Identification of a novel DIg2 isoform differentially expressed in IFNβ-producing plasmacytoid dendritic cells

Background: The murine discs large homolog 2 (DLG2; post synaptic density 93 (PSD-93); Chapsyn-110) is a member of the membrane-associated guanylate kinase (MAGUK) protein family involved in receptor assembly and associated with signaling enzymes on cell membranes. In neurons, DLG2 protein isoforms derived from alternatively spliced transcripts have been described to bind to NMDA (N-methyl-aspartate) receptors and K channels and to mediate clustering of these channels in the postsynaptic membrane. In myeloid cells of the immune system, such as dendritic cells (DCs), a lack of data exists on the expression or function of DLG2. In cDNA microarray transcriptome analyses, we found DIg2 highly expressed in a subpopulation of plasmacytoid DCs (pDCs) stimulated to produce type I interferons (IFNs) such as IFN beta. Results: Using RACE- and RT-PCR as well as immunoprecipitation followed by Western blotting we characterised the differential expression of the Dlg2 splice variants in IFN beta-producing pDCs. Besides Dlg2 gamma this cell population expressed a novel short Dlg2 eta transcript we termed Dlg2 eta 3. Our expression data were integrated into information from genome databases to obtain a novel and comprehensive overview of the mouse Dlg2 gene architecture. To elucidate the intracellular localisation pattern of protein isoforms, ectopical expression analysis of fluorescently tagged DLG2 splice variants was performed. Here we found an enrichment of the larger isoform DLG2 alpha 1 at the plasma membrane while the newly identified shorter (DLG2 eta) isoform as well as DLG2 gamma were equally distributed throughout the cytoplasm. Additionally, DLG2 eta was also found in the nucleus. Analysis of Dlg2-knockout mice previously generated by deleting exon 9 surprisingly revealed that the protein for the novel DLG2 eta isoform was still expressed in the brain and in bone marrow-derived pDCs from mice carrying the homozygous deletion (DIg2(Delta E9/)(Delta E9)). Conclusion: We describe a novel splice variant of the mouse DIg2 gene termed Dlg2 eta and define the differential expression pattern of DLG2 isoforms in IFN beta-producing pDCs. The presence of DLG2 eta protein in the CNS of DIg2(Delta E9/Delta E9) mice might influence the phenotype of these mice and has to be taken into account in the interpretation of results regarding the functional role of DLG2 in neuronal postsynaptic membranes.