Myosin 2 maintains an open exocytic fusion pore in secretory epithelial cells

Fusion pore dynamics of large secretory vesicles define a distinct mechanism of exocytosis

Exocrine cells utilize large secretory vesicles (LSVs) up to 10 μm in diameter. LSVs fuse with the apical surface, often recruiting actomyosin to extrude their content through dynamic fusion pores. The molecular mechanism regulating pore dynamics remains largely uncharacterized. We observe that the fusion pores of LSVs in the Drosophila larval salivary glands expand, stabilize, and constrict. Arp2/3 is essential for pore expansion and stabilization, while myosin II is essential for pore constriction. We identify several Bin-Amphiphysin-Rvs (BAR) homology domain proteins that regulate fusion pore expansion and stabilization. We show that the I-BAR protein Missing-in-Metastasis (MIM) localizes to the fusion site and is essential for pore expansion and stabilization. The MIM I-BAR domain is essential but not sufficient for localization and function. We conclude that MIM acts in concert with actin, myosin II, and additional BAR-domain proteins to control fusion pore dynamics, mediating a distinct mode of exocytosis, which facilitates actomyosin-dependent content release that maintains apical membrane homeostasis during secretion.

Real-time visualization of exo- and endocytosis membrane dynamics with confocal and super-resolution microscopy

Real-time confocal and super-resolution imaging reveals membrane dynamics of exo- and endocytosis, including hemi-fusion, fusion pore opening, expansion, constriction, closure (kiss-and-run), fused-vesicle shrinking (shrink fusion), and flat membrane transition to vesicles via intermediate Λ- and Ω-shape structures. Here, we describe a protocol for imaging these membrane dynamics, including primary culture of bovine adrenal chromaffin cells, fluorescent probe application, patch-clamp to deliver depolarization and evoke exo- and endocytosis, electron microscopy (EM), and real-time confocal and stimulated emission depletion (STED) microscopy. For complete details on the use and execution of this protocol, please refer to Zhao et al. (2016), Shin et al. (2018), and Shin et al. (2021).

Sequential compound fusion and kiss-and-run mediate exo- and endocytosis in excitable cells

Vesicle fusion at preestablished plasma membrane release sites releases transmitters and hormones to mediate fundamental functions like neuronal network activities and fight-or-flight responses. This half-a-century-old concept-fusion at well-established release sites in excitable cells-needs to be modified to include the sequential compound fusion reported here-vesicle fusion at previously fused Ω-shaped vesicular membrane. With superresolution STED microscopy in excitable neuroendocrine chromaffin cells, we real-time visualized sequential compound fusion pore openings and content releases in generating multivesicular and asynchronous release from single release sites, which enhances exocytosis strength and dynamic ranges in excitable cells. We also visualized subsequent compound fusion pore closure, a new mode of endocytosis termed compound kiss-and-run that enhances vesicle recycling capacity. These results suggest modifying current exo-endocytosis concepts by including rapid release-site assembly at fused vesicle membrane, where sequential compound fusion and kiss-and-run take place to enhance exo-endocytosis capacity and dynamic ranges.

Brain-derived neurotrophic factor regulates LYN kinase-mediated myosin light chain kinase activation to modulate nonmuscle myosin II activity in hippocampal neurons

Myosins belong to a large superfamily of actin-dependent molecular motors. Nonmuscle myosin II (NM II) is involved in the morphology and function of neurons, but little is known about how NM II activity is regulated. Brain-derived neurotrophic factor (BDNF) is a prevalent neurotrophic factor in the brain that encourages growth and differentiation of neurons and synapses. In this study, we report that BDNF upregulates the phosphorylation of myosin regulatory light chain (MLC2), to increases the activity of NM II. The role of BDNF on modulating the phosphorylation of MLC2 was validated by using Western blotting in primary cultured hippocampal neurons. This result was confirmed by injecting BDNF into the dorsal hippocampus of mice and detecting the phosphorylation level of MLC2 by Western blotting. We further perform coimmunoprecipitation assay to confirm that this process depends on the activation of the LYN kinase through binding with tyrosine kinase receptor B, the receptor of BDNF, in a kinase activity-dependent manner. LYN kinase subsequently phosphorylates MLCK, further promoting the phosphorylation of MLC2. Taken together, our results suggest a new molecular mechanism by which BDNF regulates MLC2 activity, which provides a new perspective for further understanding the functional regulation of NM II in the nervous system.

Oncogenic Kras-Mediated Cytokine CCL15 Regulates Pancreatic Cancer Cell Migration and Invasion through ROS

Simple Summary

Oncogenic Kras^G12D and tumor inflammation are critical components of the development and dissemination of pancreatic ductal adenocarcinoma (PDAC). The aim of this study is to investigate a lesser-known cytokine, CCL15, that functions as a new downstream target of Kras^G12D with the purpose of regulating PDAC cell migration and invasion. We showed increased levels of CCL15 as well as the presence of its receptors, including CCR1 and CCR3, in PDAC tissues and cell lines. The knockdown of CCL15 diminished metastatic Panc-1 cell migration, whereas the treatment of CCL15 in non-metastatic BxPC-3 cells promoted BxPC-3 cell motility. Similar results were verified using murine metastatic PDAC KP-2 cells. Furthermore, we demonstrated that CCL15-modulated PDAC cell migration through the upregulation of cellular reactive oxygen species (ROS) levels and the knockdown of Kras^G12D resulted in a decrease in CCL15. Altogether, our data unveiled a new mechanism of oncogenic Kras^G12D in modulating PDAC inflammation and spreading.

Abstract

Pancreatic ductal adenocarcinoma (PDAC) is well known for its high death rate due to prompt cancer metastasis caused by cancer cell migration and invasion within the early stages of its development. Here, we reveal a new function of cytokine CCL15, namely the upregulation of PDAC cell migration and invasion. We showed increased levels of CCL15 transcripts and protein expressions in human PDAC tissue samples, as well as in cultured cell lines. Furthermore, PDAC cells also expressed CCL15 receptors, including CCR1 and CCR3. Murine PDAC cell lines and tissues strengthened this finding. The manipulation of CCL15 in metastatic Panc-1 cells through CCL15 knockdown or CCL15 neutralization decreased Panc-1 cell motility and invasiveness. In addition, treating non-metastatic BxPC-3 cells with recombinant CCL15 accelerated the cell migration of BxPC-3. A reduction in the levels of reactive oxygen species (ROS) by either N-Acetyl-L-Cysteine treatment or p22phox knockdown led to a decrease in Panc-1 cell migration and a reversed effect on recombinant CCL15-promoted BxPC-3 cell movement. Importantly, the knockdown of oncogenic Kras in Panc-1 cells abolished CCL15 protein expression and impeded cell migration without affecting PDAC cell growth. Altogether, our work elucidates an additional molecular pathway of oncogenic Kras to promote PDAC metastasis through the upregulation of cell migration and invasion by the Kras downstream CCL15, a lesser-known cytokine within the cancer research field.

Actin and Myosin in Non-Neuronal Exocytosis

Cellular secretion depends on exocytosis of secretory vesicles and discharge of vesicle contents. Actin and myosin are essential for pre-fusion and post-fusion stages of exocytosis. Secretory vesicles depend on actin for transport to and attachment at the cell cortex during the pre-fusion phase. Actin coats on fused vesicles contribute to stabilization of large vesicles, active vesicle contraction and/or retrieval of excess membrane during the post-fusion phase. Myosin molecular motors complement the role of actin. Myosin V is required for vesicle trafficking and attachment to cortical actin. Myosin I and II members engage in local remodeling of cortical actin to allow vesicles to get access to the plasma membrane for membrane fusion. Myosins stabilize open fusion pores and contribute to anchoring and contraction of actin coats to facilitate vesicle content release. Actin and myosin function in secretion is regulated by a plethora of interacting regulatory lipids and proteins. Some of these processes have been first described in non-neuronal cells and reflect adaptations to exocytosis of large secretory vesicles and/or secretion of bulky vesicle cargoes. Here we collate the current knowledge and highlight the role of actomyosin during distinct phases of exocytosis in an attempt to identify unifying molecular mechanisms in non-neuronal secretory cells.

Toward a unified picture of the exocytotic fusion pore

Neurotransmitter and hormone release involve calcium-triggered fusion of a cargo-loaded vesicle with the plasma membrane. The initial connection between the fusing membranes, called the fusion pore, can evolve in various ways, including rapid dilation to allow full cargo release, slow expansion, repeated opening-closing and resealing. Pore dynamics determine the kinetics of cargo release and the mode of vesicle recycling, but how these processes are controlled is poorly understood. Previous reconstitutions could not monitor single pores, limiting mechanistic insight they could provide. Recently developed nanodisc-based fusion assays allow reconstitution and monitoring of single pores with unprecedented detail and hold great promise for future discoveries. They recapitulate various aspects of exocytotic fusion pores, but comparison is difficult because different approaches suggested very different exocytotic fusion pore properties, even for the same cell type. In this Review, I discuss how most of the data can be reconciled, by recognizing how different methods probe different aspects of the same fusion process. The resulting picture is that fusion pores have broadly distributed properties arising from stochastic processes which can be modulated by physical constraints imposed by proteins, lipids and membranes.

The Actomyosin Cytoskeleton Drives Micron-Scale Membrane Remodeling In Vivo Via the Generation of Mechanical Forces to Balance Membrane Tension Gradients

The remodeling of biological membranes is crucial for a vast number of cellular activities and is an inherently multiscale process in both time and space. Seminal work has provided important insights into nanometer-scale membrane deformations, and highlighted the remarkable variation and complexity in the underlying molecular machineries and mechanisms. However, how membranes are remodeled at the micron-scale, particularly in vivo, remains poorly understood. Here, we discuss how using regulated exocytosis of large (1.5-2.0 μm) membrane-bound secretory granules in the salivary gland of live mice as a model system, has provided evidence for the importance of the actomyosin cytoskeleton in micron-scale membrane remodeling in physiological conditions. We highlight some of these advances, and present mechanistic hypotheses for how the various biochemical and biophysical properties of distinct actomyosin networks may drive this process.

Concerted actions of distinct nonmuscle myosin II isoforms drive intracellular membrane remodeling in live animals

Membrane remodeling plays a fundamental role during a variety of biological events. However, the dynamics and the molecular mechanisms regulating this process within cells in mammalian tissues in situ remain largely unknown. In this study, we use intravital subcellular microscopy in live mice to study the role of the actomyosin cytoskeleton in driving the remodeling of membranes of large secretory granules, which are integrated into the plasma membrane during regulated exocytosis. We show that two isoforms of nonmuscle myosin II, NMIIA and NMIIB, control distinct steps of the integration process. Furthermore, we find that F-actin is not essential for the recruitment of NMII to the secretory granules but plays a key role in the assembly and activation of NMII into contractile filaments. Our data support a dual role for the actomyosin cytoskeleton in providing the mechanical forces required to remodel the lipid bilayer and serving as a scaffold to recruit key regulatory molecules.

ALPK1 phosphorylates myosin IIA modulating TNF-α trafficking in gout flares

Gout is characterized by the monosodium urate monohydrate (MSU)-induced arthritis. Alpha kinase-1 (ALPK1) has shown to be associated with MSU-induced inflammation and gout. Here, we used bioinformatics, proteomics, cell models, and twenty in vitro human assays to clarify some of its role in the inflammatory response to MSU. We found myosin IIA to be a frequent interacting protein partner of ALPK1, binding to its N-terminal and forming a protein complex with calmodulin and F-actin, and that MSU-induced ALPK1 phosphorylated the myosin IIA. A knockdown of endogenous ALPK1 or myosin IIA significantly reduced the MSU-induced secretion of tumour necrosis factor (TNF)-α. Furthermore, all gouty patients expressed higher basal protein levels of ALPK1, myosin IIA, and plasma TNF-α, however those medicated with colchicine has shown reduced myosin IIA and TNF-α but not ALPK1. The findings suggest ALPK1 is a kinase that participates in the regulation of Golgi-derived TNF-α trafficking through myosin IIA phosphorylation in the inflammation of gout. This novel pathway could be blocked at the level of myosin by colchicine in gout treatment.

The skeleton in the closet: actin cytoskeletal remodeling in β-cell function

Over the last few decades, biomedical research has considered not only the function of single cells but also the importance of the physical environment within a whole tissue, including cell-cell and cell-extracellular matrix interactions. Cytoskeleton organization and focal adhesions are crucial sensors for cells that enable them to rapidly communicate with the physical extracellular environment in response to extracellular stimuli, ensuring proper function and adaptation. The involvement of the microtubular-microfilamentous cytoskeleton in secretion mechanisms was proposed almost 50 years ago, since when the evolution of ever more sensitive and sophisticated methods in microscopy and in cell and molecular biology have led us to become aware of the importance of cytoskeleton remodeling for cell shape regulation and its crucial link with signaling pathways leading to β-cell function. Emerging evidence suggests that dysfunction of cytoskeletal components or extracellular matrix modification influences a number of disorders through potential actin cytoskeleton disruption that could be involved in the initiation of multiple cellular functions. Perturbation of β-cell actin cytoskeleton remodeling could arise secondarily to islet inflammation and fibrosis, possibly accounting in part for impaired β-cell function in type 2 diabetes. This review focuses on the role of actin remodeling in insulin secretion mechanisms and its close relationship with focal adhesions and myosin II.

Vesicular control of fusion pore expansion

Exocytic post-fusion events play an important role determining the composition and quantity of cellular secretion. In particular, Ca²⁺-dependent regulation of fusion pore dilation/closure is a key regulator for fine-tuning vesicle content secretion. This requires a tight temporal and spatial integration of vesicle fusion with the PM, Ca²⁺ signals and translation of the Ca²⁺ signal into fusion pore dilation via auxiliary factors. Yet, it is still mostly elusive how this is achieved in slow and non-excitable secretory cells, where initial Ca²⁺ signals triggering fusions will abate before onset of the post-fusion phase. New results suggest, that the vesicles themselves provide the necessary itinerary to sense and link vesicle fusion to generation of local Ca²⁺ signals and fusion pore expansion.

Actin depolymerisation and crosslinking join forces with myosin II to contract actin coats on fused secretory vesicles

In many secretory cells actin and myosin are specifically recruited to the surface of secretory granules following their fusion with the plasma membrane. Actomyosin-dependent compression of fused granules is essential to promote active extrusion of cargo. However, little is known about molecular mechanisms regulating actin coat formation and contraction. Here, we provide a detailed kinetic analysis of the molecules regulating actin coat contraction on fused lamellar bodies in primary alveolar type II cells. We demonstrate that ROCK1 and myosin light chain kinase 1 (MLCK1, also known as MYLK) translocate to fused lamellar bodies and activate myosin II on actin coats. However, myosin II activity is not sufficient for efficient actin coat contraction. In addition, cofilin-1 and α-actinin translocate to actin coats. ROCK1-dependent regulated actin depolymerisation by cofilin-1 in cooperation with actin crosslinking by α-actinin is essential for complete coat contraction. In summary, our data suggest a complementary role for regulated actin depolymerisation and crosslinking, and myosin II activity, to contract actin coats and drive secretion.

Synaptotagmin-7 links fusion-activated Ca²⁺ entry and fusion pore dilation

Ca(2+)-dependent regulation of fusion pore dilation and closure is a key mechanism determining the output of cellular secretion. We have recently described 'fusion-activated' Ca(2+) entry (FACE) following exocytosis of lamellar bodies in alveolar type II cells. FACE regulates fusion pore expansion and facilitates secretion. However, the mechanisms linking this locally restricted Ca(2+) signal and fusion pore expansion were still elusive. Here, we demonstrate that synaptotagmin-7 (Syt7) is expressed on lamellar bodies and links FACE and fusion pore dilation. We directly assessed dynamic changes in fusion pore diameters by analysing diffusion of fluorophores across fusion pores. Expressing wild-type Syt7 or a mutant Syt7 with impaired Ca(2+)-binding to the C2 domains revealed that binding of Ca(2+) to the C2A domain facilitates FACE-induced pore dilation, probably by inhibiting translocation of complexin-2 to fused vesicles. However, the C2A domain hampered Ca(2+)-dependent exocytosis of lamellar bodies. These findings support the hypothesis that Syt7 modulates fusion pore expansion in large secretory organelles and extend our picture that lamellar bodies contain the necessary molecular inventory to facilitate secretion during the exocytic post-fusion phase. Moreover, regulating Syt7 levels on lamellar bodies appears to be essential in order that exocytosis is not impeded during the pre-fusion phase.

A new role for myosin II in vesicle fission

An endocytic vesicle is formed from a flat plasma membrane patch by a sequential process of invagination, bud formation and fission. The scission step requires the formation of a tubular membrane neck (the fission pore) that connects the endocytic vesicle with the plasma membrane. Progress in vesicle fission can be measured by the formation and closure of the fission pore. Live-cell imaging and sensitive biophysical measurements have provided various glimpses into the structure and behaviour of the fission pore. In the present study, the role of non-muscle myosin II (NM-2) in vesicle fission was tested by analyzing the kinetics of the fission pore with perforated-patch clamp capacitance measurements to detect single vesicle endocytosis with millisecond time resolution in peritoneal mast cells. Blebbistatin, a specific inhibitor of NM-2, dramatically increased the duration of the fission pore and also prevented closure during large endocytic events. Using the fluorescent markers FM1-43 and pHrodo Green dextran, we found that NM-2 inhibition greatly arrested vesicle fission in a late phase of the scission event when the pore reached a final diameter of ∼ 5 nm. Our results indicate that loss of the ATPase activity of myosin II drastically reduces the efficiency of membrane scission by making vesicle closure incomplete and suggest that NM-2 might be especially relevant in vesicle fission during compound endocytosis.

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

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.

Critical role of cortical vesicles in dissecting regulated exocytosis: overview of insights into fundamental molecular mechanisms

Regulated exocytosis is one of the defining features of eukaryotic cells, underlying many conserved and essential functions. Definitively assigning specific roles to proteins and lipids in this fundamental mechanism is most effectively accomplished using a model system in which distinct stages of exocytosis can be effectively separated. Here we discuss the establishment of sea urchin cortical vesicle fusion as a model to study regulated exocytosis-a system in which the docked, release-ready, and late Ca(2+)-triggered steps of exocytosis are isolated and can be quantitatively assessed using the rigorous coupling of functional and molecular assays. We provide an overview of the insights this has provided into conserved molecular mechanisms and how these have led to and integrate with findings from other regulated exocytotic cells.

Non-muscle myosin IIA is involved in focal adhesion and actin remodelling controlling glucose-stimulated insulin secretion

AIMS/HYPOTHESIS

Actin and focal adhesion (FA) remodelling are essential for glucose-stimulated insulin secretion (GSIS). Non-muscle myosin II (NM II) isoforms have been implicated in such remodelling in other cell types, and myosin light chain kinase (MLCK) and Rho-associated coiled-coil-containing kinase (ROCK) are upstream regulators of NM II, which is known to be involved in GSIS. The aim of this work was to elucidate the implication and regulation of NM IIA and IIB in beta cell actin and FA remodelling, granule trafficking and GSIS.

METHODS

Inhibitors of MLCK, ROCK and NM II were used to study NM II activity, and knockdown of NM IIA and IIB to determine isoform specificity, using sorted primary rat beta cells. Insulin was measured by radioimmunoassay. Protein phosphorylation and subcellular distribution were determined by western blot and confocal immunofluorescence. Dynamic changes were monitored by live cell imaging and total internal reflection fluorescence microscopy using MIN6B1 cells.

RESULTS

NM II and MLCK inhibition decreased GSIS, associated with shortening of peripheral actin stress fibres, and reduced numbers of FAs and insulin granules in close proximity to the basal membrane. By contrast, ROCK inhibition increased GSIS and caused disassembly of glucose-induced central actin stress fibres, resulting in large FAs without any effect on FA number. Only glucose-induced NM IIA reorganisation was blunted by MLCK inhibition. NM IIA knockdown decreased GSIS, levels of FA proteins and glucose-induced extracellular signal-regulated kinase 1/2 phosphorylation.

CONCLUSIONS/INTERPRETATION

Our data indicate that MLCK-NM IIA may modulate translocation of secretory granules, resulting in enhanced insulin secretion through actin and FA remodelling, and regulation of FA protein levels.

Multiple roles for the actin cytoskeleton during regulated exocytosis

Regulated exocytosis is the main mechanism utilized by specialized secretory cells to deliver molecules to the cell surface by virtue of membranous containers (i.e., secretory vesicles). The process involves a series of highly coordinated and sequential steps, which include the biogenesis of the vesicles, their delivery to the cell periphery, their fusion with the plasma membrane, and the release of their content into the extracellular space. Each of these steps is regulated by the actin cytoskeleton. In this review, we summarize the current knowledge regarding the involvement of actin and its associated molecules during each of the exocytic steps in vertebrates, and suggest that the overall role of the actin cytoskeleton during regulated exocytosis is linked to the architecture and the physiology of the secretory cells under examination. Specifically, in neurons, neuroendocrine, endocrine, and hematopoietic cells, which contain small secretory vesicles that undergo rapid exocytosis (on the order of milliseconds), the actin cytoskeleton plays a role in pre-fusion events, where it acts primarily as a functional barrier and facilitates docking. In exocrine and other secretory cells, which contain large secretory vesicles that undergo slow exocytosis (seconds to minutes), the actin cytoskeleton plays a role in post-fusion events, where it regulates the dynamics of the fusion pore, facilitates the integration of the vesicles into the plasma membrane, provides structural support, and promotes the expulsion of large cargo molecules.

α-catenin and IQGAP regulate myosin localization to control epithelial tube morphogenesis in Dictyostelium

Apical actomyosin activity in animal epithelial cells influences tissue morphology and drives morphogenetic movements during development. The molecular mechanisms leading to myosin II accumulation at the apical membrane and its exclusion from other membranes are poorly understood. We show that in the nonmetazoan Dictyostelium discoideum, myosin II localizes apically in tip epithelial cells that surround the stalk, and constriction of this epithelial tube is required for proper morphogenesis. IQGAP1 and its binding partner cortexillin I function downstream of α- and β-catenin to exclude myosin II from the basolateral cortex and promote apical accumulation of myosin II. Deletion of IQGAP1 or cortexillin compromises epithelial morphogenesis without affecting cell polarity. These results reveal that apical localization of myosin II is a conserved morphogenetic mechanism from nonmetazoans to vertebrates and identify a hierarchy of proteins that regulate the polarity and organization of an epithelial tube in a simple model organism.

Real-time measurement of F-actin remodelling during exocytosis using Lifeact-EGFP transgenic animals

F-actin remodelling is essential for a wide variety of cell processes. It is important in exocytosis, where F-actin coats fusing exocytic granules. The purpose of these F-actin coats is unknown. They may be important in stabilizing the fused granules, they may play a contractile role and promote expulsion of granule content and finally may be important in endocytosis. To elucidate these functions of F-actin remodelling requires a reliable method to visualize F-actin dynamics in living cells. The recent development of Lifeact-EGFP transgenic animals offers such an opportunity. Here, we studied the characteristics of exocytosis in pancreatic acinar cells obtained from the Lifeact-EGFP transgenic mice. We show that the time-course of agonist-evoked exocytic events and the kinetics of each single exocytic event are the same for wild type and Lifeact-EGFP transgenic animals. We conclude that Lifeact-EGFP animals are a good model to study of exocytosis and reveal that F-actin coating is dependent on the de novo synthesis of F-actin and that development of actin polymerization occurs simultaneously in all regions of the granule. Our insights using the Lifeact-EGFP mice demonstrate that F-actin coating occurs after granule fusion and is a granule-wide event.

Nonmuscle myosin-2: mix and match

Members of the nonmuscle myosin-2 (NM-2) family of actin-based molecular motors catalyze the conversion of chemical energy into directed movement and force thereby acting as central regulatory components of the eukaryotic cytoskeleton. By cyclically interacting with adenosine triphosphate and F-actin, NM-2 isoforms promote cytoskeletal force generation in established cellular processes like cell migration, shape changes, adhesion dynamics, endo- and exo-cytosis, and cytokinesis. Novel functions of the NM-2 family members in autophagy and viral infection are emerging, making NM-2 isoforms regulators of nearly all cellular processes that require the spatiotemporal organization of cytoskeletal scaffolding. Here, we assess current views about the role of NM-2 isoforms in these activities including the tight regulation of NM-2 assembly and activation through phosphorylation and how NM-2-mediated changes in cytoskeletal dynamics and mechanics affect cell physiological functions in health and disease.

Actin coating and compression of fused secretory vesicles are essential for surfactant secretion--a role for Rho, formins and myosin II

Secretion of vesicular contents by exocytosis is a fundamental cellular process. Increasing evidence suggests that post-fusion events play an important role in determining the composition and quantity of the secretory output. In particular, regulation of fusion pore dilation and closure is considered a key regulator of the post-fusion phase. However, depending on the nature of the cargo, additional mechanisms might be essential to facilitate effective release. We have recently described that in alveolar type II (ATII) cells, lamellar bodies (LBs), which are secretory vesicles that store lung surfactant, are coated with actin following fusion with the plasma membrane. Surfactant, a lipoprotein complex, does not readily diffuse out of fused LBs following opening and dilation of the fusion pore. Using fluorescence microscopy, atomic force microscopy and biochemical assays, we present evidence that actin coating and subsequent contraction of the actin coat is essential to facilitate surfactant secretion. Latrunculin B prevents actin coating of fused LBs and inhibits surfactant secretion almost completely. Simultaneous imaging of the vesicle membrane and the actin coat revealed that contraction of the actin coat compresses the vesicle following fusion. This leads to active extrusion of vesicle contents. Initial actin coating of fused vesicles is dependent on activation of Rho and formin-dependent actin nucleation. Actin coat contraction is facilitated by myosin II. In summary, our data suggest that fusion pore opening and dilation itself is not sufficient for release of bulky vesicle cargos and that active extrusion mechanisms are required.

Measuring calcium signals and exocytosis in tissues

BACKGROUND

Since the 1960s it has been clear that calcium is a key regulator of exocytosis. Early experiments directly showed that the secretory output was calcium dependent. But it has taken improvements in technology and clever experimentation to determine the relationships between the calcium signal and exocytosis. Today controversies still remain because of limitations in our ability to record both the calcium responses within the local domains that control secretion and in the methods used to record exocytosis.

SCOPE OF REVIEW

Here the techniques used to measure calcium and exocytosis are reviewed with a distinction being drawn between measurements in excitable cells versus measurements in non-excitable cells. The review has a focus on techniques that are relevant to in vitro studies of native tissues and recent in vivo recordings.

MAJOR CONCLUSIONS

There are a range of methods used to study the stimulus-secretion pathway. Each presents their own advantages and drawbacks. These are discussed with reference to the latest work determining the factors controlling exocytosis in tissues.

GENERAL SIGNIFICANCE

Stimulus-secretion coupling is the fundamental step in the control of neurotransmitter release, hormone secretion and protein secretion. Understanding secretory control is therefore important in understanding the physiological regulation of processes ranging from learning and memory to pancreatic secretion. Recent technological advances are now enabling us to study stimulus-secretion coupling within native tissues. This is helping us to understand the physiological complexities of secretory control. This article is part of a Special Issue entitled Biochemical, biophysical and genetic approaches to intracellular calcium signalling.

Regulated exocytosis: novel insights from intravital microscopy

Regulated exocytosis is a fundamental process that every secretory cell uses to deliver molecules to the cell surface and the extracellular space by virtue of membranous carriers. This process has been extensively studied using various approaches such as biochemistry, electrophysiology and electron microscopy. However, recent developments in time-lapse light microscopy have made possible imaging individual exocytic events, hence, advancing our understanding of this process at a molecular level. In this review, we focus on intravital microscopy (IVM), a light microscopy-based approach that enables imaging subcellular structures in live animals, and discuss its recent application to study regulated exocytosis. IVM has revealed differences in regulation and modality of regulated exocytosis between in vitro and in vivo model systems, unraveled novel aspects of this process that can be appreciated only in in vivo settings and provided valuable and novel information on its molecular machinery. In conclusion, we make the case for IVM being a mature technique that can be used to investigate the molecular machinery of several intracellular events under physiological conditions.

The human submandibular gland: immunohistochemical analysis of SNAREs and cytoskeletal proteins

Submandibular acinar glands secrete numerous proteins such as digestive enzymes and defense proteins on the basis of the exocrine secretion mode. Exocytosis is a complex process, including a soluble NSF attachment protein receptor (SNARE)-mediated membrane fusion of vesicles and target membrane and the additional activation of cytoskeletal proteins. Relevant data are available predominantly for animal salivary glands, especially of the rat parotid acinar cells. The authors investigated the secretory molecular machinery of acinar (serous) cells in the human submandibular gland by immunohistochemistry and immunofluorescence and found diverse proteins associated with exocytosis for the first time. SNAP-23, syntaxin-2, syntaxin-4, and VAMP-2 were localized at the luminal plasma membrane; syntaxin-2 and septin-2 were expressed in vesicles in the cytoplasm. Double staining of syntaxin-2 and septin-2 revealed a colocalization on the same vesicles. Lactoferrin and α-amylase served as a marker for secretory vesicles and were labeled positively together with syntaxin-2 and septin-2 in double-staining procedures. Cytoskeletal components such as actin, myosin II, cofilin, and profilin are concentrated at the apical plasma membrane of acinar submandibular glands. These observations complement the understanding of the complex exocytosis mechanisms.

Myosin motor proteins are involved in the final stages of the secretory pathways

In eukaryotes, the final steps in both the regulated and constitutive secretory pathways can be divided into four distinct stages: (i) the 'approach' of secretory vesicles/granules to the PM (plasma membrane), (ii) the 'docking' of these vesicles/granules at the membrane itself, (iii) the 'priming' of the secretory vesicles/granules for the fusion process, and, finally, (iv) the 'fusion' of vesicular/granular membranes with the PM to permit content release from the cell. Recent work indicates that non-muscle myosin II and the unconventional myosin motor proteins in classes 1c/1e, Va and VI are specifically involved in these final stages of secretion. In the present review, we examine the roles of these myosins in these stages of the secretory pathway and the implications of their roles for an enhanced understanding of secretion in general.

Actomyosin II contractility expels von Willebrand factor from Weibel-Palade bodies during exocytosis

High-resolution microscopy reveals how discrete actin cytoskeletal functions inhibit or promote specific exocytic steps during regulated secretion.

The study of actin in regulated exocytosis has a long history with many different results in numerous systems. A major limitation on identifying precise mechanisms has been the paucity of experimental systems in which actin function has been directly assessed alongside granule content release at distinct steps of exocytosis of a single secretory organelle with sufficient spatiotemporal resolution. Using dual-color confocal microscopy and correlative electron microscopy in human endothelial cells, we visually distinguished two sequential steps of secretagogue-stimulated exocytosis: fusion of individual secretory granules (Weibel–Palade bodies [WPBs]) and subsequent expulsion of von Willebrand factor (VWF) content. Based on our observations, we conclude that for fusion, WPBs are released from cellular sites of actin anchorage. However, once fused, a dynamic ring of actin filaments and myosin II forms around the granule, and actomyosin II contractility squeezes VWF content out into the extracellular environment. This study therefore demonstrates how discrete actin cytoskeleton functions within a single cellular system explain actin filament–based prevention and promotion of specific exocytic steps during regulated secretion.

Role for the actomyosin complex in regulated exocytosis revealed by intravital microscopy

The regulation and the dynamics of membrane trafficking events have been studied primarily in in vitro models that often do not fully reflect the functional complexity found in a living multicellular organism. Here we used intravital microscopy in the salivary glands of live rodents to investigate regulated exocytosis, a fundamental process in all of the secretory organs. We found that β-adrenergic stimulation elicits exocytosis of large secretory granules, which gradually collapse with the apical plasma membrane without any evidence of compound exocytosis, as was previously described. Furthermore, we show that the driving force required to complete the collapse of the granules is provided by the recruitment of F-actin and nonmuscle myosin II on the granule membranes that is triggered upon fusion with the plasma membrane. Our results provide information on the machinery controlling regulated secretion and show that intravital microscopy provides unique opportunities to address fundamental questions in cell biology under physiological conditions.

Vesicle-associated membrane protein 8 (VAMP8) is a SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) selectively required for sequential granule-to-granule fusion

Compound exocytosis is found in many cell types and is the major form of regulated secretion in acinar and mast cells. Its key characteristic is the homotypic fusion of secretory granules. These then secrete their combined output through a single fusion pore to the outside. The control of compound exocytosis remains poorly understood. Although soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) such as syntaxin 2, SNAP23 (synaptosome-associated protein of 23 kDa), and SNAP25 have been suggested to play a role, none has been proven. Vesicle-associated membrane protein 8 (VAMP8) is a SNARE first associated with endocytic processes but more recently has been suggested as an R-SNARE in regulated exocytosis. Secretion in acinar cells is reduced when VAMP8 function is inhibited and is less in VAMP8 knock-out mice. Based on electron microscopy experiments, it was suggested that VAMP8 may be involved in compound exocytosis. Here we have tested the hypothesis that VAMP8 controls homotypic granule-to-granule fusion during sequential compound exocytosis. We use a new assay to distinguish primary fusion events (fusion with the cell membrane) from secondary fusion events (granule-granule fusion). Our data show the pancreatic acinar cells from VAMP8 knock-out animals have a specific reduction in secondary granule fusion but that primary granule fusion is unaffected. Furthermore, immunoprecipitation experiments show syntaxin 2 association with VAMP2, whereas syntaxin 3 associates with VAMP8. Taken together our data indicate that granule-to-granule fusion is regulated by VAMP8 containing SNARE complexes distinct from those that regulate primary granule fusion.

Myosin light chain kinase and Src control membrane dynamics in volume recovery from cell swelling

The expansion of the plasma membrane, which occurs during osmotic swelling of epithelia, must be retrieved for volume recovery, but the mechanisms are unknown. Here we have identified myosin light chain kinase (MLCK) as a regulator of membrane internalization in response to osmotic swelling in a model liver cell line. On hypotonic exposure, we found that there was time-dependent phosphorylation of the MLCK substrate myosin II regulatory light chain. At the sides of the cell, MLCK and myosin II localized to swelling-induced membrane blebs with actin just before retraction, and MLCK inhibition led to persistent blebbing and attenuated cell volume recovery. At the base of the cell, MLCK also localized to dynamic actin-coated rings and patches upon swelling, which were associated with uptake of the membrane marker FM4-64X, consistent with sites of membrane internalization. Hypotonic exposure evoked increased biochemical association of the cell volume regulator Src with MLCK and with the endocytosis regulators cortactin and dynamin, which colocalized within these structures. Inhibition of either Src or MLCK led to altered patch and ring lifetimes, consistent with the concept that Src and MLCK form a swelling-induced protein complex that regulates volume recovery through membrane turnover and compensatory endocytosis under osmotic stress.

Regulation of fusion pore closure and compound exocytosis in neuroendocrine PC12 cells by SCAMP1

During exocytosis, neuroendocrine cells can achieve partial release of stored secretory products from dense core vesicles (DCVs) by coupling endocytosis directly at fusion sites and without full discharge. The physiological role of partial secretion is of substantial interest. Much is known about SNARE-mediated initiation of exocytosis and dynamin-mediated completion of endocytosis, but little is known about coupling events. We have used real-time microscopy to examine the role of secretory carrier membrane protein SCAMP1 in exo-endocytic coupling in PC12 cells. While reduced SCAMP1 expression is known to impede dilation of newly opened fusion pores during onset of DCV exocytosis, we now show that SCAMP1 deficiency also inhibits closure of fusion pores after they have opened. Inhibition causes accumulation of fusion figures at the plasma membrane. Closure is recovered by restoring expression and accelerated slightly by overexpression. Interestingly, inhibited pore closure resulting from loss of SCAMP1 appears to increase secondary fusion of DCVs to already-fused DCVs (compound exocytosis). Unexpectedly, reinternalization of expanded DCV membranes following compound exocytosis appears to proceed normally in SCAMP1-deficient cells. SCAMP1's apparent dual role in facilitating dilation and closure of fusion pores implicates its function in exo-endocytic coupling and in the regulation of partial secretion. Secondarily, SCAMP1 may serve to limit the extent of compound exocytosis.

Real-time cellular impedance measurements detect Ca(2+) channel-dependent oscillations of morphology in human H295R adrenoma cells

Endocrine cells, such as H295R have been widely used to study secretion of steroid and other hormones. Exocytosis-dependent hormone release is accompanied by an increase in plasma membrane surface area and a decrease in vesicle content. Recovery of vesicles and decrease in plasma membrane area is achieved by endocytotic processes. These changes in the extent of the surface area lead to morphological changes which can be determined by label-free real-time impedance measurements. Exo- and endocytosis have been described to be triggered by activation of L-type Ca(2+) channels. The present study demonstrates that activation of L-type calcium channels induces prolonged oscillating changes in cellular impedance. The data support the hypothesis that a tight regulation of the intracellular Ca(2+) concentration is a prerequisite for the observed cellular impedance oscillations. Furthermore evidence is presented for a mechanism in which the oscillations depend on a Ca(2+)-triggered calmodulin-dependent cascade involving myosin light chain kinase, nonmuscle myosin II and ultimately actin polymerization, a known determinant for cell shape changes and exocytosis in secretory cells. The described assay provides a method to determine continuously prolonged changes in cellular morphology such as exo/endocytosis cycles. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.

Myosin VI and its binding partner optineurin are involved in secretory vesicle fusion at the plasma membrane

During constitutive secretion, proteins synthesized at the endoplasmic reticulum (ER) are transported to the Golgi complex for processing and then to the plasma membrane for incorporation or extracellular release. This study uses a unique live-cell constitutive secretion assay to establish roles for the molecular motor myosin VI and its binding partner optineurin in discrete stages of secretion. Small interfering RNA-based knockdown of myosin VI causes an ER-to-Golgi transport delay, suggesting an unexpected function for myosin VI in the early secretory pathway. Depletion of myosin VI or optineurin does not affect the number of vesicles leaving the trans-Golgi network (TGN), indicating that these proteins do not function in TGN vesicle formation. However, myosin VI and optineurin colocalize with secretory vesicles at the plasma membrane. Furthermore, live-cell total internal reflection fluorescence microscopy demonstrates that myosin VI or optineurin depletion reduces the total number of vesicle fusion events at the plasma membrane and increases both the proportion of incomplete fusion events and the number of docked vesicles in this region. These results suggest a novel role for myosin VI and optineurin in regulation of fusion pores formed between secretory vesicles and the plasma membrane during the final stages of secretion.

Regulation of acinar cell function in the pancreas

PURPOSE OF REVIEW

This review identifies and puts into context the recent articles which have advanced understanding of the functions of pancreatic acinar cells and the mechanisms by which these functions are regulated.

RECENT FINDINGS

Receptors present on acinar cells, particularly those for cholecystokinin and secretin, have been better characterized as to the molecular nature of the ligand-receptor interaction. Other reports have described the potential regulation of acinar cells by GLP-1 and cannabinoids. Intracellular Ca2+ signaling remains at the center of stimulus secretion coupling and its regulation has been further defined. Recent studies have identified specific channels mediating Ca2+ release from intracellular stores and influx across the plasma membrane. Work downstream of intracellular mediators has focused on molecular mechanisms of exocytosis particularly involving small G proteins, SNARE proteins and chaperone molecules. In addition to secretion, recent studies have further defined the regulation of pancreatic growth both in adaptive regulation to diet and hormones in the regeneration that occurs after pancreatic damage. Lineage tracing has been used to show the contribution of different cell types. The importance of specific amino acids as signaling molecules to activate the mTOR pathway is being elucidated.

SUMMARY

Understanding the mechanisms that regulate pancreatic acinar cell function is contributing to knowledge of normal pancreatic function and alterations in disease.

An emerging role for IQGAP1 in regulating protein traffic

IQGAP1, an effector of CDC42p GTPase, is a widely conserved, multifunctional protein that bundles F-actin through its N-terminus and binds microtubules through its C-terminus to modulate the cell architecture. It has emerged as a potential oncogene associated with diverse human cancers. Therefore, IQGAP1 has been heavily investigated; regardless, its precise cellular function remains unclear. Work from yeast suggests that IQGAP1 plays an important role in directed cell growth, which is a conserved feature crucial to morphogenesis, division axis, and body plan determination. New evidence suggests a conserved role for IQGAP1 in protein synthesis and membrane traffic, which may help to explain the diversity of its cellular functions. Membrane traffic mediates infections by intracellular pathogens and a range of degenerative human diseases arise from dysfunctions in intracellular traffic; thus, elucidating the mechanisms of cellular traffic will be important in order to understand the basis of a wide range of inherited and acquired human diseases. Recent evidence suggests that IQGAP1 plays its role in cell growth through regulating the conserved mTOR pathway. The mTOR signaling cascade has been implicated in membrane traffic and is activated in nearly all human cancers, but clinical response to the mTOR-specific inhibitor rapamycin has been disappointing. Thus, understanding the regulators of this pathway will be crucial in order to identify predictors of rapamycin sensitivity. In this review, I discuss emerging evidence that supports a potential role of IQGAP1 in regulating membrane traffic via regulating the mTOR pathway.

Secretory control: evidence for agonist regulation of post-fusion vesicle behaviour

1. Here, we review recent work on vesicular secretion, with a focus on the control of post-fusion events as a means of regulating secretory output. 2. In the classical model of secretion, each fused vesicle releases the entirety of its content in an all-or-none manner. In this way, the secretory output of a cell is controlled by regulating the numbers of fused vesicles. The realisation that post-fusion events can control secretory output leads to a distinct model of partial release of vesicle content. 3. Recent work shows that post-fusion events are under cellular control. Further, new data from our laboratory demonstrates agonist-dependent regulation of fusion pore behaviour. 4. We conclude that post-fusion events are not epiphenomena, but are likely an important mechanism of secretory control.

Cholesterol, regulated exocytosis and the physiological fusion machine

Exocytosis is a highly conserved and essential process. Although numerous proteins are involved throughout the exocytotic process, the defining membrane fusion step appears to occur through a lipid-dominated mechanism. Here we review and integrate the current literature on protein and lipid roles in exocytosis, with emphasis on the multiple roles of cholesterol in exocytosis and membrane fusion, in an effort to promote a more molecular systems-level view of the as yet poorly understood process of Ca2+-triggered membrane mergers.

Cortical F-actin, the exocytic mode, and neuropeptide release in mouse chromaffin cells is regulated by myristoylated alanine-rich C-kinase substrate and myosin II

Adrenal medullary chromaffin cells are innervated by the sympathetic splanchnic nerve and translate graded sympathetic firing into a differential hormonal exocytosis. Basal sympathetic firing elicits a transient kiss-and-run mode of exocytosis and modest catecholamine release, whereas elevated firing under the sympathetic stress response results in full granule collapse to release catecholamine and peptide transmitters into the circulation. Previous studies have shown that rearrangement of the cell actin cortex regulates the mode of exocytosis. An intact cortex favors kiss-and-run exocytosis, whereas disrupting the cortex favors the full granule collapse mode. Here, we investigate the specific roles of two actin-associated proteins, myosin II and myristoylated alanine-rich C-kinase substrate (MARCKS) in this process. Our data demonstrate that MARCKS phosphorylation under elevated cell firing is required for cortical actin disruption but is not sufficient to elicit peptide transmitter exocytosis. Our data also demonstrate that myosin II is phospho-activated under high stimulation conditions. Inhibiting myosin II activity prevented disruption of the actin cortex, full granule collapse, and peptide transmitter release. These results suggest that phosphorylation of both MARCKS and myosin II lead to disruption of the actin cortex. However, myosin II, but not MARCKS, is required for the activity-dependent exocytosis of the peptide transmitters.