Plasma membrane deformation by circular arrays of ESCRT-III protein filaments

Human ESCRT-III polymers assemble on positively curved membranes and induce helical membrane tube formation

Endosomal sorting complexes for transport-III (ESCRT-III) assemble in vivo onto membranes with negative Gaussian curvature. How membrane shape influences ESCRT-III polymerization and how ESCRT-III shapes membranes is yet unclear. Human core ESCRT-III proteins, CHMP4B, CHMP2A, CHMP2B and CHMP3 are used to address this issue in vitro by combining membrane nanotube pulling experiments, cryo-electron tomography and AFM. We show that CHMP4B filaments preferentially bind to flat membranes or to tubes with positive mean curvature. Both CHMP2B and CHMP2A/CHMP3 assemble on positively curved membrane tubes. Combinations of CHMP4B/CHMP2B and CHMP4B/CHMP2A/CHMP3 are recruited to the neck of pulled membrane tubes and reshape vesicles into helical "corkscrew-like" membrane tubes. Sub-tomogram averaging reveals that the ESCRT-III filaments assemble parallel and locally perpendicular to the tube axis, highlighting the mechanical stresses imposed by ESCRT-III. Our results underline the versatile membrane remodeling activity of ESCRT-III that may be a general feature required for cellular membrane remodeling processes.

A helical assembly of human ESCRT-I scaffolds reverse-topology membrane scission

The ESCRT complexes drive membrane scission in HIV-1 release, autophagosome closure, MVB biogenesis, cytokinesis, and other cell processes. ESCRT-I is the most upstream complex and bridges the system to HIV-1 Gag in virus release. The crystal structure of the headpiece of human ESCRT-I comprising TSG101–VPS28–VPS37B–MVB12A was determined, revealing an ESCRT-I helical assembly with a 12 molecule repeat. Electron microscopy confirmed that ESCRT-I subcomplexes form helical filaments in solution. Mutation of VPS28 helical interface residues blocks filament formation in vitro and autophagosome closure and HIV-1 release in human cells. Coarse grained simulations of ESCRT assembly at HIV-1 budding sites suggest that formation of a 12-membered ring of ESCRT-I molecules is a geometry-dependent checkpoint during late stages of Gag assembly and HIV-1 budding, and templates ESCRT-III assembly for membrane scission. These data show that ESCRT-I is not merely a bridging adaptor, but has an essential scaffolding and mechanical role in its own right.

Further information on experimental design is available in the Nature Research Reporting Summary linked to this article.

Plasma membrane damage causes NLRP3 activation and pyroptosis during Mycobacterium tuberculosis infection

Mycobacterium tuberculosis is a global health problem in part as a result of extensive cytotoxicity caused by the infection. Here, we show how M. tuberculosis causes caspase-1/NLRP3/gasdermin D-mediated pyroptosis of human monocytes and macrophages. A type VII secretion system (ESX-1) mediated, contact-induced plasma membrane damage response occurs during phagocytosis of bacteria. Alternatively, this can occur from the cytosolic side of the plasma membrane after phagosomal rupture in infected macrophages. This damage causes K⁺ efflux and activation of NLRP3-dependent IL-1β release and pyroptosis, facilitating the spread of bacteria to neighbouring cells. A dynamic interplay of pyroptosis with ESCRT-mediated plasma membrane repair also occurs. This dual plasma membrane damage seems to be a common mechanism for NLRP3 activators that function through lysosomal damage.

Inflammasome activation is a response to bacterial infection but can cause damage and spread infection. Here, the authors use live single-cell imaging to show two mechanisms by which M. tuberculosis causes damage to human macrophage cell plasma membranes, resulting in activation of the NLRP3 inflammasome, pyroptosis and release of infectious particles.

Proteomic Profiling of Small Extracellular Vesicles Secreted by Human Pancreatic Cancer Cells Implicated in Cellular Transformation

Extracellular vesicles secreted from tumor cells are functional vehicles capable of contributing to intercellular communication and metastasis. A growing number of studies have focused on elucidating the role that tumor-derived extracellular vesicles play in spreading pancreatic cancer to other organs, due to the highly metastatic nature of the disease. We recently showed that small extracellular vesicles secreted from pancreatic cancer cells could initiate malignant transformation of healthy cells. Here, we analyzed the protein cargo contained within these vesicles using mass spectrometry-based proteomics to better understand their makeup and biological characteristics. Three different human pancreatic cancer cell lines were compared to normal pancreatic epithelial cells revealing distinct differences in protein cargo between cancer and normal vesicles. Vesicles from cancer cells contain an enrichment of proteins that function in the endosomal compartment of cells responsible for vesicle formation and secretion in addition to proteins that have been shown to contribute to oncogenic cell transformation. Conversely, vesicles from normal pancreatic cells were shown to be enriched for immune response proteins. Collectively, results contribute to what we know about the cargo contained within or excluded from cancer cell-derived extracellular vesicles, supporting their role in biological processes including metastasis and cancer progression.

The ubiquitin hydrolase Doa4 directly binds Snf7 to inhibit recruitment of ESCRT-III remodeling factors in S. cerevisiae

The ESCRT-III protein complex executes reverse-topology membrane scission. The scission mechanism is unclear but is linked to remodeling of ESCRT-III complexes at the membrane surface. At endosomes, ESCRT-III mediates the budding of intralumenal vesicles (ILVs). In Saccharomyces cerevisiae, ESCRT-III activity at endosomes is regulated through an unknown mechanism by Doa4, an ubiquitin hydrolase that deubiquitylates transmembrane proteins sorted into ILVs. We report that the non-catalytic N-terminus of Doa4 binds Snf7, the predominant ESCRT-III subunit. Through this interaction, Doa4 overexpression alters Snf7 assembly status and inhibits ILV membrane scission. In vitro, the Doa4 N-terminus inhibits association of Snf7 with Vps2, which functions with Vps24 to arrest Snf7 polymerization and remodel Snf7 polymer structure. In vivo, Doa4 overexpression inhibits Snf7 interaction with Vps2 and also with the ATPase Vps4, which is recruited by Vps2 and Vps24 to remodel ESCRT-III complexes by catalyzing subunit turnover. Our data suggest a mechanism by which the deubiquitylation machinery regulates ILV biogenesis by interfering with ESCRT-III remodeling.

Membrane constriction and thinning by sequential ESCRT-III polymerization

The endosomal sorting complexes required for transport (ESCRTs) mediate diverse membrane remodeling events. These typically require ESCRT-III proteins to stabilize negatively curved membranes; however, recent work has indicated that certain ESCRT-IIIs also participate in positive-curvature membrane-shaping reactions. ESCRT-IIIs polymerize into membrane-binding filaments, but the structural basis for negative versus positive membrane remodeling by these proteins remains poorly understood. To learn how certain ESCRT-IIIs shape positively curved membranes, we determined structures of human membrane-bound CHMP1B-only, membrane-bound CHMP1B + IST1, and IST1-only filaments by cryo-EM. Our structures show how CHMP1B first polymerizes into a single-stranded helical filament, shaping membranes into moderate-curvature tubules. Subsequently, IST1 assembles a second strand on CHMP1B, further constricting the membrane tube and reducing its diameter nearly to the fission point. Each step of constriction thins the underlying bilayer, lowering the barrier to membrane fission. Our structures reveal how a two-component, sequential polymerization mechanism drives membrane tubulation, constriction and bilayer thinning.

Biochemical Approaches to Studying Caenorhabditis elegans ESCRT Functions In Vitro

Our fundamental understanding of the roles played by the endosomal sorting complex required for transport (ESCRT) machinery in cells comes from interdisciplinary approaches that combine numerous in vivo and in vitro techniques. Here, we focus on methods used to biochemically characterize Caenorhabditis elegans ESCRT components in vitro, including the production and characterization of recombinant ESCRT complexes and their use in membrane interaction studies. Key methodologies used include gel filtration chromatography, glycerol density gradient analysis, multi-angle light scattering, liposome co-flotation, and single-liposome fluorescence microscopy. Collectively, these studies have enabled us to define subunit stoichiometry of soluble C. elegans ESCRT complexes and to demonstrate that the late-acting ESCRT-III complex facilitates membrane bending and remodeling, at least in part by virtue of its ability to sense the curvature of lipid bilayers.

The many functions of ESCRTs

Cellular membranes can form two principally different involutions, which either exclude or contain cytosol. The 'classical' budding reactions, such as those occurring during endocytosis or formation of exocytic vesicles, involve proteins that assemble on the cytosol-excluding face of the bud neck. Inverse membrane involution occurs in a wide range of cellular processes, supporting cytokinesis, endosome maturation, autophagy, membrane repair and many other processes. Such inverse membrane remodelling is mediated by a heteromultimeric protein machinery known as endosomal sorting complex required for transport (ESCRT). ESCRT proteins assemble on the cytosolic (or nucleoplasmic) face of the neck of the forming involution and cooperate with the ATPase VPS4 to drive membrane scission or sealing. Here, we review similarities and differences of various ESCRT-dependent processes, with special emphasis on mechanisms of ESCRT recruitment.

Changes in ESCRT-III filament geometry drive membrane remodelling and fission in silico

BACKGROUND

ESCRT-III is a membrane remodelling filament with the unique ability to cut membranes from the inside of the membrane neck. It is essential for the final stage of cell division, the formation of vesicles, the release of viruses, and membrane repair. Distinct from other cytoskeletal filaments, ESCRT-III filaments do not consume energy themselves, but work in conjunction with another ATP-consuming complex. Despite rapid progress in describing the cell biology of ESCRT-III, we lack an understanding of the physical mechanisms behind its force production and membrane remodelling.

RESULTS

Here we present a minimal coarse-grained model that captures all the experimentally reported cases of ESCRT-III driven membrane sculpting, including the formation of downward and upward cones and tubules. This model suggests that a change in the geometry of membrane bound ESCRT-III filaments-from a flat spiral to a 3D helix-drives membrane deformation. We then show that such repetitive filament geometry transitions can induce the fission of cargo-containing vesicles.

CONCLUSIONS

Our model provides a general physical mechanism that explains the full range of ESCRT-III-dependent membrane remodelling and scission events observed in cells. This mechanism for filament force production is distinct from the mechanisms described for other cytoskeletal elements discovered so far. The mechanistic principles revealed here suggest new ways of manipulating ESCRT-III-driven processes in cells and could be used to guide the engineering of synthetic membrane-sculpting systems.

A charged multivesicular body protein (CHMP4B) is required for lens growth and differentiation

Charged multivesicular body protein 4B (CHMP4B) functions as a core component of the endosome sorting complex required for transport-III (ESCRT-III) machinery that facilitates diverse membrane remodeling and scission processes in eukaryotes. Mutations in the human CHMP4B gene underlie rare, inherited forms of early-onset lens opacities or cataract. Here we have characterized the lens phenotypes of mutant (knock-in) mice harboring a human cataract-associated mutation (p.D129V) in CHMP4B (Chmp4b-mutant) and conditional knockdown mice deficient in lens CHMP4B (Chmp4b-CKD). In situ hybridization localized Chmp4b transcripts to lens epithelial cells and elongating fiber cells at the lens equator. Heterozygous Chmp4b-mutant (D/V) mice were viable and fertile with lenses grossly similar to those of wild-type. However, homozygous Chmp4b-mutant (V/V) mice died by embryonic day 15.5 (E15.5) with grossly abnormal eye and brain histology. Chmp4b-CKD mice displayed variable degrees of lens dysmorphology including lens ablation. Immuno-localization of aquaporin-0 (AQP0) revealed lens fiber cell degeneration in homozygous Chmp4b-mutant (V/V) mouse embryos and in embryonic and postnatal Chmp4b-CKD mice. DNA fragmentation (TUNEL) analysis revealed global cell death in homozygous Chmp4b-mutant (V/V) embryos, whereas, cell death was confined to the lens of Chmp4b-CKD mice. Immuno-localization of the monocyte/macrophage marker macrosialin (CD68) suggested that severe lens degeneration in Chmp4b-CKD mice resulted in an ocular immune cell response. Collectively, these mouse data suggest that (1) heterozygous, germ-line mutations in Chmp4b may not manifest as cataract, (2) homozygous, germ-line mutations in Chmp4b are embryonic lethal, and (3) conditional loss of Chmp4b results in arrest of lens growth and differentiation.

Comprehensive analysis of yeast ESCRT-III composition in single ESCRT-III deletion mutants

The endosomal sorting complex required for transport (ESCRT)-III is associated with a multitude of cellular processes involving membrane remodeling and abscission. The exact composition of ESCRT-III and the contribution of individual ESCRT-III family members to these diverse functions is unclear. Most of the currently available information about ESCRT-III was obtained with tagged, largely non-functional proteins, which may not correctly reflect the in vivo situation. Here, we performed a comprehensive biochemical analysis of ESCRT-III localization and composition in yeast under purely native conditions. Most of our findings are in line with the current concepts about ESCRT-III, but some findings are unexpected and call for adjustments to the model. In particular, our data suggest that the distinction between bona fide ESCRT-III components and ESCRT-III associated proteins is not justified. We detected a single complex containing all ESCRT-III members (except of Chm7) with Did2 as its main component. The classical core components were present in equimolar amounts. Our analysis of the impact of single deletions on the composition of ESCRT-III confirmed the central role of Snf7 for ESCRT-III assembly. For the other ESCRT-III family members predictions could be made about their role in ESCRT-III assembly. Furthermore, our cell fractionation points to a role of Vps20 at the endoplasmic reticulum.

A primer on resolving the nanoscale structure of the plasma membrane with light and electron microscopy

Taraska reviews the imaging methods that are being used to understand the structure of the plasma membrane at the molecular level.

The plasma membrane separates a cell from its external environment. All materials and signals that enter or leave the cell must cross this hydrophobic barrier. Understanding the architecture and dynamics of the plasma membrane has been a central focus of general cellular physiology. Both light and electron microscopy have been fundamental in this endeavor and have been used to reveal the dense, complex, and dynamic nanoscale landscape of the plasma membrane. Here, I review classic and recent developments in the methods used to image and study the structure of the plasma membrane, particularly light, electron, and correlative microscopies. I will discuss their history and use for mapping the plasma membrane and focus on how these tools have provided a structural framework for understanding the membrane at the scale of molecules. Finally, I will describe how these studies provide a roadmap for determining the nanoscale architecture of other organelles and entire cells in order to bridge the gap between cellular form and function.

Electrostatic lateral interactions drive ESCRT-III heteropolymer assembly

Self-assembly of ESCRT-III complex is a critical step in all ESCRT-dependent events. ESCRT-III hetero-polymers adopt variable architectures, but the mechanisms of inter-subunit recognition in these hetero-polymers to create flexible architectures remain unclear. We demonstrate in vivo and in vitro that the Saccharomyces cerevisiae ESCRT-III subunit Snf7 uses a conserved acidic helix to recruit its partner Vps24. Charge-inversion mutations in this helix inhibit Snf7-Vps24 lateral interactions in the polymer, while rebalancing the charges rescues the functional defects. These data suggest that Snf7-Vps24 assembly occurs through electrostatic interactions on one surface, rather than through residue-to-residue specificity. We propose a model in which these cooperative electrostatic interactions in the polymer propagate to allow for specific inter-subunit recognition, while sliding of laterally interacting polymers enable changes in architecture at distinct stages of vesicle biogenesis. Our data suggest a mechanism by which interaction specificity and polymer flexibility can be coupled in membrane-remodeling heteropolymeric assemblies.

Seeking Closure: How Do Herpesviruses Recruit the Cellular ESCRT Apparatus?

The Herpesviridae are structurally complex DNA viruses whose capsids undergo primary envelopment at the inner nuclear membrane and secondary envelopment at organelles in the cytoplasm. In both locations, there is evidence that envelope formation and scission involve the participation of multiple viral proteins and also the cellular ESCRT apparatus. It nevertheless appears that the best-understood viral strategies for ESCRT recruitment, those adopted by the retroviruses and many other families of enveloped RNA viruses, are not utilized by the Herpesviridae, at least during envelopment in the cytoplasm. Thus, although a large number of herpesvirus proteins have been assigned roles in envelopment, there is a dearth of candidates for the acquisition of the ESCRT complex and the control of envelope scission. This review summarizes our current understanding of ESCRT association by enveloped viruses, examines what is known of herpesvirus ESCRT utilization in the nucleus and cytoplasm, and identifies candidate cellular and viral proteins that could link enveloping herpesviruses to cellular ESCRT components.

VPS4 triggers constriction and cleavage of ESCRT-III helical filaments

Many cellular processes such as endosomal vesicle budding, virus budding, and cytokinesis require extensive membrane remodeling by the endosomal sorting complex required for transport III (ESCRT-III). ESCRT-III protein family members form spirals with variable diameters in vitro and in vivo inside tubular membrane structures, which need to be constricted to proceed to membrane fission. Here, we show, using high-speed atomic force microscopy and electron microscopy, that the AAA-type adenosine triphosphatase VPS4 constricts and cleaves ESCRT-III CHMP2A-CHMP3 helical filaments in vitro. Constriction starts asymmetrically and progressively decreases the diameter of CHMP2A-CHMP3 tubular structure, thereby coiling up the CHMP2A-CHMP3 filaments into dome-like end caps. Our results demonstrate that VPS4 actively constricts ESCRT-III filaments and cleaves them before their complete disassembly. We propose that the formation of ESCRT-III dome-like end caps by VPS4 within a membrane neck structure constricts the membrane to set the stage for membrane fission.

Growth factor stimulation promotes multivesicular endosome biogenesis by prolonging recruitment of the late-acting ESCRT machinery

The formation of multivesicular endosomes (MVEs) mediates the turnover of numerous integral membrane proteins and has been implicated in the down-regulation of growth factor signaling, thereby exhibiting properties of a tumor suppressor. The endosomal sorting complex required for transport (ESCRT) machinery plays a key role in MVE biogenesis, enabling cargo selection and intralumenal vesicle (ILV) budding. However, the spatiotemporal pattern of endogenous ESCRT complex assembly and disassembly in mammalian cells remains poorly defined. By combining CRISPR/Cas9-mediated genome editing and live cell imaging using lattice light sheet microscopy (LLSM), we determined the native dynamics of both early- and late-acting ESCRT components at MVEs under multiple growth conditions. Specifically, our data indicate that ESCRT-0 accumulates quickly on endosomes, typically in less than 30 seconds, and its levels oscillate in a manner dependent on the downstream recruitment of ESCRT-I. Similarly, levels of the ESCRT-I complex also fluctuate on endosomes, but its average residency time is more than fivefold shorter compared with ESCRT-0. Vps4 accumulation is the most transient, however, suggesting that the completion of ILV formation occurs rapidly. Upon addition of epidermal growth factor (EGF), both ESCRT-I and Vps4 are retained at endosomes for dramatically extended periods of time, while ESCRT-0 dynamics are only modestly affected. Our findings are consistent with a model in which growth factor stimulation stabilizes late-acting components of the ESCRT machinery at endosomes to accelerate the rate of ILV biogenesis and attenuate signal transduction initiated by receptor activation.

The role of VPS4 in ESCRT-III polymer remodeling

The endosomal sorting complex required for transport-III (ESCRT-III) and VPS4 catalyze a variety of membrane-remodeling processes in eukaryotes and archaea. Common to these processes is the dynamic recruitment of ESCRT-III proteins from the cytosol to the inner face of a membrane neck structure, their activation and filament formation inside or at the membrane neck and the subsequent or concomitant recruitment of the AAA-type ATPase VPS4. The dynamic assembly of ESCRT-III filaments and VPS4 on cellular membranes induces constriction of membrane necks with large diameters such as the cytokinetic midbody and necks with small diameters such as those of intraluminal vesicles or enveloped viruses. The two processes seem to use different sets of ESCRT-III filaments. Constriction is then thought to set the stage for membrane fission. Here, we review recent progress in understanding the structural transitions of ESCRT-III proteins required for filament formation, the functional role of VPS4 in dynamic ESCRT-III assembly and its active role in filament constriction. The recent data will be discussed in the context of different mechanistic models for inside-out membrane fission.

ATP-dependent force generation and membrane scission by ESCRT-III and Vps4

The endosomal sorting complexes required for transport (ESCRTs) catalyze reverse-topology scission from the inner face of membrane necks in HIV budding, multivesicular endosome biogenesis, cytokinesis, and other pathways. We encapsulated ESCRT-III subunits Snf7, Vps24, and Vps2 and the AAA+ ATPase (adenosine triphosphatase) Vps4 in giant vesicles from which membrane nanotubes reflecting the correct topology of scission could be pulled. Upon ATP release by photo-uncaging, this system generated forces within the nanotubes that led to membrane scission in a manner dependent upon Vps4 catalytic activity and Vps4 coupling to the ESCRT-III proteins. Imaging of scission revealed Snf7 and Vps4 puncta within nanotubes whose presence followed ATP release, correlated with force generation and nanotube constriction, and preceded scission. These observations directly verify long-standing predictions that ATP-hydrolyzing assemblies of ESCRT-III and Vps4 sever membranes.

Topography design in model membranes: Where biology meets physics

IMPACT STATEMENT

Artificial membranes with complex topography aid the understanding of biological processes where membrane geometry plays a key regulatory role. In this review, we highlight how emerging material and engineering technologies have been employed to create minimal models of cell signaling pathways, in vitro. These artificial systems allow life scientists to answer ever more challenging questions with regards to mechanisms in cellular biology. In vitro reconstitution of biology is an area that draws on the expertise and collaboration between biophysicists, material scientists and biologists and has recently generated a number of high impact results, some of which are also discussed in this review.

Endosomal trafficking of yeast membrane proteins

Various membrane trafficking pathways transport molecules through the endosomal system of eukaryotic cells, where trafficking decisions control the localisation and activity of a diverse repertoire of membrane protein cargoes. The budding yeast Saccharomyces cerevisiae has been used to discover and define many mechanisms that regulate conserved features of endosomal trafficking. Internalised surface membrane proteins first localise to endosomes before sorting to other compartments. Ubiquitination of endosomal membrane proteins is a signal for their degradation. Ubiquitinated cargoes are recognised by the endosomal sorting complex required for transport (ESCRT) apparatus, which mediate sorting through the multivesicular body pathway to the lysosome for degradation. Proteins that are not destined for degradation can be recycled to other intracellular compartments, such as the Golgi and the plasma membrane. In this review, we discuss recent developments elucidating the mechanisms that drive membrane protein degradation and recycling pathways in yeast.

Structures, Functions, and Dynamics of ESCRT-III/Vps4 Membrane Remodeling and Fission Complexes

The endosomal sorting complexes required for transport (ESCRT) pathway mediates cellular membrane remodeling and fission reactions. The pathway comprises five core complexes: ALIX, ESCRT-I, ESCRT-II, ESCRT-III, and Vps4. These soluble complexes are typically recruited to target membranes by site-specific adaptors that bind one or both of the early-acting ESCRT factors: ALIX and ESCRT-I/ESCRT-II. These factors, in turn, nucleate assembly of ESCRT-III subunits into membrane-bound filaments that recruit the AAA ATPase Vps4. Together, ESCRT-III filaments and Vps4 remodel and sever membranes. Here, we review recent advances in our understanding of the structures, activities, and mechanisms of the ESCRT-III and Vps4 machinery, including the first high-resolution structures of ESCRT-III filaments, the assembled Vps4 enzyme in complex with an ESCRT-III substrate, the discovery that ESCRT-III/Vps4 complexes can promote both inside-out and outside-in membrane fission reactions, and emerging mechanistic models for ESCRT-mediated membrane fission.

Adaptor functions of the Ca2+-binding protein ALG-2 in protein transport from the endoplasmic reticulum

Apoptosis-linked gene 2 (ALG-2) is a Ca²⁺-binding protein with five repetitive EF-hand motifs, named penta-EF-hand (PEF) domain. It interacts with various target proteins and functions as a Ca²⁺-dependent adaptor in diverse cellular activities. In the cytoplasm, ALG-2 is predominantly localized to a specialized region of the endoplasmic reticulum (ER), called the ER exit site (ERES), through its interaction with Sec31A. Sec31A is an outer coat protein of coat protein complex II (COPII) and is recruited from the cytosol to the ERES to form COPII-coated transport vesicles. I will overview current knowledge of the physiological significance of ALG-2 in regulating ERES localization of Sec31A and the following adaptor functions of ALG-2, including bridging Sec31A and annexin A11 to stabilize Sec31A at the ERES, polymerizing the Trk-fused gene (TFG) product, and linking MAPK1-interacting and spindle stabilizing (MISS)-like (MISSL) and microtubule-associated protein 1B (MAP1B) to promote anterograde transport from the ER.

Resolving ESCRT-III Spirals at the Intercellular Bridge of Dividing Cells Using 3D STORM

The ESCRT machinery mediates membrane fission in a variety of processes in cells. According to current models, ESCRT-III proteins drive membrane fission by assembling into helical filaments on membranes. Here, we used 3D STORM imaging of endogenous ESCRT-III component IST1 to reveal the evolution of the structural organization of ESCRT-III in mammalian cytokinetic abscission. Using this approach, ESCRT-III ring and spiral assemblies were resolved and characterized at different stages of abscission. Visualization of IST1 structures in cells lacking the microtubule-severing enzyme spastin and in cells depleted of specific ESCRT-III components or the ATPase VPS4 demonstrated the contribution of these components to the organization and function of ESCRTs in cells. This work provides direct evidence that ESCRT-III proteins form helical filaments to mediate their function in cells and raises new mechanistic scenarios for ESCRT-driven cytokinetic abscission.

Timing of ESCRT-III protein recruitment and membrane scission during HIV-1 assembly

The Endosomal Sorting Complexes Required for Transport III (ESCRT-III) proteins are critical for cellular membrane scission processes with topologies inverted relative to clathrin-mediated endocytosis. Some viruses appropriate ESCRT-IIIs for their release. By imaging single assembling viral-like particles of HIV-1, we observed that ESCRT-IIIs and the ATPase VPS4 arrive after most of the virion membrane is bent, linger for tens of seconds, and depart ~20 s before scission. These observations suggest that ESCRT-IIIs are recruited by a combination of membrane curvature and the late domains of the HIV-1 Gag protein. ESCRT-IIIs may pull the neck into a narrower form but must leave to allow scission. If scission does not occur within minutes of ESCRT departure, ESCRT-IIIs and VPS4 are recruited again. This mechanistic insight is likely relevant for other ESCRT-dependent scission processes including cell division, endosome tubulation, multivesicular body and nuclear envelope formation, and secretion of exosomes and ectosomes.

CHMP1B is a target of USP8/UBPY regulated by ubiquitin during endocytosis

Integration and down-regulation of cell growth and differentiation signals rely on plasma membrane receptor endocytosis and sorting towards either recycling vesicles or degradative lysosomes via multivesicular bodies (MVB). In this process, the endosomal sorting complex-III required for transport (ESCRT-III) controls membrane deformation and scission triggering intraluminal vesicle (ILV) formation at early endosomes. Here, we show that the ESCRT-III member CHMP1B can be ubiquitinated within a flexible loop known to undergo conformational changes during polymerization. We demonstrate further that CHMP1B is deubiquitinated by the ubiquitin specific protease USP8 (syn. UBPY) and found fully devoid of ubiquitin in a ~500 kDa large complex that also contains its ESCRT-III partner IST1. Moreover, EGF stimulation induces the rapid and transient accumulation of ubiquitinated forms of CHMP1B on cell membranes. Accordingly, CHMP1B ubiquitination is necessary for CHMP1B function in both EGF receptor trafficking in human cells and wing development in Drosophila. Based on these observations, we propose that CHMP1B is dynamically regulated by ubiquitination in response to EGF and that USP8 triggers CHMP1B deubiquitination possibly favoring its subsequent assembly into a membrane-associated ESCRT-III polymer.

In multicellular organisms, the interpretation and transmission of cell growth and differentiation signals strongly rely on plasma membrane receptors. Once activated by their ligands, these receptors activate downstream signaling cascades and are rapidly internalized into intracellular vesicles that fuse inside the cell to form the endosomal compartment. From there, the receptors are sorted towards either recycling vesicles or degradative lysosomes via multivesicular bodies. Receptors sorting therefore plays a crucial role in the integration and regulation of intracellular signals during development and numerous physio-pathological processes. It requires extensive membrane remodeling and scission events at the level of the endosomal compartment by so-called ESCRT proteins, including CHMP1B. In this study, we provide evidence for dynamic regulation of CHMP1B function and subcellular localization by ubiquitin linkage. We also show the contribution of the ubiquitin specific protease USP8 in this regulation, which is a known actor of intracellular trafficking and Cushing’s disease.

Molecular Genetics of Frontotemporal Dementia Elucidated by Drosophila Models-Defects in Endosomal⁻Lysosomal Pathway

Frontotemporal dementia (FTD) is the second most common senile neurodegenerative disease. FTD is a heterogeneous disease that can be classified into several subtypes. A mutation in CHMP2B locus (CHMP2B^intron5), which encodes a component of endosomal sorting complex required for transport-III (ESCRT-III), is associated with a rare hereditary subtype of FTD linked to chromosome 3 (FTD-3). ESCRT is involved in critical cellular processes such as multivesicular body (MVB) formation during endosomal–lysosomal pathway and autophagy. ESCRT mutants causes diverse physiological defects primarily due to accumulation of endosomes and defective MVBs resulting in misregulation of signaling pathways. Charged multivesicular body protein 2B (CHMP2B) is important for neuronal physiology which especially rely on precise regulation of protein homeostasis due to their post-mitotic status. Drosophila has proven to be an excellent model for charaterization of mechanistic underpinning of neurodegenerative disorders including FTD. In this review, current understanding of various FTD-related mutations is discussed with a focus on Drosophila models of CHMP2B^intron5-associated FTD.

Triggered recruitment of ESCRT machinery promotes endolysosomal repair

Endolysosomes can be damaged by diverse materials. Terminally damaged compartments are degraded by lysophagy, but pathways that repair salvageable organelles are poorly understood. Here we found that the endosomal sorting complex required for transport (ESCRT) machinery, known to mediate budding and fission on endolysosomes, also plays an essential role in their repair. ESCRTs were rapidly recruited to acutely injured endolysosomes through a pathway requiring calcium and ESCRT-activating factors that was independent of lysophagy. We used live-cell imaging to demonstrate that ESCRTs responded to small perforations in endolysosomal membranes and enabled compartments to recover from limited damage. Silica crystals that disrupted endolysosomes also triggered ESCRT recruitment. ESCRTs thus provide a defense against endolysosomal damage likely to be relevant in physiological and pathological contexts.

ESCRTs offer repair service

A cellular membrane remodeling machinery repairs damaged organelles

All roads lead to the vacuole-autophagic transport as part of the endomembrane trafficking network in plants

Plants regulate their development and response to the changing environment by sensing and interpreting environmental signals. Intracellular trafficking pathways including endocytic-, vacuolar-, and autophagic trafficking are important for the various aspects of responses in plants. Studies in the last decade have shown that the autophagic transport pathway uses common key components of endomembrane trafficking as well as specific regulators. A number of factors previously described for their function in endosomal trafficking have been discovered to be involved in the regulation of autophagy in plants. These include conserved endocytic machineries, such as the endosomal sorting complex required for transport (ESCRT), subunits of the HOPS and exocyst complexes, SNAREs, and RAB GTPases as well as plant-specific proteins. Defects in these factors have been shown to cause impairment of autophagosome formation, transport, fusion, and degradation, suggesting crosstalk between autophagy and other intracellular trafficking processes. In this review, we focus mainly on possible functions of endosomal trafficking components in autophagy.

Dividing the Archaeal Way: The Ancient Cdv Cell-Division Machinery

Cell division in most prokaryotes is mediated by the well-studied fts genes, with FtsZ as the principal player. In many archaeal species, however, division is orchestrated differently. The Crenarchaeota phylum of archaea features the action of the three proteins, CdvABC. This Cdv system is a unique and less-well-studied division mechanism that merits closer inspection. In vivo, the three Cdv proteins form a composite band that contracts concomitantly with the septum formation. Of the three Cdv proteins, CdvA is the first to be recruited to the division site, while CdvB and CdvC are thought to participate in the active part of the Cdv division machinery. Interestingly, CdvB shares homology with a family of proteins from the eukaryotic ESCRT-III complex, and CdvC is a homolog of the eukaryotic Vps4 complex. These two eukaryotic complexes are key factors in the endosomal sorting complex required for transport (ESCRT) pathway, which is responsible for various budding processes in eukaryotic cells and which participates in the final stages of division in Metazoa. There, ESCRT-III forms a contractile machinery that actively cuts the membrane, whereas Vps4, which is an ATPase, is necessary for the turnover of the ESCRT membrane-abscission polymers. In contrast to CdvB and CdvC, CdvA is unique to the archaeal Crenarchaeota and Thaumarchaeota phyla. The Crenarchaeota division mechanism has often been suggested to represent a simplified version of the ESCRT division machinery thus providing a model system to study the evolution and mechanism of cell division in higher organisms. However, there are still many open questions regarding this parallelism and the division mechanism of Crenarchaeota. Here, we review the existing data on the role of the Cdv proteins in the division process of Crenarchaeota as well as concisely review the ESCRT system in eukaryotes. We survey the similarities and differences between the division and abscission mechanisms in the two cases. We suggest that the Cdv system functions differently in archaea than ESCRT does in eukaryotes, and that, unlike the eukaryotic case, the Cdv system's main function may be related to surplus membrane invagination and cell-wall synthesis.

VPS4 is a dynamic component of the centrosome that regulates centrosome localization of γ-tubulin, centriolar satellite stability and ciliogenesis

The hexameric AAA ATPase VPS4 facilitates ESCRT III filament disassembly on diverse intracellular membranes. ESCRT III components and VPS4 have been localized to the ciliary transition zone and spindle poles and reported to affect centrosome duplication and spindle pole stability. How the canonical ESCRT pathway could mediate these events is unclear. We studied the association of VPS4 with centrosomes and found that GFP-VPS4 was a dynamic component of both mother and daughter centrioles. A mutant, VPS4^EQ, which can’t hydrolyze ATP, was less dynamic and accumulated at centrosomes. Centrosome localization of the VPS4^EQ mutant, caused reduced γ-tubulin levels at centrosomes and consequently decreased microtubule growth and altered centrosome positioning. In addition, preventing VPS4 ATP hydrolysis nearly eliminated centriolar satellites and paused ciliogensis after formation of the ciliary vesicle. Zebrafish embryos injected with GFP-VPS4^EQ mRNA were less viable, exhibited developmental defects and had fewer cilia in Kupffer’s vesicle. Surprisingly, ESCRT III proteins seldom localized to centrosomes and their depletion did not lead to these phenotypes. Our data support an ESCRT III-independent function for VPS4 at the centrosome and reveal that this evolutionary conserved AAA ATPase influences diverse centrosome functions and, as a result, global cellular architecture and development.

Wnt and Hedgehog: Secretion of Lipid-Modified Morphogens

Morphogens are signaling molecules produced by a localized source, specifying cell fate in a graded manner. The source secretes morphogens into the extracellular milieu to activate various target genes in an autocrine or paracrine manner. Here we describe various secreted forms of two canonical morphogens, the lipid-anchored Hedgehog (Hh) and Wnts, indicating the involvement of multiple carriers in the transport of these morphogens. These different extracellular secreted forms are likely to have distinct functions. Here we evaluate newly identified mechanisms that morphogens use to traverse the required distance to activate discrete paracrine signaling.

Microvesicle Involvement in Shiga Toxin-Associated Infection

Shiga toxin is the main virulence factor of enterohemorrhagic Escherichia coli, a non-invasive pathogen that releases virulence factors in the intestine, causing hemorrhagic colitis and, in severe cases, hemolytic uremic syndrome (HUS). HUS manifests with acute renal failure, hemolytic anemia and thrombocytopenia. Shiga toxin induces endothelial cell damage leading to platelet deposition in thrombi within the microvasculature and the development of thrombotic microangiopathy, mostly affecting the kidney. Red blood cells are destroyed in the occlusive capillary lesions. This review focuses on the importance of microvesicles shed from blood cells and their participation in the prothrombotic lesion, in hemolysis and in the transfer of toxin from the circulation into the kidney. Shiga toxin binds to blood cells and may undergo endocytosis and be released within microvesicles. Microvesicles normally contribute to intracellular communication and remove unwanted components from cells. Many microvesicles are prothrombotic as they are tissue factor- and phosphatidylserine-positive. Shiga toxin induces complement-mediated hemolysis and the release of complement-coated red blood cell-derived microvesicles. Toxin was demonstrated within blood cell-derived microvesicles that transported it to renal cells, where microvesicles were taken up and released their contents. Microvesicles are thereby involved in all cardinal aspects of Shiga toxin-associated HUS, thrombosis, hemolysis and renal failure.

Ist1 regulates ESCRT-III assembly and function during multivesicular endosome biogenesis in Caenorhabditis elegans embryos

Degradation of most integral membrane proteins is directed by the endosomal sorting complex required for transport (ESCRT) machinery, which selectively targets ubiquitin-modified cargoes into intralumenal vesicles (ILVs) within multivesicular endosomes (MVEs). To better understand the mechanisms underlying ESCRT-mediated formation of ILVs, we exploited the rapid, de novo biogenesis of MVEs during the oocyte-to-embryo transition in C. elegans. In contrast to previous models suggesting that ILVs form individually, we demonstrate that they remain tethered to one another subsequent to internalization, arguing that they bud continuously from stable subdomains. In addition, we show that membrane bending and ILV formation are directed specifically by the ESCRT-III complex in vivo in a manner regulated by Ist1, which promotes ESCRT-III assembly and inhibits the incorporation of upstream ESCRT components into ILVs. Our findings underscore essential actions for ESCRT-III in membrane remodeling, cargo selection, and cargo retention, which act repetitively to maximize the rate of ILV formation.

Control of Phagocytosis by Microbial Pathogens

Phagocytosis is a fundamental process of cells to capture and ingest foreign particles. Small unicellular organisms such as free-living amoeba use this process to acquire food. In pluricellular organisms, phagocytosis is a universal phenomenon that all cells are able to perform (including epithelial, endothelial, fibroblasts, etc.), but some specialized cells (such as neutrophils and macrophages) perform this very efficiently and were therefore named professional phagocytes by Rabinovitch. Cells use phagocytosis to capture and clear all particles larger than 0.5 µm, including pathogenic microorganisms and cellular debris. Phagocytosis involves a series of steps from recognition of the target particle, ingestion of it in a phagosome (phagocytic vacuole), maturation of this phagosome into a phagolysosome, to the final destruction of the ingested particle in the robust antimicrobial environment of the phagolysosome. For the most part, phagocytosis is an efficient process that eliminates invading pathogens and helps maintaining homeostasis. However, several pathogens have also evolved different strategies to prevent phagocytosis from proceeding in a normal way. These pathogens have a clear advantage to perpetuate the infection and continue their replication. Here, we present an overview of the phagocytic process with emphasis on the antimicrobial elements professional phagocytes use. We also summarize the current knowledge on the microbial strategies different pathogens use to prevent phagocytosis either at the level of ingestion, phagosome formation, and maturation, and even complete escape from phagosomes.

Recruitment dynamics of ESCRT-III and Vps4 to endosomes and implications for reverse membrane budding

The ESCRT machinery mediates reverse membrane scission. By quantitative fluorescence lattice light-sheet microscopy, we have shown that ESCRT-III subunits polymerize rapidly on yeast endosomes, together with the recruitment of at least two Vps4 hexamers. During their 3–45 s lifetimes, the ESCRT-III assemblies accumulated 75–200 Snf7 and 15–50 Vps24 molecules. Productive budding events required at least two additional Vps4 hexamers. Membrane budding was associated with continuous, stochastic exchange of Vps4 and ESCRT-III components, rather than steady growth of fixed assemblies, and depended on Vps4 ATPase activity. An all-or-none step led to final release of ESCRT-III and Vps4. Tomographic electron microscopy demonstrated that acute disruption of Vps4 recruitment stalled membrane budding. We propose a model in which multiple Vps4 hexamers (four or more) draw together several ESCRT-III filaments. This process induces cargo crowding and inward membrane buckling, followed by constriction of the nascent bud neck and ultimately ILV generation by vesicle fission.

ESCRT-dependent control of membrane remodelling during cell division

The Endosomal Sorting Complex Required for Transport (ESCRT) proteins form an evolutionarily conserved membrane remodelling machinery. Identified originally for their role in cargo sorting and remodelling of endosomal membranes during yeast vacuolar sorting, an extensive body of work now implicates a sub-complex of this machinery (ESCRT-III), as a transplantable membrane fission machinery that is dispatched to various cellular locations to achieve a topologically unique membrane separation. Surprisingly, several ESCRT-III-regulated processes occur during cell division, when cells undergo a dramatic and co-ordinated remodelling of their membranes to allow the physical processes of division to occur. The ESCRT machinery functions in regeneration of the nuclear envelope during open mitosis and in the abscission phase of cytokinesis, where daughter cells are separated from each other in the last act of division. Roles for the ESCRT machinery in cell division are conserved as far back as Archaea, suggesting that the ancestral role of these proteins was as a membrane remodelling machinery that facilitated division and that was co-opted throughout evolution to perform a variety of other cell biological functions. Here, we will explore the function and regulation of the ESCRT machinery in cell division.

Growing functions of the ESCRT machinery in cell biology and viral replication

The vast expansion in recent years of the cellular processes promoted by the endosomal sorting complex required for transport (ESCRT) machinery has reinforced its identity as a modular system that uses multiple adaptors to recruit the core membrane remodelling activity at different intracellular sites and facilitate membrane scission. Functional connections to processes such as the aurora B-dependent abscission checkpoint also highlight the importance of the spatiotemporal regulation of the ESCRT machinery. Here, we summarise the role of ESCRTs in viral budding, and what we have learned about the ESCRT pathway from studying this process. These advances are discussed in the context of areas of cell biology that have been transformed by research in the ESCRT field, including cytokinetic abscission, nuclear envelope resealing and plasma membrane repair.

A Consensus View of ESCRT-Mediated Human Immunodeficiency Virus Type 1 Abscission

The strong dependence of retroviruses, such as human immunodeficiency virus type 1 (HIV-1), on host cell factors is no more apparent than when the endosomal sorting complex required for transport (ESCRT) machinery is purposely disengaged. The resulting potent inhibition of retrovirus release underscores the importance of understanding fundamental structure-function relationships at the ESCRT-HIV-1 interface. Recent studies utilizing advanced imaging technologies have helped clarify these relationships, overcoming hurdles to provide a range of potential models for ESCRT-mediated virus abscission. Here, we discuss these models in the context of prior work detailing ESCRT machinery and the HIV-1 release process. To provide a template for further refinement, we propose a new working model for ESCRT-mediated HIV-1 release that reconciles disparate and seemingly conflicting studies.

Effects of Inhibiting VPS4 Support a General Role for ESCRTs in Extracellular Vesicle Biogenesis

Extracellular vesicles (EVs) are proposed to play important roles in intercellular communication. Two classes of EVs can be distinguished based on their intracellular origin. Exosomes are generated within endosomes and released when these fuse with the plasma membrane, whereas ectosomes bud directly from the plasma membrane. Studies of EV function have been hindered by limited understanding of their biogenesis. Components of the endosomal sorting complex required for transport (ESCRT) machinery play essential roles in topologically equivalent processes at both the endosome and the plasma membrane and are consistently recovered in EVs, but whether they are generally required to produce EVs is still debated. Here, we study the effects of inhibiting the ESCRT-associated AAA+ ATPase VPS4 on EV release from cultured cells using two methods for EV recovery, differential centrifugation and polyethylene glycol precipitation followed by lectin affinity chromatography. We find that inhibiting VPS4 in HEK293 cells decreases release of EV-associated proteins and miRNA as well as the overall number of EV particles. The tetraspanins CD63 and CD9 are among the most frequently monitored EV proteins, but they differ in their subcellular localization, with CD63 primarily in endosomes and CD9 on the plasma membrane. We find that CD63 and CD9 are enriched in separable populations of EVs that are both sensitive to VPS4 inhibition. Serum stimulation increases release of both types of EVs and is also reduced by inhibiting VPS4. Taken together, our data indicate that VPS4 activity is important for generating exosomes and ectosomes, thereby generally implicating the ESCRT machinery in EV biogenesis.

Dynamic subunit turnover in ESCRT-III assemblies is regulated by Vps4 to mediate membrane remodelling during cytokinesis

The Endosomal Sorting Complex Required for Transport (ESCRT)-III mediates membrane fission in fundamental cellular processes, including cytokinesis. ESCRT-III is thought to form persistent filaments that over time increase their curvature to constrict membranes. Unexpectedly, we found that ESCRT-III at the midbody of human cells rapidly turns over subunits with cytoplasmic pools while gradually forming larger assemblies. ESCRT-III turnover depended on the ATPase VPS4, which accumulated at the midbody simultaneously with ESCRT-III subunits, and was required for assembly of functional ESCRT-III structures. In vitro, the Vps2/Vps24 subunits of ESCRT-III formed side-by-side filaments with Snf7 and inhibited further polymerization, but the growth inhibition was alleviated by the addition of Vps4 and ATP. High-speed atomic force microscopy further revealed highly dynamic arrays of growing and shrinking ESCRT-III spirals in presence of Vps4. Continuous ESCRT-III remodeling by subunit turnover might facilitate shape adaptions to variable membrane geometries, with broad implications for diverse cellular processes.

Phagocytosis: A Fundamental Process in Immunity

One hundred years have passed since the death of Élie Metchnikoff (1845-1916). He was the first to observe the uptake of particles by cells and realized the importance of this process for the host response to injury and infection. He also was a strong advocate of the role of phagocytosis in cellular immunity, and with this he gave us the basis for our modern understanding of inflammation and the innate and acquired immune responses. Phagocytosis is an elegant but complex process for the ingestion and elimination of pathogens, but it is also important for the elimination of apoptotic cells and hence fundamental for tissue homeostasis. Phagocytosis can be divided into four main steps: (i) recognition of the target particle, (ii) signaling to activate the internalization machinery, (iii) phagosome formation, and (iv) phagolysosome maturation. In recent years, the use of new tools of molecular biology and microscopy has provided new insights into the cellular mechanisms of phagocytosis. In this review, we present a general view of our current knowledge on phagocytosis. We emphasize novel molecular findings, particularly on phagosome formation and maturation, and discuss aspects that remain incompletely understood.

The role of extracellular vesicles in malaria biology and pathogenesis

In the past decade, research on the functions of extracellular vesicles in malaria has expanded dramatically. Investigations into the various vesicle types, from both host and parasite origin, has revealed important roles for extracellular vesicles in disease pathogenesis and susceptibility, as well as cell–cell communication and immune responses. Here, work relating to extracellular vesicles in malaria is reviewed, and the areas that remain unknown and require further investigations are highlighted.

Regulation of yeast ESCRT-III membrane scission activity by the Doa4 ubiquitin hydrolase

Doa4 is the ubiquitin hydrolase in yeast that deubiquitinates transmembrane proteins sorted by ESCRTs. Results support a model for bidirectional regulation between Doa4 and the ESCRT-III complex.

ESCRT-III executes membrane scission during the budding of intralumenal vesicles (ILVs) at endosomes. The scission mechanism is unknown but appears to be linked to the cycle of assembly and disassembly of ESCRT-III complexes at membranes. Regulating this cycle is therefore expected to be important for determining the timing of ESCRT-III–mediated membrane scission. We show that in Saccharomyces cerevisiae, ESCRT-III complexes are stabilized and ILV membrane scission is delayed by Doa4, which is the ubiquitin hydrolase that deubiquitinates transmembrane proteins sorted as cargoes into ILVs. These results suggest a mechanism to delay ILV budding while cargoes undergo deubiquitination. We further show that deubiquitination of ILV cargoes is inhibited via Doa4 binding to Vps20, which is the subunit of ESCRT-III that initiates assembly of the complex. Current models suggest that ESCRT-III complexes surround ubiquitinated cargoes to trap them at the site of ILV budding while the cargoes undergo deubiquitination. Thus our results also propose a mechanism to prevent the onset of ILV cargo deubiquitination at the initiation of ESCRT-III complex assembly.