RESUMO
Vertebrate hindbrain segmentation is an evolutionarily conserved process that involves a complex interplay of transcription factors and signalling pathways. Fibroblast growth factor (FGF) signalling plays a major role, notably by controlling the expression of the transcription factor Krox20 (Egr2), which is required for the formation and specification of two segmental units: rhombomeres (r) 3 and 5. Here, we explore the molecular mechanisms downstream of FGF signalling and the function of Sprouty 4 (Spry4), a negative-feedback regulator of this pathway, in zebrafish. We show that precise modulation of FGF signalling by Spry4 is required to determine the appropriate onset of krox20 transcription in r3 and r5 and, ultimately, rhombomere size in the r3-r5 region. FGF signalling acts by modulating the activity of krox20 initiator enhancer elements B and C; in r5, we show that this regulation is mediated by direct binding of the transcription factor MafB to element B. By contrast, FGF signalling does not control the krox20 autoregulatory element A, which is responsible for amplification and maintenance of krox20 expression. Therefore, early krox20 transcription sets the blueprint for r3-r5 patterning. This work illustrates the necessity for fine-tuning in a common and fundamental patterning process, based on a bistable cell-fate choice involving the coupling of an extracellular gradient with a positive-feedback loop. In this mode of patterning, precision and robustness can be achieved by the introduction of a negative-feedback loop, which, in the hindbrain, is mediated by Spry4.
