Increased IL-6 expression in osteoclasts is necessary but not sufficient for the development of Paget's disease of bone

Osteocytes and Paget's Disease of Bone

PURPOSE OF REVIEW

To describe the contributions of osteocytes to the lesions in Paget's disease, which are characterized by locally overactive bone resorption and formation.

RECENT FINDINGS

Osteocytes, the most abundant cells in bone, are altered in Paget's disease lesions, displaying increased size, decreased canalicular length, incomplete differentiation, and less sclerostin expression compared to controls in both patients and mouse models. Pagetic lesions show increased senescent osteocytes that express RANK ligand, which drives osteoclastic bone resorption. Abnormal osteoclasts in Paget's disease secrete abundant IGF1, which enhances osteocyte senescence, contributing to lesion formation. Recent data suggest that osteocytes contribute to lesion formation in Paget's disease by responding to high local IGF1 released from abnormal osteoclasts. Here we describe the characteristics of osteocytes in Paget's disease and their role in bone lesion formation based on recent results with mouse models and supported by patient data.

Osteoclast-derived IGF1 induces RANKL production in osteocytes and contributes to pagetic lesion formation

We previously reported that measles virus nucleocapsid protein (MVNP) expression in osteoclasts (OCLs) of patients with Paget disease (PD) or targeted to the OCL lineage in MVNP-transgenic mice (MVNP mice) increases IGF1 production in osteoclasts (OCL-IGF1) and leads to development of PD OCLs and pagetic bone lesions (PDLs). Conditional deletion of Igf1 in OCLs of MVNP mice fully blocked development of PDLs. In this study, we examined whether osteocytes (OCys), key regulators of normal bone remodeling, contribute to PD. OCys in PDLs of patients and of MVNP mice expressed less sclerostin, and had increased RANKL expression compared with OCys in bones from WT mice or normal patients. To test whether increased OCL-IGF1 is sufficient to induce PDLs and PD phenotypes, we generated TRAP-Igf1 (T-Igf1) transgenic mice to determine whether increased IGF1 expression in the absence of MVNP in OCLs is sufficient to induce PDLs and pagetic OCLs. We found that T-Igf1 mice at 16 months of age developed PD OCLs, PDLs, and OCys, with decreased sclerostin and increased RANKL, similar to MVNP mice. Thus, pagetic phenotypes could be induced by OCLs expressing increased IGF1. OCL-IGF1 in turn increased RANKL production in OCys to induce PD OCLs and PDLs.

Inhibition of NFAM1 suppresses phospho-SAPK/JNK signaling during osteoclast differentiation and bone resorption

We have recently demonstrated NFAT activating protein with ITAM motif 1 (NFAM1) signaling increases osteoclast (OCL) formation/bone resorption associated with the Paget's disease of bone, however, the underlying molecular mechanisms of the NFAM1 regulation of OCL differentiation and bone resorption remains unclear. Here, we showed that RANK ligand stimulation enhances NFAM1 expression in preosteoclast cells. Conditioned media collected from RANKL stimulated RAW264.7 NFAM1 knockdown (KD) stable cells showed inhibition of interleukin-6 (2.5-fold), tumour necrosis factor-α (2.2-fold) and CXCL-5 (3-fold) levels compared to wild-type (WT) cells. Further, RANKL stimulation significantly increased p-STAT6 expression (5.5-fold) in WT cells, but no significant effect was observed in NFAM1-KD cells. However, no changes were detected in signal transducer and activator of transcription 3 levels in either of cell groups. Interestingly, NFAM1-KD suppressed the RANKL stimulated c-fos, p-c-Jun and c-Jun N-terminal kinase (JNK) activity in preosteoclasts. We further showed that the suppression of JNK activity is through inhibition of p-SAPK/JNK in these cells. In addition, NFATc1 expression, a critical transcription factor associated with osteoclastogenesis is significantly inhibited in NFAM1-KD preosteoclast cells. Interestingly, NFAM1 inhibition suppressed the OCL differentiation and bone resorption capacity in mouse bone marrow cell cultures. We also demonstrated inhibition of tartrate-resistant acid phosphatase expression in RANKL stimulated NFAM1-KD preosteoclast cells. Thus, our results suggest that NFAM1 control SAPK/JNK signaling to modulate osteoclast differentiation and bone resorption.

Transgenic inhibition of interleukin-6 trans-signaling does not prevent skeletal pathologies in mucolipidosis type II mice

Severe skeletal alterations are common symptoms in patients with mucolipidosis type II (MLII), a rare lysosomal storage disorder of childhood. We have previously reported that progressive bone loss in a mouse model for MLII is caused by an increased number of bone-resorbing osteoclasts, which is accompanied by elevated expression of the cytokine interleukin-6 (IL-6) in the bone microenvironment. In the present study we addressed the question, if pharmacological blockade of IL-6 can prevent the low bone mass phenotype of MLII mice. Since the cellular IL-6 response can be mediated by either the membrane-bound (classic signaling) or the soluble IL-6 receptor (trans-signaling), we first performed cell culture assays and found that both pathways can increase osteoclastogenesis. We then crossed MLII mice with transgenic mice expressing the recombinant soluble fusion protein sgp130Fc, which represents a natural inhibitor of IL-6 trans-signaling. By undecalcified histology and bone-specific histomorphometry we found that high circulating sgp130Fc levels do not affect skeletal growth or remodeling in wild-type mice. Most importantly, blockade of IL-6 trans-signaling did neither reduce osteoclastogenesis, nor increase bone mass in MLII mice. Therefore, our data clearly demonstrate that the bone phenotype of MLII mice cannot be corrected by blocking the IL-6 trans-signaling.

Increased S1P expression in osteoclasts enhances bone formation in an animal model of Paget's disease

Paget's disease (PD) is characterized by increased numbers of abnormal osteoclasts (OCLs) that drive exuberant bone formation, but the mechanisms responsible for the increased bone formation remain unclear. We previously reported that OCLs from 70% of PD patients express measles virus nucleocapsid protein (MVNP), and that transgenic mice with targeted expression of MVNP in OCLs (MVNP mice) develop bone lesions and abnormal OCLs characteristic of PD. In this report, we examined if OCL-derived sphingosine-1-phosphate (S1P) contributed to the abnormal bone formation in PD, since OCL-derived S1P can act as a coupling factor to increase normal bone formation via binding S1P-receptor-3 (S1PR3) on osteoblasts (OBs). We report that OCLs from MVNP mice and PD patients expressed high levels of sphingosine kinase-1 (SphK-1) compared with wild-type (WT) mouse and normal donor OCLs. SphK-1 production by MVNP-OCLs was interleukin-6 (IL-6)-dependent since OCLs from MVNP/IL-6^-/- mice expressed lower levels of SphK-1. Immunohistochemistry of bone biopsies from a normal donor, a PD patient, WT and MVNP mice confirmed increased expression levels of SphK-1 in OCLs and S1PR3 in OBs of the PD patient and MVNP mice compared with normal donor and WT mice. Further, MVNP-OCLs cocultured with OBs from MVNP or WT mice increased OB-S1PR3 expression and enhanced expression of OB differentiation markers in MVNP-OBs precursors compared with WT-OBs, which was mediated by IL-6 and insulin-like growth factor 1 secreted by MVNP-OCLs. Finally, the addition of an S1PR3 antagonist (VPC23019) to WT or MVNP-OBs treated with WT and MVNP-OCL-conditioned media (CM) blocked enhanced OB differentiation of MVNP-OBs treated with MVNP-OCL-CM. In contrast, the addition of the SIPR3 agonist, VPC24191, to the cultures enhanced osterix and Col-1A expression in MVNP-OBs treated with MVNP-OCL-CM compared with WT-OBs treated with WT-OCL-CM. These results suggest that IL-6 produced by PD-OCLs increases S1P in OCLs and S1PR3 on OBs, to increase bone formation in PD.

Long term effects on biochemical bone markers of a single infusion of zoledronic acid in Paget disease of bone

BACKGROUND

The aim of the present study is to evaluate long term biochemical response to a single dose of zoledronic acid in patients with Paget disease of bone, as well as evaluating the value of bone turnover markers in diagnosis and follow-up.

METHODS

This is an observational, descriptive and prospective study. Included patients received a single-dose intravenous infusion of 5 mg zoledronic acid. Bone turnover markers were measured at baseline, and in every follow up visit.

RESULTS

Thirty-nine patients with a mean follow-up of 56.49 months were included. At the time Paget disease was diagnosed, all of the patients (100%) had high serum procollagen type 1 amino-terminal propeptide values, but not all patients had high serum C-terminal telopeptide and alkaline phosphatase values (85% and 89% respectively). Biochemical response to therapy occurred in 38 out of 39 patients (97%). Two patients had partial response at 6 months but complete response thereafter. Only one patient relapsed (nadir procollagen type 1 amino-terminal propeptide 35.06 μg/l, value at relapse 75.2 μg/l) 4.5 years after treatment. Values of serum C-terminal telopeptide and alkaline phosphatase of this patient were normal despite P1NP relapse.

CONCLUSIONS

We hence conclude that zoledronic acid is effective in inducing and maintaining biochemical remission and that procollagen type 1 amino-terminal propeptide is a better diagnostic and prognostic marker in PDB when compared to C-terminal telopeptide and alkaline phosphatase.

Cytokines and Bone: Osteoimmunology

Cytokines and hematopoietic growth factors have traditionally been thought of as regulators of the development and function of immune and blood cells. However, an ever-expanding number of these factors have been discovered to have major effects on bone cells and the development of the skeleton in health and disease (Table 1). In addition, several cytokines have been directly linked to the development of osteoporosis in both animal models and in patients. In order to understand the mechanisms regulating bone cells and how this may be dysregulated in disease states, it is necessary to appreciate the diverse effects that cytokines and inflammation have on osteoblasts, osteoclasts, and bone mass. This chapter provides a broad overview of this topic with extensive references so that, if desired, readers can access specific references to delve into individual topics in greater detail.

Global deletion of Optineurin results in altered type I IFN signaling and abnormal bone remodeling in a model of Paget's disease

Genome-wide association studies (GWAS) have identified Optineurin (OPTN) as genetically linked to Paget's disease of the bone (PDB), a chronic debilitating bone remodeling disorder characterized by localized areas of increased bone resorption and abnormal bone remodeling. However, only ~10% of mouse models with a mutation in Optn develop PDB, thus hindering the mechanistic understanding of the OPTN-PDB axis. Here, we reveal that 100% of aged Optn global knockout (Optn^-/-) mice recapitulate the key clinical features observed in PDB patients, including polyostotic osteolytic lesions, mixed-phase lesions, and increased serum levels of alkaline phosphatase (ALP). Differentiation of primary osteoclasts ex vivo revealed that the absence of Optn resulted in an increased osteoclastogenesis. Mechanistically, Optn-deficient osteoclasts displayed a significantly decreased type I interferon (IFN) signature, resulting from both defective production of IFNβ and impaired signaling via the IFNα/βR, which acts as a negative feedback loop for osteoclastogenesis and survival. These data highlight the dual roles of OPTN in the type I IFN response to restrain osteoclast activation and bone resorption, offering a novel therapeutic target for PDB. Therefore, our study describes a novel and essential mouse model for PDB and define a key role for OPTN in osteoclast differentiation.

Mechanism of Action of Icariin in Bone Marrow Mesenchymal Stem Cells

Osteoporosis, femoral head necrosis, and congenital bone defects are orthopedic disorders characterized by reduced bone generation and insufficient bone mass. Bone regenerative therapy primarily relies on the bone marrow mesenchymal stem cells (BMSCs) and their ability to differentiate osteogenically. Icariin (ICA) is the active ingredient of Herba epimedii, a common herb used in traditional Chinese medicine (TCM) formulations, and can effectively enhance BMSC proliferation and osteogenesis. However, the underlying mechanism of ICA action in BMSCs is not completely clear. In this review, we provide an overview of the studies on the role and mechanism of action of ICA in BMSCs, to provide greater insights into its potential clinical use in bone regeneration.

Regulated proteolysis of p62/SQSTM1 enables differential control of autophagy and nutrient sensing

The multidomain scaffold protein p62 (also called sequestosome-1) is involved in autophagy, antimicrobial immunity, and oncogenesis. Mutations in SQSTM1, which encodes p62, are linked to hereditary inflammatory conditions such as Paget's disease of the bone, frontotemporal dementia (FTD), amyotrophic lateral sclerosis, and distal myopathy with rimmed vacuoles. Here, we report that p62 was proteolytically trimmed by the protease caspase-8 into a stable protein, which we called p62^TRM We found that p62^TRM, but not full-length p62, was involved in nutrient sensing and homeostasis through the mechanistic target of rapamycin complex 1 (mTORC1). The kinase RIPK1 and caspase-8 controlled p62^TRM production and thus promoted mTORC1 signaling. An FTD-linked p62 D329G polymorphism and a rare D329H variant could not be proteolyzed by caspase-8, and these noncleavable variants failed to activate mTORC1, thereby revealing the detrimental effect of these mutations. These findings on the role of p62^TRM provide new insights into SQSTM1-linked diseases and mTORC1 signaling.

NFAM1 signaling enhances osteoclast formation and bone resorption activity in Paget's disease of bone

Paget's disease of bone (PDB) is marked by the focal activity of abnormal osteoclasts (OCLs) with excess bone resorption. We previously detected measles virus nucleocapsid protein (MVNP) transcripts in OCLs from patients with PDB. Also, MVNP stimulates pagetic OCL formation in vitro and in vivo. However, the mechanism by which MVNP induces excess OCLs/bone resorption activity in PDB is unclear. Microarray analysis identified MVNP induction of NFAM1 (NFAT activating protein with ITAM motif 1) expression. Therefore, we hypothesize that MVNP induction of NFAM1 enhances OCL differentiation and bone resorption in PDB. MVNP transduced normal human PBMC showed an increased NFAM1 mRNA expression without RANKL treatment. Further, bone marrow cells from patients with PDB demonstrated elevated levels of NFAM1 mRNA expression. Interestingly, shRNA suppression of NFAM1 inhibits MVNP induced OCL differentiation and bone resorption activity in mouse bone marrow cultures. Live cell widefield fluorescence microscopy analysis revealed that MVNP induced intracellular Ca²⁺ oscillations and levels were significantly reduced in NFAM1 suppressed preosteoclasts. Further, western blot analysis demonstrates that shRNA against NFAM1 inhibits MVNP stimulated PLCγ, calcineurin, and Syk activation in preosteoclast cells. Furthermore, NFAM1 expression controls NFATc1, a critical transcription factor expression and nuclear translocation in MVNP transuded preosteoclast cells. Thus, our results suggest that MVNP modulation of the NFAM1 signaling axis plays an essential role in pagetic OCL formation and bone resorption activity.

Epidemiology and pathology of Paget's disease of bone - a review

Paget's disease of bone (PDB) is a noninflammatory, metabolic, skeletal disorder characterized by localized excessive osteoclastic bone resorption that is followed by compensatory increased osteoblastic activity leading to unstructured, fibroblastic, and biomechanically unstable bone. As a result, there is deformity and enlargement of the bone with a defective and disorganized pattern. Here, we review the epidemiology, etiology, pathology, macrostructure, histology, and quantitative histomorphometry findings of PDB. Hyperosteoclastosis and poor definition of the boundary between cortical and medullary bone are the main histological findings in PDB. Additionally, Pagetic bone is also characterized by hypertrophy and alteration of trabecular parameters.

Measles virus nucleocapsid protein increases osteoblast differentiation in Paget's disease

Paget's disease (PD) is characterized by focal and dramatic bone resorption and formation. Treatments that target osteoclasts (OCLs) block both pagetic bone resorption and formation; therefore, PD offers key insights into mechanisms that couple bone resorption and formation. Here, we evaluated OCLs from 3 patients with PD and determined that measles virus nucleocapsid protein (MVNP) was expressed in 70% of these OCLs. Moreover, transgenic mice with OCL-specific expression of MVNP (MVNP mice) developed PD-like bone lesions that required MVNP-dependent induction of high IL-6 expression levels in OCLs. In contrast, mice harboring a knockin of p62P394L (p62-KI mice), which is the most frequent PD-associated mutation, exhibited increased bone resorption, but not formation. Evaluation of OCLs from MVNP, p62-KI, and WT mice revealed increased IGF1 expression in MVNP-expressing OCLs that resulted from the high IL-6 expression levels in these cells. IL-6, in turn, increased the expression of coupling factors, specifically ephrinB2 on OCLs and EphB4 on osteoblasts (OBs). IGF1 enhanced ephrinB2 expression on OCLs and OB differentiation. Importantly, ephrinB2 and IGF1 levels were increased in MVNP-expressing OCLs from patients with PD and MVNP-transduced human OCLs compared with levels detected in controls. Further, anti-IGF1 or anti-IGF1R blocked Runx2 and osteocalcin upregulation in OBs cocultured with MVNP-expressing OCLs. These results suggest that in PD, MVNP upregulates IL-6 and IGF1 in OCLs to increase ephrinB2-EphB4 coupling and bone formation.

Paget's disease of bone-genetic and environmental factors

Paget's disease of bone is generally diagnosed in individuals aged >50 years, usually manifests in one or several bones and is initiated by osteoclast-induced osteolytic lesions. Subsequently, over a period of many years, osteoblastic activity can result in sclerosis and deformation of bone. The prevalence of Paget's disease is highest in the UK and in countries where a large number of residents have ancestors from the UK. Currently, in many countries, the prevalence of the disorder has decreased. A considerable number of affected patients have a family history of Paget's disease and the disorder has an autosomal dominant pattern of inheritance but with incomplete penetrance. A large number of mutations in SQSTM1 (which encodes sequestosome-1; also known as ubiquitin-binding protein p62) seem to account for the susceptibility to develop Paget's disease in some families; the involvement of other genes is currently under investigation. In addition to a genetic cause, environmental factors have been proposed to have a role in the pathogenesis of Paget's disease. Although most evidence has been presented for measles virus as an aetiologic factor, some studies have not confirmed its involvement. The decreasing incidence of Paget's disease, which could be attributed to measles vaccination along with the measles virus nucleocapsid protein induction of Paget's disease lesions in transgenic mice, supports an aetiologic role of the virus.

Osteoclast derivation from mouse bone marrow

Osteoclasts are highly specialized cells that are derived from the monocyte/macrophage lineage of the bone marrow. Their unique ability to resorb both the organic and inorganic matrices of bone means that they play a key role in regulating skeletal remodeling. Together, osteoblasts and osteoclasts are responsible for the dynamic coupling process that involves both bone resorption and bone formation acting together to maintain the normal skeleton during health and disease. As the principal bone-resorbing cell in the body, changes in osteoclast differentiation or function can result in profound effects in the body. Diseases associated with altered osteoclast function can range in severity from lethal neonatal disease due to failure to form a marrow space for hematopoiesis, to more commonly observed pathologies such as osteoporosis, in which excessive osteoclastic bone resorption predisposes to fracture formation. An ability to isolate osteoclasts in high numbers in vitro has allowed for significant advances in the understanding of the bone remodeling cycle and has paved the way for the discovery of novel therapeutic strategies that combat these diseases. Here, we describe a protocol to isolate and cultivate osteoclasts from mouse bone marrow that will yield large numbers of osteoclasts.

Detection of SQSTM1/P392L post-zygotic mutations in Paget's disease of bone

Paget’s disease of bone (PDB) is transmitted, in one-third of cases, in an autosomal dominant mode of inheritance with incomplete penetrance. The SQSTM1/P392L germinal mutation is the most common mutation associated with PDB. Given the focal nature of PDB, one team of investigators showed that SQSTM1/P392L somatic mutations could occur in pagetic bone lesions in the absence of germinal mutations detectable in the peripheral blood. The objectives of this study were to develop a reliable method to detect SQSTM1/P392L post-zygotic mutations, by optimizing a polymerase chain reaction (PCR)-clamping method reported to be effective in detecting post-zygotic mutations in peripheral blood from patients with fibrous dysplasia; and to evaluate the frequency of this post-zygotic mutation in PDB patients. We used a locked nucleic acid (LNA) specifically designed for the SQSTM1/P392L mutation, which blocks the wild-type allele amplification during the PCR. DNA from 376 pagetic patients and 297 controls, all without any SQSTM1/P392L germinal mutation, was analyzed. We found that 4.8 % of PDB patients and 1.4 % of controls were carriers of this post-zygotic mutation [p = 0.013, OR 3.68 (1.23; 11.00)]. PDB patient carriers of a post-zygotic mutation had a lower number of affected bones and Renier’s index than patients carrying a germinal mutation, suggesting a lower disease extension. We also demonstrated that this post-zygotic mutation was restricted to the monocytic lineage. These results confirmed that LNA PCR clamping is effective for the detection of SQSTM1/P392L post-zygotic mutations, which may occur in patients with PDB.

Pathobiology of Paget's Disease of Bone

Paget's disease of bone is characterized by highly localized areas of increased bone resorption accompanied by exuberant, but aberrant new bone formation with the primary cellular abnormality in osteoclasts. Paget's disease provides an important paradigm for understanding the molecular mechanisms regulating both osteoclast formation and osteoclast-induced osteoblast activity. Both genetic and environmental etiologies have been implicated in Paget's disease, but their relative contributions are just beginning to be defined. To date, the only gene with mutations in the coding region linked to Paget's disease is sequestosome-1 (SQSTM1), which encodes the p62 protein, and these mutations lead to elevated cytokine activation of NF-B in osteoclasts but do not induce a "pagetic osteoclast" phenotype. Further, genetic mutations linked to Paget's appear insufficient to cause Paget's disease and additional susceptibility loci or environmental factors may be required. Among the environmental factors suggested to induce Paget's disease, chronic measles (MV) infection has been the most studied. Expression of the measles virus nucleocapsid gene (MVNP) in osteoclasts induces pagetic-like osteoclasts and bone lesions in mice. Further, mice expressing both MVNP in osteoclasts and germline mutant p62 develop dramatic pagetic bone lesions that were strikingly similar to those seen in patients with Paget's disease. Thus, interactions between environmental and genetic factors appear important to the development of Paget's disease. In this article we review the mechanisms responsible for the effects of mutant p62 gene expression and MVNP on osteoclast and osteoblast activity, and how they may contribute to the development of Paget's disease of bone.