PALS1 Is Essential for Retinal Pigment Epithelium Structure and Neural Retina Stratification

Abstract

The membrane-associated palmitoylated protein 5 (MPP5 or PALS1) is thought to organize intracellular PALS1-CRB-MUPP1 protein scaffolds in the retina that are involved in maintenance of photoreceptor-Müller glia cell adhesion. In humans, the Crumbs homolog 1 (CRB1) gene is mutated in progressive types of autosomal recessive retinitis pigmentosa and Leber congenital amaurosis. However, there is no clear genotype-phenotype correlation for CRB1 mutations, which suggests that other components of the CRB complex may influence the severity of retinal disease. Therefore, to understand the physiological role of the Crumbs complex proteins, especially PALS1, we generated and analyzed conditional knockdown mice for Pals1. Small irregularly shaped spots were detected throughout the PALS1 deficient retina by confocal scanning laser ophthalmoscopy and spectral domain optical coherence tomography. The electroretinography a-and b-wave was severely attenuated in the aged mutant retinas, suggesting progressive degeneration of photoreceptors. The histological analysis showed abnormal retinal pigment epithelium structure, ectopic photoreceptor nuclei in the subretinal space, an irregular outer limiting membrane, half rosettes of photoreceptors in the outer plexiform layer, and a thinner photoreceptor synaptic layer suggesting improper photoreceptor cell layering during retinal development. The PALS1 deficient retinas showed reduced levels of Crumbs complex proteins adjacent to adherens junctions, upregulation of glial fibrillary acidic protein indicative of gliosis, and persisting programmed cell death after retinal maturation. The phenotype suggests important functions of PALS1 in the retinal pigment epithelium in addition to the neural retina.

Targeted ablation of Crb2 in photoreceptor cells induces retinitis pigmentosa

“…Similar defects in the lamination of these cells were also observed in CRB2 null retinas with half of the amount of CRB1 (Crb1 +/2 Crb2Chx10 cKO) (17). These results are in line with (i) previous zebrafish data where Crb2a is required for retinal cell patterning and lamination (30) and (ii) mouse data where the CRB-interacting protein PALS1 affected the correct patterning of photoreceptor cells (31,32).…”

“…Here, we demonstrated that removal of CRB2 from either Müller glial or photoreceptors has similar effect on the apical complexes. This is partially in consonance with observations in PALS1 mutant mice (31,32), and consistent with the epithelial polarity and adhesion defects seen in both the Drosophila Crb mutant and the zebrafish Crb2a (ome) mutant (12,35,36). Short-term depletion of CRB2 in the adult retinas demonstrated that CRB2 is required for maintenance of the retinal structure.…”

AP-2-complex-mediated endocytosis of Drosophila Crumbs regulates polarity by antagonizing Stardust

“…Similarly, vertebrate Pals1 stabilizes Crb1 and Crb3 on the plasma membrane of photoreceptor cells and Madin-Darbine canine kidney (MDCK) epithelial cells in culture (Makarova et al, 2003;Park et al, 2011;Roh et al, 2003;van Rossum et al, 2006). Here, we showed that both Pals1 and the AP-2 complex require the same region of mCrb2 for interaction (Fig.…”

Transgenic Expression of Constitutively Active RAC1 Disrupts Mouse Rod Morphogenesis

“…Establishing and maintaining proper polarity and OLM integrity are factors crucial for photoreceptor morphogenesis. [29][30][31]33,[45][46][47] In mammals, the classical epithelial polarity PAR6 complex (PAR3/PAR6/aPKCk) localizes to the tight junctions in epithelial cells and to the adherens junctions (AJs) in neuroblasts, and it is a central player in the formation of the junctions and polarization in these cell types. 34,39,41,48,49 A recent study in Drosophila photoreceptors demonstrates that PAR6 and aPKCk form a protein complex that colocalizes to the rhabdomere stalk (equivalent to the IS in mammalian photoreceptors) and that PAR3 localizes to the AJs.…”