Molecular cloning and functional characterization of MYC transcription factor in pathogen-challenged Apostichopus japonicus

Vibrio splendidus infection promotes circRNA-FGL1-regulated coelomocyte apoptosis via competitive binding to Myc with the deubiquitinase OTUB1 in Apostichopus japonicus

Circular RNAs (circRNAs) are involved in various physiological and pathological processes in both vertebrates and invertebrates. However, most studies on circRNAs have focused on their roles as endogenous competitive RNAs. Here, we report a novel function of circRNA derived from the Fibrinogen-like protein 1 gene (circ-FGL1) that inhibits coelomocyte apoptosis via competing with the deubiquitinase AjOTUB1 to bind AjMyc in Apostichopus japonicus during Vibrio splendidus infection. The results showed that circ-FGL1 is significantly downregulated in coelomocytes of V. splendidus-induced A. japonicus and negatively regulates coelomocyte apoptosis through the AjBax-AjCyt c pathway. Mechanistically, the deubiquitinase AjOTUB1 and circ-FGL1 could interact with the transcription factor protein AjMyc in the same region with circ-FGL1/AjMyc having greater affinity. Under normal conditions, high levels of circ-FGL1 bind directly to AjMyc, inhibiting the deubiquitylation of AjMyc by AjOTUB1 and leading to the degradation of AjMyc. After V. splendidus infection, AjMyc disassociates from the depressed expression of circ-FGL1, promoting its deubiquitylation by binding to the induced deubiquitinase AjOTUB1 to inhibit its degradation. AjMyc is then transferred to the nucleus and promotes the transcription of AjCyt c and AjBax to induce coelomocyte apoptosis. The new finding will expand our present outstanding on the functional role of circRNAs and suggest new therapeutic targets for the treatment of echinoderms during bacterial invasion.

Comparative analysis for immune response of coelomic fluid from coelom and polian vesicle in Apostichopus japonicus to Vibrio splendidus infection

The polian vesicle and coelom of sea cucumber Apostichopus japonicus were full of coelomic fluid in which many types of coelomocytes with different functions were suspended. Our previous work has indicated the differences of coelomocytes between two sites mainly in subtype proportion, non-specific immune enzymes activities and several immune-related genes expression levels in healthy A. japonicus. However, the functional similarities and differences of coelomic fluid in two sites including the coelom and polian vesicle after pathogenic infection still remain unclear. Here, we investigated the changes of the total coelomocyte density (TCD) and differential coelomocyte density (DCD) after pathogen infection by Vibrio splendidus in coelom and polian vesicle. After infected by V. splendidus, the TCD in the coelom and polian vesicle rapidly declined at 12 h, and then the TCD in the coelom showed a stably ascending trend, while the TCD in the polian vesicle reached a peak at 24 h post infection (hpi), and then showed a continuously decline trend from 24 hpi to 72 hpi followed by a slow elevation until recovering the normal level from 72 hpi to 96 hpi. Then the activities of acidic phosphatase (ACP), alkaline phosphatase (AKP), catalase (CAT) and superoxide dismutase (SOD) were determined to evaluate the response of cell-free coelomic fluid to V. splendidus infection. The activities of ACP, AKP and CAT showed similar trends in the coelom and polian vesicle. The SOD activity significantly increased in the polian vesicle, whereas it exhibited a decreasing trend in the coelom. Finally, the expression profiles of nine immune-related genes including Aj-MyD88, Aj-IRAK4, Aj-i-Lys, Aj-Rel, Aj-p50, Aj-DMBT1, Aj-CDC, Aj-Rrp15 and Aj-Fibrinogen C were detected after V. splendidus challenge. The results suggested all the detected genes were significantly up-regulated both in the coelom and polian vesicle, and the expression levels of these genes in two sites shared similar trends except Aj-MyD88 and Aj-DMBT1. This research provides a new insight into the differentially immune roles of coelomic fluid and coelomocytes in polian vesicle and coelom response to bacterial infections and supplements comprehensive resources for better understanding the innate immune response of A. japonicus.

MYC drives autophagy to adapt to stress in Penaeus vannamei

MYC proto-oncogene (MYC), a first oncogenic nuclear transcription factor isolated from the human genome, belongs to the helix loop helix/leucine zipper protein family (bHLHzip). MYC plays an important part in the process of various physiological and biochemical of vertebrate, such as cell growth, proliferation, cycle, and autophagy. However, its molecular regulation mechanism and function in invertebrates are still unclear. In this study, a novel transcription factor MYC gene was screened, cloned, and characterized from Penaeus vannamei. The open reading frame of PvMYC was 1593bp, encode a polypeptide of 530 amino acids with molecular weight of 58.5 kDa, and a theoretical PI of 5.75. The results of tissue distribution showed that PvMYC was constitutively expressed in all detected tissues, and highest expression in hepatopancreas. The expression level of PvMYC up-regulated significantly and responded to low temperature stress by nuclear ectopic after low temperature stress. Overexpression of PvMYC in shrimp hemocytes negatively regulated the expression of Beclin-1 and reduced the conversion from LC3I to LC3II, yet p62 was decreased significantly. Meanwhile, RAPA eliminated the inhibition of autophagy caused by overexpression of PvMYC. ROS levels and autophagy flux showed the similar trend under low temperature stress after silencing PvMYC. The expression levels of Beclin-1, key ATG gene and LC3II increased significantly, while p62 decreased significantly under the same conditions. In addition, the Total hemocyte count (THC) decreased sharply, and accelerated the injury of hepatopancreas under low temperature stress after silencing PvMYC. Collectively, these results suggest that PvMYC has vital role in the cold adaptation mechanism of P. vannamei by negatively regulating autophagy.

4-Hydroxyphenylpyruvate dioxygenase from sea cucumber Apostichopus japonicus negatively regulates reactive oxygen species production

As a wide distribution molecule, 4-hydroxyphenylpyruvate dioxygenase (4-HPPD) catalyzes the second step in the tyrosine catabolism pathway. This process commonly occurs in all aerobic life forms. The broad distribution of these metabolites suggests that they have an important role in many organisms. A portion of the 4-HPPD homology sequence was also identified in Apostichopus japonicus transcriptome. However, the functional roles of A. japonicus 4-HPPD remain unclear. In the current study, a 4-HPPD homolog was cloned from A. japonicus (designated as AjHPPD). The nucleotide sequence analysis showed that the open reading frame of AjHPPD was 1149 bp and encoded a 382-amino-acid residue polyprotein with glyoxalase_4 (residues 20-133) and glyoxalase (residues 180-335) domains. The spatial expression analysis revealed that AjHPPD was ubiquitously expressed in all examined tissues with large-magnitude in the respiratory tree and was minimally expressed in coelomocytes. Compared with a control group, the significant increase in transcription of AjHPPD mRNA in the Vibrio splendidus-challenged sea cucumber was 2.10-fold (p < 0.01) at 48 h and returned to the normal level at 72 and 96 h. Similarly, compared with a control group, the significant increase in the transcription of AjHPPD mRNA was 3.36-fold (p < 0.01) at 24 h after stimulation with 10 mg mL^-1 of LPS. On the one hand, silencing AjHPPD in vitro could inhibit the expression of pentose phosphate pathway (PPP) flux enzyme glucose-6-phosphate dehydrogenase (G6PD) at the mRNA level and prevent the clearance of reactive oxygen species (ROS) in sea cucumbers. On the other hand, interference of AjHPPD by using specific siRNA can result in the significant promotion of coelomocyte apoptosis with a 1.61-fold increase in vitro. AjHPPD negatively regulated ROS levels by modulating tyrosine catabolism on AjG6PD expression and coelomocyte apoptosis in response to pathogen infection.