A BMP-FGF morphogen toggle switch drives the ultrasensitive expression of multiple genes in the developing forebrain

Area of Science:

• Developmental biology
• Cell signaling
• Neuroscience

Background:

• Tissue borders are crucial for organizing developing tissues into functional units.
• Understanding how morphogen gradients translate into distinct tissue borders is a key challenge in developmental biology.
• Extracellular morphogens driving ultrasensitive cellular responses for border formation remain poorly understood.

Purpose of the Study:

• To investigate the molecular mechanisms underlying border formation in the dorsal telencephalon.
• To elucidate how Bone Morphogenetic Protein (BMP) and Fibroblast Growth Factor (FGF) signaling interact to generate ultrasensitivity and tissue borders.
• To identify the specific mechanism converting morphogen gradients into switch-like cellular responses.

Main Methods:

• Manipulation of BMP and FGF signaling in dorsal telencephalon explants.
• Analysis of cellular responses in dissociated cultures to confirm cross-inhibition.
• Mathematical modeling to design experiments and rule out alternative mechanisms.
• Identification of a cross-inhibitory positive feedback (CIPF) mechanism.

Main Results:

• BMP and FGF signaling manipulations caused border defects, indicating cross-inhibition within single cells.
• A cross-inhibitory positive feedback (CIPF) mechanism, acting as a "toggle switch", was identified upstream of transcriptional targets.
• CIPF explained threshold tuning, ultrasensitivity, and hysteresis in cellular responses.
• CIPF links graded morphogen signaling to switch-like responses, enabling border formation and pattern scaling.

Conclusions:

• A novel cross-inhibitory positive feedback (CIPF) mechanism regulates border formation in the dorsal telencephalon.
• CIPF converts continuous morphogen gradients into discrete tissue borders through ultrasensitive cellular responses.
• This mechanism provides a framework for understanding pattern formation and may be relevant in other developmental systems.