Identification and characterization of a phospholipid scramblase encoded by planarian Dugesia japonica

Planarians (Platyhelminthes)-An Emerging Model Organism for Investigating Innate Immune Mechanisms

An organism responds to the invading pathogens such as bacteria, viruses, protozoans, and fungi by engaging innate and adaptive immune system, which functions by activating various signal transduction pathways. As invertebrate organisms (such as sponges, worms, cnidarians, molluscs, crustaceans, insects, and echinoderms) are devoid of an adaptive immune system, and their defense mechanisms solely rely on innate immune system components. Investigating the immune response in such organisms helps to elucidate the immune mechanisms that vertebrates have inherited or evolved from invertebrates. Planarians are non-parasitic invertebrates from the phylum Platyhelminthes and are being investigated for several decades for understanding the whole-body regeneration process. However, recent findings have emerged planarians as a useful model for studying innate immunity as they are resistant to a broad spectrum of bacteria. This review intends to highlight the research findings on various antimicrobial resistance genes, signaling pathways involved in innate immune recognition, immune-related memory and immune cells in planarian flatworms.

Two phospholipid scramblase 1-related proteins (PLSCR1like-a & -b) from Liza haematocheila: Molecular and transcriptional features and expression analysis after immune stimulation

Phospholipid scramblases (PLSCRs) are a family of transmembrane proteins known to be responsible for Ca²⁺-mediated bidirectional phospholipid translocation in the plasma membrane. Apart from the scrambling activity of PLSCRs, recent studies revealed their diverse other roles, including antiviral defense, tumorigenesis, protein-DNA interactions, apoptosis regulation, and cell activation. Nonetheless, the biological and transcriptional functions of PLSCRs in fish have not been discovered to date. Therefore, in this study, two new members related to the PLSCR1 family were identified in the red lip mullet (Liza haematocheila) as MuPLSCR1like-a and MuPLSCR1like-b, and their characteristics were studied at molecular and transcriptional levels. Sequence analysis revealed that MuPLSCR1like-a and MuPLSCR1like-b are composed of 245 and 228 amino acid residues (aa) with the predicted molecular weights of 27.82 and 25.74 kDa, respectively. A constructed phylogenetic tree showed that MuPLSCR1like-a and MuPLSCR1like-b are clustered together with other known PLSCR1 and -2 orthologues, thus pointing to the relatedness to both PLSCR1 and PLSCR2 families. Two-dimensional (2D) and 3D graphical representations illustrated the well-known 12-stranded β-barrel structure of MuPLSCR1like-a and MuPLSCR1like-b with transmembrane orientation toward the phospholipid bilayer. In analysis of tissue-specific expression, the highest expression of MuPLSCR1like-a was observed in the intestine, whereas MuPLSCR1like-b was highly expressed in the brain, indicating isoform specificity. Of note, we found that the transcription of MuPLSCR1like-a and MuPLSCR1like-b was significantly upregulated when the fish were stimulated with poly(I:C), suggesting that such immune responses target viral infections. Overall, this study provides the first experimental insight into the characteristics and immune-system relevance of PLSCR1-related genes in red lip mullets.

Identification and characterization of an atypical RIG-I encoded by planarian Dugesia japonica and its essential role in the immune response

Retinoic acid-inducible gene I (RIG-I), an RNA sensor with a conserved structure, activates the host interferon (IFN) system to produce IFNs and cytokines for eliminating pathogens upon recognizing PAMPs. However, the biological functions and the mechanism by which RIG-I regulates the innate immunity response in invertebrates are still unknown at present. Here we identified an atypical RIG-I in planarian Dugesia japonica. Sequence analysis, 3D structure modeling and phylogenetic analysis showed that this atypical protein was clustered into a single clade at the base of the tree in invertebrates, suggesting that DjRIG-I is an ancient and unique protein of the RIG-I-like receptors (RLRs). In situ hybridization analysis revealed that the DjRIG-I mRNAs were predominantly expressed in the pharynx and head of the adult and regenerative planarians. Stimulation with PAMPs induced the over-expression of DjRIG-I in planarians. The molecular simulation demonstrated that DjRIG-I formed a large hole-structure for the docking of dsRNAs, and the pull-down assay confirmed the interaction between DjRIG-I and viral analog poly(I:C). Importantly, some representative antiviral/antibacterial genes in the RIG-I-mediated IFN and P38 signaling pathway, TBK1, IRF-3, Mx, and P38, were significantly upregulated in planarians stimulated with PAMPs. Interference of the DjRIG-I expression by RNAi, inhibited the PAMPs-induced over-expression, suggesting that DjRIG-I is a key player for downstream signaling events. These results indicate that DjRIG-I triggered the intracellular signaling cascades independent of the classical CARD domains and played an essential role in the virus/bacteria-induced innate immunity of planarian.

Identification and characterization of a TNF receptor-associated factor in Dugesia japonica

The tumor necrosis factor (TNF) superfamily consists of a wide variety of inflammatory cytokine, including cell-bound and secreted proteins. These TNFs function through binding and activation of the TNF receptors for modulating TNF-associated intracellular signals. A set of mammalian TNF receptor-associated factors (TRAFs) that have emerged as the major signal transducers for the TNF receptor superfamily, play an important role in both adaptive and innate immunity. However, the existence of TRAFs and their biological functions in planarian are still unknown. In this study, a new member of TRAFs, DjTRAF2, was identified in planarian Dugesia japonica. Phylogenetic analysis revealed that DjTRAF2 could be a new member of the invertebrate TRAF2 family. Sequence analysis showed that the open reading frame of DjTRAF2 had 1353 bp in length and encoded a putative protein of 450 amino acids with a predicted molecular mass of ~51.8 kDa and an isoelectric point of 7.052. Whole-mount in situ hybridization showed that DjTRAF2 was predominantly expressed in adult and regenerative pharynx, which is an important immune organ of planarian. Quantitative real-time PCR revealed that the transcriptional level of DjTRAF2 was significantly up-regulated after induced by pathogen-associated molecular patterns (polyinosinic-polycytidylic acid, lipopolysaccharide, peptidoglycan and β-glucan), suggesting that DjTRAF2 is involved in the immune response against pathogen invasion. Collectively, these results demonstrated that DjTRAF2 might play important roles in the innate immunity of planarian.