A mouse homologue of Drosophila pins can asymmetrically localize and substitute for pins function in Drosophila neuroblasts

The evolutionary modifications of a GoLoco motif in the AGS protein facilitate micromere formation in the sea urchin embryo

The evolutionary introduction of asymmetric cell division (ACD) into the developmental program facilitates the formation of a new cell type, contributing to developmental diversity and, eventually, species diversification. The micromere of the sea urchin embryo may serve as one of those examples: an ACD at the 16-cell stage forms micromeres unique to echinoids among echinoderms. We previously reported that a polarity factor, activator of G-protein signaling (AGS), plays a crucial role in micromere formation. However, AGS and its associated ACD factors are present in all echinoderms and across most metazoans. This raises the question of what evolutionary modifications of AGS protein or its surrounding molecular environment contributed to the evolutionary acquisition of micromeres only in echinoids. In this study, we learned that the GoLoco motifs at the AGS C-terminus play critical roles in regulating micromere formation in sea urchin embryos. Further, other echinoderms' AGS or chimeric AGS that contain the C-terminus of AGS orthologs from various organisms showed varied localization and function in micromere formation. In contrast, the sea star or the pencil urchin orthologs of other ACD factors were consistently localized at the vegetal cortex in the sea urchin embryo, suggesting that AGS may be a unique variable factor that facilitates ACD diversity among echinoderms. Consistently, sea urchin AGS appears to facilitate micromere-like cell formation and accelerate the enrichment timing of the germline factor Vasa during early embryogenesis of the pencil urchin, an ancestral type of sea urchin. Based on these observations, we propose that the molecular evolution of a single polarity factor facilitates ACD diversity while preserving the core ACD machinery among echinoderms and beyond during evolution.

Diversity and structural-functional insights of alpha-solenoid proteins

Alpha-solenoids are a significant and diverse subset of structured tandem repeat proteins (STRPs) that are important in various domains of life. This review examines their structural and functional diversity and highlights their role in critical cellular processes such as signaling, apoptosis, and transcriptional regulation. Alpha-solenoids can be classified into three geometric folds: low curvature, high curvature, and corkscrew, as well as eight subfolds: ankyrin repeats; Huntingtin, elongation factor 3, protein phosphatase 2A, and target of rapamycin; armadillo repeats; tetratricopeptide repeats; pentatricopeptide repeats; Pumilio repeats; transcription activator-like; and Sel-1 and Sel-1-like repeats. These subfolds represent distinct protein families with unique structural properties and functions, highlighting the versatility of alpha-solenoids. The review also discusses their association with disease, highlighting their potential as therapeutic targets and their role in protein design. Advances in state-of-the-art structure prediction methods provide new opportunities and challenges in the functional characterization and classification of this kind of fold, emphasizing the need for continued development of methods for their identification and proper data curation and deposition in the main databases.

The evolutionary modifications of a GoLoco motif in the AGS protein facilitate micromere formation in the sea urchin embryo

The evolutionary introduction of asymmetric cell division (ACD) into the developmental program facilitates the formation of a new cell type, contributing to developmental diversity and, eventually, to species diversification. The micromere of the sea urchin embryo may serve as one of those examples: An ACD at the 16-cell stage forms micromeres unique to echinoids among echinoderms. We previously reported that a polarity factor, Activator of G-protein Signaling (AGS), plays a crucial role in micromere formation. However, AGS and its associated ACD factors are present in all echinoderms and across most metazoans. This raises the question of what evolutionary modifications of AGS protein or its surrounding molecular environment contributed to the evolutionary acquisition of micromeres only in echinoids. In this study, we learned that the GoLoco motifs at the AGS C-terminus play critical roles in regulating micromere formation in sea urchin embryos. Further, other echinoderms' AGS or chimeric AGS that contain the C-terminus of AGS orthologs from various organisms showed varied localization and function in micromere formation. In contrast, the sea star or the pencil urchin orthologs of other ACD factors were consistently localized at the vegetal cortex in the sea urchin embryo, suggesting that AGS may be a unique variable factor that facilitates ACD diversity among echinoderms. Consistently, sea urchin AGS appears to facilitate micromere-like cell formation and accelerate the enrichment timing of the germline factor Vasa during early embryogenesis of the pencil urchin, an ancestral type of sea urchin. Based on these observations, we propose that the molecular evolution of a single polarity factor facilitates ACD diversity while preserving the core ACD machinery among echinoderms and beyond during evolution.

A missense mutation in human INSC causes peripheral neuropathy

PAR3/INSC/LGN form an evolutionarily conserved complex required for asymmetric cell division in the developing brain, but its post-developmental function and disease relevance in the peripheral nervous system (PNS) remains unknown. We mapped a new locus for axonal Charcot-Marie-Tooth disease (CMT2) and identified a missense mutation c.209 T > G (p.Met70Arg) in the INSC gene. Modeling the INSCM70R variant in Drosophila, we showed that it caused proprioceptive defects in adult flies, leading to gait defects resembling those in CMT2 patients. Cellularly, PAR3/INSC/LGN dysfunction caused tubulin aggregation and necrotic neurodegeneration, with microtubule-stabilizing agents rescuing both morphological and functional defects of the INSCM70R mutation in the PNS. Our findings underscore the critical role of the PAR3/INSC/LGN machinery in the adult PNS and highlight a potential therapeutic target for INSC-associated CMT2.

RGS12 polarizes the GPSM2-GNAI complex to organize and elongate stereocilia in sensory hair cells

Inhibitory G proteins (GNAI/Gαi) bind to the scaffold G protein signaling modulator 2 (GPSM2) to form a conserved polarity complex that regulates cytoskeleton organization. GPSM2 keeps GNAI in a guanosine diphosphate (GDP)-bound state, but how GPSM2-GNAI is generated or relates to heterotrimeric G protein signaling remains unclear. We find that RGS12, a GTPase-activating protein (GAP), is required to polarize GPSM2-GNAI at the hair cell apical membrane and to organize mechanosensory stereocilia in rows of graded heights. Accordingly, RGS12 and the guanine nucleotide exchange factor (GEF) DAPLE are asymmetrically co-enriched at the hair cell apical junction, and Rgs12 mouse mutants are deaf. GPSM2 and RGS12 share GoLoco motifs that stabilize GNAI(GDP), and GPSM2 outcompetes RGS12 to bind GNAI. Our results suggest that polarized GEF/GAP junctional activity might dissociate heterotrimeric G proteins, generating free GNAI(GDP) for GPSM2 at the adjacent apical membrane. GPSM2-GNAI(GDP), in turn, imparts asymmetry to the forming stereocilia to enable sensory function in hair cells.

Diversity of activator of G-protein signaling (AGS)-family proteins and their impact on asymmetric cell division across taxa

Asymmetric cell division (ACD) is a cellular process that forms two different cell types through a cell division and is thus critical for the development of all multicellular organisms. Not all but many of the ACD processes are mediated by proper orientation of the mitotic spindle, which segregates the fate determinants asymmetrically into daughter cells. In many cell types, the evolutionarily conserved protein complex of Gαi/AGS-family protein/NuMA-like protein appears to play critical roles in orienting the spindle and/or generating the polarized cortical forces to regulate ACD. Studies in various organisms reveal that this conserved protein complex is slightly modified in each phylum or even within species. In particular, AGS-family proteins appear to be modified with a variable number of motifs in their functional domains across taxa. This apparently creates different molecular interactions and mechanisms of ACD in each developmental program, ultimately contributing to developmental diversity across species. In this review, we discuss how a conserved ACD machinery has been modified in each phylum over the course of evolution with a major focus on the molecular evolution of AGS-family proteins and its impact on ACD regulation.

Insc:LGN tetramers promote asymmetric divisions of mammary stem cells

Asymmetric cell divisions balance stem cell proliferation and differentiation to sustain tissue morphogenesis and homeostasis. During asymmetric divisions, fate determinants and niche contacts segregate unequally between daughters, but little is known on how this is achieved mechanistically. In Drosophila neuroblasts and murine mammary stem cells, the association of the spindle orientation protein LGN with the stem cell adaptor Inscuteable has been connected to asymmetry. Here we report the crystal structure of Drosophila LGN in complex with the asymmetric domain of Inscuteable, which reveals a tetrameric arrangement of intertwined molecules. We show that Insc:LGN tetramers constitute stable cores of Par3–Insc-LGN-Gαi^GDP complexes, which cannot be dissociated by NuMA. In mammary stem cells, the asymmetric domain of Insc bound to LGN:Gαi^GDP suffices to drive asymmetric fate, and reverts aberrant symmetric divisions induced by p53 loss. We suggest a novel role for the Insc-bound pool of LGN acting independently of microtubule motors to promote asymmetric fate specification.

During asymmetric divisions fate determinants and niche contacts segregate unequally between daughter cells, but the mechanism is unclear. Here the authors show that Insc:LGN tetramers promote assembly of Par3-Insc-LGN-GαiGDP complexes and asymmetric fate specification independently of microtubule motors.

Loss of the canonical spindle orientation function in the Pins/LGN homolog AGS3

In many cell types, mitotic spindle orientation relies on the canonical "LGN complex" composed of Pins/LGN, Mud/NuMA, and Gα_i subunits. Membrane localization of this complex recruits motor force generators that pull on astral microtubules to orient the spindle. Drosophila Pins shares highly conserved functional domains with its two vertebrate homologs LGN and AGS3. Whereas the role of Pins and LGN in oriented divisions is extensively documented, involvement of AGS3 remains controversial. Here, we show that AGS3 is not required for planar divisions of neural progenitors in the mouse neocortex. AGS3 is not recruited to the cell cortex and does not rescue LGN loss of function. Despite conserved interactions with NuMA and Gα_iin vitro, comparison of LGN and AGS3 functional domains in vivo reveals unexpected differences in the ability of these interactions to mediate spindle orientation functions. Finally, we find that Drosophila Pins is unable to substitute for LGN loss of function in vertebrates, highlighting that species-specific modulations of the interactions between components of the Pins/LGN complex are crucial in vivo for spindle orientation.

Molecular mechanisms of asymmetric divisions in mammary stem cells

Stem cells have the remarkable ability to undergo proliferative symmetric divisions and self-renewing asymmetric divisions. Balancing of the two modes of division sustains tissue morphogenesis and homeostasis. Asymmetric divisions of Drosophila neuroblasts (NBs) and sensory organ precursor (SOP) cells served as prototypes to learn what we consider now principles of asymmetric mitoses. They also provide initial evidence supporting the notion that aberrant symmetric divisions of stem cells could correlate with malignancy. However, transferring the molecular knowledge of circuits underlying asymmetry from flies to mammals has proven more challenging than expected. Several experimental approaches have been used to define asymmetry in mammalian systems, based on daughter cell fate, unequal partitioning of determinants and niche contacts, or proliferative potential. In this review, we aim to provide a critical evaluation of the assays used to establish the stem cell mode of division, with a particular focus on the mammary gland system. In this context, we will discuss the genetic alterations that impinge on the modality of stem cell division and their role in breast cancer development.

Proliferation control in neural stem and progenitor cells

Neural circuit function can be drastically affected by variations in the number of cells that are produced during development or by a reduction in adult cell number owing to disease. For this reason, unique cell cycle and cell growth control mechanisms operate in the developing and adult brain. In Drosophila melanogaster and in mammalian neural stem and progenitor cells, these mechanisms are intricately coordinated with the developmental age and the nutritional, metabolic and hormonal state of the animal. Defects in neural stem cell proliferation that result in the generation of incorrect cell numbers or defects in neural stem cell differentiation can cause microcephaly or megalencephaly.

Cell Division Modes and Cleavage Planes of Neural Progenitors during Mammalian Cortical Development

During mammalian brain development, neural progenitor cells undergo symmetric proliferative divisions followed by asymmetric neurogenic divisions. The division mode of these self-renewing progenitors, together with the cell fate of their progeny, plays critical roles in determining the number of neurons and, ultimately, the size of the adult brain. In the past decade, remarkable progress has been made toward identifying various types of neuronal progenitors. Recent technological advances in live imaging and genetic manipulation have enabled us to link dynamic cell biological events to the molecular mechanisms that control the asymmetric divisions of self-renewing progenitors and have provided a fresh perspective on the modes of division of these progenitors. In addition, comparison of progenitor repertoires between species has provided insight into the expansion and the development of the complexity of the brain during mammalian evolution.

Dlg1 controls planar spindle orientation in the neuroepithelium through direct interaction with LGN

Dlg1 recruits LGN to the cortex of cells in the chick neuroepithelium and may provide instructive cues that drive planar spindle orientation.

Oriented cell divisions are necessary for the development of epithelial structures. Mitotic spindle orientation requires the precise localization of force generators at the cell cortex via the evolutionarily conserved LGN complex. However, polarity cues acting upstream of this complex in vivo in the vertebrate epithelia remain unknown. In this paper, we show that Dlg1 is localized at the basolateral cell cortex during mitosis and is necessary for planar spindle orientation in the chick neuroepithelium. Live imaging revealed that Dlg1 is required for directed spindle movements during metaphase. Mechanistically, we show that direct interaction between Dlg1 and LGN promotes cortical localization of the LGN complex. Furthermore, in human cells dividing on adhesive micropatterns, homogenously localized Dlg1 recruited LGN to the mitotic cortex and was also necessary for proper spindle orientation. We propose that Dlg1 acts primarily to recruit LGN to the cortex and that Dlg1 localization may additionally provide instructive cues for spindle orientation.

Going vertical: functional role and working principles of the protein Inscuteable in asymmetric cell divisions

Coordinating mitotic spindle dynamics with cortical polarity is essential for stem cell asymmetric divisions. Over the years, the protein Inscuteable (Insc) has emerged as a key element determining the spindle orientation in asymmetric mitoses. Its overexpression increases differentiative divisions in systems as diverse as mouse keratinocytes and radial glial cells. To date, the molecular explanation to account for this phenotype envisioned Insc as an adaptor molecule bridging between the polarity proteins Par3:Par6:aPKC and the spindle pulling machines assembled on NuMA:LGN:Gαi. However, recent biochemical and structural data revealed that Insc and NuMA are competitive interactors of LGN, challenging the simplistic idea of a single apical macromolecular complex, and demanding a revision of the actual working principles of Insc.

Drosophila neuroblasts: a model for stem cell biology

Drosophila neuroblasts, the stem cells of the developing fly brain, have emerged as a key model system for neural stem cell biology and have provided key insights into the mechanisms underlying asymmetric cell division and tumor formation. More recently, they have also been used to understand how neural progenitors can generate different neuronal subtypes over time, how their cell cycle entry and exit are coordinated with development, and how proliferation in the brain is spared from the growth restrictions that occur in other organs upon starvation. In this Primer, we describe the biology of Drosophila neuroblasts and highlight the most recent advances made using neuroblasts as a model system.

Structural and biochemical characterization of the interaction between LGN and Frmpd1

The tetratricopeptide repeat (TPR) motif-containing protein LGN binds multiple targets and regulates their subcellular localizations and functions during both asymmetric and symmetric cell divisions. Here, we characterized the interaction between LGN-TPR motifs and FERM and PDZ domain containing 1 (Frmpd1) and reported the crystal structure of the complex at 2.4Å resolution. A highly conserved fragment at the center of Frmpd1 of ~20 residues was found to be necessary and sufficient to bind to LGN-TPR. This Frmpd1 fragment forms an extended structure and runs along the concave channel of the TPR superhelix in an antiparallel manner in the complex. Structural comparisons and biochemical studies of LGN/Frmpd1 and other known LGN/target interactions demonstrate that the LGN-TPR motifs are versatile and capable of recognizing multiple targets via diverse binding modes. Nevertheless, a conserved "E/QxEx4-5E/D/Qx1-2K/R" motif in LGN/Pins (partner of inscuteable) TPR binding proteins has been identified.

Mouse inscuteable induces apical-basal spindle orientation to facilitate intermediate progenitor generation in the developing neocortex

Neurons in the mammalian neocortex arise from asymmetric divisions of progenitors residing in the ventricular zone. While in most progenitor divisions, the mitotic spindle is parallel to the ventricular surface, some progenitors reorient the spindle and divide in oblique orientations. Here, we use conditional deletion and overexpression of mouse Inscuteable (mInsc) to analyze the relevance of spindle reorientation in cortical progenitors. Mutating mInsc almost abolishes oblique and vertical mitotic spindles, while mInsc overexpression has the opposite effect. Our data suggest that oblique divisions are essential for generating the correct numbers of neurons in all cortical layers. Using clonal analysis, we demonstrate that spindle orientation affects the rate of indirect neurogenesis, a process where progenitors give rise to basal progenitors, which in turn divide symmetrically into two differentiating neurons. Our results indicate that the orientation of progenitor cell divisions is important for correct lineage specification in the developing mammalian brain.

Structural basis for interaction between the conserved cell polarity proteins Inscuteable and Leu-Gly-Asn repeat-enriched protein (LGN)

Interaction between the mammalian cell polarity proteins mInsc (mammalian homologue of Inscuteable) and Leu-Gly-Asn repeat-enriched protein (LGN), as well as that between their respective Drosophila homologues Inscuteable and Partner of Inscuteable (Pins), plays crucial roles in mitotic spindle orientation, a process contributing to asymmetric cell division. Here, we report a crystal structure of the LGN-binding domain (LBD) of human mInsc complexed with the N-terminal tetratricopeptide repeat (TPR) motifs of human LGN at 2.6-Å resolution. In the complex, mInsc-LBD adopts an elongated structure with three binding modules--an α-helix, an extended region, and a β-sheet connected with a loop--that runs antiparallel to LGN along the concave surface of the superhelix formed by the TPRs. Structural analysis and structure-based mutagenesis define residues that are critical for mInsc-LGN association, and reveal that the activator of G-protein signaling 3 (AGS3)-binding protein Frmpd1 [4.1/ezrin/radixin/moesin (FERM) and PSD-95/Dlg/ZO-1 (PDZ) domain-containing protein 1] and its relative Frmpd4 interact with LGN via a region homologous to a part of mInsc-LBD, whereas nuclear mitotic apparatus protein (NuMA) and the C terminus of LGN recognize the TPR domain in a manner different from that by mInsc. mInsc binds to LGN with the highest affinity (K(D) ≈ 2.4 nM) and effectively replaces the Frmpd proteins, NuMA, and the LGN C terminus, suggesting the priority of mInsc in binding to LGN. We also demonstrate, using mutant proteins, that mInsc-LGN interaction is vital for stabilization of LGN and for intracellular localization of mInsc.

Structural determinants of affinity enhancement between GoLoco motifs and G-protein alpha subunit mutants

GoLoco motif proteins bind to the inhibitory G(i) subclass of G-protein α subunits and slow the release of bound GDP; this interaction is considered critical to asymmetric cell division and neuro-epithelium and epithelial progenitor differentiation. To provide protein tools for interrogating the precise cellular role(s) of GoLoco motif/Gα(i) complexes, we have employed structure-based protein design strategies to predict gain-of-function mutations that increase GoLoco motif binding affinity. Here, we describe fluorescence polarization and isothermal titration calorimetry measurements showing three predicted Gα(i1) point mutations, E116L, Q147L, and E245L; each increases affinity for multiple GoLoco motifs. A component of this affinity enhancement results from a decreased rate of dissociation between the Gα mutants and GoLoco motifs. For Gα(i1)(Q147L), affinity enhancement was seen to be driven by favorable changes in binding enthalpy, despite reduced contributions from binding entropy. The crystal structure of Gα(i1)(Q147L) bound to the RGS14 GoLoco motif revealed disorder among three peptide residues surrounding a well defined Leu-147 side chain. Monte Carlo simulations of the peptide in this region showed a sampling of multiple backbone conformations in contrast to the wild-type complex. We conclude that mutation of Glu-147 to leucine creates a hydrophobic surface favorably buried upon GoLoco peptide binding, yet the hydrophobic Leu-147 also promotes flexibility among residues 511-513 of the RGS14 GoLoco peptide.

Asymmetric cell division of T cells upon antigen presentation uses multiple conserved mechanisms

Asymmetric cell division is a potential means by which cell fate choices during an immune response are orchestrated. Defining the molecular mechanisms that underlie asymmetric division of T cells is paramount for determining the role of this process in the generation of effector and memory T cell subsets. In other cell types, asymmetric cell division is regulated by conserved polarity protein complexes that control the localization of cell fate determinants and spindle orientation during division. We have developed a tractable, in vitro model of naive CD8(+) T cells undergoing initial division while attached to dendritic cells during Ag presentation to investigate whether similar mechanisms might regulate asymmetric division of T cells. Using this system, we show that direct interactions with APCs provide the cue for polarization of T cells. Interestingly, the immunological synapse disseminates before division even though the T cells retain contact with the APC. The cue from the APC is translated into polarization of cell fate determinants via the polarity network of the Par3 and Scribble complexes, and orientation of the mitotic spindle during division is orchestrated by the partner of inscuteable/G protein complex. These findings suggest that T cells have selectively adapted a number of evolutionarily conserved mechanisms to generate diversity through asymmetric cell division.

Structural basis for self-renewal of neural progenitors in cortical neurogenesis

In mammalian brain development, neuroepithelial cells act as progenitors that produce self-renewing and differentiating cells. Recent technical advances in live imaging and gene manipulation now enable us to investigate how neural progenitors generate the 2 different types of cells with unprecedented accuracy and resolution, shedding new light on the roles of epithelial structure in cell fate decisions and also on the plasticity of neurogenesis.

Neuroepithelial progenitors undergo LGN-dependent planar divisions to maintain self-renewability during mammalian neurogenesis

During mammalian development, neuroepithelial cells function as mitotic progenitors, which self-renew and generate neurons. Although spindle orientation is important for such polarized cells to undergo symmetric or asymmetric divisions, its role in mammalian neurogenesis remains unclear. Here we show that control of spindle orientation is essential in maintaining the population of neuroepithelial cells, but dispensable for the decision to either proliferate or differentiate. Knocking out LGN, (the G protein regulator), randomized the orientation of normally planar neuroepithelial divisions. The resultant loss of the apical membrane from daughter cells frequently converted them into abnormally localized progenitors without affecting neuronal production rate. Furthermore, overexpression of Inscuteable to induce vertical neuroepithelial divisions shifted the fate of daughter cells. Our results suggest that planar mitosis ensures the self-renewal of neuroepithelial progenitors by one daughter inheriting both apical and basal compartments during neurogenesis.

Control of planar divisions by the G-protein regulator LGN maintains progenitors in the chick neuroepithelium

The spatio-temporal regulation of symmetrical as opposed to asymmetric cell divisions directs the fate and location of cells in the developing CNS. In invertebrates, G-protein regulators control spindle orientation in asymmetric divisions, which generate progeny with different identities. We investigated the role of the G-protein regulator LGN (also called Gpsm2) in spindle orientation and cell-fate determination in the spinal cord neuroepithelium of the developing chick embryo. We show that LGN is located at the cell cortex and spindle poles of neural progenitors, and that it regulates spindle movements and orientation. LGN promotes planar divisions in the early spinal cord. Interfering with LGN function randomizes the plane of division. Notably, this does not affect cell fate, but frequently leads one daughter of proliferative symmetric divisions to exit the neuroepithelium prematurely and to proliferate aberrantly in the mantle zone. Hence, tight control of planar spindle orientation maintains neural progenitors in the neuroepithelium, and regulates the proper development of the nervous system.

Spindle orientation, asymmetric division and tumour suppression in Drosophila stem cells

Recent genetic studies in flies have added further support to an increasing body of evidence that suggests that stem cells might be the cell-of-origin of certain tumours. Malfunction of the mechanisms that control the division of stem cells and the developmental fate of the two resulting daughters could be one of the initial events that steers cells into malignant transformation. These studies suggest a role for controlled spindle orientation in suppressing stem-cell overgrowth. In parallel, the machinery that drives asymmetry in stem cells has been further characterized, identifying new components and uncovering the unique, highly sophisticated behaviour of centrosomes in these cells.

Spindle regulation in neural precursors of flies and mammals

The mitotic spindle is the cellular scaffold that facilitates proper segregation of genetic material during cell division. Far from being static, the spindle is a dynamically regulated tool that can alter its size, shape and position during mitosis. Work in both insect and vertebrate systems has shown that regulation of this structure involves an array of highly conserved proteins. Moreover, it is now clear that tight regulation of the spindle during the process of neurogenesis is paramount to proper cell division and generation of the nervous system as a whole.

Activator of G-protein signaling in asymmetric cell divisions of the sea urchin embryo

An asymmetric fourth cell division in the sea urchin embryo results in formation of daughter cells, macromeres and micromeres, with distinct sizes and fates. Several lines of functional evidence presented here, including pharmacological interference and dominant negative protein expression, indicate that heterotrimeric G protein Gi and its interaction partner, activator of G-protein signaling (AGS), are necessary for this asymmetric cell division. Inhibition of Gi signaling by pertussis toxin interferes with micromere formation and leads to defects in embryogenesis. AGS was isolated in a yeast two-hybrid screen with G alpha i as bait and was expressed in embryos localized to the cell cortex at the time of asymmetric divisions. Introduction of exogenous dominant-negative AGS protein, containing only G-protein regulatory (GPR) domains, selectively prevented the asymmetric division in normal micromere formation. These results support the growing evidence that AGS is a universal regulator of asymmetric cell divisions in embryos.

Accessory proteins for G proteins: partners in signaling

Accessory proteins involved in signal processing through heterotrimeric G proteins are generally defined as proteins distinct from G protein-coupled receptor (GPCR), G protein, or classical effectors that regulate the strength/efficiency/specificity of signal transfer upon receptor activation or position these entities in the right microenvironment, contributing to the formation of a functional signal transduction complex. A flurry of recent studies have implicated an additional class of accessory proteins for this system that provide signal input to heterotrimeric G proteins in the absence of a cell surface receptor, serve as alternative binding partners for G protein subunits, provide unexpected modes of G protein regulation, and have introduced additional functional roles for G proteins. This group of accessory proteins includes the recently discovered Activators of G protein Signaling (AGS) proteins identified in a functional screen for receptor-independent activators of G protein signaling as well as several proteins identified in protein interaction screens and genetic screens in model organisms. These accessory proteins may influence GDP dissociation and nucleotide exchange at the G(alpha) subunit, alter subunit interactions within heterotrimeric G(alphabetagamma) independent of nucleotide exchange, or form complexes with G(alpha) or G(betagamma) independent of the typical G(alphabetagamma) heterotrimer. AGS and related accessory proteins reveal unexpected diversity in G protein subunits as signal transducers within the cell.

Mammalian Inscuteable Regulates Spindle Orientation and Cell Fate in the Developing Retina

During mammalian neurogenesis, progenitor cells can divide with the mitotic spindle oriented parallel or perpendicular to the surface of the neuroepithelium. Perpendicular divisions are more likely to be asymmetric and generate one progenitor and one neuronal precursor. Whether the orientation of the mitotic spindle actually determines their asymmetric outcome is unclear. Here, we characterize a mammalian homolog of Inscuteable (mInsc), a key regulator of spindle orientation in Drosophila. mInsc is expressed temporally and spatially in a manner that suggests a role in orienting the mitotic spindle in the developing nervous system. Using retroviral RNAi in rat retinal explants, we show that downregulation of mInsc inhibits vertical divisions. This results in enhanced proliferation, consistent with a higher frequency of symmetric divisions generating two proliferating cells. Our results suggest that the orientation of neural progenitor divisions is important for cell fate specification in the retina and determines their symmetric or asymmetric outcome.

Activator of G protein signaling type 3 mRNA is widely distributed in the rat brain and is particularly abundant in the subventricular zone-olfactory bulb system of neural precursor cell proliferation, migration and differentiation

Activator of G protein signaling 3 (AGS3) is a guanine nucleotide dissociation inhibitor (GDI) to heterotrimeric G proteins of the Galphai/o class. Previous studies have described the tissue distribution and expression changes of AGS3 in brain extracts; however the precise localization of AGS3 in intact tissue has not yet been determined. The aim of the present study is therefore to map in detail the expression of AGS3 mRNA in the rat brain by using in situ hybridization (ISH) histochemistry with a (33)P-labeled riboprobe. Hybridized sections were analyzed at the regional level after exposure to autoradiography films and at the cellular level after coating with a photographic emulsion. Our results confirm a broad distribution for AGS3 which is expressed throughout the brain. Although most regions show a low level of expression, our results reveal a high abundance of AGS3 mRNA in the cerebellum as well as in regions important for neural precursor cell proliferation (subventricular zone of the anterior lateral ventricle), migration (rostral migratory stream) and differentiation (olfactory bulb). In particular, the high abundance of AGS3 in the subventricular zone-olfactory bulb system further documents the potential role of AGS3 in neural precursor cell division, migration and/or differentiation.

Mammalian Pins is a conformational switch that links NuMA to heterotrimeric G proteins

During asymmetric cell divisions, mitotic spindles align along the axis of polarization. In invertebrates, spindle positioning requires Pins or related proteins and a G protein alpha subunit. A mammalian Pins, called LGN, binds Galphai and also interacts through an N-terminal domain with the microtubule binding protein NuMA. During mitosis, LGN recruits NuMA to the cell cortex, while cortical association of LGN itself requires the C-terminal Galpha binding domain. Using a FRET biosensor, we find that LGN behaves as a conformational switch: in its closed state, the N and C termini interact, but NuMA or Galphai can disrupt this association, allowing LGN to interact simultaneously with both proteins, resulting in their cortical localization. Overexpression of Galphai or YFP-LGN causes a pronounced oscillation of metaphase spindles, and NuMA binding to LGN is required for these spindle movements. We propose that a related switch mechanism might operate in asymmetric cell divisions in the fly and nematode.

Asymmetric localization of LGN but not AGS3, two homologs ofDrosophilapins, in dividing human neural progenitor cells

Human neural progenitor cells (hNPCs) can be recovered from postmortem human brains and used to study the molecular basis of neurogenesis. Human NPCs are being used to investigate the molecular basis of cell fate determination during stem cell divisions, based on comparison with the Drosophila model system. Drosophila neuroblasts and sensory organ precursors undergo well-defined asymmetric cell divisions (ACD), under the control of a genetically defined set of apical and basal determinants that are localized tightly and dynamically during division. We show by indirect immunofluorescence, confocal microscopy, and time-lapse video-microscopy that LGN and AGS3, two human homologs of the Drosophila ACD determinant Pins, have distinct patterns of localization in hNPCs. When cells are grown under conditions favoring proliferation, LGN is distributed asymmetrically in a cell cycle-dependent manner; it localizes to one side of the dividing cell and segregates into one of the daughter cells. When the cells are grown under conditions favoring differentiation, LGN accumulates in double foci similar to those containing the mitotic apparatus protein NuMA, and in a pattern shown previously for LGN and NuMA in differentiated cells. AGS3, a slightly more distant Pins homolog than LGN, does not show asymmetric localization in these cells. The progenitor cell marker nestin also localizes asymmetrically in colcemid-treated hNPCs and colocalizes with LGN. The results suggest that hNPCs undergo ACD and that similar molecular pathways may underlie these divisions in Drosophila and human cells.

The planar cell polarity protein Strabismus promotes Pins anterior localization during asymmetric division of sensory organ precursor cells in Drosophila

Cell fate diversity is generated in part by the unequal segregation of cell-fate determinants during asymmetric cell division. In the Drosophila bristle lineage, the sensory organ precursor (pI) cell is polarized along the anteroposterior (AP) axis by Frizzled (Fz) receptor signaling. We show here that Fz localizes at the posterior apical cortex of the pI cell prior to mitosis, whereas Strabismus (Stbm) and Prickle (Pk), which are also required for AP polarization of the pI cell, co-localize at the anterior apical cortex. Thus, asymmetric localization of Fz, Stbm and Pk define two opposite cortical domains prior to mitosis of the pI cell. At mitosis, Stbm forms an anterior crescent that overlaps with the distribution of Partner of Inscuteable (Pins) and Discs-large (Dlg), two components of the anterior Dlg-Pins-Galphai complex that regulates the localization of cell-fate determinants. At prophase, Stbm promotes the anterior localization of Pins. By contrast, Dishevelled (Dsh) acts antagonistically to Stbm by excluding Pins from the posterior cortex. We propose that the Stbm-dependent recruitment of Pins at the anterior cortex of the pI cell is a novel read-out of planar cell polarity.

Subcellular localization of LGN during mitosis: evidence for its cortical localization in mitotic cell culture systems and its requirement for normal cell cycle progression

Mammalian LGN/AGS3 proteins and their Drosophila Pins orthologue are cytoplasmic regulators of G-protein signaling. In Drosophila, Pins localizes to the lateral cortex of polarized epithelial cells and to the apical cortex of neuroblasts where it plays important roles in their asymmetric division. Using overexpression studies in different cell line systems, we demonstrate here that, like Drosophila Pins, LGN can exhibit enriched localization at the cell cortex, depending on the cell cycle and the culture system used. We find that in WISH, PC12, and NRK but not COS cells, LGN is largely directed to the cell cortex during mitosis. Overexpression of truncated protein domains further identified the Galpha-binding C-terminal portion of LGN as a sufficient domain for cortical localization in cell culture. In mitotic COS cells that normally do not exhibit cortical LGN localization, LGN is redirected to the cell cortex upon overexpression of Galpha subunits of heterotrimeric G-proteins. The results also show that the cortical localization of LGN is dependent on microfilaments and that interfering with LGN function in cultured cell lines causes early disruption to cell cycle progression.