Short amino acid chains form effective skin protection in Apalone spinifera turtles

An Italian research team found that peptides As-BD-1 to 4, BD standing for beta-defensin, which are present in the epidermis, may be the cause of the resistance to bacterial penetration in Apalone spinifera turtles.


Apalone spinifera (or Trionyx spiniferus) is the spiny soft-shell freshwater turtle, named for its short spiny projections on the anterior carapace edge. From a general point of view, turtles are characterized by their shell, which is corneous and supported in the dermis by dermal bones, protecting them as a shield. The upper (or dorsal) part is called the carapace, the lower (ventral) part the plastron - this is the part that encloses the belly. As to classification, turtles belong to the phylum Chordata, subphylum Vertebrata (= Chordata with backbones, with currently over 62,000 described species ).

In many vertebrates, particular peptides of ten to eighty amino acids inhibit or kill pathogens, mostly by disrupting their cell membrane. The beta-defensins are a large group of antimicrobial peptides that are localized in granulocyte white blood cells and in oral and epidermal keratinocytes. The peptides contain six Cys amino acids that form disulfide groups at specific locations, which form the characteristic motif for the beta-defensins.

Epithelial tissues function as the very first line of defense against bacteria and other microorganisms. Freshwater turtles have their skin directly exposed to the dirt environment; their carapace and plastron often become the grow sites for algae and bacteria, possibly depleting the protective corneous epidermis layer.
Turtles' shells are made of an internal bony dermal-endoskeleton layer and an external corneous epidermis (stratum corneum; horned layer), comprising numerous corneocytes. Corneocytes can be referred to as terminally modified keratinocytes that are regularly replaced, by skin peeling and renewal from inner layers. They are an essential element in skin barrier protection.

If bacteria are present among the corneocytes - extracellular as well as intracellular - then how can turtles be resistant to infections? Exactly that question led four Italian researchers from both the Universities of Padova and Bologna to the present study .

Extracting RNA from carapace tissue led to the preparation of a cDNA library, which was sequenced by 454 Sequencing Technology . "At the end we had a database containing the sequences of the transcripts [RNAs, red.] present in the carapace tissue", Dr. Dalla Valle says. "This database then was analyzed using bioinformatic programs like the BLAST tools on the website of the National Center for Biotechnology Information (NCBI) ". The BLAST algorithms consider similarities between the sequence of interest and (specified) query sequences already known. In this study the queries were beta-defensin sequences from other vertebrate species (birds, reptiles, and mammals).

The BLAST analysis resulted in the characterization of four full-length transcripts (RNA sequences) for beta-defensin-like peptides, leading to the conclusion that these molecules are most likely to be expressed in this carapace tissue. This counts for the neck, digits and tail, as well.
At least four genes coding for beta-defensin-like peptides are present in the Apalone spinifera turtle. The corresponding transcripts are expressed in the skin but not in liver or intestine, the other two tissues analysed. Finally, turtle beta-defensin-like peptides present the characteristic Cys motif identified before in lizard, mammal, and avian beta-defensins.

Another research used one beta-defensin (As-BD-1) to figure out the phylogenetic relationships of turtles. It appears that turtles may be closer related to birds than to any other lizard or snake. (Some lizard beta-defensin-like peptides are essentially related to venomous peptides of snakes and lizards, suggesting the both have a common ancestor).

Reference:
Benato F, Dalla Valle L, Skobo T & L Alibardi L 2013. Biomolecular identification of beta-defensin-like peptides from the skin of the soft-shelled turtle Apalone spinifera. Journal of Experimental Zoology (Molecular and Developmental Evolution) 9999B: 1-8.