Inhibition of RNA lariat debranching enzyme suppresses TDP-43 toxicity in ALS disease models

Abstract

Amyotrophic Lateral Sclerosis (ALS) is a devastating neurodegenerative disease that primarily affects upper and lower motor neurons and results in paralysis and death within 3-5 years of disease onset. Currently there is no cure. Although many factors contribute to disease pathogenesis, TDP-43 aggregation was found to play a major role in the pathogenesis of familial and sporadic cases of ALS and frontotemporal lobar degeneration with ubiquitin-positive inclusions (FTLD-U). TDP-43 is primarily nuclear, but in the disease state it is found in cytoplasmic inclusions in affected brain regions of patients with ALS and FTD. Therefore strategies that will target cytoplasmic TDP-43 and not interfere with essential nuclear functions will be important for designing of therapeutics. To gain insight into disease pathogenesis we used a yeast proteinopathy model system that recapitulates two basic features of the disease: TDP-43 aggregation and toxicity. To define modifiers of TDP-43 mediated toxicity we previously performed a deletion screen in yeast. Here we report the results from this screen and we further focus our studies on a particular gene that has important implications for therapeutics, which is Dbr1. Dbr1 is a lariat debranching enzyme that debranches the intronic lariats that accumulate during splicing in yeast and mammals. Here we show that in the absence of Dbr1 activity intronic lariats accumulate at high levels in the cytoplasm and serve as decoys for TDP-43 aggregates. Since loss of RNA binding capacity of TDP-43 is important for toxicity we propose a model in which cytoplasmic TDP-43 becomes toxic by sequestering RNAs or other RNA binding proteins that are important for cell viability. Importantly, we show that this approach has an effect on TDP-43 mediated toxicity in mammalian cells and primary rat neurons