ABSTRACT
The graded expression of transcription factor interferon regulatory factor 4 (IRF4) regulates B cell development and is critical for plasma cell differentiation. However, the mechanisms, by which IRF4 elicits its crucial tasks, are largely unknown. To characterize the molecular targets of IRF4 in B cells, we established an IRF4-deficient DT40 B cell line. We found that in the absence of IRF4, the expression of several molecules involved in BCR signalling was altered. For example, the expression of B cell adaptor for PI3K (BCAP) was upregulated, whereas the SHIP (SH2-containing Inositol 5?-Phosphatase) expression was downregulated. These molecular unbalances were accompanied by increased BCR-induced calcium signalling, attenuated B cell linker protein (BLNK) and ERK activity and enhanced activity of PI3K/protein kinase B (Akt) pathway. Further, the IRF4-deficient cells showed dramatically diminished cytoskeletal responses to anti-IgM cross-linking. Our results show that IRF4 has an important role in the regulation of BCR signalling and help to shed light on the molecular mechanisms of B cell development and germinal centre response.
Subject(s)
Avian Proteins/metabolism , B-Lymphocytes/physiology , Interferon Regulatory Factors/metabolism , Receptors, Antigen, B-Cell/metabolism , Actin Cytoskeleton/metabolism , Adaptor Proteins, Signal Transducing/genetics , Adaptor Proteins, Signal Transducing/metabolism , Animals , Avian Proteins/genetics , Calcium Signaling/genetics , Cell Line , Chickens , Extracellular Signal-Regulated MAP Kinases/metabolism , Gene Expression Regulation/genetics , Gene Knockout Techniques , Interferon Regulatory Factors/genetics , Intracellular Signaling Peptides and Proteins/metabolism , Oncogene Protein v-akt/metabolism , Phosphatidylinositol 3-Kinases/metabolism , Phosphoric Monoester Hydrolases/genetics , Phosphoric Monoester Hydrolases/metabolism , Phosphorylation/genetics , Protein-Tyrosine Kinases/metabolism , Syk KinaseABSTRACT
The actin cytoskeleton is essential for cellular remodeling and many developmental and morphological processes. Twinfilin is a ubiquitous actin monomer-binding protein whose biological function has remained unclear. We discovered and cloned the Drosophila twinfilin homologue, and show that this protein is ubiquitously expressed in different tissues and developmental stages. A mutation in the twf gene leads to a number of developmental defects, including aberrant bristle morphology. This results from uncontrolled polymerization of actin filaments and misorientation of actin bundles in developing bristles. In wild-type bristles, twinfilin localizes diffusively to cytoplasm and to the ends of actin bundles, and may therefore be involved in localization of actin monomers in cells. We also show that twinfilin and the ADF/cofilin encoding gene twinstar interact genetically in bristle morphogenesis. These results demonstrate that the accurate regulation of size and dynamics of the actin monomer pool by twinfilin is essential for a number of actin-dependent developmental processes in multicellular eukaryotes.
