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Discoidin Domain Receptor 1 (DDR1) is a receptor tyrosine kinase that binds to and is activated by collagen(s), including collagen type I. Ddr1 deletion in osteoblasts and chondrocytes has previously demonstrated the importance of this receptor in bone development. In this study, we examined the effect of DDR1 ablation on bone architecture and mechanics as a function of aging. Femurs were collected from female global Ddr1 knockout (KO) and wild-type (WT) mice at 2, 6, and 12 mo of age and analyzed by high-resolution micro-computed tomography (μCT), mechanical testing, and histology. Primary monocytes were collected for in vitro osteoclastogenesis assays. Our studies on younger (2 mo) mice revealed no significant differences between the two genotypes and the microarchitectural and mechanical features had a similar trend as those reported earlier for osteoblast or chondrocyte specific Ddr1 knockdown. At an advanced age (12 mo), significant differences were noted across the two genotypes. μCT analysis showed a decrease in medullary cavity area as well as increased trabeculation in cortical and trabecular bone in the Ddr1 KO vs. WT mice. In addition, Ddr1 KO mouse bones exhibited reduced mechanical properties (lower peak load, yield load, and energy to yield) at 12 mo. Histological analysis revealed reduced osteoclast count in Ddr1 KO femurs at 12 mo with no significant difference in osteocyte count between the genotypes. In vitro, osteoclastogenesis was impaired in Ddr1 KO bone marrow derived cells. These results suggest that DDR1 deficiency adversely impacts osteoclast differentiation and bone remodeling in an age-dependent manner. 

Introduction: Vascular diseases like abdominal aortic aneurysms (AAA) are characterized by a drastic remodeling of the vessel wall, accompanied with changes in the elastin and collagen content. At the macromolecular level, the elastin fibers in AAA have been reported to undergo significant structural alterations. While the undulations (waviness) of the collagen fibers is also reduced in AAA, very little is understood about changes in the collagen fibril at the sub-fiber level in AAA as well as in other vascular pathologies. Materials and Methods: In this study we investigated structural changes in collagen fibrils in human AAA tissue extracted at the time of vascular surgery and in aorta extracted from angiotensin II (AngII) infused ApoE−/− mouse model of AAA. Collagen fibril structure was examined using transmission electron microscopy and atomic force microscopy. Images were analyzed to ascertain length and depth of D-periodicity, fibril diameter and fibril curvature. Tissues were also stained using collagen hybridizing peptide (CHP) and analyzed using fluorescent microscopy and second harmonic generation (SHG) microscopy to locate regions of healthy and degraded collagen. Results: Abnormal collagen fibrils with compromised D-periodic banding were observed in the excised human tissue and in remodeled regions of AAA in AngII infused mice (Figure 1). These abnormal fibrils were characterized by statistically significant reduction in depths of D-periods and an increased curvature of collagen fibrils. These features were more pronounced in human AAA as compared to murine samples. Additionally, regions of abnormal collagen were located within the remodeled areas of AAA tissue and were distinct from healthy collagen regions as ascertained using CHP staining and SHG (Figure 1). Thoracic aorta from Ang II-infused mice, abdominal aorta from saline-infused mice, and abdominal aorta from non-AAA human controls did not contain abnormal collagen fibrils. Conclusions: The structural alterations in abnormal collagen fibrils appear similar to those reported for collagen fibrils subjected to mechanical overload or chronic inflammation in other tissues. Detection of abnormal collagen could be utilized to better understand the functional properties of the underlying extracellular matrix in vascular as well as other pathologies.