Hitchhiking carbon nanotubes reveal how cell stir themselves
In a research project partly funded by the FOM Foundation, an international collaboration of biophysicists from the VU University Amsterdam, Georg-August Universität Göttingen and chemical engineers from Rice University have successfully tracked single molecules inside living cells using carbon nanotubes which emit fluorescent light. Applying this newly developed method, the researchers found that cells stir their interior using the same motor proteins that serve in muscle contraction. The study is published in the 30 May edition of Science.
The research sheds new light on biological transport mechanisms in cells. To obtain the results, the team developed and applied a new method to visualize and track single molecules inside the cytoplasm of living cells. They attached extremely thin carbon filaments, known as nanotubes, to transport molecules in the cells. These hitchhiking hollow nanotubes have walls that are one-atom-thick sheets of carbon. They emit fluorescent light, and serve as little beacons attached to a molecule that researchers can track over long periods of time with high precision to investigate the small random motions in the cells.
Stirring cells and contracting muscles
Using this new research method, the researchers investigated transport within the cell's interior. For long-distance transport cells usually employ motor proteins that are tied to lipid vesicles, the cell's 'cargo containers'. An example is the transport of proteins along the long axons of nerve cells. This process involves considerable logistics: cargo, such as proteins synthesized elsewhere in the cell, needs to packed, attached to motor proteins and sent off in the right direction. This research has helped to uncover an additional, much simpler mechanism for transport within the cell interior: cells vigorously 'stir' themselves, much in the way a chemist would accelerate a reaction by shaking a test tube.
The researchers showed that this stirring is done by the same type of motor proteins that are used for muscle contraction. They reached this conclusion after they exposed the cells to drugs that specifically target these motor proteins, and saw that the drugs suppressed the stirring as well.
The mechanical cytoskeleton of cells consists of networks of filamentous proteins such as actin. Within the cell, the motor protein myosin forms little bipolar bundles that actively contract this actin network for short periods of time. The random pinching of the elastic actin network by many of such myosin bundles results in the global internal stirring of the cell. Both actin and myosin play a similar role in muscle contraction.
Active materials
These highly accurate measurements of the internal fluctuations in the cells can be beautifully explained by a theoretical model developed by the Amsterdam group, using the elastic properties of the cytoskeleton and the force-generation characteristics of the motors. The new discovery not only promotes our understanding of cell dynamics, but also points to interesting possibilities in designing 'active' technical materials.
Contact
Prof.dr. Fred MacKintosh, VU University Amsterdam, +31 (0)20 598 78 57.
Reference
Fakhri, N., Wessel, A.D., Willms, C., Pasquali, M., Klopfenstein, D.R., MacKintosh, F.C., & Schmidt, C.F. (2014). High-resolution mapping of intracellular fluctuations using carbon nanotubes. Science, 30 May 2014.