Arthropod Down syndrome cell adhesion molecules (Dscam) may sometimes function as hypervariable pathogen recognition receptors. They consist of an extracellular region and a cytoplasmic tail, both of which are highly variable. In shrimp, tail-less Dscam proteins (Dscams) have recently been identified, and these appear to be unlike other arthropod extracellular Dscams that are released from the cell membrane by proteolytic cleavage. Here we investigate the properties of these unique shrimp proteins and show that they can be directly secreted from transfected cells. We also investigate the diverse cytoplasmic tail variants of membrane-bound shrimp Dscams, and show that elements E1A and E3 seem to be related to Dscam immune function. Challenge with Vibrio harveyi not only enhanced total Dscam and the immune-related cytoplasmic tail variants, but also induced expression of certain Ig2 + Ig3 combinations. A pathogen binding assay with these Ig2 + Ig3 extracellular variants showed that both the V. harveyi-induced Dscams and Dscams induced by buffer injection could be either pathogen-specific or specific only for Gram-negative pathogens, while other "general" Dscam variants were sensitive to a wide range of pathogens. The same assay also suggested that shrimp Dscam isoforms show a stronger response to the host's natural pathogens.
Keywords: Down syndrome cell adhesion molecule; Shrimp; Specific pathogen recognition; Vibrio harveyi.
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