Human Molecular Genetics

Human Molecular Genetics 2006 15(Review Issue 2):R235-R243; doi:10.1093/hmg/ddl195

This Article Abstract FREE Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Add to My Personal Archive Download to citation manager Request Permissions Google Scholar Articles by Richard, M. Articles by Cremers, F. P.M. Search for Related Content PubMed PubMed Citation Articles by Richard, M. Articles by Cremers, F. P.M. Social Bookmarking          What's this?

© The Author 2006. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

Towards understanding CRUMBS function in retinal dystrophies

Mélisande Richard^1,*, Ronald Roepman^2,3, Wendy M. Aartsen⁴, Agnes G.S.H. van Rossum⁴, Anneke I. den Hollander², Elisabeth Knust¹, Jan Wijnholds⁴ and Frans P.M. Cremers²

¹ Institut für Genetik, Heinrich Heine Universität Düsseldorf, Universitätsstrasse 1, 40225 Düsseldorf, Germany, ² Department of Human Genetics, Radboud University Nijmegen Medical Centre, PO Box 9101, 6500 HB, Nijmegen, The Netherlands, ³ Nijmegen Centre for Molecular Life Sciences, Nijmegen, The Netherlands and ⁴ Department of Neuromedical Genetics, The Netherlands Institute for Neuroscience, Royal Netherlands Academy of Arts and Sciences (KNAW), Meibergdreef 47, 1105 BA Amsterdam, The Netherlands

^* To whom correspondence should be addresed. Tel: +49 2118113504; Fax: +49 2118112279; Email: melisande.richard{at}uni-duesseldorf.de

Received June 30, 2006; Accepted July 26, 2006

	ABSTRACT

TOP ABSTRACT CRB1 MUTATIONS CAUSE RETINAL... CRB AND ITS ORTHOLOGUES... DROSOPHILA CRB COMPLEX IS... VERTEBRATE CRB COMPLEX IS... CONCLUSION AND PERSPECTIVES REFERENCES

 
Mutations in the Crumbs homologue 1 (CRB1) gene cause autosomal recessive retinitis pigmentosa (arRP) and autosomal Leber congenital amaurosis (arLCA). The crumbs (crb) gene was originally identified in Drosophila and encodes a large transmembrane protein required for maintenance of apico-basal cell polarity and adherens junction in embryonic epithelia. Human CRB1 and its two paralogues, CRB2 and CRB3, are highly conserved throughout the animal kingdom. Both in Drosophila and in vertebrates, the short intracellular domain of Crb/CRB organizes an evolutionary conserved protein scaffold. Several lines of evidence, obtained both in Drosophila and in mouse, show that loss-of-function of crb/CRB1 or some of its intracellular interactors lead to morphological defects and light-induced degeneration of photoreceptor cells, features comparable to those observed in patients lacking CRB1 function. In this review, we describe how understanding Crb complex function in fly and vertebrate retina enhances our knowledge of basic cell biological processes and might lead to new therapeutic approaches for patients affected with retinal dystrophies caused by mutations in the CRB1 gene.

	CRB1 MUTATIONS CAUSE RETINAL DYSTROPHIES

TOP ABSTRACT CRB1 MUTATIONS CAUSE RETINAL... CRB AND ITS ORTHOLOGUES... DROSOPHILA CRB COMPLEX IS... VERTEBRATE CRB COMPLEX IS... CONCLUSION AND PERSPECTIVES REFERENCES

 
Autosomal recessive retinitis pigmentosa (arRP) and autosomal Leber congenital amaurosis (arLCA) are clinically and genetically heterogeneous disorders. ArRP is characterized by a progressive degeneration of photoreceptors. LCA is the earliest and most severe form of all inherited retinal dystrophies, with blindness or severe visual impairment at birth. So far, mutations in 19 genes have been associated with arRP and mutations in nine genes cause arLCA (http://www.sph.uth.tmc.edu/Retnet/), but many more defective genes remain to be identified.

Up to 4% of arRP patients carry mutations in the CRB1 gene (1,2). They experience night blindness from early childhood on and a progressive loss of visual field due to dystrophy of the retina and retinal pigment epithelium (RPE), though the RPE is frequently preserved close to retinal arterioles (3) (Fig. 1). ArRP patients with CRB1 mutations are at risk to develop Coats-like exudative vasculopathy (4), a rare complication of RP characterized by vascular abnormalities, extravascular lipid depositions and retinal detachment. Mutations in the CRB1 gene explain 10–15% of cases with arLCA (4–8).

View larger version (108K): [in this window] [in a new window] [Download PowerPoint slide]   Figure 1. Fundus photograph of a patient with retinitis pigmentosa and mutations in the CRB1 gene. Note the preservation of the RPE adjacent to the retinal arterioles (arrows).

 
There is no clear genotype–phenotype correlation for CRB1 mutations in arRP and arLCA. Environmental factors or genetic modifiers very likely influence the severity of the disease (1). Two recent observations suggest a role for genetic modifiers. First, a few arLCA patients carry a homozygous defect in CRB1, which is causal, together with a heterozygous defect in another gene, which modifies the arLCA phenotype through an additive effect (9). Secondly, the majority of heterozygous carriers of LCA-associated mutations can have functionally detectable, but subclinical defects of visual processes (8,10).

CRB1 mutations found in arRP and arLCA patients exhibit an autosomal recessive inheritance pattern. However, a dominant inheritance pattern was found in one family, in which a CRB1 variant co-segregated with pigmented paravenous chorioretinal atrophy (11).

Using in vivo high-resolution microscopy, it was shown that retinas of arLCA patients carrying CRB1 mutations are remarkably thick and lack the distinct layers of normal adult retina, in contrast to retinal degenerations caused by other mutations. The abnormal retinal architecture resembles that of the immature normal retina, suggesting that disruption of CRB1 function disturbs the development of normal human retinal organization (12).

In contrast to the restricted expression pattern of CRB1 (brain and retina), its paralogues CRB2 and CRB3 are expressed in a wider range of tissues, including the retina (13–15). No disease-causing variants were found in CRB2 and CRB3 in patients with arRP and arLCA so far (14) (Anneke I. den Hollander and Frans P.M. Cremers, unpublished data). Nevertheless, it cannot be ruled out that five CRB2 sequence variants detected in arLCA and arRP patients may be involved in digenic or polygenic disease mechanisms. Alternatively, a more complex clinical phenotype or lethality may be associated with the loss or altered function of CRB2 or CRB3.

	CRB AND ITS ORTHOLOGUES ORGANIZE AN INTRACELLULAR COMPLEX OF SCAFFOLDING PROTEINS

TOP ABSTRACT CRB1 MUTATIONS CAUSE RETINAL... CRB AND ITS ORTHOLOGUES... DROSOPHILA CRB COMPLEX IS... VERTEBRATE CRB COMPLEX IS... CONCLUSION AND PERSPECTIVES REFERENCES

 
Human CRB1, CRB2 and CRB3 are highly conserved throughout the animal kingdom (Fig. 2). Drosophila Crumbs, first characterized (16), contains a short intracellular domain of 37 amino acids exhibiting a C-terminal PDZ (PSD-95, Discs large, ZO-1)-binding motif, a juxtamembrane domain containing a FERM-binding motif (17) and a putative DaPKC phosphorylation motif (18) (Fig. 2A and B). The large extracellular portion includes 30 epidermal growth factor (EGF)-like repeats (some of which can bind calcium) and four laminin A globular domain-like repeats (Fig. 2A). Mammalian CRB1/Crb1 (19) and CRB2/Crb2 (14) show a conserved domain architecture of both the extra and the intracellular domains, whereas CRB3/Crb3 lacks the conserved extracellular domain (13) (Fig. 2A). Different splice variants of CRB1 and CRB2 have been described, encoding proteins lacking the transmembrane and intracellular domains, and are therefore predicted to be secreted (14,20,21) (e.g. CRB1b) (Fig. 2A). In Caenorhabditis elegans, two orthologues are described: CRB1, very similar to DCrb, and CRL-1 (Crumbs like-1) (22). In zebrafish, highly conserved Crumbs orthologues were recently identified: Crb1, Crb2a, Crb2b, Crb3a and Crb3b (23) (Fig. 2B and C).

View larger version (184K): [in this window] [in a new window] [Download PowerPoint slide]   Figure 2. Protein structure and evolutionary conservation of CRB orthologues. (A) Comparison of the major isoforms within different species. The extracellular region contains a signal sequence, EGF-like repeats and laminin A globular domains. The CRB3 variant only contains a signal peptide in its short extracellular domain, whereas C.elegans CRL-1 [a.k.a. EAT-20 (64)] lacks the laminin A-like domains and contains only three EGF-like repeats. To date, this isoform is only present in nematodes. Some splicing variants encode proteins either lacking the transmembrane and intracellular domains (H.s.CRB1b) or even a large part of the extracellular domain (M.m.Crb1s). (B and C) Different variants of CRB proteins could be detected in 18 species using Ensembl (65) and the UCSC Genome Browser database (66). Using a CLUSTALW multiple sequence alignment (67) of the C-terminal 70 amino acids containing the transmembrane and intracellular domain (B), a rooted phylogenetic tree was drawn using DRAWGRAM (68), showing the putative descent of the CRB proteins (C). The small intracellular domains contain a highly conserved C-terminal PDZ-binding motif and a FERM-binding domain (B). Human (Homo sapiens, H.s.), Chimpanzee (Pan troglodytes, P.t.), Rhesus macaque (Macaca mulatta, Ma.m.), Cow (Bos taurus, B.t.), Dog (Canis familiaris, C.f.), Opossum (Monodelphis domestica, M.d.), Rat (Rattus norvegicus, R.n.), Mouse (Mus musculus, M.m.), Chicken (Gallus gallus, G.g.), Frog (Xenopus tropicalis, X.t.), Spotted green pufferfish (Tetraodon nigroviridis, T.n.), Japanese pufferfish (Takifugu rubripes, T.r.), Zebrafish (Danio rerio, D.r.), Honeybee (Apis mellifera, A.f.), Fruit fly (Drosophila melanogaster, D.m.), Malaria mosquito (Anopheles gambiae, A.g.), Roundworm (Caenorhabditis elegans, C.e.), Sea squirt (Ciona intestinalis, C.i).

 
Despite the fact that most arLCA- or arRP-associated mutations in CRB1 reside in the extracellular domain, its function remains to be elucidated. Conversely, the short intracellular domain was found to be a docking site for a highly conserved protein scaffold (Fig. 3). This so-called ‘CRB/Crb complex’ localizes apical to the adherens junctions (AJs) in cells of epithelial origin (these include photoreceptor cells in flies and vertebrates as well as vertebrate Müller glia cells). Several studies highlight its importance in maintenance of epithelial polarity (24–30) as well as its role in morphogenesis and maintenance of the vertebrate and invertebrate retina (described subsequently).

View larger version (18K): [in this window] [in a new window] [Download PowerPoint slide]   Figure 3. Core components of the CRB/Crb complex. The intracellular domain of CRB/Crb (–ERLI) organizes an intracellular complex of proteins in the retina. The black vertical bar represents the cell membrane with the intracellular side on the right. Names of Drosophila proteins are marked in grey, mammalian proteins in black. Note that DPATJ is depicted with its four PDZ domains, whereas mammalian Patj contains 10. Only interactors present both in Drosophila and in mammalian systems have been depicted (see text for details).

 
The C-terminal amino acids of Drosophila Crb (–ERLI), conserved between all known orthologues, specifically bind to the PDZ domain of Stardust, a member of the membrane associated guanylate kinase (MAGUK) protein family (29,30). This interaction is conserved in mammalian cells, where the Stardust orthologue Mpp5/Pals1 (membrane protein palmitoylated 5/Protein associated with Lin-seven-1) was found to bind to mouse Crb1 (31), mouse Crb3 (15) and human CRB2 (Ronald Roepman, unpublished data) (Fig. 3). Human MPP5/PALS1 was the only interactor identified from a yeast two-hybrid screen of a retinal cDNA library, using the intracellular domain of CRB1 as bait (32).

Mpp5/Pals1 and Stardust were found to interact with multiple proteins. Their C- and N-terminal L27 (Lin-2/Lin-7) domains bind, respectively, to the L27 domain of Veli3/Lin-7 (33,34) and to that of the multi-PDZ protein Patj (31) [Pals1-associated tight junction protein, also called protein associated with tight junctions (13)] (Özlem Kempkens and Elisabeth Knust, unpublished data) (Fig. 3).

Using immunoprecipitation from murine retinal lysates, direct in vivo associations have been demonstrated between Mpp5/Pals1 and Mupp1 (a Patj homologue; multiple PDZ domain protein-1) and between Mpp5/Pals1 and the MAGUK protein Mpp3 but not between Mpp5/Pals1 and Mpp4 or Mpp3 and Mpp4, though in vitro data and co-immunoprecipitation experiments suggest that Mpp4 is part of the Crb complex in murine retinas (32,35–38).

Several lines of evidence demonstrate a direct connection between the CRB/Crb complex and the Par6–Par3–aPKC complex, another apically localized protein complex involved in epithelial polarity (18,39–43). It thus seems that the CRB/Crb complex is not a static entity, and it is clear that its constituents can undergo various interactions depending on the cell type or the developmental stage of a given cell.

	DROSOPHILA CRB COMPLEX IS INVOLVED IN RETINAL MORPHOGENESIS AND PROTECTS AGAINST LIGHT-INDUCED RETINAL DEGENERATION

TOP ABSTRACT CRB1 MUTATIONS CAUSE RETINAL... CRB AND ITS ORTHOLOGUES... DROSOPHILA CRB COMPLEX IS... VERTEBRATE CRB COMPLEX IS... CONCLUSION AND PERSPECTIVES REFERENCES

 
Crb complex function was examined in the Drosophila adult eye that consists of a regular array of approximately 750 ommatidia. Each ommatidial unit contains eight photoreceptor cells and 11 accessory cells, e.g. pigment cells (44) (Fig. 4A). The apical side of each photoreceptor is differentiated into a photosensitive rhabdomere, a highly pleated actin-rich array of microvilli, and stalk membranes, located basally to the rhabdomere and apically to the AJs (Fig. 4B and C). The Crb complex localizes to the stalks (Fig. 4C) (42,45–50). Correct localization of Crb at the stalks depends on the function of (i) mbt (mushroom bodies tiny), a protein binding to active monomeric GTPase Cdc42 and probably involved in actin cytoskeleton reorganization (51), and (ii) Moesin (52), an ERM (ezrin–radixin–moesin) protein and potential binding partner for Crb (53). In addition, lack of any component of the Crb complex leads to delocalization and/or dysfunction of the whole complex in the Drosophila eye (42,45,46,48,49).

View larger version (76K): [in this window] [in a new window] [Download PowerPoint slide]   Figure 4. Morphology of the Drosophila compound eye and the vertebrate retina. (A–C) Drosophila. (A and B) Electron microscopic cross-section of a wild-type eye showing several ommatidia separated by pigment cells (A) and one ommatidum displaying seven photoreceptors with their apical specializations, the rhabdomeres (rh) [shown in (B)]. (C) Schematic representation of one ommatidium in cross-section. The apical side of a photoreceptor is differentiated in rhabdomere (green) and stalk membranes (red), where the Crb complex localizes, above the AJ (blue). Adapted from Johnson et al.(47). (D–F) Mouse. (D) Light microscopic picture of retinal layers from a wild-type mouse (technovit section stained with Toluidine blue). Photoreceptor outer and inner segments (OS–IS) localize under the RPE. Basally to the OLM, photoreceptor cell bodies make up the outer nuclear layer (ONL). Phototransduction is integrated through bipolar, amacrine and horizontal cells of the INL and further transmitted to the brain by the ganglion cell layer (69). The confocal image in the inserted frame displays Crb1 protein localization at the OLM. (E) Electron microscopic section at the level of the OLM showing AJs between photoreceptor and Müller glia cells. (F) Schematic illustration of the AJs and SAR, where the CRB/Crb complex is localized. v, microvilli; PR, photoreceptor cell; MGC, Müller glia cell; SAR, subapical region.

 
Analysis of the internal structure of adult crb, sdt and DPATJ mutant ommatidia reveals essential roles for the complex in shaping rhabdomeres and maintaining AJ integrity as well as in forming stalk membranes (42,45–50) (Fig. 5A–C). Rhabdomeres of crb or sdt mutant photoreceptors extend only 40–60% of their normal distal to proximal length and are confined to the distal portion of the retina. The shortened rhabdomeres are bulkier and often contact each other (Fig. 5A). These defects are associated with a failure to maintain the integrity of the rapidly expanding AJ in an early phase during photoreceptor morphogenesis, a phenotype also observed in DPATJ synthetic mutants (50).

View larger version (130K): [in this window] [in a new window] [Download PowerPoint slide]   Figure 5. Morphogenetic defects and light-induced mutant crb/Crb1 eye phenotypes. (A–D) Drosophila. Electron microscopic section of a WT (B) or crb11A22 (A and C) adult eye kept in the dark for 7 days. Morphogenetic defects include thicker rhabdomeres [(A), compare with Fig. 4B], as well as shortening of the stalk membranes [compare (B and C); arrows, AJs; sm, stalk membrane]. Upon 7 days of continuous light exposure, photoreceptor cell degeneration is obvious (D). Adapted from Johnson et al.(47). (E and F) Mouse. Light microscopic picture of a section through a 3-month old Crb1–/– mouse retina kept under normal light conditions (12 h dark/12 h light 100 lux) (E) or kept under normal light conditions and exposed for 72 h to continuous light (3000 lux) (F). Arrows show ingression or protrusion of photoreceptors. Significantly more photoreceptors became displaced after light exposure. Adapted from van de Pavert et al.(37).

 
In crb, sdt and DPATJ mutant eyes, stalk membrane length is reduced by 40–50% (45–49) (Fig. 5B and C). Regulation of stalk membrane length seems to be a unique function for members of the Crb complex. The physiological function of this membrane is unknown, although it has been postulated to be a preferred location for endocytosis (45). Interestingly, PTEN mutant eyes display misshapen rhabdomeres, some of them being split and containing supernumerary stalk membranes on the apical surface (54). PTEN codes for a phosphatase involved in phosphoinositide turnover. Whether Crb and its complex play a role in phosphoinositide turnover, and thereby influence membrane turnover, remains to be elucidated.

Under a short-term constant illumination, crb^11A22 mutant photoreceptors show massive degeneration, a phenotype that is also observed in DPATJ mutant flies (47,49) (Fig. 5D). Degeneration involves devolution of rhabdomeric microvilli as well as cytoplasm and nucleoplasm condensation, features reminiscent of apoptosis.

Light-dependent degeneration in Drosophila can result from two classes of mutations (55). In the first class of mutations, an excessive activation of the phototransduction cascade is thought to result in massive Ca²⁺ influx and leads to necrotic death of photoreceptors (56,57). In the second model, complexes between Arrestin (a protein required for phototransduction termination) and an active phosphorylated form of Rhodopsin are abnormally stabilized in the rhabdomeric membrane. These complexes are then removed from the membrane by endocytosis, and apoptotic cell death of photoreceptors is triggered by an unknown pathway (58,59). Crb mutations are likely to induce cell death via the second pathway (47).

	VERTEBRATE CRB COMPLEX IS REQUIRED FOR MAINTENANCE OF AJs AND PREVENTION OF LIGHT-ACCELERATED RETINAL DISORGANIZATION

TOP ABSTRACT CRB1 MUTATIONS CAUSE RETINAL... CRB AND ITS ORTHOLOGUES... DROSOPHILA CRB COMPLEX IS... VERTEBRATE CRB COMPLEX IS... CONCLUSION AND PERSPECTIVES REFERENCES

 
In the vertebrate retina, the photoreceptor cell layer is embedded in the RPE (Fig. 4D). The apical side of each photoreceptor is characterized by a photosensitive outer segment supported by an inner segment, located above the AJ (Fig. 4E and F). The CRB/Crb complex localizes to the outer limiting membrane (OLM) at the subapical region (SAR), apically to the AJs that connect photoreceptor cells with each other and with Müller glia cells (32,35–38,60) (Fig. 4F). Immuno-electron microscopy of murine retinas revealed strong Crb1 immunoreactivity at the SAR in Müller glia cells but hardly in photoreceptors, whereas Crb2, Crb3, Patj, Pals1 and Mupp1 are present in both cell types (61).

There are two Crb1-mutant mouse models. The naturally occurring rd8 mouse has a 1 bp deletion in Crb1, predicted to encode a truncated protein containing the most N-terminal 15 EGF-like repeats plus 47 novel amino acids (60). It is unknown whether the Crb1^rd8 protein retains partial function or degrades immediately. In rd8 eyes, focal regions of retinal disorganization consisting of photoreceptor loss and retinal folds can be visualized as irregular white spots on fundus pictures (37,60). Already in 2-week-old rd8 retinas, the OLM is disrupted. After 4 weeks, loss of AJs (as shown by a fragmented ß-catenin staining) is found throughout the retina, causing structural alterations in both photoreceptors and Müller glia cells (60).

In Crb1^–/– mice, the Crb1 gene was targeted by deleting the promoter region and the first exon encoding the N-terminus of Crb1. Loss of Crb1 expression led to photoreceptor displacement and was accompanied by the appearance of (half-)rosettes due to focal loss of adhesion between photoreceptors and Müller glia cells (37) (Fig. 5E). In affected retinal areas, staining with markers for the SAR and AJs indicated loss of integrity of the OLM (37,60).

Light microscopy revealed changes in length of inner and outer segments in these areas, comparable to that of the rd8 mice (60), the shortened inner segments of crb2b knockdown zebrafish (23) and the shortened rhabdomeres observed in the Drosophila eye (45–47).

In ageing Crb1^–/– mice, giant half-rosettes appear and in some areas the complete outer nuclear layer (ONL), inner nuclear layer (INL) and RPE are eventually lost (37). Eventually, the retinal phenotype is less severe in rd8 than that in Crb1^–/– mice, probably due to the differences in residual function of the secreted Crb1^rd8 protein and/or genetic background. The latter is supported by the observation that the severity of the rd8 mouse phenotype is lower on a C57Bl/6 background (60), whereas the 50% C57Bl/6 background might explain the late onset of the Crb1^–/– phenotype.

As adult Crb1^–/– mice exposed to continuous white light (3000 lux) for 3 days showed significantly more foci of retinal disorganization (37) (Fig. 5F), it is tempting to speculate that light could accelerate retinal degeneration in arRP or arLCA patients carrying mutations in the CRB1 gene.

In vertebrates, structural integrity of the ONL is maintained by the OLM. It is still unknown how alterations in protein function or organization of CRB/Crb complex members could lead to loss of adhesion between Müller glia and photoreceptor cells or between photoreceptors themselves, ultimately leading to a pathological state. RNAi-induced silencing of Mpp5/Pals1 in Müller glia cells from cultured wild-type mouse retinas resulted in delocalization of Crb1, Crb2, Crb3, Mupp1 and Veli3 from the SAR, demonstrating that Mpp5/Pals1 expression is required for correct localization of several Crb complex proteins (61). In zebrafish, the Mpp5/Pals1 orthologue nagie oko (nok) is essential for a proper adhesion of photoreceptors (62). In addition, the zebrafish CRB2 orthologue Crb2a is involved in the oko meduzy (ome) phenotype that resembles the disrupted neuronal patterning of nagie oko (23,63).

	CONCLUSION AND PERSPECTIVES

TOP ABSTRACT CRB1 MUTATIONS CAUSE RETINAL... CRB AND ITS ORTHOLOGUES... DROSOPHILA CRB COMPLEX IS... VERTEBRATE CRB COMPLEX IS... CONCLUSION AND PERSPECTIVES REFERENCES

 
Loss-of-function of members of the CRB/Crb complex in Drosophila and vertebrate retina results in typical architectural disorganization and light-induced degeneration. A more detailed understanding of the extra- and intracellular CRB/Crb protein complexes will enhance our understanding of retinal disease in a significant subset of patients with arRP and arLCA, which will pave the way for studies aimed to restore Crb1/CRB1 function in mutant mouse models and in humans with retinal dystrophies. In this context, introduction of CRB1 into the Müller glia cell network represents a possible therapeutic approach, as demonstrated by proper targeting of human CRB1 at the SAR in primary Crb1^–/– mouse retinas (61).

	ACKNOWLEDGEMENTS

 
We thank Dr L.I. van den Born for providing the fundus photograph of a patient with CRB1 mutations. We thank Ilse Gosens for critical comments on the manuscript and Natalia Bulgakova for helpful discussions. This work was supported in part by an EC grant (QLG3-CT-2002-01266 to F.P.M.C., E.K. and J.W.) and ZonMW-NWO (912-02-018 to F.P.M.C. and J.W.) and grants of the Deutsche Forschungsgemeinschaft to E.K. M.R. was an EMBO fellow (ALTF-2002-322).

Conflict of Interest statement. None declared.

	REFERENCES

TOP ABSTRACT CRB1 MUTATIONS CAUSE RETINAL... CRB AND ITS ORTHOLOGUES... DROSOPHILA CRB COMPLEX IS... VERTEBRATE CRB COMPLEX IS... CONCLUSION AND PERSPECTIVES REFERENCES

 

1. den Hollander A.I., Davis J., van der Velde-Visser S.D., Zonneveld M.N., Pierrottet C.O., Koenekoop R.K., Kellner U., Van den Born L.I., Heckenlively J.R., Hyong C.B., Handford P.A., Roepman R., Cremers F.P. (2004) CRB1 mutation spectrum in inherited retinal dystrophies. Hum. Mutat. 24:355–369.[CrossRef][Web of Science][Medline]

2. Bernal S., Calaf M., Garcia-Hoyos M., Garcia-Sandoval B., Rosell J., Adan A., Ayuso C., Baiget M. (2003) Study of the involvement of the RGR, CRPB1 and CRB1 genes in the pathogenesis of autosomal recessive retinitis pigmentosa. J. Med. Genet. 40:e89.[Free Full Text]

3. Heckenlively J.R. (1982) Preserved para-arteriole retinal pigment epithelium (PPRPE) in retinitis pigmentosa. Br. J. Ophthalmol. 66:26–30.[Abstract/Free Full Text]

4. den Hollander A.I., Heckenlively J.R., van den Born L.I., de Kok Y.J., van der Velde-Visser S.D., Kellner U., Jurklies B., van Schooneveld M.J., Blankenage l.A., Rohrschneider K., et al. (2001) Leber congenital amaurosis and retinitis pigmentosa with Coats-like exudative vasculopathy are associated with mutations in the crumbs homologue 1 (CRB1) gene. Am. J. Hum. Genet. 69:198–203.[CrossRef][Web of Science][Medline]

5. Lotery A.J., Jacobson S.G., Fishman G.A., Weleber R.G., Fulton A.B., Namperumalsamy P., Heon E., Levin A.V., Grover S., Rosenow J.R., et al. (2001) Mutations in the CRB1 gene cause Leber congenital amaurosis. Arch. Ophthalmol. 119:415–420.[Abstract/Free Full Text]

6. Hanein S., Perrault I., Gerber S., Tanguy G., Barbet F., Ducroq D., Calvas P., Dollfus H., Hamel C., Lopponen T., et al. (2004) Leber congenital amaurosis: comprehensive survey of the genetic heterogeneity, refinement of the clinical definition, and genotype–phenotype correlations as a strategy for molecular diagnosis. Hum. Mutat. 23:306–317.[CrossRef][Web of Science][Medline]

7. Galvin J.A., Fishman G.A., Stone E.M., Koenekoop R.K. (2005) Evaluation of genotype–phenotype associations in leber congenital amaurosis. Retina 25:919–929.[CrossRef][Web of Science][Medline]

8. Yzer S., Leroy B.P., De Baere E., de Ravel T.J., Zonneveld M.N., Voesenek K., Kellner U., Ciriano J.P., de Faber J.T., Rohrschneider K., Roepman R., Den Hollander A.I., Cruysberg J.R., Meire F., Casteels I., Van Moll-Ramirez N.G., Allikmets R., Van den Born L.I., Cremers F.P. (2006) Microarray-based mutation detection and phenotypic characterization of patients with Leber congenital amaurosis. Invest. Ophthalmol. Vis. Sci. 47:1167–1176.[Abstract/Free Full Text]

9. Zernant J., Kulm M., Dharmaraj S., den Hollander A.I., Perrault I., Preising M.N., Lorenz B., Kaplan J., Cremers F.P., Maumenee I., et al. (2005) Genotyping microarray (disease chip) for Leber congenital amaurosis: detection of modifier alleles. Invest. Ophthalmol. Vis. Sci. 46:3052–3059.[Abstract/Free Full Text]

10. Galvin J.A., Fishman G.A., Stone E.M., Koenekoop R.K. (2005) Clinical phenotypes in carriers of Leber congenital amaurosis mutations. Ophthalmology 112:349–356.[CrossRef][Web of Science]

11. McKay G.J., Clarke S., Davis J.A., Simpson D.A., Silvestri G. (2005) Pigmented paravenous chorioretinal atrophy is associated with a mutation within the crumbs homolog 1 (CRB1) gene. Invest. Ophthalmol. Vis. Sci. 46:322–328.[Abstract/Free Full Text]

12. Jacobson S.G., Cideciyan A.V., Aleman T.S., Pianta M.J., Sumaroka A., Schwartz S.B., Smilko E.E., Milam A.H., Sheffield V.C., Stone E.M. (2003) Crumbs homolog 1 (CRB1) mutations result in a thick human retina with abnormal lamination. Hum. Mol. Genet. 12:1073–1078.[Abstract/Free Full Text]

13. Lemmers C., Medina E., Delgrossi M.-H., Didier M., Arsanto J.-P., Le Bivic A. (2002) hINAD1/PATJ, a homolog of Discs Lost, interacts with Crumbs and localizes to tight junctions in human epithelial cells. J. Biol. Chem. 277:25408–25415.[Abstract/Free Full Text]

14. van den Hurk J.A., Rashbass P., Roepman R., Davis J., Voesenek K.E., Arends M.L., Zonneveld M.N., van Roekel M.H., Cameron K., Rohrschneider K., et al. (2005) Characterization of the Crumbs homolog 2 (CRB2) gene and analysis of its role in retinitis pigmentosa and Leber congenital amaurosis. Mol. Vision 11:263–273.[Web of Science][Medline]

15. Makarova O., Roh M.H., Liu C.-J., Laurinec S., Margolis B. (2003) Mammalian Crumbs3 is a small transmembrane protein liked to protein associated with Lin-7 (Pals1). Gene 302:21–29.[CrossRef][Web of Science][Medline]

16. Tepass U., Theres C., Knust E. (1990) Crumbs encodes an EGF-like protein expressed on apical membranes of Drosophila epithelial cells and required for organization of epithelia. Cell 61:787–799.[CrossRef][Web of Science][Medline]

17. Klebes A. and Knust E. (2000) A conserved motif in Crumbs is required for E-cadherin localisation and zonula adherens formation in Drosophila. Curr. Biol. 10:76–85.[CrossRef][Web of Science][Medline]

18. Sotillos S., Díaz-Meco M.T., Caminero E., Moscat J., Campuzano S. (2004) DaPKC-dependent phosphorylation of Crumbs is required for epithelial cell polarity in Drosophila. J. Cell Biol. 166:549–557.[Abstract/Free Full Text]

19. den Hollander A.I., ten Brink J.B., de Kok Y.J., van Soest S., van den Born L.I., van Driel M.A., van de Pol D.J., Payne A.M., Bhattacharya S.S., Kellner U., et al. (1999) Mutations in a human homologue of Drosophila crumbs cause retinitis pigmentosa (RP12). Nat. Genet. 23:217–221.[CrossRef][Web of Science][Medline]

20. Katoh M. and Katoh M. (2004) Identification and characterization of Crumbs homolog 2 gene at human chromosome 9q33.3. Int. J. Oncol. 24:743–749.[Web of Science][Medline]

21. Watanabe T., Miyatani S., Katoh I., Kobayashi S., Ikawa Y. (2004) Expression of a novel secretory form (Crb1s) of mouse Crumbs homologue Crb1 in skin development. Biochem. Biophys. Res. Commun. 313:263–270.[CrossRef][Web of Science][Medline]

22. Bossinger O., Klebes A., Segbert C., Theres C., Knust E. (2001) Zonula adherens formation in Caenorhabditis elegans requires dlg-1, the homologue of the Drosophila gene discs large. Dev. Biol. 230:29–42.[CrossRef][Web of Science][Medline]

23. Omori Y. and Malicki J. (2006) oko meduzy and Related crumbs genes are determinants of apical cell features in the vertebrate embryo. Curr. Biol. 23:945–957.

24. Knust E. and Bossinger O. (2002) Composition and formation of intercellular junctions in epithelial cells. Science 298:1955–1995.[Abstract/Free Full Text]

25. Roh M.H., Fan S., Liu C.-J., Margolis B. (2003) The Crumbs3-Pals1 complex participates in the establishement of polarity in mammalian epithelial cells. J. Cell Sci. 116:2895–2906.[Abstract/Free Full Text]

26. Straight S.W., Shin K., Fogg V.C., Fan S., Liu C.J., Roh M., Margolis B. (2004) Loss of PALS1 expression leads to tight junction and polarity defects. Mol. Biol. Cell 15:1981–1990.[Abstract/Free Full Text]

27. Michel D., Arsanto J.P., Massey-Harroche D., Beclin C., Wijnholds J., Le Bivic A. (2005) PATJ connects and stabilizes apical and lateral components of tight junctions in human intestinal cells. J. Cell Sci. 118:4049–4057.[Abstract/Free Full Text]

28. Wodarz A., Hinz U., Engelbert M., Knust E. (1995) Expression of Crumbs confers apical character on plasma membrane domains of ectodermal epithelia of Drosophila. Cell 82:67–76.[CrossRef][Web of Science][Medline]

29. Bachmann A., Schneider M., Grawe F., Theilenberg E., Knust E. (2001) Drosophila Stardust is a partner of Crumbs in the control of epithelial cell polarity. Nature 414:638–643.[CrossRef][Web of Science][Medline]

30. Hong Y., Stronach B., Perrimon N., Jan L.Y., Jan Y.N. (2001) Drosophila Stardust interacts with Crumbs to control polarity of epithelia but not neuroblasts. Nature 414:634–638.[CrossRef][Web of Science][Medline]

31. Roh M.H., Makarova O., Liu C.J., Shin K., Lee S., Laurinec S., Goyal M., Wiggins R., Margolis B. (2002) The Maguk protein, Pals1, functions as an adapter linking mammalian homologues of Crumbs and Discs Lost. J. Cell Biol. 157:161–172.[Abstract/Free Full Text]

32. Kantardzhieva A., Gosens I., Alexeeva S., Punte I.M., Versteeg I., Krieger E., Neefjes-Mol C.A., den Hollander A.I., Letteboer S.J., Klooster J., et al. (2005) MPP5 recruits MPP4 to the CRB1 complex in photoreceptors. Invest. Ophthalmol. Vis. Sci. 46:2192–2201.[Abstract/Free Full Text]

33. Kamberov E., Makarova O., Roh M., Liu A., Karnak D., Straight S., Margolis B. (2000) Molecular cloning and characterization of Pals, proteins associated with mLin-7. J Biol Chem 275:11425–11431.[Abstract/Free Full Text]

34. Bachmann A., Timmer M., Sierralta J., Pietrini G., Gundelfinger E.D., Knust E., Thomas U. (2004) Cell type-specific recruitment of Drosophila Lin-7 to distinct MAGUK-based protein complexes defines novel roles for Sdt and Dlg-S97. J. Cell Sci. 117:1899–1909.[Abstract/Free Full Text]

35. Kantardzhieva A., Alexeeva S., Versteeg I., Wijnholds J. (2006) MPP3 is recruited to the MPP5 protein scaffold at the retinal outer limiting membrane. FEBS J. 273:1152–1165.[CrossRef][Medline]

36. Aartsen W.M., Kantardzhieva A., Klooster J., van Rossum A.G., van de Pavert S.A., Versteeg I., Cardozo B.N., Tonagel F., Beck S.C., Tanimoto N., et al. (2006) Mpp4 recruits Psd95 and Veli3 towards the photoreceptor synapse. Hum. Mol. Genet. 15:1291–1302.[Abstract/Free Full Text]

37. van de Pavert S.A., Kantardzhieva A., Malysheva A., Meuleman J., Versteek I., Levelt C., Klooster J., Geiger S., Seeliger M.W., Rashbass P., et al. (2004) Crumbs homologue 1 is required for maintenance of photoreceptor cell polarization and adhesion during light exposure. J. Cell Sci. 117:4169–4177.[Abstract/Free Full Text]

38. Stohr H., Molday L.L., Molday R.S., Weber B.H., Biedermann B., Reichenbach A., Kramer F. (2005) Membrane-associated guanylate kinase proteins MPP4 and MPP5 associate with Veli3 at distinct intercellular junctions of the neurosensory retina. J. Comp. Neurol. 481:31–41.[CrossRef][Web of Science][Medline]

39. Lemmers C., Michel D., Lane-Guermonprez L., Delgrossi M.-H., Médina E., Arsanto J.-P., Le Bivic A. (2004) CRB3 binds directly to Par6 and regulates the morphogenesis of the tight junctions in mammalian epithelial cells. Mol. Biol. Cell 15:1324–1333.[Abstract/Free Full Text]

40. Hurd T.W., Gao L., Roh M.H., Macara I.G., Margolis B. (2003) Direct interaction of two polarity complexes implicated in epithelial tight junction assembly. Nat. Cell Biol. 5:137–142.[CrossRef][Web of Science][Medline]

41. Wang Q., Hurd T.W., Margolis B. (2004) Tight junction protein Par6 interacts with an evolutionarily conserved region in the amino terminus pf PALS1/Stardust. J. Biol. Chem. 279:30715–30721.[Abstract/Free Full Text]

42. Nam S.-C. and Choi W. (2003) Interaction of Par-6 and Crumbs complexes is essential for photoreceptor morphogenesis in Drosophila. Development 130:4363–4372.[Abstract/Free Full Text]

43. Kempkens O., Medina E., Fernandez-Ballester G., Ozuyaman S., Le Bivic A., Serrano L., Knust E. (2006) Computer modelling in combination with in vitro studies reveals similar binding affinities of Drosophila Crumbs for the PDZ domains of Stardust and DmPar-6. Eur J Cell Biol 85:753–767.[CrossRef][Web of Science][Medline]

44. Wolff T. and Ready D.F. Pattern formation in the Drosophila retina. In Bate M. and Arias A.M. (Eds.). The Development of Drosophila melanogaster, New York Vol. II: pp. 1277–1325.

45. Pellikka M., Tanentzapf G., Pinto M., Smith C., McGlade C.J., Ready D.F., Tepass U. (2002) Crumbs, the Drosophila homologue of human CRB1/RP12, is essential for photoreceptor morphogenesis. Nature 721:143–149.

46. Izaddoost S., Nam S.-C., Bhat M.A., Bellen H.J., Choi W. (2002) Drosophila Crumbs is a positional cue in photoreceptor adherens junctions and rhabdomeres. Nature 720:178–183.

47. Johnson K., Grawe F., Grzeschik N., Knust E. (2002) Drosophila Crumbs is required to inhibit light-induced photoreceptor degeneration. Curr. Biol. 12:1675–1680.[CrossRef][Web of Science][Medline]

48. Hong Y., Ackerman L., Jan L.Y., Jan N. (2003) Distinct roles of Bazooka and Stardust in the specification of Drosophila photoreceptor membrane architecture. Proc. Natl Acad. Sci. USA 100:12712–12717.[Abstract/Free Full Text]

49. Richard M., Grawe F., Knust E. (2006) DPATJ plays a role in retinal morphogenesis and protects against light-dependent degeneration of photoreceptor cells in the Drosophila eye. Dev. Dyn. 235:895–907.[CrossRef][Web of Science][Medline]

50. Nam S.-C. and Choi W. (2006) Domain-specific early and late function of Dpatj in Drosophila photoreceptor cells. Dev. Dyn. 235:1501–1507.[CrossRef][Web of Science][Medline]

51. Schneeberger D. and Raabe T. (2003) Mbt, a Drosophila PAK protein, combines with Cdc42 to regulate photoreceptor cell morphogenesis. Development 130:427–437.[Abstract/Free Full Text]

52. Karagiosis S.A. and Ready D.F. (2004) Moesin contributes an essential structural role in Drosophila photoreceptor morphogenesis. Development 131:725–732.[Abstract/Free Full Text]

53. Medina E., Williams J., Klipfell E., Zarnescu D., Thomas G., Le Bivic A. (2002) Crumbs interacts with moesin and ß_Heavy-spectrin in the apical membrane skeleton of Drosophila. J. Cell Biol. 158:941–951.[Abstract/Free Full Text]

54. Pinal N., Goberdhan D.C., Collinson L., Fujita Y., Cox I.M., Wilson C., Pichaud F. (2006) Regulated and polarized PtdIns(3,4,5)P3 accumulation is essential for apical membrane morphogenesis in photoreceptor epithelial cells. Curr. Biol. 16:140–149.[CrossRef][Web of Science][Medline]

55. Minke B. and Hardie R.C. (2000) Genetic dissection of Drosophila phototransduction. In Stavenga D.G., De Grip W.J., Pugh E.N. Jr (Eds.). Handbook of Biological Physics(Elsevier, Amsterdam) Vol. 3: pp. 449–525.

56. Bentrop J. (1998) Rhodopsin mutations as the cause of retinal degeneration. Classification of degeneration phenotypes in the model system Drosophila melanogaster. Acta Anat. (Basel) 162:85–94.[CrossRef][Web of Science][Medline]

57. Yoon J., Ben-Ami H.C., Hong Y.S., Park S., Strong L.L., Bowman J., Geng C., Baek K., Minke B., Pak W.L. (2000) Novel mechanism of massive photoreceptor degeneration caused by mutations in the trp gene of Drosophila. J Neuroscience 20:649–659.[Abstract/Free Full Text]

58. Alloway P.G., Howard L., Dolph P.J. (2000) The formation of stable rhodopsin–arrestin complexes induces apoptosis and photoreceptor cell degeneration. Neuron 28:129–138.[CrossRef][Web of Science][Medline]

59. Kiselev A., Socolich M., Vinós J., Hardy R.W., Zuker C.S., Ranganathan R. (2000) A molecular pathway for light-dependent photoreceptor apoptosis in Drosophila. Neuron 28:139–152.[CrossRef][Web of Science][Medline]

60. Mehalow A.K., Kameya S., Smith R.S., Hawes N.L., Denegre J.M., Young J.A., Bechtold L., Haider N.B., Tepass U., Heckenlively J.R., et al. (2003) CRB1 is essential for external limiting membrane integrity and photoreceptor morphogenesis in the mammalian retina. Hum Mol Genet 12:2179–2189.[Abstract/Free Full Text]

61. van Rossum A.G., Aartsen W.M., Meuleman J., Klooster J., Malysheva A., Versteeg I., Arsanto J.-P., Le Bivic A., Wijnholds J. (2006) Pals1/Mpp5 is required for correct localization of Crb1 at the sub-apical region in polarized Müller glia cells. Hum. Mol. Genet. in press. http://dx.doi.org/doi:10.1093/hmg/ddl194.

62. Wei X. and Malicki J. (2002) nagie oko, encoding a MAGUK-family protein, is essential for cellular patterning of the retina. Nat Genet 31:150–157.[CrossRef][Web of Science][Medline]

63. Malicki J. and Driever W. (1999) oko meduzy mutations affect neuronal patterning in the zebrafish retina and reveal cell–cell interactions of the retinal neuroepithelial sheet. Development 126:1235–1246.[Abstract]

64. Shibata Y., Fujii T., Dent J.A., Fujisawa H., Takagi S. (2000) EAT-20, a novel transmembrane protein with EGF motifs, is required for efficient feeding in Caenorhabditis elegans. Genetics 154:635–646.[Abstract/Free Full Text]

65. Hubbard T., Andrews D., Caccamo M., Cameron G., Chen Y., Clamp M., Clarke L., Coates G., Cox T., Cunningham F., et al. (2005) Ensembl 2005. Nucleic Acids Res. 33:D447–D453.[Abstract/Free Full Text]

66. Karolchik D., Baertsch R., Diekhans M., Furey T.S., Hinrichs A., Lu Y.T., Roskin K.M., Schwartz M., Sugnet C., Thomas D.J., et al. (2003) The UCSC Genome Browser database. Nucleic Acids Res. 31:51–54.[Abstract/Free Full Text]

67. Thompson J.D., Higgins D.G., Gibson T.J. (1994) CLUSTAL W: improving the sensitivity of progressive multiple sequence alignment through sequence weighting, position-specific gap penalties and weight matrix choice. Nucleic Acids Res. 22:4673–4680.[Abstract/Free Full Text]

68. Felsenstein J. (1989) PHYLIP—phylogeny inference package (version 3.2). Cladistics 5:164–166.

69. Masland R.H. (2001) The fundamental plan of the retina. Nat. Neurosci. 4:877–886.[CrossRef][Web of Science][Medline]

CiteULike    Connotea    Del.icio.us    What's this?

This article has been cited by other articles:

				N. A. Bulgakova and E. Knust The Crumbs complex: from epithelial-cell polarity to retinal degeneration J. Cell Sci., August 1, 2009; 122(15): 2587 - 2596. [Abstract] [Full Text] [PDF]

				N. A. Bulgakova, O. Kempkens, and E. Knust Multiple domains of Stardust differentially mediate localisation of the Crumbs-Stardust complex during photoreceptor development in Drosophila J. Cell Sci., June 15, 2008; 121(12): 2018 - 2026. [Abstract] [Full Text] [PDF]

				J. A. Davis, P. A. Handford, and C. Redfield The N1317H Substitution Associated with Leber Congenital Amaurosis Results in Impaired Interdomain Packing in Human CRB1 Epidermal Growth Factor-like (EGF) Domains J. Biol. Chem., September 28, 2007; 282(39): 28807 - 28814. [Abstract] [Full Text] [PDF]

				I. Gosens, E. van Wijk, F. F.J. Kersten, E. Krieger, B. van der Zwaag, T. Marker, S. J.F. Letteboer, S. Dusseljee, T. Peters, H. A. Spierenburg, et al. MPP1 links the Usher protein network and the Crumbs protein complex in the retina Hum. Mol. Genet., August 15, 2007; 16(16): 1993 - 2003. [Abstract] [Full Text] [PDF]

				S. Berger, N. A. Bulgakova, F. Grawe, K. Johnson, and E. Knust Unraveling the Genetic Complexity of Drosophila stardust During Photoreceptor Morphogenesis and Prevention of Light-Induced Degeneration Genetics, August 1, 2007; 176(4): 2189 - 2200. [Abstract] [Full Text] [PDF]

				D. Massey-Harroche, M.-H. Delgrossi, L. Lane-Guermonprez, J.-P. Arsanto, J.-P. Borg, M. Billaud, and A. Le Bivic Evidence for a molecular link between the tuberous sclerosis complex and the Crumbs complex Hum. Mol. Genet., March 1, 2007; 16(5): 529 - 536. [Abstract] [Full Text] [PDF]

This Article Abstract FREE Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Add to My Personal Archive Download to citation manager Request Permissions Google Scholar Articles by Richard, M. Articles by Cremers, F. P.M. Search for Related Content PubMed PubMed Citation Articles by Richard, M. Articles by Cremers, F. P.M. Social Bookmarking          What's this?

Online ISSN 1460-2083 - Print ISSN 0964-6906

Copyright © 2009 Oxford University Press

Oxford Journals Oxford University Press

• Site Map
• Privacy Policy
• Frequently Asked Questions

Other Oxford University Press sites: Oxford University Press Oxford Journals China Oxford Journals Japan American National Biography Booksellers' Information Service Children's Fiction and Poetry Children's Reference Corporate & Special Sales Dictionaries Dictionary of National Biography Digital Reference English Language Teaching Higher Education Textbooks Humanities International Education Unit Law Medicine Music Online Products Oxford English Dictionary Reference Rights and Permissions Science School Books Social Sciences Very Short Introductions World's Classics