Introduction

Prion diseases are a rare group of neurodegenerative and transmissible disorders that affect both humans and animals1,2. Human prion diseases include sporadic Creutzfeldt-Jakob disease (sCJD), genetic CJD, Gerstmann-Sträussler-Scheinker syndrome, fatal familial insomnia, variant CJD, and kuru. Animal prion diseases include scrapie in sheep and goats, bovine spongiform encephalopathy in cattle, and chronic wasting disease in cervids. Their defining molecular event is the conformational conversion of host-encoded PrP into assemblies that template further conversion and can acquire infectivity.

No disease-modifying treatment is currently approved or curative, despite evaluation of antibodies, small molecules, and gene-silencing strategies. Genetic and animal-model evidence strongly supports PrP lowering, whereas many aggregation-directed compounds have failed because of inadequate central nervous system exposure, toxicity, suboptimal timing, or insufficient target engagement3,4. Symptomatic prion disease often progresses rapidly, reinforcing the need for intervention at an early molecular stage.

Biomolecular condensates are non-stoichiometric assemblies that can concentrate proteins, nucleic acids, and metabolites without a delimiting membrane. They are typically formed through multivalent weak interactions and can exhibit liquid-like fusion, deformation, and molecular exchange5,6. Because condensates can also mature into gels, glasses, or amyloid-enriched solids, phase behavior provides a plausible mechanism linking reversible protein organization with irreversible aggregation7. This framework is particularly relevant to PrP: its N-terminal region is intrinsically disordered and contains polybasic and aromatic motifs, whereas its C-terminal region is glycosylated and tethered to the outer leaflet of the plasma membrane by a glycosylphosphatidylinositol (GPI) anchor8,9.

Notably, evidence does not support the claim that PrP LLPS is the sole or obligatory source of PrP in vivo. Most direct observations derive from recombinant proteins, engineered truncations, or experimental cell-topology models. Accordingly, this review distinguishes established phase behavior from mechanistic inference and from demonstrated prion infectivity. It aims to define the molecular grammar of PrP condensation, characterize the transition from dynamic droplets to amyloid-competent states, map analytical methods to the readouts they actually provide, and evaluate therapeutic strategies within an explicit evidence hierarchy8,9.

Search Strategy and Study Selection

This structured narrative review was prepared in accordance with the quality domains emphasized by the Scale for the Assessment of Narrative Review Articles (SANRA), including justification of importance, statement of aims, description of the search, appropriate referencing, scientific reasoning, and presentation of relevant data10. PubMed/MEDLINE, PubMed Central, Crossref-indexed bibliographic metadata, and publisher databases were searched from January 2000 to 5 August 2026; seminal earlier work on prion biology and immunotherapy was retained when directly relevant. The core Boolean string was: ("prion protein" OR PrP OR PRNP) AND ("liquid-liquid phase separation" OR LLPS OR condensate* OR coacerv* OR "liquid-to-solid" OR "phase transition"). This was combined with method terms (FRAP, fluorescence correlation spectroscopy, Raman, AFM, electron microscopy, cryo-EM, RT-QuIC, PMCA) and intervention terms (chaperone, Clusterin, Ydj1, aptamer, antibody, antisense oligonucleotide, RNA interference, membrane anchor).

Eligible evidence included peer-reviewed primary studies of mammalian PrP phase behavior, aggregation, or prion propagation; mechanistic studies of PrP domains, membrane topology, ligands, or proteolysis; preclinical therapeutic studies; and authoritative reviews. Studies of artificial prion-like proteins were excluded unless they provided a method or conceptual principle explicitly identified as non-PrP evidence. When a peer-reviewed article was available, corresponding preprints were excluded. Each source cited in Tables 1 and 2 was checked against the methods and findings of the original paper; references were removed when the cited paper did not actually evaluate the stated technology, therapeutic category, or outcome. Additional primary studies were identified by reference-list screening and forward citation chaining. Because the literature was heterogeneous and predominantly mechanistic, no meta-analysis was performed; findings were synthesized narratively.

Molecular and Biophysical Principles of Biomolecular Condensates

Structural Basis and Pathological Progression of PrP Phase Behavior

Domain architecture and molecular interaction grammar

The human PrP is synthesized as a 253-residue precursor. Residues 1-22 form the N-terminal signal peptide that directs entry into the secretory pathway, while the C-terminal GPI-signal sequence (approximately residues 232-253) is removed during maturation. Mature PrP comprises residues 23-231 and is displayed on the extracellular surface of the plasma membrane. Residues 23-120 are predominantly disordered, whereas residues 121-231 form a globular domain containing three alpha-helices, a short antiparallel beta-sheet, a Cys179-Cys214 disulfide bond, and two N-linked glycosylation sites at Asn181 and Asn1979,11.

The disordered N terminus contains two lysine-rich polybasic clusters (residues 23-31 and approximately 100-110), an octarepeat region enriched in regularly spaced tryptophan residues, and a hydrophobic segment extending toward the folded domain. Deletion and substitution studies showed that the N-terminal region is required and sufficient for PrP LLPS under the tested conditions12. More recent work refined this interaction grammar by showing that conserved polybasic motifs and octarepeat tryptophans promote phase separation, consistent with electrostatic and cation-pi contacts. Importantly, the octarepeat contribution was independent of its histidines and of copper binding in that experimental system13. Thus, the octarepeat should not be described as a copper-dependent LLPS switch.

The folded C-terminal domain influences the conformational consequences of condensation. In vitro, LLPS can precede the emergence of protease-resistant, beta-rich conformations in full-length recombinant PrP14. In Y145Stop, loss of most of the globular domain yields a highly charged intrinsically disordered chain that forms dynamic droplets and subsequently self-seeding amyloid fibrils15. These systems show that phase separation can locally concentrate PrP and create an environment conducive to nucleation; they do not, by themselves, establish that the resulting material is equivalent to all infectious PrP strains.

From dynamic condensates to arrested and amyloid-rich assemblies

PrP can progress from reversible condensation to arrested assemblies through several routes. First, disease-associated truncation can remove structural elements that restrain aggregation. Y145Stop provides direct evidence that an intrinsically disordered PrP fragment can transition from LLPS to self-seeding amyloid15. Second, regulated beta-cleavage can generate C2-PrP. In a reconstituted system, beta-cleavage produced immobile C2-PrP aggregates only when full-length PrP had undergone LLPS before proteolysis; C2-PrP remained soluble when cleavage occurred in non-phase-separated PrP17. This context dependence indicates that dense-phase concentration or molecular organization can expose latent aggregation pathways.

Third, nucleic acids and heterotypic protein partners can stabilize or destabilize condensates. A high-affinity DNA aptamer altered PrP phase separation and fibrillation in a sequence- and stoichiometry-dependent manner18. Electrostatic complementarity between the positively charged PrP N terminus and the acidic alpha-synuclein C terminus enables heterotypic condensate formation; RNA can reorganize these assemblies into multiphasic condensates and alter amyloid conversion19. Such findings argue against labeling all condensates as uniformly protective or toxic. Outcome depends on material state, composition, stoichiometry, residence time, and the order of molecular events.

Fourth, cellular stressors can alter PrP concentration, redox state, membrane trafficking, proteolysis, and ligand availability. Oxidative stress, pH shifts, metal dyshomeostasis, and proteostasis failure are therefore plausible modifiers of phase boundaries, although their causal role in endogenous neuronal PrP condensation is less directly established than the recombinant-protein evidence8,9. A rigorous model should therefore distinguish three evidence levels: direct measurements of PrP condensates, orthogonal evidence of amyloid or seeded conversion, and inference from broader condensate biology.

Advanced Techniques for Studying PrP Condensates and Prion Conversion

No single technique simultaneously establishes the liquid-to-solid transition, amyloid structure, and disease-relevant infectivity. Table 1 cross-tabulates each method against the PrP readout that has actually been demonstrated and its principal limitation. This evidence-matched approach avoids attributing methods such as X-ray photon correlation spectroscopy, optogenetic control, proximity labeling, or cryo-electron tomography to studies that did not use those techniques.

Therapeutic Strategies: Direct Condensate Modulation versus Indirect Prion Control

A therapeutically relevant condensate-directed strategy should preserve essential PrP biology, reach the appropriate extracellular or endocytic compartment in the brain, and act before irreversible fibrils predominate. The evidence is therefore most clearly organized by proximity to the proposed mechanism. Direct condensate modifiers have experimentally altered PrP phase behavior, whereas indirect strategies change PrP abundance or prion propagation without demonstrating condensate-specific target engagement (Table 2).

Translational Challenges and Validation Framework

Five interconnected challenges shape translation. First, target engagement must be demonstrated in the relevant material state: a compound that dissolves recombinant droplets may not act on membrane-associated, glycosylated, or seeded assemblies in vivo. Second, selectivity is essential because physiological PrP interactions and proteolytic processing cannot be indiscriminately disrupted. Third, cellular compartmentalization and blood-brain barrier exposure differ substantially among small molecules, antibodies, aptamers, and ASOs. Fourth, biomarkers are needed both for presymptomatic detection and for distinguishing reduced PrP abundance from altered condensate dynamics or reduced seeding activity. Fifth, prion strain and PRNP genotype may influence structural routes to aggregation, making a universally effective phase modifier unlikely3,4,16,18,22,23.

These risks can be addressed through a staged validation pipeline. Candidate interventions should first be assessed with orthogonal phase, mobility, and structural assays, including target-engagement studies using full-length glycosylated or membrane-tethered PrP and disease-linked PrP variants. Promising candidates should then be tested in prion-infected cells and brain-derived seeding assays, followed by pharmacokinetic and efficacy studies in at least two prion models. Biomarkers should include PrP concentration, RT-QuIC lag time or seeding dose, markers of neurodegeneration, and, where feasible, imaging or biochemical indicators of assembly state. This sequence helps prevent a favorable condensation phenotype from being mistaken for genuine anti-prion activity.

Future Directions

Priority experiments should establish whether endogenous PrP forms reversible mesoscale assemblies in neurons, where such assemblies occur, and whether they precede pathological PrP accumulation in infected brain. Artefacts caused by overexpression could be reduced through endogenous tagging, single-particle tracking, super-resolution imaging, spatial proteomics, and rapid fixation or cryogenic methods. Phase diagrams should incorporate membrane attachment, native glycans, lipid composition, pH, redox conditions, and physiologically relevant RNA and protein partners. Finally, causal perturbations, not descriptive colocalization alone, should demonstrate that altering condensate material properties changes prion seeding, neurotoxicity, and survival.

Combination strategies should also be considered during therapeutic development. PrP lowering can reduce substrate; an extracellular chaperone or small molecule may stabilize early assemblies; and an antibody may clear residual pathological species. Such combinations will require careful sequencing and pharmacodynamic biomarkers. The objective is not to relabel all aggregates as condensates, but to identify a reversible, measurable state that lies before irreversible conversion and can be therapeutically targeted.

Conclusion

PrP has an established capacity to undergo LLPS, encoded largely by its N-terminal disordered region and regulated by polybasic motifs, octarepeat tryptophans, heterotypic ligands, proteolysis, and membrane topology. Under defined experimental conditions, condensates can mature into immobile or amyloid-rich assemblies, and chaperones can redirect this trajectory. These findings make phase behavior a plausible mechanistic and therapeutic framework for prion disease. However, condensate formation has not been demonstrated to be an obligatory intermediate for every infectious prion in vivo. An evidence-graded approach therefore remains the most rigorous path forward: dynamic and structural assays should define phase state, seeded-conversion and infection models should establish prion relevance, and methods or therapeutic effects should not be attributed to citations that did not test them. In this framework, condensate biology complements rather than replaces established approaches such as PrP lowering and immunotherapy.

Declarations