Mouse model of adult-onset, isolated, GH-deficiency

说明(申请人提供)：理论基础：在成年人中，生长激素(GH)已被证明具有抗脂肪、促脂肪分解和蛋白质合成代谢作用，而胰岛素样生长因子I(IGF-I)的代谢作用类似于胰岛素。GH和IGF-I也被报道对胰腺2-细胞质量、心血管功能、骨重建和免疫功能有积极作用。因此，随着年龄的增长，循环中GH/IGF-I水平的下降被预测在各种与年龄相关的疾病的病理生理学和进展中发挥作用，包括肥胖/糖尿病、骨质疏松症、心肌病和骨质疏松症。尽管GH/IGF-I对成人有潜在的积极作用，但也有证据表明，GH/IGF-I会加剧癌症并缩短寿命。因此，在正常衰老情况下观察到的GH/IGF-I的下降实际上可能是一种保护机制。问题：很难准确地确定生长激素在成人生理学中的重要性，因为我们的大部分知识都来自于1)发育性生长激素缺乏症的人类和啮齿动物模型，这可能不同于导致性成熟后发生的生长激素缺乏症和2)成人起病的生长激素缺乏症，后者通常与全垂体功能减退有关。由于这些原因，成人起病的孤立性生长激素缺乏症(AOiGHD)模型将是一种独特的、信息丰富的工具，可以更准确地定义内源性GH在与年龄相关的健康和疾病变化中的重要性。解决方案：本实验室最近开发并验证了一种在垂体前叶生长激素产生细胞(生长激素分泌细胞)中表达Cre重组酶(Cre)的转基因小鼠(rGHp-Cre)。RGHp-Cre小鼠是研究促生长激素功能各个方面的通用工具，因为它可以与携带基因工程等位基因的小鼠杂交，这些基因工程等位基因包含关键的DNA元素，两侧是独特的Cre识别序列，称为loxP位点；从而允许Cre介导的这些基因的改变(激活或失活)仅在脑下垂体前叶的生长激素基因群中。为了建立成人发病、孤立的生长激素缺乏模型，rGHp-Cre小鼠与现有的携带Cre诱导的白喉毒素受体转基因(IDTR)的小鼠杂交。初步结果表明，双转基因后代(Cre、iDTR)是正常的。在性成熟后，这些小鼠被注射低剂量的白喉毒素(DT)，它与生长激素上表达的DTR结合，允许毒素内化并随后导致生长激素群体的凋亡，导致AOiGHD。具体目标：1.完成对DT治疗的CRE、iDTR雄性和雌性AOiGHD成年小鼠组织的分析，以a)确认生长激素破坏的特异性，以及b)提供GH/IGF-I减少对代谢基因表达影响的初步评估。测试替代策略以最大化DT介导的生长激素破坏，以实现AOiGHD的“完全”模型，III.提供AOiGHD对代谢终点影响的更详细的活体分析。评估AOiGHD对寿命的影响。公共卫生相关性：这项建议将描述一种新的成人型孤立性GHD(AOiGHD)小鼠模型，并研究AOiGHD对身体成分、新陈代谢和寿命的影响。这一模型将提供一个独特的和信息丰富的工具，以更准确地确定内源性生长激素在与年龄相关的健康和疾病变化中的重要性。

DESCRIPTION (provided by applicant): Rationale: In adults, growth hormone (GH) has been shown to have anti-lipogenic, pro-lipolytic and protein anabolic effects, while the metabolic actions of insulin-like growth factor I (IGF-I) resemble those of insulin. GH and IGF-I are also reported to have positive effects on pancreatic 2-cell mass, cardiovascular performance, bone remodeling and immune function. Therefore, the fall in circulating GH/IGF-I levels observed with age is predicted to play a role in the pathophysiology and progression of a variety of age-associated diseases including obesity/diabetes, sarcopenia, cardiomyopathy and osteoporosis. Despite the potential positive effects of GH/IGF-I in adults, there is also evidence that GH/IGF-I can exacerbate cancer and reduce lifespan. Therefore the fall in GH/IGF-I observed with normal aging might, in fact, serve a protective mechanism. Problem: It is difficult to accurately determine the importance of GH in adult physiology because the bulk of our knowledge has been derived from humans and rodent models of 1) developmental GH-deficiency, which may differ from that resulting in GH-deficiency occuring after sexual maturation and 2) adult-onset, GH- deficiency, which is typically associated with panhypopituitarism. For these reasons, a model of adult-onset, isolated GH-deficiency (AOiGHD) would be a unique and informative tool to more accurately define the importance of endogenous GH in age-associated changes in health and disease. Solution: Our laboratory has recently developed and validated a transgenic mouse line that expresses Cre recombinase (Cre) in the GH-producing cells (somatotropes) of the anterior pituitary gland (rGHp-Cre). The rGHp-Cre mouse represents a versatile tool to study various aspects of somatotrope function in that it can be crossbred to mice carrying genetically engineered alleles that contained critical DNA elements flanked by unique Cre recognition sequences referred to as loxP sites; thereby allowing for the Cre-mediated alteration of these genes (activation or inactivation) only in the somatotrope population of the anterior pituitary gland. To generate a model of adult-onset, isolated, GH-deficiency, the rGHp-Cre mice have been crossbred to an existing mouse model carrying a Cre-inducible diphtheria toxin receptor transgene (iDTR). Preliminary results demonstrate that the double transgenic offspring (Cre,iDTR) are normal. After sexual maturation, these mice were injected with a low dose of diphtheria toxin (DT), which binds to the DTR expressed on somatotropes allowing for internalization of the toxin and subsequent apoptosis of the somatotrope population, resulting in AOiGHD. Specific Aims: I. Finalize the analysis of tissues taken from DT-treated Cre,iDTR male and female adult mice with "partial" AOiGHD, in order to a) confirm the specificity of somatotrope destruction, and b) provide an initial assessment of the impact of GH/IGF-I reduction on metabolic gene expression. II. Test alternative strategies to maximize DT-mediated somatotrope destruction, to achieve a model of "complete" AOiGHD, III. Provide a more detailed in vivo analysis of the impact of AOiGHD on metabolic endpoints. IV. Assess the impact of AOiGHD on lifespan. PUBLIC HEALTH RELEVANCE: This proposal will characterize a novel mouse model of adult-onset, isolated GHD (AOiGHD) and examine the effect of AOiGHD on body composition, metabolism and longevity. This model will provide a unique and informative tool to more accurately define the importance of endogenous GH in age-associated changes in health and disease.