Detection of single fluorescent microtubules and methods for determining their dynamics in living cells

The ability to tag biological molecules fluorescently and to detect their distribution in living cells has promoted the study of cytoplasmic organization in general and microtubule dynamics in particular. The techniques that we have selected and developed allowed the determination of spatial and temporal changes of the microtubule network in living fibroblasts at the level of individual microtubules. We have employed two general approaches for determining pattern changes: direct video microscopy and photobleaching and subsequent observation. Direct observation of fluorescent microtubules by high-definition video microscopy provided good spatial resolution at several time points, but was limited to the less congested and thinner periphery of the cell. This approach was made possible by a relatively bright, photostable reporter, xrhodamine-tubulin, and showed that microtubules underwent rounds of assembly and disassembly from their ends. Bleaching and subsequent observation of lysed cells improved the signal to noise ratio by extracting soluble chromophore and permitted observations in congested areas, but was limited to a single time interval. This approach demonstrated that microtubule domains were replaced one by one and that turnover was most rapid at the cell periphery. Antibodies specific for nonbleached chromophore can be used to enhance the signal to noise ratio further or to extend spatial resolution by the use of immunoelectron microscopy. Direct video microscopy and photobleaching are two approaches to the study of dynamics that have complementary strengths and wide application to the biology of living cells.