Trafficking/sorting and granule biogenesis in the beta-cell

Coding variants identified in diabetic patients alter PICK1 BAR domain function in insulin granule biogenesis

Bin/amphiphysin/Rvs (BAR) domains are positively charged crescent-shaped modules that mediate curvature of negatively charged lipid membranes during remodeling processes. The BAR domain proteins PICK1, ICA69, and the arfaptins have recently been demonstrated to coordinate the budding and formation of immature secretory granules (ISGs) at the trans-Golgi network. Here, we identify 4 coding variants in the PICK1 gene from a whole-exome screening of Danish patients with diabetes that each involve a change in positively charged residues in the PICK1 BAR domain. All 4 coding variants failed to rescue insulin content in INS-1E cells upon knock down of endogenous PICK1. Moreover, 2 variants showed dominant-negative properties. In vitro assays addressing BAR domain function suggested that the coding variants compromised BAR domain function but increased the capacity to cause fission of liposomes. Live confocal microscopy and super-resolution microscopy further revealed that PICK1 resides transiently on ISGs before egress via vesicular budding events. Interestingly, this egress of PICK1 was accelerated in the coding variants. We propose that PICK1 assists in or complements the removal of excess membrane and generic membrane trafficking proteins, and possibly also insulin, from ISGs during the maturation process; and that the coding variants may cause premature budding, possibly explaining their dominant-negative function.

Inside the Insulin Secretory Granule

The pancreatic β-cell is purpose-built for the production and secretion of insulin, the only hormone that can remove glucose from the bloodstream. Insulin is kept inside miniature membrane-bound storage compartments known as secretory granules (SGs), and these specialized organelles can readily fuse with the plasma membrane upon cellular stimulation to release insulin. Insulin is synthesized in the endoplasmic reticulum (ER) as a biologically inactive precursor, proinsulin, along with several other proteins that will also become members of the insulin SG. Their coordinated synthesis enables synchronized transit through the ER and Golgi apparatus for congregation at the trans-Golgi network, the initiating site of SG biogenesis. Here, proinsulin and its constituents enter the SG where conditions are optimized for proinsulin processing into insulin and subsequent insulin storage. A healthy β-cell is continually generating SGs to supply insulin in vast excess to what is secreted. Conversely, in type 2 diabetes (T2D), the inability of failing β-cells to secrete may be due to the limited biosynthesis of new insulin. Factors that drive the formation and maturation of SGs and thus the production of insulin are therefore critical for systemic glucose control. Here, we detail the formative hours of the insulin SG from the luminal perspective. We do this by mapping the journey of individual members of the SG as they contribute to its genesis.

A Cellular Automaton Model as a First Model-Based Assessment of Interacting Mechanisms for Insulin Granule Transport in Beta Cells

In this paper a first model is derived and applied which describes the transport of insulin granules through the cell interior and at the membrane of a beta cell. A special role is assigned to the actin network, which significantly influences the transport. For this purpose, microscopically measured actin networks are characterized and then further ones are artificially generated. In a Cellular Automaton model, phenomenological laws for granule movement are formulated and implemented. Simulation results are compared with experiments, primarily using TIRF images and secretion rates. In this respect, good similarities are already apparent. The model is a first useful approach to describe complex granule transport processes in beta cells, and offers great potential for future extensions. Furthermore, the model can be used as a tool to validate hypotheses and associated mechanisms regarding their effect on exocytosis or other processes. For this purpose, the source code for the model is provided online.

RILP Restricts Insulin Secretion Through Mediating Lysosomal Degradation of Proinsulin

Insulin secretion is tightly regulated by membrane trafficking. RILP (Rab7 interacting lysosomal protein) regulates the endocytic trafficking, but its role in insulin secretion has not been investigated. In this study, we found that overexpression of RILP inhibited insulin secretion in both the β-cell lines and freshly isolated islets. Consequently, the expression of RILP in islets suppressed the ability to recover the glucose homeostasis in type 1 diabetes mice upon transplantation. Of physiological relevance is that RILP expression was upregulated in the diabetic mouse islets. Mechanistically, overexpression of RILP induced insulin granule clustering, decreased the number of proinsulin-containing granules in β-cells, and significantly promoted proinsulin degradation. Conversely, RILP depletion sustained proinsulin and increased insulin secretion. The proinsulin degradation induced by RILP expression was inhibited by lysosomal inhibitors and was Rab7-dependent. Finally, we showed that RILP interacts with insulin granule-associated Rab26 to restrict insulin secretion. This study presents a new pathway regulating insulin secretion and mechanically demonstrates a novel function of RILP in modulating insulin secretion through mediating the lysosomal degradation of proinsulin.

Activity of the SPCA1 Calcium Pump Couples Sphingomyelin Synthesis to Sorting of Secretory Proteins in the Trans-Golgi Network

How the principal functions of the Golgi apparatus-protein processing, lipid synthesis, and sorting of macromolecules-are integrated to constitute cargo-specific trafficking pathways originating from the trans-Golgi network (TGN) is unknown. Here, we show that the activity of the Golgi localized SPCA1 calcium pump couples sorting and export of secreted proteins to synthesis of new lipid in the TGN membrane. A secreted Ca2+-binding protein, Cab45, constitutes the core component of a Ca2+-dependent, oligomerization-driven sorting mechanism whereby secreted proteins bound to Cab45 are packaged into a TGN-derived vesicular carrier whose membrane is enriched in sphingomyelin, a lipid implicated in TGN-to-cell surface transport. SPCA1 activity is controlled by the sphingomyelin content of the TGN membrane, such that local sphingomyelin synthesis promotes Ca2+ flux into the lumen of the TGN, which drives secretory protein sorting and export, thereby establishing a protein- and lipid-specific secretion pathway.

Fusion pore in exocytosis: More than an exit gate? A β-cell perspective

Secretory vesicle exocytosis is a fundamental biological event and the process by which hormones (like insulin) are released into the blood. Considerable progress has been made in understanding this precisely orchestrated sequence of events from secretory vesicle docked at the cell membrane, hemifusion, to the opening of a membrane fusion pore. The exact biophysical and physiological regulation of these events implies a close interaction between membrane proteins and lipids in a confined space and constrained geometry to ensure appropriate delivery of cargo. We consider some of the still open questions such as the nature of the initiation of the fusion pore, the structure and the role of the Soluble N-ethylmaleimide-sensitive-factor Attachment protein REceptor (SNARE) transmembrane domains and their influence on the dynamics and regulation of exocytosis. We discuss how the membrane composition and protein-lipid interactions influence the likelihood of the nascent fusion pore forming. We relate these factors to the hypothesis that fusion pore expansion could be affected in type-2 diabetes via changes in disease-related gene transcription and alterations in the circulating lipid profile. Detailed characterisation of the dynamics of the fusion pore in vitro will contribute to understanding the larger issue of insulin secretory defects in diabetes.

An endosomal syntaxin and the AP-3 complex are required for formation and maturation of candidate lysosome-related secretory organelles (mucocysts) in Tetrahymena thermophila

Lysosome-related organelles (LROs) are secretory organelles formed by convergence between secretory and endosomal trafficking pathways. In Tetrahymena, secretory vesicles that resemble dense core granules are a new class of LROs whose synthesis depends on a conserved syntaxin required for heterotypic fusion and AP-3 for maturation.

The ciliate Tetrahymena thermophila synthesizes large secretory vesicles called mucocysts. Mucocyst biosynthesis shares features with dense core granules (DCGs) in animal cells, including proteolytic processing of cargo proteins during maturation. However, other molecular features have suggested relatedness to lysosome-related organelles (LROs). LROs, which include diverse organelles in animals, are formed via convergence of secretory and endocytic trafficking. Here we analyzed Tetrahymena syntaxin 7-like 1 (Stx7l1p), a Qa-SNARE whose homologues in other lineages are linked with vacuoles/LROs. Stx7l1p is targeted to both immature and mature mucocysts and is essential in mucocyst formation. In STX7L1-knockout cells, the two major classes of mucocyst cargo proteins localize independently, accumulating in largely nonoverlapping vesicles. Thus initial formation of immature mucocysts involves heterotypic fusion, in which a subset of mucocyst proteins is delivered via an endolysosomal compartment. Further, we show that subsequent maturation requires AP-3, a complex widely implicated in LRO formation. Knockout of the µ-subunit gene does not impede delivery of any known mucocyst cargo but nonetheless arrests mucocyst maturation. Our data argue that secretory organelles in ciliates may represent a new class of LROs and reveal key roles of an endosomal syntaxin and AP-3 in the assembly of this complex compartment.

Receptor-mediated sorting of soluble vacuolar proteins: myths, facts, and a new model

To prevent their being released to the cell exterior, acid hydrolases are recognized by receptors at some point in the secretory pathway and diverted towards the lytic compartment of the cell (lysosome or vacuole). In animal cells, the receptor is called the mannosyl 6-phosphate receptor (MPR) and it binds hydrolase ligands in the trans-Golgi network (TGN). These ligands are then sequestered into clathrin-coated vesicles (CCVs) because of motifs in the cytosolic tail of the MPR which interact first with monomeric adaptors (Golgi-localized, Gamma-ear-containing, ARF-binding proteins, GGAs) and then with tetrameric (adaptin) adaptor complexes. The CCVs then fuse with an early endosome, whose more acidic lumen causes the ligands to dissociate. The MPRs are then recycled back to the TGN via retromer-coated carriers. Plants have vacuolar sorting receptors (VSRs) which were originally identified in CCVs isolated from pea (Pisum sativum L.) cotyledons. It was therefore assumed that VSRs would have an analogous function in plants to MPRs in animals. Although this dogma has enjoyed wide support over the last 20 years there are many inconsistencies. Recently, results have been published which are quite contrary to it. It now emerges that VSRs and their ligands can interact very early in the secretory pathway, and dissociate in the TGN, which, in contrast to its mammalian counterpart, has a pH of 5.5. Multivesicular endosomes in plants lack proton pump complexes and consequently have an almost neutral internal pH, which discounts them as organelles of pH-dependent receptor-ligand dissociation. These data force a critical re-evaluation of the role of CCVs at the TGN, especially considering that vacuolar cargo ligands have never been identified in them. We propose that one population of TGN-derived CCVs participate in retrograde transport of VSRs from the TGN. We also present a new model to explain how secretory and vacuolar cargo proteins are effectively separated after entering the late Golgi/TGN compartments.

An aspartyl cathepsin, CTH3, is essential for proprotein processing during secretory granule maturation in Tetrahymena thermophila

In animal cells, the assembly of dense cores in secretory granules is controlled by proteolytic processing of proproteins. The same phenomenon occurs in the ciliate Tetrahymena thermophila, but the proteases involved appear to be highly unrelated, suggesting that similar regulatory mechanisms have different molecular origins.

In Tetrahymena thermophila, peptides secreted via dense-core granules, called mucocysts, are generated by proprotein processing. We used expression profiling to identify candidate processing enzymes, which localized as cyan fluorescent protein fusions to mucocysts. Of note, the aspartyl cathepsin Cth3p plays a key role in mucocyst-based secretion, since knockdown of this gene blocked proteolytic maturation of the entire set of mucocyst proproteins and dramatically reduced mucocyst accumulation. The activity of Cth3p was eliminated by mutation of two predicted active-site mutations, and overexpression of the wild-type gene, but not the catalytic-site mutant, partially rescued a Mendelian mutant defective in mucocyst proprotein processing. Our results provide the first direct evidence for the role of proprotein processing in this system. Of interest, both localization and the CTH3 disruption phenotype suggest that the enzyme provides non–mucocyst-related functions. Phylogenetic analysis of the T. thermophila cathepsins, combined with prior work on the role of sortilin receptors in mucocyst biogenesis, suggests that repurposing of lysosomal enzymes was an important step in the evolution of secretory granules in ciliates.

PICK1 and ICA69 control insulin granule trafficking and their deficiencies lead to impaired glucose tolerance

PICK1 and ICA69, proteins containing a BAR domain, regulate the biogenesis and maturation of insulin granules in mice.

Diabetes is a metabolic disorder characterized by hyperglycemia. Insulin, which is secreted by pancreatic beta cells, is recognized as the critical regulator of blood glucose, but the molecular machinery responsible for insulin trafficking remains poorly defined. In particular, the roles of cytosolic factors that govern the formation and maturation of insulin granules are unclear. Here we report that PICK1 and ICA69, two cytosolic lipid-binding proteins, formed heteromeric BAR-domain complexes that associated with insulin granules at different stages of their maturation. PICK1-ICA69 heteromeric complexes associated with immature secretory granules near the trans-Golgi network (TGN). A brief treatment of Brefeldin A, which blocks vesicle budding from the Golgi, increased the amount of PICK1 and ICA69 at TGN. On the other hand, mature secretory granules were associated with PICK1 only, not ICA69. PICK1 deficiency in mice caused the complete loss of ICA69 and led to increased food and water intake but lower body weight. Glucose tolerance tests demonstrated that these mutant mice had high blood glucose, a consequence of insufficient insulin. Importantly, while the total insulin level was reduced in PICK1-deficient beta cells, proinsulin was increased. Lastly, ICA69 knockout mice also displayed similar phenotype as the mice deficient in PICK1. Together, our results indicate that PICK1 and ICA69 are key regulators of the formation and maturation of insulin granules.

AP-1 and clathrin are essential for secretory granule biogenesis in Drosophila

Regulated secretion of hormones, digestive enzymes, and other biologically active molecules requires the formation of secretory granules. Clathrin and the clathrin adaptor protein complex 1 (AP-1) are necessary for maturation of exocrine, endocrine, and neuroendocrine secretory granules. However, the initial steps of secretory granule biogenesis are only minimally understood. Powerful genetic approaches available in the fruit fly Drosophila melanogaster were used to investigate the molecular pathway for biogenesis of the mucin-containing "glue granules" that form within epithelial cells of the third-instar larval salivary gland. Clathrin and AP-1 colocalize at the trans-Golgi network (TGN) and clathrin recruitment requires AP-1. Furthermore, clathrin and AP-1 colocalize with secretory cargo at the TGN and on immature granules. Finally, loss of clathrin or AP-1 leads to a profound block in secretory granule formation. These findings establish a novel role for AP-1- and clathrin-dependent trafficking in the biogenesis of mucin-containing secretory granules.

Effect of glucocorticoid on the biosynthesis of growth hormone-containing secretory granules in pituitary cells

Recent studies have suggested that treatment of glucocorticoid to immature growth hormone (GH)-producing cell line, MtT/S cells, dramatically induced the accumulation of GH-containing secretory granules in the cytosol and differentiated into mature GH-producing cells. However, the molecular mechanism of glucocorticoid-induced GH-containing secretory granule biogenesis in the MtT/S cells remains unknown. In the present study, we found that GH mRNA expression was facilitated by application of glucocorticoid. We artificially increased GH synthesis by transfection of green fluorescent protein-tagged GH (GH-GFP) gene. We found that the artificial elevation of GH expression in the cells did not accumulate the secretory granules in the cytosol, whereas glucocorticoid-induced the biogenesis of granules in GH-GFP-expressing MtT/S cells. We next performed DNA microarray and real-time RT-PCR analysis and found that glucocorticoid significantly altered the expression of membrane trafficking-related protein, syntaxin11 (Syx11). Immunocytochemical analysis further demonstrated that Syx11 positive structures were well colocalized with GH-containing granules in both MtT/S cells and rat anterior pituitary gland. Our findings indicate that glucocorticoid regulate the expression of Syx11 and facilitate the biogenesis and the trafficking of GH-containing granules in the MtT/S cells.

Cathepsin L occupies a vacuolar compartment and is a protein maturase within the endo/exocytic system of Toxoplasma gondii

Regulated exocytosis allows the timely delivery of proteins and other macromolecules precisely when they are needed to fulfil their functions. The intracellular parasite Toxoplasma gondii has one of the most extensive regulated exocytic systems among all unicellular organisms, yet the basis of protein trafficking and proteolytic modification in this system is poorly understood. We demonstrate that a parasite cathepsin protease, TgCPL, occupies a newly recognized vacuolar compartment (VAC) that undergoes dynamic fragmentation during T. gondii replication. We also provide evidence that within the VAC or late endosome this protease mediates the proteolytic maturation of proproteins targeted to micronemes, regulated secretory organelles that deliver adhesive proteins to the parasite surface during cell invasion. Our findings suggest that processing of microneme precursors occurs within intermediate endocytic compartments within the exocytic system, indicating an extensive convergence of the endocytic and exocytic pathways in this human parasite.

Defective secretion of islet hormones in chromogranin-B deficient mice

Granins are major constituents of dense-core secretory granules in neuroendocrine cells, but their function is still a matter of debate. Work in cell lines has suggested that the most abundant and ubiquitously expressed granins, chromogranin A and B (CgA and CgB), are involved in granulogenesis and protein sorting. Here we report the generation and characterization of mice lacking chromogranin B (CgB-ko), which were viable and fertile. Unlike neuroendocrine tissues, pancreatic islets of these animals lacked compensatory changes in other granins and were therefore analyzed in detail. Stimulated secretion of insulin, glucagon and somatostatin was reduced in CgB-ko islets, in parallel with somewhat impaired glucose clearance and reduced insulin release, but normal insulin sensitivity in vivo. CgB-ko islets lacked specifically the rapid initial phase of stimulated secretion, had elevated basal insulin release, and stored and released twice as much proinsulin as wildtype (wt) islets. Stimulated release of glucagon and somatostatin was reduced as well. Surprisingly, biogenesis, morphology and function of insulin granules were normal, and no differences were found with regard to beta-cell stimulus-secretion coupling. We conclude that CgB is not required for normal insulin granule biogenesis or maintenance in vivo, but is essential for adequate secretion of islet hormones. Consequentially CgB-ko animals display some, but not all, hallmarks of human type-2 diabetes. However, the molecular mechanisms underlying this defect remain to be determined.

Secretogranin III: a bridge between core hormone aggregates and the secretory granule membrane

Secretory granules in endocrine cells selectively store bioactive peptide hormones and amines, which are secreted in a regulated manner upon appropriate stimulation. In addition to bioactive substances, various proteins and lipids characteristic of secretory granules are likely recruited to a restricted space at the trans-Golgi Network (TGN), and the space then matures to the secretory granule. Although experimental findings so far have strongly suggested that aggregation- and receptor-mediated processes are essential for the formation of secretory granules, the putative link between these two processes remains to be clarified. Recently, secretogranin III (SgIII) has been identified as a specific binding protein for chromogranin A (CgA), a representative constituent of the core aggregate within secretory granules, and it was later revealed that SgIII can also bind to the cholesterol-rich membrane domain at the TGN. Based on its multifaceted binding properties, SgIII may act as a central player in the formation of cholesterol-rich membrane platforms. Upon these platforms, essential processes for secretory granule biogenesis coordinately occur; that is, selective recruitment of prohormones, processing and modifying of prohormones, and condensation of mature hormones as an aggregate. This review summarizes the findings and theoretical concepts on the issue to date and then focuses on the putative role of SgIII in secretory granule biogenesis in endocrine cells.

Insulin granule biogenesis, trafficking and exocytosis

It is becoming increasingly apparent that beta cell dysfunction resulting in abnormal insulin secretion is the essential element in the progression of patients from a state of impaired glucose tolerance to frank type 2 diabetes (Del Prato, 2003; Del Prato and Tiengo, 2001). Although extensive studies have examined the molecular, cellular and physiologic mechanisms of insulin granule biogenesis, sorting, and exocytosis the precise mechanisms controlling these processes and their dysregulation in the developed of diabetes remains an area of important investigation. We now know that insulin biogenesis initiates with the synthesis of preproinsulin in rough endoplastic reticulum and conversion of preproinsulin to proinsulin. Proinsulin begins to be packaged in the Trans-Golgi Network and is sorting into immature secretory granules. These immature granules become acidic via ATP-dependent proton pump and proinsulin undergoes proteolytic cleavage resulting the formation of insulin and C-peptide. During the granule maturation process, insulin is crystallized with zinc and calcium in the form of dense-core granules and unwanted cargo and membrane proteins undergo selective retrograde trafficking to either the constitutive trafficking pathway for secretion or to degradative pathways. The newly formed mature dense-core insulin granules populate two different intracellular pools, the readily releasable pools (RRP) and the reserved pool. These two distinct populations are thought to be responsible for the biphasic nature of insulin release in which the RRP granules are associated with the plasma membrane and undergo an acute calcium-dependent release accounting for first phase insulin secretion. In contrast, second phase insulin secretion requires the trafficking of the reserved granule pool to the plasma membrane. The initial trigger for insulin granule fusion with the plasma membrane is a rise in intracellular calcium and in the case of glucose stimulation results from increased production of ATP, closure of the ATP-sensitive potassium channel and cellular depolarization. In turn, this opens voltage-dependent calcium channels allowing increased influx of extracellular calcium. Calcium is thought to bind to members of the fusion regulatory proteins synaptogamin that functionally repressors the fusion inhibitory protein complexin. Both complexin and synaptogamin interact as well as several other regulatory proteins interact with the core fusion machinery composed of the Q- or t-SNARE proteins syntaxin 1 and SNAP25 in the plasma membrane that assembles with the R- or v-SNARE protein VAMP2 in insulin granules. In this chapter we will review the current progress of insulin granule biogenesis, sorting, trafficking, exocytosis and signaling pathways that comprise the molecular basis of glucose-dependent insulin secretion.

Stem cell therapy to treat diabetes mellitus

Transplantation of pancreatic islets offers a direct treatment for type 1 diabetes and in some cases, insulin-dependent type 2 diabetes. However, its widespread use is hampered by a shortage of donor organs. Many extant studies have focused on deriving beta-cell progenitors from pancreas and pluripotent stem cells. Efforts to generate beta-cells in vitro will help elucidate the mechanisms of beta-cell formation and thus provide a versatile in vivo system to evaluate the therapeutic potential of these cells to treat diabetes. Various successful experiments using beta-cells in animal models have generated extensive interest in using human embryonic stem cells to restore normoglycemia in diabetic patients. While new techniques are continually unveiled, the success of beta-cell generation rests upon successful manipulation of culture conditions and the induction of key regulatory genes implicated in pancreas development. In this review, we compare successfully conducted protocols, highlight essential steps and identify some of the remarkable shortfalls common to these methods. In addition, we discuss recent advancements in the derivation of patient-specific pluripotent stem cells that may facilitate the use of autologous beta-cells in stem cell therapy.

Maturation of secretory granules

Exocrine, endocrine, and neuroendocrine cells store hormones and neuropeptides in secretory granules (SGs), which undergo regulated exocytosis in response to an appropriate stimulus. These cargo proteins are sorted at the trans-Golgi network into forming immature secretory granules (ISGs). ISGs undergo maturation while they are transported to and within the F-actin-rich cortex. This process includes homotypic fusion of ISGs, acidification of their lumen, processing, and aggregation of cargo proteins as well as removal of excess membrane and missorted cargo. The resulting mature secretory granules (MSGs) are stored in the F-actin-rich cell cortex, perhaps as segregated pools exhibiting specific responses to stimuli for regulated exocytosis. During the last decade our understanding of the maturation of ISGs advanced substantially. The use of biochemical approaches led to the identification of membrane molecules mechanistically involved in this process. Furthermore, live cell imaging in combination with fluorescently tagged marker proteins of SGs provided insights into the dynamics of maturing ISGs, and the functional implications of cytoskeletal elements and motor proteins.

Distinct linkage between post-translational processing and differential secretion of progastrin derivatives in endocrine cells

Prohormones often undergo extensive cellular processing prior to secretion. These post-translational processing events occur in organelles of the constitutive or regulated secretory pathway. The aim of this study was to examine the relationship between post-translational modifications and the secretory pathways taken by peptides derived from progastrin, the prohormone of gastrin, which in vivo is secreted by cells of the pyloric glands and stimulates the release of gastric acid. Targeting progastrin to compartments of the early secretory pathway shows that endoproteolytic processing is initiated in a pre-trans-Golgi network compartment of endocrine but not non-endocrine cells. The resulting N-terminal fragments of progastrin are secreted via the constitutive pathway, whereas endoproteolytically processed C-terminal fragments are secreted via the regulated or constitutive-like pathways. C-terminal fragments derived from progastrin differ in characteristic manners in levels and patterns of carboxyamidation and tyrosine sulfation in accordance with the secretory pathway taken. Point mutations introduced into a sorting motif disrupt these patterns, suggesting that differences in post-translational modifications are attributable to differential intracellular sorting of precursors. The results suggest a two-step sorting mechanism for progastrin leading to differential secretion of processed fragments via different secretory pathways.

Glucose recruits K(ATP) channels via non-insulin-containing dense-core granules

beta cells rely on adenosine triphosphate-sensitive potassium (K(ATP)) channels to initiate and end glucose-stimulated insulin secretion through changes in membrane potential. These channels may also act as a constituent of the exocytotic machinery to mediate insulin release independent of their electrical function. However, the molecular mechanisms whereby the beta cell plasma membrane maintains an appropriate number of K(ATP) channels are not known. We now show that glucose increases K(ATP) current amplitude by increasing the number of K(ATP) channels in the beta cell plasma membrane. The effect was blocked by inhibition of protein kinase A (PKA) as well as by depletion of extracellular or intracellular Ca(2+). Furthermore, glucose promoted recruitment of the potassium inward rectifier 6.2 to the plasma membrane, and intracellular K(ATP) channels localized in chromogranin-positive/insulin-negative dense-core granules. Our data suggest that glucose can recruit K(ATP) channels to the beta cell plasma membrane via non-insulin-containing dense-core granules in a Ca(2+)- and PKA-dependent manner.

Insulin granule trafficking in beta-cells: mathematical model of glucose-induced insulin secretion

A mathematical model that represents the dynamics of intracellular insulin granules in beta-cells is proposed. Granule translocation and exocytosis are controlled by signals assumed to be essentially related to ATP-to-ADP ratio and cytosolic Ca(2+) concentration. The model provides an interpretation of the roles of the triggering and amplifying pathways of glucose-stimulated insulin secretion. Values of most of the model parameters were inferred from available experimental data. The numerical simulations represent a variety of experimental conditions, such as the stimulation by high K(+) and by different time courses of extracellular glucose, and the predicted responses agree with published experimental data. Model capacity to represent data measured in a hyperglycemic clamp was also tested. Model parameter changes that may reflect alterations of beta-cell function present in type 2 diabetes are investigated, and the action of pharmacological agents that bind to sulfonylurea receptors is simulated.

Proteomics analysis of insulin secretory granules

Insulin secretory granules (ISGs) are cytoplasmic organelles of pancreatic beta-cells. They are responsible for the storage and secretion of insulin. To date, only about 30 different proteins have been clearly described to be associated with these organelles. However, data from two-dimensional gel electrophoresis analyses suggested that almost 150 different polypeptides might be present within ISGs. The elucidation of the identity and function of the ISG proteins by proteomics strategies would be of considerable help to further understand some of the underlying mechanisms implicated in ISG biogenesis and trafficking. Furthermore it should give the bases to the comprehension of impaired insulin secretion observed during diabetes. A proteomics analysis of an enriched insulin granule fraction from the rat insulin-secreting cell line INS-1E was performed. The efficacy of the fractionation procedure was assessed by Western blot and electron microscopy. Proteins of the ISG fraction were separated by SDS-PAGE, excised from consecutive gel slices, and tryptically digested. Peptides were analyzed by nano-LC-ESI-MS/MS. This strategy identified 130 different proteins that were classified into four structural groups including intravesicular proteins, membrane proteins, novel proteins, and other proteins. Confocal microscopy analysis demonstrated the association of Rab37 and VAMP8 with ISGs in INS-1E cells. In conclusion, the present study identified 130 proteins from which 110 are new proteins associated with ISGs. The elucidation of their role will further help in the understanding of the mechanisms governing impaired insulin secretion during diabetes.

Polymeric gene transfection on insulin-secreting cells: sulfonylurea receptor-mediation and transfection medium effect

PURPOSE

In vitro transfection of secreting cells is regarded as one strategy for improved cell engineering/ transplantation. Insulin-secreting insulinoma cell lines or pancreatic beta-cells could be genetically engineered using designed polymeric vectors which are safer than viral vectors. This study investigates the effects of the constituents in transfection media on polymeric transfection.

METHODS

Polyplexes conjugated with sulfonylurea (SU) were evaluated under different transfection conditions for gene transfection and their effects on cytotoxicity and insulin secretion. Several components in transfection media specifically associated with the insulin secretion pathway were amino acids, vitamins, Ca2+ and K+. The interactions of the polyplexes with insulin were monitored by surface charge and particle size to monitor how insulin as a protein influences transfection.

RESULTS

For an insulin-secreting cell line (RINm5F), polyplexes in Ca2+--containing KRH medium (Ca2+(+)KRH) enhanced transfection and did not cause damage to biological functions. When adding amino acids, vitamins, or K+ or depleting Ca2+ from Ca2+(+)KRH, poly(L-lysine)/DNA complexes showed a greater reduction in transfection than SU receptor (SUR)-targeting polyplexes (SU-polyplex). Positively charged polyplexes interacted with insulin, developing a negative surface charge, and these interactions may cause a decrease in transfection.

CONCLUSION

The findings suggest that in vitro and ex vivo polymeric transfection of insulin-secreting cells can be modulated and enhanced by adjusting the transfection conditions.

A cleavable propeptide influences Toxoplasma infection by facilitating the trafficking and secretion of the TgMIC2-M2AP invasion complex

Propeptides regulate protein function and trafficking in many eukaryotic systems and have emerged as important features of regulated secretory proteins in parasites of the phylum Apicomplexa. Regulated protein secretion from micronemes and host cell invasion are inextricably linked and essential processes for the apicomplexan parasite Toxoplasma gondii. TgM2AP is a propeptide-containing microneme protein found in a heterohexameric complex with the microneme protein TgMIC2, a protein that has a demonstrated fundamental role in gliding motility and invasion. TgM2AP function is also central to these processes, because disruption of TgM2AP (m2apKO) results in secretory retention of TgMIC2, leading to reduced TgMIC2 secretion from the micronemes and impaired invasion. Because the TgM2AP propeptide is predicted to be processed in an intracellular site near where TgMIC2 is retained in m2apKO parasites, we hypothesized that the propeptide and its proteolytic removal influence trafficking and secretion of the complex. We found that proTgM2AP traffics through endosomal compartments and that deletion of the propeptide leads to defective trafficking of the complex within or near this site, resulting in aberrant processing and decreased secretion of TgMIC2, impaired invasion, and reduced virulence in vivo, mirroring the phenotypes observed in m2apKO parasites. In contrast, mutation of several cleavage site residues resulted in normal localization, but it affected the stability and secretion of the complex from the micronemes. Therefore, the propeptide and its cleavage site influence distinct aspects of TgMIC2-M2AP function, with both impacting the outcome of infection.

The integration of cell and chemical biology in protein folding

Eukaryotic cells are specialized, interdependent functional units of complex tissues that are composed of metabolically integrated systems defined by chemically distinct organelles that operate as reaction vessels. It is now clear that the small-molecule and polymer-based composition of these organelles plays a crucial role in generating and maintaining protein folds and functions through the systems chemistry of the local environments.

Cytological and immunocytochemical characterization of the insulin secreting insulinoma cell line RINm5F

The rat insulinoma cell line RINm5F, an insulin secreting pancreatic beta cell line, has been used as an attractive model for basic studies of the mechanisms of insulin secretion and, more recently, as a model for the development of alternative methods for the treatment of diabetes. To elucidate the cytological properties and expression patterns of hormones of the gastro-entero-pancreatic system, suspensions of RINm5F cells were investigated by various methods including immunocytochemistry on serial semithin sections, quantitative immunocytochemistry, routine electron microscopy, immuno-electron microscopy, in situ hybridization, and TUNEL technique. At the ultrastructural level, several phenotypes of RIm5F cells were characterized by differences in the number, shape, size, and density of their secretory granules. The most common type contained a mixture of round granules varying in size and electron density. A second type predominantly contained relatively large, moderately dense granules. Moreover, a minority of cells was characterized by the occurrence of polymorphous electron dense granules or the complete absence of any secretory granules. The immunohistochemical data showed that, among the established islet hormones, insulin was present in more than 50% of cells, whereas glucagon and somatostatin occurred only sporadically. Though cells positive for pancreatic polypeptide (PP) were not found, PP-related peptides (NPY and PYY) however could be detected in a minority of cells. The great majority of RINm5F cells were immunoreactive for chromogranin B (CgB), followed by insulin, chromogranin A (CgA), and serotonin (5-HT). In addition to intercellular differences in the density of immunostaining, numerous colocalizations of immunoreactivities were found, suggesting that RINm5F cells represent a mixture of subtypes concerning the individual pattern of hormone expression. The present results reveal a wide range of heterogeneity with respect to the morphology and especially the hormone content between individual RINm5F cells.

Is there structural specificity in the reversible protein aggregates that are stored in secretory granules?

There are several steps that must occur for secretory granules to form: (1) Secretory proteins that make up the dense cores of the granules must be concentrated; (2) membrane proteins necessary for granule function must accumulate in the correct location; and (3) inappropriate membrane proteins and excess membrane must be removed. Reversible aggregation of secretory granule proteins provides a mechanism for concentrating and sorting these proteins. There is specificity in the way secretory granule proteins are treated in cells that make granules. The specificity has been shown in some cases to occur after the aggregation process, so that granules containing different aggregates function differently. An explanation could be that a property of the aggregate, such as a surface motif, might influence the accumulation of membrane proteins necessary for granule function. Such a conclusion implies that the aggregates are not amorphous but have structure. Use of NMR spectroscopy to investigate changes in the environment of amino acid residues in secretory granule proteins as they form oligomers by using 15N relaxation times might provide a means to determine which residues are specifically involved in aggregation.

Insulin resistance, impaired early insulin response, and insulin propeptides as predictors of the development of type 2 diabetes: a population-based, 7-year follow-up study in 70-year-old men

OBJECTIVE

Defects in insulin secretion and insulin action are the major abnormalities in the development of type 2 diabetes. In middle-aged subjects, elevated plasma proinsulin has been found to predict type 2 diabetes. Therefore, our aim was to study the longitudinal relationships between baseline determinations of insulin sensitivity index (S(i)) assessed by euglycemic insulin clamp, the early insulin response (EIR) at an oral glucose tolerance test (OGTT), fasting intact proinsulin, 32-33 split proinsulin and specific insulin, and the development of type 2 diabetes in a population-based cohort of 70-year-old nondiabetic men (n = 667) with 7-year follow-up.

RESEARCH DESIGN AND METHODS

A euglycemic insulin clamp study and a 75-g OGTT were performed at baseline, and fasting peptide concentrations were measured using specific two-site immunometric assays. Results from logistic regression models are presented as odds ratios (ORs) with 95% CIs for a 1-SD increase in the predictor variable.

RESULTS

In separate multivariate analyses adjusted for EIR (OR 0.72, 95% CI 0.59-0.89) and S(i) (0.68, 0.58-0.88), 32-33 split proinsulin (1.49, 1.18-1.88) or intact proinsulin (1.30, 1.04-1.63) were significantly associated with the development of type 2 diabetes, whereas specific insulin (1.24, 0.91-1.66) was not. The significant associations between 32-33 split or intact proinsulin and the development of type 2 diabetes were unaltered after adjustment for BMI and glucose tolerance.

CONCLUSIONS

Insulin propeptides predicted type 2 diabetes over a 7-year period in elderly men, independent of the EIR and S(i).

Adaptor protein complex 1 mediates the transport of lysosomal proteins from a Golgi-like organelle to peripheral vacuoles in the primitive eukaryote Giardia lamblia

Giardia lamblia is an early branching protist that possesses peripheral vacuoles (PVs) with characteristics of lysosome-like organelles, located underneath the plasma membrane. In more evolved cells, lysosomal protein trafficking is achieved by cargo recognition involving adaptor protein (AP) complexes that recognize specific amino acid sequences (tyrosine and/or dileucine motifs) within the cytoplasmic tail of membrane proteins. Previously, we reported that Giardia has a tyrosine-based sorting system, which mediates the targeting of a membrane-associated cysteine protease (encystation-specific cysteine protease, ESCP) to the PVs. Here, we show that Giardia AP1 mediates the transport of ESCP and the soluble acid phosphatase (AcPh) to the PVs. By using the yeast two-hybrid assay we found that the ESCP tyrosine-based motif interacts specifically with the medium subunit of AP1 (Gimicroa). Hemagglutinin-tagged Gimicroa colocalizes with ESCP and AcPh and coimmunoprecipitates with clathrin, suggesting that protein trafficking toward the PVs is clathrin-adaptin dependent. Targeted disruption of Gimicroa results in mislocalization of ESCP and AcPh but not of variant-specific surface proteins. Our results suggest that, unlike mammalian cells, only AP1 is involved in anterograde protein trafficking to the PVs in Giardia. Moreover, even though Giardia trophozoites lack a morphologically discernible Golgi apparatus, the presence of a clathrin-adaptor system suggests that this parasite possess a primitive secretory organelle capable of sorting proteins similar to that of more evolved cells.

Insulin expressing cells from differentiated embryonic stem cells are not beta cells

AIM/HYPOTHESIS

Embryonic stem (ES) cells have been proposed as a potential source of tissue for transplantation for the treatment of Type 1 diabetes. However, studies showing differentiation of beta cells from ES cells are controversial. The aim of this study was to characterise the insulin-expressing cells differentiated in vitro from ES cells and to assess their suitability for the treatment of diabetes.

METHODS

ES cell-derived insulin-expressing cells were characterised by means of immunocytochemistry, RT-PCR and functional analyses. Activation of the Insulin I promoter during ES-cell differentiation was assessed in ES-cell lines transfected with a reporter gene. ES cell-derived cultures were transplanted into STZ-treated SCID-beige mice and blood glucose concentrations of diabetic mice were monitored for 3 weeks.

RESULTS

Insulin-stained cells differentiated from ES cells were devoid of typical beta-cell granules, rarely showed immunoreactivity for C-peptide and were mostly apoptotic. The main producers of proinsulin/insulin in these cultures were neurons and neuronal precursors and a reporter gene under the control of the insulin I promoter was activated in cells with a neuronal phenotype. Insulin was released into the incubation medium but the secretion was not glucose-dependent. When the cultures were transplanted in diabetic mice they formed teratomas and did not reverse the hyperglycaemic state.

CONCLUSIONS/INTERPRETATION

Our studies show that insulin-positive cells in vitro-differentiated from ES cells are not beta cells and suggest that alternative protocols, based on enrichment of ES cell-derived cultures with cells of the endodermal lineage, should be developed to generate true beta cells for the treatment of diabetes.

Insulin constitutively secreted by beta-cells is necessary for glucose-stimulated insulin secretion

Four hypotheses have been posited on the role of insulin in glucose-stimulated insulin secretion; available evidence has supported insulin as being 1) essential, 2) a positive modulator, 3) a negative modulator, or 4) not necessary. Because circulating insulin levels in mice, before or after intraperitoneal glucose injection, are sufficient to elicit insulin responses in insulin-sensitive tissues, it is likely that beta-cell insulin receptors are continuously exposed to stimulating concentrations of insulin. To determine whether constitutively secreted insulin is necessary for glucose-stimulated insulin secretion, CD1 male mouse islets were incubated for 30 min at 4 degrees C in the absence (control) or presence of anti-insulin (1 micro g/ml) or anti-IgG (1 micro g/ml). Then islets were exposed to 3, 11, or 25 mmol/l glucose or to 20 mmol/l arginine. Nontreated islets exhibited first- and second-phase glucose-stimulated insulin secretion. Control and anti-IgG-treated islets, after a 5-min lag phase, increased their insulin secretion in 25 mmol/l glucose. Anti-insulin-treated islets secreted insulin at a basal rate in 3 or 25 mmol/l glucose buffers. Insulin secretion stimulated by 20 mmol/l arginine was the same in islets pretreated with either antibody and showed no lag phase. Taken together, these data suggest that constitutively secreted insulin is required and sufficient for beta-cells to maintain sensitivity to glucose.

Mutant proinsulin that cannot be converted is secreted efficiently from primary rat beta-cells via the regulated pathway

Prohormones are directed from the trans-Golgi network to secretory granules of the regulated secretory pathway. It has further been proposed that prohormone conversion by endoproteolysis may be necessary for subsequent retention of peptides in granules and to prevent their release by the so-called "constitutive-like" pathway. To address this directly, mutant human proinsulin (Arg/Gly(32):Lys/Thr(64)), which cannot be cleaved by conversion endoproteases, was expressed in primary rat islet cells by recombinant adenovirus. The handling of the mutant proinsulin was compared with that of wild-type human proinsulin. Infected islet cells were pulse labeled and both basal and stimulated secretion of radiolabeled products followed during a chase. Labeled products were quantified by high-performance liquid chromatography. As expected, the mutant proinsulin was not converted at any time. Basal (constitutive and constitutive-like) secretion was higher for the mutant proinsulin than for wild-type proinsulin/insulin, but amounted to <1% even during a prolonged (6-h) period of basal chase. There was no difference in stimulated (regulated) secretion of mutant and wild-type proinsulin/insulin at any time. Thus, in primary islet cells, unprocessed (mutant) proinsulin is sorted to the regulated pathway and then retained in secretory granules as efficiently as fully processed insulin.

New class of cargo protein in Tetrahymena thermophila dense core secretory granules

Regulated exocytosis of dense core secretory granules releases biologically active proteins in a stimulus-dependent fashion. The packaging of the cargo within newly forming granules involves a transition: soluble polypeptides condense to form water-insoluble aggregates that constitute the granule cores. Following exocytosis, the cores generally disassemble to diffuse in the cell environment. The ciliates Tetrahymena thermophila and Paramecium tetraurelia have been advanced as genetically manipulatable systems for studying exocytosis via dense core granules. However, all of the known granule proteins in these organisms condense to form the architectural units of lattices that are insoluble both before and after exocytosis. Using an approach designed to detect new granule proteins, we have now identified Igr1p (induced during granule regeneration). By structural criteria, it is unrelated to the previously characterized lattice-forming proteins. It is distinct in that it is capable of dissociating from the insoluble lattice following secretion and therefore represents the first diffusible protein identified in ciliate granules.

Mechanisms for storage of prolactin and growth hormone in secretory granules

There are three steps in the formation of secretory granules: aggregation of proteins to form the dense cores of granules, accumulation of appropriate membrane proteins necessary for function of the granules, and removal of extraneous membrane and inappropriate proteins by small vesicles. Formation of protein aggregates may be the initial step in this process, which is not well understood. Assays of aggregation of human prolactin and growth hormone in neuroendocrine cells indicate that acidic intracellular compartments are necessary, and Zn2+ and Cu2+ may facilitate aggregation through low affinity binding sites. There is more than one way to make proteins aggregate in solution; precipitates of human prolactin formed in "crowded" conditions most closely resemble what is likely to occur in cells. Understanding the properties of aggregates formed in cells may be important, as there are several examples of granules with different contents that function differently; human R183H-growth hormone, a mutant that causes autosomal dominant isolated growth hormone deficiency, also appears to be an example. Recognition of surface motifs on aggregates of proteins may be important to localize correctly membrane proteins necessary for function, an explanation for the means by which granule content may influence function.

The biology of cortical granules

An egg-that took weeks to months to make in the adult-can be extraordinarily transformed within minutes during its fertilization. This review will focus on the molecular biology of the specialized secretory vesicles of fertilization, the cortical granules. We will discuss their role in the fertilization process, their contents, how they are made, and the molecular mechanisms that regulate their secretion at fertilization. This population of secretory vesicles has inherent interest for our understanding of the fertilization process. In addition, they have import because they enhance our understanding of the basic processes of secretory vesicle construction and regulation, since oocytes across species utilize this vesicle type. Here, we examine diverse animals in a comparative approach to help us understand how these vesicles function throughout phylogeny and to establish conserved themes of function.

Gene and cell-replacement therapy in the treatment of type 1 diabetes: how high must the standards be set?

Recent advances in molecular and cell biology may allow for the development of novel strategies for the treatment and cure of type 1 diabetes. In particular, it is now possible to envisage restoration of insulin secretion by gene or cell-replacement therapy. The beta-cell is, however, remarkably sophisticated, and many of the features of this highly differentiated secretory cell will have to be faithfully mimicked in surrogate cells. In particular, insulin is normally secreted in a well-regulated fashion in rapid response to the metabolic needs of the individual and most specifically (but not exclusively) to changes in circulating levels of glucose. Such regulated secretion will be indispensable in order to avoid both hyper- and hypoglycemic episodes and depends on the ability of cells to store insulin in secretory granules before exocytosis in response to physiological stimuli. Furthermore, any newly created insulin-secreting cell will have to be able to adapt to alterations in insulin requirements that accompany changes with exercise, body weight, and aging. Fine tuning of insulin secretion over the longer term will also be important to avoid "clinical shifting" that could be caused by over-insulinization, including increased adiposity and cardiovascular disease. Finally, it will be necessary to ensure that newly created or implanted (surrogate) beta-cells are protected in some way from recognition by the immune system and in particular from autoimmune destruction.