Translational regulation of milk protein synthesis

加拿大有960,600头奶牛，分布在12,529个农场，它们生产国内消费的所有液态奶。奶牛的饲料是由饲料公司聘请的高素质营养学家或独立顾问制定的。奶牛日粮配方的目标之一是最大限度地提高牛奶中蛋白质的捕获效率。奶牛通常每天消耗超过4公斤的粗蛋白质，其中约25%的粗蛋白质分泌到牛奶中。牛奶中摄入的75%的蛋白质未被捕获，以含氮化合物的形式在粪便中排泄，这既是环境损失，也是经济损失。高达45%的蛋白质捕获效率是可能的，但我们对如何持续获得如此高的效率的理解是不够的。我的研究计划的长期目标是提高对奶牛乳腺乳蛋白产量控制的认识，以便定量预测对饮食操纵的反应。我们以前已经发现，牛奶蛋白的产量不能通过知道血液中氨基酸的浓度来预测。相反，控制mRNA转化为牛奶蛋白的速率的因素似乎是重要的。这些因素包括mRNA本身的结构，以及与mRNA结合的某些蛋白质的活性状态。我们将研究这两个因素。我们将使用无细胞翻译实验来产生关于mRNA结构如何解释奶牛在人类和奶牛中产生的4种酪蛋白的翻译效率差异的假设。我们将培养含有mRNA翻译途径抑制剂的乳腺上皮细胞，以测试它们在乳蛋白合成的营养激活中的作用。在细胞和细胞裂解物中的发现将导致体内试验，我们将通过在一半奶牛的乳房内注入抑制剂来抑制信号通路，并评估它们在多大程度上阻止营养物质对乳蛋白产量的刺激。这些结果将使我们能够确定控制牛奶蛋白产量的途径。这些信息将被用来参数化一个牛奶蛋白合成的计算机模型，该模型可以预测不同日粮的蛋白质产量。牛奶蛋白的高效生产是可取的，因为牛奶中的蛋白质为婴儿的生长提供了理想的氨基酸谱。此外，从牛奶蛋白发酵或消化中提取的生物活性肽在治疗和预防人类代谢综合征（包括肥胖、动脉粥样硬化、动脉高血压和2型糖尿病）的症状方面具有功能价值。

There are 960,600 dairy cows in Canada on 12,529 farms and they produce all the fluid milk consumed domestically. The cows’ diets are formulated by highly qualified nutritionists employed by feed companies or as independent consultants. One of the goals of ration formulation for dairy cows is to maximize the efficiency of dietary protein capture in milk. Cows typically consume over 4 kg/d of crude protein and secrete approximately 25% of that into milk. The 75% of protein intake that is not captured in milk is excreted as nitrogenous compounds in manure and represents both an environmental and financial loss. Protein capture efficiencies of up to 45% are possible but our understanding of how to consistently obtain such high efficiency is inadequate. The long-term goal of my research program is to improve understanding of the control of milk protein yield in the mammary glands of dairy cows so that responses to dietary manipulation can be predicted quantitatively. We have previously found that milk protein yields cannot be predicted from knowing the concentrations of amino acids in blood. Instead, it appears that factors that control the rate of mRNA translation into milk protein are important. These factors include the structure of the mRNA itself, and the activity state of certain proteins that bind to the mRNA. We will study both factors. We will use a cell-free translation assay to generate hypotheses about how mRNA structure accounts for differences in translational efficiency of the 4 caseins produced by cows, in both humans and cows. We will culture mammary epithelial cells with inhibitors of mRNA translational pathways to test their role in the nutritional activation of milk protein synthesis. The findings in cells and cell lysates will lead to tests in vivo, where we will inhibit signalling pathways in one udder half of cows by intramammary infusion of inhibitors, and evaluate how much they prevent the stimulation of milk protein yield by nutrients. These results will allow us to identify the pathways responsible for control of milk protein yield. The information will be used to parametize a computer model of milk protein synthesis that predicts protein yields on different diets. Efficient production of milk protein is desirable because the protein of milk supplies an ideal amino acid profile for infant growth. In addition, bioactive peptides derived from fermentation or digestion of milk proteins possess functional value in treatment and prevention of symptoms of the human metabolic syndrome including obesity, atherogenesis, arterial hypertension, and type 2 diabetes.