Regulation of Follistatin Expression by Activin

描述（由申请人提供）：本研究计划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功能，促进促性腺激素FSH和LH两种促性腺激素的差异表达和分泌中起关键作用。卵泡listatin是一种分泌的糖蛋白，在细胞外通过反馈回路结合和调节激活素和TGF-¿家族其他相关配体的局部生物利用度。与其他组织一样，在垂体中，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。遗传模型证实了卵泡抑素的重要性，表明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，而卵泡抑素过表达与不同程度的不孕有关。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和叉头转录因子家族成员FoxL2的协调作用介导的。功能和生化实验表明，在促性腺激素来源的细胞类型中，FoxL2是Smad3激活素依赖性转录卵泡抑素基因的必需伴侣。FoxL2在有限数量的组织中表达，FoxL2缺乏的人类患者表现出与眼睑缺陷和卵巢早衰相关的眼睑-上睑下垂-内眦赘肉倒置综合征（BPES）。FoxL2敲除小鼠表现出类似的表型。FoxL2在成年雄性和雌性小鼠的垂体中表达，并与a-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介导的敲低策略表明FoxL2是激活关键转录靶点的关键介质，这些靶点目前已被确定，如卵泡抑素、FSH¿和GnRH-R。通过利用互补的体外和体内模型，本研究旨在更好地了解FoxL2在协调Smad3依赖性促性腺激素激活素程序中的核心作用，阐明FoxL2和Smad3之间伙伴关系的机制，并确定FoxL2功能如何促进垂体激活素/卵泡抑制素网络的局部控制。这种机制可能与其他foxl2表达组织有关。了解FoxL2的作用机制可以阐明Smad3靶点的差异控制策略，并发现与生殖和其他内分泌疾病相关的新分子靶点。

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. 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.