Small immune effectors coordinate peptidoglycan-derived immunity to regulate intestinal bacteria in shrimp

Double-stranded RNA induces antiviral transcriptional response through the Dicer-2/Ampk/FoxO axis in an arthropod

Invertebrates mainly rely on sequence-specific RNA interference (RNAi) to resist viral infections. Increasing studies show that double-stranded RNA (dsRNA) can induce sequence-independent protection and that Dicer-2, the key RNAi player that cleaves long dsRNA into small interfering RNA (siRNA), is necessary for this protection. However, how this protection occurs remains unknown. Herein, we report that it is caused by adenosine triphosphate (ATP)-hydrolysis accompanying the dsRNA-cleavage. Dicer-2 helicase domain is ATP-dependent; therefore, the cleavage consumes ATP. ATP depletion activates adenosine monophosphate-activated protein kinase (Ampk) and induces nuclear localization of Fork head box O (FoxO), a key transcriptional factor for dsRNA-induced genes. siRNAs that do not require processing cannot activate the transcriptional response. This study reveals a unique nonspecific antiviral mechanism other than the specific RNAi in shrimp. This mechanism is functionally similar to, but mechanistically different from, the dsRNA-activated antiviral response in vertebrates and suggests an interesting evolution of innate antiviral immunity.

YTHDC1/CRM1 Facilitates m6A-Modified circRNA388 Nuclear Export to Induce Coelomocyte Autophagy via the miR-2008/ULK Axis in Apostichopus japonicus

N 6-methyladenosine (m6A), the most prevalent internal modification in eukaryotic RNA, was able to mediate circular RNA (circRNA) function in many immune processes. Nevertheless, the functional role of m6A-modified circRNAs in innate immunity of invertebrates remained unclear. In this study, we identified m6A-modified circRNA388 from cultured sea cucumber (Apostichopus japonicus) coelomocytes, which was mainly detected in cytoplasm after Vibrio splendidus infection. A knockdown assay indicated that cytoplasm circRNA388 promoted coelomocyte autophagy and decreased the number of intracellular V. splendidus. Mechanistically, the circRNA388 in the cytoplasm directly sponged miR-2008 to block its interaction with Unc-51-like kinase 1 from A. japonicus (AjULK) and further promoted autophagy to resist V. splendidus infection. More importantly, we found that m6A modification was vital to circRNA388 nuclear export with YTH domain-containing protein 1 from A. japonicus (AjYTHDC1) as the reader. AjYTHDC1 facilitated the nuclear export of m6A-modified circRNA388 via interaction with exportin-1 (chromosomal maintenance 1) from A. japonicus (AjCRM1). Knockdown of AjCRM1 could significantly decrease the content of cytoplasm circRNA388. Overall, our results provide the first evidence that nuclear export of m6A-modified circRNA388 is dependent on the novel AjCRM1 to our knowledge, which was further promoted coelomocyte autophagy by miR-2008/AjULK axis to clear intracellular V. splendidus.

Dual oxidase 2 (duox 2) participates in the intestinal antibacterial innate immune responses of Procambarus clarkii by regulating ROS levels

Dual oxidase (Duox) a member of the nicotinamide adenine dinucleotide phosphate oxidase (NOX) family can induce the production of reactive oxygen species (ROS). In vertebrates, the duox gene was indicated to be associated with the mucosal immunity. The roles of the duox gene in invertebrates were mainly studied in insects for the function of maintaining intestinal flora balance. In recent years, some studies have reported that Duox is involved in regulating the production of ROS and plays an important role in defending against the intestinal pathogen infection. However, the molecular mechanism has not been fully illuminated. In this study, a duox 2 involved in the production of H2O2 was identified for the first time in P. clarkii. Mature Pc-Duox 2 is a 7-transmembrane protein molecule that includes PHD, FAD, and NAD domains. Pc-duox 2 was mainly expressed in hemocytes and intestinal tissue. Its expression levels were obviously upregulated after intramuscular or oral infection with V. harveyi. In the RNAi assay, the upregulated trends of H2O2 and total ROS levels in crayfish intestine were significantly suppressed when Pc-duox 2 was knocked down. Compared with the slightly affected SOD activity, the upregulated CAT activity was suppressed more obviously in the crayfish intestine. Furthermore, Pc-duox 2 had an important effect on the maintenance of the structural stability of crayfish the intestine. Further research revealed that the knockdown of Pc-duox 2 could cause an obvious suppression in the upregulated levels of Toll signalling pathway-related genes, including Pc-toll 1, Pc-toll 3, Pc-dorsal, Pc-ALF 5, Pc-crustin 1, and Pc-lysozyme. Ultimately, these changes triggered the accelerated death of crayfish. Overall, we speculated that Pc-duox 2 played an important role in antibacterial innate immunity in the crayfish intestine by regulating the total ROS level.

A CTL - Lys immune function maintains insect metamorphosis by preventing gut bacterial dysbiosis and limiting opportunistic infections

BACKGROUND

Gut bacteria are beneficial to the host, many of which must be passed on to host offspring. During metamorphosis, the midgut of holometabolous insects undergoes histolysis and remodeling, and thus risks losing gut bacteria. Strategies employed by holometabolous insects to minimize this risk are obscure. How gut bacteria affect host insects after entering the hemocoel and causing opportunistic infections remains largely elusive.

RESULTS

We used holometabolous Helicoverpa armigera as a model and found low Lactobacillus load, high level of a C-type lectin (CTL) gene CD209 antigen-like protein 2 (CD209) and its downstream lysozyme 1 (Lys1) in the midgut of the wandering stage. CD209 or Lys1 depletion increased the load of midgut Lactobacillus, which further translocate to the hemocoel. In particular, CD209 or Lys1 depletion, injection of Lactobacillus plantarum, or translocation of midgut L. plantarum into the hemocoel suppressed 20-hydroxyecdysone (20E) signaling and delayed pupariation. Injection of L. plantarum decreased triacylglycerol and cholesterol storage, which may result in insufficient energy and 20E available for pupariation. Further, Lysine-type peptidoglycan, the major component of gram-positive bacterial cell wall, contributed to delayed pupariation and decreased levels of triacylglycerols, cholesterols, and 20E, in both H. armigera and Drosophila melanogaster.

CONCLUSIONS

A mechanism by which (Lactobacillus-induced) opportunistic infections delay insect metamorphosis was found, namely by disturbing the homeostasis of lipid metabolism and reducing 20E production. Moreover, the immune function of CTL - Lys was characterized for insect metamorphosis by maintaining gut homeostasis and limiting the opportunistic infections.

Symbiotic hemolymph bacteria reduce hexavalent chromium to protect the host from chromium toxicity in Procambarus clarkii

Hexavalent chromium (Cr(VI)) is a cytotoxic heavy metal pollutant that adversely affects all life forms. Interestingly, the crustacean Procambarus clarkii exhibits a relatively high tolerance to heavy metals. The underlying mechanisms remain unclear. In this study, we investigated the role of symbiotic bacteria in P. clarkii in alleviating Cr(VI)-induced damage and explored their potential mechanisms of action. Through transcriptomic analysis, we observed that Cr(VI) activated P. clarkii's antimicrobial immune responses and altered the bacterial composition in the hemolymph. After antibiotic treatment to reduce bacterial populations, Cr(VI)-induced intestinal and liver damage worsened, and crayfish exhibited lower levels of GSH/CAT/SOD activity. The Exiguobacterium, the symbiotic bacteria in the hemolymph of P. clarkii, were proved to be primary contributor to Cr(VI) tolerance. Further investigation suggested that it resists Cr(VI) through the activation of the ABC transporter system and the reduction of Cr(VI) via the reductase gene nfsA. To validate the role of Exiguobacterium in Cr(VI) tolerance, crayfish treated with antibiotics then supplemented with Exiguobacterium H6 and recombinant E. coli (with the nfsA gene), reduced Cr(VI)-induced ovarian damage. Overall, this study revealed that the symbiotic bacteria Exiguobacterium can absorb and reduce hexavalent chromium, mitigating Cr(VI)-induced damage in P. clarkii. These findings provide new insights into hexavalent chromium tolerance mechanisms in crustaceans.

Shrimp microbiome and immune development in the early life stages

With its contribution to nutrition, development, and disease resistance, gut microbiome has been recognized as a crucial component of the animal's health and well-being. Microbiome in the gastrointestinal tract constantly interacts with the host animal's immune systems as part of the normal function of the intestines. Interactions between the microbiome and the immune system are complex and dynamic, with the microbiome shaping immune development and function. In contrast, the immune system modulates the composition and activity of the microbiome. In shrimp, as with all other aquatic animals, the interaction between the microbiome and the animals occurs at the early developmental stages. This early interaction is likely essential to the development of immune responses of the animal as well as many key physiological developments that further contribute to the health of shrimp. This review provides background knowledge on the early developmental stage of shrimp and its microbiome, examines the interaction between the microbiome and the immune system in the early life stage of shrimp, and discusses potential pitfalls and challenges associated with microbiome research. Understanding the interaction between the microbiome and shrimp immune system at this crucial developmental stage could have the potential to aid in the establishment of a healthy microbiome, improve shrimp survival, and provide ways to shape the microbiome with feed supplements or other strategies.

N6-methyladenosine helps Apostichopus japonicus resist Vibrio splendidus infection by targeting coelomocyte autophagy via the AjULK-AjYTHDF/AjEEF-1α axis

N6-Methyladenosine (m6A) modification is one of the most abundant post-transcriptional modifications that can mediate autophagy in various pathological processes. However, the functional role of m6A in autophagy regulation is not well-documented during Vibrio splendidus infection of Apostichopus japonicus. In this study, the inhibition of m6A level by knockdown of methyltransferase-like 3 (AjMETTL3) significantly decreased V. splendidus-induced coelomocyte autophagy and led to an increase in the intracellular V. splendidus burden. In this condition, Unc-51-like kinase 1 (AjULK) displayed the highest differential expression of m6A level. Moreover, knockdown of AjULK can reverse the V. splendidus-mediated autophagy in the condition of AjMETTL3 overexpression. Furthermore, knockdown of AjMETTL3 did not change the AjULK mRNA transcript levels but instead decreased protein levels. Additionally, YTH domain-containing family protein (AjYTHDF) was identified as a reader protein of AjULK and promoted AjULK expression in an m6A-dependent manner. Furthermore, the AjYTHDF-mediated AjULK expression depended on its interaction with translation elongation factor 1-alpha (AjEEF-1α). Altogether, our findings suggest that m6A is involved in resisting V. splendidus infection via facilitating coelomocyte autophagy in AjULK-AjYTHDF/AjEEF-1α-dependent manner, which provides a theoretical basis for disease prevention and therapy in A. japonicus.