Louise Cheng

Temporal Transcription Factors and Their Targets Schedule the End of Neural Proliferation in Drosophila

The timing mechanisms responsible for terminating cell proliferation toward the end of development remain unclear. In the Drosophila CNS, individual progenitors called neuroblasts are known to express a series of transcription factors endowing daughter neurons with different temporal identities. Here we show that Castor and Seven-Up, members of this temporal series, regulate key events in many different neuroblast lineages during late neurogenesis. First, they schedule a switch in the cell size and identity of neurons involving the targets Chinmo and Broad Complex. Second, they regulate the time at which neuroblasts undergo Prospero-dependent cell-cycle exit or Reaper/Hid/Grim-dependent apoptosis. Both types of progenitor termination require the combined action of a late phase of the temporal series and indirect feedforward via Castor targets such as Grainyhead and Dichaete. These studies identify the timing mechanism ending CNS proliferation and reveal how aging progenitors transduce bursts of transcription factors into long-lasting changes in cell proliferation and cell identity.

Regulating neural proliferation in the Drosophila CNS

Neural stem and progenitor cells generate the central nervous system (CNS) in organisms as diverse as insects and mammals. In Drosophila, multipotent asymmetrically dividing progenitors called neuroblasts produce neurons and glia throughout the developing CNS. Nevertheless, the time-windows of mitotic activity, the division modes, the termination mechanisms and the lineage sizes of individual neuroblasts all vary considerably from region-to-region. Recent studies shed light on some of the mechanisms underlying this neuroblast diversity and, in particular, how proliferation is boosted in two brain regions. In the central brain, some specialised neuroblasts generate intermediate neural progenitors that can each divide multiple times, thus increasing overall lineage size. In the optic lobe, an alternative expansion strategy involves symmetrically dividing neuroepithelial cells generating large numbers of asymmetrically dividing neuroblasts. Evidence is also emerging for a cell-intrinsic timer that alters the properties of each neuroblast with increasing developmental age. The core mechanism corresponds to a series of transcription factors that coordinates temporal changes in neuronal/glial identity with transitions in neuroblast cell-cycle speed, entry into quiescence and, ultimately, with termination.

Minibrain and Wings apart control organ growth and tissue patterning through down-regulation of Capicua.

Significance The transcriptional repressor protein Capicua (Cic) is a conserved regulator of organ growth and tissue patterning, and mutations in the CIC gene in humans result in the brain cancer oligodendroglioma. Cic activity is controlled by the receptor tyrosine kinase (RTK) signaling pathway. Here, we identify the kinase Minibrain (Mnb) and its adaptor Wings apart (Wap) as Cic regulators. Mnb and Wap bind to and phosphorylate the Cic protein, and inhibit the ability of Cic to repress gene expression. Mnb-dependent down-regulation of Cic is necessary for the proper growth of multiple organs and correct the patterning of tissues. Our results uncover a previously unknown mechanism of Cic regulation that acts in parallel to other growth-controlling pathways. The transcriptional repressor Capicua (Cic) controls tissue patterning and restricts organ growth, and has been recently implicated in several cancers. Cic has emerged as a primary sensor of signaling downstream of the receptor tyrosine kinase (RTK)/extracellular signal-regulated kinase (ERK) pathway, but how Cic activity is regulated in different cellular contexts remains poorly understood. We found that the kinase Minibrain (Mnb, ortholog of mammalian DYRK1A), acting through the adaptor protein Wings apart (Wap), physically interacts with and phosphorylates the Cic protein. Mnb and Wap inhibit Cic function by limiting its transcriptional repressor activity. Down-regulation of Cic by Mnb/Wap is necessary for promoting the growth of multiple organs, including the wings, eyes, and the brain, and for proper tissue patterning in the wing. We have thus uncovered a previously unknown mechanism of down-regulation of Cic activity by Mnb and Wap, which operates independently from the ERK-mediated control of Cic. Therefore, Cic functions as an integrator of upstream signals that are essential for tissue patterning and organ growth. Finally, because DYRK1A and CIC exhibit, respectively, prooncogenic vs. tumor suppressor activities in human oligodendroglioma, our results raise the possibility that DYRK1A may also down-regulate CIC in human cells.

The transcription factor Nerfin-1 prevents reversion of neurons into neural stem cells.

Cellular dedifferentiation is the regression of a cell from a specialized state to a more multipotent state and is implicated in cancer. However, the transcriptional network that prevents differentiated cells from reacquiring stem cell fate is so far unclear. Neuroblasts (NBs), the Drosophila neural stem cells, are a model for the regulation of stem cell self-renewal and differentiation. Here we show that the Drosophila zinc finger transcription factor Nervous fingers 1 (Nerfin-1) locks neurons into differentiation, preventing their reversion into NBs. Following Prospero-dependent neuronal specification in the ganglion mother cell (GMC), a Nerfin-1-specific transcriptional program maintains differentiation in the post-mitotic neurons. The loss of Nerfin-1 causes reversion to multipotency and results in tumors in several neural lineages. Both the onset and rate of neuronal dedifferentiation in nerfin-1 mutant lineages are dependent on Myc- and target of rapamycin (Tor)-mediated cellular growth. In addition, Nerfin-1 is required for NB differentiation at the end of neurogenesis. RNA sequencing (RNA-seq) and chromatin immunoprecipitation (ChIP) analysis show that Nerfin-1 administers its function by repression of self-renewing-specific and activation of differentiation-specific genes. Our findings support the model of bidirectional interconvertibility between neural stem cells and their post-mitotic progeny and highlight the importance of the Nerfin-1-regulated transcriptional program in neuronal maintenance.

Hypaxial muscle development

Chordate larvae show a surprisingly uniform “bauplan”, with a front end carrying the sense organs plus the gill and feeding apparatus, and a posterior end used for locomotion (reviewed in Goodrich 1958; Young 1962). Although adult forms frequently give up this organisation when they switch to sessile life styles, motility based on trunk muscles is maintained in acrania, and both in jaw-less and jawed vertebrates (agnathans and gnathostomes). The mesoderm on either side of the neural canal is subdivided into metameric blocks of muscle. As the notochord, and in vertebrates the vertebral column, prevent telescoping of the body, the serial action of the muscles on either side leads to an undulating movement.

Engrailed controls epaxial-hypaxial muscle innervation and the establishment of vertebrate three-dimensional mobility

Chordates are characterised by contractile muscle on either side of the body that promotes movement by side-to-side undulation. In the lineage leading to modern jawed vertebrates (crown group gnathostomes), this system was refined: body muscle became segregated into distinct dorsal (epaxial) and ventral (hypaxial) components that are separately innervated by the medial and hypaxial motors column, respectively, via the dorsal and ventral ramus of the spinal nerves. This allows full three-dimensional mobility, which in turn was a key factor in their evolutionary success. How the new gnathostome system is established during embryogenesis and how it may have evolved in the ancestors of modern vertebrates is not known. Vertebrate Engrailed genes have a peculiar expression pattern as they temporarily demarcate a central domain of the developing musculature at the epaxial-hypaxial boundary. Moreover, they are the only genes known with this particular expression pattern. The aim of this study was to investigate whether Engrailed genes control epaxial-hypaxial muscle development and innervation. Investigating chick, mouse and zebrafish as major gnathostome model organisms, we found that the Engrailed expression domain was associated with the establishment of the epaxial-hypaxial boundary of muscle in all three species. Moreover, the outgrowing epaxial and hypaxial nerves orientated themselves with respect to this Engrailed domain. In the chicken, loss and gain of Engrailed function changed epaxial-hypaxial somite patterning. Importantly, in all animals studied, loss and gain of Engrailed function severely disrupted the pathfinding of the spinal motor axons, suggesting that Engrailed plays an evolutionarily conserved role in the separate innervation of vertebrate epaxial-hypaxial muscle.

Understanding how differentiation is maintained: lessons from the Drosophila brain

The ability to maintain cells in a differentiated state and to prevent them from reprogramming into a multipotent state has recently emerged as a central theme in neural development as well as in oncogenesis. In the developing central nervous system (CNS) of the fruit fly Drosophila, several transcription factors were recently identified to be required in postmitotic cells to maintain differentiation, and in their absence, mature neurons undergo dedifferentiation, giving rise to proliferative neural stem cells and ultimately to tumor growth. In this review, we will highlight the current understanding of dedifferentiation and cell plasticity in the Drosophila CNS.

02-P005 A mechanism for “brain sparing” during dietary restriction in Drosophila

phology. We have generated a Tg(phd3:GFP)/+; vhl1/+ transgenic line, which exhibit robust GFP expression in the vhl1/vhl1 background, essentially acting as an in vivo fluorescent reporter to identify the mutant cells/embryos. To induce VHL kidney tumors in fish, we have utilized this transgenic line to generate donor embryos and transplanted the mutant blastomeres to the wild type host embryos, specifically targeting these mutant cells to the renal primordium. We report that Tg(phd3:GFP) line is a live reporter for monitoring hypoxia and the induction of HIF signaling. We shall present our observations on the fate of the vhl1 mutant cells in the chimeric fish and also our analysis for the possible occurrence of neoplasia in these chimeric fish.