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Keratocytes pull with similar forces on their dorsal and ventral surfaces.

Galbraith CG, Sheetz MP - J. Cell Biol. (1999)

Bottom Line: Borisy. 1997.Cell Biol. 139:397-415).Similar forces were generated on both the ventral (0.2 nN/microm(2)) and the dorsal (0.4 nN/microm(2)) surfaces of the lamella, suggesting that dorsal matrix contacts are as effectively linked to the force-generating cytoskeleton as ventral contacts.

View Article: PubMed Central - PubMed

Affiliation: Department of Cell Biology, Duke University Medical Center, Durham, North Carolina 27710, USA.

ABSTRACT
As cells move forward, they pull rearward against extracellular matrices (ECMs), exerting traction forces. However, no rearward forces have been seen in the fish keratocyte. To address this discrepancy, we have measured the propulsive forces generated by the keratocyte lamella on both the ventral and the dorsal surfaces. On the ventral surface, a micromachined device revealed that traction forces were small and rearward directed under the lamella, changed direction in front of the nucleus, and became larger under the cell body. On the dorsal surface of the lamella, an optical gradient trap measured rearward forces generated against fibronectin-coated beads. The retrograde force exerted by the cell on the bead increased in the thickened region of the lamella where myosin condensation has been observed (Svitkina, T.M., A.B. Verkhovsky, K.M. McQuade, and G. G. Borisy. 1997. J. Cell Biol. 139:397-415). Similar forces were generated on both the ventral (0.2 nN/microm(2)) and the dorsal (0.4 nN/microm(2)) surfaces of the lamella, suggesting that dorsal matrix contacts are as effectively linked to the force-generating cytoskeleton as ventral contacts. The correlation between the level of traction force and the density of myosin suggests a model for keratocyte movement in which myosin condensation in the perinuclear region generates rearward forces in the lamella and forward forces in the cell rear.

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Model of keratocyte migration. Actin–myosin condensation in the perinuclear region generated rearward traction forces in the lamella by pulling on the orthogonal actin in this region. The condensation also generates forward forces that push the cell body forward. Finally, the perinuclear bundles generate the pincer forces that are orthogonal to the direction of migration. a, Orientation of traction forces measured by micromachined substrata and optical gradient trap. b, Orientation of actin and myosin network in keratocyte cytoskeleton (based on data from Svitkina et al. 1997).
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Figure 10: Model of keratocyte migration. Actin–myosin condensation in the perinuclear region generated rearward traction forces in the lamella by pulling on the orthogonal actin in this region. The condensation also generates forward forces that push the cell body forward. Finally, the perinuclear bundles generate the pincer forces that are orthogonal to the direction of migration. a, Orientation of traction forces measured by micromachined substrata and optical gradient trap. b, Orientation of actin and myosin network in keratocyte cytoskeleton (based on data from Svitkina et al. 1997).

Mentions: A model of keratocyte migration must account for the pattern of traction forces. The model needs to explain the rearward traction force in the lamella that increases in magnitude from the leading edge to the perinuclear region where it changes direction, and the model must provide a mechanism for generating the strong inward traction forces located on either side of the nucleus. We propose a model in which the actin–myosin condensation in the perinuclear region (Svitkina et al. 1997) generates the rearward traction forces under the lamella and forward movement of the cell body (Fig. 10). This mechanism is different from the dynamic network contraction model because it postulates that the condensing myosin pulls on the orthogonal array of lamella actin that is stationary with respect to the substratum. Support for this model comes from the polarity of the peripheral lamella actin (Svitkina et al. 1997) indicating that it could generate a rearward force, the continuity of the lamella actin with the condensed perinuclear fibers (Svitkina et al. 1997), and the magnitude of the traction force correlating well with the density of the myosin clusters. The proposed model also suggests that the forces generated in the condensation region are responsible for the change in traction force direction. The lamella generates rearward traction forces, and the periodic contractions of the condensation zone (Waterman-Storer, C., unpublished results) may exert sufficient force to pull the rear of the cell forward. Finally, the condensed perinuclear bundles probably generate the large forces on either side of the nucleus that are oriented perpendicular to the direction of migration. The perinuclear bundles are organized with their barbed ends oriented toward the cell periphery (Svitkina et al. 1997), which is the correct organization for myosin to generate the strong traction forces that are orthogonal to the direction of migration. These traction forces need to be stronger than the lamella traction forces to overcome the tighter adhesions to the substratum in these regions (Lee and Jacobson 1997). Moreover, the mixed polarity of these filaments in the center of the cell indicates that the mechanism of actin–myosin interaction is not sarcomeric, but is a contraction similar to that used to move fibroblasts on their ventral actin filaments (Cramer et al. 1997; Galbraith and Sheetz 1997). Thus, the condensation of actin and myosin in the perinuclear region may provide the force necessary for rearward lamellar traction force while organizing the longitudinal fibers that generate the traction forces orthogonal to the direction of migration.


Keratocytes pull with similar forces on their dorsal and ventral surfaces.

Galbraith CG, Sheetz MP - J. Cell Biol. (1999)

Model of keratocyte migration. Actin–myosin condensation in the perinuclear region generated rearward traction forces in the lamella by pulling on the orthogonal actin in this region. The condensation also generates forward forces that push the cell body forward. Finally, the perinuclear bundles generate the pincer forces that are orthogonal to the direction of migration. a, Orientation of traction forces measured by micromachined substrata and optical gradient trap. b, Orientation of actin and myosin network in keratocyte cytoskeleton (based on data from Svitkina et al. 1997).
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Related In: Results  -  Collection

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Figure 10: Model of keratocyte migration. Actin–myosin condensation in the perinuclear region generated rearward traction forces in the lamella by pulling on the orthogonal actin in this region. The condensation also generates forward forces that push the cell body forward. Finally, the perinuclear bundles generate the pincer forces that are orthogonal to the direction of migration. a, Orientation of traction forces measured by micromachined substrata and optical gradient trap. b, Orientation of actin and myosin network in keratocyte cytoskeleton (based on data from Svitkina et al. 1997).
Mentions: A model of keratocyte migration must account for the pattern of traction forces. The model needs to explain the rearward traction force in the lamella that increases in magnitude from the leading edge to the perinuclear region where it changes direction, and the model must provide a mechanism for generating the strong inward traction forces located on either side of the nucleus. We propose a model in which the actin–myosin condensation in the perinuclear region (Svitkina et al. 1997) generates the rearward traction forces under the lamella and forward movement of the cell body (Fig. 10). This mechanism is different from the dynamic network contraction model because it postulates that the condensing myosin pulls on the orthogonal array of lamella actin that is stationary with respect to the substratum. Support for this model comes from the polarity of the peripheral lamella actin (Svitkina et al. 1997) indicating that it could generate a rearward force, the continuity of the lamella actin with the condensed perinuclear fibers (Svitkina et al. 1997), and the magnitude of the traction force correlating well with the density of the myosin clusters. The proposed model also suggests that the forces generated in the condensation region are responsible for the change in traction force direction. The lamella generates rearward traction forces, and the periodic contractions of the condensation zone (Waterman-Storer, C., unpublished results) may exert sufficient force to pull the rear of the cell forward. Finally, the condensed perinuclear bundles probably generate the large forces on either side of the nucleus that are oriented perpendicular to the direction of migration. The perinuclear bundles are organized with their barbed ends oriented toward the cell periphery (Svitkina et al. 1997), which is the correct organization for myosin to generate the strong traction forces that are orthogonal to the direction of migration. These traction forces need to be stronger than the lamella traction forces to overcome the tighter adhesions to the substratum in these regions (Lee and Jacobson 1997). Moreover, the mixed polarity of these filaments in the center of the cell indicates that the mechanism of actin–myosin interaction is not sarcomeric, but is a contraction similar to that used to move fibroblasts on their ventral actin filaments (Cramer et al. 1997; Galbraith and Sheetz 1997). Thus, the condensation of actin and myosin in the perinuclear region may provide the force necessary for rearward lamellar traction force while organizing the longitudinal fibers that generate the traction forces orthogonal to the direction of migration.

Bottom Line: Borisy. 1997.Cell Biol. 139:397-415).Similar forces were generated on both the ventral (0.2 nN/microm(2)) and the dorsal (0.4 nN/microm(2)) surfaces of the lamella, suggesting that dorsal matrix contacts are as effectively linked to the force-generating cytoskeleton as ventral contacts.

View Article: PubMed Central - PubMed

Affiliation: Department of Cell Biology, Duke University Medical Center, Durham, North Carolina 27710, USA.

ABSTRACT
As cells move forward, they pull rearward against extracellular matrices (ECMs), exerting traction forces. However, no rearward forces have been seen in the fish keratocyte. To address this discrepancy, we have measured the propulsive forces generated by the keratocyte lamella on both the ventral and the dorsal surfaces. On the ventral surface, a micromachined device revealed that traction forces were small and rearward directed under the lamella, changed direction in front of the nucleus, and became larger under the cell body. On the dorsal surface of the lamella, an optical gradient trap measured rearward forces generated against fibronectin-coated beads. The retrograde force exerted by the cell on the bead increased in the thickened region of the lamella where myosin condensation has been observed (Svitkina, T.M., A.B. Verkhovsky, K.M. McQuade, and G. G. Borisy. 1997. J. Cell Biol. 139:397-415). Similar forces were generated on both the ventral (0.2 nN/microm(2)) and the dorsal (0.4 nN/microm(2)) surfaces of the lamella, suggesting that dorsal matrix contacts are as effectively linked to the force-generating cytoskeleton as ventral contacts. The correlation between the level of traction force and the density of myosin suggests a model for keratocyte movement in which myosin condensation in the perinuclear region generates rearward forces in the lamella and forward forces in the cell rear.

Show MeSH
Related in: MedlinePlus