Novel insights on the effect of sclerostin on bone and other organs.

As a key regulator of bone homeostasis, sclerostin has garnered a lot of interest over the last two decades. Although sclerostin is primarily expressed by osteocytes and is well known for its role in bone formation and remodelling, it is also expressed by a number of other cells and potentially plays a role in other organs. Herein, we aim to bring together recent sclerostin research and discuss the effect of sclerostin on bone, cartilage, muscle, liver, kidney and the cardiovascular and immune systems. Particular focus is placed on its role in diseases, such as osteoporosis and myeloma bone disease, and the novel development of sclerostin as a therapeutic target. Anti-sclerostin antibodies have recently been approved for the treatment of osteoporosis. However, a cardiovascular signal was observed, prompting extensive research into the role of sclerostin in vascular and bone tissue crosstalk. The study of sclerostin expression in chronic kidney disease was followed by the investigation of its role in liver-lipid-bone interactions, and the recent discovery of sclerostin as a myokine prompted new research into sclerostin within the bone-muscle relationship. Potentially, the effects of sclerostin reach beyond that of bone alone. We further summarise recent developments in the use of sclerostin as a potential therapeutic for osteoarthritis, osteosarcoma and sclerosteosis. Overall, these new treatments and discoveries illustrate progress within the field, however, also highlight remaining gaps in our knowledge.

Recombinant sclerostin inhibits bone formation in vitro and in a mouse model of sclerosteosis.

BACKGROUND: Sclerosteosis, a severe autosomal recessive sclerosing skeletal dysplasia characterised by excessive bone formation, is caused by absence of sclerostin, a negative regulator of bone formation that binds LRP5/6 Wnt co-receptors. Current treatment is limited to surgical management of symptoms arising from bone overgrowth. This study investigated the effectiveness of sclerostin replacement therapy in a mouse model of sclerosteosis. METHODS: Recombinant wild type mouse sclerostin (mScl) and novel mScl fusion proteins containing a C-terminal human Fc (mScl hFc), or C-terminal human Fc with a poly-aspartate motif (mScl hFc PD), were produced and purified using mammalian expression and standard chromatography methods. In vitro functionality and efficacy of the recombinant proteins were evaluated using three independent biophysical techniques and an in vitro bone nodule formation assay. Pharmacokinetic properties of the proteins were investigated in vivo following a single administration to young female wild type (WT) or SOST knock out (SOST-/-) mice. In a six week proof-of-concept in vivo study, young female WT or SOST-/- mice were treated with 10 mg/kg mScl hFc or mScl hFc PD (weekly), or 4.4 mg/kg mScl (daily). The effect of recombinant sclerostin on femoral cortical and trabecular bone parameters were assessed by micro computed tomography (µCT). RESULTS: Recombinant mScl proteins bound to the extracellular domain of the Wnt co-receptor LRP6 with high affinity (nM range) and completely inhibited matrix mineralisation in vitro. Pharmacokinetic assessment following a single dose administered to WT or SOST-/- mice indicated the presence of hFc increased protein half-life from less than 5 min to at least 1.5 days. Treatment with mScl hFc PD over a six week period resulted in modest but significant reductions in trabecular volumetric bone mineral density (vBMD) and bone volume fraction (BV/TV), of 20% and 15%, respectively. CONCLUSION: Administration of recombinant mScl hFc PD partially corrected the high bone mass phenotype in SOST-/- mice, suggesting that bone-targeting of sclerostin engineered to improve half-life was able to negatively regulate bone formation in the SOST-/- mouse model of sclerosteosis. THE TRANSLATIONAL POTENTIAL OF THIS ARTICLE: These findings support the concept that exogenous sclerostin can reduce bone mass, however the modest efficacy suggests that sclerostin replacement may not be an optimal strategy to mitigate excessive bone formation in sclerosteosis, hence alternative approaches should be explored.

Improved antiviral efficacy using TALEN-mediated homology directed recombination to introduce artificial primary miRNAs into DNA of hepatitis B virus.

Chronic infection with hepatitis B virus (HBV) remains an important global health problem. Currently licensed therapies have modest curative efficacy, which is as a result of their transient effects and limited action on the viral replication intermediate comprising covalently closed circular DNA (cccDNA). Gene editing with artificial HBV-specific endonucleases and use of artificial activators of the RNA interference pathway have shown anti-HBV therapeutic promise. Although results from these gene therapies are encouraging, maximizing durable antiviral effects is important. To address this goal, a strategy that entails combining gene editing with homology-directed DNA recombination (HDR), to introduce HBV-silencing artificial primary microRNAs (pri-miRs) into HBV DNA targets, is reported here. Previously described transcription activator-like effector nucleases (TALENs) that target the core and surface sequences of HBV were used to introduce double stranded breaks in the viral DNA. Simultaneous administration of donor sequences encoding artificial promoterless anti-HBV pri-miRs, with flanking arms that were homologous to sequences adjoining the TALENs' targets, augmented antiviral efficacy. Analysis showed targeted integration and the length of the flanking homologous arms of donor DNA had a minimal effect on antiviral efficiency. These results support the notion that gene editing and silencing may be combined to effect improved inhibition of HBV gene expression.