Effects of Extracellular Matrix Deformation on Autocrine Signaling

Annals of Biomedical Engineering, Vol. 32, No. 10, October 2004 (© 2004) pp. 1319–1335 A Model for Mechanotransduction in Cardiac Muscle: Effects of Extracellular Matrix Deformation on Autocrine Signaling , IVA N V. M ALY,1 RICHARD T. LEE,1 2 and DOUGLAS A. LAUFFENBURGER1 1Biological Engineering Division, Massachusetts Institute of Technology, Cambridge, MA and 2Cardiovascular Division, Brigham and Women’s Hospital, Boston, MA (Received 26 May 2004; accepted 27 June 2004) Abstract—We present a computational model and analysis of the growth factor receptor (EGFR). G protein-coupled recep- dynamic behavior of epidermal growth factor receptor (EGFR) tors can mediate activation of shedding of heparin-binding signaling in cardiac muscle tissue, with the aim of exploring epidermal growth factor (HB-EGF) from the surface of transduction of mechanical loading into cellular signaling that 3 could lead to cardiac hypertrophy. For this purpose, we integrated cardiomyocytes, as they do in other cell types, in response recently introduced models for ligand dynamics within compliant to the pressure overload. The released autocrine HB-EGF intercellular spaces and for the spatial dynamics of intracellular binds to the EGFR and, through the downstream effects of signaling with a positive feedback autocrine circuit. These kinetic receptor activation, triggers the hypertrophic response.1,17 models are here considered in the setting of a tissue consisting In the present study, we performed a computational anal- of cardiomyocytes and blood capillaries as a structural model for the myocardium. We show that autocrine EGFR signaling ysis of the behavior of the EGFR signaling network in can be induced directly by mechanical deformation of the tissue cardiomyocytes, with the aim of exploring transduction of and demonstrate the possibility of self-organization of signaling mechanical loading into cellular signaling that could lead that is anisotropic on the tissue level and can reflect anisotropy to cardiac hypertrophy. of the mechanical deformation. These predictions point to the Recently we have found autocrine signaling through the potential capabilities of the EGFR autocrine signaling circuit in mechanotransduction and suggest a new perspective on the cardiac EGFR involved also in mechanosignaling in lung epithe- hypertrophic response. lium, which is possibly implicated in the transformation of this tissue in response to asthmatic bronchoconstriction.32 The central event of the translation of the mechanical stress Keywords—Spatially distributed models, EGFR, Signaling into the cellular signaling in this case was compression of networks. the intercellular space into which the autocrine ligands of EGFR were shed. A several-fold reduction of the intercellu- INTRODUCTION lar volume was registered by two-photon microscopy, and Heart muscle adapts to increases in mechanical load a mathematical model of the shedding and diffusion in the such as high blood pressure in part by undergoing growth, compliant intercellular space predicted that the compres- or hypertrophy, which may serve to normalize stress in sion would lead to an equivalent several-fold increase of the tissue. Although this can be an adaptive physiologi- the concentration of the ligands at steady state. The elevated cal process, for example in response to exercise, chronic concentration of ligands to which the receptors on the cell surface are exposed then sets off the intracellular signaling overload-induced hypertrophy such as that induced by hy- 32 pertension can progress to heart failure.16 At the molecular cascade. level, the hypertrophic response often occurs with reex- In our new work here, we proceed further to consider the pression of embryonic genes and increased synthesis of potential involvement of a similar mechanism of mechan- contractile proteins,11 but the signaling events that lead to otransduction in the heart muscle by means of modeling the these downstream changes are relatively poorly understood. distribution of autocrine EGFR ligands in the heart muscle In particular, it remains largely unclear what mechanism, tissue subjected to mechanical stress. Our recent model for spatial self-organization of autocrine signaling in a single or rather multiple mechanisms, translate the mechanical 18 load into cell signaling. One of these mechanosignaling isolated cell incorporated the effects of spatial distribution pathways involves autocrine signaling through epidermal of all the components of the EGFR signaling network, as well as its nonlinear kinetic properties. Figure 1 shows the elements and processes involved in the EGFR autocrine sig- Address correspondence to I. V. Maly, Biological Engineering Divi- sion, NE47-320, Massachusetts Institute of Technology, Cambridge, MA naling network according to our model. They include shed- 02139. Electronic mail: [email protected] ding of EGF-like ligand from the cell surface, its diffusion 1319 0090-6964/04/1000-1319/1 C 2004 Biomedical Engineering Society 1320 MALY et al. FIGURE 1. Signaling interactions in the EGFR autocrine network. The shaded area is the cytoplasm; the white area is extracellular space. in the extracellular space, and binding to a transmembrane pate in the signaling complex formation.15 For the pur- receptor. A ligand-bound receptor recruits the cytoplasmic poses of studying mechanosignaling on the tissue level, this adaptor protein Grb2, which in turn recruits Sos, the actual model complements the model for deformation-dependent activator of signaling downstream from the receptor.25 The accumulation of autocrine ligands in the intercellular fully assembled signaling complex of EGF-family ligand, spaces.32 EGF receptor, Grb2, and Sos causes phosphorylation of Raf Our present model addressing myocardial mechan- on the cytoplasmic side of the membrane. This transforma- otransduction examines potential effects of combining the tion of Raf is reversed in the cytoplasm. The phosphory- previous model for EGFR ligand dynamics in the compliant lated Raf is in its active form, and itself phosphorylates intercellular spaces32 with the spatial kinetic model of intra- cytoplasmic MEK sequentially at two sites. Each phos- cellular EGFR signaling with a positive feedback circuit,18 phorylation step is reversible. The double-phosphorylated now in the setting of a tissue consisting of cardiomyocytes MEK similarly activates ERK, and the positive feedback and blood capillaries. We show that the signaling can be loop is completed by activation of the ligand shedding by triggered directly by mechanical deformation of the tis- , , ERK.7 4 19 There is also a negative feedback loop in this sue, and demonstrate the possibility of self-organization of model, which operates through phosphorylation of Sos that autocrine signaling that is anisotropic on the tissue level is promoted by ERK and renders Sos unable to partici- and reflects the anisotropy of the mechanical deformation. Mechanotransduction in Cardiac Muscle 1321 These results of a computational analysis of spatial organization of the EGFR signaling on the tissue level point to the potential capabilities of the EGFR signaling network and suggest a novel perspective for studying not only the cardiac hypertrophic response but more generally other tissue mechanotransduction applications. MODEL The nonspatial aspects of the model for the EGFR autocrine circuit that we analyze here are the same as in our previously described model for self-polarization of a single autocrine cell.18 This nonspatial kinetic description is built, as from modules, from models that were developed by Kholodenko and colleagues for the assembly of the receptor signaling complex14 and the phosphorylation cascade.12 The model for the complex assembly was op- timized against experimental measurements. For the pur- poses of spatial modeling,18 the essential feature displayed by the model for the phosphorylation cascade is hyper- sensitivity of the output to the input signal.9 This type of signal transduction through the MAP kinase cascade, of which the Raf-MEK-ERK cascade concerned here is rep- resentative, has been observed in experiments and modeled similarly.10 These modules are embedded, providing more mechanistic detail necessary for spatially distributed modeling, in the overall kinetic framework for autocrine EGFR signaling with the positive and negative feedback that was developed by Shvartsman and colleagues.28 The latter model incorporated the measured kinetics of the receptor–ligand interactions and its nonlinear behavior was validated against experiments on tissue culture cells. No parameter values have been changed here compared to our previous model for an individual cell. However, it should be noted that as the size of cardiomyocytes is quite different from the size of the cells at which our previous modeling was aimed, the total concentrations of proteins in the cell are assumed to be conserved between the two models rather than the total numbers of proteins per cell. The total concentration in the cell is defined as the num- ber of protein molecules of a given species in the cell di- vided by the volume of the cytoplasm. While this quantity can be expressed in nM, for example, it does not imply that the spatial distribution of this protein in the cell is uniform. A unique aspect of the model presented here is that the FIGURE 2. Structure of the myocardium. (A) Fluorescence kinetics of EGFR autocrine signaling network is consid- microphotograph of normal mouse heart muscle. Blue, ered as distributed

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