Identification And Characterization Of A Second Fibronectin Gene In Zebrafish

Fibronectin (FN) is a high molecular weight glycoprotein . Being a major component of ECM, fibronectin interacts with a variety of other molecules and is involved in many cellular processes, such as migration, adhesion, growth, apoptosis and differentiation, therefore it plays important roles in many developmental and physiological processes. Molecular and biochemical characterizations of FNs have been well studied in mammals, birds and amphibians.In zebrafish, it was reported that it possess a single FN gene (FN1a). Here we report the presence of a second zebrafish FN gene (FN1b) that encodes a protein with a predicted primary structure that is similar to FNs identified in other vertebrates possessing 12 type Ⅰ, 2 type Ⅱ and 17 type Ⅲ repeats including two alternative splice sites (EⅢA and EⅢB) and a variable region (Ⅴ). Zebrafish FNlb exhibits 62.0% amino acid identity with zebrafish FN1a, 54% with human and 55% with Xenopus laevis FNs respectively. Employing RT-PCR analysis, we demonstrate that EⅢB— and V~- isoforms are produced by alternative splicing of a single FN1b transcript. The FNlb EⅢA~- isoform was not detected in zebrafish embryos or adult tissues nor were EIIIA, EIIIB or V region splice variants of FN1a found. FN1b mRNA was detected by RT-PCR in embryos at the gastrula-stage (8hpf, 8hour post fertilization) through 72hpf and in various adult tissues. EIIIB~- and V~+ are the predominant forms of FN1b present in the zebrafish embryo. Unlike FN1a, which is present at a relatively high amount at the embryonic stages before gastrulation, the FN1b isoforms are present at very low amounts at the early cleavage stage. The presence of multiple isoforms of FN1b along with a different pattern of expression compared to FN1a indicates that the two FN genes have separate roles in zebrafish development.