Maresin 1 resolves aged-associated macrophage inflammation to improve bone regeneration.

Inflammaging is associated with poor tissue regeneration observed in advanced age. Specifically, protracted inflammation after acute injury has been associated with decreased bone fracture healing and increased rates of nonunion in elderly patients. Here, we investigated the efficacy of using Maresin 1 (MaR1), an omega-3 fatty acid-derived pro-resolving agent, to resolve inflammation after tibial fracture injury and subsequently improving aged bone healing. Aged (24-month-old mice) underwent tibial fracture surgery and were either treated with vehicle or MaR1 3 days after injury. Fracture calluses were harvested 7 days, 14 days, 21 days, and 28 days after injury to investigate inflammatory response, cartilage development, bone deposition, and mechanical integrity, respectively. Healing bones from MaR1-treated mice displayed decreased cartilage formation and increased bone deposition which resulted in increased structural stiffness and increased force to fracture in the later stages of repair. In the early stages, MaR1 treatment decreased the number of pro-inflammatory macrophages within the fracture callus and decreased the level of inflammatory biomarkers in circulation. In tissue culture models, MaR1 treatment of bone marrow-derived macrophages from aged mice protected cells form a pro-inflammatory phenotype and induced an anti-inflammatory fate. Furthermore, the secretome of MaR1-treated bone marrow-derived macrophages was identified as osteoinductive, enhancing osteoblast differentiation of bone marrow stromal cells. Our findings here identify resolution of inflammation, and MaR1 itself, to be a point of intervention to improve aged bone healing.

Sunlight triggers cutaneous lupus through a CSF-1-dependent mechanism in MRL-Fas(lpr) mice.

Sunlight (UVB) triggers cutaneous lupus erythematosus (CLE) and systemic lupus through an unknown mechanism. We tested the hypothesis that UVB triggers CLE through a CSF-1-dependent, macrophage (Mø)-mediated mechanism in MRL-Fas(lpr) mice. By constructing mutant MRL-Fas(lpr) strains expressing varying levels of CSF-1 (high, intermediate, none), and use of an ex vivo gene transfer to deliver CSF-1 intradermally, we determined that CSF-1 induces CLE in lupus-susceptible MRL-Fas(lpr) mice, but not in lupus-resistant BALB/c mice. UVB incites an increase in Møs, apoptosis in the skin, and CLE in MRL-Fas(lpr), but not in CSF-1-deficient MRL-Fas(lpr) mice. Furthermore, UVB did not induce CLE in BALB/c mice. Probing further, UVB stimulates CSF-1 expression by keratinocytes leading to recruitment and activation of Møs that, in turn, release mediators, which induce apoptosis in keratinocytes. Thus, sunlight triggers a CSF-1-dependent, Mø-mediated destructive inflammation in the skin leading to CLE in lupus-susceptible MRL-Fas(lpr) but not lupus-resistant BALB/c mice. Taken together, CSF-1 is envisioned as the match and lupus susceptibility as the tinder leading to CLE.

Lowering circulating apolipoprotein E levels improves aged bone fracture healing.

Age is a well-established risk factor for impaired bone fracture healing. Here, we identify a role for apolipoprotein E (ApoE) in age-associated impairment of bone fracture healing and osteoblast differentiation, and we investigate the mechanism by which ApoE alters these processes. We identified that, in both humans and mice, circulating ApoE levels increase with age. We assessed bone healing in WT and ApoE-/- mice after performing tibial fracture surgery: bone deposition was higher within fracture calluses from ApoE-/- mice. In vitro recombinant ApoE (rApoE) treatment of differentiating osteoblasts decreased cellular differentiation and matrix mineralization. Moreover, this rApoE treatment decreased osteoblast glycolytic activity while increasing lipid uptake and fatty acid oxidation. Using parabiosis models, we determined that circulating ApoE plays a strong inhibitory role in bone repair. Using an adeno-associated virus-based siRNA system, we decreased circulating ApoE levels in 24-month-old mice and demonstrated that, as a result, fracture calluses from these aged mice displayed enhanced bone deposition and mechanical strength. Our results demonstrate that circulating ApoE as an aging factor inhibits bone fracture healing by altering osteoblast metabolism, thereby identifying ApoE as a new therapeutic target for improving bone repair in the elderly.

Osteoclast deficiency results in disorganized matrix, reduced mineralization, and abnormal osteoblast behavior in developing bone.

UNLABELLED: Studies of the influence of the osteoclast on bone development, in particular on mineralization and the formation of the highly organized lamellar architecture of cortical bone by osteoblasts, have not been reported. We therefore examined the micro- and ultrastructure of the developing bones of osteoclast-deficient CSF-1R-nullizygous mice (Csf1r(-/-) mice). INTRODUCTION: Colony-stimulating factor-1 receptor (CSF-1R)-mediated signaling is critical for osteoclastogenesis. Consequently, the primary defect in osteopetrotic Csf1r(-/-) mice is severe osteoclast deficiency. Csf1r(-/-) mice therefore represent an ideal model system in which to investigate regulation by the osteoclast of osteoblast-mediated bone formation during development. MATERIALS AND METHODS: Bones of developing Csf1r(-/-) mice and their littermate controls were subjected to X-ray analysis, histological examination by light microscopy and transmission electron microscopy, and a three-point bending assay to test their biomechanical strength. Bone mineralization in embryonic and postnatal bones was visualized by double staining with alcian blue and alizarin red. Bone formation by osteoblasts in these mice was also examined by double-calcein labeling and in femoral anlagen transplantation experiments. RESULTS AND CONCLUSIONS: Frequent spontaneous fractures and decreased strength parameters (ultimate load, yield load, and stiffness) in a three-point bending assay showed the biomechanical weakness of long bones in Csf1r(-/-) mice. Histologically, these bones have an expanded epiphyseal chondrocyte region, a poorly formed cortex with disorganized collagen fibrils, and a severely disturbed matrix structure. The mineralization of their bone matrix at secondary sites of ossification is significantly reduced. While individual osteoblasts in Csf1r(-/-) mice have preserved their typical ultrastructure and matrix depositing activity, the layered organization of osteoblasts on the bone-forming surface and the direction of their matrix deposition toward the bone surface have been lost, resulting in their abnormal entrapment by matrix. Moreover, we also found that (1) osteoblasts do not express CSF-1R, (2) the bone defects in Csf1r(-/-) embryos develop later than the development of osteoclasts in normal embryos, and (3) the transplanted Csf1r(-/-) femoral anlagen develop normally in the presence of wildtype osteoclasts. These results suggest that the dramatic bone defects in Csf1r(-/-) mice are caused by a deficiency of the osteoclast-mediated regulation of osteoblasts and that the osteoclast plays an important role in regulating osteoblastic bone formation during development, in particular, in the formation of lamellar bone.

Critical roles for macrophages in islet angiogenesis and maintenance during pancreatic degeneration.

OBJECTIVE: Chronic pancreatitis, characterized by pancreatic exocrine tissue destruction with initial maintenance of islets, eventually leads to insulin-dependent diabetes in most patients. Mice deficient for the transcription factors E2F1 and E2F2 suffer from a chronic pancreatitis-like syndrome and become diabetic. Surprisingly, onset of diabetes can be prevented through bone marrow transplantation. The goal of the described studies was to determine the hematopoietic cell type responsible for maintaining islets and the associated mechanism of this protection. RESEARCH DESIGN AND METHODS: Mouse models of acute and chronic pancreatitis, together with mice genetically deficient for macrophage production, were used to determine roles for macrophages in islet angiogenesis and maintenance. RESULTS: We demonstrate that macrophages are essential for preventing endocrine cell loss and diabetes. Macrophages expressing matrix metalloproteinase-9 migrate to the deteriorating pancreas. E2f1/E2f2 mutant mice transplanted with wild-type, but not macrophage-deficient colony stimulating factor 1 receptor mutant (Csf1r(-/-)), bone marrow exhibit increased angiogenesis and proliferation within islets, coinciding with increased islet mass. A similar macrophage dependency for islet and islet vasculature maintenance is observed during caerulein-induced pancreatitis. CONCLUSIONS: These findings demonstrate that macrophages promote islet angiogenesis and protect against islet loss during exocrine degeneration, could explain why most patients with chronic pancreatitis develop diabetes, and suggest an avenue for preventing pancreatitis-associated diabetes.

Incomplete restoration of colony-stimulating factor 1 (CSF-1) function in CSF-1-deficient Csf1op/Csf1op mice by transgenic expression of cell surface CSF-1.

The primary macrophage growth factor, colony-stimulating factor 1 (CSF-1), is expressed as a secreted glycoprotein or proteoglycan found in the circulation or as a biologically active cell surface glycoprotein (csCSF-1). To investigate the in vivo roles of csCSF-1, we created mice that exclusively express csCSF-1, in a normal tissue-specific and developmental manner, by transgenic expression of csCSF-1 in the CSF-1-deficient osteopetrotic (Csf1(op)/Csf1(op)) background. The gross defects of Csf1(op)/Csf1(op) mice, including growth retardation, failure of tooth eruption, and abnormal male and female reproductive functions were corrected. Macrophage densities in perinatal liver, bladder, sublinguinal salivary gland, kidney cortex, dermis, and synovial membrane were completely restored, whereas only partial or no restoration was achieved in adult liver, adrenal gland, kidney medulla, spleen, peritoneal cavity, and intestine. Residual osteopetrosis, significantly delayed trabecular bone resorption in the subepiphyseal region of the long bone, and incomplete correction of the hematologic abnormalities in the peripheral blood, bone marrow, and spleens of CSF-1-deficient mice were also found in mice exclusively expressing csCSF-1. These data suggest that although csCSF-1 alone is able to normalize several aspects of development in Csf1(op)/Csf1(op) mice, it cannot fully restore in vivo CSF-1 function, which requires the presence of the secreted glycoprotein and/or proteoglycan forms.