Apoptotic and non-apoptotic roles of caspases in placenta physiology and pathology.

Caspases, a family of cysteine proteases, are pivotal regulators of apoptosis, the tightly controlled cell death process crucial for eliminating excessive or unnecessary cells during development, including placental development. Collecting research has unveiled the multifaceted roles of caspases in the placenta, extending beyond apoptosis. Apart from their involvement in placental tissue remodeling via apoptosis, caspases actively participate in essential regulatory processes, such as trophoblast fusion and differentiation, significantly influencing placental growth and functionality. In addition, growing evidence indicates an elevation in caspase activity under pathological conditions like pre-eclampsia (PE) and intrauterine growth restriction (IUGR), leading to excessive cell death as well as inflammation. Drawing from advancements in caspase research and placental development under both normal and abnormal conditions, we examine the significance of caspases in both cell death (apoptosis) and non-cell death-related processes within the placenta. We also discuss potential therapeutics targeting caspase-related pathways for placenta disorders.

Nuclear Binding Protein 2/Nesfatin-1 Affects Trophoblast Cell Fusion during Placental Development via the EGFR-PLCG1-CAMK4 Pathway.

Previous studies have shown that nuclear binding protein 2 (NUCB2) is expressed in the human placenta and increases with an increase in the syncytialization of trophoblast cells. This study aimed to investigate the role of NUCB2 in the differentiation and fusion of trophectoderm cells. In this study, the expression levels of NUCB2 and E-cadherin in the placentas of rats at different gestation stages were investigated. The results showed that there was an opposite trend between the expression of placental NUCB2 and E-cadherin in rat placentas in different trimesters. When primary human trophoblast (PHT) and BeWo cells were treated with high concentrations of Nesfatin-1, the trophoblast cell syncytialization was significantly inhibited. The effects of NUCB2 knockdown in BeWo cells and Forskolin-induced syncytialization were investigated. These cells showed a significantly decreased cell fusion rate. The mechanism underlying NUCB2-regulated trophoblast cell syncytialization was explored using RNA-Seq and the results indicated that the epidermal growth factor receptor (EGFR)-phospholipase C gamma 1 (PLCG1)-calmodulin-dependent protein kinase IV (CAMK4) pathway might be involved. The results suggested that the placental expression of NUCB2 plays an important role in the fusion of trophoblasts during differentiation via the EGFR-PLCG1-CAMK4 pathway.

27-Hydroxycholesterol inhibits trophoblast fusion during placenta development by activating PI3K/AKT/mTOR signaling pathway.

Trophoblast cell syncytialization is essential for placental and fetal development. Abnormal trophoblast cell fusion leads to pregnancy pathologies, such as preeclampsia (PE), intrauterine growth restriction (IUGR), and miscarriage. 27-hydroxycholesterol (27-OHC) is the most abundant oxysterol in human peripheral blood synthesized by sterol 27-hydroxylase (CYP27A1) and is considered a critical mediator between hypercholesterolemia and a variety of related disorders. Gestational hypercholesterolemia was associated with spontaneous preterm delivery and low birth weight (LBW) in term infants, yet the mechanism is unclear. In this study, two trophoblast cell models and CD-1 mice were used to evaluate the effects of 27-OHC on trophoblast fusion during placenta development. Two different kinds of trophoblast cells received a dosage of 2.5, 5, or 10 uM 27-OHC. Three groups of pregnant mice were randomly assigned: control, full treatment (E0.5-E17.5), or late treatment (E13.5-E17.5). All mice received daily intraperitoneal injections of saline (control group) and 27-OHC (treatment group; 5.5 mg/kg). In vitro experiments, we found that 27-OHC inhibited trophoblast cell fusion in primary human trophoblasts (PHT) and forskolin (FSK)-induced BeWo cells. 27-OHC up-regulated the expression of the PI3K/AKT/mTOR signaling pathway-related proteins. Moreover, the PI3K inhibitor LY294002 rescued the inhibitory effect of 27-OHC. Inhibition of trophoblast cell fusion by 27-OHC was also observed in CD-1 mice. Furthermore, fetal weight and placental efficiency decreased and fetal blood vessel development was inhibited in pregnant mice treated with 27-OHC. This study was the first to prove that 27-OHC inhibits trophoblast cell fusion by Activating PI3K/AKT/mTOR signaling pathway. This study reveals a novel mechanism by which dyslipidemia during pregnancy results in adverse pregnancy outcomes.

Downregulation of p53 drives autophagy during human trophoblast differentiation.

The placental barrier is crucial for the supply of nutrients and oxygen to the developing fetus and is maintained by differentiation and fusion of mononucleated cytotrophoblasts into the syncytiotrophoblast, a process only partially understood. Here transcriptome and pathway analyses during differentiation and fusion of cultured trophoblasts yielded p53 signaling as negative upstream regulator and indicated an upregulation of autophagy-related genes. We further showed p53 mRNA and protein levels decreased during trophoblast differentiation. Reciprocally, autophagic flux increased and cytoplasmic LC3B-GFP puncta became more abundant, indicating enhanced autophagic activity. In line, in human first trimester placenta p53 protein mainly localized to the cytotrophoblast, while autophagy marker LC3B as well as late autophagic compartments were predominantly detectable in the syncytiotrophoblast. Importantly, ectopic overexpression of p53 reduced levels of LC3B-II, supporting a negative regulatory role on autophagy in differentiating trophoblasts. This was also shown in primary trophoblasts and human first trimester placental explants, where pharmacological stabilization of p53 decreased LC3B-II levels. In summary our data suggest that differentiation-dependent downregulation of p53 is a prerequisite for activating autophagy in the syncytiotrophoblast.

Spatiotemporal regulation of cAMP signaling controls the human trophoblast fusion.

During human placentation, mononuclear cytotrophoblasts fuse to form multinucleated syncytia ensuring hormonal production and nutrient exchanges between the maternal and fetal circulation. Syncytial formation is essential for the maintenance of pregnancy and for fetal growth. The cAMP signaling pathway is the major route to trigger trophoblast fusion and its activation results in phosphorylation of specific intracellular target proteins, in transcription of fusogenic genes and assembly of macromolecular protein complexes constituting the fusogenic machinery at the plasma membrane. Specificity in cAMP signaling is ensured by generation of localized pools of cAMP controlled by cAMP phosphodiesterases (PDEs) and by discrete spatial and temporal activation of protein kinase A (PKA) in supramolecular signaling clusters inside the cell organized by A-kinase-anchoring proteins (AKAPs) and by organization of signal termination by protein phosphatases (PPs). Here we present original observations on the available components of the cAMP signaling pathway in the human placenta including PKA, PDE, and PP isoforms as well as AKAPs. We continue to discuss the current knowledge of the spatiotemporal regulation of cAMP signaling triggering trophoblast fusion.