Proteoglycans are major components of most tissues and contribute to a multitude of different functions ranging from structural support to cell surface receptors. Most proteoglycan consist of a core protein which is decorated with a variety of sugar chains. The variety in core protein structure and size along with the diverse composition, length, and number of the sugar chains provide an almost unlimited combination of versatile functions. Proteoglycans can be present on the cell surface to assist in cell signaling mechanisms or secreted into the extracellular space where they can participate in modifying the local ECM.
The core proteins can vary in size, composition, and contain functional motifs which participate in binding of other ECM components and cell surface receptors. Furthermore, certain proteases can cleave some core proteins so they perform additional functions. NG2 is a small transmembrane cell surface proteoglycan found on oligodendrocyte precursor cells that can be cleaved by α-secretase to release the extracellular domain into the ECM. The residual cytosolic fragment can be processed further by γ-secretase producing an integral membrane protein fragment and a cytosolic fragment, both can participate in signal transduction. Versican, a common ECM proteoglycan found throughout most mammalian tissues, contains several proteolytic sequences that can be cleaved by a variety of proteases which modify cell surface binding affinities, many of which are implicated in tumor metastases. Aggrecan, versican, neurocan, and brevacan all contain a C-terminal sequence that has a high binding affinity for the glycosaminoglycan (GAG), hyaluronin. This feature allows these proteoglycans to aggregate together forming complex structures which can give cartilage its supple and durable nature. The core protein of perlecan, a major component of the basement membrane, contains five distinct domains including extended IgG-like motif domains and several epidermal growth factor-like motif structures which can mimic ligand response for cell surface receptors. Meanwhile, the small leucine rich proteoglycan, decorin, can interact with collagen fibrils through the GAG chain or the core protein, providing various means to support the ECM. While the variety of core protein structures can provide clues to the function of certain proteoglycans, some of the most dynamic properties belong to the attached GAG sugar chains.
Proteoglycans differ from glycoproteins in the ratio of sugar moieties to protein in molecular mass. While glycoproteins consist of short and often branched sugar chains that decorate a core protein, the major contributors to the molecular mass of proteoglycans are the long, unbranched GAG sugar chains. For example, a fully glycosylated aggrecan proteoglycan can have a molecular mass of over 3,000 kDa while a deglycosylated core protein is about 400 kDa. Most of these GAG chains consist of different repeating disaccharide units of a hexuronic acid and a modified hexosamine linked in a β1-3 glycosidic bond. Hyaluronic acid (HA), a GAG chain without a core protein, consists of a glucuronic acid in a β1-3 bond with a N-acetylglucosamine (GlcA β1-3 NGlcAc) disaccharide subunit while the chondroitin sulfate (CS) disaccharide subunit consists of a glucuronic acid β1-3 N-acetylgalactosamine (GlcA β1-3 NGalAc). Dermatan sulfate (DS) differs slightly from CS through an epimerization the 5-carbon on the hexuronic acid unit (GlcA) to form an iduronic acid (IdoA) as well as the modification of the glycosidic bond from a β1-3 to an α1-3 (IdoA α1-3 NGalAc). Keratin sulfate (KS) consists of a galactose β1-3 N-acetylglucosamine (Gal β1-3 NGlcAc) disaccharide subunit while heparan sulfate (HS) contains glucuronic acid β1-4 N-acetylglucosamine (GlcA β1-4 NGlcAc) or the epimerized GlcA and deacetylated-Nsulfated glucosamine (IdoA α1-4 NGlcSO) subunits. Since KS and HA are beyond the focus of this dissertation, the synthesis and subsequent ECM interactions will be limited in the remainder of the document.