H-Ras, a new regulator of bone integrity - the mechanistic base of osteoporosis triggered by a constitutive active H-Ras GTPase

骨稳态是一个由骨形成细胞和骨吸收细胞调控的动态过程。骨吸收增加会引发骨质疏松症，其特征是骨量低，骨折风险增加。骨质疏松症通常与年龄增加、绝经后和长期使用类固醇有关，但也有一些罕见的疾病以骨质疏松症为表型特征。其中一种罕见的疾病是科斯特洛综合征(CS)，由H-RAS癌基因的胚系突变引发。成人患者的特点是过早衰老，骨质疏松症是一个突出的表型病理。我们的初步工作表明，在含有H-RAS G12V结构性活性突变的CS小鼠模型中，骨量减少，破骨细胞生成和破骨细胞活性增加。体外破骨细胞分化对重组M-CSF和RANKL的反应表明，高活性的H-RAS以细胞自主的方式增加破骨细胞的数量，这证实了我们的体内研究。同时，我们也不能排除破骨细胞与成骨细胞、骨细胞与基质细胞之间的相互作用失衡所导致的细胞非自主机制的影响。高活性的H-RAS是一种已知的多种细胞类型的衰老诱导剂，因此成骨细胞、骨细胞和基质细胞可能通过其分泌的表型而经历衰老，衰老的成骨细胞、骨细胞和基质细胞可能促进破骨细胞的分化和活性，从而导致CS小鼠的骨丢失。为此，我们计划(I)详细描述CS小鼠的骨质疏松表型，以确定骨质疏松的早期发病；(Ii)揭示CS小鼠和CS患者来源的破骨细胞中H-RAS控制的信号通路和失调的生物学过程；以及(Iii)研究破骨细胞与成骨细胞、骨细胞和基质细胞之间的相互作用，并了解它是否导致CS小鼠检测到的破骨细胞生成增加。预期的结果将提供对H-RAS控制的分子机制的见解，这些分子机制是导致CS患者和其他RAS-MAPK发育综合征(Rasopathies)骨质疏松的原因，并将增进我们对破骨细胞生物学的总体理解。

Bone homeostasis is a dynamic process that is regulated by the bone forming and bone resorbing cells. An increase in bone resorption triggers osteoporosis, that is characterized by a low bone mass and an increased fracture risk. Osteoporosis is generally associated with increased aging, post-menopause and long-term steroid use, but there are some rare diseases that have osteoporosis as phenotypic feature. One of these rare disorders is the Costello syndrome (CS), triggered by germline mutations in the H-Ras oncogene. Adult patients are characterized by a premature aging phenotype, with osteoporosis as one prominent phenotypic pathology. Our preliminary work demonstrated that in a CS mouse model harbouring an H-Ras G12V constitutive active mutation, bone mass is reduced and osteoclastogenesis and osteoclast activity are increased. In vitro osteoclasts differentiation in response to recombinant M-CSF and RANKL showed that hyperactive H-Ras increases osteoclasts number in a cell autonomous manner, confirming our in vivo studies. At the same time, we cannot exclude the effects of cell non-autonomous mechanisms induced by the imbalance in the cross-talk between osteoclasts and osteoblasts, osteocytes and stromal cells. Hyperactive H-Ras is a known senescence inducer in various cell types, therefore osteoblasts, osteocytes and stromal cells may undergo senescence Through their secreted phenotype, senescent osteoblasts, osteocytes and stromal cells may enhance osteoclasts differentiation and activity, thus contributing to the bone loss observed in CS mice. To this end, we plan (I) to characterize in detail the osteoporotic phenotype in CS mice in order to identify the early onset of osteoporosis, (II) to reveal H-Ras-controlled signalling pathways and dysregulated biological processes in CS mouse- and CS patients-derived osteoclasts and (III) to study the cross-talk between osteoclasts and osteoblasts, osteocytes and stromal cells and understand whether it contributes to the increased osteoclastogenesis detected in CS mouse. The expected results will provide insights into H-Ras-controlled molecular mechanisms that are responsible for osteoporosis in CS patients and other Ras-MAPK developmental syndromes (RASopathies) and will enhance our general understanding of osteoclast biology.