Roles of the cytoskeleton and motor proteins in endocytic sorting

Intracellular traffic and fate of protein transduction domains HIV-1 TAT peptide and octaarginine. Implications for their utilization as drug delivery vectors

Transduction domains such as those derived from the HIV-TAT protein are candidate vectors for intracellular delivery of therapeutic macromolecules such as DNA and proteins. The mechanism by which they enter cells is controversial, and very little spatial information regarding the downstream fate of these peptides from the plasma membrane is available. We studied endocytic traffic of fluorescent conjugates of HIV-TAT peptide and octaarginine in human hematopoietic cell lines K562 (CD34-) and KG1a (CD34+) and substantiated our findings in epithelia cells. Both peptides were efficiently internalized to endocytic pathways of both hematopoietic cell lines; however, comparative analysis of the intracellular location of the peptides with endocytic probes revealed major differences in spatial organization of their endocytic organelles and their interaction with the peptides at low temperatures. Double labeling confocal microscopy demonstrates that prelabeled lysosomes of all the tested cells are accessible to internalized peptides within 60 min of endocytic uptake. Incubation of cells with nocodazole and cytochalasin D inhibited peptide traffic from early to late endosomal structures, demonstrating a cytoskeletal requirement for lysosomal delivery. Disruption of Golgi and endoplasmic reticulum dynamics was without effect on peptide localization, suggesting that endosomes and lysosomes rather than these organelles are the major acceptor compartments for these molecules.

The Fps/Fes kinase regulates the inflammatory response to endotoxin through down-regulation of TLR4, NF-kappaB activation, and TNF-alpha secretion in macrophages

Fps/Fes and Fer are members of a distinct subfamily of cytoplasmic protein tyrosine kinases that have recently been implicated in the regulation of innate immunity. Previous studies showed that mice lacking Fps/Fes are hypersensitive to systemic LPS challenge, and Fer-deficient mice displayed enhanced recruitment of leukocytes in response to local LPS challenge. This study identifies physiological, cellular, and molecular defects that contribute to the hyperinflammatory phenotype in Fps/Fes null mice. Plasma TNF-alpha levels were elevated in LPS challenged Fps/Fes null mice as compared with wild-type mice and cultured Fps/Fes null peritoneal macrophages treated with LPS showed increased TNF-alpha production. Cultured Fps/Fes null macrophages also displayed prolonged LPS-induced degradation of IkappaB-alpha, increased phosphorylation of the p65 subunit of NF-kappaB, and defective TLR4 internalization, compared with wild-type macrophages. Together, these observations provide a likely mechanistic basis for elevated proinflammatory cytokine secretion by Fps/Fes null macrophages and the increased sensitivity of Fps/Fes null mice to endotoxin. We posit that Fps/Fes modulates the innate immune response of macrophages to LPS, in part, by regulating internalization and down-regulation of the TLR4 receptor complex.

Role of cellular FKBP52 protein in intracellular trafficking of recombinant adeno-associated virus 2 vectors

We have reported that tyrosine-phosphorylated forms of a cellular protein, FKBP52, inhibit the second-strand DNA synthesis of adeno-associated virus 2 (AAV), leading to inefficient transgene expression from recombinant AAV vectors. To further explore the role of FKBP52 in AAV-mediated transduction, we established murine embryo fibroblasts (MEFs) cultures from FKBP52 wild-type (WT), heterozygous (HE), and knockout (KO) mice. Conventional AAV vectors failed to transduce WT MEFs efficiently, and the transduction efficiency was not significantly increased in HE or KO MEFs. AAV vectors failed to traffic efficiently to the nucleus in these cells. Treatment with hydroxyurea (HU) increased the transduction efficiency of conventional AAV vectors by approximately 25-fold in WT MEFs, but only by approximately 4-fold in KO MEFs. The use of self-complementary AAV (scAAV) vectors, which bypass the requirement of viral second-strand DNA synthesis, revealed that HU treatment increased the transduction efficiency approximately 23-fold in WT MEFs, but only approximately 4-fold in KO MEFs, indicating that the lack of HU treatment-mediated increase in KO MEFs was not due to failure of AAV to undergo viral second-strand DNA synthesis. Following HU treatment, approximately 59% of AAV genomes were present in the nuclear fraction from WT MEFs, but only approximately 28% in KO MEFs, indicating that the pathway by which HU treatment mediates nuclear transport of AAV was impaired in KO MEFs. When KO MEFs were stably transfected with an FKBP52 expression plasmid, HU treatment-mediated increase in the transduction efficiency was restored in these cells, which correlated directly with improved intracellular trafficking. Intact AAV particles were also shown to interact with FKBP52 as well as with dynein, a known cellular protein involved in AAV trafficking. These studies suggest that FKBP52, being a cellular chaperone protein, facilitates intracellular trafficking of AAV, which has implications in the optimal use of recombinant AAV vectors in human gene therapy.

Rab22a regulates the sorting of transferrin to recycling endosomes

Rab22a is a member of the Rab family of small GTPases that localizes in the endocytic pathway. In CHO cells, expression of canine Rab22a (cRab22a) causes a dramatic enlargement of early endocytic compartments. We wondered whether transferrin recycling is altered in these cells. Expression of the wild-type protein and a GTP hydrolysis-deficient mutant led to the redistribution of transferrin receptor to large cRab22a-positive structures in the periphery of the cell and to a significant decrease in the plasma membrane receptor. Kinetic analysis of transferrin uptake indicates that internalization and early recycling were not affected by cRab22a expression. However, recycling from large cRab22a-positive compartments was strongly inhibited. A similar effect on transferrin transport was observed when human but not canine Rab22a was expressed in HeLa cells. After internalization for short periods of time (5 to 8 min) or at a reduced temperature (16 degrees C), transferrin localized with endogenous Rab22a in small vesicles that did not tubulate with brefeldin A, suggesting that the endogenous protein is present in early/sorting endosomes. Rab22a depletion by small interfering RNA disorganized the perinuclear recycling center and strongly inhibited transferrin recycling. We speculate that Rab22a controls the transport of the transferrin receptor from sorting to recycling endosomes.

Steady State Analysis and Experimental Validation of a Model for Hepatic High-Density Lipoprotein Transport

Transport of high-density lipoprotein (HDL) in the hepatocyte plays a fundamental role in reverse cholesterol transport and regulation of plasma HDL levels. On the basis of a recently developed kinetic model, the steady state distribution of HDL was analyzed. Fractional fluorescence of labeled HDL in the basolateral membrane, sorting endosomes (SE), the subapical compartment/ apical recycling compartment, the biliary canaliculus and in late endosomes and lysosomes (LE/LYS) including expected standard deviation is predicted. Improved parameter estimation was obtained by including kinetic data of apical endocytosis of fluorescent markers for LE/LYS, asialoorosomucoid and Rhodamine-dextran, in the regression. Predicted values using the refined kinetic parameters are in good agreement with experimental values of compartmental steady state fluorescence of Alexa488-HDL in polarized hepatic HepG2 cells. From calculated steady state fluxes, it is suggested that export of HDL from basolateral SE is the key step for determining the transport of HDL through the hepatocyte. The analysis provides testable predictions for high-throughput fluorescence microscopy screening experiments on potential inhibitors of hepatic HDL processing. By quantitative fluorescence imaging and model analysis, it is shown that the phosphoinositide kinase inhibitor wortmannin prevents apical transport of fluorescent HDL from basolateral SE. The results support that endosomes of polarized hepatic cells have different sorting functions and that apical endocytosis is an integrative trafficking step in hepatocytes.

Drug transporters in pharmacokinetics

This review deals with the drug transporters allowing drugs to enter and leave cells by carrier-mediated pathways. Emphasis is put on liver transporters but systems in gut, kidney, and blood-brain barrier are mentioned as well. Drug-drug interactions on carriers may provoke significant modification in pharmacokinetics as do carrier gene polymorphisms yielding functional carrier protein mutations. An integrated phase concept should reflect the interplay between drug metabolism and drug transport.

Microtubular integrity differentially modifies the saturated and unsaturated fatty acid metabolism in cultured Hep G2 human hepatoma cells

The influence of cytoskeleton integrity on the metabolism of saturated and unsaturated FA was studied in surface cultures and cell suspensions of human Hep G2 hepatoma cells. We found that colchicine (COL), nocodazol, and vinblastin produced a significant inhibition in the incorporation of labeled saturated FA, whereas incorporation of the unsaturated FA remained unaltered. These microtubule-disrupting drugs also diminished Delta9-, Delta5-, and Delta6-desaturase capacities. The effects produced by COL were dose (0-50 microM) and time (0-300 min) dependent, and were antagonized by stabilizing agents (phalloidin and DMSO). Dihydrocytochalasin B (20 microM) was tested as a microfilament-disrupting drug and produced no changes in either the incorporation of [14C] FA or the desaturase conversion of the substrates. We hypothesized that the interactions between cytoskeleton and membrane proteins such as FA desaturases may explain the functional organization, facilitating both substrate channeling and regulation of unsaturated FA biosynthesis.

NMDA receptors mediate calcium-dependent, bidirectional changes in dendritic PICK1 clustering

AMPA receptor (AMPAR) trafficking at CNS synapses is regulated by several receptor-binding proteins. One model of AMPAR endocytosis entails the cotargeting of the GluR2-interacting protein PICK1 and activated PKC to synapses. We demonstrate that NMDA receptor (NMDAR) activation mediates bidirectional changes in surface AMPARs through two additional forms of PICK1 redistribution. In neurons, NMDAR activation, which induces AMPAR endocytosis, increases endosomal PICK1 clustering. In contrast, stronger NMDAR activation rapidly reduces PICK1 clustering accompanied by decreases in PICK1/GluR2 association and increases in surface AMPAR levels. PICK1-siRNA similarly increases surface AMPARs and occludes the NMDAR-mediated effect, demonstrating the role of PICK1 in this process. Bidirectional NMDAR-mediated changes in PICK1 localization are determined by the magnitude of receptor-activated dendritic calcium signals. Our results show that PICK1 localization in dendrites is subject to multiple forms of regulation that contribute to surface AMPAR expression, likely by modulating the numbers of AMPARs maintained in intracellular compartments.

Interferon-gamma inhibits T84 epithelial cell migration by redirecting transcytosis of beta1 integrin from the migrating leading edge

Intestinal inflammation is associated with epithelial damage and formation of mucosal wounds. Epithelial cells migration is required for wound closure. In inflammatory status, migrating epithelial cells are exposed to proinflammatory cytokines such as IFN-gamma. However, influence of such cytokines on intestinal epithelial wound closure remains unknown. The present study was designed to investigate the effect of IFN-gamma on migration of model T84 intestinal epithelial cells and recovery of epithelial wounds. IFN-gamma significantly inhibited rate of T84 cell migration and closure of epithelial wounds. This effect was accompanied by the formation of large aberrant lamellipodia at the leading edge as well as significant decrease in the number of beta(1) integrin containing focal adhesions. IFN-gamma exposure increased endocytosis of beta(1) integrin and shifted its accumulation from early/recycling endosomes at the leading edge to a yet unidentified compartment at the cell base. This redirection in beta(1) integrin transcytosis was inhibited by depolymerization of microtubules with nocodazole and was unaffected by stabilization of microtubules with docetaxel. These results suggest that IFN-gamma attenuates epithelial wound closure by microtubule-dependent redirection of beta(1) integrin transcytosis from the leading edge of migrating cells thereby inhibiting adequate turnover of focal adhesion complexes and cell migration.

A model for intracellular trafficking of adenoviral vectors

Here we develop an integrative computational framework to model biophysical processes involved in viral gene delivery. The model combines reaction-diffusion-advection equations that describe intracellular trafficking with kinetic equations that describe transcription and translation of the exogenous DNA. It relates molecular-level trafficking events to whole-cell distribution of viruses. The approach makes use of the current understanding of cellular processes and data from single-particle single-cell imaging experiments. The model reveals two important parameters that characterize viral transport at the population level, namely, the effective velocity, V(eff), and the effective diffusion coefficient, D(eff). V(eff) measures virus's net movement rate and D(eff) represents the total dispersion rate. We employ the model to study the influence of microtubule-mediated movements on nuclear targeting and gene expression of adenoviruses of type 2 and type 5 in HeLa and A549 cells. Effects of microtubule organization and the presence of microtubule-destabilizing drugs on viral transport were analyzed and quantified. Model predictions agree well with experimental data available in literature. The paper serves as a guide for future theoretical and experimental efforts to understand viral gene delivery.

A novel method to isolate and map endothelial membrane proteins from pulmonary vasculature

Vascular endothelium has attracted extensive attention due to its important role in many physiological and pathological processes. Many methods have been developed to study the components and their functions in vascular endothelium. Here we report a novel approach to investigate vascular endothelium using normal rat lungs as the model. We perfused lung vascular beds with sulfosuccinimidyl-6-(biotinamido) hexanoate, a biotin analog, to label endothelial membrane proteins. The biotinylated proteins were isolated from lung homogenate with immobilized monomeric avidin and confirmed to be highly pure endothelial membrane proteins with little contamination of intracellular proteins. These biotinylated proteins were used as immunogens for development of monoclonal antibodies. Indeed, newly generated monoclonal antibodies have revealed different expression patterns of proteins across tissues. Some proteins were found highly specifically expressed to capillary vessels of pulmonary vasculature. This method has also been proven useful for investigating vasculature of other organs, as this study explored.

Microtubule-dependent movement of late endocytic vesicles in vitro: requirements for Dynein and Kinesin

Our previous studies demonstrated that fluorescent early endocytic vesicles prepared from rat liver after injection of Texas red asialoorosomucoid contain asialoglycoprotein and its receptor and move and undergo fission along microtubules using kinesin I and KIFC2, with Rab4 regulating KIFC2 activity (J. Cell Sci. 116, 2749, 2003). In the current study, procedures to prepare fluorescent late endocytic vesicles were devised. In addition, flow cytometry was utilized to prepare highly purified fluorescent endocytic vesicles, permitting validation of microscopy-based experiments as well as direct biochemical analysis. These studies revealed that late vesicles bound to and moved along microtubules, but in contrast to early vesicles, did not undergo fission. As compared with early vesicles, late vesicles had reduced association with receptor, Rab4, and kinesin I but were highly associated with dynein, Rab7, dynactin, and KIF3A. Dynein and KIF3A antibodies inhibited late vesicle motility, whereas kinesin I and KIFC2 antibodies had no effect. Dynamitin antibodies prevented the association of late vesicles with microtubules. These results indicate that acquisition and exchange of specific motor and regulatory proteins characterizes and may regulate the transition of early to late endocytic vesicles. Flow cytometric purification should ultimately facilitate detailed proteomic analysis and mapping of endocytic vesicle-associated proteins.