\"Viscotaxis\" - Directed Migration of Mesenchymal Stem Cells in Response to Loss Modulus Gradient

Directed cell migration in response to chemical and mechanical gradients plays a crucial role in physiological and pathological conditions. One such mechanical cues that is known to influences cell migration is the gradient of substrate elastic modulus (E). However, the elastic modulus alone cannot fully define the material properties of the cellular microenvironment, which often has both elastic and viscous characteristics. In this study, we investigated the influence of the gradient of viscous nature, as defined by loss modulus, G", on cell migration. We cultured human mesenchymal stem cells (hMSCs) on a collagen-coated polyacrylamide gel with constant elastic property, as defined by the storage modulus G9, but with the gradient of loss modulus G". We found hMSCs to migrate from high to low loss modulus. We have termed this, thus far unreported, directional cellular migration as "Viscotaxis". We have confirmed uniform collagen density and constant storage modulus of the gel by fluorescence microscopy and atomic force microscopy to eliminate the possibilities of haptotaxis and durotaxis. We hypothesize that material creep in the high loss modulus region hinders the building up of the cellular traction, leading to a force asymmetry that drives the observed viscotaxis. To verify our hypothesis, we estimated the cellular traction on gels with high and low loss moduli. We indeed found that cells apply higher traction force on more elastic materials i.e. materials with low loss modulus. On the disruption of actomyosin contractility with myosin inhibitor blebbistatin and ROCK inhibitor Y27632, directional migration was lost. Further, we showed that cells can maintain a stable morphology on a low loss modulus substrate but due to its inability to build up stable cellular traction on a substrate with high loss modulus, the cell spreading remains in a dynamic state. Our findings in this paper highlight the importance of considering the viscous modulus while preparing stiffness-based substrates for the field of tissue engineering.