Cells CRAWL from Soft to Stiff Surfaces as Myosin-II Polarizes the Cytoskeleton

Abstract

Scarring in the heart after a myocardial infarction, or scarring in the skin after wounding - lead to rigidification of tissue through extensive collagen crosslinking and can also lead to homing of adherent mesenchymal stem cells (MSCs). ‘Durotaxis’ describes the tendency for a cell to crawl from a soft, collagen-coated gel to an adjacent stiff matrix, but clear evidence for accumulation of any cell type has been lacking as has insight into molecular mechanisms. We cultured human MSCs on matrices with scar-like gradients in elastic modulus (stiffness) that are on order of 10 Pa/micron and document a bias in migration toward the stiff matrix with proliferation-independent accumulation taking just a couple of days. As found with other cell types, MSCs on stiff substrates show myosin-IIB is polarized toward the rear while the centrosome and microtubule (MT)-network are polarized toward the front, but such polarization is surprisingly absent from cells on soft substrates. With myosin-II inhibition, we find cells on stiff matrix crawl faster whereas cells on soft matrix are initially impeded but then transition to motile cells as their centrosomes and MT-networks polarize as they would on stiff matrix. While myosin-II is required for contractility but not migration, MTs are required for any migration - including durotaxis - but contractility on gels remain intact after destabilization of MTs. The model gel results thus show that the progressive polarization of myosin-II on stiff substrates is particularly key to durotaxis. The broader relevance of this conclusion is tested with decellularized heart tissue that permits an examination of MSC adhesion, contraction, and migration. Decellularized heart tissue was stiffened with chemical crosslinking to increase elastic modulus measured with atomic force microscopy. We see that this increased stiffness alters cell migration behavior.