Acyl-CoA:wax alcohol acyltransferase 2 modulates the cone visual cycle in mouse retina

The daylight and color vision of diurnal vertebrates depends on cone photoreceptors. The capability of cones to operate and respond to changes in light brightness even under high illumination is attributed to their fast rate of recovery to the ground photosensitive state. This process requires the rapid replenishing of photoisomerized visual chromophore (11-cis-retinal) to regenerate cone visual pigments. Recently, several gene candidates have been proposed to contribute to the cone-specific retinoid metabolism, including acyl-CoA wax alcohol acyltransferase 2 (AWAT2, aka MFAT). Here, we evaluated the role of AWAT2 in the regeneration of visual chromophore by the phenotypic characterization of Awat2^-/- mice. The global absence of AWAT2 enzymatic activity did not affect gross retinal morphology or the rate of visual chromophore regeneration by the canonical RPE65-dependent visual cycle. Analysis of Awat2 expression indicated the presence of the enzyme throughout the murine retina, including the retinal pigment epithelium (RPE) and Müller cells. Electrophysiological recordings revealed reduced maximal rod and cone dark-adapted responses in AWAT2-deficient mice compared to control mice. While rod dark adaptation was not affected by the lack of AWAT2, M-cone dark adaptation both in isolated retina and in vivo was significantly suppressed. Altogether, these results indicate that while AWAT2 is not required for the normal operation of the canonical visual cycle, it is a functional component of the cone-specific visual chromophore regenerative pathway.