Skeletal Response to Leptin

描述(由申请人提供)： 瘦素是Ob基因的蛋白质产物，作用于包括骨骼在内的多个器官。瘦素缺陷ob/ob小鼠和瘦素受体缺陷db/db小鼠表现出马赛克骨骼表型；与野生型小鼠相比，突变小鼠的骨长和总骨量减少，但松质骨的部位特异性增加。这些异常表明，瘦素的充足对于正常的骨生长、转换和功能是必不可少的。然而，瘦素调节骨代谢从而产生这些变化的确切机制只有一部分人知道，部分原因是瘦素可能通过多条途径影响骨细胞；一条间接途径涉及下丘脑中继站，另一条直接途径涉及瘦素与软骨和骨细胞上的瘦素受体结合。我们假设瘦素对骨代谢的调节比目前所了解的更复杂。我们认为，骨骼对瘦素作用的明显抵抗是通过骨感觉神经元的传入信号和下丘脑通过交感神经元的传出信号的负反馈回路的结果。此外，第二个负反馈回路涉及对脂肪细胞分化的调节。具体地说，脂肪细胞产生与其数量和大小成比例的瘦素，但下丘脑瘦素拮抗骨髓基质细胞向脂肪细胞的分化，同时促进成骨细胞分化。我们将在ob/ob和db/db小鼠身上测试这些假设，以实现以下特定目标：1)确定外周(骨骼产生的血清瘦素和瘦素)和下丘脑瘦素对骨生长和转换的各自作用，以及2)确定瘦素是否调节骨髓基质细胞向脂肪细胞和成骨细胞的分化。目的1将验证瘦素对骨骼具有外周和下丘脑介导作用的假设。此外，它还将检验下丘脑激素的作用是自限的假设，因为负反馈环包括通过感觉神经元的传入信号和通过交感神经元的传出信号。重要的是，这项拟议的研究将确定下丘脑瘦素对生长板、皮质骨和松质骨的短暂隔室特异性作用是否可以通过假设的相反的外周和下丘脑介导的作用来解释。目的2验证下丘脑和/或外周瘦素调节骨髓基质细胞分化为成骨细胞或脂肪细胞的假说。这些研究将确定瘦素对基质细胞分化的外周作用是否与下丘脑介导的激素作用相反。我们研究的长期目标是了解生活方式因素如何与遗传因素相互作用来确定峰值骨量。这项拟议的研究将阐明瘦素诱导的骨生长和结构调节的细胞机制。了解瘦素在骨生长中的作用及其复杂的作用机制非常重要，因为低峰值体重是骨质疏松的危险因素。

DESCRIPTION (provided by applicant): Leptin, the protein product of the Ob gene, acts on multiple organs, including bone. Leptin deficient ob/ob mice and leptin receptor-deficient db/db mice exhibit a mosaic skeletal phenotype; compared to wild-type mice, mutant mice have reduced bone length and overall bone mass but exhibit site-specific increases in cancellous bone. These abnormalities suggest that leptin sufficiency is essential for normal bone growth, turnover and function. However, the precise mechanisms by which leptin regulates bone metabolism to produce these changes are only partially known, in part, because leptin has the potential to affect bone cells through multiple pathways; an indirect pathway involving a hypothalamic relay and a direct pathway involving the binding of leptin to its receptors on cartilage and bone cells. We hypothesize that the regulation of bone metabolism by leptin is even more complex than currently appreciated. We propose that the apparent skeletal resistance to the hypothalamic actions of leptin occurs as a result of a negative feedback loop involving afferent signaling via sensory neurons in bone and efferent signaling from the hypothalamus via sympathetic neurons. Furthermore, a second negative feedback loop involves regulation of adipocyte differentiation. Specifically, adipocytes produce leptin in proportion to their number and size but hypothalamic leptin antagonizes differentiation of bone marrow stromal cells to adipocytes while enhancing osteoblast differentiation. We will test these hypotheses in ob/ob and db/db mice by accomplishing the following specific aims; 1) determine the respective roles of peripheral (serum leptin and leptin produced by the skeleton) and hypothalamic leptin on bone growth and turnover, and 2) determine if leptin regulates the differentiation of bone marrow-derived stromal cells to adipocytes and osteoblasts. Aim 1 will test the hypothesis that leptin has peripheral-mediated as well as hypothalamic-mediated actions on bone. Additionally, it will test the hypothesis that the hypothalamic actions of the hormone are self limiting because of a negative feed back loop involving afferent signaling via sensory neurons and efferent signaling via sympathetic neurons. Importantly, the proposed studies will establish whether the transient compartment-specific effects of hypothalamic leptin on growth plate, cortical bone and cancellous bone can be explained by the hypothesized opposing peripheral- and hypothalamic-mediated actions of the hormone. Aim 2 will test the hypothesis that hypothalamic and/or peripheral leptin regulates the differentiation of bone marrow stromal cells to form osteoblasts or adipocytes. These studies will establish whether the peripheral effects of leptin on stromal cell differentiation are opposed by hypothalamic-mediated actions of the hormone. The long-term goal of our research is to understand how lifestyle factors interact with genetics to determine peak bone mass. The proposed research will clarify the cellular mechanisms for leptin- induced regulation of bone growth and architecture. An understanding of the role of leptin and its complex mechanisms of action on bone growth is important because a low peak mass is a risk factor for osteoporosis.