Inositol 1,4,5-trisphosphate receptors are essential for fetal-maternal connection and embryo viability.

Author summary Inositol 1,4,5-trisphosphate receptors (IP(3)Rs) are a family of intracellular Ca2+ release channels located on the ER membrane, which in mammals consist of 3 different subtypes (IP(3)R1, IP(3)R2, and IP(3)R3). IP3R-mediated Ca2+ signaling has been proposed to play an essential role in regulating cardiovascular development, and IP(3)R1 and IP(3)R2 double knockout (DKO) mice has been shown to develop cardiovascular defects and embryonic lethality. However, our present study using conditional cell-specific gene deletion strategy revealed that deletion of both genes in cardiomyocytes, endothelial / hematopoietic cells, and early precursors of the cardiovascular lineages in mice could not result in similar lethal phenotypes. By contrast, we observed allantoic-placental defects including reduced sizes of umbilical vessels and reduced depth of placental labyrinth in DKO embryos, which could occur independently from other phenotypes in DKO embryos. We further found that deletion of both IP(3)R1 and IP(3)R2 in epiblast also resulted in embryonic lethality and similar allantoic-placental defects. Our results demonstrated that IP(3)R1 and IP(3)R2 play an essential and redundant role in maintaining the integrity of fetal-maternal connection and embryonic viability. Inositol 1,4,5-trisphosphate receptors (IP(3)Rs) are a family of intracellular Ca2+ release channels located on the ER membrane, which in mammals consist of 3 different subtypes (IP(3)R1, IP(3)R2, and IP(3)R3) encoded by 3 genes, Itpr1, Itpr2, and Itpr3, respectively. Studies utilizing genetic knockout mouse models have demonstrated that IP(3)Rs are essential for embryonic survival in a redundant manner. Deletion of both IP(3)R1 and IP(3)R2 has been shown to cause cardiovascular defects and embryonic lethality. However, it remains unknown which cell types account for the cardiovascular defects in IP(3)R1 and IP(3)R2 double knockout (DKO) mice. In this study, we generated conditional IP(3)R1 and IP(3)R2 knockout mouse models with both genes deleted in specific cardiovascular cell lineages. Our results revealed that deletion of IP(3)R1 and IP(3)R2 in cardiomyocytes by TnT-Cre, in endothelial / hematopoietic cells by Tie2-Cre and Flk1-Cre, or in early precursors of the cardiovascular lineages by Mesp1-Cre, resulted in no phenotypes. This demonstrated that deletion of both IP3R genes in cardiovascular cell lineages cannot account for the cardiovascular defects and embryonic lethality observed in DKO mice. We then revisited and performed more detailed phenotypic analysis in DKO embryos, and found that DKO embryos developed cardiovascular defects including reduced size of aortas, enlarged cardiac chambers, as well as growth retardation at embryonic day (E) 9.5, but in varied degrees of severity. Interestingly, we also observed allantoic-placental defects including reduced sizes of umbilical vessels and reduced depth of placental labyrinth in DKO embryos, which could occur independently from other phenotypes in DKO embryos even without obvious growth retardation. Furthermore, deletion of both IP(3)R1 and IP(3)R2 by the epiblast-specific Meox2-Cre, which targets all the fetal tissues and extraembryonic mesoderm but not extraembryonic trophoblast cells, also resulted in embryonic lethality and similar allantoic-placental defects. Taken together, our results demonstrated that IP(3)R1 and IP(3)R2 play an essential and redundant role in maintaining the integrity of fetal-maternal connection and embryonic viability.