Homeodomain Transcription Factors: Integral Modulators of Human Decidualization
Abstract Ineffective embryo implantation accounts for a significant percentage of female infertility, and often renders IVF procedures unsuccessful. Decidualization, the dramatic uterine morphological response to ovarian hormone exposure, is a prerequisite for embryo implantation. Despite its significance in reproduction, the genetic framework of decidualization was not systematically studied until our recent development of a suitable high-throughput screening tool, immortalized human endometrial stromal cells (hESCs) that carry the yellow fluorescent protein gene under the control of the progesterone-sensitive prolactin promoter (PRL-Y cells). We recently used PRL-Y cells to perform a genome-wide siRNA functional screen and results revealed that 36 members of the homeodomain-containing family of transcription factors (HDTFs) are modulators of human decidualization. To determine which HDTFs are transcriptionally sensitive to ovarian hormone exposure, RT-PCR was performed on wildtype hESCs for the 36 HDTF hits over a 72-hour time course of E2/P4/cAMP exposure. Twenty HDTF hits (55%) were both detectable by PCR and showed variable expression in response to ovarian hormone treatment. Interestingly, all of these homeodomain factors, with a few distinct exceptions, exhibited decreased transcriptional expression in response to ovarian hormone treatment. This suggests that precious energy is used to transcribe these factors during the pre-decidualized phase, and that they may be required to maintain homeostasis during times of low hormone exposure. Because siRNA is not fully efficient, in order to confirm which HDTFs are required for normal decidualization, we generated a doxycyline-inducible Cas9-expressing hESC clone in order to subsequently generate individual knockout hESC lines for each HDTF hit. Cas9 expression was turned on 5 days prior to crRNA and trRNA transfection targeting the first exon of each HDTF. Five days after transfection, the cells were treated for 72 hours with ovarian hormone induction medium before RNA was isolated for gene expression analysis. As a pool of cells prior to any cloning (which likely includes knockout and wildtype cells in different ratios) the results indicate that several HDTFs are required for proper decidualization. The reporter transcripts of PRL and EREG are significantly abrogated or entirely undetectable in certain knockout lines. Interestingly these include some original siRNA HDTF hits whose expression is undetectably low by PCR. Sequencing validation will be necessary to confirm that knocking out such low levels of these transcripts genuinely has the robust effect on the human decidualization reaction that we are witnessing in these results. Together these findings comprise significant initial steps in characterizing the intricate upstream roles of HDTFs in human decidualization and female fertility.
