ROLE OF BETAGLYCAN IN INHIBIN ANTAGONISM OF ACTIVIN

激活素和某些BMP是转化生长因子-β超家族的成员，它们通过垂体前叶调节FSH的产生。它们及其受体在生殖、内分泌和其他系统中也有不同的内分泌、旁分泌和自分泌作用，在发育和病理生理过程中发挥关键作用。抑制素对抗激活素的部分但不是全部作用，对生存至关重要，抑制素α缺失小鼠的致命表型说明了这一点，这些小鼠患有性腺和肾上腺肿瘤以及肝脏坏死。竭尽全力 在细胞作用下，激活素(β-β二聚体)与其II型受体丝氨酸激酶(ActRII或ActRIIB)结合，然后招募I型受体丝氨酸激酶(ALK4)形成活性复合体，启动下游信号传递。抑制素(α-β二聚体)与激活素竞争结合ActRII，但抑制素/ActRII复合体不招募ALK4或促进信号转导。抑制素是激活素的强大功能拮抗剂，但与激活素相比，抑制素对ActRII的亲和力相对较低，这不符合简单的竞争模型。因此，我们寻找了其他成分，在过去的赠款期间，我们发现Betaglycan(转化生长因子-βIII型受体)是一种高亲和力的抑制素结合蛋白(共受体)，促进抑制素与ActRII的结合，从而大大增加抑制素对抗激活素信号的效力。在模型系统中，我们已经证明抑制素可以拮抗某些BMP的反应。在Aim I中，这些发现将扩展到测试抑制素是否拮抗BMPs对脑垂体细胞的影响。我们将继续研究抑制素、β-聚糖和II型受体ActRII、ActRIIB和BMPRII之间的结合作用。我们将确定抑制素结合所需的β-多聚糖残基和区域，并将产生用于生化和结构研究(与项目II)的最小限度的可溶性β-多聚糖形式。识别这些蛋白质的关键相互作用表面和区域将有助于设计 调节抑制素反应的策略。目的II建议阻断β-多糖或干扰其在抑制素反应的细胞类型中的表达，包括垂体促性腺激素。体外和体内实验将测试β-多糖作为抑制素作用的必备介体的重要性。最后，在AIM III下，我们将寻找更多的抑制素辅助受体。我们和其他人有证据表明存在多种抑制素结合蛋白，包括可能的Betaglycan变体，我们建议对其进行表征。这些研究将提高我们对抑制素作用机制的理解，并将有助于确定控制生育和管理生殖障碍的目标。

Activins and certain BMPs are members of the TGF-beta superfamily that regulate FSH production by the anterior pituitary. They and their receptors also have diverse endocrine, paracrine, and autocrine actions throughout the reproductive, endocrine and other systems and play key roles in development and in pathophysiologic processes. Inhibins oppose some, but not all actions of activin and are crucial for survival as illustrated by the lethal phenotype of inhibin alpha null mice, which suffer gonadal and adrenal tumors and liver necrosis. To exert cellular effects, activins (beta-beta dimers) bind their type II receptor serine kinases (ActRII or ActRIIB) and then recruit the type I receptor serine kinase (ALK4) to form active complexes that initiate downstream signaling. Inhibins (alpha-beta dimers) compete with activins for binding to ActRII but the inhibin/ActRII complexes do not recruit ALK4 or promote signaling. Inhibins are potent functional antagonists of activin yet the relatively low affinity of inhibins compared to activin for ActRII was not in keeping with a simple competition model. Thus other components were sought and during the past grant period we identified betaglycan (TGF-beta type III receptor) as a high affinity inhibin binding protein (co-receptor) that facilitates inhibin binding to ActRII and, thereby, greatly increases the potency of inhibin to antagonize activin signaling. In model systems we have demonstrated that inhibin can antagonize responses to some BMPs. Under Aim I, these findings will be extended to test if inhibins antagonize the effects of BMPs on pituitary cells. We will continue to characterize the binding interactions between inhibins, betaglycan, and the type II receptors, ActRII, ActRIIB and BMPRII. We will identify the betaglycan residues and regions required for inhibin binding and will produce minimized soluble betaglycan forms that will be used for biochemical and structural (with Project II) studies. Identification of the critical interactive surfaces and regions of these proteins will facilitate efforts to devise strategies for modulating inhibin responses. Aim II proposes to block betaglycan or disrupt its expression in inhibin-responsive cell types including pituitary gonadotropes. In vitro and in vivo experiments will test the importance of betaglycan as an obligate mediator of inhibin actions. Finally, under Aim III we will search for additional inhibin co-receptors. We and others have evidence for the existence of multiple inhibin binding proteins including probable variants of betaglycan, which we propose to characterize. These studies will improve our understanding of the mechanisms of inhibin action and will help identify targets for the control of fertility and management of reproductive disorders.