Poster P5 - Pan-tissue percolation scaling of viscoelasticity with tissue collagen reflects contractile strain-inhibited fibril degradation

Introduction:

Introduction: Fibrillar collagens are the most abundant proteins in animals and form networks of different densities in animal tissues. Extremely soft tissues such as embryonic hearts and adult brain possess far less collagen than highly stiff adult mouse tendons and sustain lower mechanical stress. We investigated fibrillar collagen networks and their percolation limits in diverse tissues including heart, brain, kidney, lung, muscle, and tendon derived from embryonic chick, mouse, and even adult human (for heart). We conclude that mechanisms of collagen accumulation are determined in part by strain inhibited degradation, and across the studied tissues, stiffness follows characteristic power-law scaling in polymer network density and reflects an underlying gelation transition.

Materials and

Methods: Materials and

Methods: For studying fibrillar collagen levels and changes with time in fresh tissues ex vivo, we primarily use label-free second harmonic generation (SHG) imaging to track any kinetics over ~hr timescales. Tissues are progressively degraded by collagenase (Fig.1) or else exposed to cellular contractility inhibitor(s) (Fig.2) in combination or not with inhibitors of endogenous matrix metalloproteinases (MMP). Mass-spectrometry (MS) complements some of the SHG measurements. We employ a custom-designed environmentally controlled mechano-bioreactor for some measurements, particularly for tendon subjected to external strain plus purified MMP-1 or collagenase. Micropipette aspiration is used to measure multiple tissue physical properties: elasticity, viscosity, and plasticity.

Results, Conclusions, and Discussions: Results and

Discussion: When we combine many tissue SHG imaging with physical measurements our selected tissues as well as with median tissue stiffness values from the literature, we discover a pan-tissue percolation exponent (z ~ 1.4) that also predicts a similar power-law exponent for scaling of elasticity, viscosity, and plasticity with collagen-fiber density across adult and embryonic tissues. Pan-tissue scaling of tissue stiffness with Collagen-I levels from MS measurements relates closely to SHG results and yields a sub-linear scaling that nonetheless aligns with past results from cellularized gels of Collagen-I but not acellular gels. Verification of collagen-I protein scaling with SHG signal forms the basis for a contractile-strain stabilized feedback loop, which predicts scaling of tissue stiffness with collagen-I RNA. Importantly, the inhibition of tissue contractile strains fits the scaling, and combination with inhibitors of matrix metalloproteinases (MMPs) show collagenase activity is strain - not stress - suppressed in tissues when tissues are subjected to physiological strains ~5% limit.

Conclusions: Universality of collagen-fiber percolation together with stiffness scaling based on 'use-it-or-lose-it' kinetics not only provides matrix-based metrics for live tissue health, pathology, and regeneration but also provides insight into organ-size scaling with body-weight while suggesting contractility-driven therapeutics. Mechano-bioreactor designs proposed in the present work can provide simultaneous measurements of tissue mechanical strains and structure of micro-scale samples in a high-throughput manner when compared to conventional mechanical testing techniques. Key questions in ongoing work relate to age of the animal studied and the 'use-it-or-lose-it' responsiveness of Collagen-I protein levels.

Acknowledgements (Optional):