Establishment of chemically oligomerizable TAR DNA-binding protein-43 which mimics amyotrophic lateral sclerosis pathology in mammalian cells

The Role of Liquid-Liquid Phase Separation in the Accumulation of Pathological Proteins: New Perspectives on the Mechanism of Neurodegenerative Diseases

It is widely accepted that living organisms form highly dynamic membrane-less organelles (MLOS) with various functions through phase separation, and the indispensable role that phase separation plays in the mechanisms of normal physiological functions and pathogenesis is gradually becoming clearer. Pathological aggregates, regarded as hallmarks of neurodegenerative diseases, have been revealed to be closely related to aberrant phase separation. Specific proteins are assembled into condensates and transform into insoluble inclusions through aberrant phase separation, contributing to the development of diseases. In this review, we present an overview of the progress of phase separation research, involving its biological mechanisms and the status of research in neurodegenerative diseases, focusing on five main disease-specific proteins, tau, TDP-43, FUS, α-Syn and HTT, and how exactly these proteins reside within dynamic liquid-like compartments and thus turn into solid deposits. Further studies will yield new perspectives for understanding the aggregation mechanisms and potential therapeutic strategies, and future research directions are anticipated.

Recombinant Full-Length TDP-43 Oligomers Retain Their Ability to Bind RNAs, Are Not Toxic, and Do Not Seed TDP-43 Aggregation in Vitro

TAR DNA-binding protein with 43 kD (TDP-43) is a partially disordered protein that misfolds and accumulates in the brains of patients affected by several neurodegenerative diseases. TDP-43 oligomers have been reported to form due to aberrant misfolding or self-assembly of TDP-43 monomers. However, very little is known about the molecular and structural basis of TDP-43 oligomerization and the toxic properties of TDP-43 oligomers due to several reasons, including the lack of conditions available for isolating native TDP-43 oligomers or producing pure TDP-43 oligomers in sufficient quantities for biophysical, cellular, and in vivo studies. To address these challenges, we developed new protocols to generate different stable forms of unmodified and small-molecule-induced TDP-43 oligomers. Our results showed that co-incubation of TDP-43 with small molecules, such as epigallocatechin gallate (EGCG), dopamine, and 4-hydroxynonenal (4-HNE), increased the production yield of TDP-43 stable oligomers, which could be purified by size-exclusion chromatography. Interestingly, despite significant differences in the morphology and size distribution of the TDP-43 oligomer preparations revealed by transmission electron microscopy (TEM) and dynamic light scattering (DLS), they all retained the ability to bind to nucleotide DNA. Besides, circular dichroism (CD) analysis of these oligomers did not show much difference in the secondary structure composition. Surprisingly, none of these oligomer preparations could seed the aggregation of TDP-43 core peptide 279-360. Finally, we showed that all four types of TDP-43 oligomers exert very mild cytotoxicity to primary neurons. Collectively, our results suggest that functional TDP-43 oligomers can be selectively stabilized by small-molecule compounds. This strategy may offer a new approach to halt TDP-43 aggregation in various proteinopathies.

How can we interpret the relationship between liquid-liquid phase separation and amyotrophic lateral sclerosis?

One of the critical definitions of neurodegenerative diseases is the formation of insoluble intracellular inclusion body. These inclusions are found in various neurodegenerative diseases such as Alzheimer's disease, amyotrophic lateral sclerosis (ALS), Huntington's disease, Parkinson's disease, and frontotemporal dementia (FTD). Each inclusion body contains disease-specific proteins and is also resistant to common detergent treatments. These aggregates are generally ubiquitinated and thus recognized as misfolded by the organism. They are observed in residual neurons at the affected sites in each disease, suggesting a contribution to disease pathogenesis. The molecular mechanisms for the formation of these inclusion bodies remain unclear. Some proteins, such as superoxide dismutase 1 (SOD1) mutant that causes familial ALS, are highly aggregative due to altered folding caused by point mutations. Still, the aggregates observed in neurodegenerative diseases contain wild-type proteins. In recent years, it has been reported that the proteins responsible for neurodegenerative diseases undergo liquid-liquid phase separation (LLPS). In particular, the ALS/FTD causative proteins such as TAR DNA-binding protein 43 kDa (TDP-43) and fused-in-sarcoma (FUS) undergo LLPS. LLPS increases the local concentration of these proteins, and these proteins eventually change their phase from liquid to solid (liquid-solid phase transition) due to abnormal folding during repetitive separation cycles into two phases and recovery to one phase. In addition to the inclusion body formation, sequestration of essential proteins into the LLPS droplets or changes in the LLPS status can directly impair neural functions and cause diseases. In this review, we will discuss the relationship between the LLPS observed in ALS causative proteins and the pathogenesis of the disease and outline potential therapeutic approaches.