Developmental expression of the homeobox protein Distal-less 3 and its relationship to progesterone production in mouse placenta.

Distal-less 3 (Dlx3) is a homeobox factor that functions as a placental-specific transcriptional regulator. Dlx3 null mice (-/-) have compromised placental development and do not survive in utero past embryonic day (E) 9.5. The current studies were undertaken to examine the expression of Dlx3 in mouse placenta during gestation, and to determine whether Dlx3 was involved in placental progesterone production. Dlx3 was not detectable at E8.5 but was detected in E9.5 placenta with continuing but diminished expression through E15.5. Dlx3 immuno-localization was restricted to the labyrinth, was nuclear and was found in cytokeratin-positive cells. Previous studies in choriocarcinoma cell lines support the conclusion that Dlx3 is required for expression of 3'-hydroxysteroid dehydrogenase VI (3betaHSD VI), an obligate enzyme in the production of progesterone by trophoblast giant cells. In a rat trophoblast stem cell line (Rcho-1), Dlx3 expression was non-detectable in Rcho-1 cells induced to differ-entiate using mitogen withdrawal. In vitro progesterone production in placental cultures and 3betaHSD VI mRNA from Dlx3 (+/+), (+/-) and (-/-) mice were equivalent. In situ hybridization for 3betaHSD VI revealed mRNA expression restricted to trophoblast giants cells with no detectable expression in the labyrinth suggesting that Dlx3 and 3betaHSD VI were not colocalized within the placenta. These studies support the conclusion that Dlx3 protein expression is restricted to the labyrinth region of the murine placenta into late gestation and that Dlx3 does not appear to be expressed in trophoblast giant cells. Further, loss of Dlx3 was not correlated with synthesis of progesterone from E9.5 mouse placentas.