A BMP-FGF Morphogen Toggle Switch Drives the Ultrasensitive Expression of Multiple Genes in the Developing Forebrain

Author Summary During development, morphogen gradients play a crucial role in transforming a uniform field of cells into regions with distinct cell identities (marked by the expression of specific genes). Finding mechanisms that convert morphogen gradients into sharp borders of gene expression, however, remains a challenge. Cellular ultrasensitivity mechanisms that convert a linear stimulus into an on-off target response offer a good solution for making such borders. In this paper, we show how a cross-inhibitory positive feedback or toggle switch mechanism driven by two extracellular morphogens - BMP and FGF - produces ultrasensitivity in forebrain cells. Experiments with cells and explanted brain tissue reveal that BMPs and FGFs cross inhibit each other's signaling pathway. Such cross inhibition could occur through four possible mechanisms. By an iterative combination of modeling and experiment, we show the toggle switch to be the mechanism underlying cross inhibition, the ultrasensitive expression of multiple genes, and hysteresis in forebrain cells. As the toggle switch explicitly links extracellular morphogens to cellular ultrasensitivity, it provides a mechanism for making multiple sharp borders that can also scale with tissue size - an important issue in pattern formation. This might explain the abundance of BMP-FGF cross inhibition during development. Borders are important as they demarcate developing tissue into distinct functional units. A key challenge is the discovery of mechanisms that can convert morphogen gradients into tissue borders. While mechanisms that produce ultrasensitive cellular responses provide a solution, how extracellular morphogens drive such mechanisms remains poorly understood. Here, we show how Bone Morphogenetic Protein (BMP) and Fibroblast Growth Factor (FGF) pathways interact to generate ultrasensitivity and borders in the dorsal telencephalon. BMP and FGF signaling manipulations in explants produced border defects suggestive of cross inhibition within single cells, which was confirmed in dissociated cultures. Using mathematical modeling, we designed experiments that ruled out alternative cross inhibition mechanisms and identified a cross-inhibitory positive feedback (CIPF) mechanism, or toggle switch, which acts upstream of transcriptional targets in dorsal telencephalic cells. CIPF explained several cellular phenomena important for border formation such as threshold tuning, ultrasensitivity, and hysteresis. CIPF explicitly links graded morphogen signaling in the telencephalon to switch-like cellular responses and has the ability to form multiple borders and scale pattern to size. These benefits may apply to other developmental systems.