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

骨稳态是由骨形成细胞和骨吸收细胞调节的动态过程。骨吸收的增加引发骨质疏松症，其特征在于低骨量和骨折风险增加。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，但也有一些罕见疾病以骨质疏松症为表型特征。这些罕见的疾病之一是Costello综合征（CS），由H-Ras癌基因的生殖细胞突变引发。成年患者的特征在于过早衰老表型，骨质疏松症是一种突出的表型病理。我们的初步工作表明，在CS小鼠模型窝藏H-Ras G12 V组成型活性突变，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数量，证实了我们的体内研究。同时，我们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。过度活跃的H-Ras是已知的多种细胞类型中的衰老诱导剂，因此成骨细胞、骨细胞和基质细胞可以经历衰老。通过它们的分泌表型，衰老的成骨细胞、骨细胞和基质细胞可以增强破骨细胞的分化和活性，从而有助于在CS小鼠中观察到的骨丢失。为此，我们计划（I）详细描述CS小鼠的骨质疏松表型，以确定骨质疏松症的早期发作，（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）研究破骨细胞和成骨细胞之间的串扰，骨细胞和基质细胞，并了解它是否有助于增加破骨细胞在CS小鼠检测。预期的结果将提供洞察H-Ras控制的分子机制，负责骨质疏松症的CS患者和其他Ras-MAPK发育综合征（RASopathies），并将提高我们的破骨细胞生物学的一般理解。

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.