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Quantitative elucidation of a distinct spatial gradient-sensing mechanism in fibroblasts.

Schneider IC, Haugh JM - J. Cell Biol. (2005)

Bottom Line: 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.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.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemical and Biomolecular Engineering, North Carolina State University, Raleigh, NC 27695, USA.

ABSTRACT
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.

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Sensitivity of the PDGF gradient–sensing mechanism: mathematical modeling predictions. (A–C) Predictions based on the quasi–steady-state model of receptor activation (equation 7) and pseudo-equilibrium binding of PI 3-kinase (equation 8). (A) Dimensionless PI 3-kinase recruitment, e, as a function of midpoint PDGF concentration, with a 50% gradient across the cell, at the front (closed circle) and back (open circle) relative to the gradient and averaged over the cell membrane (solid line); the receptor activation level, 〈d〉, is also shown (dotted line). The adjustable parameters are the maximum activated receptor/PI 3-kinase ratio (αdmax = 10) and dimensionless dissociation constant of the receptor/PI 3-kinase interaction (κE = 0.1). These values yield saturable PI 3-kinase activation, matching the population dose responses reported in Park et al. (2003). (B) Difference in e between the front and back, Δe, for the parameter values in A (solid curve). Also shown are results assuming stoichiometric binding (κE = 0) with αdmax values of 1 and 10 (dashed line) and in the limit of infinite αdmax (dotted line); the latter is equivalent to the relative receptor activation gradient, Δd/〈d〉. (C) Values of the relative gradient δ and midpoint PDGF concentration that yield a given Δe; αdmax = 10 and κE = 0.1. (D) Front (closed circle), back (open circle), and average (solid line) 3′ PI levels were calculated as a function of time using a kinetic receptor activation model (see supplemental Modeling details, available at http://www.jcb.org/cgi/content/full/jcb.200509028/DC1) under conditions that mimic our experimental protocol. A 50% gradient of varying midpoint PDGF concentration, as indicated, was administered for 20 min followed by a uniformly saturating dose (10 nM) for another 10 min. Thereafter, PI 3-kinase activity was turned off as if inhibited by wortmannin (wort).
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fig1: Sensitivity of the PDGF gradient–sensing mechanism: mathematical modeling predictions. (A–C) Predictions based on the quasi–steady-state model of receptor activation (equation 7) and pseudo-equilibrium binding of PI 3-kinase (equation 8). (A) Dimensionless PI 3-kinase recruitment, e, as a function of midpoint PDGF concentration, with a 50% gradient across the cell, at the front (closed circle) and back (open circle) relative to the gradient and averaged over the cell membrane (solid line); the receptor activation level, 〈d〉, is also shown (dotted line). The adjustable parameters are the maximum activated receptor/PI 3-kinase ratio (αdmax = 10) and dimensionless dissociation constant of the receptor/PI 3-kinase interaction (κE = 0.1). These values yield saturable PI 3-kinase activation, matching the population dose responses reported in Park et al. (2003). (B) Difference in e between the front and back, Δe, for the parameter values in A (solid curve). Also shown are results assuming stoichiometric binding (κE = 0) with αdmax values of 1 and 10 (dashed line) and in the limit of infinite αdmax (dotted line); the latter is equivalent to the relative receptor activation gradient, Δd/〈d〉. (C) Values of the relative gradient δ and midpoint PDGF concentration that yield a given Δe; αdmax = 10 and κE = 0.1. (D) Front (closed circle), back (open circle), and average (solid line) 3′ PI levels were calculated as a function of time using a kinetic receptor activation model (see supplemental Modeling details, available at http://www.jcb.org/cgi/content/full/jcb.200509028/DC1) under conditions that mimic our experimental protocol. A 50% gradient of varying midpoint PDGF concentration, as indicated, was administered for 20 min followed by a uniformly saturating dose (10 nM) for another 10 min. Thereafter, PI 3-kinase activity was turned off as if inhibited by wortmannin (wort).

Mentions: From an analysis of the model equations, which relate the difference in PI 3-kinase enzyme recruitment between the front and back of the cell (Δe) to the corresponding difference in receptor dimerization/activation (Δd) at quasi–steady state, one predicts three distinct regimes of gradient sensitivity (Fig. 1, A–C). At low midpoint concentrations of PDGF, most of the PI 3-kinase remains in the cytosol, and PI 3-kinase recruitment is simply proportional to the local density of activated receptors. In other words, gradient sensing is absolute: (1)\documentclass[10pt]{article}\usepackage{amsmath}\usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy}\usepackage{mathrsfs}\usepackage{pmc}\usepackage[Euler]{upgreek}\pagestyle{empty}\oddsidemargin -1.0in\begin{document}\begin{equation*}{\mathrm{{\Delta}}}e{\mathrm{{\propto}{\Delta}}}d\end{equation*}\end{document}


Quantitative elucidation of a distinct spatial gradient-sensing mechanism in fibroblasts.

Schneider IC, Haugh JM - J. Cell Biol. (2005)

Sensitivity of the PDGF gradient–sensing mechanism: mathematical modeling predictions. (A–C) Predictions based on the quasi–steady-state model of receptor activation (equation 7) and pseudo-equilibrium binding of PI 3-kinase (equation 8). (A) Dimensionless PI 3-kinase recruitment, e, as a function of midpoint PDGF concentration, with a 50% gradient across the cell, at the front (closed circle) and back (open circle) relative to the gradient and averaged over the cell membrane (solid line); the receptor activation level, 〈d〉, is also shown (dotted line). The adjustable parameters are the maximum activated receptor/PI 3-kinase ratio (αdmax = 10) and dimensionless dissociation constant of the receptor/PI 3-kinase interaction (κE = 0.1). These values yield saturable PI 3-kinase activation, matching the population dose responses reported in Park et al. (2003). (B) Difference in e between the front and back, Δe, for the parameter values in A (solid curve). Also shown are results assuming stoichiometric binding (κE = 0) with αdmax values of 1 and 10 (dashed line) and in the limit of infinite αdmax (dotted line); the latter is equivalent to the relative receptor activation gradient, Δd/〈d〉. (C) Values of the relative gradient δ and midpoint PDGF concentration that yield a given Δe; αdmax = 10 and κE = 0.1. (D) Front (closed circle), back (open circle), and average (solid line) 3′ PI levels were calculated as a function of time using a kinetic receptor activation model (see supplemental Modeling details, available at http://www.jcb.org/cgi/content/full/jcb.200509028/DC1) under conditions that mimic our experimental protocol. A 50% gradient of varying midpoint PDGF concentration, as indicated, was administered for 20 min followed by a uniformly saturating dose (10 nM) for another 10 min. Thereafter, PI 3-kinase activity was turned off as if inhibited by wortmannin (wort).
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC2171296&req=5

fig1: Sensitivity of the PDGF gradient–sensing mechanism: mathematical modeling predictions. (A–C) Predictions based on the quasi–steady-state model of receptor activation (equation 7) and pseudo-equilibrium binding of PI 3-kinase (equation 8). (A) Dimensionless PI 3-kinase recruitment, e, as a function of midpoint PDGF concentration, with a 50% gradient across the cell, at the front (closed circle) and back (open circle) relative to the gradient and averaged over the cell membrane (solid line); the receptor activation level, 〈d〉, is also shown (dotted line). The adjustable parameters are the maximum activated receptor/PI 3-kinase ratio (αdmax = 10) and dimensionless dissociation constant of the receptor/PI 3-kinase interaction (κE = 0.1). These values yield saturable PI 3-kinase activation, matching the population dose responses reported in Park et al. (2003). (B) Difference in e between the front and back, Δe, for the parameter values in A (solid curve). Also shown are results assuming stoichiometric binding (κE = 0) with αdmax values of 1 and 10 (dashed line) and in the limit of infinite αdmax (dotted line); the latter is equivalent to the relative receptor activation gradient, Δd/〈d〉. (C) Values of the relative gradient δ and midpoint PDGF concentration that yield a given Δe; αdmax = 10 and κE = 0.1. (D) Front (closed circle), back (open circle), and average (solid line) 3′ PI levels were calculated as a function of time using a kinetic receptor activation model (see supplemental Modeling details, available at http://www.jcb.org/cgi/content/full/jcb.200509028/DC1) under conditions that mimic our experimental protocol. A 50% gradient of varying midpoint PDGF concentration, as indicated, was administered for 20 min followed by a uniformly saturating dose (10 nM) for another 10 min. Thereafter, PI 3-kinase activity was turned off as if inhibited by wortmannin (wort).
Mentions: From an analysis of the model equations, which relate the difference in PI 3-kinase enzyme recruitment between the front and back of the cell (Δe) to the corresponding difference in receptor dimerization/activation (Δd) at quasi–steady state, one predicts three distinct regimes of gradient sensitivity (Fig. 1, A–C). At low midpoint concentrations of PDGF, most of the PI 3-kinase remains in the cytosol, and PI 3-kinase recruitment is simply proportional to the local density of activated receptors. In other words, gradient sensing is absolute: (1)\documentclass[10pt]{article}\usepackage{amsmath}\usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy}\usepackage{mathrsfs}\usepackage{pmc}\usepackage[Euler]{upgreek}\pagestyle{empty}\oddsidemargin -1.0in\begin{document}\begin{equation*}{\mathrm{{\Delta}}}e{\mathrm{{\propto}{\Delta}}}d\end{equation*}\end{document}

Bottom Line: 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.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.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemical and Biomolecular Engineering, North Carolina State University, Raleigh, NC 27695, USA.

ABSTRACT
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.

Show MeSH
Related in: MedlinePlus