Effect of methionine in membrane traffic for milk secretion in the goat's mammary epithelial cell.
Effect of methionine in membrane traffic for milk secretion in the goat’s mammary epithelial cell.
M. Boutinaud*1, A. Leduc1, S. Lemosquet1, L. Bahloul2. 1INRAE, Agrocampus Ouest, PEGASE Saint-Gilles, France, 2Centre of Expertise and Research in Nutrition, Adisseo France S.A.S Commentry, France.
Methionine (Met) supplementation increases milk, protein and fat yields in cow. We investigated whether this could be partly explained by an increasing flow of milk components in the secretory pathways of mammary epithelial cells. Multiparous Alpine goats at mid lactation (n = 48), grouped by levels of expression of the CSN1S1, were assigned to 4 treatments in a randomized complete block design. Goats were fed a fixed amount of hay and a low (LE, 1.47 Mcal/kg DM) or adequate (AE, 1.54 Mcal/kg DM) energy concentrate combined with 2 levels of metabolizable Met: unbalanced vs. balanced using isopropyl ester of 2-hydroxy-4-methylthio butanoic acid (HMBi 0.24% concentrate DM) to cover 100% of Met requirement, based on cow requirement (INRA, 2007). Treatments were: LE, LEMet (LE, balanced Met), AE and AEMet (AE, balanced Met) for 5 weeks. Goats (23) were slaughtered and mammary tissue was processed for Western blotting using secretory compartment specific markers of membrane traffic. Milk protein yield (P = 0.01) and casein content (P = 0.01) increased in goats fed the Met balanced diets. The amount of the endoplasmic reticulum (ER) markers, Cnx and ERLIN2, decreased (20%, P ≤ 0.05) in goats fed the LE diet. Met balanced diets had the opposite effect on both markers (20%, P ≤ 0.05), as well as on protein disulfide isomerase (45%, P ≤ 0.05). These observations are in agreement with a positive effect of Met on the activity of the ER, the site where protein and lipid are synthesized. On the other hand, a specific marker of the exit site of the Golgi apparatus and secretory vesicles formation (AP1) decreased with the LE diet (25%, P ≤ 0.05) and its highest level was found in goats fed Met balanced diet at AE supply. The higher ßCOP (P = 0.01), a marker of intra Golgi transport, clearly reflected a decrease in membrane transport of LE diets. These data strongly suggest that energy level has a direct impact on membrane traffic in the secretory pathway of mammary epithelial cell while Met improve ER activity and has the tendency to further promote intracellular transport of milk components and, ultimately, their secretion.
Keywords: amino acid supplementation, dairy goats.