Bi-directional trafficking between the trans-Golgi network and the endosomal/lysosomal system

Protein transport in the secretory and endocytic pathways of eukaryotic cells is mediated by vesicular transport intermediates. Their formation is a tightly controlled multistep process in which coat components are recruited onto specific membranes, and cargo, as well as targeting molecules, become segregated into nascent vesicles. At the trans-Golgi network, two transport systems deliver cargo molecules to the endosomal system. They can be distinguished with regard to coat components that select cargo molecules. AP-1 assembly proteins mediate transport of MPRs and furin, whereas AP-3 adaptors mediate transport of lysosomal membrane glycoproteins to the endosomal/lysosomal system. The molecular basis for protein-specific sorting lies within sorting signals that are present in the cytoplasmic tails of cargo proteins and allow specific interactions with individual coat components. In order to maintain cellular homeostasis, some proteins are retrieved from endosomal compartments and transported back to the trans-Golgi network. Distinct points for protein retrieval exist within the endosomal system, retrieval occurring from either early or late endosomes. Whereas significant progress has been made in recent years in identifying anterograde and retrograde transport pathways, the molecular mechanisms underlying protein sorting and retrieval are only poorly defined. Recently, however, novel vesicle coats (e.g. AP-4) and proteins that might be involved in sorting (e.g. PACS-1 and TIP47) have been described, and the interactions between assembly proteins and sorting signals are becoming increasingly well defined.

Targeting of lysosomal proteins

Proper cell homeostasis requires the efficient transport of a large variety of soluble acid hydrolases and transmembrane proteins from the trans-Golgi network (TGN) to lysosomes. While most of these molecules reach this degradative compartment, some transmembrane proteins, in particular, the acid hydrolase receptors are retrieved to the TGN. This bidirectional transport process involves the formation of several vesicular transport intermediates in which cargo molecules are selectively packaged. This review summarizes our current understanding of the molecular mechanisms leading to the proper targeting of lysosomal proteins.

Role of the prohormone convertase PC3 in the processing of proglucagon to glucagon-like peptide 1

Proglucagon is processed differentially in pancreatic alpha-cells and intestinal endocrine L cells to release either glucagon or glucagon-like peptide-1-(7-36amide) (tGLP-1), two peptide hormones with opposing biological actions. Previous studies have demonstrated that the prohormone convertase PC2 is responsible for the processing of proglucagon to glucagon, and have suggested that the related endoprotease PC3 is involved in the formation of tGLP-1. To understand better the biosynthetic pathway of tGLP-1, proglucagon processing was studied in the mouse pituitary cell line AtT-20, a cell line that mimics the intestinal pathway of proglucagon processing and in the rat insulinoma cell line INS-1. In both of these cell lines, proglucagon was initially cleaved to glicentin and the major proglucagon fragment (MPGF) at the interdomain site Lys70-Arg71. In both cell lines, MPGF was cleaved successively at the monobasic site Arg77 and then at the dibasic site Arg109-Arg110, thus releasing tGLP-1, the cleavages being less extensive in INS-1 cells. Glicentin was completely processed to glucagon in INS-1 cells, but was partially converted to oxyntomodulin and very low levels of glucagon in AtT-20 cells in the face of generation of tGLP-1. Adenovirus-mediated co-expression of PC3 and proglucagon in GH4C1 cells (normally expressing no PC2 or PC3) resulted in the formation of tGLP-1, glicentin, and oxyntomodulin, but no glucagon. When expressed in alphaTC1-6 (transformed pancreatic alpha-cells) or in rat primary pancreatic alpha-cells in culture, PC3 converted MPGF to tGLP-1. Finally, GLP-1-(1-37) was cleaved to tGLP-1 in vitro by purified recombinant PC3. Taken together, these results indicate that PC3 has the same specificity as the convertase that is responsible for the processing of proglucagon to tGLP-1, glicentin and oxyntomodulin in the intestinal L cell, and it is concluded that this enzyme is thus able to act alone in this processing pathway.

Role of the prohormone convertase PC2 in the processing of proglucagon to glucagon

Proglucagon is alternatively processed to glucagon in pancreatic alpha-cells, or to glucagon-like peptide-1 in intestinal L cells. Here, the specificity of PC2, the major prohormone convertase of alpha-cells, was examined both in vivo and in vitro. Adenovirus-mediated co-expression of proglucagon and PC2 in GH4C1 cells resulted in a pattern of processing products very similar to that observed in alpha-cells. Oxyntomodulin, an intermediate in the processing of proglucagon, was quantitatively converted to glucagon in vitro by purified recombinant PC2, in combination with carboxypeptidase E. It is concluded that PC2 is able to act alone in the pancreatic pathway of proglucagon processing.

Adaptive evolution of water homeostasis regulation in amphibians: vasotocin and hydrins

Volemia and osmolality homeostasis is ensured in vertebrates through neuroendocrine reflexes, involving an afferent neural branch from baro- and osmo-receptors to hypothalamus and an efferent endocrine branch from secretory neurons to target hydroosmotic cells equipped with receptors and effectors. Whereas the osmoregulatory system in the tadpole comprises three organs, namely gut, kidney and gills, as in freshwater fishes, the adult displays a quaternary strategy with gut, kidney, urinary bladder and skin. In particular, the cutaneous permeability entails a great evaporative water loss when the animal is in the open air, loss that must be compensated by water reabsorption through the nephron and the urinary bladder and mainly by water uptake through the skin. Adaptation occurred at the level of these organs by regulation of their permeability through neurohypophysial hormones. Aside from vasotocin, active on the three organs, all anuran Amphibia possess hydrin 2 (vasotocinyl-Gly), a peptide resulting from a down-regulation of provasotocin processing. Exceptionally Xenopus laevis, a permanent aquatic toad, has hydrin 1 (vasotocinyl-Gly-Lys-Arg) instead of hydrin 2. Hydrins are somewhat more active than vasotocin on water permeation of skin and bladder but are devoid of antidiuretic activity. Adaptive evolution has created, along with the vasotocin-nephron system, preserved in all terrestrial non-mammalian tetrapods, additional functions such as the hydrin-skin and hydrin-bladder rehydration mechanisms. Specific hydrin receptors might exist in the skin and the bladder, different from those of vasotocin in the kidney. It is assumed that the water channel recruitment mechanism, found for vasopressin acting on the collecting duct principal cells in mammals, is also involved when vasotocin and hydrins stimulate their hydroosmotic target cells and that hormone-regulated aquaporin 2-like proteins could be identified in the three osmoregulatory organs of amphibians.

Defective prohormone processing and altered pancreatic islet morphology in mice lacking active SPC2

The prohormone convertase SPC2 (PC2) participates in the processing of proinsulin, proglucagon, and a variety of other neuroendocrine precursors, acting either alone or in conjunction with the structurally related dense-core granule convertase SPC3 (PC3/PC1). We have generated a strain of mice lacking active SPC2 by introducing the neomycin resistance gene (Neor) into the third exon of the mSPC2 gene. This gene insertion results in the synthesis of an exon 3-deleted form of SPC2 that does not undergo autoactivation and is not secreted. The homozygous mutant mice appear to be normal at birth. However, they exhibit a small decrease in rate of growth. They also have chronic fasting hypoglycemia and a reduced rise in blood glucose levels during an intraperitoneal glucose tolerance test, which is consistent with a deficiency of circulating glucagon. The processing of proglucagon, prosomatostatin, and proinsulin in the alpha, delta, and beta cells, respectively, of the pancreatic islets is severely impaired. The islets in mutant mice at 3 months of age show marked hyperplasia of alpha and delta cells and a relative diminution of beta cells. SPC2-defective mice offer many possibilities for further delineating neuroendocrine precursor processing mechanisms and for exploring more fully the physiological roles of many neuropeptides and peptide hormones.

The role of prohormone convertases in insulin biosynthesis: evidence for inherited defects in their action in man and experimental animals

The hormone insulin remains the cornerstone of diabetic therapy since it is required for almost all cases of Type 1 and many cases of Type 2 diabetes. Since the discovery of insulin in 1921, much has been learned about its chemistry, structure and action as well as its production in the beta cell. Insulin is formed through a series of precursors, beginning with preproinsulin, the protein encoded in the insulin gene. These precursors direct the prohormone into the secretory pathway and ultimately into the secretory granules where it is converted into insulin and C-peptide. These products are stored and secreted together in a highly regulated manner in response to glucose and other stimuli. This review focuses on the recently discovered prohormone convertases, PC2 and PC3 (PC1), the enzymes responsible for the endoproteolytic processing of proinsulin to insulin and C-peptide in the beta cell as well as for the selective processing of proglucagon to glucagon in the alpha cell or GLP1 in intestinal L-cells. PC2 and PC3 are calcium-dependent serine proteases related to the bacterial enzyme subtilisin. They cleave selectively at Lys-Arg or Arg-Arg sites in precursors, generating products with C-terminal basic residues that are then removed by carboxypeptidase E, an exopeptidase. All 3 enzymes are expressed mainly in secretory granules of neuroendocrine cells throughout the body and in the brain. Inherited defects affecting the prohormone-processing enzymes have recently been found in association with unusual syndromes of obesity and other metabolic disorders.

Differential processing of proglucagon by the subtilisin-like prohormone convertases PC2 and PC3 to generate either glucagon or glucagon-like peptide

Proglucagon is processed differently in the islet alpha cells and the intestinal endocrine L cells to release either glucagon or glucagon-like peptide 1-(7-37) (GLP1-(7-37)), peptide hormones with opposing actions in vivo. In previous studies with a transformed alpha cell line (alpha TC1-6) we demonstrated that the kexin/subtilisin-like prohormone convertase, PC2 (SPC2), is responsible for generating the typical alpha cell pattern of proglucagon processing, giving rise to glucagon and leaving unprocessed the entire C-terminal half-molecule known as major proglucagon fragment or MPGF (Rouillé, Y., Westermark, G., Martin, S. K., Steiner. D. F. (1994) Proc. Natl. Acad. Sci. U.S.A. 91, 3242-3246). Here we present evidence, using mouse pituitary AtT-20 cells infected with a vaccinia viral vector encoding proglucagon, that PC3 (SPC3), the major neuroendocrine prohormone convertase in these cells, reproduces the intestinal L cell processing phenotype, in which MPGF is processed to release two glucagon-related peptides, GLP1 and GLP2, while the glucagon-containing N-terminal half-molecule (glicentin) is only partially processed to oxyntomodulin and small amounts of glucagon. Moreover, in AtT-20 cells stably transfected with PC2 (AtT-20/PC2 cells), glicentin was efficiently processed to glucagon, providing further support for the conclusion that PC2 is the enzyme responsible for the alpha cell processing phenotype. In other cell lines expressing both PC2 and PC3 (STC-1 and beta TC-3), proglucagon was also processed extensively to both glucagon and GLP1-(7-37), although STC-1 cells express lower levels of PC2 and processed the N-terminal domain to glucagon less efficiently. In contrast, GH4C1 and COS 7 cells, which express very little or no PC2 or PC3, failed to process proglucagon, aside from a low level of interdomain cleavage which occurred only in the GH4C1 cells. In vitro PC3 did not cleave at the single Arg residue in GLP1 to generate GLP1-(7-37), its truncated biologically active form, indicating the likelihood that another convertase is required for this cleavage.

Proteolytic processing mechanisms in the biosynthesis of neuroendocrine peptides: the subtilisin-like proprotein convertases

The recent discovery of a novel family of precursor processing endoproteases has greatly accelerated progress in understanding the complex mechanisms underlying the maturation of prohormones, neuropeptides, and many other precursor-derived proteins. At least six members of this family have been found thus far in mammalian species, several having alternatively spliced isoforms, and related enzymes have been identified in many invertebrates, including molluscs, insects, nematodes, and coelenterates. The proprotein convertases are all dependent on calcium for activity and all possess highly conserved subtilisin-like domains with the characteristic catalytic triad of this serine protease (ordered Asp, His, and Ser along the polypeptide chain). Two members of this family, PC2 (SPC2) and PC1/PC3 (SPC3), appear to play a preeminent role in neuroendocrine precursor processing. Both convertases are expressed only in the brain and in the extended neuroendocrine system, while another important family member--furin/PACE (SPC1)--is expressed more ubiquitously, in almost all tissues, and at high levels in liver. SPC2 and SPC3 exhibit acidic pH optima and other properties which enhance their activity in the acidic, calcium-enriched environment of the dense-core secretory granules of the regulated pathway in neuroendocrine cells, while furin has a neutral pH optimum and is localized predominantly to the trans Golgi network where it is retained by a C-terminal transmembrane domain. Furin processes a wide variety of precursors in the constitutive pathway, such as those of growth factors, receptors, coagulation factors, and viral glycoproteins. Recent findings on the processing of proopiomelanocortin, proinsulin, proglucagon, and several other neuroendocrine precursors by SPC2 and SPC3 are discussed, along with information on the structure, properties, evolution, developmental expression, and regulation of the convertases. An inherited defect in the fat/fat mouse which affects the processing of proinsulin, and probably also many other prohormones, due to a point mutation in carboxypeptidase E has recently been identified and has begun to provide new insights into the functional integration of the individual processing steps.

Proglucagon is processed to glucagon by prohormone convertase PC2 in alpha TC1-6 cells

Proglucagon is processed differentially in the pancreatic alpha cells and the intestinal L cells to yield either glucagon or glucagon-like peptide 1, respectively, structurally related hormones with opposing metabolic actions. Here, we have studied the processing of proglucagon in alpha TC1-6 cells, an islet-cell line transformed by simian virus 40 large tumor (T) antigen, a model of the pancreatic alpha cell. We found that these cells process proglucagon at certain dibasic cleavage sites to release glucagon and only small amounts of glucagon-like peptide 1, as demonstrated by both continuous and pulse-chase labeling experiments. Both normal islet alpha cells and alpha TC1-6 cells were shown to express the prohormone convertase PC2 at high levels, but not the related protease PC3. Expression of PC2 antisense RNA in alpha TC1-6 cells inhibited both PC2 production and proglucagon processing concomitantly. We conclude that PC2 is the key endoprotease responsible for proglucagon processing in cells with the alpha-cell phenotype.

A neurosecretory granule Lys-Arg Ca(2+)-dependent endopeptidase putatively involved in prooxytocin and provasopressin processing

A Ca(2+)-dependent endopeptidase cleaving at the carboxyl side of the paired Lys-Arg residues has been found in the neurosecretory granules of the rat neurointermediate pituitary. The specificity pattern on synthetic fluorogenic substrates, the inhibitor profile, the pH optimum of 5.0 and the Ca(2+)-dependence are compatible with an involvement of this enzyme in the prooxytocin and the provasopressin processing within the granules. The enzymatic features of the neurohypophysial granule endopeptidase resemble those of the insulinoma granule type II endopeptidase and suggest that the same Ca(2+)-dependent protease or closely related enzymes could be involved in processing Lys-Arg-containing prohormones in neuroendocrine cells.

Study of frog (Rana esculenta) proopiomelanocortin processing in the intermediate pituitary. Identification of alpha-melanotropin, beta-melanotropin, Lys-gamma-melanotropin, and corticotropin-like intermediate lobe peptide

The proteolytic processing of frog (Rana esculenta) proopiomelanocortin in melanotropic cells of the intermediate pituitary gland has been examined through purification of the mature fragments by reverse-phase high-pressure liquid chromatography and microsequencing of isolated peptides. alpha-Melanotropin, beta-melanotropin, Lys-gamma-melanotropin, corticotropin-like intermediate lobe peptide, and hinge peptide have been isolated and chemically characterized. The results show a high preservation in the processing sites of frog proopiomelanotropin when compared to bovine counterparts. They reveal also a great conservation of the processing enzyme equipment of melanotropic cells in tetrapods species. Identification of Lys-gamma-melanotropin suggests the occurrence of an endopeptidase able to cleave between two basic residues. On the other hand alpha-melanotropin does not appear to be N-acetylated, as previously found in the clawed-toad Xenopus laevis, and this feature might distinguish amphibian from mammalian proopiomelanocortin processing.

Heterologue conversion of amphibian hydrin 2 into vasotocin through bovine granule alpha-amidating enzyme

Abstract Hydrin 2 (vasotocinyl-Gly), found along with vasotocin in the neurohypophysis of frogs and toads but not of other vasotocin-bearers such as birds and reptiles, is believed to act on water permeability of frog skin, whereas vasotocin mainly fulfils the antidiuretic function on the kidney. In order to understand the peculiar regulation of provasotocin differential processing in amphibians, conversion of hydrin 2 into vasotocin has been attempted using bovine pituitary granule alpha-amidating enzyme. Generated vasotocin has pharmacological properties and Chromatographic behaviour in high-performance liquid chromatography identical to those of synthetic vasotocin. However, the low yield of conversion (5% to 10%) suggests that additional factor(s) might be involved in the physiological processing.

Evolutionary specificity of hydrins, new hydroosmotic neuropeptides: occurrence of hydrin 2 (vasotocinyl-Gly) in the toad Bufo marinus but not in the viper Vipera aspis

Hydrin 2 (vasotocinyl-Gly), a hydroosmotic peptide resulting from differential processing of provasotocin and recently identified in frog neurohypophysis, has been looked for in the pituitary gland of an exotic toad (Bufo marinus) and of a reptile (Vipera aspis). Hydrin 2 has been found in the amphibian but not in the reptile. This result confirms the evolutionary specificity of hydrin 2 that has been identified only in frogs and toads but not in birds and reptiles. Occurrence of hydrin 2 is explained by its regulatory function on the water permeability of the skin of anurans.

Identification of two types of neurophysins in Xenopus laevis neurointermediate pituitary homologous to mammalian MSEL- and VLDV-neurophysins

Xenopus laevis neurophysins have been purified from neurointermediate pituitaries through high-pressure reverse-phase liquid chromatography and their N-terminal amino acid sequences have been determined by microsequencing. Two types of neurophysins, corresponding to mammalian MSEL- and VLDV-neurophysins, have been distinguished. A strong homology exists between neurophysins of Xenopus (Pipidae), frog (Ranidae) and toad (Bufonidae). Xenopus MSEL-neurophysin, as frog MSEL-neurophysin, has a high molecular mass suggesting that the C-terminal domain of the vasotocin precursor is not processed in contrast to the two-step processing observed for mammalian vasopressin precursor.

ABBREVIATIONS

Mammalian neurophysins are termed MSEL- and VLDV-neurophysins according to the nature of residues in positions 2, 3, 6 and 7 (one-letter symbols for amino acids).

Occurrence of hydrin 2 (vasotocinyl-Gly), a new hydroosmotic neurohypophyseal peptide, in secretory granules isolated from the frog (Rana esculenta) neurointermediate pituitary

Neurohypophyseal secretory granules have been purified from the frog (Rana esculenta) neurointermediate pituitary gland by sucrose gradient centrifugation, and their polypeptide content has been analyzed by reverse-phase high-pressure liquid chromatography. Aside from vasotocin, mesotocin, and their associated neurophysins, hydrin 2 (vasotocinyl-Gly), previously identified in hydrochloric acid extracts, has been recognized. This finding supports the previous suggestion that hydrin 2, a peptide active on the water permeability of frog bladder and frog skin, is a secreted hormone involved in osmoregulation specific to amphibians. Hydrin 2 has not been found in neurosecretory granules of birds such as the goose.

Hydrins, hydroosmotic neurohypophysial peptides: osmoregulatory adaptation in amphibians through vasotocin precursor processing

From neurointermediate pituitary glands of Xenopus laevis and Rana esculenta, previously unreported peptides termed hydrins, active on water permeability of frog urinary bladder and frog skin (Brunn or "water-balance" effect), have been isolated and sequenced. These peptides seem to be derived from the pro-vasotocin-neurophysin precursor. Hydrin 1, found in Xenopus, has been identified as vasotocin C-terminally extended with the Gly-Lys-Arg sequence; hydrin 2, found in Rana, has been identified as vasotocin C-terminally extended with glycine. Hydrin 2 has been detected in several Ranidae (R. esculenta, Rana temporaria, Rana pipiens) and Bufonidae (Bufo bufo, Bufo ictericus) and appears to have a large distribution in terrestrial or semiaquatic anurans. Hydrins, in contrast to vasotocin, are not active on rat uterus or rat blood pressure. They are absent from other vasotocin-bearers such as birds and could be involved specifically in water-electrolyte regulation of amphibians.

Isolation of neurosecretory granules containing vasotocin, mesotocin, MSEL- and VLDV-neurophysins from goose neurohypophysis

Neurosecretory granules have been isolated from goose posterior pituitaries and their contents have been analyzed by reverse-phase high-pressure liquid chromatography and polyacrylamide gel electrophoresis. Vasotocin and mesotocin have been identified by their biological activities and their retention times compared with those of synthetic peptides. MSEL- and VLDV-neurophysins have been characterized by their N-terminal sequences, their electrophoretic migrations and their retention times, compared with those of purified goose neurophysins. In contrast to the two-step processing of mammalian provasopressin, processing of the vasotocin - MSEL-neurophysin precursor appears to involve only one cleavage giving the hormone and a "big" MSEL-neurophysin homologous to mammalian MSEL-neurophysin extended by copeptin.

Particular processing of pro-opiomelanocortin in Xenopus laevis intermediate pituitary. Sequencing of alpha- and beta-melanocyte-stimulating hormones

alpha- and beta-melanocyte-stimulating hormones (alpha-MSH and beta-MSH) have been isolated from Xenopus laevis neurointermediate pituitary and microsequenced. Intracellular alpha-MSH is not N-acetylated after proteolytic processing of pro-opiomelanocortin in contrast to mammalian alpha-MSHs. There is a high preservation of the melanotropic amino acid sequence common to all MSHs although in Xenopus beta-MSH a histidine residue replaces the glutamic acid residue found in position 8 of mammalian beta-MSHs.

Dual duplication of neurohypophysial hormones in an Australian marsupial: mesotocin, oxytocin, lysine vasopressin and arginine vasopressin in a single gland of the northern bandicoot (Isoodon macrourus)

Neurohypophysial hormones of an Australian marsupial, the Northern bandicoot (Isoodon macrourus), have been identified by their retention times in high-pressure reverse-phase liquid chromatography using two solvent systems and by their molar pressor or uterotonic activities. Two pressor peptides, arginine vasopressin and lysipressin, and two uterotonic peptides, mesotocin and oxytocin, have been characterized. Because mesotocin and arginine vasopressin have been identified in three other Australian marsupial families, it is assumed that a duplication of each ancestral gene occurred in Peramelidae and subsequent mutations in one copy led to the additional oxytocin and lysipressin. A similar dual duplication of neurohypophysial hormones has previously been discovered in the North-American opossum (Didelphis virginiana) so that the duplication propensity seems peculiar to marsupials in contrast to placental mammals.

Isolation of neurosecretory granules containing vasopressin and MSEL-neurophysin from guinea pig neurointermediate pituitary

Neurosecretory granules have been isolated from rat and guinea pig neurointermediate pituitaries and their contents have been analyzed by reverse-phase high-pressure liquid chromatography and polyacrylamide gel electrophoresis. Granule components have been compared with synthetic neurohypophysial hormones and chemically characterized neurophysins. In rat granules, oxytocin, arginine vasopressin and MSEL- and VLDV-neurophysins have been identified. In isolated guinea pig granules, only arginine vasopressin and mature MSEL-neurophysin have been found. From these results it can be concluded that both the "dibasic" cleavage between vasopressin and MSEL-neurophysin and the "monobasic" cleavage between MSEL-neurophysin and copeptin occur within the granule compartment. Previous isolation from frozen guinea pig glands of a partially processed precursor encompassing MSEL-neurophysin and copeptin suggests a two-step processing of the three-domain vasopressin precursor, each involving a distinct enzymic system.

Guinea pig neurohypophysial hormones. Peculiar processing of the three-domain vasopressin precursor

Guinea pig neurohypophysial hormones have been purified by two procedures, one involving molecular sieving and paper chromatoelectrophoresis, the other high-pressure reverse-phase liquid chromatography. Arginine vasopressin and oxytocin have been identified by their amino acid compositions and their retention times in HPLC determined through their biological properties. No partially processed precursor, including a neurohormone and a neurophysin, has been detected. Because the cleavage of the three-domain vasopressin-neurophysin-copeptin precursor is apparently complete between the first two domains, whereas it is not between the second and the third, it is supposed that two distinct enzymic systems are involved in the processing.