Figure 4.

Comparison of the shrimp IMD pathways (A) and Drosophila IMD pathways (B) and Human TNFR signaling (C). Homologies between signaling components are depicted by similar shapes and colors. Shrimp and Drosophila IMD pathway-signaling events resemble the Human tumor necrosis factor receptor (TNFR) signaling pathway. In shrimp, some pivotal components of the IMD pathway, such as the receptors, DREDD, and FADD, are still unknown. Similar to Drosophila, pathogen challenges can initiate intracellular signaling events involving IMD, TAB2, and TAK1, which in turn induce the activation of MAPK-AP-1 and IKK-Relish branches. The activation of Relish is required to be phosphorylated by the IKK complex and is cleaved by unknown factors in shrimp. In the MAPK-AP-1 branch of shrimp IMD pathways, the signaling cascade is more similar to that of Human than that of Drosophila, as manifested by the finding that shrimp p38 can be phosphorylated by MKK4, but Drosophila p38 cannot. In shrimp, activation of both of the two branches of IMD pathways can induce the expression of AMPs that confer protection from Gram-negative and Gram-positive bacterial infection.