Monocarboxylate transporter 8 modulates the viability and invasive capacity of human placental cells and fetoplacental growth in mice

Exogenous corticosterone administration during pregnancy in mice alters placental and fetal thyroid hormone availability in females

INTRODUCTION

Maternal prenatal psychological stress is associated with adverse pregnancy outcomes and increased risk of adverse health outcomes in children. While the molecular mechanisms that govern these associations has not been fully teased apart, stress-induced changes in placental function can drive sex-specific phenotypes in offspring. We sought to identify and examine molecular pathways in the placenta that are altered in response to maternal prenatal stress.

METHODS

We previously employed a mouse model of maternal prenatal stress where pregnant dams were treated with stress hormone (CORT) beginning in mid-gestation. Using this model, we conducted RNAseq analysis of whole placenta at E18.5. We used qRT-PCR to validate gene expression changes in the placenta and in a trophoblast cell line. ELISAs were used to measure the abundance of thyroid hormones in maternal and fetal serum and in the placenta.

RESULTS

Dio2 was amongst the top differentially expressed genes in response to exogenous stress hormone. Dio2 expression was more downregulated in placenta of female fetuses from CORT-treated dams than both control placenta from females and placenta from male fetuses. Consistent with Dio2's role in production of bioactive thyroid hormone (T3), we found that there was a reduction of T3 in placenta and serum of female embryos from CORT-treated dams at E18.5. Both T3 and T4 were reduced in the fetal compartment of the placenta of female fetuses from CORT-treated dams at E16.5. Exogenous stress hormone induced reduction in thyroid hormone in females was independent of circulating levels of TH in the dams.

DISCUSSION

The placental thyroid hormone synthesis pathway may be a target of elevated maternal stress hormone and modulate fetal programming of health and disease of offspring in a sex-specific fashion.

Exogenous corticosterone administration during pregnancy alters placental and fetal thyroid hormone availability in females

Introduction

Maternal prenatal stress is associated with adverse pregnancy outcomes and predisposition to long-term adverse health outcomes in children. While the molecular mechanisms that govern these associations has not been fully teased apart, stress-induced changes in placental function can drive sex-specific phenotypes in offspring. We sought to identify and examine molecular pathways in the placenta that are altered in response to maternal prenatal stress.

Methods

Using a mouse model of maternal prenatal stress, we conducted RNA-seq analysis of whole placenta at E18.5. We used qRT-PCR to validate gene expression changes in the placenta and in a trophoblast cell line. ELISAs were used to measure the abundance of thyroid hormones in maternal and fetal serum and in the placenta.

Results

Dio2 was amongst the top differentially expressed genes in response to elevated maternal stress hormone. Dio2 expression was more downregulated in female placenta from stressed dams than both female control and male placenta. Consistent with Dio2's role in production of bioactive thyroid hormone (T3), we found that there was a reduction of T3 in placenta and serum of female embryos from stressed dams at E18.5. Both T3 and T4 were reduced in the fetal compartment of the female placenta from stressed dams at E16.5. Stress hormone induced reduction in thyroid hormone in females was independent of circulating levels of TH in the dams.

Discussion

The placental thyroid hormone synthesis pathway may be a target of maternal stress and modulate fetal programming of health and disease of offspring in a sex-specific fashion.

Thyroid hormones: Metabolism and transportation in the fetoplacental unit

The thyroid hormones (THs), thyroxine (T4) and triiodothyronine (T3), are of vital importance for fetal development. The concentration of THs in fetal circulation varies throughout gestation and differs from the concentration in the maternal serum, indicating the presence of maternal-fetal thyroid homeostasis regulatory mechanisms in the placenta. The passage of THs from maternal circulation to fetal circulation is modulated by plasma membrane transporters, enzymes, and carrier proteins. Monocarboxylate transporter 8, iodothyronine deiodinases (DIO2 and DIO3), and transthyretin are especially involved in this maternal-fetal thyroid modulation, shown by a greater expression in the placenta. THs also play a role in placental development and as expected, abnormal variations in TH levels are associated with pregnancy complications and can result in damage to the fetus. Although new evidence regarding TH regulation during pregnancy and its effects in the mother, placenta, and fetus has been published, many aspects of these interactions are still poorly understood. The objective of this review is to provide an evidence-based update, drawn from current data, on the metabolism and transport of THs in the placenta and their vital role in the maternal-fetal relationship.

Gaps in the knowledge of thyroid hormones and placental biology

Thyroid hormones (THs) are required for the growth and development of the foetus, stimulating anabolism and oxygen consumption from the early stages of pregnancy to the period of foetal differentiation close to delivery. Maternal changes in the hypothalamic–pituitary thyroid axis are also well known. In contrast, several open questions remain regarding the relationships between the placenta and the maternal and foetal TH systems. The exact mechanism by which the placenta participates in regulating the TH concentration in the foetus and mother and the role of TH in the placenta are still poorly studied. In this review, we aim to summarize the available data in the area and highlight significant gaps in our understanding of the ontogeny and cell-specific localization of TH transporters, TH receptors and TH metabolic enzymes in the placenta in both human and rodent models. Significant deficiencies also exist in knowledge of the contribution of genomic and nongenomic effects of TH on the placenta and finally how the placenta reacts during pregnancy when the mother has thyroid disease. By addressing these key knowledge gaps, improved pregnancy outcomes and management of women with thyroid alterations may be possible.

Thyroid Hormone Transporters

Thyroid hormone transporters at the plasma membrane govern intracellular bioavailability of thyroid hormone. Monocarboxylate transporter (MCT) 8 and MCT10, organic anion transporting polypeptide (OATP) 1C1, and SLC17A4 are currently known as transporters displaying the highest specificity toward thyroid hormones. Structure-function studies using homology modeling and mutational screens have led to better understanding of the molecular basis of thyroid hormone transport. Mutations in MCT8 and in OATP1C1 have been associated with clinical disorders. Different animal models have provided insight into the functional role of thyroid hormone transporters, in particular MCT8. Different treatment strategies for MCT8 deficiency have been explored, of which thyroid hormone analogue therapy is currently applied in patients. Future studies may reveal the identity of as-yet-undiscovered thyroid hormone transporters. Complementary studies employing animal and human models will provide further insight into the role of transporters in health and disease. (Endocrine Reviews 41: 1 - 55, 2020).

Thyroid hormones and female reproduction

Thyroid hormones are vital for the proper functioning of the female reproductive system, since they modulate the metabolism and development of ovarian, uterine, and placental tissues. Therefore, hypo- and hyperthyroidism may result in subfertility or infertility in both women and animals. Other well-documented sequelae of maternal thyroid dysfunctions include menstrual/estral irregularity, anovulation, abortion, preterm delivery, preeclampsia, intrauterine growth restriction, postpartum thyroiditis, and mental retardation in children. Several studies have been carried out involving prospective and retrospective studies of women with thyroid dysfunction, as well as in vivo and in vitro assays of hypo- and hyperthyroidism using experimental animal models and/or ovarian, uterine, and placental cell culture. These studies have sought to elucidate the mechanisms by which thyroid hormones influence reproduction to better understand the physiology of the reproductive system and to provide better therapeutic tools for reproductive dysfunctions that originate from thyroid dysfunctions. Therefore, this review aims to summarize and update the available information related to the role of thyroid hormones in the morphophysiology of the ovary, uterus, and placenta in women and animals and the effects of hypo- and hyperthyroidism on the female reproductive system.

Regulation of T3 Availability in the Developing Brain: The Mouse Genetics Contribution

Alterations in maternal thyroid physiology may have deleterious consequences on the development of the fetal brain, but the underlying mechanisms remain elusive, hampering the development of appropriate therapeutic strategies. The present review sums up the contribution of genetically modified mouse models to this field. In particular, knocking out genes involved in thyroid hormone (TH) deiodination, transport, and storage has significantly improved the picture that we have of the economy of TH in the fetal brain and the underlying genetic program. These data pave the way for future studies to bridge the gap in knowledge between thyroid physiology and brain development.

Disorder of thyroid hormone transport into the tissues

Transport of thyroid hormone (TH) across the plasma membrane is essential for intracellular TH metabolism and action, and this is mediated by specific transporter proteins. During the last two decades several transporters capable of transporting TH have been identified, including monocarboxylate transporter 8 (MCT8), MCT10 and organic anion transporting polypeptide 1C1 (OATP1C1). In particular MCT8 and OATP1C1 are important for the regulation of local TH activity in the brain and thus for brain development. MCT8 is a protein containing 12 transmembrane domains, and is encoded by the SLC16A2 gene located on the X chromosome. It facilitates both TH uptake and efflux across the cell membrane. Male subjects with hemizygous mutations in MCT8 are afflicted with severe intellectual and motor disability, also known as the Allan-Herndon-Dudley syndrome (AHDS), which goes together with low serum T4 and high T3 levels. This review concerns molecular and clinical aspects of MCT8 function.

RNA-seq analysis of the rat placentation site reveals maternal obesity-associated changes in placental and offspring thyroid hormone signaling

INTRODUCTION

In animal models, maternal obesity (OB) leads to augmented risk of offspring OB. While placental function is influenced by maternal habitus, the effect of maternal obesity on the interacting zones of the placenta [the labyrinth (LZ), junctional (JZ) and metrial gland (MG)] remains unknown.

METHODS

Using a rat maternal obesity model, we conducted transcriptomic profiling of the utero-placental compartments and fetal liver (FL) at dpc 18.5, in conjunction with analyses of mRNA expression of key thyroid hormone (TH) signaling genes in the placenta, fetus and weanling offspring.

RESULTS AND DISCUSSION

Gene expression analysis of placenta and offspring revealed that each utero-placental compartment responds distinctly to maternal OB with changes in inflammatory signaling, lipid metabolism and hormone stimulus being the predominant effects. OB-induced alterations in 17 genes were confirmed by qPCR, including reductions in thyrotropin-releasing hormone (Trh) in JZ. We further characterized mRNA and protein expression of TH signaling regulators including deiodinases (Dio), TH receptors (Tr), and downstream targets (uncoupling proteins (Ucp)). A concerted down-regulation of multiple facets of thyroid hormone signaling in the JZ and FL was observed. JZ expression of thyroid hormone signaling components Trh, Dio2, Trα, and Ucp2 were negatively associated with maternal leptin. mRNA expression of TRH, TRβ and UCP1 were also decreased in term placenta from OB women. Finally, our studies identified persistent impairments in expression of TH related genes in tissues from offspring of obese dams.

CONCLUSIONS

The role of lower placental thyroid expression is worthy of further study as a possible pathway that leads to low energy metabolism and obesity in animals born to obese mothers.

Molecular basis of thyrotropin and thyroid hormone action during implantation and early development

BACKGROUND

Implantation and early embryo development are finely regulated processes in which several molecules are involved. Evidence that thyroid hormones (TH: T4 and T3) might be part of this machinery is emerging. An increased demand for TH occurs during gestation, and any alteration in maternal thyroid physiology has significant implications for both maternal and fetal health. Not only overt but also subclinical hypothyroidism is associated with infertility as well as with obstetric complications, including disruptions and disorders of pregnancy, labor, delivery, and troubles in early neonatal life.

METHODS

We searched the PubMed and Google Scholar databases for articles related to TH action on ovary, endometrium, trophoblast maturation and embryo implantation. In addition, articles on the regulation of TH activity at cellular level have been reviewed. The findings are hereby summarized and critically discussed.

RESULTS

TH have been shown to influence endometrial, ovarian and placental physiology. TH receptors (TR) and thyrotropin (thyroid-stimulating hormone: TSH) receptors (TSHR) are widely expressed in the feto-maternal unit during implantation, and both the endometrium and the trophoblast might be influenced by TH either directly or through TH effects on the synthesis and activity of implantation-mediating molecules. Interestingly, due to the multiplicity of mechanisms involved in TH action (e.g. differential expression of TR isoforms, heterodimeric receptor partners, interacting cellular proteins, and regulating enzymes), the TH concentration in blood is not always predictive of their cellular availability and activity at both genomic and nongenomic level.

CONCLUSIONS

In addition to the known role of TH on the hormonal milieu of the ovarian follicle cycle, which is essential for a woman's fertility, evidence is emerging on the importance of TH signaling during implantation and early pregnancy. Based on recent observations, a local action of TH on female reproductive organs and the embryo during implantation appears to be crucial for a successful pregnancy. Furthermore, an imbalance in the spatio-temporal expression of factors involved in TH activity might induce early arrest of pregnancy in women considered as euthyroid, based on their hormonal blood concentration. In conclusion, alterations of the highly regulated local activity of TH may play a crucial, previously underestimated, role in early pregnancy and pregnancy loss. Further studies elucidating this topic should be encouraged.

Expression of organic anion transporting polypeptide 1c1 and monocarboxylate transporter 8 in the rat placental barrier and the compensatory response to thyroid dysfunction

Thyroid hormones (THs) must pass from mother to fetus for normal fetal development and require the expression of placental TH transporters. We investigate the compensatory effect of placental organic anion transporting polypeptide 1c1 (Oatp1c1) and monocarboxylate transporter 8 (Mct8) on maternal thyroid dysfunction. We describe the expressions of these two transporters in placental barriers and trophoblastic cell populations in euthyroidism and thyroid dysfunction resulting from differential iodine nutrition at gestation day (GD) 16 and 20, that is, before and after the onset of fetal thyroid function. Immunohistochemistry revealed that in the blood-placenta barrier, these two TH transporters were strongly expressed in the villous interstitial substance and were weakly expressed in trophoblast cells. Levels of Oatp1c1 protein obviously increased in the placental fetal portion during maternal thyroid deficiency at GD16. Under maternal thyroid deficiency after the production of endogenous fetal TH, quantitative PCR analysis revealed down-regulation of Oatp1c1 occurred along with up-regulation of Mct8 in trophoblast cell populations isolated by laser capture microdissection (LCM); this was consistent with the protein levels in the fetal portion of the placenta. In addition, decreased D3 mRNA at GD16 and increased D2 mRNA on two gestational days were observed in trophoblast cells with thyroid dysfunction. However, levels of Oatp1c1 mRNA at GD16 and D3 mRNA at GD20 were too low to be detectable in trophoblast cells. In conclusion, placental Oatp1c1 plays an essential compensatory role when the transplacental passage of maternal THs is insufficient at the stage before the fetal TH production. In addition, the coordinated effects of Oatp1c1, Mct8, D2 and D3 in the placental barrier may regulate both transplacental TH passage and the development of trophoblast cells during thyroid dysfunction throughout the pregnancy.

Triiodothyronine regulates angiogenic growth factor and cytokine secretion by isolated human decidual cells in a cell-type specific and gestational age-dependent manner

STUDY QUESTION

Does triiodothyronine (T3) regulate the secretion of angiogenic growth factors and cytokines by human decidual cells isolated from early pregnancy?

SUMMARY ANSWER

T3 modulates the secretion of specific angiogenic growth factors and cytokines, with different regulatory patterns observed amongst various isolated subpopulations of human decidual cells and with a distinct change between the first and second trimesters of pregnancy.

WHAT IS KNOWN ALREADY

Maternal thyroid dysfunction during early pregnancy is associated with complications of malplacentation including miscarriage and pre-eclampsia. T3 regulates the proliferation and apoptosis of fetal-derived trophoblasts, as well as promotes the invasive capability of extravillous trophoblasts (EVT). We hypothesize that T3 may also have a direct impact on human maternal-derived decidual cells, which are known to exert paracrine regulation upon trophoblast behaviour and vascular development at the uteroplacental interface.

STUDY DESIGN, SIZE, DURATION

This laboratory-based study used human decidua from first (8–11 weeks; n = 18) and second (12–16 weeks; n = 12) trimester surgical terminations of apparently uncomplicated pregnancies.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Primary cultures of total decidual cells, and immunomagnetic bead-isolated populations of stromal-enriched (CD10+) and stromal-depleted (CD10−) cells, uterine natural killer cells (uNK cells; CD56+) and macrophages (CD14+) were assessed for thyroid hormone receptors and transporters by immunocytochemistry. Each cell population was treated with T3 (0, 1, 10, 100 nM) and assessments were made of cell viability (MTT assay) and angiogenic growth factor and cytokine secretion (immunomediated assay). The effect of decidual cell-conditioned media on EVT invasion through Matrigel^® was evaluated.

MAIN RESULTS AND THE ROLE OF CHANCE

Immunocytochemistry showed the expression of thyroid hormone transporters (MCT8, MCT10) and receptors (TRα1, TRβ1) required for thyroid hormone-responsiveness in uNK cells and macrophages from the first trimester. The viability of total decidual cells and the different cell isolates were unaffected by T3 so changes in cell numbers could not account for any observed effects. In the first trimester, T3 decreased VEGF-A secretion by total decidual cells (P < 0.05) and increased angiopoietin-2 secretion by stromal-depleted cells (P < 0.05) but in the second trimester total decidual cells showed only increased angiogenin secretion (P < 0.05). In the first trimester, T3 reduced IL-10 secretion by total decidual cells (P < 0.05), and reduced granulocyte macrophage colony stimulating factor (P < 0.01), IL-8 (P < 0.05), IL-10 (P < 0.01), IL-1β (P < 0.05) and monocyte chemotactic protein -1 (P < 0.001) secretion by macrophages, but increased tumour necrosis factor-α secretion by stromal-depleted cells (P < 0.05) and increased IL-6 by uNK cells (P < 0.05). In contrast, in the second trimester T3 increased IL-10 secretion by total decidual cells (P < 0.01) but did not affect cytokine secretion by uNK cells and macrophages. Conditioned media from first trimester T3-treated total decidual cells and macrophages did not alter EVT invasion compared with untreated controls. Thus, treatment of decidual cells with T3 resulted in changes in both angiogenic growth factor and cytokine secretion in a cell type-specific and gestational age-dependent manner, with first trimester decidual macrophages being the most responsive to T3 treatment, but these changes in decidual cell secretome did not affect EVT invasion in vitro.

LIMITATIONS, REASONS FOR CAUTION

Our results are based on in vitro findings and we cannot be certain if a similar response occurs in human pregnancy in vivo.

WIDER IMPLICATIONS OF THE FINDINGS

Optimal maternal thyroid hormone concentrations could play a critical role in maintaining a balanced inflammatory response in early pregnancy to prevent fetal immune rejection and promote normal placental development through the regulation of the secretion of critical cytokines and angiogenic growth factors by human decidual cells. Our data suggest that there is an ontogenically determined regulatory ‘switch’ in T3 responsiveness between the first and second trimesters, and support the notion that the timely and early correction of maternal thyroid dysfunction is critical in influencing pregnancy outcomes.

STUDY FUNDING/COMPETING INTEREST(S)

This study is funded by Wellbeing of Women (RG/1082/09 to S.Y.C., M.D.K., J.A.F., L.S.L., G.E.L.) and Action Medical Research – Henry Smith Charity (SP4335 to M.D.K., S.Y.C., L.S.L., J.A.F.). The authors have no conflicts of interest to disclose.

Deciphering direct and indirect influence of thyroid hormone with mouse genetics

T3, the active form of thyroid hormone, binds nuclear receptors that regulate the transcription of a large number of genes in many cell types. Unraveling the direct and indirect effect of this hormonal stimulation, and establishing links between these molecular events and the developmental and physiological functions of the hormone, is a major challenge. New mouse genetics tools, notably those based on Cre/loxP technology, are suitable to perform a multiscale analysis of T3 signaling and achieve this task.

MCT8 expression in human fetal cerebral cortex is reduced in severe intrauterine growth restriction

The importance of the thyroid hormone (TH) transporter, monocarboxylate transporter 8 (MCT8), to human neurodevelopment is highlighted by findings of severe global neurological impairment in subjects with MCT8 (SLC16A2) mutations. Intrauterine growth restriction (IUGR), usually due to uteroplacental failure, is associated with milder neurodevelopmental deficits, which have been partly attributed to dysregulated TH action in utero secondary to reduced circulating fetal TH concentrations and decreased cerebral thyroid hormone receptor expression. We postulate that altered MCT8 expression is implicated in this pathophysiology; therefore, in this study, we sought to quantify changes in cortical MCT8 expression with IUGR. First, MCT8 immunohistochemistry was performed on occipital and parietal cerebral cortex sections obtained from appropriately grown for gestational age (AGA) human fetuses between 19 weeks of gestation and term. Secondly, MCT8 immunostaining in the occipital cortex of stillborn IUGR human fetuses at 24-28 weeks of gestation was objectively compared with that in the occipital cortex of gestationally matched AGA fetuses. Fetuses demonstrated widespread MCT8 expression in neurons within the cortical plate and subplate, in the ventricular and subventricular zones, in the epithelium of the choroid plexus and ependyma, and in microvessel wall. When complicated by IUGR, fetuses showed a significant fivefold reduction in the percentage area of cortical plate immunostained for MCT8 compared with AGA fetuses (P<0.05), but there was no significant difference in the proportion of subplate microvessels immunostained. Cortical MCT8 expression was negatively correlated with the severity of IUGR indicated by the brain:liver weight ratios (r(2)=0.28; P<0.05) at post-mortem. Our results support the hypothesis that a reduction in MCT8 expression in the IUGR fetal brain could further compromise TH-dependent brain development.