Microglial response to neuronal expression of aberrant TDP-43

Abstract

The TAR DNA-binding protein (TDP-43) is a ubiquitous RNA/DNA binding protein encoded by the TARDBP gene. TDP-43 pathology, characterised by the mislocalization of TDP-43 from the nucleus to the cytoplasm and the formation of hyperphosphorylated and ubiquitinated cytoplasmic inclusions/aggregates, is observed in 97% of Amyotrophic lateral sclerosis (ALS) and 50% of frontotemporal dementia (FTD) cases as well as in many other neurodegenerative diseases. There is a growing body of literature suggesting TDP-43 abnormality could interfere with neuronal health through multiple routes. Meanwhile, several recent studies indicated the involvement of microglia in disease progression by taking up pathological TDP-43 from affected neurons. The microglial response to neuronal TDP-43 pathology was investigated. Adeno-assosciated virus (AAV) transduction was used to express human wild type TDP-43 (hTDP-43), cytoplasmic TDP-43 (hTDP-43 ΔNLS) with mutation in nuclear localization sequence and cytoplasmic TDP-43 with RNA binding deficient mutation (hTDP-43 ΔNLS-5FL) in the primary motor cortex (M1) of CX3CR1 and C57BL/6J mice. Microglia morphology analysis revealed that microglia are more amoeboid cell-like in hTDP-43 and hTDP-43 ΔNLS-5FL injected mice compared to control (mCherry) and hTDP-43 ΔNLS injected mice. Using in vivo imaging to assess microglia process motility, it was demonstrated that microglia in hTDP-43 ΔNLS-5FL injected mice are more motile compared to microglia in mCherry-expressing controls. Further, the effect of these injections on motor skills was also investigated employing the pole test and cylinder test. The results from the pole test suggested that hTDP-43 injected mice display some motor dysfunction compared to hTDP-43 ΔNLS injected mice after 4 weeks of neuronal expression. Overall, this study suggests that microglia strongly respond and becoming ‘active’ in the presence of nuclear overexpression of hTDP-43 and to cytoplasmic aggregates of TDP-43 by transforming into amoeboid cell and increasing their process motility. Interestingly, the mere cytoplasmic mislocalization per se seems to not drive microglia reactivity. We thus conclude that the TDP-43 forms, primarily toxic to neurons, are nuclear overexpression and cytoplasmic aggregation.