Transduction of graded Hedgehog signaling by a combination of Gli2 and Gli3 activator functions in the developing spinal cord

The three vertebrate Gli proteins play a central role in mediating Hedgehog (Hh)-dependent cell fate specification in the developing spinal cord; however, their individual contributions to this process have not been fully characterized. In this paper, we have addressed this issue by examining patterning in the spinal cord of Gli2;Gli3 double mutant embryos, and in chick embryos transfected with dominant activator forms of Gli2 and Gli3. In double homozygotes, Gli1 is also not expressed; thus, all Gli protein activities are absent in these mice. We show that Gli3 contributes activator functions to ventral neuronal patterning, and plays a redundant role with Gli2 in the generation of V3 interneurons. We also show that motoneurons and three classes of ventral neurons are generated in the ventral spinal cord in double mutants, but develop as intermingled rather than discrete populations. Finally, we provide evidence that Gli2 and Gli3 activators control ventral neuronal patterning by regulating progenitor segregation. Thus, multiple ventral neuronal types can develop in the absence of Gli function, but require balanced Gli protein activities for their correct patterning and differentiation.     

Gli2 and Gli3 play distinct roles in the dorsoventral patterning of the mouse hindbrain

Sonic Hedgehog (Shh) signaling plays a critical role during dorsoventral (DV) patterning of the developing neural tube by modulating the expression of neural patterning genes. Overlapping activator functions of Gli2 and Gli3 have been shown to be required for motoneuron development and correct neural patterning in the ventral spinal cord. However, the role of Gli2 and Gli3 in ventral hindbrain development is unclear. In this paper, we have examined DV patterning of the hindbrain of Shh(-/-), Gli2(-/-) and Gli3(-/-) embryos, and found that the respective role of Gli2 and Gli3 is not only different between the hindbrain and spinal cord, but also at distinct rostrocaudal levels of the hindbrain. Remarkably, the anterior hindbrain of Gli2(-/-) embryos displays ventral patterning defects as severe as those observed in Shh(-/-) embryos suggesting that, unlike in the spinal cord and posterior hindbrain, Gli3 cannot compensate for the loss of Gli2 activator function in Shh-dependent ventral patterning of the anterior hindbrain. Loss of Gli3 also results in a distinct patterning defect in the anterior hindbrain, including dorsal expansion of Nkx6.1 expression. Furthermore, we demonstrate that ventral patterning of rhombomere 4 is less affected by loss of Gli2 function revealing a different requirement for Gli proteins in this rhombomere. Taken together, these observations indicate that Gli2 and Gli3 perform rhombomere-specific function during DV patterning of the hindbrain.