Molecular and cellular mechanisms of receptor-mediated endocytosis

Comparative transcriptomic analysis of gill reveals genes belonging to mTORC1 signaling pathway associated with the resistance trait of shrimp to VPAHPND

Selective breeding for acute hepatopancreatic necrosis disease (AHPND) resistant shrimp is an effective way to deal with heavy losses to shrimp aquaculture caused by AHPND. However, knowledge about the molecular mechanism of susceptibility or resistance to AHPND is very limited. We herein performed a comparative transcriptomic analysis of gill tissue between AHPND susceptible and resistant families of the white Pacific shrimp Litopenaeus vannamei during Vibrio parahaemolyticus (VP_AHPND) infection. A total of 5,013 genes that were differentially expressed between the two families at 0 and 6 h post-infection, and 1,124 DEGs were shared for both two time points. Both GO and KEGG analyses in each or two time point's comparisons showed DEGs involved in endocytosis, protein synthesis and cell inflammation were significantly enriched. Several immune DEGs including PRRs, antioxidants and AMPs were also identified. The susceptible shrimp showed enhanced endocytosis, higher aminoacyl-tRNA ligase activity and occurrence of inflammatory response, while the resistant shrimp had much more strong ability in ribosome biogenesis, antioxidant activity and pathogen recognition and clearance. These genes and processes were mostly associated with mTORC1 signaling pathway, which could reflect differences in cell growth, metabolism and immune response between the two families. Our findings indicate a close link between mTORC1 signaling-related genes and Vibrio-resistance phenotype of shrimp, and provide new clues for further research on resistance strategy of shrimp to AHPND.

Self-Reinforced Cancer Targeting (SRCT) Depending on Reciprocally Enhancing Feedback between Targeting and Therapy

Conventional cancer targeting methodology needs to be reformed to overcome the intrinsic barriers responsible for poor targeting efficiency. This study describes a concept of self-reinforced cancer targeting (SRCT) by correlating targeting with therapy in a reciprocally enhancing manner. SRCT is achieved on the basis of two prerequisites: (1) target molecules have to be expressed on cancer cell membranes but not on normal cells, and (2) notably, their expression on cancer cells must be actively upregulated in response to cellular attack by cancer treatments. As a proof-of-concept, a GRP78-targeting nanovehicle for chemotherapy was designed. Resultant data showed that chemotherapeutic drugs could effectively elevate GRP78 expression on the plasma membranes of cancer cells while having minimal influence on normal cells. DOX pretreatment of cancer cells and tumor tissues can greatly increase the targeting efficacy and therapeutic performance of the prepared GRP78-targeting nanomedicine while somewhat disfavoring the nontargeting counterpart. In vivo and in vitro results demonstrated that this GRP78-targeting nanomedicine could accurately target cancer cells to not only implement chemotherapy but also induce GRP78 upregulation on cancer cells, eventually benefiting continuous cancer-cell-targeted attack by the nanomedicines remaining in the blood circulation or administered in the next dose. The GRP78-targeting nanomedicine displays much better antitumor performance compared with the nontargeting counterpart. SRCT is expected to advance cancer-targeted therapy based on the positive dependency between targeting and therapeutic modalities.

Targeting Non-coding RNA for Glioblastoma Therapy: The Challenge of Overcomes the Blood-Brain Barrier

Glioblastoma (GBM) is the most malignant form of all primary brain tumors, and it is responsible for around 200,000 deaths each year worldwide. The standard therapy for GBM treatment includes surgical resection followed by temozolomide-based chemotherapy and/or radiotherapy. With this treatment, the median survival rate of GBM patients is only 15 months after its initial diagnosis. Therefore, novel and better treatment modalities for GBM treatment are urgently needed. Mounting evidence indicates that non-coding RNAs (ncRNAs) have critical roles as regulators of gene expression. Long non-coding RNAs (lncRNAs) and microRNAs (miRNAs) are among the most studied ncRNAs in health and disease. Dysregulation of ncRNAs is observed in virtually all tumor types, including GBMs. Several dysregulated miRNAs and lncRNAs have been identified in GBM cell lines and GBM tumor samples. Some of them have been proposed as diagnostic and prognostic markers, and as targets for GBM treatment. Most ncRNA-based therapies use oligonucleotide RNA molecules which are normally of short life in circulation. Nanoparticles (NPs) have been designed to increase the half-life of oligonucleotide RNAs. An additional challenge faced not only by RNA oligonucleotides but for therapies designed for brain-related conditions, is the presence of the blood-brain barrier (BBB). The BBB is the anatomical barrier that protects the brain from undesirable agents. Although some NPs have been derivatized at their surface to cross the BBB, optimal NPs to deliver oligonucleotide RNA into GBM cells in the brain are currently unavailable. In this review, we describe first the current treatments for GBM therapy. Next, we discuss the most relevant miRNAs and lncRNAs suggested as targets for GBM therapy. Then, we compare the current drug delivery systems (nanocarriers/NPs) for RNA oligonucleotide delivery, the challenges faced to send drugs through the BBB, and the strategies to overcome this barrier. Finally, we categorize the critical points where research should be the focus in order to design optimal NPs for drug delivery into the brain; and thus move the Oligonucleotide RNA-based therapies from the bench to the clinical setting.

pH-responsive antibodies for therapeutic applications

Therapeutic antibodies are instrumental in improving the treatment outcome for certain disease conditions. However, to enhance their efficacy and specificity, many efforts are continuously made. One of the approaches that are increasingly explored in this field are pH-responsive antibodies capable of binding target antigens in a pH-dependent manner. We reviewed suitability and examples of these antibodies that are functionally modulated by the tumor microenvironment. Provided in this review is an update about antigens targeted by pH-responsive, sweeping, and recycling antibodies. Applicability of the pH-responsive antibodies in the engineering of chimeric antigen receptor T-cells (CAR-T) and in improving drug delivery to the brain by the enhanced crossing of the blood-brain barrier is also discussed. The pH-responsive antibodies possess strong treatment potential. They emerge as next-generation programmable engineered biologic drugs that are active only within the targeted biological space. Thus, they are valuable in targeting acidified tumor microenvironment because of improved spatial persistence and reduced on-target off-tumor toxicities. We predict that the programmable pH-dependent antibodies become powerful tools in therapies of cancer.

Organ-based drug delivery

The phenomenal advances in pharmaceutical sciences over the last few decades have led to the development of new therapeutics like peptides, proteins, RNAs, DNAs and highly potent small molecules. Fruitful applications of these therapeutics have been challenged by several anatomical and physiological barriers that limit adequate drug disposition at the site-of-action and by off-target drug distribution to undesired tissues, which together result in the reduced effectiveness and increased side effects of therapeutic agents. As such, the development of drug delivery and targeting systems has been recognised as a cornerstone for future drug development. Research in pharmaceutical sciences is now devoted to tackling delivery challenges through engineering delivery systems that move beyond conventional dosage forms and regimens into state-of-the-art targeted drug delivery tailored toward specific therapeutic needs. Modern drug delivery systems comprise passive and active targeting approaches. While passive targeting relies on the natural course of distribution of drugs or drug carriers in the body, as governed by their physicochemical properties, active targeting often exploits targeting moieties that home preferentially into target tissues. Here, we provide an overview of theories of and approaches to passive and active drug delivery. As the design of drug delivery is dependent on the unique structure of target tissues and organs, we present our discussion in an organ-specific manner with the aim to inspire the development of new strategies for curing disease with high accuracy and efficiency.

CSPG4: A Target for Selective Delivery of Human Cytolytic Fusion Proteins and TRAIL

Chondroitin-sulfate proteoglycan 4 (CSPG4) is a transmembrane glycoprotein overexpressed on malignant cells in several cancer types with only limited expression on normal cells. CSPG4 is implicated in several signaling pathways believed to drive cancer progression, particularly proliferation, motility and metastatic spread. Expression may serve as a prognostic marker for survival and risk of relapse in treatment-resistant malignancies including melanoma, triple negative breast cancer, rhabdomyosarcoma and acute lymphoblastic leukemia. This tumor-associated overexpression of CSPG4 points towards a highly promising therapeutic target for antibody-guided cancer therapy. Monoclonal αCSPG4 antibodies have been shown to inhibit cancer progression by blocking ligand access to the CSPG4 extracellular binding sites. Moreover, CSPG4-directed antibody conjugates have been shown to be selectively internalized by CSPG4-expressing cancer cells via endocytosis. CSPG4-directed immunotherapy may be approached in several ways, including: (1) antibody-based fusion proteins for the selective delivery of a pro-apoptotic factors such as tumor necrosis factor-related apoptosis-inducing ligand to agonistic death receptors 4 and 5 on the cell surface; and (2) CSPG4-specific immunotoxins which bind selectively to diseased cells expressing CSPG4, are internalized by them and induce arrest of biosynthesis, closely followed by initiation of apoptotic signaling. Here we review various methods of exploiting tumor-associated CSPG4 expression to improve targeted cancer therapy.

Pressure-overload-induced angiotensin-mediated early remodeling in mouse heart

Our previous work on angiotensin II-mediated electrical-remodeling in canine left ventricle, in connection with a long history of other studies, suggested the hypothesis: increases in mechanical load induce autocrine secretion of angiotensin II (A2), which coherently regulates a coterie of membrane ion transporters in a manner that increases contractility. However, the relation between load and A2 secretion was correlative. We subsequently showed a similar or identical system was present in murine heart. To investigate whether the relation between mechanical load and A2-mediated electrical remodeling was causal, we employed transverse aortic constriction in mice to subject the left ventricle to pressure overload for short-term (1 to 2 days) or long-term (1 to 2 weeks) periods. Heart-to-body weight ratios and cell capacitance measurements were used to determine hypertrophy. Whole-cell patch clamp recordings of the predominant repolarization currents Ito,fast and IK,slow were used to assess electrical remodeling. Hearts or myocytes subjected to long-term load displayed significant hypertrophy, which was not evident in short-term load. However, short-term load induced significant reductions in Ito,fast and IK,slow. Incubation of these myocytes with the angiotensin II type 1 receptor inhibitor saralasin for 2 hours restored Ito,fast and IK,slow to control levels. The number of Ito.fast or IK,slow channels did not change with A2 or long-term load, however the hypertrophic increase in membrane area reduced the current densities for both channels. For Ito,fast but not IK,slow there was an additional reduction that was reversed by inhibition of angiotensin receptors. These results suggest increased load activates an endogenous renin angiotensin system that initially reduces Ito,fast and IK,slow prior to the onset of hypertrophic growth. However, there are functional interactions between electrical and anatomical remodeling. First, hypertrophy tends to reduce all current densities. Second, the hypertrophic program can modify signaling between the angiotensin receptor and target current.

Transcriptome using Illumina sequencing reveals the traits of spermatogenesis and developing testes in Eriocheir sinensis

Chinese mitten crab (Eriocheir sinensis) has the spermatozoa with typical aflagellate, decondensed chromatin, cup-shaped nuclei, and radial arms. However, the mechanism of spermatogenesis during which the specific spermatozoa are generated in this species is yet unclear. Here, the transcriptome of developing testis in E. sinensis was analyzed using the ways of RNA-seq and bioinformatics analysis to identify candidate genes potentially involved in development of testis and spermatogenesis. The Illumina HiSeq2500 sequencing of three replicons of samples produced a total of 145.19 M clean reads representing with a total of 21.34 Gb bases and 45.48% GC content. 56.30% clean reads were mapped to the draft genome of E. sinensis. The assembly of the transcriptome yielded contigs of 5691802 sequences and unigenes of 406527 sequences. Total 24246 and 40793 transcripts were annotated using Swissprot and Nr database, respectively. There were 48213 (70.31%) and 7858 (46.25%) transcripts with identity of more than 99 matching to mature testis unigenes in the databases of Nr and EST, respectively. The analytic results of KOG, GO and KEGG showed wide potential molecular functions of transcripts in the developing testes. KEGG analysis of unigenes yielded total 9422 predicted genes. Those predicted genes were involved in total 216 KEGG pathways related to the physiological activities of developing testis. 1975 predicted genes were involved in cellular and subcellular structural alteration of male germ cells. There were important roles of some pathways in the processes of morphological and structural biogenesis pertaining to testis development and spermatogenesis. Other 583 unigenes encoding the genetic and epigenetic factors also be found, which might contribute to the decondensation and stability of decondensed nuclei in the spermatozoa. These predicted events provide a view of the potential molecular mechanisms of development of testis and spermatogenesis in E. sinensis.

Angiotensin II Type 1 Receptor-Mediated Electrical Remodeling in Mouse Cardiac Myocytes

We recently characterized an autocrine renin angiotensin system (RAS) in canine heart. Activation of Angiotensin II Type 1 Receptors (AT1Rs) induced electrical remodeling, including inhibition of the transient outward potassium current Ito, prolongation of the action potential (AP), increased calcium entry and increased contractility. Electrical properties of the mouse heart are very different from those of dog heart, but if a similar system existed in mouse, it could be uniquely studied through genetic manipulations. To investigate the presence of a RAS in mouse, we measured APs and Ito in isolated myocytes. Application of angiotensin II (A2) for 2 or more hours reduced Ito magnitude, without affecting voltage dependence, and prolonged APs in a dose-dependent manner. Based on dose-inhibition curves, the fast and slow components of Ito (Ito,fast and IK,slow) appeared to be coherently regulated by [A2], with 50% inhibition at an A2 concentration of about 400 nM. This very high K0.5 is inconsistent with systemic A2 effects, but is consistent with an autocrine RAS in mouse heart. Pre-application of the microtubule destabilizing agent colchicine eliminated A2 effects on Ito and AP duration, suggesting these effects depend on intracellular trafficking. Application of the biased agonist SII ([Sar1-Ile4-Ile8]A2), which stimulates receptor internalization without G protein activation, caused Ito reduction and AP prolongation similar to A2-induced changes. These data demonstrate AT1R mediated regulation of Ito in mouse heart. Moreover, all measured properties parallel those measured in dog heart, suggesting an autocrine RAS may be a fundamental feedback system that is present across species.

Distinct subcellular trafficking resulting from monomeric vs multimeric targeting to endothelial ICAM-1: implications for drug delivery

Ligand-targeted, receptor-mediated endocytosis is commonly exploited for intracellular drug delivery. However, cells-surface receptors may follow distinct endocytic fates when bound by monomeric vs multimeric ligands. Our purpose was to study this paradigm using ICAM-1, an endothelial receptor involved in inflammation, to better understand its regulation and potential for drug delivery. Our procedure involved fluorescence microscopy of human endothelial cells to determine the endocytic behavior of unbound ICAM-1 vs ICAM-1 bound by model ligands: monomeric (anti-ICAM) vs multimeric (anti-ICAM biotin–streptavidin conjugates or anti-ICAM coated onto 100 nm nanocarriers). Our findings suggest that both monomeric and multimeric ligands undergo a similar endocytic pathway sensitive to amiloride (∼50% inhibition), but not inhibitors of clathrin-pits or caveoli. After 30 min, ∼60–70% of both ligands colocalized with Rab11a-compartments. By 3–5 h, ∼65–80% of multimeric anti-ICAM colocalized with perinuclear lysosomes with ∼60–80% degradation, while 70% of monomeric anti-ICAM remained associated with Rab11a at the cell periphery and recycled to and from the cell-surface with minimal (<10%) lysosomal colocalization and minimal (≤15%) degradation. In the absence of ligands, ICAM-1 also underwent amiloride-sensitive endocytosis with peripheral distribution, suggesting that monomeric (not multimeric) anti-ICAM follows the route of this receptor. In conclusion, ICAM-1 can mediate different intracellular itineraries, revealing new insight into this biological pathway and alternative avenues for drug delivery.

Cellular elements of the blood-brain barrier

The Blood-brain-barrier (BBB) provides both anatomical and physiological protection for the central nervous system (CNS), shielding the brain for toxic substances in the blood, supplying brain tissues with nutrients and filtering harmful compounds from the brain back to the bloodstream. The BBB is composed of four main cellular elements: endothelial cells (ECs), astrocyte end-feet, microglial cells, and pericytes. Transport across the BBB is limited by both physical and metabolic barriers (enzymes, and different transport systems). Tight junctions (TJs) present between ECs form an important barrier against diffusion, excluding most blood-borne substances for entering the brain.

Blood-brain barrier transport of therapeutics via receptor-mediation

Drug delivery to the brain is hindered by the presence of the blood-brain barrier (BBB). Although the BBB restricts the passage of many substances, it is actually selectively permeable to nutrients necessary for healthy brain function. To accomplish the task of nutrient transport, the brain endothelium is endowed with a diverse collection of molecular transport systems. One such class of transport system, known as a receptor-mediated transcytosis (RMT), employs the vesicular trafficking machinery of the endothelium to transport substrates between blood and brain. If appropriately targeted, RMT systems can also be used to shuttle a wide range of therapeutics into the brain in a noninvasive manner. Over the last decade, there have been significant developments in the arena of RMT-based brain drug transport, and this review will focus on those approaches that have been validated in an in vivo setting.

PTP1B-dependent insulin receptor phosphorylation/residency in the endocytic recycling compartment of CHO-IR cells

Insulin binds to the alpha subunit of the insulin receptor (IR) on the cell surface. The insulin-IR complex is subsequently internalized and trafficked within the cell. Endocytosed receptors, devoid of insulin, recycle back to the plasma membrane through the endocytic recycling compartment (ERC). Using a high content screening system, we investigate the intracellular trafficking of the IR and its phosphorylation state, within the ERC, in response to protein tyrosine phosphatase-1B (PTP1B) inhibition. Insulin stimulates, in a time- and dose-dependent manner, the accumulation of phosphorylated IR (pY(1158,1162,1163 IR) in the ERC of CHO-IR cells. Treatment of CHO-IR cells with PTP1B-specific inhibitors or siRNA leads to dose-dependent increases in IR residency and phosphorylation within the ERC. The results also demonstrate that PTP1B redistributes within CHO-IR cells upon insulin challenge. The established system will allow for efficient screening of candidate inhibitors for the modulation of PTP1B activity.

Photodynamic effects of novel XF porphyrin derivatives on prokaryotic and eukaryotic cells

The worldwide rise in the rates of antibiotic resistance of bacteria underlines the need for alternative antibacterial agents. A promising approach to the killing of gram-positive antibiotic-resistant bacteria of the skin uses light in combination with a photosensitizer to induce a phototoxic reaction. Different concentrations (0 to 100 microM) of porphyrin-based photosensitizers (CTP1, XF70, and XF73) and different incubation times (5 min, 1 h, and 4 h) were used to determine phototoxicity against two methicillin-resistant Staphylococcus aureus strains, one methicillin-sensitive S. aureus strain, one methicillin-resistant Staphylococcus epidermidis strain, one Escherichia coli strain, and human keratinocytes and fibroblasts. Incubation with 0.005 microM XF70 or XF73, followed by illumination, yielded a 3-log10 (> or = 99.9%) decrease in the viable cell numbers of all staphylococcal strains, indicating that the XF drugs have high degrees of potency against gram-positive bacteria and also that the activities of these novel drugs are independent of the antibiotic resistance pattern of the staphylococci examined. CTP1 was less potent against the staphylococci under the same conditions. At 0.005 microM, XF70 and XF73 demonstrated no toxicity toward fibroblasts or keratinocytes. No inactivation of E. coli was detected at this concentration. XF73 was confirmed to act via a reactive oxygen species from the results of studies with sodium azide (a quencher of singlet oxygen), which reduced the killing of both eukaryotic and prokaryotic cells. When a quencher of superoxide anion and the hydroxyl radical was used, cell killing was not inhibited. These results demonstrate that the porphyrin-based photosensitizers had concentration-dependent differences in their efficacies of killing of methicillin-resistant staphylococcal strains via reactive oxygen species without harming eukaryotic cells at the same concentrations.

pH dependence and stoichiometry of binding to the Fc region of IgG by the herpes simplex virus Fc receptor gE-gI

Herpes simplex virus type 1 encodes two glycoproteins, gE and gI, that form a heterodimer on the surface of virions and infected cells. The gE-gI heterodimer has been implicated in cell-to-cell spread of virus and is a receptor for the Fc fragment of IgG. Previous studies localized the gE-gI-binding site on human IgG to a region near the interface between the C(H)2 and C(H)3 domains of Fc, which also serves as the binding site for bacterial and mammalian Fc receptors. Although there are two potential gE-gI-binding sites per Fc homodimer, only one gE-gI heterodimer binds per IgG in gel filtration experiments. Here we report production of recombinant human Fc molecules that contain zero, one, or two potential gE-gI-binding sites and use them in analytical ultracentrifugation experiments to show that two gE-gI heterodimers can bind to each Fc. Further characterization of the gE-gI interaction with Fc reveals a sharp pH dependence of binding, with K(D) values of approximately 340 and approximately 930 nm for the first and second binding events, respectively, at the slightly basic pH of the cell surface (pH 7.4), but undetectable binding at pH 6.0. This strongly pH-dependent interaction suggests a physiological role for gE-gI dissociation from IgG within acidic intracellular compartments, consistent with a mechanism whereby herpes simplex virus promotes intracellular degradation of anti-viral antibodies.

Axonal transport and neuronal transcytosis of trophic factors, tracers, and pathogens

Neurons can specifically internalize macromolecules, such as trophic factors, lectins, toxins, and other pathogens. Upon internalization in terminals, proteins can move retrogradely along axons, or, upon internalization at somatodendritic domains, they can move into an anterograde axonal transport pathway. Release of internalized proteins from neurons after either retrograde or anterograde axonal transport results in transcytosis and trafficking of proteins across multiple synapses. Recent studies of binding properties of several such proteins suggest that pathogens and lectins may utilize existing transport machineries designed for trafficking of trophic factors. Specific pathways may protect trophic factors, pathogens, and toxins from degradation after internalization and may target the trophic or pathogenic cargo for transcytosis after either retrograde or anterograde transport along axons. Elucidating the molecular mechanisms of sorting steps and transport pathways will further our understanding of trophic signaling and could be relevant for an understanding and possible treatment of neurological diseases such as rabies, Alzheimer's disease, and prion encephalopathies. At present, our knowledge is remarkably sparse about the types of receptors used by pathogens for trafficking, the signals that sort trophins or pathogens into recycling or degradation pathways, and the mechanisms that regulate their release from somatodendritic domains or axon terminals. This review intends to draw attention to potential convergences and parallels in trafficking of trophic and pathogenic proteins. It discusses axonal transport/trafficking mechanisms that may help to understand and eventually treat neurological diseases by targeted drug delivery.

Isopentenyl pyrophosphate, a mycobacterial non-peptidic antigen, triggers delayed and highly sustained signaling in human gamma delta T lymphocytes without inducing eown-modulation of T cell antigen receptor

The Vgamma9Vdelta2 T cell subset, which represents up to 90% of the circulating gammadelta T cells in humans, was shown to be activated, via the T cell receptor (TcR), by non-peptidic phosphorylated small organic molecules. These phosphoantigens, which are not presented by professional antigen-presenting cells, induce production of high amounts of interferon-gamma and tumor necrosis factor (TNF-alpha). To date, the specific signals triggered by these antigens have not been characterized. Here we analyze proximal and later intracellular signals triggered by isopentenyl pyrophosphate (IPP), a mycobacterial antigen that specifically stimulates Vgamma9Vdelta2 T cells, and compare these to signals induced by the non-physiological model using an anti-CD3 antibody. During antigenic stimulation we noticed that, except for the proximal p56(lck) signal, which is triggered early, the signals appear to be delayed and highly sustained. This delay, which likely accounts for the delay observed in TNF-alpha production, is discussed in terms of the ability of the antigen to cross-link and recruit transducing molecules mostly anchored to lipid rafts. Moreover, we demonstrate that, in contrast to anti-CD3 antibody, IPP does not induce down-modulation of the TcR.CD3 complex, which likely results in the highly sustained signaling and release of high levels of TNF-alpha.

Recycling and resensitization of delta opioid receptors

Exposure to opioids results in the activation of opioid receptors; this is followed by receptor endocytosis. Previously, we showed that delta opioid receptors undergo rapid agonist-mediated internalization and that mutations in the C-tail result in a substantial loss of agonist-mediated internalization. In this study, we investigated the fate of receptors following rapid internalization. We found that the majority of the wild type receptors recycled back to the surface after acute agonist treatment. The kinetics of internalization and recycling of the receptor were virtually identical to the kinetics of internalization and recycling of the radiolabeled agonist. In contrast, the kinetics of internalization and recycling of a C-tail mutant receptor were substantially altered, suggesting an involvement of the C-tail in the recycling process. It is possible that in addition to agonist-mediated internalization, opioid receptors undergo constitutive, agonist-independent internalization. We directly examined this possibility using an antibody-prebinding assay. The wild type delta opioid receptors exhibited agonist-independent internalization via the clathrin-coated pit pathway. We also examined the role of receptor internalization and recycling in the modulation of its function by quantitating the level of opioid-stimulated phosphorylation of MAP kinase (MAPK) under conditions of receptor internalization and recycling. We found that agonist treatment caused a rapid increase in the level of phosphorylated MAPK that was rapidly desensitized. The removal of the agonist, which results in receptor recycling, led to the resensitization of the receptor, as evidenced by the agonist's ability to reinduce MAPK phosphorylation. Mutant receptors that underwent rapid recycling exhibited enhanced resensitization, suggesting a role for receptor recycling in the resensitization process. Taken together, these results indicate that agonist-mediated internalization and recycling modulate opioid receptor function and that the receptor C-tail plays an important role in both processes.

Serial TCR Engagement and Down-Modulation by Peptide:MHC Molecule Ligands: Relationship to the Quality of Individual TCR Signaling Events

In the present study, we examined the relationships among quantitative aspects of TCR engagement as measured by receptor down-modulation, functional responses, and biochemical signaling events using both mouse and human T cell clones. For T cells from both species, ligands that are more potent in inducing functional responses promote TCR down-modulation more efficiently than weaker ligands. At low ligand density, the number of down-modulated TCR exceeds the number of available ligands by as much as 80–100:1 in the optimal human case, confirming the previous description of serial ligand engagement of TCR (Valitutti, et al. 1995. Nature 375:148–151). A previously unappreciated relationship involving TCR down-modulation, the pattern of proximal TCR signaling, and the extent of serial engagement was revealed by analyzing different ligands for the same TCR. Functionally, more potent ligands induce a higher proportion of fully tyrosine phosphorylated ζ-chains and a greater amount of phosphorylated ZAP-70 than less potent ligands, and the number of TCR down-modulated per available ligand is higher with ligands showing this full agonist-like pattern. The large number of receptors showing partial ζ phosphorylation following exposure to weak ligands indicates that the true extent of TCR engagement and signaling, and thus the amount of sequential engagement, is underestimated by measurement of TCR down-modulation alone, which depends on full receptor activation. These data provide new insight into T cell activation by revealing a clear relationship among intrinsic ligand quality, signal amplification by serial engagement, functional T cell responses, and observable TCR clearance from the cell surface.

A TrkA-selective, fast internalizing nerve growth factor-antibody complex induces trophic but not neuritogenic signals

Nerve growth factor (NGF) is a neurotrophin that induces neuritogenic and trophic signals by binding to TrkA and/or p75 receptors. We report a comparative study of the binding, internalization, and biological activity of NGF versus that of NGF in association with an anti-NGF monoclonal antibody (mAb NGF30), directed against the C termini of NGF. NGF.mAb complexes do not bind p75 effectively but bind TrkA with high affinity. After binding, NGF. mAb complexes stimulate internalization faster and to a larger degree than NGF. NGF.mAb-induced activation of TrkA, Shc, and MAPK is transient compared with NGF-induced activation; yet NGF and NGF. mAb afford identical trophic responses. In contrast, NGF induces Suc-1-associated neurotrophic activating protein phosphorylation and neuritogenic differentiation, but NGF.mAb does not. Thus, an absolute separation of trophic and neuritogenic function is seen for NGF.mAb, suggesting that biological response modifiers of neurotrophins can afford ligands with selected activities.

CD77-dependent retrograde transport of CD19 to the nuclear membrane: functional relationship between CD77 and CD19 during germinal center B-cell apoptosis

A region of the N-terminal extracellular domain of the B-cell restricted cell differentiation antigen, CD19, has high amino acid sequence similarity to the receptor binding subunit B of verotoxin 1 (VT), an Escherichia coli elaborated cytotoxin, which specifically binds to the cell surface glycolipid, globotriaosylceramide, also known as the germinal center (GC) B-cell differentiation antigen, CD77. We have previously provided evidence of the association of CD19 and CD77 on the cell surface and in CD19-mediated homotypic adhesion of the Daudi Burkitt Lymphoma cell line, one normal counterpart of which is a subset of GC B cells. Evidence for the role of CD77 in CD19-induced apoptosis is now presented. Initial cell surface distribution, antibody-induced redistribution, internalization, and intracellular routing of CD19 were studied by confocal microscopy, IF, and postembedding IEM in CD77+ve and CD77-ve cells to investigate the possible role of CD77 in CD19 internalization and signaling. Daudi Burkitt's lymphoma cells were used as CD77+ve cells and as CD77-ve cells, Daudi mutant VT500 cells, and Daudi cells treated with PPMP, an inhibitor of CD77 synthesis, were used. Antibody ligated CD19 surface redistribution, internalization, and subcellular distribution of internalized CD19 was found to be different in CD77+ve and CD77-ve cells. A delay in internalization of antibody-CD19 complex was observed in CD77-ve cells. Internalized CD19 was targeted to the nuclear envelope in CD77+ve cells in a manner similar to that reported for VT, but not in CD77-ve cells. Internalization of CD77 by ligation with verotoxin prevented the internalization of ligated CD19. Induction of apoptosis following crosslinking of cell surface CD19 was greater in CD77+ve cells than in CD77-ve cells. The nuclear targeting of internalized CD19 and induction of apoptosis following CD19 crosslinking only in CD77+ve cells indicates a role for CD77-dependent CD19 retrograde transport from the B cell surface via the ER to the nuclear envelope in CD19-mediated signal transduction for apoptosis.

Structural basis of pH-dependent antibody binding by the neonatal Fc receptor

BACKGROUND

The neonatal Fc receptor (FcRn) mediates the transcytosis of maternal immunoglobulin G (IgG) across fetal and/or neonatal tissues for the acquisition of passive immunity. In adults, FcRn is involved in the maintenance of high serum IgG levels. Both processes are mediated by pH-dependent IgG binding to FcRn-FcRn binds to IgG with nanomolar affinity at pH 6, but shows no detectable binding at pH 7.5. At pH 6, FcRn is more thermally stable and the dissociation rate of its light chain is an order of magnitude slower than at pH 8.0. Comparison of the structures of FcRn at pH 6.5 and pH 8 allows an analysis of the structural basis for the receptor's pH-dependent ligand binding and stability.

RESULTS

We have determined the structure of FcRn at pH 8 and compared it to a further refined version of the structure at pH 6.5. An extensive ordered carbohydrate structure is observed at both pH values. The two structures are very similar; thus the pH dependence of FcRn stability and affinity for IgG can be attributed to chemical properties of the structures themselves, rather than mechanisms that rely on conformational changes. The pH-dependent properties are mediated by electrostatic interactions involving histidine residues, which are more favorable for the protonated form of histidine that predominates at acidic pH values.

CONCLUSIONS

No major conformational change is observed between the pH 6.5 and pH 8 structures of FcRn that could account for the differences in affinity for IgG. The pH dependence of IgG binding to FcRn can therefore primarily be attributed to titration of histidine residues on Fc that interact with anionic pockets on the receptor. The FcRn dimer, which is required for high affinity binding of IgG, is itself stabilized at acidic pH by histidine-mediated salt bridges and a sidechain rearrangement that creates a more favorable interaction with an anionic pocket at pH 6.5 relative to pH 8. FcRn dimerization is facilitated by reciprocal interactions in which carbohydrate from one receptor molecule binds to protein residues from the dimer-related receptor molecule to form a 'carbohydrate handshake'.

Binding and internalization of the melanocyte stimulating hormone receptor ligand [Nle4, D-Phe7] alpha-MSH in B16 melanoma cells

The current study aims to ascertain the fate of the melanocyte stimulating hormone (MSH) receptor and its ligand [Nle4, D-Phe7] alpha-MSH (NDP-MSH) following binding to murine B16 melanoma cells. Cells were incubated with [125I]-NDP-MSH for up to 180 min and binding, internalization and degradation determined. Intracellular trafficking of the radiolabel was assessed using Percoll density gradient centrifugation of homogenized cells. Receptor down-regulation and receptor mRNA levels were also measured over 96 hr after exposure to 1 microM ligand. NDP-MSH accumulation increased with time in a temperature-dependent manner and was inhibited by excess peptide. The ligand was rapidly internalized and translocated to the lysosomal compartment where it was degraded. Internalization was accompanied by a loss or down-regulation of cell surface receptors, suggesting internalization of the NDP-MSH-receptor complex. No recycling of the receptors between the plasma membrane and intracellular compartments could be detected in this cell-line. Approximately 15% of the surface receptors were resistant to down-regulation, possibly indicating receptor heterogeneity. Down-regulation persisted for up to 96 hr and was accompanied by a decrease in MSH receptor mRNA levels 48 hr after treatment. However, before this time, transcript levels were the same in treated and control cells. In contrast to what was seen with NDP-MSH, cell surface receptors removed with trypsin were rapidly replaced. These results show that NDP-MSH not only induced MSH receptor internalization but also inhibited receptor turnover, resulting in a prolonged down-regulation. It is concluded that, in B16 cells, the MSH receptor undergoes ligand-dependent internalization, resulting in a prolonged down-regulation.

Human cell receptor CD46 is down regulated through recognition of a membrane-proximal region of the cytoplasmic domain in persistent measles virus infection

Monkey cells persistently infected by measles virus (MV) Biken strain (Biken-CV-1 cells) showed no cytopathic effects and lacked surface expression of a homolog of human cell receptor, membrane cofactor protein CD46. Transfection of a human CD46 gene into these cells induced extensive cell fusion, indicating that down regulation of the endogenous CD46 homolog was essential for the maintenance of a noncytopathic mode of infection. Surface expression of the exogenously introduced human CD46 was also drastically down regulated in the persistently infected cells compared with uninfected cells. The down regulation was specific for CD46 and did not affect surface expression of exogenously introduced CD4. Exogenous human CD46 was synthesized efficiently in the persistently infected cells, but it did not accumulate on the cell surface. Fusion of Biken-CV-1 cells required the extracellular hemagglutinin (H-protein)-binding domain but not the cytoplasmic domain. Replacing the transmembrane and cytoplasmic domains of CD46 with a glycosylphosphatidylinositol anchor did not prevent cell fusion but completely alleviated down regulation of the glycosylphosphatidylinositol-anchored CD46 in Biken-CV-1 cells. Deletion analyses revealed that the membrane-distal sequences of the CD46 cytoplasmic domain were not only unnecessary but also inhibitory for CD46 down regulation. By contrast, the six amino acid residues proximal to the membrane contained a sequence required for CD46 down regulation in the persistently infected cells. These results indicate that CD46 is down regulated in the persistently infected cells by a mechanism that recognizes a membrane-proximal sequence in the CD46 cytoplasmic domain.

c-neu oncoprotein in developing rostral cerebral cortex: relationship to epidermal growth factor receptor

The c-neu oncoprotein, p185c-neu, is a transmembrane tyrosine kinase that shares structural similarities with the receptor for epidermal growth factor (EGFr). We used immunoblots, immunoprecipitation, and immunohistochemistry 1) to test the hypothesis that p185c-neu and EGFr are coordinately expressed in central nervous system tissue and 2) to assess the spatiotemporal expression of both the c-neu oncoprotein and EGFr in the rostral cerebral cortex. In nondenaturing gels, anti-c-neu antibody identified high molecular weight proteins (about 300-400 kDa) that were reduced by EDTA to a molecular weight of 180-200 kDa. Sodium dodecylsulfate polyacrylamide gel electrophoresis broke down this protein into an array of smaller peptides, which were expressed prenatally, transiently during the first three postnatal weeks, or in the adult. Perinatally, c-neu immunoreactivity was evident in subplate neurons, ascending processes of neurons in the cortical plate, and ventricular zone cells. During the second postnatal week, cells throughout cortex expressed somatodendritic immunostaining, but, in the adult, c-neu immunoreactivity was expressed only by pyramidal neurons in layer V and by glia in the white matter and ependyma. EGFr-positive proteins behaved in the nondenaturing gels as did c-neu-positive oncoproteins, suggesting that both proteins naturally formed dimers. This contention was supported by the EGFr-or c-neu immunolabeling of tissue that was previously immunoprecipitated with anti-c-neu or anti-EGFr, respectively. The pattern of EGFr immunolabeling in the developing and mature cortex was virtually identical to that described for c-neu immunoreactivity. Cortical neurons express the c-neu oncoprotein and EGFr, probably as heterodimers. The specific immunolabeling of layer V neurons in the adult cortex with anti-c-neu and anti-EGFr suggests that the p185c-neu ligand and EGF regulate the activity of corticofugal systems. The expression of different c-neu- and EGFr-positive peptides is developmentally defined and may be related to specific ontogenetic events.

Microtubule-based peroxisome movement

The association of peroxisomes with cytoskeletal structures was investigated both by electron microscopy and by kinetic analysis of peroxisome movement. The morphological studies indicated distinct interactions of peroxisomes with microtubules and frequently revealed multiple contact sites. The kinetic approach utilised microinjection and import of fluorescein-labeled luciferase in order to mark and track peroxisomes in vivo. Peroxisomal motility was analysed by time-lapse imaging and fluorescence microscopy. According to their movement peroxisomes were classified into two groups. Group 1 peroxisomes comprising the majority of organelles at 37 degrees C moved slowly with an average velocity of 0.024 +/- 0.012 micron/second whereas the movement of group 2 peroxisomes, 10–15% of the total population, was saltatory exhibiting an average velocity of 0.26 +/- 0.17 micron/second with maximal values of more than 2 microns/second. Saltations were completely abolished by the microtubule-depolymerising drug nocodazole and were slightly reduced by about 25% by cytochalasin D which disrupts the actin microfilament system. Double fluorescence labeling of both peroxisomes and microtubules revealed peroxisome saltations linked to distinct microtubule tracks. Cellular depletion of endogenous levels of NTPs as well as the use of 5′-adenylylimidodiphosphate, a nonhydrolysable ATP analog, applied to a permeabilised cell preparation both completely blocked peroxisomal movement. These data suggest an ATPase dependent, microtubule-based mechanism of peroxisome movement. Both the intact and the permeabilised cell system presented in this paper for the first time allow kinetic measurements on peroxisomal motility and thus will be extremely helpful in the biochemical characterisation of the motor proteins involved.

Endocytosis and vacuolar degradation of the plasma membrane-localized Pdr5 ATP-binding cassette multidrug transporter in Saccharomyces cerevisiae

Multidrug resistance (MDR) to different cytotoxic compounds in the yeast Saccharomyces cerevisiae can arise from overexpression of the Pdr5 (Sts1, Ydr1, or Lem1) ATP-binding cassette (ABC) multidrug transporter. We have raised polyclonal antibodies recognizing the yeast Pdr5 ABC transporter to study its biogenesis and to analyze the molecular mechanisms underlying MDR development. Subcellular fractionation and indirect immunofluorescence experiments showed that Pdr5 is localized in the plasma membrane. In addition, pulse-chase radiolabeling of cells and immunoprecipitation indicated that Pdr5 is a short-lived membrane protein with a half-life of about 60 to 90 min. A dramatic metabolic stabilization of Pdr5 was observed in delta pep4 mutant cells defective in vacuolar proteinases, and indirect immunofluorescence showed that Pdr5 accumulates in vacuoles of stationary-phase delta pep4 mutant cells, demonstrating that Pdr5 turnover requires vacuolar proteolysis. However, Pdr5 turnover does not require a functional proteasome, since the half-life of Pdr5 was unaffected in either pre1-1 or pre1-1 pre2-1 mutants defective in the multicatalytic cytoplasmic proteasome that is essential for cytoplasmic protein degradation. Immunofluorescence analysis revealed that vacuolar delivery of Pdr5 is blocked in conditional end4 endocytosis mutants at the restrictive temperature, showing that endocytosis delivers Pdr5 from the plasma membrane to the vacuole.

CD5-mediated specific delivery of DNA to T lymphocytes: compartmentalization augmented by adenovirus

Specific DNA delivery has been achieved via interactions between an asialoorosomucoid-polylysine conjugate and the asialoglycoprotein receptor. We have now extended this technology to another cell type. In order to achieve DNA delivery uniquely to T cells, we have employed an antibody-polylysine conjugate which binds and is internalized via CD5. Binding analyses of the T101 monoclonal antibody to Jurkat cells and freshly isolated human peripheral T lymphocytes were performed and Scatchard plots revealed Kd values of 1.4 and 1.2 pM, respectively. To introduce DNA into the T cell, a complex of T101-polylysine and the luciferase plasmid was formed (T101-PL-DNA). 125I-labeled antibody alone or T101-PL-DNA complexes were both shown to internalize. Subcellular fractionation indicated that the complex remained in the endosomal compartment of the cell for up to 90 min. However, with the addition of adenovirus particles, there was a decrease of labeled complex in the endosomal fraction over time suggesting it was no longer 'tethered' to the endosome vesicle. In vitro transfections confirmed this result showing the addition of adenovirus particles during incubation resulted in increased expression of the luciferase protein. Without adenovirus, there was limited expression of the transduced gene. These data revealed that T101 can deliver DNA via an antibody-PL conjugate. The addition of adenovirus allowed the DNA to escape the endosome enabling expression of the reporter gene.

Immunocytochemical investigation of the in vivo endocytosis by renal tubular epithelial cells

The internalization and degradation of glomerular filtered serum proteins by the proximal tubular epithelium has been extensively studied by microperfusion methods. By using a cationic probe that easily traverses the glomerular wall into the urinary space, we have performed a morpho-cytochemical and quantitative study of the in vivo endocytotic activity of the proximal tubular epithelial cell. Bovine serum albumin (BSA) was tagged with dinitrophenol (DNP) and cationized to pI over 8. It was introduced into the circulation of normal mice for 5, 10, and 30 minutes and the distribution of the labeling was determined by protein A-gold immunocytochemistry, using specific antiDNP antibodies on tissue sections of routinely aldehyde-fixed, osmiumpostfixed, and Epon-embedded kidneys. Cationic BSA-DNP was detected at the endothelial and epithelial sides of the glomerular basement membrane, and over capillary and tubular basement membranes. In the proximal tubular epithelial cell, labeling was present over microvilli as well as over endosomal and lysosomal compartments, with labeling intensities varying from one compartment to the other. Morphometric evaluations of the labeling demonstrated a progressive incorporation of the probe from microvilli and endocytic compartments at 5 minutes to endocytic and lysosomal compartments at 10 and then 30 minutes. When considering labeling densities, no significant differences were found on microvilli and basolateral membranes between times of circulation; however, the labeling density over endosomal and lysosomal compartments was very intense at 10 minutes compared with 5 minutes, decreasing at 30 minutes. Results from this study validate the cationic albumin tagged with DNP as a tool in the study of the quantitative aspects of protein endocytosis at the ultrastructural level, in the kidney tubular epithelium.

Thyrotropin internalization is directed by a highly conserved motif in the seventh transmembrane region of its receptor

The thyrotropin (TSH) receptor is a member of G protein-coupled seven-transmembrane-segment receptors. It is characterized by a large extracellular domain linked to the seven transmembrane segments and ending with a cytoplasmic tail. Sequence alignment shows that a highly conserved motif, NPXXY where X is any amino acid, exists at the boundary between the seventh transmembrane domain and proximal part of the cytoplasmic tail of virtually all G protein-coupled receptors. This motif has been implicated as an internalization signal for several cell surface receptors, such as the low density lipoprotein (LDL), insulin and insulin-like growth factor-1 (IGF-1) receptors. The potential effects of this motif on the TSH receptor signal transduction and receptor-mediated TSH internalization was analysed by replacement of the tyrosine(678) residue with an alanine residue. This mutation does not impair high affinity TSH binding, but completely abolishes the ability of cAMP response upon TSH stimulation. It also significantly reduces TSH internalization. The role of the cytoplasmic tail of the TSH receptor in receptor-mediated internalization was also assessed. Deletion of up to 56 amino acids from the C-terminus of the cytoplasmic tail enhances TSH internalization as compared to the wild-type receptor. We conclude that tyrosine(678) in the NPXXY motif is required for efficient receptor-mediated TSH internalization and G protein coupling. The cytoplasmic tail of the TSH receptor may contain sequence domains which could modulate the effects of the NPXXY internalization signal.

Phosphorylation and internalization of gp130 occur after IL-6 activation of Jak2 kinase in hepatocytes

Recent evidence has shown that members of the Jak kinase family are activated after IL-6 binds to its receptor complex, leading to a tyrosine phosphorylation of gp130, the IL-6 signal-transducing subunit. The different members of the IL-6 cytokine subfamily induce distinct patterns of Jak-Tyk phosphorylation in different cell types. Using monospecific antibodies to gp130, Jak2 kinase, and phosphotyrosine, we investigated the kinetics of IL-6 stimulation of members of this pathway in primary hepatocytes. Our findings show that Jak 2 is maximally activated within 2 min of exposure to IL-6, followed by gp130 phosphorylation that reaches its peak in another 2 min then declines to basal level by 60 min. In vitro phosphorylation experiments show that activated Jak 2 is able to phosphorylate both native gp130 and a fusion peptide containing its cytoplasmic domain, demonstrating gp130 is a direct substrate of Jak 2 kinase. Experiments designed to explore the cell surface expression of gp130 show that > or = 2 h are required to get a second round of phosphorylation after the addition of more cytokines. This finding suggests that activated gp130 is internalized from the cell surface after IL-6 stimulation. Additional experiments using protein synthesis inhibitors reveal that new protein synthesis is required to get a second cycle of gp130 phosphorylation indicating gp130 must be synthesized de novo and inserted into the membrane. These findings provide strong evidence that down regulation of the IL-6 signal in hepatocytes involves the internalization and cytosol degradation of gp130.

Failure of immunization with trypanosome endocytotic vesicle membrane proteins to provide nonvariant immunoprotection against Trypanosoma brucei

Purified trypanosome endocytotic vesicles were subjected to Triton X-114 phase separation to obtain a fraction enriched in putative parasite receptors for adsorptive endocytosis. Rabbits immunized with this material produced antibodies that recognized many parasite proteins, including nonvarying epitopes on the parasite's endocytotic surface, the flagellar pocket membrane, as well as on membranes of endosomes and lysosome-like structures. These antibodies were unable to stimulate in vitro complement-mediated lysis of trypanosomes, and in an in vitro test of parasite growth inhibition they actually marginally enhanced parasite proliferation. No effect was observed on the parasite prepatent period or parasitemia in mice injected with antibody purified from the rabbit antisera, but their survival with the infection was significantly shortened. Finally, little difference was detected in parasitologic or hematologic parameters between immunized and control rabbits upon challenge with T. brucei infection.

Characterization of the hepatic insulin receptor undergoing internalization through clathrin-coated vesicles and endosomes

Administration of insulin to rats caused a transient increase in the amount of hepatic insulin receptor present in clathrin-coated vesicles and endosomes. However, the total 'in vitro' insulin stimulated tyrosine kinase activity of the receptor present in endosomes did not vary when expressed per mg of protein and decreased when expressed per beta-subunit content. A decrease in the endogenous phospho tyrosine content of the receptor beta-subunit was observed in endosomes in response to insulin. This indicates that a fraction of the internalized receptor is dephosphorylated in endosomes, which renders it unable to become stimulated by insulin 'in vitro'.

Inhibition of the degradation of the precursor and of the mature beta 1 integrin subunit by different protein synthesis inhibitors and by ATP depletion

A stabilization of both the precursor and the mature beta 1 integrin subunit was observed in metabolically labeled human skin fibroblasts and Molt-4 T lymphocytes upon addition of protein synthesis inhibitors or by ATP depletion. Differential effects of protein synthesis inhibitors are reported since the slow degradation of the mature beta 1 subunit was sensitive to cycloheximide but not to puromycin. We also show that the half-life of the mature subunit was not dependent on intracellular lysosomal degradation or on ubiquitination suggesting that VLA turn-over occurs at the cell surface and might involve proteins or proteases with short half-life.

Porphyromonas gingivalis invades oral epithelial cells in vitro

The aim of the present study was to analyze the adhesive and invasive potential of a number of P. gingivalis strains, in an in vitro system utilizing cultures of human oral epithelial cells (KB cell line, ATCC CCL 17). P. gingivalis strains W50 and FDC 381 (laboratory strains) and OMGS 1738, 1743 and 1439 (clinical isolates) as well as E. coli strain HB 101 (non-adhering, non-invasive control) were used. Adherence was assessed by means of scintillation counting and light microscopy, after incubation of radiolabelled bacteria with epithelial cells. In the invasion assay, monolayers were infected with the P. gingivalis and E. coli strains and further incubated with an antibiotic mixture (metronidazole 0.1 mg/ml and gentamicin 0.5 mg/ml). Invasion was evaluated by (i) assessing presence of bacteria surviving the antibiotic treatment, and (ii) electron microscopy. All P. gingivalis strains adhered to and entered into the oral epithelial cells. After 3 hours of incubation, bacteria were frequently identified intracellularly by means of electron microscopy. The cellular membranes, encapsulating the microorganisms in early stages of the invasive process, appeared later to disintegrate. The presence of coated pits on the epithelial cell surfaces suggested that internalization of P. gingivalis was associated with receptor-mediated endocytosis (RME). Formation of outer membrane vesicles (blebs) by intracellular bacteria indicated that internalized P. gingivalis was able to retain its viability. E. coli strain HB 101 neither adhered to nor invaded epithelial cells.

WIN 52035-2 inhibits both attachment and eclipse of human rhinovirus 14

WIN compounds inhibit attachment of human rhinovirus 14 by binding to a hydrophobic pocket within the capsid and inducing conformational changes in the canyon floor, the region that binds the cellular receptor. To study the basis of drug resistance, we isolated and characterized a family of human rhinovirus 14 mutants resistant to WIN 52035-2. Thermostabilization data and single-cycle growth curves provided evidence for two classes of resistant mutants. One class, here called exclusion mutants, showed a marked decrease in drug-binding affinity and was characterized by substitution to bulkier amino acid side chains at two sites lining the hydrophobic pocket. The other class, called compensation mutants, displayed single-amino-acid substitutions in the drug-deformable regions of the canyon; these mutants were able to attach to cells despite the presence of bound drug. A delay in the rise period of the growth curves of compensation mutants indicated a second locus of drug action. WIN 52035-2 was found to inhibit the first step of uncoating, release of VP4. Attempts to identify this site of drug action by using single-step growth curves were obscured by abortive elution of a major fraction of cell-attached virus. The drug had no effect on the rate of this process but did affect the spectrum of particles produced.

Effect of chlorpromazine on kinetics of injected monoclonal antibody in MoAb-induced glomerular injury

The effect of chlorpromazine, one of several calmodulin antagonists that inhibit cytoskeletal movement, on the local kinetics of injected proteinuria-inducing MoAb 5-1-6 was examined to test the hypothesis that proteinuria is inhibited if the antigen recognized by MoAb 5-1-6 or injected MoAb remains on the surface of epithelial foot processes. MoAb 5-1-6 was injected into both chlorpromazine-treated (5 mg/100 g body weight) and untreated rats. As a positive control for the chlorpromazine treatment, anti-Fx1A serum was also injected into other chlorpromazine-treated and untreated rats. Chlorpromazine inhibited neither the change in localization of injected MoAb 5-1-6 nor proteinuria, although it showed an inhibitory effect on redistribution of immune complex and the fixation of complement in passive Heymann glomerulonephritis induced by injection of anti-Fx1A serum. We conclude that the kinetics of bound MoAb 5-1-6 are regulated by a system different from that operating in passive Heymann glomerulonephritis.

The effect of staurosporine, a protein kinase inhibitor, on asialoglycoprotein receptor endocytosis

Receptor-mediated endocytosis via coated pits is modulated by the activity of protein kinases and protein phosphorylation. We examined the effects of the potent protein kinase inhibitor staurosporine (SSP) on endocytosis of the asialoglycoprotein (ASGP) receptor in HepG2 cells. Staurosporine caused a rapid (< 2 min) inhibition of ligand internalization from the cell surface. In contrast the rate of receptor exocytosis from intracellular compartments to the cell surface was not altered (t1/2 = 8 min). This resulted in increased ASGP receptors at the plasma membrane (140% of control) while the total number of receptors per cell was unchanged. Receptor up-regulation was half-maximal at 30 nM SSP. At this concentration staurosporine also inhibited the internalization of iodinated transferrin by HepG2 cells and SK Hep-1 cells, another human hepatoma-derived cell line. Staurosporine was without effect on the non-receptor-mediated uptake of Lucifer yellow by pinocytosis. We investigated the possible involvement of protein kinase C in the inhibitory effects of staurosporine on receptor endocytosis. The active protein kinase C inhibitor H7 did not inhibit ASGP receptor internalization. Furthermore depletion of cellular protein kinase C by overnight incubation with 1 microM phorbol myristate acetate did not abrogate the SSP effect. Together these data suggest that the mechanism of SSP action is independent of the inhibition of protein kinase C. In conclusion staurosporine is a potent and rapid inhibitor of receptor trafficking which is specific for receptor internalization from the plasma membrane.