TDP-43 and C9orf72-Derived Dipeptide Repeats as DAMPs in ALS
1 Department of Neurodegenerative Diseases and Geriatric Psychiatry, University of Bonn, Bonn, Germany
TDP-43 protein and C9orf72-derived dipeptide repeats (DPRs) are characteristic hallmarks of amyotrophic lateral sclerosis (ALS). TDP-43 exists as monomers and fibrils and is the main component of intraneuronal inclusions in sporadic ALS cases. The majority of familial ALS forms is caused by a repeat expansion in the C9orf72 gene. This leads to the generation of five dipeptide repeats: proline-alanine (PA), glycine-proline (GP), glycine-alanine (GA), glycine-arginine (GR) and proline-arginine (PR).
Here, we explore whether TDP-43 and DPRs act as danger-associated molecular patterns (DAMPs) and activate the NLRP3-inflammasome.
For this purpose, murine and human macrophage cell lines (J774.2 and THP-1), primary microglia and post-mortem tissues from ALS patients were analyzed by ELISA, Western Blot, FACS and immunohistochemistry. Stainings included NLRP3-components, microglial markers and TDP-43. Stimulations were done with TDP-43 and DPR proteins, respectively.
Results show that monomeric, but not fibrillary TDP-43 activates the NLRP3-inflammasome. Toll-like receptor (TLR) blockade inhibits this. TDP-43 protein aggregates and a high density of microglia are found in tissue from ALS patients.
When DPR proteins are used for cell stimulation, GR and PR are taken up strongly by macrophage cells, while GA, PA and GP are not.
We propose that monomeric but not fibrillary TDP-43 acts as a DAMP and induces NLRP3-inflammasome activation. In addition, C9orf72-related GR and PR, but not the other DPR species are taken up by macrophages and have thus a proinflammatory effect. Further research is necessary to fully characterize these effects. In summary our study suggests that ALS associated protein species can drive neuroinflammation.