Essential role for MFG-E8 as ligand for αvβ5 integrin in diurnal retinal phagocytosis

The integrin receptor alpha v beta 5 controls two independent forms of interactions of the retinal pigment epithelium (RPE) with adjacent photoreceptor outer segments that are essential for vision. alpha v beta 5 localizes specifically to apical microvilli of the RPE and contributes to retinal adhesion that maintains RPE contacts with intact outer segments at all times. Additionally, alpha v beta 5 synchronizes diurnal bursts of RPE phagocytosis that clear photoreceptor outer segment fragments (POS) shed in a circadian rhythm. Dependence of retinal phagocytosis and adhesion on alpha v beta 5 receptors suggests that the extracellular matrix ensheathing RPE microvilli contains ligands for this integrin. Here we studied mice lacking expression of functional MFG-E8 to test the contribution of this integrin ligand to alpha v beta 5 functions in the retina. Lack of MFG-E8 only minimally reduced retinal adhesion. In contrast, lack of MFG-E8, like lack of alpha v beta 5 receptor, eliminated alpha v beta 5 downstream signaling involving the engulfment receptor MerTK and peak POS phagocytosis, both of which follow light onset in wild-type retina. MFG-E8-deficient RPE in primary culture retained normal epithelial morphology and levels of apical alpha v beta 5 receptors, but showed impaired binding and engulfment of isolated POS. Soluble or POS-bound recombinant MFG-E8 was sufficient to fully restore phagocytosis by IMFG-E8-deficient RPE. Furthermore, MFG-E8 supplementation strongly increased POS binding by wild-type and MerTK-deficient RPE, but did not affect POS binding by RPE lacking alpha v beta 5. Thus, MFG-E8 stimulates rhythmic POS phagocytosis by ligating apical alpha v beta 5 receptors of the RPE. These results identify MFG-E8 as the first extracellular ligand in the retina that is essential for diurnal POS phagocytosis.