Cell secretion - finally sees the light

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.

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 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.

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.