Hysteresis vs. graded responses: the connections make all the difference

Area of Science:

• Systems biology
• Molecular biology
• Biophysics

Background:

• Biological regulatory systems can exhibit graded or switch-like responses.
• Understanding the design principles governing these responses is crucial but remains rudimentary.
• Recent experimental and theoretical studies offer new insights into these principles.

Purpose of the Study:

• To review recent experimental and theoretical studies on system design principles in biological regulation.
• To explore how feedback loops and gene expression dynamics influence system responses.
• To elucidate the conditions favoring graded, bistable, or multistable behaviors.

Main Methods:

• Analysis of theoretical models and experimental data on biological regulatory systems.
• Investigation of positive and negative feedback loops in genetic systems.
• Examination of multistability arising from combined regulatory effects or individual bistable components.

Main Results:

• Overt positive or double-negative feedback loops can yield bistable or multistable systems, or graded responses.
• Bistability in genetic systems is favored by high kinetic order repression and significant expression rate differences.
• Multistability arises from combined regulators or individual bistable regulators; enzymatic systems can also exhibit bistability without overt feedback.

Conclusions:

• Feedback loops and specific kinetic properties are key determinants of biological system response types (graded vs. switch-like).
• Understanding these design principles is essential for predicting and engineering biological functions.
• Bistability and multistability are emergent properties arising from network topology and component characteristics.