For decades, mastitis has caused large-scale economic losses worldwide in dairy farming due to treatment costs, discarded milk, reduced milk yield, and increased culling rates[23–28]. A recent study from Canada estimated costs on typical dairy farms to be 662 Canadian Dollars per milking cow per year, in which nearly half of the costs were associated with subclinical mastitis[29]. Circular RNAs are a novel class of endogenous RNAs with covalently closed loop structures. They are generated during RNA splicing and arise from exons (exonic circular RNAs or circRNAs), introns (intronic circular RNAs or ciRNAs), or a combination of both (EIciRNAs)[30]. With the advent of high-throughput sequencing and novel computational approaches for non-polyadenylated RNA transcripts, thousands of circular RNAs have been successfully identified in various species[31]. Furthermore, circRNAs have been implicated in tissue and organ development and may play a role in various disease processes, such as neurodegeneration and cancer development[32, 33]. Circular RNA CircKIAA plays a regulatory role in cow mastitis epithelial cells. circRNAs are connected with hepatocellular carcinoma (HCC) according to the latest research. It is found that circ_0001649 expression is lower in HCC than in the adjacent tissues, and its expression is related to tumor size and tumor embolus[34]. Zhao et al. investigated the expression profile of circRNAs in early-stage lung adenocarcinoma tissues versus the normal tissues[35]. They identified a total of 357 circRNAs that were dysregulated in the early-stage lung adenocarcinoma, including 204 up-regulated circRNAs and 152 down-regulated circRNAs. CircRNA_404833 and circRNA_406483 displayed significant differences in their expression between LC tissues and normal tissue[35].
The function of circRNAs remains largely unknown. A handful of circRNAs are involved in post-transcriptional regulation by functioning as "sponges" of miRNAs, reducing their ability to target mRNAs[36, 37]. Pu test results show hsa_circ_0000092 impaired miR-338-3p-mediated HN1 inhibition to aggravate the development of HCC, indicating that hsa_circ_0000092 is a potential candidate marker and therapeutic target for HCC[38]. In Zhong's experiment, it was concluded that circ_PUM1 is capable of binding to miR-136 and up-regulating its target gene NOTCH3, which can be reversed by overexpression of miR-136. Circ_PUM1 can compete with miR-136, leading to up-regulation of NOTCH3, and thereby promote the development of endometrial cancer[39]. According to his experiments, He found that circFUT8 functions as a tumor suppressor in BCa cells by targeting the miR-570-3p/KLF10 axis and may serve as a potential biomarker and therapeutic target for the management of BCa patients with LN metastasis[40]. In Hu's study of the mammary glands,he showed that circRNA-0001283 positively regulated HIPK3 expression by sponging miR-187. The results reveal a new functional circRNA-0001283 in breast cancer and may provide targets for developing novel therapeutic strategies for breast cancer[41]. In Hu's study of the mammary glands༌he showed that circRNA-0001283 positively regulated HIPK3 expression by sponging miR-187. The results reveal a new functional circRNA-0001283 in breast cancer and may provide targets for developing novel therapeutic strategies for breast cancer[42]. Cao's findings illustrate the critical role of circRNF20 / mir-487 a/hif-1 / HK2 axis in breast cancer progression and Warburg effect, providing an interesting insight into BC (breast cancer) neogenesis[43]. In Jia's experiments, circ_0007255 inhibited tumor growth in vivo. Circ_0007255 is the sponge of mir-335-5p regulating SIX2 expression in the BC process. Circ_0007255 functioned as a will oncogene in the progression of BC by regulating miR − 335-5 p/SIX2 axis and took a be a promising biomarker for BC treatment[44]. Circ-TFF1 is a facilitator in breast cancer relying on TFF1 by absorbing miR-326, providing a novel promising target for BC treatment[45]. However, the majority of functions and mechanisms of circRNAs remain unknown, which suggests that circRNAs may be a promising avenue to explore in medical research[46].
We obtained 217 terms from GO enrichment analysis, including 106 biological processes, 62 molecular functions, and 49 cellular components. Transepithelial ammonium transport, transcriptional activator activity, RNA polymerase II trans, RNA polymerase II transcription factor binding, regulation of actin filament length, protein localization to cell junction, positive regulation of prolactin signaling pathway, NMDA selective glutamate receptor complex, negative regulation of glucocorticoid-mediated signaling pathway, lactosylceramide biosynthetic process, histone methyltransferase activity H3-K9 specific, elastic fiber assembly, dimethylglycine dehydrogenase activity, choline catabolic process, cerebral cortex development, cation: chloride symporter activity, C2H2 zinc finger domain binding, ATF6-mediated unfolded protein response, ammonium transport, ammonium transmembrane transporter activity and adherence junction maintenance pathways are very significant for cow mastitis. Among them, the RNA polymerase II transcription factor binding pathway had the most significant effect on cow mastitis. Sodium houttuyfonate (SH) has been indicated to play an important anti-inflammatory role[47]. SH significantly inhibited LPS-induced TLR4 expression and NF-ĸB activation. In summary, these results suggested that SH inhibited LPS-induced inflammatory response by inhibiting TLR4/NF-ĸB signaling pathway. SH is a potential agent for the treatment of mastitis[48]. The nuclear factor-kappa B (NF-κB) pathway proteins are key players in controlling both innate and adaptive immunity. However, the information on NF-κB pathway genes is very limited in mastitis resistance and milk production of Chinese Holstein cows[49]. Our data also enriched these pathways, it further illustrates the importance of these circRNAs in cow mastitis.
The expression of circRNAs has been appropriately correlated with an abundance of genes in different animal tissues[50–53]. PCR-SSCP was applied to analyze the polymorphisms of CXCR2 gene and its relationships with milk quality and mastitis in 160 cattle samples including Holstein dairy cow, Simmental dairy cow and Tongjiang cattle. The results showed that the gene had a significant effect on cow mastitis[54]. The authors examined a possible association of polymorphism of the ATP1A1 gene with somatic cell score and 305-day milk yields. Individuals with genotype CC in ATP1A1 had significantly lower somatic cell scores and 305-day milk yields than those with genotype CA. We also examined changes in Na(+), K(+)-ATPase activity of red cell membranes. The Na(+), K(+)-ATPase activity was significantly higher in dairy cows with genotype CC compared to the other two genotypes, and the Na(+), K(+)-ATPase activity of the resistant group was significantly higher than that of the susceptible group in dairy cows. We conclude that this polymorphism has potential as a marker for mastitis resistance in dairy cattle[55]. To test the possibility of gene therapy for treating dairy cow mastitis, two eukaryotic expression vectors harboring human lysozyme(hLYZ) cDNA, called pTLYZ and p205C3LYZ, were injected into milk pools of the mammary gland with mastitis, and their effectiveness was demonstrated by CMT assay. After the treatments, total bacterial numbers, as well as representative causative bacterial numbers(staphylococcus, streptococcus and E.coli), of the milk samples decreased and the milk yields increased[56]. There are MBL1 and MBL2 genes that encode the MBL-A and MBL-C proteins, respectively. This study was carried out to investigate the relationship between the variants of the bovine MBL2 gene and milk production traits, mastitis, serum MBL-C levels, and hemolytic complement activity in both classical pathways (CH50) and alternative pathway (ACH50) in Chinese Holstein cattle. Four single-nucleotide polymorphisms (SNPs) in exon 1 of the MBL2 gene in Chinese Holstein cattle and Luxi yellow cattle were identified by the direct sequencing method. MBL2 gene has a significant effect on cow mastitis[57].