Extracellular Matrix of Echinoderms.

This review considers available data on the composition of the extracellular matrix (ECM) in echinoderms. The connective tissue in these animals has a rather complex organization. It includes a wide range of structural ECM proteins, as well as various proteases and their inhibitors. Members of almost all major groups of collagens, various glycoproteins, and proteoglycans have been found in echinoderms. There are enzymes for the synthesis of structural proteins and their modification by polysaccharides. However, the ECM of echinoderms substantially differs from that of vertebrates by the lack of elastin, fibronectins, tenascins, and some other glycoproteins and proteoglycans. Echinoderms have a wide variety of proteinases, with serine, cysteine, aspartic, and metal peptidases identified among them. Their active centers have a typical structure and can break down various ECM molecules. Echinoderms are also distinguished by a wide range of proteinase inhibitors. The complex ECM structure and the variety of intermolecular interactions evidently explain the complexity of the mechanisms responsible for variations in the mechanical properties of connective tissue in echinoderms. These mechanisms probably depend not only on the number of cross-links between the molecules, but also on the composition of ECM and the properties of its proteins.

Muscle Regeneration in Holothurians without the Upregulation of Muscle Genes.

The holothurian Eupentacta fraudatrix is capable of fully restoring its muscles after transverse dissection. Although the regeneration of these structures is well studied at the cellular level, the molecular basis of the process remains poorly understood. To identify genes that may be involved in the regulation of muscle regeneration, the transcriptome of the longitudinal muscle band of E. fraudatrix has been sequenced at different time periods post-injury. An analysis of the map of biological processes and pathways has shown that most genes associated with myogenesis decrease their expression during the regeneration. The only exception is the genes united by the GO term "heart valve development". This may indicate the antiquity of mechanisms of mesodermal structure transformation, which was co-opted into various morphogeneses in deuterostomes. Two groups of genes that play a key role in the regeneration have been analyzed: transcription factors and matrix metalloproteinases. A total of six transcription factor genes (Ef-HOX5, Ef-ZEB2, Ef-RARB, Ef-RUNX1, Ef-SOX17, and Ef-ZNF318) and seven matrix metalloproteinase genes (Ef-MMP11, Ef-MMP13, Ef-MMP13-1, Ef-MMP16-2, Ef-MMP16-3, Ef-MMP24, and Ef-MMP24-1) showing differential expression during myogenesis have been revealed. The identified genes are assumed to be involved in the muscle regeneration in holothurians.

Matrix Metalloproteinases and Tissue Inhibitors of Metalloproteinases in Echinoderms: Structure and Possible Functions.

Echinoderms are one of the most ancient groups of invertebrates. The study of their genomes has made it possible to conclude that these animals have a wide variety of matrix metalloproteinases (MMPs) and tissue inhibitors of metalloproteinases (TIMPs). The phylogenetic analysis shows that the MMPs and TIMPs underwent repeated duplication and active divergence after the separation of Ambulacraria (Echinodermata+Hemichordata) from the Chordata. In this regard the homology of the proteinases and their inhibitors between these groups of animals cannot be established. However, the MMPs of echinoderms and vertebrates have a similar domain structure. Echinoderm proteinases can be structurally divided into three groups-archetypal MMPs, matrilysins, and furin-activatable MMPs. Gelatinases homologous to those of vertebrates were not found in genomes of studied species and are probably absent in echinoderms. The MMPs of echinoderms possess lytic activity toward collagen type I and gelatin and play an important role in the mechanisms of development, asexual reproduction and regeneration. Echinoderms have a large number of genes encoding TIMPs and TIMP-like proteins. TIMPs of these animals, with a few exceptions, have a structure typical for this class of proteins. They contain an NTR domain and 10-12 conservatively located cysteine residues. Repeated duplication and divergence of TIMP genes of echinoderms was probably associated with an increase in the functional importance of the proteins encoded by them in the physiology of the animals.