Meta-Biol

• Pathways

All-trans-retinol (atROL), the product of the reduction of all-trans-retinal (atRAL) released from rod and cone opsins, needs to be regenerated back to the visual chromophore 11-cis-retinal (11cRAL). For the regenerative steps, rods transports atROL back into the retinal pigment epithelium (RPE) while cones utilise Muller cells. Although interphotoreceptor retinoid-binding protein (RBP3, IRBP) (Fong & Bridges 1988, Fong et al. 1990) is not thought to be required to move all-trans-retinol (atROL) from photoreceptor cells to the retinal pigment epithelium (RPE) or Muller cells, it may function to regulate retinoid trafficking and possibly protect retinoids from biochemical damage. RBP3 is secreted by photoreceptor cells into the interphotoreceptor matrix (IPM), where, being a larger protein (135kDa) than the IPM space, becomes trapped (see mini-review Gonzalez-Fernandez & Ghosh 2008). It is through this space that retinoids move between Muller cells and cone photoreceptor outer segments during the cone retinoid cycle.