main variations detected are in the molecular weight (MW) of the polymers. In the body, most of the HA is in the form of salt, reaching high concentrations in connective tissues such as skin, synovial fluid, and vitreous humor. Adult skin accounts for approximately 50% of the total body HA. Most of the cutaneous
HA is localized in the dermis, reaching concentrations of 0.5 mg/kg; while epidermal HA has been estimated to be around 0.1 mg/kg. Due to its rheological properties, cutaneous HA modulates the overall skin quality, hydration, permeability, and immune barrier function. In addition, its unique viscoelastic
properties provide skin cells protection from mechanical damage, increasing cell survival (response to injury and wound healing) and promoting proliferation, migration, signal transduction,
and immune surveillance.3-6
HYALURONIC ACID STRUCTURE AND PROPERTIES
Hyaluronic acid is a carbohydrate that is synthetized as a large linear polymer of alternating repeating disaccharide units composed of glucuronic acid and N-acetylglucosamine. These saccharides are linked together through alternating beta-1,4 and beta-1,3 glycosidic bonds (Figure 1a). The number of repetitions
in a complete HA molecule can reach close to 10,000 or more with a molecular mass of about 4 million daltons (Da) with an average length of 1 nm that may reach 10 μm if stretched.
Hyaluronan – a term that encompasses the different forms of this carbohydrate such as acid (HA) and salts (hyaluronates) – forms particularly stable tertiary structures in aqueous solution with remarkable hydrodynamic properties, including non-Newtonian viscosity and water retention. HA solutions show very unusual rheological properties as well as high lubricious and hydrophilic properties. Structural studies have shown that in solution HA polymer chains form expanded random coils that, at lower concentrations,
entangle with each other, trapping 1,000 times their weight in water during the process.7 At elevated concentrations, HA solutions are sheer-thinning, which allows these gel-like solutions
to flow easily when under pressure (ie, through a needle).
HYALURONIC ACID SYNTHESIS AND DEGRADATION: KEEPING THE BALANCE
Hyaluronan can bind and hold large amounts of moisture (approximately
6 liters of water per gram of HA); therefore, young or youthful skin, which is well hydrated, contains large amounts of HA. Endogenous HA in skin exhibits a high turnover rate. It has been estimated that HA half-life in the dermis is < 1 day,8,9 while in the epidermis is only about 2 to 3 hours.10 The net amount of HA in the skin is regulated at different levels: synthesis,
deposit, association with hyaladherin, or other components of the extracellular matrix (ECM), and degradation. While most of the glycosaminoglycans (GAGs) are synthetized in the Golgi apparatus, HA is mainly synthetized at the cell surface by a set of enzymes named hyaluronic acid synthases (HAS), a class of membrane-integrated glycosyltransferases (Figure 1).
Although it has been suggested that HA-synthesis also occurs in the cytosol, the role and significance of this unique pathway is yet to be determined.11 The extension of the HA polymer occurs
while extruding through the plasma membrane. Recent data suggest the dimerization in parallel orientation of HAS on the cell surface12 and/or the potential formation of multi-enzyme constructs that may form pores for the extraction of the HA molecules.
10 There are 3 HAS isoforms (HAS1, HAS2, and HAS3), and they have different tissue and cell-specific expression patterns and Km values.13 HASs incorporate uridine diphosphate (UDP) sugars into the non-reducing end of the growing sugar chains, forming different sizes of polysaccharide chains (Figure 1). HAS1 and HAS2 produce chains of 2-4 x 106 Da, whereas HAS3 synthesizes
shorter chains (0.4-2.5 x 105 Da). HAS enzymes activities depend on post-transcriptional modifications such as ubiquitination,
phosphorylation, and N-glycosylation.14 Transcription of HAS genes is stimulated by growth factors including epidermal growth factor, platelet-derived growth factors, and transforming growth factors-beta (TGF-β). In vitro studies suggest that HAS2 is the main HA synthase in dermal fibroblasts and responsible for the bulk of dermal HA production. HAS1, the activity of which depends on elevated intracellular levels of UDP, is the least active of these enzymes under physiological conditions, but plays a key role in HA synthesis during inflammation and glycemic stress15,16 as well as after TGF-β stimulation.17 HAS2 and HAS3 show comparable
levels of activity in keratinocytes In vitro.
Degradation of extracellular HA is initiated by the release of HA from its interaction with ECM component or hyaladherins, which strongly links the decrease in skin HA levels to aging and photo-aging. Enzymatic catabolism of HA is achieved by a family of enzymes called hyaluronidases (HYAL). For a long time the HYAL family received little attention due mainly to the technical difficulties
in its isolation, purification, and stabilization. Seven HYAL have been described, although differential enzymatic activity has not been determined for all of them. HYAL mechanism of action is associated with the hydrolysis of the hexosaminidic β (1-4) linkages between N-acetyl-D-glucosamine and D-glucuronic acid residues in HA (Figure 1). HYAL1 and HYAL2 are considered the more active members of the HYAL family, generating tetra-saccharides and HA fragments of 10-20 x 103 Da, respectively. HYAL1 is a lysosomal (acidic) active enzyme, while HYAL2 is a GPI-anchored plasma membrane protein that can be also localized
in the lysosomes.18,19 The role of HYAL-3 in HA degradation
