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The mRNA expression of the classical genes of enzymes involved in milk fatty acid synthesis does not explain milk fat depression in dairy cows
Feeding polyunsaturated fatty acids (PUFA) to dairy cows results in milk fat depression (MFD) in some, but not all cows. The objective of this research was to compare the mRNA expression of enzymes involved in fatty acid (FA) synthesis in the mammary gland of cows resistant or sensitive to MFD resulting from feeding PUFA. Four cows were selected from a dairy farm after a switch from a control diet to a linseed-rich diet: two were resistant to MFD and had high milk fat content before and after the change (R-MFD); and two were sensitive to MFD and milk fat content decreased after the change into the linseed diet (S-MFD). Fresh milk samples were collected from each cow the week before and two weeks after the diet change, and analyzed for milk fat content, milk FA profile and transcriptional profiling of mRNA by Illumina RNA-sequencing technology. The study focused on the enzymes reported to be involved in MFD: acetyl-CoA carboxylase (ACACA), fatty acid synthase (FASN), glycerol-3-phosphate acyltransferase (GPAT), acylglycerol-3-phosphate acyltransferase (AGPAT), stearoyl-CoA desaturase (SCD1), fatty acid binding protein (FABP), lipoprotein lipase (LPL), sterol regulatory element binding protein (SREBP1) and spot 14 (S14). When S-MFD cows were fed linseed, milk fat decreased from 3.56 to 2.54%, the unsaturated:saturated FA ratio increased from 41/59 to 45/55 and short- and medium-chain FA proportion (C4 to C14 and 50% C16; SMFA) decreased from 33.9 to 32.8 %, as expected, but there were no significant changes in the mRNA expression of the genes tested. When R-MFD cows were fed linseed, milk fat unexpectedly increased from 4.06 to 4.36 %, the unsaturated:saturated FA ratio increased from 35/65 to 39/61, SMFA proportion did not change (average of 31.4%) and only the expression of ACACA decreased and that of AGPAT6 and FABP3 increased. The unexpected over expression of AGPAT6 and FABP3 transcripts in R-MFD cows may be potential candidate genes involved in the MFD-resistance. Other genes that may also be involved need to be identified.
Keywords: milk fat depression, mRNA expression, extruded linseed.