Intramuscular connective tissue (IMCT) collagen is an important factor in meat quality. This study analyzed the characteristics of type I and III collagen in the IMCT of the semitendinosus (SD) and longissimus dorsi (LD) of Wuzhumuqin sheep at different growth stages (6, 9, 12, and 18 months). Utilizing sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and Fourier transform infrared spectroscopy (FTIR), collagen types I and III were successfully isolated and shown to contain an intact triple helix structure. Immunofluorescence revealed that these collagens were located in the endomysium and perimysium. Collagen-related genes were significantly expressed in sheep aged 9 and 12 months. The amino acid content increased with age in type I collagen whereas it decreased in type III collagen. Furthermore, type III collagen contained more hydroxyproline (Hyd) than type I collagen. Differential scanning calorimetry (DSC) revealed that the thermal stability of collagen increased with age, accompanied by a decrease in solubility. Semitendinosus muscle had more collagen cross-linkages than LD muscle due to the high pyridinoline (Pyr) content in the endomysium. Finally, a correlation analysis highlighted the multiple correlations between characteristics in different types of collagen during sheep growth. In summary, the collagen characteristics in the IMCT of sheep were impacted by collagen type, muscle type, and age. Furthermore, the various correlations between these characteristics may play an important role in the development of IMCT.

Adipose tissue is a crucial economically significant trait that significantly influences the meat quality and growth performance of domestic animals. To reveal the changes in adipose tissue metabolism during the growth of naturally grazing sheep, we evaluated the thickness, adipocyte morphology, fatty acid profile, and metabolite profile of subcutaneous adipose tissue (SAT) from naturally grazing Sunit sheep at 6, 18, and 30 months of age (referred to as Mth-6, Mth-18, and Mth-30, respectively). The fat thickness and adipocyte number were significantly increased with the growth of the sheep (p < 0.05), and the increase of which from Mth-18 to Mth-30 was less than that from Mth-6 to Mth-18. Additionally, the alpha-linolenic acid metabolism was enhanced and fatty acid (FA) elongation increased with growth. The metabolomic analysis revealed 76 differentially expressed metabolites (DEMs) in the SAT in different growth stages. Interestingly, we observed elongation of FAs in lipids correlated with sheep growth. Furthermore, the expression of acylcarnitines was downregulated, and fatty acid amides, aspartic acid, acetic acid and phosphocholine were upregulated in Mth-18 and Mth-30 compared to Mth-6. Altogether, the study found that the difference in SAT in Mth-6 was great compared to Mth-18 and Mth-30. An increase in fat deposition via adipocyte proliferation with the growth of the sheep in naturally grazing. The DEMs of acylcarnitines, fatty acid amides, aspartic acid, acetic acid, and phosphocholine emerged as potential key regulators of adipose tissue metabolism. These findings illustrate the variation in and metabolic mechanism of sheep adipose tissue development under natural grazing, thus providing valuable insights into improving the edible quality of sheep meat and developing the mutton sheep industry.

This study elucidated the regulatory mechanisms of age-related meat flavor precursors in naturally grazed Sunit sheep of different ages (6, 18, and 30 months) by analyzing their metabolite and mRNA profiles. The longissimus dorsi muscle was sampled from each group and subjected to metabolomics and transcriptomics analyses. A total of 395 differential metabolites (DMs) and 1482 differentially expressed genes (DEGs) were detected across the age groups. As the age increased, the expression levels of GOT1 and GLUL increased, activating arginine biosynthesis and alanine, aspartate, and glutamate metabolism pathways, which promoted the accumulation of umami compounds (L-glutamate and L-glutamine). Meanwhile, the expression level of LPIN1 increased with age, promoting glycerophospholipid metabolism and contributing to the development of lipid-related aroma. FADS1 and FADS2 expressed the highest levels at age Mth_18. This pattern influenced the unsaturated fatty acid biosynthesis pathway and consequently had a regulatory effect on the DHA levels. An amino acid metabolic regulatory network that involved arginine biosynthesis, alanine, aspartate and glutamate metabolisms, and arginine and proline metabolisms was established. This study provided insights into the variations in meat flavor precursors among sheep of different ages and elucidated the underlying regulatory mechanisms.

Adipose tissue metabolism plays a crucial role in sheep meat quality and the optimization of adipose tissue utilization. To reveal the molecular mechanisms of adipose tissue metabolism during growth in naturally grazing sheep, we investigated the mRNA and miRNA profiles in subcutaneous adipose tissue (SAT) from naturally grazing Sunit sheep at 6, 18, and 30 months of age (Mth-6, Mth-18, and Mth-30). We identified 927 differentially expressed (DE) genes and 134 DE miRNAs in the SAT of sheep at different growth stages. Specifically, the expressions of ACACA, FASN, DGAT2, GPAM, SCD, ELOVL6, HSD17B12, TECR, PKM, TKT, PCK1, CD44, and THBS2S genes were significantly upregulated in Mth-18 and Mth-30 compared to that in Mth-6. These genes promoted fatty acid synthesis, triglyceride synthesis, gluconeogenesis, and extracellular matrix–receptor interaction and decreased glycolysis, leading to increased adipocyte proliferation and fat deposition. Notably, our findings suggested that the reduced activity of the AMPK signaling pathway may be regulated by CAMKK2 and PP2A during sheep growth. Furthermore, our results revealed several DE miRNAs, mml-miR-320b, chi-miR-1388-3p, bta-miR-6715, oar-miR-143, and miR-424, that potentially influence fat metabolism. Overall, this study provides a theoretical basis and new insights into the molecular mechanisms of adipose tissue metabolism during growth in naturally grazing sheep.

Objective: This study aimed to elucidate the lipid composition, characteristics, and metabolic mechanisms of subcutaneous adipose tissue (SAT) in natural grazing Mongolian cattle during its development.Methods: The experimental animals consisted of castrated Mongolian cattle, divided into two groups: 10 months old (10 M, n = 12, average weight: 113.26±0.87 kg) and 34 months old (34 M, n = 12, average weight: 390.44±1.23 kg). All animals were selected from the same herd in the Eren Nur grasslands, Xilingol, Inner Mongolia, China. Browning-related genes in SAT were determined using quantitative reverse transcription polymerase chain reaction with 3 replicates for each cattle, and six animals from each group were randomly selected for non-targeted lipidomic techniques to analyze the changes in lipid metabolism with 6 replicates.Results: The yellowness value and relative expression of mitochondrial biogenesis-specific markers, thermogenic markers, uncoupling protein-2 (UCP2), and beige adipocyte marker genes in the SAT increased with age. Multivariate analysis revealed 172 significantly different lipids (SDLs). Triacylglycerols and phosphatidylcholines showed high relative contents. Glycerol and sterol lipid levels were predominantly upregulated during development, whereas phospholipids and sphingolipids were primarily upregulated at 10 M. Developmental time greatly influenced the fatty acid composition of neutral lipids, phospholipids, and free fatty acids. Pathway analysis demonstrated that the 172 SDLs were primarily involved in glycerophospholipid, glycerolipid, ether lipid, and sphingolipid metabolism.Conclusion: The developmental stage significantly affected the lipid quantity, composition, and metabolism of SAT in Mongolian cattle. Certain functional lipids, such as mediumlong-chain triglycerides, fatty acid ester of hydroxyl fatty acid, and vitamin D, were detected in the yellow fat of natural grazing Mongolian cattle. These findings provide a comprehensive reference for the metabolic characteristics of yellow fat in Mongolian cattle, contributing to the meat industry chain.

While circular RNAs (circRNAs) are known to play crucial roles in adipose tissue metabolism, their regulatory mechanisms in naturally grazing Mongolian sheep remain poorly understood. This study characterized the circRNA expression profiles in the subcutaneous adipose tissue (SAT) of castrated rams (Mongolian sheep) at 6, 18, and 30 months after of birth (n = 3). A total of 18,581 circRNAs were identified in the SAT of these sheep, among which 203 were differentially expressed (DE) across different growth stages, including circRNA7989, circRNA2263, circRNA4685, and circRNA4786. The host genes of DE circRNAs were enriched in lipid metabolism, amino acid metabolism, and glucose metabolism. Moreover, competing endogenous RNA network analysis combining miRNA and mRNA data revealed that circRNA4744, circRNA12148, circRNA10725, and circRNA4895 potentially modulate adipocyte hyperplasia, TG synthesis, and fat deposition by regulating miRNAs that target PDPN, CYP26B1, COL24A1, and SCD. The results of the present study suggest that circRNAs and the ceRNA network play a critical role in SAT metabolism during the growth of naturally grazing sheep, offering a theoretical foundation for breeding strategies and meat quality regulation, for example, by modulating key genes such as circRNA4744, circRNA12148, circRNA10725, and circRNA4895.