Co-ordinated regulation of ovarian follicle assembly by Activin A and FoxL2

女性的生殖寿命是在胎儿时期通过在发育中的卵巢中形成一组原始卵泡而确定的。原始卵泡由一层前颗粒细胞包围的卵（母细胞）组成，这些细胞支持、维持和控制卵母细胞的生长。女性出生时卵泡数量有限，一生中卵泡数量会减少，出生后不会形成新的卵泡。当卵泡储备耗尽时，更年期就会发生，这通常发生在50岁左右。然而，在1%的女性中，绝经发生在40岁之前，这种情况被称为卵巢早衰（POI）。除了对尚未组建家庭的妇女来说是一种毁灭性的诊断外，POI还与严重的绝经后健康风险有关，如骨质疏松症和心血管疾病。此外，随着社会趋势越来越倾向于晚育，卵巢中卵母细胞的供应和质量随着年龄的增长而下降，经历生育能力低下的妇女人数可能会增加。尽管卵泡形成过程对人类和其他哺乳动物（如牲畜）的生殖寿命至关重要，但我们对胎儿卵巢中卵泡的形成机制知之甚少。如果我们要研究遗传、环境或生活方式因素是如何破坏卵泡形成和缩短生殖寿命的，那么提高我们对这一基本过程的认识是必不可少的。细胞间信号分子激活素A （ActA）可能是卵泡形成的关键调节因子。胎儿/新生小鼠卵巢中升高的ActA信号导致更多原始卵泡的形成。发生这种情况的机制尚不清楚，但可能涉及颗粒前细胞数量或行为的变化，因为ActA仅向发育中的卵巢中的这种细胞类型发出信号。颗粒前细胞也表达一种叫做FoxL2的蛋白质，它能激活对卵巢发育很重要的基因。在人类中，FOXL2基因的突变会导致POI、不孕症和颗粒细胞肿瘤（一种罕见的卵巢癌）。在小鼠中，缺乏FoxL2的颗粒前细胞不能正常工作，因此不能形成卵泡（或形成不能成熟的卵泡）。在脑垂体中，ActA和FoxL2共同激活细胞响应和产生生殖激素所需的基因，而FoxL2/ActA的类似相互作用激活了成年卵巢颗粒细胞中的基因。然而，ActA和FoxL2是否共同调节胎儿卵巢颗粒前细胞功能和卵泡形成尚不清楚。本项目的目的是解决这个问题，使用新生小鼠卵巢和人颗粒前细胞为模型。我们将确定卵巢中形成的表达foxl2的前颗粒细胞的数量是否限制了卵泡的形成数量，并确定激活素是否通过改变前颗粒细胞池的大小来增加卵泡数量。为了深入了解它们在颗粒前细胞中调节的生化过程，我们将确定ActA和FoxL2共同控制的基因，并研究其中任何一个因子的缺失是否会阻止这些基因的正确开启或关闭。最后，我们将研究为什么原始卵泡不能在foxl2缺陷小鼠卵巢中正确形成，确定激活素（或其他信号）的异常信号是否是导致这种缺陷的原因，并确定是否可以通过恢复细胞间正常水平的信号传导来纠正这种缺陷。这些研究将揭示卵泡数量（以及女性生殖寿命）是如何建立的，为人类FOXL2突变如何导致不孕症提供新的见解，并为未来的研究提供信息，以开发新的生育管理策略。

Female reproductive lifespan is established during fetal life through the formation of a cohort of primordial follicles in the developing ovary. Primordial follicles consist of an egg (oocyte) surrounded by a layer of pre-granulosa cells, which support, maintain and control the growth of the oocyte. Women are born with a finite number of follicles which decline in number throughout life, and no new follicles are formed after birth. The menopause occurs when the follicular reserve is exhausted, which normally happens around the age of 50. However, in 1% of women the menopause occurs before the age of 40, a condition known as premature ovarian insufficiency (POI). In addition to being a devastating diagnosis for women yet to complete their families, POI is associated with significant post-menopausal health risks, such as osteoporosis and cardiovascular disease. Furthermore, as societal trends move increasingly towards starting families later in life, and the supply and quality of oocytes in the ovary declines with age, the number of women experiencing sub-fertility is likely to rise. Despite the critical importance of the follicle formation process in establishing reproductive lifespan in humans, and other mammals (such as livestock species), we know very little about how the stock of follicles is formed in the fetal ovary. Improving our knowledge of this fundamental process is essential if we are to investigate how genetic, environmental or lifestyle factors can disrupt follicle formation and curtail reproductive lifespan. The cell to cell signalling molecule Activin A (ActA) is a possible key regulator of follicle formation. Elevated ActA signalling in fetal/neonatal mouse ovaries results in the formation of greater numbers of primordial follicles. The mechanism by which this occurs is unclear, but is likely to involve changes to pre-granulosa cell number or behavior, as ActA only signals to this cell type in the developing ovary. Pre-granulosa cells also express a protein called FoxL2, which activates genes important for ovarian development. In humans, mutations in the FOXL2 gene cause POI, infertility and granulosa cell tumours (a rare type of ovarian cancer). In mice, pre-granulosa cells that lack FoxL2 cannot function correctly, and so fail to form follicles (or form follicles that cannot mature). In the pituitary gland, ActA and FoxL2 work together to activate genes that are required for cells to respond to, and produce, reproductive hormones, and a similar FoxL2/ActA interaction activates genes in the granulosa cells of the adult ovary. Whether ActA and FoxL2 work together to regulate pre-granulosa cell function and follicle formation in the fetal ovary is not known, however. The aim of this project is to address this question, using the neonatal mouse ovary and human pre-granulosa cells as models. We will establish whether the number of FoxL2-expressing pre-granulosa cells formed in the ovary limits the number of follicles that can be formed, and determine whether activin increases follicle number by altering the size of the pre-granulosa cell pool. To gain insight into the biochemical processes they regulate in pre-granulosa cells, we will identify the genes that ActA and FoxL2 jointly-control, and investigate whether loss of either factor prevents these genes from being switched on or off correctly. Finally, we will investigate why primordial follicles fail to form correctly in FoxL2-deficient mouse ovaries, determine whether aberrant signalling by activin (or other signals) is the cause of this defect, and establish whether this can be corrected by restoring normal levels of signalling between cells. These studies will shed new light on how the number of follicles (and thus female reproductive lifespan) is established, provide insight into how mutations in FOXL2 in humans lead to infertility, and inform future studies to develop of new strategies to manage fertility.