WNT and NOTCH signaling in human trophoblast development and differentiation

Abstract

Correct development of the human placenta and its differentiated epithelial cells, syncytial trophoblasts (STBs) and extravillous trophoblasts (EVTs), is crucial for a successful pregnancy outcome. STBs develop by cell fusion of mononuclear cytotrophoblasts (CTBs) in placental floating villi, whereas migratory EVTs originate from specialized villi anchoring to the maternal decidua. Defects in trophoblast differentiation have been associated with severe pregnancy disorders such as early-onset preeclampsia and fetal growth restriction. However, the evolutionary pathways underlying normal and adverse placentation are poorly understood. Herein, we discuss Wingless (WNT) and NOTCH signaling, two pathways that play pivotal roles in human placenta and trophoblast development. Whereas WNT is necessary for expansion of trophoblast progenitors and stem cells, NOTCH1 is required for proliferation and survival of EVT precursors. Differentiation of the latter is orchestrated by a switch in NOTCH receptor expression as well as by changes in WNT ligands and their downstream effectors.

Keywords: NOTCH signaling; Placental development; Trophoblast differentiation; Trophoblast stem cells; WNT signaling.

Fig. 1

Key components of the canonical WNT signaling pathway. a Posttranslational modification and secretion of WNT ligands. Porcupine conjugates palmitoleate residues to WNTs, whereas NOTUM removes them. b ON-and OFF-state of canonical WNT signaling. In the absence of WNT ligands β-catenin is kept in the degradosome, while WNT activation provokes formation of a signalosome at the membrane, elevation of cytoplasmic β-catenin levels, its nuclear recruitment and formation of the β-catenin-dependent TCF/LEF enhanceosome. APC adenomatous polyposis coli, BCL-9 B-cell lymphoma 9, BRG1 Brahma-related gene-1, CBP CREB-binding protein, CK1 casein kinase 1, CtBP C-terminal-binding protein, DKK Dickkopf, DVL Dishevelled, HDAC histone deacetylase, GSK3 glycogen synthase kinase 3, LEF lymphoid enhancer-binding factor, LRP5/6 low density lipoprotein receptor-related protein 5/6, sFRP secreted Frizzled-related protein, Pygo Pygopus, TCF T-cell factor, TLE transducin-like Enhancer of split, WNT Wingless, WIF WNT inhibitory factor

Fig. 2

Schematic depiction of the canonical NOTCH signaling pathway. Interaction of membrane-bound NOTCH ligand with the NOTCH extracellular domain (NECD) of the dimeric NOTCH receptor provokes two sequential cleavage steps in the NOTCH transmembrane and intracellular domain (NTMICD) performed by ADAM proteins (NEXT) and γ-secretase (NICD). As a consequence, NOTCH intracellular domain (NICD) is released from membranes and translocates into the nucleus. NICD binds and converts the transcriptional repressor recombination signal binding protein for immunoglobulin kappa J (RBPJκ) into an activator. ADAM a disintegrin and metalloproteinase, BRG1 Brahma-related gene-1, CIR CBF-interacting co-repressor, MAML Mastermind-like, NEXT NOTCH extracellular truncation, PCAF CBP-associated factor, SKIP Ski-interacting protein, SMRT silencing mediator of retinoic acid and thyroid hormone receptor

Fig. 3

mRNA expression patterns of WNTs, FZDs, NOTCH ligands and receptors in the different cell types of first trimester placental villi. a Distribution of transcripts encoding WNTs and FZDs in trophoblast organoids (TB-ORG), villous cytotrophoblast (vCTB), syncytiotrophoblast (STB), placental fibroblast (pF) and placental macrophage (pM). b Expression of NOTCH ligands and receptors in the above mentioned cell types. Font sizes and colors indicate relative expression levels. Blue: low; green: medium; red: high

Fig. 4

mRNA expression patterns of WNTs, FZDs, NOTCH ligands and receptors in the different cell types of first trimester anchoring villus and decidua. a Distribution of transcripts encoding WNTs and FZDs in placental EVT (pEVT), decidual interstitial EVT (iEVT), decidual fibroblast (dF) and decidual macrophage (dM). b Expression of NOTCH ligands and receptors in the above mentioned cell types. Font sizes and colors indicate relative expression levels. Blue: low; green: medium; red: high

Fig. 5

The role of WNT and NOTCH signaling in human trophoblast development and differentiation. Black arrows indicate autocrine (circular arrow) and paracrine (straight arrow) activation of progenitor expansion and differentiation, based on functional analyses in relevant primary trophoblast models. Red arrows depict possible interactions between cell types that still require functional evidence. WNT and NOTCH signaling components depicted in black indicate genes that have been experimentally tested in primary CTBs/EVTs, villous explant cultures, TSCs or TB-ORGs. Others, shown in red, illustrate receptors, ligands and downstream effectors that have only been analyzed at level of gene expression (protein and/or mRNA). SA spiral artery