Conserved expression control and shared activity between cognate T-box genes Tbx2 and Tbx3 in connection with Sonic hedgehog signaling during Xenopus eye development

Tbx2 and Tbx3 are considered to be cognate genes within a Tbx2/3/4/5 subfamily of T-box genes and are expressed in closely overlapping areas in a variety of tissues, including the eye. Herein, we show that misexpression of Tbx2 and Tbx3 in Xenopus embryos gave rise to defective eye morphogenesis, which was reminiscent of the defect caused by attenuated Sonic hedgehog (Shh) signaling. Indeed, Tbx2/3 misexpression suppressed Gli1, Gli2, Ptc2 and Pax2, mediators or targets of Hedgehog (Hh) signals. From these data, Tbx2/3 may have a shared function in inhibiting Gli-dependent Shh signaling during eye development. Conversely, the expression of Tbx2/3 was severely affected by both Shh and a putative dominant negative form of Hh, as well as by both transactivator and transrepressor forms of Gli-fusion proteins, suggesting that the expression of Tbx2/3 may be regulated by a Gli-dependent Hh signal transduction pathway. Because the Shh signal has been considered to play crucial roles in the formation of the proximal-distal and dorsal-ventral axes in the eyes, these findings about the mutual regulatory mechanism between Tbx2/3 and Gli-dependent Hh signaling provide valuable insight into the cause of the localized expression of Tbx2/3 and their role during the formation of these axes. In addition, our findings also imply the conserved regulation and shared activity between the cognate genes of Tbx2 and Tbx3.