The F-actin cortex in chromaffin granule dynamics and fusion: a minireview.

Chromaffin granules are restrained in a dense cortical cytoskeleton before releasing their complex mix of active substances in response to cell stimulation. In recent years, the complex organization and dynamics of the chromaffin cell cortex has been unveiled through its analysis with a range of techniques to visualize this structure, including confocal fluorescence, transmitted light, and evanescent field microscopy. Accordingly, it has become apparent that the cortex is a dense F-actin mesh that contains open polygonal spaces through which vesicles can access the submembrane space. In addition to its retentive role, this structure also influences vesicle motion in both the resting state and during cell stimulation with secretagogues. During secretion, the chromaffin cell cortex undergoes a complex reorganization, helping to replenish the empty fast releasable pool of vesicles. Such changes in the cortical cytoskeleton and in the vesicle motion are governed by the activity of molecular motors, such as myosins II and Va. Interestingly, the F-actin/myosin II network also affects the final stages of exocytosis, which involve the opening and expansion of the fusion pore, and the extrusion of the vesicles contents.