On the pericellular zone of some mammalian cells in vitro

Multifunctional activities of human fibroblast 34-kDa hyaluronic acid-binding protein

We have already reported that human fibroblast 34-kDa hyaluronic acid-binding protein (HABP) is identical with P32, the protein co-purified with splicing factor SF-2 [Deb and Datta (1996) J. Biol. Chem. 271, 2206-2212]. Data search further revealed that it has 92% sequence homology with a murine protein YL2 which interacts with HIV1 Rev. In this paper we have successfully demonstrated that HIV1 Rev binds with labeled 34-kDa HABP which can be competed with excess unlabeled HABP, suggesting this protein can be a cellular factor promoting HIV1 Rev to function. Interestingly, the multifunctional nature of HABP has been elucidated as it has 100% homology with another protein gC1q, the complement protein. The distinct non-overlapping binding motifs for HA and gC1q have been identified in the same protein, suggesting that either the protein can function independently or its activity is regulated by ligand binding, wherein its binding to one of the ligands may modulate the receptor activity of the other ligand.

Molecular cloning and characterization of a cDNA encoding the third putative mammalian hyaluronan synthase

We report the isolation of a cDNA encoding the third putative hyaluronan synthase, HAS3. Partial cDNAs and genomic fragments of mouse Has3 were obtained using a degenerate polymerase chain reaction approach. Partial clones facilitated the isolation of genomic and cDNA clones representing the mouse Has3 open reading frame. The open reading frame of 554 amino acids predicted a protein of 63.3 kDa with multiple transmembrane domains and several consensus HA binding motifs. Sequence comparisons indicated that mouse Has3 is most closely related to Has2 (71% amino acid identity) and also related to Has1 (57% identity), Xenopus laevis DG42 (56% identity), and Streptococcus pyogenes HasA (28% identity). Isolation of a genomic fragment of human HAS3 indicated high conservation between mouse and human sequences, similar to those observed for HAS1 and HAS2. Expression of the mouse Has3 open reading frame in transfected COS-1 cells led to high levels of hyaluronan synthesis, as determined through a classical particle exclusion assay, and by in vitro HA synthase assays. These results suggest that there are three putative mammalian hyaluronan synthases encoded by three separate but related genes which comprise a mammalian hyaluronan synthase (HAS) gene family.

IL-1 beta, a major stimulator of hyaluronan synthesis in vitro of human peritoneal mesothelial cells: relevance to peritonitis in CAPD

The effect of several different growth factors and cytokines on the synthesis of hyaluronan (HA) by human peritoneal mesothelial cells (HPMC) was investigated. Growth arrested HPMC synthesized low levels of HA, but co-culture with PDGF-bb, TGF-beta 1, TNF-alpha, and IL-6 at a concentration of 10 ng/ml all increased HA synthesis between two- to three-fold. At the same concentration IL-1 beta significantly increased the synthesis eight-fold (N = 3; P < 0.05). The effect of IL-1 beta was also dose- and time-dependent and could be totally negated with interleukin-1 receptor antagonist (IL-1 beta RcA). Non-infected and infected dialysate from patients receiving CAPD was also found to stimulate HA synthesis by HPMC. The levels found with non-infected fluid were 4 x 10(4) dpm/ml (N = 6) and 12.9 x 10(4) dpm/ml (N = 6; P < 0.002) and 8.7 x 10(4) dpm/ml (N = 6; P < 0.003) for infected fluid collected one and two days after the commencement of peritonitis. IL-1 beta RcA dramatically reduced the effect of infected but not non-infected dialysate. These results provide new insights into the manner in which HA synthesis is controlled in the mesothelium and suggest that IL-1 beta is a key cytokine in the inflammatory response in CAPD patients.

Molecular cloning and characterization of a putative mouse hyaluronan synthase

We report the isolation of a novel mouse gene which encodes a putative hyaluronan synthase. The cDNA was identified using degenerate reverse transcriptase-polymerase chain reaction. Degenerate primers were designed based upon an alignment of the amino acid sequences of Streptococcus pyogenes HasA, Xenopus laevis DG42, and Rhizobium meliloti NodC. A mouse embryo cDNA library was screened with the resultant polymerase chain reaction product, and multiple cDNA clones spanning 3 kilobase pairs (kb) were isolated. The open reading frame predicted a 63-kDa protein with several transmembrane sequences, multiple consensus phosphorylation sites, and four putative hyaluronan binding motifs. The amino acid sequence displayed 55% identity to mouse HAS, 56% identity to Xenopus DG42, and 21% identity to Streptococcus HasA. Northern analysis identified transcripts of 4.8 kb and 3.2 kb, which were expressed highly in the developing mouse embryo and at lower levels in adult mouse heart, brain, spleen, lung, and skeletal muscle. Transfection experiments demonstrated that mouse Has2 could direct hyaluronan coat biosynthesis in transfected COS cells, as evidenced by a classical particle exclusion assay. These results suggest that mammalian HA synthase activity is regulated by at least two related genes. Accordingly, we propose the name Has2 for this gene.

The structure and function of hyaluronan: An overview

Hyaluronan is a major component of synovial tissue and fluid as well as other soft connective tissues. It is a high-Mr polysaccharide which forms entangled networks already at dilute concentrations (< 1 mg/mL) and endows its solutions with unique rheological properties. Physiological functions of hyaluronan (lubrication, water homeostasis, macromolecular filtering, exclusion, etc.) have been ascribed to the properties of these networks. Recently a number of specific interactions between hyaluronan and a group of proteins named hyaladherins have also pointed towards a role of hyaluronan in recognition and the regulation of cellular activities. Many more or less well documented hypotheses have been proposed for the function of hyaluronan in joints, for example, that it should lubricate, protect cartilage surfaces, scavenge free radicals and debris, keep the joint cavities open, form flow barriers in the synovium and prevent capillary growth.

Chondroitin sulfate proteoglycan modulates the permeability of hyaluronan-containing coats around normal human mesothelial cells

The composition and permeability of the pericellular coat surrounding normal human mesothelial (NHM) cells have been studied in vitro. NHM cells were grown in the presence of 3H-glucosamine and the amount of label recovered in hyaluronan and chondroitin sulfate was determined after selective enzymatic digestion of the polysaccharides in medium, pericellular, and intracellular pools. For comparison a similar analysis was carried out on mesothelioma cells (Mero-14). Of the labeled polysaccharides in the medium and pericellular pools of NHM cells about 80-90% could be ascribed to hyaluronan and only 3-5% to chondroitin sulfate. In contrast, Mero-14 synthesized only minute amounts of hyaluronan whereas chondroitin sulfate corresponded to 61% of the total glycosaminoglycans in the culture. The results exclude a structure of the pericellular layer of NHM cells similar to the hyaluronan-proteoglycan aggregates found in cartilage. The permeability of the pericellular layer was tested by the exclusion of polystyrene microspheres and bacteria of diameter 0.1-3.0 microns, as well as erythrocytes of diameter 7 microns. While the erythrocytes were excluded the smaller particles penetrated the coat. By adding 0.5 mg/ml of aggregating cartilage proteoglycan to the medium particles of 0.3 microns or larger were also excluded. Thus exogenous proteoglycans can reinforce the structure of the pericellular layer.

Production of hyaluronan-dependent pericellular matrix by embryonic rat glial cells

The extracellular matrix of brain is largely composed of aggregates formed by assembly of many proteoglycan and link protein molecules along a hyaluronan polymer backbone. Some cell types construct large, highly hydrated, pericellular matrices or 'coats' from these hyaluronan-mediated aggregates. We show here that embryonic glial cells produce such hyaluronan-dependent pericellular matrices in response to addition of serum or basic fibroblast growth factor plus transforming growth factor-beta. It is proposed that such a matrix is a significant component of the extracellular milieu of the brain, especially during morphogenesis within the developing brain, and that basic fibroblast growth factor and transforming growth factor-beta regulate its production.

The distal tendon of the biceps brachii. Structure and clinical correlations

The structure and blood supply of 42 distal biceps tendons were investigated by means of light and electron microscopy as well as by immunohistochemistry. Possible structural causes for the rupture of the tendon are discussed. The distal biceps tendon wraps around the radius during pronation of the forearm. In this area the tendon is exposed to pressure and shearing forces in addition to those caused by tension. Two fibrocartilaginous areas were regularly observed. Large chondrocyte-like cells were found inside the fibrocartilage. As an expression of strain, the extracellular matrix is rich in acidic glycosaminoglycans and stains intensely with toluidine blue at pH 1. Electron microscopy showed a granular pericellular matrix that increases in size towards the gliding surface. Type I collagen is the main component of the distal biceps tendon. Type II collagen is found in tendon fibrocartilage but not in traction tendons. The gliding surface of the tendon is made up of reticular fibres that are equivalent to type III collagen. Monoclonal antibodies revealed the presence of dermatan-sulfate, keratansulfate and chondroitin-4- as well as chondroitin-6-sulfate. Blood vessels are usually absent in fibrocartilage, as was shown with a polyclonal antibody against the basement membrane component laminine. There are significant differences between the extracellular matrix of traction and gliding tendons, which may be responsible for the location of tendon rupture.

Inter-alpha-inhibitor is required for the formation of the hyaluronan-containing coat on fibroblasts and mesothelial cells

Cultured cells of various origins have been shown to be surrounded by a hyaluronan-containing coat, a structure that can be visualized by its ability to exclude large particles such as erythrocytes. When cultured in medium with no or low concentrations of serum, the cells lose their coats, although they still produce hyaluronan; upon the addition of serum, the coats are formed again. Here, we show that the serum protein inter-alpha-inhibitor can replace whole serum as an inducer of the formation of the coats on fibroblasts and mesothelial cells. The physiological role of inter-alpha-inhibitor has so far been unclear; our findings, together with those obtained with cumulus cell-oocyte complexes (Chen, L., Mao, S.J., and Larsen, W. J. (1992) J. Biol. Chem. 267, 12380-12386), suggest that inter-alpha-inhibitor and related proteins have a general function as stabilizers of hyaluronan-containing pericellular coats.

Hyaluronan-mediated protective effect against cell damage caused by enzymatically produced hydroxyl (OH.) radicals is dependent on hyaluronan molecular mass

Hyaluronan (HA) protected tendon fibroblasts against cell damage mediated by hydroxyl radicals (OH.) as demonstrated by release of 51Cr from labelled cells. Protection afforded by high molecular mass (M(r)) HA (1218 kDa) was much more effective than that provided by lower (176 kDa and 668 kDa) M(r) HA. OH. was generated by coupling H2O2 produced by glucose oxidase:glucose to [Fe(2+)-EDTA] chelate in a Fenton-type system. The flux of OH. was measured by a spectrofluorimetric assay of salicylate produced by the reaction of benzoate with OH.. Cell damage caused by the OH. generating system was prevented in the presence of catalase, which destroyed H2O2. Damage caused in a standard incubation time increased with increased amounts of glucose oxidase. Protection against OH.-mediated cell damage increased with increasing concentration of HA. The presence of HA did not interfere with the enzyme-Fenton system, as monitored by production of gluconate. On the other hand, HA scavenged OH. produced by the enzyme-Fenton system, as shown by competition with benzoate, which produced less salicylate in the spectrofluorimetric assay in the presence of HA. The reaction of OH. with HA was measured directly by a pulse radiolysis technique in which a hydrated electron (eaq-) produced OH. by the reaction with nitrous oxide. Second order rate constants obtained in distilled H2O or in phosphate buffer showed no dependence on HA M(r). Similarly, fluorimetric assay of the flux of in the enzyme-Fenton system confirmed that HA competed with benzoate, thus lowering salicylate production, and the flux was also independent of the molecular mass of HA.(ABSTRACT TRUNCATED AT 250 WORDS)

The dynamic structure of the pericellular matrix on living cells

Although up to several microns thick, the pericellular matrix is an elusive structure due to its invisibility with phase contrast or DIC microscopy. This matrix, which is readily visualized by the exclusion of large particles such as fixed red blood cells is important in embryonic development and in maintenance of cartilage. While it is known that the pericellular matrix which surrounds chondrocytes and a variety of other cells consists primarily of proteoglycans and hyaluronan with the latter binding to cell surface receptors, the macromolecular organization is still speculative. The macromolecular organization previously could not be determined because of the collapse of the cell coat with conventional fixation and dehydration techniques. Until now, there has been no way to study the dynamic arrangement of hyaluronan with its aggregated proteoglycans on living cells. In this study, the arrangement and mobility of hyaluronan-aggrecan complexes were directly observed in the pericellular matrix of living cells isolated from bovine articular cartilage. The complexes were labeled with 30- to 40-nm colloidal gold conjugated to 5-D-4, an antibody to keratan sulfate, and visualized with video-enhanced light microscopy. From our observations of the motion of pericellular matrix macromolecules, we report that the chondrocyte pericellular matrix is a dynamic structure consisting of individual tethered molecular complexes which project outward from the cell surface. These complexes undergo restricted rotation or wobbling. When the cells were cultured with ascorbic acid, which promotes production of matrix components, the size of the cell coat and the position of the gold probes relative to the plasma membrane were not changed. However, the rapidity and extent of the tethered motion were reduced. Treatment with Streptomyces hyaluronidase removed the molecules that displayed the tethered motion. Addition of hyaluronan and aggrecan to hyaluronidase-treated cells yielded the same labeling pattern and tethered motion observed with native cell coats. To determine if aggrecan was responsible for the extended configuration of the complexes, only hyaluronan was added to the hyaluronidase-treated cells. The position and mobility of the hyaluronan was detected using biotinylated hyaluronan binding region (b-HABR) and gold streptavidin. The gold-labeled b-HABR was found only near the cell surface. Based on these observations, the hyaluronan-aggrecan complexes composing the cell coat are proposed to be extended in a brush-like configuration in an analogous manner to that previously described for high density, grafted polymers in good solvents.

Assembly of pericellular matrices by COS-7 cells transfected with CD44 lymphocyte-homing receptor genes

The capacity to assemble and retain a pericellular matrix is correlated with the expression of the cell surface binding sites specific for the extracellular matrix macromolecule hyaluronan. These binding proteins have been termed hyaluronan receptors. The lymphocyte-homing receptor CD44 may have identity with these hyaluronan receptors. To determine whether hyaluronan receptors function independently in this capacity for matrix assembly, mammalian cells were transfected with cDNA encoding the putative hyaluronan receptor CD44. After transfection with CD44 cDNA, COS cells gained the capacity to assemble hyaluronan-dependent pericellular matrices in the presence of exogenously added hyaluronan and proteoglycan. Thus, CD44 receptors do function as matrix-organizing, matrix-anchoring hyaluronan-binding proteins. In addition, the expression of CD44/hyaluronan receptors alone is sufficient to direct this matrix assembly. If matrix assembly is a function of cells in vivo that express hyaluronan receptors, this raises interesting possibilities for the role of the receptors in cell migration, when new extracellular matrix environments are encountered.

Hyaluronan receptor-directed assembly of chondrocyte pericellular matrix

Initial assembly of extracellular matrix occurs within a zone immediately adjacent to the chondrocyte cell surface termed the cell-associated or pericellular matrix. Assembly within the pericellular matrix compartment requires specific cell-matrix interactions to occur, that are mediated via membrane receptors. The focus of this study is to elucidate the mechanisms of assembly and retention of the cartilage pericellular matrix proteoglycan aggregates important for matrix organization. Assembly of newly synthesized chondrocyte pericellular matrices was inhibited by the addition to hyaluronan hexasaccharides, competitive inhibitors of the binding of hyaluronan to its cell surface receptor. Fully assembled chondrocyte pericellular matrices were displaced using hyaluronan hexasaccharides as well. When exogenous hyaluronan was added to matrix-free chondrocytes in combination with aggrecan, a pericellular matrix equivalent in size to an endogenous matrix formed within 30 min of incubation. Addition of hyaluronan and aggrecan to glutaraldehyde-fixed chondrocytes resulted in matrix assembly comparable to live chondrocytes. These matrices could be inhibited from assembling by the addition of excess hyaluronan hexasaccharides or displaced once assembled by subsequent incubation with hyaluronan hexasaccharides. The results indicate that the aggrecanrich chondrocyte pericellular matrix is not only on a scaffolding of hyaluronan, but actually anchored to the cell surface via the interaction between hyaluronan and hyaluronan receptors.

Unconfined lateral diffusion and an estimate of pericellular matrix viscosity revealed by measuring the mobility of gold-tagged lipids

Nanovid (video-enhanced) microscopy was used to determine whether lateral diffusion in the plasma membrane of colloidal gold-tagged lipid molecules is confined or is unrestricted. Confinement could be produced by domains within the plane of the plasma membrane or by filamentous barriers within the pericellular matrix. Fluorescein-phosphatidylethanolamine (F1-PE), incorporated into the plasma membranes of cultured fibroblasts, epithelial cells and keratocytes, was labeled with 30-nm colloidal gold conjugated to anti-fluorescein (anti-F1). The trajectories of the gold-labeled lipids were used to compute diffusion coefficients (DG) and to test for restricted motion. On the cell lamella, the gold-labeled lipids diffused freely in the plasma membrane. Since the gold must move through the pericellular matrix as the attached lipid diffuses in the plasma membrane, this result suggests that any extensive filamentous barriers in the pericellular matrix are at least 40 nm from the plasma membrane surface. The average diffusion coefficients ranged from 1.1 to 1.7 x 10(-9) cm2/s. These values were lower than the average diffusion coefficients (DF) (5.4 to 9.5 x 10(-9) cm2/s) obtained by FRAP. The lower DG is partially due to the pericellular matrix as demonstrated by the result that heparinase treatment of keratocytes significantly increased DG to 2.8 x 10(-9) cm2/s, but did not affect DF. Pericellular matrix viscosity was estimated from the frictional coefficients computed from DG and DF and ranged from 0.5 to 0.9 poise for untreated cells. Heparinase treatment of keratocytes decreased the apparent viscosity to approximately 0.1 poise. To evaluate the presence of domains or barriers, the trajectories and corresponding mean square displacement (MSD) plots of gold-labeled lipids were compared to the trajectories and MSD plots resulting from computer simulations of random walks within corrals. Based on these comparisons, we conclude that, if there are domains limiting the diffusion of F1-PE, most are larger than 5 microns in diameter.

Hyaluronate binding and CD44 expression in human glioblastoma cells and astrocytes

CD44 is an integral membrane glycoprotein of approximately 90 kDa which has been implicated in the binding of hyaluronate to the cell surface. The expression of CD44 in astrocytes was investigated by means of indirect immunofluorescence on cultured cells. The vast majority of these cells were found to express CD44. Western blot analysis of these cells revealed a highly polydisperse species having an M(r) corresponding to 74-86 kDa. In order to visualize hyaluronate-binding cells, living cultures were probed with fluorescein-conjugated hyaluronate (FI-HA). Some astrocytes were able to bind FI-HA, provided that they were first treated with hyaluronidase. Streptomyces hyaluronidase, which is hyaluronate-specific, was effective in exposing the hyaluronate-binding capacity of these cells. This leads one to conclude that hyaluronate is bound to the surface of these cells and that it masks their capacity to bind hyaluronate. Provided that they were first treated with hyaluronidase, the U-87 MG (glioblastoma-astrocytoma), U-373 MG (glioblastoma), and Hs 683 (glioma) cell lines were also able to bind FI-HA. The U-138 MG (glioblastoma) cell line was unable to bind FI-HA, with or without prior hyaluronidase treatment. A quantitative assay was developed with the use of [3H]hyaluronate ([3H]HA). This revealed the binding to be highly specific, inasmuch as the addition of unlabeled hyaluronate, but not other glycosaminoglycans, was effective in inhibiting the binding of the [3H]HA. An anti-CD44 monoclonal antibody, 50B4, was able to inhibit the binding of the [3H]HA to the U-373 MG cell line. In this cell line, then, CD44 functions as a hyaluronate receptor and one may infer that this is also the case in some astrocytes.

The role of hyaluronan-binding protein in assembly of pericellular matrices

Hyaluronan-dependent pericellular matrices or "coats" are expressed by a variety of cell types in culture and modulation of their expression may be important in regulation of cell interactions in vivo during development. Monoclonal antibody IVd4, which recognizes hyaluronan-binding protein with the properties of a hyaluronan receptor, was shown to block formation of these coats by a variety of cells. Using rat fibrosarcoma cells, it was found that the antibody not only blocked initial formation of the coats but also caused their loss when added subsequent to formation. The loss of preformed coats in the presence of antibody occurred at 4 degrees and 37 degrees, implying that the function of hyaluronan-binding protein in coat formation is not in mediating metabolic processes. The antibody also had no significant effect on hyaluronan production by the fibrosarcoma cells. In addition, hyaluronan hexasaccharide, a competitive inhibitor of the interaction between polymeric hyaluronan and its cell surface receptor, was found to inhibit coat formation. Thus it is concluded that a hyaluronan-binding protein with the properties of a hyaluronan receptor is required for pericellular matrix formation.

Hyaluronan-dependent pericellular coats of chick embryo limb mesodermal cells: induction by basic fibroblast growth factor

Basic fibroblast growth factor (FGF) has been shown previously to be present in the chick embryonic limb during early stages of its development, at which time the limb mesodermal cells are proliferating within a hyaluronan-rich extracellular matrix. In this study, basic FGF was found to stimulate hyaluronan synthesis and production of hyaluronan-dependent pericellular coats by mesodermal cells from the chick embryo limb; acidic FGF, platelet-derived growth factor, epidermal growth factor, and retinoic acid either had a much smaller effect than basic FGF or an inhibitory effect. Transforming growth factor-beta stimulated hyaluronan synthesis and coat formation but, unlike basic FGF, this factor also stimulated chondroitin sulfate production by the mesodermal cells.

Release of hyaluronate from eukaryotic cells

The mechanism of hyaluronate shedding from eukaryotic cell lines was analysed. All cell lines shed identical sizes of hyaluronate as were retained on the surface. They differed in the amount of hyaluronate synthesized and in the proportions of hyaluronate which were released and retained. A method was developed which could discriminate between shedding due to intramolecular degradation and that due to dissociation as intact macromolecules. This method was applied to B6 and SV3T3 cells in order to study the mechanism of hyaluronate release in more detail. The cells were pulse-labelled to form hyaluronate chains with labelled and unlabelled segments, and the sizes of labelled hyaluronate released into the medium during the pulse extension period were determined by gel filtration. B6 cells released identical sizes of hyaluronate at all labelled segment lengths, indicating that no intramolecular degradation occurred. When chain elongation was blocked by periodate-oxidized UDP-glucuronic acid, hyaluronate release was simultaneously inhibited. These results indicated that B6 cells dissociated hyaluronate as an intact macromolecule. In contrast, SV3T3 cells released hyaluronate of varying molecular mass distributions during extension of the labelled segment, suggesting partial degradation. Exogenous hyaluronate added to SV3T3 cultures was also degraded. This degradation could be prevented by the presence of radical scavengers such as superoxide dismutase and tocopherol. Degradation of endogenous hyaluronate could be inhibited by salicylate. These results led to the conclusion that SV3T3 cells released hyaluronate not only by dissociation, but also by radical-induced degradation.

Hyaluronate appears to be covalently linked to the cell surface

The purpose of this study was to examine the nature of the linkage between cell-surface hyaluronate and the plasma membrane. To accomplish this, rat fibrosarcoma cells were cultured in the presence of [3H]-acetate to isotopically label the hyaluronate, and then fixed with glutaraldehyde, which cross-links proteins but does not react directly with hyaluronate. The glutaraldehyde fixation stabilized the cells so that they could be manipulated in ways which would otherwise destroy cells. The fixed cells were then subjected to various treatments, and the amount of hyaluronate remaining on the cell surface was assayed via exhaustive digestion with Streptomyces hyaluronidase. Using this technique, we found that 1) cell-surface hyaluronate was quite stable for extended periods of time even in the presence of a large excess of non-labeled hyaluronate; 2) 4 M guanidine HCl and detergents did not extract a significant portion of cell-surface hyaluronate; 3) solutions of varying ionic strength (0-1 M NaCl) had no effect on the retention of hyaluronate; 4) the cell coat was stable in the range of pH 4-11, but outside this range a significant amount of hyaluronate was released; and 5) treatment with proteases released cell-surface hyaluronate. These results are consistent with the possibility that hyaluronate is covalently linked to a protein associated with the plasma membrane. Further support for this model came from experiments with the detergent Triton X-114, which can be used to separate soluble proteins from hydrophobic proteins. When nonfixed rat fibrosarcoma cells were extracted with this detergent and then partitioned by centrifugation, approximately 30 times as much hyaluronate was present in the detergent fraction which contained the hydrophobic proteins, as compared to the extracts pretreated with trypsin prior to phase separation. Again, these results suggest that cell-surface hyaluronate is directly linked to a hydrophobic core protein intercalated in the plasma membrane.

Size-dependent hyaluronate degradation by cultured cells

Hyaluronate degradation was examined in cultures of vascular wall cells (bovine aortic endothelial cells, rat aortic smooth muscle cells) and in nonvascular cells (chick embryo fibroblasts). The three cell types examined all produced hyaluronidase activity in culture which had a strict acidic pH requirement for activity. This suggested that the enzyme was active only within an acidic intracellular compartment and therefore that hyaluronate degradation occurred at an intracellular site. This was supported by the observation that the presence of hyaluronidase activity alone was not sufficient to ensure degradation of extracellular hyaluronate. Rather, the key limiting factor in this process appeared to be hyaluronate internalization, and this was found to be hyaluronate size-dependent and to a degree, cell-specific. The relationship of these results to morphogenesis and tissue remodeling is discussed.

Biochemistry of hyaluronan

Hyaluronan (hyaluronic acid) is a linear polysaccharide formed from disaccharide units containing N-acetylglucosamine and glucuronic acid. It is ubiquitously distributed in the organism but is found in the highest concentrations in soft connective tissues. The molecular weight of hyaluronan is usually in the order of 10(6) to 10(7). Due to hydrogen bonding, the chain is rather stiff and the molecule behaves in solution as an extended, randomly kinked coil. Molecules of hyaluronan start to entangle already at concentrations of less than 1 g/l and form a continuous polymer network. Some of the functions of the polysaccharide have been connected with the unique physical chemical characteristics of the network such as its rheological properties, flow resistance, osmotic pressure, exclusion properties and filter effect. Hyaluronan is synthesized in the cell membrane by adding monosaccharides to the reducing end of the chain. The precursors are UDP-glucuronic acid and UDP-N-acetylglucosamine. The polysaccharide grows out from the cell surface and it can be shown that fibroblasts, for example, surround themselves with a coat of hyaluronan. The rate of biosynthesis is regulated by various factors, such as growth factors, hormones, inflammatory mediators, etc. The responsible enzyme, hyaluronan synthase, is a phosphoprotein and the regulation of the synthetic rate is apparently via phosphorylation. The hyaluronan is at least partly carried by lymph flow from the tissues. Part of the material is taken up and degraded in the lymph nodes. Another part is carried to the general circulation and taken up in the endothelial cells in the liver sinusoids. These cells have specific receptors for hyaluronan, which also recognize chondroitin sulphate. The uptake in the liver of high-molecular weight hyaluronan is very efficient and its normal half-life in serum is only in the order of 2 to 5 min. The polysaccharide is rapidly degraded in the lysosomes to low-molecular weight products, lactate and acetate. The total turnover of hyaluronan in serum is in the order of 10-100 mg/24 h. The normal concentration of hyaluronan in serum is less than 100 micrograms/l with a mean of 30-40 micrograms/l. High serum levels have been noted in liver cirrhosis (impaired uptake in the liver) and rheumatoid arthritis (increased synthesis in the tissues). Hyaluronan has been shown to interact specifically with certain proteins and cell surfaces. It binds to proteoglycans in cartilage and other tissues and fills an important structural role in the organization of the extra-cellular matrix.(ABSTRACT TRUNCATED AT 400 WORDS)

Ultrastructural features of the lymphocyte-stimulated halos produced by human glioma-derived cells in vitro

Many glioma-derived cell lines have the capability of escaping cell-mediated immune attack. One mechanism of escape is the secretion of a hyaluronidase-sensitive mucopolysaccharide coat by these cells. This coat prevents contact and tumor cell killing by specific cytolytic allogeneic lymphocytes. The production of the coat by the tumor cells is stimulated by a macromolecular factor released by peripheral blood mononuclear (PBMC) cells in culture. We have examined the morphologic and ultrastructural features of this extracellular matrix. Three coat-producing lines were studied. Under phase contrast light microscopy, the coat is a clear pericellular 'halo'. To stain this zone, ruthenium red and Alcian Blue 8 G stains, which bind to acid mucopolysaccharides (to a large extent, hyaluronic acid), were used. The two stains produced similar results. With light microscopy, a weblike pattern of stain was evident throughout the halo region. With transmission electron microscopy, staining was found along the plasma membrane of the glioma cells and their microvilli, stretching in long, branching filaments from these surfaces and, in some instances, from one microvillus to the next. Since mucopolysaccharide matrices have a large aqueous component, it was necessary to determine whether dehydration alters the stain pattern. Therefore, undehydrated ruthenium red stained specimens from each culture were embedded in Quetal 651 (Ted Pella, Inc., Tustin, CA), a water soluble plastic. No morphologic differences were noted between the hydrated and dehydrated specimens. This study indicates that numerous long microvilli and a secreted mucopolysaccharide matrix are important structural elements of the lymphocyte-stimulated tumor cell halo in vitro. The mechanism by which the PBMC factor stimulates coat formation and the importance of the coat in in vivo tumor defenses remain to be elucidated.

Changes in the pericellular matrix during differentiation of limb bud mesoderm

Mesodermal cells in the developing chick embryo limb bud appear morphologically homogeneous until stage 21. At stage 22 the prechondrogenic and premyogenic areas begin to condense, culminating in the appearance of cartilage and muscle by stage 25-26. We have examined changes in the hyaluronate-dependent pericellular matrices elaborated by mesodermal cells of the limb bud from different developmental stages and the corresponding changes in production of cell surface-associated and secreted glycosaminoglycans. When placed in culture, most early mesodermal cells (stage 17 lateral plate and stage 19 limb bud) exhibited pericellular coats as visualized by the exclusion of particles. These coats were removed by treatment of the cultures with Streptomyces hyaluronidase. Cells from stage 20-21 limb buds (precondensation) had smaller coats, whereas cells derived from stage 22, 24, and 26 limb buds (condensed chondrogenic and myogenic regions) lacked coats. However, coats were reformed during subsequent cytodifferentiation of chondrocytes; chondrocytes from stage 28 and 30 limb buds, and more mature chondrocytes from stage 38 tibiae, had pericellular coats. Thus, cytodifferentiation of cartilage is accompanied by extensive intercellular matrix accumulation in vivo and reacquisition of pericellular coats in vitro. Although their structure was still dependent on hyaluronate, chondrocyte coats were associated with increased proteoglycan content compared to the coats of early mesodermal cells. The amount of incorporation of [3H]acetate into cell surface hyaluronate remained relatively constant from stages 17 to 38, whereas in the medium compartment, incorporation into hyaluronate was more than 4-fold greater by stage 17 and 19 mesodermal cells than by cells from stages between 20 and 38. However, there was a progressive increase in incorporation into cell surface and medium chondroitin sulfate throughout these developmental stages. Thus, at the time of cellular condensation in the limb bud in vivo, we have observed a reduction in size of hyaluronate-dependent pericellular coats and a dramatic change in the relative proportion of hyaluronate and chondroitin sulfate produced by the mesodermal cells in vitro.

Localization of hyaluronate and hyaluronate-binding protein on motile and non-motile fibroblasts

The distribution of a hyaluronate-binding (HABP) and rhodamine B-isothiocyanate (RITC)-labeled hyaluronate (HA) were studied on both actively motile and stationary chick heart fibroblasts to assess the relationship of these molecules to each other, to other extracellular matrix molecules, to membrane protrusions and to adhesion sites. RITC-HA and HABP, detected by indirect immunofluorescence, were concentrated in the perinuclear region, the leading lamella and retraction processes of actively motile cells, although RITC-HA also occurred diffusely over the rest of the cell body. Double immunofluorescence confirmed that HA and HABP co-localized in the former three regions, suggesting that, at these locations, the HABP may act as a cell surface-binding site for HA. With increasing culture confluency and consequent slowing of fibroblast motility, the localization of both polymers changed to a uniform and diffuse distribution over the cell body and processes. On actively motile cells, RITC-HA and HABP did not co-distribute with fibronectin, heparan sulfate proteoglycan or laminin. Areas coated with RITC-HA and HABP often contained specialized adhesion sites as determined by interference reflection microscopy (IRM) but neither polymer appeared to particularly localize to adhesion sites. However, the occurrence of RITC-HA and HABP in the leading lamellae of motile cells consistently coincided with ruffling activity. These results are discussed with respect to a possible instructive role of HA in cell motility.

Loss of hyaluronate-dependent coat during myoblast fusion

Cultured myoblasts were found to exhibit extensive, Streptomyces hyaluronidase-sensitive pericellular coats as revealed by exclusion of particles (fixed red blood cells). These coats are not discernible subsequent to fusion of the myoblasts to form myotubes. The myoblasts contained 2.5 times more hyaluronate attached to their cell surface than myotubes when the data was expressed per unit of protein, but no change in hyaluronate was evident on a per DNA basis. Hyaluronidase activities in the cultures were equivalent when expressed per unit of protein. We conclude that, although the myotubes accumulate larger amounts of protein than myoblasts, there is no compensatory increase in hyaluronate.

Pericellular coat of chick embryo chondrocytes: structural role of hyaluronate

Chondrocytes produce large pericellular coats in vitro that can be visualized by the exclusion of particles, e.g., fixed erythrocytes, and that are removed by treatment with Streptomyces hyaluronidase, which is specific for hyaluronate. In this study, we examined the kinetics of formation of these coats and the relationship of hyaluronate and proteoglycan to coat structure. Chondrocytes were isolated from chick tibia cartilage by collagenase-trypsin digestion and were characterized by their morphology and by their synthesis of both type II collagen and high molecular weight proteoglycans. The degree of spreading of the chondrocytes and the size of the coats were quantitated at various times subsequent to seeding by tracing phase-contrast photomicrographs of the cultures. After seeding, the chondrocytes attached themselves to the tissue culture dish and exhibited coats within 4 h. The coats reached a maximum size after 3-4 d and subsequently decreased over the next 2-3 d. Subcultured chondrocytes produced a large coat only if passaged before 4 d. Both primary and first passage cells, with or without coats, produced type II collagen but not type I collagen as determined by enzyme-linked immunosorbent assay. Treatment with Streptomyces hyaluronidase (1.0 mU/ml, 15 min), which completely removed the coat, released 58% of the chondroitin sulfate but only 9% of the proteins associated with the cell surface. The proteins released by hyaluronidase were not digestible by bacterial collagenase. Monensin and cycloheximide (0.01-10 microM, 48 h) caused a dose-dependent decrease in coat size that was linearly correlated to synthesis of cell surface hyaluronate (r = 0.98) but not chondroitin sulfate (r = 0.2). We conclude that the coat surrounding chondrocytes is dependent on hyaluronate for its structure and that hyaluronate retains a large proportion of the proteoglycan in the coat.

Hyaluronate coat formation and cell spreading in rat fibrosarcoma cells

Hyaluronate-containing pericellular coats have been demonstrated around rat fibrosarcoma cells by exclusion of particles (fixed red blood cells). The cell coats normally form during spreading of the rat fibrosarcoma cells subsequent to subculturing. Monensin, a drug which disrupts the Golgi and which also inhibits hyaluronate synthesis in these cells, inhibits the regeneration of these coats after hyaluronidase or trypsin treatment but does not inhibit cell spreading. Cycloheximide, a drug which inhibits protein but not hyaluronate synthesis does not prevent coat regeneration but partially inhibits cell spreading. Thus by exploiting the opposing effects of cycloheximide and monensin on coat regeneration and cell spreading, we have been able to dissociate these two phenomena.

Deposits of alpha 2M in the rheumatoid synovial membrane

Synovial tissue from patients with rheumatoid arthritis, systemic lupus erythematosus, osteoarthritis, and having menisectomies was examined by immunofluorescence for deposits of alpha-2-macroglobulin (alpha 2M). In inflammed tissues, alpha 2M was found in the synovial lining cells and in perivascular cells. The amount of alpha 2M correlated with the degree of inflammation. Similarly, free lining cells obtained by trypsination of the intact synovial membrane contained identical inclusions. alpha 2M was not detected in the menisectomy cases and in the less inflammatory osteoarthritic specimens. In-vitro studies demonstrated uptake of alpha 2M-trypsin complexes but not of native alpha 2M by most of the cultured synovial cells whether they came from rheumatoid patients or controls. The internalised complexes disappeared within 12 hours of culture. The results suggest that alpha 2M-proteinase complexes formed in the joint are taken up by phagocytic and perivascular cells in a similar way to immune complexes.

Transformation-dependent loss of the hyaluronate-containing coats of cultured cells

The sizes of hyaluronate-containing coats on the surfaces of parent and virus-transformed cell lines (3T3 vs. SV-3T3; BHK vs. PY-BHK) were compared according to the method of Clarris and Fraser (1968, Exp. Cell Res., 49: 181-193) in which fixed red blood cells were allowed to settle slowly on the surface of culture dishes containing the cells. The coats were seen as areas devoid of red blood cells surrounding each of the cultured cells and could be destroyed by the addition of small amounts of streptomyces hyaluronidase, an enzyme specific for hyaluronate. In the case of the parent cell lines (3T3 and BHK), the coats were clearly visible, whereas for their virus-transformed counterparts (SV-3T3 and PY-BHK), the coats were either greatly reduced or absent. To confirm these observations, the amount of hyaluronate associated with each of the cell lines was measured using a direct chemical assay and shown to be significantly greater for the parent cell lines than for their virus-transformed counterparts. In addition, the parent cell lines secreted greater amounts of hyaluronate into the medium and retained a larger fraction of the total amount of hyaluronate at the cell surface than the virus-transformed cells. Thus the larger amount of hyaluronate on the surfaces of the parent cell types may be the result of both a faster rate of production and a decreased rate of release.

Activation of human synovial cells by cholera enterotoxin: correlation of morphological responses with adenylate cyclase activities, and the reversing effects of hyaluronidase

Previously described morphological changes in human synovial cell cultures due to cholera enterotoxin (CT) were studied in relation to activation of adenylate cyclase. A single pulse of CT at nanomolar concentration or less induced at least two-fold activation of adenylate cyclase, which persisted for 7 days or more. The enzyme hyaluronidase was found to cause a rapid reversal of the morphological effects of CT. There was also a reduction in adenylate cyclase activity but only with hyaluronidase concentrations greater than those required to produce maximum reversal of the CT-induced morphological changes. Removal of hyaluronidase was followed by reappearance of the CT-associated morphological effects and a slower reactivation of adenylate cyclase. The mechanism by which hyaluronidase produces the observed changes in synovial cells is not known, but might be related to the dispersal of hyaluronic acid gels bound to the surface of these cells.

Patterns of induced variation in the morphology, hyaluronic acid secretion, and lysosomal enzyme activity of cultured human synovial cells

In contrast with newly isolated cells or early primary cultures, synovial cell lines in standardised growth conditions assume a rather uniform fibroblast-like appearance. However, 2 distinct variations in the cytological pattern can be induced at this stage. The first is characterised primarily by increased numbers of small phase-dense organelles that show the distinctive fluorescence of lysosomes after supravital staining, and are interspersed with vacuoles. The associated functional changes include increased enzyme activity and decreased net synthesis of hyaluronic acid. This variation can be induced by exposure to indigestible neutral sugars, adenosine, or its 5' nucleotides. The second variation consists of a striking reorganisation of cytoplasm by condensation into dense ridges or a dendritic network of processes. It is accompanied by increased hyaluronic acid secretion and is induced by agents that enhance intracellular activity of cyclic adenosine monophosphate, such as dibutyryl cyclic adenosine monophosphate and cholera enterotoxin. It appears possible to direct differentiation in synovial cell lines to correspond at least in part with the presumed functions of the different cell types in the parent tissue. The 2 patterns may be useful markers to correlate with other aspects of synovial cell function in vitro.

Hyaluronidase-sensitive halos around adherent cells. Their role in blocking lymphocyte-mediated cytolysis

A variety of adherent sarcoma, carcinoma and normal cells are surrounded in vitro by thick, transparent zones (approximately equal to 9 micron thick) that spleen cells and a variety of other cells and particles cannot penetrate. Seven lymphoblastoid cell lines did not possess such halos. The presence of these halos around adherent fibrosarcoma cells appeared to protect them from lymphocyte-mediated cytolysis. Hyaluronidase treatment, which destroyed the halo and allowed lymphocytes to approach the tumor cell membrane, enhanced the cytotoxic action of immune but not of normal spleen cells. These observations, in addition to highlighting a little-known feature of the cell surface, may also be of general relevance to the in vitro and in vivo killing of tumor cells by immune effector cells.

Modification of mucin clots by serum

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