Water relations of tetrapod integument

Comparison of Hydric and Thermal Physiology in an Environmentally Diverse Clade of Caribbean Anoles

As the world becomes warmer and precipitation patterns less predictable, organisms will experience greater heat and water stress. It is crucial to understand the factors that predict variation in thermal and hydric physiology among species. This study focuses on investigating the relationships between thermal and hydric diversity and their environmental predictors in a clade of Hispaniolan anole lizards, which are part of a broader Caribbean adaptive radiation. This clade, the "cybotoid" anoles, occupies a wide range of thermal habitats (from sea level to several kilometers above it) and hydric habitats (such as xeric scrub, broadleaf forest, and pine forest), setting up the possibility for ecophysiological specialization among species. Among the thermal traits, only cold tolerance is correlated with environmental temperature, and none of our climate variables are correlated with hydric physiology. Nevertheless, we found a negative relationship between heat tolerance (critical thermal maximum) and evaporative water loss at higher temperatures, such that more heat-tolerant lizards are also more desiccation-tolerant at higher temperatures. This finding hints at shared thermal and hydric specialization at higher temperatures, underscoring the importance of considering the interactive effects of temperature and water balance in ecophysiological studies. While ecophysiological differentiation is a core feature of the anole adaptive radiation, our results suggest that close relatives in this lineage do not diverge in hydric physiology and only diverge partially in thermal physiology.

Amphibians Exhibit Extremely High Hydric Costs of Respiration

Terrestrial environments pose many challenges to organisms, but perhaps one of the greatest is the need to breathe while maintaining water balance. Breathing air requires thin, moist respiratory surfaces, and thus the conditions necessary for gas exchange are also responsible for high rates of water loss that lead to desiccation. Across the diversity of terrestrial life, water loss acts as a universal cost of gas exchange and thus imposes limits on respiration. Amphibians are known for being vulnerable to rapid desiccation, in part because they rely on thin, permeable skin for cutaneous respiration. Yet, we have a limited understanding of the relationship between water loss and gas exchange within and among amphibian species. In this study, we evaluated the hydric costs of respiration in amphibians using the transpiration ratio, which is defined as the ratio of water loss (mol H2O d-1) to gas uptake (mol O2 d-1). A high ratio suggests greater hydric costs relative to the amount of gas uptake. We compared the transpiration ratio of amphibians with that of other terrestrial organisms to determine whether amphibians had greater hydric costs of gas uptake relative to plants, insects, birds, and mammals. We also evaluated the effects of temperature, humidity, and body mass on the transpiration ratio both within and among amphibian species. We found that hydric costs of respiration in amphibians were two to four orders of magnitude higher than the hydric costs of plants, insects, birds, and mammals. We also discovered that larger amphibians had lower hydric costs than smaller amphibians, at both the species- and individual-level. Amphibians also reduced the hydric costs of respiration at warm temperatures, potentially reflecting adaptive strategies to avoid dehydration while also meeting the demands of higher metabolic rates. Our results suggest that cutaneous respiration is an inefficient mode of respiration that produces the highest hydric costs of respiration yet to be measured in terrestrial plants and animals. Yet, amphibians largely avoid these costs by selecting aquatic or moist environments, which may facilitate more independent evolution of water loss and gas exchange.

A frog in hot water: the effect of temperature elevation on the adrenal stress response of an African amphibian

Amphibians, with their unique physiology and habitat requirements, are especially vulnerable to changes in environmental temperatures. While the activation of the physiological stress response can help to mitigate the impact of such habitat alteration, chronic production of elevated glucocorticoid levels can be deleterious in nature. There is no empirical evidence indicating the physiological response of African amphibians to temperature changes, where individuals are unable to emigrate away from potential stressors. To rectify this, we used the edible bullfrog (Pyxicephalus edulis) as a model species to determine the effect of elevated temperature on the adrenocortical response of the species using a recently established matrix. While a control group was kept at a constant temperature (25 °C) throughout the study period, an experimental group was exposed to control (25 °C) and elevated temperatures (30 °C). Mucous swabs were collected throughout the study period to determine dermal glucocorticoid (dGC) concentrations, as a proxy for physiological stress. In addition to this, individual body mass measurements were collected. The results showed that individuals within the experimental group who experienced increased temperatures had significantly elevated dGC levels compared to the control animals. Furthermore, there was a significant difference in the percentage mass change between experimental and control animals . These findings indicate the physiological sensitivity of the edible bullfrog to a thermal stressor in captivity. While this study shows the importance of proper amphibian management within the captive environment, it also highlights the coming danger of global climate change to this and similar amphibian species.

Complexity in the timing of the first postnatal ecdysis in snakes

Lepidosaurian reptiles, particularly snakes, periodically shed the outer epidermal layers of their skin (ecdysis) to restore or enhance vital functions such as regulating water and gaseous exchange, growth, and protection against insult, infection or physical injury. Although many studies have focused on the nature and mechanisms of skin shedding, little attention has been paid to the timing of the first ecdysis in neonates following birth or hatching. A recent study investigated patterns of the time to first postnatal ecdysis in snakes based on a large dataset taken from the literature. The analysis demonstrated patterns in the time to first postnatal ecdysis related to phylogeny as well as several life history traits. While this assessment provides important advances in our knowledge of this topic, data on known biophysical drivers of ecdysis - temperature and humidity - were largely unavailable and were not evaluated. The first postnatal ecdysis of neonatal snakes can be viewed as an adaptive adjustment to the transition from the aqueous environment of the embryo to the aerial environment of the newborn. Hence, the timing of the first postnatal ecdysis is logically influenced by the aerial environment into which a newborn snake or hatchling finds itself. Therefore, in this Commentary, we first emphasize the putative plasticity of ecdysis with respect to epidermal lipids that structure the water permeability barrier and are established or renewed during ecdysis to reduce transepidermal evaporative water loss. We then discuss the likely importance of biophysical variables as influential covariates that need future investigation as potential co-determinants of the timing of first postnatal ecdysis.

Lipids in the American Alligator stratum corneum provide insights into the evolution of vertebrate skin

In terrestrial vertebrates, the outermost layer of the skin, the stratum corneum (SC), provides a durable and flexible interface with the environment and is comprised of corneocytes embedded in lipids. However, the morphology and lipid composition of the SC varies throughout evolutionary history. Because crocodilians and birds phylogenetically bracket the Archosaurian clade, lipid composition in crocodilian SC may be compared with that of birds and other vertebrates to make inferences about broader phylogenetic patterns within Archosaurs while highlighting adaptations in vertebrate skin. We identified and quantified lipid classes in the SC of the American Alligator (Alligator mississippiensis) from three skin regions varying in mobility. Our results find similarities in lipid composition between alligator and avian SC, including a high percentage of cerebrosides, a polar lipid previously found only in the SC of birds and bats. Furthermore, polar lipids were more abundant in the most mobile region of the SC. Because polar lipids bind with water to increase skin hydration and therefore its pliability under physical stress, we hypothesize that selection for lipids in Archosaurian SC was driven by the unique distribution of proteins in the SC of this clade, and cerebrosides may have served as pre-adaptations for flight.

Tissue characterization using axicon probe-assisted common-path optical coherence tomography

In this work, a common-path optical coherence tomography (OCT) system is demonstrated for characterizing the tissue in terms of some optical properties. A negative axicon structure chemically etched inside the fiber tip is employed as optical probe in the OCT. This probe generates a quality Bessel beam owning a large depth-of-field, ∼700 µm and small central spot size, ∼3 µm. The OCT system is probing the sample without using any microscopic lens. For experimental validation, the OCT imaging of chicken tissue has been obtained along with estimation of its refractive index and optical attenuation coefficient. Afterwards, the cancerous tissue is differentiated from the normal tissue based on the OCT imaging, refractive index, and optical attenuation coefficient. The respective tissue samples are collected from the human liver and pancreas. This probe could be a useful tool for endoscopic or minimal-invasive inspection of malignancy inside the tissue either at early-stage or during surgery.

Limited contribution of photoenzymatic DNA repair in mitigating carry-over effects from larval UVB exposure: Implications for frog recruitment

Solar ultraviolet-B (UVB) radiation has increased due to stratospheric ozone depletion, climate and ecosystem changes and is a driver of amphibian population declines. Photoenzymatic repair (PER) is a critical mechanism for limiting UVB lethality in amphibian larvae. However, the link between PER and the UVB-induced effects remains understudied through long-term investigations in vivo. Here, we assessed how larval PER determines the lethal and sublethal effects induced by environmentally relevant acute UVB exposure until the juvenile phase in the Neotropical frog Odontophrynus americanus. We conducted laboratory-based controlled experiments in which tadpoles were or were not exposed to UVB and subsequently were exposed to light (for PER activation) or dark treatments. Results showed that the rates of mortality and apoptosis observed in post-UVB dark treatment are effectively limited in post-UVB light treatment, indicating PER (and not dark repair, i.e. nucleotide excision repair) is critical to limit the immediate genotoxic impact of UVB-induced pyrimidine dimers. Nonetheless, even tadpoles that survived UVB exposure using PER showed sublethal complications that extended to the juvenile phase. Tadpole responses included alterations in morphology, chromosomal instability, increased skin susceptibility to fungal proliferation, as well as increased generation of reactive oxygen species. The short-term effects were carried over to later stages of life because metamorphosis time increased and juveniles were smaller. No body abnormalities were visualized in tadpoles, metamorphs, and juveniles, suggesting that O. americanus is UVB-resistant concerning these responses. This study reveals that even frog species equipped with an effective PER are not immune to carry-over effects from early UVB exposure, which are of great ecological relevance as late metamorphosis and smaller juveniles may impact individual performance and adult recruitment to breeding. Future ecological risk assessments and conservation and management efforts for amphibian species should exercise caution when linking PER effectiveness to UVB resistance.

Cutaneous Evaporative Water Loss in Lizards Changes Immediately with Temperature

AbstractCutaneous evaporative water loss (CEWL) makes up a significant portion of total evaporative water loss in many terrestrial vertebrates. CEWL changes on evolutionary and acclimatory timescales in response to temperature and humidity. However, the lability of CEWL on acute timescales is unknown. To examine this, we increased or decreased body temperatures of western fence lizards (Sceloporus occidentalis) over a 15-min period while continuously recording CEWL with a handheld evaporimeter. CEWL increased in response to heating and decreased in response to cooling on the order of seconds. However, CEWL was different between heating and cooling groups at a common body temperature. We observed the same positive relationship between CEWL and body temperature, as well as the difference in CEWL between treatments, for deceased lizards that we opportunistically measured. However, deceased lizards had more extreme CEWL values for any given body temperature and treatment. Overall, our results suggest that both structural traits and active physiological processes likely influence the rates and plasticity of CEWL.

Ontogenetic Expression of Aquaporins in the Kidney and Urinary Bladder of the Japanese Tree Frog, Dryophytes japonicus

For adult anuran amphibians, the kidney and urinary bladder play important osmoregulatory roles through water reabsorption. In the present study, we have examined ontogenetic expression of aquaporins, i.e., AQP2, AQPamU (AQP6ub, AQPa2U), and AQP3, in these organs using the Japanese tree frog, Dryophytes japonicus. Immunohistochemistry using the metamorphosing larvae at stages 40-43 localized AQP2 protein to the collecting ducts in the dorsal zone of the mesonephric kidney. At prometamorphic stages 40 and 41, labelling of AQP2 protein was observed in the apical/ subapical regions of the collecting duct cells. At climax stages 42 and 43, labels for AQP2 and AQP3 became observed in the apical/subapical regions and basolateral membrane of the collecting duct cells, respectively, as seen in the adults. As for the urinary bladder, immuno-positive labels for AQPamU were localized to the apical/subapical regions of granular cells in the mucosal epithelium at stages 40-43. On the other hand, AQP3 immunoreactivity was hardly observed in the urinary bladder at stage 40, and weakly appeared in many granular cells at stage 41. Thereafter, labels for AQP3 became evident along the basolateral membrane of granular cells at stages 42 and 43, together with AQPamU in the apical/subapical regions. These results suggest that the kidney and urinary bladder might be capable of water reabsorption, via AQP2, AQPamU, and AQP3, at stage 42, contributing to the acclimation of the tree frogs to terrestrial environments.

Paleozoic cave system preserves oldest-known evidence of amniote skin

The richest and most diverse assemblage of early terrestrial tetrapods is preserved within the infilled cave system of Richards Spur, Oklahoma (289-286 Mya1). Some of the oldest-known terrestrial amniotes2,3 are exquisitely preserved here because of early impregnation and encasement of organic material by oil-seep hydrocarbons within rapidly deposited clay-rich cave sediments under toxic anoxic conditions.4 This phenomenon has also afforded the preservation of exceedingly rare integumentary soft tissues, reported here, providing critical first evidence into the anatomical changes marking the transition from the aquatic and semiaquatic lifestyles of anamniotes to the fully terrestrial lifestyles of early amniotes. This is the first record of a skin-cast fossil (3D carbonization of the skin proper) from the Paleozoic Era and the earliest known occurrence of epidermal integumentary structures. We also report on several compression fossils (carbonized skin impressions), all demonstrating similar external morphologies to extant crocodiles. A variety of previously unknown ossifications, as well as what are likely palpebral ossifications of the deeper dermis layer of the skin, are also documented. These fossils also serve as invaluable references for paleontological reconstructions. Chromatographic analysis of extractable hydrocarbons from bone and cave samples indicates that the source rock is the Devonian age Woodford Shale. Hydrocarbons derived from ancient marine organisms interacting with geologically younger terrestrial vertebrates have therefore resulted in the oldest-known preservation of amniote skin proper.

A review of sebum in mammals in relation to skin diseases, skin function, and the skin microbiome

Diseases vary among and within species but the causes of this variation can be unclear. Immune responses are an important driver of disease variation, but mechanisms on how the body resists pathogen establishment before activation of immune responses are understudied. Skin surfaces of mammals are the first line of defense against abiotic stressors and pathogens, and skin attributes such as pH, microbiomes, and lipids influence disease outcomes. Sebaceous glands produce sebum composed of multiple types of lipids with species-specific compositions. Sebum affects skin barrier function by contributing to minimizing water loss, supporting thermoregulation, protecting against pathogens, and preventing UV-induced damage. Sebum also affects skin microbiome composition both via its antimicrobial properties, and by providing potential nutrient sources. Intra- and interspecific variation in sebum composition influences skin disease outcomes in humans and domestic mammal species but is not well-characterized in wildlife. We synthesized knowledge on sebum function in mammals in relation to skin diseases and the skin microbiome. We found that sebum composition was described for only 29 live, wild mammalian species. Sebum is important in dermatophilosis, various forms of dermatitis, demodicosis, and potentially white-nose syndrome. Sebum composition likely affects disease susceptibility, as lipid components can have antimicrobial functions against specific pathogens. It is unclear why sebum composition is species-specific, but both phylogeny and environmental effects may drive differences. Our review illustrates the role of mammal sebum function and influence on skin microbes in the context of skin diseases, providing a baseline for future studies to elucidate mechanisms of disease resistance beyond immune responses.

Keratinocytes of the Upper Epidermis and Isthmus of Hair Follicles Express Hemoglobin mRNA and Protein

The epidermis, the keratinized stratified squamous epithelium surrounding the body surface, offers a valuable framework to investigate how terrestrial animals overcome environmental stresses. However, the mechanisms underlying epidermal barrier function remain nebulous. In this study, we examined genes highly expressed in the human and mouse upper epidermis, the outer frontier that induces various barrier-related genes. Transcriptome analysis revealed that the messenger RNA level of hemoglobin α (HBA), an oxygen carrier in erythroid cells, was enriched in the upper epidermis compared with that in the whole epidermis. Immunostaining analysis confirmed HBA protein expression in human and mouse keratinocytes (KCs) of the stratum spinosum and stratum granulosum. HBA was also expressed in hair follicle KCs in the isthmus region; its expression levels were more prominent than those in interfollicular KCs. HBA expression was not observed in noncutaneous keratinized stratified squamous epithelia of mice, for example, the vagina, esophagus, and forestomach. HBA expression was upregulated in human epidermal KC cultures after UV irradiation, a major cause of skin-specific oxidative stress. Furthermore, HBA knockdown increased UV-induced production of ROS in primary KCs. Our findings suggest that epidermal HBA expression is induced by oxidative stress and acts as an antioxidant, contributing to skin barrier function.

Host macrocyclic acylcarnitines mediate symbiotic interactions between frogs and their skin microbiome

The host-microbiome associations occurring on the skin of vertebrates significantly influence hosts' health. However, the factors mediating their interactions remain largely unknown. Herein, we used integrated technical and ecological frameworks to investigate the skin metabolites sustaining a beneficial symbiosis between tree frogs and bacteria. We characterize macrocyclic acylcarnitines as the major metabolites secreted by the frogs' skin and trace their origin to an enzymatic unbalance of carnitine palmitoyltransferases. We found that these compounds colocalize with bacteria on the skin surface and are mostly represented by members of the Pseudomonas community. We showed that Pseudomonas sp. MPFS isolated from frogs' skin can exploit acylcarnitines as its sole carbon and nitrogen source, and this metabolic capability is widespread in Pseudomonas. We summarize frogs' multiple mechanisms to filter environmental bacteria and highlight that acylcarnitines likely evolved for another function but were co-opted to provide nutritional benefits to the symbionts.

Label-Free High-Resolution Photothermal Optical Infrared Spectroscopy for Spatiotemporal Chemical Analysis in Fresh, Hydrated Living Tissues and Embryos

Label-free chemical imaging of living and functioning systems is the holy grail of biochemical research. However, existing techniques often require extensive sample preparation to remove interfering molecules such as water, rendering many molecular imaging techniques unsuitable for in situ structural studies. Here, we examined freshly extracted tissue biopsies and living small vertebrates at submicrometer resolution using optical photothermal infrared (O-PTIR) microspectroscopy and demonstrated the following major advances: (1) O-PTIR can be used for submicrometer structural analysis of unprocessed, fully hydrated tissue biopsies extracted from diverse organs, including living brain and lung tissues. (2) O-PTIR imaging can be performed on living organisms, such as salamander embryos, without compromising their further development. (3) Using O-PTIR, we tracked the structural changes of amyloids in functioning brain tissues over time, observing the appearance of newly formed amyloids for the first time. (4) Amyloid structures appeared altered following standard fixation and dehydration procedures. Thus, we demonstrate that O-PTIR enables time-resolved submicrometer in situ investigation of chemical and structural changes in diverse biomolecules in their native conditions, representing a technological breakthrough for in situ molecular imaging of biological samples.

Hydration and evaporative water loss of lizards change in response to temperature and humidity acclimation

Testing acclimation plasticity informs our understanding of organismal physiology and applies to conservation management amidst our rapidly changing climate. Although there is a wealth of research on the plasticity of thermal and hydric physiology in response to temperature acclimation, there is a comparative gap for research on acclimation to different hydric regimes, as well as the interaction between water and temperature. We sought to fill this gap by acclimating western fence lizards (Sceloporus occidentalis) to experimental climate conditions (crossed design of hot or cool, dry or humid) for 8 days, and measuring cutaneous evaporative water loss (CEWL), plasma osmolality, hematocrit and body mass before and after acclimation. CEWL changed plastically in response to the different climates, with lizards acclimated to hot humid conditions experiencing the greatest increase in CEWL. Change in CEWL among individuals was negatively related to treatment vapor pressure deficit and positively related to treatment water vapor pressure. Plasma osmolality, hematocrit and body mass all showed greater changes in response to temperature than to humidity or vapor pressure deficit. CEWL and plasma osmolality were positively related across treatment groups before acclimation and within treatment groups after acclimation, but the two variables showed different responses to acclimation, suggesting that they are interrelated but governed by different mechanisms. This study is among few that assess more than one metric of hydric physiology and that test the interactive effects of temperature and humidity. Such measurements will be essential for predictive models of activity and survival for animals under climate change.

Male spiny frogs enter the underwater battlefield with loose skin exhibiting enhanced penetration of capillaries into the epidermis

The skin has multiple functions, and capillaries can penetrate the epidermis to shorten the diffusion path while allowing maintenance of overall epidermal thickness for nonrespiratory roles. However, a method for quantifying the capillary penetration extent is lacking. Such a method may facilitate making comparisons and detecting associations, potentially making the extent a useful variable in biological studies. We quantified the extent as the ratio of the average minimum thickness of epidermis overlying each capillary to the average epidermal thickness along a skin section and then explored its performance in the Emei mustache toad, Leptobrachium boringii, a species in which breeding males with loose skin call and fight each other with maxillary spines underwater. The ratio showed informative associations with other variables, such as perfused capillary density. It displayed small intragroup variation and could be more sensitive than other variables in revealing structural differences in the skin. The ratio estimates were lowest and were correlated with epidermal and stratum compactum thicknesses in breeding males, i.e., a covariation but not reinforcement against stabbing, constituting early evidence consistent with the increased extensibility of loose skin conferring a defensive advantage during combat in amphibians. In addition, our results lead to the hypothesis that high hemoglobin density along subepidermal capillaries favors the maintenance of low blood partial oxygen pressure and hence increases cutaneous oxygen uptake. We also provide evidence supporting the new idea that the cooccurrence of loose skin and underwater calling found in some frogs can be explained by the latter benefiting from a large functional respiratory surface area. Awareness of the usefulness of the ratio may promote its application and the quantification of the penetration. Regarding exchange surface design, these findings for L. boringii imply a case in which looseness increases surface area as well as prevents damage.

Differential effect of dehydration on the voluntary activity of a diurnal toad

Anuran amphibians' ability to maintain their activity at high temperatures or low humidity depends on their capacity to face dehydration, especially when they display diurnal and terrestrial life habits. Melanophryniscus rubriventris is a diurnal and terrestrial toad from humid Yungas Andean forests that breeds in temporary ponds. It is exposed to the recurrent risk of dehydration because of pond desiccations during the breeding season. Here, we study how M. rubriventris males behaviorally respond to dehydration by measuring their voluntary activity under an ex-situ experiment. Toads with different hydration levels were exposed to a circular track for voluntary activity measurements. Dehydrated males of M. rubriventris toads did not adopt a water-conserving posture staying active during the test and increasing walking under severe dehydration. Certain tolerance to dehydration would allow performing daily activities under challenging diurnal conditions. The increased walking under severe dehydration suggests water or shelter-seeking behavior that would be crucial for diurnal and terrestrial toads to overcome the unpredictable hydric environment during the breeding season.

In Vitro Investigation of the Antibacterial Activity of Salamander Skin Peptides

Given the current and future costs of antibiotic-resistant bacteria to human health and economic productivity, there is an urgent need to develop new antimicrobial compounds. Antimicrobial peptides are a promising alternative to conventional antibiotics and other antimicrobials. Amphibian skin is a rich source of bioactive compounds, but the antibacterial properties of salamander skin peptides have been neglected. Here, we examined the in vitro ability of skin peptides from 9 species of salamander representing 6 salamander families to inhibit the growth of ESKAPE pathogens, which are bacteria that have developed resistance to conventional antibiotics. We also examined whether the skin peptides caused lysis of human red blood cells. Skin peptides from Amphiuma tridactylum had the greatest antimicrobial properties, completely inhibiting the growth of all bacterial strains except for Enterococcus faecium. Likewise, skin peptides from Cryptobranchus alleganiensis completely inhibited the growth of several of the bacterial strains. In contrast, skin peptide mixtures from Ambystoma maculatum, Desmognathus fuscus, Eurycea bislineata, E. longicauda, Necturus beyeri, N. maculosus, and Siren intermedia did not completely inhibit bacterial growth even at the highest concentrations. Finally, none of the skin peptide mixtures caused lysis of human red blood cells. Together, we demonstrate that salamander skin produces peptides with potent antibacterial properties. It remains to elucidate the peptide sequences and their antibacterial mechanisms.

Histological changes of the skin during postembryonic development of the crested newt Triturus ivanbureschi (Urodela, Salamandridae)

BACKGROUND

Amphibian skin has been studied for many decades, especially the metamorphic changes in the skin of frogs. Less attention has been paid to salamander skin. Here, we describe changes in the skin structure during postembryonic development in a salamandrid species, the Balkan crested newt Triturus ivanbureschi.

METHOD

Using traditional histological techniques we examined the skin in the trunk region of three premetamorphic larval stages (hatchling, mid larval and late larval) and two postmetamorphic stages (juvenile, just after metamorphosis, and adult).

RESULTS

In larval stages, skin consists only of the epidermis, which gradually develops from the single epithelial cell layer in hatchlings, to a stratified epidermis with gland nests and characteristic Leydig cells at the late larval stage. During metamorphosis, Leydig cells disappear, and the dermal layer develops. In postmetamorphic stages, skin is differentiated on stratified epidermis and the dermis with well-developed glands. Three types of glands were observed in the skin of the postmetamorphic stages: mucous, granular and mixed. Gland composition appears to be stage- and sex-specific, with juveniles and adult female being more similar to each other. In juveniles and adult female, there are a similar proportion of glands in both dorsal and ventral skin, whereas in adult male granular glands dominated the dorsal skin, while mixed glands dominated the ventral skin.

CONCLUSION

Our results provide a baseline for future comparative research of skin anatomy in salamanders.

Estimating dermal contact soil exposure for amphibians

Chemical exposure estimation through the dermal route is an underemphasized area of ecological risk assessment for terrestrial animals. Currently, there are efforts to create exposure models to estimate doses from this pathway for use in ecological risk assessment. One significant limitation has been insufficient published data to characterize exposure and to support the selection and parameterization of appropriate models, particularly for amphibians in terrestrial habitats. Recent publications measuring pesticide doses to terrestrial-phase amphibians have begun to rectify this situation. We collated and summarized available measurements of terrestrial amphibian dermal exposure to pesticides from 11 studies in which researchers measured tissue concentrations associated with known pesticide experimental application rates. This data set included tissue concentrations in 11 amphibian species and 14 different pesticides. We then compared the results of two screening exposure models that differed based on surface area scaling approaches as a function of body weight (one based on birds as surrogates for amphibians and another amphibian-specific) to the measured tissue residue concentrations. We define a false-negative rate for each screening model as the proportion of amphibians for which the predicted concentration is less than the observed concentration (i.e., underestimate), contrary to the intent of screening models, which are intended to have a bias for higher exposure concentrations. The screening model that uses birds as surrogates did not have any instances where estimated expected avian doses were less than measured amphibian body burdens. When using the amphibian-specific exposure model that corrected for differences between avian and amphibian surface area, measured concentrations were greater than model estimates for 11.3% of the 1158 comparisons. The database of measured pesticide concentrations in terrestrial amphibians is provided for use in calculating bioconcentration factors and for future amphibian dermal exposure model development. Integr Environ Assess Manag 2023;19:9-16. © 2022 SETAC. This article has been contributed to by U.S. Government employees and their work is in the public domain in the USA.

A lipid transfer protein ensures nematode cuticular impermeability

The cuticle of C. elegans is impermeable to chemicals, toxins, and pathogens. However, increased permeability is a desirable phenotype because it facilitates chemical uptake. Surface lipids contribute to the permeability barrier. Here, we identify the lipid transfer protein GMAP-1 as a critical element setting the permeability of the C. elegans cuticle. A gmap-1 deletion mutant increases cuticular permeability to sodium azide, levamisole, Hoechst, and DiI. Expressing GMAP-1 in the hypodermis or transiently in the adults is sufficient to rescue this gmap-1 permeability phenotype. GMAP-1 protein is secreted from the hypodermis to the aqueous fluid filling the space between collagen fibers of the cuticle. In vitro, GMAP-1 protein binds phosphatidylserine and phosphatidylcholine while in vivo, GMAP-1 sets the surface lipid composition and organization. Altogether, our results suggest GMAP-1 secreted by hypodermis shuttles lipids to the surface to form the permeability barrier of C. elegans.

•

GMAP-1 is secreted by the hypodermis toward the cuticle of Caenorhabditis elegans

•

GMAP-1 binds and shuttle phosphoglycerides

•

GMAP-1 sets the lipid composition of the cuticle

•

While healthy, gmap-1 mutant displays high cuticular permeability

Cetacean epidermal specialization: A review

Cetacean skin continues to be the investigative focus of researchers from several different scientific disciplines. Yet, most research on the basic functions of lipo-keratinocytes, which constitute most of the cetacean epidermis, providing the first layer of protection against various environmental aggressors (including an ever-increasing level of pollutants), is restricted to specialized literature on the permeability barrier only. In this review, we have attempted to bring together much of the recent research on the functional biology of cetacean skin, including special adaptations at the cellular, genetic and molecular level. We have correlated these data with the cetacean permeability barrier's unique structural and metabolic adaptations to fully aquatic life, including the development of secondary barriers to ward off challenges such as biofouling as well as exposure to extreme cold for the epidermis, which is outside of the insulation provided by blubber. An apparent contradiction exists between some of the reported gene loss for lipogenic enzymes in cetacean skin and the high degree of cetacean epidermal lipogenesis, as well as loss of desmocollin 1 and desmoplakin genes [while immunolocalization of these proteins is reported (Journal of Anatomy, 234, 201)] warrants a re-evaluation of the gene loss data.

Empirical estimation of skin resistance to water loss in amphibians: agar evaluation as a non-resistance model to evaporation

Total resistance (RT) to evaporative water loss (EWL) in amphibians is given by the sum of the boundary layer (rb) and the skin resistance (rs). Thus, rs can be determined if the rb component is defined (rs=RT - rb). The use of agar models has become the standard technique to estimate rb under the assumption that agar surface imposes no barrier to evaporation (rs=0). We evaluated this assumption by determining EWL rates and rb values from exposed surfaces of free water, a physiological solution mimicking the osmotic properties of a generalized amphibian, and agar gels prepared at various concentrations either using water or physiological solution as diluent. Water evaporation was affected by both, the presence of solutes and agar concentration. Models prepared with agar at 5% concentration in water provided the most practical and appropriate proxy for the estimation of rb.

Anurans against SARS-CoV-2: A review of the potential antiviral action of anurans cutaneous peptides

At the end of 2019, in China, clinical signs and symptoms of unknown etiology have been reported in several patients whose sample sequencing revealed pneumonia caused by the SARS-CoV-2 virus. COVID-19 is a disease triggered by this virus, and in 2020, the World Health Organization declared it a pandemic. Since then, efforts have been made to find effective therapeutic agents against this disease. Identifying novel natural antiviral drugs can be an alternative to treatment. For this reason, antimicrobial peptides secreted by anurans' skin have gained attention for showing a promissory antiviral effect. Hence, this review aimed to elucidate how and which peptides secreted by anurans' skin can be considered therapeutic agents to treat or prevent human viral infectious diseases. Through a literature review, we attempted to identify potential antiviral frogs' peptides to combat COVID-19. As a result, the Magainin-1 and -2 peptides, from the Magainin family, the Dermaseptin-S9, from the Dermaseptin family, and Caerin 1.6 and 1.10, from the Caerin family, are molecules that already showed antiviral effects against SARS-CoV-2 in silico. In addition to these peptides, this review suggests that future studies should use other families that already have antiviral action against other viruses, such as Brevinins, Maculatins, Esculentins, Temporins, and Urumins. To apply these peptides as therapeutic agents, experimental studies with peptides already tested in silico and new studies with other families not tested yet should be considered.

Time changes everything: a multispecies analyses of temporal patterns in evaporative water loss

Increasing trends in environmental temperatures and changes in the hydrological cycle caused by climate change are affecting organisms around the world. Water balance is an important feature of organisms as warmer and drier environments increase water vapour pressure deficit and cutaneous water loss. The aim of our study was to understand the hourly kinetics of losses under water-restricted conditions. In general, these kinetics are thought to be negligible, and values from water loss studies have often been summed or averaged for population and species comparisons. We compiled a cross-species data set of instantaneous water loss (EWL_i) of lizards in which weight was recorded every hour for 12 h (a period considered ecologically relevant for lizards) under standardized conditions with minimal disturbance. We statistically tested whether the duration of the experiment affected the values and found that the duration affected 83% of the data sets. In most cases, water loss was significantly higher during the first hour. This is likely a result of increased activity due to acclimation to the experimental conditions. However, as the experiment progressed, other temporal patterns of EWL_i occurred that were not consistent between species. In addition, the average value and the first and last values of EWL_i changed significantly for most species as the experiment progressed. These results suggest that the hourly kinetics of water loss should not be ignored, while experimental procedures in which EWL is recorded continuously rather than only at the beginning and end of the experiment should be used to find finer temporal variations.

Habitat aridity as a determinant of the trade-off between water conservation and evaporative heat loss in bats

The maintenance of water balance in arid environments might represent a formidable challenge for Chiroptera, since they have high surface-to-volume ratios. In deserts, bats conserve water, for example, using daily torpor, but they also might experience episodic heat bouts, when they may need to increase total evaporative water loss (TEWL) to thermoregulate. We hypothesized that in bats, habitat aridity and its variability determine a trade-off between water conservation and thermoregulation via evaporative means. To test this hypothesis, we collated data from the literature of 22 species of bats on TEWL, body temperature and resting metabolic rate, in torpor and euthermy. We also collected data on ambient temperature (T_a) and precipitation of the locations where bats were captured, calculated an aridity index, and built an index of variability of the environment. After correcting for phylogeny, we found that, as aridity and variability of the environment increased, bats had lower values of TEWL, but the rate at which TEWL increases with T_a was higher, supporting our hypothesis. These results suggest that at high T_a there is a trade-off between water conservation and evaporative heat loss in bats. The evolution of physiological mechanisms that allow water conservation and tolerance to conditions of high T_a without access to free water might thus be crucial to explain the distribution of desert bats.

Response to thermal and hydric regimes point to differential inter- and intraspecific vulnerability of tropical amphibians to climate warming

In Puerto Rico, an island threatened by climate warming, only one of two species of frogs that share part of their distribution has undergone a recent range contraction to higher elevations. We questioned if differences in their physiological response to temperature and dehydration might explain this distributional change. We studied a lowland and a highland population of Eleutherodactylus coqui, a widespread generalist, and E. portoricensis, an endangered species that is currently found only above 600 m. We compared various physiological aspects: operative temperature; temperature selection; critical temperatures; and their response to jumping performance tests at various thermal and hydric regimes. Results revealed that E. portoricensis had the highest CT_min and lowest CT_max and selected a cooler range of temperatures from the experimental gradient. Jumping performance increased with temperature for the three populations until attaining maximum performance. Afterwards, performance dropped drastically until reaching CT_max. Dehydration had a negative effect on performance for both species, particularly on maximum performance. This effect was greatest for E. portoricensis, followed by high-elevation E. coqui. The significantly greater thermo-hydric physiological limitations of E. portoricensis may explain its recent range contraction, potentially, as a response to climate warming. Low-elevation E. coqui had the lowest operative warming tolerance and was the only population to select temperatures like those encountered in their environment, indicating it may be narrowly adapted to local thermal conditions and thus, also vulnerable to climate change. Our results point towards plasticity in the response of E. coqui to varying climatic conditions, and present evidence of different physiological responses between closely related species at the same locality. This work highlights the importance of studying the combined effects of temperature and hydration to understand the response of ectotherms to warming environments and presents further evidence that desiccation may be a limiting factor determining which species may survive.

Novel spikey ionocytes are regulated by cortisol in the skin of an amphibious fish

Cortisol is a major osmoregulatory hormone in fishes. Cortisol acts upon the gills, the primary site of ionoregulation, through modifications to specialized ion-transporting cells called ionocytes. We tested the hypothesis that cortisol also acts as a major regulator of skin ionocyte remodelling in the amphibious mangrove rivulus (Kryptolebias marmoratus) when gill function ceases during the water-to-land transition. When out of water, K. marmoratus demonstrated a robust cortisol response, which was linked with the remodelling of skin ionocytes to increase cell cross-sectional area and Na+-K+-ATPase (NKA) content, but not when cortisol synthesis was chemically inhibited by metyrapone. Additionally, we discovered a novel morphology of skin-specific ionocyte that are spikey with multiple cell processes. Spikey ionocytes increased in density, cell cross-sectional area and NKA content during air exposure, but not in metyrapone-treated fish. Our findings demonstrate that skin ionocyte remodelling during the water-to-land transition in amphibious fish is regulated by cortisol, the same hormone that regulates gill ionocyte remodelling in salinity-challenged teleosts, suggesting conserved hormonal function across diverse environmental disturbances and organs in fishes.

Repeatability of water loss and gain rates of Plethodon cinereus

Water loss and gain rates of amphibians are important to understanding their ecology, especially for plethodontid salamanders. We report the first estimates of repeatability of relative water loss and gain rates of the two major colour morphs of the Eastern Red-backed Salamander, Plethodon cinereus, in the fall and spring seasons. Repeatability of relative water loss in P. cinereus was >0 in the fall but not in the spring. Repeatability of relative water gain was significant for all salamanders pooled in the fall, and was not repeatable in the spring. There were no apparent differences in repeatability of relative water loss or gain between the two colour morphs. Our results suggest that the repeatability of relative water loss and gain rates varies by season, but not by colour morph.

Getting ready for a long bath: skin permeability decreases prior to aquatic breeding in male toads

Vertebrate skin regulates exchanges between the organism and its environment and notably plays a fundamental role in regulating water fluxes. Dynamic changes of skin resistance to water fluxes are expected to occur in species that regularly shift between habitat types especially if these habitats differ in their hydric properties (e.g., terrestrial versus aquatic). We investigated changes of skin permeability using a study system (terrestrial toads) where reproduction induces a transition from terrestrial to freshwater habitats and a prolonged immersion that can last several weeks in males. In this system, the simultaneity between skin shedding and the onset of breeding suggests that the production of new integument layers prior to immersion for reproduction may regulate water influxes. We found that the skin permeability of male toads decreases significantly prior to breeding, suggesting that skin shedding at the onset of breeding regulates water fluxes to alleviate osmotic costs of immersion during reproduction. The continued decrease of skin permeability detected during breeding suggests that additional mechanisms interact with skin structure to further decrease permeability to water during a prolonged immersion. Future studies are required to assess whether changes in skin permeability to water tradeoffs with other skin characteristics (gas exchanges) relevant to aquatic breeding amphibians.

Lipid composition of the stratum corneum in different regions of the body of Kuhl's pipistrelle from the Negev Desert, Israel

The most superficial epidermal layer in endotherms is the stratum corneum (SC), which is composed of dead corneocytes embedded in a lipid matrix with free fatty acids, cholesterol, ceramides, and cerebrosides; the lipid composition of the SC determines its permeability to water vapor. Lipids that are more polar, have longer hydrocarbon chains, and are less bulky are often packed in more ordered phase states to slow cutaneous evaporative water loss (CEWL); these lipids also resist transitions to more disordered phases at high ambient temperatures (T_a). In bats, wing and tail membranes (wing patagia and tail uropatagium, respectively) allow powered flight, but increase surface area, and hence CEWL, with implications for survival in arid environments. We captured Pipistrellus kuhlii from an arid habitat and measured the lipid composition of the SC of the plagiopatagium in the wing, the uropatagium, and the non-membranous region (NMR) of the body using thin layer chromatography and reversed phase high performance liquid chromatography coupled with atmospheric pressure photoionization mass spectrometry. The patagia contained more cholesterol and shorter-chained ceramides, and fewer cerebrosides than the NMR, indicating that the lipid phase transition temperature in the patagia is lower than in the NMR. Thus, at moderate T_a the lipids in the SC in all body regions will remain in an ordered phase state, allowing water conservation; but as T_a increases, the lipids in the SC of the patagia will more easily transition into a disordered phase, resulting in increased CEWL from the patagia facilitating efficient heat dissipation in hot environments.

How to define the experimental conditions of skin permeation assays for drugs presenting biopharmaceutical limitations? The experience with testosterone

Cutaneous permeation assays are crucial to attest the performance or bioequivalence of topical or transdermal products. Although the official guidelines (e.g., FDA/EMA) play a key role in harmonizing the experimental design, alternative methods are often proposed by the scientific community, which makes it difficult to compare results from different studies. In this review, permeation assays with testosterone (TST) were selected to show this high variability in drug transport rate. The main sources of variation discussed were tissue thickness, animal model, donor and receptor fluid constitution, type of solubilizing agent used in aqueous fluids, drug concentration, degree of supersaturation, skin lipid content, number of experimental times and the physical-chemical stability of the molecule in test fluids. This variation becomes even more critical for molecules that present biopharmaceutical limitations such as TST. In addition, the skin presents specific receptors for this hormone due to its physiological action in this region of the body, which makes the evaluation of the TST transport rate in this tissue even more challenging. The impact of each experimental parameter mentioned above on the flux or permeation coefficient of TST is discussed in detail in the review. Assays used to evaluate tissue integrity are also presented.

Ontogeny, phylogeny and mechanisms of adaptive changes in evaporative water loss in geckos

Body size dependence of metabolic rate, body surface and scale morphology complicate disentangling the contribution of these characteristics to adaptive changes in total evaporative water loss (TEWL) of reptiles. To separate adaptive changes from size-related dependence, we compared intra- and interspecific scaling of several candidate traits in eyelid geckos (Eublepharidae), a group exhibiting large variation in body size and TEWL. The intraspecific allometry of TEWL of a eublepharid species fits the geometric surface-mass relationship. However, evolutionary shifts to both higher and lower evaporation were strongly correlated with habitat aridity and cannot be explained by shifts in body size alone. The intraspecific allometry of standard metabolic rate is nearly the same as the interspecific allometry. Unlike for mammals and birds, this pattern rules out respiratory water loss as a driver of the adaptive changes in TEWL among eublepharids. Scale morphology was independent of TEWL variation as well, but the correlation between cutaneous water loss and TEWL suggests a crucial role of skin permeability in adaptation to habitat aridity. Our analyses demonstrate how powerful a comparison between intra- and interspecific allometries can be for detecting body size-dependent mechanisms of adaptive changes in ecophysiological traits correlated with body size.

Bearded dragons (Pogona vitticeps) with reduced scalation lose water faster but do not have substantially different thermal preferences

Whether scales reduce cutaneous evaporative water loss in lepidosaur reptiles (Superorder Lepidosauria) such as lizards and snakes has been a contentious issue for nearly half a century. Furthermore, while many studies have looked at whether dehydration affects thermal preference in lepidosaurs, far fewer have examined whether normally hydrated lepidosaurs can assess their instantaneous rate of evaporative water loss and adjust their thermal preference to compensate in an adaptive manner. We tested both of these hypotheses using three captive-bred phenotypes of bearded dragon (Pogona vitticeps) sourced from the pet trade: 'wild-types' with normal scalation, 'leatherbacks' exhibiting scales of reduced prominence, and scaleless bearded dragons referred to as 'silkbacks'. Silkbacks on average lost water evaporatively at about twice the rate that wild-types did. Leatherbacks on average were closer in their rates of evaporative water loss to silkbacks than they were to wild-types. Additionally, very small (at most ∼1°C) differences in thermal preference existed between the three phenotypes that were not statistically significant. This suggests a lack of plasticity in thermal preference in response to an increase in the rate of evaporative water loss, and may be reflective of a thermal 'strategy' as employed by thermoregulating bearded dragons that prioritises immediate thermal benefits over the threat of future dehydration. The results of this study bolster an often-discounted hypothesis regarding the present adaptive function of scales and have implications for the applied fields of animal welfare and conservation.

Genome size influences adaptive plasticity of water loss, but not metabolic rates, in lungless salamanders

Many expressions of phenotype, such as physiological performance, integrate multiple underlying traits to function. Linking component traits to adaptive physiology thus gives insight into mechanisms of selection acting on performance. Genome size (C-value) is a trait that influences physiology in multiple taxa by exerting a nucleotypic effect, constraining cell size and cellular physiology such that whole-organism mass-specific metabolism is reduced with increasing C-value. We tested for this mechanism of C-value function acting in lungless salamanders, plus an unexplored potential mechanism of C-value effects constraining water transport across the body surface to influence cutaneous water loss rates. We found no evidence for a nucleotypic effect on metabolic rates, but we demonstrate a relationship between C-value and water loss physiology. Under warmer experimental conditions, C-value was inversely correlated with water loss and positively correlated with resistance to water loss, which demonstrated adaptive plasticity at higher temperatures. We hypothesize that this pattern results from differences in cell size constraining diffusion and evaporation of water from the skin under warm conditions when cutaneous perfusion is reduced. Testing this hypothesis may confirm a previously unappreciated adaptive role for C-value variation in this group, and reveals the possibility that genome size influences physiological exchange across transport barriers more broadly.

Molecular mechanisms of local adaptation for salt-tolerance in a treefrog

Salinization is a global phenomenon affecting ecosystems and forcing freshwater organisms to deal with increasing levels of ionic stress. However, our understanding of mechanisms that permit salt tolerance in amphibians is limited. This study investigates mechanisms of salt tolerance in locally adapted, coastal populations of a treefrog, Hyla cinerea. Using a common garden experiment, we (i) determine the extent that environment (i.e., embryonic and larval saltwater exposure) or genotype (i.e., coastal vs. inland) affects developmental benchmarks and transcriptome expression, and (ii) identify genes that may underpin differences in saltwater tolerance. Differences in gene expression, survival, and plasma osmolality were most strongly associated with genotype. Population genetic analyses on expressed genes also delineated coastal and inland groups based on genetic similarity. Coastal populations differentially expressed osmoregulatory genes including ion transporters (atp1b1, atp6V1g2, slc26a), cellular adhesion components (cdh26, cldn1, gjb3, ocln), and cytoskeletal components (odc1-a, tgm3). Several of these genes are the same genes expressed by euryhaline fish after exposure to freshwater, which is a novel finding for North American amphibians and suggests that these genes may be associated with local salinity adaptation. Coastal populations also highly expressed glycerol-3-phosphate dehydrogenase 1 (gpd1), which indicates they use glycerol as a compatible osmolyte to reduce water loss - another mechanism of saltwater tolerance previously unknown in frogs. These data signify that Hyla cinerea inhabiting coastal, brackish wetlands have evolved a salt-tolerant ecotype, and highlights novel candidate pathways that can lead to salt tolerance in freshwater organisms facing habitat salinization.

Hydrothermal physiology and climate vulnerability in amphibians

Concerns over the consequences of global climate change for biodiversity have spurred a renewed interest in organismal thermal physiology. However, temperature is only one of many environmental axes poised to change in the future. In particular, hydrologic regimes are also expected to shift concurrently with temperature in many regions, yet our understanding of how thermal and hydration physiology jointly affect performance and fitness is still limited for most taxonomic groups. Here, we investigated the relationship between functional performance, hydration state and temperature in three ecologically distinct amphibians, and compare how temperature and water loss can concurrently limit activity under current climate conditions. We found that performance was maintained across a broad range of hydration states in all three species, but then declines abruptly after a threshold of 20-30% mass loss. This rapid performance decline was accelerated when individuals were exposed to warmer temperatures. Combining our empirical hydrothermal performance curves with species-specific biophysical models, we estimated that dehydration can increase restrictions on species' activity by up to 60% compared to restriction by temperature alone. These results illustrate the importance of integrating species' hydration physiology into forecasts of climate vulnerability, as omitting this axis may significantly underestimate the effects of future climate change on Earth's biological diversity.

Nature or Nurture: Can Prey-Based Diets Influence Species-Specific Physiological Performance Traits of Epidermal Lipid Content and Cutaneous Water Loss?

Epidermal lipids serve as the primary barrier to cutaneous water loss (CWL) and play a significant role in water conservation and homeostasis. Previous studies have shown the correlation between increased aridity of habitats and the amount of epidermal lipids among species. Generally, increased amounts of epidermal lipids lower skin permeability. Species-specific differences in CWL and prey preferences between two sympatric snake species, the Northern Cottonmouth (Agkistrodon piscivorus) and the Eastern Copperhead (Agkistrodon contortrix), motivated us to question if prey-base can result in these observed species-specific differences in CWL. We experimentally controlled the diets for a captive colony of Northern Cottonmouths (A. piscivorus) by feeding either fish (Notemigonus crysoleucas) or mice (Mus musculus) to investigate if diet can affect the quantity and quality of epidermal lipids and the rates of CWL. Snakes fed mice gained consistently more mass, but diet treatments did not affect growth rate. We found no significant differences in quantitative lipid content or rates of CWL between diet treatments. An analysis for qualitative lipid content using infrared spectrophotometry also showed no diet effect, thus suggesting that lipid content and CWL are strong species-specific physiological performance traits not influenced by recent dietary history. While there is some evidence that epidermal permeability may be variable under certain environmental conditions (e.g., humidity), our findings show that diet has no effect and that a shift in prey preference may not influence or enhance physiological performance for decreasing CWL.

Genomic and physiological mechanisms underlying skin plasticity during water to air transition in an amphibious fish

The terrestrial radiation of vertebrates required changes in skin that resolved the dual demands of maintaining a mechanical and physiological barrier while also facilitating ion and gas transport. Using the amphibious killifish Kryptolebias marmoratus, we found that transcriptional regulation of skin morphogenesis was quickly activated upon air exposure (1 h). Rapid regulation of cell-cell adhesion complexes and pathways that regulate stratum corneum formation was consistent with barrier function and mechanical reinforcement. Unique blood vessel architecture and regulation of angiogenesis likely supported cutaneous respiration. Differences in ionoregulatory transcripts and ionocyte morphology were correlated with differences in salinity acclimation and resilience to air exposure. Evolutionary analyses reinforced the adaptive importance of these mechanisms. We conclude that rapid plasticity of barrier, respiratory and ionoregulatory functions in skin evolved to support the amphibious lifestyle of K. marmoratus; similar processes may have facilitated the terrestrial radiation of other contemporary and ancient fishes.

Xela DS2 and Xela VS2: Two novel skin epithelial-like cell lines from adult African clawed frog (Xenopus laevis) and their response to an extracellular viral dsRNA analogue

The skin epithelial layer acts as an important immunological barrier against pathogens and is capable of recognizing and responding to pathogen-associated molecular patterns (PAMPs) in human and mouse models. Although presumed, it is unknown whether amphibian skin epithelial cells exhibit the ability to respond to PAMPs such as viral double-stranded RNA (dsRNA). To address this, two cell lines from the dorsal skin (Xela DS2) and ventral skin (Xela VS2) of the African clawed frog (Xenopus laevis) were established. Xela DS2 and Xela VS2 cells have an epithelial-like morphology, express genes associated with epithelial cells, and lack senescence-associated beta-galactosidase activity. Cells grow optimally in 70% Leibovitz's L-15 medium supplemented with 15% fetal bovine serum at 26 °C. Upon treatment with poly(I:C), a synthetic analogue of viral dsRNA and known type I interferon inducer, Xela DS2 and Xela VS2 exhibit marked upregulation of key antiviral and pro-inflammatory transcripts suggesting frog epithelial cells participate in the recognition of extracellular viral dsRNA and production of local inflammatory signals; similar to human and mouse models. Currently, these are the only known Xenopus laevis skin epithelial-like cell lines and will be important for future research in amphibian epithelial cell biology, initial host-pathogen interactions, and rapid screening of the effects of environmental stressors, including contaminants, on frog skin epithelial cells.

Heat and water loss versus shelter: a dilemma in thermoregulatory decision making for a retreat-dwelling nocturnal gecko

Understanding the interaction between upper voluntary thermal limit (VTmax) and water loss may aid in predicting responses of ectotherms to increasing temperatures within microhabitats. However, the temperature at which climate heating will force cool-climate nocturnal lizards to abandon daytime retreats remains poorly understood. Here, we developed a new laboratory protocol for determining VTmax in the retreat-dwelling, viviparous Woodworthia 'Otago/Southland' gecko, based on escape behaviour (abandonment of heated retreat). We compared the body temperature (Tb) at VTmax, and duration of heating, between two source groups with different thermal histories, and among three reproductive groups. We also examined continuous changes in Tb (via an attached biologger) and total evaporative water loss (EWL) during heating. In the field, we measured Tb and microhabitat thermal profiles to establish whether geckos reach VTmax in nature. We found that VTmax and duration of heating varied between source groups (and thus potentially with prior thermal experience), but not among reproductive groups. Moreover, geckos reached a peak temperature slightly higher than VTmax before abandoning the retreat. Total EWL increased with increasing VTmax and with the duration of heating. In the field, pregnant geckos with attached biologgers reached VTmax temperature, and temperatures of some separately monitored microhabitats exceeded VTmax in hot weather implying that some retreats must be abandoned to avoid overheating. Our results suggest that cool-climate nocturnal lizards that inhabit daytime retreats may abandon retreats more frequently if climate warming persists, implying a trade-off between retention of originally occupied shelter and ongoing water loss due to overheating.

Cholesterol derivatives make large part of the lipids from epidermal molts of the desert-adapted Gila monster lizard (Heloderma suspectum)

In order to understand the cutaneous water loss in the desert-adapted and venomous lizard Heloderma suspectum, the microscopic structure and lipid composition of epidermal molts have been examined using microscopic, spectroscopic and chemical analysis techniques. The molt is formed by a variably thick, superficial beta-layer, an extensive mesos-region and few alpha-cells in its lowermost layers. The beta-layer contains most corneous beta proteins while the mesos-region is much richer in lipids. The proteins in the mesos-region are more unstructured than those located in the beta-layer. Most interestingly, among other lipids, high contents of cholesteryl-β-glucoside and cholesteryl sulfate were detected, molecules absent or present in traces in other species of squamates. These cholesterol derivatives may be involved in the stabilization and compaction of the mesos-region, but present a limited permeability to water movements. The modest resistance to cutaneous water-loss of this species is compensated by adopting other physiological strategies to limit thermal damage and water transpiration as previous eco-physiological studies have indicated. The increase of steroid derivatives may also be implicated in the heat shock response, influencing the relative behavior in this desert-adapted lizard.

Insights into the skin of caecilian amphibians from gene expression profiles

Background

Gene expression profiles can provide insights into the molecular machinery behind tissue functions and, in turn, can further our understanding of environmental responses, and developmental and evolutionary processes. During vertebrate evolution, the skin has played a crucial role, displaying a wide diversity of essential functions. To unravel the molecular basis of skin specialisations and adaptations, we compared gene expression in the skin with eight other tissues in a phylogenetically and ecologically diverse species sample of one of the most neglected vertebrate groups, the caecilian amphibians (order Gymnophiona).

Results

The skin of the five studied caecilian species showed a distinct gene expression profile reflecting its developmental origin and showing similarities to other epithelial tissues. We identified 59 sequences with conserved enhanced expression in the skin that might be associated with caecilian dermal specialisations. Some of the up-regulated genes shared expression patterns with human skin and potentially are involved in skin functions across vertebrates. Variation trends in gene expression were detected between mid and posterior body skin suggesting different functions between body regions. Several candidate biologically active peptides were also annotated.

Conclusions

Our study provides the first atlas of differentially expressed sequences in caecilian tissues and a baseline to explore the molecular basis of the skin functions in caecilian amphibians, and more broadly in vertebrates.