Discovery of the 'porosome'; the universal secretory machinery in cells

The role of heat shock proteins (HSPs) in type 2 diabetes mellitus pathophysiology

Type 2 diabetes mellitus (T2DM) is a metabolic disorder characterized by sustained hyperglycemia caused by impaired insulin signaling and secretion. Metabolic stress, caused by an inappropriate diet, is one of the major hallmarks provoking inflammation, endoplasmic reticulum (ER) stress, and mitochondrial dysfunction. Heat shock proteins (HSPs) are a group of highly conserved proteins that have a crucial role in chaperoning damaged and misfolded proteins to avoid disruption of cellular homeostasis under stress conditions. To do this, HSPs interact with diverse intra-and extracellular pathways among which are the insulin signaling, insulin secretion, and apoptosis pathways. Therefore, HSP dysfunction, e.g. HSP70, may lead to disruption of the pathways responsible for insulin secretion and uptake. Consistently, the altered expression of other HSPs and genetic polymorphisms in HSP-producing genes in diabetic subjects has made HSPs hot research in T2DM. This paper provides a comprehensive overview of the role of different HSPs in T2DM pathogenesis, affected cellular pathways, and the potential therapeutic strategies targeting HSPs in T2DM.

The neuronal porosome complex in health and disease

Cup-shaped secretory portals at the cell plasma membrane called porosomes mediate the precision release of intravesicular material from cells. Membrane-bound secretory vesicles transiently dock and fuse at the base of porosomes facing the cytosol to expel pressurized intravesicular contents from the cell during secretion. The structure, isolation, composition, and functional reconstitution of the neuronal porosome complex have greatly progressed, providing a molecular understanding of its function in health and disease. Neuronal porosomes are 15 nm cup-shaped lipoprotein structures composed of nearly 40 proteins, compared to the 120 nm nuclear pore complex composed of >500 protein molecules. Membrane proteins compose the porosome complex, making it practically impossible to solve its atomic structure. However, atomic force microscopy and small-angle X-ray solution scattering studies have provided three-dimensional structural details of the native neuronal porosome at sub-nanometer resolution, providing insights into the molecular mechanism of its function. The participation of several porosome proteins previously implicated in neurotransmission and neurological disorders, further attest to the crosstalk between porosome proteins and their coordinated involvement in release of neurotransmitter at the synapse.

Encyclopedia of bacterial gene circuits whose presence or absence correlate with pathogenicity--a large-scale system analysis of decoded bacterial genomes

BACKGROUND

Bacterial infections comprise a global health challenge as the incidences of antibiotic resistance increase. Pathogenic potential of bacteria has been shown to be context dependent, varying in response to environment and even within the strains of the same genus.

RESULTS

We used the KEGG repository and extensive literature searches to identify among the 2527 bacterial genomes in the literature those implicated as pathogenic to the host, including those which show pathogenicity in a context dependent manner. Using data on the gene contents of these genomes, we identified sets of genes highly abundant in pathogenic but relatively absent in commensal strains and vice versa. In addition, we carried out genome comparison within a genus for the seventeen largest genera in our genome collection. We projected the resultant lists of ortholog genes onto KEGG bacterial pathways to identify clusters and circuits, which can be linked to either pathogenicity or synergy. Gene circuits relatively abundant in nonpathogenic bacteria often mediated biosynthesis of antibiotics. Other synergy-linked circuits reduced drug-induced toxicity. Pathogen-abundant gene circuits included modules in one-carbon folate, two-component system, type-3 secretion system, and peptidoglycan biosynthesis. Antibiotics-resistant bacterial strains possessed genes modulating phagocytosis, vesicle trafficking, cytoskeletal reorganization, and regulation of the inflammatory response. Our study also identified bacterial genera containing a circuit, elements of which were previously linked to Alzheimer's disease.

CONCLUSIONS

Present study produces for the first time, a signature, in the form of a robust list of gene circuitry whose presence or absence could potentially define the pathogenicity of a microbiome. Extensive literature search substantiated a bulk majority of the commensal and pathogenic circuitry in our predicted list. Scanning microbiome libraries for these circuitry motifs will provide further insights into the complex and context dependent pathogenicity of bacteria.

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 in the Exocrine Pancreas: A Detailed EM Study suppressor

A major question in cell biology that accumulation of partially empty vesicles in cells following secretion is seen, while it is believed that secretion occurs via the complete merger of secretory vesicles with the cell plasma membrane. This important question was solved nearly two decades ago, with the discovery of the Porosome. Porosomes are cup-shaped lipoprotein structures found at the plasma membrane of all cells. Secretory vesicles dock and transiently fuse at the porosome base to form a continuous channel or fusion pore to release the pressurized vesicle contents to the outside. In a decade-long study by our group, we carefully examined using electron microscopy, the detailed structure of the porosome complex in acinar cells of the exocrine pancreas. Besides conformation of earlier findings, our study provides in much greater detail, the in situ morphology of the porosome complex in the exocrine pancreas. The discovery of the detailed morphology of the exocrine pancreas porosome complex in my laboratory is one of the major highlights of my academic career spanning nearly 50 years. Results from our EM studies, reveal for the first time the presence of tethers or cables, which are likely t-SNAREs, present at the porosome base. These EM studies further demonstrate for the first time the docking of a single secretory vesicle or zymogen granule at the base of more than one porosome complex. Detailed spoke-like elements lining the porosome cup were also observed for the first time in these studies, greatly advancing our understanding of the molecular architecture and physiology of the porosome in the exocrine pancreas.

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.

3D organization and function of the cell: Golgi budding and vesicle biogenesis to docking at the porosome complex

Insights into the three-dimensional (3D) organization and function of intracellular structures at nanometer resolution, holds the key to our understanding of the molecular underpinnings of cellular structure-function. Besides this fundamental understanding of the cell at the molecular level, such insights hold great promise in identifying the disease processes by their altered molecular profiles, and help determine precise therapeutic treatments. To achieve this objective, previous studies have employed electron microscopy (EM) tomography with reasonable success. However, a major hurdle in the use of EM tomography is the tedious procedures involved in fixing, high-pressure freezing, staining, serial sectioning, imaging, and finally compiling the EM images to obtain a 3D profile of sub-cellular structures. In contrast, the resolution limit of EM tomography is several nanometers, as compared to just a single or even sub-nanometer using the atomic force microscope (AFM). Although AFM has been hugely successful in 3D imaging studies at nanometer resolution and in real time involving isolated live cellular and isolated organelles, it has had limited success in similar studies involving 3D imaging at nm resolution of intracellular structure-function in situ. In the current study, using both AFM and EM on aldehyde-fixed and semi-dry mouse pancreatic acinar cells, new insights on a number of intracellular structure-function relationships and interactions were achieved. Golgi complexes, some exhibiting vesicles in the process of budding were observed, and small vesicles were caught in the act of fusing with larger vesicles, possibly representing either secretory vesicle biogenesis or vesicle refilling following discharge, or both. These results demonstrate the power and scope of the combined engagement of EM and AFM imaging of fixed semi-dry cells, capable of providing a wealth of new information on cellular structure-function and interactions.

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.

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.

Porosome: the secretory portal in cells

Porosomes are supramolecular, cup-shaped lipoprotein structures at the cell plasma membrane, where membrane-bound secretory vesicles dock and fuse to release intravesicular contents to the outside during cell secretion. The porosome opening to the outside ranges from 150 nm in diameter in acinar cells of the exocrine pancreas to 12 nm in neurons. In the past decade, the composition of the porosome, its structure and dynamics at nanometer resolution in real time, and its functional reconstitution into an artificial lipid membrane have been described. Discovery of the universal secretory machinery in cells, the porosome, came as no surprise since porosome-like "canaliculi" structures for secretion from human platelets, the secretory machinery in single-cell organisms like the secretion apparatus in bacteria and Toxoplasma gondii, and the contractile vacuole in paramecium have been demonstrated. In this review, the discovery of the porosome complex and the molecular mechanism of its function and how this information provides a new understanding of cell secretion are discussed.

Cell secretion: an update

This past decade has witnessed the publication of a flurry of scientific papers and reports on the subject of cell secretion, following discovery of a permanent plasma membrane structure termed ‘porosome’ and its determination as the universal secretory machinery in cells. This discovery has led to a paradigm shift in our understanding of the secretory process, demonstrating that membrane-bound secretory vesicles transiently dock and fuse at the porosome base to release their contents to the cell exterior. The regulated release of intravesicular contents during cell secretion is governed by dilation of the porosome opening to the outside, and the extent of vesicle swelling. In agreement, a great number of articles have been written and studies performed, which are briefly discussed in this article.

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.

Paleontological evidence for membrane fusion between a unit membrane and a half-unit membrane

Membrane fusion is of fundamental importance for many biological processes and has been a topic of intensive research in past decades with several models being proposed for it. Fossils had previously not been considered relevant to studies on membrane fusion. But here two different membrane fusion patterns are reported in the same well-preserved fossil plant from the Miocene (15-20 million years old) at Clarkia, Idaho, US. Scanning electron microscope, transmission electron microscope, and traditional studies reveal the vesicles in various states (even transient semi-fusion) of membrane fusion, and thus shed new light on their membrane structure and fusion during exocytoses. The new evidence suggests that vesicles in plant cells may have not only a unit membrane but also a half-unit membrane, and that a previously overlooked membrane fusion pattern exists in plant cells. This unexpected result from an unexpected material not only marks the first evidence of on-going physiological activities in fossil plants, but also raises questions on membrane fusion in recent plants.

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.