Department of Anatomy, Cell Biology & Physiology, Indiana University School of Medicine, Indianapolis, IN, USA.
Program for Skeletal Disease and Tumor Microenvironment, Center for Cancer and Cell Biology, Van Andel Research Institute, Grand Rapids, MI, USA.
Bone. 2021 Feb;143:115708. doi: 10.1016/j.bone.2020.115708. Epub 2020 Oct 24.
The cysteine knot protein sclerostin is an osteocyte-derived secreted inhibitor of the Wnt co-receptors LRP5 and LRP6. LRP5 plays a dominant role in bone homeostasis, but we previously reported that Sost/sclerostin suppression significantly increased osteogenesis regardless of Lrp5 presence or absence. Those observations suggested that the bone forming effects of sclerostin inhibition can occur through Lrp6 (when Lrp5 is suppressed), or through other yet undiscovered mechanisms independent of Lrp5/6. To distinguish between these two possibilities, we generated mice with compound deletion of Lrp5 and Lrp6 selectively in bone, and treated them with sclerostin monoclonal antibody (Scl-mAb). All mice were homozygous flox for both Lrp5 and Lrp6 (Lrp5; Lrp6), and varied only in whether or not they carried the Dmp1-Cre transgene. Positive (Cre+) and negative (Cre-) mice were injected with Scl-mAb or vehicle from 4.5 to 14 weeks of age. Vehicle-treated Cre+ mice exhibited significantly reduced skeletal properties compared to vehicle-treated Cre- mice, as assessed by DXA, μCT, pQCT, and histology, indicating that Lrp5/6 deletions were effective and efficient. Scl-mAb treatment improved nearly every bone-related parameter among Cre- mice, but the same treatment in Cre+ mice resulted in little to no improvement in skeletal properties. For the few endpoints where Cre+ mice responded to Scl-mAb, it is likely that antibody-induced promotion of Wnt signaling occurred in cell types earlier in the mesenchymal/osteoblast differentiation pathway than the Dmp1-expressing stage. This latter conclusion was supported by changes in some histomorphometric parameters. In conclusion, unlike with the deletion of Lrp5 alone, the bone-selective late-stage co-deletion of Lrp5 and Lrp6 significantly impairs or completely nullifies the osteogenic action of Scl-mAb, and highlights a major role for both Lrp5 and Lrp6 in the mechanism of action for the bone-building effects of sclerostin antibody.
卷曲相关蛋白聚糖（sclerostin）是一种由骨细胞分泌的 Wnt 共受体 LRP5 和 LRP6 的抑制剂。LRP5 在骨稳态中起着主导作用，但我们之前的研究表明，Sost/sclerostin 抑制显著增加成骨作用，无论 Lrp5 是否存在。这些观察结果表明，sclerostin 抑制的骨形成作用可以通过 Lrp6（当 Lrp5 被抑制时）发生，或者通过其他尚未发现的独立于 Lrp5/6 的机制发生。为了区分这两种可能性，我们生成了 Lrp5 和 Lrp6 在骨中选择性缺失的复合缺失小鼠，并对其进行 sclerostin 单克隆抗体（Scl-mAb）治疗。所有小鼠均为 Lrp5 和 Lrp6 的纯合 flox （Lrp5; Lrp6），并且仅在是否携带 Dmp1-Cre 转基因上有所不同。阳性（Cre+）和阴性（Cre-）小鼠从 4.5 至 14 周龄时接受 Scl-mAb 或载体注射。载体处理的 Cre+ 小鼠的骨骼特性明显低于载体处理的 Cre- 小鼠，这通过 DXA、μCT、pQCT 和组织学评估，表明 Lrp5/6 缺失是有效的和高效的。Scl-mAb 治疗改善了 Cre- 小鼠几乎所有与骨骼相关的参数，但在 Cre+ 小鼠中，这种治疗对骨骼特性几乎没有改善。对于 Cre+ 小鼠对 Scl-mAb 有反应的少数终点，抗体诱导的 Wnt 信号转导可能发生在间充质/成骨细胞分化途径中比 Dmp1 表达阶段更早的细胞类型中。这一结论得到了一些组织形态计量学参数变化的支持。总之，与单独缺失 Lrp5 不同，Lrp5 和 Lrp6 的骨选择性晚期复合缺失显著削弱或完全消除了 Scl-mAb 的成骨作用，突出了 Lrp5 和 Lrp6 在 sclerostin 抗体的骨形成作用的作用机制中的主要作用。
