Biglycan evokes autophagy in macrophages via a novel CD44/Toll-like receptor 4 signaling axis in ischemia/reperfusion injury.

Biglycan, a small leucine-rich proteoglycan, acts as a danger signal and is classically thought to promote macrophage recruitment via Toll-like receptors (TLR) 2 and 4. We have recently shown that biglycan signaling through TLR 2/4 and the CD14 co-receptor regulates inflammation, suggesting that TLR co-receptors may determine whether biglycan-TLR signaling is pro- or anti-inflammatory. Here, we sought to identify other co-receptors and characterize their impact on biglycan-TLR signaling. We found a marked increase in the number of autophagic macrophages in mice stably overexpressing soluble biglycan. In vitro, stimulation of murine macrophages with biglycan triggered autophagosome formation and enhanced the flux of autophagy markers. Soluble biglycan also promoted autophagy in human peripheral blood macrophages. Using macrophages from mice lacking TLR2 and/or TLR4, CD14, or CD44, we demonstrated that the pro-autophagy signal required TLR4 interaction with CD44, a receptor involved in adhesion, migration, lymphocyte activation, and angiogenesis. In vivo, transient overexpression of circulating biglycan at the onset of renal ischemia/reperfusion injury (IRI) enhanced M1 macrophage recruitment into the kidneys of Cd44+/+ and Cd44-/- mice but not Cd14-/- mice. The biglycan-CD44 interaction increased M1 autophagy and the number of renal M2 macrophages and reduced tubular damage following IRI. Thus, CD44 is a novel signaling co-receptor for biglycan, an interaction that is required for TLR4-CD44-dependent pro-autophagic activity in macrophages. Interfering with the interaction between biglycan and specific TLR co-receptors could represent a promising therapeutic intervention to curtail kidney inflammation and damage.

Immune-Intrinsic Myd88 Directs the Production of Antibodies With Specificity for Extracellular Matrix Components in Primary Sjögren's Syndrome.

Primary Sjögren's syndrome is an autoimmune disease that is predominantly seen in women. The disease is characterized by exocrine gland dysfunction in combination with serious systemic manifestations. At present, the causes of pSS are poorly understood. Pulmonary and renal inflammation are observed in pSS mice, reminiscent of a subset of pSS patients. A growing body of evidence indicates that inflammation mediated by Damage-Associated Molecular Patterns (DAMPs) contributes to autoimmunity, although this is not well-studied in pSS. Degraded extracellular matrix (ECM) constituents can serve as DAMPs by binding pattern-recognition receptors and activating Myd88-dependent signaling cascades, thereby exacerbating and perpetuating inflammatory cascades. The ECM components biglycan (Bgn) and decorin (Dcn) mediate sterile inflammation and both are implicated in autoimmunity. The objective of this study was to determine whether these ECM components and anti-ECM antibodies are altered in a pSS mouse model, and whether this is dependent on Myd88 activation in immune cells. Circulating levels of Bgn and Dcn were similar among pSS mice and controls and tissue expression studies revealed pSS mice had robust expression of both Bgn and Dcn in the salivary tissue, saliva, lung and kidney. Sera from pSS mice displayed increased levels of autoantibodies directed against ECM components when compared to healthy controls. Further studies using sera derived from conditional knockout pSS mice demonstrated that generation of these autoantibodies relies, at least in part, on Myd88 expression in the hematopoietic compartment. Thus, this study demonstrates that ECM degradation may represent a novel source of chronic B cell activation in the context of pSS.

Serum amyloid A-containing HDL binds adipocyte-derived versican and macrophage-derived biglycan, reducing its antiinflammatory properties.

The ability of HDL to inhibit inflammation in adipocytes and adipose tissue is reduced when HDL contains serum amyloid A (SAA) that is trapped by proteoglycans at the adipocyte surface. Because we recently found that the major extracellular matrix proteoglycan produced by hypertrophic adipocytes is versican, whereas activated adipose tissue macrophages produce mainly biglycan, we further investigated the role of proteoglycans in determining the antiinflammatory properties of HDL. The distributions of versican, biglycan, apolipoprotein A1 (the major apolipoprotein of HDL), and SAA were similar in adipose tissue from obese mice and obese human subjects. Colocalization of SAA-enriched HDL with versican and biglycan at the cell surface of adipocyte and peritoneal macrophages, respectively, was blocked by silencing these proteoglycans, which also restored the antiinflammatory property of SAA-enriched HDL despite the presence of SAA. Similar to adipocytes, normal HDL exerted its antiinflammatory function in macrophages by reducing lipid rafts, reactive oxygen species generation, and translocation of Toll-like receptor 4 and NADPH oxidase 2 into lipid rafts, effects that were not observed with SAA-enriched HDL. These findings imply that SAA present in HDL can be trapped by adipocyte-derived versican and macrophage-derived biglycan, thereby blunting HDL's antiinflammatory properties.

Biglycan is a new high-affinity ligand for CD14 in macrophages.

Sterile inflammation is a therapeutic target in many diseases where it represents an important initiator of disease progression. However, the detailed mechanisms underlying its evolution and biological relevance are not yet completely elucidated. Biglycan, a prototype extracellular matrix-derived damage-associated molecular pattern, mediates sterile inflammation in macrophages through Toll-like receptor (TLR) 2 and/or TLR4-dependent signaling pathways. Here we discovered that soluble biglycan is a novel high-affinity ligand for CD14, a well-known GPI-anchored co-receptor for TLRs. CD14 is required for all biglycan-mediated TLR2/4 dependent inflammatory signaling pathways in macrophages. By binding to CD14 and choosing different TLR signaling branches, biglycan induced TNF-α and CCL2 via TLR2/4, HSP70 through TLR2, and CCL5 via TLR4. Mechanistically, biglycan evoked phosphorylation and subsequent nuclear translocation of p38, p44/42, and NF-κB, and these effects were due to a specific, high-affinity interaction between biglycan protein core and CD14. Finally, we provide proof-of-principle for the requirement of CD14, by transiently overexpressing biglycan in a mouse model of renal ischemia/reperfusion injury performed in Cd14-/- mice. Lack of Cd14 prevented biglycan-mediated cytokine expression, recruitment of macrophages, M1 macrophage polarization as well as mitigated the tubular damage and serum creatinine levels, thereby improving renal function. Thus, CD14 inhibition could lead to the reduction in the activation of biglycan-TLR2/4 signaling pathways and could be a novel therapeutic approach in inflammatory kidney diseases.

Small Leucine-Rich Proteoglycans in Renal Inflammation: Two Sides of the Coin.

It is now well-established that members of the small leucine-rich proteoglycan (SLRP) family act in their soluble form, released proteolytically from the extracellular matrix (ECM), as danger-associated molecular patterns (DAMPs). By interacting with Toll-like receptors (TLRs) and the inflammasome, the two SLRPs, biglycan and decorin, autonomously trigger sterile inflammation. Recent data indicate that these SLRPs, besides their conventional role as pro-inflammatory DAMPs, additionally trigger anti-inflammatory signaling pathways to tightly control inflammation. This is brought about by selective employment of TLRs, their co-receptors, various adaptor molecules, and through crosstalk between SLRP-, reactive oxygen species (ROS)-, and sphingolipid-signaling. In this review, the complexity of SLRP signaling in immune and kidney resident cells and its relevance for renal inflammation is discussed. We propose that the dichotomy in SLRP signaling (pro- and anti-inflammatory) allows for fine-tuning the inflammatory response, which is decisive for the outcome of inflammatory kidney diseases.

Immunolocalization and distribution of proteoglycans in carious dentine.

BACKGROUND: Collagen type I, proteoglycans (PG) and non-collagenous proteins represent important building blocks of the dentine matrix. While different PGs have been identified in dentine, changes in the distribution of these macromolecules with the progression of caries have been poorly characterized. The aim of this study was to compare the immunolocalization of three small collagen-binding PGs (biglycan, fibromodulin and lumican) as well as collagen (types I and VI) in healthy versus carious dentine. METHODS: Longitudinal demineralized sections of extracted teeth were stained with antibodies recognizing specific PG core proteins and collagens, as well as glycosaminoglycans (GAGs) with toluidine blue. RESULTS: In healthy dentine, PGs appeared to be more abundant near the tubule walls and directly under the cusps. Conversely, in carious dentine, specific locations appeared to be more prone to PG degradation than others. These degradation patterns were well correlated with the progression of caries into the tissue, and also appeared to trigger interesting morphological changes in the tissue structure, such as the deformation of dentine tubules near highly infected areas and the lower concentration of PG in tertiary dentine. CONCLUSIONS: This study presents new insights into the involvement of PGs in the progression of caries.

BMP-2 and TGF-ß1 mediate biglycan-induced pro-osteogenic reprogramming in aortic valve interstitial cells.

UNLABELLED: Biglycan accumulates in aortic valves affected by calcific aortic valve disease (CAVD), and soluble biglycan upregulates BMP-2 expression in human aortic valve interstitial cells (AVICs) via Toll-like receptor (TLR) 2 and induces AVIC pro-osteogenic reprogramming, characterized by elevated pro-osteogenic activities. We sought to identify the factors responsible for biglycan-induced pro-osteogenic reprogramming in human AVICs. Treatment of AVICs with recombinant biglycan induced the secretion of BMP-2 and TGF-ß1, but not BMP-4 or BMP-7. Biglycan upregulated TGF-ß1 expression in a TLR4-dependent fashion. Neutralization of BMP-2 or TGF-ß1 attenuated the expression of alkaline phosphatase (ALP), osteopontin, and runt-related transcription factor 2 (Runx2) in cells exposed to biglycan. However, neutralization of both BMP-2 and TGF-ß1 abolished the expression of these osteogenic biomarkers and calcium deposition. Phosphorylated Smad1 and Smad3 were detected in cells exposed to biglycan, and knockdown of Smad1 or Smad3 attenuated the effect of biglycan on the expression of osteogenic biomarkers. While BMP-2 and TGF-ß1 each upregulated the expression of osteogenic biomarkers, an exposure to BMP-2 plus TGF-ß1 induced a greater upregulation and results in calcium deposition. We conclude that concurrent upregulation of BMP-2 and TGF-ß1 is responsible for biglycan-induced pro-osteogenic reprogramming in human AVICs. The Smad 1/3 pathways are involved in the mechanism of AVIC pro-osteogenic reprogramming. KEY MESSAGE: Biglycan upregulates BMP-2 and TGF-ß1 in human aortic valve cells through TLRs. Both BMP-2 and TGF-ß1 are required for aortic valve cell pro-osteogenic reprogramming. Smad signaling pathways are involved in mediating the pro-osteogenic effects of biglycan.

Biological interplay between proteoglycans and their innate immune receptors in inflammation.

An emerging body of evidence indicates that secreted proteoglycans act as signaling molecules, in addition to their canonical function in maintaining and regulating the architecture of various extracellular matrices. Proteoglycans interact with a number of receptors that regulate growth, motility and immune response. In part, as a consequence of their complex structure, proteoglycans can induce crosstalk among various families of receptors and can also interact with natural receptor ligands, often blocking and sequestering their bioactivity. In their soluble form, originating from either partial proteolytic processing or through de novo synthesis by activated cells, some proteoglycans can become potent danger signals, denoting tissue stress and injury. Recently, it has been shown that proteoglycans, especially those belonging to the small leucine-rich and hyaluronan-binding gene families as well as the glycosaminoglycan hyaluronan, act as endogenous ligands of the toll-like receptors, a group of central receptors regulating innate immunity. Furthermore, proteoglycans can activate intracellular inflammasomes and trigger sterile inflammation. In this review, we critically assess the signaling events induced by the proteoglycans biglycan, decorin, lumican and versican as well as hyaluronan during inflammation. We discuss the intriguing emerging notion that, in spite of structural diversity of biglycan, decorin, versican and hyaluronan, all of them signal through the same toll-like receptors, albeit triggering differential responses and biological outcomes. Finally, we review the modes of action of these endogenous ligands of toll-like receptors and their ability to specifically modify the final signaling events and the inflammatory response.

Non-enzymatic decomposition of collagen fibers by a biglycan antibody and a plausible mechanism for rheumatoid arthritis.

Rheumatoid arthritis (RA) is a systemic autoimmune inflammatory and destructive joint disorder that affects tens of millions of people worldwide. Normal healthy joints maintain a balance between the synthesis of extracellular matrix (ECM) molecules and the proteolytic degradation of damaged ones. In the case of RA, this balance is shifted toward matrix destruction due to increased production of cleavage enzymes and the presence of (autoimmune) immunoglobulins resulting from an inflammation induced immune response. Herein we demonstrate that a polyclonal antibody against the proteoglycan biglycan (BG) causes tissue destruction that may be analogous to that of RA affected tissues. The effect of the antibody is more potent than harsh chemical and/or enzymatic treatments designed to mimic arthritis-like fibril de-polymerization. In RA cases, the immune response to inflammation causes synovial fibroblasts, monocytes and macrophages to produce cytokines and secrete matrix remodeling enzymes, whereas B cells are stimulated to produce immunoglobulins. The specific antigen that causes the RA immune response has not yet been identified, although possible candidates have been proposed, including collagen types I and II, and proteoglycans (PG's) such as biglycan. We speculate that the initiation of RA associated tissue destruction in vivo may involve a similar non-enzymatic decomposition of collagen fibrils via the immunoglobulins themselves that we observe here ex vivo.