Exploring the Effect of RNA Binding on TDP‐43 Liquid‐Liquid Phase Separation

Abstract

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease that is characterized by progressive deterioration of nerve cells in the brain and spinal cord. Cytoplasmic aggregates of TDP‐43 have been observed in the majority of ALS cases (~97%). In addition to ALS, TDP‐43 aggregates have been associated with other neurodegenerative disease pathologies such as frontotemporal dementia (FTD) and limbic‐predominant age‐related TDP‐43 encephalopathy (LATE). Besides forming aggregates, TDP‐43 can form liquid‐like droplets termed ‘condensates’. TDP‐43 condensates are a consequence of liquid‐liquid phase separation (LLPS), a physical process in which a solution de‐mixes into two phases, a dense phase, and a dilute phase. Aberrant LLPS of TDP‐43 is shown to be neurotoxic in the disease pathology of ALS/FTD. In recent literature, yeast models that are predisposed to form cytoplasmic aggregates of TDP‐43 have increased viability in comparison to their droplet forming counterparts. This suggests that TDP‐43 aggregates may act as a cellular defense mechanism against toxic liquid‐like condensates. GU‐rich RNAs have displayed binding of TDP‐43 that mitigate neurotoxicity by inhibition of LLPS. Specifically, RNA binding of TDP‐43's RNA recognition motif (RRM) domains. Based on these findings, I focus on the dynamics between RNA binding and TDP‐43 phase separation, and the importance of TDP‐43 RRMs in LLPS. Depending on characteristics of RNAs, RNA binding on TDP‐43 RRMs may enhance the impedance of droplet formation. We can better understand how TDP‐43's RRMs facilitate and stabilize RNA binding interface by the introduction of inhibitory residue mutations on TDP‐43's RRM 1 & 2. Through the screening of TDP‐43 RRM mutants and wild‐type with various RNA sequences, we can reveal how RNA interactions are crucial for preventing neurotoxic droplets.