David Ish-Horowicz

Groucho is required for Drosophila neurogenesis, segmentation, and sex determination and interacts directly with hairy-related bHLH proteins.

We have used the interaction trap, a yeast two-hybrid system, to identify proteins interacting with hairy, a basic-helix-loop-helix (bHLH) protein that represses transcription during Drosophila embryonic segmentation. We find that the groucho (gro) protein binds specifically to hairy and also to hairy-related bHLH proteins encoded by deadpan and the Enhancer of split complex. The C-terminal WRPW motif present in all these bHLH proteins is essential for this interaction. We demonstrate that these associations reflect in vivo maternal requirements for gro during neurogenesis, segmentation, and sex determination, three processes regulated by the above bHLH proteins, and we propose that gro is a transcriptional corepressor recruited to specific target promoters by hairy-related bHLH proteins.

Transcription pattern of the Drosophila segmentation gene hairy

Segmentation of the Drosophila embryo requires expression of the pair-rule genes, mutations of which cause reiterated deletions in alternate segments along the antero-posterior body axis. We find that transcripts of one such gene, hairy, accumulate in eight distinct regions of the early embryo. This pattern of expression is compared with that of another pair-rule gene, fushi tarazu, and its dependence on maternally expressed genes is described.

X:A ratio, the primary sex-determining signal in Drosophila, is transduced by helix-loop-helix proteins.

Drosophila determines its sex by counting X chromosomes. We show that premature expression of the pair-rule segmentation gene hairy interferes with this process, resulting in female-specific lethality by inhibiting initiation of the master control gene Sex-lethal (Sxl). The female-specific lethality can be suppressed by a constitutive Sxl allele or by extra copies of X-linked counting elements. These results are best explained by competition between hairy and other helix-loop-helix transcription factors that act in chromosome counting. We have confirmed this model by showing that misexpression of the achaete-scute T4 gene induces ectopic Sxl expression and male-specific lethality, confirming that achaete-scute T4 is the sisterless-b counting element. We propose that X chromosomes are counted through heterodimers of helix-loop-helix transcription factors that act synergistically to initiate Sxl expression.

Pattern abnormalities induced by ectopic expression of the Drosophila gene hairy are associated with repression of ftz transcription

The pair-rule segmentation gene hairy (h) is expressed and required in alternate metameres in the early Drosophila embryo. We show that h expression outside these domains, driven by an hsp70 promoter, suppresses the expression of fushi tarazu (ftz). The kinetics of action favor h acting directly as a transcriptional repressor. The resulting pattern defects, and the patterns of en and Ubx activity, can be explained if h acts via ftz and other pair-rule genes in the establishment of stable en domains. We present evidence that ftz is required for the initiation of Ubx transcription, but not for its maintenance.

Autocatalytic ftz activation and metameric instability induced by ectopic ftz expression

Inappropriate expression of the Drosophila pair-rule gene, fushi tarazu (ftz), causes cuticular pattern deletions apparently complementary to those in ftz larvae. We show that the two patterns actually originate similarly, in both cases affecting the even-numbered parasegmental boundaries. The reciprocal cuticular patterns derive from differing patterns of selector gene expression (homoeotic transformations). The primary effect of ectopic ftz activity is to broaden ftz domains by autocatalytic activation of endogenous ftz expression in an additional anterior cell. This activates engrailed (en) and represses wingless (wg) expression, consistent with their proposed combinatorial control by ftz (and other pair-rule genes) to define parasegmental primordia. We propose that the anterior margin of each ftz stripe is normally defined by the posterior even-skipped (eve) boundary.

Imp and Syp RNA-binding proteins govern decommissioning of Drosophila neural stem cells

The termination of the proliferation of Drosophila neural stem cells, also known as neuroblasts (NBs), requires a ‘decommissioning’ phase that is controlled in a lineage-specific manner. Most NBs, with the exception of those of the mushroom body (MB), are decommissioned by the ecdysone receptor and mediator complex, causing them to shrink during metamorphosis, followed by nuclear accumulation of Prospero and cell cycle exit. Here, we demonstrate that the levels of Imp and Syp RNA-binding proteins regulate NB decommissioning. Descending Imp and ascending Syp expression have been shown to regulate neuronal temporal fate. We show that Imp levels decline slower in the MB than in other central brain NBs. MB NBs continue to express Imp into pupation, and the presence of Imp prevents decommissioning partly by inhibiting the mediator complex. Late-larval induction of transgenic Imp prevents many non-MB NBs from decommissioning in early pupae. Moreover, the presence of abundant Syp in aged NBs permits Prospero accumulation that, in turn, promotes cell cycle exit. Together, our results reveal that progeny temporal fate and progenitor decommissioning are co-regulated in protracted neuronal lineages. Highlighted Article: Temporal progression of Imp/Syp gradients determines the timing of neuroblast decommissioning and cell cycle exit in addition to progeny temporal fate, allowing proper completion of a neuronal lineage.

Single molecule fluorescence in situ hybridisation for quantitating post-transcriptional regulation in Drosophila brains.

Highlights • Simple and rapid smFISH protocol suitable for medium throughput.• Sensitive mRNA detection deep in whole-mount larval and adult Drosophila brains.• Multiplexed detection of RNA in combination with antibody staining.• Quantitation of primary transcription and post-transcriptional mRNA levels.• Reliable cell type markers in a whole-mount brain complementary to antibody markers.