Hox Function in Mammalian Kidney Development.

Abstract

The Hox gene complexes encode an evolutionarily conserved set of transcription factors important for the anteroposterior (AP) patterning of the body plan, and are essential for the proper development of many organ systems, including the kidney. The purpose of my work was to delineate the mechanisms involved in Hox patterning of the kidney. The Hox11 genes are necessary for the initial stage of metanephric kidney development. Six2 and Gdnf, factors required for ureteric bud outgrowth into the kidney mesenchyme, are absent in the metanephric mesenchyme of Hox11 mutant animals. Using genetic, molecular, and biochemical assays, we showed that Hox11 proteins interact with Pax2 and Eya1, which are novel Hox cofactors, to directly regulate the expression of Six2 and Gdnf. We also identified a single enhancer site, a Hox response element (HRE), in the Six2 promoter that is necessary for Hox11-Pax2-Eya1 mediated activation of Six2 in vitro and in vivo. Transgenic mutation of the Six2 enhancer additionally demonstrated that the HRE required for activation of Six2 in the kidney is also necessary for repression of Six2 in the head mesenchyme, a phenotype previously reported in Hoxa2 loss-of-function mutants. Thus, Hoxa11 activation and Hoxa2 repression use the same HRE to regulate Six2 expression. To investigate how Hox proteins confer differential activation and repression of Six2 we used a combination of protein domain swaps and cell culture reporter analyses. We demonstrated that DNA binding is required for Hox protein function, but the regions N- and C-terminal to the homeodomain confer the differential activities of Hoxa2 and Hoxa11. Concurrent with these molecular analyses of the function of Hox11 in kidney organogenesis, I have also demonstrated that the Hox10 paralogous genes are necessary for proper kidney development. Morphological analyses and gene expression studies showed that the Hox10 genes are necessary for patterning the cortical stromal cell compartment in the developing kidney and regulate the formation of the renal capsule.