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We compared the transcriptome in the developing notochord of Xenopus laevis embryos with that of other embryonic regions. A coordinated and intense activation of a large set of secretory pathway genes was observed in the notochord, but not in notochord precursors in the axial mesoderm at early gastrula stage. The genes encoding Xbp1 and Creb3l2 were also activated in the notochord. These two transcription factors are implicated in the activation of secretory pathway genes during the unfolded protein response, where cells react to the stress of a build-up of unfolded proteins in their endoplasmic reticulum. Xbp1 and Creb3l2 are differentially expressed but not differentially activated in the notochord. Reduction of expression of Xbp1 or Creb3l2 by injection of antisense morpholinos led to strong deficits in notochord but not somitic muscle development. In addition, the expression of some, but not all, genes encoding secretory proteins was inhibited by injection of xbp1 morpholinos. Furthermore, expression of activated forms of Xbp1 or Creb3l2 in animal explants could activate a similar subset of secretory pathway genes. We conclude that coordinated activation of a battery of secretory pathway genes mediated by Xbp1 and Creb/ATF factors is a characteristic and necessary feature of notochord formation.
Fig. 2. In situ hybridization of selected secretory pathway genes to Xenopus embryos. (A-K′) Embryos at stage 12.5 (A-K), stages 15-17 (A′-K′) and stages 28-32 (A′-K′). All of these genes show preferential expression in the notochord, and some also show expression in the pronephros (red triangle).
Fig. 3. XPB1 and Creb3l2 are involved in notochord formation. (A,B) In situ hybridization at stage 17 shows the preferential notochord expression of xbp1 (A) and creb3l2 (B). (C-J) Embryos were injected with control MO (C,E,G,I), a mixture of xbp1(n) and xbp1(s) MOs (D,H), or a mixture of creb3l2 A and creb3l2 B MOs (F,J). Embryos were stained with the notochord-specific antibody MZ15 (C-F), or the muscle-specific antibody 12-101 (G-J).
large1 (LARGE xylosyl- and glucuronyltransferase 1) gene expression in Xenopus laevis embryo, assayed via in situ hybridization, NF stage 28, lateral view, anteriorleft, dorsal up.
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