Evolutionary co-option of endogenous retroviruses: syncytins as regulators of placental development and disease

Syncytins are envelope proteins of retroviral origin that have been evolutionarily co-opted to play essential roles in placental biology. Primarily recognized for mediating the fusion of cytotrophoblasts into the syncytiotrophoblast, a multinucleated epithelium, critical for nutrient transport and maternal-fetal immune tolerance. Syncytins also contribute to broader aspects of placental development through diverse mechanistic pathways. This review focuses on the evolutionary origins and functional mechanisms of Syncytin-1 (encoded by HERV-W) and Syncytin-2 (encoded by HERV-FRD) in orchestrating placental morphogenesis and homeostasis. We further discuss the clinical significance of aberrant syncytin expression, which is associated with adverse pregnancy outcomes including preeclampsia, intrauterine growth restriction, gestational diabetes mellitus, and trophoblastic disease. Additionally, we examine how exogenous viral infections, such as cytomegalovirus and SARS-CoV-2, may disrupt syncytin transcriptional regulation and compromise placental integrity. Collectively, syncytins represent a paradigm of evolutionary viral domestication, wherein pathogenic genetic elements have been repurposed into indispensable mediators of human reproduction. Their precise spatiotemporal regulation is paramount for optimal placental function and maternal-fetal health. Future research leveraging advanced omics technologies and placental organoid systems will be instrumental in elucidating underlying molecular mechanisms and developing targeted therapeutic interventions for pregnancy-related disorders.