Trophoblast Fusion Deficiency in Placenta-Related Pregnancy Disorders

Trophoblast cell fusion is a vital developmental process that enables the formation of the multinucleated syncytiotrophoblast (STB), which plays a central role in placental function and maternal-fetal exchange. Fusion defects in this lineage are closely associated with pregnancy complications such as preeclampsia (PE) and fetal growth restriction (FGR). Although the fusion of trophoblasts is a highly coordinated event, it involves multiple interdependent steps, including transcriptional programming, membrane remodeling, cytoskeletal rearrangement, metabolic adaptation, and immune regulation. Recent studies have uncovered critical molecular mediators at each of these levels. Transcription factors such as Glial cells missing transcription factor 1(GCM1), Krüppel-like factor 6(KLF6), and Transcription factor EB(TFEB) govern differentiation timing; fusion proteins, including Syncytin-1/2 and Mfn2, facilitate membrane merger; polarity regulators and actin-associated proteins like Par6 and CNN3 organize cytoskeletal architecture; metabolic reprogramming, particularly a shift from oxidative phosphorylation to glycolysis, supplies energy and biosynthetic precursors; and immune modulators such as pregnancy-induced factor 1 and Interleukin-10(IL-10) ensure a permissive environment for fusion at the maternal-fetal interface. Epigenetic mechanisms, including DNA methylation and histone modifications, further fine-tune the expression of fusion-related genes. Alongside mechanistic discoveries, a wide range of experimental models has been developed to investigate trophoblast fusion in vitro. These include traditional monolayer cell lines (e.g., BeWo), primary human trophoblasts, placental explants, trophoblast stem cells, and trophoblast organoids. Each model system provides distinct advantages in recapitulating aspects of syncytialization and placental physiology. Moreover, the integration of multi-omics technologies-such as single-cell and spatial transcriptomics, proteomics, metabolomics, and epigenomics-has expanded our understanding of the spatiotemporal dynamics and molecular complexity underlying trophoblast fusion. Despite these advances, several key challenges remain unresolved, including the lack of models that fully recapitulate the structural and functional features of the human maternal-fetal interface and the limited understanding of posttranslational and spatiotemporal regulatory mechanisms. Addressing these gaps will be essential for translating basic insights into diagnostic and therapeutic innovations for placental diseases.