INRA, UMR1313 Unité Génétique Animale et Biologie Intégrative, F-78350 Jouy-en-Josas, France.
J Dairy Sci. 2012 Nov;95(11):6215-29. doi: 10.3168/jds.2011-5233. Epub 2012 Aug 23.
Milk fat secretion is a complex process that initiates in the endoplasmic reticulum of the mammary epithelial cell by the budding of lipid droplets. Lipid droplets are finally released as fat globules in milk enveloped by the apical plasma membrane of the mammary epithelial cell. The milk fat globule membrane (MFGM) thus comprises membrane-specific proteins and polar lipids (glycerophospholipids and sphingolipids) surrounding a core of neutral lipids (mainly triacylglycerols and cholesterol esters). We have recently described major proteins of the MFGM in the goat and we have highlighted prominent differences between goats and bovine species, especially regarding lactadherin, a major MFGM protein. Here, we show that, in the goat species, the well-documented genetic polymorphism at the α(s1)-casein (CSN1S1) locus affects both structure and composition of milk fat globules. We first evidenced that both milk fat globule size and ζ-potential are related to the α(s1)-casein genotype. At midlactation, goats displaying strong genotypes for α(s1)-casein (A/A goats) produce larger fat globules than goats with a null genotype at the CSN1S1 locus (O/O goats). A linear relationship (R(2)=0.75) between fat content (g/kg) in the milk and diameter of fat globules (μm) was established. Moreover, we found significant differences with regard to MFGM composition (including both polar lipids and MFGM proteins) from goats with extreme genotype at the CSN1S1 locus. At midlactation, the amount of polar lipids is significantly higher in the MFGM from goats with null genotypes for α(s1)-casein (O/O goats; 5.97±0.11mg/g of fat; mean ± standard deviation) than in the MFGM from goats with strong genotypes for α(s1)-casein (A/A goats; 3.96±0.12mg/g of fat; mean ± standard deviation). Two MFGM-associated proteins, namely lactadherin and stomatin, are also significantly upregulated in the MFGM from goats with null genotype for α(s1)-casein at early lactation. Our findings are discussed with regard to techno-functional properties and nutritional value of goat milk. In addition, the genetic polymorphism in the goat species appears to be a tool to provide clues to the lipid secretion pathways in the mammary epithelial cell.
乳脂分泌是一个复杂的过程，始于乳腺上皮细胞内质网中脂滴的出芽。脂滴最终以被乳腺上皮细胞顶膜包裹的脂肪球形式释放到牛奶中。因此，乳脂肪球膜 (MFGM) 由膜特异性蛋白和极性脂质（甘油磷脂和鞘脂）组成，周围环绕着中性脂质（主要是三酰甘油和胆固醇酯）的核心。我们最近描述了山羊 MFGM 的主要蛋白质，并强调了山羊和牛种之间的显著差异，特别是在乳贴蛋白（一种主要的 MFGM 蛋白）方面。在这里，我们表明，在山羊物种中，α(s1)-酪蛋白 (CSN1S1) 基因座的已充分记录的遗传多态性不仅影响乳脂肪球的结构，还影响其组成。我们首先证明，乳脂肪球的大小和 ζ-电位都与 α(s1)-酪蛋白基因型有关。在泌乳中期，具有 CSN1S1 基因座强基因型的α(s1)-酪蛋白（A/A 山羊）产生的脂肪球比具有该基因座 null 基因型的山羊（O/O 山羊）更大。建立了脂肪含量（g/kg）与脂肪球直径（μm）之间的线性关系（R(2)=0.75）。此外，我们发现，CSN1S1 基因座极端基因型的山羊的 MFGM 组成（包括极性脂质和 MFGM 蛋白）存在显著差异。在泌乳中期，具有α(s1)-酪蛋白 null 基因型（O/O 山羊；5.97±0.11mg/g 脂肪；平均值 ± 标准偏差）的山羊的 MFGM 中的极性脂质含量明显高于具有强基因型的山羊（A/A 山羊；3.96±0.12mg/g 脂肪；平均值 ± 标准偏差）。在泌乳早期，具有α(s1)-酪蛋白 null 基因型的山羊的 MFGM 中，两种与 MFGM 相关的蛋白，即乳贴蛋白和 stomatin，也显著上调。我们的研究结果与山羊奶的技术功能特性和营养价值进行了讨论。此外，山羊种的遗传多态性似乎是揭示乳腺上皮细胞中脂质分泌途径的工具。
