Polypyrimidine tract-binding protein is critical for the turnover and subcellular distribution of CD40 ligand mRNA in CD4+ T cells.
CD40L (CD154) is regulated at the posttranscriptional level by an activation-induced process that results in a highly stable transcript at extended times of T cell activation. Transcript stability is mediated by polypyrimidine tract-binding protein (PTB)-containing complexes (complex I and II) that bind to three adjacent CU-rich sequences within the 3' untranslated region. To assess the role of PTB in the expression and distribution of CD40L mRNA, PTB was targeted using short hairpin RNA in both primary T cells and a T cell line that recapitulates the stability phase of regulated CD40L mRNA decay. PTB knockdown resulted in a marked decrease in the mRNA stability that resulted in lowered CD40L surface expression. PTB was also critical for appropriate distribution of CD40L mRNA between the nucleus and cytoplasm and in the cytoplasm between the cytosol and the translating polysomes. The activation-induced formation of PTB-specific ribonucleoprotein complexes was observed only with cytoplasmic and not nuclear PTB indicating functional differences in the protein defined by cellular localization. Finally, we observed that cytoplasmic and nuclear PTB isoforms were differentially modified relative to each other and that the changes in cytoplasmic PTB were consistent with activation-induced phosphorylation. Together this work suggests that differentially modified PTB regulates CD40L expression at multiple steps by 1) retaining CD40L mRNA in the nucleus, 2) directly regulating mRNA stability at late times of activation, and 3) forming a ribonuclear complex that preferentially associates with translating ribosomes thus leading to an enhanced level of CD40L protein.
Department of Cell Biology and Neuroscience, Rutgers University, Piscataway, NJ 08854, USA.
Rodrigo
R
Matus-Nicodemos
Stefano
S
Vavassori
Moraima
M
Castro-Faix
Anibal
A
Valentin-Acevedo
Karnail
K
Singh
Valentina
V
Marcelli
Lori R
LR
Covey
D015496:CD4-Positive T-Lymphocytes / Q000276:immunology* / Q000378:metabolism*; D023201:CD40 Ligand / Q000235:genetics* / Q000378:metabolism*; D002478:Cells, Cultured; D003593:Cytoplasm / Q000235:genetics / Q000378:metabolism; D057809:HEK293 Cells; D006801:Humans; D019169:Jurkat Cells; D009687:Nuclear Proteins / Q000235:genetics / Q000378:metabolism / Q000502:physiology; D038941:Polypyrimidine Tract-Binding Protein / Q000502:physiology*; D020033:Protein Isoforms / Q000235:genetics / Q000378:metabolism / Q000502:physiology; D021381:Protein Transport / Q000235:genetics / Q000276:immunology; D020871:RNA Stability / Q000235:genetics / Q000276:immunology; D012333:RNA, Messenger / Q000378:metabolism*; D013347:Subcellular Fractions / Q000378:metabolism; D014158:Transcription, Genetic / Q000276:immunology
D009687:Nuclear Proteins; D020033:Protein Isoforms; D012333:RNA, Messenger; D038941:Polypyrimidine Tract-Binding Protein; D023201:CD40 Ligand
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