Invertebrate data predict an early emergence of vertebrate fibrillar collagen clades and an anti-incest model.

Fibrillar collagens are involved in the formation of striated fibrils and are present from the first multicellular animals, sponges, to humans. Recently, a new evolutionary model for fibrillar collagens has been suggested (Boot-Handford, R. P., Tuckwell, D. S., Plumb, D. A., Farrington Rock, C., and Poulsom, R. (2003) J. Biol. Chem. 278, 31067-31077). In this model, a rare genomic event leads to the formation of the founder vertebrate fibrillar collagen gene prior to the early vertebrate genome duplications and the radiation of the vertebrate fibrillar collagen clades (A, B, and C). Here, we present the modular structure of the fibrillar collagen chains present in different invertebrates from the protostome Anopheles gambiae to the chordate Ciona intestinalis. From their modular structure and the use of a triple helix instead of C-propeptide sequences in phylogenetic analyses, we were able to show that the divergence of A and B clades arose early during evolution because alpha chains related to these clades are present in protostomes. Moreover, the event leading to the divergence of B and C clades from a founder gene arose before the appearance of vertebrates; altogether these data contradict the Boot-Handford model. Moreover, they indicate that all the key steps required for the formation of fibrils of variable structure and functionality arose step by step during invertebrate evolution.

Distinct maturations of N-propeptide domains in fibrillar procollagen molecules involved in the formation of heterotypic fibrils in adult sea urchin collagenous tissues.

We have characterized the primary structure of a new sea urchin fibrillar collagen, the 5alpha chain, including nine repeats of the sea urchin fibrillar module in its N-propeptide. By Western blot and immunofluorescence analyses, we have shown that 5alpha is co-localized in adult collagenous ligaments with the 2alpha fibrillar collagen chain and fibrosurfin, two other extracellular matrix proteins possessing sea urchin fibrillar modules. At the ultrastructural level, the 5alpha N-propeptide is detected at the surface of fibrils, suggesting the retention of this domain in mature collagen molecules. Biochemical characterization of pepsinized collagen molecules extracted from the test tissue (the endoskeleton) together with a matrix-assisted laser desorption ionization time-of-flight analysis allowed us to determine that 5alpha is a quantitatively minor fibrillar collagen chain in comparison with the 1alpha and 2alpha chains. Moreover, 5alpha forms heterotrimeric molecules with two 1alpha chains. Hence, as in vertebrates, sea urchin collagen fibrils are made up of quantitatively major and minor fibrillar molecules undergoing distinct maturation of their N-propeptide regions and participating in the formation of heterotypic fibrils.

Sequence and embryonic expression of collagen XVIII NC1 domain (endostatin) in the zebrafish.

Endostatin, located in the NC1 domain of the collagen XVIII, is believed to inhibit the migration and proliferation of endothelial cells (Fed. Am. Soc. Exp. Biol. J. 15 (2001) 1044) and to play a role in axon guidance in Caenorhabditis elegans (J. Cell Biol. 152 (2001) 1219). Zebrafish is an attractive vertebrate model to determine the role of endostatin and the entire molecule of collagen XVIII during vertebrate development. Therefore, we have investigated the expression pattern of COL18A1 in zebrafish embryos from the segmentation to the hatching period stages.

Phylogenetic analysis of vertebrate fibrillar collagen locates the position of zebrafish alpha3(I) and suggests an evolutionary link between collagen alpha chains and hox clusters.

Type I collagen in tetrapods is usually a heterotrimeric molecule composed of two alpha1 and one alpha2 chains. In some teleosts, a third alpha chain has been identified by chromatography, suggesting that type I collagen should also exist as an alpha1(I)alpha2(I)alpha3(I) heterotrimer. We prepared, from zebrafish, three distinct cDNAs identified to be those of the collagen alpha1(I), alpha2(I), and alpha3(I) chains. In this study on the evolution of fibrillar collagen alpha chains and their relationships, an exhaustive phylogenetic analysis, using vertebrate fibrillar collagen sequences, showed that each alpha chain constitutes a monophyletic cluster. Results obtained with the newly isolated sequences of the zebrafish showed that the alpha3(I) chain is phylogenetically close to the alpha1(I) chain and support the hypothesis that the alpha3(I) chain arose from a duplication of the alpha1(I) gene. The duplication might occur during the duplication of the actinopterygian genome, soon after the divergence of actinopterygians and sarcopterygians, a hypothesis supported by the demonstration of a syntenic evolution between a set of fibrillar collagen genes and Hox clusters in mammals. An evolutionary scenario is proposed in which phylogenetic relationships of the alpha chains of fibrillar collagens of vertebrates could be related to Hox cluster history.

Evolution of collagens.

The extracellular matrix is often defined as the substance that gives multicellular organisms (from plants to vertebrates) their structural integrity, and is intimately involved in their development. Although the general functions of extracellular matrices are comparable, their compositions are quite distinct. One of the specific components of metazoan extracellular matrices is collagen, which is present in organisms ranging from sponges to humans. By comparing data obtained in diploblastic, protostomic, and deuterostomic animals, we have attempted to trace the evolution of collagens and collagen-like proteins. Moreover, the collagen story is closely involved with the emergence and evolution of metazoa. The collagen triple helix is one of numerous modules that arose during the metazoan radiation which permit the formation of large multimodular proteins. One of the advantages of this module is its involvement in oligomerization, in which it acts as a structural organizer that is not only relatively resistant to proteases but also permits the creation of multivalent supramolecular networks.

NG2 proteoglycan mediates beta1 integrin-independent cell adhesion and spreading on collagen VI.

Collagens V and VI have been previously identified as specific extracellular matrix (ECM) ligands for the NG2 proteoglycan. In order to study the functional consequences of NG2/collagen interactions, we have utilized the GD25 cell line, which does not express the major collagen-binding beta(1) integrin heterodimers. Use of these cells has allowed us to study beta(1) integrin-independent phenomena that are mediated by binding of NG2 to collagens V and VI. Heterologous expression of NG2 in the GD25 line endows these cells with the capability of attaching to surfaces coated with collagens V and VI. The specificity of this effect is emphasized by the failure of NG2-positive GD25 cells to attach to other collagens or to laminin-1. More importantly, NG2-positive GD25 cells spread extensively on collagen VI. beta(1) integrin-independent extension of ruffling lamellipodia demonstrates that engagement of NG2 by the collagen VI substratum triggers signaling events that lead to rearrangement of the actin cytoskeleton. In contrast, even though collagens V and VI each bind to the central segment of the NG2 ectodomain, collagen V engagement of NG2 does not trigger cell spreading. The distinct morphological consequences of NG2/collagen VI and NG2/collagen V interaction indicate that closely-related ECM ligands for NG2 differ in their ability to initiate transmembrane signaling via engagement of the proteoglycan.

Structure and spatio temporal expression of the full length DNA complementary to RNA coding for alpha2 type I collagen of zebrafish.

Twenty distinct genetic types of collagen have been identified up to now. Their structure and function are not completely elucidated. We have chosen zebrafish as a model to bring information about the role of collagen during embryogenesis. In the present study, we isolated four overlapping DNA complementary to RNA clones covering the 4879 nucleotides of a zebrafish messenger RNA (mRNA) encoding a fibrillar procollagen chain. The comparison of its primary structure with known other vertebrate collagens allowed to conclude that it encodes collagen pro-alpha2(I) chain. The 5' untranslated region showed a typical stem-loop structure with three ATG codons which is found in mammals types I and III collagen chains (but not in type II), which are expressed in the same tissues. This suggests that the supposed regulatory role of the stem loop structure could be tissue specific. The comparison of the Gly-Gly doublets found along the helical domain of several species allowed to speculate that the Gly-Gly repeats could be a poikilotherm feature. Expression of pro-alpha2(I) was examined during zebrafish development by reverse transcriptase-polymerase chain reaction and in situ hybridization on whole embryo and tissue section. Col1a2 was expressed as early as stage10 h post fertilization (hpf) and two peaks of expression were observed at 20 and 48 hpf. alpha2 mRNAs, whose presence suggests a collagen synthesis, were detected principally in the superficial cell layers surrounding 20-72 hpf embryos which are characterized by an acellular collagen stratum. At 26-30 days, fibroblasts invade the dermis and take over from the epithelial cells to synthesize collagen. This suggests a fine regulation of collagen synthesis in these cells that remains to be elucidated. alpha2 mRNA were also detected in other tissues such as the tail fin primordium and the notochord primordium suggesting a participation of type I collagen in a pathway for notochord and tail formation.

Ultrastructure of siliceous spicules and microsclerocytes in the marine sponge Neofibularia irata N. SP.

The marine sponge Neofibularia irata contains four different categories of siliceous spicules. These spicules are evident in the tissues as distinct bundles that act to increase the structural rigidity of the sponge. All spicules have a normal structural morphology with silica deposition around a hexagonal axial canal containing a crystalline axial filament. The megasclere strongyles are secreted in typical megasclerocytes. The sigma and raphid microscleres are secreted in individual microsclerocytes that are grouped together in parallel to form loose bundles. However, the microxea microscleres are apparently secreted in distinct tight bundles (trichodragmas) within a single cell. These cells, containing between 13 and 39 spicules, are grouped to form large packets of bundles of spicules.