Function and specificity of synthetic Hox transcription factors in vivo
Dimitrios K. PapadopoulosVladana VukojevićYoshitsugu AdachiLars TereniusRudolf RiglerWalter J. Gehring
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Homeotic ( Hox ) genes encode transcription factors that confer segmental identity along the anteroposterior axis of the embryo. However the molecular mechanisms underlying Hox -mediated transcription and the differential requirements for specificity in the regulation of the vast number of Hox -target genes remain ill-defined. Here we show that synthetic Sex combs reduced ( Scr ) genes that encode the Scr C terminus containing the homedomain (HD) and YPWM motif (Scr-HD) are functional in vivo. Synthetic Scr-HD peptides can induce ectopic salivary glands in the embryo and homeotic transformations in the adult fly, act as transcriptional activators and repressors during development, and participate in protein-protein interactions. Their transformation capacity was found to be enhanced over their full-length counterpart and mutations known to transform the full-length protein into constitutively active or inactive variants behaved accordingly in the synthetic peptides. Our results show that synthetic Scr -HD genes are sufficient for homeotic function in Drosophila and suggest that the N terminus of Scr has a role in transcriptional potency, rather than specificity. We also demonstrate that synthetic peptides behave largely in a predictable way, by exhibiting Scr -specific phenotypes throughout development, which makes them an important tool for synthetic biology.Keywords:
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Abstract The development of diverse structures on the anterior–posterior axis of animal embryos is dependent on homeotic or Hox genes. The expression and function of the Hox genes varies during evolution and underlies some of the morphological differences in different animal taxa.
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The Hox gene products are transcription factors involved in specifying regional identity along the anteroposterior body axis. In the mouse, several single mutants for Hox genes show variably penetrant, partial homeotic transformations of vertebrae at their anterior limits of expression, suggesting that compound Hox mutants might show more complete transformations with greater penetrance than the single Hox mutants. Compound mutants for the paralogous group 3 genes, hoxa-3 and hoxd-3, show deletion of a cervical vertebrae, which is not readily interpretable in terms of an alteration in regional identity. Here, we report the skeletal phenotypes of compound mutants in the group 4 Hox genes, hoxa-4, hoxb-4, and hoxd-4. Mice mutant for each of these genes were intercrossed to generate the three possible double mutant combinations and the triple mutant. In contrast to the hoxa-3, hoxd-3 double mutants, group 4 Hox compound mutants displayed clear alterations in regional identity, including a nearly complete transformation of the second cervical vertebrae toward the morphology of the first cervical vertebra in one double mutant combination. In comparing the types of homeotic transformations observed, different double mutant combinations showed different degrees of synergism. These results suggest a certain degree of functional redundancy among paralogous genes in specifying regional identity. Furthermore, there was a remarkable dose-dependent increase in the number of vertebrae transformed to a first cervical vertebra identity, including the second through the fifth cervical vertebrae in the triple mutant. Thus, these genes are required in a larger anteroposterior domain than is revealed by the single mutant phenotypes alone, such that multiple mutations in these genes result in transformations of vertebrae that are not at their anterior limit of expression.
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Ultrabithorax
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Abstract Hox genes establish regional identity along the anterior-posterior axis in diverse animals. Changes in Hox expression can induce striking homeotic transformations, where one region of the body is transformed into another. Previous work in Drosophila has demonstrated that Hox cross-regulatory interactions are crucial for maintaining proper Hox expression. One major mechanism is the phenomenon of “posterior prevalence”, wherein anterior Hox genes are repressed by more posterior Hox genes. Loss of posterior Hox expression under this model would predict posterior-to-anterior transformations, as is frequently observed in Drosophila . While posterior prevalence is thought to occur in many animals, studies of such Hox cross-regulation have focused on a limited number of organisms. In this paper, we examine the cross-regulatory interactions of three Hox genes, Ultrabithorax (Ubx), abdominal-A (abd-A) , and Abdominal-B (Abd-B) in patterning thoracic and abdominal appendages in the amphipod crustacean Parhyale hawaiensis . Studies of Hox function in Parhyale have previously revealed two striking phenotypes which differed markedly from what a “posterior prevalence” model would predict, including non-contiguous and anterior-to-posterior transformations. We probe the logic of Parhyale Hox cross-regulation by using CRISPR/Cas9 to systematically examine all combinations of Ubx, abd-A , and Abd-B loss of function in Parhyale . By analyzing homeotic phenotypes and examining the expression of additional Hox genes, we reveal Hox cross-regulatory interactions in Parhyale . From these data, we also demonstrate that some Parhyale Hox genes function combinatorially to specify posterior limb identity, rather than abiding by a posterior prevalence mechanism. These results provide evidence that combinatorial Hox interactions may be responsible for the tremendous body plan diversity of crustaceans. Graphical Abstract
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Vertebral column
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Homeobox A1
Homeobox protein Nkx-2.5
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ABSTRACT Comparisons between Hox genes in different arthropods suggest that the diversity of Antennapedia-class homeotic genes present in modern insects had already arisen before the divergence of insects and crustaceans, probably during the Cambrian. Hox gene duplications are therefore unlikely to have occurred concomitantly with trunk segment diversification in the lineage leading to insects. Available data suggest that domains of homeotic gene expression are also generally conserved among insects, but changes in Hox gene regulation may have played a significant role in segment diversification. Differences that have been documented alter specific aspects of Hox gene regulation within segments and correlate with alterations in segment morphology rather than overt homeotic transformations. The Drosophila Hox cluster contains several homeobox genes that are not homeotic genes -bicoid, fushi-tarazu and zen. The role of these genes during early development has been studied in some detail. It appears to be without parallel among the vertebrate Hox genes. No well conserved homologues of these genes have been found in other taxa, suggesting that they are evolving faster than the homeotic genes. Relatively divergent Antp-class genes isolated from other insects are probably homologues of fushi-tarazu, but these are almost unrecognisable outside of their homeodomains, and have accumulated approximately 10 times as many changes in their homeodomains as have homeotic genes in the same comparisons. They show conserved patterns of expression in the nervous system, but not during early development.
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Many common sets of genes are used to generate diverse animal body plans. One set of these genes are the Hox genes, transcription factors that specify segmental identity along the anterior-posterior axis of animals in early development. Many studies have been carried out to uncover how the evolution of Hox genes and Hox gene function may have precipitated the evolution of diverse body plans. I carried out functional assays in Drosophila melanogaster embryos to explore whether changes in protein sequence may have facilitated the divergence of six-legged insects from multi-legged crustaceans. I developed fluorescent immunohistochemistry and double in situ hybridization methods in the crustacean, Artemia franciscana, to further clarify the HOX expression patterns in the trunk. From these studies, I found an example of a Hox gene capable of homeotic function, but inhibited from expression and presumably, inhibited from conferring segmental identity. This loss of segment identity function may contribute to the overall morphological body plan of Artemia to ensure the development of limbs throughout the trunk
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Abstract The development of diverse structures on the anterior–posterior axis of animal embryos is dependent on homeotic or Hox genes. The expression and function of the Hox genes varies during evolution and underlies some of the morphological differences in different animal taxa.
Body plan
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Significant changes have occurred in the developmental role of Hox genes, even within groups of arthropods that already have complex body plans and many different segment types. This is hard to reconcile with the 'selector gene' model for Hox gene function. Selector genes act as stable binary switches that direct lineages of cells to adopt alternative developmental fates. This model suggests that the regulation of selector genes can only evolve through mutations that alter the identity of whole developmental compartments -in the case of Hox genes, whole segments. Once segments have evolved distinct morphology and function, such mutations will result in dramatic homeotic transformations that are unlikely to be tolerated by natural selection. Thus we would expect the developmental role of these "master control genes" to become frozen as body plans become more complex. I argue for a revised model for the role and regulation of the Hox genes. This provides alternative mechanisms for evolutionary change, that may lead to incremental changes in segment morphology. The summation of such changes over long periods of time would result in differences in Hox gene function between taxa comparable to the effects of gross homeotic mutations, without the need to invoke the selective advantage of hopeful monsters.
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Evolutionary developmental biology
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