Lund, F. W.4; Lomholt, M. A.5; Solanko, L. M.4; Bittman, R.3; Wustner, D.4
1 Department of Biochemistry and Molecular Biology, Faculty of Science, SDU2 Department of Physics, Chemistry and Pharmacy, Faculty of Science, SDU3 unknown4 Department of Biochemistry and Molecular Biology, Faculty of Science, SDU5 Department of Physics, Chemistry and Pharmacy, Faculty of Science, SDU
Background: Cholesterol is an important membrane component, but our knowledge about its transport in cells is sparse. Previous imaging studies using dehydroergosterol (DHE), an intrinsically fluorescent sterol from yeast, have established that vesicular and non-vesicular transport modes contribute to sterol trafficking from the plasma membrane. Significant photobleaching, however, limits the possibilities for in-depth analysis of sterol dynamics using DHE. Co-trafficking studies with DHE and the recently introduced fluorescent cholesterol analog BODIPY-cholesterol (BChol) suggested that the latter probe has utility for prolonged live-cell imaging of sterol transport. Results: We found that BChol is very photostable under two-photon (2P)-excitation allowing the acquisition of several hundred frames without significant photobleaching. Therefore, long-term tracking and diffusion measurements are possible. Two-photon temporal image correlation spectroscopy (2P-TICS) provided evidence for spatially heterogeneous diffusion constants of BChol varying over two orders of magnitude from the cell interior towards the plasma membrane, where D similar to 1.3 mu m(2)/s. Number and brightness (N&B) analysis together with stochastic simulations suggest that transient partitioning of BChol into convoluted membranes slows local sterol diffusion. We observed sterol endocytosis as well as fusion and fission of sterol-containing endocytic vesicles. The mobility of endocytic vesicles, as studied by particle tracking, is well described by a model for anomalous subdiffusion on short time scales with an anomalous exponent alpha similar to 0.63 and an anomalous diffusion constant of D-alpha = 1.95 x 10(-3) mu m(2)/s(a). On a longer time scale (t > similar to 5 s), a transition to superdiffusion consistent with slow directed transport with an average velocity of v similar to 6 x 10(-3) mu m/s was observed. We present an analytical model that bridges the two regimes and fit this model to vesicle trajectories from control cells and cells with disrupted microtubule or actin filaments. Both treatments reduced the anomalous diffusion constant and the velocity by similar to 40-50%. Conclusions: The mobility of sterol-containing vesicles on the short time scale could reflect dynamic rearrangements of the cytoskeleton, while directed transport of sterol vesicles occurs likely along both, microtubules and actin filaments. Spatially varying anomalous diffusion could contribute to fine-tuning and local regulation of intracellular sterol transport.