The Drosophila eye - a model to study the genetic and cellular basis of RP12 and LCA

The Drosophila compound eye is composed of about 750 individual units, called ommatidia, which are arranged in a highly stereotypic pattern (Figure 6).

Figure 6: Scanning electron micrographs of an adult fly eyes. Eyes show a regular, hexagonal arrangement of ommatidia. Bristles are found at alternating vertices of the hexagonal array.

Each ommatidium is composed of eight photoreceptor cells, four cone cells and two primary pigment cells. Ommatidia are separated from each other by secondary and tertiary pigment cells, and the number and arrangement of these defines the precise honeycomb-like arrangement of the ommatidia. (Figures 7 and 8A).

The adult compound eye develops from a single-layered epithelium, the eye imaginal disc. During the third larval instar, the eight photoreceptor cells and four lens-secreting cone cells are specified by a stereotypic sequence of inductive interactions, while the other support cells are recruited in the first half of pupal development. Also during pupal development, photoreceptors undergo a tremendous morphogenetic cell shape change, resulting in the apposition of their apical surfaces towards the inner of the ommatidium. This is followed by expansion of the apical surface by the formation of a huge number of microvilli, which finally form the rhabdomere that carries the photopigment rhodopsin and the components of the light-induced signal cascade.

Figure 7: Schematic representation of one ommatidium. Left: longitudinal section, right: transverse sections at different levels. (from Cagan & Ready, 1989)

The Drosophila genes crumbs and stardust are essential for the maintenance of polarity and tissue integrity of many ectodermally derived epithelia of the embryo. The MAGUK (membrane-associated guanylate kinase) protein Stardust (Sdt) organises a protein scaffold in the apical cytocortex of embryonic epithelial cells by recruiting the transmembrane protein Crumbs (Crb) and the PDZ-domain proteins DPATJ and DLin-7. This complex, in turn, stabilises the zonula adherens, a belt like structure encircling the apex of cells, which mediates adhesive interactions and thus guarantees the integritiy of the epithelial tissue.

In addition crumbs and stardust are required later in the photoreceptor cells. In these highly polarised cells, which develop from epithelial cells, a similar protein complex composed of Crumbs, Stardust, DPATJ and DLin-7 is localised at the stalk membrane, which topologically corresponds to the subapical region of epithelia cells (Figure 8).

Figure 8: Figure 8: Expression of Crumbs (magenta) in adult eyes. The rhabdomeres are marked in green. Crumbs is localised on the stalk membrane.

Loss of crumbs or stardust results in defective morphogenesis and in light dependent retinal degeneration (Figure 9).

Degeneration can be prevented by reduction in rhodopsin levels. In human, mutations in CRB1 are associated with Retinitis pigmentosa type 12 (RP12) and Leber congenital amaurosis (LCA), the two severe forms of retinal dystrophy. Similarly, mice lacking Crb1 exhibit light-dependent degeneration of photoreceptors. CRB1 co-localises with Pals1/Mpp5, the mammalian homologue of Stardust, at the outer limiting membrane of mouse photoreceptor cells. This striking similarity makes Drosophila an ideal model to study the genetic and molecular basis of RP12 and LCA.

Figure 9: Transverse sections of wild-type eyes (A, C) and eyes mutant for crb^11A22, kept in the dark (B, D) or in the light (E, F). A, B: The wild-type eye is characterised by the regular arrangement of hexagonal ommatidia, which are separated from each other by pigment cells. The dark structures in the center are the rhabdomeres, which carry the photopigment rhodopsin. B, D: crb^11A22, kept in the dark. The mutant ommatidia are recognised by the lack of pigment. Note the thicker rhabdomeres. E, F: crb^11A22, kept for five days in constant light. Nearly all photoreceptor cells are degenerated.