dachshund: Biological Overview | Evolutionary Homologs | Regulation | Targets of Activity | Developmental Biology | Effects of Mutation | References

Gene name - dachshund

Synonyms -

Cytological map position - 36A1--36A2

Function - transcriptional co-repressor

Keywords - leg, eye

Symbol - dac

FlyBase ID: FBgn0005677

Genetic map position - 2-[52]

Classification - Dac and Ski/Sno DS domain

Cellular location - nuclear



NCBI links: Precomputed BLAST | Entrez Gene | HomoloGene | UniGene

Recent literature
Bras-Pereira, C., Potier, D., Jacobs, J., Aerts, S., Casares, F. and Janody, F. (2016). dachshund potentiates Hedgehog signaling during Drosophila retinogenesis. PLoS Genet 12: e1006204. PubMed ID: 27442438
Summary:
Proper organ patterning depends on a tight coordination between cell proliferation and differentiation. The patterning of Drosophila retina occurs both very fast and with high precision. This process is driven by the dynamic changes in signaling activity of the conserved Hedgehog (Hh) pathway, which coordinates cell fate determination, cell cycle and tissue morphogenesis. This study shows that during Drosophila retinogenesis, the retinal determination gene dachshund (dac) is not only a target of the Hh signaling pathway, but is also a modulator of its activity. Using developmental genetics techniques, dac was demonstrated to enhance Hh signaling by promoting the accumulation of the Gli transcription factor Cubitus interruptus (Ci) parallel to or downstream of fused. In the absence of dac, all Hh-mediated events associated to the morphogenetic furrow are delayed. One of the consequences is that, posterior to the furrow, dac- cells cannot activate a Roadkill-Cullin3 negative feedback loop that attenuates Hh signaling and which is necessary for retinal cells to continue normal differentiation. Therefore, dac is part of an essential positive feedback loop in the Hh pathway, guaranteeing the speed and the accuracy of Drosophila retinogenesis.

BIOLOGICAL OVERVIEW

The mutant phenotype of dachshund inspired its gene name: the legs of mutant flies, in comparison to wild-type and in relation to body length (as in the breed of dog), are extremely short. The wild-type leg is composed of ten discrete segments. Moving from proximal (nearest the body) to distal, they are: the coxa, trochanter, femur, tibia, plus five tarsal segments and the claw. While the proximal and distal morphology of these segments in dac mutant legs appears to be normal, the intermediate segments are fused and condensed. Upon eclosion from the pupal cases, these mutants are unable to locomote normally; if allowed, they will quickly fall into their food and die. However, if kept away from wet medium, dac homozygotes can remain, unable to move about, for several days before dying, presumably from dehydration. These helpless homozygotes are able to flail their misshapen legs, albeit to no avail, indicating that at least a portion of the leg neuromusculature develops normally and is functional (Mardon, 1994).

The expression pattern of dachshund during larval leg disc development is consistent with the mutant phenotype of the adult leg. The leg imaginal disc is composed of concentric folds of epithelia such that the outermost portions of the disc give rise to the most proximal segments (i.e. the coxa) in the adult leg while the most distal structures are derived from the central portion of the disc. dachshund is expressed specifically in the presumptive epithelium that is fated to give rise to the femur, tibia and proximal tarsal segments, the same structures most severely affected in dac mutant legs. Although the imaginal discs from dac mutants appear morphologically normal, a significant increase in cell death in dac mutant leg discs is apparent. Elongation, the outgrowth of the leg disc that forms the extended structure of the adult leg, does not occur in mutant flies. Thus there is a failure of morphogenesis in dac mutant leg discs during larval and early pupal development (Mardon, 1994).

dachshund was originally isolated as a dominant suppressor of the Ellipse mutation of the Epidermal growth factor receptor. Ellipse is a hypermorphic allele of Egfr, a dominant hyperactive receptor function that causes a rough eye phenotype. Mutations of the Egfr gene prevent normal spacing and differentiation of photoreceptor cells in the developing eye. These results suggest that loss-of-function mutations in dac, which produce the defective leg phenotype, reduce the eye specific activity of the hyperactive Elp allele of Egfr. The eyes of dac mutant homozygotes are reduced and roughened. In contrast to the compound eye, the external morphology of the adult ocelli appears normal in all dac mutants (Mardon, 1994).

Photoreceptor development is prevented in dachshund mutant eye discs, suggesting a role for dac in ommatidial assembly. Since dac is a nuclear protein, it may be a candidate for regulating the Egfr gene, which is required for normal photoreceptor determination. A highly variable, but reduced number of photoreceptors is present in mutant eyes and just a few Elav positive clusters are formed. Elav is normally expressed in all neurons, and its expression is indicative of the degree of neural maturation of photorecepter precursors. dachshund does not appear to be required specifically for neural differentiation, instead dac function is required for normal movement of the morphogenetic furrow. In the absence of furrow movement, cells in dac mutants fail to adopt a neural fate, remain in an undifferentiated state and eventually die (Mardon, 1994).

The morphogenetic furrow, is a self propagating line of shortened cells that coordinates gene expression and initiation of neural differentiation in the developing eye. dachshund is required for the initiation of the furrow and for the normal movement of the furrow across the eye disc. Clonal analysis reveals that dac is cell autonomously required for initiation of movement of the away from the posterior margin of the eye disc where the furrow originates. This may reflect a direct requirement for dac in furrow initiation (Mardon, 1994).

Dac may have an even more fundamental role in eye development (see Specification of the eye disc primordium and establishment of dorsal/ventral asymmetry). Ectopic expression of dac is sufficient to induce ectopic retinal development in a variety of tissues, including the adult head, thorax and legs. This result is similar to that observed with Drosophila's pax6 homolog, eyeless. The external morphology of dac-induced ectopic eyes closely resembles that of normal adult eyes. Three independent results suggest that dac functions downstream of eyeless:

  1. Misexpression of eyeless in imaginal discs is sufficient to induce ectopic dac expression.
  2. Targeted eyeless expression is unable to induce ectopic eye formation in a dac null mutant, again suggesting that eyeless cannot compensate for the absence of dac. If eyeless functions downstream of dac ectopic eyeless should be able to direct eye differentiation in the absence of dac.
  3. eyeless is still expressed in dac mutant eye discs, demonstrating that dac is not essential for eyeless expression.

These results suggest that dac functions downstream of eyeless and, considering that dac can induce ectopic retinal development, are constent with the idea thad dac may be a direct target of eyeless. If this is true, then dac can also function in eyeless independent pathways, as in the leg, for example. This result points to the likelihood that dac functions in independent combinatorial pathways and highlights the complexity of interactions that may characterize some gene action in development (Shen, 1997).

Since dac is able to induce ectopic eyeless, dac can function as a positive regulator of eyeless. However, neither gene is able to induce photoreceptor in all cells in which it is expressed. For example, ectopic dac expression is unable to efficiently induce retinal development along any part of the A-P compartment boundary of the wing disc. Both dac and eyeless are unable to act alone in the control of gene expression or retinal cell-fate specification. Instead, these genes are likely to require other factors that are expressed in a spatially or temporally restricted pattern during development. Genes acting early in retinal development are potential candidates for such factors, including sine oculis, coding for a homeodomain protein, and clift/eyes absent, coding for a novel protein that like Dachshund, acts in multiple tissues (Shen, 1997).

Mushroom bodies (MBs) are the centers for olfactory associative learning and elementary cognitive functions in the Drosophila brain. By high-resolution neuroanatomy, it has been shown that eyeless, twin of eyeless, and dachshund, which are implicated in eye development, also are expressed in the developing MBs. Mutations of ey completely disrupt the MB neuropils, and a null mutation of dac results in marked disruption and aberrant axonal projections. Genetic analyses demonstrate that, whereas ey and dac synergistically control the structural development of the MBs, the two genes are regulated independently in the course of MB development. These data argue for a distinct combinatorial code of regulatory genes for MBs as compared with eye development and suggest conserved roles of Pax6 homologs in the genetic programs of the olfactory learning centers of complex brains (Kurusu, 2000).

Mushroom bodies (MBs) are a pair of prominent neuropil structures in the insect brain that are implicated as centers for higher-order behaviors including olfactory associative learning and elementary cognitive functions. Anatomically, each MB comprises a large number of densely packed parallel fibers organized into distinct neuronal structures in the brain. The MB cell bodies, Kenyon cells, are located at the dorsal cortex, extending their dendrites into the calyx and their axonal projections through the peduncles, which split dorsally into two lobes, alpha and alpha', and medially into three lobes, beta, beta', and gamma. The calyces of MBs receive olfactory information from the antennal lobes via the prominent antennoglomerular tracts. The peduncles and lobes send neural commands through their connections to the major brain regions including the lateral protocerebrum. These anatomical structures are consistent with the putative MB function: that MBs integrate various sensory information to compute behavioral outputs (Kurusu, 2000).

A Gal4 MB marker, 238Y, identifies the MB primordia in the embryonic brain. Neuroanatomical examination of the developing brains double-stained for 238Y and the Ey protein reveals that Ey is expressed in the embryonic MB primordia. High-resolution imaging shows that Ey is expressed in the MB neuroblasts, ganglion mother cells, and their progenies, suggesting pivotal functions of Ey in various stages of cell differentiation in MB development. In addition to ey, studies on Drosophila eye development have revealed a cascade of regulatory genes that function synergistically in the early specification of eye primordia. Among such regulatory genes involved in eye development, toy also is expressed in the embryonic MBs. Moreover, dac, another gene involved in eye development, also is expressed in the embryonic MBs. However, the expression of the Dac protein is rather confined to ganglion mother cells and embryonic Kenyon cells. Yet, in contrast to the eye development cascade, neither sine oculis (so) nor eyes absent (eya) is expressed in the embryonic MBs, though Eya is detected in nearby cell clusters in the anterior region of the embryonic brain (Kurusu, 2000).

The characteristic expression of ey, toy, and dac in the developing MBs is maintained in the larval brain. Ey is expressed in all of the larval MB cells at a significant level whereas expression of a Gal4 MB marker, 201Y, is absent in the central cells. Expression of toy is also evident in the Kenyon cells. As with ey, toy is expressed in all of the MB cells. On the other hand, Dac is not expressed in the central cells, including neuroblasts and ganglion mother cells, whereas it is clearly detected in distantly located cells. Double staining for Dac and Gal4 MB markers, 201Y, c831 and 238Y, demonstrated that the Gal4 MB markers are expressed in outer cells, which are located several cells diameters away from the central cells. Neither so nor eya is expressed in the larval MBs though they are expressed in nearby cells (Kurusu, 2000).

The distinctive expression profiles of ey, toy, and dac in the embryonic and larval MBs suggest combinatorial regulatory mechanisms in the initial formation and structural development of the MBs. To examine functional significance of these genes in the MBs, the neural structures of the developing MBs were examined in mutant backgrounds of either ey or dac. The larval MBs are topologically similar to the adult MBs but have only two orthogonal lobes, alphaL and betaL. Internally, the peduncles and lobes have simple concentric organization, in which the FAS II proteins are expressed homogeneously except for the central, unstained core. Mutational inactivation of ey results in moderate defects in the larval MBs in all the cases examined, with weak but consistent suppression of FAS II in the peduncles and lobes. The distribution of FAS II also is affected: the globular end of the alphaL-lobe is often devoid of FAS II. In contrast, a null mutation of dac (dac4) barely affects the larval MBs. However, 50% reduction of dac activity in heterozygous larvae enhances the structural defects of ey mutants, suggesting synergistic regulatory functions of the two genes in the development of the MB structures. In the double mutant for ey and dac, most parts of the peduncles and lobes showed clear symptoms of neural degeneration including significant degeneration of the alphaL-lobe in many cases (10%-20%). Furthermore, FAS II expression is markedly suppressed, leaving uneven residual expression in the peduncles and remaining lobes (Kurusu, 2000).

The significance of ey and dac in MB development was examined further in the early pupal stage, in which MBs undergo massive degeneration and reorganization to form the complex adult MB structures. Fifty hours after puparium formation, most of the MB structures are reorganized into the adult architecture, in which FAS II is strongly expressed in the alpha/beta-lobes and peduncles and moderately in the gamma-lobe. In addition, it is heavily expressed in the ellipsoid body, which belongs to the central complex. On the other hand, DIF is strongly expressed in the gamma-lobe and weakly in the other lobes and the peduncle (Kurusu, 2000).

Mutations of ey abolished all the neuropil structures of the pupal MBs in all the cases examined, whereas Kenyon cells expressing Dac are retained. Notably, the ellipsoid body also is disrupted in the mutant. The dac4 mutation disrupts most of the neuropil structures of the pupal MBs, leaving Kenyon cells expressing Ey protein intact. Occasionally dac4 causes ectopic projections of peduncles. In these cases, the structural profile of the FAS II expression resembles that of the larval MB structures, with homogeneous concentric patterns suggesting failure of reorganization of the MB structures at the onset of pupation. Thus, these results clearly demonstrate the functional importance of ey and dac in the structural formation of the adult MBs in the course of the massive neural reorganization in the early pupal stage. Studies of eye development have revealed a combinatorial network of key regulatory genes, in which toy acts upstream of ey, which initiates the regulatory feedback loop that additionally includes so, eya, and dac. These nuclear regulatory genes then synergistically control the subsequent stages of eye development. To dissect the regulatory network of MB development, an examination was made of the expression of ey, toy, and dac in various mutant backgrounds (Kurusu, 2000).

Whereas Ey and Dac are clearly coexpressed in the embryonic primordia, ey expression is not affected by the loss of dac activity and vice versa. Likewise, Ey and Dac expression is independent of one another's activity in the larval MBs. Ey and Dac are coexpressed in most of the Kenyon cells at the pupal stage except for the central cells, which express only Ey. Again, mutation of ey does not alter the Dac expression though the number of the Kenyon cells is slightly reduced. Mutation of dac does not alter Ey expression at all with the normal number of Kenyon cells (Kurusu, 2000).

Expression of toy is initiated from the cellular blastoderm stage earlier than the onset of ey and dac in both the eye and brain. Consistent with this temporal order of gene expression, neither ey nor dac mutation affects the expression of toy in the developing MBs. Moreover, Dac expression was examined in nullo 4 embryos, which lack both ey and toy genes because of the loss of the fourth chromosome. Despite the fact that the brain is largely deformed in nullo 4 embryos, characteristic MB neuroblasts expressing a nuclear marker are found at a dorsolateral position of each brain hemisphere with Dac-expressing progenies. Taken together, in contrast to the intricate feedback cascade in eye development, these results argue for distinct parallel cascades for the regulation of ey and dac in the developing MBs (Kurusu, 2000).

In vertebrates, Pax6 is expressed in various regions of forebrain, including the anlagen of the olfactory bulb, piriform cortex, and amygdala, which are important to olfactory information processing and emotional learning. Mutations of Pax6 result in profound defects in these forebrain structures as well as other telencephalon regions. Intriguingly, a mouse dac homolog also is expressed in the developing telencephalon in overlapping regions with the Pax6 gene. The findings that, in both Drosophila and mouse, homologs of Pax6 genes are expressed in and required for the development of the neural structures that are important to the olfactory perception and learning raises the possibility that these structures arose very early in brain evolution (Kurusu, 2000).


GENE STRUCTURE

Genomic length - 20 kb

Exons - 12


PROTEIN STRUCTURE

Amino Acids - 1081

Structural Domains

Dachshund is a novel protein which shares no significant similarity to any sequence in the databases (Mardon, 1994).

Ski/Sno proteins share structural homology with the Dach protein. The Dach N-BOX (the dac and ski/sno DS domain) consists of ~100 amino acids conserved with various Sno/Ski family members, predicted to form a highly organized structure of alpha-helices and ß-strands. This domain comprises the critical region of Ski responsible for its oncogenic potential. From the crystallographic analysis of the DACH1 N-BOX (DS domain), it has been proposed that DACH1 might have a general and/or specific DNA binding activity, an idea further supported by studies of DACH1 interaction with chromatin-complexed and naked DNA. Repression by DACH1 requires a conserved DS domain that binds the transcriptional co-repressor NCoR (Wu, 2003 and references therein)

Dachshund (Dac) is a highly conserved nuclear protein that is distantly related to the Ski/Sno family of corepressor proteins. In Drosophila, Dac is necessary and sufficient for eye development and, along with Eyeless (Ey), Sine oculis (So), and Eyes absent (Eya), forms the core of the retinal determination (RD) network. In vivo and in vitro experiments suggest that members of the RD network function together in one or more complexes to regulate the expression of downstream targets. For example, Dac and Eya synergize in vivo to induce ectopic eye formation and they physically interact through conserved domains. Dac contains two highly conserved domains, named DD1 and DD2, but no function has been assigned to either of them in an in vivo context. Structure-function studies were performed to understand the relationship between the conserved domains of Dac and the rest of the protein and to determine the function of each domain during development. Only DD1 is essential for Dac function and while DD2 facilitates DD1, it is not absolutely essential in spite of more than 500 million years of conservation. Moreover, the physical interaction between Eya and DD2 is not required for the genetic synergy between the two proteins. Finally, DD1 also plays a central role for nuclear localization of Dac (Tavsanli, 2003).


dachshund: |
Evolutionary Homologs | Regulation | Targets of Activity | Developmental Biology | Effects of Mutation | References

date revised: 20 December 2006
  

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