A Drosophila melanogaster homologue of Caenorhabditis elegans par-1 acts at an early step in embryonic-axis formation

Drosophila ensconsin promotes productive recruitment of Kinesin-1 to microtubules

Ensconsin is a conserved microtubule-associated protein (MAP) that interacts dynamically with microtubules, but its cellular function has remained elusive. We show that Drosophila ensconsin is required for all known kinesin-1-dependent processes in the polarized oocyte without detectable effects on microtubules. ensconsin is also required in neurons. Using a single molecule assay for kinesin-1 motility in Drosophila ovary extract, we show that recruitment to microtubules and subsequent motility is severely impaired without ensconsin. Ensconsin protein is enriched at the oocyte anterior and apically in polarized epithelial cells, although required for localization of posterior determinants. Par-1 is required for ensconsin localization and directly phosphorylates it at conserved sites. Our results reveal an unexpected function of a MAP, promoting productive recruitment of a specific motor to microtubules, and an additional level of kinesin regulation. Furthermore, spatial control of motor recruitment can provide additional regulatory control in Par-1 and microtubule-dependent cell polarity.

Par-1 and Tau regulate the anterior-posterior gradient of microtubules in Drosophila oocytes

The formation of an anterior-posterior (AP) gradient of microtubules in Drosophila oocytes is essential for specification of the AP axis. Proper microtubule organization in the oocyte requires the function of serine/threonine kinase Par-1. The N1S isoform of Par-1 is enriched at the posterior cortex of the oocyte from stage 7 of oogenesis. Here we report that posterior restriction of Par-1 (N1S) kinase activity is critical for microtubule AP gradient formation. Egg chambers with excessive and ectopic Par-1 (N1S) kinase activity in the germline cells display phenotypes similar to those of egg chambers treated with the microtubule-depolymerizing drug colcemid: depolymerization of microtubules in the oocyte and disruption of oocyte nucleus localization. A phosphorylation target of Par-1, the microtubule-associated protein Tau, is also involved in oocyte polarity formation, and overexpression of Tau alleviates the phenotypes caused by ectopic Par-1 (N1S) kinase activity, suggesting that Par-1 regulates oocyte polarity at least partly through Tau. Our findings reveal that maintaining proper levels of Par-1 at correct position in the oocyte is key to oocyte polarity formation and that the conserved role of Par-1 and Tau is crucial for the establishment of an AP gradient of microtubules and for AP axis specification.

The actin-binding protein Lasp promotes Oskar accumulation at the posterior pole of the Drosophila embryo

During Drosophila oogenesis, Oskar mRNA is transported to the posterior pole of the oocyte, where it is locally translated and induces germ-plasm assembly. Oskar protein recruits all of the components necessary for the establishment of posterior embryonic structures and of the germline. Tight localization of Oskar is essential, as its ectopic expression causes severe patterning defects. Here, we show that the Drosophila homolog of mammalian Lasp1 protein, an actin-binding protein previously implicated in cell migration in vertebrate cell culture, contributes to the accumulation of Oskar protein at the posterior pole of the embryo. The reduced number of primordial germ cells in embryos derived from lasp mutant females can be rescued only with a form of Lasp that is capable of interacting with Oskar, revealing the physiological importance of the Lasp-Oskar interaction.

PIP5K-dependent production of PIP2 sustains microtubule organization to establish polarized transport in the Drosophila oocyte

The attachment of the cytoskeleton to the plasma membrane is crucial in controlling the polarized transport of cell-fate-determining molecules. Attachment involves adaptor molecules, which have the capacity to bind to both the plasma membrane and elements of the cytoskeleton, such as microtubules and actin filaments. Using the Drosophila oocyte as a model system, we show that the type I phosphatidylinositol 4-phosphate 5-kinase (PIP5K), Skittles, is necessary to sustain the organization of microtubules and actin cytoskeleton required for the asymmetric transport of oskar, bicoid and gurken mRNAs and thereby controls the establishment of cell polarity. We show that Skittles function is crucial to synthesize and maintain phosphatidylinositol 4,5 bisphosphate (PIP2) at the plasma membrane in the oocyte. Reduction of Skittles activity impairs activation at the plasma membrane of Moesin, a member of the ERM family known to link the plasma membrane to the actin-based cytoskeleton. Furthermore, we provide evidence that Skittles, by controlling the localization of Bazooka, Par-1 and Lgl, but not Lkb1, to the cell membrane, regulates PAR polarity proteins and the maintenance of specific cortical domains along the anteroposterior axis.

The JAK/STAT pathway positively regulates DPP signaling in the Drosophila germline stem cell niche

The stem cell niche, formed by surrounding stromal cells, provides extrinsic signals that maintain stem cell self-renewal. However, it remains unclear how these extrinsic signals are regulated. In the Drosophila female germline stem cell (GSC) niche, Decapentaplegic (DPP) is an important niche factor for GSC self-renewal. The exact source of the DPP and how its transcription is regulated in this niche remain unclear. We show that dpp is expressed in somatic cells of the niche including the cap cells, a subtype of niche cells. Furthermore, our data show that the Janus kinase/signal transducer and activator of transcription (JAK/STAT) pathway positively regulates dpp expression in the cap cells, suggesting that JAK/STAT activity is required in somatic niche cells to prevent precocious GSC differentiation. Our data suggest that the JAK/STAT pathway functions downstream/independently of cap cell formation induced by Notch signaling. JAK/STAT signaling may also regulate dpp expression in the male GSC niche, suggesting a common origin of female and male GSC niches.

The PAR proteins: fundamental players in animal cell polarization

The par genes were discovered in genetic screens for regulators of cytoplasmic partitioning in the early embryo of C. elegans, and encode six different proteins required for asymmetric cell division by the worm zygote. Some of the PAR proteins are localized asymmetrically and form physical complexes with one another. Strikingly, the PAR proteins have been found to regulate cell polarization in many different contexts in diverse animals, suggesting they form part of an ancient and fundamental mechanism for cell polarization. Although the picture of how the PAR proteins function remains incomplete, cell biology and biochemistry are beginning to explain how PAR proteins polarize cells.

PAR1 specifies ciliated cells in vertebrate ectoderm downstream of aPKC

Partitioning-defective 1 (PAR1) and atypical protein kinase C (aPKC) are conserved serine/threonine protein kinases implicated in the establishment of cell polarity in many species from yeast to humans. Here we investigate the roles of these protein kinases in cell fate determination in Xenopus epidermis. Early asymmetric cell divisions at blastula and gastrula stages give rise to the superficial (apical) and the deep (basal) cell layers of epidermal ectoderm. These two layers consist of cells with different intrinsic developmental potential, including superficial epidermal cells and deep ciliated cells. Our gain- and loss-of-function studies demonstrate that aPKC inhibits ciliated cell differentiation in Xenopus ectoderm and promotes superficial cell fates. We find that the crucial molecular substrate for aPKC is PAR1, which is localized in a complementary domain in superficial ectoderm cells. We show that PAR1 acts downstream of aPKC and is sufficient to stimulate ciliated cell differentiation and inhibit superficial epidermal cell fates. Our results suggest that aPKC and PAR1 function sequentially in a conserved molecular pathway that links apical-basal cell polarity to Notch signaling and cell fate determination. The observed patterning mechanism may operate in a wide range of epithelial tissues in many species.

Microtubule polarity and axis formation in the Drosophila oocyte

The body axes of the fruit fly are established in mid-oogenesis by the localization of three mRNA determinants, bicoid, oskar, and gurken, within the oocyte. General mechanisms of RNA localization and cell polarization, applicable to many cell types, have emerged from investigation of these determinants in Drosophila oogenesis. Localization of these RNAs is dependent on the germline microtubules, which reorganize to form a polarized array at mid-oogenesis in response to a signaling relay between the oocyte and the surrounding somatic follicle cells. Here we describe what is known about this microtubule reorganization and the signaling relay that triggers it. Recent studies have identified a number of ubiquitous RNA binding proteins essential for this process. So far, no targets for any of these proteins have been identified, and future work will be needed to illuminate how they function to reorganize microtubes and whether similar mechanisms also exist in other cell types.

AMPK links energy status to cell structure and mitosis

AMP-activated protein kinase (AMPK) is known as an important cellular energy sensor, but its in vivo role has not been fully understood. Recent studies provided surprising results that AMPK regulates cell polarity and mitosis under the control of tumour suppressor LKB1. Moreover, these newly found in vivo functions of AMPK are regulated by energy status in a cell autonomous manner. These findings provide novel insights into the physiological function of AMPK and the treatment of AMPK-related diseases such as cancer and diabetes.

The molecular chaperone Hsp90 is required for mRNA localization in Drosophila melanogaster embryos

Localization of maternal nanos mRNA to the posterior pole is essential for development of both the abdominal segments and primordial germ cells in the Drosophila embryo. Unlike maternal mRNAs such as bicoid and oskar that are localized by directed transport along microtubules, nanos is thought to be trapped as it swirls past the posterior pole during cytoplasmic streaming. Anchoring of nanos depends on integrity of the actin cytoskeleton and the pole plasm; other factors involved specifically in its localization have not been described to date. Here we use genetic approaches to show that the Hsp90 chaperone (encoded by Hsp83 in Drosophila) is a localization factor for two mRNAs, nanos and pgc. Other components of the pole plasm are localized normally when Hsp90 function is partially compromised, suggesting a specific role for the chaperone in localization of nanos and pgc mRNAs. Although the mechanism by which Hsp90 acts is unclear, we find that levels of the LKB1 kinase are reduced in Hsp83 mutant egg chambers and that localization of pgc (but not nos) is rescued upon overexpression of LKB1 in such mutants. These observations suggest that LKB1 is a primary Hsp90 target for pgc localization and that other Hsp90 partners mediate localization of nos.

Rab6 mediates membrane organization and determinant localization during Drosophila oogenesis

The Drosophila melanogaster body axes are defined by the precise localization and the restriction of molecular determinants in the oocyte. Polarization of the oocyte during oogenesis is vital for this process. The directed traffic of membranes and proteins is a crucial component of polarity establishment in various cell types and organisms. Here, we investigate the role of the small GTPase Rab6 in the organization of the egg chamber and in asymmetric determinant localization during oogenesis. We show that exocytosis is affected in rab6-null egg chambers, which display a loss of nurse cell plasma membranes. Rab6 is also required for the polarization of the oocyte microtubule cytoskeleton and for the posterior localization of oskar mRNA. We show that, in vivo, Rab6 is found in a complex with Bicaudal-D, and that Rab6 and Bicaudal-D cooperate in oskar mRNA localization. Thus, during Drosophila oogenesis, Rab6-dependent membrane trafficking is doubly required; first, for the general organization and growth of the egg chamber, and second, more specifically, for the polarization of the microtubule cytoskeleton and localization of oskar mRNA. These findings highlight the central role of vesicular trafficking in the establishment of polarity and in determinant localization in Drosophila.

PAR-1 kinase phosphorylates Dlg and regulates its postsynaptic targeting at the Drosophila neuromuscular junction

Targeting of synaptic molecules to their proper location is essential for synaptic differentiation and plasticity. PSD-95/Dlg proteins have been established as key components of the postsynapse. However, the molecular mechanisms regulating the synaptic targeting, assembly, and disassembly of PSD-95/Dlg are not well understood. Here we show that PAR-1 kinase, a conserved cell polarity regulator, is critically involved in controlling the postsynaptic localization of Dlg. PAR-1 is prominently localized at the Drosophila neuromuscular junction (NMJ). Loss of PAR-1 function leads to increased synapse formation and synaptic transmission, whereas overexpression of PAR-1 has the opposite effects. PAR-1 directly phosphorylates Dlg at a conserved site and negatively regulates its mobility and targeting to the postsynapse. The ability of a nonphosphorylatable Dlg to largely rescue PAR-1-induced synaptic defects supports the idea that Dlg is a major synaptic substrate of PAR-1. Control of Dlg synaptic targeting by PAR-1-mediated phosphorylation thus constitutes a critical event in synaptogenesis.

Solution structure of the kinase-associated domain 1 of mouse microtubule-associated protein/microtubule affinity-regulating kinase 3

Microtubule-associated protein/microtubule affinity-regulating kinases (MARKs)/PAR-1 are common regulators of cell polarity that are conserved from nematode to human. All of these kinases have a highly conserved C-terminal domain, which is termed the kinase-associated domain 1 (KA1), although its function is unknown. In this study, we determined the solution structure of the KA1 domain of mouse MARK3 by NMR spectroscopy. We found that approximately 50 additional residues preceding the previously defined KA1 domain are required for its proper folding. The newly defined KA1 domain adopts a compact alpha+beta structure with a betaalphabetabetabetabetaalpha topology. We also found a characteristic hydrophobic, concave surface surrounded by positively charged residues. This concave surface includes the highly conserved Glu-Leu-Lys-Leu motif at the C terminus, indicating that it is important for the function of the KA1 domain.

An oskar-dependent positive feedback loop maintains the polarity of the Drosophila oocyte

The localization of oskar mRNA to the posterior of the Drosophila oocyte defines the site of assembly of the pole plasm, which contains the abdominal and germline determinants 1, 2, 3. oskar mRNA localization requires the polarization of the microtubule cytoskeleton, which depends on the recruitment of PAR-1 to the posterior cortex in response to a signal from the follicle cells, where it induces an enrichment of microtubule plus ends 4, 5, 6, 7. Here, we show that overexpressed oskar mRNA localizes to the middle of the oocyte, as well as the posterior. This ectopic localization depends on the premature translation of Oskar protein, which recruits PAR-1 and microtubule-plus-end markers to the oocyte center instead of the posterior pole, indicating that Oskar regulates the polarity of the cytoskeleton. Oskar also plays a role in the normal polarization of the oocyte; mutants that disrupt oskar mRNA localization or translation strongly reduce the posterior recruitment of microtubule plus ends. Thus, oskar mRNA localization is required to stabilize and amplify microtubule polarity, generating a positive feedback loop in which Oskar recruits PAR-1 to the posterior to increase the microtubule cytoskeleton's polarization, which in turn directs the localization of more oskar mRNA.

Extending from PARs in Caenorhabditis elegans to homologues in Haemonchus contortus and other parasitic nematodes

Signal transduction molecules play key roles in the regulation of developmental processes, such as morphogenesis, organogenesis and cell differentiation in all organisms. They are organized into 'pathways' that represent a coordinated network of cell-surface receptors and intracellular molecules, being involved in sensing environmental stimuli and transducing signals to regulate or modulate cellular processes, such as gene expression and cytoskeletal dynamics. A particularly important group of molecules implicated in the regulation of the cytoskeleton for the establishment and maintenance of cell polarity is the PAR proteins (derived from partition defective in asymmetric cell division). The present article reviews salient aspects of PAR proteins involved in the early embryonic development and morphogenesis of the free-living nematode Caenorhabditis elegans and some other organisms, with an emphasis on the molecule PAR-1. Recent advances in the knowledge and understanding of PAR-1 homologues from the economically important parasitic nematode, Haemonchus contortus, of small ruminants is summarized and discussed in the context of exploring avenues for future research in this area for parasitic nematodes.

A translation-independent role of oskar RNA in early Drosophila oogenesis

The Drosophila maternal effect gene oskar encodes the posterior determinant responsible for the formation of the posterior pole plasm in the egg, and thus of the abdomen and germline of the future fly. Previously identified oskar mutants give rise to offspring that lack both abdominal segments and a germline, thus defining the ;posterior group phenotype'. Common to these classical oskar alleles is that they all produce significant amounts of oskar mRNA. By contrast, two new oskar mutants in which oskar RNA levels are strongly reduced or undetectable are sterile, because of an early arrest of oogenesis. This egg-less phenotype is complemented by oskar nonsense mutant alleles, as well as by oskar transgenes, the protein-coding capacities of which have been annulled. Moreover, we show that expression of the oskar 3' untranslated region (3'UTR) is sufficient to rescue the egg-less defect of the RNA null mutant. Our analysis thus reveals an unexpected role for oskar RNA during early oogenesis, independent of Oskar protein. These findings indicate that oskar RNA acts as a scaffold or regulatory RNA essential for development of the oocyte.

Metastasis-associated kinase modulates Wnt signaling to regulate brain patterning and morphogenesis

Wnt signaling is a major pathway regulating cell fate determination, cell proliferation and cell movements in vertebrate embryos. Distinct branches of this pathway activate beta-catenin/TCF target genes and modulate morphogenetic movements in embryonic tissues by reorganizing the cytoskeleton. The selection of different molecular targets in the pathway is driven by multiple phosphorylation events. Here, we report that metastasis-associated kinase (MAK) is a novel regulator of Wnt signaling during morphogenetic movements, and eye and brain development in Xenopus embryos. Injected MAK RNA suppressed Wnt transcriptional reporters and activated Jun N-terminal kinase. Furthermore, MAK was recruited to the cell membrane by Frizzled 3, formed a complex with Dishevelled and phosphorylated Dsh in vitro. The regional brain markers Otx2, En2 and Gbx2 were affected in embryos with modulated MAK activity in a manner consistent with a role for MAK in midbrain-hindbrain boundary formation. Confirming the inhibitory role for this kinase in Wnt/beta-catenin signaling, the midbrain patterning defects in embryos depleted of MAK were rescued by the simultaneous depletion of beta-catenin. These findings indicate that MAK may function in different developmental processes as a switch between the canonical and non-canonical branches of Wnt signaling.

The C. elegans MELK ortholog PIG-1 regulates cell size asymmetry and daughter cell fate in asymmetric neuroblast divisions

In the nematode Caenorhabditis elegans, neurons are generated from asymmetric divisions in which a mother cell divides to produce daughters that differ in fate. Here, we demonstrate that the gene pig-1 regulates the asymmetric divisions of neuroblasts that divide to produce an apoptotic cell and either a neural precursor or a neuron. In pig-1 mutants, these neuroblasts divide to produce daughters that are more equal in size, and their apoptotic daughters are transformed into their sisters, leading to the production of extra neurons. PIG-1 is orthologous to MELK, a conserved member of the polarity-regulating PAR-1/Kin1/SAD-1 family of serine/threonine kinases. Although MELK has been implicated in regulating the cell cycle, our data suggest that PIG-1, like other PAR-1 family members, regulates cell polarity.

Drosophila Anterior-Posterior Polarity Requires Actin-Dependent PAR-1 Recruitment to the Oocyte Posterior

The Drosophila anterior-posterior axis is established at stage 7 of oogenesis when the posterior follicle cells signal to polarize the oocyte microtubule cytoskeleton. This requires the conserved PAR-1 kinase, which can be detected at the posterior of the oocyte in immunostainings from stage 9. However, this localization depends on Oskar localization, which requires the earlier PAR-1-dependent microtubule reorganization, indicating that Oskar-associated PAR-1 cannot establish oocyte polarity. Here we analyze the function of the different PAR-1 isoforms and find that only PAR-1 N1 isoforms can completely rescue the oocyte polarity phenotype. Furthermore, PAR-1 N1 is recruited to the posterior cortex of the oocyte at stage 7 in response to the polarizing follicle cell signal, and this requires actin, but not microtubules. This suggests that posterior PAR-1 N1 polarizes the microtubule cytoskeleton. PAR-1 N1 localization is mediated by a cortical targeting domain and a conserved anterior-lateral exclusion signal in its C-terminal linker domain. PAR-1 is also required for the polarization of the C. elegans zygote and is recruited to the posterior cortex in an actin-dependent manner. Our results therefore identify a molecular parallel between axis formation in Drosophila and C. elegans and make Drosophila PAR-1 N1 the earliest known marker for the polarization of the oocyte.

Haemonchus contortus: prokaryotic expression and enzyme activity of recombinant HcSTK, a serine/threonine protein kinase

Members of the PAR-1/MARK serine/threonine protein kinase (STK) subfamily are important regulators of the cytoskeleton, and their characterization can provide insights into a number of critical processes relating to the development and survival of an organism. We previously investigated the mRNA expression for and organization of a gene (hcstk) representing HcSTK, an STK from the parasitic nematode Haemonchus contortus. In the present study, a recombinant form of HcSTK was expressed and characterized. Affinity-purified anti-HcSTK antibodies reacted with native HcSTK in protein homogenates extracted from third-stage larvae (L3) of H. contortus and were also used to immunolocalize the protein around the nuclei of ovarian and intestinal tissues of adult H. contortus. The enzyme activity of the recombinant HcSTK protein was also demonstrated. The findings show that recombinant HcSTK is a functional protein kinase, with activity directed to KXGS motifs, consistent with other members of the PAR-1/MARK STK subfamily.

Par-1 kinase establishes cell polarity and functions in Notch signaling in theDrosophilaembryo

The Drosophila protein kinase Par-1 is expressed throughout Drosophila development, but its function has not been extensively characterized because of oocyte lethality of null mutants. In this report, we have characterized the function of Par-1 in embryonic and post-embryonic epithelia. Par-1 protein is dynamically localized during embryonic cell polarization, transiently restricted to the lateral membrane domain, followed by apicolateral localization. We depleted maternal and zygotic par-1 by RNAi and revealed a requirement for Par-1 in establishing cell polarity. Par-1 restricts the coalescing adherens junction to an apicolateral position and prevents its widespread formation along the lateral domain. Par-1 also promotes the localization of lateral membrane proteins such as Delta. These activities are important for the further development of cell polarity during gastrulation. By contrast, Par-1 is not essential to maintain epithelial polarity once it has been established. However, it still has a maintenance role since overexpression causes severe polarity disruption. Additionally, we find a novel role for Par-1 in Notch signal transduction during embryonic neurogenesis and retina determination. Epistasis analysis indicates that Par-1 functions upstream of Notch and is critical for proper localization of the Notch ligand Delta.

Structure of the Catalytic and Ubiquitin-Associated Domains of the Protein Kinase MARK/Par-1

The Ser/Thr kinase MARK2 phosphorylates tau protein at sites that cause detachment from microtubules in Alzheimer neurofibrillary degeneration. Homologs of MARK2 include Par-1 in C. elegans and Drosophila, which generates embryonic polarity. We report the X-ray structure of the catalytic and ubiquitin-associated domains (UBA) of human MARK2. The activity was altered by mutations in the ATP binding site and/or activation loop. The catalytic domain shows the small and large lobes typical of kinases. The substrate cleft is in an inactive, open conformation in the inactivated and the wild-type structure. The UBA domain is attached via a taut linker to the large lobe of the kinase domain and leans against a hydrophobic patch on the small lobe. The UBA structure is unusual because the orientation of its third helix is inverted, relative to previous structures. Possible implications of the structure for the regulation of kinase activity are discussed.

BicD-dependent localization processes: from Drosophilia development to human cell biology

Many eukaryotic cells depend on proper cell polarization for their development and physiological function. The establishment of these polarities often involve the subcellular localization of a specific subset of proteins, RNAs and organelles. In Drosophila, the microtubule-dependent BicD (BicaudalD) localization machinery is involved in the proper localization of mRNA during oogenesis and embryogenesis and the proper positioning of the oocyte and photoreceptor nuclei. BicD acts together with the minus-end directed motor dynein as well as Egl and Lis-1. The finding that the mammalian homologs of BicD function in retrograde Golgi-to-ER transport has supported the view that BicD may be part of a repeatedly used and evolutionary conserved localization machinery. In this review we focus on the various processes in which BicD is involved during Drosophilian development and in mammals. In addition, we evaluate the interactions between BicD, the dynein localization machinery and associated factors.

Microtubule affinity-regulating kinase 1 (MARK1) is activated by electroconvulsive shock in the rat hippocampus

Electroconvulsive shock (ECS) induces phosphorylation and dephosphorylation of many signaling molecules in the rat brain. While studying phosphorylated proteins in the rat brain after ECS, we observed a 100-kDa protein that cross-reacted with anti-phospho-p70 S6 kinase antibody, which was subsequently purified and identified as microtubule affinity-regulating kinase 1 (MARK1). Purified MARK1 was phosphorylated at the Ser and Thr residues. MARK1 activation and subsequent Tau phosphorylation in the hippocampus after ECS was confirmed by an in-gel kinase assay using tau protein as a substrate. MARK1 was maximally activated between 2 and 5 min after ECS, and Tau phosphorylation at Ser262 was also increased at 2 min and lasted to 1 h after ECS. Taken together, we concluded that ECS activated MARK1 and subsequently phosphorylated Tau at Ser262. Both MARK1 activity and Tau phosphorylation were increased in the rat hippocampus after chronic ECS where axonal remodeling was apparent. In order to investigate the physiologic stimuli which are involved in the activation of MARK1, SH-SY 5Y cells were treated with brain-derived neurotrophic factor or 60 mm KCl. Both stimuli were capable of inducing MARK activation.

Understanding and treating neurodegeneration: insights from the flies

Drosophila has recently emerged as a model system for studying mechanisms of neurodegeneration. Genetic models for most of the major neurodegenerative diseases, including Alzheimer's disease (AD), Parkinson's disease (PD), polyglutamine diseases, and tauopathies, have been successfully established. Pharmacological models of some of these diseases have also been created. Genetic modifier screens using these models have uncovered previously implicated mechanisms and molecules as well as novel ones. Fly models have turned out to be excellent system for the in vivo testing of therapeutic potentials of candidate compounds. It is anticipated that further exploration of the fly models will not only provide novel insights into mechanisms of neurodegeneration but also lead to the development of rational treatment of those debilitating degenerative diseases.

Distinct PAR-1 Proteins Function in Different Branches of Wnt Signaling during Vertebrate Development

The kinase PAR-1 plays conserved roles in cell polarity. PAR-1 has also been implicated in axis establishment in C. elegans and Drosophila and in Wnt signaling, but its role in vertebrate development is unclear. Here we report that PAR-1 has two distinct and essential roles in axial development in Xenopus mediated by different PAR-1 isoforms. Depletion of PAR-1A or PAR-1BX causes dorsoanterior deficits, reduced Spemann organizer gene expression, and inhibition of canonical Wnt-beta-catenin signaling. By contrast, PAR-1BY depletion inhibits cell movements and localization of Dishevelled protein to the cell cortex, processes associated with noncanonical Wnt signaling. PAR-1 phosphorylation sites in Dishevelled are required for this translocation, but not for canonical Wnt signaling. We conclude that PAR-1BY is required in the PCP branch and mediates Dsh membrane localization while PAR-1A and PAR-1BX are essential for canonical signaling to beta-catenin, possibly via targets other than Dishevelled.

Asymmetric distribution of PAR proteins in the mouse embryo begins at the 8-cell stage during compaction

In many organisms, like Caenorhabditis elegans and Drosophila melanogaster, establishment of spatial patterns and definition of cell fate are driven by the segregation of determinants in response to spatial cues, as early as oogenesis or fertilization. In these organisms, a family of conserved proteins, the PAR proteins, is involved in the asymmetric distribution of cytoplasmic determinants and in the control of asymmetric divisions. In the mouse embryo, it is only at the 8-cell stage during compaction that asymmetries, leading to cellular diversification and blastocyst morphogenesis, are first observed. However, it has been suggested that developmentally relevant asymmetries could be established already in the oocyte and during fertilization. This led us to study the PAR proteins during the early stages of mouse development. We observed that the homologues of the different members of the PAR/aPKC complex and PAR1 are expressed in the preimplantation mouse embryo. During the first embryonic cleavages, before compaction, PARD6b and EMK1 are observed on the spindle. The localization of these two proteins becomes asymmetric during compaction, when blastomeres flatten upon each other and polarize. PARD6b is targeted to the apical pole, whereas EMK1 is distributed along the baso-lateral domain. The targeting of EMK1 is dependent upon cell-cell interactions while the apical localization of PARD6b is independent of cell contacts. At the 16-cell stage, aPKCzeta colocalizes with PARD6b and a colocalization of the three proteins (PARD6b/PARD3/aPKCzeta can occur in blastocysts, only at tight junctions. This choreography suggests that proteins of the PAR family are involved in the setting up of blastomere polarity and blastocyst morphogenesis in the early mammalian embryo although the interactions between the different players differ from previously studied systems. Finally, they reinforce the idea that the first developmentally relevant asymmetries are set up during compaction.

The Drosophila PAR-1 spacer domain is required for lateral membrane association and for polarization of follicular epithelial cells

The Ser/Thr kinases of the PAR-1/MARK/Kin1 family are conserved regulators of polarity in epithelial and non-epithelial cells . Drosophila PAR-1 localizes laterally in the follicular epithelium of the ovary , where it has been shown to function at two distinct levels: It stabilizes the cytoskeleton and it regulates apical-basal polarity by directly inhibiting lateral assembly of the apical aPKC/Bazooka/PAR-6 complex . However, it has been unclear how lateral localization of Drosophila PAR-1 is achieved and whether this localization contributes to epithelial polarity in vivo. Here we show that, through its spacer domain, Drosophila PAR-1 accumulates on the lateral plasma membrane (PM) in cells of the follicular epithelium (FE). Rescue experiments indicate that in FE cells PAR-1 kinase activity is essential for all the described functions of PAR-1. In contrast, the spacer domain of PAR-1 is required for apical-basal polarity and growth control but is dispensable for microtubule (MT) stabilization. Our data indicate that the spacer domain of PAR-1 is required for lateral PM localization of PAR-1 kinase and for development of a polarized FE.

Signaling pathways in intestinal development and cancer

The study of the epithelium of the adult mammalian intestine touches upon many modern aspects of biology. The epithelium is in a constant dialogue with the underlying mesenchyme to control stem cell activity, proliferation in transit-amplifying compartments, lineage commitment, terminal differentiation and, ultimately, cell death. There are spatially distinct compartments dedicated to each of these events. The Wnt, TGF-beta, BMP, Notch, and Par polarity pathways are the major players in homeostatic control of the adult epithelium. Several hereditary cancer syndromes deregulate these same signaling cascades through mutational (in)activation. Moreover, these mutations often also occur in sporadic tumors. Thus symmetry exists between the roles that these signaling pathways play in physiology and in cancer of the intestine. This is particularly evident for the Wnt/APC pathway, for which the mammalian intestine has become one of the most-studied paradigms. Here, we integrate recent knowledge of the molecular inner workings of the prototype signaling cascades with their specific roles in intestinal epithelial homeostasis and in neoplastic transformation of the epithelium.

The yeast par-1 homologs kin1 and kin2 show genetic and physical interactions with components of the exocytic machinery

Kin1 and Kin2 are Saccharomyces cerevisiae counterparts of Par-1, the Caenorhabditis elegans kinase essential for the establishment of polarity in the one cell embryo. Here, we present evidence for a novel link between Kin1, Kin2, and the secretory machinery of the budding yeast. We isolated KIN1 and KIN2 as suppressors of a mutant form of Rho3, a Rho-GTPase acting in polarized trafficking. Genetic analysis suggests that KIN1 and KIN2 act downstream of the Rab-GTPase Sec4, its exchange factor Sec2, and several components of the vesicle tethering complex, the Exocyst. We show that Kin1 and Kin2 physically interact with the t-SNARE Sec9 and the Lgl homologue Sro7, proteins acting at the final stage of exocytosis. Structural analysis of Kin2 reveals that its catalytic activity is essential for its function in the secretory pathway and implicates the conserved 42-amino acid tail at the carboxy terminal of the kinase in autoinhibition. Finally, we find that Kin1 and Kin2 induce phosphorylation of t-SNARE Sec9 in vivo and stimulate its release from the plasma membrane. In summary, we report the finding that yeast Par-1 counterparts are associated with and regulate the function of the exocytic apparatus via phosphorylation of Sec9.

Genomic organization and expression analysis for hcstk, a serine/threonine protein kinase gene of Haemonchus contortus, and comparison with Caenorhabditis elegans par-1

The organization and expression of a putative serine/threonine kinase gene (designated hcstk), proposed to relate to a conserved eukaryotic signal transduction pathway, was characterized for the socio-economically important pathogen Haemonchus contortus (Nematoda). The entire hcstk gene is approximately 26.7 kb in size, has 26 exons and is inferred to produce multiple isoforms via alternative splicing in its N-terminal header and spacer domains. Comparison of hcstk with its Caenorhabditis elegans homologue, par-1, revealed major differences in genomic organization, exon number and inferred mRNA processing. The expression of hcstk transcripts was highest in the first- and late-fourth-stage larvae of the parasite compared with other developmental stages, somewhat distinct from par-1 in C. elegans. In spite of a substantial amino acid sequence identity in the functional domains between the predicted proteins HcSTK and PAR-1, overall, the findings suggest a unique functional role for each molecule.

LKB1 tumor suppressor protein: PARtaker in cell polarity

The LKB1 (also called serine/threonine kinase 11) tumor suppressor gene was cloned in 1998 by linkage analysis of Peutz-Jeghers cancer syndrome patients. Mammalian LKB1 has been implicated as a regulator of multiple biological processes and signaling pathways, including the control of cell-cycle arrest, p53-mediated apoptosis, Wnt signaling, transforming growth factor (TGF)-beta signaling, ras-induced cell transformation, and energy metabolism. The Caenorhabditis elegans and Drosophila melanogaster LKB1 homologs, termed PAR4 and dLKB1, respectively, regulate cell polarity. Recently, mammalian LKB1 was found to be active only in a complex with two other proteins--STRAD and MO25--and to induce complete polarization of intestinal epithelial cells in a cell-autonomous fashion. In this article, we summarize the findings regarding LKB1 over the past six years. In addition, we discuss LKB1 in polarity in the context of both the other PAR proteins and its tumor suppressive activities.

Par-1 regulates bicoid mRNA localisation by phosphorylating Exuperantia

The Ser/Thr kinase Par-1 is required for cell polarisation in diverse organisms such as yeast, worms, flies and mammals. During Drosophilaoogenesis, Par-1 is required for several polarisation events, including localisation of the anterior determinant bicoid. To elucidate the molecular pathways triggered by Par-1, we have performed a genome-wide,high-throughput screen for Par-1 targets. Among the targets identified in this screen was Exuperantia (Exu), a mediator of bicoid mRNA localisation. We show that Exu is a phosphoprotein whose phosphorylation is dependent on Par-1 in vitro and in vivo. We identify two motifs in Exu that are phosphorylated by Par-1, and show that their mutation abolishes bicoid mRNA localisation during mid-oogenesis. Interestingly, exu mutants in which Exu phosphorylation is specifically affected can to some extent recover from these bicoid mRNA localisation defects during late oogenesis. These results demonstrate that Par-1 establishes polarity in the oocyte by activating a mediator of bicoid mRNA localisation. Furthermore, our analysis reveals two phases of Exu-dependent bicoid mRNA localisation: an early phase that is strictly dependent on Exu phosphorylation and a late phase that is less phosphorylation dependent.

Cryptococcus neoformans Kin1 protein kinase homologue, identified through a Caenorhabditis elegans screen, promotes virulence in mammals

Cryptococcal infections are a global cause of significant morbidity and mortality. Recent studies support the hypothesis that virulence of Cryptococcus neoformans may have evolved via survival selection in environmental hosts, such as amoebae and free-living nematodes. We used killing of the nematode Caenorhabditis elegans by C. neoformans as an assay to screen a library of random C. neoformans insertion mutants. Of 350 mutants tested, seven were identified with attenuated virulence that persisted after crossing the mutation back into a wild-type strain. Genetic analysis of one strain revealed an insertion in a gene homologous to Saccharomyces cerevisiae KIN1, which encodes a serine/threonine protein kinase. C. neoformans kin1 mutants exhibited significant defects in virulence in murine inhalation and haematogenous infection models and displayed increased binding to alveolar and peritoneal macrophages. The kin1 mutant phenotypes were complemented by the wild-type KIN1 gene. These findings show that the C. neoformans Kin1 kinase homologue is required for full virulence in disparate hosts and that C. elegans can be used as a substitute host to identify novel factors involved in fungal pathogenesis in mammals.

aPKC Acts Upstream of PAR-1b in Both the Establishment and Maintenance of Mammalian Epithelial Polarity

BACKGROUND

aPKC and PAR-1 are required for cell polarity in various contexts. In mammalian epithelial cells, aPKC localizes at tight junctions (TJs) and plays an indispensable role in the development of asymmetric intercellular junctions essential for the establishment and maintenance of apicobasal polarity. On the other hand, one of the mammalian PAR-1 kinases, PAR-1b/EMK1/MARK2, localizes to the lateral membrane in a complimentary manner with aPKC, but little is known about its role in apicobasal polarity of epithelial cells as well as its functional relationship with aPKC.

RESULTS

We demonstrate that PAR-1b is essential for the asymmetric development of membrane domains of polarized MDCK cells. Nonetheless, it is not required for the junctional localization of aPKC nor the formation of TJs, suggesting that PAR-1b works downstream of aPKC during epithelial cell polarization. On the other hand, aPKC phosphorylates threonine 595 of PAR-1b and enhances its binding with 14-3-3/PAR-5. In polarized MDCK cells, T595 phosphorylation and 14-3-3 binding are observed only in the soluble form of PAR-1b, and okadaic acid treatment induces T595-dependent dissociation of PAR-1b from the lateral membrane. Furthermore, T595A mutation induces not only PAR-1b leakage into the apical membrane, but also abnormal development of membrane domains. These results suggest that in polarized epithelial cells, aPKC phosphorylates PAR-1b at TJs, and in cooperation with 14-3-3, promotes the dissociation of PAR-1b from the lateral membrane to regulate PAR-1b activity for the membrane domain development.

CONCLUSIONS

These results suggest that mammalian aPKC functions upstream of PAR-1b in both the establishment and maintenance of epithelial cell polarity.

A clean start: degradation of maternal proteins at the oocyte-to-embryo transition

In many organisms, the transition from oocyte to embryo occurs in the absence of mRNA transcription. Therefore, early developmental programs rely on maternal mRNAs and proteins that are synthesized during oogenesis. The regulated translation of maternal RNAs is essential for the proper deployment of regulatory factors during early embryogenesis. Recent studies suggest that the degradation of maternal proteins by the ubiquitin-proteasome pathway is also crucial for the oocyte-to-embryo transition. In this article, we explore the hypothesis that the coordinated degradation of germline proteins is essential for remodeling the oocyte into a totipotent zygote that is capable of somatic development.

Hrp48, a Drosophila hnRNPA/B homolog, binds and regulates translation of oskar mRNA

Establishment of the Drosophila embryonic axes provides a striking example of RNA localization as an efficient mechanism for protein targeting within a cell. oskar mRNA encodes the posterior determinant and is essential for germline and abdominal development in the embryo. Tight restriction of Oskar activity to the posterior is achieved by mRNA localization-dependent translational control, whereby unlocalized mRNA is translationally repressed and repression is overcome upon mRNA localization. Here we identify the previously reported oskar RNA binding protein p50 as Hrp48, an abundant Drosophila hnRNP. Analysis of three hrp48 mutant alleles reveals that Hrp48 levels are crucial for polarization of the oocyte during mid-oogenesis. Our data also show that Hrp48, which binds to the 5' and 3' regions of oskar mRNA, plays an important role in restricting Oskar activity to the posterior of the oocyte, by repressing oskar mRNA translation during transport.

PKA-R1 spatially restricts Oskar expression for Drosophila embryonic patterning

Targeting proteins to specific domains within the cell is central to the generation of polarity, which underlies many processes including cell fate specification and pattern formation during development. The anteroposterior and dorsoventral axes of the Drosophila melanogaster embryo are determined by the activities of localized maternal gene products. At the posterior pole of the oocyte, Oskar directs the assembly of the pole plasm, and is thus responsible for formation of abdomen and germline in the embryo. Tight restriction of oskar activity is achieved by mRNA localization, localization-dependent translation, anchoring of the RNA and protein, and stabilization of Oskar at the posterior pole. Here we report that the type 1 regulatory subunit of cAMP-dependent protein kinase (Pka-R1) is crucial for the restriction of Oskar protein to the oocyte posterior. Mutations in PKA-R1 cause premature and ectopic accumulation of Oskar protein throughout the oocyte. This phenotype is due to misregulation of PKA catalytic subunit activity and is suppressed by reducing catalytic subunit gene dosage. These data demonstrate that PKA mediates the spatial restriction of Oskar for anteroposterior patterning of the Drosophila embryo and that control of PKA activity by PKA-R1 is crucial in this process.

Polarisation des oeufs et des embryons : principes communs

Embryonic development depends on the establishment of polarities which define the axial characteristics of the body. In a small number of cases such as the embryo of the fly drosophila, developmental axes are established well before fertilization while in other organisms such as the nematode worm C. elegans these axes are set up only after fertilization. In most organisms the egg posesses a primary (A-V, Animal-Vegetal) axis acquired during oogenesis which participates in the establishment of the embryonic axes. Such is the case for the eggs of ascidians or the frog Xenopus whose AV axes are remodelled by sperm entry to yield the embryonic axes. Embryos of different species thus acquire an anterior end and a posterior end (Antero-Posterior, A-P axis), dorsal and ventral sides (D-V axis) and then a left and a right side.

Cloning and developmental expression of MARK/Par-1/MELK-related protein kinase xMAK-V in Xenopus laevis

MAK-V/Hunk is a recently identified MARK/Par-1-related mammalian protein kinase. Although the precise function of this protein kinase is yet to be established, available data suggest its involvement in animals' development and in the physiology of the nervous system. Here we report characterization of a cDNA encoding Xenopus laevis orthologue of MAK-V/Hunk protein kinase, xMAK-V. The in silico analysis also revealed MAK-V/Hunk orthologues in the fish Fugu rubripes and primitive chordate Ciona intestinalis but not in invertebrate species such as Drosophila melanogaster and Caenorhabditis elegans, suggesting that MAK-V/Hunk is a chordate-specific protein kinase. The expression of xmak-v in X. laevis embryos was analyzed using whole-mount in situ hybridization. Expression of xmak-v has been detected in all developmental stages studied including maternal expression in unfertilized eggs. The xmak-v mRNA has a predominant occurrence on the animal hemisphere of the egg, and this pattern of expression is sustained throughout cleavage and blastula stages. At the gastrula stage xmak-v expression is restricted to the ectoderm. In the later stage embryos xmak-v is expressed over the entire embryonic surface including the open neural plate at stage 15 and also in neural tube at stage 22. At tadpole stage xmak-v expression is strong in embryonic epidermis, nervous system and sensory organs, and is also obvious in perisomitic mesoderm and brachial arches.

PAR-1 Kinase Plays an Initiator Role in a Temporally Ordered Phosphorylation Process that Confers Tau Toxicity in Drosophila

Multisite hyperphosphorylation of tau has been implicated in the pathogenesis of neurodegenerative diseases including Alzheimer's disease (AD). However, the phosphorylation events critical for tau toxicity and mechanisms regulating these events are largely unknown. Here we show that Drosophila PAR-1 kinase initiates tau toxicity by triggering a temporally ordered phosphorylation process. PAR-1 directly phosphorylates tau at S262 and S356. This phosphorylation event is a prerequisite for the action of downstream kinases, including glycogen synthase kinase 3 (GSK-3) and cyclin-dependent kinase-5 (Cdk5), to phosphorylate several other sites and generate disease-associated phospho-epitopes. The initiator role of PAR-1 is further underscored by the fact that mutating PAR-1 phosphorylation sites causes a much greater reduction of overall tau phosphorylation and toxicity than mutating S202, one of the downstream sites whose phosphorylation depends on prior PAR-1 action. These findings begin to differentiate the effects of various phosphorylation events on tau toxicity and provide potential therapeutic targets.

Roles of Bifocal, Homer, and F-actin in anchoring Oskar to the posterior cortex of Drosophila oocytes

Transport, translation, and anchoring of osk mRNA and proteins are essential for posterior patterning of Drosophila embryos. Here we show that Homer and Bifocal act redundantly to promote posterior anchoring of the osk gene products. Disruption of actin microfilaments, which causes delocalization of Bifocal but not Homer from the oocyte cortex, severely disrupts anchoring of osk gene products only when Homer (not Bifocal) is absent. Our data suggest that two processes, one requiring Bifocal and an intact F-actin cytoskeleton and a second requiring Homer but independent of intact F-actin, may act redundantly to mediate posterior anchoring of the osk gene products.

Mammalian PAR-1 determines epithelial lumen polarity by organizing the microtubule cytoskeleton

Epithelial differentiation involves the generation of luminal surfaces and of a noncentrosomal microtubule (MT) network aligned along the polarity axis. Columnar epithelia (e.g., kidney, intestine, and Madin-Darby canine kidney [MDCK] cells) generate apical lumina and orient MT vertically, whereas liver epithelial cells (hepatocytes and WIFB9 cells) generate lumina at cell–cell contact sites (bile canaliculi) and orient MTs horizontally. We report that knockdown or inhibition of the mammalian orthologue of Caenorhabditis elegans Par-1 (EMK1 and MARK2) during polarization of cultured MDCK and WIFB9 cells prevented development of their characteristic lumen and nonradial MT networks. Conversely, EMK1 overexpression induced the appearance of intercellular lumina and horizontal MT arrays in MDCK cells, making EMK1 the first known candidate to regulate the developmental branching decision between hepatic and columnar epithelial cells. Our experiments suggest that EMK1 primarily promotes reorganization of the MT network, consistent with the MT-regulating role of this gene product in other systems, which in turn controls lumen formation and position.

Drosophila PAR-1 and 14-3-3 inhibit Bazooka/PAR-3 to establish complementary cortical domains in polarized cells

PAR-1 kinases are required for polarity in diverse cell types, such as epithelial cells, where they localize laterally. PAR-1 activity is believed to be transduced by binding of 14-3-3 proteins to its phosphorylated substrates, but the relevant targets are unknown. We show that PAR-1 phosphorylates Bazooka/PAR-3 on two conserved serines to generate 14-3-3 binding sites. This inhibits formation of the Bazooka/PAR-6/aPKC complex by blocking Bazooka oligomerization and binding to aPKC. In epithelia, this complex localizes apically and defines the apical membrane, whereas Bazooka lacking PAR-1 phosphorylation/14-3-3 binding sites forms ectopic lateral complexes. Lateral exclusion by PAR-1/14-3-3 cooperates with apical anchoring by Crumbs/Stardust to restrict Bazooka localization, and loss of both pathways disrupts epithelial polarity. PAR-1 also excludes Bazooka from the posterior of the oocyte, and disruption of this regulation causes anterior-posterior polarity defects. Thus, antagonism of Bazooka by PAR-1/14-3-3 may represent a general mechanism for establishing complementary cortical domains in polarized cells.

The protein kinase kin1, the fission yeast orthologue of mammalian MARK/PAR-1, localises to new cell ends after mitosis and is important for bipolar growth

The kin1 protein kinase of the fission yeast Schizosaccharomyces pombe is a member of the PAR-1/MARK (partitioning-defective 1/microtubule-associated protein/microtubule affinity-regulating kinase) family important in eukaryotic cell polarity and cytoskeletal dynamics. We show here that kin1 plays a role in establishing the characteristic rod-shaped morphology of fission yeast. Cells in which kin1 was deleted are viable but are impaired in growth, and are rounded at one end or both ends. They are monopolar because after mitosis they fail to activate bipolar growth, and are delayed in cytokinesis, resulting in a high proportion of septated cells often with multiple septa. This phenotype can be partially rescued by heterologous expression of human MARKs, which restore bipolar growth in most cells, but do not correct the delay in cytokinesis. Using chromosomal epitope tagging, we show that kin1p localises to the cell ends, except during mitosis when it disappears from cell ends. After mitosis, kin1p first reappears at the new cell end. Overexpression of kin1 results in a loss of polarity, with partially or fully rounded cells. From these results we suggest that kin1 is required to direct the growth machinery to the cell ends.

MARKK, a Ste20-like kinase, activates the polarity-inducing kinase MARK/PAR-1

MARK, a kinase family related to PAR-1 involved in establishing cell polarity, phosphorylates microtubule-associated proteins (tau/MAP2/MAP4) at KXGS motifs, causes detachment from microtubules, and their disassembly. The sites are prominent in tau from Alzheimer's disease brains. We studied the activation of MARK and identified the upstream kinase, MARKK, a member of the Ste20 kinase family. It phosphorylates MARK within the activation loop (T208 in MARK2). A fraction of MARK in brain tissue is doubly phosphorylated (at T208/S212), reminiscent of the activation of MAP kinase; however, the phosphorylation of the second site in MARK (S212) is inhibitory. In cells the activity of MARKK enhances microtubule dynamics through the activation of MARK and leads to phosphorylation and detachment of tau or equivalent MAPs from microtubules. Overexpression of MARK eventually leads to microtubule breakdown and cell death, but in neuronal cells the primary effect is to allow the development of neurites during differentiation.

MARK4 is a novel microtubule-associated proteins/microtubule affinity-regulating kinase that binds to the cellular microtubule network and to centrosomes

The MARK protein kinases were originally identified by their ability to phosphorylate a serine motif in the microtubule-binding domain of tau that is critical for microtubule binding. Here, we report the cloning and expression of a novel human paralog, MARK4, which shares 75% overall homology with MARK1-3 and is predominantly expressed in brain. Homology is most pronounced in the catalytic domain (90%), and MARK4 readily phosphorylates tau and the related microtubule-associated protein 2 (MAP2) and MAP4. In contrast to the three paralogs that all exhibit uniform cytoplasmic localization, MARK4 colocalizes with the centrosome and with microtubules in cultured cells. Overexpression of MARK4 causes thinning out of the microtubule network, concomitant with a reorganization of microtubules into bundles. In line with these findings, we show that a tandem affinity-purified MARK4 protein complex contains alpha-, beta-, and gamma-tubulin. In differentiated neuroblastoma cells, MARK4 is localized prominently at the tips of neurite-like processes. We suggest that although the four MARK/PAR-1 kinases might play multiple cellular roles in concert with different targets, MARK4 is likely to be directly involved in microtubule organization in neuronal cells and may contribute to the pathological phosphorylation of tau in Alzheimer's disease.

The role of PAR-1 in regulating the polarised microtubule cytoskeleton in the Drosophila follicular epithelium

The PAR-1 kinase plays a conserved role in cell polarity in C. elegans, Drosophila and mammals. We have investigated the role of PAR-1 in epithelial polarity by generating null mutant clones in the Drosophila follicular epithelium. Large clones show defects in apicobasal membrane polarity, but small clones induced later in development usually have a normal membrane polarity. However, all cells that lack PAR-1 accumulate spectrin and F-actin laterally, and show a strong increase in the density of microtubules. This is consistent with the observation that the mammalian PAR-1 homologues, the MARKs, dramatically reduce the number of microtubules, when overexpressed in tissue culture cells. The MARKs have been proposed to destabilize microtubules by inhibiting the stabilizing activity of the Tau family of microtubule-associated proteins. This is not the case in Drosophila, however, as null mutations in the single tau family member in the genome have no effect on the microtubule organisation in the follicle cells. Furthermore, PAR-1 activity stabilises microtubules, as microtubules in mutant cells depolymerise much more rapidly after cold or colcemid treatments. Loss of PAR-1 also disrupts the basal localisation of the microtubule plus ends, which are mislocalised to the centre of mutant cells. Thus, Drosophila PAR-1 regulates the density, stability and apicobasal organisation of microtubules. Although the direct targets of PAR-1 are unknown, we suggest that it functions by regulating the plus ends, possibly by capping them at the basal cortex.

The identification of novel genes required for Drosophila anteroposterior axis formation in a germline clone screen using GFP-Staufen

The anteroposterior axis of Drosophila is defined during oogenesis, when the polarisation of the oocyte microtubule cytoskeleton directs the localisation of bicoid and oskar mRNAs to the anterior and posterior poles, respectively. Although maternal-effect lethal and female-sterile screens have identified many mutants that disrupt these processes, these screens could not recover mutations in essential genes. Here we describe a genetic screen in germline clones for mutants that disrupt the localisation of GFP-Staufen in living oocytes, which overcomes this limitation. As Staufen localises to the posterior with oskar mRNA and to the anterior with bicoid mRNA, it acts as a marker for both poles of the oocyte, allowing the identification of mutants that affect the localisation of either mRNA, as well as mutants that disrupt oocyte polarity. Using this approach, we have identified 23 novel complementation groups on chromosome 3R that disrupt anteroposterior axis formation. Analyses of new alleles of spn-E and orb show that both SPN-E and ORB proteins are required to organise the microtubule cytoskeleton at stage 9, and to prevent premature cytoplasmic streaming. Furthermore, yps mutants partially suppress the premature cytoplasmic streaming of orb mutants. As orb, yps and spn-E encode RNA-binding proteins, they may regulate the translation of unidentified RNAs necessary for the polarisation of the microtubule cytoskeleton.

LKB1, a protein kinase regulating cell proliferation and polarity

LKB1 is a serine-threonine protein kinase mutated in patients with an autosomal dominantly inherited cancer syndrome predisposing to multiple benign and malignant tumours, termed Peutz-Jeghers syndrome. Since its discovery in 1998, much research has focused on identification and characterisation of its cellular roles and analysing how LKB1 might be regulated. In this review we discuss exciting recent advances indicating that LKB1 functions as a tumour suppressor perhaps by controlling cell polarity. We also outline the current understanding of the molecular mechanisms by which LKB1 is regulated in vivo, through interaction with other proteins as well as by protein phosphorylation and prenylation.