The C-terminus of prohormone convertase 2 is sufficient and necessary for Raft association and sorting to the regulated secretory pathway

Cell-specific secretory granule sorting mechanisms: the role of MAGEL2 and retromer in hypothalamic regulated secretion

Intracellular protein trafficking and sorting are extremely arduous in endocrine and neuroendocrine cells, which synthesize and secrete on-demand substantial quantities of proteins. To ensure that neuroendocrine secretion operates correctly, each step in the secretion pathways is tightly regulated and coordinated both spatially and temporally. At the trans-Golgi network (TGN), intrinsic structural features of proteins and several sorting mechanisms and distinct signals direct newly synthesized proteins into proper membrane vesicles that enter either constitutive or regulated secretion pathways. Furthermore, this anterograde transport is counterbalanced by retrograde transport, which not only maintains membrane homeostasis but also recycles various proteins that function in the sorting of secretory cargo, formation of transport intermediates, or retrieval of resident proteins of secretory organelles. The retromer complex recycles proteins from the endocytic pathway back to the plasma membrane or TGN and was recently identified as a critical player in regulated secretion in the hypothalamus. Furthermore, melanoma antigen protein L2 (MAGEL2) was discovered to act as a tissue-specific regulator of the retromer-dependent endosomal protein recycling pathway and, by doing so, ensures proper secretory granule formation and maturation. MAGEL2 is a mammalian-specific and maternally imprinted gene implicated in Prader-Willi and Schaaf-Yang neurodevelopmental syndromes. In this review, we will briefly discuss the current understanding of the regulated secretion pathway, encompassing anterograde and retrograde traffic. Although our understanding of the retrograde trafficking and sorting in regulated secretion is not yet complete, we will review recent insights into the molecular role of MAGEL2 in hypothalamic neuroendocrine secretion and how its dysregulation contributes to the symptoms of Prader-Willi and Schaaf-Yang patients. Given that the activation of many secreted proteins occurs after they enter secretory granules, modulation of the sorting efficiency in a tissue-specific manner may represent an evolutionary adaptation to environmental cues.

Maturing secretory granules: Where secretory and endocytic pathways converge

Secretory granules (SGs) are specialized organelles responsible for the storage and regulated release of various biologically active molecules from the endocrine and exocrine systems. Thus, proper SG biogenesis is critical to normal animal physiology. Biogenesis of SGs starts at the trans-Golgi network (TGN), where immature SGs (iSGs) bud off and undergo maturation before fusing with the plasma membrane (PM). How iSGs mature is unclear, but emerging studies have suggested an important role for the endocytic pathway. The requirement for endocytic machinery in SG maturation blurs the line between SGs and another class of secretory organelles called lysosome-related organelles (LROs). Therefore, it is important to re-evaluate the differences and similarities between SGs and LROs.

Insulin granule biogenesis and exocytosis

Insulin is produced by pancreatic β-cells, and once released to the blood, the hormone stimulates glucose uptake and suppresses glucose production. Defects in both the availability and action of insulin lead to elevated plasma glucose levels and are major hallmarks of type-2 diabetes. Insulin is stored in secretory granules that form at the trans-Golgi network. The granules undergo extensive modifications en route to their release sites at the plasma membrane, including changes in both protein and lipid composition of the granule membrane and lumen. In parallel, the insulin molecules also undergo extensive modifications that render the hormone biologically active. In this review, we summarize current understanding of insulin secretory granule biogenesis, maturation, transport, docking, priming and eventual fusion with the plasma membrane. We discuss how different pools of granules form and how these pools contribute to insulin secretion under different conditions. We also highlight the role of the β-cell in the development of type-2 diabetes and discuss how dysregulation of one or several steps in the insulin granule life cycle may contribute to disease development or progression.

Cholesterol-enriched membrane rafts and insulin secretion

The failure of pancreatic β‐cells to supply insulin in quantities sufficient to maintain euglycemia is a hallmark of type 2 diabetes. Perturbation of β‐cell cholesterol homeostasis, culminating in elevated intracellular cholesterol levels, impairs insulin secretion and has therefore been proposed as a mechanism contributing to β‐cell dysfunction. The manner in which this occurs, however, is unclear. Cholesterol is an essential lipid, as well as a major component of membrane rafts, and numerous proteins critical for the regulation of insulin secretion have been reported to associate with these domains. Although this suggests that alterations in membrane rafts could partially account for the reduction in insulin secretion observed when β‐cell cholesterol accumulates, this has not yet been demonstrated. In this review, we provide a brief overview of recent work implicating membrane rafts in some of the basic molecular mechanisms of insulin secretion, and discuss the insight it provides into the β‐cell dysfunction characteristic of type 2 diabetes. (J Diabetes Invest, doi: 10.1111/j.2040‐1124.2012.00200.x, 2012)

Two dipolar α-helices within hormone-encoding regions of proglucagon are sorting signals to the regulated secretory pathway

Proglucagon is expressed in pancreatic α cells, intestinal L cells, and some hypothalamic and brainstem neurons. Tissue-specific processing of proglucagon yields three major peptide hormones as follows: glucagon in the α cells and glucagon-like peptides (GLP)-1 and -2 in the L cells and neurons. Efficient sorting and packaging into the secretory granules of the regulated secretory pathway in each cell type are required for nutrient-regulated secretion of these proglucagon-derived peptides. Our previous work suggested that proglucagon is directed into granules by intrinsic sorting signals after initial processing to glicentin and major proglucagon fragment (McGirr, R., Guizzetti, L., and Dhanvantari, S. (2013) J. Endocrinol. 217, 229-240), leading to the hypothesis that sorting signals may be present in multiple domains. In the present study, we show that the α-helices within glucagon and GLP-1, but not GLP-2, act as sorting signals by efficiently directing a heterologous secretory protein to the regulated secretory pathway. Biophysical characterization of these peptides revealed that glucagon and GLP-1 each encode a nonamphipathic, dipolar α-helix, whereas the helix in GLP-2 is not dipolar. Surprisingly, glicentin and major proglucagon fragment were sorted with different efficiencies, thus providing evidence that proglucagon is first sorted to granules prior to processing. In contrast to many other prohormones in which sorting is directed by ordered prodomains, the sorting determinants of proglucagon lie within the ordered hormone domains of glucagon and GLP-1, illustrating that each prohormone has its own sorting "signature."

Attenuation of insulin signalling contributes to FSN-1-mediated regulation of synapse development

A neuronal F-box protein FSN-1 regulates Caenorhabditis elegans neuromuscular junction development by negatively regulating DLK-mediated MAPK signalling. In the present study, we show that attenuation of insulin/IGF signalling also contributes to FSN-1-dependent synaptic development and function. The aberrant synapse morphology and synaptic transmission in fsn-1 mutants are partially and specifically rescued by reducing insulin/IGF-signalling activity in postsynaptic muscles, as well as by reducing the activity of EGL-3, a prohormone convertase that processes agonistic insulin/IGF ligands INS-4 and INS-6, in neurons. FSN-1 interacts with, and potentiates the ubiquitination of EGL-3 in vitro, and reduces the EGL-3 level in vivo. We propose that FSN-1 may negatively regulate insulin/IGF signalling, in part, through EGL-3-dependent insulin-like ligand processing.

The sorting of proglucagon to secretory granules is mediated by carboxypeptidase E and intrinsic sorting signals

Proglucagon is expressed in pancreatic alpha cells, intestinal L cells and brainstem neurons. Tissue-specific processing of proglucagon yields the peptide hormones glucagon in the alpha cell and glucagon-like peptide (GLP)-1 and GLP-2 in L cells. Both glucagon and GLP-1 are secreted in response to nutritional status and are critical for regulating glycaemia. The sorting of proglucagon to the dense-core secretory granules of the regulated secretory pathway is essential for the appropriate secretion of glucagon and GLP-1. We examined the roles of carboxypeptidase E (CPE), a prohormone sorting receptor, the processing enzymes PC1/3 and PC2 and putative intrinsic sorting signals in proglucagon sorting. In Neuro 2a cells that lacked CPE, PC1/3 and PC2, proglucagon co-localised with the Golgi marker p115 as determined by quantitative immunofluorescence microscopy. Expression of CPE, but not of PC1/3 or PC2, enhanced proglucagon sorting to granules. siRNA-mediated knockdown of CPE disrupted regulated secretion of glucagon from pancreatic-derived alphaTC1-6 cells, but not of GLP-1 from intestinal cell-derived GLUTag cells. Mutation of the PC cleavage site K70R71, the dibasic R17R18 site within glucagon or the alpha-helix of glucagon, all significantly affected the sub-cellular localisation of proglucagon. Protein modelling revealed that alpha helices corresponding to glucagon, GLP-1 and GLP-2, are arranged within a disordered structure, suggesting some flexibility in the sorting mechanism. We conclude that there are multiple mechanisms for sorting proglucagon to the regulated secretory pathway, including a role for CPE in pancreatic alpha cells, initial cleavage at K70R71 and multiple sorting signals.

Multiple roles for the actin cytoskeleton during regulated exocytosis

Regulated exocytosis is the main mechanism utilized by specialized secretory cells to deliver molecules to the cell surface by virtue of membranous containers (i.e., secretory vesicles). The process involves a series of highly coordinated and sequential steps, which include the biogenesis of the vesicles, their delivery to the cell periphery, their fusion with the plasma membrane, and the release of their content into the extracellular space. Each of these steps is regulated by the actin cytoskeleton. In this review, we summarize the current knowledge regarding the involvement of actin and its associated molecules during each of the exocytic steps in vertebrates, and suggest that the overall role of the actin cytoskeleton during regulated exocytosis is linked to the architecture and the physiology of the secretory cells under examination. Specifically, in neurons, neuroendocrine, endocrine, and hematopoietic cells, which contain small secretory vesicles that undergo rapid exocytosis (on the order of milliseconds), the actin cytoskeleton plays a role in pre-fusion events, where it acts primarily as a functional barrier and facilitates docking. In exocrine and other secretory cells, which contain large secretory vesicles that undergo slow exocytosis (seconds to minutes), the actin cytoskeleton plays a role in post-fusion events, where it regulates the dynamics of the fusion pore, facilitates the integration of the vesicles into the plasma membrane, provides structural support, and promotes the expulsion of large cargo molecules.

Exploring the membrane topology of prohormone convertase 1 in AtT20 Cells: in situ analysis by immunofluorescence microscopy

Prohormone convertase 1 (PC1) was previously characterized as a partially transmembrane protein in purified chromaffin granules of bovine adrenal medulla1. This was challenged with experiments on transfected PC1 in COS1 cells, a non-endocrine cell line2. To address this issue, we undertook to analyze its extraction properties  in vitro and its immunocytochemical localization  in situ in AtT20 cells, an endocrine cell line that expresses PC1. Most of the 87 kDa form of PC1 was resistant to carbonate extraction suggesting that it had properties of a transmembrane protein. Under semi-permeabilized conditions whereby only the plasma membrane was permeabilized, the carboxy-terminus of PC1 was specifically immunostained whereas the amino-terminus was not. These results indicate that the amino-terminus of PC1 was within the lumen of the Golgi and granules, and some of the C-terminus was exposed to the cytosol. Thus, endogenous PC1 can assume a transmembrane orientation  in situ in AtT20 cells.

Revisiting the regulated secretory pathway: from frogs to human

The regulated secretory pathway is a hallmark of endocrine and neuroendocrine cells. This process comprises different sequential steps, including ER-associated protein synthesis, ER-to-Golgi protein transport, Golgi-associated posttranslational modification, sorting and packing of secretory proteins into carrier granules, cytoskeleton-based granule transport towards the plasma membrane and tethering, docking and fusion of granules with specialized releasing zones in the plasma membrane. Each one of these steps is tightly regulated by a large number of factors that function in a spatially and temporarily coordinated fashion. During the past three decades, much effort has been devoted to characterize the precise role of the yet-known proteins participating in the different steps of this process and to identify new regulatory factors in order to obtain a unifying picture of the secretory pathway. In spite of this and given the enormous complexity of the process, certain steps are not fully understood yet and many players remain to be identified. In this review, we offer a summary of the current knowledge on the main molecular mechanisms that govern and ensure the correct release of secretory proteins. In addition, we have integrated the advance on the field made possible by studies carried out in non-mammalian vertebrates, which, although not very numerous, have substantially contributed to acquire a mechanistic understanding of the regulated secretory pathway.

A dileucine determinant in the carboxyl terminal sequence of the LHβ subunit is implicated in the regulated secretion of lutropin from transfected GH3 cells

LH and FSH are essential for control of gonadal function. They are synthesized in the same gonadotrope but differ in their mode of secretion. LH release is regulated, while FSH is secreted constitutively. One unique feature of LHβ is a carboxyl terminal hydrophobic heptapeptide. We demonstrated that deleting the heptapeptide diverted the truncated LH dimer to the constitutive pathway in vitro. To examine if the residues of this heptapeptide play a role in LH sorting, leucines 118-119 were substituted with alanine (L118A and L119A, respectively). The intracellular pool of the L118A mutant protein decreased with a corresponding increase in constitutive secretion. Moreover, immunofluorescence microscopy revealed that the L118A mutant exhibited fewer puncta as compared to wild-type LH. L119A behaved similar to wild-type LH, indicating that a single leucine residue at position 118, rather than a dileucine motif, contributes to the process that sorts LH into the regulated pathway.

An analysis of the Caenorhabditis elegans lipid raft proteome using geLC-MS/MS

Lipid rafts are microdomains of the phospholipid bilayer, proposed to form semi-stable "islands" that act as a platform for several important cellular processes; major classes of raft-resident proteins include signalling proteins and glycosylphosphatidylinositol (GPI)-anchored proteins. Proteomic studies into lipid rafts have been mainly carried out in mammalian cell lines and single cell organisms. The nematode Caenorhabditis elegans, the model organism with a well-defined developmental profile, is ideally suited for the study of this subcellular locale in a complex developmental context. A study of the lipid raft proteome of C. elegans is presented here. A total of 44 proteins were identified from the lipid raft fraction using geLC-MS/MS, of which 40 have been determined to be likely raft proteins after analysis of predicted functions. Prediction of GPI-anchoring of the proteins found 21 to be potentially modified in this way, two of which were experimentally confirmed to be GPI-anchored. This work is the first reported study of the lipid raft proteome in C. elegans. The results show that raft proteins, including numerous GPI-anchored proteins, may have a variety of potentially important roles within the nematode, and will hopefully lead to C. elegans becoming a useful model for the study of lipid rafts.

Chromogranin A as a crucial factor in the sorting of peptide hormones to secretory granules

Chromogranin A (CgA) is a soluble glycoprotein stored along with hormones and neuropeptides in secretory granules of endocrine cells. In the last four decades, intense efforts have been concentrated to characterize the structure and the biological function of CgA. Besides, CgA has been widely used as a diagnostic marker for tumors of endocrine origin, essential hypertension, various inflammatory diseases, and neurodegenerative disorders such as amyotrophic lateral sclerosis and Alzheimer's disease. CgA displays peculiar structural features, including numerous multibasic cleavage sites for prohormone convertases as well as a high proportion of acidic residues. Thus, it has been proposed that CgA represents a precursor of biologically active peptides, and a "granulogenic protein" that plays an important role as a chaperone for catecholamine storage in adrenal chromaffin cells. The widespread distribution of CgA throughout the neuroendocrine system prompted several groups to investigate the role of CgA in peptide hormone sorting to the regulated secretory pathway. This review summarizes the findings and theoretical concepts around the molecular machinery used by CgA to exert this putative intracellular function. Since CgA terminal regions exhibited strong sequence conservation through evolution, our work focused on the implication of these domains as potential functional determinants of CgA. Characterization of the molecular signals implicating CgA in the intracellular traffic of hormones represents a major biological issue that may contribute to unraveling the mechanisms defining the secretory competence of neuroendocrine cells.

Cholesterol biosynthesis pathway intermediates and inhibitors regulate glucose-stimulated insulin secretion and secretory granule formation in pancreatic beta-cells

Cholesterol is reportedly abundant in the endocrine secretory granule (SG) membrane. In this study, we examined the involvement of cholesterol biosynthesis intermediates and inhibitors in insulin secretion and SG formation mechanisms. There are two routes for the supply of cholesterol to the cells: one via de novo biosynthesis and the other via low-density lipoprotein receptor-mediated endocytosis. We found that insulin secretion and content are diminished by β-hydroxy-β-methylglutaryl-coenzyme A inhibitor lovastatin but not by lipoprotein depletion from the culture medium in MIN6 β-cells. Cholesterol biosynthesis intermediates mevalonate, squalene, and geranylgeranyl pyrophosphate enhanced glucose-stimulated insulin secretion, and the former two increased insulin content. The glucose-stimulated insulin secretion-enhancing effect of geranylgeranyl pyrophosphate was also confirmed in perifusion with rat islets. Morphologically, mevalonate and squalene increased the population of SGs without affecting their size. In contrast, lovastatin increased the SG size with reduction of insulin-accumulating dense cores, leading to a decrease in insulin content. Furthermore, insulin was secreted in a constitutive manner, indicating disruption of regulated insulin secretion. Because secretogranin III, a cholesterol-binding SG-residential granin-family protein, coincides with SG localization based on the cholesterol composition, secretogranin III may be associated with insulin-accumulating mechanisms. Although the SG membrane exhibits a high cholesterol composition, we could not find detergent-resistant membrane regions using a lipid raft-residential protein flotillin and a fluorescent cholesterol-Si-pyrene probe as markers on a sucrose-density gradient fractionation. We suggest that the high cholesterol composition of SG membrane with 40-50 mol% is crucial for insulin secretion and SG formation functions.

Sorting of neuropeptides and neuropeptide receptors into secretory pathways

There are two major secretory pathways in neurons, the regulated pathway and the constitutive pathway. Neuropeptides and other regulated secretory proteins are known to be sorted into large dense-core vesicles of the regulated pathway in the trans-Golgi network and are secreted upon stimulus-induced increases in intracellular Ca(2+). The newly synthesized cell surface receptors are usually sorted into microvesicles of the constitutive pathway and inserted into the plasma membrane by spontaneous exocytosis. Small-diameter sensory neurons in dorsal root ganglia and pheochromocytoma cells express neuropeptides (e.g., substance P) and several neuropeptide receptors including opioid receptors. The mu-opioid receptors are delivered to the cell surface through the constitutive pathway, whereas another type of opioid receptor, the delta-opioid receptor, is often found in the membrane of large dense-core vesicles and can be inserted into the plasma membrane when exocytosis occurs. Recent studies show that sequences with opposite electrical polarity within the prohormones of substance P are essential for their sorting into large dense-core vesicles. Moreover, the delta-opioid receptor is sorted into large dense-core vesicles by its interaction with protachykinin, a prohormone of substance P. These findings provide insight into the molecular mechanisms that determine the sorting and trafficking of neuropeptides and neuropeptide receptors in neurons.

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.

Short elements with charged amino acids form clusters to sort protachykinin into large dense-core vesicles

The sorting of neuropeptide tachykinins into large dense-core vesicles (LDCVs) is a key step in their regulated secretion from neurons. However, the sorting mechanism for protachykinin has not yet to be clearly resolved. In this study, we report that the clustered short elements with charged amino acids regulate the efficiency of protachykinin sorting into LDCVs. A truncation experiment showed that the propeptide and the mature peptide-containing sequence of protachykinin were sorted into LDCVs. These two regions exhibit a polarized distribution of charged amino acids. The LDCV localization of the propeptide was gradually decreased with an increasing number of neutral amino acids. Furthermore, the short element with four to five amino acids containing two charged residues was found to be a basic unit for LDCV sorting that enables regulated secretion. In the native propeptide sequence, these charged short elements were clustered to enhance the intermolecular aggregation by electrostatic interaction and produce a gradual and additive effect on LDCV sorting. The optimal conditions for intermolecular aggregation of protachykinin were at millimolar Ca(2+) concentrations and pH 5.5-6.0. These results demonstrate that the charged short elements are clustered such that they serve as aggregative signals and regulate the efficiency of protachykinin sorting into LDCVs. These findings reveal a novel mechanism for the sorting of neuropeptides into a regulated secretory pathway.

7B2 prevents unfolding and aggregation of prohormone convertase 2

Prohormone convertase 2 (PC2) requires interaction with the neuroendocrine protein 7B2 for the production of an activatable zymogen; the mechanism for this effect is unknown. 7B2 could act proactively to generate an activation-competent form of pro-PC2 during synthesis, or block spontaneous generation of activation-incompetent forms. We here demonstrate that addition of exogenous recombinant 7B2 to CHO cells expressing pro-PC2 prevented the unfolding and aggregation of secreted PC2 forms in a dose-dependent manner, as assessed by aggregation assays, activity assays, cross-linking experiments, and sucrose density gradients. Intracellular pro-PC2 was also found to exist in part as higher-order oligomers that were reduced in the presence of coexpressed 7B2. 7B2 addition did not result in the acquisition of enzymatic competence unless added before or very rapidly after pro-PC2 secretion, indicating that an activation-competent structure cannot be maintained in the absence of 7B2. Velocity sedimentation experiments showed that addition of extracellular 7B2 solubilized three different PC2 species from a precipitable aggregate: two activatable pro-PC2 species, the intact zymogen and a zymogen with a partially cleaved propeptide, and an inactive 66-kDa form. Our results suggest that 7B2 possesses chaperone activity that blocks partially unfolded pro-PC2 forms from losing catalytic competence and then aggregating. The loss of the catalytically competent conformer appears to represent the earliest indicator of pro-PC2 unfolding and is followed on a slower time scale by the appearance of aggregates. Because 7B2 expression is not confined to areas expressing pro-PC2, 7B2 may represent a general intracellular and extracellular secretory chaperone.

How peptide hormone vesicles are transported to the secretion site for exocytosis

Post-Golgi transport of peptide hormone-containing vesicles from the site of genesis at the trans-Golgi network to the release site at the plasma membrane is essential for activity-dependent hormone secretion to mediate various endocrinological functions. It is known that these vesicles are transported on microtubules to the proximity of the release site, and they are then loaded onto an actin/myosin system for distal transport through the actin cortex to just below the plasma membrane. The vesicles are then tethered to the plasma membrane, and a subpopulation of them are docked and primed to become the readily releasable pool. Cytoplasmic tails of vesicular transmembrane proteins, as well as many cytosolic proteins including adaptor proteins, motor proteins, and guanosine triphosphatases, are involved in vesicle budding, the anchoring of the vesicles, and the facilitation of movement along the transport systems. In addition, a set of cytosolic proteins is also necessary for tethering/docking of the vesicles to the plasma membrane. Many of these proteins have been identified from different types of (neuro)endocrine cells. Here, we summarize the proteins known to be involved in the mechanisms of sorting various cargo proteins into regulated secretory pathway hormone-containing vesicles, movement of these vesicles along microtubules and actin filaments, and their eventual tethering/docking to the plasma membrane for hormone secretion.

A large form of secretogranin III functions as a sorting receptor for chromogranin A aggregates in PC12 cells

Granin-family proteins, including chromogranin A and secretogranin III, are sorted to the secretory granules in neuroendocrine cells. We previously demonstrated that secretogranin III binds chromogranin A and targets it to the secretory granules in pituitary corticotrope-derived AtT-20 cells. However, secretogranin III has not been identified in adrenal chromaffin and PC12 cells, where chromogranin A is correctly sorted to the secretory granules. In this study, low levels of a large and noncleaved secretogranin III have been identified in PC12 cells and rat adrenal glands. Although the secretogranin III expression was limited in PC12 cells, when the FLAG-tagged secretogranin III lacking the secretory granule membrane-binding domain was expressed excessively, hemagglutinin-tagged chromogranin A was unable to target to the secretory granules at the tips and shifted to the constitutive secretory pathway. Secretogranin III was able to bind the aggregated form of chromogranin A, suggesting that a small quantity of secretogranin III is enough to carry a large quantity of chromogranin A. Furthermore, secretogranin III bound adrenomedullin, a major peptide hormone in chromaffin cells. Indeed, small interfering RNA-directed secretogranin III depletion impaired intracellular retention of chromogranin A and adrenomedullin, suggesting that they are constitutively released to the medium. We suggest that the sorting function of secretogranin III for chromogranin A is common in PC12 and chromaffin cells as well as in other endocrine cells, and a small amount of secretogranin III is able to sort chromogranin A aggregates together with adrenomedullin to secretory granules.

Electrical Stimulation-Induced Release of β-Endorphin from Genetically Modified Neuro-2a Cells

The quantity of therapeutic gene products released from genetically engineered cells can be controlled externally at different levels. The widely used approach of controlling expression, however, generally has the disadvantage that chemical substances must be applied for stimulation. An alternative strategy aims at controlling gene products at posttranslational levels such as secretion. The secretion of a therapeutic agent can be regulated if the agent is targeted to the regulated secretory pathway and stored in the secretory granules until its release. In this article we address the question of whether the release of β-endorphin, an opioid with a potent analgesic effect, could be induced by electrically stimulating stably transfected Neuro-2a cells. Throughout this study we used the human proopiomelanocortin (POMC) gene, which is the precursor molecule for human β-endorphin. We analyzed its subcellular localization and found it in the regulated secretory pathway in Neuro-2a cells. Using electrical field stimulation we were able to identify a stimulation pattern that significantly increased the release of β-endorphin-immunoreactive material, although to a limited extent. This result indicates that electrical stimulation of secretion could be used to manipulate the amount of a therapeutic agent released from transplanted cells.

A transient forward-targeting element for microneme-regulated secretion in Toxoplasma gondii

BACKGROUND INFORMATION

Accurate sorting of proteins to the three types of secretory granules in Toxoplasma gondii is crucial for successful cell invasion by this obligate intracellular parasite. As in other eukaryotic systems, propeptide sequences are a common yet poorly understood feature of proteins destined for regulated secretion, which for Toxoplasma occurs through two distinct invasion organelles, rhoptries and micronemes. Microneme discharge during parasite apical attachment plays a pivotal role in cell invasion by delivering adhesive proteins for host receptor engagement.

RESULTS

We show here that the small micronemal proprotein MIC5 (microneme protein-5) undergoes proteolytic maturation at a site beyond the Golgi, and only the processed form of MIC5 is secreted via the micronemes. Proper cleavage of the MIC5 propeptide relies on an arginine residue in the P1' position, although P1' mutants are still cleaved to a lesser extent at an alternative site downstream of the primary site. Nonetheless, this aberrantly cleaved species still correctly traffics to the micronemes, indicating that correct cleavage is not necessary for micronemal targeting. In contrast, a deletion mutant lacking the propeptide was retained within the secretory system, principally in the ER (endoplasmic reticulum). The MIC5 propeptide also supported correct trafficking when exchanged for the M2AP propeptide, which was recently shown to also be required for micronemal trafficking of the TgMIC2 (T. gondii MIC2)-M2AP complex [Harper, Huynh, Coppens, Parussini, Moreno and Carruthers (2006) Mol. Biol. Cell 17, 4551-4563].

CONCLUSION

Our results illuminate common and unique features of micronemal propeptides in their role as trafficking facilitators.

A carboxyl-terminal sequence in the lutropin beta subunit contributes to the sorting of lutropin to the regulated pathway

Although synthesized in the same pituitary gonadotropes, the secretion profiles of lutropin (LH) and follitropin (FSH) differ. LH is secreted through a regulated pathway and associated with a bolus release at mid-estrous cycle. In contrast, the majority of FSH is secreted constitutively with an incremental increase until ovulation. Both share an identicalalpha subunit, and thus thebeta subunit contains determinants for sorting into the regulated pathway. Previously, we demonstrated that a hydrophobic carboxyl-terminal heptapeptide of the LHbeta subunit (Leu-Ser-Gly-Leu-Leu-Phe-Leu), not found in the FSHbeta subunit, influences the intracellular behavior of the LH dimer. To test the hypothesis that the peptide contributes to differential sorting, we monitored the fates of LH and LHDeltaT (LHbeta subunit lacking the carboxyl-terminal seven amino acids) dimers in the rat somatotrope-derived GH(3) cell line in which both the regulated and constitutive secretory pathways operate. Pulse-chase labeling demonstrated that the LHDeltaT dimer was diverted to the constitutive pathway, resulting in a significant decrease in the corresponding intracellular pool. Forskolin stimulated LH dimer release 3-fold, which was accompanied by a parallel decrease of intracellular LH; only marginal forskolin stimulation of LHDeltaT was seen. Immunofluorescence after cycloheximide treatment demonstrated decreased retention of LHDeltaT compared with LH, consistent with increased constitutive secretion of LHDeltaT. We also demonstrated that fusing the heptapeptide to the carboxyl terminus of the FSHbeta subunit resulted in an increased regulated secretion of this FSH analog compared with wild-type FSH. These data are the first to identify a novel structural determinant responsible for the sorting of a member of the glycoprotein hormone family into the regulated secretory pathway.

PC1/3, PC2 and PC5/6A are targeted to dense core secretory granules by a common mechanism

There are seven members of the proprotein convertase (PC) family of secreted serine proteases that cleave their substrates at basic amino acids, thereby activating a variety of hormones, growth factors, and viruses. PC1/3, PC2 and PC5/6A are the only members of the PC family that are targeted to dense core secretory granules, where they carry out the processing of proteins that are secreted from the cell in a regulated manner. Previous studies have identified alpha-helices in the C-termini of the PC1/3 and PC2 proteases that are required for this subcellular targeting. In the current study, we demonstrate that a predicted alpha-helix in the C-terminus of PC5/6A is also critical for the ability of this domain to target a heterologous protein to the regulated secretory pathway of mouse endocrine AtT-20 cells. Analysis of the subcellular distribution of fusion proteins containing the C-terminal domains of PC1/3, PC2 and PC5/6A confirmed that all three domains have the capacity to redirect a constitutively secreted protein to the granule-containing cytoplasmic extensions. Analysis of the predicted structures formed by these three granule-sorting helices shows a correlation between their granule-sorting efficiency and the clustering of hydrophobic amino acids in their granule-targeting helices.

Sending proteins to dense core secretory granules: still a lot to sort out

The intracellular sorting of peptide hormone precursors to the dense core secretory granules (DCSGs) is essential for their bioactivation. Despite the fundamental importance of this cellular process, the nature of the sorting signals for entry of proteins into DCSGs remains a source of vigorous debate. This review highlights recent discoveries that are consistent with a model in which several protein domains, acting in a cell-specific fashion and at different steps in the sorting process, act in concert to regulate the entry of proteins into DCSGs.

A hydrophobic patch in a charged alpha-helix is sufficient to target proteins to dense core secretory granules

Many endocrine and neuroendocrine cells contain specialized secretory organelles called dense core secretory granules. These organelles are the repository of proteins and peptides that are secreted in a regulated manner when the cell receives a physiological stimulus. The targeting of proteins to these secretory granules is crucial for the generation of certain peptide hormones, including insulin and ACTH. Although previous work has demonstrated that proteins destined to a variety of cellular locations, including secretory granules, contain targeting sequences, no single consensus sequence for secretory granule-sorting signals has emerged. We have shown previously that alpha-helical domains in the C-terminal tail of the prohormone convertase PC1/3 play an important role in the ability of this region of the protein to direct secretory granule targeting (Jutras, I. Seidah, N. G., and Reudelhuber, T. L. (2000) J. Biol. Chem. 275, 40337-40343). In this study, we show that a variety of alpha-helical domains are capable of directing a heterologous secretory protein to granules. By testing a series of synthetic alpha-helices, we also demonstrate that the presence of charged (either positive or negative) amino acids spatially segregated from a hydrophobic patch in the alpha-helices of secretory proteins likely plays a critical role in the ability of these structures to direct secretory granule sorting.

Molecular probes for sensing the cholesterol composition of subcellular organelle membranes

Neuroendocrine cells contain two types of secretagogue-regulated acidic compartments: secretory granules (SGs) and synaptic-like microvesicles (SLMVs), which can be identified by acidotropic probes such as acridine orange (AO) and DAMP. We investigated the accumulation of these probes in SGs and SLMVs as a function of glucose levels in the culture media using a pancreatic beta-cell line MIN6. AO was accumulated in the low-glucose condition, but not in the high-glucose condition. The AO accumulation correlated well with the SLMV dynamics by glucose and DAMP was localized in the SGs. Because SG membranes are reportedly high in cholesterol, we prepared liposomes with increasing cholesterol levels. AO is well incorporated into liposomes having a 20 to 40 mol% cholesterol composition, whereas DAMP was so in those having over 40 mol% cholesterol levels. Indeed, when cholesterol was depleted from MIN6 SG membranes, DAMP incorporation decreased, instead AO was incorporated. In PC12 cells, AO incorporation into SGs was significant but DAMP incorporation was limited. Consistently, the cholesterol composition was found 37 to 39 mol% in the SG membrane of PC12 cells. We suggest that cholesterol-sensing probes, AO and DAMP, are useful tools for investigating cholesterol compositions in acidic organelle membranes.

Modulation of Secretory Granule-targeting Efficiency by Cis and Trans Compounding of Sorting Signals

Several protein domains acting through seemingly different mechanisms have been reported to have the capacity to target proteins to dense core secretory granules. Because proteins enter secretory granules with different efficiencies and because some of these proteins contain more than one granule-targeting motif, we have investigated whether compounding sorting signals could alter the efficiency of protein entry into secretory granules. In the current study we demonstrate that a paired basic cleavage site from human prorenin and an alpha-helix-containing secretory granule-sorting signal from the prohormone convertase PC1/3 can synergize to increase granule-sorting efficiency not only when located on the same protein, but also when located on distinct proteins that associate in the secretory pathway.