Hippo pathway and Bonus control developmental cell fate decisions in the Drosophila eye.

The canonical function of the Hippo signaling pathway is the regulation of organ growth. How this pathway controls cell-fate determination is less well understood. Here, we identify a function of the Hippo pathway in cell-fate decisions in the developing Drosophila eye, exerted through the interaction of Yorkie (Yki) with the transcriptional regulator Bonus (Bon), an ortholog of mammalian transcriptional intermediary factor 1/tripartite motif (TIF1/TRIM) family proteins. Instead of controlling tissue growth, Yki and Bon promote epidermal and antennal fates at the expense of the eye fate. Proteomic, transcriptomic, and genetic analyses reveal that Yki and Bon control these cell-fate decisions by recruiting transcriptional and post-transcriptional co-regulators and by repressing Notch target genes and activating epidermal differentiation genes. Our work expands the range of functions and regulatory mechanisms under Hippo pathway control.

Drosophila yellow-h encodes dopaminechrome tautomerase: A new enzyme in the eumelanin biosynthetic pathway.

Melanin is a widely distributed phenolic pigment that is biosynthesized from tyrosine and its hydroxylated product, dopa, in all animals. However, recent studies reveal a significant deviation from this paradigm, as insects appear to use dopamine rather than dopa as the major precursor of melanin. This observation calls for a reconsideration of the insect melanogenic pathway. While phenoloxidases and laccases can oxidize dopamine for dopaminechrome production, the fate of dopaminechrome remains undetermined. Dopachrome decarboxylase/tautomerase, encoded by yellow-f/f2 of Drosophila melanogaster, can convert dopaminechrome into 5,6-dihydroxyindole, but the same enzyme from other organisms does not act on dopaminechrome, suggesting the existence of a specific dopaminechrome tautomerase (DPT). We now report the identification of this novel enzyme that biosynthesizes 5,6-dihydroxyindole from dopaminechrome in Drosophila. Dopaminechrome tautomerase acted on both dopaminechrome and N-methyl dopaminechrome but not on dopachrome or other aminochromes tested. Our biochemical and molecular studies reveal that this enzyme is encoded by the yellow-h gene, a member of the yellow gene family, and advance our understanding of the physiological functions of this gene family. Identification and characterization of DPT clarifies the precursor for melanin biosynthetic pathways and proves the existence of an independent melanogenic pathway in insects that utilizes dopamine as the primary precursor.

The ecdysone receptor coactivator Taiman links Yorkie to transcriptional control of germline stem cell factors in somatic tissue.

The Hippo pathway is a conserved signaling cascade that modulates tissue growth. Although its core elements are well defined, factors modulating Hippo transcriptional outputs remain elusive. Here we show that components of the steroid-responsive ecdysone (Ec) pathway modulate Hippo transcriptional effects in imaginal disc cells. The Ec receptor coactivator Taiman (Tai) interacts with the Hippo transcriptional coactivator Yorkie (Yki) and promotes expression of canonical Yki-responsive genes. Tai enhances Yki-driven growth, while Tai loss, or a form of Tai unable to bind Yki, suppresses Yki-driven tissue growth. This growth suppression is not correlated with impaired induction of canonical Hippo-responsive genes but with suppression of a distinct pro-growth program of Yki-induced/Tai-dependent genes, including the germline stem cell factors nanos and piwi. These data reveal Hippo/Ec pathway crosstalk in the form a Yki-Tai complex that collaboratively induces germline genes as part of a transcriptional program that is normally repressed in developing somatic epithelia.

Short-Form CDYLb but not long-form CDYLa functions cooperatively with histone methyltransferase G9a in hepatocellular carcinomas.

In hepatocellular carcinomas (HCCs), the levels of histone H3 dimethylation at lysine 9 (H3K9me2) and its corresponding histone methyltransferase G9a are significantly elevated. Recently, G9a was reported to form a complex with the H3K9 methylation effector protein CDYL, but little is known about the expression of CDYL in HCC patients. The human CDYL gene produces two transcripts, a long form (CDYLa) and a short form (CDYLb), but it is unclear whether the protein products have different functions. The aim of this study was to investigate the distinctions between CDYLa and CDYLb and their expression levels in HCC tissues. We first examined binding abilities of the different CDYL forms with methylated H3 peptides by a pull-down assay. Human CDYLb (h-CDYLb) specifically recognized H3Kc9me2 and H3Kc9me3 modifications, whereas human CDYLa (h-CDYLa) did not interact with any methylated H3 peptides. Similarly, mouse CDYLb (m-CDYLb) specifically bound with di- and tri-methylated H3Kc9 peptides, while mouse CDYLa (m-CDYLa) lacked that ability. Affinity purification also was used to identify the distinct composition of the h-CDYLa or h-CDYLb protein complex. h-CDYLb was found in a multiprotein complex with G9a and GLP, while the h-CDYLa complex did not contain these two enzymes. Consistent with the protein complex composition, h-CDYLb and G9a were both upregulated in HCC tissues, compared with adjacent non-cancerous liver tissues. Furthermore, the positive correlation between expression levels of h-CDYLb and G9a was statistically significant. In contrast, h-CDYLa showed no enrichment in HCC tissues. These findings suggest that h-CDYLb and G9a are cooperatively involved in HCC.