Hong Wa Yung

Over a quarter of human pregnancies are associated with complications, including fetal growth restriction, pre-eclampsia and gestational diabetes. These are major causes of maternal and fetal morbidity and mortality, and also lead to a 3–5-fold increased risk of subsequent development of cardio-metabolic diseases. Although the mechanistic details remain elusive, a dysfunctional placenta is central to the pathophysiology of these conditions. The placenta ensures sufficient nutrient supply to the fetus without compromising maternal wellbeing. This balance is achieved by the secretion of large quantities of placental-derived peptide hormones into the maternal circulation. Consequently, the placenta is susceptible to endoplasmic reticulum (ER) stress, and we were the first to demonstrate the presence of ER stress in placentas from complicated pregnancies. The mouse placenta provides an ideal model for studying the impact of ER stress as it is composed of two distinct regions, an endocrine zone and a transport zone. Therefore, perturbation of placental endocrine function by ER stress can be generated without directly affecting its capacity for nutrient exchange. In this review, we summarise the current literature on how transgenic mouse models enhance our understanding of ER stress-mediated perturbation of placental endocrine function, and its contribution to the pathophysiology of pregnancy complications and life-long health.

Zika virus (ZIKV) is an emerging flavivirus primarily transmitted by Aedes mosquitoes. It has gained significant attention due to its potential teratogenic effects during pregnancy, particularly the increased risk of severe congenital outcomes such as microcephaly and fetal mortality. ZIKV's ability to breach the fetal-maternal interface and compromise placental function distinguishes it from other arboviruses. This mini-review explores how ZIKV targets key placental cells, including cytotrophoblasts, extravillous trophoblasts, syncytiotrophoblasts, and Hofbauer cells, to establish infection in the placenta. We first explore the molecular mechanisms of ZIKV replication, focusing on viral entry, replication strategies, and evasion of host immunity in trophoblasts. We then discuss how ZIKV infection triggers cellular stress responses, such as oxidative stress, endoplasmic reticulum stress, and inflammatory pathways, along with metabolic reprogramming. These processes ultimately lead to placental insufficiency and adverse pregnancy outcomes. Drawing on recent findings from trophoblast organoids and in vivo models, we highlight novel therapeutic opportunities, including vasoactive intestinal peptide, endoplasmic reticulum stress modulators, and immunomodulatory interventions that may protect pregnancy. Finally, we propose future directions for mitigating ZIKV's impact on maternal-fetal health, emphasizing the need for comprehensive research into viral pathogenesis, advanced model systems, and vector control programs to prevent this devastating infection.

The mechanisms that drive placental dysfunction in pregnancies complicated by hypoxia and fetal growth restriction remain poorly understood. Changes to mitochondrial respiration contribute to cellular dysfunction in conditions of hypoxia and have been implicated in the pathoaetiology of pregnancy complications, such as pre‐eclampsia. We used bespoke isobaric hypoxic chambers and a combination of functional, molecular and imaging techniques to study cellular metabolism and mitochondrial dynamics in sheep undergoing hypoxic pregnancy. We show that hypoxic pregnancy in sheep triggers a shift in capacity away from β‐oxidation and complex I‐mediated respiration, while maintaining total oxidative phosphorylation capacity. There are also complex‐specific changes to electron transport chain composition and a switch in mitochondrial dynamics towards fission. Hypoxic placentas show increased activation of the non‐canonical mitochondrial unfolded protein response pathway and enhanced insulin like growth factor 2 signalling. Combined, therefore, the data show that the hypoxic placenta undergoes significant metabolic and morphological adaptations to maintain cellular energy balance. Chronic hypoxia during pregnancy in sheep activated placental mitochondrial stress pathways, leading to alterations in mitochondrial respiration, mitochondrial energy metabolism and mitochondrial dynamics, as seen in the placenta of women with pre‐eclampsia. image Key points: Hypoxia shifts mitochondrial respiration away from β‐oxidation and complex I. Complex‐specific changes occur in the electron transport chain composition. Activation of the non‐canonical mitochondrial unfolded protein response pathway is heightened in hypoxic placentas. Enhanced insulin like growth factor 2 signalling is observed in hypoxic placentas. Hypoxic placentas undergo significant functional adaptations for energy balance.

BACKGROUND: The chronic hypoxia of high-altitude residence poses challenges for tissue oxygen supply and metabolism. Exposure to high altitude during pregnancy increases the incidence of hypertensive disorders of pregnancy and fetal growth restriction and alters placental metabolism. High-altitude ancestry protects against altitude-associated fetal growth restriction, indicating hypoxia tolerance that is genetic in nature. Yet, not all babies are protected and placental pathologies associated with fetal growth restriction occur in some Andean highlanders. METHODS: We examined placental metabolic function in 79 Andeans (18–45 years; 39 preeclamptic and 40 normotensive) living in La Paz, Bolivia (3600–4100 m) delivered by unlabored Cesarean section. Using a selection-nominated approach, we examined links between putatively adaptive genetic variation and phenotypes related to oxygen delivery or placental metabolism. RESULTS: Mitochondrial oxidative capacity was associated with fetal oxygen delivery in normotensive but not preeclamptic placenta and was also suppressed in term preeclamptic pregnancy. Maternal haplotypes in or within 200 kb of selection-nominated genes were associated with lower placental mitochondrial respiratory capacity ( PTPRD [protein tyrosine phosphatase receptor-δ]), lower maternal plasma erythropoietin ( CPT2 [carnitine palmitoyl transferase 2], proopiomelanocortin, and DNMT3 [DNA methyltransferase 3]), and lower VEGF (vascular endothelial growth factor) in umbilical venous plasma ( TBX5 [T-box transcription factor 5]). A fetal haplotype within 200 kb of CPT2 was associated with increased placental mitochondrial complex II capacity, placental nitrotyrosine, and GLUT4 (glucose transporter type 4) protein expression. CONCLUSIONS: Our findings reveal novel associations between putatively adaptive gene regions and phenotypes linked to oxygen delivery and placental metabolic function in highland Andeans, suggesting that such effects may be of genetic origin. Our findings also demonstrate maladaptive metabolic mechanisms in the context of preeclampsia, including dysregulation of placental oxygen consumption.

Hyperhomocysteinemia may arise from folate/vitamin B-12 deficiency, genetic polymorphisms, kidney disease, or hypothyroidism. It is associated with an increased risk of early pregnancy loss and placenta-related complications of pregnancy, including pre-eclampsia and fetal growth restriction. While the majority of studies of hyperhomocysteinemia focus on epigenetic changes secondary to metabolic disruption, the effects of homocysteine toxicity on placental development remain unexplored. Here, we investigated the influence of hyperhomocysteinemia on early blastocyst development and trophoblast differentiation. Exposure of cultured blastocysts to high homocysteine levels reduces cell number in the trophectoderm layer, most likely through increased apoptosis. Homocysteine also promotes differentiation of a trophoblast stem cell line. Both effects diminish the stem cell pool, and are mediated in an endoplasmic reticulum (ER) unfolded protein response (UPRER)-dependent manner. Targeted alleviation of UPRER may therefore provide a new therapeutic intervention to improve pregnancy outcome in women with hyperhomocysteinemia.

Key points: Endoplasmic reticulum (ER) stress promotes placental dysmorphogenesis and is associated with poor pregnancy outcomes. We show that unfolded protein response signalling pathways located in the ER drive differentiation of mouse trophoblast stem cells into trophoblast subtypes involved in development of the placental labyrinth zone and trophoblast invasion. In a mouse model of chronic ER stress (Eif2s1^tm1RjK), higher ER stress in homozygous blastocysts is accompanied by reduced trophectoderm cell number and developmental delay and also is associated with an increased incidence of early pregnancy loss. Administration of the chemical chaperone, tauroursodeoxycholic acid, to Eif2s1^+/tm1RjK heterozygous females during pregnancy alleviated ER stress in the mutant placenta, restored normal trophoblast populations and reduced the frequency of early pregnancy loss. Our results suggest that alleviation of intrauterine ER stress could provide a potential therapeutic target to improve pregnancy outcome in women with pre‐gestational metabolic or gynaecological conditions. Women with pre‐gestational health conditions (e.g. obesity, diabetes) or gynaecological problems (e.g. endometriosis) are at increased risk of adverse pregnancy outcomes including miscarriage, pre‐eclampsia and fetal growth restriction. Increasing evidence suggests that unfavourable intrauterine conditions leading to poor implantation and/or defective placentation are a possible causative factor. The endoplasmic reticulum (ER) unfolded protein response (UPR^ER) signalling pathways are a convergence point of various physiological stress stimuli that can be triggered by an unfavourable intrauterine environment. Therefore, we explored the impact of ER stress on mouse trophoblast differentiation in vitro, mouse blastocyst formation and early placenta development in the Eif2s1^tm1RjK mutant mouse model of chronic ER stress. Chemically‐manipulated ER stress or activation of UPR^ER pathways in a mouse trophoblast stem cell line promoted lineage‐specific differentiation. Co‐treatment with specific UPR^ER pathway inhibitors rescued this effect. Although the inner cell mass was unaffected, the trophectoderm of homozygous Eif2s1^tm1RjK blastocysts exhibited ER stress associated with a reduced cell number. Furthermore, one‐third of Eif2s1^tm1RjK homozygous blastocysts exhibited severe developmental defects. We have previously reported a reduced trophoblast population and premature trophoblast differentiation in Eif2s1^tm1RjK homozygous placentas at mid‐gestation. Here, we demonstrate that treatment of Eif2s1^+/tm1RjK heterozygous pregnant females with the chemical chaperone tauroursodeoxycholic acid alleviated ER stress, restored the trophoblast population and reduced the frequency of embryonic lethality. Our data suggest that therapeutic targeting of ER stress may improve pregnancy outcome in women with pre‐gestational metabolic or gynaecological conditions.

The mechanism behind transgenerational epigenetic inheritance (TEI) is unclear, particularly through the maternal grandparental line. We previously showed that disruption of folate metabolism in mice by the Mtrr ^gt hypomorphic mutation results in TEI of congenital malformations. Either maternal grandparent can initiate this phenomenon, which persists for at least four wildtype generations. In this work, we use a genome-wide approach to reveal genetic stability in the Mtrr ^gt model and epigenome-wide differential DNA methylation in the germline of Mtrr ^+/gt maternal grandfathers. While epigenetic reprogramming occurs, wildtype grandprogeny and great grandprogeny exhibit transcriptional memory of germline methylation defects. One region encompasses the Hira gene, which is misexpressed in embryos at least until the F3 generation in a manner that distinguishes Hira transcript expression as a biomarker of maternal phenotypic inheritance.

Significance: Preeclampsia endangers the lives and well-being of mother and baby. The syndrome is associated with placental dysfunction. High demand for energy to support active nutrient transport and hormone production increases placental susceptibility to mitochondrial stress. Here, we investigate mitochondrial activity and explore stress-response pathways in preeclamptic placentas. We demonstrate activation of noncanonical mitochondrial unfolded protein response (UPR ^mt ) pathways associated with reduced CLPP, a key protease in UPR ^mt signalling, that compromises mitochondrial respiration. The changes can be recapitulated in trophoblast cells by hypoxia–reoxygenation. Either activation of UPR ^mt or knockdown of CLPP can sufficiently reduce mitochondrial respiration. Translation of CLPP is negatively regulated by the endoplasmic reticulum UPR pathway. Understanding mitochondrial stress provides new insights into the pathophysiology of early-onset preeclampsia.