Cdc42 acts downstream of Bazooka to regulate neuroblast polarity through Par-6 aPKC

Cdc42 recruits Par-6-aPKC to establish cell polarity from worms to mammals. Although Cdc42 is reported to have no function in Drosophila neuroblasts, a model for cell polarity and asymmetric cell division, we show that Cdc42 colocalizes with Par-6-aPKC at the apical cortex in a Bazooka-dependent manner, and is required for Par-6-aPKC localization. Loss of Cdc42 disrupts neuroblast polarity: cdc42 mutant neuroblasts have cytoplasmic Par-6-aPKC, and this phenotype is mimicked by neuroblast-specific expression of a dominant-negative Cdc42 protein or a Par-6 protein that lacks Cdc42-binding ability. Conversely, expression of constitutively active Cdc42 leads to ectopic Par-6-aPKC localization and corresponding cell polarity defects. Bazooka remains apically enriched in cdc42 mutants. Robust Cdc42 localization requires Par-6, indicating the presence of feedback in this pathway. In addition to regulating Par-6-aPKC localization, Cdc42 increases aPKC activity by relieving Par-6 inhibition. We conclude that Cdc42 regulates aPKC localization and activity downstream of Bazooka, thereby directing neuroblast cell polarity and asymmetric cell division.

Galphai generates multiple Pins activation states to link cortical polarity and spindle orientation in Drosophila neuroblasts

Drosophila neuroblasts divide asymmetrically by aligning their mitotic spindle with cortical cell polarity to generate distinct sibling cell types. Neuroblasts asymmetrically localize Galphai, Pins, and Mud proteins; Pins/Galphai direct cortical polarity, whereas Mud is required for spindle orientation. It is currently unknown how Galphai-Pins-Mud binding is regulated to link cortical polarity with spindle orientation. Here, we show that Pins forms a "closed" state via intramolecular GoLoco-tetratricopeptide repeat (TPR) interactions, which regulate Mud binding. Biochemical, genetic, and live imaging experiments show that Galphai binds to the first of three Pins GoLoco motifs to recruit Pins to the apical cortex without "opening" Pins or recruiting Mud. However, Galphai and Mud bind cooperatively to the Pins GoLocos 2/3 and tetratricopeptide repeat domains, respectively, thereby restricting Pins-Mud interaction to the apical cortex and fixing spindle orientation. We conclude that Pins has multiple activity states that generate cortical polarity and link it with mitotic spindle orientation.

Stem Cell Self-Renewal: Centrosomes on the Move

Three recent studies show that centrosome asymmetry correlates with self-renewal of Drosophila neural and germline stem cells and that equalizing centrosomes disrupts asymmetric cell division.

Polarity and intracellular compartmentalization of Drosophila neurons

Background

Proper neuronal function depends on forming three primary subcellular compartments: axons, dendrites, and soma. Each compartment has a specialized function (the axon to send information, dendrites to receive information, and the soma is where most cellular components are produced). In mammalian neurons, each primary compartment has distinctive molecular and morphological features, as well as smaller domains, such as the axon initial segment, that have more specialized functions. How neuronal subcellular compartments are established and maintained is not well understood. Genetic studies in Drosophila have provided insight into other areas of neurobiology, but it is not known whether flies are a good system in which to study neuronal polarity as a comprehensive analysis of Drosophila neuronal subcellular organization has not been performed.

Results

Here we use new and previously characterized markers to examine Drosophila neuronal compartments. We find that: axons and dendrites can accumulate different microtubule-binding proteins; protein synthesis machinery is concentrated in the cell body; pre- and post-synaptic sites localize to distinct regions of the neuron; and specializations similar to the initial segment are present. In addition, we track EB1-GFP dynamics and determine microtubules in axons and dendrites have opposite polarity.

Conclusion

We conclude that Drosophila will be a powerful system to study the establishment and maintenance of neuronal compartments.

Microtubule-induced cortical cell polarity

Most cells are polarized. Embryonic and stem cells can use their polarity to generate cell diversity by asymmetric cell division, whereas differentiated cells use their polarity to execute specific functions. For example, fibroblasts form an actin-rich leading edge required for cell migration, neurons form distinctive axonal and dendritic compartments important for directional signaling, and epithelial cells have apical and basolateral cortical domains necessary for maintaining tissue impermeability. It is well established that actin and actin-associated proteins are essential for generating molecular and morphological cell polarity, but only recently has it become accepted that microtubules can induce and/or maintain polarity. One common feature among different cell types is that microtubules can establish the position of cortical polarity, but are not required for cortical polarity per se. In this review, we discuss how different cell types utilize microtubules and microtubule-associated signaling pathways to generate cortical cell polarity, highlight common mechanisms, and discuss open questions for directing future research.

Drosophila Aurora-A kinase inhibits neuroblast self-renewal by regulating aPKC/Numb cortical polarity and spindle orientation

Regulation of stem cell self-renewal versus differentiation is critical for embryonic development and adult tissue homeostasis. Drosophila larval neuroblasts divide asymmetrically to self-renew, and are a model system for studying stem cell self-renewal. Here we identify three mutations showing increased brain neuroblast numbers that map to the aurora-A gene, which encodes a conserved kinase implicated in human cancer. Clonal analysis and time-lapse imaging in aurora-A mutants show single neuroblasts generate multiple neuroblasts (ectopic self-renewal). This phenotype is due to two independent neuroblast defects: abnormal atypical protein kinase C (aPKC)/Numb cortical polarity and failure to align the mitotic spindle with the cortical polarity axis. numb mutant clones have ectopic neuroblasts, and Numb overexpression partially suppresses aurora-A neuroblast overgrowth (but not spindle misalignment). Conversely, mutations that disrupt spindle alignment but not cortical polarity have increased neuroblasts. We conclude that Aurora-A and Numb are novel inhibitors of neuroblast self-renewal and that spindle orientation regulates neuroblast self-renewal.

Regulation of spindle orientation and neural stem cell fate in the Drosophila optic lobe

Background

The choice of a stem cell to divide symmetrically or asymmetrically has profound consequences for development and disease. Unregulated symmetric division promotes tumor formation, whereas inappropriate asymmetric division affects organ morphogenesis. Despite its importance, little is known about how spindle positioning is regulated. In some tissues cell fate appears to dictate the type of cell division, whereas in other tissues it is thought that stochastic variation in spindle position dictates subsequent sibling cell fate.

Results

Here we investigate the relationship between neural progenitor identity and spindle positioning in the Drosophila optic lobe. We use molecular markers and live imaging to show that there are two populations of progenitors in the optic lobe: symmetrically dividing neuroepithelial cells and asymmetrically dividing neuroblasts. We use genetically marked single cell clones to show that neuroepithelial cells give rise to neuroblasts. To determine if a change in spindle orientation can trigger a neuroepithelial to neuroblast transition, we force neuroepithelial cells to divide along their apical/basal axis by misexpressing Inscuteable. We find that this does not induce neuroblasts, nor does it promote premature neuronal differentiation.

Conclusion

We show that symmetrically dividing neuroepithelial cells give rise to asymmetrically dividing neuroblasts in the optic lobe, and that regulation of spindle orientation and division symmetry is a consequence of cell type specification, rather than a mechanism for generating cell type diversity.

Chinmo and neuroblast temporal identity

Although spatial patterning during embryonic development is well characterized, a corresponding framework for temporal patterning has not been established. In this issue, Zhu et al. (2006) identify the Chinmo protein as conferring temporal identity on the neural progeny of Drosophila neuroblasts, revealing appealing parallels with spatial patterning.

Pdm and Castor specify late-born motor neuron identity in the NB7-1 lineage

Embryonic development requires generating cell types at the right place (spatial patterning) and the right time (temporal patterning). Drosophila neuroblasts undergo stem cell-like divisions to generate an ordered sequence of neuronal progeny, making them an attractive system to study temporal patterning. Embryonic neuroblasts sequentially express Hunchback, Krüppel, Pdm1/Pdm2 (Pdm), and Castor (Cas) transcription factors. Hunchback and Krüppel specify early-born temporal identity, but the role of Pdm and Cas in specifying temporal identity has never been addressed. Here we show that Pdm and Cas regulate late-born motor neuron identity within the NB7-1 lineage: Pdm specifies fourth-born U4 motor neuron identity, while Pdm/Cas together specify fifth-born U5 motor neuron identity. We conclude that Pdm and Cas specify late-born neuronal identity; that Pdm and Cas act combinatorially to specify a temporal identity distinct from either protein alone, and that Cas repression of pdm expression regulates the generation of neuronal diversity.

Regulation of neuroblast competence: multiple temporal identity factors specify distinct neuronal fates within a single early competence window

Cellular competence is an essential but poorly understood aspect of development. Is competence a general property that affects multiple signaling pathways (e.g., chromatin state), or is competence specific for each signaling pathway (e.g., availability of cofactors)? Here we find that Drosophila neuroblast 7-1 (NB7-1) has a single early window of competence to respond to four different temporal identity genes (Hunchback, Krüppel, Pdm, and Castor); that each of these factors specifies distinct motor neuron identities within this competence window but not outside it; and that progressive restriction to respond to Hunchback and Krüppel occurs within this window. Our work raises the possibility that multiple competence windows may allow the same factors to generate different cell types within the same lineage.

The NuMA-related Mud protein binds Pins and regulates spindle orientation in Drosophila neuroblasts

Asymmetric cell division generates cell diversity during development and regulates stem-cell self-renewal in Drosophila and mammals. In Drosophila, neuroblasts align their spindle with a cortical Partner of Inscuteable (Pins)-G alpha i crescent to divide asymmetrically, but the link between cortical polarity and the mitotic spindle is poorly understood. Here, we show that Pins directly binds, and coimmunoprecipitates with, the NuMA-related Mushroom body defect (Mud) protein. Pins recruits Mud to the neuroblast apical cortex, and Mud is also strongly localized to centrosome/spindle poles, in a similar way to NuMA. In mud mutants, cortical polarity is normal, but the metaphase spindle frequently fails to align with the cortical polarity axis. When spindle orientation is orthogonal to cell polarity, symmetric division occurs. We propose that Mud is a functional orthologue of mammalian NuMA and Caenorhabditis elegans Lin-5, and that Mud coordinates spindle orientation with cortical polarity to promote asymmetric cell division.

Brat is a Miranda cargo protein that promotes neuronal differentiation and inhibits neuroblast self-renewal

An important question in stem cell biology is how a cell decides to self-renew or differentiate. Drosophila neuroblasts divide asymmetrically to self-renew and generate differentiating progeny called GMCs. Here, we report that the Brain tumor (Brat) translation repressor is partitioned into GMCs via direct interaction with the Miranda scaffolding protein. In brat mutants, another Miranda cargo protein (Prospero) is not partitioned into GMCs, GMCs fail to downregulate neuroblast gene expression, and there is a massive increase in neuroblast numbers. Single neuroblast clones lacking Prospero have a similar phenotype. We conclude that Brat suppresses neuroblast stem cell self-renewal and promotes neuronal differentiation.

Microtubule-induced Pins/Galphai cortical polarity in Drosophila neuroblasts

Cortical polarity regulates cell division, migration, and differentiation. Microtubules induce cortical polarity in yeast, but few examples are known in metazoans. We show that astral microtubules, kinesin Khc-73, and Discs large (Dlg) induce cortical polarization of Pins/Galphai in Drosophila neuroblasts; this cortical domain is functional for generating spindle asymmetry, daughter-cell-size asymmetry, and distinct sibling fates. Khc-73 localizes to astral microtubule plus ends, and Dlg/Khc-73 and Dlg/Pins coimmunoprecipitate, suggesting that microtubules induce Pins/Galphai cortical polarity through Dlg/Khc-73 interactions. The microtubule/Khc-73/Dlg pathway acts in parallel to the well-characterized Inscuteable/Par pathway, but each provides unique spatial and temporal information: The Inscuteable/Par pathway initiates at prophase to coordinate neuroblast cortical polarity with CNS tissue polarity, whereas the microtubule/Khc-73/Dlg pathway functions at metaphase to coordinate neuroblast cortical polarity with the mitotic spindle axis. These results identify a role for microtubules in polarizing the neuroblast cortex, a fundamental step for generating cell diversity through asymmetric cell division.

Zfh1, a somatic motor neuron transcription factor, regulates axon exit from the CNS

Motor neurons are defined by their axon projections, which exit the CNS to innervate somatic or visceral musculature, yet remarkably little is known about how motor axons are programmed to exit the CNS. Here, we describe the role of the Drosophila Zfh1 transcription factor in promoting axon exit from the CNS. Zfh1 is detected in all embryonic somatic motor neurons, glia associated with the CNS surface and motor axons, and one identified interneuron. In zfh1 mutants, ventral projecting motor axons often stall at the edge of the CNS, failing to enter the muscle field, despite having normal motor neuron identity. Conversely, ectopic Zfh1 induces a subset of interneurons--all normally expressing two or more "ventral motor neuron transcription factors" (e.g. Islet, Hb9, Nkx6, Lim3)--to project laterally and exit the CNS. We conclude that Zfh1 is required for ventral motor axon exit from the CNS.

Extrinsic cues orient the cell division axis in Drosophila embryonic neuroblasts

Cell polarity must be integrated with tissue polarity for proper development. The Drosophila embryonic central nervous system (CNS) is a highly polarized tissue; neuroblasts occupy the most apical layer of cells within the CNS, and lie just basal to the neural epithelium. Neuroblasts are the CNS progenitor cells and undergo multiple rounds of asymmetric cell division, ;budding off' smaller daughter cells (GMCs) from the side opposite the epithelium, thereby positioning neuronal/glial progeny towards the embryo interior. It is unknown whether this highly stereotypical orientation of neuroblast divisions is controlled by an intrinsic cue (e.g. cortical mark) or an extrinsic cue (e.g. cell-cell signal). Using live imaging and in vitro culture, we find that neuroblasts in contact with epithelial cells always ;bud off' GMCs in the same direction, opposite from the epithelia-neuroblast contact site, identical to what is observed in vivo. By contrast, isolated neuroblasts 'bud off' GMCs at random positions. Imaging of centrosome/spindle dynamics and cortical polarity shows that in neuroblasts contacting epithelial cells, centrosomes remained anchored and cortical polarity proteins localize at the same epithelia-neuroblast contact site over subsequent cell cycles. In isolated neuroblasts, centrosomes drifted between cell cycles and cortical polarity proteins showed a delay in polarization and random positioning. We conclude that embryonic neuroblasts require an extrinsic signal from the overlying epithelium to anchor the centrosome/centrosome pair at the site of epithelial-neuroblast contact and for proper temporal and spatial localization of cortical Par proteins. This ensures the proper coordination between neuroblast cell polarity and CNS tissue polarity.

Lgl, Pins and aPKC regulate neuroblast self-renewal versus differentiation

How a cell chooses to proliferate or to differentiate is an important issue in stem cell and cancer biology. Drosophila neuroblasts undergo self-renewal with every cell division, producing another neuroblast and a differentiating daughter cell, but the mechanisms controlling the self-renewal/differentiation decision are poorly understood. Here we tested whether cell polarity genes, known to regulate embryonic neuroblast asymmetric cell division, also regulate neuroblast self-renewal. Clonal analysis in larval brains showed that pins mutant neuroblasts rapidly fail to self-renew, whereas lethal giant larvae (lgl) mutant neuroblasts generate multiple neuroblasts. Notably, lgl pins double mutant neuroblasts all divide symmetrically to self-renew, filling the brain with neuroblasts at the expense of neurons. The lgl pins neuroblasts show ectopic cortical localization of atypical protein kinase C (aPKC), and a decrease in aPKC expression reduces neuroblast numbers, suggesting that aPKC promotes neuroblast self-renewal. In support of this hypothesis, neuroblast-specific overexpression of membrane-targeted aPKC, but not a kinase-dead version, induces ectopic neuroblast self-renewal. We conclude that cortical aPKC kinase activity is a potent inducer of neuroblast self-renewal.

Live imaging of Drosophila brain neuroblasts reveals a role for Lis1/dynactin in spindle assembly and mitotic checkpoint control

Lis1 is required for nuclear migration in fungi, cell cycle progression in mammals, and the formation of a folded cerebral cortex in humans. Lis1 binds dynactin and the dynein motor complex, but the role of Lis1 in many dynein/dynactin-dependent processes is not clearly understood. Here we generate and/or characterize mutants for Drosophila Lis1 and a dynactin subunit, Glued, to investigate the role of Lis1/dynactin in mitotic checkpoint function. In addition, we develop an improved time-lapse video microscopy technique that allows live imaging of GFP-Lis1, GFP-Rod checkpoint protein, green fluorescent protein (GFP)-labeled chromosomes, or GFP-labeled mitotic spindle dynamics in neuroblasts within whole larval brain explants. Our mutant analyses show that Lis1/dynactin have at least two independent functions during mitosis: first promoting centrosome separation and bipolar spindle assembly during prophase/prometaphase, and subsequently generating interkinetochore tension and transporting checkpoint proteins off kinetochores during metaphase, thus promoting timely anaphase onset. Furthermore, we show that Lis1/dynactin/dynein physically associate and colocalize on centrosomes, spindle MTs, and kinetochores, and that regulation of Lis1/dynactin kinetochore localization in Drosophila differs from both Caenorhabditis elegans and mammals. We conclude that Lis1/dynactin act together to regulate multiple, independent functions in mitotic cells, including spindle formation and cell cycle checkpoint release.

Genetic screens for enhancers of brahma reveal functional interactions between the BRM chromatin-remodeling complex and the delta-notch signal transduction pathway in Drosophila

The Drosophila trithorax group gene brahma (brm) encodes the ATPase subunit of a 2-MDa chromatin-remodeling complex. brm was identified in a screen for transcriptional activators of homeotic genes and subsequently shown to play a global role in transcription by RNA polymerase II. To gain insight into the targeting, function, and regulation of the BRM complex, we screened for mutations that genetically interact with a dominant-negative allele of brm (brm(K804R)). We first screened for dominant mutations that are lethal in combination with a brm(K804R) transgene under control of the brm promoter. In a distinct but related screen, we identified dominant mutations that modify eye defects resulting from expression of brm(K804R) in the eye-antennal imaginal disc. Mutations in three classes of genes were identified in our screens: genes encoding subunits of the BRM complex (brm, moira, and osa), other proteins directly involved in transcription (zerknullt and RpII140), and signaling molecules (Delta and vein). Expression of brm(K804R) in the adult sense organ precursor lineage causes phenotypes similar to those resulting from impaired Delta-Notch signaling. Our results suggest that signaling pathways may regulate the transcription of target genes by regulating the activity of the BRM complex.

Drosophila neuroblast 7-3 cell lineage: a model system for studying programmed cell death, Notch/Numb signaling, and sequential specification of ganglion mother cell identity

Cell lineage studies provide an important foundation for experimental analysis in many systems. Drosophila neural precursors (neuroblasts) sequentially generate ganglion mother cells (GMCs), which generate neurons and/or glia, but the birth order, or cell lineage, of each neuroblast is poorly understood. The best-characterized neuroblast is NB7-3, in which GMC-1 makes the EW1 serotonergic interneuron and GW motoneuron; GMC-2 makes the EW2 serotonergic interneuron and a programmed cell death; and GMC-3 gives rise to the EW3 interneuron. However, the end of this lineage has not been determined. Here, we use positively marked genetic clones, bromodeoxyuridine (BrdU) labeling, mutations that affect Notch signaling, and antibody markers to further define the end of the cell lineage of NB7-3. We provide evidence that GMC-3 directly differentiates into EW3 and that the sibling neuroblast undergoes programmed cell death. Our results confirm and extend previous work on the early portion of the NB7-3 lineage (Novotny et al. [2002] Development 129:1027-1036; Lundell et al. [ 2003] Development 130:4109-4121).

Regulation of Temporal Identity Transitions in Drosophila Neuroblasts

Temporal patterning is an important aspect of embryonic development, but the underlying molecular mechanisms are not well understood. Drosophila neuroblasts are an excellent model for studying temporal identity: they sequentially express four genes (hunchback --> Kruppel --> pdm1 --> castor) whose temporal regulation is essential for generating neuronal diversity. Here we show that hunchback --> Kruppel timing is regulated transcriptionally and requires neuroblast cytokinesis, consistent with asymmetric partitioning of transcriptional regulators during neuroblast division or feedback signaling from the neuroblast progeny. Surprisingly, Kruppel --> pdm1 --> castor timing occurs normally in isolated or G(2)-arrested neuroblasts, and thus involves a neuroblast-intrinsic timer. Finally, we find that Hunchback potently regulates the neuroblast temporal identity timer: prolonged Hunchback expression keeps the neuroblast "young" for multiple divisions, and subsequent downregulation allows resumption of Kruppel --> pdm1 --> castor expression and the normal neuroblast lineage. We conclude that two distinct "timers" regulate neuroblast gene expression: a hunchback --> Kruppel timer requiring cytokinesis, and a Kruppel --> pdm1 --> castor timer which is cell cycle independent.

Scribble protein domain mapping reveals a multistep localization mechanism and domains necessary for establishing cortical polarity

The Drosophila tumor suppressor protein Scribble is required for epithelial polarity, neuroblast polarity, neuroblast spindle asymmetry and limiting cell proliferation. It is a member of the newly described LAP protein family, containing 16 leucine rich repeats (LRRs), four PDZ domains and an extensive carboxyl-terminal (CT) domain. LRR and PDZ domains mediate protein-protein interactions, but little is know about their function within LAP family proteins. We have determined the role of the LRR, PDZ and CT domains for Scribble localization in neuroblasts and epithelia, and for Scribble function in neuroblasts. We found that the LRR and PDZ domains are both required for proper targeting of Scribble to septate junctions in epithelia; that the LRR domain is necessary and sufficient for cortical localization in mitotic neuroblasts, and that the PDZ2 domain is required for efficient cortical and apical localization of Scribble in neuroblasts. In addition, we show that the LRR domain is sufficient to target Miranda protein to the neuroblast cortex, but that LRR+PDZ will exclude Miranda from the cortex. Our results highlight the importance of both LRR and PDZ domains for the proper localization and function of Scribble in neuroblasts.

Baz, Par-6 and aPKC are not required for axon or dendrite specification in Drosophila

Par-3/Baz, Par-6, and aPKC are evolutionarily conserved regulators of cell polarity, and overexpression experiments implicate them as axon determinants in vertebrate hippocampal neurons. Here we examined their mutant and overexpression phenotypes in Drosophila melanogaster. We found that mutants neurons had normal axon and dendrite morphology and remodeled axons correctly in metamorphosis, and that overexpression did not affect axon or dendrite specification. Baz/Par-6/aPKC are therefore not essential for axon specification in Drosophila.

SPECIFICATION OF TEMPORAL IDENTITY IN THE DEVELOPING NERVOUS SYSTEM

The nervous system of higher organisms exhibits extraordinary cellular diversity owing to complex spatial and temporal patterning mechanisms. The role of spatial patterning in generating neuronal diversity is well known; here we discuss how neural progenitors change over time to contribute to cell diversity within the central nervous system (CNS). We focus on five model systems: the vertebrate retina, cortex, hindbrain, spinal cord, and Drosophila neuroblasts. For each, we address the following questions: Do multipotent progenitors generate different neuronal cell types in an invariant order? Do changes in progenitor-intrinsic factors or progenitor-extrinsic signals regulate temporal identity (i.e., the sequence of neuronal cell types produced)? What is the mechanism that regulates temporal identity transitions; i.e., what triggers the switch from one temporal identity to the next? By applying the same criteria to analyze each model system, we try to highlight common themes, point out unique attributes of each system, and identify directions for future research.

Zebrafish and fly Nkx6 proteins have similar CNS expression patterns and regulate motoneuron formation

Genes belonging to the Nkx, Gsh and Msx families are expressed in similar dorsovental spatial domains of the insect and vertebrate central nervous system (CNS), suggesting the bilaterian ancestor used this genetic program during CNS development. We have investigated the significance of these similar expression patterns by testing whether Nkx6 proteins expressed in ventral CNS of zebrafish and flies have similar functions. In zebrafish, Nkx6.1 is expressed in early-born primary and later-born secondary motoneurons. In the absence of Nkx6.1, there are fewer secondary motoneurons and supernumerary ventral interneurons, suggesting Nkx6.1 promotes motoneuron and suppresses interneuron formation. Overexpression of fish or fly Nkx6 is sufficient to generate supernumerary motoneurons in both zebrafish and flies. These results suggest that one ancestral function of Nkx6 proteins was to promote motoneuron development.

Abstrakt, a DEAD box protein, regulates Insc levels and asymmetric division of neural and mesodermal progenitors

Asymmetric cell division generates cell diversity in bacteria, yeast, and higher eukaryotes. In Drosophila, both neural and muscle progenitors divide asymmetrically. In these cells the Inscuteable (Insc) protein complex coordinates cell polarity and spindle orientation. Abstrakt (Abs) is a DEAD-box protein that regulates aspects of cell polarity in oocytes and embryos. We use a conditional allele of abs to investigate its role in neural and muscle progenitor cell polarity. In neuroblasts we observe loss of apical Insc crescents, failure in basal protein targeting, and defects in spindle orientation. In the GMC4-2a cell we observe loss of apical Insc crescents, defects in basal protein targeting, and equalization of sibling neuron fates; muscle precursors show a similar equalization of sibling cell fates. These phenotypes resemble those of insc mutants; indeed, abs mutants show a striking loss of Insc protein levels but no change of insc RNA levels. Furthermore, we find that the Abs protein physically interacts with insc RNA. Our results demonstrate a novel role for Abs in the posttranscriptional regulation of insc expression, which is essential for proper cell polarity, spindle orientation, and the establishment of distinct sibling cell fates within embryonic neural and muscle progenitors.