Mannose-specific endocytosis receptor of alveolar macrophages: demonstration of two functionally distinct intracellular pools of receptor and their roles in receptor recycling

Human Intelectin-1 Promotes Cellular Attachment and Neutrophil Killing of Streptococcus pneumoniae in a Serotype-Dependent Manner

Human intelectin-1 (hIntL-1) is a secreted glycoprotein capable of binding exocyclic 1,2-diols within surface glycans of human pathogens such as Streptococcus pneumoniae, Vibrio cholerae, and Helicobacter pylori. For the latter, lectin binding was shown to cause bacterial agglutination and increased phagocytosis, suggesting a role for hIntL-1 in pathogen surveillance. In this study, we investigated the interactions between hIntL-1 and S. pneumoniae, the leading cause of bacterial pneumonia. We show that hIntL-1 also agglutinates S. pneumoniae serotype 43, which displays an exocyclic 1,2-diol moiety in its capsular polysaccharide but is unable to kill in a complement-dependent manner or to promote bacterial killing by peripheral blood mononuclear cells. In contrast, hIntL-1 not only significantly increases serotype-specific S. pneumoniae killing by neutrophils but also enhances the attachment of these bacteria to A549 lung epithelial cells. Taken together, our results suggest that hIntL-1 participates in host surveillance through microbe sequestration and enhanced targeting to neutrophils.

Effect of mannose targeting of hydroxyl PAMAM dendrimers on cellular and organ biodistribution in a neonatal brain injury model

Neurotherapeutics for the treatment of central nervous system (CNS) disorders must overcome challenges relating to the blood-brain barrier (BBB), brain tissue penetration, and the targeting of specific cells. Neuroinflammation mediated by activated microglia is a major hallmark of several neurological disorders, making these cells a desirable therapeutic target. Building on the promise of hydroxyl-terminated generation four polyamidoamine (PAMAM) dendrimers (D4-OH) for penetrating the injured BBB and targeting activated glia, we explored if conjugation of targeting ligands would enhance and modify brain and organ uptake. Since mannose receptors [cluster of differentiation (CD) 206] are typically over-expressed on injured microglia, we conjugated mannose to the surface of multifunctional D4-OH using highly efficient, atom-economical, and orthogonal Cu(I)-catalyzed alkyne-azide cycloaddition (CuAAC) click chemistry and evaluated the effect of mannose conjugation on the specific cell uptake of targeted and non-targeted dendrimers both in vitro and in vivo. In vitro results indicate that the conjugation of mannose as a targeting ligand significantly changes the mechanism of dendrimer internalization, giving mannosylated dendrimer a preference for mannose receptor-mediated endocytosis as opposed to non-specific fluid phase endocytosis. We further investigated the brain uptake and biodistribution of targeted and non-targeted fluorescently labeled dendrimers in a maternal intrauterine inflammation-induced cerebral palsy (CP) rabbit model using quantification methods based on fluorescence spectroscopy and confocal microscopy. We found that the conjugation of mannose modified the distribution of D4-OH throughout the body in this neonatal rabbit CP model without lowering the amount of dendrimer delivered to injured glia in the brain, even though significantly higher glial uptake was not observed in this model. Mannose conjugation to the dendrimer modifies the dendrimer's interaction with cells, but does not minimize its inherent inflammation-targeting abilities.

Endocytosis and Trafficking of Natriuretic Peptide Receptor-A: Potential Role of Short Sequence Motifs

The targeted endocytosis and redistribution of transmembrane receptors among membrane-bound subcellular organelles are vital for their correct signaling and physiological functions. Membrane receptors committed for internalization and trafficking pathways are sorted into coated vesicles. Cardiac hormones, atrial and brain natriuretic peptides (ANP and BNP) bind to guanylyl cyclase/natriuretic peptide receptor-A (GC-A/NPRA) and elicit the generation of intracellular second messenger cyclic guanosine 3',5'-monophosphate (cGMP), which lowers blood pressure and incidence of heart failure. After ligand binding, the receptor is rapidly internalized, sequestrated, and redistributed into intracellular locations. Thus, NPRA is considered a dynamic cellular macromolecule that traverses different subcellular locations through its lifetime. The utilization of pharmacologic and molecular perturbants has helped in delineating the pathways of endocytosis, trafficking, down-regulation, and degradation of membrane receptors in intact cells. This review describes the investigation of the mechanisms of internalization, trafficking, and redistribution of NPRA compared with other cell surface receptors from the plasma membrane into the cell interior. The roles of different short-signal peptide sequence motifs in the internalization and trafficking of other membrane receptors have been briefly reviewed and their potential significance in the internalization and trafficking of NPRA is discussed.

Evolution of an expanded mannose receptor gene family

Sequences of peptides from a protein specifically immunoprecipitated by an antibody, KUL01, that recognises chicken macrophages, identified a homologue of the mammalian mannose receptor, MRC1, which we called MRC1L-B. Inspection of the genomic environment of the chicken gene revealed an array of five paralogous genes, MRC1L-A to MRC1L-E, located between conserved flanking genes found either side of the single MRC1 gene in mammals. Transcripts of all five genes were detected in RNA from a macrophage cell line and other RNAs, whose sequences allowed the precise definition of spliced exons, confirming or correcting existing bioinformatic annotation. The confirmed gene structures were used to locate orthologues of all five genes in the genomes of two other avian species and of the painted turtle, all with intact coding sequences. The lizard genome had only three genes, one orthologue of MRC1L-A and two orthologues of the MRC1L-B antigen gene resulting from a recent duplication. The Xenopus genome, like that of most mammals, had only a single MRC1-like gene at the corresponding locus. MRC1L-A and MRC1L-B genes had similar cytoplasmic regions that may be indicative of similar subcellular migration and functions. Cytoplasmic regions of the other three genes were very divergent, possibly indicating the evolution of a new functional repertoire for this family of molecules, which might include novel interactions with pathogens.

Interaction of members of the heat shock protein-70 family with the macrophage mannose receptor

The macrophage MR has been the subject of investigation for over 20 years, and several important physiological functions have been described. However, the molecular mechanisms that regulate MR signaling and trafficking during these processes still remain elusive. The focus of the current paper was to identify potential cellular MR-interacting proteins. An initial screen of binding proteins in MR-expressing cells was performed using coimmunoprecipitation, followed by identification of matching peptide sequences using proteomics and MS. The major class of binding proteins identified belonged to the heat shock family of proteins. The specific interaction of the MR with HSP70 family members was validated by Western blot analysis, ligand binding assays, and intracellular colocalization using confocal microscopy. Additional studies indicated that inhibition of the HSP BiP by treatment of cells with EGCG reduced BiP interaction with and surface expression of the MR. Studies of possible motifs within the cytoplasmic tail of the receptor suggested that a juxtamembrane dibasic sequence may contribute to the interaction with BiP. These findings suggest that the molecular association of the MR with HSP70 family members via the receptor cytoplasmic tail may contribute to MR trafficking in macrophages.

Antibodies and carbohydrate ligands binding to DC-SIGN differentially modulate receptor trafficking

DCs are regarded as key APCs that initiate humoral and cellular immune responses. Consequently, targeted delivery of Ag toward DC-specific receptors enhances vaccine efficacy. DC-SIGN is a C-type lectin receptor that facilitates DC-specific delivery of Ag. This is accomplished by conjugating Ag to receptor-specific Ab or carbohydrate ligands that bind to its carbohydrate recognition domain. Here, we investigated the fate of DC-SIGN following receptor triggering with Ab. Both whole and single-chain Ab induced rapid internalization of about half of the surface receptor molecules. Biochemical studies showed that about half of the receptor molecules were still intracellular after 3 h, while minimal or no resurfacing of internalized or newly synthesized unbound DC-SIGN molecules was observed. Prolonged exposure of DCs to DC-SIGN Ab, but not carbohydrate ligands, resulted in reduced receptor expression levels, which lasted up to 2 days following removal of the Ab. In addition, exposure to DC-SIGN Ab reduced the ability of the receptor to internalize. Consequently, DC-SIGN showed a poor ability to accumulate targeting Abs within DCs. Vaccine efficacy may therefore be enhanced by strategies increasing the amount of Ag entering via a single receptor molecule, such as the use of targeting moieties allowing DC-SIGN recycling or Ab-coated vaccine carriers.

Cell-surface receptors on macrophages and dendritic cells for attachment and entry of influenza virus

Review of interactions between influenza A virus and C‐type lectin receptors on macrophages and dendritic cells that may result in virus entry and infection.

Airway MΦ and DCs are important components of innate host defense and can play a critical role in limiting the severity of influenza virus infection. Although it has been well established that cell‐surface SA acts as a primary attachment receptor for IAV, the particular receptor(s) or coreceptor(s) that mediate IAV entry into any cell, including MΦ and DC, have not been clearly defined. Identifying which receptors are involved in attachment and entry of IAV into immune cells may have important implications in regard to understanding IAV tropism and pathogenesis. Recent evidence suggests that specialized receptors on MΦ and DCs, namely CLRs, can act as capture and/or entry receptors for many viral pathogens, including IAV. Herein, we review the early stages of infection of MΦ and DC by IAV. Specifically, we examine the potential role of CLRs expressed on MΦ and DC to act as attachment and/or entry receptors for IAV.

The increase in mannose receptor recycling favors arginase induction and Trypanosoma cruzi survival in macrophages

The macrophage mannose receptor (MR) is a pattern recognition receptor of the innate immune system that binds to microbial structures bearing mannose, fucose and N-acetylglucosamine on their surface. Trypanosoma cruzi antigen cruzipain (Cz) is found in the different developmental forms of the parasite. This glycoprotein has a highly mannosylated C-terminal domain that participates in the host-antigen contact. Our group previously demonstrated that Cz-macrophage (Mo) interaction could modulate the immune response against T. cruzi through the induction of a preferential metabolic pathway. In this work, we have studied in Mo the role of MR in arginase induction and in T. cruzi survival using different MR ligands. We have showed that pre-incubation of T. cruzi infected cells with mannose-Bovine Serum Albumin (Man-BSA, MR specific ligand) biased nitric oxide (NO)/urea balance towards urea production and increased intracellular amastigotes growth. The study of intracellular signals showed that pre-incubation with Man-BSA in T. cruzi J774 infected cells induced down-regulation of JNK and p44/p42 phosphorylation and increased of p38 MAPK phosphorylation. These results are coincident with previous data showing that Cz also modifies the MAPK phosphorylation profile induced by the parasite. In addition, we have showed by confocal microscopy that Cz and Man-BSA enhance MR recycling. Furthermore, we studied MR behavior during T. cruzi infection in vivo. MR was up-regulated in F4/80+ cells from T. cruzi infected mice at 13 and 15 days post infection. Besides, we investigated the effect of MR blocking antibody in T. cruzi infected peritoneal Mo. Arginase activity and parasite growth were decreased in infected cells pre-incubated with anti-MR antibody as compared with infected cells treated with control antibody. Therefore, we postulate that during T. cruzi infection, Cz may contact with MR, increasing MR recycling which leads to arginase activity up-regulation and intracellular parasite growth.

Engagement of the mannose receptor by tumoral mucins activates an immune suppressive phenotype in human tumor-associated macrophages

Tumor-Associated Macrophages (TAMs) are abundantly present in the stroma of solid tumors and modulate several important biological processes, such as neoangiogenesis, cancer cell proliferation and invasion, and suppression of adaptive immune responses. Myeloid C-type lectin receptors (CLRs) constitute a large family of transmembrane carbohydrate-binding receptors that recognize pathogens as well as endogenous glycoproteins. Several lines of evidence demonstrate that some CLRs can inhibit the immune response. In this study we investigated TAM-associated molecules potentially involved in their immune suppressive activity. We found that TAMs isolated from human ovarian carcinoma samples predominantly express the CLRs Dectin-1, MDL-1, MGL, DCIR, and most abundantly the Mannose Receptor (MR). Components of carcinomatous ascites and purified tumoral mucins (CA125 and TAG-72) bound the MR and induced its internalization. MR engagement by tumoral mucins and by an agonist anti-MR antibody modulated cytokine production by TAM toward an immune-suppressive profile: increase of IL-10, absence of IL-12, and decrease of the Th1-attracting chemokine CCL3. This study highlights that tumoral mucin-mediated ligation of the MR on infiltrating TAM may contribute to their immune suppressive phenotype.

Ligand-mediated endocytosis and intracellular sequestration of guanylyl cyclase/natriuretic peptide receptors: role of GDAY motif

The guanylyl cyclase/natriuretic peptide receptor-A (GC-A/NPRA), also referred to as GC-A, is a single polypeptide molecule having a critical function in blood pressure regulation and cardiovascular homeostasis. GC-A/NPRA, which resides in the plasma membrane, consists of an extracellular ligand-binding domain, a single transmembrane domain, and an intracellular cytoplasmic region containing a protein kinase-like homology domain (KHD) and a guanylyl cyclase (GC) catalytic domain. After binding with atrial and brain natriuretic peptides (ANP and BNP), GC-A/NPRA is internalized and sequestered into intracellular compartments. Therefore, GC-A/NPRA is a dynamic cellular macromolecule that traverses different subcellular compartments through its lifetime. This review describes the roles of short-signal sequences in the internalization, trafficking, and intracellular redistribution of GC-A/NPRA from cell surface to cell interior. Evidence indicates that, after internalization, the ligand-receptor complexes dissociate inside the cell and a population of GC-A/NPRA recycles back to the plasma membrane. Subsequently, the disassociated ligands are degraded in the lysosomes. However, a small percentage of the ligand escapes the lysosomal degradative pathway, and is released intact into culture medium. Using pharmacologic and molecular perturbants, emphasis has been placed on the cellular regulation and processing of ligand-bound GC-A/NPRA in terms of receptor trafficking and down-regulation in intact cells. The discussion is concluded by examining the functions of short-signal sequence motifs in the cellular life-cycle of GC-A/NPRA, including endocytosis, trafficking, metabolic processing, inactivation, and/or down-regulation in model cell systems.

Receptor-Mediated Delivery of Antigens to Dendritic Cells: Anticancer Applications

Recently, there has been a surge of interest in the use of ex vivo antigen-pulsed dendritic cells (DCs) in the immunotherapy for cancer. DCs are powerful adjuvants with the ability to prime naive CD4+ and CD8+ T cells. As antigen sources, various preparations, including peptides, proteins, crude tumor lysates, and DCs transfected or transformed with various viruses, have been used. These procedures that involve the isolation of patient DCs and reintroduction after in vitro manipulation are time-consuming and expensive. The DC populations used frequently in ex vivo clinical studies are IL-4 and GM-CSF cultured DCs that may not represent the in vivo DC populations. An attractive method of targeting in vivo DCs is to utilize various ligands or antibodies that bind discrete populations of DCs. These cell surface receptors will direct the antigen to different antigen processing pathways depending on the targeted receptor to induce cytotoxic T cell or T helper responses. This review will discuss the various approaches and receptors that have been used for antigen targeting that may eventually be translated to alternative DC-based immunotherapies.

Mannose receptor-targeted vaccines

Targeting antigens to endocytic receptors on professional antigen-presenting cells (APCs) represents an attractive strategy to enhance the efficacy of vaccines. Such APC-targeted vaccines have an exceptional ability to guide exogenous protein antigens into vesicles that efficiently process the antigen for major histocompatibility complex class I and class II presentation. Efficient targeting not only requires high specificity for the receptor that is abundantly expressed on the surface of APCs, but also the ability to be rapidly internalised and loaded into compartments that contain elements of the antigen-processing machinery. The mannose receptor (MR) and related C-type lectin receptors are particularly designed to sample antigens (self and non-self), much like pattern recognition receptors, to integrate the innate with adaptive immune responses. In fact, a variety of approaches involving delivery of antigens to the MR have demonstrated effective induction of potent cellular and humoral immune responses. Yet, although several lines of evidence in diverse experimental systems attest to the efficacy of targeted vaccine strategies, it is becoming increasingly clear that additional signals, such as those afforded by adjuvants, may be critical to elicit sustained immunity. Therefore, MR-targeted vaccines are likely to be most efficacious in vivo when combined with agents that elicit complementary activation signals. Certainly, a better understanding of the mechanism associated with the induction of immune responses as a result of targeting antigens to the MR, will be important in exploiting MR-targeted vaccines not only for mounting immune defenses against cancer and infectious disease, but also for specific induction of tolerance in the treatment of autoimmune disease.

Internalization and trafficking of guanylyl cyclase/natriuretic peptide receptor-A

One of the principal loci involved in the regulatory action of atrial and brain natriuretic peptides (ANP and BNP) is guanylyl cyclase/natriuretic peptide receptor-A (GC-A/NPRA), whose ligand-binding efficiency and GC catalytic activity vary remarkably in different target cells and tissues. In its mature form, NPRA resides in the plasma membrane and contains an extracellular ligand-binding domain, a single transmembrane region, and the intracellular protein kinase-like homology domain (KHD) and guanylyl cyclase (GC) catalytic domain. NPRA is a dynamic cellular macromolecule that traverses through different compartments of the cell through its lifetime. Binding of ligand to NPRA triggers a complex array of signal transduction events and accelerates the endocytosis. The endocytic transport is important in regulating signal transduction, formation of specialized signaling complexes, and modulation of specific components of internalization events. The present review describes the experiments which reveal the internalization of ligand-receptor complexes of NPRA, receptor trafficking and recycling, and delivery of both ligand-receptor molecules into subcellular compartments. The ligand-receptor complexes of NPRA are finally degraded within the lysosomes. The experimental evidence provides a consensus forum, which establishes the endocytosis, cellular trafficking, sequestration, and metabolic processing of ANP/NPRA complexes in the intact cells. The discussion is afforded to address the experimental insights into the mechanisms that cells utilize in modulating the delivery and metabolic processing of ligand-bound NPRA into the cell interior.

Carrier-based strategies for targeting protein and peptide drugs to the lungs

With greater interest in delivery of protein and peptide-based drugs to the lungs for topical and systemic activity, a range of new devices and formulations are being investigated. While a great deal of recent research has focused on the development of novel devices, attention must now be paid to the formulation of these macromolecular drugs. The emphasis in this review will be on targeting of protein/peptide drugs by inhalation using carriers and ligands.

HIV-1 Nef mediates post-translational down-regulation and redistribution of the mannose receptor

Human immunodeficiency virus (HIV) has derived a variety of means to evade the host immune response. HIV-derived proteins, including Tat, Nef, and Env, have all been reported to decrease expression of host molecules such as CD4 and major histocompatibility complex I, which would assist in limiting viral replication. The mannose receptor (MR) on the surface of macrophages and dendritic cells (DC) has been proposed to function as an effective antigen-capture molecule, as well as a receptor for entering pathogens such as Mycobacterium tuberculosis and Pneumocystis carinii. Regulation of this receptor would therefore benefit HIV in removing an additional arm of the innate immune system. Previous work has shown that MR function is reduced in alveolar macrophages from HIV-infected patients and that surface MR levels are decreased by the HIV-derived protein Nef in DC. In addition, several laboratories have shown that CD4 is removed from the surface of T cells in a manner that might be applicable to decreased MR surface expression in macrophages. In the current study, we have investigated the role of Nef in removing MR from the cell surface. We have used a human macrophage cell line stably expressing the MR as well as human epithelial cells transiently expressing CD4 and a unique CD4/MR chimeric molecule constructed from the extracellular and transmembrane domains of CD4 and the cytoplasmic tail portion of the MR. We show that the MR is reduced on the cell surface by approximately 50% in the presence of Nef and that the MR cytoplasmic tail can confer susceptibility to Nef in the CD4/MR chimera. These data suggest that the MR is a potential intracellular target of Nef and that this regulation may represent a mechanism to further cripple the host innate immune system.

Identification of a naturally processed cyclin D1 T-helper epitope by a novel combination of HLA class II targeting and differential mass spectrometry

T-helper (Th) cells play an important role in orchestrating the effector function of CTL in anti-tumor immunity. However, only a limited number of Th cell epitopes has been characterized. Here we describe a novel approach for identifying naturally processed and presented peptides derived from chosen antigens. This method combines a transfection step of antigen-presenting cells with a vector encoding a fusion protein between the Ii chain and the antigen of interest, elution of the HLA-bound peptides and identification of the antigen-derived peptides by mass spectrometric comparison to the non-transfected cells. In vitro-stimulated Th cells against the identified peptide of interest specifically recognize transfectants overexpressing the cognate antigen. Using this approach, we were able to identify the HLA-DR4-restricted Th cell epitope NPPSMVAAGSVVAAV derived from cyclin D1, which is frequently overexpressed in tumors. This method will help in identifying peptide candidates for vaccination studies for tumor immunotherapy.

Scavenger receptors facilitate protein transport in the trophotaenial placenta of the goodeid fish, Ameca splendens (Teleostei: Atheriniformes)

In the goodeid placental analogue, trophotaeniae provide extraembryonic gut-derived exchange surfaces. Ameca splendens embryos possess endocytosing trophotaeniae that are capable of absorbing a dazzling array of proteinaceous substances. The iron core protein, native ferritin (NF), and several radioiodinated proteinaceous substances were used to study ligand and binding site pathways in the trophotaenial absorptive cells (TACs). Time sequence analysis of NF trafficking indicated an exclusively lysosomal pathway. Binding to TACs of NF was completely inhibitable by proteins containing multiple lysine residues such as apoferritin, bovine serum albumin (BSA), human transferrin (HTf), fetuin, hemoglobin, myoglobin, cytochrome c, ubiquitin, parvalbumin as well as the random copolymers, poly(Glu,Lys,Tyr)6:3:1 and poly(D-Glu,D-Lys)6:4. Peptide hormones and pepsin that contains only one lysine residue did not produce inhibitory effects. Radiolabels such as (125)I-BSA, (125)I-HTf and (125)I-poly(Glu,Lys,Tyr) bound to trophotaeniae in a specific saturable manner. Any two proteins were shown to hinder one another in getting hold of a binding site. Concentration-dependent (125)I-BSA binding and Scatchard analysis of the data revealed both low- and medium-affinity binding with apparent dissociation constants, K(d)s, of 3.4 x 10(-5) M and 2 x 10(-7) M, respectively. Binding of NF and radioiodinated proteins was inhibited in the presence of a large excess of L-Lys, D-Lys, and several dipeptides containing Lys. Both Ca(2+)-depletion and low pH dramatically reduced the TACs' capacity to bind proteins. The effects of acidotropic agents included a reversible loss of surface protein binding sites, tremendous vacuolation, and the arrest of lysosomal degradation. Collectively, present results demonstrate that TACs bind and absorb multiple proteinaceous substances through a mechanism satisfying the criteria of receptor-mediated endocytosis. It is concluded that scavenger protein binding sites are used to ingest proteins for lysosomal degradation, helping to meet the embryos' amino acid requirement.

Functional characterization of mannose receptor expressed by immunocompetent mouse microglia

The mannose receptor is a pattern-recognition receptor involved in innate and adaptive immunity. The receptor is mainly expressed by macrophages and, within the brain, by astrocytes and microglia. This study reports for the first time the effects of two classical proinflammatory (interferon-gamma, IFNgamma) and anti-inflammatory (interleukin-4, IL-4) cytokines on the levels of expression and activity of the mannose receptor expressed by mouse microglia, the brain resident macrophages. As observed for macrophages, IFNgamma treatment led to a decrease and IL-4 to an increase of mannose receptor expression. Consequently, the rates of pinocytosis were strongly upregulated by IL-4 and inhibited by IFNgamma. This latter, however, resumed with time and reached again the constitutive rate of pinocytosis. This recovery resulted from an increased pinocytic activity of the few mannose receptor molecules still expressed by IFNgamma-treated microglia. This may suggest a brain-specific regulation of the effects of IFNgamma since such a phenomenon has not been observed in macrophages. Together, these observations demonstrate that cytokine-stimulated immunocompetent microglia express a functional mannose receptor.

Pulmonary surfactant protein A up-regulates activity of the mannose receptor, a pattern recognition receptor expressed on human macrophages

Inhaled particulates and microbes are continually cleared by a complex array of lung innate immune determinants, including alveolar macrophages (AMs). AMs are unique cells with an enhanced capacity for phagocytosis that is due, in part, to increased activity of the macrophage mannose receptor (MR), a pattern recognition receptor for various microorganisms. The local factors that "shape" AM function are not well understood. Surfactant protein A (SP-A), a major component of lung surfactant, participates in the innate immune response and can enhance phagocytosis. Here we show that SP-A selectively enhances MR expression on human monocyte-derived macrophages, a process involving both the attached sugars and collagen-like domain of SP-A. The newly expressed MR is functional. Monocyte-derived macrophages on an SP-A substrate demonstrated enhanced pinocytosis of mannose BSA and phagocytosis of Mycobacterium tuberculosis lipoarabinomannan-coated microspheres. The newly expressed MR likely came from intracellular pools because: 1) up-regulation of the MR by SP-A occurred by 1 h, 2) new protein synthesis was not necessary for MR up-regulation, and 3) pinocytosis of mannose BSA via MR recycling was increased. AMs from SP-A(-/-) mice have reduced MR expression relative to SP-A(+/+). SP-A up-regulation of MR activity provides a mechanism for enhanced phagocytosis of microbes by AMs, thereby enhancing lung host defense against extracellular pathogens or, paradoxically, enhancing the potential for intracellular pathogens to enter their intracellular niche. SP-A contributes to the alternative activation state of the AM in the lung.

Ligand-regulated internalization, trafficking, and down-regulation of guanylyl cyclase/atrial natriuretic peptide receptor-A in human embryonic kidney 293 cells

We examined the kinetics of internalization, trafficking, and down-regulation of recombinant guanylyl cyclase/natriuretic peptide receptor-A (NPRA) utilizing stably transfected 293 cells expressing a very high density of receptors. After atrial natriuretic peptide (ANP) binding to NPRA, ligand-receptor complexes are internalized, processed intracellularly, and sequestered into subcellular compartments, which provided an approach to examining directly the dynamics of metabolic turnover of NPRA in intact cells. The translocation of ligand-receptor complexes from cell surface to intracellular compartments seems to be linked to ANP-dependent down-regulation of NPRA. Using tryptic proteolysis of cell surface receptors, it was found that approximately 40-50% of internalized ligand-receptor complexes recycled back to the plasma membrane with an apparent t(12) = 8 min. The recycling of NPRA was blocked by the lysosomotropic agent chloroquine, the energy depleter dinitrophenol, and also by low temperature, suggesting that recycling of the receptor is an energy- and temperature-dependent process. Data suggest that approximately 70-80% of internalized (125)I-ANP is processed through a lysosomal degradative pathway; however, 20-25% of internalized ligand is released intact into the cell exterior through an alternative mechanism involving an chloroquine-insensitive pathway. It is implied that internalization and processing of bound ANP-NPRA complexes may play an important role in mediating the biological action of hormone and the receptor protein. In retrospect, this could occur at the level of receptor regulation or through the initiation of ANP mediated signals. It is envisioned that the endocytotic pathway of ligand-receptor complexes of ANP-NPRA would lead to termination and/or diminished responsiveness of ANP in target cells.

Intracellular trafficking and metabolic turnover of ligand-bound guanylyl cyclase/atrial natriuretic peptide receptor-A into subcellular compartments

Atrial natriuretic peptide (ANP) is the first described member of the natriuretic peptide hormone family. ANP elicits natriuretic, diuretic, vasorelaxant and antiproliferative effects, important factors in the control of blood pressure homeostasis. One of the principal loci involved in the regulatory action of ANP is the guanylyl cyclase-linked ANP-receptor which has been designated as NPRA, also referred to as GC-A, whose ANP-binding efficiency and guanylyl cyclase activity vary remarkably in different target tissues. However, the cellular and molecular basis of these activities and the functional expression and regulation of NPRA are not well understood. The mature form of receptor resides in the plasma membrane and consists of an extracellular ligand-binding domain, a single transmembrane-spanning region, and intracellular protein kinase-like homology and guanylyl cyclase catalytic domains. In this review, emphasis has been placed on the interaction ofANP with NPRA, the ligand-mediated endocytosis, trafficking, and subcellular distribution of ligand-receptor complexes from cell surface to the intracellular compartments. Furthermore, it is implicated that after internalization, the ANP/NPRA complexes dissociate into the subcellular compartments and a population of receptor recycles back to the plasma membrane. This is an interesting area of research in the natriuretic peptide receptor field because there is currently debate over whether ANP/NPRA complexes internalize at all or whether cell utilizes some other mechanisms to release ANP from the bound receptor molecules. Indeed, controversy exist since it has been previously reported by default that among the three natriuretic peptide receptors only NPRC internalizes with bound ligand. Hence, from a thematic standpoint it is clearly evident that there is a current need to review this subject and provide a consensus forum that establishes the cellular trafficking, sequestration and processing of ANP/NPRA complexes in intact cells. Towards this aim the cellular life-cycle of NPRA will be described in the context ofANP-binding, internalization, metabolic processing, and/or inactivation, down-regulation, and degradation of ligand-receptor complexes in model cell systems.

Chloroquine decreases cell-surface expression of tumour necrosis factor receptors in human histiocytic U-937 cells

Proinflammatory cytokine tumour necrosis factor (TNF) mediates its diverse effects through cell surface receptors. A variety of inflammatory signals are known to modulate TNF activities by changing expression and shedding of cell-surface TNF receptors. We have examined the effects of anti-rheumatic drug chloroquine on the expression of cell surface and soluble TNF receptors in human histiocytic U-937 cells. Chloroquine partially reduced production of soluble p55 and p75 TNF receptors in cells stimulated with phorbol 12-myristate 13-acetate (PMA). In these cells, induction of both TNF receptor mRNA was not changed and the levels of cell-associated TNF receptors were rather increased by chloroquine. Flow cytometric analysis revealed that chloroquine does not inhibit the PMA-triggered shedding of TNF receptors from cell surface, while it was suppressed by a metalloproteinase inhibitor BB-3103. Treatment of U-937 cells with chloroquine significantly reduced the level of cell surface TNF receptors and a similar effect was observed with human peripheral blood monocytes. Other weak-base amines, including hydroxychloroquine, ammonium chloride and methylamine, also induced reduction of cell surface TNF receptors, whereas lysosomal proteinase inhibitor, leupeptin, and BB-3013 were without effect. Our results suggest that chloroquine down-regulates cell surface TNF receptors by retarding their transport to the cell surface, while cleavage of cell surface receptors is not inhibited by chloroquine.

Bacterial ghosts as carrier and targeting systems

Bacterial ghosts are empty cell envelopes originating from Gram-negative bacteria. They have a natural outer surface make-up which provides them with the original targeting functions of the bacteria they are derived from and are thus able to bind to and/or are taken up by specific cells or tissues of animal, human or plant origin. The extended bacterial ghost system represents a platform technology for creating new qualities in non-living carriers which can be used for the specific targeting of drugs, DNA or other compounds to overcome toxic or non-desired obstacles. Freeze dried bacterial ghosts are stable without the requirement of a cold chain and can be effectively administered orally and aerogenically as drug carriers. The new system is an alternative to liposomes and may have an advantage due to its higher specificity for targeting specific tissues, its easy method of production and its versatility in entrapping and packaging various compounds in different compartments of the carriers.

Dynamics of internalization and sequestration of guanylyl cyclase/atrial natriuretic peptide receptor-A

The guanylyl cyclase/natriuretic peptide receptor-A (NPRA), also referred to as GC-A, is a single polypeptide molecule. In its mature form, NPRA resides in the plasma membrane and consists of an extracellular ligand-binding domain, a single transmembrane-spanning region, and intracellular cytoplasmic domain that contains a protein kinase-like homology domain (KHD) and a guanylyl cyclase (GC) catalytic active site. The binding of atrial natriuretic peptide (ANP) to NPRA occurs at the plasma membrane; the receptor is synthesized on the polyribosomes of the endoplasmic reticulum, and is presumably degraded within the lysosomes. It is apparent that NPRA is a dynamic cellular macromolecule that traverses through different compartments of the cell through its lifetime. This review describes the experiments addressing the interaction of ANP with the NPRA, the receptor-mediated internalization and stoichiometric distribution of ANP-NPRA complexes from cell surface to cell interior, and its release into culture media. It is hypothesized that after internalization, the ligand-receptor complexes dissociate inside the cell and a population of NPRA recycles back to plasma membrane. Subsequently, some of the dissociated ligand molecules escape the lysosomal degradative pathway and are released intact into culture media, which reenter the cell by retroendocytotic mechanisms. By utilizing the pharmacologic and physiologic perturbants, the emphasis has been placed on the cellular regulation and processing of ligand-receptor complexes in intact cells. I conclude the discussion by examining the data available on the utilization of deletion mutations of NPRA cDNA, which has afforded experimental insights into the mechanisms the cell utilizes in modulating the expression and functioning of NPRA.Key words: atrial natriuretic peptide receptor-A, guanylyl cyclase receptors, ANP-binding, internalization and recycling of receptor, lysosomal hydrolysis.

Acemannan purified from Aloe vera induces phenotypic and functional maturation of immature dendritic cells

Acemannan, a major carbohydrate fraction of Aloe vera gel, has been known to have antiviral and antitumoral activities in vivo through activation of immune responses. The present study was set out to define the immunomodulatory activity of acemannan on dendritic cells (DCs), which are the most important accessory cells for the initiation of primary immune responses. Immature DCs were generated from mouse bone marrow (BM) cells by culturing in a medium supplemented with GM-CSF and IL-4, and then stimulated with acemannan, sulfated acemannan, and LPS, respectively. The resultant DCs were examined for phenotypic and functional properties. Phenotypic analysis for the expression of class II MHC molecules and major co-stimulatory molecules such as B7-1, B7-2, CD40 and CD54 confirmed that acemannan could induce maturation of immature DCs. Functional maturation of immature DCs was supported by increased allogeneic mixed lymphocyte reaction (MLR) and IL-12 production. The differentiation-inducing activity of acemannan was almost completely abolished by chemical sulfation. Based on these results, we propose that the adjuvant activity of acemannan is at least in part due to its capacity to promote differentiation of immature DCs.

Dermal microvascular endothelial cells express the 180-kDa macrophage mannose receptor in situ and in vitro

Expression of the 180-kDa mannose receptor (MR) is mainly found on cells of the macrophage lineage. MR mediates the uptake of micro-organisms and host-derived glycoproteins. We demonstrate that endothelium of the human skin in situ and dermal microvascular endothelial cells (DMEC) in vitro expressed MR at both the protein and mRNA levels. In contrast, HUVEC were consistently negative for MR expression. DMEC internalized dextran as well as Escherichia coli by the way of MR into acidic phagosomes, only a few of which fused with CD63- and lysosomal-associated membrane glycoprotein-2-positive lysosomes. This contrasts with the situation in monocyte-derived dendritic cells, where almost all of the MR-Ag complexes reached CD63- and lysosomal-associated membrane glycoprotein-2-positive compartments, indicating differences in the phagolysosomal fusion rate between DMEC and dendritic cells. In conclusion, DMEC express functional MR, a finding that corroborates a role of skin endothelium in Ag capture/clearing.

A di-aromatic motif in the cytosolic tail of the mannose receptor mediates endosomal sorting

The mannose receptor (MR), the prototype of a new family of multilectin receptor proteins important in innate immunity, undergoes rapid internalization and recycling from the endosomal system back to the cell surface. Sorting of the MR in endosomes prevents the receptor from entering lysosomes where it would be degraded. Here, we focused on a diaromatic sequence (Tyr(18)-Phe(19)) in the MR cytoplasmic tail as an endosomal sorting signal. The subcellular distribution of chimeric constructs between the MR and the cation-dependent mannose 6-phosphate receptor was assessed by Percoll density gradients and cell surface assays. Unlike the wild type constructs, mutant receptors with alanine substitutions of Tyr(18)-Phe(19) were highly missorted to lysosomes, indicating that the di-aromatic motif of the MR cytoplasmic tail mediates sorting in endosomes. Within this sequence Tyr(18) is the key residue with Phe(19) contributing to this function. Moreover, Tyr(18) was also found to be essential for internalization, consistent with the presence of overlapping signals for internalization and endosomal sorting in the cytosolic tail of the MR. A di-aromatic amino acid sequence in the cytosolic tail has now been shown to function in two receptors known to be internalized from the plasma membrane, the MR and the cation-dependent mannose 6-phosphate receptor. This feature therefore appears to be a general determinant for endosomal sorting.

In vivo macrophage activation in chickens with Acemannan, a complex carbohydrate extracted from Aloe vera

Acemannan (ACM 1), a beta-(1,4) -acetylated mannan isolated from Aloe vera, can be used as an effective adjuvant in vaccination against some avian viral diseases. Our results demonstrate a quick and lasting in vivo priming effect of ACM 1 on macrophage response after intramuscular inoculation in chickens (500 microg per 2-month-old bird). In response to IFN-gamma in vitro, monocytes from ACM 1-treated chickens exhibited a strong enhancement of NO production from 3 to 9 days p.i., but a weaker effect on MHC II cell surface antigen expression on day 3 p.i. A stimulating effect of ACM 1 treatment was also observed on spontaneous and inducible NO production for splenocytes only on day 3 p.i. By that time, splenocytes exhibited a strong higher capacity to proliferate in response to the T cell-mitogen PHA. At the same time, the in vivo capacity to produce NO, measured by the (NO(-)(2)+NO(-)(3)) serum level after intravenous LPS injection, increased greatly from 3 to 9 days p.i. In conclusion, ACM 1 was able efficiently and durably to increase the activation capacity of macrophages from the systemic immune compartment (in particular from the blood and spleen after an intramuscular injection) in chickens, especially for NO production. These findings provide a better understanding of the adjuvant activity of ACM 1 for viral and tumoral diseases.

A cardiomyocyte mannose receptor system is involved in Trypanosoma cruzi invasion and is down-modulated after infection

Mannosyl binding sites were detected "in vitro" on cardiomyocytes (CM) surface using horseradish peroxidase (HRP) as the ligand. Binding assays revealed a specific recognition system, which was time- and concentration-dependent. The binding required physiological pH and was inhibited by EDTA and trypsin treatments. HRP binding was reduced by pre-incubations with low concentrations of D-mannose. Ultrastructural analysis of the endocytic process was followed using HRP coupled to colloidal gold particles (HRP-Au). The tracer was found within caveolae characterizing early steps of the receptor-mediated endocytosis. The addition of 10 mM D-mannose to the interaction medium blocked Trypanosoma cruzi uptake by CM. The labeling of CM with a subsaturating concentration of HRP-Au before their infection showed, by ultrastructural studies, that its association with trypomastigote forms occurred frequently near to HRP-gold particles that could also be seen to comprise the parasitophorous vacuole. After infection of CM with T. cruzi, a considerable reduction on HRP binding was noticed. Binding was almost completely restored by treating the infected cultures with the trypanocidal drug Nifurtimox. Our "in vitro" findings suggest that cardiomyocyte's mannose receptors localized at the sarcolemma mediates T. cruzi recognition and can be down-modulated by parasite infection.

Endocytosis of Heparin-Binding Protein (CAP37) Is Essential for the Enhancement of Lipopolysaccharide-Induced TNF-α Production in Human Monocytes

Heparin-binding protein (HBP), also known as CAP37, is a proteolytically inactive serine protease homologue that is released from activated granulocytes. However, HBP is not a biologically inactive molecule but rather a multifunctional protein with properties that include the enhancement of LPS-induced TNF-α production from monocytes. We have previously demonstrated that HBP is internalized in monocytes. In the current study, we hypothesize that HBP is internalized in monocytes via endocytosis, and this internalization is an important mechanism by which HBP enhances LPS-induced TNF-α release. Using whole blood from healthy donors and flow cytometry, we found that colchicine (0.1–10 mM), cytochalasin D (1000 μM), NH4Cl (10–50 mM), and bafilomycin A1 (0.1–3 μM) significantly reduced the affinity of FITC-HBP for CD14-positive monocytes. Using isolated human monocytes and ELISA, we found that colchicine (0.1 mM), cytochalasin D (30 and 300 μM), NH4Cl (30 mM), and bafilomycin A1 (1 μM) significantly reduced the effect of HBP (10 μg/ml) to enhance LPS (10 ng/ml)-induced TNF-α release after 24 h. These findings demonstrate that internalization of HBP in monocytes is essential for the enhancement of LPS-induced TNF-α release. Transport of HBP to an activating compartment depends on intact F-actin polymerization and endosomal acidification, an important mechanism for endosomal protein sorting and trafficking.

Ligand-induced endocytosis of the asialoglycoprotein receptor: evidence for heterogeneity in subunit oligomerization

The hepatic asialoglycoprotein receptor, a noncovalent hetero-oligomer of two subunits, is a constitutively cycling endocytic receptor. However, the ligand asialoorosomucoid caused downregulation of up to 40% of surface binding sites and a twofold increase in internalization rate. This was not the result of receptor crosslinking, since monovalent ligands had the same effect. Ligand binding thus appears to transmit a signal to the cytosolic portion of the receptor not unlike in signaling receptors. The two subunits were endocytosed at different average rates lower than that of ligand, indicating heterogeneity in oligomer formation and potentially in ligand specificity.

The mannose receptor is a pattern recognition receptor involved in host defense

The mannose receptor recognizes the patterns of carbohydrates that decorate the surfaces and cell walls of infectious agents. This macrophage and dendritic cell pattern-recognition receptor mediates endocytosis and phagocytosis. The mannose receptor is the prototype of a new family of multilectin receptor proteins (membrane-spanning receptors containing eight-ten lectin-like domains, which appear to play a key role in host defense) and provides a link between innate and adaptive immunity. Recent advances include the identification of three new members of the mannose receptor family, additional work on defining the molecular requirements for sugar binding, a role for the mannose receptor in antigen presentation of lipoglycan antigens and evidence that the mannose receptor is associated with a signal transduction pathway leading to cytokine production.

Transport from late endosomes to lysosomes, but not sorting of integral membrane proteins in endosomes, depends on the vacuolar proton pump

Endocytosed proteins are sorted in early endosomes to be recycled to the plasma membrane or transported further into the degradative pathway. We studied the role of endosomes acidification on the endocytic trafficking of the transferrin receptor (TfR) as a representative for the recycling pathway, the cation-dependent mannose 6-phosphate receptor (MPR) as a prototype for transport to late endosomes, and fluid-phase endocytosed HRP as a marker for transport to lysosomes. Toward this purpose, bafilomycin A1 (Baf), a specific inhibitor of the vacuolar proton pump, was used to inhibit acidification of the vacuolar system. Microspectrofluorometric measurement of the pH of fluorescein-rhodamine-conjugated transferrin (Tf)-containing endocytic compartments in living cells revealed elevated endosomal pH values (pH > 7.0) within 2 min after addition of Baf. Although recycling of endocytosed Tf to the plasma membrane continued in the presence of Baf, recycled Tf did not dissociate from its receptor, indicating failure of Fe3+ release due to a neutral endosomal pH. In the presence of Baf, the rates of internalization and recycling of Tf were reduced by a factor of 1.40 +/- 0.08 and 1.57 +/- 0.25, respectively. Consequently, little if any in TfR expression at the cell surface was measured during Baf treatment. Sorting between endocytosed TfR and MPR was analyzed by the HRP-catalyzed 3,3'-diaminobenzidine cross-linking technique, using transferrin conjugated to HRP to label the endocytic pathway of the TfR. In the absence of Baf, endocytosed surface 125I-labeled MPR was sorted from the TfR pathway starting at 10 min after uptake, reaching a plateau of 40% after 45 min. In the presence of Baf, sorting was initiated after 20 min of uptake, reaching approximately 40% after 60 min. Transport of fluid-phase endocytosed HRP to late endosomes and lysosomes was measured using cell fractionation and immunogold electron microscopy. Baf did not interfere with transport of HRP to MPR-labeled late endosomes, but nearly completely abrogated transport to cathepsin D-labeled lysosomes. From these results, we conclude that trafficking through early and late endosomes, but not to lysosomes, continued upon inactivation of the vacuolar proton pump.

Mannosylated lipoarabinomannan interacts with phagocytes

Infection by Mycobacterium tuberculosis first involves its adhesion to mononuclear host phagocytes. Various macrophage opsonic and non-opsonic receptors are known to mediate this adhesion, with some specificity of mannosyl receptors for the more virulent strains. Mannosylated lipoarabinomannan, a major component of cell walls from M. tuberculosis and Mycobacterium bovis BCG, is endowed with mannooligosaccharide units that could mediate its binding to these latter receptors. To explore its interaction with murine immune cells by flow cytometry, we report a new procedure to fluorescently tag the polysaccharide molecules. We covalently labeled mannosylated lipoarabinomannan from M. bovis BCG with biotin, allowing formation of stable complexes with streptavidin coupled to a fluorochrome. In this work, we demonstrated that this major carbohydrate antigen interacts selectively with murine phagocytes, i.e. granulocytes and macrophages. This binding was affected by temperature and was serum- and divalent-cation-dependent. It also appears to involve a metabolically recycling protein receptor on the phagocyte surface and mannosyl aggretopes on the mannosylated lipoarabinomannan molecule. Thus, the latter may provide a means for mycobacteria to bind to and invade their host phagocytes. This molecule could constitute one of the early factors of mycobacterial virulence.

Kinetic modelling of liposome degradation in peritoneal macrophages

The objective of this study was to quantify and model the degradation process of liposomes in peritoneal macrophages (PMs). Iodinated albumin (125I-alb) was chosen to be the marker of liposome degradation. The time course of the degradation of free 125I-alb after pinocytosis by PMs followed first-order kinetics with a half-life of 23 min. The degradation of liposomally encapsulated 125I-alb was also quantified. Kinetic modelling of liposome degradation indicated the existence of two kinetically different processes, one with a half-life of 13 min and the other with a half-life of 7.5 h. Comparing the degradation of liposomal and free 125I-alb suggested that 125I-alb was delivered to lysosomes much faster through phagocytosis than pinocytosis. These results indicate that the intracellular degradation kinetics of pinosomes and phagosomes is different. This method can quantify the rate and extent of liposomal degradation in macrophages and provide kinetic information on the intracellular destiny of liposomally encapsulated compounds.

Evidence that the thyrotropin-releasing hormone receptor and its ligand are recycled dissociated from each other

We have examined the trafficking of the thyrotropin-releasing hormone receptor (TRHR) and its ligand, after TRHR-TRH internalization in rat pituitary GH4C1 cells. After rapid ligand-induced receptor sequestration, the cell surface receptor pool was replenished. Replenishment was insensitive to inhibition of protein synthesis and was dependent on the duration of internalization; therefore, the replenished receptors were not newly synthesized but recycled. The total amount of recycled receptors decreased with increasing internalization time, resulting in only partial replenishment of the cell-surface receptor pool after prolonged incubation with ligand. Thus, in addition to a receptor recycling pathway, a non-cycling route exists for TRHR sorting; this route became dominant with increasing internalization periods. TRHR entry into these pathways was not determined by the affinity of the receptor-ligand interaction, because the extent of receptor recycling was similar after TRH- and methyl-TRH (MeTRH)-induced internalization. Unlike results with the TRHR, the TRH recycling pool was not depleted by the noncycling pathway. After multiple rounds of [3H]MeTRH internalization, the amount of cell-associated radioactivity increased with increasing internalization time due to accumulation of the ligand or its metabolites in a non-cycling pathway, but the absolute amount of recycled ligand remained constant after short or long internalization times. The difference in the proportion of TRHR and MeTRH that were diverted into a noncycling pathway indicated intracellular dissociation of the internalized TRHR-TRH complex. Dissociation of the internalized TRHR-TRH complex was dependent on the acidic pH in an intracellular compartment. Although extracellular acidic pH did not enhance cell-surface receptor-ligand (RL) dissociation, bafilomycin A1 inhibited both receptor and ligand recycling. We conclude that the TRHR-TRH system is unique among recycling receptors because, after RL sequestration, the TRHR-TRH complex becomes dissociated intracellularly via a bafilomycin A1-sensitive, acidic pH-dependent mechanism, and both the unoccupied TRHR and TRH recycle disassociated from each other.

Endocytosis, partial degradation and release of heparan sulfate by elicited mouse peritoneal macrophages

Interactions between glycosaminoglycans (GAGs) and low density lipoprotein (LDL) are thought to influence the progression of atherogenesis. In an effort to gauge whether macrophages mediate GAG-LDL interaction by GAG modification, we have investigated the endocytosis, degradation and retro-endocytosis of the GAG heparan sulfate (HS) by mouse peritoneal macrophages. Radiolabelled HS was produced by derivatization with sulfosuccinimidyl-3-(4-hydroxyphenyl) propionate and radio-iodination by the chloramine T method. The amount of 125I-HS internalized by cultures of thioglycollate-elicited macrophages rose over a 24 h time period in proportion to the amount of tracer added to the wells (2-2500 ng ml-1). Analysis of GAG molecular weight was performed using gel filtration chromatography and polyacrylamide gel electrophoresis. After a 24 h pulse period, the 125I-HS in the intracellular fraction of the cultured cells was of smaller molecular weight than for control material. During a 24 h cold chase, fragments of 125I-HS were released into the medium. These fragments had lower affinity for Polybrene-Sepharose but did not appear significantly N-desulfated as determined by low pH nitrous acid treatment. The NADPH oxidase inhibitor diphenylene iodonium, although minimizing basal and phorbol ester-triggered radical output, did not inhibit 125I-HS depolymerization. These data indicate that elicited macrophages can interact with and reduce the polymer length of HS without extensively desulfating the molecule. They are consistent with a mechanism by which the macrophage internalizes and partially degrades HS by endoglucuronidase activity rather than NADPH oxidase-generated free radicals, followed by release of the products into the extracellular milieu.

Characterization of the high mannose asparagine-linked oligosaccharides synthesized by microfilariae of Dirofilaria immitis

This report describes the structures of high-mannose-type N-linked oligosaccharides in glycoproteins synthesized by the microfilariae of Dirofilaria immitis. Microfilariae of D. immitis were incubated in vitro in media containing 2-[3H] mannose to allow metabolic radiolabeling of the oligosaccharide moieties of newly synthesized glycoproteins. Glycopeptides were prepared from the radiolabeled glycoproteins by digestion with pronase and fractionation by chromatography on concanavalin A-Sepharose. Thirty eight percent of 2-[3H] mannose incorporated into the microfilariae of D. immitis glycopeptides was recovered in high mannose-type asparagine-linked oligosaccharides which were bound to the immobilized lectin. Upon treatment of 2-[3H] mannose labeled glycopeptides with endo-beta-N-acetylglucosaminidase H, the high mannosetype chains were released and their structures were determined by high performance liquid chromatography and exoglycosidase digestion. The major species of high mannosetype chains synthesized by microfilariae of D. immitis have the composition Man5GlcNAc2, Man6ClcNAc2, Man7GlcNAc2, and Man8GlcNAc2. Structural analyses indicate that these oligosaccharides are similar to high mannose-type chains synthesized by vertebrates.

Mechanism of trimethylamine-induced inhibition of macromolecular synthesis by mouse embryos in culture

The effects of trimethylamine (TMA) on uptake mechanisms and lysosomal function were studied in mouse embryos, isolated yolk sacs and limb buds. TMA at 0.75 mM did not inhibit uptake of [14C]sucrose by yolk sacs of day 9 embryos or by day 15 isolated yolk sacs but did inhibit uptake of 125I-labelled bovine serum albumin ([125I]BSA) by day 15 isolated yolk sacs. Concentrations of TMA up to 2.5 mM did not inhibit lysosomal degradation of [125I]BSA by isolated yolk sacs, as judged by the release of trichloroacetic acid (TCA)-soluble radioactivity into the culture media. The inhibition of [125I]BSA uptake induced by TMA was reversible on removal of TMA. When day 8 embryos were cultured in serum containing [3H]leucine-labelled proteins, uptake and incorporation of radioactivity in 0.75 mM TMA-treated embryos was 47 and 44%, respectively, of that in untreated controls. TMA at 0.75 mM did not inhibit the uptake and incorporation of free [3H]leucine into embryonic protein nor the amount of free [3H]leucine taken up or incorporated into protein by day 12 isolated limb buds. It is concluded that the reduced macromolecular synthesis in embryos exposed to TMA is due to an inhibition of receptor-mediated uptake of nutrients by the yolk sac.

Targeted delivery of superoxide dismutase to macrophages via mannose receptor-mediated mechanism

Human recombinant superoxide dismutase (SOD) was modified into a mannosylated form (Man-SOD), and its cellular uptake and inhibitory effect on superoxide anion release were studied in vitro, using cultured mouse peritoneal macrophages. [111In]Man-SOD was taken up by the macrophages to a great extent, whereas no significant uptake was observed for native and galactosylated SOD. The uptake of Man-SOD was inhibited significantly at a low temperature and by the presence of mannan, mannose and colchicine, demonstrating the targeted delivery of Man-SOD via mannose receptor-mediated endocytosis. Man-SOD exhibited a superior inhibitory effect on superoxide anion release from inflammatory macrophages stimulated by phorbol-myristate acetate. The present study suggested the potential of Man-SOD as a therapeutic agent for the inflammatory disease mediated by superoxide anions generated by macrophages.

A novel cycle involving fatty acyl-coenzyme A regulates asialoglycoprotein receptor activity in permeable hepatocytes

Asialoglycoprotein receptors (ASGP-Rs) in permeable rat hepatocytes can be inactivated in the absence of ligand. This cytosol-independent effect is relatively slow (t1/2 approximately 12 min) and is temperature and ATP dependent. Here we show that in the absence of cytosol, the addition of palmitoyl-CoA (Pal-CoA) rapidly (t1/2 < 0.4 min) and quantitatively reactivates the inactivated receptors. Receptor reactivation was half-maximal at approximately 10-12 microM free Pal-CoA at 37 degrees C. Although substantially higher total concentrations were used, much of the added Pal-CoA was cell associated and not free. The effects of Pal-CoA were eliminated by bovine serum albumin at concentrations sufficient to bind all free monomeric fatty acyl-CoA, suggesting that micellar effects are not responsible for the ability to reactivate ASGP-Rs. Also, palmitoyl-carnitine did not substitute for Pal-CoA. The initial ASGP-R inactivation is not affected by treating cells with N-ethylmaleimide or by a KCl wash but is inhibited by sodium orthovanadate or high Ca2+ levels. Myristoyl-CoA (C14) was also able to reactivate inactive ASGP-Rs about as well as Pal-CoA. Fatty acyl-CoAs with chain lengths of C12 (lauroyl) or C18 (steroyl) were < 50% as active. The ligand binding activity of these receptors can subsequently be modulated within minutes by the further addition of ATP or Pal-CoA to achieve additional rounds of ASGP-R inactivation or reactivation, respectively. These in vitro data demonstrate the occurrence of a novel asialoglycoprotein receptor inactivation-reactivation cycle that could regulate receptor activity during endocytosis and receptor recycling.

The complement- but not mannose receptor-mediated phagocytosis is involved in the hepatic uptake of cetylmannoside-modified liposomes in situ

In the elimination of injected liposomes in vivo, it is considered that several serum components play an important role on hepatic uptake of them. This study was conducted to clarify the hepatic uptake mechanism of cetylmannoside (Man)-modified multilamellar vesicles (Man-MLV) using perfused rat liver. In the presence of serum, Man-MLV was taken up by the liver depending on the serum concentration, and it showed an approximately two-fold higher accumulation than MLV without any surface modifications (PC-MLV). These hepatic uptakes of liposomes were obviously inhibited by preheating the serum at 56 degrees C for thirty minutes or by the treatment with anti-rat C3 antiserum. Further, SDS-PAGE followed by immunoblot analysis showed the deposition of iC3b on the opsonized Man-MLV. These results obtained in the present study suggested that hepatic uptake of Man-MLV was mainly mediated by complement receptor rather than mannose receptor on Kupffer cells in vivo.

Endosome acidification and receptor trafficking: bafilomycin A1 slows receptor externalization by a mechanism involving the receptor's internalization motif

To examine the relationship between endosome acidification and receptor trafficking, transferrin receptor trafficking was characterized in Chinese hamster ovary cells in which endosome acidification was blocked by treatment with the specific inhibitor of the vacuolar H(+)-ATPase, bafilomycin A1. Elevating endosome pH slowed the receptor externalization rate to approximately one-half of control but did not affect receptor internalization kinetics. The slowed receptor externalization required the receptor's cytoplasmic domain and was largely eliminated by substitutions replacing either of two aromatic amino acids within the receptor's cytoplasmic YTRF internalization motif. These results confirm, using a specific inhibitor of the vacuolar proton pump, that proper endosome acidification is necessary to maintain rapid recycling of intracellular receptors back to the plasma membrane. Moreover, receptor return to the plasma membrane is slowed in the absence of proper endosome acidification by a signal-dependent mechanism involving the receptor's cytoplasmic tyrosine-containing internalization motif. These results, in conjunction with results from other studies, suggest that the mechanism for clustering receptors in plasma membrane clathrin-coated pits may be an example of a more general mechanism that determines the dynamic distribution of membrane proteins among various compartments with luminal acidification playing a crucial role in this process.