The Impact of the Extracellular Matrix Environment on Sost Expression by the MLO-Y4 Osteocyte Cell Line

Bone is a dynamic organ that can adapt its structure to meet the demands of its biochemical and biophysical environment. Osteocytes form a sensory network throughout the tissue and orchestrate tissue adaptation via the release of soluble factors such as a sclerostin. Osteocyte physiology has traditionally been challenging to investigate due to the uniquely mineralized extracellular matrix (ECM) of bone leading to the development of osteocyte cell lines. Importantly, the most widely researched and utilized osteocyte cell line: the MLO-Y4, is limited by its inability to express sclerostin (Sost gene) in typical in-vitro culture. We theorised that culture in an environment closer to the in vivo osteocyte environment could impact on Sost expression. Therefore, this study investigated the role of composition and dimensionality in directing Sost expression in MLO-Y4 cells using collagen-based ECM analogues. A significant outcome of this study is that MLO-Y4 cells, when cultured on a hydroxyapatite (HA)-containing two-dimensional (2D) film analogue, expressed Sost. Moreover, three-dimensional (3D) culture within HA-containing collagen scaffolds significantly enhanced Sost expression, demonstrating the impact of ECM composition and dimensionality on MLO-Y4 behaviour. Importantly, in this bone mimetic ECM environment, Sost expression was found to be comparable to physiological levels. Lastly, MLO-Y4 cells cultured in these novel conditions responded accordingly to fluid flow stimulation with a decrease in expression. This study therefore presents a novel culture system for the MLO-Y4 osteocyte cell line, ensuring the expression of an important osteocyte specific gene, Sost, overcoming a major limitation of this model.

Suppression of sost/sclerostin and dickkopf-1 promote intervertebral disc structure in mice

Intervertebral disc (IVD) degeneration is a leading cause of low back pain and characterized by accelerated extracellular matrix breakdown and IVD height loss but there is no approved pharmacological therapeutic. Deletion of Wnt signaling receptor Lrp5 induces IVD degeneration and suggests that Wnt signaling in the IVD may be responsive to inhibition of Wnt signaling inhibitors sost(gene)/sclerostin(protein) or dickkopf-1 (dkk1). Anti-sclerostin antibody (Scl-Ab) is an FDA-approved bone therapeutic that activates Wnt signaling. We (1) determined if pharmacological neutralization of sclerostin, dkk1 or their combination stimulate Wnt signaling and promote IVD structure and (2) determined the extent of the response of the IVD to global, persistent deletion of sost. Nine-week-old C57Bl/6J female mice (n=6-7/grp) were subcutaneously injected 2x/wk for 5.5 wk with Scl-Ab (25 mg/kg), Dkk1-Ab (25 mg/kg), 3:1 Scl-Ab/Dkk1-Ab (18.75:6.25 mg/kg) or vehicle (Veh). Separately, IVD of sost KO and WT (wildtype) mice (n=8, grp) were harvested at 16 weeks of age. First, compared to vehicle, Scl-Ab, Dkk1-Ab and 3:1 Scl-Ab/Dkk1-Ab similarly increased lumbar IVD height and β-catenin gene expression. Despite these similarities, Scl-Ab decreased cellular stress-related heat shock protein gene expressions while neither Dkk1-Ab nor Scl-Ab/Dkk1-Ab altered the same. Genetically and compared to WT, sost KO increased MRI-determined hydration and proteoglycan staining in the IVD. Notably, persistent deletion of sost was compensated by upregulation of Dkk1, which consequently reduced the cellular nuclear fraction for Wnt signaling transcription factor β-catenin in whole IVD. Lastly, RNA-sequencing pathway analysis confirmed the suppression of Wnt signaling and cellular stress pathways. Together, suppression of sost/sclerostin or dkk1 each promote IVD structure by stimulating Wnt signaling, but sclerostin and dkk1 may differentially regulate cellular stress pathways. Ultimately, postmenopausal women prescribed Scl-Ab to prevent vertebral fracture may also benefit from a restoration of IVD height and health.

The rs1634330 Polymorphisms in the SOST Gene Are Associated with Body Composition in Chinese Nuclear Families with Male Offspring

Objective. The purpose of this study was to explore the effect of the SOST gene polymorphisms on body composition in Chinese nuclear families with male offspring. Methods. 1,016 individuals were recruited from 335 Chinese nuclear families with male offspring. The nuclear families consist of at least one male offspring aged 18 to 44. We genotyped the 10 tagged single-nucleotide polymorphisms (SNPs) in SOST gene (rs7220711, rs865429, rs851057, rs1708635, rs2023794, rs1234612, rs74252774, rs1634330, rs851058, and rs1513670) in all the above people. We used dual-energy X-ray absorptiometry to measure the composition of the human body. The quantitative transmission disequilibrium test (QTDT) was used to analyze the associations of the SNPs with the body composition. Results. QTDT analysis showed that rs1634330 was significantly associated with trunk LM P < 0.05 . However, haplotypes were not found to be significantly associated with the body composition in the within-family association. The 1000 permutations were consistent with these within-family association results. Conclusions. Our results showed that the genetic variation in the SOST gene may contribute to variations in the body composition of Chinese male offspring.

A Novel Mutation in SOST Gene Causes Sclerosteosis

Background: Sclerostin is a SOST gene product that inhibits osteoblasts activity and prevents excessive bone formation by antagonizing Wnt signaling pathway. Sclerosteosis has been linked to the loss of function mutation in SOST gene. It is a rare autosomal recessive disorder characterized by craniotubular hyperostosis leading to gigantism, cranial nerves entrapment, and fatal cerebellar herniation. Objectives To report a novel mutation of SOST gene in a patient with sclerosteosis. Clinical Case: A 25-year-old female was referred to the endocrine clinic for suspected GH excess. The patient noted the onset of headache, progressive bilateral blurred vision and hearing disturbance, irregular menses, and generalized arthralgia; at the age of 23 years. Subsequently, she observed a progressive increase in the size of shoes and hands, proptosis, and protrusion of the chin. She was the second of seven siblings from non-consanguineous parents with normal antenatal and neonatal history except for syndactyly. All family members were phenotypically normal except for a sister with similar physical appearance who had cranial decompression 20 years back. MRI pituitary was done initially due the suspicion of pituitary adenoma and it revealed an enlarged sella turcica with normal pituitary gland. Surprisingly, the MRI showed diffuse osseous thickening with narrowing of skull base foramina, narrowing of optic and internal auditory canals, secondary compression of cerebral parenchyma and bilateral cerebellar tonsillar herniation. Further image revealed extremely increased bone mass density with Z-score values of +12, generalized increase cortical thickness, vertebral end plates sclerosis, and deformed left index finger. Biochemical and endocrine tests revealed normal GH, IGF -1, TSH, prolactin, short Synacthen test, FSH, LH, estradiol, calcium, phosphorus, PTH and alkaline phosphatase. Due to progressive worsening of vision with compressive optic neuropathy, optic nerve fenestration with decompression hemicraniotomy was performed. Sclerosteosis was suspected due to the predominant craniotubular hyperostosis with syndactyly. There was no definite therapy. Management aimed at relieving symptoms and preventing complications, so she was commenced on calcitriol and prednisolone to suppress the osteoclasts. Genomic sequencing of the SOST was performed. We identified a novel deletion mutation in SOST gene (c.387delG, p.D131fs*) which disrupts the sclerostin function causing sclerosteosis in this patient. Conclusion: We describe a novel mutation in the SOST gene in a patient with sclerosteosis in Saudi Arabia, that has not been previously described. Closing the gap between the genomic knowledge and clinical applications will add the benefit of success in development of targeted therapies in such a fatal disease.

Identification of Sclerostin as a Putative New Myokine Involved in the Muscle-to-Bone Crosstalk

Bone and muscle have been recognized as endocrine organs since they produce and secrete “hormone-like factors” that can mutually influence each other and other tissues, giving rise to a “bone–muscle crosstalk”. In our study, we made use of myogenic (C2C12 cells) and osteogenic (2T3 cells) cell lines to investigate the effects of muscle cell-produced factors on the maturation process of osteoblasts. We found that the myogenic medium has inhibitory effects on bone cell differentiation and we identified sclerostin as one of the myokines produced by muscle cells. Sclerostin is a secreted glycoprotein reportedly expressed by bone/cartilage cells and is considered a negative regulator of bone growth due to its role as an antagonist of the Wnt/β-catenin pathway. Given the inhibitory role of sclerostin in bone, we analyzed its expression by muscle cells and how it affects bone formation and homeostasis. Firstly, we characterized and quantified sclerostin synthesis by a myoblast cell line (C2C12) and by murine primary muscle cells by Western blotting, real-time PCR, immunofluorescence, and ELISA assay. Next, we investigated in vivo production of sclerostin in distinct muscle groups with different metabolic and mechanical loading characteristics. This analysis was done in mice of different ages (6 weeks, 5 and 18 months after birth) and revealed that sclerostin expression is dynamically modulated in a muscle-specific way during the lifespan. Finally, we transiently expressed sclerostin in the hind limb muscles of young mice (2 weeks of age) via in vivo electro-transfer of a plasmid containing the SOST gene in order to investigate the effects of muscle-specific overproduction of the protein. Our data disclosed an inhibitory role of the muscular sclerostin on the bones adjacent to the electroporated muscles. This observation suggests that sclerostin released by skeletal muscle might synergistically interact with osseous sclerostin and potentiate negative regulation of osteogenesis possibly by acting in a paracrine/local fashion. Our data point out a role for muscle as a new source of sclerostin.