Reinforced microtubules

Heart contractions cause periodic wavy buckling of microtubles. M uch of the mechanical strength of micro-tubules comes from the cytoskeleton surrounding them, report Brangwynne et al. on page 733. When compressive forces push on a microtubule in a cell, such as when a growing polymer butts up against the cell edge, the fi ber bends in multiple short wavelength curves like a snake. By contrast, when the end of an isolated microtubule is pushed with even small forces , the fi ber compresses and bends in a single large arch. The minor forces necessary to bend isolated microtubules call into question the importance of the fi bers in determining cell shape and strength. Brangwynne et al. found that if they pressed on the end of a microtubule inside a cell with a micro-needle, short wavelength bending occurred. Moreover, contraction of the actin–myosin cytoskeleton induced such buckling in rhythmically contracting cardiac my-ocytes, and neighboring microtubules bent in a coordinated pattern. To fi nd out if an elastic medium surrounding a fi ber could cause a shift from long to short wavelength bending, the team compressed a thin plastic rod fi rst in aqueous solution and then in a gelatin matrix. In water, the rod formed a long arc, but when constrained by gelatin, which the rod had to push out of the way, it bent in a shorter sinusoidal pattern. Mathematical modeling showed that the wavelength of bending in response to compression resulted from the combined strength of the fi ber and the resistance of the medium. When the team disrupted the actin–myosin matrix with cytochalasin and then compressed micro-tubules with a microneedle, they saw that the micro-tubule now bent in a longer arc than occurred when the actin fi bers were intact. The team concluded that the surrounding network adds substantial strength to the microtubules. Furthermore , by increasing the reinforcement in particular regions, the cell can hold one part of a microtubule straight while allowing small wavelength bends in other regions. Thus, microtubules can withstand and generate the forces necessary to support motility and tissue development. An unused license to fire N ot all available origins of replication fi re during a normal S-phase. But when replication is perturbed, otherwise dormant origins go to work, Woodward et al. show on page 673. Cells initially respond to slowed replication by turning on the ATR-dependent checkpoint, which prevents other origins from …