The complex array of bradykinin-related peptides (BRPs) in the peptidome of pickerel frog (Rana palustris) skin secretion is the product of transcriptional economy

Identification and Functional Analysis of Novel Bradykinin-Related Peptides (BRPs) from Skin Secretions of Five Asian Frogs

In recent decades, various types of bioactive substances have been identified from amphibian skin and its secretions. Bradykinin-related peptides (BRPs) are among these compounds that make up the host defence system of amphibians. In the present study, we identified six novel BRPs, amolopkinin-GN1, amolopkinin-RK1, amolopkinin-TR1, amolopkinin-LF1, ranakinin-MS1, and ranakinin-MS2, from five East Asian amphibians, Amolops granulosus, Amolops ricketti, Amolops torrentis, Amolops lifanensis, and Hylarana maosonensis. This is the first report on BRPs in the skin of these species. Physiological assays reveal that these peptides have a contractive effect on the smooth muscle of rat ileum.

A review on bradykinin-related peptides isolated from amphibian skin secretion

Amphibian skin secretion has great potential for drug discovery and contributes hundreds of bioactive peptides including bradykinin-related peptides (BRPs). More than 50 BRPs have been reported in the last two decades arising from the skin secretion of amphibian species. They belong to the families Ascaphidae (1 species), Bombinatoridae (3 species), Hylidae (9 speices) and Ranidae (25 species). This paper presents the diversity of structural characteristics of BRPs with N-terminal, C-terminal extension and amino acid substitution. The further comparison of cDNA-encoded prepropeptides between the different species and families demonstrated that there are various forms of kininogen precursors to release BRPs and they constitute important evidence in amphibian evolution. The pharmacological activities of isolated BRPs exhibited unclear structure–function relationships, and therefore the scope for drug discovery and development is limited. However, their diversity shows new insights into biotechnological applications and, as a result, comprehensive and systematic studies of the physiological and pharmacological activities of BRPs from amphibian skin secretion are needed in the future.

The diversity and evolution of anuran skin peptides

Amphibians exhibit various, characteristic adaptations related to their "incomplete" shift from the aquatic to the terrestrial habitat. In particular, the integument was subject to a number of specialized modifications during the evolution of these animals. In this review, we place special emphasis on endogenous host-defence skin peptides from the cuteanous granular glands anuran amphibians (frogs and toads). The overview on the two broad groups of neuroactive and antimicrobial peptides (AMPs) goes beyond a simple itemization in that we provide a new perspective into the evolution and function of anuran AMPs. Briefly, these cationic, amphipathic and α-helical peptides are traditionally viewed as being part of the innate immune system, protecting the moist skin against invading microorganisms through their cytolytic action. However, the complete record of anuran species investigated to date suggests that AMPs are distributed sporadically (i.e., non-universally) across Anura. Together with the intriguing observation that virtually all anurans known to produce neuropeptides in their granular glands also co-secrete cytolytic peptides, we call the traditional role for AMPs as being purely antimicrobial into question and present an alternative scenario. We hypothesize AMPs to assist neuroactive peptides in their antipredator role through their cytolytic action increasing the delivery of the latter to the endocrine and nervous system of the predator. Thus, AMPs are more accurately viewed as cytolysins and their contribution to the immune system is better regarded as an accessory benefit.

Low structural variation in the host-defense peptide repertoire of the dwarf clawed frog Hymenochirus boettgeri (Pipidae)

THE skin secretion of many amphibians contains peptides that are able to kill a broad range of microorganisms (antimicrobial peptides: AMPs) and potentially play a role in innate immune defense. Similar to the toxin arsenals of various animals, amphibian AMP repertoires typically show major structural variation, and previous studies have suggested that this may be the result of diversifying selection in adaptation to a diverse spectrum of pathogens. Here we report on transcriptome analyses that indicate a very different pattern in the dwarf clawed frog H. boettgeri. Our analyses reveal a diverse set of transcripts containing two to six tandem repeats, together encoding 14 distinct peptides. Five of these have recently been identified as AMPs, while three more are shown here to potently inhibit the growth of gram-negative bacteria, including multi-drug resistant strains of the medically important Pseudomonas aeruginosa. Although the number of predicted peptides is similar to the numbers of related AMPs in Xenopus and Silurana frog species, they show significantly lower structural variation. Selection analyses confirm that, in contrast to the AMPs of other amphibians, the H. boettgeri peptides did not evolve under diversifying selection. Instead, the low sequence variation among tandem repeats resulted from purifying selection, recent duplication and/or concerted gene evolution. Our study demonstrates that defense peptide repertoires of closely related taxa, after diverging from each other, may evolve under differential selective regimes, leading to contrasting patterns of structural diversity.

Origin and functional diversification of an amphibian defense peptide arsenal

The skin secretion of many amphibians contains an arsenal of bioactive molecules, including hormone-like peptides (HLPs) acting as defense toxins against predators, and antimicrobial peptides (AMPs) providing protection against infectious microorganisms. Several amphibian taxa seem to have independently acquired the genes to produce skin-secreted peptide arsenals, but it remains unknown how these originated from a non-defensive ancestral gene and evolved diverse defense functions against predators and pathogens. We conducted transcriptome, genome, peptidome and phylogenetic analyses to chart the full gene repertoire underlying the defense peptide arsenal of the frog Silurana tropicalis and reconstruct its evolutionary history. Our study uncovers a cluster of 13 transcriptionally active genes, together encoding up to 19 peptides, including diverse HLP homologues and AMPs. This gene cluster arose from a duplicated gastrointestinal hormone gene that attained a HLP-like defense function after major remodeling of its promoter region. Instead, new defense functions, including antimicrobial activity, arose by mutation of the precursor proteins, resulting in the proteolytic processing of secondary peptides alongside the original ones. Although gene duplication did not trigger functional innovation, it may have subsequently facilitated the convergent loss of the original function in multiple gene lineages (subfunctionalization), completing their transformation from HLP gene to AMP gene. The processing of multiple peptides from a single precursor entails a mechanism through which peptide-encoding genes may establish new functions without the need for gene duplication to avoid adaptive conflicts with older ones.

Expression analysis and identification of antimicrobial peptide transcripts from six North American frog species

Frogs secrete antimicrobial peptides onto their skin. We describe an assay to preserve and analyze antimicrobial peptide transcripts from field-collected skin secretions that will complement existing methods for peptide analysis. We collected skin secretions from 4 North American species in the field in California and 2 species in the laboratory. Most frogs appeared healthy after release; however, Rana boylii in the Sierra Nevada foothills, but not the Coast Range, showed signs of morbidity and 2 died after handling. The amount of total RNA extracted from skin secretions was higher in R. boylii and R. sierrae compared to R. draytonii, and much higher compared to Pseudacris regilla. Interspecies variation in amount of RNA extracted was not explained by size, but for P. regilla it depended upon collection site and date. RNA extracted from skin secretions from frogs handled with bare hands had poor quality compared to frogs handled with gloves or plastic bags. Thirty-four putative antimicrobial peptide precursor transcripts were identified. This study demonstrates that RNA extracted from skin secretions collected in the field is of high quality suitable for use in sequencing or quantitative PCR (qPCR). However, some species do not secrete profusely, resulting in very little extracted RNA. The ability to measure transcript abundance of antimicrobial peptides in field-collected skin secretions complements proteomic analyses and may provide insight into transcriptional mechanisms that could affect peptide abundance.

Amolopkinins W1 and W2--novel bradykinin-related peptides (BRPs) from the skin of the Chinese torrent frog, Amolops wuyiensis: antagonists of bradykinin-induced smooth muscle contraction of the rat ileum

Bradykinin-related peptides (BRPs) represent one of the most widespread and closely studied families of amphibian defensive skin secretion peptides. Apart from canonical bradykinin (RPPGFSPFR) that was first reported in skin extracts of the European brown frog, Rana temporaria, many additional site-substituted, N- and/or C-terminally extended peptides have been isolated from skin extracts and secretions from representative species of the families Ranidae, Hylidae, Bombinatoridae and Leiopelmatidae. The most diverse range of BRPs has been found in ranid frog skin secretions and this probably reflects the diversity and number of species studied and their associated life histories within this taxon. Amolops (torrent or cascade frogs) is a genus within the Ranidae that has been poorly studied. Here we report the presence of two novel BRPs in the skin secretions of the Chinese Wuyi Mountain torrent frog (Amolops wuyiensis). Amolopkinins W1 and W2 are dodecapeptides differing in only one amino acid residue at position 2 (Val/Ala) that are essentially (Leu(1), Thr(6))-bradykinins extended at the N-terminus by either RVAL (W1) or RAAL (W2). Amolopkinins W1 and W2 are structurally similar to amolopkinin L1 from Amolops loloensis and the major BRP (Leu(1), Thr(6), Trp(8))-bradykinin from the skin of the Japanese frog, Rana sakuraii. A. wuyiensis amolopkinins were separately encoded as single copies within discrete precursors of 61 amino acid residues as deduced from cloned skin cDNA. Synthetic replicates of both peptides were found to potently antagonize the contractile effects of canonical bradykinin on isolated rat ileum smooth muscle preparations. Amolopkinins thus appear to represent a novel sub-family of ranid frog skin secretion BRPs.

Cloning of the kininogen gene from Lampetra japonica provides insights into its phylogeny in vertebrates

Kininogens, the precursors of bradykinins, ubiquitously exist in vertebrates, including mammals, birds, amphibians, and fishes. To elucidate the phylogeny of kininogen genes in early vertebrates, we cloned the full-length cDNA of kininogen gene from the liver of Lampetra japonica. The open reading frame of this sequence contained 546 bp and encoded 181 amino acids, including a cystatin domain without the canonical binding site for cysteine proteinases and a bradykinin domain. Our results suggested that in lampreys and most of other vertebrates, there might be only one kininogen gene, which was fused by certain sequences during vertebrate evolution and encoded proteins with more functions; however, another special kininogen gene, only encoding the bradykinin domain with multiple copies in some species, arose only in amphibians for adapting themselves to the unique environment. Using reverse transcription PCR, kininogen mRNA was also detected in lamprey gut, kidney, and leukocyte, but absent in lamprey buccal gland. Our findings may provide insights into the phylogeny of kininogen genes as well as other gene families in vertebrates.

Skin bradykinin-related peptides (BRPs) and their biosynthetic precursors (kininogens): comparisons between various taxa of Chinese and North American ranid frogs

Bradykinins and related peptides (BRPs) occur in the defensive skin secretions of many amphibians. Here we report the structures of BRPs and their corresponding biosynthetic precursor cDNAs from the Chinese brown frog, Rana chensinensis, and the North American leopard frog, Lithobates pipiens. R. chensinensis skin contained four transcripts each encoding a different kininogen whose organizations and spectrum of encoded BRPs were similar to those reported for the pickerel frog, Lithobates palustris. In contrast, from L. pipiens, a single skin kininogen was cloned whose structural organization and spectrum of mature BRPs were similar to those reported for the Chinese piebald odorous frog, Huia schmackeri. These data also implied that the endogenous precursor processing proteases in each species pair have identical site-directed specificities, which in part may be dictated by the primary structures of encoded BRPs. Thus the spectra of skin BRPs and the organization of their biosynthetic precursors are not consistent with recent taxonomy. The natural selective pressures that mould the primary structures of amphibian skin secretion peptides are thought to be related to the spectrum of predators encountered within their habitats. Thus similarities and differences in skin bradykinins may be reflective of predator spectra rather than indicative of species relatedness.

Phylogenetic analysis of vertebrate kininogen genes

Kininogens, the precursors of bradykinins, vary extremely in both structure and function among different taxa of animals, in particular between mammals and amphibians. This includes even the most conserved bradykinin domain in terms of biosynthesis mode and structure. To elucidate the evolutionary dynamics of kininogen genes, we have identified 19 novel amino acid sequences from EST and genomic databases (for mammals, birds, and fishes) and explored their phylogenetic relationships using combined amino acid sequence and gene structure as markers. Our results show that there were initially two paralogous kininogen genes in vertebrates. During their evolution, the original gene was saved with frequent multiplication in amphibians, but lost in fishes, birds, and mammals, while the novel gene was saved with multiple functions in fishes, birds, and mammals, but became a pseudogene in amphibians. We also propose that the defense mechanism against specific predators in amphibian skin secretions has been bradykinin receptor dependent. Our findings may provide a foundation for identification and structural, functional, and evolutionary analyses of more kininogen genes and other gene families.

Cryptides: buried secrets in proteins

The proteome originally described the entire set of proteins expressed by a genome, tissue or organism. Subsequently this term was limited to all the expressed proteins at a given time under defined conditions. Hence, specializations such as functional proteome, cancer proteome, liver proteome and so forth have arisen. One particular proteome that has been recently described is the cryptome, a unique subset of already known proteins that has the ability of generating bioactive peptides and proteins when submitted to proteolytic cleavage, rather than the classical processing pathways. This is an idea in agreement with the concept that evolution is not related to the amount of genes or putative proteins that could be secreted by an organism, but to the way these proteins are processed. These 'new' molecules may have related or increased properties when compared to the 'original' molecule or possess completely unrelated biological effects, thus increasing the array of biological roles that can be associated to one given protein (or gene). In this work, we review this recent concept and put it into the toxinology field as well, an area in which the diversity of functional molecules (and roles) is essential for the survival of a given organism.