Molecular Motors Involved in Chromaffin Cell Secretion

Abstract

Neurosecretory cells, including chromaffin cells, possess a mesh of filamentous actin underneath the plasma membrane. It has been proposed that filamentous actin network separates the secretory vesicles into two compartments: the reserve pool and the release-ready vesicle pool. Disassembly of chromaffin cell cortical filamentous actin in response to stimulation allows the movement of vesicles from the reserve pool into the release-ready vesicle pool. Electron microscopy of cytoskeletons revealed the presence of polygonal areas almost devoid of actin filaments in stimulated cells. The percentage of stimulated cells showing disrupted cytoskeleton correlates well with the increase in secretion in these cells. Fine filaments also remain in these areas of disassembly, and these reacted with actin antibodies, as demonstrated by immunogold staining. In addition, the movement of vesicles between pools requires Ca(2+) and ATP, a condition for activation of a molecular motor. Confocal microscopy images demonstrated colocalization of myosin Va with dopamine-beta-hydroxylase. Cell depolarization induced the dissociation of myosin Va from chromaffin vesicles. 2,3-Butadione-2-monoxime (BDM), an inhibitor of myosin ATPase, inhibited secretion, suggesting a blockage for chromaffin vesicle transport between the reserve pool and the release-ready vesicle pool. On the other hand, myosin II subcellular distribution was not affected by cell depolarization. Confocal microscopy images show myosin II to be localized in the cell cortex and in some perinuclear structures. Chromaffin vesicles were not stained by myosin II antibody.

Uncoated Endocytic Vesicles Require the Unconventional Myosin, Myo6, for Rapid Transport through Actin Barriers

“…In these systems, actin serves a scaffolding role and is required to anchor secretory vesicles as a readily releasable pool (Sankaranarayanan et al, 2003). In some cases, this anchoring requires a myosin (Rose et al, 2002;Rudolf et al, 2002), whereas in other cases, such as GLUT4 delivery to the adipocyte plasma membrane, a plus-enddirected unconventional myosin, myo1C, may play a role in vesicle delivery across the barrier toward the plasma membrane (Bose et al, 2002). In peripheral regions there is a higher percentage of plus ends facing outward so a plusend-directed myosin would be appropriate for vesicle trafficking outward for exocytosis.…”

Myosin II Activation and Actin Reorganization Regulate the Mode of Quantal Exocytosis in Mouse Adrenal Chromaffin Cells

“…Indeed, chemical stimulation causes the regulatory subunit of MLCK to become phosphorylated in a calcium-dependent manner proportional to the magnitude of the stimulus (Nicotera et al, 1986;Gutierrez et al, 1989). Furthermore, granule recruitment to the readily releasable pool has been shown to require phosphorylation of MLCK and activation of myosin (Kumakura et al, 1994;Rosé et al, 2002). Recent work found that expression of a phosphorylation-incompetent form of myosin II regulatory light chain decreased the kinetics of catecholamine release from single granules.…”

Membrane traffic in the secretory pathway

“…However, Myo2 is also associated with the Golgi complex, vesicles emanating from the TGN and secretory granules, suggesting that it participates in constitutive and stimulated secretion [60,64,86].…”