Effect Of Low Protein Diet Supplemented With Methionine On Milk Protein Synthesis In Lactating Mice

Amino acids are not only substrates for protein synthesis,but also directly regulate protein synthesis as biological signals.Our team’s previous experiments showed that under low protein diet conditions,supplemented with rumen--protected Met or supplemented with four kinds of rumen-protected essential amino acids Met,Leu,Ile and Thr can significantly increase milk production,milk protein synthesis and milk N efficiency.But the mechanism of action of Met is still unclear.In this project,we used lactating mice as a model to study the effects of Met on milk production and milk protein synthesis,in order to accumulate data for signal amino acid regulation theory,and provide experiments for the study of supplemental feeding programs with promotion value in dairy cows.Thirty-eight SPF Kunming clean-grade female pregnant mice with similar delivery dates(17~19d)were randomly divided into 4 treatment groups.12 mice in each group,fed in a single cage.Four diets were fed from the day of delivery: low protein diet(LP,dietary protein level 16%),high protein diet(HP,dietary protein level 21%),low protein diet plus 1 time Met(M1,adding 1.86 g Met)or 2 times Met(M2,adding 3.72 g Met).Energy levels of the 4 diets were same,and dams were fed the test diets from parturition through 17 d of lactation when the studies were ended.The effects on feed intake of dams,litter weight gain,milk protein production,plasma AA concentration,hormone concentration and mammary signaling pathway were observed.The results showed that there was no significant effect on the feed intake and the weight gain of the litter in HP group,the group,M1 group and M2 group,but the feed intake and the weight gain of the litter in M1 and M2 groups were significantly higher than those in LP group.The expression of β-casein mRNA did not change significantly in each treatment group,but the expression of β-casein mRNA in M1 group increased compared with the LP group.And the concentration of urea(UREA)in M1 group and M2 group was significantlylower than that in HP group,and there was no significant difference compared with the LP group.The plasma albumin levels were significantly increased by addition of Met,but glucose,total protein,alanine aminotransferase,and aspartate aminotransferase were not changed significantly.Compared with LP group,the levels of insulin,IGF-1,growth hormone,prolactin,glucagon,and NO in plasma were significantly lower in M1 group,the concentration of these hormones and NO in M2 group was significantly higher than that in M1 group.Adding Met dose-dependently down-regulated the concentration of alanine(Ala),increased the concentration of Met and isoleucine(Ile),but had no significant effect on other amino acid concentrations in plasma.Compared with LP group,the expression levels of mTOR and its downstream signaling molecules eEF2 and eIF2α mRNA in M1 group and M2 group were significantly decreased,but the expression level of S6K1 mRNA was not significantly different among the groups.There were no significant difference in the total protein and phosphorylation levels of mTOR and its downstream signaling proteins between the groups;however,compared with LP group,the phosphorylation levels of these proteins were increased in M1 and M2 groups.In summary,the addition of Met to low-protein diets can increase the milk production of lactating mice and significantly reduce the amount of urea in the blood.The addition of Met to low-protein diets affects the synthesis of milk proteins by altering the levels of insulin,IGF-1,growth hormone,prolactin,glucagon,and NO,as well as the mTOR signaling pathway.