The cortical acto-Myosin network: from diffusion barrier to functional gateway in the transport of neurosecretory vesicles to the plasma membrane

Abstract

Dysregulation of regulated exocytosis is linked to an array of pathological conditions, including neurodegenerative disorders, asthma, and diabetes. Understanding the molecular mechanisms underpinning neuroexocytosis including the processes that allow neurosecretory vesicles to access and fuse with the plasma membrane and to recycle post-fusion, is therefore critical to the design of future therapeutic drugs that will efficiently tackle these diseases. Despite considerable efforts to determine the principles of vesicular fusion, the mechanisms controlling the approach of vesicles to the plasma membrane in order to undergo tethering, docking, priming, and fusion remain poorly understood. All these steps involve the cortical actin network, a dense mesh of actin filaments localized beneath the plasma membrane. Recent work overturned the long-held belief that the cortical actin network only plays a passive constraining role in neuroexocytosis functioning as a physical barrier that partly breaks down upon entry of Ca(2+) to allow secretory vesicles to reach the plasma membrane. A multitude of new roles for the cortical actin network in regulated exocytosis have now emerged and point to highly dynamic novel functions of key myosin molecular motors. Myosins are not only believed to help bring about dynamic changes in the actin cytoskeleton, tethering and guiding vesicles to their fusion sites, but they also regulate the size and duration of the fusion pore, thereby directly contributing to the release of neurotransmitters and hormones. Here we discuss the functions of the cortical actin network, myosins, and their effectors in controlling the processes that lead to tethering, directed transport, docking, and fusion of exocytotic vesicles in regulated exocytosis.

Keywords: cdc42; cortical actin; myosin; phosphoinositides; regulated exocytosis; secretory vesicles.

Figure 1

Imaging the actin network in neurosecretory cells. (A) Electron micrograph of a bovine chromaffin cell region attached to the thermanox support. Note the presence of a filamentous cortical region that is devoid of SG. Bar, 1 μm [adapted from Ref. (19)]. (B) Confocal images showing the mid section of bovine chromaffin cells expressing lifeact-RFP and counter stained with FITC-conjugated phalloidin in the presence or absence of nicotine (50 μM). (C) Maximum intensity projection of the footprint of a chromaffin cell. (D) TIRF images showing actin lengthening in a chromaffin cell expressing lifeact-GFP (pseudocolor) after the addition of PI3Kδ inhibitor IC87114. (B–D) Adapted from Ref. (2).

Figure 2

Schematic diagrams of the myosin heavy chains involved in regulated exocytosis. All myosins consist of a head (motor) domain (blue), a neck that contains one or more IQ motifs for light chain and CaM binding (black), and a tail domain with coiled-coil regions (green) and membrane/cargo-binding domains (orange). The small insert of myosin VI, shown to be essential for the tethering of SGs to the cortical actin network, and the DYD-Src phosphorylation motif are highlighted. Cargo-binding induced dimerization of myosin VI is likely to be mediated by the coiled-coil regions and the cargo-binding domains. Adapted from Ref. (83).

Figure 3

The roles of myosins and accessory proteins involved in regulated exocytosis. Myosins are involved in several steps of regulated exocytosis. Myosin 1C (yellow), myosin 1E (burgundy), myosin II (orange), and myosin Va (green) are involved in secretory vesicle transport. In contrast, myosin VI (red) recruits SGs to the cortical actin network. Myosin 1C interacts with SG through cysteine string proteins and myosin Va binds to MyRIP (purple) on the membrane of SGs. Myosin 1C can be recruited to membranes through PIP₂ interaction. The effector that mediates binding between myosin VI and SGs (light green) is currently unknown. Myosin 1E is also involved in regulating actin polymerization through interaction with WASP/Arp 2/3. Cdc42 as well WASP/Arp 2/3 regulate actin polymerization in an activity-dependent manner. Myosin II also regulates size and duration of fusion pore opening.