Calcium transients induce spatially coordinated increases in traction force during the movement of fish keratocytes

Ca2+ influx through mechanosensitive channels inhibits neurite outgrowth in opposition to other influx pathways and release from intracellular stores

Ca2+ signals are known to be important regulators of neurite outgrowth and steering. Here we show that inhibiting Ca2+ influx through stretch-activated channels using various compounds, including a highly specific peptide isolated from Grammostola spatulata spider venom (GsMTx4), strongly accelerates the rate of neurite extension on diverse substrata and within the intact spinal cord. Consistent with the presence of stretch-activated channels, we show that Ca2+ influx is triggered by hypotonic solutions, which can be partially blocked by GsMTx4. Finally, chelating local, but not global, Ca2+ signals prevents the acceleration that is normally produced by GsMTx4. Blocking Ca2+ influx through other channel types has little or opposite effects, but release from intracellular stores is required for maximal acceleration. Together, our data suggest that Ca2+ functions at distinct microdomains in growth cones, with influx through mechanosensitive channels acting to inhibit outgrowth in opposition to influx through other plasma membrane channels and release from stores.

Substrate rigidity and force define form through tyrosine phosphatase and kinase pathways

Cell forces define cell morphology, alterations in which are caused by tyrosine kinase and phosphatase mutations, which implies a causal linkage. Recent studies have shown that phosphotyrosine signaling is involved in force sensing for cells on flat surfaces. Early force-dependent activation of Src family kinases by phosphatases or cytoskeleton stretch leads to the activation of downstream signaling. In addition, force generation by cells depends on a feedback mechanism between matrix rigidity or force generation and myosin contractility. Components of the force-sensing pathway are linked to the integrin-cytoskeleton complex at sites of force application and serve as scaffolds for signaling processes. Thus, early events in force detection are mechanically induced cytoskeletal changes that result in biochemical signals to mechanoresponsive pathways that then regulate cell form.

Organizing moving groups during morphogenesis

The directed migration of cells drives the formation of many complex organ systems. Although in this morphogenetic context cells display a strong preference for migrating in organized, cohesive groups, little is known about the mechanisms that coordinate their movements. Recent studies on several model systems have begun to dissect the organization of these migrating tissues in vivo and have shown that cell guidance is mediated by a combination of chemical and mechanical cues.

Tissue cells feel and respond to the stiffness of their substrate

Normal tissue cells are generally not viable when suspended in a fluid and are therefore said to be anchorage dependent. Such cells must adhere to a solid, but a solid can be as rigid as glass or softer than a baby's skin. The behavior of some cells on soft materials is characteristic of important phenotypes; for example, cell growth on soft agar gels is used to identify cancer cells. However, an understanding of how tissue cells-including fibroblasts, myocytes, neurons, and other cell types-sense matrix stiffness is just emerging with quantitative studies of cells adhering to gels (or to other cells) with which elasticity can be tuned to approximate that of tissues. Key roles in molecular pathways are played by adhesion complexes and the actinmyosin cytoskeleton, whose contractile forces are transmitted through transcellular structures. The feedback of local matrix stiffness on cell state likely has important implications for development, differentiation, disease, and regeneration.

Direct mechanical force measurements during the migration of Dictyostelium slugs using flexible substrata

We use the flexible substrate method to study how and where mechanical forces are exerted during the migration of Dictyostelium slugs. This old and contentious issue has been left poorly understood so far. We are able to identify clearly separate friction forces in the tip and in the tail of the slug, traction forces mostly localized in the inner slug/surface contact area in the prespore region and large perpendicular forces directed in the outward direction at the outline of contact area. Surprisingly, the magnitude of friction and traction forces is decreasing with slug velocity indicating that these quantities are probably related to the dynamics of cell/substrate adhesion complexes. Contrary to what is always assumed in models and simulations, friction is not of fluid type (viscous drag) but rather close to solid friction. We suggest that the slime sheath confining laterally the cell mass of the slug experiences a tension that in turn is pulling out the elastic substrate in the direction tangential to the slug profile where sheath is anchored. In addition, we show in the appendix that the iterative method we developed is well adapted to study forces over large and continuous fields when the experimental error is sufficiently low and when the plane of recorded bead deformations is close enough to the elastomer surface, requirements fulfilled in this experimental study of Dictyostelium slugs.

Direct comparison of the spread area, contractility, and migration of balb/c 3T3 fibroblasts adhered to fibronectin- and RGD-modified substrata

Native proteins are often substituted by short peptide sequences. These peptides can recapitulate key, but not all biofunctional properties of the native proteins. Here, we quantify the similarities and differences in spread area, contractile activity, and migration speed for balb/c 3T3 fibroblasts adhered to fibronectin- (FN) and Arg-Gly-Asp (RGD)-modified substrata of varying surface density. In both cases spread area has a biphasic dependence on surface ligand density (sigma) with a maximum at sigma approximately 200 molecules/microm2, whereas the total traction force increases and reaches a plateau as a function of sigma. In addition to these qualitative similarities, there are significant quantitative differences between fibroblasts adhered to FN and RGD. For example, fibroblasts on FN have a spread area that is on average greater by approximately 200 microm2 over a approximately 40-fold change in sigma. In addition, fibroblasts on FN exert approximately 3-5 times more total force, which reaches a maximum at a value of sigma approximately 5 times less than for cells adhered to RGD. The data also indicate that the differences in traction are not simply a function of the degree of spreading. In fact, fibroblasts on FN (sigma approximately 2000 microm(-2)) and RGD (sigma approximately 200 microm(-2)) have both similar spread area (approximately 600 microm2) and migration speed (approximately 11 microm/h), yet the total force production is five times higher on FN than RGD (approximately 0.05 dyn compared to approximately 0.01 dyn). Thus, the specific interactions between fibroblasts and FN molecules must inherently allow for higher traction force generation in comparison to the interactions between fibroblasts and RGD.

Cyclic changes in keratocyte speed and traction stress arise from Ca2+-dependent regulation of cell adhesiveness

The activation of stretch-activated calcium channels (SACs) in keratocytes can induce spatially coordinated increases in traction stress that promote protrusion at the cell front, while simultaneously inducing retraction at the rear. To investigate how this occurs, we correlated calcium-induced changes in traction stress with alterations in cell speed and shape. Cyclic changes in these parameters were associated with each calcium transient. In addition, an inverse relationship was found between traction stress and cell speed, suggesting that alternating changes in adhesiveness were occurring at the rear. We investigated this further by inhibiting or inducing calcium transients and observing the effects on traction stress, cell speed and shape. Inhibition of calcium transients prevented retraction and led to a slow increase in traction stress. In addition, large aggregates of vinculin developed at the lateral rear edges of treated keratocytes, consistent with an increase in adhesiveness. Induction of a calcium transient resulted in a rapid retraction, involving both increased traction stress and adhesion disassembly at the rear. We also found that keratocytes exhibiting frequent transients generated larger traction stress and moved significantly faster than other cells. Together, these data suggest that calcium transients coordinate changes in adhesiveness with SAC-mediated cycles of mechano-chemical feedback.

Slipping or gripping? Fluorescent speckle microscopy in fish keratocytes reveals two different mechanisms for generating a retrograde flow of actin

Fish keratocytes can generate rearward directed traction forces within front portions of the lamellipodium, suggesting that a retrograde flow of actin may also occur here but this was not detected by previous photoactivation experiments. To investigate the relationship between retrograde flow and traction force generation, we have transfected keratocytes with GFP-actin and used fluorescent speckle microscopy, to observe speckle flow. We detected a retrograde flow of actin within the leading lamellipodium that is inversely proportional to both protrusion rate and cell speed. To observe the effect of reducing contractility, we treated transfected cells with ML7, a potent inhibitor of myosin II. Surprisingly, ML7 treatment led to an increase in retrograde flow rate, together with a decrease in protrusion and cell speed, but only in rapidly moving cells. In slower moving cells, retrograde flow decreased, whereas protrusion rate and cell speed increased. These results suggest that there are two mechanisms for producing retrograde flow. One involves slippage between the cytoskeleton and adhesions, that decreases traction force production. The other involves slippage between adhesions and the substratum, which increases traction force production. We conclude that a biphasic relationship exists between retrograde actin flow and adhesiveness in moving keratocytes.