Drosophila CK2 phosphorylates Deadpan, a member of the HES family of basic-helix-loop-helix (bHLH) repressors

New neurons for injured brains? The emergence of new genetic model organisms to study brain regeneration

Neuronal circuits in the adult brain have long been viewed as static and stable. However, research in the past 20 years has shown that specialized regions of the adult brain, which harbor adult neural stem cells, continue to produce new neurons in a wide range of species. Brain plasticity is also observed after injury. Depending on the extent and permissive environment of neurogenic regions, different organisms show great variability in their capacity to replace lost neurons by endogenous neurogenesis. In Zebrafish and Drosophila, the formation of new neurons from progenitor cells in the adult brain was only discovered recently. Here, we compare properties of adult neural stem cells, their niches and regenerative responses from mammals to flies. Current models of brain injury have revealed that specific injury-induced genetic programs and comparison of neuronal fitness are implicated in brain repair. We highlight the potential of these recently implemented models of brain regeneration to identify novel regulators of stem cell activation and regenerative neurogenesis.

Evidence that the C-terminal domain (CtD) autoinhibits neural repression by Drosophila E(spl)M8

Analysis of the retinal defects of a CK2 phosphomimetic variant of E(spl)M8 (M8S(159)D) and the truncated protein M8* encoded by the E(spl)D allele, suggest that the nonphosphorylated CtD "autoinhibits" repression. We have investigated this model by testing for inhibition (in "trans") by the CtD fragment in its nonphosphorylated (M8-CtD) and phosphomimetic (M8SD-CtD) states. In N(+) flies, ectopic M8-CtD compromises lateral inhibition, i.e., elicits supernumerary bristles as with loss of N signaling. This antimorphic activity of M8-CtD strongly rescues the reduced eye and/or bristle loss phenotypes that are elicited by ectopic M8SD or wild type M8. Additionally, the severely reduced eye of N(spl)/Y; E(spl)D/+ flies is also rescued by M8-CtD. Rescue is specific to the time and place, the morphogenetic furrow, where "founding" R8 photoreceptors are specified. In contrast, the phosphomimetic M8SD-CtD that is predicted to be deficient for autoinhibition, exhibits significantly attenuated or negligible activity. These studies provide evidence that autoinhibition by the CtD regulates M8 activity in a phosphorylation-dependent manner.

Genetically Derepressed Nucleoplasmic Stellate Protein in Spermatocytes of D. melanogaster interacts with the catalytic subunit of protein kinase 2 and carries histone-like lysine-methylated mark

SUMMARY

The X-chromosome-linked clusters of the tandemly repeated testis-specific Stellate genes of Drosophila melanogaster, encoding proteins homologous to the regulatory beta-subunit of the protein kinase casein kinase 2 (CK2), are repressed in wild-type males. Derepression of Stellate genes in the absence of the Y chromosome or Y-linked crystal locus (crystal line) causes accumulation of abundant protein crystals in testes and different meiotic abnormalities, which lead to partial or complete male sterility. To understand the cause of abnormalities in chromosome behavior owing to Stellate overexpression, we studied subcellular localization of Stellate proteins by biochemical fractionation and immunostaining of whole testes. We showed that, apart from the known accumulation of Stellate in crystalline form, soluble Stellate was located exclusively in the nucleoplasm, whereas Stellate crystals were located mainly in the cytoplasm. Coimmunoprecipitation experiments revealed that the alpha-subunit of the protein kinase CK2 (CK2alpha) was associated with soluble Stellate. Interaction between soluble Stellate and CK2alpha in the nucleus could lead to modulations in the phosphorylation of nuclear targets of CK2 and abnormalities in the meiotic segregation of chromosomes. We also observed that Stellate underwent lysine methylation and mimicked trimethyl-H3K9 epigenetic modification of histone H3 tail.

Maternal Groucho and bHLH repressors amplify the dose-sensitive X chromosome signal in Drosophila sex determination

In Drosophila, XX embryos are fated to develop as females, and XY embryos as males, because the diplo-X dose of four X-linked signal element genes, XSEs, activates the Sex-lethal establishment promoter, SxlPe, whereas the haplo-X XSE dose leaves SxlPe off. The threshold response of SxlPe to XSE concentrations depends in part on the bHLH repressor, Deadpan, present in equal amounts in XX and XY embryos. We identified canonical and non-canonical DNA-binding sites for Dpn at SxlPe and found that cis-acting mutations in the Dpn-binding sites caused stronger and earlier Sxl expression than did deletion of dpn implicating other bHLH repressors in Sxl regulation. Maternal Hey encodes one such bHLH regulator but the E(spl) locus does not. Elimination of the maternal corepressor Groucho also caused strong ectopic Sxl expression in XY, and premature Sxl activation in XX embryos, but Sxl was still expressed differently in the sexes. Our findings suggest that Groucho and associated maternal and zygotic bHLH repressors define the threshold XSE concentrations needed to activate SxlPe and that they participate directly in sex signal amplification. We present a model in which the XSE signal is amplified by a feedback mechanism that interferes with Gro-mediated repression in XX, but not XY embryos.