Sequence of zebrafish fibulin-1 and its expression in developing heart and other embryonic organs

Using a homology-based cDNA cloning strategy we have identified a member of the fibulin family in the zebrafish Danio rerio. The deduced sequence of this protein is highly homologous to mammalian fibulins. Two variants of this protein have been identified and they were found to correspond to variants C and D of mammalian fibulin-1. RT-PCR showed that variant C is expressed as early as the blastula period, whereas variant D was first detected during gastrulation. By in situ hybridization with a probe detecting both variants, fibulin-1 could first be detected during the late gastrula period. During the segmentation period, fibulin-1 transcripts are restricted to the most posterior presomitic mesoderm and to cells lying at the lateral boundary of the embryo proper and the yolk. Subsequently, whilst the tail is budding and elongating, fibulin-1 transcripts are localized in the most posterior part of the extending tail tip and in the region of somite formation. At the early hatching period, highly restricted expression of fibulin-1 mRNA is observed in the heart at the region where the valves are forming. Fibulin-1 mRNA is also present in the mesenchymal cells underlying the apical ectodermal ridge in the pectoral fins and in the inner three quarters of the tail fin. In addition, high levels of fibulin-1 expression are seen in the developing swim bladder, which by rapid contraction and expansion regulates swimming altitude. It is thus notable that fibulin-1 is prominently expressed in regions where tissue compartments are continuously moving in relation to each other. These and previous observations of expression of fibulins in heart valves in mouse and chicken suggest that fibulins evolved as specialized extracellular matrix components for tissues involved in continuous stretching both during embryogenesis and at later stages.