30 Investigation into the relationship of mitochondrial haplotype on respiratory complex I activity in beef cattle skeletal muscle.

Abstract Mitochondria regulate energy metabolism by converting nutrients into ATP through the process of oxidative phosphorylation. A series of protein complexes in the inner mitochondrial membrane facilitate ATP production via electron transfer, with respiratory complex I (i.e., NADH dehydrogenase) transferring electrons from NADH to build the proton gradient that facilitates ATP synthesis. Efficient complex I activity supports ATP production in muscle tissue, sustaining mitochondrial function and metabolic processes essential for muscle efficiency and overall animal performance. Despite mitochondria’s key role in energy production, mitochondrial DNA (mtDNA) variation has been largely overlooked in livestock breeding, with its impact on economically relevant traits remaining largely unexplored due to the focus on paternal genetics and nuclear DNA. Therefore, the objective of this study was to investigate the relationship between mitochondrial haplotype and complex I activity in skeletal muscle to determine if genetic variation in mtDNA influences complex I function. Haplotype was determined by performing low-pass sequencing (average 55X mitochondrial coverage) on DNA of 84 beef steers from the composite University of Nebraska-Lincoln herd. Sequencing data were trimmed and aligned to the ARS-UCD2.0 genome using BWA-MEM and variants called using GATK. Mitochondrial haplotypes were defined by considering only nonsynonymous variants. Nine haplotypes were considered with 4-25 (average = 9) cattle represented per group. Calorimetric assays were used to quantify citrate synthase and complex I activities of sternomandibularis muscle homogenates collected at harvest. Complex I activity was normalized to citrate synthase activity to account for differences in mitochondrial content. Citrate synthase activity averaged 176.40 ± 83.60 nmol/min/mg of protein. Complex I activity normalized to citrate synthase activity averaged 0.99 ± 0.67. A linear mixed model was implemented with complex I activity normalized to citrate synthase as the dependent variable to assess the effect of mitochondrial haplotype on complex I activity. Fixed effects included mitochondrial haplotype, contemporary group (birth and harvest cohort), and age at sampling. A random animal effect, modeled using a genomic relationship matrix based on ~90,000 nuclear genomic markers, accounted for polygenic influences. Mitochondrial haplotype was not associated with complex I activity for these steers. Age and contemporary group also had no effect on complex I activity, suggesting that variation in these animals was driven by factors other than mitochondrial haplotype. Given that mitochondrial respiratory complexes are derived from both mitochondrial and nuclear-encoded genes, nuclear genetic variation or the interaction of nuclear and mitochondrial genes may have been more influential than complex I haplotype alone in determining the complex’s efficiency. Future analysis of data from more animals and consideration of complex I activity’s relationship to growth traits (e.g., average daily gain, dry matter intake, and feed-to-gain ratio) will further characterize its role in beef cattle production traits.