The appearance and modulation of osteocyte marker expression during calcification of vascular smooth muscle cells

Novel insights into osteogenesis and matrix remodelling associated with calcific uraemic arteriolopathy

BACKGROUND

Calcific uraemic arteriolopathy (CUA) or calciphylaxis is a rare, life-threatening disease predominantly occurring in patients with end-stage renal disease. Its pathogenesis has been suggested to include ectopic osteogenesis in soft tissue and the vasculature associated with extracellular matrix (ECM) remodelling.

METHODS

To gain further insights into the pathogenesis of CUA, we performed systematic analyses of skin specimens obtained from seven CUA patients including histology, immunohistochemistry, electron microscopy, electron dispersive X-ray analysis (EDX) and quantitative real-time RT-PCR. Skin specimens of (i) seven patients without chronic kidney disease and without CUA and (ii) seven dialysis patients without CUA served as controls.

RESULTS

In the CUA skin lesions, we observed a significant upregulation of bone morphogenic protein 2 (BMP-2), its target gene Runx2 and its indirect antagonist sclerostin. Furthermore, we detected an increased expression of inactive uncarboxylated matrix Gla protein (Glu-MGP). The upregulation of osteogenesis-associated markers was accompanied by an increased expression of osteopontin, fibronectin, laminin and collagen I indicating an extensive remodelling of the subcutaneous ECM. EDX analysis revealed calcium/phosphate accumulations in the subcutis of all CUA patients with a molar ratio of 1.68 ± 0.06 matching that of hydroxyapatite mineral. Widespread media calcification in cutaneous arterioles was associated with destruction of the endothelial layer and partial exfoliation of the endothelial cells (ECs). CD31 immunostaining revealed aggregates of ECs contributing to intraluminal obstruction and consecutive malperfusion resulting in the clinical picture of ulcerative necrosis in all seven patients.

CONCLUSIONS

Our data indicate that CUA is an active osteogenic process including the upregulation of BMP-2 signalling, hydroxyapatite deposition and extensive matrix remodelling of the subcutis.

New insights into NPP1 function: lessons from clinical and animal studies

The recent elucidation of rare human genetic disorders resulting from mutations in ectonucleotide pyrophosphotase/phosphodiesterase (ENPP1), also known as plasma cell membrane glycoprotein 1 (PC-1), has highlighted the vital importance of this molecule in human health and disease. Generalised arterial calcification in infants (GACI), a frequently lethal disease, has been reported in recessive inactivating mutations in ENPP1. Recent findings have also linked hypophosphataemia to a lack of NPP1 function. A number of human genetic studies have indicated that NPP1 is a vital regulator that influences a wide range of tissues through various signalling pathways and when disrupted can lead to significant pathology. The function of Enpp1 has been widely studied in rodent models, where both the mutant tiptoe walking (ttw/ttw) mouse and genetically engineered Enpp1(-/-) mice show significant alterations in skeletal and soft tissue mineralisation, calcium/phosphate balance and glucose homeostasis. These models therefore provide important tools with which to study the potential mechanisms underpinning the human diseases associated with altered NPP1. This review will focus on the recent advances in our current knowledge of the actions of NPP1 in relation to bone disease, cardiovascular pathologies and diabetes. A fuller understanding of the mechanisms through which NPP1 exerts its pathological effects may stimulate the development of novel therapeutic strategies for patients at risk from the devastating clinical outcomes associated with disrupted NPP1 function.

Sclerostin and Dickkopf-1 as therapeutic targets in bone diseases

The processes of bone growth, modeling, and remodeling determine the structure, mass, and biomechanical properties of the skeleton. Dysregulated bone resorption or bone formation may lead to metabolic bone diseases. The Wnt pathway plays an important role in bone formation and regeneration, and expression of two Wnt pathway inhibitors, sclerostin and Dickkopf-1 (DKK1), appears to be associated with changes in bone mass. Inactivation of sclerostin leads to substantially increased bone mass in humans and in genetically manipulated animals. Studies in various animal models of bone disease have shown that inhibition of sclerostin using a monoclonal antibody (Scl-Ab) increases bone formation, density, and strength. Additional studies show that Scl-Ab improves bone healing in models of bone repair. Inhibition of DKK1 by monoclonal antibody (DKK1-Ab) stimulates bone formation in younger animals and to a lesser extent in adult animals and enhances fracture healing. Thus, sclerostin and DKK1 are emerging as the leading new targets for anabolic therapies to treat bone diseases such as osteoporosis and for bone repair. Clinical trials are ongoing to evaluate the effects of Scl-Ab and DKK1-Ab in humans for the treatment of bone loss and for bone repair.

Calcium scoring in patients with a history of Kawasaki disease

OBJECTIVES

The goal of this study was to assess coronary artery calcification in patients ≥10 years or age with a history of Kawasaki disease (KD).

BACKGROUND

Patients with a history of KD and coronary artery aneurysms are at risk for late morbidity from coronary artery events. It is unknown whether patients with KD with acutely normal or transiently dilated coronary arteries also have increased risk of late coronary artery complications. Coronary calcium scoring using noncontrast computed tomography is a well-established tool for risk-stratifying patients with atherosclerotic coronary artery disease, but there are limited data on its role in evaluating patients with a history of KD.

METHODS

We performed coronary artery calcium (CAC) volume scoring using a low radiation dose computed tomography protocol on 70 patients (median age 20.0 years) with a remote history of KD (median interval from acute KD to imaging 14.8 years). Forty-four (63%) patients had no history of coronary dilation, 12 (17%) had a history of transient dilation, and 14 (20%) had coronary aneurysms.

RESULTS

All of the patients with normal coronary artery internal diameter during the acute phase of KD and 11 of 12 patients with transient dilation had CAC scores of zero. Coronary calcification was observed in 10 of the 14 patients with coronary aneurysms, with the degree of calcification ranging from mild to severe and occurring years after the patients' acute KD.

CONCLUSIONS

Coronary calcification was not observed in patients with a history of KD and normal coronary arteries during the acute phase. Therefore, CAC scanning may be a useful tool to screen patients with a remote history of KD or suspected KD and unknown coronary artery status. Coronary calcification, which may be severe, occurs late in patients with coronary aneurysms. The pathophysiology and clinical implications of coronary calcification in patients with aneurysms are currently unknown and warrant further study.

Intercellular cross-talk among bone cells: new factors and pathways

Intercellular communication within the bone microenvironment is critical for the maintenance of normal bone structure. Osteoblast-lineage cells at all stages of differentiation, from pluripotent precursors to matrix-embedded osteocytes, produce regulatory factors that modulate the differentiation and activity of both bone-forming osteoblasts and bone-resorbing osteoclasts. Osteoclasts can also release factors that feed back to regulate osteoblast activity. Intercellular cross-talk within the bone microenvironment is not restricted only to these bone cells. Other cells within the bone marrow microenvironment, including adipocytes, T cells, and macrophages, play key roles that influence the processes of bone formation and resorption. This review discusses recent work that provides new insights into some of these communication networks and the factors involved, including osteocytic production of receptor activator of nuclear factor-κB ligand (RANKL) and sclerostin, osteoblastic production of interleukin-33, osteoclast-derived Semaphorin 4D, ephrin signaling, and signals from T helper cells and resident osteal macrophages (osteomacs).

Circulating sclerostin and Dickkopf-1 (DKK1) in predialysis chronic kidney disease (CKD): relationship with bone density and arterial stiffness

Abnormalities of bone metabolism and increased vascular calcification are common in chronic kidney disease (CKD) and important causes of morbidity and mortality. The Wnt signaling pathway may play a role in the bone and vascular disturbances seen in CKD, termed collectively "CKD-MBD." The aim of the study was to investigate the possible association of circulating concentrations of the secreted Wnt signaling inhibitors DKK1 and sclerostin with BMD and arterial stiffness in predialysis CKD. Seventy-seven patients (48 M, 29 F), mean age 57 (SD = 14) years with CKD stages 3B (n = 32) and 4 (n = 45) were studied. Sclerostin, DKK1, PTH, and 1,25(OH)(2)D were analyzed. BMD was measured at the lumbar spine (LS), femoral neck (FN), total hip (TH), and forearm (FARM). Arterial stiffness index was determined by contour analysis of digital volume pulse (SI(DVP)). There was a positive correlation between sclerostin and age (r = 0.47, p < 0.000). Sclerostin was higher in men than women (p = 0.013). Following correction for age and gender, there was a negative association between GFR and sclerostin (p = 0.002). We observed a positive association between sclerostin and BMD at the LS (p = 0.0001), FN (p = 0.004), and TH (p = 0.002). In contrast, DKK1 was negatively associated with BMD at the FN (p = 0.038). A negative association was seen between DKK1 and SI(DVP) (p = 0.027). Our data suggest that the Wnt pathway may play a role in CKD-MBD. Prospective studies are required to establish the clinical relevance of sclerostin and DKK1 as serological markers in CKD.

Extracellular matrix mineralization promotes E11/gp38 glycoprotein expression and drives osteocytic differentiation

Osteocytes are terminally differentiated osteoblasts which reside in a mineralized extracellular matrix (ECM). The factors that regulate this differentiation process are unknown. We have investigated whether ECM mineralization could promote osteocyte formation. To do this we have utilised MLO-A5 pre-osteocyte-like cells and western blotting and comparative RT-PCR to examine whether the expression of osteocyte-selective markers is elevated concurrently with the onset of ECM mineralization. Secondly, if mineralization of the ECM is indeed a driver of osteocyte formation, we reasoned that impairment of ECM mineralization would result in a reversible inhibition of osteocyte formation. Supplementation of MLO-A5 cell cultures with ascorbic acid and phosphate promoted progressive ECM mineralization as well as temporally associated increases in expression of the osteocyte-selective markers, E11/gp38 glycoprotein and sclerostin. Consistent with a primary role for ECM mineralization in osteocyte formation, we also found that inhibition of ECM mineralization, by omitting phosphate or adding sodium pyrophosphate, a recognized inhibitor of hydroxyapatite formation, resulted in a 15-fold decrease in mineral deposition that was closely accompanied by lower expression of E11 and other osteocyte markers such as Dmp1, Cd44 and Sost whilst expression of osteoblast markers Ocn and Col1a increased. To rule out the possibility that such restriction of ECM mineralization may produce an irreversible modification in osteoblast behaviour to limit E11 expression and osteocytogenesis, we also measured the capacity of MLO-A5 cells to re-enter the osteocyte differentiation programme. We found that the mineralisation process was re-initiated and closely allied to increased expression of E11 protein after re-administration of phosphate or omission of sodium pyrophosphate, indicating an ECM mineralization-induced restoration in osteocyte formation. These results emphasise the importance of cell-ECM interactions in regulating osteoblast behaviour and, more importantly, suggest that ECM mineralization exerts pivotal control during terminal osteoblast differentiation and acquisition of the osteocyte phenotype.

Mechanisms and clinical consequences of vascular calcification

Vascular calcification has severe clinical consequences and is considered an accurate predictor of future adverse cardiovascular events, including myocardial infarction and stroke. Previously vascular calcification was thought to be a passive process which involved the deposition of calcium and phosphate in arteries and cardiac valves. However, recent studies have shown that vascular calcification is a highly regulated, cell-mediated process similar to bone formation. In this article, we outline the current understanding of key mechanisms governing vascular calcification and highlight the clinical consequences. By understanding better the molecular pathways and genetic circuitry responsible for the pathological mineralization process novel drug targets may be identified and exploited to combat and reduce the detrimental effects of vascular calcification on human health.