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

Abstract

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.

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

“…This result is what would be expected if Ddr2 was expressed in skeletal progenitor cells (SPCs) whose progeny became the mature cells of each skeletal lineage (hypertrophic chondrocytes for the cartilage lineage, osteocytes for the osteoblast lineage). Consistent with this concept, a high degree of colocalization between DDR2 and the skeletal progentitor/stem cell marker, GLI1 (59,60), was observed by immunofluorescence in cranial sutures, synchondroses and tibial growth plates (52,53). Also, CD140α + /CD51 + SPCs purified from bone marrow by FACS were enriched in Ddr2 mRNA (52).…”

“…In contrast, skulls from Ddr2-deficient mice are dramatically shorter in the anterior-posterior direction with a more spherical skull shape associated with increased anterior skull width as well as reduced nasal bone length. Subsequent analysis of this phenotype identified a defect in proliferation of synchondrosis chondrocytes, particularly in the intersphenoid synchondrosis, in the absence of changes in apoptosis (53). These changes were associated with a characteristic expansion of the synchondrosis resting zone, possibly related to the defective conversion of these cells into proliferating chondrocytes.…”

“…Early in situ hybridization studies localized Ddr2 expression to tibial growth plates (51). Subsequent more detailed analysis using Ddr2 +/LacZ mice stained for β-galactosidase activity, first detected Ddr2 expression in bone rudiments at E11.5 (52,53). Analysis from E13.5 through adulthood showed strong staining in all developing skeletal elements in the appendicular, axial and cranial skeletons including growth plate cartilage, metaphyses, periosteum, cranial sutures and cranial base synchondroses.…”

“…To gain further insight into the lineage of Ddr2-expressing cells, Ddr2 mer−icre−mer ; ROSA26 LSLtdTomato mice were developed (52,53). After tamoxifen-induced recombination, Ddr2-expressing cells are labelled with tdTomato fluorescent protein, thereby allowing these cells to be followed over time.…”

“…These changes were associated with a characteristic expansion of the synchondrosis resting zone, possibly related to the defective conversion of these cells into proliferating chondrocytes. Ddr2-deficient skulls also have open fontanelles at birth, thinning of frontal bones and defects in frontal suture fusion that persist into adulthood (53,65).…”