Twins/PP2A regulates aPKC to control neuroblast cell polarity and self-renewal

Protein phosphatase 2A regulates blood cell proliferation and differentiation in Drosophila larval lymph glands

Protein phosphatase 2A (PP2A), one of the most abundant protein phosphatases, has divergent functions in multiple types of cells. Its inactivation has been closely associated with leukemia diseases. However, the physiological function of PP2A for hematopoiesis has been poorly understood in organisms. Drosophila hematopoiesis parallels the vertebrate counterpart in developmental and functional features but involves a much simpler hematopoietic system. Here, utilizing the Drosophila major larval hematopoietic organ lymph gland, we studied the function of PP2A for hematopoiesis in vivo. By knocking down the expression of Pp2A-29B that encodes the scaffold subunit of the PP2A holoenzyme complex, we found that PP2A silencing in the differentiating hemocytes resulted in their excessive proliferation. Furthermore, this PP2A inhibition downregulated the expression of Smoothened (Smo), a crucial component in the Hedgehog pathway, and smo overexpression was able to rescue the phenotypes of PP2A depletion, indicating that Smo functions as a downstream effector of PP2A to restrict the hemocyte proliferation. PDGF/VEGF-receptor (Pvr) overexpression also restored the Smo expression and lymph gland morphology of PP2A silencing, suggesting a PP2A-Pvr-Smo axis to regulate lymph gland growth and hemocyte proliferation. Moreover, inhibiting PP2A activity in the blood progenitor cells promoted their differentiation, but which was independent with Smo. Together, our data suggested that PP2A plays a dual role in the Drosophila lymph gland by preserving the progenitor population and restraining the hemocyte proliferation, to properly regulate the hematopoietic process.

Parafibromin governs cell polarity and centrosome assembly in Drosophila neural stem cells

Neural stem cells (NSCs) divide asymmetrically to balance their self-renewal and differentiation, an imbalance in which can lead to NSC overgrowth and tumor formation. The functions of Parafibromin, a conserved tumor suppressor, in the nervous system are not established. Here, we demonstrate that Drosophila Parafibromin/Hyrax (Hyx) inhibits ectopic NSC formation by governing cell polarity. Hyx is essential for the asymmetric distribution and/or maintenance of polarity proteins. hyx depletion results in the symmetric division of NSCs, leading to the formation of supernumerary NSCs in the larval brain. Importantly, we show that human Parafibromin rescues the ectopic NSC phenotype in Drosophila hyx mutant brains. We have also discovered that Hyx is required for the proper formation of interphase microtubule-organizing center and mitotic spindles in NSCs. Moreover, Hyx is required for the proper localization of 2 key centrosomal proteins, Polo and AurA, and the microtubule-binding proteins Msps and D-TACC in dividing NSCs. Furthermore, Hyx directly regulates the polo and aurA expression in vitro. Finally, overexpression of polo and aurA could significantly suppress ectopic NSC formation and NSC polarity defects caused by hyx depletion. Our data support a model in which Hyx promotes the expression of polo and aurA in NSCs and, in turn, regulates cell polarity and centrosome/microtubule assembly. This new paradigm may be relevant to future studies on Parafibromin/HRPT2-associated cancers.

This study shows that the conserved tumor suppressor Parafibromin plays an important role in Drosophila neural stem cell function, regulating the expression of the centrosomal proteins Polo and AurA, modulating centrosome and microtubule assembly, and ultimately influencing neural stem cell polarity during cell division.

A Novel Role of PP2A Methylation in the Regulation of Tight Junction Assembly and Integrity

Tight junctions (TJs) are multiprotein complexes essential for cell polarity and the barrier function of epithelia. The major signaling molecule, protein serine/threonine phosphatase 2A (PP2A), interacts with the TJ and modulates the phosphorylation state of TJ proteins. An important PP2A regulatory mechanism involves leucine carboxyl methyltransferase-1 (LCMT1)-dependent methylation and protein phosphatase methylesterase-1 (PME1)-mediated demethylation of its catalytic subunit on Leu309. Here, using MDCK cells, we show that overexpression of LCMT1, which enhances cellular PP2A methylation, inhibits TJ formation, induces TJ ruffling, and decreases TJ barrier function. Conversely, overexpression of PME1 accelerates TJ assembly and enhances TJ barrier function. PME1-dependent PP2A demethylation increases during early Ca²⁺-dependent junctional assembly. Inhibition of endogenous PME1 delays the initial Ca²⁺-mediated redistribution of TJ proteins to cell-cell contacts and affects TJ morphology and barrier function. Manipulating one-carbon metabolism modulates TJ assembly, at least in part by affecting PP2A methylation state. The integrity of PP2A methylation is critical for proper targeting of PP2A to the TJ. It is necessary for PP2A complex formation with the TJ proteins, occludin and ZO-1, and proteins of the PAR complex, Par3 and atypical protein kinase C ζ (aPKCζ), which play a key role in development of cell polarity. Expression of a methylation incompetent PP2A mutant induces defects in TJ assembly and barrier function. aPKCζ-mediated Par3 phosphorylation is also required for targeting of the PP2A ABαC holoenzyme to the TJ. Our findings provide the first evidence for a role of LCMT1, PME1 and PP2A methylation/demethylation processes in modulating TJ assembly and functional integrity. They also position PP2A at the interface of one-carbon metabolism and the regulation of key TJ and polarity proteins that become deregulated in many human diseases.

Border cell polarity and collective migration require the spliceosome component Cactin

Border cells are an in vivo model for collective cell migration. Here, we identify the gene cactin as essential for border cell cluster organization, delamination, and migration. In Cactin-depleted cells, the apical proteins aPKC and Crumbs (Crb) become abnormally concentrated, and overall cluster polarity is lost. Apically tethering excess aPKC is sufficient to cause delamination defects, and relocalizing apical aPKC partially rescues delamination. Cactin is conserved from yeast to humans and has been implicated in diverse processes. In border cells, Cactin's evolutionarily conserved spliceosome function is required. Whole transcriptome analysis revealed alterations in isoform expression in Cactin-depleted cells. Mutations in two affected genes, Sec23 and Sec24CD, which traffic Crb to the apical cell surface, partially rescue border cell cluster organization and migration. Overexpression of Rab5 or Rab11, which promote Crb and aPKC recycling, similarly rescues. Thus, a general splicing factor is specifically required for coordination of cluster polarity and migration, and migrating border cells are particularly sensitive to splicing and cell polarity disruptions.

Re-visiting the principles of apicobasal polarity in Drosophila neural stem cells

The ability of stem cells to divide asymmetrically is crucial for cell-type diversity and tissue homeostasis. Drosophila neural stem cells, also knowns as neuroblasts, utilize asymmetric cell division to self-renew and give rise to differentiated daughter cells. Drosophila neuroblasts relies on the polarized protein complexes on the apical and basal cortex to govern cell polarity and asymmetry. Here, we review recent advances in our understanding of the neuroblast polarity focusing on how actin cytoskeleton, phosphoinositide lipids and liquid-liquid phase separation regulate the asymmetric cell division of Drosophila neuroblasts.

Proteasome α6 Subunit Negatively Regulates the JAK/STAT Pathway and Blood Cell Activation in Drosophila melanogaster

JAK/STAT signaling regulates central biological functions such as development, cell differentiation and immune responses. In Drosophila, misregulated JAK/STAT signaling in blood cells (hemocytes) induces their aberrant activation. Using mass spectrometry to analyze proteins associated with a negative regulator of the JAK/STAT pathway, and by performing a genome-wide RNAi screen, we identified several components of the proteasome complex as negative regulators of JAK/STAT signaling in Drosophila. A selected proteasome component, Prosα6, was studied further. In S2 cells, Prosα6 silencing decreased the amount of the known negative regulator of the pathway, ET, leading to enhanced expression of a JAK/STAT pathway reporter gene. Silencing of Prosα6 in vivo resulted in activation of the JAK/STAT pathway, leading to the formation of lamellocytes, a specific hemocyte type indicative of hemocyte activation. This hemocyte phenotype could be partially rescued by simultaneous knockdown of either the Drosophila STAT transcription factor, or MAPKK in the JNK-pathway. Our results suggest a role for the proteasome complex components in the JAK/STAT pathway in Drosophila blood cells both in vitro and in vivo.

STRIPAK Members Orchestrate Hippo and Insulin Receptor Signaling to Promote Neural Stem Cell Reactivation

Adult stem cells reactivate from quiescence to maintain tissue homeostasis and in response to injury. How the underlying regulatory signals are integrated is largely unknown. Drosophila neural stem cells (NSCs) also leave quiescence to generate adult neurons and glia, a process that is dependent on Hippo signaling inhibition and activation of the insulin-like receptor (InR)/PI3K/Akt cascade. We performed a transcriptome analysis of individual quiescent and reactivating NSCs harvested directly from Drosophila brains and identified the conserved STRIPAK complex members mob4, cka, and PP2A (microtubule star, mts). We show that PP2A/Mts phosphatase, with its regulatory subunit Widerborst, maintains NSC quiescence, preventing premature activation of InR/PI3K/Akt signaling. Conversely, an increase in Mob4 and Cka levels promotes NSC reactivation. Mob4 and Cka are essential to recruit PP2A/Mts into a complex with Hippo kinase, resulting in Hippo pathway inhibition. We propose that Mob4/Cka/Mts functions as an intrinsic molecular switch coordinating Hippo and InR/PI3K/Akt pathways and enabling NSC reactivation.

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Transcriptional profiling of reactivating versus quiescent NSCs identifies STRIPAK members

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PP2A/Mts phosphatase inhibits Akt activation, maintaining NSC quiescence

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Mob4 and Cka target Mts to Hippo to inhibit its activity and promote NSC reactivation

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Mob4/Cka/Mts coordinate Hippo and InR/PI3K/Akt signaling in NSCs

Mutations in ANKLE2, a ZIKA Virus Target, Disrupt an Asymmetric Cell Division Pathway in Drosophila Neuroblasts to Cause Microcephaly

The apical Par complex, which contains atypical protein kinase C (aPKC), Bazooka (Par-3), and Par-6, is required for establishing polarity during asymmetric division of neuroblasts in Drosophila, and its activity depends on L(2)gl. We show that loss of Ankle2, a protein associated with microcephaly in humans and known to interact with Zika protein NS4A, reduces brain volume in flies and impacts the function of the Par complex. Reducing Ankle2 levels disrupts endoplasmic reticulum (ER) and nuclear envelope morphology, releasing the kinase Ballchen-VRK1 into the cytosol. These defects are associated with reduced phosphorylation of aPKC, disruption of Par-complex localization, and spindle alignment defects. Importantly, removal of one copy of ballchen or l(2)gl suppresses Ankle2 mutant phenotypes and restores viability and brain size. Human mutational studies implicate the above-mentioned genes in microcephaly and motor neuron disease. We suggest that NS4A, ANKLE2, VRK1, and LLGL1 define a pathway impinging on asymmetric determinants of neural stem cell division.

Protein Phosphatase 2A: More Than a Passenger in the Regulation of Epithelial Cell-Cell Junctions

Cell–cell adhesion plays a key role in the maintenance of the epithelial barrier and apicobasal cell polarity, which is crucial for homeostasis. Disruption of cell–cell adhesion is a hallmark of numerous pathological conditions, including invasive carcinomas. Adhesion between apposing cells is primarily regulated by three types of junctional structures: desmosomes, adherens junctions, and tight junctions. Cell junctional structures are highly regulated multiprotein complexes that also serve as signaling platforms to control epithelial cell function. The biogenesis, integrity, and stability of cell junctions is controlled by complex regulatory interactions with cytoskeletal and polarity proteins, as well as modulation of key component proteins by phosphorylation/dephosphorylation processes. Not surprisingly, many essential signaling molecules, including protein Ser/Thr phosphatase 2A (PP2A) are associated with intercellular junctions. Here, we examine how major PP2A enzymes regulate epithelial cell–cell junctions, either directly by associating with and dephosphorylating component proteins, or indirectly by affecting signaling pathways that control junctional integrity and cytoskeletal dynamics. PP2A deregulation has severe consequences on the stability and functionality of these structures, and disruption of cell–cell adhesion and cell polarity likely contribute to the link between PP2A dysfunction and human carcinomas.

Genome-Wide Screen for New Components of the Drosophila melanogaster Torso Receptor Tyrosine Kinase Pathway

Patterning of the Drosophila embryonic termini by the Torso (Tor) receptor pathway has long served as a valuable paradigm for understanding how receptor tyrosine kinase signaling is controlled. However, the mechanisms that underpin the control of Tor signaling remain to be fully understood. In particular, it is unclear how the Perforin-like protein Torso-like (Tsl) localizes Tor activity to the embryonic termini. To shed light on this, together with other aspects of Tor pathway function, we conducted a genome-wide screen to identify new pathway components that operate downstream of Tsl. Using a set of molecularly defined chromosomal deficiencies, we screened for suppressors of ligand-dependent Tor signaling induced by unrestricted Tsl expression. This approach yielded 59 genomic suppressor regions, 11 of which we mapped to the causative gene, and a further 29 that were mapped to <15 genes. Of the identified genes, six represent previously unknown regulators of embryonic Tor signaling. These include twins (tws), which encodes an integral subunit of the protein phosphatase 2A complex, and α-tubulin at 84B (αTub84B), a major constituent of the microtubule network, suggesting that these may play an important part in terminal patterning. Together, these data comprise a valuable resource for the discovery of new Tor pathway components. Many of these may also be required for other roles of Tor in development, such as in the larval prothoracic gland where Tor signaling controls the initiation of metamorphosis.

Vibrator and PI4KIIIα govern neuroblast polarity by anchoring non-muscle myosin II

A central feature of most stem cells is the ability to self-renew and undergo differentiation via asymmetric division. However, during asymmetric division the role of phosphatidylinositol (PI) lipids and their regulators is not well established. Here, we show that the sole type I PI transfer protein, Vibrator, controls asymmetric division of Drosophilaneural stem cells (NSCs) by physically anchoring myosin II regulatory light chain, Sqh, to the NSC cortex. Depletion of vib or disruption of its lipid binding and transfer activities disrupts NSC polarity. We propose that Vib stimulates PI4KIIIα to promote synthesis of a plasma membrane pool of phosphatidylinositol 4-phosphate [PI(4)P] that, in turn, binds and anchors myosin to the NSC cortex. Remarkably, Sqh also binds to PI(4)P in vitro and both Vib and Sqh mediate plasma membrane localization of PI(4)P in NSCs. Thus, reciprocal regulation between Myosin and PI(4)P likely governs asymmetric division of NSCs.

Stem cells are cells that can both make copies of themselves and make new cells of various types. They can either divide symmetrically to produce two identical new cells, or they can divide asymmetrically to produce two different cells. Asymmetric division happens because the two new cells contain different molecules. Stem cells drive asymmetric division by moving key molecules to one end of the cell before they divide.

Asymmetric division is key to how neural stem cells produce new brain cells. Many studies have used the developing brain of the fruit fly Drosophila melanogaster to understand this process. Errors in asymmetric division can lead to too many stem cells or not enough brain cells. This can contribute to brain tumors and other neurological disorders. Fat molecules called phosphatidylinositol lipids are some of chemicals that cause asymmetry in neural stem cells. Yet, it is not clear how these lipid molecules affect cell behavior to turn stem cells into brain cells.

The production of phosphatidylinositol lipids involves proteins called Vibrator and PI4KIIIα. Koe et al. examined the role of these two proteins in asymmetric cell division of neural stem cells in fruit flies. The results show that Vibrator activates PI4KIIIα, which leads to high levels of a phosphatidylinositol lipid called PI(4)P within the cell. These lipids act as an anchor for a group of proteins called myosin, part of the machinery that physically divides the cell. Hence, myosin and phosphatidylinositol lipids together control asymmetric division of neural stem cells.

Further experiments used mouse proteins to compensate for defects in the equivalent fly proteins. The results suggest that the same mechanisms are likely to hold true in mammalian brains, although this still needs to be proven. Nevertheless, given that human equivalents of Vibrator and PI4KIIIα are associated with neurodegenerative disorders, schizophrenia or cancers, these new findings are likely to help scientists better to understand several human diseases.

Machine Learning Analysis Identifies Drosophila Grunge/Atrophin as an Important Learning and Memory Gene Required for Memory Retention and Social Learning

High-throughput experiments are becoming increasingly common, and scientists must balance hypothesis-driven experiments with genome-wide data acquisition. We sought to predict novel genes involved in Drosophila learning and long-term memory from existing public high-throughput data. We performed an analysis using PILGRM, which analyzes public gene expression compendia using machine learning. We evaluated the top prediction alongside genes involved in learning and memory in IMP, an interface for functional relationship networks. We identified Grunge/Atrophin (Gug/Atro), a transcriptional repressor, histone deacetylase, as our top candidate. We find, through multiple, distinct assays, that Gug has an active role as a modulator of memory retention in the fly and its function is required in the adult mushroom body. Depletion of Gug specifically in neurons of the adult mushroom body, after cell division and neuronal development is complete, suggests that Gug function is important for memory retention through regulation of neuronal activity, and not by altering neurodevelopment. Our study provides a previously uncharacterized role for Gug as a possible regulator of neuronal plasticity at the interface of memory retention and memory extinction.

Regulation of protein phosphatase 2A during embryonic diapause process in the silkworm, Bombyx mori

Regulation of protein phosphorylation requires coordinated interactions between protein kinases and protein phosphatases. In the present study, we investigated regulation of protein phosphatase 2A (PP2A) during the embryonic diapause process of B. mori. An immunoblotting analysis showed that Bombyx eggs contained a catalytic C subunit, a major regulatory B subunit (B55/PR55 subunit), and a structural A subunit, with the A and B subunits undergoing differential changes between diapause and non-diapause eggs during embryonic process. In non-diapause eggs, eggs whose diapause initiation was prevented by HCl, and eggs in which diapause had been terminated by chilling of diapausing eggs at 5°C for 70days and then were transferred to 25°C, protein levels of the A and B subunits of PP2A gradually increased toward embryonic development. However, protein levels of the A and B subunits in diapause eggs remained at low levels during the first 8days after oviposition. The direct determination of PP2A enzymatic activity showed that the activity remained at low levels in diapause eggs during the first 8days after oviposition. However, in non-diapause eggs, eggs whose diapause initiation was prevented by HCl, and eggs in which diapause had been terminated by chilling, PP2A enzymatic activity sharply increased during the first several days, reached a peak during the middle embryonic development, and then greatly decreased 3 or 4days before hatching. Examination of temporal changes in mRNA expression levels of the catalytic β subunit and regulatory subunit of PP2A showed high levels in eggs whose diapause initiation was prevented by HCl compared to those in diapause eggs. These results demonstrate that the higher PP2A gene expression and PP2A A and B subunit protein levels and increased enzymatic activity are related to embryonic development of B. mori.

Drosophila melanogaster Neuroblasts: A Model for Asymmetric Stem Cell Divisions

Asymmetric cell division (ACD) is a fundamental mechanism to generate cell diversity, giving rise to daughter cells with different developmental potentials. ACD is manifested in the asymmetric segregation of proteins or mRNAs, when the two daughter cells differ in size or are endowed with different potentials to differentiate into a particular cell type (Horvitz and Herskowitz, Cell 68:237-255, 1992). Drosophila neuroblasts, the neural stem cells of the developing fly brain, are an ideal system to study ACD since this system encompasses all of these characteristics. Neuroblasts are intrinsically polarized cells, utilizing polarity cues to orient the mitotic spindle, segregate cell fate determinants asymmetrically, and regulate spindle geometry and physical asymmetry. The neuroblast system has contributed significantly to the elucidation of the basic molecular mechanisms underlying ACD. Recent findings also highlight its usefulness to study basic aspects of stem cell biology and tumor formation. In this review, we will focus on what has been learned about the basic mechanisms underlying ACD in fly neuroblasts.

Arl2- and Msps-dependent microtubule growth governs asymmetric division

Drosophila Arl2 governs neuroblast asymmetric cell division through regulation of microtubule growth and localization of Msps to centrosomes.

Asymmetric division of neural stem cells is a fundamental strategy to balance their self-renewal and differentiation. It is long thought that microtubules are not essential for cell polarity in asymmetrically dividing Drosophila melanogaster neuroblasts (NBs; neural stem cells). Here, we show that Drosophila ADP ribosylation factor like-2 (Arl2) and Msps, a known microtubule-binding protein, control cell polarity and spindle orientation of NBs. Upon arl2 RNA intereference, Arl2-GDP expression, or arl2 deletions, microtubule abnormalities and asymmetric division defects were observed. Conversely, overactivation of Arl2 leads to microtubule overgrowth and depletion of NBs. Arl2 regulates microtubule growth and asymmetric division through localizing Msps to the centrosomes in NBs. Moreover, Arl2 regulates dynein function and in turn centrosomal localization of D-TACC and Msps. Arl2 physically associates with tubulin cofactors C, D, and E. Arl2 functions together with tubulin-binding cofactor D to control microtubule growth, Msps localization, and NB self-renewal. Therefore, Arl2- and Msps-dependent microtubule growth is a new paradigm regulating asymmetric division of neural stem cells.

Protein phosphatase 2A promotes the transition to G0 during terminal differentiation in Drosophila

Protein phosphatase type 2A complex (PP2A) has been known as a tumor suppressor for over two decades, but it remains unclear exactly how it suppresses tumor growth. Here, we provide data indicating a novel role for PP2A in promoting the transition to quiescence upon terminal differentiation in vivo. Using Drosophila eyes and wings as a model, we find that compromising PP2A activity during the final cell cycle prior to a developmentally controlled cell cycle exit leads to extra cell divisions and delays entry into quiescence. By systematically testing the regulatory subunits of Drosophila PP2A, we find that the B56 family member widerborst (wdb) is required for the role of PP2A in promoting the transition to quiescence. Cells in differentiating tissues with compromised PP2A retain high Cdk2 activity when they should be quiescent, and genetic epistasis tests demonstrate that ectopic Cyclin E/Cdk2 activity is responsible for the extra cell cycles caused by PP2A inhibition. The loss of wdb/PP2A function cooperates with aberrantly high Cyclin E protein levels, allowing cells to bypass a robust G0 late in development. This provides an example of how loss of PP2A can cooperate with oncogenic mutations in cancer. We propose that the PP2A complex plays a novel role in differentiating tissues to promote developmentally controlled quiescence through the regulation of Cyclin E/Cdk2 activity.

A genetic toolkit for tagging intronic MiMIC containing genes

Previously, we described a large collection of Minos-Mediated Integration Cassettes (MiMICs) that contain two phiC31 recombinase target sites and allow the generation of a new exon that encodes a protein tag when the MiMIC is inserted in a codon intron (Nagarkar-Jaiswal et al., 2015). These modified genes permit numerous applications including assessment of protein expression pattern, identification of protein interaction partners by immunoprecipitation followed by mass spec, and reversible removal of the tagged protein in any tissue. At present, these conversions remain time and labor-intensive as they require embryos to be injected with plasmid DNA containing the exon tag. In this study, we describe a simple and reliable genetic strategy to tag genes/proteins that contain MiMIC insertions using an integrated exon encoding GFP flanked by FRT sequences. We document the efficiency and tag 60 mostly uncharacterized genes.

aPKC phosphorylates p27Xic1, providing a mechanistic link between apicobasal polarity and cell-cycle control

During the development of the nervous system, apicobasally polarized stem cells are characterized by a shorter cell cycle than nonpolar progenitors, leading to a lower differentiation potential of these cells. However, how polarization might be directly linked to the kinetics of the cell cycle is not understood. Here, we report that apicobasally polarized neuroepithelial cells in Xenopus laevis have a shorter cell cycle than nonpolar progenitors, consistent with mammalian systems. We show that the apically localized serine/threonine kinase aPKC directly phosphorylates an N-terminal site of the cell-cycle inhibitor p27Xic1 and reduces its ability to inhibit the cyclin-dependent kinase 2 (Cdk2), leading to shortening of G1 and S phases. Overexpression of activated aPKC blocks the neuronal differentiation-promoting activity of p27Xic1. These findings provide a direct mechanistic link between apicobasal polarity and the cell cycle, which may explain how proliferation is favored over differentiation in polarized neural stem cells.

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aPKC shortens G1 and S phases of cell cycle by phosphorylating p27Xic1

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Phosphorylated p27Xic1 exhibits weaker binding to and inhibition of Cdk2

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p27Xic1 promotes neuronal differentiation and elongates cell cycle via G1 phase

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Effects of p27Xic1 on neuronal differentiation are rescued by activated aPKC

Phosphotyrosyl phosphatase activator facilitates Miranda localization through dephosphorylation in dividing neuroblasts

The mechanism for the basal targeting of the Miranda (Mira) complex during the asymmetric division of Drosophila neuroblasts (NBs) is yet to be fully understood. We have identified conserved Phosphotyrosyl Phosphatase Activator (PTPA) as a novel mediator for the basal localization of the Mira complex in larval brain NBs. In ptpa NBs, Mira remains cytoplasmic during early mitosis where its basal localization is delayed until anaphase. Detailed analyses indicate that PTPA acts independently of, and prior to, aPKC activity to localize Mira. Mechanistically, our data show that the phosphorylation status of the Thr591 (T591) residue determines the subcellular localization of Mira and that PTPA facilitates the dephosphorylation of T591. Furthermore, PTPA associates with the Protein Phosphatase 4 complex to mediate Mira localization. Based on these results, a two-step process for Mira basal localization during NB division is revealed where PTPA/PP4-mediated cortical association followed by apical aPKC-mediated basal restriction.

Drosophila neuroblasts as a new model for the study of stem cell self-renewal and tumour formation

Drosophila larval brain stem cells (neuroblasts) have emerged as an important model for the study of stem cell asymmetric division and the mechanisms underlying the transformation of neural stem cells into tumour-forming cancer stem cells. Each Drosophila neuroblast divides asymmetrically to produce a larger daughter cell that retains neuroblast identity, and a smaller daughter cell that is committed to undergo differentiation. Neuroblast self-renewal and differentiation are tightly controlled by a set of intrinsic factors that regulate ACD (asymmetric cell division). Any disruption of these two processes may deleteriously affect the delicate balance between neuroblast self-renewal and progenitor cell fate specification and differentiation, causing neuroblast overgrowth and ultimately lead to tumour formation in the fly. In this review, we discuss the mechanisms underlying Drosophila neural stem cell self-renewal and differentiation. Furthermore, we highlight emerging evidence in support of the notion that defects in ACD in mammalian systems, which may play significant roles in the series of pathogenic events leading to the development of brain cancers.

The Brm-HDAC3-Erm repressor complex suppresses dedifferentiation in Drosophila type II neuroblast lineages

The control of self-renewal and differentiation of neural stem and progenitor cells is a crucial issue in stem cell and cancer biology. Drosophila type II neuroblast lineages are prone to developing impaired neuroblast homeostasis if the limited self-renewing potential of intermediate neural progenitors (INPs) is unrestrained. Here, we demonstrate that Drosophila SWI/SNF chromatin remodeling Brahma (Brm) complex functions cooperatively with another chromatin remodeling factor, Histone deacetylase 3 (HDAC3) to suppress the formation of ectopic type II neuroblasts. We show that multiple components of the Brm complex and HDAC3 physically associate with Earmuff (Erm), a type II-specific transcription factor that prevents dedifferentiation of INPs into neuroblasts. Consistently, the predicted Erm-binding motif is present in most of known binding loci of Brm. Furthermore, brm and hdac3 genetically interact with erm to prevent type II neuroblast overgrowth. Thus, the Brm-HDAC3-Erm repressor complex suppresses dedifferentiation of INPs back into type II neuroblasts.

DOI:http://dx.doi.org/10.7554/eLife.01906.001

Stem cells show great promise for repairing damaged tissue, and maybe even generating new organs, but stem cell therapies will only be successful if researchers can understand and control the behaviour of stem cells in the lab. Neural stem cells or ‘neuroblasts’ from the brains of larval fruit flies have become a popular model for studying these processes, and one type of neuroblast—known as a ‘type II’ neuroblast—is similar to mammalian neural stem cells in many ways.

When type II neuroblasts divide, they generate another neuroblast and a second cell called an intermediate neural progenitor (INP) cell. This progenitor cell then matures and undergoes a limited number of divisions to generate more INP cells and cells called ganglion mother cells. The process by which stem cells and INP cells become specific types of cells is known as differentiation. However, under certain circumstances, the INP cells can undergo the opposite process, which is called dedifferentiation, and become ‘ectopic neuroblasts’. This can give rise to tumors, so cells must employ a mechanism to prevent dedifferentiation. Researchers have known that a protein specifically expressed in INP cells called Earmuff is involved in this process, but many of the details have remained hidden.

Now, Koe et al. have discovered that a multi-protein complex containing Earmuff and a number of other proteins—Brahma and HDAC3—have important roles in preventing dedifferentiation. All three proteins are involved in different aspects of gene expression: Earmuff is a transcription factor that controls the process by which the genes in DNA are transcribed to make molecules of messenger RNA; Brahma and HDAC3 are both involved in a process called chromatin remodeling. The DNA inside cells is packaged into a compact structure known as chromatin, and chromatin remodeling involves partially unpacking this structure so that transcription factors and other proteins can have access to the DNA.

Koe et al. also showed that Earmuff, Brahma and HDAC3 combine to form a complex that prevents dedifferentiation. An immediate priority is to identify those genes whose expression is regulated by this complex in order to prevent dedifferentiation.

DOI:http://dx.doi.org/10.7554/eLife.01906.002

Clueless regulates aPKC activity and promotes self-renewal cell fate in Drosophila lgl mutant larval brains

Asymmetric cell division of Drosophila neural stem cells or neuroblasts is an important process which gives rise to two different daughter cells, one of which is the stem cell itself and the other, a committed or differentiated daughter cell. During neuroblast asymmetric division, atypical Protein Kinase C (aPKC) activity is tightly regulated; aberrant levels of activity could result in tumorigenesis in third instar larval brain. We identified clueless (clu), a genetic interactor of parkin (park), as a novel regulator of aPKC activity. It preferentially binds to the aPKC/Bazooka/Partition Defective 6 complex and stabilizes aPKC levels. In clu mutants, Miranda (Mira) and Numb are mislocalized in small percentages of dividing neuroblasts. Adult mutants are short-lived with severe locomotion defects. Clu promotes tumorigenesis caused by loss of function of lethal(2) giant larvae (lgl) in the larval brain. Removal of clu in lgl mutants rescues Mira and Numb mislocalization and restores the enlarged brain size. Western blot analyses indicate that the rescue is due to the down-regulation of aPKC levels in the lgl clu double mutant. Interestingly, the phenotype of the park mutant, which causes Parkinson's Disease-like symptoms in adult flies, is reminiscent of that of clu in neuroblast asymmetric division. Our study provides the first clue for the potential missing pathological link between temporally separated neurogenesis and neurodegeneration events; the minor defects during early neurogenesis could be a susceptible factor contributing to neurodegenerative diseases at later stages of life.

Asymmetric cell division of stem and progenitor cells during homeostasis and cancer

Stem and progenitor cells are characterized by their ability to self-renew and produce differentiated progeny. A fine balance between these processes is achieved through controlled asymmetric divisions and is necessary to generate cellular diversity during development and to maintain adult tissue homeostasis. Disruption of this balance may result in premature depletion of the stem/progenitor cell pool, or abnormal growth. In many tissues, including the brain, dysregulated asymmetric divisions are associated with cancer. Whether there is a causal relationship between asymmetric cell division defects and cancer initiation is as yet not known. Here, we review the cellular and molecular mechanisms that regulate asymmetric cell divisions in the neural lineage and discuss the potential connections between this regulatory machinery and cancer.

Initial neurogenesis in Drosophila

Early neurogenesis comprises the phase of nervous system development during which neural progenitor cells are born. In early development, the embryonic ectoderm is subdivided by a conserved signaling mechanism into two main domains, the epidermal ectoderm and the neurectoderm. Subsequently, cells of the neurectoderm are internalized and form a cell layer of proliferating neural progenitors. In vertebrates, the entire neurectoderm folds into the embryo to give rise to the neural tube. In Drosophila and many other invertebrates, a subset of neurectodermal cells, called neuroblasts (NBs), delaminates and forms the neural primordium inside the embryo where they divide in an asymmetric, stem cell-like mode. The remainder of the neurectodermal cells that stay behind at the surface loose their neurogenic potential and later give rise to the ventral part of the epidermis. The genetic and molecular analysis of the mechanisms controlling specification and proliferation of NBs in the Drosophila embryo, which played a significant part in pioneering the field of modern developmental neurobiology, represents the topic of this review.

Mechanisms of asymmetric progenitor divisions in the Drosophila central nervous system

The Drosophila central nervous system develops from polarised asymmetric divisions of precursor cells, called neuroblasts. Decades of research on neuroblasts have resulted in a substantial understanding of the factors and molecular events responsible for fate decisions of neuroblasts and their progeny. Furthermore, the cell-cycle dependent mechanisms responsible for asymmetric cortical protein localisation, resulting in the unequal partitioning between daughters, are beginning to be exposed. Disruption to the appropriate partitioning of proteins between neuroblasts and differentiation-committed daughters can lead to supernumerary neuroblast-like cells and the formation of tumours. Many of the factors responsible for regulating asymmetric division of Drosophila neuroblasts are evolutionarily conserved and, in many cases, have been shown to play a functionally conserved role in mammalian neurogenesis. Recent genome-wide studies coupled with advancements in live-imaging technologies have opened further avenues of research into neuroblast biology. We review our current understanding of the molecular mechanisms regulating neuroblast divisions, a powerful system to model mammalian neurogenesis and tumourigenesis.

Uncovering the link between malfunctions in Drosophila neuroblast asymmetric cell division and tumorigenesis

Asymmetric cell division is a developmental process utilized by several organisms. On the most basic level, an asymmetric division produces two daughter cells, each possessing a different identity or fate. Drosophila melanogaster progenitor cells, referred to as neuroblasts, undergo asymmetric division to produce a daughter neuroblast and another cell known as a ganglion mother cell (GMC). There are several features of asymmetric division in Drosophila that make it a very complex process, and these aspects will be discussed at length. The cell fate determinants that play a role in specifying daughter cell fate, as well as the mechanisms behind setting up cortical polarity within neuroblasts, have proved to be essential to ensuring that neurogenesis occurs properly. The role that mitotic spindle orientation plays in coordinating asymmetric division, as well as how cell cycle regulators influence asymmetric division machinery, will also be addressed. Most significantly, malfunctions during asymmetric cell division have shown to be causally linked with neoplastic growth and tumor formation. Therefore, it is imperative that the developmental repercussions as a result of asymmetric cell division gone awry be understood.

Apicobasal polarity and cell proliferation during development

Cell polarization and cell division are two fundamental cellular processes. The mechanisms that establish and maintain cell polarity and the mechanisms by which cells progress through the cell cycle are now fairly well understood following decades of experimental work. There is also increasing evidence that the polarization state of a cell affects its proliferative properties. The challenge now is to understand how these two phenomena are mechanistically connected. The aim of the present chapter is to provide an overview of the evidence of cross-talk between apicobasal polarity and proliferation, and the current state of knowledge of the precise mechanism by which this cross-talk is achieved.

Eya1 protein phosphatase regulates tight junction formation in lung distal epithelium

Little is known about the regulatory mechanisms underlying lung epithelial tight junction (TJ) assembly, which is inextricably linked to the preservation of epithelial polarity, and is highly coordinated by proteins that regulate epithelial cell polarity, such as aPKCζ. We recently reported that Eya1 phosphatase functions through aPKCζ-Notch1 signaling to control cell polarity in the lung epithelium. Here, we have extended these observations to TJ formation to demonstrate that Eya1 is crucial for the maintenance of TJ protein assembly in the lung epithelium, probably by controlling aPKCζ phosphorylation levels, aPKCζ-mediated TJ protein phosphorylation and Notch1-Cdc42 activity. Thus, TJs are disassembled after interfering with Eya1 function in vivo or during calcium-induced TJ assembly in vitro. These effects are reversed by reintroduction of wild-type Eya1 or partially inhibiting aPKCζ in Eya1siRNA cells. Moreover, genetic activation of Notch1 rescues Eya1(-/-) lung epithelial TJ defects. These findings uncover novel functions for the Eya1-aPKCζ-Notch1-Cdc42 pathway as a crucial regulatory mechanism of TJ assembly and polarity of the lung epithelium, providing a conceptual framework for future mechanistic and translational studies in this area.

A link between mitotic entry and membrane growth suggests a novel model for cell size control

Addition of new membrane to the cell surface by membrane trafficking is necessary for cell growth. In this paper, we report that blocking membrane traffic causes a mitotic checkpoint arrest via Wee1-dependent inhibitory phosphorylation of Cdk1. Checkpoint signals are relayed by the Rho1 GTPase, protein kinase C (Pkc1), and a specific form of protein phosphatase 2A (PP2A(Cdc55)). Signaling via this pathway is dependent on membrane traffic and appears to increase gradually during polar bud growth. We hypothesize that delivery of vesicles to the site of bud growth generates a signal that is proportional to the extent of polarized membrane growth and that the strength of the signal is read by downstream components to determine when sufficient growth has occurred for initiation of mitosis. Growth-dependent signaling could explain how membrane growth is integrated with cell cycle progression. It could also control both cell size and morphogenesis, thereby reconciling divergent models for mitotic checkpoint function.

Balancing self-renewal and differentiation by asymmetric division: insights from brain tumor suppressors in Drosophila neural stem cells

Balancing self-renewal and differentiation of stem cells is an important issue in stem cell and cancer biology. Recently, the Drosophila neuroblast (NB), neural stem cell has emerged as an excellent model for stem cell self-renewal and tumorigenesis. It is of great interest to understand how defects in the asymmetric division of neural stem cells lead to tumor formation. Here, we review recent advances in asymmetric division and the self-renewal control of Drosophila NBs. We summarize molecular mechanisms of asymmetric cell division and discuss how the defects in asymmetric division lead to tumor formation. Gain-of-function or loss-of-function of various proteins in the asymmetric machinery can drive NB overgrowth and tumor formation. These proteins control either the asymmetric protein localization or mitotic spindle orientation of NBs. We also discuss other mechanisms of brain tumor suppression that are beyond the control of asymmetric division.

Dronc caspase exerts a non-apoptotic function to restrain phospho-Numb-induced ectopic neuroblast formation in Drosophila

Drosophila neuroblasts have served as a model to understand how the balance of stem cell self-renewal versus differentiation is achieved. Drosophila Numb protein regulates this process through its preferential segregation into the differentiating daughter cell. How Numb restricts the proliferation and self-renewal potentials of the recipient cell remains enigmatic. Here, we show that phosphorylation at conserved sites regulates the tumor suppressor activity of Numb. Enforced expression of a phospho-mimetic form of Numb (Numb-TS4D) or genetic manipulation that boosts phospho-Numb levels, attenuates endogenous Numb activity and causes ectopic neuroblast formation (ENF). This effect on neuroblast homeostasis occurs only in the type II neuroblast lineage. We identify Dronc caspase as a novel binding partner of Numb, and demonstrate that overexpression of Dronc suppresses the effects of Numb-TS4D in a non-apoptotic and possibly non-catalytic manner. Reduction of Dronc activity facilitates ENF induced by phospho-Numb. Our findings uncover a molecular mechanism that regulates Numb activity and suggest a novel role for Dronc caspase in regulating neural stem cell homeostasis.

Phosphorylation and dephosphorylation events allow for rapid segregation of fate determinants during Drosophila neuroblast asymmetric divisions

Drosophila neuroblasts display remarkable asymmetry throughout mitosis. The most prominent asymmetry is the size difference between daughter cells at cytokinesis. The larger cell retains stem cell identity, i.e., remains a neuroblast while the smaller cell, called a ganglion mother cell (GMC), will generate differentiated neural and glial progeny. Preceding this size difference, several protein complexes localize to opposite sides of the neuroblast cortex (apical and basal in the embryo and, by analogy, referred to as such in larval neuroblasts although their asymmetry no longer correlates with such axis). The plane of division is coordinated with this molecular asymmetry such that apical and basal complexes are unequally partitioned between the two daughter cells: apical complexes are inherited by the self-renewing neuroblast while basal complexes are inherited by the GMC. This unequal segregation has been extensively shown to be functionally significant. Apical complexes contain factors required for cellular selfrenewal and basal complexes contain factors required for the differentiation of the GMC progeny. Curiously, however, some "basal" neuroblast proteins such as the scaffold protein Miranda (Mira) and its associated fate determinant Prospero (Pros), are initially apically localized prior to translocating to the opposite side of the cell cortex by the onset of mitosis. This is because mira mRNA is apically enriched, where it remains throughout the cell cycle, suggesting that Mira protein is translated within the apical environment.1,2 The transition from apical to basal enrichment of Mira and Pros takes place within minutes.2 Here, we summarize the known phosphorylation events and roles during neuroblast asymmetric divisions, as well as very recent work, including our own, identifying the first protein phosphatases implicated in this process. We then discuss models previously proposed, as well as a new model, for apical-to-basal transition of the Mira complex in light of our new results.

Elaborating polarity: PAR proteins and the cytoskeleton

Cell polarity is essential for cells to divide asymmetrically, form spatially restricted subcellular structures and participate in three-dimensional multicellular organization. PAR proteins are conserved polarity regulators that function by generating cortical landmarks that establish dynamic asymmetries in the distribution of effector proteins. Here, we review recent findings on the role of PAR proteins in cell polarity in C. elegans and Drosophila, and emphasize the links that exist between PAR networks and cytoskeletal proteins that both regulate PAR protein localization and act as downstream effectors to elaborate polarity within the cell.

Mitochondrial dysfunction and oxidative stress contribute to the pathogenesis of spinocerebellar ataxia type 12 (SCA12)

Spinal cerebellar ataxia type 12 (SCA12) has been attributed to the elevated expression of ppp2r2b. To better elucidate the pathomechanism of the neuronal disorder and to search for a pharmacological treatment, Drosophila models of SCA12 were generated by overexpression of a human ppp2r2b and its Drosophila homolog tws. Ectopic expression of ppp2r2b or tws caused various pathological features, including neurodegeneration, apoptosis, and shortened life span. More detailed analysis revealed that elevated ppp2r2b and tws induced fission of mitochondria accompanied by increases in cytosolic reactive oxygen species (ROS), cytochrome c, and caspase 3 activity. Transmission electron microscopy revealed that fragmented mitochondria with disrupted cristae were engulfed by autophagosomes in photoreceptor neurons of flies overexpressing tws. Additionally, transgenic flies were more susceptible to oxidative injury induced by paraquat. By contrast, ectopic Drosophila Sod2 expression and antioxidant treatment reduced ROS and caspase 3 activity and extended the life span of the SCA12 fly model. In summary, our study demonstrates that oxidative stress induced by mitochondrial dysfunction plays a causal role in SCA12, and reduction of ROS is a potential therapeutic intervention for this neuropathy.

Interplay between the transcription factor Zif and aPKC regulates neuroblast polarity and self-renewal

How a cell decides to self-renew or differentiate is a critical issue in stem cell and cancer biology. Atypical protein kinase C (aPKC) promotes self-renewal of Drosophila larval brain neural stem cells, neuroblasts. However, it is unclear how aPKC cortical polarity and protein levels are regulated. Here, we have identified a zinc-finger protein, Zif, which is required for the expression and asymmetric localization of aPKC. aPKC displays ectopic cortical localization with upregulated protein levels in dividing zif mutant neuroblasts, leading to neuroblast overproliferation. We show that Zif is a transcription factor that directly represses aPKC transcription. We further show that Zif is phosphorylated by aPKC both in vitro and in vivo. Phosphorylation of Zif by aPKC excludes it from the nucleus, leading to Zif inactivation in neuroblasts. Thus, reciprocal repression between Zif and aPKC act as a critical regulatory mechanism for establishing cell polarity and controlling neuroblast self-renewal.

Dividing cellular asymmetry: asymmetric cell division and its implications for stem cells and cancer

Cell division is commonly thought to involve the equal distribution of cellular components into the two daughter cells. During many cell divisions, however, proteins, membrane compartments, organelles, or even DNA are asymmetrically distributed between the two daughter cells. Here, we review the various types of asymmetries that have been described in yeast and in animal cells. Asymmetric segregation of protein determinants is particularly relevant for stem cell biology. We summarize the relevance of asymmetric cell divisions in various stem cell systems and discuss why defects in asymmetric cell division can lead to the formation of tumors.

Protein phosphatase 2A negatively regulates aPKC signaling by modulating phosphorylation of Par-6 in Drosophila neuroblast asymmetric divisions

Drosophila neural stem cells or neuroblasts undergo typical asymmetric cell division. An evolutionally conserved protein complex, comprising atypical protein kinase C (aPKC), Bazooka (Par-3) and Par-6, organizes cell polarity to direct these asymmetric divisions. Aurora-A (AurA) is a key molecule that links the divisions to the cell cycle. Upon its activation in metaphase, AurA phosphorylates Par-6 and activates aPKC signaling, triggering the asymmetric organization of neuroblasts. Little is known, however, about how such a positive regulatory cue is counteracted to coordinate aPKC signaling with other cellular processes. During a mutational screen using the Drosophila compound eye, we identified microtubule star (mts), which encodes a catalytic subunit of protein phosphatase 2A (PP2A), as a negative regulator for aPKC signaling. Impairment of mts function causes defects in neuroblast divisions, as observed in lethal (2) giant larvae (lgl) mutants. mts genetically interacts with par-6 and lgl in a cooperative manner in asymmetric neuroblast division. Furthermore, Mts tightly associates with Par-6 and dephosphorylates AurA-phosphorylated Par-6. Our genetic and biochemical evidence indicates that PP2A suppresses aPKC signaling by promoting Par-6 dephosphorylation in neuroblasts, which uncovers a novel balancing mechanism for aPKC signaling in the regulation of asymmetric cell division.