Master control genes in development and evolution

 

Homeotic genes still keep revealing fascinating and unexpected facets. For example the essential differences between the homeotic Antennapedia and Sex combs reduced genes can be narrowed down to two single residues in the N-terminal arm of their homeodomains, which in Sex combs reduced can be phosphorylated. The phosphorylated form is inactive, and protein phosphatase PP2a is capable of specific dephosphorylation of the N-terminal arm and activation of the homeodomain (Berry and Gehring, 2000).

A novel concept emerged from our analysis of the role of Antennapedia in the inhibition of eye development which is under the control of eyeless (ey) a homolog of Pax 6. Antennapedia (Antp) antagonizes the ey gene cascade without interfering with transcription nor translation of ey. Using both in vitro and in vivo experiments, we demonstrated that this inhibitory mechanism involves direct protein-protein interactions between the homeodomain of Antp on the one hand, and the paired domain and the homeodomain of ey on the other (Plaza et al., 2001). These findings indicate that the paired- and homeodomains are not only DNA-binding domains, but also specific protein-protein interaction domains and lead to a new concept of gene regulatory networks, in which superimposed on the transcriptional network, a network of protein-protein interactions plays a crucial role.

Homeotic genes can induce cell fate changes only under certain preconditions. In the paper by Kurata et al. (2000) we show that Notch plays an important role in these cell fate changes. The Notch signaling pathway defines an evolutionarily conserved cell-cell interaction mechanism which throughout development controls the ability of precursor cells to respond to developmental signals. We were able to show that Notch signaling regulates the expression of the master control genes eyeless, vestigial, and distal-less, which in combination with homeotic genes induce the formation of either eyes, wings, antennae or legs on the head of the fly. Therefore, Notch is involved in a common regulatory pathway for the determination of the various appendages in Drosophila.

 

The genetic control of eye development and its evolutionary implications

Optix is a more recently discovered gene of the sine oculis family and presumably the homolog of Six 3 in mice. Gain-of-function experiments indicate that optix plays an important role in eye development, which is clearly distinct from sine oculis, which is a direct target gene of ey. Optix is expressed independently of ey, and in contrast to sine oculis, optix alone is capable of inducing ectopic eyes in the antennal disc. These results suggest that optix is involved in eye-morphogenesis by an eyeless-independent mechanism (Seimiya & Gehring, 2000). The further analysis of optix has to await the isolation of loss-of-function mutants. The evolutionary studies have been extended to the base of the metazoan pedigree: Whereas a clear cut ortholog of Pax 6 has been found in planarians and nemerteans, the situation in cnidarians is more difficult to interpret (Miller et al. 2000; Gröger et al. 2000). However, medusae with well developed eyes remain to be studied.

The development of methods for RNA interference offers evolution biologists new possibilities to perform a functional genetic analysis. One might call this method also “poor man’s genetics”. In any case, in collaboration with Emili Salo we have succeeded in knocking out sine oculis RNA in Dugesia, a flatworm, which renders these worms incapable of regenerating their eyes. This indicates that the gene cascade for eye morphogenesis in flatworms is similar to the one of insects and vertebrates.

 

Eye Organogenesis Programme

After many years of basic science in Drosophila and other model systems, I decided to try some possible medical applications. Seed money provided by the University was successfully used to launch a programme on age-related macula degeneration, a degenerative disease affecting a very large fraction of the aging population leading to blindness. We joined forces with the ophthalmologists in Lausanne (Munier and Schorderet) and in Zurich (Remé and Grimm), and try to transfer our knowledge gained on flies and mice to possible application in humans.