Cloning of SsAkt in Sebastes schlegelii and Its Expression Pattern after Bacterial Stimulation

Sebastes schlegelii is a cold temperate ovoviviparous fish found near the seabed layer and is widely distributed in the Yellow Sea and Bohai Sea of China, as well as in the coasts of South Korea and Japan. It is characterized by its tender meat, delicious taste, rich nutrition, and high medicinal value, thus it is an economically important marine fish species in the northern coastal areas of China. In recent years, deep-water cage cultures have rapidly developed in China. Among the economically important fish species suitable for cage culture in northern China, S. schlegelii is the most important local breed with excellent quality and characteristics. However, with the expansion of the scale of the aquaculture industry, aquaculture diseases including those caused by Vibrio harveyi, Vibrio anguillarum, and Photobacterium damselae subsp. damselae, occur frequently. The authors isolated and identified a dominant pathogenic bacterium, the P5W of V. harveyi, from the lesions of diseased S. schlegelii and studied the response of S. schlegelii to V. harveyi infection. Multitissue transcriptome analysis screened the immune differential gene protein kinase B. Protein kinase B, also known as Akt protein, is a serine or threonine protein kinase, which mainly conducts signal transduction by phosphorylating other proteins. It plays an important role in various biological processes such as cell metabolism, transcriptional regulation, cell cycle regulation, immune defense, and embryonic development. Many studies have shown that during viral infection of cells, the Akt pathway is activated, thereby inhibiting viral proliferation. To clarify the sequence information, evolutionary characteristics, and biological functions of the Akt gene in S. schlegelii and to understand the immune response mechanism of S. schlegelii to bacterial invasion, the full-length cDNA of the SsAkt gene was obtained using molecular biology, and the sequence characteristics, tissue expression rules of the gene, and the response rules to Micrococcus luteus and V. anguillarum were studied. The results showed that the length of the open reading frame was 1 440 bp. The predicted relative molecular mass of the SsAkt protein was 55.80 kDa and the isoelectric point was 5.64. Sequence analysis revealed that the SsAkt protein contains three conserved domains: the PH domain (residues 6–109), serine/threonine protein kinase catalytic (S_TKc) domain (residues 148–405), and serine/threonine protein kinase (S_TK_X) domain (residues 406–475). The conserved amino acid site appears in the center of the S_TKc domain and at the end of the S_TK_ X domain: Thr305 and Ser472. Homologous alignment showed that the amino acid sequence of this gene was highly conserved among different species. Phylogenetic tree analysis showed that SsAkt is closely related to Akt in vertebrates, such as Sebastes umbrosus, Lates calcarifer, Larimichthys crocea, and Cyprinus carpio. The relative expression of SsAkt in different tissues of S. schlegelii was detected by real-time fluorescence quantitative PCR. The results showed that SsAkt was expressed in all tested healthy tissues of S. schlegelii, which indicated the relative expression of the SsAkt gene in S. schlegelii. This expression has distinct tissue specificity. The expression of SsAkt in the kidney is the highest, and in the blood and brain, it is also higher than that in other tissues. Therefore, it is speculated that higher expression of SsAkt in the kidney and brain is closely related to immune defense, neurodevelopment, and other processes. To study the role of SsAkt in the immune response of S. schlegelii, we designed a bacterial challenge experiment. The results showed that the transcription level of SsAkt changed significantly, and the relative expression of SsAkt in the three tissues increased after stimulation with M. luteus and V. anguillarum compared with the control group (PBS). After infection with M. luteus, its expression only increased sharply in the kidney, whereas its expression increased slowly in the blood and liver. In the experimental group infected with V. anguillarum, the expression increased significantly in the liver and kidney, and the two relative expression peaks in the kidney occurred at 12 h and 72 h after infection, respectively. The results revealed that the SsAkt gene may play an important role in the immune defense of S. schlegelii, which can further enrich the sequence information and evolutionary data of the Akt gene in marine fish and provide reference materials for an in-depth study of the biological function of this gene. In follow-up experiments, we will explore the specific mechanism of SsAkt in the innate immune defense of S. schlegelii to provide theoretical guidance for the study of the immune signaling pathway of S. schlegelii and to lay a theoretical foundation for revealing the role of SsAkt in other important biological functions in growth and development.