In 1934, Meyer and Palmer isolated a novel, high M r glycosaminoglycan from the vitreous of the eye (1). They showed that this substance contained a hexuronic acid, an amino sugar, and no sulfoesters and proposed the name hyaluronic acid (hyaluronan, HA), 1 from the Greek hyaloid (vitreous) and uronic acid. It took 20 years before Weissmann and Meyer (2) finally established the precise structure of the repeating disaccharide unit of hyaluronic acid (GlcA␤(133)GlcNAc␤ (134)). The number of repeating disaccharides in an HA molecule can exceed 30,000, a M r Ͼ10 7 . MedLine surveys for reports describing the structure, synthesis, degradation, and biology of HA reveal a steadily increasing interest in this biopolymer during the four decades following the determination of its structure: 790 papers published from 1966 to 1975; 2200 from 1976 to 1985; over 3300 from 1986 to 1996. During this time, HA has been identified in virtually every tissue in vertebrates and has achieved widespread use in various clinical applications, most notably and appropriately as an intra-articular matrix supplement (3) and in eye surgery. This period has also seen a transition from the original perception that HA is primarily a passive structural component in the matrix of a few connective tissues and in the capsule of certain strains of bacteria to a recognition that this ubiquitous macromolecule is dynamically involved in many biological processes: from modulating cell migration and differentiation during embryogenesis (4) to regulation of extracellular matrix organization and metabolism (5) to important roles in the complex processes of metastasis, wound healing, and inflammation (6, 7). Further, it is becoming clear that HA is highly metabolically active and that cells focus much attention on the processes of its synthesis and catabolism. For example, the half-life of HA in tissues ranges from 1 to 3 weeks in cartilage (8) to Ͻ1 day in epidermis (9). In this report, we describe recent advances that provide exciting new insights into the biosynthetic side of these metabolic processes. HA BiosynthesisIt is now clear that a single protein utilizes both sugar substrates to synthesize HA (10). The abbreviation HAS, for the HA synthase, has gained widespread support for designating this class of enzymes and should now be accepted as standard nomenclature. Markovitz et al. (11) successfully characterized the HAS activity from Streptococcus pyogenes and discovered the enzyme's membrane localization and its requirements for sugar nucleotide precursors and Mg 2ϩ. Prehm (12) found that elongating HA, made by B6 cells, was digested by hyaluronidase added to the medium and proposed that HAS resides at the plasma membrane. Philipson and Schwartz (13) also showed that HAS activity cofractionated with plasma membrane markers in mouse oligodendroglioma cells. HAS assembles high M r HA that is simultaneously extruded through the membrane into the extracellular space (or to make the cell capsule in the case of bacteria) as glycosaminoglycan synthesis ...

Hyaluronan synthases (HASs) are glycosyltransferases that catalyze polymerization of hyaluronan found in vertebrates and certain microbes. HASs transfer two distinct monosaccharides in different linkages and, in certain cases, participate in polymer transfer out of the cell. In contrast, the vast majority of glycosyltransferases form only one sugar linkage. Although our understanding of HAS biochemistry is still incomplete, very good progress has been made since the first genetic identification of a HAS in 1993. New enzymes have been discovered, and some molecular details have emerged. Important findings are the lipid dependence of Class I HASs, the function of HASs as protein monomers, and the elucidation of mechanisms of synthesis by Class II HAS. We propose three classes of HASs based on differences in protein sequences, predicted membrane topologies, potential architectures, mechanisms, and direction of polymerization.

Rat liver sinusoidal endothelial cells (LECs) express two hyaluronan (HA) receptors, of 175 and 300 kDa, responsible for the endocytic clearance of HA. We have characterized eight monoclonal antibodies (mAbs) raised against the 175-kDa HA receptor partially purified from rat LECs. These mAbs also cross-react with the 300-kDa HA receptor. The 175-kDa HA receptor is a single protein, whereas the 300-kDa species contains three subunits, ␣, ␤, and ␥ at 260, 230, and 97 kDa, respectively (Zhou, B., Oka, J. A., and Weigel, P. H. (1999) J. Biol. Chem. 274, 33831-33834). The 97-kDa subunit was not recognized by any of the mAbs in Western blots. Based on their cross-reactivity with these mAbs, the 175-, 230-, and 260-kDa proteins appear to be related. Two of the mAbs inhibit 125 I-HA binding and endocytosis by LECs at 37°C. All of these results confirm that the mAbs recognize the bone fide LEC HA receptor. Indirect immunofluoresence shows high protein expression in liver sinusoids, the venous sinuses of the red pulp in spleen, and the medullary sinuses of lymph nodes. Because the tissue distribution for this endocytic HA receptor is not unique to liver, we propose the name HARE (HA receptor for endocytosis). HA1 is an important and often abundant extracellular matrix component of all tissues, in particular cartilage, skin, and vitreous humor (1). HA plays a key role in development, morphogenesis, and differentiation, in cell adhesion and proliferation, and in inflammation and wound healing (1-4). In humans the total body turnover of HA is several grams per day (1). Although local turnover of HA occurs in avascular tissues, particularly cartilage (5, 6), two major clearance systems are responsible for HA degradation and removal in the body (4). The first is the lymphatic system, which accounts for ϳ85% of the HA turnover, and the second is in the liver, which accounts for the other ϳ15% of the total body HA turnover. Throughout the body, HA is continuously synthesized and degraded in almost all tissues. At the same time, chondroitin sulfate and other glycosaminoglycans are also released from the cleavage of proteoglycans, especially aggregating proteoglycans associated with HA. Large native HA molecules (ϳ10 7 Da) are partially degraded to large fragments (ϳ10 6 Da) that are released from the matrix and enter the lymphatic system, flowing to lymph nodes.The lymph nodes completely degrade the majority of HA (ϳ85%) by unknown mechanisms. Neither the responsible cell type, the receptor involved, nor the location in lymph nodes at which HA uptake and degradation occurs has been determined. The remaining HA (ϳ15%) that escapes degradation in the lymph nodes ultimately flows into the blood at the thoracic duct. Since HA is an exceptionally viscous polysaccharide in solution, it would be deleterious for the blood concentration of HA, even at relatively low molecular weight, to increase. Clearance of this circulating HA and the other glycosaminoglycan degradation fragments is presumably important for normal health (1, 4). Elevate...

The hasA gene from Streptococcus equisimilis, which encodes the enzyme hyaluronan synthase, has been expressed in Bacillus subtilis, resulting in the production of hyaluronic acid (HA) in the 1-MDa range. Artificial operons were assembled and tested, all of which contain the hasA gene along with one or more genes encoding enzymes involved in the synthesis of the UDP-precursor sugars that are required for HA synthesis. It was determined that the production of UDP-glucuronic acid is limiting in B. subtilis and that overexpressing the hasA gene along with the endogenous tuaD gene is sufficient for high-level production of HA. In addition, the B. subtilis-derived material was shown to be secreted and of high quality, comparable to commercially available sources of HA.

No abstract

No abstract

The two hyaluronan synthases (HASs) from Streptococcus pyogenes (spHAS) and Streptococcus equisimilis (seHAS) were expressed in Escherichia coli as recombinant proteins containing His 6 tails. Both enzymes were expressed as major membrane proteins, accounting for ϳ5-8% of the total membrane protein. Using nickel chelate affinity chromatography, the HASs were purified to homogeneity from n-dodecyl ␤-D-maltoside extracts. High levels of HAS activity could be achieved only if the purified enzymes were supplemented with either bovine or E. coli cardiolipin (CL), although bovine CL gave consistently greater activity. Mass spectroscopic analysis revealed that the fatty acid compositions of these two CL preparations did not overlap. The two HAS enzymes showed similar but distinct activation profiles with the 10 other lipids tested. For example, phosphatidic acid and phosphatidylethanolamine stimulated seHAS, but not spHAS. Phosphatidylserine stimulated both enzymes. spHAS appears to be more CL-specific than se-HAS, although both purified enzymes still contain endogenous CL that can not easily be removed. Both seHAS and spHAS were inhibited by phosphatidylcholine, sphingomyelin, and sulfatides and were not substantially stimulated by cerebrosides, phosphatidylglycerol, or phosphatidylinositol. With both HASs, CL increased the K m for UDP-GlcUA, but decreased the K m for UDP-GlcNAc and gave an overall stimulation of V max . A kinetic characterization of the two membrane-bound and purified HASs is presented in the accompanying paper (Tlapak-Simmons, V. L., Baggenstoss, B. A., Kumari, K., Heldermon, C., and Weigel, P. H. (1999) J. Biol. Chem. 274, 4246 -4253). Both purified HASs became inactive after storage for ϳ5 days at 4°C. Both purified enzymes also lost activity over 4 -5 days when stored at -80°C in the presence of CL, but reached a level of activity that then slowly decreased over a period of months. Although the purified enzymes stored in the absence of CL at ؊80°C were much less active, the enzymes retained this same low level of activity for at least 5 weeks. When both spHAS and seHAS were stored without CL at ؊80°C, even after 2 months, they could be stimulated by the addition of bovine CL to ϳ60% of the initial activity of the freshly purified enzyme.Since the discovery of HA 1 over 60 years ago (1), this saccharide polymer, which contains repeating disaccharide units of GlcUA␤(1,3)GlcNAc␤(1,4), has been shown to have numerous biological functions. For example, HA provides the viscous lubrication of synovial fluid in joints and provides cartilage with its viscoelastic properties. HA is involved in a wide variety of cellular functions and behaviors, including cell migration (2, 3) development (4 -6), differentiation (7-9), phagocytosis (6), and proteoglycan synthesis (2, 4). As well as being a major structural component of the matrix, HA has wound healing, pharmaceutical, and analgesic effects (10 -14) and is also being used as a vehicle for drug delivery (15,16).Although cell-free HA biosynthesis was achieved ...

The hyaluronic acid (HA) receptor for endocytosis (HARE; also designated stabilin-2 and FEEL-2) mediates systemic clearance of glycosaminoglycans from the circulatory and lymphatic systems via coated pit-mediated uptake. HARE is primarily found as two isoforms ( The glycosaminoglycan (GAG)2 hyaluronic acid (HA) is a protein-free polymer of disaccharide units containing glucuronic acid and N-acetylglucosamine (1, 2). HA is involved in many physiological processes (3), such as wound healing, development, and metastasis of some cancers (4 -8). The typical molecular mass of the polysaccharide ranges from just a few thousand Da (tens of sugars) that are thought to be important in cellular signaling (6) to several million Da (tens of thousands of sugars). These larger forms of HA are present throughout the body and are particularly concentrated within the bursa of major joints, such as the knee, where they help to provide shock absorbance in cartilage or lubrication in synovial fluid (9, 10), and the eye, where HA maintains structural integrity of the vitreous humor (11). The adult human body contains ϳ15 g of HA, of which about 5 g are turned over daily (12). Partially degraded HA perfuses from extracellular matrices (ECMs) and enters the lymphatic and vascular circulation systems, where it is catabolized to shorter fragments. This active maintenance of HA turnover must be efficient in order to maintain homeostatic conditions for total body HA.All of the other GAGs, including the chondroitin sulfates (CSs), heparan sulfate (HS), and keratan sulfate, are linked to core proteins (as proteoglycans) that help to form ECMs, such as the basement membranes of tissues, or structural components of organs, such as the vitreous humor. There are over 30 known core proteins that are essential for a diverse array of functions, such as neural development, growth factor signaling, and pathogen recognition (13). These core proteins are found as prevalent components of tissue ECMs or as specialized components needed for the development of microenvironments that interface a specialized tissue cell type with the ECM. Both the proteoglycans and their attached GAG chains may combinatorially interact with ligands and contribute to modulation of the functional aspects of a particular microenvironment (e.g. CS interacting with apolipoprotein E for uptake of ␤-very low density lipoprotein in hippocampal neurons) (14). Although numerous studies have focused on how the inhibition of some CS proteoglycans enhances neural development, especially in injured spinal cord models, there is very little information on how CS and HS are catabolized. The current model is that extracellular chondroitinases, heparinases, and proteases initially break down these GAGs and proteoglycans, and their final digestion can then take place intracellularly at the local tissue * This research was supported by NIGMS, National Institutes of Health, Grant GM69961. The costs of publication of this article were defrayed in part by the payment of page charges. This article ...