Spectral Tuning in Mammalian Melanopsins

Area of Science:

• Molecular biology
• Vision science
• Biophysics

Background:

• Melanopsin, a G-protein coupled receptor, mediates crucial visual and non-visual behaviors in mammals.
• Human melanopsin's peak sensitivity (λmax) is around 480 nm, a trait presumed common across mammals.
• The molecular basis for melanopsin's spectral sensitivity and its variation is poorly understood.

Purpose of the Study:

• To determine the spectral sensitivity of melanopsins from diverse mammalian species.
• To investigate the molecular mechanisms underlying melanopsin spectral tuning.
• To compare these mechanisms with those found in other opsin families.

Main Methods:

• Heterologous action spectroscopy was employed to measure melanopsin spectral sensitivity.
• Melanopsins from 8 diverse mammalian species were analyzed.
• Engineered mutants of mouse melanopsin were created to probe spectral tuning mechanisms.

Main Results:

• Melanopsin λmax showed limited variation (23 nm) across the tested mammalian species.
• Spectral sensitivity was not strongly predicted by phylogeny, retinal physiology, or lighting niche.
• Mutagenesis altered mouse melanopsin spectral sensitivity, but mechanisms differed from known opsin tuning principles.

Conclusions:

• Mammalian melanopsin spectral sensitivity is highly conserved.
• The molecular mechanisms governing melanopsin spectral tuning are distinct from those in vertebrate visual and invertebrate opsins.
• These findings highlight unique evolutionary constraints on melanopsin function.