Particulate endocytosis mediates biological responses of human mesenchymal stem cells to titanium wear debris.

Continual loading and articulation cycles undergone by metallic (e.g., titanium) alloy arthroplasty prostheses lead to liberation of a large number of metallic debris particulates, which have long been implicated as a primary cause of periprosthetic osteolysis and postarthroplasty aseptic implant loosening. Long-term stability of total joint replacement prostheses relies on proper integration between implant biomaterial and osseous tissue, and factors that interfere with this integration are likely to cause osteolysis. Because multipotent mesenchymal stem cells (MSCs) located adjacent to the implant have an osteoprogenitor function and are critical contributors to osseous tissue integrity, when their functions or activities are compromised, osteolysis will most likely occur. To date, it is not certain or sufficiently confirmed whether MSCs endocytose titanium particles, and if so, whether particulate endocytosis has any effect on cellular responses to wear debris. This study seeks to clarify the phenomenon of titanium endocytosis by human MSCs (hMSCs), and investigates the influence of endocytosis on their activities. hMSCs incubated with commercially pure titanium particles exhibited internalized particles, as observed by scanning electron microscopy and confocal laser scanning microscopy, with time-dependent reduction in the number of extracellular particles. Particulate endocytosis was associated with reduced rates of cellular proliferation and cell-substrate adhesion, suppressed osteogenic differentiation, and increased rate of apoptosis. These cellular effects of exposure to titanium particles were reduced when endocytosis was inhibited by treatment with cytochalasin D, and no significant effect was seen when hMSCs were treated only with conditioned medium obtained from particulate-treated cells. These findings strongly suggest that the biological responses of hMSCs to wear debris are triggered primarily by the direct endocytosis of titanium particulates, and not mediated by secreted soluble factors. In this manner, therapeutical approaches that suppress particle endocytosis could reduce the bioreactivity of hMSCs to particulates, and enhance long-term orthopedic implant prognosis by minimizing wear-debris periprosthethic osteolysis.     

Hepatic Disposition Characteristics of Electrically Charged Macromolecules in Rat in Vivo and in the Perfused Liver

The effect of electric charge on the hepatic disposition of macromolecules was studied in the rat. Charged derivatives of dextran (T-70) and bovine serum albumin (BSA), mitomycin C–dextran conjugates (MMC-D), and lactosaminated BSA (Lac-BSA) were employed as model macromolecules. After intravenous injection, cationic macromolecules were rapidly eliminated from plasma because of their extensive hepatic uptake, while anionic and neutral macromolecules were slowly eliminated. Cationic macromolecules were recovered from parenchymal and nonparenchymal hepatic cells at a cellular uptake (per unit cell number) ratio of 1.4–3.2, while that of Lac-BSA was 14. During liver perfusion using a single-pass constant infusion mode, cationic macromolecules were continuously extracted by the liver, with extraction ratios at steady-state (E _ss) ranging between 0.03 and 0.54, whereas anionic and neutral macromolecules were almost completely recovered in the outflow at steady state. The E _ss for cationized BSA (Cat-BSA) and cationic MMC-Dcat were concentration dependent and decreased at low temperatures and in the presence of colchicine and cytochalasin B. The possible participation of the internalization process in the uptake of cationic macromolecules by hepatocytes was suggested.     

Fibronectin inhibits endocytosis of calcium oxalate crystals by renal tubular cells

BACKGROUND: Fibronectin (FN; 230 kDa) is a multifunctional alpha2-glycoprotein distributed throughout the extracellular matrix and body fluids. We recently reported that FN has a protective effect against injury of renal tubular cells by exposure to oxalate and calcium oxalate (CaOX) crystals and inhibits the adhesion of CaOX crystals to renal tubular cells. In the study presented here, we investigated whether FN has inhibitory effect on crystal endocytosis by renal tubular cells. METHODS: The inhibitory effect of FN on endocytosis of CaOX crystals by MDCK cells was examined by using a radioactivity uptake assay. Also, crystal endocytosis by cells was morphologically assessed by means of transmission electron microscopy (TEM). RESULTS: FN had inhibitory effects on CaOX crystal endocytosis by MDCK cells. The morphological TEM study showed that few crystals were taken into cells when FN was added compared to the number of crystals when FN was not added. CONCLUSION: We found that FN had the inhibitory effects on the interaction between crystals and renal tubular cells, including the adhesion or endocytosis of crystals by cells.     

Short-term enzyme replacement in the murine model of Sanfilippo syndrome type B

The Sanfilippo syndrome type B (MPS III B) is an autosomal recessive disease caused by deficiency of alpha-N-acetylglucosaminidase (EC 3. 2.1.50), one of the lysosomal enzymes required for the degradation of heparan sulfate. The disease is characterized by profound neurodegeneration but relatively mild somatic manifestations, and is usually fatal in the second decade. A mouse model had been generated by disruption of the Naglu gene in order to facilitate the study of pathogenesis and the development of therapy for this currently untreatable disease. Recombinant human alpha-N-acetylglucosaminidase (rhNAGLU) was prepared from secretions of Lec1 mutant Chinese hamster ovary cells. The enzyme, which has only unphosphorylated high-mannose carbohydrate chains, was endocytosed by mouse peritoneal macrophages via mannose receptors, with half-maximal uptake at ca. 10(-7) M. When administered intravenously to 3 month-old mice, rhNAGLU was taken up avidly by liver and spleen but marginally if at all by thymus, lung, kidney, heart, and brain (in order of diminishing uptake). The half-life of the enzyme was 2.5 days in liver and spleen. Immunohistochemistry and electron microscopy showed that only macrophages were involved in enzyme uptake and correction in these two organs, yet the storage of glycosaminoglycan was reduced to almost normal levels. The results show that the macrophage-targeted rhNAGLU can substantially reduce the body burden of glycosaminoglycan storage in the mouse model of Sanfilippo syndrome III B.     

Binding of nucleosomes to a cell surface receptor: Redistribution and endocytosis in the presence of lupus antibodies

In the present study, we sought evidence for a surface nucleosome receptor in the fibroblastic cell line CV-1, and questioned whether anti-double-stranded (ds)DNA and/or anti-histone autoantibodies could recognized and influence the fate of cell surface-bound nucleosomes. ¹²⁵I-labeled mononucleosomes were shown to bind to the cell layer in a specific, concentration-dependent and a saturable manner. Scatchard analysis revealed the presence of two binding sites: a high-affinity site with a Kd of ～ 7nM and a low-affinity site (Kd ～ 400 nM) with a high capacity of 9 × 10⁷ sites. Visualization of bound mononucleosomes by fluorescence revealed staining on both the cell surface and the extracellular matrix (ECM). Purified mononucleosome-derived dsDNA (180–200 bp) was found to compete for binding of ¹²⁵I-mononucleosomes on the low-affinity site, to stain exclusively the ECM in immunofluorescence, and to precipitate three specific proteins of 43, 180 and 240 kDa from ¹²⁵-I-labeled cell lysates. Nucleosomes were found to precipitate not only the 180-kDa dsDNA-reactive component, but also a unique protein of 50 kDa, suggesting that this protein is a cell surface receptor for nucleosomes on these fibroblasts. Once bound on the cell surface, mononucleosomes were recognized and secondarily complexed by lupus anti-dsDNA or anti-histone antibodies (i.e. anti-nucleosome antibodies), thus forming immune complexes in situ. The presence of these complexing auto-antibodies was found dramatically to enhance the kinetics of mononucleosome internalization. Following the internalization of the nucleosome-anti-nucleosome complexes by immunofluorescence, we observed the formation of vesicles at the edge of the cells by 5–10 min which moved toward the perinuclear region by 20–30 min. By means of double-fluorescence labeling and proteolytic treatment, these fluorescent vesicles were shown to be in the cytoplasm, suggesting true endocytosis of nucleosome-anti-nucleosome immune complexes. As shown by confocal microscopy, at no stage of this endocytic process was there any indication that coated pits or coated vesicles participated. Co-distribution of the endocytic vesicles with regions rich in actin filaments and inhibition of endocytosis of nucleosome-anti-nucleosome complexes by disruption of the micro-filament network with cytochalasin D suggest a mechanism mediated by the cytoskeleton. Taken together, our data provide evidence for the presence of a surface nucleosome receptor. We also show that anti-dsDNA and anti-histone antibodies can form nucleosome-anti-nucleosome immune complexes in situ at the cell surface, and thus dramatically enhance the kinetics of nucleosome endocytosis.     

Distinct pathways for constitutive endocytosis of fully conformed and non-conformed L(d) molecules

PROBLEM: To characterize the constitutive internalization of major histocompatibility complex (MHC) class I molecules, we have studied the expression of completely conformed (full) and unconformed (empty) L(d) molecules on non-polarized murine P815 cells. METHODS OF STUDY: Spontaneous endocytosis of L(d) molecules was induced by cycloheximide, an inhibitor of protein synthesis, and their disappearance from the cell surface was determined by flow cytometry. In order to investigate the mechanism of internalization, a palette of inhibitors of endocytosis and vesicular transport was used. RESULTS: Inhibitors of clathrine endocytosis did not influence the internalization of L(d) molecules. Inhibitors of caveolar endocytosis and inhibitors of endolysosomal degradation prevented down-regulation of empty, but not of full L(d) molecules. CONCLUSIONS: Empty L(d) molecules are internalized mostly by caveolar endocytosis and full L(d) molecules use a different pathway, neither clathrine-mediated nor caveolar. After internalization, full L(d) molecules are probably degraded and empty L(d) molecules recycle between endosomal compartment and the cell surface before they enter into the degradation compartment.     

SIGN-R1, a novel C-type lectin expressed by marginal zone macrophages in spleen,mediates uptake of the polysaccharide dextran

The marginal zone macrophages of the spleen are implicated in the clearance of polysaccharides, but underlying mechanisms need to be pinpointed. SIGN-R1 is one of five recently identified mouse genes that are homologous to human DC-SIGN and encode a single, external, C-terminal C-type lectin domain. We find that a polyclonal antibody to a specific SIGN-R1 peptide reacts primarily and strongly with a subset of macrophages in the marginal zone of spleen and lymph node medulla. In both sites, SIGN-R1 exists primarily in an aggregated form, resistant to dissociation into monomers upon boiling in SDS under reducing conditions. Upon transfection into three different cell lines, high-mol.-wt forms bearing SIGN-R1 are expressed, as well as reactivity with ER-TR9, a mAb previously described to react selectively with marginal zone macrophages. SIGN-R1-expressing macrophages preferentially sequester dextrans following i.v. injection. Likewise, when phagocytic cells are enriched from spleen and tested in culture, dextran is selectively endocytosed by a subset of very large SIGN-R1(+) cells representing approximately 5% of total released macrophages. Uptake of FITC-dextran by these macrophages in vivo and in vitro is blocked by ER-TR9 and polyclonal anti-SIGN-R1 antibodies. Following transfection with SIGN-R1, cell lines become competent to endocytose dextrans. The dextran localizes primarily to compartments lacking transferrin receptor and the LAMP-1 CD107a panlysosomal antigen. Therefore, SIGN-R1 mediates the uptake of dextran polysaccharides, and it is predominantly expressed in the macrophages of the splenic marginal zone and lymph node medulla.     

Histone H1-mediated transfection: role of calcium in the cellular uptake and intracellular fate of H1-DNA complexes

Previously, we have shown that the transgene expression in the endothelial cell line ECV 304 strongly depends on the presence of low concentrations of Ca2+. However, it remained unclear, which transfection steps are controlled by Ca2+ ions. In the present study, we constructed transfection complexes of digoxigenin-labelled DNA and FITC-labelled histone H1. We monitored the pathway of these complexes with the use of anti-digoxigenin and anti-cathepsin B antibodies and immunofluorescence microscopy. Double labelling of DNA and cathepsin B permitted the localization of transfection complexes into endosomes/lysosomes which suggests an uptake of transfection complexes via endocytosis. It was also found that the uptake of transfection complexes by the cells was independent of the presence or absence of Ca2+ ions in the transfection medium. On the other hand, the presence of Ca2+ in the transfection medium dramatically changed the composition of the transfection complexes inside the endosome/lysosome compartment, which resulted in a strong reduction of H1 binding to DNA. Presence of Ca2+ in the postincubation medium for 24 h resulted in release of the transfection complexes with reduced H1 content from the endosomes/lysosomes into the cytosol. In the absence of Ca2+ the transfection complexes practically disappeared. These results allow us to come to the following conclusions: Ca2+ ions control the reorganization of the transfection complexes in endosomes/lysosomes and their release into the cytosol, which is an important prerequisite for transgene expression, whereas uptake of transfection complexes by the cells is not dependent on Ca2+.     

Solute Absorption from the Airways of the Isolated Rat Lung. IV. Mechanisms of Absorption of Fluorophore-Labeled Poly-α,β-[N(2-Hydroxyethyl)-DL-Aspartamide]

The pulmonary absorption kinetics of a single molecular weight distribution (MWD) of fluorophore-labeled poly-,-[N(2-hydroxyethyl)-DL-aspartamide] (F-PHEA), a hydrophilic and biocompatible synthetic polypeptide, were studied in the isolated, perfused rat lung (iprl) as functions of administered polymer concentration, dose, vehicle, and presence and absence of fluorophore. The MWD was characterized before and after absorption by measurement of weight- and number-averaged molecular weights (M _wand M _n, respectively) using high-performance gel-permeation chromatography. Values for M _w and M _n were 8.6 and 5.3 kD before, and 6.7 and 4.7 kD after, absorption into the perfusate; there was no significant metabolism and the MWD of the absorbed polymer was independent of both dose and sampling time over a 3-hr period. F-PHEA failed to show any evidence of aggregation in solution or changes in dose distribution within the airways as functions of increasing polymer concentration and dose. A concentration ranging study indicated the presence of a saturable, carrier-mediated transport process for F-PHEA with a maximum absorption rate, V _max, of approximately 180 µg or 0.027 µmol/hr. Coadministration of fluorophore-free PHEA was capable of depressing the absorption of F-PHEA. The transport process for F-PHEA appeared to have a molecular weight limit of about 7 kD for this hydrophilic polymer.     

Transmission routes of HIV-1 gp120 from brain to lymphoid tissues

The blood-brain barrier (BBB) restricts the entry of antiviral agents into the CNS thereby facilitating the creation of a reservoir of HIV that could potentially reinfect peripheral tissues. We characterized the efflux from brain of radioactively labeled viral coat HIV-1 gp120 (I-gp120) after intracerebroventricular (i.c.v.) injection. The half-time disappearance rate of I-gp120 from brain was 12.6 min, which was faster than could be explained by the reabsorption of cerebrospinal fluid into blood but could not be explained by a saturable transporter. After i.c.v. injection, I-gp120 appeared in the serum and was sequestered by spleen and the cervical nodes, demonstrating a potential for virus within the CNS to reinfect peripheral tissues. However, the amount of I-gp120 appearing in serum was less than that expected based on the efflux rate, whereas uptake by the cervical nodes was much greater after i. c.v. than after i.v. injection of I-gp120. These findings were explained by drainage from the brain directly to the cervical lymph nodes through the brain's primitive lymphatic system. These lymphatics potentially provide a pathway through which CNS reservoirs of HIV-1 could directly reinfect lymphoid tissue without being exposed to circulating antiviral agents.