Development of TDP-43 granule inhibitors as potential amyotrophic lateral sclerosis and frontotemporal lobar degeneration therapies
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The 43 kDa TAR DNA binding protein (TDP-43) has been identified as one of the major proteins that accumulates in the cytoplasm of brain and spinal cord from the patients affected with amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD). Under basal conditions, TDP-43 localizes in nucleus functioning as an RNA binding protein to regulate different aspects of RNA metabolism, such as alternative splicing of messenger RNA. In ALS/FTLD brains and spinal cords, TDP-43 forms well-defined cytoplasmic granules, the behavior very similar to stress granule (SG) proteins, but the mechanisms are poorly understood. To investigate the mechanism of TDP-43 granule formation and to identify potential therapeutic targets by inhibiting the granule formation, our laboratory screened a chemical library of 75,000 compounds using the inducible PC12 cells that express EGFP-tagged wild-type human TDP-43. We used the biological effect of cycloheximide on SGs as a basis for the screen, since it is known to prevent the formation of SGs and TDP-43 granules, pointing to a novel biological pathway that regulates TDP-43 granule formation. One of the candidate compounds, Compound 8 (C8) and its analog C8j dose- dependently decreased the arsenite-induced TDP-43 granules without cytotoxicity, and reduced the protein levels of full-length, truncated, high molecular weight and phosphorylated TDP-43. In addition, we found C8j reduced the phosphorylation at novel, previously unknown Thr103-Ser104 amino acid residues of human TDP-43 under arsenite stress. The phospho-mimetic mutations at these sites induced spontaneous intracellular TDP-43 granules, indicating their regulatory role in TDP-43 granule formation. We also performed a series of gene expression analysis combined with the systems biology algorithm, mode of action by network identification (MNI), to identify the mode of action of C8, and found C8 potentially targets protein metabolism and modification processes to reduce the TDP-43 granules. Our study identified a family of non-cytotoxic chemical compounds that reduces the formation of arsenite-induced TDP-43 granules and their potential mode of action. Furthermore, we identified previously unknown TDP-43 phosphorylation sites Thr103- Ser104 that are involved in the TDP-43 granule formation. We anticipate this study will elucidate the biological pathways regulating TDP-43 aggregation and potential therapeutics for ALS/FTLD-U.