Osteocyte control of bone remodeling through the PTH receptor

描述（由申请人提供）： PI领导的最新研究表明，骨细胞中PTH受体信号传导的激活足以增加骨量和重塑速率，这是PTH对骨骼的两种公认作用;将骨细胞定位为激素的关键靶细胞。本研究的长期目标是确定骨细胞对骨骼稳态调节的贡献。该建议的具体目标是揭示骨细胞诱导骨吸收对PTH的反应的机制。用PTH处理或在骨细胞中表达组成型活性PTH受体的小鼠表现出破骨细胞增加和骨吸收升高。相反，骨细胞中缺乏PTH受体的小鼠表现出骨吸收减少。PTH诱导的吸收是由于RANKL基因的上调，需要基质金属蛋白酶（MMP）降解细胞外基质。尽管人们认识到PTH的促破骨细胞生成作用是由成骨细胞谱系的细胞介导的，但PTH靶细胞的确切分化阶段仍然未知。最近的证据表明，骨细胞是RANKL的重要来源;它们分泌MMP 13，可能促进破骨细胞募集和再吸收，并表达膜结合MMP 14（或MT 1-MMP），重塑细胞周围基质，从而维持骨细胞小管网络，并诱导RANKL脱落。基于这一证据和发现，表明RANKL、MMP 13和MMP 14的表达通过激活骨细胞中的PTH受体信号传导而升高，我们假设PTH对骨细胞的直接作用增加RANKL和MMP，其反过来作为骨细胞衍生因子增加破骨细胞形成和活性以及骨吸收。这一假设将通过追求两个特定的目标，联合收割机体内和体外的方法，并使用新的基因修饰小鼠和小鼠和人骨细胞和小鼠原代骨细胞进行测试。目的1将检查膜结合和可溶性RANKL对骨细胞中PTHR 1活化诱导的吸收的贡献。目的2将确定骨细胞源性MMPs对PTH诱导的骨吸收的贡献。成功完成拟议的研究将通过推进我们对骨细胞中PTH受体信号传导调节重塑骨、破骨细胞和成骨细胞的细胞功能的机制的理解，对基础骨生物学和病理生理学产生积极影响，在健康和疾病中。

DESCRIPTION (provided by applicant): Recent studies led by the PI demonstrated that activation of PTH receptor signaling in osteocytes is sufficient to increase bone mass and the rate of remodeling, two recognized actions of PTH on the skeleton; positioning osteocytes as crucial target cells of the hormone. The long term goal of this research is to determine the contribution of osteocytes to the regulation of skeletal homeostasis. The specific goal of this proposal is to reveal the mechanisms by which osteocytes induce bone resorption in response to PTH. Mice treated with PTH or expressing a constitutively active PTH receptor in osteocytes exhibit increased osteoclasts and elevated bone resorption. Conversely, mice lacking the PTH receptor in osteocytes exhibit decreased bone resorption. PTH-induced resorption is due to upregulation of the RANKL gene and requires extracellular matrix degradation by matrix metalloproteinases (MMPs). Although it is recognized that the pro-osteoclastogenic action of PTH is mediated by cells of the osteoblastic lineage, the precise stage of differentiation of the PTH target cell remains unknown. Recent evidence indicates that osteocytes are an important source of RANKL; and that they secrete MMP13, which might facilitate osteoclast recruitment and resorption, and express membrane-bound MMP14 (or MT1-MMP), which remodels the pericellular matrix thus maintaining the osteocyte canalicular network and also induces RANKL shedding. Based on this evidence and findings showing that the expression of RANKL, MMP13 and MMP14 is elevated by activation of PTH receptor signaling in osteocytes, we hypothesize that direct effects of PTH on osteocytes increase RANKL and MMPs, which in turn act as osteocyte-derived factors to increase osteoclast formation and activity, and bone resorption. This hypothesis will be tested by pursuing two specific aims that combine in vivo and in vitro approaches and use novel genetically modified mice and murine and human osteocytic cells and murine primary osteocytes. Aim 1 will examine the contribution of membrane-bound and soluble RANKL to resorption induced by PTHR1 activation in osteocytes. Aim 2 will determine the contribution of osteocyte-derived MMPs to PTH-induced resorption. Successful completion of the proposed studies will positively impact basic bone biology and pathophysiology by advancing our understanding of the mechanisms by which PTH receptor signaling in osteocytes regulates the function of the cells that remodel bone, osteoclasts and osteoblasts, in health and disease.