Lethal Giant Larvae Controls the Localization of Notch-Signaling Regulators Numb, Neuralized, and Sanpodo in Drosophila Sensory-Organ Precursor Cells

Drosophila Hey is a target of Notch in asymmetric divisions during embryonic and larval neurogenesis

bHLH-O proteins are a subfamily of the basic-helix-loop-helix transcription factors characterized by an 'Orange' protein-protein interaction domain. Typical members are the Hairy/E(spl), or Hes, proteins, well studied in their ability, among others, to suppress neuronal differentiation in both invertebrates and vertebrates. Hes proteins are often effectors of Notch signalling. In vertebrates, another bHLH-O protein group, the Hey proteins, have also been shown to be Notch targets and to interact with Hes. We have studied the single Drosophila Hey orthologue. We show that it is primarily expressed in a subset of newly born neurons, which receive Notch signalling during their birth. Unlike in vertebrates, however, Hey is not expressed in precursor cells and does not block neuronal differentiation. It rather promotes one of two alternative fates that sibling neurons adopt at birth. Although in the majority of cases Hey is a Notch target, it is also expressed independently of Notch in some lineages, most notably the larval mushroom body. The availability of Hey as a Notch readout has allowed us to study Notch signalling during the genesis of secondary neurons in the larval central nervous system.

Numb independently antagonizes Sanpodo membrane targeting and Notch signaling in Drosophila sensory organ precursor cells

How Numb regulates Notch signaling following asymmetric cell division is unclear. Numb directly binds and blocks membrane localization of Sanpodo, a protein essential for Notch signaling in Drosophila. Uncoupling Sanpodo from Numb results in accumulation of Sanpodo at the membrane, but this surprisingly does not appear to promote Notch signaling.

In Drosophila, mitotic neural progenitor cells asymmetrically segregate the cell fate determinant Numb in order to block Notch signaling in only one of the two daughter cells. Sanpodo, a membrane protein required for Notch signaling in asymmetrically dividing cells, is sequestered from the plasma membrane to intracellular vesicles in a Numb-dependent way after neural progenitor cell mitosis. However, the significance of Numb-dependent Sanpodo regulation is unclear. In this study, we conducted a structure–function analysis to identify the determinants of Sanpodo targeting in vivo. We identified an NPAF motif in the amino-terminal cytoplasmic tail of Sanpodo, which is conserved among insect Sanpodo homologues. The Sanpodo NPAF motif is predicted to bind directly to the Numb phosphotyrosine-binding domain and is critical for Numb binding in vitro. Deletion or mutation of the NPAF motif results in accumulation of Sanpodo at the plasma membrane in Numb-positive cells in vivo. Genetic analysis of Sanpodo NPAF mutants shows that Numb-dependent Sanpodo endocytic targeting can be uncoupled from Notch signaling regulation. Our findings demonstrate that Sanpodo contains an evolutionarily conserved motif that has been linked to Numb-dependent regulation in vertebrates and further support the model that Numb regulates Notch signaling independently of Sanpodo membrane trafficking in neural progenitor cells.

Endocytosis and intracellular trafficking of Notch and its ligands

Notch signaling occurs through direct interaction between Notch, the receptor, and its ligands, presented on the surface of neighboring cells. Endocytosis has been shown to be essential for Notch signal activation in both signal-sending and signal-receiving cells, and numerous genes involved in vesicle trafficking have recently been shown to act as key regulators of the pathway. Defects in vesicle trafficking can lead to gain- or loss-of-function defects in a context-dependent manner. Here, we discuss how endocytosis and vesicle trafficking regulate Notch signaling in both signal-sending and signal-receiving cells. We will introduce the key players in different trafficking steps, and further illustrate how they impact the signal outcome. Some of these players act as general factors and modulate Notch signaling in all contexts, whereas others modulate signaling in a context-specific fashion. We also discuss Notch signaling during mechanosensory organ development in the fly to exemplify how endocytosis and vesicle trafficking are effectively used to determine correct cell fates. In summary, endocytosis plays an essential role in Notch signaling, whereas intracellular vesicle trafficking often plays a context-dependent or regulatory role, leading to divergent outcomes in different developmental contexts.

Sanpodo: a context-dependent activator and inhibitor of Notch signaling during asymmetric divisions

Asymmetric cell divisions generate sibling cells of distinct fates ('A', 'B') and constitute a fundamental mechanism that creates cell-type diversity in multicellular organisms. Antagonistic interactions between the Notch pathway and the intrinsic cell-fate determinant Numb appear to regulate asymmetric divisions in flies and vertebrates. During these divisions, productive Notch signaling requires sanpodo, which encodes a novel transmembrane protein. Here, we demonstrate that Drosophila sanpodo plays a dual role to regulate Notch signaling during asymmetric divisions - amplifying Notch signaling in the absence of Numb in the 'A' daughter cell and inhibiting Notch signaling in the presence of Numb in the 'B' daughter cell. In so doing, sanpodo ensures the asymmetry in Notch signaling levels necessary for the acquisition of distinct fates by the two daughter cells. These findings answer long-standing questions about the restricted ability of Numb and Sanpodo to inhibit and to promote, respectively, Notch signaling during asymmetric divisions.

At the crossroads of polarity, proliferation and apoptosis: the use of Drosophila to unravel the multifaceted role of endocytosis in tumor suppression

Endocytosis is an important regulator of cell-cell signaling and endocytic trafficking has been increasingly implicated in control of tumor suppression. Recent insights from Drosophila indicate that impairment of multiple trafficking steps which lead to receptor degradation can cause tumor formation in epithelial organs. These tumors are characterized by sustained activation of a number of mitogenic signaling pathways, and by subversion of epithelial polarity and the apoptotic response. Cooperation between such alterations, as well as tumor-host interactions, is also observed. The recapitulation of several hallmarks of human cancers in fly tumors provides a framework to understand the role of defective endocytosis in cancer.

3D image stack reconstruction in live cell microscopy of Drosophila muscles and its validation

Rapid movements of live tissues during the acquisition of 3D image stacks can result in misalignments between successive image slices. The remodeling of the muscles in Drosophila metamorphosis is an example where sporadic motion during image acquisition impede image analysis and volume visualization. Most of the image stack registration algorithms applied in microscopy are aimed at the linear alignment of fixed histological sections. However, live muscles are nonrigid objects and their contractions and relaxations represent nonlinear transformations that cannot be properly rectified by applying purely linear registration methods. We developed a fully automated area-based nonrigid stack registration (NSR) method that minimizes the mean square error of intensities between successive image slices. The mapping function is formulated using the thin plate spline (TPS). A hierarchical linear to nonlinear, coarse to fine matching strategy is applied to ensure stability and fast convergence. Topological structure is preserved by constraining the step size of the nonlinear transformation. To assess the accuracy of 3D reconstruction, we propose a new benchmarking method that measures geometrical features of restored nuclei. We tested our algorithm on image stacks generated by laser scanning confocal microscopy that show live muscles during the prepupal stage of Drosophila metamorphosis. Our registration algorithm is able to restore image stacks that are distorted by periodic contraction of muscles. Quantitative assessment of registration performance agrees well with qualitative visual inspection. Our NSR method is able to restore image stacks for the purpose of visualization and quantitative analysis of Drosophila metamorphosis and, potentially, various other processes in developmental biology studied by 3D live cell microscopy.

Endocytosis, asymmetric cell division, stem cells and cancer: unus pro omnibus, omnes pro uno

Carcinogenesis can be initiated in adult stem cells, suggesting that tumours arise as a consequence of stem-cell dysfunction. In the fruitfly, cancer arises in stem cells that fail to undergo asymmetric cell division. In flies and mammals, a specific regulation of the endocytic trafficking machinery allows stem cells to self-renew and generate the differentiating cells required to form and maintain mature organs. We review recent findings suggesting that an understanding of the relationship between endocytosis, asymmetric cell division, stem cells and cancer will be crucial to unravel the cell biological basis of tumourigenesis.

Control of T helper cell differentiation through cytokine receptor inclusion in the immunological synapse

The antigen recognition interface formed by T helper precursors (Thps) and antigen-presenting cells (APCs), called the immunological synapse (IS), includes receptors and signaling molecules necessary for Thp activation and differentiation. We have recently shown that recruitment of the interferon-gamma receptor (IFNGR) into the IS correlates with the capacity of Thps to differentiate into Th1 effector cells, an event regulated by signaling through the functionally opposing receptor to interleukin-4 (IL4R). Here, we show that, similar to IFN-gamma ligation, TCR stimuli induce the translocation of signal transducer and activator of transcription 1 (STAT1) to IFNGR1-rich regions of the membrane. Unexpectedly, STAT1 is preferentially expressed, is constitutively serine (727) phosphorylated in Thp, and is recruited to the IS and the nucleus upon TCR signaling. IL4R engagement controls this process by interfering with both STAT1 recruitment and nuclear translocation. We also show that in cells with deficient Th1 or constitutive Th2 differentiation, the IL4R is recruited to the IS. This observation suggest that the IL4R is retained outside the IS, similar to the exclusion of IFNGR from the IS during IL4R signaling. This study provides new mechanistic cues for the regulation of lineage commitment by mutual immobilization of functionally antagonistic membrane receptors.

Directional Delta and Notch trafficking in Sara endosomes during asymmetric cell division

Endocytosis has a crucial role during Notch signalling after the asymmetric division of fly sensory organ precursors (SOPs): directional signalling is mediated by differential endocytosis of the ligand Delta and the Notch effector Sanpodo in one of the SOP daughters, pIIb. Here we show a new mechanism of directional signalling on the basis of the trafficking of Delta and Notch molecules already internalized in the SOP and subsequently targeted to the other daughter cell, pIIa. Internalized Delta and Notch traffic to an endosome marked by the protein Sara. During SOP mitosis, Sara endosomes containing Notch and Delta move to the central spindle and then to pIIa. Subsequently, in pIIa (but not in pIIb) Notch appears cleaved in Sara endosomes in a gamma-secretase- and Delta internalization-dependent manner, indicating that the release of the intracellular Notch tail to activate Notch target genes has occurred. We thus uncover a new mechanism to bias signalling even before asymmetric endocytosis of Sanpodo and Delta takes place in the daughter cells: already during SOP mitosis, asymmetric targeting of Delta and Notch-containing Sara endosomes will increase Notch signalling in pIIa and decrease it in pIIb.

Frequent unanticipated alleles of lethal giant larvae in Drosophila second chromosome stocks

Forty years ago, a high frequency of lethal giant larvae (lgl) alleles in wild populations of Drosophila melanogaster was reported. This locus has been intensively studied for its roles in epithelial polarity, asymmetric neural divisions, and restriction of tissue proliferation. Here, we identify a high frequency of lgl alleles in the Bloomington second chromosome deficiency kit and the University of California at Los Angeles Bruinfly FRT40A-lethal P collection. These unrecognized aberrations confound the use of these workhorse collections for phenotypic screening or genetic mapping. In addition, we determined that independent alleles of insensitive, reported to affect asymmetric cell divisions during sensory organ development, carry lgl deletions that are responsible for the observed phenotypes. Taken together, these results encourage the routine testing of second chromosome stocks for second-site alleles of lgl.

Cell proliferation in the Drosophila adult brain revealed by clonal analysis and bromodeoxyuridine labelling

BACKGROUND

The production of new neurons during adulthood and their subsequent integration into a mature central nervous system have been shown to occur in all vertebrate species examined to date. However, the situation in insects is less clear and, in particular, it has been reported that there is no proliferation in the Drosophila adult brain.

RESULTS

We report here, using clonal analysis and 5'-bromo-2'-deoxyuridine (BrdU) labelling, that cell proliferation does occur in the Drosophila adult brain. The majority of clones cluster on the ventrolateral side of the antennal lobes, as do the BrdU-positive cells. Of the BrdU-labelled cells, 86% express the glial gene reversed polarity (repo), and 14% are repo negative.

CONCLUSION

We have observed cell proliferation in the Drosophila adult brain. The dividing cells may be adult stem cells, generating glial and/or non-glial cell types.

Van Gogh and Frizzled act redundantly in the Drosophila sensory organ precursor cell to orient its asymmetric division

Drosophila sensory organ precursor cells (SOPs) divide asymmetrically along the anterior-posterior (a-p) body axis to generate two different daughter cells. Planar Cell Polarity (PCP) regulates the a-p orientation of the SOP division. The localization of the PCP proteins Van Gogh (Vang) and Frizzled (Fz) define anterior and posterior apical membrane domains prior to SOP division. Here, we investigate the relative contributions of Vang, Fz and Dishevelled (Dsh), a membrane-associated protein acting downstream of Fz, in orienting SOP polarity. Genetic and live imaging analyses suggest that Dsh restricts the localization of a centrosome-attracting activity to the anterior cortex and that Vang is a target of Dsh in this process. Using a clone border assay, we provide evidence that the Vang and fz genes act redundantly in SOPs to orient its polarity axis in response to extrinsic local PCP cues. Additionally, we find that the activity of Vang is dispensable for the non-autonomous polarizing activity of fz. These observations indicate that both Vang and Fz act as cues for downstream effectors orienting the planar polarity axis of dividing SOPs.

Upsides and downsides to polarity and asymmetric cell division in leukemia

The notion that polarity regulators can act as tumor suppressors in epithelial cells is now well accepted. The function of these proteins in lymphocytes is less well explored, and their possible function as suppressors of leukemia has had little attention so far. We review the literature on lymphocyte polarity and the growing recognition that polarity proteins have an important function in lymphocyte function. We then describe molecular relationships between the polarity network and signaling pathways that have been implicated in leukemogenesis, which suggest mechanisms by which the polarity network might impact on leukemogenesis. We particularly focus on the possibility that disruption of polarity might alter asymmetric cell division (ACD), and that this might be a leukemia-initiating event. We also explore the converse possibility that leukemic stem cells might be produced or maintained by ACD, and therefore that Dlg, Scribble and Lgl might be important regulators of this process.

Control of tumourigenesis by the Scribble/Dlg/Lgl polarity module

The neoplastic tumour suppressors, Scribble, Dlg and Lgl, originally discovered in the vinegar fly Drosophila melanogaster, are currently being actively studied for their potential role in mammalian tumourigenesis. In Drosophila, these tumour suppressors function in a common genetic pathway to regulate apicobasal cell polarity and also play important roles in the control of cell proliferation, survival, differentiation and in cell migration/invasion. The precise mechanism by which Scribble, Dlg and Lgl function is not clear; however, they have been implicated in the regulation of signalling pathways, vesicle trafficking and in the Myosin II-actin cytoskeleton. We review the evidence for the involvement of Scribble, Dlg, and Lgl in cancer, and how the various functions ascribed to these tumour suppressors in Drosophila and mammalian systems may impact on the process of tumourigenesis.

Notch: From fly wings to human hematological tumors

Notch history begins in 1919 with Thomas Hunt Morgan studies on fruit fly mutants. From then, this gene aroused lively interest in the scientific community since it is involved in a wide variety of processes, including morphogenesis, tissue homeostasis, and stem cell maintenance. Deregulation of Notch signaling characterizes several human tumors. Hematopoietic system is affected by mutations of Notch receptors, Notch ligands, and proteins controlling their stability. Approximately 60% T acute lymphoblastic leukemia (T-ALL) patients carry activating Notch1 mutations prompting blasts growth. In addition, multiple myeloma is characterized by Notch signaling hyper-activation due to an abnormal expression of the Jagged2 ligand; this affects not only myeloma cells, but also their interaction with bone marrow microenvironment, influencing tumor burden and bone disease. These findings make Notch a rational target of a therapeutic approach. Inhibitors of the Notch activating enzyme, ?-Secretase, have been successfully used in vitro and in vivo and are currently under clinical trials for T-ALL and breast cancer. Yet a wide use of these inhibitors is prevented by frequently occurring drug resistance. To elucidate the mechanism underlying this phenomenon, a number of pathways have been identified mediating Notch biological effects: AKT and c-Myc are frequently deregulated in leukemic patients and account for resistance to ?-Secretase inhibitors by acting downstream Notch receptor. Therefore, the interaction of Notch with other cancer-associated proteins should be clarified to predict the biological outcome of a Notch targeted therapy and possibly, to exploit combined treatments against the key deregulated elements in Notch-associated cancers.

Linking cell cycle to asymmetric division: Aurora-A phosphorylates the Par complex to regulate Numb localization

Drosophila neural precursor cells divide asymmetrically by segregating the Numb protein into one of the two daughter cells. Numb is uniformly cortical in interphase but assumes a polarized localization in mitosis. Here, we show that a phosphorylation cascade triggered by the activation of Aurora-A is responsible for the asymmetric localization of Numb in mitosis. Aurora-A phosphorylates Par-6, a regulatory subunit of atypical protein kinase C (aPKC). This activates aPKC, which initially phosphorylates Lethal (2) giant larvae (Lgl), a cytoskeletal protein that binds and inhibits aPKC during interphase. Phosphorylated Lgl is released from aPKC and thereby allows the PDZ domain protein Bazooka to enter the complex. This changes substrate specificity and allows aPKC to phosphorylate Numb and release the protein from one side of the cell cortex. Our data reveal a molecular mechanism for the asymmetric localization of Numb and show how cell polarity can be coupled to cell-cycle progression.

Overexpression of partner of numb induces asymmetric distribution of the PI4P 5-Kinase Skittles in mitotic sensory organ precursor cells in Drosophila

Unequal segregation of cell fate determinants at mitosis is a conserved mechanism whereby cell fate diversity can be generated during development. In Drosophila, each sensory organ precursor cell (SOP) divides asymmetrically to produce an anterior pIIb and a posterior pIIa cell. The Par6-aPKC complex localizes at the posterior pole of dividing SOPs and directs the actin-dependent localization of the cell fate determinants Numb, Partner of Numb (Pon) and Neuralized at the opposite pole. The plasma membrane lipid phosphatidylinositol (4,5)-bisphosphate (PIP2) regulates the plasma membrane localization and activity of various proteins, including several actin regulators, thereby modulating actin-based processes. Here, we have examined the distribution of PIP2 and of the PIP2-producing kinase Skittles (Sktl) in mitotic SOPs. Our analysis indicates that both Sktl and PIP2 reporters are uniformly distributed in mitotic SOPs. In the course of this study, we have observed that overexpression of full-length Pon or its localization domain (LD) fused to the Red Fluorescent Protein (RFP::Pon^LD) results in asymmetric distribution of Sktl and PIP2 reporters in dividing SOPs. Our observation that Pon overexpression alters polar protein distribution is relevant because RFP::Pon^LD is often used as a polarity marker in dividing progenitors.

Polarized subcellular localization of Jak/STAT components is required for efficient signaling

Three protein complexes control polarization of epithelial cells: the apicolateral Crumbs and Par-3 complexes and the basolateral Lethal giant larvae complex. Polarization results in the specific localization of proteins and lipids to different membrane domains. The receptors of the Notch, Hedgehog, and WNT pathways are among the proteins that are polarized, with subcellular receptor localization representing an important aspect of signaling regulation. For example, in the WNT pathway, differential DFz2 receptor localization results in activation of either the canonical or the planar polarity pathway. Despite the large body of research on the vertebrate JAK/STAT pathway, there are no reports indicating polarized signaling. By using the conserved Drosophila JAK/STAT pathway as a system, we find that the receptor and its associated kinase are located in the apical membrane of epithelial cells. Unexpectedly, the transcription factor STAT is enriched in the apicolateral membrane domain of ectoderm epithelial cells in a Par-3-dependent manner. Our results indicate that preassembly of STAT and the Receptor/JAK complex to specific membrane domains is a key aspect for signaling efficiency. Our results also suggest that receptor polarization in the ectoderm cell membrane restricts the cell's response to ligands provided by neighboring cells.

Signaling networks during development: the case of asymmetric cell division in the Drosophila nervous system

Remarkable progress in genetics and molecular biology has made possible the sequencing of the genomes from numerous species. In the post-genomic era, technical developments in the fields of proteomics and bioinformatics are poised to further catapult our understanding of protein structure, function and organization into complex signaling networks. One of the greatest challenges in the field now is to unravel the functional signaling networks and their spatio-temporal regulation in living cells. Here, the need for such in vivo system-wide level approach is illustrated in relation to the mechanisms that underlie the biological process of asymmetric cell division. Genomic, post-genomic and live imaging techniques are reviewed in light of the huge impact they are having on this field for the discovering of new proteins and for the in vivo analysis of asymmetric cell division. The proteins, signals and the emerging networking of functional connections that is arising between them during this process in the Drosophila nervous system will be also discussed.

Notch signaling--constantly on the move

The Notch pathway is a highly conserved and ubiquitous signaling system that functions in determining a diverse array of cell fates and regulates many cellular processes during embryonic development and throughout adulthood. Links to cancer, stroke and Alzheimer's disease underscore the need to define the molecular basis of Notch activation. Notch signaling is induced through direct cell-cell interactions that promote receptor activation following engagement with a membrane-bound Delta, Serrate, Lag-2 (DSL) ligand on adjacent cells. Cells take on distinct fates because Notch signaling is consistently activated in only one of the two interacting cells, highlighting the importance of establishing and maintaining signaling polarity. Studies in flies and worms have identified positive and negative transcriptional feedback mechanisms that amplify small differences in Notch and DSL ligand expression to bias which cells send or receive signals. However, endocytosis by signal-sending and signal-receiving cells also appears critical for directing and regulating Notch activation. In particular, endocytosis and membrane trafficking of DSL ligands, Notch and modulators can determine the competence of cells to send or receive signals that ensure reproducibility in generating cell types regulated by Notch signaling.

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.

aPKC-mediated phosphorylation regulates asymmetric membrane localization of the cell fate determinant Numb

In Drosophila, the partition defective (Par) complex containing Par3, Par6 and atypical protein kinase C (aPKC) directs the polarized distribution and unequal segregation of the cell fate determinant Numb during asymmetric cell divisions. Unequal segregation of mammalian Numb has also been observed, but the factors involved are unknown. Here, we identify in vivo phosphorylation sites of mammalian Numb and show that both mammalian and Drosophila Numb interact with, and are substrates for aPKC in vitro. A form of mammalian Numb lacking two protein kinase C (PKC) phosphorylation sites (Numb2A) accumulates at the cell membrane and is refractory to PKC activation. In epithelial cells, mammalian Numb localizes to the basolateral membrane and is excluded from the apical domain, which accumulates aPKC. In contrast, Numb2A is distributed uniformly around the cell cortex. Mutational analysis of conserved aPKC phosphorylation sites in Drosophila Numb suggests that phosphorylation contributes to asymmetric localization of Numb, opposite to aPKC in dividing sensory organ precursor cells. These results suggest a model in which phosphorylation of Numb by aPKC regulates its polarized distribution in epithelial cells as well as during asymmetric cell divisions.

Detection of GFP during nervous system development in Drosophila melanogaster

Using the vital marker GFP and its spectral variants, it is possible to visualize multiple proteins in individual cells and thereby monitor embryonic development on a cellular and molecular level. In the following chapter we describe how to prepare Drosophila embryos or larvae for live imaging or immunohistochemical staining and provide some guidelines for optimal GFP detection.

Endocytosis, endosome trafficking, and the regulation of Drosophila development

Endocytosis and endosome trafficking regulate cell signaling in unexpected ways. Here we review the contribution that Drosophila research has made to this exciting field. In addition to attenuating signaling, endocytosis shapes morphogen gradients, activates ligands, and regulates spatially receptor activation within a single cell. Moreover, some receptors signal from within endosomes, and the ability of a specific type of endosome to form controls the ability of cells to signal. Experiments in Drosophila reveal that through regulation of a variety of cell signaling pathways, endocytosis controls cell patterning and cell fate.

Notch signalling: a simple pathway becomes complex

A small number of signalling pathways are used iteratively to regulate cell fates, cell proliferation and cell death in development. Notch is the receptor in one such pathway, and is unusual in that most of its ligands are also transmembrane proteins; therefore signalling is restricted to neighbouring cells. Although the intracellular transduction of the Notch signal is remarkably simple, with no secondary messengers, this pathway functions in an enormous diversity of developmental processes and its dysfunction is implicated in many cancers.

Neuronal polarity in CNS development

The diversity of neuronal morphologies and the complexity of synaptic connections in the mammalian brain provide striking examples of cell polarity. Over the past decade, the identification of the PAR (for partitioning-defective) proteins, their function in polarity in the Caenorhabditis elegans zygote, and the conservation of polarity proteins related to the PAR polarity complex in Drosophila and vertebrates, kindled intense interest in polarity pathways. Although the existence of a conserved polarity protein complex does not prove that these proteins function the same way in different systems, the emergence of an evolutionarily conserved mechanism that regulates cell polarity provides an exciting opportunity to define the role of polarity proteins in the generation of the diverse array of cell types and patterns of connections in the developing mammalian brain. This review addresses emerging genetic, molecular genetic, biochemical, and cell biological approaches and mechanisms that control neuronal polarity, focusing on recent studies using the neonatal cerebellum and hippocampus as model systems.

PAR3 acts as a molecular organizer to define the apical domain of chick neuroepithelial cells

Neural progenitors in the vertebrate nervous system are fully polarized epithelial cells, with intercellular junctions at the apical region. These progenitor cells remain within the neuroepithelium throughout neurogenesis, and will ultimately give rise to all the neurons in the mature nervous system. We have addressed the role of the PAR polarity complex in vertebrate neuroepithelial polarity and show that PAR3 functions as the initial scaffold to assemble and organize the PAR complex at the apical region of neuroepithelial cells, coordinating also the recruitment of additional polarity complexes and junction-associated proteins to the same region, while restricting other polarity proteins to the basolateral membrane. We propose that PAR3 acts as a molecular organizer to connect the acquisition of apico-basal polarity with the positioning and formation of junctional structures in neuroepithelial cells, a function of upmost importance for the morphogenesis of embryonic neural tissue and the process of neurogenesis.

Monomeric red fluorescent protein variants used for imaging studies in different species

Fluorescent proteins have proven to be excellent tools for live-cell imaging studies. In addition to green fluorescent protein (GFP) and its variants, recent progress was achieved in the development of monomeric red fluorescent proteins (mRFPs) that show improved properties in respect to maturation and intracellular fluorescence. mRFPmars, a red fluorescent protein designed especially for the use in Dictyostelium, has been employed to tag different proteins for live-cell investigations in Dictyostelium. mRFPruby, which differs in sequence from mRFPmars in four amino acids, has a codon usage optimised for the application in mammalian cells. Here, we show that both mRFP variants can also be applied for localisation studies in other organisms. mRFPmars was expressed in Hydra and fused to the Bcl-2 family protein Bax. mRFPruby in combination with histone 2B was expressed in Drosophila S2 cells to monitor mitosis. Using mouse cell lines, mRFPruby fused to beta-actin was assayed with high spatial resolution to study details of actin cytoskeleton dynamics. In addition, we demonstrate that both mRFP variants are also suitable for dual-colour microscopy in the different species.

Dynamic regulation of mammalian numb by G protein-coupled receptors and protein kinase C activation: Structural determinants of numb association with the cortical membrane

The cell fate determinant Numb is a membrane-associated adaptor protein involved in both development and intracellular vesicular trafficking. It has a phosphotyrosine-binding (PTB) domain and COOH-terminal endocytic-binding motifs for alpha-adaptin and Eps15 homology domain-containing proteins. Four isoforms of Numb are expressed in vertebrates, two of which selectively associate with the cortical membrane. In this study, we have characterized a cortical pool of Numb that colocalizes with AP2 and Eps15 at substratum plasma membrane punctae and cortical membrane-associated vesicles. Green fluorescent protein (GFP)-tagged mutants of Numb were used to identify the structural determinants required for localization. In addition to the previously described association of the PTB domain with the plasma membrane, we show that the AP2-binding motifs facilitate the association of Numb with cortical membrane punctae and vesicles. We also show that agonist stimulation of G protein-coupled receptors (GPCRs) that are linked to phospholipase Cbeta and protein kinase C (PKC) activation causes redistribution of Numb from the cortical membrane to the cytosol. This effect is correlated with Numb phosphorylation and an increase in its Triton X-100 solubility. Live-imaging analysis of mutants identified two regions within Numb that are independently responsive to GPCR-mediated lipid hydrolysis and PKC activation: the PTB domain and a region encompassing at least three putative PKC phosphorylation sites. Our data indicate that membrane localization of Numb is dynamically regulated by GPCR-activated phospholipid hydrolysis and PKC-dependent phosphorylation events.

Endosome dynamics during development

Endocytosis has traditionally been studied in isolated cells. More recently, however, the analysis of protein trafficking in whole organisms has revealed that it plays exciting roles during development. Endocytic trafficking of cell adhesion molecules regulates epithelial polarity and cell migration. Developmental signaling pathways are regulated by the trafficking of receptors and their ligands through the endocytic pathway. Finally, impairment of the endocytic machinery can affect proliferation control and contribute to tumor development.

Quantitative analysis of protein dynamics during asymmetric cell division

In dividing Drosophila sensory organ precursor (SOP) cells, the fate determinant Numb and its associated adaptor protein Pon localize asymmetrically and segregate into the anterior daughter cell, where Numb influences cell fate by repressing Notch signaling. Asymmetric localization of both proteins requires the protein kinase aPKC and its substrate Lethal (2) giant larvae (Lgl). Because both Numb and Pon localization require actin and myosin, lateral transport along the cell cortex has been proposed as a possible mechanism for their asymmetric distribution. Here, we use quantitative live analysis of GFP-Pon and Numb-GFP fluorescence and fluorescence recovery after photobleaching (FRAP) to characterize the dynamics of Numb and Pon localization during SOP division. We demonstrate that Numb and Pon rapidly exchange between a cytoplasmic pool and the cell cortex and that preferential recruitment from the cytoplasm is responsible for their asymmetric distribution during mitosis. Expression of a constitutively active form of aPKC impairs membrane recruitment of GFP-Pon. This defect can be rescued by coexpression of nonphosphorylatable Lgl, indicating that Lgl is the main target of aPKC. We propose that a high-affinity binding site is asymmetrically distributed by aPKC and Lgl and is responsible for asymmetric localization of cell-fate determinants during mitosis.

Lethal giant puzzle of Lgl

Cell polarity is one of the most basic properties of all normal cells and loss of polarity is a hallmark of cancer. While multiple proteins have been implicated in the maintenance of cell polarity, the functionally related neoplastic tumor suppressors Lethal giant larvae (Lgl), Scribble and Disks large comprise a unique group of molecules that are not only involved in the maintenance of cell polarity, but also in the regulation of cell proliferation and cancer. Lgl is the first identified member of this group. Loss of Lgl leads to massive tissue disorganization, tumor-like growth and lethal phenotypes in both Drosophila and mice. Lgl mutant cells display disruption of cell polarity, failure of asymmetric cell division, deregulation of Notch signaling and loss of proper cell fate determination. Lgl is a critical downstream target of the Par6/aPKC cell polarity complex; however, the functional role of Lgl itself and, specifically, the mechanisms of Lgl function in cell polarity and regulation of cell proliferation remain enigmatic. This minireview summarizes available information and discusses potential mechanisms of Lgl function.

Drosophila Ric-8 regulates Galphai cortical localization to promote Galphai-dependent planar orientation of the mitotic spindle during asymmetric cell division

Localization and activation of heterotrimeric G proteins have a crucial role during asymmetric cell division. The asymmetric division of the Drosophila sensory precursor cell (pl) is polarized along the antero-posterior axis by Frizzled signalling and, during this division, activation of Galphai depends on Partner of Inscuteable (Pins). We establish here that Ric-8, which belongs to a family of guanine nucleotide-exchange factors for Galphai, regulates cortical localization of the subunits Galphai and Gbeta13F. Ric-8, Galphai and Pins are not necessary for the control of the anteroposterior orientation of the mitotic spindle during pl cell division downstream of Frizzled signalling, but they are required for maintainance of the spindle within the plane of the epithelium. On the contrary, Frizzled signalling orients the spindle along the antero-posterior axis but also tilts it along the apico-basal axis. Thus, Frizzled and heterotrimeric G-protein signalling act in opposition to ensure that the spindle aligns both in the plane of the epithelium and along the tissue polarity axis.