Kinase and phosphatase inhibitors cause rapid alterations in microtubule dynamic instability in living cells

To examine whether microtubule dynamic instability can be rapidly regulated during interphase, we used video-enhanced differential interference contrast (DIC) microscopy to observe individual microtubules at the periphery of living newt lung epithelial cells. Microtubules were observed before and after perfusion with either the phosphatase inhibitor okadaic acid or the kinase inhibitors staurosporine or olomoucine. Addition of these inhibitors caused rapid changes in dynamic instability. Thirty to sixty seconds after perfusion with 0.2-1 μM okadaic acid, a 1.5-fold increase in elongation velocity and small increases in catastrophe and rescue frequencies were observed. In contrast, treatment with 40-200 nM staurosporine decreased microtubule elongation and shortening velocities ~2-fold, and catastrophes were slightly more frequent. Olomoucine, at 100 μM, had similar effects. Transition dynamics were further examined by probabilistic analysis, which showed that microtubules become more likely to undergo catastrophe as they elongated and more likely to undergo rescue as they shortened, an effect previously called microtubule 'memory.' This memory effect for catastrophes was observed in untreated and okadaic acid-treated cells but was abolished by staurosporine or olomoucine. In contrast, the memory effect for rescue was unaffected by these treatments, suggesting that catastrophe and rescue proceed via distinct, multistep mechanisms. Overall, these results demonstrate that microtubule assembly regulators can be altered rapidly by inhibition of either kinases or phosphatases and suggest that, in the absence of inhibitors, these regulators exist in a dynamic equilibrium between phosphorylated and dephosphorylated states.