Structural organization and dynamics of FCHo2 docking on membranes

Saddle curvature association of nsP1 facilitates the replication complex assembly of Chikungunya virus in cells

Positive-sense RNA viruses, including SARS-CoV-1 and -2, DENV, and CHIKV, replicate in curved membrane compartments within host cells. Non-structural proteins (nsPs) critically regulate these nanoscale membrane structures, yet their curvature-dependent assembly remains elusive due to the challenges of imaging nanoscale interaction on curved surfaces. Using vertically aligned nanostructures to generate pre-defined membrane curvatures, we here investigate the impact of curvature on nsPs assembly. Taking CHIKV as a model, we reveal that nsP1 preferentially binds and stabilizes on positively curved membranes, with stronger accumulation at radii ≤150 nm. This is driven by hydrophobic residues in the membrane association (MA) loops of individual nsP1. Molecular dynamics simulations further confirm the improved binding stability of nsP1 on curved membranes, particularly when it forms a dodecamer ring. Together, nsP1 supports a strong saddle curvature association, with flexible MA loops sensing a range of positive curvatures in the x-z plane while the rigid dodecamer stabilizing fixed negative curvature in the x-y plane - crucial for constraining the membrane spherule neck during replication progression. Moreover, CHIKV replication enriches on patterned nanoring structures, underscoring the curvature-guided assembly of the viral replication complex. Our findings highlight membrane curvature as a key regulator of viral nsPs organization, opening new avenues for studying membrane remodeling in viral replication.

Graft-derived extracellular vesicles transport miRNAs to modulate macrophage polarization after heart transplantation

Heart transplantation, a crucial intervention for saving lives of those with end-stage cardiac failure, often faces complications from acute allograft rejection. This study focuses on the intricate dynamics of immune cell interactions and specific communication pathways between organs, which are not yet well understood. Our study investigates this interplay using a murine heterotopic transplant model, using single-cell RNA sequencing to examine CD45+ immune cells from both the heart grafts and spleens. We conduct a comprehensive analysis focused on functional enrichment, cell trajectory, and interorgan communication in heart transplants, highlighting dynamic interactions between monocyte/macrophage subtypes that is mediated by extracellular vesicles (EVs). We use unsupervised clustering and elucidate the complex cellular interactions that influence allograft outcomes. Notably, we discovered that microRNA-363 and microRNA-709, carried by EVs from CD63+ graft macrophages, can induce M1 polarization within the recipient's spleen via the Fcho2/Notch1 signaling pathway. These insights illuminate the nuanced immune responses during acute cardiac rejection and suggest that targeting EVs from graft-resident macrophages may offer a new strategy to mitigate transplant rejection.

SGIP1 Deletion in Mice Attenuates Mechanical Hypersensitivity Elicited by Inflammation

Background: Activation of cannabinoid receptor 1 (CB1R) in the nervous system modulates the processing of acute and chronic pain. CB1R activity is regulated by desensitization and internalization. SH3-containing GRB2-like protein 3-interacting protein 1 (SGIP1) inhibits the internalization of CB1R. This causes increased and prolonged association of the desensitized receptor with G protein-coupled receptor kinase 3 (GRK3) and beta-arrestin on the cell membrane and results in decreased activation of extracellular signal-regulated kinase 1/2 (ERK1/2) pathway. Genetic deletion of SGIP1 in mice leads to altered CB1R-related functions, such as decreased anxiety-like behaviors, modified cannabinoid tetrad behaviors, reduced acute nociception, and increased sensitivity to analgesics. In this work, we asked if deletion of SGIP1 affects chronic nociception and analgesic effect of Δ9-tetrahydrocannabinol (THC) and WIN 55,212-2 (WIN) in mice. Methods: We measured tactile responses of hind paws to increasing pressure in wild-type and SGIP1 knock-out mice. Inflammation in the paw was induced by local injection of carrageenan. To determine the mechanical sensitivity, the paw withdrawal threshold (PWT) was measured using an electronic von Frey instrument with the progression of the applied force. Results: The responses to mechanical stimuli varied depending on the sex, genotype, and treatment. SGIP1 knock-out male mice exhibited lower PWT than wild-type males. On the contrary, the female mice exhibited comparable PWT. Following THC or WIN treatment in male mice, SGIP1 knock-out males exhibited PWT lower than wild-type males. THC treatment in SGIP1 knock-out females resulted in PWT higher than after THC treatment of wild-type females. However, SGIP1 knock-out and wild-type female mice exhibited similar PWT after WIN treatment. Conclusions: We provide evidence that SGIP1, possibly by interacting with CB1R, is involved in processing the responses to chronic pain. The absence of SGIP1 results in enhanced sensitivity to mechanical stimuli in males, but not females. The antinociceptive effect of THC is superior to that of WIN in SGIP1 knock-out mice in the carrageenan-induced model of chronic pain.

Septin assemblies promote the lipid organization of membranes

Cytoskeletal-mediated membrane compartmentalization is essential to support cellular functions, from signaling to cell division, migration, or phagocytosis. Septins are cytoskeletal proteins that directly interact with membranes, acting as scaffolds to recruit proteins to cellular locations and as structural diffusion barriers. How septins interact with and remodel the lipid organization of membranes is unclear. Here, we combined minimal reconstituted systems and yeast cell imaging to study septin-mediated membrane organization. Our results show that at low concentrations membrane-diffusive septins self-assemble into sub-micrometric patches that co-exist with the septin collar at the division site. We found that patches are made of short septin filaments and that are able to modulate the lipid organization of membranes. Furthermore, we show that the polybasic domain of Cdc11 influences the membrane-organizing and curvature-sensing properties of septins. Collectively, our work provides understanding of the molecular mechanisms by which septins can support cellular functions intimately linked to membranes.

Sequential recruitment of F-BAR proteins controls cytoskeletal crosstalk at the yeast bud neck

In vivo functions of the septin and actin cytoskeletons are closely intertwined, yet the mechanisms underlying septin-actin crosstalk have remained poorly understood. Here, we show that the yeast-bud-neck-associated Fes/CIP4 homology Bar-amphiphysin-Rvs (F-BAR) protein suppressor of yeast profilin 1 (Syp1)/FCHo uses its intrinsically disordered region (IDR) to directly bind and bundle filamentous actin (F-actin) and to physically link septins and F-actin. Interestingly, the only other F-BAR protein found at the neck during bud development, Hof1, has related activities and also potently inhibits the bud-neck-associated formin Bnr1. However, we find that Syp1 enhances rather than inhibits Bnr1-mediated actin assembly and fully overcomes Hof1-mediated inhibition of Bnr1. Further, during bud development, Syp1 and Hof1 show reciprocal patterns of arrival and departure from the bud neck, and in vitro Syp1 and Hof1 compete for septin binding. Together, our observations suggest that as the bud grows, the relative levels of these two F-BAR proteins at the bud neck invert, driving changes in septin organization, septin-actin linkage, and formin activity. More broadly, our findings expand the functional roles of Syp1/FCHo family proteins and our understanding of the working relationships among F-BAR proteins in cytoskeletal regulation.

Tuning the Immune Cell Response through Surface Nanotopography Engineering

Dendritic cells (DCs) are central regulators of the immune response by detecting inflammatory signals, aberrant cells, or pathogens. DC-mediated immune surveillance requires morphology changes to adapt to the physical and biochemical cues of the external environment. These changes are assisted by a dynamic actin cytoskeleton-membrane interface connected to surface receptors that will trigger signaling cascades. In recent years, the development of synthetic immune environments has allowed to investigate the impact of the external environment in the immune cell response. In this direction, the bioengineering of functional topographical features should make it possible to establish how membrane morphology modulates specific cellular functions in DCs. Herein, the engineering of one-dimensional nanostructured SiO2 surfaces by soft-nanoimprint lithography to manipulate the membrane morphology of ex vivo human DCs is reported. Super-resolution microscopy and live-cell imaging studies show that vertical pillar topographies promote the patterning and stabilization of adhesive actin-enriched structures in DCs. Furthermore, vertical topographies stimulate the spatial organization of innate immune receptors and regulate the Syk- and ERK-mediated signaling pathways across the cell membrane. In conclusion, engineered SiO2 surface topographies can modulate the cellular response of ex vivo human immune cells by imposing local plasma membrane nano-deformations.

Structure and dynamics of ESCRT-III membrane remodeling proteins by high-speed atomic force microscopy

Endosomal Sorting Complex Required for Transport (ESCRT) proteins assemble on the cytoplasmic leaflet of membranes and remodel them. ESCRT is involved in biological processes where membranes are bent away from the cytosol, constricted, and finally severed, such as in multi-vesicular body formation (in the endosomal pathway for protein sorting) or abscission during cell division. The ESCRT system is hijacked by enveloped viruses to allow buds of nascent virions to be constricted, severed and released. ESCRT-III proteins, the most downstream components of the ESCRT system, are monomeric and cytosolic in their autoinhibited conformation. They share a common architecture, a four-helix bundle with a fifth helix that interacts with this bundle to prevent polymerizing. Upon binding to negatively charged membranes, the ESCRT-III components adopt an activated state that allows them to polymerize into filaments and spirals, and to interact with the AAA-ATPase Vps4 for polymer remodeling. ESCRT-III has been studied with electron microscopy (EM) and fluorescence microscopy (FM); these methods provided invaluable information about ESCRT assembly structures or their dynamics, respectively, but neither approach provides detailed insights into both aspects simultaneously. High-speed atomic force microscopy (HS-AFM) has overcome this shortcoming, providing movies at high spatio-temporal resolution of biomolecular processes, significantly increasing our understanding of ESCRT-III structure and dynamics. Here, we review the contributions of HS-AFM in the analysis of ESCRT-III, focusing on recent developments of non-planar and deformable HS-AFM supports. We divide the HS-AFM observations into four sequential steps in the ESCRT-III lifecycle: 1) polymerization, 2) morphology, 3) dynamics, and 4) depolymerization.

The Syp1/FCHo2 protein induces septin filament bundling through its intrinsically disordered domain

The septin collar of budding yeast is an ordered array of septin filaments that serves a scaffolding function for the cytokinetic machinery at the bud neck and compartmentalizes the membrane between mother and daughter cell. How septin architecture is aided by septin-binding proteins is largely unknown. Syp1 is an endocytic protein that was implicated in the timely recruitment of septins to the newly forming collar through an unknown mechanism. Using advanced microscopy and in vitro reconstitution assays, we show that Syp1 is able to align laterally and tightly pack septin filaments, thereby forming flat bundles or sheets. This property is shared by the Syp1 mammalian counterpart FCHo2, thus emphasizing conserved protein functions. Interestingly, the septin-bundling activity of Syp1 resides mainly in its intrinsically disordered region. Our data uncover the mechanism through which Syp1 promotes septin collar assembly and offer another example of functional diversity of unstructured protein domains.

Bending over backwards: BAR proteins and the actin cytoskeleton in mammalian receptor-mediated endocytosis

The role of the actin cytoskeleton during receptor-mediated endocytosis (RME) has been well characterized in yeast for many years. Only more recently has the interplay between the actin cytoskeleton and RME been extensively explored in mammalian cells. These studies have revealed the central roles of BAR proteins in RME, and have demonstrated significant roles of BAR proteins in linking the actin cytoskeleton to this cellular process. The actin cytoskeleton generates and transmits mechanical force to promote the extension of receptor-bound endocytic vesicles into the cell. Many adaptor proteins link and regulate the actin cytoskeleton at the sites of endocytosis. This review will cover key effectors, adaptors and signalling molecules that help to facilitate the invagination of the cell membrane during receptor-mediated endocytosis, including recent insights gained on the roles of BAR proteins. The final part of this review will explore associations of alterations to genes encoding BAR proteins with cancer.

Structural basis of an endocytic checkpoint that primes the AP2 clathrin adaptor for cargo internalization

Clathrin-mediated endocytosis (CME) is the main route of internalization from the plasma membrane. It is known that the heterotetrameric AP2 clathrin adaptor must open to simultaneously engage membrane and endocytic cargo, yet it is unclear how transmembrane cargos are captured to catalyze CME. Using cryogenic-electron microscopy, we discover a new way in which mouse AP2 can reorganize to expose membrane- and cargo-binding pockets, which is not observed in clathrin-coated structures. Instead, it is stimulated by endocytic pioneer proteins called muniscins, which do not enter vesicles. Muniscin-engaged AP2 is primed to rearrange into the vesicle-competent conformation on binding the tyrosine cargo internalization motif (YxxΦ). We propose adaptor priming as a checkpoint to ensure cargo internalization.