Search for inhibitors of endocytosis: Intended specificity and unintended consequences

Intelligent substance delivery into cells using cell-penetrating peptides

Cell-penetrating peptides (CPPs) are oligopeptides that can permeate the cell membrane. The use of a CPP-mediated transport system could be an excellent method for delivering cell-impermeable substances such as proteins, antibodies, antisense oligonucleotides, siRNAs, plasmids, drugs, fluorescent compounds, and nanoparticles as covalently or noncovalently conjugated cargo into cells. Nonetheless, the mechanisms through which CPPs are internalized remain unclear. Endocytosis and direct translocation through the membrane are the generally accepted routes. Internalization via both pathways can occur simultaneously, depending on cellular conditions. However, the peculiar property of CPPs has attracted many researchers, especially in drug discovery or development, who intend to deliver impermeable substances into cells through the cell membrane. The delivery of drugs using CPPs may non-invasively solve the problem of drug penetration into cells with the added benefit of low cytotoxicity. Moreover, macromolecules can also be delivered by this transport system. In this review, I discuss the possibilities and advantages of substance delivery into cells using CPPs.

Clathrin localization and dynamics in Aspergillus nidulans

Cell growth necessitates extensive membrane remodeling events including vesicle fusion or fission, processes that are regulated by coat proteins. The hyphal cells of filamentous fungi concentrate both exocytosis and endocytosis at the apex. This investigation focuses on clathrin in Aspergillus nidulans, with the aim of understanding its role in membrane remodeling in growing hyphae. We examined clathrin heavy chain (ClaH-GFP) which localized to three distinct subcellular structures: late Golgi (trans-Golgi equivalents of filamentous fungi), which are concentrated just behind the hyphal tip but are intermittently present throughout all hyphal cells; the region of concentrated endocytosis just behind the hyphal apex (the "endocytic collar"); and small, rapidly moving puncta that were seen trafficking long distances in nearly all hyphal compartments. ClaH localized to distinct domains on late Golgi, and these clathrin "hubs" dispersed in synchrony after the late Golgi marker PH^OSBP . Although clathrin was essential for growth, ClaH did not colocalize well with the endocytic patch marker fimbrin. Tests of FM4-64 internalization and repression of ClaH corroborated the observation that clathrin does not play an important role in endocytosis in A. nidulans. A minor portion of ClaH puncta exhibited bidirectional movement, likely along microtubules, but were generally distinct from early endosomes.

Comparative Endocytosis Mechanisms and Anticancer Effect of HPMA Copolymer- and PAMAM Dendrimer-MTCP Conjugates for Photodynamic Therapy

Polymer architecture can influence biodistribution and the mode of presentation of bioactive agents to cells. Herein delivery, loading efficiency, and mode of cellular entry of polymer conjugates of the photosensitizer Meso-Tetra (4-Carboxyphenyl) Porphyrine (MTCP) are examined when attached to hyperbranched amine terminated poly(amido amine) (PAMAM) dendrimer or random coil linear N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer containing free amines in the side chains. The in vitro dark cytotoxicity and phototoxicity of MTCP and related conjugates are assessed on mouth epidermal carcinoma (KB) and human adenocarcinoma alveolar basal epithelial (A549) cells. Phototoxicity of polymeric conjugates increases by ≈100 and 4000 fold in KB and A549 cells compared with nonconjugated MTCP. The increase in phototoxicity activity is shown to result from increased rate of cellular uptake, whereas, cellular internalization of MTCP is negligible in comparison with the conjugated forms. The results of this study suggest the superiority of amine-terminated HPMA copolymer versus PAMAM dendrimer under study for delivery of MTCP. Treatment with various pharmacological inhibitors of endocytosis shows that polymer architecture influences the mechanism of cellular uptake of the conjugated photosensitizer. Results show that polymeric conjugates of MTCP improve solubility, influence the route and the rate of cellular internalization, and drastically enhance the uptake of the photosensitizer.

Two-Step Mechanism of Cellular Uptake of Cationic Gold Nanoparticles Modified by (16-Mercaptohexadecyl)trimethylammonium Bromide

Cationic colloidal gold nanorods (GNRs) have a great potential as a theranostic tool for diverse medical applications. GNRs' properties such as cellular internalization and stability are determined by physicochemical characteristics of their surface coating. GNRs modified by (16-mercaptohexadecyl)trimethylammonium bromide (MTAB), ^MTABGNRs, show excellent cellular uptake. Despite their promise for biomedicine, however, relatively little is known about the cellular pathways that facilitate the uptake of GNRs, their subcellular fate and intracellular persistence. Here we studied the mechanism of cellular internalization and long-term fate of GNRs coated with MTAB, for which the synthesis was optimized to give higher yield, in various human cell types including normal diploid versus cancerous, and dividing versus nondividing (senescent) cells. The process of ^MTABGNRs internalization into their final destination in lysosomes proceeds in two steps: (1) fast passive adhesion to cell membrane mediated by sulfated proteoglycans occurring within minutes and (2) slower active transmembrane and intracellular transport of individual nanorods via clathrin-mediated endocytosis and of aggregated nanorods via macropinocytosis. The expression of sulfated proteoglycans was the major factor determining the extent of uptake by the respective cell types. Upon uptake into proliferating cells, ^MTABGNRs were diluted equally and relatively rapidly into daughter cells; however, in nondividing/senescent cells the loss of ^MTABGNRs was gradual and very modest, attributable mainly to exocytosis. Exocytosed ^MTABGNRs can again be internalized. These findings broaden our knowledge about cellular uptake of gold nanorods, a crucial prerequisite for future successful engineering of nanoparticles for biomedical applications such as photothermal cancer therapy or elimination of senescent cells as part of the emerging rejuvenation approach.

PGE2 /EP4 Signaling Controls the Transfer of the Mammary Stem Cell State by Lipid Rafts in Extracellular Vesicles

Prostaglandin E₂ (PGE₂ )-initiated signaling contributes to stem cell homeostasis and regeneration. However, it is unclear how PGE₂ signaling controls cell stemness. This study identifies a previously unknown mechanism by which PGE₂ /prostaglandin E receptor 4 (EP₄ ) signaling regulates multiple signaling pathways (e.g., PI3K/Akt signaling, TGFβ signaling, Wnt signaling, EGFR signaling) which maintain the basal mammary stem cell phenotype. A shift of basal mammary epithelial stem cells (MaSCs) from a mesenchymal/stem cell state to a non-basal-MaSC state occurs in response to prostaglandin E receptor 4 (EP₄ ) antagonism. EP₄ antagonists elicit release of signaling components, by controlling their trafficking into extracellular vesicles/exosomes in a lipid raft/caveolae-dependent manner. Consequently, EP₄ antagonism indirectly inactivates, through induced extracellular vesicle/exosome release, pathways required for mammary epithelial stem cell homeostasis, e.g. canonical/noncanonical Wnt, TGFβ and PI3K/Akt pathways. EP₄ antagonism causes signaling receptors and signaling components to shift from non-lipid raft fractions to lipid raft fractions, and to then be released in EP₄ antagonist-induced extracellular vesicles/exosomes, resulting in the loss of the stem cell state by mammary epithelial stem cells. In contrast, luminal mammary epithelial cells can acquire basal stem cell properties following ingestion of EP₄ antagonist-induced stem cell extracellular vesicles/exosomes, and can then form mammary glands. These findings demonstrate that PGE₂ /EP₄ signaling controls homeostasis of mammary epithelial stem cells through regulating extracellular vesicle/exosome release. Reprogramming of mammary epithelial cells can result from EP₄ -mediated stem cell property transfer by extracellular vesicles/exosomes containing caveolae-associated proteins, between mammary basal and luminal epithelial cells. Stem Cells 2017;35:425-444.

Cell-penetrating self-nanoemulsifying drug delivery systems (SNEDDS) for oral gene delivery

OBJECTIVE

The aim of study was to investigate whether cell-penetrating peptides could amplify cellular uptake of plasmid DNA (pDNA) loaded self-nanoemulsifying drug delivery systems (SNEDDS) by mucosal epithelial cells, thereby enhancing transfection efficiency.

METHODS

HIV-1 Tat peptide-oleoyl conjugate (TAT-OL) was synthesized through amide bond formation between HIV-1 Tat-protein 49-57 (TAT) and oleoyl-chloride (OL). SNEDDS formulation contained 29.7% each of Cremophor EL, Capmul MCM and Crodamol, 9.9% propylene glycol and 1% TAT-OL. SNEDDS with OL instead of TAT-OL served as control.

RESULTS

Fluorescent-microscopy demonstrated 0.5% (m/v) nanoemulsions were suitable for subsequent studies. Mucus diffusion of nanoemulsion loaded with fluorescein diacetate (FDA) was 1.5-fold increased by incorporation of TAT-OL. Confocal microscopy confirmed that droplets of nanoemulsions were successfully internalized. Furthermore, quantitative analysis showed that addition of TAT-OL increases uptake of nanoemulsions by 2.3- and 2.6-folds after 2 and 4 hours of incubation, respectively. Cellular internalization pathways were found with substantial decrease in uptake in presence of indomethacin and chlorpromazine. Transfection efficiency investigated on HEK-293-cells was found to be 1.7- and 1.8-fold higher for SNEDDS loaded with TAT-OL compared to Lipofectin and control, respectively.

CONCLUSION

In comparison to prevailing lipid and polymer-based delivery systems, these novel cell-penetrating SNEDDS likely represent most effective, simplistic and expedite dosage form for mucosal gene delivery.

Probing the effect of an inhibitor of an ATPase domain of Hsc70 on clathrin-mediated endocytosis

Hsc70 is known to be involved in clathrin-mediated endocytosis (CME) by which cells take up various extracellular materials. More specifically, this protein promotes the disassembly of clathrin-coated vesicles (CCVs) by directly binding to clathrin during CME. As the ATPase activity of Hsc70 is required for its association with clathrin, we have investigated the effect of an inhibitor (apoptozole, Az) of an ATPase domain of Hsc70 on CME. The results of biochemical studies show that Az binds to Hsc70 and Hsp70 without binding to other types of heat shock proteins. Structure-activity relationship studies provide information on the structural features responsible for the inhibition of the ATPase activity of Hsc70. The results obtained from cell experiments reveal that Az disrupts the interaction of Hsc70 with clathrin in cells, thereby leading to the accumulation of transferrin in CCVs and suppression of release of free Fe(3+) from CCVs during transferrin receptor-mediated endocytosis.

Insight into the relationship between the cell culture model, cell trafficking and siRNA silencing efficiency

Despite research efforts, cell uptake processes determining siRNA silencing efficiency remain unclear. Here, we examine the relationship between in vitro cell culture models, cellular trafficking and siRNA silencing efficiency to provide a mechanistic insight on siRNA delivery system design. Model siRNA-polyplexes, based on chitosan as a ‘classical’ condensing agent, were applied to a panel of lung epithelial cell lines, H1299, A549 and Calu-3 and cell internalization levels, trafficking pathways and gene silencing assessed on exposure to pharmacological inhibitors. The data reveal striking differences in the internalization behaviour and gene silencing efficiency in the tested cell lines, despite their common lung epithelial origins. The model system’s silencing was lower where clathrin internalization pathway predominated in Calu-3, relative to silencing in H1299 cells where a non-clathrin internalization appears dominant. Increased silencing on endosomal disruption was apparent in Calu-3 cells, but absent when cellular internalization was not predominantly clathrin-mediated in A549 cells. This highlights that identifying cell trafficking pathways before incorporation of functional components to siRNA delivery systems (e.g. endosomolytic compounds) is crucial. The study hence stresses the importance of selection of appropriate cell culture model, relevant to in vivo target, to assess the gene silencing efficiency and decide which functionalities the ‘stratified siRNA silencing vector’ requires.

•

Relationship between cell type, uptake path and silencing examined to inform siRNA vector design.

•

Notable differences observed in cell uptake pathways and silencing despite cells’ common origin.

•

Addition of endosomolytic functionality shows no effect when non-clathrin pathways dominate.

•

Cell model important to assess silencing and decide which functionalities siRNA vector requires.

Discovery of the cell-penetrating function of A2 domain derived from LTA subunit of Escherichia coli heat-labile enterotoxin

Heat-labile enterotoxin (LT) is a protein toxin produced by enterotoxigenic Escherichia coli (ETEC). As a bacterial toxin, LT holotoxin can enter intestinal epithelial cells and cause diarrhea. In addition, LT is also a powerful mucosal adjuvant capable of enhancing the strong immune responses to co-administered antigens. However, the LT immunological mechanism is still not clear in some aspects, especially with the respect to how the LTA subunit functions alone. Here, we discovered that the A2 domain of LTA could carry a fluorescent protein into cells, whose function is similar to a cell-penetrating peptide. The transmembrane-transporting ability of the A2 domain is non-specific in its cell-penetrating function, which was shown through testing with different cell types. Moreover, the LTA2 fusion protein penetrated a fluorescently labeled cell membrane that identified LTA2 internalization through membrane transport pathways, and showed it finally localized in the endoplasmic reticulum. Furthermore, low-temperature stress and pharmacological agent treatments showed that the LTA2 internalization route is a temperature-dependent process involving the clathrin-mediated endocytosis and the macropinocytosis pathways. These results could explain the internalization of the LTA subunit alone without the LTB pentamer, contributing to a better understanding of LTA working as a mucosal adjuvant; they also suggest that the A2 domain could be used as a novel transport vehicle for research and treatment of disease.

Viral entry pathways: the example of common cold viruses

For infection, viruses deliver their genomes into the host cell. These nucleic acids are usually tightly packed within the viral capsid, which, in turn, is often further enveloped within a lipid membrane. Both protect them against the hostile environment. Proteins and/or lipids on the viral particle promote attachment to the cell surface and internalization. They are likewise often involved in release of the genome inside the cell for its use as a blueprint for production of new viruses. In the following, I shall cursorily discuss the early more general steps of viral infection that include receptor recognition, uptake into the cell, and uncoating of the viral genome. The later sections will concentrate on human rhinoviruses, the main cause of the common cold, with respect to the above processes. Much of what is known on the underlying mechanisms has been worked out by Renate Fuchs at the Medical University of Vienna.

Combined sodium ion sensitivity in agonist binding and internalization of vasopressin V1b receptors

Reducing Na⁺ in the extracellular environment may lead to two beneficial effects for increasing agonist binding to cell surface G-protein coupled receptors (GPCRs): reduction of Na⁺-mediated binding block and reduce of receptor internalization. However, such combined effects have not been explored. We used Chinese Hamster Ovary cells expressing vasopressin V1b receptors as a model to explore Na⁺ sensitivity in agonist binding and receptor internalization. Under basal conditions, a large fraction of V1b receptors is located intracellularly, and a small fraction is in the plasma membrane. Decreases in external Na⁺ increased cell surface [³H]AVP binding and decreased receptor internalization. Substitution of Na⁺ by Cs⁺ or NH₄⁺ inhibited agonist binding. To suppress receptor internalization, the concentration of NaCl, but not of CsCl, had to be less than 50 mM, due to the high sensitivity of the internalization machinery to Na⁺ over Cs⁺. Iso-osmotic supplementation of glucose or NH₄Cl maintained internalization of the V1b receptor, even in a low-NaCl environment. Moreover, iodide ions, which acted as a counter anion, inhibited V1b agonist binding. In summary, we found external ionic conditions that could increase the presence of high-affinity state receptors at the cell surface with minimum internalization during agonist stimulations.

Combination Approach of YSA Peptide Anchored Docetaxel Stealth Liposomes with Oral Antifibrotic Agent for the Treatment of Lung Cancer

Therapeutic efficacy of nanocarriers can be amplified by active targeting and overcoming the extracellular matrix associated barriers of tumors. The aim of the present study was to investigate the effect of oral antifibrotic agent (telmisartan) on tumor uptake and anticancer efficacy of EphA2 receptor targeted liposomes. Docetaxel loaded PEGylated liposomes (DPL) functionalized with nickel chelated phospholipid were prepared using a modified hydration method. DPL were incubated with various concentrations of histidine tagged EphA2 receptor specific peptide (YSA) to optimize particle size, zeta potential, and percentage YSA binding. Cellular uptake studies using various endocytosis blockers revealed that a caveolae dependent pathway was the major route for internalization of YSA anchored liposomes of docetaxel (YDPL) in A549 lung cancer cell line. Hydrodynamic diameter and zeta potential of optimized YDPL were 157.3 ± 11.8 nm and -3.64 mV, respectively. Orthotopic lung tumor xenograft (A549) bearing athymic nude mice treated with oral telmisartan (5 mg/kg) for 2 days showed significantly (p < 0.05) higher uptake of YDPL in tumor tissues compared to healthy tissue. Average lung tumor weight of the YDPL + telmisartan treated group was 4.8- and 3.8-fold lower than that of the DPL and YDPL treated groups (p < 0.05). Substantially lower expression (p < 0.05) of EphA2 receptor protein, proliferating cell nuclear antigen (PCNA), MMP-9, and collagen 1A level with increased E-cadherin and TIMP-1 levels in immunohistochemistry and Western blot analysis of lung tumor samples of the combination group confirmed antifibrotic effect with enhanced anticancer activity. Active targeting and ECM remodeling synergistically contributed to anticancer efficacy of YDPL in orthotopic lung cancer.

Human Host Defense Cathelicidin Peptide LL-37 Enhances the Lipopolysaccharide Uptake by Liver Sinusoidal Endothelial Cells without Cell Activation

The liver is a major organ that removes waste substances from the blood, and liver sinusoidal endothelial cells (LSECs) are professional scavenger cells, which incorporate and degrade various endogenous and exogenous molecules including pathogenic factor LPS. Mammalian cells express a number of peptide antibiotics that function as effectors in the innate host defense systems. LL-37, a human cathelicidin antimicrobial peptide, has a potent LPS-neutralizing activity and exhibits protective actions on various infection models. However, the effect of LL-37 on the LPS clearance has not been clarified. In this study, to further understand the host-protective mechanism of LL-37, we evaluated the effect of LL-37 on the LPS clearance in vitro. LL-37 enhanced the LPS uptake by human LSECs. Of interest, LL-37 was similarly incorporated into LSECs both in the presence and the absence of LPS, and the incorporated LPS and LL-37 were colocalized in LSECs. Importantly, the uptake of LPS and LL-37 was inhibited by endocytosis inhibitors, heparan sulfate proteoglycan analogs, and glycosaminoglycan lyase treatment of the cells. Moreover, the uptake of LL-37-LPS did not activate TLR4 signaling in both MyD88-dependent and -independent pathways. In addition, the incorporated LL-37-LPS was likely transported to the lysosomes in LSECs. Together these observations suggest that LL-37 enhances the LPS uptake by LSECs via endocytosis through the complex formation with LPS and the interaction with cell-surface heparan sulfate proteoglycans, thereby facilitating the intracellular incorporation and degradation of LPS without cell activation. In this article, we propose a novel function of LL-37 in enhancing LPS clearance.

Cysteine and arginine-rich peptides as molecular carriers

A number of linear and cyclic peptides containing alternative arginine and cysteine residues, namely linear (CR)3, linear (CR)4, linear (CR)5, cyclic [CR]4, and cyclic [CR]5, were synthesized. The peptides were evaluated for their ability to deliver two molecular cargos, fluorescence-labeled cell-impermeable negatively charged phosphopeptide (F'-GpYEEI) and fluorescence-labeled lamivudine (F'-3TC), intracellularly in human leukemia cancer (CCRF-CEM) cells. We investigated the role of cyclization and the number of amino acids in improving the transporting ability of the peptides. The flow cytometry studies suggested that the synthesized peptides were able to work efficiently as transporters for both cargos. Among all compounds, cyclic [CR]4 was found to be the most efficient peptide in transporting the cargo into cells. For instance, the cellular uptake of F'-3TC (5μM) and F'-GpYEEI (5μM) was enhanced by 16- and 20-fold, respectively, in the presence of cyclic [CR]4 compared to that of the parent compound alone. The mechanism of F'-GpYEEI uptake by cells was found to be energy-independent. The results showed that the number of amino acids and their cyclic nature can impact the efficiency of the peptide in transporting the molecular cargos.

In-depth study on the gene silencing capability of silica nanoparticles with different pore sizes: degree and duration of RNA interference

The mesoporous silica nanoparticles (MSN) having different pore sizes were synthesized and utilized for siRNA delivery system capable of controlling kinetics of RNA interference.

Different oligoarginine modifications alter endocytic pathways and subcellular trafficking of polymeric nanoparticles

Different length of oligoarginine ligands alter both endocytic pathways and subcellular trafficking of PEG-b-PCL nanoparticles.

Involvement of an intracellular vesicular transport process in naked-sgRNA-mediated TRUE gene silencing

tRNase ZL-utilizing efficacious gene silencing (TRUE gene silencing) is an RNA-mediated gene expression control technology with therapeutic potential. Recently, our group demonstrated that a heptamer, mh1 (Bcl‑2), targeting human Bcl-2 mRNA, can be taken up by cells without the use of any transfection reagents and can induce the apoptosis of leukemia cells. However, little is known regarding the mechanism of naked small guide (sg)RNA uptake by cultured cells. Therefore, in the present study the effects of various inhibitors on the induction of apoptosis by naked sgRNA treatment were investigated in order to identify the uptake pathway required for sgRNA function in cultured cells. Addition of the endocytosis inhibitors chlorpromazine, nystatin or methyl‑β‑cyclodextrin together with naked effective sgRNA was unable to diminish the apoptosis‑inducing effects of naked sgRNA or the reduction in target mRNA, suggesting that functional uptake of sgRNA by cells is clathrin‑, caveolae‑ and raft‑independent. Next, chloroquine, an inhibitor of lysosome acidification, and brefeldin A, an inhibitor that blocks protein transport from the Golgi apparatus to the endoplasmic reticulum were administered. In the presence of these compounds, the apoptosis‑inducing effects of naked sgRNA were reduced. These results suggest that a vesicular transport process is involved in sgRNA‑mediated TRUE gene silencing. A greater understanding of how naked sgRNAs enter cells and how they reach their target RNAs may aid in the design of more specifically‑targeted and potent sgRNA drugs.

Discovery of a non-cationic cell penetrating peptide derived from membrane-interacting human proteins and its potential as a protein delivery carrier

Cell penetrating peptides (CPPs) are peptides that can be translocated into cells and used as a carrier platform for the intracellular uptake of cargo molecules. Subject to the source of CPP sequences and their positively charged nature, the cytotoxicity and immunogenicity of conventional CPPs needs to be optimized to expand their utility for biomedical applications. In addition to these safety issues, the stability of CPPs needs to be addressed since their positively charged residues are prone to interact with the biological milieu. As an effort to overcome these limitations of the current CPP technology, we isolated CPP candidate sequences and synthesized peptides from twelve isoforms of annexin, a family of membrane-interacting human proteins. The candidate screen returned a CPP rich in hydrophobic residues that showed more efficient cellular uptake than TAT-CPP. We then investigated the uptake mechanism, subcellular localization, and biophysical properties of the newly found CPP, verifying low cytotoxicity, long-term serum stability, and non-immunogenicity. Finally, model proteins conjugated to this peptide were successfully delivered into mammalian cells both in vitro and in vivo, indicating a potential use of the peptide as a carrier for the delivery of macromolecular cargos.

Oral delivery of nanoparticles containing anticancer SN38 and hSET1 antisense for dual therapy of colon cancer

An oral delivery system intended for treatment of colon cancer in HT29 cancerous cells was investigated by encapsulating hSET1 antisense and SN38 anticancer in nanoparticles based on cysteine trimethyl chitosan (cysTMC) and carboxymethyl dextran (CMD). Studies have shown hSET1 as the main type of histone methyltransferase (HMT) complex, is significantly overexpressed in malignant cells. In this study, hSET1 antisense was employed to inhibit gene expression. Additionally, SN38 was incorporated into nanoparticles to enhance the efficiency of the system by inhibition of topoisomerase 1. CysTMC was synthetized and characterized by (1)H NMR and FTIR. Nanoparticles were prepared through complexation of CMD and cysTMC. Particle size and surface charge was 100-150 nm and 17-21 mV respectively with drug content of around 2.6%. Gel electrophoresis assay proved the stability of antisense in simulated gastric and intestinal fluids. Nanoparticles showed high mucoadhesion and glutathione responsive release. Cellular uptake was observed by confocal microscopy and quantified by flow cytometry. Cytotoxicity of NPs was assessed using MTT assay. Results showed hSET1/SN38 nanoparticles had significantly higher cytotoxicity against HT29 cells compared with nanoparticles containing SN38, free SN38 or naked hSET1. Therefore, present system could be considered an effective combination therapy of highly hydrophobic SN38 and hSET1.

Understanding nanoparticle cellular entry: A physicochemical perspective

Understanding interactions between nanoparticles (NPs) with biological matter, particularly cells, is becoming increasingly important due to their growing application in medicine and materials, and consequent biological and environmental exposure. For NPs to be utilised to their full potential, it is important to correlate their functional characteristics with their physical properties, which may also be used to predict any adverse cellular responses. A key mechanism for NPs to impart toxicity is to gain cellular entry directly. Many parameters affect the behaviour of nanomaterials in a cellular environment particularly their interactions with cell membranes, including their size, shape and surface chemistry as well as factors such as the cell type, location and external environment (e.g. other surrounding materials, temperature, pH and pressure). Aside from in vitro and in vivo experiments, model cell membrane systems have been used in both computer simulations and physicochemical experiments to elucidate the mechanisms for NP cellular entry. Here we present a brief overview of the effects of NPs physical parameters on their cellular uptake, with focuses on 1) related research using model membrane systems and physicochemical methodologies; and 2) proposed physical mechanisms for NP cellular entrance, with implications to their nanotoxicity. We conclude with a suggestion that the energetic process of NP cellular entry can be evaluated by studying the effects of NPs on lipid mesophase transitions, as the molecular deformations and thus the elastic energy cost are analogous between such transitions and endocytosis. This presents an opportunity for contributions to understanding nanotoxicity from a physicochemical perspective.

Beclin 1 regulates growth factor receptor signaling in breast cancer

Beclin 1 is a haploinsufficient tumor suppressor that is decreased in many human tumors. The function of beclin 1 in cancer has been attributed primarily to its role in the degradative process of macroautophagy. However, beclin 1 is a core component of the vacuolar protein sorting 34 (Vps34)/class III phosphatidylinositoI-3 kinase (PI3KC3) and Vps15/p150 complex that regulates multiple membrane-trafficking events. In the current study, we describe an alternative mechanism of action for beclin 1 in breast cancer involving its control of growth factor receptor signaling. We identify a specific stage of early endosome maturation that is regulated by beclin 1, the transition of APPL1-containing phosphatidyIinositol 3-phosphate-negative (PI3P(-)) endosomes to PI3P(+) endosomes. Beclin 1 regulates PI3P production in response to growth factor stimulation to control the residency time of growth factor receptors in the PI3P(-)/APPL(+)-signaling-competent compartment. As a result, suppression of BECN1 sustains growth factor-stimulated AKT and ERK activation resulting in increased breast carcinoma cell invasion. In human breast tumors, beclin 1 expression is inversely correlated with AKT and ERK phosphorylation. Our data identify a novel role for beclin 1 in regulating growth factor signaling and reveal a mechanism by which loss of beclin 1 expression would enhance breast cancer progression.

Oddershede L, Loeschner K, Larsen EH, Loft S, Møller P. Uptake of gold nanoparticles in primary human endothelial cells

Single-particle resolution techniques show that endothelial cells internalise 80 nm unmodified gold nanoparticles by endocytosis with subsequent transport to vesicles.

A Multilayered Cell Culture Model for Transport Study in Solid Tumors: Evaluation of Tissue Penetration of Polyethyleneimine Based Cationic Micelles

Limited drug distribution is partially responsible for the efficacy gap between preclinical and clinical studies of nano-sized drug carriers for cancer therapy. In this study, we examined the transport behavior of cationic micelles formed from a triblock copolymer of poly(D,L-lactide-co-glycolide)-block-branched polyethyleneimine-block-poly(D,L-lactide-co-glycolide) using a unique in vitro tumor model composed of a multilayered cell culture (MCC) and an Ussing chamber system. The Cy3-labeled cationic micelles showed remarkable Cy3 distribution in the MCC whereas charge-shielded micelles with a poly(ethylene glycol) surface accumulated on the surface of the MCC. Penetration occurred against convectional flow caused by a hydraulic pressure gradient. The study using fluorescence resonance energy transfer (FRET) showed that the cationic micelles dissociate at the interface between the culture media and the MCC or possibly inside of the first-layer cells and penetrates into the MCC as unimers. The penetration and distribution were energy-dependent and suppressed by various endocytic inhibitors. These suggest that cationic unimers mainly utilized clathrin-mediated endocytosis and macropinocytosis for cellular entry and a significant fraction were exocytosed by an unknown mechanism.

Identification and characterisation of small molecule inhibitors of feline coronavirus replication

•

Chloroquine, mefloquine, and hexamethylene amiloride demonstrated marked inhibition of FIPV viral replication.

•

All three compounds acted at an early stage of viral replication.

•

A cost effective resazurin-based cytopathic effect inhibition assay was developed for screening compounds.

•

These antiviral compounds, warrant further investigation for clinical use in cats with FIP.

Feline infectious peritonitis (FIP), a feline coronavirus (FCoV) induced disease, is almost invariably fatal with median life expectancy measured in days. Current treatment options are, at best, palliative. The objectives of this study were to evaluate a panel of nineteen candidate compounds for antiviral activity against FCoV in vitro to determine viable candidates for therapy. A resazurin-based cytopathic effect inhibition assay, which detects viable cells through their reduction of the substrate resazurin to fluorescent resorufin, was developed for screening compounds for antiviral efficacy against FCoV. Plaque reduction and virus yield reduction assays were performed to confirm antiviral effects of candidate compounds identified during screening, and the possible antiviral mechanisms of action of these compounds were investigated using virucidal suspension assays and CPE inhibition and IFA-based time of addition assays. Three compounds, chloroquine, mefloquine, and hexamethylene amiloride demonstrated marked inhibition of virus induced CPE at low micromolar concentrations. Orthogonal assays confirmed inhibition of CPE was associated with significant reductions in viral replication. Selectivity indices calculated based on in vitro cytotoxicity screening and reductions in extracellular viral titre were 217, 24, and 20 for chloroquine, mefloquine, and hexamethylene amiloride respectively. Preliminary experiments performed to inform the antiviral mechanism of the compounds demonstrated all three acted at an early stage of viral replication. These results suggest that these direct acting antiviral compounds, or their derivatives, warrant further investigation for clinical use in cats with FIP.

Flipping the Switch on Clathrin-Mediated Endocytosis using Thermally Responsive Protein Microdomains

A ubiquitous approach to study protein function is to knock down activity (gene deletions, siRNA, small molecule inhibitors, etc) and study the cellular effects. Using a new methodology, this manuscript describes how to rapidly and specifically switch off cellular pathways using thermally responsive protein polymers. A small increase in temperature stimulates cytosolic elastin-like polypeptides (ELPs) to assemble microdomains. We hypothesize that ELPs fused to a key effector in a target macromolecular complex will sequester the complex within these microdomains, which will bring the pathway to a halt. To test this hypothesis, we fused ELPs to clathrin-light chain (CLC), a protein associated with clathrin-mediated endocytosis. Prior to thermal stimulation, the ELP fusion is soluble and clathrin-mediated endocytosis remains 'on.' Increasing the temperature induces the assembly of ELP fusion proteins into organelle-sized microdomains that switches clathrin-mediated endocytosis 'off.' These microdomains can be thermally activated and inactivated within minutes, are reversible, do not require exogenous chemical stimulation, and are specific for components trafficked within the clathrin-mediated endocytosis pathway. This temperature-triggered cell switch system represents a new platform for the temporal manipulation of trafficking mechanisms in normal and disease cell models and has applications for manipulating other intracellular pathways.

Major histocompatibility complex class I downregulation induced by equine herpesvirus type 1 pUL56 is through dynamin-dependent endocytosis

Equine herpesvirus type 1 (EHV-1) downregulates cell surface expression of major histocompatibility complex class I (MHC-I) in infected cells. We have previously shown that pUL56 encoded by the EHV-1 ORF1 gene regulates the process (G. Ma, S. Feineis, N. Osterrieder, and G. R. Van de Walle, J. Virol. 86:3554-3563, 2012, doi:http://dx.doi.org/10.1128/JVI.06994-11). Here, we report that cell surface MHC-I in EHV-1-infected cells is internalized and degraded in the lysosomal compartment in a pUL56-dependent fashion. pUL56-induced MHC-I endocytosis required dynamin and tyrosine kinase but was independent of clathrin and caveolin-1, the main constituents of the clathrin- and raft/caveola-mediated endocytosis pathways, respectively. Downregulation of cell surface MHC-I was significantly inhibited by the ubiquitin-activating enzyme E1 inhibitor PYR41, indicating that ubiquitination is essential for the process. Finally, we show that downregulation is not specific for MHC-I and that other molecules, including CD46 and CD63, are also removed from the cell surface in a pUL56-dependent fashion.

IMPORTANCE

We show that alphaherpesvirus induces MHC-I downregulation through endocytosis, which is mediated by pUL56. The dynamin-dependent endocytic pathway is responsible for MHC-I internalization in infected cells. Furthermore, we discovered that this endocytic process can be disrupted by the inhibiting ubiquitin-activating E1 enzyme, which is indispensable for ubiquitination. Finally, pUL56 action extends to a number of cell surface molecules that are significant for host immunity. Therefore, the protein may exert a more general immunomodulatory effect.

Localization of APOL1 protein and mRNA in the human kidney: nondiseased tissue, primary cells, and immortalized cell lines

Although APOL1 gene variants are associated with nephropathy in African Americans, little is known about APOL1 protein synthesis, uptake, and localization in kidney cells. To address these questions, we examined APOL1 protein and mRNA localization in human kidney and human kidney-derived cell lines. Indirect immunofluorescence microscopy performed on nondiseased nephrectomy cryosections from persons with normal kidney function revealed that APOL1 protein was markedly enriched in podocytes (colocalized with synaptopodin and Wilms' tumor suppressor) and present in lower abundance in renal tubule cells. Fluorescence in situ hybridization detected APOL1 mRNA in glomeruli (podocytes and endothelial cells) and tubules, consistent with endogenous synthesis in these cell types. When these analyses were extended to renal-derived cell lines, quantitative RT-PCR did not detect APOL1 mRNA in human mesangial cells; however, abundant levels of APOL1 mRNA were observed in proximal tubule cells and glomerular endothelial cells, with lower expression in podocytes. Western blot analysis revealed corresponding levels of APOL1 protein in these cell lines. To explain the apparent discrepancy between the marked abundance of APOL1 protein in kidney podocytes observed in cryosections versus the lesser abundance in podocyte cell lines, we explored APOL1 cellular uptake. APOL1 protein was taken up readily by human podocytes in vitro but was not taken up efficiently by mesangial cells, glomerular endothelial cells, or proximal tubule cells. We hypothesize that the higher levels of APOL1 protein in human cryosectioned podocytes may reflect both endogenous protein synthesis and APOL1 uptake from the circulation or glomerular filtrate.

Enhanced cellular uptake of maleimide-modified liposomes via thiol-mediated transport

With a small amount of maleimide modification on the liposome surface, enhanced cellular uptake of liposomes and drug-delivery efficiency can be obtained both in vitro and in vivo. Herein, we describe the mechanisms underlying this enhanced cellular uptake. Suppression of the cellular uptake of maleimide-modified liposomes (M-GGLG, composed of 1,5-dihexadecyl N,N-diglutamyl-lysyl-L-glutamate [GGLG]/cholesterol/poly(ethylene glycol) - 1,2-distearoyl-sn-glycero-3-phosphoethanolamine [PEG₅₀₀₀-DSPE]/maleimide [M]-PEG₅₀₀₀-Glu2C18 at a molar ratio of 5:5:0.03:0.03) caused by temperature block and addition of serum was alleviated compared with that of liposomes without maleimide modification (GGLG liposomes, composed of GGLG/cholesterol/PEG₅₀₀₀-DSPE/PEG₅₀₀₀-Glu2C₁₈ at a molar ratio of 5:5:0.03:0.03). When 0.01 nM N-ethylmaleimide was used to pre-block cellular thiols, the cellular uptake of M-GGLG liposomes was decreased to approximately 70% in HeLa, HCC1954, MDA-MB-468, and COS-7 cell lines. Moreover, inhibition of a thiol-related reductase such as protein disulfide isomerase resulted in a 15%-45% inhibition of the cellular uptake of M-GGLG liposomes, whereas GGLG liposomes were not influenced. Further, single and mixed inhibitors of clathrin-mediated endocytosis, caveolae-mediated endocytosis, and macropinocytosis did not efficiently inhibit the cellular uptake of M-GGLG liposomes. Using confocal microscopy, we verified that M-GGLG liposomes were localized partially in lysosomes after inhibition of the mentioned conventional endocytic pathways. Therefore, it was hypothesized that the mechanisms underlying the enhanced cellular uptake of liposomes by maleimide modification was thiol-mediated membrane trafficking, including endocytosis and energy-independent transport.

Cytadherence of Mycoplasma pneumoniae induces inflammatory responses through autophagy and toll-like receptor 4

Mycoplasma pneumoniae causes pneumonia, tracheobronchitis, pharyngitis, and asthma in humans. The pathogenesis of M. pneumoniae infection is attributed to excessive immune responses. We previously demonstrated that M. pneumoniae lipoproteins induced inflammatory responses through Toll-like receptor 2 (TLR2). In the present study, we demonstrated that M. pneumoniae induced strong inflammatory responses in macrophages derived from TLR2 knockout (KO) mice. Cytokine production in TLR2 KO macrophages was increased compared with that in the macrophages of wild-type (WT) mice. Heat-killed, antibiotic-treated, and overgrown M. pneumoniae failed to induce inflammatory responses in TLR2 KO macrophages. 3-Methyladenine and chloroquine, inhibitors of autophagy, decreased the induction of inflammatory responses in TLR2 KO macrophages. These inflammatory responses were also inhibited in macrophages treated with the TLR4 inhibitor VIPER and those obtained from TLR2 and TLR4 (TLR2/4) double-KO mice. Two mutants that lacked the ability to induce inflammatory responses in TLR2 KO macrophages were obtained by transposon mutagenesis. The transposons were inserted in atpC encoding an ATP synthase F0F1 ε subunit and F10_orf750 encoding hypothetical protein MPN333. These mutants showed deficiencies in cytadherence. These results suggest that cytadherence of M. pneumoniae induces inflammatory responses through TLR4 and autophagy.

Prolonged activity of the pestiviral RNase Erns as an interferon antagonist after uptake by clathrin-mediated endocytosis

The RNase activity of the envelope glycoprotein E(rns) of the pestivirus bovine viral diarrhea virus (BVDV) is required to block type I interferon (IFN) synthesis induced by single-stranded RNA (ssRNA) and double-stranded RNA (dsRNA) in bovine cells. Due to the presence of an unusual membrane anchor at its C terminus, a significant portion of E(rns) is also secreted. In addition, a binding site for cell surface glycosaminoglycans is located within the C-terminal region of E(rns). Here, we show that the activity of soluble E(rns) as an IFN antagonist is not restricted to bovine cells. Extracellularly applied E(rns) protein bound to cell surface glycosaminoglycans and was internalized into the cells within 1 h of incubation by an energy-dependent mechanism that could be blocked by inhibitors of clathrin-dependent endocytosis. E(rns) mutants that lacked the C-terminal membrane anchor retained RNase activity but lost most of their intracellular activity as an IFN antagonist. Surprisingly, once taken up into the cells, E(rns) remained active and blocked dsRNA-induced IFN synthesis for several days. Thus, we propose that E(rns) acts as an enzymatically active decoy receptor that degrades extracellularly added viral RNA mainly in endolysosomal compartments that might otherwise activate intracellular pattern recognition receptors (PRRs) in order to maintain a state of innate immunotolerance.

IMPORTANCE

The pestiviral RNase E(rns) was previously shown to inhibit viral ssRNA- and dsRNA-induced interferon (IFN) synthesis. However, the localization of E(rns) at or inside the cells, its species specificity, and its mechanism of interaction with cell membranes in order to block the host's innate immune response are still largely unknown. Here, we provide strong evidence that the pestiviral RNase E(rns) is taken up within minutes by clathrin-mediated endocytosis and that this uptake is mostly dependent on the glycosaminoglycan binding site located within the C-terminal end of the protein. Remarkably, the inhibitory activity of E(rns) remains for several days, indicating the very potent and prolonged effect of a viral IFN antagonist. This novel mechanism of an enzymatically active decoy receptor that degrades a major viral pathogen-associated molecular pattern (PAMP) might be required to efficiently maintain innate and, thus, also adaptive immunotolerance, and it might well be relevant beyond the bovine species.

Non-specificity of Pitstop 2 in clathrin-mediated endocytosis

Small molecule inhibitors of clathrin-mediated endocytosis are highly desired for the dissection of membrane trafficking pathways in the lab and for potential use as anti-infectives in the clinic. One inhibition strategy is to prevent clathrin from contacting adaptor proteins so that clathrin-mediated endocytosis cannot occur. “Pitstop” compounds have been developed that block only one of the four functional interaction sites on the N-terminal domain of clathrin heavy chain. Despite this limitation, Pitstop 2 causes profound inhibition of clathrin-mediated endocytosis. In this study, we probed for non-specific activity of Pitstop 2 by examining its action in cells expressing clathrin heavy chain harbouring mutations in the N-terminal domain interaction sites. We conclude that the inhibition observed with this compound is due to non-specificity, i.e. it causes inhibition away from its proposed mode of action. We recommend that these compounds be used with caution in cells and that they should not be used to conclude anything of the function of clathrin's N-terminal domain.

Enteroviruses harness the cellular endocytic machinery to remodel the host cell cholesterol landscape for effective viral replication

Cholesterol is a critical component of cellular membranes, regulating assembly and function of membrane-based protein/lipid complexes. Many RNA viruses, including enteroviruses, remodel host membranes to generate organelles with unique lipid blueprints on which they assemble replication complexes and synthesize viral RNA. Here we find that clathrin-mediated endocytosis (CME) is harnessed by enteroviruses to traffic cholesterol from the plasma membrane (PM) and extracellular medium to replication organelles, where cholesterol then regulates viral polyprotein processing and facilitates genome synthesis. When CME is disrupted, cellular cholesterol pools are instead stored in lipid droplets, cholesterol cannot be trafficked to replication organelles, and replication is inhibited. In contrast, replication is stimulated in cholesterol-elevated cells like those lacking caveolins or those from Niemann-Pick disease patients. Our findings indicate cholesterol as a critical determinant for enteroviral replication and outline roles for the endocytic machinery in both the enteroviral life cycle and host cell cholesterol homeostasis.

Lysosomal enzymes may cross the blood-brain-barrier by pinocytosis: implications for enzyme replacement therapy

Here we hypothesized that the water-soluble lysosomal enzymes may cross the blood-brain-barrier and reach the brain using the mechanism of unspecific fluid-phase endocytosis. We also highlight studies that show that, at higher serum concentrations, a fraction of these proteins can reach the brain after intravenous injection, and we suggest some experiments to study this hypothesis. Finally we discuss the implications of this for treatments such as enzyme replacement of lysosomal storage disorders.