Abstract
Many organelles and vesicles move in an intermittent (start-stop) manner in live cells when observed by video microscopy. We propose that such vesicles travel in one of two states, a driven state in which they are being actively pulled along a cytoskeletal fiber by motor proteins, and a Brownian state in which they are detached from the fiber and obey the laws of diffusion in the cytoplasm. Using variational Bayesian analysis, we analyze the tracks of peroxisomes, lysosomes, and other organelles in PC12 and HME cells to reveal the probability that the vesicle is in the driven or the Brownian state at each time (frame). Instantaneous vesicle velocity and directional persistence are the input data for our hidden Markov, Gaussian mixture model Bayesian analysis. This analysis evaluates the most probable velocity as well the variance of the velocity of each state. It also determines the most probable times at which the state changes. Further analysis of the motions of the vesicles during the Brownian episodes may permit a cleaner assessment of the viscoelastic properties of cytoplasm. This research is supported by the National Science Foundation under Grant No. CMMI-1106105. AMS is supported by the NIGMS of the National Institutes of Health under award number T32GM095440.
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