Discovery of the Porosome: revealing the molecular mechanism of secretion and membrane fusion in cells

Porosomes in uterine epithelial cells: Ultrastructural identification and characterization during early pregnancy

Porosomes are plasma membrane structures in secretory cells that allow transient docking and/or partial fusion of vesicles during which they release their content then disengage. This is referred to as "kiss and run" exocytosis. During early pregnancy, at the time of receptivity, there is a high level of vesicle activity in uterine epithelial cells (UECs). One of the secretory pathways for these vesicles could be via porosomes, which have yet to be identified in UECs. This study identified porosomes in the apical plasma membrane of UECs for the first time. These structures were present on days 1, 5.5, and 6 of early pregnancy, where they likely facilitate partial secretion via "kiss and run" exocytosis. The porosomes were measured and quantified on days 1, 5.5, and 6, which showed there are significantly more porosomes on day 5.5 (receptive) compared to day 1 (nonreceptive) of pregnancy. This increase in porosome numbers may reflect major morphological and molecular changes in the apical plasma membrane at this time such as increased cholesterol and soluble NSF attachment protein receptor proteins, as these are structural and functional components of the porosome complex assembly. Porosomes were observed in both resting (inactive) and dilated (active) states on days 1, 5.5, and 6 of early pregnancy. Porosomes on day 5.5 are significantly more active than on day 1 as demonstrated by the dilation of their base diameter. Further two-way ANOVA analysis of base diameter in resting and dilated states found a significant increase in porosome activity in day 5.5 compared to day 1. This study therefore indicates an increase in the number and activity of porosomes at the time of uterine receptivity in the rat, revealing a mechanism by which the UECs modify the uterine luminal environment at this time.

Fifty Years of the Fluid–Mosaic Model of Biomembrane Structure and Organization and Its Importance in Biomedicine with Particular Emphasis on Membrane Lipid Replacement

The Fluid–Mosaic Model has been the accepted general or basic model for biomembrane structure and organization for the last 50 years. In order to establish a basic model for biomembranes, some general principles had to be established, such as thermodynamic assumptions, various molecular interactions, component dynamics, macromolecular organization and other features. Previous researchers placed most membrane proteins on the exterior and interior surfaces of lipid bilayers to form trimolecular structures or as lipoprotein units arranged as modular sheets. Such membrane models were structurally and thermodynamically unsound and did not allow independent lipid and protein lateral movements. The Fluid–Mosaic Membrane Model was the only model that accounted for these and other characteristics, such as membrane asymmetry, variable lateral movements of membrane components, cis- and transmembrane linkages and dynamic associations of membrane components into multimolecular complexes. The original version of the Fluid–Mosaic Membrane Model was never proposed as the ultimate molecular description of all biomembranes, but it did provide a basic framework for nanometer-scale biomembrane organization and dynamics. Because this model was based on available 1960s-era data, it could not explain all of the properties of various biomembranes discovered in subsequent years. However, the fundamental organizational and dynamic aspects of this model remain relevant to this day. After the first generation of this model was published, additional data on various structures associated with membranes were included, resulting in the addition of membrane-associated cytoskeletal, extracellular matrix and other structures, specialized lipid–lipid and lipid–protein domains, and other configurations that can affect membrane dynamics. The presence of such specialized membrane domains has significantly reduced the extent of the fluid lipid membrane matrix as first proposed, and biomembranes are now considered to be less fluid and more mosaic with some fluid areas, rather than a fluid matrix with predominantly mobile components. However, the fluid–lipid matrix regions remain very important in biomembranes, especially those involved in the binding and release of membrane lipid vesicles and the uptake of various nutrients. Membrane phospholipids can associate spontaneously to form lipid structures and vesicles that can fuse with various cellular membranes to transport lipids and other nutrients into cells and organelles and expel damaged lipids and toxic hydrophobic molecules from cells and tissues. This process and the clinical use of membrane phospholipid supplements has important implications for chronic illnesses and the support of healthy mitochondria, plasma membranes and other cellular membrane structures.

A Brief Introduction to Some Aspects of the Fluid-Mosaic Model of Cell Membrane Structure and Its Importance in Membrane Lipid Replacement

Early cell membrane models placed most proteins external to lipid bilayers in trimolecular structures or as modular lipoprotein units. These thermodynamically untenable structures did not allow lipid lateral movements independent of membrane proteins. The Fluid-Mosaic Membrane Model accounted for these and other properties, such as membrane asymmetry, variable lateral mobilities of membrane components and their associations with dynamic complexes. Integral membrane proteins can transform into globular structures that are intercalated to various degrees into a heterogeneous lipid bilayer matrix. This simplified version of cell membrane structure was never proposed as the ultimate biomembrane description, but it provided a basic nanometer scale framework for membrane organization. Subsequently, the structures associated with membranes were considered, including peripheral membrane proteins, and cytoskeletal and extracellular matrix components that restricted lateral mobility. In addition, lipid-lipid and lipid-protein membrane domains, essential for cellular signaling, were proposed and eventually discovered. The presence of specialized membrane domains significantly reduced the extent of the fluid lipid matrix, so membranes have become more mosaic with some fluid areas over time. However, the fluid regions of membranes are very important in lipid transport and exchange. Various lipid globules, droplets, vesicles and other membranes can fuse to incorporate new lipids or expel damaged lipids from membranes, or they can be internalized in endosomes that eventually fuse with other internal vesicles and membranes. They can also be externalized in a reverse process and released as extracellular vesicles and exosomes. In this Special Issue, the use of membrane phospholipids to modify cellular membranes in order to modulate clinically relevant host properties is considered.

Molecular Machinery and Mechanism of Cell Secretion

Secretion occurs in all living cells and involves the delivery of intracellular products to the cell exterior. Secretory products are Packaged and stored in membranous sacs or vesicles within the cell. When the cell needs to secrete these products, the secretory vesicles containing them dock and fuse at plasma membrane-associated supramolecular structures, called poro-somes, to release their contents. Specialized cells for neurotransmission, enzyme secretion, or hormone release use a highly regulated secretory process. Similar to other fundamen-tal cellular processes, cell secretion is precisely regulated. During secretion, swelling of secretory vesicles results in a build-up of intravesicular pressure, allowing expulsion of vesicular contents. The extent of vesicle swelling dictates the amount of vesicular contents expelled. The discovery of the Porosome as the universal secretory machinery, its isolation, its structure and dynamics at nanometer resolution and in real time, and its biochemical composition and functional reconstitution into artificial lipid membrane have been determined. The Molecular mechanism of secretory vesicle swelling and the fusion of opposing bilayers, that is, the fusion of secretory vesicle membrane at the base of the porosome membrane, have also been resolved. These findings reveal, for the first time, the universal molecular machinery and mechanism of secretion in cells.

Functional Reconstitution of the Insulin-Secreting Porosome Complex in Live Cells

Supramolecular cup-shaped lipoprotein structures called porosomes embedded in the cell plasma membrane mediate fractional release of intravesicular contents from cells during secretion. The presence of porosomes, have been documented in many cell types including neurons, acinar cells of the exocrine pancreas, GH-secreting cells of the pituitary, and insulin-secreting pancreatic β-cells. Functional reconstitution of porosomes into artificial lipid membranes, have also been accomplished. Earlier studies on mouse insulin-secreting Min6 cells report 100-nm porosome complexes composed of nearly 30 proteins. In the current study, porosomes have been functionally reconstituted for the first time in live cells. Isolated Min6 porosomes reconstituted into live Min6 cells demonstrate augmented levels of porosome proteins and a consequent increase in the potency and efficacy of glucose-stimulated insulin release. Elevated glucose-stimulated insulin secretion 48 hours after reconstitution, reflects on the remarkable stability and viability of reconstituted porosomes, documenting the functional reconstitution of native porosomes in live cells. These results, establish a new paradigm in porosome-mediated insulin secretion in β-cells.

Proteome of the insulin-secreting Min6 cell porosome complex: involvement of Hsp90 in its assembly and function

Porosomes are secretory portals located at the cell plasma membrane involved in the regulated release of intravesicular contents from cells. Porosomes have been immunoisolated from a number of cells including the exocrine pancreas and neurons, biochemically characterized, and functionally reconstituted into an artificial lipid membrane. In the current study, the proteome of the porosome complex in mouse insulinoma Min6 cells was determined, demonstrating among other proteins, the presence of 30 core proteins including the heat shock protein Hsp90. Half maximal inhibition of Hsp90 using the specific inhibitor 17-demethoxy-17-(2-prophenylamino) geldanamycin, results in the loss of proteins, including the calcium-transporting ATPase type 2C and the potassium channel subfamily K member 2 from the Min6 porosome. This loss of porosome proteins is reflected in the observed inhibition of glucose stimulated insulin release from Min6 cells exposed to the Hsp90 specific inhibitor. Results from the study implicate Hsp90 in the assembly and function of the porosome complex.

BIOLOGICAL SIGNIFICANCE

In the present study, the porosome proteome in the insulin-secreting mouse β-cell line Min6 has been determined. Nearly 30 core proteins including the heat shock protein Hsp90 are found to compose the Min6 porosome complex. Results from the study implicate Hsp90 in the assembly of the Min6 porosome. These new findings will facilitate understanding of the porosome assembly and its function in insulin secretion.

Porosome: a membrane microdomain acting as the universal secretory portal in exocytosis

Most, if not all, cells in the organism, at least in some period of their lifetime, secrete materials that are produced within the cell. Cell secretion is a phenomenon requiring membrane fusion at a specialized plasma membrane structure called the 'porosome,' which allows the material stored within secretory vesicles to be delivered to the cell's exterior environment. This is achieved when the secretory vesicles fuse at the base of the porosome complex, establishing a fusion pore or fluid continuity between the vesicle interior and the cell's exterior. Besides cell secretion, membrane fusion is necessary for intracellular membrane traffic and vesicular transport from one endomembrane bound structure to another. In addition to cell secretion, membrane fusion is necessary for intracellular membrane trafficking and vesicle transport from one intracellular membrane to another. We suggest that the debate about whether to use the term 'porosome' or 'fusion pore' to describe this process is unnecessary, since both of these terms are useful in describing aspects of the last event of cell secretion, namely exocytosis. In this review, we will summarize the information related to the discovery of the porosome, a universal secretory portal for exocytosis, and discuss porosome molecular organization and function. Finally, we will develop the notion that the porosome is a specialized plasma membrane microdomain.

Proteome of the porosome complex in human airway epithelia: interaction with the cystic fibrosis transmembrane conductance regulator (CFTR)

The surface of the airways is coated with a thin film of mucus composed primarily of mucin, which is under continuous motion via ciliary action. Mucin not only serves to lubricate the airways epithelia, but also functions as a trap for foreign particles and pathogens, thereby assisting in keeping the airways clean and free of particulate matter and infections. Altered mucin secretion especially increased mucin viscosity, results in mucin stagnation due to the inability of the cilia to propel them, leading to infections and diseases such as cystic fibrosis (CF). Since porosomes have been demonstrated to be the secretory portals at the cell plasma membrane in cells, their presence, structure, and composition in the mucin-secreting human airway epithelial cell line Calu-3 expressing CF transmembrane receptor (CFTR), were investigated. Atomic force microscopy (AFM) of Calu-3 cells demonstrates the presence of approximately 100nm in diameter porosome openings at the plasma membrane surface. Electron microscopy confirms the AFM results, and tandem mass spectrometry and immunoanalysis performed on isolated Calu-3 porosomes, reveal the association of CFTR with the porosome complex. These new findings will facilitate understanding of CFTR-porosome interactions influencing mucous secretion, and provide critical insights into the etiology of CF disease.

BIOLOGICAL SIGNIFICANCE

In the present study, the porosome proteome in human airway epithelia has been determined. The interaction between the cystic fibrosis transmembrane conductance regulator (CFTR) and the porosome complex in the human airway epithelia is further demonstrated. The possible regulation by CFTR on the quality of mucus secretion via the porosome complex at the cell plasma membrane is hypothesized. These new findings will facilitate understanding of CFTR-porosome interactions influencing mucous secretion, and provide critical insights into the etiology of CF disease.

Membrane tubulovesicular extensions (cytonemes): secretory and adhesive cellular organelles

In this review, we summarized data on the formation and structure of the long and highly adhesive membrane tubulovesicular extensions (TVEs, membrane tethers or cytonemes) observed in human neutrophils and other mammalian cells, protozoan parasites and bacteria. We determined that TVEs are membrane protrusions characterized by a uniform diameter (130-250 nm for eukaryotic cells and 60-90 nm for bacteria) along the entire length, an outstanding length and high rate of development and a high degree of flexibility and capacity for shedding from the cells. This review represents TVEs as protrusions of the cellular secretory process, serving as intercellular adhesive organelles in eukaryotic cells and bacteria. An analysis of the physical and chemical approaches to induce TVEs formation revealed that disrupting the actin cytoskeleton and inhibiting glucose metabolism or vacuolar-type ATPase induces TVE formation in eukaryotic cells. Nitric oxide is represented as a physiological regulator of TVE formation.

Porosome: the secretory portal

'It seems terribly wasteful that, during the release of hormones and neurotransmitters from a cell, the membrane of a vesicle should merge with the plasma membrane to be retrieved for recycling only seconds or minutes later.' - Erwin Neher, Nature 1993;363:497-498. This insightful statement so appropriately put, clearly reflected on the perception that secretory vesicles completely merge at the cell plasma membrane, failing to justify the generation of partially empty secretory vesicles in cells following secretion. A rational cellular mechanism would employ the transient fusion of secretory vesicles at the cell plasma membrane without compromising vesicle integrity, combined with vesicle retrieval following partial discharge of contents, to generate such partially empty vesicles following secretion. This hypothesis was finally confirmed with the serendipitous discovery of the porosome almost 16 years ago. The porosome has been demonstrated to be the universal secretory portal in cells and is present at the cell plasma membrane. In the past decade, the composition of the porosome, its dynamics, its structure at nanometer resolution in realtime using atomic force and electron microscopy, and its functional reconstitution into artificial lipid membrane, has resulted in a paradigm shift and a molecular understanding of the secretory process in cells. A brief background on porosome discovery, and our current understanding of its structure and function is summarized in this Minireview.

Identification of new structural elements within 'porosomes' of the exocrine pancreas: a detailed study using high-resolution electron microscopy

In the past two decades, great progress has been made in our knowledge of how cells secrete. This progress has been possible primarily due to discovery of the 'porosome', the universal secretory portals at the plasma membrane in cells. Porosomes are permanent cup-shaped lipoprotein structures at the cell plasma membrane, where membrane-bounded secretory vesicles temporarily dock and fuse to expel all or part of their contents during cellular secretion. Porosomes have been found in neurons, in neuroendocrine cells, as well as in the exocrine pancreas. Furthermore, porosomes have been isolated, functionally reconstituted, and their composition determined. Although, the neuronal porosome has been exhaustively investigated, the detailed morphology of porosomes in the exocrine pancreas in situ remains to be further explored. The current study was carried out to determine the detailed morphology of the porosome in rat exocrine pancreas using high-resolution electron microscopy. Results from our study, demonstrate for the first time the presence of tethers or cables (which could be t-SNAREs) associated at the base of porosomes. Furthermore, for the first time our studies demonstrate the docking of a single secretory vesicle at the base of more than one porosome complex. Detailed spoke-like elements lining the porosome cup are also demonstrated for the first time in our study, providing a better understanding of the molecular architecture and physiology of this important cellular organelle.

Neuronal porosome proteome: Molecular dynamics and architecture

Porosomes are the universal secretory portals at the cell plasma membrane, where membrane-bound secretory vesicles transiently dock and fuse to expel intravesicular contents to the outside during cell secretion. In the past decade, the neuronal porosome complex, a 10-15nm cup-shaped lipoprotein structure has been isolated, its partial composition and 3D contour map determined, and it has been functionally reconstituted into artificial lipid membrane. Here we further determine the composition of the neuronal porosome proteome using immunoisolation and gel filtration chromatography, followed by tandem mass spectrometry. Results from the study demonstrate nearly 40 proteins to constitute the neuronal porosome proteome. Furthermore, interaction of proteins within the porosome and their resulting arrangement is predicted. The association and dissociation of proteins at the porosome following stimulation of cell secretion demonstrate the dynamic nature of the organelle.

Porosome: the secretory NanoMachine in cells

Cells synthesize and store within membranous sacs products such as hormones, growth factors, neurotransmitters, or digestive enzymes, for release on demand. As recently as just 15 years ago, it was believed that during cell secretion, membrane-bound secretory vesicles completely merge at the cell plasma membrane resulting in the diffusion of intravesicular contents to the cell exterior and the compensatory retrieval of the excess membrane by endocytosis. This explanation, however, failed to explain the generation of partially empty vesicles observed in electron micrographs following secretion. Logically therefore, in a 1993 News and Views article in the journal Nature, Prof. Erwin Neher wrote "It seems terribly wasteful that, during the release of hormones and neurotransmitters from a cell, the membrane of a vesicle should merge with the plasma membrane to be retrieved for recycling only seconds or minutes later." The discovery of permanent secretory portals or nanomachines at the cell plasma membrane called POROSOMES, where membrane-bound secretory vesicles transiently dock and fuse to release intravesicular contents to the cell exterior, has finally resolved this conundrum. Following this discovery, the composition of the porosome, its structure and dynamics visualized with high-resolution imaging techniques atomic force and electron microscopy, and its functional reconstitution into artificial lipid membrane have provided a molecular understanding of cell secretion. In agreement, it has been demonstrated that "secretory granules are recaptured largely intact after stimulated exocytosis in cultured endocrine cells" (Proc Natl Acad Sci U S A 100:2070-2075, 2003); that "single synaptic vesicles fuse transiently and successively without loss of identity" (Nature 423:643-647, 2003); and that "zymogen granule exocytosis is characterized by long fusion pore openings and preservation of vesicle lipid identity" (Proc Natl Acad Sci U S A 101:6774-6779, 2004). It made no sense all these years to argue that mammalian cells possess an "all or none" mechanism of cell secretion resulting from complete vesicle merger at the cell plasma membrane, when even single-cell organisms have developed specialized and sophisticated secretory machinery, such as the secretion apparatus of Toxoplasma gondii, contractile vacuoles in paramecium, and different types of secretory structures in bacteria. The discovery of the porosome and its functional reconstitution in artificial lipid membrane, and an understanding of its morphology, composition, and dynamics, has resulted in a paradigm shift in our understanding of the secretory process in cells.

Identification of the porosome complex in the hair cell

Porosomes are proposed to be the universal secretory machinery of the cell plasma membrane, where membrane-bound secretory vesicles transiently dock and fuse to expel their contents to the extracellular space during cell secretion. In neurons, porosomes are manifested as cup-shaped lipoprotein structures in the presynaptic membrane, 12-17 nm in diameter and possessing a central plug. Hair cells of hearing and balance secrete transmitter from synaptic vesicles in sensory signal transduction, but it has not previously been demonstrated that these mechanosensory cells possess porosome structures that could participate in the secretory process. In the current study, we provide, for the first time, evidence obtained using transmission electron microscopy that porosome structures indeed exist in the hair cell, suggesting a mechanism of hair-cell transmitter secretion markedly different from that of the exocytotic process currently proposed.

Compartmentalization of pancreatic secretory zymogen granules as revealed by low-voltage transmission electron microscopy

Low-voltage (5-kV) transmission electron microscopy revealed a novel aspect of the pancreatic acinar cell secretory granules not previously detected by conventional (80-kV) transmission electron microscopy. Examination of ultra-thin (30-nm) sections of non-osmicated, stain-free pancreatic tissue sections by low-voltage electron microscopy revealed the existence of granules with non-homogeneous matrix and sub-compartments having circular or oval profiles of different electron densities and sizes. Such partition is completely masked when observing tissues after postfixation with osmium tetroxide by low-voltage transmission electron microscopy at 5 kV and/or when thicker sections (70 nm) are examined at 80 kV. This morphological partition reflects an internal compartmentalization of the granule content that was previously predicted by morphological, physiological, and biochemical means. It corresponds to the segregation of the different secretory proteins inside the granule as demonstrated by high-resolution immunocytochemistry and reflects a well-organized aggregation of the secretory proteins at the time of granule formation in the trans-Golgi. Such partition of the granule matrix undergoes changes under experimental conditions known to alter the secretory process such as stimulation of secretion or diabetes.

Palaeontological evidence of membrane relationship in step-by-step membrane fusion

Studies on membrane fusion in living cells indicate that initiation of membrane fusion is a transient and hard to capture process. Despite previous research, membrane behaviour at this point is still poorly understood. Recent palaeobotanical research has revealed snapshots of membrane fusion in a 15-million-year-old fossil pinaceous cone. To reveal the membrane behaviour during the fusion, we conducted more observations on the same fossil material. Several discernible steps of membrane fusion have been fixed naturally and observed in the fossil material. This observation provides transmission electron microscope (TEM) images of the transient intermediate stage and clearly shows the relationship between membranes. Observing such a transient phenomenon in fossil material implies that the fixing was most likely accomplished quickly by a natural process. The mechanism behind this phenomenon is clearly worthy of further enquiry.

Applications of atomic force microscopy in biophysical chemistry of cells

This article addresses the question of what information and new insights atomic force microscopy (AFM) provides that are of importance and relevance to cellular biophysical chemistry research. Three enabling aspects of AFM are discussed: (a) visualization of membrane structural features with nanometer resolution, such as microvilli, ridges, porosomes, lamellapodia, and filopodia; (b) revealing structural evolution associated with cellular signaling pathways by time-dependent and high-resolution imaging of the cellular membrane in correlation with intracellular components from simultaneous optical microscopy; and (c) qualitative and quantitative measurements of single cell mechanics by acquisition of force-deformation profiles and extraction of Young's moduli for the membrane as well as cytoskeleton. A future prospective of AFM is also presented.

Functional organization of the porosome complex and associated structures facilitating cellular secretion

Porosomes, the universal secretory machinery at the cell plasma membrane, are cup-shaped supramolecular lipoprotein structures, where membrane-bound vesicles transiently dock and fuse to release intravesicular contents during cell secretion. In this review, the discovery of the porosome and its structure, dynamics, composition, and functional reconstitution are outlined. Furthermore, the architecture of porosome-like structures such as the "canaliculi system" in human platelets and various associated structures such as the T-bars at the Drosophila synapse or the "beams," "ribs," and "pegs" at the frog neuromuscular junction, each organized to facilitate a certain specialized secretory activity, are briefly discussed.

Water channel proteins (later called aquaporins) and relatives: past, present, and future

Water channels or water channel proteins (WCPs) are transmembrane proteins that have a specific three-dimensional structure with a pore that can be permeated by water molecules. WCPs are large families (over 450 members) that are present in all kingdoms of life. The first WCP was discovered in the human red blood cell (RBC) membrane in 1980s. In 1990s other WCPs were discovered in plants, microorganisms, various animals, and humans; and it became obvious that the WCPs belong to the superfamily of major intrinsic proteins (MIPs, over 800 members). WCPs include three subfamilies: (a) aquaporins (AQPs), which are water specific (or selective water channels); (b) aquaglyceroporins (and glycerol facilitators), which are permeable to water and/or other small molecules; and (c) "superaquaporins" or subcellular AQPs. WCPs (and MIPs) have several structural characteristics which were better understood after the atomic structure of some MIPs was deciphered. The structure-function relationships of MIPs expressed in microorganisms (bacteria, archaea, yeast, and protozoa), plants, and some multicellular animal species [nematodes, insects, fishes, amphibians, mammals (and humans)] are described. A synthetic overview on the WCPs from RBCs from various species is provided. The physiological roles of WCPs in kidney, gastrointestinal system, respiratory apparatus, central nervous system, eye, adipose tissue, skin are described, and some implications of WCPs in various diseases are briefly presented. References of detailed reviews on each topic are given. This is the first review providing in a condensed form an overview of the whole WCP field that became in the last 20 years a very hot area of research in biochemistry and molecular cell biology, with wide and increasing implications.

Atomic force microscopy: Unraveling the fundamental principles governing secretion and membrane fusion in cells

The story of cell secretion and membrane fusion is as old as life itself. Without these fundamental cellular processes known to occur in yeast to humans, life would cease to exist. In the last 15 years, primarily using the atomic force microscope, a detailed understanding of the molecular process and of the molecular machinery and mechanism of secretion and membrane fusion in cells has come to light. This has led to a paradigm shift in our understanding of the underlying mechanism of cell secretion. The journey leading to the discovery of a new cellular structure the 'porosome',-the universal secretory machinery in cells, and the contributions of the AFM in our understanding of the general molecular machinery and mechanism of cell secretion and membrane fusion, is briefly discussed in this article.

Impact of actin rearrangement and degranulation on the membrane structure of primary mast cells: a combined atomic force and laser scanning confocal microscopy investigation

Degranulation of bone marrow-derived mast cells (BMMCs) triggered by antigens (e.g., 2,4-dinitrophenylated bovine serum albumin (DNP-BSA) and secretagogues (e.g., poly-L-lysine) was investigated by combined atomic force microscopy (AFM) and laser scanning confocal microscopy (LSCM). This combination enables the simultaneous visualization and correlation of membrane morphology with cytoskeletal actin arrangement and intracellular granules. Two degranulation mechanisms and detailed membrane structures that directly corresponded to the two stimuli were revealed. In DNP-BSA triggered activation, characteristic membrane ridges formed in accordance with the rearrangement of underlying F-actin networks. Individual granules were visualized after they released their contents, indicating a "kiss-and-run" pathway. In BMMCs stimulated by poly-L-lysine, lamellopodia and filopodia were observed in association with the F-actin assemblies at and near the cell periphery, whereas craters were observed on the central membrane lacking F-actin. These craters represent a new membrane feature resulting from the "kiss-and-merge" granule fusion. This work provides what we believe is important new insight into the local membrane structures in correlation with the cytoskeleton arrangement and detailed degranulation processes.

Extracellular dynamics at nm resolution in live cells

We are all voyagers in time and space, and throughout history of human civilization, our quest to understand Nature has fueled our imagination to make the necessary inventions that further our perception of Nature, perceptions beyond the natural limits of our senses. For example, the invention of various telescopes for observing distant objects, and microscopes for perceiving the very small, has enabled discoveries of distant galaxies and planets light years away, and of the micrometer-size unit of life-the "Cell," and of its nanometer-size subcellular organelles. The story of cell secretion, a fundamental process as old as life itself, occurs in all organisms-from the simple yeast to cells in humans. In the last 15 years, primarily using the atomic force microscope-a force spectroscope, a detailed understanding of the molecular machinery and mechanism of secretion in cells has come to light. This has led to a paradigm shift in our understanding of the underlying mechanism of cell secretion. The journey leading to the discovery of the "porosome," a nanometer-size structure at the cell plasma membrane-the universal secretory machinery, and its structure and dynamics in live cells, is briefly discussed in this chapter.

Physiology and pathophysiology of potassium channels in gastrointestinal epithelia

Epithelial cells of the gastrointestinal tract are an important barrier between the "milieu interne" and the luminal content of the gut. They perform transport of nutrients, salts, and water, which is essential for the maintenance of body homeostasis. In these epithelia, a variety of K(+) channels are expressed, allowing adaptation to different needs. This review provides an overview of the current literature that has led to a better understanding of the multifaceted function of gastrointestinal K(+) channels, thereby shedding light on pathophysiological implications of impaired channel function. For instance, in gastric mucosa, K(+) channel function is a prerequisite for acid secretion of parietal cells. In epithelial cells of small intestine, K(+) channels provide the driving force for electrogenic transport processes across the plasma membrane, and they are involved in cell volume regulation. Fine tuning of salt and water transport and of K(+) homeostasis occurs in colonic epithelia cells, where K(+) channels are involved in secretory and reabsorptive processes. Furthermore, there is growing evidence for changes in epithelial K(+) channel expression during cell proliferation, differentiation, apoptosis, and, under pathological conditions, carcinogenesis. In the future, integrative approaches using functional and postgenomic/proteomic techniques will help us to gain comprehensive insights into the role of K(+) channels of the gastrointestinal tract.

High-resolution three-dimensional imaging of the rich membrane structures of bone marrow-derived mast cells

Atomic force microscopy (AFM) enables high-resolution three-dimensional (3D) imaging of cultured bone marrow-derived mast cells. Cells were immobilized by a quick centrifugation and fixation to preserve their transient cellular morphologies followed by AFM characterization in buffer. This "fix-and-look" approach preserves the structural integrity of individual cells. Well-known membrane morphologies, such as ridges and microvilli, are visualized, consistent with prior electron microscopy observations. Additional information including the 3D measurements of these characteristic features are attained from AFM topographs. Filopodia and lamellopodia, associated with cell spreading, were captured and visualized in three dimensions. New morphologies are also revealed, such as high-density ridges and micro-craters. This investigation demonstrates that the "fix-and-look" approach followed by AFM imaging provides an effective means to characterize the membrane structure of hydrated cells with high resolution. The quantitative imaging and measurements pave the way for systematic correlation of membrane structural features with the biological status of individual cells.

Role of aquaporins and regulation of secretory vesicle volume in cell secretion

In exocrine glands, secretory proteins synthesized in the rough endoplasmic reticulum (RER) exhibit vectorial transport from ER through a succession of membrane-bounded components such as Golgi complex, condensing vacuoles and secretory granules. The secretory granules migrate to particular locations within the cell close to the apical membrane prior to the release of their contents into the acinar lumen. Currently, to release intragranular contents, secretory granules have been demonstrated to transiently dock and fuse at ‘porosome’, a permanent cup-shaped structures at the cell membranes. Then swelling of secretory granules occurs to allow explusion of intragranular contents. In this process, water and ion fluxes in the granule membrane appear to contribute to maintain secretory granule integrity and morphology via osmoregulation in secretory granules. Aquaporins (AQPs) are a family of small, hydrophobic, integral membrane proteins, which function as channels to permeate water and small solutes. The AQPs reside constitutively at the plasma membrane in most cell types. However, recent studies have demonstrated that the AQPs are present in secretory granules in exocrine glands, synaptic vesicles and intracellular vesicles in liver and kidney, implying that AQPs in secretory granules and vesicles are involved in their volume regulation. This paper reviews the possible role of AQPs on secretory granules, especially in exocrine glands, in secretory function.

Landmark discoveries in intracellular transport and secretion

Cellular protein transport and secretion is fundamental to the very existence of an organism, regulating important physiological functions such as reproduction, digestion, energy production, growth, neurotransmission, hormone release, water and ion transport, etc., all required for the survival and maintenance of homeostasis within an organism. Molecular understanding of transport and secretion of intracellular product has therefore been of paramount importance and aggressively investigated for over six decades. Only in the last 20 years, the general molecular mechanism of the process has come to light, following discovery of key proteins involved in ER-Golgi transport, and discovery of the porosome the universal secretion machinery in cells.

Cellular secretion studied by force microscopy

Using the optical microscope, real adventures in cellular research began in earnest in the latter half of the nineteenth century. With the development of the electron microscope, ultramicroscopy, and improved cell staining techniques, significant advances were made in defining intracellular structures at the nanometer level. The invention of force microscopy, the atomic force microscope (AFM) in the mid 1980s, and the photonic force microscope (PFM) in the mid 1990s, finally provided the opportunity to study live cellular structure-function at the nanometer level. Working with the AFM, dynamic cellular and subcellular events at the molecular level were captured in the mid 1990s, and a new cellular structure 'the porosome' in the plasma membrane of all secretory cells has been defined, where specific docking and fusion of secretory vesicles occur. The molecular mechanism of fusion of the secretory vesicle membrane at the base of the porosome membrane in cells, and the regulated release of intravesicular contents through the porosome opening to the extracellular space, has been determined. These seminal discoveries provide for the first time a molecular mechanism of cell secretion, and the possibility to ameliorate secretory defects in disease states.

Membrane fusion in cells: molecular machinery and mechanisms

Membrane fusion is a sine qua non process for cell physiology. It is critical for membrane biogenesis, intracellular traffic, and cell secretion. Although investigated for over a century, only in the last 15 years, the molecular machinery and mechanism of membrane fusion has been deciphered. The membrane fusion event elicits essentially three actors on stage: anionic phospholipids - phosphatidylinositols, phosphatidyl serines, specific membrane proteins, and the calcium ions, all participating in a well orchestrated symphony. Three soluble N-ethylmaleimide-sensitive factor (NSF)-attachment protein receptors (SNAREs) have been implicated in membrane fusion. Target membrane proteins, SNAP-25 and syntaxin (t-SNARE) and secretory vesicle-associated membrane protein (v-SNARE) or VAMP were discovered in the 1990's and suggested to be the minimal fusion machinery. Subsequently, the molecular mechanism of SNARE-induced membrane fusion was discovered. It was demonstrated that when t-SNARE-associated lipid membrane is exposed to v-SNARE-associated vesicles in the presence of Ca²⁺, the SNARE proteins interact in a circular array to form conducting channels, thus establishing continuity between the opposing bilayers. Further it was proved that SNAREs bring opposing bilayers close to within a distance of 2-3 Å, allowing Ca²⁺ to bridge them. The bridging of bilayers by Ca²⁺ then leads to the expulsion of water between the bilayers at the contact site, allowing lipid mixing and membrane fusion. Calcium bridging of opposing bilayers leads to the release of water, both from the water shell of hydrated Ca²⁺ ions, as well as the displacement of loosely coordinated water at the phosphate head groups in the lipid membrane. These discoveries provided for the first time, the molecular mechanism of SNARE-induced membrane fusion in cells. Some of the seminal discoveries are briefly discussed in this minireview.

The story of cell secretion: events leading to the discovery of the 'porosome' - the universal secretory machinery in cells

Cell secretio has come age, and a century old quest has been elegantly solved. We have come a long way since earlier observations of what appeared to be ‘fibrillar regions’ at teh cell plasama membrance, and electrophysological studies suggesting the presence of ‘fusion pores’ at the cell plasma membrane where secretion occurs. Finally, the fusion pore or ‘porosome’ has been discovered, and its morpholgy and dynamics determined at nm resolution and in real time in live secretory cells. The porosome has been isolated, its omposition determined at nm resolution and in real time in live secretory cells. The porosme has been isolated, its composition determmined and it has been jkboth structurally and functionally reconsituted n artificial lipid membrance. The discoversy of the porosome as the univeral secretory machinery in cell and the discovery of the molecular mechaninsm of vesicular content expulsion during cell secretin have fially enabled a clear understanding of this important cellular process. This review outlines the fascinating and exciting joumey leding to the dicovery of the porosme, ultimately solving one of the most difficut, significant, and fundamental cellular process- cell seretion.

AQPs and control of vesicle volume in secretory cells

Aquaporins (AQPs) are a family of small, hydrophobic, integral membrane proteins. In mammals, they are expressed in many epithelia and endothelia and function as channels that permit water or small solutes to pass. Although the AQPs reside constitutively at the plasma membrane in most cell types, the presence of AQPs in intracellular organelles such as secretory granules and vesicles has currently been demonstrated. The secretory granules and vesicles contain secretory proteins, migrate to particular locations within the cell close to the plasma membrane and release their contents to the outside. During the process, including exocytosis, regulation of secretory granule or vesicle volume is important. This paper reviews the possible role of AQPs in secretory granules and vesicles.

Porosome: the fusion pore revealed by multiple imaging modalities

Secretion occurs in all cells of multicellular organisms and involves the delivery of secretory products packaged in membrane-bound vesicles to the cell exterior. Specialized cells for neurotransmission, enzyme secretion, or hormone release utilize a highly regulated secretory process. Secretory vesicles are transported to specific sites at the plasma membrane, where they dock and fuse to release their contents. Similar to other cellular processes, cell secretion is found to be highly regulated and a precisely orchestrated event. It has been demonstrated that membrane-bound secretory vesicles dock and fuse at porosomes, which are specialized supramolecular structures at the cell plasma membrane. Swelling of secretory vesicles results in a buildup of pressure, allowing expulsion of intravesicular contents. The extent of secretory vesicle swelling dictates the amount of intravesicular contents expelled during secretion. The discovery of the porosome, its isolation, its structure and dynamics at nanometer resolution and in real time, and its biochemical composition and functional reconstitution into artificial lipid membrane have been determined. The molecular mechanism of secretory vesicle fusion at the base of porosomes and vesicle swelling have also been resolved. These findings reveal the molecular machinery and mechanism of cell secretion. In this chapter, the discovery of the porosome, its isolation, its structure and dynamics at nanometer resolution and in real time, and its biochemical composition and functional reconstitution into artificial lipid membrane are discussed.

Cell secretion machinery: studies using the AFM

A new field in biology, 'nano-cell biology', has emerged from the successful use of force microscopy in understanding the structure and dynamics of cells and biomolecules, at nm resolution and in real time. Atomic force microscopy, in combination with conventional tools and approaches (electron microscopy, electrophysiology, X-ray diffraction, photon correlation spectroscopy, mass spectroscopy, biochemistry, and molecular biology), has revealed for the first time, the universal molecular machinery and mechanism of secretion in cells. Secretion occurs in all living cells and involves the delivery of intracellular products to the cell exterior. Secretory products are packaged and stored in membranous sacs or vesicles within the cell. When the cell needs to secrete these products, the secretory vesicles containing them, dock and fuse at plasma membrane-associated supramolecular structures called Porosome, to release their contents. Specialized cells for neurotransmission, enzyme secretion, or hormone release utilize a highly regulated secretory process. During secretion, swelling of secretory vesicles results in a build-up of intravesicular pressure, allowing expulsion of vesicular contents. The extent of vesicle swelling dictates the amount of vesicular contents expelled. The discovery of the porosome as the universal secretory machinery, its isolation, its structure and dynamics at nm resolution and in real time, its biochemical composition and functional reconstitution into artificial lipid membrane, have been determined. The molecular mechanism of secretory vesicle swelling, and the fusion of opposing bilayers, i.e., the fusion of secretory vesicle membrane at the base of the porosome membrane, has also been resolved.

Physiology of ghrelin and related peptides

Growth hormone (GH) released from pituitary under direct control of hypothalamic releasing (i.e., GHRH) and inhibiting (i.e., sst or SRIF) hormones is an anabolic hormone that regulates metabolism of proteins, fats, sugars and minerals in mammals. Cyril Bowers' discovery of GH-releasing peptide (GHRP-6) was followed by a search for synthetic peptide and nonpeptide GH-secretagogues (GHSs) that stimulate GH release, as well as a receptor(s) unique from GHRH receptor. GHRH and GHSs operate through distinct G protein-coupled receptors to release GH. Signal transduction pathways activated by GHS increase intracellular Ca2+ concentration in somatotrophs, whereas GHRH increases cAMP. Isolation and characterization of ghrelin, the natural ligand for GHS receptor, has opened a new era of understanding to physiology of anabolism, feeding behavior, and nutritional homeostasis for GH secretion and gastrointestinal motility through gut-brain interactions. Other peptide hormones (i.e., motilin, TRH, PACAP, GnRH, leptin, FMRF amide, galanin, NPY, NPW) from gut, brain and other tissues also play a role in modulating GH secretion in livestock and lower vertebrate species. Physiological processes, such as neurotransmission, and secretion of hormones or enzymes, require fusion of secretory vesicles at the cell plasma membrane and expulsion of vesicular contents. This process for GH release from porcine somatotrophs was revealed by atomic force microscopy (AFM), transmission electron microscopy (TEM) and immunohistochemical distribution of the cells in pituitary during stages of development.

Cell secretion and membrane fusion

Secretion occurs in all cells of multicellular organisms and involves the delivery of secretory products packaged in membrane-bound vesicles to the cell exterior. Specialized cells for neurotransmission, enzyme secretion or hormone release utilize a highly regulated secretory process. Secretory vesicles are transported to specific sites at the plasma membrane, where they dock and fuse to release their contents. Similar to other cellular processes, cell secretion is found to be highly regulated and a precisely orchestrated event. It has been demonstrated that membrane-bound secretory vesicles dock and fuse at porosomes, which are specialized supramolecular structures at the cell plasma membrane. Swelling of secretory vesicles results in a build-up of pressure, allowing expulsion of intravesicular contents. The extent of secretory vesicle swelling dictates the amount of intravesicular contents expelled during secretion. The discovery of the porosome, its isolation, its structure and dynamics at nm resolution and in real time, its biochemical composition and functional reconstitution into artificial lipid membrane, have been determined. The molecular mechanism of secretory vesicle fusion at the base of porosomes, and vesicle swelling, has also been resolved. These findings reveal the molecular mechanism of cell secretion.

Patch clamped single pancreatic zymogen granules: direct measurements of ion channel activities at the granule membrane

BACKGROUND/AIM

Pancreatic acinar cells are involved in the secretion of digestive enzymes. Digestive enzymes in pancreatic acinar cells are stored in membrane-bound secretory vesicles called zymogen granules (ZGs). The swelling of ZGs is implicated in the regulation of the expulsion of intravesicular contents during secretion. The molecular mechanism of ZG swelling has been previously elucidated. It has been further demonstrated that the water channel aquaporin-1, the potassium channel IRK-8, and the chloride channel CLC-2, are present in the ZG membrane and involved in ZG swelling. However, a direct measurement of these ion channels at the ZG membrane in intact ZGs had not been performed. The aim of this study was to investigate the electrical activity of single ZGs and verify the types of channels found within their membrane.

METHODS

ZGs from pancreatic acinar cells were isolated from the pancreas of Sprague-Dawley rats. Direct measurements of whole vesicle currents, in the presence and absence of ion channel blockers (quinine, glyburide and DIDS), were recorded following successful patching of single ZGs.

CONCLUSION

In this study, we were able, for the first time, to patch single ZGs and study ion channels in their membrane. We were able to record currents across the ZG membrane and, utilizing ion channel blockers, confirm the presence of the chloride channels CLC-2 and the potassium channel IRK-8 (Kir6.1), and additionally demonstrate the presence of a second chloride channel CLC-3.

The v-ATPase V0 subunit a1 is required for a late step in synaptic vesicle exocytosis in Drosophila

The V(0) complex forms the proteolipid pore of an ATPase that acidifies vesicles. In addition, an independent function in membrane fusion has been proposed largely based on yeast vacuolar fusion experiments. We have isolated mutations in the largest V(0) component vha100-1 in flies in an unbiased genetic screen for synaptic malfunction. The protein is only required in neurons, colocalizes with markers for synaptic vesicles as well as active zones, and interacts with t-SNAREs. Loss of vha100-1 leads to vesicle accumulation in synaptic terminals, suggesting a deficit in release. The amplitude of spontaneous release events and release with hypertonic stimulation indicate normal levels of neurotransmitter loading, yet mutant embryos display severe defects in evoked synaptic transmission and FM1-43 uptake. Our data suggest that Vha100-1 functions downstream of SNAREs in synaptic vesicle fusion.

Elucidation of cell secretion: pancreas led the way

Secretion is a basic process in all cells and is required for several important functions such as neurotransmission, the secretion of digestive juices from the exocrine pancreas and the release of hormones from endocrine and neuroendocrine cells. Due to these important functions, the mechanism of cell secretion has been intensely investigated for over half a century. However, it is only in the last decade, with the discovery of a new cellular structure, the 'porosome' or 'fusion pore', and the elucidation of SNARE-induced membrane fusion, that has finally provided us with an understanding of cell secretion at the molecular level. The 'porosome', a supramolecular structure at the cell plasma membrane, was first discovered in the exocrine pancreas, and subsequently in endocrine/neuroendocrine cells and in neurons. The structure and dynamics of the 'porosome' in live cells at nanometer resolution and in real-time, its composition and functional reconstitution in lipid membrane, have all been determined. These findings have fundamentally changed our understanding of cell secretion and provide a clear understanding of this highly regulated process in cells.