Prion diseases are characterized by prion protein (PrP) transmissible aggregation and neurodegeneration, which has been linked to oxidative stress. The physiological function of PrP seems related to sequestering of redox-active Cu ²⁺ , and Cu ²⁺ dyshomeostasis is observed in prion disease brain. It is unclear whether Cu ²⁺ contributes to PrP aggregation, recently shown to be mediated by PrP condensation. This study indicates that Cu ²⁺ promotes PrP condensation in live cells at the cell surface and in vitro through copartitioning. Molecularly, Cu ²⁺ inhibited PrP β-structure and hydrophobic residues exposure. Oxidation, induced by H ₂ O ₂ , triggered liquid-to-solid transition of PrP:Cu ²⁺ condensates and promoted amyloid-like PrP aggregation. In cells, overexpression of PrP ^C initially protected against Cu ²⁺ cytotoxicity but led to PrP ^C aggregation upon extended copper exposure. Our data suggest that PrP condensates function as a buffer for copper that prevents copper toxicity but can transition into PrP aggregation at prolonged oxidative stress.

Abnormal phase transitions have been implicated in the occurrence of proteinopathies. Disordered proteins with nucleic acid binding ability drive the formation of reversible micron-sized condensates capable of controlling nucleic acid processing/transport. This mechanism, achieved via liquid-liquid phase separation (LLPS), underlies the formation of long-studied membraneless organelles (e.g., nucleolus) and various transient condensates formed by driver proteins. The prion protein (PrP) is not a classical nucleic acid-binding protein. However, it binds nucleic acids with high affinity, undergoes nucleocytoplasmic shuttling, contains a long intrinsically disordered region rich in glycines and evenly spaced aromatic residues, among other biochemical/biophysical properties of bona fide drivers of phase transitions. Because of this, our group and others have characterized LLPS of recombinant PrP. In vitro phase separation of PrP is modulated by nucleic acid aptamers, and, depending on the aptamer conformation, the liquid droplets evolve to solid-like species. Herein we discuss recent studies and previous evidence supporting PrP phase transitions. We focus on the central role of LLPS related to PrP physiology and pathology, with a special emphasis on the interaction of PrP with different ligands, such as proteins and nucleic acids, which can play a role in prion disease pathogenesis. Finally, we comment on therapeutic strategies directed at the nonfunctional phase separation that could potentially tackle prion diseases or other protein misfolding disorders.

LMO7 is a multifunctional PDZ–LIM protein that can interact with different molecular partners and is found in several intracellular locations. The aim of this work was to shed light on LMO7 evolution, alternative transcripts, protein structure and gene regulation through multiple in silico analyses. We also explored the intracellular distribution of the LMO7 protein in chicken and zebrafish embryonic skeletal muscle cells by means of confocal fluorescence microscopy. Our results revealed a single LMO7 gene in mammals, sauropsids, Xenopus and in the holostean fish spotted gar while two lmo7 genes (lmo7a and lmo7b) were identified in teleost fishes. In addition, several different transcripts were predicted for LMO7 in human and in major vertebrate model organisms (mouse, chicken, Xenopus and zebrafish). Bioinformatics tools revealed several structural features of the LMO7 protein including intrinsically disordered regions. We found the LMO7 protein in multiple intracellular compartments in chicken and zebrafish skeletal muscle cells, such as membrane adhesion sites and the perinuclear region. Curiously, the LMO7 protein was detected within the nuclei of muscle cells in chicken but not in zebrafish. Our data showed that a conserved regulatory element may be related to muscle-specific LMO7 expression. Our findings uncover new and important information about LMO7 and open new challenges to understanding how the diverse regulation, structure and distribution of this protein are integrated into highly complex vertebrate cellular milieux, such as skeletal muscle cells.

Prion diseases are characterized by prion protein (PrP) transmissible aggregation and toxicity in the brain. The physiological function of PrP seems related to sequestering and internalization of redox-active Cu ²⁺ . It is unclear whether Cu ²⁺ contributes to PrP aggregation, recently shown to be mediated by PrP condensation. We investigated the role of Cu ²⁺ and oxidation in PrP condensation and aggregation using multiple biophysical and biochemical methods. We find that Cu ²⁺ promotes PrP condensation at the cell surface and in vitro through co-partitioning. Molecularly, Cu ²⁺ inhibited PrP β-structure and hydrophobic residues exposure. Oxidation, induced by H ₂ O ₂ , triggered liquid-to-solid transition of PrP:Cu ²⁺ condensates and promoted amyloid-like PrP aggregation. In cells, overexpression of PrP ^C initially protected against Cu ²⁺ cytotoxicity but led to PrP ^C aggregation upon extended copper exposure. Our data suggest that PrP condensates function as a buffer for copper that prevent copper toxicity but can transition into PrP aggregation at prolonged oxidative stress.