Molecular interactions between Doxycycline (and derivatives) and cell membrane models

Neurodegenerative diseases, such as Parkinson’s disease, are characterized by the progressive loss of dopaminergic neurons and the formation of abnormal protein aggregates, particularly α-synuclein. Evidence indicates that doxycycline (DOX), in addition to its antimicrobial activity, exhibits neuroprotective potential by modulating neuroinflammation and inhibiting protein aggregation. In parallel, proteins associated with neurodegeneration can form biomolecular condensates through phase separation. These are dynamic structures whose dysregulation is associated with pathological states, including the so-called condensatopathies. In this context, α-synuclein stands out as a relevant target, as its condensates may act as intermediates in the formation of amyloid aggregates. DOX has emerged as a promising candidate for the treatment of neurodegenerative diseases. Structural derivatives of DOX, such as DDOX, have been developed to reduce antibiotic activity while enhancing its neuroprotective effects. Here, we intend to investigate the molecular interactions of DOX and DDOX, with: (1) model cell membranes and (2) disease-associated protein structures, including amyloid fibers and biomolecular condensates. In case (1), we explored whether these molecules alter membrane organization, mimicking their intracellular trafficking and incorporation into cells. We used differential scanning calorimetry (DSC), fluorescence spectroscopy, electron paramagnetic resonance (EPR), and optical and fluorescence microscopy. The results indicate that DOX and DDOX modulate lipid membrane organization by differentially affecting the structural and dynamic properties of lipid membranes, altering membrane thermotropic behavior, packing, hydration, and lipid dynamics. Overall, DDOX promoted more pronounced changes, suggesting a distinct mode of interaction with the lipid bilayer compared with DOX. In case (2), the focus is on determining how the molecules may alter, modulate, and control the assembly, dissolution, and aging processes of condensates associated with neurodegenerative diseases. Preliminary experiments with α-synuclein biomolecular condensates revealed that both compounds affect condensate formation and stability, demonstrating that their modulatory effects are not restricted to lipid membranes.