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　Interaction between the ureteric-bud epithelium and the metanephric mesenchyme is important for kidney development (Fig. 1). Six1 and Six4 are the mammalian homologs of Drosophila sine oculis, and they are coexpressed in the nephrogenic mesenchyme. Six1 -deficient mice show varying kidney defects, while Six4 -deficient mice have no apparent abnormalities. Here, we report Six1/Six4 -deficient mice that we generated in order to elucidate the functions of Six4 in Six1 -deficient kidney development. The Six1/Six4 -deficient mice exhibited more severe kidney phenotypes than the Six1 -deficient mice; kidney and ureter agenesis was observed in all the neonates examined. The Six1/Six4 -deficient metanephric mesenchyme cells were directed toward kidney lineage but failed to express Pax2 , Pax8 , or Gdnf , whereas the expression of these genes was partially reduced or unchanged in the case of Six1 deficiency (Fig. 2). Thus, Six4 cooperates with Six1 in the metanephric mesenchyme to regulate the level of Gdnf expression; this could explain the absence of the ureteric bud in the Six1/Six4 -deficient mice (Fig. 1). In contrast, Six1 deficiency alone caused defects in mesonephric-tubule formation, and these defects were not exacerbated in the Six1/Six4 -deficient mesonephros. These results highlight the fact that Six1 and Six4 have collaborative functions in the metanephros but not in the mesonephros.

 

Fig. 1. Gdnf attracts the ureteric bud into the metanephric mesenchyme Six1 and Six4 regulate Gdnf expression cooperatively.

Fig. 2. Six1 and Six4 are essential for Gdnf expression and the ureteric bud attraction in the kidney development Upper row ： Kidney phenotypes are exacerbated in Six1/Six4 -deficient mice. ad ； adrenal, kid ； kidney, ur ； urinary duct.
Middle row ： Ureteric buds are not detected in the Six1 / Six4 -deficient embryos. wd ； Wolffian duct, arrow ； ureteric bud.
Lower row ： Gdnf expression is absent in the Six1 / Six4 -deficient embryos. yellow circle ； Wolffian duct, arrowhead ； metanephric mesenchyme.