BAG2 mediates coelomocyte apoptosis in Vibrio splendidus challenged sea cucumber Apostichopus japonicus

Comparative analysis of BAG1 and BAG2: Insights into their structures, functions and implications in disease pathogenesis

As BAG family members, Bcl-2 associated athanogene family protein 1 (BAG1) and 2 (BAG2) are implicated in multiple cellular processes, including apoptosis, autophagy, protein folding and homeostasis. Although structurally similar, they considerably differ in many ways. Unlike BAG2, BAG1 has four isoforms (BAG1L, BAG1M, BAG1S and BAG1 p29) displaying different expression features and functional patterns. BAG1 and BAG2 play different cellular functions by interacting with different molecules to participate in the regulation of various diseases, including cancer/tumor and neurodegenerative diseases. Commonly, BAG1 acts as a protective factor to predict a good prognosis of patients with some types of cancer or a risk factor in some other cancers, while BAG2 is regarded as a risk factor to promote cancer/tumor progression. In neurodegenerative diseases, BAG2 commonly acts as a neuroprotective factor. In this review, we summarized the differences in molacular structure and biological function between BAG1 and BAG2, as well as the influences of them on pathogenesis of diseases, and explore the prospects for their clinical therapy application by specifying the activators and inhibitors of BAG1 and BAG2, which might provide a better understanding of the underlying pathogenesis and developing the targeted therapy strategies for diseases.

Pathogen-induced apoptosis in echinoderms: A review

Echinoderms possess unique biological traits that make them valuable models in immunology, regeneration, and developmental biology studies. As a class rich in active substances with significant nutritional and medicinal value, echinoderms face threats from marine pathogens, including bacteria, viruses, fungi, protozoa, and parasites, which have caused substantial economic losses in echinoderm aquaculture. Echinoderms counteract pathogen invasion through innate immunity and programmed cell death, in particular, with apoptosis being essential for eliminating infected or damaged cells and maintaining homeostasis in many echinoderm cell types. Despite the importance of this process, there is a lack of comprehensive and updated reviews on this topic. This review underscores that echinoderm apoptotic pathways exhibit a complexity comparable to that of vertebrates, featuring proteins with unique domains that may indicate the presence of novel signaling mechanisms. We synthesize current knowledge on how echinoderms utilize diverse transcriptional and post-transcriptional mechanisms to regulate apoptosis in response to pathogen infections and explore how pathogens have evolved strategies to manipulate echinoderm apoptosis, either by inhibiting it to create survival niches or by inducing excessive apoptosis to weaken the host. By elucidating the primary apoptotic pathways in echinoderms and the host-pathogen interactions that modulate these pathways, this review aims to reveal new mechanisms of apoptosis in animal immune defense and provide insights into the evolutionary arms race between hosts and pathogens.

Calnexin interacts with B-cell receptor-associated protein 31 (Bap31) to mediate coelomocyte phagocytosis and Vibrio splendidus clearance in Apostichopus japonicus

Calnexin serves as a lectin chaperone located on the endoplasmic reticulum membrane and functions in glycoprotein folding and synthesis quality control, as well as in Ca2+ storage. Calnexin is extensively documented to participate in host immunity in the endoplasmic reticulum. However, the functions and fundamental mechanisms of calnexin in the invertebrate innate defence remain largely unknown. In this research, the complete cDNA sequence for calnexin from Apostichopus japonicus (Ajcalnexin) was cloned, revealing a 1779 bp open reading frame that codes for 592 amino acids, 113 bp 5'-Untranslated Region (UTR), and 3251 bp 3'-UTR. Upon Vibrio splendidus infection, both AjCalnexin mRNA and protein levels were significantly increased in coelomocytes. Knocking down Ajcalnexin with specific siRNAs significantly decreased coelomocyte phagocytosis, reducing the intracellular load of V. splendidus. By contrast, overexpression of AjCalnexin using recombinant AjCalnexin protein (rAjCalnexin) had the opposite effect. Moreover, B-cell receptor-associated protein 31 of A. japonicus (AjBap31) was identified as an interacting partner of AjCalnexin, which positively regulates AjBap31 expression. Silencing Ajbap31 also decreased coelomocyte phagocytosis and inhibited the intracellular load of V. splendidus. Furthermore, phagocytosis levels and intracellular loads of V. splendidus in the coelomocytes of sea cucumbers treated with rAjCalnexin and siAjBap31 were significantly lower than those in rAjCalnexin- and siNC-treated sea cucumbers. Collectively, we provide the first functional evidence that the AjCalnexin-AjBap31 axis plays a crucial role in host immune defence by mediating coelomocyte phagocytosis in A. japonicus during V. splendidus infection. These findings enhance understanding of the regulatory mechanism of phagocytosis in echinoderms and offer theoretical insights for preventing and controlling skin ulcer syndrome in sea cucumbers.

CircRNA254 functions as the miR-375 sponge to inhibit coelomocyte apoptosis via targeting BAG2 in V. splendidus-challenged Apostichopus japonicus

Circular RNAs (circRNAs) function as immune regulators in many biological processes in mammals, while their function and underlying mechanisms in invertebrates are largely unexplored. In this study, the competing endogenous RNA (ceRNA) mechanism of circRNA that sponges miR-375 and thus regulates AjBAG2-mediated coelomocyte apoptosis was evaluated in Apostichopus japonicus. The results showed that circRNA254 (circ254) was significantly down-regulated in the intestines and coelomocytes after Vibrio splendidus challenge or Lipopolysaccharide exposure, which matched the RNA-seq results in A. japonicus within skin ulceration syndrome. Dual-luciferase and RNA FISH assays indicated that circ254 could directly combine with miR-375, in which circ254 possesses three binding sites of miR-375. Moreover, circ254 knockdown significantly promoted the coelomocyte apoptosis levels upon pathogen infection in vivo and in vitro. Furthermore, circ254 silencing could also down-regulate AjBAG2 expression and thereby promoting the levels of coelomocyte apoptosis levels and the expression of caspase 3, which the phenomenon could be reversed by treatment with miR-375 inhibitors. Taken together, our results confirmed that circ254 functions as a ceRNA of AjBAG2 by sponging miR-375, resulting in the inhibition of coelomocyte apoptosis in A. japonicus.

circRNA432 enhances the coelomocyte phagocytosis via regulating the miR-2008-ELMO1 axis in Vibrio splendidus-challenged Apostichopus japonicus

Circular RNAs (circRNAs) are a kind of extensive and diverse covalently closed circular endogenous RNA, which exert crucial functions in immune regulation in mammals. However, the functions and mechanisms of circRNAs in invertebrates are largely unclarified. In our previous work, 261 differentially expressed circRNAs including circRNA432 (circ432) were identified from skin ulcer syndrome (SUS) diseased sea cucumber Apostichopus japonicus by RNA-seq. To better address the functional role of sea cucumber circRNAs, circ432 was first found to be significantly induced by Vibrio splendidus challenge and LPS exposure in this study. Knock-down circ432 could depress the V. splendidus-induced coelomocytes phagocytosis. Moreover, circ432 is validated to serve as the sponge of miR-2008, a differential expressed miRNA in SUS-diseased sea cucumbers, by Argonaute 2-RNA immunoprecipitation (AGO2-RIP) assay, luciferase reporter assay and RNA fluorescence in situ hybridization (FISH) in vitro. Engulfment and cell motility protein 1 (AjELMO1) is further demonstrated to be the target of miR-2008, and silencing AjELMO1 inhibits the V. splendidus-induced coelomocytes phagocytosis, and this phenomenon could be further suppressed by supplementing with miR-2008 mimics, suggesting that circ432 might regulate coelomocytes phagocytosis via miR-2008-AjELMO1 axis. We further confirm that the depressed coelomocytes' phagocytosis by circ432 silencing is consistent with the decreased abundance of AjELMO1, and could be recovered by miR-2008 inhibitors transfection. All our results provide the evidence that circ432 is involved in regulating pathogen-induced coelomocyte phagocytosis via sponge miR-2008 and promotes the abundance of AjELMO1. These findings will enrich the regulatory mechanism of phagocytosis in echinoderm and provide theoretical data for SUS disease prevention and control in sea cucumbers.