Signal integration by a bHLH circuit enables fate choice in neural stem cells
Stem cells integrate information from multiple signals in their environment to make fate decisions. It is unclear how signal integration is linked to the coordinated activation of a target fate program and simultaneous inactivation of competing fates. Here, we investigated this question in mouse neural stem cells, which differentiate synergistically into astrocytes in response to combined treatment with Bone Morphogenetic Protein (BMP) and Leukemia Inhibitory Factor (LIF) at the expense of alternative neuronal or oligodendrocyte fates. Analysis of the expression dynamics of Glial Fibrillary Acidic Protein (GFAP), an early astrocyte marker, showed that its synergistic activation in BMP and LIF reflects early activation by LIF which is sustained by a delayed contribution to its expression from BMP. In parallel, multiplexed RNA-FISH analysis of 14 basic helix-loop-helix (bHLH) transcription factors, known to regulate alternative fates, showed that LIF and BMP individually control different subsets of bHLHs, but together suppress all bHLHs known to promote alternative fates. Ectopic expression experiments showed that these bHLHs also inhibit GFAP induction, suggesting that suppression of alternative fates by BMP + LIF simultaneously relieves GFAP inhibition. In particular, BMP primarily affected inhibitory bHLHs indirectly, through induction of Id factors, explaining why it has a delayed contribution to GFAP transcription compared to LIF. These results show that a circuit of bHLH factors enables both synergistic astrocytic differentiation and suppression of alternative fates in NSCs. Signal integration by bHLH circuits for fate choice could be broadly relevant, given the widespread utilization of these and other bHLH factors across diverse developmental contexts.