Discoidin Domain Receptor 2, a Potential Therapeutic Target in Lung Fibrosis

Comprehensive analysis of the effects of P4ha1 and P4ha2 deletion on post-translational modifications of fibrillar collagens in mouse skin

Introduction

Collagens, the most abundant proteins in mammals, play pivotal roles in maintaining tissue structure, functions, cell-to-cell communication, cellular migration, cellular behavior, and growth. Structures of collagens are highly complex due to the presence of dynamic post-translational modifications (PTMs), such as hydroxylations (on prolines and lysine residues) and O-glycosylation (on hydroxylysines) enzymatically catalyzed during biosynthesis in the endoplasmic reticulum. Collagen PTMs are essential for maintaining structural stability, elasticity, and different functions of collagens. The most prevalent modification in fibrillar collagens is prolyl 4-hydroxylation catalyzed by collagen prolyl 4-hydroxylases (C-P4Hs). Prolyl 4-hydroxylation on collagens plays a critical role in collagen biosynthesis, thermostability, and cell-collagen interactions. Collagens are large proteins. Different regions of collagen perform different functions, so the presence or absence of a PTM on a particular collagen site can affect its functioning. However, comprehensive site-specific identification of these PTMs on fibrillar collagen chains of mice skin has not been performed yet. Furthermore, the effects of prolyl 4-hydroxylase alpha 1 (P4HA1) and P4HA2 on 3-hydroxyproline, 5-hydroxylysine, and O-glycosylation sites of fibrillar collagen chains have not yet been explored.

Methodology

This study presents a comprehensive PTM analysis of fibrillar collagen chains extracted from the skin of different mutants of C-P4Hs (P4ha1 +/- ; P4ha2-/-, P4ha1 +/+ ; P4ha2-/-, P4ha1 +/- ; P4ha2 +/- , P4ha1 +/+ ; P4ha2 +/- ) and wild-type mice. In this study, proteomics-based comprehensive PTM site identification by MS2 level ions from raw mass spectrometry data was performed, and MS1-level quantification was performed for PTM occupancy percentage analysis.

Results and discussion

A total of 421 site-specific PTMs were identified on fibrillar collagen chains (COL1A1, COL1A2, and COL3A1) extracted from wild-type mice skin. A total of 23 P4HA1-specific and seven P4HA2-specific 4-hydroxyproline sites on fibrillar collagen chains were identified. Moreover, it was found that the P4ha1 and P4ha2 deletion can affect the 3-hydroxyproline occupancy percentages in mice skin. Interestingly, increased levels of lysyl 5-hydroxylation were detected upon partial deletion of P4ha1 and full deletion of P4ha2. These findings show that the effects of deletion of prolyl 4-hydroxylases are not limited to less 4-hydroxylation on some specific proline sites, but it can also modulate the prolyl 3-hydroxylation, lysyl 5-hydroxylation, and O-glycosylation occupancy percentages in the fibrillar collagen chains in a site-specific manner.

Inhibition of discoidin domain receptor 2 reveals kinase-dependent and kinase-independent functions in regulating fibroblast activity

Progressive pulmonary fibrosis is a devastating condition and current treatment is suboptimal. There has been considerable interest in the role of tyrosine kinase signaling as mediators of pro- and antifibrotic processes. Nintedanib is a nonspecific tyrosine kinase that has been shown to have therapeutic benefit in lung fibrosis. However, the precise mechanism of action remains unclear because nintedanib inhibits several tyrosine kinases, which are often expressed on multiple cell types with different activities during fibrosis. Discoidin domain receptor 2 (DDR2) has been suggested as a potential target of nintedanib. DDR2 is a receptor tyrosine kinase that is activated by fibrillar collagens such as type I collagen. DDR2 is primarily expressed by fibroblasts. The effectiveness of specifically targeting DDR2 signaling during fibrosis remains undefined. In the present study, we show that nintedanib acts as a direct and indirect inhibitor of DDR2. We then utilize a novel allosteric inhibitor of DDR2, WRG-28, which blocks ligand binding and activation of DDR2. We find that WRG-28 augments fibroblast apoptosis and attenuates fibrosis. Finally, we show that fibroblast type I collagen autocrine signaling is regulated by DDR2 through both kinase-dependent and kinase-independent functions of DDR2. These findings highlight the importance of type I collagen autocrine signaling by fibroblasts during fibrosis and demonstrate that DDR2 has a central role in this pathway making it a potential therapeutic target.NEW & NOTEWORTHY Type I collagen is a major component of fibrosis and can signal through cell surface receptors such as discoidin domain receptor 2 (DDR2). DDR2 activation can lead to further collagen deposition by fibroblasts setting up a profibrotic positive feedback loop. In this report, we find that inhibition of DDR2 with nintedanib or a specific DDR2 inhibitor, WRG-28, can disrupt this cycle and prevent fibrosis through augmented fibroblast apoptosis and inhibited activation.

Exploring the cardiac ECM during fibrosis: A new era with next-gen proteomics

Extracellular matrix (ECM) plays a critical role in maintaining elasticity in cardiac tissues. Elasticity is required in the heart for properly pumping blood to the whole body. Dysregulated ECM remodeling causes fibrosis in the cardiac tissues. Cardiac fibrosis leads to stiffness in the heart tissues, resulting in heart failure. During cardiac fibrosis, ECM proteins get excessively deposited in the cardiac tissues. In the ECM, cardiac fibroblast proliferates into myofibroblast upon various kinds of stimulations. Fibroblast activation (myofibroblast) contributes majorly toward cardiac fibrosis. Other than cardiac fibroblasts, cardiomyocytes, epithelial/endothelial cells, and immune system cells can also contribute to cardiac fibrosis. Alteration in the expression of the ECM core and ECM-modifier proteins causes different types of cardiac fibrosis. These different components of ECM culminated into different pathways inducing transdifferentiation of cardiac fibroblast into myofibroblast. In this review, we summarize the role of different ECM components during cardiac fibrosis progression leading to heart failure. Furthermore, we highlight the importance of applying mass-spectrometry-based proteomics to understand the key changes occurring in the ECM during fibrotic progression. Next-gen proteomics studies will broaden the potential to identify key targets to combat cardiac fibrosis in order to achieve precise medicine-development in the future.

Discovery of a novel DDRs kinase inhibitor XBLJ-13 for the treatment of idiopathic pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is a chronic fatal lung disease characterized by destruction of lung parenchyma and deposition of extracellular matrix in interstitial and alveolar spaces. But known drugs for IPF are far from meeting clinical demands, validation of drug targets against pulmonary fibrosis is in urgent demand. Tyrosine kinase receptor DDRs has been considered as a potential therapeutic target for pulmonary fibrosis due to its pathological collagen binding property and the roles in regulating extracellular matrix remodeling. In this study we designed and synthesized a new indazole derivative XBLJ-13, and identified XBLJ-13 as a highly specific and potent DDRs inhibitor with anti-inflammation and anti-fibrosis activities. We first demonstrated that DDR1/2 was highly expressed in the lung tissues of IPF patients. Then we showed that XBLJ-13 potently inhibited DDR1 and DDR2 kinases with IC₅₀ values of 17.18 nM and 15.13 nM, respectively. Among a panel of 34 kinases tested, XBLJ-13 displayed relatively high selectivity for DDRs with minimal inhibitory effect on PDGFR family and FGFR1, as well as Abl kinase that had high homology with DDRs. Extensive profiling of XBLJ-13 revealed that the new inhibitor had much lower toxicity than nintedanib and better pharmacokinetic properties in mice. Furthermore, pharmacodynamic evaluation conducted in bleomycin-induced pulmonary fibrosis mice showed that administration of XBLJ-13 (30, 60, 90 mg·kg^-1·d^-1, i.g.) for 12 days significantly and dose-dependently ameliorated lung inflammation and fibrosis. Together, this study confirms that DDRs kinase is a potential target for PF, Particularly, compound XBLJ-13 is a highly potent and specific DDRs inhibitor, along with good pharmacokinetics profiles, and preferable in vivo efficacy, suggesting that it is a potential candidate for the treatment of PF.

Discoidin domain receptors (DDRs): Potential implications in periodontitis

Periodontitis is a chronic inflammatory disease leading to the destruction of periodontal tissues associated with high prevalence and significant economic burden. As special collagen-binding tyrosine kinase receptors, the discoidin domain receptors (DDRs) can control cell migration, adhesion, proliferation, and extracellular matrix remodeling. DDRs are constitutively expressed and widely distributed in periodontal tissues which are rich in collagen. Ddr1/2 knockout mice showed significant periodontal defects including connective tissue destruction, alveolar bone loss, and even tooth loss. It has been demonstrated that bone homeostasis, inflammation, matrix metalloproteinases, and autophagy are crucial characteristics involved in the pathogenesis of periodontitis. Of note, DDRs have been reported to participate in the above pathophysiological processes, implicating the potential roles of DDRs in periodontitis. In this review article, we aim to illustrate the possible roles of DDRs in periodontitis in an attempt to explore their potential value as therapeutic targets for periodontitis.