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.     

Gli proteins encode context-dependent positive and negative functions: implications for development and disease.

Several lines of evidence implicate zinc finger proteins of the Gli family in the final steps of Hedgehog signaling in normal development and disease. C-terminally truncated mutant GLI3 proteins are also associated with human syndromes, but it is not clear whether these C-terminally truncated Gli proteins fulfil the same function as full-length ones. Here, structure-function analyses of Gli proteins have been performed using floor plate and neuronal induction assays in frog embryos, as well as induction of alkaline phosphatase (AP) in SHH-responsive mouse C3H10T1/2 (10T1/2) cells. These assays show that C-terminal sequences are required for positive inducing activity and cytoplasmic localization, whereas N-terminal sequences determine dominant negative function and nuclear localization. Analyses of nuclear targeted Gli1 and Gli2 proteins suggest that both activator and dominant negative proteins are modified forms. In embryos and COS cells, tagged Gli cDNAs yield C-terminally deleted forms similar to that of Ci. These results thus provide a molecular basis for the human Polydactyly type A and Pallister-Hall Syndrome phenotypes, derived from the deregulated production of C-terminally truncated GLI3 proteins. Analyses of full-length Gli function in 10T1/2 cells suggest that nuclear localization of activating forms is a regulated event and show that only Gli1 mimics SHH in inducing AP activity. Moreover, full-length Gli3 and all C-terminally truncated forms act antagonistically whereas Gli2 is inactive in this assay. In 10T1/2 cells, protein kinase A (PKA), a known inhibitor of Hh signaling, promotes Gli3 repressor formation and inhibits Gli1 function. Together, these findings suggest a context-dependent functional divergence of Gli protein function, in which a cell represses Gli3 and activates Gli1/2 prevents the formation of repressor Gli forms to respond to Shh. Interpretation of Hh signals by Gli proteins therefore appears to involve a fine balance of divergent functions within each and among different Gli proteins, the misregulation of which has profound biological consequences.     

Gli1 can rescue the in vivo function of Gli2.

In mice, three Gli genes are thought to mediate sonic hedgehog (Shh) signaling collectively. Mis-expression studies and analysis of null mutants for each gene have indicated that the Gli proteins have different functions. In particular, Gli1 appears to be a constitutive activator, and Gli2 and Gli3 have repressor functions. To determine the precise functional differences between Gli1 and Gli2, we have expressed Gli1 in place of Gli2 from the endogenous Gli2 locus in mice. Strikingly, a low level of Gli1 can rescue all the Shh signaling defects in Gli2 mutants; however, only in the presence of a wild-type Shh gene. These studies demonstrate that only the activator function of Gli2 is actually required, and indicates that in specific situations, Shh can modulate the ability of Gli1 to activate target genes. Furthermore, expression of both copies of Gli1 in place of Gli2 does not disrupt spinal cord patterning, but does result in new gain-of-function defects that lead to lethality. We show that the defects are enhanced when Gli3 function is reduced, demonstrating that an important difference between Gli1 and Gli2 is the ability of Gli1 to antagonize Gli3 function.