Fold-change detection in biological systems

Many sensory systems in cells and organisms share a recurring property called fold-change detection (FCD). FCD describes a system whose dynamics – including amplitude and response time – are determined only by the relative change in input signal, rather than its absolute change. FCD entails two important features – exact adaptation and the Weber–Fechner law. Systems with FCD include bacterial and eukaryotic chemotaxis, signaling pathways inmammalian cells suchasNF-kB,Wnt andTgf-b, and organismal vision, hearing and olfaction. Here, we review circuits that can provide FCD such as the incoherent type 1 feedforward loop, the non-linear integral feedback loop, and logarithmic sensor. We review experimental ways to test for FCD and differentiate between FCD mechanisms, and highlight theoretical studies that begin to map the space of FCD circuits and the functions they canprovide. Finally, wediscussopenquestionson the structure and function of FCD systems. Addresses Department of Molecular Cell Biology, Weizmann Institute of Science, Rehovot, 76100, Israel Corresponding author: Alon, Uri ([email protected]) Current Opinion in Systems Biology 2018, 8:81–89 This review comes from a themed issue on Regulatory and metabolic networks (2018) Edited by Bas Teusink and Uwe Sauer For a complete overview see the Issue and the Editorial Available online 20 December 2017 https://doi.org/10.1016/j.coisb.2017.12.005 2452-3100/© 2017 The Authors. Published by Elsevier Ltd. This is an open access ar ticle under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).