Molecular cloning and cellular localization of a furin-like prohormone convertase from the atrial gland of Aplysia

Big defensins, a diverse family of antimicrobial peptides that follows different patterns of expression in hemocytes of the oyster Crassostrea gigas

Background

Big defensin is an antimicrobial peptide composed of a highly hydrophobic N-terminal region and a cationic C-terminal region containing six cysteine residues involved in three internal disulfide bridges. While big defensin sequences have been reported in various mollusk species, few studies have been devoted to their sequence diversity, gene organization and their expression in response to microbial infections.

Findings

Using the high-throughput Digital Gene Expression approach, we have identified in Crassostrea gigas oysters several sequences coding for big defensins induced in response to a Vibrio infection. We showed that the oyster big defensin family is composed of three members (named Cg-BigDef1, Cg-BigDef2 and Cg-BigDef3) that are encoded by distinct genomic sequences. All Cg-BigDefs contain a hydrophobic N-terminal domain and a cationic C-terminal domain that resembles vertebrate β-defensins. Both domains are encoded by separate exons. We found that big defensins form a group predominantly present in mollusks and closer to vertebrate defensins than to invertebrate and fungi CSαβ-containing defensins. Moreover, we showed that Cg-BigDefs are expressed in oyster hemocytes only and follow different patterns of gene expression. While Cg-BigDef3 is non-regulated, both Cg-BigDef1 and Cg-BigDef2 transcripts are strongly induced in response to bacterial challenge. Induction was dependent on pathogen associated molecular patterns but not damage-dependent. The inducibility of Cg-BigDef1 was confirmed by HPLC and mass spectrometry, since ions with a molecular mass compatible with mature Cg-BigDef1 (10.7 kDa) were present in immune-challenged oysters only. From our biochemical data, native Cg-BigDef1 would result from the elimination of a prepropeptide sequence and the cyclization of the resulting N-terminal glutamine residue into a pyroglutamic acid.

Conclusions

We provide here the first report showing that big defensins form a family of antimicrobial peptides diverse not only in terms of sequences but also in terms of genomic organization and regulation of gene expression.

Regulatory actions of neuropeptides and peptide hormones on the reproduction of molluscsThe present review is one of a series of occasional review articles that have been invited by the Editors and will feature the broad range of disciplines and expertise represented in our Editorial Advisory Board

Reproductive success of individual animals is essential for the survival of any species. Molluscs have adapted to a wide variety of environments (freshwater, brackish water, seawater, and terrestrial habits) and have evolved unique tactics for reproduction. Both of these features attract the academic interests of scientists. Because neuropeptides and peptide hormones play critical roles in neural and neurohormonal regulation of physiological functions and behaviors in this animal group, the regulatory actions of these messengers in reproduction have been extensively investigated. In this review, we will briefly summarize how peptidergic messengers are involved in various aspects of reproduction, using some peptides such as egg-laying hormone, caudo-dorsal cell hormone, APGWamide, and gonadotropin-releasing hormone as typical examples.

Structure, localization and potential role of a novel molluscan trypsin inhibitor in Lymnaea

Eggs and egg masses of the freshwater gastropod mollusc Lymnaea provide a microenvironment for developing embryos. Secretions of the exocrine albumen gland of Lymnaea are packaged in the eggs of an egg mass before the eggs are laid externally. The perivitelline fluid that directly surrounds individual oocytes is the main source of nutrition for developing embryos. During early stages of development, the perivitelline fluid is initially internalized by pinocytosis and degraded by lysosomes; in later stages, the embryo ingests the fluid. We previously found that the albumen gland produces large amounts of Lymnaea epidermal growth factor. The albumen gland also appears to produce significant amounts of a novel Lymnaea trypsin inhibitor (LTI), a second peptide that was purified and characterized from Lymnaea albumen gland extracts. The primary structure was determined by microsequence analysis, mass spectrometry, and C-terminal sequence analysis, and showed that LTI is a 57-residue glycosylated peptide. Comparison of the LTI sequence with other known serine protease inhibitors indicates that LTI is a member of the bovine pancreatic trypsin inhibitor family. Reverse phase-high performance liquid chromatography, microsequence analysis, mass spectrometry, and immunocytochemistry demonstrated that abundant amounts of intact LTI are packaged in egg masses. The presence of a trypsin inhibitor in the perivitelline fluid compartment of the egg mass may minimize digestion of peptides and proteins in the perivitelline fluid that are important for the development of the embryo, for example, Lymnaea epidermal growth factor.

Neuropeptide amidation: cloning of a bifunctional alpha-amidating enzyme from Aplysia

One of the most common mechanisms of posttranslational modifications to generate biologically active (neuro)peptides is the process of peptide alpha-amidation. The only enzyme known to catalyze this important modification is peptidylglycine alpha-amidating monooxygenase (PAM): a (bifunctional) zymogen, giving rise to a monooxygenase (PHM) and a lyase (PAL). The highly peptidergic central nervous system and endocrine system of the marine mollusk Aplysia has homologs of various mammalian peptide processing enzymes, including furin, Afurin2, prohormone convertase 1 (PC1), PC2, carboxypeptidase E (CPE) and CPD. Previously, it has been shown that the abdominal ganglion of Aplysia, which contains approximately 800 peptidergic bag cell neurons, contains the highest specific alpha-amidating activity. We have identified and cloned multiple overlapping central nervous system and bag cell cDNAs that encode a predicted 748-residue protein that is a member of the PAM family. The protein sequence contains the contiguous sequence of the catalytic domains of PHM and PAL, clearly demonstrating the existence of bifunctional Aplysia PAM, the first invertebrate PAM zymogen with an organization similar to that in vertebrates. None of the characterized clones encoded the so-called exon A domain between the PHM and PAL domains. Furthermore, in a specific search by reverse transcription-polymerase chain reaction of RNA from multiple tissues we could only detect exon A-less transcripts. PAM expression was detected in the central nervous system, and in several endocrine and exocrine organs. Aplysia PAM is a candidate prohormone processing enzyme that plays an important role in the processing of Aplysia prohormones in the secretory pathway.

Interaction of Drosophila melanogaster prohormone convertase 2 and 7B2. Insect cell-specific processing and secretion

The prohormone convertases (PCs) are an evolutionarily ancient group of proteases required for the maturation of neuropeptide and peptide hormone precursors. In Drosophila melanogaster, the homolog of prohormone convertase 2, dPC2 (amontillado), is required for normal hatching behavior, and immunoblotting data indicate that flies express 80- and 75-kDa forms of this protein. Because mouse PC2 (mPC2) requires 7B2, a helper protein for productive maturation, we searched the fly data base for the 7B2 signature motif PPNPCP and identified an expressed sequence tag clone encoding the entire open reading frame for this protein. dPC2 and d7B2 cDNAs were subcloned into expression vectors for transfection into HEK-293 cells; mPC2 and rat 7B2 were used as controls. Although active mPC2 was detected in medium in the presence of either d7B2 or r7B2, dPC2 showed no proteolytic activity upon coexpression of either d7B2 or r7B2. Labeling experiments showed that dPC2 was synthesized but not secreted from HEK-293 cells. However, when dPC2 and either d7B2 or r7B2 were coexpressed in Drosophila S2 cells, abundant immunoreactive dPC2 was secreted into the medium, coincident with the appearance of PC2 activity. Expression and secretion of dPC2 enzyme activity thus appears to require insect cell-specific posttranslational processing events. The significant differences in the cell biology of the insect and mammalian enzymes, with 7B2 absolutely required for secretion of dPC2 and zymogen conversion occurring intracellularly in the case of dPC2 but not mPC2, support the idea that the Drosophila enzyme has specific requirements for maturation and secretion that can be met only in insect cells.

Evolution of the prohormone convertases: identification of a homologue of PC6 in the protochordate amphioxus

Many of the protein precursors traversing the secretory pathway undergo cleavage at multibasic sites to generate their bioactive forms. The proprotein convertases (PCs), a family of subtilisin-like proteases, are the major endoproteases that serve this function. Genes encoding seven distinct members of this family have so far been characterized in vertebrates: furin, PC2, PC1/PC3, PC4, PACE4, PC5/PC6 and PC7/PC8/LPC. Multiple PC genes have also been cloned from a number of invertebrates, including Drosophila melanogaster and Caenorhabditis elegans. These findings suggest that gene duplication and diversification of the PCs have occurred throughout metazoan evolution. To investigate the structural and functional changes which have occurred during vertebrate development, we have analyzed the expression of PC genes in the protochordate amphioxus. We have previously shown that amphioxus express homologous PC2 and PC1/PC3 genes [Proc. Natl. Acad. Sci. USA 92 (1995) 3591]. Here we report the characterization of amphioxus cDNAs encoding proteases with a high degree of similarity to mammalian PC6. Three cDNAs encoding three PC6 isoforms differing only in their carboxy-terminal sequences were found, derived by alternative splicing. Two isoforms appear to be soluble enzymes, whereas the third contains a transmembrane hydrophobic segment and thus is likely to be membrane-bound. All three variants contain many repeats of a cysteine-rich motif that is found in several other PC family members. Thus, amphioxus, like the vertebrates, expresses two types of PCs, e.g., PC2 and PC1/PC3 which function in the regulated secretory pathway in neuroendocrine cells, and the more widely expressed PC6 which functions mainly in the constitutive pathway.

Cloning and expression of Aplysia carboxypeptidase D, a candidate prohormone-processing enzyme

Many peptide hormones in a variety of species are produced from larger precursors by limited proteolysis at basic amino acid-containing sites. The marine mollusc Aplysia has homologs of mammalian peptide-processing enzymes, including furin, prohormone convertase 1 (PC1), PC2, and carboxypeptidase E (CPE). A novel neuronal Aplysia enzyme was recently identified that was most closely related to carboxypeptidase D (CPD; Fan and Nagle, DNA Cell Biol. 15, 937-945, 1996), a second carboxypeptidase thought to be present in the secretory pathway and to contribute to peptide hormone processing. We have identified and cloned multiple overlapping bag-cell neuron cDNAs that encode two proteins that are members of the CPD family. Sequence analyses demonstrate that the longer CPD protein (1446 residues) contains an N-terminal signal peptide and four carboxypeptidase-like domains; the third and fourth domains are not predicted to form active enzymes, as several critical residues are absent. The shorter CPD protein is predicted to contain two active carboxypeptidase-like domains. Northern blot analysis identified a major Aplysia CPD mRNA (5.3 kb) and several smaller minor transcripts in central nervous system tissue. The CPD was purified from Aplysia ovotestis using a method previously developed for mammalian CPD. The purified Aplysia CPD binds antisera raised against regions of the protein encoded by the Aplysia cDNA clone, as well as an antiserum raised against duck CPD. The enzymatic properties of purified Aplysia CPD are generally similar to those of mammalian CPD. Aplysia CPD is a candidate prohormone-processing enzyme that may play a role in the processing of Aplysia prohormones in the secretory pathway.

Identification and characterization of spodoptera frugiperda furin: a thermostable subtilisin-like endopeptidase

Spodoptera frugiperda (Sf9) cells are widely employed for high-level expression of heterologous recombinant genes from baculovirus vectors. Using a plasmid library encoding cDNA of Sf9 cells we have identified here the Spodoptera frugiperda analog of the proprotein convertase furin which plays an important role in posttranslational protein processing. Spodoptera frugiperda furin (Sfurin) is closest related to Drosophila melanogasterfurin with which it shares an extended cysteine-rich domain, whereas mammalian furin shows high homology only in the catalytic domain. Mammalian furin and Sfurin were further compared by expression from baculovirus vectors. Substrate specificity and inhibitor profiles are identical for Sfurin and mammalian furin, whereas calcium-dependence, pH-optimum, and thermostability differ. Cleavage of recombinant influenza virus hemagglutinin was significantly enhanced in Sf9 cells after overexpression of Sfurin.

Molecular cloning of a cDNA encoding the neuropeptides APGWamide and cerebral peptide 1: localization of APGWamide-like immunoreactivity in the central nervous system and male reproductive organs of Aplysia

While much is known about the neural and endocrine mechanisms that control egg laying in the gastropod mollusk Aplysia, relatively little is known about the regulation of male reproductive activity in this simultaneous hermaphrodite. In the present study, we have cloned and sequenced a cDNA that encodes a precursor protein, the predicted posttranslational processing of which presumably generates nine copies of the neuropeptide Ala-Pro-Gly-Trp-NH2 (APGWamide), five connecting peptide sequences, and a C-terminal peptide. The sequence of one connecting peptide is identical to the previously characterized cerebral peptide 1. Northern blot analysis identified two major APGWamide mRNA transcripts (approximately 1.3 kb, approximately 2.4 kb), which were present in central nervous system ganglia, but were most abundant in the right cerebral and right pedal ganglia. Immunohistochemical studies using sexually mature Aplysia demonstrated that the vast majority of APGWamide-like immunoreactivity was localized in 30-40 neurons along the anterior and medial margins of the right cerebral ganglion and in a cluster of 15-20 neurons in the right pedal ganglion. A total of only about ten immunoreactive neurons were located in other ganglia. Immunohistochemistry also demonstrated that APGWamide was present in the reproductive organs that participate in the storage or transport of sperm, including the small hermaphroditic duct (site of sperm storage before mating), the white hemiduct (also known as the copulatory duct), and penial complex. As a group, these data suggest that APGWamide may play a role in regulating male reproductive function in Aplysia, as it does in other gastropods.

Expression and genetic variation of the Aplysia egg-laying hormone gene family in the atrial gland

We have screened an Aplysia atrial gland cDNA library using an egg-laying hormone (ELH) precursor probe and have isolated and characterized five different clones, four of which are full-length and approximately 0.8 kb in size. The characterization of these cDNA clones firmly established the genetic variation of the ELH-related precursors expressed in the atrial gland and provided a rational basis for their revised nomenclature proposed herein. The five precursor ELH-related cDNA sequences obtained predicted the following genetically distinct polypeptide precursors designated as: A, [Asp143]A, [Glu94,Gln139]A, [Pro25]B, and [Phe96,Asp107]BT. The [Phe96,Asp107]Br cDNA sequence predicted a truncated form of a B-type precursor. Northern blot analysis of atrial gland RNA identified two transcripts of about equal intensity of 0.9 kb and 1.1 kb. Polymerase chain reaction of genomic DNA, together with DNA sequence analysis, resolved previously reported discrepancies between genomic and cDNA sequences of the ELH-related precursors. Taken together the results obtained identified the expression of five ELH-related precursor genes in the atrial gland of Aplysia from at least two genetic loci per haploid genome.

Molecular cloning of Aplysia neuronal cDNAs that encode carboxypeptidases related to mammalian prohormone processing enzymes

The bag cell neurons of Aplysia synthesize an egg-laying hormone (ELH) precursor that initially is cleaved into two fragments in the Golgi apparatus, and the fragments are differentially packaged in separate granule populations and further processed. Aplysia Afurin, Afurin2, prohormone convertase 1 (PC1), and PC2 are thought to be involved in the posttranslational processing of the ELH prohormone. In the present study, we have cloned Aplysia neuronal cDNAs that encode an enzyme most closely related to mammalian carboxypeptidase E (CPE), a peptide hormone processing enzyme that removes basic residues during prohormone processing. Northern blot analysis identified a single Aplysia CPE mRNA (approximately 5.2 kb) in central nervous system tissue. The C-terminal region of Aplysia CPE contains amphiphilic alpha-helices that may serve as a hydrophobic membrane anchor. A novel neuronal Aplysia enzyme was also identified by the polymerase chain reaction that was most closely related to the carboxypeptidase D (CPD)-related duck protein gp180 and the Drosophila silver gene carboxypeptidases. Aplysia CPE and the CPD-related enzyme are candidate processing enzymes that may play a role in the processing of the ELH prohormone and other Aplysia prohormones.

Limulus kexin: a new type of Kex2-like endoprotease specifically expressed in hemocytes of the horseshoe crab

A Kex2-like protease was identified in hemocytes of the horseshoe crab (Tachypleus tridentatus), named limulus kexin, and a full-length cDNA was obtained from a hemocyte cDNA library. The deduced amino acid sequence contains 752 residues, composed of five domains with a signal sequence, a propeptide, a catalytic domain, a Ser/Thr-rich domain, and a transmembrane domain. The domain organization is very similar to that of the yeast Kex2 except that limulus kexin does not have a cytoplasmic tail. The catalytic domain exhibits striking sequence identities with those of furins, especially Drosophila furin1 (79%). Northern blotting showed specific expression of limulus kexin in hemocytes, suggesting the involvement in proteolytic processing of the granule components of hemocytes.

Molecular cloning of prohormone convertase 1 from the atrial gland of Aplysia

We have screened an Aplysia atrial gland cDNA library using a prohormone convertase (PC)1 probe prepared by polymerase chain reaction (PCR) and have isolated an Aplysia PC1-related full-length 3.6-kb cDNA clone. The cDNA sequence (3,565 bp) encoded a putative preproendoprotease (APC1) of 703 amino acid residues that showed considerable sequence identity with other eukaryotic PC1s, and indicated a high degree of sequence identity with an Aplysia nervous system PC sequence (aPC1B). Northern blot analysis of atrial gland RNA identified two APC1 transcripts of 3.9 kb and 5.0 kb. APC1 is a candidate PC that may play an important role in the processing of egg-laying hormone (ELH)-related precursors in atrial gland secretory cells and represents one of the first examples of PC1 expression in an exocrine tissue.

Furin/PACE/SPC1: a convertase involved in exocytic and endocytic processing of precursor proteins

One of the most exciting breakthroughs of the 90's in the fields of biochemistry, cell biology and neuroendocrinology is the identification of a novel family of proteolytic enzymes called mammalian subtilisin-like convertases. This family is comprised so far of seven distinct endoproteases responsible for the proteolytic excision of biologically active polypeptides from inactive precursor proteins. Six years after the initial observation of a structural conservation between a characterized yeast enzyme (kexin) and a human gene product (furin), it is now well accepted that one of these convertases, furin, has the enzymatic capabilities to efficiently and correctly process a great variety of precursors. Furin's ability to cleave precursors within both the exocytic and endocytic pathways will require sustained efforts in order to delineate all of its physiological roles.