Liver Mitochondrial Transcriptomic Responses to Dietary Crude Protein and Phosphorus Deficiencies and Feed Restriction in Wethers.

Background/Objectives: Seasonal crude protein (CP) and phosphorus (P) deficiency in northern Australian pastures reduces feed intake and growth of grazing ruminants, but the hepatic mitochondrial mechanisms underlying this response remain unclear. We characterized the hepatic mitochondrial transcriptome of sheep exposed to CP-P deficiency or matched-intake feed restriction. Methods: Merino wethers were assigned for 63 d to one of three treatments (n = 8/group): High CP-P, Low CP-P, or Restricted, in which High CP-P feed was offered at the same energy intake as the Low CP-P group. Liver RNA was sequenced, and transcripts encoding mitochondrial proteins were identified using MitoCarta 3.0. Differentially expressed genes (DEGs) were defined as adjusted p < 0.05 and |log2FC| ≥ 0.585. Results: Of 804 mitochondrial genes detected, 83 were differentially expressed in at least one pairwise comparison. The greatest transcriptional response occurred in contrasts against High CP-P (Low CP-P vs. High CP-P: 38 DEGs in 8 enriched pathways; Restricted vs. High CP-P: 37 DEGs in 10 enriched pathways). In both low-intake treatments, ALDH1L2, ALDH1L1, SHMT2, and DMGDH were upregulated, suggesting altered folate-mediated one-carbon metabolism. Restricted sheep also showed higher expression of several SLC25A transporters (SLC25A4, SLC25A28, SLC25A29, SLC25A33, and SLC25A34), indicative of enhanced mitochondrial nucleotide and metabolite exchange under CP-P adequate energy restriction. In contrast, Low CP-P sheep showed higher expression of SLC25A15 and SLC25A25 relative to either High CP-P or Restricted sheep, a nutrient-deficiency specific transporter response. CKMT2 expression was also higher in Restricted sheep than in both other groups. Conclusions: These findings suggest that reduced metabolizable energy intake was associated with the bulk of the hepatic mitochondrial transcriptional response, particularly in folate-mediated one-carbon metabolism, whereas CP-P deficiency was associated with a smaller but distinct transporter signature. The liver mitochondrial transcriptome may provide mechanistic insight into nutritional adaptation under CP and P deficiency in grazing sheep.