Sclerostin and the Unloading of Bone

It is now well accepted that, whereas increases in the loading of bone are associated with increased bone mass, the unloading of bone (disuse) induces profound osteopenia. Without doubt, the skeleton perceives and responds to a variety of general input(s) generated, or lost, as a consequence of mechanical unloading of bone that are distinct from habitual activity, a process referred to as functional adaptation.⁽¹⁾ However, even in the 21st century, the mechanisms regulating bone loss remain poorly understood. In the current issue of the Journal of Bone and Mineral Research, Lin et al.⁽²⁾ describe experiments showing that the loss of bone induced by hind-limb unloading in mice is associated with increased expression of the SOST gene and decreased expression of Wnt target genes. In addition, Wnt signaling is not altered by unloading, and no bone is lost in unloaded mice lacking the SOST gene.

Sclerostin, the product of the SOST gene, is a secreted glycoprotein with a C-terminal cysteine knot-like (CTCK) domain and sequence similarity to the DAN (differential screening-selected gene aberrative in neuroblastoma) family of bone morphogenetic protein (BMP) antagonists.⁽³⁾ Loss-of-function mutations in SOST are associated with an autosomal-recessive disorder, sclerosteosis, which causes progressive bone overgrowth.^(4,5) A deletion downstream of this gene, which results in reduced sclerostin expression, is associated with a milder form of the disorder called van Buchem disease.⁽⁶⁾ The important role of SOST in the regulation of bone mass was subsequently confirmed by demonstration of a high bone mass phenotype in SOST-null mice.⁽⁷⁾ Because sclerostin is constitutively and almost exclusively expressed in bone by osteoblasts/osteocytes, it is not surprising to hypothesize that the control of SOST may also control the mechanical response of the skeleton.

Osteocytes have mechanosensory properties, and mechanical loading triggers them to modulate bone homeostasis. In vivo mechanical loading of bone was found to reduce sclerostin expression, thereby providing a mechanism by which mechanical loading increased bone formation.⁽⁸⁾ The paper by Lin et al.⁽²⁾ adds considerable weight to this idea and provides genetic evidence showing a lack of disuse osteopenia in the absence of SOST, suggesting that Wnt/β-catenin signaling is a key local signaling pathway mediating the effect of mechanical unloading. The data clearly show that SOST-deficient mice express higher levels of anti-apoptotic proteins with significantly reduced osteoblast and osteocyte apoptosis that contributes to their high bone mass phenotype and the resistance to hind-limb suspension induced bone loss.

The data in this paper confirm that SOST deletion results in increased bone mass and modulates bone remodeling. However, although the strong bone formation defect and gene expression data suggest a Wnt/β catenin effect in bone, they do not specifically provide direct evidence of such an effect. The genes examined are not unique to β catenin signaling and thus only provide indirect evidence supporting a role for Wnt/β catenin signaling in the mechanical response of bone to disuse. Indeed, further examination is required and is likely the target of future studies.

Secreted Wnt family members bind to co-receptors comprised of LRP5 (low-density lipoprotein receptor-related protein 5) or LRP6 and a member of the Frizzled family of receptors. This interaction results in the stimulation of an intracellular signaling cascade leading to stabilization of β catenin, in association with lymphoid enhancer binding factor/T cell factor (LEF/TCF) transcription factors, that activate the expression of specific target genes.⁽⁹⁾

An important and quite timely aspect of the work of Lin et al. is that it suggests a link between Wnt/β catenin signaling and the mechanical adaptation of bone, perhaps providing evidence of an important role for the antagonism of LRP5 in osteoblasts/osteocytes in the regulation of bone mass. In contrast, Yadav et al.⁽¹⁰⁾ recently reported the regulation of gut-produced serotonin by LRP5 and the deleterious effect of serotonin on bone mass and proposed a critical role for gut-derived LRP5 and not bone-derived LRP5. The importance of bone-derived LRP5 versus gut-derived LRP5 remains controversial, and further study is needed to confirm the observations of both groups and to determine the relative contributions of both receptor sources to the regulation of bone mass. However, it is conceivable that site-specific regulation of the skeleton occurs, with normal bone homeostasis the result of gut-derived serotonin and LRP5 activation, whereas mechanical regulation occurs at the level of the osteocyte through SOST and LRP5.

In any case, much remains to be learned about SOST, LRP5 (and other LRPs), and Wnt signaling and their relative roles in the regulation of bone formation. For example, it is possible that SOST impacts the production of other osteocyte secretory products that provide an alternative explanation of the inhibitory effect of SOST; thus, a conditional knockout of SOST (perhaps using DMP1-cre) in osteocytes or in osteoblasts (OC-cre) may help to resolve this fundamental and unanswered issue. Similarly, the mechanistic details mediating SOST antagonism of Wnt signaling in osteoblasts remains unclear.

It is also interesting to consider how the observations of Lin et al.⁽²⁾ intersect with the elegant studies of Lee et al.,⁽¹¹⁾ showing that estrogen receptor α mediates the skeleton's adaptive response to mechanical loading. It would seem that the strain-sensitive mechanism(s) that enable the mammalian skeleton to adapt to increased loads (load bearing) are distinct from the mechanism(s) involved in the response of the skeleton to decreased mechanical loads (disuse). In some as yet undefined fashion, these distinct mechanisms exploit the respective signaling pathway's ability to influence bone's adaptive remodeling activity. The integration of the signals mediated by this fascinating signaling paradigm regulating unloading and loading responses of bone may culminate in the identification of the mechanism that the late Harold Frost referred to as the mechanostat.⁽¹²⁾ Given the extremely high incidence of signaling cross-talk in modern biology, interactions between SOST and estrogen receptor signaling are likely.

In fact, evidence that estrogen has a protective effect against glucocorticoid-induced osteocyte apoptosis was suggested by Gu et al.⁽¹³⁾ More recently, genetic evidence of a direct mechanistic link between the control of BMD through estrogen receptor α signaling pathways and SOST was identified using gene-wide SNP-based association analyses.⁽¹⁴⁾ The regulation of BMD was associated with a cis-regulatory polymorphism rs1230399 (T/C) in the SOST upstream enhancer region. Collectively, the extent of the interactions that exist between estrogen receptor regulation of bone loading and SOST regulation of bone unloading is only now being uncovered.

Therapeutically, specific targeting of Wnt activation to increase bone mass using small molecules targeting LRP5/6, frizzled, or downstream enzymes that are regulated by these receptors has proven futile. Given the relatively exclusive expression of SOST in osteoblasts/osteocytes in bone, the inhibition of sclerostin is a valid therapeutic target to treat patients with low bone mass. This idea is directly confirmed by the studies of Lin et al.⁽²⁾ and significantly enriched by the suggestion that, not only is antagonism of SOST anabolic, but that any bone gained by the lack of SOST function is not lost by disuse; this effect is not noted with other anabolic agents, such as PTH. Thus, the systemic administration of a SOST antagonist (or recombinant SOST antibody) should affect SOST activity only in the skeleton. The result would be to locally relieve endogenous Wnt inhibition and activation of bone formation, without affecting Wnt pathways in other non-bone sites. The increased understanding of the role of SOST and the further elucidation of the signaling pathways involved in SOST anabolism make it particularly amenable to pharmacological intervention for the stimulation of bone mass.