Gene expression during newt lens regeneration and cephalopod eye evolution

Konstantinos Sousounis


Patterning plays a crucial role during organogenesis. Axial patterning transforms a single sheet of organ primordial cells to a three dimensional organ by defining the Dorsal-Ventral (DV), Anterior-Posterior (AP) and Proximal-Distal (PD) axis. In the Drosophila eye, DV patterning is the first lineage restriction event, which results in formation of the dorsal and ventral compartments of the eye. Loss-of-function (LOF) of genes involved in DV patterning results in loss of the developing eye field. Understanding the mechanism of this crucial process is far from complete, as there is a need to identify more genetic components of this pathway. We have identified defective proventriculus (dve) as a new member of the DV patterning gene hierarchy using the Drosophila eye model. We have shown that dve is expressed in the dorsal domain of the developing eye imaginal disc and induces a downstream target gene wingless (wg), to promote head specific fate and thereby define the boundary between the eye and the head vertex region. Loss of Wg signaling, within the domain of dve expression results in ectopic eye formation. Ectopic eyes seen in the region that forms the head cuticle and antenna of the adult fly (where dve is not expressed) explains the non-autonomous eyes and dorsal eye enlargements seen by blocking dve mediated regulation of Wg. We propose that the ectopic eyes observed in the dve loss-of-function phenotype is due to downregulation of the highly conserved Wg signaling pathway. Interestingly, change of cell fates also involves the role of highly conserved signaling pathways like Wg, Decapentaplegic (Dpp) and Hedgehog (Hh). We will investigate, if dve can regulate morphogen gradients of these signaling molecules to allocate a fate within the developing eye, head and antennal field. Genetic epistasis shows that dve acts downstream of pannier (pnr) to regulate Wg expression in the dorsal eye.We have characterized the novel role of a K50 homeodomain transcription factor, Dve, in regulating Wg expression in the developing eye. dve has a human ortholog, SATB1 (special AT rich sequence binding protein). We found that Dve expressing cells are the sites for expression of Wg in the dorsal head vertex region of eye imaginal disc. Furthermore, we found that dve is involved in generating the Wg gradient in the eye to determine eye versus head fate. This mechanism may also be conserved in other insects like Lucilia sericata and Phormina regina that display sexual dimorphic traits and differential expression of dve may contribute towards this trait. During development, gene regulation occurs by complex transcriptional networks that drive cell-specific patterns of gene expression. At a molecular level, transcriptional programs are orchestrated by the recruitment of transcription factors (TFs) to enhancer elements or cis-regulatory modules (CRMs) that act as modular units, giving rise to a specific spatial-temporal output of gene expression. Using the enhancer library, a valuable tool to identify enhancers we have been able to identify two eye specific enhancers and one wing specific enhancer of dve. We would further use these tissue-specific enhancer lines of dve to identify the TF binding sites that regulate dve expression. Our studies have provided insights into the molecular genetic mechanism by which dve regulates delineation of an eye versus head fate during development and assigned it to the dorsal eye gene hierarchy as a new DV patterning gene.