RESUMEN
Bone-lining tissues contain a population of resident macrophages termed osteomacs that interact with osteoblasts in vivo and control mineralization in vitro. The role of osteomacs in bone repair was investigated using a mouse tibial bone injury model that heals primarily through intramembranous ossification and progresses through all major phases of stabilized fracture repair. Immunohistochemical studies revealed that at least two macrophage populations, F4/80(+) Mac-2(-/low) TRACP(-) osteomacs and F4/80(+) Mac-2(hi) TRACP(-) inflammatory macrophages, were present within the bone injury site and persisted throughout the healing time course. In vivo depletion of osteomacs/macrophages (either using the Mafia transgenic mouse model or clodronate liposome delivery) or osteoclasts (recombinant osteoprotegerin treatment) established that osteomacs were required for deposition of collagen type 1(+) (CT1(+)) matrix and bone mineralization in the tibial injury model, as assessed by quantitative immunohistology and micro-computed tomography. Conversely, administration of the macrophage growth factor colony-stimulating factor 1 (CSF-1) increased the number of osteomacs/macrophages at the injury site significantly with a concurrent increase in new CT1(+) matrix deposition and enhanced mineralization. This study establishes osteomacs as participants in intramembranous bone healing and as targets for primary anabolic bone therapies.
Asunto(s)
Macrófagos/metabolismo , Tibia/lesiones , Tibia/patología , Cicatrización de Heridas , Fosfatasa Ácida/metabolismo , Animales , Matriz Ósea/efectos de los fármacos , Matriz Ósea/metabolismo , Calcificación Fisiológica/efectos de los fármacos , Ácido Clodrónico/administración & dosificación , Ácido Clodrónico/farmacología , Modelos Animales de Enfermedad , Inflamación/patología , Isoenzimas/metabolismo , Liposomas/administración & dosificación , Factor Estimulante de Colonias de Macrófagos/farmacología , Macrófagos/efectos de los fármacos , Membranas/efectos de los fármacos , Membranas/patología , Ratones , Ratones Endogámicos C57BL , Ratones Transgénicos , Osteoblastos/efectos de los fármacos , Osteoblastos/patología , Osteogénesis/efectos de los fármacos , Osteoprotegerina/farmacología , Propiedades de Superficie/efectos de los fármacos , Fosfatasa Ácida Tartratorresistente , Tibia/efectos de los fármacos , Cicatrización de Heridas/efectos de los fármacosRESUMEN
Osteoimmunological interactions are central to maintaining bone homeostasis and are key mechanisms in bone pathology. Macrophages are highly adaptable cells with pleiotropic actions. They have important roles in development, homeostasis and both innate and adaptive immunity. Macrophages can have broad ranging effects on bone, particularly in pathologic situations, but they are most commonly considered for their in vitro potential as an osteoclast precursor. We have recently shown that, like most tissues, the endosteum and periosteum contain a population of resident tissue macrophages (OsteoMacs) that impact on the bone formation process and are likely to play important roles in the bone niche. This review discusses the wider impact of macrophages in bone homeostasis and disease and proposes novel roles for OsteoMacs in bone modelling and remodelling.
Asunto(s)
Huesos/citología , Macrófagos/citología , Remodelación Ósea , Calcificación Fisiológica , HumanosRESUMEN
Resident macrophages are an integral component of many tissues and are important in homeostasis and repair. This study examines the contribution of resident tissue macrophages to bone physiology. Using immunohistochemistry, we showed that a discrete population of resident macrophages, OsteoMacs, was intercalated throughout murine and human osteal tissues. OsteoMacs were distributed among other bone lining cells within both endosteum and periosteum. Furthermore, OsteoMacs were coisolated with osteoblasts in murine bone explant and calvarial preparations. OsteoMacs made up 15.9% of calvarial preparations and persisted throughout standard osteoblast differentiation cultures. Contrary to previous studies, we showed that it was OsteoMacs and not osteoblasts within these preparations that responded to pathophysiological concentrations of LPS by secreting TNF. Removal of OsteoMacs from calvarial cultures significantly decreased osteocalcin mRNA induction and osteoblast mineralization in vitro. In a Transwell coculture system of enriched osteoblasts and macrophages, we demonstrated that macrophages were required for efficient osteoblast mineralization in response to the physiological remodeling stimulus, elevated extracellular calcium. Notably, OsteoMacs were closely associated with areas of bone modeling in situ, forming a distinctive canopy structure covering >75% of mature osteoblasts on diaphyseal endosteal surfaces in young growing mice. Depletion of OsteoMacs in vivo using the macrophage-Fas-induced apoptosis (MAFIA) mouse caused complete loss of osteoblast bone-forming surface at this modeling site. Overall, we have demonstrated that OsteoMacs are an integral component of bone tissues and play a novel role in bone homeostasis through regulating osteoblast function. These observations implicate OsteoMacs, in addition to osteoclasts and osteoblasts, as principal participants in bone dynamics.
Asunto(s)
Huesos/fisiología , Macrófagos/fisiología , Osteoblastos/fisiología , Animales , Huesos/citología , Calcificación Fisiológica , Calcio/metabolismo , Diferenciación Celular , Células Cultivadas , Expresión Génica , Humanos , Lipopolisacáridos/inmunología , Macrófagos/citología , Macrófagos/inmunología , Ratones , Ratones Endogámicos C57BL , Ratones Transgénicos , Osteoblastos/citología , Osteocalcina/genética , Osteocalcina/metabolismo , OsteogénesisRESUMEN
Microphthalmia transcription factor (MITF) regulates bone homeostasis by inducing expression of critical genes associated with osteoclast function. Gpnmb is a macrophage-enriched gene that has also been shown to be expressed in osteoblasts. Here, we have shown gpnmb to be highly induced in maturing murine osteoclasts. Microarray expression profile analysis identified gpnmb as a potential target of MITF in RAW264.7 cells, subclone C4 (RAW/C4), that overexpress this transcription factor. Electrophoretic mobility shift assays identified a MITF-binding site (M-box) in the gpnmb promoter that is conserved in different mammalian species. Anti-MITF antibody supershifted the DNA-MITF complex for the promoter site while MITF binding was abolished by mutation of this site. The gpnmb promoter was transactivated by co-expression of MITF in reporter gene assays while mutation of the gpnmb M-box prevented MITF transactivation. The induction of gpnmb expression during osteoclastogenesis was shown to exhibit similar kinetics to the known MITF targets, acp5 and clcn7. GPNMB expressed in RAW/C4 cells exhibited distinct subcellular distribution at different stages of osteoclast differentiation. At days 5 and 7, GPNMB protein co-localised with the osteoclast/macrophage lysosomal/endocytic marker MAC-3/LAMP-2, suggesting that GPNMB resides in the endocytic pathway of mature macrophages and is possibly targeted to the plasma membrane of bone-resorbing osteoclasts. The inclusion of gpnmb in the MITF regulon suggests a role for GPNMB in mature osteoclast function.