Par-1 regulates stability of the posterior determinant Oskar by phosphorylation

Developmental and behavioural toxicity induced by acrylamide exposure and amelioration using phytochemicals in Drosophila melanogaster

Acrylamide, an environmental pollutant, is known to occur in food substances cooked at high temperatures. Studies on various models indicate acrylamide to cause several physiological conditions such as neuro- and reproductive toxicity, and carcinogenesis. In our study, exposure of Drosophila melanogaster (Oregon K strain) to acrylamide via their diet resulted in a concentration and time-dependent mortality, while the surviving flies exhibited significant locomotor deficits, most likely due to oxidative stress-induced neuronal damage. Also, Drosophila embryos exhibited signs of developmental toxicity as evidenced by the alteration in the migration of border cells and cluster cells during the developmental stages, concomitant to modulation in expression of gurken and oskar genes. Curcumin, a known antioxidant has been widely studied for its neuroprotective effects against acrylamide; however; very few studies focus on thymoquinone for its role against food toxicant. Our research focuses on the toxicity elicited by acrylamide and the ability of the antioxidants: thymoquinone, curcumin and combination of thereof, in reversing the same.

The PIWI protein Aubergine recruits eIF3 to activate translation in the germ plasm

Piwi-interacting RNAs (piRNAs) and PIWI proteins are essential in germ cells to repress transposons and regulate mRNAs. In Drosophila, piRNAs bound to the PIWI protein Aubergine (Aub) are transferred maternally to the embryo and regulate maternal mRNA stability through two opposite roles. They target mRNAs by incomplete base pairing, leading to their destabilization in the soma and stabilization in the germ plasm. Here, we report a function of Aub in translation. Aub is required for translational activation of nanos mRNA, a key determinant of the germ plasm. Aub physically interacts with the poly(A)-binding protein (PABP) and the translation initiation factor eIF3. Polysome gradient profiling reveals the role of Aub at the initiation step of translation. In the germ plasm, PABP and eIF3d assemble in foci that surround Aub-containing germ granules, and Aub acts with eIF3d to promote nanos translation. These results identify translational activation as a new mode of mRNA regulation by Aub, highlighting the versatility of PIWI proteins in mRNA regulation.

Par-1 controls the composition and growth of cortical actin caps during Drosophila embryo cleavage

The cell cortex is populated by various proteins, but it is unclear how they interact to change cell shape. Jiang and Harris find that the kinase Par-1 is required for Diaphanous-based actin bundles, and that these bundles intersperse with separately induced Arp2/3 networks to form an actin cap that grows into a metaphase compartment of the syncytial Drosophila embryo.

Cell structure depends on the cortex, a thin network of actin polymers and additional proteins underlying the plasma membrane. The cell polarity kinase Par-1 is required for cells to form following syncytial Drosophila embryo development. This requirement stems from Par-1 promoting cortical actin caps that grow into dome-like metaphase compartments for dividing syncytial nuclei. We find the actin caps to be a composite material of Diaphanous (Dia)-based actin bundles interspersed with independently formed, Arp2/3-based actin puncta. Par-1 and Dia colocalize along extended regions of the bundles, and both are required for the bundles and for each other’s bundle-like localization, consistent with an actin-dependent self-reinforcement mechanism. Par-1 helps establish or maintain these bundles in a cortical domain with relatively low levels of the canonical formin activator Rho1-GTP. Arp2/3 is required for displacing the bundles away from each other and toward the cap circumference, suggesting interactions between these cytoskeletal components could contribute to the growth of the cap into a metaphase compartment.

MARK4 regulates NLRP3 positioning and inflammasome activation through a microtubule-dependent mechanism

Excessive activation of the NLR family pyrin domain containing 3 (NLRP3) inflammasome is involved in many chronic inflammatory diseases, including cardiovascular and Alzheimer's disease. Here we show that microtubule-affinity regulating kinase 4 (MARK4) binds to NLRP3 and drives it to the microtubule-organizing centre, enabling the formation of one large inflammasome speck complex within a single cell. MARK4 knockdown or knockout, or disruption of MARK4-NLRP3 interaction, impairs NLRP3 spatial arrangement and limits inflammasome activation. Our results demonstrate how an evolutionarily conserved protein involved in the regulation of microtubule dynamics orchestrates NLRP3 inflammasome activation by controlling its transport to optimal activation sites, and identify a targetable function for MARK4 in the control of innate immunity.

Regulation of Cell Polarity by PAR-1/MARK Kinase

PAR-1/MARK kinases are conserved serine/threonine kinases that are essential regulators of cell polarity. PAR-1/MARK kinases localize and function in opposition to the anterior PAR proteins to control the asymmetric distribution of factors in a wide variety polarized cells. In this review, we discuss the mechanisms that control the localization and activity of PAR-1/MARK kinases, including their antagonistic interactions with the anterior PAR proteins. We focus on the role PAR-1 plays in the asymmetric division of the Caenorhabditis elegans zygote, in the establishment of the anterior/posterior axis in the Drosophila oocyte and in the control of microtubule dynamics in mammalian neurons. In addition to conserved aspects of PAR-1 biology, we highlight the unique ways in which PAR-1 acts in these distinct cell types to orchestrate their polarization. Finally, we review the connections between disruptions in PAR-1/MARK function and Alzheimer's disease and cancer.

Oskar is targeted for degradation by the sequential action of Par-1, GSK-3, and the SCF⁻Slimb ubiquitin ligase

Translation of oskar messenger RNA (mRNA) is activated at the posterior of the Drosophila oocyte, producing Long Oskar, which anchors the RNA, and Short Oskar, which nucleates the pole plasm, containing the posterior and germline determinants. Here, we show that Oskar is phosphorylated by Par-1 and GSK-3/Shaggy to create a phosphodegron that recruits the SCF(-Slimb) ubiquitin ligase, which targets Short Oskar for degradation. Phosphorylation site mutations cause Oskar overaccumulation, leading to an increase in pole cell number and embryonic patterning defects. Furthermore, the nonphosphorylatable mutant produces bicaudal embryos when oskar mRNA is mislocalized. Thus, the Par-1/GSK-3/Slimb pathway plays important roles in limiting the amount of pole plasm posteriorly and in degrading any mislocalized Oskar that results from leaky translational repression. These results reveal that Par-1 controls the timing of pole plasm assembly by promoting the localization of oskar mRNA but inhibiting the accumulation of Short Oskar protein.

Regulation of glucose homeostasis by KSR1 and MARK2

Protein scaffolds control the intensity and duration of signaling and dictate the specificity of signaling through MAP kinase pathways. KSR1 is a molecular scaffold of the Raf/MEK/ERK MAP kinase cascade that regulates the intensity and duration of ERK activation. Relative to wild-type mice, ksr1^-/- mice are modestly glucose intolerant, but show a normal response to exogenous insulin. However, ksr1^-/- mice also demonstrate a three-fold increase in serum insulin levels in response to a glucose challenge, suggesting a role for KSR1 in insulin secretion. The kinase MARK2 is closely related to C-TAK1, a known regulator of KSR1. Mice lacking MARK2 have an increased rate of glucose disposal in response to exogenous insulin, increased glucose tolerance, and are resistant to diet-induced obesity. mark2^-/-ksr1^-/- (DKO) mice were compared to wild type, mark2^-/-, and ksr1^-/- mice for their ability to regulate glucose homeostasis. Here we show that disruption of KSR1 in mark2^-/- mice reverses the increased sensitivity to exogenous insulin resulting from MARK2 deletion. DKO mice respond to exogenous insulin similarly to wild type and ksr1^-/- mice. These data suggest a model whereby MARK2 negatively regulates insulin sensitivity in peripheral tissue through inhibition of KSR1. Consistent with this model, we found that MARK2 binds and phosphorylates KSR1 on Ser392. Phosphorylation of Ser392 is a critical regulator of KSR1 stability, subcellular location, and ERK activation. These data reveal an unexpected role for the molecular scaffold KSR1 in insulin-regulated glucose metabolism.

Aubergine is a component of a nanos mRNA localization complex

Localization of nanos (nos) mRNA to the posterior pole of the Drosophila oocyte is essential for abdominal segmentation and germline development during embryogenesis. Posterior localization is mediated by a complex cis-acting localization signal in the nos 3' untranslated region that comprises multiple partially redundant elements. Genetic analysis suggests that this signal is recognized by RNA-binding proteins and associated factors that package nos mRNA into a localization competent ribonucleoprotein complex. However, functional redundancy among localization elements has made the identification of individual localization factors difficult. Indeed, only a single direct-acting nos localization factor, Rumpelstiltskin (Rump), has been identified thus far. Through a sensitized genetic screen, we have now identified the Argonaute family member Aubergine (Aub) as a nos localization factor. Aub interacts with nos mRNA in vivo and co-purifies with Rump in an RNA-dependent manner. Our results support a role for Aub, independent of its function in RNA silencing, as a component of a nos mRNA localization complex.

Symmetry breaking during Drosophila oogenesis

The orthogonal axes of Drosophila are established during oogenesis through a hierarchical series of symmetry-breaking steps, most of which can be traced back to asymmetries inherent in the architecture of the ovary. Oogenesis begins with the formation of a germline cyst of 16 cells connected by ring canals. Two of these 16 cells have four ring canals, whereas the others have fewer. The first symmetry-breaking step is the selection of one of these two cells to become the oocyte. Subsequently, the germline cyst becomes surrounded by somatic follicle cells to generate individual egg chambers. The second symmetry-breaking step is the posterior positioning of the oocyte within the egg chamber, a process mediated by adhesive interactions with a special group of somatic cells. Posterior oocyte positioning is accompanied by a par gene-dependent repolarization of the microtubule network, which establishes the posterior cortex of the oocyte. The next two steps of symmetry breaking occur during midoogenesis after the volume of the oocyte has increased about 10-fold. First, a signal from the oocyte specifies posterior follicle cells, polarizing a symmetric prepattern present within the follicular epithelium. Second, the posterior follicle cells send a signal back to the oocyte, which leads to a second repolarization of the oocyte microtubule network and the asymmetric migration of the oocyte nucleus. This process again requires the par genes. The repolarization of the microtubule network results in the transport of bicoid and oskar mRNAs, the anterior and posterior determinants, respectively, of the embryonic axis, to opposite poles of the oocyte. The asymmetric positioning of the oocyte nucleus defines a cortical region of the oocyte where gurken mRNA is localized, thus breaking the dorsal-ventral symmetry of the egg and embryo.

The molecular machinery of germ line specification

Germ cells occupy a unique position in animal reproduction, development, and evolution. In sexually reproducing animals, only they can produce gametes and contribute genetically to subsequent generations. Nonetheless, germ line specification during embryogenesis is conceptually the same as the specification of any somatic cell type: germ cells must activate a specific gene regulatory network in order to differentiate and go through gametogenesis. While many genes with critical roles in the germ line have been characterized with respect to expression pattern and genetic interactions, it is the molecular interactions of the relevant gene products that are ultimately responsible for germ cell differentiation. This review summarizes the current state of knowledge on the molecular functions and biochemical connections between germ line gene products. We find that homologous genes often interact physically with the same conserved molecular partners across the metazoans. We also point out cases of nonhomologous genes from different species whose gene products play analogous biological roles in the germ line. We suggest a preliminary molecular definition of an ancestral "pluripotency module" that could have been modified during metazoan evolution to become specific to the germ line.

PAR-1 phosphorylates Mind bomb to promote vertebrate neurogenesis

Generation of neurons in the vertebrate central nervous system requires a complex transcriptional regulatory network and signaling processes in polarized neuroepithelial progenitor cells. Here we demonstrate that neurogenesis in the Xenopus neural plate in vivo and mammalian neural progenitors in vitro involves intrinsic antagonistic activities of the polarity proteins PAR-1 and aPKC. Furthermore, we show that Mind bomb (Mib), a ubiquitin ligase that promotes Notch ligand trafficking and activity, is a crucial molecular substrate for PAR-1. The phosphorylation of Mib by PAR-1 results in Mib degradation, repression of Notch signaling, and stimulation of neuronal differentiation. These observations suggest a conserved mechanism for neuronal fate determination that might operate during asymmetric divisions of polarized neural progenitor cells.

The tau of MARK: a polarized view of the cytoskeleton

Microtubule-affinity regulating kinases (MARKs) were originally discovered by their ability to phosphorylate tau protein and related microtubule-associated proteins (MAPs), and thereby to regulate microtubule dynamics in neurons. Members of the MARK (also known as partition-defective [Par]-1 kinase) family were subsequently found to be highly conserved and to have key roles in cell processes such as determination of polarity, cell-cycle control, intracellular signal transduction, transport and cytoskeleton. This is important for neuronal differentiation, but is also prominent in neurodegenerative 'tauopathies' such as Alzheimer's disease. The identified functions of MARK/Par-1 are diverse and require accurate regulation. Recent discoveries including the x-ray structure of human MARKs contributed to an increased understanding of the mechanisms that control the kinase activity and, thus, the actin and microtubule cytoskeleton.

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.

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.

PAR-3 and PAR-1 inhibit LET-99 localization to generate a cortical band important for spindle positioning in Caenorhabditis elegans embryos

The conserved PAR proteins are localized in asymmetric cortical domains and are required for the polarized localization of cell fate determinants in many organisms. In Caenorhabditis elegans embryos, LET-99 and G protein signaling act downstream of the PARs to regulate spindle positioning and ensure asymmetric division. PAR-3 and PAR-2 localize LET-99 to a posterior cortical band through an unknown mechanism. Here we report that LET-99 asymmetry depends on cortically localized PAR-1 and PAR-4 but not on cytoplasmic polarity effectors. In par-1 and par-4 embryos, LET-99 accumulates at the entire posterior cortex, but remains at low levels at the anterior cortex occupied by PAR-3. Further, PAR-3 and PAR-1 have graded cortical distributions with the highest levels at the anterior and posterior poles, respectively, and the lowest levels of these proteins correlate with high LET-99 accumulation. These results suggest that PAR-3 and PAR-1 inhibit the localization of LET-99 to generate a band pattern. In addition, PAR-1 kinase activity is required for the inhibition of LET-99 localization, and PAR-1 associates with LET-99. Finally, examination of par-1 embryos suggests that the banded pattern of LET-99 is critical for normal posterior spindle displacement and to prevent spindle misorientation caused by cell shape constraints.

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.

Ribonucleoprotein remodeling during RNA localization

Cytoplasmic RNA localization is a means to create polarity by restricting protein expression to a discrete subcellular location. RNA localization is a multistep process that begins with the recognition of cis-acting sequences within the RNA by specific trans-factors, and RNAs are localized in ribonucleoprotein (RNP) complexes that contain both the RNA and numerous protein components. Components of the localization machinery transport the RNP complex, usually in a translationally repressed state, to a distinct subcellular region, resulting in spatially restricted gene expression. Recent efforts to identify both the cis- and trans-factors required for RNA localization have elucidated RNA-protein interactions that are remodeled during localization.

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.

Common critical pathways in embryogenesis and cancer

Cancer may arise because the developmental programs that create the dramatic alterations in form and structure in embryonic development are potentially corrupted. The cells in our bodies retain memories of these processes and cancer can occur later in life if imperfections occur in the fidelity of these pathways. This article is particularly interested in the phenomenon of epithelial to mesenchymal transition, which occurs in embryogenesis. Also reviewed are the small molecules and pathways that are involved both in homeostasis in adult epithelium and embryogenesis in utero. There are five such pathways in particular selected for review in this article: the Wnt pathway, Hedgehog, Notch, PAR and Bone morphogenetic peptide/TGF beta. These are usually conserved throughout mammalian evolution. Though they have been arbitrarily separated in this article they are not exclusive from one another. Their pathologically altered expression is found especially frequently in childhood tumours where they may recapitulate their developmental role, and in tumours that resemble primitive precursor cells. These pathways are important for selecting cell fates, cellular rearrangements, cytological context and morphologic design in embryology as well as participating in epithelial function in adults.

Oskar allows nanos mRNA translation in Drosophila embryos by preventing its deadenylation by Smaug/CCR4

Anteroposterior patterning of the Drosophila embryo depends on a gradient of Nanos protein arising from the posterior pole. This gradient results from both nanos mRNA translational repression in the bulk of the embryo and translational activation of nanos mRNA localized at the posterior pole. Two mechanisms of nanos translational repression have been described, at the initiation step and after this step. Here we identify a novel level of nanos translational control. We show that the Smaug protein bound to the nanos 3' UTR recruits the deadenylation complex CCR4-NOT, leading to rapid deadenylation and subsequent decay of nanos mRNA. Inhibition of deadenylation causes stabilization of nanos mRNA, ectopic synthesis of Nanos protein and head defects. Therefore, deadenylation is essential for both translational repression and decay of nanos mRNA. We further propose a mechanism for translational activation at the posterior pole. Translation of nanos mRNA at the posterior pole depends on oskar function. We show that Oskar prevents the rapid deadenylation of nanos mRNA by precluding its binding to Smaug, thus leading to its stabilization and translation. This study provides insights into molecular mechanisms of regulated deadenylation by specific proteins and demonstrates its importance in development.

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.

Proteolysis: anytime, any place, anywhere?

Proteolysis via the ubiquitin-proteasome system (UPS) is a rapid and effective method of degrading a specific protein at a specific time, and in many cases a protein is degraded only in response to a particular cellular signal or event. However, an added dimension to the control of protein degradation is possible because the ubiquitin system can be spatially regulated. Controlling where a protein is degraded can enhance the specificity and timing of proteolysis, generate asymmetry and maintain sub-compartments even in the mitotic cell. Here, we discuss this aspect of the UPS.

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.

Maternal determinants and mRNAs in the cortex of ascidian oocytes, zygotes and embryos

The peripheral region of ascidian oocytes and zygotes contains five determinants for morphogenesis and differentiation of the embryo. The determinant for the 24 primary muscle cells of the tadpole, macho1, is one of several cortical mRNAs localized in a gradient along the animal-vegetal axis in the oocyte. After fertilization these mRNAs, together with cortical endoplasmic reticulum (cER) and a subcortical mitochondria-rich domain (myoplasm), relocate in two major reorganization phases forming the posterior plasm (postplasm) of the zygote. At the 8-cell stage cortical mRNAs concentrate in a macroscopic cortical structure called the centrosome-attracting body (CAB), forming a characteristic posterior end mark (PEM) in the two posterior vegetal blastomeres. We propose to call the numerous mRNAs showing this particular cortical localization in the posterior region of the embryo postplasmic/PEM RNAs and suggest a nomemclature. We do not know how postplasmic/PEM RNAs reach their polarized distribution in the oocyte cortex but at least PEM1 and macho1 (and probably others) bind to the network of cER retained in isolated cortical fragments. We propose that after fertilization, these postplasmic/PEM mRNAs move in the zygote cortex together with the cER network (cER/mRNA domain) via microfilament- and microtubule-driven translocations. The cER/mRNA domain is localized posteriorly at the time of first cleavage and distributed equally between the first two blastomeres. After the third cleavage, the cER/mRNA domain and dense particles compact to form the CAB in posterior vegetal blastomeres of the 8-cell stage. We discuss the identity of postplasmic/PEM RNAs, how they localize, anchor, relocate and may be translated. We also examine their roles in unequal cleavage and as a source of posterior morphogenetic and differentiation factors.

RNA localization mechanisms in oocytes

In many animals, normal development depends on the asymmetric distribution of maternal determinants, including various coding and noncoding RNAs, within the oocyte. The temporal and spatial distribution of localized RNAs is determined by intricate mechanisms that regulate their movement and anchoring. These mechanisms involve cis-acting sequences within the RNA molecules and a multitude of trans-acting factors, as well as a polarized cytoskeleton, molecular motors and specific transporting organelles. The latest studies show that the fates of localized RNAs within the oocyte cytoplasm are predetermined in the nucleus and that nuclear proteins, some of them deposited on RNAs during splicing, together with the components of the RNA-silencing pathway, dictate the proper movement, targeting, anchoring and translatability of localized RNAs.

Drosophila valois encodes a divergent WD protein that is required for Vasa localization and Oskar protein accumulation

valois (vls) was identified as a posterior group gene in the initial screens for Drosophila maternal-effect lethal mutations. Despite its early genetic identification, it has not been characterized at the molecular level until now. We show that vls encodes a divergent WD domain protein and that the three available EMS-induced point mutations cause premature stop codons in the vls ORF. We have generated a null allele that has a stronger phenotype than the EMS mutants. The vlsnull mutant shows that vls+ is required for high levels of Oskar protein to accumulate during oogenesis, for normal posterior localization of Oskar in later stages of oogenesis and for posterior localization of the Vasa protein during the entire process of pole plasm assembly. There is no evidence for vls being dependent on an upstream factor of the posterior pathway, suggesting that Valois protein (Vls) instead acts as a co-factor in the process. Based on the structure of Vls, the function of similar proteins in different systems and our phenotypic analysis, it seems likely that vls may promote posterior patterning by facilitating interactions between different molecules.

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.

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.

Interaction with eIF5B is essential for Vasa function during development

The DEAD-box RNA helicase Vasa (Vas) is required for germ cell development and function, as well as for embryonic somatic posterior patterning. Vas interacts with the general translation initiation factor eIF5B (cIF2, also known as dIF2), and thus may regulate translation of specific mRNAs. In order to investigate which functions of Vas are related to translational control, we have analyzed the effects of site-directed vas mutations that reduce or eliminate interaction with eIF5B. Reduction in Vas-eIF5B interaction during oogenesis leads to female sterility, with phenotypes similar to a vas null mutation. Accumulation of Gurken (Grk) protein is greatly reduced when Vas-eIF5B interaction is reduced, suggesting that this interaction is crucial for translational regulation of grk. In addition, we show that reduction in Vas-eIF5B interaction virtually abolishes germ cell formation in embryos, while producing a less severe effect on somatic posterior patterning. We conclude that interaction with the general translation factor eIF5B is essential for Vas function during development.

Splicing of oskar RNA in the nucleus is coupled to its cytoplasmic localization

oskar messenger RNA localization at the posterior pole of the Drosophila oocyte is essential for germline and abdomen formation in the future embryo. The nuclear shuttling proteins Y14/Tsunagi and Mago nashi are required for oskar mRNA localization, and they co-localize with oskar mRNA at the posterior pole of the oocyte. Their human homologues, Y14/RBM8 and Magoh, are core components of the exon-exon junction complex (EJC). The EJC is deposited on mRNAs in a splicing-dependent manner, 20-24 nucleotides upstream of exon-exon junctions, independently of the RNA sequence. This indicates a possible role of splicing in oskar mRNA localization, challenging the established notion that the oskar 3' untranslated region (3'UTR) is sufficient for this process. Here we show that splicing at the first exon-exon junction of oskar RNA is essential for oskar mRNA localization at the posterior pole. We revisit the issue of sufficiency of the oskar 3'UTR for posterior localization and show that the localization of unrelated transcripts bearing the oskar 3'UTR is mediated by endogenous oskar mRNA. Our results reveal an important new function for splicing: regulation of messenger ribonucleoprotein complex assembly and organization for mRNA cytoplasmic localization.

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.

C. elegans PAR Proteins Function by Mobilizing and Stabilizing Asymmetrically Localized Protein Complexes

BACKGROUND

The PAR proteins are part of an ancient and widely conserved machinery for polarizing cells during animal development. Here we use a combination of genetics and live imaging methods in the model organism Caenorhabditis elegans to dissect the cellular mechanisms by which PAR proteins polarize cells.

RESULTS

We demonstrate two distinct mechanisms by which PAR proteins polarize the C. elegans zygote. First, we show that several components of the PAR pathway function in intracellular motility, producing a polarized movement of the cell cortex. We present evidence that this cortical motility may drive the movement of cellular components that must become asymmetrically distributed, including both germline-specific ribonucleoprotein complexes and cortical domains containing the PAR proteins themselves. Second, PAR-1 functions to refine the asymmetric localization of germline ribonucleoprotein complexes by selectively stabilizing only those complexes that reach the PAR-1-enriched posterior cell cortex during the period of cortical motility.

CONCLUSIONS

These results identify two cellular mechanisms by which the PAR proteins polarize the C. elegans zygote, and they suggest mechanisms by which PAR proteins may polarize cells in diverse animal systems.

The Drosophila SDE3 homolog armitage is required for oskar mRNA silencing and embryonic axis specification

Polarization of the microtubule cytoskeleton during early oogenesis is required to specify the posterior of the Drosophila oocyte, which is essential for asymmetric mRNA localization during mid-oogenesis and for embryonic axis specification. The posterior determinant oskar mRNA is translationally silent until mid-oogenesis. We show that mutations in armitage and three components of the RNAi pathway disrupt oskar mRNA translational silencing, polarization of the microtubule cytoskeleton, and posterior localization of oskar mRNA. armitage encodes a homolog of SDE3, a presumptive RNA helicase involved in posttranscriptional gene silencing (RNAi) in Arabidopsis, and is required for RNAi in Drosophila ovaries. Armitage forms an asymmetric network associated with the polarized microtubule cytoskeleton and is concentrated with translationally silent oskar mRNA in the oocyte. We conclude that RNA silencing is essential for establishment of the cytoskeletal polarity that initiates embryonic axis specification and for translational control of oskar mRNA.

Live imaging of nuage and polar granules: evidence against a precursor-product relationship and a novel role for Oskar in stabilization of polar granule components

Nuage, a germ line specific organelle, is remarkably conserved between species, suggesting that it has an important germline cell function. Very little is known about the specific role of this organelle, but in Drosophila three nuage components have been identified, the Vasa, Tudor and Aubergine proteins. Each of these components is also present in polar granules, structures that are assembled in the oocyte and specify the formation of embryonic germ cells. We used GFP-tagged versions of Vasa and Aubergine to characterize and track nuage particles and polar granules in live preparations of ovaries and embryos. We found that perinuclear nuage is a stable structure that maintains size, seldom detaches from the nuclear envelope and exchanges protein components with the cytoplasm. Cytoplasmic nuage particles move rapidly in nurse cell cytoplasm and passage into the oocyte where their movements parallel that of the bulk cytoplasm. These particles do not appear to be anchored at the posterior or incorporated into polar granules, which argues for a model where nuage particles do not serve as the precursors of polar granules. Instead, Oskar protein nucleates the formation of polar granules from cytoplasmic pools of the components shared with nuage. Surprisingly, Oskar also appears to stabilize at least one shared component, Aubergine, and this property probably contributes to the Oskar-dependent formation of polar granules. We also find that Bruno, a translational control protein, is associated with nuage, which is consistent with a model in which nuage facilitates post transcriptional regulation by promoting the formation or reorganization of RNA-protein complexes.

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.

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.

Germ plasm: protein degradation in the soma

Germ plasm is a specialized cytoplasm that is physically segregated to the germline cells during early embryogenesis. Recent results suggest that, in Caenorhabditis elegans, germ plasm is also prevented from accumulating in somatic lineages by a ubiquitin ligase that targets germ plasm proteins for degradation.

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.

Exclusion of germ plasm proteins from somatic lineages by cullin-dependent degradation

In many animals, establishment of the germ line depends on segregation of a specialized cytoplasm, or 'germ plasm', to a small number of germline precursor cells during early embryogenesis. Germ plasm asymmetry involves targeting of RNAs and proteins to a specific region of the oocyte and/or embryo. Here we demonstrate that germ plasm asymmetry also depends on degradation of germline proteins in non-germline (somatic) cells. We show that five CCCH finger proteins, components of the Caenorhabditis elegans germ plasm, are targeted for degradation by the novel CCCH-finger-binding protein ZIF-1. ZIF-1 is a SOCS-box protein that interacts with the E3 ubiquitin ligase subunit elongin C. Elongin C, the cullin CUL-2, the ring finger protein RBX-1 and the E2 ubiquitin conjugation enzyme UBC5 (also known as LET-70) are all required in vivo for CCCH finger protein degradation. Degradation is activated in somatic cells by the redundant CCCH finger proteins MEX-5 and MEX-6, which are counteracted in the germ line by the PAR-1 kinase. We propose that segregation of the germ plasm involves both stabilization of germline proteins in the germ line and cullin-dependent degradation in the soma.

Orb and a long poly(A) tail are required for efficient oskar translation at the posterior pole of the Drosophila oocyte

During Drosophila oogenesis, the posterior determinant, Oskar, is tightly localized at the posterior pole of the oocyte. The exclusive accumulation of Oskar at this site is ensured by localization-dependent translation of oskar mRNA: translation of oskar mRNA is repressed during transport and activated upon localization at the posterior cortex. Previous studies have suggested that oskar translation is poly(A)-independent. We show that a long poly(A) tail is required for efficient oskar translation, both in vivo and in vitro, but is not sufficient to overcome BRE-mediated repression. Moreover, we show that accumulation of Oskar activity requires the Drosophila homolog of Cytoplasmic Polyadenylation Element Binding protein (CPEB), Orb. As posterior localization of oskar mRNA is an essential prerequisite for its translation, it was critical to identify an allele of orb that does localize oskar mRNA to the posterior pole of the oocyte. We show that flies bearing the weak mutation orb(mel) localize oskar transcripts with a shortened poly(A) that fails to enhance oskar translation, resulting in reduced Oskar levels and posterior patterning defects. We conclude that Orb-mediated cytoplasmic polyadenylation stimulates oskar translation to achieve the high levels of Oskar protein necessary for posterior patterning and germline differentiation.

Bruno regulates gurken during Drosophila oogenesis

Translational regulation of localized transcripts is a powerful mechanism to control the precise timing and localization of protein expression within a cell. In the Drosophila germline, oskar transcript must be translationally repressed until its localization at the posterior pole of the oocyte, as ectopic production of Oskar causes severe patterning defects. Translational repression of oskar mRNA is mediated by the RNA-binding protein Bruno, which binds to specific motifs in the oskar 3'UTR. Here we show that Bruno over-expression causes defects in antero-posterior and dorso-ventral patterning, consistent with a role of Bruno in both oskar and gurken mRNA regulation. We also show that Bruno and gurken interact genetically. Finally, we show that Bruno binds specifically to the gurken 3'UTR and that the dorso-ventral defects caused by Bruno over-expression are due to a reduction of Gurken levels in the oocyte. We conclude that Bruno plays similar roles in translational regulation of gurken and oskar.

Control of cell polarity and mitotic spindle positioning in animal cells

Cell polarity is an essential feature of many animal cells. It is critical for epithelial formation and function, for correct partitioning of fate-determining molecules, and for individual cells to chemotax or grow in a defined direction. For some of these processes, the position and orientation of the mitotic spindle must be coupled to cell polarity for correct positioning of daughter cells and inheritance of localised molecules. Recent work in several different systems has led to the realisation that similar mechanisms dictate the establishment of polarity and subsequent spindle positioning in many animal cells. Microtubules and conserved PAR proteins are essential mediators of cell polarity, and mitotic spindle positioning depends on heterotrimeric G protein signalling and the microtubule motor protein dynein.

Runnin' with the Dvl: proteins that associate with Dsh/Dvl and their significance to Wnt signal transduction

Wnt proteins transmit myriad intercellular signals crucial for the development and homeostasis of metazoan animals from Hydra to human. Abnormal Wnt signaling causes a growing number of diseases, including cancer and osteoporosis. Depending on the context, a given Wnt signal may denote: cell proliferation or apoptosis; cell fate determination, differentiation, or stem cell maintenance; a variety of changes in cell behavior; and/or coordinated interactions with its neighbors. Which event(s) occur in Wnt-responsive cells depends critically on the ability of Dishevelled (Dsh)/Dvl proteins to interpret distinct types of intracellular, receptor-generated stimuli and transmit them to at least two distinct sets of effector molecules, all while apparently ignoring a third type of Wnt-generated Ca(2+) signal. The three conserved domains present in Dsh/Dvl proteins uniquely function in each Wnt pathway, in part by association with 18 (and counting) Dsh/Dvl-associated proteins. The latest data suggest that Dsh/Dvl proteins organize dynamic, pathway-specific subcellular signaling complexes that ensure correct information routing, signal amplification, and dynamic control through feedback regulation. The biochemical and cell biological mechanisms by which Dsh/Dvl proteins accomplish these remarkable tasks remain obscure.

PAR-1 is required for morphogenesis of the Caenorhabditis elegans vulva

The Caenorhabditis elegans vulva provides a simple model for the genetic analysis of pattern formation and organ morphogenesis during metazoan development. We have discovered an essential role for the polarity protein PAR-1 in the development of the vulva. Postembryonic RNA interference of PAR-1 causes a protruding vulva phenotype. We found that depleting PAR-1 during the development of the vulva has no detectable effect on fate specification or precursor proliferation, but instead seems to specifically alter morphogenesis. Using an apical junction-associated GFP marker, we discovered that PAR-1 depletion causes a failure of the two mirror-symmetric halves of the vulva to join into a single, coherent organ. The cells that normally form the ventral vulval rings fail to make contact or adhere and consequently form incomplete toroids, and dorsal rings adopt variably abnormal morphologies. We also found that PAR-1 undergoes a redistribution from apical junctions to basolateral domains during morphogenesis. Despite a known role for PAR-1 in cell polarity, we have observed no detectable differences in the distribution of various markers of epithelial cell polarity. We propose that PAR-1 activity at the cell cortex is critical for mediating cell shape changes, cell surface composition, or cell signaling during vulval morphogenesis.