Molecular cloning, cDNA sequence, and localization of a prohormone convertase (PC2) from the Aplysia atrial gland

Molecular characterization and spatiotemporal expression of prohormone convertase 2 in the Pacific abalone, Haliotis discus hannai

Prohormone convertases (PCs) are subtilisin-like proteases responsible for the intracellular processing of prohormones and proneuropeptides in vertebrates and invertebrates. The full-length PC2 cDNA sequence was cloned from pleuropedal ganglion of Haliotis discus hannai, consisted of 2254-bp with an open reading frame of 1989-bp and encoded a protein of 662 amino acid residues. The architecture of Hdh PC2 displayed key features of PCs, including a signal peptide, a pro-segment domain with sites for autocatalytic activation, a catalytic domain, and a pro-protein domain (P-domain). It shares the highest homology of its amino acid sequence with the PC2 from H. asinina and to lesser extent with that of Homo sapiens and Rana catesbeiana PC2. Sequence alignment analysis indicated that Hdh PC2 was highly conserved in the catalytic domain, including a catalytic triad of serine proteinases of the subtilisin family at positions Asp-195, His-236, and Ser-412. The cloned sequence contained a canonical integrin binding sequence, and four cysteine residues involved in the formation of an intramolecular disulfide link. Phylogenetic analysis revealed that the Hdh PC2 is robustly clustered with the Has PC2. Quantitative PCR assay demonstrated that the Hdh PC2 was predominantly expressed in the pleuropedal ganglion rather than in other examined tissues. Although PC2 mRNA was expressed throughout the gametogenetic cycle of male and female abalone, the expression level was significantly higher in the ripening stage of female abalone. Also, a significantly higher expression was observed in the pleuropedal ganglion and gonadal tissues at a higher effective accumulative temperature (1000°C). In situ hybridization revealed that the PC2 mRNA expressing neurosecretory cells were distributed in the cortex region of the pleuropedal ganglion. According to the results, it can be concluded that pleuropedal ganglion is the highest site of PC2 activity, and this enzyme might be involved in the abalone reproduction process.

Molecular insights into land snail neuropeptides through transcriptome and comparative gene analysis

Background

Snails belong to the molluscan class Gastropoda, which inhabit land, freshwater and marine environments. Several land snail species, including Theba pisana, are crop pests of major concern, causing extensive damage to agriculture and horticulture. A deeper understanding of their molecular biology is necessary in order to develop methods to manipulate land snail populations.

Results

The present study used in silico gene data mining of T. pisana tissue transcriptomes to predict 24,920 central nervous system (CNS) proteins, 37,661 foot muscle proteins and 40,766 hepatopancreas proteins, which together have 5,236 unique protein functional domains. Neuropeptides, metabolic enzymes and epiphragmin genes dominated expression within the CNS, hepatopancreas and muscle, respectively. Further investigation of the CNS transcriptome demonstrated that it might contain as many as 5,504 genes that encode for proteins destined for extracellular secretion. Neuropeptides form an important class of cell-cell messengers that control or influence various complex metabolic events. A total of 35 full-length neuropeptide genes were abundantly expressed within T. pisana CNS, encoding precursors that release molluscan-type bioactive neuropeptide products. These included achatin, allototropin, conopressin, elevenin, FMRFamide, LFRFamide, LRFNVamide, myomodulins, neurokinin Y, PKYMDT, PXFVamide, sCAPamides and several insulin-like peptides. Liquid chromatography-mass spectrometry of neural ganglia confirmed the presence of many of these neuropeptides.

Conclusions

Our results provide the most comprehensive picture of the molecular genes and proteins associated with land snail functioning, including the repertoire of neuropeptides that likely play significant roles in neuroendocrine signalling. This information has the potential to expedite the study of molluscan metabolism and potentially stimulate advances in the biological control of land snail pest species.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1510-8) contains supplementary material, which is available to authorized users.

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.

Molecular cloning, ontogeny and tissue distribution of zebrafish (Danio rerio) prohormone convertases: pcsk1 and pcsk2

Prohormone convertase subtilisin/kexin (PCSK) enzymes are a family of nine related serine proteases, found in a multitude of tissues, and responsible for the maturation of a variety of protein and peptide precursors. Pcsk1 and Pcsk2 are found within dense core secretory granules in endocrine and neuroendocrine cells and are responsible for cleaving several hormones and neuropeptide precursors. In this work, we cloned and sequenced the cDNA of pcsk1 and pcsk2 from zebrafish (Danio rerio). pcsk1 is a 2268bp ORF, whose 755 amino acid protein product is identical to that predicted from the genome sequence. pcsk2 is a 1941bp ORF, encoding a 646 amino acid peptide. Both Pcsk1 and Pcsk2 display high degrees of similarity to their counterparts in other species, including the conservation of the catalytic triad and other essential residues. The brain contained the highest expression levels of both pcsk1 (1.49+/-0.21) (displayed as ratio to EF-1a), and pcsk2 (0.23+/-0.04). Both transcripts were also detectable in the fore, mid and distal gut. pcsk1 and 2 were detectable at 4.5h post-fertilization, and while pcsk1 expression increased throughout development (0.12+/-0.01 maximum at 3 days post-fertilization), pcsk2 expression was highest at day 5 post-fertilization (0.03+/-0.01), and decreased prior. For the first time, we have identified and characterized a pcsk1 transcript in fish. We have also identified and characterized the pcsk2 transcript in zebrafish, and have assessed the tissue distribution and ontogeny of both.

Mollusk-derived growth factor: cloning and developmental expression in the central nervous system and reproductive tract of Aplysia

We have isolated and characterized an atrial gland cDNA that corrects the previously reported sequence for Aplysia atrial gland granule-specific antigen (AGSA), a glycoprotein of unknown function. We designated the protein mollusk-derived growth factor (MDGF) to distinguish the revised sequence from AGSA and to emphasize its similarity to an insect-derived growth factor (IDGF). We describe MDGF mRNA expression that suggests a possible role during embryonic development and CNS injury repair.

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.

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 of specific mRNAs affected by treatments producing long-term facilitation in Aplysia

Neural correlates of long-term sensitization of defensive withdrawal reflexes in Aplysia occur in sensory neurons in the pleural ganglia and can be mimicked by exposure of these neurons to serotonin (5-HT). Studies using inhibitors indicate that transcription is necessary for production of long-term facilitation by 5-HT. Several mRNAs that change in response to 5-HT have been identified, but the molecular events responsible for long-term facilitation have not yet been fully described. To detect additional changes in mRNAs, we investigated the effects of 5-HT (1.5 hr) on levels of mRNA in pleural-pedal ganglia using in vitro translation. Four mRNAs were affected by 5-HT, three of which were identified as calmodulin (CaM), phosphoglycerate kinase (PGK), and a novel gene product (protein 3). Using RNase protection assays, we found that 5-HT increased all three mRNAs in the pleural sensory neurons. CaM and protein 3 mRNAs were also increased in the sensory neurons by sensitization training. Furthermore, stimulation of peripheral nerves of pleural-pedal ganglia, an in vitro analog of sensitization training, increased the incorporation of labeled amino acids into CaM, PGK, and protein 3. These results indicate that increases in CaM, PGK, and protein 3 are part of the early response of sensory neurons to stimuli that produce long-term facilitation, and that CaM and protein 3 could have a role in the generation of long-term sensitization.

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 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.

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

Prohormone convertases (PCs) are Ca(2+)-dependent subtilisin-related endoproteases that have been implicated in the post-translational processing of prohormones and other proproteins. Furin is an ubiquitously expressed PC that has been shown to hydrolyze a wide variety of precursor proteins in secretory pathways. We have screened an Aplysia atrial gland cDNA library using a furin probe prepared by polymerase chain reaction (PCR) and have isolated an Aplysia furin-related 6.7-kb cDNA partial clone that was truncated on the 5' end. The remaining 5' atrial gland furin nucleotide sequence was obtained by two stages of reverse transcription PCR. The final composite nucleotide sequence of the atrial gland furin cDNA was 7,837 bp in length. This sequence encoded a putative preproendoprotease (Afurin2) of 824 amino acid residues that was related to other eukaryotic furins, and showed a high sequence identity with a recently reported Aplysia nervous system furin-like sequence. In situ hybridization demonstrated extensive expression of Afurin2 in atrial gland secretory cells. The cDNA clone contained a relatively long 3' untranslated region of 5,230 nucleotides that included a microsatellite repeat region (TG)n. The characterized Aplysia Afurin2 is a candidate PC that may play an important role in the processing of egg-laying hormone (ELH)-related precursors in the secretory cells of the atrial gland. In addition, comparative structural studies of Afurin2, together with previously reported localization studies, argue for the occurrence of a furin-like convertase within secretory granules.