Placental hypoxia: What have we learnt from small animal models?

Assessment of feto-placental oxygenation and perfusion in a rat model of placental insufficiency using T2* mapping and 3D dynamic contrast-enhanced MRI

INTRODUCTION

Placental insufficiency may lead to preeclampsia and fetal growth restriction. There is no cure for placental insufficiency, emphasizing the need for monitoring fetal and placenta health. Current monitoring methods are limited, underscoring the necessity for imaging techniques to evaluate fetal-placental perfusion and oxygenation. This study aims to use MRI to evaluate placental oxygenation and perfusion in the reduced uterine perfusion pressure (RUPP) model of placental insufficiency.

METHODS

Pregnant rats were randomized to RUPP (n = 11) or sham surgery (n = 8) on gestational day 14. On gestational day 19, rats imaged using a 7T MRI scanner to assess oxygenation and perfusion using T2* mapping and 3D-DCE MRI sequences, respectively. The effect of the RUPP on the feto-placental units were analyzed from the MRI images.

RESULTS

RUPP surgery led to reduced oxygenation in the labyrinth (24.7 ± 1.8 ms vs. 28.0 ± 2.1 ms, P = 0.002) and junctional zone (7.0 ± 0.9 ms vs. 8.1 ± 1.1 ms, P = 0.04) of the placenta, as indicated by decreased T2* values. However, here were no significant differences in fetal organ oxygenation or placental perfusion between RUPP and sham animals.

DISCUSSION

The reduced placental oxygenation without a corresponding decrease in perfusion suggests an adaptive response to placental ischemia. While acute reduction in placental perfusion may cause placental hypoxia, persistence of this condition could indicate chronic placental insufficiency after ischemic reperfusion injury. Thus, placental oxygenation may be a more reliable biomarker for assessing fetal condition than perfusion in hypertensive disorders of pregnancies including preeclampsia and FGR.

Smoking during pregnancy and its effect on placental weight: a Mendelian randomization study

BACKGROUND

The causal relationship between maternal smoking in pregnancy and reduced offspring birth weight is well established and is likely due to impaired placental function. However, observational studies have given conflicting results on the association between smoking and placental weight. We aimed to estimate the causal effect of newly pregnant mothers quitting smoking on their placental weight at the time of delivery.

METHODS

We used one-sample Mendelian randomization, drawing data from the Avon Longitudinal Study of Parents and Children (ALSPAC) (N = 690 to 804) and the Norwegian Mother, Father and Child Cohort Study (MoBa) (N = 4267 to 4606). The sample size depends on the smoking definition used for different analyses. The analysis was performed in pre-pregnancy smokers only, due to the specific role of the single-nucleotide polymorphism (SNP) rs1051730 (CHRNA5 - CHRNA3 - CHRNB4) in affecting smoking cessation but not initiation.

RESULTS

Fixed effect meta-analysis showed a 182 g [95%CI: 29,335] higher placental weight for pre-pregnancy smoking mothers who continued smoking at the beginning of pregnancy, compared with those who stopped smoking. Using the number of cigarettes smoked per day in the first trimester as the exposure, the causal effect on placental weight was 11 g [95%CI: 1,21] per cigarette per day. Similarly, smoking at the end of pregnancy was causally associated with higher placental weight. Using the residuals of birth weight regressed on placental weight as the outcome, we showed evidence of lower offspring birth weight relative to the placental weight, both for continuing smoking at the start of pregnancy as well as continuing smoking throughout pregnancy (change in z-score birth weight adjusted for z-score placental weight: -0.8 [95%CI: -1.6,-0.1]).

CONCLUSION

Our results suggest that continued smoking during pregnancy causes higher placental weights.

Gestational intermittent hypoxia induces endothelial dysfunction and hypertension in pregnant rats: role of endothelin type B receptor†

Obstructive sleep apnea is a recognized risk factor for gestational hypertension, yet the exact mechanism behind this association remains unclear. Here, we tested the hypothesis that intermittent hypoxia, a hallmark of obstructive sleep apnea, induces gestational hypertension through perturbed endothelin-1 signaling. Pregnant Sprague-Dawley rats were subjected to normoxia (control), mild intermittent hypoxia (10.5% O2), or severe intermittent hypoxia (6.5% O2) from gestational days 10-21. Blood pressure was monitored. Plasma was collected and mesenteric arteries were isolated for myograph and protein analyses. The mild and severe intermittent hypoxia groups demonstrated elevated blood pressure, reduced plasma nitrate/nitrite, and unchanged endothelin-1 levels compared to the control group. Western blot analysis revealed decreased expression of endothelin type B receptor and phosphorylated endothelial nitric oxide synthase, while the levels of endothelin type A receptor and total endothelial nitric oxide synthase remained unchanged following intermittent hypoxia exposure. The contractile responses to potassium chloride, phenylephrine, and endothelin-1 were unaffected in endothelium-denuded arteries from mild and severe intermittent hypoxia rats. However, mild and severe intermittent hypoxia rats exhibited impaired endothelium-dependent vasorelaxation responses to endothelin type B receptor agonist IRL-1620 and acetylcholine compared to controls. Endothelium denudation abolished IRL-1620-induced vasorelaxation, supporting the involvement of endothelium in endothelin type B receptor-mediated relaxation. Treatment with IRL-1620 during intermittent hypoxia exposure significantly attenuated intermittent hypoxia-induced hypertension in pregnant rats. This was associated with elevated circulating nitrate/nitrite levels, enhanced endothelin type B receptor expression, increased endothelial nitric oxide synthase activation, and improved vasodilation responses. Our data suggested that intermittent hypoxia exposure during gestation increases blood pressure in pregnant rats by suppressing endothelin type B receptor-mediated signaling, providing a molecular mechanism linking intermittent hypoxia and gestational hypertension.

Smoking during pregnancy and its effect on placental weight: A Mendelian randomization study

Background

The causal relationship between maternal smoking in pregnancy and reduced offspring birth weight is well established and is likely due to impaired placental function. However, observational studies have given conflicting results on the association between smoking and placental weight. We aimed to estimate the causal effect of newly pregnant mothers quitting smoking on their placental weight at the time of delivery.

Methods

We used one-sample Mendelian randomization, drawing data from the Avon Longitudinal Study of Parents and Children (ALSPAC) (up to N = 805) and the Norwegian Mother, Father and Child Cohort Study (MoBa) (up to N = 4475). The analysis was performed in pre-pregnancy smokers only, due to the specific role of the genetic instrument SNP rs1051730 (CHRNA5 - CHRNA3 - CHRNB4) in affecting smoking cessation but not initiation.

Results

Fixed effect meta-analysis showed a 175 g [95%CI: 16, 334] higher placental weight for pre-pregnancy smoking mothers who continued smoking at the beginning of pregnancy, compared with those who stopped smoking. Using the number of cigarettes smoked per day in the first trimester as the exposure, the causal estimate was a 12 g [95%CI: 2,22] higher placental weight per cigarette per day. Results were similar when the smoking exposures were measured at the end of pregnancy. Using the residuals of birth weight regressed on placental weight as the outcome, we showed weak evidence of lower offspring birth weight relative to the placental weight for continuing smoking.

Conclusion

Our results suggest that continued smoking during pregnancy causes higher placental weights.

From Molecules to Imaging: Assessment of Placental Hypoxia Biomarkers in Placental Insufficiency Syndromes

Placental hypoxia poses significant risks to both the developing fetus and the mother during pregnancy, underscoring the importance of early detection and monitoring. Effectively identifying placental hypoxia and evaluating the deterioration in placental function requires reliable biomarkers. Molecular biomarkers in placental tissue can only be determined post-delivery and while maternal blood biomarkers can be measured over time, they can merely serve as proxies for placental function. Therefore, there is an increasing demand for non-invasive imaging techniques capable of directly assessing the placental condition over time. Recent advancements in imaging technologies, including photoacoustic and magnetic resonance imaging, offer promising tools for detecting and monitoring placental hypoxia. Integrating molecular and imaging biomarkers may revolutionize the detection and monitoring of placental hypoxia, improving pregnancy outcomes and reducing long-term health complications. This review describes current research on molecular and imaging biomarkers of placental hypoxia both in human and animal studies and aims to explore the benefits of an integrated approach throughout gestation.

The effects of COVID-19 on placental morphology

The impact of the COVID-19 infection, caused by Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), during the pandemic has been considerably more severe in pregnant women than non-pregnant women. Therefore, a review detailing the morphological alterations and physiological changes associated with COVID-19 during pregnancy and the effect that these changes have on the feto-placental unit is of high priority. This knowledge is crucial for these mothers, their babies and clinicians to ensure a healthy life post-pandemic. Hence, we review the placental morphological changes due to COVID-19 to enhance the general understanding of how pregnant mothers, their placentas and unborn children may have been affected by this pandemic. Based on current literature, we deduced that COVID-19 pregnancies were oxygen deficient, which could further result in other pregnancy-related complications like preeclampsia and IUGR. Therefore, we present an up-to-date review of the COVID-19 pathophysiological implications on the placenta, covering the function of the placenta in COVID-19, the effects of this virus on the placenta, its functions and its link to other gestational complications. Furthermore, we highlight the possible effects of COVID-19 therapeutic interventions on pregnant mothers and their unborn children. Based on the literature, we strongly suggest that consistent surveillance for the mothers and infants from COVID-19 pregnancies be prioritised in the future. Though the pandemic is now in the past, its effects are long-term, necessitating the monitoring of clinical manifestations in the near future.

Maternal and Fetal PI3K-p110α Deficiency Induces Sex-Specific Changes in Conceptus Growth and Placental Mitochondrial Bioenergetic Reserve in Mice

Fetal growth is reliant on placental formation and function, which, in turn, requires the energy produced by the mitochondria. Prior work has shown that both mother and fetus operate via the phosphoinositol 3-kinase (PI3K)-p110α signalling pathway to modify placental development, function, and fetal growth outcomes. This study in mice used genetic inactivation of PI3K-p110α (α/+) in mothers and fetuses and high resolution respirometry to investigate the influence of maternal and fetal PI3K-p110α deficiency on fetal and placental growth, in relation to placental mitochondrial bioenergetics, for each fetal sex. The effect of PI3K-p110α deficiency on maternal body composition was also determined to understand more about the maternal-driven changes in feto-placental development. These data show that male fetuses were more sensitive than females to fetal PI3K-p110α deficiency, as they had greater reductions in fetal and placental weight, when compared to their WT littermates. Placental weight was also altered in males only of α/+ dams. In addition, α/+ male, but not female, fetuses showed an increase in mitochondrial reserve capacity, when compared to their WT littermates in α/+ dams. Finally, α/+ dams exhibited reduced adipose depot masses, compared to wild-type dams. These findings, thus, demonstrate that maternal nutrient reserves and ability to apportion nutrients to the fetus are reduced in α/+ dams. Moreover, maternal and fetal PI3K-p110α deficiency impacts conceptus growth and placental mitochondrial bioenergetic function, in a manner dependent on fetal sex.

Prenatal interventions for fetal growth restriction in animal models: A systematic review

Fetal growth restriction (FGR) in human pregnancy is associated with perinatal mortality, short- and long-term morbidities. No prenatal therapy is currently established despite decades of research. We aimed to review interventions in animal models for prenatal FGR treatment, and to seek the next steps for an effective clinical therapy. We registered our protocol and searched MEDLINE, Embase, and The Cochrane Library with no language restrictions, in accordance with the PRISMA guideline. We included all studies that reported the effects of any prenatal intervention in animal models of induced FGR. From 3257 screened studies, 202 describing 237 interventions were included for the final synthesis. Mice and rats were the most used animals (79%) followed by sheep (16%). Antioxidants (23%), followed by vasodilators (18%), nutrients (14%), and immunomodulators (12%) were the most tested therapy. Two-thirds of studies only reported delivery or immediate neonatal outcomes. Adverse effects were rarely reported (11%). Most studies (73%), independent of the intervention, showed a benefit in fetal survival or birthweight. The risk of bias was high, mostly due to the lack of randomization, allocation concealment, and blinding. Future research should aim to describe both short- and long-term outcomes across various organ systems in well-characterized models. Further efforts must be made to reduce selection, performance, and detection bias.

Hypoxia ameliorates maternal diet-induced insulin resistance during pregnancy while having a detrimental effect on the placenta

Maternal overweight/obesity contributes significantly to the development of gestational diabetes, which causes risks to both mother and fetus and is increasing sharply in prevalence worldwide. Since hypoxia reprograms energy metabolism and can alleviate weight gain, adiposity, insulin resistance (IR), and dyslipidemia, we set out to study the potential of sustained reduced ambient oxygen tension (15% O₂ ) during pregnancy for alleviating the detrimental effects of diet-induced IR in C57Bl/6N mice, taking normal chow-fed and normoxia (21% O₂ ) groups as controls. Our data show that hypoxic intervention reduced maternal weight gain, adiposity, and adipose tissue inflammation, and ameliorated maternal glucose metabolism and IR during gestation in diet-induced IR relative to normoxia. Where diet-induced IR reduced maternal hemoglobin and increased serum erythropoietin levels, hypoxic intervention compensated for these changes. Diet-induced IR reduced fetal growth in normoxia, and even more in hypoxia. Hypoxic intervention reduced liver weight gain during pregnancy in the dams with diet-induced IR, maternal liver weight being positively associated with embryo number. In case of diet-induced IR, the hypoxic intervention compromised placental energy metabolism and vascularization and increased end-pregnancy placental necrosis. Altogether, these data show that although hypoxic intervention mediates several beneficial effects on maternal metabolism, the combination of it with diet-induced IR is even more detrimental to the placental and fetal outcome than diet-induced IR alone.

Effect of Hypoxia on Glucose Transporter 1 and 3 Gene Expression in Placental Mesenchymal Stem Cells Derived from Growth-Restricted Fetuses

(1) Background: Glucose is transferred from maternal blood to the fetus by glucose transporters. What is the effect of hypoxia on the gene expression of placenta glucose transporter 1 (GLUT1) and glucose transporter 3 (GLUT3) in growth-restricted fetus is interesting. (2) Methods: The gene expression of GLUT1 and GLUT3 and the protein expression of HIF-1α were evaluated under nonhypoxic conditions and after 4 and 8 h under hypoxic conditions in placental mesenchymal stem cells derived from monochorionic twin pregnancies with selective intrauterine growth restriction. (3) Results: The gene expressions of GLUT1 and GLUT3 under hypoxia conditions were higher in placental mesenchymal stem cells derived from appropriate-for-gestational-age fetuses than in those from selective intrauterine growth-restricted fetuses. However, the protein expression of hypoxia induced factor-1 α (HIF-1α) at hypoxia condition was not lower in placenta mesenchymal stem cells from selective intrauterine growth-restricted fetuses than in placental mesenchymal stem cells from appropriate-for-gestational-age fetuses. (4) Conclusions: Hypoxia-induced upregulation of GLUT1 and GLUT3 expression was decreased in placental mesenchymal stem cells from selective intrauterine growth-restricted fetuses but not due to decreased HIF-1α expression. Selective growth-restricted fetuses have less capacity for hypoxia-induced upregulation of placental glucose transport.

Are there sex differences in fetal growth strategies and in the long-term effects of pregnancy complications on cognitive functioning?

Males and females have been proposed to have different prenatal growth strategies, whereby males invest more in fetal growth and less in placental development, leaving them more susceptible to early-life adversity. We tested predictions of this hypothesis using data from the National Collaborative Perinatal Project. Male newborns were heavier than females, but there was no difference in placental weight, adjusting for birthweight. Among infants born prior to 33 weeks, the difference in birthweight between males and females was greater among those who did not survive than among those who did, potentially reflecting a strategy whereby males maintained growth in the face of prenatal insults, while females adjusted growth. However, there was no significant difference in mortality between the sexes. Being born small-for-gestational age or very preterm (prior to 33 weeks) was associated with significantly reduced performance for most of the cognitive traits examined at 7 years, although maternal preeclampsia was associated with reduced performance in fewer traits. Generally, these effects of early-life adversity (poor fetal growth, prematurity, and preeclampsia) did not differ between the sexes. However, analyzing the sexes separately (rather than testing the interaction between sex and adversity) resulted in numerous spurious sex-specific effects, whereby the effect of early-life adversity appeared to be significant in one sex but not the other. Overall, we found little support for the hypothesis that males prioritize growth more than females, and that this makes them more susceptible to early-life adversity. Furthermore, our results show that analyzing the sexes separately, rather than testing the adversity by sex interaction, can be highly misleading.

Placental pathology: Pathways leading to or associated with perinatal brain injury in experimental neurology, special issue: Placental mediated mechanisms of perinatal brain injury

Perinatal brain injury is a multifactorial process. In utero placental physiology plays a major role in neuroprotection and the normal development of the fetal central nervous system. Advances in placental pathology have clarified several specific mechanisms of injury and the histologic lesions most strongly associated with them. This review provides an updated summary of the relevant placental anatomy and physiology, the specific placental pathways leading to brain injury, the revised Amsterdam classification system for placental pathology, and the known associations of specific placental lesions with subtypes of adverse neurologic outcomes.