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异性作用的机制。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/旁分泌机制起作用。它们在垂体局部调节促性腺激素功能和促进两种促性腺激素FSH和LH的差异表达和分泌中起关键作用。卵泡抑素是分泌的糖蛋白，其在细胞外起作用以通过反馈环结合和调节活化素和TGF-β家族的其他相关配体的局部生物利用度。在垂体中，与在其他组织中一样，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/卵泡抑素张力的动态控制是通过激活素和卵泡抑素的相互作用以及对卵泡抑素表达施加控制的激活素的自调节作用来实现的。遗传模型已经证实了卵泡抑素的重要性，通过证明卵泡抑素基因敲除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基因。在促性腺激素细胞中，这种作用是通过激活素介体Smad 3和转录因子叉头家族成员FoxL 2的协调作用介导的。功能和生物化学实验表明，FoxL 2是Smad 3的一个强制性的合作伙伴，促性腺激素衍生的细胞类型中的卵泡抑素基因的激活素依赖性转录。FoxL 2在有限数量的组织中表达，并且具有FoxL 2缺陷的人类患者在受影响的女性亚组中显示与眼睑缺陷和卵巢早衰相关的睑裂-上睑下垂-内眦赘皮倒位综合征（BPES）。FoxL 2敲除小鼠表现出相似的表型。FoxL 2在成年雄性和雌性小鼠的垂体中表达，并与a-糖蛋白和FSH â亚基共定位。FoxL 2蛋白在促性腺激素衍生的细胞系中也容易检测到，并且siRNA介导的敲低策略已经证明FoxL 2是激活素对迄今为止已经鉴定的关键转录靶点（如卵泡抑素、FSH和GnRH-R）的作用的关键介导剂。通过利用互补的体外和体内模型，本申请提出的研究旨在更好地理解FoxL 2在协调促性腺激素的Smad 3依赖性激活素程序中的中心作用，阐明FoxL 2和Smad 3之间伙伴关系的潜在机制，并确定FoxL 2功能如何有助于垂体的激活素/卵泡抑素网络的局部控制，这些机制可能被证明与其他表达FoxL 2的组织相关。了解FoxL 2作用的机制可以阐明Smad 3靶点差异控制的策略，并确定与生殖和其他内分泌疾病相关的新分子靶点。 公共卫生相关性：激活素是普遍存在的生长因子，已知调节许多细胞过程，包括垂体促性腺激素的差异FSH表达和分泌。激活素部分地通过对称为卵泡抑素的激活素结合蛋白的表达施加积极作用来控制其自身的生物活性，卵泡抑素也是普遍存在的并且通常与激活素共定位。通过更好地了解激活素如何调节促性腺激素中滤泡素抑制素和其他关键靶点的表达，这些研究将有助于识别新的靶点，这些靶点有可能选择性地靶向与生殖相关的激活素的组织特异性作用和/或其他内分泌病理学。

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. PUBLIC HEALTH RELEVANCE: Activins are ubiquitous growth factors known to regulate many cellular processes, including differential FSH expression and secretion from pituitary gonadotropes. Activins control their own bioactivity in part by exerting positive effects on the expression of activin-binding proteins known as follistatins, which are also ubiquitous and often co-localize with activins. Through a better understanding of how activin regulates the expression of follistatin and other key targets in gonadotropes, these studies will facilitate the identification of novel targets that have the potential to selectively target the tissue-specific actions of activin relevant to reproductive and/or other endocrine pathologies.