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

描述(由申请人提供)：这项建议的主要目标是通过循环胰岛素样生长因子-I(IGF-I)来确定骨获取的时间和空间调节。有几条证据支持IGF-I在骨重建过程中发挥重要作用的信条。人体研究表明，血清IGF-I与骨密度(BMD)之间存在较强的相关性。然而，循环IGF-I影响骨骼获得和维持的确切机制和时间尚未阐明。尽管在研究IGF-I对骨骼的作用方面取得了显著的进展，但还没有研究解决循环中IGF-I的时间作用。我们之前使用Cre/loxP系统培育了肝脏特异性IGF-I缺陷(LID)小鼠。LID小鼠血清IGF-I降低75%，GH分泌增加四倍，肝外组织中IGF-I的表达没有变化。LID小鼠表现出骨膜周长、皮质厚度和总体积骨密度的减少。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，这个模型有局限性：1-IGF-I缺乏从出生就开始；2-生长激素有深刻的和永久的继发性增加。因此，生长和成年动物骨骼的发育顺序和解剖变化对血清IGF-I的变化尚不清楚。我们最近建立了一种诱导性肝脏IGF-I缺陷小鼠模型(ILID)。ILID模型基于Cre/loxP系统，在该系统中，Cre重组酶在抗胰酶-11启动子的作用下在肝脏中特异性表达，并且可以由一次他莫昔芬注射诱导，否则不会影响骨骼。这一模型使我们能够剖析循环中的IGF-I对骨骼获取和维持的时间贡献，并允许我们假设内分泌IGF-I在骨骼生长、成年期和衰老过程中对骨形成和骨吸收的差异性调节，主要是通过其对成骨细胞招募、分化和存活的影响。因此，提出了两个主要目标：1-确定生长早期、青春期和青春期后(2、4、8和12周)血清IGF-I耗竭对骨获取峰值的影响。我们将确定IGF-I缺失导致的骨量减少是由于骨积累减少还是由于骨吸收增加。我们将利用生长关键时期短暂耗竭的ILID小鼠来确定IGF-I对骨骼作用的高峰期。2-确定暂时性血清IGF-I耗竭对成年小鼠骨骼维持的影响。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减少，并确定这是由于成骨前体细胞减少、成骨细胞基质合成改变或早期成骨细胞凋亡所致。我们将在16周、20周和32周耗尽血清IGF-I，并检查Ilid小鼠长达52周的骨骼。公共卫生相关性：这项建议的主要目标是通过循环胰岛素样生长因子-I(IGF-I)来确定骨获取的时间和空间调节。我们将利用我们最近开发的三苯氧胺诱导的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优化骨骼健康的基本调控途径的作用机制提供重要的洞察，并应该确定血清IGF-I水平降低是否代表着峰值骨获取的损害和/或加速的骨丢失的风险。

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.