The adrenal paraneurone: tubulin organization

When chromaffin cells from the bovine adrenal medulla are maintained in culture, they develop neuritelike processes which end with growth-cone-like structures. Chromaffin granules were found to migrate from the cell body to the neurite endings. Thus, the intracellular transport of secretory granules, existing in vivo, seems to occur in an exaggerated way in the cultured cells. These cells offer an excellent model for studying the mechanism of transport, particularly the role of microtubules. By immunofluorescent staining, we observed that tubulin antibodies decorate a complex network visible along the neurites. Colchicine treatment induced the disappearance of this network followed by a return of granules in the cell body and a retraction of neurites. To test the presence of tubulin in the chromaffin granule membrane, we used two-dimensional gel electrophoresis and a radioimmunoassay. Our results indicate that tubulin is not a significant component of chromaffin granules. However, binding experiments show that granule membranes are able to bind tubulin through high affinity binding sites. These results show that microtubules appear involved in neurite formation and probably in granule transport. Tubulin is not an integral constituent of the granule membrane, but is present as a result of a reversible specific binding. This insertion of tubulin into the membrane might represent a step in the association between microtubules and secretory granules.