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

Transcriptomic Profiling Reveals Extraordinary Diversity of Venom Peptides in Unexplored Predatory Gastropods of the Genus Clavus

Predatory gastropods of the superfamily Conoidea number over 12,000 living species. The evolutionary success of this lineage can be explained by the ability of conoideans to produce complex venoms for hunting, defense, and competitive interactions. Whereas venoms of cone snails (family Conidae) have become increasingly well studied, the venoms of most other conoidean lineages remain largely uncharacterized. In the present study, we present the venom gland transcriptomes of two species of the genus Clavus that belong to the family Drilliidae. Venom gland transcriptomes of two specimens of Clavus canalicularis and two specimens of Clavus davidgilmouri were analyzed, leading to the identification of a total of 1,176 putative venom peptide toxins (drillipeptides). Based on the combined evidence of secretion signal sequence identity, entire precursor similarity search (BLAST), and the orthology inference, putative Clavus toxins were assigned to 158 different gene families. The majority of identified transcripts comprise signal, pro-, mature peptide, and post-regions, with a typically short (<50 amino acids) and cysteine-rich mature peptide region. Thus, drillipeptides are structurally similar to conotoxins. However, convincing homology with known groups of Conus toxins was only detected for very few toxin families. Among these are Clavus counterparts of Conus venom insulins (drillinsulins), porins (drilliporins), and highly diversified lectins (drillilectins). The short size of most drillipeptides and structural similarity to conotoxins were unexpected, given that most related conoidean gastropod families (Terebridae and Turridae) possess longer mature peptide regions. Our findings indicate that, similar to conotoxins, drillipeptides may represent a valuable resource for future pharmacological exploration.

Myticalins: A Novel Multigenic Family of Linear, Cationic Antimicrobial Peptides from Marine Mussels (Mytilus spp.)

The application of high-throughput sequencing technologies to non-model organisms has brought new opportunities for the identification of bioactive peptides from genomes and transcriptomes. From this point of view, marine invertebrates represent a potentially rich, yet largely unexplored resource for de novo discovery due to their adaptation to diverse challenging habitats. Bioinformatics analyses of available genomic and transcriptomic data allowed us to identify myticalins, a novel family of antimicrobial peptides (AMPs) from the mussel Mytilus galloprovincialis, and a similar family of AMPs from Modiolus spp., named modiocalins. Their coding sequence encompasses two conserved N-terminal (signal peptide) and C-terminal (propeptide) regions and a hypervariable central cationic region corresponding to the mature peptide. Myticalins are taxonomically restricted to Mytiloida and they can be classified into four subfamilies. These AMPs are subject to considerable interindividual sequence variability and possibly to presence/absence variation. Functional assays performed on selected members of this family indicate a remarkable tissue-specific expression (in gills) and broad spectrum of activity against both Gram-positive and Gram-negative bacteria. Overall, we present the first linear AMPs ever described in marine mussels and confirm the great potential of bioinformatics tools for the de novo discovery of bioactive peptides in non-model organisms.

Myticalins: A Novel Multigenic Family of Linear, Cationic Antimicrobial Peptides from Marine Mussels (Mytilus spp.)

The application of high-throughput sequencing technologies to non-model organisms has brought new opportunities for the identification of bioactive peptides from genomes and transcriptomes. From this point of view, marine invertebrates represent a potentially rich, yet largely unexplored resource for de novo discovery due to their adaptation to diverse challenging habitats. Bioinformatics analyses of available genomic and transcriptomic data allowed us to identify myticalins, a novel family of antimicrobial peptides (AMPs) from the mussel Mytilus galloprovincialis, and a similar family of AMPs from Modiolus spp., named modiocalins. Their coding sequence encompasses two conserved N-terminal (signal peptide) and C-terminal (propeptide) regions and a hypervariable central cationic region corresponding to the mature peptide. Myticalins are taxonomically restricted to Mytiloida and they can be classified into four subfamilies. These AMPs are subject to considerable interindividual sequence variability and possibly to presence/absence variation. Functional assays performed on selected members of this family indicate a remarkable tissue-specific expression (in gills) and broad spectrum of activity against both Gram-positive and Gram-negative bacteria. Overall, we present the first linear AMPs ever described in marine mussels and confirm the great potential of bioinformatics tools for the de novo discovery of bioactive peptides in non-model organisms.

Optimized deep-targeted proteotranscriptomic profiling reveals unexplored Conus toxin diversity and novel cysteine frameworks

Cone snails are predatory marine gastropods characterized by a sophisticated venom apparatus responsible for the biosynthesis and delivery of complex mixtures of cysteine-rich toxin peptides. These conotoxins fold into small highly structured frameworks, allowing them to potently and selectively interact with heterologous ion channels and receptors. Approximately 2,000 toxins from an estimated number of >70,000 bioactive peptides have been identified in the genus Conus to date. Here, we describe a high-resolution interrogation of the transcriptomes (available at www.ddbj.nig.ac.jp) and proteomes of the diverse compartments of the Conus episcopatus venom apparatus. Using biochemical and bioinformatic tools, we found the highest number of conopeptides yet discovered in a single Conus specimen, with 3,305 novel precursor toxin sequences classified into 9 known superfamilies (A, I1, I2, M, O1, O2, S, T, Z), and identified 16 new superfamilies showing unique signal peptide signatures. We were also able to depict the largest population of venom peptides containing the pharmacologically active C-C-CC-C-C inhibitor cystine knot and CC-C-C motifs (168 and 44 toxins, respectively), as well as 208 new conotoxins displaying odd numbers of cysteine residues derived from known conotoxin motifs. Importantly, six novel cysteine-rich frameworks were revealed which may have novel pharmacology. Finally, analyses of codon usage bias and RNA-editing processes of the conotoxin transcripts demonstrate a specific conservation of the cysteine skeleton at the nucleic acid level and provide new insights about the origin of sequence hypervariablity in mature toxin regions.

New roles of carboxypeptidase E in endocrine and neural function and cancer

Carboxypeptidase E (CPE) or carboxypeptidase H was first discovered in 1982 as an enkephalin-convertase that cleaved a C-terminal basic residue from enkephalin precursors to generate enkephalin. Since then, CPE has been shown to be a multifunctional protein that subserves many essential nonenzymatic roles in the endocrine and nervous systems. Here, we review the phylogeny, structure, and function of CPE in hormone and neuropeptide sorting and vesicle transport for secretion, alternative splicing of the CPE transcript, and single nucleotide polymorphisms in humans. With this and the analysis of mutant and knockout mice, the data collectively support important roles for CPE in the modulation of metabolic and glucose homeostasis, bone remodeling, obesity, fertility, neuroprotection, stress, sexual behavior, mood and emotional responses, learning, and memory. Recently, a splice variant form of CPE has been found to be an inducer of tumor growth and metastasis and a prognostic biomarker for metastasis in endocrine and nonendocrine tumors.

ConoServer: updated content, knowledge, and discovery tools in the conopeptide database

ConoServer (http://www.conoserver.org) is a database specializing in the sequences and structures of conopeptides, which are toxins expressed by marine cone snails. Cone snails are carnivorous gastropods, which hunt their prey using a cocktail of toxins that potently subvert nervous system function. The ability of these toxins to specifically target receptors, channels and transporters of the nervous system has attracted considerable interest for their use in physiological research and as drug leads. Since the founding publication on ConoServer in 2008, the number of entries in the database has nearly doubled, the interface has been redesigned and new annotations have been added, including a more detailed description of cone snail species, biological activity measurements and information regarding the identification of each sequence. Automatically updated statistics on classification schemes, three-dimensional structures, conopeptide-bearing species and endoplasmic reticulum signal sequence conservation trends, provide a convenient overview of current knowledge on conopeptides. Transcriptomics and proteomics have began generating massive numbers of new conopeptide sequences, and two dedicated tools have been recently implemented in ConoServer to standardize the analysis of conopeptide precursor sequences and to help in the identification by mass spectrometry of toxins whose sequences were predicted at the nucleic acid level.

The crystal structure of the inhibitor-complexed carboxypeptidase D domain II and the modeling of regulatory carboxypeptidases

The three-dimensional crystal structure of duck carboxypeptidase D domain II has been solved in a complex with the peptidomimetic inhibitor, guanidinoethylmercaptosuccinic acid, occupying the specificity pocket. This structure allows a clear definition of the substrate binding sites and the substrate funnel-like access. The structure of domain II is the only one available from the regulatory carboxypeptidase family and can be used as a general template for its members. Here, it has been used to model the structures of domains I and III from the former protein and of human carboxypeptidase E. The models obtained show that the overall topology is similar in all cases, the main differences being local and because of insertions in non-regular loops. In both carboxypeptidase D domain I and carboxypeptidase E slightly different shapes of the access to the active site are predicted, implying some kind of structural selection of protein or peptide substrates. Furthermore, emplacement of the inhibitor structure in the active site of the constructed models showed that the inhibitor fits very well in all of them and that the relevant interactions observed with domain II are conserved in domain I and carboxypeptidase E but not in the non-active domain III because of the absence of catalytically indispensable residues in the latter protein. However, in domain III some of the residues potentially involved in substrate binding are well preserved, together with others of unknown roles, which also are highly conserved among all carboxypeptidases. These observations, taken together with others, suggest that domain III might play a role in the binding and presentation of proteins or peptide substrates, such as the pre-S domain of the large envelope protein of duck hepatitis B virus.

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.

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.

Regulation of c-fos expression by EGF and GnRH in specific anterior pituitary cells from proestrous female rats

C-fos is an early expression oncogene that can be stimulated by a variety of regulators. It is expressed by subsets of all pituitary cells, with increased expression seen in proestrous rats. However, in freshly dispersed pituitary cells studied during different stages of the cycle, there is limited expression of fos by luteinizing hormone (LH) cells and little basal expression by cells with follicle-stimulating hormone (FSH) antigens. Proestrus is a time during which pituitary gonadotropes express peak levels of receptors for gonadotropin-releasing hormone (GnRH) and epidermal growth factor (EGF). We hypothesized that if GnRH or EGF stimulated fos activity in gonadotropes they would be most effective during the peak expression of their receptors. Anterior pituitaries were removed, cut into small pieces, and stimulated for 30 min. Total RNA was then collected and analyzed by Northern analysis. Both EGF and GnRH caused an increase in c-fos mRNA levels in the anterior pituitary gland compared with unstimulated pituitary glands assayed immediately after removal from the pituitary. However, the stimulatory effects were no greater than those seen with medium alone. This suggested that fos expression could be stimulated by local factors either in the pituitary or the medium itself. The second phase of the study focused on pituitary cells plated for 1 hr and then stimulated with EGF and GnRH for 15 min. Dual immunocytochemistry was done to learn which cell types expressed the fos proteins. After 15 min, EGF and GnRH both increased the percentages of fos-bearing cells above levels seen in medium alone. EGF stimulated fos proteins in subsets of FSH, adrenocorticotropin (ACTH), and growth hormone (GH) cells. GnRH increased fos proteins in subsets of ACTH and GH cells. These results suggest that EGF and GnRH may regulate fos expression, but not necessarily in gonadotropes. They also highlight the need for carefully timed experiments because endogenous factors in the pituitary itself may stimulate immediate early gene expression. (J Histochem Cytochem 46:935-943, 1998)

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.

Molecular cloning of a cDNA encoding a potential water-borne pheromonal attractant released during Aplysia egg laying

Recently deposited egg cordons are a source of water-borne pheromones that attract the marine mollusk Aplysia into breeding aggregations and coordinate male and female reproductive behavior within the aggregation. A potential pheromonal attractant has been isolated from egg cordon eluates and the peptide partially characterized [S.D. Painter, B. Clough, X. Fan, G.T. Nagle, Soc. Neurosci. Abstr., Vol. 22 (1996) 837]. Using this information, we have cloned an Aplysia albumen gland cDNA that encodes a precursor protein containing a single copy of the full-length peptide, and demonstrated that there are abundant levels of pheromone mRNA transcripts (0.8 and 2.5 kb) in the albumen gland. This is consistent with the reported function of the gland (i.e. packaging the eggs into a cordon for deposition), with behavioral studies showing that the albumen gland is a potential source of attractants, and more recent biochemical studies in which the full-length peptide has been isolated from the albumen gland. This is the first candidate peptide pheromone in mollusks and the first in invertebrates. The pheromonal regulatory system in Aplysia may provide a model system for examining the structural characteristics of peptide pheromones.

Characterization of Aplysia carboxypeptidase E

Carboxypeptidase E (CPE) is involved in the biosynthesis of peptide hormones and neurotransmitters. To determine whether a recently reported Aplysia californica cDNA encodes a CPE-like enzyme, this cDNA was expressed in the baculovirus system. The Aplysia CPE is optimal at pH 5.5-6.5 and is inhibited by chelating agents and by the sulfhydryl reagent p-chloromercuriphenyl sulfonate. The effect of divalent cations and active site-directed inhibitors on enzyme activity are generally similar for Aplysia and rat CPE. Western blot analysis using antisera to the N- and C-terminal regions of the Aplysia CPE show that the Aplysia CPE is present in atrial glands and ovotestis. This Aplysia CPE is purified on a p-aminobenzoyl-Arg Sepharose affinity column under conditions that selectively purify rat CPE. Taken together, these results suggest that the previously cloned cDNA represents a CPE-like enzyme that is expressed in Aplysia tissue.