Discoidin Receptor 2 Controls Bone Formation and Marrow Adipogenesis

Area of Science:

• Molecular and Cellular Biology
• Skeletal Biology
• Biochemistry

Background:

• Cell-extracellular matrix (ECM) interactions regulate progenitor cell fate.
• Discoidin domain receptor 2 (DDR2), a receptor tyrosine kinase, mediates cell interactions with fibrillar collagens.
• DDR2 signaling through ERK1/2 and p38 MAP kinase influences osteoblast differentiation and bone formation.

Purpose of the Study:

• To investigate the role of DDR2 in skeletal development and marrow progenitor cell differentiation.
• To determine DDR2's impact on osteoblastogenesis and adipogenesis.
• To elucidate the molecular mechanisms underlying DDR2's function in bone metabolism.

Main Methods:

• Analysis of skeletal defects in Smallie mice (Ddr2^slie/slie) with a nonfunctional Ddr2 allele.
• Micro-computed tomography (micro-CT) to assess bone structure (trabecular and cortical bone).
• In vitro studies using bone marrow stromal cells (BMSCs) under osteogenic and adipogenic conditions.
• Western blotting and reporter assays to evaluate transcription factor activity (RUNX2, PPARγ) and phosphorylation.

Main Results:

• Ddr2^slie/slie mice exhibit significant skeletal defects, including reduced bone volume, altered trabecular structure, and widened calvarial sutures.
• Bone loss in Ddr2^slie/slie mice is due to decreased bone formation, not increased osteoclast activity.
• Marrow adipogenesis and adipocyte-specific gene expression are significantly elevated in Ddr2^slie/slie animals.
• Ddr2^slie/slie BMSCs show impaired osteoblast differentiation and accelerated adipogenesis.
• DDR2 signaling modulates RUNX2 and PPARγ phosphorylation, affecting their transcriptional activity.

Conclusions:

• DDR2 is essential for skeletal development and maintaining osteoblast activity.
• DDR2 plays a critical role in suppressing marrow adipogenesis.
• DDR2's functions are mediated through MAPK-dependent phosphorylation of RUNX2 and PPARγ.