Trafficking defects and loss of ligand binding are the underlying causes of all reported DDR2 missense mutations found in SMED-SL patients

Multi-gene panel sequencing in highly consanguineous families and patients with congenital forms of skeletal dysplasias

Skeletal dysplasias (SKDs) are a heterogeneous group of more than 750 genetic disorders characterized by abnormal development, growth, and maintenance of bones or cartilage in the human skeleton. SKDs are often caused by variants in early patterning genes and in many cases part of multiple malformation syndromes and occur in combination with non-skeletal phenotypes. The aim of this study was to investigate the underlying genetic cause of congenital SKDs in highly consanguineous Pakistani families, as well as in sporadic and familial SKD cases from India using multigene panel sequencing analysis. Therefore, we performed panel sequencing of 386 bone-related genes in 7 highly consanguineous families from Pakistan and 27 cases from India affected with SKDs. In the highly consanguineous families, we were able to identify the underlying genetic cause in five out of seven families, resulting in a diagnostic yield of 71%. Whereas, in the sporadic and familial SKD cases, we identified 12 causative variants, corresponding to a diagnostic yield of 44%. The genetic heterogeneity in our cohorts was very high and we were able to detect various types of variants, including missense, nonsense, and frameshift variants, across multiple genes known to cause different types of SKDs. In conclusion, panel sequencing proved to be a highly effective way to decipher the genetic basis of SKDs in highly consanguineous families as well as sporadic and or familial cases from South Asia. Furthermore, our findings expand the allelic spectrum of skeletal dysplasias.

Novel insights into the role of Discoidin domain receptor 2 (DDR2) in cancer progression: a new avenue of therapeutic intervention

Discoidin domain receptors (DDRs), including DDR1 and DDR2, are a unique class of receptor tyrosine kinases (RTKs) activated by collagens at the cell-matrix boundary interface. The peculiar mode of activation makes DDRs as key cellular sensors of microenvironmental changes, with a critical role in all physiological and pathological processes governed by collagen remodeling. DDRs are widely expressed in fetal and adult tissues, and experimental and clinical evidence has shown that their expression is deregulated in cancer. Strong findings supporting the role of collagens in tumor progression and metastasis have led to renewed interest in DDRs.  However, despite an increasing number of studies, DDR biology remains poorly understood, particularly the less studied DDR2, whose involvement in cancer progression mechanisms is undoubted. Thus, the understanding of a wider range of DDR2 functions and related molecular mechanisms is expected. To date, several lines of evidence support DDR2 as a promising target in cancer therapy. Its involvement in key functions in the tumor microenvironment makes DDR2 inhibition particularly attractive to achieve simultaneous targeting of tumor and stromal cells, and tumor regression, which is beneficial for improving the response to different types of anti-cancer therapies, including chemo- and immunotherapy. This review summarizes current research on DDR2, focusing on its role in cancer progression through its involvement in tumor and stromal cell functions, and discusses findings that support the rationale for future development of direct clinical strategies targeting DDR2.

Expression of the Discoidin Domain Receptor Family Depended on Glucose and Their High Expression in Arterial Tissues in the Rat Model of Type 2 Diabetes

The active form of discoidin domain receptors (DDRs) is expressed in cell surface and regulated post-translationally by glucose. The DDR2 and DDR1 transfected in HEK293 cells were expressed mainly in their active forms with sizes of 130 and 120 kDa, respectively. DDRs were observed predominantly as 100 kDa proteins in glucose-depleted culture conditions. However, transfection of endothelial growth factor receptor (EGFR) in HEK293 cells resulted in the expression of only one form regardless of glucose concentration. Vascular smooth muscle cells, HT1080s, and MDA-MB-231 cancer cells expressed DDRs in their active forms in high glucose concentrations, which did not occur with EGFR. In diabetic rats, DDRs were expressed at high levels in arterial tissue but EGFR was not highly expressed. Taken together, these results suggest that DDRs expression depends on glucose concentration it may cooperate in the development of atherosclerosis and kidney fibroblasts, promoting nephropathy in diabetic rats.

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.

A case of neonatal osteofibrous dysplasia with novel CDK12 and DDR2 mutations

Osteofibrous dysplasia [OFD] is a rare, benign pediatric fibro-osseous lesion that exclusively arises in the lower limbs. Apart from the limited number of familial OFD cases with MET mutation, no other genetic aberrations have been identified. Herein, we report a case of OFD in a four-month- old girl's leg with novel cyclin-dependent kinase 12 and discoidin domain receptor 2 gene mutations. Further studies to understand their role in the pathogenesis and clinical utility are needed.

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.

Spondylo-meta-epiphyseal dysplasia (SMED), short limb-hand abnormal calcification type: Further expanding the mutational spectrum and dental findings of three new patients

Genetic skeletal disorders are clinically and genetically heterogeneous group of disorders that affect the normal development, growth, and maintenance of the human skeleton. Spondylo-meta-epiphyseal dysplasia, short limb-abnormal calcification type (SMED-SL/AC; MIM# 271665) is a rare autosomal recessive genetic skeletal disorder characterized by distinctive facial features, disproportionate short stature, vertebral, metaphyseal, and epiphyseal abnormalities. This unique phenotype is caused by biallelic loss-of-function variants in Discoidin domain receptor 2 gene (DDR2, MIM# 191311). To date, only 10 pathogenic variants (six missense, two nonsense, one deletion, and one splice site) in DDR2 have been reported in patients with SMED-SL/AC. Dental anomalies related to skeletal dysplasia can include various abnormalities in the number, shape, and position of teeth in the jaw, as well as enamel hypoplasia and dentinogenesis imperfecta. Although abnormal dentition has previously been reported, orodental findings were described in only six patients with SMED-SL/AC. This study aimed to define the clinical, dental, radiological, and molecular findings of three new SMED-SL/AC patients from three unrelated families. Three DDR2 variants, two of which were novel, were detected with the aid of Sanger sequencing. Interestingly, one of the patients was diagnosed with Wilson's disease (WD) during the follow-up, a co-occurrence that has never been reported in patients with SMED-SL/AC so far.

Control of craniofacial development by the collagen receptor, discoidin domain receptor 2

Development of the craniofacial skeleton requires interactions between progenitor cells and the collagen-rich extracellular matrix (ECM). The mediators of these interactions are not well-defined. Mutations in the discoidin domain receptor 2 gene (DDR2), which encodes a non-integrin collagen receptor, are associated with human craniofacial abnormalities, such as midface hypoplasia and open fontanels. However, the exact role of this gene in craniofacial morphogenesis is not known. As will be shown, Ddr2-deficient mice exhibit defects in craniofacial bones including impaired calvarial growth and frontal suture formation, cranial base hypoplasia due to aberrant chondrogenesis and delayed ossification at growth plate synchondroses. These defects were associated with abnormal collagen fibril organization, chondrocyte proliferation and polarization. As established by localization and lineage-tracing studies, Ddr2 is expressed in progenitor cell-enriched craniofacial regions including sutures and synchondrosis resting zone cartilage, overlapping with GLI1 + cells, and contributing to chondrogenic and osteogenic lineages during skull growth. Tissue-specific knockouts further established the requirement for Ddr2 in GLI +skeletal progenitors and chondrocytes. These studies establish a cellular basis for regulation of craniofacial morphogenesis by this understudied collagen receptor and suggest that DDR2 is necessary for proper collagen organization, chondrocyte proliferation, and orientation.

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.

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.

Spondylometaepiphyseal Dysplasia Short Limb-Abnormal Calcification Type in Turkish Patients Reveals a Novel Mutation and New Features

Spondylometaepiphyseal dysplasia short limb-abnormal calcification type (SMED-SL/AC) is a rare autosomal recessive disorder. It is a severe dwarfism syndrome with a characteristic feature of progressive calcification of epiphyseal and other cartilaginous tissues. It is caused by pathogenic variants in the DDR2 gene encoding the discoidin domain receptor tyrosine kinase 2. Thus far, 37 cases and 8 pathogenic variants have been reported. Most of the reported cases are of Middle Eastern and Puerto Rican origins. Only one Turkish case has been reported previously with a novel truncating variant p.(R489*). Here, we report 2 new cases, 1 with a novel variant p.(S311G) and 1 with a splice site variant c.2283+1G>A. In addition, we reviewed a previously reported case, and sequencing of stored DNA revealed the recently reported nonsense variant p.(R489*) as the underlying cause. Therefore, our data increase the number of SMED-SL/AC Turkish patients with molecular results to 4. Furthermore, we compared the features of Turkish patients with other reported cases and expanded the characteristics of the disorder with new features such as triventricular hydrocephalus, intracranial hemorrhage, hypopigmentation of hair, dry and scaly skin, arthralgia, and hypocalcemia. We also compared the pathogenic variants of Turkish patients with other variants, aiming to explain the mechanism leading to a more severe and early fatal course in Turkish patients.

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.

The Journey of DDR1 and DDR2 Kinase Inhibitors as Rising Stars in the Fight Against Cancer

Discoidin domain receptor (DDR) is a collagen-activated receptor tyrosine kinase that plays critical roles in regulating essential cellular processes such as morphogenesis, differentiation, proliferation, adhesion, migration, invasion, and matrix remodeling. As a result, DDR dysregulation has been attributed to a variety of human cancer disorders, for instance, non-small-cell lung carcinoma (NSCLC), ovarian cancer, glioblastoma, and breast cancer, in addition to some inflammatory and neurodegenerative disorders. Since the target identification in the early 1990s to date, a lot of efforts have been devoted to the development of DDR inhibitors. From a medicinal chemistry perspective, we attempted to reveal the progress in the development of the most promising DDR1 and DDR2 small molecule inhibitors covering their design approaches, structure-activity relationship (SAR), biological activity, and selectivity.

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.

Clinical and genetic evaluation of six children with diffuse capillary malformation and undergrowth

BACKGROUND

Diffuse capillary malformation with overgrowth (DCMO) has been well described. However, capillary malformation with undergrowth (CMU) has been less reported in the literature.

OBJECTIVES

We sought to describe the clinical features and determine associated somatic mutations in patients with CMU.

METHODS

We searched our multidisciplinary vascular anomalies clinic database for patients with CMU. Girth and length limb measurements were performed. In case of discrepancies in length, long leg radiograph studies were obtained. Whole-exome sequencing of blood and involved tissue DNA was carried out.

RESULTS

We included six patients with CM and soft-tissue and bone undergrowth. CMs were patchy, reticulated, segmental, poorly demarcated, pink-red stains affecting the lower limb (five patients) or the whole hemibody (one patient). In five patients, the stain was diffuse, affecting more than one anatomic region. Prominent superficial veins were observed in three patients. Five patients presented with lower limb girth discrepancy; in three of them, there was also lower limb length discrepancy. In the remaining patient, only lower limb length discrepancy was found. Whole-exome sequencing from DNA tissue/blood detected previously described pathogenic somatic mutations on DDR2 (c.314G > A; p.Arg105His), GRHL2 (c.791A > G; p.Glu264Gly), and PIK3CA (c.2740G > A; p.Gly914Arg) genes.

CONCLUSION

We propose the term "diffuse capillary malformation with undergrowth" for extensive reticular CMs associated with proportionate undergrowth. All our patients had a favorable outcome, and no genotype-phenotype association was found.

ECM signaling in cartilage development and endochondral ossification

During cartilage development chondrocytes undergo a multi-step process characterized by consecutive changes in cell morphology and gene expression. Cell proliferation, polarity, differentiation, and migration are influenced by chemical and mechanical signaling between the extracellular matrix (ECM) and the cell. Several structurally diverse transmembrane receptors such as integrins, discoidin domain receptor 2 (DDR 2), and CD44 mediate the crosstalk between cells and their ECM. However, the contribution of cell-matrix interactions during early chondrogenesis and further cartilage development through cell receptors and their signal transduction pathways is still not fully understood. Determination of receptor signaling pathways and the function of downstream targets will aid in a better understanding of musculoskeletal pathologies such as chondrodysplasia, and the development of new approaches for the treatment of cartilage disorders. We will summarize recent findings, linking cell receptors and their potential signaling pathways to the control of chondrocyte behavior during early chondrogenesis and endochondral ossification.

Recurrent, Activating Variants in the Receptor Tyrosine Kinase DDR2 Cause Warburg-Cinotti Syndrome

We have investigated a distinct disorder with progressive corneal neovascularization, keloid formation, chronic skin ulcers, wasting of subcutaneous tissue, flexion contractures of the fingers, and acro-osteolysis. In six affected individuals from four families, we found one of two recurrent variants in discoidin domain receptor tyrosine kinase 2 (DDR2): c.1829T>C (p.Leu610Pro) or c.2219A>G (p.Tyr740Cys). DDR2 encodes a collagen-responsive receptor tyrosine kinase that regulates connective-tissue formation. In three of the families, affected individuals comprise singleton adult individuals, and parental samples were not available for verification of the de novo occurrence of the DDR2 variants. In the fourth family, a mother and two of her children were affected, and the c.2219A>G missense variant was proven to be de novo in the mother. Phosphorylation of DDR2 was increased in fibroblasts from affected individuals, suggesting reduced receptor autoinhibition and ligand-independent kinase activation. Evidence for activation of other growth-regulatory signaling pathways was not found. Finally, we found that the protein kinase inhibitor dasatinib prevented DDR2 autophosphorylation in fibroblasts, suggesting an approach to treatment. We propose this progressive, fibrotic condition should be designated as Warburg-Cinotti syndrome.

Further expansion of the mutational spectrum of spondylo-meta-epiphyseal dysplasia with abnormal calcification

Spondylo-meta-epiphyseal dysplasia, short limb-abnormal calcification type, is a rare autosomal recessive disorder of the skeleton characterized by disproportionate short stature with narrow chest and dysmorphic facial features. The skeletal manifestations include platyspondyly, short flared ribs, short tubular bones with abnormal metaphyses and epiphyses, severe brachydactyly, and premature stippled calcifications in the cartilage. The abnormal calcifications are so distinctive as to point to the definitive diagnosis. However, they may be too subtle to attract diagnostic attention in infancy. Homozygous variants in DDR2 cause this disorder. We report on a 5-year-old girl with the classic phenotype of SMED, SL-AC in whom a novel homozygous nonsense mutation in DDR2 was detected using exome sequencing.

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

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

Incorporation of DDR2 clusters into collagen matrix via integrin-dependent posterior remnant tethering

Cell-matrix interactions play critical roles in cell adhesion, tissue remodeling and cancer metastasis. Discoidin domain receptor 2 (DDR2) is a collagen receptor belonging to receptor tyrosine kinase (RTK) family. It is a powerful regulator of collagen deposition in the extracellular matrix (ECM). Although the oligomerization of DDR extracellular domain (ECD) proteins can affect matrix remodeling by inhibiting fibrillogenesis, it is still unknown how cellular DDR2 is incorporated into collagen matrix. Using 3-dimentional (3D) imaging for migrating cells, we identified a novel mechanism that explains how DDR2 incorporating into collagen matrix, which we named as posterior remnant tethering. We followed the de novo formation of these remnants and identified that DDR2 clusters formed at the retracting phase of a pseudopodium, then these clusters were tethered to fibrillar collagen and peeled off from the cell body to generate DDR2 containing posterior remnants. Inhibition of β1-integrin or Rac1 activity abrogated the remnant formation. Thus, our findings unveil a special cellular mechanism for DDR2 clusters incorporating into collagen matrix in an integrin-dependent manner.

A B3GALT6 variant in patient originally described as Al-Gazali syndrome and implicating the endoplasmic reticulum quality control in the mechanism of some β3GalT6-pathy mutations

Al-Gazali syndrome encompasses several clinical features including prenatal growth retardation, large joints contractures with camptodactyly, bilateral talipes equinovarus, small mouth, anterior segment anomalies of the eyes, and early lethality. Recently, a baby with features very similar to Al-Gazali syndrome was found to have compound heterozygous variants in B3GALT6. This gene encodes Beta-1,3-galactosyltransferase 6 (β3GalT6), an essential component of the glycosaminoglycan synthesis pathway. Pathogenic variants in B3GALT6 have also been shown to cause Ehlers-Danlos syndrome spondylodysplastic type (spEDS-B3GALT6) and spondyloepimetaphyseal dysplasia with joint laxity type I (SEMD-JL1). In 2017, a new international classification of EDS included these 2 conditions together with the child reported to have features similar to Al-Gazali syndrome under spondylodysplastic EDS (spEDS). We report a disease-causing variant c.618C > G, p.(Cys206Trp) in 1 patient originally described as Al-Gazali syndrome and reported in 1999. We evaluated the involvement of the endoplasmic reticulum-associated protein degradation, in the pathogenesis of 13 B3GALT6 variants. Retention in endoplasmic reticulum was evident in 6 of them while the c.618C > G, p.(Cys206Trp) and the other 6 variants trafficked normally. Our findings confirm the involvement of B3GALT6 in the pathogenesis of Al-Gazali syndrome and suggest that Al-Gazali syndrome represents the severe end of the spectrum of the phenotypes caused by pathogenic variants in this gene.

Collagen induces activation of DDR1 through lateral dimer association and phosphorylation between dimers

The collagen-binding receptor tyrosine kinase DDR1 (discoidin domain receptor 1) is a drug target for a wide range of human diseases, but the molecular mechanism of DDR1 activation is poorly defined. Here we co-expressed different types of signalling-incompetent DDR1 mutants (‘receiver’) with functional DDR1 (‘donor’) and demonstrate phosphorylation of receiver DDR1 by donor DDR1 in response to collagen. Making use of enforced covalent DDR1 dimerisation, which does not affect receptor function, we show that receiver dimers are phosphorylated in trans by the donor; this process requires the kinase activity of the donor but not that of the receiver. The receiver ectodomain is not required, but phosphorylation in trans is abolished by mutation of the transmembrane domain. Finally, we show that mutant DDR1 that cannot bind collagen is recruited into DDR1 signalling clusters. Our results support an activation mechanism whereby collagen induces lateral association of DDR1 dimers and phosphorylation between dimers.

DOI:http://dx.doi.org/10.7554/eLife.25716.001

The membrane surrounding each living cell contains a variety of proteins that carry out different roles. For example, proteins called receptor tyrosine kinases help a cell to receive signals from its external environment. Receptor tyrosine kinases span the membrane so that one part of the protein known as the ectodomain sticks out from the surface of the cell, while another part (called the kinase domain) sits inside the cell.

When a signalling molecule binds to the ectodomain, the kinase domain becomes active and starts to add chemical groups called phosphates to other proteins. This process, known as phosphorylation, changes the protein’s activity, which in turn influences the cell’s behaviour. In most cases, the signalling molecule causes two receptor tyrosine kinase proteins to bind to each other and form a “dimer” in which the kinase domains are able to phosphorylate, and thus activate, each other.

Female mammals need a receptor tyrosine kinase called DDR1 to develop mammary glands (the glands that produce milk). DDR1 is activated when a signalling molecule called collagen binds to its ectodomain. Unlike many other receptor tyrosine kinases, DDR1 exists as a dimer even before it binds to collagen, so it is not clear how collagen activates DDR1. One possibility is that collagen causes several DDR1 dimers to form clusters on the membrane so that kinases on neighbouring dimers can phosphorylate each other. Juskaite et al. explored this idea by pairing up normal DDR1 proteins with mutant versions that are unable to bind to collagen.

The experiments show that when collagen binds to the normal DDR1 molecules, DDR1 dimers do indeed form clusters. This enables the normal protein molecules in neighbouring dimers to phosphorylate each other as well as the mutant proteins. In this way, the clustered DDR1 dimers can become active even if the clusters contain one or more mutant versions that are unable to detect collagen. Further experiments show that specific contacts need to form between neighbouring dimers for this phosphorylation to occur.

Abnormal DDR1 activity is associated with several diseases including cancer, inflammation and fibrosis. The findings of Juskaite et al. suggest that developing new drugs that can prevent DDR1 from forming clusters may help to treat people with these conditions. Further work is also needed to analyse the size and structure of DDR1 clusters and investigate if other proteins also associate with the clusters.

DOI:http://dx.doi.org/10.7554/eLife.25716.002

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.

A molecular classification of human mesenchymal stromal cells

Mesenchymal stromal cells (MSC) are widely used for the study of mesenchymal tissue repair, and increasingly adopted for cell therapy, despite the lack of consensus on the identity of these cells. In part this is due to the lack of specificity of MSC markers. Distinguishing MSC from other stromal cells such as fibroblasts is particularly difficult using standard analysis of surface proteins, and there is an urgent need for improved classification approaches. Transcriptome profiling is commonly used to describe and compare different cell types; however, efforts to identify specific markers of rare cellular subsets may be confounded by the small sample sizes of most studies. Consequently, it is difficult to derive reproducible, and therefore useful markers. We addressed the question of MSC classification with a large integrative analysis of many public MSC datasets. We derived a sparse classifier (The Rohart MSC test) that accurately distinguished MSC from non-MSC samples with >97% accuracy on an internal training set of 635 samples from 41 studies derived on 10 different microarray platforms. The classifier was validated on an external test set of 1,291 samples from 65 studies derived on 15 different platforms, with >95% accuracy. The genes that contribute to the MSC classifier formed a protein-interaction network that included known MSC markers. Further evidence of the relevance of this new MSC panel came from the high number of Mendelian disorders associated with mutations in more than 65% of the network. These result in mesenchymal defects, particularly impacting on skeletal growth and function. The Rohart MSC test is a simple in silico test that accurately discriminates MSC from fibroblasts, other adult stem/progenitor cell types or differentiated stromal cells. It has been implemented in the www.stemformatics.org resource, to assist researchers wishing to benchmark their own MSC datasets or data from the public domain. The code is available from the CRAN repository and all data used to generate the MSC test is available to download via the Gene Expression Omnibus or the Stemformatics resource.

Novel DDR2 mutation identified by whole exome sequencing in a Moroccan patient with spondylo-meta-epiphyseal dysplasia, short limb-abnormal calcification type

Spondylo-meta-epiphyseal dysplasia (SMED), short limb-abnormal calcification type (SMED, SL-AC), is a very rare autosomal recessive disorder with various skeletal changes characterized by premature calcification leading to severe disproportionate short stature. Twenty-two patients have been reported until now, but only five mutations (four missense and one splice-site) in the conserved sequence encoding the tyrosine kinase domain of the DDR2 gene has been identified. We report here a novel DDR2 missense mutation, c.370C > T (p.Arg124Trp) in a Moroccan girl with SMED, SL-AC, identified by whole exome sequencing. Our study has expanded the mutational spectrum of this rare disease and it has shown that exome sequencing is a powerful and cost-effective tool for the diagnosis of clinically heterogeneous disorders such as SMED.

New therapeutic targets in rare genetic skeletal diseases

Introduction: Genetic skeletal diseases (GSDs) are a diverse and complex group of rare genetic conditions that affect the development and homeostasis of the skeleton. Although individually rare, as a group of related diseases, GSDs have an overall prevalence of at least 1 per 4,000 children. There are currently very few specific therapeutic interventions to prevent, halt or modify skeletal disease progression and therefore the generation of new and effective treatments requires novel and innovative research that can identify tractable therapeutic targets and biomarkers of these diseases.

Areas covered: Remarkable progress has been made in identifying the genetic basis of the majority of GSDs and in developing relevant model systems that have delivered new knowledge on disease mechanisms and are now starting to identify novel therapeutic targets. This review will provide an overview of disease mechanisms that are shared amongst groups of different GSDs and describe potential therapeutic approaches that are under investigation.

Expert opinion: The extensive clinical variability and genetic heterogeneity of GSDs renders this broad group of rare diseases a bench to bedside challenge. However, the evolving hypothesis that clinically different diseases might share common disease mechanisms is a powerful concept that will generate critical mass for the identification and validation of novel therapeutic targets and biomarkers.

Improved plasma membrane expression of the trafficking defective P344R mutant of muscle, skeletal, receptor tyrosine kinase (MuSK) causing congenital myasthenic syndrome

Muscle, skeletal, receptor tyrosine kinase (MuSK) is a key organizer at the postsynaptic membrane and critical for proper development and maintenance of the neuromuscular junction. Mutations in MUSK result in congenital myasthenic syndrome (CMS). We hypothesized that the CMS-causing missense mutation (P344R), found within the cysteine-rich domain of the protein, will affect its conformational tertiary structure. Consequently, the protein will misfold, get retained in the endoplasmic reticulum (ER) and lose its biological function through degradation by the highly conserved ER associated degradation (ERAD) machinery. We report that P344R-MuSK mutant is trafficking-deficient when expressed at 37°C in HeLa, COS-7 and HEK293 cell lines. It colocalized with the ER marker calnexin in contrast to wild-type MuSK which localized to the plasma membrane. The N-glycosylation status of P344R-MuSK is that of an immature and not properly post-translationally modified protein. Inhibition of protein synthesis showed that the P344R mutant's half-life is shorter than wild-type MuSK protein. Proteasomal inhibition resulted in the stabilization of the mutant protein. The mutant protein is highly ubiquitinated compared to wild-type confirming targeting for proteasomal degradation. The mutant showed around 50% of its in vivo autophosphorylation activity. P344R-MuSK mutant's trafficking defect is correctable by culturing the expressing cells at 27°C. Moreover, chemical compounds namely 2.5% glycerol, 1% dimethyl sulfoxide, 10 μM thapsigargin and 1 μM curcumin improved the maturation and exit of the mutant protein from the ER. These findings open perspectives for potential therapeutic intervention for patients with CMS harboring the P344R-MuSK mutation.

Typical and atypical domain combinations in human protein kinases: functions, disease causing mutations and conservation in other primates

A twist in the evolution of human kinases resulting in kinases with hybrid and rogue properties.

Defective Ca(2+) binding in a conserved binding site causes incomplete N-glycan processing and endoplasmic reticulum trapping of discoidin domain receptors

An X-ray crystallographic study has suggested that vertebrate discoidin domain receptors (DDRs) have a conserved Ca(2+) binding site. DDR1 and DDR2 transfected in HEK293 cells were expressed mainly as 120 and 130 kDa forms, respectively, as they are sufficiently N-glycosylated. However, both of them showed the molecular weight of 110 kDa predominantly in the cells cultured with Ca(2+)-depleted media. DDR2-carrying D234A mutation at the conserved Ca(2+)-binding site expressed the 110 kDa form dominantly even in normal culture condition. DDR2 becomes 100 kDa form in glucose-depleted culture condition and its molecular weight increases up to 130 kDa with re-feeding glucose. However, in the mutant DDR2, the increase came to a halt at 110 kDa. The 110 kDa form had premature N-glycosyl carbohydrates and located predominantly within the endoplasmic reticulum. These results suggest that DDRs require Ca(2+)-binding to complete their N-glycan processing and generate the form targeted to cell membrane.

Discoidin domain receptor functions in physiological and pathological conditions

The discoidin domain receptors, DDR1 and DDR2, are nonintegrin 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 remodeling of extracellular matrices. This review summarizes 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.

Impaired trafficking of the very low density lipoprotein receptor caused by missense mutations associated with dysequilibrium syndrome

Dysequilibrium syndrome (DES, OMIM 224050) is a genetically heterogeneous condition that combines autosomal recessive non-progressive cerebellar ataxia with mental retardation. The subclass dysequilibrium syndrome type 1 (CAMRQ1) has been attributed to mutations in the VLDLR gene encoding the very low density lipoprotein receptor (VLDLR). This receptor is involved in the Reelin signaling pathway that guides neuronal migration in the cerebral cortex and cerebellum. Three missense mutations (c.1459G>T; p.D487Y, c.1561G>C; p.D521H and c.2117G>T; p.C706F) have been previously identified in VLDLR gene in patients with DES. However, the functional implications of those mutations are not known and therefore we undertook detailed functional analysis to elucidate the cellular mechanisms underlying their pathogenicity. The mutations have been generated by site-directed mutagenesis and then expressed in cultured cell lines. Confocal microscopy and biochemical analysis have been employed to examine the subcellular localization and functional activities of the mutated proteins relative to wild type. Our results indicate that the three missense mutations lead to defective intracellular trafficking and ER retention of the mutant VLDLR protein. This trafficking impairment prevents the mutants from reaching the plasma membrane and binding exogenous Reelin, the initiating event in Reelin signaling. Collectively, our results provide evidence that ER quality control is involved in the functional inactivation and underlying pathogenicity of these DES-associated mutations in the VLDLR.

Assessment of DDR2, BRAF, EGFR and KRAS mutations as therapeutic targets in non-adenocarcinoma lung cancer patients

Molecular-targeted therapy has not been established in non-adenocarcinoma lung cancer (non-AdLC), as no targets that affect the clinical efficacy of molecular-targeted drugs have yet been identified. In this study, we investigated the frequency of genetic variations in discoidin domain receptor 2 (DDR2), v-raf murine sarcoma viral oncogene homolog B1 (BRAF), epidermal growth factor receptor (EGFR) and v-Ki-ras2 Kirsten rat sarcoma viral oncogene homolog (KRAS) in non-AdLC patients, in order to evaluate the possibility of genetic mutations in these genes being used as therapeutic targets for the treatment of patients with non-AdLC. For this purpose, we enrolled 150 non-AdLC patients who had undergone surgery at the Gunma University Hospital between December, 2003 and December, 2012. Genetic mutations in the EGFR, KRAS, DDR2 and BRAF genes were detected by a sequencing method or probe assay using DNA derived from cancer tissues. No somatic mutations in DDR2 or BRAF were detected in non-AdLC patients. Conversely, genetic mutations in EGFR exon 19 were found in 3 squamous cell carcinoma (SCC) and 3 adenosquamous carcinoma patients, whereas KRAS codon 12 mutations were also found in 3 SCC patients and 1 large-cell neuroendocrine carcinoma patient. EGFR and KRAS mutations were mutually exclusive. This study indicated that, although DDR2 and BRAF mutations may only rarely be used as therapeutic targets, EGFR and KRAS mutations may represent candidate therapeutic targets, at least in the non-AdLC patients investigated.

A novel mutation in DDR2 causing spondylo-meta-epiphyseal dysplasia with short limbs and abnormal calcifications (SMED-SL) results in defective intra-cellular trafficking

Background

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.

Methods

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.

Results

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.

Conclusions

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.

Prenatal diagnosis of proximal partial trisomy 1q confirmed by comparative genomic hybridization array: molecular cytogenetic analysis, fetal pathology and review of the literature

BACKGROUND

Partial trisomy of the long arm of chromosome 1 (1q) is an exceptionally rare chromosomal abnormality and most of the prenatally diagnosed cases are associated with either complete (q11-qter) or large (q21-qter) duplications with pre- or perinatal demise of all reported cases. The most common sonographic findings associated with this karyotype abnormality include ventriculomegaly, increased nuchal translucency or nuchal fold, renal and cardiac abnormalities, craniofacial dysmorphism, and limb deformities. However, there is a wide spectrum of clinical manifestations due to the great variability in the extent of the duplication size and the possible contribution of additional genetic rearrangements in the final phenotype.

CASE REPORT

We report on a female fetus with sole partial trisomy 1q presenting with multiple structural malformations in the second trimester scan. Standard karyotyping demonstrated a large duplication on the proximal end of chromosome 1 [46,XX,dup(1)(pter→q31::q31→q12::q31→qter)] and further application of comparative genomic hybridization array confirmed the diagnosis and offered a precise characterization of the genetic defect.

CONCLUSION

A fetus with nonmosaic partial trisomy 1q that was prenatally diagnosed upon multiple abnormal ultrasound findings is presented. A detailed review of the currently available literature on the prenatal diagnostic approach of partial trisomy 1q in terms of fetal sonographic assessment and molecular cytogenetic investigation is also provided. The use of novel molecular techniques such comparative genomic hybridization array could shed further light on the correlation between the genes identified in the chromosomal region of interest and the resultant phenotype.

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

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

Discoidin domain receptors in disease

Discoidin domain receptors, DDR1 and DDR2, lie at the intersection of two large receptor families, namely the extracellular matrix and tyrosine kinase receptors. As such, DDRs are uniquely positioned to function as sensors for extracellular matrix and to regulate a wide range of cell functions from migration and proliferation to cytokine secretion and extracellular matrix homeostasis/remodeling. While activation of DDRs by extracellular matrix collagens is required for normal development and tissue homeostasis, aberrant activation of these receptors following injury or in disease is detrimental. The availability of mice lacking DDRs has enabled us to identify key roles played by these receptors in disease initiation and progression. DDR1 promotes inflammation in atherosclerosis, lung fibrosis and kidney injury, while DDR2 contributes to osteoarthritis. Furthermore, both DDRs have been implicated in cancer progression. Yet the mechanisms whereby DDRs contribute to disease progression are poorly understood. In this review we highlight the mechanisms whereby DDRs regulate two important processes, namely inflammation and tissue fibrosis. In addition, we discuss the challenges of targeting DDRs in disease. Selective targeting of these receptors requires understanding of how they interact with and are activated by extracellular matrix, and whether their cellular function is dependent on or independent of receptor kinase activity.

Rare mutations in non-small-cell lung cancer

In the last decade, new insights in molecular biology have changed the therapeutic landscape of non-small-cell lung cancer. Since 2004, when activating mutations of the EGFR were firstly identified, several genetic aberrations have been discovered, mainly in adenocarcinoma. EGFR mutations are a relatively frequent event in non-small-cell lung cancer, generally consisting of exon 19 deletion or exon 21 substitution. In adenocarcinoma, additional rare mutations are detectable in the EGFR gene, as well as in other genes, including ALK, ROS1, RET, HER2 and BRAF. Recent studies in squamous cell carcinoma identified TP53 as the most frequent mutation, followed by additional more rare mutations, including PI3KCA, PTEN, DDR2 and FGFR. The aim of the present review is to analyze the potential prognostic and predictive role of rare mutations.

Canine chondrodysplasia caused by a truncating mutation in collagen-binding integrin alpha subunit 10

The skeletal dysplasias are disorders of the bone and cartilage tissues. Similarly to humans, several dog breeds have been reported to suffer from different types of genetic skeletal disorders. We have studied the molecular genetic background of an autosomal recessive chondrodysplasia that affects the Norwegian Elkhound and Karelian Bear Dog breeds. The affected dogs suffer from disproportionate short stature dwarfism of varying severity. Through a genome-wide approach, we mapped the chondrodysplasia locus to a 2-Mb region on canine chromosome 17 in nine affected and nine healthy Elkhounds (p_raw = 7.42×10⁻⁶, p_genome-wide = 0.013). The associated locus contained a promising candidate gene, cartilage specific integrin alpha 10 (ITGA10), and mutation screening of its 30 exons revealed a nonsense mutation in exon 16 (c.2083C>T; p.Arg695*) that segregated fully with the disease in both breeds (p = 2.5×10⁻²³). A 24% mutation carrier frequency was indicated in NEs and an 8% frequency in KBDs. The ITGA10 gene product, integrin receptor α10-subunit combines into a collagen-binding α10β1 integrin receptor, which is expressed in cartilage chondrocytes and mediates chondrocyte-matrix interactions during endochondral ossification. As a consequence of the nonsense mutation, the α10-protein was not detected in the affected cartilage tissue. The canine phenotype highlights the importance of the α10β1 integrin in bone growth, and the large animal model could be utilized to further delineate its specific functions. Finally, this study revealed a candidate gene for human chondrodysplasias and enabled the development of a genetic test for breeding purposes to eradicate the disease from the two dog breeds.

Discoidin domain receptor 2 (DDR2) regulates body size and fat metabolism in mice

Discoidin domain receptor 2 (DDR2) is a receptor tyrosine kinase that is activated by fibrillar collagens, which act as its endogenous ligand. DDR2 regulates cell proliferation, cell adhesion, migration, extracellular matrix remodeling and reproductive functions. Both DDR2 null allele mice and mice with a recessive, loss-of-function allele for Ddr2 exhibit dwarfing and a reduction in body weight. However, the detailed mechanisms by which DDR2 exerts its positive systemic regulation of whole body size, local skeletal size and fat tissue volume remain to be clarified. To investigate the systemic role of DDR2 in body size regulation, we produced transgenic mice in which the DDR2 protein is overexpressed, then screened the transgenic mice for abnormalities using systematic mouse abnormality screening. The modified-SHIPRA screen revealed that only the parameter of body size was significantly different among the genotypes. We also discovered that the body length was significantly increased, while the body weight was significantly decreased in transgenic mice compared to their littermate controls. We also found that the epididymal fat pads were significantly decreased in transgenic mice compared to normal littermate mice, which may have been the cause of the leptin decrement in the transgenic mice. The new insight that DDR2 might promote metabolism in adipocyte cells is very interesting, but more experiments will be needed to elucidate the direct relation between DDR2 and adipose-derived hormones. Taken together, our data demonstrated that DDR2 might play a systemic role in the regulation of body size thorough skeletal formation and fat metabolism.

Discoipyrroles A-D: isolation, structure determination, and synthesis of potent migration inhibitors from Bacillus hunanensis

Discoidin domain receptor 2 (DDR2) is a receptor tyrosine kinase involved in a variety of cellular response pathways, including regulation of cell growth, proliferation, and motility. Using a newly developed platform to identify the signaling pathway/molecular target of natural products, we identified a family of alkaloid natural products, discoipyrroles A-D (1-4), from Bacillus hunanensis that inhibit the DDR2 signaling pathway. The structure of 1-4, determined by detailed two-dimensional (2D) NMR methods and confirmed by X-ray crystallographic analysis has an unusual 3H-benzo[d]pyrrolo][1,3]oxazine-3,5-dione core. Discoipyrroles A-D potently inhibit DDR2 dependent migration of BR5 fibroblasts and show selective cytotoxicity to DDR2 mutant lung cancer cell lines (IC50 120-400 nM). Examination of the biosynthesis has led to the conclusion that the discoipyrroles are formed through a nonenzymatic process, leading to a one-pot total synthesis of 1.

Low stability and a conserved N-glycosylation site are associated with regulation of the discoidin domain receptor family by glucose via post-translational N-glycosylation

Cell-surface expression of the discoidin domain receptor (DDR) tyrosine kinase family in high molecular mass form was controlled sensitively by the glucose concentration through a post-translational N-glycosylation process. Cycloheximide time-course experiments revealed that the high-molecular-mass forms of DDR1 and DDR2 were significantly less stable than control receptor tyrosine kinases. Site-directed mutational analysis of the consensus N-glycosylation sites of the DDRs revealed that mutations of asparagine 213 of DDR2 and asparagine 211 of DDR1, a conserved N-glycosylation site among vertebrate DDRs, inhibited the generation of the high-molecular-mass isoform. Taken together, these results suggest a mechanism to control the activity of the DDR family by regulating its cell-surface expression. Due to low stability, the steady-state population of functional DDR proteins in the cell surface depends sensitively on its maturation process via post-translational N-glycosylation, which is controlled by the glucose supply and the presence of a conserved N-glycosylation site.

Delineation of the clinical, molecular and cellular aspects of novel JAM3 mutations underlying the autosomal recessive hemorrhagic destruction of the brain, subependymal calcification, and congenital cataracts

We have recently shown that the hemorrhagic destruction of the brain, subependymal, calcification, and congenital cataracts is caused by biallelic mutations in the gene encoding junctional adhesion molecule 3 (JAM3) protein. Affected members from three new families underwent detailed clinical examination including imaging of the brain. Affected individuals presented with a distinctive phenotype comprising hemorrhagic destruction of the brain, subependymal calcification, and congenital cataracts. All patients had a catastrophic clinical course resulting in death. Sequencing the coding exons of JAM3 revealed three novel homozygous mutations: c.2T>G (p.M1R), c.346G>A (p.E116K), and c.656G>A (p.C219Y). The p.M1R mutation affects the start codon and therefore is predicted to impair protein synthesis. Cellular studies showed that the p.C219Y mutation resulted in a significant retention of the mutated protein in the endoplasmic reticulum, suggesting a trafficking defect. The p.E116K mutant traffics normally to the plasma membrane as the wild-type and may have lost its function due to the lack of interaction with an interacting partner. Our data further support the importance of JAM3 in the development and function of the vascular system and the brain.

New insights into adhesion signaling in bone formation

Mineralized tissues that are protective scaffolds in the most primitive species have evolved and acquired more specific functions in modern animals. These are as diverse as support in locomotion, ion homeostasis, and precise hormonal regulation. Bone formation is tightly controlled by a balance between anabolism, in which osteoblasts are the main players, and catabolism mediated by the osteoclasts. The bone matrix is deposited in a cyclic fashion during homeostasis and integrates several environmental cues. These include diffusible elements that would include estrogen or growth factors and physicochemical parameters such as bone matrix composition, stiffness, and mechanical stress. Therefore, the microenvironment is of paramount importance for controlling this delicate equilibrium. Here, we provide an overview of the most recent data highlighting the role of cell-adhesion molecules during bone formation. Due to the very large scope of the topic, we focus mainly on the role of the integrin receptor family during osteogenesis. Bone phenotypes of some deficient mice as well as diseases of human bones involving cell adhesion during this process are discussed in the context of bone physiology.

Discoidin domain receptors promote α1β1- and α2β1-integrin mediated cell adhesion to collagen by enhancing integrin activation

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.