RT Journal Article SR Electronic T1 Arp2/3 complex activity is necessary for mouse ESC differentiation, times formative pluripotency, and enables lineage specification JF bioRxiv FD Cold Spring Harbor Laboratory SP 2021.11.02.465785 DO 10.1101/2021.11.02.465785 A1 Aloisio, Francesca M. A1 Barber, Diane L. YR 2021 UL http://biorxiv.org/content/early/2021/11/04/2021.11.02.465785.abstract AB Mouse embryonic stem cells (mESCs), a model for differentiation into primed epiblast-like cells (EpiLCs), have revealed transcriptional and epigenetic control of early embryonic development. The control and significance of morphological changes, however, remain less defined. We show marked changes in morphology and actin architectures during differentiation that depend on Arp2/3 complex but not formin activity. Inhibiting Arp2/3 complex activity pharmacologically or genetically does not block exit from naive pluripotency but attenuates increases in EpiLC markers. We find that inhibiting Arp2/3 complex activity delays formative pluripotency and causes globally defective lineage specification as indicated by RNA-sequencing, with significant effects on TBX3-depedendent transcriptional programs. We also identify two previously unreported indicators of mESC differentiation; MRTF and FHL2, which have inverse Arp2/3 complex-dependent nuclear translocation. Our findings on Arp2/3 complex activity in differentiation and the established role of formins in EMT indicate that these two actin nucleators regulate distinct modes of epithelial plasticity.HighlightsArp2/3 complex activity is necessary for morphology changes during differentiationArp2/3 complex activity regulates transcriptional markers of differentiationInhibiting Arp2/3 complex activity delays entry into formative pluripotencyArp2/3 complex activity-dependent shuttling of FHL2 and MRTF occurs in mESCsCompeting Interest StatementThe authors have declared no competing interest.