title: Retinal pigment epithelial cells use a MerTK-dependent mechanism to limit the phagocytic particle binding activity of αvβ5 integrin.
abstract: BACKGROUND
  αvβ5 integrin and Mer tyrosine kinase (MerTK) receptors reside at the apical surface of the retinal pigment epithelium (RPE) in the eye to promote the diurnal, synchronised phagocytosis of shed photoreceptor outer segment fragments (POS) that is critical for vision. Phagocytosis assays studying RPE cells in culture have defined roles for αvβ5 in POS surface binding and for MerTK in engulfment of bound POS. Both receptors have thus far only been studied separately. It is therefore unknown if αvβ5 integrin activity in POS binding is independent of the engulfment function of RPE cells. This study investigates how increasing αvβ5 receptor levels affect POS binding and internalisation by wild-type (wt), αvβ5- or MerTK-deficient RPE.
  
  
  RESULTS
  β5 integrin-green fluorescent protein (β5-GFP) fusion proteins formed heterodimeric receptors with endogenous αv integrin subunits at the apical surface of mouse or rat RPE cells that co-immunoprecipitated focal adhesion kinase and redistributed with bound POS such as endogenous αvβ5 receptors. In β5(-/-)  RPE cells, de novo formation of αvβ5-GFP receptors restored POS binding and internalisation up to, but not, above wt POS uptake levels. In wt RPE cells, increasing levels of αvβ5 surface receptors by over-expressing β5-GFP only moderately stimulated POS binding, even if POS internalisation was inhibited pharmacologically or by lowering incubation temperatures. In contrast, the same increase in αvβ5 receptor levels dramatically enhanced POS binding of RPE cells lacking MerTK. Furthermore, decreasing MerTK expression by RNA interference increased POS binding to endogenous αvβ5 receptors of wt RPE cells.
  
  
  CONCLUSIONS
  Expressing β5-GFP is sufficient to reverse phagocytic deficiencies of RPE cells derived from β5(-/-)  mice, indicating that these cells do not irreversibly lose other components of the phagocytic machinery. RPE cells expressing the engulfment receptor MerTK control POS binding by limiting activity of endogenous αvβ5 and αvβ5-GFP integrins, although they reside at the apical, phagocytic surface. In contrast, RPE cells permanently or transiently losing MerTK expression lack this regulatory mechanism and bind excess POS via surface αvβ5 receptors. Taken together, these data reveal a novel feedback mechanism that restricts binding of POS to surface αvβ5 integrin receptors in RPE cells.
authors.affiliation: Department of Ophthalmology, Margaret M. Dyson Vision Research Institute, Weill Medical College of Cornell University, New York, NY 10065, USA.
authors.forename: Emeline F
authors.initials: EF
authors.lastname: Nandrot
authors.forename: Kathryn E
authors.initials: KE
authors.lastname: Silva
authors.forename: Christina
authors.initials: C
authors.lastname: Scelfo
authors.forename: Silvia C
authors.initials: SC
authors.lastname: Finnemann
mesh_terms: D000818:Animals; D002417:Cattle; D002460:Cell Line; D002478:Cells, Cultured; D051416:Focal Adhesion Protein-Tyrosine Kinases / Q000032:analysis / Q000378:metabolism; D017353:Gene Deletion; D005786:Gene Expression Regulation; D049452:Green Fluorescent Proteins / Q000032:analysis / Q000378:metabolism; D006801:Humans; D039641:Integrin beta Chains / Q000032:analysis / Q000235:genetics / Q000378:metabolism; D051379:Mice; D010587:Phagocytosis; D011518:Proto-Oncogene Proteins / Q000235:genetics / Q000378:metabolism*; D051381:Rats; D020794:Receptor Protein-Tyrosine Kinases / Q000235:genetics / Q000378:metabolism*; D019030:Receptors, Vitronectin / Q000032:analysis / Q000235:genetics / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000032:analysis / Q000378:metabolism; D055214:Retinal Photoreceptor Cell Outer Segment / Q000378:metabolism*; D055213:Retinal Pigment Epithelium / Q000166:cytology* / Q000378:metabolism; D013552:Swine; D000076202:c-Mer Tyrosine Kinase
chemical_list: D039641:Integrin beta Chains; D011518:Proto-Oncogene Proteins; D019030:Receptors, Vitronectin; D011993:Recombinant Fusion Proteins; C064660:integrin alphaVbeta5; C084344:integrin beta5; D049452:Green Fluorescent Proteins; C490767:Mertk protein, mouse; D020794:Receptor Protein-Tyrosine Kinases; D000076202:c-Mer Tyrosine Kinase; D051416:Focal Adhesion Protein-Tyrosine Kinases