Drosophila myc regulates cellular growth during development

Abstract

Transcription factors of the Myc proto-oncogene family promote cell division, but how they do this is poorly understood. Here we address the functions of Drosophila Myc (dMyc) during development. Using mosaic analysis in the fly wing, we show that loss of dMyc retards cellular growth (accumulation of cell mass) and reduces cell size, whereas dMyc overproduction increases growth rates and cell size. dMyc-induced growth promotes G1/S progression but fails to accelerate cell division because G2/M progression is independently controlled by Cdc25/String. We also show that the secreted signal Wingless patterns growth in the wing primordium by modulating dMyc expression. Our results indicate that dMyc links patterning signals to cell division by regulating primary targets involved in cellular growth and metabolism.

Figure 1.

Molecular and Genetic Characterization of dmyc Mutants (A) Map of genomic sequences surrounding the dmyc locus, located at 3D5 on the X chromosome. The dmyc^P0/P1 P element is inserted within less than 100 bp upstream of the putative transcription start site. Also shown is the insertion site, within the first intron of dmyc, of the gypsy element giving rise to the dmyc^dm1. (B) Adult male wild-type (left) and dmyc^P0 mutant (right) flies, illustrating their size difference. (C) Scanning electron micrographs of the dorsal thorax of wild-type (left) and dmyc^P0 mutant (right) males. Note the shortened and more slender bristles of the mutant compared to the wild type.

Figure 2.

Analysis of Cell Size in dmyc Mutant Wing Discs (A) Representative forward scatter (FSC) distributions of control (green), dmyc^P0 (red), and dmyc^P1 (blue) wing disc cells from flow cytometric analysis. Three independent experiments were performed with similar results. The mean FSC height of each genotype (and FSC values for cell cycle phases) is, control, 117 (G1 = 105.9, S = 116.7, G2 = 129.3); dmyc^P0, 97 (G1 = 81.8, S = 91.3, G2 = 105.3); and dmyc^P1, 79 (G1 = 67, S = 72, G2 = 91). (B) DNA profiles from the flow cytometric analysis in (A). The traces for each genotype are color coded as in (A).

Figure 3.

Cell Competition of dmyc Mutant Cells (A) Cell competition assay. Right, confocal image of a dmyc^P0+/− wing disc in which mitotic recombination was induced, yielding pairs of dmyc^P0−/− clones (black; arrows) and +/+ wild-type twins (white). Note that the mutant clones (arrows) are much smaller than their sister wild-type twins. Left, graphs showing the relative sizes (clone areas, in pixels) of individual pairs of dmyc^P0−/− clones (black bars) and +/+ twins (gray bars). Left, pairs of −/− clones and +/+ twins located in the anterior (A) compartment of the wing disc; right, pairs of −/− clones and +/+ twins located in the posterior (P) compartment of the wing disc. Below is a tabulation of the data, divided into anterior and posterior. Clones in the posterior of the disc grow even more poorly than in the anterior (see text). n, number of clones or twin spots. Similar results were obtained with dmyc^P1. (B) Engrailed-Gal4 driving expression of UAS-dMyc (En>dMyc) in the posterior compartment of dmyc^P0+/− discs confers a partial rescue of the growth disadvantage of dmyc^P0−/− clones. Right, confocal image of dmyc^P0, En-Gal4>UAS-dMyc-expressing disc stained for anti-Myc antibody and showing dMyc protein in the posterior compartment. Posterior cells, to the right of the dashed line, are expressing dMyc. Left, as in (A), the graphs represent pairs of −/− clones and +/+ twins, grouped into anterior and posterior. The number of +/+ twin spots with accompanying −/− clones is increased relative to control posterior cells (B), as is the relative size (area, in pixels) of the −/− clones (compare bold number here with the bold number in [A]). A −/− clone that is rescued is indicated by the white arrow.

Figure 4.

Overexpression of dMyc in Wild-Type Wing Discs (A) FSC plots showing the relative size of control cells versus dMyc-expressing wing disc cells. Left panel, cells are expressing dMyc in random cell clones using Act>Gal4. dMyc-expressing cells also express GFP (green); control, GFP-negative cells are from the same disc (red). Right panel, FSC plot of En>dMyc, GFP-expressing discs. Green trace, posterior cells coexpressing GFP and dMyc; red trace, control, non-GFP-expressing anterior cells. (B) Rhodamine-phalloidin staining of actin in a normal wing disc, showing that anterior and posterior cells are very similar in size at this stage of development. Arrow points to the boundary between anterior and posterior cells. A, anterior; P, posterior. (C) Actin staining of a wing disc expressing dMyc in posterior cells (right part of disc) under En>Gal4 control. Inset shows magnified detail of the A/P border, showing the large, dMyc-expressing cells in the posterior. These cells have a mean FSC value 16% larger than anterior control cells (see [A]).

Figure 5.

Growth of Clones Overexpressing dMyc (A) Two dMyc-expressing Act>Gal4 cell clones (right) or three control clones (left), showing that the area encompassing dMyc-expressing clones is larger than that of control clones. Clones were induced at 48 – 4 hr AED and fixed for analysis at 118 – 4 hr AED. (B) Quantitation of the areas of control Act>Gal4 clones and clones expressing dMyc. Clones were induced at 48 or 72 hr AED and analyzed at 118 hr AED. dMyc-expressing clone areas were larger than controls at both time points. Mass doubling times (mass DT; see Experimental Procedures) for 70 hr clones (48–118 hr AED), Act>GFP control = 12 hr; Act>dMyc = 9.4 hr. Very similar mass DTs were obtained for clones induced at 72 hr and fixed at 118 hr AED. n, number of clones analyzed. The asterisk indicates p ≤ .001 relative to control.

Figure 6.

Cell and Mass Doubling Times of Clones Overexpressing dMyc or dMyc+Stg (A) Left, cell cycle profile from FACS analysis of cells expressing Act>dMyc and GFP (green trace) and from internal control (GFP-negative cells; red trace). dMyc expression results in a smaller fraction of G1 cells and an increase in the S and G2 fractions relative to the controls. Right, cell cycle profile of clones of cells expressing both dMyc and Stg under Act>Gal4 control. The presence of the mitotic inducer Stg prevents the reduction of G1 cells and the increase of G2 cells by dMyc. (B) Act>dMyc, Act>Stg, or Act>dMyc+Stg cell clones were induced at 72 hr and analyzed at 120 hr AED. Left panels, cells in each clone were counted, and cell doubling times for each genotype are indicated. Clones are displayed as the distribution of clonal cell number, with the range reflecting the inherent variability of cell division rates within the wing disc. Cell doubling times (cell DT) were calculated from the median size of the clone. Although the distribution of clonal cell number is similar for control, Stg alone, and dMyc alone, the median number of cells expressing dMyc+Stg is larger, as a result of a faster cell DT. Right panels, analysis of mass doubling times (mass DT) of the cell clones represented on the left (Act>GFP and Act>dMyc clones are a subset of the clones on left). The total number of pixels within each clone (the clone area) was measured and converted to μM², where 1 pixel = 0.782 μM². The mass doubling time was calculated as described in the Experimental Procedures, from the median clone area. n, number of clones scored.

Figure 7.

dmyc Expression in the Wing Disc and Its Effect on the Cell Cycle (A) In situ hybridization of a wing disc to dmyc mRNA. dmyc mRNA is present at high levels in the wing pouch and at lower, variable levels throughout the rest of the disc but is not expressed in cells flanking the dorsoventral (D/V) boundary of the disc (arrow). These cells make up the zone of nonproliferating cells (ZNC). In these images, anterior is to the left and dorsal is up. (B) In situ hybridization of dmyc^P1 mutant wing disc, showing that dmyc mRNA is undetectable in all regions of the disc. A similar result was observed for dmyc^P0. (C) The absence of dmyc mRNA in the ZNC requires the activity of Wingless. Wing disc in which a dominant-negative form of TCF (dnTCF) is expressed specifically in the ZNC under control of C96>Gal4. dnTCF blocks Wingless activity in the ZNC and results in the induction of dmyc mRNA (arrow). (D) Control wing disc labeled with the S phase marker BrdU, showing the ZNC as a population of arrested cells surrounding the D/V boundary (bracket). (E) BrdU-labeled wing disc in which dMyc is expressed specifically in the ZNC with C96>Gal4. Many of the cells at the D/V boundary have incorporated BrdU, indicating they are not arrested (bracket). The cell cycle arrest of the ZNC includes a G2 arrest in anterior cells flanking the D/V boundary and a G1 arrest in the anterior cells at the D/V border and in all posterior cells (see Johnston and Edgar, 1998). Interestingly, ectopic expression of dMyc prevents only the G1 arrest. Inset shows pattern of C96>Gal4 with expression of GFP. (F) Model for regulation of cellular growth and cell division by dMyc in wing disc cells. Extracellular signaling molecules such as those regulating metabolism (e.g., insulin) or patterning (e.g., Wingless) signal to dMyc to regulate cellular growth. Cyclin E activity is modulated in response to the altered growth rates and controls the G1/S transition. Although our data suggest that dMyc controls Cyclin E posttranscriptionally, we cannot rule out a more direct influence (broken arrow). However, indirect regulation of Cyclin E activity by Myc has been documented in vertebrate cells (Amati, 1998). The length of the cell cycle is also limited by the availability of Stg/Cdc25. Stg/Cdc25 expression is independently controlled by the patterning signals (e.g., Johnston and Edgar, 1998); thus, cell cycle rates can be controlled at both G1/S (by cellular growth) and G2/M (by Stg/Cdc25).