Regulation of Follistatin Expression by Activin

描述(由申请人提供)：本研究提案的目的是了解激活素对垂体促性腺激素关键靶点的细胞类型特定作用的潜在机制。激活素是多种组织和细胞功能的多效性调节剂，通常通过自分泌/旁分泌机制发挥作用。它们在脑下垂体局部调节促性腺激素功能，促进两种促性腺激素FSH和LH的差异表达和分泌方面起着关键作用。卵泡抑素是分泌的糖蛋白，其功能是在细胞外通过反馈环结合和调节激活素和其他相关的转化生长因子-β家族配体的局部生物利用度。在脑垂体中，如同在其他组织中一样，对激活素/卵泡抑素网络的精确控制有助于维持靶细胞的功能完整性，并防止病理的发展。激活素/卵泡抑素的动态调控是通过激活素和卵泡抑素的相互作用以及激活素对卵泡抑素表达的自我调节作用来实现的。遗传模型已经证实了卵泡抑素的重要性，证明了卵泡抑素缺失的小鼠表现出许多胚胎缺陷并在出生后不久死亡，而卵泡抑素的过度表达与不同程度的不孕症有关。卵泡抑素基因是激活素的转录靶点。在促性腺激素中，这种作用是通过激活素介质SMAD3和转录因子FOXL2家族中的一个成员的协调作用来实现的。功能和生化实验表明，FOXL2是Smad3在促性腺激素来源的细胞类型中依赖激活素转录卵泡抑素基因的必备伙伴。FOXL2在有限的组织中表达，患有FOXL2缺陷的人类患者在一组受影响的女性中表现出与眼皮缺陷和卵巢早衰相关的青春痘-上睑下垂-内翻内翻综合征(BPES)。FOXL2基因敲除小鼠表现出类似的表型。FOXL2在成年雄性和雌性小鼠脑垂体中均有表达，并与α-糖蛋白和FSH？亚基共定位。FOXL2蛋白在促性腺激素来源的细胞系中也很容易检测到，siRNA介导的敲除策略表明FOXL2是激活素作用于关键转录靶点的关键中介，这些关键转录靶标包括卵泡生成素、FSH和GnRH-R。通过利用互补的体外和体内模型，这一应用的拟议研究旨在更好地了解FOXL2在协调Smad3依赖的促性腺激素激活素程序中的核心作用，阐明FOXL2和Smad3之间合作的机制，并确定FOXL2功能如何有助于局部控制垂体激活素/卵泡抑素网络，这些机制可能被证明与其他FOXL2表达的组织相关。了解FOXL2作用的机制可以阐明对SMAD3靶点的差异化控制策略，并识别与生殖和其他内分泌疾病相关的新的分子靶点。

DESCRIPTION (provided by applicant): The objective of this research proposal is to gain an understanding of the mechanisms underlying cell-type specific actions of activin on key targets of pituitary gonadotropes. Activins are pleiotropic regulators of diverse tissues and cellular functions, often acting through autocrine/paracrine mechanisms. They play a critical role in the pituitary to locally regulate gonadotrope function and promote the differential expression and secretion of the two gonadotropins, FSH and LH. Follistatins are secreted glycoproteins that function extracellularly to bind and modulate the local bioavailability of activin and other related ligands of the TGF-¿ family through a feedback loop. In the pituitary, as in other tissues, the precise control of the activin/follistatin network helps maintain the functional integrity of target cells and prevents the development of pathologies. The dynamic control of activin/follistatin tone is achieved by the reciprocal actions of activin and follistatin and the self- modulating action of activin that exerts control on follistatin expression. Genetic models have substantiated the importance of follistatin by demonstrating that mice null for follistatin exhibit many embryonic defects and die shortly after birth while follistatin over-expression is associated with varying degrees of infertility. The follistatin gene is a transcriptional target of activin. In gonadotropes, this effect is mediated through the coordinated actions of the activin mediator, Smad3, and a member of the forkhead family of transcription factors, FoxL2. Functional and biochemical experiments have illustrated that FoxL2 is an obligatory partner of Smad3 for activin-dependent transcription of the follistatin gene in gonadotrope-derived cell types. FoxL2 is expressed in a limited number of tissues and human patients with FoxL2 deficiency display the Blepherophimosis-Ptosis- Epicanthus Inversus syndrome (BPES) associated with eyelid defects and premature ovarian failure in a sub- set of affected women. FoxL2 knockout mice exhibit a similar phenotype. FoxL2 is expressed in the pituitary of adult male and female mice and co-localizes with a-glycoprotein and FSH¿ subunits. FoxL2 protein is also readily detectable in gonadotrope-derived cell lines and siRNA-mediated knockdown strategies have demonstrated that FoxL2 is a critical mediator of activin actions on key transcriptional targets that have thus far been identified such as follistatin, FSH¿ and GnRH-R. By utilizing complimentary in vitro and in vivo models, the proposed studies of this application aim to provide a better understanding of the central role of FoxL2 in coordinating the Smad3-dependent activin program of gonadotropes, elucidate the mechanism underlying the partnership between FoxL2 and Smad3 and determine how FoxL2 function contributes to the local control of the activin/follistatin network of the pituitary, mechanisms that might prove to be relevant to other FoxL2-expressing tissues. Understanding the mechanism underlying FoxL2 action could illuminate strategies for the differential control of Smad3 targets and identify novel molecular targets with relevance to reproductive and other endocrine disorders.