Carina Wollnik1, Benjamin Eltzner2, Kwang-Rae Kim2, Ina  Schachtschneider2, Carsten Gottschlich2, Stephan Huckemann2 and Florian Rehfeldt1

1 Third Institute of Physics - Biophysics, Georg-August-University, Göttingen, Germany
2 Institute for Mathematical Stochastics, Georg-August-University, Göttingen, Germany

Adult stem cells like human mesenchymal stem cells (hMSCs) from bone marrow differentiate into a variety of different cell types like nerve, bone, muscle, cartilage or fat cells. For this process mechanical cues as well as biochemical ones need to orchestrate. Strikingly, even substrate stiffness is enough to guide hMSCs towards different lineages in the absence of additional biochemical stimuli [1].
Stress fibres composed of actin filaments, actin binding- and crosslinking-proteins as well as non-muscle myosin motor proteins, generate and transmit forces within the cell and to the extracellular matrix. Inhibition of motor proteins shuts down the cell differentiation process [1], demonstrating that stress fibre mechanical tension is crucial for differentiaton.
Though the differentiation process takes weeks, early characteristic reorganisation of the actin-cytoskeleton can be detected within the first 24 hours and can be used as an early morphological marker [2].
To gain a deeper understanding of the role stress fibres play in the early stage of stem cell differentiation, we trace stress fibres during the first 24 hours after seeding the cells on substrates of different stiffness. We use life-cell imaging of RFP-LifeAct transfected hMSCs and analyse acto-myosin stress fibre behaviour with novel sophisticated filament tracking algorithms (based on [3]) to gain a better understanding of the dynamics of stress fibre formation that leads to a non-monotonic dependence of stress fibre polarization (quantified by order parameter S)
on the Youngs modulus of the underlying substrate [2].

References

[1] A. J. Engler, S. Sen, H. L. Sweeney and D E. Discher, Matrix elasticity directs stem cell lineage specication Cell (2006) 126 677-689

[2] A. Zemel, F. Rehfeldt, A.E.X. Brown, D. E. Discher and S. A. Safran, Optimal matrix rigidity for stress-fibre polarization in stem cells Nature Physics (2010) 6 468-473.

[3] C. Gottschlich, P. Mihailescu and A. Munk, Robust Orientation Field Estimation and Extrapolation Using Semilocal Line Sensors IEEE Transactions on Information Forensics and Security (2009) 4 802-811.