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The Mechanics of Intracellular Transport Developmental Cell Previews Cutting through the Noise: The Mechanics of Intracellular Transport Samantha Stam1,2 and Margaret L. Gardel2,3,* 1Biophysical Sciences Graduate Program, University of Chicago, Chicago, IL 60637, USA 2James Franck Institute and Institute for Biophysical Dynamics, University of Chicago, Chicago, IL 60637, USA 3Department of Physics, University of Chicago, Chicago, IL 60637, USA *Correspondence: [email protected] http://dx.doi.org/10.1016/j.devcel.2014.08.013 Intracellular transport of organelles and proteins is driven by multiple ATP-dependent processes. Recently in Cell, Guo et al. (2014) developed a technique, force-spectrum microscopy, to measure intracellular forces and demonstrate that large motion of cellular components can be produced by random ATP-dependent fluc- tuations within the cytoplasm. Intracellular transport is crucial to diverse mechanisms, and they overcome the Here, Guo et al. (2014) provide the physiological tasks. The cell employs limitations of diffusive transport in at first measurements to directly charac- multiple mechanisms to meet the de- least two distinct ways. One well-appre- terize these ATP-dependent yet random mands of rapidly transporting cell con- ciated mechanism is that molecular forces within the cytoplasm. The authors tents of varying size over large distances, motor proteins drive directed transport measured the mechanics of the cyto- ranging from microns to up to a meter, to of attached cargo along filament tracks plasm using optical tweezers to apply support specific physiological tasks (Figure 1C, blue and black) (Howard, forces to inert particles microinjected (Figure 1A). In a recent issue of Cell, Guo 2001). Motors transport cargo along into the cytoplasm. The authors found et al. (2014) provide insights into the cytoskeletal tracks with a characteristic that the cytoplasm was predominately molecular forces governing intracellular velocity resulting in an MSD that is elastic, meaning that particles tend to transport. described as ‘‘ballistic’’ or superdiffusive. return to their original position after the There are two main types of intracellular In addition, recent studies have demon- force is removed. In the presence of solely transport: ATP-driven processes and strated that mechanoenzymatic activity thermal forces, this would result in con- diffusion. Thermal energy drives random within the cytoplasm can also generate strained motion of large beads within the movements of atoms, small molecules, random, incoherent forces in a stochas- cytoplasm, as depicted in Figure 1B. proteins, and micron-sized organelles. tic manner (Brangwynne et al., 2007; However, when the authors measured The random nature of thermal forces Hoffman et al., 2006; Parry et al., 2014). the spontaneous (e.g., in the absence of generates movements of particles known These forces can drive movements of external force) motion of particles and as a random walk. This type of motion embedded particles that appear diffusive organelles within the cytoplasm, they ob- produces a characteristic mean-squared but are highly ATP dependent. This type served motion that appeared diffusive displacement (MSD), which is a param- of ATP-dependent motion has been (Figure 1D, dark blue). eter describing particle displacement observed in both bacteria (Parry et al., This is a quite surprising result: how within a specific time interval. Specifically, 2014; Robert et al., 2010; Weber et al., is diffusive-like motion produced in an the MSD rises linearly with the magnitude 2012) and eukaryotic cells (Brangwynne elastic network? This diffusive trans- of the time interval (Figure 1B, black). et al., 2007). These stochastic forces port could be explained by a model in Thermal-based diffusion is considered have been shown to vary with cell meta- which active forces build and release insufficient to facilitate intracellular trans- bolic state, suggesting that they may stochastically within an elastic network port for larger organelles, which can have important effects in regulating (MacKintosh and Levine, 2008), as has become stuck within the cytoskeleton as biochemical interactions (Parry et al., been previously observed in reconsti- they become larger than the average 2014). In addition, these forces provide tuted actomyosin networks (Mizuno spacing between cytoskeletal filaments, a potential mechanism to ‘‘actively stir’’ et al., 2007). While the forces still retain resulting in a mean-squared displace- the cytoplasm (Fakhri et al., 2014) the random nature of thermal forces, ment that is constant at long times without requiring ordered tracks or tar- the magnitude of their effect on object (Figure 1B, blue). Recent data indicate geted localization of motor proteins. An displacement is much larger. In order that globular proteins within the cyto- outstanding challenge is to understand to test this idea, the authors developed plasm can be sufficiently crowded to how this ‘‘active stirring’’ or ‘‘active diffu- a technique, termed force-spectrum restrict the diffusion of organelles (Parry sion’’ is regulated. Simply, what is the microscopy (FSM), allowing them to et al., 2014). interplay between the ATP-dependent utilize optical tweezer-based force mea- ATP-dependent mechanoenzymes forces and the local cytoplasmic me- surements in combination with tracing within the cytoplasm comprise the sec- chanics to regulate this novel mode of spontaneous particle movements to ond main class of intracellular transport intracellular transport? deconstruct the stochastic force Developmental Cell 30, August 25, 2014 ª2014 Elsevier Inc. 365 Developmental Cell Previews Figure 1. Expected Motions for Small and Large Intracellular Particles (A) Schematic of the crowded intracellular environment, including actin networks at the cell cortex (red rods) and microtubules (green). (B) Large particles in crowded environments (top left) or cytoskeletal networks (top right) are confined. Their MSD reaches a plateau at long timescales (bottom, solid cyan curve). Small particles may diffuse with random trajectories within gaps, as shown by dashed gray lines (top panels). This behavior produces an MSD that rises linearly with time (bottom, dashed black curve). (C) Molecular motors moving along tracks may transport objects along straight trajectories. In this case, the MSD is bal- listic and rises as the square of the time delay (bottom). Similar motion is produced for large and small particles. (D) Addition of random ATP-driven fluctuations modifies the motion of objects from (B). Large particles are no longer trapped but may also show diffusive-like MSDs due to the random motions (bottom, solid cyan curves). Small particles still display diffusive-like behavior, but the larger amplitude of active compared to thermal fluctuations produces faster transport (bottom, dashed curves). components that govern particle dis- larger random movements in malignant REFERENCES placement. The authors demonstrated cells. This suggests that these types of that these diffusive-like motions are random active forces may influence Brangwynne, C.P., MacKintosh, F.C., and Weitz, D.A. (2007). Proc. Natl. Acad. Sci. USA 104, highly ATP dependent and are elimi- physiological processes and contribute 16128–16133. nated upon depletion of the chemical to disease progression. Fakhri, N., Wessel, A.D., Willms, C., Pasquali, M., energy source. Clarifying how the material properties Klopfenstein, D.R., MacKintosh, F.C., and Guo et al. then made important con- of the cytoplasm are tuned to allow Schmidt, C.F. (2014). Science 344, 1031–1035. nections to the transport of proteins transport in response to active forces Guo, M., et al. (2014). Cell 158, 822–832. and organelles within the cytoplasm. while maintaining mechanical integrity Using FSM, they found that the motions of the cell is a subject for future work. Hoffman, B.D., Massiera, G., Van Citters, K.M., of endogenous organelles and vesicles The cytoplasm of cells examined in and Crocker, J.C. (2006). Proc. Natl. Acad. Sci. USA 103, 10259–10264. matched those of microinjected par- Guo et al. (2014) has mechanical proper- ticles. This indicates that this type of ties of an elastic solid yet retains (or im- Howard, J. (2001). Mechanics of Motor Proteins random active motion may drive the proves upon) the ability of a fluid to and the Cytoskeleton. (Sunderland: Sinauer Asso- ciates). movements of large organelles within transport objects via diffusion-like mo- the cell body. Moreover, they observed tion. Identification of the ATPases that MacKintosh, F.C., and Levine, A.J. (2008). Phys. Rev. Lett. 100, 018104. that the diffusion coefficient of a soluble regulate cytoplasmic noise may provide globular protein was also ATP depen- further insight into the regulation of this Mizuno, D., Tardin, C., Schmidt, C.F., and Mackin- dent, indicating that similar processes behavior in healthy and malignant cell tosh, F.C. (2007). Science 315, 370–373. may affect transport at much smaller types. For instance, such effects could Parry, B.R., Surovtsev, I.V., Cabeen, M.T., O’Hern, scales, which may have implications alter cell metabolism or invasion. These C.S., Dufresne, E.R., and Jacobs-Wagner, C. 156 for biochemical regulation (Figure 1D, results provide a new understanding of (2014). Cell , 183–194. dashed black). Intriguingly, they found how cells make use of random ATP- Robert, D., Nguyen, T.-H., Gallet, F., and Wilhelm, that a malignant epithelial cell exerted dependent forces and cytoskeletal ma- C. (2010). PLoS ONE 5, e10046. higher stochastic forces than normal terials to support robust intracellular Weber, S.C., Spakowitz, A.J., and Theriot, J.A. epithelial cells, with particles showing transport. (2012). Proc. Natl. Acad. Sci. USA 109, 7338–7343. 366 Developmental Cell 30, August 25, 2014 ª2014 Elsevier Inc..
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