Sorting and recycling of cell surface receptors and endocytosed ligands: the asialoglycoprotein and transferrin receptors

The Molecular Aspects of Functional Activity of Macrophage-Activating Factor GcMAF

Group-specific component macrophage-activating factor (GcMAF) is the vitamin D3-binding protein (DBP) deglycosylated at Thr420. The protein is believed to exhibit a wide range of therapeutic properties associated with the activation of macrophagal immunity. An original method for GcMAF production, DBP conversion to GcMAF, and the analysis of the activating potency of GcMAF was developed in this study. Data unveiling the molecular causes of macrophage activation were obtained. GcMAF was found to interact with three CLEC10A derivatives having molecular weights of 29 kDa, 63 kDa, and 65 kDa. GcMAF interacts with high-molecular-weight derivatives via Ca2+-dependent receptor engagement. Binding to the 65 kDa or 63 kDa derivative determines the pro- and anti-inflammatory direction of cytokine mRNA expression: 65 kDa-pro-inflammatory (TNF-α, IL-1β) and 63 kDa-anti-inflammatory (TGF-β, IL-10). No Ca2+ ions are required for the interaction with the canonical 29 kDa CLEC10A. Both forms, DBP protein and GcMAF, bind to the 29 kDa CLEC10A. This interaction is characterized by the stochastic mRNA synthesis of the analyzed cytokines. Ex vivo experiments have demonstrated that when there is an excess of GcMAF ligand, CLEC10A forms aggregate, and the mRNA synthesis of analyzed cytokines is inhibited. A schematic diagram of the presumable mechanism of interaction between the CLEC10A derivatives and GcMAF is provided. The principles and elements of standardizing the GcMAF preparation are elaborated.

Competition for cysteine acylation by C16:0 and C18:0 derived lipids is a global phenomenon in the proteome

S-acylation is a reversible posttranslational protein modification consisting of attachment of a fatty acid to a cysteine via a thioester bond. Research over the last few years has shown that a variety of different fatty acids, such as palmitic acid (C16:0), stearate (C18:0), or oleate (C18:1), are used in cells to S-acylate proteins. We recently showed that GNAI proteins can be acylated on a single residue, Cys3, with either C16:0 or C18:1, and that the relative proportion of acylation with these fatty acids depends on the level of the respective fatty acid in the cell's environment. This has functional consequences for GNAI proteins, with the identity of the acylating fatty acid affecting the subcellular localization of GNAIs. Unclear is whether this competitive acylation is specific to GNAI proteins or a more general phenomenon in the proteome. We perform here a proteome screen to identify proteins acylated with different fatty acids. We identify 218 proteins acylated with C16:0 and 308 proteins acylated with C18-lipids, thereby uncovering novel targets of acylation. We find that most proteins that can be acylated by C16:0 can also be acylated with C18-fatty acids. For proteins with more than one acylation site, we find that this competitive acylation occurs on each individual cysteine residue. This raises the possibility that the function of many different proteins can be regulated by the lipid environment via differential S-acylation.

Exploring Receptor Binding Affinities and Hepatic Cell Association of N-Acetyl-d-Galactosamine-Modified β-Cyclodextrin-Based Polyrotaxanes for Liver-Targeted Therapies

Acid-degradable polyrotaxanes (PRXs) containing threading β-cyclodextrins (β-CDs) are promising candidates for therapeutic applications of β-CDs in metabolic diseases with cholesterol overload or imbalance. To improve cellular uptake specificity and efficiency of PRXs in hepatocytes, N-acetyl-d-galactosamine (GalNAc)-modified PRXs were developed to facilitate asialoglycoprotein receptor (ASGR)-mediated endocytosis. Binding affinity studies revealed that the dissociation constant (K_D) values between recombinant ASGR and GalNAc-PRXs decreased with an increase in the number of modified GalNAc units. Additionally, the K_D values for GalNAc-PRXs were smaller than those for GalNAc-modified β-CD and amylose, suggesting that the PRX backbone structure improves the binding affinity with ASGR. However, the intracellular uptake levels of GalNAc-PRXs in HepG2 cells increased with a decrease in the number of modified GalNAc units, which was opposite to the trend observed in the binding affinity study. We found that GalNAc-PRXs had a large number of GalNAc units localized in recycling endosomes, resulting in the low intracellular uptake. The cholesterol-reducing abilities of GalNAc-PRXs were assessed using cholesterol-overloaded HepG2 cells. GalNAc-PRXs with a small number of GalNAc units were demonstrated to show superior cholesterol-reducing effects compared to previously designed acid-degradable PRX and clinically tested β-CD derivatives. Thus, we conclude that GalNAc modification is a promising molecular design for the therapeutic application of β-CD-threaded PRXs in various metabolic diseases with cholesterol overload or imbalance in the liver.

In vitro galactose-targeted study of RSPP050-loaded micelles against liver hepatocellular carcinoma

Andrographolide is in a group of diterpenoid lactone isolated from Andrographis paniculata (Burm.f.) NEES. One of the analogs is 19-O-triphenylmethylandrographolide (RSPP050) which possesses anticancer activity. In seeking to capitalise on the last property, we have investigated the in vitro tumour targeting capabilities and MRI imaging for hepatocellular carcinoma. In this study, we have designed galactose-targeted and non-targeted micelles comprised of poly(ethylene glycol)-b-poly(lactide) that enveloped RSPP050 as an anticancer agent and superparamagnetic iron oxide (SPIO) as a contrast agent. The targeting abilities were endeavored by examining the cellular uptake with MTT assay, fluorescence microscopy, Prussian blue staining, and in vitro MRI. Targeted SPIO micelles as a T₂* contrast agent decreased the relative T2* MRI intensity at 3 h. Results revealed that galactose micelles displayed 10.91 ± 0.19% drug loading content, -37.17 ± 0.63 mV zeta potential, and these micelles at the concentration of 0.5 µg/ml exhibited higher cytotoxicity than non-targeted micelles and free RSPP050 after incubation for 24 h. Fluorescence microscopy and Prussian blue staining at 3 h demonstrated significant cellular uptake by HepG2 cells. Thus, anticancer activity of RSPP050 could be improved using galactose as a targeting ligand and theranostic function was achieved using SPIO.

ASGR1 and Its Enigmatic Relative, CLEC10A

The large family of C-type lectin (CLEC) receptors comprises carbohydrate-binding proteins that require Ca²⁺ to bind a ligand. The prototypic receptor is the asialoglycoprotein receptor-1 (ASGR1, CLEC4H1) that is expressed primarily by hepatocytes. The early work on ASGR1, which is highly specific for N-acetylgalactosamine (GalNAc), established the foundation for understanding the overall function of CLEC receptors. Cells of the immune system generally express more than one CLEC receptor that serve diverse functions such as pathogen-recognition, initiation of cellular signaling, cellular adhesion, glycoprotein turnover, inflammation and immune responses. The receptor CLEC10A (C-type lectin domain family 10 member A, CD301; also called the macrophage galactose-type lectin, MGL) contains a carbohydrate-recognition domain (CRD) that is homologous to the CRD of ASGR1, and thus, is also specific for GalNAc. CLEC10A is most highly expressed on immature DCs, monocyte-derived DCs, and alternatively activated macrophages (subtype M2a) as well as oocytes and progenitor cells at several stages of embryonic development. This receptor is involved in initiation of T_H1, T_H2, and T_H17 immune responses and induction of tolerance in naïve T cells. Ligand-mediated endocytosis of CLEC receptors initiates a Ca²⁺ signal that interestingly has different outcomes depending on ligand properties, concentration, and frequency of administration. This review summarizes studies that have been carried out on these receptors.

A TfR-Binding Cystine-Dense Peptide Promotes Blood-Brain Barrier Penetration of Bioactive Molecules

The impenetrability of the blood-brain barrier (BBB) to most conventional drugs impedes the treatment of central nervous system (CNS) disorders. Interventions for diseases like brain cancer, neurodegeneration, or age-associated inflammatory processes require varied approaches to CNS drug delivery. Cystine-dense peptides (CDPs) have drawn recent interest as drugs or drug-delivery vehicles. Found throughout the phylogenetic tree, often in drug-like roles, their size, stability, and protein interaction capabilities make CDPs an attractive mid-size biologic scaffold to complement conventional antibody-based drugs. Here, we describe the identification, maturation, characterization, and utilization of a CDP that binds to the transferrin receptor (TfR), a native receptor and BBB transporter for the iron chaperone transferrin. We developed variants with varying binding affinities (K_D as low as 216 pM), co-crystallized it with the receptor, and confirmed murine cross-reactivity. It accumulates in the mouse CNS at ~25% of blood levels (CNS blood content is only ~1%-6%) and delivers neurotensin, an otherwise non-BBB-penetrant neuropeptide, at levels capable of modulating CREB signaling in the mouse brain. Our work highlights the utility of CDPs as a diverse, easy-to-screen scaffold family worthy of inclusion in modern drug discovery strategies, demonstrated by the discovery of a candidate CNS drug delivery vehicle ready for further optimization and preclinical development.

Surface engineering of nanoparticles with ligands for targeted delivery to osteosarcoma

Osteosarcoma is one of the most common malignant bone tumors which affect adolescents. Neoadjuvant chemotherapy followed by operation has become recommended for osteosarcoma treatment. Whereas, the effects of conventional chemotherapy are unsatisfactory because of multidrug resistance, fast clearance rate, nontargeted delivery, side effects and so on. Accordingly, Nanoparticle-mediated targeted drug delivery system (NTDDS) is recommended to be a novel treatment strategy for osteosarcoma. NTDDS can overcome the above obstacles by enhanced permeability and retention effect and active targeting. The active targeting of the delivery system is mainly based on ligands. In this study, we investigate and summarize the most common ligands used in the latest NTDDS for osteosarcoma. It might provide new insights into nanomedicine for osteosarcoma treatment.

Transferrin receptors-targeting nanocarriers for efficient targeted delivery and transcytosis of drugs into the brain tumors: a review of recent advancements and emerging trends

Treatment of glioblastoma multiforme (GBM) is a predominant challenge in chemotherapy due to the existence of blood-brain barrier (BBB) which restricts delivery of chemotherapeutic agents to the brain together with the problem of drug penetration through hard parenchyma of the GBM. With the structural and mechanistic elucidation of the BBB under both physiological and pathological conditions, it is now viable to target central nervous system (CNS) disorders utilizing the presence of transferrin (Tf) receptors (TfRs). However, overexpression of these TfRs on the GBM cell surface can also help to avoid restrictions of GBM cells to deliver chemotherapeutic agents within the tumor. Therefore, targeting of TfR-mediated delivery could counteract drug delivery issues in GBM and create a delivery system that could cross the BBB effectively to utilize ligand-conjugated drug complexes through receptor-mediated transcytosis. Hence, approach towards successful delivery of antitumor agents to the gliomas has been making possible through targeting these overexpressed TfRs within the CNS and glioma cells. This review article presents a thorough analysis of current understanding on Tf-conjugated nanocarriers as efficient drug delivery system.

Mitochondrial dysfunction in the neuro-degenerative and cardio-degenerative disease, Friedreich's ataxia

Mitochondrial homeostasis is essential for maintaining healthy cellular function and survival. The detrimental involvement of mitochondrial dysfunction in neuro-degenerative diseases has recently been highlighted in human conditions, such as Parkinson's, Alzheimer's and Huntington's disease. Friedreich's ataxia (FA) is another neuro-degenerative, but also cardio-degenerative condition, where mitochondrial dysfunction plays a crucial role in disease progression. Deficient expression of the mitochondrial protein, frataxin, is the primary cause of FA, which leads to adverse alterations in whole cell and mitochondrial iron metabolism. Dys-regulation of iron metabolism in these compartments, results in the accumulation of inorganic iron deposits in the mitochondrial matrix that is thought to potentiate oxidative damage observed in FA. Therefore, the maintenance of mitochondrial homeostasis is crucial in the progression of neuro-degenerative conditions, particularly in FA. In this review, vital mitochondrial homeostatic processes and their roles in FA pathogenesis will be discussed. These include mitochondrial iron processing, mitochondrial dynamics (fusion and fission processes), mitophagy, mitochondrial biogenesis, mitochondrial energy production and calcium metabolism.

Early Endosome Morphology in Health and Disease

Early endosomes are organelles that receive macromolecules and solutes from the extracellular environment. The major function of early endosomes is to sort these cargos into recycling and degradative compartments of the cell. Degradation of the cargo involves maturation of early endosomes into late endosomes, which, after acquisition of hydrolytic enzymes, form lysosomes. Endosome maturation involves recruitment of specific proteins and lipids to the early endosomal membrane, which drives changes in endosome morphology. Defects in early endosome maturation are generally accompanied by alterations in morphology, such as increase in volume and/or number. Enlarged early endosomes have been observed in Alzheimer's disease and Niemann Pick Disease type C, which also exhibit defects in endocytic sorting. This article discusses the mechanisms that regulate early endosome morphology and highlights the potential importance of endosome maturation in the retinal pigment epithelium.

Targeting human liver cancer cells with lactobionic acid-G(4)-PAMAM-FITC sorafenib loaded dendrimers

Reported here is the synthesis and biological evaluation of the asialoglycoprotein receptor (ASGP-R) targeted fourth generation poliamidoamine dendrimer (G(4)-PAMAM) loaded with sorafenib. The ASGP-R targeted dendrimer was obtained by conjugation of Lactobionic acid (La) to the G(4)-PAMAM dendrimer, followed by acetylation (Ac) of the free amino groups in order to reduce the non-specific interactions with the cell membrane. Moreover, by additionally grafting fluorescein (FITC), it was easy to characterize the internalization pathway and the intracellular fate of the targeted dendrimer Ac-La-G(4)-PAMAM-FITC. In vitro experiments performed on HepG-2 and HLE cell lines, allowed to study the ability of the dendrimers to affect the cell vitality. Confocal microscopy and cytofluorimetric analysis confirmed higher binding and uptake ability of the Ac-La-G(4)-PAMAM-FITC dendrimer in well differentiated and ASGP-R expressing human liver cancer cell line HepG-2 compared non-expressing HLE cells. Ac-La-G(4)-PAMAM-FITC dendrimer loaded with sorafenib was stable and showed sustained sorafenib release. As evidenced by the cytotoxicity studies, sorafenib included in the dendrimer maintained its effectiveness, and was able to produce a longer lasting effect over the time compared to molar equivalent doses of free sorafenib. This new targeted dendrimer appears to be a suitable carrier for the delivery of sorafenib to liver cancer cells expressing ASGP-R.

Asialoglycoprotein receptors as important mediators of plasma lipids and atherosclerosis

PURPOSE OF REVIEW

This review seeks to describe the role of the asialoglycoprotein receptor (ASGR) in modulating non-HDL lipoprotein levels, platelet numbers and atherosclerosis.

RECENT FINDINGS

Genetics studies have revealed that ASGR haplodeficiency provides protection from atherosclerosis. The potential interactions of ASGR with LDL receptor may regulate the rate of LDL uptake and as a result may lower plasma non-HDL cholesterol. ASGR clears senescent platelets and induces the expression of hepatic thrombopoietin. Platelet activity promotes plaque formation and atherosclerosis.

SUMMARY

ASGR is a multifunctional receptor specializing in clearance of desialylated glycoproteins and platelets. The impact of ASGR on the levels of plasma non-HDL cholesterol makes it a potential therapeutic target for the treatment of hypercholesterolemia and atherosclerosis.

Cell surface recycling in yeast: mechanisms and machineries

Sorting internalized proteins and lipids back to the cell surface controls the supply of molecules throughout the cell and regulates integral membrane protein activity at the surface. One central process in mammalian cells is the transit of cargo from endosomes back to the plasma membrane (PM) directly, along a route that bypasses retrograde movement to the Golgi. Despite recognition of this pathway for decades we are only beginning to understand the machinery controlling this overall process. The budding yeastSaccharomyces cerevisiae, a stalwart genetic system, has been routinely used to identify fundamental proteins and their modes of action in conserved trafficking pathways. However, the study of cell surface recycling from endosomes in yeast is hampered by difficulties that obscure visualization of the pathway. Here we briefly discuss how recycling is likely a more prevalent process in yeast than is widely appreciated and how tools might be built to better study the pathway.

Ligand-based targeted therapy: a novel strategy for hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is the most common primary liver cancer with high morbidity and mortality worldwide. Chemotherapy is recommended to patients with intermediate or advanced stage cancer. However, the conventional chemotherapy yields low desired response rates due to multidrug resistance, fast clearance rate, nonspecific delivery, severe side effects, low drug concentration in cancer cells, and so on. Nanoparticle-mediated targeted drug delivery system can surmount the aforementioned obstacles through enhanced permeability and retention effect and active targeting as a novel approach of therapeutics for HCC in recent years. The active targeting is triggered by ligands on the delivery system, which recognize with and internalize into hepatoma cells with high specificity and efficiency. This review focuses on the latest targeted delivery systems for HCC and summarizes the ligands that can enhance the capacity of active targeting, to provide some insight into future research in nanomedicine for HCC.

Sialidase Unmasks Mucin Domain Epitopes of Lubricin

Lubricin is a secreted, mucin-like glycoprotein and proteoglycan abundant in synovial fluid that provides boundary lubrication and prevents cell adhesion in synovial joints. The antilubricin S6.79 monoclonal antibody recognizes an O-linked glycopeptide epitope in lubricin's mucin domain. The central, long mucin domain of lubricin is extensively O-glycosylated with Gal(β1-3)GalNAc-O-Ser/Thr, and about two thirds of the O-glycosylated sites are capped with sialic acid. Our aim was to determine whether removal of sialic acid by sialidase could improve the detection of lubricin in a number of human tissues using the S6.79 monoclonal antibody. Sialidase treatment caused a dramatic increase in antibody reactivity in human pericardium, splenic capsule and trabeculae, plasma, serum, eye sleep extract, and liver sinusoids. Sialidase had minimal effect on S6.79 antibody reactivity with lubricin in synovial fluid and synovial tissue. These observations suggest that the origin of lubricin in blood may be different from that in synovial fluid and that desialylation of lubricin is essential for unmasking epitopes within the mucin domain.

Endosome-mitochondria interactions are modulated by iron release from transferrin

Using superresolution and quantitative fluorescence microscopy, Das et al. have revealed that iron-transferrin–containing endosomes directly interact with mitochondria, facilitating iron transfer in epithelial cells. Their findings further enrich the repertoire of organelle–organelle direct interactions to accomplish a functional significance.

Transient “kiss and run” interactions between endosomes containing iron-bound transferrin (Tf) and mitochondria have been shown to facilitate direct iron transfer in erythroid cells. In this study, we used superresolution three-dimensional (3D) direct stochastic optical reconstruction microscopy to show that Tf-containing endosomes directly interact with mitochondria in epithelial cells. We used live-cell time-lapse fluorescence microscopy, followed by 3D rendering, object tracking, and a distance transformation algorithm, to track Tf-endosomes and characterize the dynamics of their interactions with mitochondria. Quenching of iron sensor RDA-labeled mitochondria confirmed functional iron transfer by an interacting Tf-endosome. The motility of Tf-endosomes is significantly reduced upon interaction with mitochondria. To further assess the functional role of iron in the ability of Tf-endosomes to interact with mitochondria, we blocked endosomal iron release by using a Tf K206E/K534A mutant. Blocking intraendosomal iron release led to significantly increased motility of Tf-endosomes and increased duration of endosome–mitochondria interactions. Thus, intraendosomal iron regulates the kinetics of the interactions between Tf-containing endosomes and mitochondria in epithelial cells.

A convenient method for hTfR1 inclusion body purification

Human transferrin receptor, referred as hTfR1, is ubiquitously expressed at low levels in most normal human tissues; however, the expression level of hTfR1 at the blood-brain barrier (BBB) and in tumor tissues is relatively higher. hTfR1 is a type II homodimeric transmembrane protein. The extracellular domain of hTfR1 consists of three domains: helical domain, apical, and protease-like domain. In order to prepare hTfR1 antibody, which can be utilized to deliver drugs across BBB through receptor-mediated endocytosis, we began to express the nonligand binding domain of hTfR1 in Escherichia coli BL21 Transetta (DE3). The TfR1 gene was first obtained from HepG2 cells by reverse-transcription polymerase chain reaction (RT-PCR) and then inserted into pET 32a(c+) vector. The protein was expressed in the form of inclusion body with extremely high purity by the E. coli BL21 Transetta (DE3), and the purity was further improved by size-exclusion chromatography. The Western blot test indicated that the recombinant protein was TfR1 as expected. Above all, this report provided a convenient protocol that could be fulfilled in order to prepare hTfR1 inclusion body, which failed to be purified by an Ni(2+) affinity column.

Enhancing Glioblastoma-Specific Penetration by Functionalization of Nanoparticles with an Iron-Mimic Peptide Targeting Transferrin/Transferrin Receptor Complex

Treatment of glioblastoma (GBM) remains to be the most formidable challenge because of the hindrance of the blood-brain barrier (BBB) along with the poor drug penetration into the glioma parenchyma. Nanoparticulate drug delivery systems (DDS) utilizing transferrin (Tf) as the targeting ligand to target the glioma-associated transferrin receptor (TfR) had met the problem of loss of specificity in biological environment due to the high level of endogenous Tf. Here we conjugated CRT peptide, an iron-mimicry moiety targeting the whole complex of Tf/TfR, to poly(ethylene glycol)-poly(l-lactic-co-glycolic acid) nanoparticles (CRT-NP), to open a new route to overcome such obstacle. High cellular associations, advanced transport ability through the BBB model, and penetration in 3-dimensional C6 glioma spheroids in vitro had preliminarily proved the advantages of CRT-NP over Tf-nanoparticle conjugates (Tf-NP). Compared with Tf-NP, NP, and Taxol, paclitaxel-loaded CRT-NP (CRT-NP-PTX) displayed a superior antiproliferation effect on C6 glioma cells and stronger inhibitory effect on glioma spheroids. Favored pharmacokinetics behavior and enhanced accumulation in glioma foci was observed, together with a much deeper distribution pattern in glioma parenchyma compared with unmodified nanoparticles and Tf-NP. Eventually, mice treated with CRT-NP-PTX showed a remarkably prolonged median survival compared to those treated with Taxol, NP, or Tf-NP. In conclusion, the modification of CRT to nanoparticles holds great promise for enhancement of antiglioma therapy.

Genetic investigation of MHC-independent missing-self recognition by mouse NK cells using an in vivo bone marrow transplantation model

MHC-I-specific receptors play a vital role in NK cell-mediated "missing-self" recognition, which contributes to NK cell activation. In contrast, MHC-independent NK recognition mechanisms are less well characterized. In this study, we investigated the role of NKR-P1B:Clr-b (Klrb1:Clec2d) interactions in determining the outcome of murine hematopoietic cell transplantation in vivo. Using a competitive transplant assay, we show that Clr-b(-/-) bone marrow (BM) cells were selectively rejected by wild-type B6 recipients, to a similar extent as H-2D(b-/-) MHC-I-deficient BM cells. Selective rejection of Clr-b(-/-) BM cells was mitigated by NK depletion of recipient mice. Competitive rejection of Clr-b(-/-) BM cells also occurred in allogeneic transplant recipients, where it was reversed by selective depletion of NKR-P1B(hi) NK cells, leaving the remaining NKR-P1B(lo) NK subset and MHC-I-dependent missing-self recognition intact. Moreover, competitive rejection of Clr-b(-/-) hematopoietic cells was abrogated in Nkrp1b-deficient recipients, which lack the receptor for Clr-b. Of interest, similar to MHC-I-deficient NK cells, Clr-b(-/-) NK cells were hyporesponsive to both NK1.1 (NKR-P1C)-stimulated and IL-12/18 cytokine-primed IFN-γ production. These findings support a unique and nonredundant role for NKR-P1B:Clr-b interactions in missing-self recognition of normal hematopoietic cells and suggest that optimal BM transplant success relies on MHC-independent tolerance mechanisms. These findings provide a model for human NKR-P1A:LLT1 (KLRB1:CLEC2D) interactions in human hematopoietic cell transplants.

The small GTPase Rab11 co-localizes with α-synuclein in intracellular inclusions and modulates its aggregation, secretion and toxicity

Alpha-synuclein (aSyn) misfolding and aggregation are pathological features common to several neurodegenerative diseases, including Parkinson's disease (PD). Mounting evidence suggests that aSyn can be secreted and transferred from cell to cell, participating in the propagation and spreading of pathological events. Rab11, a small GTPase, is an important regulator in both endocytic and secretory pathways. Here, we show that Rab11 is involved in regulating aSyn secretion. Rab11 knockdown or overexpression of either Rab11a wild-type (Rab11a WT) or Rab11a GDP-bound mutant (Rab11a S25N) increased secretion of aSyn. Furthermore, we demonstrate that Rab11 interacts with aSyn and is present in intracellular inclusions together with aSyn. Moreover, Rab11 reduces aSyn aggregation and toxicity. Our results suggest that Rab11 is involved in modulating the processes of aSyn secretion and aggregation, both of which are important mechanisms in the progression of aSyn pathology in PD and other synucleinopathies.

Site-specific glycan microheterogeneity of inter-alpha-trypsin inhibitor heavy chain H4

Inter-alpha-trypsin inhibitor heavy chain H4 (ITIH4) is a 120 kDa acute-phase glycoprotein produced primarily in the liver, secreted into the blood, and identified in serum. ITIH4 is involved in liver development and stabilization of the extracellular matrix (ECM), and its expression is altered in liver disease. In this study, we aimed to characterize glycosylation of recombinant and serum-derived ITIH4 using analytical mass spectrometry. Recombinant ITIH4 was analyzed to optimize glycopeptide analyses, followed by serum-derived ITIH4. First, we confirmed that the four ITIH4 N-X-S/T sequons (N81, N207, N517, and N577) were glycosylated by treating ITIH4 tryptic/GluC glycopeptides with PNGaseF in the presence of (18)O water. Next, we performed glycosidase-assisted LC-MS/MS analysis of ITIH4 trypsin-GluC glycopeptides enriched via hydrophilic interaction liquid chromatography to characterize ITIH4 N-glycoforms. While microheterogeneity of N-glycoforms differed between ITIH4 protein expressed in HEK293 cells and protein isolated from serum, occupancy of N-glycosylation sites did not differ. A fifth N-glycosylation site was discovered at N274 with the rare nonconsensus NVV motif. Site N274 contained high-mannose N-linked glycans in both serum and recombinant ITIH4. We also identified isoform-specific ITIH4 O-glycoforms and documented that utilization of O-glycosylation sites on ITIH4 differed between the cell line and serum.

Fixing frataxin: 'ironing out' the metabolic defect in Friedreich's ataxia

The metabolically active and redox-active mitochondrion appears to play a major role in the cellular metabolism of the transition metal, iron. Frataxin, a mitochondrial matrix protein, has been identified as playing a key role in the iron metabolism of this organelle due to its iron-binding properties and is known to be essential for iron-sulphur cluster formation. However, the precise function of frataxin remains elusive. The decrease in frataxin expression, as seen in the inherited disorder Friedreich's ataxia, markedly alters cellular and mitochondrial iron metabolism in both the mitochondrion and the cell. The resulting dysregulation of iron trafficking damages affects tissues leading to neuro- and cardiodegeneration. This disease underscores the importance of iron homeostasis in the redox-active environment of the mitochondrion and the molecular players involved. Unravelling the mechanisms of altered iron metabolism in Friedreich's ataxia will help elucidate a biochemical function for frataxin. Consequently, this will enable the development of more effective and rationally designed treatments. This review will focus on the emerging function of frataxin in relation to the observed alterations in mitochondrial iron metabolism in Friedreich's ataxia. Tissue-specific alterations due to frataxin loss will also be discussed, as well as current and emerging therapeutic strategies.

Assessing the transport of receptor-mediated drug-delivery devices across cellular monolayers

Receptor-mediated endocytosis (RME) has been extensively studied as a method for augmenting the transport of therapeutic devices across monolayers. These devices range from simple ligand-therapeutic conjugates to complex ligand-nanocarrier systems. However, characterizing the uptake of these carriers typically relies on their comparisons to the native therapeutic, which provides no understanding of the ligand or cellular performance. To better understand the potential of the RME pathway, a model for monolayer transport was designed based on the endocytosis cycle of transferrin, a ligand often used in RME drug-delivery devices. This model established the correlation between apical receptor concentration and transport capability. Experimental studies confirmed this relationship, demonstrating an upper transport limit independent of the applied dose. This contrasts with the dose-proportional pathways that native therapeutics rely on for transport. Thus, the direct comparison of these two transport mechanisms can produce misleading results that change with arbitrarily chosen doses. Furthermore, transport potential was hindered by repeated use of the RME cycle. Future studies should base the success of this technology not on the performance of the therapeutic itself, but on the capabilities of the cell. Using receptor-binding studies, we were able to demonstrate how these capabilities can be predicted and potentially adopted for high-throughput screening methods.

Galactosylated manganese ferrite nanoparticles for targeted MR imaging of asialoglycoprotein receptor

Cancer cells can express specific biomarkers, such as cell membrane proteins and signaling factors. Thus, finding biomarkers and delivering diagnostic agents are important in the diagnosis of cancer. In this study, we investigated a biomarker imaging agent for the diagnosis of hepatic cancers. The asialoglycoprotein receptor (ASGPr) was selected as a biomarker for hepatoma cells and the ASGPr-targetable imaging agent bearing a galactosyl group was prepared using manganese ferrite nanoparticles (MFNP) and galactosylgluconic acid. The utility of the ASGPr-targetable imaging agent, galactosylated MFNP (G-MFNP) was assessed by several methods in ASGPr-expressing HepG2 cells as target cells and ASGPr-deficient MCF7 cells. Physical and chemical properties of G-MFNP were examined using Fourier-transform infrared spectroscopy, dynamic light scattering, zeta potential analysis, and transmission electron microscopy. No significant cytotoxicity was observed in either cell line. Targeting ability was assessed using flow cytometry, magnetic resonance imaging, inductively coupled plasma atomic emission spectroscopy, absorbance analysis, dark-field microscopy, Prussian blue staining, and transmission electron microscopy. We demonstrated that G-MFNP target successfully and bind to ASGPr-expressing HepG2 cells specifically. We suggest that these results will be useful in strategies for cancer diagnoses based on magnetic resonance imaging.

Iron homeostasis in the liver

Iron is an essential nutrient that is tightly regulated. A principal function of the liver is the regulation of iron homeostasis. The liver senses changes in systemic iron requirements and can regulate iron concentrations in a robust and rapid manner. The last 10 years have led to the discovery of several regulatory mechanisms in the liver that control the production of iron regulatory genes, storage capacity, and iron mobilization. Dysregulation of these functions leads to an imbalance of iron, which is the primary cause of iron-related disorders. Anemia and iron overload are two of the most prevalent disorders worldwide and affect over a billion people. Several mutations in liver-derived genes have been identified, demonstrating the central role of the liver in iron homeostasis. During conditions of excess iron, the liver increases iron storage and protects other tissues, namely, the heart and pancreas from iron-induced cellular damage. However, a chronic increase in liver iron stores results in excess reactive oxygen species production and liver injury. Excess liver iron is one of the major mechanisms leading to increased steatohepatitis, fibrosis, cirrhosis, and hepatocellular carcinoma.

Nanomedicines for back of the eye drug delivery, gene delivery, and imaging

Treatment and management of diseases of the posterior segment of the eye such as diabetic retinopathy, retinoblastoma, retinitis pigmentosa, and choroidal neovascularization is a challenging task due to the anatomy and physiology of ocular barriers. For instance, traditional routes of drug delivery for therapeutic treatment are hindered by poor intraocular penetration and/or rapid ocular elimination. One possible approach to improve ocular therapy is to employ nanotechnology. Nanomedicines, products of nanotechnology, having at least one dimension in the nanoscale include nanoparticles, micelles, nanotubes, and dendrimers, with and without targeting ligands. Nanomedicines are making a significant impact in the fields of ocular drug delivery, gene delivery, and imaging, the focus of this review. Key applications of nanotechnology discussed in this review include a) bioadhesive nanomedicines; b) functionalized nanomedicines that enhance target recognition and/or cell entry; c) nanomedicines capable of controlled release of the payload; d) nanomedicines capable of enhancing gene transfection and duration of transfection; f) nanomedicines responsive to stimuli including light, heat, ultrasound, electrical signals, pH, and oxidative stress; g) diversely sized and colored nanoparticles for imaging, and h) nanowires for retinal prostheses. Additionally, nanofabricated delivery systems including implants, films, microparticles, and nanoparticles are described. Although the above nanomedicines may be administered by various routes including topical, intravitreal, intravenous, transscleral, suprachoroidal, and subretinal routes, each nanomedicine should be tailored for the disease, drug, and site of administration. In addition to the nature of materials used in nanomedicine design, depending on the site of nanomedicine administration, clearance and toxicity are expected to differ.

Lethal iron deprivation induced by non-neutralizing antibodies targeting transferrin receptor 1 in malignant B cells

A number of antibodies have been developed that induce lethal iron deprivation (LID) by targeting the transferrin receptor 1 (TfR1/CD71) and either neutralizing transferrin (Tf) binding, blocking internalization of the receptor and/or inducing its degradation. We have developed recombinant antibodies targeting human TfR1 (ch128.1 and ch128.1Av), which induce receptor degradation and are cytotoxic to certain malignant B-cells. We now show that internalization of TfR1 bound to these antibodies can lead to its sequestration and degradation, as well as reduced Tf uptake, and the induction of a transcriptional response consistent with iron deprivation, which is mediated in part by downstream targets of p53. Cells resistant to these antibodies do not sequester and degrade TfR1 after internalization of the antibody/receptor complex, and accordingly maintain their ability to internalize Tf. These findings are expected to facilitate the rational design and clinical use of therapeutic agents targeting iron import via TfR1 in hematopoietic malignancies.

Hepatocyte-targeted psiRNA delivery mediated by galactosylated poly(ethylene glycol)-graft-polyethylenimine in vitro

Gene silencing in liver disease could be achieved by delivering siRNA with nonviral vectors. However, the transfection efficiency of plasmid siRNA (psiRNA) applied through this approach in hepatocytes is generally low. Based on the fact that the asialoglycoprotein receptors present on hepatocytes can recognize galactose, we synthesized galactosylated poly(ethylene glycol)-graft-polyethylenimine (Gal-PEG-PEI) as a nonviral psiRNA carrier for hepatocyte targeting. Our results indicate that 0.2% (molar percentage) of amine groups of PEI was conjugated with PEG having galactose on its distal end. Increasing the molar ratios of Gal-PEG-PEI to psiRNA in complexation led to a decrease in particle size but an increase in zeta potential of complexes. The transfection efficiency of nanocomplexes, that is, Gal-PEG-PEI/psiRNA, in HepG2 cell line depends on the N/P value, which reflects the molar ratio of Gal-PEG-PEI to psiRNA in the complex. The highest transfection efficiency was 37.34%, which was obtained at N/P 8. At the same N/P value, the transfection efficiency with the nontargeting PEG-PEI/psiRNA or Lipofectamine 2000/psiRNA was much lower. The transfection efficiency of Gal-PEG-PEI/psiRNA dropped to 3.60% from 37.34% after an excessive amount of free galactose was added into the medium for HepG2 cell incubation. By contrast, the similar phenomenon was observed neither when using PEG-PEI or Lipofectamine 2000 as a delivery vector nor in human embryonic kidney 293 cell line lacking ASGR. Real-time PCR analysis and western blot assay demonstrate that the knockdown of HLA-E gene expression by psiRNA/Gal-PEG-PEI (N/P 8) can reach about 60% in HepG2 cells after a 48-h transfection.

The transferrin receptor and the targeted delivery of therapeutic agents against cancer

BACKGROUND

Traditional cancer therapy can be successful in destroying tumors, but can also cause dangerous side effects. Therefore, many targeted therapies are in development. The transferrin receptor (TfR) functions in cellular iron uptake through its interaction with transferrin. This receptor is an attractive molecule for the targeted therapy of cancer since it is upregulated on the surface of many cancer types and is efficiently internalized. This receptor can be targeted in two ways: 1) for the delivery of therapeutic molecules into malignant cells or 2) to block the natural function of the receptor leading directly to cancer cell death.

SCOPE OF REVIEW

In the present article we discuss the strategies used to target the TfR for the delivery of therapeutic agents into cancer cells. We provide a summary of the vast types of anti-cancer drugs that have been delivered into cancer cells employing a variety of receptor binding molecules including Tf, anti-TfR antibodies, or TfR-binding peptides alone or in combination with carrier molecules including nanoparticles and viruses.

MAJOR CONCLUSIONS

Targeting the TfR has been shown to be effective in delivering many different therapeutic agents and causing cytotoxic effects in cancer cells in vitro and in vivo.

GENERAL SIGNIFICANCE

The extensive use of TfR for targeted therapy attests to the versatility of targeting this receptor for therapeutic purposes against malignant cells. More advances in this area are expected to further improve the therapeutic potential of targeting the TfR for cancer therapy leading to an increase in the number of clinical trials of molecules targeting this receptor. This article is part of a Special Issue entitled Transferrins: molecular mechanisms of iron transport and disorders.

Spermine grafted galactosylated chitosan for improved nanoparticle mediated gene delivery

Despite multitude of beneficial features, chitosan has poor water solubility and transfection ability which affect its gene delivery efficacy. The two features are improved when certain chemical modifications are incorporated into the chitosan parent backbone. This strategy is adopted here, by coupling galactose and spermine into the chitosan backbone. The conjugation was determined with FTIR and (1)H NMR and nanoparticle morphology was assessed by TEM and AFM techniques. Particle size, zeta potential, buffering capacity and DNA binding ability gave encouraging result of enhanced solubility and stability. In vitro studies of GCSM in HepG2 cell lines displayed low cytotoxicity and improved transfection. We also identified the preference of receptor mediated internalization for nanoparticles cellular uptake by treating with cellular uptake inhibitors. The results evidently led us to comprehend that galactosylated chitosan-g-spermine could be considered as a promising chitosan derivative for conducting nanoparticle mediated gene delivery.

CD317/tetherin is enriched in the HIV-1 envelope and downregulated from the plasma membrane upon virus infection

CD317/Bst-2/tetherin is a host factor that restricts the release of human immunodeficiency virus type 1 (HIV-1) by trapping virions at the plasma membrane of certain producer cells. It is antagonized by the HIV-1 accessory protein Vpu. Previous light microscopy studies localized CD317 to the plasma membrane and the endosomal compartment and showed Vpu induced downregulation. In the present study, we performed quantitative immunoelectron microscopy of CD317 in cells producing wild-type or Vpu-defective HIV-1 and in control cells. Double-labeling experiments revealed that CD317 localizes to the plasma membrane, to early and recycling endosomes, and to the trans-Golgi network. CD317 largely relocated to endosomes upon HIV-1 infection, and this effect was partly counteracted by Vpu. Unexpectedly, CD317 was enriched in the membrane of viral buds and cell-associated and cell-free viruses compared to the respective plasma membrane, and this enrichment was independent of Vpu. These results suggest that the tethering activity of CD317 critically depends on its density at the cell surface and appears to be less affected by its density in the virion membrane.

Evidence for an asialoglycoprotein receptor on nonparenchymal cells for O-linked glycoproteins

B cell-activating factor receptor 3 (BR3)-Fc is an IgG1-receptor dimeric fusion protein that has multiple O-linked glycosylation sites and sialylation levels that can vary in the manufacturing process. Increased sialic acid levels resulted from increased site occupancy with the O-linked N-acetylgalactosamine (GalNAc-Gal), but because the ratio of sialic acid per mole of oligosaccharide remained approximately 1, this led to increased asialo terminal GalNAc. Previous studies have demonstrated an effect of terminal asialo Gal or GalNAc on the clearance of glycoproteins due to uptake and degradation by lectin receptors in the liver. However, the previous studies examined N-linked oligosaccharides, and there are less data regarding O-linked oligosaccharides. The objective of these studies was to determine the effects on the pharmacokinetics and distribution of the asialo terminal GalNAc and varying amounts of sialic acid residues on BR3-Fc. The results of the data presented here suggest that exposed Gal on the desialylated BR3-Fc led to rapid clearance due to uptake and degradation in the liver that was associated with nonparenchymal cells. It is interesting to note that the data indicated a decreased clearance and increased exposure of BR3-Fc as the sialic acid levels increased, even though increased sialic acid was associated with increased asialo GalNAc. Therefore, the exposed GalNAc did not seem to play a role in the clearance of BR3-Fc; although the Gal linked to the hydroxyl group at position 3 may have prevented an interaction. Because we did not see uptake of desialylated BR3-Fc in hepatocytes where the asialoglycoprotein receptor is localized, this nonparenchymal cell lectin may have preference for O-linked glycoproteins.

The human hyaluronan receptor for endocytosis (HARE/Stabilin-2) is a systemic clearance receptor for heparin

The hyaluronic acid receptor for endocytosis (HARE; also designated Stabilin-2) mediates systemic clearance of hyaluronan and chondroitin sulfates from the vascular and lymphatic circulations. The internalized glycosaminoglycans are degraded in lysosomes, thus completing their normal turnover process. Sinusoidal endothelial cells of human liver, lymph node, and spleen express two HARE isoforms of 315 and 190 kDa. Here we report that the 190- and 315-kDa HARE isoforms, expressed stably either in Flp-In 293 cell lines or as soluble ectodomains, specifically bind heparin (Hep). The K(d) for Hep binding to purified 190- and 315-kDa HARE ectodomains was 17.2 +/- 4.9 and 23.4 +/- 5.3 nm, respectively. Cells expressing HARE readily and specifically internalized (125)I-streptavidin-biotin-Hep complexes, which was inhibited >70% by hyperosmolar conditions, confirming that uptake is mediated by the clathrin-coated pit pathway. Internalization of Hep occurred for many hours with an estimated HARE recycling time of approximately 12 min. Internalized fluorescent streptavidin-biotin-Hep was present in a typical endocytic vesicular pattern and was delivered to lysosomes. We conclude that HARE in the sinusoidal endothelial cells of lymph nodes and liver likely mediates the efficient systemic clearance of Hep and many different Hep-binding protein complexes from the lymphatic and vascular circulations.

Kinetics of internalization and degradation of N-type voltage-gated calcium channels: Role of the α2/δ subunit

The contribution of voltage-gated calcium channels to excitable cell function depends, critically, upon the mechanisms that control their expression at the cell surface. While co-assembly of the pore forming alpha(1) and auxiliary beta subunits enhances channel surface expression, the levels are still only 30-40% of those seen with the core alpha(1B)/beta(1b)/alpha(2)delta calcium channel complex. To rationalize this observation, it has been suggested that the alpha(2)/delta subunit might stabilize calcium channel expression at the cell surface. To test this notion, we have resolved the effect of the alpha(2)/delta subunit on the rates of binding, internalization and degradation of defined N-type calcium channel surface complexes expressed in HEK293 cells, through pulse-labeling with the selective, cell impermeable, radioligand [(125)I]-omega-CgTx. Through detailed kinetic and sensitivity analysis we show that alpha(1B)/beta(1b)/alpha(2)delta complexes are internalized slowly (k(int) 0.4/h), whereupon, most become degraded (k(deg) 0.02/h). In contrast, alpha(1B)/beta(1b) complexes are internalized more rapidly (k(int) 0.8/h), following which they are either quickly degraded (k(deg) 0.1/h) or are sequestered slowly (k(tra) 0.1/h) to a pool that is metabolically stable within the time-frame of our experiments (24h). In neither case did we find evidence for recycling via the cell surface. Thus, our data argue for a novel mechanism where complexes lacking an alpha(2)/delta subunit are cleared from the cell surface and are rapidly degraded or stored, possibly for further attempts at complexation as new alpha(2)/delta subunits become available. The slower rate of internalization of complexes containing the alpha(2)/delta subunit rationalizes the stabilizing effect this subunit has upon calcium channel surface expression and suggests a mechanism by which alpha(2)delta mutations may cause severe neurological deficits.

A single point mutation in the low-density lipoprotein receptor switches the degradation of its mature protein from the proteasome to the lysosome

Pathogenic mutations in the low-density lipoprotein receptor prevent cholesterol uptake and cause familial hypercholesterolemia. In comparison to the biogenesis and endocytic trafficking of this receptor and some of its mutants, their degradation mechanisms are not well understood. Therefore, to gain some insights into this aspect, we analyzed the effects of proteasomal and lysosomal inhibitors on the levels of the wild type low-density lipoprotein receptor and a mutant form, C358Y, which was prevalent in a sample of Spanish familial hypercholesterolemia patients. In transfected cells, the mutant C358Y exhibited lower activity than the wild type receptor, as well as retarded post-translational processing of its precursor to the mature form. Interestingly, about 30% of the mutant precursor was degraded by a lysosomal pathway. Moreover, its mature form was more rapidly degraded than the wild type receptor (half lives of 5.3 and 10.9 h, respectively) and its degradation was exclusively dependent on a lysosomal pathway. In contrast, the mature form of the wild type receptor was mainly degraded by proteasomes and, to a minor extent (30%), by lysosomes. We conclude that a single mutation in the low-density lipoprotein receptor switches the degradation of the mature receptor from a proteasomal to a lysosomal pathway which degrades the protein at a faster rate. This suggests cooperation of proteasomes and lysosomes in the degradation of the low-density lipoprotein receptor and adds an intriguing new aspect to our understanding of receptor-mediated endocytosis.

Crosstalk between Rab GTPases and cell junctions

For the past several years, studies from other laboratories, as well as ours, have begun to unravel the mechanism of germ cell movement in the testis by using several in vitro and in vivo models of tight and adherens junction assembly and disassembly, two cellular phenomena that confer cell movement. However, for cell movement to be fully appreciated, the importance of "intracellular" cell movements, such as those involving actin and microtubule filaments, must be better understood. Recent research on Rab GTPases has shown that members of this superfamily function in the trafficking of vesicles containing cargo to distinct subcellular sites such as the plasma membrane while utilizing actin and microtubule filaments as tracks. In this mini-review, we provide an overview of Rab GTPase structure, function, and regulation, while placing added emphasis on the role of Rabs in cell junction dynamics in the testis.

Chemo-Enzymatic Synthesis of Raffinose-Branched Polyelectrolytes and Self-Assembly Application in Microcapsules

A novel biocompatible polyelectrolyte poly(vinyl raffinose-co-acrylic acid) (PRCA) containing a raffinose branch was prepared via redox polymerization using Fe(2+)/K(2)S(2)O(8)/H(2)O(2) starting from enzymatically-synthesized monomer: 1-O-vinyldecanedioyl raffinose. Copolymers with different monomer feed ratios were prepared and characterized with IR, NMR, and GPC. PRCA can be alternated with polycation to form microcapsules on a crystals template by electrostatic layer-by-layer technique. The multilayers of PRCA/poly(methacryloyloxyethyl dimethylbenzyl ammonium chloride) (PMBA) on quartz slides and PRCA/poly(dimethyldiallyl ammonium chloride) (PDDA) on acyclovir crystals template were fabricated and characterized with UV-Vis spectra, the microelectrophoretic measurement, and TEM. Hollow capsules can be formed after the removal of acyclovir crystals template in a buffer solution. The nano-capsule-carrying galactose residue is a potential targeting drug-controlled delivery systems.

Non-stoichiometric relationship between clathrin heavy and light chains revealed by quantitative comparative proteomics of clathrin-coated vesicles from brain and liver

We used tandem mass spectrometry with peptide counts to identify and to determine the relative levels of expression of abundant protein components of highly enriched clathrin-coated vesicles (CCVs) from rat liver. The stoichiometry of stable protein complexes including clathrin heavy chain and clathrin light chain dimers and adaptor protein (AP) heterotetramers was assessed. We detected a deficit of clathrin light chain compared with clathrin heavy chain in non-brain tissues, suggesting a level of regulation of clathrin cage formation specific to brain. The high ratio of AP-1 to AP-2 in liver CCVs is reversed compared with brain where there is more AP-2 than AP-1. Despite this, general endocytic cargo proteins were readily detected in liver but not in brain CCVs, consistent with the previous demonstration that a major function for brain CCVs is recycling synaptic vesicles. Finally we identified 21 CCV-associated proteins in liver not yet characterized in mammals. Our results further validate the peptide accounting approach, reveal new information on the properties of CCVs, and allow for the use of quantitative proteomics to compare abundant components of organelles under different experimental and pathological conditions.

Galactosylated chitosan/DNA nanoparticles prepared using water-soluble chitosan as a gene carrier

Water-soluble chitosan (WSC) was used to increase the stability of chitosan in water and decrease the cytotoxicity induced by acetic acid. Lactobionic acid (LA) bearing galactose group was coupled with WSC for hepatocytes specificity. The composition of galactose in galactosylated chitosan (GC) was determined by NMR spectroscopy. The GC was complexed with plasmid DNA in various GC/DNA (N/P) charge ratios and the resulting complex was characterized by dynamic light scattering, gel retardation, and turbidity to determine the particle sizes, complex formation, and complex stability, respectively. Cytotoxicity and transfection efficiency of GC were also studied in cultured HepG2 human hepatoblastoma cell line and HeLa human cervix epithelial carcinoma cells. The complete GC/DNA complex was formed at the charge ratio of 5 and the GC/DNA complex to DNase I resistance was obtained. Particle sizes decreased with increasing charge ratio of GC to DNA and had a minimum value around 120 nm at the charge ratio of 5. And no significant difference in particle sizes from the charge ratio of 5-20 was found. The suspension of GC/DNA complexes exhibited no significant change in turbidity at the charge ratios of 10, indicating the complete shielding of DNA charge. Cytotoxicity study showed that GC prepared by the water-soluble chitosan had no cytotoxic effects on cells. And transfection efficiency into HepG2, which has asialoglycoprotein receptors (ASGP-R), was higher than that into HeLa without ASGP-R.

Hepatitis C virus targets DC-SIGN and L-SIGN to escape lysosomal degradation

Hepatitis C virus (HCV) is a major health problem. However, the mechanism of hepatocyte infection is largely unknown. We demonstrate that the dendritic cell (DC)-specific C-type lectin DC-SIGN and its liver-expressed homologue L-SIGN/DC-SIGNR are important receptors for HCV envelope glycoproteins E1 and E2. Mutagenesis analyses demonstrated that both HCV E1 and E2 bind the same binding site on DC-SIGN as the pathogens human immunodeficiency virus type 1 (HIV-1) and mycobacteria, which is distinct from the cellular ligand ICAM-3. HCV virus-like particles are efficiently captured and internalized by DCs through binding of DC-SIGN. Antibodies against DC-SIGN specifically block HCV capture by both immature and mature DCs, demonstrating that DC-SIGN is the major receptor on DCs. Interestingly, internalized HCV virus-like particles were targeted to nonlysosomal compartments within immature DCs, where they are protected from lysosomal degradation in a manner similar to that demonstrated for HIV-1. Lewis X antigen, another ligand of DC-SIGN, was internalized to lysosomes, demonstrating that the internalization pathway of DC-SIGN-captured ligands may depend on the structure of the ligand. Our results suggest that HCV may target DC-SIGN to "hide" within DCs and facilitate viral dissemination. L-SIGN, expressed by THP-1 cells, internalized HCV particles into similar nonlysosomal compartments, suggesting that L-SIGN on liver sinusoidal endothelial cells may capture HCV from blood and transmit it to hepatocytes, the primary target for HCV. We therefore conclude that both DCs and liver sinusoidal endothelial cells may act as reservoirs for HCV and that the C-type lectins DC-SIGN and L-SIGN, as important HCV receptors, may represent a molecular target for clinical intervention in HCV infection.

Acidification and protein traffic

Acidification of some organelles, including the Golgi complex, lysosomes, secretory granules, and synaptic vesicles, is important for many of their biochemical functions. In addition, acidic pH in some compartments is also required for the efficient sorting and trafficking of proteins and lipids along the biosynthetic and endocytic pathways. Despite considerable study, however, our understanding of how pH modulates membrane traffic remains limited. In large part, this is due to the diversity of methods to perturb and monitor pH, as well as to the difficulties in isolating individual transport steps within the complex pathways of membrane traffic. This review summarizes old and recent evidence for the role of acidification at various steps of biosynthetic and endocytic transport in mammalian cells. We describe the mechanisms by which organelle pH is regulated and maintained, as well as how organelle pH is monitored and quantitated. General principles that emerge from these studies as well as future directions of interest are discussed.