Expression patterns of alpha-2 macroglobulin reveal potential immune functions in brood pouch of the lined seahorse Hippocampus erectus

Abstract

Seahorses (family Syngnathidae) are ovoviviparous fish whose embryos can obtain paternal immunoprotection through the male's brood pouch, which is a specialized organ for pregnancy. Previous studies have revealed that seahorses have a unique immune system and alpha-2 macroglobulin (α2M) may be involved in male pregnancy. To ascertain the significance of α2M in the immune system of the brood pouch, the characteristics of the α2M gene were first investigated in the lined seahorse (Hippocampus erectus). In this study, α2M showed a broad expression pattern, with the highest expression in the liver and brood pouch. Interestingly, during the six-month seahorse culture, expression level of α2M in the liver was particularly decreased during brood pouch formation (fourth and fifth months), but was restored the complete development of this brood pouch (sixth months), and a similar expression occurred in the brood pouch. This suggests that the seahorse might be able to regulate its immune response while a new organ develops. Further investigation also examined the immune regulation of α2M in brood pouches by injecting lipopolysaccharide and polyinosinic-polycytidylic acid. The results showed that, after the challenge experiment, α2M expression in the inner pseudoplacenta was upregulated, along with other important immune factors, such as Toll-like receptors (TLRs), tumor necrosis factor-alpha (TNFα), interleukin-6 (IL-6) and interferon-gamma (IFNγ). Taken together, our results reveal that α2M plays an important role in immunoprotection of the brood pouch, and may provide new insights into studies of male pregnancy regulation in seahorses.