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Altered tissue mechanics is an important signature of invasive solid tumors. While the phenomena have been extensively studied by measuring the bulk rheology of the extracellular matrix (ECM) surrounding tumors, micromechanical remodeling at the cellular scale remains poorly understood. By combining holographic optical tweezers and confocal microscopy on in vitro tumor models, we show that the micromechanics of collagen ECM surrounding an invading tumor demonstrate directional anisotropy, spatial heterogeneity and significant variations in time as tumors invade. To test the cellular mechanisms of ECM micromechanical remodeling, we construct a simple computational model and verify its predictions with experiments. We find that collective force generation of a tumor stiffens the ECM and leads to anisotropic local mechanics such that the extension direction is more rigid than the compression direction. ECM degradation by cell-secreted matrix metalloproteinase softens the ECM, and active traction forces from individual disseminated cells re-stiffen the matrix. Together, these results identify plausible biophysical mechanisms responsible for the remodeled ECM micromechanics surrounding an invading tumor.
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MICROMECHANICAL MODEL OF MECHANOSENSITIVE COLLAGEN TISSUES
The mechanical behavior of soft collagenous tissues is largely influenced by the reinforcing collagen fiber microstructure. The anisotropic collagen microstructure can remodel in response to changes in mechanical loading, which can dramatically alter the mechanical properties of the tissues and the mechanical environment of the resident cells. It is important to study the remodeling mechanisms of collagen tissues to understand the pathophysiology of various connective tissue diseases. We hypothesize that the collagen structure actively changes in response to mechanical stimuli through concurrent processes of collagen deposition and degradation and that the rates of these processes are altered by collagen mechanochemistry, mechanosensitive collagen production, and cellular contraction. In prior studies, we developed micromechanical models of collagen tissues to investigate the role of collagen mechanochemistry and mechanosensitive collagen production in remodeling the collagen fiber structure and tissue growth.[1,2] We found that stress inhibition of enzymatic degradation and stimulation of collagen production can explain many phenomena, including remodeling the anisotropic collagen structure along the directions of the maximum principal stress and the development of stress homeostasis. The goal of this study is to investigate the effect of mechanical loading on the active behavior of the cells. Our approach uses a model 3D microtissue systems, self-assembled on a magnetically actuated two-pillar system (µTUG), to investigate these cell-collagen interactions and effects of mechanical loading. The micropillar support allows for measurement of the active cellular contraction, while the magnetic tweezer allows for mechanical testing of the microtissue under a controlled stress rate. Digital image analysis is applied to measure the local two-dimensional (2D) strain field. To analyze the mechanical measurements for mechanical properties of the collagen structure and active behavior of the cells, we developed a micromechanical model for the mechanical behavior of the microtissue. The micromechanical model includes the elastic behavior of the anisotropic collagen structure and the anisotropic active behavior of the cells. To describe mechanosensitive cellular contraction, we assume concurrent polymerization/depolymerization of actin filaments, where the polymerization rate increases with the fiber stress. In this paper, we will briefly summarize the model and describe an initial model validation by comparing to µTUG experiments measuring the stress-strain behavior of the microtissue to load-unload tests.
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- Award ID(s):
- 2032922
- PAR ID:
- 10548645
- Publisher / Repository:
- Summer Biomechanics, Bioengineering and Biotransport Conference
- Date Published:
- Subject(s) / Keyword(s):
- mechanobiology constitutive modeling viscoelasticity
- Format(s):
- Medium: X
- Location:
- Vail, Colorado, USA
- Sponsoring Org:
- National Science Foundation
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Introduction— In response to external stress, cells alter their morphology, metabolic activity, and functions to mechanically adapt to the dynamic, local environment through cell allostasis. To explore mechanotransduction in cellular allostasis, we applied an integrated micromechanical system that combines an ‘ultrasound tweezers’-based mechanical stressor and a Förster resonance energy transfer (FRET)-based molecular force biosensor, termed “actinin-sstFRET,” to monitor in situ single-cell allostasis in response to transient stimulation in real time. Methods— The ultrasound tweezers utilize 1 Hz, 10-second transient ultrasound pulses to acoustically excite a lipid-encapsulated microbubble, which is bound to the cell membrane, and apply a pico- to nano-Newton range of forces to cells through an RGD-integrin linkage. The actinin-sstFRET molecular sensor, which engages the actin stress fibers in live cells, is used to map real-time actomyosin force dynamics over time. Then, the mechanosensitive behaviors were examined by profiling the dynamics in Ca2+ influx, actomyosin cytoskeleton (CSK) activity, and GTPase RhoA signaling to define a single-cell mechanical allostasis. Results—By subjecting a 1 Hz, 10-second physical stress, single vascular smooth muscle cells (VSMCs) were observed to remodeled themselves in a biphasic mechanical allostatic manner within 30 minutes that caused them to adjust their contractility and actomyosin activities. The cellular machinery that underscores the vital role of CSK equilibrium in cellular mechanical allostasis, includes Ca2+ influx, remodeling of actomyosin CSK and contraction, and GTPase RhoA signaling. Mechanical allostasis was observed to be compromised in VSMCs from patients with type II diabetes mellitus (T2DM), which could potentiate an allostatic maladaptation. Conclusions— By integrating tools that simultaneously permit localized mechanical perturbation and map actomyosin forces, we revealed distinct cellular mechanical allostasis profiles in our micromechanical system. Our findings of cell mechanical allostasis and maladaptation provide the potential for mechanophenotyping cells to reveal their pathogenic contexts and their biophysical mediators that underlie multi-etiological diseases such as diabetes, hypertension, or aging.more » « less
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Tissues grow and remodel in response to mechanical cues, extracellular and intracellular signals experienced through various biological events, from the developing embryo to disease and aging. The macroscale response of soft tissues is typically nonlinear, viscoelastic anisotropic, and often emerges from the hierarchical structure of tissues, primarily their biopolymer fiber networks at the microscale. The adaptation to mechanical cues is likewise a multiscale phenomenon. Cell mechanobiology, the ability of cells to transform mechanical inputs into chemical signaling inside the cell, and subsequent regulation of cellular behavior through intra- and inter-cellular signaling networks, is the key coupling at the microscale between the mechanical cues and the mechanical adaptation seen macroscopically. To fully understand mechanics of tissues in growth and remodeling as observed at the tissue level, multiscale models of tissue mechanobiology are essential. In this review, we summarize the state-of-the art modeling tools of soft tissues at both scales, the tissue level response, and the cell scale mechanobiology models. To help the interested reader become more familiar with these modeling frameworks, we also show representative examples. Our aim here is to bring together scientists from different disciplines and enable the future leap in multiscale modeling of tissue mechanobiology.more » « less
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Accepted Manuscript1.0
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