09:05-09:30
Macropinocytosis and cellular growth control
Professor Joel Swanson, University of Michigan, USA
Abstract
Macropinocytosis has long been implicated as a mechanism for growing cells to assimilate extracellular nutrients. Studies of murine macrophages and embryonic fibroblasts showed that activation of the growth-associated metabolic regulator mechanistic target of rapamycin complex-1 (mTORC1) in response to growth factors and extracellular amino acids requires internalization of amino acids by macropinocytosis. This led the group to propose that macropinocytosis is necessary for mTORC1-dependent growth of metazoan cells, as both a route for nutrient delivery into lysosomes and a platform for growth factor-dependent signalling to mTORC1 via PI 3-kinase (PI3K) and Akt. A functional actin cytoskeleton is required for macropinocytic cup formation and activation of mTORC1 by growth factors and amino acids. Although activation of Akt by some growth factors occurs independent of the actin cytoskeleton, maximal activation of Akt by stimuli that elicit weak Akt responses requires that cells be capable of forming macropinocytic cups or circular dorsal ruffles. This indicates that growth factors and chemokines which trigger low maximal levels of Akt activity require actin-based macropinocytic cup formation for localized amplification of PI3K-dependent responses. In this way, ruffles and cups could organize signalling by extracellular ligands into stochastic, structure-dependent cascades of chemical reactions that stimulate cell growth or orient cell migration.
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Professor Joel Swanson, University of Michigan, USA
Professor Joel Swanson, University of Michigan, USA
Joel Swanson, Professor of Microbiology and Immunology, trained at Rutgers University (BA, Biology), Ohio State University (M.S., Botany), and Princeton University (PhD, Biology), followed by post-doctoral studies at Rockefeller University and Columbia College of Physicians and Surgeons. He served on the faculty at Harvard Medical School and the University of Michigan Medical School. His laboratory has a longstanding interest of heterophagy, which is the cellular ingestion of extracellular particles and solutes by macropinocytosis, phagocytosis and endocytosis, and the subsequent degradation of ingested materials in lysosomes. His approach emphasizes molecular and microscopic studies of the spatial organization of cytoplasm in cells. Recent work has demonstrated roles for macropinocytosis in the regulation of cell growth in macrophages and other cell types.
09:45-10:15
Constitutive vs inducible macropinocytosis in macrophages
Professor Sergio Grinstein, The Hospital for Sick Children, Canada
Abstract
Like other cells, macrophages respond to growth promoters by ruffling their membrane, which in turn promotes macropinocytosis. However, the surveillance role of macrophages requires ongoing sampling of their environment, which is performed by constitutive macropinocytosis. The latter occurs continuously, even in the presence of growth promoters, generating smaller macropinosomes. Constitutive macropinocytosis requires extracellular calcium and is mediated by calcium-sensing receptors. Both inducible and constitutive macropinocytosis depend on actin and on phosphatidylinositol 3,4,5-trisphosphate (PIP3), but only the inducible form is sensitive to amiloride analogues. Constitutive macropinocytosis is active in anti-inflammatory macrophages, but negligible in pro-inflammatory macrophages, such as those polarized by treatment with GM-CSF, LPS and/or IFNγ. Inflammatory macrophages have reduced levels of PIP3, which accounts at least in part for their inability to perform constitutive macropinocytosis.
Oxidized LDL (oxLDL), the source of the vascular plaque responsible for heart attacks and strokes, is taken up by macrophages via scavenger receptors. Unlike conventional receptor-mediated endocytosis, oxLDL uptake requires actin polymerization. The evidence indicates that constitutive macropinocytosis is largely responsible for oxLDL internalization by non-inflammatory macrophages, in a scavenger receptor-dependent manner. The group has termed this process 'receptor-assisted macropinocytosis'.
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Professor Sergio Grinstein, The Hospital for Sick Children, Canada
Professor Sergio Grinstein, The Hospital for Sick Children, Canada
Dr Sergio Grinstein completed his PhD in 1976 at the Centro de Investigacion y Estudios Avanzados, in Mexico City. He then spent two years as a post-doctoral fellow at the Hospital For Sick Children in Toronto, followed by a year in the Department of Biochemistry at the Federal Institute of Technology in Zurich. He is currently working at the Hospital For Sick Children in Toronto and has been Professor of Biochemistry at the University of Toronto since 1988.
Dr Grinstein is interested in the cell physiology and biophysics of innate immunity, particularly phagocytosis and host-pathogen interactions.
11:00-11:30
Macropinocytosis and membrane recycling
Professor Julie Donaldson, National Institutes of Health, USA
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Professor Julie Donaldson, National Institutes of Health, USA
Professor Julie Donaldson, National Institutes of Health, USA
Julie Donaldson received her PhD from the University of Maryland in 1988 and did her postdoctoral research training at NIH before starting her own lab in 1995 at the National Heart, Lung and Blood Institute at the NIH. She is currently a Senior Investigator in the Cell Biology and Physiology Center. Her research program focuses on understanding the mechanism and physiological function of clathrin-independent endocytosis (CIE) and the subsequent routing and fate of endocytosed membrane proteins. CIE is the entry mechanism for many cell surface proteins including cell adhesion molecules, ion channels, nutrient transporters and microbial pathogens. Macropinocytosis is another form of CIE that provides a unique platform for following membrane dynamics.
11:45-12:15
Learning how to form and process macropinosomes with Dictyostelium amoeba
Dr Jason King, University of Sheffield, UK
Abstract
Macropinocytosis requires the co-ordinated regulation of both the cytoskeleton to form the cup-shaped protrusions, as well as the vesicular trafficking machinery to process them after internalisation. The group is trying to understand both these processes using Dictyostelium amoeba, which use macropinocytosis for feeding.
Macropinocytic cups are able to self-assemble stochastically, without any external physical scaffolds or localised signals. This requires cells to generate a ring of protrusion, driven by localised actin polymerisation, that encircles a static membrane domain that defines the cup interior. Dr King will discuss a new mechanism by which the Ras and Rac small GTPases differentially regulated across the protrusive rim and cup interior, in order to spatially modulate the cytoskeleton and generate protrusions that efficiently engulf fluid.
After engulfment cells must rapidly recycle any surface components before they are degraded, whilst orchestrating a complex series of trafficking steps that ensure the efficient processing of the internalised material. Dr King will therefore also discuss how the group is using the amoeba model to dissect these processes to provide a mechanistic understanding of macropinosome maturation.
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Dr Jason King, University of Sheffield, UK
Dr Jason King, University of Sheffield, UK
Jason King is currently a Royal Society University Research Fellow at the University of Sheffield, having moved there in 2013 to start his independent group. Their main interests are understanding how cells both form, and regulate the maturation of macropinosomes and phagosomes. Prior to this Dr King worked as a postdoc in the laboratory of Robert Insall at the Beatson Institute for Cancer Research in Glasgow, UK, studying the cytoskeleton and cell migration, before developing interests in autophagy and trafficking. This followed on from his PhD work in the lab of Adrian Harwood at Cardiff University, studying how lithium treatment (still widely used to tread bipolar disorder) affects cellular signalling and in particular phosphinositide signalling. The group’s current work on macropinocytosis brings all these interests together in order to determine how both the cytoskeleton and vesicular trafficking are coordinated to facilitate fluid uptake and processing.