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Review
. 2014 Apr 23:8:96.
doi: 10.3389/fncel.2014.00096. eCollection 2014.

Hox gene regulation in the central nervous system of Drosophila

Maheshwar Gummalla  1 , Sandrine Galetti  2 , Robert K Maeda  2 , François Karch  2
Affiliations
Review

Hox gene regulation in the central nervous system of Drosophila

Maheshwar Gummalla et al. Front Cell Neurosci. .

Abstract

Hox genes specify the structures that form along the anteroposterior (AP) axis of bilateria. Within the genome, they often form clusters where, remarkably enough, their position within the clusters reflects the relative positions of the structures they specify along the AP axis. This correspondence between genomic organization and gene expression pattern has been conserved through evolution and provides a unique opportunity to study how chromosomal context affects gene regulation. In Drosophila, a general rule, often called "posterior dominance," states that Hox genes specifying more posterior structures repress the expression of more anterior Hox genes. This rule explains the apparent spatial complementarity of Hox gene expression patterns in Drosophila. Here we review a noticeable exception to this rule where the more-posteriorly expressed Abd-B Hox gene fails to repress the more-anterior abd-A gene in cells of the central nervous system (CNS). While Abd-B is required to repress ectopic expression of abd-A in the posterior epidermis, abd-A repression in the posterior CNS is accomplished by a different mechanism that involves a large 92 kb long non-coding RNA (lncRNA) encoded by the intergenic region separating abd-A and Abd-B (the iab8ncRNA). Dissection of this lncRNA revealed that abd-A is repressed by the lncRNA using two redundant mechanisms. The first mechanism is mediated by a microRNA (mir-iab-8) encoded by intronic sequence within the large iab8-ncRNA. Meanwhile, the second mechanism seems to involve transcriptional interference by the long iab-8 ncRNA on the abd-A promoter. Recent work demonstrating CNS-specific regulation of genes by ncRNAs in Drosophila, seem to highlight a potential role for the iab-8-ncRNA in the evolution of the Drosophila Hox complexes.

Keywords: Hox genes; abd-A; bithorax-complex; miRNA; ncRNA.

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Figures

FIGURE 1
FIGURE 1
Synopsis of the BX-C. The genomic region of the BX-C is marked off in kilobases according to the numbering of (Martin et al., 1995). The three transcription units Ubx, abd-A, and Abd-B with their exons marked as thick lines and the arrows showing the transcription polarity are drawn below the DNA map. The horizontal and colored brackets above the DNA line indicate the extends of the segment-specific cis- regulatory regions with the following color code. Orange and red (abx/bx and bxd/pb) regulate expression of Ubx in PS5/T3 and PS6/A1, respectively. The blue iab-2, iab-3, and iab-4 regions regulate abd-A expression in PS7/A2, PS8/A3, and PS9/A4. Finally, the green iab-5, iab-6, iab-7, and iab-8 regulate Abd-B expression in PS10/A5, PS11/A6, PS12/A7, and PS13/A8, respectively. These segmental boundaries are depicted with the same colors on the fly above the BX-C map. Note that the parasegmental boundaries are visible in the thoracic segments where PS5 corresponds to the posterior part of T2 and the anterior part of T3. PS6 corresponds to posterior T3 and anterior A1.
FIGURE 2
FIGURE 2
abd-A and Abd-B are expressed in broad domains. Panels A, B, and C show pelts of stage 13 embryos. In these preparations, embryos were cut along the dorsal midline and flattened on a slide. Anterior is at the top. In stage 13 embryos, Hox gene expression is mostly visible in the epidermis with abd-A displayed in red and Abd-B in green. In panel A, Abd-B appears in a graded fashion from PS10 to PS13 (parasegments are marked by brackets). In these parasegments, Abd-B is produced from promoter A under the regulation of, respectively the iab-5, iab-6, iab-7, and iab-8 regulatory regions (see also text). In PS14 an alternative form of Abd-B is produced from promoters B, C, and γ. The abd-A expression pattern in PS7 to PS12 is shown in panel B. Both abd-A and Abd-B are displayed in panel C. Note that their overall expression domains appear complementary to each other. Original observations published in (Celniker et al., 1990; Karch et al., 1990; Gummalla et al., 2012) The abdominal part of the BX-C is shown in panel D with the same map coordinates as in Figure 1 (Martin et al., 1995) and with the same color code for the abd-A, Abd-B genes and their respective regulatory domains. The structure of the iab-8 ncRNA is shown in red under te DNA map. Introns are numbered with latin numbers and exons with regular numbering. Note that the polarity of transcription is the same as that for abd-A and Abd-B. Note also the presence of one exon for each of the iab cis-regulatory regions to the exception of 2 exons in iab-3. The location of miRiab-4/iab-8 in intron V is shown.
FIGURE 3
FIGURE 3
abd-A and Abd-B are both co-expressed in some cells of the central nervous system. CNS of stage 15 embryos stained for abd-A (red) and Abd-B (green) were dissected and mounted on a slide with anterior on top. Parasegments boundaries are shown. Note the presence of neurons in PS10 to PS12 expressing both proteins as seen by the yellow color. Often the neurons expressing high level of abd-A also express Abd-B. (original observation published in Gummalla et al., 2012).
FIGURE 4
FIGURE 4
abd-A expansion in PS13 in Abd-B mutant context is restricted to the epidermis. Pelts of stage 15 embryos stained for abd-A were prepared as in Figure 1. The WT expression pattern from PS7 to PS12 is shown in panel A. Panel B shows the pattern of abd-A expression in a homozygous Abd-BD16 mutant embryo. Note the expansion of abd-A expression in PS13 in the epidermis. In the CNS, however, (circled) there is no expansion. Panel C shows a homozygous Df(3R)C4 mutant embryo in which abd-A expansion in PS13 occurs in both epidermis and CNS (circled; original observation published in Gummalla et al., 2012). Panel D depicts the extend of the various deficiencies we used in our unsuccessful attempts to locate a second discrete repressive mechanism (see page 13). Panel E, same as panel D in Figure 2.
FIGURE 5
FIGURE 5
abd-Ais still repressed in the CNS of Abd-BD14 mutant embryo. Abd-BD14 removes the promoter A of the Abd-B transcription unit (indicated above panel B). As the A promoter is regulated by the iab-5, iab-6, iab-7, and iab-8 regulatory domains, there is no Abd-B expression in PS10 to PS13 (see panel A). In PS14, however, a truncated version of Abd-B (cross-reacting with the antibody) is expressed in PS14 from the B, C, and γ promoters (panel A). This result indicates the existence of alternate mechanism(s) (than Abd-B repression) to keep abd-A off in PS13 (original observation published in Gummalla et al., 2012).
FIGURE 6
FIGURE 6
Chromosomal breaks to the right of the miR-iab-8 fail to complement ΔmiRNA. Panel A shows the genomic map of the abdominal region of the BX-C as described in Figure 2. Panel B symbolizes the two homologs chromosomes of heterozygotes between ΔmiRNA and various rearrangement breakpoints that disrupt the abdominal region of the BX-C. Breaks in red fail to complement the sterility phenotype of ΔmiRNA, while break in green are fully fertile over ΔmiRNA. The Fab-864 deletion removing the promoter of the iab-8 ncRNA is indicated by red brackets.
FIGURE 7
FIGURE 7
Expression pattern of the iab-8 ncRNA. Embryos were hybridized with a strand-specific probe derived from the iab-6 region, to detect transcription in the same polarity than abd-A and Abd-B. Panel A shows an embryo 3 h after fertilization at the cellular blastoderm stage. A uniform band is visible at the posterior end of the embryo (shown by the thick oblique bar). At this stage, transient transcription from the iab-6 regulatory regions is detectable in PS11 (oblique arrow). At the elongated germ band stage (B), transcription is visible in the epidermis in PS13 and PS14. Panel C show a stage 15 embryo, after germ band contraction. Transcription is restricted to the CNS in PS13 and PS14 (original observation published in Bender, 2008).
FIGURE 8
FIGURE 8
abd-A is only de-repressed in a few cells in PS13 in ΔmiRNA. Panel A shows the genomic map of the abdominal region of the BX-C as described in Figure 2 with the ΔmiRNA deletion drawn above. CNSs were dissected out from stage 15 embryos Note in panel B that abd-A is de-repressed in only few neurons in PS13. Panel C show the abd-A(red) and Abd-B (green) expression patterns in WT and Fab-864 homozygotes. Note the complete de-repression of abd-A in PS13 (original observation published in Gummalla et al., 2012).
FIGURE 9
FIGURE 9
abd-A expression in the CNS in mutant that truncate the iab-8 ncRNA. Panel A, show the molecular map of the abdominal region of the BX-C as in the figure above. The various rearrangement breaks truncating the iab-8ncRNA are shown below the map, along with the ΔmiRNA. Panel B show the posterior CNS of embryos that were stained for abd-A (red) and engrailed (en, green). The engrailed stripes mark each of the parasegments. Note that rearrangements disrupting the iab-8ncRNA upstream from miR-iab-8 lead to a complete de-repression of abd-A in the CNS in PS13 (iab-6186, iab-7SGA62). Rearrangements breaks disrupting the iab-8-ncRNA downstream from the site of miR-iab-8 result in only a partial de-repression of abd-A in PS13. A ΔmiRNA CNS is also shown for comparisons (original observation published in Gummalla et al., 2012).
FIGURE 10
FIGURE 10
Haplo-insufficiency of breaks disrupting the iab-8 ncRNA. Panel A show CNSs stained for abd-A (red) and engrailed (green) from embryos heterozygous for mutations disrupting the iab-8ncRNA. Panel B dispalys a CNS from a heterozygous ΔmiRNA/+ embryo. Note that while one dose of miR-iab-8 is sufficient to keep abd-A repressed in PS13 (B), de-repression of abd-A in PS13 is observed in each of the four genotypes displayed in panel A. Panel C summarizes the relative positions of the trans-acting repression mechanism (miRiab-8) and cis-acting repression mechanism symbolized as a cloud. The level of de-repressions depends on the position of the disrupting break (upstream or downstream of miRiab-8). De-repression increases when the disrupting break is over ΔmiRNA. In iab-4186/ΔmiRNA PS13 abd-A expression reaches a level as if only one of the two homologs produces abd-A.
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References

    1. Akam M. E., Martinez-Arias A. (1985). The distribution of Ultrabithorax transcripts in Drosophila embryos. EMBO J. 4 1689–1700 - PMC - PubMed
    1. Bae E., Calhoun V. C., Levine M., Lewis E. B., Drewell R. A. (2002). Characterization of the intergenic RNA profile at abdominal-A and abdominal-B in the Drosophila bithorax complex. Proc. Natl. Acad. Sci. U.S.A. 99 16847–1685210.1073/pnas.222671299 - DOI - PMC - PubMed
    1. Barges S., Mihaly J., Galloni M., Hagstrom K., Müller M., Shanower G., et al. (2000). The Fab-8 boundary defines the distal limit of the bithorax complex iab-7 domain and insulates iab-7 from initiation elements and a PRE in the adjacent iab-8 domain. Development 127 779–790 - PubMed
    1. Beachy P. A., Helfand S. L., Hogness D. S. (1985). Segmental distribution of bithorax complex proteins during Drosophila development. Nature 313 545–55110.1038/313545a0 - DOI - PubMed
    1. Bello B. C., Hirth F., Gould A. P. (2003). A pulse of the Drosophila Hox protein abdominal-A schedules the end of neural proliferation via neuroblast apoptosis. Neuron 37 209–21910.1016/S0896-6273(02)01181-9 - DOI - PubMed

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