Transcriptome Comparison Reveals the Difference in Liver Fat Metabolism between Different Sheep Breeds

Simple Summary Fat metabolism is an important research topic in sheep, and tail fat tissue is usually used as the study material in the fat metabolism of sheep. The liver is also a major fat metabolism organ, and there are many studies on fat metabolism in the livers of chickens, ducks, pigs, and cattle, but studies on the livers of sheep are still fairly rudimentary. In this study, we investigated the liver transcriptome difference between Hu sheep and Tibetan sheep. We reported the differentially expressed genes in the livers of two sheep breeds using RNA-seq technology and identified fat-metabolism-related genes, including ACSLs, CPT1s, and FGF21. We further verified the relative expression levels of genes that are significant and stably expressed on liver fat metabolism at the mRNA and protein levels; then, we described the details of how these genes regulate fat metabolism. The results revealed the difference in fat metabolism between the two sheep breeds. Hu sheep and Tibetan sheep are two commonly raised local sheep breeds in China, and they have different morphological characteristics, such as tail type and adaptability to extreme environments. A fat tail in sheep is the main adipose depot in sheep, whereas the liver is an important organ for fat metabolism, with the uptake, esterification, oxidation, and secretion of fatty acids (FAs). Meanwhile, adaptations to high-altitude and arid environments also affect liver metabolism. Therefore, in this study, RNA-sequencing (RNA-seq) technology was used to characterize the difference in liver fat metabolism between Hu sheep and Tibetan sheep. We identified 1179 differentially expressed genes (DEGs) (Q-value < 0.05) between the two sheep breeds, including 25 fat-metabolism-related genes. Through Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis, 16 pathways were significantly enriched (Q-value < 0.05), such as the proteasome, glutamatergic synapse, and oxidative phosphorylation pathways. In particular, one of these pathways was enriched to be associated with fat metabolism, namely the thermogenesis pathway, to which fat-metabolism-related genes such as ACSL1, ACSL4, ACSL5, CPT1A, CPT1C, SLC25A20, and FGF21 were enriched. Then, the expression levels of ACSL1, CPT1A, and FGF21 were verified in mRNA and protein levels via qRT-PCR and Western blot analysis between the two sheep breeds. The results showed that the mRNA and protein expression levels of these three genes were higher in the livers of Tibetan sheep than those of Hu sheep. The above genes are mainly related to FAs oxidation, involved in regulating the oxidation of liver FAs. So, this study suggested that Tibetan sheep liver has a greater FAs oxidation level than Hu sheep liver. In addition, the significant enrichment of fat-metabolism-related genes in the thermogenesis pathway appears to be related to plateau-adaptive thermogenesis in Tibetan sheep, which may indicate that liver- and fat-metabolism-related genes have an impact on adaptive thermogenesis.

Automated summary

This study investigates the difference in liver fat metabolism between two sheep breeds, Hu sheep and Tibetan sheep, using RNA-seq technology. Differentially expressed genes and enriched pathways related to fat metabolism were identified, and the expression levels of several genes were validated at both the mRNA and protein levels. The results reveal the difference in fat metabolism between the two sheep breeds, suggesting that Tibetan sheep have a higher level of fatty acid oxidation in the liver compared to Hu sheep. The enrichment of fat-metabolism-related genes in the thermogenesis pathway indicates their impact on adaptive thermogenesis in Tibetan sheep.