Introduction Abercrombie formulated: a cell’s relationship to the substratum is the key to cell crawling, and it is the changing adhesion of a cell to the substratum that moves a cell along (Abercrombie, 1978). Movement itself also requires traction, and the need for contractility to provide traction for cell movement was clear to the earliest investigators (Weiss, 1959). We now know that traction is exerted on the substrate via specialised adhesion sites that interface with the actin cytoskeleton. At least two classes of adhesion sites can be distinguished: focal adhesions, which interface with actin stress fibre bundles, and smaller, focal complexes, which are associated with actin filament networks and filopodia (reviewed in Burridge and Chrzanowska-Wodnicka, 1996; Small et al., 1999a). Focal complexes can serve as precursors of focal adhesions through a transition effected by changes in the balance of activities of members of the Rho GTPase family (Nobes and Hall, 1995; Rottner et al., 1999a). And both focal complexes and focal adhesions require contractility in the actin cytoskeleton for their formation and maintenance (Chrzanowska-Wodnicka and Burridge, 1996; Rottner et al., 1999a). Contractility, in turn, probably plays an important part in inside-out signalling events (Shyy and Chien, 1997), possibly through force-induced conformational changes in molecules of the extracellular matrix (Zhong et al., 1998), integrins (Vinogradova et al., 2000; Takagi et al., 2001) and cytoskeleton-associated components (Yamada and Geiger, 1997). Cell locomotion not only involves the mutual and dynamic reorganisation of the actin cytoskeleton and its associated adhesion sites but also mechanisms that confer polarity on these structural changes. Only then can traction forces in the actin cytoskeleton be converted into net movement (e.g. Beningo et al., 2001). Earlier findings attributed this polarising function to microtubules (Vasiliev and Gelfand, 1976), and more recent studies have revealed how they may achieve this role (Kaverina et al., 1998; Kaverina et al., 1999; Kaverina et al., 2000). It has been shown in fibroblasts that microtubules specifically target substrate adhesion sites and that these targeting events are followed by the turnover of adhesion sites or their dislocation from the substrate (Kaverina et al., 1998; Kaverina et al., 1999). Given the dependence of adhesion site maintenance on contractility, it was speculated that microtubules destabilise adhesions by delivering signals that antagonise the contractility pathway. This contention was supported by the demonstration that dissociation of adhesion sites at the cell edge could be mimicked by the local application of inhibitors of actomyosin contractility (Kaverina et al., 2000). The question addressed in the present study is what is the mechanism by which microtubules are guided to adhesion sites. We formerly showed that the local application of contractility inhibitors to a cell edge produced a rapid and local depolymerisation of microtubules towards the cell centre. In the present work, we have used alternative approaches to modulate radial stress at the cell periphery. By this means, we demonstrate that microtubules specifically invade regions where stress is locally increased. These findings highlight a stress-dependent feedback mechanism that presumably plays an important role in the selection of adhesion sites for targeted modulation via microtubules. Materials and Methods Cells and expression constructs Cells of the mouse melanoma line B16F1 (ATCC) were maintained 2283 Cell motility is driven by the sum of asymmetric traction forces exerted on the substrate through adhesion foci that interface with the actin cytoskeleton. Establishment of this asymmetry involves microtubules, which exert a destabilising effect on adhesion foci via targeting events. Here, we demonstrate the existence of a mechano-sensing mechanism that signals microtubule polymerisation and guidance of the microtubules towards adhesion sites under increased stress. Stress was applied either by manipulating the body of cells moving on glass with a microneedle or by stretching a flexible substrate that cells were migrating on. We propose a model for this mechano-sensing phenomenon whereby microtubule polymerisation is stimulated and guided through the interaction of a microtubule tip complex with actin filaments under tension. Movies available on-line Key words: Microtubules, Actin cytoskeleton, Adhesion, Tension, Mechanosensor Summary Tensile stress stimulates microtubule outgrowth in living cells Irina Kaverina 1 , Olga Krylyshkina 1 , Karen Beningo 2 , Kurt Anderson 3 , Yu-Li Wang 2 and J. Victor Small 1, * 1 Institute of Molecular Biology of the Austrian Academy of Sciences, A-5020, Austria 2 Department of Physiology, University of Massachusetts Medical School, Worcester, Massachusetts 01605, USA 3 Max Planck Institute for Molecular Cell Biology and Genetics, Pfotenhauerstr. 108, Dresden, D-01307, Germany *Author for correspondence (e-mail: jvsmall@imb.oeaw.ac.at) Accepted 21 March 2002 Journal of Cell Science 115, 2283-2291(2002) © The Company of Biologists Ltd Research Article