(390f) Derivation of a Kinematic Model Coupling Stress Fiber Dynamics with JNK Activation In Response to Matrix Stretching

Adherent cells in many tissues, including arteries, bladder, muscle, and tendon, to name a few, are subjected to stretch as the tissue deforms in response to external forces. Mechanical stretch regulates many cellular functions, including proliferation, apoptosis, migration and morphology that occur in response to stretch-induced changes in intracellular signaling and gene expression (Haga et al., 2007). While the mechanisms that regulate mechanotransduction remain unclear, focal adhesions and the actin cytoskeleton clearly play a role (Bershadsky et al., 2003). It is well established that focal adhesions and stress fibers are interdependent. This is supported by the fact that focal adhesions anchor stress fibers to the matrix and the integrity of the actin cytoskeleton is necessary to maintain focal adhesions (Volberg et al., 1994). Integrins have been implicated as mechanosensors responsible for stretch-induced intracellular signaling (Thodeti et al., 2009). Katsumi et al. (2005) reported that c-jun N-terminal kinase (JNK) activation in response to stretch requires the formation of new integrin–matrix bonds. We have shown that cells, along with their stress fibers and focal adhesions, orient perpendicular to the direction of cyclic uniaxial stretch (Kaunas et al., 2005). The time course of cyclic stretch-induced JNK activation corresponds with that of stress fiber realignment (Kaunas et al., 2006). Specifically, cyclic uniaxial stretch causes the transient activation of JNK that subsides as the stress fibers become oriented perpendicular to the direction of stretch. Stretch-induced stress fiber alignment occurs via disassembly and reassembly of stress fibers (Hayakawa et al., 2001), which would result in the formation of new integrin–matrix bonds at the associated focal adhesions. Together, these studies suggest that stretch-induced JNK activation is upregulated during times when stress fibers and their associated integrin–matrix adhesions are undergoing relatively high rates of disassembly and reassembly. It is of interest to investigate intracellular signaling that is associated with JNK activation in response to different patterns of stretch stimulation using a mathematical modeling approach, as it can provide systems-level insight into the complex relationships between matrix stretch pattern, stress fiber dynamics, integrin turnover, and JNK signaling.

In order to link the kinetics of intracellular signaling with different patterns of stretch, a model is needed that describes the temporal and spatial evolution of stress fiber organization in response to stretch. Recently, computational modeling has been used to elucidate the complex interplay between matrix deformation, cytoskeletal and integrin bond dynamics and stress fiber reorganization. For example, Kaunas and Hsu (2009) have formulated a computational model of stress fiber dynamic reorganization based on constrained mixture theory (Humphrey, 2008) that successfully describes published time courses of stress fiber reorientation in response to cyclic uniaxial and equibiaxial stretches. Based on the interdependence of stress fibers, integrins, and JNK activation, we coupled the stress fiber dynamics model presented by Kaunas and Hsu (2009) to a kinetic model of JNK activation in this work. Specifically, we developed a mathematical model coupling the dynamic disassembly and reassembly of actin stress fibers and associated focal adhesions to the activation of JNK in cells attached to deformable matrices. The model is based on the assumptions that stress fibers are pre-extended to a preferred level under static conditions and that perturbations from this preferred level destabilize the stress fibers. Numerical solutions of the developed model equations predict that different patterns of matrix stretch result in distinct temporal patterns in JNK activation that compare well with published experimental results. In the case of cyclic uniaxial stretching, stretch-induced JNK activation slowly subsides as stress fibers gradually reorient perpendicular to the stretch direction. In contrast, JNK activation is chronically elevated in response to cyclic equibiaxial stretch. A step change in either uniaxial or equibiaxial stretch results in a short, transient upregulation in JNK that quickly returns to the basal level as overly stretched stress fibers disassemble and are replaced by fibers assembled at the preferred level of stretch. In summary, the model describes a mechanism by which the dynamic properties of the actin cytoskeleton allow cells to adapt to applied forces through turnover and reorganization to modulate intracellular signaling.

References

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