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

Abstract

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.

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

“…Although an early study that measured DDR1 binding sites in mice using DDR1 extracellular domain fused with alkaline phosphatase showed binding to all skeletal structures, skin and the urogenital tract because of their high collagen content (46), studies that actually measured the tissue distribution of DDR1 protein or mRNA are quite limited. In neonatal and adult mice, DDR1 was localized by immunohistochemistry to proliferating and hypertrophic chondrocytes of long bone growth plates, cortical and trabecular bone osteocytes, periosteum, and articular chondrocytes (47)(48)(49). In situ hybridization analysis was conducted in oral tissues using a Ddr1 probe (50).…”

“…Chondrocyte or osteoblast-selective inactivation of Ddr1 was achieved by crossing Ddr1 fl/fl mice with Col2a1 CreERT or Col1a1 CreERT mice (47)(48)(49). Chondrocyte-selective knockout of Ddr1 in tamoxifen-treated Col2a1 CreERT ; Ddr1 fl/fl mice led to a 10-20 percent decrease in body weight and length and delayed formation of a secondary ossification center (47).…”

“…These changes were accompanied by a loss of mechanical properties and inhibition of osteoblast markers such as RUNX2, ALPL, BGLAP and COLIA1. In a second study with Col1a1 CreERT ; Ddr1 fl/fl mice, the same group examined the consequences of Ddr1 inactivation in adults over extended periods (49). In this case, modest changes in trabecular parameters were noted together with reductions in cortical thickness, osteoblast differentiation markers and cortical bone formation rate.…”

The Potential of Natural Compounds Regulating Autophagy in the Treatment of Osteoporosis

“…Intriguingly, the knockout of the DDR1 gene has been shown to markedly diminish autophagy levels while concurrently triggering OBs apoptosis. 41 Heat shock protein 60 (HSP60) has been recognized as a key regulator in preserving protein functionality within the cellular microenvironment. Notably, in OBs subjected to glucocorticoid treatment, HSP60 maintains the expression of specific autophagy-related genes, such as Atg4 and Atg12, and promotes autophagic flux.…”

Effects of E’Jiao on Skeletal Mineralisation, Osteocyte and WNT Signalling Inhibitors in Ovariectomised Rats

“…Bone remodelling is important in maintaining the integrity and functions of bone tissues, whereby osteoblasts, osteoclasts and osteocytes work sequentially in governing this biological process throughout life [ 7 ]. Osteocytes are the master regulator of bone remodelling and the most populous cells in the bone [ 8 , 9 ]. They could sense mechanical loading and mediate bone formation by osteoblasts and bone resorption by osteoclasts [ 10 ].…”