Triggering receptor expressed on myeloid cell 2 (TREM2) is a surface receptor that, in the central nervous system, is exclusively expressed on microglia. TREM2 variants have been linked to increased risk for neurodegenerative diseases, but the functional effects of microglial TREM2 remain largely unknown. To this end, we investigated TAR-DNA binding protein 43 kDa (TDP-43)-related neurodegenerative disease via viral-mediated expression of human TDP-43 protein (hTDP-43) in neonatal and adult mice or inducible expression of hTDP43 with defective nuclear localization signals in transgenic mice. We found that TREM2 deficiency impaired microglia phagocytic clearance of pathological TDP-43, and enhanced neuronal damage and motor function impairments. Mass cytometry analysis revealed that hTDP-43 induced a TREM2-dependent subpopulation of microglia with high CD11c expression and higher phagocytic ability. Using mass spectrometry, we further demonstrated an interaction between TDP-43 and TREM2, in vitro and in vivo, in hTDP-43-expressing transgenic mouse brains. We computationally identified the region within hTDP-43 that interacts with TREM2 and observed the potential interaction in ALS patient tissues. Our data reveal the novel interaction between TREM2 and TDP-43, highlighting that TDP-43 is a potential ligand for microglial TREM2 and the interaction mediates neuroprotection of microglial TREM2 in TDP-43-related neurodegeneration.

Repeat-associated non-AUG (RAN) translation is a noncanonical translation initiation event that occurs at nucleotiderepeat expansion mutations that are associated with several neurodegenerative diseases, including fragile X-associated tremor ataxia syndrome (FXTAS), ALS, and frontotemporal dementia (FTD). Translation of expanded repeats produces toxic proteins that accumulate in human brains and contribute to disease pathogenesis. Consequently, RAN translation constitutes a potentially important therapeutic target for managing multiple neurodegenerative disorders. Here, we adapted a previously developed RAN translation assay to a high-throughput format to screen 3,253 bioactive compounds for inhibition of RAN translation of expanded CGG repeats associated with FXTAS. We identified five diverse small molecules that dosedependently inhibited CGG RAN translation, while relatively sparing canonical translation. All five compounds also inhibited RAN translation of expanded GGGGCC repeats associated with ALS and FTD. Using CD and native gel analyses, we found evidence that three of these compounds, BIX01294, CP-31398, and propidium iodide, bind directly to the repeat RNAs. These findings provide proof-of-principle supporting the development of selective small-molecule RAN translation inhibitors that act across multiple disease-causing repeats. This work was supported by Veterans Affairs Biomedical Laboratory Research and Development Grants 1I21BX001841 and 1I01BX003231, National Institutes of Health Grants R01NS099280 and R01NS086810, and the Michigan Alzheimer's Disease Center and Protein Folding Disease Initiative (to P. K. T., M. I. I., and A. B. S.). P. K. T. served as a consultant with Denali Therapeutics, and he, K. M. G., and M. G. K. licensed technology through the University of Michigan to Denali Therapeutics that is based on the work published here. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health. This article contains Tables S1-S3 and Figs. S1-S7.

Triggering receptor expressed on myeloid cell 2 (TREM2) is a surface receptor that, in the central nervous system, is exclusively expressed on microglia. TREM2 variants have been linked to increased risk for neurodegenerative diseases, but the functional effects of microglial TREM2 remain largely unknown. To this end, we investigated TAR-DNA binding protein 43 kDa (TDP-43)-related neurodegenerative disease via viral-mediated expression of human TDP-43 protein (hTDP-43) in mice or inducible expression of hTDP43 with defective nuclear localization signals in transgenic mice. We found that TREM2 deficiency impaired microglia phagocytic clearance of pathological TDP-43, and enhanced neuronal damage and motor function impairments. Mass cytometry analysis revealed that hTDP-43 induced a TREM2-dependent subpopulation of microglia with high CD11c expression and higher phagocytic ability. Using mass spectrometry, we further demonstrated an interaction between TDP-43 and TREM2, in vitro and in vivo, in hTDP-43-expressing transgenic mouse brains. We computationally identified the region within hTDP-43 that interacts with TREM2 and observed the potential interaction in ALS patient tissues. Our data reveal the novel interaction between TREM2 and TDP-43, highlighting that TDP-43 is a potential ligand for microglial TREM2 and the interaction mediates neuroprotection of microglial TREM2 in TDP-43-related neurodegeneration.

Arginine-rich dipeptide repeat proteins (R-DPRs), abnormal translational products of a GGGGCC hexanucleotide repeat expansion in C9ORF72 , play a critical role in C9ORF72 -related amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), the most common genetic form of the disorders (c9ALS/FTD). R-DPRs form liquid condensates in vitro, induce stress granule formation in cultured cells, aggregate, and sometimes coaggregate with TDP-43 in postmortem tissue from patients with c9ALS/FTD. However, how these processes are regulated is unclear. Here, we show that loss of poly(ADP-ribose) (PAR) suppresses neurodegeneration in c9ALS/FTD fly models and neurons differentiated from patient-derived induced pluripotent stem cells. Mechanistically, PAR induces R-DPR condensation and promotes R-DPR–induced stress granule formation and TDP-43 aggregation. Moreover, PAR associates with insoluble R-DPR and TDP-43 in postmortem tissue from patients. These findings identified PAR as a promoter of R-DPR toxicity and thus a potential target for treating c9ALS/FTD.