Actin polymerization promotes invagination of flat clathrin-coated lattices in mammalian cells by pushing at lattice edges

CCDC32 stabilizes clathrin-coated pits and drives their invagination

Clathrin-mediated endocytosis (CME) is essential for maintaining cellular homeostasis. Previous studies have reported more than 50 CME accessory proteins; however, the mechanism driving the invagination of clathrin-coated pits (CCPs) remains elusive. We show by quantitative live cell imaging that siRNA-mediated knockdown of CCDC32, a poorly characterized endocytic accessory protein, leads to the accumulation of unstable flat clathrin assemblies. CCDC32 interacts with the alpha-appendage domain (AD) of AP2 in vitro and with full length AP2 complexes in cells. Deletion of aa78-98 in CCDC32, corresponding to a predicted alpha-helix, abrogates AP2 binding and CCDC32's early function in CME. Furthermore, clinically observed nonsense mutations in CCDC32, which result in C-terminal truncations that lack aa78-98, are linked to the development of cardio-facio-neuro-developmental syndrome (CFNDS). Overall, our data demonstrate the function of a novel endocytic accessory protein, CCDC32, in regulating CCP stabilization and invagination, critical early stages of CME.

Elucidating Molecular Mechanisms Governing TNF-Alpha-Mediated Regulation of Amyloid Beta 42 Uptake in Blood-Brain Barrier Endothelial Cells

Cerebrovascular inflammation is prevalent in a majority of Alzheimer's patients. Inflammatory cytokines, such as tumor necrosis factor-alpha (TNF-alpha), circulating in the plasma have been shown to cause the inflammation of blood-brain barrier (BBB) endothelium lining the cerebral microvasculature. The BBB inflammation has been implicated in the increase of toxic Aβ accumulation within Alzheimer's disease (AD) brain. TNF-alpha in the peripheral circulation can aggravate the accumulation of amyloid-beta (Aβ) peptides in Alzheimer's disease brain. In the current study, we have shown that the exposure to TNF-alpha leads to an increase in Aβ42 accumulation in mice and BBB endothelial cells in vitro. Moreover, dynamic SPECT/CT imaging in wild-type (WT) mice infused with TNF-alpha increased the permeability and influx of Aβ42 into the mice brain. In addition, our results show that TNF-alpha modifies the expression of cofilin, actin, and dynamin, which are critical components for Aβ endocytosis by BBB endothelial cells. These results offer a mechanistic understanding of how TNF-alpha may promote Aβ accumulation at the BBB and the underlying interactions between inflammation and Aβ exposure that drives BBB dysfunction. Hence, a therapeutic intervention aimed at addressing cerebrovascular inflammation in Alzheimer's disease may potentially reduce Aβ induced cerebrovascular toxicity in Alzheimer's disease brain.

Significance statement

Increased levels of TNF-alpha circulating in the plasma are considered significant factors in the consequences of Aβ pathology in Alzheimer's disease, where it can promote cerebrovascular inflammation and BBB dysfunction. However, the role of TNF-alpha, in exacerbating Aβ pathology by increasing Aβ accumulation at the BBB endothelial cells remains only partially understood. In this study, we demonstrated that TNF-alpha enhances Aβ42 accumulation in the BBB endothelium by altering the expression of the BBB endocytosis machinery, specifically cofilin, actin, and dynamin. These findings are anticipated to contribute to the development of therapeutic approaches aimed at addressing elevated cytokine levels in Alzheimer's disease.

SuperResNET - single-molecule network analysis detects changes to clathrin structure induced by small-molecule inhibitors

SuperResNET is a network analysis pipeline for the analysis of point cloud data generated by single-molecule localization microscopy (SMLM). Here, we applied SuperResNET network analysis of SMLM direct stochastic optical reconstruction microscopy (dSTORM) data to determine how the clathrin endocytosis inhibitors pitstop 2, dynasore and latrunculin A (LatA) alter the morphology of clathrin-coated pits. SuperResNET analysis of HeLa and Cos7 cells identified three classes of clathrin structures: small oligomers (class I), pits and vesicles (class II), and larger clusters corresponding to fused pits or clathrin plaques (class III). Pitstop 2 and dynasore treatment induced distinct homogeneous populations of class II structures in HeLa cells, suggesting that they arrest endocytosis at different stages. Inhibition of endocytosis was not via actin depolymerization, as the actin-depolymerizing agent LatA induced large, heterogeneous clathrin structures. Ternary analysis of SuperResNET shape features presented a distinct more planar profile for blobs from pitstop 2-treated cells, which aligned with clathrin pits identified with high-resolution minimal photon fluxes (MINFLUX) microscopy, whereas control structures resembled MINFLUX clathrin vesicles. SuperResNET analysis therefore showed that pitstop 2 arrests clathrin pit maturation at early stages of pit formation, representing an approach to detect the effect of small molecules on target structures in situ in the cell from SMLM datasets.

Focal adhesions, reticular adhesions, flat clathrin lattices: what divides them, what unites them?

The majority of cells within multicellular organisms requires anchorage to their surroundings in the form of cell-cell or cell-matrix adhesions. In regards to cell-matrix adhesions, the transmembrane receptors of the integrin family have long been recognized as the central scaffold around which these adhesion complexes are built. Via their extracellular domains integrins bind extracellular matrix ligands while their intracellular tails interact with a plethora of proteins that link integrin-based adhesions to the cytoskeleton and turn them also into important signaling platforms. Depending on the specific intracellular interactome of the integrins, different types of integrin adhesion complexes have been classified. The best-studied ones are the focal adhesions, in which integrins become firmly linked to contractile actomyosin fibers, allowing force transduction. But integrins also form an integral part of adhesion structures that lack the strong actomyosin link and are enriched in endocytic proteins. These have been named reticular adhesions, flat clathrin lattices, or clathrin plaques. Initially, the different types of integrin adhesion complexes have been viewed as discrete entities with their own separate life cycles. However, in the past years it has become more and more apparent how closely intertwined they are. In fact, it was shown that they can trigger each other's biogenesis or can even directly convert into each other. Here, we describe similarities as well as differences between integrin adhesion complexes, focusing on the versatile αvβ5 integrins, and discuss the recently discovered close links and interconversion modes between the different αvβ5 integrin adhesion types.

The actin-spectrin submembrane scaffold restricts endocytosis along proximal axons

Clathrin-mediated endocytosis has characteristic features in neuronal dendrites and presynapses, but how membrane proteins are internalized along the axon shaft remains unclear. We focused on clathrin-coated structures and endocytosis along the axon initial segment (AIS) and their relationship to the periodic actin-spectrin scaffold that lines the axonal plasma membrane. A combination of super-resolution microscopy and platinum-replica electron microscopy on cultured neurons revealed that AIS clathrin-coated pits form within "clearings", circular areas devoid of actin-spectrin mesh. Actin-spectrin scaffold disorganization increased clathrin-coated pit formation. Cargo uptake and live-cell imaging showed that AIS clathrin-coated pits are particularly stable. Neuronal plasticity-inducing stimulation triggered internalization of the clathrin-coated pits through polymerization of branched actin around them. Thus, spectrin and actin regulate clathrin-coated pit formation and scission to control endocytosis at the AIS.

Mechanistic divergences of endocytic clathrin-coated vesicle formation in mammals, yeasts and plants

Clathrin-coated vesicles (CCVs), generated by clathrin-mediated endocytosis (CME), are essential eukaryotic trafficking organelles that transport extracellular and plasma membrane-bound materials into the cell. In this Review, we explore mechanisms of CME in mammals, yeasts and plants, and highlight recent advances in the characterization of endocytosis in plants. Plants separated from mammals and yeast over 1.5 billion years ago, and plant cells have distinct biophysical parameters that can influence CME, such as extreme turgor pressure. Plants can therefore provide a wider perspective on fundamental processes in eukaryotic cells. We compare key mechanisms that drive CCV formation and explore what these mechanisms might reveal about the core principles of endocytosis across the tree of life. Fascinatingly, CME in plants appears to more closely resemble that in mammalian cells than that in yeasts, despite plants being evolutionarily further from mammals than yeast. Endocytic initiation appears to be highly conserved across these three systems, requiring similar protein domains and regulatory processes. Clathrin coat proteins and their honeycomb lattice structures are also highly conserved. However, major differences are found in membrane-bending mechanisms. Unlike in mammals or yeast, plant endocytosis occurs independently of actin, highlighting that mechanistic assumptions about CME across different systems should be made with caution.

Clathrin assemblies at a glance

Clathrin assembles into honeycomb-like lattices at the plasma membrane but also on internal membranes, such as at the Golgi and tubular endosomes. Clathrin assemblies primarily regulate the intracellular trafficking of different cargoes, but clathrin also has non-endocytic functions in cell adhesion through interactions with specific integrins, contributes to intraluminal vesicle formation by forming flat bilayered coats on endosomes and even assembles on kinetochore k-fibers during mitosis. In this Cell Science at a Glance article and the accompanying poster, we review our current knowledge on the different types of canonical and non-canonical membrane-associated clathrin assemblies in mammalian cells, as observed by thin-section or platinum replica electron microscopy in various cell types, and discuss how the structural plasticity of clathrin contributes to its functional diversity.

Dexamethasone targets actin cytoskeleton signaling and inflammatory mediators to reverse sulfur mustard-induced toxicity in rabbit corneas

PURPOSE

Sulfur mustard (SM), a bi-functional alkylating agent, was used during World War I and the Iran-Iraq war. SM toxicity is ten times higher in eyes than in other tissues. Cornea is exceptionally susceptible to SM-injuries due to its anterior positioning and mucous-aqueous interphase. Ocular SM exposure induces blepharitis, photosensitivity, dry eye, epithelial defects, limbal ischemia and stem cell deficiency, and mustard gas keratopathy leading to temporary or permanent vision impairments. We demonstrated that dexamethasone (Dex) is a potent therapeutic intervention against SM-induced corneal injuries; however, its mechanism of action is not well known. Investigations employing proteomic profiling (LC-MS/MS) to understand molecular mechanisms behind SM-induced corneal injury and Dex efficacy were performed in the rabbit cornea exposed to SM and then received Dex treatment. PEAKS studio was used to extract, search, and summarize peptide identity. Ingenuity Pathway Analysis was used for pathway identification. Validation was performed using immunofluorescence. One-Way ANOVA (FDR < 0.05; p < 0.005) and Student's t-test (p < 0.05) were utilized for analyzing proteomics and IF data, respectively. Proteomic analysis revealed that SM-exposure upregulated tissue repair pathways, particularly actin cytoskeleton signaling and inflammation. Prominently dysregulated proteins included lipocalin2, coronin1A, actin-related protein2, actin-related protein2/3 complex subunit2, actin-related protein2/3 complex subunit4, cell division cycle42, ezrin, bradykinin/kininogen1, moesin, and profilin. Upregulated actin cytoskeleton signaling increases F-actin formation, dysregulating cell shape and motility. Dex reversed SM-induced increases in the aforementioned proteins levels to near control expression profiles. Dex aids corneal wound healing and improves corneal integrity via actin cytoskeletal signaling and anti-inflammatory effects following SM-induced injuries.

Fibrous topology promoted pBMP2-activated matrix on titanium implants boost osseointegration

Titanium (Ti) implants have been extensively used after surgical operations. Its surface bioactivity is of importance to facilitate integration with surrounding bone tissue, and ultimately ensure stability and long-term functionality of the implant. The plasmid DNA-activated matrix (DAM) coating on the surface could benefit osseointegration but is still trapped by poor transfection for further application, especially on the bone marrow mesenchymal stem cells (BMSCs) in vivo practical conditions. Herein, we constructed a DAM on the surface of fibrous-grained titanium (FG Ti) composed of phase-transition lysozyme (P) as adhesive, cationic arginine-rich lipid (RLS) as the transfection agent and plasmid DNA (pDNA) for bone morphology protein 2 (BMP2) expression. The cationic lipid RLS improved up to 30-fold higher transfection than that of commercial reagents (Lipofectamine 2000 and polyethyleneimine) on MSC. And importantly, Ti surface topology not only promotes the DAM to achieve high transfection efficiency (∼75.7% positive cells) on MSC due to the favorable combination but also reserves its contact induction effect for osteoblasts. Upon further exploration, the fibrous topology on FG Ti could boost pDNA uptake for gene transfection, and cell migration in MSC through cytoskeleton remodeling and induce contact guidance for enhanced osteointegration. At the same time, the cationic RLS together with adhesive P were both antibacterial, showing up to 90% inhibition rate against Escherichia coli and Staphylococcus aureus with reduced adherent microorganisms and disrupted bacteria. Finally, the FG Ti-P/pBMP2 implant achieved accelerated bone healing capacities through highly efficient gene delivery, aligned surface topological structure and increased antimicrobial properties in a rat femoral condylar defect model.

The amount of Nck rather than N-WASP correlates with the rate of actin-based motility of Vaccinia virus

IMPORTANCE

Vaccinia virus is a large double-stranded DNA virus and a close relative of Mpox and Variola virus, the causative agent of smallpox. During infection, Vaccinia hijacks its host's transport systems and promotes its spread into neighboring cells by recruiting a signaling network that stimulates actin polymerization. Over the years, Vaccinia has provided a powerful model to understand how signaling networks regulate actin polymerization. Nevertheless, we still lack important quantitative information about the system, including the precise number of viral and host molecules required to induce actin polymerization. Using quantitative fluorescence microscopy techniques, we have determined the number of viral and host signaling proteins accumulating on virions during their egress. Our analysis has uncovered two unexpected new aspects of this process: the number of viral proteins in the virion is not fixed and the velocity of virus movement depends on the level of a single adaptor within the signaling network.

Polymerization force-regulated actin filament-Arp2/3 complex interaction dominates self-adaptive cell migrations

Cells migrate by adapting their leading-edge behaviors to heterogeneous extracellular microenvironments (ECMs) during cancer invasions and immune responses. Yet it remains poorly understood how such complicated dynamic behaviors emerge from millisecond-scale assembling activities of protein molecules, which are hard to probe experimentally. To address this gap, we establish a spatiotemporal "resistance-adaptive propulsion" theory based on the interactions between Arp2/3 complexes and polymerizing actin filaments and a multiscale dynamic modeling system spanning from molecular proteins to the cell. We quantitatively find that cells can accurately self-adapt propulsive forces to overcome heterogeneous ECMs via a resistance-triggered positive feedback mechanism, dominated by polymerization-induced actin filament bending and the bending-regulated actin-Arp2/3 binding. However, for high resistance regions, resistance triggers a negative feedback, hindering branched filament assembly, which adapts cellular morphologies to circumnavigate the obstacles. Strikingly, the synergy of the two opposite feedbacks not only empowers the cell with both powerful and flexible migratory capabilities to deal with complex ECMs but also enables efficient utilization of intracellular proteins by the cell. In addition, we identify that the nature of cell migration velocity depending on ECM history stems from the inherent temporal hysteresis of cytoskeleton remodeling. We also show that directional cell migration is dictated by the competition between the local stiffness of ECMs and the local polymerizing rate of actin network caused by chemotactic cues. Our results reveal that it is the polymerization force-regulated actin filament-Arp2/3 complex binding interaction that dominates self-adaptive cell migrations in complex ECMs, and we provide a predictive theory and a spatiotemporal multiscale modeling system at the protein level.

Self-assembly of CIP4 drives actin-mediated asymmetric pit-closing in clathrin-mediated endocytosis

Clathrin-mediated endocytosis is pivotal to signal transduction pathways between the extracellular environment and the intracellular space. Evidence from live-cell imaging and super-resolution microscopy of mammalian cells suggests an asymmetric distribution of actin fibres near the clathrin-coated pit, which induces asymmetric pit-closing rather than radial constriction. However, detailed molecular mechanisms of this 'asymmetricity' remain elusive. Herein, we used high-speed atomic force microscopy to demonstrate that CIP4, a multi-domain protein with a classic F-BAR domain and intrinsically disordered regions, is necessary for asymmetric pit-closing. Strong self-assembly of CIP4 via intrinsically disordered regions, together with stereospecific interactions with the curved membrane and actin-regulating proteins, generates a small actin-rich environment near the pit, which deforms the membrane and closes the pit. Our results provide mechanistic insights into how disordered and structured domain collaboration promotes spatio-temporal actin polymerisation near the plasma membrane.

Exosomal transmission of viruses, a two-edged biological sword

As a common belief, most viruses can egress from the host cells as single particles and transmit to uninfected cells. Emerging data have revealed en bloc viral transmission as lipid bilayer-cloaked particles via extracellular vesicles especially exosomes (Exo). The supporting membrane can be originated from multivesicular bodies during intra-luminal vesicle formation and autophagic response. Exo are nano-sized particles, ranging from 40-200 nm, with the ability to harbor several types of signaling molecules from donor to acceptor cells in a paracrine manner, resulting in the modulation of specific signaling reactions in target cells. The phenomenon of Exo biogenesis consists of multiple and complex biological steps with the participation of diverse constituents and molecular pathways. Due to similarities between Exo biogenesis and virus replication and the existence of shared pathways, it is thought that viruses can hijack the Exo biogenesis machinery to spread and evade immune cells. To this end, Exo can transmit complete virions (as single units or aggregates), separate viral components, and naked genetic materials. The current review article aims to scrutinize challenges and opportunities related to the exosomal delivery of viruses in terms of viral infections and public health. Video Abstract.