How multifunctional signals combine to specify unique cell fates during pattern formation is not well understood. Here, we demonstrate that together with the transcription factor Lozenge, the nuclear effectors of the EGFR and Notch signaling pathways directly regulate D-Pax2 transcription in cone cells of the Drosophila eye disc. Moreover, the specificity of D-Pax2 expression can be altered upon genetic manipulation of these inputs. Thus, a relatively small number of temporally and spatially controlled signals received by a set of pluripotent cells can create the unique combinations of activated transcription factors required to regulate target genes and ultimately specify distinct cell fates within this group. We expect that similar mechanisms may specify pattern formation in vertebrate developmental systems that involve intercellular communication.

SUMMARY Transcriptional control requires epigenetic changes directed by mitochondrial tricarboxylic acid (TCA) cycle metabolites. In the mouse embryo, global epigenetic changes occur during the zygotic genome activation (ZGA) at a 2-cell stage. Pyruvate is essential for development beyond this stage, which is at odds with the low activity of mitochondria in this period. We now show that a number of enzymatically active mitochondrial enzymes associated with the TCA cycle essential for epigenetic remodeling are transiently and partially localized to the nucleus. Pyruvate is essential for this nuclear localization, and a failure of TCA cycle enzymes to enter the nucleus correlates with loss of specific histone modifications and a block in ZGA. At later stages, however, these enzymes are exclusively mitochondrial. In humans, the enzyme pyruvate dehydrogenase is transiently nuclear at the 4/8-cell stage coincident with timing of human embryonic genome activation, suggesting a conserved metabolic control mechanism underlying early pre-implantation development.

The Notch and Epidermal Growth Factor Receptor (EGFR) pathways both regulate proliferation and differentiation, and the cellular response to each is often influenced by the other. Here, we describe a mechanism that links them in a sequential fashion, in the developing compound eye of Drosophila. EGFR activation induces photoreceptor (R cell) differentiation and promotes their expression of Delta. This Notch ligand then induces neighboring cells to become nonneuronal cone cells. ebi and strawberry notch (sno) regulate EGFR-dependent Delta transcription by antagonizing a repressor function of Suppressor of Hairless (Su(H)). Sno binds to Su(H), and Ebi, an F-box/WD40 protein, forms a complex with Su(H) and the corepressor SMRTER. EGFR-activated transcriptional derepression requires ebi and sno, is proteasome-dependent, and correlates with the translocation of SMRTER to the cytoplasm.

The binding of Hedgehog to its receptor Patched causes de-repression of Smoothened resulting in the activation of the Hedgehog pathway. Here, we show that Smo activation is dependent on the levels of phospholipid, Phosphatidyl Inositol-4 Phosphate (PI4P). Loss of STT4 kinase required for the generation of PI4P exhibits hh-loss of function phenotypes while loss of Sac1 phosphatase required for the degradation of PI4P results in hh-gain of function phenotypes in multiple setting during Drosophila development. Furthermore, loss of Ptc function which results in the activation of Hedgehog pathway also causes an increase in PI4P levels. Sac1 functions downstream of STT4 and Ptc in the regulation of Smo membrane localization and Hh pathway activation. Taken together, our results suggest a model in which Ptc directly or indirectly functions to suppress the accumulation of PI4P. Binding of Hh to Ptc derepresses the levels of PI4P, which in turn promotes Smo activation.

Highlights d TE formation in mouse embryos is controlled by glucose metabolism activating CDX2 d Glycolysis is dispensable, and glucose is not required for amino acid or lipid synthesis d Nucleotides synthesized from glucose, along with S1P, activate mTOR and TFAP2C d Glucose metabolism by the HBP controls YAP1, which, together with TFAP2C, activates CDX2