In murine placentas, peroxisome proliferator-activated receptor (PPAR) γ1, a nuclear receptor, is abundant at the late stage of pregnancy (E15-E16), but its functional roles are still elusive because PPARγ-full knockout embryos die early (E10). We generated mice disrupted in only Pparγ1, one of the two major mRNA splicing variants of PPARγ1. Pparγ1knockout embryos developed normally until 15.5 dpc, but their growth was retarded thereafter and they did not survive. At 15.5 dpc, in the wild-type placentas, intense PPARγimmunostaining was detected in sinusoidal trophoblast giant cells (sTGCs), a cell lineage that coordinates the maternal blood microcirculation in the labyrinth, whereas they were absent in the knockouts. Although Pparγ1-knockout placentas were normal in morphology, we observed severely dilated maternal blood sinuses in the labyrinth. The Pparγ1-knockout sTGCs had abnormally large nuclei, an enhanced endocycling phenotype, indicating insufficient differentiation. RNA-sequencing of the placentas showed increased expression of genes coding for nucleosome assembly factors. Labyrinthine gene expressions for atypical E2Fs and cyclin E, key drivers for endocycling, were increased >3-fold. These findings suggested that PPARγ1 plays a key role in endocycle termination.
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