Osteocyte control of bone remodeling through the PTH receptor

描述(由申请人提供)： 最近由PI领导的研究表明，激活骨细胞中的PTH受体信号足以增加骨量和重塑速度，这是PTH对骨骼的两个公认的作用；将骨细胞定位为激素的关键靶细胞。这项研究的长期目标是确定骨细胞对骨骼动态平衡的调节作用。这项建议的具体目标是揭示骨细胞在甲状旁腺激素刺激下诱导骨吸收的机制。甲状旁腺素治疗的小鼠或在骨细胞中表达具有构成活性的甲状旁腺素受体的小鼠表现出破骨细胞增加和骨吸收增加。相反，在骨细胞中缺乏甲状旁腺素受体的小鼠表现出骨吸收减少。甲状旁腺激素诱导的骨吸收是由于RANKL基因上调所致，需要基质金属蛋白酶(MMPs)降解细胞外基质。虽然已认识到甲状旁腺素的破骨作用是由成骨细胞系的细胞介导的，但甲状旁腺素靶细胞的确切分化阶段仍不清楚。最近的证据表明，骨细胞是RANKL的重要来源，它们分泌MMP13，促进破骨细胞的募集和吸收，并表达膜结合的MMP14(或MT1-MMPs)，重塑细胞周围基质，从而维持骨细胞管状网络，并诱导RANKL脱落。根据这一证据和研究结果表明，PTH受体信号通路的激活会增加RANKL、MMP13和MMP14的表达，我们推测PTH对骨细胞的直接作用会增加RANKL和MMPs，进而作为骨细胞衍生因子促进破骨细胞的形成和活性，并促进骨吸收。这一假设将通过追求两个具体目标来验证，这两个目标结合了体内和体外的方法，并使用了新的转基因小鼠和小鼠、人和人的骨细胞以及小鼠的原代骨细胞。目的1研究膜结合RANKL和可溶性RANKL在PTHR1活化诱导骨细胞吸收中的作用。目的2将确定骨细胞来源的MMPs在甲状旁腺激素诱导的吸收中的作用。这些研究的成功完成将对基础骨生物学和病理生理学产生积极的影响，因为这将促进我们对骨细胞中的甲状旁腺素受体信号调节破骨细胞和成骨细胞在健康和疾病中重塑骨的功能的机制的理解。

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.