title: Circadian-clock driven cone-like photoreceptor phagocytosis in the neural retina leucine zipper gene knockout mouse.
abstract: OBJECTIVE
  Whereas much information is available on rod outer segment phagocytosis by the retinal pigmented epithelium (RPE), corresponding data for cones are quite limited, especially in laboratory models of normal rats and mice with very low cone numbers. To characterize the light and circadian control of cone photoreceptor phagocytosis in mice, we capitalized on the blue cone-like phenotype of neural retina leucine zipper gene (Nrl) null mice (Nrl(-/-)).
  
  
  METHODS
  Nrl(-/-) mice were maintained under standard cyclic light (12h:12h light-dark [LD] cycle; light=300 lux) for one month, then divided into two groups: 1) continued maintenance in LD (36 mice); or 2) transferred to constant darkness (DD; 21 mice) for 36 h. Animals were sacrificed every 3 h over 24 h, and their eyes were rapidly enucleated and fixed. Cryosections were stained using specific cone short-wavelength opsin antibodies. Phagosome numbers in the RPE were quantified with a morphometric system. We monitored the expression of c-mer proto-oncogene tyrosine kinase (MerTK) in wild-type and knockout mice using a specific MerTK antibody.
  
  
  RESULTS
  In LD, cone phagocytosis showed a statistically significant peak of activity 1 h after light onset, 2-3 fold higher than at other times. In constant darkness, the temporal phagocytic profile resembled that of LD (significant peak at 1 h of subjective day), but the number of phagosomes was decreased at all time points. Immunostaining of MerTK in wild-type and Nrl(-/-) mice showed expression at the apical surface of the RPE.
  
  
  CONCLUSIONS
  Cone-like outer segment phagocytosis in Nrl(-/-) mice shows a similar profile to that of rods in normal mice and other species. These data are the first to quantify blue cone-like photoreceptor phagocytosis under different lighting conditions in mice, and suggest this model may constitute a valuable system for investigating circadian regulation of cone function.
authors.affiliation: Département de Neurobiologie des Rythmes, CNRS UPR 3212, Institut des Neurosciences Cellulaires et Intégratives, Strasbourg, France.
authors.forename: Arthur
authors.initials: A
authors.lastname: Krigel
authors.forename: Marie-Paule
authors.initials: MP
authors.lastname: Felder-Schmittbuhl
authors.forename: David
authors.initials: D
authors.lastname: Hicks
mesh_terms: D000818:Animals; D050976:Basic-Leucine Zipper Transcription Factors / Q000172:deficiency* / Q000235:genetics* / Q000378:metabolism; D057906:Circadian Clocks / Q000528:radiation effects; D005136:Eye Proteins / Q000235:genetics* / Q000378:metabolism; D007150:Immunohistochemistry; D008027:Light; D051379:Mice; D018345:Mice, Knockout; D010587:Phagocytosis / Q000528:radiation effects; D010588:Phagosomes / Q000378:metabolism / Q000528:radiation effects; D011518:Proto-Oncogene Proteins / Q000378:metabolism; D051381:Rats; D020794:Receptor Protein-Tyrosine Kinases / Q000378:metabolism; D017949:Retinal Cone Photoreceptor Cells / Q000166:cytology* / Q000378:metabolism* / Q000528:radiation effects; D055213:Retinal Pigment Epithelium / Q000166:cytology / Q000201:enzymology / Q000528:radiation effects; D017948:Retinal Rod Photoreceptor Cells / Q000166:cytology / Q000378:metabolism / Q000528:radiation effects; D000076202:c-Mer Tyrosine Kinase
chemical_list: D050976:Basic-Leucine Zipper Transcription Factors; D005136:Eye Proteins; C084884:Nrl protein, mouse; D011518:Proto-Oncogene Proteins; C490767:Mertk protein, mouse; D020794:Receptor Protein-Tyrosine Kinases; D000076202:c-Mer Tyrosine Kinase