Down Syndrome Cell Adhesion Molecule Triggers Membrane-to-Nucleus Signaling-Regulated Hemocyte Proliferation against Bacterial Infection in Invertebrates.

Down syndrome cell adhesion molecule (Dscam) generates tens of thousands of isoforms by alternative splicing, thereby providing crucial functions during immune responses. In this study, a novel Dscam signaling pathway was investigated in crab, which remains poorly characterized in invertebrates. Bacterial infection induced the cytoplasmic cleavage of Dscam intracellular domains (ICDs) by γ-secretase, and then the released ICDs carrying specific alternatively spliced exons could directly interact with IPO5 to facilitate nuclear translocation. Nuclear imported ICDs thus promoted hemocyte proliferation and protect the host from bacterial infection. Protein-interaction studies revealed that the ectodomain of Dscam bound to a disintegrin and metalloprotease domain 10 (ADAM10) rather than ADAM17. Inhibition or overexpression of ADAM10 impaired or accelerated Dscam shedding activity post-bacterial stimulation, respectively. Moreover, the shedding signal then mediated Dscam with an intact cytoplasmic domain to promote the cleavage of ICDs by γ-secretase. Furthermore, the transcription of ADAM10 was regulated by Dscam-induced canonical signaling, but not nuclear imported ICDs, to serve as a feedback regulation between two different Dscam pathways. Thus, membrane-to-nuclear signaling of Dscam regulated hemocyte proliferation in response to bacterial infection.

Identification and characterization of a Reeler domain containing protein in Procambarus clarkii provides new insights into antibacterial immunity in crustacean.

Crayfish, as an invertebrate, relies only on the innate immune system to resist external pathogens. In this study, a molecule containing a single Reeler domain was identified from red swamp crayfish Procambarus clarkii (named as PcReeler). Tissue distribution analysis showed that PcReeler was highly expressed in gills and its expression was induced by bacterial stimulation. Inhibiting the expression of PcReeler by RNA interference led to a significant increase in the bacterial abundance in the gills of crayfish, and a significant increase in the crayfish mortality. Silencing of PcReeler influenced the stability of the microbiota in the gills revealed by 16S rDNA high-throughput sequencing. Recombinant PcReeler showed the ability to bind microbial polysaccharide and bacteria and to inhibit the formation of bacterial biofilms. These results provided direct evidence for the involvement of PcReeler in the antibacterial immune mechanism of P. clarkii.