Regulation of growth and cell proliferation during eye development

Fly-CURE, a multi-institutional CURE using Drosophila, increases students' confidence, sense of belonging, and persistence in research

The Fly-CURE is a genetics-focused multi-institutional Course-Based Undergraduate Research Experience (CURE) that provides undergraduate students with hands-on research experiences within a course. Through the Fly-CURE, undergraduate students at diverse types of higher education institutions across the United States map and characterize novel mutants isolated from a genetic screen in Drosophila melanogaster. To date, more than 20 mutants have been studied across 20 institutions, and our scientific data have led to eleven publications with more than 500 students as authors. To evaluate the impact of the Fly-CURE experience on students, we developed and validated assessment tools to identify students' perceived research self-efficacy, sense of belonging in science, and intent to pursue additional research opportunities. Our data, collected over three academic years and involving 14 institutions and 480 students, show gains in these metrics after completion of the Fly-CURE across all student subgroups analyzed, including comparisons of gender, academic status, racial and ethnic groups, and parents' educational background. Importantly, our data also show differential gains in the areas of self-efficacy and interest in seeking additional research opportunities between Fly-CURE students with and without prior research experience, illustrating the positive impact of research exposure (dosage) on student outcomes. Altogether, our data indicate that the Fly-CURE experience has a significant impact on students' efficacy with research methods, sense of belonging to the scientific research community, and interest in pursuing additional research experiences.

Fly-CURE, a Multi-institutional CURE using Drosophila, Increases Students' Confidence, Sense of Belonging, and Persistence in Research

The Fly-CURE is a genetics-focused multi-institutional Course-Based Undergraduate Research Experience (CURE) that provides undergraduate students with hands-on research experiences within a course. Through the Fly-CURE, undergraduate students at diverse types of higher education institutions across the United States map and characterize novel mutants isolated from a genetic screen in Drosophila melanogaster. To evaluate the impact of the Fly-CURE experience on students, we developed and validated assessment tools to identify students' perceived research self-efficacy, sense of belonging in science, and intent to pursue additional research opportunities. Our data show gains in these metrics after completion of the Fly-CURE across all student subgroups analyzed, including comparisons of gender, academic status, racial and ethnic groups, and parents' educational background. Importantly, our data also show differential gains in the areas of self-efficacy and interest in seeking additional research opportunities between Fly-CURE students with and without prior research experience, illustrating the positive impact of research exposure (dosage) on student outcomes. Altogether, our data indicate that the Fly-CURE experience has a significant impact on students' efficacy with research methods, sense of belonging to the scientific community, and interest in pursuing additional research experiences.

The gap junction protein Innexin3 is required for eye disc growth in Drosophila

The Drosophila compound eye develops from a bilayered epithelial sac composed of an upper peripodial epithelium layer and a lower disc proper, the latter giving rise to the eye itself. During larval stages, complex signalling events between the layers contribute to the control of cell proliferation and differentiation in the disc. Previous work in our lab established the gap junction protein Innexin2 (Inx2) as crucial for early larval eye disc growth. By analysing the contribution of other Innexins to eye size control, we have identified Innexin3 (Inx3) as an important growth regulator. Depleting inx3 during larval eye development reduces eye size, while elevating inx3 levels increases eye size, thus phenocopying the inx2 loss- and gain-of-function situation. As demonstrated previously for inx2, inx3 regulates disc cell proliferation and interacts genetically with the Dpp pathway, being required for the proper activation of the Dpp pathway transducer Mad at the furrow and the expression of Dpp receptor Punt in the eye disc. At the developmental timepoint corresponding to eye disc growth, Inx3 colocalises with Inx2 in disc proper and peripodial epithelium cell membranes. In addition, we show that Inx3 protein levels critically depend on inx2 throughout eye development and that inx3 modulates Inx2 protein levels in the larval eye disc. Rescue experiments demonstrate that Inx3 and Inx2 cooperate functionally to enable eye disc growth in Drosophila. Finally, we demonstrate that expression of Inx3 and Inx2 is not only needed in the disc proper but also in the peripodial epithelium to regulate growth of the eye disc. Our data provide a functional demonstration that putative Inx2/Inx3 heteromeric channels regulate organ size.

Drosophila eye size is determined by Innexin 2-dependent Decapentaplegic signalling

Organogenesis relies on specific genetic and molecular programmes, which orchestrate growth and cellular differentiation over developmental time. This is particularly important during Drosophila eye development in which cell-cell inductive events and long-range signalling have to be integrated to regulate proper cell proliferation, differentiation and morphogenesis. How these processes are coordinated is still not very well understood. Here we identify the gap junction protein Innexin2 (Inx2) as an important regulator of eye development. Depleting inx2 during eye development reduces eye size whereas elevating inx2 levels increases eye size. Loss- and gain-of-function experiments demonstrate that inx2 is required functionally in larval eye disc cells where it localises apico-laterally. inx2 regulates disc cell proliferation as well as morphogenetic furrow movement and as a result the amount of differentiated photoreceptors. inx2 interacts genetically with the Dpp pathway and we find that proper activation of the Dpp pathway transducer Mad at the furrow and expression of Dpp receptors Thickveins and Punt in the anterior disc compartment require inx2. We further show that inx2 is required for the transcriptional activation of dpp and punt in the eye disc. Our results highlight the crucial role of gap junction proteins in regulating morphogen-dependent organ size determination.

Developmental regulation of nucleolus size during Drosophila eye differentiation

When cell cycle withdrawal accompanies terminal differentiation, biosynthesis and cellular growth are likely to change also. In this study, nucleolus size was monitored during cell fate specification in the Drosophila eye imaginal disc using fibrillarin antibody labeling. Nucleolus size is an indicator of ribosome biogenesis and can correlate with cellular growth rate. Nucleolar size was reduced significantly during cell fate specification and differentiation, predominantly as eye disc cells entered a cell cycle arrest that preceded cell fate specification. This reduction in nucleolus size required Dpp and Hh signaling. A transient enlargement of the nucleolus accompanied cell division in the Second Mitotic Wave. Nucleoli continued to diminish in postmitotic cells following fate specification. These results suggest that cellular growth is regulated early in the transition from proliferating progenitor cells to terminal cell fate specification, contemporary with regulation of the cell cycle, and requiring the same extracellular signals.

Notch activity opposes Ras-induced differentiation during the Second Mitotic Wave of the developing Drosophila eye

Background

EGF receptor acts through Ras and the MAPK cascade to trigger differentiation and maintain survival of most of cell types in the Drosophila retina. Cell types are specified sequentially by separate episodes of EGFR activity. All the cell types differentiate in G1 phase of the cell cycle. Before differentiating, many cells pass through the cell cycle in the "Second Mitotic Wave" in response to Notch activity, but no cell fates are specified during the Second Mitotic Wave. It is not known how fate specification is limited to G1-arrested cells.

Results

Competence to differentiate in response to activated RasV12 was diminished during the Second Mitotic Wave accounting for the failure to recruit cell fates from cycling cells. Competence was not restored by blocking cell cycle progression, but was restored by reduced Notch activity.

Conclusion

Competence to differentiate does not depend on cell cycle progression per se, but on the same receptor activity that also induces cell cycle entry. Dual effects of Notch on the cell cycle and on differentiation help ensure that only G1 phase cells undergo fate specification.

Notch signaling controls proliferation through cell-autonomous and non-autonomous mechanisms in the Drosophila eye

During Drosophila eye development, localized Notch signaling at the dorsal ventral (DV)-midline promotes growth of the entire eye field. This long-range action of Notch signaling may be mediated through the diffusible ligand of the Jak/STAT pathway, Unpaired (Upd), which was recently identified as a downstream target of Notch. However, Notch activity has not been shown to be cell-autonomously required for Upd expression and therefore yet another diffusible signal may be required for Notch activation of Upd. Our results clarify the Notch requirement, demonstrating that Notch activity at the DV-midline leads to cell-autonomous expression of Upd as monitored in loss and gain-of-function Notch clones. In addition, mutations in the Jak/STAT pathway interact genetically with the Notch pathway by suppressing Notch mediated overgrowth. N(act) clones show non-autonomous effects on the cell cycle anterior to the furrow, indicating function of the Jak/STAT pathway. However, cell-autonomous effects of Notch within and posterior to the furrow are independent of Upd. Here, Notch autonomously maintains cells in a proliferative state and blocks photoreceptor differentiation.

Extracellular Signals Responsible for Spatially Regulated Proliferation in the Differentiating Drosophila Eye

Spatially and temporally choreographed cell cycles accompany the differentiation of the Drosophila retina. The extracellular signals that control these patterns have been identified through mosaic analysis of mutations in signal transduction pathways. All cells arrest in G1 prior to the start of neurogenesis. Arrest depends on Dpp and Hh, acting redundantly. Most cells then go through a synchronous round of cell division before fate specification and terminal cell cycle exit. Cell cycle entry is induced by Notch signaling and opposed in subsets of cells by EGF receptor activity. Unusually, Cyclin E levels are not limiting for retinal cell cycles. Rbf/E2F and the Cyclin E antagonist Dacapo are important, however. All retinal cells, including the postmitotic photoreceptor neurons, continue dividing when rbf and dacapo are mutated simultaneously. These studies identify the specific extracellular signals that pattern the retinal cell cycles and show how differentiation can be uncoupled from cell cycle exit.