br Nanoscale Cortical Actin Remodeling in Regulated Exocytos

Nanoscale Cortical Actin Remodeling in Regulated Exocytosis The network of cortical plerixafor is formed by numerous fine actin filaments, which are only about 10nm in diameter [52]. Although the microscale role of the cortical actin network in vesicle exocytosis is well established, the function of the ultrafine actin filaments has been underappreciated due to the diffraction limit of fluorescence imaging technology. Recently, the emerging development of SR technologies has allowed the visualization of the remodeling of these nanoscale actin filaments as they control the immediate surroundings of the secretory granules as they undergo docking, priming, and fusion in exocytosis hot spots 19, 24, 26. Using these SR techniques, such as structured illumination microscopy (SIM) or electron microscopy, the fine spatiotemporal details of actin remodeling could be visualized over the entire exocytosis process [26]. It was proposed that the nanoscale remodeling of the actin filaments surrounding secretory vesicles dynamically controls the fusion of primed vesicles 25, 53, 54. Such exquisite remodeling is independent from the oscillation and relaxation of the microscale cortical network 8, 12, 55. This actin-based nanostructure around vesicles not only physically facilitates exocytosis by caging vesicles at the fusion site or crosslinking vesicles to the plasma membrane, but also provides a scaffold on which various regulators of exocytosis are recruited, thereby orchestrating the final stage of exocytosis [23] (Figure 2).
Mesoscopic Properties of the Cortical Actin Network The identification of the nanoscale functions of the actin network demonstrates that cortical actin can regulate exocytosis behavior both on the larger scale of the cortical actin network and the nanoscale level around secretory vesicles and at the boundary between the plasma and vesicular membranes. Paradoxically, these mesoscopic remodeling processes share some effectors, such as myosin IIa, which could provide the basis for their coupling and synchronicity to ensure complete and efficient exocytotic events 12, 56, 75. Such synchronization lies not only in the dynamics of the actin structure but also in the molecular preparation for proper function on the nanoscale level.
Temporal and Spatial Micro/Nanoscale Coupling Recently, the oscillatory nature and activity-dependent relaxation of the cortical actin network in triggering vesicle translocation revealed a functional cortical actin layer that is non-homogenous in acting as a physical barrier. Indeed, the pre-existing actin filaments in certain sites help the approach of vesicles to the proximity of the plasma membrane 12, 24. This effect requires, at the microscale level, a low local density of cortical actin. However, a lower density of the actin cytoskeleton is conducive to higher vesicle mobility, which is detrimental to vesicle docking and fusion. Therefore, while most of the cortical actin network is cleared or relaxed, the ultrafine actin filaments are activated and form the basis of the nanoscale function of the network. These reorganized ultrafine actin filaments help to precisely trap and anchor vesicles, and translocate them to the peri-membrane region, which is necessary for their subsequent docking and fusions [25]. Meanwhile, the nanoscale actin structure around each caged vesicle interacts with lipid/lipid binding proteins and recruits actin bundling and signaling transduction proteins, building a molecular nanoenvironment conducive to priming and fusion 25, 51, 54, 56. Such a mesoscopic function of the cortical actin structure may allow the coupling of the large-scale recruitment of vesicles with their docking and priming at hotspots on the plasma membrane during the sequential steps of exocytosis. However the precise mechanisms need to be further investigated.
Concluding Remarks The application of recent imaging technologies has revealed new aspects in the role of the actin cortex in regulated exocytosis, leading to new hypotheses that may resolve several disparate or even opposing results from the literature. Despite this progress, the majority of studies in exocytosis have focused on a few hundreds of nanometers from adherent secretory cells in culture. The challenge will be to pursue these investigations in extending localization microscopy into thicker 3D samples, thereby allowing in vivo monitoring of this process. Currently, SR modalities are being combined to yield novel methodologies, such as light sheet microscopy with two photon excitation 88, 89 and structured illumination microscopy with two photon excitation to allow for improved spatial resolution and the ability to image deep into thick samples 19, 21, 90.