FSTL3: A Crucial Regulator of Sertoli Cell Proliferation

尽管目前世界人口不孕不育，但人类的不孕不育率仍在上升。每10对夫妇中就有一对不育，在大约三分之一的病例中，缺陷来自男性。睾丸缺陷是男性生殖缺陷的主要原因之一。也许由于环境的影响，动物中的男性不育症也在上升。这很可能是导致许多野生动物物种数量减少的原因。因此，了解调节睾丸发育和功能的机制对于开发治疗策略以缓解男性不育疾病至关重要。睾丸的两个主要功能是在整个物种的生殖生命中产生睾酮和精子。一般来说，这从青春期开始，此时睾丸大小增加。睾丸体积增大是精子生成和睾丸功能活跃的指标。然而，需要与年龄相关的睾丸功能衰退，以便随着年龄积累的突变不会在人群中传播。对于季节性繁殖者，一个额外的要求是随着每个繁殖季节的开始和结束，睾丸的周期性增加和减少。睾丸中的支持细胞(SC)允许产生精子的细胞的复制和发育，通常产生的生殖细胞总数取决于睾丸中SC的数量。这个数字在生命的早期是固定的，通常不会随着年龄的增长而变化。尚不清楚的是SC的数量是如何确定的，以及睾丸中的SC数量是否可以在以后的生命中增加。也不清楚睾丸大小缩小是如何以年龄或季节相关的方式控制的。我们发现，一种天然的细胞产物--卵泡抑素样蛋白3(FSTL3)可能在调节睾丸发育中起关键作用。我们在所有细胞中都缺乏FSTL3的转基因小鼠中的发现显示，睾丸大小增加，并且随着年龄的增长，睾丸大小没有减小。此外，在睾丸内，SC数量增加，产生精子的细胞也相应增加。因此，我们假设，睾丸FSTL3的作用对于限制睾丸大小和与年龄相关的睾丸功能下降是必不可少的。只有两个其他小鼠模型的睾丸大小增加，但这两个模型都没有阻止睾丸大小随年龄的减少。因此，我们的FSTL3缺失小鼠模型允许我们提出几个关于睾丸发育和功能的关键问题。在这里，我们将首先确定是否可以通过减少小鼠和组织培养细胞中的FSTL3来增加SC的增殖。然后，我们将调查是否可以通过仅删除SC中的FSTL3来增加SC编号。这将证明SC数量是否由SC中表达的本地FSTL3控制，或者是否存在对FSTL3生产的系统性要求。此外，我们将解决在青春期后移除FSTL3是否会增加SC数量。这将证明干细胞是否可以在出生后的最初几天内繁殖，从而提供了一种治疗策略，即在睾丸有限小的不育男性中诱导干细胞生长和生育。最后，我们将确定FSTL3可能会影响哪些基因和细胞信号通路来限制睾丸大小，并帮助实现随年龄减少睾丸大小。因此，我们的研究将确定FSTL3如何调节睾丸的发育和功能，并帮助限制随年龄增长的生殖。我们将确定这些作用的分子机制，FSTL3依赖的SC数量调节是否仅通过其在SC中的表达来实现，以及是否可以在出生后的任何时候通过去除FSTL3来诱导SC生长。这项工作对我们理解睾丸发育过程的贡献可能会支持更有效地处理男性不育的预防和治疗方法的发展。

Notwithstanding current world population infertility in humans is on the rise. 1 in 10 couples is infertile and in about a third of the cases, the defect arises from the male. Testicular defects are among the principal causes of reproductive deficit in males. Perhaps due to environmental effects male infertility in animals is also on the rise. This has most likely contributed to the reduction in numbers of many animal species in the wild. Understanding mechanisms that regulate testicular development and function is therefore essential for development of therapeutic strategies to alleviate male infertility disorders.The two major functions of the testis are to produce testosterone and sperm throughout the reproductive life of the species. Generally this begins at puberty when testicular size increases. Increased testicular size is an indicator of active generation of sperm and testicular function. There is, however, a need for age-related waning of testis function so that mutations accumulated with age are not propagated in the population. For seasonal breeders an additional requirement is the cyclical increase and decrease of the testis with the beginning and end of each breeding season. Sertoli cells (SC) in the testis allow for the duplications and development of the cells that give rise to sperm and generally the total number of germ cells produced depends on the number of SC in a testis. This number is fixed very early in life and normally does not change with age.What is not clear is how the number of SC is determined and whether SC numbers in a testis can be increased later on in life. It is also unclear how testis size reduction is controlled in an age or season dependent manner. We have found that a natural cell product, follistatin-like 3 (FSTL3) might be crucial in regulating testicular development. Our findings in genetically modified mice lacking FSTL3 in all cells show increased testis size, and a lack of testis size reduction with age. Also, within the testis there is an increase in SC numbers and related increase in cells that give rise to sperm. We, therefore, hypothesise that testicular FSTL3 action is essential for limiting testicular size and age-related decline in testicular function. There are only two other mouse models with increased testis size, but neither of these have a block in age-dependent reduction of testicular size. Our FSTL3 deletion mouse model, therefore, allows us to ask several crucial questions regarding testis development and function. Here we will first determine whether SC multiplication can be increased by reducing FSTL3 in mice and in tissue culture cells. We will then investigate whether SC numbers, can be increased by deleting FSTL3 only in the SC. This will demonstrate whether SC numbers are controlled by local FSTL3 expressed in the SC or whether there is a systemic requirement for FSTL3 production. Furthermore, we will address whether removing FSTL3 after puberty increases SC numbers. This will demonstrate whether SCs can multiply beyond the first few days after birth, therefore providing the possibility of a therapeutic strategy of inducing SC growth and fertility in infertile males with limitingly small testes. Finally, we will identify which genes and cellular signalling pathways FSTL3 may influence to limit testicular size and help achieve reduction of testis size with age. Our research will, therefore, identify how FSTL3 regulates testis development and function and helps limit reproduction with age. We will be identify the molecular mechanisms of these actions, whether FSTL3 dependent SC number regulation is achieved through its expression solely in SC and whether SC can be induced to grow, by removing FSTL3 at any time after birth. The contributions from this work to our understanding of the processes underlying testis development may support development of preventative and therapeutic approaches to dealing more effectively with male infertility.