic peptides from cow and human proteins with the sequence inferred from the cDNA for mouse ADRP. The putative ADRP of lipid globules from cow, human and rat milk was recognized specifically by antisera raised against a peptide synthesized to duplicate the N-terminal 26 residues of the mouse protein. In homogenates of lactating mammary gland, ADRP was found only in endoplasmic reticulum and in lipid droplet fractions. ADRP was modified, apparently post-translationally, and one modification apparently was acylation, primarily with C14, C16 and C18 fatty acids. Two isoelectric variants of ADRP were present in cow globule membrane material. In vitro, ADRP served as a substrate for protein kinases associated with milk lipid globule membrane, but this protein did not seem to become phosphorylated intracellularly.

Composition and morphology of lipid droplets from oviduct epithelial cells

This study was undertaken to determine the composition and morphology of lipid droplets in situ and isolated from oviductal epithelial cells and oviductal fluid. Oviductal epithelial cells were harvested enzymatically from oviducts of cows in either the luteal or the follicular stages of the ovarian cycle. Lipid droplets were isolated from cellular homogenates and characterized biochemically using thin layer chromatography. The morphology of lipid droplets in oviductal epithelial cells and in fractions isolated by ultracentrifugation from cellular homogenates was examined by electron microscopy. Lipid droplets isolated from oviduct epithelial cells varied in composition with the ovarian cycle and the oviductal region. There was more total lipid in droplets isolated from cows in the luteal than follicular phase of the ovarian cycle. Most of this difference was due to large amounts of esterified cholesterol present in the samples from luteal-stage animals. The most esterified cholesterol was found in droplets isolated from the oviductal isthmus of luteal cows. Droplets similar in lipid composition to those isolated from epithelial cells were found in oviductal fluid. Four distinct types of lipid inclusions were evident in electron micrographs of oviductal epithelia and characterized as osmiophilic droplets, lipofuscin-like clusters, lamellar structures, and composite bodies. All of the lipid inclusions were found in droplet isolates except for the extracted lipid portion of the composite body. The presence and diversity of oviduct epithelial lipid inclusions suggest that the oviductal epithelium may be very active in lipid metabolism, particularly cholesterol dynamics.

Neutral lipid droplets in bovine oviductal epithelium and lipid composition of epithelial cell homogenates

This study was undertaken to locate and to quantify lipids within the oviduct epithelial cells that might serve as a source of luminal lipids. Oviduct epithelial cells were analyzed from 12 cows in either the follicular or luteal stage of the ovarian cycle. Histochemical differences in neutral lipid droplets and phospholipids containing choline were detected among the oviductal regions. Neutral lipid staining was greatest in the preampulla and the ampulla and least in the isthmus. Staining of phospholipids containing choline was similar in preampullary and ampullary epithelia but was concentrated in isthmic crypts. Neutral lipid droplets, detected fluorescently with Nile Red, were present in a greater percentage of preampullary epithelial cells (76.8 +/- 1.8) than ampullary (42.1 +/- 2.1) or isthmic cells (12.2 +/- 1.3). Neither ovarian cycle stage nor side of ovulation affected the concentration of epithelial lipid droplets. Colorimetric lipid assays determined that concentrations of free cholesterol and glycerides in the preampulla were higher than in other regions. Most esterified cholesterol was detected in the isthmus. These findings indicate that the bovine oviduct epithelium exhibits regional differences in phospholipid and neutral lipid distribution. Because many of the lipids studied affect gamete and early embryo membranes, localization differences may affect the functional variability of the oviductal regions.

The composition of milk fat

Bovine milk contains about 3.5 to 5% total lipid, existing as emulsified globules 2 to 4 microns in diameter and coated with a membrane derived from the secreting cell. In homogenized milk, the coat is mostly casein. About 98% or more of the lipid is triacylglycerol, which is found in the globule. Phospholipids are about .5 to 1% of total lipids, and sterols are .2 to .5%. These are mostly located in the globule membrane. Cholesterol is the major sterol at 10 to 20 mg/dl. Data are presented on the membrane and the emulsion. Bovine milk contains substantial quantities of C4:0 to C10:0, about 2% each of C18:2 and trans-C18:1, and almost no other long-chain polyunsaturated fatty acids. The fatty acid composition is not altered by ordinary changes in diet. The triacylglycerol structure is unique, with much of the C4:0 to C10:0 at sn-3. The effects of milk cholesterol and fatty acids on human blood cholesterol levels and nonnutritive roles of some microlipids are discussed.