Adaptations in Tricarboxylic Acid Cycle and Branched-Chain Amino Acid Degradation in Embryonic and Neonatal Chicken Muscle

A dramatic metabolic switch from lipid oxidation to lipid accumulation is a characteristic feature of the embryonic-to-neonatal transition in chickens. While this metabolic switch is well characterized in the liver, it is unclear how muscle mitochondrial energetics adapt during embryonic-to-neonatal transition. Further, emerging evidence suggests a tight correlation between changes in lipid metabolism and branched-chain amino acid (BCAA) metabolism. We hypothesized that the activity of the tricarboxylic acid (TCA) cycle in the muscle will progressively increase during the embryonic-to-neonatal transition and that this rate of increase will be positively correlated with rates of BCAA degradation. Leg muscles were dissected and flash frozen in liquid nitrogen from broiler (Strain: Ross 708; flock age 25-30 wks.) embryonic (E) days 9, 11, 13, 15, and 18, the day of hatch, and 2- and 4 days post-hatch (PH). Gas-chromatography mass-spectrometry-based targeted metabolomics were utilized to determine tissue levels of TCA cycle intermediates, essential and non-essential amino acids, beta-hydroxybutyrate, and the corresponding keto-acids of BCAAs. Consistent with the higher rates of lipid oxidation during embryonic stages, beta-hydroxybutyrate (µg/g muscle ± SEM) was significantly higher in embryonic muscles vs. their post-hatch counterparts (E18; 847±35 vs. PH2; 87±6, p ≤ 0.001). On the contrary, several of the TCA cycle intermediates in the muscle (µg/g muscle ± SEM) were significantly higher during the post-hatch stages compared to their embryonic counterparts (e.g., Succinate; E18; 95±16 vs. PH2; 262±41, p ≤ 0.001; Malate; E18; 110±11 vs. PH2; 309±18, p ≤ 0.001). All three keto-acid intermediates (µg/g muscle ± SEM) of BCAA transamination were higher in the embryonic muscle compared to their post-hatch counterparts (e.g., α-ketoisovaleric acid of valine; KIV; E18; 4.52±0.55 vs. PH2; 1.29±0.17, p ≤ 0.01). The significantly lower keto-acid to BCAA ratios in the post-hatch muscles (e.g., KIV/Valine; E18; 0.022±0.003 vs. PH2; 0.005±0.001, p ≤ 0.001) further highlighted the decreasing rates of BCAA degradation with embryonic-to-neonatal transition. More interestingly, the BCAA keto-acids in the muscle showed a negative correlation with several intermediates of the TCA cycle (e.g., Succinate vs. KIV; r (55) = -0.56, p ≤ 0.001) suggesting a possible crosstalk between BCAA degradation and mitochondrial TCA cycle metabolism. In conclusion, these results demonstrate that an induction in the activity of TCA cycle metabolism coexists with the downregulation of BCAA degradation during the embryonic-to-neonatal transition in chicken muscle. Identifying whether the changes in BCAA degradation can modulate TCA cycle activity will provide avenues to nutritionally or molecularly enhance and optimize muscle mitochondrial function. AFRI-USDA-NIFA, Grant # 2021-67015-33387 This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.