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

Relative contributions of osteal macrophages and osteoclasts to postnatal bone development in CSF1R-deficient rats and phenotype rescue following wild-type bone marrow cell transfer

Macrophage and osteoclast proliferation, differentiation and survival are regulated by colony-stimulating factor 1 receptor (CSF1R) signaling. Osteopetrosis associated with Csf1 and Csf1r mutations has been attributed to the loss of osteoclasts and deficiency in bone resorption. Here, we demonstrate that homozygous Csf1r mutation in rat leads to delayed postnatal skeletal ossification associated with substantial loss of osteal macrophages in addition to osteoclasts. Osteosclerosis and site-specific skeletal abnormalities were reversed by intraperitoneal transfer of wild-type bone marrow cells (bone marrow cell transfer, BMT) at weaning. Following BMT, IBA1+ macrophages were detected before TRAP+ osteoclasts at sites of ossification restoration. These observations extend evidence that osteal macrophages independently contribute to bone anabolism and are required for normal postnatal bone growth and morphogenesis.

Increased bone mass but delayed mineralization: in vivo and in vitro study for zoledronate in bone regeneration

BACKGROUND

Bisphosphonates (BPs) are widely used to inhibit excessive osteoclast activity. However, the potential to compromise bone defect healing has limited their broader application. To better understand the influence of BPs on bone regeneration, we established a bone grafting model with Zoledronate administration, aiming to deepen the understanding of bone remodeling and mineralization processes.

METHODS

A bone grafting model was established in the distal femurs of male Sprague-Dawley rats. The experimental group received systemic administration of Zoledronate (ZOL, 0.2 mg/kg, administered twice). Histological analysis and immunohistochemistry (IHC) were employed to assess osteoblastic and macrophage activity, tartrate-resistant acid phosphatase (TRAP) staining was used to evaluate osteoclastogenesis. Mineralization was assessed through Micro-CT analysis, Raman spectroscopy, and back-scatter scanning electron microscopy (BSE-SEM). Additionally, the in vitro effects of ZOL on osteoblast and osteoclast activity were investigated to further elucidate its impact on bone regeneration.

RESULTS

In vivo, the ZOL group showed increased bone mass, as observed in histological and radiological assessments. However, Micro-CT, Raman spectroscopy, and BSE-SEM detection revealed lower mineralization levels in ZOL group's regenerated bone. Acid-etched SEM analysis showed abnormal osteocyte characteristics in ZOL-group's regenerated bone. Simultaneously, elevated osteopontin (OPN), F4/80 expression along with reduced TRAP expressing was found in the grafting region of ZOL group. In vitro, ZOL did not negatively impact osteogenetic activity (ALP, BMP4, OCN expression) at the tested concentrations (0.02-0.5 g/ml) but significantly impaired mineralization and inhibited osteoclast formation, even at the lowest concentration.

CONCLUSIONS

This study highlights a less recognized negative effect of ZOL on bone mineralization during bone regeneration. More research is needed to elucidate the underlying mechanism.

Osteoporosis: interferon-gamma-mediated bone remodeling in osteoimmunology

As the world population ages, osteoporosis, the most common disease of bone metabolism, affects more than 200 million people worldwide. The etiology is an imbalance in bone remodeling process resulting in more significant bone resorption than bone remodeling. With the advent of the osteoimmunology field, the immune system's role in skeletal pathologies is gradually being discovered. The cytokine interferon-gamma (IFN-γ), a member of the interferon family, is an important factor in the etiology and treatment of osteoporosis because it mediates bone remodeling. This review starts with bone remodeling process and includes the cellular and key signaling pathways of bone remodeling. The effects of IFN-γ on osteoblasts, osteoclasts, and bone mass are discussed separately, while the overall effects of IFN-γ on primary and secondary osteoporosis are summarized. The net effect of IFN-γ on bone appears to be highly dependent on the environment, dose, concentration, and stage of cellular differentiation. This review focuses on the mechanisms of bone remodeling and bone immunology, with a comprehensive discussion of the relationship between IFN-γ and osteoporosis. Finding the paradoxical balance of IFN-γ in bone immunology and exploring the potential of its clinical application provide new ideas for the clinical treatment of osteoporosis and drug development.

Embryonic inhibition of colony-stimulating factor 1 receptor induces enlarged cartilaginous zone of the midpalatal suture in postnatal mice

OBJECTIVES

The midpalatal suture acts as the growth centre of the maxilla. Colony-stimulating factor 1 receptor (CSF1R) is essential for osteoclastogenesis. Deletion of CSF1R, and its ligand, results in significant craniofacial phenotypes but has not been studied in detail in the midpalatal suture.

MATERIALS AND METHODS

Pregnant ICR mice were treated with the CSF1R inhibitor PLX5622 at embryo Day 14.5 (E14.5) to E17.5. Pups at E18.5, postnatal Day 3 (P3) and P7 were collected for skeletal and histological staining. Osteoclasts were labelled using TRAP staining. PHH3 and TUNEL were employed to detect cell proliferation and apoptosis. Sox9, Ihh, and Col10a1 and Runx2, Col1a1, and DMP1 were used to detect chondrogenic differentiation and osteogenic differentiation, respectively. CD31, MMP9 and CTSK were utilized to assess vascular invasion and osteoclast secretion enzymes, respectively.

RESULTS

Embryonic inhibition of CSF1R resulted in a depletion of TRAP-positive cells and an enlarged cartilage zone of the midpalatal suture of postnatal mice. Compared to those in the control group, Sox9, Ihh, Col10a1, Runx2 and Col1a1 were upregulated, whereas TUNEL and DMP1 were decreased in this zone. In the trabecular region, Col10a1 was upregulated, while TUNEL, Col1a1 and DMP1 were downregulated. Moreover, the expression of MMP9, CTSK and CD31 was decreased, and invasion into the cartilage zone was delayed.

CONCLUSIONS

Embryonic inhibition of CSF1R led to an abnormally enlarged cartilaginous zone in the midpalatal suture, potentially due to delayed endochondral ossification caused by the depletion of osteoclasts. Additionally, we established a novel model of midpalatal suture dysplasia, offering prospects for future research.

Temporal dynamics of immune-stromal cell interactions in fracture healing

Bone fracture repair is a complex, multi-step process that involves communication between immune and stromal cells to coordinate the repair and regeneration of damaged tissue. In the US, 10% of all bone fractures do not heal properly without intervention, resulting in non-union. Complications from non-union fractures are physically and financially debilitating. We now appreciate the important role that immune cells play in tissue repair, and the necessity of the inflammatory response in initiating healing after skeletal trauma. The temporal dynamics of immune and stromal cell populations have been well characterized across the stages of fracture healing. Recent studies have begun to untangle the intricate mechanisms driving the immune response during normal or atypical, delayed healing. Various in vivo models of fracture healing, including genetic knockouts, as well as in vitro models of the fracture callus, have been implemented to enable experimental manipulation of the heterogeneous cellular environment. The goals of this review are to (1): summarize our current understanding of immune cell involvement in fracture healing (2); describe state-of-the art approaches to study inflammatory cells in fracture healing, including computational and in vitro models; and (3) identify gaps in our knowledge concerning immune-stromal crosstalk during bone healing.

Osteopetrosis: Gene-based nosology and significance Dysosteosclerosis

Dysosteosclerosis (DSS) refers to skeletal dysplasias that radiographically feature focal appendicular osteosclerosis with variable platyspondyly. Genetic heterogeneity is increasingly reported for the DSS phenotype and now involves mutations of SLC29A3, TNFRSF11A, TCIRG1, LRRK1, and CSF1R. Typical radiological findings are widened radiolucent long bones with thin cortices yet dense irregular metaphyses, flattened vertebral bodies, dense ribs, and multiple fractures. However, the radiographic features of DSS evolve, and the metaphyseal and/or appendicular osteosclerosis variably fades with increasing patient age, likely due to some residual osteoclast function. Fractures are the principal presentation of DSS, and may even occur in infancy with SLC29A3-associated DSS. Cranial base sclerosis can lead to cranial nerve palsies such as optic atrophy, and may be the initial presentation, though not observed with SLC29A3-associated DSS. Gene-specific extra-skeletal features can be the main complication in some forms of DSS such as CSF1R- associated DSS. Further genetic heterogeneity is likely, especially for X-linked recessive DSS and cases currently with an unknown genetic defect. Distinguishing DSS can be challenging due to variable clinical and radiological features and an evolving phenotype. However, defining the DSS phenotype is important for predicting complications, prognosis, and instituting appropriate health surveillance and treatment.

CSF1R as a Therapeutic Target in Bone Diseases: Obvious but Not so Simple

PURPOSE OF REVIEW

The purpose of the review is to summarize the expression and function of CSF1R and its ligands in bone homeostasis and constraints on therapeutic targeting of this axis.

RECENT FINDINGS

Bone development and homeostasis depends upon interactions between mesenchymal cells and cells of the mononuclear phagocyte lineage (MPS), macrophages, and osteoclasts (OCL). The homeostatic interaction is mediated in part by the systemic and local production of growth factors, macrophage colony-stimulating factor (CSF1), and interleukin 34 (IL34) that interact with a receptor (CSF1R) expressed exclusively by MPS cells and their progenitors. Loss-of-function mutations in CSF1 or CSF1R lead to loss of OCL and macrophages and dysregulation of postnatal bone development. MPS cells continuously degrade CSF1R ligands via receptor-mediated endocytosis. As a consequence, any local or systemic increase or decrease in macrophage or OCL abundance is rapidly reversible. In principle, both CSF1R agonists and antagonists have potential in bone regenerative medicine but their evaluation in disease models and therapeutic application needs to carefully consider the intrinsic feedback control of MPS biology.

Endocannabinoids modulate production of osteoclastogenic factors by stem cells of the apical papilla in vitro

INTRODUCTION

Many mediators are produced during pulp inflammation and necrosis, including endocannabinoids (ECbs), which might affect the function of stem cells of the apical papilla (SCAP), cells of paramount importance for root formation and regenerative endodontic treatment (RET). The aim of this study was to evaluate the production of osteoclastogenesis-related mediators by SCAP, modulated by ECbs and lipopolysaccharide (LPS) in vitro.

METHODS

SCAP were cultured and treated with ECbs anandamide (AEA), 2-A arachidonoylglycerol or N-arachidonoylaminophenol (AM404). All groups were incubated in the presence of vehicle or LPS and the antagonist of transient receptor potential cation channel subfamily V member 1 (TRPV-1), capsazepine (CPZ). After 24 h, the culture medium supernatants were collected for further quantification of tumor necrosis factor (TNF)-α, CCL2, macrophage colony-stimulating factor (M-CSF), osteoprotegerin (OPG), and receptor activator of nuclear factor kappa-Β ligand (RANKL).

RESULTS

Small amounts of TNF-α and RANKL were detected in SCAP supernatants, and none of the experimental conditions altered their production. A downregulation in constitutive CCL2 production was observed in the AEA group compared to that in the LPS group. The production of M-CSF was significantly increased in all groups treated with AEA compared to the control and LPS-treated groups. OPG was significantly increased by AEA alone and by 2AG and AM404 in presence of LPS and CPZ.

CONCLUSIONS

AEA modulate some of the osteoclastogenic factors produced by SCAP in a bone resorption-protective fashion.

Modeling CSF-1 receptor deficiency diseases - how close are we?

The role of colony-stimulating factor-1 receptor (CSF-1R) in macrophage and organismal development has been extensively studied in mouse. Within the last decade, mutations in the CSF1R have been shown to cause rare diseases of both pediatric (Brain Abnormalities, Neurodegeneration, and Dysosteosclerosis, OMIM #618476) and adult (CSF1R-related leukoencephalopathy, OMIM #221820) onset. Here we review the genetics, penetrance, and histopathological features of these diseases and discuss to what extent the animal models of Csf1r deficiency currently available provide systems in which to study the underlying mechanisms involved.

Upregulation of Runt related transcription factor 1 (RUNX1) contributes to tendon-bone healing after anterior cruciate ligament reconstruction using bone mesenchymal stem cells

Background

Anterior cruciate ligament (ACL) injury could lead to functional impairment along with disabilities. ACL reconstruction often fails owing to the regeneration failure of tendon–bone interface. Herein, we aimed to investigate the effects of Runt related transcription factor 1 (RUNX1) on tendon–bone healing after ACL reconstruction using bone mesenchymal stem cells (BMSCs).

Methods

BMSCs were isolated from the marrow cavity of rat femur, followed by the modification of RUNX1 with lentiviral system. Then, an ACL reconstruction model of rats was established with autografts.

Results

Results of flow cytometry exhibited positive-antigen CD44 and CD90, as well as negative-antigen CD34 and CD45 of the BMSCs. Then, we found that RUNX1-upregulated BMSCs elevated the decreased biomechanical strength of the tendon grafts after ACL reconstruction. Moreover, based on the histological observation, upregulation of RUNX1 was linked with better recovery around the bone tunnel, a tighter tendon–bone interface, and more collagen fibers compared to the group of BMSCs infected with LV-NC. Next, RUNX1-upregulated BMSCs promoted osteogenesis after ACL reconstruction, as evidenced by the mitigation of severe loss and erosion of the cartilage and bone in the tibial and femur area, as well as the increased number of osteoblasts identified by the upregulation of alkaline phosphatase, osteocalcin, and osteopontin in the tendon–bone interface.

Conclusion

Elevated expression of RUNX1 contributed to tendon–bone healing after ACL reconstruction using BMSCs.

Sequential activation of heterogeneous macrophage phenotypes is essential for biomaterials-induced bone regeneration

Macrophage has been gradually recognized as a central regulator in tissue regeneration, and the study of how macrophage mediates biomaterials-induced bone regeneration through immunomodulatory pathway becomes popular. However, the current understanding on the roles of different macrophage phenotypes in regulating bone tissue regeneration remains controversial. In this study, we demonstrate that sequential infiltration of heterogeneous phenotypes of macrophages triggered by bio-metal ions effectively facilitates bone healing in bone defect. Indeed, M1 macrophages promote the recruitment and early commitment of osteogenic and angiogenic progenitors, while M2 macrophages and osteoclasts support the deposition and mineralization of the bone matrix, as well as the maturation of blood vessels. Moreover, we have identified a group of bone biomaterial-related multinucleated cells that behave similarly to M2 macrophages with wound-healing features rather than participate in the bone resorption cascade similarly to osteoclasts. Our study shows how sequential activation of macrophage-osteoclast lineage contribute to a highly orchestrated immune response in the bone tissue microenvironment around biomaterials to regulate the complex biological process of bone healing. Therefore, we believe that the temporal activation pattern of heterogeneous macrophage phenotypes should be considered when the next generation of biomaterials for bone regeneration is engineered.

CSF1R-dependent macrophages control postnatal somatic growth and organ maturation

Homozygous mutation of the Csf1r locus (Csf1rko) in mice, rats and humans leads to multiple postnatal developmental abnormalities. To enable analysis of the mechanisms underlying the phenotypic impacts of Csf1r mutation, we bred a rat Csf1rko allele to the inbred dark agouti (DA) genetic background and to a Csf1r-mApple reporter transgene. The Csf1rko led to almost complete loss of embryonic macrophages and ablation of most adult tissue macrophage populations. We extended previous analysis of the Csf1rko phenotype to early postnatal development to reveal impacts on musculoskeletal development and proliferation and morphogenesis in multiple organs. Expression profiling of 3-week old wild-type (WT) and Csf1rko livers identified 2760 differentially expressed genes associated with the loss of macrophages, severe hypoplasia, delayed hepatocyte maturation, disrupted lipid metabolism and the IGF1/IGF binding protein system. Older Csf1rko rats developed severe hepatic steatosis. Consistent with the developmental delay in the liver Csf1rko rats had greatly-reduced circulating IGF1. Transfer of WT bone marrow (BM) cells at weaning without conditioning repopulated resident macrophages in all organs, including microglia in the brain, and reversed the mutant phenotypes enabling long term survival and fertility. WT BM transfer restored osteoclasts, eliminated osteopetrosis, restored bone marrow cellularity and architecture and reversed granulocytosis and B cell deficiency. Csf1rko rats had an elevated circulating CSF1 concentration which was rapidly reduced to WT levels following BM transfer. However, CD43hi non-classical monocytes, absent in the Csf1rko, were not rescued and bone marrow progenitors remained unresponsive to CSF1. The results demonstrate that the Csf1rko phenotype is autonomous to BM-derived cells and indicate that BM contains a progenitor of tissue macrophages distinct from hematopoietic stem cells. The model provides a unique system in which to define the pathways of development of resident tissue macrophages and their local and systemic roles in growth and organ maturation.

Role of OSCAR Signaling in Osteoclastogenesis and Bone Disease

Formation of mature bone-resorbing cells through osteoclastogenesis is required for the continuous remodeling and repair of bone tissue. In aging and disease this process may become aberrant, resulting in excessive bone degradation and fragility fractures. Interaction of receptor-activator of nuclear factor-κB (RANK) with its ligand RANKL activates the main signaling pathway for osteoclastogenesis. However, compelling evidence indicates that this pathway may not be sufficient for the production of mature osteoclast cells and that co-stimulatory signals may be required for both the expression of osteoclast-specific genes and the activation of osteoclasts. Osteoclast-associated receptor (OSCAR), a regulator of osteoclast differentiation, provides one such co-stimulatory pathway. This review summarizes our present knowledge of osteoclastogenesis signaling and the role of OSCAR in the normal production of bone-resorbing cells and in bone disease. Understanding the signaling mechanism through this receptor and how it contributes to the production of mature osteoclasts may offer a more specific and targeted approach for pharmacological intervention against pathological bone resorption.

Further expanding the mutational spectrum of brain abnormalities, neurodegeneration, and dysosteosclerosis: A rare disorder with neurologic regression and skeletal features

Colony stimulating factor 1 receptor (CSF1R, MIM# 164770) encodes a tyrosine-kinase receptor playing an important role in development of osteoclasts and microglia. Heterozygous CSF1R variants have been known to cause hereditary diffuse leukoencephalopathy with spheroids (HDLS, MIM# 221820), an adult-onset leukoencephalopathy characterized by loss of motor functions and cognitive decline. Recently, a new phenotype characterized by brain abnormalities, neurodegeneration, and dysosteosclerosis (BANDDOS) with biallelic CSF1R pathogenic variants in the etiology has been described. BANDDOS differs from HDLS by early-onset neurodegenerative changes with additional structural brain abnormalities and skeletal findings resembling dysosteosclerosis (DOS). Described skeletal findings of the disease are highly variable ranging from absence of a skeletal phenotype and milder Pyle disease-like to osteopetrosis and DOS. To date, only a few patients carrying biallelic CSF1R variants have been reported. In this clinical report, we describe three siblings with variable skeletal findings along with neurological symptoms ranging from mild to severe in whom exome sequencing revealed a novel homozygous splice site variant in canonical splice donor site of intron 21 adjacent to an exon, which encoding part of kinase domain of CSF1R along with a review of the literature.

Variants Affecting the C-Terminal of CSF1R Cause Congenital Vertebral Malformation Through a Gain-of-Function Mechanism

CSF1R encodes the colony-stimulating factor 1 receptor which regulates the proliferation, differentiation, and biological activity of monocyte/macrophage lineages. Pathogenic variants in CSF1R could lead to autosomal dominant adult-onset leukoencephalopathy with axonal spheroids and pigmented glia or autosomal recessive skeletal dysplasia. In this study, we identified three heterozygous deleterious rare variants in CSF1R from a congenital vertebral malformation (CVM) cohort. All of the three variants are located within the carboxy-terminal region of CSF1R protein and could lead to an increased stability of the protein. Therefore, we established a zebrafish model overexpressing CSF1R. The zebrafish model exhibits CVM phenotypes such as hemivertebral and vertebral fusion. Furthermore, overexpression of the mutated CSF1R mRNA depleted of the carboxy-terminus led to a higher proportion of zebrafish with vertebral malformations than wild-type CSF1R mRNA did (p = 0.03452), implicating a gain-of-function effect of the C-terminal variant. In conclusion, variants affecting the C-terminal of CSF1R could cause CVM though a potential gain-of-function mechanism.

Effects of an Environmentally Relevant Mixture of Organophosphate Esters Derived From House Dust on Endochondral Ossification in Murine Limb Bud Cultures

Organophosphate esters (OPEs) are used widely as flame retardants and plasticizers but much remains unknown about their potential toxicity. Previously, we reported that 4 individual OPEs suppress endochondral ossification in murine limb bud cultures. However, real-life exposure is to complex OPE mixtures. In the present study, we tested the hypothesis that a Canadian household dust-based OPE mixture will affect endochondral ossification in gestation day 13 CD1 mouse embryo limb buds expressing fluorescent markers for the major cell populations involved in the process: collagen type II alpha 1-enhanced cyan fluorescent protein (proliferative chondrocytes), collagen type X alpha 1-mCherry (hypertrophic chondrocytes), and collagen type I alpha 1-yellow fluorescent protein (osteoblasts). Limbs were cultured for 6 days in the presence of vehicle or dilutions of the OPE mixture (1/1 000 000, 1/600 000, and 1/300 000). All 3 OPE mixture dilutions affected cartilage template development and the progression of endochondral ossification, as indicated by the fluorescent markers. The expression of Sox9, the master regulator of chondrogenesis, was unchanged, but the expression of Runx2 and Sp7, which drive chondrocyte hypertrophy and osteoblastogenesis, was dilution-dependently suppressed. RNA-seq revealed that exposure to the 1/300 000 dilution of the OPE mixture for 24 h downregulated 153 transcripts and upregulated 48 others by at least 1.5-fold. Downregulated transcripts were enriched for those related to the immune system and bone formation. In contrast, upregulated transcripts were enriched for those with stress response functions known to be regulated by ATF4 activation. Thus, exposure to the mixture of OPEs commonly found in house dust may have adverse effects on bone formation.

An osteoconductive PLGA scaffold with bioactive β-TCP and anti-inflammatory Mg(OH)2 to improve in vivo bone regeneration

Poly(lactic-co-glycolic acid) (PLGA) has been widely used as a biomaterial for pharmaceutical and medical applications. However, the decomposition products of PLGA are known to acidify the surrounding tissue of the implanted site, causing an inflammatory response. Previously, we developed PLGA/inorganic nanocomposites and optimized the amounts of inorganic compounds, β-tricalcium phosphate (β-TCP) and magnesium hydroxide [Mg(OH)2], in terms of osteogenesis of normal human osteoblasts and anti-inflammatory responses of preosteoclastic cells in vitro. In this study, the potential of the optimized PLGA/β-TCP/Mg(OH)2 nanocomposite (TCP/MH) to promote bone repair through osteoinductive, osteoconductive, and anti-inflammatory abilities was assessed using a bone defect in a rat humeral defect model. PLGA nanocomposites with or without inorganic compounds, PLGA, β-TCP, MH, and TCP/MH were prepared through one-step bulk modification using a twin-screw extruder. The resulting TCP/MH nanocomposite successfully enhanced the bone regeneration rate for allowing complete bone defect healing with significantly suppressed inflammatory responses. Taken together, the organic and inorganic bioactive nanocomposite developed in this study, TCP/MH, is a promising material in orthopedic implantation.

The M-CSF receptor in osteoclasts and beyond

Colony-stimulating factor 1 receptor (CSF1R, also known as c-FMS) is a receptor tyrosine kinase. Macrophage colony-stimulating factor (M-CSF) and IL-34 are ligands of CSF1R. CSF1R-mediated signaling is crucial for the survival, function, proliferation, and differentiation of myeloid lineage cells, including osteoclasts, monocytes/macrophages, microglia, Langerhans cells in the skin, and Paneth cells in the intestine. CSF1R also plays an important role in oocytes and trophoblastic cells in the female reproductive tract and in the maintenance and maturation of neural progenitor cells. Given that CSF1R is expressed in a wide range of myeloid cells, altered CSF1R signaling is implicated in inflammatory, neoplastic, and neurodegenerative diseases. Inhibiting CSF1R signaling through an inhibitory anti-CSF1R antibody or small molecule inhibitors that target the kinase activity of CSF1R has thus been a promising therapeutic strategy for those diseases. In this review, we cover the recent progress in our understanding of the various roles of CSF1R in osteoclasts and other myeloid cells, highlighting the therapeutic applications of CSF1R inhibitors in disease conditions.

Drugs directed at a key signaling receptor involved in breaking down bone tissue could help treat diseases marked by pathological bone loss and destruction. In a review article, Kyung-Hyun Park-Min and colleagues from the Hospital for Special Surgery in New York, USA, discuss the essential roles played by the colony-stimulating factor 1 receptor (CSF1R) protein in the survival, function, proliferation and differentiation of myeloid lineage stem cells in the bone marrow, including bone-resorbing osteoclasts. They explore the links between the CSF1R-mediated signaling pathway and diseases such as cancer and neurodegeneration. The authors largely focus on bone conditions, highlighting mouse studies in which CSF1R-blocking drugs were shown to ameliorate bone loss and inflammatory symptoms in models of arthritis, osteoporosis and metastatic cancer. Clinical trials are ongoing to test therapeutic applications.

Development of a new macrophage-specific TRAP mouse (MacTRAP) and definition of the renal macrophage translational signature

Tissue macrophages play an important role in organ homeostasis, immunity and the pathogenesis of various inflammation-driven diseases. One major challenge has been to selectively study resident macrophages in highly heterogeneous organs such as kidney. To address this problem, we adopted a Translational Ribosome Affinity Purification (TRAP)- approach and designed a transgene that expresses an eGFP-tagged ribosomal protein (L10a) under the control of the macrophage-specific c-fms promoter to generate c-fms-eGFP-L10a transgenic mice (Mac^TRAP). Rigorous characterization found no gross abnormalities in Mac^TRAP mice and confirmed transgene expression across various organs. Immunohistological analyses of Mac^TRAP kidneys identified eGFP-L10a expressing cells in the tubulointerstitial compartment which stained positive for macrophage marker F4/80. Inflammatory challenge led to robust eGFP-L10a upregulation in kidney, confirming Mac^TRAP responsiveness in vivo. We successfully extracted macrophage-specific polysomal RNA from Mac^TRAP kidneys and conducted RNA sequencing followed by bioinformatical analyses, hereby establishing a comprehensive and unique in vivo gene expression and pathway signature of resident renal macrophages. In summary, we created, validated and applied a new, responsive macrophage-specific TRAP mouse line, defining the translational profile of renal macrophages and dendritic cells. This new tool may be of great value for the study of macrophage biology in different organs and various models of injury and disease.

IL-17 promotes osteoblast differentiation, bone regeneration, and remodeling in mice

Bone homeostasis is maintained by concerted actions of bone-forming osteoblasts and bone-resorbing osteoclasts. A wide range of evidence indicates that a proinflammatory cytokine IL-17 promotes osteoclastogenesis. However, the role of IL-17 in osteoblasts is less well-understood. In the current study, the effect of IL-17 on osteogenic differentiation was investigated in mouse calvarial cells. IL-17 stimulated osteoblast differentiation, mineralization, proliferation, motility, and osteoblast-dependent osteoclastogenesis in vitro. The pro-osteogenic role of IL-17 was dependent on Act1 and the generation of reactive oxygen species. In a critical size calvarial defect model, IL-17 significantly augmented bone regeneration. Importantly, IL-17 also remarkably increased bone remodeling and restored osteoclastogenesis in zoledronate-treated mice. Furthermore, IL-17 conspicuously stimulated the formation of lamellar bones. These data not only provide a clue to understand the role of IL-17 in bone metabolism but also suggest possible applications in bone augmentation therapies.

Osteoclasts may contribute bone substitute materials remodeling and bone formation in bone augmentation

Bone augmentation is increasingly important in implantology. Bone substitute materials exert essential roles during bone augmentation process. However, accelerating bone substitute materials remodeling and acquiring high bone architecture quality was still the challenges of bone augmentation. Accumulated studies had suggested osteoclasts is the key cell type to resorb bone or bone substitute materials. Our previous study and other studies suggested osteoclasts contributed to bone formation by promoting osteoblast function and facilitate angiogenesis. We hypothesized that bone substitute materials loaded osteoclastogenic cytokines or osteoclast progenitors will help to bone substitute materials rapid remodeling and subsequent bone formation. Our hypothesis could help to lessen long-term post-bone augmentation period and acquire better bone quality for osseointegration.

Phenotypic impacts of CSF1R deficiencies in humans and model organisms

Mϕ proliferation, differentiation, and survival are controlled by signals from the Mϕ CSF receptor (CSF1R). Mono-allelic gain-of-function mutations in CSF1R in humans are associated with an autosomal-dominant leukodystrophy and bi-allelic loss-of-function mutations with recessive skeletal dysplasia, brain disorders, and developmental anomalies. Most of the phenotypes observed in these human disease states are also observed in mice and rats with loss-of-function mutations in Csf1r or in Csf1 encoding one of its two ligands. Studies in rodent models also highlight the importance of genetic background and likely epistatic interactions between Csf1r and other loci. The impacts of Csf1r mutations on the brain are usually attributed solely to direct impacts on microglial number and function. However, analysis of hypomorphic Csf1r mutants in mice and several other lines of evidence suggest that primary hydrocephalus and loss of the physiological functions of Mϕs in the periphery contribute to the development of brain pathology. In this review, we outline the evidence that CSF1R is expressed exclusively in mononuclear phagocytes and explore the mechanisms linking CSF1R mutations to pleiotropic impacts on postnatal growth and development.

Bi-allelic CSF1R Mutations Cause Skeletal Dysplasia of Dysosteosclerosis-Pyle Disease Spectrum and Degenerative Encephalopathy with Brain Malformation

Colony stimulating factor 1 receptor (CSF1R) plays key roles in regulating development and function of the monocyte/macrophage lineage, including microglia and osteoclasts. Mono-allelic mutations of CSF1R are known to cause hereditary diffuse leukoencephalopathy with spheroids (HDLS), an adult-onset progressive neurodegenerative disorder. Here, we report seven affected individuals from three unrelated families who had bi-allelic CSF1R mutations. In addition to early-onset HDLS-like neurological disorders, they had brain malformations and skeletal dysplasia compatible to dysosteosclerosis (DOS) or Pyle disease. We identified five CSF1R mutations that were homozygous or compound heterozygous in these affected individuals. Two of them were deep intronic mutations resulting in abnormal inclusion of intron sequences in the mRNA. Compared with Csf1r-null mice, the skeletal and neural phenotypes of the affected individuals appeared milder and variable, suggesting that at least one of the mutations in each affected individual is hypomorphic. Our results characterized a unique human skeletal phenotype caused by CSF1R deficiency and implied that bi-allelic CSF1R mutations cause a spectrum of neurological and skeletal disorders, probably depending on the residual CSF1R function.

Anti-osteoclastic effects of C-glucosidic ellagitannins mediated by actin perturbation

Actin subunits assemble into actin filaments whose dynamics and three-dimensional architectures are further regulated by a variety of cellular factors to establish the functional actin cytoskeleton. The C-glucosidic ellagitannin vescalagin and its simpler analogue vescalin, affect both the dynamics and the ultrastructure of the actin cytoskeleton by directly binding to F-actin. Herein, we show that in vitro, the two compounds induce the formation of distinct F-actin networks characterized by different superstructures and dynamics. In living mature osteoclasts, highly specialized bone-degrading cells that constantly remodel their cytoskeleton, vescalagin and vescalin alter actin dynamics at podosomes and compromise the integrity of the podosome belt that forms the bone-degrading apparatus. Both compounds target the bone-resorbing activity at concentrations that preserve osteoclastic maturation and survival and with no detectable impact on the behaviour of bone-forming osteoblastic cells. This anti-osteoclastic activity of vescalagin and vescalin reveals the potential of targeting actin dynamics as a new therapeutic opportunity and, in this case, as a plausible approach for the local treatment of osteoporosis.

Developing a Customized Perfusion Bioreactor Prototype with Controlled Positional Variability in Oxygen Partial Pressure for Bone and Cartilage Tissue Engineering

Skeletal development is a multistep process that involves the complex interplay of multiple cell types at different stages of development. Besides biochemical and physical cues, oxygen tension also plays a pivotal role in influencing cell fate during skeletal development. At physiological conditions, bone cells generally reside in a relatively oxygenated environment whereas chondrocytes reside in a hypoxic environment. However, it is technically challenging to achieve such defined, yet diverse oxygen distribution on traditional in vitro cultivation platforms. Instead, engineered osteochondral constructs are commonly cultivated in a homogeneous, stable environment. In this study, we describe a customized perfusion bioreactor having stable positional variability in oxygen tension at defined regions. Further, engineered collagen constructs were coaxed into adopting the shape and dimensions of defined cultivation platforms that were precasted in 1.5% agarose bedding. After cultivating murine embryonic stem cells that were embedded in collagen constructs for 50 days, mineralized constructs of specific dimensions and a stable structural integrity were achieved. The end-products, specifically constructs cultivated without chondroitin sulfate A (CSA), showed a significant increase in mechanical stiffness compared with their initial gel-like constructs. More importantly, the localization of osteochondral cell types was specific and corresponded to the oxygen tension gradient generated in the bioreactor. In addition, CSA in complementary with low oxygen tension was also found to be a potent inducer of chondrogenesis in this system. In summary, we have demonstrated a customized perfusion bioreactor prototype that is capable of generating a more dynamic, yet specific cultivation environment that could support propagation of multiple osteochondral lineages within a single engineered construct in vitro. Our system opens up new possibilities for in vitro research on human skeletal development.

Effects of single or combination therapy of teriparatide and anti-RANKL monoclonal antibody on bone defect regeneration in mice

OBJECTIVE

The purpose of this study is to investigate the effects of a single or combination therapy of teriparatide (TPTD) and anti-RANKL Ab (anti-murine receptor activator of nuclear factor κB ligand monoclonal antibody) on the regeneration of both cancellous and cortical bone.

METHODS

Nine-week-old mice underwent bone defect surgery on the left femoral metaphysis (cancellous-bone healing model) and right femoral mid-diaphysis (cortical-bone healing model). After surgery, the mice were assigned to 1 of 4 groups to receive 1) saline (5 times a week; CNT group), 2) TPTD (40μg/kg 5 times a week; TPTD group), 3) anti-RANKL Ab (5mg/kg once; Ab group), or 4) a combination of TPTD and anti-RANKL Ab (COMB group). The following analyses were performed: Time-course microstructural analysis of healing in both cancellous and cortical bone in the bone defect, the volumetric bone mineral density of the tibia with micro-computed tomography, histological, histomorphometrical, and biomechanical analysis of regenerated bone.

RESULTS

Regeneration of cancellous bone volume in the COMB group was the highest among the 4 groups, and this combined administration prompted medullary callus formation in the early phase of bone regeneration. On the other hand, regeneration of cortical bone volume in the COMB group was significantly higher than in the Ab group and was almost same as in the TPTD group. Histological analysis showed remaining woven bones, cartilage matrix, and immature lamellar bone in the COMB and Ab groups. However, biomechanical analysis showed that hardness and Young's modulus of regenerated cortical bone in the COMB group was not lower than in both the CNT and TPTD groups. Volumetric bone mineral density in the tibia was significantly increased in the COMB group compared with the other 3 groups.

CONCLUSION

In the early phase of bone regeneration, the combination of TPTD and anti-RANKL Ab accelerates regeneration of cancellous bone in bone defects and increases cancellous bone mass in the tibia more effectively than either agent does individually, but these additive effects are not observed in the regeneration of cortical bone.

Genetic Models of Macrophage Depletion

Macrophages are a heterogeneous population of innate immune cells and are distributed in most adult tissues. Certain tissue-resident macrophages with a prenatal origin, together with postnatal monocyte-derived macrophages, serve as the host scavenger system to eliminate invading pathogens, malignant cells, senescent cells, dead cells, cellular debris, and other foreign substances. As a key member of the mononuclear phagocyte system, macrophages play essential roles in regulation of prenatal development, tissue homeostasis, and disease progression. Over the past two decades, considerable efforts have been made to generate genetic models of macrophage ablation in mice. These models support investigations of the precise functions of tissue-specific macrophages under physiological and pathological conditions. Herein, we overview the currently available mouse strains for in vivo genetic ablation of macrophages and discuss their respective advantages and limitations.

mTORC1 Inhibits NF-κB/NFATc1 Signaling and Prevents Osteoclast Precursor Differentiation, In Vitro and In Mice

The mechanistic target of rapamycin complex 1 (mTORC1) is a critical sensor for bone homeostasis and bone formation; however, the role of mTORC1 in osteoclast development and the underlying mechanisms have not yet been fully established. Here, we found that mTORC1 activity declined during osteoclast precursors differentiation in vitro and in vivo. We further targeted deletion of Raptor (mTORC1 key component) or Tsc1 (mTORC1 negative regulator) to constitutively inhibit or activate mTORC1 in osteoclast precursors (monocytes/macrophages), using LyzM-cre mice. Osteoclastic formation was drastically increased in cultures of Raptor deficient bone marrow monocytes/macrophages (BMMs), and Raptor-deficient mice displayed osteopenia with enhanced osteoclastogenesis. Conversely, BMMs lacking Tsc1 exhibited a severe defect in osteoclast-like differentiation and absorptive function, both of which were restored following rapamycin treatment. Importantly, expression of nuclear factor κ-light-chain-enhancer of activated B cells (NF-κB) and nuclear factor of activated T cells, cytoplasmic 1 (NFATc1), transcription factors that are essential for osteoclast differentiation was negatively regulated by mTORC1 in osteoclast lineages. These results provide evidence that mTORC1 plays as a critical role as an osteoclastic differentiation-limiting signal and suggest a potential drawback in treating bone loss-related diseases with mTOR inhibitors clinically. © 2017 American Society for Bone and Mineral Research.

Cathepsin K Inhibitors for Osteoporosis: Biology, Potential Clinical Utility, and Lessons Learned

Cathepsin K is a cysteine protease member of the cathepsin lysosomal protease family. Although cathepsin K is highly expressed in osteoclasts, lower levels of cathepsin K are also found in a variety of other tissues. Secretion of cathepsin K from the osteoclast into the sealed osteoclast-bone cell interface results in efficient degradation of type I collagen. The absence of cathepsin K activity in humans results in pycnodysostosis, characterized by increased bone mineral density and fractures. Pharmacologic cathepsin K inhibition leads to continuous increases in bone mineral density for ≤5 years of treatment and improves bone strength at the spine and hip. Compared with other antiresorptive agents, cathepsin K inhibition is nearly equally efficacious for reducing biochemical markers of bone resorption but comparatively less active for reducing bone formation markers. Despite multiple efforts to develop cathepsin K inhibitors, potential concerns related to off-target effects of the inhibitors against other cathepsins and cathepsin K inhibition at nonbone sites, including skin and perhaps cardiovascular and cerebrovascular sites, prolonged the regulatory approval process. A large multinational randomized, double-blind phase III study of odanacatib in postmenopausal women with osteoporosis was recently completed. Although that study demonstrated clinically relevant reductions in fractures at multiple sites, odanacatib was ultimately withdrawn from the regulatory approval process after it was found to be associated with an increased risk of cerebrovascular accidents. Nonetheless, the underlying biology and clinical effects of cathepsin K inhibition remain of considerable interest and could guide future therapeutic approaches for osteoporosis.

Progranulin regulates neurogenesis in the developing vertebrate retina

We evaluated the expression and function of the microglia‐specific growth factor, Progranulin‐a (Pgrn‐a) during developmental neurogenesis in the embryonic retina of zebrafish. At 24 hpf pgrn‐a is expressed throughout the forebrain, but by 48 hpf pgrn‐a is exclusively expressed by microglia and/or microglial precursors within the brain and retina. Knockdown of Pgrn‐a does not alter the onset of neurogenic programs or increase cell death, however, in its absence, neurogenesis is significantly delayed—retinal progenitors fail to exit the cell cycle at the appropriate developmental time and postmitotic cells do not acquire markers of terminal differentiation, and microglial precursors do not colonize the retina. Given the link between Progranulin and cell cycle regulation in peripheral tissues and transformed cells, we analyzed cell cycle kinetics among retinal progenitors following Pgrn‐a knockdown. Depleting Pgrn‐a results in a significant lengthening of the cell cycle. These data suggest that Pgrn‐a plays a dual role during nervous system development by governing the rate at which progenitors progress through the cell cycle and attracting microglial progenitors into the embryonic brain and retina. Collectively, these data show that Pgrn‐a governs neurogenesis by regulating cell cycle kinetics and the transition from proliferation to cell cycle exit and differentiation. © 2017 The Authors. Developmental Neurobiology Published by Wiley Periodicals, Inc. Develop Neurobiol 77: 1114–1129, 2017

Regulation of Embryonic and Postnatal Development by the CSF-1 Receptor

Macrophages are found in all tissues and regulate tissue morphogenesis during development through trophic and scavenger functions. The colony stimulating factor-1 (CSF-1) receptor (CSF-1R) is the major regulator of tissue macrophage development and maintenance. In combination with receptor activator of nuclear factor κB (RANK), the CSF-1R also regulates the differentiation of the bone-resorbing osteoclast and controls bone remodeling during embryonic and early postnatal development. CSF-1R-regulated macrophages play trophic and remodeling roles in development. Outside the mononuclear phagocytic system, the CSF-1R directly regulates neuronal survival and differentiation, the development of intestinal Paneth cells and of preimplantation embryos, as well as trophoblast innate immune function. Consistent with the pleiotropic roles of the receptor during development, CSF-1R deficiency in most mouse strains causes embryonic or perinatal death and the surviving mice exhibit multiple developmental and functional deficits. The CSF-1R is activated by two dimeric glycoprotein ligands, CSF-1, and interleukin-34 (IL-34). Homozygous Csf1-null mutations phenocopy most of the deficits of Csf1r-null mice. In contrast, Il34-null mice have no gross phenotype, except for decreased numbers of Langerhans cells and microglia, indicating that CSF-1 plays the major developmental role. Homozygous inactivating mutations of the Csf1r or its ligands have not been reported in man. However, heterozygous inactivating mutations in the Csf1r lead to a dominantly inherited adult-onset progressive dementia, highlighting the importance of CSF-1R signaling in the brain.

Osteoclast TGF-β Receptor Signaling Induces Wnt1 Secretion and Couples Bone Resorption to Bone Formation

Osteoblast-mediated bone formation is coupled to osteoclast-mediated bone resorption. These processes become uncoupled with age, leading to increased risk for debilitating fractures. Therefore, understanding how osteoblasts are recruited to sites of resorption is vital to treating age-related bone loss. Osteoclasts release and activate TGF-β from the bone matrix. Here we show that osteoclast-specific inhibition of TGF-β receptor signaling in mice results in osteopenia due to reduced osteoblast numbers with no significant impact on osteoclast numbers or activity. TGF-β induced osteoclast expression of Wnt1, a protein crucial to normal bone formation, and this response was blocked by impaired TGF-β receptor signaling. Osteoclasts in aged murine bones had lower TGF-β signaling and Wnt1 expression in vivo. Ex vivo stimulation of osteoclasts derived from young or old mouse bone marrow macrophages showed no difference in TGF-β-induced Wnt1 expression. However, young osteoclasts expressed reduced Wnt1 when cultured on aged mouse bone chips compared to young mouse bone chips, consistent with decreased skeletal TGF-β availability with age. Therefore, osteoclast responses to TGF-β are essential for coupling bone resorption to bone formation, and modulating this pathway may provide opportunities to treat age-related bone loss.

Targeting a novel bone degradation pathway in primary bone cancer by inactivation of the collagen receptor uPARAP/Endo180

In osteosarcoma, a primary mesenchymal bone cancer occurring predominantly in younger patients, invasive tumour growth leads to extensive bone destruction. This process is insufficiently understood, cannot be efficiently counteracted and calls for novel means of treatment. The endocytic collagen receptor, uPARAP/Endo180, is expressed on various mesenchymal cell types and is involved in bone matrix turnover during normal bone growth. Human osteosarcoma specimens showed strong expression of this receptor on tumour cells, along with the collagenolytic metalloprotease, MT1-MMP. In advanced tumours with ongoing bone degeneration, sarcoma cells positive for these proteins formed a contiguous layer aligned with the degradation zones. Remarkably, osteoclasts were scarce or absent from these regions and quantitative analysis revealed that this scarcity marked a strong contrast between osteosarcoma and bone metastases of carcinoma origin. This opened the possibility that sarcoma cells might directly mediate bone degeneration. To examine this question, we utilized a syngeneic, osteolytic bone tumour model with transplanted NCTC-2472 sarcoma cells in mice. When analysed in vitro, these cells were capable of degrading the protein component of surface-labelled bone slices in a process dependent on MMP activity and uPARAP/Endo180. Systemic treatment of the sarcoma-inoculated mice with a mouse monoclonal antibody that blocks murine uPARAP/Endo180 led to a strong reduction of bone destruction. Our findings identify sarcoma cell-resident uPARAP/Endo180 as a central player in the bone degeneration of advanced tumours, possibly following an osteoclast-mediated attack on bone in the early tumour stage. This points to uPARAP/Endo180 as a promising therapeutic target in osteosarcoma, with particular prospects for improved neoadjuvant therapy.

A distinct regulatory region of the Bmp5 locus activates gene expression following adult bone fracture or soft tissue injury

Bone morphogenetic proteins (BMPs) are key signaling molecules required for normal development of bones and other tissues. Previous studies have shown that null mutations in the mouse Bmp5 gene alter the size, shape and number of multiple bone and cartilage structures during development. Bmp5 mutations also delay healing of rib fractures in adult mutants, suggesting that the same signals used to pattern embryonic bone and cartilage are also reused during skeletal regeneration and repair. Despite intense interest in BMPs as agents for stimulating bone formation in clinical applications, little is known about the regulatory elements that control developmental or injury-induced BMP expression. To compare the DNA sequences that activate gene expression during embryonic bone formation and following acute injuries in adult animals, we assayed regions surrounding the Bmp5 gene for their ability to stimulate lacZ reporter gene expression in transgenic mice. Multiple genomic fragments, distributed across the Bmp5 locus, collectively coordinate expression in discrete anatomic domains during normal development, including in embryonic ribs. In contrast, a distinct regulatory region activated expression following rib fracture in adult animals. The same injury control region triggered gene expression in mesenchymal cells following tibia fracture, in migrating keratinocytes following dorsal skin wounding, and in regenerating epithelial cells following lung injury. The Bmp5 gene thus contains an "injury response" control region that is distinct from embryonic enhancers, and that is activated by multiple types of injury in adult animals.

Genetic regulation of bone metabolism in the chicken: similarities and differences to Mammalian systems

Birds have a unique bone physiology, due to the demands placed on them through egg production. In particular their medullary bone serves as a source of calcium for eggshell production during lay and undergoes continuous and rapid remodelling. We take advantage of the fact that bone traits have diverged massively during chicken domestication to map the genetic basis of bone metabolism in the chicken. We performed a quantitative trait locus (QTL) and expression QTL (eQTL) mapping study in an advanced intercross based on Red Junglefowl (the wild progenitor of the modern domestic chicken) and White Leghorn chickens. We measured femoral bone traits in 456 chickens by peripheral computerised tomography and femoral gene expression in a subset of 125 females from the cross with microarrays. This resulted in 25 loci for female bone traits, 26 loci for male bone traits and 6318 local eQTL loci. We then overlapped bone and gene expression loci, before checking for an association between gene expression and trait values to identify candidate quantitative trait genes for bone traits. A handful of our candidates have been previously associated with bone traits in mice, but our results also implicate unexpected and largely unknown genes in bone metabolism. In summary, by utilising the unique bone metabolism of an avian species, we have identified a number of candidate genes affecting bone allocation and metabolism. These findings can have ramifications not only for the understanding of bone metabolism genetics in general, but could also be used as a potential model for osteoporosis as well as revealing new aspects of vertebrate bone regulation or features that distinguish avian and mammalian bone.

In this work we seek to further the understanding of bone genetics by mapping bone traits and gene expression in the chicken. Bone in female birds is special due to egg production. In this study, we combine the genetic mapping of bone traits with bone gene expression to find candidate quantitative trait genes that explain the differences between wild and domestic chickens in terms of bone production. The concept of combining genetic mapping and gene expression mapping is not new, and has already been successful in isolating bone-related genes in mammals, however this is the first time it has been applied to an avian system with such unique bone modelling processes. We aim to reveal new molecular mechanisms of bone regulation, and many of the candidates we find are new, highlighting the potential this technique has to identify the potential differences between avian and mammalian bone biology.

Phenotypic characterization of a Csf1r haploinsufficient mouse model of adult-onset leukodystrophy with axonal spheroids and pigmented glia (ALSP)

Mutations in the colony stimulating factor-1 receptor (CSF1R) that abrogate the expression of the affected allele or lead to the expression of mutant receptor chains devoid of kinase activity have been identified in both familial and sporadic cases of ALSP. To determine the validity of the Csf1r heterozygous mouse as a model of adult-onset leukoencephalopathy with axonal spheroids and pigmented glia (ALSP) we performed behavioral, radiologic, histopathologic, ultrastructural and cytokine expression studies of young and old Csf1r+/- and control Csf1r+/+ mice. Six to 8-month old Csf1r+/- mice exhibit cognitive deficits, and by 9-11 months develop sensorimotor deficits and in male mice, depression and anxiety-like behavior. MRIs of one year-old Csf1r+/- mice reveal lateral ventricle enlargement and thinning of the corpus callosum. Ultrastructural analysis of the corpus callosum uncovers dysmyelinated axons as well as neurodegeneration, evidenced by the presence of axonal spheroids. Histopathological examination of 11-week-old mice reveals increased axonal and myelin staining in the cortex, increase of neuronal cell density in layer V and increase of microglial cell densities throughout the brain, suggesting that early developmental changes contribute to disease. By 10-months of age, the neuronal cell density normalizes, oligodendrocyte precursor cells increase in layers II-III and V and microglial densities remain elevated without an increase in astrocytes. Also, the age-dependent increase in CSF-1R+ neurons in cortical layer V is reduced. Moreover, the expression of Csf2, Csf3, Il27 and Il6 family cytokines is increased, consistent with microglia-mediated inflammation. These results demonstrate that the inactivation of one Csf1r allele is sufficient to cause an ALSP-like disease in mice. The Csf1r+/- mouse is a model of ALSP that will allow the critical events for disease development to be determined and permit rapid evaluation of therapeutic approaches. Furthermore, our results suggest that aberrant activation of microglia in Csf1r+/- mice may play a central role in ALSP pathology.

A novel microRNA regulates osteoclast differentiation via targeting protein inhibitor of activated STAT3 (PIAS3)

MicroRNAs (miRNAs) involve in the regulation of a wide range of physiological processes. Recent studies suggested that miRNAs might play a role in osteoclast differentiation. Here, we identify a new miRNA (miR-9718) in primary mouse osteoclasts that promotes osteoclast differentiation by repressing protein inhibitor of activated STAT3 (PIAS3) at the post-transcriptional level. MiR-9718 was found to be transcribed during osteoclastogenesis, which was induced by macrophage colony stimulating factor (M-CSF) and receptor activator of nuclear factor-κB ligand (RANKL). Overexpression of miR-9718 in RAW 264.7 cells promoted M-CSF and RANKL-induced osteoclastogenesis, whereas inhibition of miR-9718 attenuated it. PIAS3 was predicted to be a target of miR-9718. Luciferase reporter gene validated the prediction. Transfection of pre-miR-9718 in RAW 264.7 cells induced by both M-CSF and RANKL inhibited expression of PIAS3 protein, while the mRNA levels of PIAS3 were not attenuated. In vivo, our study showed that silencing of miR-9718 using a specific antagomir inhibited bone resorption and increased bone mass in mice receiving ovariectomy (OVX) and in sham-operated control mice. Thus, our study showed that miR-9718 played an important role in osteoclast differentiation via targeting PIAS3 both in vitro and in vivo.

Osteoclasts: more than 'bone eaters'

As the only cells definitively shown to degrade bone, osteoclasts are key mediators of skeletal diseases including osteoporosis. Bone-forming osteoblasts, and hematopoietic and immune system cells, each influence osteoclast formation and function, but the reciprocal impact of osteoclasts on these cells is less well appreciated. We highlight here the functions that osteoclasts perform beyond bone resorption. First, we consider how osteoclast signals may contribute to bone formation by osteoblasts and to the pathology of bone lesions such as fibrous dysplasia and giant cell tumors. Second, we review the interaction of osteoclasts with the hematopoietic system, including the stem cell niche and adaptive immune cells. Connections between osteoclasts and other cells in the bone microenvironment are discussed within a clinically relevant framework.

A comparison of osteoclast-rich and osteoclast-poor osteopetrosis in adult mice sheds light on the role of the osteoclast in coupling bone resorption and bone formation

Osteopetrosis due to lack of acid secretion by osteoclasts is characterized by abolished bone resorption, increased osteoclast numbers, but normal or even increased bone formation. In contrast, osteoclast-poor osteopetrosis appears to have less osteoblasts and reduced bone formation, indicating that osteoclasts are important for regulating osteoblast activity. To illuminate the role of the osteoclast in controlling bone remodeling, we transplanted irradiated skeletally mature 3-month old wild-type mice with hematopoietic stem cells (HSCs) to generate either an osteoclast-rich or osteoclast-poor adult osteopetrosis model. We used fetal liver HSCs from (1) oc/oc mice, (2) RANK KO mice, and (3) compared these to wt control cells. TRAP5b activity, a marker of osteoclast number and size, was increased in the oc/oc recipients, while a significant reduction was seen in the RANK KO recipients. In contrast, the bone resorption marker CTX-I was similarly decreased in both groups. Both oc/oc and Rank KO recipients developed a mild osteopetrotic phenotype. However, the osteoclast-rich oc/oc recipients showed higher trabecular bone volume (40 %), increased bone strength (66 %), and increased bone formation rate (54 %) in trabecular bone, while RANK KO recipients showed only minor trends compared to control recipients. We here show that maintaining non-resorbing osteoclasts, as opposed to reducing the osteoclasts, leads to increased bone formation, bone volume, and ultimately higher bone strength in vivo, which indicates that osteoclasts are sources of anabolic molecules for the osteoblasts.

Interkeukin-34, a cytokine crucial for the differentiation and maintenance of tissue resident macrophages and Langerhans cells

IL-34 is a recently discovered cytokine that acts on tissue resident macrophages and Langerhans cells upon binding the receptor for CSF-1, CSF-1R. The existence of two ligands for CSF-1R, IL-34, and CSF-1, raises several intriguing questions. Are IL-34 and CSF-1 redundant or does each perform temporally and spatially distinct functions? Is IL-34 involved in human pathology? Would therapeutic strategies based on selective inhibition or administration of either IL-34 or CSF-1 be advantageous for preventing human pathology? Recent in vivo studies indicate that IL-34 promotes the development, survival, and function of microglia and Langerhans cells; therefore, this cytokine may predominately function in brain and skin biology. Here, we review the evidence for IL-34 as a key cytokine in the development and function of these two diverse cell types and discuss its potential role in pathological conditions.

Implications of osteoblast-osteoclast interactions in the management of osteoporosis by antiresorptive agents denosumab and odanacatib

Antiresorptive agents, used in the treatment of osteoporosis, inhibit either osteoclast formation or function. However, with these approaches, osteoblast activity is also reduced because of the loss of osteoclast-derived coupling factors that serve to stimulate bone formation. This review discusses how osteoclast inhibition influences osteoblast function, comparing the actions of an inhibitor of osteoclast formation [anti-RANKL/Denosumab (DMAB)] with that of a specific inhibitor of osteoclastic cathepsin K activity [Odanacatib (ODN)]. Denosumab rapidly and profoundly, but reversibly, reduces bone formation. In contrast, preclinical studies and clinical trials of ODN showed that bone formation at some skeletal sites was preserved although resorption was reduced. This preservation of bone formation appears to be due to effects of coupling factors, secreted by osteoclasts and released from demineralized bone matrix. This indicates that bone resorptive activities of osteoclasts are separable from their coupling activities.

IL-34 and CSF-1: similarities and differences

Colony-stimulating factor-1 (CSF-1) is widely expressed and considered to regulate the development, maintenance, and function of mononuclear phagocyte lineage cells such as monocytes, macrophages, dendritic cells (DCs), Langerhans cells (LCs), microglia, and osteoclasts. Interleukin-34 (IL-34) was recently identified as an alternative ligand for the CSF-1 receptor (CSF-1R) through functional proteomics experiments. It is well established that the phenotype of CSF-1R-deficient (CSF-1R⁻/⁻) mice is more severe than that of mice bearing a spontaneous null mutation in CSF-1 (CSF-1(op/op)). CSF-1R⁻/⁻ mice are severely depleted of macrophages and completely lack LCs, microglia, and osteoclasts during their lifetime. In contrast, CSF-1(op/op) mice exhibit late-onset macrophage development and osteoclastogenesis, whereas they show modestly reduced numbers of microglia and a relatively normal LC development. In contrast, IL-34-deficient (IL-34⁻/⁻) mice show a marked reduction of LCs and a decrease in microglia. IL-34 and CSF-1 display different spatiotemporal expression patterns and have distinct biological functions. In this review, we focus on the functional similarities and differences between IL-34 and CSF-1 in vivo.

Receptor-type protein-tyrosine phosphatase ζ is a functional receptor for interleukin-34

Interleukin-34 (IL-34) is highly expressed in brain. IL-34 signaling via its cognate receptor, colony-stimulating factor-1 receptor (CSF-1R), is required for the development of microglia. However, the differential expression of IL-34 and the CSF-1R in brain suggests that IL-34 may signal via an alternate receptor. By IL-34 affinity chromatography of solubilized mouse brain membrane followed by mass spectrometric analysis, we identified receptor-type protein-tyrosine phosphatase ζ (PTP-ζ), a cell surface chondroitin sulfate (CS) proteoglycan, as a novel IL-34 receptor. PTP-ζ is primarily expressed on neural progenitors and glial cells and is highly expressed in human glioblastomas. IL-34 selectively bound PTP-ζ in CSF-1R-deficient U251 human glioblastoma cell lysates and inhibited the proliferation, clonogenicity, and motility of U251 cells in a PTP-ζ-dependent manner. These effects were correlated with an increase in tyrosine phosphorylation of the previously identified PTP-ζ downstream effectors focal adhesion kinase and paxillin. IL-34 binding to U251 cells was abrogated by chondroitinase ABC treatment, and CS competed with IL-34 for binding to the extracellular domain of PTP-ζ and to the cells, indicating a dependence of binding on PTP-ζ CS moieties. This study identifies an alternate receptor for IL-34 that may mediate its action on novel cellular targets.

miR-148a regulates osteoclastogenesis by targeting V-maf musculoaponeurotic fibrosarcoma oncogene homolog B

MicroRNAs (miRNAs) play crucial roles in bone metabolism. In the present study, we found that miR-148a is dramatically upregulated during osteoclastic differentiation of circulating CD14+ peripheral blood mononuclear cells (PBMCs) induced by macrophage colony stimulating factor (M-CSF) and receptor activator of nuclear factor-κB ligand (RANKL). Overexpression of miR-148a in CD14+ PBMCs promoted osteoclastogenesis, whereas inhibition of miR-148a attenuated osteoclastogenesis. V-maf musculoaponeurotic fibrosarcoma oncogene homolog B (MAFB) is a transcription factor negatively regulating RANKL-induced osteoclastogenesis. miR-148a directly targeted MAFB mRNA by binding to the 3' untranslated region (3'UTR) and repressed MAFB protein expression. In vivo, our study showed that silencing of miR-148a using a specific antagomir-inhibited bone resorption and increased bone mass in mice receiving ovariectomy (OVX) and in sham-operated control mice. Furthermore, our results showed that miR-148a levels significantly increased in CD14+ PBMCs from lupus patients and resulted in enhanced osteoclastogenesis, which contributed to the lower bone mineral density (BMD) in lupus patients compared with normal controls. Thus, our study provides a new insight into the roles of miRNAs in osteoclastogenesis, and contributes to a new therapeutic pathway for osteoporosis.

CSF-1 receptor-dependent colon development, homeostasis and inflammatory stress response

The colony stimulating factor-1 (CSF-1) receptor (CSF-1R) directly regulates the development of Paneth cells (PC) and influences proliferation and cell fate in the small intestine (SI). In the present study, we have examined the role of CSF-1 and the CSF-1R in the large intestine, which lacks PC, in the steady state and in response to acute inflammation induced by dextran sulfate sodium (DSS). As previously shown in mouse, immunohistochemical (IHC) analysis of CSF-1R expression showed that the receptor is baso-laterally expressed on epithelial cells of human colonic crypts, indicating that this expression pattern is shared between species. Colons from Csf1r null and Csf1(op/op) mice were isolated and sectioned for IHC identification of enterocytes, enteroendocrine cells, goblet cells and proliferating cells. Both Csf1r(-/-) and Csf1(op/op) mice were found to have colon defects in enterocytes and enteroendocrine cell fate, with excessive goblet cell staining and reduced cell proliferation. In addition, the gene expression profiles of the cell cycle genes, cyclinD1, c-myc, c-fos, and c-myb were suppressed in Csf1r(-/-) colonic crypt, compared with those of WT mice and the expression of the stem cell marker gene Lgr5 was markedly reduced. However, analysis of the proliferative responses of immortalized mouse colon epithelial cells (lines; Immorto-5 and YAMC) indicated that CSF-1R is not a major regulator of colonocyte proliferation and that its effects on proliferation are indirect. In an examination of the acute inflammatory response, Csf1r(+/-) male mice were protected from the adverse affects of DSS-induced colitis compared with WT mice, while Csf1r(+/-) female mice were significantly less protected. These data indicate that CSF-1R signaling plays an important role in colon homeostasis and stem cell gene expression but that the receptor exacerbates the response to inflammatory challenge in male mice.

PSTPIP2 deficiency in mice causes osteopenia and increased differentiation of multipotent myeloid precursors into osteoclasts

Missense mutations that reduce or abrogate myeloid cell expression of the F-BAR domain protein, proline serine threonine phosphatase-interacting protein 2 (PSTPIP2), lead to autoinflammatory disease involving extramedullary hematopoiesis, skin and bone lesions. However, little is known about how PSTPIP2 regulates osteoclast development. Here we examined how PSTPIP2 deficiency causes osteopenia and bone lesions, using the mouse PSTPIP2 mutations, cmo, which fails to express PSTPIP2 and Lupo, in which PSTPIP2 is dysfunctional. In both models, serum levels of the pro-osteoclastogenic factor, MIP-1α, were elevated and CSF-1 receptor (CSF-1R)-dependent production of MIP-1α by macrophages was increased. Treatment of cmo mice with a dual specificity CSF-1R and c-Kit inhibitor, PLX3397, decreased circulating MIP-1α and ameliorated the extramedullary hematopoiesis, inflammation, and osteopenia, demonstrating that aberrant myelopoiesis drives disease. Purified osteoclast precursors from PSTPIP2-deficient mice exhibit increased osteoclastogenesis in vitro and were used to probe the structural requirements for PSTPIP2 suppression of osteoclast development. PSTPIP2 tyrosine phosphorylation and a functional F-BAR domain were essential for PSTPIP2 inhibition of TRAP expression and osteoclast precursor fusion, whereas interaction with PEST-type phosphatases was only required for suppression of TRAP expression. Thus, PSTPIP2 acts as a negative feedback regulator of CSF-1R signaling to suppress inflammation and osteoclastogenesis.

Osteogenic and chondrogenic potential of biomembrane cells from the PMMA-segmental defect rat model

A layer of cells (the "biomembrane") has been identified in large segmental defects between bone and surgically placed methacrylate spacers or antibiotic-impregnated cement beads. We hypothesize that this contains a pluripotent stem cell population with potential valuable applications in orthopedic tissue engineering. Objectives using biomembranes harvested from rat segmental defects were to: (1) Culture biomembrane cells in specialized media to direct progenitor cells along bone or cartilage cell differentiation lineages; (2) evaluate harvested biomembranes for mesenchymal stem cell markers, and (3) define relevant gene expression patterns in harvested biomembranes using microarray analysis. Culture in osteogenic media produced mineralized nodules; culture in chondrogenic media produced masses containing chondroitin sulfate/sulfated proteoglycans. Molecular analysis of biomembrane cells versus control periosteum showed significant upregulation of key genes functioning in mesenchymal stem cell differentiation, development, maintenance, and proliferation. Results identified significant upregulation of WNT receptor signaling pathway genes and significant upregulation of BMP signaling pathway genes. Findings confirm that the biomembrane has a pluripotent stem cell population. The ability to heal large bone defects is clinically challenging, and novel tissue engineering uses of the biomembrane hold great promise in treating non-unions, open fractures with large bone loss and/or infections, and defects associated with tumor resection.

Mice lacking the calcineurin inhibitor Rcan2 have an isolated defect of osteoblast function

Calcineurin-nuclear factor of activated T cells signaling controls the differentiation and function of osteoclasts and osteoblasts, and regulator of calcineurin-2 (Rcan2) is a physiological inhibitor of this pathway. Rcan2 expression is regulated by T(3), which also has a central role in skeletal development and bone turnover. To investigate the role of Rcan2 in bone development and maintenance, we characterized Rcan2(-/-) mice and determined its skeletal expression in T(3) receptor (TR) knockout and thyroid-manipulated mice. Rcan2(-/-) mice had normal linear growth but displayed delayed intramembranous ossification, impaired cortical bone formation, and reduced bone mineral accrual during development as well as increased mineralization of adult bone. These abnormalities resulted from an isolated defect in osteoblast function and are similar to skeletal phenotypes of mice lacking the type 2 deiodinase thyroid hormone activating enzyme or with dominant-negative mutations of TRα, the predominant TR isoform in bone. Rcan2 mRNA was expressed in primary osteoclasts and osteoblasts, and its expression in bone was differentially regulated in TRα and TRβ knockout and thyroid-manipulated mice. However, in primary osteoblast cultures, T(3) treatment did not affect Rcan2 mRNA expression or nuclear factor of activated T cells c1 expression and phosphorylation. Overall, these studies establish that Rcan2 regulates osteoblast function and its expression in bone is regulated by thyroid status in vivo.