Proteomics of Galápagos Marine Iguanas Links Function of Femoral Gland Proteins to the Immune System

Comparative proteomic analysis of tail regeneration in the green anole lizard, Anolis carolinensis

As amniote vertebrates, lizards are the most closely related organisms to humans capable of appendage regeneration. Lizards can autotomize, or release their tails as a means of predator evasion, and subsequently regenerate a functional replacement. Green anoles (Anolis carolinensis) can regenerate their tails through a process that involves differential expression of hundreds of genes, which has previously been analyzed by transcriptomic and microRNA analysis. To investigate protein expression in regenerating tissue, we performed whole proteomic analysis of regenerating tail tip and base. This is the first proteomic data set available for any anole lizard. We identified a total of 2,646 proteins - 976 proteins only in the regenerating tail base, 796 only in the tail tip, and 874 in both tip and base. For over 90% of these proteins in these tissues, we were able to assign a clear orthology to gene models in either the Ensembl or NCBI databases. For 13 proteins in the tail base, 9 proteins in the tail tip, and 10 proteins in both regions, the gene model in Ensembl and NCBI matched an uncharacterized protein, confirming that these predictions are present in the proteome. Ontology and pathways analysis of proteins expressed in the regenerating tail base identified categories including actin filament-based process, ncRNA metabolism, regulation of phosphatase activity, small GTPase mediated signal transduction, and cellular component organization or biogenesis. Analysis of proteins expressed in the tail tip identified categories including regulation of organelle organization, regulation of protein localization, ubiquitin-dependent protein catabolism, small GTPase mediated signal transduction, morphogenesis of epithelium, and regulation of biological quality. These proteomic findings confirm pathways and gene families activated in tail regeneration in the green anole as well as identify uncharacterized proteins whose role in regrowth remains to be revealed. This study demonstrates the insights that are possible from the integration of proteomic and transcriptomic data in tail regrowth in the green anole, with potentially broader application to studies in other regenerative models.

Carbonic anhydrase IV in lizard chemical signals

The evolution of chemical signals is subject to environmental constraints. A multicomponent signal may combine semiochemical molecules with supporting compounds able to enhance communication efficacy. Carbonic anhydrases (CAs) are ubiquitous enzymes catalysing the reversible hydration of carbon dioxide, a reaction involved in a variety of physiological processes as it controls the chemical environment of the different tissues or cellular compartments, thus contributing to the overall system homeostasis. CA-IV isoform has been recently identified by mass spectrometry in the femoral gland secretions (FG) of the marine iguana, where it has been hypothesized to contribute to the chemical stability of the signal, by regulating blend pH. Lizards, indeed, use FG to communicate by delivering the waxy secretion on bare substrate, where it is exposed to environmental stressors. Therefore, we expect that some molecules in the mixture may play supporting functions, enhancing the stability of the chemical environment, or even conferring homeostatic properties to the blend. CA-IV may well represent an important candidate to this hypothesized supporting/homeostatic function, and, therefore, we can expect it to be common in FG secretions of other lizard species. To evaluate this prediction and definitely validate CA identity, we analysed FG secretions of eight species of wall lizards (genus Podarcis), combining mass spectrometry, immunoblotting, immunocytochemistry, and transmission electron microscopy. We demonstrate CA-IV to actually occur in the FG of seven out of the eight considered species, providing an immunochemistry validation of mass-spectrometry identifications, and localizing the enzyme within the secretion mass. The predicted structure of the identified CA is compatible with the known enzymatic activity of CA-IV, supporting the hypothesis that CA play a signal homeostasis function and opening to new perspective about the role of proteins in vertebrate chemical communication.

Protein-lipid Association in Lizard Chemical Signals

Chemical communication in terrestrial vertebrates is often built on complex blends, where semiochemical and structural compounds may form an integrated functional unit. In lizards, many species have specialized epidermal glands whose secretions are waxy, homogeneous blends of lipids and proteins, both active in communication. The intimate co-occurrence of such compounds allows us to hypothesize that they should undergo a certain degree of covariation, considering both their semiochemical role and the support-to-lipid function hypothesized for the protein fraction. In order to assess the occurrence and level of protein-lipid covariation, we compared the composition and complexity of the two fractions in the femoral gland secretions of 36 lizard species, combining phylogenetically-informed analysis with tandem mass spectrometry. We found the composition and complexity of the two fractions to be strongly correlated. The composition of the protein fraction was mostly influenced by the relative proportion of cholestanol, provitamin D₃, stigmasterol, and tocopherol, while the complexity of the protein pattern increased with that of lipids. Additionally, two identified proteins (carbonic anhydrase and protein disulfide isomerase) increased their concentration as provitamin D₃ became more abundant. Although our approach does not allow us to decrypt the functional relations between the proteinaceous and lipid components, nor under the semiochemical or structural hypothesis, the finding that the proteins involved in this association were enzymes opens up to new perspectives about protein role: They may confer dynamic properties to the blend, making it able to compensate predictable variation of the environmental conditions. This may expand the view about proteins in the support-to-lipid hypothesis, from being a passive and inert component of the secretions to become an active and dynamic one, thus providing cues for future research.

Functional Protein Composition in Femoral Glands of Sand Lizards (Lacerta agilis)

Proteins are ubiquitous macromolecules that display a vast repertoire of chemical and enzymatic functions, making them suitable candidates for chemosignals, used in intraspecific communication. Proteins are present in the skin gland secretions of vertebrates but their identity, and especially, their functions, remain largely unknown. Many lizard species possess femoral glands, i.e., epidermal organs primarily involved in the production and secretion of chemosignals, playing a pivotal role in mate choice and intrasexual communication. The lipophilic fraction of femoral glands has been well studied in lizards. In contrast, proteins have been the focus of only a handful of investigations. Here, we identify and describe inter-individual expression patterns and the functionality of proteins present in femoral glands of male sand lizards (Lacerta agilis) by applying mass spectrometry-based proteomics. Our results show that the total number of proteins varied substantially among individuals. None of the identified femoral gland proteins could be directly linked to chemical communication in lizards, although this result hinges on protein annotation in databases in which squamate semiochemicals are poorly represented. In contrast to our expectations, the proteins consistently expressed across individuals were related to the immune system, antioxidant activity and lipid metabolism as their main functions, showing that proteins in reptilian epidermal glands may have other functions besides chemical communication. Interestingly, we found expression of the Major Histocompatibility Complex (MHC) among the multiple and diverse biological processes enriched in FGs, tentatively supporting a previous hypothesis that MHC was coopted for semiochemical function in sand lizards, specifically in mate recognition. Our study shows that mass spectrometry-based proteomics are a powerful tool for characterizing and deciphering the role of proteins secreted by skin glands in non-model vertebrates.