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1.  A novel mutation in DDR2 causing spondylo-meta-epiphyseal dysplasia with short limbs and abnormal calcifications (SMED-SL) results in defective intra-cellular trafficking 
BMC Medical Genetics  2014;15:42.
The rare autosomal genetic disorder, Spondylo-meta-epiphyseal dysplasia with short limbs and abnormal calcifications (SMED-SL), is reported to be caused by missense or splice site mutations in the human discoidin domain receptor 2 (DDR2) gene. Previously our group has established that trafficking defects and loss of ligand binding are the underlying cellular mechanisms of several SMED-SL causing mutations. Here we report the clinical characteristics of two siblings of consanguineous marriage with suspected SMED-SL and identification of a novel disease-causing mutation in the DDR2 gene.
Clinical evaluation and radiography were performed to evaluate the patients. All the coding exons and splice sites of the DDR2 gene were sequenced by Sanger sequencing. Subcellular localization of the mutated DDR2 protein was determined by confocal microscopy, deglycosylation assay and Western blotting. DDR2 activity was measured by collagen activation and Western analysis.
In addition to the typical features of SMED-SL, one of the patients has an eye phenotype including visual impairment due to optic atrophy. DNA sequencing revealed a novel homozygous dinucleotide deletion mutation (c.2468_2469delCT) on exon 18 of the DDR2 gene in both patients. The mutation resulted in a frameshift leading to an amino acid change at position S823 and a predicted premature termination of translation (p.S823Cfs*2). Subcellular localization of the mutant protein was analyzed in mammalian cell lines, and it was found to be largely retained in the endoplasmic reticulum (ER), which was further supported by its N-glycosylation profile. In keeping with its cellular mis-localization, the mutant protein was found to be deficient in collagen-induced receptor activation, suggesting protein trafficking defects as the major cellular mechanism underlying the loss of DDR2 function in our patients.
Our results indicate that the novel mutation results in defective trafficking of the DDR2 protein leading to loss of function and disease. This confirms our previous findings that DDR2 missense mutations occurring at the kinase domain result in retention of the mutant protein in the ER.
PMCID: PMC4001364  PMID: 24725993
DDR2; Spondylo-meta-epiphyseal dysplasia; Trafficking defect; SMED-SL; ERAD; Optic atrophy
2.  Glycosylation at Asn211 Regulates the Activation State of the Discoidin Domain Receptor 1 (DDR1)* 
The Journal of Biological Chemistry  2014;289(13):9275-9287.
Background: DDR1 is a receptor tyrosine kinase that signals in response to collagen.
Results: Mutagenesis at the 211NDS glycosylation site enhances receptor dimerization and results in ligand-independent receptor autophosphorylation.
Conclusion: N-Glycosylation of DDR1 plays a critical role in maintenance of the inactive state of the receptor dimers.
Significance: These studies highlight a new structural feature that regulates DDR1 activation.
Discoidin domain receptor 1 (DDR1) belongs to a unique family of receptor tyrosine kinases that signal in response to collagens. DDR1 undergoes autophosphorylation in response to collagen binding with a slow and sustained kinetics that is unique among members of the receptor tyrosine kinase family. DDR1 dimerization precedes receptor activation suggesting a structural inhibitory mechanism to prevent unwarranted phosphorylation. However, the mechanism(s) that maintains the autoinhibitory state of the DDR1 dimers is unknown. Here, we report that N-glycosylation at the Asn211 residue plays a unique role in the control of DDR1 dimerization and autophosphorylation. Using site-directed mutagenesis, we found that mutations that disrupt the conserved 211NDS N-glycosylation motif, but not other N-glycosylation sites (Asn260, Asn371, and Asn394), result in collagen I-independent constitutive phosphorylation. Mass spectrometry revealed that the N211Q mutant undergoes phosphorylation at Tyr484, Tyr520, Tyr792, and Tyr797. The N211Q traffics to the cell surface, and its ectodomain displays collagen I binding with an affinity similar to that of the wild-type DDR1 ectodomain. However, unlike the wild-type receptor, the N211Q mutant exhibits enhanced receptor dimerization and sustained activation upon ligand withdrawal. Taken together, these data suggest that N-glycosylation at the highly conserved 211NDS motif evolved to act as a negative repressor of DDR1 phosphorylation in the absence of ligand. The presence of glycan moieties at that site may help to lock the collagen-binding domain in the inactive state and prevent unwarranted signaling by receptor dimers. These studies provide a novel insight into the structural mechanisms that regulate DDR activation.
PMCID: PMC3979393  PMID: 24509848
Collagen; Glycosylation; Mutagenesis Site-specific; Receptor Regulation; Receptor Tyrosine Kinase
3.  Structural Mechanisms Determining Inhibition of the Collagen Receptor DDR1 by Selective and Multi-Targeted Type II Kinase Inhibitors 
Journal of Molecular Biology  2014;426(13):2457-2470.
The discoidin domain receptors (DDRs), DDR1 and DDR2, form a unique subfamily of receptor tyrosine kinases that are activated by the binding of triple-helical collagen. Excessive signaling by DDR1 and DDR2 has been linked to the progression of various human diseases, including fibrosis, atherosclerosis and cancer. We report the inhibition of these unusual receptor tyrosine kinases by the multi-targeted cancer drugs imatinib and ponatinib, as well as the selective type II inhibitor DDR1-IN-1. Ponatinib is identified as the more potent molecule, which inhibits DDR1 and DDR2 with an IC50 of 9 nM. Co-crystal structures of human DDR1 reveal a DFG-out conformation (DFG, Asp-Phe-Gly) of the kinase domain that is stabilized by an unusual salt bridge between the activation loop and αD helix. Differences to Abelson kinase (ABL) are observed in the DDR1 P-loop, where a β-hairpin replaces the cage-like structure of ABL. P-loop residues in DDR1 that confer drug resistance in ABL are therefore accommodated outside the ATP pocket. Whereas imatinib and ponatinib bind potently to both the DDR and ABL kinases, the hydrophobic interactions of the ABL P-loop appear poorly satisfied by DDR1-IN-1 suggesting a structural basis for its DDR1 selectivity. Such inhibitors may have applications in clinical indications of DDR1 and DDR2 overexpression or mutation, including lung cancer.
Graphical Abstract
•DDR kinases bind potently to clinically relevant type II kinase inhibitors.•Co-crystal structures of DDR1 reveal the inhibitor binding modes.•Structural differences to other kinases allow for DDR-selective inhibitors.•Type II kinase inhibitors have potential to treat DDR-associated diseases.
PMCID: PMC4058747  PMID: 24768818
A-loop, activation loop; CML, chronic myeloid leukemia; DDR, discoidin domain receptor; IGF1R, insulin-like growth factor 1 receptor; ITC, isothermal titration calorimetry; KID, kinase insert domain; RTKs, receptor tyrosine kinases; TCEP, tris(2-carboxyethyl)phosphine; TKI, tyrosine kinase inhibitor; TrkB, tropomyosin-related kinase B; phosphorylation; crystallography; drug design; oncology; gleevec
4.  Discoidin Domain Receptor 1 Tyrosine Kinase Has an Essential Role in Mammary Gland Development 
Molecular and Cellular Biology  2001;21(8):2906-2917.
Various types of collagen have been identified as potential ligands for the two mammalian discoidin domain receptor tyrosine kinases, DDR1 and DDR2. Here, we used a recombinant fusion protein between the extracellular domain of DDR1 and alkaline phosphatase to detect specific receptor binding sites during mouse development. Major sites of DDR1-binding activity, indicative of ligand expression, were found in skeletal bones, the skin, and the urogenital tract. Ligand expression in the uterus during implantation and in the mammary gland during pregnancy colocalized with the expression of the DDR1 receptor. The generation of DDR1-null mice by gene targeting yielded homozygous mutant animals that were viable but smaller in size than control littermates. The majority of mutant females were unable to bear offspring due to a lack of proper blastocyst implantation into the uterine wall. When implantation did occur, the mutant females were unable to lactate. Histological analysis showed that the alveolar epithelium failed to secrete milk proteins into the lumen of the mammary gland. The lactational defect appears to be caused by hyperproliferation and abnormal branching of mammary ducts. These results suggest that DDR1 is a key mediator of the stromal-epithelial interaction during ductal morphogenesis in the mammary gland.
PMCID: PMC86919  PMID: 11283268
5.  Collagen binding specificity of the discoidin domain receptors: Binding sites on collagens II and III and molecular determinants for collagen IV recognition by DDR1 
Matrix Biology  2011;30(1):16-26.
The discoidin domain receptors, DDR1 and DDR2 are cell surface receptor tyrosine kinases that are activated by triple-helical collagen. While normal DDR signalling regulates fundamental cellular processes, aberrant DDR signalling is associated with several human diseases. We previously identified GVMGFO (O is hydroxyproline) as a major DDR2 binding site in collagens I–III, and located two additional DDR2 binding sites in collagen II. Here we extend these studies to the homologous DDR1 and the identification of DDR binding sites on collagen III. Using sets of overlapping triple-helical peptides, the Collagen II and Collagen III Toolkits, we located several DDR2 binding sites on both collagens. The interaction of DDR1 with Toolkit peptides was more restricted, with DDR1 mainly binding to peptides containing the GVMGFO motif. Triple-helical peptides containing the GVMGFO motif induced DDR1 transmembrane signalling, and DDR1 binding and receptor activation occurred with the same amino acid requirements as previously defined for DDR2. While both DDRs exhibit the same specificity for binding the GVMGFO motif, which is present only in fibrillar collagens, the two receptors display distinct preferences for certain non-fibrillar collagens, with the basement membrane collagen IV being exclusively recognised by DDR1. Based on our recent crystal structure of a DDR2-collagen complex, we designed mutations to identify the molecular determinants for DDR1 binding to collagen IV. By replacing five amino acids in DDR2 with the corresponding DDR1 residues we were able to create a DDR2 construct that could function as a collagen IV receptor.
PMCID: PMC3034869  PMID: 21044884
DDR, discoidin domain receptor; DS, discoidin homology; HEK, human embryonic kidney; RTK, receptor tyrosine kinase; VWF, von Willebrand factor; Collagen receptor; Discoidin domain receptor; Receptor tyrosine kinase; Cell–extracellular matrix interaction; Collagen binding specificity
6.  Discovery of a Potent and Selective DDR1 Receptor Tyrosine Kinase Inhibitor 
ACS Chemical Biology  2013;8(10):2145-2150.
The DDR1 receptor tyrosine kinase is activated by matrix collagens and has been implicated in numerous cellular functions such as proliferation, differentiation, adhesion, migration, and invasion. Here we report the discovery of a potent and selective DDR1 inhibitor, DDR1-IN-1, and present the 2.2 Å DDR1 co-crystal structure. DDR1-IN-1 binds to DDR1 in the ‘DFG-out’ conformation and inhibits DDR1 autophosphorylation in cells at submicromolar concentrations with good selectivity as assessed against a panel of 451 kinases measured using the KinomeScan technology. We identified a mutation in the hinge region of DDR1, G707A, that confers >20-fold resistance to the ability of DDR1-IN-1 to inhibit DDR1 autophosphorylation and can be used to establish what pharmacology is DDR1-dependent. A combinatorial screen of DDR1-IN-1 with a library of annotated kinase inhibitors revealed that inhibitors of PI3K and mTOR such as GSK2126458 potentiate the antiproliferative activity of DDR1-IN-1 in colorectal cancer cell lines. DDR1-IN-1 provides a useful pharmacological probe for DDR1-dependent signal transduction.
PMCID: PMC3800496  PMID: 23899692
7.  Discovery of a Potent and Selective DDR1 Receptor Tyrosine Kinase Inhibitor 
ACS chemical biology  2013;8(10):2145-2150.
The DDR1 receptor tyrosine kinase is activated by matrix collagens and has been implicated in numerous cellular functions such as proliferation, differentiation, adhesion, migration and invasion. Here we report the discovery of a potent and selective DDR1 inhibitor, DDR1-IN-1, and present the 2.2 Å DDR1 co-crystal structure. DDR1-IN-1 binds to DDR1 in the ‘DFG-out’ conformation and inhibits DDR1 auto-phosphorylation in cells at sub-micromolar concentrations with good selectivity as assessed against a panel of 451 kinases measured using the KinomeScan™ technology. We identified a mutation in the hinge region of DDR1, G707A that confers over 20-fold resistance to the ability of DDR1-IN-1to inhibit DDR1 autophosphorylation that can be used to establish what pharmacology isDDR1-dependent. A combinatorial screen of DDR1-IN-1 with a library of annotated kinase inhibitors revealed that inhibitors of PI3K and mTOR such as GSK2126458 potentiate the anti-proliferative activity of DDR1-IN-1 in colorectal cancer cell lines. DDR1-IN-1 provides a useful pharmacological probe for DDR1-dependent signal transduction.
PMCID: PMC3800496  PMID: 23899692
Cancer Metastasis Reviews  2012;31(1-2):295-321.
Almost all human cancers display dysregulated expression and/or function of one or more receptor tyrosine kinases (RTKs). The strong causative association between altered RTK function and cancer progression has translated into novel therapeutic strategies that target these cell surface receptors in the treatment of cancer. Yet, the full spectrum of RTKs that may alter the oncogenic process is not completely understood. Accumulating evidence suggests that a unique set of RTKs known as the Discoidin Domain Receptors (DDRs) play a role in cancer progression by regulating the interactions of tumor cells with their surrounding collagen matrix. The DDRs are the only RTKs that specifically bind to, and are activated by collagen. Hence, the DDRs are part of the signaling networks that translate information from the extracellular matrix thereby acting as key regulators of cell-matrix interactions. Under physiological conditions, DDRs control cell and tissue homeostasis by acting as collagen sensors, transducing signals that regulate cell polarity, tissue morphogenesis, and cell differentiation. In cancer, DDRs are hijacked by tumor cells to disrupt normal cell-matrix communication and initiate pro-migratory and pro-invasive programs. Importantly, several cancer types exhibit DDR mutations, which are thought to alter receptor function and contribute to cancer progression. Other evidence suggests that the actions of DDRs in cancer are complex, either promoting or suppressing tumor cell behavior in a DDR type/isoform specific and context dependent manner. Thus, there is still a considerable gap in our knowledge of DDR actions in cancer tissues. This review summarizes the current knowledge on DDR expression and function in cancer and discusses the potential implications of DDRs in cancer biology. It is hoped that this effort will encourage more research into these poorly understood but unique RTKs, which have the potential of becoming novel therapeutics targets in cancer.
PMCID: PMC3351584  PMID: 22366781
discoidin domain receptor; tyrosine kinase; collagen; extracellular matrix; signaling; cell migration; metastasis
9.  Discoidin Domain Receptors Promote α1β1- and α2β1-Integrin Mediated Cell Adhesion to Collagen by Enhancing Integrin Activation 
PLoS ONE  2012;7(12):e52209.
The discoidin domain receptors, DDR1 and DDR2, are receptor tyrosine kinases that bind to and are activated by collagens. Similar to collagen-binding β1 integrins, the DDRs bind to specific motifs within the collagen triple helix. However, these two types of collagen receptors recognize distinct collagen sequences. While GVMGFO (O is hydroxyproline) functions as a major DDR binding motif in fibrillar collagens, integrins bind to sequences containing Gxx’GEx”. The DDRs are thought to regulate cell adhesion, but their roles have hitherto only been studied indirectly. In this study we used synthetic triple-helical collagen-derived peptides that incorporate either the DDR-selective GVMGFO motif or integrin-selective motifs, such as GxOGER and GLOGEN, in order to selectively target either type of receptor and resolve their contributions to cell adhesion. Our data using HEK293 cells show that while cell adhesion to collagen I was completely inhibited by anti-integrin blocking antibodies, the DDRs could mediate cell attachment to the GVMGFO motif in an integrin-independent manner. Cell binding to GVMGFO was independent of DDR receptor signalling and occurred with limited cell spreading, indicating that the DDRs do not mediate firm adhesion. However, blocking the interaction of DDR-expressing cells with collagen I via the GVMGFO site diminished cell adhesion, suggesting that the DDRs positively modulate integrin-mediated cell adhesion. Indeed, overexpression of the DDRs or activation of the DDRs by the GVMGFO ligand promoted α1β1 and α2β1 integrin-mediated cell adhesion to medium- and low-affinity integrin ligands without regulating the cell surface expression levels of α1β1 or α2β1. Our data thus demonstrate an adhesion-promoting role of the DDRs, whereby overexpression and/or activation of the DDRs leads to enhanced integrin-mediated cell adhesion as a result of higher integrin activation state.
PMCID: PMC3527415  PMID: 23284937
10.  Collagen Type I Selectively Activates Ectodomain Shedding of the Discoidin Domain Receptor 1: Involvement of Src Tyrosine Kinase 
Journal of cellular biochemistry  2006;98(3):672-684.
The discoidin domain receptor 1 (DDR1) is a receptor tyrosine kinase that is highly expressed in breast carcinoma cells. Upon binding to collagen, DDR1 undergoes autophosphorylation followed by limited proteolysis to generate a tyrosine phosphorylated C-terminal fragment (CTF). Although it was postulated that this fragment is formed as a result of shedding of the N-terminal ectodomain, collagen-dependent release of the DDR1 extracellular domain has not been demonstrated. We now report that, in conjunction with CTF formation, collagen type I stimulates concentration-dependent, saturable shedding of the DDR1 ectodomain from two carcinoma cell lines, and from transfected cells. In contrast, collagen did not promote cleavage of other transmembrane proteins including the amyloid precursor protein (APP), ErbB2, and E-cadherin. Collagen-dependent tyrosine phosphorylation and proteolysis of DDR1 in carcinoma cells were reduced by a pharmacologic Src inhibitor. Moreover, expression of a dominant negative Src mutant protein in human embryonic kidney cells inhibited collagen-dependent phosphorylation and shedding of co-transfected DDR1. The hydroxamate-based metalloproteinase inhibitor TAPI-1 (tumor necrosis factor-α protease inhibitor-1), and tissue inhibitor of metalloproteinase (TIMP)-3, also blocked collagen-evoked DDR1 shedding, but did not reduce levels of the phosphorylated CTF. Neither shedding nor CTF formation were affected by the γ-secretase inhibitor, L-685,458. The results demonstrate that collagen-evoked ectodomain cleavage of DDR1 is mediated in part by Src-dependent activation or recruitment of a matrix- or disintegrin metalloproteinase, and that CTF formation can occur independently of ectodomain shedding. Delayed shedding of the DDR1 ectodomain may represent a mechanism that limits DDR1-dependent cell adhesion and migration on collagen matrices.
PMCID: PMC2593136  PMID: 16440311
zinc-dependent metalloproteinase; amyloid precursor protein; tyrosine phosphorylation; TIMP (tissue inhibitor of metalloproteinase)
11.  Oligomerization of DDR1 ECD affects receptor-ligand binding 
Journal of structural biology  2013;183(3):495-500.
Discoidin Domain Receptor 1 (DDR1) is a widely expressed receptor tyrosine kinase (RTK) which regulates cell differentiation, proliferation and migration and remodeling of the extracellular matrix. Collagen(s) are the only known ligand for DDR1. We have previously reported that collagen stimulation leads to oligomerization of the full length receptor. In this study we investigated the effect of oligomerization of the DDR1 extracellular domain (ECD) pre and post ligand binding. Solid phase binding assays showed that oligomers of recombinant DDR1-Fc bound more strongly to collagen compared to dimeric DDR1-Fc alone. In addition, DDR1-Fc itself could oligomerize upon in-vitro binding to collagen when examined using atomic force microscopy. Inhibition of dynamin mediated receptor endocytosis could prevent ligand induced endocytosis of DDR1-YFP in live cells. However inhibition of receptor endocytosis did not affect DDR1 oligomerization. In summary our results demonstrate that DDR1 ECD plays a crucial role in receptor oligomerization which mediates high-affinity interactions with its ligand.
PMCID: PMC3785576  PMID: 23810922
Collagen; discoidin domain receptor; extracellular domain; oligomerization; atomic force microscopy
12.  Normal Activation of Discoidin Domain Receptor 1 Mutants with Disulfide Cross-links, Insertions, or Deletions in the Extracellular Juxtamembrane Region 
The Journal of Biological Chemistry  2014;289(19):13565-13574.
Background: DDR1 is a constitutively dimeric receptor tyrosine kinase that is activated by collagen. The mechanism of transmembrane signaling is unknown.
Results: DDR1 activation is unaffected by disulfide cross-links, insertions, or deletions in the extracellular juxtamembrane region.
Conclusion: The extracellular juxtamembrane region of DDR1 does not transmit a conformational change across the cell membrane.
Significance: The activation mechanism of DDR1 appears to be unique among receptor tyrosine kinases.
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.
PMCID: PMC4036362  PMID: 24671415
Collagen; Mutagenesis Site-specific; Receptor Regulation; Receptor Structure-Function; Receptor Tyrosine Kinase; Signal Transduction; Activation Mechanism; Cysteine Scanning
13.  Phosphoproteomics of collagen receptor networks reveals SHP-2 phosphorylation downstream of wild-type DDR2 and its lung cancer mutants 
Biochemical Journal  2013;454(Pt 3):501-513.
Collagen is an important extracellular matrix component that directs many fundamental cellular processes including differentiation, proliferation and motility. The signalling networks driving these processes are propagated by collagen receptors such as the β1 integrins and the DDRs (discoidin domain receptors). To gain an insight into the molecular mechanisms of collagen receptor signalling, we have performed a quantitative analysis of the phosphorylation networks downstream of collagen activation of integrins and DDR2. Temporal analysis over seven time points identified 424 phosphorylated proteins. Distinct DDR2 tyrosine phosphorylation sites displayed unique temporal activation profiles in agreement with in vitro kinase data. Multiple clustering analysis of the phosphoproteomic data revealed several DDR2 candidate downstream signalling nodes, including SHP-2 (Src homology 2 domain-containing protein tyrosine phosphatase 2), NCK1 (non-catalytic region of tyrosine kinase adaptor protein 1), LYN, SHIP-2 [SH2 (Src homology 2)-domain-containing inositol phosphatase 2], PIK3C2A (phosphatidylinositol-4-phosphate 3-kinase, catalytic subunit type 2α) and PLCL2 (phospholipase C-like 2). Biochemical validation showed that SHP-2 tyrosine phosphorylation is dependent on DDR2 kinase activity. Targeted proteomic profiling of a panel of lung SCC (squamous cell carcinoma) DDR2 mutants demonstrated that SHP-2 is tyrosine-phosphorylated by the L63V and G505S mutants. In contrast, the I638F kinase domain mutant exhibited diminished DDR2 and SHP-2 tyrosine phosphorylation levels which have an inverse relationship with clonogenic potential. Taken together, the results of the present study indicate that SHP-2 is a key signalling node downstream of the DDR2 receptor which may have therapeutic implications in a subset of DDR2 mutations recently uncovered in genome-wide lung SCC sequencing screens.
The present study characterizes integrin and DDR2 signalling networks activated by collagen. Using clustering approaches, DDR2-specific signalling components such as SHP-2 were identified. We further demonstrate that SHP-2 is phosphorylated by a subset of DDR2 lung cancer mutants.
PMCID: PMC3893797  PMID: 23822953
cell signalling; collagen; discoidin domain receptor; lung cancer; mass spectrometry; phosphoproteomics; CDK1, cyclin-dependent kinase 1; DDR, discoidin domain receptor; DMEM, Dulbecco’s modified Eagle’s medium; DYRK1A, dual-specificity tyrosine-phosphorylation-regulated kinase 1A; EGFR, epidermal growth factor receptor; ERK, extracellular-signal-regulated kinase; EV, empty vector; GO, Gene Ontology; HEK, human embryonic kidney; HRP, horseradish peroxidase; IL, interleukin; IMAC, immobilized metal-ion-affinity chromatography; KD, kinase domain; mAb, monoclonal antibody; MCAM, multiple clustering analysis methodology; NCK1, non-catalytic region of tyrosine kinase adaptor protein 1; PIK3C2A, phosphatidylinositol-4-phosphate 3-kinase, catalytic subunit type 2α; PLCL2, phospholipase C-like 2; RFB, radiometric filter binding; RTK, receptor tyrosine kinase; SCC, squamous cell carcinoma; SHIP-2, SH2 (Src homology 2)-domain-containing inositol phosphatase 2; SHP-2, Src homology 2 domain-containing protein tyrosine phosphatase 2; SRM, selective reaction monitoring; TDA, template-directed assembly; TEAB, triethylammonium bicarbonate; TFA, trifluoroacetic acid
14.  Mapping of DDR1 Distribution and Oligomerization on the Cell Surface by FRET Microscopy 
Journal of molecular biology  2008;385(2):432-445.
Activation of discoidin domain receptor (DDR) 1 by collagen is reported to regulate cell migration and survival processes. While the oligomeric state of DDR1 is reported to play a significant role in collagen binding, not much is known about the effect of collagen binding on DDR1 oligomerization and cellular distribution. Using fluorescence resonance energy transfer (FRET) microscopy, we monitored the interaction between DDR1 tagged with cyan fluorescent protein and DDR1 tagged with yellow fluorescent protein in live cells. Significant FRET signal indicative of receptor dimerization was found even in the absence of collagen stimulation. Collagen stimulation induced aggregation of DDR1, followed by a sharp increase in FRET signal, localized in the regions of aggregated receptor. Further analysis of DDR1 aggregation revealed that DDR1 undergoes cytoplasmic internalization and incorporation into the early endosome. We found the kinetics of DDR1 internalization to be fast, with a significant percentage of the receptor population being internalized in the first few minutes of collagen stimulation. Our results indicate that collagen stimulation induces the aggregation and internalization of DDR1 dimers at timescales much before receptor activation. These findings provide new insights into the cellular redistribution of DDR1 following its interaction with collagen type I.
PMCID: PMC2786213  PMID: 19007791
DDR1; oligomerization; FRET; collagen; internalization
15.  Discoidin Domain Receptor 1 Contributes to Tumorigenesis through Modulation of TGFBI Expression 
PLoS ONE  2014;9(11):e111515.
Discoidin domain receptor 1 (DDR1) is a member of the receptor tyrosine kinase family. The receptor is activated upon binding to its ligand, collagen, and plays a crucial role in many fundamental processes such as cell differentiation, adhesion, migration and invasion. Although DDR1 is expressed in many normal tissues, upregulated expression of DDR1 in a variety of human cancers such as lung, colon and brain cancers is known to be associated with poor prognosis. Using shRNA silencing, we assessed the oncogenic potential of DDR1. DDR1 knockdown impaired tumor cell proliferation and migration in vitro and tumor growth in vivo. Microarray analysis of tumor cells demonstrated upregulation of TGFBI expression upon DDR1 knockdown, which was subsequently confirmed at the protein level. TGFBI is a TGFβ-induced extracellular matrix protein secreted by the tumor cells and is known to act either as a tumor promoter or tumor suppressor, depending on the tumor environment. Here, we show that exogenous addition of recombinant TGFBI to BXPC3 tumor cells inhibited clonogenic growth and migration, thus recapitulating the phenotypic effect observed from DDR1 silencing. BXPC3 tumor xenografts demonstrated reduced growth with DDR1 knockdown, and the same xenograft tumors exhibited an increase in TGFBI expression level. Together, these data suggest that DDR1 expression level influences tumor growth in part via modulation of TGFBI expression. The reciprocal expression of DDR1 and TGFBI may help to elucidate the contribution of DDR1 in tumorigenesis and TGFBI may also be used as a biomarker for the therapeutic development of DDR1 specific inhibitors.
PMCID: PMC4219757  PMID: 25369402
16.  Impaired dermal wound healing in discoidin domain receptor 2-deficient mice associated with defective extracellular matrix remodeling 
The wounding response relies on tightly regulated crosstalk between recruited fibroblasts and the collagenous extracellular matrix (ECM). Discoidin domain receptor 2 (DDR2) is a tyrosine kinase receptor for fibrillar collagen expressed during pathologic scarring, for example wound healing, arthritis and cancer. We have previously shown that DDR2 phosphorylation drives key wounding responses in skin fibroblasts including proliferation, chemotactic migration and secretion of both metalloproteinases and fibrillar collagen. In this study we compared healing of cutaneous wounds in DDR2+/+ and DDR2-/- mice and analyzed specific fibroblast responses.
Cutaneous wound healing was significantly delayed in DDR2-/- mice compared with DDR2+/+ animals. Reduced α-smooth muscle actin (αSMA) expression and matrix metalloproteinase 2 (MMP2) activity in the DDR2-/- wound extracts indicated defective recruitment of skin fibroblasts. DDR2-/- wounds showed decreased tensile strength during healing, which correlated with a significant reduction in collagen content and defective collagen crosslinking. Non-wounded skin in DDR2-/- mice expressed less mRNA of the crosslinking enzymes lysyl oxidase (LOX), lysyl hydroxylase1 (LH1) and matricellular 'secreted protein, acidic and rich in cysteine' (SPARC; also known as osteonectin). Skin fibroblasts isolated from DDR2-/- mice displayed altered mRNA expression of a cluster of collagens, proteoglycans, integrins and MMPs that have been previously correlated with DDR2 expression, and reduced LOX, LH1 and SPARC mRNA levels and proteins. Stable reconstitution of wild-type DDR2 by retroviral infection restored LOX, LH1 and SPARC mRNA and protein levels in DDR2-/- fibroblasts. Contraction of collagen gels was reduced in DDR2-/- fibroblasts, accompanied by significantly reduced phosphorylated SrcY418. Inhibition of either LOX activity by β-aminoproprionitrile or MMP activity by N-[(2R)-2-(hydroxamido carbonylmethyl)-4-methylpentanoyl]-l-tryptophan methylamide (GM6001) reduced collagen gel contraction by skin fibroblasts after DDR2 induction with soluble collagen type I.
DDR2 contributes to skin fibroblast responses during tissue injury. Defective synthesis of collagen type I, crosslinking molecules and MMP2 predispose DDR2-/- mice to defective dermal wounding.
PMCID: PMC3044108  PMID: 21288331
17.  Tissue absence initiates regeneration through Follistatin-mediated inhibition of Activin signaling 
eLife  2013;2:e00247.
Regeneration is widespread, but mechanisms that activate regeneration remain mysterious. Planarians are capable of whole-body regeneration and mount distinct molecular responses to wounds that result in tissue absence and those that do not. A major question is how these distinct responses are activated. We describe a follistatin homolog (Smed-follistatin) required for planarian regeneration. Smed-follistatin inhibition blocks responses to tissue absence but does not prevent normal tissue turnover. Two activin homologs (Smed-activin-1 and Smed-activin-2) are required for the Smed-follistatin phenotype. Finally, Smed-follistatin is wound-induced and expressed at higher levels following injuries that cause tissue absence. These data suggest that Smed-follistatin inhibits Smed-Activin proteins to trigger regeneration specifically following injuries involving tissue absence and identify a mechanism critical for regeneration initiation, a process important across the animal kingdom.
eLife digest
Most animals can respond to injury with some form of tissue regeneration. In mammals, this is limited to wound healing, whereas other vertebrates—such as salamanders and zebrafish—can regenerate parts of internal organs and even entire appendages. The planarian, a flatworm, is even more remarkable, being able to regenerate its head or tail following amputation, and even a whole animal from just a small body fragment. This is particularly impressive given that planarians have a complex internal anatomy, which includes muscles, intestines, a system similar to kidneys, and a central nervous system with a brain.
How is such regeneration accomplished? Why are planarians able to regenerate their bodies so extensively, whereas humans cannot? To what extent are the mechanisms of planarian regeneration common to other animals? These questions have driven the study of planarian regeneration for more than a century, but it is only in recent years that the tools needed to address these questions at the molecular level have become available.
Planarian regeneration proceeds over several days and involves multiple processes, including gene expression, cell division and cell death. Importantly, it has recently been shown that planarians activate different responses depending on whether an injury results in significant tissue loss—and therefore requires regeneration for repair—or if simple wound healing will be sufficient. The mechanisms behind these different responses to injury have, however, remained a mystery.
Now, Gaviño et al. have identified a key mechanism in the initiation of regeneration following tissue loss. This is centered on the gene follistatin, which is expressed following wounding. When genetic techniques are used to disrupt the expression of follistatin, regeneration is completely blocked. However, the animal’s ability to routinely replace old cells via a stem-cell mediated mechanism is unaffected. This indicates that follistatin is specifically required for the replacement of cells lost through injury. Gaviño et al. further demonstrate that the protein encoded by follistatin likely initiates tissue regeneration upon substantial tissue loss through inhibition of proteins called Activins.
Activin and Follistatin proteins are broadly conserved in evolution, and are also expressed in mammals, raising the possibility that similar molecular circuits may govern regenerative responses in many species.
PMCID: PMC3771573  PMID: 24040508
planarian regeneration; wound signaling; Follistatin; Activin; Other
18.  Collagen Advanced Glycation Inhibits Its Discoidin Domain Receptor 2 (DDR2)-Mediated Induction of Lysyl Oxidase in Osteoblasts 
Bone  2013;58:10.1016/j.bone.2013.10.001.
Diabetes increases the risk of bone fracture. Organic and inorganic bone extracellular matrix components determine bone strength. Previous studies indicate that in diabetes, glycation of collagen causes abnormal arrangements of collagen molecules and fragile bones. Diabetic bone fragility is additionally attributed to reduced levels of lysyl oxidase enzyme-dependent collagen cross-links. The mechanism underlying the presence of lower enzymatic collagen cross-links in diabetic bone has not been directly investigated. Here we determine in primary osteoblast cultures the regulation of lysyl oxidase protein by type I collagen and collagen modified by carboxymethylation (CML-collagen), a form of advanced glycation endproducts. Data indicate that non-glycated collagen up-regulates lysyl oxidase levels both in primary non-differentiated and in differentiating mouse and rat osteoblast cultures, while CML-collagen fails to regulate lysyl oxidase in these cells. Collagen binding to Discoidin Domain Receptor-2 (DDR2) mediates lysyl oxidase increases, determined in DDR2 shRNA knockdown studies. DDR2 binding and activation were disrupted by collagen glycation, pointing to a mechanism for the diminished levels of lysyl oxidase and consequent low lysyl oxidase-derived cross-links in diabetic bone. Our studies indicate that collagen-integrin interactions may not play a major role in up-regulating lysyl oxidase. Furthermore, non-collagenous ligands for the receptor for advanced glycation end products (RAGE) failed to alter lysyl oxidase levels. Taken together with published studies a new understanding emerges in which diabetes- and age-dependent inhibition of normal collagen-stimulated DDR2- and integrin-signaling, and independent advanced glycation-stimulated RAGE-signaling, each contributes to different aspects of diabetic osteopenia.
PMCID: PMC3851576  PMID: 24120383
Discoidin Domain Receptor 2; Lysyl Oxidase; Osteoblasts; Advanced Glycation Endproducts; Diabetes
19.  A Discoidin Domain Receptor 1/SHP-2 Signaling Complex Inhibits α2β1-Integrin–mediated Signal Transducers and Activators of Transcription 1/3 Activation and Cell Migration 
Molecular Biology of the Cell  2006;17(6):2839-2852.
Regulation of cell migration is an important step for the development of branching tubule morphogenesis in collagen gel. Here, we showed that discoidin domain receptor (DDR) 1a/b inhibited collagen-induced tyrosine phosphorylation of signal transducers and activators of transcription (Stat) 1/3 and cell migration triggered by α2β1-integrin. Overexpression of DDR1a/b increased the interaction of DDR1 with SHP-2 and up-regulated the tyrosine phosphatase activity of SHP-2. Expression of catalytically inactive SHP-2 in DDR1-transfected cells restored the tyrosine phosphorylation of Stat3 and cell migration. We demonstrated that the Src homology-2 (SH2)-SH2 and phosphotyrosyl phosphatase (PTP) domains of SHP-2 were responsible for interaction with DDR1 and that both tyrosine phosphorylation sites 703 and 796 of DDR1 were essential for it to bind with SHP-2. Mutation of tyrosine 703 or 796 of DDR1 abolished the ability of DDR1 to inhibit the tyrosine phosphorylation of Stat1 and Stat3 and restored collagen-induced cell migration and hepatocyte growth factor-induced branching tubulogenesis in collagen gel. Together, these results demonstrate that SHP-2 is required for the DDR1-induced suppression of Stat1 and Stat3 tyrosine phosphorylation, cell migration, and branching tubulogenesis.
PMCID: PMC1474786  PMID: 16611743
20.  Discoidin domain receptor functions in physiological and pathological conditions 
The discoidin domain receptors, DDR1 and DDR2, are non-integrin collagen receptors that are members of the receptor tyrosine kinase family. Both DDRs bind a number of different collagen types and play important roles in embryo development. Dysregulated DDR function is associated with progression of various human diseases, including fibrosis, arthritis and cancer. By interacting with key components of the extracellular matrix and displaying distinct activation kinetics, the DDRs form a unique subfamily of receptor tyrosine kinases. DDR-facilitated cellular functions include cell migration, cell survival, proliferation and differentiation, as well as remodelling of extracellular matrices. This review summarises the current knowledge of DDR-ligand interactions, DDR-initiated signal pathways and the molecular mechanisms that regulate receptor function. Also discussed are the roles of DDRs in development and disease progression.
PMCID: PMC4021107  PMID: 24725424
transmembrane collagen receptor; receptor tyrosine kinase; cell-matrix interactions; cell signaling; receptor activation; therapeutic target
21.  Mutational analysis of the tyrosine kinome in serous and clear cell endometrial cancer uncovers rare somatic mutations in TNK2 and DDR1 
BMC Cancer  2014;14(1):884.
Endometrial cancer (EC) is the 8th leading cause of cancer death amongst American women. Most ECs are endometrioid, serous, or clear cell carcinomas, or an admixture of histologies. Serous and clear ECs are clinically aggressive tumors for which alternative therapeutic approaches are needed. The purpose of this study was to search for somatic mutations in the tyrosine kinome of serous and clear cell ECs, because mutated kinases can point to potential therapeutic targets.
In a mutation discovery screen, we PCR amplified and Sanger sequenced the exons encoding the catalytic domains of 86 tyrosine kinases from 24 serous, 11 clear cell, and 5 mixed histology ECs. For somatically mutated genes, we next sequenced the remaining coding exons from the 40 discovery screen tumors and sequenced all coding exons from another 72 ECs (10 clear cell, 21 serous, 41 endometrioid). We assessed the copy number of mutated kinases in this cohort of 112 tumors using quantitative real time PCR, and we used immunoblotting to measure expression of these kinases in endometrial cancer cell lines.
Overall, we identified somatic mutations in TNK2 (tyrosine kinase non-receptor, 2) and DDR1 (discoidin domain receptor tyrosine kinase 1) in 5.3% (6 of 112) and 2.7% (3 of 112) of ECs. Copy number gains of TNK2 and DDR1 were identified in another 4.5% and 0.9% of 112 cases respectively. Immunoblotting confirmed TNK2 and DDR1 expression in endometrial cancer cell lines. Three of five missense mutations in TNK2 and one of two missense mutations in DDR1 are predicted to impact protein function by two or more in silico algorithms. The TNK2P761Rfs*72 frameshift mutation was recurrent in EC, and the DDR1R570Q missense mutation was recurrent across tumor types.
This is the first study to systematically search for mutations in the tyrosine kinome in clear cell endometrial tumors. Our findings indicate that high-frequency somatic mutations in the catalytic domains of the tyrosine kinome are rare in clear cell ECs. We uncovered ten new mutations in TNK2 and DDR1 within serous and endometrioid ECs, thus providing novel insights into the mutation spectrum of each gene in EC.
Electronic supplementary material
The online version of this article (doi:10.1186/1471-2407-14-884) contains supplementary material, which is available to authorized users.
PMCID: PMC4258955  PMID: 25427824
Endometrial; Cancer; Mutation; TNK2; ACK1; DDR1; Copy number; Tyrosine kinase; Tyrosine kinome
22.  Activation of WIP1 Phosphatase by HTLV-1 Tax Mitigates the Cellular Response to DNA Damage 
PLoS ONE  2013;8(2):e55989.
Genomic instability stemming from dysregulation of cell cycle checkpoints and DNA damage response (DDR) is a common feature of many cancers. The cancer adult T cell leukemia (ATL) can occur in individuals infected with human T cell leukemia virus type 1 (HTLV-1), and ATL cells contain extensive chromosomal abnormalities, suggesting that they have defects in the recognition or repair of DNA damage. Since Tax is the transforming protein encoded by HTLV-1, we asked whether Tax can affect cell cycle checkpoints and the DDR. Using a combination of flow cytometry and DNA repair assays we showed that Tax-expressing cells exit G1 phase and initiate DNA replication prematurely following damage. Reduced phosphorylation of H2AX (γH2AX) and RPA2, phosphoproteins that are essential to properly initiate the DDR, was also observed in Tax-expressing cells. To determine the cause of decreased DDR protein phosphorylation in Tax-expressing cells, we examined the cellular phosphatase, WIP1, which is known to dephosphorylate γH2AX. We found that Tax can interact with Wip1 in vivo and in vitro, and that Tax-expressing cells display elevated levels of Wip1 mRNA. In vitro phosphatase assays showed that Tax can enhance Wip1 activity on a γH2AX peptide target by 2-fold. Thus, loss of γH2AX in vivo could be due, in part, to increased expression and activity of WIP1 in the presence of Tax. siRNA knockdown of WIP1 in Tax-expressing cells rescued γH2AX in response to damage, confirming the role of WIP1 in the DDR. These studies demonstrate that Tax can disengage the G1/S checkpoint by enhancing WIP1 activity, resulting in reduced DDR. Premature G1 exit of Tax-expressing cells in the presence of DNA lesions creates an environment that tolerates incorporation of random mutations into the host genome.
PMCID: PMC3566092  PMID: 23405243
23.  Expression and mutation analysis of the discoidin domain receptors 1 and 2 in non-small cell lung carcinoma 
British Journal of Cancer  2007;96(5):808-814.
The discoidin domain receptors, (DDR)1 and DDR2, have been linked to numerous human cancers. We sought to determine expression levels of DDRs in human lung cancer, investigate prognostic determinates, and determine the prevalence of recently reported mutations in these receptor tyrosine kinases. Tumour samples from 146 non-small cell lung carcinoma (NSCLC) patients were analysed for relative expression of DDR1 and DDR2 using quantitative real-time PCR (qRT-PCR). An additional 23 matched tumour and normal tissues were tested for differential expression of DDR1 and DDR2, and previously reported somatic mutations. Discoidin domain receptor 1 was found to be significantly upregulated by 2.15-fold (P=0.0005) and DDR2 significantly downregulated to an equivalent extent (P=0.0001) in tumour vs normal lung tissue. Discoidin domain receptor 2 expression was not predictive for patient survival; however, DDR1 expression was significantly associated with overall (hazard ratio (HR) 0.43, 95% CI=0.22–0.83, P=0.014) and disease-free survival (HR=0.56, 95% CI=0.33–0.94, P=0.029). Multivariate analysis revealed DDR1 is an independent favourable predictor for prognosis independent of tumour differentiation, stage, histology, and patient age. However, contrary to previous work, we did not observe DDR mutations. We conclude that whereas altered expression of DDRs may contribute to malignant progression of NSCLC, it is unlikely that this results from mutations in the DDR1 and DDR2 genes that we investigated.
PMCID: PMC2360060  PMID: 17299390
lung cancer; DDR1; DDR2; mutation
24.  The Zn Finger protein Iguana impacts Hedgehog signaling by promoting ciliogenesis 
Developmental biology  2009;337(1):148-156.
Hedgehog signaling is critical for metazoan development and requires cilia for pathway activity. The gene iguana was discovered in zebrafish as required for Hedgehog signaling, and encodes a novel Zn finger protein. Planarians are flatworms with robust regenerative capacities and that utilize epidermal cilia for locomotion. RNA interference of Smed-iguana in the planarian S. mediterranea caused cilia loss and failure to regenerate new cilia, but did not cause defects similar to those observed in hedgehog(RNAi) animals. Smed-iguana gene expression was also similar in pattern to the expression of multiple other ciliogenesis genes, but was not required for expression of these ciliogenesis genes. iguana-defective zebrafish had too few motile cilia in pronephric ducts and in Kupffer's vesicle. Kupffer's vesicle promotes left-right asymmetry and iguana mutant embryos had left-right asymmetry defects. Finally, human Iguana proteins (dZIP1 and dZIP1L) localize to the basal bodies of primary cilia and, together, are required for primary cilia formation. Our results indicate that a critical and broadly conserved function for Iguana is in ciliogenesis and that this function has come to be required for Hedgehog signaling in vertebrates.
PMCID: PMC2799895  PMID: 19852954
25.  Smed454 dataset: unravelling the transcriptome of Schmidtea mediterranea 
BMC Genomics  2010;11:731.
Freshwater planarians are an attractive model for regeneration and stem cell research and have become a promising tool in the field of regenerative medicine. With the availability of a sequenced planarian genome, the recent application of modern genetic and high-throughput tools has resulted in revitalized interest in these animals, long known for their amazing regenerative capabilities, which enable them to regrow even a new head after decapitation. However, a detailed description of the planarian transcriptome is essential for future investigation into regenerative processes using planarians as a model system.
In order to complement and improve existing gene annotations, we used a 454 pyrosequencing approach to analyze the transcriptome of the planarian species Schmidtea mediterranea Altogether, 598,435 454-sequencing reads, with an average length of 327 bp, were assembled together with the ~10,000 sequences of the S. mediterranea UniGene set using different similarity cutoffs. The assembly was then mapped onto the current genome data. Remarkably, our Smed454 dataset contains more than 3 million novel transcribed nucleotides sequenced for the first time. A descriptive analysis of planarian splice sites was conducted on those Smed454 contigs that mapped univocally to the current genome assembly. Sequence analysis allowed us to identify genes encoding putative proteins with defined structural properties, such as transmembrane domains. Moreover, we annotated the Smed454 dataset using Gene Ontology, and identified putative homologues of several gene families that may play a key role during regeneration, such as neurotransmitter and hormone receptors, homeobox-containing genes, and genes related to eye function.
We report the first planarian transcript dataset, Smed454, as an open resource tool that can be accessed via a web interface. Smed454 contains significant novel sequence information about most expressed genes of S. mediterranea. Analysis of the annotated data promises to contribute to identification of gene families poorly characterized at a functional level. The Smed454 transcriptome data will assist in the molecular characterization of S. mediterranea as a model organism, which will be useful to a broad scientific community.
PMCID: PMC3022928  PMID: 21194483

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