An atypical degenerative osteoarthropathy in Hyp mice is characterized by a loss in the mineralized zone of articular cartilage

Biomechanical Impact of Phosphate Wasting on Articular Cartilage Using the Murine Hyp Model of X-linked hypophosphatemia

Degenerative osteoarthritis (OA) is recognized as an early-onset comorbidity of X-linked hypophosphatemia (XLH), contributing to pain and stiffness and limiting range of motion and activities of daily living. Here, we extend prior findings describing biochemical and cellular changes of articular cartilage (AC) in the phosphate-wasting environment of XLH to determine the impact of these changes on the biomechanical properties of AC in compression and potential role in the etiology of OA. We hypothesize that despite increased proteoglycan biosynthesis, disruption of the mineralized zone of AC impacts the mechanical properties of cartilage that function to accommodate loads and that therapeutic restoration of this zone will improve the mechanical properties of AC. Data were compared between three groups: wild type (WT), Hyp, and Hyp mice treated with calcitriol and oral phosphate. EPIC microCT confirmed AC mineral deficits and responsiveness to therapy. MicroCT of the Hyp subchondral bone plate revealed that treatment improved trabecular bone volume (BV/TV) but remained significantly lower than WT mice in other trabecular microstructures (p < 0.05). Microindentation AC studies revealed that, compared with WT mice, the mean stiffness of tibial AC was significantly lower in untreated Hyp mice (2.65 ± 0.95 versus 0.87 ± 0.33 N/mm, p < 0.001) and improved with therapy (2.15 + 0.38 N/mm) to within WT values. Stress relaxation of AC under compressive loading displayed similar biphasic relaxation time constants (Taufast and Tauslow) between controls and Hyp mice, although Tauslow trended toward slowed relaxation times. In addition, Taufast and Tauslow times correlated with peak load in WT mice (r = 0.80; r = 0.78, respectively), whereas correlation coefficient values for Hyp mice (r = 0.46; r = 0.21) improved with treatment (r = 0.71; r = 0.56). These data provide rationale for therapies that both preserve AC stiffness and recovery from compression. The Hyp mouse also provides unique insight into determinants of structural stiffness and the viscoelastic properties of AC in the progression of OA. © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Evaluation and Application of Silk Fibroin Based Biomaterials to Promote Cartilage Regeneration in Osteoarthritis Therapy

Osteoarthritis (OA) is a common joint disease characterized by cartilage damage and degeneration. Traditional treatments such as NSAIDs and joint replacement surgery only relieve pain and do not achieve complete cartilage regeneration. Silk fibroin (SF) biomaterials are novel materials that have been widely studied and applied to cartilage regeneration. By mimicking the fibrous structure and biological activity of collagen, SF biomaterials can promote the proliferation and differentiation of chondrocytes and contribute to the formation of new cartilage tissue. In addition, SF biomaterials have good biocompatibility and biodegradability and can be gradually absorbed and metabolized by the human body. Studies in recent years have shown that SF biomaterials have great potential in treating OA and show good clinical efficacy. Therefore, SF biomaterials are expected to be an effective treatment option for promoting cartilage regeneration and repair in patients with OA. This article provides an overview of the biological characteristics of SF, its role in bone and cartilage injuries, and its prospects in clinical applications to provide new perspectives and references for the field of bone and cartilage repair.

Anticipated effects of burosumab treatment on long-term clinical sequelae in XLH: expert perspectives

X-linked hypophosphatemia (XLH) is a rare, progressive, genetic disease with multisystem impact that typically begins to manifest in early childhood. Two treatment options exist: oral phosphate in combination with active vitamin D ("conventional therapy") and a fully human monoclonal anti-FGF23 antibody, burosumab. The clinical benefit of conventional therapy in adults is limited, and poor tolerance and complications are common. Burosumab was first approved as a treatment for XLH in 2018 and its disease-modifying benefits in clinical trials in children suggest burosumab treatment could also alter the disease course in adults. Without long-term clinical data on multiple XLH-related sequelae available, the results of an elicitation exercise are reported, in which eight global experts in XLH posited how long-term treatment with burosumab is anticipated to impact the life course of clinical sequelae in adults with XLH. Based on their clinical experiences, the available evidence and their disease understanding, the experts agreed that some long-term benefits of using burosumab are likely in adults with XLH even if they have a misaligned skeleton from childhood. Burosumab treatment is anticipated to reduce the incidence of fractures and halt the progression of clinical sequelae associated with conventional therapy. While the trajectories for established dental abscesses are not expected to improve with burosumab treatment, dental abscess development may be prevented. Starting treatment with burosumab in childhood to increase the likelihood of an aligned skeleton and continuation into and throughout adulthood to maintain euphosphatemia may optimize patient outcomes, although future real-world investigation is required to support this hypothesis.

FGF receptor inhibitor BGJ398 partially rescues osteoarthritis-like phenotype in older high molecular weight FGF2 transgenic mice via multiple mechanisms

We have used Basic Fibroblast Growth Factor (FGF2) transgenic mice as experimental models for human X-linked hypophosphatemia (XLH)-related degenerative osteoarthritis (OA) to investigate the pathogenesis of the disease and to test potential pharmacotherapies for treatment. This study tested the efficacy of BJG398, a small molecule fibroblast growth factor receptor tyrosine kinase (FGFRTK) inhibitor, to rescue the knee joint osteoarthritis phenotype in High Molecular Weight fibroblast growth factor 2 transgenic (HMWTgFGF2) mice. BJG398 was administered in vivo to 8-month-old female HMWTgFGF2 mice for six weeks. Histomorphometry, immunohistochemistry and micro-CT were used to examine the knee joints in BGJ398-treated and control mice. We assessed: Fibroblast Growth Factor 23 (FGF23) expression and FGFR1 activity; Matrix metalloproteinase 13 (MMP13) and Aggrecanase2 (ADAMTS5) expression; then signaling by SMAD1/5/8-pSMAD6, pERK1/2 and Runt-related transcription factor 2 (RUNX2). Using PrimePCR arrays, we identified a contributing role for major target genes in the TGFB/BMP2 signaling pathway that were regulated by BGJ398. BGJ398 inhibited HMWFGF2/FGF23-induced increase in bone morphogenic protein receptor-1, bone morphogenic protein-2 and 4 and Serine peptidase inhibitor, clade E, member 1. The results from Micro-CT and histology show BGJ398 treatment rescued the OA changes in subchondral bone and knee articular cartilage of HMWTgFGF2 mice. The gene expression and signal transduction results provide convincing evidence that HMWFGF2 generates OA through FGFRTK with characteristic downstream signaling that defines OA, namely: increased FGF23-FGFR1 activity with BMP-BMPR, activation of pSMAD1/5/8-RUNX2 and pERK signaling pathways, then upregulation of MMP13 and ADAMTS5 to degrade matrix. BGJ398 treatment effectively reversed these OA molecular phenotypes, providing further evidence that the OA generated by HMWFGF2 in the transgenic mice is FGFR-mediated and phenocopies the OA found in the Hyp mouse homolog of XLH with a spontaneous mutation in the Phex (phosphate regulating endopeptidase on the X chromosome) gene and human XLH-OA. Overall, the results obtained here explain how the pleotropic effects of FGF2 emanate from the different functions of HMW protein isoforms for cartilage and bone homeostasis, and the pathogenesis of XLH-degenerative osteoarthropathy. BGJ398 inhibits HMWFGF2-induced osteoarthritis via multiple mechanisms. These results provided important scientific evidence for the potential application of BGJ398 as a therapeutic agent for osteoarthritis in XLH.

The Enthesopathy of XLH Is a Mechanical Adaptation to Osteomalacia: Biomechanical Evidence from Hyp Mice

A major comorbidity of X-linked hypophosphatemia (XLH) is fibrocartilaginous tendinous insertion site mineralization resulting in painful enthesophytes that contribute to the adult clinical picture and significantly impact physical function. Enthesophytes in Hyp mice, a murine model of XLH are the result of a hyperplastic expansion of resident alkaline phosphatase, Sox9-positive mineralizing fibrochondrocytes. Here, we hypothesized hyperplasia as a compensatory physical adaptation to aberrant mechanical stresses at the level of the entheses interface inserting into pathologically soft bone. To test this hypothesis, we examined the Achilles insertion of the triceps surae developed under normal and impaired loading conditions in Hyp and WT mice. Tensile stiffness, ultimate strength, and maximum strain were measured and compared. Biomechanical testing revealed that under normal loading conditions, despite inserting into a soft bone matrix, both the enthesophyte development (9 weeks) and progression (6-8 months) of Hyp mice were equivalent to the mechanical properties of WT mice. Unloading the insertion during development significantly reduced alkaline phosphatase, Sox9-positive fibrochondrocytes. In WT mice, this correlated with a decrease in stiffness and ultimate strength relative to the control limb, confirming the critical role of mechanical loading in the development of the enthesis. Most significantly, in response to unloading, maximum strain was increased in tensile tests only in the setting of subchondral osteomalacia of Hyp mice. These data suggest that mineralizing fibrochondrocyte expansion in XLH occurs as a compensatory adaptation to the soft bone matrix.

An Evidence-based Physical Therapy Prescription for Adults With X-linked Hypophosphatemia

Context

X-linked hypophosphatemia (XLH) is a rare and progressive metabolic phosphate-wasting disorder characterized by lifelong musculoskeletal comorbidities. Despite considerable physical disability, there are currently no disease-specific physical therapy (PT) recommendations for XLH designed to improve engagement and confidence in performing activities of daily living (ADL).

Objective

The objective of this patient-centered study was to develop an evidence-based PT program to address gaps in the management of adult XLH without imposing unintended harm.

Methods

Creation of the program was informed by a prior controlled clinical study to evaluate the physical and functional effect of XLH on adulthood, and guided by the physical presentation of participants, subjective data and patient goals acquired at intake, and by performance on multiple active range of motion (ROM) movements from the standing position. A weekly standardized interview process was used to assess progression of physical and functional abilities, gains and concerns, and to obtain timely feedback to inform future exercise modifications. Outcomes were evaluated using validated functional tools and subjective data obtained throughout the study.

Results

A remote 12-week PT program was created based on collected data. Open and closed kinetic-chain exercises were developed and implemented. Functional improvements were documented, and weekly surveys indicated improved abilities and confidence to engage in ADL. Minimal improvements were observed in active upper and lower extremity ROM, reflective of substantial bony restrictions characteristic of XLH.

Conclusion

This study represents the first disease-specific PT recommendations for XLH to mitigate the unique physical challenges of the adult disorder that can be modified to adapt to the current progression status of the adult disorder.

Impact of Early Conventional Treatment on Adult Bone and Joints in a Murine Model of X-Linked Hypophosphatemia

X-linked hypophosphatemia (XLH) is the most common form of genetic rickets. Mainly diagnosed during childhood because of growth retardation and deformities of the lower limbs, the disease affects adults with early enthesopathies and joint structural damage that significantly alter patient quality of life. The conventional treatment, based on phosphorus supplementation and active vitamin D analogs, is commonly administered from early childhood to the end of growth; unfortunately, it does not allow complete recovery from skeletal damage. Despite adequate treatment during childhood, bone and joint complications occur in adults and become a dominant feature in the natural history of the disease. Our previous data showed that the Hyp mouse is a relevant model of XLH for studying early enthesophytes and joint structural damage. Here, we studied the effect of conventional treatment on the development of bone and joint alterations in this mouse model during growth and young adulthood. Mice were supplemented with oral phosphorus and calcitriol injections, following two timelines: (i) from weaning to 3 months of age and (ii) from 2 to 3 months to evaluate the effects of treatment on the development of early enthesophytes and joint alterations, and on changes in bone and joint deformities already present, respectively. We showed that early conventional treatment improved bone microarchitecture, and partially prevented bone and joint complications, but with no noticeable improvement in enthesophytes. In contrast, later administration had limited efficacy in ameliorating bone and joint alterations. Despite the improvement in bone microarchitecture, the conventional treatment, early or late, had no effect on osteoid accumulation. Our data underline the usefulness of the Hyp murine model for preclinical studies on skeletal and extraskeletal lesions. Although the early conventional treatment is important for the improvement of bone microarchitecture, the persistence of osteomalacia implies seeking new therapeutic strategies, in particular anti-FGF23 approach, in order to optimize the treatment of XLH.

Giving Credence to the Experience of X-Linked Hypophosphatemia in Adulthood: An Interprofessional Mixed-Methods Study

Purpose

X-linked hypophosphatemia (XLH) is a rare X-linked dominant metabolic bone disease, often diagnosed in childhood but causing increasing physical debilitation and pain in adulthood. Physical comorbidities of XLH in adulthood include pervasive and early-onset degenerative arthritis, mineralizing enthesophytes and osteophytes, osteomalacia and pseudofracture, dental abscesses, and hearing loss.

Methods

This mixed-methods analysis included physical findings, diagnostic imaging, occupational and physical therapy assessments, and semi-structured interviews by social work to understand the functional outcomes and lived experience of XLH in adulthood, through connections between qualitative data obtained by social work and occupational therapy with the quantitative findings from other disciplines.

Results

Three primary themes of chronic pain, fear of falling, and lack of credence given by health care providers emerged from qualitative data and could be correlated with physical findings from functional outcome assessments These concerns contrasted with resilience, adaption, and compensation demonstrated by participants.

Conclusions

While new treatments may ease the burden of disease for adults with XLH, further research, increased provider awareness and education, and further development of standards of care are needed to promote accurate and adequate assessment and intervention for adults with XLH.

Osteoarthritis, Osteophytes, and Enthesophytes Affect Biomechanical Function in Adults With X-linked Hypophosphatemia

CONTEXT

X-Linked hypophosphatemia (XLH) is a lifelong metabolic disease with musculoskeletal comorbidities that dominate the adult clinical presentation.

OBJECTIVE

The adult XLH disorder has yet to be quantified on the basis of the physical and functional limitations that can affect activities of daily living. Our goal was to report the impact of the musculoskeletal manifestations on physical function.

DESIGN AND SETTING

Musculoskeletal function was evaluated by validated questionnaires and in an interdisciplinary clinical space where participants underwent full-body radiologic imaging, goniometric range of motion (ROM) measurements, general performance tests, and kinematic gait analysis.

PATIENTS

Nine adults younger than 60 years with a diagnosis of XLH and self-reported musculoskeletal disability, but able to independently ambulate, were selected to participate. Passive ROM and gait analysis were also performed on age-approximated controls to account for differences between individual laboratory instrumentation.

RESULTS

Enthesophytes, degenerative arthritis, and osteophytes were found to be consistently bilateral and diffusely present at the spine and synovial joints across participants, with predominance at weight-bearing joints. Passive ROM in adults with XLH was decreased at the cervical spine, hip, knee, and ankle compared to controls. Gait analysis relative to controls revealed increased step width, markedly increased lateral trunk sway, and physical restriction at the hip, knees, and ankle joints that translated into limitations through the gait cycle.

CONCLUSIONS

The functional impact of XLH musculoskeletal comorbidities supports the necessity for creating an interprofessional health-care team with the goal of establishing a longitudinal plan of care that considers the manifestations of XLH across the lifespan.

Disrupted Protein Expression and Altered Proteolytic Events in Hypophosphatemic Dentin Can Be Rescued by Dentin Matrix Protein 1

Dentin, one of the four mineralized tissues of the craniofacial complex, forms sequentially from the deposition of an organic matrix to the nucleation of an inorganic phase within the matrix scaffold. Several promoters and inhibitors of mineralization support and regulate mineral nucleation. Clinical and experimental evidence suggest that dentin matrix protein 1 (DMP1) and phosphate-regulating neutral endopeptidase (PHEX) cooperate and are necessary for the formation of a cohesive dentin layer. The following study investigates the effect of PHEX loss-of-function on dentin matrix formation preceding mineralization. Using the Hyp mouse, an animal model for X-linked hypophosphatemia (XLH), we identified an irregular distribution of dentin extracellular matrix proteins. Likewise, dental pulp stem cells (DPSCs) from XLH patients exhibited altered proteolytic events with disrupted extracellular matrix deposition. Further differentiation assays demonstrated that XLH DPSCs exhibited impaired matrix mineralization. Overexpression of DMP1 in XLH DPSCs restored the irregular protein processing patterns to near-physiological levels. Our results support the hypothesis that hypophosphatemia resulting from PHEX loss-of-function affects the integrity of the organization of the dentin matrix and suggests that exogenous DMP1 can restore physiological processing of matrix proteins, in addition to its canonical role in mineralization.

Inhibition of FGFR Signaling Partially Rescues Osteoarthritis in Mice Overexpressing High Molecular Weight FGF2 Isoforms

Fibroblast growth factor 2 (FGF2) and fibroblast growth factor receptors (FGFRs) are key regulatory factors in osteoarthritis (OA). HMWTg mice overexpress the high molecular weight FGF2 isoforms (HMWFGF2) in osteoblast lineage and phenocopy both Hyp mice (which overexpress the HMWFGF2 isoforms in osteoblasts and osteocytes) and humans with X-linked hypophosphatemia (XLH). We previously reported that, similar to Hyp mice and XLH subjects who develop OA, HMWTg mice also develop an OA phenotype associated with increased degradative enzymes and increased FGFR1 compared with VectorTg mice. Therefore, in this study, we examined whether in vivo treatment with the FGFR tyrosine kinase inhibitor NVP-BGJ398 (BGJ) would modulate development of the OA phenotype in knee joints of HMWTg mice. VectorTg and HMWTg mice (21 days of age) were treated with vehicle or BGJ for 13 weeks. Micro-computed tomography images revealed irregular shape and thinning of the subchondral bone with decreased trabecular number and thickness within the epiphyses of vehicle-treated HMWTg knees, which was partially rescued following BGJ treatment. Articular cartilage thickness was decreased in vehicle-treated HMWTg mice, and was restored to the cartilage thickness of VectorTg mice in the BGJ-treated HMWTg group. Increased OA degradative enzymes present in HMWTg vehicle-treated joints decreased after BGJ treatment. OA in HMWTg mice was associated with increased Wnt signaling that was rescued by BGJ treatment. This study demonstrates that overexpression of the HMWFGF2 isoforms in preosteoblasts results in osteoarthropathy that can be partially rescued by FGFR inhibitor via reduction in activated Wnt signaling.

FGF23 and its role in X-linked hypophosphatemia-related morbidity

Background

X-linked hypophosphatemia (XLH) is an inherited disease of phosphate metabolism in which inactivating mutations of the Phosphate Regulating Endopeptidase Homolog, X-Linked (PHEX) gene lead to local and systemic effects including impaired growth, rickets, osteomalacia, bone abnormalities, bone pain, spontaneous dental abscesses, hearing difficulties, enthesopathy, osteoarthritis, and muscular dysfunction. Patients with XLH present with elevated levels of fibroblast growth factor 23 (FGF23), which is thought to mediate many of the aforementioned manifestations of the disease. Elevated FGF23 has also been observed in many other diseases of hypophosphatemia, and a range of animal models have been developed to study these diseases, yet the role of FGF23 in the pathophysiology of XLH is incompletely understood.

Methods

The role of FGF23 in the pathophysiology of XLH is here reviewed by describing what is known about phenotypes associated with various PHEX mutations, animal models of XLH, and non-nutritional diseases of hypophosphatemia, and by presenting molecular pathways that have been proposed to contribute to manifestations of XLH.

Results

The pathophysiology of XLH is complex, involving a range of molecular pathways that variously contribute to different manifestations of the disease. Hypophosphatemia due to elevated FGF23 is the most obvious contributor, however localised fluctuations in tissue non-specific alkaline phosphatase (TNAP), pyrophosphate, calcitriol and direct effects of FGF23 have been observed to be associated with certain manifestations.

Conclusions

By describing what is known about these pathways, this review highlights key areas for future research that would contribute to the understanding and clinical treatment of non-nutritional diseases of hypophosphatemia, particularly XLH.

Joint replacement in X-linked hypophosphatemia

X-linked hypophosphatemia (XLH) is associated with a pervasive, severe degenerative osteoarthritis. We conducted a retrospective chart review/patient survey using the Knee or Hip Osteoarthritis Outcome Score Physical Function Short Form. Fourteen total knee arthroplasties and 7 total hip arthroplasties among 11 patients were included. The mean KOOS-PS score was 31.4 ± 9.7 with a mean follow up of 6.9 years. Mean HOOS-PS score was 14.8 ± 12.9 at a mean follow up of 7.6 years. One knee failed due to aseptic loosening and one hip was revised due to polyethylene wear. In conclusion, total joint arthroplasty is beneficial in XLH.

Vibrational spectroscopic analysis of hydroxyapatite in HYP mice and individuals with X-linked hypophosphatemia

BACKGROUND

X-linked hypophosphatemia (XLH) is the most common form of familial phosphate-wasting disorders, due to an inactivating mutation in the phosphate-regulating neutral endopeptidase, X-linked gene. Persistent osteomalacia, enthesophytes, osteophytes, degenerative arthritis and dental abscesses/periodontal disease dominate the adult disorder. However, the impact of insufficient phosphate on hydroxyapatite composition, the major inorganic component of bone and teeth, is unknown in individuals with XLH.

METHODS

Using Raman spectroscopy, the carbonate (CO3 2-) to phosphate (PO4 3-) ion ratio was measured in HYP and wild-type mice and in primary and permanent teeth from XLH individuals and unaffected controls.

RESULTS

There was a significant difference in carbonate ion substitution between the HYP and wild-type femoral cortical bone (0.36 ± 0.08 versus 0.24 ± 0.04; p < 0.001). Carbonate ion substitution levels were also higher in permanent XLH teeth compared with unaffected individuals (0.39 ± 0.12 versus 0.23 ± 0.04; p < 0.001), but not in primary teeth (0.29 ± 0.11 versus 0.26 ± 0.02; p = 0.29). Complementary Fourier transform infrared analyses demonstrated higher relative intensities of the four major vibrational bands originating from the carbonate anion in XLH teeth compared with unaffected controls.

CONCLUSION

Ionic substitution within the crystal lattice is a common feature of hydroxyapatite and one that confers the physiological properties of bone that impact mechanical strength and the process of bone remodeling. Our data demonstrating anionic substitution in human dentin from individuals with XLH validate the use of dentin as a proxy for bone and to better understand the molecular adaptations that occur in the biochemical milieu of XLH.

FGF23 Regulates Wnt/β-Catenin Signaling-Mediated Osteoarthritis in Mice Overexpressing High-Molecular-Weight FGF2

Although humans with X-linked hypophosphatemia (XLH) and the Hyp mouse, a murine homolog of XLH, are known to develop degenerative joint disease, the exact mechanism that drives the osteoarthritis (OA) phenotype remains unclear. Mice that overexpress high-molecular-weight fibroblast growth factor (FGF) 2 isoforms (HMWTg mice) phenocopy both XLH and Hyp, including OA with increased FGF23 production in bone and serum. Because HMWTg cartilage also has increased FGF23 and there is cross-talk between FGF23-Wnt/β-catenin signaling, the purpose of this study was to determine if OA observed in HMWTg mice is due to FGF23-mediated canonical Wnt signaling in chondrocytes, given that both pathways are implicated in OA pathogenesis. HMWTg OA joints had decreased Dkk1, Sost, and Lrp6 expression with increased Wnt5a, Wnt7b, Lrp5, Axin2, phospho-GSK3β, Lef1, and nuclear β-catenin, as indicated by immunohistochemistry or quantitative PCR analysis. Chondrocytes from HMWTg mice had enhanced alcian blue and alkaline phosphatase staining as well as increased FGF23, Adamts5, Il-1β, Wnt7b, Wnt16, and Wisp1 gene expression and phospho-GSK3β protein expression as indicated by Western blot, compared with chondrocytes of vector control and chondrocytes from mice overexpressing the low-molecular-weight isoform, which were protected from OA. Canonical Wnt inhibitor treatment rescued some of those parameters in HMWTg chondrocytes, seemingly delaying the initially accelerated chondrogenic differentiation. FGF23 neutralizing antibody treatment was able to partly ameliorate OA abnormalities in subchondral bone and reduce degradative/hypertrophic chondrogenic marker expression in HMWTg joints in vivo. These results demonstrate that osteoarthropathy of HMWTg is at least partially due to FGF23-modulated Wnt/β-catenin signaling in chondrocytes.

FGF2 High Molecular Weight Isoforms Contribute to Osteoarthropathy in Male Mice

Humans with X-linked hypophosphatemia (XLH) and Hyp mice, the murine homolog of the disease, develop severe osteoarthropathy and the precise factors that contribute to this joint degeneration remain largely unknown. Fibroblast growth factor 2 (FGF2) is a key regulatory growth factor in osteoarthritis. Although there are multiple FGF2 isoforms the potential involvement of specific FGF2 isoforms in joint degradation has not been investigated. Mice that overexpress the high molecular weight FGF2 isoforms in bone (HMWTg mice) phenocopy Hyp mice and XLH subjects and Hyp mice overexpress the HMWFGF2 isoforms in osteoblasts and osteocytes. Given that Hyp mice and XLH subjects develop osteoarthropathies we examined whether HMWTg mice also develop knee joint degeneration at 2, 8, and 18 mo compared with VectorTg (control) mice. HMWTg mice developed spontaneous osteoarthropathy as early as age 2 mo with thinning of subchondral bone, osteophyte formation, decreased articular cartilage thickness, abnormal mineralization within the joint, increased cartilage degradative enzymes, hypertrophic markers, and angiogenesis. FGF receptors 1 and 3 and fibroblast growth factor 23 were significantly altered compared with VectorTg mice. In addition, gene expression of growth factors and cytokines including bone morphogenetic proteins, Insulin like growth factor 1, Interleukin 1 beta, as well as transcription factors Sex determining region Y box 9, hypoxia inducible factor 1, and nuclear factor kappa B subunit 1 were differentially modulated in HMWTg compared with VectorTg. This study demonstrates that overexpression of the HMW isoforms of FGF2 in bone results in catabolic activity in joint cartilage and bone that leads to osteoarthropathy.

Dmp1 Null Mice Develop a Unique Osteoarthritis-like Phenotype

Patients with hypophosphatemia rickets (including DMP1 mutations) develop severe osteoarthritis (OA), although the mechanism is largely unknown. In this study, we first identified the expression of DMP1 in hypertrophic chondrocytes using immunohistochemistry (IHC) and X-gal analysis of Dmp1-knockout-lacZ-knockin heterozygous mice. Next, we characterized the OA-like phenotype in Dmp1 null mice from 7-week-old to one-year-old using multiple techniques, including X-ray, micro-CT, H&E staining, Goldner staining, scanning electronic microscopy, IHC assays, etc. We found a classical OA-like phenotype in Dmp1 null mice such as articular cartilage degradation, osteophyte formation, and subchondral osteosclerosis. These Dmp1 null mice also developed unique pathological changes, including a biphasic change in their articular cartilage from the initial expansion of hypertrophic chondrocytes at the age of 1-month to a quick diminished articular cartilage layer at the age of 3-months. Further, these null mice displayed severe enlarged knees and poorly formed bone with an expanded osteoid area. To address whether DMP1 plays a direct role in the articular cartilage, we deleted Dmp1 specifically in hypertrophic chondrocytes by crossing the Dmp1-loxP mice with Col X Cre mice. Interestingly, these conditional knockout mice didn't display notable defects in either the articular cartilage or the growth plate. Because of the hypophosphatemia remained in the entire life span of the Dmp1 null mice, we also investigated whether a high phosphate diet would improve the OA-like phenotype. A 8-week treatment of a high phosphate diet significantly rescued the OA-like defect in Dmp1 null mice, supporting the critical role of phosphate homeostasis in maintaining the healthy joint morphology and function. Taken together, this study demonstrates a unique OA-like phenotype in Dmp1 null mice, but a lack of the direct impact of DMP1 on chondrogenesis. Instead, the regulation of phosphate homeostasis by DMP1 via the axis of “FGF23-renal phosphorus reabsorption” is vital for maintaining a healthy joint.

Phlpp1 facilitates post-traumatic osteoarthritis and is induced by inflammation and promoter demethylation in human osteoarthritis

OBJECTIVE

Osteoarthritis (OA) is the most common form of arthritis and a leading cause of disability. OA is characterized by articular chondrocyte deterioration, subchondral bone changes and debilitating pain. One strategy to promote cartilage regeneration and repair is to accelerate proliferation and matrix production of articular chondrocytes. We previously reported that the protein phosphatase Phlpp1 controls chondrocyte differentiation by regulating the activities of anabolic kinases. Here we examined the role of Phlpp1 in OA progression in a murine model. We also assessed PHLPP1 expression and promoter methylation.

DESIGN

Knee joints of WT and Phlpp1(-/-) mice were surgically destabilized by transection of the medial meniscal ligament (DMM). Mice were assessed for signs of OA progression via radiographic and histological analyses, and pain assessment for mechanical hypersensitivity using the von Frey assay. Methylation of the PHLPP1 promoter and PHLPP1 expression were evaluated in human articular cartilage and chondrocyte cell lines.

RESULTS

Following DMM surgeries, Phlpp1 deficient mice showed fewer signs of OA and cartilage degeneration. Mechanical allodynia associated with DMM surgeries was also attenuated in Phlpp1(-/-) mice. PHLPP1 was highly expressed in human articular cartilage from OA patients, but was undetectable in cartilage specimens from femoral neck fractures (FNFxs). Higher PHLPP1 levels correlated with less PHLPP1 promoter CpG methylation in cartilage from OA patients. Blocking cytosine methylation or treatment with inflammatory mediators enhanced PHLPP1 expression in human chondrocyte cell lines.

CONCLUSION

Phlpp1 deficiency protects against OA progression while CpG demethylation and inflammatory cytokines promote PHLPP1 expression.

Novel anatomic adaptation of cortical bone to meet increased mineral demands of reproduction

The goal of this study was to investigate the effects of reproductive adaptations to mineral homeostasis on the skeleton in a mouse model of compromised mineral homeostasis compared to adaptations in control, unaffected mice. During pregnancy, maternal adaptations to high mineral demand include more than doubling intestinal calcium absorption by increasing calcitriol production. However, calcitriol biosynthesis is impaired in HYP mice, a murine model of X-linked hypophosphatemia (XLH). In addition, there is a paucity of mineralized trabecular bone, a primary target of bone resorption during pregnancy and lactation. Because the highest density of mineral is in mature cortical bone, we hypothesized that mineral demand is met by utilizing intracortical mineral reserves. Indeed, analysis of HYP mice revealed dramatic increases in intracortical porosity characterized by elevated serum PTH and type-I collagen matrix-degrading enzyme MMP-13. We discovered an increase in carbonate ion substitution in the bone mineral matrix during pregnancy and lactation of HYP mice, suggesting an alternative mechanism of bone remodeling that maintains maternal bone mass during periods of high mineral demand. This phenomenon is not restricted to XLH, as increased carbonate in the mineral matrix also occurred in wild-type mice during lactation. Taken together, these data suggest that increased intracortical perilacunar mineral turnover also contributes to maintaining phosphate levels during periods of high mineral demand. Understanding the mechanisms of skeletal contribution to mineral homeostasis is important to improving the treatment and prevention of fracture risk and bone fragility for female patients with XLH, but also provides important insight into the role and unique adaptations of the maternal skeleton to the demands of fetal development and the needs of postnatal nutrition.

Hypophosphatemic rickets: lessons from disrupted FGF23 control of phosphorus homeostasis

Fibroblast growth factor-23 (FGF23) regulates phosphate reabsorption in the kidney and therefore plays an essential role in phosphate balance in humans. There is a host of defects that ultimately lead to excess FGF23 levels and thereby cause renal phosphate wasting and hypophosphatemic rickets. We describe the genetic, pathophysiologic, and clinical aspects of this group of disorders with a focus on X-linked hypophosphatemia (XLH), the best characterized of these abnormalities. We also discuss autosomal dominant hypophosphatemic rickets (ADHR), autosomal recessive hypophosphatemic rickets (ARHR) and tumor-induced osteomalacia (TIO) in addition to other rarer FGF23-mediated conditions. We contrast the FGF23-mediated disorders with FGF23-independent hypophosphatemia, specifically hypophosphatemic rickets with hypercalciuria (HHRH). Errant diagnosis of hypophosphatemic disorders is common. This review aims to enhance the recognition and appropriate diagnosis of hypophosphatemia and to guide appropriate treatment.

Contrast-enhanced microCT (EPIC-μCT) ex vivo applied to the mouse and human jaw joint

OBJECTIVES

The temporomandibular joint (TMJ) is susceptive to the development of osteoarthritis (OA). More detailed knowledge of its development is essential to improve our insight into TMJ-OA. It is imperative to have a standardized reliable three-dimensional (3D) imaging method that allows for detailed assessment of both bone and cartilage in healthy and diseased joints. We aimed to determine the applicability of a contrast-enhanced microCT (µCT) technique for ex vivo research of mouse and human TMJs.

METHODS

Equilibrium partitioning of an ionic contrast agent via µCT (EPIC-µCT) was previously applied for cartilage assessment in the knee joint. The method was ex vivo, applied to the mouse TMJ and adapted for the human TMJ.

RESULTS

EPIC-µCT (30-min immersion time) was applied to mouse mandibular condyles, and 3D imaging revealed an average cartilage thickness of 110 ± 16 µm. These measurements via EPIC-µCT were similar to the histomorphometric measures (113 ± 19 µm). For human healthy OA-affected TMJ samples, the protocol was adjusted to an immersion time of 1 h. 3D imaging revealed a significant thicker cartilage layer in joints with early signs of OA compared with healthy joints (414.2 ± 122.6 and 239.7 ± 50.5 µm, respectively). A subsequent significant thinner layer was found in human joints with late signs of OA (197.4 ± 159.7 µm).

CONCLUSIONS

The EPIC-µCT technique is effective for the ex vivo assessment of 3D cartilage morphology in the mouse as well as human TMJ and allows bone-cartilage interaction research in TMJ-OA.

Mineralizing enthesopathy is a common feature of renal phosphate-wasting disorders attributed to FGF23 and is exacerbated by standard therapy in hyp mice

We have previously confirmed a paradoxical mineralizing enthesopathy as a hallmark of X-linked hypophosphatemia. X-linked hypophosphatemia is the most common of the phosphate-wasting disorders mediated by elevated fibroblast growth factor 23 (FGF23) and occurs as a consequence of inactivating mutations of the PHEX gene product. Despite childhood management of the disease, these complications of tendon and ligament insertion sites account for a great deal of the disease's morbidity into adulthood. It is unclear whether the enthesopathy occurs in other forms of renal phosphate-wasting disorders attributable to high FGF23 levels. Here we describe two patients with autosomal recessive hypophosphatemic rickets due to the Met1Val mutation in dentin matrix acidic phosphoprotein 1 (DMP1). In addition to the biochemical and skeletal features of long-standing rickets with elevated FGF23 levels, these individuals exhibited severe, debilitating, generalized mineralized enthesopathy. These data suggest that enthesophytes are a feature common to FGF23-mediated phosphate-wasting disorders. To address this possibility, we examined a murine model of FGF23 overexpression using a transgene encoding the secreted form of human FGF23 (R176Q) cDNA (FGF23-TG mice). We report that FGF23-TG mice display a similar mineralizing enthesopathy of the Achilles and plantar facial insertions. In addition, we examined the impact of standard therapy for phosphate-wasting disorders on enthesophyte progression. We report that fibrochondrocyte hyperplasia persisted in Hyp mice treated with oral phosphate and calcitriol. In addition, treatment had the untoward effect of further exacerbating the mineralization of fibrochondrocytes that define the bone spur of the Achilles insertion. These studies support the need for newer interventions targeted at limiting the actions of FGF23 and minimizing both the toxicities and potential morbidities associated with standard therapy.

The expanding family of hypophosphatemic syndromes

Investigation of X-linked hypophosphatemia (XLH) has led to the identification of a novel phosphate-regulating homeostatic system. Initially considered vitamin D-refractory rickets, renal phosphate wasting was identified as the cardinal biochemical feature of XLH and several related disorders. Current therapy employs calcitriol and phosphate, which usually improves, but does not completely heal deformities and short stature. Later complications of XLH include development of osteophytes, entheses, and osteoarthritis. The mutated gene in XLH, PHEX, is expressed in osteocytes, but its role in the pathogenesis of phosphate wasting is poorly understood. Many hypophosphatemic disorders are mediated by FGF23, a unique fibroblast growth factor with endocrine properties. Renal action of FGF23 leads to reduced expression of type II sodium-phosphate co-transporters, as well as reduced expression of CYP27B1, which encodes vitamin D 1α-hydroxylase. FGF23-mediated hypophosphatemia is characterized by inappropriately normal circulating 1,25-dihydroxyvitamin D together with renal phosphate wasting. The FGF23 system serves as a novel mechanism by which the mineralizing skeleton can communicate phosphate supply to the kidney and thereby mediate excretion or conservation of this important skeletal component. Other forms of FGF23-mediated hypophosphatemia represent various aberrations in this axis. Secretion of excess FGF23 (as in tumor-induced osteomalacia), and mutations preventing proteolytic cleavage of FGF23 result in similar clinical features. Other hypophosphatemic disorders are discussed.