Impaired primitive erythropoiesis and defective vascular development in Trim71-KO embryos.

The transition of an embryo from gastrulation to organogenesis requires precisely coordinated changes in gene expression, but the underlying mechanisms remain unclear. The RNA-binding protein Trim71 is essential for development and serves as a potent regulator of post-transcriptional gene expression. Here, we show that global deficiency of induces severe defects in mesoderm-derived cells at the onset of organogenesis. Murine -KO embryos displayed impaired primitive erythropoiesis, yolk sac vasculature, heart function, and circulation, explaining the embryonic lethality of these mice. conditional knockout did not induce strong defects, showing that Trim71 expression in endothelial cells and their immediate progenitors is dispensable for embryonic survival. scRNA-seq of E7.5 global -KO embryos revealed that transcriptomic changes arise already at gastrulation, showing a strong up-regulation of the mesodermal pioneer transcription factor Eomes. We identify Eomes as a direct target of Trim71-mediated mRNA repression via the NHL domain, demonstrating a functional link between these important regulatory genes. Taken together, our data suggest that Trim71-dependent control of gene expression at gastrulation establishes a framework for proper development during organogenesis.