Morphology of the granular secretory glands in skin of poison-dart frogs (Dendrobatidae)

Binding and sequestration of poison frog alkaloids by a plasma globulin

Alkaloids are important bioactive molecules throughout the natural world, and in many animals they serve as a source of chemical defense against predation. Dendrobatid poison frogs bioaccumulate alkaloids from their diet to make themselves toxic or unpalatable to predators. Despite the proposed roles of plasma proteins as mediators of alkaloid trafficking and bioavailability, the responsible proteins have not been identified. We use chemical approaches to show that a ~50 kDa plasma protein is the principal alkaloid-binding molecule in blood of poison frogs. Proteomic and biochemical studies establish this plasma protein to be a liver-derived alkaloid-binding globulin (ABG) that is a member of the serine-protease inhibitor (serpin) family. In addition to alkaloid-binding activity, ABG sequesters and regulates the bioavailability of 'free' plasma alkaloids in vitro. Unexpectedly, ABG is not related to saxiphilin, albumin, or other known vitamin carriers, but instead exhibits sequence and structural homology to mammalian hormone carriers and amphibian biliverdin-binding proteins. ABG represents a new small molecule binding functionality in serpin proteins, a novel mechanism of plasma alkaloid transport in poison frogs, and more broadly points toward serpins acting as tunable scaffolds for small molecule binding and transport across different organisms.

A potential cost of evolving epibatidine resistance in poison frogs

BACKGROUND

Some dendrobatid poison frogs sequester the toxin epibatidine as a defense against predators. We previously identified an amino acid substitution (S108C) at a highly conserved site in a nicotinic acetylcholine receptor β2 subunit of dendrobatid frogs that decreases sensitivity to epibatidine in the brain-expressing α4β2 receptor. Introduction of S108C to the orthologous high-sensitivity human receptor similarly decreased sensitivity to epibatidine but also decreased sensitivity to acetylcholine, a potential cost if this were to occur in dendrobatids. This decrease in the acetylcholine sensitivity manifested as a biphasic acetylcholine concentration-response curve consistent with the addition of low-sensitivity receptors. Surprisingly, the addition of the β2 S108C into the α4β2 receptor of the dendrobatid Epipedobates anthonyi did not change acetylcholine sensitivity, appearing cost-free. We proposed that toxin-bearing dendrobatids may have additional amino acid substitutions protecting their receptors from alterations in acetylcholine sensitivity. To test this, in the current study, we compared the dendrobatid receptor to its homologs from two non-dendrobatid frogs.

RESULTS

The introduction of S108C into the α4β2 receptors of two non-dendrobatid frogs also does not affect acetylcholine sensitivity suggesting no additional dendrobatid-specific substitutions. However, S108C decreased the magnitude of neurotransmitter-induced currents in Epipedobates and the non-dendrobatid frogs. We confirmed that decreased current resulted from fewer receptors in the plasma membrane in Epipedobates using radiolabeled antibodies against the receptors. To test whether S108C alteration of acetylcholine sensitivity in the human receptor was due to (1) adding low-sensitivity binding sites by changing stoichiometry or (2) converting existing high- to low-sensitivity binding sites with no stoichiometric alteration, we made concatenated α4β2 receptors in stoichiometry with only high-sensitivity sites. S108C substitutions decreased maximal current and number of immunolabeled receptors but no longer altered acetylcholine sensitivity.

CONCLUSIONS

The most parsimonious explanation of our current and previous work is that the S108C substitution renders the β2 subunit less efficient in assembling/trafficking, thereby decreasing the number of receptors in the plasma membrane. Thus, while β2 S108C protects dendrobatids against sequestered epibatidine, it incurs a potential physiological cost of disrupted α4β2 receptor function.

A potential cost of evolving epibatidine resistance in poison frogs

Background

Some poison arrow frogs sequester the toxin epibatidine as a defense against predators. We previously identified a single amino acid substitution (S108C) at a highly conserved site in a neuronal nicotinic acetylcholine receptor (nAChR) ß2 subunit that prevents epibatidine from binding to this receptor. When placed in a homologous mammalian nAChR this substitution minimized epibatidine binding but also perturbed acetylcholine binding, a clear cost. However, in the nAChRs of poison arrow frogs, this substitution appeared to have no detrimental effect on acetylcholine binding and, thus, appeared cost-free.

Results

The introduction of S108C into the α4β2 nAChRs of non-dendrobatid frogs also does not affect ACh sensitivity, when these receptors are expressed in Xenopus laevis oocytes. However, α4β2 nAChRs with C108 had a decreased magnitude of neurotransmitter-induced currents in all species tested ( Epipedobates anthonyi , non-dendrobatid frogs, as well as human), compared with α4β2 nAChRs with the conserved S108. Immunolabeling of frog or human α4β2 nAChRs in the plasma membrane using radiolabeled antibody against the β2 nAChR subunit shows that C108 significantly decreased the number of cell-surface α4β2 nAChRs, compared with S108.

Conclusions

While S108C protects these species against sequestered epibatidine, it incurs a potential physiological cost of disrupted α4β2 nAChR function. These results may explain the high conservation of a serine at this site in vertebrates, as well as provide an example of a tradeoff between beneficial and deleterious effects of an evolutionary change. They also provide important clues for future work on assembly and trafficking of this important neurotransmitter receptor.

Noninvasive Detection of Chemical Defenses in Poison Frogs Using the MasSpec Pen

Poison frogs are well-known for their fascinating ability to store alkaloids in their skin as chemical defense against predators. Chemical methods used to study these alkaloids are limited by requirements for euthanasia or stress during sampling. Here, we demonstrate sensitive and biocompatible alkaloid detection and monitoring in vivo using the MasSpec Pen, a handheld, noninvasive chemical detection device coupled to a mass spectrometer. The MasSpec Pen allowed rapid (<15 s), gentle, and consecutive molecular analysis without harm or undue stress to the animals. Through a month-long alkaloid-feeding study with the dyeing poison frog, we observed temporal dynamics of chemical sequestration in vivo by comparing frogs fed the alkaloid decahydroquinoline (DHQ) to vehicle-fed frogs. We also demonstrate the feasibility of the MasSpec Pen for the untargeted detection of rich alkaloid profiles from skin extracts of the Diablito poison frog, collected from two distinct geographical populations in Ecuador. The results obtained in this study demonstrate the utility of the MasSpec Pen for direct, rapid, and biocompatible analysis of poison frog alkaloids.

Transcriptomic analysis reveals potential candidate pathways and genes involved in toxin biosynthesis in true toads

Synthesized chemical defenses have broadly evolved across countless taxa and are important in shaping evolutionary and ecological interactions within ecosystems. However, the underlying genomic mechanisms by which these organisms synthesize and utilize their toxins are relatively unknown. Herein, we use comparative transcriptomics to uncover potential toxin synthesizing genes and pathways, as well as interspecific patterns of toxin synthesizing genes across ten species of North American true toads (Bufonidae). Upon assembly and annotation of the ten transcriptomes, we explored patterns of relative gene expression and possible protein-protein interactions across the species to determine what genes and/or pathways may be responsible for toxin synthesis. We also tested our transcriptome dataset for signatures of positive selection to reveal how selection may be acting upon potential toxin producing genes. We assembled high quality transcriptomes of the bufonid parotoid gland, a tissue not often investigated in other bufonid related RNAseq studies. We found several genes involved in metabolic and biosynthetic pathways (e.g. steroid biosynthesis, terpenoid backbone biosynthesis, isoquinoline biosynthesis, glucosinolate biosynthesis) that were functionally enriched and/or relatively expressed across the ten focal species that may be involved in the synthesis of alkaloid and steroid toxins, as well as other small metabolic compounds that cause distastefulness in bufonids. We hope that our study lays a foundation for future studies to explore the genomic underpinnings and specific pathways of toxin synthesis in toads, as well as at the macroevolutionary scale across numerous taxa that produce their own defensive toxins.

How Phylogenetics Can Elucidate the Chemical Ecology of Poison Frogs and Their Arthropod Prey

The sequestration by neotropical poison frogs (Dendrobatidae) of an amazing array of defensive alkaloids from oribatid soil mites has motivated an exciting research theme in chemical ecology, but the details of mite-to-frog transfer remain hidden. To address this, McGugan et al. (2016, Journal of Chemical Ecology 42:537-551) used the little devil poison frog (Oophaga sylvatica) and attempted to simultaneously characterize the prey mite alkaloids, the predator skin alkaloids, and identify the mites using DNA sequences. Heethoff et al. (2016, Journal of Chemical Ecology 42:841-844) argued that none of the mite families to which McGugan et al. allocated the prey was thought to possess alkaloids. Heethoff et al. concluded from analyses including additional sequences that the mite species were unlikely to be close relatives of the defended mites. We re-examine this by applying more appropriate phylogenetic methods to broader and denser taxonomic samples of mite sequences using the same gene (CO1). We found, over trees based on CO1 datasets, only weak support (except in one case) for branches critical to connecting the evolution of alkaloid sequestration with the phylogeny of mites. In contrast, a well-supported analysis of the 18S ribosomal gene suggests at least two independent evolutionary origins of oribatid alkaloids. We point out impediments in the promising research agenda, namely a paucity of genetic, chemical, and taxonomic information, and suggest how phylogenetics can elucidate at a broader level the evolution of chemical defense in prey arthropods, sequestration by predators, and the impact of alkaloids on higher-order trophic interactions.

Structure and evolution of the sexually dimorphic integumentary swelling on the hands of dendrobatid poison frogs and their relatives (Amphibia: Anura: Dendrobatoidea)

The hands of adult males of many dendrobatid poison frogs and their relatives possess swelling formed by glandular tissue hypothesized to secrete courtship pheromones delivered to the female during cephalic amplexus. Variation in the occurrence and external morphology of the swelling of finger IV has provided important evidence for dendrobatoid systematics for decades, but its underlying structure has not been investigated. We undertook a detailed comparative analysis of the integument of the hand, including both external morphology and histology, of 36 species representing the diversity of dendrobatoid frogs and several close relatives. The swelling is caused by four densely packed, hypertrophic, morpho-histochemical types of specialized mucous glands (SMGs). We observed type I SMGs on fingers II-V and the wrist, including areas that are not swollen, types II and IV exclusively on finger IV, and type III on finger IV and the wrist. Type I SMGs occur either in isolation or together with types II, III, or IV; types II, III, and IV never occur together or without type I. We delimited 15 characters to account for the variation in external morphology and the occurrence of SMGs. Our data suggest that type I SMGs are a new synapomorphy for Dendrobatoidea and that type II SMGs originated in either the most recent common ancestor (MRCA) of Dendrobatidae or independently in the MRCAs of Aromobatidae and Colostethinae, respectively, while types III and IV are restricted to Anomaloglossus. The discovery of these SMGs adds a new dimension to studies of poison frog reproductive biology, which have investigated acoustic, visual, and tactile cues in courtship, mating, and parental care across the diversity of Dendrobatoidea for decades but have almost entirely overlooked the possible role of chemical cues.

First characterization of toxic alkaloids and volatile organic compounds (VOCs) in the cryptic dendrobatid Silverstoneia punctiventris

Background

Poison frogs are known for the outstanding diversity of alkaloid-based chemical defences with promising therapeutic applications. However, current knowledge about chemical defences in Dendrobatoidea superfamily has two sources of bias. First, cryptic, brown-colored species have been neglected in comparison to those conspicuously colored, and second, there has been little interest in characterizing metabolites other than alkaloids mediating defensive functions. In an effort to contribute to fill the gap of knowledge about cryptic species and broadening the spectrum of compounds analyzed we have applied head-space solid phase microextraction coupled to gas chromatography and mass spectrometry (HS-SPME/GC-MS) for extracting amphibian alkaloids and volatile organic compounds (VOCs) from Silverstoneia punctiventris.

Results

Using the skin from 8 specimens in 4 biological replicates we have found 33 different compounds. Twenty of them were classified as VOCs into 15 chemical classes including alkanes, alcohols, carbonyl compounds, methylpyridines, benzothiazoles, N-alkylpyrrolidines, pyrazines, and sesquiterpenoids, some of which were previously reported as repellents, defence compounds or defence pheromones in other organisms, and as sex pheromones in a treefrog. Interestingly, six of the remaining compounds were identified as alkaloids previously reported in other toxic/unpalatable dendrobatid frogs.

Conclusions

This is the first report of alkaloids and VOCs found in the Silverstoneia genus, which has been assumed for decades as non-chemically defended. This study establishes HS-SPME/GC-MS as a new application for a simultaneous approach to amphibian alkaloids and VOCs in poison frogs while opens up new research questions to assess the co-occurrence of both type of compounds and to investigate the evolutionary significance of a defence gradient that includes olfactory avoidance, unpalatability, and toxicity in dendrobatids. In addition, our results show that amphibian alkaloids could have a dual function (olfactory at distance, taste by contact) never explored before neither in Silverstonaeia nor in any other dendrobatid species.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12983-021-00420-1.

Rapid toxin sequestration modifies poison frog physiology

Poison frogs sequester chemical defenses from their diet of leaf litter arthropods for defense against predation. Little is known about the physiological adaptations that confer this unusual bioaccumulation ability. We conducted an alkaloid-feeding experiment with the Diablito poison frog (Oophaga sylvatica) to determine how quickly alkaloids are accumulated and how toxins modify frog physiology using quantitative proteomics. Diablito frogs rapidly accumulated the alkaloid decahydroquinoline within 4 days, and dietary alkaloid exposure altered protein abundance in the intestines, liver and skin. Many proteins that increased in abundance with decahydroquinoline accumulation are plasma glycoproteins, including the complement system and the toxin-binding protein saxiphilin. Other protein classes that change in abundance with decahydroquinoline accumulation are membrane proteins involved in small molecule transport and metabolism. Overall, this work shows that poison frogs can rapidly accumulate alkaloids, which alter carrier protein abundance, initiate an immune response, and alter small molecule transport and metabolism dynamics across tissues.

Use of whole-body cryosectioning and desorption electrospray ionization mass spectrometry imaging to visualize alkaloid distribution in poison frogs

Ambient mass spectrometry is useful for analyzing compounds that would be affected by other chemical procedures. Poison frogs are known to sequester alkaloids from their diet, but the sequestration pathway is unknown. Here, we describe methods for whole-body cryosectioning of frogs and use desorption electrospray ionization mass spectrometry imaging (DESI-MSI) to map the orally administered alkaloid histrionicotoxin 235A in a whole-body section of the poison frog Dendrobates tinctorius. Our results show that whole-body cryosectioning coupled with histochemical staining and DESI-MSI is an effective technique to visualize alkaloid distribution and help elucidate the mechanisms involved in alkaloid sequestration in poison frogs.

Transcriptomic Signatures of Experimental Alkaloid Consumption in a Poison Frog

In the anuran family Dendrobatidae, aposematic species obtain their toxic or unpalatable alkaloids from dietary sources, a process known as sequestering. To understand how toxicity evolved in this family, it is paramount to elucidate the pathways of alkaloid processing (absorption, metabolism, and sequestering). Here, we used an exploratory skin gene expression experiment in which captive-bred dendrobatids were fed alkaloids. Most of these experiments were performed with Dendrobates tinctorius, but some trials were performed with D. auratus, D. leucomelas and Allobates femoralis to explore whether other dendrobatids would show similar patterns of gene expression. We found a consistent pattern of up-regulation of genes related to muscle and mitochondrial processes, probably due to the lack of mutations related to alkaloid resistance in these species. Considering conserved pathways of drug metabolism in vertebrates, we hypothesize alkaloid degradation is a physiological mechanism of resistance, which was evidenced by a strong upregulation of the immune system in D. tinctorius, and of complement C2 across the four species sampled. Probably related to this strong immune response, we found several skin keratins downregulated, which might be linked to a reduction of the cornified layer of the epidermis. Although not conclusive, our results offer candidate genes and testable hypotheses to elucidate alkaloid processing in poison frogs.

Diversity and evolution of the parotoid macrogland in true toads (Anura: Bufonidae)

Skin glands in amphibians are either distributed throughout the skin or aggregated in multiglandular structures such as the parotoids typical of most species of Bufonidae. Although many early divergent and derived bufonids lack a discrete parotoid in the postorbital–supratympanic (PoSt) region, they have a great macroscopic diversity in the skin morphology of this region. To understand the origin and evolution of this diversity, in particular of the parotoids, we describe the histomorphology of the skin of the PoSt and dorsal regions in 17 species of bufonids, with or without external evident parotoid, and compare it with previously published descriptions. The survey results in 27 characters that were optimized on a phylogenetic hypothesis of Bufonidae. Our results reveal that the PoSt region has a noteworthy morphological diversity of types of glands, spatial organization and differences in the secretion products. Some morphological characters represent putative synapomorphies of internal clades of Bufonidae and are related to the progressive differentiation towards defined structures (macroglands, parotoids). These morphological results, along with published information on the toxicity of the skin secretions and defensive behaviours in some representative species, allow us to infer possible relationships between these features.

Form and Function of the skin glands in the Himalayan newt Tylototriton verrucosus

BACKGROUND

Amphibians have evolved a remarkable diversity of defensive mechanisms against predators. One of the most conspicuous components in their defense is related to their ability to produce and store a high variety of bioactive (noxious to poisonous) substances in specialized skin glands. Previous studies have shown that T. verrucosus is poisonous with the potential to truly harm or even kill would-be predators by the effect of its toxic skin secretions. However, little is known on form and function of the skin glands responsible for production and release of these secretions.

RESULTS

By using light- and scanning electron microscopy along with confocal laser scanning microscopy, we show that T. verrucosus exhibits three different multicellular skin glands: one mucous- and two granular glands. While mucous glands are responsible for the production of the slippery mucus, granular glands are considered the production site of toxins. The first type of granular glands (GG1) is found throughout the skin, though its average size can vary between body regions. The second type of granular glands (GG2) can reach larger dimensions compared with the former type and is restricted to the tail region. Despite their different morphology, all three skin gland types are enwrapped by a distinct myoepithelial sheath that is more prominently developed in the granular (i.e. poison-) glands compared to the mucous glands. The myoepithelial sheath consists of one layer of regularly arranged slender myoepithelial cells that run from the gland pore to the basal gland pole.

CONCLUSIONS

This study shows that the skin in the Himalayan newt T. verrucosus displays one mucus- and two poison gland types enwrapped by a myoepithelial sheath. Contraction of the myoepithelium squeezes the glands and glandular content is released upon the skin surface where the secretion can deploy its defensive potential.

Diversity within diversity: Parasite species richness in poison frogs assessed by transcriptomics

Symbionts (e.g., endoparasites and commensals) play an integral role in their host's ecology, yet in many cases their diversity is likely underestimated. Although endoparasites are traditionally characterized using morphology, sequences of conserved genes, and shotgun metagenomics, host transcriptomes constitute an underused resource to identify these organisms' diversity. By isolating non-host transcripts from host transcriptomes, individual host tissues can now simultaneously reveal their endoparasite species richness (i.e., number of different taxa) and provide insights into parasite gene expression. These approaches can be used in host taxa whose endoparasites are mostly unknown, such as those of tropical amphibians. Here, we focus on the poison frogs (Dendrobatidae) as hosts, which are a Neotropical clade known for their bright coloration and defensive alkaloids. These toxins are an effective protection against vertebrate predators (e.g., snakes and birds), bacteria, and skin-biting ectoparasites (e.g., mosquitoes); however, little is known about their deterrence against eukaryotic endoparasites. With de novo transcriptomes of dendrobatids, we developed a bioinformatics pipeline for endoparasite identification that uses host annotated RNA-seq data and set of a priori parasite taxonomic terms, which are used to mine for specific endoparasites. We found a large community of helminths and protozoans that were mostly restricted to the digestive tract and a few systemic parasites (e.g., Trypanosoma). Contrary to our expectations, all dendrobatid frogs regardless of the presence of alkaloid defenses have endoparasites, with their highest species richness located in the frog digestive tract. Some of these organisms (e.g., roundworms) might prove to be generalists, as they were not found to be co-diversifying with their frog hosts. We propose that endoparasites may escape poison frogs' chemical defenses by colonizing tissues with fewer alkaloids than the frog's skin, where most toxins are stored.

Ant and Mite Diversity Drives Toxin Variation in the Little Devil Poison Frog

Poison frogs sequester chemical defenses from arthropod prey, although the details of how arthropod diversity contributes to variation in poison frog toxins remains unclear. We characterized skin alkaloid profiles in the Little Devil poison frog, Oophaga sylvatica (Dendrobatidae), across three populations in northwestern Ecuador. Using gas chromatography/mass spectrometry, we identified histrionicotoxins, 3,5- and 5,8-disubstituted indolizidines, decahydroquinolines, and lehmizidines as the primary alkaloid toxins in these O. sylvatica populations. Frog skin alkaloid composition varied along a geographical gradient following population distribution in a principal component analysis. We also characterized diversity in arthropods isolated from frog stomach contents and confirmed that O. sylvatica specialize on ants and mites. To test the hypothesis that poison frog toxin variability reflects species and chemical diversity in arthropod prey, we (1) used sequencing of cytochrome oxidase 1 to identify individual prey specimens, and (2) used liquid chromatography/mass spectrometry to chemically profile consumed ants and mites. We identified 45 ants and 9 mites in frog stomachs, including several undescribed species. We also showed that chemical profiles of consumed ants and mites cluster by frog population, suggesting different frog populations have access to chemically distinct prey. Finally, by comparing chemical profiles of frog skin and isolated prey items, we traced the arthropod source of four poison frog alkaloids, including 3,5- and 5,8-disubstituted indolizidines and a lehmizidine alkaloid. Together, the data show that toxin variability in O. sylvatica reflects chemical diversity in arthropod prey.

Odorous secretions in anurans: morphological and functional assessment of serous glands as a source of volatile compounds in the skin of the treefrog Hypsiboas pulchellus (Amphibia: Anura: Hylidae)

Serous (granular or venom) glands occur in the skin of almost all species of adult amphibians, and are thought to be the source of a great diversity of chemical compounds. Despite recent advances in their chemistry, odorous volatile substances are compounds that have received less attention, and until now no study has attempted to associate histological data with the presence of these molecules in amphibians, or in any other vertebrate. Given the recent identification of 40 different volatile compounds from the skin secretions of H. pulchellus (a treefrog species that releases a strong odour when handled), we examined the structure, ultrastructure, histochemistry, and distribution of skin glands of this species. Histological analysis from six body regions reveals the presence of two types of glands that differ in their distribution. Mucous glands are homogeneously distributed, whereas serous glands are more numerous in the scapular region. Ultrastructural results indicate that electron-translucent vesicles observed within granules of serous glands are similar to those found in volatile-producing glands from insects and also with lipid vesicles from different organisms. Association among lipids and volatiles is also evidenced from chemical results, which indicate that at least some of the volatile components in H. pulchellus probably originate within the metabolism of fatty acids or the mevalonate pathway. As odorous secretions are often considered to be secreted under stress situations, the release of glandular content was assessed after pharmacological treatments, epinephrine administrated in vivo and on skin explants, and through surface electrical stimulation. Serous glands responded to all treatments, generally through an obvious contraction of myoepithelial cells that surround their secretory portion. No response was observed in mucous glands. Considering these morpho-functional results, along with previous identification of volatiles from H. pulchellus and H. riojanus after electrical stimulation, we suggest that the electron-translucent inclusions found within the granules of serous glands likely are the store sites of volatile compounds and/or their precursors. Histochemical and glandular distribution analyses in five other species of frogs of the hylid tribe Cophomantini, revealed a high lipid content in all the species, whereas a heterogeneous distribution of serous glands is only observed in species of the H. pulchellus group. The distribution pattern of serous glands in members of this species group, and the odorous volatile secretions are probably related to defensive functions.

Convergent Substitutions in a Sodium Channel Suggest Multiple Origins of Toxin Resistance in Poison Frogs

Complex phenotypes typically have a correspondingly multifaceted genetic component. However, the genotype–phenotype association between chemical defense and resistance is often simple: genetic changes in the binding site of a toxin alter how it affects its target. Some toxic organisms, such as poison frogs (Anura: Dendrobatidae), have defensive alkaloids that disrupt the function of ion channels, proteins that are crucial for nerve and muscle activity. Using protein-docking models, we predict that three major classes of poison frog alkaloids (histrionicotoxins, pumiliotoxins, and batrachotoxins) bind to similar sites in the highly conserved inner pore of the muscle voltage-gated sodium channel, Nav1.4. We predict that poison frogs are somewhat resistant to these compounds because they have six types of amino acid replacements in the Nav1.4 inner pore that are absent in all other frogs except for a distantly related alkaloid-defended frog from Madagascar, Mantella aurantiaca. Protein-docking models and comparative phylogenetics support the role of these replacements in alkaloid resistance. Taking into account the four independent origins of chemical defense in Dendrobatidae, phylogenetic patterns of the amino acid replacements suggest that 1) alkaloid resistance in Nav1.4 evolved independently at least five times in these frogs, 2) variation in resistance-conferring replacements is likely a result of differences in alkaloid exposure across species, and 3) functional constraint shapes the evolution of the Nav1.4 inner pore. Our study is the first to demonstrate the genetic basis of autoresistance in frogs with alkaloid defenses.

Arthropod predation in a dendrobatid poison frog: does frog life stage matter?

Frogs in the family Dendrobatidae are well known for their conspicuous colors and variable alkaloid-based chemical defenses. The aposematic coloration in dendrobatid frogs appears to deter predators with color vision, but relatively little is known about how these frogs are protected and their defenses are perceived by non-color vision dominated predators. The neotropical bullet ant Paraponera clavata and the red-legged banana spider Cupiennius coccineus are predators that avoid adults of the dendrobatid Oophaga pumilio, but readily consume non-toxic frogs. Juvenile O. pumilio possess the same warning coloration as adult O. pumilio, but may be more palatable given that they have lower quantities of defensive chemicals. This may provide juvenile O. pumilio protection from color-sighted predators, while leaving them susceptible to predators that use chemoreception. To test this hypothesis, we presented juveniles and adults of both O. pumilio and the non-chemically defended frog Craugastor bransfordii to bullet ants and banana spiders. Both bullet ants and banana spiders preyed upon C. bransfordii significantly more than on O. pumilio. Adult and juvenile C. bransfordii experienced similar predation rates by both predators. The life stage of O. pumilio significantly predicted predation by bullet ants, with juveniles being consumed significantly more often than adults. However, the life stage of O. pumilio did not predict predation by banana spiders, as no adults or juveniles were consumed. Our study provides evidence that bullet ants can detect differences in chemical defenses between juvenile and adult O. pumilio, resulting in differential predation on the more palatable juvenile frogs. The avoidance of both adults and juveniles by C. coccineus suggests the alkaloids in O. pumilio act as an effective chemical deterrent to banana spiders, regardless of quantity. Overall, our results suggest that differences in alkaloid defenses among life stages in O. pumilio correspond to differences in relative palatability to at least one arthropod predator.

Dietary source for skin alkaloids of poison frogs (Dendrobatidae)?

A wide range of alkaloids, many of which are unknown elsewhere in nature, occur in skin of frogs. Major classes of such alkaloids in dendrobatid frogs are the batrachotoxins, pumiliotoxins, histrionicotoxins, gephyrotoxins, and decahydroquinolines. Such alkaloids are absent in skin of frogs (Dendrobates auratus) raised in Panama on wingless fruit flies in indoor terraria. Raised on leaf-litter arthropods that were collected in a mainland site, such terraria-raised frogs contain tricyclic alkaloids including the beetle alkaloid precoccinelline, 1,4-disubstituted quinolizidines, pyrrolizidine oximes, the millipede alkaloid nitropolyzonamine, a decahydroquinoline, a gephyrotoxin, and histrionicotoxins. The profiles of these alkaloids in the captive-raised frogs are closer to the mainland population ofDendrobates auratus at the leaf-litter site than to the parent population ofDendrobates auratus from a nearby island site. Extracts of a seven-month sampling of leaf-litter insects contained precoccinelline, pyrrolizidine oxime236 (major), and nitropolyzonamine (238). The results indicate a dietary origin for at least some "dendrobatid alkaloids," in particular the pyrrolizidine oximes, the tricyclic coccinellines, and perhaps the histrionicotoxins and gephyrotoxins.

Serous cutaneous glands in anurans: Fourier transform analysis of the repeating secretory granule substructure

A combined transmission electron microscopy (TEM) and Fourier transform analysis has been performed on the secretory granules storing active peptides/proteins in serous cutaneous glands of n = 12 anuran species. Previous TEM investigation showed that the granules are provided with remarkable repeating substructures based on discrete subunits, arranged into a consistent framework. Furthermore, TEM findings revealed that this recurrent arrangement is acquired during a prolonged post-Golgian (or maturational) processing that affects the secretory product. Maturation leads to a variety of patterns depending on the degree of subunit clustering. This variety of recurrent patterns has been plotted into a range of frequency spectra. Through this quantitative approach, we found that the varying granule substructure can be reduced to a single mechanism of peptide/protein aggregation.

An endogenous bile acid and dietary sucrose from skin secretions of alkaloid-sequestering poison frogs

The skins of Madagascar poison frogs (Mantella) and certain Neotropical poison frogs (Epipedobates, Dendrobates) secrete the new bile acid tauromantellic acid (1), which was found in both wild-caught and captive-born frogs. This is the first molecule of endogenous origin detected in skin secretions from these taxa. Sucrose was also detected in secretions from wild-caught Mantella but not in captive-born frogs, suggesting a dietary origin.

Amphibian immune defenses against chytridiomycosis: impacts of changing environments

Eco-immunology is the field of study that attempts to understand the functions of the immune system in the context of the host's environment. Amphibians are currently suffering devastating declines and extinctions in nearly all parts of the world due to the emerging infectious disease chytridiomycosis caused by the chytrid fungus, Batrachochytrium dendrobatidis. Because chytridiomycosis is a skin infection and remains confined to the skin, immune defenses of the skin are critical for survival. Skin defenses include secreted antimicrobial peptides and immunoglobulins as well as antifungal metabolites produced by symbiotic skin bacteria. Low temperatures, toxic chemicals, and stress inhibit the immune system and may impair natural defenses against B. dendrobatidis. Tadpoles' mouth parts can be infected by B. dendrobatidis. Damage to the mouth parts can impair growth, and the affected tadpoles maintain the pathogen in the environment even when adults have dispersed. Newly metamorphosing frogs appear to be especially vulnerable to infection and to the lethal effects of this pathogen because the immune system undergoes a dramatic reorganization at metamorphosis, and postmetamorphic defenses are not yet mature. Here we review our current understanding of amphibian immune defenses against B. dendrobatidis and the ability of the pathogen to resist those defenses. We also briefly review what is known about the impacts of temperature, environmental chemicals, and stress on the host-pathogen interactions and suggest future directions for research.

Interactions between Batrachochytrium dendrobatidis and its amphibian hosts: a review of pathogenesis and immunity

The fungus Batrachochytrium dendrobatidis (Bd) causes a lethal skin disease of amphibians, chytridiomycosis, which has caused catastrophic amphibian die-offs around the world. This review provides a summary of host characteristics, pathogen characteristics and host-pathogen responses to infection that are important for understanding disease development.

The chemical and evolutionary ecology of tetrodotoxin (TTX) toxicity in terrestrial vertebrates

Tetrodotoxin (TTX) is widely distributed in marine taxa, however in terrestrial taxa it is limited to a single class of vertebrates (Amphibia). Tetrodotoxin present in the skin and eggs of TTX-bearing amphibians primarily serves as an antipredator defense and these taxa have provided excellent models for the study of the evolution and chemical ecology of TTX toxicity. The origin of TTX present in terrestrial vertebrates is controversial. In marine organisms the accepted hypothesis is that the TTX present in metazoans results from either dietary uptake of bacterially produced TTX or symbiosis with TTX producing bacteria, but this hypothesis may not be applicable to TTX-bearing amphibians. Here I review the taxonomic distribution and evolutionary ecology of TTX in amphibians with some attention to the origin of TTX present in these taxa.

Chapter 6: cubic membranes the missing dimension of cell membrane organization

Biological membranes are among the most fascinating assemblies of biomolecules: a bilayer less than 10 nm thick, composed of rather small lipid molecules that are held together simply by noncovalent forces, defines the cell and discriminates between “inside” and “outside”, survival, and death. Intracellular compartmentalization—governed by biomembranes as well—is a characteristic feature of eukaryotic cells, which allows them to fulfill multiple and highly specialized anabolic and catabolic functions in strictly controlled environments. Although cellular membranes are generally visualized as flat sheets or closely folded isolated objects, multiple observations also demonstrate that membranes may fold into “unusual”, highly organized structures with 2D or 3D periodicity. The obvious correlation of highly convoluted membrane organizations with pathological cellular states, for example, as a consequence of viral infection, deserves close consideration. However, knowledge about formation and function of these highly organized 3D periodic membrane structures is scarce, primarily due to the lack of appropriate techniques for their analysis in vivo. Currently, the only direct way to characterize cellular membrane architecture is by transmission electron microscopy (TEM). However, deciphering the spatial architecture solely based on two-dimensionally projected TEM images is a challenging task and prone to artifacts. In this review, we will provide an update on the current progress in identifying and analyzing 3D membrane architectures in biological systems, with a special focus on membranes with cubic symmetry, and their potential role in physiological and pathophysiological conditions. Proteomics and lipidomics approaches in defined experimental cell systems may prove instrumental to understand formation and function of 3D membrane morphologies.

Spatial and temporal patterns of alkaloid variation in the poison frog Oophaga pumilio in Costa Rica and Panama over 30 years

A total of 232 alkaloids, representing 21 structural classes were detected in skin extracts from the dendrobatid poison frog Oophaga pumilio, collected from 53 different populations from over 30 years of research. The highly toxic pumiliotoxins and allopumiliotoxins, along with 5,8-disubstitiuted and 5,6,8-trisubstituted indolizidines, all of which are proposed to be of dietary mite origin, were common constituents in most extracts. One decahydroquinoline (DHQ), previously shown be of ant origin, occurred in many extracts often as a major alkaloid, while other DHQs occurred rather infrequently. Histrionicotoxins, thought to be of ant origin, did not appear to possess a specific pattern of occurrence among the populations, but when present, were usually found as major components. Certain 3,5-disubstituted pyrrolizidines and indolizidines, known to be of ant origin, did occur in extracts, but infrequently. Alkaloid composition differed with regard to geographic location of frog populations, and for populations that were sampled two or more times during the 30-year period significant changes in alkaloid profiles sometimes occurred. The results of this study indicate that chemical defense in a dendrobatid poison frog is dependent on geographic location and habitat type, which presumably controls the abundance and nature of alkaloid-containing arthropods.

Lack of bufadienolides in the skin secretion of red bellied toads, Melanophryniscus spp. (Anura, Bufonidae), from Uruguay

The South-American red bellied toads (Melanophryniscus spp.) belonging to the Bufonidae family contain toxic alkaloids in their skin, predominantly of the pumiliotoxin group. Whole animal methanolic extracts of individual specimens of three species (Melanophryniscus atroluteus, M. devincenzii, and M. montevidensis) were analyzed for the presence of toad specific bufadienolides and indolalkylamines (serotonin derivatives) by HPLC-electrospray (ESI)-MS-TOF. No bufadienolides, but few bufotenines, mainly dehydrobufotenine, were detected in the extracts in variable amounts. The concentration of the dehydrobufotenine in the extracts seems to be species specific. Whereas M. atroluteus and M. montevidensis contain very low or trace amounts, M. devincenzii specimens exhibit high concentrations of this indolalkylamine. In comparison, analysis of extracts from Bufo arenarum (Uruguay) and from B. bufo (Germany) confirmed the presence of bufadienolides as well as of bufotenine derivatives. Tadpoles of both species exhibited a different pattern: extracts from B. arenarum tadpoles contained only dehydrobufotenine, but those from B. bufo tadpoles bufotoxin and two alkylamines. Melanophryniscus toads appear not to be able to compensate the high variability of toxic skin alkaloids by producing defensive bufadienolides.

Serous cutaneous glands in the South American horned frogCeratophrys ornata (Leptodactylformes, Chthonobatrachia, Ceratophrydae): Ultrastructural expression of poison biosynthesis and maturation

Serous cutaneous glands are described in newly metamorphosed and juvenile specimens of the horned frog Ceratophrys ornata using light and transmission electron microscopy. We report patterns of biosynthesis and maturation of the specific product of the gland secretory unit. The syncytial, secretory compartment possesses a complex of endoplasmic reticulum (predominantly smooth endoplasmic reticulum after metamorphosis) and Golgi stacks. The serous product is weak in density and is contained in vesicles involved in repeating merging processes. During this maturation activity, secondary lysosomes are observed, which derive from autophagic processes (crinophagy) involving the secretory materials. Ceratophrys ornata, a species representative of the type genus of the family Ceratophrydae, belongs to the heterogeneous group of anurans that, possibly as the result of convergence, all produce cutaneous poisons consisting of vesicles or faint density granules.

Serous cutaneous glands of the Pacific tree-frog Hyla regilla (Anura, Hylidae): Patterns of secretory release induced by nor-epinephrine

The serous (poison) cutaneous glands of the Pacific tree-frog Hyla regilla were induced to release their product by 10(-3)M nor-epinephrine stimulation. After discharge structural and ultrastructural features of the cutaneous glands involved in release were observed. Furthermore, the discharged product, consisting of discrete, secretory granules, was collected and processed for transmission electron microscope analysis. As indicated by patterns found in the myoepithelium encircling the syncytial secretory unit, gland discharge is caused by contraction of the peripheral myocytes. Muscle cell compression dramatically affects the syncytium and results in degenerative changes, including expulsion of the secretory unit nuclei. Therefore, the structural collapse in depleted glands has been ascribed to the mechanical activity performed by the myoepithelium during discharge, rather than cytoplasm involution described in conventional, holocrine glands. TEM investigation revealed that the secretory granules collected after discharge maintain their peculiar traits: they consist of recurrent patterns of thin subunits, acquired during serous maturation and provided with remarkable structural stability.

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.

Studies on the poisonous skin secretion of individual red bellied toads, Melanophryniscus montevidensis (Anura, Bufonidae), from Uruguay

Toads belonging to the genus Melanophryniscus contain toxic alkaloids in their skin. From six locations in south-eastern Uruguay 81 specimens of Melanophryniscusmontevidensis were collected. In whole animal methanolic extracts of individual specimens, alkaloids of the pumiliotoxin (PTX) group and indolizidines were identified by gas chromatography/mass spectrometry; the predominant component PTX 251D was assayed quantitatively. The PTX-content of the various toad populations was found to be highly variable among individual specimens as well as among the populations. Very high levels of PTX 251D were detected in toads of the western part of the collection area, whereas very low levels of this alkaloid were assayed in toads near the Brazilian border. Remarkably high concentrations of the non-alkaloid hydroquinone were found to be present in all toads. The analysis of extracts from 125 arthropod samples (Arachnida and Insecta, including termites, ants and beetles), which may represent a potential food source, revealed no alkaloids of the PTX group.

Convergent evolution of chemical defense in poison frogs and arthropod prey between Madagascar and the Neotropics

With few exceptions, aposematically colored poison frogs sequester defensive alkaloids, unchanged, from dietary arthropods. In the Neotropics, myrmicine and formicine ants and the siphonotid millipede Rhinotus purpureus are dietary sources for alkaloids in dendrobatid poison frogs, yet the arthropod sources for Mantella poison frogs in Madagascar remained unknown. We report GC-MS analyses of extracts of arthropods and microsympatric Malagasy poison frogs (Mantella) collected from Ranomafana, Madagascar. Arthropod sources for 11 "poison frog" alkaloids were discovered, 7 of which were also detected in microsympatric Mantella. These arthropod sources include three endemic Malagasy ants, Tetramorium electrum, Anochetus grandidieri, and Paratrechina amblyops (subfamilies Myrmicinae, Ponerinae, and Formicinae, respectively), and the pantropical tramp millipede R. purpureus. Two of these ant species, A. grandidieri and T. electrum, were also found in Mantella stomachs, and ants represented the dominant prey type (67.3% of 609 identified stomach arthropods). To our knowledge, detection of 5,8-disubstituted (ds) indolizidine iso-217B in T. electrum represents the first izidine having a branch point in its carbon skeleton to be identified from ants, and detection of 3,5-ds pyrrolizidine 251O in A. grandidieri represents the first ponerine ant proposed as a dietary source of poison frog alkaloids. Endemic Malagasy ants with defensive alkaloids (with the exception of Paratrechina) are not closely related to any Neotropical species sharing similar chemical defenses. Our results suggest convergent evolution for the acquisition of defensive alkaloids in these dietary ants, which may have been the critical prerequisite for subsequent convergence in poison frogs between Madagascar and the Neotropics.

Variability of alkaloids in the skin secretion of the European fire salamander (Salamandra salamadra terrestris)

The two major alkaloids, samandarine and samandarone, were identified in the skin secretion of individual specimens from two populations of the European fire salamander (Salamandra salamandra terrestris) by gas chromatography/mass spectrometry. High intraspecific variability in the ratio of both alkaloids was observed, but also in individual specimens over a period of 4 months suggesting separate metabolic pathways of the compounds. Alkaloid synthesis appears to take place also in liver, testes and ovaries, whereas the larvae of the salamanders are entirely free of alkaloids.

Evolution of Dietary Specialization and Chemical Defense in Poison Frogs (Dendrobatidae): A Comparative Analysis

Defensive mechanisms, including noxious or toxic substances, are favored by predation-driven natural selection. The acquisition of noxious/toxic substances can be either endogenous, in which the substances are produced by the organism, or exogenous, in which the substances are produced by another organism and are sequestered. Evidence indicates that the defensive skin alkaloids of Neotropical poison frogs (Dendrobatidae) have an exogenous source: a diet of ants and other small alkaloid-containing arthropods, which we term the diet-toxicity hypothesis. A critical prediction of the diet-toxicity hypothesis is that independent origins of dietary specialization will be found to be correlated with independent origins of skin alkaloids. We tested this prediction in an integrated framework using comparative methods with new and published data on feeding ecology and chemical defense for 15 species of dendrobatids in five genera. We found a significant correlation between alkaloid profiles and degree of dietary specialization. This reveals a recurring association of dietary specialization and alkaloid sequestration in dendrobatids, which suggests parallel evolutionary trends in the origins of defensive mechanisms.

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.