Efficient targeting to storage granules of human proinsulins with altered propeptide domain

Secretory protein trafficking: genetic and biochemical analysis

Acute insulin secretion from stimulated pancreatic beta-cells is derived from the intracellular pool of insulin secretory granules wherein insulin is packaged in a highly concentrated (and in some species, crystalline) state. Here we review experimental work, principally from our laboratory, on the question of biogenesis of mature secretory granules within the broader context of intracellular protein trafficking. Events occurring in the lumen of organelles at various stages of intracellular transport within the secretory pathway and events at the limiting membrane of newly forming secretory granules each contribute to formation of the insulin storage compartment comprising the readily releasable pool.

Origins of the regulated secretory pathway

Modes of transport of soluble (or luminal) secretory proteins synthesized in the endoplasmic reticulum (ER) could be divided into two groups. The socalled constitutive secretory pathway (CSP) is common to all eukaryotic cells, constantly delivering constitutive soluble secretory proteins (CSSPs) linked to the rate of protein synthesis but largely independent of external stimuli. In regulated secretion, protein is sorted from the Golgi into storage/secretory granules (SGs) whose contents are released when stimuli trigger their final fusion with the plasma membrane (Hannah et al. 1999).

Sorting ourselves out: seeking consensus on trafficking in the beta-cell

Biogenesis of the regulated secretory pathway in the pancreatic beta-cell involves packaging of products, notably proinsulin, into immature secretory granules derived from the trans-Golgi network. Proinsulin is converted to insulin and C-peptide as granules mature. Secretory proteins not entering granules are conveyed by transport intermediates directly to the plasma membrane for constitutive secretion. One of the co-authors, Peter Arvan, has proposed that in addition, small vesicles bud from granules to traffic to the endosomal system. From there, some proteins are secreted by a (post-granular) constitutive-like pathway. He argues that retention in granules is facilitated by condensation, rendering soluble products (notably C-peptide and proinsulin) more available for constitutive-like secretion. Thus he argues that prohormone conversion is potentially important in secretory granule biogenesis. The other co-author, Philippe Halban, argues that the post-granular secretory pathway is not of physiological relevance in primary beta-cells, and contests the importance of proinsulin conversion for retention in granules. Both, however, agree that trafficking from granules to endosomes is important, purging granules of unwanted newly synthesized proteins and allowing their traffic to other destinations. In this Traffic Interchange, the two co-authors attempt to reconcile their differences, leading to a common vision of proinsulin trafficking in primary and transformed cells.

Disruption of a receptor-mediated mechanism for intracellular sorting of proinsulin in familial hyperproinsulinemia

In familial hyperproinsulinemia, specific mutations in the proinsulin gene are linked with a profound increase in circulating plasma proinsulin levels. However, the molecular and cellular basis for this disease remains uncharacterized. Here we investigated how these mutations may disrupt the sorting signal required to target proinsulin to the secretory granules of the regulated secretory pathway, resulting in the unregulated release of proinsulin. Using a combination of molecular modeling and site-directed mutagenesis, we have identified structural molecular motifs in proinsulin that are necessary for correct sorting into secretory granules of endocrine cells. We show that membrane carboxypeptidase E (CPE), previously identified as a prohormone-sorting receptor, is essential for proinsulin sorting. This was demonstrated through short interfering RNA-mediated depletion of CPE and transfection with a dominant negative mutant of CPE in a beta-cell line. Mutant proinsulins found in familial hyperproinsulinemia failed to bind to CPE and were not sorted efficiently. These findings provide evidence that the elevation of plasma proinsulin levels found in patients with familial hyperproinsulinemia is caused by the disruption of CPE-mediated sorting of mutant proinsulins to the regulated secretory pathway.

Role of the connecting peptide in insulin biosynthesis

In single-chain insulins (SCIs), the C terminus of the insulin B-chain is contiguous with the N terminus of the A-chain, connected by a short bioengineered linker sequence. SCIs have been proposed to offer potential benefit for gene therapy of diabetes (Lee, H. C., Kim, S. J., Kim, K. S., Shin, H. C., and Yoon, J. W. (2000) Nature 408, 483-488) yet relatively little is known about their folding, intracellular transport, or secretion from mammalian cells. Because SCIs can be considered as mutant proinsulin (with selective shortening of the 35-amino acid connecting peptide that normally includes two sets of flanking dibasic residues), they offer insights into understanding the role of the connecting peptide in insulin biosynthesis. Herein we have explored the relationship of the linker sequence to SCI biosynthesis, folding, and intracellular transport in transiently transfected HEK293 or Chinese hamster ovary cells or in stably transfected AtT20 cells. Despite previous reports that direct linkage of B- and A-chains produces a structure isomorphous with authentic two-chain insulin, we find that constructs with short linkers tend to be synthesized at lower levels, with a significant fraction of molecules exhibiting improper disulfide bonding. Nevertheless, disulfide-mispaired isoforms from a number of different SCI constructs are secreted. While this suggests that a novel folded state goes unrecognized by secretory pathway quality control, we find that misfolded SCIs are detected at higher levels in Chinese hamster ovary cells with artificially activated unfolded protein response mediated by inducible overexpression of active ATF-6. Such a maneuver allows analysis of more seriously misfolded mutants with further foreshortening of the linker sequence or loss (by mutation) of the insulin interchain disulfide bonds.

Proinsulin endoproteolysis confers enhanced targeting of processed insulin to the regulated secretory pathway

Recently, two different prohormone-processing enzymes, prohormone convertase 1 (PC1) and carboxypeptidase E, have been implicated in enhancing the storage of peptide hormones in endocrine secretory granules. It is important to know the extent to which such molecules may act as "sorting receptors" to allow the selective trafficking of cargo proteins from the trans-Golgi network into forming granules, versus acting as enzymes that may indirectly facilitate intraluminal storage of processed hormones within maturing granules. GH4C1 cells primarily store prolactin in granules; they lack PC1 and are defective for intragranular storage of transfected proinsulin. However, proinsulin readily enters the immature granules of these cells. Interestingly, GH4C1 clones that stably express modest levels of PC1 store more proinsulin-derived protein in granules. Even in the presence of PC1, a sizable portion of the proinsulin that enters granules goes unprocessed, and this portion largely escapes granule storage. Indeed, all of the increased granule storage can be accounted for by the modest portion converted to insulin. These results are not unique to GH4C1 cells; similar results are obtained upon PC1 expression in PC12 cells as well as in AtT20 cells (in which PC1 is expressed endogenously at higher levels). An in vitro assay of protein solubility indicates a difference in the biophysical behavior of proinsulin and insulin in the PC1 transfectants. We conclude that processing to insulin, facilitated by the catalytic activities of granule proteolytic enzymes, assists in the targeting (storage) of the hormone.

The role of assembly in insulin's biosynthesis

Insulin is synthesised as a single-chain precursor, preproinsulin, that contains an N-terminal signal sequence and a connecting peptide linking the A and B chains of the insulin molecule. Nascent proinsulin is directed into the regulated secretory pathway, converted to insulin and stored as microcrystals. These processes exploit assembly to the zinc-containing hexamer. Structural, chemical and genetic studies, and experiments with transgenic animals and transfected cells are providing new details about the molecular events in insulin's biosynthesis.

Essential role of the disulfide-bonded loop of chromogranin B for sorting to secretory granules is revealed by expression of a deletion mutant in the absence of endogenous granin synthesis

Sorting of regulated secretory proteins in the TGN to immature secretory granules (ISG) is thought to involve at least two steps: their selective aggregation and their interaction with membrane components destined to ISG. Here, we have investigated the sorting of chromogranin B (CgB), a member of the granin family present in the secretory granules of many endocrine cells and neurons. Specifically, we have studied the role of a candidate structural motif implicated in the sorting of CgB, the highly conserved NH2-terminal disulfide- bonded loop. Sorting to ISG of full-length human CgB and a deletion mutant of human CgB (Deltacys-hCgB) lacking the 22-amino acid residues comprising the disulfide-bonded loop was compared in the rat neuroendocrine cell line PC12. Upon transfection, i.e., with ongoing synthesis of endogenous granins, the sorting of the deletion mutant was only slightly impaired compared to full-length CgB. To investigate whether this sorting was due to coaggregation of the deletion mutant with endogenous granins, we expressed human CgB using recombinant vaccinia viruses, under conditions in which the synthesis of endogenous granins in the infected PC12 cells was shut off. In these conditions, Deltacys-hCgB, in contrast to full-length hCgB, was no longer sorted to ISG, but exited from the TGN in constitutive secretory vesicles. Coexpression of full-length hCgB together with Deltacys-hCgB by double infection, using the respective recombinant vaccinia viruses, rescued the sorting of the deletion mutant to ISG. In conclusion, our data show that (a) the disulfide-bonded loop is essential for sorting of CgB to ISG and (b) the lack of this structural motif can be compensated by coexpression of loop-bearing CgB. Furthermore, comparison of the two expression systems, transfection and vaccinia virus-mediated expression, reveals that analyses under conditions in which host cell secretory protein synthesis is blocked greatly facilitate the identification of sequence motifs required for sorting of regulated secretory proteins to secretory granules.

Targeting of leptin to the regulated secretory pathway in pituitary AtT-20 cells

Leptin, a key regulator of fat homeostasis, is the product of the obese gene [1-3], and is secreted from adipocytes and binds to receptor sites in the choroid plexus [4-5]. Several studies have implicated serum insulin levels in the upregulation of leptin gene expression [6-8]. It is currently not known whether leptin levels are also subject to regulation at the level of secretion. Leptin is normally produced in adipocytes, the secretory pathways of which are not well characterized. Here, we used pituitary AtT-20 cells, which serve as a model system for both regulated and constitutive secretory pathways, to examine the intracellular targeting and secretion of leptin. Confocal immunofluorescence analysis of AtT-20 cells expressing an epitope-tagged human leptin (FLAG-leptin) demonstrated that FLAG-leptin colocalized with endogenous adrenocorticotrophic hormone (ACTH) at the tips of processes extended from these cells, where regulated secretory granules accumulate. FLAG-leptin secretion was increased in the presence of 8-Br-cAMP, which stimulates the secretion of ACTH. For FLAG-leptin, the calculated sorting index, a quantitative measure of the efficiency of protein sorting to the regulated pathway, was similar to those of other regulated secretory proteins. These results demonstrate that FLAG-leptin behaves like a regulated protein in cells with a biosynthetic regulated secretory pathway.

PACE4: a subtilisin-like endoprotease with unique properties

PACE4 is one of the neuroendocrine-specific mammalian subtilisin-related endoproteases believed to function in the secretory pathway. The biosynthesis and secretion of PACE4 have been studied using transfected neuroendocrine and fibroblast cell lines. as well as primary pituitary cultures. ProPACE4 (approx. 106 kDa) is cleaved intracellularly before secretion of PACE4 (approx. 97 kDa); the N-terminal propeptide cleavage is accelerated in a truncated form of PACE4 lacking the Cys-rich C-terminal region (PACE4s). Neither PACE4 nor PACE4s is stored in regulated neuroendocrine secretory granules, whereas pro-opiomelanocortin-derived peptides and prohormone convertase I enter the regulated secretory pathway efficiently. The relatively slow cleavage of the proregion of proPACE4 in primary anterior pituitary cells, followed by rapid secretion of PACE4, is similar to the results for proPACE4 in transfected cell lines. The enzyme activity of PACE4 is distinct from furin and prohormone convertases, both in the marked sensitivity of PACE4 to inhibition by leupeptin and the relative insensitivity of PACE4 to inhibition by Ca2+ chelators and dithiothreitol; PACE4 is not inhibited by the alpha1-antitrypsin Portland variant that is very potent at inhibiting furin. The unique biosynthetic and enzymic patterns seen for PACE4 suggest a role for this neuroendocrine-specific subtilisin-like endoprotease outside the pathway for peptide biosynthesis.

Selective localization and regulated release of calcitonin gene-related peptide from dense-core vesicles in engineered PC12 cells

Introduction of the gene for calcitonin into the neuroendocrine PC12 cell line resulted in the expression of the neuronal-specific splice product, calcitonin gene-related peptide (CGRP). Expression of this neuropeptide did not require treatment of the PC12 cells with NGF. By all available criteria, including biochemical, immunological, and morphological analysis, we have determined that the CGRP in stably transfected PC12 cells is sorted selectively into the large, dense-core catecholamine-containing secretory vesicles. Conversely, the CGRP is excluded from the small, synaptophysin-rich vesicles present in the same cells. Stimulation conditions that trigger the release of catecholamines cause a parallel burst in the release of CGRP. In all these respects, the engineered PC12 cells process the foreign CGRP in a manner similar to that seen in spinal motor neurons in vivo. These results indicate that this small (37 amino acids) peptide contains sorting information sufficient for targeting to large, dense-core vesicles in heterologous cells, placing very narrow constraints on the possible location of sorting signals. In addition, this CGRP-expressing cell line opens the possibility of studying the physiological role of CGRP in the establishment and maintenance of neuromuscular contacts.

Peptidergic transmission: from morphological correlates to functional implications

Like non-peptidergic transmitters, neuropeptides and their receptors display a wide distribution in specific cell types of the nervous system. The peptides are synthesized, typically as part of a larger precursor molecule, on the rough endoplasmic reticulum in the cell body. In the trans-Golgi network, they are sorted to the regulated secretory pathway, packaged into so-called large dense-core vesicles, and concentrated. Large dense-core vesicles are preferentially located at sites distant from active zones of synapses. Exocytosis may occur not only at synaptic specializations in axonal terminals but frequently also at nonsynaptic release sites throughout the neuron. Large dense-core vesicles are distinguished from small, clear synaptic vesicles, which contain "classical' transmitters, by their morphological appearance and, partially, their biochemical composition, the mode of stimulation required for release, the type of calcium channels involved in the exocytotic process, and the time course of recovery after stimulation. The frequently observed "diffuse' release of neuropeptides and their occurrence also in areas distant to release sites is paralleled by the existence of pronounced peptide-peptide receptor mismatches found at the light microscopic and ultrastructural level. Coexistence of neuropeptides with other peptidergic and non-peptidergic substances within the same neuron or even within the same vesicle has been established for numerous neuronal systems. In addition to exerting excitatory and inhibitory transmitter-like effects and modulating the release of other neuroactive substances in the nervous system, several neuropeptides are involved in the regulation of neuronal development.

Processing of mutated human proinsulin to mature insulin in the non-endocrine cell line, CHO

Heterologous genes encoding proproteins, including proinsulin, generally produce mature protein when expressed in endocrine cells while unprocessed or partially processed protein is produced in non-endocrine cells. Proproteins, which are normally processed in the regulated pathway restricted to endocrine cells, do not always contain the recognition sequence for cleavage by furin, the endoprotease specific to the constitutive pathway, the principal protein processing pathway in non-endocrine cells. Human proinsulin consists of B-Chain-C-peptide-A-Chain and cleavage at the B/C and C/A junctions is required for processing. The B/C, but not the C/A junction, is recognised and cleaved in the constitute pathway. We expressed a human proinsulin and a mutated proinsulin gene with an engineered furin recognition sequence at the C/A junction and compared the processing efficiency of the mutant and native proinsulin in Chinese Hamster Ovary cells. The processing efficiency of the mutant proinsulin was 56% relative to 0.7% for native proinsulin. However, despite similar levels of mRNA being expressed in both cell lines, the absolute levels of immunoreactive insulin, normalized against mRNA levels, were 18-fold lower in the mutant proinsulin-expressing cells. As a result, there was only a marginal increase in absolute levels of insulin produced by these cells. This unexpected finding may result from preferential degradation of insulin in non-endocrine cells which lack the protection offered by the secretory granules found in endocrine cells.

Insulin secretory granule biogenesis and the proinsulin-processing endopeptidases

The insulin storage granule of the pancreatic beta cell is assembled within the trans Golgi network from around 50 or so gene products many of which are synthesized coordinately with the major component, proinsulin. An important contribution to our understanding of the regulation of this process has come from studies of the post-translational processing of proinsulin and of other proteins which are stored in the granule, particularly the processing enzymes themselves. The present review focusses on recent insights into the molecular nature of the processing machinery, and the granule Ca(2+)-dependent subtilisin-related endopeptidases which catalyse the initial rate-limiting step in the enzymic conversion of proinsulin.

Sorting and processing of secretory proteins

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Differential trafficking of soluble and integral membrane secretory granule-associated proteins

The posttranslational processing enzyme peptidylglycine alpha-amidating monooxygenase (PAM) occurs naturally in integral membrane and soluble forms. With the goal of understanding the targeting of these proteins to secretory granules, we have compared the maturation, processing, secretion, and storage of PAM proteins in stably transfected AtT-20 cells. Integral membrane and soluble PAM proteins exit the ER and reach the Golgi apparatus with similar kinetics. Biosynthetic labeling experiments demonstrated that soluble PAM proteins were endoproteolytically processed to a greater extent than integral membrane PAM; this processing occurred in the regulated secretory pathway and was blocked by incubation of cells at 20 degrees C. 16 h after a biosynthetic pulse, a larger proportion of soluble PAM proteins remained cell-associated compared with integral membrane PAM, suggesting that soluble PAM proteins were more efficiently targeted to storage granules. The nonstimulated secretion of soluble PAM proteins peaked 1-2 h after a biosynthetic pulse, suggesting that release was from vesicles which bud from immature granules during the maturation process. In contrast, soluble PAM proteins derived through endoproteolytic cleavage of integral membrane PAM were secreted in highest amount during later times of chase. Furthermore, immunoprecipitation of cell surface-associated integral membrane PAM demonstrated that very little integral membrane PAM reached the cell surface during early times of chase. However, when a truncated PAM protein lacking the cytoplasmic tail was expressed in AtT-20 cells, > 50% of the truncated PAM-1 protein reached the cell surface within 3 h. We conclude that the trafficking of integral membrane and soluble secretory granule-associated enzymes differs, and that integral membrane PAM proteins are less efficiently retained in maturing secretory granules.

Targeting of pro-opiomelanocortin to the regulated secretory pathway may involve cooperation between different protein domains

The structure of pro-opiomelanocortin (POMC) can be divided into three main domains: an NH2-terminal domain formed by the NH2-terminal glycopeptide and the joining peptide, a central domain corresponding to the adrenocorticotropin sequences and a COOH-terminal domain containing the beta-lipotropin sequences. Expression of POMC in neuroendocrine cell lines such as the mouse neuroblastoma Neuro2A cells results in its targeting to the regulated secretory pathway of these cells. Intracellular targeting of proteins along non default pathways are widely believed to involve the recognition of specific structural features by a sorting machinery. To understand the nature of the signal involved in targeting prohormone to the regulated secretory pathway, we have constructed mutants of POMC in which sequences from the NH2-terminal, the central and the COOH-terminal domains were deleted and examined the sorting of these mutant POMC molecules in Neuro2A cells by immunofluorescence and immunoelectron microscopy. Our results indicate that POMC NH2-terminal glycopeptide or beta-LPH domain do not contain sufficient information for targeting to the regulated pathway since these peptides are not sorted to secretory vesicles when expressed in Neuro2A cells: Similarly, the ACTH domain does not contain essential targeting information since POMC mutants lacking these sequences were sorted to secretory vesicles. Mutant POMCs containing the sequences of more than one of the main protein domains were, however, correctly targeted to the regulated secretory pathway. Our results indicate that POMC is not targeted to the regulated secretory pathway through recognition of a unique continuous 'molecular address'.(ABSTRACT TRUNCATED AT 250 WORDS)

Secretory protein traffic. Chromogranin A contains a dominant targeting signal for the regulated pathway

Secretory proteins are targeted into either constitutive (secreted upon synthesis) or regulated (stored in vesicles and released in response to a secretagogue) pathways. To investigate mechanisms of protein targeting into catecholamine storage vesicles (CSV), we stably expressed human chromogranin A (CgA), the major soluble protein in human CSV, in the rat pheochromocytoma PC-12 cell line. Chromaffin cell secretagogues (0.1 mM nicotinic cholinergic agonist, 55 mM K+, or 2 mM Ba++) caused cosecretion of human CgA and catecholamines from human CgA-expressing cells. Sucrose gradients colocalized human CgA and catecholamines to subcellular particles of the same buoyant density. Chimeric proteins, in which human CgA (either full-length [457 amino acids] or truncated [amino-terminal 226 amino acids]) was fused in-frame to the ordinarily nonsecreted protein chloramphenicol acetyltransferase (CAT), were expressed transiently in PC-12 cells. Both constructs directed CAT activity into regulated secretory vesicles, as judged by secretagogue-stimulated release. These data demonstrate that human CgA expressed in PC-12 cells is targeted to regulated secretory vesicles. In addition, human CgA can divert an ordinarily non-secreted protein into the regulated secretory pathway, consistent with the operation of a dominant targeting signal for the regulated pathway within the peptide sequence of CgA.

Expression and localization of two low molecular weight GTP-binding proteins, Rab8 and Rab10, by epitope tag

Small GTP-binding proteins of the YPT/SEC4/Rab family have been shown to play an essential role in intracellular membrane trafficking. In mammals, Rab8 and Rab10 are the two small GTP-binding proteins identified so far that are closest to SEC4, an essential gene product involved in post-Golgi constitutive secretion in the yeast Saccharomyces cerevisiae. To study the localization of Rab proteins, we have expressed the cDNAs with an influenza virus hemagglutinin (HA) epitope tag at the N terminus. The feasibility of this method was tested by using yeast SEC4. HA-tagged SEC4 functionally complemented a temperature-sensitive sec4 mutant similarly to wild-type SEC4, indicating that the modified protein retained functional integrity. Monoclonal antibody 12CA5, raised against the HA tag, was used to determine the expression and localization of HA-tagged proteins after transfection. In stably transfected CHO and Swiss 3T3 cells, HA-tagged Rab8 was localized to the cell periphery, with the highest concentration in the ruffling areas. In contrast, epitope-tagged Rab10 expressed in CHO and BHK cells was concentrated on membranes in the perinuclear region. By light microscopy, the staining partially overlapped with that of a Golgi marker, beta-COP. Thus, despite the high degree homology of Rab8 and Rab10 (66% identity), the two proteins are localized to distinct cellular compartments. This approach should provide a general tool for the analyses of other members of the YPT/SEC4/rab gene family.

Protein targeting to dense-core secretory granules

Regulated secretory proteins are stored within specialized vesicles known as secretory granules. It is not known how proteins are sorted into these organelles. Regulated proteins may possess targeting signals which interact with specific sorting receptors in the lumen of the trans-Golgi network (TGN) prior to their aggregation to form the characteristic dense-core of the granule. Alternatively, sorting may occur as the result of specific aggregation of regulated proteins in the TGN. Aggregates may be directed to secretory granules by interaction of a targeting signal on the surface with a sorting receptor. Novel targeting signals which confer on regulated proteins a tendency to aggregate under certain conditions, and in so doing cause them to be incorporated into secretory granules, have been implicated. Specific targeting signals may also play a role in directing membrane proteins to secretory granules.

Deletions of the synenkephalin domain which do not alter cell-specific proteolytic processing or secretory targeting of human proenkephalin

To identify signals that direct the proteolytic processing and regulated secretion of human proenkephalin (hPE), we have transfected the hPE gene or minigene constructs into pituitary tumor cells, either rat GH4Cl cells or mouse AtT-20 cells. Cells transfected with either the hPE gene or minigene contained similar levels of methionine-enkephalin (ME)-containing peptides and hPE mRNA. In the GH4Cl clones, ME was present predominantly in high-molecular-mass forms (5-25 kDa). In contrast, the AtT-20 clones contained almost exclusively free ME and low-molecular-mass forms (< 5 kDa), with very little high-molecular-mass species present. Thus, among pituitary cells, corticotroph-derived cells appear better equipped to process hPE than lactotroph-derived cells. Despite limited proteolytic processing, GH4Cl clones secreted large amounts of unprocessed (> 20 kDa) hPE into the medium, making up to 10% of endogenous rat prolactin secretion. Both precursor and processed forms of ME were cosecreted acutely (< 1 h) with rat prolactin, and release of both polypeptides was stimulated up to 12-fold by secretagogues. Thus, complete proteolytic processing was not required for accurate targeting of hPE to the regulated secretory pathway. When transfected with constructs bearing deletions of amino-terminal amino acids 2-43 or 2-67, i.e., part or nearly all of the synenkephalin moiety, GH4Cl cells handled the modified protein much like cells expressing the complete protein. They did not process the modified hPE extensively, but the protein was correctly targeted to the regulated secretory pathway. AtT-20 cells transfected with truncated hPE cDNA constructs expressed and processed the protein as efficiently as cells expressing unmodified hPE and expressed predominantly low-molecular-mass forms of ME. Therefore, the structural features required for correct targeting and processing are not present in the cysteine-rich amino-terminal third of the prohormone. It is interesting that the deletions did not include the SHLL peptide motif in synenkephalin, a motif that has been proposed as a sorting signal.

Sequence requirements for processing of proinsulin in transfected mouse pituitary AtT20 cells

To investigate the sequence requirements for proteolytic processing of prohormones at pairs of basic amino acids, normal and mutant proinsulins were expressed in the mouse pituitary corticotrophic cell line AtT20. The extent of processing was determined by h.p.l.c. analysis of insulin-like immunoreactivity secreted into the media of transfected cells. In this model system, normal proinsulin was efficiently processed to insulin. The mutant des-38-62-proinsulin, in which all but six amino acids of the C-peptide were deleted, was also processed to insulin but less efficiently than the wild-type. The mutant Lys64-Arg65 to Thr64-Arg65 was partially processed to insulin, while the mutant Arg31-Arg32 to Arg31-Gly32 was not processed at either site. These results indicate: (i) that a six-amino-acid spacer between the two pairs of basic amino acids in proinsulin is sufficient to permit processing at both sites; (ii) that the endoproteinase responsible for cleavage at the Lys64-Arg65 site will also recognize Thr64-Arg65; (iii) that the endoproteinase responsible for cleavage at the Arg31-Arg32 site will not recognize Arg31-Gly32; and (iv) that the change Arg31-Arg32 to Arg31-Gly32 affects processing at the Lys64-Arg65 site.

Protein targeting via the "constitutive-like" secretory pathway in isolated pancreatic islets: passive sorting in the immature granule compartment

We have suggested the existence of a novel "constitutive-like" secretory pathway in pancreatic islets, which preferentially conveys a fraction of newly synthesized C-peptide, insulin, and proinsulin, and is related to the presence of immature secretory granules (IGs). Regulated exocytosis of IGs results in an equimolar secretion of C-peptide and insulin; however an assay of the constitutive-like secretory pathway recently demonstrated that this route conveys newly synthesized C-peptide in molar excess of insulin (Arvan, P., R. Kuliawat, D. Prabakaran, A.-M. Zavacki, D. Elahi, S. Wang, and D. Pilkey. J. Biol. Chem. 266:14171-14174). We now use this assay to examine the kinetics of constitutive-like secretion. Though its duration is much shorter than the life of mature granules under physiologic conditions, constitutive-like secretion appears comparatively slow (t1/2 approximately equal to 1.5 h) compared with the rate of proinsulin traffic through the ER and Golgi stacks. We have examined whether this slow rate is coupled to the rate of IG exit from the trans-Golgi network (TGN). Escape from the 20 degrees C temperature block reveals a t1/2 less than or equal to 12 min from TGN exit to stimulated release of IGs; the time required for IG formation is too rapid to be rate limiting for constitutive-like secretion. Further, conditions are described in which constitutive-like secretion is blocked yet regulated discharge of IGs remains completely intact. Thus, constitutive-like secretion appears to represent an independent secretory pathway that is kinetically restricted to a specific granule maturation period. The data support a model in which passive sorting due to insulin crystallization results in enrichment of C-peptide in membrane vesicles that bud from IGs to initiate the constitutive-like secretory pathway.

P-selectin, a granule membrane protein of platelets and endothelial cells, follows the regulated secretory pathway in AtT-20 cells

P-selectin (PADGEM, GMP-140, CD62) is a transmembrane protein specific to alpha granules of platelets and Weibel-Palade bodies of endotheial cells. Upon stimulation of these cells, P-selectin is translocated to the plasma membrane where it functions as a receptor for monocytes and neutrophils. To investigate whether the mechanism of targeting of P- selectin to granules is specific for megakaryocytes and endothelial cells and/or dependent on von Willebrand factor, a soluble adhesive protein that is stored in the same granules, we have expressed the cDNA for P-selectin in AtT-20 cells. AtT-20 cells are a mouse pituitary cell line that can store proteins in a regulated fashion. By double-label immunofluorescence, P-selectin was visible as a punctate pattern at the tips of cell processes. This pattern closely resembled the localization of ACTH, the endogenous hormone produced and stored by the AtT-20 cells. Fractionation of the transfected cells resulted in the codistribution of P-selectin and ACTH in cellular compartments of the same density. Immunoelectron microscopy using a polyclonal anti-P- selectin antibody demonstrated immunogold localization in dense granules, morphologically indistinguishable from the ACTH granules. Binding experiments with radiolabeled monoclonal antibody to P-selectin indicated that there was also surface expression of P-selectin on the AtT-20 cells. After stimulation with the secretagogue 8-Bromo-cAMP the surface expression increased twofold, concomitant with the release of ACTH. In contrast, the surface expression of P-selectin transfected into CHO cells, which do not have a regulated pathway of secretion, did not change with 8-Br-cAMP treatment. In conclusion, we provide evidence for the regulated secretion of a transmembrane protein (P-selectin) in a heterologous cell line, which indicates that P-selectin contains an independent sorting signal directing it to storage granules.

Investigation of a possible role of the amino-terminal pro-region of proopiomelanocortin in its processing and targeting to secretory granules

Proopiomelanocortin (POMC) is a polyprotein which is targeted to the regulated secretory pathway of neuroendocrine cells where it undergoes tissue-specific proteolysis to yield peptides such as adrenocorticotropic hormone, beta-lipotropin and beta-endorphin. The pro-region of POMC is 49 amino acid long with two disulfide bonds between cysteine residues 2 and 24 and 8 and 20. These cysteine residues are conserved across the species. The pro-region contains no known hormonal sequence. Sorting to the regulated secretory pathway is thought to involve targeting signals encoded in the structure of secretory proteins. In the present study, we have examined the possibility that the disulfide bridges located in the NH2-terminal portion of the pro-region of POMC are essential for maintaining a determinant involved in the sorting of POMC to the regulated secretory pathway. Using site-directed and deletion mutagenesis of the porcine POMC cDNA, we created mutants in which one or both disulfide bridges were disrupted or in which the first 26 amino acid residues of the pro-region were deleted. Recombinant retroviruses carrying the mutated POMC cDNAs were used to infect Neuro2A cells. Immunofluorescence and immunoelectron microscopy studies performed on infected cells revealed that the unmutated and mutated POMC-immunoreactive peptides were localized in dense-core vesicles at the tips of cellular extensions. Analysis of the POMC-immunoreactive peptides extracted from the infected Neuro2A cells indicated that the mutated precursors in which one disulfide bridge was disrupted (POMC-S2 or POMC-S8) were stored and processed as efficiently as the unmutated POMC. By contrast, the mutated precursor in which both disulfide bridges were disrupted (POMC-S2,8) did not accumulate in intracellular compartments to the same extent as unmutated POMC. Moreover, this mutant was very inefficiently processed and no release could be observed upon stimulation of the cells with K+/Ca2+. These results suggest that POMC-S2,8 entered the regulated secretory pathway less efficiently than the unmutated precursor. However, when both disulfide bridges were removed from the precursor from the precursor by deletion of the first 26 amino acid residues of POMC, the truncated precursor (POMC delta 1-26) behaved as the unmutated POMC. Taken together our results indicate that the NH2-terminal portion of the pro-region including both disulfide bridges can be deleted without affecting the targeting of the molecule to secretory granules. However, when the entire POMC sequence is expressed in Neuro2A cells, the proper folding of the NH2-terminal region might be important for efficient processing and targeting.

Structural domains and molecular lifestyles of insulin and its precursors in the pancreatic beta cell

Insulin is both produced and degraded within the pancreatic Beta cell. Production involves the synthesis of the initial insulin precursor preproinsulin, which is converted to proinsulin shortly after (or during) translocation into the lumen of the rough endoplasmic reticulum. Proinsulin is then transported to the trans-cisternae of the Golgi complex where it is directed towards nascent secretory granules. Conversion of proinsulin to insulin and C-peptide arises within secretory granules, and is dependent upon their acidification. Granule contents are discharged by exocytosis in response to an appropriate stimulus. This represents the regulated secretory pathway to which more than 99% of proinsulin is directed in Beta cells of a healthy individual. An alternative route also exists in the Beta cell, the constitutive secretory pathway. It involves the rapid transfer of products from the Golgi complex to the plasma membrane for immediate release, with, it is supposed, little occasion for prohormone conversion. Even if delivered appropriately to secretory granules, not all insulin is released; some is degraded by fusion of granules with lysosomes (crinophagy). Each event in the molecular lifestyles of insulin and its precursors in the Beta cell will be seen to be governed by their own discrete functional domains. The identification and characterisation of these protein domains will help elucidate the steps responsible for delivery of proinsulin to secretory granules and conversion to insulin. Understanding the molecular mechanism of these steps may, in turn, help to explain defective insulin production in certain disease states including diabetes mellitus.

Secretory granule and synaptic vesicle formation

Sorting of secreted proteins into dense-core secretory granules may involve selective aggregation of regulated secretory proteins, rather than a conventional sortase. Synaptic vesicles, which mediate paracrine communication between adjacent cells, appear to arise by a modification of the early endosome pathway. Targeting to the cell surface involves the actin-based cytoskeleton and small GTP-binding proteins.

Intracellular transport and sorting of mutant human proinsulins that fail to form hexamers

Human proinsulin and insulin oligomerize to form dimers and hexamers. It has been suggested that the ability of prohormones to self associate and form aggregates may be responsible for the sorting process at the trans-Golgi. To examine whether insulin oligomerization is required for proper sorting into regulated storage granules, we have constructed point mutations in human insulin B chain that have been previously shown to prevent formation of insulin hexamers (Brange, J., U. Ribel, J. F. Hansen, G. Dodson, M. T. Hansen, S. Havelund, S. G. Melberg, F. Norris, K. Norris, L. Snel, A. R. Sorensen, and H. O. Voight. 1988. Nature [Lond.]. 333:679-682). One mutant (B10His----Asp) allows formation of dimers but not hexamers and the other (B9Ser----Asp) prevents formation of both dimers and hexamers. The mutants were transfected into the mouse pituitary AtT-20 cells, and their ability to be sorted into regulated secretory granules was compared to wild-type insulin. We found that while B10His----Asp is sorted somewhat less efficiently than wild-type insulin as reported previously (Carroll, R. J., R. E. Hammer, S. J. Chan, H. H. Swift, A. H. Rubenstein, and D. F. Steiner. 1988. Proc. Natl. Acad. Sci. USA. 85:8943-8947; Gross, D. J., P. A. Halban, C. R. Kahn, G. C. Weir, and L. Villa-Kumaroff. 1989. Proc. Natl. Acad. Sci. USA. 86:4107-4111). B9Ser----Asp is targeted to granules as efficiently as wild-type insulin. These results indicate that self association of proinsulin into hexamers is not required for its targeting to the regulated secretory pathway.

Human prorenin

Human prorenin is the enzymatically inactive biosynthetic precursor of renin. Recent interest has focused on the posttranslational sorting and processing of prorenin to renin since markedly increased levels of circulating prorenin have been associated with both physiological and pathological changes. These observations raise the question of whether prorenin processing may be a regulatory event in renin production in the kidney. In the juxtaglomerular cells of the kidney, prorenin can be sorted to either of two pathways: 1) the regulated pathway, which is mediated by secretory granules, where a thiol protease resembling cathepsin B processes prorenin to renin by cleavage of the amino terminal 43-amino acid prosegment, which allows exposure of the active site of renin, or 2) the constitutive pathway, which is not regulated and does not involve conversion of prorenin to renin. Studies in which segments of prorenin are modified by site-directed mutagenesis suggest that the prosegment and glycosylation are not required for sorting, although they may influence or participate in sorting, or both. Certain areas in the prosegment are important determinants of enzyme activity and ability to cleave the prosegment. Further structural analysis of prorenin will be useful to assess details of its sorting and processing. In addition, a number of extrarenal tissues such as uterine lining, ovarian theca, corpus luteum, pituitary, and adrenal, express the renin gene. These tissues have different capabilities to sort and process prorenin compared with kidney, and some tissues secrete only prorenin. Whether prorenin-to-renin conversion is necessary to activate these local renin-angiotensin systems is a key issue.(ABSTRACT TRUNCATED AT 250 WORDS)

A motif found in propeptides and prohormones that may target them to secretory vesicles

Sorting of prohormones and propeptides into secretory vesicles at the trans-Golgi face probably depends on a signal contained within the amino acid sequence of the peptide. To date no consensus sequence has been identified in prohormones or propeptides that might serve such a targeting function. In this report, we have analyzed the amino acid sequences and secondary structures of 15 prohormones and propeptides that have been shown experimentally to be sorted to secretory vesicles when the corresponding cDNA is transfected into mouse pituitary AtT20 cells. From these analyses, we have identified a motif that is shared by all of these diverse propeptides and might serve as a vesicular targeting sequence. This motif is degenerate and consists of two or more leucines occupying one side of a highly amphipattic alpha helix with a serine (or rarely threonine) positioned N-terminal to the leucines and projecting to the same side of the helix.

Induction of specific storage organelles by von Willebrand factor propolypeptide

Endothelial cells store the multimeric adhesive glycoprotein von Willebrand factor (vWf), which promotes the formation of a platelet plug at the site of vessel injury. To investigate the packaging of vWf into the granules called Weibel-Palade bodies, we expressed pro-vWf cDNA and cDNA lacking the prosequence in a variety of cell lines. Storage granules formed only in cells that contain a regulated pathway of secretion. Furthermore, packaging required the prosequence. Pro-vWf, lacking the C-terminal region involved in interchain disulfide bonding, formed granules. We conclude that the signal for storage is universal in that an adhesive glycoprotein can be stored by a hormone-secreting cell; the storage of vWf is independent of its covalent multimeric structure; the unusual rod shape of Weibel-Palade bodies is due to vWf; and the vWf propolypeptide is necessary for the formation of vWf storage granules.

Effects of propeptide deletion on human renin secretion from mouse pituitary AtT-20 cells

To study the role of the N-terminal propeptide in the secretory process of renin, mouse pituitary AtT-20 cells were transfected with expression plasmids of human preprorenin and a mutant deleted of its propeptide. The transfectant of the native construct secreted inactive prorenin and active renin, and renin secretion was stimulated by a secretagogue, 8-Br-cAMP. On the contrary, the transfectant of the deleted construct secreted only active renin, whose release was also stimulated by the secretagogue. The amount of renin molecule secreted from the latter transfectant was lower than that from the former one, although a significant amount of fully active renin could be produced. These results suggest that the propeptide plays an important role in the secretory process of renin, probably folding and/or stabilizing the renin molecule, but it does not contain the signal for intracellular sorting to target renin to secretory granules.

Proinsulin trafckin and processing in the pancreatic B cell

Insulin is synthesized as a precursor, preproinsulin, in the rough endoplasmic reticulum of the pancreatic B cell. The combination of precursor processing and the movement of products in vesicles from one subcellular compartment to the next results in insulin becoming stored in secretory granules ready for release in response to a secretagogue.

The propeptide of preprosomatostatin mediates intracellular transport and secretion of alpha-globin from mammalian cells

We have investigated the role of the somatostatin propeptide in mediating intracellular transport and sorting to the regulated secretory pathway. Using a retroviral expression vector, two fusion proteins were expressed in rat pituitary (GH3) cells: a control protein consisting of the beta-lactamase signal peptide fused to chimpanzee alpha-globin (142 amino acids); and a chimera of the somatostatin signal peptide and proregion (82 amino acids) fused to alpha-globin. Control globin was translocated into the endoplasmic reticulum as determined by accurate cleavage of its signal peptide; however, alpha-globin was not secreted but was rapidly and quantitatively degraded intracellularly with a t 1/2 of 4-5 min. Globin degradation was insensitive to chloroquine, a drug which inhibits lysosomal proteases, but was inhibited at 16 degrees C suggesting proteolysis occurred during transport to the cis-Golgi apparatus. In contrast to the control globin, approximately 30% of the somatostatin propeptide-globin fusion protein was transported to the distal elements of the Golgi apparatus where it was endoproteolytically processed. Processing of the chimera occurred in an acidic intracellular compartment since cleavage was inhibited by 25 microM chloroquine. 60% of the transported chimera was cleaved at the Arg-Lys processing site in native prosomatostatin yielding "mature" alpha-globin. Most significantly, approximately 50% of processed alpha-globin was sorted to the regulated pathway and secreted in response to 8-Br-cAMP. We conclude that the somatostatin propeptide mediated transport of alpha-globin from the endoplasmic reticulum to the trans-Golgi network by protecting molecules from degradation and in addition, facilitated packaging of alpha-globin into vesicles whose secretion was stimulated by cAMP.

Amino-terminal sequences of prosomatostatin direct intracellular targeting but not processing specificity

Rat preprosomatostatin (rPPSS) is processed to two bioactive peptides, somatostatin-14 and somatostatin-28. In anglerfish islets, the two peptides are synthesized by distinct cell types and are derived from different precursors, anglerfish preprosomatostatin-1 (a(I)PPSS) and anglerfish preprosomatostatin-2 (a(II)PPSS). To determine the basis of the differential processing, we introduced a(I)PPSS or a(II)PPSS expression vectors into mammalian endocrine cell lines that can accomplish both patterns of processing. Both precursors were processed identically, indicating that cellular factors must determine the processing pattern. Although similar processing sites are present in both precursors, high levels of unprocessed anglerfish prosomatostatin-2 were secreted constitutively from the transfected cells. A hybrid protein containing the leader sequence and a portion of the pro-region of rPPSS fused to the carboxy-terminal third of a(II)PPSS was processed and secreted via a regulated pathway. We conclude that the amino-terminal 78 residues of rPPSS contain sufficient information to correct the targeting deficiency of a(II)PPSS in mammalian endocrine cell lines.