A novel endocytic pathway induced by clustering endothelial ICAM-1 or PECAM-1

Utilization of I-domain of LFA-1 to Target Drug and Marker Molecules to Leukocytes

The long-term objective of this project is to utilize the I-domain protein for the α-subunit of LFA-1 to target drugs to lymphocytes by binding to ICAM receptors on the cell surface. The short-term goal is to provide proof-of-concept that I-domain conjugated to small molecules can still bind to and uptake by ICAM-1 on the surface of lymphocytes (i.e., Raji cells). To accomplish this goal, the I-domain protein was labeled with FITC at several lysine residues to produce the FITC-I-domain and CD spectroscopy showed that the FITC-I-domain has a secondary structure similar to that of the parent I-domain. The FITC-I-domain was taken up by Raji cells via receptor-mediated endocytosis and its uptake can be blocked by anti-I-domain mAb but not by its isotype control. Antibodies to ICAM-1 enhance the binding of I-domain to ICAM-1, suggesting it binds to ICAM-1 at different sites than the antibodies. The results indicate that fluorophore modification does not alter the binding and uptake properties of the I-domain protein. Thus, I-domain could be useful as a carrier of drug to target ICAM-1-expressing lymphocytes.

Overcoming in vivo barriers to targeted nanodelivery

Nanoparticles have been investigated as promising nanocarriers for delivery of imaging and therapeutic agents for several decades, but have met with limited success. Although enormous progress in the fields of nanotechnology and nanoscience has been achieved, basic discoveries have not yet translated into effective targeted therapies. Nanoparticles can potentially improve the pharmacokinetics and pharmacodynamics of drugs; however, the complexity of in vivo systems imposes multiple barriers that severely inhibit efficiency and have to be overcome to fully exploit the theoretical potential of nanoparticles. Here, we address two major challenges to effective systemic nanodelivery. Both limited penetration across the vascular endothelium and uptake by the reticuloendothelial system (RES) substantially impede effectiveness of nanoparticle delivery into tissues. Although the design of nanoparticles with extended circulation half-life is essential, it is not sufficient for effective penetration of nanoparticles across the formidable barrier formed by the vascular endothelium. Current nanodelivery systems rely on passive transvascular exchange and tissue accumulation. They require high dosages to create large concentration gradients that drive nanoparticles passively across the blood-tissue interface. However, passive accumulation has resulted in only a fractional dosage of nanoparticles penetrating into target tissue. This inevitably diminishes therapeutic efficacy and aggravates potential side effects. Although there are multiple ways to augment passive delivery, active delivery of targeted nanoparticles across the vascular endothelium could significantly increase the therapeutic index and decrease side effects of nanoparticle-based drug delivery systems. Use of active transendothelial transport pathways, such as caveolae, may provide an effective solution to both target and deliver nanoparticles.

Polymer-based therapeutics: nanoassemblies and nanoparticles for management of atherosclerosis

Coronary arterial disease, one of the leading causes of adult mortality, is triggered by atherosclerosis. A disease with complex etiology, atherosclerosis results from the progressive long-term combination of atherogenesis, the accumulation of modified lipoproteins within blood vessel walls, along with vascular and systemic inflammatory processes. The management of atherosclerosis is challenged by the localized flare-up of several multipronged signaling interactions between activated monocytes, atherogenic macrophages and inflamed or dysfunctional endothelial cells. A new generation of approaches is now emerging founded on multifocal, targeted therapies that seek to reverse or ameliorate the atheroinflammatory cascade within the vascular intima. This article reviews the various classes and primary examples of bioactive configurations of nanoscale assemblies. Of specific interest are polymer-based or polymer-lipid micellar assemblies designed as multimodal receptor-targeted blockers or drug carriers whose activity can be tuned by variations in polymer hydrophobicity, charge, and architecture. Also reviewed are emerging reports on multifunctional nanoassemblies and nanoparticles for improved circulation and enhanced targeting to atheroinflammatory lesions and atherosclerotic plaques.

Catalase and superoxide dismutase conjugated with platelet-endothelial cell adhesion molecule antibody distinctly alleviate abnormal endothelial permeability caused by exogenous reactive oxygen species and vascular endothelial growth factor

Reactive oxygen species (ROS) superoxide anion (O(2)()) and hydrogen peroxide (H(2)O(2)) produced by activated leukocytes and endothelial cells in sites of inflammation or ischemia cause endothelial barrier dysfunction that may lead to tissue edema. Antioxidant enzymes (AOEs) catalase and superoxide dismutase (SOD) conjugated with antibodies to platelet-endothelial cell adhesion molecule-1 (PECAM-1) specifically bind to endothelium, quench the corresponding ROS, and alleviate vascular oxidative stress and inflammation. In the present work, we studied the effects of anti-PECAM/catalase and anti-PECAM/SOD conjugates on the abnormal permeability manifested by transendothelial electrical resistance decline, increased fluorescein isothiocyanate-dextran influx, and redistribution of vascular endothelial-cadherin in human umbilical vein endothelial cell (HUVEC) monolayers. Anti-PECAM/catalase protected HUVEC monolayers against H(2)O(2)-induced endothelial barrier dysfunction. Polyethylene glycol-conjugated catalase exerted orders of magnitude lower endothelial uptake and no protective effect, similarly to IgG/catalase. Anti-PECAM/catalase, but not anti-PECAM/SOD, alleviated endothelial hyperpermeability caused by exposure to hypoxanthine/xanthine oxidase, implicating primarily H(2)O(2) in the disruption of the endothelial barrier in this model. Thrombin-induced endothelial permeability was not affected by treatment with anti-PECAM/AOEs or the NADPH oxidase inhibitor apocynin or overexpression of AOEs, indicating that the endogenous ROS play no key role in thrombin-mediated endothelial barrier dysfunction. In contrast, anti-PECAM/SOD, but not anti-PECAM/catalase, inhibited a vascular endothelial growth factor (VEGF)-induced increase in endothelial permeability, identifying a key role of endogenous O(2)() in the VEGF-mediated regulation of endothelial barrier function. Therefore, AOEs targeted to endothelial cells provide versatile molecular tools for testing the roles of specific ROS in vascular pathology and may be translated into remedies for these ROS-induced abnormalities.

Targeted modulation of reactive oxygen species in the vascular endothelium

'Endothelial cells lining vascular luminal surface represent an important site of signaling and injurious effects of reactive oxygen species (ROS) produced by other cells and endothelium itself in ischemia, inflammation and other pathological conditions. Targeted delivery of ROS modulating enzymes conjugated with antibodies to endothelial surface molecules (vascular immunotargeting) provides site-specific interventions in the endothelial ROS, unattainable by other formulations including PEG-modified enzymes. Targeting of ROS generating enzymes (e.g., glucose oxidase) provides ROS- and site-specific models of endothelial oxidative stress, whereas targeting of antioxidant enzymes SOD and catalase offers site-specific quenching of superoxide anion and H(2)O(2). These targeted antioxidant interventions help to clarify specific role of endothelial ROS in vascular and pulmonary pathologies and provide basis for design of targeted therapeutics for treatment of these pathologies. In particular, antibody/catalase conjugates alleviate acute lung ischemia/reperfusion injury, whereas antibody/SOD conjugates inhibit ROS-mediated vasoconstriction and inflammatory endothelial signaling. Encapsulation in protease-resistant, ROS-permeable carriers targeted to endothelium prolongs protective effects of antioxidant enzymes, further diversifying the means for targeted modulation of endothelial ROS.

Recycling and LFA-1-dependent trafficking of ICAM-1 to the immunological synapse

Little is known about how adhesion molecules on APCs accumulate at immunological synapses. We show here that ICAM-1 on APCs is continuously internalized and rapidly recycled back to the interface after antigen-priming T-cell contact. The internalization rate is high in APCs, including Raji B cells and dendritic cells, but low in endothelial cells. Internalization is significantly reduced by inhibitors of Na(+)/H(+) exchangers (NHEs), suggesting that members of the NHE-family regulate this process. Once internalized, ICAM-1 is co-localized with MHC class II in the polarized recycling compartment. Surprisingly, not only ICAM-1, but also MHC class II, is targeted to the immunological synapse through LFA-1-dependent adhesion. Cytosolic ICAM-1 is highly mobile and forms a tubular structure. Inhibitors of microtubule or actin polymerization can reduce ICAM-1 mobility, and thereby block accumulation at immunological synapses. Membrane ICAM-1 also moves to the T-cell contact zone, presumably through an active, cytoskeleton-dependent mechanism. Collectively, these results demonstrate that ICAM-1 can be transported to the immunological synapse through the recycling compartment. Furthermore, the high-affinity state of LFA-1 on T cells is critical to induce targeted movements of both ICAM-1 and MHC class II to the immunological synapse on APCs.

Nanoparticles targeting dendritic cell surface molecules effectively block T cell conjugation and shift response

Dendritic cells (DCs) are potent professional antigen presenting cells (APC) that activate naïve T cells. Interaction of ICAM-1 and LFA-1 molecules on each cell is required for T cell conjugation to DCs, which leads to naïve CD4+ T cell activation and proliferation. Nanoparticles capable of blocking LFA-1/ICAM-1 interaction were studied as inhibitors of T cell conjugation to DCs. Primary DCs were primed with ovalbumin, then treated with a peptide that binds ICAM-1 (LABL), a peptide that binds LFA-1 (cIBR), or the same peptides covalently linked to the surface of poly(dl-lactic-co-glycolic acid) nanoparticles (NPs). LABL-NPs and cIBR-NPs rapidly bound to DCs and inhibited T cell conjugation to DCs to a greater extent than the free peptides, unconjugated nanoparticles (NPs), anti-ICAM-1 antibodies, and anti-LFA-1 antibodies. In addition, DCs treated with NPs or with cIBR-NPs stimulated the proliferation of T cells, but DCs treated with LABL-NPs did not stimulate T cell proliferation. Nanoparticles targeting ICAM-1 or LFA-1 also altered cytokine production by DC cocultured with T cells when compared to free ligands, suggesting that these NPs may offer a unique tool for shaping T cell response.

Optimizing endothelial targeting by modulating the antibody density and particle concentration of anti-ICAM coated carriers

Targeting of drug carriers to cell adhesion molecules expressed on endothelial cells (ECs) may improve treatment of diseases involving the vascular endothelium. This is the case for carriers targeted to intercellular adhesion molecule 1 (ICAM-1), an endothelial surface protein overexpressed in many pathologies. In order to optimize our design of anti-ICAM carriers, we have explored in this study the influence of two carrier design parameters on specific and efficient endothelial targeting in vitro and in vivo: carrier dose and density of targeting molecules (antibodies-Ab) on the carrier surface. Using radioisotope tracing we assessed the role of these parameters on the biodistribution of model polymer carriers targeted to ICAM-1 ((125)I-anti-ICAM carriers) in mice. Increasing the carrier dose enhanced specific accumulation in the lung vasculature (a preferential endothelial target) and decreased non-specific hepatic and splenic uptake. Increasing the Ab density enhanced lung accumulation with minimally reduced liver and spleen uptake. These studies account for the influence of blood hydrodynamic forces on carrier binding to endothelium, relevant to arterioles, venules and larger vessels. Yet, carriers may rather bind to the extensive capillary bed where shear stress is minimal. We used fluorescence microscopy to determine binding kinetics of FITC-labeled anti-ICAM carriers in static conditions, at the threshold found in vivo and conditions mimicking low vs high ICAM-1 expression on quiescent vs activated ECs. Binding to activated ECs reached similar saturation with all tested Ab densities and carrier concentrations. In quiescent cells, carriers reached ~3-fold lower binding saturation, even at high carrier concentration and Ab density, and carriers with low Ab density did not reach saturation, reflecting avidity below threshold. Binding kinetics was positively regulated by anti-ICAM carrier concentration and Ab density. Counterintuitively, binding was faster in quiescent ECs (except for carriers with high Ab density and concentration), likely due to fast saturation of fewer binding sites on these cells. These results will guide optimization of ICAM-1-targeted carriers, e.g., in the context of targeting healthy vs diseased endothelium for prophylactic vs therapeutic interventions.

Enhanced endothelial delivery and biochemical effects of α-galactosidase by ICAM-1-targeted nanocarriers for Fabry disease

Fabry disease, due to the deficiency of α-galactosidase A (α-Gal), causes lysosomal accumulation of globotriaosylceramide (Gb3) in multiple tissues and prominently in the vascular endothelium. Although enzyme replacement therapy (ERT) by injection of recombinant α-Gal improves the disease outcome, the effects on the vasculopathy associated with life-threatening cerebrovascular, cardiac and renal complications are still limited. We designed a strategy to enhance the delivery of α-Gal to organs and endothelial cells (ECs). We targeted α-Gal to intercellular adhesion molecule 1 (ICAM-1), a protein expressed on ECs throughout the vasculature, by loading this enzyme on nanocarriers coated with anti-ICAM (anti-ICAM/α-Gal NCs). In vitro radioisotope tracing showed efficient loading of α-Gal on anti-ICAM NCs, stability of this formulation under storage and in model physiological fluids, and enzyme release in response to lysosome environmental conditions. In mice, the delivery of (125)I-α-Gal was markedly enhanced by anti-ICAM/(125)I-α-Gal NCs in brain, kidney, heart, liver, lung, and spleen, and transmission electron microscopy showed anti-ICAM/α-Gal NCs attached to and internalized into the vascular endothelium. Fluorescence microscopy proved targeting, endocytosis and lysosomal transport of anti-ICAM/α-Gal NCs in macro- and micro-vascular ECs and a marked enhancement of Gb3 degradation. Therefore, this ICAM-1-targeting strategy may help improve the efficacy of therapeutic enzymes for Fabry disease.

PECAM-targeted delivery of SOD inhibits endothelial inflammatory response

Elevated generation of reactive oxygen species (ROS) by endothelial enzymes, including NADPH-oxidase, is implicated in vascular oxidative stress and endothelial proinflammatory activation involving exposure of vascular cell adhesion molecule-1 (VCAM-1). Catalase and superoxide dismutase (SOD) conjugated with antibodies to platelet/endothelial cell adhesion molecule 1 (PECAM-1) bind specifically to endothelium and inhibit effects of corresponding ROS, H(2)O(2), and superoxide anion. In this study, anti-PECAM/SOD, but not anti-PECAM/catalase or nontargeted enzymes, including polyethylene glycol (PEG)-SOD, inhibited 2- to 3-fold VCAM expression caused by tumor necrosis factor (TNF), interleukin-1β, and lipopolysaccharide. Anti- PECAM/SOD, but not nontargeted counterparts, accumulated in vascular endothelium after intravenous injection, localized in endothelial endosomes, and inhibited by 70% lipopolysaccharide-caused VCAM-1 expression in mice. Anti-PECAM/SOD colocalized with EEA-1-positive endothelial vesicles and quenched ROS produced in response to TNF. Inhibitors of NADPH oxidase and anion channel ClC3 blocked TNF-induced VCAM expression, affirming that superoxide produced and transported by these proteins, respectively, mediates inflammatory signaling. Anti-PECAM/SOD abolished VCAM expression caused by poly(I:C)-induced activation of toll-like receptor 3 localized in intracellular vesicles. These results directly implicate endosomal influx of superoxide in endothelial inflammatory response and suggest that site-specific interception of this signal attained by targeted delivery of anti-PECAM/SOD into endothelial endosomes may have anti-inflammatory effects.

Poly(glycoamidoamine) vehicles promote pDNA uptake through multiple routes and efficient gene expression via caveolae-mediated endocytosis

The use of synthetic polymers for the delivery of nucleic acids holds considerable promise for understanding and treating disease at the molecular level. This work aims to decipher the cellular internalization mechanisms for a series of synthetic glycopolymer DNA delivery vehicles we have termed poly(glycoamidoamine)s (PGAAs). To this end, we have performed cellular delivery experiments in the presence of pharmacological endocytosis inhibitors. Confocal microscopy analysis showed colocalization of labeled pDNA in polyplexes with immunolabeled endocytic molecules to identify the cellular internalization pathways in HeLa cells. Direct membrane penetration was also investigated through various methods, including cellular energy depletion and leakage of a cytosolic enzyme from the cell. The data suggests that the cellular internalization of PGAA polyplexes occurs through a multifaceted internalization mechanism primarily involving caveolae, yet clathrin-coated vesicles and macropinosomes were also involved to a lesser degree. The primary mechanism that leads to efficient nuclear delivery and transgene expression appears to be caveolae/raft-mediated endocytosis. The cellular internalization pathways for PGAAs were not identical to those for polyethylenimine, illustrating that differences in the chemical structure of materials directly impacts the cellular internalization mechanisms.

Shear-regulated uptake of nanoparticles by endothelial cells and development of endothelial-targeting nanoparticles

The purpose of this research project was to develop nanoparticles with improved targeting, adhesion, and cellular uptake to activated or inflamed endothelial cells (ECs) under physiological flow conditions. Our hypothesis is that by mimicking platelet binding to activated ECs through the interaction between platelet glycoprotein Ibalpha (GP Ibalpha) and P-selectin on activated endothelial cells, GP Ibalpha-conjugated nanoparticles could exhibit increased targeting and higher cellular uptake in injured or activated endothelial cells under physiological flow conditions. To test this hypothesis, fluorescent-carboxylated polystyrene nanoparticles were selected for the study as a model particle because of its narrow size distribution as a "proof-of-concept." Using confocal microscopy, fluorescent measurements, and protein assays, cellular uptake properties were characterized for these polystyrene nanoparticles. The study also found that conjugation of 100-nm polystyrene nanoparticles with glycocalicin (the extracellular segment of GP Ibalpha) significantly increased the particle adhesion on P-selectin-coated surfaces and cellular uptake of nanoparticles by activated endothelial cells under physiological flow conditions. The results demonstrate that these novel endothelial-targeting nanoparticles could be the first step toward developing a targeted and sustained drug delivery system that can improve shear-regulated particle adhesion and cellular uptake.

Targeting antioxidant and antithrombotic biotherapeutics to endothelium

The endothelium is one of the key targets for pharmacological interventions in oxidative stress and thrombosis, two conditions that are notoriously difficult to treat due to limited efficacy and precision of action of current drugs. Design of molecular and nano-devices that deliver potent antioxidant and antithrombotic therapeutic enzymes to the endothelium holds promise to improve the potency, localization, timing, specificity, safety, and mechanistic precision of these interventions. In particular, cell adhesion molecules expressed on the surface of resting and pathologically altered endothelial cells can be used for drug delivery to the endothelial surface (preferable for thrombolytics) and into intracellular compartments (preferable for antioxidants). Drug delivery platforms including protein conjugates, recombinant fusion constructs, and stealth polymer carriers designed to target these drugs to endothelium are reviewed in this article.

Multiple pathways involved in porcine parvovirus cellular entry and trafficking toward the nucleus

Porcine parvovirus (PPV) is a major cause of reproductive failure in swine. The mechanisms implicated in the first steps of infection that lead to the delivery of the PPV genome to the nucleus are poorly understood. In the present work, a panel of chemical inhibitors was used to dissect the cellular mechanisms involved in establishing a PPV infection. The results demonstrated that following binding to sialic acids on cell surface glycoproteins, the virus used both clathrin-mediated endocytosis and macropinocytosis pathways to gain access into cells. Virus obtained from infected cells was present either as isolated particles or as aggregates, and these two forms could be separated by low-speed centrifugation. Isolated and purified particles strongly preferred entry by clathrin-mediated endocytosis, whereas aggregates clearly favored macropinocytosis. Subsequent endosomal acidification and traffic to the late endosomes were also shown to be essential for infection. The microtubule network was found to be important during the first 10 h of infection, whereas an intact actin network was required for almost the whole viral cycle. Proteasome processing was found to be essential, and capsid proteins were ubiquitinated relatively early during infection. Taken together, these results provided new insights into the first steps of PPV infection, including the use of alternative entry pathways, unique among members of this viral family.

Targeting of ICAM-1-directed immunoliposomes specifically to activated endothelial cells with low cellular uptake: use of an optimized procedure for the coupling of low concentrations of antibody to liposomes

Targeted delivery of therapeutics to the endothelium is an important goal in the treatment of inflammatory diseases. The aim of this work was to exploit the overexpression of intercellular adhesion molecule-1 (ICAM-1) on activated endothelial cells for the targeting of anti-ICAM-1-coupled immunoliposomes with the intent for further use as drug carriers. Immunoliposomes were prepared from using an optimized method for the coupling of low concentrations of antibody to liposomes, thereby preventing the loss of antibody through the derivatization, extraction, and activation process. This is especially suitable for limiting ligand conjugates that are isolated or synthesized in small quantities, such as monoclonal antibodies (mAbs). To investigate the functionality of the resulting immunoliposomes, the specificity of binding and cellular internalization studies of liposomes, either nonconjugated or conjugated with mAbs to ICAM-1 or to irrelevant IgG to high endothelial venule (HEV) cells, were analyzed by fluorescence microplate spectroscopy at 4 and 37°C. Immunoliposomes specifically directed against ICAM-1 were shown to bind selectively and specifically to tumor necrosis factor alpha-activated endothelial cells in vitro, with minimal cellular internalization. This study provides a novel delivery system that has the potential for targeting therapeutics to inflammatory tissue.

New biotechnological and nanomedicine strategies for treatment of lysosomal storage disorders

This review discusses the multiple bio- and nanotechnological strategies developed in the last few decades for treatment of a group of fatal genetic diseases termed lysosomal storage disorders. Some basic foundation on the biomedical causes and social and clinical relevance of these diseases is provided. Several treatment modalities, from those currently available to novel therapeutic approaches under development, are also discussed; these include gene and cell therapies, substrate reduction therapy, chemical chaperones, enzyme replacement therapy, multifunctional chimeras, targeting strategies, and drug carrier approaches.

Human rhinovirus 14 enters rhabdomyosarcoma cells expressing icam-1 by a clathrin-, caveolin-, and flotillin-independent pathway

Intercellular adhesion molecule 1 (ICAM-1) mediates binding and entry of major group human rhinoviruses (HRVs). Whereas the entry pathway of minor group HRVs has been studied in detail and is comparatively well understood, the pathway taken by major group HRVs is largely unknown. Use of immunofluorescence microscopy, colocalization with specific endocytic markers, dominant negative mutants, and pharmacological inhibitors allowed us to demonstrate that the major group virus HRV14 enters rhabdomyosarcoma cells transfected to express human ICAM-1 in a clathrin-, caveolin-, and flotillin-independent manner. Electron microscopy revealed that many virions accumulated in long tubular structures, easily distinguishable from clathrin-coated pits and caveolae. Virus entry was strongly sensitive to the Na(+)/H(+) ion exchange inhibitor amiloride and moderately sensitive to cytochalasin D. Thus, cellular uptake of HRV14 occurs via a pathway exhibiting some, but not all, characteristics of macropinocytosis and is similar to that recently described for adenovirus 3 entry via alpha(v) integrin/CD46 in HeLa cells.

Cell-penetrating peptide exploited syndecans

Cell-penetrating peptides (CPPs) are short peptides capable of translocating across the plasma membrane of live cells and transporting conjugated compounds intracellularly. Fifteen years after discovering the first model cationic CPPs, penetratin and TAT, CPP internalization is still challenging many questions. Particularly it has been unknown whether CPPs enter the cells with or without mediation of a specific surface receptor. Here we report that syndecan-4, the universally expressed isoform of the syndecan family of transmembrane proteoglycans, binds and mediates transport of the three most frequently utilized cationic CPPs (penetratin, octaarginine and TAT) into the cells. Quantitative uptake studies and mutational analyses demonstrate that attachment of the cationic CPPs is mediated by specific interactions between the heparan sulfate chains of syndecan-4 and the CPPs. Protein kinase C alpha is also heavily involved in the uptake mechanism. The collected data give the first direct evidence on the receptor-mediated uptake of cationic CPPs and may replace the long-thought, but already contradicted membrane penetration hypothesis. Thus our study might give an answer for a decade long debate and foster the development of rationalized, syndecan-4 targeted novel delivery technologies.

Flow dynamics, binding and detachment of spherical carriers targeted to ICAM-1 on endothelial cells

Vascular drug delivery by administration of carriers targeted to endothelial surface determinants, such as intercellular adhesion molecule (ICAM-1), holds considerable promise to improve disease treatment. As a model to define elusive factors controlling the interplay between carrier motion in the bloodstream and its interactions with molecular targets in the endothelial wall, we used 1 mum beads coated with ICAM-1 monoclonal antibody (Ab) at 370, 1100 or 4100 Ab/microm2. Carriers were perfused at two shear rates over resting or activated endothelial cells, expressing minimum vs. maximum ICAM-1 levels, to determine carrier rolling, binding and detachment. Even at 0.1 Pa and 4100 Ab/microm2, carriers attached only to activated cells (21 fold increase over resting cells), ideal for specific drug targeting to sites of pathology. Binding was increased by raising the Ab surface density on the carrier, e.g., 59.4+/-11.1% increase for carriers having 4100 vs. 1100 Ab/microm2, as a consequence of decreased rolling velocity. Carrier binding was stable even under a high shear stress: carriers with 1100 and 4100 Ab/microm2 withstand shear stress over 3 Pa without detaching from the cells. This is further supported by theoretical modeling. These results will guide vascular targeting of drug carriers via rational design of experimentally tunable parameters.

The regulation of leucocyte transendothelial migration by endothelial signalling events

Leucocytes use sophisticated mechanisms to cross the endothelium lining the vasculature. This is initiated by chemokine- and adhesion molecule-induced intracellular signalling that controls adhesion, spreading, and motility. At the same time, adherent leucocytes trigger the endothelium, manipulating the barrier to promote their transmigration into the underlying tissues. Over the past years, our insights in the associated signalling events within the endothelium have increased considerably, albeit the order of events, their crosstalk, and the consequences for endothelial cells and leucocytes are only partially resolved. Here, we briefly review endothelial signalling that is initiated at the apical endothelial membrane, where the first contact with the leucocytes takes place and signal transduction is induced. In addition, we discuss subsequent events at endothelial cell-cell junctions insofar as they have been linked to transendothelial migration. Finally, we briefly touch upon the modulation of endothelial signalling by infectious pathogens, since these have developed additional, elegant ways to manipulate the endothelium and transendothelial migration that may provide new, relevant insights into this process.

Picornaviruses

The picornavirus family consists of a large number of small RNA viruses, many of which are significant pathogens of humans and livestock. They are amongst the simplest of vertebrate viruses comprising a single stranded positive sense RNA genome within a T = 1 (quasi T = 3) icosahedral protein capsid of approximately 30 nm diameter. The structures of a number of picornaviruses have been determined at close to atomic resolution by X-ray crystallography. The structures of cell entry intermediate particles and complexes of virus particles with receptor molecules or antibodies have also been obtained by X-ray crystallography or at a lower resolution by cryo-electron microscopy. Many of the receptors used by different picornaviruses have been identified, and it is becoming increasingly apparent that many use co-receptors and alternative receptors to bind to and infect cells. However, the mechanisms by which these viruses release their genomes and transport them across a cellular membrane to gain access to the cytoplasm are still poorly understood. Indeed, detailed studies of cell entry mechanisms have been made only on a few members of the family, and it is yet to be established how broadly the results of these are applicable across the full spectrum of picornaviruses. Working models of the cell entry process are being developed for the best studied picornaviruses, the enteroviruses. These viruses maintain particle integrity throughout the infection process and function as genome delivery modules. However, there is currently no model to explain how viruses such as cardio- and aphthoviruses that appear to simply dissociate into subunits during uncoating deliver their genomes into the cytoplasm.

Nanocarriers' entry into the cell: relevance to drug delivery

Nanocarriers offer unique possibilities to overcome cellular barriers in order to improve the delivery of various drugs and drug candidates, including the promising therapeutic biomacromolecules (i.e., nucleic acids, proteins). There are various mechanisms of nanocarrier cell internalization that are dramatically influenced by nanoparticles' physicochemical properties. Depending on the cellular uptake and intracellular trafficking, different pharmacological applications may be considered. This review will discuss these opportunities, starting with the phagocytosis pathway, which, being increasingly well characterized and understood, has allowed several successes in the treatment of certain cancers and infectious diseases. On the other hand, the non-phagocytic pathways encompass various complicated mechanisms, such as clathrin-mediated endocytosis, caveolae-mediated endocytosis and macropinocytosis, which are more challenging to control for pharmaceutical drug delivery applications. Nevertheless, various strategies are being actively investigated in order to tailor nanocarriers able to deliver anticancer agents, nucleic acids, proteins and peptides for therapeutic applications by these non-phagocytic routes.

Endocytosis and intracellular trafficking of human natural killer cell receptors

Natural killer (NK) cells play a vital role in the defense against viral infections and tumor development. NK cell function is primarily regulated by the sum of signals from a broad array of activation and inhibitory receptors. Key to generating the input level of either activating or inhibitory signals is the maintenance of receptor expression levels on the cell surface. Although the mechanisms of endocytosis and trafficking for some cell surface receptors, such as transferrin receptor and certain immune receptors, are very well known, that is not the situation for receptors expressed by NK cells. Recent studies have uncovered that endocytosis and trafficking routes characteristic for specific activation and inhibitory receptors can regulate the functional responses of NK cells. In this review, we summarize the current knowledge of receptor endocytosis and trafficking, and integrate this with our current understanding of NK cell receptor trafficking.

CD82 endocytosis and cholesterol-dependent reorganization of tetraspanin webs and lipid rafts

Tetraspanin CD82 suppresses cell migration, tumor invasion, and tumor metastasis. To determine the mechanism by which CD82 inhibits motility, most studies have focused on the cell surface CD82, which forms tetraspanin-enriched microdomains (TEMs) with other transmembrane proteins, such as integrins. In this study, we found that CD82 undergoes endocytosis and traffics to endosomes and lysosomes. To determine the endocytic mechanism of CD82, we demonstrated that dynamin and clathrin are not essential for CD82 internalization. Depletion or sequestration of sterol in the plasma membrane markedly inhibited the endocytosis of CD82. Despite the demand on Cdc42 activity, CD82 endocytosis is distinct from macropinocytosis and the documented dynamin-independent pinocytosis. As a TEM component, CD82 reorganizes TEMs and lipid rafts by redistributing cholesterol into these membrane microdomains. CD82-containing TEMs are characterized by the cholesterol-containing microdomains in the extreme light- and intermediate-density fractions. Moreover, the endocytosis of CD82 appears to alleviate CD82-mediated inhibition of cell migration. Taken together, our studies demonstrate that lipid-dependent endocytosis drives CD82 trafficking to late endosomes and lysosomes, and CD82 reorganizes TEMs and lipid rafts through redistribution of cholesterol.

Dual roles for RHOA/RHO-kinase in the regulated trafficking of a voltage-sensitive potassium channel

Kv1.2 is a member of the Shaker family of voltage-sensitive potassium channels and contributes to regulation of membrane excitability. The electrophysiological activity of Kv1.2 undergoes tyrosine kinase-dependent suppression in a process involving RhoA. We report that RhoA elicits suppression of Kv1.2 ionic current by modulating channel endocytosis. This occurs through two distinct pathways, one clathrin-dependent and the other cholesterol-dependent. Activation of Rho kinase (ROCK) via the lysophosphatidic acid (LPA) receptor elicits clathrin-dependent Kv1.2 endocytosis and consequent attenuation of its ionic current. LPA-induced channel endocytosis is blocked by the ROCK inhibitor Y27632 or by clathrin RNA interference. In contrast, steady-state endocytosis of Kv1.2 in unstimulated cells is cholesterol dependent. Inhibition of basal ROCK signaling with Y27632 increased surface Kv1.2, an effect that persists in the presence of clathrin small interfering RNA and that is not additive to the increase in surface channel levels elicited by the cholesterol sequestering drug filipin. Temperature block experiments show that ROCK affects cholesterol-dependent trafficking by modulating the recycling of endocytosed channel back to the plasma membrane. Both receptor-stimulated and steady-state Kv1.2 trafficking modulated by RhoA/ROCK required the activation of dynamin as well as the ROCK effector Lim-kinase, indicating a key role for actin remodeling in RhoA-dependent Kv1.2 regulation.

Anchoring fusion thrombomodulin to the endothelial lumen protects against injury-induced lung thrombosis and inflammation

RATIONALE

Endothelial thrombomodulin (TM) regulates thrombosis and inflammation. Diverse forms of pulmonary and vascular injury are accompanied by down-regulation of TM, which aggravates tissue injury. We postulated that anchoring TM to the endothelial surface would restore its protective functions.

OBJECTIVES

To design an effective and safe strategy to treat pulmonary thrombotic and inflammatory injury.

METHODS

We synthesized a fusion protein, designated scFv/TM, by linking the extracellular domain of mouse TM to a single-chain variable fragment of an antibody to platelet endothelial cell adhesion molecule-1 (PECAM-1). The targeting and protective functions of scFv/TM were tested in mouse models of lung ischemia-reperfusion and acute lung injury (ALI) caused by intratracheal endotoxin and hyperoxia, both of which caused approximately 50% reduction in the endogenous expression of TM.

MEASUREMENTS AND MAIN RESULTS

Biochemical assays showed that scFv/TM accelerated protein C activation by thrombin and bound mouse PECAM-1 and cytokine high mobility group-B1. After intravenous injection, scFv/TM preferentially accumulated in the mouse pulmonary vasculature. In a lung model of ischemia-reperfusion injury, scFv/TM attenuated elevation of early growth response-1, inhibited pulmonary deposition of fibrin and leukocyte infiltration, and preserved blood oxygenation more effectively than soluble TM. In an ALI model, scFv/TM, but not soluble TM, suppressed activation of nuclear factor-kappaB, inflammation and edema in the lung and reduced mortality without causing hemorrhage.

CONCLUSIONS

Targeting TM to the endothelium using an scFv anchor enhances its antithrombotic and antiinflammatory effectiveness in models of ALI.

Matrix metalloproteinase 12 silencing: a therapeutic approach to treat pathological lung tissue remodeling?

Among the large matrix metalloproteinases (MMPs) family, MMP-12, also referred to as macrophage elastase, plays a significant role in chronic pulmonary pathologies characterized by an intense tissue remodeling such as asthma and COPD. This review will summarize knowledge about MMP-12 structure, functions and mechanisms of activation and regulation, including potential MMP-12 modulation by microRNA. As MMP-12 is involved in many tissue remodeling diseases, efforts have been made to develop specific synthetic inhibitors. However, at this time, very few chemical inhibitors have proved to be efficient and specific to a particular MMP. The relevance of silencing MMP-12 by RNA interference is highlighted. The specificity of this approach using siRNA or shRNA and the strategies to deliver these molecules in the lung are discussed.

Polymeric carriers: role of geometry in drug delivery

The unique properties of synthetic nanostructures promise a diverse set of applications as carriers for drug delivery, which are advantageous in terms of biocompatibility, pharmacokinetics, targeting and controlled drug release. Historically, more traditional drug delivery systems have focused on spherical carriers. However, there is a growing interest in pursuing non-spherical carriers, such as elongated or filamentous morphologies, now available due to novel formulation strategies. Unique physiochemical properties of these supramolecular structures offer distinct advantages as drug delivery systems. In particular, results of recent studies in cell cultures and lab animals indicate that rational design of carriers of a given geometry (size and shape) offers an unprecedented control of their longevity in circulation and targeting to selected cellular and subcellular locations. This article reviews drug delivery aspects of non-spherical drug delivery systems, including material selection and formulation, drug loading and release, biocompatibility, circulation behavior, targeting and subcellular addressing.

Endocytosis as a mechanism of regulating natural killer cell function: unique endocytic and trafficking pathway for CD94/NKG2A

Natural killer (NK) cells are lymphocytes generally recognized as sentinels of the innate immune system due to their inherent capacity to deal with diseased (stressed) cells, including malignant and infected. This ability to recognize many potentially pathogenic situations is due to the expression of a diverse panel of activation receptors. Because NK cell activation triggers an aggressive inflammatory response, it is important to have a means of throttling this response. Hence, NK cells also express a panel of inhibitory receptors that recognize ligands expressed by "normal" cells. Little or nothing is known about the endocytosis and trafficking of NK cell receptors, which are of great relevance to understanding how NK cells maintain the appropriate balance of activating and inhibitory receptors on their cell surface. In this review, we focus on the ITIM-containing inhibitory receptor CD94/NKG2A showing that it is endocytosed by a previously undescribed macropinocytic-like process that may be related to the maintenance of its surface expression.

Surface-expressed viral proteins in feline infectious peritonitis virus-infected monocytes are internalized through a clathrin- and caveolae-independent pathway

Infection with feline infectious peritonitis virus (FIPV), a feline coronavirus, frequently leads to death in spite of a strong humoral immune response. In previous work, we reported that infected monocytes, thein vivotarget cells of FIPV, express viral proteins in their plasma membranes. These proteins are quickly internalized upon binding of antibodies. As the cell surface is cleared from viral proteins, internalization might offer protection against antibody-dependent cell lysis. Here, the internalization and subsequent trafficking of the antigen–antibody complexes were characterized using biochemical, cell biological and genetic approaches. Internalization occurred through a clathrin- and caveolae-independent pathway that did not require dynamin, rafts, actin or rho-GTPases. These findings indicate that the viral antigen–antibody complexes were not internalized through any of the previously described pathways. Further characterization showed that this internalization process was independent from phosphatases and tyrosine kinases but did depend on serine/threonine kinases. After internalization, the viral antigen–antibody complexes passed through the early endosomes, where they resided only briefly, and accumulated in the late endosomes. Between 30 and 60 min after antibody addition, the complexes left the late endosomes but were not degraded in the lysosomes. This study reveals what is probably a new internalization pathway into primary monocytes, confirming once more the complexity of endocytic processes.

Targeted delivery of therapeutics to endothelium

The endothelium is a target for therapeutic and diagnostic interventions in a plethora of human disease conditions including ischemia, inflammation, edema, oxidative stress, thrombosis and hemorrhage, and metabolic and oncological diseases. Unfortunately, drugs have no affinity to the endothelium, thereby limiting the localization, timing, specificity, safety, and effectiveness of therapeutic interventions. Molecular determinants on the surface of resting and pathologically altered endothelial cells, including cell adhesion molecules, peptidases, and receptors involved in endocytosis, can be used for drug delivery to the endothelial surface and into intracellular compartments. Drug delivery platforms such as protein conjugates, recombinant fusion constructs, targeted liposomes, and stealth polymer carriers have been designed to target drugs and imaging agents to these determinants. We review endothelial target determinants and drug delivery systems, describe parameters that control the binding of drug carriers to the endothelium, and provide examples of the endothelial targeting of therapeutic enzymes designed for the treatment of acute vascular disorders including ischemia, oxidative stress, inflammation, and thrombosis.

Centrifugal enhancement of Kaposi's sarcoma-associated virus infection of human endothelial cells in vitro

In order to improve the efficiency of infection of primary human endothelial cells in vitro of Kaposi's sarcoma-associated herpesvirus (KSHV), the effect of low speed centrifugation was investigated. The recombinant KSHV, BAC36, was used to examine the centrifugal enhancement of KSHV. Infectivity was estimated by green fluorescent protein (GFP) expression and real-time RT-PCR. The enhancement of infectivity was dependent upon the time and force of centrifugation in human umbilical vein endothelial cells (HUVECs). Centrifugation enhanced the infectivity of KSHV by up to 70 fold compared to non-centrifugal control infection for the same period of time; viral mRNA expression was also enhanced by centrifugation. HUVECs that were centrifuged before infection with KSHV displayed no enhancement in infectivity; therefore, enhancement is believed to occur during centrifugation. In addition, the mechanisms of infection including the initial viral attachment to cells, lipid rafts, and clathrin-mediated and caveolae endocytosis appear to be similar in KSHV infection with and without centrifugal enhancement. These results show that low speed centrifugation could be a useful tool for improving the efficiency of KSHV infection in vitro.

HPV16 and BPV1 infection can be blocked by the dynamin inhibitor dynasore

The initial entry of papillomaviruses into their target cells has been shown to occur by clathrin-mediated endocytosis and caveolae-mediated endocytosis. These mechanisms entail the formation of nascent-coated vesicles at the plasma membrane. Such coated vesicles, clathrin or caveolin, form and pinch-off in a controlled mechanism that involves several proteins including dynamin. Dynamin is a GTPase that forms a dynamin ring at the stem connecting the nascent vesicle to the plasma membrane. In a still not fully characterized mechanism, dynamin's contraction and twisting results in the scission of the vesicle. In an effort to better characterize the role and molecular mechanisms of dynamin's function, researchers have identified dynasore, a dynamin GTPase inhibitor that prevents the scission of dynamin-dependent endocytic vesicles. Here, we have tested if infection by pseudovirus corresponding to the oncogenic human papillomavirus type 16 and bovine papillomavirus type 1 can be blocked by dynasore. We present data demonstrating that dynasore can block infection of human papillomavirus type 16 and bovine papillomavirus type 1 pseudovirions in a dose- and time-dependent manner with equal efficiency. Presently, there is no available therapy that can block infection by a wide range of papillomavirus regardless of species or genotypes. Targeting dynamin may lead to the rational design of drug able to prevent infection by papillomaviruses, and by other infectious agents dependent on this protein for initial internalization into target cells. Whether such an approach will prove successful needs further investigation.

Oxygen microscopy by two-photon-excited phosphorescence

High-resolution images of oxygen distributions in microheterogeneous samples are obtained by two-photon laser scanning microscopy (2P LSM), using a newly developed dendritic nanoprobe with internally enhanced two-photon absorption (2PA) cross-section. In this probe, energy is harvested by a 2PA antenna, which passes excitation onto a phosphorescent metalloporphyrin via intramolecular energy transfer. The 2P LSM allows sectioning of oxygen gradients with near diffraction-limited resolution, and lifetime-based acquisition eliminates dependence on the local probe concentration. The technique is validated on objects with a priori known oxygen distributions and applied to imaging of pO(2) in cells.

Site-specific labeling of enveloped viruses with quantum dots for single virus tracking

This study reports a general method of labeling enveloped viruses with semiconductor quantum dots (QDs) for use in single virus trafficking studies. Retroviruses, including human immunodeficiency virus (HIV), could be successfully tagged with QDs through the membrane incorporation of a short acceptor peptide (AP) that is susceptible to site-specific biotinylation and attachment of streptavidin-conjugated QDs. It was found that this AP tag-based QD labeling had little effect on the viral infectivity and allowed for the study of the kinetics of the internalization of the recombinant lentivirus enveloped with vesicular stomatitis virus glycoprotein (VSVG) into the early endosomes. It also allows for the live cell imaging of the trafficking of labeled virus to the Rab5(+) endosomal compartments. This study further demonstrated by direct visualization of QD-labeled virus that VSVG-pseudotyped lentivirus enters cells independent of clatherin- and caveolin-pathways, while the entry of VSVG-pseudotyped retrovirus occurs via the clathrin pathway. The studies monitoring HIV particles using QD-labeling showed that we could detect single virions on the surface of target cells expressing either CD4/CCR5 or DC-SIGN. Further internalization studies of QD-HIV evidently showed that the clathrin pathway is the major route for DC-SIGN-mediated uptake of viruses. Taken together, our data demonstrate the potential of this QD-labeling for visualizing the dynamic interactions between viruses and target cell structures.

Differential intra-endothelial delivery of polymer nanocarriers targeted to distinct PECAM-1 epitopes

Coupling drug carriers to antibodies for targeting endothelial cells (ECs) may improve treatment of vascular and pulmonary diseases. Selecting antibodies that deliver carriers to the cell surface or intracellularly may further optimize specificity of interventions. We studied antibody-directed targeting of nanocarriers to platelet-endothelial cell adhesion molecule (PECAM)-1, an endothelial glycoprotein containing 6 Ig-like extracellular domains. PECAM-1 antibodies bind to ECs without internalization, but ECs internalize by endocytosis nanocarriers carrying multiple copies of anti-PECAM (anti-PECAM/NCs). To determine whether binding and intracellular transport of anti-PECAM/NCs depend on the epitope engaged, we targeted five PECAM-1 epitopes: mAb35, mAb37 and mAb62 (membrane-distal Ig domain 1), mAbGi34 (Ig domains 2/3), and mAb4G6 (membrane-proximal Ig domain 6). The antibodies bound to ECs regardless of the epitope proximity to the plasmalemma, whereas 130 nm diameter nanocarriers only targeted effectively distal domains (mAb4G6/NCs did not bind to ECs). ECs internalized mAb35, mAb62, and mAbGi34 carriers regardless of their size (0.13 to 5 microm diameter), yet they did not internalize mAb37/NCs. After internalization, mAb62/NCs trafficked to lysosomes within 2-3 h, whereas mAb35/NCs had prolonged residence in pre-lysosomal vesicles. Therefore, endothelial binding, endocytosis, and intracellular transport of anti-PECAM/NCs are epitope-specific. This paradigm will guide the design of endothelial drug delivery systems providing specific cellular localizations.

Control of endothelial targeting and intracellular delivery of therapeutic enzymes by modulating the size and shape of ICAM-1-targeted carriers

Endocytosis in endothelial cells (ECs) is important for many biomedical applications, including drug delivery by nano- and microscale carriers. However, little is known about how carrier geometry influences endothelial drug targeting, intracellular trafficking, and effects. We studied this using prototype polymer carriers of various sizes (0.1-10 mum) and shapes (spheres versus elliptical disks). Carriers were targeted to intercellular adhesion molecule 1 (ICAM-1), a transmembrane glycoprotein that is upregulated in many pathologies and used as a target for intraendothelial drug delivery. ECs internalized anti-ICAM-coated carriers of up to several microns in size via cell adhesion molecule-mediated endocytosis. This pathway is distinct from caveolar and clathrin endocytosis that operate for submicron-size objects. Carrier geometry was found to influence endothelial targeting in the vasculature, and the rate of endocytosis and lysosomal transport within ECs. Disks had longer half-lives in circulation and higher targeting specificity in mice, whereas spheres were endocytosed more rapidly. Micron-size carriers had prolonged residency in prelysosomal compartments, beneficial for endothelial antioxidant protection by delivered catalase. Submicron carriers trafficked to lysosomes more readily, optimizing effects of acid sphingomyelinase (ASM) enzyme replacement in a model of lysosomal storage disease. Therefore, rational design of carrier geometry will help optimize endothelium-targeted therapeutics.

Targeted delivery of nanoparticles for the treatment of lung diseases

Targeted delivery of drug molecules to organs or special sites is one of the most challenging research areas in pharmaceutical sciences. By developing colloidal delivery systems such as liposomes, micelles and nanoparticles a new frontier was opened for improving drug delivery. Nanoparticles with their special characteristics such as small particle size, large surface area and the capability of changing their surface properties have numerous advantages compared with other delivery systems. Targeted nanoparticle delivery to the lungs is an emerging area of interest. This article reviews research performed over the last decades on the application of nanoparticles administered via different routes of administration for treatment or diagnostic purposes. Nanotoxicological aspects of pulmonary delivery are also discussed.

Uncommon endocytic and trafficking pathway of the natural killer cell CD94/NKG2A inhibitory receptor

The CD94/NKG2A inhibitory receptor, expressed by natural killer and T cells, is constantly exposed to its HLA-E ligand expressed by surrounding cells. Ligand exposure often induces receptor downregulation. For CD94/NKG2A, this could potentiate activation receptor(s) induced responses to normal bystander cells. We investigated CD94/NKG2A endocytosis and found that it occurs by an amiloride-sensitive, Rac1-dependent macropinocytic-like process; however, it does not require clathrin, dynamin, ADP ribosylation factor-6, phosphoinositide-3 kinase or the actin cytoskeleton. Once endocytosed, CD94/NKG2A traffics to early endosomal antigen 1(+), Rab5(+) early endosomes. It does appear in Rab4(+) early/sorting endosome, but, in the time period examined, fails to reach Rab11(+) recycling or Rab7(+) late endosomes or lysosome-associated membrane protein-1(+) lysosomes. These results indicate that CD94/NKG2A utilizes a previously undescribed endocytic mechanism coupled with an abbreviated trafficking pattern, perhaps to insure surface expression.

Activation of peroxisome proliferator-activated receptor gamma (PPARgamma) suppresses Rho GTPases in human brain microvascular endothelial cells and inhibits adhesion and transendothelial migration of HIV-1 infected monocytes

Under inflammatory conditions (including HIV-1 encephalitis and multiple sclerosis), activated brain endothelium enhances the adhesion and transmigration of monocytes across the blood-brain barrier (BBB). Synthetic ligands that activate the peroxisome proliferator-activated receptors (PPARs) have anti-inflammatory properties, and PPAR stimulation prevents the interaction of leukocytes with cytokine stimulated-endothelium. However, the mechanism underlying these effects of PPAR ligands and their ability to intervene with leukocyte adhesion and migration across brain endothelial cells has yet to be explored. For the first time, using primary human brain endothelial cells (BMVEC), we demonstrated that monocyte adhesion and transendothelial migration across inflamed endothelium were markedly reduced by PPARgamma activation. In contrast to non-brain-derived endothelial cells, PPARalpha activation in the BMVEC had no significant effect on monocyte-endothelial interaction. Previously, our work indicated a critical role of Rho GTPases (like RhoA) in BMVEC to control migration of HIV-1 infected monocytes across BBB. In this study, we show that in the BMVEC PPARgamma stimulation prevented activation of two GTPases, Rac1 and RhoA, which correlated with decreased monocyte adhesion to and migration across brain endothelium. Relevant to HIV-1 neuropathogenesis, enhanced adhesion and migration of HIV-1 infected monocytes across the BBB were significantly reduced when BMVEC were treated with PPARgamma agonist. These findings indicate that Rac1 and RhoA inhibition by PPARgamma agonists could be a new approach for treatment of neuroinflammation by preventing monocyte migration across the BBB.

Delivery of acid sphingomyelinase in normal and niemann-pick disease mice using intercellular adhesion molecule-1-targeted polymer nanocarriers

Type B Niemann-Pick disease (NPD) is a multiorgan system disorder caused by a genetic deficiency of acid sphingomyelinase (ASM), for which lung is an important and challenging therapeutic target. In this study, we designed and evaluated new delivery vehicles for enzyme replacement therapy of type B NPD, consisting of polystyrene and poly(lactic-coglycolic) acid polymer nanocarriers targeted to intercellular adhesion molecule (ICAM)-1, an endothelial surface protein up-regulated in many pathologies, including type B NPD. Real-time vascular imaging using intravital microscopy and postmortem imaging of mouse organs showed rapid, uniform, and efficient binding of fluorescently labeled ICAM-1-targeted ASM nanocarriers (anti-ICAM/ASM nanocarriers) to endothelium after i.v. injection in mice. Fluorescence microscopy of lung alveoli actin, tissue histology, and 125I-albumin blood-to-lung transport showed that anti-ICAM nanocarriers cause neither detectable lung injury, nor abnormal vascular permeability in animals. Radioisotope tracing showed rapid disappearance from the circulation and enhanced accumulation of anti-ICAM/125I-ASM nanocarriers over the nontargeted naked enzyme in kidney, heart, liver, spleen, and primarily lung, both in wild-type and ASM knockout mice. These data demonstrate that ICAM-1-targeted nanocarriers may enhance enzyme replacement therapy for type B NPD and perhaps other lysosomal storage disorders.

RhoA activation and actin reorganization involved in endothelial CAM-mediated endocytosis of anti-PECAM carriers: critical role for tyrosine 686 in the cytoplasmic tail of PECAM-1

Platelet-endothelial cell adhesion molecule-1 (PECAM-1), a transmembrane glycoprotein involved in leukocyte transmigration, represents a good target for endothelial drug delivery (eg, using antibody-directed nanocarriers, anti-PECAM/NCs). Although endothelial cells do not internalize PECAM antibodies, PECAM-1 engagement by multivalent anti-PECAM conjugates and nanocarriers causes endocytosis via a nonclassic CAM-mediated pathway. We found that endothelial uptake of multivalent anti-PECAM complexes is associated with PECAM-1 phosphorylation. Using model REN cells expressing a series of PECAM-1 deletion and point mutants, we found that the PECAM-1 cytoplasmic domain and, more precisely, PECAM-1 tyrosine 686, is critical in mediating RhoA activation and recruitment of EGFP-RhoA to anti-PECAM/NC binding sites at the plasmalemma, actin polymerization into phalloidin-positive stress fibers, and finally CAM endocytosis of anti-PECAM/NCs. Endothelial targeting and endocytosis of anti-PECAM/NCs were markedly efficient and did not compromise endothelial barrier function in vitro (determined by immunostaining of VE-cadherin and (125)I-albumin transport across endothelial monolayers) or in vivo (determined by electron microscopy imaging of pulmonary capillaries and (125)I-albumin transport from the blood into the lung tissue after intravenous injection of anti-PECAM/NCs in mice). These results reveal PECAM-1 signaling and interactions with the cytoskeleton, which are required for CAM-endocytosis, and may provide safe intra-endothelial drug delivery by anti-PECAM/NCs.

In vivo imaging of 64Cu-labeled polymer nanoparticles targeted to the lung endothelium

Nanoparticles (NPs) targeting the intercellular adhesion molecule 1 (ICAM-1) hold promise as a mean of delivering therapeutics to the pulmonary endothelium in patients with acute and chronic respiratory diseases. As these new materials become available, strategies are needed to understand their behavior in vivo. We have evaluated the use of (64)Cu and PET to noninvasively image the lung uptake and distribution of NPs coated with an anti-ICAM antibody.

METHODS

Model fluorescent NPs were coated with a mixture of an anti-ICAM antibody (or nonspecific IgG) and (64)Cu-DOTA-IgG (where DOTA is 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid). Biodistribution and small-animal PET and CT studies were performed in healthy mice and in mice pretreated with lipopolysaccharides (LPSs). Metabolism studies were also performed to evaluate the stability of (64)Cu-labeled NPs in lungs in vivo.

RESULTS

The lungs of mice administered anti-ICAM NPs labeled with (64)Cu were clearly imaged by small-animal PET 1, 4, and 24 h after administration. Both biodistribution and small-animal imaging showed a 3- to 4-fold higher uptake in the lungs of mice injected with ICAM-targeted NPs relative to that of the control group. Lung uptake was further enhanced by pretreating the mice with LPS, presumably because of ICAM-1 upregulation. However, an approximately 2-fold decrease in lung signal was observed in each experimental group over 24 h. Metabolism studies in lung tissues harvested from mice injected with (64)Cu-labeled anti-ICAM NPs showed considerable release of a small (64)Cu-radiometabolite from the NPs beginning as early as 1 h after injection. A decrease in lung fluorescence was also observed, most likely reflecting partial release of NPs from the lungs in vivo.

CONCLUSION

The use of small-animal PET to track (64)Cu-labeled nanostructures in vivo shows potential as a strategy for the preclinical screening of new NP drug delivery agents targeting the lung endothelium and other tissues. Future design optimization to prolong the stability of the radiolabel in vivo will further improve this promising approach.

Clathrin-independent internalization and recycling

The functionality of receptor and channel proteins depends directly upon their expression level on the plasma membrane. Therefore, the ability to selectively adjust the surface level of a particular receptor or channel protein is pivotal to many cellular signalling events. The internalization and recycling pathway plays a major role in the regulation of protein surface level, and thus has been a focus of research for many years. Although several endocytic pathways have been identified, most of our knowledge has come from the clathrin-dependent pathway, while the other pathways remain much less well defined. Considering that clathrin-independent internalization may account for as much as 50% of the total endocytic activity in the cell, the lack of such knowledge constitutes a major gap in our efforts to understand how different internalization pathways are utilized and co-ordinated. Recent studies have provided valuable insights into this area, yet many more questions still remain. In this review, we will give a panoramic introduction to the current knowledge of various internalization and recycling pathways, with an emphasis on the latest findings that have broadened our view of the clathrin-independent pathways. We will also dedicate one section to the emerging studies of the clathrin-independent internalization pathways in neuronal cells.

Regulation of raft-dependent endocytosis

Raft-dependent endocytosis is in large part defined as the cholesterol-sensitive, clathrin-independent internalization of ligands and receptors from the plasma membrane. It encompasses the endocytosis of caveolae, smooth plasmalemmal vesicles that form a subdomain of cholesterol and sphingolipid-rich lipid rafts and that are enriched for caveolin-1. While sharing common mechanisms, like cholesterol sensitivity, raft endocytic routes show differential regulation by various cellular components including caveolin-1, dynamin-2 and regulators of the actin cytoskeleton. Dynamin-dependent raft pathways, mediated by caveolae and morphologically equivalent non-caveolin vesicular intermediates, are referred to as caveolae/raft-dependent endocytosis. In contrast, dynamin-independent raft pathways are mediated by non-caveolar intermediates. Raft-dependent endocytosis is regulated by tyrosine kinase inhibitors and, through the regulation of the internalization of various ligands, receptors and effectors, is also a determinant of cellular signaling. In this review, we characterize and discuss the regulation of raft-dependent endocytic pathways and the role of key regulators such as caveolin-1.

The Role of Glycocalyx in Nanocarrier-Cell Adhesion Investigated Using a Thermodynamic Model and Monte Carlo Simulations

We present an equilibrium model for quantifying the effect of glycocalyx in mediating the interaction of functionalized nanocarriers with endothelial cells. In this model, nanocarrier adhesion is governed by the interplay between three physical parameters, namely, glycocalyx resistance, flexural rigidity of receptors, and receptor-ligand bond stiffness. We describe a procedure to rationally determine the values of these crucial parameters based on several independent (single molecule and cell-based) characterizing experiments. Using our model and independent derivation of the parameter values in conjunction with Monte Carlo simulations, we describe the binding of nanocarriers to endothelial cells at equilibrium. We show that we can quantitatively reproduce the experimental binding affinities with zero fitting to binding data. Additionally, our simulations provide quantitative descriptions for the multivalency in nanocarrier binding, as well as for the degree of clustering of antigens. Our study identifies two interesting parameters: glycocalyx resistance and antigen flexural rigidity, both of which reduce binding of nanocarriers and alter the sensitivity of the nanocarrier binding constant to changes in temperature. Collectively, our model, parameter estimations, simulations, and sensitivity analyses help provide unified molecular and energetic analyses of the nanocarrier binding process.