Real-time dynamics of the F-actin cytoskeleton during secretion from chromaffin cells

Transmitted light images showed an intricate and dynamic cytoplasmic structural network in cultured bovine chromaffin cells observed under high magnification. These structures were sensitive to chemicals altering F-actin-myosin and colocalised with peripheral F-actin, beta-actin and myosin II. Interestingly, secretagogues induced a Ca2+-dependent, rapid (>10 second) and transitory (60-second cycle) disassembling of these cortical structures. The simultaneous formation of channel-like structures perpendicular to the plasmalemma conducting vesicles to the cell limits and open spaces devoid of F-actin in the cytoplasm were also observed. Vesicles moved using F-actin pathways and avoided diffusion in open, empty zones. These reorganisations representing F-actin transfer from the cortical barrier to the adjacent cytoplasmic area have been also confirmed by studying fluorescence changes in cells expressing GFP-beta-actin. Thus, these data support the function of F-actin-myosin II network acting simultaneously as a barrier and carrier system during secretion, and that transmitted light images could be used as an alternative to fluorescence in the study of cytoskeleton dynamics in neuroendocrine cells.