title: Transgenic expression of constitutively active RAC1 disrupts mouse rod morphogenesis.
abstract: OBJECTIVE
  Dominant-active RAC1 rescues photoreceptor structure in Drosophila rhodopsin-null mutants, indicating an important role in morphogenesis. This report assesses the morphogenetic effect of activated RAC1 during mammalian rod photoreceptor development using transgenic mice that express constitutively active (CA) RAC1.
  
  
  METHODS
  Transgenic mice were generated by expressing CA RAC1 under control of the Rhodopsin promoter, and morphological features of the photoreceptors were evaluated by histology, immunohistochemistry, and transmission electron microscopy. Function was evaluated by electroretinography. Potential protein partners of CA RAC1 were identified by co-immunoprecipitation of retinal extracts.
  
  
  RESULTS
  Constitutively active RAC1 expression in differentiating rods disrupted outer retinal lamination as early as postnatal day (P)6, and many photoreceptor cell nuclei were displaced apically into the presumptive subretinal space. These photoreceptors did not develop normal inner and outer segments and had abnormal placement of synaptic elements. Some photoreceptor nuclei were also mislocalized into the inner nuclear layer. Extensive photoreceptor degeneration was subsequently observed in the adult animal. Constitutively active RAC1 formed a complex with the polarity protein PAR6 and with microtubule motor dynein in mouse retina. The normal localization of the PAR6 complex was disrupted in CA RAC1-expressing rod photoreceptors.
  
  
  CONCLUSIONS
  Constitutively active RAC1 had a profound negative effect on mouse rod cell viability and development. Rod photoreceptors in the CA RAC1 retina exhibited a defect in polarity and migration. Constitutively active RAC1 disrupted rod morphogenesis and gave a phenotype resembling that found in the Crumbs mutant. PAR6 and dynein are two potential downstream effectors that may be involved in CA RAC1-mediated defective mouse photoreceptor morphogenesis.
authors.affiliation: Section for Translational Research in Retinal and Macular Degeneration, National Institute on Deafness and Other Communication Disorders, National Institutes of Health, Bethesda, Maryland, United States.
authors.forename: Hongman
authors.initials: H
authors.lastname: Song
authors.forename: Ronald A
authors.initials: RA
authors.lastname: Bush
authors.forename: Camasamudram
authors.initials: C
authors.lastname: Vijayasarathy
authors.forename: Robert N
authors.initials: RN
authors.lastname: Fariss
authors.forename: Sten
authors.initials: S
authors.lastname: Kjellstrom
authors.forename: Paul A
authors.initials: PA
authors.lastname: Sieving
mesh_terms: D000818:Animals; D015153:Blotting, Western; D004247:DNA / Q000235:genetics*; D004596:Electroretinography; D018507:Gene Expression Regulation, Developmental; D007150:Immunohistochemistry; D051379:Mice; D008822:Mice, Transgenic; D046529:Microscopy, Electron, Transmission; D009024:Morphogenesis / Q000235:genetics*; D009479:Neuropeptides / Q000096:biosynthesis / Q000235:genetics*; D010641:Phenotype; D012162:Retinal Degeneration / Q000235:genetics* / Q000378:metabolism / Q000473:pathology; D017948:Retinal Rod Photoreceptor Cells / Q000378:metabolism* / Q000648:ultrastructure; D020830:rac1 GTP-Binding Protein / Q000096:biosynthesis / Q000235:genetics*
chemical_list: D009479:Neuropeptides; C484602:Rac1 protein, mouse; D004247:DNA; D020830:rac1 GTP-Binding Protein
keywords: RAC1; lamination; mouse rod morphogenesis; photoreceptor degeneration; positioning and polarity