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

Structural extracellular matrix-mediated molecular signaling in wound repair and tissue regeneration

The extracellular matrix (ECM) is a complex, non-cellular network of molecules that offers structural support for cells and tissues. The ECM is composed of various structural components, including collagen, fibronectin, laminin, perlecan, nidogen, tenascin, and fibulin, which are capable of binding to each other and to cell-to-adhesion receptors, endowing the ECM with unique physical and biochemical properties that are essential for its function in maintaining health and managing disease. Over the past three decades, extensive research has shown that the core of the ECM can significantly impact cellular events at the molecular level. Structural modifications have also been strongly associated with tissue repair. Through interactions with cells, matrix proteins regulate critical processes such as cell proliferation and differentiation, migration, and apoptosis, essential for maintaining tissue homeostasis, formation, and regeneration. This review emphasizes the interlocking networks of ECM macromolecules and their primary roles in tissue regeneration and wound repair. Through studying ECM dynamics, researchers have discovered molecular signaling pathways that demonstrate how the ECM influences protein patterns and open up more possibilities for developing therapeutics that target the ECM to enhance wound repair and tissue regeneration.

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.

Cavalcante R, Ge C, Franceschi RT, Ma PX. Synthetic helical peptides on nanofibers to activate cell-surface receptors and synergistically enhance critical-sized bone defect regeneration

More than 500,000 bone grafting procedures are performed annually in the USA. Considering the significant limitations of available bone grafts, we previously invented a phase-separation technology to generate nanofibrous poly(l-lactic acid) (PLLA) scaffolds that mimic the bone matrix collagen in nanofiber geometry and enhance bone regeneration. Here we report the development of nanofibrous scaffolds with covalently attached synthetic peptides that mimic native collagen peptides to activate the two main collagen receptors in bone cells, discoidin domain receptor 2 (DDR2) and β1 integrins. We synthesized a PLLA-based graft-copolymer to enable covalent peptide conjugation via a click reaction. Using PLLA and the graft-copolymer, we developed 3D scaffolds with interconnected pores and peptides-containing nanofibers to activate DDR2 and β1 integrins of osteogenic cells. The degradation rate and mechanical properties of the scaffolds are tunable. The peptides-decorated nanofibrous scaffolds demonstrated 7.8 times more mineralized bone regeneration over the control scaffolds without the peptides in a critical-sized bone defect regeneration model after 8 weeks of implantation, showing a synergistic effect of the two peptides. This study demonstrates the power of scaffolds to mimic ECM at both nanometer and molecular levels, activating cell surface receptors to liberate the innate regenerative potential of host stem/progenitor cells.

Are osteoblasts multiple cell types? A new diversity in skeletal stem cells and their derivatives

Only in the past decade have skeletal stem cells (SSCs), a cell type displaying formal evidence of stemness and serving as the ultimate origin of mature skeletal cell types such as osteoblasts, been defined. Here, we discuss a pair of recent reports that identify that SSCs do not represent a single cell type, but rather a family of related cells that each have characteristic anatomic locations and distinct functions tailored to the physiology of those sites. The distinct functional properties of these SSCs in turn provide a basis for the diseases of their respective locations. This concept emerges from one report identifying a distinct vertebral skeletal stem cell driving the high rate of breast cancer metastasis to the spine over other skeletal sites and a report identifying 2 SSCs in the calvaria that interact to mediate both physiologic calvarial mineralization and pathologic calvarial suture fusion in craniosynostosis. Despite displaying functional differences, these SSCs are each united by shared features including a shared series of surface markers and parallel differentiation hierarchies. We propose that this diversity at the level of SSCs in turn translates into a similar diversity at the level of mature skeletal cell types, including osteoblasts, with osteoblasts derived from different SSCs each displaying different functional and transcriptional characteristics reflecting their cell of origin. In this model, osteoblasts would represent not a single cell type, but rather a family of related cells each with distinct functions, paralleling the functional diversity in SSCs.

Skeletal stem and progenitor cells in bone development and repair

Bone development, growth, and repair are complex processes involving various cell types and interactions, with central roles played by skeletal stem and progenitor cells. Recent research brought new insights into the skeletal precursor populations that mediate intramembranous and endochondral bone development. Later in life, many of the cellular and molecular mechanisms determining development are reactivated upon fracture, with powerful trauma-induced signaling cues triggering a variety of postnatal skeletal stem/progenitor cells (SSPCs) residing near the bone defect. Interestingly, in this injury context, the current evidence suggests that the fates of both SSPCs and differentiated skeletal cells can be considerably flexible and dynamic, and that multiple cell sources can be activated to operate as functional progenitors generating chondrocytes and/or osteoblasts. The combined implementation of in vivo lineage tracing, cell surface marker-based cell selection, single-cell molecular analyses, and high-resolution in situ imaging has strongly improved our insights into the diversity and roles of developmental and reparative stem/progenitor subsets, while also unveiling the complexity of their dynamics, hierarchies, and relationships. Albeit incompletely understood at present, findings supporting lineage flexibility and possibly plasticity among sources of osteogenic cells challenge the classical dogma of a single primitive, self-renewing, multipotent stem cell driving bone tissue formation and regeneration from the apex of a hierarchical and strictly unidirectional differentiation tree. We here review the state of the field and the newest discoveries in the origin, identity, and fates of skeletal progenitor cells during bone development and growth, discuss the contributions of adult SSPC populations to fracture repair, and reflect on the dynamism and relationships among skeletal precursors and differentiated cell lineages. Further research directed at unraveling the heterogeneity and capacities of SSPCs, as well as the regulatory cues determining their fate and functioning, will offer vital new options for clinical translation toward compromised fracture healing and bone regenerative medicine.

Craniofacial chondrogenesis in organoids from human stem cell-derived neural crest cells

Knowledge of cell signaling pathways that drive human neural crest differentiation into craniofacial chondrocytes is incomplete, yet essential for using stem cells to regenerate craniomaxillofacial structures. To accelerate translational progress, we developed a differentiation protocol that generated self-organizing craniofacial cartilage organoids from human embryonic stem cell-derived neural crest stem cells. Histological staining of cartilage organoids revealed tissue architecture and staining typical of elastic cartilage. Protein and post-translational modification (PTM) mass spectrometry and snRNA-seq data showed that chondrocyte organoids expressed robust levels of cartilage extracellular matrix (ECM) components: many collagens, aggrecan, perlecan, proteoglycans, and elastic fibers. We identified two populations of chondroprogenitor cells, mesenchyme cells and nascent chondrocytes, and the growth factors involved in paracrine signaling between them. We show that ECM components secreted by chondrocytes not only create a structurally resilient matrix that defines cartilage, but also play a pivotal autocrine cell signaling role in determining chondrocyte fate.

Extracellular matrix dynamics: A key regulator of cell migration across length-scales and systems

The interactions between cells and their surrounding extracellular matrix (ECM) are dynamic and play critical roles in cell migration during development, health, and diseases. Recent advances have highlighted the complexity and diversity of ECM compositions, or "matrisomes", of tissues resulting in ECMs of different physical, mechanical, and biochemical properties. Investigating the effects of these properties on cell-ECM interactions in the context of cell migration have led to a better understanding of the principles underlying tissue morphogenesis, wound healing, immune response, or cancer metastasis. These new insights into the interplay between ECM dynamics and cell migration can lead to the identification of unique opportunities for therapeutic interventions.

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.

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.