Volume 10 Number 1 2026
Liquid–Liquid Phase Separation as a Unifying Biophysical Driver of Proteinopathy in ALS and Frontotemporal Dementia: Mechanisms, Molecular Grammar, and Therapeutic Vulnerabilities
Betty Fitriyasti 1* Md Abdur Rahman Biswash2, Md Abu Bakar Siddique3, Md Mostafizur Rahman3, Moushumi Afroza Mou3, Asim Debnath4
Integrative Biomedical Research 10 (1) 1-8 https://doi.org/10.25163/biomedical.10110919
Submitted: 20 November 2025 Revised: 12 January 2026 Accepted: 22 January 2026 Published: 24 January 2026
Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) form a single clinicopathological spectrum united by the cytoplasmic mislocalisation and aggregation of RNA-binding proteins, most notably TDP-43 and FUS, alongside dipeptide-repeat proteins (DPRs) translated from the C9orf72 hexanucleotide expansion. A growing body of biophysical evidence indicates that these pathogenic assemblies do not arise directly from folded, native proteins but instead emerge from an intermediate, reversible state: liquid-liquid phase separation (LLPS). We conducted a narrative-structured synthesis of the primary biophysical literature on LLPS and the liquid-to-solid transition (LST) in ALS/FTD, drawing on in vitro reconstitution, coarse-grained molecular dynamics, nuclear magnetic resonance, single-molecule and cellular imaging studies of TDP-43, FUS, C9orf72-derived DPRs, TIA1, UBQLN2, and MATR3. Sequence-specific determinants-namely intrinsically disordered, low-complexity, and prion-like domains-govern the propensity of each protein to condense. TDP-43 phase separation depends on a conserved, transiently folding α-helix (residues 321-340), whose disruption by ALS mutations abolishes reversible LLPS and favours direct solid-state aggregation. FUS phase behaviour bifurcates according to mutation class, with arginine substitutions freezing static RNA contacts and glycine substitutions accelerating loss of fluidity. C9orf72 DPRs display divergent biophysical grammars: poly-GA undergoes hydrophobic homotypic demixing above a critical length of approximately fifty repeats, whereas arginine-rich poly-GR and poly-PR undergo heterotypic complex coacervation with polyanionic partners above roughly twenty-five repeats, closely matching clinical toxicity thresholds. Across systems, nucleation of amyloid fibrils is preferentially catalysed at the condensate-solvent interface, and chaperones such as Karyopherin-β2 and ubiquitin can reverse aberrant transitions. LLPS constitutes a mechanistic bridge between the genetics and neuropathology of ALS/FTD, and the biophysical rules identified here delineate concrete, druggable nodes-charge neutralisation, interface stabilisation, and chaperone enhancement-for future disease-modifying therapy.
Keywords: liquid–liquid phase separation; amyotrophic lateral sclerosis; frontotemporal dementia; TDP-43; FUS; C9orf72 dipeptide repeat proteins; liquid-to-solid transition
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