An essential role of discoidin domain receptor 2 (DDR2) in osteoblast differentiation and chondrocyte maturation via modulation of Runx2 activation

Discoid Domain Receptors Signaling in Macrophages-Mediated Diseases

Macrophages, as a crucial component of the body's immune system, play a vital role in the onset, progression, and outcome of diseases. Discoidin domain receptors (DDRs), important members of the novel receptor tyrosine kinase superfamily, exhibit unique functions in macrophage physiology. Through interactions with the extracellular matrix, DDRs activate signaling pathways such as p38 MAPK and NF-κB, regulating macrophage adhesion, migration, and secretory functions, thereby influencing their behavior in diseases. Recent studies have indicated a direct correlation between DDRs and the progression of various diseases, including inflammation, cancer, and fibrosis. However, there remain numerous knowledge gaps regarding the specific mechanisms by which DDRs function in macrophage-mediated diseases. This article provides an in-depth summary of the regulatory mechanisms of DDRs on macrophages, detailing their modulatory roles in various diseases through macrophages and their underlying mechanisms. The aim is to offer new insights into biomedical therapies targeting DDRs and the development of novel drugs.

Discoidin domain receptor 2 is an important modulator of BMP signaling during heterotopic bone formation

Bone morphogenetic proteins are essential for bone regeneration/fracture healing but can also induce heterotopic ossification (HO). Understanding accessory factors modulating BMP signaling would provide both a means of enhancing BMP-dependent regeneration while preventing HO. This study focuses on the ability of the collagen receptor, discoidin domain receptor 2 (DDR2), to regulate BMP activity. As will be shown, induction of bone formation by subcutaneous BMP2 implants is severely compromised in Ddr2-deficient mice. In addition, Ddr2 deficiency attenuates HO in mice expressing the ACVR1 mutation associated with human fibrodysplasia ossificans progressiva. In cells migrating into BMP2 implants, DDR2 is co-expressed with GLI1, a skeletal stem cell marker, and DDR2/GLI1-positive cells participate in BMP2-induced bone formation where they contribute to chondrogenic and osteogenic lineages. Consistent with this distribution, conditional knockout of Ddr2 in Gli1-expressing cells inhibited bone formation to the same extent seen in globally Ddr2-deficient animals. This response was explained by selective inhibition of Gli1+ cell proliferation without changes in apoptosis. The basis for this DDR2 requirement was explored further using bone marrow stromal cells. Although Ddr2 deficiency inhibited BMP2-dependent chondrocyte and osteoblast differentiation and in vivo, bone formation, early BMP responses including SMAD phosphorylation remained largely intact. Instead, Ddr2 deficiency reduced the nuclear/cytoplasmic ratio of the Hippo pathway intermediates, YAP and TAZ. This suggests that DDR2 regulates Hippo pathway-mediated responses to the collagen matrix, which subsequently affect BMP responsiveness. In summary, DDR2 is an important modulator of BMP signaling and a potential therapeutic target both for enhancing regeneration and treating HO.

Relationship between arginine methylation and vascular calcification

In patients on maintenance hemodialysis (MHD), vascular calcification (VC) is an independent predictor of cardiovascular disease (CVD), which is the primary cause of death in chronic kidney disease (CKD). The main component of VC in CKD is the vascular smooth muscle cells (VSMCs). VC is an ordered, dynamic activity. Under the stresses of oxidative stress and calcium-‑phosphorus imbalance, VSMCs undergo osteogenic phenotypic transdifferentiation, which promotes the formation of VC. In addition to traditional epigenetics like RNA and DNA control, post-translational modifications have been discovered to be involved in the regulation of VC in recent years. It has been reported that the process of osteoblast differentiation is impacted by catalytic histone or non-histone arginine methylation. Its function in the osteogenic process is comparable to that of VC. Thus, we propose that arginine methylation regulates VC via many signaling pathways, including as NF-B, WNT, AKT/PI3K, TGF-/BMP/SMAD, and IL-6/STAT3. It might also regulate the VC-related calcification regulatory factors, oxidative stress, and endoplasmic reticulum stress. Consequently, we propose that arginine methylation regulates the calcification of the arteries and outline the regulatory mechanisms involved.

DDR2 signaling and mechanosensing orchestrate neuroblastoma cell fate through different transcriptome mechanisms

The extracellular matrix (ECM) regulates carcinogenesis by interacting with cancer cells via cell surface receptors. Discoidin Domain Receptor 2 (DDR2) is a collagen-activated receptor implicated in cell survival, growth, and differentiation. Dysregulated DDR2 expression has been identified in various cancer types, making it as a promising therapeutic target. Additionally, cancer cells exhibit mechanosensing abilities, detecting changes in ECM stiffness, which is particularly important for carcinogenesis given the observed ECM stiffening in numerous cancer types. Despite these, whether collagen-activated DDR2 signaling and ECM stiffness-induced mechanosensing exert similar effects on cancer cell behavior and whether they operate through analogous mechanisms remain elusive. To address these questions, we performed bulk RNA sequencing (RNA-seq) on human SH-SY5Y neuroblastoma cells cultured on collagen-coated substrates. Our results show that DDR2 downregulation induces significant changes in the cell transcriptome, with changes in expression of 15% of the genome, specifically affecting the genes associated with cell division and differentiation. We validated the RNA-seq results by showing that DDR2 knockdown redirects the cell fate from proliferation to senescence. Like DDR2 knockdown, increasing substrate stiffness diminishes cell proliferation. Surprisingly, RNA-seq indicates that substrate stiffness has no detectable effect on the transcriptome. Furthermore, DDR2 knockdown influences cellular responses to substrate stiffness changes, highlighting a crosstalk between these two ECM-induced signaling pathways. Based on our results, we propose that the ECM could activate DDR2 signaling and mechanosensing in cancer cells to orchestrate their cell fate through distinct mechanisms, with or without involving gene expression, thus providing novel mechanistic insights into cancer progression.

Cross species systems biology discovers glial DDR2, STOM, and KANK2 as therapeutic targets in progressive supranuclear palsy

Progressive supranuclear palsy (PSP) is a neurodegenerative parkinsonian disorder characterized by cell-type-specific tau lesions in neurons and glia. Prior work uncovered transcriptome changes in human PSP brains, although their cell-specificity is unknown. Further, systematic data integration and experimental validation platforms to prioritize brain transcriptional perturbations as therapeutic targets in PSP are currently lacking. In this study, we combine bulk tissue (n = 408) and single nucleus RNAseq (n = 34) data from PSP and control brains with transcriptome data from a mouse tauopathy and experimental validations in Drosophila tau models for systematic discovery of high-confidence expression changes in PSP with therapeutic potential. We discover, replicate, and annotate thousands of differentially expressed genes in PSP, many of which reside in glia-enriched co-expression modules and cells. We prioritize DDR2, STOM, and KANK2 as promising therapeutic targets in PSP with striking cross-species validations. We share our findings and data via our interactive application tool PSP RNAseq Atlas ( https://rtools.mayo.edu/PSP_RNAseq_Atlas/ ). Our findings reveal robust glial transcriptome changes in PSP, provide a cross-species systems biology approach, and a tool for therapeutic target discoveries in PSP with potential application in other neurodegenerative diseases.

Exploring the Cellular and Molecular Mechanism of Discoidin Domain Receptors (DDR1 and DDR2) in Bone Formation, Regeneration, and Its Associated Disease Conditions

The tyrosine kinase family receptor of discoidin domain receptors (DDR1 and DDR2) is known to be activated by extracellular matrix collagen catalytic binding protein receptors. They play a remarkable role in cell proliferation, differentiation, migration, and cell survival. DDR1 of the DDR family regulates matrix-metalloproteinase, which causes extracellular matrix (ECM) remodeling and reconstruction during unbalanced homeostasis. Collagenous-rich DDR1 triggers the ECM of cartilage to regenerate the cartilage tissue in osteoarthritis (OA) and temporomandibular disorder (TMD). Moreover, DDR2 is prominently present in the fibroblasts, smooth muscle cells, myofibroblasts, and chondrocytes. It is crucial in generating and breaking collagen vital cellular activities like proliferation, differentiation, and adhesion mechanisms. However, the deficiency of DDR1 rather than DDR2 was detrimental in cases of OA and TMDs. DDR1 stimulated the ECM cartilage and improved bone regeneration. Based on the above information, we made an effort to outline the advancement of the utmost promising DDR1 and DDR2 regulation in bone and cartilage, also summarizing their structural, biological activity, and selectivity.

Synthetic peptides activating discoidin domain receptor 2 and collagen-binding integrins cooperate to stimulate osteoblast differentiation of skeletal progenitor cells

Skeletal progenitor: collagen interactions are critical for bone development and regeneration. Both collagen-binding integrins and discoidin domain receptors (DDR1 and DDR2) function as collagen receptors in bone. Each receptor is activated by a distinct collagen sequence; GFOGER for integrins and GVMGFO for DDRs. Specific triple helical peptides containing each of these binding domains were evaluated for ability to stimulate DDR2 and integrin signaling and osteoblast differentiation. GVMGFO peptide stimulated DDR2 Y740 phosphorylation and osteoblast differentiation as measured by induction of osteoblast marker mRNAs and mineralization without affecting integrin activity. In contrast, GFOGER peptide stimulated focal adhesion kinase (FAK) Y397 phosphorylation, an early measure of integrin activation, and to a lesser extent osteoblast differentiation without affecting DDR2-P. Significantly, the combination of both peptides cooperatively enhanced both DDR2 and FAK signaling and osteoblast differentiation, a response that was blocked in Ddr2-deficient cells. These studies suggest that the development of scaffolds containing DDR and integrin-activating peptides may provide a new route for promoting bone regeneration. STATEMENT OF SIGNIFICANCE: A method for stimulating osteoblast differentiation of skeletal progenitor cells is described that uses culture surfaces coated with a collagen-derived triple-helical peptide to selectively activate discoidin domain receptors. When this peptide is combined with an integrin-activating peptide, synergistic stimulation of differentiation is seen. This approach of combining collagen-derived peptides to stimulate the two main collagen receptors in bone (DDR2 and collagen-binding integrins) provides a route for developing a new class of tissue engineering scaffolds for bone regeneration.

Discoidin domain receptors; an ancient family of collagen receptors has major roles in bone development, regeneration and metabolism

The extracellular matrix (ECM) niche plays a critical role in determining cellular behavior during bone development including the differentiation and lineage allocation of skeletal progenitor cells to chondrocytes, osteoblasts, or marrow adipocytes. As the major ECM component in mineralized tissues, collagen has instructive as well as structural roles during bone development and is required for bone cell differentiation. Cells sense their extracellular environment using specific cell surface receptors. For many years, specific β1 integrins were considered the main collagen receptors in bone, but, more recently, the important role of a second, more primordial collagen receptor family, the discoidin domain receptors, has become apparent. This review will specifically focus on the roles of discoidin domain receptors in mineralized tissue development as well as related functions in abnormal bone formation, regeneration and metabolism.

Ablation of myeloid discoidin domain receptor 2 exacerbates arthritis and high fat diet induced inflammation

Chronic systemic inflammation leads to sever disorders and diseases. It is of great importance to explore novel target for effective treatment. Discoidin domain receptor 2 (Ddr2) is a member of receptor tyrosine kinase (RTK) family and is implicated in skeletal and fat hemostasis. However, the role of Ddr2 in myeloid cells remains obscure. In this study, we conditionally deleted Ddr2 in myeloid lineage cells to generate cKO mice to investigate the role of Ddr2 in myeloid lineage cells. We found that cKO mice exhibited more severe inflammation both in collagen antibody-induced arthritis (CAIA) and high-fat diet (HFD)-induced obesity, indicating the protective role of Ddr2 against inflammation. Mechanistically, Ddr2 promotes macrophage repolarization from the M1 to M2 phenotype, and protect against systemic inflammation. Our study reveals for the first time that Ddr2 modulates macrophage repolarization and plays critical roles in macrophage-mediated inflammation, providing potential target for the intervention of inflammation and related diseases.

Metformin Attenuates Hyperglycaemia-Stimulated Pro-Fibrotic Gene Expression in Adventitial Fibroblasts via Inhibition of Discoidin Domain Receptor 2

Molecular mechanisms underlying the diverse therapeutic effects of anti-diabetic metformin, beyond its anti-hyperglycaemic effects, remain largely unclear. Metformin is reported to reduce the long-term complications of diabetes, including cardiovascular fibrosis and remodelling. Our recent investigations show that Discoidin Domain Receptor 2 (DDR2), a Collagen receptor tyrosine kinase, has an obligate regulatory role in Collagen type I gene expression in cardiac and vascular adventitial fibroblasts, and that it may be a molecular link between arterial fibrosis and metabolic syndrome in rhesus monkeys. Using gene knockdown and overexpression approaches, the present study examined whether DDR2 is a target of metformin and whether, by targeting DDR2, it inhibits Fibronectin and Collagen type I expression in rat aortic adventitial fibroblasts exposed to hyperglycaemic conditions. Metformin was found to attenuate hyperglycaemia-induced increase in DDR2 mRNA and protein expression by inhibiting TGF-β1/SMAD2/3 signalling that mediates the stimulatory effect of hyperglycaemia on DDR2 expression. Metformin also inhibited DDR2-dependent expression of Fibronectin and Collagen type I, indicating that it regulates these matrix proteins via DDR2 inhibition. The findings identify DDR2, a mediator of cardiovascular remodelling, as a molecular target of metformin, thereby uncovering the molecular basis of its protective role in vascular fibrosis and possibly cardiac fibrosis associated with diabetic cardiomyopathy.

Discoidin domain receptor 2 regulates aberrant mesenchymal lineage cell fate and matrix organization

Extracellular matrix (ECM) interactions regulate both the cell transcriptome and proteome, thereby determining cell fate. Traumatic heterotopic ossification (HO) is a disorder characterized by aberrant mesenchymal lineage (MLin) cell differentiation, forming bone within soft tissues of the musculoskeletal system following traumatic injury. Recent work has shown that HO is influenced by ECM-MLin cell receptor signaling, but how ECM binding affects cellular outcomes remains unclear. Using time course transcriptomic and proteomic analyses, we identified discoidin domain receptor 2 (DDR2), a cell surface receptor for fibrillar collagen, as a key MLin cell regulator in HO formation. Inhibition of DDR2 signaling, through either constitutive or conditional Ddr2 deletion or pharmaceutical inhibition, reduced HO formation in mice. Mechanistically, DDR2 perturbation alters focal adhesion orientation and subsequent matrix organization, modulating Focal Adhesion Kinase (FAK) and Yes1 Associated Transcriptional Regulator and WW Domain Containing Transcription Regulator 1 (YAP/TAZ)-mediated MLin cell signaling. Hence, ECM-DDR2 interactions are critical in driving HO and could serve as a previously unknown therapeutic target for treating this disease process.

Special issue "The advance of solid tumor research in China": Discoidin domain receptor 2 promotes colorectal cancer metastasis by regulating epithelial mesenchymal transition via activating AKT signaling

Tumor metastasis is one of the main reasons for the high mortality rate associated with colorectal cancer (CRC). However, its underlying mechanisms have not been fully understood. Here, we reported that the expression of discoidin domain receptor 2 (DDR2) was significantly upregulated in CRC tissues compared with that in normal adjacent tissues. The expression level of DDR2 was negatively associated with prognosis of CRC patients. Therefore, DDR2 may play an oncogenic role in CRC development. Furthermore, DDR2 induced epithelial mesenchymal transition in CRC cells and regulated their invasive and metastatic capacity in vitro and in vivo. Mechanistically, increased DDR2 expression level activated the AKT/GSK-3β/Slug signaling pathway. In conclusion, these findings showed that DDR2 promoted CRC metastasis and DDR2 inhibition might represent an effective therapeutic strategy for local advanced and metastatic CRC treatment. This article is protected by copyright. All rights reserved.

Targeting adipocytic discoidin domain receptor 2 impedes fat gain while increasing bone mass

Obesity is closely associated with low-bone-mass disorder. Discoidin domain receptor 2 (DDR2) plays essential roles in skeletal metabolism, and is probably involved in fat metabolism. To test the potential role of DDR2 in fat and fat-bone crosstalk, Ddr2 conditional knockout mice (Ddr2^Adipo) were generated in which Ddr2 gene is exclusively deleted in adipocytes by Adipoq Cre. We found that Ddr2^Adipo mice are protected from fat gain on high-fat diet, with significantly decreased adipocyte size. Ddr2^Adipo mice exhibit significantly increased bone mass and mechanical properties, with enhanced osteoblastogenesis and osteoclastogenesis. Marrow adipocyte is diminished in the bone marrow of Ddr2^Adipo mice, due to activation of lipolysis. Fatty acid in the bone marrow was reduced in Ddr2^Adipo mice. RNA-Seq analysis identified adenylate cyclase 5 (Adcy5) as downstream molecule of Ddr2. Mechanically, adipocytic Ddr2 modulates Adcy5-cAMP-PKA signaling, and Ddr2 deficiency stimulates lipolysis and supplies fatty acid for oxidation in osteoblasts, leading to the enhanced osteoblast differentiation and bone mass. Treatment of Adcy5 specific inhibitor abolishes the increased bone mass gain in Ddr2^Adipo mice. These observations establish, for the first time, that Ddr2 plays an essential role in the crosstalk between fat and bone. Targeting adipocytic Ddr2 may be a potential strategy for treating obesity and pathological bone loss simultaneously.

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.

The collagen receptor, discoidin domain receptor 2, functions in Gli1-positive skeletal progenitors and chondrocytes to control bone development

Discoidin Domain Receptor 2 (DDR2) is a collagen-activated receptor kinase that, together with integrins, is required for cells to respond to the extracellular matrix. Ddr2 loss-of-function mutations in humans and mice cause severe defects in skeletal growth and development. However, the cellular functions of Ddr2 in bone are not understood. Expression and lineage analysis showed selective expression of Ddr2 at early stages of bone formation in the resting zone and proliferating chondrocytes and periosteum. Consistent with these findings, Ddr2⁺ cells could differentiate into hypertrophic chondrocytes, osteoblasts, and osteocytes and showed a high degree of colocalization with the skeletal progenitor marker, Gli1. A conditional deletion approach showed a requirement for Ddr2 in Gli1-positive skeletal progenitors and chondrocytes but not mature osteoblasts. Furthermore, Ddr2 knockout in limb bud chondroprogenitors or purified marrow-derived skeletal progenitors inhibited chondrogenic or osteogenic differentiation, respectively. This work establishes a cell-autonomous function for Ddr2 in skeletal progenitors and cartilage and emphasizes the critical role of this collagen receptor in bone development.

Discoidin Domain Receptor 2: A New Target in Cancer

BACKGROUND

Discoidin domain receptor is a new and unique type of receptor tyrosine kinases, which binds to collagen, the main compose of an extracellular matrix. DDR1 was identified to mediate cell aggregation, and dysregulation of DDR2 has also been shown to be involved in tumor pathogenesis, although its role in cancer development and progression remains controversial.

SUMMARY

Abnormal expression and mutations of DDR2 have been reported in several cancer types and its participation in different aspects of tumor progression, including proliferation, migration, invasion, metastasis, epithelial-mesenchymal transition, and chemotherapy resistance. Moreover, novel DDR2 inhibitors have been designed and indicate a therapeutic effect for the cancer treatment. Key Messages: In this review, we summarize the current knowledge on the role of DDR2 in cancer promotion and the potential therapeutic value of targeting DDR2.

Discoidin domain receptor 2 activation of p38 mitogen-activated protein kinase as an important pathway for osteonectin-regulating osteoblast mineralization

Objective

The present study aimed to determine the role of the discoidin domain receptor 2 (DDR2) in the osteonectin (ON) regulation of osteoblast mineralization through the activation of p38 mitogen-activated protein kinase (MAPK).

Methods

Four groups were established: the ON group, the inhibitor group, the Ddr2-small interfering ribonucleic acid (siRNA) group, and the control group. Osteoblasts from the parietal bones of neonatal Sprague–Dawley rats were isolated and cultured. In the ON group, 1 µg/mL ON was added to the osteoblasts. The gene expressions of collagen 1 (Col 1) and Ddr2 were detected using reverse transcription-quantitative polymerase chain reaction (RT-qPCR). In the inhibitor group, the osteoblasts were added to WRG-28 (a specific DDR2 inhibitor), and in the Ddr2-siRNA group, the osteoblasts were transfected with Ddr2-siRNA. The gene and protein expressions of DDR2, bone sialoprotein, osteocalcin, osteopontin, and p38 MAPK were determined using RT-qPCR and western blot analysis. Alizarin red staining and transmission electron microscopy were used to detect mineralization.

Results

The results showed that ON enhanced the osteoblast Col 1 and Ddr2 gene expressions, while the use of a Ddr2-siRNA/DDR2-blocker decreased the OPN, BSP, OCN, and P38 gene and protein expressions and reduced osteoblast cellular activity and mineralized nodules.

Conclusion

The present study demonstrated that DDR2 activation of p38 MAPK is an important approach to ON-regulating osteoblast mineralization.

Targeting Discoidin Domain Receptor 2 for the Development of Disease-Modifying Osteoarthritis Drugs

One of the most pressing issues in osteoarthritis (OA) research is the development of disease-modifying OA drugs (DMOADs), as currently there are no such drugs available. The paucity of suitable DMOADs is mostly due to the lack of approved ideal therapeutic targets necessary for the development of these drugs. However, based on recent discoveries from our laboratory and other independent laboratories, it is indicated that a cell surface receptor tyrosine kinase for collagen type II, discoidin domain receptor 2 (DDR2), may be an ideal therapeutic target for the development of DMOADs. In this article, we review the current status of research in understanding roles of DDR2 in the development of OA.

Identification of Chimeric RNAs in Pig Skeletal Muscle and Transcriptomic Analysis of Chimeric RNA TNNI2-ACTA1 V1

Chimeric RNA was considered a special marker of cancer. However, recent studies have demonstrated that chimeric RNAs also exist in non-cancerous cells and tissues. Here, we analyzed and predicted jointly 49 chimeric RNAs by Star-Fusion and FusionMap. One chimeric RNA, we named TNNI2-ACTA1, and its eight transcript variants were identified by reverse transcriptase–polymerase chain reaction. The overexpression of TNNI2-ACTA1 V1 inhibited the proliferation of porcine skeletal muscle satellite cells through down-regulating the mRNA expression levels of cell cycle–related genes cyclinD1. However, as parental genes, there is no such effect in the TNNI2 and ACTA1. To explore the underlying mechanism for this phenomenon, we used RNA-seq to profile the transcriptomes of PSCs with overexpression. Compared with the negative control group, 1,592 differentially expressed genes (DEGs) were upregulated and 1,077 DEGs downregulated in TNNI2 group; 1,226 DEGs were upregulated and 902 DEGs downregulated in ACTA1 group; and 13 DEGs were upregulated and 16 DEGs downregulated in TNNI2-ACTA1 V1 group, respectively. Compared with the parental gene groups, three specific genes were enriched in the TNNI2-ACTA1 V1 group (NCOA3, Radixin, and DDR2). These three genes may be the key to TNNI2-ACTA1 V1 regulating cell proliferation. Taken together, our study explores the role of chimeric RNAs in normal tissues. In addition, our study as the first research provides the foundation for the mechanism of chimeric RNAs regulating porcine skeletal muscle growth.

Role of Discoidin Domain Receptor 2 in Craniofacial Bone Regeneration

Bone loss caused by trauma, neoplasia, congenital defects, or periodontal disease is a major cause of disability and human suffering. Skeletal progenitor cell-extracellular matrix interactions are critical for bone regeneration. Discoidin domain receptor 2 (DDR2), an understudied collagen receptor, plays an important role in skeletal development. Ddr2 loss-of-function mutations in humans and mice cause severe craniofacial and skeletal defects, including altered cranial shape, dwarfing, reduced trabecular and cortical bone, alveolar bone/periodontal defects, and altered dentition. However, the role of this collagen receptor in craniofacial regeneration has not been examined. To address this, calvarial subcritical-size defects were generated in wild-type (WT) and Ddr2-deficient mice. The complete bridging seen in WT controls at 4 wk postsurgery was not observed in Ddr2-deficient mice even after 12 wk. Quantitation of defect bone area by micro-computed tomography also revealed a 50% reduction in new bone volume in Ddr2-deficient mice. Ddr2 expression during calvarial bone regeneration was measured using Ddr2-LacZ knock-in mice. Expression was restricted to periosteal surfaces of uninjured calvarial bone and, after injury, was detected in select regions of the defect site by 3 d postsurgery and expanded during the healing process. The impaired bone healing associated with Ddr2 deficiency may be related to reduced osteoprogenitor or osteoblast cell proliferation and differentiation since knockdown/knockout of Ddr2 in a mesenchymal cell line and primary calvarial osteoblast cultures reduced osteoblast differentiation while Ddr2 overexpression was stimulatory. In conclusion, Ddr2 is required for cranial bone regeneration and may be a novel target for therapy.

Tyrosine kinases regulate chondrocyte hypertrophy: promising drug targets for Osteoarthritis

Osteoarthritis (OA) is a major health problem worldwide that affects the joints and causes severe disability. It is characterized by pain and low-grade inflammation. However, the exact pathogenesis remains unknown and the therapeutic options are limited. In OA articular chondrocytes undergo a phenotypic transition becoming hypertrophic, which leads to cartilage damage, aggravating the disease. Therefore, a therapeutic agent inhibiting hypertrophy would be a promising disease-modifying drug. The therapeutic use of tyrosine kinase inhibitors has been mainly focused on oncology, but the Food and Drug Administration (FDA) approval of the Janus kinase inhibitor Tofacitinib in Rheumatoid Arthritis has broadened the applicability of these compounds to other diseases. Interestingly, tyrosine kinases have been associated with chondrocyte hypertrophy. In this review, we discuss the experimental evidence that implicates specific tyrosine kinases in signaling pathways promoting chondrocyte hypertrophy, highlighting their potential as therapeutic targets for OA.

Enhanced osteogenic differentiation of alendronate-conjugated nanodiamonds for potential osteoporosis treatment

Background

Alendronate (Alen) is promising material used for bone-targeted drug delivery due to its high bone affinity and therapeutic effects on bone diseases. In addition, Alen can enhance the osteogenic differentiation of osteoblastic cell. Recently, nanodiamonds (NDs) with hardness, non-toxicity, and excellent biocompatibility are employed as promising materials for carrier systems and osteogenic differentiation. Therefore, we prepared Alen-conjugated NDs (Alen-NDs) and evaluated their osteogenic differentiation performances.

Methods

Alen-NDs were synthesized using DMTMM as a coupling reagent. Morphological change of Mouse calvaria-derived preosteoblast (MC3T3-E1) treated with Alen-NDs was observed using the confocal microscope. The osteogenic differentiation was confirmed by cell proliferation, alkaline phosphatase (ALP), calcium deposition, and real-time polymerase chain reaction assay.

Results

Alen-NDs were prepared to evaluate their effect on the proliferation and differentiation of osteoblastic MC3T3-E1 cells. The Alen-NDs had a size of about 100 nm, and no cytotoxicity at less than 100 μg/mL of concentration. The treatment of NDs and Alen-NDs reduced the proliferation rate of MC3T3-E1 cells without cell death. Confocal microscopy images confirmed that the treatment of NDs and Alen-NDs changed the cellular morphology from a fibroblastic shape to a cuboidal shape. Flow cytometry, alkaline phosphatase (ALP) activity, calcium deposition, and real-time polymerase chain reaction (RT-PCR) confirmed the higher differentiation of MC3T3-E1 cells treated by Alen-NDs, compared to the groups treated by osteogenic medium and NDs. The higher concentration of Alen-ND treated in MC3T3-E1 resulted in a higher differentiation level.

Conclusions

Alen-NDs can be used as potential therapeutic agents for osteoporosis treatment by inducing osteogenic differentiation.

Parallels between the extracellular matrix roles in developmental biology and cancer biology

Interaction of a tumor with its microenvironment is an emerging field of investigation, and the crosstalk between cancer cells and the extracellular matrix is of particular interest, since cancer patients with abundant and stiff extracellular matrices display a poorer prognosis. At the post-juvenile stage, the extracellular matrix plays predominantly a structural role by providing support to cells and tissues; however, during development, matrix proteins exert a plethora of diverse signals to guide the movement and determine the fate of pluripotent cells. Taking a closer look at the communication between the extracellular matrix and cells of a developing body may bring new insights into cancer biology and identify cancer weaknesses. This review discusses parallels between the extracellular matrix roles during development and tumor growth.

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.

Active MT1-MMP is tethered to collagen fibers in DDR2-containing remnants

Type I collagen is a major extracellular matrix (ECM) component in the interstitial stroma of solid tumors, and it represents the first barrier against tumor cell invasion after basement-membrane degradation. The collagen receptors that convey molecular signals into the cells are collagen-binding discoidin domain receptors (DDRs) and integrins. Collagen-activated DDR2 clusters form DDR2-containing remnants in an integrin-dependent manner in three-dimensional (3D) collagen matrix. Although DDR2-containing remnants in the collagen matrix may generate sustained perturbation to ECM remodeling, the molecular components and function of the remnants are largely unknown. Here we determined the interaction and co-localization between DDR2 and membrane type I-matrix metalloproteinase (MT1-MMP) in the cells and the DDR2-containing remnants on collagen fibers, and we found that MT1-MMP was co-tethered to collagen fibers in the remnants. These collagen fiber-associated MT1-MMP remained active. Furthermore, DDR2 enhanced MT1-MMP proteolytic activity. These results demonstrate that DDR2 ensures the remnant-associated MT1-MMP to continue the degradation of ECM in addition to pericellular ECM degradation mediated by cell surface tethered MT1-MMP. Thus, our findings reveal a new alternative ECM degradation mechanism mediated by MT1-MMP in the DDR2-containing remnants.

The expression of two collagen receptor subfamilies, integrins and discoidin domains during osteogenic and chondrogenic differentiation of human mesenehymal stem cells

BACKGROUND

Collagen receptors are characterized by binding to and being activated by collagens. We know little about the molecular mechanism by which the integrins and discoidin domains (DDRs) recognize collagen.

OBJECTIVE

The aim of this study was to investigate the expression of two main collagen receptor subfamilies, integrins and DDRs, during osteogenic and chondrogenic differentiation of human mesenehymal stem cells (hMSCs).

METHODS

Using qRT-PCR, Western blots and FACS, the levels of DDR1, DDR2, integrin subunits β1, α1, α2, α10 and α11 receptors on hMSCs, were assessed upon activation by collagen type I, as well as during osteogenic and chondrogenic differentiation.

RESULTS

The expression of DDR2 and integrin α11β1 was altered compared with other receptors when the cells were cultured under undifferentiated conditions. During osteogenic and chondrogenetic differentiation, DDR2 and α11 were up-regulated during early stages (6 day) of osteogenesis and chondrogenesis, respectively. The expression and activation of DDR2 was concomitant with another receptor integrin subunit β1 during osteogenetic differentiation.

CONCLUSIONS

The results suggested that DDR2 was more specific for osteogenesis than chondrogenesis, while integrin α11β1 was more specific in chondrogenesis. DDR2 and α11 may play a role in the regulation of osteogenesis and chondrogenesis based on the differential expression of these receptors during lineage-dependent changes.

Complex roles of discoidin domain receptor tyrosine kinases in cancer

Discoidin domain receptors, DDR1 and DDR2 are members of the receptor tyrosine kinase (RTK) family that serves as a non-integrin collagen receptor and were initially identified as critical regulators of embryonic development and cellular homeostasis. In recent years, numerous studies have focused on the role of these receptors in disease development, in particular, cancer where they have been reported to augment ECM remodeling, invasion, drug resistance to facilitate tumor progression and metastasis. Interestingly, accumulating evidence also suggests that DDRs promote apoptosis and suppress tumor progression in various human cancers due to which their functions in cancer remain ill-defined and presents a case of an interesting therapeutic target. The present review has discussed the role of DDRs in tumorigenesis and the metastasis.

Discoidin Domain Receptors 1 Inhibition Alleviates Osteoarthritis via Enhancing Autophagy

We recently reported that the chondrocyte-specific knockout of discoidin domain receptors 1 (Ddr1) delayed endochondral ossification (EO) in the growth plate by reducing the chondrocyte hypertrophic terminal differentiation, and apoptosis. The biologic and phenotypic changes in chondrocytes in the articular cartilage with osteoarthritis (OA) are similar to the phenomena observed in the process of EO. Additionally, autophagy can promote chondrocyte survival and prevent articular cartilage from degradation in OA. On this basis, we explored the effect of Ddr1 inhibition on OA prevention and further investigated the roles of autophagy in treating OA with a Ddr1 inhibitor (7 rh). The anterior cruciate ligament transection (ACLT)-OA model was used to investigate the role of 7 rh in vivo. Forty 8-week-old mice were randomly assigned to four groups, including the sham group, ACLT group, and two treated groups (ACLT with 7 rh 6.9 nM or 13.8 nM). According to the study design, normal saline or 7 rh were intra-articular (IA) injected into studied knees 3 times per week for 2 weeks and then once per week for 4 weeks. The results showed that 7 rh treatment significantly improved the functional performances (the weight-bearing ability and the running endurance), decreased cartilage degradation, and also reduced the terminal differentiation markers (collagen type X, Indian hedgehog, and matrix metalloproteinase 13). Moreover, 7 rh decreased chondrocyte apoptosis by regulating chondrocyte autophagy through reducing the expression of the mammalian target of rapamycin and enhancing the light chain 3 and beclin-1 expression. These results demonstrated that the IA injection of 7 rh could reduce the chondrocyte apoptosis and promote chondrocyte autophagy, leading to the attenuation of cartilage degradation. Our observations suggested that the IA injection of 7 rh could represent a potential disease-modifying therapy to prevention OA progression.

DDR2, a discoidin domain receptor, is a marker of periosteal osteoblast and osteoblast progenitors

INTRODUCTION

The periosteum has a bilayered structure that surrounds cortical bone. The outer layer is rich in connective tissue and fibroblasts, while the inner layer in contact with the cortical surface of the bone predominantly consists of osteoblasts and osteoblast progenitors. The identification of cell-specific surface markers of the bilayered structure of the periosteum is important for the purpose of tissue regeneration.

MATERIALS AND METHODS

We investigated the expression of the discoidin domain tyrosine kinase receptor DDR2, fibroblast specific protein-1 (FSP-1) and alkaline phosphatase (ALP) in the periosteum of cortical bone by immunohistochemistry. Osteogenic differentiation was compared between DDR2- and FSP-1-expressing cells flow-sorted from the periosteum.

RESULTS

We showed that DDR2 predominantly labeled osteogenic cells residing in the inner layer of the periosteum and that Pearson's coefficient of colocalization indicated a significant correlation with the expression of ALP. The mineralization of DDR2-expressing osteogenic cells isolated from the periosteum was significantly induced. In contrast, FSP-1 predominantly labeled the outer layer of periosteal fibroblasts, and Pearson's coefficient of colocalization indicated that FSP-1 was poorly correlated with the expression of DDR2 and ALP. FSP-1-expressing periosteal fibroblasts did not exhibit osteogenic differentiation for the induction of bone mineralization.

CONCLUSION

DDR2 is a novel potential cell surface marker for identifying and isolating osteoblasts and osteoblast progenitors within the periosteum that can be used for musculoskeletal regenerative therapies.

Blockade of Discoidin Domain Receptor 2 as a Strategy for Reducing Inflammation and Joint Destruction in Rheumatoid Arthritis Via Altered Interleukin-15 and Dkk-1 Signaling in Fibroblast-Like Synoviocytes

OBJECTIVE

This study was undertaken to uncover the pathophysiologic role of discoidin domain receptor 2 (DDR-2), a putative fibrillar collagen receptor, in inflammation promotion and joint destruction in rheumatoid arthritis (RA).

METHODS

In synovial tissue from patients with RA and from mice with collagen antibody-induced arthritis (CAIA) (using Ddr2^-/- and DBA/1 mice), gene and protein expression levels of DDR-2, interleukin-15 (IL-15), and Dkk-1 were measured by quantitative reverse transcription-polymerase chain reaction, Western blotting, and immunohistochemistry. Gene knockdown of DDR2 in human RA fibroblast-like synoviocytes (FLS) was conducted via small interfering RNA. Interaction between the long noncoding RNA H19 and microRNA 103a (miR-103a) was assessed in RA FLS using RNA pulldown assays. Cellular localization of H19 was examined using fluorescence in situ hybridization assays. Chromatin immunoprecipitation and dual luciferase reporter assays were applied to verify H19 transcriptional and posttranscriptional regulation by miR-103a.

RESULTS

DDR2 messenger RNA (mRNA) expression was significantly associated with the levels of IL-15 and Dkk-1 mRNA in the synovial tissue of RA patients (r² = 0.2022-0.3293, all P < 0.05; n = 33) and with the serum levels of IL-15 and Dkk-1 in mice with CAIA (P < 0.05). In human RA FLS, activated DDR-2 induced the expression of H19 through c-Myc. Moreover, H19 directly interacted with and promoted the degradation of miR-103a.

CONCLUSION

These results indicate a novel role for activated DDR-2 in RA FLS, showing that DDR-2 is responsible for regulating the expression of IL-15 and Dkk-1 in RA FLS and is involved in the promotion of inflammation and joint destruction during pathophysiologic development of RA. Moreover, DDR-2 inhibition, acting through the H19-miR-103a axis, leads to reductions in the inflammatory reaction and severity of joint destruction in mice with CAIA, suggesting that inhibition of DDR-2 may be a potential therapeutic strategy for RA.

The Role of Discoidin Domain Receptor 2 in Tooth Development

Collagen signaling is critical for proper bone and tooth formation. Discoidin domain receptor 2 (DDR2) is a collagen-activated tyrosine kinase receptor shown to be essential for skeletal development. Patients with loss of function mutations in DDR2 develop spondylo-meta-epiphyseal dysplasia (SMED), a rare, autosomal recessive disorder characterized by short stature, short limbs, and craniofacial anomalies. A similar phenotype was observed in Ddr2-deficient mice, which exhibit dwarfism and defective bone formation in the axial, appendicular, and cranial skeletons. However, it is not known if Ddr2 has a role in tooth formation. We first defined the expression pattern of Ddr2 during tooth formation using Ddr2-LacZ knock-in mice. Ddr2 expression was detected in the dental follicle/sac and dental papilla mesenchyme of developing teeth and in odontoblasts and the periodontal ligament (PDL) of adults. No LacZ staining was detected in wild-type littermates. This Ddr2 expression pattern suggests a potential role in the tooth and surrounding periodontium. To uncover the function of Ddr2, we used Ddr2^slie/slie mice, which contain a spontaneous 150-kb deletion in the Ddr2 locus to produce an effective null. In comparison with wild-type littermates, Ddr2^slie/slie mice displayed disproportional tooth size (decreased root/crown ratio), delayed tooth root development, widened PDL space, and interradicular alveolar bone defects. Ddr2^slie/slie mice also had abnormal collagen content associated with upregulation of periostin levels within the PDL. The delayed root formation and periodontal abnormalities may be related to defects in RUNX2-dependent differentiation of odontoblasts and osteoblasts; RUNX2-S319-P was reduced in PDLs from Ddr2^slie/slie mice, and deletion of Ddr2 in primary cell cultures from dental pulp and PDL inhibited differentiation of cells to odontoblasts or osteoblasts, respectively. Together, our studies demonstrate odontoblast- and PDL-specific expression of Ddr2 in mature and immature teeth, as well as indicate that DDR2 signaling is important for normal tooth formation and maintenance of the surrounding periodontium.

Dosage analysis of the 7q11.23 Williams region identifies BAZ1B as a major human gene patterning the modern human face and underlying self-domestication

We undertook a functional dissection of chromatin remodeler BAZ1B in neural crest (NC) stem cells (NCSCs) from a uniquely informative cohort of typical and atypical patients harboring 7q11.23 copy number variants. Our results reveal a key contribution of BAZ1B to NCSC in vitro induction and migration, coupled with a crucial involvement in NC-specific transcriptional circuits and distal regulation. By intersecting our experimental data with new paleogenetic analyses comparing modern and archaic humans, we found a modern-specific enrichment for regulatory changes both in BAZ1B and its experimentally defined downstream targets, thereby providing the first empirical validation of the human self-domestication hypothesis and positioning BAZ1B as a master regulator of the modern human face. In so doing, we provide experimental evidence that the craniofacial and cognitive/behavioral phenotypes caused by alterations of the Williams-Beuren syndrome critical region can serve as a powerful entry point into the evolution of the modern human face and prosociality.

Insulin/IGF signaling and discoidin domain receptors: An emerging functional connection

The insulin/insulin-like growth factor system (IIGFs) plays a fundamental role in the regulation of prenatal and postnatal growth, metabolism and homeostasis. As a consequence, dysregulation of this axis is associated with growth disturbance, type 2 diabetes, chronic inflammation and tumor progression. A functional crosstalk between IIGFs and discoidin domain receptors (DDRs) has been recently discovered. DDRs are non-integrin collagen receptors that canonically undergo slow and long-lasting autophosphorylation after binding to fibrillar collagen. While both DDR1 and DDR2 functionally interact with IIGFs, the crosstalk with DDR1 is so far better characterized. Notably, the IIGFs-DDR1 crosstalk presents a feed-forward mechanism, which does not require collagen binding, thus identifying novel non-canonical action of DDR1. Further studies are needed to fully explore the role of this IIGFs-DDRs functional loop as potential target in the treatment of inflammatory and neoplastic disorders.

Proteomic Analysis of Vocal Fold Fibroblasts Exposed to Cigarette Smoke Extract: Exploring the Pathophysiology of Reinke's Edema

Reinke's edema is a smoking-associated, benign, mostly bilateral lesion of the vocal folds leading to difficulties in breathing and voice problems. Pronounced histological changes such as damaged microvessels or immune cell infiltration have been described in the vocal fold connective tissue, the lamina propria Thus, vocal fold fibroblasts, the main cell type of the lamina propria, have been postulated to play a critical role in disease mediation. Yet information about the pathophysiology is still scarce and treatment is only surgical, i.e. symptomatic. To explore the pathophysiology of Reinke's edema, we exposed near-primary human vocal fold fibroblasts to medium conditioned with cigarette smoke extract for 24 h as well as 4 days followed by quantitative mass spectrometry.Proteomic analyses after 24 h revealed that cigarette smoke increased proteins previously described to be involved in oxidative stress responses in other contexts. Correspondingly, gene sets linked to metabolism of xenobiotics and reactive oxygen species were significantly enriched among cigarette smoke-induced proteins. Among the proteins most downregulated by cigarette smoke, we identified fibrillar collagens COL1A1 and COL1A2; this reduction was validated by complementary methods. Further, we found a significant increase of UDP-glucose 6-dehydrogenase, generating a building block for biosynthesis of hyaluronan, another crucial component of the vocal fold lamina propria In line with this result, hyaluronan levels were significantly increased because of cigarette smoke exposure. Long term treatment of 4 days did not lead to significant changes.The current findings corroborate previous studies but also reveal new insights in possible disease mechanisms of Reinke's edema. We postulate that changes in the composition of the vocal folds' extracellular matrix -reduction of collagen fibrils, increase of hyaluronan- may lead to the clinical findings. This might ease the identification of better, disease-specific treatment options.

Histone Arginine Methylation-Mediated Epigenetic Regulation of Discoidin Domain Receptor 2 Controls the Senescence of Human Bone Marrow Mesenchymal Stem Cells

The application of human bone marrow mesenchymal stem cells (hBM-MSCs) in cell-based clinical therapies is hindered by the limited number of cells remaining after the initial isolation process and by cellular senescence following in vitro expansion. Understanding the process of in vitro senescence in hBM-MSCs would enable the development of strategies to maintain their vitality after cell culture. Herein, we compared the gene expression profiles of human embryonic stem cells and human BM-MSCs from donors of different ages. We first found that the expression of discoidin domain receptor 2 (DDR2) in adult donor-derived hBM-MSCs was lower than it was in the young donor-derived hBM-MSCs. Moreover, in vitro cultured late-passage hBM-MSCs showed significant downregulation of DDR2 compared to their early-passage counterparts, and siRNA inhibition of DDR2 expression recapitulated features of senescence in early-passage hBM-MSCs. Further, we found through knockdown and overexpression approaches that coactivator-associated arginine methyltransferase 1 (CARM1) regulated the expression level of DDR2 and the senescence of hBM-MSCs. Finally, chromatin immunoprecipitation analysis confirmed direct binding of CARM1 to the DDR2 promoter region with a high level of H3R17 methylation in early-passage hBM-MSCs, and inhibition of CARM1-mediated histone arginine methylation decreased DDR2 expression and led to cellular senescence. Taken together, our findings suggest that DDR2 plays a major role in regulating the in vitro senescence of hBM-MSCs and that CARM1-mediated histone H3 methylation might be the upstream regulatory mechanism controlling this function of DDR2.

Allogeneic adipose-derived stem cell transplantation on knee osteoarthritis rats and its effect on MMP-13 and DDR2

This study investigated the efficacy of transplantation of allogeneic adipose-derived stem cells (ADMSCs) in rats with knee osteoarthritis (KOA) and the effect on the expression of matrix metalloproteinase 13 (MMP-13) and discoid domain receptor 2 (DDR2). In total, sixty rats were randomly selected. Eleven rats were selected as the blank group. Forty-four rat KOA models were established, and the remaining 5 rats were used for stem cell extraction. The rats were randomly divided into two groups, and the transplantation group was treated with ADMSCs transplantation. The KOA group was intragastrically administered with saline. The expressions of MMP-13 mRNA and DDR2 in rats were detected by RT-qPCR and immunohistochemistry. Correlation analysis was performed in MMP-13 mRNA and DDR2 expression levels in the KOA rats. After treatment, the indexes of Lequesne MG knee joints, MMP-13 mRNA and DDR2 in the transplanted rats were significantly lower than those in the KOA group (P<0.05). In the KOA rats, MMP-13 mRNA and DDR2 was positively correlated (r=0.830, P<0.001). Therefore, the transplantation of ADMSCs has a significant effect on the KOA rats, which can effectively improve the knee joint function of KOA rats and reduce the expression of MMP-13 mRNA and DDR2 in rats, and it is worthy of clinical promotion.

A catalog of single nucleotide changes distinguishing modern humans from archaic hominins

Throughout the past decade, studying ancient genomes has provided unique insights into human prehistory, and differences between modern humans and other branches like Neanderthals can enrich our understanding of the molecular basis of unique modern human traits. Modern human variation and the interactions between different hominin lineages are now well studied, making it reasonable to go beyond fixed genetic changes and explore changes that are observed at high frequency in present-day humans. Here, we identify 571 genes with non-synonymous changes at high frequency. We suggest that molecular mechanisms in cell division and networks affecting cellular features of neurons were prominently modified by these changes. Complex phenotypes in brain growth trajectory and cognitive traits are likely influenced by these networks and other non-coding changes presented here. We propose that at least some of these changes contributed to uniquely human traits, and should be prioritized for experimental validation.

Cell-Matrix Interactions and Matricrine Signaling in the Pathogenesis of Vascular Calcification

Vascular calcification is a complex pathological process occurring in patients with atherosclerosis, type 2 diabetes, and chronic kidney disease. The extracellular matrix, via matricrine-receptor signaling plays important roles in the pathogenesis of calcification. Calcification is mediated by osteochondrocytic-like cells that arise from transdifferentiating vascular smooth muscle cells. Recent advances in our understanding of the plasticity of vascular smooth muscle cell and other cells of mesenchymal origin have furthered our understanding of how these cells transdifferentiate into osteochondrocytic-like cells in response to environmental cues. In the present review, we examine the role of the extracellular matrix in the regulation of cell behavior and differentiation in the context of vascular calcification. In pathological calcification, the extracellular matrix not only provides a scaffold for mineral deposition, but also acts as an active signaling entity. In recent years, extracellular matrix components have been shown to influence cellular signaling through matrix receptors such as the discoidin domain receptor family, integrins, and elastin receptors, all of which can modulate osteochondrocytic differentiation and calcification. Changes in extracellular matrix stiffness and composition are detected by these receptors which in turn modulate downstream signaling pathways and cytoskeletal dynamics, which are critical to osteogenic differentiation. This review will focus on recent literature that highlights the role of cell-matrix interactions and how they influence cellular behavior, and osteochondrocytic transdifferentiation in the pathogenesis of cardiovascular calcification.

Secretome proteins regulate comparative osteogenic and adipogenic potential in bone marrow and dental stem cells

INTRODUCTION

Dental stem cells (DMSC) have been studied extensively since their early discovery. However, the data regarding osteogenic potential of DMSC with other cell types is sparse and the secretome proteins underlying these differences have not been explored. In this study, we have compared the osteogenic and adipogenic potential of DMSC with Bone Marrow Stem cells (BMSC) and reported the contribution of secretome proteins in controlling their differentiation.

METHODS

Osteogenic potential of these stem cells was compared by mineralization assay, alkaline phosphatase (ALP) assay, immunofluorescence of dentine sialo phosphoprotein (DSPP) & qPCR for osteogenic genes. Adipogenic potential was compared by Oil Red O staining and qPCR for PPAR-γ, leptin & adipsin. Proteomic analysis of secretome was performed by employing WATERS nano Lc-MS/MS system.

RESULTS

We observed a higher osteogenic potential in DMSC, especially dental pulp stem cells (DPSC) as compared to BMSC population but adipogenic potential was found to be better in BMSC as compared to DMSC. Deeper investigations into secretome of these cells by Lc-MS/MS revealed the presence of proteins pertaining to osteogenic and adipogenic lineage. Presence of some important proteins regulating osteogenic (DSPP, BMP7, DDR2, USP9X) and adipogenic differentiation (NCOA2, PEG10, LPA) in secretome of BMSC and DMSC reflected the role of paracrine factors during differentiation.

CONCLUSION

Our study provides first evidence regarding regulation of osteogenic/adipogenic potential by secretome proteins in DMSC and BMSC. DMSC especially DPSC and its secretome show an inherent tendency for higher osteogenic differentiation and lower adipogenic differentiation, these may be potential candidates for effective future therapy in osteoporosis where disturbance of osteocyte/adipocyte homeostasis is reported.

The Physical and Biochemical Properties of the Extracellular Matrix Regulate Cell Fate

The extracellular matrix is a complex network of hydrated macromolecular proteins and sugars that, in concert with bound soluble factors, comprise the acellular stromal microenvironment of tissues. Rather than merely providing structural information to cells, the extracellular matrix plays an instructive role in development and is critical for the maintenance of tissue homeostasis. In this chapter, we review the composition of the extracellular matrix and summarize data illustrating its importance in embryogenesis, tissue-specific development, and stem cell differentiation. We discuss how the biophysical and biochemical properties of the extracellular matrix ligate specific transmembrane receptors to activate intracellular signaling that alter cell shape and cytoskeletal dynamics to modulate cell growth and viability, and direct cell migration and cell fate. We present examples describing how the extracellular matrix functions as a highly complex physical and chemical entity that regulates tissue organization and cell behavior through a dynamic and reciprocal dialogue with the cellular constituents of the tissue. We suggest that the extracellular matrix not only transmits cellular and tissue-level force to shape development and tune cellular activities that are key for coordinated tissue behavior, but that it is itself remodeled such that it temporally evolves to maintain the integrated function of the tissue. Accordingly, we argue that perturbations in extracellular matrix composition and structure compromise key developmental events and tissue homeostasis, and promote disease.

Selective Role of Discoidin Domain Receptor 2 in Murine Temporomandibular Joint Development and Aging

Temporomandibular joint (TMJ) disorders are often associated with development of osteoarthritis-like changes in the mandibular condyle. Discoidin domain receptor 2 (DDR2), a collagen receptor preferentially activated by type I and III collagen found in the TMJ and other fibrocartilages, has been associated with TMJ degeneration, but its role in normal joint development has not been previously examined. Using Ddr2 LacZ-tagged mice and immunohistochemistry, we found that DDR2 is preferentially expressed and activated in the articular zone of TMJs but not knee joints. To assess the requirement for Ddr2 in TMJ development, studies were undertaken to compare wild-type and smallie ( slie) mice, which contain a spontaneous deletion in Ddr2 to produce an effective null allele. Analysis of TMJs from newborn Ddr2^slie/slie mice revealed a developmental delay in condyle mineralization, as measured by micro-computed tomography and histologic analysis. In marked contrast, knee joints of Ddr2^slie/slie mice were normal. Analysis of older Ddr2^slie/slie mice (3 and 10 mo) revealed that the early developmental delay led to a dramatic and progressive loss of TMJ articular integrity and osteoarthritis-like changes. Mutant condyles had a rough and flattened bone surface, accompanied by a dramatic loss of bone mineral density. Mankin scores showed significantly greater degenerative changes in the TMJs of 3- and 10-mo-old Ddr2^slie/slie mice as compared with wild-type controls. No DDR2-dependent degenerative changes were seen in knees. Analysis of primary cultures of TMJ articular chondrocytes from wild-type and Ddr2^slie/slie mice showed defects in chondrocyte maturation and mineralization in the absence of Ddr2. These studies demonstrate that DDR2 is necessary for normal TMJ condyle development and homeostasis and that these DDR2 functions are restricted to TMJ fibrocartilage and not seen in the hyaline cartilage of the knee.

DDR2 controls the epithelial-mesenchymal-transition-related gene expression via c-Myb acetylation upon matrix stiffening

Increasing matrix stiffness caused by the extracellular matrix (ECM) deposition surrounding cancer cells is accompanied by epithelial-mesenchymal transition (EMT). Here, we show that expression levels of EMT marker genes along with discoidin domain receptor 2 (DDR2) can increase upon matrix stiffening. DDR2 silencing by short hairpin RNA downregulated EMT markers. Promoter analysis and chromatin immunoprecipitation revealed that c-Myb and LEF1 may be responsible for DDR2 induction during cell culture on a stiff matrix. Mechanistically, c-Myb acetylation by p300, which is upregulated on the stiff matrix, seems to be necessary for the c-Myb-and-LEF1-mediated DDR2 expression. Finally, we found that the c-Myb-DDR2 axis is crucial for lung cancer cell line proliferation and expression of EMT marker genes in a stiff environment. Thus, our results suggest that DDR2 regulation by p300 expression and/or c-Myb acetylation upon matrix stiffening may be necessary for regulation of EMT and invasiveness of lung cancer cells.

Toxicity of cadmium and its health risks from leafy vegetable consumption

Cadmium (Cd) is a highly toxic heavy metal and has spread widely in the environment in recent decades. This review summarizes current knowledge about Cd contamination of leafy vegetables, its toxicity, exposure, health risks, and approaches to reducing its toxicity in humans. Leafy vegetable consumption has been identified as a dominant exposure pathway of Cd in the human body. An overview of Cd pollution in leafy vegetables as well as the main sources of Cd is given. Notable estimated daily intakes and health risks of Cd exposure through vegetable consumption for humans are revealed in occupational exposure areas and even in some reference areas. Vegetable consumption is one of the most significant sources of exposure to Cd, particularly in occupational exposure regions. Therefore, numerous approaches have been developed to minimize the accumulation of Cd in leafy vegetables, among which the breeding of Cd pollution-safe cultivars is one of the most effective tools. Furthermore, dietary supplements from leafy vegetables perform positive roles in alleviating Cd toxicity in humans with regard to the effects of essential mineral elements, vitamins and phytochemicals taken into the human body via leafy vegetable consumption.

Control of the Osteoblast Lineage by Mitogen-Activated Protein Kinase Signaling

PURPOSE OF THE REVIEW

This review will provide a timely assessment of MAP kinase actions in bone development and homeostasis with particular emphasis on transcriptional control of the osteoblast lineage.

RECENT FINDINGS

ERK and p38 MAP kinases function as transducers of signals initiated by the extracellular matrix, mechanical loading, TGF-β, BMPs and FGF2. MAPK signals may also affect and/or interact with other important pathways such as WNT and HIPPO. ERK and p38 MAP kinase pathways phosphorylate specific osteogenic transcription factors including RUNX2, Osterix, ATF4 and DLX5. For RUNX2, phosphorylation at specific serine residues initiates epigenetic changes in chromatin necessary for decondensation and increased transcription. MAPK also suppresses marrow adipogenesis by phosphorylating and inhibiting PPARγ, which may explain the well-known relationship between reduced skeletal loading and marrow fat accumulation.

SUMMARY

MAPKs transduce signals from the extracellular environment to the nucleus allowing bone cells to respond to changes in hormonal/growth factor signaling and mechanical loading thereby optimizing bone structure to meet physiological and mechanical needs of the body.

DDR2-CYR61-MMP1 Signaling Pathway Promotes Bone Erosion in Rheumatoid Arthritis Through Regulating Migration and Invasion of Fibroblast-Like Synoviocytes

Regulation of matrix metalloproteinases (MMPs) by collagen in the fibroblast-like synoviocytes (FLSs) plays a critical role in joint destruction in rheumatoid arthritis (RA). Our previous study indicated that discoidin receptor 2 (DDR2) mediated collagen upregulation of MMPs. However, the precise underlying mechanism remains unclear. We report here that CYR61, a secreted, extracellular matrix-associated signaling protein which is capable of regulating a broad range of cellular activities, including cell adhesion, migration, proliferation, and apoptosis, is significantly upregulated in collagen II-stimulated RA FLS. Further studies found that collagen II-activated phosphorylated-DDR2 induces CYR61 through activation of transcription factor activator protein 1 (AP-1). The elevated CYR61, in turn, accelerates MMP1 production via ETS1 (ETS proto-oncogene 1). In addition, CYR61 significantly promotes FLS invasion and migration. Blockade of CYR61 by an adenovirus expressing CYR61 shRNA (Ad-shCYR61) in vivo remarkably ameliorated the severity of arthritis, reduced inflammatory cytokine secretion, and attenuated bone erosion as detected by micro-computed tomography (μCT), in collagen-induced arthritis (CIA) rats. Taken together, we uncovered the Collagen II-DDR2-AP-1-CYR61-ETS1-MMP1 loop in RA FLS. In which, CYR61 acts as a hinge to promote cartilage damage through regulating FLS invasion, migration, and MMP1 production and the inflammatory cascade in RA. Thus, CYR61 may be a promising diagnostic and therapeutic target for RA treatment. © 2016 American Society for Bone and Mineral Research.

Discoidin Receptor 2 Controls Bone Formation and Marrow Adipogenesis

Cell-extracellular matrix (ECM) interactions play major roles in controlling progenitor cell fate and differentiation. The receptor tyrosine kinase, discoidin domain receptor 2 (DDR2), is an important mediator of interactions between cells and fibrillar collagens. DDR2 signals through both ERK1/2 and p38 MAP kinase, which stimulate osteoblast differentiation and bone formation. Here we show that DDR2 is critical for skeletal development and differentiation of marrow progenitor cells to osteoblasts while suppressing marrow adipogenesis. Smallie mice (Ddr2slie/slie ), which contain a nonfunctional Ddr2 allele, have multiple skeletal defects. A progressive decrease in tibial trabecular bone volume/total volume (BV/TV) was observed when wild-type (WT), Ddr2wt/slie , and Ddr2slie/slie mice were compared. These changes were associated with reduced trabecular number (Tb.N) and trabecular thickness (Tb.Th) and increased trabecular spacing (Tb.Sp) in both males and females, but reduced cortical thickness only in Ddr2slie/slie females. Bone changes were attributed to decreased bone formation rather than increased osteoclast activity. Significantly, marrow fat and adipocyte-specific mRNA expression were significantly elevated in Ddr2slie/slie animals. Additional skeletal defects include widened calvarial sutures and reduced vertebral trabecular bone. To examine the role of DDR2 signaling in cell differentiation, bone marrow stromal cells (BMSCs) were grown under osteogenic and adipogenic conditions. Ddr2slie/slie cells exhibited defective osteoblast differentiation and accelerated adipogenesis. Changes in differentiation were related to activity of runt-related transcription factor 2 (RUNX2) and PPARγ, transcription factors that are both controlled by MAPK-dependent phosphorylation. Specifically, the defective osteoblast differentiation in calvarial cells from Ddr2slie/slie mice was associated with reduced ERK/MAP kinase and RUNX2-S319 phosphorylation and could be rescued with a constitutively active phosphomimetic RUNX2 mutant. Also, DDR2 was shown to increase RUNX2-S319 phosphorylation and transcriptional activity while also increasing PPARγ-S112 phosphorylation, but reducing its activity. DDR2 is, therefore, important for maintenance of osteoblast activity and suppression of marrow adipogenesis in vivo and these actions are related to changes in MAPK-dependent RUNX2 and PPARγ phosphorylation. © 2016 American Society for Bone and Mineral Research.

Discoidin Domain Receptor 2 as a Potential Therapeutic Target for Development of Disease-Modifying Osteoarthritis Drugs

Osteoarthritis (OA) is the most common form of arthritis disorders, but the identification of therapeutic targets to effectively prevent OA has been increasingly difficult. The goal of this investigation is to provide experimental evidence that discoidin domain receptor 2 (DDR2) may be an ideal target for the development of disease-modifying OA drugs. Ddr2 was conditionally deleted from articular cartilage of adult mouse knee joints. Aggrecan-CreERT2;floxed Ddr2 mice, which were generated by crossing Aggrecan-CreERT2 mice with floxed Ddr2 mice, then received tamoxifen injections at the age of 8 weeks. The mice were then subjected to destabilization of the medial meniscus (DMM) surgery. At 8 and 16 weeks after DMM, mice were euthanized for the collection of knee joints. In a separate experiment, Aggrecan-CreERT2;floxed Ddr2 mice were subjected to DMM at the age of 10 weeks. The mice then received tamoxifen injections at 8 weeks after DMM. The mice were euthanized for the collection of knee joints at 16 weeks after DMM. The progressive process of articular cartilage degeneration was significantly delayed in the knee joints of Ddr2-deficient mice in comparison to their control littermates. Articular cartilage damage in the knee joints of the mice was associated with increased expression profiles of both Ddr2 and matrix metalloproteinase 13. These findings suggest that DDR2 may be an ideal target for the development of disease-modifying OA drugs.

Targeting of Discoidin Domain Receptor 2 (DDR2) Prevents Myofibroblast Activation and Neovessel Formation During Pulmonary Fibrosis

Idiopathic pulmonary fibrosis (IPF) is a lethal human disease with short survival time and few treatment options. Herein, we demonstrated that discoidin domain receptor 2 (DDR2), a receptor tyrosine kinase that predominantly transduces signals from fibrillar collagens, plays a critical role in the induction of fibrosis and angiogenesis in the lung. In vitro cell studies showed that DDR2 can synergize the actions of both transforming growth factor (TGF)-β and fibrillar collagen to stimulate lung fibroblasts to undergo myofibroblastic changes and vascular endothelial growth factor (VEGF) expression. In addition, we confirmed that late treatment of the injured mice with specific siRNA against DDR2 or its kinase inhibitor exhibited therapeutic efficacy against lung fibrosis. Thus, this study not only elucidated novel mechanisms by which DDR2 controls the development of pulmonary fibrosis, but also provided candidate target for the intervention of this stubborn disease.