Sclerostin inhibition reverses skeletal fragility in an Lrp5-deficient mouse model of OPPG syndrome

Affinity targeting of therapeutic proteins to the bone surface-local delivery of sclerostin-neutralizing antibody enhances efficacy

Currently available biotherapeutics for the treatment of osteoporosis lack explicit mechanisms for bone localization, potentially limiting efficacy and inducing off-target toxicities. While various strategies have been explored for targeting the bone surface, critical aspects remain poorly understood, including the optimal affinity ligand, the role of binding avidity and circulation time, and, most importantly, whether or not this strategy can enhance the functional activity of clinically relevant protein therapeutics. To investigate, we generated fluorescent proteins (eg, mCherry) with site-specifically attached small molecule (bisphosphonate) or peptide (deca-aspartate, D10) affinity ligands. While both affinity ligands successfully anchored fluorescent protein to the bone surface, quantitative radiotracing revealed only modest femoral and vertebral accumulation and suggested a need for enhanced circulation time. To achieve this, we fused mCherry to the Fc fragment of human IgG1 and attached D10 peptides to each C-terminus. The mCherry-Fc-D10 demonstrated an ~80-fold increase in plasma exposure and marked increases in femoral and vertebral accumulation (13.6% ± 1.4% and 11.4% ± 1.3% of the injected dose/g [%ID/g] at 24 h, respectively). To determine if bone surface targeting could enhance the efficacy of a clinically relevant therapeutic, we generated a bone-targeted sclerostin-neutralizing antibody, anti-sclerostin-D10. The targeted antibody demonstrated marked increases in bone accumulation and retention (20.9 ± 2.5% and 19.5 ± 2.5% ID/g in femur and vertebrae at 7 days) and enhanced effects in a murine model of ovariectomy-induced bone loss (bone volume/total volume, connectivity density, and structure model index all increased [P < .001] vs untargeted anti-sclerostin). Collectively, our results indicate the importance of both bone affinity and circulation time in achieving robust targeting of therapeutic proteins to the bone surface and suggest that this approach may enable lower doses and/or longer dosing intervals without reduction in biotherapeutic efficacy. Future studies will be needed to determine the translational potential of this strategy and its potential impact on off-site toxicities.

Deletion of the auxiliary α2δ1 voltage sensitive calcium channel subunit in osteocytes and late-stage osteoblasts impairs femur strength and load-induced bone formation in male mice

Osteocytes sense and respond to mechanical force by controlling the activity of other bone cells. However, the mechanisms by which osteocytes sense mechanical input and transmit biological signals remain unclear. Voltage-sensitive calcium channels (VSCCs) regulate calcium (Ca2+) influx in response to external stimuli. Inhibition or deletion of VSCCs impairs osteogenesis and skeletal responses to mechanical loading. VSCC activity is influenced by its auxiliary subunits, which bind the channel's α1 pore-forming subunit to alter intracellular Ca2+ concentrations. The α2δ1 auxiliary subunit associates with the pore-forming subunit via a glycosylphosphatidylinositol anchor and regulates the channel's calcium-gating kinetics. Knockdown of α2δ1 in osteocytes impairs responses to membrane stretch, and global deletion of α2δ1 in mice results in osteopenia and impaired skeletal responses to loading in vivo. Therefore, we hypothesized that the α2δ1 subunit functions as a mechanotransducer, and its deletion in osteocytes would impair skeletal development and load-induced bone formation. Mice (C57BL/6) with LoxP sequences flanking Cacna2d1, the gene encoding α2δ1, were crossed with mice expressing Cre under the control of the Dmp1 promoter (10 kb). Deletion of α2δ1 in osteocytes and late-stage osteoblasts decreased femoral bone quantity (P < .05) by DXA, reduced relative osteoid surface (P < .05), and altered osteoblast and osteocyte regulatory gene expression (P < .01). Cacna2d1f/f, Cre + male mice displayed decreased femoral strength and lower 10-wk cancellous bone in vivo micro-computed tomography measurements at the proximal tibia (P < .01) compared to controls, whereas Cacna2d1f/f, Cre + female mice showed impaired 20-wk cancellous and cortical bone ex vivo micro-computed tomography measurements (P < .05) vs controls. Deletion of α2δ1 in osteocytes and late-stage osteoblasts suppressed load-induced calcium signaling in vivo and decreased anabolic responses to mechanical loading in male mice, demonstrating decreased mechanosensitivity. Collectively, the α2δ1 auxiliary subunit is essential for the regulation of osteoid-formation, femur strength, and load-induced bone formation in male mice.

Severe Osteoporosis With Pathogenic LRP5 Variant

A 24-year-old female patient was diagnosed with osteoporosis after presenting with numerous fractures throughout her childhood and adolescence. Risk factors included chronic constipation, severe vitamin D deficiency, and long-term high-dose steroid use for severe eczema. Metabolic bone disorder clinical exome screening (limited panel of metabolic bone disorders and gastrointestinal disorders) was undertaken and revealed a class 4 likely pathogenic variant in the LRP5 gene known to cause osteoporosis. Optimal treatment for patients with this variant is not well defined. A literature review of the condition and potential treatment options is discussed.

Loss of the auxiliary α2δ1 voltage-sensitive calcium channel subunit impairs bone formation and anabolic responses to mechanical loading

Voltage-sensitive calcium channels (VSCCs) influence bone structure and function, including anabolic responses to mechanical loading. While the pore-forming (α1) subunit of VSCCs allows Ca2+ influx, auxiliary subunits regulate the biophysical properties of the pore. The α2δ1 subunit influences gating kinetics of the α1 pore and enables mechanically induced signaling in osteocytes; however, the skeletal function of α2δ1 in vivo remains unknown. In this work, we examined the skeletal consequences of deleting Cacna2d1, the gene encoding α2δ1. Dual-energy X-ray absorptiometry and microcomputed tomography imaging demonstrated that deletion of α2δ1 diminished bone mineral content and density in both male and female C57BL/6 mice. Structural differences manifested in both trabecular and cortical bone for males, while the absence of α2δ1 affected only cortical bone in female mice. Deletion of α2δ1 impaired skeletal mechanical properties in both sexes, as measured by three-point bending to failure. While no changes in osteoblast number or activity were found for either sex, male mice displayed a significant increase in osteoclast number, accompanied by increased eroded bone surface and upregulation of genes that regulate osteoclast differentiation. Deletion of α2δ1 also rendered the skeleton insensitive to exogenous mechanical loading in males. While previous work demonstrates that VSCCs are essential for anabolic responses to mechanical loading, the mechanism by which these channels sense and respond to force remained unclear. Our data demonstrate that the α2δ1 auxiliary VSCC subunit functions to maintain baseline bone mass and strength through regulation of osteoclast activity and also provides skeletal mechanotransduction in male mice. These data reveal a molecular player in our understanding of the mechanisms by which VSCCs influence skeletal adaptation.

Multimodal spatiotemporal transcriptomic resolution of embryonic palate osteogenesis

The terminal differentiation of osteoblasts and subsequent formation of bone marks an important phase in palate development that leads to the separation of the oral and nasal cavities. While the morphogenetic events preceding palatal osteogenesis are well explored, major gaps remain in our understanding of the molecular mechanisms driving the formation of this bony union of the fusing palate. Through bulk, single-nucleus, and spatially resolved RNA-sequencing analyses of the developing secondary palate, we identify a shift in transcriptional programming between embryonic days 14.5 and 15.5 pinpointing the onset of osteogenesis. We define spatially restricted expression patterns of key osteogenic marker genes that are differentially expressed between these developmental timepoints. Finally, we identify genes in the palate highly expressed by palate nasal epithelial cells, also enriched within palatal osteogenic mesenchymal cells. This investigation provides a relevant framework to advance palate-specific diagnostic and therapeutic biomarker discovery.

Integrated spatiotemporal transcriptomic resolution of embryonic palate osteogenesis

The differentiation of osteoblasts and the subsequent formation of bone marks an important terminal phase in palate formation that leads to the separation of the oral and nasal cavities. While the developmental events that precede palatal osteogenesis are well explored, major gaps remain in our understanding of the molecular mechanisms that lead to the bony union of fusing palatal shelves. Herein, the timeline of osteogenic transcriptional programming is unveiled in the embryonic palate by way of integrated bulk, single-cell, and spatially resolved RNA-seq analyses. We define spatially restricted expression patterns of key marker genes, both regulatory and structural, that are differentially expressed during palatal fusion, including the identification of several novel genes ( Deup1, Dynlrb2, Lrrc23 ) spatially restricted in expression to the palate, providing a relevant framework for future studies that identify new candidate genes for cleft palate anomalies in humans as well as the timing of mammalian embryonic palatal osteogenesis.

Synthetic Multivalent Disulfide-Constrained Peptide Agonists Potentiate Wnt1/β-Catenin Signaling via LRP6 Coreceptor Clustering

Wnt ligands are critical for tissue homeostasis and form a complex with LRP6 and frizzled coreceptors to initiate Wnt/β-catenin signaling. Yet, how different Wnts achieve various levels of signaling activation through distinct domains on LRP6 remains elusive. Developing tool ligands that target individual LRP6 domains could help elucidate the mechanism of Wnt signaling regulation and uncover pharmacological approaches for pathway modulation. We employed directed evolution of a disulfide constrained peptide (DCP) to identify molecules that bind to the third β-propeller domain of LRP6. The DCPs antagonize Wnt3a while sparing Wnt1 signaling. Using PEG linkers with different geometries, we converted the Wnt3a antagonist DCPs to multivalent molecules that potentiated Wnt1 signaling by clustering the LRP6 coreceptor. The mechanism of potentiation is unique as it occurred only in the presence of extracellular secreted Wnt1 ligand. While all DCPs recognized a similar binding interface on LRP6, they displayed different spatial orientations that influenced their cellular activities. Moreover, structural analyses revealed that the DCPs exhibited new folds that were distinct from the parent DCP framework they were evolved from. The multivalent ligand design principles highlighted in this study provide a path for developing peptide agonists that modulate different branches of cellular Wnt signaling.

Role of Wnt signaling and sclerostin in bone and as therapeutic targets in skeletal disorders

Wnt signaling and its bone tissue-specific inhibitor sclerostin are key regulators of bone homeostasis. The therapeutic potential of anti-sclerostin antibodies (Scl-Abs), for bone mass recovery and fragility fracture prevention in low bone mass phenotypes, has been supported by animal studies. The Scl-Ab romosozumab is currently used for osteoporosis treatment.

INTRODUCTION

Wnt signaling is a key regulator of skeletal development and homeostasis; germinal mutations affecting genes encoding components, inhibitors, and enhancers of the Wnt pathways were shown to be responsible for the development of rare congenital metabolic bone disorders. Sclerostin is a bone tissue-specific inhibitor of the Wnt/β-catenin pathway, secreted by osteocytes, negatively regulating osteogenic differentiation and bone formation, and promoting osteoclastogenesis and bone resorption.

PURPOSE AND METHODS

Here, we reviewed current knowledge on the role of sclerostin and Wnt pathways in bone metabolism and skeletal disorders, and on the state of the art of therapy with sclerostin-neutralizing antibodies in low-bone-mass diseases.

RESULTS

Various in vivo studies on animal models of human low-bone-mass diseases showed that targeting sclerostin to recover bone mass, restore bone strength, and prevent fragility fracture was safe and effective in osteoporosis, osteogenesis imperfecta, and osteoporosis pseudoglioma. Currently, only treatment with romosozumab, a humanized monoclonal anti-sclerostin antibody, has been approved in human clinical practice for the treatment of osteoporosis, showing a valuable capability to increase BMD at various skeletal sites and reduce the occurrence of new vertebral, non-vertebral, and hip fragility fractures in treated male and female osteoporotic patients.

CONCLUSIONS

Preclinical studies demonstrated safety and efficacy of therapy with anti-sclerostin monoclonal antibodies in the preservation/restoration of bone mass and prevention of fragility fractures in low-bone-mass clinical phenotypes, other than osteoporosis, to be validated by clinical studies for their approved translation into prevalent clinical practice.

Got WNTS? Insight into bone health from a WNT perspective

WNT signaling, essential for many aspects of development, is among the most commonly altered pathways associated with human disease. While initially studied in cancer, dysregulation of WNT signaling has been determined to be essential for skeletal development and the maintenance of bone health throughout life. In this review, we discuss the role of Wnt signaling in bone development and disease with a particular focus on two areas. First, we discuss the roles of WNT signaling pathways in skeletal development, with an emphasis on congenital and idiopathic skeletal syndromes and diseases that are associated with genetic variations in WNT signaling components. Next, we cover a topic that has long been an interest of our laboratory, how high and low levels of WNT signaling affects the establishment and maintenance of healthy bone mass. We conclude with a discussion of the status of WNT-based therapeutics in the treatment of skeletal disease.

Single cell cortical bone transcriptomics define novel osteolineage gene sets altered in chronic kidney disease

INTRODUCTION

Due to a lack of spatial-temporal resolution at the single cell level, the etiologies of the bone dysfunction caused by diseases such as normal aging, osteoporosis, and the metabolic bone disease associated with chronic kidney disease (CKD) remain largely unknown.

METHODS

To this end, flow cytometry and scRNAseq were performed on long bone cells from Sost-cre/Ai9⁺ mice, and pure osteolineage transcriptomes were identified, including novel osteocyte-specific gene sets.

RESULTS

Clustering analysis isolated osteoblast precursors that expressed Tnc, Mmp13, and Spp1, and a mature osteoblast population defined by Smpd3, Col1a1, and Col11a1. Osteocytes were demarcated by Cd109, Ptprz1, Ramp1, Bambi, Adamts14, Spns2, Bmp2, WasI, and Phex. We validated our in vivo scRNAseq using integrative in vitro promoter occupancy via ATACseq coupled with transcriptomic analyses of a conditional, temporally differentiated MSC cell line. Further, trajectory analyses predicted osteoblast-to-osteocyte transitions via defined pathways associated with a distinct metabolic shift as determined by single-cell flux estimation analysis (scFEA). Using the adenine mouse model of CKD, at a time point prior to major skeletal alterations, we found that gene expression within all stages of the osteolineage was disturbed.

CONCLUSION

In sum, distinct populations of osteoblasts/osteocytes were defined at the single cell level. Using this roadmap of gene assembly, we demonstrated unrealized molecular defects across multiple bone cell populations in a mouse model of CKD, and our collective results suggest a potentially earlier and more broad bone pathology in this disease than previously recognized.

Notum Deletion From Late-Stage Skeletal Cells Increases Cortical Bone Formation and Potentiates Skeletal Effects of Sclerostin Inhibition

Wnt signaling plays a vital role in the cell biology of skeletal patterning, differentiation, and maintenance. Notum is a secreted member of the α/β-hydrolase superfamily that hydrolyzes the palmitoleoylate modification on Wnt proteins, thereby disrupting Wnt signaling. As a secreted inhibitor of Wnt, Notum presents an attractive molecular target for improving skeletal health. To determine the cell type of action for Notum's effect on the skeleton, we generated mice with Notum deficiency globally (Notum-/- ) and selectively (Notumf/f ) in limb bud mesenchyme (Prx1-Cre) and late osteoblasts/osteocytes (Dmp1-Cre). Late-stage deletion induced increased cortical bone properties, similar to global mutants. Notum expression was enhanced in response to sclerostin inhibition, so dual inhibition (Notum/sclerostin) was also investigated using a combined genetic and pharmacologic approach. Co-suppression increased cortical properties beyond either factor alone. Notum suppressed Wnt signaling in cell reporter assays, but surprisingly also enhanced Shh signaling independent of effects on Wnt. Notum is an osteocyte-active suppressor of cortical bone formation that is likely involved in multiple signaling pathways important for bone homeostasis © 2021 American Society for Bone and Mineral Research (ASBMR).

Sclerostin Depletion Induces Inflammation in the Bone Marrow of Mice

Romosozumab, a humanized monoclonal antibody specific for sclerostin (SOST), has been approved for treatment of postmenopausal women with osteoporosis at a high risk for fracture. Previous work in sclerostin global knockout (Sost^-/-) mice indicated alterations in immune cell development in the bone marrow (BM), which could be a possible side effect in romosozumab-treated patients. Here, we examined the effects of short-term sclerostin depletion in the BM on hematopoiesis in young mice receiving sclerostin antibody (Scl-Ab) treatment for 6 weeks, and the effects of long-term Sost deficiency on wild-type (WT) long-term hematopoietic stem cells transplanted into older cohorts of Sost^-/- mice. Our analyses revealed an increased frequency of granulocytes in the BM of Scl-Ab-treated mice and WT→Sost^-/- chimeras, indicating myeloid-biased differentiation in Sost-deficient BM microenvironments. This myeloid bias extended to extramedullary hematopoiesis in the spleen and was correlated with an increase in inflammatory cytokines TNFα, IL-1α, and MCP-1 in Sost^-/- BM serum. Additionally, we observed alterations in erythrocyte differentiation in the BM and spleen of Sost^-/- mice. Taken together, our current study indicates novel roles for Sost in the regulation of myelopoiesis and control of inflammation in the BM.

Sclerostin antibody increases trabecular bone and bone mechanical properties by increasing osteoblast activity damaged by whole-body irradiation in mice

Irradiation therapy causes bone deterioration and increased risk for skeletal-related events. Irradiation interferes with trabecular architecture through increased osteoclastic activity, decreased osteoblastic activity, and increased adipocyte expansion in the bone marrow (BM), which further compounds bone-related disease. Neutralizing antibodies to sclerostin (Scl-Ab) increase bone mass and strength by increasing bone formation and reducing bone resorption. We hypothesized that treatment with Scl-Ab would attenuate the adverse effects of irradiation by increasing bone volume and decreasing BM adipose tissue (BMAT), resulting in better quality bone. In this study, 12-week-old female C57BL/6J mice were exposed to 6 Gy whole-body irradiation or were non-irradiated, then administered Scl-Ab (25 mg/kg) or vehicle weekly for 5 weeks. Femoral μCT analysis confirmed that the overall effect of IR significantly decreased trabecular bone volume/total volume (Tb.BV/TV) (2-way ANOVA, p < 0.0001) with a -43.8% loss in Tb.BV/TV in the IR control group. Scl-Ab independently increased Tb.BV/TV by 3.07-fold in non-irradiated and 3.6-fold in irradiated mice (2-way ANOVA, p < 0.0001). Irradiation did not affect cortical parameters, although Scl-Ab increased cortical thickness and area significantly in both irradiated and non-irradiated mice (2-way ANOVA, p < 0.0001). Femoral mechanical testing confirmed Scl-Ab significantly increased bending rigidity and ultimate moment independently of irradiation (2-way ANOVA, p < 0.0001). Static and dynamic histomorphometry of the femoral metaphysis revealed osteoblast vigor, not number, was significantly increased in the irradiated mice treated with Scl-Ab. Systemic alterations were assessed through serum lipidomic analysis, which showed that Scl-Ab normalized lipid profiles in the irradiated group. This data supports the theory of sclerostin as a novel contributor to the regulation of osteoblast activity after irradiation. Overall, our data support the hypothesis that Scl-Ab ameliorates the deleterious effects of whole-body irradiation on bone and adipose tissue in a mouse model. Our findings suggest that future research into localized and systemic therapies after irradiation exposure is warranted.

Improving Bone Health by Optimizing the Anabolic Action of Wnt Inhibitor Multitargeting

Sclerostin antibody (romosozumab) was recently approved for clinical use in the United States to treat osteoporosis. We and others have explored Wnt‐based combination therapy to disproportionately improve the anabolic effects of sclerostin inhibition, including cotreatment with sclerostin antibody (Scl‐mAb) and Dkk1 antibody (Dkk1‐mAb). To determine the optimal ratio of Scl‐mAb and Dkk1‐mAb for producing maximal anabolic action, the proportion of Scl‐mAb and Dkk1‐mAb were systematically varied while holding the total antibody dose constant. A 3:1 mixture of Scl‐mAb to Dkk1‐mAb produced two to three times as much cancellous bone mass as an equivalent dose of Scl‐mAb alone. Further, a 75% reduction in the dose of the 3:1 mixture was equally efficacious to a full dose of Scl‐mAb in the distal femur metaphysis. The Scl‐mAb/Dkk1‐mAb combination approach was highly efficacious in the cancellous bone mass, but the cortical compartment was much more subtly affected. The osteoanabolic effects of Wnt pathway targeting can be made more efficient if multiple antagonists are simultaneously targeted. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC. on behalf of American Society for Bone and Mineral Research.

Osteoporosis Treatment with Anti-Sclerostin Antibodies-Mechanisms of Action and Clinical Application

Osteoporosis is characterized by reduced bone mass and disruption of bone architecture, resulting in increased risk of fragility fractures and significant long-term disability. Although both anti-resorptive treatments and osteoanabolic drugs, such as parathyroid hormone analogues, are effective in fracture prevention, limitations exist due to lack of compliance or contraindications to these drugs. Thus, there is a need for novel potent therapies, especially for patients at high fracture risk. Romosozumab is a monoclonal antibody against sclerostin with a dual mode of action. It enhances bone formation and simultaneously suppresses bone resorption, resulting in a large anabolic window. In this opinion-based narrative review, we highlight the role of sclerostin as a critical regulator of bone mass and present human diseases of sclerostin deficiency as well as preclinical models of genetically modified sclerostin expression, which led to the development of anti-sclerostin antibodies. We review clinical studies of romosozumab in terms of bone mass accrual and anti-fracture activity in the setting of postmenopausal and male osteoporosis, present sequential treatment regimens, and discuss its safety profile and possible limitations in its use. Moreover, an outlook comprising future translational applications of anti-sclerostin antibodies in diseases other than osteoporosis is given, highlighting the clinical significance and future scopes of Wnt signaling in these settings.

Co-deletion of Lrp5 and Lrp6 in the skeleton severely diminishes bone gain from sclerostin antibody administration

The cysteine knot protein sclerostin is an osteocyte-derived secreted inhibitor of the Wnt co-receptors LRP5 and LRP6. LRP5 plays a dominant role in bone homeostasis, but we previously reported that Sost/sclerostin suppression significantly increased osteogenesis regardless of Lrp5 presence or absence. Those observations suggested that the bone forming effects of sclerostin inhibition can occur through Lrp6 (when Lrp5 is suppressed), or through other yet undiscovered mechanisms independent of Lrp5/6. To distinguish between these two possibilities, we generated mice with compound deletion of Lrp5 and Lrp6 selectively in bone, and treated them with sclerostin monoclonal antibody (Scl-mAb). All mice were homozygous flox for both Lrp5 and Lrp6 (Lrp5^f/f; Lrp6^f/f), and varied only in whether or not they carried the Dmp1-Cre transgene. Positive (Cre+) and negative (Cre-) mice were injected with Scl-mAb or vehicle from 4.5 to 14 weeks of age. Vehicle-treated Cre+ mice exhibited significantly reduced skeletal properties compared to vehicle-treated Cre- mice, as assessed by DXA, μCT, pQCT, and histology, indicating that Lrp5/6 deletions were effective and efficient. Scl-mAb treatment improved nearly every bone-related parameter among Cre- mice, but the same treatment in Cre+ mice resulted in little to no improvement in skeletal properties. For the few endpoints where Cre+ mice responded to Scl-mAb, it is likely that antibody-induced promotion of Wnt signaling occurred in cell types earlier in the mesenchymal/osteoblast differentiation pathway than the Dmp1-expressing stage. This latter conclusion was supported by changes in some histomorphometric parameters. In conclusion, unlike with the deletion of Lrp5 alone, the bone-selective late-stage co-deletion of Lrp5 and Lrp6 significantly impairs or completely nullifies the osteogenic action of Scl-mAb, and highlights a major role for both Lrp5 and Lrp6 in the mechanism of action for the bone-building effects of sclerostin antibody.

Generation and Characterization of Mouse Models for Skeletal Disease

Our laboratories have used genetically engineered mouse models (GEMMs) to assess genetic contributions to skeletal diseases such as osteoporosis and osteoarthritis. Studies on the genetic contributions to OA are often done by assessing how GEMMs respond to surgical methods that induce symptoms modeling OA. Here, we will describe protocols outlining the induction of experimental OA in mice as well as detailed descriptions of methods for analyzing skeletal phenotypes using micro-computerized tomography and skeletal histomorphometry.

Sensitive detection of Cre-mediated recombination using droplet digital PCR reveals Tg(BGLAP-Cre) and Tg(DMP1-Cre) are active in multiple non-skeletal tissues

In humans, somatic activating mutations in PIK3CA are associated with skeletal overgrowth. In order to determine if activated PI3K signaling in bone cells causes overgrowth, we used Tg(BGLAP-Cre) and Tg(DMP1-Cre) mouse strains to somatically activate a disease-causing conditional Pik3ca allele (Pik3ca^H1047R) in osteoblasts and osteocytes. We observed Tg(BGLAP-Cre);Pik3ca^H1047R/+ offspring were born at the expected Mendelian frequency. However, these mice developed cutaneous lymphatic malformations and died before 7 weeks of age. In contrast, Tg(DMP1-Cre);Pik3ca^H1047R/+ offspring survived and had no cutaneous lymphatic malformations. Assuming that Cre-activity outside of the skeletal system accounted for the difference in phenotype between Tg(BGLAP-Cre);Pik3ca^H1047R/+ and Tg(DMP1-Cre);Pik3ca^H1047R/+ mice, we developed sensitive and specific droplet digital PCR (ddPCR) assays to search for and quantify rates of Tg(BGLAP-Cre)- and Tg(DMP1-Cre)-mediated recombination in non-skeletal tissues. We observed Tg(BGLAP-Cre)-mediated recombination in several tissues including skin, muscle, artery, and brain; two CNS locations, hippocampus and cerebellum, exhibited Cre-mediated recombination in >5% of cells. Tg(DMP1-Cre)-mediated recombination was also observed in muscle, artery, and brain. Although we cannot preclude that differences in phenotype between mice with Tg(BGLAP-Cre)- and Tg(DMP1-Cre)-mediated PIK3CA activation are due to Cre-recombination being induced at different stages of osteoblast differentiation, differences in recombination at non-skeletal sites are the more likely explanation. Since unanticipated sites of recombination can affect the interpretation of data from experiments involving conditional alleles, we recommend ddPCR as a good first step for assessing efficiency, leakiness, and off-targeting in experiments that employ Cre-mediated or Flp-mediated recombination.

Rheumatoid Arthritis in the View of Osteoimmunology

Rheumatoid arthritis is characterized by synovial inflammation and irreversible bone erosions, both highlighting the immense reciprocal relationship between the immune and bone systems, designed osteoimmunology two decades ago. Osteoclast-mediated resorption at the interface between synovium and bone is responsible for the articular bone erosions. The main triggers of this local bone resorption are autoantibodies directed against citrullinated proteins, as well as pro-inflammatory cytokines and the receptor activator of nuclear factor-κB ligand, that regulate both the formation and activity of the osteoclast, as well as immune cell functions. In addition, local bone loss is due to the suppression of osteoblast-mediated bone formation and repair by inflammatory cytokines. Similarly, inflammation affects systemic bone remodeling in rheumatoid arthritis with the net increase in bone resorption, leading to systemic osteoporosis. This review summarizes the substantial progress that has been made in understanding the pathophysiology of systemic and local bone loss in rheumatoid arthritis.

Single-Cell RNA Sequencing of Calvarial and Long-Bone Endocortical Cells

Single-cell RNA sequencing (scRNA-Seq) is emerging as a powerful technology to examine transcriptomes of individual cells. We determined whether scRNA-Seq could be used to detect the effect of environmental and pharmacologic perturbations on osteoblasts. We began with a commonly used in vitro system in which freshly isolated neonatal mouse calvarial cells are expanded and induced to produce a mineralized matrix. We used scRNA-Seq to compare the relative cell type abundances and the transcriptomes of freshly isolated cells to those that had been cultured for 12 days in vitro. We observed that the percentage of macrophage-like cells increased from 6% in freshly isolated calvarial cells to 34% in cultured cells. We also found that Bglap transcripts were abundant in freshly isolated osteoblasts but nearly undetectable in the cultured calvarial cells. Thus, scRNA-Seq revealed significant differences between heterogeneity of cells in vivo and in vitro. We next performed scRNA-Seq on freshly recovered long bone endocortical cells from mice that received either vehicle or sclerostin-neutralizing antibody for 1 week. We were unable to detect significant changes in bone anabolism-associated transcripts in immature and mature osteoblasts recovered from mice treated with sclerostin-neutralizing antibody; this might be a consequence of being underpowered to detect modest changes in gene expression, because only 7% of the sequenced endocortical cells were osteoblasts and a limited portion of their transcriptomes were sampled. We conclude that scRNA-Seq can detect changes in cell abundance, identity, and gene expression in skeletally derived cells. In order to detect modest changes in osteoblast gene expression at the single-cell level in the appendicular skeleton, larger numbers of osteoblasts from endocortical bone are required. © 2020 American Society for Bone and Mineral Research.

The mTORC2 Component Rictor Is Required for Load-Induced Bone Formation in Late-Stage Skeletal Cells

Bone relies on mechanical cues to build and maintain tissue composition and architecture. Our understanding of bone cell mechanotransduction continues to evolve, with a few key signaling pathways emerging as vital. Wnt/β‐catenin, for example, is essential for proper anabolic response to mechanical stimulation. One key complex that regulates β‐catenin activity is the mammalian target of rapamycin complex 2 (mTORc2). mTORc2 is critical for actin cytoskeletal reorganization, an indispensable component in mechanotransduction in certain cell types. In this study, we probed the impact of the mTORc2 signaling pathway in osteocyte mechanotransduction by conditionally deleting the mTORc2 subunit Rictor in Dmp1‐expressing cells of C57BL/6 mice. Conditional deletion of the Rictor was achieved using the Dmp1–Cre driver to recombine Rictor floxed alleles. Rictor mutants exhibited a decrease in skeletal properties, as measured by DXA, μCT, and mechanical testing, compared with Cre‐negative floxed littermate controls. in vivo axial tibia loading conducted in adult mice revealed a deficiency in the osteogenic response to loading among Rictor mutants. Histological measurements of osteocyte morphology indicated fewer, shorter cell processes in Rictor mutants, which might explain the compromised response to mechanical stimulation. In summary, inhibition of the mTORc2 pathway in late osteoblasts/osteocytes leads to decreased bone mass and mechanically induced bone formation. © 2020 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

An osteocalcin-deficient mouse strain without endocrine abnormalities

Osteocalcin (OCN), the most abundant noncollagenous protein in the bone matrix, is reported to be a bone-derived endocrine hormone with wide-ranging effects on many aspects of physiology, including glucose metabolism and male fertility. Many of these observations were made using an OCN-deficient mouse allele (Osc^–) in which the 2 OCN-encoding genes in mice, Bglap and Bglap2, were deleted in ES cells by homologous recombination. Here we describe mice with a new Bglap and Bglap2 double-knockout (dko) allele (Bglap/2^{p.Pro25fs17Ter}) that was generated by CRISPR/Cas9-mediated gene editing. Mice homozygous for this new allele do not express full-length Bglap or Bglap2 mRNA and have no immunodetectable OCN in their serum. FTIR imaging of cortical bone in these homozygous knockout animals finds alterations in the collagen maturity and carbonate to phosphate ratio in the cortical bone, compared with wild-type littermates. However, μCT and 3-point bending tests do not find differences from wild-type littermates with respect to bone mass and strength. In contrast to the previously reported OCN-deficient mice with the Osc⁻allele, serum glucose levels and male fertility in the OCN-deficient mice with the Bglap/2^{pPro25fs17Ter} allele did not have significant differences from wild-type littermates. We cannot explain the absence of endocrine effects in mice with this new knockout allele. Possible explanations include the effects of each mutated allele on the transcription of neighboring genes, or differences in genetic background and environment. So that our findings can be confirmed and extended by other interested investigators, we are donating this new Bglap and Bglap2 double-knockout strain to the Jackson Laboratories for academic distribution.

Cells that make and maintain bone express proteins that function either locally or systemically. The former proteins, such as type 1 collagen, affect the material properties of the skeleton, while the latter, such as fibroblast growth factor 23, enable the skeleton to communicate with other organ systems. Mutations that affect the functions of most bone-cell-expressed proteins cause diseases that have similar features in humans and other mammals such as mice, for example, brittle bone diseases for type 1 collagen mutations and hypophosphatemic rickets for mutations in fibroblast growth factor 23. Our study focuses on another bone-cell-expressed protein, osteocalcin, which has been suggested to function locally to affect bone strength and systemically as a hormone. Studies using osteocalcin knockout mice led other investigators to suggest endocrine roles for osteocalcin in regulating blood glucose, male fertility, muscle mass, brain development, behavior, and cognition. We therefore decided to generate a new strain of osteocalcin knockout mice that could also be used to investigate these nonskeletal effects. To our surprise, the osteocalcin knockout mice we created did not significantly differ from wild-type mice for the three phenotypes we examined: bone strength, blood glucose, and male fertility. Our data are consistent with findings from osteocalcin knockout rats but are inconsistent with data from the original osteocalcin knockout mice. Because we do not know why our new strain fails to recapitulate the phenotypes previously reported for another knockout mouse stain, we have donated our mice to a public repository so that they can be easily obtained and studied in other academic laboratories.

The evolving therapeutic landscape of genetic skeletal disorders

BACKGROUND

Rare bone diseases account for 5% of all birth defects yet very few have personalised treatments. Developments in genetic diagnosis, molecular techniques and treatment technologies however, are leading to unparalleled therapeutic advance. This review explores the evolving therapeutic landscape of genetic skeletal disorders (GSDs); the key conditions and there key differentials.

METHODS

A retrospective literature based review was conducted in December 2018 using a systematic search strategy for relevant articles and trials in Pubmed and clinicaltrials.gov respectively. Over 140 articles and 80 trials were generated for review.

RESULTS

Over 20 personalised therapies are discussed in addition to several novel disease modifying treatments in over 25 GSDs. Treatments discussed are at different stages from preclinical studies to clinical trials and approved drugs, including; Burosumab for X-linked hypophosphatemia, Palovarotene for Hereditary Multiple Exostoses, Carbamazepine for Metaphyseal Chondrodysplasia (Schmid type), Lithium carbonate and anti-sclerostin therapy for Osteoporosis Pseudoglioma syndrome and novel therapies for Osteopetrosis. We also discuss therapeutic advances in Achondroplasia, Osteogenesis Imperfecta (OI), Hypophosphotasia (HPP), Fibrodysplasia Ossificans Progressiva, and RNA silencing therapies in preclinical studies for OI and HPP.

DISCUSSION

It is an exciting time for GSD therapies despite the challenges of drug development in rare diseases. In discussing emerging therapies, we explore novel approaches to drug development from drug repurposing to in-utero stem cell transplants. We highlight the improved understanding of bone pathophysiology, genetic pathways and challenges of developing gene therapies for GSDs.

Salt-inducible kinases dictate parathyroid hormone 1 receptor action in bone development and remodeling

The parathyroid hormone 1 receptor (PTH1R) mediates the biologic actions of parathyroid hormone (PTH) and parathyroid hormone-related protein (PTHrP). Here, we showed that salt-inducible kinases (SIKs) are key kinases that control the skeletal actions downstream of PTH1R and that this GPCR, when activated, inhibited cellular SIK activity. Sik gene deletion led to phenotypic changes that were remarkably similar to models of increased PTH1R signaling. In growth plate chondrocytes, PTHrP inhibited SIK3, and ablation of this kinase in proliferating chondrocytes rescued perinatal lethality of PTHrP-null mice. Combined deletion of Sik2 and Sik3 in osteoblasts and osteocytes led to a dramatic increase in bone mass that closely resembled the skeletal and molecular phenotypes observed when these bone cells express a constitutively active PTH1R that causes Jansen's metaphyseal chondrodysplasia. Finally, genetic evidence demonstrated that class IIa histone deacetylases were key PTH1R-regulated SIK substrates in both chondrocytes and osteocytes. Taken together, our findings establish that SIK inhibition is central to PTH1R action in bone development and remodeling. Furthermore, this work highlights the key role of cAMP-regulated SIKs downstream of GPCR action.

Wnt1 is an Lrp5-independent bone-anabolic Wnt ligand

WNT1 mutations in humans are associated with a new form of osteogenesis imperfecta and with early-onset osteoporosis, suggesting a key role of WNT1 in bone mass regulation. However, the general mode of action and the therapeutic potential of Wnt1 in clinically relevant situations such as aging remain to be established. Here, we report the high prevalence of heterozygous WNT1 mutations in patients with early-onset osteoporosis. We show that inactivation of Wnt1 in osteoblasts causes severe osteoporosis and spontaneous bone fractures in mice. In contrast, conditional Wnt1 expression in osteoblasts promoted rapid bone mass increase in developing young, adult, and aged mice by rapidly increasing osteoblast numbers and function. Contrary to current mechanistic models, loss of Lrp5, the co-receptor thought to transmit extracellular WNT signals during bone mass regulation, did not reduce the bone-anabolic effect of Wnt1, providing direct evidence that Wnt1 function does not require the LRP5 co-receptor. The identification of Wnt1 as a regulator of bone formation and remodeling provides the basis for development of Wnt1-targeting drugs for the treatment of osteoporosis.

Autosomal-Recessive Mutations in MESD Cause Osteogenesis Imperfecta

Osteogenesis imperfecta (OI) comprises a genetically heterogeneous group of skeletal fragility diseases. Here, we report on five independent families with a progressively deforming type of OI, in whom we identified four homozygous truncation or frameshift mutations in MESD. Affected individuals had recurrent fractures and at least one had oligodontia. MESD encodes an endoplasmic reticulum (ER) chaperone protein for the canonical Wingless-related integration site (WNT) signaling receptors LRP5 and LRP6. Because complete absence of MESD causes embryonic lethality in mice, we hypothesized that the OI-associated mutations are hypomorphic alleles since these mutations occur downstream of the chaperone activity domain but upstream of ER-retention domain. This would be consistent with the clinical phenotypes of skeletal fragility and oligodontia in persons deficient for LRP5 and LRP6, respectively. When we expressed wild-type (WT) and mutant MESD in HEK293T cells, we detected WT MESD in cell lysate but not in conditioned medium, whereas the converse was true for mutant MESD. We observed that both WT and mutant MESD retained the ability to chaperone LRP5. Thus, OI-associated MESD mutations produce hypomorphic alleles whose failure to remain within the ER significantly reduces but does not completely eliminate LRP5 and LRP6 trafficking. Since these individuals have no eye abnormalities (which occur in individuals completely lacking LRP5) and have neither limb nor brain patterning defects (both of which occur in mice completely lacking LRP6), we infer that bone mass accrual and dental patterning are more sensitive to reduced canonical WNT signaling than are other developmental processes. Biologic agents that can increase LRP5 and LRP6-mediated WNT signaling could benefit individuals with MESD-associated OI.

Twist1 Inactivation in Dmp1-Expressing Cells Increases Bone Mass but Does Not Affect the Anabolic Response to Sclerostin Neutralization

Wnt signaling plays a major role in bone metabolism. Advances in our understanding of secreted regulators of Wnt have yielded several therapeutic targets to stimulate osteoanabolism—the most promising of which is the Wnt inhibitor sclerostin. Sclerostin antibody recently gained approval for clinical use to treat osteoporosis, but the biology surrounding sclerostin antagonism is still incompletely understood. Numerous factors regulate the efficacy of sclerostin inhibition on bone formation, a process known as self-regulation. In previous communications we reported that the basic helix-loop-helix transcription factor Twist1—a gene know to regulate skeletal development—is highly upregulated among the osteocyte cell population in mice treated with sclerostin antibody. In this communication, we tested the hypothesis that preventing Twist1 upregulation by deletion of Twist1 from late-stage osteoblasts and osteocytes would increase the efficacy of sclerostin antibody treatment, since Twist1 is known to restrain osteoblast activity in many models. Twist1-floxed loss-of-function mice were crossed to the Dmp1-Cre driver to delete Twist1 in Dmp1-expressing cells. Conditional Twist1 deletion was associated with a mild but significant increase in bone mass, as assessed by dual energy x-ray absorptiometry (DXA) and microCT (µCT) for many endpoints in both male and female mice. Biomechanical properties of the femur were not affected by conditional mutation of Twist1. Sclerostin antibody improved all bone properties significantly, regardless of Twist1 status, sex, or endpoint examined. No interactions were detected when Twist1 status and antibody treatment were examined together, suggesting that Twist1 upregulation in the osteocyte population is not an endogenous mechanism that restrains the osteoanabolic effect of sclerostin antibody treatment. In summary, Twist1 inhibition in the late-stage osteoblast/osteocyte increases bone mass but does not affect the anabolic response to sclerostin neutralization.

RNA-seq in Skeletal Biology

PURPOSE OF REVIEW

The goal of this paper is to review state-of-the-art transcriptome profiling methods and their recent applications in the field of skeletal biology.

RECENT FINDINGS

Next-generation sequencing of mRNA (RNA-seq) methods have been established and routinely used in skeletal biology research. RNA-seq has led to the identification of novel genes and transcription factors involved in skeletal development and disease, through its application in small and large animal models, as well as human tissue and cells. With the availability of advanced techniques such as single-cell RNA-seq, novel cell types in skeletal tissues are being identified. As the sequencing technologies are rapidly evolving, the exciting discoveries supported by transcriptomics will continue to emerge and improve our understanding of the biology of the skeleton.

Differential changes in bone strength of two inbred mouse strains following administration of a sclerostin-neutralizing antibody during growth

Administration of sclerostin-neutralizing antibody (Scl-Ab) treatment has been shown to elicit an anabolic bone response in growing and adult mice. Prior work characterized the response of individual mouse strains but did not establish whether the impact of Scl-Ab on whole bone strength would vary across different inbred mouse strains. Herein, we tested the hypothesis that two inbred mouse strains (A/J and C57BL/6J (B6)) will show different whole bone strength outcomes following sclerostin-neutralizing antibody (Scl-Ab) treatment during growth (4.5–8.5 weeks of age). Treated B6 femurs showed a significantly greater stiffness (S) (68.8% vs. 46.0%) and maximum load (ML) (84.7% vs. 44.8%) compared to A/J. Although treated A/J and B6 femurs showed greater cortical area (Ct.Ar) similarly relative to their controls (37.7% in A/J and 41.1% in B6), the location of new bone deposition responsible for the greater mass differed between strains and may explain the greater whole bone strength observed in treated B6 mice. A/J femurs showed periosteal expansion and endocortical infilling, while B6 femurs showed periosteal expansion. Post-yield displacement (PYD) was smaller in treated A/J femurs (-61.2%, p < 0.001) resulting in greater brittleness compared to controls; an effect not present in B6 mice. Inter-strain differences in S, ML, and PYD led to divergent changes in work-to-fracture (Work). Work was 27.2% (p = 0.366) lower in treated A/J mice and 66.2% (p < 0.001) greater in treated B6 mice relative to controls. Our data confirmed the anabolic response to Scl-Ab shown by others, and provided evidence suggesting the mechanical benefits of Scl-Ab administration may be modulated by genetic background, with intrinsic growth patterns of these mice guiding the location of new bone deposition. Whether these differential outcomes will persist in adult and elderly mice remains to be determined.

Osteoimmunology of Bone Loss in Inflammatory Rheumatic Diseases

Over the past two decades, the field of osteoimmunology has emerged in response to a range of evidence demonstrating the reciprocal relationship between the immune system and bone. In particular, localized bone loss, in the form of joint erosions and periarticular osteopenia, as well as systemic osteoporosis, caused by inflammatory rheumatic diseases including rheumatoid arthritis, the prototype of inflammatory arthritis has highlighted the importance of this interplay. Osteoclast-mediated resorption at the interface between synovium and bone is responsible for the joint erosion seen in patients suffering from inflammatory arthritis. Clinical studies have helped to validate the impact of several pathways on osteoclast formation and activity. Essentially, the expression of pro-inflammatory cytokines as well as Receptor Activator of Nuclear factor κB Ligand (RANKL) is, both directly and indirectly, increased by T cells, stimulating osteoclastogenesis and resorption through a crucial regulator of immunity, the Nuclear factor of activated T-cells, cytoplasmic 1 (NFATc1). Furthermore, in rheumatoid arthritis, autoantibodies, which are accurate predictors both of the disease and associated structural damage, have been shown to stimulate the differentiation of osteoclasts, resulting in localized bone resorption. It is now also evident that osteoblast-mediated bone formation is impaired by inflammation both in joints and the skeleton in rheumatoid arthritis. This review summarizes the substantial progress that has been made in understanding the pathophysiology of bone loss in inflammatory rheumatic disease and highlights therapeutic targets potentially important for the cure or at least an alleviation of this destructive process.

High Fidelity of Mouse Models Mimicking Human Genetic Skeletal Disorders

The 2019 International Skeletal Dysplasia Society nosology update lists 441 genes for which mutations result in rare human skeletal disorders. These genes code for enzymes (33%), scaffolding proteins (18%), signal transduction proteins (16%), transcription factors (14%), cilia proteins (8%), extracellular matrix proteins (5%), and membrane transporters (4%). Skeletal disorders include aggrecanopathies, channelopathies, ciliopathies, cohesinopathies, laminopathies, linkeropathies, lysosomal storage diseases, protein-folding and RNA splicing defects, and ribosomopathies. With the goal of evaluating the ability of mouse models to mimic these human genetic skeletal disorders, a PubMed literature search identified 260 genes for which mutant mice were examined for skeletal phenotypes. These mouse models included spontaneous and ENU-induced mutants, global and conditional gene knockouts, and transgenic mice with gene over-expression or specific base-pair substitutions. The human X-linked gene ARSE and small nuclear RNA U4ATAC, a component of the minor spliceosome, do not have mouse homologs. Mouse skeletal phenotypes mimicking human skeletal disorders were observed in 249 of the 260 genes (96%) for which comparisons are possible. A supplemental table in spreadsheet format provides PubMed weblinks to representative publications of mutant mouse skeletal phenotypes. Mutations in 11 mouse genes (Ccn6, Cyp2r1, Flna, Galns, Gna13, Lemd3, Manba, Mnx1, Nsd1, Plod1, Smarcal1) do not result in similar skeletal phenotypes observed with mutations of the homologous human genes. These discrepancies can result from failure of mouse models to mimic the exact human gene mutations. There are no obvious commonalities among these 11 genes. Body BMD and/or radiologic dysmorphology phenotypes were successfully identified for 28 genes by the International Mouse Phenotyping Consortium (IMPC). Forward genetics using ENU mouse mutagenesis successfully identified 37 nosology gene phenotypes. Since many human genetic disorders involve hypomorphic, gain-of-function, dominant-negative and intronic mutations, future studies will undoubtedly utilize CRISPR/Cas9 technology to examine transgenic mice having genes modified to exactly mimic variant human sequences. Mutant mice will increasingly be employed for drug development studies designed to treat human genetic skeletal disorders.

SIGNIFICANCE

Great progress is being made identifying mutant genes responsible for human rare genetic skeletal disorders and mouse models for genes affecting bone mass, architecture, mineralization and strength. This review organizes data for 441 human genetic bone disorders with regard to heredity, gene function, molecular pathways, and fidelity of relevant mouse models to mimic the human skeletal disorders. PubMed weblinks to citations of 249 successful mouse models are provided.

Clinical advantages and disadvantages of anabolic bone therapies targeting the WNT pathway

The WNT signalling pathway is a key regulator of bone metabolism, particularly bone formation, which has helped to define the role of osteocytes - the most abundant bone cells - as orchestrators of bone remodelling. Several molecules involved in the control of the WNT signalling pathway have been identified as potential targets for the development of bone-building therapeutics for patients with osteoporosis. Several of these molecules have been investigated in animal models, but only inhibitors of sclerostin (which is produced by osteocytes) have been investigated in phase III clinical studies. Here, we review the rationale for these developments and the specificity and potential off-target actions of WNT-based therapeutics. We also describe the available preclinical and clinical studies and discuss the benefits and risks of using sclerostin inhibitors for the management of patients with osteoporosis.

Strontium ranelate inhibits wear particle-induced aseptic loosening in mice

The imbalance between bone formation and osteolysis plays a key role in the pathogenesis of aseptic loosening. Strontium ranelate (SR) can promote bone formation and inhibit osteolysis. The aim of this study was to explore the role and mechanism of SR in aseptic loosening induced by wear particles. Twenty wild-type (WT) female C57BL/6j mice and 20 sclerostin-/- female C57BL/6j mice were used in this study. Mice were randomly divided into four groups: WT control group, WT SR group, knockout (KO) control group, and KO SR group. We found that SR enhanced the secretion of osteocalcin (0.72±0.007 in WT control group, 0.98±0.010 in WT SR group, P=0.000), Runx2 (0.34±0.005 in WT control group, 0.47±0.010 in WT SR group, P=0.000), β-catenin (1.04±0.05 in WT control group, 1.22±0.02 in WT SR group, P=0.000), and osteoprotegerin (OPG) (0.59±0.03 in WT control group, 0.90±0.02 in WT SR group, P=0.000). SR significantly decreased the level of receptor activator for nuclear factor-κB ligand (RANKL) (1.78±0.08 in WT control group, 1.37±0.06 in WT SR group, P=0.000) and improved the protein ratio of OPG/RANKL, but these effects were not observed in sclerostin-/- mice. Our findings demonstrated that SR enhanced bone formation and inhibited bone resorption in a wear particle-mediated osteolysis model in wild-type mice, and this effect relied mainly on the down-regulation of sclerostin levels to ameliorate the inhibition of the canonical Wnt pathway.

Sclerostin neutralization unleashes the osteoanabolic effects of Dkk1 inhibition

The WNT pathway has become an attractive target for skeletal therapies. High-bone-mass phenotypes in patients with loss-of-function mutations in the LRP5/6 inhibitor Sost (sclerosteosis), or in its downstream enhancer region (van Buchem disease), highlight the utility of targeting Sost/sclerostin to improve bone properties. Sclerostin-neutralizing antibody is highly osteoanabolic in animal models and in human clinical trials, but antibody-based inhibition of another potent LRP5/6 antagonist, Dkk1, is largely inefficacious for building bone in the unperturbed adult skeleton. Here, we show that conditional deletion of Dkk1 from bone also has negligible effects on bone mass. Dkk1 inhibition increases Sost expression, suggesting a potential compensatory mechanism that might explain why Dkk1 suppression lacks anabolic action. To test this concept, we deleted Sost from osteocytes in, or administered sclerostin neutralizing antibody to, mice with a Dkk1-deficient skeleton. A robust anabolic response to Dkk1 deletion was manifest only when Sost/sclerostin was impaired. Whole-body DXA scans, μCT measurements of the femur and spine, histomorphometric measures of femoral bone formation rates, and biomechanical properties of whole bones confirmed the anabolic potential of Dkk1 inhibition in the absence of sclerostin. Further, combined administration of sclerostin and Dkk1 antibody in WT mice produced a synergistic effect on bone gain that greatly exceeded individual or additive effects of the therapies, confirming the therapeutic potential of inhibiting multiple WNT antagonists for skeletal health. In conclusion, the osteoanabolic effects of Dkk1 inhibition can be realized if sclerostin upregulation is prevented. Anabolic therapies for patients with low bone mass might benefit from a strategy that accounts for the compensatory milieu of WNT inhibitors in bone tissue.

Anabolic Therapy for the Treatment of Osteoporosis in Childhood

PURPOSE OF REVIEW

Numerous forms of osteoporosis in childhood are characterized by low bone turnover (for example, osteoporosis due to neuromuscular disorders and glucocorticoid exposure). Anti-resorptive therapy, traditionally used to treat osteoporosis in the young, is associated with further reductions in bone turnover, raising concerns about the long-term safety and efficacy of such therapy. These observations have led to increasing interest in the role of anabolic therapy to treat pediatric osteoporosis.

RECENT FINDINGS

While growth hormone and androgens appears to be relatively weak anabolic modulators of bone mass, emerging therapies targeting bone formation pathways (anti-transforming growth factor beta antibody and anti-sclerostin antibody) hold considerable promise. Teriparatide remains an attractive option that merits formal study for patients post-epiphyseal fusion, although it must be considered that adult studies have shown its effect is blunted when administered following bisphosphonate therapy. Mechanical stimulation of bone through whole body vibration therapy appears to be much less effective than bisphosphonate therapy for treating osteoporosis in children. New anabolic therapies which target important pathways in skeletal metabolism merit further study in children, including their effects on fracture risk reduction and after treatment discontinuation.

The high bone mass phenotype of Lrp5-mutant mice is not affected by megakaryocyte depletion

Bone remodeling is a continuously ongoing process mediated by bone-resorbing osteoclasts and bone-forming osteoblasts. One key regulator of bone formation is the putative Wnt co-receptor Lrp5, where activating mutations in the extracellular domain cause increased bone formation in mice and humans. We have previously reported that megakaryocyte numbers are increased the bone marrow of mice carrying a high bone mass mutation (HBM) of Lrp5 (Lrp5^G170V). Since megakaryocytes can promote bone formation, we addressed the question, if the bone remodeling phenotype of Lrp5^G170V mice is affected by megakaryocyte depletion. For that purpose we took advantage of a mouse model carrying a mutation of the Mpl gene, encoding the thrombopoietin receptor. These mice (Mpl^hlb219) were crossed with Lrp5^G170V mice to generate animals carrying both mutations in a homozygous state. Using μCT, undecalcified histology and bone-specific histomorphometry of 12 weeks old littermates we observed that megakaryocyte number was remarkably decreased in Mpl^hlb219/Lrp5^G170V mice, yet the high bone mass phenotype of Lrp5^G170V mice was not significantly affected by the homozygous Mpl mutation. Finally, when we analyzed 24 weeks old wildtype and Mpl^hlb219 mice we did not observe a statistically significant alteration of bone remodeling in the latter ones. Taken together, our results demonstrate that an increased number of bone marrow megakaryocytes does not contribute to the increased bone formation caused by Lrp5 activation.

WNT-mediated Modulation of Bone Metabolism: Implications for WNT Targeting to Treat Extraskeletal Disorders

The WNT-signaling pathway is involved in cellular and tissue functions that control such diverse processes as body axis patterning, cellular proliferation, differentiation, and life span. The long list of molecules that can participate or modify WNT signaling makes this pathway one of the most complex in cell biology. In bone tissues, WNT signaling is required for proper skeletal development, and human mutations in various components of the cascade revealed insights into pharmacologic targeting that can be harnessed to improve skeletal health. In particular, mutations in genes that code for the WNT-signaling inhibitor sclerostin or the WNT coreceptor lipoprotein receptor-related protein 5 have highlighted the potential therapeutic value of recapitulating those effects in patients with low bone mass. A constant challenge in this area is selectively modifying WNT components in the tissue of interest, as WNT has manifold effects in nearly every tissue.

LRP5: From bedside to bench to bone

A role for low-density lipoprotein-related receptor 5 (LRP5) in human bone was first established by the identification of genetic alterations that led to dramatic changes in bone mass. Shortly thereafter, mutations that altered the function of the sclerostin (SOST) gene were also associated with altered human bone mass. Subsequent studies of LRP5 and sclerostin have provided important insights into the mechanisms by which these proteins regulate skeletal homeostasis. Sclerostin normally binds to LRP5 and the related LRP6 protein and prevents their activation by Wnts, the LRP5/LRP6 ligands. The interaction of sclerostin with LRP5 or LRP6 is facilitated by the LRP4 protein. Loss of LRP5 leads to defective osteoblast function and low bone mass, while loss of SOST or mutations in LRP5, which produce a protein that can no longer be bound by SOST, result in high bone mass. Insights gained from the use of genetically engineered mouse models are presented, as well as a brief summary of the status of antibodies in clinical trials that block the function of SOST as a mechanism to increase bone mass.

Low-Density Lipoprotein Receptor-Related Proteins in Skeletal Development and Disease

The identification of the low-density lipoprotein receptor (LDLR) provided a foundation for subsequent studies in lipoprotein metabolism, receptor-mediated endocytosis, and many other fundamental biological functions. The importance of the LDLR led to numerous studies that identified homologous molecules and ultimately resulted in the description of the LDL-receptor superfamily, a group of proteins that contain domains also found in the LDLR. Subsequent studies have revealed that members of the LDLR-related protein family play roles in regulating many aspects of signal transduction. This review is focused on the roles of selected members of this protein family in skeletal development and disease. We present background on the identification of this subgroup of receptors, discuss the phenotypes associated with alterations in their function in human patients and mouse models, and describe the current efforts to therapeutically target these proteins to treat human skeletal disease.

[SOST knockdown promotes differentiation of osteoblasts MG63 and mesenchymal stem cells C3H10 in an in vitro model of bone metastasis of breast cancer]

OBJECTIVE

To investigate whether SOST is involved in breast cancer MDA-MB-231 cells-induced suppression of differentiation of osteoblast MG63 cells and mesenchymal stem C3H10 cells.

METHODS

SOST-specific small interfering RNA (siRNA) was transfected into breast cancer MDA-MB-231 cells, and the interfering efficiency was verified by RT-PCR. The supernatants were collected from MDA-MB-231 cells in routine culture, cells transfected with SOST siRNA via adenovirus, and cells transfected with empty adenoviral vectors and added in MG63 or C3H10 cell cultures. The changes in the expressions of OPG, OCN, OPN and IBSP in MG63 and C3H10 cells were detected using quantitative real-time PCR, and ALP activity was detected with ALP reading and ALP staining with the cells cultured in routine culture medium and cells in osteogenic induction medium as the negative and positive controls.

RESULTS

The adenovirus Ad-siSOST effectively knocked down the expression of SOST in MDA-MB-231 cells. MG63 cells and C3H10 cells cultured in osteogenic medium showed significantly upregulated expressions of the osteoblast markers OPG, OPN, OCN and IBSP (P<0.01), while co-culture with the supernatant of MDA-MB-231 cells obviously reduced the expressions of the osteoblast markers (P<0.01); the expression of the markers increased again in MG63 and C3H10 cells after treatment with the supernatant of MDA-MB-231 cells transfected with ad-siSOST (P<0.01). ALP activity in MG63 and C3H10 cells exhibited a similar pattern of variations in response to the treatments (P<0.01).

CONCLUSION

In the in vitro model of bone metastasis of breast cancer, the differentiation of MG63 or C3H10 cells is suppressed, which can be partly reversed by knocking down the expression of SOST in the bone metastasis microenvironment.

Bone mechanobiology in mice: toward single-cell in vivo mechanomics

Mechanically driven bone (re)modeling is a multiscale process mediated through complex interactions between multiple cell types and their microenvironments. However, the underlying mechanisms of how cells respond to mechanical signals are still unclear and are at the focus of the field of bone mechanobiology. Traditionally, this complex process has been addressed by reducing the system to single scales and cell types. It is only recently that more integrative approaches have been established to study bone mechanobiology across multiple scales in which mechanical load at the organ level is related to molecular responses at the cellular level. The availability of mouse loading models and imaging techniques with improved spatial and temporal resolution has made it possible to track dynamic bone (re)modeling at the tissue and cellular level in vivo. Coupled with advanced computational models, the (re)modeling activities at the tissue scale can be associated with the mechanical microenvironment. However, methods are lacking to link the molecular responses of different cell types to their local mechanical microenvironment and bone (re)modeling activities occurring at the tissue scale. With recent improvements in "omics" technologies and single-cell molecular biology, it is now possible to sequence the complete genome and transcriptome of single cells. These technologies offer unique opportunities to comprehensively investigate the cellular transcriptional profiles within their specific microenvironment. By combining single-cell "omics" technologies with well-established tissue-scale models of bone mechanobiology, we propose a mechanomics approach to locally analyze the transcriptome of single cells with respect to their local 3D mechanical in vivo environment.

Time course of disassociation of bone formation signals with bone mass and bone strength in sclerostin antibody treated ovariectomized rats

Sclerostin antibodies increase bone mass by stimulating bone formation. However, human and animal studies show that bone formation increases transiently and returns to pre-treatment level despite ongoing antibody treatment. To understand its mechanism of action, we studied the time course of bone formation, correlating the rate and extent of accrual of bone mass and strength after sclerostin antibody treatment. Ovariectomized (OVX) rats were treated with a sclerostin-antibody (Scle-ab) at 20mg/kg sc once weekly and sacrificed at baseline and 2, 3, 4, 6, and 8weeks post-treatment. In Scle-ab treated rats, serum PINP and OCN rapidly increased at week 1, peaked around week 3, and returned to OVX control levels by week 6. Transcript analyses from the distal femur revealed an early increase in bone formation followed by a sustained decrease in bone resorption genes. Lumbar vertebral (LV) osteoblast surface increased 88% by week 2, and bone formation rate (BFR/BS) increased 138% by week 4. Both parameters were below OVX control by week 8. Bone formation was primarily a result of modeling based formation. Endocortical and periosteal BFR/BS peaked around week 4 at 313% and 585% of OVX control, respectively. BFR/BS then declined but remained higher than OVX control on both surfaces through week 8. Histomorphometric analyses showed LV-BV/TV did not further increase after week 4, while BMD continued to increase at LV, mid femur (MF), and femoral neck (FN) through week 8. Biomechanical tests showed a similar improvement in bone strength through 8weeks in MF and FN, but bone strength plateaued between weeks 6 and 8 for LV. Our data suggest that bone formation with Scle-ab treatment is rapid and modeling formation dominated in OVX rats. Although transient, the bone formation response persists longer in cortical than trabecular bone.

Effects of sclerostin antibodies in animal models of osteoporosis

There is an unmet need for therapies that can restore bone strength and reduce fracture risk among patients at high risk of osteoporotic fracture. To address this need, bone-forming therapies that increase osteoblast activity are required to help restore bone structure and strength. Sclerostin is now recognized as a target for osteoporosis therapy. Sclerostin is predominantly secreted by the osteocyte and acts as an extracellular inhibitor of canonical Wnt signaling by binding to the receptors lipoprotein receptor-related protein-4, 5 and 6. Monoclonal antibodies to sclerostin (Scl-Ab) have been used in both clinical and in preclinical studies of osteoporosis with beneficial outcomes for bone density, structure, strength and fracture risk reduction. In this review paper, we summarize the current literature describing the effects of Scl-Ab in animal models of osteoporosis. In addition, we report new pharmacologic data from three animal studies of Scl-Ab: 1) a 12-month study evaluating bone quality in ovariectomized (OVX) rats; 2) a 6-month study evaluating bone structure and strength in adolescent cynomolgus monkeys; and 3) the effects of transition from Scl-Ab to vehicle or the RANKL inhibitor osteoprotegerin-Fc in OVX rats. Together, these results demonstrate that inhibition of sclerostin by Scl-Ab increased bone formation, and decreased bone resorption, leading to improved bone structure, bone mass and bone strength while maintaining bone quality in multiple animal models of osteoporosis. Further, gains in bone mass induced by Scl-Ab treatment were preserved by antiresorptive agents such as a RANKL inhibitor as a follow-on therapy. The bone-forming effects of Scl-Ab were unaffected by pre- or co-treatment with a bisphosphonate, and were restored following a treatment-free period after initial dosing. These data support the clinical development of Scl-Ab for treatment of conditions with low bone mass such as postmenopausal and male osteoporosis.

Application of anti-Sclerostin therapy in non-osteoporosis disease models

Sclerostin, a known inhibitor of the low density lipoprotein related protein 5 and 6 (LRP5 and LRP6) cell surface signaling receptors, is integral in the maintenance of normal bone mass and strength. Patients with loss of function mutations in SOST or missense mutations in LRP5 that prevent Sclerostin from binding and inhibiting the receptor, have significantly increased bone mass. This observation leads to the development of Sclerostin neutralizing therapies to increase bone mass and strength. Anti-Sclerostin therapy has been shown to be effective at increasing bone density and strength in animal models and patients with osteoporosis. Loss of function of Sost or treatment with a Sclerostin neutralizing antibody improves bone properties in animal models of Osteoporosis Pseudoglioma syndrome (OPPG), likely due to action through the LRP6 receptor, which suggests patients may benefit from these therapies. Sclerostin antibody is effective at improving bone properties in mouse models of Osteogenesis Imperfecta, a genetic disorder of low bone mass and fragility due to type I collagen mutations, in as little as two weeks after initiation of therapy. However, these improvements are due to increases in bone quantity as the quality (brittleness) of bone remains unaffected. Similarly, Sclerostin antibody treatment improves bone density in animal models of other diseases. Sclerostin neutralizing therapies are likely to benefit many patients with genetic disorders of bone, as well as other forms of metabolic bone disease.

Pharmacological and biological therapeutic strategies for osteogenesis imperfecta

Osteogenesis imperfecta (OI) is a connective tissue disorder characterized by bone fragility, low bone mass, and bone deformities. The majority of cases are caused by autosomal dominant pathogenic variants in the COL1A1 and COL1A2 genes that encode type I collagen, the major component of the bone matrix. The remaining cases are caused by autosomal recessively or dominantly inherited mutations in genes that are involved in the post-translational modification of type I collagen, act as type I collagen chaperones, or are members of the signaling pathways that regulate bone homeostasis. The main goals of treatment in OI are to decrease fracture incidence, relieve bone pain, and promote mobility and growth. This requires a multi-disciplinary approach, utilizing pharmacological interventions, physical therapy, orthopedic surgery, and monitoring nutrition with appropriate calcium and vitamin D supplementation. Bisphosphonate therapy, which has become the mainstay of treatment in OI, has proven beneficial in increasing bone mass, and to some extent reducing fracture risk. However, the response to treatment is not as robust as is seen in osteoporosis, and it seems less effective in certain types of OI, and in adult OI patients as compared to most pediatric cases. New pharmacological treatments are currently being developed, including anti-resorptive agents, anabolic treatment, and gene- and cell-therapy approaches. These therapies are under different stages of investigation from the bench-side, to pre-clinical and clinical trials. In this review, we will summarize the recent findings regarding the pharmacological and biological strategies for the treatment of patients with OI. © 2016 Wiley Periodicals, Inc.

Analysis of SOST expression using large minigenes reveals the MEF2C binding site in the evolutionarily conserved region (ECR5) enhancer mediates forskolin, but not 1,25-dihydroxyvitamin D3 or TGFβ1 responsiveness

Transcribed from the SOST gene, sclerostin is an osteocyte-derived negative regulator of bone formation that inhibits osteoblastogenesis via antagonism of the Wnt pathway. Sclerostin is a promising therapeutic target for low bone mass diseases and neutralizing antibody therapies that target sclerostin are in development. Diverse stimuli regulate SOST including the vitamin D hormone, forskolin (Fsk), bone morphogenic protein 2 (BMP-2), oncostatin M (OSM), dexamethasone (Dex), and transforming growth factor (TGFβ₁). To explore the mechanisms by which these compounds regulate SOST expression, we examined their ability to regulate a SOST reporter minigene containing the entire SOST locus including the downstream regionor mutant minigenes containing a deletion of the -1kb to -2kb promoter proximal region (-1kb), ECR2, ECR5, or two point mutations in the MEF2 binding site of ECR5 (ECR5/MEF2). Previous reports suggest that both the PTH and TGFβ₁ effects on SOST are mediated through ECR5 and that the action of PTH is mediated specifically via the MEF2 binding site at ECR5. Consistent with these reports, the suppressive effects of Fsk were abrogated following both ECR5 deletion and ECR5/MEF2 mutation. In contrast, we found that TGFβ₁ negatively regulated SOST and that neither ECR5 nor ECR5/MEF2 was involved. Surprisingly, none of these four deletions/mutations abrogated the suppressive effects of the vitamin D hormone, OSM, Dex, or TGFβ₁, or the positive effects of BMP-2. These data suggest that we need to move beyond ECR5 to understand SOST regulation.

New Genetic Forms of Childhood-Onset Primary Osteoporosis

Recent developments in genetic technology have given us the opportunity to look at diseases in a new and more detailed way. This Mini Review discusses monogenetic forms of childhood-onset primary osteoporosis, with the main focus on osteoporosis caused by mutations in WNT1 and PLS3, two of the most recently discovered genes underlying early-onset osteoporosis. The importance of WNT1 in the accrual and maintenance of bone mass through activation of canonical WNT signaling was recognized in 2013. WNT1 was shown to be a key ligand for the WNT-signaling pathway, which is of major importance in the regulation of bone formation. More recently, mutations in PLS3, located on the X chromosome, were shown to be the cause of X-linked childhood-onset primary osteoporosis affecting mainly males. The function of PLS3 in bone metabolism is still not completely understood, but it has been speculated to have an important role in mechanosensing by osteocytes and in matrix mineralization. In this new era of genetics, our knowledge on genetic causes of childhood-onset osteoporosis expands constantly. These discoveries bring new possibilities, but also new challenges. Guidelines are needed to implement this new genetic knowledge to clinical patient care and to guide genetic investigations in affected families.

Postnatal β-catenin deletion from Dmp1-expressing osteocytes/osteoblasts reduces structural adaptation to loading, but not periosteal load-induced bone formation

Mechanical signal transduction in bone tissue begins with load-induced activation of several cellular pathways in the osteocyte population. A key pathway that participates in mechanotransduction is Wnt/Lrp5 signaling. A putative downstream mediator of activated Lrp5 is the nucleocytoplasmic shuttling protein β-catenin (βcat), which migrates to the nucleus where it functions as a transcriptional co-activator. We investigated whether osteocytic βcat participates in Wnt/Lrp5-mediated mechanotransduction by conducting ulnar loading experiments in mice with or without chemically induced βcat deletion in osteocytes. Mice harboring βcat floxed loss-of-function alleles (βcat(f/f)) were bred to the inducible osteocyte Cre transgenic (10)(kb)Dmp1-CreERt2. Adult male mice were induced to recombine the βcat alleles using tamoxifen, and intermittent ulnar loading sessions were applied over the following week. Although adult-onset deletion of βcat from Dmp1-expressing cells reduced skeletal mass, the bone tissue was responsive to mechanical stimulation as indicated by increased relative periosteal bone formation rates in recombined mice. However, load-induced improvements in cross sectional geometric properties were compromised in recombined mice. The collective results indicate that the osteoanabolic response to loading can occur on the periosteal surface when β-cat levels are significantly reduced in Dmp1-expressing cells, suggesting that either (i) only low levels of β-cat are required for mechanically induced bone formation on the periosteal surface, or (ii) other additional downstream mediators of Lrp5 might participate in transducing load-induced Wnt signaling.