Cytoskeletal control of vesicle transport and exocytosis in chromaffin cells

Abstract

Chromaffin cell exocytosis is a fascinating interplay between secretory vesicles and cellular components. One of these components is the cytoskeleton and its associated regulatory proteins. Transport of chromaffin secretory granules from their site of biosynthesis towards the active site of exocytosis requires both F-actin fine remodelling as well as microtubule trails. At least two molecular motors, myosins II and V, seem to play a crucial role in the control of F-actin dynamics and vectorial vesicle displacement respectively. Vesicle movement experiences spatial restrictions as they approach the cell cortical region, where the F-actin meshwork constitutes a barrier-limiting vesicle access to the plasmalemma. During secretion, cortical F-actin is locally disrupted providing access of vesicles to release sites on the plasmalemma. Removal of the stimulus restores cortical F-actin. Two pathways (Ca2+-scinderin and PKC-MARCKS) control F-actin changes during the secretory cycle . Furthermore, GTPases such as RhoA, that controls F-actin network integrity, and Cdc42 signalling which induces the formation of local actin filaments at active sites, provide additional evidence on the importance of F-actin as a key element in vesicle transport and in the exocytotic machinery of chromaffin cells.

Activity-driven relaxation of the cortical actomyosin II network synchronizes Munc18-1-dependent neurosecretory vesicle docking

“…By maintaining tension of the cortical actin network 18,19,40 in unstimulated cells, myosin II retains most SVs away from the plasma membrane. This is in good agreement with previous reports showing that the cortical actin network acts as a barrier, preventing the majority of vesicles from accessing the plasma membrane in resting conditions 2,9 . To the best of our knowledge, our study is the first to demonstrate that relaxation of the cortical actin network is the main driving force that allows SVs to be translocated and recruited to the plasma membrane.…”

“…It is worth noting that even though blebbistatin blocks the initial burst of catecholamine release events, a slower wave of release is unaffected by myosin II inhibition. This is probably due to other cytoskeletal effects, as described previously 9 . It is also important to note that myosin II is involved in regulating fusion pore dynamics, which also affects the release process 12,17,47 .…”

“…During stimulation, the initial round of release is achieved by the fusion of a small number of already docked or primed vesicles [3][4][5][6] , but prolonged stimulation requires the translocation of SVs that are not in the immediate vicinity of the plasma membrane. The cortical actin network plays a major role in regulated exocytosis 2,7 and has been shown to interact with SVs in various ways, in particular through the action of myosin molecular motors 8,9 . SVs can be tethered to the cortical actin network by myosin VI in an activity-dependent manner 10 , and can also be transported along actin fibres by myosin Va 11 .…”

Newcomer insulin secretory granules as a highly calcium-sensitive pool

“…Our assumption of PKC-enhanced mobilization is supported by the fact that SNAP-25 phosphorylation increases the rate of granule delivery (57), possibly because of SNAP-25-actin interactions (60), or more speculatively, because of a requirement for SNAP-25 in the weak tethering process described by Toonen et al (35). Moreover, PKC is well known to have effects on the submembrane actin barrier because of activation of proteins involved in the remodeling of the actin network such as MARCKS (42). Such actin remodeling allows granules to arrive at the cell membrane and is important for second-phase insulin secretion (24,43,60).…”

“…The larger total number of releasable granules (HCSP plus RRP) suggests that the rate of recruitment to the membrane is increased. PKC is known to participate in remodeling of the actin network below the cell membrane in chromaffin cells (42), a crucial step controlling recruitment of granules from the reserve pool, including in ␤-cells (43). If there were no other effects on rates, this would lead to a proportional increase in size of the various pools in our model.…”