In the human placenta, antiviral immune responses must be strictly regulated because they could disrupt the mechanisms that maintain maternal–fetal tolerance during pregnancy. If the placenta permits viral transmission, viral infections can lead to adverse pregnancy outcomes (e.g., miscarriage and fetal congenital anomalies) (Racicot and Mor, 2017, Yockey et al., 2020). The human placenta has complex antiviral defense mechanisms to protect the fetus. Villous trophoblasts, including the syncytiotrophoblast (STB) covering the chorionic villi, constitute a barrier to viral infection at the maternal blood–placental interface, and the decidua, a maternal immune-cell-rich layer, is a barrier at the uterine–placental interface (Yockey et al., 2020, Megli and Coyne, 2022). Villous trophoblasts defend against infecting viruses by producing interferons (IFNs) and antiviral microRNAs (miRNAs). IFNs induce the expression of target genes in an autocrine manner, several of which function to suppress virus replication (Bayer et al., 2018, Wells and Coyne, 2018, Ding et al., 2022). Villous trophoblasts constitutively produce antiviral miRNAs, which are derived from chromosome 19 miRNA cluster (C19MC) genes and restrict virus replication (Delorme-Axford et al. 2013).

DROSHA, a class II ribonuclease III (RNase III) nuclease, is a key component of the miRNA biogenesis pathway in vertebrates (Nguyen et al., 2015, Kwon et al., 2016). DROSHA is localized mainly to the nucleus and initiates miRNA biogenesis by cleaving hairpin-containing long pri-miRNAs (> 1000 nucleotides [nt]), thereby generating short hairpin pre-miRNAs (40–160 nt) (Treiber et al. 2019). Aside from its canonical function in the miRNA pathway, DROSHA regulates mRNAs by cleaving hairpin structures, destabilizing them in a miRNA-independent manner (e.g., DGCR8 and NFIB) (Han et al., 2009, Rolando et al., 2016). Upon virus infection, DROSHA translocates from the nucleus into the cytoplasm, where it exerts a miRNA-independent antiviral effect (Aguado et al. 2017).

Although in villous trophoblast DROSHA is devoted to pri-miRNA processing, little is known of its functions other than in miRNA biogenesis. In this study, we performed formaldehyde crosslinking, immunoprecipitation, and sequencing (fCLIP-seq) of trophoblasts in terminal chorionic villi of full-term placenta to identify DROSHA-binding RNAs. In villous trophoblasts in the human placenta, DROSHA may be involved in the metabolism of hairpin-containing RNAs, as well as miRNAs. We also evaluated the in vivo distribution of DROSHA in human villous trophoblasts by immunohistochemistry; DROSHA was abundant in the cytoplasm. We also investigated whether virus infection induces cytoplasmic translocation of DROSHA in BeWo trophoblasts in vitro. In villous trophoblasts, DROSHA may mediate RNA recognition to exert, in conjunction with the IFN and placental miRNA systems, an antiviral effect.