IntroductionHyaluronan, or hyaluronic acid (HA) is a linear, highmolecular-weight (mega-Dalton) polymer comprised of repeating disaccharide units of (β1→3)D-glucuronate-(β1→4)N-acetyl-D-glucosamine. HA is synthesized by integral plasma membrane glycosyltransferases and is exported directly into the extracellular space (1, 2). Although HA is chemically homogeneous, there are three distinct mammalian HA synthases (designated Has1, Has2, and Has3), encoded by related but nonlinked genes (3-9). Each synthase has distinct catalytic properties, and the distribution and abundance of each varies during development of the mouse (6, 10). These observations suggest that the different Has enzymes play distinct roles.HA binds salt and water, expanding the extracellular space (11)(12)(13)(14). HA is especially prominent at sites where cell migration occurs, such as pathways of neural crest cell migration and in the developing cardiovascular system. In vivo, HA interacts with other extracellular matrix molecules, typically via an HA-binding domain called the link module (15). These interactions create a supramolecular architecture of the extracellular matrix, i.e., the composite matrix network of HA, link protein, and aggrecan that plays a critical role in load-bearing articular cartilage (16)(17)(18).In addition to its important physical properties, the overexpression of Has genes results in increased anchorage-independent growth and metastasis of transformed cells (19,20), suggesting a link between HA and transformation. HA is also implicated in receptor-mediated cell adhesion and intracellular signaling (21,22). Taken together, such observations suggest that HA plays a vital role in diverse cellular events, including cell migration, tissue remodeling, and metastasis. However, the near-ubiquitous distribution of HA in vivo, the biological activity of HA fragments released by degradative enzymes (23), and the inability to inhibit HA synthesis in vivo have hindered definitive analysis of the physiological roles of HA. Accordingly, we used a genetic approach to investigate the roles of HA in vivo and to identify the HA synthase that is critical during embryogenesis.Expression of Has2 appeared to correlate with expansion of cardiac cushion tissue and subsequent transformation of endocardial cells into mesenchyme. The tar- We identified hyaluronan synthase-2 (Has2) as a likely source of hyaluronan (HA) during embryonic development, and we used gene targeting to study its function in vivo. Has2 -/-embryos lack HA, exhibit severe cardiac and vascular abnormalities, and die during midgestation (E9.5-10). Heart explants from Has2 -/-embryos lack the characteristic transformation of cardiac endothelial cells into mesenchyme, an essential developmental event that depends on receptor-mediated intracellular signaling. This defect is reproduced by expression of a dominant-negative Ras in wild-type heart explants, and is reversed in Has2 -/-explants by gene rescue, by administering exogenous HA, or by expressing activated Ras. Conversely, ...

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Hyaluronan is particularly attractive for tissue engineering and repair because it: (1) is a normal component of the extracellular matrices of most mammalian tissues; (2) contributes to the biological and physical functions of these tissues; and (3) possesses excellent biocompatibility and physiochemical properties. In the present study, we characterize a two-step enzymatic cross-linking chemistry for production of tyramine-based hyaluronan hydrogels using fluorophore-assisted carbohydrate electrophoresis, enzymatic digestion, and spectroscopy including absorbance, fluorescence and (1)H NMR. Substitution on hyaluronan of tyramine and other adducts from unproductive side reactions depends on the molar ratio of tyramine to carbodiimide used during the substitution (step 1) reaction. Results indicate that relatively low tyramine substitution is required to form stable hydrogels, leaving the majority of hyaluronan disaccharides unmodified. Sufficient native HA structure is maintained to allow recognition and binding by b-HABP, a HA binding complex typically found in normal cartilage biology. Hydrogels were formed from tyramine-substituted hyaluronan through a peroxidase-dependent cross-linking (step 2) reaction at hyaluronan concentrations of 2.5 mg/ml and above. Uncross-linked tyramine-substituted hyaluronan was characterized after hyaluronidase SD digestion. Cross-linked hydrogels showed increased resistance to digestion by testicular hyaluronidase and hyaluronidase SD with increasing hyaluronan concentration. Cells directly encapsulated within the hydrogels during hydrogel cross-linking remained metabolically active during 7 days of culture similar to cells cultured in monolayer.

Hyaluronan and chondroitin/dermatan sulfate are glycosaminoglycans that play major roles in the biomechanical properties of a wide variety of tissues, including cartilage. A chondroitin/dermatan sulfate chain can be divided into three regions: (1) a single linkage region oligosaccharide, through which the chain is attached to its proteoglycan core protein, (2) numerous internal repeat disaccharides, which comprise the bulk of the chain, and (3) a single nonreducing terminal saccharide structure. Each of these regions of a chondroitin/dermatan sulfate chain has its own level of microheterogeneity of structure, which varies with proteoglycan class, tissue source, species, and pathology. We have developed rapid, simple, and sensitive protocols for detection, characterization and quantitation of the saccharide structures from the internal disaccharide and nonreducing terminal regions of hyaluronan and chondroitin/dermatan sulfate chains. These protocols rely on the generation of saccharide structures with free reducing groups by specific enzymatic treatments (hyaluronidase/chondroitinase) which are then quantitatively tagged though their free reducing groups with the fluorescent reporter, 2-aminoacridone. These saccharide structures are further characterized by modification through additional enzymatic (sulfatase) or chemical (mercuric ion) treatments. After separation by fluorophore-assisted carbohydrate electrophoresis, the relative fluorescence in each band is quantitated with a cooled, charge-coupled device camera for analysis. Specifically, the digestion products identified are (1) unsaturated internal Deltadisaccharides including DeltaDiHA, DeltaDi0S, DeltaDi2S, DeltaDi4S, DeltaDi6S, DeltaDi2,4S, DeltaDi2,6S, DeltaDi4,6S, and DeltaDi2,4,6S; (2) saturated nonreducing terminal disaccharides including DiHA, Di0S, Di4S and Di6S; and (3) nonreducing terminal hexosamines including glcNAc, galNAc, 4S-galNAc, 6S-galNAc, and 4, 6S-galNAc.