Goat αs1-casein genotype affects milk fat globule physicochemical properties and the composition of the milk fat globule membrane

Abstract

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 αs1-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 αs1-casein genotype. At midlactation, goats displaying strong genotypes for αs1-casein (A/A goats) produce larger fat globules than goats with a null genotype at the CSN1S1 locus (O/O goats). A linear relationship (R2=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 αs1-casein (O/O goats; 5.97±0.11mg/g of fat; mean ± standard deviation) than in the MFGM from goats with strong genotypes for αs1-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 αs1-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.