Sclerostin and Gsalpha signaling in osteoblasts

骨质疏松症是一种由骨形成与骨吸收不平衡引起的退行性疾病，估计有50%的50岁以上的美国人患有骨质疏松症。虽然骨折会让人想起童年时打石膏的肢体，在短暂的康复期里乞求签名和同情，但老年人的脆性骨折要危险得多。50岁以上髋部骨折患者一年的死亡率高达惊人的24%，只有三分之一的患者能够恢复骨折前的独立活动能力。随着寿命的延长和人口的老龄化，骨质疏松性骨折的费用——每年已经超过180亿美元——肯定会上升。目前治疗骨质疏松症的方法大多是抗吸收的，因此不能治愈。随着最近重组人甲状旁腺激素（teriparatide, PTH[1-34]）的批准，一种对骨形成具有强大刺激作用的合成代谢剂现已问世。然而，由于肠外给药的局限性和对成骨恶性肿瘤的潜在担忧，都表明对骨质疏松症的额外治疗的持久需求。典型的Wnt信号通路在骨形成中也起着关键作用，因此靶向Wnt通路是治疗骨质疏松症的一个有吸引力的选择。最近的研究强调了硬化蛋白的重要作用，硬化蛋白是Wnt信号的抑制剂，PTH抑制硬化蛋白表达的证据表明这两条通路之间存在有意义的串扰。异三聚体G蛋白Gs1是通过PTH/PTH相关肽受体介导PTH信号传导的主要下游介质。我发现小鼠成骨细胞谱系中Gs1的消融（Gs1 KO小鼠）导致深度骨质疏松症；在这些小鼠中，硬化蛋白的表达显著增加，导致典型Wnt信号的减少。由于PTH和Wnt通路可能对骨形成有重叠和不同的作用，在本应用中，我建议确定硬化蛋白对Wnt信号的抑制如何导致Gs1 KO小鼠骨量的急剧减少。我建议将Gs1 KO小鼠与缺乏硬化蛋白的小鼠（SOST KO小鼠）杂交。在Aim 1中，我将研究硬化蛋白在Gs1 KO小鼠胚胎骨骼发育中的作用。在目标2中，我将延迟骨祖细胞中Gs1的消融直到出生，特别关注硬化蛋白在出生后骨骼稳态中的功能。在Aim 3中，我将从单和双KO小鼠中获取骨髓基质细胞和颅骨成骨细胞，以确定PTH和Wnt信号通路如何相互作用以调节间充质谱系承诺和成骨分化。了解PTH和Wnt信号合成代谢作用的分子机制可能最终为骨质疏松症的治疗提供额外的靶点。

DESCRIPTION (provided by applicant): Project Summary Osteoporosis, a degenerative disease resulting from an imbalance of bone formation relative to bone resorption, will strike an estimated 50% of Americans over the age of 50. Although fractures call forth images from childhood of a limb in a cast, soliciting autographs and sympathy during a brief period of recovery, fragility fractures in the elderly are far more sinister. The one-year mortality rate for patients over the age of 50 with a hip fracture is an astounding 24%, and only one in three regain their pre-fracture ability to function independently. As lifespans lengthen and the population ages, the costs of osteoporotic fractures - already over $18 billion annually - will surely rise. Current therapies for osteoporosis are mostly anti-resorptive, and therefore not curative. With the recent approval of recombinant human parathyroid hormone (teriparatide, PTH[1-34]), an anabolic agent with potent stimulatory effects on bone formation is now available. However, limitations due to parenteral administration and a potential concern for osteogenic malignancies all point to an enduring need for additional therapies for osteoporosis. The canonical Wnt signaling pathway also has a critical role in bone formation, thus targeting the Wnt pathway is an attractive option for treating osteoporosis. Recent studies have highlighted an important role for sclerostin, an inhibitor of Wnt signaling, and evidence that PTH suppresses sclerostin expression suggests that meaningful cross-talk exists between these two pathways. The heterotrimeric G protein Gs1 is a major downstream mediator of PTH signaling via the PTH/PTH-related peptide receptor. I have found that ablation of Gs1 in the osteoblast lineage in mice (Gs1 KO mice) leads to profound osteoporosis; in these mice sclerostin expression is markedly increased, with a resultant decrease in canonical Wnt signaling. Since the PTH and Wnt pathways likely have both overlapping and distinct actions on bone formation, in this application I propose to determine how inhibition of Wnt signaling by sclerostin contributes to the dramatic reduction of bone mass in Gs1 KO mice. I propose to cross Gs1 KO mice with mice lacking sclerostin (SOST KO mice). In Aim 1 I will examine the role of sclerostin in embryonic skeletal development of Gs1 KO mice. In Aim 2 I will delay ablation of Gs1 in osteoprogenitors until birth, to specifically focus on the function of sclerostin in postnatal skeletal homeostasis. In Aim 3 I will harvest bone marrow stromal cells and calvarial osteoblasts from single and double KO mice to establish how PTH and Wnt signaling pathways interact to regulate mesenchymal lineage commitment and osteogenic differentiation. Understanding the molecular mechanisms underlying the anabolic effects of PTH and Wnt signaling may eventually provide additional targets for the treatment of osteoporosis. PUBLIC HEALTH RELEVANCE: Project Narrative Osteoporosis is a common degenerative disease of aging, and treatments available to cure this debilitating disease are lacking. Parathyroid hormone and the Wnt signaling pathway both act to control bone formation. This proposal seeks to understand how these two pathways interact at the molecular level, as greater insights into the processes by which bone mass is increased may ultimately result in novel therapies for osteoporosis.