Immunocytochemical evidence of the localization of the Crumbs homologue 3 protein (CRB3) in the developing and mature mouse retina

Diverse functions and pathogenetic role of Crumbs in retinopathy

The Crumbs protein (CRB) family plays a crucial role in maintaining the apical-basal polarity and integrity of embryonic epithelia. The family comprises different isoforms in different animals and possesses diverse structural, localization, and functional characteristics. Mutations in the human CRB1 or CRB2 gene may lead to a broad spectrum of retinal dystrophies. Various CRB-associated experimental models have recently provided mechanistic insights into human CRB-associated retinopathies. The knowledge obtained from these models corroborates the importance of CRB in retinal development and maintenance. Therefore, complete elucidation of these models can provide excellent therapeutic prospects for human CRB-associated retinopathies. In this review, we summarize the current animal models and human-derived models of different CRB family members and describe the main characteristics of their retinal phenotypes.

Distribution of intracellular Ca2+-ATPases in the mouse retina and their involvement in light-induced cone degeneration

Calcium signalling is involved in many processes in mammalian retina, from development to mature functions and neurodegeneration. Although proteins involved in Ca2+ entry in retinal cells have been well studied, less is known about Ca2+-clearance. Among the Ca2+ pumps, plasma membrane Ca2+-ATPases (PMCAs) have been identified as key proteins extruding Ca2+ across the plasma membrane with specific distribution in developing and adult retina. However, the two main isoforms of intracellular Ca2+-ATPases in the central nervous system, the sarco(endo)plasmic reticulum (ER) Ca2+-ATPase 2b (SERCA2b) and the secretory pathway Ca2+-ATPase 1 (SPCA1), which remove cytosolic Ca2+ into intracellular stores, have been less or not at all analysed, respectively. In this study, we described for the first time the SPCA1 localisation in adult mouse retina and we report differential distributions of SERCA2b and SPCA1 transporters within various classes of retinal neurons and distinct subcellular localisations. In addition, we studied the expression and localisation of both Ca2+ pumps in 661W cells, a cone photoreceptor-derived cell line. Since continuous exposure to high light intensity induces photodegeneration, we analysed the effect of LED light exposure on these cells and SERCA2b and SPCA1 distribution. We found that continuous mild LED-light exposure compromised cell survival and produced stress in the ER and Golgi, the Ca2+ stores where the two pumps are localised. These effects were reversed after halting light exposure and washing. This study demonstrates that Ca2+ signalling may be involved in light-induced photoreceptor cell damage and points to previously unrecognised functions of intracellular Ca2+-ATPases in retina physiology.

CRB3 navigates Rab11 trafficking vesicles to promote γTuRC assembly during ciliogenesis

The primary cilium plays important roles in regulating cell differentiation, signal transduction, and tissue organization. Dysfunction of the primary cilium can lead to ciliopathies and cancer. The formation and organization of the primary cilium are highly associated with cell polarity proteins, such as the apical polarity protein CRB3. However, the molecular mechanisms by which CRB3 regulates ciliogenesis and the location of CRB3 remain unknown. Here, we show that CRB3, as a navigator, regulates vesicle trafficking in γ-tubulin ring complex (γTuRC) assembly during ciliogenesis and cilium-related Hh and Wnt signaling pathways in tumorigenesis. Crb3 knockout mice display severe defects of the primary cilium in the mammary ductal lumen and renal tubule, while mammary epithelial-specific Crb3 knockout mice exhibit the promotion of ductal epithelial hyperplasia and tumorigenesis. CRB3 is essential for lumen formation and ciliary assembly in the mammary epithelium. We demonstrate that CRB3 localizes to the basal body and that CRB3 trafficking is mediated by Rab11-positive endosomes. Significantly, CRB3 interacts with Rab11 to navigate GCP6/Rab11 trafficking vesicles to CEP290, resulting in intact γTuRC assembly. In addition, CRB3-depleted cells are unresponsive to the activation of the Hh signaling pathway, while CRB3 regulates the Wnt signaling pathway. Therefore, our studies reveal the molecular mechanisms by which CRB3 recognizes Rab11-positive endosomes to facilitate ciliogenesis and regulates cilium-related signaling pathways in tumorigenesis.

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.

Loss of CRB2 in Müller glial cells modifies a CRB1-associated retinitis pigmentosa phenotype into a Leber congenital amaurosis phenotype

Variations in the human Crumbs homolog-1 (CRB1) gene lead to an array of retinal dystrophies including early onset of retinitis pigmentosa (RP) and Leber congenital amaurosis (LCA) in children. To investigate the physiological roles of CRB1 and CRB2 in retinal Müller glial cells (MGCs), we analysed mouse retinas lacking both proteins in MGC. The peripheral retina showed a faster progression of dystrophy than the central retina. The central retina showed retinal folds, disruptions at the outer limiting membrane, protrusion of photoreceptor nuclei into the inner and outer segment layers and ingression of photoreceptor nuclei into the photoreceptor synaptic layer. The peripheral retina showed a complete loss of the photoreceptor synapse layer, intermingling of photoreceptor nuclei within the inner nuclear layer and ectopic photoreceptor cells in the ganglion cell layer. Electroretinography showed severe attenuation of the scotopic a-wave at 1 month of age with responses below detection levels at 3 months of age. The double knockout mouse retinas mimicked a phenotype equivalent to a clinical LCA phenotype due to loss of CRB1. Localization of CRB1 and CRB2 in non-human primate (NHP) retinas was analyzed at the ultrastructural level. We found that NHP CRB1 and CRB2 proteins localized to the subapical region adjacent to adherens junctions at the outer limiting membrane in MGC and photoreceptors. Our data suggest that loss of CRB2 in MGC aggravates the CRB1-associated RP-like phenotype towards an LCA-like phenotype.

Photoreceptor actin dysregulation in syndromic and non-syndromic retinitis pigmentosa

Retinitis pigmentosa (RP) is the leading cause of inherited blindness. RP is a genetically heterogeneous disorder, with more than 100 different causal genes identified in patients. Central to disease pathogenesis is the progressive loss of retinal photoreceptors. Photoreceptors are specialised sensory neurons that exhibit a complex and highly dynamic morphology. The highly polarised and elaborated architecture of photoreceptors requires precise regulation of numerous cytoskeletal elements. In recent years, significant work has been placed on investigating the role of microtubules (specifically, the acetylated microtubular axoneme of the photoreceptor connecting cilium) and their role in normal photoreceptor function. This has been driven by the emerging field of ciliopathies, human diseases arising from mutations in genes required for cilia formation or function, of which RP is a frequently reported phenotype. Recent studies have highlighted an intimate relationship between cilia and the actin cystoskeleton. This review will focus on the role of actin in photoreceptors, examining the connection between actin dysregulation in RP.

Expression and localization of the polarity protein CRB2 in adult mouse brain: a comparison with the CRB1rd8 mutant mouse model

Acquisition of cell polarization is essential for the performance of crucial functions, like a successful secretion and appropriate cell signaling in many tissues, and it depends on the correct functioning of polarity proteins, including the Crumbs complex. The CRB proteins, CRB1, CRB2 and CRB3, identified in mammals, are expressed in epithelial-derived tissues like brain, kidney and retina. CRB2 has a ubiquitous expression and has been detected in embryonic brain tissue; but currently there is no data regarding its localization in the adult brain. In our study, we characterized the presence of CRB2 in adult mice brain, where it is particularly enriched in cortex, hippocampus, hypothalamus and cerebellum. Double immunofluorescence analysis confirmed that CRB2 is a neuron-specific protein, present in both soma and projections where colocalizes with certain populations of exocytic and endocytic vesicles and with other members of the Crumbs complex. Finally, in the cortex of CRB1^rd8 mutant mice that contain a mutation in the Crb1 gene generating a truncated CRB1 protein, there is an abnormal increase in the expression levels of the CRB2 protein which suggests a possible compensatory mechanism for the malfunction of CRB1 in this mutant background.

Common and distinctive localization patterns of Crumbs polarity complex proteins in the mammalian eye

Crumbs polarity complex proteins are essential for cellular and tissue polarity, and for adhesion of epithelial cells. In epithelial tissues deletion of any of three core proteins disrupts localization of the other proteins, indicating structural and functional interdependence among core components. Despite previous studies of function and co-localization that illustrated the properties that these proteins share, it is not known whether an individual component of the complex plays a distinct role in a unique cellular and developmental context. In order to investigate this question, we primarily used confocal imaging to determine the expression and subcellular localization of the core Crumbs polarity complex proteins during ocular development. Here we show that in developing ocular tissues core Crumbs polarity complex proteins, Crb, Pals1 and Patj, generally appear in an overlapping pattern with some exceptions. All three core complex proteins localize to the apical junction of the retinal and lens epithelia. Pals1 is also localized in the Golgi of the retinal cells and Patj localizes to the nuclei of the apically located subset of progenitor cells. These findings suggest that core Crumbs polarity complex proteins exert common and independent functions depending on cellular context.

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.