Keywords

1 Introduction

The skeleton, while strong, isn’t made of static tissue. It is a highly dynamic organ that constantly undergoes changes and regeneration. A continuous change is taking place, as osteoclasts degrade bone and osteoblasts rebuild new bone. This ongoing skeletal adaptation is greatly influenced by the amount of mechanical strain that the skeleton senses as a result of everyday movement and physical activity. However, many burning questions were, at least until recently, without an answer. In particular, was how does the skeleton “feel” mechanical strain and maybe most importantly how does it turn this information into the act of making more or less bone?

The answer to this question seems to be related to the nerve-like osteocyte network embedded throughout bone acting as a mechano sensor that allows the skeleton to “feel” and respond to mechanical strain. This network produces a powerful and cryptic inhibitory signal which most likely represents a master regulator of the skeleton. This master regulatory molecule, called sclerostin, is a glycoprotein (22 kDa) product of the SOST gene, which is localized at chromosome region 17q 12-p21 [1]. Inactivating mutations of this gene lead to a rare genetic disease characterized by high bone mass, namely sclerosteosis. The tremendous increase in bone mass and bone mineral density (BMD) that is observed in these patients is similar to what is seen in another autosomal recessive, inherited high bone mass disorder, Van Buchem disease. In the Van Buchem disease SOST itself is not mutated; however, there is a 52-kb deletion in the downstream region of the SOST gene that results in the absence of postnatal sclerostin production. Thus, both sclerosteosis and Van Buchem disease are causes by sclerostin deficiency, leading to the conclusion that sclerostin must be a natural brake for bone formation, preventing the body from making too much bone. When mechanical forces are applied to the bone, the osteocytes stop secreting sclerostin and bone formation is initiated on the bone surface.

Wnt/B-catenin signaling pathway is a critical regulator of skeletal development and mass, working in part through the stimulation of Runx2 gene expression. Activation of the canonical Wnt signaling involves the formation of a complex between Wnt proteins, frizzled and low density lipoprotein receptor-related protein 5 (LRP5) or LRP6 receptors. Osteocytes are the predominant cellular source of the Wnt antagonist sclerostin, a limiting factor for osteoblast generation and bone mass accrual that mediates the homeostatic adaptation of bone to mechanical loading. Sclerostin is a negative regulator of Wnt signaling. It binds to both LRP5 and LRP6 and prevents activation of the Wnt receptor complex, resulting in inhibition of bone formation. In addition to sclerostin, the DKK family members, particularly DKK-1 (Dickkopf-1), inhibit the Wnt pathway by binding to the LPR-5/6 receptor. Wnt signaling can also be blocked by other proteins, such as soluble frizzled-related protein, that bind to Wnt ligands.

Osteocytes effectively act as mechanoreceptors for bone formation, and sclerostin was shown to play a key role in the development of osteoporosis associated with lack of mechanical stimulation, as observed in weightless astronauts or in patients confined to bed for a long period of time. Most studies, in both the general and the osteoporotic populations, sustain this hypothesis by reporting a positive association between circulating sclerostin levels and bone mineral density (BMD).

Several clinical and biological variables have been described as determinants of sclerostin secretion. Among the most important of them, age and CKD have been found to be directly associated with increased circulating sclerostin concentrations, whereas an inverse correlation has been observed between circulating sclerostin and parathyroid hormone (PTH) levels and other bone biomarkers [2].

2 Sclerostin in CKD

In the setting of CKD, circulating sclerostin concentrations clearly increase as glomerular filtration rate (GFR) decreases reaching an almost four times higher serum sclerostin level in predialysis patients with CKD stage V than in participants with normal renal function [3]; whether this is due to reduced renal clearance, increased skeletal production, or both is still a subject of debate. Recently, Cejka et al. showed that excretion of sclerostin increases with declining renal function [4] thus invalidating the hypothesis that increasing serum levels of sclerostin in CKD patients are related only to renal retention. The reason for increased circulating levels of sclerostin is therefore linked to an increase in its production; this hypothesis has been also suggested by previous research of Sabbagh et al. using immunohistochemical staining of sclerostin in bone biopsies from CKD patients [5]. Thus, in an experimental study of mice experiencing progressive CKD, the repression of the Wnt/b-catenin pathway and its inhibitor sclerostin was associated with increased osteoclast activity and repression of bone formation suggesting a possible implication in pathogenesis of renal osteodystrophy [5]. However, the exact underlying mechanism of increased production of sclerostin in CKD is still a matter of debate. It has been suggested that PTH, which is a known repressor of SOST gene expression and an inhibitor of sclerostin production in normal situations [6] might have a role. Indeed, it is well known that uremia is associated with a renal and skeletal resistance to the actions of PTH [7], which may in some extent be related to the increased production of sclerostin in CKD patients. This finding may open new possible therapeutic strategies in which anti-sclerostin antibodies which are currently in development [8], might ameliorate bone formation rates especially in elderly osteoporotic subjects with some degree of renal impairment.

However, the PTH-sclerostin correlation is not consistent through all the studies. Thus, Kanbay et al. suggest a possible role of other factors including phosphorus and FGF23 in the regulation of sclerostin through a PTH-independent mechanism in CKD patients treated by hemodialysis (HD) [9]. Moreover, sclerostin at least partly regulates bone matrix mineralization through a signaling pathway involving phosphate regulators—the phosphate regulating neutral endopeptidase on chromosome X (PHEX) and the matrix extracellular phosphoglycoprotein (MEPE) axis [10]. However, the mechanism underlying the positive association between serum sclerostin levels and serum phosphate levels remains unclear. They seem to interact via another phosphate regulator like FGF23, PHEX or MEPE and thus regulating bone turnover, bone mineralization, and renal mineral homeostasis [10, 11].

In peritoneal dialysis patients, as in HD patients, there is also a higher than normal serum level of sclerostin which is inversely correlated with the degree of bone formation rate [12]. According to the KDIGO (Kidney Disease Improving Global Outcome) guidelines [13] and other studies [12], the most frequent pattern of renal osteodystrophy in PD is characterized by a low bone turnover, with the leading entity being adynamic bone disease. Sclerostin is therefore one potential “actor” that may play a role in the pathophysiology of adynamic bone disease.

In renal transplanted patients, sclerostin acknowledge a rapid decrease to normal or even subnormal values shortly after transplantation in contrast with the persistent elevation of PTH and FGF23 [14]. This decrease of sclerostin is probably due to the improvement of renal function, increased physical activity and use of glucocorticoids. Subsequently, in the first year after renal transplantation there is a gradual increase in serum sclerostin levels towards normal values; this rise is not influenced by the GFR, but paralleled the reduction of PTH and the normalization of serum calcium, phosphate and vitamin D concentrations [14].

Although preliminary data suggest that sclerostin may be a promising biomarker in assessing bone health in CKD patients, it is not clear whether it has any added value compared with existing bone biomarkers in predicting bone turnover and/or BMD. Its clinical utility in determining hard clinical end points such as fracture is unknown. Indeed, given that global bone strength is determined both by qualitative changes in bone (for instance, mineralization and turnover) and by quantitative changes in bone volume and density it is perhaps unrealistic to expect a single biomarker to predict such outcomes. Therefore the biological significance and interpretation of circulating sclerostin levels in CKD remain uncertain.

3 Sclerostin and Vascular Calcification in CKD

Vascular calcifications (VCs) are recognized as a strong predictor of all-cause and cardiovascular mortality in CKD patients [15]. The discovery of CKD bone-vascular axis, addressing the complex interactions between bone and vessel which share similar underlying mechanisms, let bone turnover inhibitors emerge as potential risk factors for VC. More recently, attention has focused on sclerostin, a novel candidate for the bone-vascular axis.

Vascular smooth muscles cells undergo osteo/chondrogenic transdifferentiation in a pro-calcifying environment. In the late phase of VC, sclerostin is expressed. This can be interpreted as a defensive response that aims to block the Wnt pathway in order to reduce the mineralization in the vascular tissue. Sclerostin may spill over to the circulation and may reciprocally inhibit bone metabolism [16].

Several studies report a positive association between sclerostin and VC [15, 17] (Table 11.1); furthermore, expression of sclerostin has also been demonstrated in the vascular wall, in the calcification site [18]. However, once again other authors reported discordant results describing an inverse correlation between sclerostin and VC. Thus, in a cohort of hemodialyzed patients, those with more severe aortic calcifications had significantly lower serum sclerostin levels. In addition, low levels of sclerostin remained a significant predictor of cardiovascular outcome even after adjusting for age and gender, suggesting that Wnt/β-catenin signaling plays an additional role in uremic VC beyond aging [19].

Table 11.1 Association between sclerostin with vascular calcification
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4 Sclerostin and Mortality in CKD

Even if experimental and clinical studies suggest that the Wnt pathway may also play a role in atherosclerosis and vascular calcification, the association between sclerostin and mortality in CKD patients remains so far inconsistent (Table 11.2). In a post-hoc analysis in 100 prevalent HD patients, Viaene et al. [16], found a positive association between higher circulating sclerostin levels (defined as values superior to the median) and survival after a median follow-up time of 637 days. The authors link this survival benefit to the possible attenuation of the progression of VC in the setting of high sclerostin [16]. However, within a fully adjusted model including bone-specific alkaline phosphatase the association between survival and sclerostin lost statistical significance [16]. In the same line, a very recent prospective study, from The (Netherlands) (the NECOSAD cohort), Drechsler et al. found that high or intermediate levels of circulating sclerostin were strongly associated with lower risk factor for future all-cause and cardiovascular mortality in 637 incident dialysis patients, particularly in the short term follow-up (18 months) [20]. The results were quite impressive, cardiovascular mortality being 70 % lower in patients of the highest tertile of sclerostin within 18 months when compared with patients of the lowest tertile. In addition, compared with Viaene et al. study, these results remained consistent even in the fully adjusted model. In contrast with the results previously reported, our group [9] found in 173 non-dialyzed patients with CKD stages 3–5 that higher sclerostin values were associated with fatal and non-fatal cardiovascular events after a mean follow-up of 26 months even after multiple adjustments [9]. The discrepancy with previous studies might be related first of all with the different study population (hemodialysed vs. non-dialysed CKD patients) with different serum vitamin D, PTH, or calcium levels and thus with possible different underlying bone disease. Unfortunately neither study performed direct bone histomorphometry in order to confirm this hypothesis. To further complicate the understanding of the impact of sclerostin on mortality, Delanaye et al. [21] and Nowak et al. [22] found no correlation between serum sclerostin level and mortality in hemodialysis patients.

Table 11.2 Studies reporting the correlation between circulating sclerostin levels and mortality
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The potential reasons for these differences remain somewhat unclear. Some possible explanations are related to the heterogeneity of patients includes in those studies. Thus, the study of Viaene et al. included a higher proportion of diabetics and had an overall shorter dialysis vintage in the hemodialysis; the NECOSAD study by Drechsler et al. included incident (and not prevalent) patients of younger age; meanwhile, Kanbay et al. enrolled non-dialyzed CKD patients. Moreover, it is highly probable that all the patients included in the aforementioned studies had different medication like vitamin D or phosphate binders and different cardiovascular comorbidities which may thus confer a different impact on survival. Furthermore, different assays were used to measure the circulating sclerostin levels; even in healthy individuals these kits are not in perfect agreement; possible binding of sclerostin fragments may occur, and this could partly explain different values obtained.

5 Vitamin D and Sclerostin

Vitamin D is of paramount importance to skeletal development, integrity and health. Vitamin D homeostasis is typically deranged in a number of chronic conditions, of which CKD is one of the most important. The vitamin D or calcitriol receptor is not the classic, membrane-bound type, but rather a member of the nuclear receptor family located within the cytoplasm of the cell, and when activated by its ligand, travels into the nucleus and activates specific areas on the genome. Other members of this family include the steroid, sex hormone, retinoid, and thyroid hormone receptors. What makes these nuclear receptors unique is that each tissue containing these receptors has its own mix of regulatory proteins that repress or activate the receptor once it has been engaged by the ligand, causing it to change shape. The final conformational change induced by the natural ligand or its analogues will determine whether or not the receptor is active or inactive in that tissue. This is one of the reasons why the different analogues of active vitamin D will have different effects on various tissues from that of calcitriol and each other.

The effects of vitamin D on bone tissue as a whole are not yet fully understood but are likely due to a combination of direct effects via VDREs, downstream effects of the induced gene expression and effects at specific stages of bone cell proliferation and differentiation. One direct action that has been clearly demonstrated is the ability of plasma 1,25 vitamin D to stimulate bone resorption by the activation of osteoclast [23]. Furthermore, 1,25 vitamin D is capable of regulating osteoblast gene transcription, proliferation, differentiation and mineralization as was shown in in vitro studies [24, 25].

Vitamin D may also modulate the bone homeostasis by affecting key osteocytic genes and thus interfere with the sclerostin secretion. While it has not been studied in detail, it is likely that osteocytes are responsible for the majority of osteocalcin synthesis, which may be under the control of 1,25 vitamin D. The direct regulatory effect of 1,25 vitamin D on sclerostin was demonstrated by several studies. Thus, in one of them, treatment of human primary osteoblasts, including cells differentiated to an osteocyte-like stage, with 1,25 vitamin D resulted in the dose-dependent increased expression of SOST mRNA which may in turn increase the secretion of sclerostin [26]. The association between sclerostin and vitamin D was also reported in a cohort of patients suffering from hypercalciuria in which a positive correlation between sclerostin expression by osteocytes and serum 1,25D levels was depicted [27]. However, other in-vivo study reported a decrease in serum sclerostin level after vitamin D3 treatment in vitamin D-deficient young adult females [28]. This inconsistency with previous studies might be attributable to the fact that vitamin D deficiency affects osteocytes and thus alters serum sclerostin levels. Understanding the effects of vitamin D on serum sclerostin may improve knowledge of bone physiology; future studies should further investigate this complex physiological relationship.

6 Conclusions and Future Perspectives

In the last years, the Wnt/sclerostin pathway has been the focus of intense basic and clinical research in the bone field because of its importance in skeletal development and maintenance of bone mass. Disturbances in Wnt/sclerostin pathway can be added to the whole spectrum of changes associated with the CKD-MBD progression. However, the relative impact of this pathway on the progression of cardiovascular calcification and importantly on cardiovascular mortality requires additional investigation.

Growing evidence confers sclerostin a central role in the pathogenesis of CKD-MBD. This fact, all along with the rather disappointing results of current therapeutic strategies on hard-outcomes in CKD-MBD opens new perspectives for targeted therapy, including pharmacological neutralization of sclerostin or DKK1 by monoclonal antibodies. In the next years, sclerostin inhibition will probably become a possible approach in the bone anabolic treatment of osteoporosis. This is supported by several experimental studies in which anti-sclerostin antibodies were associated with improved bone properties [29] and decreased vascular calcification in combination with phosphate binders [30]. A potential concern is that the use of therapies to promote bone anabolism might have a negative impact on the cardiovascular disease. Conversely, improvement of bone health may reduce other risk factors that have higher impact on cardiac disease such as serum phosphate and FGF23. Additional studies are thus required to define determinants of Wnt inhibitors in CKD and to evaluate the efficacy and safety of recently introduced pharmaceuticals inhibitors. Other questions related to the normal range of sclerostin through the different stages of CKD, to the standardization of current laboratory testing methods or to the effect of dialysis or other concurrent medication on sclerostin serum level need a definite answer before introducing sclerostin into day-by-day clinical practice.