Organization and Dynamics of Endomembrane Pathways and Organelles

Eunice Kennedy Shriver National Institute Of Child Health & Human Development

Investigators

Linked publications, trials & patents

Abstract

One major area of investigation of our lab has been defining inter-organelle association and communication. In one project, we developed a model of endosomal membrane sorting using macropinocytic cells. Macropinocytosis is an actin driven form of endocytosis that generates enlarged endosomes that can be unambiguously tracked using live-cell microscopy. Upon formation and internalization into the cell interior, membrane and proteins on the limiting membrane of the endosome can be transported to other organelles such as Golgi or lysosomes, or recycled back to the cell surface. Due to the large size of nascent endosomes, macropinocytic cells provide a unique model system to study sorting machinery and trafficking pathways with increased resolution. We have been able to resolve protein-specific, sorting events on individual endosomes, as well as observe recruitment of retromer machinery to isolated endosomal domains. We have employed a variety of microscopy techniques in these studies, including correlative light/electron microscopy (CLEM), live cell Airyscan super-resolution microscopy, photoactivation and photobleaching. We are further developing fluorescent probes to track protein delivery from the endosome to specific organelle compartments with the goal of better visualizing inter-organelle trafficking. In a second project, we have been studying the ability of the Hippo signaling effector, YAP/TAZ, to sense and respond to cellular signals such as cytosolic volume changes. Mouse embryonic stem cells can grow in discrete pluiropotency states (nave and primed) which display differences in cell volume, shape, and confinement in cell culture. Nave ESCs grow clumped together in spheroid colonies, while primed ESCs flatten out into a more traditional monolayer. In comparing the two distinct pluripotency states of ESCs we demonstrated not only differences in baseline metabolism, with primed ESCs mainly depending on glycolysis and nave ESCs utilizing both glycolysis and oxidative phosphorylation, but also a differential YAP/TAZ nuclear localization in these two cell states. To understand if the YAP/TAZ localization could be a response to volumetric differences in the cell states, we tested YAP/TAZ localization in response to cytosolic volume reduction induced by hyperosmotic shock in multiple cell lines. Not only did YAP/TAZ increase its nuclear localization during hyperosmotic shock, but we observed its localization to phase-separated liquid droplets dependent upon the YAP1 transcription activation domain. We are further characterizing the effects of downstream signaling due to YAP/TAZ accumulation in liquid droplets. A third project has focused on characterizing the relationship between calcium signaling and vesicular movements in cultured neuronal cells. Local calcium spikes have been reported to inhibit the movement of mitochondria in cultured neurons, which is important for neuronal activity-dependent recruitment of mitochondria to distinct locations. Using live-cell confocal imaging in astrocytes and neurons overexpressing the calcium sensor GCaMP6f, we further demonstrated the ability of local calcium spikes to inhibit movement of endososomes and lysosomes as well, indicating a broader role for calcium signals in organizing localization of multiple organelles in neurons. We plan to further explore the broader role of calcium-mediated organelle recruitment during brain organoid differentiation and development from iPSC cell lines. In a fourth project, we have been investigating highly regulated biological processes by which intracellular organic crystals can be formed in specialized cells. It has been known for many years that a variety of animals, such as the chameleon and different kinds of marine organisms, can form purine crystals within specialized skin cells to create crystalline-based structural coloring. While studies have clearly shown dynamic color changes due to altering the spacing between these intracellular crystals, the cellular machinery underlying this process is not well understood. Using iridophores (specialized guanine crystal forming cells) in the zebrafish skin as a model system, we are attempting to characterize the early stages of bio-organic crystallization and identify the cellular organelle in which crystallization takes place. Our approaches utilize a combination of Cryo-SEM, super-resolution fluorescence microscopy, Raman microscopy, and mass spectroscopy to identify the cellular bio-molecules involved in this process, and will use genetic manipulations within Zebrafish to verify their role. A better understating of these highly regulated processes is likely to provide a better understanding of similar bio-organic pathological crystallization in which similar crystals can grow under no biological control, including that in human diseases like gout and nephrolithiasis.