Microtubule-targetable fluorescent probe: site-specific detection and super-resolution imaging of ultratrace tubulin in microtubules of living cancer cells.

Tubulins in microtubules have been recognized as potential targets in cancer chemotherapy for several years. However, their detection and imaging in living cells, especially following exposure to anticancer drugs, remains difficult to achieve. This difficulty is due to the very small cross section of microtubules and the very small changes in tubulin concentration involved. Photoswitchable fluorescent probes combined with the "super-resolution" fluorescence imaging technique present an exciting opportunity for site-specific detection and super-resolution imaging of specific microscopic populations, such as tubulin. In this study, a tubulin specific photoswitchable fluorescent probe (Tu-SP), that labels and detects ultratrace levels of tubulin in microtubules of living biosystems, was designed and evaluated. To realize super-resolution fluorescence imaging, the spiropyran derivative (SP), a classic photoswitch, was introduced to Tu-SP as a fluorophore. To detect ultratrace tubulin, Tu-SP employed the tubulin inhibitor, alkaloid colchicine (Tu), as a recognition unit. Tu-SP exhibited nearly nonintrinsic fluorescence before binding to tubulin, even if there were divalent metal ions and 375 nm lasers, respectively. After binding to tubulin, a dramatic increase in fluorescence was detected within milliseconds when irradiated at 375 nm, this increase is a result of the transformation of Tu-SP into a colored merocyanine (Tu-SP-1) with fluorescence. Tu-SP was successfully used for site-specific imaging of tubulin at a resolution of 20 ± 5 nm in microtubules of living cancer cells. More importantly, the probe was suitable for site-specific and quantitative detection of trace tubulin in microtubules of living biological samples.