Chemodetection in fluctuating environments: Receptor coupling, buffering, and antagonism

Abstract

Variability in the chemical composition of the extracellular environment can significantly degrade the ability of cells to detect rare cognate ligands. Using concepts from statistical detection theory, we formalize the generic problem of detection of small concentrations of ligands in a fluctuating background of biochemically similar ligands binding to the same receptors. We discover that in contrast with expectations arising from considerations of signal amplification, inhibitory interactions between receptors can improve detection performance in the presence of substantial environmental variability, providing an adaptive interpretation to the phenomenon of ligand antagonism. Our results suggest that the structure of signaling pathways responsible for chemodetection in fluctuating and heterogeneous environments might be optimized with respect to the statistics and dynamics of environmental composition. The developed formalism stresses the importance of characterizing nonspecific interactions to understand function in signaling pathways.I nformation transmission within biological networks has recently been subject to intense scrutiny. Quantities such as mutual information appear to be biologically relevant and optimized in many contexts (1), such as neural coding (2) and development (3, 4). In these situations, the nature of the signal is unambiguous: only uncertainty in the value of the input limits information content. In other realistic biological problems, the input consists of a complex mixture, with the signal of interest buried in a sea of nonspecific interactions. For instance, multiple different molecules could bind to a cellular receptor, in which case the signaling pathway downstream needs to find a way to discriminate between correct and spurious signals. An example is immune ligand detection: T cells need to detect foreign ligands at the surface of antigen presenting cells (APC) but there are many other nonagonist ligands interacting with receptors in charge of detection (5).The consideration of heterogeneous environments, with multiple ligand types binding to a receptor, corresponds to a departure from past theoretical analyses of bounds on the performance of cellular measurements (6, 7) and requires specific treatment (8). Predefined mixtures of ligands were previously considered in ref. 9 with an emphasis on optimum decision time for given ligand compositions. Here, we use statistical detection theory to formalize the general problem of detection of a chemical signal in a time-varying mixture. We compare the detection performance of optimal proofreading networks of independent receptors to receptors with inhibitory coupling whose strength increases with the input. We find that despite antagonism and signal attenuation, coupled receptors outperform independent receptors in strongly varying environments. Intuitively, a negative feedback growing with the input size damps rare large fluctuations in the spurious ligand concentration, while retaining sensitivity to situations where few correct ligands ...

Antagonism in olfactory receptor neurons and its implications for the perception of odor mixtures

“…The latter quantifies the lowered affinity of cAMP for the CNG channels due to masking by reducing the activation efficacy η that appears in (2). It follows that the maximal firing rate is thereby reduced (see (19) in Methods).…”

“…The hallmark of inhibitory feedback is a strongly nonlinear response to mixtures of antigens. The advantage of inhibition is the taming of a dominating background ; its drawback is the reduction in response amplitudes [18,19].…”

The Case for Absolute Ligand Discrimination: Modeling Information Processing and Decision by Immune T Cells

“…While antagonism is reduced by addition of proofreading steps, there are however other trade-offs appearing in the system: for instance adding too many proofreading steps might reduce the final concentration of the output too much, which creates several downstream problems in terms of response times very similar to what happens in McKeithan's KPR model (see Supplement of [41] for discussions of this effect).…”

“…An even more general problem is chemodetection of ligands of different qualities in fluctuating environments, where mixture composition can vary with time. If there is a high probability of observing ligands just subthresholds, antagonism is helpful to essentially buffer spurious fluctuations of ligands [41], reminiscent of what is observed in olfaction [63]. Finally, some models have recently explored theoretically the possible network topologies maximizing information transmission on ligand mixtures, and have pointed out the need of both proofreading and internal feedbacks [49,57].…”

Receptor crosstalk improves concentration sensing of multiple ligands

“…Our results help rationalize the observed ubiquity of receptor crosstalk in molecular sensing. * These authors are listed alphabetically † Corresponding authors derstanding the limits in cellular sensing for single receptors with spatial [10-14] and temporal gradients [15,16], for multiple receptors [17][18][19] and even for cells that can communicate [20][21][22]. The thermodynamic cost [23][24][25][26][27] and the trade-offs between different resources for sensing [28][29][30] has also been explored at large.…”