Intracellular fluid flow in rapidly moving cells

Abstract

Cytosolic fluid dynamics have been implicated in cell motility 1-5 because of the hydrodynamic forces they induce and because of their influence on transport of components of the actin machinery to the leading edge. To investigate the existence and the direction of fluid flow in rapidly moving cells, we introduced inert quantum dots into the lamellipodia of fish epithelial keratocytes and analysed their distribution and motion. Our results indicate that fluid flow is directed from the cell body towards the leading edge in the cell frame of reference, at about 40% of cell speed. We propose that this forward-directed flow is driven by increased hydrostatic pressure generated at the rear of the cell by myosin contraction, and show that inhibition of myosin II activity by blebbistatin reverses the direction of fluid flow and leads to a decrease in keratocyte speed. We present a physical model for fluid pressure and flow in moving cells that quantitatively accounts for our experimental data.Various indirect results 4,6 suggest that fluid influx at the leading edge could play an active part in actin-based cell motility by generating hydrodynamic forces that oppose the membrane load and thus increase the rate of actin polymerization at a protruding edge. Notably, the expression of aquaporins, which are highly enriched at the leading edge and increase water permeability of the membrane, accelerates motility and increases the metastatic potential of melanoma cells4 , 7. Alternatively, it has been suggested that intracellular fluid flow towards the leading edge might contribute to motility by expediting transport of actin and other soluble proteins to the leading edge8. Direct measurements of fluid flow in the lamellipodia of moving cells are © 2009 Macmillan Publishers Limited. All rights reserved.7Correspondence should be addressed to K. K. (kinneret@ph.technion.ac.il). AUTHOR CONTRIBUTIONS K.K. and J.A.T. conceived and designed the experiments; K.K. performed the experiments and analysed the data; A.M. and K.K. developed the model; P.T.Y. made the initial observation of enhancement of large probes at the leading edge; A.K. contributed to the single-particle tracking experiments; K.K., A.M., P.T.Y. and J.A.T. discussed the results and wrote the paper.Note: Supplementary Information is available on the Nature Cell Biology website. COMPETING INTERESTSThe authors declare that they have no competing financial interest. Fish keratocytes are a simple and widely used model system for studying the dynamics of the motility process. They are among the fastest moving animal cells, with average speeds of about 0.3 μm s -1 , yet their motion is extremely persistent, with hardly any change in cell shape, speed or direction over many minutes12 ,13 . In these cells, any intracellular fluid flow associated with motility should be persistent because of the steady-state nature of their motion. Furthermore, the broad, flat and extremely thin (~100-200 nm) 10 lamellipodia of these cells alone are sufficient for persistent motility12...

Actin disassembly clock determines shape and speed of lamellipodial fragments

“…Myosin contraction can contribute to rear retraction as well as promote motility by enhancing actin disassembly (7) and inducing forward directed fluid flow (27). However, it is not required; knockouts and inhibition experiments have shown that although myosin II typically promotes movement and increases cell speed, it is not essential for motility in many cell types including keratocytes (5,7,27), Acanthamoeba (28), and Dictyostelium discoideum (29). Here we show that myosin II has a negligible role in rear retraction in keratocyte fragments (Fig.…”

Hydrodynamic property of the cytoplasm is sufficient to mediate cytoplasmic streaming in the Caenorhabiditis elegans embryo

“…Cytoplasmic streaming in the green algae Chara and Nitella has been proposed to have a role in mixing nutrients (4). In migrating cells, cytoplasmic streaming is considered to facilitate the transport of actin monomers in the direction of cell movement (5,6).…”

Asymmetry between pushing and pulling for crawling cells

“…Instead, we assume that depolymerization takes place everywhere in the bulk of the cell as it is suggested by observations [66][67][68][69].…”

“…We therefore assume a smaller value v + 1 which is also plausible in view of [28,31,67,68]. In the presence of elasticity penalizing infinite stretching such re-scaling is not necessary.…”