Hspa5 Functions In Early Development Of The Zebrafish Embryo

Hspa5 belongs to a member of the Hsp70 family. It plays important roles in facilitating fold and assembly of newly synthesized proteins and their translocation across the endoplasmic reticulum membrane as a molecular chaperone, helping refold stress-denatured soluble proteins and regulating signal transduction pathways by modulating the stability and activity of protein kinases and transcriptional factors. During early mouse embryogenesis, Hspa5 is able to protect the inner cell mass from apoptosis. This project aims to investigate expression and regulation of Hspa5 during zebrafish embryogenesis and its function in embryonic development. Transcripts of zebrafish Hspa5 are present in fertilized eggs, suggesting their maternal origin. Zygotic expression of Hspa5 starts during midgastrulation and occurs in the hypoblast cells. It is specifically expressed in the hatching gland and the notochord during segmentation. The transcription of Hspa5 in zebrafish embryos is enhanced when translation of endogenous Hspa5 mRNA is blocked, suggesting a feed-back regulatory mechanism. Upon heat shock, embryos show higher levels and expansion of Hspa5 expression, indicating that Hspa5 expression is inducible by heat shock. Although overexpression of Hspa5 does not cause detectable defects, its knockdown by injecting Hspa5 morpholino antisense oligonucleotides results in slower epiboly during gastrulation and dorsalized phenotypes at 24 hpf, including smaller and curled tail and absence of caudal ventral fin. When Hspa5 expresison in embryos is knockdown, the expression of the dorsal markers gsc and ntl is enhanced while the expression of ventral markers is reduced. These results suggest that Hspa5 is required for epibolic movement and is involved in controlling of dorsal development in zebrafish embryos. Furthermore, knockdown of Hspa5 rescues ventralized phenotypes induced by overexpression of Bmp2b; on the other hand, dorsalized phenotypes caused by Hspa5 knockdown disappear when Bmp2b mRNA is coinjected. It is likely that involvement of Hspa5 in dorsoventral patterning is associated with Bmp signals. However, its underlying mechanisms remain unclear.