Deletion of the P2Y2 receptor aggravates internal elastic lamina calcification in chronic kidney disease mice through upregulation of alkaline phosphatase and lipocalin-2.

Calcification of the medial layer, inducing arterial stiffness, contributes significantly to cardiovascular mortality in patients with chronic kidney disease (CKD). Extracellular nucleotides block the mineralization of arteries by binding to purinergic receptors including the P2Y2 receptor. This study investigates whether deletion of the P2Y2 receptor influences the development of arterial media calcification in CKD mice. Animals were divided into: (i) wild type mice with normal renal function (control diet) (n = 8), (ii) P2Y2 R-/- mice with normal renal function (n = 8), (iii) wild type mice with CKD (n = 27), and (iv) P2Y2 R-/- mice with CKD (n = 22). To induce CKD, animals received an alternating (0.2-0.3%) adenine diet for 7 weeks. All CKD groups developed a similar degree of chronic renal failure as reflected by high serum creatinine and phosphorus levels. Also, the presence of CKD induced calcification in the heart and medial layer of the aortic wall. However, deletion of the P2Y2 receptor makes CKD mice more susceptible to the development of calcification in the heart and aorta (aortic calcium scores (median ± IQR), CKD-wild type: 0.34 ± 4.3 mg calcium/g wet tissue and CKD-P2Y2 R-/- : 4.0 ± 13.2 mg calcium/g wet tissue). As indicated by serum and aortic mRNA markers, this P2Y2 R-/- mediated increase in CKD-related arterial media calcification was associated with an elevation of calcification stimulators, including alkaline phosphatase and inflammatory molecules interleukin-6 and lipocalin 2. The P2Y2 receptor should be considered as an interesting therapeutic target for tackling CKD-related arterial media calcification.

Endothelial dysfunction aggravates arterial media calcification in warfarin administered rats.

Arterial media calcification is an active cell process. This encompasses osteochondrogenic transdifferentiation of vascular smooth muscle cells followed by the deposition of calcium-phosphate crystals. Increasing evidence suggests a significant role for endothelial cells (ECs) in the development of arterial media calcification. This manuscript explores a role for endothelial dysfunction in the disease progression of arterial media calcification. Male rats were randomly assigned to four different groups. The first group received standard chow. The second group was given L-NAME (≈50 mg kg-1 · d-1 ), to induce endothelial dysfunction, in addition to standard chow. The third group and fourth group received a warfarin-supplemented diet to induce mild calcification and the latter group was co-administered L-NAME. Prior to sacrifice, non-invasive measurement of aortic distensibility was performed. Animals were sacrificed after 6 weeks. Arterial media calcification was quantified by measuring aortic calcium and visualized on paraffin-embedded slices by the Von Kossa method. Arterial stiffness and aortic reactivity was assessed on isolated carotid segments using specialized organ chamber setups. Warfarin administration induced mineralization. Simultaneous administration of warfarin and L-NAME aggravated the arterial media calcification process. Through organ chamber experiments an increased vessel tonus was found, which could be linked to reduced basal NO availability, in arteries of warfarin-treated animals. Furthermore, increased calcification because of L-NAME administration was related to a further compromised endothelial function (next to deteriorated basal NO release also deteriorated stimulated NO release). Our findings suggest early EC changes to impact the disease progression of arterial media calcification.

Metformin and Canagliflozin Are Equally Renoprotective in Diabetic Kidney Disease but Have No Synergistic Effect.

Diabetic Kidney Disease (DKD) is a major microvascular complication for diabetic patients and is the most common cause of chronic kidney disease (CKD) and end-stage renal disease. Antidiabetic drugs, such as metformin and canagliflozin, have been shown to exert renoprotective effects. Additionally, quercetin recently showed promising results for the treatment of DKD. However, the molecular pathways through which these drugs exert their renoprotective effects remain partly unknown. The current study compares the renoprotective potential of metformin, canagliflozin, metformin + canagliflozin, and quercetin in a preclinical rat model of DKD. By combining streptozotocin (STZ) and nicotinamide (NAD) with daily oral N(ω)-Nitro-L-Arginine Methyl Ester (L-NAME) administration, DKD was induced in male Wistar Rats. After two weeks, rats were assigned to five treatment groups, receiving vehicle, metformin, canagliflozin, metformin + canagliflozin, or quercetin for a period of 12 weeks by daily oral gavage. Non-diabetic vehicle-treated control rats were also included in this study. All rats in which diabetes was induced developed hyperglycemia, hyperfiltration, proteinuria, hypertension, renal tubular injury and interstitial fibrosis, confirming DKD. Metformin and canagliflozin, alone or together, exerted similar renoprotective actions and similar reductions in tubular injury and collagen accumulation. Renoprotective actions of canagliflozin correlated with reduced hyperglycemia, while metformin was able to exert these effects even in the absence of proper glycemic control. Gene expression revealed that the renoprotective pathways may be traced back to the NF-κB pathway. No protective effect was seen with quercetin. In this experimental model of DKD, metformin and canagliflozin were able to protect the kidney against DKD progression, albeit in a non-synergistic way. These renoprotective effects may be attributable to the inhibition of the NF-κB pathway.

A Proteomic Screen to Unravel the Molecular Pathways Associated with Warfarin-Induced or TNAP-Inhibited Arterial Calcification in Rats.

Arterial media calcification refers to the pathological deposition of calcium phosphate crystals in the arterial wall. This pathology is a common and life-threatening complication in chronic kidney disease, diabetes and osteoporosis patients. Recently, we reported that the use of a TNAP inhibitor, SBI-425, attenuated arterial media calcification in a warfarin rat model. Employing a high-dimensionality unbiased proteomic approach, we also investigated the molecular signaling events associated with blocking arterial calcification through SBI-425 dosing. The remedial actions of SBI-425 were strongly associated with (i) a significant downregulation of inflammatory (acute phase response signaling) and steroid/glucose nuclear receptor signaling (LXR/RXR signaling) pathways and (ii) an upregulation of mitochondrial metabolic pathways (TCA cycle II and Fatty Acid ß-oxidation I). Interestingly, we previously demonstrated that uremic toxin-induced arterial calcification contributes to the activation of the acute phase response signaling pathway. Therefore, both studies suggest a strong link between acute phase response signaling and arterial calcification across different conditions. The identification of therapeutic targets in these molecular signaling pathways may pave the way to novel therapies against the development of arterial media calcification.

N-acetylcysteine (NAC) differentially affects arterial medial calcification and bone formation: The role of l-cysteine and hydrogen sulphide.

Arterial medial calcification (AMC) is the deposition of calcium phosphate in the arteries. AMC is widely thought to share similarities with physiological bone formation; however, emerging evidence suggests several key differences between these processes. N-acetylcysteine (NAC) displays antioxidant properties and can generate hydrogen sulphide (H2 S) and glutathione (GSH) from its deacetylation to l-cysteine. This study found that NAC exerts divergent effects in vitro, increasing osteoblast differentiation and bone formation by up to 5.5-fold but reducing vascular smooth muscle cell (VSMC) calcification and cell death by up to 80%. In vivo, NAC reduced AMC in a site-specific manner by 25% but had no effect on the bone. The actions of l-cysteine and H2 S mimicked those of NAC; however, the effects of H2 S were much less efficacious than NAC and l-cysteine. Pharmacological inhibition of H2 S-generating enzymes did not alter the actions of NAC or l-cysteine; endogenous production of H2 S was also unaffected. In contrast, NAC and l-cysteine increased GSH levels in calcifying VSMCs and osteoblasts by up to 3-fold. This suggests that the beneficial actions of NAC are likely to be mediated via the breakdown of l-cysteine and the subsequent GSH generation. Together, these data show that while the molecular mechanisms driving the actions of NAC appear similar, the downstream effects on cell function differ significantly between osteoblasts and calcifying VSMCs. The ability of NAC to exert these differential actions further supports the notion that there are differences between the development of pathological AMC and physiological bone formation. NAC could represent a therapeutic option for treating AMC without exerting negative effects on bone.

Progression of established non-diabetic chronic kidney disease is halted by metformin treatment in rats.

Current treatment strategies for chronic kidney disease (CKD) mainly focus on controlling risk factors. Metformin, a first-line drug for type 2 diabetes, exerts beneficial pleiotropic actions beyond its prescribed use and incipient data have revealed protective effects against the development of kidney impairment. This study evaluated the therapeutic efficacy of metformin and canagliflozin, a sodium-glucose cotransporter-2 (SGLT2) inhibitor recently approved by the United States Food and Drug Administration to treat diabetic nephropathy, in slowing the progression of established non-diabetic CKD. Rats with adenine-induced CKD were assigned to different treatment groups to receive either 200 mg/kg metformin, four or five weeks after the start of the adenine diet (established mild-moderate CKD), or 25 mg/kg canagliflozin four weeks after the start of the diet, by daily oral gavage administered during four weeks. Each treatment group was compared to a vehicle group. Chronic adenine dosing resulted in severe CKD in vehicle-treated rats as indicated by a marked rise in serum creatinine levels, a marked decrease in creatinine clearance, and a disturbed mineral metabolism. Metformin, but not canagliflozin, halted functional kidney decline. Additionally, kidneys of metformin-treated animals showed less interstitial area and inflammation as compared to the vehicle group. Proteomic analyses revealed that metformin's kidney-protective effect was associated with the activation of the Hippo signaling pathway, a highly conserved multiprotein kinase cascade that controls tissue development, organ size, cell proliferation, and apoptosis. Thus, metformin demonstrated therapeutic efficacy by halting the progression of established CKD in a rat model.

Runx2 (Runt-Related Transcription Factor 2) Links the DNA Damage Response to Osteogenic Reprogramming and Apoptosis of Vascular Smooth Muscle Cells.

[Figure: see text].

Nutritional Calcium Supply Dependent Calcium Balance, Bone Calcification and Calcium Isotope Ratios in Rats.

Serum calcium isotopes (δ44/42Ca) have been suggested as a non-invasive and sensitive Ca balance marker. Quantitative δ44/42Ca changes associated with Ca flux across body compartment barriers relative to the dietary Ca and the correlation of δ44/42CaSerum with bone histology are unknown. We analyzed Ca and δ44/42Ca by mass-spectrometry in rats after two weeks of standard-Ca-diet (0.5%) and after four subsequent weeks of standard- and of low-Ca-diet (0.25%). In animals on a low-Ca-diet net Ca gain was 61 ± 3% and femur Ca content 68 ± 41% of standard-Ca-diet, bone mineralized area per section area was 68 ± 15% compared to standard-Ca-diet. δ44/42Ca was similar in the diets, and decreased in feces and urine and increased in serum in animals on low-Ca-diet. δ44/42CaBone was higher in animals on low-Ca-diet, lower in the diaphysis than the metaphysis and epiphysis, and unaffected by gender. Independent of diet, δ44/42CaBone was similar in the femora and ribs. At the time of sacrifice, δ44/42CaSerum inversely correlated with intestinal Ca uptake and histological bone mineralization markers, but not with Ca content and bone mineral density by µCT. In conclusion, δ44/42CaBone was bone site specific, but mechanical stress and gender independent. Low-Ca-diet induced marked changes in feces, serum and urine δ44/42Ca in growing rats. δ44/42CaSerum inversely correlated with markers of bone mineralization.

Non-oxidized parathyroid hormone (PTH) measured by current method is not superior to total PTH in assessing bone turnover in chronic kidney disease.

Parathyroid hormone (PTH) is a key regulator of bone turnover but can be oxidized in vivo, which impairs biological activity. Variable PTH oxidation may account for the rather poor correlation of PTH with indices of bone turnover in chronic kidney disease. Here, we tested whether non-oxidized PTH is superior to total PTH as a marker of bone turnover in 31 patients with kidney failure included from an ongoing prospective observational bone biopsy study and selected to cover the whole spectrum of bone turnover. Receiver Operating Characteristic (ROC) curves, Spearman correlation and regression analysis of non-oxidized PTH, total PTH and bone turnover markers (bone-specific alkaline phosphatase, procollagen N-terminal pro-peptide and tartrate-resistant acid phosphatase 5b) were used to assess the capability of non-oxidized PTH vs. total PTH to discriminate low from non-low and high from non-high bone turnover, as assessed quantitatively by bone histomorphometry. Serum levels of non-oxidized PTH and total PTH were strongly and significantly correlated. Histomorphometric parameters of bone turnover and the circulating bone turnover markers showed similar correlation coefficients with non-oxidized PTH and total PTH. The area under the ROC (AUROC) values for discriminating between low/non-low turnover for non-oxidized PTH and total PTH were significant and comparable (0.82 and 0.79, respectively). For high/non-high turnover the AUROCs were also significant and of the same magnitude (0.76 and 0.80, respectively). Thus, measuring non-oxidized PTH using the currently available method provides no added value compared to total PTH as an indicator of bone turnover in patients with kidney failure.

The Role of Bone Biopsy in the Management of CKD-MBD.

A bone biopsy is still considered the gold standard for diagnosis of renal osteodystrophy. It allows to measure both static and dynamic parameters of bone remodeling and is the only method able to evaluate mineralization and allows analysis of both cortical and trabecular bone. Although bone volume can be measured indirectly by dual-energy X-ray absorptiometry, mineralization defects, bone metal deposits, cellular number/activity, and even turnover abnormalities are difficult to determine by techniques other than qualitative bone histomorphometry. In this review, we evaluate the role of bone biopsy in the clinical practice.

Inhibition of tissue non-specific alkaline phosphatase; a novel therapy against arterial media calcification?

Arterial media calcification refers to ectopic mineralization in the arterial wall and favors arterial stiffness and cardiovascular events. Patients with chronic kidney disease (CKD), diabetes, or osteoporosis are highly vulnerable to the development of arterial media calcifications. Tissue non-specific alkaline phosphatase (TNAP) is upregulated in calcified arteries and plays a key role in the degradation of the calcification inhibitor pyrophosphate into inorganic phosphate ions. A recent study published in The Journal of Pathology showed that an oral dosage of 10 or 30 mg/kg/day SBI-425, a selective TNAP inhibitor, inhibited the development of arterial media calcification in mice with CKD, without affecting bone mineralization. Their results indicated that SBI-425 is an effective and safe treatment for arterial media calcification. However, additional studies regarding the effect of TNAP-inhibitor SBI-425 on the progression and even the reversion of pre-existing pathological arterial media calcifications without affecting physiological bone mineralization are deserved. Furthermore, investigating the extent to which SBI-425 inhibits arterial calcification in a non-CKD context would be of particular interest to treat this comorbidity in diabetes and osteoporosis patients. © 2019 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Conditional mouse models support the role of SLC39A14 (ZIP14) in Hyperostosis Cranialis Interna and in bone homeostasis.

Hyperostosis Cranialis Interna (HCI) is a rare bone disorder characterized by progressive intracranial bone overgrowth at the skull. Here we identified by whole-exome sequencing a dominant mutation (L441R) in SLC39A14 (ZIP14). We show that L441R ZIP14 is no longer trafficked towards the plasma membrane and excessively accumulates intracellular zinc, resulting in hyper-activation of cAMP-CREB and NFAT signaling. Conditional knock-in mice overexpressing L438R Zip14 in osteoblasts have a severe skeletal phenotype marked by a drastic increase in cortical thickness due to an enhanced endosteal bone formation, resembling the underlying pathology in HCI patients. Remarkably, L438R Zip14 also generates an osteoporotic trabecular bone phenotype. The effects of osteoblastic overexpression of L438R Zip14 therefore mimic the disparate actions of estrogen on cortical and trabecular bone through osteoblasts. Collectively, we reveal ZIP14 as a novel regulator of bone homeostasis, and that manipulating ZIP14 might be a therapeutic strategy for bone diseases.

L-NAME Administration Enhances Diabetic Kidney Disease Development in an STZ/NAD Rat Model.

One of the most important risk factors for developing chronic kidney disease (CKD) is diabetes. To assess the safety and efficacy of potential drug candidates, reliable animal models that mimic human diseases are crucial. However, a suitable model of diabetic kidney disease (DKD) is currently not available. The aim of this study is to develop a rat model of DKD by combining streptozotocin and nicotinamide (STZ/NAD) with oral N(ω)-Nitro-L-Arginine Methyl Ester (L-NAME) administration. Diabetes was induced in male Wistar rats by intravenous injection of 65 mg/kg STZ, 15 min after intraperitoneal injection of 230 mg/kg NAD. Rats were assigned to different groups receiving L-NAME (100 mg/kg/day) (STZ/NAD/L-NAME) or vehicle (STZ/NAD) for a period of 9 or 12 weeks by daily oral gavage. All rats developed hyperglycemia. Hyperfiltration was observed at the start of the study, whereas increased serum creatinine, albumin-to-creatinine ratio, and evolving hypofiltration were detected at the end of the study. Daily L-NAME administration caused a rapid rise in blood pressure. Histopathological evaluation revealed heterogeneous renal injury patterns, which were most severe in the STZ/NAD/L-NAME rats. L-NAME-induced NO-deficiency in STZ/NAD-induced diabetic rats leads to multiple characteristic features of human DKD and may represent a novel rat model of DKD.

Chronic interstitial nephritis in agricultural communities is a toxin-induced proximal tubular nephropathy.

Almost 30 years after the detection of chronic interstitial nephritis in agricultural communities (CINAC) its etiology remains unknown. To help define this we examined 34 renal biopsies from Sri Lanka, El Salvador, India and France of patients with chronic kidney disease 2-3 and diagnosed with CINAC by light and electron microscopy. In addition to known histopathology, we identified a unique constellation of proximal tubular cell findings including large dysmorphic lysosomes with a light-medium electron-dense matrix containing dispersed dark electron-dense non-membrane bound "aggregates". These aggregates associated with varying degrees of cellular/tubular atrophy, apparent cell fragment shedding and no-weak proximal tubular cell proliferative capacity. Identical lysosomal lesions, identifiable by electron microscopy, were observed in 9% of renal transplant implantation biopsies, but were more prevalent in six month (50%) and 12 month (67%) protocol biopsies and in indication biopsies (76%) of calcineurin inhibitor treated transplant patients. The phenotype was also found associated with nephrotoxic drugs (lomustine, clomiphene, lithium, cocaine) and in some patients with light chain tubulopathy, all conditions that can be directly or indirectly linked to calcineurin pathway inhibition or modulation. One hundred biopsies of normal kidneys, drug/toxin induced nephropathies, and overt proteinuric patients of different etiologies to some extent could demonstrate the light microscopic proximal tubular cell changes, but rarely the electron microscopic lysosomal features. Rats treated with the calcineurin inhibitor cyclosporine for four weeks developed similar proximal tubular cell lysosomal alterations, which were absent in a dehydration group. Overall, the finding of an identical proximal tubular cell (lysosomal) lesion in CINAC and calcineurin inhibitor nephrotoxicity in different geographic regions suggests a common paradigm where CINAC patients undergo a tubulotoxic mechanism similar to calcineurin inhibitor nephrotoxicity.

Renoprotective effects of sucroferric oxyhydroxide in a rat model of chronic renal failure.

INTRODUCTION: Sucroferric oxyhydroxide (PA21) is an efficacious, well-tolerated iron-based phosphate binder and a promising alternative to existing compounds. We compared the effects of PA21 with those of a conventional phosphate binder on renal function, mineral homeostasis and vascular calcification in a chronic kidney disease-mineral and bone disorder (CKD-MBD) rat model. METHODS: To induce stable renal failure, rats were administered a 0.25% adenine diet for 8 weeks. Concomitantly, rats were treated with vehicle, 2.5 g/kg/day PA21, 5.0 g/kg/day PA21 or 3.0 g/kg/day calcium carbonate (CaCO3). Renal function and calcium/phosphorus/iron metabolism were evaluated during the study course. Renal fibrosis, inflammation, vascular calcifications and bone histomorphometry were quantified. RESULTS: Rats treated with 2.5 or 5.0 g/kg/day PA21 showed significantly lower serum creatinine and phosphorus and higher ionized calcium levels after 8 weeks of treatment compared with vehicle-treated rats. The better preserved renal function with PA21 went along with less severe anaemia, which was not observed with CaCO3. Both PA21 doses, in contrast to CaCO3, prevented a dramatic increase in fibroblast growth factor (FGF)-23 and significantly reduced the vascular calcium content while both compounds ameliorated CKD-related hyperparathyroid bone. CONCLUSIONS: PA21 treatment prevented an increase in serum FGF-23 and had, aside from its phosphate-lowering capacity, a beneficial impact on renal function decline (as assessed by the renal creatinine clearance) and related disorders. The protective effect of this iron-based phosphate binder on the kidney in rats, together with its low pill burden in humans, led us to investigate its use in patients with impaired renal function not yet on dialysis.

Indoxyl Sulfate and p-Cresyl Sulfate Promote Vascular Calcification and Associate with Glucose Intolerance.

BACKGROUND: Protein-bound uremic toxins indoxyl sulfate (IS) and p-cresyl sulfate (PCS) have been associated with cardiovascular morbidity and mortality in patients with CKD. However, direct evidence for a role of these toxins in CKD-related vascular calcification has not been reported. METHODS: To study early and late vascular alterations by toxin exposure, we exposed CKD rats to vehicle, IS (150 mg/kg per day), or PCS (150 mg/kg per day) for either 4 days (short-term exposure) or 7 weeks (long-term exposure). We also performed unbiased proteomic analyses of arterial samples coupled to functional bioinformatic annotation analyses to investigate molecular signaling events associated with toxin-mediated arterial calcification. RESULTS: Long-term exposure to either toxin at serum levels similar to those experienced by patients with CKD significantly increased calcification in the aorta and peripheral arteries. Our analyses revealed an association between calcification events, acute-phase response signaling, and coagulation and glucometabolic signaling pathways, whereas escape from toxin-induced calcification was linked with liver X receptors and farnesoid X/liver X receptor signaling pathways. Additional metabolic linkage to these pathways revealed that IS and PCS exposure engendered a prodiabetic state evidenced by elevated resting glucose and reduced GLUT1 expression. Short-term exposure to IS and PCS (before calcification had been established) showed activation of inflammation and coagulation signaling pathways in the aorta, demonstrating that these signaling pathways are causally implicated in toxin-induced arterial calcification. CONCLUSIONS: In CKD, both IS and PCS directly promote vascular calcification via activation of inflammation and coagulation pathways and were strongly associated with impaired glucose homeostasis.

The Role of Sclerostin in Bone and Ectopic Calcification.

Sclerostin, a 22-kDa glycoprotein that is mainly secreted by the osteocytes, is a soluble inhibitor of canonical Wnt signaling. Therefore, when present at increased concentrations, it leads to an increased bone resorption and decreased bone formation. Serum sclerostin levels are known to be increased in the elderly and in patients with chronic kidney disease. In these patient populations, there is a high incidence of ectopic cardiovascular calcification. These calcifications are strongly associated with cardiovascular morbidity and mortality. Although data are still controversial, it is likely that there is a link between ectopic calcification and serum sclerostin levels. The main question, however, remains whether sclerostin exerts either a protective or deleterious role in the ectopic calcification process.

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.

Association of Serum Sclerostin with Bone Sclerostin in Chronic Kidney Disease is Lost in Glucocorticoid Treated Patients.

The osteocytic protein sclerostin inhibits bone turnover. Serum sclerostin rises early in chronic kidney disease (CKD), but if this reflects osteocyte sclerostin production is unclear, since sclerostin is also expressed in extra-skeletal tissue. Glucocorticoid treatment impacts on serum sclerostin, but the effect on the association between serum and bone sclerostin is unknown. We sought to determine whether serum sclerostin reflects bone sclerostin in different CKD stages and how this association is influenced by glucocorticoid treatment. In a cross-sectional analysis, we investigated serum sclerostin, bone sclerostin by immunohistochemistry, and bone histomorphometry in iliac crest bone biopsies from 43 patients with CKD 3-5D, including 14 dialysis patients and 22 transplanted patients (18 kidney, 4 other). Thirty-one patients were on glucocorticoid treatment at time of biopsy. Patients with low bone turnover (bone formation rate < 97 µm²/mm²/day; N = 13) had higher median serum sclerostin levels (224.7 vs. 141.7 pg/ml; P = 0.004) and higher bone sclerostin, expressed as sclerostin positive osteocytes per bone area (12.1 vs. 5.0 Scl+ osteocytes/B.Ar; P = 0.008), than patients with non-low bone turnover (N = 28). In linear regression analyses, correcting for age, gender, dialysis status and PTH, serum sclerostin was only associated with bone sclerostin in patients not treated with glucocorticoids (r2 = 0.6, P = 0.018). For the first time, we describe that female CKD patients have higher median bone sclerostin than males (11.7 vs. 5.7 Scl+ osteocytes/B.Ar, P = 0.046), despite similar serum sclerostin levels and bone histo-morphometric parameters. We conclude that glucocorticoid treatment appears to disrupt the association of serum sclerostin with bone sclerostin in CKD.

Sclerostin in chronic kidney disease-mineral bone disorder think first before you block it!

Canonical Wnt signalling activity is a major player in physiological and adaptive bone metabolism. Wnt signalling is regulated by soluble inhibitors, with sclerostin being the most widely studied. Sclerostin's main origin is the osteocyte and its major function is blockade of osteoblast differentiation and function. Therefore, sclerostin is a potent inhibitor of bone formation and mineralization. Consequently, blocking sclerostin via human monoclonal antibodies (such as romosozumab) represents a promising perspective for the treatment of (postmenopausal) osteoporosis. However, sclerostin's physiology and the effects of sclerostin monoclonal antibody treatment are not limited to the skeleton. Specifically, the potential roles of sclerostin in chronic kidney disease (CKD) and associated pathologies covered by the term chronic kidney disease and mineral bone disorder (CKD-MBD), which also includes accelerated cardiovascular calcification, warrant specific attention. CKD-MBD is a complex disease condition in which sclerostin antibodies may interfere at different levels and influence the multiform interplay of hyperparathyroidism, renal osteodystrophy and vascular calcification, but the clinical sequelae remain obscure. The present review summarizes the potential effects of sclerostin blockade in CKD-MBD. We will address and summarize the urgent research targets that are being identified and that need to be addressed before a valid risk-benefit ratio can be established in the clinical setting of CKD.