Mammalian sperm preservation: mechanism sperm protection by milk and development of novel extenders

公牛精液冷冻保存是继人工授精技术问世后，奶牛养殖业取得的最重要成果之一。这两种方法使所需基因系能够以合理的成本在全球范围内分布和使用。加拿大占全球经济总量的3%，但却提供了全球牛精液出口的20%(约7000万美元/年)。卵黄和脱脂牛奶是冷冻哺乳动物精子最常用的保护剂。安永和牛奶都是动物来源的产品，存在着精液受到污染的潜在风险，而且它们的成分也不一致。因此，迫切需要开发不含动物来源产品的新介质来保存精液。然而，很难找到安永或牛奶的替代品，因为安永和牛奶保护精子免受冷却、冷冻和储存损害的机制尚不清楚。在过去的六年中，我们提供的证据表明，EY对精子的保护涉及到EY低密度脂蛋白(LDL)与精浆蛋白之间的联系，这些蛋白对精子存储不利。这些最初在牛精浆中发现的有害蛋白质(因此BSP蛋白)具有从精子膜中提取脂质的潜力，从而对精子的保存能力产生不利影响。然而，当这些BSP蛋白与低密度脂蛋白相关时，它们就失去了从精子膜中提取脂肪的能力，而精子膜是精子保护的起源(新概念)。有趣的是，脱脂牛奶不含脂蛋白，在储存过程中也能保护精子。因此，我们推测，牛奶的主要成分酪蛋白胶束和乳清蛋白与BSP蛋白结合，这种相互作用是保护精子的关键。目前的计划旨在了解卵黄和牛奶保护精子的机制，这将有助于我们开发不含动物产品的新型商业应用扩展剂。

The cryopreservation of bull semen represents one of the most important achievements in dairy farming after the introduction of artificial insemination. These two approaches have enabled the worldwide distribution and use of desired genetic lines at a reasonable cost. Canada represents <3% of the global economy but provides >20% of world bovine semen exports (~70 million dollars/annum). Egg yolk (EY) and skimmed milk are the most used protective agents to freeze mammalian sperm. EY and milk, being products of animal origin, represent a potential risk of contamination of semen and moreover, their composition is not uniform. Thus, there is an urgent need of developing novel media free of products from animal origin to conserve semen. However, it is difficult to find replacement component for EY or milk because the mechanisms involved in sperm protection by EY and milk against cooling, freezing and storage damages remains unclear. Over the past six years we provided evidences, which indicate that the sperm protection by EY involves an association of EY low-density lipoproteins (LDL) with seminal plasma proteins that are detrimental to sperm storage. These detrimental proteins originally discovered in bovine seminal plasma (hence BSP proteins) have the potential to extract lipids from the sperm membrane thereby adversely affecting the ability of sperm to be preserved. However, when these BSP proteins are associated with LDL, they lose their ability to extract lipid from the sperm membrane, which is the origin for sperm protection (novel concept). Interestingly, skimmed milk, which does not contain lipoproteins, also protects sperm during storage. Thus we hypothesize that the casein micelles and whey proteins, which are the major components of milk bind to BSP proteins and this interaction is the key for sperm protection. The current program is aimed at understanding of the mechanisms involved in sperm protection by egg yolk and milk, which would help us to develop novel extenders for commercial application that do not contain animal products.