Defects in protein transport within vertebrate photoreceptors can result in photoreceptor degeneration. In developing and mature photoreceptors, proteins targeted to the outer segment are transported through the connecting cilium via the process of intraflagellar transport (IFT). In studies of vertebrate IFT, mutations in any component of the IFT particle typically abolish ciliogenesis, suggesting that IFT proteins are equally required for IFT. To determine whether photoreceptor outer segment formation depends equally on individual IFT proteins, we compared the retinal phenotypes of IFT57 and IFT88 mutant zebrafish. IFT88 mutants failed to form outer segments, whereas IFT57 mutants formed short outer segments with reduced amounts of opsin. Our phenotypic analysis revealed that IFT57 is not essential for IFT, but is required for efficient IFT. In co-immunoprecipitation experiments from whole-animal extracts, we determined that kinesin II remained associated with the IFT particle in the absence of IFT57, but IFT20 did not. Additionally, kinesin II did not exhibit ATP-dependent dissociation from the IFT particle in IFT57 mutants. We conclude that IFT20 requires IFT57 to associate with the IFT particle and that IFT57 and/or IFT20 mediate kinesin II dissociation.

Genetic analysis in zebrafish has been instrumental in identifying genes necessary for visual system development and function. Recently, a large-scale retroviral insertional mutagenesis screen, in which 315 different genes were mutated, that resulted in obvious phenotypic defects by 5 days postfertilization was completed. That the disrupted gene has been identified in each of these mutants provides unique resource through which the formation, function, or physiology of individual organ systems can be studied. To that end, a screen for visual system mutants was performed on 250 of the mutants in this collection, examining each of them histologically for morphological defects in the eye and behaviorally for overall visual system function. Forty loci whose disruption resulted in defects in eye development and/or visual function were identified. The mutants have been divided into the following phenotypic classes that show defects in: (1) morphogenesis, (2) growth and central retinal development, (3) the peripheral marginal zone, (4) retinal lamination, (5) the photoreceptor cell layer, (6) the retinal pigment epithelium, (7) the lens, (8) retinal containment, and (9) behavior. The affected genes in these mutants highlight a diverse set of proteins necessary for the development, maintenance, and function of the vertebrate visual system. T HE zebrafish has been an important model through apparent by the 18-19 SS. The first postmitotic neurons of the retina are generated at 28 hr postfertilization which genes necessary for visual system development and function have been identified (reviewed in (hpf) and by 72 hpf the retina is functional (Easter and Nicola 1996; Hu and Easter 1999; Schmitt and Easter and Malicki 2002 and Neuhauss 2003). Zebrafish eyes are large, easily accessible, and structurally Dowling 1999). Retinas of many fish and amphibians also possess a specialized region at their margins, termed similar to the human eye. Eye formation in zebrafish is analogous to that observed in other vertebrate embryos, peripheral or ciliary marginal zones, that perpetually adds cells to the retina during the lifetime of the animal thus providing an excellent model system with which the understanding of vertebrate eye development can ( Johns 1977). Several generations of chemically based forward gebe advanced. Additionally, many disrupted genes and pathways identified as integral to the formation of the netic screens have been undertaken in zebrafish (Driever et al. 1996;Haffter et al. 1996; Matsuda and Mishina zebrafish eye produce phenotypes that resemble disorders of the human visual system. Thus, characterization 2004), some of which have focused on eye development and function (Malicki et al. 1996; Fadool et al. 1997; of the molecular mechanisms of eye development in zebrafish should facilitate a better understanding of these hu- Neuhauss et al. 1999). While these chemically based screens have been instrumental in generating interesting man pathologies (Goldsmith and Harris 2003).Eye development in zebrafish ...

Intraflagellar Transport (IFT) refers to a highly conserved process occurring in eukaryotic ciliated structures. In vertebrate photoreceptors, IFT mediates protein trafficking to the outer segments. The IFT particle is a multi-subunit complex and mutations in many individual components causes photoreceptor defects. In zebrafish, mutations in the ift57, ift88 and ift172 genes result in retinal degeneration by 5 days post fertilization (dpf). Although the effects of these mutations on photoreceptor survival have been described, early developmental morphogenesis remains poorly understood. We used transmission electron microscopy and immunohistochemistry to examine these mutants at 60, 72, and 96 hours post fertilization (hpf) and describe early photoreceptor morphogenesis defects.

Cilia formation requires intraflagellar transport (IFT) proteins. Recent studies indicate that mammalian Hedgehog (Hh) signaling requires cilia. It is unclear, however, if the requirement for cilia and IFT proteins in Hh signaling represents a general rule for all vertebrates. Here we examine zebrafish ift57, ift88, and ift172 mutants and morphants for defects in Hh signaling. Although ift57 and ift88 mutants and morphants contained residual maternal protein, the cilia were disrupted. In contrast to previous genetic studies in mouse, mutations in zebrafish IFT genes did not affect the expression of Hh target genes in the neural tube and forebrain and had no quantitative effect on Hh target gene expression. Zebrafish IFT mutants also exhibited no dramatic changes in the craniofacial skeleton, somite formation, or motor neuron patterning. Thus, our data indicate the requirement for cilia in the Hh signal transduction pathway may not represent a universal mechanism in vertebrates.

Choroideremia is an X-linked hereditary retinal degeneration resulting from mutations in the Rab escort protein-1 (REP1). The Rep1 protein facilitates posttranslational modification of Rab proteins, which regulate intracellular trafficking in the retinal pigment epithelium (RPE) and photoreceptors and are likely involved in the removal of outer segment disk membranes by the RPE. A critical question for potential treatment of choroideremia is whether photoreceptor degeneration results from autonomous defects in opsin transport within the photoreceptor or as a nonautonomous and secondary consequence of RPE degeneration. To address this question, we have characterized the retinal pathology in zebrafish rep1 mutants, which carry a recessive nonsense mutation in the REP1 gene. Zebrafish rep1 mutants exhibit degeneration of the RPE and photoreceptors and complete loss of visual function as measured by electroretinograms. In the mutant RPE, photoreceptor outer segment material was not effectively eliminated, and large vacuoles were observed. However, opsin trafficking in photoreceptors occurred normally. Mosaic analysis revealed that photoreceptor degeneration was nonautonomous and required contact with the mutant RPE as mutant photoreceptors were rescued in wild-type hosts and wild-type photoreceptors degenerated in mutant hosts. We conclude that mutations in REP1 disrupt cellular processes in the RPE, which causes photoreceptor death as a secondary consequence. These results suggest that therapies that correct the RPE may successfully rescue photoreceptor loss in choroideremia.retinal degeneration ͉ retinal pigment epithelium C horoideremia (CHM) is an X-linked form of retinal degeneration caused by mutations in the gene for Rab escort protein 1 (Rep1) (1, 2), a protein found in all tissues and highly expressed in the outer retina and retinal pigment epithelium (RPE). CHM causes night blindness in children and progresses to complete loss of vision in adults. CHM is one of the few hereditary blindness disorders that can be clinically identified before significant loss of visual function (3), suggesting that diagnosis and intervention during childhood may prevent further loss of vision.Rep proteins play an essential role in the posttranslational modification of Rab proteins, the small GTP-binding proteins that are essential for many aspects of intracellular transport. Rep proteins bind newly synthesized Rab proteins and facilitate the addition of geranyl-geranyl groups, a modification essential for Rab function in intracellular trafficking (reviewed in ref. 4). In humans, Rep1 and its homolog, Rep2, are ubiquitously expressed and exhibit overlapping substrate specificity (5). Mutations in Rep1 prevent the modification of Rab proteins, thereby disrupting Rab-mediated intracellular trafficking in photoreceptors and the RPE. Because patients with CHM only experience age-related blindness, Rep2 appears to effectively compensate for the loss of Rep1 in all tissues except the eye (6). Interestingly, zebrafish do not contain ...

PurposeJoubert syndrome (JBTS) is an autosomal recessive ciliopathy with considerable phenotypic variability. In addition to central nervous system abnormalities, a subset of JBTS patients exhibit retinal dystrophy and/or kidney disease. Mutations in the AHI1 gene are causative for approximately 10% of all JBTS cases. The purpose of this study was to generate ahi1 mutant alleles in zebrafish and to characterize the retinal phenotypes.MethodsZebrafish ahi1 mutants were generated using transcription activator-like effector nucleases (TALENs). Expression analysis was performed by whole-mount in situ hybridization. Anatomic and molecular characterization of photoreceptors was investigated by histology, electron microscopy, and immunohistochemistry. The optokinetic response (OKR) behavior assay was used to assess visual function. Kidney cilia were evaluated by whole-mount immunostaining.ResultsThe ahi1lri46 mutation in zebrafish resulted in shorter cone outer segments but did not affect visual behavior at 5 days after fertilization (dpf). No defects in rod morphology or rhodopsin localization were observed at 5 dpf. By 5 months of age, cone degeneration and rhodopsin mislocalization in rod photoreceptors was observed. The connecting cilium formed normally and Cc2d2a and Cep290 localized properly. Distal pronephric duct cilia were absent in mutant fish; however, only 9% of ahi1 mutants had kidney cysts by 5 dpf, suggesting that the pronephros remained largely functional.ConclusionsThe results indicate that Ahi1 is required for photoreceptor disc morphogenesis and outer segment maintenance in zebrafish.

No abstract

ESCRTing lenses away from senescence ESCRT proteins control membrane fusion in various key cellular processes, but the mechanisms involved are still incompletely understood. Gulluni et al . report that ESCRT recruitment at the cytokinetic bridge is mediated by the binding of an ESCRT-II subunit to the signaling lipid phosphatidylinositol 3,4-bisphosphate (see the Perspective by Brill and Wilde). This pathway acts in parallel to a known cascade driven by a protein called ALIX, but its failure is sufficient to lead to premature senescence in the lens of fish, mouse, and human eyes, where ALIX is expressed at lower levels. These results point to an evolutionarily conserved pathway for the cell-specific control of cytokinesis that serves to protect from senescence and the early onset of cataracts. —SMH