Transcription Factor Spr2 Regulates Mesodermal Formation And Neuroectodermal Posteriorization

Many signaling pathways play important roles in the formation of mesoderm and patterning of the neuroectoderm during early development of vertebrate embryos. This study has identified a zebrafish transcription factor Spr2, a member of Sp1-like family. Chromosomal mapping and phylogenetic analysis reveal that zebrafish spr2 and bts1 are duplicated genes and the human SPR2 was lost while human SP5/Bts1 retains during evolution. The expression of spr2, detected by whole-mount in situ hybridization, initiates on the future dorsal side of the late blastulas, and occurs during early gastrulation in both epiblast and hypoblast layers of the blastoderm margin, including cells committed to mesodermal and neuroectoderm fates. During segmentation, spr2 transcripts are detected in the trunk neural crest precursors, the developing somites, the tail bud and lateral mesoderm tissues. Injection of spr2 mRNA enhances expression of ntl, a mesodermal marker, and alleviates inhibitory effect on ntl expression of XFD that inhibits FGF signaling. In contrast, injection with antisense morpholino that targets spr2 mRNA inhibits ntl expression and lessens induction of ntl by FGF. It is also demonstrated that FGF signaling relays mesoderm induction activity of Nodal signaling and Spr2 is involved in this signal relay process. The correct spatial expression of spr2 requires FGF, Wnt and Nodal signals. These data suggest that expression of spr2 is an immediate-early response to mesoderm induction by FGF and in turn regulate expression of effector genes involved in development of mesodermal tissues. Patterning of the neuroectoderm in vertebrate embryos is regulated by FGF, Wnt and Nodal signals that generally posteriorize the neuroectoderm. Injection of spr2 mRNA can enhance the expression of markers for posterior brain and the spinal cord, including hoxb1b, and alleviate inhibitory effect on these marker genes of XFD, Axin and Lefty1. In contrast, overexpression of spr2 can inhibit expression of otx1and other anterior brain markers and counteract the inductive effect on these markers of XFD, Axin and Lefty1. On the other hand, morpholino-mediated knockdown of Spr2 leads to decrease of hoxb1b expression and antagonizes inductive activity of ectopic FGF, Wnt and Nodal signals on hoxb1b. However, knockdown of Spr2 increases otx1 expression and counteracts inhibitory effect of ectopic FGF, Wnt and Nodal signals on otx1. It is concluded that Spr2 mediates the roles of FGF, Wnt and Nodal signals in posteriorizing the neuroectoderm. Moreover, it is demonstrated that FGF and Wnt signals are required for function of Nodal signal in posteriorization of the neuroectoderm.