The Agrin/Perlecan-Related Protein Eyes Shut Is Essential for Epithelial Lumen Formation in the Drosophila Retina

Abstract

The formation of epithelial lumina is a fundamental process in animal development. Each ommatidium of the Drosophila retina forms an epithelial lumen, the interrhabdomeral space, which has a critical function in vision as it optically isolates individual photoreceptor cells. Ommatidia containing an interrhabdomeral space have evolved from ancestral insect eyes that lack this lumen, as seen, for example, in bees. In a genetic screen, we identified eyes shut (eys) as a gene that is essential for the formation of matrix-filled interrhabdomeral space. Eys is closely related to the proteoglycans agrin and perlecan and secreted by photoreceptor cells into the interrhabdomeral space. The honeybee ortholog of eys is not expressed in photoreceptors, raising the possibility that recruitment of eys expression has made an important contribution to insect eye evolution. Our findings show that the secretion of a proteoglycan into the apical matrix is critical for the formation of epithelial lumina in the fly retina.

Mechanotransduction and auditory transduction in Drosophila

“…4). The extracellular, agrin-related protein Eyes shut, which is secreted into the scolopale cavity and concentrated over the middle part of the cilium [46], could possibly interact with the TRPV channel, but its distribution is not altered in nan or iav mutants (our unpublished data). Alternatively, the NAN/IAV channel could be activated by tension in the ciliary membrane or by movements of the axoneme.…”

“…The scolopale that encapsulates the cilia first appears as a cleft, which expands into two concentric cavities that ultimately coalesce into a membrane-lined space [13,14], marked as apical by the Crumbs protein [46]. The cavity walls are reinforced by thick, cytoskeletal rods composed of filamentous actin and tropomyosin with some embedded microtubules [106].…”

“…The distal end of the neuronal sensory process and the ciliary outer segments are shaded in pink and the distal parts of the sheath and scolopale cells in gray; microtubules are indicated as blue lines. Cilia in both es and ch organs can be divided into proximal and distal sections based on the non-overlapping locations of the dendritic cap protein NOMPA [17] and EYS/SPAM, the MAb21A6 antigen [46]. The location of the nompC TRPN channel is unknown, but may be in the outer segment of es organs, and by analogy, the distal part of ch cilia.…”

“…Mutants lacking this antigen have recently been identified. The affected gene, spacemaker [111] /eyes shut [46], encodes an agrinrelated proteoglycan that is also expressed in the eye, where it fills the extracellular, intraommatidial space, optically isolating individual photoreceptors. However, spam/eys mutants do not show the severe uncoordination typical of mechanosensory bristle mutants, so it does not appear to be essential for bristle function.…”

Opening the “Black Box”: The Genetic and Biochemical Basis of Eye Evolution

“…Open rhabdomes, where photoreceptor cells that are spread out within each facet of the eye, as opposed to tightly packed, are present in the compound eye of flies. The open rhabdome is associated with expression of the gene eyes shut, also called spam (Husain et al 2006;Zelhof et al 2006). This correlation was tested by mis-expression of the spam gene in an eye with tightly packed receptors.…”