Cytotrophoblast, Not Syncytiotrophoblast, Dominates Glycolysis and Oxidative Phosphorylation in Human Term Placenta

Abstract

The syncytiotrophoblast (SCT) at the maternal-fetal interface has been presumed to be the primary driver of placental metabolism, and the underlying progenitor cytotrophoblast cells (CTB) an insignificant contributor to placental metabolic activity. However, we now show that the metabolic rate of CTB is much greater than the SCT. The oxygen consumption and extracellular acidification rate, a measure of glycolysis, are both greater in CTB than in SCT in vitro (CTB: 96 ± 16 vs SCT: 46 ± 14 pmol O 2 × min −1 × 100 ng DNA −1 , p < 0.001) and (CTB: 43 ± 6.7 vs SCT 1.4 ± 1.0 ∆mpH × min −1 × 100 ng DNA −1 , p < 0.0001). Mitochondrial activity, as determined by using the mitochondrial activity-dependent dye Mitotracker CM-H 2 TMRosa, is higher in CTB than in SCT in culture and living explants. These data cast doubt on the previous supposition that the metabolic rate of the placenta is dominated by the SCT contribution. Moreover, differentiation into SCT leads to metabolic suppression. The normal suppression of metabolic activity during CTB differentiation to SCT is prevented with a p38 MAPK signaling inhibitor and epidermal growth factor co-treatment. We conclude that the undifferentiated CTB, in contrast to the SCT, is highly metabolically active, has a high level of fuel flexibility, and contributes substantially to global metabolism in the late gestation human placenta.The primary function of the human placenta is to ensure that the fetus it serves continuously receives maternally-derived nutrients that are necessary for optimal growth. The placenta is a powerful organ that produces hormones, alters nutrients biochemically as needed, and transports them to the developing fetus. Each of these processes exacts a metabolic cost. Thus, the placenta has an extraordinarily high metabolic rate, consuming approximately 40% of the oxygen used by the entire conceptus while accounting for less than 20% of its mass [1][2][3][4] . The metabolic status of the placenta is evidently important because several maternal disorders including pre-eclampsia and gestational diabetes mellitus, are associated with metabolic abnormalities [5][6][7] .The materno-placental barrier is composed of a monolayer of terminally differentiated trophoblast cells, the syncytialized trophoblast (SCT), the underlying layer of progenitor cells, the cytotrophoblast (CTB) and the endothelium of the fetal vasculature. Throughout gestation, the SCT is continually repaired by CTB cells 8 . The SCT has been presumed to account for the largest proportion of placental metabolic activity because it is in direct contact with maternal blood, synthesizes and secretes large quantities of protein and steroid hormones, and is the primary transport organ for all nutrients acquired by the fetus. Furthermore, it has been assumed that because the CTB layer gradually disappears with gestational age, it plays an increasingly diminished biological role as term approaches 9 .Freshly isolated CTB and SCT can be studied in vitro. Isolated CTB are known to differentiate and fuse over...

Proteomic Analysis of Placental Mitochondria Following Trophoblast Differentiation

“…Understanding trophoblast biology is key to understanding major gestational complications of pregnancy, and central to this is trophoblast mitochondrial function. Previous studies have indicated that the syncytiotrophoblast is less metabolically active than cytotrophoblasts, and that this might be critical for directing nutrients to the developing fetus whilst maintaining placental homeostasis (Kolahi et al, 2017). This study would support such a hypothesis, as many proteins involved in key stages of carbohydrate, fatty acid and amino acid metabolism are decreased in syncytiotrophoblast mitochondria.…”

“…Phosphoenolpyruvate is an important metabolic intermediate in the conversion of pyruvate to oxaloacetate and alterations in the activity of PC and PCK2 would alter the supply of substrates from glucose metabolism to the citric acid cycle (TCA), potentially impairing Syncytio-Mito metabolism (Figure 2). This is supported by a recent publication assessing cytotrophoblasts and the syncytiotrophoblast, which found higher levels of ATP, and greater rates of respiration and glycolysis, in cytotrophoblasts when compared to syncytiotrophoblasts (Kolahi et al, 2017). As gestation proceeds, the underlying cytotrophoblast layer diminishes as it continually fuses to form the syncytium; therefore these metabolic changes may be important to fetal development as they imply that carbohydrate reserves would be utilized less by the placenta as gestation proceeds and be more readily available to the rapidly developing fetus.…”

“…However, mitochondria in the syncytium appear smaller, spherical and punctate with little to no definition in cristae structure. What drives this process at a molecular level remains unclear although it is known that alterations in morphology are accompanied by changes in mitochondrial bioenergetics, changes in membrane potential and ATP production (Kolahi et al, 2017;Fisher et al, 2019).…”

Placental mitochondria adapt developmentally and in response to hypoxia to support fetal growth

“…The lower rates of Lz mitochondrial O 2 consumption, particularly with Py toward term, suggest O 2 and glucose may be spared by the placenta for transfer to the fetus during its rapid growth phase. Indeed, OXPHOS rates are lower for the transporting syncytiotrophoblast than the proliferative cytotrophoblast in the term human placenta (28). Since Lz Pal P was unaffected by gestational age, while Pal L declined, fatty acids may become more important substrates for meeting the energy demands placed on the Lz for transport by the rapidly growing fetus (29).…”

Changes in mitochondrial respiration in the human placenta over gestation

“…Cytotrophoblast mitochondria have a more classical morphology, whereas syncytiotrophoblast mitochondria are smaller and have an irregular shape and cristae structure potential related to their role in progesterone production [7][8][9][10]. Both the syncytiotrophoblast and cytotrophoblasts also have important roles in metabolic reactions and transport to support the fetus, and mitochondria are likely to be critical organelles in these processes [11].…”