Endocytic trafficking of membrane-bound cargo: a flotillin point of view

Targeting ABCD1-ACOX1-MET/IGF1R axis suppresses multiple myeloma

Multiple myeloma (MM) remains an incurable hematological malignancy that necessitates the identification of novel therapeutic strategies. Here, we report that intracellular levels of very long chain fatty acids (VLCFAs) control the cytotoxicity of MM chemotherapeutic agents. Inhibition of VLCFA biosynthesis reduced cell death in MM cells caused by the proteasome inhibitor, bortezomib. Conversely, inhibition of VLCFA degradation via suppression of peroxisomal acyl-CoA oxidase 1 (ACOX1) increased the cytotoxicity of bortezomib, its next-generation analog, carfilzomib, and the immunomodulatory agent lenalidomide. Furthermore, treatment with an orally available ACOX1 inhibitor cooperated with bortezomib in suppressing the growth of bortezomib-resistant MM xenografts in mice. Increased VLCFA levels caused by genetic or pharmacological inhibition of VLCFA degradation reduced the activity of two major kinases involved in MM pathogenesis, MET proto-oncogene (MET) and insulin-like growth factor 1 receptor (IGF1R). Mechanistically, inhibition of ACOX1 promoted the accumulation of VLCFA-containing cerebrosides, altered MET and IGF1R interaction with a cerebroside analog, and selectively inhibited the association of these kinases with the plasma membrane signaling platforms, importantly, without disrupting the platforms' integrity. Our study revealed a specific metabolic vulnerability of MM cells and identified a targetable axis linking VLCFA metabolism to the regulation of MET and IGF1R activity.

SNX19 Interacts with Caveolin-1 and Flotillin-1 to Regulate D1R Endocytosis and Signaling

Background: Sorting nexin 19 (SNX19) is important in the localization and trafficking of the dopamine D1 receptor (D1R) to lipid raft microdomains. However, the interaction between SNX19 and the lipid raft components caveolin-1 or flotillin-1 and, in particular, their roles in the cellular endocytosis and cell membrane trafficking of the D1R have not been determined. Methods: Caveolin-1 and flotillin-1 motifs were analyzed by in silico analysis; colocalization was observed by confocal immunofluorescence microscopy; protein-protein interaction was determined by co-immunoprecipitation. Results: In silico analysis revealed the presence of putative caveolin-1 and flotillin-1 binding motifs within SNX19. In mouse and human renal proximal tubule cells (RPTCs), SNX19 was localized mainly in lipid rafts. In mouse RPTCs transfected with wild-type (WT) Snx19, fenoldopam (FEN), a D1-like receptor agonist, increased the colocalization of SNX19 with caveolin-1 and flotillin-1. FEN also increased the co-immunoprecipitation of SNX19 with caveolin-1 and flotillin-1, effects that were prevented by SCH39166, a D1-like receptor antagonist. The FEN-mediated increase in the residence of SNX19 in lipid rafts and the colocalization of the D1R with caveolin-1 and flotilin-1 were attenuated by the deletion of a caveolin-1 (YHTVNRRYREF) (ΔCav1) or a flotillin-1 (EEGPGTETETGLPVS) (ΔFlot1) binding motif. The FEN-mediated increase in intracellular cAMP production was also impaired by the deletion of either the flotillin-1 or caveolin-1 binding motif. Nocodazole, a microtubule depolymerization inhibitor, interfered with the FEN-mediated increase in the colocalization between SNX19 and D1R. Conclusion: SNX19 contains caveolin-1 and flotillin-1 binding motifs, which play an important role in D1R endocytosis and signaling.

RNAs in tumour-derived extracellular vesicles and their significance in the tumour microenvironment

Small extracellular vesicles (sEVs) secreted by various types of cells serve as crucial mediators of intercellular communication within the complex tumour microenvironment (TME). Tumour-derived small extracellular vesicles (TDEs) are massively produced and released by tumour cells, recapitulating the specificity of their cell of origin. TDEs encapsulate a variety of RNA species, especially messenger RNAs, microRNAs, long non-coding RNAs, and circular RNAs, which release to the TME plays multifaced roles in cancer progression through mediating cell proliferation, invasion, angiogenesis, and immune evasion. sEVs act as natural delivery vehicles of RNAs and can serve as useful targets for cancer therapy. This review article provides an overview of recent studies on TDEs and their RNA cargo, with emphasis on the role of these RNAs in carcinogenesis.

Extracellular vesicle isolation methods identify distinct HIV-1 particles released from chronically infected T-cells

The current study analyzed the intersecting biophysical, biochemical, and functional properties of extracellular particles (EPs) with the human immunodeficiency virus type-1 (HIV-1) beyond the currently accepted size range for HIV-1. We isolated five fractions (Frac-A through Frac-E) from HIV-infected cells by sequential differential ultracentrifugation (DUC). All fractions showed a heterogeneous size distribution with median particle sizes greater than 100 nm for Frac-A through Frac-D but not for Frac-E, which contained small EPs with an average size well below 50 nm. Synchronized and released cultures contained large infectious EPs in Frac-A, with markers of amphisomes and viral components. Additionally, Frac-E uniquely contained EPs positive for CD63, HSP70, and HIV-1 proteins. Despite its small average size, Frac-E contained membrane-protected viral integrase, detectable only after SDS treatment, indicating that it is enclosed in vesicles. Single particle analysis with dSTORM further supported these findings as CD63, HIV-1 integrase, and the viral surface envelope (Env) glycoprotein (gp) colocalized on the same Frac-E particles. Surprisingly, Frac-E EPs were infectious, and infectivity was significantly reduced by immunodepleting Frac-E with anti-CD63, indicating the presence of this protein on the surface of infectious small EPs in Frac-E. To our knowledge, this is the first time that extracellular vesicle (EV) isolation methods have identified infectious small HIV-1 particles (smHIV-1) that are under 50 nm. Collectively, our data indicate that the crossroads between EPs and HIV-1 potentially extend beyond the currently accepted biophysical properties of HIV-1, which may have further implications for viral pathogenesis.

Low Density Lipoprotein Cholesterol Decreases the Expression of Adenosine A2A Receptor and Lipid Rafts-Protein Flotillin-1: Insights on Cardiovascular Risk of Hypercholesterolemia

High blood levels of low-density lipoprotein (LDL)-cholesterol (LDL-C) are associated with atherosclerosis, mainly by promoting foam cell accumulation in vessels. As cholesterol is an essential component of cell plasma membranes and a regulator of several signaling pathways, LDL-C excess may have wider cardiovascular toxicity. We examined, in untreated hypercholesterolemia (HC) patients, selected regardless of the cause of LDL-C accumulation, and in healthy participants (HP), the expression of the adenosine A2A receptor (A2AR), an anti-inflammatory and vasodilatory protein with cholesterol-dependent modulation, and Flotillin-1, protein marker of cholesterol-enriched plasma membrane domains. Blood cardiovascular risk and inflammatory biomarkers were measured. A2AR and Flotillin-1 expression in peripheral blood mononuclear cells (PBMC) was lower in patients compared to HP and negatively correlated to LDL-C blood levels. No other differences were observed between the two groups apart from transferrin and ferritin concentrations. A2AR and Flotillin-1 proteins levels were positively correlated in the whole study population. Incubation of HP PBMCs with LDL-C caused a similar reduction in A2AR and Flotillin-1 expression. We suggest that LDL-C affects A2AR expression by impacting cholesterol-enriched membrane microdomains. Our results provide new insights into the molecular mechanisms underlying cholesterol toxicity, and may have important clinical implication for assessment and treatment of cardiovascular risk in HC.

Acylation of MLKL Impacts Its Function in Necroptosis

Mixed lineage kinase domain-like (MLKL) is a key signaling protein of necroptosis. Upon activation by phosphorylation, MLKL translocates to the plasma membrane and induces membrane permeabilization, which contributes to the necroptosis-associated inflammation. Membrane binding of MLKL is initially initiated by electrostatic interactions between the protein and membrane phospholipids. We previously showed that MLKL and its phosphorylated form (pMLKL) are S-acylated during necroptosis. Here, we characterize the acylation sites of MLKL and identify multiple cysteines that can undergo acylation with an interesting promiscuity at play. Our results show that MLKL and pMLKL undergo acylation at a single cysteine, with C184, C269, and C286 as possible acylation sites. Using all-atom molecular dynamic simulations, we identify differences that the acylation of MLKL causes at the protein and membrane levels. Through investigations of the S-palmitoyltransferases that might acylate pMLKL in necroptosis, we showed that zDHHC21 activity has the strongest effect on pMLKL acylation, inactivation of which profoundly reduced the pMLKL levels in cells and improved membrane integrity. These results suggest that blocking the acylation of pMLKL destabilizes the protein at the membrane interface and causes its degradation, ameliorating the necroptotic activity. At a broader level, our findings shed light on the effect of S-acylation on MLKL functioning in necroptosis and MLKL-membrane interactions mediated by its acylation.

Biogenesis and delivery of extracellular vesicles: harnessing the power of EVs for diagnostics and therapeutics

Extracellular vesicles (EVs) are membrane-enclosed particles secreted by a variety of cell types. These vesicles encapsulate a diverse range of molecules, including proteins, nucleic acids, lipids, metabolites, and even organelles derived from their parental cells. While EVs have emerged as crucial mediators of intercellular communication, they also hold immense potential as both biomarkers and therapeutic agents for numerous diseases. A thorough understanding of EV biogenesis is crucial for the development of EV-based diagnostic developments since the composition of EVs can reflect the health and disease status of the donor cell. Moreover, when EVs are taken up by target cells, they can exert profound effects on gene expression, signaling pathways, and cellular behavior, which makes these biomolecules enticing targets for therapeutic interventions. Yet, despite decades of research, the intricate processes underlying EV biogenesis by donor cells and subsequent uptake by recipient cells remain poorly understood. In this review, we aim to summarize current insights and advancements in the biogenesis and uptake mechanisms of EVs. By shedding light on the fundamental mechanisms governing EV biogenesis and delivery, this review underscores the potential of basic mechanistic research to pave the way for developing novel diagnostic strategies and therapeutic applications.

Human brain microvascular endothelial cells release different types of P-glycoprotein-containing extracellular vesicles upon exposure to doxorubicin

In the brain, the efflux transporter P-glycoprotein (Pgp) is predominantly located on the luminal membrane of microvascular endothelial cells (BMECs) that form the blood-brain barrier. In addition, Pgp is localized in intracellular organelles involved in Pgp traffic and cycling and, by the release of extracellular vesicles (EVs), in intercellular Pgp transfer to cells with low Pgp expression. We recently described that drug exposure of a human BMEC line (hCMEC/D3) induces the release of Pgp-EGFP-containing EVs; however, the nature of the Pgp-enriched vesicles was not characterized. The two main categories of EVs are exosomes and microvesicles, which differ in origin, size, and molecular cargo. In the present study, we performed similar experiments with hCMEC/D3 cells in the absence and presence of doxorubicin and isolated and characterized the EVs released by the cells during the experiments by differential ultracentrifugation with/without subsequent sucrose gradient fractionation of EV pellets, proteomic profiling, EV size analysis, and confocal fluorescence microscopy. Using cocultures of hCMEC/D3 wildtype cells and cells transduced with MDR1-EGFP or monocultures of hCMEC/D3-MDR1-EGFP cells, we found release of both Pgp-enriched exosomes and microvesicles but analysis of the exosomal marker protein Rab7 indicated that doxorubicin increased particularly the release of exosomes. Transfer experiments with isolated EVs demonstrated EV endocytosis by recipient cells. EV release from BMECs in response to anticancer drugs such as doxorubicin likely serves different functions, including non-genetic intercellular transfer of a resistance phenotype to neighboring BMECs and a mechanism of drug extrusion that contributes to brain protection against potentially toxic chemotherapeutic drugs.

Acylation of MLKL impacts its function in necroptosis

Mixed lineage kinase domain-like (MLKL) is a key signaling protein of necroptosis. Upon activation by phosphorylation, MLKL translocates to the plasma membrane and induces membrane permeabilization which contributes to the necroptosis-associated inflammation. Membrane binding of MLKL is initially initiated by the electrostatic interactions between the protein and membrane phospholipids. We previously showed that MLKL and its phosphorylated form (pMLKL) are S-acylated during necroptosis. Here, we characterize acylation sites of MLKL and identify multiple cysteines that can undergo acylation with an interesting promiscuity at play. Our results show that MLKL and pMLKL undergo acylation at a single cysteine, C184, C269 and C286 are the possible acylation sites. Using all atom molecular dynamic simulations, we identify differences that the acylation of MLKL causes at the protein and membrane level. Through systematic investigations of the S-palmitoyltransferases that might acylate MLKL in necroptosis, we showed that zDHHC21 activity has the strongest effect on pMLKL acylation, inactivation of which profoundly reduced the pMLKL levels in cells and improved membrane integrity. These results suggest that blocking the acylation of pMLKL destabilizes the protein at the membrane interface and causes its degradation, ameliorating necroptotic activity. At a broader level, our findings shed light on the effect of S-acylation on MLKL functioning in necroptosis and MLKL-membrane interactions mediated by its acylation.

Hexa-Histidine, a Peptide with Versatile Applications in the Study of Amyloid-β(1-42) Molecular Mechanisms of Action

Amyloid β (Aβ) oligomers are the most neurotoxic forms of Aβ, and Aβ(1-42) is the prevalent Aβ peptide found in the amyloid plaques of Alzheimer's disease patients. Aβ(25-35) is the shortest peptide that retains the toxicity of Aβ(1-42). Aβ oligomers bind to calmodulin (CaM) and calbindin-D28k with dissociation constants in the nanomolar Aβ(1-42) concentration range. Aβ and histidine-rich proteins have a high affinity for transition metal ions Cu2+, Fe3+ and Zn2+. In this work, we show that the fluorescence of Aβ(1-42) HiLyteTM-Fluor555 can be used to monitor hexa-histidine peptide (His6) interaction with Aβ(1-42). The formation of His6/Aβ(1-42) complexes is also supported by docking results yielded by the MDockPeP Server. Also, we found that micromolar concentrations of His6 block the increase in the fluorescence of Aβ(1-42) HiLyteTM-Fluor555 produced by its interaction with the proteins CaM and calbindin-D28k. In addition, we found that the His6-tag provides a high-affinity site for the binding of Aβ(1-42) and Aβ(25-35) peptides to the human recombinant cytochrome b5 reductase, and sensitizes this enzyme to inhibition by these peptides. In conclusion, our results suggest that a His6-tag could provide a valuable new tool to experimentally direct the action of neurotoxic Aβ peptides toward selected cellular targets.

Proteomic Investigation of Differential Interactomes of Glypican 1 and a Putative Disease-Modifying Variant of Ataxia

In a currently 13-year-old girl of consanguineous Turkish parents, who developed unsteady gait and polyneuropathy at the ages of 3 and 6 years, respectively, we performed whole genome sequencing and identified a biallelic missense variant c.424C>T, p.R142W in glypican 1 (GPC1) as a putative disease-associated variant. Up to date, GPC1 has not been associated with a neuromuscular disorder, and we hypothesized that this variant, predicted as deleterious, may be causative for the disease. Using mass spectrometry-based proteomics, we investigated the interactome of GPC1 WT and the missense variant. We identified 198 proteins interacting with GPC1, of which 16 were altered for the missense variant. This included CANX as well as vacuolar ATPase (V-ATPase) and the mammalian target of rapamycin complex 1 (mTORC1) complex members, whose dysregulation could have a potential impact on disease severity in the patient. Importantly, these proteins are novel interaction partners of GPC1. At 10.5 years, the patient developed dilated cardiomyopathy and kyphoscoliosis, and Friedreich's ataxia (FRDA) was suspected. Given the unusually severe phenotype in a patient with FRDA carrying only 104 biallelic GAA repeat expansions in FXN, we currently speculate that disturbed GPC1 function may have exacerbated the disease phenotype. LC-MS/MS data are accessible in the ProteomeXchange Consortium (PXD040023).

Upregulation of endocytic protein expression in the Alzheimer's disease male human brain

Amyloid-beta (Aβ) is produced from amyloid precursor protein (APP) primarily after APP is internalised by endocytosis and clathrin-mediated endocytic processes are altered in Alzheimer's disease (AD). There is also evidence that cholesterol and flotillin affect APP endocytosis. We hypothesised that endocytic protein expression would be altered in the brains of people with AD compared to non-diseased subjects which could be linked to increased Aβ generation. We compared protein expression in frontal cortex samples from men with AD compared to age-matched, non-diseased controls. Soluble and insoluble Aβ40 and Aβ42, the soluble Aβ42/Aβ40 ratio, βCTF, BACE1, presenilin-1 and the ratio of phosphorylated:total GSK3β were significantly increased while the insoluble Aβ42:Aβ40 ratio was significantly decreased in AD brains. Total and phosphorylated tau were markedly increased in AD brains. Significant increases in clathrin, AP2, PICALM isoform 4, Rab-5 and caveolin-1 and 2 were seen in AD brains but BIN1 was decreased. However, using immunohistochemistry, caveolin-1 and 2 were decreased. The results obtained here suggest an overall increase in endocytosis in the AD brain, explaining, at least in part, the increased production of Aβ during AD.

ELMOD3-Rab1A-Flotillin2 cascade regulates lumen formation via vesicle trafficking in Ciona notochord

Lumen development is a crucial phase in tubulogenesis, although its molecular mechanisms are largely unknown. In this study, we discovered an ELMO domain-containing 3 (ELMOD3), which belongs to ADP-ribosylation factor GTPase-activating protein family, was necessary to form the notochord lumen in Ciona larvae. We demonstrated that ELMOD3 interacted with lipid raft protein Flotillin2 and regulated its subcellular localization. The loss-of-function of Flotillin2 prevented notochord lumen formation. Furthermore, we found that ELMOD3 also interacted with Rab1A, which is the regulatory GTPase for vesicle trafficking and located at the notochord cell surface. Rab1A mutations arrested the lumen formation, phenocopying the loss-of-function of ELMOD3 and Flotillin2. Our findings further suggested that Rab1A interactions influenced Flotillin2 localization. We thus identified a unique pathway in which ELMOD3 interacted with Rab1A, which controlled the Flotillin2-mediated vesicle trafficking from cytoplasm to apical membrane, required for Ciona notochord lumen formation.

Insight into the role of clathrin-mediated endocytosis inhibitors in SARS-CoV-2 infection

Emergence of SARS-CoV-2 variants warrants sustainable efforts to upgrade both the diagnostic and therapeutic protocols. Understanding the details of cellular and molecular basis of the virus-host cell interaction is essential for developing variant-independent therapeutic options. The internalization of SARS-CoV-2, into lung epithelial cells, is mediated by endocytosis, especially clathrin-mediated endocytosis (CME). Although vaccination is the gold standard strategy against viral infection, selective inhibition of endocytic proteins, complexes, and associated adaptor proteins may present a variant-independent therapeutic strategy. Although clathrin and/or dynamins are the most important proteins involved in CME, other endocytic mechanisms are clathrin and/or dynamin independent and rely on other proteins. Moreover, endocytosis implicates some subcellular structures, like plasma membrane, actin and lysosomes. Also, physiological conditions, such as pH and ion concentrations, represent an additional factor that mediates these events. Accordingly, endocytosis related proteins are potential targets for small molecules that inhibit endocytosis-mediated viral entry. This review summarizes the potential of using small molecules, targeting key proteins, participating in clathrin-dependent and -independent endocytosis, as variant-independent antiviral drugs against SARS-CoV-2 infection. The review takes two approaches. The first outlines the potential role of endocytic inhibitors in preventing endocytosis-mediated viral entry and its mechanism of action, whereas in the second computational analysis was implemented to investigate the selectivity of common inhibitors against endocytic proteins in SARS-CoV-2 endocytosis. The analysis revealed that remdesivir, methyl-β-cyclodextrin, rottlerin, and Bis-T can effectively inhibit clathrin, HMG-CoA reductase, actin, and dynamin I GTPase and are more potent in inhibiting SARS-CoV-2 than chloroquine. CME inhibitors for SARS-CoV-2 infection remain understudied.

Cross-linking of the endolysosomal system reveals potential flotillin structures and cargo

Lysosomes are well-established as the main cellular organelles for the degradation of macromolecules and emerging as regulatory centers of metabolism. They are of crucial importance for cellular homeostasis, which is exemplified by a plethora of disorders related to alterations in lysosomal function. In this context, protein complexes play a decisive role, regulating not only metabolic lysosomal processes but also lysosome biogenesis, transport, and interaction with other organelles. Using cross-linking mass spectrometry, we analyze lysosomes and early endosomes. Based on the identification of 5376 cross-links, we investigate protein-protein interactions and structures of lysosome- and endosome-related proteins. In particular, we present evidence for a tetrameric assembly of the lysosomal hydrolase PPT1 and a heterodimeric structure of FLOT1/FLOT2 at lysosomes and early endosomes. For FLOT1-/FLOT2-positive early endosomes, we identify >300 putative cargo proteins and confirm eleven substrates for flotillin-dependent endocytosis, including the latrophilin family of adhesion G protein-coupled receptors.

The new general biological property of stem-like tumor cells Part I. Peculiarities of the process of the double-stranded DNA fragments internalization into stem-like tumor cells

Stem-like tumor cells of ascites carcinoma Krebs-2 and Epstein-Barr virus-induced B-lymphoma were shown to possess the innate capability of binding and internalizing the TAMRA-labeled double-stranded DNA (dsDNA) probe. The process of binding and internalizing is rather complicated and composed of the following successive stages: 1) initiating electrostatic interaction and contact of a negatively charged dsDNA molecule with a positively charged molecule(s) on the surface of a stem-like tumor cell; 2) binding of the dsDNA probe to a tumor stem cell surface protein(s) via the formation of a strong chemical/molecular bond; and 3) the very internalization of dsDNA into the cell. Binding of DNA to cell surface proteins is determined by the presence of heparin/polyanion-binding sites within the protein structure, which can be competitively blocked by heparin and/or dextran sulfate, wherein heparin blocks only the binding, while dextran sulfate abrogates both binding and internalization. The abrogation of internalization by dextran sulfate implies the role of scavenger receptors in this process. Cells were shown to uptake DNA in amounts constituting ∼0.008% of the haploid genome. Inhibitors of caveolae-dependent internalization abrogate the DNA uptake in Krebs-2 cells, and inhibitors of the clathrin/caveolar mechanism block the internalization in B-lymphoma cells. In the present report, it is shown for the first time that in contrast to the majority of committed tumor cells, stem-like tumor cells of Krebs-2 and B-lymphoma carry a general positive charge on their surface.

Selective endocytosis controls slit diaphragm maintenance and dynamics in Drosophila nephrocytes

The kidneys generate about 180 l of primary urine per day by filtration of plasma. An essential part of the filtration barrier is the slit diaphragm, a multiprotein complex containing nephrin as major component. Filter dysfunction typically manifests with proteinuria and mutations in endocytosis regulating genes were discovered as causes of proteinuria. However, it is unclear how endocytosis regulates the slit diaphragm and how the filtration barrier is maintained without either protein leakage or filter clogging. Here, we study nephrin dynamics in podocyte-like nephrocytes of Drosophila and show that selective endocytosis either by dynamin- or flotillin-mediated pathways regulates a stable yet highly dynamic architecture. Short-term manipulation of endocytic functions indicates that dynamin-mediated endocytosis of ectopic nephrin restricts slit diaphragm formation spatially while flotillin-mediated turnover of nephrin within the slit diaphragm is needed to maintain filter permeability by shedding of molecules bound to nephrin in endosomes. Since slit diaphragms cannot be studied in vitro and are poorly accessible in mouse models, this is the first analysis of their dynamics within the slit diaphragm multiprotein complex. Identification of the mechanisms of slit diaphragm maintenance will help to develop novel therapies for proteinuric renal diseases that are frequently limited to symptomatic treatment.

Comparing extracellular vesicles and cell membranes as biocompatible coatings for gold nanorods: Implications for targeted theranostics

Extracellular vesicles (EVs) and cell membrane nanoghosts are excellent coatings for nanomaterials, providing enhanced delivery in the target sites and evasion of the immune system. These cell-derived coatings allow the exploration of the delivery properties of the nanoparticles without stimulation of the immune system. Despite the advances reported on the use of EVs and cell-membrane coatings for nanomedicine applications, there are no standards to compare the benefits and main differences between these technologies. Here we investigated macrophage-derived EVs and cell membranes-coated gold nanorods and compared both systems in terms of target delivery in cancer and stromal cells. Our results reveal a higher tendency of EV-coated nanorods to interact with macrophages yet both EV and cell membrane-coated nanorods were internalized in the metastatic breast cancer cells. The main differences between these nanoparticles are related to the presence or absence of CD47 in the coating material, not usually addressed in EVs characterization. Our findings highlight important delivery differences exhibited by EVs- or cell membranes- coated nanorods which understanding may be important to the design and development of theragnostic nanomaterials using these coatings for target delivery.

The differential role of the lipid raft-associated protein flotillin 2 for progression of myeloid leukemia

Lipid raft-associated proteins play a vital role in membrane-mediated processes. The lipid microdomain-associated protein flotillin 2 (FLOT2), which has a scaffolding function, is involved in polarization, as well as in actin cytoskeletal organization of primitive and mature hematopoietic cells and has been associated with different malignancies. However, its involvement in myeloid leukemias is not well studied. Using murine transplantation models, we show here that the absence of FLOT2 from leukemia-initiating cells (LICs) altered the disease course of BCR-ABL1+ chronic myeloid leukemia (CML), but not of MLL-AF9-driven acute myeloid leukemia (AML). While FLOT2 was required for expression of the adhesion molecule CD44 on both CML- and AML-LIC, a defect in the cytoskeleton, cell polarity, and impaired homing ability of LIC was only observed in FLOT2-deficient BCR-ABL1+ compared with MLL-AF9+ cells. Downstream of CD44, BCR-ABL1 kinase-independent discrepancies were observed regarding expression, localization, and activity of cell division control protein 42 homolog (CDC42) between wild-type (WT) and FLOT2-deficient human CML and AML cells. Inhibition of CDC42 by ML141 impaired the homing of CML LIC and, thereby, CML progression. This suggested that alteration of both CD44 and CDC42 may be causative of impaired CML progression in the absence of FLOT2. In summary, our data suggest a FLOT2-CD44-CDC42 axis, which differentially regulates CML vs AML progression, with deficiency of FLOT2 impairing the development of CML.

Endocytic trafficking of GAS6-AXL complexes is associated with sustained AKT activation

AXL, a TAM receptor tyrosine kinase (RTK), and its ligand growth arrest-specific 6 (GAS6) are implicated in cancer metastasis and drug resistance, and cellular entry of viruses. Given this, AXL is an attractive therapeutic target, and its inhibitors are being tested in cancer and COVID-19 clinical trials. Still, astonishingly little is known about intracellular mechanisms that control its function. Here, we characterized endocytosis of AXL, a process known to regulate intracellular functions of RTKs. Consistent with the notion that AXL is a primary receptor for GAS6, its depletion was sufficient to block GAS6 internalization. We discovered that upon receptor ligation, GAS6–AXL complexes were rapidly internalized via several endocytic pathways including both clathrin-mediated and clathrin-independent routes, among the latter the CLIC/GEEC pathway and macropinocytosis. The internalization of AXL was strictly dependent on its kinase activity. In comparison to other RTKs, AXL was endocytosed faster and the majority of the internalized receptor was not degraded but rather recycled via SNX1-positive endosomes. This trafficking pattern coincided with sustained AKT activation upon GAS6 stimulation. Specifically, reduced internalization of GAS6–AXL upon the CLIC/GEEC downregulation intensified, whereas impaired recycling due to depletion of SNX1 and SNX2 attenuated AKT signaling. Altogether, our data uncover the coupling between AXL endocytic trafficking and AKT signaling upon GAS6 stimulation. Moreover, our study provides a rationale for pharmacological inhibition of AXL in antiviral therapy as viruses utilize GAS6–AXL-triggered endocytosis to enter cells.

Circadian regulation of protein cargo in extracellular vesicles

The circadian clock controls many aspects of physiology, but it remains undescribed whether extracellular vesicles (EVs), including exosomes, involved in cell-cell communications between tissues are regulated in a circadian pattern. We demonstrate a 24-hour rhythmic abundance of individual proteins in small EVs using liquid chromatography-mass spectrometry in circadian-synchronized tendon fibroblasts. Furthermore, the release of small EVs enriched in RNA binding proteins was temporally separated from those enriched in cytoskeletal and matrix proteins, which peaked during the end of the light phase. Last, we targeted the protein sorting mechanism in the exosome biogenesis pathway and established (by knockdown of circadian-regulated flotillin-1) that matrix metalloproteinase 14 abundance in tendon fibroblast small EVs is under flotillin-1 regulation. In conclusion, we have identified proteomic time signatures for small EVs released by tendon fibroblasts, which supports the view that the circadian clock regulates protein cargo in EVs involved in cell-cell cross-talk.

Intracellular Dynamin Elastin-like Polypeptides Assemble into Rodlike, Spherical, and Reticular Dynasomes

Dynamin (DNM) is a family of large GTPases possessing a unique mechanical ability to "pinch" off vesicles entering cells. DNM2 is the most ubiquitously expressed member of the DNM family. We developed a novel tool based on elastin-like polypeptide (ELP) technology to quickly, precisely, and reversibly modulate the structure of DNM2. ELPs are temperature-sensitive biopolymers that self-assemble into microdomains above sharp transition temperatures. When linked together, DNM2 and a temperature-sensitive ELP fusion organize into a range of distinct temperature-dependent structures above a sharp transition temperature, which were not observed with wild-type DNM2 or a temperature-insensitive ELP fusion control. The structures comprised three different morphologies, which were prevalent at different temperature ranges. The size of these structures was influenced by an inhibitor of the DNM2 GTPase activity, dynasore; furthermore, they appear to entrap co-expressed cytosolic ELPs. Having demonstrated an unexpected diversity of morphologically distinct structures, DNM2-ELP fusions may have applications in the exploration of dynamin-dependent biology.

Selective Surface and Intraluminal Localization of Wnt Ligands on Small Extracellular Vesicles Released by HT-22 Hippocampal Neurons

The Wnt signaling pathway induces various responses underlying the development and maturation of the nervous system. Wnt ligands are highly hydrophobic proteins that limit their diffusion through an aqueous extracellular medium to a target cell. Nevertheless, their attachment to small extracellular vesicles-like exosomes is one of the described mechanisms that allow their transport under this condition. Some Wnt ligands in these vehicles are expected to be dependent on post-translational modifications such as acylation. The mechanisms determining Wnt loading in exosomes and delivery to the target cells are largely unknown. Here, we took advantage of a cell model that secret a highly enriched population of small extracellular vesicles (sEVs), hippocampal HT-22 neurons. First, to establish the cell model, we characterized the morphological and biochemical properties of an enriched fraction of sEVs obtained from hippocampal HT-22 neurons that express NCAM-L1, a specific exosomal neuronal marker. Transmission electron microscopy showed a highly enriched fraction of exosome-like vesicles. Next, the exosomal presence of Wnt3a, Wnt5a, and Wnt7a was confirmed by western blot analysis and electron microscopy combined with immunogold. Also, we studied whether palmitoylation is a necessary post-translational modification for the transport Wnt in these vesicles. We found that proteinase-K treatment of exosomes selectively decreased their Wnt5a and Wnt7a content, suggesting that their expression is delimited to the exterior membrane surface. In contrast, Wnt3a remained attached, suggesting that it is localized within the exosome lumen. On the other hand, Wnt-C59, a specific inhibitor of porcupine O-acyltransferase (PORCN), decreased the association of Wnt with exosomes, suggesting that Wnt ligand acylation is necessary for them to be secreted by exosomes. These findings may help to understand the action of the Wnt ligands in the target cell, which could be defined during the packaging of the ligands in the secretory cell sEVs.

Different Ability of Multidrug-Resistant and -Sensitive Counterpart Cells to Release and Capture Extracellular Vesicles

Cancer multidrug resistance (MDR) is one of the main challenges for cancer treatment efficacy. MDR is a phenomenon by which tumor cells become resistant to several unrelated drugs. Some studies have previously described the important role of extracellular vesicles (EVs) in the dissemination of a MDR phenotype. EVs’ cargo may include different players of MDR, such as microRNAS and drug-efflux pumps, which may be transferred from donor MDR cells to recipient drug-sensitive counterparts. The present work aimed to: (i) compare the ability of drug-sensitive and their MDR counterpart cells to release and capture EVs and (ii) study and relate those differences with possible distinct fate of the endocytic pathway in these counterpart cells. Our results showed that MDR cells released more EVs than their drug-sensitive counterparts and also that the drug-sensitive cells captured more EVs than their MDR counterparts. This difference in the release and capture of EVs may be associated with differences in the endocytic pathway between drug-sensitive and MDR cells. Importantly, manipulation of the recycling pathway influenced the response of drug-sensitive cells to doxorubicin treatment.

Polycation-Mediated Transfection: Mechanisms of Internalization and Intracellular Trafficking

Polyplex-mediated gene transfection is now in its' fourth decade of serious research, but the promise of polyplex-mediated gene therapy has yet to fully materialize. Only approximately one in a million applied plasmids actually expresses. A large part of this is due to an incomplete understanding of the mechanism of polyplex transfection. There is an assumption that internalization must follow a canonical mechanism of receptor mediated endocytosis. Herein, we present arguments that untargeted (and most targeted) polyplexes do not utilize these routes. By incorporating knowledge of syndecan-polyplex interactions, we can show that syndecans are the "target" for polyplexes. Further, it is known that free polycations (which disrupt cell-membranes by acid-catalyzed hydrolysis of phospholipid esters) are necessary for (untargeted) endocytosis. This can be incorporated into the model to produce a novel mechanism of endocytosis, which fits the observed phenomenology. After membrane translocation, polyplex containing vesicles reach the endosome after diffusing through the actin mesh below the cell membrane. From there, they are acidified and trafficked toward the lysosome. Some polyplexes are capable of escaping the endosome and unpacking, while others are not. Herein, it is argued that for some polycations, as acidification proceeds the polyplexes excluding free polycations, which disrupt the endosomal membrane by acid-catalyzed hydrolysis, allowing the polyplex to escape. The polyplex's internal charge ratio is now insufficient for stability and it releases plasmids which diffuse to the nucleus. A small proportion of these plasmids diffuse through the nuclear pore complex (NPC), with aggregation being the major cause of loss. Those plasmids that have diffused through the NPC will also aggregate, and this appears to be the reason such a small proportion of nuclear plasmids express mRNA. Thus, the structural features which promote unpacking in the endosome and allow for endosomal escape can be determined, and better polycations can be designed.

Single-nucleotide polymorphisms in Orai1 associated with atopic dermatitis inhibit protein turnover, decrease calcium entry and disrupt calcium-dependent gene expression

Loss-of function mutations in Orai1 Ca²⁺ channels lead to a form of severe combined immunodeficiency, auto-immunity, muscle hypotonia and defects in dental enamel production and sweat gland function. Two single-nucleotide polymorphisms (SNPs) in Orai1 have been found and localize to the second extracellular loop. These polymorphisms associate with atopic dermatitis but how they affect Ca²⁺ signalling and cell function is unknown. Here, we find that Orai1–SNPs turnover considerably more slowly than wild type Orai1 and are more abundantly expressed in the plasma membrane. We show a central role for flotillin in the endocytotic recycling of Orai1 channels and that endocytosed wild type Orai1 is trafficked to Rab 7-positive late endosomes for lysosomal degradation. Orai1–SNPs escape the degradation pathway and instead enter Rab 11-positive recycling endosomes, where they are returned to the surface membrane through Arf6-dependent exocytosis. We find that Orai1–SNPs escape late endosomes through endosomal pH regulation of interaction between the channel and flotillin. We identify a pH-sensitive electrostatic interaction between positively charged arginine in extracellular loop 2 (K210) and a negatively charged aspartate (D112) in extracellular loop 1 that helps determine Orai1 turnover. The increase in membrane Orai1–SNP leads to a mis-match in Orai1–STIM stoichiometry, resulting in inhibition of Ca²⁺ entry and Ca²⁺-dependent gene expression. Our results identify new strategies for targeting atopic dermatitis.

Proteomics of purified lamellocytes from Drosophila melanogaster HopTum-l identifies new membrane proteins and networks involved in their functions

In healthy Drosophila melanogaster larvae, plasmatocytes and crystal cells account for 95% and 5% of the hemocytes, respectively. A third type of hemocytes, lamellocytes, are rare, but their number increases after oviposition by parasitoid wasps. The lamellocytes form successive layers around the parasitoid egg, leading to its encapsulation and melanization, and finally the death of this intruder. However, the total number of lamellocytes per larva remains quite low even after parasitoid infestation, making direct biochemical studies difficult. Here, we used the Hop^Tum-l mutant strain that constitutively produces large numbers of lamellocytes to set up a purification method and analyzed their major proteins by 2D gel electrophoresis and their plasma membrane surface proteins by 1D SDS-PAGE after affinity purification. Mass spectrometry identified 430 proteins from 2D spots and 344 affinity-purified proteins from 1D bands, for a total of 639 unique proteins. Known lamellocyte markers such as PPO3 and the myospheroid integrin were among the components identified with specific chaperone proteins. Affinity purification detected other integrins, as well as a wide range of integrin-associated proteins involved in the formation and function of cell-cell junctions. Overall, the newly identified proteins indicate that these cells are highly adapted to the encapsulation process (recognition, motility, adhesion, signaling), but may also have several other physiological functions (such as secretion and internalization of vesicles) under different signaling pathways. These results provide the basis for further in vivo and in vitro studies of lamellocytes, including the development of new markers to identify coexisting populations and their respective origins and functions in Drosophila immunity.

Molecular Mechanisms of Borrelia burgdorferi Phagocytosis and Intracellular Processing by Human Macrophages

Lyme disease is the most common vector-borne illness in North America and Europe. Its causative agents are spirochetes of the Borrelia burgdorferi sensu latu complex. Infection with borreliae can manifest in different tissues, most commonly in the skin and joints, but in severe cases also in the nervous systems and the heart. The immune response of the host is a crucial factor for preventing the development or progression of Lyme disease. Macrophages are part of the innate immune system and thus one of the first cells to encounter infecting borreliae. As professional phagocytes, they are capable of recognition, uptake, intracellular processing and final elimination of borreliae. This sequence of events involves the initial capture and internalization by actin-rich cellular protrusions, filopodia and coiling pseudopods. Uptake into phagosomes is followed by compaction of the elongated spirochetes and degradation in mature phagolysosomes. In this review, we discuss the current knowledge about the processes and molecular mechanisms involved in recognition, capturing, uptake and intracellular processing of Borrelia by human macrophages. Moreover, we highlight interactions between macrophages and other cells of the immune system during these processes and point out open questions in the intracellular processing of borreliae, which include potential escape strategies of Borrelia.

Flotillin-Dependent Membrane Microdomains Are Required for Functional Phagolysosomes against Fungal Infections

Lipid rafts form signaling platforms on biological membranes with incompletely characterized role in immune response to infection. Here we report that lipid-raft microdomains are essential components of phagolysosomal membranes of macrophages and depend on flotillins. Genetic deletion of flotillins demonstrates that the assembly of both major defense complexes vATPase and NADPH oxidase requires membrane microdomains. Furthermore, we describe a virulence mechanism leading to dysregulation of membrane microdomains by melanized wild-type conidia of the important human-pathogenic fungus Aspergillus fumigatus resulting in reduced phagolysosomal acidification. We show that phagolysosomes with ingested melanized conidia contain a reduced amount of free Ca²⁺ ions and that inhibition of Ca²⁺-dependent calmodulin activity led to reduced lipid-raft formation. We identify a single-nucleotide polymorphism in the human FLOT1 gene resulting in heightened susceptibility for invasive aspergillosis in hematopoietic stem cell transplant recipients. Collectively, flotillin-dependent microdomains on the phagolysosomal membrane play an essential role in protective antifungal immunity.

Understanding nanoparticle endocytosis to improve targeting strategies in nanomedicine

Nanoparticles (NPs) have attracted considerable attention in various fields, such as cosmetics, the food industry, material design, and nanomedicine. In particular, the fast-moving field of nanomedicine takes advantage of features of NPs for the detection and treatment of different types of cancer, fibrosis, inflammation, arthritis as well as neurodegenerative and gastrointestinal diseases. To this end, a detailed understanding of the NP uptake mechanisms by cells and intracellular localization is essential for safe and efficient therapeutic applications. In the first part of this review, we describe the several endocytic pathways involved in the internalization of NPs and we discuss the impact of the physicochemical properties of NPs on this process. In addition, the potential challenges of using various inhibitors, endocytic markers and genetic approaches to study endocytosis are addressed along with the principal (semi) quantification methods of NP uptake. The second part focuses on synthetic and bio-inspired substances, which can stimulate or decrease the cellular uptake of NPs. This approach could be interesting in nanomedicine where a high accumulation of drugs in the target cells is desirable and clearance by immune cells is to be avoided. This review contributes to an improved understanding of NP endocytic pathways and reveals potential substances, which can be used in nanomedicine to improve NP delivery.

Protein acylation by saturated very long chain fatty acids and endocytosis are involved in necroptosis

Necroptosis is a form of cell death characterized by receptor-interacting protein kinase activity and plasma membrane permeabilization via mixed-lineage kinase-like protein (MLKL). This permeabilization is responsible for the inflammatory properties of necroptosis. We previously showed that very long chain fatty acids (VLCFAs) are functionally involved in necroptosis, potentially through protein fatty acylation. Here, we define the scope of protein acylation by saturated VLCFAs during necroptosis. We show that MLKL and phosphoMLKL, key for membrane permeabilization, are exclusively acylated during necroptosis. Reducing the levels of VLCFAs decreases their membrane recruitment, suggesting that acylation by VLCFAs contributes to their membrane localization. Acylation of phosphoMLKL occurs downstream of phosphorylation and oligomerization and appears to be, in part, mediated by ZDHHC5 (a palmitoyl transferase). We also show that disruption of endosomal trafficking increases cell viability during necroptosis, possibly by preventing recruitment, or removal, of phosphoMLKL from the plasma membrane.

The Gb3-enriched CD59/flotillin plasma membrane domain regulates host cell invasion by Pseudomonas aeruginosa

The opportunistic pathogen Pseudomonas aeruginosa has gained precedence over the years due to its ability to develop resistance to existing antibiotics, thereby necessitating alternative strategies to understand and combat the bacterium. Our previous work identified the interaction between the bacterial lectin LecA and its host cell glycosphingolipid receptor globotriaosylceramide (Gb3) as a crucial step for the engulfment of P. aeruginosa via the lipid zipper mechanism. In this study, we define the LecA-associated host cell membrane domain by pull-down and mass spectrometry analysis. We unraveled a predilection of LecA for binding to saturated, long fatty acyl chain-containing Gb3 species in the extracellular membrane leaflet and an induction of dynamic phosphatidylinositol (3,4,5)-trisphosphate (PIP₃) clusters at the intracellular leaflet co-localizing with sites of LecA binding. We found flotillins and the GPI-anchored protein CD59 not only to be an integral part of the LecA-interacting membrane domain, but also majorly influencing bacterial invasion as depletion of either of these host cell proteins resulted in about 50% reduced invasiveness of the P. aeruginosa strain PAO1. In summary, we report that the LecA-Gb3 interaction at the extracellular leaflet induces the formation of a plasma membrane domain enriched in saturated Gb3 species, CD59, PIP₃ and flotillin thereby facilitating efficient uptake of PAO1.

Extracellular Vesicle Transportation and Uptake by Recipient Cells: A Critical Process to Regulate Human Diseases

Emerging evidence highlights the relevance of extracellular vesicles (EVs) in modulating human diseases including but not limited to cancer, inflammation, and neurological disorders. EVs can be found in almost all types of human body fluids, suggesting that their trafficking may allow for their targeting to remote recipient cells. While molecular processes underlying EV biogenesis and secretion are increasingly elucidated, mechanisms governing EV transportation, target finding and binding, as well as uptake into recipient cells remain to be characterized. Understanding the specificity of EV transport and uptake is critical to facilitating the development of EVs as valuable diagnostics and therapeutics. In this mini review, we focus on EV uptake mechanisms and specificities, as well as their implications in human diseases.

Specialised endocytic proteins regulate diverse internalisation mechanisms and signalling outputs in physiology and cancer

Although endocytosis was first described as the process mediating macromolecule or nutrient uptake through the plasma membrane, it is now recognised as a critical component of the cellular infrastructure involved in numerous processes, ranging from receptor signalling, proliferation and migration to polarity and stem cell regulation. To realise these varying roles, endocytosis needs to be finely regulated. Accordingly, multiple endocytic mechanisms exist that require specialised molecular machineries and an array of endocytic adaptor proteins with cell-specific functions. This review provides some examples of specialised functions of endocytic adaptors and other components of the endocytic machinery in different cell physiological processes, and how the alteration of these functions is linked to cancer. In particular, we focus on: (i) cargo selection and endocytic mechanisms linked to different adaptors; (ii) specialised functions in clathrin-mediated versus non-clathrin endocytosis; (iii) differential regulation of endocytic mechanisms by post-translational modification of endocytic proteins; (iv) cell context-dependent expression and function of endocytic proteins. As cases in point, we describe two endocytic protein families, dynamins and epsins. Finally, we discuss how dysregulation of the physiological role of these specialised endocytic proteins is exploited by cancer cells to increase cell proliferation, migration and invasion, leading to anti-apoptotic or pro-metastatic behaviours.

Parasitoid wasp venom vesicles (venosomes) enter Drosophila melanogaster lamellocytes through a flotillin/lipid raft-dependent endocytic pathway

Venosomes are extracellular vesicles found in the venom of Leptopilina endoparasitoids wasps, which transport and target virulence factors to impair the parasitoid egg encapsulation by the lamellocytes of their Drosophila melanogaster host larva. Using the co-immunolocalization of fluorescent L. boulardi venosomes and one of the putative-transported virulence factors, LbGAP, with known markers of cellular endocytosis, we show that venosomes endocytosis by lamellocytes is not a process dependent on clathrin or macropinocytosis and internalization seems to bypass the early endosomal compartment Rab5. After internalization, LbGAP colocalizes strongly with flotillin-1 and the GPI-anchored protein Atilla/L1 (a lamellocyte surface marker) suggesting that entry occurs via a flotillin/lipid raft-dependent pathway. Once internalized, venosomes reach all intracellular compartments, including late and recycling endosomes, lysosomes, and the endoplasmic reticulum network. Venosomes therefore enter their target cells by a specific mechanism and the virulence factors are widely distributed in the lamellocytes' compartments to impair their functions.

Clathrin- and dynamin-dependent endocytosis limits canonical NF-κB signaling triggered by lymphotoxin β receptor

Background

Lymphotoxin β receptor (LTβR) is a member of tumor necrosis factor receptor (TNFR) superfamily which regulates the immune response. At the cellular level, upon ligand binding, the receptor activates the pro-inflammatory NF-κB and AP-1 pathways. Yet, the intracellular distribution of LTβR, the routes of its endocytosis and their connection to the signaling activation are not characterized. Here, we investigated the contribution of LTβR internalization to its signaling potential.

Methods

Intracellular localization of LTβR in unstimulated and stimulated cells was analyzed by confocal microscopy. Endocytosis impairment was achieved through siRNA- or CRISPR/Cas9-mediated depletion, or chemical inhibition of proteins regulating endocytic routes. The activation of LTβR-induced signaling was examined. The levels of effector proteins of the canonical and non-canonical branches of the NF-κB pathway, and the phosphorylation of JNK, Akt, ERK1/2, STAT1 and STAT3 involved in diverse signaling cascades, were measured by Western blotting. A transcriptional response to LTβR stimulation was assessed by qRT-PCR analysis.

Results

We demonstrated that LTβR was predominantly present on endocytic vesicles and the Golgi apparatus. The ligand-bound pool of the receptor localized to endosomes and was trafficked towards lysosomes for degradation. Depletion of regulators of different endocytic routes (clathrin-mediated, dynamin-dependent or clathrin-independent) resulted in the impairment of LTβR internalization, indicating that this receptor uses multiple entry pathways. Cells deprived of clathrin and dynamins exhibited enhanced activation of canonical NF-κB signaling represented by increased degradation of IκBα inhibitor and elevated expression of LTβR target genes. We also demonstrated that clathrin and dynamin deficiency reduced to some extent LTβR-triggered activation of the non-canonical branch of the NF-κB pathway.

Conclusions

Our work shows that the impairment of clathrin- and dynamin-dependent internalization amplifies a cellular response to LTβR stimulation. We postulate that receptor internalization restricts responsiveness of the cell to subthreshold stimuli.

Cellular Prion Protein (PrPc): Putative Interacting Partners and Consequences of the Interaction

Cellular prion protein (PrPc) is a small glycosylphosphatidylinositol (GPI) anchored protein most abundantly found in the outer leaflet of the plasma membrane (PM) in the central nervous system (CNS). PrPc misfolding causes neurodegenerative prion diseases in the CNS. PrPc interacts with a wide range of protein partners because of the intrinsically disordered nature of the protein’s N-terminus. Numerous studies have attempted to decipher the physiological role of the prion protein by searching for proteins which interact with PrPc. Biochemical characteristics and biological functions both appear to be affected by interacting protein partners. The key challenge in identifying a potential interacting partner is to demonstrate that binding to a specific ligand is necessary for cellular physiological function or malfunction. In this review, we have summarized the intracellular and extracellular interacting partners of PrPc and potential consequences of their binding. We also briefly describe prion disease-related mutations at the end of this review.

Stomatin is highly expressed in exosomes of different origin and is a promising candidate as an exosomal marker

Proteins involved in the organizing of lipid rafts can be found in exosomes, as shown for caveolin-1, and they could contribute to exosomal cargo sorting, as shown for flotillins. Stomatin belongs to the same stomatin/prohibitin/flotillin/HflK/C family of lipid rafts proteins, but it has never been studied in exosomes except for extracellular vesicles (EVs) originating from blood cells. Here we first show the presence of stomatin in exosomes produced by epithelial cancer cells (non-small cell lung cancer, breast, and ovarian cancer cells) as well as in EVs from biological fluids, including blood plasma, ascitic fluids, and uterine flushings. A high abundance of stomatin in EVs of various origins and its enrichment in exosomes make stomatin a promising exosomal marker. Comparison with other lipid raft proteins and exosomal markers showed that the level of stomatin protein in exosomes from different sources corresponds well to that of CD9, while it differs essentially from flotillin-1 and flotillin-2 homologs, which in turn are present in exosomes in nearly equal proportions. In contrast, the level of vesicular caveolin-1 as well as its EV-to-cellular ratio vary drastically depending on cell type.

Targeting positive feedback between BASP1 and EGFR as a therapeutic strategy for lung cancer progression

Rationale: Brain metastasis in patients with lung cancer is life-threatening. However, the molecular mechanism for this catastrophic disease remains elusive, and few druggable targets are available. Therefore, this study aimed to identify and characterize proteins that could be used as therapeutic targets.

Methods: Proteomic analyses were conducted to identify differentially expressed membrane proteins between brain metastatic lung cancer cells and primary lung cancer cells. A neuronal growth-associated protein, brain acid soluble protein 1 (BASP1), was chosen for further investigation. The clinical relevance of BASP1 in lung adenocarcinoma was first assessed. Tyrosine kinase activity assays and in vitro and in vivo functional assays were conducted to explore the oncogenic mechanisms of BASP1.

Results: The protein levels of BASP1 were positively associated with tumor progression and poor prognosis in patients with lung adenocarcinoma. Membrane-bound BASP1 increased EGFR signaling and stabilized EGFR proteins by facilitating their escape from the ubiquitin-proteasome pathway. Reciprocally, activation of EGFR recruited more BASP1 to the plasma membrane, generating a positive feedback loop between BASP1 and EGFR. Moreover, the synergistic therapeutic effects of EGFR tyrosine kinase inhibitor and arsenic trioxide led to a reduction in the level of BASP1 protein observed in lung cancer cells with acquired resistance to EGFR inhibitors.

Conclusions: The reciprocal interaction between BASP1 and EGFR facilitates EGFR signaling in brain metastatic lung cancer. Targeting the newly identified BASP1-EGFR interaction could open new venues for lung cancer treatment.

The Heat Shock Protein 27 Immune Complex Enhances Exosomal Cholesterol Efflux

Previously, we demonstrated that Heat Shock Protein 27 (HSP27) reduces the inflammatory stages of experimental atherogenesis, is released by macrophage (MΦ) exosomes and lowers cholesterol levels in atherosclerotic plaques. Recently, we discovered that natural autoantibodies directed against HSP27 enhance its signaling effects, as HSP27 immune complexes (IC) interact at the cell membrane to modulate signaling. We now seek to evaluate the potential role of the HSP27 IC on MΦ exosomal release and cholesterol export. First, in human blood samples, we show that healthy control subjects have 86% more exosomes compared to patients with coronary artery disease (p < 0.0001). Treating human THP-1 MΦ with rHSP27 plus a validated anti-HPS27 IgG antibody increased the abundance of exosomes in the culture media (+98%; p < 0.0001) as well as expression of Flotillin-2, a marker reflective of exosomal release. Exosome cholesterol efflux was independent of Apo-A1. THP-1 MΦ loaded with NBD-labeled cholesterol and treated with the HSP27 IC showed a 22% increase in extracellular vesicles labeled with NBD and a 95% increase in mean fluorescent intensity. In conclusion, exosomal abundance and secretion of cholesterol content increases in response to HSP27 IC treatment, which may represent an important therapeutic option for diseases characterized by cholesterol accumulation.

Vacuolins and myosin VII are required for phagocytic uptake and phagosomal membrane recycling in Dictyostelium discoideum

Flotillins are lipid raft residents involved in membrane trafficking and recycling of plasma membrane proteins. Dictyostelium discoideum uses phagocytosis to kill, digest and feed on bacteria. It possesses three flotillin-like vacuolins that are strongly associated with membranes and that gradually accumulate on maturing phagosomes. Absence of vacuolins reduced adhesion and particle recognition resulting in a drastic reduction in the uptake of various types of particles. This was caused by a block in the recycling of plasma membrane components and the absence of their specific cortex-associated proteins. In addition, absence of vacuolins also impaired phagolysosome biogenesis, without significantly impacting killing and digestion of a range of bacteria. Strikingly, both absence and overexpression of vacuolins induced a strong downregulation of myosin VII (also known as MyoI) expression, as well as its binding partner talin A. Episomal expression of myosin VII fully rescued defects in uptake and adhesion but not in phagosome maturation. These results suggest a dual role for vacuolins: a novel mechanism involving membrane microdomains and myosin VII-talin A in clustering phagosomal receptors and adhesion molecules at the plasma membrane, and a role in phagolysosomal biogenesis.

Macropinocytosis and Clathrin-Dependent Endocytosis Play Pivotal Roles for the Infectious Entry of Puumala Virus

Viruses from the family Hantaviridae are encountered as emerging pathogens causing two life-threatening human zoonoses: hemorrhagic fever with renal syndrome (HFRS) and hantavirus cardiopulmonary syndrome (HCPS), with case fatality rates of up to 50%. Here, we comprehensively investigated entry of the Old World hantavirus Puumala virus (PUUV) into mammalian cells, showing that upon treatment with pharmacological inhibitors of macropinocytosis and clathrin-mediated endocytosis, PUUV infections are greatly reduced. We demonstrate that the inhibitors did not interfere with viral replication and that RNA interference, targeting cellular mediators of macropinocytosis, decreases PUUV infection levels significantly. Moreover, we established lipophilic tracer staining of PUUV particles and show colocalization of stained virions and markers of macropinosomes. Finally, we report a significant increase in the fluid-phase uptake of cells infected with PUUV, indicative of a virus-triggered promotion of macropinocytosis.IMPORTANCE The family Hantaviridae comprises a diverse group of virus species and is considered an emerging global public health threat. Individual hantavirus species differ considerably in terms of their pathogenicity but also in their cell biology and host-pathogen interactions. In this study, we focused on the most prevalent pathogenic hantavirus in Europe, Puumala virus (PUUV), and investigated the entry and internalization of PUUV into mammalian cells. We show that both clathrin-mediated endocytosis and macropinocytosis are cellular pathways exploited by the virus to establish productive infections and demonstrate that pharmacological inhibition of macropinocytosis or a targeted knockdown using RNA interference significantly reduced viral infections. We also found indications of an increase of macropinocytic uptake upon PUUV infection, suggesting that the virus triggers specific cellular mechanisms in order to stimulate its own internalization, thus facilitating infection.

The role of lipid species in membranes and cancer-related changes

Several studies have demonstrated interactions between the two leaflets in membrane bilayers and the importance of specific lipid species for such interaction and membrane function. We here discuss these investigations with a focus on the sphingolipid and cholesterol-rich lipid membrane domains called lipid rafts, including the small flask-shaped invaginations called caveolae, and the importance of such membrane structures in cell biology and cancer. We discuss the possible interactions between the very long-chain sphingolipids in the outer leaflet of the plasma membrane and the phosphatidylserine species PS 18:0/18:1 in the inner leaflet and the importance of cholesterol for such interactions. We challenge the view that lipid rafts contain a large fraction of lipids with two saturated fatty acyl groups and argue that it is important in future studies of membrane models to use asymmetric membrane bilayers with lipid species commonly found in cellular membranes. We also discuss the need for more quantitative lipidomic studies in order to understand membrane function and structure in general, and the importance of lipid rafts in biological systems. Finally, we discuss cancer-related changes in lipid rafts and lipid composition, with a special focus on changes in glycosphingolipids and the possibility of using lipid therapy for cancer treatment.

Lipid Rafts in Exosome Biogenesis

Exosomes (secreted extracellular vesicles formed in the intracellular vesicular transport system) play a crucial role in distant cell-cell communication. Exosomes transfer active forms of various biomolecules; the molecular composition of the exosomal cargo is a result of targeted selection and depends on the type of producer cells. The mechanisms underlying exosome formation and cargo selection are poorly understood. It is believed that there are several pathways for exosome biogenesis, although the questions about their independence and simultaneous coexistence in the cell still remain open. The least studied topic is the recently discovered mechanism of exosome formation associated with lipid rafts, or membrane lipid microdomains. Here, we present modern concepts and basic hypotheses on the mechanisms of exosome biogenesis and secretion and summarize current data on the involvement of lipid rafts and their constituent molecules in these processes. Special attention is paid to the analysis of possible role in the exosome formation of raft-forming proteins of the SPFH family, components of planar rafts, and caveolin, the main component of caveolae.

Fourier-transform Infrared (FT-IR) spectroscopy fingerprints subpopulations of extracellular vesicles of different sizes and cellular origin

Identification of extracellular vesicle (EV) subpopulations remains an open challenge. To date, the common strategy is based on searching and probing set of molecular components and physical properties intended to be univocally characteristics of the target subpopulation. Pitfalls include the risk to opt for an unsuitable marker set - which may either not represent the subpopulation or also cover other unintended subpopulations - and the need to use different characterization techniques and equipment. This approach focused on specific markers may result inadequate to routinely deal with EV subpopulations that have an intrinsic high level of heterogeneity. In this paper, we show that Fourier-transform Infrared (FT-IR) spectroscopy can provide a collective fingerprint of EV subpopulations in one single experiment. FT-IR measurements were performed on large (LEVs, ~600 nm), medium (MEVs, ~200 nm) and small (SEVs ~60 nm) EVs enriched from two different cell lines medium: murine prostate cancer (TRAMP-C2) and skin melanoma (B16). Spectral regions between 3100-2800 cm-1 and 1880-900 cm-1, corresponding to functional groups mainly ascribed to lipid and protein contributions, were acquired and processed by Principal Component Analysis (PCA). LEVs, MEVs and SEVs were separately grouped for both the considered cell lines. Moreover, subpopulations of the same size but from different sources were assigned (with different degrees of accuracy) to two different groups. These findings demonstrate that FT-IR has the potential to quickly fingerprint EV subpopulations as a whole, suggesting an appealing complement/alternative for their characterization and grading, extendable to healthy and pathological EVs and fully artificial nanovesicles.

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.

Monitoring Therapy Efficiency in Cancer through Extracellular Vesicles

Extracellular vesicles (EVs) are a heterogeneous group of membrane-enclosed vesicles made of a phospholipid bilayer and are secreted by all cell types. EVs are present in a variety of body fluids containing proteins, DNA, RNA species, and lipids, and play an important role in cell- to-cell communication and are worth being considered as biomarkers for both early diagnosis of cancer patients and real-time monitoring of treatment response. Recently, emerging evidence verified EVs to have crucial roles in cancer progression and metastasis and a great potential in therapeutic applications. In this review, we discuss the potential of EVs in monitoring the efficacy of cancer therapies.

Targeted enhancement of flotillin-dependent endocytosis augments cellular uptake and impact of cytotoxic drugs

Cellular uptake is limiting for the efficacy of many cytotoxic drugs used to treat cancer. Identifying endocytic mechanisms that can be modulated with targeted, clinically-relevant interventions is important to enhance the efficacy of various cancer drugs. We identify that flotillin-dependent endocytosis can be targeted and upregulated by ultrasound and microbubble (USMB) treatments to enhance uptake and efficacy of cancer drugs such as cisplatin. USMB involves targeted ultrasound following administration of encapsulated microbubbles, used clinically for enhanced ultrasound image contrast. USMB treatments robustly enhanced internalization of the molecular scaffold protein flotillin, as well as flotillin-dependent fluid-phase internalization, a phenomenon dependent on the protein palmitoyltransferase DHHC5 and the Src-family kinase Fyn. USMB treatment enhanced DNA damage and cell killing elicited by the cytotoxic agent cisplatin in a flotillin-dependent manner. Thus, flotillin-dependent endocytosis can be modulated by clinically-relevant USMB treatments to enhance drug uptake and efficacy, revealing an important new strategy for targeted drug delivery for cancer treatment.

Protein phosphatase 2A-mediated flotillin-1 dephosphorylation up-regulates endothelial cell migration and angiogenesis regulation

Endothelial cells have key functions in endothelial barrier integrity and in responses to angiogenic signals that promote cell proliferation, cell migration, cytoskeletal reorganization, and formation of new blood vessels. These functions highly depend on protein-protein interactions in cell-cell junction and cell attachment complexes and on interactions with cytoskeletal proteins. Protein phosphatase 2A (PP2A) dephosphorylates several target proteins involved in cytoskeletal dynamics and cell adhesion. Our goal was to find new interacting and substrate proteins of the PP2A-B55α holoenzyme in bovine pulmonary endothelial cells. Using LC-MS/MS analysis, we identified flotillin-1 as a protein that binds recombinant GSH S-transferase-tagged PP2A-B55α. Immunoprecipitation experiments, proximity ligation assays, and immunofluorescent staining confirmed the interaction between these two endogenous proteins in endothelial cells. Originally, flotillins were described as regulatory proteins for axon regeneration, but they appear to function in many cellular processes, such as membrane receptor signaling, endocytosis, and cell adhesion. Ser³¹⁵ is a known PKC-targeted site in flotillin-1. Utilizing phosphomutants of flotillin-1 and the NanoBiT luciferase assay, we show here that phosphorylation/dephosphorylation of Ser³¹⁵ in flotillin-1 significantly affects its interaction with PP2A-B55α and that PP2A-B55α dephosphorylates phospho-Ser³¹⁵ Spreading, attachment, migration, and in vitro tube formation rates of S315A variant-overexpressing cells were faster than those of nontransfected or S315D-transfected cells. These results indicate that the PP2A-flotillin-1 interaction identified here affects major physiological activities of pulmonary endothelial cells.