BiP, GRP94, calreticulin and calnexin contribute to development of the notochord in medaka fish

The accumulation of unfolded or misfolded proteins in the endoplasmic reticulum (ER) activates the unfolded protein response (UPR) to maintain the homeostasis of the ER. The UPR consists of the IRE1, PERK and ATF6 pathways in vertebrates. Knockout of the IRE1 and PERK pathways causes defects in liver and pancreatic β cells, respectively, in mice, whereas knockout of the ATF6 pathway causes very early embryonic lethality in mice and medaka fish, a vertebrate model organism. We previously showed that ATF6 knockout in medaka causes a defect in the development of the notochord-the notochord becomes shorter-but that transient overexpression of the ER chaperone BiP via microinjection of BiP mRNA into one-cell stage embryos of these ATF6 knockout rescues this defect. Here, we microinjected mRNA encoding various ER chaperones and found that GRP94, calreticulin and calnexin also partially rescued this defect. Thus, BiP/GRP94 and calreticulin/calnexin greatly contribute to the development of the notochord by controlling the quality of collagens and N-glycosylated proteins (such as laminin and fibrillin), respectively, which have been confirmed necessary for the formation of the notochord in zebrafish.Key words: endoplasmic reticulum, protein folding, molecular chaperone, collagen, glycoprotein.