Architecture and control of exocytosis and endocytosis in excitable cells

可兴奋细胞胞吐作用和内吞作用的结构和控制

• 批准号: 9157411
• 负责人: Justin Taraska
• 金额: $ 179.11万
• 依托单位: NATIONAL HEART, LUNG, AND BLOOD INSTITUTE
• 依托单位国家: 美国
• 资助国家: 美国
• 起止时间: 至
• 项目状态: 未结题
• 来源: https://reporter.nih.gov/project-details/9157411
• 关键词: Accounting; Architecture; Behavior; Beta Cell; Biochemistry; Calcium; Caliber; Cell membrane; Cell surface; Cells; Chemicals; Clathrin; Color; Complex; Cytoplasmic Granules; Dense Core Vesicle; Diffuse; Diffusion; Docking; Dynamin; Electron Microscopy; Electrophysiology (science); Endocrine system; Endocytic Vesicle; Endocytosis; Environment; Event; Exocytosis; Future; Genetic; Goals; Hela Cells; Heterogeneity; Image; Image Analysis; Individual; Insulin; Life; Location; Mammalian Cell; Maps; Measures; Membrane; Membrane Fusion; Methods; Microscopy; Modeling; Molecular; Nervous System Physiology; Nervous system structure; Neuroendocrine Cell; Neurosciences; Neurosecretory Systems; Neurotransmitters; Organelles; PC12 Cells; Pathology; Peptides; Physiology; Platinum; Play; Positioning Attribute; Process; Protein Dynamics; Proteins; Recycling; Regulation; Resolution; Role; Running; Secretory Vesicles; Signal Pathway; Site; Structural Models; Structure; System; Time; Total Internal Reflection Fluorescent; Transcription Factor AP-2 Alpha; Transmission Electron Microscopy; Vesicle; Work; acetylcholine transporter; amphiphysin; cell growth regulation; cell type; cellular imaging; density; epsin; light microscopy; nano; nanoscale; photoactivation; protein distribution; response; small molecule; three dimensional structure; trafficking; ultra high resolution

Aim 1 Classical neuroscience has proposed two competing models for membrane fusion. In the first, vesicles completely merge with the plasma membrane, dispersing the entirety of their contents. This full fusion model of exocytosis predicts that vesicle contents will spill into the membrane and diffuse away from the site of fusion. In the second, vesicles transiently connect with the plasma membrane and release only a subset of their components. This kiss-and-run model predicts that the vesicle contents will remain within a vesicle cavity and then will be recaptured into the cell mostly intact. To determine which of these two models occurs in neuroendocrine cells, we have imaged single fluorescently-tagged vesicles in living PC12 cells with total internal reflection fluorescent microscopy (TIRF). This method allows us to track and measure the behavior of individual secretory vesicles in real time in living cells. By watching the diffusive behavior of vesicle components before, during, and after fusion, we will determine if (or which of) the two classical models of fusion fit triggered exocytosis of vesicles in PC12 cells. Through these studies we hope to measure the behavior of individual vesicles to determine the heterogeneity of vesicle fusion behaviors, their topology, relationships, and regulation by cellular signaling pathway and pathologies. Using two-color total internal reflection microscopy we have shown that the dominant mode of fusion for SLMV in PC12 cells is the full fusion model. As such, vesicle transporters, including the vesicular acetylcholine transporter, diffuse into the plasma membrane within seconds. A surprising finding of this work, however, is that the material that exits vesicles is rapidly captured on preformed clusters on the cells surface. These clusters composed of the endocytic protein clathrin and AP2 inhibit the free diffusion of the transporter across the plasma membrane. To further investigate the density and topology of the structures responsible for capturing VAChT on the cell surface, we used three forms of ultra-high resolution imaging: 1) photo-activation localization microscopy, 2) ground state depletion (GSD) super-resolution imaging, and 2) electron microscopy. The combinations of these methods have shown that the density of endocytic clathrin-coated structures in PC12 cells is very high. The density approaches 2 structures per square micron. The structures are randomly distributed across the surface of the cell, and produce a network of endocytic nano-traps capable of rapidly capturing material that escapes from exocytic vesicles. We propose that this system can account for the rapidly recycling of vesicle material in highly excitable cells necessary for the continued function of the nervous and neuroendocrine system. We have additionally begun to dynamically map how dozens of proteins associate with another type of exocytic vesicle called dense core granules in cultured insulin-secreting beta cells. These vesicles are larger (150 nm in diameter) and release protein cargo and chemicals in response to increased intracellular calcium. We have imaged the dynamics of multiple proteins at the moment of fusion and were able to detect the rapid recruitment of several important cytosolic endocytic proteins to sites of exocytosis. These proteins include dynamin, amphiphysin, syndapin, and endophilin. These proteins could be regulating the expansion of the fusion pore to limit the amount of cargo released during each exocytic fusion event. Future work is aimed at determining the exact role these proteins play in the regulation of protein release from dense core vesicles. Aim 2 Dozens of proteins control the docking, fusion, and then recapture of vesicles in excitable cells. The identity and functional roles of many of these proteins have been discovered through a combination of genetics, biochemistry, and electrophysiology. However, the architecture, structure, and structural dynamics of these proteins and their complexes have yet to be determined. In this aim we have mapped the location, architecture, and dynamics of proteins proposed to act during exocytosis and endocytosis. To accomplish this, we have used a combination of live cell imaging, super-resolution, and electron microscopy. Through this multi-modal approach, the location, and dynamics of individual proteins are being compared to the underlying cellular architecture that organizes exocytic and endocytic sites at the nanometer scale. This allows us to map the architecture of the plasma membrane along with protein components responsible for vesicle trafficking. These studies are determining the complex three dimensional structure of the exocytic and endocytic protein machinery in intact cells. We have specifically been using two-color TIRF microscopy and a form of high-throughput image analysis to detect and characterize over 80 proteins that are associated with both endocytic and exocytic vesicles. These studies have rapidly mapped the occupancy and distribution of these proteins at the plasma membrane. Our studies are developing a general topographic map of the endocytic and exocytic machinery in living cells at the plasma membrane. These studies provide a network systems level analysis of the machinery responsible for vesicle fusion and recapture in cells of the nervous system. Along with these studies we have developed a super-resolution correlative light and electron microscopy method. This allows us to image the nanometer-scale location of proteins in the context of the cellular environment. Specifically, we have succeeded in developing a robust pipeline for imaging the plasma membrane of mamallian cells with both iPALM (interferometric photoactivation localization microcopy) and 3D transmission electron microscopy (TEM) of platinum replicas. In these studies we have examined the position of the endocytic protein Epsin at single clathrin-coated structures. We have continued to map dozens of other critical components in several mammalian cell types including SKMEL, Hela, PC12, and Ins-1 cells. These studies are allowing us to build functional structural models for how proteins are organized at single organelles and regulate endocytosis, a central process for all living cells.

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