Tracking chromaffin granules on their way through the actin cortex

Quantitative time-lapse evanescent-wave imaging of individual fluorescently labelled chromaffin granules was used for kinetic analysis of granule trafficking through a approximately 300-nm(1/e2) optical section beneath the plasma membrane. The mean squared displacement (MSD) was used to estimate the three-dimensional diffusion coefficient (D(3)). We calculated the granules' speed, frame-to-frame displacement and direction and their autocorrelation to identify different stages of approach to the membrane. D(3) was about 10,000 times lower than expected for free diffusion. Granules located approximately 60 nm beneath the plasma membrane moved on random tracks (D(3) approximately 10(-10) cm(2)s(-1)) with several reversals in direction before they approached their docking site at angles larger than 45 degrees. Docking was observed as a loss of vesicle mobility by two orders of magnitude within <100 ms. For longer observation times the MSD saturated, as if the granules' movement was confined to a volume only slightly larger than the granule. Rarely, the local random motion was superimposed with a directed movement in a plane beneath the membrane. Stimulation of exocytosis selectively depleted the immobile, near-membrane granule population and caused a recruitment of distant granules to sites at the plasma membrane. Their absolute mobility levels were not significantly altered. Application of latrunculin or jasplakinolide to change F-actin polymerisation caused a change in D(3) of the mobile granule population as well as a reduction of the rate of release, suggesting that granule mobility is constrained by the filamentous actin meshwork and that stimulation-dependent changes in actin viscosity propel granules through the actin cortex.