Crumbs controls epithelial integrity by inhibiting Rac1 and PI3K

Drosophila Crumbs (Crb) and its mammalian ortholog CRB3 control epithelial polarity through poorly understood molecular mechanisms. Elucidating these mechanisms is crucial, because the physiology of epithelia largely depends on the polarized architecture of individual epithelial cells. In addition, loss of CRB3 favors tumor cell growth, metastasis and epithelial to mesenchymal transition (EMT). Using Drosophila embryos, we report that Rac1 sustains PI3K signaling, which is required for Rac1 activation. Crb represses this positive-feedback loop. Notably, this property confers to Crb its ability to promote epithelial integrity in vivo, because attenuation of either Rac1 or PI3K activity rescues the crb mutant phenotype. Moreover, inhibition of Rac1 or PI3K results in Crb-dependent apical membrane growth, whereas Rac1 activation restricts membrane localization of Crb and interferes with apical domain formation. This illustrates that Crb and the Rac1-PI3K module are antagonists, and that the fine balance between the activities of these proteins is crucial to maintain epithelial organization and an appropriate apical to basolateral ratio. Together, our results elucidate a mechanism that mediates Crb function and further define the role of PI3K and Rac1 in epithelial morphogenesis, allowing for a better understanding of how distinct membrane domains are regulated in polarized epithelial cells.

Generation of Rab-based transgenic lines for in vivo studies of endosome biology in zebrafish

The Rab family of small GTPases function as molecular switches regulating membrane and protein trafficking. Individual Rab isoforms define and are required for specific endosomal compartments. To facilitate in vivo investigation of specific Rab proteins, and endosome biology in general, we have generated transgenic zebrafish lines to mark and manipulate Rab proteins. We also developed software to track and quantify endosome dynamics within time-lapse movies. The established transgenic lines ubiquitously express EGFP fusions of Rab5c (early endosomes), Rab11a (recycling endosomes), and Rab7 (late endosomes) to study localization and dynamics during development. Additionally, we generated UAS-based transgenic lines expressing constitutive active (CA) and dominant-negative (DN) versions for each of these Rab proteins. Predicted localization and functional consequences for each line were verified through a variety of assays, including lipophilic dye uptake and Crumbs2a localization. In summary, we have established a toolset for in vivo analyses of endosome dynamics and functions.

Time-specific regulation of airway clearance by the Drosophila J-domain transmembrane protein Wurst

At the end of embryogenesis, Drosophila and mammalian airways convert from liquid-filled to air-filled tubes. This process is regulated by Clathrin-mediated endocytosis. However, these molecular mechanisms are poorly understood. In Drosophila, the DnaJ transmembrane protein Wurst interacts with Clathrin and Hsc70 to mediate early steps of endocytosis. Wurst is expressed in epithelial tissues from early stages onwards. Here we show time- and tissue-specific requirement of Wurst in airway liquid-clearance and air-filling. RNAi experiments demonstrate that Wurst activity is specifically required at the final stage 17 of embryogenesis. Furthermore, we show that the apical membrane organizer Crumbs regulates Wurst-mediated airway liquid-air-transition.

Apical deficiency triggers JNK-dependent apoptosis in the embryonic epidermis of Drosophila

Epithelial homeostasis and the avoidance of diseases such as cancer require the elimination of defective cells by apoptosis. Here, we investigate how loss of apical determinants triggers apoptosis in the embryonic epidermis of Drosophila. Transcriptional profiling and in situ hybridisation show that JNK signalling is upregulated in mutants lacking Crumbs or other apical determinants. This leads to transcriptional activation of the pro-apoptotic gene reaper and to apoptosis. Suppression of JNK signalling by overexpression of Puckered, a feedback inhibitor of the pathway, prevents reaper upregulation and apoptosis. Moreover, removal of endogenous Puckered leads to ectopic reaper expression. Importantly, disruption of the basolateral domain in the embryonic epidermis does not trigger JNK signalling or apoptosis. We suggest that apical, not basolateral, integrity could be intrinsically required for the survival of epithelial cells. In apically deficient embryos, JNK signalling is activated throughout the epidermis. Yet, in the dorsal region, reaper expression is not activated and cells survive. One characteristic of these surviving cells is that they retain discernible adherens junctions despite the apical deficit. We suggest that junctional integrity could restrain the pro-apoptotic influence of JNK signalling.

Emerging role for epithelial polarity proteins of the Crumbs family as potential tumor suppressors

Defects in apical-basal polarity regulation are associated with tissue overgrowth and tumorogenesis, yet the molecular mechanisms linking epithelial polarity regulators to hyperplasia or neoplasia remain elusive. In addition, exploration of the expression and function of the full complement of proteins required for the polarized architecture of epithelial cells in the context of cancer is awaited. This paper provides an overview of recent studies performed on Drosophila and vertebrates showing that apical polarity proteins of the Crumbs family act to repress tissue growth and epithelial to mesenchymal transition. Thus, these proteins emerge as potential tumor suppressors. Interestingly, analysis of the molecular function of Crumbs proteins reveals a function for these polarity regulators in junctional complexes stability and control of signaling pathways regulating proliferation and apoptosis. Thereby, these studies provide a molecular basis explaining how regulation of epithelial polarity is coupled to tumorogenesis.

Integrating developmental signals: a Hippo in the (path)way

The Hippo pathway, a signaling cascade that controls cell cycle progression, apoptosis and cell differentiation, has emerged as a fundamental regulator of many physiological and pathological processes. Recent studies have revealed a complex network of interactions directing Hippo pathway activity, and have connected this pathway with other key signaling pathways. Such crosstalk has uncovered novel roles for Hippo signaling, including regulation of TGFβ/SMAD and WNT/β-catenin pathways. This review highlights some of the recent findings in the Hippo field with an emphasis on how the Hippo pathway is integrated with other pathways to mediate diverse processes.

Human CRB1-associated retinal degeneration: comparison with the rd8 Crb1-mutant mouse model

PURPOSE

To investigate the human disease due to CRB1 mutations and compare results with the Crb1-mutant rd8 mouse.

METHODS

Twenty-two patients with CRB1 mutations were studied. Function was assessed with perimetry and electroretinography (ERG) and retinal structure with optical coherence tomography (OCT). Cortical structure and function were quantified with magnetic resonance imaging (MRI). Rd8 mice underwent ERG, OCT, and retinal histopathology.

RESULTS

Visual acuities ranged from 20/25 to light perception. Rod ERGs were not detectable; small cone signals were recordable. By perimetry, small central visual islands were separated by midperipheral scotomas from far temporal peripheral islands. The central islands were cone mediated, whereas the peripheral islands retained some rod function. With OCT, there were small foveal islands of thinned outer nuclear layer (ONL) surrounded by thick delaminated retina with intraretinal hyperreflective lesions. MRI showed structurally normal optic nerves and only subtle changes to occipital lobe white and gray matter. Functional MRI indicated that whole-brain responses from patients were of reduced amplitude and spatial extent compared with those of normal controls. Rd8 mice had essentially normal ERGs; OCT and histopathology showed patchy retinal disorganization with pseudorosettes more pronounced in ventral than in dorsal retina. Photoreceptor degeneration was associated with dysplastic regions.

CONCLUSIONS

CRB1 mutations lead to early-onset severe loss of vision with thickened, disorganized, nonseeing retina. Impaired peripheral vision can persist in late disease stages. Rd8 mice also have a disorganized retina, but there is sufficient photoreceptor integrity to produce largely normal retinal function. Differences between human and mouse diseases will complicate proof-of-concept studies intended to advance treatment initiatives.

Molecular regulation of lumen morphogenesis

The asymmetric polarization of cells allows specialized functions to be performed at discrete subcellular locales. Spatiotemporal coordination of polarization between groups of cells allowed the evolution of metazoa. For instance, coordinated apical-basal polarization of epithelial and endothelial cells allows transport of nutrients and metabolites across cell barriers and tissue microenvironments. The defining feature of such tissues is the presence of a central, interconnected luminal network. Although tubular networks are present in seemingly different organ systems, such as the kidney, lung, and blood vessels, common underlying principles govern their formation. Recent studies using in vivo and in vitro models of lumen formation have shed new light on the molecular networks regulating this fundamental process. We here discuss progress in understanding common design principles underpinning de novo lumen formation and expansion.

The Ran importin system in cilia trafficking

Cilia are microtubule-based organelles that arise from the centrosome and project from the surface of many cells. Defects in cilia-localized proteins are felt to lead to polycystic kidney disease as well as ciliopathies with multiple organ involvement. Movement of proteins along mammalian cilia is a specialized process that is highly related to the intraflagellar movement of proteins in lower organisms. Entry of proteins into the cilia appears to be a tightly regulated process. Several cilia-targeting sequences have been identified that appear to mediate the movement of proteins into cilia, although the molecular basis through which these sequences operate is still being elucidated. Entry of proteins into cilia appears to be regulated at the base of the cilia at a region known as the transition zone. It has been proposed that a ciliary pore exists in this zone that controls entry of proteins into the cilia, similar to the nuclear pore that controls entry of proteins into the nucleus. Our group at the University of Michigan has found that proteins important in nuclear import appear to function similarly in cilia entry. In particular, we have identified roles for the small GTPase, Ran and its binding partners, the importins, in regulating cilia entry of specific proteins.

Retromer controls epithelial cell polarity by trafficking the apical determinant Crumbs

The evolutionarily conserved apical determinant Crumbs (Crb) is essential for maintaining apicobasal polarity and integrity of many epithelial tissues [1]. Crb levels are crucial for cell polarity and homeostasis, yet strikingly little is known about its trafficking or the mechanism of its apical localization. Using a newly established, liposome-based system described here, we determined Crb to be an interaction partner and cargo of the retromer complex. Retromer is essential for the retrograde transport of numerous transmembrane proteins from endosomes to the trans-Golgi network (TGN) and is conserved between plants, fungi, and animals [2]. We show that loss of retromer function results in a substantial reduction of Crb in Drosophila larvae, wing discs, and the follicle epithelium. Moreover, loss of retromer phenocopies loss of crb by preventing apical localization of key polarity molecules, such as atypical protein kinase C (aPKC) and Par6 in the follicular epithelium, an effect that can be rescued by overexpression of Crb. Additionally, loss of retromer results in multilayering of the follicular epithelium, indicating that epithelial integrity is severely compromised. Our data reveal a mechanism for Crb trafficking by retromer that is vital for maintaining Crb levels and localization. We also show a novel function for retromer in maintaining epithelial cell polarity.

Aberrant expression of the polarity complex atypical PKC and non-muscle myosin IIA in active and inactive inflammatory bowel disease

Epithelial barrier function is contingent on appropriate polarization of key protein components. Work in intestinal epithelial cell cultures and animal models of bowel inflammation suggested that atypical PKC (aPKC), the kinase component of the Par3–Par6 polarity complex, is downregulated by pro-inflammatory signaling. Data from other laboratories showed the participation of myosin light chain kinase in intestinal inflammation, but there is paucity of evidence for assembly of its major target, non-muscle myosin II, in inflammatory bowel disease (IBD). In addition, we showed before that non-muscle myosin IIA (nmMyoIIA) is upregulated in intestinal inflammation in mice and TNFα-treated Caco-2 cells. Thus far, it is unknown if a similar phenomena occur in patients with IBD. Moreover, it is unclear whether aPKC downregulation is directly correlated with local mucosal inflammation or occurs in uninvolved areas. Frozen sections from colonoscopy material were stained for immunofluorescence with extensively validated specific antibodies against phosphorylated aPKC turn motif (active form) and nmMyoIIA. Inflammation was scored for the local area from where the material was obtained. We found a significant negative correlation between the expression of active aPKC and local inflammation, and a significant increase in the apical expression of nmMyoIIA in surface colon epithelia in inflamed areas, but not in non-inflamed mucosa even in the same patients. Changes in aPKC and nmMyoIIA expression are likely to participate in the pathogenesis of epithelial barrier function in response to local pro-inflammatory signals. These results provide a rationale for pursuing mechanistic studies on the regulation of these proteins.

Atypical protein kinase C controls sea urchin ciliogenesis

The distribution and function of aPKC are examined during sea urchin ciliogenesis. The kinase concentrates in a ring at the transition zone between the basal body and the elongating axoneme. Inhibition of aPKC results in mislocalization of the kinase and defective ciliogenesis. Thus aPKC controls the growth of motile cilia in invertebrate embryos.

The atypical protein kinase C (aPKC) is part of the conserved aPKC/PAR6/PAR3 protein complex, which regulates many cell polarity events, including the formation of a primary cilium at the apical surface of epithelial cells. Cilia are highly organized, conserved, microtubule-based structures involved in motility, sensory processes, signaling, and cell polarity. We examined the distribution and function of aPKC in the sea urchin embryo, which forms a swimming blastula covered with motile cilia. We found that in the early embryo aPKC is uniformly cortical and becomes excluded from the vegetal pole during unequal cleavages at the 8- to 64-cell stages. During the blastula and gastrula stages the kinase localizes at the base of cilia, forming a ring at the transition zone between the basal body and the elongating axoneme. A dose-dependent and reversible inhibition of aPKC results in mislocalization of the kinase, defective ciliogenesis, and lack of swimming. Thus, as in the primary cilium of differentiated mammalian cells, aPKC controls the growth of motile cilia in invertebrate embryos. We suggest that aPKC might function to phosphorylate kinesin and so activate the transport of intraflagellar vesicles.

Elaborating polarity: PAR proteins and the cytoskeleton

Cell polarity is essential for cells to divide asymmetrically, form spatially restricted subcellular structures and participate in three-dimensional multicellular organization. PAR proteins are conserved polarity regulators that function by generating cortical landmarks that establish dynamic asymmetries in the distribution of effector proteins. Here, we review recent findings on the role of PAR proteins in cell polarity in C. elegans and Drosophila, and emphasize the links that exist between PAR networks and cytoskeletal proteins that both regulate PAR protein localization and act as downstream effectors to elaborate polarity within the cell.

The Hippo pathway in biological control and cancer development

The Hippo pathway is an evolutionally conserved protein kinase cascade involved in regulating organ size in vivo and cell contact inhibition in vitro by governing cell proliferation and apoptosis. Deregulation of the Hippo pathway is linked to cancer development. Its first core kinase Warts was identified in Drosophila more than 15 years ago, but it gained much attention when other core components of the pathway were identified 8 years later. Major discoveries of the pathway were made during past several years. The core kinase components Hippo, Salvador, Warts, and Mats in the fly and Mst1/2, WW45, Lats1/2, and Mob1 in mammals phosphorylate and inactivate downstream transcriptional co-activators Yorkie in the fly, Yes-associated protein (YAP) and transcriptional co-activator with PDZ-binding motif (TAZ) in mammals, respectively. Phosphorylated Yorkie, YAP, and TAZ are sequestered in the cytoplasm by interaction with 14-3-3 proteins. Here we review recent progresses of this pathway by focusing on how these proteins communicate with each other and how loss of regulation results in cancers.

800 facets of retinal degeneration

In today's world of genomics and large computational analyses, rapid progress has been made in identifying genes associated with human retinal diseases. Nevertheless, before significant advances toward effective therapeutic intervention is made, a clearer understanding of the molecular and cellular role of these gene products in normal and diseased photoreceptor cell biology is required. Given the complexity of the vertebrate retina, these advancements are unlikely to be revealed in isolated human cell lines, but instead, will require the use of numerous model systems. Here, we describe several parallels between vertebrate and invertebrate photoreceptor cell biology that are beginning to emerge and advocate the use of Drosophila melanogaster as a powerful genetic model system for uncovering molecular mechanisms of human retinal pathologies, in particular photoreceptor neurodegeneration.

Regulated Crb accumulation controls apical constriction and invagination in Drosophila tracheal cells

Many epithelial tissues undergo extensive remodelling during morphogenesis. How their epithelial features, such as apicobasal polarity or adhesion, are maintained and remodelled and how adhesion and polarity proteins contribute to morphogenesis are two important questions in development. Here, we approach these issues by investigating the role of the apical determinant protein Crumbs (Crb) during the morphogenesis of the embryonic Drosophila tracheal system. Crb accumulates differentially throughout tracheal development and is required for different tracheal events. The earliest requirement for Crb is for tracheal invagination, which is preceded by an enhanced accumulation of Crb in the invagination domain. There, Crb, acting in parallel with the epidermal growth factor receptor (Egfr) pathway, is required for tracheal cell apical constriction and for organising an actomyosin complex, which we propose is mediated by Crb recruitment of moesin (Moe). The ability of a Crb isoform unable to rescue polarity in crb mutants to otherwise rescue their invagination phenotype, and the converse inability of a FERM-binding domain mutant Crb to rescue faulty invagination, support our hypothesis that it is the absence of Crb-dependent Moe enrichment, and not the polarity defect, that mainly underlies the crb invagination phenotype. This hypothesis is supported by the phenotype of lethal giant larvae (lgl); crb double mutants. These results unveil a link between Crb and the organisation of the actin cytoskeleton during morphogenesis.

Evolution of microRNAs and the diversification of species

MicroRNAs (miRNAs) are ancient, short noncoding RNA molecules that regulate the transcriptome through post-transcriptional mechanisms. miRNA riboregulation is involved in a diverse range of biological processes, and misregulation is implicated in disease. It is generally thought that miRNAs function to canalize cellular outputs, for instance as “fail-safe” repressors of gene misexpression. Genomic surveys in humans have revealed reduced genetic polymorphism and the signature of negative selection for both miRNAs themselves and the target sequences to which they are predicted to bind. We investigated the evolution of miRNAs and their binding sites across cichlid fishes from Lake Malawi (East Africa), where hundreds of diverse species have evolved in the last million years. Using low-coverage genome sequence data, we identified 100 cichlid miRNA genes with mature regions that are highly conserved in other animal species. We computationally predicted target sites on the 3′-untranslated regions (3′-UTRs) of cichlid genes to which miRNAs may bind and found that these sites possessed elevated single nucleotide polymorphism (SNP) densities. Furthermore, polymorphic sites in predicted miRNA targets showed higher minor allele frequencies on average and greater genetic differentiation between Malawi lineages when compared with a neutral expectation and nontarget 3′-UTR SNPs. Our data suggest that divergent selection on miRNA riboregulation may have contributed to the diversification of cichlid species and may similarly play a role in rapid phenotypic evolution of other natural systems.

Formation of a Bazooka-Stardust complex is essential for plasma membrane polarity in epithelia

Recruitment of the Crumbs–Stardust polarity complex depends on interactions between Bazooka and the Stardust PDZ domain and is regulated by aPKC-mediated phosphorylation.

Apical–basal polarity in Drosophila melanogaster epithelia depends on several evolutionarily conserved proteins that have been assigned to two distinct protein complexes: the Bazooka (Baz)–PAR-6 (partitioning defective 6)–atypical protein kinase C (aPKC) complex and the Crumbs (Crb)–Stardust (Sdt) complex. These proteins operate in a functional hierarchy, in which Baz is required for the proper subcellular localization of all other proteins. We investigated how these proteins interact and how this interaction is regulated. We show that Baz recruits Sdt to the plasma membrane by direct interaction between the Postsynaptic density 95/Discs large/Zonula occludens 1 (PDZ) domain of Sdt and a region of Baz that contains a phosphorylation site for aPKC. Phosphorylation of Baz causes the dissociation of the Baz–Sdt complex. Overexpression of a nonphosphorylatable version of Baz blocks the dissociation of Sdt from Baz, causing phenotypes very similar to those of crb and sdt mutations. Our findings provide a molecular mechanism for the phosphorylation-dependent interaction between the Baz–PAR-3 and Crb complexes during the establishment of epithelial polarity.

Cortical progenitor expansion, self-renewal and neurogenesis-a polarized perspective

Neural stem and progenitor cells giving rise to neurons in developing mammalian neocortex fall into two principal classes with regard to location of mitosis-apical and basal, and into three principal classes in terms of cell polarity during mitosis-bipolar, monopolar, and nonpolar. Insight has been gained into how inheritance of polarized, apical and basal, cell constituents is related to symmetric versus asymmetric divisions of these progenitors, and how this inheritance is linked to their expansion, self-renewal, and neurogenesis. Retention and inheritance of the basal process emerge as key for self-renewal, notably for the monopolar progenitors of prospective gyrencephalic neocortex that undergo asymmetric mitoses at basal locations. The resulting expansion of the neocortex during evolution is proposed to be associated with an increased cone-shape of radial units.

Antagonistic functions of two stardust isoforms in Drosophila photoreceptor cells

Two Stardust isoforms are expressed in adult Drosophila photoreceptors, which associate with Crumbs and PATJ, but form distinct complexes. Sdt-H and Sdt-D have antagonistic functions on stalk membrane length and light-dependent retinal degeneration, suggesting a fine-tuned balance of different Crumbs complexes regulating photoreceptor homeostasis.

Membrane-associated guanylate kinases (MAGUKs) are scaffolding proteins that organize supramolecular protein complexes, thereby partitioning the plasma membrane into spatially and functionally distinct subdomains. Their modular organization is ideally suited to organize protein complexes with cell type- or stage-specific composition, or both. Often more than one MAGUK isoform is expressed by one gene in the same cell, yet very little is known about their individual in vivo functions. Here, we show that two isoforms of Drosophila stardust, Sdt-H (formerly called Sdt-B2) and Sdt-D, which differ in their N terminus, are expressed in adult photoreceptors. Both isoforms associate with Crumbs and PATJ, constituents of the conserved Crumbs–Stardust complex. However, they form distinct complexes, localized at the stalk, a restricted region of the apical plasma membrane. Strikingly, Sdt-H and Sdt-D have antagonistic functions. While Sdt-H overexpression increases stalk membrane length and prevents light-dependent retinal degeneration, Sdt-D overexpression reduces stalk length and enhances light-dependent retinal degeneration. These results suggest that a fine-tuned balance of different Crumbs complexes regulates photoreceptor homeostasis.

A local autocrine axis in the testes that regulates spermatogenesis

Spermiation--the release of mature spermatozoa from Sertoli cells into the seminiferous tubule lumen--occurs by the disruption of an anchoring device known as the apical ectoplasmic specialization (apical ES). At the same time, the blood-testis barrier (BTB) undergoes extensive restructuring to facilitate the transit of preleptotene spermatocytes. While these two cellular events take place at opposite ends of the Sertoli cell epithelium, the events are in fact tightly coordinated, as any disruption in either process will lead to infertility. A local regulatory axis exists between the apical ES and the BTB in which biologically active laminin fragments produced at the apical ES by the action of matrix metalloproteinase 2 can regulate BTB restructuring directly or indirectly via the hemidesmosome. Equally important, polarity proteins play a crucial part in coordinating cellular events within this apical ES-BTB-hemidesmosome axis. Additionally, testosterone and cytokines work in concert to facilitate BTB restructuring, which enables the transit of spermatocytes while maintaining immunological barrier function. Herein, we will discuss this important autocrine-based cellular axis that parallels the hormonal-based hypothalamic-pituitary-testicular axis that regulates spermatogenesis. This local regulatory axis is the emerging target for male contraception.

Crumbs regulates Salvador/Warts/Hippo signaling in Drosophila via the FERM-domain protein Expanded

BACKGROUND

Altered expression of apicobasal polarity factors is associated with cancer in vertebrates and tissue overgrowth in invertebrates, yet the mechanisms by which these factors affect growth-regulatory pathways are not well defined. We have tested the basis of an overgrowth phenotype driven by the Drosophila protein Crumbs (Crb), which nucleates an apical membrane complex that functionally interacts with the Par6/Par3/aPKC and Scrib/Dlg/Lgl apicobasal polarity complexes.

RESULTS

We find that Crb-driven growth is dependent upon the Salvador/Warts/Hippo (SWH) pathway and its transcriptional effector Yorkie (Yki). Expression of the Crb intracellular domain elevates Yki activity, and this correlates in tissues and cultured cells with loss of Expanded (Ex), an apically localized SWH component that inhibits Yki. Reciprocally, loss of crb elevates Ex levels, although this excess Ex does not concentrate to its normal location at apical junctions. The Ex-regulatory domain of Crb maps to the juxtamembrane FERM-binding motif (JM), a cytoskeletal interaction domain distinct from the PDZ-binding motif (PBM) through which Crb binds polarity factors. Expression of Crb-JM drives Yki activity and organ growth with little effect on tissue architecture, while Crb-PBM reciprocally produces tissue architectural defects without significant effect on Yki activity.

CONCLUSIONS

These studies identify Crb as a novel SWH regulator via JM-dependent effects on Ex levels and localization and suggest that discrete domains within Crb may allow it to integrate junctional polarity signals with a conserved growth pathway.

A role for the extracellular domain of Crumbs in morphogenesis of Drosophila photoreceptor cells

Morphogenesis of Drosophila photoreceptor cells includes the subdivision of the apical membrane into the photosensitive rhabdomere and the associated stalk membrane, as well as a considerable elongation of the cell. Drosophila Crumbs (Crb), an evolutionarily conserved transmembrane protein, organizes an apical protein scaffold, which is required for elongation of the photoreceptor cell and extension of the stalk membrane. To further elucidate the role played by different Crb domains during eye morphogenesis, we performed a structure-function analysis in the eye. The analysis showed that the three variants tested, namely full-length Crb, the membrane-bound intracellular domain and the extracellular domain were able to rescue the elongation defects of crb mutant rhabdomeres. However, only full-length Crb and the membrane-bound intracellular domain could partially restore the length of the stalk membrane, while the extracellular domain failed to do so. This failure was associated with the inability of the extracellular domain to recruit beta(Heavy)-spectrin to the stalk membrane. These results highlight the functional importance of the extracellular domain of Crb in the Drosophila eye. They are in line with previous observations, which showed that mutations in the extracellular domain of human CRB1 are associated with retinitis pigmentosa 12 and Leber congenital amaurosis, two severe forms of retinal dystrophy.