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Background: Remdesivir is an antiviral drug approved for the treatment of COVID‐19, whose developmental toxicity remains unclear. More information about the safety of remdesivir is urgently needed for people of childbearing potential, who are affected by the ongoing pandemic. Morphogenetic embryoid bodies (MEBs) are three‐dimensional (3D) aggregates of pluripotent stem cells that recapitulate embryonic body patterning in vitro, and have been used as effective embryo models to detect the developmental toxicity of chemical exposures specifically and sensitively. Methods: MEBs were generated from mouse P19C5 and human H9 pluripotent stem cells, and used to examine the effects of remdesivir. The morphological effects were assessed by analyzing the morphometric parameters of MEBs after exposure to varying concentrations of remdesivir. The molecular impact of remdesivir was evaluated by measuring the transcript levels of developmental regulator genes. Results: The mouse MEB morphogenesis was impaired by remdesivir at 1–8 μM. Remdesivir affected MEBs in a manner dependent on metabolic conversion, and its potency was higher than GS‐441524 and GS‐621763, presumptive anti‐COVID‐19 drugs that act similarly to remdesivir. The expressions of developmental regulator genes, particularly those involved in axial and somite patterning, were dysregulated by remdesivir. The early stage of MEB development was more vulnerable to remdesivir exposure than the later stage. The morphogenesis and gene expression profiles of human MEBs were also impaired by remdesivir at 1–8 μM. Conclusions: Remdesivir impaired mouse and human MEBs at concentrations that are comparable to the therapeutic plasma levels in humans, urging further investigation into the potential impact of remdesivir on developing embryos. [ABSTRACT FROM AUTHOR]

Embryonic cell specification and pattern formation in the ascidian embryo are controlled by prelocalized egg cytoplasmic determinants. In previous studies, we showed that overexpression of a maternal gene, posterior end mark (pem), whose transcript localizes to posterior-vegetal cytoplasm of the fertilized egg, causes a loss of the anterior and dorsal structures of the larva (Yoshida et al., Development 122, 2005-2012, 1996). In the present study, first we observed that lithium treatment resulted in reduction of the larval tail. Lineage tracing analyses revealed that descendants of the A4.1 blastomere of the 8-cell-stage embryo (which forms the greater part of notochord and nerve cord) were missing from the tail region, that they were translocated anteriorly into the trunk region, and that the fate of the A4.1-line notochord cells had changed to endoderm. These results suggest that lithium treatment affects the trunk-tail patterning during embryogenesis by changing the cell fate of specific cell lineages. Second, we showed that lithium treatment could rescue the anterior and dorsal structures in pem-overexpressed larvae. This result suggests that peru plays a role in the patterning of the ascidian embryo via a signaling cascade that is affected by lithium. Third, we isolated an ascidian [Beta]-catenin gene and found that overexpression of [Beta]-catenin in the A4.1 blastomere had effects very similar to lithium treatment, such as reduction of the tail and anterior translocation of A4.1 descendants. These results suggest that the target of lithium is, at least in part, the Wnt-signaling cascade and that peru may also function via this cascade. Key Words: ascidian; lithium; trunk-tail patterning; peru; localized maternal factor; [Beta]-catenin.

The Xenopus egg contains maternal dorsal determinants that are specifically located at the vegetal cortex. To study physical and functional properties of the dorsal determinants, we took advantage of the animal-vegetal reversed embryo. The animal-vegetal reversed embryo is produced by inversion of the fertilized egg, which results in formation of ectoderm and endoderm from the unpigmented and the pigmented halves, respectively [Neff et al. (1983). Dev. Biol. 97, 103-112; Black and Gerhart (1985). Dev. Biol. 108, 310-324]. We demonstrated by cytoplasmic transplantation that the dorsal activity was specifically localized to the unpigmented cortical cytoplasm of the inverted egg, which is segregated into the future ectodermal lineage. This result suggests that the dorsal determinants are associated with the unpigmented cortex and are not dislodged by the inversion. In addition, we found that two vegetally localized transcripts, Xcat2 and Vg1 mRNAs, were present in the reversed animal pole of the inverted egg, suggesting their association with the unpigmented cortex. In order to compare the dorsal determinant activity with known dorsalizing molecules, we examined the expression pattern of Xnr3 and Siamois in the reversed embryo because these two genes are activated by the Wnt.pathway activators (Xwnt-8, [Beta]-catenin, etc.) but not by other dorsalizing molecules (noggin, BVg1, etc.). Animal cap of the reversed embryo, which received the unpigmented cortex of the egg, expressed Xnr3 and Siamois. However, Mix.1, a marker expressed in endoderm and mesoderm in the normal embryo in response to mesodermal inducers, was not detected in the animal cap of the reversed embryo. In addition, we found that [Beta]-catenin protein accumulated in nuclei of unpigmented animal pole cells of the reversed embryo. These results suggest that the maternal dorsal determinants behave more similarly to the Wnt-pathway activators than noggin or BVg1.

cDNA clones of two epidermis-specific genes HrEpiA and HrEpiB are isolated from ascidian Halocynthia roretzi with the genes exhibiting zygotic expression. All embryonic cells derived from epidermal cells expressing blastomeres encode the two specific genes. Cleavage-repressed one-celled embryos also express the specific genes indicating that epidermal lineage is a self-sustained developing system.

An approach is formulated to differentiate into specific egg fragments the cytoplasmic factors causing differentiation of larval muscle cells in the eggs of ascidians. Unfertilized eggs of Ciona savignyi were treated with cytochalasin B, after which centrifugation was performed. The muscle determinants were concentrated into black fragments and removed from red fragments, and are contained in unfertilized eggs.