Cocoon and epidermis of Australian Cyclorana frogs differ in composition of lipid classes that affect water loss

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.

Animal secretory endolysosome channel discovery

Secretory pore-forming proteins (PFPs) have been identified in organisms from all kingdoms of life. Our studies with the toad species Bombina maxima found an interaction network among aerolysin family PFPs (af-PFPs) and trefoil factors (TFFs). As a toad af-PFP, BmALP1 can be reversibly regulated between active and inactive forms, with its paralog BmALP3 acting as a negative regulator. BmALP1 interacts with BmTFF3 to form a cellular active complex called βγ-CAT. This PFP complex is characterized by acting on endocytic pathways and forming pores on endolysosomes, including stimulating cell macropinocytosis. In addition, cell exocytosis can be induced and/or modulated in the presence of βγ-CAT. Depending on cell contexts and surroundings, these effects can facilitate the toad in material uptake and vesicular transport, while maintaining mucosal barrier function as well as immune defense. Based on experimental evidence, we hereby propose a secretory endolysosome channel (SELC) pathway conducted by a secreted PFP in cell endocytic and exocytic systems, with βγ-CAT being the first example of a SELC protein. With essential roles in cell interactions and environmental adaptations, the proposed SELC protein pathway should be conserved in other living organisms.

Permeability of frog skin to chemicals: effect of penetration enhancers

Rarely do commercial chemical products contain solely the active chemical/ingredient. It is therefore important to consider whether ingredients other than the active may: 1) alter absorption of the active chemical, or 2) be absorbed themselves, resulting in systemic effects. Frogs have highly permeable skin and are routinely exposed to commercial chemical products in the environment or therapeutically. Ethanol and propylene glycol (PG), which have known penetration-enhancing effects, are commonly included in such products. The current study has therefore investigated the in vitro absorption kinetics through Rh. marina skin of three model chemicals – caffeine, benzoic acid, and ibuprofen – formulated individually as solutions containing: 1%, 10% or 30% v/v ethanol, or 20% v/v PG. Differential scanning calorimetry and histology were used to characterise fresh frog skin, investigate the mechanism of these enhancers in frog skin, and to determine whether these enhancers significantly affected skin structure. Results showed that the extent of absorption enhancement was influenced by chemical, enhancer and skin region, and that enhancement was generally not consistent for individual enhancers or skin regions. The exception was 1% v/v ethanol, which did not significantly alter flux across the skin for any of the chemicals evaluated. Caffeine absorption was not enhanced by any of the investigated penetration enhancers, and was in fact significantly reduced by 30% v/v ethanol and PG. Ethanol caused concentration-dependant changes in skin morphology and should be avoided in concentrations ≥10% v/v. PG, however, caused minimal changes to the skin and consistently improved absorption of benzoic acid and ibuprofen through all skin regions. Owing to the significant changes in skin structure following ≥10% v/v ethanol exposure, it is recommended to avoid its use in frogs. For enhancement of penetration of moderately-to-highly lipophilic chemicals, this study has identified 20% v/v PG should to be the enhancer of choice.

Percutaneous absorption of chemicals: developing an understanding for the treatment of disease in frogs

The permeable nature of frog skin presents an alternative route for the delivery of therapeutic chemicals to treat disease in frogs. However, although therapeutic chemicals are often topically applied to the skin of frogs, their pharmacokinetics have rarely been reported. To provide evidence to guide both candidate drug and formulation selection, we highlight factors expected to influence percutaneous absorption through frog skin, including the anatomy and physiology of the skin and the physicochemical properties of applied therapeutic chemicals. Importantly, we also highlight the effects of the formulation on percutaneous absorption, especially the inclusion of potential penetration enhancers as excipients. Finally, we collate empirical data on the topical application of various therapeutic chemicals in postmetamorphic frogs and show that, in contrast to mammalian species, even large chemicals (i.e. >500 Da) and those with a wide range of log P values (-4 through +6) are likely to be absorbed percutaneously. Topical application in frogs thus promises a convenient and effective method for delivering systemic treatments of a diverse range of chemicals; however, further experimental quantification is required to ensure optimal outcomes.

Field hydration state varies among tropical frog species with different habitat use

We have previously shown that ecological habit (e.g., arboreal, terrestrial, amphibious) correlates with thermoregulatory behaviors and water balance physiology among species of hylid frogs in northern Australia. We hypothesized that these frogs would be different with respect to their field hydration states because of the challenges associated with the different ecological habits. There are very few data on the hydration levels that frogs maintain in the field, and the existing data are from disparate species and locations and do not relate hydration state to habit or changes in seasonal water availability. We measured the hydration state of 15 species of frogs from tropical northern Australia to determine the influences of ecological habit and season on the hydration state that these frogs maintain. As predicted, frogs were significantly less hydrated in the dry season than they were in the wet season and showed significantly higher variation among individuals, suggesting that maintaining hydration is more challenging in the dry season. In the wet season, terrestrial species were significantly less hydrated than arboreal or amphibious species. During the dry season, amphibious species that sought refuge in cracking mud after the pond dried were significantly less hydrated than terrestrial or arboreal species. These data suggest that hydration behaviors and voluntary tolerance of dehydration vary with habitat use, even within closely related species in the same family or genus. Terrestrial and arboreal species might be expected to be the most vulnerable to changes in water availability, because they are somewhat removed from water sources, but the physiological characteristics of arboreal frogs that result in significant cutaneous resistance to water loss allow them to reduce the effects of their dehydrating microenvironment.