Discoidin Domain Receptor 2, β1 Integrins and ECM Control of Bone Formation

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Discoidin domain receptor 2 (DDR2) is a non-integrin collagen receptor having important but poorly understood skeletal functions. Inactivating mutations in DDR2 cause spondylo-meta-epiphyseal dysplasia (SMED-short limb type), a human disorder with severe craniofacial and skeletal abnormalities. Ddr2-deficient mice have a similar phenotype including abnormal skull shape, delayed suture fusion and defective cartilage growth/orientation in synchondroses at the base of the skull, shortened long bone growth plates and reduced trabecular and cortical bone mass. Preliminary studies where Ddr2 was selectively inactivated in skeletal progenitors indicate distinct functions in cells of the bone lineage. Studies will test the following hypothesis: DDR2 is an important regulator of bone formation that functions in bone lineage cells to sense the collagenous ECM niche and thereby stimulate SPC lineage commitment and differentiation; on activation by fibrillar collagens, DDR2 synergistically interacts with β1 integrins to stimulate osteoblast differentiation. Aims are: 1. Establish the distribution, fates and gene expression profile of Ddr2-positive cells during craniofacial and skeletal development. Hypothesis: During development, Ddr2 is expressed in SPCs as well as more mature bone cells. Studies will: a) establish the time course and pattern of Ddr2 expression during skeletal development, b) define the lineage of Ddr2-expressing cells, c) Define the molecular signature of Ddr2-expressing cells and their progeny using single-cell RNAseq. 2. Define the cellular sites of action of DDR2 in the craniofacial and appendicular skeleton. Hypothesis: Ddr2 functions in both SPCs and committed chondrogenic and osteogenic cells to stimulate bone formation. It is not known which skeletal cells require DDR2 for normal function; skeletal SCs, differentiated cells or both. a) To define cellular functions of DDR2, a targeted deletion approach will be used to inactivate Ddr2 in Gli+ craniofacial/skeletal progenitors, chondrocytes and osteoblasts. b) To determine if Ddr2 controls the differentiation of SPCs, the effect of Ddr2 deficiency on the lineage of Gli1+ SPCs will be examined. 3. Examine the mechanistic basis for interactions between DDR2 and collagen-binding β1 integrins. Hypothesis: DDR2 and β1 integrins synergistically interact to stimulate cellular activity. Collagen-binding β1 integrins and DDR2 synergistically interact. The mechanistic basis for this synergy will be established using in vitro and in vivo approaches as follows: a. Examine interactions between DDR2 and integrin signaling during osteoblast differentiation. b. Use a genetic approach to detect Ddr2 and β1 integrin interactions in vivo. Proposed studies will elucidate the skeletal functions of DDR2 and its interactions with β1 integrins. This is a highly significant but understudied area with important implications for understanding the role of cell- collagen interactions in bone diseases including SMED, osteoporosis and diabetes. In addition, bone-targeted DDR2 agonists if developed could be a new class of bone anabolic agents.