Differential selection within the Drosophila retinal determination network and evidence for functional divergence between paralog pairs

Allocation of distinct organ fates from a precursor field requires a shift in expression and function of gene regulatory networks

A common occurrence in metazoan development is the rise of multiple tissues/organs from a single uniform precursor field. One example is the anterior forebrain of vertebrates, which produces the eyes, hypothalamus, diencephalon, and telencephalon. Another instance is the Drosophila wing disc, which generates the adult wing blade, the hinge, and the thorax. Gene regulatory networks (GRNs) that are comprised of signaling pathways and batteries of transcription factors parcel the undifferentiated field into discrete territories. This simple model is challenged by two observations. First, many GRN members that are thought to control the fate of one organ are actually expressed throughout the entire precursor field at earlier points in development. Second, each GRN can simultaneously promote one of the possible fates choices while repressing the other alternatives. It is therefore unclear how GRNs function to allocate tissue fates if their members are uniformly expressed and competing with each other within the same populations of cells. We address this paradigm by studying fate specification in the Drosophila eye-antennal disc. The disc, which begins its development as a homogeneous precursor field, produces a number of adult structures including the compound eyes, the ocelli, the antennae, the maxillary palps, and the surrounding head epidermis. Several selector genes that control the fates of the eye and antenna, respectively, are first expressed throughout the entire eye-antennal disc. We show that during early stages, these genes are tasked with promoting the growth of the entire field. Upon segregation to distinct territories within the disc, each GRN continues to promote growth while taking on the additional roles of promoting distinct primary fates and repressing alternate fates. The timing of both expression pattern restriction and expansion of functional duties is an elemental requirement for allocating fates within a single field.

A battery of transcription factors collectively called the retinal determination (RD) network controls the earliest steps in the specification of the fruit fly compound eye. Loss-of-function mutations lead to the loss of the compound eyes while over-expression of RD network members in non-retinal tissues induces the formation of ectopic eyes. These observations suggest that the network governs the growth, specification, and patterning of the eye field. Recent studies have also shown that the RD network represses the fates of the non-ocular tissues that are also derived from the disc such as the antenna, maxillary palp, and head epidermis. One inconsistency in the model for how this network controls eye specification is that many of its members are expressed throughout the entire eye-antennal disc. In this study, we show that early in development, the RD network is expressed throughout and promotes the growth of the entire eye-antennal disc. After the initial growth phase, the expression of these genes is restricted to just the eye field. This temporal and spatial limiting of the RD network to the developing eye is essential so that its role can expand to include promoting eye specification and repressing non-ocular fates.

Distinct evolutionary rate in the eye field transcription factors found by estimation of ancestral protein structure

Eye-field transcription factors (EFTFs) are a set of genes that compose a regulatory network for eye development in animals, which are highly conserved among various animal phyla. To investigate the processes of conservation and diversification of the transcription factors for eye development, we examined the structural changes in the EFTF proteins by estimating the ancestral sequences with the available genome information. Among the different types of EFTFs, we selected otx2, tbx3, rx1, pax6, six3/6, lhx2 and nr2e1 because they are highly conserved in bilaterian animals. We searched the genome sequences of representative animal phyla for EFTF protein sequences. With deduced ancestral sequences and three-dimensional structures of EFTFs, we traced the evolutionary changes in amino acid residues and found that the DNA-binding domains were always more conserved than other regions, and that the other regions showed distinct evolutionary rates. The EFTF rx1, which resides at the pivotal part of the EFTF network, had a faster evolutionary rate than the others. These results indicated that the evolutionary rates of each protein in the EFTF network, which were expected to be consistent with each other to maintain the interactions in the network, were not constant among or within the factors, but rather, varied to a significant extent.

The tiptop/teashirt genes regulate cell differentiation and renal physiology in Drosophila

The physiological activities of organs are underpinned by an interplay between the distinct cell types they contain. However, little is known about the genetic control of patterned cell differentiation during organ development. We show that the conserved Teashirt transcription factors are decisive for the differentiation of a subset of secretory cells, stellate cells, in Drosophila melanogaster renal tubules. Teashirt controls the expression of the water channel Drip, the chloride conductance channel CLC-a and the Leukokinin receptor (LKR), all of which characterise differentiated stellate cells and are required for primary urine production and responsiveness to diuretic stimuli. Teashirt also controls a dramatic transformation in cell morphology, from cuboidal to the eponymous stellate shape, during metamorphosis. teashirt interacts with cut, which encodes a transcription factor that underlies the differentiation of the primary, principal secretory cells, establishing a reciprocal negative-feedback loop that ensures the full differentiation of both cell types. Loss of teashirt leads to ineffective urine production, failure of homeostasis and premature lethality. Stellate cell-specific expression of the teashirt paralogue tiptop, which is not normally expressed in larval or adult stellate cells, almost completely rescues teashirt loss of expression from stellate cells. We demonstrate conservation in the expression of the family of tiptop/teashirt genes in lower insects and establish conservation in the targets of Teashirt transcription factors in mouse embryonic kidney.

Dual transcriptional activities of SIX proteins define their roles in normal and ectopic eye development

The SIX family of homeodomain-containing DNA-binding proteins play crucial roles in both Drosophila and vertebrate retinal specification. In flies, three such family members exist, but only two, Sine oculis (So) and Optix, are expressed and function within the eye. In vertebrates, the homologs of Optix (Six3 and Six6) and probably So (Six1 and Six2) are also required for proper eye formation. Depending upon the individual SIX protein and the specific developmental context, transcription of target genes can either be activated or repressed. These activities are thought to occur through physical interactions with the Eyes absent (Eya) co-activator and the Groucho (Gro) co-repressor, but the relative contribution that each complex makes to overall eye development is not well understood. Here, we attempt to address this issue by investigating the role that each complex plays in the induction of ectopic eyes in Drosophila. We fused the VP16 activation and Engrailed repressor domains to both So and Optix, and attempted to generate ectopic eyes with these chimeric proteins. Surprisingly, we find that So and Optix must initially function as transcriptional repressors to trigger the formation of ectopic eyes. Both factors appear to be required to repress the expression of non-retinal selector genes. We propose that during early phases of eye development, SIX proteins function, in part, to repress the transcription of non-retinal selector genes, thereby allowing induction of the retina to proceed. This model of repression-mediated induction of developmental programs could have implications beyond the eye and might be applicable to other systems.

A dissection of the teashirt and tiptop genes reveals a novel mechanism for regulating transcription factor activity

In the Drosophila eye the retinal determination (RD) network controls both tissue specification and cell proliferation. Mutations in network members result in severe reductions in the size of the eye primordium and the transformation of the eye field into head cuticle. The zinc-finger transcription factor Teashirt (Tsh) plays a role in promoting cell proliferation in the anterior most portions of the eye field as well as in inducing ectopic eye formation in forced expression assays. Tiptop (Tio) is a recently discovered paralog of Tsh. It is distributed in an identical pattern to Tsh within the retina and can also promote ectopic eye development. In a previous study we demonstrated that Tio can induce ectopic eye formation in a broader range of cell populations than Tsh and is also a more potent inducer of cell proliferation. Here we have focused on understanding the molecular and biochemical basis that underlies these differences. The two paralogs are structurally similar but differ in one significant aspect: Tsh contains three zinc finger motifs while Tio has four such domains. We used a series of deletion and chimeric proteins to identify the zinc finger domains that are selectively used for either promoting cell proliferation or inducing eye formation. Our results indicate that for both proteins the second zinc finger is essential to the proper functioning of the protein while the remaining zinc finger domains appear to contribute but are not absolutely required. Interestingly, these domains antagonize each other to balance the overall activity of the protein. This appears to be a novel internal mechanism for regulating the activity of a transcription factor. We also demonstrate that both Tsh and Tio bind to C-terminal Binding Protein (CtBP) and that this interaction is important for promoting both cell proliferation and eye development. And finally we report that the physical interaction that has been described for Tsh and Homothorax (Hth) do not occur through the zinc finger domains.