The cortical actin filaments are arranged in

The cortical CHIR-98014 filaments are arranged in a network abuting the plasma membrane with a thickness of ∼190nm in living cells, that displays highly dynamic behaviors 15, 16, 17. Technical challenges, however, have hampered investigation of the detailed function dynamic interactions between the cortical actin network and secretory granules in regulated exocytosis. Previous observations have been obtained under conventional light microscopy using drugs or toxins that induce global changes in the actin cytoskeleton, with seemingly contradictory conclusions being drawn on the role of the cortical actin network in regulated exocytosis [9]. Recently, the development of super-resolution (SR) microscopy combined with the generation of novel specific probes has led to new insights regarding the role of the cortical actin network in regulated exocytosis 8, 9, 11, 12, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25. Importantly, advanced SR techniques not only image fixed specimens, but also visualize fluorescent molecules with high-speed and low-illumination intensities at <50nm resolution for multiple frames, enabling time-lapse measurements of both actin dynamics and granule movements before and during exocytosis [26]. As such, the spatiotemporal interactions between the actin–myosin (actomyosin) network and secretory granules have been imaged in unprecedented detail at both the micro- 8, 12, 13, 14, 18, and nanoscale 19, 20, 21, 22, 23, 24, 25 level during single exocytic events. Spatiotemporal analyses show that individual vesicle fusion events occur in hotspots that are closely associated with the cortical cytoskeleton 13, 14. SR imaging has further demonstrated that granule secretion occurs in a microscale region where the cortical density is low 19, 21, but relies on the nanoscale structure of the actin-related scaffold around secretory granules, which contributes to granule translocation and exocytosis 23, 24, 25, 27. Furthermore, emerging lines of evidence suggest that microscale actin remodeling in the secretory hot spots is coupled to nanoscale actin remodeling around exocytosing granules 8, 23, 24, 25, 28, although the micro/nano coupling and the relationship among F-actin remodeling, granule movement, and degranulation events sometimes appear to be complex and nonlinear [11]. Such a coupling between the micro- and nanoscale layers of the cortical actin network could potentially coordinate events such as vesicular translocation, docking/priming, and fusion/repulsion of cargoes in regulated exocytosis.
Microscale Cortical Actin Remodeling in Regulated Exocytosis Recent investigations have revealed that dynamic local changes in the cortical actin filamentous network play a critical role in mobilizing vesicles in readiness for fusion [29]. The close proximity of this network to the plasma membrane creates a compartmentalization of the membrane 30, 31, 32. The actin-based cytoskeleton generates ‘fences’ beneath the plasma membrane, with these fences serving as anchors that ‘pickets’ transmembrane proteins in the underlying membrane 30, 33. One of the early descriptions of the function of the actin cytoskeleton was in chromaffin cells, where its role in preventing most cellular organelles from contacting the plasmalemma was highlighted 34, 35. It was therefore proposed that a major purpose of the meshwork was to act as a diffusional barrier that prevents access to vesicle fusion sites (Figure 1) 36, 37; a notion that was confirmed based on studies that used chemicals to stabilize or prevent actin polymerization 38, 39. However, stimulated exocytosis has more recently been shown to alter the organization and the tensile strength of the underlying actin network, allowing vesicle to first bind to the cortical actin network via myosin VI small insert isoform and then translocate to the plasma membrane via myosin-II-dependent relaxation of the actin meshwork 8, 12, 18, 40. Recent imaging systems have revealed periodic changes in the cortical actin network in controlling granule translocation and subsequent exocytosis 8, 12.