Glycosylation at Asn211 regulates the activation state of the discoidin domain receptor 1 (DDR1)

Research progress of DDR1 inhibitors in the treatment of multiple human diseases

Discoidin domain receptor 1 (DDR1) is a collagen-activated receptor tyrosine kinase (RTK) and plays pivotal roles in regulating cellular functions such as proliferation, differentiation, invasion, migration, and matrix remodeling. DDR1 is involved in the occurrence and progression of many human diseases, including cancer, fibrosis, and inflammation. Therefore, DDR1 represents a highly promising therapeutic target. Although no selective small-molecule inhibitors have reached clinical trials to date, many molecules have shown therapeutic effects in preclinical studies. For example, BK40143 has demonstrated significant promise in the therapy of neurodegenerative diseases. In this context, our perspective aims to provide an in-depth exploration of DDR1, encompassing its structure characteristics, biological functions, and disease relevance. Furthermore, we emphasize the importance of understanding the structure-activity relationship of DDR1 inhibitors and highlight the unique advantages of dual-target or multitarget inhibitors. We anticipate offering valuable insights into the development of more efficacious DDR1-targeted drugs.

Blockade of DDR1/PYK2/ERK signaling suggesting SH2 superbinder as a novel autophagy inhibitor for pancreatic cancer

Pancreatic cancer is highly lethal, of which 90% is pancreatic ductal adenocarcinoma (PDAC), with a 5-year survival rate of less than 12%, lacking effective treatment options and late diagnosis. Furthermore, the tumors show an intense resistance to cytotoxic chemotherapies. As autophagy is elevated in PDAC, targeting the autophagic pathway is regarded as a promising strategy for cancer treatment. Immunofluorescence and transmission electron microscopy were utilized to assess the autophagic flux. Label-free quantitative phosphoproteomics was used to figure out critically altered tyrosine phosphorylation of the proteins. Tumor-bearing mice were used to validate that SH2 TrM-(Arg)9 restrained the growth of tumor cells. SH2 TrM-(Arg)9 inhibited collagen-induced autophagy via blocking the DDR1/PYK2/ERK signaling cascades. SH2 TrM-(Arg)9 improved the sensitivity of PANC-1/GEM cells to gemcitabine (GEM). Inhibition of autophagy by SH2 TrM-(Arg)9 may synergized with chemotherapy and robusted tumor suppression in pancreatic cancer xenografts. SH2 TrM-(Arg)9 could enter into PDAC cells and blockade autophagy through inhibiting DDR1/PYK2/ERK signaling and may be a new treatment strategy for targeted therapy of PDAC.

Genetic and pharmacological tools to study the role of discoidin domain receptors in kidney disease

Following injury the kidney undergoes a repair process, which results in replacement of the injured tissue with little evidence of damage. However, repetitive injuries or inability of the kidney to stop the repair process result in abnormal deposition of extracellular matrix (ECM) components leading to fibrosis and organ dysfunction. The synthesis/degradation of ECM components is finely regulated by several factors, including discoidin domain receptors (DDRs). These are receptor tyrosine kinases that are activated by collagens. Upon activation, DDRs control several cell functions that, when exacerbated, contribute to kidney injury and fibrosis. DDRs are undetectable in healthy kidney, but become rapidly upregulated in several kidney fibrotic conditions, thus making them attractive anti-fibrotic targets. DDRs contribute to kidney injury and fibrosis by promoting apoptosis of injured kidney cells, stimulating the production of pro-inflammatory cytokines, and regulating the production of ECM components. They achieve these effects by activating canonical intracellular molecules or by directly interacting with nuclear chromatin and promoting the transcription of pro-fibrotic genes. The goal of this review is to highlight canonical and non-canonical mechanisms whereby DDRs contribute to kidney injury/fibrosis. This review will summarize key findings obtained using cells and mice lacking DDRs and it will discuss the discovery and development of targeted DDR small molecule- and antisense-based inhibitors. Understanding the molecular mechanisms whereby DDRs control kidney injury and fibrosis might enable us to not only develop more selective and potent inhibitors, but to also determine when DDR inhibition needs to be achieved to prevent and/or halt the development of kidney fibrosis.

Discoidin domain receptor 1 promotes lung adenocarcinoma migration via the AKT/snail signaling axis

BACKGROUND

Discoidin domain receptor 1 (DDR1), a member of receptor tyrosine kinase, has been implicated in tumor progression. However, the function and underlying mechanism of DDR1 in lung adenocarcinoma (LUAD) progression is unclear. Thus, we explored the molecular regulatory mechanism of DDR1 in the migration of LUAD.

METHODS

Transwell assays, wound healing assays and xenograft tumor assays were performed to study the function of DDR1 in the progression of LUAD. Immunoblotting and quantitative real-time polymerase chain reaction (RT-qPCR) were used to detect the expression levels of genes. Co-immunoprecipitation (co-IP) assays were performed to detect the interaction between DDR1 and AKT. Immunofluorescence and immunohistochemistry assays were used to determine the expression level of proteins in cells and tissues, respectively.

RESULTS

DDR1 expression was significantly higher in LUAD tissues than in normal lung tissues, and the level of DDR1 was inversely correlated with prognosis in patients. We found that DDR1 promoted the migration and invasion of LUAD cells in vitro. Furthermore, ectopic expression of DDR1 in LUAD cells altered EMT-related markers expression. Importantly, the DDR1 protein interacted with AKT and phosphorylated AKT. The AKT inhibitor MK2206 interrupted Snail upregulation in DDR1-overexpressing LUAD cells. Finally, our study revealed that depletion of DDR1 attenuated LUAD cell migration in a tumor xenograft mouse model.

CONCLUSION

Our findings uncovered that a high abundance of DDR1 increased the migration and invasion capability of LUAD cells via the AKT/Snail signaling axis and indicated that DDR1 could be a potential target for treating LUAD.

Stiffness-Responsive Feedback Autoregulation of DDR1 Expression is Mediated by a DDR1-YAP/TAZ Axis

OBJECTIVE

Increased matrix stiffness is sensed by the collagen-binding receptor tyrosine kinase discoidin domain receptor 1 (DDR1). We have previously shown that DDR1 stimulates a positive feedback loop to increase its own expression in vascular smooth muscle cells (VSMCs). The transcriptional co-factors YAP/TAZ are stiffness sensing molecules that have not previously been investigated in DDR1 signaling. Here, we test the hypothesis that DDR1 signals through YAP/TAZ to auto-regulate its own expression.

APPROACH AND RESULTS

We used vascular smooth muscle cells (VSMCs) from wild-type and DDR1 knockout mice stimulated with collagen and/or substrates of different stiffness. We show that DDR1 controls YAP/TAZ nuclear localization and activity, whereas knockdown of YAP/TAZ attenuates DDR1 expression. In response to increased substrate stiffness, collagen stimulation, or RhoA activation, YAP/TAZ translocate to the nucleus and bind to chromatin. Finally, collagen stimulation promotes increased YAP/TAZ association with the Ddr1 promoter.

CONCLUSIONS

These findings reveal the mechanism by which DDR1 regulates YAP/TAZ activity which can then mediate positive feedback regulation of DDR1 expression by promoting transcription of the DDR1 gene.

The Molecular Interaction of Collagen with Cell Receptors for Biological Function

Collagen, an extracellular protein, covers the entire human body and has several important biological functions in normal physiology. Recently, collagen from non-human sources has attracted attention for therapeutic management and biomedical applications. In this regard, both land-based animals such as cow, pig, chicken, camel, and sheep, and marine-based resources such as fish, octopus, starfish, sea-cucumber, and jellyfish are widely used for collagen extraction. The extracted collagen is transformed into collagen peptides, hydrolysates, films, hydrogels, scaffolds, sponges and 3D matrix for food and biomedical applications. In addition, many strategic ideas are continuously emerging to develop innovative advanced collagen biomaterials. For this purpose, it is important to understand the fundamental perception of how collagen communicates with receptors of biological cells to trigger cell signaling pathways. Therefore, this review discloses the molecular interaction of collagen with cell receptor molecules to carry out cellular signaling in biological pathways. By understanding the actual mechanism, this review opens up several new concepts to carry out next level research in collagen biomaterials.

Expression and subcellular localization of Discoidin Domain Receptor 1 (DDR1) define prostate cancer aggressiveness

BACKGROUND

The Discoidin Domain Receptor 1 (DDR1) is one of the two members of a unique family of receptor tyrosine kinase receptors that signal in response to collagen, which has been implicated in cancer progression. Here, we examined the expression of DDR1 in prostate cancer (PCa), and assessed its potential value as a prognostic marker, as a function of grade, stage and other clinicopathologic parameters.

METHODS

We investigated the association between the expression level and subcellular localization of DDR1 protein and PCa aggressiveness by immunohistochemistry, using tissue microarrays (TMAs) encompassing 200 cases of PCa with various Gleason scores (GS) and pathologic stages with matched normal tissue, and a highly specific monoclonal antibody.

RESULTS

DDR1 was found to be localized in the membrane, cytoplasm, and nuclear compartments of both normal and cancerous prostate epithelial cells. Analyses of DDR1 expression in low GS (≤ 7[3 + 4]) vs high GS (≥ 7[4 + 3]) tissues showed no differences in nuclear or cytoplasmic DDR1in either cancerous or adjacent normal tissue cores. However, relative to normal-matched tissue, the percentage of cases with higher membranous DDR1 expression was significantly lower in high vs. low GS cancers. Although nuclear localization of DDR1 was consistently detected in our tissue samples and also in cultured human PCa and normal prostate-derived cell lines, its presence in that site could not be associated with disease aggressiveness. No associations between DDR1 expression and overall survival or biochemical recurrence were found in this cohort of patients.

CONCLUSION

The data obtained through multivariate logistic regression model analysis suggest that the level of membranous DDR1 expression status may represent a potential biomarker of utility for better determination of PCa aggressiveness.

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.

Quantitative phosphoproteomics uncovers dysregulated kinase networks in Alzheimer’s disease

Alzheimer's disease (AD) is a form of dementia characterized by amyloid-β plaques and tau neurofibrillary tangles that progressively disrupt neural circuits in the brain. The signaling networks underlying AD pathological changes are poorly characterized at the phosphoproteome level. Using mass spectrometry, we analyzed the proteome and tyrosine, serine and threonine phosphoproteomes of temporal cortex tissue from patients with AD and aged-matched controls. We identified cocorrelated peptide clusters that were linked to varying levels of phospho-tau, oligodendrocyte, astrocyte, microglia and neuron pathologies. We found that neuronal synaptic protein abundances were strongly anti-correlated with markers of microglial reactivity. We also observed that phosphorylation sites on kinases targeting tau and other new signaling factors were correlated with these peptide modules. Finally, we used data-driven statistical modeling to identify individual peptides and peptide clusters that were predictive of AD histopathologies. Together, these results build a map of pathology-associated phosphorylation signaling events occurring in AD.

Glycosylation and raft endocytosis in cancer

Changes in glycosylation on proteins or lipids are one of the hallmarks of tumorigenesis. In many cases, it is still not understood how glycan information is translated into biological function. In this review, we discuss at the example of specific cancer-related glycoproteins how their endocytic uptake into eukaryotic cells is tuned by carbohydrate modifications. For this, we not only focus on overall uptake rates, but also illustrate how different uptake processes-dependent or not on the conventional clathrin machinery-are used under given glycosylation conditions. Furthermore, we discuss the role of certain sugar-binding proteins, termed galectins, to tune glycoprotein uptake by inducing their crosslinking into lattices, or by co-clustering them with glycolipids into raft-type membrane nanodomains from which the so-called clathrin-independent carriers (CLICs) are formed for glycoprotein internalization into cells. The latter process has been termed glycolipid-lectin (GL-Lect) hypothesis, which operates in a complementary manner to the clathrin pathway and galectin lattices.

Discoidin domain receptor 1 activation links extracellular matrix to podocyte lipotoxicity in Alport syndrome

Background

Discoidin domain receptor 1 (DDR1) is a receptor tyrosine kinase that is activated by collagens that is involved in the pathogenesis of fibrotic disorders. Interestingly, de novo production of the collagen type I (Col I) has been observed in Col4a3 knockout mice, a mouse model of Alport Syndrome (AS mice). Deletion of the DDR1 in AS mice was shown to improve survival and renal function. However, the mechanisms driving DDR1-dependent fibrosis remain largely unknown.

Methods

Podocyte pDDR1 levels, Collagen and cluster of differentiation 36 (CD36) expression was analyzed by Real-time PCR and Western blot. Lipid droplet accumulation and content was determined using Bodipy staining and enzymatic analysis. CD36 and DDR1 interaction was determined by co-immunoprecipitation. Creatinine, BUN, albuminuria, lipid content, and histological and morphological assessment of kidneys harvested from AS mice treated with Ezetimibe and/or Ramipril or vehicle was performed.

Findings

We demonstrate that Col I-mediated DDR1 activation induces CD36-mediated podocyte lipotoxic injury. We show that Ezetimibe interferes with the CD36/DDR1 interaction in vitro and prevents lipotoxicity in AS mice thus preserving renal function similarly to ramipril.

Interpretation

Our study suggests that Col I/DDR1-mediated lipotoxicity contributes to renal failure in AS and that targeting this pathway may represent a new therapeutic strategy for patients with AS and with chronic kidney diseases (CKD) associated with Col4 mutations.

Funding

This study is supported by the NIH grants R01DK117599, R01DK104753, R01CA227493, U54DK083912, UM1DK100846, U01DK116101, UL1TR000460 (Miami Clinical Translational Science Institute, National Center for Advancing Translational Sciences and the National Institute on Minority Health and Health Disparities), F32DK115109, Hoffmann-La Roche and Alport Syndrome Foundation.

DDR1 autophosphorylation is a result of aggregation into dense clusters

The collagen receptor DDR1 is a receptor tyrosine kinase that promotes progression of a wide range of human disorders. Little is known about how ligand binding triggers DDR1 kinase activity. We previously reported that collagen induces DDR1 activation through lateral dimer association and phosphorylation between dimers, a process that requires specific transmembrane association. Here we demonstrate ligand-induced DDR1 clustering by widefield and super-resolution imaging and provide evidence for a mechanism whereby DDR1 kinase activity is determined by its molecular density. Ligand binding resulted in initial DDR1 reorganisation into morphologically distinct clusters with unphosphorylated DDR1. Further compaction over time led to clusters with highly aggregated and phosphorylated DDR1. Ligand-induced DDR1 clustering was abolished by transmembrane mutations but did not require kinase activity. Our results significantly advance our understanding of the molecular events underpinning ligand-induced DDR1 kinase activity and provide an explanation for the unusually slow DDR1 activation kinetics.

N-Glycosylation of the Discoidin Domain Receptor Is Required for Axon Regeneration in Caenorhabditis elegans

Axon regeneration following neuronal injury is an important repair mechanism that is not well understood at present. In Caenorhabditis elegans, axon regeneration is regulated by DDR-2, a receptor tyrosine kinase (RTK) that contains a discoidin domain and modulates the Met-like SVH-2 RTK-JNK MAP kinase signaling pathway. Here, we describe the svh-10/sqv-3 and svh-11 genes, which encode components of a conserved glycosylation pathway, and show that they modulate axon regeneration in C. elegans Overexpression of svh-2, but not of ddr-2, can suppress the axon regeneration defect observed in svh-11 mutants, suggesting that SVH-11 functions between DDR-2 and SVH-2 in this glycosylation pathway. Furthermore, we found that DDR-2 is N-glycosylated at the Asn-141 residue located in its discoidin domain, and mutation of this residue caused an axon regeneration defect. These findings indicate that N-linked glycosylation plays an important role in axon regeneration in C. elegans.

The Extracellular Matrix Receptor Discoidin Domain Receptor 1 Regulates Collagen Transcription by Translocating to the Nucleus

BACKGROUND

The discoidin domain receptor 1 (DDR1) is activated by collagens, upregulated in injured and fibrotic kidneys, and contributes to fibrosis by regulating extracellular matrix production, but how DDR1 controls fibrosis is poorly understood. DDR1 is a receptor tyrosine kinase (RTK). RTKs can translocate to the nucleus via a nuclear localization sequence (NLS) present on the receptor itself or a ligand it is bound to. In the nucleus, RTKs regulate gene expression by binding chromatin directly or by interacting with transcription factors.

METHODS

To determine whether DDR1 translocates to the nucleus and whether this event is mediated by collagen-induced DDR1 activation, we generated renal cells expressing wild-type or mutant forms of DDR1 no longer able to bind collagen. Then, we determined the location of the DDR1 upon collagen stimulation. Using both biochemical assays and immunofluorescence, we analyzed the steps involved in DDR1 nuclear translocation.

RESULTS

We show that although DDR1 and its natural ligand, collagen, lack an NLS, DDR1 is present in the nucleus of injured human and mouse kidney proximal tubules. We show that DDR1 nuclear translocation requires collagen-mediated receptor activation and interaction of DDR1 with SEC61B, a component of the Sec61 translocon, and nonmuscle myosin IIA and β-actin. Once in the nucleus, DDR1 binds to chromatin to increase the transcription of collagen IV, a major collagen upregulated in fibrosis.

CONCLUSIONS

These findings reveal a novel mechanism whereby activated DDR1 translates to the nucleus to regulate synthesis of profibrotic molecules.

Discoidin domain receptor 1 interactions with myosin motors contribute to collagen remodeling and tissue fibrosis

Discoidin Domain Receptor (DDR) genes and their homologues have been identified in sponges, worms and flies. These genes code for proteins that are implicated in cell adhesion to matrix proteins. DDRs are now recognized as playing central regulatory roles in several high prevalence human diseases, including invasive cancers, atherosclerosis, and organ fibrosis. While the mechanisms by which DDRs contribute to these diseases are just now being delineated, one of the common themes involves cell adhesion to collagen and the assembly and organization of collagen fibers in the extracellular matrix. In mammals, the multi-functional roles of DDRs in promoting cell adhesion to collagen fibers and in mediating collagen-dependent signaling, suggest that DDRs contribute to multiple pathways of extracellular matrix remodeling, which are centrally important processes in health and disease. In this review we consider that interactions of the cytoplasmic domains of DDR1 with cytoskeletal motor proteins may contribute to matrix remodeling by promoting collagen fiber alignment and compaction. Poorly controlled collagen remodeling with excessive compaction of matrix proteins is a hallmark of fibrotic lesions in many organs and tissues that are affected by infectious, traumatic or chemical-mediated injury. An improved understanding of the mechanisms by which DDRs mediate collagen remodeling and collagen-dependent signaling could suggest new drug targets for treatment of fibrotic diseases.

Discoidin domain receptors: Micro insights into macro assemblies

Assembly of cell-surface receptors into specific oligomeric states and/or clusters before and after ligand binding is an important feature governing their biological function. Receptor oligomerization can be mediated by specific domains of the receptor, ligand binding, configurational changes or other interacting molecules. In this review we summarize our understanding of the oligomeric state of discoidin domain receptors (DDR1 and DDR2), which belong to the receptor tyrosine kinase family (RTK). DDRs form an interesting system from an oligomerization perspective as their ligand collagen(s) can also undergo supramolecular assembly to form fibrils. Even though DDR1 and DDR2 differ in the domains responsible to form ligand-free dimers they share similarities in binding to soluble, monomeric collagen. However, only DDR1b forms globular clusters in response to monomeric collagen and not DDR2. Interestingly, both DDR1 and DDR2 are assembled into linear clusters by the collagen fibril. Formation of these clusters is important for receptor phosphorylation and is mediated in part by other membrane components. We summarize how the oligomeric status of DDRs shares similarities with other members of the RTK family and with collagen receptors. Unraveling the multiple macro-molecular configurations adopted by this receptor-ligand pair can provide novel insights into the intricacies of cell-matrix interactions.

Clustering, Spatial Distribution, and Phosphorylation of Discoidin Domain Receptors 1 and 2 in Response to Soluble Collagen I

Discoidin domain receptors (DDR1 and DDR2) are receptor tyrosine kinases that signal in response to collagen. We had previously shown that collagen binding leads to clustering of DDR1b, a process partly mediated by its extracellular domain (ECD). In this study, we investigated (i) the impact of the oligomeric state of DDR2 ECD on collagen binding and fibrillogenesis, (ii) the effect of collagen binding on DDR2 clustering, and (iii) the spatial distribution and phosphorylation status of DDR1b and DDR2 after collagen stimulation. Studies were conducted using purified recombinant DDR2 ECD proteins in monomeric, dimeric or oligomeric state, and MC3T3-E1 cells expressing full-length DDR2-GFP or DDR1b-YFP. We show that the oligomeric form of DDR2 ECD displayed enhanced binding to collagen and inhibition of fibrillogenesis. Using atomic force and fluorescence microscopy, we demonstrate that unlike DDR1b, DDR2 ECD and DDR2-GFP do not undergo collagen-induced receptor clustering. However, after prolonged collagen stimulation, both DDR1b-YFP and DDR2-GFP formed filamentous structures consistent with spatial re-distribution of DDRs in cells. Immunocytochemistry revealed that while DDR1b clusters co-localized with non-fibrillar collagen, DDR1b/DDR2 filamentous structures associated with collagen fibrils. Antibodies against a tyrosine phosphorylation site in the intracellular juxtamembrane region of DDR1b displayed positive signals in both DDR1b clusters and filamentous structures. However, only the filamentous structures of both DDR1b and DDR2 co-localized with antibodies directed against tyrosine phosphorylation sites within the receptor kinase domain. Our results uncover key differences and similarities in the clustering abilities and spatial distribution of DDR1b and DDR2 and their impact on receptor phosphorylation.

Signaling by discoidin domain receptor 1 in cancer metastasis

Collagen is the most abundant component of tumor extracellular matrix (ECM). ECM collagens are known to directly interact with the tumor cells via cell surface receptor and play crucial role in tumor cell survival and promote tumor progression. Collagen receptor DDR1 is a member of receptor tyrosine kinase (RTK) family with a unique motif in the extracellular domain resembling Dictyostelium discoideum protein discoidin-I. DDR1 displays delayed and sustained activation upon interaction with collagen and recent findings have demonstrated that DDR1-collagen signaling play important role in cancer progression. In this review, we discuss the current knowledge on the role of DDR1 in cancer metastasis and possibility of a potential therapeutic approach of DDR1 targeted therapy in cancer.

Status Report on the High-Throughput Characterization of Complex Intact O-Glycopeptide Mixtures

A very complex mixture of intact, human N- and O-glycopeptides, enriched from the tryptic digest of urinary proteins of three healthy donors using a two-step lectin affinity enrichment, was analyzed by LC-MS/MS, leading to approximately 45,000 glycopeptide EThcD spectra. Two search engines, Byonic and Protein Prospector, were used for the interpretation of the data, and N- and O-linked glycopeptides were assigned from separate searches. The identification rate was very low in all searches, even when results were combined. Thus, we investigated the reasons why was it so, to help to improve the identification success rate. Focusing on O-linked glycopeptides, we noticed that in EThcD, larger glycan oxonium ions better survive the activation than those in HCD. These fragments, combined with reducing terminal Y ions, provide important information about the glycan(s) present, so we investigated whether filtering the peaklists for glycan oxonium ions indicating the presence of a tetra- or hexasaccharide structure would help to reveal all molecules containing such glycans. Our study showed that intact glycans frequently do not survive even mild supplemental activation, meaning one cannot rely on these oxonium ions exclusively. We found that ETD efficiency is still a limiting factor, and for highly glycosylated peptides, the only information revealed in EThcD was related to the glycan structures. The limited overlap of results delivered by the two search engines draws attention to the fact that automated data interpretation of O-linked glycopeptides is not even close to being solved. Graphical abstract ᅟ.

Spatial localisation of Discoidin Domain Receptor 2 (DDR2) signalling is dependent on its collagen binding and kinase activity

Discoidin Domain Receptor 2 (DDR2) is a collagen-binding receptor tyrosine kinase that initiates delayed and sustained tyrosine phosphorylation signalling. To understand the molecular basis of this unique phosphorylation profile, here we utilise fluorescence microscopy to map the spatiotemporal localisation of DDR2 and tyrosine phosphorylated proteins upon stimulation with collagen. We show that cellular phosphorylated proteins are localised to the interface where DDR2 is in contact with collagen and not in the early endosomes or lysosomes. We find that DDR2 localisation is independent of integrin activation and the key DDR2 signalling effector SHC1. Structure-function analysis reveals that DDR2 mutants defective for collagen binding or kinase activity are unable to localise to the cell surface, demonstrating for the first time that both collagen binding and kinase functions are required for spatial localisation of DDR2. This study provides new insights into the underlying structural features that control DDR2 activation in space and time.

Mechanical signaling through the discoidin domain receptor 1 plays a central role in tissue fibrosis

The preservation of tissue and organ architecture and function depends on tightly regulated interactions of cells with the extracellular matrix (ECM). These interactions are maintained in a dynamic equilibrium that balances intracellular, myosin-generated tension with extracellular resistance conferred by the mechanical properties of the extracellular matrix. Disturbances of this equilibrium can lead to the development of fibrotic lesions that are associated with a wide repertoire of high prevalence diseases including obstructive cardiovascular diseases, muscular dystrophy and cancer. Mechanotransduction is the process by which mechanical cues are converted into biochemical signals. At the core of mechanotransduction are sensory systems, which are frequently located at sites of cell-ECM and cell-cell contacts. As integrins (cell-ECM junctions) and cadherins (cell-cell contacts) have been extensively studied, we focus here on the properties of the discoidin domain receptor 1 (DDR1), a tyrosine kinase that mediates cell adhesion to collagen. DDR1 expression is positively associated with fibrotic lesions of heart, kidney, liver, lung and perivascular tissues. As the most common end-point of all fibrotic disorders is dysregulated collagen remodeling, we consider here the mechanical signaling functions of DDR1 in processing of fibrillar collagen that lead to tissue fibrosis.

Carboxypeptidase X-1 (CPX-1) is a secreted collagen-binding glycoprotein

Carboxypeptidase X-1 (CPX-1) is an atypical member of the carboxypeptidase (CP) family of proteins involved in a variety of physiological and pathological processes. However, unlike most other family members CPX-1 lacks catalytic activity making its biological function unclear. CPX-1 contains a 160 amino acid discoidin domain (DSD) that serves as a binding domain in other proteins prompting us to investigate a putative functional role for this domain in CPX-1. Sequence alignment confirmed the overarching homology between the DSD of CPX-1 and other DSDs whilst more detailed analysis revealed conservation of the residues known to form the collagen-binding trench within the DSD of the discoidin domain receptors (DDRs) 1 and 2. Biochemical characterisation of transiently expressed human CPX-1 revealed that CPX-1 was secreted in an N-glycosylation-dependent manner as treatment with the N-glycosylation inhibitor tunicamycin inhibited secretion concomitant with a reduction in CPX-1 mobility on Western blot. Using a collagen pull-down assay we found that secreted CPX-1 bound collagen and this appeared independent of N-glycosylation as treatment with PNGaseF did not affect binding. Further analysis under non-reducing and reducing (+DTT) conditions revealed that CPX-1 was secreted in both monomeric and dimeric forms and only the former bound collagen. Finally, mutation of a key residue situated within the putative collagen-binding trench within the DSD of CPX-1 (R192A) significantly reduced secretion and collagen-binding by 40% and 60%, respectively. Collectively these results demonstrate that CPX-1 is a secreted collagen-binding glycoprotein and provide a foundation for future studies investigating the function of CPX-1.

Discoidin domain receptor 1 (DDR1) kinase as target for structure-based drug discovery

Discoidin domain receptor (DDR) 1 and 2 are transmembrane receptors that belong to the family of receptor tyrosine kinases (RTK). Upon collagen binding, DDRs transduce cellular signaling involved in various cell functions, including cell adhesion, proliferation, differentiation, migration, and matrix homeostasis. Altered DDR function resulting from either mutations or overexpression has been implicated in several types of disease, including atherosclerosis, inflammation, cancer, and tissue fibrosis. Several established inhibitors, such as imatinib, dasatinib, and nilotinib, originally developed as Abelson murine leukemia (Abl) kinase inhibitors, have been found to inhibit DDR kinase activity. As we review here, recent discoveries of novel inhibitors and their co-crystal structure with the DDR1 kinase domain have made structure-based drug discovery for DDR1 amenable.

Discoidin domain receptors: a proteomic portrait

The discoidin domain receptors (DDRs) are collagen-binding receptor tyrosine kinases that have been implicated in a number of fundamental biological processes ranging from growth and development to immunoregulation. In this review, we examine how recent proteomic technologies have enriched our understanding of DDR signaling mechanisms. We provide an overview on the use of large-scale proteomic profiling and chemical proteomics to reveal novel insights into DDR therapeutics, signaling networks, and receptor crosstalk. A perspective of how proteomics may be harnessed to answer outstanding fundamental questions including the dynamic regulation of receptor activation kinetics is presented. Collectively, these studies present an emerging molecular portrait of these unique receptors and their functional role in health and disease.

Normal activation of discoidin domain receptor 1 mutants with disulfide cross-links, insertions, or deletions in the extracellular juxtamembrane region: mechanistic implications

The discoidin domain receptors, DDR1 and DDR2, are receptor tyrosine kinases that are activated by collagen. DDR activation does not appear to occur by the common mechanism of ligand-induced receptor dimerization: the DDRs form stable noncovalent dimers in the absence of ligand, and ligand-induced autophosphorylation of cytoplasmic tyrosines is unusually slow and sustained. Here we sought to identify functionally important dimer contacts within the extracellular region of DDR1 by using cysteine-scanning mutagenesis. Cysteine substitutions close to the transmembrane domain resulted in receptors that formed covalent dimers with high efficiency, both in the absence and presence of collagen. Enforced covalent dimerization did not result in constitutive activation and did not affect the ability of collagen to induce receptor autophosphorylation. Cysteines farther away from the transmembrane domain were also cross-linked with high efficiency, but some of these mutants could no longer be activated. Furthermore, the extracellular juxtamembrane region of DDR1 tolerated large deletions as well as insertions of flexible segments, with no adverse effect on activation. These findings indicate that the extracellular juxtamembrane region of DDR1 is exceptionally flexible and does not constrain the basal or ligand-activated state of the receptor. DDR1 transmembrane signaling thus appears to occur without conformational coupling through the juxtamembrane region, but requires specific receptor interactions farther away from the cell membrane. A plausible mechanism to explain these findings is signaling by DDR1 clusters.