After discovering that lack of Sost/sclerostin expression is the cause of the high bone mass human syndromes Van Buchem disease and sclerosteosis, extensive animal experimentation and clinical studies demonstrated that sclerostin plays a critical role in bone homeostasis and that its deficiency or pharmacological neutralization increases bone formation. Dysregulation of sclerostin expression also underlies the pathophysiology of skeletal disorders characterized by loss of bone mass as well as the damaging effects of some cancers in bone. Thus, sclerostin has quickly become a promising molecular target for the treatment of osteoporosis and other skeletal diseases, and beneficial skeletal outcomes are observed in animal studies and clinical trials using neutralizing antibodies against sclerostin. However, the anabolic effect of blocking sclerostin decreases with time, bone mass accrual is also accompanied by anti-catabolic effects, and there is bone loss over time after therapy discontinuation. Further, the cellular source of sclerostin in the bone/bone marrow microenvironment under physiological and pathological conditions, the pathways that regulate sclerostin expression and the mechanisms by which sclerostin modulates the activity of osteocytes, osteoblasts, and osteoclasts remain unclear. In this review, we highlight the current knowledge on the regulation of Sost/sclerotin expression and its mechanism(s) of action, discuss novel observations regarding its role in signaling pathways activated by hormones and mechanical stimuli in bone, and propose future research needed to understand the full potential of therapeutic interventions that modulate Sost/sclerostin expression.

Osteocytes, >90% of the cells in bone, lie embedded within the mineralized matrix and coordinate osteoclast and osteoblast activity on bone surfaces by mechanisms still unclear. Bone anabolic stimuli activate Wnt signaling, and human mutations of components along this pathway underscore its crucial role in bone accrual and maintenance. However, the cell responsible for orchestrating Wnt anabolic actions has remained elusive. We show herein that activation of canonical Wnt signaling exclusively in osteocytes [dominant active (da)βcat Ot mice] induces bone anabolism and triggers Notch signaling without affecting survival. These features contrast with those of mice expressing the same daß-catenin in osteoblasts, which exhibit decreased resorption and perinatal death from leukemia. daßcat Ot mice exhibit increased bone mineral density in the axial and appendicular skeleton, and marked increase in bone volume in cancellous/trabecular and cortical compartments compared with littermate controls. daßcat Ot mice display increased resorption and formation markers, high number of osteoclasts and osteoblasts in cancellous and cortical bone, increased bone matrix production, and markedly elevated periosteal bone formation rate. Wnt and Notch signaling target genes, osteoblast and osteocyte markers, and proosteoclastogenic and antiosteoclastogenic cytokines are elevated in bones of daßcat Ot mice. Further, the increase in RANKL depends on Sost/sclerostin. Thus, activation of osteocytic β-catenin signaling increases both osteoclasts and osteoblasts, leading to bone gain, and is sufficient to activate the Notch pathway. These findings demonstrate disparate outcomes of β-catenin activation in osteocytes versus osteoblasts and identify osteocytes as central target cells of the anabolic actions of canonical Wnt/β-catenin signaling in bone.osteocytes | canonical Wnt | beta-catenin | bone anabolism | notch signaling

In multiple myeloma (MM) increased numbers of monoclonal plasma cells in the bone marrow induce localized osteolytic lesions that rarely heal, due to increased bone resorption and suppressed bone formation. Numerous studies reported the contributions that different cell types in the MM microenvironment make to MM growth and bone disease, but the role of matrix-embedded osteocytes in MM, which comprise >95% of bone cells and are major regulators of osteoclast and osteoblast activity, is unclear. We report that osteocytes in MM-bearing bones physically interact with MM cells in vivo, undergo caspase3-dependent apoptosis, and express higher RANKL and Sclerostin levels than osteocytes from control mice. Mechanistic studies revealed that osteocyte apoptosis is initiated by activation of Notch signaling in osteocytes through direct contact with MM cells, and is further amplified by MM cell-secreted TNFα. This Notch/TNFα induced osteocyte apoptosis increases osteocytic Rankl expression, the osteocytic Rankl/Opg ratio and the ability of osteocytes to attract osteoclast precursors to induce local bone resorption. Further, osteocytes in contact with MM cells express high levels of Sost/Sclerostin that decrease Wnt signaling in osteoblasts and inhibit osteoblast differentiation. Importantly, direct contact between osteocytes and MM cells reciprocally activates Notch signaling and increases Notch receptor expression in MM cells, in particular Notch3 and 4, and stimulates MM cell growth. These studies reveal a previously unknown role for bidirectional Notch signaling between MM cells and osteocytes that enhances MM growth and bone disease, and suggest the potential of targeting osteocyte-MM cell interactions as a novel MM treatment.

In differentiated cells, aging is associated with hypermethylation of DNA regions enriched in repressive histone posttranslational modifications. However, the chromatin marks associated with changes in DNA methylation in adult stem cells during lifetime are still largely unknown. Here, DNA methylation profiling of mesenchymal stem cells (MSCs) obtained from individuals aged 2 to 92 yr identified 18,735 hypermethylated and 45,407 hypomethylated CpG sites associated with aging. As in differentiated cells, hypermethylated sequences were enriched in chromatin repressive marks. Most importantly, hypomethylated CpG sites were strongly enriched in the active chromatin mark H3K4me1 in stem and differentiated cells, suggesting this is a cell type-independent chromatin signature of DNA hypomethylation during aging. Analysis of scedasticity showed that interindividual variability of DNA methylation increased during aging in MSCs and differentiated cells, providing a new avenue for the identification of DNA methylation changes over time. DNA methylation profiling of genetically identical individuals showed that both the tendency of DNA methylation changes and scedasticity depended on nongenetic as well as genetic factors. Our results indicate that the dynamics of DNA methylation during aging depend on a complex mixture of factors that include the DNA sequence, cell type, and chromatin context involved and that, depending on the locus, the changes can be modulated by genetic and/or external factors.