Spatial Bistability Generates hunchback Expression Sharpness in the Drosophila Embryo

Abstract

During embryonic development, the positional information provided by concentration gradients of maternal factors directs pattern formation by providing spatially dependent cues for gene expression. In the fruit fly, Drosophila melanogaster, a classic example of this is the sharp on–off activation of the hunchback (hb) gene at midembryo, in response to local concentrations of the smooth anterior–posterior Bicoid (Bcd) gradient. The regulatory region for hb contains multiple binding sites for the Bcd protein as well as multiple binding sites for the Hb protein. Some previous studies have suggested that Bcd is sufficient for properly sharpened Hb expression, yet other evidence suggests a need for additional regulation. We experimentally quantified the dynamics of hb gene expression in flies that were wild-type, were mutant for hb self-regulation or Bcd binding, or contained an artificial promoter construct consisting of six Bcd and two Hb sites. In addition to these experiments, we developed a reaction–diffusion model of hb transcription, with Bcd cooperative binding and hb self-regulation, and used Zero Eigenvalue Analysis to look for multiple stationary states in the reaction network. Our model reproduces the hb developmental dynamics and correctly predicts the mutant patterns. Analysis of our model indicates that the Hb sharpness can be produced by spatial bistability, in which hb self-regulation produces two stable levels of expression. In the absence of self-regulation, the bistable behavior vanishes and Hb sharpness is disrupted. Bcd cooperative binding affects the position where bistability occurs but is not itself sufficient for a sharp Hb pattern. Our results show that the control of Hb sharpness and positioning, by hb self-regulation and Bcd cooperativity, respectively, are separate processes that can be altered independently. Our model, which matches the changes in Hb position and sharpness observed in different experiments, provides a theoretical framework for understanding the data and in particular indicates that spatial bistability can play a central role in threshold-dependent reading mechanisms of positional information.

Identifying ultrasensitive HGF dose-response functions in a 3D mammalian system for synthetic morphogenesis

“…In addition, ultrasensitivity can increase the robustness of bistable responses141516 and can stabilise biological oscillations17181920212223242526. Finally, ultrasensitivity has been shown to play a major role in converting spatial gradients into sharp boundaries in morphogenesis and developmental patterning27282930313233. In the latter context, our previous work showed that steeper dose-response functions better supported Turing pattern formation in a theoretical developmental model28.…”

Sequential construction of a model for modular gene expression control, applied to spatial patterning of theDrosophilagenehunchback

“…52,67 This makes modeling of P1 and P2 output somewhat conjectural (though the technology is available to distinguish the transcripts 16 ). Given this, however, our approach o®ers a means whereby the knowledge we have of the hb cis-architecture (CRMs, transcription start sites) can be turned into a dynamic model for expression patterning.…”

Experimental and Modeling Approaches for Understanding the Effect of Gene Expression Noise in Biological Development

“…For instance, a stochastic model of Bcd and Hb (self) activation of the anterior hb expression domain [43], developed from an earlier deterministic version [44], predicted that reporter constructs with less than wild-type numbers of Bcd BSs should show increased variability of the mid-embryo boundary, and that loss of hb autoregulation should decrease correlation of the FISH signal between the two hb gene copies (dots). See also Sanchez et al and Monteoliva et al [40,45] on the effect of the number of BSs on transcriptional noise.…”