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

Association of the proprotein convertase subtilisin/kexin-type 2 (PCSK2) gene with type 2 diabetes in an African American population

In a genome-wide scan for type 2 diabetes (T2DM) in African American (AA) families, ordered subsets analysis (OSA) provided evidence for linkage to chromosome 20p in a subset with later age at diagnosis (max LOD 2.57, P=0.008). The proprotein convertase subtilisin/kexin-type 2 (PCSK2) gene is within the LOD-1 interval of this linkage peak. Twenty-nine single nucleotide polymorphisms (SNPs) were genotyped across this gene in 380 unrelated AA individuals with T2DM and end-stage renal disease (T2DM-ESRD), 278 AA controls, 96 European Americans (EA) and 120 Yoruba Nigerian (YRI) controls. In addition, 22 ancestry-informative markers (AIMs) were genotyped in all AA subjects, 120 YRI, and 282 EA controls. ADMIXMAP was used to model the distributions of admixture and generate score tests of allelic and haplotypic association. Association with T2DM was observed among 4 SNPs: rs2021785 (admixture-adjusted Pa=0.00014), rs1609659 (Pa=0.028), rs4814597 (Pa=0.039) and rs2269023 (Pa=0.043). None of the PCSK2 SNPs were associated with age at T2DM diagnosis. A variant in the PCKS2 gene, rs2021785, appears to play a role in susceptibility to T2DM in this AA population.

Regulation of prohormone convertases in hypothalamic neurons: implications for prothyrotropin-releasing hormone and proopiomelanocortin

Recent evidence demonstrated that posttranslational processing of neuropeptides is critical in the pathogenesis of obesity. Leptin or other physiological changes affects the biosynthesis and processing of many peptides hormones as well as the regulation of the family of prohormone convertases responsible for the maturation of these hormones. Regulation of energy balance by leptin involves regulation of several proneuropeptides such as proTRH and proopiomelanocortin. These proneuropeptide precursors require for their maturation proteolytic cleavage by the prohormone convertases 1 and 2 (PC1/3 and PC2). Because biosynthesis of mature peptides in response to leptin requires prohormone processing, it is hypothesized that leptin might regulate hypothalamic PC1/3 and PC2 expression, ultimately leading to coordinated processing of prohormones into mature peptides. Leptin has been shown to increase PC1/3 and PC2 promoter activities, and starvation of rats, leading to low serum leptin levels, resulted in a decrease in PC1/3 and PC2 gene and protein expression in the paraventricular and arcuate nucleus of the hypothalamus. Changes in nutritional status also changes proopiomelanocortin processing in the nucleus of the solitary tract, but this is not reversed by leptin. The PCs are also physiologically regulated by states of hyperthyroidism, hyperglycemia, inflammation, and suckling, and a recently discovered nescient helix-loop-helix-2 transcription factor is the first one to show an ability to regulate the transcription of PC1/3 and PC2. Therefore, the coupled regulation of proneuropeptide/processing enzymes may be a common process, by which cells generate more effective processing of prohormones into mature peptides.

Altered neuropeptide profile of Caenorhabditis elegans lacking the chaperone protein 7B2 as analyzed by mass spectrometry

Cellular synthesis of naturally occurring, bioactive peptides requires the proprotein convertase PC2/EGL-3 for cleavage from the larger peptide precursors. A neuroendocrine chaperone 7B2 is needed for the proteolytical activation of proPC2, as extensively studied in mouse models. To determine the role of its orthologue in Caenorhabditis elegans, we analyzed wild-type and 7B2-null strains by HPLC and matrix-assisted laser desorption ionization time-of-flight mass spectrometry, which allowed the identification of a novel neuropeptide gene, flp-33. The presence and/or absence of some neuropeptides in 7B2-null animals strongly differs form the peptide profile in wild-type, suggesting a specific and determined action of 7B2 in C. elegans.

Impaired processing of FLP and NLP peptides in carboxypeptidase E (EGL-21)-deficient Caenorhabditis elegans as analyzed by mass spectrometry

Biologically active peptides are synthesized from inactive pre-proproteins or peptide precursors by the sequential actions of processing enzymes. Proprotein convertases cleave the precursor at pairs of basic amino acids, which are then removed from the carboxyl terminus of the generated fragments by a specific carboxypeptidase. Caenorhabditis elegans strains lacking proprotein convertase EGL-3 display a severely impaired neuropeptide profile (Husson et al. 2006, J. Neurochem.98, 1999-2012). In the present study, we examined the role of the C. elegans carboxypeptidase E orthologue EGL-21 in the processing of peptide precursors. More than 100 carboxy-terminally extended neuropeptides were detected in egl-21 mutant strains. These findings suggest that EGL-21 is a major carboxypeptidase involved in the processing of FMRFamide-like peptide (FLP) precursors and neuropeptide-like protein (NLP) precursors. The impaired peptide profile of egl-3 and egl-21 mutants is reflected in some similar phenotypes. They both share a severe widening of the intestinal lumen, locomotion defects, and retention of embryos. In addition, egl-3 animals have decreased intestinal fat content. Taken together, these results suggest that EGL-3 and EGL-21 are key enzymes for the proper processing of neuropeptides that control egg-laying, locomotion, fat storage and the nutritional status.

Immunohistochemical expression and colocalization of somatostatin, carboxypeptidase-E and prohormone convertases 1 and 2 in rat brain

The processing of many peptides for their maturation in target tissue depends upon the presence of sorting receptor. Several previous studies have predicted that carboxypeptidase-E (CPE), prohormone convertase 1 (PC1) and prohormone convertase 2 (PC2) may function as sorting elements for somatostatin (SST) for its maturation and processing to appropriate targets. However, nothing is currently known about whether brain, neuronal culture or even endocrine cells express SST, CPE, PC1 and PC2 and exhibit colocalization. Accordingly, in the present study using peroxidase immunohistochemistry, double-labeled indirect immunofluorescence immunohistochemistry and Western blot analysis, we mapped the distributional pattern of SST, CPE, PC1 and PC2 in different rat brain regions. Additionally, we also determined the colocalization of SST with CPE, PC1 and PC2 as well as colocalization of CPE with PC1 and PC2. The localization of SST, CPE, PC1 and PC2 reveals a distinct and region specific distribution pattern in the rat brain. Using an indirect double-label immunofluorescence method we observed selective neuron specific colocalization in a region specific manner in cortex, striatum and hippocampus. These studies provide the first evidence for colocalization between SST, CPE, PC1 and PC2 as well as CPE with PC1 and PC2. SST in cerebral cortex colocalized in pyramidal and non-pyramidal neurons with CPE, PC1 and PC2. Most importantly, in striatum and hippocampus colocalization was mostly observed selectively and preferentially in interneurons. CPE is also colocalized with PC1 and PC2 in a region specific manner. The data presented here provide a new insight into the distribution and colocalization of SST, CPE, PC1 and PC2 in rat brain. Taken together, our data anticipate the possibility that CPE, PC1 and PC2 might be potential target for the maturation of SST.

Identification of positional candidate genes for body weight and adiposity in subcongenic mice

We previously constructed a congenic mouse, B6.S-D2Mit194-D2Mit311 (B6.S-2) with 27 Mb of SPRET/Ei donor DNA on distal chromosome 2 in a C57BL/6J background that captured an obesity quantitative trait locus (QTL). Mice homozygous for SPRET/Ei alleles at the donor region had decreased body weight and obesity-related phenotypes (Diament AL, Farahani P, Chiu S, Fisler J, Warden CH. Mamm Genome 15: 452-459, 2004). In this study, we constructed five overlapping subcongenics with smaller SPRET/Ei donor regions to fine map the underlying gene(s). One of the five subcongenic lines derived from the B6.S-2 founding congenic, B6.S-2A, captured the body weight and adiposity phenotypes in a donor region with a maximum size of 7.4 Mb. Homozygous SPRET/Ei donor alleles in both the founding congenic and the derived B6.S-2A subcongenic exhibited significant decreases in body weight, multiple fat pad weights, and adiposity index (total fat pad weight divided by body weight). Interval-specific microarray analysis in four tissues for donor region genes from the founding B6.S-2 congenic identified several differentially expressed genes mapping to the B6.S-2A subcongenic donor region, including prohormone convertase 2 (PC2; gene name: Pcsk2). Quantitative real-time PCR confirmed a modest decrease of PC2 expression in brains of mice homozygous for SPRET/Ei donor alleles. Analysis of the relative levels of mRNA for B6 and SPRET/Ei in heterozygous congenic mice showed differentially higher expression of the C57BL/6J allele over the SPRET/Ei allele, indicating a cis regulation of differential expression. Using subcongenic mapping, we successfully narrowed a body weight and obesity QTL interval and identified PC2 as a positional candidate gene.

Neuropeptidergic signaling in the nematode Caenorhabditis elegans

The nematode Caenorhabditis elegans joins the menagerie of behavioral model systems next to the fruit fly Drosophila melanogaster, the marine snail Aplysia californica and the mouse. In contrast to Aplysia, which contains 20,000 neurons having cell bodies of hundreds of microns in diameter, C. elegans harbors only 302 tiny neurons from which the cell lineage is completely described, as is the case for all the other somatic cells. As such, this nervous system appears at first sight incommensurable with those of higher organisms, although genome-wide comparison of predicted C. elegans genes with their counterparts in vertebrates revealed many parallels. Together with its short lifespan and ease of cultivation, suitability for high-throughput genetic screenings and genome-wide RNA interference approaches, access to an advanced genetic toolkit and cell-ablation techniques, it seems that this tiny transparent organism of only 1mm in length has nothing to hide. Recently, highly exciting developments have occurred within the field of neuropeptidergic signaling in C. elegans, not only because of the availability of a sequenced genome since 1998, but especially because of state of the art post genomic technologies, that allow for molecular characterization of the signaling molecules. Here, we will focus on endogenous, bioactive (neuro)peptides and mainly discuss biosynthesis, peptide sequence information, localization and G-protein coupled receptors of the three major peptide families in C. elegans.

Biology of incretins: GLP-1 and GIP

This review focuses on the mechanisms regulating the synthesis, secretion, biological actions, and therapeutic relevance of the incretin peptides glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1). The published literature was reviewed, with emphasis on recent advances in our understanding of the biology of GIP and GLP-1. GIP and GLP-1 are both secreted within minutes of nutrient ingestion and facilitate the rapid disposal of ingested nutrients. Both peptides share common actions on islet beta-cells acting through structurally distinct yet related receptors. Incretin-receptor activation leads to glucose-dependent insulin secretion, induction of beta-cell proliferation, and enhanced resistance to apoptosis. GIP also promotes energy storage via direct actions on adipose tissue, and enhances bone formation via stimulation of osteoblast proliferation and inhibition of apoptosis. In contrast, GLP-1 exerts glucoregulatory actions via slowing of gastric emptying and glucose-dependent inhibition of glucagon secretion. GLP-1 also promotes satiety and sustained GLP-1-receptor activation is associated with weight loss in both preclinical and clinical studies. The rapid degradation of both GIP and GLP-1 by the enzyme dipeptidyl peptidase-4 has led to the development of degradation-resistant GLP-1-receptor agonists and dipeptidyl peptidase-4 inhibitors for the treatment of type 2 diabetes. These agents decrease hemoglobin A1c (HbA1c) safely without weight gain in subjects with type 2 diabetes. GLP-1 and GIP integrate nutrient-derived signals to control food intake, energy absorption, and assimilation. Recently approved therapeutic agents based on potentiation of incretin action provide new physiologically based approaches for the treatment of type 2 diabetes.

The role of alpha-cell dysregulation in fasting and postprandial hyperglycemia in type 2 diabetes and therapeutic implications

The hyperglycemic activity of pancreatic extracts was encountered some 80 yr ago during efforts to optimize methods for the purification of insulin. The hyperglycemic substance was named "glucagon," and it was subsequently determined that glucagon is a 29-amino acid peptide synthesized and released from pancreatic alpha-cells. This article begins with a brief overview of the discovery of glucagon and the contributions that somatostatin and a sensitive and selective assay for pancreatic (vs. gut) glucagon made to understanding the physiological and pathophysiological roles of glucagon. Studies utilizing these tools to establish the function of glucagon in normal nutrient homeostasis and to document a relative glucagon excess in type 2 diabetes mellitus (T2DM) and precursors thereof are then discussed. The evidence that glucagon excess contributes to the development and maintenance of fasting hyperglycemia and that failure to suppress glucagon secretion contributes to postprandial hyperglycemia is then reviewed. Although key human studies are emphasized, salient animal studies highlighting the importance of glucagon in normal and defective glucoregulation are also described. The past eight decades of research in this area have led to development of new therapeutic approaches to treating T2DM that have been shown to, or are expected to, improve glycemic control in patients with T2DM in part by improving alpha-cell function or by blocking glucagon action. Accordingly, this review ends with a discussion of the status and therapeutic potential of glucagon receptor antagonists, alpha-cell selective somatostatin agonists, glucagon-like peptide-1 agonists, and dipeptidyl peptidase-IV inhibitors. Our overall conclusions are that there is considerable evidence that relative hyperglucagonemia contributes to fasting and postprandial hyperglycemia in patients with T2DM, and there are several new and emerging pharmacotherapies that may improve glycemic control in part by ameliorating the hyperglycemic effects of this relative glucagon excess.

Nestin expression in pancreatic endocrine and exocrine cells of mice lacking glucagon signaling

Nestin, a marker of neural stem cells, is also expressed by cells located in the epithelium of the pancreatic primordium and by a subpopulation of exocrine cells but not by endocrine cells. These findings raised the possibility that the pancreatic epithelium is heterogeneous and comprised of subpopulations of exocrine/nestin-positive and endocrine/nestin-negative precursor cells. We examined this issue in two mutant mouse models characterized by protracted expression of several embryonal properties in islet cells. One mutant line comprises mice lacking mature glucagon due to abrogation of proprotein convertase-2 (PC2(-/-)), responsible for the conversion of proglucagon into glucagon, while the second line consists of mice with a global deletion of the glucagon receptor (Gcgr(-/-)). We demonstrate that nestin is transiently expressed by acinar cells and by insulin and glucagon cells of islets of both lines of mice. In addition, the lack of glucagon signaling increased nestin mRNA levels in pancreas of mutant embryos and adult mice. We conclude that nestin+ cells located in the pancreatic primordium generate the cells of the endocrine and exocrine lineages. Furthermore, our results suggest that nestin expression is regulated by glucagon signaling.

Pro-protein convertases in intermediary metabolism: islet hormones, brain/gut hormones and integrated physiology

Many peptide hormones implicated in the regulation of intermediary metabolism arise from larger precursors called prohormones. These precursors are cut into pieces by proprotein convertases, more precisely those called prohormone convertases (PCs) that cleave at the C terminus of basic doublets. The remaining basic amino acids are eliminated by a specialized carboxypeptidase, leading to the active hormone. This processing may provide, from a single precursor, several peptides with different biological activities depending on the site(s) of cleavage on the precursor. When the processing is tissue-specific, this mechanism allows to produce, from a single protein, different sets of hormones depending on the tissue considered, leading to novel regulatory processes. The archetype of such a pluripotent prohormone in the field of intermediary metabolism is pro-glucagon that, when cut by PC1 in intestinal L cells, produces four different peptides with different specificities [glicentin, oxyntomodulin (OXM), glucagon-like peptide-1, and glucagon-like peptide-2], whereas, when cut by PC2 in the alpha cells of the endocrine pancreas, glucagon is produced and, through the supplementary action of NRD convertase, a fragment of glucagon (miniglucagon) with original properties.

Strain-specific steroidal control of pituitary function

We have previously shown that 7B2 null mice on the 129/SvEvTac (129) genetic background die at 5 weeks of age with hypercorticosteronemia due to a Cushing's-like disease unless they are rescued by adrenalectomy; however, 7B2 nulls on the C57BL/6NTac (B6) background remain healthy, with normal steroid levels. Since background exerts such a profound influence on the phenotype of this mutation, we have evaluated whether these two different mouse strains respond differently to high circulating steroids by chronically treating wild-type 129 and B6 mice with the synthetic steroid dexamethasone (Dex). Dex treatment decreased the dopamine content of the neurointermediate lobes (NIL) of 129 mice, leading to NIL enlargement and increased total D(2)R mRNA in the 129, but not the B6, NIL. Despite the decrease in this inhibitory transmitter, Dex-treated 129 mice exhibited reduced circulating alpha-melanocyte-stimulating hormone (alpha-MSH) along with reduced POMC-derived peptides compared with controls, possibly due to reduced POMC content in the NIL. In contrast, Dex-treated B6 mice showed lowered cellular ACTH, unchanged alpha-MSH and beta-endorphin, and increased circulating alpha-MSH, most likely due to increased cleavage of NIL ACTH by increased PC2. Dex-treated 129 mice exhibited hyperinsulinemia and lowered blood glucose, whereas Dex-treated B6 mice showed slightly increased glucose levels despite their considerably increased insulin levels. Taken together, our results suggest that the endocrinological response of 129 mice to chronic Dex treatment is very different from that of B6 mice. These strain-dependent differences in steroid sensitivity must be taken into account when comparing different lines of transgenic or knockout mice.

Alpha-cells of the endocrine pancreas: 35 years of research but the enigma remains

Glucagon, a hormone secreted from the alpha-cells of the endocrine pancreas, is critical for blood glucose homeostasis. It is the major counterpart to insulin and is released during hypoglycemia to induce hepatic glucose output. The control of glucagon secretion is multifactorial and involves direct effects of nutrients on alpha-cell stimulus-secretion coupling as well as paracrine regulation by insulin and zinc and other factors secreted from neighboring beta- and delta-cells within the islet of Langerhans. Glucagon secretion is also regulated by circulating hormones and the autonomic nervous system. In this review, we describe the components of the alpha-cell stimulus secretion coupling and how nutrient metabolism in the alpha-cell leads to changes in glucagon secretion. The islet cell composition and organization are described in different species and serve as a basis for understanding how the numerous paracrine, hormonal, and nervous signals fine-tune glucagon secretion under different physiological conditions. We also highlight the pathophysiology of the alpha-cell and how hyperglucagonemia represents an important component of the metabolic abnormalities associated with diabetes mellitus. Therapeutic inhibition of glucagon action in patients with type 2 diabetes remains an exciting prospect.

Cardiovascular and renal actions of melanocyte-stimulating hormone peptides

PURPOSE OF REVIEW

Melanocyte stimulating hormones (MSHs, melanocortins) have important roles in feeding and energy metabolism and in inflammation. Recent observations have uncovered major functions for these peptides, particularly gamma-MSH, in cardiovascular regulation and sodium metabolism.

RECENT FINDINGS

Both alpha- and gamma-MSH acutely elevate blood pressure and heart rate through central stimulation of sympathetic nervous outflow. This action of alpha-MSH is mediated by the melanocortin 4 receptor (MC4R), whereas sympathetic nervous stimulation by gamma-MSH does not involve its receptor MC3R but rather is likely due to activation of a sodium channel in the central nervous system. In contrast, gamma-MSH deficiency in rodents, or disruption of MC3R, leads to marked salt-sensitive hypertension, again through a central mechanism: a small dose of exogenous peptide delivered into the cerebroventricular system of mice with gamma-MSH deficiency restores blood pressure to normal. This salt-sensitive hypertension is accompanied by the development of insulin resistance; the mechanism linking these two consequences of a high-salt diet is not yet known but may involve activation of the sympathetic nervous system.

SUMMARY

The study of MSH peptides in blood pressure regulation offers a new opportunity to gain insight into the mechanisms underlying salt sensitivity and its link to insulin resistance, and to new therapies.

Unique neuronal functions of cathepsin L and cathepsin B in secretory vesicles: biosynthesis of peptides in neurotransmission and neurodegenerative disease

Proteases are required for the production of peptide neurotransmitters and toxic peptides in neurodegenerative diseases. Unique roles of the cysteine proteases cathepsin L and cathepsin B in secretory vesicles for the production of biologically active peptides have been demonstrated in recent studies. Secretory vesicle cathepsin L participates in the proteolytic conversion of proenkephalin into the active enkephalin, an opioid peptide neurotransmitter that mediates pain relief. Moreover, recent findings provide evidence that cathepsin B in regulated secretory vesicles participates in the production of toxic beta-amyloid peptides that are known to accumulate extracellularly in Alzheimer's disease brains. The neurobiological functions of cathepsins L and B demonstrate that these secretory vesicle cysteine proteases produce biologically active peptides. These results demonstrate newly identified roles for cathepsins L and B in neurosecretory vesicles in the production of biologically active peptides.

On the Processing of Proghrelin to Ghrelin

The orexigenic hormone ghrelin is a 28-amino-acid peptide derived from a 99-amino-acid precursor and acylated at Ser-3, which was initially isolated from rat stomach. In addition to stimulating appetite and growth, it also plays various important roles in energy homeostasis and in the cardiovascular and immune systems. Although analysis of its physiological effects has yielded many significant results, much less information is available on its biosynthesis and the mechanism of its acylation. In this report, we have studied the endoproteolytic processing of this molecule from its precursor (proghrelin) into mature ghrelin in various prohormone convertase null mouse strains generated in our laboratory and have identified the convertase responsible for this event. Using Western blotting, mass spectrometry, and immunocytochemical methods, we have demonstrated that (a) in mouse stomach, prohormone convertase 1/3 (PC1/3) is the endoprotease responsible for the conversion of proghrelin to ghrelin, (b) the acylation of this peptide is processing-independent, and (c) the expression of proghrelin mRNA is increased in the processing-deficient (PC1/3 null) mouse.

Glucagon receptor knockout mice display increased insulin sensitivity and impaired beta-cell function

In previous studies, glucagon receptor knockout mice (Gcgr(-/-)) display reduced blood glucose and increased glucose tolerance, with hyperglucagonemia and increased levels of glucagon-like peptide (GLP)-1. However, the role of glucagon receptor signaling for the regulation of islet function and insulin sensitivity is unknown. We therefore explored beta-cell function and insulin sensitivity in Gcgr(-/-) and wild-type mice. The steady-state glucose infusion rate during hyperinsulinemic-euglycemic clamp was elevated in Gcgr(-/-) mice, indicating enhanced insulin sensitivity. Furthermore, the acute insulin response (AIR) to intravenous glucose was higher in Gcgr(-/-) mice. The augmented AIR to glucose was blunted by the GLP-1 receptor antagonist, exendin-3. In contrast, AIR to intravenous administration of other secretagogues was either not affected (carbachol) or significantly reduced (arginine, cholecystokinin octapeptide) in Gcgr(-/-) mice. In islets isolated from Gcgr(-/-) mice, the insulin responses to glucose and several insulin secretagogues were all significantly blunted compared with wild-type mice. Furthermore, glucose oxidation was reduced in islets from Gcgr(-/-) mice. In conclusion, the present study shows that glucagon signaling is required for normal beta-cell function and that insulin action is improved when disrupting the signal. In vivo, augmented GLP-1 levels compensate for the impaired beta-cell function in Gcgr(-/-) mice.

RNAi-mediated silencing of prohormone convertase (PC) 5/6 expression leads to impairment in processing of cocaine- and amphetamine-regulated transcript (CART) precursor

Understanding the functions of the widely expressed PCs (prohormone/proprotein convertases), including PC5/6, furin and PACE4 (paired basic amino acid cleaving enzyme 4), in animal models is difficult since individual knockouts of these PCs in mice exhibit early embryonic lethality. To investigate the roles of PC5/6 in processing pro-CART (pro-cocaine- and amphetamine-regulated transcript), an important anorexigenic peptide precursor, we have generated GH3 cells silenced for PC5/6 expression by RNAi (RNA interference). We show, following transient knockdown of PC5/6 in these neuroendocrine cells, that generation of the two bioactive forms, CART I (amino acids 42-89/55-102) and CART II (amino acids 49-89/62-102), from pro-CART is impaired due to a lack particularly of the A isoform of PC5/6. The results indicate that PC5/6A shares specificities primarily with PC2 (PC5/6A<PC2) in cleaving the pairs of basic residues, KR (40, 41 //53, 54/) and KK (47, 48//60, 61/), within the pro-CART isoforms [see Dey, Zhu, Carroll, Turck, Stein and Steiner (2003) J. Biol. Chem. 278, 15007-15014]. We do not find any significant role of PC5/6A in processing the RXXR (29-32/) site for production of intermediate CART (amino acids 33-102) from long pro-CART. The findings taken altogether indicate that PC5/6 participates in normal processing of pro-CART.

Defective processing of neuropeptide precursors in Caenorhabditis elegans lacking proprotein convertase 2 (KPC-2/EGL-3): mutant analysis by mass spectrometry

Biologically active peptides are synthesized as larger inactive proprotein peptide precursors which are processed by the concerted action of a cascade of enzymes. Among the proprotein convertases, PC2 is widely expressed in neuro-endocrine tissues and has been proposed to be the major convertase involved in the biosynthesis of neuropeptides. In this study, we have examined the role of the Caenorhabditis elegans orthologue PC2/EGL-3 in the processing of proprotein peptide precursors. We recently isolated and identified 60 endogenous peptides in the nematode C. elegans by two-dimensional nanoscale liquid chromatography - quadrupole time-of-flight tandem mass spectrometry. In the present study, we compare the peptide profile of different C. elegans strains, including PC2/EGL-3 mutants. For this purpose, we used an offline approach in which HPLC fractions are analysed by a matrix-assisted laser desorption ionisation - time of flight mass spectrometer. This differential peptidomic approach unambiguously provides evidence for the role of PC2/EGL-3 in the processing of FMRFamide-like peptide (FLP) precursors and neuropeptide-like protein (NLP) precursors in nematodes.

Glucagon-like Peptide-2

Multiple peptide hormones produced within the gastrointestinal system aid in the regulation of energy homeostasis and metabolism. Among these is the intestinotrophic peptide glucagon-like peptide-2 (GLP-2), which is released following food intake and plays a significant role in the adaptive regulation of bowel mass and mucosal integrity. The discovery of GLP-2's potent growth-promoting and cytoprotective effects in the gastrointestinal (GI) tract stimulated interest in its use as a therapeutic agent for the treatment of GI diseases involving malabsorption, inflammation, and/or mucosal damage. Current research has focused on determining the physiological mechanisms contributing to the effects of GLP-2 and factors regulating its biological mechanisms of action. This chapter provides an overview of the biology of GLP-2 with a focus on the most recent findings on the role of this peptide hormone in the normal and diseased GI tract.

Deletion of the gene encoding the ubiquitously expressed glucose-6-phosphatase catalytic subunit-related protein (UGRP)/glucose-6-phosphatase catalytic subunit-beta results in lowered plasma cholesterol and elevated glucagon

In liver, glucose-6-phosphatase catalyzes the hydrolysis of glucose-6-phosphate (G6P) to glucose and inorganic phosphate, the final step in the gluconeogenic and glycogenolytic pathways. Mutations in the glucose-6-phosphatase catalytic subunit (G6Pase) give rise to glycogen storage disease (GSD) type 1a, which is characterized in part by hypoglycemia, growth retardation, hypertriglyceridemia, hypercholesterolemia, and hepatic glycogen accumulation. Recently, a novel G6Pase isoform was identified, designated UGRP/G6Pase-beta. The activity of UGRP relative to G6Pase in vitro is disputed, raising the question as to whether G6P is a physiologically important substrate for this protein. To address this issue we have characterized the phenotype of UGRP knock-out mice. G6P hydrolytic activity was decreased by approximately 50% in homogenates of UGRP(-/-) mouse brain relative to wild type tissue, consistent with the ability of UGRP to hydrolyze G6P. In addition, female, but not male, UGRP(-/-) mice exhibit growth retardation as do G6Pase(-/-) mice and patients with GSD type 1a. However, in contrast to G6Pase(-/-) mice and patients with GSD type 1a, UGRP(-/-) mice exhibit no change in hepatic glycogen content, blood glucose, or triglyceride levels. Although UGRP(-/-) mice are not hypoglycemic, female UGRP(-/-) mice have elevated ( approximately 60%) plasma glucagon and reduced ( approximately 20%) plasma cholesterol. We hypothesize that the hyperglucagonemia prevents hypoglycemia and that the hypocholesterolemia is secondary to the hyperglucagonemia. As such, the phenotype of UGRP(-/-) mice is mild, indicating that G6Pase is the major glucose-6-phosphatase of physiological importance for glucose homeostasis in vivo.

Increased stress-induced analgesia in mice lacking the proneuropeptide convertase PC2

Many neuropeptides involved in pain perception are generated by endoproteolytic cleavages of their precursor proteins by the proprotein convertases PC1 and PC2. To investigate the role of PC2 in nociception and analgesia, we tested wild-type and PC2-null mice for their responses to mechanical and thermal nociceptive stimuli, before and after a short swim in cold or warm water. Basal responses and responses after a cold swim were similar between the two groups. However, after a short forced swim in warm water, PC2-null mice were significantly less responsive to the stimuli than wild-type mice, an indication of increased opioid-mediated stress-induced analgesia. The enhanced analgesia in PC2-null mice may be caused by an accumulation of opioid precursor processing intermediates with potent analgesic effects, or by loss of anti-opioid peptides.

Proprotein convertases: lessons from knockouts

The physiological role of the subtilisin/kexin-like proprotein convertases (PCs) in rodents has been examined through the use of knockout mice. This review will summarize the major in vivo defects that result from the disruption of the expression of their genes. This includes abnormal embryonic development, hormonal disorder, infertility, and/or modified lipid/sterol metabolism. Members of the PC family play a central role in the processing of various protein precursors ranging from hormones and growth factors to bacterial toxins and viral glycoproteins. Proteolysis occurring at basic residues is mediated by the basic amino acid-specific proprotein convertases, namely: PC1/3, PC2, furin, PACE4, PC4, PC5/6, and PC7. In contrast, proteolysis at nonbasic residues is performed by the subtilisin/kexin-like isozyme-1 (SKI-1/S1P) and the newly identified neural apoptosis-regulated convertase-1 (PCSK9/NARC-1). In addition to their requirement for many physiological processes, these enzymes are also involved in various pathologies such as cancer, obesity, diabetes, lipid disorders, infectious diseases, atherosclerosis and neurodegenerative diseases.

Ablation of the glucagon receptor gene increases fetal lethality and produces alterations in islet development and maturation

Although glucagon (GLU) plays a pivotal role in glucose homeostasis, its role in the regulation of fetal growth and maturation is poorly understood. These issues were examined in a line of mice with a global deletion of the GLU receptor (Gcgr-/-), which are characterized by lower blood glucose levels and by alpha- and delta-cell hyperplasia in adults. Ablation of Gcgr was deleterious to fetal survival; it delayed beta-cell differentiation and perturbed the proportion of beta- to alpha-cells in embryonic islets. In adults, the mutation inhibited the progression of alpha-cells to maturity, affected the expression of several beta-cell-specific genes, and resulted in an augmentation of the alpha-, beta-, and delta-cell mass. This increase was due to an augmentation in both islet number and in the rate of proliferation of cells expressing GLU or insulin. These findings suggest that GLU participates in a feedback loop that regulates the proportion of the different endocrine cell types in islets, the number of islets per pancreas, and development of the mature alpha-cell phenotype.

The role of prohormone convertase-2 in hypothalamic neuropeptide processing: a quantitative neuropeptidomic study

Prohormone convertase (PC) 1/3 and 2 are involved in the generation of neuropeptides from their precursors. A quantitative peptidomic approach was used to explore the role PC2 plays in the processing of hypothalamic peptides. In this approach, extracts from mice lacking PC2 activity and from wild-type littermates were labeled with isotopic tags, combined, fractionated on a reverse phase HPLC column, and analyzed by electrospray ionization mass spectrometry. Altogether, 53 neuropeptides or other peptides derived from secretory pathway proteins were identified and sequenced using tandem mass spectrometry. These peptides arise from 21 distinct proteins: proenkephalin, proopiomelanocortin, prodynorphin, protachykinin A and B, procholecystokinin, promelanin-concentrating hormone, proneurotensin, proneuropeptide Y, provasopressin, pronociceptin/orphanin, prothyrotropin-releasing hormone, cocaine- and amphetamine-regulated transcript, chromogranin A and B, secretogranin II, prohormone convertase 1 and 2, propeptidyl-amidating monooxygenase, and proteins designated proSAAS and VGF. Approximately one third of the peptides found in wild-type mice were not detectable in PC2 knock-out mice, and another third were present at levels ranging from 25 to 75% of wild-type levels. Comparison of the cleavage sites suggests that sequences with a Trp, Tyr and/or Pro in the P1' or P2' position, or a basic residue in the P3 position, are preferentially cleaved by PC2 and not by other enzymes present in the secretory pathway.

Processing of cocaine- and amphetamine-regulated transcript (CART) precursor proteins by prohormone convertases (PCs) and its implications

Cocaine- and amphetamine-regulated transcript (CART) peptides are expressed in several neuroendocrine tissues, including hypothalamus, pituitary, gut, adrenal and pancreas, and are involved in regulating important biological processes including feeding/appetite, drug reward and stress. CART is synthesized as larger, inactive peptide precursors (pro-CART) that require endoproteolytic processing to generate smaller, active forms. Prohormone/proprotein convertases (PCs), a family of calcium-dependent, serine endoproteases, have been shown to cleave many protein precursors in the regulated/constitutive secretory pathway to generate smaller fragments. In our previous studies, we have demonstrated the important roles of the two neuroendocrine-specific PCs, PC2 and PC1/3, in processing the two pro-CART isoforms, long (102aa) and short (89aa), to generate the bioactive CART peptides, I (55-102/42-89) and II (62-102/49-89) as well as the intermediate fragments, 10-89 and 33-102. Our subsequent studies have revealed the participation of another PC family member, PC5/6A (the soluble isoform of a widely expressed PC, PC5/6), in cleaving both precursor isoforms. We conclude that PC5/6A contributes to the normal efficient processing of pro-CART and is functionally more redundant with PC2 than PC1/3 in generating both CART I and II.

Impaired NH2-terminal processing of human proislet amyloid polypeptide by the prohormone convertase PC2 leads to amyloid formation and cell death

Islet amyloid, formed by aggregation of islet amyloid polypeptide (IAPP; amylin), is a pathological characteristic of the pancreas in type 2 diabetes and may contribute to the progressive loss of beta-cells in this disease. We tested the hypothesis that impaired processing of the IAPP precursor proIAPP contributes to amyloid formation and cell death. GH3 cells lacking the prohormone convertase 1/3 (PC1/3) and IAPP and with very low levels of prohormone convertase 2 (PC2) were transduced with adenovirus (Ad) expressing human or rat (control) proIAPP linked to green fluorescent protein, with or without Ad-PC2 or Ad-PC1/3. Expression of human proIAPP increased the number of transferase-mediated dUTP nick-end labeling (TUNEL)-positive cells 96 h after transduction (+hIAPP 8.7 +/- 0.4% vs. control 3.0 +/- 0.4%; P < 0.05). COOH-terminal processing of human proIAPP by PC1/3 increased (hIAPP+PC1/3 10.4 +/- 0.7%; P < 0.05), whereas NH(2)-terminal processing of proIAPP by addition of PC2 markedly decreased (hIAPP+PC2 5.5 +/- 0.5%; P < 0.05) the number of apoptotic GH3 cells. Islets from mice lacking PC2 and with beta-cell expression of human proIAPP (hIAPP(+/+)/PC2(-/-)) developed amyloid associated with beta-cell death during 2-week culture. Rescue of PC2 expression by ex vivo transduction with Ad-PC2 restored NH(2)-terminal processing to mature IAPP and decreased both the extent of amyloid formation and the number of TUNEL-positive cells (-PC2 26.5 +/- 4.1% vs. +PC2 16.1 +/- 4.3%; P < 0.05). These findings suggest that impaired NH(2)-terminal processing of proIAPP leads to amyloid formation and cell death and that accumulation of the NH(2)-terminally extended human proIAPP intermediate may be a critical initiating step in amyloid formation.

Gut hormones, and short bowel syndrome: The enigmatic role of glucagon-like peptide-2 in the regulation of intestinal adaptation

Short bowel syndrome (SBS) refers to the malabsorption of nutrients, water, and essential vitamins as a result of disease or surgical removal of parts of the small intestine. The most common reasons for removing part of the small intestine are due to surgical intervention for the treatment of either Crohn's disease or necrotizing enterocolitis. Intestinal adaptation following resection may take weeks to months to be achieved, thus nutritional support requires a variety of therapeutic measures, which include parenteral nutrition. Improper nutrition management can leave the SBS patient malnourished and/or dehydrated, which can be life threatening. The development of therapeutic strategies that reduce both the complications and medical costs associated with SBS/long-term parenteral nutrition while enhancing the intestinal adaptive response would be valuable.

Currently, therapeutic options available for the treatment of SBS are limited. There are many potential stimulators of intestinal adaptation including peptide hormones, growth factors, and neuronally-derived components. Glucagon-like peptide-2 (GLP-2) is one potential treatment for gastrointestinal disorders associated with insufficient mucosal function. A significant body of evidence demonstrates that GLP-2 is a trophic hormone that plays an important role in controlling intestinal adaptation. Recent data from clinical trials demonstrate that GLP-2 is safe, well-tolerated, and promotes intestinal growth in SBS patients. However, the mechanism of action and the localization of the glucagon-like peptide-2 receptor (GLP-2R) remains an enigma. This review summarizes the role of a number of mucosal-derived factors that might be involved with intestinal adaptation processes; however, this discussion primarily examines the physiology, mechanism of action, and utility of GLP-2 in the regulation of intestinal mucosal growth.

Protease pathways in peptide neurotransmission and neurodegenerative diseases

1. Recent research demonstrates the critical importance of neuroproteases for the production of peptide neurotransmitters, and for the production of toxic peptides in major neurodegenerative diseases that include Alzheimer's (AD) and Huntington's diseases. This review describes the strategies utilized to identify the appropriate proteases responsible for producing active peptides for neurotransmission, with application of such approaches for defining protease mechanisms in neurodegenerative diseases. 2. Integration of multidisciplinary approaches in neurobiology, biochemistry, chemistry, proteomics, molecular biology, and genetics has been utilized for neuroprotease studies. These investigations have identified secretory vesicle cathepsin L for the production of the enkephalin opioid peptide neurotransmitter and other neuropeptides. Furthermore, new results using these strategies have identified secretory vesicle cathepsin B for the production of beta-amyloid (Abeta) in the major regulated secretory pathway that provides activity-dependent secretion of Abeta peptides, which accumulate in AD. 3. CNS neuroproteases that participate in peptide neurotransmission and in neurodegenerative diseases represent new candidate drug targets that may be explored in future research for the development of novel therapeutic agents for neurological conditions.

Thyroid hormones selectively regulate the posttranslational processing of prothyrotropin-releasing hormone in the paraventricular nucleus of the hypothalamus

Over the last few years, our laboratory has demonstrated that different physiological conditions or stressors affect the posttranslational processing of hypophysiotropic and nonhypophysiotropic proTRH and, consequently, the output of TRH and other proTRH-derived peptides. These alterations in proTRH processing are generally associated with parallel changes in the levels of two members of the family of prohormone convertases 1/3 and 2 (PC1/3 and PC2). An important regulator of proTRH is thyroid hormone, which is the peripheral end product of the hypothalamic (TRH)-pituitary (TSH)-thyroid (T3/4) (HPT) axis. In this study we investigated the effect of thyroid status on the processing of proTRH inside and outside the HPT axis. Our data showed that high levels of thyroid hormone down-regulated PC1/3 and PC2 and TRH synthesis, which led to an accumulation of intermediate forms of proTRH processing. Conversely, low levels of thyroid hormone up-regulated proTRH synthesis and PC1/3 and PC2 levels. Control of the activity of PCs and proTRH processing occurred specifically in the paraventricular nucleus, whereas no change due to thyroid status was found in the lateral hypothalamus or preoptic area. The posttranslational regulation of proTRH processing in the paraventricular nucleus by thyroid status is a novel aspect of the regulation of the HPT axis, which may have important implications for the pathophysiology of hypo- and hyperthyroidism.

The endoproteolytic maturation of progastrin and procholecystokinin

The homologous brain-gut propeptides, procholecystokinin (proCCK) and progastrin, both undergo extensive posttranslational maturation in specific neuroendocrine cells. The process comprises multiple endoproteolytic cleavages at mono- and dibasic sites, in addition to exoproteolytic trimmings and amino acid derivatizations. Knockout of prohormone convertases (PCs) in mice and studies in cell lines indicate that PC1, PC2 and, to a minor extent, PC5, are responsible for most of the endoproteolytic cleavages of both prohormones. Progastrin in antral G-cells is cleaved by PC1 at two di-Arg sites, R36R37 and R73R74, whereas, PC2 only cleaves at the single di-Lys site, K53K54. Pituitary corticotrophs and intestinal TG-cells, both of which express gastrin, do not cleave K53K54 due to lack of PC2. In proCCK five monobasic (R25, R44, R50, K61 and R75) as well as a single dibasic site (R85R86) can all be cleaved by both PC1 and PC2. But the cleavage differs in a cell-specific manner in that PC1 is responsible for the entire endoproteolytic cleavage in intestinal endocrine I-cells, except for perhaps the K61 site. In contrast PC2 is responsible for most endoproteolysis of proCCK in the cerebral CCK-neurons, which do not express PC1 in significant amounts. Moreover, PC5 appears to contribute to a minor extent to the neuronal proCCK and to the antral progastrin processing. This review emphasizes that prohormone convertases play a decisive but substrate and cell-specific role in the biosynthetic maturation of gastrin and CCK.

Deletion of the gene encoding proprotein convertase 5/6 causes early embryonic lethality in the mouse

PC5 belongs to the proprotein convertase family and activates precursor proteins by cleavage at basic sites during their transit through the secretory pathway and/or at the cell surface. These precursors include prohormones, proreceptors, growth factors, adhesion molecules, and viral glycoproteins. The Pcsk5 gene encodes two alternatively spliced isoforms, the soluble PC5A and transmembrane PC5B. We have carefully analyzed the expression of PC5 in the mouse during development and in adulthood by in situ hybridization, as well as in mouse tissues and various cell lines by quantitative reverse transcription-PCR. The data show that adrenal cortex and intestine are the richest sources of PC5A and PC5B, respectively. To better define the specific physiological roles of PC5, we have generated a mouse Pcsk5(Delta4)-deficient allele missing exon 4 that encodes the catalytic Asp173. While Delta4/+ heterozygotes were healthy and fertile, genotyping of progeny obtained from Delta4/+ interbreeding indicated that Delta4/Delta4 embryos died between embryonic days 4.5 and 7.5. These data demonstrate that Pcsk5 is an essential gene.

Effect of chronic hyperghrelinemia on ingestive action of ghrelin

The stomach hormone ghrelin is the endogenous ligand for the growth hormone secretagogue receptor (GHS-R). Systemic administration of ghrelin will cause elevations in growth hormone (GH) secretion, food intake, adiposity, and body growth. Ghrelin also affects insulin secretion, gastric acid secretion, and gastric motility. Several reports indicate that repeated or continuous activation of GHS-R by exogenous GHSs or ghrelin results in a diminished GH secretory response. The purpose of this study was to examine the extent to which the acute stimulation of food intake by exogenous ghrelin is altered by chronic hyperghrelinemia in transgenic mice that overexpress the human ghrelin gene. The present findings show that the orexigenic action of exogenous ghrelin is not diminished by a chronic hyperghrelinemia and indicate that the food ingestive pathway of the GHS-R is not susceptible to desensitization. In contrast, the epididymal fat pad growth response, like the GH response, to exogenous ghrelin is blunted in ghrelin transgenic mice with chronic hyperghrelinemia.

Prohormone convertase 1/3 is essential for processing of the glucose-dependent insulinotropic polypeptide precursor

The physiology of the incretin hormones, glucagon-like peptide 1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP), and their role in type 2 diabetes currently attract great interest. Recently we reported an essential role for prohormone convertase (PC) 1/3 in the cleavage of intestinal proglucagon, resulting in formation of GLP-1, as demonstrated in PC1/3-deficient mice. However, little is known about the endoproteolytic processing of the GIP precursor. This study investigates the processing of proGIP in PC1/3 and PC2 null mice and in cell lines using adenovirus-mediated overexpression. Supporting a role for PC1/3 in proGIP processing, we found co-localization of GIP and PC1/3 but not PC2 in intestinal sections by immunohistochemistry, and analysis of intestinal extracts from PC1/3-deficient animals demonstrated severely impaired processing to GIP, whereas processing to GIP was unaltered in PC2-deficient mice. Accordingly, overexpression of preproGIP in the neuroendocrine AtT-20 cell line that expresses high levels of endogenous PC1/3 and negligible levels of PC2 resulted in production of GIP. Similar results were obtained after co-expression of preproGIP and PC1/3 in GH4 cells that express no PC2 and only low levels of PC1/3. In addition, studies in GH4 cells and the alpha-TC1.9 cell line, expressing PC2 but not PC1/3, indicate that PC2 can mediate processing to GIP but also to other fragments not found in intestinal extracts. Taken together, our data indicate that PC1/3 is essential and sufficient for the production of the intestinal incretin hormone GIP, whereas PC2, although capable of cleaving proGIP, does not participate in intestinal proGIP processing and is not found in intestinal GIP-expressing cells.

Differences of pancreatic expression of 7B2 between C57BL/6J and C3H/HeJ mice and genetic polymorphisms at its locus (Sgne1)

C57BL/6 (B6) mice develop glucose intolerance with age, whereas C3H/He (C3H) mice do not. In this study, we examined whether this differential glucose homeostasis was associated with differences of proteolytic activation of pancreatic prohormones. Radioimmunoassays showed comparable levels of fasting plasma insulin between the two strains but a significantly lower glucagon level in B6 mice. Pulse-chase analysis of glucagon biosynthesis in isolated pancreatic islets revealed that proglucagon was less efficiently processed in B6 mice. Because proprotein convertase (PC)2 and its 7B2 helper protein are required for this processing, we quantified islet mRNA levels by RT-PCR and protein levels by immunoblotting. The levels of proPC2 mRNA were similar between the two strains, but B6 protein extracts contained less of the mature PC2. In contrast, 7B2 mRNA and protein levels were both significantly lower in B6 pancreas. Sequencing of the 7B2 gene promoter and cDNA in the two strains revealed seven single nucleotide polymorphisms and one dinucleotide insertion/deletion in the cDNA as well as a single nucleotide polymorphism and two insertions/deletions in the promoter. Differential expression of 7B2 may contribute to the difference between B6 and C3H mice not only in glucagon production and secretion but also in glucose tolerance.

Neuroproteases in Peptide Neurotransmission and Neurodegenerative Diseases

The nervous system represents a key area for development of novel therapeutic agents for the treatment of neurological and neurodegenerative diseases. Recent research has demonstrated the critical importance of neuroproteases for the production of specific peptide neurotransmitters and for the production of toxic peptides in major neurodegenerative diseases that include Alzheimer, Huntington, and Parkinson diseases. This review illustrates the successful criteria that have allowed identification of proteases responsible for converting protein precursors into active peptide neurotransmitters, consisting of dual cysteine protease and subtilisin-like protease pathways in neuroendocrine cells. These peptide neurotransmitters are critical regulators of neurologic conditions, including analgesia and cognition, and numerous behaviors. Importantly, protease pathways also represent prominent mechanisms in neurodegenerative diseases, especially Alzheimer, Huntington, and Parkinson diseases. Recent studies have identified secretory vesicle cathepsin B as a novel beta-secretase for production of the neurotoxic beta-amyloid (Abeta) peptide of Alzheimer disease. Moreover, inhibition of cathepsin B reduces Abeta peptide levels in brain. These neuroproteases potentially represent new drug targets that should be explored in future pharmaceutical research endeavors for drug discovery.

Discovery of the Proprotein Convertases and their Inhibitors

The members of the convertase family play a central role in the processing of various protein precursors ranging from hormones and growth factors to viral envelope proteins and bacterial toxins. The proteolysis of these precursors that occurs at basic residues is mediated by the proprotein convertases (PCs), namely: PC1, PC2, Furin, PACE4, PC4, PC5 and PC7. The proteolysis at non-basic residues is performed by subtilisin/kexin-like isozyme-1 (S1P/SKI-1) and the newly identified neural apoptosis-regulated convertase-1 (NARC-1/PCSK9). These proteases have key roles in many physiological processes and various pathologies including cancer, obesity, diabetes, neurodegenerative diseases and autosomal dominant hypercholesterolermia. Here we summarize the discovery of the proprotein convertases and their inhibitors, discuss their properties, roles, resemblance and differences

A vitamin A-free diet results in impairment of the rat hippocampal somatostatinergic system

Previous studies have revealed the presence of retinoid specific receptors in the hippocampus and have demonstrated that vitamin A deficiency produces a severe deficit in spatial learning and memory which are linked to a proper hippocampal functioning. It is also well known that the tetradecapeptide somatostatin binds to specific receptors in the hippocampus and, when injected into this brain area, facilitates the acquisition of spatial tasks. In addition, depletion of somatostatin by cysteamine impairs acquisition of these tasks. Taken together, these studies support the idea that the hippocampal somatostatinergic system might be regulated by vitamin A. Hence, we evaluated the effects of vitamin A deprivation and subsequent administration of vitamin A on the rat hippocampal somatostatinergic system. Rats fed a vitamin A-free diet exhibited a significant reduction of somatostatin-like immunoreactivity content in the hippocampus whereas the somatostatin mRNA levels were unaltered. Vitamin A deficiency increased the somatostatin receptor density and its dissociation constant. Functional Gi activity as well as the capacity of somatostatin to inhibit basal and forskolin-stimulated adenylyl cyclase activity was decreased in vitamin A deficiency rats as compared with the control animals. All these parameters were fully restored when vitamin A was replaced in the diet. Furthermore, we found that the Gialpha1, Gialpha2 and Gialpha3 protein levels were unaltered in hippocampal membranes from rats fed a vitamin A-free diet whereas subsequent vitamin A administration to these rats caused a significant increase in the levels of Gialpha1 and Gialpha2. Altogether, the present findings suggest that dietary vitamin A levels modulate the somatostatinergic system in the rat hippocampus.

Recombinant prohormone convertase 1 and 2 cleave purified pro cholecystokinin (CCK) and a synthetic peptide containing CCK 8 Gly Arg Arg and the carboxyl-terminal flanking peptide

Purified recombinant prohormone convertase 1 and 2 (PC1 and PC2) cleave a peptide containing cholecystokinin (CCK) 8 Gly Arg Arg and the carboxyl-terminal peptide liberating CCK 8 Gly Arg Arg. PC1 and PC2 also cleave purified pro CCK, liberating the amino terminal pro-peptide while no carboxyl-terminal cleavage was detected. Under the conditions of the in vitro cleavage assay, it appears that the carboxyl-terminal cleavage site of pro CCK is not accessible to the enzymes while this site is readily cleaved in a synthetic peptide. Additional cellular proteins that unfold the prohormone may be required to expose the carboxyl-terminal site for cleavage.

Proglucagon-derived peptides: mechanisms of action and therapeutic potential

Glucagon is used for the treatment of hypoglycemia, and glucagon receptor antagonists are under development for the treatment of type 2 diabetes. Moreover, glucagon-like peptide (GLP)-1 and GLP-2 receptor agonists appear to be promising therapies for the treatment of type 2 diabetes and intestinal disorders, respectively. This review discusses the physiological, pharmacological, and therapeutic actions of the proglucagon-derived peptides, with an emphasis on clinical relevance of the peptides for the treatment of human disease.

Cholecystokinin Levels in Prohormone Convertase 2 Knock-out Mouse Brain Regions Reveal a Complex Phenotype of Region-specific Alterations

Prohormone convertase 2 is widely co-localized with cholecystokinin in rodent brain. To examine its role in cholecystokinin processing, cholecystokinin levels were measured in dissected brain regions from prohormone convertase 2 knock-out mice. Cholecystokinin levels were lower in hippocampus, septum, thalamus, mesencephalon, and pons in knock-out mice than wild-type mice. In cerebral cortex, cortex-related structures and olfactory bulb, cholecystokinin levels were higher than wild type. Female mice were more affected by the loss of prohormone convertase 2 than male mice. The decrease in cholecystokinin levels in these brain regions shows that prohormone convertase 2 is important for cholecystokinin processing. Quantitative polymerase chain reaction measurements were performed to examine the relationship between peptide levels and cholecystokinin and enzyme expression. They revealed that cholecystokinin and prohormone convertase 1 mRNA levels in cerebral cortex and olfactory bulb were actually lower in knock-out than wild type, whereas their expression in other brain regions of knock-out mouse brain was the same as wild type. Female mice frequently had higher expression of cholecystokinin and prohormone convertase 1, 2, and 5 mRNA than male mice. The loss of prohormone convertase 2 alters CCK processing in specific brain regions. This loss also appears to trigger compensatory mechanisms in cerebral cortex and olfactory bulb that produce elevated levels of cholecystokinin but do not involve increased expression of cholecystokinin, prohormone convertase 1 or 5 mRNA.

Biologic actions and therapeutic potential of the proglucagon-derived peptides

The actions of the structurally related proglucagon-derived peptides (PGDPs)-glucagon, glucagon-like peptide (GLP)-1 and GLP-2-are focused on complementary aspects of energy homeostasis. Glucagon opposes insulin action, regulates hepatic glucose production, and is a primary hormonal defense against hypoglycemia. Conversely, attenuation of glucagon action markedly improves experimental diabetes, hence glucagon antagonists may prove useful for the treatment of type 2 diabetes. GLP-1 controls blood glucose through regulation of glucose-dependent insulin secretion, inhibition of glucagon secretion and gastric emptying, and reduction of food intake. GLP-1-receptor activation also augments insulin biosynthesis, restores beta-cell sensitivity to glucose, increases beta-cell proliferation, and reduces apoptosis, leading to expansion of the beta-cell mass. Administration of GLP-1 is highly effective in reducing blood glucose in subjects with type 2 diabetes but native GLP-1 is rapidly degraded by dipeptidyl peptidase IV. A GLP-1-receptor agonist, exendin 4, has recently been approved for the treatment of type 2 diabetes in the US. Dipeptidyl-peptidase-IV inhibitors, currently in phase III clinical trials, stabilize the postprandial levels of GLP-1 and gastric inhibitory polypeptide and lower blood glucose in diabetic patients via inhibition of glucagon secretion and enhancement of glucose-stimulated insulin secretion. GLP-2 acts proximally to control energy intake by enhancing nutrient absorption and attenuating mucosal injury and is currently in phase III clinical trials for the treatment of short bowel syndrome. Thus the modulation of proglucagon-derived peptides has therapeutic potential for the treatment of diabetes and intestinal disease.

Inhibitors of proprotein convertases

The discovery of mammalian subtilases, proprotein convertases (PCs) or subtilisin-like proprotein convertases (SPCs), in 1990 was a result of sustained efforts in searching for enzyme/s responsible for maturation of inactive protein precursors. Since then, seven PCs have so far been discovered that cleave at the carboxy-terminal of a basic amino acid characterized by the consensus sequence Arg/Lys/His-X-X/Lys/Arg-Arg downward arrow, where X denotes any amino acid other than Cys. Two additional PC subtypes--called subtilisin kexin isozyme 1 (SKI-1) or site 1 protease (S1P) and neural apoptosis regulated convertase 1 (NARC-1), also known as PCSK9--that cleave at the carboxy terminus of nonbasic amino acids were discovered later. Numerous studies revealed various important functional roles of PCs in health and diseases such as tumorigenesis, diabetes, viral infections, bacterial pathogenesis, atherosclerosis, and neurodegenarative diseases such as Alzheimer's. Owing to these findings, PCs became a promising frontier for treatment of diverse pathologies. Thus modulation of PC activity with designed inhibitors is an attractive proposition not only for intervention of diseases, but also for biochemical characterization of these enzymes. Various physiological and bioengineered proteins as well as small molecules such as peptide, peptidomimetic, and nonpeptide compounds as inhibitors of PCs have been described in the literature. Among the strategies used for design of PC inhibitors, the most successful is the one based on bioengineered serpin proteins, of which the best example is alpha1-PDX, the double mutant variant of alpha1-antitrypsin (from A(355)IPM(358) to R(355)IPR(358)). Others include small peptide inhibitors with C-terminal carboxyl function modified with a potent neucleophile or those containing pseudo or isosteric peptide bond at the scissile site of a suitable peptide substrate. Among nonpeptide PC inhibitors, the number is very limited. So far, these include 20-carbon atoms containing alicyclic diterpenes of andrographolide family and heterocyclic compounds that are ligands of Zn2+ and Cu2+ ions. Overall, these molecules display only a modest enzyme inhibition; however, they may serve as important lead structures for further development of more potent and specific nonpeptide PC inhibitors as potential therapeutic agents. Many PC inhibitors display their functional properties in proliferation, fertilization, tumorigenesis, obesity, embryogenesis, or diabetes via their inhibitory action on PC activities.

Neuropeptide-processing enzymes: applications for drug discovery

Neuropeptides serve many important roles in communication between cells and are an attractive target for drug discovery. Neuropeptides are produced from precursor proteins by selective cleavages at specific sites, and are then broken down by further cleavages. In general, the biosynthetic cleavages occur within the cell and the degradative cleavages occur postsecretion, although there are exceptions where intracellular processing leads to inactivation, or extracellular processing leads to activation of a particular neuropeptide. A relatively small number of peptidases are responsible for processing the majority of neuropeptides, both inside and outside of the cell. Thus, inhibition of any one enzyme will lead to a broad effect on several different neuropeptides and this makes it unlikely that such inhibitors would be useful therapeutics. However, studies with mutant animals that lack functional peptide-processing enzymes have facilitated the discovery of novel neuropeptides, many of which may be appropriate targets for therapeutics.

Cytokines affect PDX-1 expression, insulin and proinsulin secretion from iNOS deficient murine islets

In rodent islets, exposure to interleukin-1beta (IL-1beta) and interferon-gamma (IFN-gamma) induces expression of inducible nitric oxide synthase (iNOS) and subsequent nitric oxide (NO) formation, which may inhibit islet function. However, cytokines may also induce NO-independent islet suppression. The present aim was to investigate the effect of cytokine exposure to iNOS deficient (iNOS-/-) mouse islets on various islet functions. Islets from iNOS-/- and wt mice exposed to IL-1beta or (IL-1beta + IFN-gamma) for 2-20 h showed different kinetics of glucose-stimulated insulin secretion. In iNOS-/- islets, IL-1beta at high glucose induced a delayed and prolonged stimulation of insulin secretion, and this was followed by an increase in phospholipase D mRNA expression. After 6 and 24 h, proinsulin convertase 1 and 2 (PC1 and PC2) mRNA expression was suppressed and proinsulin secretion increased from wt islets. In iNOS-/- islets, PC1 expression was recovered after 24 h, and there was no difference in proinsulin secretion. PDX-1 mRNA expression was suppressed independent of NO-formation. We conclude that cytokines induce both NO-dependent and NO-independent functional inhibition of murine beta-cells.

Glucagon-related peptide 1 (GLP-1): hormone and neurotransmitter

The interest in glucagon-like petide-1 (GLP-1) and other pre-proglucagon derived peptides has risen almost exponentially since seminal papers in the early 1990s proposed to use GLP-1 agonists as therapeutic agents for treatment of type 2 diabetes. A wealth of interesting studies covering both normal and pathophysiological role of GLP-1 have been published over the last two decades and our understanding of GLP-1 action has widened considerably. In the present review, we have tried to cover our current understanding of GLP-1 actions both as a peripheral hormone and as a central neurotransmitter. From an initial focus on glycaemic control, GLP-1 research has been diverted to study its role in energy homeostasis, neurodegeneration, cognitive functions, anxiety and many more functions. With the upcoming introduction of GLP-1 agonists on the pharmaceutical venue, we have witnessed an outstanding example of how initial ideas from basic science laboratories have paved their way to become a novel therapeutic strategy to fight diabetes.

Maintenance of the postabsorptive plasma glucose concentration: insulin or insulin plus glucagon?

The prevalent view is that the postabsorptive plasma glucose concentration is maintained within the physiological range by the interplay of the glucose-lowering action of insulin and the glucose-raising action of glucagon. It is supported by a body of evidence derived from studies of suppression of glucagon (and insulin, among other effects) with somatostatin in animals and humans, immunoneutralization of glucagon, defective glucagon synthesis, diverse mutations, and absent or reduced glucagon receptors in animals and glucagon antagonists in cells, animals, and humans. Many of these studies are open to alternative interpretations, and some lead to seemingly contradictory conclusions. For example, immunoneutralization of glucagon lowered plasma glucose concentrations in rabbits, but administration of a glucagon antagonist did not lower plasma glucose concentrations in healthy humans. Evidence that the glycemic threshold for glucagon secretion, unlike that for insulin secretion, lies below the physiological range, and the finding that selective suppression of insulin secretion without stimulation of glucagon secretion raises fasting plasma glucose concentrations in humans underscore the primacy of insulin in the regulation of the postabsorptive plasma glucose concentration and challenge the prevalent view. The alternative view is that the postabsorptive plasma glucose concentration is maintained within the physiological range by insulin alone, specifically regulated increments and decrements in insulin, and the resulting decrements and increments in endogenous glucose production, respectively, and glucagon becomes relevant only when glucose levels drift below the physiological range. Although the balance of evidence suggests that glucagon is involved in the maintenance of euglycemia, more definitive evidence is needed, particularly in humans.

Strain-dependent influences on the hypothalamo-pituitary-adrenal axis profoundly affect the 7B2 and PC2 null phenotypes

Two null mouse models have previously been created to study the role of the prohormone convertase (PC2) and its helper protein 7B2; unexpectedly, the phenotypes of these two nulls differ profoundly, with the 7B2 but not the PC2 null dying at 5 wk. The genetic backgrounds of these two models differ, with the 7B2 null in a 129/SvEv (129) background and the PC2 null in a mixed C57BL/N6:129/SvEv (B6:129) background. Because background can contribute greatly to phenotype, we have here examined strain influence on the hypothalamo-pituitary-adrenal (HPA) axis and glucose levels in wild-type, 7B2 null, and PC2 null mice. Wild-type B6 and 129 mice differed in basal corticosterone and glucose levels. When 7B2 nulls were transferred onto the B6 background, they survived and showed greatly decreased circulating corticosterone and increased blood glucose levels, most likely due to the comparatively higher adrenal resistance of the B6 strain to ACTH stimulation. Circulating ACTH levels were increased over wild-type in the B6 7B2 null but did not reach levels as high as the 129 7B2 null. Conversely, when the mixed-strain PC2 nulls were bred into the 129 background at the N6 generation, they began to exhibit the Cushing's-like phenotype characteristic of 129 7B2 null mice and died before 6 wk of age. Taken together, these results indicate that background effects are critical because they increase the phenotypic differences between the 7B2 and PC2 nulls and play a life-or-death role in the ACTH hypersecretion syndrome present in both 129 nulls.

Endocrinomic profile of neurointermediate lobe pituitary prohormone processing in PC1/3- and PC2-Null mice using SELDI-TOF mass spectrometry

Pro-vasopressin and pro-oxytocin are prohormones processed in the neurointermediate lobe pituitary to form the biologically active peptide hormones, arginine vasopressin (AVP) and oxytocin. Neurointermediate lobe pituitaries from normal (+/+), heterozygous (+/-), PC2-Null (-/-), PC1/3-Null and oxytocin-Null mice were analyzed by SELDI-TOF mass spectroscopy for the peptide hormone products, AVP, oxytocin and neurophysin I and II. Molecular ion species with masses characteristic of oxytocin, AVP, neurophysin I and II, i.e. 1009.41, 1084.5, 9677 and 9679 daltons respectively, were identified in all but the oxytocin-Null mice by comparison with synthetic standards or by C-terminal sequence analysis. Other ion species were found specifically in PC2-Null, heterozygote or normal mice. The results indicate that, in mice, both PC1/3 or PC2 enzyme activity are capable, but not required to correctly process pro-vasopressin or pro-oxytocin to their constituent active peptide hormones.

Neutral sphingomyelinase 2 (smpd3) in the control of postnatal growth and development

Neutral sphingomyelinases sphingomyelin phosphodiesterase (SMPD)2 and -3 hydrolyze sphingomyelin to phosphocholine and ceramide. smpd2 is expressed ubiquitously, and smpd3 is expressed predominantly in neurons of the CNS. Their activation and the functions of the released ceramides have been associated with signaling pathways in cell growth, differentiation, and apoptosis. However, these cellular responses remain poorly understood. Here we describe the generation and characterization of the smpd3(-/-) and smpd2(-/-)smpd3(-/-) double mutant mouse, which proved to be devoid of neutral sphingomyelinase activity. SMPD3 plays a pivotal role in the control of late embryonic and postnatal development: the smpd3-null mouse develops a novel form of dwarfism and delayed puberty as part of a hypothalamus-induced combined pituitary hormone deficiency. Our studies suggest that SMPD3 is segregated into detergent-resistant subdomains of Golgi membranes of hypothalamic neurosecretory neurons, where its transient activation modifies the lipid bilayer, an essential step in the Golgi secretory pathway. The smpd3(-/-) mouse might mimic a form of human combined pituitary hormone deficiency.