Stabilizing effects of geraniol on native and pathogenic M39R rhodopsin variants

Mutations in rhodopsin are a major cause of autosomal dominant retinitis pigmentosa, frequently due to protein misfolding and reduced structural stability. The M39^1.34R variant, located in transmembrane helix 1, is associated with sector retinitis pigmentosa and exhibits pronounced thermal and chemical destabilization.

We investigated whether geraniol, a natural monoterpenoid alcohol with reported cytoprotective properties, can act as a pharmacological stabilizer of rhodopsin. Using purified pigments from bovine rod outer segments and recombinant wild-type and M39^1.34R variants, we assessed photochemical integrity, thermal denaturation, hydroxylamine susceptibility, regeneration kinetics and Meta II fluorescence responses in the presence or absence of geraniol.

Geraniol did not interfere with chromophore binding, photobleaching, regeneration, or acid-induced Schiff base protonation. However, it significantly enhanced the thermal and chemical stability of rod outer segments-derived and recombinant wild-type rhodopsin. The M39^1.34R mutant displayed severe thermal instability and increased susceptibility to hydroxylamine, but geraniol treatment nearly doubled its thermal half-life and improved chromophore retention, although without restoring wild-type stability levels. Remarkably, geraniol induced rapid and marked quenching of Trp265^6.48 fluorescence in Meta II, indicating state-dependent interaction with the activated receptor.

These findings identify geraniol as a membrane-interacting small molecule capable of partially stabilizing structurally compromised rhodopsin variants. While incomplete, the rescue achieved supports the potential of monoterpenoid-based pharmacological chaperones for selected rhodopsin-linked retinal degenerations.