Sclerostin emerges as a novel target for bone anabolic therapy in bone diseases. Osteogenesis imperfecta (OI) and X-linked hypophosphatemia (XLH) are rare bone diseases in which therapeutic potential of sclerostin inhibition cannot be ignored. In OI, genetic/pharmacologic sclerostin inhibition promoted bone formation of mice, but responses varied by genotype and age. Serum sclerostin levels were higher in young OI-I patients, while lower in adult OI-I/III/IV. It's worth investigating whether therapeutic response of OI to sclerostin inhibition could be clinically predicted by genotype and age. In XLH, preclinical/clinical data suggested factors other than identified FGF23 contributing to XLH. Higher levels of circulating sclerostin were detected in XLH. Sclerostin inhibition promoted bone formation in Hyp mice, while restored phosphate homeostasis in age-/gender-dependent manner. The role of sclerostin in regulating phosphate metabolism deserves investigation. Sclerostin/FGF23 levels of XLH patients with/without response to FGF23-antibody warrants study to develop precise sclerostin/FGF23 inhibition strategy or synergistic/additive strategy. Notably, OI patients were associated with cardiovascular abnormalities, so were XLH patients receiving conventional therapy. Targeting sclerostin loop3 promoted bone formation without cardiovascular risks. Further, blockade of sclerostin loop3-LRP4 interaction while preserving sclerostin loop2-ApoER2 interaction could be a potential precise sclerostin inhibition strategy for OI and XLH with cardiovascular safety. The Translational Potential of this Article. Preclinical data on the molecular understanding of sclerostin inhibition in OI and therapeutic efficacy in mouse models of different genotypes, as well as clinical data on serum sclerostin levels in patients with different phenotypes of OI, were reviewed and discussed. Translationally, it would facilitate to develop clinical prediction strategies (e.g. based on genotype and age, not just phenotype) for OI patients responsive to sclerostin inhibition. Both preclinical and clinical data suggested sclerostin as another factor contributing to XLH, in addition to the identified FGF23. The molecular understanding and therapeutic effects of sclerostin inhibition on both promoting bone anabolism and improving phosphate homostasis in Hyp mice were reviewed and discussed. Translationaly, it would facilitate the development of precise sclerostin/FGF23 inhibition strategy or synergistic/additive strategy for the treatment of XLH. Cardiovascular risk could not be ruled out during sclerostin inhibition treatment, especially for OI and XLH patients with cardiovascular diseases history and cardiovascular abnormalities. Studies on the role of sclerostin in inhiting bone formation and protecting cardiovascular system were reviewed and discussed. Translationaly, blockade of sclerostin loop3-LRP4 interaction while preserving sclerostin loop2-ApoER2 interaction could be a potential precise sclerostin inhibition strategy for OI and XLH with cardiovascular safety.

Sclerostin negatively regulates bone formation. The marketed antibody against sclerostin loop2 promoted bone formation but may have caused severe cardiovascular events in clinical use. In our published studies, sclerostin loop3 was found to be involved in inhibitory effects of sclerostin on bone formation, whereas cardiovascular protective effects of sclerostin in mice were independent of loop3. It is necessary to investigate how sclerostin loop3 participates in the inhibitory effects of sclerostin on bone formation to facilitate developing precise strategies that promote bone formation without increasing cardiovascular risk. In this study, sclerostin loop3 was identified to bind to LRP4, thereby facilitating binding of sclerostin to LRP6 in osteoblasts. Blockade of sclerostin loop3-LRP4 interaction by both Lrp4 mutation ( Lrp4m ) and blocking peptide (LRP4-Pep) diminished the antagonistic effect of sclerostin on Wnt/β-catenin signaling in osteoblasts in vitro. Consistently, Lrp4m promoted bone formation in Lrp4m mice in vivo. Mechanistically, osteoblast-conditional correction of Lrp4m to wild-type Lrp4 resulted in significantly lower bone formation than Lrp4m mice, indicating that the promotive effects of Lrp4m on bone formation acted in osteoblasts in vivo. Moreover, re-expression of sclerostin dramatically inhibited bone formation in sost ^−/− mice, whilst the inhibitory effects of sclerostin were significantly weaker in sost ^−/− .Lrp4m mice. Pharmacologically, LRP4-Pep diminished the inhibitory effects of sclerostin on bone formation in SOST ^ki mice. Taken together, osteoblastic sclerostin loop3-LRP4 interaction, as an anchor, was required by sclerostin to bind to LRP6, thereby inhibiting bone formation. Translationally, blockade of sclerostin loop3-LRP4 interaction in osteoblasts would provide precise therapeutic strategies to promote bone formation without increasing cardiovascular risk.

Therapeutic antibody against sclerostin loop2 promoted bone formation in postmenopausal osteoporosis but caused severe cardiovascular events in clinical applications. The studies of atherosclerosis and aortic aneurysm in SOST ^ki .ApoE ^−/− mice and sost ^−/− . ApoE ^−/− mice collectively indicated the cardiovascular protective action of sclerostin. However, how sclerostin exerts cardiovascular protective action remains unclear. In this study, ApoER2 (LRP8) is notably identified as a novel transmembrane receptor for sclerostin in macrophages. Mechanistically, blockade of macrophagic sclerostin loop2‐ApoER2 interaction attenuates the suppressive effects of sclerostin on NF‐κB nuclear translocation, phosphorylation, and mRNA expression in macrophages, reduces the promotive effects of sclerostin on macrophage conversion to anti‐inflammatory phenotypes, and inhibits the preventive effects of sclerostin on atherosclerosis and aortic aneurysm in ApoE ^−/− mice. Together, macrophagic sclerostin loop2‐ApoER2 interaction is required by sclerostin to suppress inflammatory responses, atherosclerosis, and aortic aneurysm in ApoE ^−/− mice. Sclerostin plays a compensatory protective role in the cardiovascular system when ApoE is absent or mutated. Translationally, it provided critical pre‐clinical evidence regarding the prediction of cardiovascular risk populations (e.g. , APOE variants) for the marketed antibody against sclerostin loop2. Importantly, targeting sclerostin while preserving macrophagic sclerostin loop2‐ApoER2 interaction would offer the next generation of precise sclerostin inhibition strategy without cardiovascular safety concern, while promoting bone formation.

Despite significant progress in extracellular targeted protein degradation (eTPD), existing approaches rarely achieved tissue-specific drug accumulation while maintaining efficient systemic clearance, a critical challenge in treating bone disorders. In this study, we introduced GalNAc-Apc001, a novel aptamer-based lysosome-targeting chimera (LYTAC) that uniquely combined bone-specific retention with hepatocyte-mediated clearance through a spatiotemporally controlled mechanism. By conjugating a tri-N-acetylgalactosamine (GalNAc) moiety to a bone-homing sclerostin aptamer (Apc001), we engineered a bifunctional molecule capable of accumulating in bone via hydroxyapatite binding, capturing circulating sclerostin with high affinity and directing it to hepatocytes for ASGPR-mediated lysosomal degradation. In the absence of ASGPR-positive cells, GalNAc-Apc001 functioned via the conventional aptamer mechanism of binding inhibition, demonstrating efficacy comparable to that of Apc001 but notably lower than that of a sclerostin antibody. However, in ASGPR-positive cell coculture systems, GalNAc-Apc001 achieved a 40% greater activation of the Wnt signaling pathway compared to the sclerostin antibody, effectively reversing sclerostin-mediated inhibition (96 vs 60% recovery). Pharmacologically, GalNAc-Apc001 exhibited superior therapeutic efficacy by mitigating the suppressive effects of sclerostin on Wnt signaling, upregulating bone formation markers, and enhancing bone mass in a Col1a2 ^+/G610C osteogenesis imperfecta mouse model. These findings provided compelling mechanistic evidence that the spatiotemporal control of protein degradation could resolve the inherent trade-off between tissue targeting and systemic clearance, supporting the clinical potential of GalNAc-Apc001 in bone disorders.

Sclerostin, encoded by the SOST gene, is a novel bone anabolic target for bone diseases. Humanized anti-sclerostin antibody, romosozumab, was approved for treatment of postmenopausal osteoporosis by the US Food and Drug Administration (FDA), but with a black-box warning on cardiovascular risk. The clinical data regarding cardiovascular events from various pre-marketing and post-marketing studies of romosozumab were inconsistent. Overall, the cardiovascular risk of sclerostin inhibition could not be excluded. The restriction of romosozumab in patients with cardiovascular disease history would be necessary. Moreover, genome-wide association study (GWAS) analyses of SOST variants revealed inconsistent results of the association between SOST variations and cardiovascular diseases. Further research incorporating larger sample sizes and functional analyses are necessary. In analyses of serum/tissue sclerostin levels in patients with cardiovascular diseases, the results were controversial but indicated an association between sclerostin and the presence/severity/outcomes of cardiovascular diseases. Nonclinical studies in rodents indicated the inhibitory effect of sclerostin on inflammation, aortic aneurysm, atherosclerosis, and vascular calcification. Sclerostin loop3 participated in the inhibitory effect of sclerostin on bone formation, while the cardiovascular protective effect of sclerostin was independent of sclerostin loop3. Macrophagic sclerostin loop2–apolipoprotein E receptor 2 (ApoER2) interaction participated in the inhibitory effect of sclerostin on inflammation in vitro. Sclerostin in human aortic smooth muscle cells participated in the reduction in calcium deposition. The role of sclerostin in cardiovascular system deserves further investigation.

Sclerostin, which has three loops, inhibits bone formation and impairs whole-body lipid and glucose metabolism. The marketed therapeutic sclerostin antibody for postmenopausal osteoporosis (POP) mainly targeting loop2 promotes bone formation and improves whole-body lipid and glucose metabolism. However, FDA/EMA warns of its cardiovascular risk. We previously demonstrate that sclerostin loop3 contributes to the inhibitory effect of sclerostin on bone formation but not its cardioprotective effect. Here we find elevated serum sclerostin levels in both POP-T2DM patients and newly-diagnosed T2DM patients and further demonstrate that sclerostin loop3 participates in the impairment effect of sclerostin on whole-body lipid and glucose metabolism in vivo. Mechanistically, specific blockade of adipocytic sclerostin loop3-LRP4 interaction attenuates the impairment effect of sclerostin on lipid and glucose metabolism in vitro and in vivo. This study provides an innovative strategy, blocking adipocytic sclerostin loop3-LRP4 interaction, to normalize lipid and glucose metabolism in POP-T2DM patients, in cardiovascular safety.