Genetic and in vivo comparative analyses of Gli function in vertebrates

The Hedgehog (Hh) signaling pathway is a cardinal regulator of a diverse set of developmental processes in amniotes. In mammals, three members of the Gli family of transcription factors serve as the intracellular mediators of Hh signaling. Gli2 and Gli3 have the potential to function as either activators (Gli2/3A) or repressors (Gli2/3R) of Hh target gene expression, whereas Gli1 functions only as an activator. Interestingly, genetic analyses of Gli function in mouse have revealed that the specific combination of Gli genes, and GliA and GliR functions, required for normal development differs for each tissue type. In addition, analyses of gli function in zebrafish have provided evidence that the developmental requirements for different members of the Gli family have been differentially distributed in zebrafish and mouse. As a means of better understanding the mechanisms underlying tissue- and species-specific requirements for Gli function in vertebrates, a combination of genetic approaches were utilized, including targeted gene replacement and temporal conditional gene removal in mice. By conditionally removing Gli3 from the limb mesenchyme in different loss-of-function and gain-of-function Gli mutant backgrounds, we provide evidence for a model of anterior-posterior limb patterning that integrates the duration and dose of total Gli2 and Gli3 exposure with specific spatial requirements for GliA and GliR functions. By expressing zebrafish gli1 and gli2a from the endogenous mouse Gli2 locus we provide evidence for differences in zebrafish and mouse Gli1 and Gli2 protein functions that could account for the majority of the differences in Gli gene requirements observed in these two vertebrate species. Our preliminary data are consistent with a model in which the distribution of requirements for Gli1 or Gli2 orthologues in different tissues in zebrafish and mouse reflects the requirement for differing strengths of GliA function, such that mouse Gli2A and zebrafish Gli1A are the stronger GliAs