Sex determination requires the commitment of bipotential gonads to either a testis or an ovarian fate. Gene deletion of the kinase Map3k4 results in gonadal sex reversal in XY mice, and transgenic re-expression of Map3k4 rescues the sex reversal phenotype. Map3k4 encodes a large, multi-functional protein possessing a kinase domain and several, additional protein–protein interaction domains. Although MAP3K4 plays a critical role in male gonadal sex determination, it is unknown if the kinase activity of MAP3K4 is required. Here, we use mice expressing full-length, kinase-inactive MAP3K4 from the endogenous Map3k4 locus to examine the requirement of MAP3K4 kinase activity in sex determination. Although homozygous kinase-inactivation of MAP3K4 (Map3k4^KI/KI) is lethal, a small fraction survive to adulthood. We show Map3k4^KI/KI adults exhibit a 4:1 female-biased sex ratio. Many adult Map3k4^KI/KI phenotypic females have a Y chromosome. XY Map3k4^KI/KI adults with sex reversal display female mating behavior, but do not give rise to offspring. Reproductive organs are overtly female, but there is a broad spectrum of ovarian phenotypes, including ovarian absence, primitive ovaries, reduced ovarian size, and ovaries having follicles in all stages of development. Further, XY Map3k4^KI/KI adults are smaller than either male or female Map3k4^WT/WT mice. Examination of the critical stage of gonadal sex determination at E11.5 shows that loss of MAP3K4 kinase activity results in the loss of Sry expression in XY Map3k4^KI/KI embryos, indicating embryonic male gonadal sex reversal. Together, these findings demonstrate the essential role for kinase activity of MAP3K4 in male gonadal sex determination.

Summary sentence

Male gonadal sex determination requires the kinase activity of the multi-domain containing protein MAP3K4.

Graphical Abstract