TDP-43 phosphorylation by casein kinase iε promotes oligomerization and enhances toxicity in vivo

Dominant mutations in transactive response DNA-binding protein-43 (TDP-43) cause amyotrophic lateral sclerosis. TDP-43 inclusions occur in neurons, glia and muscle in this disease and in sporadic and inherited forms of frontotemporal lobar degeneration. Cytoplasmic localization, cleavage, aggregation and phosphorylation of TDP-43 at the Ser409/410 epitope have been associated with disease pathogenesis. TDP-43 aggregation is not a common feature of mouse models of TDP-43 proteinopathy, and TDP-43 is generally not thought to acquire an amyloid conformation or form fibrils. A number of putative TDP-43 kinases have been identified, but whether any of these functions to regulate TDP-43 phosphorylation or toxicity in vivo is not known. Here,wedemonstrate that human TDP-43^Q331K undergoes cytoplasmic localization and aggregates when misexpressed in Drosophila when compared with wild-type and M337V forms. Coexpression of Q331K with doubletime (DBT), the fly homolog of casein kinase I1 (CKI1), enhances toxicity. There is at best modest basal phosphorylation of misexpressed human TDP-43 in Drosophila, but coexpression with DBT increases Ser409/410 phosphorylation of all TDP-43 isoforms tested. Phosphorylation of TDP-43 in the fly is specific forDBT, as it is not observed using the validated tau kinases GSK-3β, PAR-1/MARK2 or CDK5. Coexpression of DBT with TDP-43^Q331K enhances the formation of high-molecularweight oligomeric species coincidentwith enhanced toxicity, and treatment of recombinant oligomeric TDP-43with rat CKI strongly enhances its toxicity inmammalian cell culture. These data identify CKIE as a potent TDP-43 kinase in vivo and implicate oligomeric species as the toxic entities in TDP-43 proteinopathies.