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Presence of calcium binding proteins in the echinoderm nervous system and their preferential distribution to the connective tissue plexi

Echinoderms have been considered an enigmatic group of deuterostome animals. One of their most interesting characteristics, referred to as mutable connective tissue, is the ability of the non‐cellular connective tissue (CT) to modulate its stiffness. These changes in the CT are known to be under nervous system control. Here we investigate the association of nerve cell and fiber populations with the CT components of the sea cucumber Holothuria glaberrima. Antibodies made against intracellular calcium‐binding proteins with EF‐hand motifs, namely, rat Calbindin‐D28k (CB) and Parvalbumin (PV) and human Calretinin (CR), labeled cells and fibers within the echinoderm nervous system. The neural specificity was verified by co‐labeling with the neural‐specific marker RN1. Immunoblotting with anti‐CB and anti‐PV showed that these markers recognized an antigen of 32 kDa in homogenates of the radial nerve cord. Bioinformatic analyses of the possible antigens being recognized by these markers showed that a CR homolog, similar to the CB‐32 previously identified in other invertebrates, is present in the echinoderms. Our findings suggest that the CB‐32 present in invertebrates is the homolog of CR, while CB‐D28k is a paralog. The immunoreactivity of these markers in the CT component provides the first evidence for a candidate protein that could be playing a direct role in the control of mutability of the CT in echinoderms.