Wingless signaling and the control of cell shape in Drosophila wing imaginal discs

The control of cell morphology is important for shaping animals during development. Here we address the role of the Wnt/Wingless signal transduction pathway and two of its target genes, vestigial and shotgun (encoding E-cadherin), in controlling the columnar shape of Drosophila wing disc cells. We show that clones of cells mutant for arrow (encoding an essential component of the Wingless signal transduction pathway), vestigial or shotgun undergo profound cell shape changes and are extruded towards the basal side of the epithelium. Compartment-wide expression of a dominant-negative form of the Wingless transducer T-cell factor (TCF/Pangolin), or double-stranded RNA targeting vestigial or shotgun, leads to abnormally short cells throughout this region, indicating that these genes act cell autonomously to maintain normal columnar cell shape. Conversely, overexpression of Wingless, a constitutively-active form of the Wingless transducer β-catenin/Armadillo, or Vestigial, results in precocious cell elongation. Co-expression of Vestigial partially suppresses the abnormal cell shape induced by dominant-negative TCF. We conclude that Wingless signal transduction plays a cell-autonomous role in promoting and maintaining the columnar shape of wing disc cells. Furthermore, our data suggest that Wingless controls cell shape, in part, through maintaining vestigial expression.     

Dally regulates Dpp morphogen gradient formation by stabilizing Dpp on the cell surface

Decapentaplegic (Dpp), a Drosophila homologue of bone morphogenetic proteins, acts as a morphogen to regulate patterning along the anterior-posterior axis of the developing wing. Previous studies showed that Dally, a heparan sulfate proteoglycan, regulates both the distribution of Dpp morphogen and cellular responses to Dpp. However, the molecular mechanism by which Dally affects the Dpp morphogen gradient remains to be elucidated. Here, we characterized activity, stability, and gradient formation of a truncated form of Dpp (Dpp^ΔN), which lacks a short domain at the N-terminus essential for its interaction with Dally. Dpp^ΔN shows the same signaling activity and protein stability as wild-type Dpp in vitro but has a shorter half-life in vivo, suggesting that Dally stabilizes Dpp in the extracellular matrix. Furthermore, genetic interaction experiments revealed that Dally antagonizes the effect of Thickveins (Tkv; a Dpp type I receptor) on Dpp signaling. Given that Tkv can downregulate Dpp signaling by receptor-mediated endocytosis of Dpp, the ability of dally to antagonize tkv suggests that Dally inhibits this process. Based on these observations, we propose a model in which Dally regulates Dpp distribution and signaling by disrupting receptor-mediated internalization and degradation of the Dpp-receptor complex.     

Loss of growth homeostasis by genetic decoupling of cell division from biomass growth: implication for size control mechanisms

Growing cells adjust their division time with biomass accumulation to maintain growth homeostasis. Size control mechanisms, such as the size checkpoint, provide an inherent coupling of growth and division by gating certain cell cycle transitions based on cell size. We describe genetic manipulations that decouple cell division from cell size, leading to the loss of growth homeostasis, with cells becoming progressively smaller or progressively larger until arresting. This was achieved by modulating glucose influx independently of external glucose. Division rate followed glucose influx, while volume growth was largely defined by external glucose. Therefore, the coordination of size and division observed in wild‐type cells reflects tuning of two parallel processes, which is only refined by an inherent feedback‐dependent coupling. We present a class of size control models explaining the observed breakdowns of growth homeostasis.     

NBT-II cell locomotion is modulated by restricting the size of focal contacts and is improved through EGF and ROCK signaling

Focal contacts, large macromolecular complexes that link the extracellular matrix and the internal cell cytoskeleton, are thought to govern cell locomotion. However, the maturation process through which focal contacts control the cellular migratory machinery by changes in size and molecular composition remain unclear. Here, we fabricated cell growth substrates that contained linear ECM strips of micron- or submicron-width in order to limit the enlargement of focal contacts. We found that NBT-II cells plated on the submicron substrate possessed smaller focal complexes that exhibited a highly dynamic turnover. These cells possessed various leading edges at multiple sites of the cell periphery, which prevented the cell from advancing. In contrast, cells grown on the micron-width substrate possessed large and stable focal adhesions. Most of these cells were elongated bipolar cells that were tethered at both ends and were immobile. Further, EGF and ROCK signaling pathways can modulate the cellular migratory responses according to the substrate guidance. On the submicron-width substrate, EGF treatment increased the focal contact size and the contractile force, causing these cells to develop one leading edge and migrate along the submicron-sized ECM paths. In contrast, inhibition of ROCK signaling decreased the focal contact size for cells plated on the micron substrate. These cells became less tethered and were able to migrate along or even across the micron-sized ECM paths. Our results indicate that formation and maturation of focal contacts is controlled by both ECM cues and intracellular signaling and they play a central role in directed cell motion.     

Effects of the medium ammonium-nitrate ratio on competence for asparagus cell division induced by phytosulfokine-α

The effects of medium ammonium-nitrate ratio on cell proliferation were investigated using a low cell-density culture of Asparagus officinalis L., which was triggered by a peptidal plant growth factor, phytosulfokine-α (PSK-α). The asparagus cells proliferated the most in a medium with an ammonium-nitrate ratio of 0:30 mM and could be maintained without significant loss of responsiveness to PSK-α for at least 96 h from the beginning of culture. In this medium, single cells gave rise to microcalli at initial densities as low as 3.2×10² cells/ml as long as PSK-α was present in the medium. Increasing the ammonium-nitrate ratio resulted in severe inhibition of cell proliferation at a low cell density, even if PSK-α was added to the medium. Received: 6 May 1997 / Revision received: 5 August 1997 / Accepted: 2 September 1997     

Isolation of cell size mutants of a fission yeast by a new selective method: characterization of mutants and implications for division control mechanisms.

Previously known cell size (wee) mutations of fission yeast suppress the mitotic block caused by a defective cdc25 allele. Some 700 revertants of cdc25-22 were obtained after ultraviolet mutagenesis and selection at the restrictive temperature. Most revertants carried the original cdc25 lesion plus a mutation in or very close to the wee1 gene. Two partial wee1 mutations of a new type were found among the revertants. Two new wee mutations mapping at the cdc2 gene (cdc2-w mutants) were also obtained. The various mutations were examined for their effects on cell division size, their efficiency as cdc25 suppressors, and their dominance relations. Full wee1 mutations were found to suppress cdc25 lesions very efficiently, whereas partial wee1 mutations were poor suppressors. The cdc25 suppression ability of cdc2-w mutations was allele specific for cdc2, suggesting bifunctionality of the gene product. The wee1 mutations were recessive for cdc25 suppression; cdc2-w mutations were dominant. A model is proposed for the genetic control of mitotic timing and cell division size, in which the cdc2+ product is needed and is rate limiting for mitosis. The cdc2+ activity is inhibited by the wee1+ product, whereas the cdc25+ product relieves this inhibition.     

XIAO is involved in the control of organ size by contributing to the regulation of signaling and homeostasis of brassinosteroids and cell cycling in rice

Organ size is determined by cell number and size, and involves two fundamental processes: cell proliferation and cell expansion. Although several plant hormones are known to play critical roles in shaping organ size by regulating the cell cycle, it is not known whether brassinosteroids (BRs) are also involved in regulating cell division. Here we identified a rice T-DNA insertion mutant for organ size, referred to as xiao, that displays dwarfism and erect leaves, typical BR-related phenotypes, together with reduced seed setting. XIAO is predicted to encode an LRR kinase. The small stature of the xiao mutant resulted from reduced organ sizes due to decreased cell numbers resulting from reduced cell division rate, as supported by the observed co-expression of XIAO with a number of genes involved in cell cycling. The xiao mutant displayed a tissue-specific enhanced BR response and greatly reduced BR contents at the whole-plant level. These results indicated that XIAO is a regulator of BR signaling and cell division. Thus, XIAO may provide a possible connection between BRs and cell-cycle regulation in controlling organ growth.