Many cellular organelles, including endosomes, show compartmentalization into distinct functional domains, which, however, cannot be resolved by diffraction‐limited light microscopy. Single molecule ...localization microscopy (SMLM) offers nanoscale resolution but data interpretation is often inconclusive when the ultrastructural context is missing. Correlative light electron microscopy (CLEM) combining SMLM with electron microscopy (EM) enables correlation of functional subdomains of organelles in relation to their underlying ultrastructure at nanometer resolution. However, the specific demands for EM sample preparation and the requirements for fluorescent single‐molecule photo‐switching are opposed. Here, we developed a novel superCLEM workflow that combines triple‐color SMLM (dSTORM & PALM) and electron tomography using semi‐thin Tokuyasu thawed cryosections. We applied the superCLEM approach to directly visualize nanoscale compartmentalization of endosomes in HeLa cells. Internalized, fluorescently labeled Transferrin and EGF were resolved into morphologically distinct domains within the same endosome. We found that the small GTPase Rab5 is organized in nanodomains on the globular part of early endosomes. The simultaneous visualization of several proteins in functionally distinct endosomal sub‐compartments demonstrates the potential of superCLEM to link the ultrastructure of organelles with their molecular organization at nanoscale resolution.
Suborganelle compartmentalization cannot be resolved by diffraction‐limited light microscopy and interpreted without knowledge of the underlying ultrastructure. This work presents a novel superCLEM workflow that combines multicolor single‐molecule localization‐microscopy with electron tomography using semi‐thin Tokuyasu thawed cryosections to map fluorescent molecules on the ultrastructure of early endosomes. superCLEM reveals that the small GTPase Rab5 is organized in nanodomains largely devoid of cargo molecules Transferrin and EGF. This method opens new possibilities to perform structure‐function analysis of organelles at the nanoscale.
The ADP‐ribosylation factor (Arf) small G proteins act as molecular switches to coordinate multiple downstream pathways that regulate membrane dynamics. Their activation is spatially and temporally ...controlled by the guanine nucleotide exchange factors (GEFs). Members of the evolutionarily conserved GBF/Gea family of Arf GEFs are well known for their roles in formation of coat protein complex I (COPI) vesicles, essential for maintaining the structure and function of the Golgi apparatus. However, studies over the past 10 years have found new functions for these GEFs, along with their substrate Arf1, in lipid droplet metabolism, clathrin‐independent endocytosis, signalling at the plasma membrane, mitochondrial dynamics and transport along microtubules. Here, we describe these different functions, focussing in particular on the emerging theme of GFB1 and Arf1 regulation of organelle movement on microtubules.
Review: Cellular organelles are highly dynamic, communicating through vesicle flux and movement along microtubule tracks. These dynamics are regulated by Arf small G proteins and their activators. GBF1 activates Arf1 at the Golgi to form coat protein complex I (COPI) vesicles, and has more recently been shown to function in lipid droplet metabolism, plasma membrane signalling, mitochondrial dynamics and transport along microtubules. Whether GBF1 and Arf1 act to integrate these different functions is not yet known, an important direction for future research.
Modern life science research is a collaborative effort. Few research groups can single‐handedly support the necessary equipment, expertise and personnel needed for the ever‐expanding portfolio of ...technologies that are required across multiple disciplines in today's life science endeavours. Thus, research institutes are increasingly setting up scientific core facilities to provide access and specialised support for cutting‐edge technologies. Maintaining the momentum needed to carry out leading research while ensuring high‐quality daily operations is an ongoing challenge, regardless of the resources allocated to establish such facilities. Here, we outline and discuss the range of activities required to keep things running once a scientific imaging core facility has been established. These include managing a wide range of equipment and users, handling repairs and service contracts, planning for equipment upgrades, renewals, or decommissioning, and continuously upskilling while balancing innovation and consolidation.
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Modern life science research is a team effort. Consequently, no single research group can support all the equipment, expertise and personnel required for today's life science research. Research institutes are setting up scientific core facilities to address this to provide access and specialised support for cutting‐edge technologies. Multiple resources exist to set up and evaluate a core facility. However, remaining cutting‐edge as a core facility while ensuring high‐quality daily operations is an ongoing challenge. Here, we discuss a range of activities required to keep things running once a scientific imaging core facility is set up. The main challenge is finding the balance between innovation and consolidation. In addition, we discuss the challenge of managing a wide range of equipment and users, handling repairs and service contracts, planning for equipment upgrades, renewals or decommissioning and continuously training personnel.
Cells are filled with macromolecules and polymer networks that set scale-dependent viscous and elastic properties to the cytoplasm. Although the role of these parameters in molecular diffusion, ...reaction kinetics, and cellular biochemistry is being increasingly recognized, their contributions to the motion and positioning of larger organelles, such as mitotic spindles for cell division, remain unknown. Here, using magnetic tweezers to displace and rotate mitotic spindles in living embryos, we uncovered that the cytoplasm can impart viscoelastic reactive forces that move spindles, or passive objects with similar size, back to their original positions. These forces are independent of cytoskeletal force generators yet reach hundreds of piconewtons and scale with cytoplasm crowding. Spindle motion shears and fluidizes the cytoplasm, dissipating elastic energy and limiting spindle recoils with functional implications for asymmetric and oriented divisions. These findings suggest that bulk cytoplasm material properties may constitute important control elements for the regulation of division positioning and cellular organization.
An evolutionarily conserved feature of cellular organelles is the distinct phospholipid composition of their bounding membranes, which is essential to their identity and function. Within eukaryotic ...cells, two major lipid territories can be discerned, one centered on the endoplasmic reticulum and characterized by membranes with lipid packing defects, the other comprising plasma-membrane-derived organelles and characterized by membrane charge. We discuss how this cellular lipid organization is maintained, how lipid flux is regulated, and how perturbations in cellular lipid homeostasis can lead to disease.
In this Review, Jackson and colleagues examine lipid organization within eukaryotic cells, highlighting the two major lipid territories that define cellular membranes. They discuss how cellular lipid organization is maintained, how lipid flux is regulated, and how perturbations in cellular lipid homeostasis can lead to disease.
One of the key questions in biology is how the metabolism of a cell responds to changes in the environment. In budding yeast, starvation causes a drop in intracellular pH, but the functional role of ...this pH change is not well understood. Here, we show that the enzyme glutamine synthetase (Gln1) forms filaments at low pH and that filament formation leads to enzymatic inactivation. Filament formation by Gln1 is a highly cooperative process, strongly dependent on macromolecular crowding, and involves back-to-back stacking of cylindrical homo-decamers into filaments that associate laterally to form higher order fibrils. Other metabolic enzymes also assemble into filaments at low pH. Hence, we propose that filament formation is a general mechanism to inactivate and store key metabolic enzymes during a state of advanced cellular starvation. These findings have broad implications for understanding the interplay between nutritional stress, the metabolism and the physical organization of a cell.
Significance Proteins carrying aggregation-prone prion-like domains cause many neurodegenerative diseases. The presence of prions in yeast has provided important insights into these disease processes ...and mechanisms of cellular proteostasis. However, it is not known whether these findings extend to other organisms. In this paper, we show that Dictyostelium discoideum has the highest content of prion-like proteins of all organisms investigated to date. Remarkably, overexpressed prion-like proteins remain soluble and are innocuous to D. discoideum , in contrast to yeast and other organisms, where they form cytotoxic cytosolic aggregates. However, when exposed to conditions that compromise proteostasis, these proteins aggregate and become cytotoxic. This indicates that D. discoideum has undergone specific adaptations in its proteostasis machinery to control its highly aggregation-prone prion-like proteome.
Many protein-misfolding diseases are caused by proteins carrying prion-like domains. These proteins show sequence similarity to yeast prion proteins, which can interconvert between an intrinsically disordered and an aggregated prion state. The natural presence of prions in yeast has provided important insight into disease mechanisms and cellular proteostasis. However, little is known about prions in other organisms, and it is not yet clear whether the findings in yeast can be generalized. Using bioinformatics tools, we show that Dictyostelium discoideum has the highest content of prion-like proteins of all organisms investigated to date, suggesting that its proteome has a high overall aggregation propensity. To study mechanisms regulating these proteins, we analyze the behavior of several well-characterized prion-like proteins, such as an expanded version of human huntingtin exon 1 (Q103) and the prion domain of the yeast prion protein Sup35 (NM), in D. discoideum. We find that these proteins remain soluble and are innocuous to D. discoideum , in contrast to other organisms, where they form cytotoxic cytosolic aggregates. However, when exposed to conditions that compromise molecular chaperones, these proteins aggregate and become cytotoxic. We show that the disaggregase Hsp101, a molecular chaperone of the Hsp100 family, dissolves heat-induced aggregates and promotes thermotolerance. Furthermore, prion-like proteins accumulate in the nucleus, where they are targeted by the ubiquitin–proteasome system. Our data suggest that D. discoideum has undergone specific adaptations that increase the proteostatic capacity of this organism and allow for an efficient regulation of its prion-like proteome.
The influenza virus (IFV) acquires its envelope by budding from host cell plasma membranes. Using quantitative shotgun mass spectrometry, we determined the lipidomes of the host Madin-Darby canine ...kidney cell, its apical membrane, and the IFV budding from it. We found the apical membrane to be enriched in sphingolipids (SPs) and cholesterol, whereas glycerophospholipids were reduced, and storage lipids were depleted compared with the whole-cell membranes. The virus membrane exhibited a further enrichment of SPs and cholesterol compared with the donor membrane at the expense of phosphatidylcholines. Our data are consistent with and extend existing models of membrane raft-based biogenesis of the apical membrane and IFV envelope.
During cell division, spindle microtubules ensure an equal repartition of chromosomes between the two daughter cells. While the kinetochore-dependent mechanisms that drive mitotic chromosome ...segregation are well understood, in oocytes of most species atypical spindles assembled in absence of centrosomes entail poorly understood mechanisms of chromosome segregation. In particular, the structure(s) responsible for force generation during meiotic chromosome separation in oocytes is unclear. Using quantitative light microscopy, electron tomography, laser-mediated ablation, and genetic perturbations in the Caenorhabditis elegans oocyte, we studied the mechanism of chromosome segregation in meiosis. We find spindle poles are largely dispensable, and in fact act as brakes for chromosome segregation. Instead, our results suggest that CLS-2-dependent microtubules of the meiotic central spindle, located between the segregating chromosomes and aligned along the axis of segregation, are essential. Our results support a model in which inter-chromosomal microtubules of the central spindle push chromosomes apart during meiotic anaphase in oocytes.
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