Colin P. Sibley

Imprinted Genes, Placental Development and Fetal Growth

In mammals, imprinted genes have an important role in feto-placental development. They affect the growth, morphology and nutrient transfer capacity of the placenta and, thereby, control the nutrient supply for fetal growth. In particular, the reciprocally imprinted Igf2–H19 gene complex has a central role in these processes and matches the placental nutrient supply to the fetal nutrient demands for growth. Comparison of Igf2P0 and complete Igf2 null mice has shown that interplay between placental and fetal Igf2 regulates both placental growth and nutrient transporter abundance. In turn, epigenetic modification of imprinted genes via changes in DNA methylation may provide a mechanism linking environmental cues to placental phenotype, with consequences for development both before and after birth. Changes in expression of imprinted genes, therefore, have major implications for developmental programming and may explain the poor prognosis of the infant born small for gestational age and the wide spectrum of adult-onset diseases that originate in utero.

Association between the Activity of the System A Amino Acid Transporter in the Microvillous Plasma Membrane of the Human Placenta and Severity of Fetal Compromise in Intrauterine Growth Restriction

Primarily, our objectives were to compare system A amino acid transporter activity in the microvillous plasma membrane (MVM) of placentas from normally grown (appropriate for gestational age, AGA) and intrauterine growth-restricted (IUGR) fetuses delivered during the third trimester, as a whole and in relation to the severity of IUGR. Ten AGA and 16 IUGR pregnancies were studied at the time of elective cesarean section performed between 28 and 40 wk of gestation. Severity of IUGR pregnancies was assessed primarily by Doppler velocimetry and fetal heart rate monitoring. Placental MVM vesicles were prepared, and system A activity in these was measured. The transporter activity was significantly lower in IUGR compared with AGA pregnancies. Within the IUGR group system A activity was only significantly lower, compared with AGA, in cases that presented with a reduction in umbilical blood flow. We conclude that placental MVM system A activity is lower in IUGR compared with AGA pregnancies delivered during the third trimester. System A activity is related to the severity of IUGR.

Placental Phenotypes of Intrauterine Growth

The placenta is essential to nutrition before birth. Recent work has shown that a range of clearly defined alterations can be found in the placentas of infants with intrauterine growth restriction (IUGR). In the mouse, a placental specific knockout of a single imprinted gene, encoding IGF-2, results in one pattern of alterations in placenta structure and function which leads to IUGR. We speculate that the alterations in the human placenta can also be grouped into patterns, or phenotypes, that are associated with specific patterns of fetal growth. Identifying the placental phenotypes of different fetal growth patterns will improve the ability of clinicians to recognize high-risk patients, of laboratory scientists to disentangle the complexities of IUGR, and of public health teams to target interventions aimed at ameliorating the long-term adverse effects of inadequate intrauterine growth.

Placental nutrient supply and fetal growth

This review considers mechanisms by which transfer across the placenta takes place and how the capacity of the placenta to supply nutrients relates to fetal growth and vice versa. Blood flow through both uterine and umbilical circulations of the placenta, the structural properties of the placental exchange barrier and its related diffusional permeability, and the expression and activity of a wide range of transporter proteins in the syncytiotrophoblast, the transporting epithelium of the placenta, all need to be taken into account in considering placental supply capacity. We discuss the evidence that each of these factors affects, and is affected by, fetal growth rate and consider the regulatory mechanisms involved, with a particular focus on data that has emerged from study of the system A amino acid transporter. We consider that future work will build on the considerable foundation of knowledge regarding placental transfer mechanisms, as well as the other aspects of placental structure and function, to develop new diagnostic and therapeutic strategies for pregnancy complications, such as fetal growth restriction or overgrowth.

Expression of folate transporters in human placenta and implications for homocysteine metabolism

Poor folate status during pregnancy can lead to elevated maternal plasma levels of homocysteine (Hcy) with associated pregnancy complications and adverse neonatal outcomes, suggesting placental metabolism of Hcy might be an important determinant in influencing fetal development. The metabolic pathways for Hcy in placenta are not well defined. In this study we examined the gene expression of key enzymes involved in Hcy metabolism in first trimester and term human placenta to determine which metabolic pathways prevail. Expression of mRNA for methionine synthase and 5,10-methylene tetrahydrofolate reductase, enzymes involved in the methionine cycle and responsible for the re-methylation of Hcy to methionine, were expressed at similar levels between first trimester and term and in comparison to human liver as positive control. In contrast, cystathionine beta-synthase mRNA expression was markedly lower than that in liver at both gestational periods. Betaine-homocysteine methyltransferase mRNA was undetectable at either gestational age. These data suggest that re-methylation of Hcy using methyl donation from 5-methyltetrahydrofolate is the prevalent pathway, indicating a marked reliance on folate availability. This led to further investigations examining the expression and localisation of folate transporters in first trimester and term placenta. Folate receptor alpha (FRalpha) was highly polarised to the microvillous plasma membrane (MVM) of the syncytiotrophoblast at both gestational periods, a distribution shared by the proton-coupled folate transporter which co-localised with FRalpha. Reduced folate carrier was distributed to both MVM and basal syncytiotrophoblast plasma membranes at term suggesting a role at both loci, and in first trimester was localised to MVM as well as cytotrophoblast plasma membranes. These data support the concept that placental folate transport is established early in pregnancy, providing folate for utilisation in placental Hcy metabolism.

Altered activity of the system A amino acid transporter in microvillous membrane vesicles from placentas of macrosomic babies born to diabetic women.

Fetal macrosomia (FM) is a well-recognized complication of diabetic pregnancy but it is not known whether placental transport mechanisms are altered. We therefore studied the activity of the system A amino acid transporter, the system L amino acid transporter, and the Na+/H+ exchanger in microvillous membrane vesicles from placentas of macrosomic babies born to diabetic women (FM group), from placentas of appropriately grown babies born to diabetic women (appropriate for gestational age group) and from placentas of appropriately grown babies of normal women (control group). Sodium-dependent uptake of [14C]-methylaminoisobutyric acid at 30 s (initial rate, a measure of system A activity) was 49% lower into FM vesicles than into control vesicles (P < 0.02); this effect was due to a decrease in Vmax of the transporter with no change in Km. There was no significant difference in system A activity between the appropriate for gestational age group and control or FM group. There was also no difference between system L transporter or Na+/H+ exchanger activity between the three groups. We conclude that the number of system A transporters per milligram of membrane protein in the placental microvillous membrane is selectively reduced in diabetic pregnancies associated with FM.

Obesity and the placenta: A consideration of nutrient exchange mechanisms in relation to aberrant fetal growth

The obesity epidemic, including childhood obesity, is rapidly gaining strength as one of the most significant challenges to the health of the global community in the 21st Century. The proportion of women who are obese at the beginning of pregnancy is also increasing. These women and their babies are at high risk of pregnancy complications, and of programming for metabolic disease in adult life. In particular, maternal obesity is associated with aberrant fetal growth, encompassing both growth restricted and large for gestational age, or macrosomic fetuses. This article considers the potential effect of obesity and adipose tissue on placental nutrient exchange mechanisms in relation to aberrant fetal growth. The review emphasizes the dearth of work on this topic to date despite its importance to current and future healthcare of the population.

Sildenafil Citrate Rescues Fetal Growth in the Catechol-O-Methyl Transferase Knockout Mouse Model

Preeclampsia and fetal growth restriction are responsible for the majority of maternal and perinatal morbidity and mortality associated with complicated pregnancies. Although their etiologies are complex and multifactorial, both are associated with increased uterine artery resistance. Sildenafil citrate is able to rescue the dysfunction observed ex vivo in uterine arteries of women with preeclampsia. The ability of sildenafil citrate to increase uterine artery vasodilation, thereby decreasing uterine artery resistance and, hence, ameliorated preeclampsia and fetal growth restriction, was tested in a mouse model of preeclampsia, the catechol-O-methyl transferase knockout mouse (COMT−/−). COMT−/− and C57BL/6J mice were treated (0.2 mg/mL in drinking water, n=6–12) from gestational day 12.5 to 18.5. Measures of pup growth, including body weight, crown/rump length, and abdominal circumference, were reduced in COMT−/− mice; this was normalized after treatment with Sildenafil. COMT−/− mice also demonstrated abnormal umbilical Doppler waveforms, including reverse arterial blood flow velocity. This was normalized after treatment with Sildenafil. Abnormal uterine artery Doppler waveforms were not demonstrated in COMT−/− mice, although ex vivo responses of uterine arteries to phenylephrine were increased; moreover, treatment with Sildenafil did improve ex vivo sensitivity to an endothelium-dependent vasodilator. The data presented here demonstrate that Sildenafil can rescue pup growth and improve abnormal umbilical Doppler waveforms, providing support for a potential new therapeutic strategy targeting fetal growth restriction.

Placental transporter activity and expression in relation to fetal growth

The question of whether there are causative or compensatory changes in placental transport physiology affecting fetal growth is considered. Reductions in uterine and umbilical blood flow in growth retardation will reduce maternofetal exchange of lipophilic solutes, such as O2 and CO2, but will not have a major effect on the transfer of hydrophilic solutes. These solutes are transferred across the placenta by paracellular diffusion, transporter protein‐mediated transport and endocytosis‐exocytosis. Neither paracellular diffusion nor endocytosis‐exocytosis has been investigated in relation to fetal growth. The weight of evidence is that there is no change in the activity and expression of the syncytiotrophoblast GI UTI glucose transporter in fetal growth retardation. However, there is strong evidence that the activity of the system A amino acid transporter, per milligram of placental membrane protein, is altered in relation to fetal growth, but in a complex manner. There is also some weaker evidence that the activity of the Na(+)‐H+ exchanger, per milligram of placental membrane protein, is directly related to birth‐weight. There are no data for other solute transporters; a considerable amount of work still remains to be done in order to understand the relationship between placental function and fetal growth rate.

Mechanisms of alphafetoprotein transfer in the perfused human placental cotyledon from uncomplicated pregnancy.

We investigated the mechanisms of alphafetoprotein (AFP) transfer across the human placenta by correlating measurements of AFP transfer with cytochemical localization of AFP. Placental cotyledons were dually perfused in vitro with either the fetal or maternal perfusate containing umbilical cord plasma as a source of AFP. Steady state AFP clearance, corrected for release of endogenous AFP, was 0.973 +/- 0.292 microliter/min per gram in the fetal to maternal direction (n = 10), significantly higher (P < 0.02) than that in the maternal to fetal direction (n = 5; 0.022 +/- 0.013 microliter/min per gram). Clearance of a similarly sized protein, horseradish peroxidase was also asymmetric but clearance of the small tracer creatinine was not. Using a monoclonal antibody, we localized AFP to fibrinoid deposits in regions of villi with discontinuities of the syncytiotrophoblast, to cytotrophoblast cells in these deposits, to syncytiotrophoblast on some villi, and to trophoblast cells in the decidua. We conclude that AFP transfer in the placenta is asymmetric and that there are two available pathways for AFP transfer: (a) from the fetal circulation into the villous core and across fibrinoid deposits at discontinuities in the villous syncytiotrophoblast to enter the maternal circulation; and (b) AFP present in the decidua could enter vessels that traverse the basal plate.