Abstract：
Pathogenic variants disrupting the binding between sclerostin (encoded by SOST) and its receptor LRP4 have previously been described to cause sclerosteosis, a rare high bone mass disorder. The sclerostin-LRP4 complex inhibits canonical WNT signaling, a key pathway regulating osteoblastic bone formation and a promising therapeutic target for common bone disorders, such as osteoporosis. In the current study, we crossed mice deficient for Sost (Sost-/-) with our p.Arg1170Gln Lrp4 knock-in (Lrp4KI/KI) mouse model to create double mutant Sost-/-;Lrp4KI/KI mice. We compared the phenotype of Sost-/- mice with that of Sost-/-;Lrp4KI/KI mice, to investigate a possible synergistic effect of the disease-causing p.Arg1170Trp variant in Lrp4 on Sost deficiency. Interestingly, presence of Lrp4KI alleles partially mitigated the Sost-/- phenotype. Cellular and dynamic histomorphometry did not reveal mechanistic insights into the observed phenotypic differences. We therefore determined the molecular effect of the Lrp4KI allele by performing bulk RNA sequencing on Lrp4KI/KI primary osteoblasts. Unexpectedly, mostly genes related to bone resorption or remodeling (Acp5, Rankl, Mmp9) were upregulated in Lrp4KI/KI primary osteoblasts. Verification of these markers in Lrp4KI/KI, Sost-/- and Sost-/-;Lrp4KI/KI mice revealed that sclerostin deficiency counteracts this Lrp4KI/KI effect in Sost-/-;Lrp4KI/KI mice. We therefore hypothesize that models with two inactivating Lrp4KI alleles rather activate bone remodeling, with a net gain in bone mass, whereas sclerostin deficiency has more robust anabolic effects on bone formation. Moreover, these effects of sclerostin and Lrp4 are stronger in female mice, contributing to a more severe phenotype than in males and more detectable phenotypic differences among different genotypes.
摘要：
先前已经描述了破坏硬化蛋白（由SOST编码）与其受体LRP4之间结合的致病变体会导致硬化，一种罕见的高骨量疾病.硬化蛋白-LRP4复合物抑制典型的WNT信号，这是调节成骨细胞骨形成的关键途径，也是常见骨疾病的有希望的治疗靶点，比如骨质疏松症。在目前的研究中,我们用p.Arg1170GlnLrp4敲入（Lrp4KI/KI）小鼠模型杂交Sost缺陷（Sost-/-）小鼠，以创建双突变Sost-/-;Lrp4KI/KI小鼠。我们比较了Sost-/-小鼠与Sost-/-;Lrp4KI/KI小鼠的表型，研究Lrp4中引起疾病的p.Arg1170Trp变体对Sost缺乏症的可能协同作用。有趣的是,Lrp4KI等位基因的存在部分减轻了Sost-/-表型。细胞和动态组织形态计量学未揭示对观察到的表型差异的机械见解。因此,我们通过对Lrp4KI/KI原代成骨细胞进行大量RNA测序来确定Lrp4KI等位基因的分子效应。出乎意料的是,大多数与骨吸收或重塑相关的基因(Acp5，Rankl，Mmp9）在Lrp4KI/KI原代成骨细胞中上调。在Lrp4KI/KI中验证这些标记，Sost-/-和Sost-/-;Lrp4KI/KI小鼠揭示了硬化素缺乏在Sost-/-;Lrp4KI/KI小鼠中抵消这种Lrp4KI/KI效应。因此，我们假设具有两个失活Lrp4KI等位基因的模型激活了骨重建，随着骨量的净增加，而硬化蛋白缺乏对骨形成有更强的合成代谢作用。此外，硬化蛋白和Lrp4的这些作用在雌性小鼠中更强，导致比男性更严重的表型和不同基因型之间更可检测的表型差异。
