CaF2 nanoparticles as surface carriers of GCAP1, a calcium sensor protein involved in retinal dystrophies

CaF2-based nanoparticles (NP) are promising biocompatible tools for nanomedicine applications. The structure of the NP crystal lattice allows for specific interactions with Ca(2+)-binding proteins through their EF-hand cation binding motifs. Here we investigated the interaction of 23 nm citrate-coated CaF2 NP with a calcium sensor protein GCAP1 that is normally expressed in photoreceptor cells and involved in the regulation of the early steps of vision. Protein-NP interactions were thoroughly investigated for the wild type (WT) GCAP1 as well as for a variant carrying the Asp 100 to Glu mutation (D100E), which prevents the binding of Ca(2+) to the highest affinity site and is linked to cone dystrophy. Circular dichroism and fluorescence spectroscopy showed that protein structure and Ca(2+)-sensing capability are conserved for both variants upon interaction with the NP surface, although the interaction mode depends on the specific occupation of Ca(2+)-binding sites. NP binding stabilizes the structure of the bound GCAP1 and occurs with nanomolar affinity, as probed by isothermal titration calorimetry. Surface plasmon resonance revealed a fully reversible binding compatible with physiologically relevant kinetics of protein release whereas biochemical assays indicated a residual capability for NP-dissociated GCAP1 to regulate the target retinal guanylate cyclase. Our study constitutes a proof of concept that CaF2 NP could be optimized to serve as biologically compatible carriers of high amounts of functional GCAP1 in photoreceptors affected by retinal dystrophies.