ATG16L1 induces the formation of phagophore-like membrane cups.

The hallmark of non-selective autophagy is the formation of cup-shaped phagophores that capture bulk cytoplasm. The process is accompanied by the conjugation of LC3B to phagophores by an E3 ligase complex comprising ATG12-ATG5 and ATG16L1. Here we combined two complementary reconstitution approaches to reveal the function of LC3B and its ligase complex during phagophore expansion. We found that LC3B forms together with ATG12-ATG5-ATG16L1 a membrane coat that remodels flat membranes into cups that closely resemble phagophores. Mechanistically, we revealed that cup formation strictly depends on a close collaboration between LC3B and ATG16L1. Moreover, only LC3B, but no other member of the ATG8 protein family, promotes cup formation. ATG16L1 truncates that lacked the C-terminal membrane binding domain catalyzed LC3B lipidation but failed to assemble coats, did not promote cup formation and inhibited the biogenesis of non-selective autophagosomes. Our results thus demonstrate that ATG16L1 and LC3B induce and stabilize the characteristic cup-like shape of phagophores.

Staying on track - Keeping things running in a high-end scientific imaging core facility.

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.

Caveolin-1 protects endothelial cells from extensive expansion of transcellular tunnel by stiffening the plasma membrane.

Large transcellular pores elicited by bacterial mono-ADP-ribosyltransferase (mART) exotoxins inhibiting the small RhoA GTPase compromise the endothelial barrier. Recent advances in biophysical modeling point toward membrane tension and bending rigidity as the minimal set of mechanical parameters determining the nucleation and maximal size of transendothelial cell macroaperture (TEM) tunnels induced by bacterial RhoA-targeting mART exotoxins. We report that cellular depletion of caveolin-1, the membrane-embedded building block of caveolae, and depletion of cavin-1, the master regulator of caveolae invaginations, increase the number of TEMs per cell. The enhanced occurrence of TEM nucleation events correlates with a reduction in cell height due to the increase in cell spreading and decrease in cell volume, which, together with the disruption of RhoA-driven F-actin meshwork, favor membrane apposition for TEM nucleation. Strikingly, caveolin-1 specifically controls the opening speed of TEMs, leading to their dramatic 5.4-fold larger widening. Consistent with the increase in TEM density and width in siCAV1 cells, we record a higher lethality in CAV1 KO mice subjected to a catalytically active mART exotoxin targeting RhoA during staphylococcal bloodstream infection. Combined theoretical modeling with independent biophysical measurements of plasma membrane bending rigidity points toward a specific contribution of caveolin-1 to membrane stiffening in addition to the role of cavin-1/caveolin-1-dependent caveolae in the control of membrane tension homeostasis.

High-fidelity 3D live-cell nanoscopy through data-driven enhanced super-resolution radial fluctuation.

Live-cell super-resolution microscopy enables the imaging of biological structure dynamics below the diffraction limit. Here we present enhanced super-resolution radial fluctuations (eSRRF), substantially improving image fidelity and resolution compared to the original SRRF method. eSRRF incorporates automated parameter optimization based on the data itself, giving insight into the trade-off between resolution and fidelity. We demonstrate eSRRF across a range of imaging modalities and biological systems. Notably, we extend eSRRF to three dimensions by combining it with multifocus microscopy. This realizes live-cell volumetric super-resolution imaging with an acquisition speed of ~1 volume per second. eSRRF provides an accessible super-resolution approach, maximizing information extraction across varied experimental conditions while minimizing artifacts. Its optimal parameter prediction strategy is generalizable, moving toward unbiased and optimized analyses in super-resolution microscopy.

MiniBAR/GARRE1 is a dual Rac and Rab effector required for ciliogenesis.

Cilia protrude from the cell surface and play critical roles in intracellular signaling, environmental sensing, and development. Reduced actin-dependent contractility and intracellular trafficking are both required for ciliogenesis, but little is known about how these processes are coordinated. Here, we identified a Rac1- and Rab35-binding protein with a truncated BAR (Bin/amphiphysin/Rvs) domain that we named MiniBAR (also known as KIAA0355/GARRE1), which plays a key role in ciliogenesis. MiniBAR colocalizes with Rac1 and Rab35 at the plasma membrane and on intracellular vesicles trafficking to the ciliary base and exhibits fast pulses at the ciliary membrane. MiniBAR depletion leads to short cilia, resulting from abnormal Rac-GTP/Rho-GTP levels and increased acto-myosin-II-dependent contractility together with defective trafficking of IFT88 and ARL13B into cilia. MiniBAR-depleted zebrafish embryos display dysfunctional short cilia and hallmarks of ciliopathies, including left-right asymmetry defects. Thus, MiniBAR is a dual Rac and Rab effector that controls both actin cytoskeleton and membrane trafficking for ciliogenesis.

Redistribution of FLAgellar Member 8 during the trypanosome life cycle: Consequences for cell fate prediction.

The single flagellum of African trypanosomes is essential in multiple aspects of the parasites' development. The FLAgellar Member 8 protein (FLAM8), localised to the tip of the flagellum in cultured insect forms of Trypanosoma brucei, was identified as a marker of the locking event that controls flagellum length. Here, we investigated whether FLAM8 could also reflect the flagellum maturation state in other parasite cycle stages. We observed that FLAM8 distribution extended along the entire flagellar cytoskeleton in mammalian-infective forms. Then, a rapid FLAM8 concentration to the distal tip occurs during differentiation into early insect forms, illustrating the remodelling of an existing flagellum. In the tsetse cardia, FLAM8 further localises to the entire length of the new flagellum during an asymmetric division. Strikingly, in parasites dividing in the tsetse midgut and in the salivary glands, the amount and distribution of FLAM8 in the new flagellum were seen to predict the daughter cell fate. We propose and discuss how FLAM8 could be considered a meta-marker of the flagellum stage and maturation state in trypanosomes.

The viral restriction factor tetherin/BST2 tethers cytokinetic midbody remnants to the cell surface.

The midbody at the center of the intercellular bridge connecting dividing cells recruits the machinery essential for the final steps of cytokinesis.1-5 Successive abscission on both sides of the midbody generates a free midbody remnant (MBR) that can be inherited and accumulated in many cancer, immortalized, and stem cells, both in culture and in vivo.6-12 Strikingly, this organelle was recently shown to contain information that induces cancer cell proliferation, influences cell polarity, and promotes dorso-ventral axis specification upon interaction with recipient cells.13-16 Yet the mechanisms by which the MBR is captured by either a daughter cell or a distant cell are poorly described.10,14 Here, we report that BST2/tetherin, a well-established restriction factor that blocks the release of numerous enveloped viruses from the surface of infected cells,17-20 plays an analogous role in retaining midbody remnants. We found that BST2 is enriched at the midbody during cytokinesis and localizes at the surface of MBRs in a variety of cells. Knocking out BST2 induces the detachment of MBRs from the cell surface, their accumulation in the extracellular medium, and their transfer to distant cells. Mechanistically, the localization of BST2 at the MBR membrane is both necessary and sufficient for the interaction between MBRs and the cell surface. We thus propose that BST2 tethers post-cytokinetic midbody remnants to the cell surface. This finding reveals new parallels between cytokinesis and viral biology21-26 that unexpectedly extend beyond the ESCRT-dependent abscission step.

Author Correction: Phosphoinositides regulate the TCR/CD3 complex membrane dynamics and activation.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

The Flemmingsome reveals an ESCRT-to-membrane coupling via ALIX/syntenin/syndecan-4 required for completion of cytokinesis.

Cytokinesis requires the constriction of ESCRT-III filaments on the side of the midbody, where abscission occurs. After ESCRT recruitment at the midbody, it is not known how the ESCRT-III machinery localizes to the abscission site. To reveal actors involved in abscission, we obtained the proteome of intact, post-abscission midbodies (Flemmingsome) and identified 489 proteins enriched in this organelle. Among these proteins, we further characterized a plasma membrane-to-ESCRT module composed of the transmembrane proteoglycan syndecan-4, ALIX and syntenin, a protein that bridges ESCRT-III/ALIX to syndecans. The three proteins are highly recruited first at the midbody then at the abscission site, and their depletion delays abscission. Mechanistically, direct interactions between ALIX, syntenin and syndecan-4 are essential for proper enrichment of the ESCRT-III machinery at the abscission site, but not at the midbody. We propose that the ESCRT-III machinery must be physically coupled to a membrane protein at the cytokinetic abscission site for efficient scission, uncovering common requirements in cytokinesis, exosome formation and HIV budding.

Intracellular offspring released from SFB filaments are flagellated.

The gut commensal segmented filamentous bacterium (SFB) attaches to the ileal epithelium and potently stimulates the host immune system. Using transmission electron microscopy (TEM), we show that mouse and rat SFB are flagellated above the concave tip at the unicellular intracellular offspring (IO) stage and that flagellation occurs prior to full IO differentiation and release of IOs from SFB filaments. This finding adds a missing link to the SFB life cycle.

CSB promoter downregulation via histone H3 hypoacetylation is an early determinant of replicative senescence.

Cellular senescence has causative links with ageing and age-related diseases, however, it remains unclear if progeroid factors cause senescence in normal cells. Here, we show that depletion of CSB, a protein mutated in progeroid Cockayne syndrome (CS), is the earliest known trigger of p21-dependent replicative senescence. CSB depletion promotes overexpression of the HTRA3 protease resulting in mitochondrial impairments, which are causally linked to CS pathological phenotypes. The CSB promoter is downregulated by histone H3 hypoacetylation during DNA damage-response. Mechanistically, CSB binds to the p21 promoter thereby downregulating its transcription and blocking replicative senescence in a p53-independent manner. This activity of CSB is independent of its role in the repair of UV-induced DNA damage. HTRA3 accumulation and senescence are partially rescued upon reduction of oxidative/nitrosative stress. These findings establish a CSB/p21 axis that acts as a barrier to replicative senescence, and link a progeroid factor with the process of regular ageing in human.

Dynamic Growth and Shrinkage of the Salmonella-Containing Vacuole Determines the Intracellular Pathogen Niche.

Salmonella is a human and animal pathogen that causes gastro-enteric diseases. The key to Salmonella infection is its entry into intestinal epithelial cells, where the bacterium resides within a Salmonella-containing vacuole (SCV). Salmonella entry also induces the formation of empty macropinosomes, distinct from the SCV, in the vicinity of the entering bacteria. A few minutes after its formation, the SCV increases in size through fusions with the surrounding macropinosomes. Salmonella also induces membrane tubules that emanate from the SCV and lead to SCV shrinkage. Here, we show that these antipodal events are utilized by Salmonella to either establish a vacuolar niche or to be released into the cytosol by SCV rupture. We identify the molecular machinery underlying dynamic SCV growth and shrinkage. In particular, the SNARE proteins SNAP25 and STX4 participate in SCV inflation by fusion with macropinosomes. Thus, host compartment size control emerges as a pathogen strategy for intracellular niche regulation.

Atlastin Endoplasmic Reticulum-Shaping Proteins Facilitate Zika Virus Replication.

The endoplasmic reticulum (ER) is the site for Zika virus (ZIKV) replication and is central to the cytopathic effects observed in infected cells. ZIKV induces the formation of ER-derived large cytoplasmic vacuoles followed by "implosive" cell death. Little is known about the nature of the ER factors that regulate flavivirus replication. Atlastins (ATL1, -2, and -3) are dynamin-related GTPases that control the structure and the dynamics of the ER membrane. We show here that ZIKV replication is significantly decreased in the absence of ATL proteins. The appearance of infected cells is delayed, the levels of intracellular viral proteins and released virus are reduced, and the cytopathic effects are strongly impaired. We further show that ATL3 is recruited to viral replication sites and interacts with the nonstructural viral proteins NS2A and NS2B3. Thus, proteins that shape and maintain the ER tubular network ensure efficient ZIKV replication.IMPORTANCE Zika virus (ZIKV) is an emerging virus associated with Guillain-Barré syndrome, and fetal microcephaly as well as other neurological complications. There is no vaccine or specific antiviral treatment against ZIKV. We found that endoplasmic reticulum (ER)-shaping atlastin proteins (ATL1, -2, and -3), which induce ER membrane fusion, facilitate ZIKV replication. We show that ATL3 is recruited to the viral replication site and colocalize with the viral proteins NS2A and NS2B3. The results provide insights into host factors used by ZIKV to enhance its replication.

Adhesion to nanofibers drives cell membrane remodeling through one-dimensional wetting.

The shape of cellular membranes is highly regulated by a set of conserved mechanisms that can be manipulated by bacterial pathogens to infect cells. Remodeling of the plasma membrane of endothelial cells by the bacterium Neisseria meningitidis is thought to be essential during the blood phase of meningococcal infection, but the underlying mechanisms are unclear. Here we show that plasma membrane remodeling occurs independently of F-actin, along meningococcal type IV pili fibers, by a physical mechanism that we term 'one-dimensional' membrane wetting. We provide a theoretical model that describes the physical basis of one-dimensional wetting and show that this mechanism occurs in model membranes interacting with nanofibers, and in human cells interacting with extracellular matrix meshworks. We propose one-dimensional wetting as a new general principle driving the interaction of cells with their environment at the nanoscale that is diverted by meningococci during infection.

Imaging Intermediate Filaments and Microtubules with 2-dimensional Direct Stochastic Optical Reconstruction Microscopy.

The cytoskeleton, composed of actin microfilaments, microtubules, and intermediate filaments (IF), plays a key role in the control of cell shape, polarity, and motility. The organization of the actin and microtubule networks has been extensively studied but that of IFs is not yet fully characterized. IFs have an average diameter of 10 nm and form a network extending throughout the cell cytoplasm. They are physically associated with actin and microtubules through molecular motors and cytoskeletal linkers. This tight association is at the heart of the regulatory mechanisms that ensure the coordinated regulation of the three cytoskeletal networks required for most cell functions. It is therefore crucial to visualize IFs alone and also together with each of the other cytoskeletal networks. However, IF networks are extremely dense in most cell types, especially in glial cells, which makes its resolution very difficult to achieve with standard fluorescence microscopy (lateral resolution of ~250 nm). Direct STochastic Optical Reconstruction Microscopy (dSTORM) is a technique allowing a gain in lateral resolution of one order of magnitude. Here, we show that lateral dSTORM resolution is sufficient to resolve the dense organization of the IF networks and, in particular, of IF bundles surrounding microtubules. Such tight association is likely to participate in the coordinated regulation of these two networks and may, explain how vimentin IFs template and stabilize microtubule organization as well as could influence microtubule dependent vesicular trafficking. More generally, we show how the observation of two cytoskeletal components with dual-color dSTORM technique brings new insight into their mutual interaction.

Fluorescence imaging host pathogen interactions: fifteen years benefit of hindsight .

We consider in review current state-of-the-art fluorescence microscopy for investigating the host-pathogen interface. Our perspective is honed from years with literally thousands of microbiologists using the variety of imaging technologies available within our dedicated BSL2/BSL3 optical imaging research service facilities at the Institut Pasteur Paris founded from scratch in 2001. During fifteen years learning from the success and failures of introducing different fluorescence imaging technologies, methods, and technical development strategies we provide here a synopsis review of our experience to date and a synthesis of how we see the future in perspective for fluorescence imaging at the host-pathogen interface.

Phosphoinositides regulate the TCR/CD3 complex membrane dynamics and activation.

Phosphoinositides (PIs) play important roles in numerous membrane-based cellular activities. However, their involvement in the mechanism of T cell receptor (TCR) signal transduction across the plasma membrane (PM) is poorly defined. Here, we investigate their role, and in particular that of phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] in TCR PM dynamics and activity in a mouse T-cell hybridoma upon ectopic expression of a PM-localized inositol polyphosphate-5-phosphatase (Inp54p). We observed that dephosphorylation of PI(4,5)P2 by the phosphatase increased the TCR/CD3 complex PM lateral mobility prior stimulation. The constitutive and antigen-elicited CD3 phosphorylation as well as the antigen-stimulated early signaling pathways were all found to be significantly augmented in cells expressing the phosphatase. Using state-of-the-art biophotonic approaches, we further showed that PI(4,5)P2 dephosphorylation strongly promoted the CD3ε cytoplasmic domain unbinding from the PM inner leaflet in living cells, thus resulting in an increased CD3 availability for interactions with Lck kinase. This could significantly account for the observed effects of PI(4,5)P2 dephosphorylation on the CD3 phosphorylation. Our data thus suggest that PIs play a key role in the regulation of the TCR/CD3 complex dynamics and activation at the PM.

α-Synuclein transfer between neurons and astrocytes indicates that astrocytes play a role in degradation rather than in spreading.

Recent evidence suggests that disease progression in Parkinson's disease (PD) could occur by the spreading of α-synuclein (α-syn) aggregates between neurons. Here we studied the role of astrocytes in the intercellular transfer and fate of α-syn fibrils, using in vitro and ex vivo models. α-Syn fibrils can be transferred to neighboring cells; however, the transfer efficiency changes depending on the cell types. We found that α-syn is efficiently transferred from astrocytes to astrocytes and from neurons to astrocytes, but less efficiently from astrocytes to neurons. Interestingly, α-syn puncta are mainly found inside the lysosomal compartments of the recipient cells. However, differently from neurons, astrocytes are able to efficiently degrade fibrillar α-syn, suggesting an active role for these cells in clearing α-syn deposits. Astrocytes co-cultured with organotypic brain slices are able to take up α-syn fibrils from the slices. Altogether our data support a role for astrocytes in trapping and clearing α-syn pathological deposits in PD.

Drosophila cells use nanotube-like structures to transfer dsRNA and RNAi machinery between cells.

Tunnelling nanotubes and cytonemes function as highways for the transport of organelles, cytosolic and membrane-bound molecules, and pathogens between cells. During viral infection in the model organism Drosophila melanogaster, a systemic RNAi antiviral response is established presumably through the transport of a silencing signal from one cell to another via an unknown mechanism. Because of their role in cell-cell communication, we investigated whether nanotube-like structures could be a mediator of the silencing signal. Here, we describe for the first time in the context of a viral infection the presence of nanotube-like structures in different Drosophila cell types. These tubules, made of actin and tubulin, were associated with components of the RNAi machinery, including Argonaute 2, double-stranded RNA, and CG4572. Moreover, they were more abundant during viral, but not bacterial, infection. Super resolution structured illumination microscopy showed that Argonaute 2 and tubulin reside inside the tubules. We propose that nanotube-like structures are one of the mechanisms by which Argonaute 2, as part of the antiviral RNAi machinery, is transported between infected and non-infected cells to trigger systemic antiviral immunity in Drosophila.

A role for septin 2 in Drp1-mediated mitochondrial fission.

Mitochondria are essential eukaryotic organelles often forming intricate networks. The overall network morphology is determined by mitochondrial fusion and fission. Among the multiple mechanisms that appear to regulate mitochondrial fission, the ER and actin have recently been shown to play an important role by mediating mitochondrial constriction and promoting the action of a key fission factor, the dynamin-like protein Drp1. Here, we report that the cytoskeletal component septin 2 is involved in Drp1-dependent mitochondrial fission in mammalian cells. Septin 2 localizes to a subset of mitochondrial constrictions and directly binds Drp1, as shown by immunoprecipitation of the endogenous proteins and by pulldown assays with recombinant proteins. Depletion of septin 2 reduces Drp1 recruitment to mitochondria and results in hyperfused mitochondria and delayed FCCP-induced fission. Strikingly, septin depletion also affects mitochondrial morphology in Caenorhabditis elegans, strongly suggesting that the role of septins in mitochondrial dynamics is evolutionarily conserved.