On the existence of endocytosis driven by membrane phase separations

Small extracellular vesicles: crucial mediators for prostate cancer

Small extracellular vesicles (sEVs) play a critical role in the progression, diagnosis, and treatment of prostate cancer (PCa), particularly within the tumor microenvironment (TME). Acting as novel biomarkers and agents for targeted biological therapy, sEVs contribute significantly to improving patient survival. These vesicles transport a variety of biomolecules, including proteins, nucleic acids, and lipids, which are instrumental in remodeling the TME, facilitating intercellular communication, and influencing key processes such as tumor growth, metastasis, and therapy resistance. A thorough understanding of sEV heterogeneity, including their biogenesis, characteristics, and potential applications, is essential. Recent advances have illuminated the origins, formation processes, and molecular cargo of PCa-derived sEVs (PCa-sEVs), enhancing our understanding of their role in disease progression. Furthermore, sEVs show promise as diagnostic markers, with potential applications in early detection and prognostic assessment in PCa. Therapeutically, natural and engineered sEVs offer versatile applications, including drug delivery, gene therapy, and immunomodulation, underscoring their potential in PCa management. This review delves into the substantial potential of sEVs in clinical practices for PCa.

Massive endocytosis mechanisms are involved in uptake of HIV-1 particles by monocyte-derived dendritic cells

Introduction

HIV-1 exploits dendritic cells (DCs) to spread throughout the body via specific recognition of gangliosides present on the viral envelope by the CD169/Siglec-1 membrane receptor. This interaction triggers the internalization of HIV-1 within a structure known as the sac-like compartment. While the mechanism underlying sac-like compartment formation remains elusive, prior research indicates that the process is clathrin-independent and cell membrane cholesterol-dependent and involves transient disruption of cortical actin. Here, we investigate the potential involvement of massive endocytosis (MEND) in this process.

Methods

We used live cell confocal imaging to measure the dimensions and dynamics of the compartment. We assessed the role of actin and cholesterol in fixed and live cells using confocal microscopy and evaluated the effect of PI3K and protein palmytoilation inhibitors during viral uptake.

Results

Our data demonstrate extensive plasma membrane invagination based on sac-like compartment dimensions (2.9 μm in diameter and 20 μm3 in volume). We showed that the cholesterol concentration doubles within the regions of viral uptake, suggesting lipid-phase separation, and that development of the sac-like compartment is accompanied by transient depolarization of cortical actin. Moreover, we observed that protein palmitoylation and PI3K inhibition reduce the sac-like compartment formation rate from 70% to 20% and 40%, respectively.

Conclusions

Our results indicate the involvement of MEND mechanisms during sac-like compartment formation.

Dynamin-dependent entry of Chlamydia trachomatis is sequentially regulated by the effectors TarP and TmeA

Chlamydia invasion of epithelial cells is a pathogen-driven process involving two functionally distinct effectors - TarP and TmeA. They collaborate to promote robust actin dynamics at sites of entry. Here, we extend studies on the molecular mechanism of invasion by implicating the host GTPase dynamin 2 (Dyn2) in the completion of pathogen uptake. Importantly, Dyn2 function is modulated by TarP and TmeA at the levels of recruitment and activation through oligomerization, respectively. TarP-dependent recruitment requires phosphatidylinositol 3-kinase and the small GTPase Rac1, while TmeA has a post-recruitment role related to Dyn2 oligomerization. This is based on the rescue of invasion duration and efficiency in the absence of TmeA by the Dyn2 oligomer-stabilizing small molecule activator Ryngo 1-23. Notably, Dyn2 also regulated turnover of TarP- and TmeA-associated actin networks, with disrupted Dyn2 function resulting in aberrant turnover dynamics, thus establishing the interdependent functional relationship between Dyn2 and the effectors TarP and TmeA.

Vesicle fusion and release in neurons under dynamic mechanical equilibrium

Vesicular fusion plays a pivotal role in cellular processes, involving stages like vesicle trafficking, fusion pore formation, content release, and membrane integration or separation. This dynamic process is regulated by a complex interplay of protein assemblies, osmotic forces, and membrane tension, which together maintain a mechanical equilibrium within the cell. Changes in cellular mechanics or external pressures prompt adjustments in this equilibrium, highlighting the system's adaptability. This review delves into the synergy between intracellular proteins, structural components, and external forces in facilitating vesicular fusion and release. It also explores how cells respond to mechanical stress, maintaining equilibrium and offering insights into vesicle fusion mechanisms and the development of neurological disorders.

Microfluidic Transfection System and Temperature Strongly Influence the Efficiency of Transient Transfection

For the process of transient transfection (TTF), DNA is often transported into the cells using polyplexes. The polyplex uptake and the subsequent transient expression of the gene of interest are of great importance for a successful transfection. In this study, we investigated a 3D-printed microfluidic system designed to facilitate direct TTF for suspension of CHO-K1 cells. The results demonstrate that this system achieves significantly better results than the manual approach. Furthermore, the effect of both post-transfection incubation time (t) and temperature (T) on polyplex uptake was explored in light of the membrane phase transitions. Attention was paid to obtaining the highest possible transfection efficiency (TFE), viability (V), and viable cell concentration (VCC). Our results show that transfection output measured as product of VCC and TFE is optimal for t = 1 h at T = 22 °C. Moreover, post-transfection incubation at T = 22 °C with short periods of increased T at T = 40 °C were observed to further increase the output. Finally, we found that around T = 19 °C, the TFE increases strongly. This is the membrane phase transition T of CHO-K1 cells, and those results therefore suggest a correlation between membrane order and permeability (and in turn, TFE).

Cell-Surface Binding of DNA Nanostructures for Enhanced Intracellular and Intranuclear Delivery

DNA nanostructures (DNs) have found increasing use in biosensing, drug delivery, and therapeutics because of their customizable assembly, size and shape control, and facile functionalization. However, their limited cellular uptake and nuclear delivery have hindered their effectiveness in these applications. Here, we demonstrate the potential of applying cell-surface binding as a general strategy to enable rapid enhancement of intracellular and intranuclear delivery of DNs. By targeting the plasma membrane via cholesterol anchors or the cell-surface glycocalyx using click chemistry, we observe a significant 2 to 8-fold increase in the cellular uptake of three distinct types of DNs that include nanospheres, nanorods, and nanotiles, within a short time frame of half an hour. Several factors are found to play a critical role in modulating the uptake of DNs, including their geometries, the valency, positioning and spacing of binding moieties. Briefly, nanospheres are universally preferable for cell surface attachment and internalization. However, edge-decorated nanotiles compensate for their geometry deficiency and outperform nanospheres in both categories. In addition, we confirm the short-term structural stability of DNs by incubating them with cell medium and cell lysate. Further, we investigate the endocytic pathway of cell-surface bound DNs and reveal that it is an interdependent process involving multiple pathways, similar to those of unmodified DNs. Finally, we demonstrate that cell-surface attached DNs exhibit a substantial enhancement in the intranuclear delivery. Our findings present an application that leverages cell-surface binding to potentially overcome the limitations of low cellular uptake, which may strengthen and expand the toolbox for effective cellular and nuclear delivery of DNA nanostructure systems.

Constitutive Plasma Membrane Turnover in T-REx293 cells via Ordered Membrane Domain Endocytosis under Mitochondrial Control

Clathrin/dynamin-independent endocytosis of ordered plasma membrane domains (ordered membrane domain endocytosis, OMDE) can become massive in response to cytoplasmic Ca elevations, G protein activation by non-hydrolyzable GTP analogs, and enhanced oxidative metabolism. In patch-clamped murine bone marrow macrophages (BMMs), cytoplasmic succinate and pyruvate, but not β-hydroxybutyrate, induce OMDE of 75% of the plasma membrane within 2 min. The responses require palmitoylation of membrane proteins, being decreased by 70% in BMMs lacking the acyltransferase, DHHC5, by treatment with carnitine to shift long-chain acyl groups from cytoplasmic to mitochondrial acyl-CoAs, by bromopalmitate/albumin complexes to block DHHCs, and by the mitochondria-specific cyclosporin, NIM811, to block permeability transition pores that may release mitochondrial coenzyme A into the cytoplasm. Using T-REx293 cells, OMDE amounts to 40% with succinate, pyruvate, or GTPγS, and it is inhibited by actin cytoskeleton disruption. Pyruvate-induced OMDE is blocked by the hydrophobic antioxidant, edaravone, which prevents permeability transition pore openings. Using fluorescent 3kD dextrans to monitor endocytosis, OMDE appears to be constitutively active in T-REx293 cells but not in BMMs. After 1 h without substrates or bicarbonate, pyruvate and hydroxybutyrate inhibit constitutive OMDE, as expected for a shift of CoA from long-chain acyl-CoAs to other CoA metabolites. In the presence of bicarbonate, pyruvate strongly enhances OMDE, which is then blocked by β-hydroxybutyrate, bromopalmitate/albumin complexes, cyclosporines, or edaravone. After pyruvate responses, T-REx293 cells grow normally with no evidence for apoptosis. Fatty acid-free albumin (15 μM) inhibits basal OMDE in T-REx293 cells, as do cyclosporines, carnitine, and RhoA blockade. Surprisingly, OMDE in the absence of substrates and bicarbonate is not inhibited by siRNA knockdown of the acyltransferases, DHHC5 or DHHC2, which are required for activated OMDE in patch clamp experiments. We verify biochemically that small CoA metabolites decrease long-chain acyl-CoAs. We verify also that palmitoylations of many PM-associated proteins decrease and increase when OMDE is inhibited and stimulated, respectively, by different metabolites. STED microscopy reveals that vesicles formed during constitutive OMDE in T-REX293 cells have 90 to 130 nm diameters. In summary, OMDE is likely a major G-protein-dependent endocytic mechanism that can be constitutively active in some cell types, albeit not BMMs. OMDE depends on different DHHC acyltransferases in different circumstances and can be limited by local supplies of fatty acids, CoA, and long-chain acyl-CoAs.

Albumin Uptake and Processing by the Proximal Tubule: Physiologic, Pathologic and Therapeutic Implications

For nearly 50 years the proximal tubule (PT) has been known to reabsorb, process, and either catabolize or transcytose albumin from the glomerular filtrate. Innovative techniques and approaches have provided insights into these processes. Several genetic diseases, nonselective PT cell defects, chronic kidney disease (CKD), and acute PT injury lead to significant albuminuria, reaching nephrotic range. Albumin is also known to stimulate PT injury cascades. Thus, the mechanisms of albumin reabsorption, catabolism, and transcytosis are being reexamined with the use of techniques that allow for novel molecular and cellular discoveries. Megalin, a scavenger receptor, cubilin, amnionless, and Dab2 form a nonselective multireceptor complex that mediates albumin binding and uptake and directs proteins for lysosomal degradation after endocytosis. Albumin transcytosis is mediated by a pH-dependent binding affinity to the neonatal Fc receptor (FcRn) in the endosomal compartments. This reclamation pathway rescues albumin from urinary losses and cellular catabolism, extending its serum half-life. Albumin that has been altered by oxidation, glycation, or carbamylation or because of other bound ligands that do not bind to FcRn traffics to the lysosome. This molecular sorting mechanism reclaims physiological albumin and eliminates potentially toxic albumin. The clinical importance of PT albumin metabolism has also increased as albumin is now being used to bind therapeutic agents to extend their half-life and minimize filtration and kidney injury. The purpose of this review is to update and integrate evolving information regarding the reabsorption and processing of albumin by proximal tubule cells including discussion of genetic disorders and therapeutic considerations.

Sterols lower energetic barriers of membrane bending and fission necessary for efficient clathrin-mediated endocytosis

Clathrin-mediated endocytosis (CME) is critical for cellular signal transduction, receptor recycling, and membrane homeostasis in mammalian cells. Acute depletion of cholesterol disrupts CME, motivating analysis of CME dynamics in the context of human disorders of cholesterol metabolism. We report that inhibition of post-squalene cholesterol biosynthesis impairs CME. Imaging of membrane bending dynamics and the CME pit ultrastructure reveals prolonged clathrin pit lifetimes and shallow clathrin-coated structures, suggesting progressive impairment of curvature generation correlates with diminishing sterol abundance. Sterol structural requirements for efficient CME include 3′ polar head group and B-ring conformation, resembling the sterol structural prerequisites for tight lipid packing and polarity. Furthermore, Smith-Lemli-Opitz fibroblasts with low cholesterol abundance exhibit deficits in CME-mediated transferrin internalization. We conclude that sterols lower the energetic costs of membrane bending during pit formation and vesicular scission during CME and suggest that reduced CME activity may contribute to cellular phenotypes observed within disorders of cholesterol metabolism.

Anderson et al. demonstrate that sterol abundance and identity play a dominant role in facilitating clathrin-mediated endocytosis. Detailed analyses of clathrin-coated pits under sterol depletion support a requirement for sterol-mediated membrane bending during multiple stages of endocytosis, implicating endocytic dysfunction within the pathogenesis of disorders of cholesterol metabolism.

Mechanisms and functions of endocytosis in T cells

Once thought of primarily as a means to neutralize pathogens or to facilitate feeding, endocytosis is now known to regulate a wide range of eukaryotic cell processes. Among these are regulation of signal transduction, mitosis, lipid homeostasis, and directed migration, among others. Less well-appreciated are the roles various forms of endocytosis plays in regulating αβ and, especially, γδ T cell functions, such as T cell receptor signaling, antigen discovery by trogocytosis, and activated cell growth. Herein we examine the contribution of both clathrin-mediated and clathrin-independent mechanisms of endocytosis to T cell biology.

TMEM16F and dynamins control expansive plasma membrane reservoirs

Cells can expand their plasma membrane laterally by unfolding membrane undulations and by exocytosis. Here, we describe a third mechanism involving invaginations held shut by the membrane adapter, dynamin. Compartments open when Ca activates the lipid scramblase, TMEM16F, anionic phospholipids escape from the cytoplasmic monolayer in exchange for neutral lipids, and dynamins relax. Deletion of TMEM16F or dynamins blocks expansion, with loss of dynamin expression generating a maximally expanded basal plasma membrane state. Re-expression of dynamin2 or its GTPase-inactivated mutant, but not a lipid binding mutant, regenerates reserve compartments and rescues expansion. Dynamin2-GFP fusion proteins form punctae that rapidly dissipate from these compartments during TMEM16F activation. Newly exposed compartments extend deeply into the cytoplasm, lack numerous organellar markers, and remain closure-competent for many seconds. Without Ca, compartments open slowly when dynamins are sequestered by cytoplasmic dynamin antibodies or when scrambling is mimicked by neutralizing anionic phospholipids and supplementing neutral lipids. Activation of Ca-permeable mechanosensitive channels via cell swelling or channel agonists opens the compartments in parallel with phospholipid scrambling. Thus, dynamins and TMEM16F control large plasma membrane reserves that open in response to lateral membrane stress and Ca influx.

Liquid-liquid phase separation as a common organizing principle of intracellular space and biomembranes providing dynamic adaptive responses

This work is devoted to the phenomenon of liquid-liquid phase separation (LLPS), which has come to be recognized as fundamental organizing principle of living cells. We distinguish separation processes with different dimensions. Well-known 3D-condensation occurs in aqueous solution and leads to membraneless organelle (MLOs) formation. 2D-films may be formed near membrane surfaces and lateral phase separation (membrane rafts) occurs within the membranes themselves. LLPS may also occur on 1D structures like DNA and the cyto- and nucleoskeleton. Phase separation provides efficient transport and sorting of proteins and metabolites, accelerates the assembly of metabolic and signaling complexes, and mediates stress responses. In this work, we propose a model in which the processes of polymerization (1D structures), phase separation in membranes (2D structures), and LLPS in the volume (3D structures) influence each other. Disordered proteins and whole condensates may provide membrane raft separation or polymerization of specific proteins. On the other hand, 1D and 2D structures with special composition or embedded IDRs can nucleate condensates. We hypothesized that environmental change may trigger a LLPS which can propagate within the cell interior moving along the cytoskeleton or as an autowave. New phase propagation quickly and using a low amount of energy adjusts cell signaling and metabolic systems to new demands. Cumulatively, the interconnected phase separation phenomena in different dimensions represent a previously unexplored system of intracellular communication and regulation which cannot be ignored when considering both physiological and pathological cell processes.

Membrane partitioning and lipid selectivity of the N-terminal amphipathic H0 helices of endophilin isoforms

Endophilin is an N-BAR protein, which is characterized by a crescent-shaped BAR domain and an amphipathic helix that contributes to the membrane binding of these proteins. The exact function of that H0 helix has been a topic of debate. In mammals, there are five different endophilin isoforms, grouped into A (three members) and B (two members) subclasses, which have been described to differ in their subcellular localization and function. We asked to what extent molecular properties of the H0 helices of these members affect their membrane targeting behavior. We found that all H0 helices of the endophilin isoforms display a two-state equilibrium between disordered and α-helical states in which the helical secondary structure can be stabilized through trifluoroethanol. The helicities in high TFE were strikingly different among the H0 peptides. We investigated H0-membrane partitioning by the monitoring of secondary structure changes via CD spectroscopy. We found that the presence of anionic phospholipids is critical for all H0 helices partitioning into membranes. Membrane partitioning is found to be sensitive to variations in membrane complexity. Overall, the H0 B subfamily displays stronger membrane partitioning than the H0 A subfamily. The H0 A peptide-membrane binding occurs predominantly through electrostatic interactions. Variation among the H0 A subfamily may be attributed to slight alterations in the amino acid sequence. Meanwhile, the H0 B subfamily displays greater specificity for certain membrane compositions, and this may link H0 B peptide binding to endophilin B's cellular function.

Mechanisms and Regulation of Cardiac CaV1.2 Trafficking

During cardiac excitation contraction coupling, the arrival of an action potential at the ventricular myocardium triggers voltage-dependent L-type Ca²⁺ (Ca_V1.2) channels in individual myocytes to open briefly. The level of this Ca²⁺ influx tunes the amplitude of Ca²⁺-induced Ca²⁺ release from ryanodine receptors (RyR2) on the junctional sarcoplasmic reticulum and thus the magnitude of the elevation in intracellular Ca²⁺ concentration and ultimately the downstream contraction. The number and activity of functional Ca_V1.2 channels at the t-tubule dyads dictates the amplitude of the Ca²⁺ influx. Trafficking of these channels and their auxiliary subunits to the cell surface is thus tightly controlled and regulated to ensure adequate sarcolemmal expression to sustain this critical process. To that end, recent discoveries have revealed the existence of internal reservoirs of preformed Ca_V1.2 channels that can be rapidly mobilized to enhance sarcolemmal expression in times of acute stress when hemodynamic and metabolic demand increases. In this review, we provide an overview of the current thinking on Ca_V1.2 channel trafficking dynamics in the heart. We highlight the numerous points of control including the biosynthetic pathway, the endosomal recycling pathway, ubiquitination, and lysosomal and proteasomal degradation pathways, and discuss the effects of β-adrenergic and angiotensin receptor signaling cascades on this process.

Dynamic Palmitoylation of the Sodium-Calcium Exchanger Modulates Its Structure, Affinity for Lipid-Ordered Domains, and Inhibition by XIP

The transmembrane sodium-calcium (Na-Ca) exchanger 1 (NCX1) regulates cytoplasmic Ca levels by facilitating electrogenic exchange of Ca for Na. Palmitoylation, the only reversible post-translational modification known to modulate NCX1 activity, controls NCX1 inactivation. Here, we show that palmitoylation of NCX1 modifies the structural arrangement of the NCX1 dimer and controls its affinity for lipid-ordered membrane domains. NCX1 palmitoylation occurs dynamically at the cell surface under the control of the enzymes zDHHC5 and APT1. We identify the position of the endogenous exchange inhibitory peptide (XIP) binding site within the NCX1 regulatory intracellular loop and demonstrate that palmitoylation controls the ability of XIP to bind this site. We also show that changes in NCX1 palmitoylation change cytosolic Ca. Our results thus demonstrate the broad molecular consequences of NCX1 palmitoylation and highlight a means to manipulate the inactivation of this ubiquitous ion transporter that could ameliorate pathologies linked to Ca overload via NCX1.

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NCX1 is dynamically palmitoylated at the cell surface by zDHHC5 and APT1

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Palmitoylation modifies the NCX1 dimer’s structure and affinity for lipid rafts

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We identify the binding site of the endogenous XIP domain in NCX1’s regulatory loop

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Palmitoylation modifies NCX1 XIP affinity and hence regulates intracellular Ca

Unconventional endocytic mechanisms

Endocytosis mediates the uptake of extracellular proteins, micronutrients and transmembrane cell surface proteins. Importantly, many viruses, toxins and bacteria hijack endocytosis to infect cells. The canonical pathway is clathrin-mediated endocytosis (CME) and is active in all eukaryotic cells to support critical house-keeping functions. Unconventional mechanisms of endocytosis exit in parallel of CME, to internalize specific cargoes and support various cellular functions. These clathrin-independent endocytic (CIE) routes use three distinct mechanisms: acute signaling-induced membrane remodeling drives macropinocytosis, activity-dependent bulk endocytosis (ADBE), massive endocytosis (MEND) and EGFR non-clathrin endocytosis (EGFR-NCE). Cargo capture and local membrane deformation by cytosolic proteins is used by fast endophilin-mediated endocytosis (FEME), IL-2Rβ endocytosis and ultrafast endocytosis at synapses. Finally, the formation of endocytic pits by clustering of extracellular lipids or cargoes according to the Glycolipid-Lectin (GL-Lect) hypothesis mediates the uptake of SV40 virus, Shiga and cholera toxins, and galectin-clustered receptors by the CLIC/GEEC and the endophilin-A3-mediated CIE.

Direct coupling of oligomerization and oligomerization-driven endocytosis of the dopamine transporter to its conformational mechanics and activity

Dopamine transporter (DAT) mediates the reuptake of synaptically released dopamine, and thus controls the duration and intensity of dopamine neurotransmission. Mammalian DAT has been observed to form oligomers, although the mechanisms of oligomerization and its role in DAT activity and trafficking remain largely unknown. We discovered a series of small molecule compounds that stabilize trimers and induce high-order oligomers of DAT and concomitantly promote its clathrin-independent endocytosis. Using a combination of chemical cross-linking, fluorescence resonance energy transfer microscopy, antibody-uptake endocytosis assay, live-cell lattice light sheet microscopy, ligand binding and substrate transport kinetics analyses, and molecular modeling and simulations, we investigated molecular basis of DAT oligomerization and endocytosis induced by these compounds. Our study showed that small molecule–induced DAT oligomerization and endocytosis are favored by the inward-facing DAT conformation and involve interactions of four hydrophobic residues at the interface between transmembrane (TM) helices TM4 and TM9. Surprisingly, a corresponding quadruple DAT mutant displays altered dopamine transport kinetics and increased cocaine-analog binding. The latter is shown to originate from an increased preference for outward-facing conformation and inward-to-outward transition. Taken together, our results demonstrate a direct coupling between conformational dynamics of DAT, functional activity of the transporter, and its oligomerization leading to endocytosis. The high specificity of such coupling for DAT makes the TM4-9 hub a new target for pharmacological modulation of DAT activity and subcellular localization.

Membrane cholesterol regulates endocytosis and trafficking of the serotonin1A receptor: Insights from acute cholesterol depletion

Endocytosis and intracellular trafficking constitute important regulatory features associated with G protein-coupled receptor (GPCR) function. GPCR endocytosis involves several remodeling events at the plasma membrane orchestrated by a concerted interplay of a large number of proteins and membrane lipids. Although considerable literature exists on the protein framework underlying GPCR endocytosis, the role of membrane lipids in this process remains largely unexplored. In order to explore the role of membrane cholesterol (an essential and important lipid in higher eukaryotes) in GPCR endocytosis, we monitored the effect of acute cholesterol depletion using methyl-β-cyclodextrin (MβCD) on endocytosis and intracellular trafficking of the serotonin_1A receptor, an important neurotransmitter GPCR. Our results show that the serotonin_1A receptor exhibits agonist-induced clathrin-mediated endocytosis with a concentration-dependent inhibition in internalization with increasing concentrations of MβCD, which was restored upon cholesterol replenishment. Interestingly, subsequent to internalization under these conditions, serotonin_1A receptors were re-routed toward lysosomal degradation, instead of endosomal recycling observed under normal conditions, thereby implicating membrane cholesterol in modulation of intracellular trafficking of the receptor. This raises the possibility of a novel cholesterol-dependent role of intracellular sorting proteins in GPCR trafficking. These results differ from our previous observations on the endocytosis of the serotonin_1A receptor upon statin-induced chronic cholesterol depletion, in terms of endocytic pathway. We conclude that analysis of complex cellular trafficking events such as GPCR endocytosis under acute and chronic cholesterol depletion conditions should be carried out with caution due to fundamental differences underlying these processes.

Minimizing isotropic and deviatoric membrane energy - An unifying formation mechanism of different cellular membrane nanovesicle types

Tiny membrane-enclosed cellular fragments that can mediate interactions between cells and organisms have recently become a subject of increasing attention. In this work the mechanism of formation of cell membrane nanovesicles (CNVs) was studied experimentally and theoretically. CNVs were isolated by centrifugation and washing of blood cells and observed by optical microscopy and scanning electron microscopy. The shape of the biological membrane in the budding process, as observed in phospholipid vesicles, in erythrocytes and in CNVs, was described by an unifying model. Taking the mean curvature h and the curvature deviator d of the membrane surface as the relevant parameters, the shape and the distribution of membrane constituents were determined theoretically by minimization of membrane free energy. Considering these results and previous results on vesiculation of red blood cells it was interpreted that the budding processes may lead to formation of different types of CNVs as regards the compartment (exo/endovesicles), shape (spherical/tubular/torocytic) and composition (enriched/depleted in particular kinds of molecules). It was concluded that the specificity of pinched off nanovesicles derives from the shape of the membrane constituents and not primarily from their chemical identity, which explains evidences on great heterogeneity of isolated extracellular vesicles with respect to composition.

One of the amazing properties of a biological membrane is the ability to undergo dramatic changes of its shape. It may exhibit very high curvature and thereby enclose nano-sized compartments that pinch off from the mother membrane and become freely moving cellular nanovesicles (CNVs). CNVs externalize the pieces of the cell and make them available to other cells within the same organism or other organisms. Therefore they have been acknowledged as mediators of communication between microorganisms, plants, animals and human. Furthernore, they dwell on the border between living and non-living things. Recent findings report on heterogeneity of the size and composition of CNVs found in isolates from different biological samples. As communication between cells is involved in many physiological and patophysiological processes, it is of importance to understand the mechanisms of CNVs formation and recognize the natural laws that mainly govern them. We point to an unifying mechanism that explains stability of differently shaped and composed CNVs by taking into account that the biological membrane tends to attain the minimum of its relevant energy. Conveniently, the procedure can be described by a mathematical model which allows for transparent comparison between experimentally induced shapes of membrane-enclosed vesicular structures and numerical calculations.

Molecular mechanism of Fast Endophilin-Mediated Endocytosis

Endocytosis mediates the cellular uptake of micronutrients and cell surface proteins. Clathrin-mediated endocytosis (CME) is the housekeeping pathway in resting cells but additional Clathrin-independent endocytic (CIE) routes, including Fast Endophilin-Mediated Endocytosis (FEME), internalize specific cargoes and support diverse cellular functions. FEME is part of the Dynamin-dependent subgroup of CIE pathways. Here, we review our current understanding of the molecular mechanism of FEME. Key steps are: (i) priming, (ii) cargo selection, (iii) membrane curvature and carrier formation, (iv) membrane scission and (v) cytosolic transport. All steps are controlled by regulatory mechanisms mediated by phosphoinositides and by kinases such as Src, LRRK2, Cdk5 and GSK3β. A key feature of FEME is that it is not constitutively active but triggered upon the stimulation of selected cell surface receptors by their ligands. In resting cells, there is a priming cycle that concentrates Endophilin into clusters on discrete locations of the plasma membrane. In the absence of receptor activation, the patches quickly abort and new cycles are initiated nearby, constantly priming the plasma membrane for FEME. Upon activation, receptors are swiftly sorted into pre-existing Endophilin clusters, which then bud to form FEME carriers within 10 s. We summarize the hallmarks of FEME and the techniques and assays required to identify it. Next, we review similarities and differences with other CIE pathways and proposed cargoes that may use FEME to enter cells. Finally, we submit pending questions and future milestones and discuss the exciting perspectives that targeting FEME may boost treatments against cancer and neurodegenerative diseases.

L-Glucose: Another Path to Cancer Cells

Cancerous tumors comprise cells showing metabolic heterogeneity. Among numerous efforts to understand this property, little attention has been paid to the possibility that cancer cells take up and utilize otherwise unusable substrates as fuel. Here we discuss this issue by focusing on l-glucose, the mirror image isomer of naturally occurring d-glucose; l-glucose is an unmetabolizable sugar except in some bacteria. By combining relatively small fluorophores with l-glucose, we generated fluorescence-emitting l-glucose tracers (fLGs). To our surprise, 2-NBDLG, one of these fLGs, which we thought to be merely a control substrate for the fluorescent d-glucose tracer 2-NBDG, was specifically taken up into tumor cell aggregates (spheroids) that exhibited nuclear heterogeneity, a major cytological feature of malignancy in cancer diagnosis. Changes in mitochondrial activity were also associated with the spheroids taking up fLG. To better understand these phenomena, we review here the Warburg effect as well as key studies regarding glucose uptake. We also discuss tumor heterogeneity involving aberrant uptake of glucose and mitochondrial changes based on the data obtained by fLG. We then consider the use of fLGs as novel markers for visualization and characterization of malignant tumor cells.

Flotillins: At the Intersection of Protein S-Palmitoylation and Lipid-Mediated Signaling

Flotillin-1 and flotillin-2 are ubiquitously expressed, membrane-associated proteins involved in multifarious cellular events from cell signaling, endocytosis, and protein trafficking to gene expression. They also contribute to oncogenic signaling. Flotillins bind the cytosolic leaflet of the plasma membrane and endomembranes and, upon hetero-oligomerization, serve as scaffolds facilitating the assembly of multiprotein complexes at the membrane-cytosol interface. Additional functions unique to flotillin-1 have been discovered recently. The membrane-binding of flotillins is regulated by S-palmitoylation and N-myristoylation, hydrophobic interactions involving specific regions of the polypeptide chain and, to some extent, also by their oligomerization. All these factors endow flotillins with an ability to associate with the sphingolipid/cholesterol-rich plasma membrane domains called rafts. In this review, we focus on the critical input of lipids to the regulation of the flotillin association with rafts and thereby to their functioning. In particular, we discuss how the recent developments in the field of protein S-palmitoylation have contributed to the understanding of flotillin1/2-mediated processes, including endocytosis, and of those dependent exclusively on flotillin-1. We also emphasize that flotillins affect directly or indirectly the cellular levels of lipids involved in diverse signaling cascades, including sphingosine-1-phosphate and PI(4,5)P₂. The mutual relations between flotillins and distinct lipids are key to the regulation of their involvement in numerous cellular processes.

Multivalent nephrin-Nck interactions define a threshold for clustering and tyrosine-dependent nephrin endocytosis

Assembly of signaling molecules into micrometer-sized clusters is driven by multivalent protein-protein interactions, such as those found within the nephrin-Nck (Nck1 or Nck2) complex. Phosphorylation on multiple tyrosine residues within the tail of the nephrin transmembrane receptor induces recruitment of the cytoplasmic adaptor protein Nck, which binds via its triple SH3 domains to various effectors, leading to actin assembly. The physiological consequences of nephrin clustering are not well understood. Here, we demonstrate that nephrin phosphorylation regulates the formation of membrane clusters in podocytes. We also reveal a connection between clustering and endocytosis, which appears to be driven by threshold levels of nephrin tyrosine phosphorylation and Nck SH3 domain signaling. Finally, we expose an in vivo correlation between transient changes in nephrin tyrosine phosphorylation, nephrin localization and integrity of the glomerular filtration barrier during podocyte injury. Altogether, our results suggest that nephrin phosphorylation determines the composition of effector proteins within clusters to dynamically regulate nephrin turnover and podocyte health.