Modeling skeletal dysplasia in Hurler syndrome using patient-derived bone marrow osteoprogenitor cells.

Dysostosis multiplex is a major cause of morbidity in Hurler syndrome (mucopolysaccharidosis type IH [MPS IH], OMIM #607014) because currently available therapies have limited success in its prevention and reversion. Unfortunately, the elucidation of skeletal pathogenesis in MPS IH is limited by difficulties in obtaining bone specimens from pediatric patients and poor reproducibility in animal models. Thus, the application of experimental systems that can be used to dissect cellular and molecular mechanisms underlying the skeletal phenotype of MPS IH patients and to identify effective therapies is highly needed. Here, we adopted in vitro/in vivo systems based on patient-derived bone marrow stromal cells to generate cartilaginous pellets and bone rudiments. Interestingly, we observed that heparan sulphate accumulation compromised the remodeling of MPS IH cartilage into other skeletal tissues and other critical aspects of the endochondral ossification process. We also noticed that MPS IH hypertrophic cartilage was characterized by dysregulation of signaling pathways controlling cartilage hypertrophy and fate, extracellular matrix organization, and glycosaminoglycan metabolism. Our study demonstrates that the cartilaginous pellet-based system is a valuable tool to study MPS IH dysostosis and to develop new therapeutic approaches for this hard-to-treat aspect of the disease. Finally, our approach may be applied for modeling other genetic skeletal disorders.

A pathogenic role for brain-derived neurotrophic factor (BDNF) in fibrous dysplasia of bone.

Brain derived neurotrophic factor (BDNF) is a neurotrophin, expressed in the central nervous system and in peripheral tissues, that is regulated by the Gsα/cAMP pathway. In bone, it regulates osteogenesis and stimulates RANKL secretion and osteoclast formation in osteolytic tumors such as Multiple Myeloma. Fibrous dysplasia (FD) of bone is a rare genetic disease of the skeleton caused by gain-of-function mutations of the Gsα gene in which RANKL-dependent enhanced bone resorption is a major cause of bone fragility and clinical morbidity. We observed that BDNF transcripts are expressed in human FD lesions. Specifically, immunolocalization studies performed on biopsies obtained from FD patients revealed the expression of BDNF in osteoblasts and, to a lower extent, in the spindle-shaped cells within the fibrous tissue. Therefore, we hypothesized that BDNF can play a role in the pathogenesis of FD by stimulating RANKL secretion and bone resorption. To test this hypothesis, we used the EF1α-GsαR201C mouse model of the human disease (FD mice). Western blot analysis revealed a higher expression of BDNF in bone segments of FD mice compared to WT mice and the immunolabeling pattern within mouse FD lesions was similar to that observed in human FD. Treatment of FD mice with a monoclonal antibody against BDNF reduced the fibrous tissue along with the number of osteoclasts and osteoblasts within femoral lesions. These results reveal BDNF as a new player in the pathogenesis of FD and a potential molecular mechanism by which osteoclastogenesis may be nourished within FD bone lesions. They also suggest that BDNF inhibition may be a new approach to reduce abnormal bone remodeling in FD.

AML alters bone marrow stromal cell osteogenic commitment via Notch signaling.

Introduction: Acute myeloid leukemia (AML) is a highly heterogeneous malignancy caused by various genetic alterations and characterized by the accumulation of immature myeloid blasts in the bone marrow (BM). This abnormal growth of AML cells disrupts normal hematopoiesis and alters the BM microenvironment components, establishing a niche supportive of leukemogenesis. Bone marrow stromal cells (BMSCs) play a pivotal role in giving rise to essential elements of the BM niche, including adipocytes and osteogenic cells. Animal models have shown that the BM microenvironment is significantly remodeled by AML cells, which skew BMSCs toward an ineffective osteogenic differentiation with an accumulation of osteoprogenitors. However, little is known about the mechanisms by which AML cells affect osteogenesis. Methods: We studied the effect of AML cells on the osteogenic commitment of normal BMSCs, using a 2D co-culture system. Results: We found that AML cell lines and primary blasts, but not normal hematopoietic CD34+ cells, induced in BMSCs an ineffective osteogenic commitment, with an increase of the early-osteogenic marker tissue non-specific alkaline phosphatase (TNAP) in the absence of the late-osteogenic gene up-regulation. Moreover, the direct interaction of AML cells and BMSCs was indispensable in influencing osteogenic differentiation. Mechanistic studies identified a role for AML-mediated Notch activation in BMSCs contributing to their ineffective osteogenic commitment. Inhibition of Notch using a γ-secretase inhibitor strongly influenced Notch signaling in BMSCs and abrogated the AML-induced TNAP up-regulation. Discussion: Together, our data support the hypothesis that AML infiltration produces a leukemia-supportive pre-osteoblast-rich niche in the BM, which can be partially ascribed to AML-induced activation of Notch signaling in BMSCs.

An in vivo humanized model to study homing and sequestration of Plasmodium falciparum transmission stages in the bone marrow.

Introduction: Recent evidence suggests that the bone marrow (BM) plays a key role in the diffusion of P. falciparum malaria by providing a "niche" for the maturation of the parasite gametocytes, responsible for human-to-mosquito transmission. Suitable humanized in vivo models to study the mechanisms of the interplay between the parasite and the human BM components are still missing. Methods: We report a novel experimental system based on the infusion of immature P. falciparum gametocytes into immunocompromised mice carrying chimeric ectopic ossicles whose stromal and bone compartments derive from human osteoprogenitor cells. Results: We demonstrate that immature gametocytes home within minutes to the ossicles and reach the extravascular regions, where they are retained in contact with different human BM stromal cell types. Discussion: Our model represents a powerful tool to study BM function and the interplay essential for parasite transmission in P. falciparum malaria and can be extended to study other infections in which the human BM plays a role.

The enhancer landscape predetermines the skeletal regeneration capacity of stromal cells.

Multipotent stromal cells are considered attractive sources for cell therapy and tissue engineering. Despite numerous experimental and clinical studies, broad application of stromal cell therapeutics is not yet emerging. A major challenge is the functional diversity of available cell sources. Here, we investigated the regenerative potential of clinically relevant human stromal cells from bone marrow (BMSCs), white adipose tissue, and umbilical cord compared with mature chondrocytes and skin fibroblasts in vitro and in vivo. Although all stromal cell types could express transcription factors related to endochondral ossification, only BMSCs formed cartilage discs in vitro that fully regenerated critical-size femoral defects after transplantation into mice. We identified cell type-specific epigenetic landscapes as the underlying molecular mechanism controlling transcriptional stromal differentiation networks. Binding sites of commonly expressed transcription factors in the enhancer and promoter regions of ossification-related genes, including Runt and bZIP families, were accessible only in BMSCs but not in extraskeletal stromal cells. This suggests an epigenetically predetermined differentiation potential depending on cell origin that allows common transcription factors to trigger distinct organ-specific transcriptional programs, facilitating forward selection of regeneration-competent cell sources. Last, we demonstrate that viable human BMSCs initiated defect healing through the secretion of osteopontin and contributed to transient mineralized bone hard callus formation after transplantation into immunodeficient mice, which was eventually replaced by murine recipient bone during final tissue remodeling.

Selective homing of CAR-CIK cells to the bone marrow niche enhances control of the acute myeloid leukemia burden.

Acute myeloid leukemia (AML) is a hematological malignancy derived from neoplastic myeloid progenitor cells characterized by abnormal clonal proliferation and differentiation. Although novel therapeutic strategies have recently been introduced, the prognosis of AML is still unsatisfactory. So far, the efficacy of chimeric antigen receptor (CAR)-T-cell therapy in AML has been hampered by several factors, including the poor accumulation of the blood-injected cells in the leukemia bone marrow (BM) niche in which chemotherapy-resistant leukemic stem cells reside. Thus, we hypothesized that overexpression of CXCR4, whose ligand CXCL12 is highly expressed by BM stromal cells within this niche, could improve T-cell homing to the BM and consequently enhance their intimate contact with BM-resident AML cells, facilitating disease eradication. Specifically, we engineered conventional CD33.CAR-cytokine-induced killer cells (CIKs) with the wild-type (wt) CXCR4 and the variant CXCR4R334X, responsible for leukocyte sequestration in the BM of patients with warts, hypogammaglobulinemia, immunodeficiency, and myelokathexis syndrome. Overexpression of both CXCR4wt and CXCR4mut in CD33.CAR-CIKs resulted in significant improvement of chemotaxis toward recombinant CXCL12 or BM stromal cell-conditioned medium, with no observed impairment of cytotoxic potential in vitro. Moreover, CXCR4-overexpressing CD33.CAR-CIKs showed enhanced in vivo BM homing, associated with a prolonged retention for the CXCR4R334X variant. However, only CD33.CAR-CIKs coexpressing CXCR4wt but not CXCR4mut exerted a more sustained in vivo antileukemic activity and extended animal survival, suggesting a noncanonical role for CXCR4 in modulating CAR-CIK functions independent of BM homing. Taken together, these data suggest that arming CAR-CIKs with CXCR4 may represent a promising strategy for increasing their therapeutic potential for AML.

Modelling skeletal pain harnessing tissue engineering.

Bone pain typically occurs immediately following skeletal damage with mechanical distortion or rupture of nociceptive fibres. The pain mechanism is also associated with chronic pain conditions where the healing process is impaired. Any load impacting on the area of the fractured bone will stimulate the nociceptive response, necessitating rapid clinical intervention to relieve pain associated with the bone damage and appropriate mitigation of any processes involved with the loss of bone mass, muscle, and mobility and to prevent death. The following review has examined the mechanisms of pain associated with trauma or cancer-related skeletal damage focusing on new approaches for the development of innovative therapeutic interventions. In particular, the review highlights tissue engineering approaches that offer considerable promise in the application of functional biomimetic fabrication of bone and nerve tissues. The strategic combination of bone and nerve tissue engineered models provides significant potential to develop a new class of in vitro platforms, capable of replacing in vivo models and testing the safety and efficacy of novel drug treatments aimed at the resolution of bone-associated pain. To date, the field of bone pain research has centred on animal models, with a paucity of data correlating to the human physiological response. This review explores the evident gap in pain drug development research and suggests a step change in approach to harness tissue engineering technologies to recapitulate the complex pathophysiological environment of the damaged bone tissue enabling evaluation of the associated pain-mimicking mechanism with significant therapeutic potential therein for improved patient quality of life. Graphical abstract: Rationale underlying novel drug testing platform development. Pain detected by the central nervous system and following bone fracture cannot be treated or exclusively alleviated using standardised methods. The pain mechanism and specificity/efficacy of pain reduction drugs remain poorly understood. In vivo and ex vivo models are not yet able to recapitulate the various pain events associated with skeletal damage. In vitro models are currently limited by their inability to fully mimic the complex physiological mechanisms at play between nervous and skeletal tissue and any disruption in pathological states. Robust innovative tissue engineering models are needed to better understand pain events and to investigate therapeutic regimes.

Safe and effective off-the-shelf immunotherapy based on CAR.CD123-NK cells for the treatment of acute myeloid leukaemia.

BACKGROUND: Paediatric acute myeloid leukaemia (AML) is characterized by poor outcomes in patients with relapsed/refractory disease, despite the improvements in intensive standard therapy. The leukaemic cells of paediatric AML patients show high expression of the CD123 antigen, and this finding provides the biological basis to target CD123 with the chimeric antigen receptor (CAR). However, CAR.CD123 therapy in AML is hampered by on-target off-tumour toxicity and a long "vein-to-vein" time. METHODS: We developed an off-the-shelf product based on allogeneic natural killer (NK) cells derived from the peripheral blood of healthy donors and engineered them to express a second-generation CAR targeting CD123 (CAR.CD123). RESULTS: CAR.CD123-NK cells showed significant anti-leukaemia activity not only in vitro against CD123+ AML cell lines and CD123+ primary blasts but also in two animal models of human AML-bearing immune-deficient mice. Data on anti-leukaemia activity were also corroborated by the quantification of inflammatory cytokines, namely granzyme B (Granz B), interferon gamma (IFN-γ) and tumour necrosis factor alpha (TNF-α), both in vitro and in the plasma of mice treated with CAR.CD123-NK cells. To evaluate and compare the on-target off-tumour effects of CAR.CD123-T and NK cells, we engrafted human haematopoietic cells (hHCs) in an immune-deficient mouse model. All mice infused with CAR.CD123-T cells died by Day 5, developing toxicity against primary human bone marrow (BM) cells with a decreased number of total hCD45+ cells and, in particular, of hCD34+CD38- stem cells. In contrast, treatment with CAR.CD123-NK cells was not associated with toxicity, and all mice were alive at the end of the experiments. Finally, in a mouse model engrafted with human endothelial tissues, we demonstrated that CAR.CD123-NK cells were characterized by negligible endothelial toxicity when compared to CAR.CD123-T cells. CONCLUSIONS: Our data indicate the feasibility of an innovative off-the-shelf therapeutic strategy based on CAR.CD123-NK cells, characterized by remarkable efficacy and an improved safety profile compared to CAR.CD123-T cells. These findings open a novel intriguing scenario not only for the treatment of refractory/resistant AML patients but also to further investigate the use of CAR-NK cells in other cancers characterized by highly difficult targeting with the most conventional T effector cells.

MPSI Manifestations and Treatment Outcome: Skeletal Focus.

Mucopolysaccharidosis type I (MPSI) (OMIM #252800) is an autosomal recessive disorder caused by pathogenic variants in the IDUA gene encoding for the lysosomal alpha-L-iduronidase enzyme. The deficiency of this enzyme causes systemic accumulation of glycosaminoglycans (GAGs). Although disease manifestations are typically not apparent at birth, they can present early in life, are progressive, and include a wide spectrum of phenotypic findings. Among these, the storage of GAGs within the lysosomes disrupts cell function and metabolism in the cartilage, thus impairing normal bone development and ossification. Skeletal manifestations of MPSI are often refractory to treatment and severely affect patients' quality of life. This review discusses the pathological and molecular processes leading to impaired endochondral ossification in MPSI patients and the limitations of current therapeutic approaches. Understanding the underlying mechanisms responsible for the skeletal phenotype in MPSI patients is crucial, as it could lead to the development of new therapeutic strategies targeting the skeletal abnormalities of MPSI in the early stages of the disease.

GsαR201C and estrogen reveal different subsets of bone marrow adiponectin expressing osteogenic cells.

The Gsα/cAMP signaling pathway mediates the effect of a variety of hormones and factors that regulate the homeostasis of the post-natal skeleton. Hence, the dysregulated activity of Gsα due to gain-of-function mutations (R201C/R201H) results in severe architectural and functional derangements of the entire bone/bone marrow organ. While the consequences of gain-of-function mutations of Gsα have been extensively investigated in osteoblasts and in bone marrow osteoprogenitor cells at various differentiation stages, their effect in adipogenically-committed bone marrow stromal cells has remained unaddressed. We generated a mouse model with expression of GsαR201C driven by the Adiponectin (Adq) promoter. Adq-GsαR201C mice developed a complex combination of metaphyseal, diaphyseal and cortical bone changes. In the metaphysis, GsαR201C caused an early phase of bone resorption followed by bone deposition. Metaphyseal bone formation was sustained by cells that were traced by Adq-Cre and eventually resulted in a high trabecular bone mass phenotype. In the diaphysis, GsαR201C, in combination with estrogen, triggered the osteogenic activity of Adq-Cre-targeted perivascular bone marrow stromal cells leading to intramedullary bone formation. Finally, consistent with the previously unnoticed presence of Adq-Cre-marked pericytes in intraosseous blood vessels, GsαR201C caused the development of a lytic phenotype that affected both cortical (increased porosity) and trabecular (tunneling resorption) bone. These results provide the first evidence that the Adq-cell network in the skeleton not only regulates bone resorption but also contributes to bone formation, and that the Gsα/cAMP pathway is a major modulator of both functions.

Morphological and Immunophenotypical Changes of Human Bone Marrow Adipocytes in Marrow Metastasis and Myelofibrosis.

The bone marrow adipose tissue constitutes more than two-thirds of the bone marrow volume in adult life and is known to have unique metabolic and functional properties. In neoplastic disorders, bone marrow adipocytes (BMAds) contribute to create a favorable microenvironment to survival and proliferation of cancer cells. Many studies explored the molecular crosstalk between BMAds and neoplastic cells, predominantly in ex-vivo experimental systems or in animal models. However, little is known on the features of BMAds in the human neoplastic marrow. The aim of our study was to analyze the in situ changes in morphology and immunophenotype of BMAds in two different types of neoplastic marrow conditions. We selected a series of archival iliac crest and vertebral bone biopsies from patients with bone marrow metastasis (MET), patients with myeloproliferative neoplasia with grade-3 myelofibrosis (MPN-MF) and age-matched controls (CTR). We observed a significant reduction in the number of BMAds in MET and MPN-MF compared to CTR. Accordingly, in the same groups, we also detected a significant reduction in the mean cell diameter and area. Immunolocalization of different adipocyte markers showed that, compared to CTR, in both MET and MPN-MF the percentages of adiponectin- and phosphorylated hormone sensitive lipase-positive BMAds were significantly reduced and increased respectively. No statistically significant difference was found between MET and MPN-MF. Interestingly, in one MET sample, "remodeled" BMAds containing a large lipid vacuole and multiple, smaller and polarized lipid droplets were identified. In conclusion, our data show that in different types of marrow cancers, BMAds undergo significant quantitative and qualitative changes, which need to be further investigated in future studies.

Inflammatory myofibroblastic tumour of the larynx: report of a case.

Only 0.3-1% of laryngeal cancer are non-squamous cell neoplasms. Of these, a rare entity is inflammatory myofibroblastic tumour (IMT), in which anaplastic lymphoma kinase-1 (ALK-1) is frequently expressed. Just 50 cases of IMT have been reported. Therefore, many otolaryngologists may be unfamiliar with this type of tumour and be prone to its over- or undertreatment.We report a case of ALK-1-negative IMT treated with transoral endoscopic excision and disease-free 6 months after surgery.

Nanostring technology on Fibrous Dysplasia bone biopsies. A pilot study suggesting different histology-related molecular profiles.

Identifying the molecular networks that underlie Fibrous Dysplasia (FD) is key to understand the pathogenesis of the disease, to refine current diagnostic approaches and to develop efficacious therapies. In this study, we used the NanoString nCounter Analysis System to investigate the gene signature of a series of nine Formalin Fixed Decalcified and Paraffin-Embedded (FFDPE) bone biopsies from seven FD patients. We analyzed the expression level of 770 genes. Unsupervised clustering analysis demonstrated partitioning into two clusters with distinct patterns of gene expression. Differentially expressed genes included growth factors, components of the Wnt signaling system, interleukins and some of their cognate receptors, ephrin ligands, matrix metalloproteinases, neurotrophins and genes encoding components of the cAMP-dependent protein kinase. Interestingly, two tissue samples obtained from the same skeletal site of one patient one year apart failed to segregate in the same cluster. Retrospective histological review of the samples revealed different microscopic aspects in the two groups. The results of our pilot study suggest that the genetic signature of FD is heterogeneous and varies according to the histology and, likely, to the age of the lesion. In addition, they show that the Nanostring technology is a valuable tool for molecular translational studies on archival FFDPE material in FD and other rare bone diseases.

Thrombospondin 1 and 2 along with PEDF inhibit angiogenesis and promote lymphangiogenesis in intrahepatic cholangiocarcinoma.

BACKGROUND & AIMS: The microenvironment of intrahepatic cholangiocarcinoma (iCCA) is hypovascularized, with an extensive lymphatic network. This leads to rapid cancer spread into regional lymph nodes and the liver parenchyma, precluding curative treatments. Herein, we investigated which factors released in the iCCA stroma drive the inhibition of angiogenesis and promote lymphangiogenesis. METHODS: Quantitative proteomics was performed on extracellular fluid (ECF) proteins extracted both from cancerous and non-cancerous tissues (NCT) of patients with iCCA. Computational biology was applied on a proteomic dataset to identify proteins involved in the regulation of vessel formation. Endothelial cells incubated with ECF from either iCCA or NCT specimens were used to assess the role of candidate proteins in 3D vascular assembly, cell migration, proliferation and viability. Angiogenesis and lymphangiogenesis were further investigated in vivo by a heterotopic transplantation of bone marrow stromal cells, along with endothelial cells in SCID/beige mice. RESULTS: Functional analysis of upregulated proteins in iCCA unveils a soluble angio-inhibitory milieu made up of thrombospondin (THBS)1, THBS2 and pigment epithelium-derived factor (PEDF). iCCA ECF was able to inhibit in vitro vessel morphogenesis and viability. Antibodies blocking THBS1, THBS2 and PEDF restored tube formation and endothelial cell viability to levels observed in NCT ECF. Moreover, in transplanted mice, the inhibition of blood vessel formation, the de novo generation of the lymphatic network and the dissemination of iCCA cells in lymph nodes were shown to depend on THBS1, THBS2 and PEDF expression. CONCLUSIONS: THBS1, THBS2 and PEDF reduce blood vessel formation and promote tumor-associated lymphangiogenesis in iCCA. Our results identify new potential targets for interventions to counteract the dissemination process in iCCA. LAY SUMMARY: Intrahepatic cholangiocarcinoma is a highly aggressive cancer arising from epithelial cells lining the biliary tree, characterized by dissemination into the liver parenchyma via lymphatic vessels. Herein, we show that the proteins THBS1, THBS2 and PEDF, once released in the tumor microenvironment, inhibit vascular growth, while promoting cancer-associated lymphangiogenesis. Therefore, targeting THBS1, THBS2 and PEDF may be a promising strategy to reduce cancer-associated lymphangiogenesis and counteract the invasiveness of intrahepatic cholangiocarcinoma.

From Stem Cells to Bone-Forming Cells.

Bone formation starts near the end of the embryonic stage of development and continues throughout life during bone modeling and growth, remodeling, and when needed, regeneration. Bone-forming cells, traditionally termed osteoblasts, produce, assemble, and control the mineralization of the type I collagen-enriched bone matrix while participating in the regulation of other cell processes, such as osteoclastogenesis, and metabolic activities, such as phosphate homeostasis. Osteoblasts are generated by different cohorts of skeletal stem cells that arise from different embryonic specifications, which operate in the pre-natal and/or adult skeleton under the control of multiple regulators. In this review, we briefly define the cellular identity and function of osteoblasts and discuss the main populations of osteoprogenitor cells identified to date. We also provide examples of long-known and recently recognized regulatory pathways and mechanisms involved in the specification of the osteogenic lineage, as assessed by studies on mice models and human genetic skeletal diseases.

Heterogeneity of the bone marrow niche in patients with myeloproliferative neoplasms: ActivinA secretion by mesenchymal stromal cells correlates with the degree of marrow fibrosis.

Mesenchymal stromal cells (MSCs) represent an essential component of the bone marrow (BM) niche and display disease-specific alterations in several myeloid malignancies. The aim of this work was to study possible MSC abnormalities in Philadelphia-negative myeloproliferative neoplasms (MPNs) in relationship to the degree of BM fibrosis. MSCs were isolated from BM of 6 healthy donors (HD) and of 23 MPN patients, classified in 3 groups according to the diagnosis and the grade of BM fibrosis: polycythemia vera and essential thrombocythemia (PV/ET), low fibrosis myelofibrosis (LF-MF), and high fibrosis MF (HF-MF). MSC cultures were established from 21 of 23 MPN patients. MPN-derived MSCs did not exhibit any functional impairment in their adipogenic/osteogenic/chondrogenic differentiation potential and displayed a phenotype similar to HD-derived MSCs but with a decreased expression of CD146. All MPN-MSC lines were negative for the patient-specific hematopoietic clone mutations (JAK2, MPL, CALR). MSCs derived from HF-MF patients displayed a reduced clonogenic potential and a lower growth kinetic compared to MSCs from HD, LF-MF, and PV/ET patients. mRNA levels of hematopoiesis regulatory molecules were unaffected in MSCs from HF-MF compared to HD. Finally, in vitro ActivinA secretion by MSCs was increased in HF-MF compared to LF-MF patients, in association with a lower hemoglobin value. Increased ActivinA immunolabeling on stromal cells and erythroid precursors was also observed in HF-MF BM biopsies. In conclusion, higher grade of BM fibrosis is associated with functional impairment of MSCs and the increased secretion of ActivinA may represent a suitable target for anemia treatment in MF patients.

Zoledronic Acid in a Mouse Model of Human Fibrous Dysplasia: Ineffectiveness on Tissue Pathology, Formation of "Giant Osteoclasts" and Pathogenetic Implications.

We compared the effects of a nitrogen-containing bisphosphonate (N-BP), zoledronic acid (ZA), and an anti-mouse RANKL antibody (anti-mRANKL Ab) on the bone tissue pathology of a transgenic mouse model of human fibrous dysplasia (FD). For comparison, we also reviewed the histological samples of a child with McCune-Albright syndrome (MAS) treated with Pamidronate for 3 years. EF1α-GsαR201C mice with FD-like lesions in the tail vertebrae were treated with either 0.2 mg/kg of ZA at day 0, 7, and 14 or with 300 µg/mouse of anti-mRANKL Ab at day 0 and 21. All mice were monitored by Faxitron and histological analysis was performed at day 42. ZA did not affect the progression of the radiographic phenotype in EF1α-GsαR201C mice. FD-like lesions in the ZA group showed the persistence of osteoclasts, easily detectable osteoclast apoptotic activity and numerous "giant osteoclasts". In contrast, in the anti-mRANKL Ab-treated mice, osteoclasts were markedly reduced/absent, the radiographic phenotype reverted and the FD-like lesions were extensively replaced by newly formed bone. Numerous "giant osteoclasts" were also detected in the samples of the child with MAS. This study supports the hypothesis that osteoclasts per se, independently of their resorptive activity, are essential for development and expansion of FD lesions.

RANKL Inhibition in Fibrous Dysplasia of Bone: A Preclinical Study in a Mouse Model of the Human Disease.

Fibrous dysplasia of bone/McCune-Albright syndrome (Polyostotic FD/MAS; OMIM#174800) is a crippling skeletal disease caused by gain-of-function mutations of Gs α. Enhanced bone resorption is a recurrent histological feature of FD and a major cause of fragility of affected bones. Previous work suggests that increased bone resorption in FD is driven by RANKL and some studies have shown that the anti-RANKL monoclonal antibody, denosumab, reduces bone turnover and bone pain in FD patients. However, the effect of RANKL inhibition on the histopathology of FD and its impact on the natural history of the disease remain to be assessed. In this study, we treated the EF1α-Gs αR201C mice, which develop an FD-like phenotype, with an anti-mouse RANKL monoclonal antibody. We found that the treatment induced marked radiographic and microscopic changes at affected skeletal sites in 2-month-old mice. The involved skeletal segments became sclerotic due to the deposition of new, highly mineralized bone within developing FD lesions and showed a higher mechanical resistance compared to affected segments from untreated transgenic mice. Similar changes were also detected in older mice with a full-blown skeletal phenotype. The administration of anti-mouse RANKL antibody arrested the growth of established lesions and, in young mice, prevented the appearance of new ones. However, after drug withdrawal, the newly formed bone was remodelled into FD tissue and the disease progression resumed in young mice. Taken together, our results show that the anti-RANKL antibody significantly affected the bone pathology and natural history of FD in the mouse. Pending further work on the prevention and management of relapse after treatment discontinuation, our preclinical study suggests that RANKL inhibition may be an effective therapeutic option for FD patients. © 2019 American Society for Bone and Mineral Research.

Clonal Analysis Delineates Transcriptional Programs of Osteogenic and Adipogenic Lineages of Adult Mouse Skeletal Progenitors.

Bone, cartilage, and marrow adipocytes are generated by skeletal progenitors, but the relationships between lineages and mechanisms controlling their differentiation are poorly understood. We established mouse clonal skeletal progenitors with distinct differentiation properties and analyzed their transcriptome. Unipotent osteogenic and adipogenic cells expressed specific transcriptional programs, whereas bipotent clones combined expression of those genes and did not show a unique signature. We tested potential regulators of lineage commitment and found that in the presence of interferon-γ (IFNγ) adipogenic clones can be induced to osteogenesis and that their adipogenic capacity is inhibited. Analysis of IFNγ-regulated genes showed that lineage signatures and fate commitment of skeletal progenitors were controlled by EGR1 and EGR2. Knockdown experiments revealed that EGR1 is a positive regulator of the adipogenic transcriptional program and differentiation capacity, whereas EGR2 inhibits the osteogenic program and potency. Therefore, our work revealed transcriptional signatures of osteogenic and adipogenic lineages and mechanism triggering cell fate.