Casein Kinase 1d Phosphorylates TDP-43 and Suppresses Its Function in Tau mRNA Processing

Background: Neurofibrillary tangle aggregated from anomalous hyperphosphorylated tau is a hallmark of Alzheimer's disease (AD). Trans-active response DNA-binding protein of 43 kDa (TDP-43) enhances the instability and exon (E) 10 inclusion of tau mRNA. Cytoplasmic inclusion of hyperphosphorylated TDP-43 in the neurons constitutes the third most prevalent proteinopathy of AD. Casein kinase 1d (CK1d) is elevated in AD brain and phosphorylates TDP-43 in vitro. Objective: To determine the roles of CK1d in phosphorylation, aggregation, and function of TDP-43 in the processing of tau mRNA. Methods: The interaction and colocalization of TDP-43 and CK1d were analyzed by co-immunoprecipitation and immunofluorescence staining. TDP-43 phosphorylation by CK1d was determined in vitro and in cultured cells. RIPA-insoluble TDP-43 aggregates obtained by ultracentrifugation were analyzed by immunoblots. The instability and E10 splicing of tau mRNA were studied by using a reporter of green fluorescence protein tailed with 3'-untranslational region of tau mRNA and a mini-tau gene and analyzed by real-time quantitative PCR and reverse transcriptional PCR. Results: We found that CK1 delta interacted and co-localized with TDP-43. TDP-43 was phosphorylated by CK1d at Ser379, Ser403/404, and Ser409/410 in vitro and in cultured cells, which was mutually enhanced. CK1d overexpression promoted the aggregation of TDP-43 and suppressed its activity in enhancing the instability and E10 inclusion of tau mRNA. Conclusion: CK1d phosphorylates TDP-43, promotes its aggregation, and inhibits its activity in promoting the instability of tau mRNA and inclusion of tau E10. Elevated CK1d in AD brain may contribute to TDP-43 and tau pathologies directly or indirectly.