Proprotein and prohormone convertases: a family of subtilases generating diverse bioactive polypeptides

Molecular characterization of the processing of arenavirus envelope glycoprotein precursors by subtilisin kexin isozyme-1/site-1 protease

A crucial step in the life cycle of arenaviruses is the biosynthesis of the mature fusion-active viral envelope glycoprotein (GP) that is essential for virus-host cell attachment and entry. The maturation of the arenavirus GP precursor (GPC) critically depends on proteolytic processing by the cellular proprotein convertase (PC) subtilisin kexin isozyme-1 (SKI-1)/site-1 protease (S1P). Here we undertook a molecular characterization of the SKI-1/S1P processing of the GPCs of the prototypic arenavirus lymphocytic choriomeningitis virus (LCMV) and the pathogenic Lassa virus (LASV). Previous studies showed that the GPC of LASV undergoes processing in the endoplasmic reticulum (ER)/cis-Golgi compartment, whereas the LCMV GPC is cleaved in a late Golgi compartment. Herein we confirm these findings and provide evidence that the SKI-1/S1P recognition site RRLL, present in the SKI-1/S1P prodomain and LASV GPC, but not in the LCMV GPC, is crucial for the processing of the LASV GPC in the ER/cis-Golgi compartment. Our structure-function analysis revealed that the cleavage of arenavirus GPCs, but not cellular substrates, critically depends on the autoprocessing of SKI-1/S1P, suggesting differences in the processing of cellular and viral substrates. Deletion mutagenesis showed that the transmembrane and intracellular domains of SKI-1/S1P are dispensable for arenavirus GPC processing. The expression of a soluble form of the protease in SKI-I/S1P-deficient cells resulted in the efficient processing of arenavirus GPCs and rescued productive virus infection. However, exogenous soluble SKI-1/S1P was unable to process LCMV and LASV GPCs displayed at the surface of SKI-I/S1P-deficient cells, indicating that GPC processing occurs in an intracellular compartment. In sum, our study reveals important differences in the SKI-1/S1P processing of viral and cellular substrates.

The proprotein convertases, 20 years later

The proprotein convertases (PCs) are secretory mammalian serine proteinases related to bacterial subtilisin-like enzymes. The family of PCs comprises nine members, PC1/3, PC2, furin, PC4, PC5/6, PACE4, PC7, SKI-1/S1P, and PCSK9 (Fig. 3.1). While the first seven PCs cleave after single or paired basic residues, the last two cleave at non-basic residues and the last one PCSK9 only cleaves one substrate, itself, for its activation. The targets and substrates of these convertases are very varied covering many aspects of cellular biology and communication. While it took more than 22 years to begin to identify the first member in 1989-1990, in less than 14 years they were all characterized. So where are we 20 years later in 2011? We have now reached a level of maturity needed to begin to unravel the mechanisms behind the complex physiological functions of these PCs both in health and disease states. We are still far away from comprehensively understanding the various ramifications of their roles and to identify their physiological substrates unequivocally. How do these enzymes function in vivo? Are there other partners to be identified that would modulate their activity and/or cellular localization? Would non-toxic inhibitors/silencers of some PCs provide alternative therapies to control some pathologies and improve human health? Are there human SNPs or mutations in these PCs that correlate with disease, and can these help define the finesses of their functions and/or cellular sorting? The more we know about a given field, the more questions will arise, until we are convinced that we have cornered the important angles. And yet the future may well reserve for us many surprises that may allow new leaps in our understanding of the fascinating biology of these phylogenetically ancient eukaryotic proteases (Fig. 3.2) implicated in health and disease, which traffic through the cells via multiple sorting pathways (Fig. 3.3).

Molecular changes associated with hippocampal long-lasting depression induced by the serine protease subtilisin-A

The serine protease subtilisin-A (SubA) induces a form of long-term depression (LTD) of synaptic transmission in the rat hippocampus, and molecular changes associated with SubA-induced LTD (SubA-LTD) were explored by using recordings of evoked postsynaptic potentials and immunoblotting. SubA-LTD was prevented by a selective inhibitor of SubA proteolysis, but the same inhibitor did not affect LTD induced by electrical stimulation or activation of metabotropic glutamate receptors. SubA-LTD was reduced by the protein kinase inhibitors genistein and lavendustin A, although not by inhibitors of p38 mitogen-activated protein kinase, glycogen synthase kinase-3, or protein phosphatases. It was also reduced by (RS)-α-methyl-4-carboxyphenylglycine, a broad-spectrum antagonist at metabotropic glutamate receptors. Inhibition of the Rho kinase enzyme Rho-associated coiled-coil kinase reduced SubA-LTD, although inhibitors of the RhoGTPase-activating enzymes farnesyl transferase and geranylgeranyl transferase did not. In addition, a late phase of SubA-LTD was dependent on new protein synthesis. There was a small, non-significant difference in SubA-LTD between wild-type and RhoB(-/-) mice. Marked decreases were seen in the levels of Unc-5H3, a protein that is intimately involved in the development and plasticity of glutamatergic synapses. Smaller changes were noted, at higher concentrations of SubA, in Unc-5H1, vesicle-associated membrane protein-1 (synaptobrevin), and actin, with no changes in the levels of synaptophysin, synaptotagmin, RhoA, or RhoB. None of these changes was associated with LTD induced electrically or by the metabotropic glutamate receptor agonist (RS)-3,5-dihydroxyphenylglycine. These results indicate that SubA induces molecular changes that overlap with other forms of LTD, but that the overall molecular profile of SubA-LTD is quite different.

Disruption of proprotein convertase 1/3 (PC1/3) expression in mice causes innate immune defects and uncontrolled cytokine secretion

The proprotein convertase 1/3 is expressed in the regulated secretory pathway of neural and endocrine cells. Its major function is in the post-translational processing and activation of precursor proteins. The PC1/3 knock-out (KO) mouse model has allowed us to elucidate its physiological functions in studies focused primarily on neuroendocrine tissues. However, PC1/3 is also expressed in cells of the immune system, mainly in macrophages. The present study explores the effects of innate immune challenge in the PC1/3 KO mouse. PC1/3 KO mice have an enlarged spleen with marked disorganization of the marginal zone and red pulp. Immunohistochemical studies using various markers demonstrate a depletion of dendritic cells in PC1/3 KO spleens. When challenged with lipopolysaccharide, PC1/3 KO mice are more susceptible to septic shock than wild-type controls or other PC KO mice, such as PC2 and PC7 null mice. Plasma levels of proinflammatory cytokines (IL-6, IL-1β, and TNF-α) were very significantly elevated in PC1/3 KO mice, consistent with a hypercytokinemia, i.e. indicative of a major systemic uncontrolled inflammatory response or cytokine storm. Peritoneal macrophages isolated from PC1/3 KO mice also demonstrate elevated cytokine secretion when treated with LPS. Electron micrographs show morphological features indicating a prolonged activation of these cells following LPS stimulation. We also present evidence that the proinflammatory T(h)1 pathway is dominant in the PC1/3 KO mouse model. We conclude that aside from its important role in neuroendocrine functions PC1/3 also has an important role in the regulation of the innate immune system, most likely through the regulation of cytokine secretion in macrophages.

Allelic clustering and ancestry-dependent frequencies of rs6232, rs6234, and rs6235 PCSK1 SNPs in a Northern Ontario population sample

The PCSK1 (proprotein convertase subtilisin/kexin type 1) locus encodes proprotein convertase 1/3, an endoprotease that converts prohormones and proneuropeptides to their active forms. Spontaneous loss-of-function mutations in the coding sequence of its gene have been linked to obesity in humans. Minor alleles of two common non-synonymous single-nucleotide polymorphisms (SNPs), rs6232 (T > C, N221D) and rs6235 (C > G, S690T), have been associated with increased risk of obesity in European populations. In this study, we compared the frequencies of the rs6232 and rs6234 (G > C, Q665E) SNPs in Aboriginal and Caucasian populations of Northern Ontario. The two SNPs were all relatively less frequent in Aboriginals: The minor allele frequency of the rs6232 SNP was 0.01 in Aboriginals and 0.08 in Caucasians (P < 4.10(-6)); for the rs6234 SNP, it was 0.20 and 0.32, respectively (P < 0.001). Resequencing revealed that the rs6234 SNP variation was tightly linked to that of the rs6235 SNP, as previously reported. Most interestingly, all carriers of the rs6232 SNP variation also carried the rs6234/rs6235 SNP clustered variations, but not the reverse, suggesting the former occurred later on an allele already carrying the latter. These data indicate that, in Northern Ontario Aboriginals, the triple-variant PCSK1 allele is relatively rare and might be of lesser significance for obesity risk in this population.

Β-site APP-cleaving enzyme 1 trafficking and Alzheimer's disease pathogenesis

β-Site APP-cleaving enzyme (BACE1) cleaves the amyloid precursor protein (APP) at the β-secretase site to initiate the production of Aβ peptides. These accumulate to form toxic oligomers and the amyloid plaques associated with Alzheimer's disease (AD). An increase of BACE1 levels in the brain of AD patients has been mostly attributed to alterations of its intracellular trafficking. Golgi-associated adaptor proteins, GGA sort BACE1 for export to the endosomal compartment, which is the major cellular site of BACE1 activity. BACE1 undergoes recycling between endosome, trans-Golgi network (TGN), and the plasma membrane, from where it is endocytosed and either further recycled or retrieved to the endosome. Phosphorylation of Ser498 facilitates BACE1 recognition by GGA1 for retrieval to the endosome. Ubiquitination of BACE1 C-terminal Lys501 signals GGA3 for exporting BACE1 to the lysosome for degradation. In addition, the retromer mediates the retrograde transport of BACE1 from endosome to TGN. Decreased levels of GGA proteins and increased levels of retromer-associated sortilin have been associated with AD. Both would promote the co-localization of BACE1 and the amyloid precursor protein in the TGN and endosomes. Decreased levels of GGA3 also impair BACE1 degradation. Further understanding of BACE1 trafficking and its regulation may offer new therapeutic approaches for the treatment of Alzheimer's disease.

Is secretoneurin a new hormone?

Numerous small potentially bioactive peptides are derived from the selective processing of the ~600 amino acid secretogranin II (SgII) precursor, but only the 31-42 amino acid segment termed secretoneurin (SN) is well-conserved from sharks to mammals. Both SNa and SNb paralogs have been identified in some teleosts, likely arising as a result of the specific genome duplication event in this lineage. Only one copy of the putative lamprey SgII (188 amino acids) could be identified which gives rise to a divergent agnathan SN that contains the signature YTPQ-X-LA-X(7)-EL sequence typical of the central core of all known SN peptides. In rodent models, SN has regulatory effects on neuroinflammation and neurotransmitter release, and possesses therapeutic potential for the induction of angiogenesis. The wide distribution of SN in neuroendocrine neurons and pituitary cells suggests important endocrine roles. The clearest example of the endocrine action of SN is the stimulatory effects on pituitary luteinizing hormone release from goldfish pituitary and mouse LβT2 gonadotroph cells, indicative of an important role in reproduction. Several lines of evidence suggest that the SN receptor is most likely a G-protein coupled protein. Microarray analysis of SN effects on dispersed goldfish pituitary cells in vitro reveals novel SN actions that include effects on genes involved in notch signaling and the guanylate cyclase pathway. Intracerebroventricular injection of SN increases feeding and locomotory behaviors in goldfish. Given that SgII appeared early in vertebrate evolution, SN is an old peptide with emerging implications as a new multifunctional hormone.

The role of ghrelin in the control of energy balance

Ghrelin is the only potent orexigenic peptide in circulation. It stimulates food intake and leads to positive energy balance, adipogenesis, and body weight gain. However, the physiological significance of ghrelin in the regulation of energy homeostasis is controversial, since loss of ghrelin function in rodents does not necessarily lead to anorexia and weight loss. In this chapter, we discuss the metabolic function of ghrelin and are highlighting recent findings including the discovery and function of ghrelin-acylating enzyme ghrelin O-acyltransferase (GOAT). Based on available published data, we conclude that ghrelin is a principally important endogenous regulator of energy balance, which however may affect both food intake and systemic metabolism via independent mechanisms. Importantly, ghrelin, when acylated by GOAT, might represent a key molecular link between the sensing of consumed calories and the neuroendocrine control of energy homeostasis. Thus, agents antagonizing the action of ghrelin may have therapeutic potential in the therapy of obesity.

Different distribution of neuromedin S and its mRNA in the rat brain: NMS peptide is present not only in the hypothalamus as the mRNA, but also in the brainstem

Neuromedin S (NMS) is a neuropeptide identified as another endogenous ligand for two orphan G protein-coupled receptors, FM-3/GPR66 and FM-4/TGR-1, which have also been identified as types 1 and 2 receptors for neuromedin U structurally related to NMS. Although expression of NMS mRNA is found mainly in the brain, spleen, and testis, the distribution of its peptide has not yet been investigated. Using a newly prepared antiserum, we developed a highly sensitive radioimmunoassay for rat NMS. NMS peptide was clearly detected in the rat brain at a concentration of 68.3 ± 3.4 fmol/g wet weight, but it was hardly detected in the spleen and testis. A high content of NMS peptide was found in the hypothalamus, midbrain, and pons-medulla oblongata, whereas abundant expression of NMS mRNA was detected only in the hypothalamus. These differing distributions of the mRNA and peptide suggest that nerve fibers originating from hypothalamic NMS neurons project into the midbrain, pons, or medulla oblongata. In addition, abundant expression of type 2 receptor mRNA was detected not only in the hypothalamus, but also in the midbrain and pons-medulla oblongata. These results suggest novel, unknown physiological roles of NMS within the brainstem.

Proprotein convertase 5/6 is critical for embryo implantation in women: regulating receptivity by cleaving EBP50, modulating ezrin binding, and membrane-cytoskeletal interactions

Establishment of endometrial receptivity is vital for successful embryo implantation; its failure causes infertility. Epithelial receptivity acquisition involves dramatic structural changes in the plasma membrane and cytoskeleton. Proprotein convertase 5/6 (PC6), a serine protease of the proprotein convertase (PC) family, is up-regulated in the human endometrium specifically at the time of epithelial receptivity and stromal cell decidualization. PC6 is the only PC member tightly regulated in this manner. The current study addressed the importance and mechanisms of PC6 action in regulating receptivity in women. PC6 was dysregulated in the endometrial epithelium during the window of implantation in infertile women of three demographically different cohorts. Its critical role in receptivity was evidenced by a significant reduction in mouse blastocyst attachment of endometrial epithelial cells after PC6 knockdown by small interfering RNA. Using a proteomic approach, we discovered that PC6 cleaved the key scaffolding protein, ezrin-radixin-moesin binding phosphoprotein 50 (EBP50), thereby profoundly affecting its interaction with binding protein ezrin (a key protein bridging actin filaments and plasma membrane), EBP50/ezrin cellular localization, and cytoskeleton-membrane connections. We further validated this novel PC6 regulation of receptivity in human endometrium in vivo in fertile vs. infertile patients. These results strongly indicate that PC6 plays a key role in regulating fundamental cellular remodeling processes, such as plasma membrane transformation and membrane-cytoskeletal interface reorganization. PC6 cleavage of a crucial scaffolding protein EBP50, thereby profoundly regulating membrane-cytoskeletal reorganization, greatly extends the current knowledge of PC biology and provides substantial new mechanistic insight into the fields of reproduction, basic cellular biology, and PC biochemistry.

Effects of rs6234/rs6235 and rs6232/rs6234/rs6235 PCSK1 single-nucleotide polymorphism clusters on proprotein convertase 1/3 biosynthesis and activity

BACKGROUND

Proprotein convertase 1/3 (PC1/3) is one of the endoproteases initiating the proteolytic activation of prohormones and proneuropeptides in the secretory pathway. It is produced as a zymogen that is subsequently modified by activity-determining cleavages at the amino and the carboxyl termini. In human, it is encoded by the PCSK1 locus on chromosome 5. Spontaneous inactivating mutations in its gene have been linked to obesity. Minor alleles of the common non-synonymous single-nucleotide polymorphisms (SNPs) rs6232 (T>C, N221D), rs6234 (G>C, Q665E) and rs6235 (C>G, S690T) have been associated with increased risk of obesity. We have shown that the variations associated with these SNPs are linked on minor PCSK1 alleles.

GOAL

In this study, we examined the impact of amino acid substitutions specified by the minor PCSK1 alleles on PC1/3 biosynthesis and prohormone processing activity in cultured cells.

METHODS

The common and variant isoforms of PC1/3 were expressed in transfected rat pituitary GH4C1 cells with or without proopiomelanocortin (POMC) as a substrate. Secreted PC1/3- or POMC-related proteins and peptides were analyzed by immunoblotting and immunoprecipitation.

RESULTS

When expressed in GH4C1 cells, the triple-variant PC1/3 underwent significantly more proteolytic processing at the amino and carboxyl termini than the common and double-variant isoforms. However, there was no detectable difference among these isoforms in their ability to process POMC in the transfected cells.

CONCLUSIONS

Since truncation of PC1/3 in its C-terminal region reportedly renders the enzyme unstable, we speculate that the accentuated processing of the triple variant in this region may, in vivo, create a subtle deficit of PC1/3 enzymatic activity in endocrine and neuroendocrine cells, causing impaired processing of prohormones and proneuropeptides to their bioactive forms.

The extended granin family: structure, function, and biomedical implications

The chromogranins (chromogranin A and chromogranin B), secretogranins (secretogranin II and secretogranin III), and additional related proteins (7B2, NESP55, proSAAS, and VGF) that together comprise the granin family subserve essential roles in the regulated secretory pathway that is responsible for controlled delivery of peptides, hormones, neurotransmitters, and growth factors. Here we review the structure and function of granins and granin-derived peptides and expansive new genetic evidence, including recent single-nucleotide polymorphism mapping, genomic sequence comparisons, and analysis of transgenic and knockout mice, which together support an important and evolutionarily conserved role for these proteins in large dense-core vesicle biogenesis and regulated secretion. Recent data further indicate that their processed peptides function prominently in metabolic and glucose homeostasis, emotional behavior, pain pathways, and blood pressure modulation, suggesting future utility of granins and granin-derived peptides as novel disease biomarkers.

On the cutting edge of proprotein convertase pharmacology: from molecular concepts to clinical applications

There is increasing interest in the therapeutic targeting of proteases for the treatment of important diseases. Additionally new protein-based therapeutic strategies have the potential to widen the available treatments against these pathologies. In the last decade, accumulated evidence has confirmed that the family of proteases known as proprotein convertases (PCs) are potential targets for viral infections, osteoarthritis, cancer and cardiovascular disease, among others. Nevertheless, there are still many unanswered questions about the relevance of targeting PCs in a therapeutic context, especially regarding the anticipated secondary effects of treatment, considering the observed embryonic lethality of some PC knockout mice. In this review, the benefits of PCs as pharmacological targets will be discussed, with focus on concepts and strategies, as well as on the state of advancement of actual and future inhibitors.

Hysteretic behavior of proprotein convertase 1/3 (PC1/3)

The proprotein convertases (PCs) are calcium-dependent proteases responsible for processing precursor proteins into their active forms in eukariotes. The PC1/3 is a pivotal enzyme of this family that participates in the proteolytic maturation of prohormones and neuropeptides inside the regulated secretory pathway. In this paper we demonstrate that mouse proprotein convertase 1/3 (mPC1/3) has a lag phase of activation by substrates that can be interpreted as a hysteretic behavior of the enzyme for their hydrolysis. This is an unprecedented observation in peptidases, but is frequent in regulatory enzymes with physiological relevance. The lag phase of mPC1/3 is dependent on substrate, calcium concentration and pH. This hysteretic behavior may have implications in the physiological processes in which PC1/3 participates and could be considered an additional control step in the peptide hormone maturation processes as for instance in the transformation of proinsulin to insulin.

Effects of magnesium ions on recombinant human furin: selective activation of hydrolytic activity upon substrates derived from virus envelope glycoprotein

Here we report a detailed analysis of magnesium (Mg²+) ion effects on furin hydrolysis of fluorescent resonance energy transfer decapeptide substrates derived from canonical R-X-K/R-R furin cleavage motifs within certain viral envelope glycoproteins and eukaryotic proproteins. Using virus-derived sequences a selective activation of furin by Mg²+) ions was observed as a result of cooperativity between furin subsites. Furin hydrolysis of the peptides Abz-SRRHKR↓FAGV-Q-EDDnp (from measles virus fusion protein F₀ and Abz-RERRRKKR↓GLFG-Q-EDDnp (from Asian avian influenza A, H5N1) was activated between 60- and 80-fold by MgCl₂. It appears that virus envelope glycoprotein mutations have been selected to increase their susceptibility to furin within cells, a location where Mg²+ is present in adequate concentrations for activation. Both the pH profile of furin and its intrinsic fluorescence were modified by Mg²+ ions, which bind to furin with a K(d) value of 1.1 mM.

Chromogranin A: a novel factor acting at the cross road between the neuroendocrine and the cardiovascular systems

Chromogranin A (CHGA) is a secretory protein stored in and released from neurons and cells of the diffuse neuroendocrine system. Cells of the adrenal medulla and adrenergic terminals are a main source of CHGA but also myocardial cells produce it under stress conditions. After secretion, CHGA is cleaved into several biologically active fragments, including vasostatins and catestatin. CHGA and its proteolytic peptides exert a broad spectrum of activities on the cardiovascular system. They act on blood pressure by controlling the vascular tone and the cardiac inotropic and chronotropic function. CHGA revealed to be a sensitive marker of myocardial dysfunction, with a high predictive power of morbidity and mortality in heart failure and ischemic heart disease. In addition, CHGA has been involved in the control of sustained endothelial inflammation and has been shown to be a good marker of persistent vascular inflammation in rheumatologic disorders affecting vessels.

Molecular aspects involved in swimming exercise training reducing anhedonia in a rat model of depression

Patients suffering from depression frequently display hyperactivity of the hypothalamic-pituitary-adrenal axis (HPA) resulting in elevated cortisol levels. One main symptom of this condition is anhedonia. There is evidence that exercise training can be used as a rehabilitative intervention in the treatment of depressive disorders. In this scenario, the aim of the present study was to assess the effect of an aerobic exercise training protocol on the depressive-like behavior, anhedonia, induced by repeated dexamethasone administration. The study was carried out on adult male Wistar rats randomly divided into four groups: the "control group" (C), "exercise group" (E), "dexamethasone group" (D) and the "dexamethasone plus exercise group" (DE). The exercise training consisted of swimming (1 h/d, 5 d/wk) for 3 weeks, with an overload of 5% of the rat body weight. Every day rats were injected with either dexamethasone (D/DE) or saline solution (C/E). Proper positive controls, using fluoxetine, were run in parallel. Decreased blood corticosterone levels, reduced adrenal cholesterol synthesis and adrenal weight (HPA disruption), reduced preference for sucrose consumption and increased immobility time (depressive-like behavior), marked hippocampal DNA oxidation, increased IL-10 and total brain-derived neurotrophic factor (BDNF; pro-plus mature-forms) and a severe loss of body mass characterized the dexamethasone-treated animals. Besides increasing testosterone blood concentrations, the swim training protected depressive rats from the anhedonic state, following the same profile as fluoxetine, and also from the dexamethasone-induced impaired neurochemistry. The data indicate that physical exercise could be a useful tool in preventing and treating depressive disorders.

VGF: an inducible gene product, precursor of a diverse array of neuro-endocrine peptides and tissue-specific disease biomarkers

The vgf gene (non-acronymic) is induced in vivo by neurotrophins including Nerve Growth Factor (NGF), Brain Derived Growth Factor (BDNF) and Glial Derived Growth Factor (GDNF), by synaptic activity and by homeostatic and other stimuli. Post-translational processing of a single VGF precursor gives raise to a varied multiplicity of neuro-endocrine peptides, some of which are secreted upon stimulation both in vitro and in vivo. Several VGF peptides, accounting for ∼20% of the VGF precursor sequence, have shown biological roles including regulation of food intake, energy balance, reproductive and homeostatic mechanisms, synaptic strengthening, long-term potentiation (LTP) and anti-depressant activity. From a further ∼50% of VGF derive multiple "fragments", largely identified in the human cerebro-spinal fluid by proteomic studies searching for disease biomarkers. These represent an important starting point for discovery of further VGF products relevant to neuronal brain functions, as well as to neurodegenerative and psychiatric disease conditions. A distinct feature of VGF peptides is their cell type specific diversity in all neuroendocrine organs studied so far. Selective differential profiles are found across the cell populations of pituitary, adrenal medulla and pancreatic islets, and in gastric neuroendocrine as well as some further mucosal cells, and are yet to be investigated in neuronal systems. At the same time, specific VGF peptide/s undergo selective modulation in response to organ or cell population relevant stimuli. Such pattern argues for a multiplicity of roles for VGF peptides, including endocrine functions, local intercellular communication, as well as the possible mediation of intracellular mechanisms.

α-TC1.9 cells--a model system for analyzing the endoproteolytic processing of POMC

The mouse α-TC1.9 endocrine cell line was used to analyze the amino acid requirements for endoproteolytic processing at the paired basic amino acid cleavage site, K(141)R(142) that is N-terminal to the ACTH sequence in the POMC proprotein of the anuran amphibian, Silurana tropicalis. Real-Time PCR analysis of non-transfected α-TC1.9 cells indicated that these cells endogenously express the pc2 (proprotein convertase 2) gene, but do not express the pc1/3 (proprotein convertase 1/3) gene or the pomc gene. In addition, immunocytochemical analysis and RIA analysis of non-transfected α-TC1.9 cells did not detect the presence of POMC-related products in these cells. For this study the open reading frame of a S. tropicalis POMC cDNA (wild-type) was placed into an expression vector and transiently transfected into α-TC1.9 cells. Two days after transfection the steady-state levels of α-MSH-related and β-endorphin-related end-products were nearly the same as the steady-state levels of these POMC-related end-products in extracts of the S. tropicalis intermediate pituitary. Transient transfection of either the R(142)/A(142)pomc construct or the K(141)/A(141)pomc construct completely blocked cleavage at this site and yielded a 6K immunoreactive product that had the ACTH(1-13)NH(2) sequence at the C-terminal end of the fusion protein. However, substitution of an alanine residue at R(137), Q(138), E(139), and N(140) had no effect on cleavage at the K(141)R(142) cleavage site. Collectively, these results indicate that secondary structure N-terminal to the K(141)R(142) does not appear to influence cleavage at this site. However, both K(141) and R(142) are required for the integrity of this cleavage site. Finally, this study indicates that α-TC1.9 cells should be useful for studying the amino acid requirements for the other endoproteolytic cleavage sites in the S. tropicalis POMC proprotein.

Observations on the ligand selectivity of the melanocortin 2 receptor

The melanocortin 2 receptor (MC2R) is unique in terms of ligand selectivity and in vitro expression in mammalian cell lines as compared to the other four mammalian MCRs. It is well established that ACTH is the only melanocortin ligand that can activate the ACTH receptor (i.e., melanocortin 2 receptor). Recent studies have provided new insights into the presence of a common binding site for the HFRW motif common to all melanocortin ligands. However, the activation of the melanocortin 2 receptor requires an additional amino acid motif that is only found in the sequence of ACTH. This mini-review will focus on these two topics and provide a phylogenetic perspective on the evolution of MC2R ligand selectivity.

Furin is the major processing enzyme of the cardiac-specific growth factor bone morphogenetic protein 10

Bone morphogenetic protein 10 (BMP10) is a member of the TGF-β superfamily and plays a critical role in heart development. In the postnatal heart, BMP10 is restricted to the right atrium. The inactive pro-BMP10 (∼60 kDa) is processed into active BMP10 (∼14 kDa) by an unknown protease. Proteolytic cleavage occurs at the RIRR(316)↓ site (human), suggesting the involvement of proprotein convertase(s) (PCs). In vitro digestion of a 12-mer peptide encompassing the predicted cleavage site with furin, PACE4, PC5/6, and PC7, showed that furin cleaves the best, whereas PC7 is inactive on this peptide. Ex vivo studies in COS-1 cells, a cell line lacking PC5/6, revealed efficient processing of pro-BMP10 by endogenous PCs other than PC5/6. The lack of processing of overexpressed pro-BMP10 in the furin- and PACE4-deficient cell line, CHO-FD11, and in furin-deficient LoVo cells, was restored by stable (CHO-FD11/Fur cells) or transient (LoVo cells) expression of furin. Use of cell-permeable and cell surface inhibitors suggested that endogenous PCs process pro-BMP10 mostly intracellularly, but also at the cell surface. Ex vivo experiments in mouse primary hepatocytes (wild type, PC5/6 knock-out, and furin knock-out) corroborated the above findings that pro-BMP10 is a substrate for endogenous furin. Western blot analyses of heart right atria extracts from wild type and PACE4 knock-out adult mice showed no significant difference in the processing of pro-BMP10, implying no in vivo role of PACE4. Overall, our in vitro, ex vivo, and in vivo data suggest that furin is the major convertase responsible for the generation of BMP10.

N-glycosylation controls trafficking, zymogen activation and substrate processing of proprotein convertases PC1/3 and subtilisin kexin isozyme-1

The limited proteolysis of proteins by the proprotein convertases (PCs) is a common means of producing bioactive proteins or peptides. The PCs are associated with numerous human pathologies and their activity can be reduced through the use of specific inhibitors. Here, we demonstrate an alternative approach to inhibiting PCs by altering their N-glycosylation. Through site-directed mutagenesis, we show that the convertase PC1/3 contains two N-glycans, only one of which is critical for its prosegment cleavage. The exact structure of PC1/3 N-glycans does not significantly affect its zymogen activation within endocrine cells, but glycosylation of Asn(146) is critical. Processing of the PC1/3's substrate proopiomelanocortin (POMC) was used in a cell-based assay to screen a collection of 45 compounds structurally related to known glycosidase inhibitors. Two 5-thiomannose-containing disaccharide derivatives were discovered to block PC1/3 and POMC processing into the analgesic peptide β-endorphin. These compounds also reduced the zymogen activation of the convertase subtilisin kexin isozyme-1 (SKI-1), blocked the processing of its substrate the sterol regulatory element-binding protein SREBP-2 and altered its glycosylation. Thus, modification of PC glycosylation may also be a means of blocking their activity, an effect which, in the case of SKI-1, may be of possible therapeutic use since SREBP-2 regulates sterol levels including cholesterol biosynthesis and its metabolism.

Genomewide analysis of rat barrel cortex reveals time- and layer-specific mRNA expression changes related to experience-dependent plasticity

Because of its anatomical organization, the rodent whisker-to-barrel system is an ideal model to study experience-dependent plasticity. Manipulation of sensory input causes changes in the properties of the barrels at the physiological, structural, and functional levels. However, much less is known about the molecular events underlying these changes. To explore such molecular events, we have used a genomewide approach to identify key genes and molecular pathways involved in experience-induced plasticity in the barrel cortex of adult rats. Given the natural tendency of rats to explore novel objects, exposure to an enriched environment (EE) was used to stimulate the activity of the whisker-to-barrel cortex in vivo. Microarray analysis at two different time points after EE revealed differential expression of genes encoding transcription factors, including nuclear receptors, as well as of genes involved in the regulation of synaptic plasticity, cell differentiation, metabolism, and, surprisingly, blood vessel morphogenesis. These expression differences reflect changes in somatosensory information processing because unilateral whisker clipping showed EE-induced differential expression patterns in the spared versus deprived barrel cortex. Finally, in situ hybridization revealed cortical layer patterns specific for each selected gene. Together, the present study offers the first genomewide exploration of the key genes regulated by somatosensory stimulation in the barrel cortex and thus provides a solid experimental framework for future in-depth analysis of the mechanisms underlying experience-dependent plasticity.

Surface expression of precursor N-cadherin promotes tumor cell invasion

The expression of N-cadherin (NCAD) has been shown to correlate with increased tumor cell motility and metastasis. However, NCAD-mediated adhesion is a robust phenomenon and therefore seems to be inconsistent with the "release" from intercellular adhesion required for invasion. We show that in the most invasive melanoma and brain tumor cells, altered posttranslational processing results in abundant nonadhesive precursor N-cadherin (proNCAD) at the cell surface, although total NCAD levels remain constant. We demonstrate that aberrantly processed proNCAD promotes cell migration and invasion in vitro. Furthermore, in human tumor specimens, we find high levels of proNCAD as well, supporting an overall conclusion that proNCAD and mature NCAD coexist on these tumor cell surfaces and that it is the ratio between these functionally antagonistic moieties that directly correlates with invasion potential. Our work provides insight into what may be a widespread mechanism for invasion and metastasis and challenges the current dogma of the functional roles played by classic cadherins in tumor progression.

Plasminogen activator inhibitor 1 is an intracellular inhibitor of furin proprotein convertase

Proprotein convertases (PCs) are a family of serine proteases that are involved in the post-translational processing and activation of a wide range of regulatory proteins. The upstream role of PCs in the control of many physiological and pathological processes generates a growing interest in understanding their regulation. Here, we demonstrate that the serine protease inhibitor plasminogen activator inhibitor 1 (PAI-1) forms an SDS-stable complex with the PC furin, which leads to the inhibition of the intra-Golgi activity of furin. It is known that elevated PAI-1 plasma levels are correlated with the occurrence of the metabolic syndrome and type 2 diabetes, and we show that PAI-1 reduces the furin-dependent maturation and activity of the insulin receptor and ADAM17: two proteins involved in the onset of these metabolic disorders. In addition to demonstrating that PAI-1 is an intracellular inhibitor of furin, this study also provides arguments in favor of an active role for PAI-1 in the development of metabolic disorders.

Molecular cloning of anti-Müllerian hormone from the American alligator, Alligator mississippiensis

Anti-Müllerian hormone (AMH) plays an important role in male sex differentiation in vertebrates. AMH produced by Sertoli cells of the fetal testis induces regression of the Müllerian duct in mammalian species. In alligators, sexual differentiation is controlled by the temperature during egg incubation, termed temperature-dependent sex determination (TSD). The TSD mechanism inducing sex differentiation is thought to be unique and different from that of genetic sex determination as no gene such as the SRY of mammals has been identified. However, many of the genes associated with gonadal differentiation in mammals also are expressed in the developing gonads of species exhibiting TSD. To clarify the molecular mechanisms associated with gonad formation during the temperature-sensitive period (TSP), we have cloned the full length AMH gene in the alligator, and quantitatively compared mRNA expression patterns in the gonad-adrenal-mesonephros (GAM) complex isolated from alligator embryos incubated at male and female producing temperatures. The deduced amino acid sequence of the alligator AMH cDNA showed high identity (59-53%) to avian AMH genes. AMH mRNA expression was high in the GAM of male alligator embryos at stage 24 (immediately after sex determination) and hatchlings, but suppressed in the GAM of estrogen-exposed hatchlings incubated at the male-producing temperature. In the alligator AMH proximal promoter, a number of transcriptional factors (for SF-1. GATA, WT-1 and SOX9) binding elements were also identified and they exhibit a conserved pattern seen in other species. SOX9 up-regulates transcriptional activity through the amAMH promoter region. These results suggested that AMH and SOX9 play important roles in TSD of the American alligator.

FurinDB: A database of 20-residue furin cleavage site motifs, substrates and their associated drugs

FurinDB (freely available online at http://www.nuolan.net/substrates.html) is a database of furin substrates. This database includes experimentally verified furin cleavage sites, substrates, species, experimental methods, original publications of experiments and associated drugs targeting furin substrates. The current database release contains 126 furin cleavage sites from three species: mammals, bacteria and viruses. A main feature of this database is that all furin cleavage sites are recorded as a 20-residue motif, including one core region (eight amino acids, P6–P2′) and two flanking solvent accessible regions (eight amino acids, P7–P14, and four amino acids, P3′–P6′), that represent our current understanding of the molecular biology of furin cleavage. This database is important for understanding the molecular evolution and relationships between sequence motifs, 3D structures, cellular functions and physical properties required by furin for cleavage, and for elucidating the molecular mechanisms and the progression of furin cleavage associated human diseases, including pathogenic infections, neurological disorders, tumorigenesis, tumor invasion, angiogenesis, and metastasis. FurinDB database will be a solid addition to the publicly available infrastructure for scientists in the field of molecular biology.

The proprotein convertase PC7: unique zymogen activation and trafficking pathways

The zymogen activation mechanism and physiological functions of the most ancient and highly conserved basic amino acid-specific proprotein convertase 7 (PC7) are not known. Herein, we characterized the biosynthesis, subcellular localization, and trafficking of the membrane-bound full-length rat and human PC7. The prosegment of PC7 is primarily secreted alone as a non-inhibitory protein via the conventional, Golgi-dependent, secretory pathway. Mature PC7 is partially sulfated and thus reaches the cell surface via the conventional route. However, a fraction of PC7 reaches the cell surface through a brefeldin A- and COPII-independent unconventional secretory pathway. The latter trafficking may explain the rapid (<10 min) transit of a fraction of PC7 from the ER to the cell surface. Electron microscopy further confirmed the localization of PC7 to the cell surface of HEK293 cells. Within the cytosolic tail, only two cysteines (Cys(699) and Cys(704)) are palmitoylated, but this modification does not affect the choice of trafficking pathway. Swapping the transmembrane-cytosolic tail (TMCT) sequences of the convertases Furin and PC7 revealed that PC7(TMCT-Furin) is much more sulfated and hence traffics more efficiently through the conventional secretory pathway. In contrast, the Furin(TMCT-PC7) is no longer sulfated and thus reaches the cell surface by the unconventional pathway. Because trafficking of PC7(CT-Furin) and Furin(CT-PC7) resemble their wild type counterparts, we deduce that the transmembrane domain of PC7 regulates the sorting of PC7 toward the unconventional secretory pathway. In conclusion, PC7 is distinct from other proprotein convertases in its zymogen activation, subcellular localization, and trafficking.

The catecholamine release-inhibitory peptide catestatin (chromogranin A344-363) modulates myocardial function in fish

Catestatin (CST), the 21-amino acid, cationic and hydrophobic peptide proteolytically derived from the ubiquitous chromogranin A (CgA), is an endogenous inhibitor of catecholamine release, a potent vasodilator in vivo and an anti-hypertensive agent in mammals, including humans. Recently, we discovered that CST also functions as an important negative modulator of heart performance in frog and rat. To gain an evolutionary perspective on CST cardiotropism in fish, we analysed the influence of bovine CST (CgA₃₄₄₋₃₆₄) on the eel heart, as well as the eventual species-specific mechanisms of its myocardial action. Experiments were carried out on fresh-water eels (Anguilla anguilla L.) using an electrically paced isolated working heart preparation. Stroke volume and stroke work were used as measures of ventricular performance. Under basal conditions, CST (from 11 nmol l⁻¹ to 165 nmol l⁻¹) caused a concentration-dependent negative inotropism, which was abolished by inhibitors of either β₁/β₂ (propranolol) or β₃ (SR₅₉₂₃₀) adrenergic receptors, or by G(i/o) protein (PTx) or nitric oxide synthase (L-NMMA), or guanylate cyclase (ODQ) blockers. This suggests a β-adrenergic receptor-G(i/o) protein-NO-cGMP-dependent mechanism. By contrast, the CST-induced cardio-suppression was not influenced by atropine, unspecific muscarinic antagonist, thus excluding cholinergic receptor involvement. CST also counteracted the adrenergic (isoproterenol)-mediated positive inotropism. Under increased preload (i.e. Frank-Starling response) conditions, CST induced a significant increase of the Frank-Starling response, which was blocked by L-NMMA and thapsigargin, but independent from guanylate cyclase. In conclusion, this is the first report in fish that CST modulates myocardial performance under basal, as well as under increased preload, conditions and counteracts the adrenergic-mediated positive inotropism, which strikingly supports the evolutionary significance and establishes the cardioactive role of this peptide.

PC6 levels in uterine lavage are closely associated with uterine receptivity and significantly lower in a subgroup of women with unexplained infertility

BACKGROUND Embryo implantation requires a healthy embryo and a receptive uterus. Uterine incompetence contributes significantly to implantation failure and infertility. To date, there are no reliable biochemical methods that can determine whether the uterus is receptive. Proprotein convertase 5/6 (PC6) is tightly regulated in the uterus and critical for receptivity and implantation; its secretory nature predicts PC6 to be secreted into the uterine cavity. The present study examines whether PC6 is detectable in uterine lavage and whether there is any correlation between secreted PC6 levels and uterine receptivity. METHODS Western blotting determined the presence of PC6 protein in uterine lavage. A sensitive and high-throughput activity assay was established and validated. This assay was applied to 103 lavages collected from different phases of the menstrual cycle from women with proven fertility or unexplained infertility. RESULTS Uterine lavage contained PC6 protein with levels paralleling enzymatic activity. PC6 levels were significantly higher in the receptive than in the non-receptive phase in fertile women, and the putative receptive phase levels in a subgroup of women with unexplained infertility were significantly lower than in the fertile counterparts. CONCLUSIONS PC6 levels in uterine lavage are significantly elevated in the luteal phase of fertile women and markedly reduced in a subgroup of women with unexplained infertility. Uterine fluid is a valuable source of material to evaluate uterine function. Detection of PC6 in uterine fluid may lead to the development of a rapid and relatively non-surgical assessment of uterine receptivity.

Proteins mediating collagen biosynthesis and accumulation in arterial repair: novel targets for anti-restenosis therapy

Events contributing to restenosis after coronary interventions include platelet aggregation, inflammatory cell infiltration, growth factor release, and accumulation of smooth muscle cells (SMCs) and extracellular matrix (ECM). The ECM is composed of various collagen subtypes and proteoglycans and over time constitutes the major component of the mature restenotic plaque. The pathophysiology of collagen accumulation in the ECM during arterial restenosis is reviewed. Factors regulating collagen synthesis and degradation, including various cytokines and growth factors involved in the process, may be targets for therapies aimed at prevention of in-stent restenosis.

Molecular mechanisms of interspecies transmission and pathogenicity of influenza viruses: Lessons from the 2009 pandemic

The emergence of the 2009 H1N1 virus pandemic was unexpected, since it had been predicted that the next pandemic would be caused by subtype H5N1. We also had to learn that a pandemic does not necessarily require the introduction of a new virus subtype into the human population, but that it may result from antigenic shift within the same subtype. The new variant was derived from human and animal viruses by genetic reassortment in the pig, supporting the concept that this animal is the mixing vessel for the generation of new human influenza viruses. Although it is generally believed that the 2009 outbreak was mild, there have been severe cases particularly among the young and the middle-aged. Pathogenicity and host range are determined to a large extent by the polymerase, the haemagglutinin and the NS1 protein of influenza A viruses. There is evidence that mutations of these proteins may change the pathogenicity of the new virus.

Proprotein convertase PC7 enhances the activation of the EGF receptor pathway through processing of the EGF precursor

Although the processing profile of the membrane-bound epidermal growth factor precursor (pro-EGF) is tissue-specific, it has not been investigated at the cellular level nor have the cognate proteinases been defined. Among the proprotein convertases (PCs), only the membrane-bound PC7, the most ancient and conserved basic amino acid-specific PC family member, induces the processing of pro-EGF into an ∼115-kDa transmembrane form (EGF-115) at an unusual VHPR(290)↓A motif. Because site-directed mutagenesis revealed that Arg(290) is not critical, the generation of EGF-115 by PC7 is likely indirect. This was confirmed by testing a wide range of protease inhibitors, which revealed that the production of EGF-115 is most probably achieved via the activation by PC7 of a latent serine and/or cysteine protease(s). EGF-115 is more abundant at the cell surface than pro-EGF and is associated with a stronger EGF receptor (EGFR) activation, as evidenced by higher levels of phosphorylated ERK1/2. This suggests that the generation of EGF-115 represents a regulatory mechanism of juxtacrine EGFR activation. Thus, PC7 is distinct from the other PCs in its ability to enhance the activation of the cell surface EGFR.

Prohormone processing in zebrafish

Proprotein convertases (PCs) are secretory proteolytic enzymes that activate precursor proteins into biologically active forms by limited proteolysis at one or multiple internal sites. PCs are implicated in the processing of multiple protein precursors, including hormones, proteases, growth factors, angiogenic factors, and receptors. PCs have been linked recently to various pathologies such as Alzheimer's disease, tumorigenesis, and infections. The zebrafish has emerged as an attractive model for studying the role of PCs not only in substrate production but also in development. Herein we describe methods that are used to characterize DNA sequences of PCs in zebrafish, as well as to evaluate the ontogeny and tissue distribution of their transcripts. We also provide information on the morpholino-mediated knockdown of proprotein convertases.

In vitro assay for protease activity of proprotein convertase subtilisin kexins (PCSKs): an overall review of existing and new methodologies

The mammalian proprotein convertase subtilisin kexins (PCSKs) previously called proprotein or prohormone convertases (PCs) are a family of Ca(+2)-dependent endoproteases in the subtilisin family. These proteolytic enzymes exert their many crucial physiological and biological functions in vivo via their ability to cleave larger inactive precursor proteins into their biologically active mature forms. This event takes place in a highly efficient and selective manner. Such actions of PCSKs either alone or in combination to cleave specific protein bonds are the hallmark events that not only define the normal functions and metabolism of the body but also may lead to a variety of diseases or disorders with associated conditions. These include among others, diabetes, obesity, cancer, cardiovascular diseases, reproduction abnormalities as well as viral bacterial infections. These conditions were the direct consequences of an enhanced level of enzymatic activity of one or more PCSKs except only PCSK9, whose protease activity in relation to its physiological substrate has yet to be characterized. Owing to this finding, a large number of research studies have been exclusively devoted to develop rapid, efficient and reliable in vitro methods for examining the protease activity of these enzymes. Several assays have been developed to monitor PCSK activity and these are widely used in chemical, biochemical, cellular and animal studies. This review will cover various methodologies and protocols that are currently available in the literature for PCSK activity assays. These include liquid phase methods using fluorogenic, chromogenic and intramolecularly quenched fluorescent substrates as well as a newly developed novel solid phase fluorescence method. This review will also highlight the usefulness of these methodologies and finally a comparative analysis has been made to examine their merits and demerits with some key examples.

A proteomic protocol to identify physiological substrates of pro-protein convertases

Proprotein convertases (PCs) convert pro-proteins into their bioactive forms through limited proteolytic cleavage, thereby regulating the temporal and spatial activation of a large number of functionally important proteins. This "converting" process is involved in a wide range of essential physiological and pathological processes, making PCs valuable therapeutic targets. One of the challenges in the field of PC research has been to identify the physiological substrates of a particular PC in a specific tissue or cellular process. Proteomics provides an unprecedented opportunity to identify novel PC substrates in a physiological context. Here we provide a detailed practical procedure utilizing two-dimensional fluorescent differential gel electrophoresis (2D-DiGE) and tandem mass spectrometry techniques, in combination with other standard molecular and biochemical methods, to identify and subsequently validate novel PC6 substrates in a critical uterine event called decidualization. This method is applicable to the study of any PC members and their relevant cellular processes.

Analyses of PCSK9 post-translational modifications using time-of-flight mass spectrometry

Post-translational modification(s) can affect a protein's function - changing its half-life/stability, its protein-protein interactions, biological activity and/or sub-cellular localization. Following translation, a protein can be modified in several ways, including (i) disulfide bridge formation, (ii) chemical conversion of its constituent amino acids (for instance, glutamine can undergo deamidation to glutamic acid), (iii) sulfation, phosphorylation, de/acetylation, and glycosylation (to name a few), (iv) addition of other proteins as occurs during sumoylation and ubiquitination, and (v) proteolytic cleavage(s). There are several techniques available to identify and monitor post-translational modifications of proteins and peptides including mass spectrometry, two-dimensional sodium dodecyl sulfate polyacrylamide electrophoresis (2D-SDS-PAGE), radiolabeling, and immunoblotting. Ciphergen's surface-enhanced laser desorption/ionization time-of-flight mass spectrometer (SELDI-TOF-MS) has been used successfully for protein/peptide profiling in disease states and for the detection of protein/peptide biomarkers (1-4). In this chapter, the secreted proprotein convertase subtilisin/kexin 9 (PCSK9), which we study in our lab, is used to demonstrate coupling of immunoprecipitation with Ciphergen's time-of-flight mass spectrometer and its ProteinChip software to detect and analyze the common post-translational modifications of phosphorylation and glycosylation. The following topics are covered (1): preparation of cell extracts/samples/spent media (2), processing of samples by immunoprecipitation including optimization of conditions and (3) data acquisition by mass spectrometry and its subsequent analyses.

Pcsk6 mutant mice exhibit progressive loss of ovarian function, altered gene expression, and formation of ovarian pathology

Bioactivation of precursor proteins by members of the proprotein convertase (PC) family is essential for normal reproduction. The Pcsk6 gene is a member of the PC family that is expressed in numerous ovarian cell types including granulosa cells and oocytes. We hypothesized that loss of PCSK6 would produce adverse effects in the mouse ovary. Mice incapable of expressing PCSK6 (Pcsk6(tm1Rob)) were obtained, and reproductive parameters (serum hormones, whelping interval, estrus cyclicity, and fertility) were compared to Pcsk6(+/+) mice. While Pcsk6(tm1Rob) female mice are fertile, they manifest reduced reproductive capacity at an accelerated rate relative to Pcsk6(+/+) mice. Reproductive senescence is typically reached by 9 months of age and is correlated with loss of estrus cyclicity, elevated serum FSH levels, and gross alterations in ovarian morphology. A wide range of ovarian morphologies were identified encompassing mild, such as an apparent reduction in follicle number, to moderate--ovarian atrophy with a complete absence of follicles--to severe, manifesting as normal ovarian structures replaced by benign ovarian tumors, including tubulostromal adenomas. Targeted gene expression profiling highlighted changes in RNA expression of molecules involved in processes such as steroidogenesis, gonadotropin signaling, transcriptional regulation, autocrine/paracrine signaling, cholesterol handling, and proprotein bioactivation. These results show that PCSK6 activity plays a role in maintaining normal cellular and tissue homeostasis in the ovary.

Replication study of SNP associations for colorectal cancer in Hong Kong Chinese

BACKGROUND

Recent genome-wide association studies of colorectal cancer (CRC) have identified common single-nucleotide polymorphisms (SNPs) mapping to 10 independent loci that confer modest increased risk. These studies have been conducted in European populations and it is unclear whether these observations generalise to populations with different ethnicities and rates of CRC.

METHODS

An association study was performed on 892 CRC cases and 890 controls recruited from the Hong Kong Chinese population, genotyping 32 SNPs, which were either associated with CRC in previous studies or are in close proximity to previously reported risk SNPs.

RESULTS

Twelve of the SNPs showed evidence of an association. The strongest associations were provided by rs10795668 on 10p14, rs4779584 on 15q14 and rs12953717 on 18q21.2. There was significant linear association between CRC risk and the number of independent risk variants possessed by an individual (P=2.29 × 10(-5)).

CONCLUSION

These results indicate that some previously reported SNP associations also impact on CRC risk in the Chinese population. Possible reasons for failure of replication for some loci include inadequate study power, differences in allele frequency, linkage disequilibrium structure or effect size between populations. Our results suggest that many associations for CRC are likely to generalise across populations.

In vivo evidence that furin from hepatocytes inactivates PCSK9

The proprotein convertase PCSK9 plays a key role in cholesterol homeostasis by binding the LDL receptor and targeting it toward degradation. PCSK9 is strongly expressed in the liver and is found in human and mouse plasma as mature (∼ 62 kDa) and inactivated (∼ 55 kDa) forms. Ex vivo data showed that human PCSK9 is inactivated by cleavage at Arg(218)↓ by the overexpressed convertases furin and PC5/6A. Analysis of the plasma of human heterozygotes for R218S and F216L mutations revealed a ∼ 50% reduction in the levels of the ∼ 55-kDa form. To identify the convertase(s) responsible for cleavage at Arg(218) in vivo, we inactivated the genes of furin and/or PC5/6 specifically in hepatocytes. The PCSK9-inactivated form was strongly reduced in mice lacking furin in hepatocytes (Fur-hKO) and only slightly reduced in PC5/6-hKO plasma. In agreement with a key role of furin in regulating PCSK9 activity in vivo, we observed an overall 26% drop in the LDL receptor protein levels of Fur-hKO livers, likely due to the compound effects of a 35% increase in PCSK9 mRNA levels and the loss of PCSK9 cleavage, suggesting a higher activity of PCSK9 in these mice. Overexpression of PCSK9 in primary hepatocytes obtained from these mice revealed that only full-length, membrane-bound, but not soluble, furin is the cognate convertase. We conclude that in hepatocytes furin regulates PCSK9 mRNA levels and is the key in vivo-inactivating protease of circulating PCSK9.

The precursor to the germ cell-specific PCSK4 proteinase is inefficiently activated in transfected somatic cells: evidence of interaction with the BiP chaperone

Proprotein convertase subtilisin/kexin type 4 (PCSK4), also known as proprotein convertase 4 (PC4), is a serine endoproteinase primarily expressed in testicular germ cells and in sperm. Inactivation of its gene in mouse causes male infertility. From studies of the biosynthesis of PCSK3/furin, its closest relative, it has been inferred that PCSK4 is synthesised in the endoplasmic reticulum as a zymogen; that it is rapidly matured by autocatalytic cleavage between the prodomain and the catalytic domain; that the cleaved prodomain remains attached to the mature enzyme; and that the enzyme is finally activated by the removal of the prodomain peptides following a secondary cleavage within the prodomain. In this study, we used human embryonic kidney 293 (HEK293) cells to study the biosynthesis of rat or human PCSK4. Our results show that the bulk of PCSK4 remains as an intracellular zymogen, presumably trapped in the endoplasmic reticulum, where it interacts with the general molecular chaperone glucose-regulated protein 78/Immunoglobulin heavy-chain binding protein (GRP78/BiP). These data suggest that, unlike other members of the convertase family, proPCSK4 cannot efficiently self-activate in somatic cells. These cells may lack the intracellular environment and the interacting molecules specific to testicular germ cells where this enzyme is normally expressed.

RALFs: peptide regulators of plant growth

Peptide signaling regulates a variety of developmental processes and environmental responses in plants. For example, the peptide systemin induces the systemic defense response in tomato and defensins are small cysteine-rich proteins that are involved in the innate immune system of plants. The CLAVATA3 peptide regulates meristem size and the SCR peptide is the pollen self-incompatibility recognition factor in the Brassicaceae. LURE peptides produced by synergid cells attract pollen tubes to the embryo sac. RALFs are a recently discovered family of plant peptides that play a role in plant cell growth.