The PBC domain contains a MEINOX domain: Coevolution of Hox and TALE homeobox genes?

 A recent survey of TALE superclass homeobox genes revealed a new domain upstream of the homeodomain that is conserved between the plant KNOX genes and the animal MEIS genes. At the same time, another paper identified the Drosophila gene homothorax (hth) as a homologue of the vertebrate MEIS genes, which prompted a reexamination of the sequences of the MEIS, KNOX (collectively named MEINOX) and PBC domains. Similarity of the complete MEINOX domain was found within the PBC domain. This suggests that the PBC class genes were also derived from the ancient MEINOX genes. Recently, it has been shown that the MEIS genes can interact with the Abd-B genes, whilst previous results have shown that the PBC genes interact with anterior Hox genes. This leads to the hypothesis that the duplication of an ancestral MEINOX gene into the PBC and MEIS genes happened at a point in time when the first two Hox cluster genes, an anterior one and a posterior one, emerged, and that subsequently these gene classes coevolved. Received: 19 January 1998 / Accepted: 11 February 1998     

Segment-specific generation of Drosophila Capability neuropeptide neurons by multi-faceted Hox cues

In the Drosophila ventral nerve cord, the three pairs of Capability neuropeptide-expressing Va neurons are exclusively found in the second, third and fourth abdominal segments (A2–A4). To address the underlying mechanisms behind such segment-specific cell specification, we followed the developmental specification of these neurons. We find that Va neurons are initially generated in all ventral nerve cord segments and progress along a common differentiation path. However, their terminal differentiation only manifests itself in A2–A4, due to two distinct mechanisms: segment-specific programmed cell death (PCD) in posterior segments, and differentiation to an alternative identity in segments anterior to A2. Genetic analyses reveal that the Hox homeotic genes are involved in the segment-specific appearance of Va neurons. In posterior segments, the Hox gene Abdominal-B exerts a pro-apoptotic role on Va neurons, which involves the function of several RHG genes. Strikingly, this role of Abd-B is completely opposite to its role in the segment-specific apoptosis of other classes of neuropeptide neurons, the dMP2 and MP1 neurons, where Abd-B acts in an anti-apoptotic manner. In segments A2–A4 we find that abdominal A is important for the terminal differentiation of Va cell fate. In the A1 segment, Ultrabithorax acts to specify an alternate Va neuron fate. In contrast, in thoracic segments, Antennapedia suppresses the Va cell fate. Thus, Hox genes act in a multi-faceted manner to control the segment-specific appearance of the Va neuropeptide neurons in the ventral nerve cord.     

Of mites and zen: expression studies in a chelicerate arthropod confirm zen is a divergent Hox gene

 We have cloned, from an oribatid mite, a gene homologous to the zerknült (zen) genes of insects and the Hox 3 genes of vertebrates. Hox genes specify cell fates in specific regions of the body in all metazoans studied and are expressed in antero-posteriorly restricted regions of the embryo. This is true of the vertebrate Hox 3 but not of the zen genes, the insect homologs, and it has been proposed that the zen genes have lost their Hox-like function in the ancestor of the insects. We studied expression of a mite Hox 3/zen homolog and found that it is expressed in a discrete antero-posterior region of the body with an anterior boundary coinciding with that of the chelicerate homolog of the Drosophila Hox gene, proboscipedia, and propose that its loss of Hox function in insects is due to functional redundancy due to this overlap with another Hox gene. Received: 23 April 1998 / Accepted: 25 August 1998     

Differential Activity of Drosophila Hox Genes Induces Myosin Expression and Can Maintain Compartment Boundaries

Compartments are units of cell lineage that subdivide territories with different developmental potential. In Drosophila, the wing and haltere discs are subdivided into anterior and posterior (A/P) compartments, which require the activity of Hedgehog, and into dorsal and ventral (D/V) compartments, needing Notch signaling. There is enrichment in actomyosin proteins at the compartment boundaries, suggesting a role for these proteins in their maintenance. Compartments also develop in the mouse hindbrain rhombomeres, which are characterized by the expression of different Hox genes, a group of genes specifying different structures along their main axis of bilaterians. We show here that the Drosophila Hox gene Ultrabithorax can maintain the A/P and D/V compartment boundaries when Hedgehog or Notch signaling is compromised, and that the interaction of cells with and without Ultrabithorax expression induces high levels of non-muscle myosin II. In the absence of Ultrabithorax there is occasional mixing of cells from different segments. We also show a similar role in cell segregation for the Abdominal-B Hox gene. Our results suggest that the juxtaposition of cells with different Hox gene expression leads to their sorting out, probably through the accumulation of non-muscle myosin II at the boundary of the different cell territories. The increase in myosin expression seems to be a general mechanism used by Hox genes or signaling pathways to maintain the segregation of different groups of cells.     

Expression of the human FUSED protein in <Emphasis Type="Italic">Drosophila</Emphasis>

The Drosophila segment polarity gene fused, which encodes a serine threonine kinase, is required to transmit the Hedgehog (Hh) signal in imaginal discs. To explore the functional homology between the human protein FUSED (hFU) and the Drosophila protein fused (dFu), we have subjected hFU to a precise and well-defined Hh signalling assay of Drosophila wing development. In the wildtype, hFU affects the expression of Hh target genes leading thus to defects in adult wings. In fu mutants, overexpression of hFU cannot rescue the fu phenotype. These results suggest that hFU in Drosophila interferes with endogenous Hh signalling probably by competing with endogenous dFu when binding its partners but cannot perform the normal Fu function.Edited by C. Desplan     

Drosophila morgue and the intersection between protein ubiquitination and programmed cell death

In Drosophila, cell survival decisions are mediated by the integrated functions of the Grim-Reaper death activators and Inhibitor-of-Apoptosis-Proteins (IAPs), such as DIAP1, to regulate caspase activities. We recently identified a gene that enhances the actions of the Grim-Reaper proteins and negatively regulates the levels of DIAP1 protein. This gene, morgue, encodes a novel protein that contains both an F box and a ubiquitin conjugase domain. Interestingly, the Morgue conjugase domain lacks the active site cysteine required for covalent linkage to ubiquitin. Morgue could target IAPs and other proteins for ubiquitination and proteasome-dependent turnover by acting either in an SCF ubiquitin E3 ligase complex, or as a ubiquitin E2 conjugase enzyme variant (UEV) in conjunction with a catalytically active E2 conjugase. Morgue is evolutionarily conserved, as a Morgue ortholog was identified from the mosquito, Anopheles gambiae. Elucidation of morgue function should provide novel insights into the mechanisms of ubiquitination and programmed cell death.     

Hox Genes from the Tapeworm Taenia asiatica (Platyhelminthes: Cestoda)

Hox genes are important in forming the anterior-posterior body axis pattern in the early developmental stage of animals. The conserved nature of the genomic organization of Hox genes is well known in diverse metazoans. To understand the Hox gene architecture in human-infecting Taenia tapeworms, we conducted a genomic survey of the Hox gene using degenerative polymerase chain reaction primers in Taenia asiatica. Six Hox gene orthologs from 276 clones were identified. Comparative analysis revealed that T. asiatica has six Hox orthologs, including two lab/Hox1, two Hox3, one Dfd/Hox4, and one Lox2/Lox4. The results suggest that Taenia Hox genes may have undergone independent gene duplication in two Hox paralogs. The failure to detect Post1/2 orthologs in T. asiatica may suggest that sequence divergence or the secondary loss of the posterior genes has occurred in the lineage leading to the cestode and trematode.     

Yeast Srp1, a nuclear protein related toDrosophila and mouse pendulin, is required for normal migration, division, and integrity of nuclei during mitosis

This paper describes genes from yeast and mouse with significant sequence similarities to aDrosophila gene that encodes the blood cell tumor suppressor pendulin. The protein encoded by the yeast gene, Srp1p, and mouse pendulin share 42% and 51% amino acid identity withDrosophila pendulin, respectively. All three proteins consist of 10.5 degenerate tandem repeats of 42 amino acids each. Similar repeats occur in a superfamily of proteins that includes theDrosophila Armadillo protein. All three proteins contain a consensus sequence for a bipartite nuclear localization signal (NLS) in the N-terminal domain, which is not part of the repeat structure. Confocal microscopic analysis of yeast cells stained with antibodies against Srp1p reveals that this protein is intranuclear throughout the cell cycle. Targeted gene disruption shows thatSRP1 is an essential gene. Despite their sequence similarities,Drosophila and mouse pendulin are unable to rescue the lethality of anSRP1 disruption. We demonstrate that yeast cells depleted of Srp1p arrest in mitosis with a G2 content of DNA. Arrested cells display abnormal structures and orientations of the mitotic spindles, aberrant segregation of the chromatin and the nuclei, and threads of chromatin emanating from the bulk of nuclear DNA. This phenotype suggests that Srplp is required for the normal function of microtubules and the spindle pole bodies, as well as for nuclear integrity. We suggest that Srp1p interacts with multiple components of the cell nucleus that are required for mitosis and discuss its functional similarities to, and differences fromDrosophila pendulin.     

Distinct Molecular Strategies for Hox-Mediated Limb Suppression in Drosophila: From Cooperativity to Dispensability/Antagonism in TALE Partnership

The emergence following gene duplication of a large repertoire of Hox paralogue proteins underlies the importance taken by Hox proteins in controlling animal body plans in development and evolution. Sequence divergence of paralogous proteins accounts for functional specialization, promoting axial morphological diversification in bilaterian animals. Yet functionally specialized paralogous Hox proteins also continue performing ancient common functions. In this study, we investigate how highly divergent Hox proteins perform an identical function. This was achieved by comparing in Drosophila the mode of limb suppression by the central (Ultrabithorax and AbdominalA) and posterior class (AbdominalB) Hox proteins. Results highlight that Hox-mediated limb suppression relies on distinct modes of DNA binding and a distinct use of TALE cofactors. Control of common functions by divergent Hox proteins, at least in the case studied, relies on evolving novel molecular properties. Thus, changes in protein sequences not only provide the driving force for functional specialization of Hox paralogue proteins, but also provide means to perform common ancient functions in distinct ways.     

Conserved boundary elements from the Hox complex of mosquito,Anopheles gambiae

The conservation of hox genes as well as their genomic organization across the phyla suggests that this system of anterior–posterior axis formation arose early during evolution and has come under strong selection pressure. Studies in the split Hox cluster of Drosophila have shown that proper expression of hox genes is dependent on chromatin domain boundaries that prevent inappropriate interactions among different types of cis-regulatory elements. To investigate whether boundary function and their role in regulation of hox genes is conserved in insects with intact Hox clusters, we used an algorithm to locate potential boundary elements in the Hox complex of mosquito, Anopheles gambiae. Several potential boundary elements were identified that could be tested for their functional conservation. Comparative analysis revealed that like Drosophila, the bithorax region in A. gambiae contains an extensive array of boundaries and enhancers organized into domains. We analysed a subset of candidate boundary elements and show that they function as enhancer blockers in Drosophila. The functional conservation of boundary elements from mosquito in fly suggests that regulation of hox genes involving chromatin domain boundaries is an evolutionary conserved mechanism and points to an important role of such elements in key developmentally regulated loci.     

Regulation of Krüppel expression in the anlage of the Malpighian tubules in the Drosophila embryo

The expression of most Drosophila segmentation genes is not limited to the early blastoderm stage, when the segmental anlagen are determined. Rather, these genes are often expressed in a variety of organs and tissues at later stages of development. In contrast to the early expression, little is known about the regulatory interactions that govern the later expression patterns. Among other tissues, the central gap gene Krüppel is expressed and required in the anlage of the Malpighian tubules at the posterior terminus of the embryo. We have studied the interaction of Krüppel with other terminal genes. The gap genes tailles and huckebein, which repress Krüppel in the central segmentation domain, activate Krüppel expression in the posterior Malpighian tubule domain. The opposite effect on the posterior Krüppel expression is achieved by the interposition of another factor, the homeotic gene fork head, which is not involved in the control of the central domain. In addition, Krüppel activates different genes in the Malpighian tubules than in the central domain. Thus, both the regulation and the function of Krüppel in the Malpighian tubules differ strikingly from its role in segmentation.