Gonzalez-Martinez, J.; Ayala, E.; Sanchez-Belmonte, A.; Garcia, A.; Nogueira, E.; Melati, A.; Gimenez, D.; Losada, A.; Ortega, S.; Malumbres, M.

Commonly expressed at developmental transitions, microRNAs operate as fine tuners of gene expression. However, how they regulate the earliest developmental transitions in early mammalian embryogenesis remains obscure. Here, we identify the microRNA miR-203 as a critical regulator of timing within the totipotency to pluripotency transition in mouse embryos. Genetically engineered mouse models show that loss of miR-203 delays developmental timing during preimplantation leading to the accumulation of delayed embryos with high expression of totipotency-associated markers. By introducing miR-203-null alleles in a new embryonic reporter (eE-Reporter) transgenic mouse carrying MERVL-Tomato and Sox2-GFP transgenes, we observed that lack of miR-203 leads to sustained expression of MERVL retroviral elements and reduced Sox2 expression in later preimplantation developmental stages. A combination of single-cell transcriptional studies and epigenetic analyses identified the central coactivator and histone acetyltransferase P300 as a major miR-203 target at the totipotency to pluripotency transition in vivo. By fine tuning P300 levels, miR-203 carves the epigenetic rewiring process needed for this developmental transition, allowing a timely and correctly paced development.