Histochemical detection of free thiols in glandular cells and tissues of different marine Polychaeta
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ORIGINAL PAPER
Histochemical detection of free thiols in glandular cells and tissues of different marine Polychaeta Cátia Gonçalves1 · Pedro M. Costa1 Accepted: 28 May 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract Either through differentiated glands or specialised individual cells, the coating epithelia of soft-bodied marine invertebrates are responsible for the secretion of a broad span of peptidic substances, from protective mucins to biocides. These secretions are characterised by the presence of cysteine-rich proteins and peptides, rendering a distinct histochemical signature of secretory epithelia. Through a histochemical procedure for fluorescence microscopy in paraffin sections, we performed a comparative assessment of the distribution of thiol-rich compounds in multiple epithelia of different species of intertidal Polychaeta, which revealed distinctive patterns of distribution that closely relate to ecology, morphoanatomy and physiology. The presence of free thiols was notorious in mucocytes and enzyme-plus toxin-secreting cells. Consequently, strong signals were recorded in the mucocytes of the parapodia of Nereis splendida, the epidermis and pharynx epithelium of Mysta picta and the venom glands of Glycera alba. The findings show an investment in mucus secretion in foragers such as Nereis and Mysta, especially the latter, which is not a native burrower, as a protective response and as lubricant for locomotion. Additionally, nereidids are believed to secret integumentary toxins for defence. On the other hand, Glycera is an ambush predatorial burrower whose behaviour entirely revolves around the delivery of venom making use of its four jaws. The results showed that the detection of thiol-rich compounds in histological sections can be a tool to identify potential toxin secretion and delivery structures, with important consequences for the bioprospecting of novel bioreactives from marine invertebrates for the purpose of drug discovery. Keywords Sulfhydryls · Histochemistry · Fluorescence microscopy · Annelida · Secretion · Paraffin sections
Introduction Thiols (also termed sulfhydryls or mercaptans) are potent nucleophiles, easily oxidised to disulphides and highly reactive. Protein thiols are present in virtually all organisms and have key roles as antioxidant and chelating agents, polymerforming structural proteins due to disulphide bonds, among others. Due to their free thiol group, cysteine residues in proteins, particularly those of heavy molecular weight like mucins, are in fact a major factor providing stability and * Cátia Gonçalves [email protected] * Pedro M. Costa [email protected] 1
UCIBIO‑Applied Molecular Biosciences Unit, Departamento de Ciências da Vida, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829‑516 Caparica, Portugal
proper folding (Trivedi et al. 2009). Metallothioneins, for instance, are small molecular mass proteins that owe their ability to chelate metal cations to the presence of about 20 cysteine residue
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