Activation-derepression synergy enables a bHLH network to coordinate a signal-specific fate response

Stem cells integrate multiple environmental signals to activate appropriate fate programs. To ensure coherent responses, alternative fates must be concomitantly inactivated. However, mechanisms that coordinate fates in a signal-specific manner are not fully understood. Here, we investigate the role of a network of basic-helix-loop-helix (bHLH) transcription factors in neural stem cells, which integrate leukemia inhibitory factor (LIF) and bone morphogenetic protein (BMP) signaling to synergistically induce glial fibrillary acidic protein (GFAP), a key astrocyte-fate determinant. Using quantitative RNA-fluorescence in situ hybridization (FISH) and ectopic expression, we find that multiple bHLHs that promote alternative fates also repress GFAP but are all suppressed by BMP and, to a lesser extent, LIF. Mathematical modeling shows that synergy arises from this coordinated derepression of GFAP combined with its activation by LIF signaling. Finally, we determine how coordinated and tunable derepression results from extensive cross-regulation among bHLHs. Activation-derepression synergy could be broadly utilized to couple signaling and fate, particularly across the numerous developmental systems regulated by bHLH factors.