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

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.