THE TEMPORAL AND SPATIAL REGULATION OF BONE ACQUISITION BY SERUM IGF-1

描述（由申请人提供）：本提案的主要目标是确定循环胰岛素样生长因子-I（IGF-I）对骨获取的时间和空间调节。几条证据支持IGF-I在骨重建过程中发挥重要作用的原则。人体研究表明，血清IGF-I和骨矿物质密度（BMD）之间存在较强的相关性。然而，循环IGF-I对骨骼获得和维持施加影响的精确机制和时间尚未阐明。尽管在研究IGF-I对骨的作用方面取得了显著进展，但还没有研究说明循环IGF-I的时间作用。我们先前使用Cre/LoxP系统产生肝脏特异性IGF-I缺陷（LID）小鼠。LID小鼠血清IGF-I降低75%，GH分泌增加4倍，肝外组织IGF-I表达无变化。LID小鼠表现出骨膜周长、皮质厚度和总体积BMD的减少。LID小鼠模型确立了内分泌IGF-I在骨骼完整性中的重要作用。然而，这种模型也有局限性：1- IGF-I缺乏从出生开始; 2-生长激素有一个深刻的和永久的继发性增加。因此，生长和成年动物的骨骼对血清IGF-I变化的反应的发育顺序和解剖学变化尚不清楚。我们最近开发了一种可诱导的肝脏IGF-I缺陷小鼠模型（iLID）。iLID模型基于Cre/loxP系统，其中Cre重组酶在抗胰蛋白酶-11启动子下在肝脏中特异性表达，并且可以通过单次他莫昔芬注射诱导，其不会影响骨骼。这个模型使我们能够解剖的时间贡献循环IGF-I骨骼收购和维护，并允许我们假设，内分泌IGF-I差异调节骨形成和骨吸收在骨骼生长，成年和老化，主要是通过其对成骨细胞的招聘，分化和生存的影响。因此，提出了两个主要目标：1-确定在早期生长、青春期和青春期后（2、4、8和12周）血清IGF-I消耗对峰值骨获得的影响。我们将确定IGF-I缺失导致的骨量减少是否是由于骨累积减少或骨吸收增强。我们将通过利用在生长关键时间具有时间消耗的iLID小鼠来确定IGF-I对骨作用的峰值时间。2-确定暂时血清IGF-I消耗对成年小鼠骨维持的影响。我们将确定老年小鼠血清IGF-I缺乏是否通过骨形成的改变导致骨小梁厚度的减少，并确定这是否是由于骨祖细胞减少，成骨细胞基质合成改变，或早期成骨细胞凋亡。我们将在16、20和32周时耗尽血清IGF-I，并检查iLID小鼠长达52周的骨骼。公共卫生相关性：该建议的主要目标是确定通过循环胰岛素样生长因子-I（IGF-I）对骨获得的时间和空间调节。我们将利用我们最近开发的他莫昔芬诱导的iLID模型，首次确定生长和成人骨骼中IGF-I缺乏的完整发育序列。这些研究的结果将为最佳骨骼健康的基本调节途径的作用机制提供重要的见解，并应确定血清IGF-I水平降低是否代表峰值骨获得受损和/或加速骨丢失的风险。

DESCRIPTION (provided by applicant): The primary goal of this proposal is to determine the temporal and spatial regulation of bone acquisition by circulating insulin like growth factor-I (IGF-I). Several lines of evidence support the tenet that IGF-I has an important role in the process of bone remodeling. Human studies have demonstrated a relatively strong correlation between serum IGF-I and bone mineral density (BMD). Yet the precise mechanism and timing whereby circulating IGF-I exerts its influence on skeletal acquisition and maintenance have not been elucidated. Notwithstanding the remarkable progress in studying IGF-I action on bone, there are no studies that address the temporal role of circulating IGF-I. We previously generated liver specific IGF-I deficient (LID) mice using the Cre/LoxP system. LID mice exhibited a 75% reduction in serum IGF-I, accompanied by a fourfold increase in GH secretion, with no changes in IGF-I expression in extra-hepatic tissues. LID mice exhibit reductions in periosteal circumference, cortical thickness and total volumetric BMD. The LID mouse model established an essential role for endocrine IGF-I in skeletal integrity. However, there are limitations to this model: 1- IGF-I deficiency starts from birth; and 2- there is a profound and perpetual secondary increase in growth hormone. Thus, the developmental sequences and the anatomical changes in the skeleton of the growing and adult animal in response to changes in serum IGF-I are not clear. We have recently developed an inducible liver IGF-I deficient mouse model (iLID). The iLID model is based on the Cre/loxP system whereby the Cre recombinase is expressed specifically in the liver under the anti-trypsin-11 promoter, and can be induced by a single tamoxifen injection that does not otherwise affect the skeleton. This model permits us to dissect the temporal contribution of circulating IGF-I to skeletal acquisition and maintenance, and allows us to hypothesize that endocrine IGF-I differentially regulates bone formation and resorption during skeletal growth, adulthood and aging, primarily through its effects on osteoblast recruitment, differentiation and survival. Thus two major aims are proposed: 1-Determine the effect of serum IGF-I depletion during early growth, puberty, and post puberty (2,4,8, and 12 weeks) on peak bone acquisition. We will determine whether reductions in bone mass with IGF-I deletions are due to reduced accrual of bone or enhanced resorption. We will ascertain the peak time of IGF-I action on bone by utilizing the iLID mouse with temporal depletions at critical times of growth. 2-Determine the effect of temporal serum IGF-I depletion on bone maintenance in the adult mouse. We will define whether serum IGF-I deficiency in older mice leads to a reduction in trabecular thickness through alterations in bone formation, and determine whether this is due to reduced osteoprogenitor cells, altered osteoblast matrix synthesis, or early osteoblast apoptosis. We will deplete serum IGF-I at 16, 20, and 32 weeks, and examine the skeleton of the iLID mice up to 52 weeks. PUBLIC HEALTH RELEVANCE: The primary goal of this proposal is to determine the temporal and spatial regulation of bone acquisition by circulating insulin like growth factor-I (IGF-I). We will capitalize on our recently developed tamoxifen inducible iLID model to determine, for the first time, the full developmental sequence of IGF-I deficiency in the growing and adult skeleton. The results of these studies will provide significant insight into the mechanisms of action of an essential regulatory pathway for optimal skeletal health and should determine whether reduced levels of serum IGF-I represent a risk for the impairment of peak bone acquisition and/or accelerated bone loss.