Collaborative Control of Cell Cycle Progression by the RNA Exonuclease Dis3 and Ras Is Conserved Across Species.

Dis3 encodes a conserved RNase that degrades or processes all RNA species via an N-terminal PilT N terminus (PIN) domain and C-terminal RNB domain that harbor, respectively, endonuclease activity and 3'-5' exonuclease activity. In Schizosaccharomyces pombe, dis3 mutations cause chromosome missegregation and failure in mitosis, suggesting dis3 promotes cell division. In humans, apparently hypomorphic dis3 mutations are found recurrently in multiple myeloma, suggesting dis3 opposes cell division. Except for the observation that RNAi-mediated depletion of dis3 function drives larval arrest and reduces tissue growth in Drosophila, the role of dis3 has not been rigorously explored in higher eukaryotic systems. Using the Drosophila system and newly generated dis3 null alleles, we find that absence of dis3 activity inhibits cell division. We uncover a conserved CDK1 phosphorylation site that when phosphorylated inhibits Dis3's exonuclease, but not endonuclease, activity. Leveraging this information, we show that Dis3's exonuclease function is required for mitotic cell division: in its absence, cells are delayed in mitosis and exhibit aneuploidy and overcondensed chromosomes. In contrast, we find that modest reduction of dis3 function enhances cell proliferation in the presence of elevated Ras activity, apparently by accelerating cells through G2/M even though each insult by itself delays G2/M. Additionally, we find that dis3 and ras genetically interact in worms and that dis3 can enhance cell proliferation under growth stimulatory conditions in murine B cells. Thus, reduction, but not absence, of dis3 activity can enhance cell proliferation in higher organisms.

The Tribolium columnar genes reveal conservation and plasticity in neural precursor patterning along the embryonic dorsal-ventral axis.

The Drosophila columnar genes are key regulators of neural precursor formation and patterning along the dorsal-ventral axis of the developing CNS and include ventral nerve cord defective (vnd), intermediate nerve cord defective (ind), muscle segment homeodomain (msh), and Epidermal growth factor receptor (Egfr). To investigate the evolution of neural pattern formation, we identified and determined the expression patterns of Tribolium vnd, ind, and msh, and found that they are expressed in the medial, intermediate, and lateral columns of the developing CNS, respectively, in patterns similar, but not identical, to their Drosophila orthologs. The pattern of Egfr activity suggests that the genetic regulatory mechanisms that initiate Tc-vnd expression are similar in Drosophila and Tribolium, whereas those that initiate Tc-ind have diverged. RNAi analyses of gene function show that Tc-vnd and Tc-ind promote the formation of medial and intermediate column neural precursors and that vnd-mediated repression of ind establishes the boundary between the medial and intermediate columns. These data suggest that columnar gene expression and function underlie neural pattern formation in Drosophila, Tribolium, and potentially all insects, but that subtle spatiotemporal differences in expression of these genes may produce species-specific morphological differences.

pannier and pointedP2 act sequentially to regulate Drosophila heart development.

The Drosophila heart consists of two major cell types: cardioblasts, which form the contractile tube of the heart; and pericardial cells, which flank the cardioblasts and are thought to filter and detoxify the blood or hemolymph of the fly. We present the completion of the entire cell lineage of all heart cells. Notably, we detect a previously unappreciated distinction between the lineages of heart cells located in the posterior seven segments relative to those located more anteriorly. Using a genetic screen, we have identified the ETS-transcription factor pointed as a key regulator of cardioblast and pericardial cell fates in the posterior seven segments of the heart. In this domain, pointed promotes pericardial cell development and opposes cardioblast development. We find that this function of pointed is carried out primarily if not exclusively by the pointedP2 isoform and, that in this context, pointedP2 may act independently of Ras/MAPK pathway activity. We go on to show that the GATA transcription factor pannier acts early in dorsal mesoderm development to promote the development of the cardiac mesoderm and thus all heart cells. Finally, we demonstrate that pannier acts upstream of pointed in a developmental pathway in which pannier promotes cardiac mesoderm formation, and pointed acts subsequently in this domain to distinguish between cardioblast and pericardial cell fates.