Disseminated arterial calcification and enhanced myogenic response are associated with abcc6 deficiency in a mouse model of pseudoxanthoma elasticum

Characterization of Atherosclerotic Mice Reveals a Sex-Dependent Susceptibility to Plaque Calcification but No Major Changes in the Lymphatics in the Arterial Wall

Lymphatics participate in reverse cholesterol transport, and their presence in the arterial wall of the great vessels and prior experimental results suggest their possible role in the development of atherosclerosis. The aim of this study was to characterize the lymphatic vasculature of the arterial wall in atherosclerosis. Tissue sections and tissue-cleared aortas of wild-type mice unveiled significant differences in the density of the arterial lymphatic network throughout the arterial tree. Male and female Ldlr-/- and ApoE-/- mice on a Western diet showed sex-dependent differences in plaque formation and calcification. Female mice on a Western diet developed more calcification of atherosclerotic plaques than males. The lymphatic vessels within the aortic wall of these mice showed no major changes regarding the number of lymphatic junctions and end points or the lymphatic area. However, female mice on a Western diet showed moderate dilation of lymphatic vessels in the abdominal aorta and exhibited indications of increased peripheral lymphatic function, findings that require further studies to understand the role of lymphatics in the arterial wall during the development of atherosclerosis.

Bone Marrow-Derived ABCC6 Is an Essential Regulator of Ectopic Calcification In Pseudoxanthoma Elasticum

Physiological calcification of soft tissues is a common occurrence in aging and various acquired and inherited disorders. ABCC6 sequence variations cause the calcification phenotype of pseudoxanthoma elasticum (PXE) as well as some cases of generalized arterial calcification of infancy, which is otherwise caused by defective ENPP1. ABCC6 is primarily expressed in the liver, which has given the impression that the liver is central to the pathophysiology of PXE/generalized arterial calcification of infancy. The emergence of inflammation as a contributor to the calcification in PXE suggested that peripheral tissues play a larger role than expected. In this study, we investigated whether bone marrow-derived ABCC6 contributes to the calcification in PXE. In Abcc6‒/‒ mice, we observed prevalent mineralization in several lymph nodes and surrounding connective tissues and an extensive network of lymphatic vessels within vibrissae, a calcified tissue in Abcc6‒/‒ mice. Furthermore, we found evidence of lymphangiogenesis in patients with PXE and mouse skin, suggesting an inflammatory process. Finally, restoring wild-type bone marrow in Abcc6‒/‒ mice produced a significant reduction of calcification, suggesting that the liver alone is not sufficient to fully inhibit mineralization. With evidence that ABCC6 is expressed in lymphocytes, we suggest that the adaptative immune system and inflammation largely contribute to the calcification in PXE/generalized arterial calcification of infancy.

The Purinergic Nature of Pseudoxanthoma Elasticum

Pseudoxanthoma Elasticum (PXE) is an inherited disease characterized by elastic fiber calcification in the eyes, the skin and the cardiovascular system. PXE results from mutations in ABCC6 that encodes an ABC transporter primarily expressed in the liver and kidneys. It took nearly 15 years after identifying the gene to better understand the etiology of PXE. ABCC6 function facilitates the efflux of ATP, which is sequentially hydrolyzed by the ectonucleotidases ENPP1 and CD73 into pyrophosphate (PPi) and adenosine, both inhibitors of calcification. PXE, together with General Arterial Calcification of Infancy (GACI caused by ENPP1 mutations) as well as Calcification of Joints and Arteries (CALJA caused by NT5E/CD73 mutations), forms a disease continuum with overlapping phenotypes and shares steps of the same molecular pathway. The explanation of these phenotypes place ABCC6 as an upstream regulator of a purinergic pathway (ABCC6 → ENPP1 → CD73 → TNAP) that notably inhibits mineralization by maintaining a physiological Pi/PPi ratio in connective tissues. Based on a review of the literature and our recent experimental data, we suggest that PXE (and GACI/CALJA) be considered as an authentic "purinergic disease". In this article, we recapitulate the pathobiology of PXE and review molecular and physiological data showing that, beyond PPi deficiency and ectopic calcification, PXE is associated with wide and complex alterations of purinergic systems. Finally, we speculate on the future prospects regarding purinergic signaling and other aspects of this disease.

Vascular deficits contributing to skeletal fragility in type 1 diabetes

Over 1 million Americans are currently living with T1D and improvements in diabetes management have increased the number of adults with T1D living into later decades of life. This growing population of older adults with diabetes is more susceptible to aging comorbidities, including both vascular disease and osteoporosis. Indeed, adults with T1D have a 2- to 3- fold higher risk of any fracture and up to 7-fold higher risk of hip fracture compared to those without diabetes. Recently, diabetes-related vascular deficits have emerged as potential risks factors for impaired bone blood flow and poor bone health and it has been hypothesized that there is a direct pathophysiologic link between vascular disease and skeletal outcomes in T1D. Indeed, microvascular disease (MVD), one of the most serious consequences of diabetes, has been linked to worse bone microarchitecture in older adults with T1D compared to their counterparts without MVD. The association between the presence of microvascular complications and compromised bone microarchitecture indicates the potential direct deleterious effect of vascular compromise, leading to abnormal skeletal blood flow, altered bone remodeling, and deficits in bone structure. In addition, vascular diabetic complications are characterized by increased vascular calcification, decreased arterial distensibility, and vascular remodeling with increased arterial stiffness and thickness of the vessel walls. These extensive alterations in vascular structure lead to impaired myogenic control and reduced nitric-oxide mediated vasodilation, compromising regulation of blood flow across almost all vascular beds and significantly restricting skeletal muscle blood flow seen in those with T1D. Vascular deficits in T1D may very well extend to bone, compromising skeletal blood flow control, and resulting in reduced blood flow to bone, thus negatively impacting bone health. Indeed, several animal and ex vivo human studies report that diabetes induces microvascular damage within bone are strongly correlated with diabetes disease severity and duration. In this review article, we will discuss the contribution of diabetes-induced vascular deficits to bone density, bone microarchitecture, and bone blood flow regulation, and review the potential contribution of vascular disease to skeletal fragility in T1D.

Cartilage calcification in osteoarthritis: mechanisms and clinical relevance

Pathological calcification of cartilage is a hallmark of osteoarthritis (OA). Calcification can be observed both at the cartilage surface and in its deeper layers. The formation of calcium-containing crystals, typically basic calcium phosphate (BCP) and calcium pyrophosphate dihydrate (CPP) crystals, is an active, highly regulated and complex biological process that is initiated by chondrocytes and modified by genetic factors, dysregulated mitophagy or apoptosis, inflammation and the activation of specific cellular-signalling pathways. The links between OA and BCP deposition are stronger than those observed between OA and CPP deposition. Here, we review the molecular processes involved in cartilage calcification in OA and summarize the effects of calcium crystals on chondrocytes, synovial fibroblasts, macrophages and bone cells. Finally, we highlight therapeutic pathways leading to decreased joint calcification and potential new drugs that could treat not only OA but also other diseases associated with pathological calcification.

A Reference Range for Plasma Levels of Inorganic Pyrophosphate in Children Using the ATP Sulfurylase Method

PURPOSE

Generalized arterial calcification of infancy, pseudoxanthoma elasticum, autosomal recessive hypophosphatemic rickets type 2, and hypophosphatasia are rare inherited disorders associated with altered plasma levels of inorganic pyrophosphate (PPi). In this study, we aimed to establish a reference range for plasma PPi in the pediatric population, which would be essential to support its use as a biomarker in children with mineralization disorders.

METHODS

Plasma samples were collected from 200 children aged 1 day to 18 years who underwent blood testing for medical conditions not affecting plasma PPi levels. PPi was measured in proband plasma utilizing a validated adenosine triphosphate (ATP) sulfurylase method.

RESULTS

The analytical sensitivity of the ATP sulfurylase assay consisted of 0.15 to 10 µM PPi. Inter- and intra-assay coefficients of variability on identical samples were below 10%. The standard range of PPi in the blood plasma of children and adolescents aged 0 to 18 years was calculated as 2.36 to 4.44 µM, with a median of 3.17 µM, with no difference between male and female probands. PPi plasma levels did not differ significantly in different pediatric age groups.

MAIN CONCLUSIONS

Our results yielded no noteworthy discrepancy to the reported standard range of plasma PPi in adults (2-5 µM). We propose the described ATP sulfurylase method as a diagnostic tool to measure PPi levels in plasma as a biomarker in the pediatric population.

Vascular Calcification in Rodent Models-Keeping Track with an Extented Method Assortment

Simple Summary

Arterial vessel diseases are the leading cause of death in the elderly and their accelerated pathogenesis is responsible for premature death in patients with chronic renal failure. Since no functioning therapy concepts exist so far, the identification of the main signaling pathways is of current research interest. To develop therapeutic concepts, different experimental rodent models are needed, which should be subject to the 3R principle of Russel and Burch: “Replace, Reduce and Refine”. This review aims to summarize the current available experimental rodent models for studying vascular calcification and their quantification methods.

Abstract

Vascular calcification is a multifaceted disease and a significant contributor to cardiovascular morbidity and mortality. The calcification deposits in the vessel wall can vary in size and localization. Various pathophysiological pathways may be involved in disease progression. With respect to the calcification diversity, a great number of research models and detection methods have been established in basic research, relying mostly on rodent models. The aim of this review is to provide an overview of the currently available rodent models and quantification methods for vascular calcification, emphasizing animal burden and assessing prospects to use available methods in a way to address the 3R principles of Russel and Burch: “Replace, Reduce and Refine”.

Regulatory Role of Sex Hormones in Cardiovascular Calcification

Sex differences in cardiovascular disease (CVD), including aortic stenosis, atherosclerosis and cardiovascular calcification, are well documented. High levels of testosterone, the primary male sex hormone, are associated with increased risk of cardiovascular calcification, whilst estrogen, the primary female sex hormone, is considered cardioprotective. Current understanding of sexual dimorphism in cardiovascular calcification is still very limited. This review assesses the evidence that the actions of sex hormones influence the development of cardiovascular calcification. We address the current question of whether sex hormones could play a role in the sexual dimorphism seen in cardiovascular calcification, by discussing potential mechanisms of actions of sex hormones and evidence in pre-clinical research. More advanced investigations and understanding of sex hormones in calcification could provide a better translational outcome for those suffering with cardiovascular calcification.

ABCC6, Pyrophosphate and Ectopic Calcification: Therapeutic Solutions

Pathological (ectopic) mineralization of soft tissues occurs during aging, in several common conditions such as diabetes, hypercholesterolemia, and renal failure and in certain genetic disorders. Pseudoxanthoma elasticum (PXE), a multi-organ disease affecting dermal, ocular, and cardiovascular tissues, is a model for ectopic mineralization disorders. ABCC6 dysfunction is the primary cause of PXE, but also some cases of generalized arterial calcification of infancy (GACI). ABCC6 deficiency in mice underlies an inducible dystrophic cardiac calcification phenotype (DCC). These calcification diseases are part of a spectrum of mineralization disorders that also includes Calcification of Joints and Arteries (CALJA). Since the identification of ABCC6 as the “PXE gene” and the development of several animal models (mice, rat, and zebrafish), there has been significant progress in our understanding of the molecular genetics, the clinical phenotypes, and pathogenesis of these diseases, which share similarities with more common conditions with abnormal calcification. ABCC6 facilitates the cellular efflux of ATP, which is rapidly converted into inorganic pyrophosphate (PPi) and adenosine by the ectonucleotidases NPP1 and CD73 (NT5E). PPi is a potent endogenous inhibitor of calcification, whereas adenosine indirectly contributes to calcification inhibition by suppressing the synthesis of tissue non-specific alkaline phosphatase (TNAP). At present, therapies only exist to alleviate symptoms for both PXE and GACI; however, extensive studies have resulted in several novel approaches to treating PXE and GACI. This review seeks to summarize the role of ABCC6 in ectopic calcification in PXE and other calcification disorders, and discuss therapeutic strategies targeting various proteins in the pathway (ABCC6, NPP1, and TNAP) and direct inhibition of calcification via supplementation by various compounds.

ABCC6 deficiency promotes dyslipidemia and atherosclerosis

ABCC6 deficiency promotes ectopic calcification; however, circumstantial evidence suggested that ABCC6 may also influence atherosclerosis. The present study addressed the role of ABCC6 in atherosclerosis using Ldlr-/- mice and pseudoxanthoma elasticum (PXE) patients. Mice lacking the Abcc6 and Ldlr genes were fed an atherogenic diet for 16 weeks before intimal calcification, aortic plaque formation and lipoprotein profile were evaluated. Cholesterol efflux and the expression of several inflammation, atherosclerosis and cholesterol homeostasis-related genes were also determined in murine liver and bone marrow-derived macrophages. Furthermore, we examined plasma lipoproteins, vascular calcification, carotid intima-media thickness and atherosclerosis in a cohort of PXE patients with ABCC6 mutations and compared results to dysmetabolic subjects with increased cardiovascular risk. We found that ABCC6 deficiency causes changes in lipoproteins, with decreased HDL cholesterol in both mice and humans, and induces atherosclerosis. However, we found that the absence of ABCC6 does not influence overall vascular mineralization induced with atherosclerosis. Decreased cholesterol efflux from macrophage cells and other molecular changes such as increased pro-inflammation seen in both humans and mice are likely contributors for the phenotype. However, it is likely that other cellular and/or molecular mechanisms are involved. Our study showed a novel physiological role for ABCC6, influencing plasma lipoproteins and atherosclerosis in a haploinsufficient manner, with significant penetrance.

Research Models for Studying Vascular Calcification

Calcification of the vessel wall contributes to high cardiovascular morbidity and mortality. Vascular calcification (VC) is a systemic disease with multifaceted contributing and inhibiting factors in an actively regulated process. The exact underlying mechanisms are not fully elucidated and reliable treatment options are lacking. Due to the complex pathophysiology, various research models exist evaluating different aspects of VC. This review aims to give an overview of the cell and animal models used so far to study the molecular processes of VC. Here, in vitro cell culture models of different origins, ex vivo settings using aortic tissue and various in vivo disease-induced animal models are summarized. They reflect different aspects and depict the (patho)physiologic mechanisms within the VC process.

Evidence of Cardiovascular Calcification and Fibrosis in Pseudoxanthoma Elasticum Mouse Models Subjected to DOCA-Salt Hypertension

Pseudoxanthoma Elasticum (PXE) is a rare disorder characterized by fragmentation and progressive calcification of elastic fibres in connective tissues. Although arterial hypertension (AHT) has been reported in PXE patients, its impact on pathological manifestations has as yet been unexplored. We investigated the consequences of experimental AHT on Abcc6−/− PXE mouse models. Experimental AHT was induced by deoxycorticosterone acetate (DOCA-salt) in uni-nephrectomised mice. Blood pressure (BP) and vascular reactivity were monitored using tail-cuff plethysmography and myography respectively. Calcium content and fibrosis were assessed using colorimetry, Von Kossa and Sirius red staining respectively. The gene expression implicated in vascular biology was measured using quantitative polymerase chain reaction. DOCA-salt induced a matching rise in BP in Abcc6−/− and WT mice. Aortic ring contraction and relaxation in vitro were comparable. Calcium accumulated in the hearts of hypertensive Abcc6−/− mice along with significant fibrosis in the myocardium and aorta by contrast with the WT mice. In hypertensive Abcc6−/− mouse aortas, these results were corroborated by gene expression patterns favouring calcification, fibrosis and extracellular matrix remodelling. Abcc6 loss-of-function is associated with greater cardiovascular calcification and fibrosis in mice subjected to DOCA-Salt hypertension. These results suggest likely cardiovascular deterioration in PXE patients with AHT, necessitating diligent BP monitoring.

Endogenous Calcification Inhibitors in the Prevention of Vascular Calcification: A Consensus Statement From the COST Action EuroSoftCalcNet

The physicochemical deposition of calcium-phosphate in the arterial wall is prevented by calcification inhibitors. Studies in cohorts of patients with rare genetic diseases have shed light on the consequences of loss-of-function mutations for different calcification inhibitors, and genetic targeting of these pathways in mice have generated a clearer picture on the mechanisms involved. For example, generalized arterial calcification of infancy (GACI) is caused by mutations in the enzyme ecto-nucleotide pyrophosphatase/phosphodiesterase-1 (eNPP1), preventing the hydrolysis of ATP into pyrophosphate (PP_i). The importance of PP_i for inhibiting arterial calcification has been reinforced by the protective effects of PP_i in various mouse models displaying ectopic calcifications. Besides PP_i, Matrix Gla Protein (MGP) has been shown to be another potent calcification inhibitor as Keutel patients carrying a mutation in the encoding gene or Mgp-deficient mice develop spontaneous calcification of the arterial media. Whereas PP_i and MGP represent locally produced calcification inhibitors, also systemic factors contribute to protection against arterial calcification. One such example is Fetuin-A, which is mainly produced in the liver and which forms calciprotein particles (CPPs), inhibiting growth of calcium-phosphate crystals in the blood and thereby preventing their soft tissue deposition. Other calcification inhibitors with potential importance for arterial calcification include osteoprotegerin, osteopontin, and klotho. The aim of the present review is to outline the latest insights into how different calcification inhibitors prevent arterial calcification both under physiological conditions and in the case of disturbed calcium-phosphate balance, and to provide a consensus statement on their potential therapeutic role for arterial calcification.

Oral administration of pyrophosphate inhibits connective tissue calcification

Various disorders including pseudoxanthoma elasticum (PXE) and generalized arterial calcification of infancy (GACI), which are caused by inactivating mutations in ABCC6 and ENPP1, respectively, present with extensive tissue calcification due to reduced plasma pyrophosphate (PPi). However, it has always been assumed that the bioavailability of orally administered PPi is negligible. Here, we demonstrate increased PPi concentration in the circulation of humans after oral PPi administration. Furthermore, in mouse models of PXE and GACI, oral PPi provided via drinking water attenuated their ectopic calcification phenotype. Noticeably, provision of drinking water with 0.3 mM PPi to mice heterozygous for inactivating mutations in Enpp1 during pregnancy robustly inhibited ectopic calcification in their Enpp1^−/− offspring. Our work shows that orally administered PPi is readily absorbed in humans and mice and inhibits connective tissue calcification in mouse models of PXE and GACI. PPi, which is recognized as safe by the FDA, therefore not only has great potential as an effective and extremely low‐cost treatment for these currently intractable genetic disorders, but also in other conditions involving connective tissue calcification.

Etidronate prevents dystrophic cardiac calcification by inhibiting macrophage aggregation

Cardiovascular calcification is associated with high risk of vascular disease. This involves macrophage infiltration of injured vascular tissue and osteoclast-related processes. Splenic monocytes from mice, that are predisposed (C3H) or resistant (B6) to calcification, were isolated and differentiated in vitro with M-CSF to generate macrophages, which aggregate to form multinucleated (MN) cells in the presence of RANKL. MN cell formation was significantly decreased in monocytes from resistant compared with calcifying mice. Conditioned media from C3H macrophages strongly induced calcification in vitro. However, medium from B6 macrophages inhibited calcification. An increase in ICAM-1 was detected in conditioned media from C3H macrophages compared with B6, suggesting a key role for this molecule in calcification processes. Due to natural genetic loss of Abcc6, the causal gene for cardiac calcification, C3H mice have reduced plasma levels of inorganic pyrophosphate (PPi), a potential calcification inhibitor. Supplementation of C3H mice with PPi or Etidronate prevented but did not completely reverse cardiac calcification. Our data provide strong evidence of the pathogenesis of macrophages and MNs during tissue calcification and suggest PPi or its analogue Etidronate as a potential inhibitor of MN formation and calcification. Furthermore, the adhesion molecule ICAM-1 was shown to play a key role in calcification.

Alteration of Extracellular Nucleotide Metabolism in Pseudoxanthoma Elasticum

Pseudoxanthoma elasticum (PXE) is a rare genetic condition primarily caused by hepatic ABCC6 transporter dysfunction. Most clinical manifestations of PXE are due to premature calcification of elastic fibers. However, the vascular impact of PXE is pleiotropic and remains ill defined. ABCC6 expression has recently been associated with cellular nucleotide export. We studied the impact of ABCC6 deficiency on blood levels of adenosine triphosphate and related metabolites and on soluble nucleotidase activities in PXE patients and Abcc6^-/- mice. In addition, we investigated the expression of genes encoding ectocellular purinergic signaling proteins in mouse liver and aorta. Plasma adenosine triphosphate and pyrophosphate levels were significantly reduced in PXE patients and in Abcc6^-/- mice, whereas adenosine concentration was not modified. Moreover, 5'-nucleotidase/CD73 activity was increased in the serum of PXE patients and Abcc6^-/- mice. Consistent with alterations of purinergic signaling, the expression of genes involved in purine and phosphate transport/metabolism was dramatically modified in Abcc6^-/- mouse aorta, with much less impact on the liver. ABCC6 deficiency causes impaired vascular homeostasis and tissue perfusion. Our findings suggest that these alterations are linked to changes in extracellular nucleotide metabolism that are remote from the liver. This opens new perspectives for the understanding of PXE pathophysiology.

Vascular Smooth Muscle Cells and Arterial Stiffening: Relevance in Development, Aging, and Disease

The cushioning function of large arteries encompasses distension during systole and recoil during diastole which transforms pulsatile flow into a steady flow in the microcirculation. Arterial stiffness, the inverse of distensibility, has been implicated in various etiologies of chronic common and monogenic cardiovascular diseases and is a major cause of morbidity and mortality globally. The first components that contribute to arterial stiffening are extracellular matrix (ECM) proteins that support the mechanical load, while the second important components are vascular smooth muscle cells (VSMCs), which not only regulate actomyosin interactions for contraction but mediate also mechanotransduction in cell-ECM homeostasis. Eventually, VSMC plasticity and signaling in both conductance and resistance arteries are highly relevant to the physiology of normal and early vascular aging. This review summarizes current concepts of central pressure and tensile pulsatile circumferential stress as key mechanical determinants of arterial wall remodeling, cell-ECM interactions depending mainly on the architecture of cytoskeletal proteins and focal adhesion, the large/small arteries cross-talk that gives rise to target organ damage, and inflammatory pathways leading to calcification or atherosclerosis. We further speculate on the contribution of cellular stiffness along the arterial tree to vascular wall stiffness. In addition, this review provides the latest advances in the identification of gene variants affecting arterial stiffening. Now that important hemodynamic and molecular mechanisms of arterial stiffness have been elucidated, and the complex interplay between ECM, cells, and sensors identified, further research should study their potential to halt or to reverse the development of arterial stiffness.

The ABCC6 Transporter: A New Player in Biomineralization

Pseudoxanthoma elasticum (PXE) is an inherited metabolic disease with autosomal recessive inheritance caused by mutations in the ABCC6 gene. Since the first description of the disease in 1896, alleging a disease involving the elastic fibers, the concept evolved with the further discoveries of the pivotal role of ectopic mineralization that is preponderant in the elastin-rich tissues of the skin, eyes and blood vessel walls. After discovery of the causative gene of the disease in 2000, the function of the ABCC6 protein remains elusive. More than 300 mutations have been now reported and the concept of a dermal disease has progressively evolved toward a metabolic disorder resulting from the remote effects caused by lack of a circulating anti-mineralization factor. Very recently, evidence has accumulated that this anti-mineralizing factor is inorganic pyrophosphate (PPi). This leads to decreased PPi/Pi (inorganic phosphate) ratio that results from the lack of extracellular ATP release by hepatocytes and probably renal cells harboring the mutant ABCC6 protein. However, the mechanism by which ABCC6 dysfunction causes diminished ATP release remains an enigma. Studies of other ABC transporters, such as ABCC7 or ABCC1 could help our understanding of what ABCC6 exact function is. Data and a hypothesis on the possible roles of ABCC6 in acquired metabolic diseases are also discussed.

Pseudoxanthoma elasticum

Pseudoxanthoma elasticum (PXE) is a genetic metabolic disease with autosomal recessive inheritance caused by mutations in the ABCC6 gene. The lack of functional ABCC6 protein leads to ectopic mineralization that is most apparent in the elastic tissues of the skin, eyes and blood vessels. The clinical prevalence of PXE has been estimated at between 1 per 100,000 and 1 per 25,000, with slight female predominance. The first clinical sign of PXE is almost always small yellow papules on the nape and sides of the neck and in flexural areas. The papules coalesce, and the skin becomes loose and wrinkled. The mid-dermal elastic fibers are short, fragmented, clumped and calcified. Dystrophic calcification of Bruch's membrane, revealed by angioid streaks, may trigger choroidal neovascularization and, ultimately, loss of central vision and blindness in late-stage disease. Lesions in small and medium-sized artery walls may result in intermittent claudication and peripheral artery disease. Cardiac complications (myocardial infarction, angina pectoris) are thought to be relatively rare but merit thorough investigation. Ischemic strokes have been reported. PXE is a metabolic disease in which circulating levels of an anti-mineralization factor are low. There is good evidence to suggest that the factor is inorganic pyrophosphate (PPi), and that the circulating low levels of PPi and decreased PPi/Pi ratio result from the lack of ATP release by hepatocytes harboring the mutant ABCC6 protein. However, the substrate(s) bound, transported or modulated by the ABCC6 protein remain unknown. More than 300 sequence variants of the ABCC6 gene have been identified. There is no cure for PXE; the main symptomatic treatments are vascular endothelial growth factor inhibitor therapy (for ophthalmic manifestations), lifestyle, lipid-lowering and dietary measures (for reducing vascular risk factors), and vascular surgery (for severe cardiovascular manifestations). Future treatment options may include gene therapy/editing and pharmacologic chaperone therapy.

Klotho suppresses high phosphate-induced osteogenic responses in human aortic valve interstitial cells through inhibition of Sox9

Elevated level of blood phosphate (Pi) associated with chronic kidney disease (CKD) is a risk factor of aortic valve calcification. Aortic valve interstitial cells (AVICs) display osteogenic responses to high Pi although the underlying mechanism is incompletely understood. Sox9 is a pro-chondrogenic factor and may play a role in ectopic tissue calcification. Circulating and kidney levels of Klotho are reduced in patients with CKD. We hypothesized that Sox9 mediates high Pi-induced osteogenic responses in human AVICs and that Klotho inhibits the responses. Treatment of human AVICs with high Pi increased protein levels of Runt-related transcription factor 2 (Runx2) and alkaline phosphatase (ALP), and a prolonged exposure to high Pi caused calcium deposition. High Pi induced Sox9 upregulation through PKD and Akt activation. Knockdown of Sox9 essentially abolished the effect of high Pi on the osteogenic responses. Lower Klotho levels were observed in calcified aortic valve tissues. Interestingly, high Pi decreased Klotho levels in AVICs from normal valves, and treatment with recombinant Klotho markedly reduced the effect of high Pi on the levels of Sox9, Runx2, and ALP and suppressed calcium deposition. We conclude that high Pi induces human AVIC osteogenic responses through Sox9. Human AVICs express Klotho, and its levels in AVICs are modulated by high Pi and valvular calcification. Importantly, Klotho suppresses the pro-osteogenic effect of high Pi on human AVICs. These novel findings indicate that modulation of Klotho may have therapeutic potential for mitigation of valvular calcification associated with CKD.

KEY MESSAGES

CAVD associated with chronic kidney disease is a significant clinical problem. High phosphate upregulates Sox9 through AKT and PKD in human AVICs. Calcified human aortic valves have lower levels of Klotho. Klotho suppresses Sox9 upregulation and intranuclear translocation. Klotho inhibits high phosphate-induced osteogenic activity in human AVICs.

Killing Me Unsoftly: Causes and Mechanisms of Arterial Stiffness

The aorta is a blood vessel that provides a low-resistance path for blood flow directed from the heart to peripheral organs and tissues. However, the aorta has another central hemodynamic function, whereby the elastic nature of the aortic wall provides a significant biomechanical buffering capacity complementing the pulsatile cardiac blood flow, and this is often referred to as Windkessel function. Stiffening of the arterial wall leads to fundamental alterations in central hemodynamics, with widespread detrimental implications for organ function. In this Recent Highlights article, we describe recent contributions in ATVB that have highlighted the novel mechanisms and consequences of arterial stiffness and the clinical conditions in which arterial stiffness occurs, with a focus on advancements in the field.

ABCC6 deficiency is associated with activation of BMP signaling in liver and kidney

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ABCC6 deficiency stimulates BMP signaling in multiple organs.

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ABCC6 deficiency causes tissue-specific induction of BMP-related genes.

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Tissue-specific targeting of BMP signaling may be needed in ABCC deficiency.

Mutations in ABCC6 (ATP-binding cassette, subfamily C, member 6), an orphan transporter expressed in the liver, are the cause of pseudoxanthoma elasticum. Since ABCC6 was reported to affect matrix Gla protein (MGP), an inhibitor of bone morphogenetic proteins (BMPs), we studied BMP signaling and expression in various tissues of mice with and without functional ABCC. Enhanced BMP signaling was found in all examined tissues in the absence of ABCC6. Despite this, the expression of particular BMP proteins varied widely between tissues. Interestingly, the expression of most BMP proteins in the liver moved in the opposite direction to the same BMP proteins in kidneys in response to ABCC6 alterations. Thus, ABCC6 deficiency stimulates BMP signaling by acting on the expression of multiple BMPs.

Vascular calcification: Mechanisms of vascular smooth muscle cell calcification

Vascular calcification is highly prevalent and, when present, is associated with major adverse cardiovascular events. Vascular smooth muscle cells play an integral role in mediating vessel calcification by undergoing differentiation to osteoblast-like cells and generating matrix vesicles that serve as a nidus for calcium-phosphate deposition in the vessel wall. Once believed to be a passive process, it is now recognized that vascular calcification is a complex and highly regulated process that involves activation of cellular signaling pathways, circulating inhibitors of calcification, genetic factors, and hormones. This review will examine several of the key mechanisms linking vascular smooth muscle cells to vessel calcification that may be targeted to reduce vessel wall mineralization and, thereby, reduce cardiovascular risk.