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| <?xml version="1.0" encoding="UTF-8"?><!DOCTYPE article PUBLIC "-//NLM//DTD JATS (Z39.96) Journal Archiving and Interchange DTD v1.1d1 20130915//EN" "JATS-archivearticle1.dtd"><article article-type="research-article" dtd-version="1.1d1" xmlns:xlink="http://www.w3.org/1999/xlink"><front><journal-meta><journal-id journal-id-type="nlm-ta">elife</journal-id><journal-id journal-id-type="hwp">eLife</journal-id><journal-id journal-id-type="publisher-id">eLife</journal-id><journal-title-group><journal-title>eLife</journal-title></journal-title-group><issn publication-format="electronic">2050-084X</issn><publisher><publisher-name>eLife Sciences Publications, Ltd</publisher-name></publisher></journal-meta><article-meta><article-id pub-id-type="publisher-id">03363</article-id><article-id pub-id-type="doi">10.7554/eLife.03363</article-id><article-categories><subj-group subj-group-type="display-channel"><subject>Short report</subject></subj-group><subj-group subj-group-type="heading"><subject>Developmental biology and stem cells</subject></subj-group></article-categories><title-group><article-title>Transient nuclear Prospero induces neural progenitor quiescence</article-title></title-group><contrib-group><contrib contrib-type="author" id="author-14043"><name><surname>Lai</surname><given-names>Sen-Lin</given-names></name><xref ref-type="aff" rid="aff1"/><xref ref-type="aff" rid="aff2"/><xref ref-type="other" rid="par-1"/><xref ref-type="fn" rid="con1"/><xref ref-type="fn" rid="conf1"/></contrib><contrib contrib-type="author" corresp="yes" id="author-14004"><name><surname>Doe</surname><given-names>Chris Q</given-names></name><xref ref-type="aff" rid="aff1"/><xref ref-type="aff" rid="aff2"/><xref ref-type="corresp" rid="cor1">*</xref><xref ref-type="other" rid="par-1"/><xref ref-type="fn" rid="con2"/><xref ref-type="fn" rid="conf1"/></contrib><aff id="aff1"><institution content-type="dept">Institute of Neuroscience</institution>, <institution>Howard Hughes Medical Institute, University of Oregon</institution>, <addr-line><named-content content-type="city">Eugene</named-content></addr-line>, <country>United States</country></aff><aff id="aff2"><institution content-type="dept">Institute of Molecular Biology</institution>, <institution>University of Oregon</institution>, <addr-line><named-content content-type="city">Eugene</named-content></addr-line>, <country>United States</country></aff></contrib-group><contrib-group content-type="section"><contrib contrib-type="editor"><name><surname>Brand</surname><given-names>Andrea</given-names></name><role>Reviewing editor</role><aff><institution>University of Cambridge</institution>, <country>United Kingdom</country></aff></contrib></contrib-group><author-notes><corresp id="cor1"><label>*</label>For correspondence: <email>cdoe@uoregon.edu</email></corresp></author-notes><pub-date date-type="pub" publication-format="electronic"><day>29</day><month>10</month><year>2014</year></pub-date><pub-date pub-type="collection"><year>2014</year></pub-date><volume>3</volume><elocation-id>e03363</elocation-id><history><date date-type="received"><day>14</day><month>05</month><year>2014</year></date><date date-type="accepted"><day>17</day><month>09</month><year>2014</year></date></history><permissions><copyright-statement>© 2014, Lai and Doe</copyright-statement><copyright-year>2014</copyright-year><copyright-holder>Lai and Doe</copyright-holder><license xlink:href="http://creativecommons.org/licenses/by/4.0/"><license-p>This article is distributed under the terms of the <ext-link ext-link-type="uri" xlink:href="http://creativecommons.org/licenses/by/4.0/">Creative Commons Attribution License</ext-link>, which permits unrestricted use and redistribution provided that the original author and source are credited.</license-p></license></permissions><self-uri content-type="pdf" xlink:href="elife03363.pdf"/><abstract><object-id pub-id-type="doi">10.7554/eLife.03363.001</object-id><p>Stem cells can self-renew, differentiate, or enter quiescence. Understanding how stem cells switch between these states is highly relevant for stem cell-based therapeutics. <italic>Drosophila</italic> neural progenitors (neuroblasts) have been an excellent model for studying self-renewal and differentiation, but quiescence remains poorly understood. In this study, we show that when neuroblasts enter quiescence, the differentiation factor Prospero is transiently detected in the neuroblast nucleus, followed by the establishment of a unique molecular profile lacking most progenitor and differentiation markers. The pulse of low level nuclear Prospero precedes entry into neuroblast quiescence even when the timing of quiescence is advanced or delayed by changing temporal identity factors. Furthermore, loss of Prospero prevents entry into quiescence, whereas a pulse of low level nuclear Prospero can drive proliferating larval neuroblasts into quiescence. We propose that Prospero levels distinguish three progenitor fates: absent for self-renewal, low for quiescence, and high for differentiation.</p><p><bold>DOI:</bold> <ext-link ext-link-type="doi" xlink:href="10.7554/eLife.03363.001">http://dx.doi.org/10.7554/eLife.03363.001</ext-link></p></abstract><abstract abstract-type="executive-summary"><object-id pub-id-type="doi">10.7554/eLife.03363.002</object-id><title>eLife digest</title><p>Stem cells provide tissues in the body with a continuing source of new cells, both when the tissues are first developing and when they are growing or repairing in adulthood. A stem cell can divide to create either another stem cell, or a cell that will mature into one of many different cell types.</p><p>Neuroblasts are a type of brain stem cell and can divide to create two new cells: another neuroblast that will continue to replicate itself and a cell called a ganglion mother cell that will go on to produce two mature cells for the nervous system. Moreover, when a neuroblast divides, it splits unequally, so that certain molecules end up predominantly in the ganglion mother cell—including a protein called Prospero. Once partitioned into the ganglion mother cell, the Prospero protein enters the nucleus, where it switches off ‘stem cell genes’ and switches on ‘neuron genes’ so the ganglion mother cell can form the mature neurons of the brain. Thus, neuroblasts must keep Prospero out of the nucleus to maintain stem cell properties, whereas ganglion mother cells must move Prospero into the nucleus to form neurons.</p><p>Now, Lai and Doe discover a new way that the Prospero protein is used to control stem cell biology. Neuroblasts, like all stem cells, can enter periods where they go dormant or quiescent—that is, they temporarily stop generating ganglion mother cells. By analyzing which proteins are present in neuroblasts during this transition to quiescence, Lai and Doe discovered that the Prospero protein was briefly detected, at low levels, in the nucleus of the neuroblast just before it became dormant.</p><p>To see whether this ‘low-level pulse’ of nuclear Prospero is linked to the cell entering a dormant state, Lai and Doe investigated two types of mutant fly in which neuroblasts become dormant either earlier or later than in normal flies. A low-level pulse of nuclear Prospero still precisely matched the start of the dormant state in both mutants. When the Prospero protein was removed altogether, the neuroblasts failed to become dormant, and instead continued dividing.</p><p>Lai and Doe propose that different levels of Prospero distinguish three different fates for neuroblasts. Neuroblasts self-replicate when Prospero is kept out of the nucleus, become dormant when exposed to low level nuclear Prospero, and produce the mature cells of the brain when nuclear Prospero levels are high. Exactly how the intermediate levels of nuclear Prospero trigger the dormant state remains a question for future work, as is the question of whether the related mammalian protein, called Prox1, has a similar function.</p><p>Understanding how stem cells switch between cell division and quiescence is important for developing effective stem cell-based therapies. It could also help us understand cancer, as cancer cells go through similar periods of inactivity, during which they do not respond to many anti-tumor drugs.</p><p><bold>DOI:</bold> <ext-link ext-link-type="doi" xlink:href="10.7554/eLife.03363.002">http://dx.doi.org/10.7554/eLife.03363.002</ext-link></p></abstract><kwd-group kwd-group-type="author-keywords"><title>Author keywords</title><kwd>quiescence</kwd><kwd>prospero</kwd><kwd>neural progenitor</kwd></kwd-group><kwd-group kwd-group-type="research-organism"><title>Research organism</title><kwd><italic>D. melanogaster</italic></kwd></kwd-group><funding-group><award-group id="par-1"><funding-source><institution-wrap><institution-id institution-id-type="FundRef">http://dx.doi.org/10.13039/100000011</institution-id><institution>Howard Hughes Medical Institute</institution></institution-wrap></funding-source><principal-award-recipient><name><surname>Lai</surname><given-names>Sen-Lin</given-names></name><name><surname>Doe</surname><given-names>Chris Q</given-names></name></principal-award-recipient></award-group><funding-statement>The funder had no role in study design, data collection and interpretation, or the decision to submit the work for publication.</funding-statement></funding-group><custom-meta-group><custom-meta><meta-name>elife-xml-version</meta-name><meta-value>2</meta-value></custom-meta><custom-meta specific-use="meta-only"><meta-name>Author impact statement</meta-name><meta-value>A low-level pulse of the nuclear Prospero protein is necessary and sufficient to cause neural progenitor cells to enter a quiescent state.</meta-value></custom-meta></custom-meta-group></article-meta></front><body><sec id="s1" sec-type="intro"><title>Introduction</title><p>Quiescent stem cells preserve the stem cell pool for activation following disease or injury. Moreover, cancer stem cells can enter remission during treatment, possibly using mechanisms similar to stem cell quiescence, and re-initiate proliferation months to years later. Thus, studying stem cell quiescence may help design clinical approaches to prevent tumor dormancy and cancer recurrence, as well as to aid in activating stem cells for tissue repair.</p><p><italic>Drosophila</italic> neuroblasts have served as a model system for identifying conserved signaling pathways that regulate stem cell proliferation and quiescence (<xref ref-type="bibr" rid="bib23">Ebens et al., 1993</xref>; <xref ref-type="bibr" rid="bib19">Datta, 1995</xref>; <xref ref-type="bibr" rid="bib7">Britton and Edgar, 1998</xref>; <xref ref-type="bibr" rid="bib21">Doe et al., 1998</xref>; <xref ref-type="bibr" rid="bib24">Egger et al., 2008</xref>; <xref ref-type="bibr" rid="bib46">Tsuji et al., 2008</xref>; <xref ref-type="bibr" rid="bib40">Sousa-Nunes et al., 2010</xref>; <xref ref-type="bibr" rid="bib29">Homem and Knoblich, 2012</xref>; <xref ref-type="bibr" rid="bib48">Weng and Cohen, 2012</xref>). <italic>Drosophila</italic> neuroblasts delaminate from neuroectoderm during early embryogenesis, and then they go through multiple rounds of asymmetric cell division before exiting cell cycle at the end of embryogenesis; upon cell cycle exit, neuroblasts can undergo apoptosis (<xref ref-type="bibr" rid="bib31a">Karcavitch and Doe, 2005</xref>; <xref ref-type="bibr" rid="bib34a">Maurange and Gould, 2005</xref>; <xref ref-type="bibr" rid="bib46a">Ulvklo et al., 2012</xref>) or enter quiescence (<xref ref-type="bibr" rid="bib45">Truman and Bate, 1988</xref>; <xref ref-type="bibr" rid="bib19">Datta, 1995</xref>; <xref ref-type="bibr" rid="bib46">Tsuji et al., 2008</xref>; <xref ref-type="bibr" rid="bib14">Chell and Brand, 2010</xref>; <xref ref-type="bibr" rid="bib41">Sousa-Nunes et al., 2011</xref>). During neuroblast asymmetric division, the scaffolding protein Miranda (Mira) is partitioned into the differentiating daughter cell, called a ganglion mother cell (GMC), and it carries at least three cargo proteins: the translational repressors Staufen and Brain tumor (Brat) and the transcriptional repressor Prospero (<xref ref-type="bibr" rid="bib28">Hirata et al., 1995</xref>; <xref ref-type="bibr" rid="bib8">Broadus and Doe, 1997</xref>; <xref ref-type="bibr" rid="bib30">Ikeshima-Kataoka et al., 1997</xref>; <xref ref-type="bibr" rid="bib9">Broadus et al., 1998</xref>; <xref ref-type="bibr" rid="bib38">Schuldt et al., 1998</xref>; <xref ref-type="bibr" rid="bib39">Shen et al., 1998</xref>; <xref ref-type="bibr" rid="bib3">Bello et al., 2006</xref>; <xref ref-type="bibr" rid="bib4">Betschinger et al., 2006</xref>; <xref ref-type="bibr" rid="bib33">Lee et al., 2006</xref>). In the GMC, Mira appears to be degraded and its cargo is released into the cytoplasm (Staufen, Brat) or the nucleus (Prospero). Prospero is an atypical homeodomain protein that directly binds and represses progenitor and cell cycle genes to initiate GMC and neuronal differentiation (<xref ref-type="bibr" rid="bib34">Li and Vaessin, 2000</xref>; <xref ref-type="bibr" rid="bib17">Choksi et al., 2006</xref>); the mammalian ortholog Prox1 has a similar role in repressing cell cycle gene expression (<xref ref-type="bibr" rid="bib22">Dyer, 2003</xref>; <xref ref-type="bibr" rid="bib25">Foskolou et al., 2013</xref>). In addition, keeping high levels of Prospero out of the neuroblast nucleus is essential to prevent neuroblast differentiation (<xref ref-type="bibr" rid="bib17">Choksi et al., 2006</xref>; <xref ref-type="bibr" rid="bib10">Cabernard and Doe, 2009</xref>; <xref ref-type="bibr" rid="bib2">Bayraktar et al., 2010</xref>).</p><p>In this study, we show that the Prospero differentiation factor is transiently detected at low levels in the neuroblast nucleus just prior to entry into quiescence. We find that loss of Prospero prevents entry into quiescence, whereas a pulse of low level nuclear Prospero drives proliferating larval neuroblasts into quiescence. We propose a model in which low levels of Prospero repress all cell cycle and progenitor genes except <italic>deadpan</italic>, whereas high levels of Prospero additionally repress <italic>deadpan</italic> to allow neural differentiation.</p></sec><sec id="s2" sec-type="results|discussion"><title>Results and discussion</title><sec id="s2-1"><title>Quiescent neuroblasts have a novel transcriptional profile lacking both progenitor and differentiation markers</title><p>To investigate the transition from a proliferating neuroblast to a quiescent neuroblast, we examined known proliferation/progenitor and differentiation marker expression during this transition. We hypothesized that loss of a progenitor marker or gain of a differentiation marker might induce neuroblast quiescence. As a model system we used the identified neuroblast 3–3 in the thoracic segments (NB3-3T) which reliably enters quiescence at embryonic stage 15 (<xref ref-type="bibr" rid="bib46">Tsuji et al., 2008</xref>). Moreover, neuroblast 3–3 in abdominal segments (NB3-3A) remains proliferative until the end of embryogenesis and provides an excellent internal control (<xref ref-type="bibr" rid="bib46">Tsuji et al., 2008</xref>). Previous work showed that the quiescent NB3-3T can be recognized by Deadpan, a basic helix-loop-helix transcription factor (<xref ref-type="bibr" rid="bib50">Zhu et al., 2008</xref>; <xref ref-type="bibr" rid="bib14">Chell and Brand, 2010</xref>). The coiled-coil Miranda protein is also reported to mark quiescent neuroblasts (<xref ref-type="bibr" rid="bib46">Tsuji et al., 2008</xref>), which we confirm here (<xref ref-type="fig" rid="fig1">Figure 1</xref>), but note that <italic>mira</italic> transcription is down-regulated in quiescent neuroblasts based lack of mRNA in the stage 17 CNS in the BDGP in situ database (<xref ref-type="bibr" rid="bib44">Tomancak et al., 2007</xref>). Interestingly, we find that all other tested progenitor markers—Worniu, Asense, and Cyclin E (CycE) (<xref ref-type="bibr" rid="bib6">Brand et al., 1993</xref>; <xref ref-type="bibr" rid="bib11">Caldwell and Datta, 1998</xref>; <xref ref-type="bibr" rid="bib1">Ashraf et al., 2004</xref>; <xref ref-type="bibr" rid="bib32">Lai et al., 2012</xref>)—are not expressed in the quiescent NB3-3T (<xref ref-type="fig" rid="fig1">Figure 1</xref>; data not shown), although they can be robustly detected in the proliferative NB3-3A (<xref ref-type="fig" rid="fig1">Figure 1</xref>). Similarly, larval quiescent neuroblasts lack expression of Worniu, Asense, CycE (and Miranda), while retaining Deadpan protein and active <italic>deadpan</italic> transcription (<xref ref-type="fig" rid="fig1s1">Figure 1—figure supplement 1</xref>). However, misexpression of Worniu (data not shown), Asense, or CycE (<xref ref-type="fig" rid="fig1s2">Figure 1—figure supplement 2</xref>) has no effect on the timing of NB3-3T quiescence.<fig-group><fig id="fig1" position="float"><object-id pub-id-type="doi">10.7554/eLife.03363.003</object-id><label>Figure 1.</label><caption><title>Quiescent neuroblasts have a novel transcriptional profile lacking both progenitor and differentiation markers.</title><p>(<bold>A</bold>–<bold>F</bold>) Proliferating NB3-3A contains nuclear Deadpan (<bold>A</bold>), cytoplasmic Mira (<bold>B</bold>), nuclear Asense (<bold>C</bold>), nuclear Worniu (<bold>D</bold>), and nuclear CycE (<bold>E</bold>) at embryonic stages 14 and 16; quantification is shown in (<bold>F</bold>). Neuroblast lineages are marked by Eg-GFP (<italic>Eg-gal4 UAS-mCD8:GFP</italic>), and the neuroblast is identified by Dpn (shown in insets) and indicated by arrows. Anterior side is up, and lateral is at right. Scale bar: 5 μm. (<bold>G</bold>–<bold>L</bold>) Proliferating NB3-3T expresses all above-mentioned neuroblast markers at stage 14 (S14). At stage 16 (S16), NB3-3T is quiescent and contains Deadpan (<bold>G</bold>) and Mira (<bold>H</bold>) but lacks Asense (<bold>I</bold>), Worniu (<bold>J</bold>) and CycE (<bold>K</bold>). Quantified in (<bold>L</bold>). Scale bar: 5 μm. (<bold>F</bold> and <bold>L</bold>) Quantification; number of neuroblasts scored shown in bar. (<bold>M</bold>) Schematic summary of neuroblast marker profiles of proliferating neuroblast, quiescent neuroblast and neuron.</p><p><bold>DOI:</bold> <ext-link ext-link-type="doi" xlink:href="10.7554/eLife.03363.003">http://dx.doi.org/10.7554/eLife.03363.003</ext-link></p></caption><graphic xlink:href="elife03363f001"/></fig><fig id="fig1s1" position="float" specific-use="child-fig"><object-id pub-id-type="doi">10.7554/eLife.03363.004</object-id><label>Figure 1—figure supplement 1.</label><caption><title>Larval quiescent neuroblasts are Deadpan-positive but lack the progenitor markers Miranda, Asense, Cyclin E, Worniu and the differentiation markers Prospero, Elav, and Repo.</title><p>(<bold>A</bold>) The newly hatched larval (NHL) central brain contains five persistently proliferative neuroblasts (the four mushroom body neuroblasts and the lateral antennal lobe neuroblast) and ∼100 quiescent central brain neuroblasts in each lobe. (<bold>B</bold>–<bold>E</bold>) Proliferative neuroblasts in the NHL contain progenitor markers Deadpan (Dpn), Asense (Ase), and Worniu (Wor) but lack the differentiation markers Prospero (Pros), Elav, and Repo and incorporate EdU (top). Quiescent neuroblasts maintain Dpn, but lack the progenitor markers and the differentiation markers, and fail to incorporate EdU (bottom).</p><p><bold>DOI:</bold> <ext-link ext-link-type="doi" xlink:href="10.7554/eLife.03363.004">http://dx.doi.org/10.7554/eLife.03363.004</ext-link></p></caption><graphic xlink:href="elife03363fs001"/></fig><fig id="fig1s2" position="float" specific-use="child-fig"><object-id pub-id-type="doi">10.7554/eLife.03363.005</object-id><label>Figure 1—figure supplement 2.</label><caption><title>Overexpressing Asense and Cyclin E does not change the timing of neuroblast quiescence.</title><p>No significant change in the number of proliferating NB3-3T in stage 15 embryos with overexpressed Asense (Ase<sup>OXN</sup>) or Cyclin E (CycE<sup>OXN</sup>). Error bars, standard deviation; n.s., not significant. Fly genotype: Ase<sup>OXN</sup>, <italic>worniu-gal4 UAS-asense</italic>; CycE<sup>OXN</sup>, <italic>worniu-gal4 UAS-cyclin E</italic>.</p><p><bold>DOI:</bold> <ext-link ext-link-type="doi" xlink:href="10.7554/eLife.03363.005">http://dx.doi.org/10.7554/eLife.03363.005</ext-link></p></caption><graphic xlink:href="elife03363fs002"/></fig></fig-group></p><p>We next turned to examining differentiation markers. We assayed nuclear Prospero, which marks differentiating GMCs and young neurons in the larval CNS (<xref ref-type="bibr" rid="bib12">Carney et al., 2013</xref>), Embryonic lethal and abnormal vision (Elav), which marks all neurons (<xref ref-type="bibr" rid="bib37">Robinow and White, 1988</xref>); and Reversed polarity (Repo), which marks all non-midline glia (<xref ref-type="bibr" rid="bib49">Xiong et al., 1994</xref>). We found that the mature quiescent NB3-3T did not express nuclear Prospero, Elav, or Repo (data not shown), nor were these differentiation markers observed in quiescent larval brain neuroblasts (<xref ref-type="fig" rid="fig1s1">Figure 1—figure supplement 1</xref>). Interestingly, we did observe transient low level nuclear Prospero in NB3-3T just prior to its entry into quiescence (see next section). We conclude that quiescent neuroblasts have a novel transcriptional profile that lacks both progenitor and differentiation markers.</p></sec><sec id="s2-2"><title>A pulse of low level nuclear Prospero marks neuroblast entry into quiescence</title><p>Here, we characterize further our observation that NB3-3T showed low level nuclear Prospero at stage 15, concurrent with the entry into quiescence. Whereas NB3-3T showed a pulse of nuclear Prospero at stage 15 concurrent with entry into quiescence (<xref ref-type="fig" rid="fig2">Figure 2A,B</xref>), NB3-3A did not show nuclear Prospero and did not enter quiescence (<xref ref-type="fig" rid="fig2">Figure 2C,D</xref>). The levels of Prospero in the NB3-3T neuroblast are clearly detectable by immunofluorescent staining, but at significantly lower levels than in differentiating GMCs (<xref ref-type="fig" rid="fig2">Figure 2B</xref>, arrow; <xref ref-type="fig" rid="fig2s1">Figure 2—figure supplement 1</xref>). To probe the correlation of transient nuclear Prospero and neuroblast quiescence in more detail, we assayed temporal identity mutations that are known to shift the timing of NB3-3T quiescence: <italic>nubbin/pdm2</italic> (<italic>pdm</italic>) mutants cause precocious NB3-3T quiescence, whereas <italic>castor</italic> (<italic>cas</italic>) mutants cause a delay in NB3-3T quiescence (<xref ref-type="bibr" rid="bib46">Tsuji et al., 2008</xref>). Strikingly, the pulse of nuclear Prospero precisely matched the timing of neuroblast quiescence: it occurred earlier in <italic>pdm</italic> mutants, and later in <italic>cas</italic> mutants (<xref ref-type="fig" rid="fig2">Figure 2E,F</xref>). We conclude that a pulse of low level nuclear Prospero marks neuroblast entry into quiescence, suggesting a functional relationship.<fig-group><fig id="fig2" position="float"><object-id pub-id-type="doi">10.7554/eLife.03363.006</object-id><label>Figure 2.</label><caption><title>Transient low level nuclear Prospero is tightly correlated with neuroblast entry into quiescence.</title><p>(<bold>A</bold>–<bold>B</bold>) NB3-3T shows transient nuclear Prospero (Pros) during entry into quiescence at stage 15 (S15) (<bold>B</bold>). Differentiated daughter cell shows strong nuclear Prospero and is indicated by the arrow. Neuroblast lineages are identified by Eg-GFP (<italic>Eg-gal4 UAS-mCD8:GFP</italic>) (<bold>A</bold>), and the neuroblast is identified by Dpn and outlined by dashed lines. Summarized in schematic below (Prospero, green); quantified in (<bold>E</bold>). Anterior side is up, and lateral is at right. Scale bar: 5 μm. (<bold>C</bold>–<bold>D</bold>) Proliferating NB3-3A does not contain nuclear Prospero. Summarized in schematic below; quantified in (<bold>E</bold>). Scale bar: 5 μm. (<bold>E</bold>) Temporal identity factors schedule the timing of neuroblast entry into quiescence and the expression of nuclear Prospero. In <italic>cas</italic> mutant (<italic>cas</italic><sup><italic>24</italic></sup>), neuroblast quiescence is delayed, as is the timing of nuclear Prospero. In the deficiency allele <italic>Df(2L)ED773</italic> which removes both <italic>nubbin/pdm2</italic> (<italic>pdm</italic> mutant), NB3-3T precociously enters quiescence, and the timing of nuclear Prospero is advanced. (<bold>F</bold>) Schematic of the timing of neuroblast entry into quiescence and the expression of nuclear Prospero.</p><p><bold>DOI:</bold> <ext-link ext-link-type="doi" xlink:href="10.7554/eLife.03363.006">http://dx.doi.org/10.7554/eLife.03363.006</ext-link></p></caption><graphic xlink:href="elife03363f002"/></fig><fig id="fig2s1" position="float" specific-use="child-fig"><object-id pub-id-type="doi">10.7554/eLife.03363.007</object-id><label>Figure 2—figure supplement 1.</label><caption><title>Nuclear Prospero levels are lower in stage 15 NB3-3T than in differentiating GMCs.</title><p>(<bold>A</bold>) Confocal images of a wild type NB3-3T (blue circle, identified by Eg-GFP and Dpn) and its adjacent GMC (yellow circle, identified by Eg-GFP and its smaller size) in stage 15 embryos. Scale bar: 5 μm. (<bold>B</bold>) Quantification of nuclear Prospero intensity in unit area in a neuroblast vs an adjacent GMC in stage 15 embryos. Prospero intensity in each confocal stack was measured with the open source software FIJI to obtain gray value and the nuclear area. The total Prospero intensity was summed from the confocal stacks, and then divided by nuclear volume to obtain the Prospero intensity per unit volume. Small triangles represent the intensity quantified from each individual neuroblast, and the large triangle is the average. Small circles represent the intensity quantified from each individual GMC, and the large circle is the average.</p><p><bold>DOI:</bold> <ext-link ext-link-type="doi" xlink:href="10.7554/eLife.03363.007">http://dx.doi.org/10.7554/eLife.03363.007</ext-link></p></caption><graphic xlink:href="elife03363fs003"/></fig></fig-group></p></sec><sec id="s2-3"><title>Prospero is required for neuroblast quiescence</title><p>Having shown a strong correlation between the timing of nuclear Prospero and neuroblast entry into quiescence, we next asked if Prospero is required for neuroblast entry into quiescence and loss of nuclear Prospero can delay the timing of neuroblast quiescence. In theory, this can be done by quantifying the number of EdU+ neuroblasts in <italic>prospero</italic> null mutant embryos. The problem is that <italic>prospero</italic> mutants show de-differentiation of GMCs into proliferating neuroblasts (<xref ref-type="bibr" rid="bib17">Choksi et al., 2006</xref>; <xref ref-type="bibr" rid="bib33">Lee et al., 2006</xref>), making it necessary to distinguish the parental neuroblasts from the de-differentiated GMCs. To resolve the issue, we used the Notch signaling reporter, mγ-LacZ (<xref ref-type="bibr" rid="bib47">Wech et al., 1999</xref>; <xref ref-type="bibr" rid="bib18">Cooper et al., 2000</xref>), to identify the parental neuroblasts. During neuroblast asymmetric cell division, Miranda-Prospero and Partner of numb (Pon)-Numb protein complexes are independently segregated into the GMC to promote differentiation (<xref ref-type="bibr" rid="bib20">Doe, 2008</xref>). In <italic>prospero</italic> mutants, Numb is still properly segregated to GMCs where it represses Notch activity (<xref ref-type="bibr" rid="bib42">Spana and Doe, 1995</xref>) and consequently the Notch reporter mγ-LacZ is restricted to parental neuroblasts. <italic>mγ-LacZ</italic> is transcribed only in neuroblasts; in wild type the protein perdures into neuroblast progeny, whereas in <italic>prospero</italic> mutants the protein is restricted to neuroblasts because the progeny proliferate without expressing mγ-LacZ and thus dilute out the protein (<xref ref-type="bibr" rid="bib17">Choksi et al., 2006</xref>; <xref ref-type="fig" rid="fig3">Figure 3A,B</xref>). Thus, we scored for EdU incorporation in the mγ-LacZ+ parental neuroblasts. Whereas in wild type nearly all neuroblasts ceased EdU incorporation by stage 16 (<xref ref-type="fig" rid="fig3">Figure 3A</xref>), many mγ-LacZ+ neuroblasts continued to incorporate EdU in <italic>prospero</italic> mutants at the same stage (<xref ref-type="fig" rid="fig3">Figure 3B</xref>). This strongly supports a model in which Prospero is required for neuroblast entry into quiescence. In addition, Prospero is unique among basal cell fate determinants in regulating neuroblast quiescence: loss of function mutations in <italic>numb</italic> or <italic>brat</italic>, or the Prospero target gene <italic>string</italic>, showed no effect on the timing of neuroblast quiescence (data not shown).<fig id="fig3" position="float"><object-id pub-id-type="doi">10.7554/eLife.03363.008</object-id><label>Figure 3.</label><caption><title>Prospero is required for neuroblast quiescence.</title><p>(<bold>A</bold>–<bold>B</bold>) Wild type and <italic>prospero</italic> mutant (<italic>pros</italic><sup><italic>17</italic></sup>) stage 16 embryos. Parental neuroblasts (NB) stain for Deadpan and mγ-LacZ, whereas ectopic ‘de-differentiated’ neuroblasts stain for Deadpan but not mγ-LacZ. Proliferating neuroblasts are marked by EdU incorporation. (<bold>A</bold>) In wild type, most thoracic neuroblasts are in quiescence and do not incorporate EdU. (<bold>B</bold>) In <italic>prospero</italic> mutants, most parental neuroblasts (mγ-LacZ+) have not entered quiescence and still incorporate EdU. Ventral side is up and anterior is at left. Dashed line, boundary of neuroblast and neuron (N) layers. Scale bars: 20 μm. (<bold>C</bold>–<bold>E</bold>) NB3-3T remains proliferative in <italic>prospero</italic> mutant stage 16 embryos. (<bold>C</bold>) In wild type, NB3-3T is Mira+ mγ-LacZ+ EdU−; (<bold>D</bold>) in <italic>prospero</italic> mutants, NB3-3T is Mira+ mγ-LacZ+ EdU+. The NB3-3T lineage was identified by Eg-GFP (<italic>Eg-gal4 UAS-mCD8:GFP</italic>) and the neuroblast is indicated by arrowheads. (<bold>E</bold>) Quantification. n.s., not significant; ****p < 0.00001.</p><p><bold>DOI:</bold> <ext-link ext-link-type="doi" xlink:href="10.7554/eLife.03363.008">http://dx.doi.org/10.7554/eLife.03363.008</ext-link></p></caption><graphic xlink:href="elife03363f003"/></fig></p><p>To test further whether Prospero is required for neuroblast quiescence, we returned to the NB3-3T model system. We used mγ-LacZ, Miranda, and EdU incorporation to assay the timing of NB3-3T quiescence in wild type and <italic>prospero</italic> mutant embryos. In both wild type and <italic>prospero</italic> haplo-insufficiency embryos, the mγ-LacZ+ NB3-3T was proliferative at stage 14 and quiescent by stage 16 (<xref ref-type="fig" rid="fig3">Figure 3C</xref>, quantified in <xref ref-type="fig" rid="fig3">Figure 3E</xref>). In <italic>prospero</italic> mutant embryos, the mγ-LacZ+ NB3-3T was proliferative at both stage 14 and 16 (<xref ref-type="fig" rid="fig3">Figure 3D</xref>, quantified in <xref ref-type="fig" rid="fig3">Figure 3E</xref>). The results suggest that the level of Prospero required for neuroblast quiescence is lower than that in <italic>prospero</italic> haplo-insufficiency and that a low level of Prospero is sufficient to promote neuroblast quiescence (see next section). We conclude that Prospero is required for neuroblast quiescence.</p></sec><sec id="s2-4"><title>Prospero is sufficient to induce neuroblast quiescence</title><p>The central brain neuroblasts are continuously proliferating during the third larval instar (<xref ref-type="bibr" rid="bib31">Ito and Hotta, 1992</xref>), and so we used this stage to determine whether transient Prospero expression could induce neuroblast quiescence. We used the TARGET method (<xref ref-type="bibr" rid="bib36">McGuire et al., 2003</xref>) to transiently induce Prospero expression for 2 hr, and then assayed for neuroblast quiescence 12 hr later (<xref ref-type="fig" rid="fig4">Figure 4</xref>). Because high levels of Prospero can induce neuroblast differentiation (<xref ref-type="fig" rid="fig4s1">Figure 4—figure supplement 1</xref>) (<xref ref-type="bibr" rid="bib17">Choksi et al., 2006</xref>; <xref ref-type="bibr" rid="bib10">Cabernard and Doe, 2009</xref>; <xref ref-type="bibr" rid="bib2">Bayraktar et al., 2010</xref>), we used Prospero levels low enough to minimize neuroblast differentiation, as shown by the persistence of most Deadpan+ neuroblasts after misexpression (<xref ref-type="fig" rid="fig4">Figure 4D</xref>). We identified quiescent neuroblasts by their failure to incorporate EdU, lack of progenitor marker Worniu, and lack of the differentiation marker Elav—together a robust signature for quiescent neuroblasts (see <xref ref-type="fig" rid="fig1">Figure 1</xref>). In wild-type larval brains, we found few or no quiescent neuroblasts, as expected (<xref ref-type="fig" rid="fig4">Figure 4E,F</xref>). In contrast, a pulse of Prospero resulted in the appearance of numerous quiescent neuroblasts (<xref ref-type="fig" rid="fig4">Figure 4E,F</xref>; 12 hr timepoint). To rule out the possibility that these ‘quiescent neuroblasts’ were early in the differentiation pathway, we reasoned that if they were quiescent they could reactivate proliferation in a nourishing environment (<xref ref-type="bibr" rid="bib14">Chell and Brand, 2010</xref>; <xref ref-type="bibr" rid="bib40">Sousa-Nunes et al., 2010</xref>) whereas if they were differentiating they would never re-enter the cell cycle. Thus, we exposed larval neuroblasts to the same 2 hr pulse of Prospero, but waited 24 hr to assay neuroblasts. We found that nearly all of the neuroblasts that were quiescent at 12 hr after induction had reactivated proliferation and re-expressed progenitor markers (<xref ref-type="fig" rid="fig4">Figure 4E,F</xref>; 24 hr timepoint). In addition, precocious low level nuclear Prospero could advance the timing of NB3-3T quiescence (<xref ref-type="fig" rid="fig4s2">Figure 4—figure supplement 2</xref>). We conclude that transient, low-level nuclear Prospero is sufficient to induce neuroblast quiescence. Interestingly, Prospero can also transiently arrest the cell cycle without inducing differentiation in embryonic longitudinal glial progenitors (<xref ref-type="bibr" rid="bib26a">Griffiths and Hidalgo, 2004</xref>), although the relative levels of Prospero have not been explored in this system.<fig-group><fig id="fig4" position="float"><object-id pub-id-type="doi">10.7554/eLife.03363.009</object-id><label>Figure 4.</label><caption><title>Prospero is sufficient to induce neuroblast quiescence.</title><p>(<bold>A</bold>–<bold>C</bold>) Transient overexpression of nuclear Prospero in neuroblasts (<italic>worniu-gal4 UAS-HA:prospero tub-gal80</italic><sup><italic>ts</italic></sup>) results in neuroblast quiescence (<bold>A</bold>) or differentiation (<bold>B</bold>). Third-instar larvae were shifted from 22°C to 30°C to inactivate Gal80, which resulted in nuclear Prospero in neuroblasts. Low levels of nuclear Prospero result in neuroblast quiescence (Dpn<sup>+</sup> Elav<sup>−</sup>) (<bold>A</bold>) whereas high levels of nuclear Prospero suppress Deadpan and activate Elav to induce differentiation (Dpn<sup>−</sup> Elav<sup>+</sup>) (<bold>B</bold>). Prospero levels were quantified in (<bold>C</bold>). Scale bars: 5 μm. (<bold>C</bold>) Quantification of Prospero levels in quiescent neuroblasts (Dpn<sup>+</sup> Elav<sup>−</sup>) or differentiated neuroblasts (Dpn<sup>−</sup> Elav<sup>+</sup>). Differentiated neuroblasts were identified by their nuclear size larger than 7 μm in diameter. Prospero intensity was determined by totaling the gray value in nucleus of confocal stacks, followed by normalization to the total gray value of DNA marker 4ʹ,6-diamidino-2-phenylindole (DAPI). Number of neuroblasts quantified shown in bar. Error bars: standard deviation. (<bold>D</bold>–<bold>F</bold>) Larval neuroblast phenotype upon transient overexpression of Prospero. (<bold>D</bold>) Neuroblasts stay Deadpan<sup>+</sup> (Dpn<sup>+</sup>) and thus do not differentiate. (<bold>E</bold>) There is an increase of quiescent neuroblasts 12 hr after Prospero induction, as determined by the elevated percentage of EdU<sup>−</sup> (<bold>D</bold>) and Wor<sup>−</sup> (<bold>E</bold>) neuroblasts. The quiescent neuroblasts re-enter the cell cycle and express the progenitor marker Wor 24 hr after induction. Error bars: standard deviation. n.s., not significant; *p < 0.05; **p < 0.01; ***p < 0.0001. (<bold>G</bold>) Model. Prospero levels distinguish three progenitor fates: absent for self-renewal, low for quiescence, and high for differentiation.</p><p><bold>DOI:</bold> <ext-link ext-link-type="doi" xlink:href="10.7554/eLife.03363.009">http://dx.doi.org/10.7554/eLife.03363.009</ext-link></p></caption><graphic xlink:href="elife03363f004"/></fig><fig id="fig4s1" position="float" specific-use="child-fig"><object-id pub-id-type="doi">10.7554/eLife.03363.010</object-id><label>Figure 4—figure supplement 1.</label><caption><title>Low nuclear Prospero induces neuroblast quiescence, whereas high Prospero induces neuroblast differentiation.</title><p>(<bold>A</bold>) Two levels of Prospero were generated by increasing time of misexpression (4 or 12 hr) with one copy of <italic>UAS-HA:prospero</italic> transgene. Whereas 2 hr can induce neuroblast quiescence (<xref ref-type="fig" rid="fig4">Figure 4</xref>), 4 hr and 12 hr can robustly induce neuroblast differentiation. (<bold>B</bold>) Two levels of Prospero were generated by misexpressing one vs two copies of the <italic>UAS-HA:prospero</italic> transgene for the same time (2 hr). A significant increase of the number of neuroblasts differentiated with two copies of <italic>UAS-HA:prospero</italic> transgene.</p><p><bold>DOI:</bold> <ext-link ext-link-type="doi" xlink:href="10.7554/eLife.03363.010">http://dx.doi.org/10.7554/eLife.03363.010</ext-link></p></caption><graphic xlink:href="elife03363fs004"/></fig><fig id="fig4s2" position="float" specific-use="child-fig"><object-id pub-id-type="doi">10.7554/eLife.03363.011</object-id><label>Figure 4—figure supplement 2.</label><caption><title>Prospero is sufficient to induce embryonic neuroblast quiescence.</title><p>(<bold>A</bold>–<bold>G</bold>) Transient overexpression of nuclear Prospero in neuroblasts (<italic>worniu-gal4 UAS-HA:prospero tub-gal80</italic><sup><italic>ts</italic></sup>) induces neuroblast cell cycle arrest. Embryos were shifted from 22°C to 30°C for 1 hr to inactivate Gal80, which resulted in transient nuclear Prospero in NB3-3T at stage 14 (A, wild type; B, transient nuclear Prospero). Scale bars: 2 μm. (<bold>C</bold>–<bold>G</bold>) Quantification of the Eg+ NB3-3T phenotype upon transient overexpression of Prospero. (<bold>C</bold>) Some neuroblasts have detectable nuclear Prospero. (<bold>D</bold>) Most neuroblasts fail to incorporate EdU. (<bold>E</bold>–<bold>G</bold>) There is a slight loss of Worniu+ (<bold>E</bold>), Asense+ (<bold>F</bold>) and CycE+ (<bold>G</bold>) neuroblasts.</p><p><bold>DOI:</bold> <ext-link ext-link-type="doi" xlink:href="10.7554/eLife.03363.011">http://dx.doi.org/10.7554/eLife.03363.011</ext-link></p></caption><graphic xlink:href="elife03363fs005"/></fig></fig-group></p><p>Our results confirm that high levels of Prospero can trigger differentiation and reveal that low levels of Prospero induce quiescence in the <italic>Drosophila</italic> neuroblasts. High levels of the vertebrate ortholog Prox1 can also trigger differentiation (<xref ref-type="bibr" rid="bib22">Dyer, 2003</xref>; <xref ref-type="bibr" rid="bib43">Takahashi et al., 2006</xref>; <xref ref-type="bibr" rid="bib25">Foskolou et al., 2013</xref>); thus it is interesting that low levels of nuclear Prospero promote neuroblast quiescence without triggering differentiation (<xref ref-type="fig" rid="fig4s1">Figure 4—figure supplement 1</xref>). Prox1 activates some target genes at high levels but represses some target genes at low levels (<xref ref-type="bibr" rid="bib15">Chen et al., 2008a</xref>, <xref ref-type="bibr" rid="bib16">2008b</xref>). We suggest a speculative model for Prospero function: low levels of Prospero are sufficient to repress most progenitor-specific genes in neuroblasts (<italic>worniu</italic>, <italic>asense</italic>, <italic>miranda</italic>, and <italic>cycE</italic>) but do not repress the expression of <italic>deadpan</italic> (perhaps due to lack of high affinity Prospero binding sites). In contrast, high levels of Prospero are sufficient to repress <italic>deadpan</italic> (perhaps via low affinity binding sites) and may even activate neuronal differentiation genes (<xref ref-type="fig" rid="fig4">Figure 4G</xref>).</p><p>In embryonic neuroblasts, the temporal transcription factors Pdm and Cas schedule the timing of neuroblast quiescence, but how they regulate Prospero nuclear import is unknown. In larval neuroblasts, Grh prevents accumulation of nuclear Prospero which would induce neuroblast cell cycle exit and differentiation (<xref ref-type="bibr" rid="bib35">Maurange et al., 2008</xref>; <xref ref-type="bibr" rid="bib13">Chai et al., 2013</xref>), again by an unknown mechanism. The protein Caliban is known to promote Prospero nuclear export in S2 cells (<xref ref-type="bibr" rid="bib5">Bi et al., 2005</xref>), but it is unknown whether Caliban has a similar role in neuroblasts or if down-regulation of Caliban levels or activity leads to increased nuclear Prospero in neuroblasts entering quiescence.</p></sec></sec><sec id="s3" sec-type="materials|methods"><title>Materials and methods</title><sec id="s3-1"><title>Fly genetics</title><p>The following flies were used in this study: (1) <italic>UAS-mCD8:Gfp;eagle-gal4</italic>; (2) <italic>cas</italic><sup><italic>24</italic></sup>/<italic>TM3 Sb Ubx-lacZ</italic>; (3) <italic>Df(2L)ED773</italic>/<italic>CyO wg-lacZ</italic>; (4) <italic>mγ-lacZ;UAS-mCD8:Gfp; eg-gal4 pros</italic><sup><italic>17</italic></sup>/<italic>TM3 Sb Ubx-lacZ</italic>; (5) <italic>UAS-HA:prospero</italic> (attP40); (5) <italic>UAS-HA:prospero</italic> (attP2); (6) <italic>wor-gal4;tubulin-gal80</italic><sup><italic>ts</italic></sup>.</p></sec><sec id="s3-2"><title>Induction of Prospero, EdU incorporation, antibody staining and imaging</title><p>Embryos with genotype <italic>UAS-HA:prospero/wor-gal4;tubulin-gal80</italic><sup><italic>ts</italic></sup><italic>/+</italic> were collected for 4 hr at 22°C, and then cultured at 22°C for 5 days. The larvae were transferred to 30°C to inactivate Gal80, which resulted in the activation of Gal4 to express HA:Prospero. After heat shock, the larvae were immediately dissected or moved back to 22°C to incubate for 12 or 24 hr for later dissection, followed by 2-hr EdU incorporation, fixation, and antibody staining. EdU incorporation was performed in PBS at the concentration of 200 μg/ml. Antibody staining and EdU detection were previously described (<xref ref-type="bibr" rid="bib32">Lai et al., 2012</xref>). Antibodies used in this study included: rabbit anti-Asense (1:1000; Cheng-Yu Lee, University of Michigan, Ann Arbor, MI), mouse anti-β-galactosidase (1:1000; Promega, Madison, WA), mouse anti-Cyclin E (1:50; Developmental Studies Hybridoma Bank (DHSB) developed under NICHD and maintained by University of Iowa), rabbit anti-Cyclin E (1:300; Santa Cruz Biotechnology, Dallas, TX); rat anti-Deadpan monoclonal (1:50; Doe lab), guinea pig anti-Deadpan (1:1000; Jim Skeath, Washington Univ., St. Louis, MO), mouse anti-Eagle (1:100; Doe lab); rabbit anti-Eagle (1:500) (<xref ref-type="bibr" rid="bib27">Higashijima et al., 1996</xref>; <xref ref-type="bibr" rid="bib26">Freeman and Doe, 2001</xref>); rat anti-Elav (1:50, DSHB); chicken anti-GFP (1:500; Aves Labs, Tigard, OR), mouse anti-HA (1:1000; Covance, Princeton, NJ), chicken anti-HA (1:1000, Bethyl Laboratories, Montgomery, TX); guinea pig anti-Miranda (1:2000; Doe lab), mouse anti-Prospero monoclonal purified IgG (1:1000; Doe lab), and rat anti-Worniu (1:2; Doe lab). Microscopy was done using a Zeiss LSM700 or LSM710; image processing and quantification were performed with the open source software FIJI.</p></sec></sec></body><back><ack id="ack"><title>Acknowledgements</title><p>We thank Travis Carney and Minoree Kohwi for critical comments on the manuscript. We also thank Jim Skeath, Cheng-Yu Lee, and Sarah Bray for reagents. 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pub-id-type="doi">10.1242/dev.010876</pub-id></element-citation></ref></ref-list></back><sub-article article-type="article-commentary" id="SA1"><front-stub><article-id pub-id-type="doi">10.7554/eLife.03363.012</article-id><title-group><article-title>Decision letter</article-title></title-group><contrib-group content-type="section"><contrib contrib-type="editor"><name><surname>Brand</surname><given-names>Andrea</given-names></name><role>Reviewing editor</role><aff><institution>University of Cambridge</institution>, <country>United Kingdom</country></aff></contrib></contrib-group></front-stub><body><boxed-text><p>eLife posts the editorial decision letter and author response on a selection of the published articles (subject to the approval of the authors). An edited version of the letter sent to the authors after peer review is shown, indicating the substantive concerns or comments; minor concerns are not usually shown. Reviewers have the opportunity to discuss the decision before the letter is sent (see <ext-link ext-link-type="uri" xlink:href="http://elifesciences.org/review-process">review process</ext-link>). Similarly, the author response typically shows only responses to the major concerns raised by the reviewers.</p></boxed-text><p>Thank you for sending your work entitled “Transient nuclear Prospero induces neural progenitor quiescence” for consideration at <italic>eLife.</italic> Your article has been favorably evaluated by K (Vijay) VijayRaghavan (Senior editor), a Reviewing editor, and 3 reviewers.</p><p>The Reviewing editor and the other reviewers discussed their comments before we reached this decision, and the Reviewing editor has assembled the following comments to help you prepare a revised submission.</p><p>The current study addresses the role of the Prospero transcription factor in inducing neuroblast quiescence in the embryonic and larval <italic>Drosophila</italic> CNS. They find that the NB3-3T lineage displays a brief pulse of weak Pros nuclear expression just prior to entering quiescence. Mutant and mis-expression analysis lends support for the notion that this brief pulse in fact induces quiescence. They also address this notion in the postembryonic neuroblasts, and find support for a similar role for Pros in quiescence. Since we know very little about the fascinating, and even potentially medically important phenomena of stem cell quiescence, this study is of importance, and would likely be interesting for a broader audience. However, the manuscript is not without problems, and several issues should be addressed prior to publication.</p><p>Substantive concerns:</p><p>1) If a weak and brief pulse of Pros protein in NB nuclei is indeed important for NB quiescence, it is possible that pros may show haplo-insufficiency. To this end, they could analyze NB3-3T quiescence in pros heterozygotes. In addition, given the role of <italic>grh</italic> in quiescence, any heterozygotic effect of pros may be enhanced in transhet with <italic>grh</italic> i.e., <italic>pros+/-; grh+/-</italic>. Any such genetic dosage effect would greatly strengthen the study.</p><p>2) In the Introduction section and <xref ref-type="fig" rid="fig1s1">Figure 1–figure supplement 1</xref>: They state that “However, loss or misexpression of Worniu, Asense, or CycE has no effect on the timing of NB3-3T quiescence (<xref ref-type="fig" rid="fig1s1">Figure 1–figure supplement 1</xref>; data not shown)”. I cannot find any data regarding that misexpression of these factors cannot affect the timing of NB3-3T quiescence. This is an important point in the paper, and some data should be provided.</p><p>3) In pros mutants GMCs are de-differentiated into NBs, and hence express NB markers, e.g. Dpn. Hence, they use mgamma-LacZ to identify NBs in pros mutants. This is a reasonable approach, given that the asymmetric localization of Numb into GMCs (which is not affected in pros) results in expression of mgamma-lacZ only in the NBs. However, given that b-gal has a half-life in embryos of 8-12 hrs, it is not clear to me how the b-gal can label only NBs. Yes, the transgene is only transcriptionally active in NBs, but surely the b-gal protein, being quite stable and not subjected to asymmetric distribution, must label daughter cells as well. We need to be convinced, in this context, that all or in fact any of the ectopically EdU labelled cells in <xref ref-type="fig" rid="fig3">Figure 3B</xref> are actually NBs. Similarly, in <xref ref-type="fig" rid="fig3">Figure 3C-D</xref>, in pros mutants there is b-gal expression (as well as Mira expression) in several cells in the NB3-3T lineage; how can the authors be sure that the EdU positive cell is the NB?</p><p>4) <xref ref-type="fig" rid="fig4s1 fig4s2">Figure 4–figure supplement 1 and 2</xref>: They mis-express pros in NB3-3T, and find that it can induce NB quiescence. This is not surprising, given the role of pros in terminating proliferation.</p><p>5) Regarding postembryonic NBs, there are at least two previous publications that have addressed the function of pros in postembryonic NBs. In the paper from the Gould lab (Maraunge et al, 2008, Supplement <xref ref-type="fig" rid="fig3">Figure 3C</xref>) they show ectopic dividing cells in pros MARCM clones. However, since the clones are quite large, and they use few markers, ectopically dividing cells may just as well be GMCs and not NBs. Similarly, in the recent paper from the Doe lab itself (Bayraktar et al., Neural Dev, 2010,) they only study GOF effects of Pros, and find no pros LOF effects in Type I NBs. In the current study, they again show that GOF of pros can trigger NB quiescence in postembryonic NBs. This is not surprising given the two previous publications. However, they provide no new LOF data supporting the idea that Pros is necessary for NB quiescence.</p><p>6) The targets of Pros that induce/repress neuroblast quiescence are unknown. Pros has been shown to repress neuroblast cell-fate genes and cell-cycle genes in GMCs (Choksi SP, et al., Dev Cell 2006). The authors mentioned that loss or misexpression of Wor, Ase or CycE has no effect on the timing of NB3-3T quiescence. Have they tested these in 3rd instar central larval brain neuroblasts, in which the transient Pros expression was shown to induce neuroblast quiescence? Also, have they tested the role of String, an important cell cycle regulator that was repressed by Pros (Choksi SP, et al., Dev Cell 2006)?</p><p>7) The authors could also discuss the potential role of other cell fate determinants (Numb and Brat) in neuroblast quiescence. It will be valuable to know if the role of Pros represents a general function of cell fate determinants in regulating neuroblast quiescence or is Pros unique among the cell fate determinants in this process.</p></body></sub-article><sub-article article-type="reply" id="SA2"><front-stub><article-id pub-id-type="doi">10.7554/eLife.03363.013</article-id><title-group><article-title>Author response</article-title></title-group></front-stub><body><p><italic>1) If a weak and brief pulse of Pros protein in NB nuclei is indeed important for NB quiescence, it is possible that pros may show haplo-insufficiency. To this end, they could analyze NB3-3T quiescence in pros heterozygotes. In addition, given the role of</italic> grh <italic>in quiescence, any heterozygotic effect of pros may be enhanced in transhet with</italic> grh <italic>i.e.,</italic> pros+/-; grh+/-<italic>. Any such genetic dosage effect would greatly strengthen the study</italic>.</p><p>We appreciate reviewers raising the point and help us to further strengthen our conclusion: “low levels of Prospero promote neuroblast quiescence.” We first performed EdU incorporation in the <italic>pros</italic> heterozygous mutants and found that the neuroblast quiescence timing was not delayed. The results suggest that the levels of Prospero required for neuroblast quiescence is lower than that in <italic>pros</italic> haplo-insufficiency, which is consistent with our conclusions from the overexpression studies. We have added the result in <xref ref-type="fig" rid="fig3">Figure 3E</xref> and revised the Results section: “In both wild type and prospero haplo-insufficiency embryos, the mγ-LacZ+ NB3-3T was proliferative at stage 14 and quiescent by stage 16 (<xref ref-type="fig" rid="fig3">Figure 3C</xref>, quantified in 3E). In prospero mutant embryos, the mγ-LacZ+ NB3-3T was proliferative at both stage 14 and 16 (<xref ref-type="fig" rid="fig3">Figure 3D</xref>, quantified in 3E). The results suggest that the level of Prospero required for neuroblast quiescence is lower than that in prospero haplo-insufficiency and that a low level of Prospero is sufficient to promote neuroblast quiescence.”</p><p>Although <italic>grh</italic> plays an important role in regulating Pros activity in larval neuroblasts (<xref ref-type="bibr" rid="bib35">Maurange et al., 2008</xref>; <xref ref-type="bibr" rid="bib13">Chai et al., 2013</xref>), it is unknown whether <italic>grh</italic> plays a similar role in embryonic neuroblasts. If <italic>grh</italic> regulates the timing of neuroblast quiescence by preventing Pronuclear accumulation, trans-heterozygous <italic>pros+/- grh+/-</italic> will show no phenotype as Grh and Pros counteract each other in regulating neuroblast quiescence. As expected, we didn’t find any phenotype regarding the timing of neuroblast quiescence in the <italic>pros+/- grh+/-</italic>trans-heterozygous mutants (data not shown). Nevertheless, the concern led us to put back the discussion we removed in the previous submission due to the limitation of space: “In embryonic neuroblasts, the temporal transcription factors Pdm and Cas schedule the timing of neuroblast quiescence, but how they regulate Prospero nuclear import is unknown. In larval neuroblasts, Grh prevents accumulation of nuclear Prospero which would induce neuroblast cell cycle exit and differentiation (<xref ref-type="bibr" rid="bib13">Chai et al., 2013</xref>; <xref ref-type="bibr" rid="bib35">Maurange et al., 2008</xref>), again by an unknown mechanism.”</p><p><italic>2) In the Introduction section and</italic> <xref ref-type="fig" rid="fig1s1"><italic>Figure 1–figure supplement 1</italic></xref><italic>: They state that “However, loss or misexpression of Worniu, Asense, or CycE has no effect on the timing of NB3-3T quiescence (</italic><xref ref-type="fig" rid="fig1s1"><italic>Figure 1–figure supplement 1</italic></xref><italic>; data not shown)”. I cannot find any data regarding that misexpression of these factors cannot affect the timing of NB3-3T quiescence. This is an important point in the paper, and some data should be provided</italic>.</p><p>We agree, and we have added a new figure with the requested data (<xref ref-type="fig" rid="fig1s2">Figure 1–figure supplement 2</xref>). In summary, when we overexpressed Ase or CycE, we didn’t observe significant change in the number of proliferating neuroblasts.</p><p><italic>3) In pros mutants GMCs are de-differentiated into NBs, and hence express NB markers, e.g. Dpn. Hence, they use mgamma-LacZ to identify NBs in pros mutants. This is a reasonable approach, given that the asymmetric localization of Numb into GMCs (which is not affected in pros) results in expression of mgamma-lacZ only in the NBs. However, given that b-gal has a half-life in embryos of 8-12 hrs, it is not clear to me how the b-gal can label only NBs. Yes, the transgene is only transcriptionally active in NBs, but surely the b-gal protein, being quite stable and not subjected to asymmetric distribution, must label daughter cells as well. We need to be convinced, in this context, that all or in fact any of the ectopically EdU labelled cells in</italic> <xref ref-type="fig" rid="fig3"><italic>Figure 3B</italic></xref> <italic>are actually NBs. Similarly, in</italic> <xref ref-type="fig" rid="fig3"><italic>Figure 3C-D</italic></xref><italic>, in pros mutants there is b-gal expression (as well as Mira expression) in several cells in the NB3-3T lineage; how can the authors be sure that the EdU positive cell is the NB?</italic></p><p>We thank the reviewers for highlighting this issue, which obviously needed a better explanation! The short answer is that <italic>pros</italic> mutant GMCs do not transcribe mγ-lacZ but continue to divide and thus dilute out mγ−lacZ, resulting in high level mγ−lacZ specifically in the parental neuroblasts. We have clarified this in the Results section: “mγ-LacZ is transcribed only in neuroblasts; in wild type the protein perdures into neuroblast progeny, whereas in prospero mutants the protein is restricted to neuroblasts because the progeny proliferate without expressing mγ-LacZ and thus dilute out the protein (Choksi et al.; <xref ref-type="fig" rid="fig3">Figure 3A,B</xref>).”</p><p>We thank the reviewers for helping us clarify this important point, which further supports our identification of NB3-3T in the <italic>prospero</italic> mutants, and our conclusion that Prospero is required for neuroblast quiescence.</p><p><italic>4)</italic> <xref ref-type="fig" rid="fig4s1 fig4s2"><italic>Figure 4–figure supplement 1 and 2</italic></xref><italic>: They mis-express pros in NB3-3T, and find that it can induce NB quiescence. This is not surprising, given the role of pros in terminating proliferation</italic>.</p><p>Yes, we agree that a major role of Prospero in neural progenitor cells is to terminate proliferation as previously reported (<xref ref-type="bibr" rid="bib34">Li and Vaessin, 2000</xref>; <xref ref-type="bibr" rid="bib17">Choksi et al., 2006</xref>). However, we want to emphasize that our new discovery is the dosage effect of Prospero in the progenitor cells: high levels of Pros will terminate proliferation and induce differentiation, while low levels of Pros transiently arrest cell cycle but are not sufficient to change the neuroblast identity, which leads to quiescence. We have added the following sentence in the Result section to further emphasize our main conclusion in the current study: “Our results confirm that high levels of Prospero can trigger differentiation, and reveal that low levels of Prospero induce quiescence in the <italic>Drosophila</italic> neuroblasts.”</p><p><italic>5) Regarding postembryonic NBs, there are at least two previous publications that have addressed the function of pros in postembryonic NBs. In the paper from the Gould lab (</italic><italic>Maraunge et al, 2008</italic><italic>, Supplement</italic> <xref ref-type="fig" rid="fig3"><italic>Figure 3C</italic></xref><italic>) they show ectopic dividing cells in pros MARCM clones. However, since the clones are quite large, and they use few markers, ectopically dividing cells may just as well be GMCs and not NBs. Similarly, in the recent paper from the Doe lab itself (Bayraktar et al., Neural</italic> <italic>Dev, 2010</italic><italic>,) they only study GOF effects of Pros, and find no pros LOF effects in Type I NBs. In the current study, they again show that GOF of pros can trigger NB quiescence in postembryonic NBs. This is not surprising given the two previous publications. However, they provide no new LOF data supporting the idea that Pros is necessary for NB quiescence</italic>.</p><p>In our Results section, we have shown that loss of Prospero can delay the timing of neuroblast quiescence (<xref ref-type="fig" rid="fig3">Figure 3</xref>). We have also shown that transient overexpression of low levels of Prospero can induce neuroblast quiescence (<xref ref-type="fig" rid="fig4">Figure 4</xref>). To help readers easily understand our studies, we have revised the first sentence in the section of “Prospero is required for neuroblast quiescence” as follows: “Having shown a strong correlation between the timing of nuclear Prospero and neuroblast entry into quiescence, we next asked if Prospero is required for neuroblast entry into quiescence and loss of nuclear Prospero can delay the timing of neuroblast quiescence.”</p><p><italic>6) The targets of Pros that induce/repress neuroblast quiescence are unknown. Pros has been shown to repress neuroblast cell-fate genes and cell-cycle genes in GMCs (Choksi SP, et al.,</italic> <italic>Dev Cell 2006</italic><italic>). The authors mentioned that loss or misexpression of Wor, Ase or CycE has no effect on the timing of NB3-3T quiescence. Have they tested these in 3rd instar central larval brain neuroblasts, in which the transient Pros expression was shown to induce neuroblast quiescence? Also, have they tested the role of String, an important cell cycle regulator that was repressed by Pros (Choksi SP, et al.,</italic> <italic>Dev Cell 2006</italic><italic>)?</italic></p><p>In our earlier paper (<xref ref-type="bibr" rid="bib32">Lai et al., 2012</xref>, Dev Cell), we showed that <italic>wor</italic> mutant larval neuroblasts prematurely differentiate or die, while overexpression of Wor results in the cell cycle arrest but not quiescence. We have also used <italic>cycE-RNAi</italic> (FBti0140127) and <italic>ase-RNAi</italic> (FBti0158728) to knock down CycE and Ase, respectively, but could not induce larval neuroblast quiescence (data not shown). We thus conclude that loss of CycE, Ase or Wor do not induce larval neuroblast quiescence.</p><p>As requested, we tested the role of String in neuroblast quiescence. We examined stg mutants for neuroblast EdU incorporation and Ase, Wor and Mira markers all are absent in quiescent neuroblasts. Interestingly, <italic>string</italic> mutant neuroblasts are EdU- as expected, but did not downregulate Ase, Wor and Mira, showing that they did not match the gene expression profile of quiescent neuroblasts. We conclude that although <italic>string</italic> is repressed by Pros, <italic>string</italic> downregulation is not sufficient to induce neuroblast quiescence. We add: “In addition, Prospero is unique among basal cell fate determinants in regulating neuroblast quiescence: loss of function mutations in numb or brat, or the Prospero target gene string, showed no effect on the timing of neuroblast quiescence (data not shown)”.</p><p><italic>7) The authors could also discuss the potential role of other cell fate determinants (Numb and Brat) in neuroblast quiescence. It will be valuable to know if the role of Pros represents a general function of cell fate determinants in regulating neuroblast quiescence or is Pros unique among the cell fate determinants in this process</italic>.</p><p>We thank the reviewers for this comment, which allowed us to add new experiments that further highlight the unique role of Pros in promoting neuroblast quiescence. We found that <italic>numb</italic> or <italic>brat</italic> null mutant embryos did not exhibit a delay or advance in neuroblast quiescence (data not shown). We add: “In addition, Prospero is unique among basal cell fate determinants in regulating neuroblast quiescence: loss of function mutations in numb or brat, or the Prospero target gene string, showed no effect on the timing of neuroblast quiescence (data not shown)”.</p></body></sub-article></article> |