Specificity of Drosophila cytonemes for distinct signaling pathways.

Cytonemes are types of filopodia in the Drosophila wing imaginal disc that are proposed to serve as conduits in which morphogen signaling proteins move between producing and target cells. We investigated the specificity of cytonemes that are made by target cells. Cells in wing discs made cytonemes that responded specifically to Decapentaplegic (Dpp) and cells in eye discs made cytonemes that responded specifically to Spitz (the Drosophila epidermal growth factor protein). Tracheal cells had at least two types: one made in response to Branchless (a Drosophila fibroblast growth factor protein, Bnl), to which they segregate the Bnl receptor, and another to which they segregate the Dpp receptor. We conclude that cells can make several types of cytonemes, each of which responds specifically to a signaling pathway by means of the selective presence of a particular signaling protein receptor that has been localized to that cytoneme.

Serial line scan encoding imaging cytometer for both adherent and suspended cells.

We present a new design of an imaging cytometer for high content bioanalysis, which is equipped with a low-cost linear complementary metal oxide semiconductor (CMOS) imager (running at ≥40kHz). The fluorescent signals are encoded in a series of line scans across the cellular body, while it streams through a precisely defined line-shaped focus spot. This bioanalysis platform enables the concurrent collection of multiple fluorescence channels, while maintaining both high resolution and excellent throughput (1000 cells/s). We develop several image processing routines for the on-the-fly quantitative analysis of subcellular structures. Finally, we characterize our prototype system by imaging both adherent cells (plate format) and suspended cells (microfluidics format).

Dependence of Drosophila wing imaginal disc cytonemes on Decapentaplegic.

The anterior/posterior (A/P) and dorsal/ventral (D/V) compartment borders that subdivide the wing imaginal discs of Drosophila third instar larvae are each associated with a developmental organizer. Decapentaplegic (Dpp), a member of the transforming growth factor-beta (TGF-beta) superfamily, embodies the activity of the A/P organizer. It is produced at the A/P organizer and distributes in a gradient of decreasing concentration to regulate target genes, functioning non-autonomously to regulate growth and patterning of both the anterior and posterior compartments. Wingless (Wg) is produced at the D/V organizer and embodies its activity. The mechanisms that distribute Dpp and Wg are not known, but proposed mechanisms include extracellular diffusion, successive transfers between neighbouring cells, vesicle-mediated movement, and direct transfer via cytonemes. Cytonemes are actin-based filopodial extensions that have been found to orient towards the A/P organizer from outlying cells. Here we show that in the wing disc, cytonemes orient towards both the A/P and D/V organizers, and that their presence and orientation correlates with Dpp signalling. We also show that the Dpp receptor, Thickveins (Tkv), is present in punctae that move along cytonemes. These observations are consistent with a role for cytonemes in signal transduction.

Slingshot cofilin phosphatase localization is regulated by receptor tyrosine kinases and regulates cytoskeletal structure in the developing Drosophila eye.

Animal development requires that positional information act on the genome to control cell fate and cell shape. The primary determinant of animal cell shape is the cytoskeleton and thus the mechanisms by which extracellular signals influence the cytoskeleton are crucial for morphogenesis. In the developing Drosophila compound eye, localized polymerization of actin functions to constrict the apical surface of epithelial cells, both at the morphogenetic furrow and later to maintain the coherence of the nascent ommatidia. As elsewhere, actin polymerization in the developing eye is regulated by ADF/cofilin ('Twinstar', or 'Tsr' in Drosophila), which is activated by Slingshot (Ssh), a cofilin phosphatase. Here we show that Ssh does act in the developing eye to limit actin polymerization in the assembling ommatidia, but not in the morphogenetic furrow. While Ssh does control cell shape, surprisingly there are no direct or immediate consequences for cell type. Ssh protein becomes apically concentrated in cells that express elevated levels of the Sevenless (Sev) receptor-tyrosine kinase (RTK), even those which receive no ligand. We interpret this as a non-signal driven, RTK-dependent localization of Ssh to allow for locally increased actin filament turnover. We suggest that there are two modes of actin remodeling in the developing eye: a non-RTK, non-Ssh mediated mechanism in the morphogenetic furrow, and an RTK and Ssh-dependent mode during ommatidial assembly.

Nuclear translocation of activated MAP kinase is developmentally regulated in the developing Drosophila eye.

In proneural groups of cells in the morphogenetic furrow of the developing Drosophila eye phosphorylated mitogen activated protein kinase (MAPK) antigen is held in the cytoplasm for hours. We have developed a reagent to detect nuclear MAPK non-antigenically and report our use of this reagent to confirm that MAPK nuclear translocation is regulated by a second mechanism in addition to phosphorylation. This "cytoplasmic hold" of activated MAPK has not been observed in cell culture systems. We also show that MAPK cytoplasmic hold has an essential function in vivo: if it is overcome, developmental patterning in the furrow is disrupted.

Retinal development in Drosophila: specifying the first neuron.

In vertebrates, a proneural basic helix-loop-helix transcription factor (Ath5, Atonal homolog 5) plays a crucial role in the specification of the first retinal neuron: the retinal ganglion cell (RGC). Math5 homozygous null mutant mice lack RGCs and have no optic nerve. Furthermore, the expression of the Ath5 protein is regulated to give a non-random dispersed pattern of RGCs. In Drosophila, retinal histogenesis is precisely coordinated and is associated with a progressive wave called the morphogenetic furrow. In the furrow, single precisely spaced cells are specified to become the first retinal neural cell type: the R8 photoreceptor cell. This Drosophila founder cell specification is coincident with and dependant upon the expression of the fly Ath5 ortholog: Atonal. Indeed, in both taxa, the process of founder cell specification may be viewed as the regulation of Atonal expression. It is now clear that, in flies, this regulation depends on the action of inductive and inhibitory signals. This review concentrates on the signaling mechanisms that produce this precise pattern of founder cells.