A comparison of single particle tracking and temporal image correlation spectroscopy for quantitative analysis of endosome motility

Single particle tracking (SPT) is becoming a standard method to extract transport parameters from time-lapse image sequences of fluorescent vesicles in living cells. Another method to obtain these data is temporal image correlation spectroscopy (TICS), but this method is less often used for measurement of intracellular vesicle transport. Here, we present an extensive comparison of SPT and TICS. First we examine the effect of photobleaching, shading and noise on SPT and TICS analysis using simulated image sequences. To this end, we developed a simple photophysical model, which relates spatially varying illumination intensity to the bleaching propensity and fluorescence intensity of the moving particles. We found that neither SPT nor TICS are affected by photobleaching per se, but the transport parameters obtained by both methods are sensitive to the signal-to-noise ratio. In addition, the number of obtained trajectories in SPT is affected by noise. Diffusion constants determined by TICS are significantly overestimated when large immobile fluorescent structures are present in the image sequences, while the opposite is true for SPT. To improve the performance of both techniques, we compare three different methods for image denoising. Appropriate denoising significantly reduced the effect of noise and of immobile structures on both methods. Shape fluctuations of simulated particles had a more pronounced effect on TICS than on SPT analysis. In denoised images of fluorescent beads or cytosolic vesicles containing fluorescent protein NPC2 in human skin fibroblast cells, the transport parameters acquired by SPT and TICS were comparable emphasizing the value of both analysis methods.