Roles of phenoloxidases in the sea cucumber Apostichopus japonicus under environmental stresses.

Sea cucumber Apostichopus japonicus is an important economic species in echinoderms; however, its culture frequently suffered from diseases. Environmental stress was found to be a dominant inducer of A. japonicus diseases, and thus, discovering the characteristics of immune system under environmental stress is of great significance for disease control in A. japonicus culture. Phenoloxidase (PO) is recognized as a crucial immune‐related enzyme in invertebrates. In order to preliminarily illustrate the different roles that A. japonicus PO isoforms play under environmental stresses, the transcriptional expression of three identified A. japonicus PO isoforms (named as AjPOI, AjPOII and AjPOIII respectively) after different in vivo environmental stress simulations was determined with quantitative real‐time PCR method. Meanwhile, the transcriptional expression analysis of other representational immune‐related factors under environmental stresses was also performed for the evaluation of relationships between AjPOs and other immune‐related factors. The correlations between AjPOs and other immune‐related factors were analysed using Pearson correlation coefficient, and the effects of AjPOs on other immune‐related factors were then tested by dsRNA interference method. The results of transcriptional expression analysis indicated that AjPOI was involved in the resistance to low salinity, low temperature and hypoxia; AjPOII and AjPOIII were both required for the resistance to low salinity, high temperature, low temperature and hypoxia; and all three AjPOs were excluded from the response against high salinity. In addition, three AjPOs were found to take part in different periods of resistance to environmental stresses, and this property made A. japonicus PO system capable of outputting resistance activities continuously. The correlation analysis combined with dsRNA interference test revealed that AjPOI was capable of regulating AjCTSD, AjI‐LYZ and AjMTF expression; AjPOII and AjPOIII both played important roles in the regulation of AjC‐LYZ expression; and AjPOIII was also able to regulate AjToll expression. These findings suggested that A. japonicus PO isoforms function in innate immunity not only via catalysing the oxidative reactions, but also by broadly regulating the expression of other immune‐related factors. On the whole, three AjPOs showed obvious differences among each other in the resistance to environmental stresses and the regulation of other immune‐related factors, but comparatively, AjPOII and AjPOIII shared more similarity. [ABSTRACT FROM AUTHOR]