Regulation of biological phase separation by molecular chaperones and related factors in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disorder characterized by the selective accumulation of abnormal protein assemblies in motor neurons. Increasing evidence indicates that dysregulated biological phase separation contributes to the aberrant condensation and aggregation of RNA-binding proteins such as FUS and TDP-43. Although previous studies have emphasized phase separation and aggregation mechanisms, less attention has been given to molecular chaperones and related regulatory systems that preserve condensate homeostasis in ALS. Under physiological conditions, low-complexity (LC) domains enable dynamic and reversible condensate formation. In ALS, these assemblies progressively lose reversibility and transition toward more stable states prone to aggregation. Condensate behavior is shaped not only by the intrinsic properties of LC domains but also by multiple layers of cellular regulation. Molecular chaperones such as HSP70 family proteins, nuclear import receptors including Kapβ2, and enzymatic components of the proteostasis network preserve condensate reversibility, promote assembly remodeling, and limit pathological hardening and aggregation. These systems therefore act as key regulators of phase transitions and condensate homeostasis in ALS. Disruption of these regulatory systems, including the impairment of nuclear import pathways by arginine-rich dipeptide repeat proteins derived from C9ORF72 repeat expansions, further promotes aberrant condensation and aggregation. Recent studies also suggest that zinc finger domains may provide an additional regulatory layer by recognizing LC domain polymers and constraining their growth. This mechanism supports a model in which compact structured domains function as modulators of higher-order assemblies and constrain polymer growth and material transitions. Regulation of biological phase transitions and condensate homeostasis appears central to ALS pathogenesis. Molecular chaperones, nuclear import receptors, and proteostasis networks help preserve reversible assemblies and limit pathological aggregation, whereas disruption of these systems promotes toxic maturation of condensates. Recognition of polymer states by structured domains may represent an additional regulatory layer, and clarifying how these mechanisms operate across ALS-associated proteins may inform future therapeutic strategies.