@article{haugh_schneider_2006, title={Effectiveness factor for spatial gradient sensing in living cells}, volume={61}, ISSN={["1873-4405"]}, DOI={10.1016/j.ces.2006.04.041}, abstractNote={We consider the steady-state pattern of messenger molecules produced in the membrane of a cell perceiving and responding to an extracellular gradient of chemoattractant, which directs cell movement towards the chemoattractant source. Specifically, we analyze the undesirable effect of lateral diffusion in blurring the intracellular messenger profile. The concept of an effectiveness factor, akin to the analysis of reactions in porous catalysts, is applied to the spatial gradient sensing problem, with the distinction that slow, not fast, diffusion is required for effective gradient sensing. Analytical effectiveness factor expressions are derived for ideal geometries and then generalized to arbitrary cell shapes. In the case of mouse fibroblasts responding to gradients of platelet-derived growth factor, we conclude that the cell morphology and orientation with respect to the gradient can dictate whether messenger diffusion obliterates gradient sensing or has very little effect. The analysis outlined here allows the effect of intracellular messenger diffusion on spatial gradient sensing to be quantified for individual cells.}, number={17}, journal={CHEMICAL ENGINEERING SCIENCE}, publisher={Elsevier BV}, author={Haugh, Jason M. and Schneider, Ian C.}, year={2006}, month={Sep}, pages={5603–5611} }
 @article{schneider_haugh_2006, title={Mechanisms of gradient sensing and chemotaxis - Conserved pathways, diverse regulation}, volume={5}, ISSN={["1551-4005"]}, DOI={10.4161/cc.5.11.2770}, abstractNote={Directed cell migration is critical for normal development, immune responses, and wound healing and plays a prominent role in tumor metastasis. In eukaryotes, cell orientation is biased by an external chemoattractant gradient through a spatial contrast in chemoattractant receptor-mediated signal transduction processes that differentially affect cytoskeletal dynamics at the cell front and rear. Mechanisms of spatial gradient sensing and chemotaxis have been studied extensively in the social amoeba Dictyostelium discoideum and mammalian leukocytes (neutrophils), which are similar in their remarkable sensitivity to shallow gradients and robustness of response over a broad range of chemoattractant concentration. Recently, we have quantitatively characterized a different gradient sensing system, that of platelet-derived growth factor-stimulated fibroblasts, an important component of dermal wound healing. The marked differences between this system and the others have led us to speculate on the diversity of gradient sensing mechanisms and their biological implications.}, number={11}, journal={CELL CYCLE}, publisher={Informa UK Limited}, author={Schneider, Ian C. and Haugh, Jason M.}, year={2006}, month={Jun}, pages={1130–1134} }
 @article{schneider_haugh_2005, title={Quantitative elucidation of a distinct spatial gradient-sensing mechanism in fibroblasts}, volume={171}, ISSN={["1540-8140"]}, DOI={10.1083/jcb.200509028}, abstractNote={Migration of eukaryotic cells toward a chemoattractant often relies on their ability to distinguish receptor-mediated signaling at different subcellular locations, a phenomenon known as spatial sensing. A prominent example that is seen during wound healing is fibroblast migration in platelet-derived growth factor (PDGF) gradients. As in the well-characterized chemotactic cells Dictyostelium discoideum and neutrophils, signaling to the cytoskeleton via the phosphoinositide 3-kinase pathway in fibroblasts is spatially polarized by a PDGF gradient; however, the sensitivity of this process and how it is regulated are unknown. Through a quantitative analysis of mathematical models and live cell total internal reflection fluorescence microscopy experiments, we demonstrate that PDGF detection is governed by mechanisms that are fundamentally different from those in D. discoideum and neutrophils. Robust PDGF sensing requires steeper gradients and a much narrower range of absolute chemoattractant concentration, which is consistent with a simpler system lacking the feedback loops that yield signal amplification and adaptation in amoeboid cells.}, number={5}, journal={JOURNAL OF CELL BIOLOGY}, publisher={Rockefeller University Press}, author={Schneider, IC and Haugh, JM}, year={2005}, month={Dec}, pages={883–892} }
 @article{schneider_parrish_haugh_2005, title={Spatial analysis of 3 ' phosphoinositide signaling in living fibroblasts, III: Influence of cell morphology and morphological polarity}, volume={89}, ISSN={["1542-0086"]}, DOI={10.1529/biophysj.105.061218}, abstractNote={Activation of phosphoinositide (PI) 3-kinase is a required signaling pathway in fibroblast migration directed by platelet-derived growth factor. The pattern of 3' PI lipids in the plasma membrane, integrating local PI 3-kinase activity as well as 3' PI diffusion and turnover, influences the spatiotemporal regulation of the cytoskeleton. In fibroblasts stimulated uniformly with platelet-derived growth factor, visualized using total internal reflection fluorescence microscopy, we consistently observed localized regions with significantly higher or lower 3' PI levels than adjacent regions (hot and cold spots, respectively). A typical cell contained multiple hot spots, coinciding with apparent leading edge structures, and at most one cold spot at the rear. Using a framework for finite-element modeling with actual cell contact area geometries, we find that although the 3' PI pattern is affected by irregular contact area shape, cell morphology alone cannot explain the presence of hot or cold spots. Our results and analysis instead suggest that these regions reflect different local 3' PI dynamics, specifically through a combination of mechanisms: enhanced PI 3-kinase activity, reduced 3' PI turnover, and possibly slow/constrained 3' PI diffusion. The morphological polarity of the cell may thus bias 3' PI signaling to promote persistent migration in fibroblasts.}, number={2}, journal={BIOPHYSICAL JOURNAL}, publisher={Elsevier BV}, author={Schneider, IC and Parrish, EM and Haugh, JM}, year={2005}, month={Aug}, pages={1420–1430} }
 @article{haugh_schneider_lewis_2004, title={On the cross-regulation of protein tyrosine phosphatases and receptor tyrosine kinases in intracellular signaling}, volume={230}, ISSN={["1095-8541"]}, DOI={10.1016/j.jtbi.2004.04.023}, abstractNote={Intracellular signaling proteins are very often regulated by site-specific phosphorylation. For example, growth factor receptors in eukaryotic cells contain intrinsic tyrosine kinase activity and use inter- and intra-molecular interactions to recruit and orient potential protein substrates for phosphorylation. Equally important in determining the magnitude and kinetics of such a response is protein dephosphorylation, catalysed by phosphatase enzymes. A growing body of evidence indicates that certain protein tyrosine phosphatases (PTPs), like tyrosine kinases, are affected by intermolecular interactions that alter the specific activity or localization of their catalytic domains. Using a detailed kinetic modeling framework, we theoretically explore the regulation of PTPs through their association with receptor tyrosine kinases, as noted for the Src homology 2-domain-containing PTPs, SHP-1 and -2. Receptor-PTP binding, in turn, is expected to influence the phosphorylation pattern of those receptors and proteins they associate with, and we show how PTPs might serve to co- or counter-regulate parallel pathways in a signaling network.}, number={1}, journal={JOURNAL OF THEORETICAL BIOLOGY}, publisher={Elsevier BV}, author={Haugh, JM and Schneider, IC and Lewis, JM}, year={2004}, month={Sep}, pages={119–132} }
 @article{haugh_schneider_2004, title={Spatial analysis of 3 ' phosphoinositide signaling in living fibroblasts: I. Uniform stimulation model and bounds on dimensionless groups}, volume={86}, ISSN={["1542-0086"]}, DOI={10.1016/S0006-3495(04)74137-5}, abstractNote={Fluorescent protein probes now permit spatial distributions of specific intracellular signaling molecules to be observed in real time. Mathematical models have been used to simulate molecular gradients and other spatial patterns within cells, and the output of such models may be compared directly with experiments if the binding of the fluorescent probe and the physics of the imaging technique are each incorporated. Here we present a comprehensive model describing the dynamics of 3' phosphoinositides (PIs), lipid second messengers produced in the plasma membrane in response to stimulation of the PI 3-kinase signaling pathway, as monitored in the cell-substratum contact area using total internal reflection fluorescence microscopy. With this technique it was previously shown that uniform stimulation of fibroblasts with platelet-derived growth factor elicits the formation of axisymmetric 3' PI gradients, which we now characterize in the context of our model. We find that upper and lower bounds on the relevant dimensionless model parameter values for an individual cell can be calculated from four well-defined fluorescence measurements. Based on our analysis, we expect that the key dimensionless group, the ratio of 3' PI turnover and diffusion rates, can be estimated within approximately 20% or less.}, number={1}, journal={BIOPHYSICAL JOURNAL}, publisher={Elsevier BV}, author={Haugh, JM and Schneider, IC}, year={2004}, month={Jan}, pages={589–598} }
 @article{schneider_haugh_2004, title={Spatial analysis of 3 ' phosphoinositide signaling in living fibroblasts: II. Parameter estimates for individual cells from experiments}, volume={86}, ISSN={["1542-0086"]}, DOI={10.1016/S0006-3495(04)74138-7}, abstractNote={Fibroblast migration is directed by gradients of platelet-derived growth factor (PDGF) during wound healing. As in other chemotactic systems, it has been shown recently that localized stimulation of intracellular phosphoinositide (PI) 3-kinase activity and production of 3' PI lipids in the plasma membrane are important events in the signaling of spatially biased motility processes. In turn, 3' PI localization depends on the effective diffusion coefficient, D, and turnover rate constant, k, of these lipids. Here we present a systematic and direct comparison of mathematical model calculations and experimental measurements to estimate the values of the effective 3' PI diffusion coefficient, D, turnover rate constant, k, and other parameters in individual fibroblasts stimulated uniformly with PDGF. In the context of our uniform stimulation model, the values of D and k in each cell were typically estimated within 10-20% or less, and the mean values across all of the cells analyzed were D = 0.37 +/- 0.25 microm2/s and k = 1.18 +/- 0.54 min(-1). In addition, we report that 3' PI turnover is not affected by PDGF receptor signaling in our cells, allowing us to focus our attention on the regulation of 3' PI production as this system is studied further.}, number={1}, journal={BIOPHYSICAL JOURNAL}, publisher={Elsevier BV}, author={Schneider, IC and Haugh, JM}, year={2004}, month={Jan}, pages={599–608} }
 @article{park_schneider_haugh_2003, title={Kinetic analysis of platelet-derived growth factor receptor/phosphoinositide 3-kinase/Akt signaling in fibroblasts}, volume={278}, ISSN={["0021-9258"]}, DOI={10.1074/jbc.M304968200}, abstractNote={Isoforms of the serine-threonine kinase Akt coordinate multiple cell survival pathways in response to stimuli such as platelet-derived growth factor (PDGF). Activation of Akt is a multistep process, which relies on the production of 3′-phosphorylated phosphoinositide (PI) lipids by PI 3-kinases. To quantitatively assess the kinetics of PDGF receptor/PI 3-kinase/Akt signaling in fibroblasts, a systematic study of this pathway was performed, and a mechanistic mathematical model that describes its operation was formulated. We find that PDGF receptor phosphorylation exhibits positive cooperativity with respect to PDGF concentration, and its kinetics are quantitatively consistent with a mechanism in which receptor dimerization is initially mediated by the association of two 1:1 PDGF/PDGF receptor complexes. Receptor phosphorylation is transient at high concentrations of PDGF, consistent with the loss of activated receptors upon endocytosis. By comparison, Akt activation responds to lower PDGF concentrations and exhibits more sustained kinetics. Further analysis and modeling suggest that the pathway is saturated at the level of PI 3-kinase activation, and that the p110α catalytic subunit of PI 3-kinase contributes most to PDGF-stimulated 3′-PI production. Thus, at high concentrations of PDGF the kinetics of 3′-PI production are limited by the turnover rate of these lipids, while the Akt response is additionally influenced by the rate of Akt deactivation.}, number={39}, journal={JOURNAL OF BIOLOGICAL CHEMISTRY}, publisher={American Society for Biochemistry & Molecular Biology (ASBMB)}, author={Park, CS and Schneider, IC and Haugh, JM}, year={2003}, month={Sep}, pages={37064–37072} }