Transcriptomic Signature and Pro-Osteoclastic Secreted Factors of Abnormal Bone-Marrow Stromal Cells in Fibrous Dysplasia.

Fibrous dysplasia (FD) is a mosaic skeletal disorder caused by somatic activating variants of GNAS encoding for Gαs and leading to excessive cyclic adenosine monophosphate signaling in bone-marrow stromal cells (BMSCs). The effect of Gαs activation in the BMSC transcriptome and how it influences FD lesion microenvironment are unclear. We analyzed changes induced by Gαs activation in the BMSC transcriptome and secretome. RNAseq analysis of differential gene expression of cultured BMSCs from patients with FD and healthy volunteers, and from an inducible mouse model of FD, was performed, and the transcriptomic profiles of both models were combined to build a robust FD BMSC genetic signature. Pathways related to Gαs activation, cytokine signaling, and extracellular matrix deposition were identified. To assess the modulation of several key secreted factors in FD pathogenesis, cytokines and other factors were measured in culture media. Cytokines were also screened in a collection of plasma samples from patients with FD, and positive correlations of several cytokines to their disease burden score, as well as to one another and bone turnover markers, were found. These data support the pro-inflammatory, pro-osteoclastic behavior of FD BMSCs and point to several cytokines and other secreted factors as possible therapeutic targets and/or circulating biomarkers for FD.

Loss of the glycosyltransferase Galnt11 affects vitamin D homeostasis and bone composition.

O-glycosylation is a conserved posttranslational modification that impacts many aspects of organismal viability and function. Recent studies examining the glycosyltransferase Galnt11 demonstrated that it glycosylates the endocytic receptor megalin in the kidneys, enabling proper binding and reabsorption of ligands, including vitamin D-binding protein (DBP). Galnt11-deficient mice were unable to properly reabsorb DBP from the urine. Vitamin D plays an essential role in mineral homeostasis and its deficiency is associated with bone diseases such as rickets, osteomalacia, and osteoporosis. We therefore set out to examine the effects of the loss of Galnt11 on vitamin D homeostasis and bone composition. We found significantly decreased levels of serum 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D, consistent with decreased reabsorption of DBP. This was accompanied by a significant reduction in blood calcium levels and a physiologic increase in parathyroid hormone (PTH) in Galnt11-deficient mice. Bones in Galnt11-deficient mice were smaller and displayed a decrease in cortical bone accompanied by an increase in trabecular bone and an increase in a marker of bone formation, consistent with PTH-mediated effects on bone. These results support a unified model for the role of Galnt11 in bone and mineral homeostasis, wherein loss of Galnt11 leads to decreased reabsorption of DBP by megalin, resulting in a cascade of disrupted mineral and bone homeostasis including decreased circulating vitamin D and calcium levels, a physiological increase in PTH, an overall loss of cortical bone, and an increase in trabecular bone. Our study elucidates how defects in O-glycosylation can influence vitamin D and mineral homeostasis and the integrity of the skeletal system.

Cell surface-bound La protein regulates the cell fusion stage of osteoclastogenesis.

Multinucleated osteoclasts, essential for skeletal remodeling in health and disease, are formed by the fusion of osteoclast precursors, where each fusion event raises their bone-resorbing activity. Here we show that the nuclear RNA chaperone, La protein has an additional function as an osteoclast fusion regulator. Monocyte-to-osteoclast differentiation starts with a drastic decrease in La levels. As fusion begins, La reappears as a low molecular weight species at the osteoclast surface, where it promotes fusion. La's role in promoting osteoclast fusion is independent of canonical La-RNA interactions and involves direct interactions between La and Annexin A5, which anchors La to transiently exposed phosphatidylserine at the surface of fusing osteoclasts. Disappearance of cell-surface La, and the return of full length La to the nuclei of mature, multinucleated osteoclasts, acts as an off switch of their fusion activity. Targeting surface La in a novel explant model of fibrous dysplasia inhibits excessive osteoclast formation characteristic of this disease, highlighting La's potential as a therapeutic target.

Fibrous dysplasia animal models: A systematic review.

BACKGROUND: Fibrous dysplasia (FD) is a rare genetic bone disorder resulting in an overproduction of cAMP leading to a structurally unsound tissue, caused by a genetic mutation in the guanine nucleotide-binding protein gene (GNAS). In order to better understand this disease, several animal models have been developed with different strategies and features. OBJECTIVE: Conduct a systematic review to analyze and compare animal models with the causative mutation and features of FD. METHODS: A PRISMA search was conducted in Scopus, PubMed, and Web of Science. Studies reporting an in vivo model of FD that expressed the causative mutation were included for analysis. Models without the causative mutation, but developed an FD phenotype and models of FD cell implantation were included for subanalysis. RESULTS: Seven unique models were identified. The models were assessed and compared for their face validity, construct validity, mosaicism, and induction methods. This was based on the features of clinical FD that were reported within the categories of: macroscopic features, imaging, histology and histomorphometry, histochemical and cellular markers, and blood/urine markers. LIMITATIONS: None of the models reported all features of FD and some features were only reported in one model. This made comparing models a challenge, but indicates areas where further research is necessary. CONCLUSION: The benefits and disadvantages of every model were assessed from a practical and scientific standpoint. While all published reports lacked complete data, the models have nonetheless informed our understanding of FD and provided meaningful information to guide researchers in bench and clinical research.

Secreted frizzled related-protein 2 (Sfrp2) deficiency decreases adult skeletal stem cell function in mice.

In a previous transcriptomic study of human bone marrow stromal cells (BMSCs, also known as bone marrow-derived "mesenchymal stem cells"), SFRP2 was highly over-represented in a subset of multipotent BMSCs (skeletal stem cells, SSCs), which recreate a bone/marrow organ in an in vivo ectopic bone formation assay. SFRPs modulate WNT signaling, which is essential to maintain skeletal homeostasis, but the specific role of SFRP2 in BMSCs/SSCs is unclear. Here, we evaluated Sfrp2 deficiency on BMSC/SSC function in models of skeletal organogenesis and regeneration. The skeleton of Sfrp2-deficient (KO) mice is overtly normal; but their BMSCs/SSCs exhibit reduced colony-forming efficiency, reflecting low SSC self-renewal/abundancy. Sfrp2 KO BMSCs/SSCs formed less trabecular bone than those from WT littermates in the ectopic bone formation assay. Moreover, regeneration of a cortical drilled hole defect was dramatically impaired in Sfrp2 KO mice. Sfrp2-deficient BMSCs/SSCs exhibited poor in vitro osteogenic differentiation as measured by Runx2 and Osterix expression and calcium accumulation. Interestingly, activation of the Wnt co-receptor, Lrp6, and expression of Wnt target genes, Axin2, C-myc and Cyclin D1, were reduced in Sfrp2-deficient BMSCs/SSCs. Addition of recombinant Sfrp2 restored most of these activities, suggesting that Sfrp2 acts as a Wnt agonist. We demonstrate that Sfrp2 plays a role in self-renewal of SSCs and in the recruitment and differentiation of adult SSCs during bone healing. SFRP2 is also a useful marker of BMSC/SSC multipotency, and a factor to potentially improve the quality of ex vivo expanded BMSC/SSC products.

Novel Majeed Syndrome-Causing LPIN2 Mutations Link Bone Inflammation to Inflammatory M2 Macrophages and Accelerated Osteoclastogenesis.

OBJECTIVE: To identify novel heterozygous LPIN2 mutations in a patient with Majeed syndrome and characterize the pathomechanisms that lead to the development of sterile osteomyelitis. METHODS: Targeted genetic analysis and functional studies assessing monocyte responses, macrophage differentiation, and osteoclastogenesis were conducted to compare the pathogenesis of Majeed syndrome to interleukin-1 (IL-1)-mediated diseases including neonatal-onset multisystem inflammatory disease (NOMID) and deficiency of the IL-1 receptor antagonist (DIRA). RESULTS: A 4-year-old girl of mixed ethnic background presented with sterile osteomyelitis and elevated acute-phase reactants. She had a 17.8-kb deletion on the maternal LPIN2 allele and a splice site mutation, p.R517H, that variably spliced out exons 10 and 11 on the paternal LPIN2 allele. The patient achieved long-lasting remission receiving IL-1 blockade with canakinumab. Compared to controls, monocytes and monocyte-derived M1-like macrophages from the patient with Majeed syndrome and those with NOMID or DIRA had elevated caspase 1 activity and IL-1ß secretion. In contrast, lipopolysaccharide-stimulated, monocyte-derived, M2-like macrophages from the patient with Majeed syndrome released higher levels of osteoclastogenic mediators (IL-8, IL-6, tumor necrosis factor, CCL2, macrophage inflammatory protein 1α/ß, CXCL8, and CXCL1) compared to NOMID patients and healthy controls. Accelerated osteoclastogenesis in the patient with Majeed syndrome was associated with higher NFATc1 levels, enhanced JNK/MAPK, and reduced Src kinase activation, and partially responded to JNK inhibition and IL-1 (but not IL-6) blockade. CONCLUSION: We report 2 novel compound heterozygous disease-causing mutations in LPIN2 in an American patient with Majeed syndrome. LPIN2 deficiency drives differentiation of proinflammatory M2-like macrophages and enhances intrinsic osteoclastogenesis. This provides a model for the pathogenesis of sterile osteomyelitis which differentiates Majeed syndrome from other IL-1-mediated autoinflammatory diseases.

Erythropoietin modulates bone marrow stromal cell differentiation.

Erythropoietin is essential for bone marrow erythropoiesis and erythropoietin receptor on non-erythroid cells including bone marrow stromal cells suggests systemic effects of erythropoietin. Tg6 mice with chronic erythropoietin overexpression have a high hematocrit, reduced trabecular and cortical bone and bone marrow adipocytes, and decreased bone morphogenic protein 2 driven ectopic bone and adipocyte formation. Erythropoietin treatment (1 200 IU·kg-1) for 10 days similarly exhibit increased hematocrit, reduced bone and bone marrow adipocytes without increased osteoclasts, and reduced bone morphogenic protein signaling in the bone marrow. Interestingly, endogenous erythropoietin is required for normal differentiation of bone marrow stromal cells to osteoblasts and bone marrow adipocytes. ΔEpoRE mice with erythroid restricted erythropoietin receptor exhibit reduced trabecular bone, increased bone marrow adipocytes, and decreased bone morphogenic protein 2 ectopic bone formation. Erythropoietin treated ΔEpoRE mice achieved hematocrit similar to wild-type mice without reduced bone, suggesting that bone reduction with erythropoietin treatment is associated with non-erythropoietic erythropoietin response. Bone marrow stromal cells from wild-type, Tg6, and ΔEpoRE-mice were transplanted into immunodeficient mice to assess development into a bone/marrow organ. Like endogenous bone formation, Tg6 bone marrow cells exhibited reduced differentiation to bone and adipocytes indicating that high erythropoietin inhibits osteogenesis and adipogenesis, while ΔEpoRE bone marrow cells formed ectopic bones with reduced trabecular regions and increased adipocytes, indicating that loss of erythropoietin signaling favors adipogenesis at the expense of osteogenesis. In summary, endogenous erythropoietin signaling regulates bone marrow stromal cell fate and aberrant erythropoietin levels result in their impaired differentiation.

Age-Related Changes and Effects of Bisphosphonates on Bone Turnover and Disease Progression in Fibrous Dysplasia of Bone.

Fibrous dysplasia (FD) is a mosaic disease in which bone is replaced with fibro-osseous tissue. Lesions expand during childhood, reaching final burden by age 15 years. In vitro data suggest that disease activity decreases in adulthood; however, there is no clinical data to support this concept. Bone turnover markers (BTMs) have been used as markers of disease activity in FD; however, the natural history of BTM changes, the effects of antiresorptive treatment, and their association to clinical outcomes have not been described. The goals of this study are to describe 1) the natural history of FD disease activity and its association with pain; 2) the impact of bisphosphonates on the natural history of BTMs; and 3) the effect of bisphosphonates on progression of FD burden during childhood. Disease burden scores and alkaline phosphatase, osteocalcin, NTx, FGF23, and RANKL levels from 178 subjects in an FD/MAS natural history study were reviewed, including 73 subjects treated with bisphosphonates. BTMs, RANKL, and FGF23 demonstrated a sustained reduction with age. Bisphosphonate treatment did not significantly impact this age-dependent decrease in BTMs. Pain was more prevalent and severe in adults compared with children and was not associated with BTMs. In children, the progression of disease burden was not affected by bisphosphonates. In conclusion, FD is associated with an age-dependent decline in bone turnover and other markers of disease activity. Pain, in contrast, is more frequent and severe in adults with FD and is not related to bone turnover. Bisphosphonate treatment does not significantly impact the age-dependent decrease in bone turnover, nor does it prevent the progression of FD disease burden in children. These findings, in association with the established adverse effects of antiresorptives, should be considered when evaluating use and response to bisphosphonates in patients being treated for FD and in any study using BTMs as surrogate endpoints. © 2019 American Society for Bone and Mineral Research.

Skeletal Consequences of Nephropathic Cystinosis.

Nephropathic cystinosis is a rare lysosomal storage disorder. Patients present in the first year of life with renal Fanconi syndrome that evolves to progressive chronic kidney disease (CKD). Despite the multiple risk factors for bone disease, the frequency and severity of skeletal disorders in nephropathic cystinosis have not been described. We performed systematic bone and mineral evaluations of subjects with cystinosis seen at the NIH (n = 30), including history and physical examination, serum and urine biochemistries, DXA, vertebral fracture assessment, skeletal radiographs, and renal ultrasound. Additionally, histomorphometric analyses are reported on six subjects seen at the UCLA Bone and Mineral Metabolism Clinic. In NIH subjects, mean age was 20 years (range, 5 to 44 years), 60% were CKD stages G1 to G4, and 40% had a renal transplant. Mean bone mineral density (BMD) Z-scores were decreased in the femoral neck, total hip, and 1/3 radius (p < 0.05). Low bone mass at one or more sites was present in 46% of subjects. Twenty-seven percent of subjects reported one or more long bone fractures. Thirty-two percent of subjects had incidental vertebral fractures, which were unrelated to transplant status. Long-bone deformity/bowing was present in 64%; 50% had scoliosis. Diffuse osteosclerosis was present in 21% of evaluated subjects. Risk factors included CKD, phosphate wasting, hypercalciuria, secondary hyperparathyroidism, hypovitaminosis D, male hypogonadism, metabolic acidosis, and glucocorticoid/immunosuppressive therapy. Sixty-one percent of the non-transplanted subjects had ultrasonographic evidence of nephrocalcinosis or nephrolithiasis. Histomorphometric analyses showed impaired mineralization in four of six studied subjects. We conclude that skeletal deformities, decreased bone mass, and vertebral fractures are common and relevant complications of nephropathic cystinosis, even before renal transplantation. Efforts to minimize risk factors for skeletal disease include optimizing mineral metabolism and hormonal status, combined with monitoring for nephrocalcinosis/nephrolithiasis. © 2018 This article is a U.S. Government work and is in the public domain in the USA.

Expression of an active Gαs mutant in skeletal stem cells is sufficient and necessary for fibrous dysplasia initiation and maintenance.

Fibrous dysplasia (FD) is a disease caused by postzygotic activating mutations of GNAS (R201C and R201H) that encode the α-subunit of the Gs stimulatory protein. FD is characterized by the development of areas of abnormal fibroosseous tissue in the bones, resulting in skeletal deformities, fractures, and pain. Despite the well-defined genetic alterations underlying FD, whether GNAS activation is sufficient for FD initiation and the molecular and cellular consequences of GNAS mutations remains largely unresolved, and there are no currently available targeted therapeutic options for FD. Here, we have developed a conditional tetracycline (Tet)-inducible animal model expressing the GαsR201C in the skeletal stem cell (SSC) lineage (Tet-GαsR201C/Prrx1-Cre/LSL-rtTA-IRES-GFP mice), which develops typical FD bone lesions in both embryos and adult mice in less than 2 weeks following doxycycline (Dox) administration. Conditional GαsR201C expression promoted PKA activation and proliferation of SSCs along the osteogenic lineage but halted their differentiation to mature osteoblasts. Rather, as is seen clinically, areas of woven bone admixed with fibrous tissue were formed. GαsR201C caused the concomitant expression of receptor activator of nuclear factor kappa-B ligand (Rankl) that led to marked osteoclastogenesis and bone resorption. GαsR201C expression ablation by Dox withdrawal resulted in FD-like lesion regression, supporting the rationale for Gαs-targeted drugs to attempt FD cure. This model, which develops FD-like lesions that can form rapidly and revert on cessation of mutant Gαs expression, provides an opportunity to identify the molecular mechanism underlying FD initiation and progression and accelerate the development of new treatment options.

Comparison of the skeletal effects induced by daily administration of PTHrP (1-36) and PTHrP (107-139) to ovariectomized mice.

We here compared the changes induced by subcutaneous injection of PTHrP (1-36) or PTHrP (107-139) (80 µg/kg/day, 5 days/week for 4 or 8 weeks) in bone histology and bone remodeling factors, and in bone marrow cells (BMCs) ex vivo, in ovariectomized (OVX) mice. We also examined the osteogenic effects of these peptides in mouse mesenchymal C3H10T1/2 cells under oxidative stress condition in vitro, which recapitulates the effects of OVX. We confirmed that PTHrP (1-36) exerts bone anabolic actions, as assessed by bone histology and osteoblast differentiation markers in the long bones and plasma from OVX mice. PTHrP (107-139) was also efficient in stimulating several bone formation parameters, and it dramatically decreased bone resorption markers. Moreover, both PTHrP peptides modulate DKK-1 and Sost/sclerostin in osteoblast-like UMR-106 cells highly expressing these Wnt pathway inhibitors, related to their osteogenic action in this in vivo scenario. Administration of either PTHrP peptide improved osteogenic differentiation in BMCs from OVX mice ex vivo and in mouse mesenchymal C3H10T1/2 cells under oxidative stress condition in vitro. These data demonstrate that PTHrP (1-36) and PTHrP (107-139) can exert similar osteogenic effects in the appendicular skeleton of OVX mice. Our results suggest that these effects might occur in part by modulating the Wnt pathway. These findings lend credence to the notion that the osteogenic action of PTHrP (107-139) is likely a consequence of its anti-resorptive and anabolic features, and further support the usefulness of PTHrP (1-36) as a bone anabolic peptide in the setting of estrogen-depletion.