Effect of high muscle and low muscle reserves on longissimus dorsi transcriptome in late gestation dairy cows and the impact of branched chain volatile fatty acid supplmentation.

The amount of muscle reserves of dairy cattle in late gestation is related to production performance including calf size and milk yield in the postpartum. Branched-chain volatile fatty acids (BCVFA) supplementation may close energy gaps experienced in late gestation dairy cows, as relative to unsupplmented control it increases blood glucose levels. Our objective was to compare longissimus dorsi muscle transcriptomes and morphology between cows with high muscle (HM) versus low muscle (LM) reserves and in response to BCVFA supplementation. At 42 d before expected calving (BEC) Holstein dairy cows were enrolled in a 2 x 2 factorial study of diet and muscle reserves, by assigning to control (CON) or BCVFA supplemented diets and HM or LM groups based on depth of longissimus dorsi at 42 d BEC. We biopsied the longisumus dorsi at 21 d BEC, isolated total RNA, ribo-depleted samples and sequenced RNA using paired end reads. After trimming, DSeq2 was used to identify differentially expressed genes (DEG). Using a raw p-value cut-off of <0.05, 713 DEG were identified between HM and LM groups and 481 DEG between BCVFA and CON. Transcriptional signatures and morphology indicated differential distribution of Type II fibers between muscle groups, with MYH1 (encods type IIx) greater in LM and MYH2 (encodes type IIA) greater in HM cattle. LM cattle relative to HM showed transcriptome evidence for activation of autophagy, ubiquitin-proteasome, and Ca2+-calpain pathways. HM cattle expressed higher levels of extracellular matrix (ECM) proteins and factors that regulate ECM proteolysis and turnover. Genes that regulate fatty acid degradation and flux of carbons into the TCA cycle as acetyl CoA were increased by BCVFA supplementation. HM and LM cattle have distinct molecular signatures that indicate differences in type II fiber distribution and potentially different metabolic strategies to close energy and amino acid gaps due to metabolic demands in late gestation, whereas BCVFA supplementation increases substrates for energy generation. Overall design: A 2 by 2 factorial of treatments of multiparous non-lactating Holstein cows assigned to high muscle (HM; >4.6 cm) or low muscle (LM; =4.6 cm) at 42 days before expected calving and branched chain volatile fatty acid (BCVFA) or soyhull pellets control (CON) supplementation of diets, resulting in HM-CON (n=13), HM-BCVFA (n=8), LM-CON (n=9), and LM-BCVFA (n=9). Muscle depth assignment was based on depth of longissimus dorsi muscle captured by ultrasound at 42 days before expected calving. BCVFA supplmented cows received isobutyrate, isovalerate and 2-methylbutyrate at 39.1 g/d, 19.6 g/d, and 19.4 g/d dry matter as a calcium salt of isoacid top dressed on the diet. Feed for CON cows was top dressed with 73 g/d DM of soyhull pellets. Cows were housed in tie stalls and released for exercise twice daily. Cows were fed for ad libitum intake by dispersing gestation diets one-time each day at 110% of the previous day's intake. Biopsy of longissimus dorsi muscle was performed at approximately 0700 h 3 weeks BEC, which was 3 weeks after beginning BCVFA supplmentation and initial assignment to HM and LM groups. Muscle biopsies were flash frozen in liquid nitrogen and sent to Azenta Life sciences for isolation of total RNA and sequencig.