Functional redundancy among Nanos proteins and a distinct role of Nanos2 during male germ cell development.
The mouse Nanos proteins, Nanos2 and Nanos3, are required for germ cell development and share a highly conserved zinc-finger domain. The expression patterns of these factors during development, however, differ from each other. Nanos3 expression in the mouse embryo commences in the primordial germ cells (PGCs) just after their formation, and a loss of this protein results in the germ cell-less phenotype in both sexes. By contrast, Nanos2 expression begins only in male PGCs after their entry into the genital ridge and a loss of this protein results in a male germ cell deficiency, irrespective of the co-expression of Nanos3 in these cells. These results indicate that these two Nanos proteins have distinct functions, which depend on the time and place of their expression. To further elucidate this, we have generated transgenic mouse lines that express Nanos2 under the control of the Oct4DeltaPE promoter and examined Nanos2 function in a Nanos3-null genetic background. We find that ectopically produced Nanos2 protein rescues the Nanos3-null defects, because the germ cells fully develop in both sexes in the transgenic mice. This result indicates that Nanos2 can substitute for Nanos3 during early PGC development. By contrast, our current data show that Nanos3 does not rescue the defects in Nanos2-null mice. Our present findings thus indicate that there are redundant functions of the Nanos proteins in early PGC development, but that Nanos2 has a distinct function during male germ cell development in the mouse.