The progressive ankylosis gene product ANK regulates extracellular ATP levels in primary articular chondrocytes

Calcium Pyrophosphate Crystal Formation and Deposition: Where Do we Stand and What Does the Future hold?

PURPOSE OF THE REVIEW

Although calcium pyrophosphate deposition (CPPD) has been known since the 1960s, our understanding of its pathogenesis remains rudimentary. This review aims to illustrate the known mechanisms underlying calcium pyrophosphate (CPP) crystal formation and deposition and explore future directions in research. By examining various perspectives, from basic research to clinical and imaging assessments, as well as new emerging methodologies, we can establish a starting point for a deeper understanding of CPPD pathogenesis.

RECENT FINDINGS

Recent years have seen significant advances in CPPD research, particularly in the clinical field with the development of the 2023 ACR/EULAR classification criteria for CPPD disease, and in imaging with the introduction of the OMERACT ultrasonographic definitions and scoring system. However, progress in basic research has been slower. New laboratory approaches, such as Raman spectroscopy and omics sciences, offer promising insights that may help piece together the puzzle of CPPD. CPPD is a common yet understudied condition. As the population ages and CPPD becomes more prevalent, there is an urgent need to better understand the disease and the mechanisms involved in crystal formation and deposition, in order to improve diagnosis and therapeutic approaches.

ENPP1 enzyme replacement therapy improves ectopic calcification but does not rescue skeletal phenotype in a mouse model for craniometaphyseal dysplasia

Craniometaphyseal dysplasia (CMD) is a rare genetic bone disorder, characterized by progressive thickening of craniofacial bones and flared metaphyses of long bones. Craniofacial hyperostosis leads to the obstruction of neural foramina and neurological symptoms such as facial palsy, blindness, deafness, or severe headache. Mutations in ANKH (mouse ortholog ANK), a transporter of small molecules such as citrate and ATP, are responsible for autosomal dominant CMD. Knock-in (KI) mice carrying an ANKF377del mutation (AnkKI/KI ) replicate many features of human CMD. Pyrophosphate (PPi) levels in plasma are significantly reduced in AnkKI/KI mice. PPi is a potent inhibitor of mineralization. To examine the extent to which restoration of circulating PPi levels may prevent the development of a CMD-like phenotype, we treated AnkKI/KI mice with the recombinant human ENPP1-Fc protein IMA2a. ENPP1 hydrolyzes ATP into AMP and PPi. Male and female Ank+/+ and AnkKI/KI mice (n ≥ 6/group) were subcutaneously injected with IMA2a or vehicle weekly for 12 wk, starting at the age of 1 wk. Plasma ENPP1 activity significantly increased in AnkKI/KI mice injected with IMA2a (Vehicle/IMA2a: 28.15 ± 1.65/482.7 ± 331.2 mOD/min; p <.01), which resulted in the successful restoration of plasma PPi levels (Ank+/+ /AnkKI/KI vehicle treatment/AnkKI/KI IMA2a: 0.94 ± 0.5/0.43 ± 0.2/1.29 ± 0.8 μM; p <.01). We examined the skeletal phenotype by X-Ray imaging and μCT. IMA2a treatment of AnkKI/KI mice did not significantly correct CMD features such as the abnormal shape of femurs, increased bone mass of mandibles, hyperostotic craniofacial bones, or the narrowed foramen magnum. However, μCT imaging showed ectopic calcification near basioccipital bones at the level of the foramen magnum and on joints of AnkKI/KI mice. Interestingly, IMA2a treatment significantly reduced the volume of calcified nodules at both sites. Our data demonstrate that IMA2a is sufficient to restore plasma PPi levels and reduce ectopic calcification but fails to rescue skeletal abnormalities in AnkKI/KI mice under our treatment conditions.

A Narrative Review of Chondrocalcinosis: Clinical Presentation, Diagnosis, and Therapies

Calcium pyrophosphate deposition disease is categorized into radiographic chondrocalcinosis, acute calcium pyrophosphate arthritis, chronic calcium pyrophosphate arthritis, and osteoarthritis with calcium pyrophosphate deposition. These entities collectively are characterized by the deposition of calcium into joints, which then may cause localized and systemic inflammation, resulting in pain and swelling in the affected joints. Patients with the ANKH gene are more susceptible to the development of CPP arthritis as are those with primary hyperparathyroidism, hypomagnesemia, and hemochromatosis. Radiographic chondrocalcinosis is asymptomatic. Acute calcium pyrophosphate arthritis results in self-limited periods of joint pain and swelling in the affected joint. Along with localized inflammation, there may also be systemic inflammation characterized by fever and elevated inflammatory markers. Chronic calcium pyrophosphate arthritis results in periods of quiescence interrupted by flares that are identical to acute periods of disease. Osteoarthritis associated calcium pyrophosphate arthritis presents with chronic pain well described in osteoarthritis with periods of acute flares. In 2023, a joint effort by the American College of Rheumatology and the European League Against Rheumatism developed guidelines meant to aid in the recognition of calcium pyrophosphate deposition diseases. The diagnosis is made if there is proof of either crowned dens syndrome or synovial fluid analysis demonstrating calcium pyrophosphate crystals or when more than 56 points are summed utilizing the criteria described in the guidelines. Radiographic chondrocalcinosis requires no therapy. Acute calcium pyrophosphate arthritis is treated with the goal of aborting the flare. Treatment options include nonsteroidal anti-inflammatory drugs (NSAIDs), colchicine, oral corticosteroids, parenteral corticosteroids, intraarticular corticosteroids, IL-1 inhibitors, or parenteral adrenocorticotropic hormone (ACTH). The goal in treatment for chronic calcium pyrophosphate arthritis is the suppression of acute flares. The drugs used for acute flare treatment may be given as maintenance therapy with the additional options of methotrexate and hydroxychloroquine.

Identification of Common Pathogenic Pathways Involved in Hemochromatosis Arthritis and Calcium Pyrophosphate Deposition Disease: a Review

OBJECTIVES

Arthritis is a common clinical manifestation of hereditary hemochromatosis (HH), and HH is one of a handful of conditions linked to calcium pyrophosphate deposition (CPPD) in joints. The connection between these two types of arthritis has not yet been fully elucidated. In light of new pathogenic pathways recently implicated in CPPD involving bone, we reviewed the literature on the etiology of hemochromatosis arthropathy (HHA) seeking shared pathogenic mechanisms.

RESULTS

Clinical observations reinforce striking similarities between HHA and CPPD even in the absence of CPP crystals. They share a similar joint distribution, low grade synovial inflammation, and generalized bone loss. Excess iron damages chondrocytes and bone cells in vitro. While direct effects of iron on cartilage are not consistently seen in animal models of HH, there is decreased osteoblast alkaline phosphatase activity, and increased osteoclastogenesis. These abnormalities are also seen in CPPD. Joint repair processes may also be impaired in both CPPD and HHA.

CONCLUSIONS

Possible shared pathogenic pathways relate more to bone and abnormal damage/repair mechanisms than direct damage to articular cartilage. While additional work is necessary to fully understand the pathogenesis of arthritis in HH and to firmly establish causal links with CPPD, this review provides some plausible hypotheses explaining the overlap of these two forms of arthritis.

Pathogenesis of calcium pyrophosphate deposition disease

Calcium pyrophosphate deposition disease is defined by the presence of calcium pyrophosphate (CPP) crystals in articular cartilage and is the fourth most common type of arthritis in adults. Despite its high prevalence, the etiology of CPPD disease remains unclear and no specific therapies currently exist. It has been known for several decades that abnormalities of cartilage pyrophosphate metabolism are common in patients with CPPD disease, and this classic work will be reviewed here. Recent studies of rare familial forms of CPPD disease have provided additional novel information about its pathophysiology. This work suggests that CPPD disease occurs through at least two unique and potentially intertwined biomolecular pathways. We are hopeful that a detailed understanding of the components and regulation of these pathways will lead to improved therapies for this common disease.

Polycystin-2 Is Required for Chondrocyte Mechanotransduction and Traffics to the Primary Cilium in Response to Mechanical Stimulation

Primary cilia and associated intraflagellar transport are essential for skeletal development, joint homeostasis, and the response to mechanical stimuli, although the mechanisms remain unclear. Polycystin-2 (PC2) is a member of the transient receptor potential polycystic (TRPP) family of cation channels, and together with Polycystin-1 (PC1), it has been implicated in cilia-mediated mechanotransduction in epithelial cells. The current study investigates the effect of mechanical stimulation on the localization of ciliary polycystins in chondrocytes and tests the hypothesis that they are required in chondrocyte mechanosignaling. Isolated chondrocytes were subjected to mechanical stimulation in the form of uniaxial cyclic tensile strain (CTS) in order to examine the effects on PC2 ciliary localization and matrix gene expression. In the absence of strain, PC2 localizes to the chondrocyte ciliary membrane and neither PC1 nor PC2 are required for ciliogenesis. Cartilage matrix gene expression (Acan, Col2a) is increased in response to 10% CTS. This response is inhibited by siRNA-mediated loss of PC1 or PC2 expression. PC2 ciliary localization requires PC1 and is increased in response to CTS. Increased PC2 cilia trafficking is dependent on the activation of transient receptor potential cation channel subfamily V member 4 (TRPV4) activation. Together, these findings demonstrate for the first time that polycystins are required for chondrocyte mechanotransduction and highlight the mechanosensitive cilia trafficking of PC2 as an important component of cilia-mediated mechanotransduction.

P2X7 Receptor Induces Pyroptotic Inflammation and Cartilage Degradation in Osteoarthritis via NF-κB/NLRP3 Crosstalk

Osteoarthritis (OA) is an urgent public health problem; however, the underlying causal mechanisms remain unclear, especially in terms of inflammatory mediators in cartilage degradation and chondrocyte imbalance. P2X7 receptor (P2X7R) is a critical inflammation switch, but few studies have examined its function and mechanisms in OA-like pyroptotic inflammation of chondrocytes. In this study, Sprague–Dawley rats were injected in the knee with monosodium iodoacetate (MIA) to induce OA, followed by multiple intra-articular injections with P2X7R antagonist A740003, P2X7R agonist BzATP, NF-κB inhibitor Bay 11-7082, and NLRP3 inhibitor CY-09. Primary rat chondrocytes were harvested and treated similarly. We assessed cell viability, damage, and death via cell viability assay, lactate dehydrogenase (LDH) release, and flow cytometry. Concentrations of adenosine triphosphate (ATP) and interleukin- (IL-) 1β in cell culture supernatant and joint cavity lavage fluid were analyzed by enzyme-linked immunosorbent assay. Changes in expression levels of P2X7 and inflammation-related indicators were analyzed by immunofluorescence, quantitative reverse-transcription polymerase chain reaction, and western blotting. Cell morphology changes and pyroptosis were observed using transmission electron microscopy. Histology, immunohistochemistry, and microcomputed tomography were used to analyze damage to bone and cartilage tissues and assess the severity of OA. Similar to MIA, BzATP reduced cell viability and collagen II expression in a dose-dependent manner. Conversely, A740003 ameliorated MIA-induced cartilage degradation and OA-like pyroptotic inflammation by rescuing P2X7, MMP13, NF-κB p65, NLRP3, caspase-1 (TUNEL-positive and active), and IL-1β upregulation. Additionally, A740003 reduced the caspase-1/propidium iodide double-positive rate, LDH concentration, and reactive oxygen species production. These effects also occurred via coincubation with Bay 11-7082 and CY-09. In conclusion, activated P2X7 promoted extracellular matrix degradation and pyroptotic inflammation in OA chondrocytes through NF-κB/NLRP3 crosstalk, thus, aggravating the symptoms of OA. The study findings suggest P2X7 as a potential target for inflammation treatment, providing new avenues for OA research and therapy.

Restriction of Dietary Phosphate Ameliorates Skeletal Abnormalities in a Mouse Model for Craniometaphyseal Dysplasia

Craniometaphyseal dysplasia (CMD), a rare genetic bone disorder, is characterized by lifelong progressive thickening of craniofacial bones and metaphyseal flaring of long bones. The autosomal dominant form of CMD is caused by mutations in the progressive ankylosis gene ANKH (mouse ortholog Ank), encoding a pyrophosphate (PPi) transporter. We previously reported reduced formation and function of osteoblasts and osteoclasts in a knockin (KI) mouse model for CMD (Ank^KI/KI) and in CMD patients. We also showed rapid protein degradation of mutant ANK/ANKH. Mutant ANK protein displays reduced PPi transport, which may alter the inorganic phosphate (Pi) and PPi ratio, an important regulatory mechanism for bone mineralization. Here we investigate whether reducing dietary Pi intake can ameliorate the CMD-like skeletal phenotype by comparing male and female Ank^+/+ and Ank^KI/KI mice exposed to a low (0.3%) and normal (0.7%) Pi diet for 13 weeks from birth. Serum Pi and calcium (Ca) levels were not significantly changed by diet, whereas PTH and 25-hydroxy vitamin D (25-OHD) were decreased by low Pi diet but only in male Ank^+/+ mice. Importantly, the 0.3% Pi diet significantly ameliorated mandibular hyperostosis in both sexes of Ank^KI/KI mice. A tendency of decreased femoral trabeculation was observed in male and female Ank^+/+ mice as well as in male Ank^KI/KI mice fed with the 0.3% Pi diet. In contrast, in female Ank^KI/KI mice the 0.3% Pi diet resulted in increased metaphyseal trabeculation. This was also the only group that showed increased bone formation rate. Low Pi diet led to increased osteoclast numbers and increased bone resorption in all mice. We conclude that lowering but not depleting dietary Pi delays the development of craniofacial hyperostosis in CMD mice without severely compromising serum levels of Pi, Ca, PTH, and 25-OHD. These findings may have implications for better clinical care of patients with CMD. © 2020 American Society for Bone and Mineral Research.

IL-6 promotes collagen-induced arthritis by activating the NLRP3 inflammasome through the cathepsin B/S100A9-mediated pathway

Rheumatoid arthritis (RA) is an inflammatory disease with symmetric polyarthritis. IL-6 and NLRP3 inflammasome in macrophages contribute to the pathogenesis of RA. This study aimed to investigate the relationship between IL-6 and the NLRP3 inflammasome in RA. Here, we found that IL-6 inhibition reduced NLRP3 inflammasome activation in mice with collage-induced arthritis (CIA). In vitro studies showed that IL-6 directly induced NLRP3 inflammasome activation via cathepsin B (CTSB) in the presence of ATP. In addition, S100A9 induced by ATP stimulation promoted the interaction of CTSB and NLRP3 to activate the NLRP3 inflammasome. Our findings show a novel mechanism of NLRP3 inflammasome activation by IL-6 that may lead to a potential therapy for RA by interrupting the interaction between IL-6 and the NLRP3 inflammasome.

Purinergic System Signaling in Metainflammation-Associated Osteoarthritis

Inflammation triggered by metabolic imbalance, also called metainflammation, is low-grade inflammation caused by the components involved in metabolic syndrome (MetS), including central obesity and impaired glucose tolerance. This phenomenon is mainly due to excess nutrients and energy, and it contributes to the pathogenesis of osteoarthritis (OA). OA is characterized by the progressive degeneration of articular cartilage, which suffers erosion and progressively becomes thinner. Purinergic signaling is involved in several physiological and pathological processes, such as cell proliferation in development and tissue regeneration, neurotransmission and inflammation. Adenosine and ATP receptors, and other members of the signaling pathway, such as AMP-activated protein kinase (AMPK), are involved in obesity, type 2 diabetes (T2D) and OA progression. In this review, we focus on purinergic regulation in osteoarthritic cartilage and how different components of MetS, such as obesity and T2D, modulate the purinergic system in OA. In that regard, we describe the critical role in this disease of receptors, such as adenosine A2A receptor (A2AR) and ATP P2X7 receptor. Finally, we also assess how nucleotides regulate the inflammasome in OA.

The membrane protein ANKH is crucial for bone mechanical performance by mediating cellular export of citrate and ATP

The membrane protein ANKH was known to prevent pathological mineralization of joints and was thought to export pyrophosphate (PPi) from cells. This did not explain, however, the presence of ANKH in tissues, such as brain, blood vessels and muscle. We now report that in cultured cells ANKH exports ATP, rather than PPi, and, unexpectedly, also citrate as a prominent metabolite. The extracellular ATP is rapidly converted into PPi, explaining the role of ANKH in preventing ankylosis. Mice lacking functional Ank (Ankank/ank mice) had plasma citrate concentrations that were 65% lower than those detected in wild type control animals. Consequently, citrate excretion via the urine was substantially reduced in Ankank/ank mice. Citrate was even undetectable in the urine of a human patient lacking functional ANKH. The hydroxyapatite of Ankank/ank mice contained dramatically reduced levels of both, citrate and PPi and displayed diminished strength. Our results show that ANKH is a critical contributor to extracellular citrate and PPi homeostasis and profoundly affects bone matrix composition and, consequently, bone quality.

Fluorescence Differentiation of ATP-related Multiple Enzymatic Activities in Synovial Fluid as a Marker of Calcium Pyrophosphate Deposition Disease using Kyoto Green

Calcium pyrophosphate deposition disease (CPPD) is a crystal induced inflammation in joints, and causes severe pain in elderly people. The accumulation of pyrophosphate (PPi) in synovial fluid (SF) results from several enzymatic reactions, especially the highly activated e-NPPs, which catalyze the conversion of ATP to PPi. This study demonstrates the detection of relative catalytic activity of 3 enzymes—ecto-nucleotide pyrophosphatase/phosphodiesterases (e-NPPs), tissue nonspecific alkaline phosphatase (TNAP), and ecto-nucleoside triphosphate diphosphohydrolases (e-NTPDases)—using a single molecular sensor called Kyoto Green. Kyoto Green exhibits excellent performance in sensing the catalytic activity of the commercial representatives of the e-NPPs, TNAP, and e-NTPDases, which are ENPP1, PPase, and apyrase, respectively, in both single-enzyme and multi-enzyme assays. Analysis of SF enzymes in 19 SF samples from human and swine revealed moderate activity of e-NPPs, high activity of e-NTPDases, and low activity of TNAP. Our newly developed method for analysis of multiple enzymatic activities using Kyoto Green in biological SF will assist improvement in accuracy of the CPPD prognosis/diagnosis, which will minimize unnecessary medical procedures.

Signaling of the Purinergic System in the Joint

The joint is a complex anatomical structure consisting of different tissues, each with a particular feature, playing together to give mobility and stability at the body. All the joints have a similar composition including cartilage for reducing the friction of the movement and protecting the underlying bone, a synovial membrane that produces synovial fluid to lubricate the joint, ligaments to limit joint movement, and tendons for the interaction with muscles. Direct or indirect damage of one or more of the tissues forming the joint is the foundation of different pathological conditions. Many molecular mechanisms are involved in maintaining the joint homeostasis as well as in triggering disease development. The molecular pathway activated by the purinergic system is one of them.The purinergic signaling defines a group of receptors and intermembrane channels activated by adenosine, adenosine diphosphate, adenosine 5’-triphosphate, uridine triphosphate, and uridine diphosphate. It has been largely described as a modulator of many physiological and pathological conditions including rheumatic diseases. Here we will give an overview of the purinergic system in the joint describing its expression and function in the synovium, cartilage, ligament, tendon, and bone with a therapeutic perspective.

Calcium crystal deposition diseases - beyond gout

The most common types of calcium-containing crystals that are associated with joint and periarticular disorders are calcium pyrophosphate dihydrate (CPP) and basic calcium phosphate (BCP) crystals. Several diverse but difficult-to-treat acute and chronic arthropathies and other clinical syndromes are associated with the deposition of these crystals. Although the pathogenic mechanism of calcium crystal deposition is partially understood, much remains to be investigated, as no drug is available to prevent crystal deposition, permit crystal dissolution or specifically target the pathogenic effects that result in the clinical manifestations. In this Review, the main clinical manifestations of CPP and BCP crystal deposition are discussed, along with the biological effects of these crystals, current therapeutic approaches and future directions in therapy.

Cellular signaling in pseudoxanthoma elasticum: an update

Pseudoxanthoma elasticum is an autosomal recessive genodermatosis with variable expression, due to mutations in the ABCC6 or ENPP1 gene. It is characterized by elastic fiber mineralization and fragmentation, resulting in skin, eye and cardiovascular symptoms. Significant advances have been made in the last 20 years with respect to the phenotypic characterization and pathophysiological mechanisms leading to elastic fiber mineralization. Nonetheless, the substrates of the ABCC6 transporter - the main cause of PXE - remain currently unknown. Though the precise mechanisms linking the ABCC6 transporter to mineralization of the extracellular matrix are unclear, several studies have looked into the cellular consequences of ABCC6 deficiency in PXE patients and/or animal models. In this paper, we compile the evidence on cellular signaling in PXE, which seems to revolve mainly around TGF-βs, BMPs and inorganic pyrophosphate signaling cascades. Where conflicting results or fragmented data are present, we address these with novel signaling data. This way, we aim to better understand the up- and down-stream signaling of TGF-βs and BMPs in PXE and we demonstrate that ANKH deficiency can be an additional mechanism contributing to decreased serum PPi levels in PXE patients.

Cartilage diseases

Hyaline cartilages, fibrocartilages and elastic cartilages play multiple roles in the human body including bearing loads in articular joints and intervertebral discs, providing joint lubrication, forming the external ears and nose, supporting the trachea, and forming the long bones during development and growth. The structure and organization of cartilage's extracellular matrix (ECM) are the primary determinants of normal function. Most diseases involving cartilage lead to dramatic changes in the ECM which can govern disease progression (e.g., in osteoarthritis), cause the main symptoms of the disease (e.g., dwarfism caused by genetically inherited mutations) or occur as collateral damage in pathological processes occurring in other nearby tissues (e.g., osteochondritis dissecans and inflammatory arthropathies). Challenges associated with cartilage diseases include poor understanding of the etiology and pathogenesis, delayed diagnoses due to the aneural nature of the tissue and drug delivery challenges due to the avascular nature of adult cartilages. This narrative review provides an overview of the clinical and pathological features as well as current treatment options available for various cartilage diseases. Late breaking advances are also described in the quest for development and delivery of effective disease modifying drugs for cartilage diseases including osteoarthritis, the most common form of arthritis that affects hundreds of millions of people worldwide.

Review: Unmet Needs and the Path Forward in Joint Disease Associated With Calcium Pyrophosphate Crystal Deposition

Calcium pyrophosphate (CPP) crystal deposition (CPPD) is prevalent and can be associated with synovitis and joint damage. The population of elderly persons predominantly affected by CPPD is growing rapidly. Since shortfalls exist in many aspects of CPPD, we conducted an anonymous survey of CPPD unmet needs, prioritized by experts from the Gout, Hyperuricemia and Crystal-Associated Disease Network. We provide our perspectives on the survey results, and we propose several CPPD basic and clinical translational research pathways. Chondrocyte and cartilage culture systems for generating CPP crystals in vitro and transgenic small animal CPPD models are needed to better define CPPD mechanism paradigms and help guide new therapies. CPPD recognition, clinical research, and care would be improved by international consensus on CPPD nomenclature and disease phenotype classification, better exploitation of advanced imaging, and pragmatic new point-of-care crystal analytic approaches for detecting CPP crystals. Clinical impacts of CPP crystals in osteoarthritis and in asymptomatic joints in elderly persons remain major unanswered questions that are rendered more difficult by current inability to therapeutically limit or dissolve the crystal deposits and assess the consequent clinical outcome. Going forward, CPPD clinical research studies should define clinical settings in which articular CPPD does substantial harm and should include analyses of diverse clinical phenotypes and populations. Clinical trials should identify the best therapeutic targets to limit CPP crystal deposition and associated inflammation and should include assessment of intraarticular agents. Our perspective is that such advances in basic and clinical science in CPPD are now within reach and can lead to better treatments for this disorder.

Mutations in osteoprotegerin account for the CCAL1 locus in calcium pyrophosphate deposition disease

OBJECTIVE

Mutations on chromosomes 5p (CCAL2) and 8q (CCAL1) have been linked to familial forms of calcium pyrophosphate deposition disease (CPDD). Mutations in the ANKH gene account for CCAL2, but the identity of CCAL1 has been elusive. Recently, a single Dutch kindred with a mutation in the Tumor Necrosis Factor Receptor Super Family member 11B (TNFRSF11B) gene coding for osteoprotegerin (OPG) was described as a gain-of-function mutation. Affected family members had premature generalized osteoarthritis (PGOA) and CPDD. As the TNFRSF11B gene is on 8q, we sought additional evidence that TNFRSF11B was CCAL1, and investigated potential disease mechanisms.

DESIGN

DNA from two novel PGOA/CPDD families was screened for sequence variants in the TNFRSF11B gene. Mutations were verified by genotype analysis of affected and unaffected family members. We also investigated effects of normal and mutant OPG on regulators of CPP crystal formation in porcine cartilage.

RESULTS

The identical TNFRSF11B mutation described in the Dutch family was present in two novel PGOA/CPDD families. ANKH was normal in affected patient fibroblasts. Exogenous OPG did not alter ANKH mRNA or protein levels, affect translocation of ANKH to the membrane, nor increase [pyrophosphate (PPi)] or other key regulators of CPDD.

CONCLUSION

We have firmly established the identity of CCAL1 as TNFRSF11B (OPG). Our findings suggest that this mutation produces disease in an ANKH-independent manner via novel mechanisms not primarily targeting cartilage. This work rationalizes further investigation of OPG pathway components as potential druggable targets for CPDD.

TRPV4-dependent induction of a novel mammalian cold-inducible protein SRSF5 as well as CIRP and RBM3

Cold-inducible RNA-binding protein (CIRP) and RNA-binding motif protein 3 (RBM3) are two evolutionarily conserved RNA-binding proteins that are structurally related to hnRNPs and upregulated in response to moderately low temperatures in mammalian cells. Although contributions of splicing efficiency, the gene promoters activated upon mild hypothermia and the transcription factor Sp1 to induction of CIRP have been reported, precise mechanisms by which hypothermia and other stresses induce the expression of mammalian cold-inducible proteins (CIPs) are poorly understood. By screening the serine/arginine-rich splicing factors (SRSFs), we report that the transcript and protein levels of SRSF5 were increased in mammalian cells cultured at 32 °C. Expression of SRSF5 as well as CIRP and RBM3 were also induced by DNA damage, hypoxia, cycloheximide and hypotonicity. Immunohistochemical studies demonstrated that SRSF5 was constitutively expressed in male germ cells and the level was decreased in human testicular germ cell tumors. SRSF5 facilitated production of p19 H-RAS, and increased sensitivity to doxorubicin in human U-2 OS cells. Induction of CIPs was dependent on transient receptor potential vanilloid 4 (TRPV4) channel protein, but seemed independent of its ion channel activity. These findings indicate a previously unappreciated role for the TRP protein in linking environmental stress to splicing.

Endogenous adenosine maintains cartilage homeostasis and exogenous adenosine inhibits osteoarthritis progression

Osteoarthritis (OA) is characterized by cartilage destruction and chondrocytes have a central role in this process. With age and inflammation chondrocytes have reduced capacity to synthesize and maintain ATP, a molecule important for cartilage homeostasis. Here we show that concentrations of ATP and adenosine, its metabolite, fall after treatment of mouse chondrocytes and rat tibia explants with IL-1β, an inflammatory mediator thought to participate in OA pathogenesis. Mice lacking A2A adenosine receptor (A2AR) or ecto-5′nucleotidase (an enzyme that converts extracellular AMP to adenosine) develop spontaneous OA and chondrocytes lacking A2AR develop an ‘OA phenotype' with increased expression of Mmp13 and Col10a1. Adenosine replacement by intra-articular injection of liposomal suspensions containing adenosine prevents development of OA in rats. These results support the hypothesis that maintaining extracellular adenosine levels is an important homeostatic mechanism, loss of which contributes to the development of OA; targeting adenosine A2A receptors might treat or prevent OA.

Osteoarthritis (OA) is a debilitating and destructive joint disease for which disease modifying drugs are not available. Here the authors show that extracellular adenosine signalling via the A2AR receptor on chondrocytes is needed to prevent OA and that liposome-bound adenosine injection can treat the pathology in rats.

The role of the progressive ankylosis protein (ANK) in adipogenic/osteogenic fate decision of precursor cells

The progressive ankylosis protein (ANK) is a transmembrane protein that transports intracellular pyrophosphate (PPi) to the extracellular milieu. In this study we show increased fatty degeneration of the bone marrow of adult ank/ank mice, which lack a functional ANK protein. In addition, isolated bone marrow stromal cells (BMSCs) isolated from ank/ank mice showed a decreased proliferation rate and osteogenic differentiation potential, and an increased adipogenic differentiation potential compared to BMSCs isolated from wild type (WT) littermates. Wnt signaling pathway PCR array analysis revealed that Wnt ligands, Wnt receptors and Wnt signaling proteins that stimulate osteoblast differentiation were expressed at markedly lower levels in ank/ank BMSCs than in WT BMSCs. Lack of ANK function also resulted in impaired bone fracture healing, as indicated by a smaller callus formed and delayed bone formation in the callus site. Whereas 5weeks after fracture, the fractured bone in WT mice was further remodeled and restored to original shape, the fractured bone in ank/ank mice was not fully restored and remodeled to original shape. In conclusion, our study provides evidence that ANK plays a critical role in the adipogenic/osteogenic fate decision of adult mesenchymal precursor cells. ANK functions in precursor cells are required for osteogenic differentiation of these cells during adult bone homeostasis and repair, whereas lack of ANK functions favors adipogenic differentiation.

Cell culture: complications due to mechanical release of ATP and activation of purinoceptors

There is abundant evidence that ATP (adenosine 5′-triphosphate) is released from a variety of cultured cells in response to mechanical stimulation. The release mechanism involved appears to be a combination of vesicular exocytosis and connexin and pannexin hemichannels. Purinergic receptors on cultured cells mediate both short-term purinergic signalling of secretion and long-term (trophic) signalling such as proliferation, migration, differentiation and apoptosis. We aim in this review to bring to the attention of non-purinergic researchers using tissue culture that the release of ATP in response to mechanical stress evoked by the unavoidable movement of the cells acting on functional purinergic receptors on the culture cells is likely to complicate the interpretation of their data.

Fluorescence Sensing of Inorganic Phosphate and Pyrophosphate Using Small Molecular Sensors and Their Applications

The aim of this contribution is to provide an introduction and a brief summary of the principle of fluorescence molecular sensors specific to inorganic phosphate (Pi) and inorganic pyrophosphate (PPi) as well as their applications. In our introduction we describe the impact of both Pi and PPi in the living organism and in the environment, followed by a description of the principle of fluorescence molecular sensors and the sensing mechanism in solution. We then focus on exciting research which has emerged in recent years on the development of fluorescent sensors specific to Pi and PPi, categorized by chemical interactions between the sensor and the target molecule, such as hydrogen bonding, coordination chemistry, displacement assay, aggregation induced emission or quenching, and chemical reactions.

The Role of ANK in Calcium Pyrophosphate Deposition Disease

The protein product of the progressive ankylosis gene, known as ANK, is a 492-amino acid multi-pass transmembrane protein. This protein is critical for the regulation of pyrophosphate, and gain of function ANK mutations is associated with calcium pyrophosphate deposition disease. Much about the structure, function, and regulation of ANK remain unstudied. This review of the current literature examines recent contributions to our understanding of ANK. We focus on new work on the function, binding partners, and regulators of ANK. A more complete understanding of this important protein may help to identify future therapeutic targets for the treatment of calcium pyrophosphate deposition disease.

Regulation of bone and cartilage by adenosine signaling

There is growing recognition that bone serves important endocrine and immunologic functions that are compromised in several disease states. While many factors are known to affect bone metabolism, recent attention has focused on investigating the role of purinergic signaling in bone formation and regulation. Adenosine is a purine nucleoside produced intracellularly and extracellularly in response to stimuli such as hypoxia and inflammation, which then interacts with P1 receptors. Numerous studies have suggested that these receptors play a pivotal role in osteoblast, osteoclast, and chondrocyte differentiation and function. This review discusses the various ways by which adenosine signaling contributes to bone and cartilage homeostasis, while incorporating potential therapeutic applications of these signaling pathways.

Adenosine and adenosine receptors in the pathogenesis and treatment of rheumatic diseases

Adenosine, a nucleoside derived primarily from the extracellular hydrolysis of adenine nucleotides, is a potent regulator of inflammation. Adenosine mediates its effects on inflammatory cells by engaging one or more cell-surface receptors. The expression and function of adenosine receptors on different cell types change during the course of rheumatic diseases, such as rheumatoid arthritis (RA). Targeting adenosine receptors directly for the treatment of rheumatic diseases is currently under study; however, indirect targeting of adenosine receptors by enhancing adenosine levels at inflamed sites accounts for most of the anti-inflammatory effects of methotrexate, the anchor drug for the treatment of RA. In this Review, we discuss the regulation of extracellular adenosine levels and the role of adenosine in regulating the inflammatory and immune responses in rheumatic diseases such as RA, psoriasis and other types of inflammatory arthritis. In addition, adenosine and its receptors are involved in promoting fibrous matrix production in the skin and other organs, and the role of adenosine in fibrosis and fibrosing diseases is also discussed.

Lessons from rare diseases of cartilage and bone

Studying severe phenotypes of rare syndromes can elucidate disease mechanisms of more common disorders and identify potential therapeutic targets. Lessons from rare bone diseases contributed to the development of the most successful class of bone active agents, the bisphosphonates. More recent research on rare bone diseases has helped elucidate key pathways and identify new targets in bone resorption and bone formation including cathepsin K and sclerostin, for which drugs are now in clinical trials. By contrast, there has been much less focus on rare cartilage diseases and osteoarthritis (OA) remains a common disease with no effective therapy. Investigation of rare cartilage syndromes is identifying new potential targets in OA including GDF5 and lubricin. Research on the arthropathy of the ultra-rare disease alkaptonuria has identified several new features of the OA phenotype, including high density mineralized protrusions (HDMPs) which constitute a newly identified mechanism of joint destruction.

Protective effect of apigenin on Freund's complete adjuvant-induced arthritis in rats via inhibiting P2X7/NF-κB pathway

To evaluate the effect of apigenin (AP) on arthritis in rats stimulated by Freund's complete adjuvant (FCA) was the main purpose of the investigation. Arthritis model was established by the administration of 0.1 ml FCA in the palmar surface. AP and diclofenac sodium (DS) were administered to explore and evidence the protective effects against adjuvant-induced arthritis (AA). Cytokines tumor necrosis factor-alpha (TNF-α), interleukin-1 beta (IL-1β) and interleukin-6 (IL-6) were detected to assess the anti-inflammatory effect of AP. Besides, pathological conditions were examined in rat paws. Related-proteins of nuclear factor kappa B (NF-κB) signal pathway activated by P2X7 were investigated to determine the molecular mechanism of AP and their expressions were measured by western blot. The data showed that AP significantly suppressed the expressions of P2X7/NF-κB signal-related proteins and alleviated inflammatory reactions. Therefore, it was assumed that AP might be a potential therapeutic candidate to treat arthritis.

Emerging targets in osteoarthritis therapy

Osteoarthritis (OA) is a destructive joint disease in which the initiation may be attributed to direct injury and mechanical disruption of joint tissues, but the progressive changes are dependent on active cell-mediated processes that can be observed or inferred during the generally long time-course of the disease. Based on clinical observations and experimental studies, it is now recognized a that it is possible for individual patients to exhibit common sets of symptoms and structural abnormalities due to distinct pathophysiological pathways that act independently or in combination. Recent research that has focused on the underlying mechanisms involving biochemical cross talk among the cartilage, synovium, bone, and other joint tissues within a background of poorly characterized genetic factors will be addressed in this review.

TRPV4 as a therapeutic target for joint diseases

Biomechanical factors play a critical role in regulating the physiology as well as the pathology of multiple joint tissues and have been implicated in the pathogenesis of osteoarthritis. Therefore, the mechanisms by which cells sense and respond to mechanical signals may provide novel targets for the development of disease-modifying osteoarthritis drugs (DMOADs). Transient receptor potential vanilloid 4 (TRPV4) is a Ca(2+)-permeable cation channel that serves as a sensor of mechanical or osmotic signals in several musculoskeletal tissues, including cartilage, bone, and synovium. The importance of TRPV4 in joint homeostasis is apparent in patients harboring TRPV4 mutations, which result in the development of a spectrum of skeletal dysplasias and arthropathies. In addition, the genetic knockout of Trpv4 results in the development of osteoarthritis and decreased osteoclast function. In engineered cartilage replacements, chemical activation of TRPV4 can reproduce many of the anabolic effects of mechanical loading to accelerate tissue growth and regeneration. Overall, TRPV4 plays a key role in transducing mechanical, pain, and inflammatory signals within joint tissues and thus is an attractive therapeutic target to modulate the effects of joint diseases. In pathological conditions in the joint, when the delicate balance of TRPV4 activity is altered, a variety of different tools could be utilized to directly or indirectly target TRPV4 activity.

Clodronate exerts an anabolic effect on articular chondrocytes mediated through the purinergic receptor pathway

OBJECTIVE

Bisphosphonates are commonly used anti-osteoporotic drugs which have controversial effects on joint diseases including osteoarthritis. Certain bisphosphonates have been shown to have anabolic effects on cartilage which could have important ramifications for their proposed effects in vivo; however, the underlying mechanisms are poorly understood. Thus, the purpose of this study was to characterize the effects of clodronate on primary articular chondrocyte metabolism and to determine the underlying signaling pathways responsible.

DESIGN

The effects of clodronate and pamidronate on extracellular matrix (ECM) biosynthesis, accumulation and MMP-13 activity were observed in high density, 3D cultures of bovine articular chondrocytes for up to 4 weeks were evaluated. Mechanisms were delineated by measuring intracellular Ca(2+) signaling and the effects of pharmacologic inhibition of the purinergic receptor pathway.

RESULTS

Clodronate (100 μM) induced an anabolic effect (increased biosynthesis by 13-14%) which resulted in an 89-90% increase in ECM accumulation after 4 weeks of culture and without an associated effect on matrix turn-over. Stimulation by clodronate resulted in a 3.3-fold increase in Ca(2+) signaling and pharmacological inhibitor experiments suggested that the anabolic effects exerted by clodronate are transduced through the purinergic receptor pathway.

CONCLUSIONS

These findings support the previous notion that certain bisphosphonates may be useful as adjunctive therapies to potentially ameliorate progression of cartilage degeneration and improve arthritis management.