Connective Tissues in Echinoderm Animals that can Reversibly Change their Stiffness and their Stiffening Protein Factors

Some connective tissues shown in echinoderm animals such as sea urchins, sea stars, and sea cucumbers can extensively and reversibly change their stiffness in a few minutes under the regulation of the nervous system. They are called 'catch connective tissues' or 'mutable collagenous tissues'. Neurotransmitters such as acetylcholine, adrenaline, and some oligopeptides cause changes in the stiffness of the living pieces dissected from these tissues. They are no more effective, however, after the treatment with detergent solubilizing cell membranes. There are some granule-containing cells in the tissues, which are assumed to secret some factors under the regulation of nerves causing changes in the tissue stiffness. The tissues contain a large amount of the extracellular matrix mainly consisting of collagen fibrils, proteoglycans and microfibrils. The unique properties of these collagenous tissues might be due to lack of permanent associations between the collagen fibrils and the surrounding extracellular matrix. It seems that cross-linking between the fibrils are formed or broken during the change of the stiffness of the tissues. Its molecular mechanisms are, however, not yet fully understood. We isolated a protein factor called ‘tensilin’ from an extract of sea cucumber body wall dermis, one of the known catch connective tissues. It stiffens the detergent-treated dermal pieces and induces aggregation of collagen fibrils isolated from the tissue. We also isolated another protein factor which stiffens the dermal pieces. It is possible that there are other factors affecting on interactions among dermal fibrils and the stiffness of the tissues. Molecular mechanisms of the stiffness changes of the catch connective tissues should be clarified by purifying and characterizing these factors.