Shared and distinct gain-of-function consequences from pathologic cytoplasmic versus nuclear TDP-43

Abstract

While predominantly nuclear localized in healthy cells, TDP-43 can transit between the nucleus and cytoplasm. In frontotemporal lobar degeneration (FTLD-TDP) and amyotrophic lateral sclerosis (ALS), TDP-43 accumulates in hyperphosphorylated cytoplasmic aggregates. However, a subset of patients develops nuclear aggregates. Expression of wild-type TDP-43 in C. elegans neurons results in nuclear protein accumulation and toxic gain of function. To enable comparative study of nuclear and cytoplasmic TDP-43 phenotypes, we generated new models with inactivating mutations in the nuclear localization sequence (NLS) of TDP-43. When compared to nuclear TDP-43 strains, similar protein levels of cytoplasmic TDP-43 cause less neuronal dysfunction in C. elegans . Using RNA sequencing, we determine that transcriptomes are largely unchanged in these models. However, we identify innate immune pathway increases in response to cytoplasmic but not nuclear TDP-43. We find phosphorylation-mediated neurotoxicity of both cytoplasmic and nuclear localized TDP-43 is controlled by the phosphatase calcineurin. We also find that overexpression of the unfolded protein response (UPR) transcription factor XBP-1s worsens outcomes for wild-type TDP-43 animals but improves TDP-43 ΔNLS, suggesting different pathways controlling toxicity and clearance of nuclear versus cytoplasmic TDP-43. Taken together, these data support shared and distinct cellular responses to nuclear versus cytoplasmic TDP-43 localization in an intact animal nervous system. This comparative approach supports a better understanding of human disease and informs therapeutic development for pathological subtypes of TDP-43 proteinopathies.