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The apical transmembrane protein Crumbs is a central regulator of epithelial apical-basal polarity in Drosophila. Loss-of-function mutations in the human homologue of Crumbs, CRB1 (RP12), cause recessive retinal dystrophies, including retinitis pigmentosa. Here we show that Crumbs and CRB1 localize to corresponding subdomains of the photoreceptor apical plasma membrane: the stalk of the Drosophila photoreceptor and the inner segment of mammalian photoreceptors. These subdomains support the morphogenesis and orientation of the photosensitive membrane organelles: rhabdomeres and outer segments, respectively. Drosophila Crumbs is required to maintain zonula adherens integrity during the rapid apical membrane expansion that builds the rhabdomere. Crumbs also regulates stalk development by stabilizing the membrane-associated spectrin cytoskeleton, a function mechanistically distinct from its role in epithelial apical-basal polarity. We propose that Crumbs is a central component of a molecular scaffold that controls zonula adherens assembly and defines the stalk as an apical membrane subdomain. Defects in such scaffolds may contribute to human CRB1-related retinal dystrophies.

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Mutations in the Notch locus affect a variety of developmental decisions in Drosophila. In this paper, we examine the role of Notch in the developing retina. We reduced Notch activity at successive intervals during development of the retina, and then examined the effect on individual cells. When Notch activity was reduced, cells responded by selecting inappropriate developmental pathways. We found that all cell types appear to require Notch when establishing their fate. To examine further Notch's role in eye development, we examined two alleles of Notch-split and facet-glossy, split flies show defects in the initial clustering of photoreceptors, whereas the defects in facet-glossy flies are due to the misrouting of presumptive primary pigment cells into the secondary pigment cell pathway. Our results suggest that Notch plays a permissive role in the cell-cell interactions used to assemble the eye.

In developing Drosophila photoreceptors, rhodopsin is trafficked to the rhabdomere, a specialized domain within the apical membrane surface. Rab11, a small GTPase implicated in membrane traffic, immunolocalizes to the trans-Golgi network, cytoplasmic vesicles and tubules, and the base of rhabdomeres. One hour after release from the endoplasmic reticulum, rhodopsin colocalizes with Rab11 in vesicles at the base of the rhabdomere. When Rab11 activity is reduced by three different genetic procedures, rhabdomere morphogenesis is inhibited and rhodopsin-bearing vesicles proliferate within the cytosol. Rab11 activity is also essential for development of MVB endosomal compartments; this is probably a secondary consequence of impaired rhabdomere development. Furthermore, Rab11 is required for transport of TRP, another rhabdomeric protein, and for development of specialized membrane structures within Garland cells. These results establish a role for Rab11 in the post-Golgi transport of rhodopsin and of other proteins to the rhabdomeric membranes of photoreceptors, and in analogous transport processes in other cells.

A stereotyped, three-dimensional network of cell-cell contacts mediated by adherens junctions and cell-extracellular matrix contacts mediated by focal adhesions defines the architecture of the Drosophila ommatidium. Developmental reconstruction shows that this network is built in an incremental and generally conservative sequence; contacts established early in eye development typically persist into adulthood. Reconstructions show that photoreceptor apical surfaces are involuted into the retinal epithelium and are subsequently elaborated to form the photosensitive rhabdomeres. Rhabdomeres become aligned to the ommatidial optical axis via their anchorage to the retinal floor at the cone cell plate, a specialized nexus of cell-cell and cell-extracellular matrix contacts. Parallel reconstructions of retinal development in integrin mutants show that several eye phenotypes trace their origin to the structural failure of the cone cell plate.

Sensory neuron terminal differentiation tasks apical secretory transport with delivery of abundant biosynthetic traffic to the growing sensory membrane. We recently showed Drosophila Rab11 is essential for rhodopsin transport in developing photoreceptors and asked here if myosin V and the Drosophila Rab11 interacting protein, dRip11, also participate in secretory transport. Reduction of either protein impaired rhodopsin transport, stunting rhabdomere growth and promoting accumulation of cytoplasmic rhodopsin. MyoV-reduced photoreceptors also developed ectopic rhabdomeres inappropriately located in basolateral membrane, indicating a role for MyoV in photoreceptor polarity. Binary yeast two hybrids and in vitro protein–protein interaction predict a ternary complex assembled by independent dRip11 and MyoV binding to Rab11. We propose this complex delivers morphogenic secretory traffic along polarized actin filaments of the subcortical terminal web to the exocytic plasma membrane target, the rhabdomere base. A protein trio conserved across eukaryotes thus mediates normal, in vivo sensory neuron morphogenesis.