Inhibition of opioid growth factor receptor (OGFR) promotes wound healing in the freshwater pearl mussel (Hyriopsis schlegelii)

Hyriopsis schlegelii, widely regarded as the most economically significant freshwater pearl mussel farmed in China, is known for producing high-quality pearls. The damage inflicted on the organism by pearl insertion is incalculable. Opioid growth factor receptor (OGFR), as a receptor for the opioid growth factor (OGF; [Met⁵]-enkephalin), plays essential roles in regulating cell proliferation and wound healing. Nonetheless, the role of OGFR in H. schlegelii wound healing remains poorly understood. In this paper, a cDNA of 3662 bp for the pearl mussel H.schlegelii OGFR (hereinafter referred to as HsOGFR) was obtained using the rapid amplification of cDNA ends (RACE) method. The complete cDNA sequence of HsOGFR contains a 2640 bp open reading frame (ORF) encoding 879 amino acids. The amino acid sequences of HsOGFR are 43–63 % similar to those of other OGFRs. The N-terminal of HsOGFR contains a conserved OGFR_N domain. Transcripts of HsOGFR were expressed in all tested tissues, with the highest expression observed in the hepatopancreas. Histological evaluation demonstrated that RNA interference-mediated HsOGFR knockdown (via dsRNAs) or pharmacological inhibition with naloxone significantly accelerated wound healing following the removal of mantle tissues in the mussels. Further qRT-PCR assays revealed that the mRNA expression of HsOGFR was down-regulated in mantle and haemocytes following wound treatment (P < 0.05). Concomitantly, HsCDKN1 and HsCDKN3 expression was moderately downregulated at the incision site post-wounding (P < 0.05). Following HsOGFR interference, the expression levels of HsKi67, HsCCND, HsCDKL1 and HsCDK6 were significantly upregulated (P < 0.05) compared with the wound group. Conversely, HsCDKN1 and HsCDKN3 expression was further downregulated after HsOGFR blockade (P < 0.05). Collectively, these findings demonstrate that HsOGFR, as a homolog of OGFRs, is significantly involved in tissue repair in mollusks, and might work through cell cycle related proteins above.