Crumbs homologue 1 is required for maintenance of photoreceptor cell polarization and adhesion during light exposure

Abstract

Loss of Crumbs homologue 1 (CRB1) function causes either the eye disease Leber congenital amaurosis or progressive retinitis pigmentosa, depending on the amount of residual CRB1 activity and the genetic background. Crb1 localizes specifically to the sub-apical region adjacent to the adherens junction complex at the outer limiting membrane in the retina. We show that it is associated here with multiple PDZ protein 1 (Mupp1), protein associated with Lin-7 (Pals1 or Mpp5) and Mpp4. We have produced Crb1-/- mice completely lacking any functional Crb1. Although the retinas are initially normal, by 3-9 months the Crb1-/- retinas develop localized lesions where the integrity of the outer limiting membrane is lost and giant half rosettes are formed. After delamination of the photoreceptor layer, neuronal cell death occurs in the inner and outer nuclear layers of the retina. On moderate exposure to light for 3 days at 3 months of age, the number of severe focal retinal lesions significantly increases in the Crb1-/- retina. Crb2, Crb3 and Crb1 interacting proteins remain localized to the sub-apical region and therefore are not sufficient to maintain cell adhesion during light exposure in Crb1-/- retinas. Thus we propose that during light exposure Crb1 is essential to maintain, but not assemble, adherens junctions between photoreceptors and Müller glia cells and prevents retinal disorganization and dystrophy. Hence, light may be an influential factor in the development of the corresponding human diseases.

Nok plays an essential role in maintaining the integrity of the outer nuclear layer in the zebrafish retina

“…As we show in Figure 6, Crb1 and Crb2 have an identical expression pattern in the zebrafish retina. This identical expression pattern and the fact that the phenotypes of Crb1 knockout mice are much less severe than that of nok mutants (van de Pavert et al, 2004) suggest that Crb1 and Crb2 are functionally redundant in terms of cell adhesion. Second, Crumbs may mediate homophilic adhesion by binding to the same Crumbs protein of neighboring photoreceptors and/or mediate heterophilic adhesion by binding to the extracellular matrix or different surface molecules of neighboring photoreceptors.…”

“…In mammalian MDCK cell line, Crumbs interacts with the PDZ domain of Nok's mammalian homolog Pals1 via the COOH terminus of Crumbs (Roh et al, 2002). The physical interaction between Pals1 and Crb has also been demonstrated in mouse retinal lysate (van de Pavert et al, 2004). Interaction between Crumbs and Stardust, the fly homolog of Nok, is also observed in fly epithelium.…”

“…The Crumbs proteins are good candidates for the mediation for the following reasons: First, Crumbs, a transmembrane protein initially identified in fly that interacts with Stardust, the fly homolog of Nok, has three homologs in vertebrates: Crb1, Crb2, and Crb3 (Tepass and Knust, 1993;Hong, et al, 2001;Makarova et al, 2003;Katoh, 2004;den Hollander, 2001b). Second, Crb1, 2, and 3 have been observed to be expressed in the ONL in the mouse retina and Crb1 physically interacts with Pals1, the mouse homolog of Nok, in retinal lysates (van de Pavert et al, 2004). Third, the extracellular domains of Crumbs contain modules similar to Liminin-A G domain and EGF polypeptide modules that are known to interact with other proteins (Stenflo et al, 2000;Hohenester et al, 1999), likely rendering Crumbs the capability to adhere to other molecules.…”

Targeted ablation of Crb2 in photoreceptor cells induces retinitis pigmentosa

“…The Crumbs proteins are associated through PALS1 with PATJ or MUPP1, forming the core of the Crumbs complex (5 -10). In the developing and adult mouse retina, the apical Crumbs complex resides at the subapical region adjacent to the adherens junctions between the retinal progenitor cells (11) or between the photoreceptors and Müller glial cells (9,12). However, in the adult mouse retina, while CRB2 protein is present in both photoreceptor and Müller glial cells, CRB1 is detected only in Müller glial cells (13).…”

“…In the mouse retina, loss of CRB1 from Müller glial cells results in retinal disorganization and dystrophy limited to one quadrant of the retina (9,14). The severity of the CRB1/Crb1 phenotype is strongly dependent on the genetic background as different mutations cause various retinal phenotypes in human and mice (9,14,15).…”