Unraveling the Molecular Mystery of Retinal Pigment Epithelium Phagocytosis

  • Nora B. Caberoy
  • Wei LiEmail author
Conference paper
Part of the Advances in Experimental Medicine and Biology book series (AEMB, volume 723)


Retinal pigment epithelium (RPE) phagocytosis is essential for clearance of shed photoreceptor outer segments (POS) to prevent retinal degeneration. Despite the importance, our understanding of the molecular mechanisms of RPE phagocytosis is relatively limited. Phagocytosis ligands, receptors, and signaling cascades are traditionally identified on case-by-case basis with challenges. It is even more daunting to identify a new signaling pathway in the absence of any molecular probe. We developed a phagocytosis-based functional cloning strategy with open reading frame (ORF) phage display and identified tubby-like protein 1 (Tulp1) as a new phagocytosis ligand in an unbiased manner. We used Tulp1 as a molecular probe and characterized the well-known MerTK as the receptor of Tulp1. Tulp1 as a genuine MerTK ligand was independently validated by co-immunoprecipitation, MerTK autophosphorylation and MerTK-dependent intracellular signaling. Moreover, Tulp1 was characterized as a bridging molecule with a C-terminal phagocytosis prey-binding domain. These results suggest that Tulp1 is a genuine RPE phagocytosis ligand and that ORF phage display is a valid technology for unbiased identification of phagocytosis ligands, which will further lead to their receptors and signaling cascades. Thus, this new strategy will facilitate the unbiased mapping of molecular mechanisms of RPE and other phagocytes.


Retinal pigment epithelium Phagocytosis Phagocytosis ligand Tubby-like protein 1 Tulp1 MerTK ORF phage display 



We thank Dr. Douglas Graham for technical help with MerTK phosphorylation. This study was supported by NIH R01EY016211, R01EY016211-05S1, P30-EY014801, and an institutional grant from Research to Prevent Blindness.


  1. Banerjee P, Kleyn PW, Knowles JA et al (1998) TULP1 mutation in two extended Dominican kindreds with autosomal recessive retinitis pigmentosa. Nat Genet 18: 177–179PubMedCrossRefGoogle Scholar
  2. Caberoy NB, Li W (2009) Unconventional secretion of tubby and tubby-like protein 1. FEBS Lett 583: 3057–3062PubMedCrossRefGoogle Scholar
  3. Caberoy NB, Maiguel D, Kim Y, Li W (2010a) Identification of tubby and tubby-like protein 1 as eat-me signals by phage display. Exp Cell Res 316: 245–257PubMedCrossRefGoogle Scholar
  4. Caberoy NB, Zhou Y, Jiang X et al (2010b) Efficient identification of tubby-binding proteins by an improved system of T7 phage display. J Mol Recognit 23: 74–83PubMedGoogle Scholar
  5. Caberoy NB, Zhou Y, Li W (2009) Can phage display be used as a tool to functionally identify endogenous eat-me signals in phagocytosis? J Biomol Screen 14: 653–661PubMedCrossRefGoogle Scholar
  6. Caberoy NB, Zhou Y, Li W (2010c) Tubby and tubby-like protein 1 are new MerTK ligands for phagocytosis. EMBO J (in press) Google Scholar
  7. Li W, Caberoy NB (2010) New perspective for phage display as an efficient and versatile technology of functional proteomics. Appl Microbiol Biotechnol 85: 909–919PubMedCrossRefGoogle Scholar
  8. Milam AH, Hendrickson AE, Xiao M et al (2000) Localization of tubby-like protein 1 in developing and adult human retinas. Invest Ophthalmol Vis Sci 41: 2352–2356PubMedGoogle Scholar
  9. Strauss O (2005) The retinal pigment epithelium in visual function. Physiol Rev 85: 845–881PubMedCrossRefGoogle Scholar
  10. Strick DJ, Feng W, Vollrath D (2009) Mertk drives myosin II redistribution during retinal pigment epithelial phagocytosis. Invest Ophthalmol Vis Sci 50: 2427–2435PubMedCrossRefGoogle Scholar

Copyright information

© Springer Science+Business Media, LLC 2012

Authors and Affiliations

  1. 1.Bascom Palmer Eye Institute, Department of OphthalmologyUniversity of Miami Miller School of MedicineMiamiUSA

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