Transport of casein submicelles and formation of secretion granules in the Golgi apparatus of epithelial cells of the lactating mammary gland of the rat

Regulatory roles of dopamine D2 receptor in milk protein production and apoptosis in mammary epithelial cells

Dopamine D2 receptors (D2Rs) play crucial roles in regulating diverse physiological functions of the central nervous system and peripheral organs. D2Rs are also expressed in mammary glands. However, which cell types express D2Rs and whether they are involved in milk production remains unclear. The present findings revealed that D2Rs are expressed in the apical regions of the lateral membranes of mammary epithelial cells (MECs) in lactating mice. We also investigated the effects of the D2R agonist bromocriptine and/or antagonist domperidone on intracellular cAMP levels, milk protein production, and apoptosis in a lactation culture model of MECs that produce major milk components like lactating MECs in vivo. We found that bromocriptine decreased intracellular cAMP levels, whereas domperidone dose-dependently neutralized this effect. Bromocriptine also inhibited casein and lactoferrin production and suppressed activities of STAT5 and glucocorticoid receptors (GRs). Domperidone neutralized the inhibition of casein production as well as STAT5 and GR inactivation induced by bromocriptine. Furthermore, D2R activation by bromocriptine induced apoptosis and inactivated ERK, a signaling molecule responsible for promoting cell proliferation and survival. Domperidone attenuated ERK inactivation and apoptosis induced by bromocriptine. These findings suggest that D2Rs play regulatory roles in milk protein production and apoptosis in MECs.

Culture Models to Investigate Mechanisms of Milk Production and Blood-Milk Barrier in Mammary Epithelial Cells: a Review and a Protocol

Mammary epithelial cells (MECs) are the only cell type that produces milk during lactation. MECs also form less-permeable tight junctions (TJs) to prevent the leakage of milk and blood components through the paracellular pathway (blood-milk barrier). Multiple factors that include hormones, cytokines, nutrition, and temperature regulate milk production and TJ formation in MECs. Multiple intracellular signaling pathways that positively and negatively regulate milk production and TJ formation have been reported. However, their regulatory mechanisms have not been fully elucidated. In addition, unidentified components that regulate milk production in MECs likely exist in foods, for example plants. Culture models of functional MECs that recapitulate milk production and TJs are useful tools for their study. Such models enable the elimination of indirect effects via cells other than MECs and allows for more detailed experimental conditions. However, culture models of MECs with inappropriate functionality may result in unphysiological reactions that never occur in lactating mammary glands in vivo. Here, I briefly review the physiological functions of alveolar MECs during lactation in vivo and culture models of MECs that feature milk production and less-permeable TJs, together with a protocol for establishment of MEC culture with functional TJ barrier and milk production capability using cell culture inserts.

α-casein micelles-membranes interaction: Flower-like lipid protein coaggregates formation

BACKGROUND

Environmental conditions regulate the association/aggregation states of proteins and their action in cellular compartments. Analysing protein behaviour in presence of lipid membranes is fundamental for the comprehension of many functional and dysfunctional processes. Here, we present an experimental study on the interaction between model membranes and α-casein. α-casein is the major component of milk proteins and it is recognised to play a key role in performing biological functions. The conformational properties of this protein and its capability to form supramolecular structures, like micelles or irreversible aggregates, are key effectors in functional and pathological effects.

METHODS

By means of quantitative fluorescence imaging and complementary spectroscopic methods, we were able to characterise α-casein association state and the course of events induced by pH changes, which regulate the interaction of this molecule with membranes.

RESULTS

The study of these complex dynamic events revealed that the initial conformation of the protein critically regulates the fate of α-casein, size and structure of the newly formed aggregates and their effect on membrane structures. Disassembly of micelles due to modification in electrostatic interactions results in increased membrane structure rigidity which accompanies the formation of protein lipid flower-like co-aggregates with protein molecules localised in the external part.

GENERAL SIGNIFICANCE

These results may contribute to the comprehension of how the initial state of a protein establishes the course of events that occur upon changes in the molecular environment. These events which may occur in cells may be essential to functional, pathological or therapeutical properties specifically associated to casein proteins.

Comparison of the Cisterna Maturation-Progression Model with the Kiss-and-Run Model of Intra-Golgi Transport: Role of Cisternal Pores and Cargo Domains

The Golgi complex is the central station of the secretory pathway. Knowledge about the mechanisms of intra-Golgi transport is inconsistent. Here, we compared the explanatory power of the cisterna maturation-progression model and the kiss-and-run model. During intra-Golgi transport, conventional cargoes undergo concentration and form cisternal distensions or distinct membrane domains that contain only one membrane cargo. These domains and distension are separated from the rest of the Golgi cisternae by rows of pores. After the arrival of any membrane cargo or a large cargo aggregate at the Golgi complex, the cis-Golgi SNAREs become enriched within the membrane of cargo-containing domains and then replaced by the trans-Golgi SNAREs. During the passage of these domains, the number of cisternal pores decreases. Restoration of the cisternal pores is COPI-dependent. Our observations are more in line with the kiss-and-run model.

Electron Tomography Revels that Milk Lipids Originate from Endoplasmic Reticulum Domains with Novel Structural Features

Lipid droplets (LD) are dynamically-regulated organelles that originate from the endoplasmic reticulum (ER), and function in the storage, trafficking and metabolism of neutral lipids. In mammary epithelial cells (MEC) of lactating animals, intact LD are secreted intact into milk to form milk lipids by a novel apocrine mechanism. The secretion of intact LD and the relatively large amounts of lipid secreted by lactating MEC increase demands on the cellular processes responsible for lipid synthesis and LD formation. As yet these processes are poorly defined due to limited understanding of LD-ER interactions. To overcome these limitations, we used rapid-freezing and freeze-substitution methods in conjunction with 3D electron tomography and high resolution immunolocalization to define interactions between LD with ER in MEC of pregnant and lactating rats. Using these approaches, we identified distinct ER domains that contribute to lipid droplet formation and stabilization and which possess unique features previously unrecognized or not fully appreciated. Our results show nascent lipid droplets within the ER lumen and the association of both forming and mature droplets with structurally unique regions of ER cisternae, characterized by the presence of perilipin-2, a protein implicated in lipid droplet formation, and enzymes involved in lipid synthesis. These data demonstrate that milk lipids originate from LD-ER domains with novel structural features and suggest a mechanism for initial droplet formation in the ER lumen and subsequent maturation of the droplets in association with ER cisternae.

Models of Intracellular Transport: Pros and Cons

Intracellular transport is one of the most confusing issues in the field of cell biology. Many different models and their combinations have been proposed to explain the experimental data on intracellular transport. Here, we analyse the data related to the mechanisms of endoplasmic reticulum-to-Golgi and intra-Golgi transport from the point of view of the main models of intracellular transport; namely: the vesicular model, the diffusion model, the compartment maturation–progression model, and the kiss-and-run model. This review initially describes our current understanding of Golgi function, while highlighting the recent progress that has been made. It then continues to discuss the outstanding questions and potential avenues for future research with regard to the models of these transport steps. To compare the power of these models, we have applied the method proposed by K. Popper; namely, the formulation of prohibitive observations according to, and the consecutive evaluation of, previous data, on the basis on the new models. The levels to which the different models can explain the experimental observations are different, and to date, the most powerful has been the kiss-and-run model, whereas the least powerful has been the diffusion model.

Role of Intracellular Transport in the Centriole-Dependent Formation of Golgi Ribbon

The intracellular transport is the most confusing issue in the field of cell biology. The Golgi complex (GC) is the central station along the secretory pathway. It contains Golgi glycosylation enzymes, which are responsible for protein and lipid glycosylation, and in many cells, it is organized into a ribbon. Position and structure of the GC depend on the position and function of the centriole. Here, we analyze published data related to the role of centriole and intracellular transport (ICT) for the formation of Golgi ribbon and specifically stress the importance of the delivery of membranes containing cargo and membrane proteins to the cell centre where centriole/centrosome is localized. Additionally, we re-examined the formation of Golgi ribbon from the point of view of different models of ICT.

Distinct roles of prolactin, epidermal growth factor, and glucocorticoids in β-casein secretion pathway in lactating mammary epithelial cells

Beta-casein is a secretory protein contained in milk. Mammary epithelial cells (MECs) synthesize and secrete β-casein during lactation. However, it remains unclear how the β-casein secretion pathway is developed after parturition. In this study, we focused on prolactin (PRL), epidermal growth factor (EGF), and glucocorticoids, which increase in blood plasma and milk around parturition. MECs cultured with PRL, EGF and dexamethasone (DEX: glucocorticoid analog) developed the β-casein secretion pathway. In the absence of PRL, MECs hardly expressed β-casein. EGF enhanced the expression and secretion of β-casein in the presence of PRL and DEX. DEX treatment rapidly increased secreted β-casein concurrent with enhancing β-casein expression. DEX also up-regulated the expression of SNARE proteins, such as SNAP-23, VAMP-8 and Syntaxin-12. Furthermore, PRL and DEX regulated the expression ratio of α_s1-, β- and κ-casein. These results indicate that PRL, EGF and glucocorticoids have distinct roles in the establishment of β-casein secretion pathway.

The membrane-associated form of α(s1)-casein interacts with cholesterol-rich detergent-resistant microdomains

Caseins, the main milk proteins, interact with colloidal calcium phosphate to form the casein micelle. The mesostructure of this supramolecular assembly markedly influences its nutritional and technological functionalities. However, its detailed molecular organization and the cellular mechanisms involved in its biogenesis have been only partially established. There is a growing body of evidence to support the concept that α(s1)-casein takes center stage in casein micelle building and transport in the secretory pathway of mammary epithelial cells. Here we have investigated the membrane-associated form of α(s1)-casein in rat mammary epithelial cells. Using metabolic labelling we show that α(s1)-casein becomes associated with membranes at the level of the endoplasmic reticulum, with no subsequent increase at the level of the Golgi apparatus. From morphological and biochemical data, it appears that caseins are in a tight relationship with membranes throughout the secretory pathway. On the other hand, we have observed that the membrane-associated form of α(s1)-casein co-purified with detergent-resistant membranes. It was poorly solubilised by Tween 20, partially insoluble in Lubrol WX, and substantially insoluble in Triton X-100. Finally, we found that cholesterol depletion results in the release of the membrane-associated form of α(s1)-casein. These experiments reveal that the insolubility of α(s1)-casein reflects its partial association with a cholesterol-rich detergent-resistant microdomain. We propose that the membrane-associated form of α(s1)-casein interacts with the lipid microdomain, or lipid raft, that forms within the membranes of the endoplasmic reticulum, for efficient forward transport and sorting in the secretory pathway of mammary epithelial cells.

Transport of soluble proteins through the Golgi occurs by diffusion via continuities across cisternae

The mechanism of transport through the Golgi complex is not completely understood, insofar as no single transport mechanism appears to account for all of the observations. Here, we compare the transport of soluble secretory proteins (albumin and α1-antitrypsin) with that of supramolecular cargoes (e.g., procollagen) that are proposed to traverse the Golgi by compartment progression-maturation. We show that these soluble proteins traverse the Golgi much faster than procollagen while moving through the same stack. Moreover, we present kinetic and morphological observations that indicate that albumin transport occurs by diffusion via intercisternal continuities. These data provide evidence for a transport mechanism that applies to a major class of secretory proteins and indicate the co-existence of multiple intra-Golgi trafficking modes.

The biology of zinc transport in mammary epithelial cells: implications for mammary gland development, lactation, and involution

Zinc plays a critical role in a vast array of cellular functions including gene transcription, protein translation, cell proliferation, differentiation, bioenergetics, and programmed cell death. The mammary gland depends upon tight coordination of these processes during development and reproduction for optimal expansion, differentiation, and involution. For example, zinc is required for activation of matrix metalloproteinases, intracellular signaling cascades such as MAPK and PKC, and the activation of both mitochondrial-mediated apoptosis and lysosomal-mediated cell death. In addition to functional needs, during lactation the mammary gland must balance providing optimal zinc for cellular requirements with the need to secrete a substantial amount of zinc into milk to meet the requirements of the developing neonate. Finally, the mammary gland exhibits the most profound example of programmed cell death, which is driven by both apoptotic and lysosomal-mediated cell death. Two families of zinc-specific transporters regulate zinc delivery for these diverse functions. Members of the ZIP family of zinc transporters (ZIP1-14) import zinc into the cytoplasm from outside the cell or from subcellular organelles, while members of the ZnT family (ZnT1-10) export zinc from the cytoplasm. Recently, the ion channel transient receptor potential mucolipin 1 (TRPML1) has also been implicated in zinc transport. Herein, we review our current understanding of the molecular mechanisms through which mammary epithelial cells utilize zinc with a focus on the transport of zinc into discrete subcellular organelles for specific cellular functions during mammary gland development, lactation, and involution.

ZnT4 provides zinc to zinc-dependent proteins in the trans-Golgi network critical for cell function and Zn export in mammary epithelial cells

Zinc (Zn) transporter 4 (ZnT4) plays a key role in mammary gland Zn metabolism. A mutation in ZnT4 (SLC30A4) that targets the protein for degradation is responsible for the "lethal milk" (lm/lm) mouse phenotype. ZnT4 protein is only detected in the secreting mammary gland, and lm/lm mice have ∼35% less Zn in milk, decreased mammary gland size, and decreased milk secretion. However, the precise contribution of ZnT4 is unknown. We used cultured mouse mammary epithelial cells (HC11) and determined that ZnT4 was localized to the trans-Golgi network (TGN) and cell membrane and transported Zn from the cytoplasm. ZnT4-mediated Zn import into the TGN directly contributed to labile Zn accumulation as ZnT4 overexpression increased FluoZin3 fluorescence. Moreover, ZnT4 provided Zn for metallation of galactosyltransferase, a Zn-dependent protein localized within the TGN that is critical for milk secretion, and carbonic anhydrase VI, a Zn-dependent protein secreted from the TGN into milk. We further noted that ZnT4 relocalized to the cell membrane in response to Zn. Together these studies demonstrated that ZnT4 transports Zn into the TGN, which is critical for key secretory functions of the mammary cell.

How to squeeze a sponge: casein micelles under osmotic stress, a SAXS study

By combining the osmotic stress technique with small-angle x-ray scattering measurements, we followed the structural response of the casein micelle to an overall increase in concentration. When the aqueous phase that separates the micelles is extracted, they behave as polydisperse repelling spheres and their internal structure is not affected. When they are compressed, the micelles lose water and shrink to a smaller volume. Our results indicate that this compression is nonaffine, i.e., some parts of the micelle collapse, whereas other parts resist deformation. We suggest that this behavior is consistent with a spongelike casein micelle having a triple hierarchical structure. The lowest level of the structure consists of the CaP nanoclusters that serve as anchors for the casein molecules. The intermediate level consists of 10- to 40-nm hard regions that resist compression and contain the nanoclusters. Those regions are connected and/or partially merged with each other, thus forming a continuous and porous material. The third level of structure is the casein micelle itself, with an average size of 100 nm. In our view, such a structure is consistent with the observation of 10- to 20-nm casein particles in the Golgi vesicles of lactating cells: upon aggregation, those particles would rearrange, fuse, and/or swell to form the spongelike micelle.

AlphaS1-casein, which is essential for efficient ER-to-Golgi casein transport, is also present in a tightly membrane-associated form

BACKGROUND

Caseins, the main milk proteins, aggregate in the secretory pathway of mammary epithelial cells into large supramolecular structures, casein micelles. The role of individual caseins in this process and the mesostructure of the casein micelle are poorly known.

RESULTS

In this study, we investigate primary steps of casein micelle formation in rough endoplasmic reticulum-derived vesicles prepared from rat or goat mammary tissues. The majority of both alphaS1- and beta-casein which are cysteine-containing casein was dimeric in the endoplasmic reticulum. Saponin permeabilisation of microsomal membranes in physico-chemical conditions believed to conserve casein interactions demonstrated that rat immature beta-casein is weakly aggregated in the endoplasmic reticulum. In striking contrast, a large proportion of immature alphaS1-casein was recovered in permeabilised microsomes when incubated in conservative conditions. Furthermore, a substantial amount of alphaS1-casein remained associated with microsomal or post-ER membranes after saponin permeabilisation in non-conservative conditions or carbonate extraction at pH11, all in the presence of DTT. Finally, we show that protein dimerisation via disulfide bond is involved in the interaction of alphaS1-casein with membranes.

CONCLUSIONS

These experiments reveal for the first time the existence of a membrane-associated form of alphaS1-casein in the endoplasmic reticulum and in more distal compartments of the secretory pathway of mammary epithelial cells. Our data suggest that alphaS1-casein, which is required for efficient export of the other caseins from the endoplasmic reticulum, plays a key role in early steps of casein micelle biogenesis and casein transport in the secretory pathway.

Regulation of secretory granule size by the precise generation and fusion of unit granules

Morphometric evidence derived from studies of mast cells, pancreatic acinar cells and other cell types supports a model in which the post-Golgi processes that generate mature secretory granules can be resolved into three steps: (1) fusion of small, Golgi-derived progranules to produce immature secretory granules which have a highly constrained volume; (2) transformation of such immature granules into mature secretory granules, a process often associated with a reduction in the maturing granule's volume, as well as changes in the appearance of its content and (3) fusion of secretory granules of the smallest size, termed 'unit granules', forming granules whose volumes are multiples of the unit granule's volume. Mutations which perturb this process can cause significant pathology. For example, Chediak-Higashi syndrome / lysosomal trafficking regulator (CHS)/(Lyst) mutations result in giant secretory granules in a number of cell types in human beings with the Chediak-Higashi syndrome and in 'beige' (Lyst(bg)/Lyst(bg)) mice. Analysis of the secretory granules of mast cells and pancreatic acinar cells in Lyst-deficient beige mice suggests that beige mouse secretory granules retain the ability to fuse randomly with other secretory granules no matter what the size of the fusion partners. By contrast, in normal mice, the pattern of granule-granule fusion occurs exclusively by the addition of unit granules, either to each other or to larger granules. The normal pattern of fusion is termed unit addition and the fusion evident in cells with CHS/Lyst mutations is called random addition. The proposed model of secretory granule formation has several implications. For example, in neurosecretory cells, the secretion of small amounts of cargo in granules constrained to a very narrow size increases the precision of the information conveyed by secretion. By contrast, in pancreatic acinar cells and mast cells, large granules composed of multiple unit granules permit the cells to store large amounts of material without requiring the amount of membrane necessary to package the same amount of cargo into small granules. In addition, the formation of mature secretory granules that are multimers of unit granules provides a mechanism for mixing in large granules the contents of unit granules which differ in their content of cargo.

Mechanisms of transport through the Golgi complex

The Golgi complex is the central sorting and processing station of the secretory pathway, ensuring that cargo proteins, which are synthesized in the endoplasmic reticulum, are properly glycosylated and packaged into carriers for transport to their final destinations. Two recent studies highlight the fact that properties of membrane lipids play key roles in Golgi structural organization and trafficking. The Antonny laboratory has demonstrated the mechanism by which a Golgi tether containing a membrane-curvature-sensing domain at one end can link highly curved and flat membranes together in a reversible manner. In this way, a strong interaction that binds membranes together in an oriented fashion can easily be disrupted as the properties of the membranes change. The Lippincott-Schwartz laboratory has developed a new model for intra-Golgi trafficking, called the rapid-partitioning model, which incorporates lipid trafficking as an integral part. Simulations reveal that the sorting of lipids into processing and export domains that are connected to each Golgi cisterna, and bidirectional trafficking throughout the Golgi to allow proteins to associate with their preferred lipid environment, is sufficient to drive protein transport through the secretory pathway. Although only a proof in principle, this model for the first time invokes lipid sorting as the driving force in intra-Golgi trafficking, and provides a framework for future experimental work.

Biogenesis of Dense-Core Secretory Granules

Dense core granules (DCGs) are vesicular organelles derived from outbound traffic through the eukaryotic secretory pathway. As DCGs are formed, the secretory pathway can also give rise to other types of vesicles, such as those bound for endosomes, lysosomes, and the cell surface. DCGs differ from these other vesicular carriers in both content and function, storing highly concentrated cores’ of condensed cargo in vesicles that are stably maintained within the cell until a specific extracellular stimulus causes their fusion with the plasma membrane. These unique features are imparted by the activities of membrane and lumenal proteins that are specifically delivered to the vesicles during synthesis. This chapter will describe the DCG biogenesis pathway, beginning with the sorting of DCG proteins from proteins that are destined for other types of vesicle carriers. In the trans-Golgi network (TGN), sorting occurs as DCG proteins aggregate, causing physical separation from non-DCG proteins. Recent work addresses the nature of interactions that produce these aggregates, as well as potentially important interactions with membranes and membrane proteins. DCG proteins are released from the TGN in vesicles called immature secretory granules (ISGs). The mechanism of ISG formation is largely unclear but is not believed to rely on the assembly of vesicle coats like those observed in other secretory pathways. The required cytosolic factors are now beginning to be identified using in vitro systems with purified cellular components. ISG transformation into a mature fusion-competent, stimulus-dependent DCG occurs as endoproteolytic processing of many DCG proteins causes continued condensation of the lumenal contents. At the same time, proteins that fail to be incorporated into the condensing core are removed by a coat-mediated budding mechanism, which also serves to remove excess membrane and membrane proteins from the maturing vesicle. This chapter will summarize the work leading to our current view of granule synthesis, and will discuss questions that need to be addressed in order to gain a more complete understanding of the pathway.

Origins of the regulated secretory pathway

Modes of transport of soluble (or luminal) secretory proteins synthesized in the endoplasmic reticulum (ER) could be divided into two groups. The socalled constitutive secretory pathway (CSP) is common to all eukaryotic cells, constantly delivering constitutive soluble secretory proteins (CSSPs) linked to the rate of protein synthesis but largely independent of external stimuli. In regulated secretion, protein is sorted from the Golgi into storage/secretory granules (SGs) whose contents are released when stimuli trigger their final fusion with the plasma membrane (Hannah et al. 1999).

Secretion and fluid transport mechanisms in the mammary gland: comparisons with the exocrine pancreas and the salivary gland

Milk is a complex fluid composed of proteins, sugars, lipids and minerals, in addition to a wide variety of bioactive molecules including vitamins, trace elements and growth factors. The composition of these components reflects the integrated activities of distinct synthetic, secretion and transport processes found in mammary epithelial cells, and mirrors the differing nutritional and developmental requirements of mammalian neonates. Five general pathways have been described for secretion of milk components. With the exception of lipids, which are secreted a unique pathway, milk components are thought to be secreted by adaptations of pathways found in other secretory organs. However little is known about the molecular and cellular mechanisms that constitute these pathways or the physiological mechanisms by which they are regulated. Comparisons of current secretion and transport models in the mammary gland, exocrine pancreas and salivary gland indicate that significant differences exist between the mammary gland and other exocrine organs in how proteins and lipids are packaged and secreted, and how fluid is transported.

Disulphide bonds in casein micelle from milk

Mammary epithelial cells synthesised and secreted caseins, the major milk proteins in most mammals, as large aggregates called micelles into the alveolar lumen they surround. We investigated the implication of the highly conserved cysteine(s) of kappa-casein in disulphide bond formation in casein micelles from several species. Dimers were found in all milks studied, confirming previous observation in ruminants. More importantly, the study of interchain disulphide bridges in mouse and rat casein micelles revealed that any casein possessing a cysteine is engaged in disulphide bond interchange; these species express four or five cysteine-containing caseins, respectively. We found that the main rodent caseins form both homo- and heterodimers. Additionally, disulphide bond formation among milk proteins was specific since the interaction of the caseins with cysteine-containing whey proteins was not observed in native casein micelles.

Intra-Golgi transport: A way to a new paradigm?

The morpho-functional principles of intra-Golgi transport are, surprisingly, still not clear, which is in marked contrast to our advanced knowledge of the underlying molecular machineries. Recently, the conceptual and technological hindrances that had delayed progress in this area have been disappearing, and a cluster of powerful morphological techniques has been revealing new glimpses of the organization of traffic in intact cells. Here, we discuss the new concepts around the present models of intra-Golgi transport.

Ca(2+)-independent phospholipase A2 participates in the vesicular transport of milk proteins

Changes in the lipid composition of intracellular membranes are believed to take part in the molecular processes that sustain traffic between organelles of the endocytic and exocytic transport pathways. Here, we investigated the participation of the calcium-independent phospholipase A2 in the secretory pathway of mammary epithelial cells. Treatment with bromoenol lactone, a suicide substrate which interferes with the production of lysophospholipids by the calcium-independent phospholipase A2, resulted in the reduction of milk proteins secretion. The inhibitor slowed down transport of the caseins from the endoplasmic reticulum to the Golgi apparatus and affected the distribution of p58 and p23, indicating that the optimal process of transport of these proteins between the endoplasmic reticulum, the endoplasmic reticulum/Golgi intermediate compartment and/or the cis-side of the Golgi was dependent upon the production of lysolipids. Moreover, bromoenol lactone was found to delay the rate of protein transport from the trans-Golgi network to the plasma membrane. Concomitantly, membrane-bound structures containing casein accumulated in the juxtanuclear Golgi region. We concluded from these results that efficient formation of post-Golgi carriers also requires the phospholipase activity. These data further support the participation of calcium-independent phospholipase A2 in membrane trafficking and shed a new light on the tubulo/vesicular transport of milk protein through the secretory pathway.

Calcium secretion into milk

Ionized calcium ([Ca(2+)]) is present in milk at concentrations around 3 mM, a concentration that drives the formation of complexes with citrate, phosphate, and casein, thereby generating compounds that carry the major portion of calcium in milk. In humans and cows, where it has been studied, changes in milk calcium appear to be regulated by the amount of citrate and casein in milk rather than changes in [Ca(2+)]. Most or all of the calcium in milk is likely derived through exocytosis of secretory vesicles derived from the Golgi compartment where a calcium ATPase mediates transport from the cytoplasm. The identity of the transporters is not yet certain but gene expression for the plasma membrane calcium ATPase, PMCA2bw, and the secretory pathway calcium ATPase, SPCA, is highly upregulated during lactation. Currently nothing appears to be known about the mechanisms that mediate transport of calcium across the basolateral membrane of the alveolar cell.

Morphodynamics of the secretory pathway

A careful scrutiny of the dynamics of secretory compartments in the entire eukaryotic world reveals many common themes. The most fundamental theme is that the Golgi apparatus and related structures appear as compartments formed by the act of transporting cargo. The second common theme is the pivotal importance for endomembrane dynamics of shifting back and forth the equilibrium between full and perforated cisternae along the pathway. The third theme is the role of a continuous membrane flow in anterograde transfer of molecules from the endoplasmic reticulum through the Golgi apparatus. The last common theme is the self-regulatory balance between anatomical continuities and discontinuities of the endomembrane system. As this balance depends on secretory activity, it provides a source of morphological variability among cell types or, for a given cell type, according to environmental conditions. Beyond this first source of variability, it appears that divergent strategies pave the evolutionary routes in different eukaryotic kingdoms. These divergent strategies primarily affect the levels of stacking, of stabilization, and of clustering of the Golgi apparatus. They presumably underscore a trade-off between versatility and stability to adapt the secretory function to the degree of environmental variability. Nonequilibrium secretory structures would provide yeasts, and plants to a lesser extent, with the required versatility to cope with ever changing environments, by contrast to the stabler milieu intérieur of homeothermic animals.

Mammary physiology and milk secretion

The presence of drugs or other potentially toxic materials in milk is an obvious public health risk, especially to infants and neonates. There is also increasing concern that human breast cancer is principally epigenetic in origin and results from environmentally produced lesions. Little is known about the mechanisms by which toxic substances enter milk or mammary tissue but knowledge of these processes is important to toxicologists and researchers involved in drug design and metabolism. Five general pathways have been described for transport of proteins, lipids, ions, nutrients and water into milk. Four of these pathways are transcellular, involving transport across at least two membrane barriers; the fifth is paracellular and allows direct exchange of interstitial and milk components. Solute transport by these pathways is mediated by a diverse, and complex array of transport and secretory processes that are regulated by hormonal, developmental, and physiological factors. Current research is beginning to define the mechanisms underlying some of these processes, however the regulation and coordination of solute transport mechanisms remains poorly understood. In this article we review our current understanding of the normal solute transport and secretory processes involved in milk production, and discuss potential regulatory mechanisms.

Proteomic analysis of two functional states of the Golgi complex in mammary epithelial cells

Organellar compartments involved in secretion are expanded during the transition from late pregnancy (basal secretory state) to lactation (maximal secretory state) to accommodate for the increased secretory function required for copious milk production in mammary epithelial cells. The Golgi complex is a major organelle of the secretory pathway and functions to sort, package, distribute, and post-translationally modify newly synthesized proteins and membrane lipids. These complex functions of the Golgi are reflected in the protein complement of the organelle. Therefore, using proteomics, the protein complements of Golgi fractions isolated at two functional states (basal and maximal) were compared to identify some of the molecular changes that occur during this transition. This global analysis has revealed that only a subset of the total proteins is upregulated from steady state during the transition. Identification of these proteins by tandem mass spectrometry has revealed several classes of proteins involved in the regulation of membrane fusion and secretion. This first installment of the functional proteomic analysis of the Golgi complex begins to define the molecular basis for the transition from basal to maximal secretion.

Megavesicles implicated in the rapid transport of intracisternal aggregates across the Golgi stack

Engineered protein aggregates ranging up to 400 nm in diameter were selectively deposited within the cis-most cisternae of the Golgi stack following a 15 degrees C block. These aggregates are much larger than the standard volume of Golgi vesicles, yet they are transported across the stack within 10 min after warming the cells to 20 degrees C. Serial sectioning reveals that during the peak of anterograde transport, about 20% of the aggregates were enclosed in topologically free "megavesicles" which appear to pinch off from the rims of the cisternae. These megavesicles can explain the rapid transport of aggregates without cisternal progression on this time scale.

Phospholipase D-dependent and -independent mechanisms are involved in milk protein secretion in rabbit mammary epithelial cells

Phospholipase D has been implicated in membrane traffic in the secretory pathway of yeast and of some mammalian cell lines. Here we investigated the involvement of phospholipase D in protein transport at various steps of the secretory pathway of mammary epithelial cells. Treatment of rabbit mammary explants with butanol, which blocks the formation of phosphatidic acid, decreased the secretion of caseins and to a lesser extent that of whey acidic protein. Butanol interfered with both the endoplasmic reticulum to Golgi complex transport of the caseins and secretory vesicle formation from the trans-Golgi network. In contrast, the transport of whey acidic protein to the Golgi was less affected. Activation of protein kinase C enhanced the overall secretion of both markers and interestingly, this stimulation of secretion was maintained for whey acidic protein in the presence of butanol. Transphosphatidylation assays demonstrated the existence of a constitutive phospholipase D activity which was stimulated by the activation of protein kinase C. We conclude that phospholipase D plays a role in casein transport from the endoplasmic reticulum to the Golgi and in the secretory vesicle formation from the trans-Golgi network. Moreover, our results suggest a differential requirement for phospholipase D in the secretion of caseins and that of whey acidic protein.

Secretory protein trafficking and organelle dynamics in living cells

Green fluorescent protein chimerae acting as reporters for protein localization and trafficking within the secretory membrane system of living cells have been used in a wide variety of applications, including time-lapse imaging, double-labeling, energy transfer, quantitation, and photobleaching experiments. Results from this work are clarifying the steps involved in the formation, translocation, and fusion of transport intermediates; the organization and biogenesis of organelles; and the mechanisms of protein retention, sorting, and recycling in the secretory pathway. In so doing, they are broadening our thinking about the temporal and spatial relationships among secretory organelles and the membrane trafficking pathways that operate between them.

The majority of clathrin coated vesicles from lactating rabbit mammary gland arises from the secretory pathway

Clathrin coated vesicles were isolated from lactating rabbit mammary gland by differential centrifugation, centrifugation on (2)H2O-sucrose cushions and Sephacryl S-1000 chromatography. Mammary epithelial cells contain an unexpectedly high quantity of clathrin coated vesicles which appear heterogeneous in size, with a mean diameter of 95.9+/-10.5 nm and a density of 1.23 g × ml(−1). Analysis of clathrin coated vesicle adaptor composition by SDS-PAGE and western blot showed that only approximately 5–10% of total APs consist of AP-2 in isolated mammary gland clathrin coated vesicles whereas it represents approximately 70% of the total APs from bovine brain clathrin coated vesicles. Cargo molecules known to be transcytosed such as IgG, IgA, and the pIgR were detected in the clathrin coated vesicles, indicating that part of this vesicle population is involved in transcytotic pathways. However, as the vast majority of the clathrin coated vesicles contained AP-1, it was likely that these clathrin coated vesicles were involved in the secretory pathway. Relatively high quantities of furin and cation-independent mannose 6-phosphate receptor were detected in mammary clathrin coated vesicles. By immuno electron microscopy, AP-1 and the cation-independent mannose 6-phosphate receptor were localized in Golgi-associated vesicles and on the membrane of secretory vesicles. The presence of AP-1 in the coat patches on the membrane of secretory vesicles containing casein micelles, and the presence of alpha(s1)-casein in mammary gland clathrin coated vesicles, support a role for AP-1 in the maturation of secretory vesicles. Our data pinpoint the importance of clathrin coated vesicles in lactating mammary epithelial cells, and suggest these vesicles are involved in the transcytotic pathway, in sorting at the trans-Golgi network and in the biogenesis of casein-containing secretory vesicles.

Alpha(S1)-casein is required for the efficient transport of beta- and kappa-casein from the endoplasmic reticulum to the Golgi apparatus of mammary epithelial cells

In lactating mammary epithelial cells, interaction between caseins is believed to occur after their transport out of the endoplasmic reticulum. We show here that, in alpha(S1)-casein-deficient goats, the rate of transport of the other caseins to the Golgi apparatus is highly reduced whereas secretion of whey proteins is not significantly affected. This leads to accumulation of immature caseins in distended rough endoplasmic reticulum cisternae. Casein micelles, nevertheless, were still observed in secretory vesicles. In contrast, no accumulation was found in mammary epithelial cells which lack beta-casein. In mammary epithelial cells secreting an intermediate amount of alpha(S1)-casein, less casein accumulated in the rough endoplasmic reticulum, and the transport of alpha(S1)-casein to the Golgi occurred with kinetics similar to that of control cells. In prolactin-treated mouse mammary epithelial HC11 cells, which do not express alpha(S)-caseins, endoplasmic reticulum accumulation of beta-casein was also observed. The amount of several endoplasmic reticulum-resident proteins increased in conjunction with casein accumulation. Finally, the permeabilization of rough endoplasmic reticulum vesicles allowed the recovery of the accumulated caseins in soluble form. We conclude that optimal export of the caseins out of the endoplasmic reticulum is dependent upon alpha(S1)-casein. Our data suggest that alpha(S1)-casein interacts with the other caseins in the rough endoplasmic reticulum and that the formation of this complex is required for their efficient export to the Golgi.

Golgi structure in three dimensions: functional insights from the normal rat kidney cell

Three-dimensional reconstructions of portions of the Golgi complex from cryofixed, freeze-substituted normal rat kidney cells have been made by dual-axis, high-voltage EM tomography at approximately 7-nm resolution. The reconstruction shown here ( approximately 1 x 1 x 4 microm3) contains two stacks of seven cisternae separated by a noncompact region across which bridges connect some cisternae at equivalent levels, but none at nonequivalent levels. The rest of the noncompact region is filled with both vesicles and polymorphic membranous elements. All cisternae are fenestrated and display coated buds. They all have about the same surface area, but they differ in volume by as much as 50%. The trans-most cisterna produces exclusively clathrin-coated buds, whereas the others display only nonclathrin coated buds. This finding challenges traditional views of where sorting occurs within the Golgi complex. Tubules with budding profiles extend from the margins of both cis and trans cisternae. They pass beyond neighboring cisternae, suggesting that these tubules contribute to traffic to and/or from the Golgi. Vesicle-filled "wells" open to both the cis and lateral sides of the stacks. The stacks of cisternae are positioned between two types of ER, cis and trans. The cis ER lies adjacent to the ER-Golgi intermediate compartment, which consists of discrete polymorphic membranous elements layered in front of the cis-most Golgi cisterna. The extensive trans ER forms close contacts with the two trans-most cisternae; this apposition may permit direct transfer of lipids between ER and Golgi membranes. Within 0.2 microm of the cisternae studied, there are 394 vesicles (8 clathrin coated, 190 nonclathrin coated, and 196 noncoated), indicating considerable vesicular traffic in this Golgi region. Our data place structural constraints on models of trafficking to, through, and from the Golgi complex.

Constitutive transport between the trans-Golgi network and the plasma membrane according to the maturation model. A hypothesis

Here we examine the application of the cisternal/carrier maturation model to describe transport of cargo proteins from the Golgi apparatus to the plasma membrane. Interpretation of the available evidence in the light of carrier maturation suggests that the transport intermediates between these stations are large pleiomorphic carriers formed by maturation of the trans-Golgi compartment, rather than vesicles, as would be postulated by the vesicular shuttle model. Mature carriers move along microtubules towards the plasma membrane via a microtubule/(kinesin)-based motor system. The maturation and vesicular transport models are compared in terms of consistency with the available literature.

Secretion of milk proteins

Mammary epithelial cells secrete milk proteins in a polarized manner from their apical surface during lactation. These secreted proteins are either synthesized by the mammary cells or are transported by transcytosis from blood plasma. The intracellular trafficking pathways by which milk proteins are secreted are known in general outline. In this review the basic cell biology of the mammary epithelial cell secretory pathway is considered in relation to what is known in more detail for other cell types. In addition, potential points of control of protein secretion are examined. The secretory biology of mammary epithelial cells has not been characterized extensively in recent years and, while some aspects are well understood, other key issues, which still remain to be resolved, have been highlighted.

Casein secretion in mammary tissue: tonic regulation of basal secretion by protein kinase A

Despite its quantitative importance in the secretion of lactoproteins, little is known about the triggering and control mechanisms that initiate, regulate and terminate the operation of the basal pathway of lactoprotein secretion throughout the lactation cycle. This study investigated the possible modulation by cAMP-mediated mechanisms, of cellular transit of newly-synthesised caseins and their basal secretion in explants of mammary tissue from lactating rats and rabbits. Enhancement of the rate of secretion of newly-synthesised caseins occurs when mammary explants are challenged in vitro with agents that activate protein kinase A (PKA). Inhibition of PKA slows casein secretion. The PKA-sensitive step(s) in casein secretion is early in the exocytosis pathway but inhibition of PKA does not impair casein maturation. Ultrastructural, immunochemical and biochemical methods locate PKA on membranes of vesicles situated in the Golgi region. Exposure of tissue to a cell-permeant PKA inhibitor results in morphological modification of these vesicular structures. We conclude that PKA mediates tonic positive regulation of the basal secretory pathway for lactoproteins in the mammary epithelial cell.

Getting through the Golgi complex

Electron micrographs of the Golgi apparatus typically show a series of flat cisternae stacked together, surrounded by numerous vesicles and tubules. Palade and colleagues established in the 1960s that secretory proteins pass through this morphologically complex organelle as they travel from the endoplasmic reticulum to the cell surface. However, the precise mechanism of transport through the cisternal stack has proven a controversial issue. Recent advances in identifying the molecules responsible for this traffic suggest that the solution may not be far away.

The Golgi apparatus: 100 years of progress and controversy

Research on the Golgi apparatus has resulted in major advances in understanding its structure and functions, but many important questions remain unanswered. The history of the Golgi apparatus has been marked by arguments and controversies, some of which have been resolved, whereas others are still ongoing. This article charts progress in understanding the role of the Golgi apparatus during the 100 years since it was discovered, highlighting major milestones and discoveries that have led to the concepts of the organization and functions of this organelle that we have today.

A cisternal maturation mechanism can explain the asymmetry of the Golgi stack

Morphological data suggest that Golgi cisternae form at the cis-face of the stack and then progressively mature into trans-cisternae. However, other studies indicate that COPI vesicles transport material between Golgi cisternae. These two observations can be reconciled by assuming that cisternae carry secretory cargo through the stack in the anterograde direction, while COPI vesicles transport Golgi enzymes in the retrograde direction. This model provides a mechanism for cisternal maturation. If Golgi enzymes compete with one another for packaging into COPI vesicles, we can account for the asymmetric distribution of enzymes across the stack.

Time-course study of the Golgi-lysosome region of rat hepatocytes during early acute phase response to inflammation

BACKGROUND

In a previous study of the hepatocyte Golgi apparatus 24 hr following the induction of the acute phase response to inflammation, it was found that the predominant secretory content was a granulofilamentous material (GFM), rather than lipoprotein particles (LP) as in controls (Thorne-Tjomsland and Jamieson, 1995, Anat. Rec., 241:439-450). The present study aimed to determine when and how this switch occurs and to monitor the Golgi-lysosome region of hepatocytes in general during early inflammation.

METHODS

Rats were injected with turpentine oil as an inflammatory agent. At 1, 2, 6, 12, and 16 hr after turpentine-injection (TI), the livers were prefixed by perfusion with cacodylate-buffered glutaraldehyde, postfixed with ferrocyanide reduced osmium, and processed for thin section transmission electron microscopy.

RESULTS

LP were processed at all time points between 1 and 16 hr post-TI, and at 1 and 2 hr post-TI, the Golgi stacks were engorged with LP and therefore apparently processed more of these particles than in controls. A GFM was processed at later time points, starting at 12 hr post-TI. At the two time points when LP and GFM were co-processed, these materials were frequently seen sorted from each other within the trans-saccule and/or the trans-Golgi network (TGN). At the level of the TGN, these materials were packaged into secretory vesicles which typically contained LP and/or GFM. Massive depletion of cytoplasmic glycogen was observed primarily at 1, 2, and 6 hr following TI. Concomitantly and temporarily, glycogen accumulated within Golgi-associated lysosome-like bodies with extensive appendages.

CONCLUSIONS

The switch in Golgi apparatus secretory content from LP to GFM occurred at around 12 hr post-TI, but was not discrete. The initial appearance of the GFM coincided with the initial increase in the serum of hepatocyte-derived acute phase reactants (see review Schreiber et al., 1989, Ann. NY Acad. Sci., 557: 61-85). An additional and surprising finding of the present study was that significant changes occurred in the Golgi-lysosome region much prior to the onset of acute phase reactant synthesis: 1 and 2 hr post-TI, the Golgi processed above-normal amounts of LP and Golgi-associated, lysosome-like bodies sequestered glycogen.

Trans-Golgi network (TGN) of different cell types: three-dimensional structural characteristics and variability

BACKGROUND

The trans-Golgi network (TGN) is generally considered as a distinct and permanent structural compartment of the Golgi apparatus of various cell types. To verify this postulate we examined and compared the three-dimensional characteristics of the TGNs of 14 different mammalian cell types as presented in our various publications since 1979 when we initially described the trans-tubular network of Sertoli cells.

METHODS

In all these studies we used low and high voltage electron microscopes on thin or thick sections of tissues fixed with glutaraldehyde and postfixed with reduced osmium. The sections were stained with uranyl acetate and lead citrate. Stereopairs, prepared from photographs of tilted specimens, permitted a direct observation of the three-dimensional structure of the various elements of the Golgi apparatus.

RESULTS

The TGNs are multilayered and extensive in cells which do not form large typical secretory granules (Sertoli cells, nonciliated cells of ductuli efferentes, spinal ganglion cells) but have an extensive lysosomal system. The TGN is absent in cells forming very large secretory granules (secretory cells of seminal vesicles and lactating mammary glands). The TGNs are small in cells producing small to medium-size secretory granules and/or appear as residual fragments on the trans aspect of the Golgi stacks (e.g., mucous cells of Brunner's gland, pancreatic acinar cells, etc.). In cells with multiple and extensive TGNs, a continuity of these tubular networks with the two or three transmost saccules of the stack is observed but there are seemingly no connections between the TGNs. Whenever the TGNs are present, they do not form a continuous structure along the Golgi ribbon. However, they do present, in all cases, configurations suggestive of desquamation and renewal.

CONCLUSIONS

The structure of the TGN varies considerably from one cell type to another, being extensive in cells not showing typical secretory granules but having an extensive lysosomal system, while in secretory cells showing small or large secretory granules the TGN is either small or even entirely absent.