title: Mertk drives myosin II redistribution during retinal pigment epithelial phagocytosis.
abstract: OBJECTIVE
  Mertk is a key phagocytic receptor in the immune, male reproductive, and visual systems. In the retinal pigment epithelium, Mertk is required for the daily ingestion of photoreceptor outer segment (OS) tips. Loss of Mertk function causes retinal degeneration in rats, mice, and humans; however, little is known about the mechanism by which Mertk regulates the ingestion phase of retinal pigment epithelial (RPE) phagocytosis. To address this, the authors sought proteins that associated with Mertk during OS phagocytosis.
  
  
  METHODS
  Lysates of RPE-J cells challenged with OS for various times were immunoprecipitated with Mertk antibody. Potential interacting proteins were identified by mass spectrometry and characterized with confocal microscopy, pharmacologic inhibition, and siRNA knockdown coupled with an in vitro phagocytic assay in primary RPE cells.
  
  
  RESULTS
  Myh9, the non-muscle myosin II-A heavy chain, was enriched in immunoprecipitates from OS-treated samples. Myosin II-A and II-B isoforms exhibited a striking redistribution in wild-type rat primary RPE cells challenged with OS, moving from the cell periphery to colocalize with ingested OS over time. In contrast, myosin II-A redistribution in response to OS was blunted in primary RPE cells from RCS rats, which lack functional Mertk. Wild-type rat primary RPE cells treated with the myosin II-specific inhibitor blebbistatin or myosin II siRNAs exhibited a significant phagocytic defect.
  
  
  CONCLUSIONS
  Mertk mobilizes myosin II from the RPE cell periphery to sites of OS engulfment, where myosin II function is essential for the normal phagocytic ingestion of OS.
authors.affiliation: Department of Genetics, Stanford University, Stanford, California 94305-5120, USA.
authors.forename: David J
authors.initials: DJ
authors.lastname: Strick
authors.forename: Wei
authors.initials: W
authors.lastname: Feng
authors.forename: Douglas
authors.initials: D
authors.lastname: Vollrath
mesh_terms: D000199:Actins / Q000378:metabolism; D000818:Animals; D002417:Cattle; D006576:Heterocyclic Compounds, 4 or More Rings / Q000494:pharmacology; D013058:Mass Spectrometry; D051379:Mice; D008810:Mice, Inbred C57BL; D018613:Microscopy, Confocal; D024507:Nonmuscle Myosin Type IIA / Q000037:antagonists & inhibitors / Q000378:metabolism; D024501:Nonmuscle Myosin Type IIB / Q000037:antagonists & inhibitors / Q000378:metabolism; D010587:Phagocytosis / Q000502:physiology*; D011518:Proto-Oncogene Proteins / Q000502:physiology*; D034741:RNA, Small Interfering / Q000502:physiology; D051381:Rats; D020318:Rats, Long-Evans; D011922:Rats, Mutant Strains; D017207:Rats, Sprague-Dawley; D020794:Receptor Protein-Tyrosine Kinases / Q000502:physiology*; D055213:Retinal Pigment Epithelium / Q000187:drug effects / Q000378:metabolism*; D012374:Rod Cell Outer Segment / Q000378:metabolism*; D024744:Skeletal Muscle Myosins / Q000037:antagonists & inhibitors / Q000378:metabolism*; D014162:Transfection; D000076202:c-Mer Tyrosine Kinase
chemical_list: D000199:Actins; D006576:Heterocyclic Compounds, 4 or More Rings; D011518:Proto-Oncogene Proteins; D034741:RNA, Small Interfering; C472645:blebbistatin; C490765:Mertk protein, rat; D020794:Receptor Protein-Tyrosine Kinases; D000076202:c-Mer Tyrosine Kinase; D024507:Nonmuscle Myosin Type IIA; D024501:Nonmuscle Myosin Type IIB; D024744:Skeletal Muscle Myosins