Mouse knockout experiments have provided evidence for a relationship between the developing heart and the placenta. Germ-line deletions for either p38α (Mapk14) or Peroxisome-proliferator activator receptor gamma (Pparγ) cause severe cardiac and placental defects, but when the placental deficiencies are selectively rescued by aggregation with wildtype tetraploid embryos, the heart defects in Mapk14−/− and Pparγ−/− embryos are corrected (Adams et al., 2000; Barak et al., 1999). These experiments demonstrate that heart defects can be a secondary consequence of impaired placentation, although the precise nature of this relationship remains ill defined. Both Mapk14 and Pparγ mutations do however affect the formation of the placental labyrinth; the site where the maternal and fetal circulations become torturously intertwined to facilitate two-way exchange, suggesting that vascularization of the placenta may be implicated.
In a previous study we reported the expression of Ly6e in the transporting trophoblast cells of the labyrinth (Hughes et al., 2013). Ly6e encodes a small molecular weight, GPI-linked glycoprotein of the Ly-6 family of cell surface proteins. Genetic deletion of Ly6e is embryonic lethal by mid-gestation, a phenotype originally attributed to heart abnormalities (Zammit et al., 2002). However, Ly6e is not appreciably expressed in heart, suggesting the resultant dilated cardiomyopathy and ventricular trabeculation defects may be secondary to placental defects. We were fortunate to obtain the Ly6e mutant mouse strain to investigate placental morphogenesis in the absence of Ly6e.
Our analysis demonstrated that Ly6e−/− placentae are abnormal, both in the gross organisation of the fetal vasculature and in the ultrastructural organisation of the interhaemal membrane, the cellular barrier separating the fetal and maternal blood compartments. In situ hybridisation using genetic markers for a number of placental trophoblast subtypes indicated that trophoblast differentiation is not affected in Ly6e−/− placenta per se, but a reduction in villous branching is clearly evident. Interestingly, a loss of Ly6e in trophoblast does appear to affect the specification of fetal endothelial cells, situated on the opposite side of the interhemal membrane. Electron microscopy of Ly6e−/− interhaemal membranes revealed areas of disrupted syncytiotrophoblast fusion and an overall increase in barrier thickness. Knockdown of Ly6e expression in differentiating trophoblast stem cells in vitro confirmed the syncytiotrophoblast fusion defect. Together our observations suggest profound deficiencies in placental function in the absence of Ly6e. Furthermore, these placental phenotypes precede the appearance of heart abnormalities, and are therefore the likely cause of both the heart defects and mid-gestational lethality in these mice.
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Hughes M, Natale BV, Simmons DG, Natale DR. (2013). Ly6e expression is restricted to syncytiotrophoblast cells of the mouse placenta. Placenta, 34, 831–835.
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