Microtubule self-healing under stress

A microfluidic device has been designed to attach and bend microtubules with hydrodynamic flows. Repeated sequences of microtubule bending revealed that microtubules soften under constraint but also that they could recover their stiffness by self-repairing.

HFSP Young Investigator Grant holders Manuel Théry and Maxence Nachury and HFSP Program Grant holder Laurent Blanchoin and colleagues
authored on Thu, 10 September 2015

Microtubules are one of the main components of cell internal architecture. Their rigidity, a hundred times higher than other cytoskeletal components, allows them to run straight through the intra-cellular space (Figure 1) thereby supporting the intracellular transport between cell centre and periphery. As a result microtubule shape and network geometry arrange intra-cellular traffic and direct cell functions. Despite this key role, almost nothing is known about the mechanisms regulating microtubules’ architecture and their unusual mechanical properties.

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Figure 1:(a) Microtubule immune-labelling in a human retinal cells. Credit: Manuel Théry (CEA)
Figure 2: Schematic illustration of tubulin dimers loss during microtubule bending and softening (background) and dimers incorporation during microtubule self-healing and stiffness recovery (foreground). Credit: Agnieszka Kawska (www.illuscientia.com)

Microtubules’ hollow tube shape is likely to support their rigidity. This well-known material property is regularly exploited as, for example, in the manufacturing of mountain-bike frames. However, the lack of well-suited devices has prevented more in depth study of microtubule mechanics. By means of an HFSP Research Grant, a team of researchers designed and used a microfluidic device to attach and apply hydrodynamic forces to microtubules (Figure 3). They found that microtubule stiffness decreases incrementally with each cycle of bending and release. Similar to other cases of material fatigue, the concentration of mechanical stresses on pre-existing defects in the microtubule lattice seemed responsible for the generation of larger-scale damage, which further decreased microtubule stiffness. Strikingly, short rest periods allowed damaged microtubules to recover their stiffness. This showed that microtubules’ structure could reorganise upon stress application and release, conferring them adaptable mechanical properties.

Figure 3: Microfluidic device comprising a micro-patterned substrate and micro-channels to lay down microtubule seeds and apply hydrodynamic laminar flows perpendicular to the microtubules. The time-lapse sequence shows microtubule bending and relaxation upon flow application and release. Credit: Laura Schaedel (CEA)

Microtubule structure is made of 13 adjacent protofilaments (Figure 2). These protofilaments polymerize from a soluble pool of tubulin dimers. To gain further insight in the mechanism of microtubule self-repair, microtubules polymerized from red-fluorescent tubulin were bent in the presence of green-fluorescent soluble dimers. Surprisingly, during the rest period, green spots appeared along microtubule shafts. These spots were genuine repair sites where surrounding dimers had been incorporated into the damaged microtubule structure (Figure 2). So the microtubule self-repair process was inherently based on microtubule protofilaments' self-assembly properties.

These findings demonstrate that microtubules are ductile materials with self-healing properties. Importantly, microtubule self-assembly does not exclusively occur at their ends, contrary to the classical belief. Tubulin dimers can be removed and added along the microtubule shaft. This process seems associated to the presence of lattice defects such as local variations in the number of protofilaments. This remarkable structural plasticity enables the microtubules' adaptation to mechanical stresses.

Reference

Microtubules self-repair in response to mechanical stress. Laura Schaedel, Karin John, Jérémie Gaillard, Maxence Nachury, Laurent Blanchoin and Manuel Théry. Nature Materials (2015) doi:10.1038/nmat4396.

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