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  • ESI-09 Notochord is a transient structure differentiating at

    2021-06-10

    Notochord is a transient structure differentiating at early stage during embryogenesis that is at the origin of vertebral bodies in all vertebrates. In zebrafish, it is composed of vacuolated ESI-09 surrounded by a unique ECM structure improperly called notochordal BM. As in chicken and amphibians, this ECM sheath consists of 3 layers: a “true” BM inner layer, a dense fibrous medial layer and a fibrous outer layer [11]. It is not thus surprising that BM notochordal cells express both typical BM zone components, such as laminins [35], col18a1 [36] and col15a1a [37], and fibrillar collagen genes, such as col2a1a [38] and col11a1 [39]. Expression of the collagen genes in the notochord is transient, decreasing by 24 hpf and ceasing after 48 hpf. We refer the interested readers to an existing excellent review on the composition, structure and function of the notochordal BM [40].
    The zebrafish collagen super family: common features and specificities Collagens are encoded by 44 genes in humans and by 58 genes in zebrafish [3]. Very recently, a fourth collagen XXVIII gene, col28a2b, was identified in the GRCz11 version of the zebrafish genome (https://zfin.org/) bringing the total number of zebrafish collagen genes to 59 (Fig. 2A). The elevated number of collagen genes contained in the zebrafish genome results from a further round of whole-genome duplication (WGD) referred to as teleost-specific-WGD (TS-WGD) that took place 320–350 millions years ago and resulted in a massive increase of protein coding genes. Duplicated collagen genes can have different fates including non-functionalization of one of the duplicated gene as a consequence of lack of selective constraint or subfunctionalization, neofunctionalization or dosage selection, that make zebrafish fascinating from a phylogenetic perspective [41]. These mechanisms act through evolution for the loss, maintenance or functionalization of duplicated collagen genes resulting in specific features of the zebrafish collagen gene repertoire [3]. All in all, 16 collagen genes are duplicated in zebrafish compared to mammals and the function of only some of them have been interrogated so far. Specifically, the zebrafish genome contains 4 collagen XXVIII genes, col28a1a, col28a1b, col28a2a and col28a2b, consequential to the duplication of the α1 and α2 branch although the α2 branch was then lost in mammals [42]. As many other teleost fish, zebrafish does not have any collagen III gene that was probably lost during early diversification of this infraclass [43]. Finally, some of the collagen paralogs have diverged during evolution and displayed distinct expression patterns and functions as we reported for collagen XV paralogs, col15a1a and col15a1b [31,37,44]. While collagen XV is expressed widely in the BM zones of various organs in mice and humans, col15a1b is specifically expressed in slow muscle precursors and col15a1a is mainly expressed in notochord [37,44]. Exploiting the existence of collagen paralogs for discovering new functions will undoubtedly bring new insights in the multiple roles of collagen genes in human development and disease. Collagen genes encode long polypeptidic chains called α-chains that contain unique structural signature, the presence in their primary structure of the [Gly-X-Y] amino-acid repeats, where X and Y are often a proline and a hydroxyproline respectively. This results in the formation of a triple helix, hallmark of the collagen superfamily (Fig. 2B). Collagens are secreted proteins and their biosynthesis ends in the extracellular space where they can form different supramolecular aggregates (banded fibrils, beaded filaments, hexagonal networks, anchoring fibrils) [4]. This main extracellular step in collagen biosynthesis is mediated by fine-tuned combination of self-assembly and cell-driven processes that are still far from being completely understood. Overall, zebrafish collagen fibrils are structurally indistinguishable from the ones of other vertebrates with TEM (Fig. 1) and they can be revealed by the classical histology and imaging technics such as SHG microscopy [[45], [46], [47], [48]]. Although collagen chain assembly is rarely studied in zebrafish, the presence of duplicated genes can lead to differences compared to mammals. Collagen I mostly occurs as a heterotrimer with two α1(I) chains and one α2(I) chain, but zebrafish contain an additional α-chain, originally called α3(I) chain (encoded by the col1a1b gene) that forms with the two other chains the heterotrimeric molecule α1(I)α2(I)α3(I). The collagen I chains are expressed in connectives tissues (skin, bones, tendons…) as in other vertebrates. In addition, the actinotrichia contains another molecular species, the homotrimeric form [α1(I)]3 [49]. Interestingly, high amounts of collagen I homotrimer was also reported in fetal calf bones [50].