Betaglycan/TGFβ type III receptor
The betaglycan belongs to the HSPG and contains two sites for the attachment of GAG chains and have a molecular mass of about 93 kDa. Betaglycan is member of TGF-β superfamily of co-receptors also known as TGF-β type III receptor (TGFB3). It has a single-pass transmembrane topology (Figure 11). The extracellular domain contains site for GAG attachment at 534 and 545 position and protease-sensitive sequences near the transmembrane domain (Figure 11). The GAG chains are important for its proper function and the TGFβ downstream signaling. The GAG chains of betaglycan regulate the cell migration by interaction with other co-receptors in the ECM (Mythreye and Blobe, 2009b). However, the mechanism by which betaglycan GAG modifications regulate cell migration is still unknown. The extracellular domain also contains a cleavage site for proteases which release the soluble form of betaglycan. The soluble form downregulate TGFβ signaling and suppress the migration of the breast cancer cell lines (Elderbroom et al., 2014). The short intracellular domain contains many serine and threonine residues which are phosphorylated by PKC (López-Casillas et al., 1991). The intracellular domain also contains PDZ-binding domain. The betaglycan regulate cell migration by interaction with β-arrestin2 via the PDZ domain and mediate activation of Cdc42 (Mythreye and Blobe, 2009b). The betaglycan knockout (Tgfbr3-/- ) mice showed embryonic lethality (Sarraj et al., 2013).
Chondroitin sulfate proteoglycans (CSPG)
Decorin belongs to a multifunctional family of PGs collectively designated as small leucine-rich PGs (SLRPs). Decorin core protein molecular mass is around 40 kDa. It contains only one GAG chain either CS or DS. The CS-type GAG chains are mostly present in cartilage, while DS-type GAG chains present in other tissues.
The core protein contains four domains. The first domain or N-terminal domain consists of a signal peptide (14-amino acids) and a propeptide (16-amino acids) and this part is cleaved before secretion of the protein. The second domain contains several cysteine residues and DEASGIG motif (Ser34) for attachment of GAG chain. The third domain contains leucine-rich repeats consisting of 10 repeats of 24 amino acids rich in leucine and three sites for the N-linked glycosylation (Asn211, Asn262 and Asn303). The fourth domain or C-terminal domain contains two cysteine residues and a conserved disulfide loop (Figure 12).
Figure 12:- Molecular structure of decorin. The decorin has four domains containing leucine rich repeats and one GAG attachment site. It also contains three N-linked glycosylation sites (Järveläinen et al., 2015).
The core protein interacts with type I and type II collagens and also with various growth factors (Figure 13). The soluble decorin is present in the normal human plasma and its secretion is increased in patients with diabetes and septicemia (Bolton et al., 2008). However, the secretion of decorin is reduced in various disease conditions such as in patients suffering acute ischemic stroke and esophageal squamous cell carcinomas (Wu et al., 2010).
Fibroblasts and myofibroblasts produce a large amount of decorin which is secrete into the ECM as soluble form. Decorin is important in collagen fibers assembly and it decorate the collagen fibers. It is involved in various physiological and pathological conditions associated with dysfunctions of the ECM. If decorin is not secreted as in decorin KO mice, collagen fibers become thinner however in the presence of decorin without GAG chains, collagen fibers become thicker (Seidler, 2012). It is produced by human dermal fibroblasts and is major PGs expressed in dermal ECM. The young skin contains longer decorin-GAG chains compared to the aged groups. This shows the variation of the structure of decorin with age (Li et al., 2013).
Figure 13:- Interaction of decorin with collagen fibers. The decorin stabilizes the collagen fibers and in its absence the collagen fibers become thin, while the lack of GAG chain on decorin led to thicker collagen fibers (Seidler, 2012).
The soluble form of decorin acts as endogenous pan-receptor tyrosine kinase (RTK) inhibitor. It downregulate tumor development, migration, and angiogenesis. The decorin core protein interacts with RTK members and influences their downstream signaling. Treatment with decorin inhibits the metastatic spreading and growth of breast carcinoma xenografts in mice. The transcriptome profiling of tumor after the treatment with decorin revealed upregulation of paternally expressed gene 3 (PEG3) which is not originally expressed in tumors. The treatment of decorin showed higher PEG3 expression leading to upregulation of the autophagic proteins like Beclin 1 (BECN1) and light chain 3 (LC3) in the vascular endothelial cells. The PEG3, BECN1 and LC3 form complex and induce the autophagy (Buraschi et al., 2013). Hence, it is suggested that decorin acts as an appetite for autophagy of vascular endothelial cells (Figure 14) (Neill et al., 2013).
Decorin interacts with kinase death receptor (KDR) to stimulate PIK3 dependent activation of LC3, BECN1 and PEG3 complex. This complex activate autophagy directly or enhance the transcription of BECN1 and LC3 to initiate autophagy. However, the siRNA against KDR and PEG3 blocks decorin dependent autophagy (Figure 13). Decorin attenuates the EGFR and suppresses downstream anti-autophagic effectors (Akt, mTOR, p70S6K), hence, stimulate autophagy markers like Bcl-2, BECN1, and LC3. The decorin also enhance the mitophagy by activating the Met receptor (Neill et al., 2014).
The overexpression of decorin in the human alveolar adenocarcinoma (A549) cells has antiproliferative effect. It produces accumulation of cells at G1 phase and counteract the TGF-β1 effects and phosphorylation of EGFR. It increases the expression of the P53 and P21 (Liang et al., 2013). Recently, it has been shown that the expression of decorin is reduced in the Non-small cell lung cancer (NSCLC) tissues and the reduction is proportional to the size, stage and metastasis of tumor. Decorin inhibits tumor metastasis by downregulation the E-cadherin in NSCLC cells (Shi et al., 2014).
The soluble form of decorin interacts with various growth factors and cytokines receptors. This includes transforming growth factor (TGF-β1) receptor, epidermal growth factor receptor (EGFR), the insulin-like growth factor receptor I (IGF-IR), and the hepatocyte growth factor receptor (Met). Therefore, decorin influence their intracellular signaling pathways. Both the decorin core protein and GAG chain are important in decorin mediated signaling, but some interactions are mediated by either decorin core protein or decorin GAG chain. Hence, it is suggested that decorin acts as guardian for the matrix (Figure 15).
Decorin also plays an important role in the mineralization of the human smooth muscle cells (VSMC). Indeed, it has been reported that the oxidized low density lipoprotein (Ox-LDL) induced mineralization of VSMC is dependent on the decorin core protein and the CS/DS GAG chain. Treatment of VSMC with Ox-LDL results in upregulation of XT-I (the enzyme that initiate the GAG synthesis) and Msx2 (marker of mineralization). It is also responsible for the activation of TGF-β dependent Smad signaling pathway to enhance the mineralization. The inhibition of XT-I suppressed decorin induced mineralization (Yan et al., 2011).
Decorin GAG chain interact with collagen fibrils and play a crucial role in matrix assembly and in the pathophysiology of angiogenesis. Decorin has an important role in angiogenesis and tumorogensis. It either promote or inhibit angiogenesis. But, it is interesting that it has inhibitory effect on tumor associated angiogenesis. Therefore, decorin based therapies have positional role in controlling tumor growth, metastasis and angiogenesis (Bi and Yang, 2013). Taken together, decorin emerged as potential therapeutic target for various tumors (Figure 16).
The biglycan belongs to the CSPG, SLRP class 1, with core protein of about 42 kDa and contains two GAG chains (Figure 17). The chondroitin or dermatan sulfate GAG chains are attached on the N-terminal. It is synthesized as precursor and BMP-I cleaves the N-terminal propeptide to release the active form. Biglycan interacts with many ECM components such as decorin, it also interacts with various growth factors and cytokines like TGF, BMP and TNF-α.
Biglycan has an important role in the signaling pathways because it act as ligand for Toll-like receptors (TLRs)-2 and -4 (Schaefer et al., 2005), P2X7/P2X4 receptors, low-density lipoprotein receptor-related protein 6 (LRP6) and receptor tyrosine kinase (MuSK ) (Amenta et al., 2012; Nastase et al., 2012). Biglycan also has an important role in the innate immunity and inflammation. Its expression is upregulated in the pulmonary inflammation and asthma. It stimulates the canonical Wnt/β-catenin signaling (Berendsen et al., 2011; Wang et al., 2015), BMP and TGFβ signaling pathways in osteoprogenitor cells to enhance the bone formation (Figure 18) (Nastase et al., 2012).
CSPG4/NG2 is a CSPG containing one CS/DS chain. The core protein of CSPG4/NG2 is about 250 kDa with a type I transmembrane topology (Figure 19). The large extracellular domain is subdivided into three subdomains, the N-terminal domain (D1 subdomain) contains two laminin-like globular (LG) repeats, the central domain (D2 subdomain) contains 15 tandem repeats and have site for the CS attached GAG chains and the juxtamembrane domain (D3 subdomain) have several sites for the protease cleavage. The LG repeats in the D1 region interact with integrins and RTKs to mediate the cell–matrix, cell–cell and cell-ligand interactions. The CS GAG chain in the D2 region is also important for cell-matrix interaction and interacts with collagen fibres. The D3 binds with integrins and galectin (Price et al., 2011). A small transmembrane domain followed by the intracellular domain. The intracellular domain contains the PDZ domain for the binding of the PDZ family of proteins. Intracellular domain is important for the phosphorylation by PKCα and ERK1/2 kinases (Iozzo and Schaefer, 2015; Price et al., 2011).
Table of contents :
1.1 Glycosaminoglycans (GAGs) chains
1.2 Classification of the PGs
1.3 Heparan sulfate proteoglycans (HSPG)
1.3.1 HS synthesis
1.3.5 Betaglycan/TGFβ type III receptor
1.4 Chondroitin sulfate proteoglycans (CSPG)
2. Biosynthesis of GAG chains
2.2 Synthesis of the tetrasaccharide linker
2.3 Elongation of the CS chains
2.4 Elongation of the HS Chains
2.4 Regulation of PG synthesis
3.1 Specificity of the xylosyltransferase
3.2 Structure of the XT-I and XT-II
3.4 Structural/Functional relationship
3.4 Purification of the XT from tissue source
3.5 Purification of the XT from cellular source as cDNA cloning
3.6 Intracellular localization
3.7 XT activity analysis
3.8 XT in health and disease
3.9 XT-I and XT-II genetic variants
3.10 Regulation of Xylosyltransferase
4.1 Structure of Fam20B
4.4 Interaction of phosphoxylose with GalT-I, GalT-II, GlcAT-I, EXTL2 and ChGn-I
5.1 GAG priming xylosides
5.2 GAG inhibiting xylosides