A new opioid peptide predicted from cloned cDNAs from skin of Pachymedusa dacnicolor and Agalychnis annae

Advancing d-amino acid-containing peptide discovery in the metazoan

The discovery of enzyme-derived d-amino acid-containing peptides (DAACPs) that have physiological importance in the metazoan challenges previous assumptions about the homochirality of animal proteins while simultaneously revealing new analytical challenges in the structural and functional characterization of peptides. Most known DAACPs have been identified though laborious activity-guided purification studies or by homology to previously identified DAACPs. Peptide characterization experiments are increasingly dominated by high throughput mass spectrometry-based peptidomics, with stereochemistry rarely considered due to the technical challenges of identifying l/d isomerization. This review discusses the prevalence of enzyme-derived DAACPs among animals and the physiological consequences of peptide isomerization. Also highlighted are the analytical methods that have been applied for structural characterization/discovery of DAACPs, including results of several recent studies using non-targeted discovery methods for revealing novel DAACPs, strongly suggesting that more DAACPs remain to be uncovered.

Novel peptide dermaseptin-PS1 exhibits anticancer activity via induction of intrinsic apoptosis signalling

Antimicrobial peptides (AMP) secreted by the granular glands of frog skin have been widely reported to exhibit strong bacteriostatic and bactericidal activities. Many of them have been documented with potent antiproliferative effects on multiple cancer cells, many studies also suggested that AMPs exert their functions via disrupting cell membranes. However, whether and how other cell death induction mechanism is involved in mammalian cancer cells has rarely been investigated. In this study, a novel AMP named Dermaseptin-PS1 was isolated and identified from Phyllomedusa sauvagei, it showed strong antimicrobial activities against three types of microorganisms. In vitro antiproliferative studies on human glioblastoma U-251 MG cells indicated that Dermaseptin-PS1 disrupted cell membranes at the concentrations of 10^-5  M and above, while the cell membrane integrity was not affected when concentrations were decreased to 10^-6  M or lower. Further examinations revealed that, at the relatively low concentration (10^-6  M), Dermaseptin-PS1 induced apoptosis through mitochondrial-related signal pathway in U-251 MG cells. Thus, for the first time, we report a novel frog skin derived AMP with anticancer property by distinct mechanisms, which largely depends on its concentration. Together, our study provides new insights into the mechanism-illustrated drug design and the optimisation of dose control for cancer treatment in clinic.

Discovery of Distinctin-Like-Peptide-PH (DLP-PH) From the Skin Secretion of Phyllomedusa hypochondrialis, a Prototype of a Novel Family of Antimicrobial Peptide

Amphibian skin secretions are an important treasure house of bioactive antimicrobial peptides (AMPs). Despite having been the focus of decades of research in this context, investigations of phyllomedusine frogs continue to identify new AMPs from their skin secretions. In this study, the prototype of a novel family of AMP distinctin-like-peptide-PH (DLP-PH) was identified from the skin secretion of the otherwise well-studied Tiger-Legged Tree Frog Phyllomedusa hypochondrialis through cloning of its precursor-encoding cDNA from a skin secretion-derived cDNA library by a 3′-rapid amplification of cDNA ends (RACE) strategy. Subsequently, the mature peptide was isolated and characterized using reverse-phase HPLC and MS/MS fragmentation sequencing. DLP-PH adopted an α-helical conformation in membrane mimetic solution and demonstrated unique structural features with two distinct domains that differed markedly in their physiochemical properties. Chemically synthesized replicates of DLP-PH showed antimicrobial activity against planktonic bacterial and yeast cells, but more potent against Escherichia coli at 32 μg/mL. However, DLP-PH showed much weaker inhibitory activity against the growth of sessile cells in biofilms. In addition, DLP-PH exhibited anti-proliferative activity against human cancer cell lines, H157, and PC3, but with no major toxicity against normal human cell, HMEC-1. These combined properties make DLP-PH deserving further study as an antimicrobial agent and further investigations of its structure-activity relationship could provide valuable new insights into drug lead candidates for antimicrobial and/or anti-cancer purposes.

Identification and Characterisation of the Antimicrobial Peptide, Phylloseptin-PT, from the Skin Secretion of Phyllomedusa tarsius, and Comparison of Activity with Designed, Cationicity-Enhanced Analogues and Diastereomers

Antimicrobial peptides belonging to the phylloseptin family are mainly found in phyllomedusine frogs. These peptides not only possess potent antimicrobial activity but exhibit low toxicity against eukaryotic cells. Therefore, they are considered as promising drug candidates for a number of diseases. In a recent study, potent antimicrobial activity was correlated with the conserved structures and cationic amphiphilic characteristics of members of this peptide family. A phylloseptin peptide precursor was discovered here in the skin secretion of Phyllomedusa tarsius and the mature peptide was validated by MS/MS sequencing, and was subsequently named phylloseptin-PT. The chemically-synthesized and purified phylloseptin-PT displayed activity against Staphylococcus aureus and Candida albicans. Nevertheless, a range of cationicity-enhanced peptide analogues of phylloseptin-PT, which contained amino acid substitutions at specific sites, exhibited significant increases in antimicrobial activity compared to native phylloseptin-PT. In addition, alternative conformers which were designed and chemically-synthesized with d-lysine, showed potent antimicrobial activity and enhanced bioavailability. These data indicate that phylloseptins may represent potential candidates for next-generation antibiotics. Thus, rational design through modification of natural antimicrobial peptide templates could provide an accelerated path to overcoming obstacles en-route to their possible clinical applications.

A stereochemical switch in the aDrs model system, a candidate for a functional amyloid

Amyloid fibrils are commonly observed to adopt multiple distinct morphologies, which eventually can have significantly different neurotoxicities, as e.g. demonstrated in case of the Alzheimer peptide. The architecture of amyloid deposits is apparently also determined by the stereochemistry of amino acids. Post-translational changes of the chirality of certain residues may thus be a factor in controlling the formation of functional or disease-related amyloids. Anionic dermaseptin (aDrs), an unusual peptide from the skin secretions of the frog Pachymedusa dacnicolor, assembles to amyloid-like fibrils in a pH-dependent manner, which could play a functional role in defense. aDrs can be enzymatically converted into the diastereomer [d-Leu2]-aDrs by an l/d-isomerase. EM and AFM on fibrils formed by these isomers have shown that their predominant morphology is controlled by the stereochemistry of residue 2, whereas kinetic and thermodynamic parameters of aggregation are barely affected. When fibrils were grown from preformed seeds, backbone stereochemistry rather than templating-effects apparently dominated the superstructural organization of the isomers. Interestingly, MD indicated small differences in the conformational propensities between the isomers. Our results demonstrate how d-amino acid substitutions could take active part in the formation of functional or disease-related amyloid. Moreover, these findings contribute to the development of amyloid-based nanomaterials.

Substrate specificity of a peptidyl-aminoacyl-L/D-isomerase from frog skin

In the skin of fire-bellied toads (Bombina species), an aminoacyl-l/d-isomerase activity is present which catalyses the post-translational isomerization of the l- to the d-form of the second residue of its substrate peptides. Previously, this new type of enzyme was studied in some detail and genes potentially coding for similar polypeptides were found to exist in several vertebrate species including man. Here, we present our studies to the substrate specificity of this isomerase using fluorescence-labeled variants of the natural substrate bombinin H with different amino acids at positions 1, 2 or 3. Surprisingly, this enzyme has a rather low selectivity for residues at position 2 where the change of chirality at the alpha-carbon takes place. In contrast, a hydrophobic amino acid at position 1 and a small one at position 3 of the substrate are essential. Interestingly, some peptides containing a Phe at position 3 also were substrates. Furthermore, we investigated the role of the amino-terminus for substrate recognition. In view of the rather broad specificity of the frog isomerase, we made a databank search for potential substrates of such an enzyme. Indeed, numerous peptides of amphibia and mammals were found which fulfill the requirements determined in this study. Expression of isomerases with similar characteristics in other species can therefore be expected to catalyze the formation of peptides containing d-amino acids.

Analgesic and anti-inflammatory effects of the amphibian neurotoxin, anntoxin

Anntoxin is the first gene-encoded neurotoxin identified from amphibians, which is a 60-residue neurotoxin peptide, acting as an inhibitor of tetrodotoxin-sensitive (TTX-S) voltage-gated sodium channel (VGSC). Sodium channels have been considered as therapeutic targets for pain. Several animal models of persistent inflammatory and neuropathic pain (tail-flick test, hot plate test, acetic acid-induced writhing test, formalin-induced paw licking, carrageenan-induced paw edema) were used to test analgesic functions of recombinant anntoxin (r-anntoxin). In all these animal models, r-anntoxin showed strong analgesic functions. R-anntoxin obviously inhibited secretions of both tumor necrosis factor alpha (TNF-α) and cyclooxygenase-2 (COX-2). Histopathological study indicated that r-anntoxin reduced the edematous epidermis induced by carrageenan. All these results indicate that r-anntoxin has strong analgesic and anti-inflammatory activities.

An orphan dermaseptin from frog skin reversibly assembles to amyloid-like aggregates in a pH-dependent fashion

Dermaseptin PD-3-7 (aDrs) from frog skin contains three aspartic acid residues resulting in a negative net charge at neutral pH, as opposed to numerous other dermaseptins which are cationic helical antimicrobial peptides. Still, this peptide can be fitted into an amphipathic alpha helix by an Edmundson wheel projection. However, folding to the proposed helix was induced to only a low extent by zwitterionic vesicles or even detergents. Furthermore, no evidence of antibacterial or cytotoxic activity from soluble aDrs could be obtained. The peptide has an inherent propensity to an extended conformation in aqueous solution and self-assembles into amyloid fibrils in a reversible pH-controlled fashion, which was studied in some detail; above pH 5, the amyloid fibrils disassemble in a cooperative manner. This is probably caused by deprotonation of both side chain and terminal carboxyl groups, which results in intermolecular electrostatic repulsion. At neutral pH, this process proceeds instantaneously to the soluble form. Within the transition interval (pH 5-6.5), however, 'backward' granular aggregates, 10-500 nm in size, are formed. Such metastable amorphous aggregates, which are quickly released from an amyloid depot by a shift in pH, can mediate a strong cytotoxic effect. This activity does not involve lysis or interference with the cellular redox status, but apparently acts via an as yet unidentified mechanism. In this study, we present a new member of an emerging class of self-assembling frog skin peptides with extraordinary self-aggregation properties, which may potentially be relevant for biological processes. Structured digital abstract: * MINT-7256467: Dermaseptin (uniprotkb:O93455) and Dermaseptin (uniprotkb:O93455) bind (MI:0407) by circular dichroism (MI:0016) * MINT-7255686: Dermaseptin (uniprotkb:O93455) and Dermaseptin (uniprotkb:O93455) bind (MI:0407) by biophysical (MI:0013) * MINT-7256439: Dermaseptin (uniprotkb:O93455) and Dermaseptin (uniprotkb:O93455) bind (MI:0407) by fluorescence microscopy (MI:0416) * MINT-7256449: Dermaseptin (uniprotkb:O93455) and Dermaseptin (uniprotkb:O93455) bind (MI:0407) by electron microscopy (MI:0040) * MINT-7256430: Dermaseptin (uniprotkb:O93455) and Dermaseptin (uniprotkb:O93455) bind (MI:0407) by fluorescence technologies (MI:0051).

The first gene-encoded amphibian neurotoxin

Many gene-encoded neurotoxins with various functions have been discovered in fish, reptiles, and mammals. A novel 60-residue neurotoxin peptide (anntoxin) that inhibited tetrodotoxin-sensitive (TTX-S) voltage-gated sodium channel (VGSC) was purified and characterized from the skin secretions of the tree frog Hyla annectans (Jerdon). This is the first gene-encoded neurotoxin found in amphibians. The IC50 of anntoxin for the TTX-S channel was about 3.4 microM. Anntoxin shares sequence homology with Kunitz-type toxins but contains only two of three highly conserved cysteine bridges, which are typically found in these small, basic neurotoxin modules, i.e. snake dendrotoxins. Anntoxin showed an inhibitory ability against trypsin with an inhibitory constant (Ki) of 0.025 microM. Anntoxin was distributed in skin, brain, stomach, and liver with a concentration of 25, 7, 3, and 2 microg/g wet tissue, respectively. H. annectans lives on trees or other plants for its entire life cycle, and its skin contains the largest amount of anntoxin, which possibly helps defend against various aggressors or predators. A low dose of anntoxin was found to induce lethal toxicity for several potential predators, including the insect, snake, bird, and mouse. The tissue distribution and functional properties of the current toxin may provide insights into the ecological adaptation of tree-living amphibians.

Amphibian skin secretomics: application of parallel quadrupole time-of-flight mass spectrometry and peptide precursor cDNA cloning to rapidly characterize the skin secretory peptidome of Phyllomedusa hypochondrialis azurea: discovery of a novel peptide family, the hyposins

This study reports the variety of peptides present in the skin secretory peptidome of Phyllomedusa hypochondrialis azurea. Peptide structures, along with post-translational modifications, were elucidated by QTOF MS/MS analysis, cDNA sequencing, or a combination of both. Twenty-two peptides, including 19 novel structures, were identified from six different structural classes, including tryptophyllins, dermorphins, and a novel group of peptides termed hyposins. The study demonstrates the power of this combined approach to mine the rich peptidome compliment of the amphibian defensive skin secretome.

The Cecropin Superfamily of Toxic Peptides

All sequenced peptide toxins of the cecropin, pleurocidin and dermaceptin/ceratotoxin families in the National Center for Biotechnology Information (NCBI) database as of May 2005 were identified and shown to comprise a single superfamily. The peptide sequences were multiply aligned, revealing conserved residues that may play roles in structure and function. Signature sequences were derived for each of the 3 constituent families. Phylogenetic analyses revealed the relationships of these peptides to each other, and average hydropathy/amphipathicity studies provided structural information. This study serves to characterize a large superfamily of toxic peptides that perform host defense functions in a range of animal kingdoms.

Post-translational amino acid racemization in the frog skin peptide deltorphin I in the secretion granules of cutaneous serous glands

The dermal glands of the South American hylid frog Phyllomedusa bicolor synthesize and expel huge amounts of cationic, alpha-helical, 24- to 33-residue antimicrobial peptides, the dermaseptins B. These glands also produce a wide array of peptides that are similar to mammalian hormones and neuropeptides, including a heptapeptide opioid containing a D-amino acid, deltorphin I (Tyr-DAla-Phe-Asp-Val-Val-Gly NH2). Its biological activity is due to the racemization of L-Ala2 to D-Ala. The dermaseptins B and deltorphins are all derived from a single family of precursor polypeptides that have an N-terminal preprosequence that is remarkably well conserved, although the progenitor sequences giving rise to mature opioid or antimicrobial peptides are markedly different. Monoclonal and polyclonal antibodies were used to examine the cellular and ultrastructural distributions of deltorphin I and dermaseptin B in the serous glands by immunofluoresence confocal microscopy and immunogold-electron microscopy. Preprodeltorphin I and preprodermaseptins B are sorted into the regulated pathway of secretion, where they are processed to give the mature products. Deltorphin I, [l-Ala2]-deltorphin I and dermaseptin B are all stored together in secretion granules which accumulate in the cytoplasm of all serous glands. We conclude that the L- to D-amino acid isomerization of the deltorphin I occurs in the secretory granules as a post-translational event. Thus the specificity of isomerization depends on the presence of structural and/or conformational determinants in the peptide N-terminus surrounding the isomerization site.

Isolation and characterisation of an unexpected class of insulinotropic peptides in the skin of the frog Agalychnis litodryas

Skin secretions of the frog Agalychnis litodryas were evaluated for the isolation and characterisation of novel insulinotropic peptides. Crude secretions obtained from young adult frogs by mild electrical stimulation of the dorsal skin surface were purified by reverse-phase high-performance liquid chromatography (HPLC) yielding 70 fractions. In acute 20-min incubations with glucose responsive BRIN-BD11 cells, fractions 39-42 (band 1) and fractions 44-46 (band 2) significantly stimulated insulin release by 2-3.5-fold compared with 5.6 mM glucose alone. Pooled fractions in band 1 and band 2 were rechromatographed to reveal 20 homogenous peptide peaks, which elicited significant 1.5-4-fold increases in insulin release. Mass spectrometry analyses indicated molecular masses of between 1649.2 and 4988.9 Da. The two peptides with the greatest insulin-releasing activity were directly subjected to N-terminal amino acid sequence analysis. The sequence of the 3020 Da peptide, called frog skin insulinotropic peptide or FSIP, was determined as AVWKDFLKNIGKAAGKAVLNSVTDMVNE, which has 79% homology with the C-terminal of the 75 amino acid dermaseptin BIV precursor. A partial N-terminal sequence was determined for the 2546.2 Da peptide as MLADVFEKIMGD... These data indicate that the skin secretions of A. litodryas frogs contain biologically active peptides which merit further evaluation as a new class of insulin secretagogues.

Novel insulin-releasing peptides in the skin of Phyllomedusa trinitatis frog include 28 amino acid peptide from dermaseptin BIV precursor

OBJECTIVE

The granular glands of amphibians have long been known to produce many biologically active compounds. The aim of this study was to isolate and characterize insulinotropic peptides from the skin of Phyllomedusa trinitatis frog.

METHODS AND RESULTS

Crude secretions obtained by mild electrical stimulation of the dorsal skin surface were purified by reverse phase HPLC yielding 80 fractions. In acute incubations with glucose-responsive BRIN-BD11 cells, fractions 39-40 (band 1) and fractions 43-46 (band 2) significantly stimulated insulin release by 1.5 to 2.5-fold. Pooled fractions in bands 1 and 2 were rechromatographed to 4 homogeneous peaks, each with insulin-releasing activity. Mass spectrometry analysis was successfully completed for 3 peptides, indicating 2996.4, 3379.9, and 8326.4 Da. The sequence of the 2996.4 Da peptide was determined as ALWKDILKNVGKAAGKAVLNTVTDMVNQ. This 28-amino-acid peptide has 100% homology with the C-terminal of the 75-amino-acid dermaseptin BIV precursor of a family of structurally related antimicrobial peptides in the skin of the Phyllomedusinae subfamily.

CONCLUSION

These data demonstrate that the defensive skin secretions of P. trinitatis contain biologically active peptides, which may have mammalian counterparts and merit further investigation as insulin secretagogues.

Cloning of maximakinin precursor cDNAs from Chinese toad, Bombina maxima, venom

Using a novel technique that we have developed for cloning of amphibian skin secretion peptide cDNAs from lyophilized samples, we report here that maximakinin (DLPKINRKGP-bradykinin) is encoded by two different cDNAs, named BMK-1 and BMK-2, containing either four tandem repeat sequences or a single copy. The open reading frames of both precursor cDNAs were found to be 152 and 116 amino acid residues, respectively. These data provide evidence that the structural diversity of peptides in amphibian skin secretions arising from molecular evolutionary events, can be mediated by parallel diversity in encoding mRNAs that in itself may reflect serial gene duplications.

Pharmacology of amphibian opiate peptides

In 1980 the skin of certain frogs belonging to the genus Phyllomedusinae was found to contain two new peptides that proved to be selective mu-opioid agonists, and named dermorphins. Since 1987 deltorphins, a family of highly selective delta-opioid peptides were identified either by cloning of the cDNA from frog skins or isolation of the peptides. The distinctive feature of opioid peptides is the presence of a naturally occurring D-enantiomer at the second position in their common N-terminal sequence, Tyr-D-Xaa-Phe. The discovery of the amphibian opiate peptides, provided new insights into the functional role of the mu- and delta-opiate systems. It also provided models for novel analgesics with enhanced therapeutic benefits and reduced toxicity.

Neurologically active plant compounds and peptide hormones: a chirality connection

The most dramatic, but seldom mentioned, difference between alkaloid and peptide opioids is the change of chirality of the alpha carbon of the tyramine moiety. We propose that the presence of Gly2 or D-Ala2 in the two most common message domains compensates this change by allowing the attainment of unusual conformations. A thorough conformational search of Tyr-D-Ala-Phe-NH-CH3 and of its isomer Tyr-L-Ala-Phe-NH-CH3 backs this view and establishes a solid link between alkaloid and peptide opioids. This finding supports the notion that morphine, like other neurologically active plant compounds, may bind to endogenous receptors in plants to regulate cell-to-cell signaling systems.

What peptides these deltorphins be

The deltorphins are a class of highly selective delta-opioid heptapeptides from the skin of the Amazonian frogs Phyllomedusa sauvagei and P. bicolor. The first of these fascinating peptides came to light in 1987 by cloning of the cDNA of from frog skins, while the other members of this family were identified either by cDNA or isolation of the peptides. The distinctive feature of deltorphins is the presence of a naturally occurring D-enantiomer at the second position in their common N-terminal sequence, Tyr-D-Xaa-Phe, comparable to dermorphin, which is the prototype of a group of mu-selective opioids from the same source. The D-amino acid and the anionic residues, either Glu or Asp, as well as their unique amino acid compositions are responsible for the remarkable biostability, high delta-receptor affinity, bioactivity and peptide conformation. This review summarizes a decade of research from many laboratories that defined which residues and substituents in the deltorphins interact with the delta-receptor and characterized pharmacological and physiological activities in vitro and in vivo. It begins with a historical description of the topic and presents general schema for the synthesis of peptide analogues of deltorphins A, B and C as a means to document the methods employed in producing a myriad of analogues. Structure activity studies of the peptides and their pharmacological activities in vitro are detailed in abundantly tabulated data. A brief compendium of the current level of knowledge of the delta-receptor assists the reader to appreciate the rationale for the design of these analogues. Discussion of the conformation of these peptides addresses how structure leads to further hypotheses regarding ligand receptor interaction. The review ends with a broad discussion of the potential applications of these peptides in clinical and therapeutic settings.