, Volume 29, Issue 2, pp 67-89

Tracking chromaffin granules on their way through the actin cortex

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Quantitative time-lapse evanescent-wave imaging of individual fluorescently labelled chromaffin granules was used for kinetic analysis of granule trafficking through a ∼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 ∼60 nm beneath the plasma membrane moved on random tracks (D (3)≈10−10 cm2 s−1) with several reversals in direction before they approached their docking site at angles larger than 45. 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.

Received: 18 November 1999 / Revised version: 26 January 2000 / Accepted: 2 February 2000