Layered Feedback Control Improves Robust Functionality across Heterogeneous Cell Populations

Abstract

Realizing homeostatic control of metabolites or proteins is one of the key goals of synthetic circuits. However, if control is only implemented internally in individual cells, cellcell heterogeneity may break the homeostasis on population level since cells do not contribute equally to the production or regulation. New control structures are needed to achieve robust functionality in heterogeneous cell populations. Quorum sensing (QS) serves as a collective mechanism by releasing and sensing small and diffusible signaling molecules for group decisionmaking. We propose a layered feedback control structure that includes a global controller using quorum sensing and a local controller via internal signal-receptor systems. We demonstrate with modeling and simulation that the global controller drives contributing cells to compensate for disturbances while the local controller governs the fail-mode performance in noncontributing cells. The layered controller can tolerate a higher portion of non-contributing cells or longer generations of mutant cells while maintaining metabolites or proteins level within a small error range, compared with only internal feedback control. We further discuss the potential of such layered structures in robust control of cell population size, population fraction and other population-dependent functions.

Analysis of Circuits for Dosage Control in Microbial Populations

“…Previous studies have utilized forms of layered feedback to achieve robust regulation of molecules in biocircuits [4, 11]. We adapted their designs to design a simple layered feed-back controller in which AHLs acted through LuxR to directly repress LuxI via a LuxR repressable promoter and to indirectly repression LuxI through activating the expression of a secondary repressor that inhibits LuxI expression.…”

“…Since quorum sensing molecules can convey information about population density, these molecules can be utilized for designing synthetic circuits for population level control [7]. For example, N-Acyl homoserine lactones (AHLs) have been used to implement synchronized genetic oscillators [8,9], control cell density [10], and maintain robustness across heterogeneous populations [11][12][13][14]. These efforts demonstrate the power of AHLs to be used as sensors and controllers in order to coordinate population level behaviors in synthetic circuits.…”

Bistable State Switch Enables Ultrasensitive Feedback Control in Heterogeneous Microbial Populations

“…In this context, the adoption of cell-cell communication systems like quorum sensing holds great potential in achieving population control and cellular coordination, as shown by recent studies aiming at population density control [32], [33]. Cell-cell communication can also improve population-level robustness through an intercellular layer of feedback [34]. Thus, we foresee that strategies like the one described here will enable engineered microbial consortia to achieve more complex functionalities, by combining cell-cell communication and feedback architectures.…”

Bistable State Switch Enables Ultrasensitive Feedback Control in Heterogeneous Microbial Populations

“…In this context, the adoption of cell-cell communication systems like quorum sensing holds great potential in achieving population control and cellular coordination, as shown by recent studies aiming at population density control [32], [33]. Cell-cell communication can also improve population-level robustness through an intercellular layer of feedback [34]. Thus, we foresee that strategies like the one described here will enable engineered microbial consortia to achieve more complex functionalities, by combining cell-cell communication and feedback architectures.…”