Ablation of Discoidin Domain Receptor 1 Provokes an Osteopenic Phenotype by Regulating Osteoblast/Osteocyte Autophagy and Apoptosis.

Discoidin domain receptor 1 (DDR1) is a collagen receptor that belongs to the receptor tyrosine kinase family. We have previously shown that DDR1 plays a crucial role during bone development, resulting in dwarfism and a short stature in osteoblast-specific knockout mice (OKO mice). However, the detailed pathophysiological effects of DDR1 on bone development throughout adulthood have remained unclear. This study aims to identify how DDR1 regulates osteoblast and osteocyte functions in vivo and in vitro during bone development in adulthood. The metabolic changes in bone tissues were analyzed using Micro-CT and immunohistochemistry staining (IHC) in vivo; the role of DDR1 in regulating osteoblasts was examined in MC3T3-E1 cells in vitro. The Micro-CT analysis results demonstrated that OKO mice showed a 10% reduction in bone-related parameters from 10 to 14 weeks old and a significant reduction in cortical thickness and diameter compared with flox/flox control mice (FF) mice. These results indicated that DDR1 knockout in OKO mice exhibiting significant bone loss provokes an osteopenic phenotype. The IHC staining revealed a significant decrease in osteogenesis-related genes, including RUNX2, osteocalcin, and osterix. We noted that DDR1 knockout significantly induced osteoblast/osteocyte apoptosis and markedly decreased autophagy activity in vivo. Additionally, the results of the gain- and loss-of-function of the DDR1 assay in MC3T3-E1 cells indicated that DDR1 can regulate the osteoblast differentiation through activating autophagy by regulating the phosphorylation of the mechanistic target of rapamycin (p-mTOR), light chain 3 (LC3), and beclin-1. In conclusion, our study highlights that the ablation of DDR1 results in cancellous bone loss by regulating osteoblast/osteocyte autophagy. These results suggest that DDR1 can act as a potential therapeutic target for managing cancellous bone loss.

The Essential Role of Stathmin in Myoblast C2C12 for Vertical Vibration-Induced Myotube Formation.

Vertical vibration (VV) is a type of whole body vibration, which induces muscle contraction through vibration to improve muscle strength and bone density. However, the mechanism of VV on muscle cell myotube formation is still unclear. In the current study, we aim to clarify the mechanism involved in VV's stimulation of myotube formation. In order to identify the molecules regulated by VV, we performed proteomics analysis including 2D electrophoresis combined with MALDI-TOF/TOF Mass. Stathmin was identified as a high potential molecule responding to VV stimulation, and we found that under VV stimulation, the expression of stathmin gene and protein increased in a time-dependent manner. In addition, we also confirmed that the increase of stathmin stimulated by VV is mediated through the PI3K/Akt pathway. Furthermore, stathmin siRNA significantly down-regulated the expression of myogenic regulatory factor (MRF) MyoD, decorin, and type I collagen (Col-I), and down-regulated the cellular process regulators such as FGF7, TGFBr1 and PAK3. Taken together, our results confirm that under the stimulation of VV, PI3K/Akt and stathmin would be activated, as well as the up-regulation of MRFs, such as FGF7, TGFBr1 and PAK3 to initiate myogenesis. It also showed that the response of MRF to VV stimulation was significantly related to stathmin expression, which also confirmed the importance of stathmin in the entire myotube formation process. This study may provide evidence of stathmin as a biological indicator of VV to increase muscle strength.

Discoidin Domain Receptor 1 Regulates Runx2 during Osteogenesis of Osteoblasts and Promotes Bone Ossification via Phosphorylation of p38.

Discoidin domain receptor 1 (Drd1) is a collagen-binding membrane protein, but its role in osteoblasts during osteogenesis remains undefined. We generated inducible osteoblast-specific Ddr1 knockout (OKOΔDdr1) mice; their stature at birth, body weight and body length were significantly decreased compared with those of control Ddr1f/f-4OHT mice. We hypothesize that Ddr1 regulates osteogenesis of osteoblasts. Micro-CT showed that compared to 4-week-old Ddr1f/f-4OHT mice, OKOΔDdr1 mice presented significant decreases in cancellous bone volume and trabecular number and significant increases in trabecular separation. The cortical bone volume was decreased in OKOΔDdr1 mice, resulting in decreased mechanical properties of femurs compared with those of Ddr1f/f-4OHT mice. In femurs of 4-week-old OKOΔDdr1 mice, H&E staining showed fewer osteocytes and decreased cortical bone thickness than Ddr1f/f-4OHT. Osteoblast differentiation markers, including BMP2, Runx2, alkaline phosphatase (ALP), Col-I and OC, were decreased compared with those of control mice. Ddr1 knockdown in osteoblasts resulted in decreased mineralization, ALP activity, phosphorylated p38 and protein levels of BMP2, Runx2, ALP, Col-I and OC during osteogenesis. Overexpression and knockdown of Ddr1 in osteoblasts demonstrated that DDR1 mediates the expression and activity of Runx2 and the downstream osteogenesis markers during osteogenesis through regulation of p38 phosphorylation.

Discoidin domain receptor 1 regulates endochondral ossification through terminal differentiation of chondrocytes.

Chondrocytes in growth plates are responsible for longitudinal growth in long bones during endochondral ossification. Discoidin domain receptor 1 (Ddr1) is expressed in chondrocytes, but the molecular mechanisms by which DDR1 regulates chondrocyte behaviors during the endochondral ossification process remain undefined. To elucidate Ddr1-mediate chondrocyte functions, we generated chondrocyte-specific Ddr1 knockout (CKOΔDdr1) mice in this study. The CKOΔDdr1 mice showed delayed development of the secondary ossification center and increased growth plate length in the hind limbs. In the tibial growth plate in CKOΔDdr1 mice, chondrocyte proliferation was reduced in the proliferation zone, and remarkable downregulation of Ihh, MMP13, and Col-X expression in chondrocytes resulted in decreased terminal differentiation in the hypertrophic zone. Furthermore, apoptotic chondrocytes were reduced in the growth plates of CKOΔDdr1 mice. We concluded that chondrocytes with Ddr1 knockout exhibit decreased proliferation, terminal differentiation, and apoptosis in growth plates, which delays endochondral ossification and results in short stature. We also demonstrated that Ddr1 regulates the Ihh/Gli1/Gli2/Col-X pathway to regulate chondrocyte terminal differentiation. These results indicate that Ddr1 is required for chondrocytes to regulate endochondral ossification in skeletal development.