Graphene Biosensor Differentiating Sensitive Interactions between Ribonucleic Acid and Dipeptide Repeats in Liquid–Liquid Phase Separation

Liquid-Liquid Phase Separation (LLPS) plays a crucial role in cell biology and is closely associated with neurodegenerative diseases like Amyotrophic Lateral Sclerosis (ALS) and Frontotemporal Dementia (FTD). Recent studies connect mutations in the C9ORF72 gene to the production of arginine-rich dipeptide repeat proteins (R-DPRs), such as poly(PR) and poly(GR). These R-DPRs disrupt LLPS in membrane-less organelles (MLOs) and contribute to disease pathology. While traditional analysis techniques like nuclear magnetic resonance (NMR), fluorescence recovery after photobleaching (FRAP), and Förster resonance energy transfer (FRET) provide insights into LLPS's role in these diseases, their ability is limited in detecting weak intermolecular interactions within LLPS droplets. This study employs graphene field-effect transistors (GFETs) for their superior sensitivity in detecting these molecular interactions. We immobilized RNA (poly-A) on GFETs and measured the electrical conductivity of GFETs to characterize shifts in the voltage of the charge neutral point in GFETs, allowing for the detection of dipeptide repeat peptides, such as (PR)₁₂, (GR)₁₂, and R₁₂. Our results show that interactions between peptides and RNA require a specific peptide concentration threshold and vary between peptide types. Notably, the minimal conductivity shift suggests that peptides containing proline residues exhibit a nonuniform spatial distribution during interactions with RNA on graphene surfaces. This finding indicates that peptide rigidity induced by prolines plays a vital role in these molecular interactions and their multivalent contacts with RNA, which agrees with findings reported in other recent works. The capability of GFETs to detect these interactions at nanomolar concentrations marks a significant advancement in sensitivity over existing methods. This research sheds light on the mechanisms of LLPS involving R-DPRs and opens avenues for further understanding of related neurodegenerative diseases.