Two pathways control chromaffin cell cortical F-actin dynamics during exocytosis.

Neurosecretory cells including chromaffin cells possess a mesh of filamentous actin underneath the plasma membrane. We have proposed that the F-actin network acts as a barrier to the secretory vesicles blocking their access to exocytotic sites at the plasma membrane. Disassembly of cortical F-actin in chromaffin cells in response to stimulation is thought to allow the free movement of secretory vesicles to exocytotic sites. Moreover, experiments by us using morphometric analysis of resting and stimulated chromaffin cells together with membrane capacitance measurements have shown that cortical F-actin controls the traffic of vesicles from the vesicle reserve compartment to the release-ready vesicle compartment. The dynamics of the cortical F-actin is controlled by two pathways: A) stimulation-induced Ca(2+) entry and scinderin activation; and B) protein kinase C (PKC) activation and MARCKS (myristoylated alanine-rich C kinase substrate) phosphorylation. When chromaffin cells are stimulated through nicotinic receptors, cortical F-actin disassembly is mainly through the intervention of pathway A, since in the presence of PKC inhibitors, F-actin disassembly in response to cholinergic stimulation is only blocked by 20%. Pathway A involves the activation of scinderin by Ca(2+) with a consequent F-actin severing. Pathway B is fully activated by phorbol esters and in this case PKC blockers inhibit by 100% the disruption of cortical F-actin. This pathway operates through MARCKS. A peptide with amino acid sequence corresponding to the phosphorylation site domain of MARCKS, which also corresponds to its actin binding site, blocks PMA potentiation of Ca(2+)-induced catecholamine release. The results suggest that under physiological conditions (i.e., nicotinic receptor stimulation) pathway A is the principal mechanism for the control of cortical F-actin dynamic changes.