Arterial Calcification Due to Deficiency of CD73 (ACDC) As One of Rheumatic Diseases Associated With Periarticular Calcification

A new perspective on intervertebral disc calcification-from bench to bedside

Disc degeneration primarily contributes to chronic low back and neck pain. Consequently, there is an urgent need to understand the spectrum of disc degeneration phenotypes such as fibrosis, ectopic calcification, herniation, or mixed phenotypes. Amongst these phenotypes, disc calcification is the least studied. Ectopic calcification, by definition, is the pathological mineralization of soft tissues, widely studied in the context of conditions that afflict vasculature, skin, and cartilage. Clinically, disc calcification is associated with poor surgical outcomes and back pain refractory to conservative treatment. It is frequently seen as a consequence of disc aging and progressive degeneration but exhibits unique molecular and morphological characteristics: hypertrophic chondrocyte-like cell differentiation; TNAP, ENPP1, and ANK upregulation; cell death; altered Pi and PPi homeostasis; and local inflammation. Recent studies in mouse models have provided a better understanding of the mechanisms underlying this phenotype. It is essential to recognize that the presentation and nature of mineralization differ between AF, NP, and EP compartments. Moreover, the combination of anatomic location, genetics, and environmental stressors, such as aging or trauma, govern the predisposition to calcification. Lastly, the systemic regulation of calcium and Pi metabolism is less important than the local activity of PPi modulated by the ANK-ENPP1 axis, along with disc cell death and differentiation status. While there is limited understanding of this phenotype, understanding the molecular pathways governing local intervertebral disc calcification may lead to developing disease-modifying drugs and better clinical management of degeneration-related pathologies.

Purines and Adenosine Receptors in Osteoarthritis

OA is a common and debilitating condition that restricts mobility and diminishes the quality of life. Recent work indicates that the generation of adenosine at the cell surface is an important mediator of chondrocyte homeostasis, and topical application of adenosine in a slow-release form (liposomes) can halt the progression of OA and diminish the pain associated with OA. Here, we review the evidence indicating that adenosine, acting at A2A receptors, plays a critical role in endogenous and exogenous treatment and reversal of OA.

Identification and functional characterization of imbalanced osteoarthritis-associated fibronectin splice variants

OBJECTIVE

To identify FN1 transcripts associated with OA pathophysiology and investigate the downstream effects of modulating FN1 expression and relative transcript ratio.

METHODS

FN1 transcriptomic data was obtained from our previously assessed RNA-seq dataset of lesioned and preserved OA cartilage samples from the Research osteoArthritis Articular Cartilage (RAAK) study. Differential transcript expression analysis was performed on all 27 FN1 transcripts annotated in the Ensembl database. Human primary chondrocytes were transduced with lentiviral particles containing short hairpin RNA (shRNA) targeting full-length FN1 transcripts or non-targeting shRNA. Subsequently, matrix deposition was induced in our 3D in vitro neo-cartilage model. Effects of changes in the FN1 transcript ratio on sulphated glycosaminoglycan (sGAG) deposition were investigated by Alcian blue staining and dimethylmethylene blue assay. Moreover, gene expression levels of 17 cartilage-relevant markers were determined by reverse transcription quantitative polymerase chain reaction.

RESULTS

We identified 16 FN1 transcripts differentially expressed between lesioned and preserved cartilage. FN1-208, encoding migration-stimulating factor, was the most significantly differentially expressed protein coding transcript. Downregulation of full-length FN1 and a concomitant increased FN1-208 ratio resulted in decreased sGAG deposition as well as decreased ACAN and COL2A1 and increased ADAMTS-5, ITGB1 and ITGB5 gene expression levels.

CONCLUSION

We show that full-length FN1 downregulation and concomitant relative FN1-208 upregulation was unbeneficial for deposition of cartilage matrix, likely due to decreased availability of the classical RGD (Arg-Gly-Asp) integrin-binding site of fibronectin.

NT5E mutation in sisters who underwent aortic valve replacements for aortic stenosis

Mutations of the NT5E gene encoding the cluster of differentiation 73 (CD73) protein have been found in patients with characteristic calcification of joints and arteries (CALJA). CD73 plays a protective role against aortic valve calcification; therefore, its deletion results in aortic valve calcification. However, to date, there are no reports of a patient with CALJA with aortic stenosis. In this study, we describe 2 extremely rare cases of sisters with identical NT5E gene mutation patterns, both of whom developed late-onset severe aortic stenosis and limb ischaemia. Both patients underwent aortic valve replacement and bilateral distal arterial bypass surgeries successfully. They were genetically diagnosed with CALJA based on the NT5E mutation. Our report suggests that NT5E mutations should be considered in patients requiring aortic valve replacement for a calcified aortic valve and bypass surgery for specific calcified and occluded arteries.

Adenosine A2A receptor null chondrocyte transcriptome resembles that of human osteoarthritic chondrocytes

Adenosine signaling plays a critical role in the maintenance of articular cartilage and may serve as a novel therapeutic for osteoarthritis (OA), a highly prevalent and morbid disease without effective therapeutics in the current market. Mice lacking adenosine A2A receptors (A2AR) develop spontaneous OA by 16 weeks of age, a finding relevant to human OA since loss of adenosine signaling due to diminished adenosine production (NT5E deficiency) also leads to development of OA in mice and humans. To better understand the mechanism by which A2AR and adenosine generation protect from OA development, we examined differential gene expression in neonatal chondrocytes from WT and A2AR null mice. Analysis of differentially expressed genes was analyzed by KEGG pathway analysis, and oPOSSUM and the flatiron database were used to identify transcription factor binding enrichment, and tissue-specific network analyses and patterns were compared to gene expression patterns in chondrocytes from patients with OA. There was a differential expression of 2211 genes (padj<0.05). Pathway enrichment analysis revealed that pro-inflammatory changes, increased metalloprotease, reduced matrix organization, and homeostasis are upregulated in A2AR null chondrocytes. Moreover, stress responses, including autophagy and HIF-1 signaling, seem to be important drivers of OA and bear marked resemblance to the human OA transcriptome. Although A2AR null mice are born with grossly intact articular cartilage, we identify here the molecular foundations for early-onset OA in these mice, further establishing their role as models for human disease and the potential use of adenosine as a treatment for human disease.

Multifocal Calcific Periarthritis with Distinctive Clinical and Radiological Features in Patients with CD73 Deficiency

OBJECTIVE

Arterial calcification due to deficiency of CD73 (ACDC) is a hereditary autosomal recessive ectopic mineralization syndrome caused by loss-of-function mutations in the 5'-nucleotidase Ecto (NT5E) gene. Periarticular calcification has been reported but the clinical characterization of arthritis as well as the microstructure and chemical composition of periarticular calcifications and synovial fluid crystals has not been systematically investigated.

METHODS

Eight ACDC patients underwent extensive rheumatological and radiological evaluation over a period of 11 years. Periarticular and synovial biopsies were obtained from four patients. Characterization of crystal composition was evaluated by compensated polarized light microscopy, Alizarin red staining for synovial fluid along with x-ray diffraction and x-ray micro tomosynthesis for periarticular calcification.

RESULTS

Arthritis in ACDC patients has a clinical presentation of mixed erosive-degenerative joint changes with a median onset of articular symptoms at 17 years of age and progresses over time to the development of fixed deformities and functional limitations of small peripheral joints with eventually, larger joint and distinct axial involvement later in life. We have identified calcium pyrophosphate (CPP) and calcium hydroxyapatite (CHA) crystals in synovial fluid specimens and determined that CHA crystals are the principal component of periarticular calcifications.

CONCLUSION

This is the largest study in ACDC patients to describe erosive peripheral arthropathy and axial enthesopathic calcifications over a period of 11 years and the first to identify the composition of periarticular calcifications and synovial fluid crystals. ACDC should be considered among the genetic causes of early-onset osteoarthritis, as musculoskeletal disease signs may often precede vascular symptoms.

Osteoarthritis in Pseudoxanthoma Elasticum Patients: An Explorative Imaging Study

Pseudoxanthoma elasticum (PXE) is a systemic disease affecting the skin, eyes, and cardiovascular system of patients. Cardiovascular disease is associated with osteoarthritis (OA), which is the most common cause of joint pain. There is a lack of systematic investigations on joint manifestations in PXE in the literature. In this explorative study, we aimed to investigate whether patients with PXE are more at risk for developing osseous signs of OA. Patients with PXE and hospital controls with whole-body low-dose CT examinations available were included. OA was assessed using the OsteoArthritis Computed Tomography (OACT)-score, which is a 4-point Likert scale, in the acromioclavicular (AC), glenohumeral (GH), facet, hip, knee, and ankle joints. Additionally, intervertebral disc degeneration was scored. Data were analyzed using ordinal logistic regression adjusted for age, body mass index (BMI), and smoking status. In total, 106 PXE patients (age 56 (48-64), 42% males, BMI 25.3 (22.7-28.2)) and 87 hospital controls (age 55 (43-67), 46% males, BMI 26.0 (22.5-29.2)) were included. PXE patients were more likely to have a higher OA score for the AC joints (OR 2.00 (1.12-3.61)), tibiofemoral joint (OR 2.63 (1.40-5.07)), and patellofemoral joint (2.22 (1.18-4.24)). For the other joints, the prevalence and severity of OA did not differ significantly. This study suggests that patients with PXE are more likely to have structural OA of the knee and AC joints, which needs clinical confirmation in larger groups and further investigation into the mechanism.

Extracellular Nucleotides Regulate Arterial Calcification by Activating Both Independent and Dependent Purinergic Receptor Signaling Pathways

Arterial calcification, the deposition of calcium-phosphate crystals in the extracellular matrix, resembles physiological bone mineralization. It is well-known that extracellular nucleotides regulate bone homeostasis raising an emerging interest in the role of these molecules on arterial calcification. The purinergic independent pathway involves the enzymes ecto-nucleotide pyrophosphatase/phosphodiesterases (NPPs), ecto-nucleoside triphosphate diphosphohydrolases (NTPDases), 5′-nucleotidase and alkaline phosphatase. These regulate the production and breakdown of the calcification inhibitor—pyrophosphate and the calcification stimulator—inorganic phosphate, from extracellular nucleotides. Maintaining ecto-nucleotidase activities in a well-defined range is indispensable as enzymatic hyper- and hypo-expression has been linked to arterial calcification. The purinergic signaling dependent pathway focusses on the activation of purinergic receptors (P1, P2X and P2Y) by extracellular nucleotides. These receptors influence arterial calcification by interfering with the key molecular mechanisms underlying this pathology, including the osteogenic switch and apoptosis of vascular cells and possibly, by favoring the phenotypic switch of vascular cells towards an adipogenic phenotype, a recent, novel hypothesis explaining the systemic prevention of arterial calcification. Selective compounds influencing the activity of ecto-nucleotidases and purinergic receptors, have recently been developed to treat arterial calcification. However, adverse side-effects on bone mineralization are possible as these compounds reasonably could interfere with physiological bone mineralization.

Biochemical analysis of ectonucleotidases on primary rat vascular smooth muscle cells and in silico investigation of their role in vascular diseases

Vascular smooth muscle cells (VSMCs) exhibit a high degree of plasticity when they undergo the progression from a normal to a disease condition, which makes them a potential target for evaluating early markers and for the development of new therapies. Purinergic signalling plays a key role in vascular tonus control, ATP being an inductor of vasoconstriction, whereas adenosine mediates a vasodilation effect antagonising the ATP actions. The control of extracellular ATP and adenosine levels is done by ectonucleotidases, which represent a potential target to be evaluated in the progression of cardiovascular diseases. In this study, we analysed the basal activity and expression of the ectonucleotidases in aortic rat VSMCs, and we further performed in silico analysis to determine the expression of those enzymes in conditions that mimicked vascular diseases. Cultured in vitro VSMCs showed a prominent expression of Entpd1 followed by Entpd2 and Nt5e (CD73) and very low levels of Entpd3. Slightly faster AMP hydrolysis was observed when compared to ATP and ADP nucleotides. In silico analysis showed that the ectonucleotidases were modulated after induction of conditions that can lead to vascular diseases such as, hypertensive and hypotensive mice models (Nt5e); exposition to high-fat (Entpd1 and Entpd2) or high-phosphate (Nt5e) diet; mechanical stretch (Entpd1, Entpd2 and Nt5e); and myocardial infarction (Entpd1). Our data show that VSMCs are able to efficiently metabolise the extracellular nucleotides generating adenosine. The modulation of Entpd1, Entdp2 and Nt5e in vascular diseases suggests these ectoenzymes as potential targets or markers to be investigated in future studies.

Dysregulation of FOXO1 (Forkhead Box O1 Protein) Drives Calcification in Arterial Calcification due to Deficiency of CD73 and Is Present in Peripheral Artery Disease

Supplemental Digital Content is available in the text.

Objective:

The recessive disease arterial calcification due to deficiency of CD73 (ACDC) presents with extensive nonatherosclerotic medial layer calcification in lower extremity arteries. Lack of CD73 induces a concomitant increase in TNAP (tissue nonspecific alkaline phosphatase; ALPL), a key enzyme in ectopic mineralization. Our aim was to investigate how loss of CD73 activity leads to increased ALPL expression and calcification in CD73-deficient patients and assess whether this mechanism may apply to peripheral artery disease calcification.

Approach and Results:

We previously developed a patient-specific disease model using ACDC primary dermal fibroblasts that recapitulates the calcification phenotype in vitro. We found that lack of CD73-mediated adenosine signaling reduced cAMP production and resulted in increased activation of AKT. The AKT/mTOR (mammalian target of rapamycin) axis blocks autophagy and inducing autophagy prevented calcification; however, we did not observe autophagy defects in ACDC cells. In silico analysis identified a putative FOXO1 (forkhead box O1 protein) binding site in the human ALPL promoter. Exogenous AMP induced FOXO1 nuclear localization in ACDC but not in control cells, and this was prevented with a cAMP analogue or activation of A2a/2b adenosine receptors. Inhibiting FOXO1 reduced ALPL expression and TNAP activity and prevented calcification. Mutating the FOXO1 binding site reduced ALPL promoter activation. Importantly, we provide evidence that non-ACDC calcified femoropopliteal arteries exhibit decreased CD73 and increased FOXO1 levels compared with control arteries.

Conclusions:

These data show that lack of CD73-mediated cAMP signaling promotes expression of the human ALPL gene via a FOXO1-dependent mechanism. Decreased CD73 and increased FOXO1 was also observed in more common peripheral artery disease calcification.

CD73 (Cluster of Differentiation 73) and the Differences Between Mice and Humans

As vascular disease is complex and the various manifestations are influenced by differences in vascular bed architecture, exposure to shear and mechanical forces, cell types involved, and inflammatory responses, in vivo models are necessary to recapitulate the complex physiology and dynamic cellular interactions during pathogenesis. Murine knockout models are commonly used tools for investigators to study the role of a specific gene or pathway in multifaceted disease traits. Although valuable, these models are not perfect, and this is particularly true in regard to CD73 (cluster of differentiation 73), the extracellular enzyme that generates adenosine from AMP. At baseline, CD73-deficient mice do not present with an overt phenotype, whereas CD73-deficient humans present with the complex phenotype of vascular calcification, arteriomegaly and tortuosity, and calcification in small joints. In this review, we highlight the differences between the mouse and human systems and discuss the potential to leverage findings in mice to inform us on the human conditions.

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.