Placental Gene Expression Reveals Reduced Angiogenesis in Primiparous Mares

Primiparous mares produce smaller and lighter foals than in subsequent pregnancies. In most studies, their placenta is also smaller, more efficient and presents structural alterations, but maternal age is a confounding factor. This study aimed to characterize the effects of maternal parity using the mare's first two pregnancies or by comparing young mares with different parities over the same gestation year. The same young mares (4–6-year-old over the study) foaled for the first time in 2018 (primiparous in 2018, P18, n=10) and for the second time in 2020 (multiparous in 2020, M20, n=10). In 2020, another group of young mares foaled for the first time (primiparous in 2020, P20, n=18). All mares were inseminated with the semen of the same stallion and were fed identically over the study. To assess the effect of primiparity, M20 were compared to P18 (effect on the same mare), or P20 (effect in the same year). Placentas were recovered at foaling, measured, weighed and sampled close to the umbilical cord. Stereology was performed using MercartorPro software on HES-stained paraffin-embedded samples and analyzed using a linear model for paired samples or not and permutations with R software. Paired-end RNA-sequencing was performed (Illumina, NextSeq500) on 6 randomly chosen snap-frozen placentas/group. Differential expression was analyzed (DESeq2) using a false discovery rate (FDR) <0.05 cutoff. Gene Set Enrichment Analysis was performed on differential expressed genes using PANTHER overrepresentation test with GOBP database. Placental weight and surface were reduced and the efficiency (foal weight/placental surface, in kg/m2) was increased in primiparous mares in both comparisons (p<0.05). Volume was not modified by parity between the consecutives gestations but reduced in P20 vs M20 (p<0.05). Only the volume of allantoic stroma was significantly reduced in P18 vs M20 (p<0.05), whereas hemotrophic trophoblast and microcotyledonary vessels’ volume tended to be reduced in P20 vs M20 placentas (p<0.1). In consecutive gestations, 118 genes were differentially expressed in placenta while in the same year, only 2 genes (IL4R and FFAR, interleukin-4 and free fatty acid receptors) were differentially expressed (FDR <0.05). Among the 118 genes differentially expressed between consecutive pregnancies, apelin signaling, angiogenesis and catalytic activity were overrepresented. All genes involved in these pathways were under-expressed in P18 mares. Both the apelin (normalized enrichment score, NES=-2.16, FDR <0.05) and its receptor (NES=-2.48, FDR <0.05) were less expressed in the placenta of P18 vs M20. This is also observed in intrauterine growth retardation in humans. Although placental morphology was lightly affected in both analyses, the inter-individual variability seemed to hide parity effects for transcriptomic analysis. Consecutive gestations, however, demonstrate that the smaller placental and foal size in primiparous mares seems to originate from reduced vascularization.