58 Evidence of Metabolic Compartmentalization or Zonation in the Bovine Placenta: Significance for the Regulation of Placental Function and Fetal Growth

Abstract

The concept of “metabolic zonation” in the liver proposes that opposing or complementary metabolic pathways are carried out within distinct regions of the liver lobule to maintain optimal metabolic homeostasis. As bovine placentomes morpho-topographically differ in mass and shape within the pregnant tract, an event more pronounced in cloned-derived concepti, the aims of this study were (a) to investigate the occurrence of metabolic zonation in the bovine placenta by placentome shape and size, and (b) to compare placenta metabolic functions between control and cloned concepti, which may relate to the abnormal conceptus growth after cloning. Bovine pregnancies established by superovulation and AI (Control group, n = 4) or by cloning by nuclear transfer (Cloned group, n = 6) were slaughtered on Day 225 of gestation, with the harvesting of maternal and conceptus tissues and fluids [Gerger et al. 2016 Reprod. Fertil. Dev. 29, 950-967; 10.1071/RD15215]. Placentomes were excised, weighed, measured, and classified by shape as types A, B, C or D. Total RNA from each placentome type per group was extracted, reverse transcribed, and used for the RT-qPCR analysis of 27 key genes related to glycolysis, gluconeogenesis, fructolysis, fructogenesis, pentose phosphate pathway (PPP), fatty acid synthesis, cholesterol synthesis, glucose transporters, apoptosis, and components of the IGF system. Data were analysed by ANOVA, the Tukey test, and a simple correlation test. Day-225 cloned concepti were 2-fold larger than controls, with 64% of placentomes being larger than the largest in the Control group. A linear positive correlation between mass, size and shape was observed in the control group, with 83% of the normal-shaped placentomes (A and B) representing 85% of the total placenta weight. Conversely, abnormal-shaped placentomes (C and D) represented 39% in the cloned group and 32% total placenta mass, for fewer type A and more type C placentomes in clones. No differences were detected in expression for key metabolic-related genes between placentas of clones and controls, irrespective of the placentome type. However, differences were verified between placentome types, with an increase in gene expression in type C in relation to type A placentomes for glucose transporters (2.2-fold), glycolysis (1.7-fold), PPP (2.1-fold), fructogenesis (1.3- to 1.6-fold), Igf2 (2.1-fold) and Igf1r (2.2-fold), for a 2.5-fold decrease in Igf2r, with no difference in fatty acid and cholesterol synthesis or apoptosis. The differential expression pattern observed only between placentome types demonstrates a potential metabolic zonation in the bovine placenta. Therefore, morpho-physiologic differences observed between clones and controls could be due to changes in size and proportions of placentome types, and not due to changes in tissue unit function per se. The more significant proportion of type C placentomes in the placenta of clones may be associated with increased fructose synthesis due to increased glucose supply, in a nutrient surplus that promotes conceptus growth.