MPP3 is recruited to the MPP5 protein scaffold at the retinal outer limiting membrane

Human CRB1 and CRB2 form homo- and heteromeric protein complexes in the retina

Crumbs homolog 1 (CRB1) is one of the key genes linked to retinitis pigmentosa and Leber congenital amaurosis, which are characterized by a high clinical heterogeneity. The Crumbs family member CRB2 has a similar protein structure to CRB1, and in zebrafish, Crb2 has been shown to interact through the extracellular domain. Here, we show that CRB1 and CRB2 co-localize in the human retina and human iPSC-derived retinal organoids. In retina-specific pull-downs, CRB1 was enriched in CRB2 samples, supporting a CRB1-CRB2 interaction. Furthermore, novel interactors of the crumbs complex were identified, representing a retina-derived protein interaction network. Using co-immunoprecipitation, we further demonstrate that human canonical CRB1 interacts with CRB1 and CRB2, but not with CRB3, which lacks an extracellular domain. Next, we explored how missense mutations in the extracellular domain affect CRB1-CRB2 interactions. We observed no or a mild loss of CRB1-CRB2 interaction, when interrogating various CRB1 or CRB2 missense mutants in vitro. Taken together, our results show a stable interaction of human canonical CRB2 and CRB1 in the retina.

Not Just Another Scaffolding Protein Family: The Multifaceted MPPs

Membrane palmitoylated proteins (MPPs) are a subfamily of a larger group of multidomain proteins, namely, membrane-associated guanylate kinases (MAGUKs). The ubiquitous expression and multidomain structure of MPPs provide the ability to form diverse protein complexes at the cell membranes, which are involved in a wide range of cellular processes, including establishing the proper cell structure, polarity and cell adhesion. The formation of MPP-dependent complexes in various cell types seems to be based on similar principles, but involves members of different protein groups, such as 4.1-ezrin-radixin-moesin (FERM) domain-containing proteins, polarity proteins or other MAGUKs, showing their multifaceted nature. In this review, we discuss the function of the MPP family in the formation of multiple protein complexes. Notably, we depict their significant role for cell physiology, as the loss of interactions between proteins involved in the complex has a variety of negative consequences. Moreover, based on recent studies concerning the mechanism of membrane raft formation, we shed new light on a possible role played by MPPs in lateral membrane organization.

Research Models and Gene Augmentation Therapy for CRB1 Retinal Dystrophies

Retinitis pigmentosa (RP) and Leber congenital amaurosis (LCA) are inherited degenerative retinal dystrophies with vision loss that ultimately lead to blindness. Several genes have been shown to be involved in early onset retinal dystrophies, including CRB1 and RPE65. Gene therapy recently became available for young RP patients with variations in the RPE65 gene. Current research programs test adeno-associated viral gene augmentation or editing therapy vectors on various disease models mimicking the disease in patients. These include several animal and emerging human-derived models, such as human-induced pluripotent stem cell (hiPSC)-derived retinal organoids or hiPSC-derived retinal pigment epithelium (RPE), and human donor retinal explants. Variations in the CRB1 gene are a major cause for early onset autosomal recessive RP with patients suffering from visual impairment before their adolescence and for LCA with newborns experiencing severe visual impairment within the first months of life. These patients cannot benefit yet from an available gene therapy treatment. In this review, we will discuss the recent advances, advantages and disadvantages of different CRB1 human and animal retinal degeneration models. In addition, we will describe novel therapeutic tools that have been developed, which could potentially be used for retinal gene augmentation therapy for RP patients with variations in the CRB1 gene.

Retinogenesis of the Human Fetal Retina: An Apical Polarity Perspective

The Crumbs complex has prominent roles in the control of apical cell polarity, in the coupling of cell density sensing to downstream cell signaling pathways, and in regulating junctional structures and cell adhesion. The Crumbs complex acts as a conductor orchestrating multiple downstream signaling pathways in epithelial and neuronal tissue development. These pathways lead to the regulation of cell size, cell fate, cell self-renewal, proliferation, differentiation, migration, mitosis, and apoptosis. In retinogenesis, these are all pivotal processes with important roles for the Crumbs complex to maintain proper spatiotemporal cell processes. Loss of Crumbs function in the retina results in loss of the stratified appearance resulting in retinal degeneration and loss of visual function. In this review, we begin by discussing the physiology of vision. We continue by outlining the processes of retinogenesis and how well this is recapitulated between the human fetal retina and human embryonic stem cell (ESC) or induced pluripotent stem cell (iPSC)-derived retinal organoids. Additionally, we discuss the functionality of in utero and preterm human fetal retina and the current level of functionality as detected in human stem cell-derived organoids. We discuss the roles of apical-basal cell polarity in retinogenesis with a focus on Leber congenital amaurosis which leads to blindness shortly after birth. Finally, we discuss Crumbs homolog (CRB)-based gene augmentation.

Yap is essential for retinal progenitor cell cycle progression and RPE cell fate acquisition in the developing mouse eye

Yap functions as a transcriptional regulator by acting together with sequence-specific DNA binding factors and transcription cofactors to mediate cell proliferation in developing epithelial tissues and tumors. An upstream kinase cascade controls nuclear localization and function in response to partially identified exogenous signals, including cell-to-cell contact. Nevertheless, its role in CNS development is poorly understood. In order to investigate Yap function in developing CNS, we characterized the cellular outcomes after selective Yap gene ablation in developing ocular tissues. When Yap was lost, presumptive retinal pigment epithelium acquired anatomical and molecular characteristics resembling those of the retinal epithelium rather than of RPE, including loss of pigmentation, pseudostratified epithelial morphology and ectopic induction of markers for retinal progenitor cells, like Chx10, and neurons, like β-Tubulin III. In addition, developing retina showed signs of progressive degeneration, including laminar folding, thinning and cell loss, which resulted from multiple defects in cell proliferation and survival, and in junction integrity. Furthermore, Yap-deficient retinal progenitors displayed decreased S-phase cells and altered cell cycle progression. Altogether, our studies not only illustrate the canonical function of Yap in promoting the proliferation of progenitors, but also shed new light on its evolutionarily conserved, instructive role in regional specification, maintenance of junctional integrity and precise regulation of cell proliferation during neuroepithelial development.

CRB2 acts as a modifying factor of CRB1-related retinal dystrophies in mice

Mutations in the CRB1 gene lead to retinal dystrophies ranging from Leber congenital amaurosis (LCA) to early-onset retinitis pigmentosa (RP), due to developmental defects or loss of adhesion between photoreceptors and Müller glia cells, respectively. Whereas over 150 mutations have been found, no clear genotype-phenotype correlation has been established. Mouse Crb1 knockout retinas show a mild phenotype limited to the inferior quadrant, whereas Crb2 knockout retinas display a severe degeneration throughout the retina mimicking the phenotype observed in RP patients associated with CRB1 mutations. Crb1Crb2 double mutant retinas have severe developmental defects similar to the phenotype observed in LCA patients associated with CRB1 mutations. Therefore, CRB2 is a candidate modifying gene of human CRB1-related retinal dystrophy. In this study, we studied the cellular localization of CRB1 and CRB2 in human retina and tested the influence of the Crb2 gene allele on Crb1-retinal dystrophies in mice. We found that in contrast to mice, in the human retina CRB1 protein was expressed at the subapical region in photoreceptors and Müller glia cells, and CRB2 only in Müller glia cells. Genetic ablation of one allele of Crb2 in heterozygote Crb1(+/-) retinas induced a mild retinal phenotype, but in homozygote Crb1 knockout mice lead to an early and severe phenotype limited to the entire inferior retina. Our data provide mechanistic insight for CRB1-related LCA and RP.

The CRB1 and adherens junction complex proteins in retinal development and maintenance

The early developing retinal neuroepithelium is composed of multipotent retinal progenitor cells that differentiate in a time specific manner, giving rise to six major types of neuronal and one type of glial cells. These cells migrate and organize in three distinct nuclear layers divided by two plexiform layers. Apical and adherens junction complexes have a crucial role in this process by the establishment of polarity and adhesion. Changes in these complexes disturb the spatiotemporal aspects of retinogenesis, leading to retinal degeneration resulting in mild or severe impairment of retinal function and vision. In this review, we summarize the mouse models for the different members of the apical and adherens junction protein complexes and describe the main features of their retinal phenotypes. The knowledge acquired from the different mutant animals for these proteins corroborate their importance in retina development and maintenance of normal retinal structure and function. More recently, several studies have tried to unravel the connection between the apical proteins, important cellular signaling pathways and their relation in retina development. Still, the mechanisms by which these proteins function remain largely unknown. Here, we hypothesize how the mammalian apical CRB1 complex might control retinogenesis and prevents onset of Leber congenital amaurosis or retinitis pigmentosa.

Targeted ablation of Crb2 in photoreceptor cells induces retinitis pigmentosa

In humans, the Crumbs homolog-1 (CRB1) gene is mutated in autosomal recessive Leber congenital amaurosis and early-onset retinitis pigmentosa. In mammals, the Crumbs family is composed of: CRB1, CRB2, CRB3A and CRB3B. Recently, we showed that removal of mouse Crb2 from retinal progenitor cells, and consequent removal from Müller glial and photoreceptor cells, results in severe and progressive retinal degeneration with concomitant loss of retinal function that mimics retinitis pigmentosa due to mutations in the CRB1 gene. Here, we studied the effects of cell-type-specific loss of CRB2 from the developing mouse retina using targeted conditional deletion of Crb2 in photoreceptors or Müller cells. We analyzed the consequences of targeted loss of CRB2 in the adult mouse retina using adeno-associated viral vectors encoding Cre recombinase and short hairpin RNA against Crb2. In vivo retinal imaging by means of optical coherence tomography on retinas lacking CRB2 in photoreceptors showed progressive thinning of the photoreceptor layer and cellular mislocalization. Electroretinogram recordings under scotopic conditions showed severe attenuation of the a-wave, confirming the degeneration of photoreceptors. Retinas lacking CRB2 in developing photoreceptors showed early onset of abnormal lamination, whereas retinas lacking CRB2 in developing Müller cells showed late onset retinal disorganization. Our data suggest that in the developing retina, CRB2 has redundant functions in Müller glial cells, while CRB2 has essential functions in photoreceptors. Our data suggest that short-term loss of CRB2 in adult mouse photoreceptors, but not in Müller glial cells, causes sporadic loss of adhesion between photoreceptors and Müller cells.

MPP3 is required for maintenance of the apical junctional complex, neuronal migration, and stratification in the developing cortex

During mammalian cortical development, division of progenitor cells occurs at the apical ventricular zone. Apical complex proteins and adherens junctions regulate the different modes of division. Here, we have identified the membrane-associated guanylate kinase protein membrane palmitoylated protein 3 (MPP3) as an essential protein for the maintenance of these complexes. MPP3 localizes at the apical membrane in which it shows partial colocalization with adherens junction proteins and apical proteins. We generated Mpp3 conditional knock-out mice and specifically ablated Mpp3 expression in cortical progenitor cells. Conditional deletion of Mpp3 during cortical development resulted in a gradual loss of the apical complex proteins and disrupted adherens junctions. Although there is cellular disorganization in the ventricular zone, gross morphology of the cortex was unaffected during loss of MPP3. However, in the ventricular zone, removal of MPP3 resulted in randomization of spindle orientation and ectopically localized mitotic cells. Loss of MPP3 in the developing cortex resulted in delayed migration of progenitor cells, whereas the rate of cell division and exit from the cell cycle was not affected. This resulted in defects in cortical stratification and ectopically localized layer II-IV pyramidal neurons and interneurons. These data show that MPP3 is required for maintenance of the apical protein complex and adherens junctions and for stratification and proper migration of neurons during the development of the cortex.

Distinct expression and localization of diacylglycerol kinase isozymes in rat retina

Recent studies have revealed that phosphoinositide (PI) signaling molecules are expressed in mammalian retinas, suggesting their importance in its signal transduction. We previously showed that diacylglycerol kinase (DGK) isozymes are expressed in distinct patterns in rat retina at the mRNA level. However, little is known about the nature and morphological aspects of DGKs in the retina. For this study, we performed immunohistochemical analyses to investigate in the retina the expression and localization of DGK isozymes at the protein level. Here, we show that both DGKβ and DGKι localize in the outer plexiform layer, within which photoreceptor cells make contact with bipolar and horizontal cells. These isozymes exhibit distinct subcellular localization patterns: DGKι localizes to the synaptic area of bipolar cells in a punctate manner, whereas DGKβ distributes diffusely in the subsynaptic and dendritic regions of bipolar and horizontal cells. However, punctate labeling for DGKε is evident in the outer limiting membrane. DGKζ and DGKα localize predominantly to the nucleus of ganglion cells. These findings show distinct expression and localization of DGK isozymes in the retina, suggesting a different role of each isozyme.

Loss of CRB2 in the mouse retina mimics human retinitis pigmentosa due to mutations in the CRB1 gene

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, we generated and analysed conditional knockout mice lacking CRB2 in the developing retina. Progressive disorganization was detected during late retinal development. Progressive thinning of the photoreceptor layer and sites of cellular mislocalization was detected throughout the CRB2-deficient retina by confocal scanning laser ophthalmoscopy and spectral domain optical coherence tomography. Under scotopic conditions using electroretinography, the attenuation of the a-wave was relatively stronger than that of the b-wave, suggesting progressive degeneration of photoreceptors in adult animals. Histological analysis of newborn mice showed abnormal lamination of immature rod photoreceptors and disruption of adherens junctions between photoreceptors, Müller glia and progenitor cells. The number of late-born progenitor cells, rod photoreceptors and Müller glia cells was increased, concomitant with programmed cell death of rod photoreceptors. The data suggest an essential role for CRB2 in proper lamination of the photoreceptor layer and suppression of proliferation of late-born retinal progenitor cells.

The dynamic architecture of photoreceptor ribbon synapses: cytoskeletal, extracellular matrix, and intramembrane proteins

Rod and cone photoreceptors possess ribbon synapses that assist in the transmission of graded light responses to second-order bipolar and horizontal cells of the vertebrate retina. Proper functioning of the synapse requires the juxtaposition of presynaptic release sites immediately adjacent to postsynaptic receptors. In this review, we focus on the synaptic, cytoskeletal, and extracellular matrix proteins that help to organize photoreceptor ribbon synapses in the outer plexiform layer. We examine the proteins that foster the clustering of release proteins, calcium channels, and synaptic vesicles in the presynaptic terminals of photoreceptors adjacent to their postsynaptic contacts. Although many proteins interact with one another in the presynaptic terminal and synaptic cleft, these protein-protein interactions do not create a static and immutable structure. Instead, photoreceptor ribbon synapses are remarkably dynamic, exhibiting structural changes on both rapid and slow time scales.

Protein composition of immunoprecipitated synaptic ribbons

The synaptic ribbon is an electron-dense structure found in hair cells and photoreceptors. The ribbon is surrounded by neurotransmitter-filled vesicles and considered to play a role in vesicle release. We generated an objective, quantitative analysis of the protein composition of the ribbon complex using a mass spectrometry-based proteomics analysis. Our use of affinity-purified ribbons and control IgG immunoprecipitations ensure that the identified proteins are indeed associated with the ribbon complex. The use of mouse tissue, where the proteome is complete, generated a comprehensive analysis of the candidates. We identified 30 proteins (comprising 56 isoforms and subunits) associated with the ribbon complex. The ribbon complex primarily comprises proteins found in conventional synapses, which we categorized into 6 functional groups: vesicle handling (38.5%), scaffold (7.3%), cytoskeletal molecules (20.6%), phosphorylation enzymes (10.6%), molecular chaperones (8.2%), and transmembrane proteins from the presynaptic membrane firmly attached to the ribbon (11.3%). The 3 CtBP isoforms represent the major protein in the ribbon whether calculated by molar amount (30%) or by mass (20%). The relatively high quantity of phosphorylation enzymes suggests a very active and regulated structure. The ribbon appears to comprise a concentrated cluster of proteins dealing with vesicle creation, retention and distribution, and consequent exocytosis.

Membrane palmitoylated proteins regulate trafficking and processing of nectins

Nectins are cell-cell adhesion molecules involved in the formation of various intercellular junctions and the establishment of apical-basal polarity at cell-cell adhesion sites. To have a better understanding of the roles of nectins in the formation of cell-cell junctions, we searched for new cytoplasmic binding partners for nectin. We report that nectin-1α associates with membrane palmitoylated protein 3 (MPP3), one of the human homologues of a Drosophila tumor suppressor gene, Disc large. Two major forms of MPP3 at 66 and 98 kDa were detected, in conjunction with nectin-1α, suggesting that an association between the two may occur in various cell types. Nectin-1α recruits MPP3 to cell-cell contact sites, mediated by a PDZ-binding motif at the carboxyl terminus of nectin-1α. Association with MPP3 increases cell surface expression of nectin-1α and enhances nectin-1α ectodomain shedding, indicating that MPP3 regulates trafficking and processing of nectin-1α. Further study showed that MPP3 interacts with nectin-3α, but not with nectin-2α, showing that the association of nectins with MPP3 is isoform-specific. MPP5, another MPP family member, interacts with nectins with varying affinity and facilitates surface expression of nectin-1α, nectin-2α, and nectin-3α. These data suggest that wide interactions between nectins and MPP family members may occur in various cell-cell junctions and that these associations may regulate trafficking and processing of nectins.

A perisynaptic ménage à trois between Dlg, DLin-7, and Metro controls proper organization of Drosophila synaptic junctions

Structural plasticity of synaptic junctions is a prerequisite to achieve and modulate connectivity within nervous systems, e.g., during learning and memory formation. It demands adequate backup systems that allow remodeling while retaining sufficient stability to prevent unwanted synaptic disintegration. The strength of submembranous scaffold complexes, which are fundamental to the architecture of synaptic junctions, likely constitutes a crucial determinant of synaptic stability. Postsynaptic density protein-95 (PSD-95)/ Discs-large (Dlg)-like membrane-associated guanylate kinases (DLG-MAGUKs) are principal scaffold proteins at both vertebrate and invertebrate synapses. At Drosophila larval glutamatergic neuromuscular junctions (NMJs) DlgA and DlgS97 exert pleiotropic functions, probably reflecting a few known and a number of yet-unknown binding partners. In this study we have identified Metro, a novel p55/MPP-like Drosophila MAGUK as a major binding partner of perisynaptic DlgS97 at larval NMJs. Based on homotypic LIN-2,-7 (L27) domain interactions, Metro stabilizes junctional DlgS97 in a complex with the highly conserved adaptor protein DLin-7. In a remarkably interdependent manner, Metro and DLin-7 act downstream of DlgS97 to control NMJ expansion and proper establishment of synaptic boutons. Using quantitative 3D-imaging we further demonstrate that the complex controls the size of postsynaptic glutamate receptor fields. Our findings accentuate the importance of perisynaptic scaffold complexes for synaptic stabilization and organization.

Restricted localization of ponli, a novel zebrafish MAGUK-family protein, to the inner segment interface areas between green, red, and blue cones

PURPOSE

The inner segments (IS) of the photoreceptors in vertebrates are enriched with polarity scaffold proteins, which maintain the integrity of many tissues by mediating cell-cell adhesion either directly or indirectly. The formation of photoreceptor mosaics may require differential adhesion among different types of photoreceptors. It is unknown whether any polarity proteins are selectively expressed in certain photoreceptors to mediate differential intercellular adhesion, which may be important for photoreceptor patterning. This study was undertaken to identify such polarity proteins.

METHODS

To identify novel MAGUK-family (membrane-associated guanylate kinase) proteins that are similar to Nagie oko (Nok), the authors performed BLAST searches of the zebrafish genome with the Nok amino acid sequence as the query. The coding sequence of one of the identified genes was obtained and verified through RT-PCR and RACE (rapid amplification of cDNA ends). Its protein expression patterns were examined by immunomicroscopy and Western blot analysis. Morpholino knockdown technology was used for loss-of-function analyses.

RESULTS

The authors cloned a novel nok homolog and designated it photoreceptor-layer-nok-like (ponli). Unlike Nok, which is expressed broadly, Ponli is only expressed at the interface areas between the IS of the green, red, and blue cones in differentiated zebrafish retina.

CONCLUSIONS

Ponli is the first identified polarity protein that is not expressed in all types of photoreceptors. Ponli's selective distribution stimulates future investigations on its functions for photoreceptor mosaic formation.

Case report: autofluorescence imaging and phenotypic variance in a sibling pair with early-onset retinal dystrophy due to defective CRB1 function

PURPOSE

To phenotype two siblings with autosomal recessive early-onset retinal dystrophy due to CRB1 mutations.

METHODS

Autofluorescence (AF) imaging, high resolution optical coherence tomography (OCT), and full-field electroretinography (ERG) were performed. The results of DNA sequencing from polymerase chain reaction (PCR) products of the CRB1 gene were obtained from hospital records.

RESULTS

Two siblings, 14 years old and 17 years old, were compound heterozygotes for 749 del Ser and C948Y mutations in the gene encoding CRB1. AF imaging documented the preservation of retinal pigment epithelium (RPE) along the arterioles. High-resolution OCT showed abnormally thick retinae with increased lamination.

CONCLUSION

Leber congenital amaurosis caused by CRB1 is a unique form of early-onset retinal dystrophy because it spares the para-arteriolar RPE and causes abnormal retinal lamination with thickening. These findings, detectable with AF imaging and high-resolution OCT, can be combined with electrophysiology and genetic testing to molecularly classify retinal degenerations efficiently.

Divergent polarization mechanisms during vertebrate epithelial development mediated by the Crumbs complex protein Nagie oko

The zebrafish MAGUK protein Nagie oko is a member of the evolutionarily conserved Crumbs protein complex and functions as a scaffolding protein involved in the stabilization of multi-protein assemblies at the tight junction. During zebrafish embryogenesis, mutations in nagie oko cause defects in both epithelial polarity and cardiac morphogenesis. We used deletion constructs of Nagie oko in functional rescue experiments to define domains essential for cell polarity, maintenance of epithelial integrity and cardiac morphogenesis. Inability of Nagie oko to interact with Crumbs proteins upon deletion of the PDZ domain recreates all aspects of the nagie oko mutant phenotype. Consistent with this observation, apical localization of Nagie oko within the myocardium and neural tube is dependent on Oko meduzy/Crumbs2a. Disruption of direct interactions with Patj or Lin-7, two other members of the Crumbs protein complex, via the bipartite L27 domains produces only partial nagie oko mutant phenotypes and does not impair correct junctional localization of the truncated Nagie oko deletion protein within myocardial cells. Similarly, loss of the evolutionarily conserved region 1 domain, which mediates binding to Par6, causes only a subset of the nagie oko mutant epithelial phenotypes. Finally, deletion of the C-terminus, including the entire guanylate kinase and the SH3 domains, renders the truncated Nagie oko protein inactive and recreates all features of the nagie oko mutant phenotype when tested in functional complementation assays. Our observations reveal a previously unknown diversity of alternative multi-protein assembly compositions of the Crumbs-Nagie-oko and Par6-aPKC protein complexes that are highly dependent on the developmental context.

Leber congenital amaurosis: genes, proteins and disease mechanisms

Leber congenital amaurosis (LCA) is the most severe retinal dystrophy causing blindness or severe visual impairment before the age of 1 year. Linkage analysis, homozygosity mapping and candidate gene analysis facilitated the identification of 14 genes mutated in patients with LCA and juvenile retinal degeneration, which together explain approximately 70% of the cases. Several of these genes have also been implicated in other non-syndromic or syndromic retinal diseases, such as retinitis pigmentosa and Joubert syndrome, respectively. CEP290 (15%), GUCY2D (12%), and CRB1 (10%) are the most frequently mutated LCA genes; one intronic CEP290 mutation (p.Cys998X) is found in approximately 20% of all LCA patients from north-western Europe, although this frequency is lower in other populations. Despite the large degree of genetic and allelic heterogeneity, it is possible to identify the causative mutations in approximately 55% of LCA patients by employing a microarray-based, allele-specific primer extension analysis of all known DNA variants. The LCA genes encode proteins with a wide variety of retinal functions, such as photoreceptor morphogenesis (CRB1, CRX), phototransduction (AIPL1, GUCY2D), vitamin A cycling (LRAT, RDH12, RPE65), guanine synthesis (IMPDH1), and outer segment phagocytosis (MERTK). Recently, several defects were identified that are likely to affect intra-photoreceptor ciliary transport processes (CEP290, LCA5, RPGRIP1, TULP1). As the eye represents an accessible and immune-privileged organ, it appears to be uniquely suitable for human gene replacement therapy. Rodent (Crb1, Lrat, Mertk, Rpe65, Rpgrip1), avian (Gucy2D) and canine (Rpe65) models for LCA and profound visual impairment have been successfully corrected employing adeno-associated virus or lentivirus-based gene therapy. Moreover, phase 1 clinical trials have been carried out in humans with RPE65 deficiencies. Apart from ethical considerations inherently linked to treating children, major obstacles for the treatment of LCA could be the putative developmental deficiencies in the visual cortex in persons blind from birth (amblyopia), the absence of sufficient numbers of viable photoreceptor or RPE cells in LCA patients, and the unknown and possibly toxic effects of overexpression of transduced genes. Future LCA research will focus on the identification of the remaining causal genes, the elucidation of the molecular mechanisms of disease in the retina, and the development of gene therapy approaches for different genetic subtypes of LCA.

Composition and function of the Crumbs protein complex in the mammalian retina

The Crumbs proteins (CRBs) are transmembrane proteins, homologous to Drosophila Crumbs, with a key role in defining the apical membrane domain in photoreceptors as well as in embryonic epithelia. Crumbs proteins are conserved between species and their intracellular domains are involved in organizing a conserved macromolecular protein scaffold with important roles in cell polarity as well as morphogenesis and maintenance of the retina. Mutations in the gene encoding human CRB1, the first one identified out of the three human orthologs, have been associated with a number of retinal dystrophies including Leber amaurosis and retinitis pigmentosa type 12. Although no other mammalian Crumbs complex members as of yet have been associated with retinal degeneration, disruption of different zebrafish and fruitfly orthologs can lead to various retinal defects. The core Crumbs complex localizes apical to the outer limiting membrane, where photoreceptors and Müller glia contact each other. Correct functioning of Crumbs ensures adhesion between these cells by an unknown mechanism. This review summarizes the current view on the composition and function of the Crumbs prsotein complex in the mammalian retina. Recently, a number of new members of the Crumbs protein complex have been identified. These include most members of the membrane palmitoylated protein family (MPP), involved in assembly of macromolecular protein complexes. Some components of the complex are found to exert a function in the photoreceptor synapses and/or at the region of the connecting cilium. Studies using polarized cell cultures or model organisms, like Drosophila and zebrafish, suggest important links of the Crumbs protein complex to several biological processes in the mammalian eye, including retinal patterning, ciliogenesis and vesicular transport.

Crb1 is a determinant of retinal apical Müller glia cell features

Mutations in the human Crumbs homologue-1 (CRB1) gene cause retinal blinding diseases, such as Leber congenital amaurosis and retinitis pigmentosa. In the previous studies we have shown that Crb1 resides in retinal Müller glia cells and that loss of Crb1 results in retinal degeneration (particularly in the inferior temporal quadrant of the mouse eye). Degeneration is increased by exposure to white light. Here, we studied the role of light and aging to gain a better understanding of the factors involved in the progress of retinal disease. Our data reveal that light is neither sufficient nor required to induce retinal disorganization and degeneration in young Crb1(-/-) mutant mice, suggesting that it rather modulates the retinal phenotype. Gene expression profiling showed that expression of five genes is altered in light-exposed Crb1(-/-) mutant retinas. Three of the five genes are involved in chromosome stabilization (Pituitary tumor transforming gene 1 or Pttg1, Establishment of cohesion 1 homolog 1 or Esco1, and a gene similar to histone H2B). In aged retinas, degeneration of photoreceptors, inner retinal neurons, and retinal pigment epithelium was practically limited to the inferior temporal quadrant. Loss of Crb1 in Müller glia cells resulted in an irregular number and size of their apical villi. We propose that Crb1 is required to regulate number and size of these Müller glia cell villi. The subsequent loss of retinal integrity resulted in neovascularization, in which blood vessels of the choroid protruded into the neural retina.

The N1317H Substitution Associated with Leber Congenital Amaurosis Results in Impaired Interdomain Packing in Human CRB1 Epidermal Growth Factor-like (EGF) Domains

The calcium-binding epidermal growth factor-like (cbEGF) domain is a widely occurring module in proteins of diverse function. Amino acid substitutions that disrupt its structure or calcium affinity have been associated with various disorders. The extracellular portion of CRB1, the human homologue of Drosophila Crumbs, exhibits a modular domain organization that includes EGF and cbEGF domains. The N1317H substitution in the 19th cbEGF domain of CRB1 is associated with the serious visual disorder Leber congenital amaurosis. We have investigated the structure and Ca(2+) binding of recombinant wild-type and N1317H CRB1 fragments (EGF18-cbEGF19) using NMR and find that Ca(2+) binding is altered, resulting in disruption of long range interactions between adjacent EGF domains in CRB1. From these observations, we propose that this substitution affects the structural integrity of CRB1 in the inter-photoreceptor matrix of the retina, where it is expressed. Furthermore, we identify disease-causing substitutions in other cbEGF-containing proteins that are likely to result in similar disruption of interdomain packing, supporting the hypothesis that the tandem cbEGF domain linkages are critical for the structure and function of proteins containing cbEGF domains.

Unraveling the genetic complexity of Drosophila stardust during photoreceptor morphogenesis and prevention of light-induced degeneration

Drosophila Stardust, a membrane-associated guanylate kinase (MAGUK), recruits the transmembrane protein Crumbs and the cytoplasmic proteins DPATJ and DLin-7 into an apically localized protein scaffold. This evolutionarily conserved complex is required for epithelial cell polarity in Drosophila embryos and mammalian cells in culture. In addition, mutations in Drosophila crumbs and DPATJ impair morphogenesis of photoreceptor cells (PRCs) and result in light-dependent retinal degeneration. Here we show that stardust is a genetically complex locus. While all alleles tested perturb epithelial cell polarity in the embryo, only a subset of them affects morphogenesis of PRCs or induces light-dependent retinal degeneration. Alleles retaining particular postembryonic functions still express some Stardust protein in pupal and/or adult eyes. The phenotypic complexity is reflected by the expression of distinct splice variants at different developmental stages. All proteins expressed in the retina contain the PSD95, Discs Large, ZO-1 (PDZ), Src homology 3 (SH3), and guanylate kinase (GUK) domain, but lack a large region in the N terminus encoded by one exon. These results suggest that Stardust-based protein scaffolds are dynamic, which is not only mediated by multiple interaction partners, but in addition by various forms of the Stardust protein itself.

A single amino acid substitution (Cys249Trp) in Crb1 causes retinal degeneration and deregulates expression of pituitary tumor transforming gene Pttg1

Different mutations in the human Crumbs homolog-1 (CRB1) gene cause a variety of retinal dystrophies, such as Leber congenital amaurosis, early onset retinitis pigmentosa (e.g., RP12), RP with Coats-like exudative vasculopathy, and pigmented paravenous retinochoroidal atrophy. Loss of Crb1 leads to displaced photoreceptors and focal degeneration of all neural layers attributable to loss of adhesion between photoreceptors and Müller glia cells. To gain insight into genotype-phenotype relationship, we generated Crb1(C249W) mice that harbor an amino acid substitution (Cys249Trp) in the extracellular sixth calcium-binding epidermal growth factor domain of Crb1. Our analysis showed that Crb1(C249W) as wild-type protein trafficked to the subapical region adjacent to adherens junctions at the outer limiting membrane (OLM). Hence, these data suggest correct trafficking of the corresponding mutant CRB1 in RP12 patients. Crb1(C249W) mice showed loss of photoreceptors in the retina, relatively late compared with mice lacking Crb1. Scanning laser ophthalmoscopy revealed autofluorescent dots that presumably represent layer abnormalities after OLM disturbance. Gene expression analyses revealed lower levels of pituitary tumor transforming gene 1 (Pttg1) transcripts in Crb1(C249W/-) knock-in and Crb1(-/-) knock-out compared with control retinas. Exposure to white light decreased levels of Pttg1 in Crb1 mutant retinas. We hypothesize deregulation of Pttg1 expression attributable to a C249W substitution in the extracellular domain of Crb1.

Towards understanding CRUMBS function in retinal dystrophies

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.

Opposite effects of PSD-95 and MPP3 PDZ proteins on serotonin 5-hydroxytryptamine2C receptor desensitization and membrane stability

PSD-95/Disc large/Zonula occludens 1 (PDZ) domain-containing proteins (PDZ proteins) play an important role in the targeting and the trafficking of transmembrane proteins. Our previous studies identified a set of PDZ proteins that interact with the C terminus of the serotonin 5-hydroxytryptamine (5-HT)(2C) receptor. Here, we show that the prototypic scaffolding protein postsynaptic density-95 (PSD-95) and another membrane-associated guanylate kinase, MAGUK p55 subfamily member 3 (MPP3), oppositely regulate desensitization of the receptor response in both heterologous cells and mice cortical neurons in primary culture. PSD-95 increased desensitization of the 5-HT(2C) receptor-mediated Ca(2+) response, whereas MPP3 prevented desensitization of the Ca(2+) response. The effects of the PDZ proteins on the desensitization of the Ca(2+) response were correlated with a differential regulation of cell surface expression of the receptor. Additional experiments were performed to assess how PDZ proteins globally modulate desensitization of the 5-HT(2C) receptor response in neurons, by using a peptidyl mimetic of the 5-HT(2C) receptor C terminus fused to the human immunodeficiency virus type-1 Tat protein transduction domain, which disrupts interaction between the 5-HT(2C) receptor and PDZ proteins. Transduction of this peptide inhibitor into cultured cortical neurons increased the desensitization of the 5-HT(2C) receptor-mediated Ca(2+) response. This indicates that, overall, interaction of 5-HT(2C) receptors with PDZ proteins inhibits receptor desensitization in cortical neurons.

Pals1/Mpp5 is required for correct localization of Crb1 at the subapical region in polarized Muller glia cells

Mutations in the human Crumbs homologue-1 (CRB1) gene cause retinal diseases including Leber's congenital amaurosis (LCA) and retinitis pigmentosa type 12. The CRB1 transmembrane protein localizes at a subapical region (SAR) above intercellular adherens junctions between photoreceptor and Müller glia (MG) cells. We demonstrate that the Crb1-/- phenotype, as shown in Crb1-/- mice, is accelerated and intensified in primary retina cultures. Immuno-electron microscopy showed strong Crb1 immunoreactivity at the SAR in MG cells but barely in photoreceptor cells, whereas Crb2, Crb3, Patj, Pals1 and Mupp1 were present in both cell types. Human CRB1, introduced in MG cells in Crb1-/- primary retinas, was targeted to the SAR. RNA interference-induced silencing of the Crb1-interacting-protein Pals1 (protein associated with Lin7; Mpp5) in MG cells resulted in loss of Crb1, Crb2, Mupp1 and Veli3 protein localization and partial loss of Crb3. We conclude that Pals1 is required for correct localization of Crb family members and its interactors at the SAR of polarized MG cells.

Mpp4 recruits Psd95 and Veli3 towards the photoreceptor synapse

Membrane-associated guanylate kinase (MAGUK) proteins function as scaffold proteins contributing to cell polarity and organizing signal transducers at the neuronal synapse membrane. The MAGUK protein Mpp4 is located in the retinal outer plexiform layer (OPL) at the presynaptic plasma membrane and presynaptic vesicles of photoreceptors. Additionally, it is located at the outer limiting membrane (OLM) where it might be involved in OLM integrity. In Mpp4 knockout mice, loss of Mpp4 function only sporadically causes photoreceptor displacement, without changing the Crumbs (Crb) protein complex at the OLM, adherens junctions or synapse structure. Scanning laser ophthalmology revealed no retinal degeneration. The minor morphological effects suggest that Mpp4 is a candidate gene for mild retinopathies only. At the OPL, Mpp4 is essential for correct localization of Psd95 and Veli3 at the presynaptic photoreceptor membrane. Psd95 labeling is absent of presynaptic membranes in both rods and cones but still present in cone basal contacts and dendritic contacts. Total retinal Psd95 protein levels are significantly reduced which suggests Mpp4 to be involved in Psd95 turnover, whereas Veli3 proteins levels are not changed. These protein changes in the photoreceptor synapse did not result in an altered electroretinograph. These findings suggest that Mpp4 coordinates Psd95/Veli3 assembly and maintenance at synaptic membranes. Mpp4 is a critical recruitment factor to organize scaffolds at the photoreceptor synapse and is likely to be associated with synaptic plasticity and protein complex transport.