Characterization of lymphocyte antigen 75 (Ly75/CD205) as a potential cell-surface marker on spermatogonia in Pacific bluefin tuna Thunnus orientalis

In spermatogonial transplantation using Pacific bluefin tuna Thunnus orientalis as a donor, enrichment of spermatogonia (SG) is expected to facilitate high colonization efficiency. Although it is desirable to establish a bluefin tuna SG enrichment procedure using cell-surface markers, a germ cell-specific cell-surface marker has not been identified to date. We previously found that Ly75 is a mitotic germ cell-specific cell-surface marker in rainbow trout, and that its amino-acid sequences are highly conserved in various teleosts. Thus, the ly75 gene is an excellent candidate cell-surface marker of SG in bluefin tuna. In this study, the bluefin tuna ly75 homolog was cloned and characterized for further use as a germ cell-specific cell-surface marker. In adult tissues, high levels of ly75 transcripts were detected in the liver, pyloric caeca, and testis. In situ hybridization analyses showed that ly75 mRNA was predominantly localized in type-A spermatogonia (A-SG), including single A-SG that contain transplantable germ cells. In contrast, ly75 mRNA was not detected in spermatocytes, spermatids, or gonadal somatic cells in testis. The expression profiles of Ly75 protein were similar to those of the mRNA. Therefore, Ly75 is appropriate for use as a cell-surface marker of SG in bluefin tuna.