Doctoral Dissertation Improvement: The Molecular Evolution of Placental Invasion in Primates

生殖系统在灵长类进化过程中经历了深刻的变化。最引人注目的生殖变化之一是高等灵长类动物（眼镜猴、猴子、猿和人类）侵入性胎盘的进化。相比之下，低等灵长类动物（狐猴和懒猴）的胎盘没有侵入性。 侵入性胎盘钻入母亲的子宫，直接接触她的血液供应。这增加了从母亲到胎儿的营养转移，并为发育胎儿器官提供能量。人类胎盘比其他灵长类动物的胎盘更具侵入性。这可能有助于人类大脑的发育。因此，人类的侵入性胎盘可能与大脑尺寸增加的进化有关，这是一种定义人类适应的进化。侵袭性胎盘形成需要许多基因的协调作用，但由基质金属蛋白酶2（MMP 2）和基质金属蛋白酶9（MMP 9）编码的蛋白质是最重要的。它们是直接负责降解子宫组织的酶。MMP 2和MMP 9的酶促能力（降解组织的能力）的增加可能是高等灵长类动物侵入性胎盘形成的分子机制。有两个过程可以增加MMP的酶促能力：第一，增加蛋白质的内在酶活性（蛋白质重塑），第二，增加它们产生的水平（基因调控变化）。(More产生的蛋白质会导致更多的组织降解。）本项目使用新的方法研究灵长类动物中MMP 2/9的蛋白质重塑和基因调控变化。自然选择有利于增加MMP 2/9酶活性，以促进胎盘入侵高等灵长类动物（特别是人类）的假设将进行测试。MMP 2/9 DNA序列将从12种灵长类动物物种中产生，以确定蛋白质重塑对MMP活性的影响。灵长类物种之间的氨基酸差异将被映射到三维蛋白质结构上，以研究DNA序列变化对酶活性的影响。一系列的实验，测量在体外胎盘细胞系统的蛋白质生产将测试是否基因调控的变化导致增加MMP 2/9的活性在高等灵长类动物。蛋白质作图和体外基因调控实验已成功地用于其他科学领域，但新的分子人类学。它们在这项研究中的使用将提供DNA序列进化变化与其编码蛋白质功能之间的关键联系。这将使我们不仅能够确定自然选择是否有助于提高高级灵长类动物中MMP 2/9的酶活性，而且还可以确定蛋白质如何改变以变得更加活跃。通过这种方式，我们将能够彻底地描述灵长类生殖进化的一个重要方面。本项目所使用的方法将为分子人类学的研究开辟重要的新途径。该项目的其他更广泛的影响是：1）通过人类学系与哈佛医学院的合作，促进人类学、进化生物学和分子生物学研究生培训的整合; 2）通过在本科课程和专业会议上展示研究成果，产生教育影响。研究结果将发表在人类学、分子进化和生殖生物学的同行评议期刊上。

The reproductive system has experienced profound change during primate evolution. One of the most striking reproductive changes is the evolution of invasive placentation in higher primates (tarsiers, monkeys, apes and humans). In contrast, the placentae of lower primates (lemurs and lorises) are not invasive. Invasive placentae burrow into the mother's uterus and make direct contact with her blood supply. This increases nutrient transfer from mother to fetus and provides energy for developing fetal organs. The human placenta is significantly more invasive than those of other primates. This may facilitate the development of the large human brain. Therefore, human invasive placentation may be linked to the evolution of increased brain size, a defining human adaptation. Invasive placentation requires the coordinated action of numerous genes, but the proteins encoded by Matrix Metalloproteinase 2 (MMP2) and Matrix Metalloproteinase 9 (MMP9) are most important. They are enzymes directly responsible for degrading uterine tissues. An increase in the enzymatic ability (the ability to degrade tissue) of MMP2 and MMP9 is the molecular mechanism that likely underlies invasive placentation in higher primates. Two processes can bring about an increase in MMP enzymatic ability: first, an increase in the intrinsic enzymatic activity of the proteins (protein remodeling), and second, an increase in the levels at which they are produced (gene regulatory change). (More protein produced would result in more tissue degraded.)This project uses novel methods to investigate protein remodeling and gene regulatory change in MMP2/9 among primates. The hypothesis that natural selection favored increased enzymatic MMP2/9 activity to facilitate placental invasion in higher primates (particularly humans) will be tested. MMP2/9 DNA sequences will be generated from twelve primate species to determine the effect of protein remodeling on MMP activity. Amino acid differences between primate species will be mapped onto three-dimensional protein structures to investigate the effect of DNA sequence change on enzymatic activity. A series of experiments that measure protein production in an in vitro placental cell system will test whether gene regulatory change caused increased MMP2/9 activity in higher primates.Protein mapping and in vitro gene regulatory experiments have been used successfully in other scientific fields but are new to molecular anthropology. Their use in this study will provide a critical link between evolutionary changes to DNA sequences and to the function of their encoded proteins. This will enable us to determine not only whether natural selection worked to increase the enzymatic activity of MMP2/9 in higher primates but also how the proteins changed to become more active. In this way we will be able to characterize an important aspect of primate reproductive evolution thoroughly.The methods used in this project will open significant new avenues of research in molecular anthropology. Additional broader impacts of the project are that it 1) promotes the integration of graduate training in anthropology, evolutionary biology, and molecular biology through collaboration between the Department of Anthropology and Harvard Medical School; and 2) will have educational impact by presenting research results in undergraduate courses and at professional conferences. Results will be published in peer-reviewed journals in anthropology, molecular evolution, and reproductive biology.