N6-Methyladenosine (m6A)-Circular RNA Pappalysin 1 (circPAPPA) from Cashmere Goats: Identification, Regulatory Network and Expression Potentially Regulated by Methylation in Secondary Hair Follicles Within the First Intron of Its Host Gene

<i>N</i>⁶-methyladenosine (m⁶A) is one of the most abundant modifications in eukaryotic RNA molecules and mediates the functional exertion of RNA molecules. We characterized the circPAPPA and validated its potential m⁶A modification sites in secondary hair follicles (SHFs) of cashmere goats. Furthermore, we generated integrated regulatory networks of the circPAPPA along with enrichment analysis of signaling pathways. We also explored the potential relationship of circPAPPA expression with the first intron methylation of the <i>PAPPA</i> gene in SHFs of cashmere goats. Host source analysis revealed that circPAPPA is derived from the complete exon 2 of the PAPPA gene, spliced in reverse orientation, and predominantly localized in the cytoplasm of SHF stem cells in cashmere goats. The circPAPPA was verified to contain at least four m⁶A modification sites in SHFs of cashmere goats, including m⁶A-450/456, m⁶A-852, m⁶A-900, and m⁶A-963. The generated regulatory network indicated complex and diverse regulatory relationships of m⁶A-circPAPPA with its putative regulatory molecules, including miRNAs, mRNAs, and proteins. Enrichment analysis of signaling pathways showed that m⁶A-circPAPPA might play multiple functional roles in the growth and development of SHF in cashmere goats through the putative regulatory network mediated by its target miRNAs and regulatory proteins. The first intron methylation of the <i>PAPPA</i> gene most likely is significantly involved in the dynamic expression of m⁶A-circPAPPA in cashmere goat SHFs. Results from this study provided novel information to elucidate the biological roles and functional regulatory pathways of m⁶A-circPAPPA in SHF development and the growth of cashmere goat fiber.