Background-Aldosteronism may account for oxi/nitrosative stress, a proinflammatory phenotype, and wasting in congestive heart failure. We hypothesized that aldosterone/1% NaCl treatment (ALDOST) in rats enhances

SUMMARYBone injury induces an inflammatory response that involves neutrophils, macrophages and other inflammatory cells. The recruitment of inflammatory cells to sites of injury occurs in response to specific signaling pathways. The CC chemokine receptor type 2 (CCR2) is crucial for recruiting macrophages, as well as regulating osteoclast function. In this study, we examined fracture healing in Ccr2-/-mice. We first demonstrated that the expression of Ccr2 transcripts and the filtration of macrophages into fracture calluses were most robust during the early phases of fracture healing. We then determined that the number of macrophages at the fracture site was significantly lower in Ccr2-/-mice compared with wild-type controls at 3 days after injury. As a result, impaired vascularization, decreased formation of callus, and delayed maturation of cartilage were observed at 7 days after injury in mutant mice. At day 14, Ccr2-/-mice had less bone in their calluses. At day 21, Ccr2-/-mice had larger calluses and more bone compared with wild-type mice, suggesting a delayed remodeling. In addition, we examined the effect of Ccr2 mutation on osteoclasts. We found that a lack of Ccr2 did not affect the number of osteoclasts within fracture calluses at 21 days after injury. However, Ccr2-/-osteoclasts exhibited a decreased ability to resorb bone compared with wild-type cells, which could contribute to the delayed remodeling of fracture calluses observed in Ccr2-/-mice. Collectively, these results indicate that a deficiency of Ccr2 reduces the infiltration of macrophages and impairs the function of osteoclasts, leading to delayed fracture healing.

Extracellular matrix (ECM) remodeling is important during bone development and repair. Because matrix metalloproteinase 13 (MMP13, collagenase-3) plays a role in long bone development, we have examined its role during adult skeletal repair. In this study we find that MMP13 is expressed by hypertrophic chondrocytes and osteoblasts in the fracture callus. We demonstrate that MMP13 is required for proper resorption of hypertrophic cartilage and for normal bone remodeling during non-stabilized fracture healing, which occurs via endochondral ossification. However, no difference in callus strength was detected in the absence of MMP13. Transplant of wild-type bone marrow, which reconstitutes cells only of the hematopoietic lineage, did not rescue the endochondral repair defect, indicating that impaired healing in Mmp13−/− mice is intrinsic to cartilage and bone. Mmp13−/− mice also exhibited altered bone remodeling during healing of stabilized fractures and cortical defects via intramembranous ossification. This indicates that the bone phenotype occurs independently from the cartilage phenotype. Taken together, our findings demonstrate that MMP13 is involved in normal remodeling of bone and cartilage during adult skeletal repair, and that MMP13 may act directly in the initial stages of ECM degradation in these tissues prior to invasion of blood vessels and osteoclasts.

Age significantly reduces the regenerative capacity of the skeleton, but the underlying causes are unknown. Here, we tested whether the functional status of inflammatory cells contributes to delayed healing in aged animals. We created chimeric mice by bone marrow transplantation after lethal irradiation. In this model, chondrocytes and osteoblasts in the regenerate are derived exclusively from host cells while inflammatory cells are derived from the donor. Using this model, the inflammatory system of middle-aged mice (12 month old) was replaced by transplanted bone marrow from juvenile mice (4 weeks old), or age-matched controls. We found that the middle-aged mice receiving juvenile bone marrow had larger calluses and more bone formation during early stages and faster callus remodeling at late stages of fracture healing, indicating that inflammatory cells derived from the juvenile bone marrow accelerated bone repair in the middle-aged animals. In contrast, transplanting bone marrow from middle-aged mice to juvenile mice did not alter the process of fracture healing in juvenile mice. Thus, the roles of inflammatory cells in fracture healing may be age-related, suggesting the possibility of enhancing fracture healing in aged animals by manipulating the inflammatory system. ß

No abstract