Dev Bio SuperGroup Meeting
Positive feedback between PU.1 and the cell cycle controls myeloid differentiation
Hao Yuan Kueh
Rothenberg and Elowitz Labs
Regulatory gene circuits with positive feedback loops are critical for mammalian stem and progenitor cell differentiation, but it has generally remained unclear how positive feedback drives fate transitions and maintains stable differentiated states. Here, we study how positive feedback on the transcription factor PU.1 controls lymphoid and myeloid differentiation from haematopoietic progenitors. Quantitative live-cell imaging revealed that developing B-cells decrease PU.1 levels by turning down PU.1 transcription, whereas developing macrophages increase PU.1 levels by lengthening their cell cycles, which decreases the rate of PU.1 dilution. Exogenous PU.1 expression in progenitors does not further activate PU.1 transcription, but induces cell-cycle lengthening to increase endogenous PU.1 levels and promote macrophage differentiation, providing evidence for a circuit architecture involving positive feedback on a transcription factor through the cell cycle. Mathematical modeling showed that this cell-cycle coupled feedback loop effectively stabilizes and maintains a growth-slowed differentiated state. Our results imply that cell cycle length regulation can have a general role in modulating cellular protein levels that affect regulatory gene circuit activity.
Brf1 Regulates Differentiation in Response to FGF/Erk MAP Kinase Signals
Fred Tan
Elowitz Lab
FGF/Erk MAP kinase signalling destabilizes pluripotency and influences early cell fate decisions in the mouse. However, the mechanisms utilized by this regulatory pathway and the identity of its targets have remained unclear. Here we show that FGF/Erk MAP kinase signalling up-regulates Brf1, a Zfp36-family RNA-binding protein (RBP), in mouse embryonic stem cells (mESCs). Brf1 binds elements in the 3'UTR of Nanog and other pluripotency associated mRNAs, rapidly targeting them for degradation. Increasing Brf1 levels perturbs the expression of core pluripotency associated genes, but does not immediately induce differentiation or disrupt self-renewal in the presence of leukemia inhibitory factor (LIF). However, upon LIF withdrawal, enhanced Brf1 expression compromises pluripotency by biasing lineage commitment toward mesoderm. Together, these studies demonstrate that stem cell development utilizes targeted mRNA degradation to enable rapid post-transcriptional regulation of differentiation decisions in response to FGF signals.