The in vitro maturation efficiency of porcine oocytes is relatively low, and this limits the production of in vitro porcine embryos. Since melatonin is involved in mammalian reproductive physiology, in this study, we have explored whether endogenously produced melatonin can help in porcine oocyte in vitro maturation. We have found, for the first time in the literature, that mitochondria are the major sites for melatonin biosynthesis in porcine oocytes. This mitochondrially originated melatonin reduces ROS production and increases the activity of the mitochondrial respiratory electron transport chain, mitochondrial biogenesis, mitochondrial membrane potential, and ATP production. Therefore, melatonin improves the quality of oocytes and their in vitro maturation. In contrast, the reduced melatonin level caused by siRNA to knockdown AANAT (siAANAT) is associated with the abnormal distribution of mitochondria, decreasing the ATP level of porcine oocytes and inhibiting their in vitro maturation. These abnormalities can be rescued by melatonin supplementation. In addition, we found that siAANAT switches the mitochondrial oxidative phosphorylation to glycolysis, a Warburg effect. This metabolic alteration can also be corrected by melatonin supplementation. All these activities of melatonin appear to be mediated by its membrane receptors since the non-selective melatonin receptor antagonist Luzindole can blunt the effects of melatonin. Taken together, the mitochondria of porcine oocytes can synthesize melatonin and improve the quality of oocyte maturation. These results provide an insight from a novel aspect to study oocyte maturation under in vitro conditions.

Melatonin is synthesized in multiple tissues and organs of pigs, and existing studies have shown the presence of the melatonin-synthesizing enzyme ASMT protein. However, the genomic information for the ASMT gene has been lacking. The aim of this study was to locate the genomic information of the ASMT gene in pigs using comparative genomics analysis and then obtain the coding region information through molecular cloning. First, using the NCBI Genome Data Viewer, we found that in most animals, the AKAP17A gene is often located next to the ASMT gene, with both genes arranged in the same direction. Similarly, the P2RY8 gene is commonly adjacent to the ASMTL gene, also in the same orientation. We also discovered that the ASMTL gene is frequently adjacent to the ASMT gene and arranged in the opposite direction. Using the “three-point localization” principle, we inferred the position of the ASMT gene based on the coordinates of AKAP17A and ASMTL in pigs. Our results revealed that on the pig X chromosome, a gene called LOC110258194 is located next to the AKAP17A and ASMTL genes, and its arrangement aligns with the ASMT gene in other species. Additionally, Ensembl contains a gene, ENSSSCG00000032659, at the same position, with completely overlapping exons, though it is not annotated as ASMT. Further analysis using the TreeFam tool from EMBL-EBI and the CDD tool from NCBI revealed that LOC110258194 and ENSSSCG00000032659 do not contain the typical Maf domain of ASMTL and, thus, should not be annotated as ASMTL, but rather as the ASMT gene. Using a slow-down PCR method for high-GC content genes, we successfully cloned the full CDS region of the pig ASMT gene and identified a new transcript missing Exon 6 and Exon 7. This transcript was submitted to NCBI and assigned the GenBank accession number MW847601. Our results represent the first successful localization of the ASMT gene in pigs, the first cloning of the ASMT gene’s coding region, and the first discovery of a new transcript of the pig ASMT gene.