HcN13, a Fibroblast Growth Factor Binding Protein, Is Associated with Shell Formation in Hyriopsis cumingii

Fibroblast growth factor binding proteins (FGF-BPs) are involved in bone formation by binding to FGFs and modulating FGF signaling in vertebrates. Herein, a novel shell matrix protein gene, HcN13, was identified from the mussels Hyriopsis cumingii. Sequence analysis indicated that HcN13 belongs to the FGF-BP1 family. Quantitative real-time PCR and in situ hybridization analysis showed that HcN13 is expressed in the dorsal epithelial cells of the mantle center, indicating that HcN13 is a shell nacreous layer matrix protein. The expression of HcN13 in the mantle significantly increased during the regeneration of the prismatic and nacreous layers. Furthermore, the suppression of HcN13 at both the transcriptional and protein levels resulted in the complete destruction of the prisms and nacre tablets in vivo. However, the addition of SUMO-HcN13 did not affect the polymorphism and morphology of the crystals in vitro. These results indicated that HcN13 may serve as a modulator that influences downstream signaling pathways to further regulate shell formation. Additionally, HcN13 was highly expressed in hemocytes during pearl nacre disorder deposition and was also highly expressed in the mantle during pearl nacre order deposition, indicating that HcN13 is essential for pearl biomineralization. This study demonstrates, for the first time, the presence of FGF-BPs in the mollusk shell, highlighting their essential role in biomineralization in invertebrates.