BI86CH14-Ismagilov ARI 18 May 2017 17:11 Conceptual and Experimental Tools to Understand Spatial ANNUAL REVIEWS Further Effects and Transport Click here to view this article's online features: • Download figures as PPT slides Phenomena in Nonlinear • Navigate linked references • Download citations • Explore related articles • Search keywords Biochemical Networks Illustrated with Patchy Switching Rebecca R. Pompano,1 Andrew H. Chiang,2 Christian J. Kastrup,3 and Rustem F. Ismagilov4 1Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904; email: [email protected] 2Department of Chemistry and Institute for Biophysical Dynamics, The University of Chicago, Chicago, Illinois 60637; email: [email protected] 3Michael Smith Laboratories and Department of Biochemistry and Molecular Biology, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada; email: [email protected] 4Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125; email: [email protected] Annu. Rev. Biochem. 2017. 86:333–56 Keywords The Annual Review of Biochemistry is online at state switching, signaling, flow, mass transfer, Damkohler¨ number, biochem.annualreviews.org microfluidics https://doi.org/10.1146/annurev-biochem- 060815-014207 Abstract Copyright c 2017 by Annual Reviews. Many biochemical systems are spatially heterogeneous and exhibit nonlinear All rights reserved behaviors, such as state switching in response to small changes in the local concentration of diffusible molecules. Systems as varied as blood clotting, intracellular calcium signaling, and tissue inflammation are all heavily in- fluenced by the balance of rates of reaction and mass transport phenomena including flow and diffusion. Transport of signaling molecules is also affected Annu. Rev. Biochem. 2017.86:333-356. Downloaded from www.annualreviews.org by geometry and chemoselective confinement via matrix binding. In this re- Access provided by California Institute of Technology on 06/28/17. For personal use only. view, we use a phenomenon referred to as patchy switching to illustrate the interplay of nonlinearities, transport phenomena, and spatial effects. Patchy switching describes a change in the state of a network when the local concen- tration of a diffusible molecule surpasses a critical threshold. Using patchy switching as an example, we describe conceptual tools from nonlinear dy- namics and chemical engineering that make testable predictions and provide a unifying description of the myriad possible experimental observations. We describe experimental microfluidic and biochemical tools emerging to test conceptual predictions by controlling transport phenomena and spatial dis- tribution of diffusible signals, and we highlight the unmet need for in vivo tools. 333 BI86CH14-Ismagilov ARI 18 May 2017 17:11 Contents INTRODUCTION . 334 TRANSPORT AND THRESHOLD KINETICS DRIVE PATCHYSWITCHING....................................................... 334 DefinitionofPatchySwitching.................................................. 334 Dynamics of Patchy Switching Predicted by the DamkohlerNumber.............¨ 336 PredictingPatchySwitchinginResponsetoPatchSize........................... 337 MICROSCALECONTROLOVERTRANSPORTPHENOMENA............... 339 Controlling Patch Size and Clustering . 339 Chemically Controlling Diffusivity . 342 Utilizing Confinement . 343 Manipulating Flow and Shear . 345 CONCLUSIONSANDOUTLOOK.............................................. 346 To Which Biological Systems Might Patchy Switching Apply? . 346 Case Study: Possible Patchy Switching in the Lesions of Multiple Sclerosis ........ 346 Outlook on Tools for Testing the Role of Patchy Switching InVitroandInVivo......................................................... 348 INTRODUCTION Biochemical reactions rarely occur in a perfectly homogeneous environment. With the exception of a well-mixed test tube, molecules and cells are dispersed unevenly through complex matrices, whether inside a cell, culture dish, or tissue. Furthermore, the reactions themselves are moving in space—molecules and cells are transported by diffusion, convection, and cellular motion, except when bound to relatively stationary support structures such as the extracellular matrix (ECM). Historically, scientists have had better tools for controlling rates of reactions than for ma- nipulating transport phenomena. Quantification of the rates of reactions affecting signaling molecules—for example, rate of enzymatic activity, rate of cellular secretion, and levels of gene expression—is standard in biological experiments, and modulating reaction rates (via small molecules, siRNA, inactivating antibodies, etc.) is common in basic science and medicine. In con- trast, characterization and especially control of transport phenomena (1)—for example, diffusion, flow, effects of spatial arrangement, and effects of binding to the ECM (2–4)—have been more challenging. Yet, when these experiments are performed, they suggest that changes in transport Annu. Rev. Biochem. 2017.86:333-356. Downloaded from www.annualreviews.org rates can be significant enough to perturb the local concentration of a signal. For example, bind- ∼ Access provided by California Institute of Technology on 06/28/17. For personal use only. ing of basic fibroblast growth factor (bFGF) to heparan sulfate (Kd 24 nM) slows its diffusion (5) through the basement membrane by >125-fold. Such a large change in local concentration can strongly affect the state of a biochemical network when the kinetics of the network depend nonlinearly on concentration (6). If a system shows threshold dependence on the concentration of the signal [e.g., in growth factor signaling (7)], this change may be sufficient to drive switching to a new state. TRANSPORT AND THRESHOLD KINETICS DRIVE PATCHY SWITCHING Definition of Patchy Switching To illustrate the interplay of nonlinear reaction kinetics, changes in local concentrations, and mass transport phenomena, in this review we focus our attention on biological systems that switch their 334 Pompano et al. BI86CH14-Ismagilov ARI 18 May 2017 17:11 Patchy switching System is OFF System is ON Threshold concentration Low [trigger] on patch High [trigger] on patch Da << 1 Da >> 1 Diffusivity Rate of reaction + Production Flow Da = – Degradation Rate of transport Geometry – Inhibition Figure 1 In patchy switching, local accumulation of diffusible triggers causes transitions between biological states, shown here for OFF and ON (red starburst) states. Triggers accumulate to high concentration (dark blue) only when their removal by transport ( green arrows) is slower than their net production (red arrows). The competition between reaction and transport is described by a unitless quantity termed the Damkohler¨ number (Da). state in response to a change in local concentrations of diffusible molecules, which we call triggers (Figure 1). Although this phenomenon may sound specialized, it is actually quite common. Intu- itive examples of state switching occur in blood coagulation (clotted/not clotted), cellular signaling networks (signaling on/off ), quorum sensing (swarming/resting), neural dynamics (firing/not fir- ing), and stem cell differentiation (pluripotent/differentiated). For a more rigorous description of state switching, we refer the reader to reviews of dynamical systems (8) and transitions between macro states in statistical thermodynamics (9). Triggers that control the state of a system as a re- sult of a change in concentration include small molecules, proteins, and enzymes. For simplicity, here we focus on molecular triggers, although such state switching may also occur in response to changes in the local concentration of larger structures, such as vesicles or cells. All of these molec- ular triggers are subject to mass transport, and therefore their local concentrations are controlled by the balance of production, degradation, and mass transport phenomena. A dramatic effect arises if two conditions are met: (a) triggers are localized or produced in a cluster we call a patch and (b) the system can switch from OFF to ON when triggers are present Annu. Rev. Biochem. 2017.86:333-356. Downloaded from www.annualreviews.org above a critical threshold concentration. When the local concentration of triggers near a patch passes through its critical threshold value, we call the state change patchy switching (Figure 1). Access provided by California Institute of Technology on 06/28/17. For personal use only. For a concrete example, consider a small injury to a blood vessel that exposes blood to a patch of subendothelial tissue. This patch of damaged vasculature contains proteins, including tissue factor (TF) and collagen, which begin adhering platelets and activating an enzymatic cascade that can lead to a blood clot (10). Clotting occurs only when activated enzymes accumulate above a threshold concentration (11, 12), so this scenario meets the two conditions described above. Whether or not a clot forms and grows from the patch of damage depends on the balance between the net rate of enzyme activation (activation minus consumption and inhibition) and the rate of removal of those enzymes by flow or diffusion (13). The concept of patchy switching is not new, though appreciation for it has remained mostly confined to specific fields. Switching driven by mass transport effects is predicted by classical theory in chemistry, including work by Turing on the chemical basis of morphogenesis and work www.annualreviews.org