(53g) Multi-Scale Dynamics of Biological Systems and Active Matter

Mechanics of the extracellular matrix (ECM) play a pivotal role in shaping diverse biological processes relevant to human development, aging, and disease. However, cells do not simply passively respond to their ECM, but also actively remodel it. These reciprocal tissue dynamics emerge from a collaboration among molecular, cellular, and collective processes that are weaved across a formidably broad cascade of time scales. Thus, there exists a crucial need for techniques that can non-invasively capture a panoramic view of the local dynamics of the ECM throughout the course of biological phenomena.

In this work, we develop a non-destructive and non-invasive microrheology technique, based on dynamic light scattering, for interrogating the hierarchical viscoelastic behavior of biological materials. This dynamic light scattering microrheology (DLSμR) illuminates a vast range of time scales, ranging from 10-6 to 102 seconds. The time scale range accessed by DLSμR encompasses a broad range of biological processes relevant to cell-ECM interactions, including the bend fluctuations of actin filaments, the power stroke of myosin motors, and the turnover of cell-matrix adhesions.

We harness these capabilities to study the reciprocal interactions that occur between mammalian cells and their ECM using 3D in vitro cell culture systems. The broadband capabilities of DLSμR enable us to develop a force spectrum analysis (FSA) technique to quantify the time scale-dependent thermal and active force fluctuation spectrum generated within the ECM. We leverage this FSA to demonstrate the existence of distinct temporal regimes over which thermal and active forces drive ECM dynamics in 3D culture of contractile stromal cells. We find that the slowly fluctuating forces driven by the contractile actin cytoskeleton produce significantly different responses in the surrounding collagenous matrix at different time scales. On time scales much shorter than the fluctuations of cell-generated forces, active cell-generated tension stiffens the ECM due to its nonlinear stress-stiffening behavior. On longer time scales, active fluctuating forces couple to the viscoelasticity of the matrix to drive flow of the ECM.

Finally, we leverage these advancements to probe how matrix remodeling impacts breast cancer spheroids during 3D invasion and morphogenesis. We find that transforming growth factor β (TGFβ) induced invasion is directly regulated by loss of basement membrane proteins and contact with collagen I. In turn TGFβ and collagen I cooperatively drive a breast cancer phenotype that actively alters matrix viscoelasticity and dynamics. Paradoxically, matrix degradation by matrix metalloproteinases, which fluidizes the matrix in its low frequency response, is absolutely required for matrix stiffening in the high-frequency viscoelastic spectrum. This MMP-mediated fluidization cooperates with the slowly fluctuating contractile force spectrum of invasive cells to drive viscoplastic remodeling that alters matrix viscoelasticity across 7 decades in time. Our results highlight the dynamic and reciprocal nature of cell matrix interactions and emphasize the multi-time scale action of the physical forces involved.