Cell Polarity Organization Research Articles

Structural regulation and dynamic behaviour of organelles during plant meiosis.

Eukaryotes use various mechanisms to maintain cell division stability during sporogenesis, and in particular during meiosis to achieve production of haploid spores. In addition to establishing even chromosome segregation in meiosis I and II, it is crucial for meiotic cells to guarantee balanced partitioning of organelles to the daughter cells, to properly inherit cellular functions. In plants, cytological studies in model systems have yielded insights into the meiotic behaviour of different organelles, i.e., clearly revealing a distinct organization at different stages throughout meiosis indicating for an active regulatory mechanism determining their subcellular dynamics. However, how, and why plant meiocytes organize synchronicity of these elements and whether this is conserved across all plant genera is still not fully elucidated. It is generally accepted that the highly programmed intracellular behaviour of organelles during meiosis serves to guarantee balanced cytoplasmic inheritance. However, recent studies also indicate that it contributes to the regulation of key meiotic processes, like the organization of cell polarity and spindle orientation, thus exhibiting different functionalities than those characterized in mitotic cell division. In this review paper, we will outline the current knowledge on organelle dynamics in plant meiosis and discuss the putative strategies that the plant cell uses to mediate this programmed spatio-temporal organization in order to safeguard balanced separation of organelles. Particular attention is thereby given to putative molecular mechanisms that underlie this dynamic organelle organization taken into account existing variations in the meiotic cell division program across different plant types. Furthermore, we will elaborate on the structural role of organelles in plant meiosis and discuss on organelle-based cellular mechanisms that contribute to the organization and molecular coordination of key meiotic processes, including spindle positioning, chromosome segregation and cell division. Overall, this review summarizes all relevant insights on the dynamic behaviour and inheritance of organelles during plant meiosis, and discusses on their functional role in the structural and molecular regulation of meiotic cell division.

Decrease of Pdzrn3 is required for heart maturation and protects against heart failure

Heart failure is the final common stage of most cardiopathies. Cardiomyocytes (CM) connect with others via their extremities by intercalated disk protein complexes. This planar and directional organization of myocytes is crucial for mechanical coupling and anisotropic conduction of the electric signal in the heart. One of the hallmarks of heart failure is alterations in the contact sites between CM. Yet no factor on its own is known to coordinate CM polarized organization. We have previously shown that PDZRN3, an ubiquitine ligase E3 expressed in various tissues including the heart, mediates a branch of the Planar cell polarity (PCP) signaling involved in tissue patterning, instructing cell polarity and cell polar organization within a tissue. PDZRN3 is expressed in the embryonic mouse heart then its expression dropped significantly postnatally corresponding with heart maturation and CM polarized elongation. A moderate CM overexpression of Pdzrn3 (Pdzrn3 OE) during the first week of life, induced a severe eccentric hypertrophic phenotype with heart failure. In models of pressure-overload stress heart failure, CM-specific Pdzrn3 knockout showed complete protection against degradation of heart function. We reported that Pdzrn3 signaling induced PKC ζ expression, c-Jun nuclear translocation and a reduced nuclear ß catenin level, consistent markers of the planar non-canonical Wnt signaling in CM. We then show that subcellular localization (intercalated disk) of junction proteins as Cx43, ZO1 and Desmoglein 2 was altered in Pdzrn3 OE mice, which provides a molecular explanation for impaired CM polarization in these mice. Our results reveal a novel signaling pathway that controls a genetic program essential for heart maturation and maintenance of overall geometry, as well as the contractile function of CM, and implicates PDZRN3 as a potential therapeutic target for the prevention of human heart failure.

Regulation of Cultured Fibroblast Cell Polarity on Adhesive Micropatterns

In cultured cell systems, fibroblastic cells select specific substrates for typical cell-substrate adhesion. However, the mechanisms that regulate orientation and polarity are not clear. In this study, using glass-bottomed culture dishes with adhesive micropatterns to regulate polarized cell spreading, we analyzed the behavior of cultured fibroblasts during the organization of cell polarity on adhesive micropatterns. When fibroblasts attached to the micropatterns, both tyrosine-phosphorylated proteins and focal adhesion kinase were strongly detected along the inner border between the adhesive micropatterns and non-adhesive glass surface. The above observation suggests that the confinement of the cells by adhesive micropatterns affects fibroblastic cell polarity. The localization of focal adhesion kinase therefore seems to play a key role in recognition of the border between the adhesive micropattern and the non-adhesive glass surface

RalGTPase Controls Cell Polarity Organization during Epithelial Tissue Remodeling

Ral, a Ras-like GTPase, is an essential component of Ras-driven oncogenesis that supports tumor initiation and progression. We explored the role of Ral in the polarity organization of epithelial tissue in Drosophila. We found that Ral is dispensable for the polarization of proliferative epithelial tissue, and that Ral is required for the maintenance of apical-basal polarity of post-mitotic epithelial cells during tissue remodeling. More precisely, lack of Ral activity results in loss of cortical apical aPKC and lateral Lgl, and in apical spreading and accumulation of Armadillo/betacatenin. Our analysis demonstrates that Ral regulates polarity organization by acting as a negative regulator of JNK and p38 MAPK signaling. Ral controls aPKC apical localization by down-regulating JNK and controls Armadillo localization at adherens junction by down-regulating p38 MAPK signaling, whereas Lgl membrane localization depends on both pathways. Finally, in the absence of Ral function aPKC becomes dispensable for Lgl basolateral localization and Armadillo is recruited to the cell membrane independently of DE-Cadherin and Bazooka localization. This suggests that additional mechanisms control Lgl and Arm distribution in polarized epithelia.

Junctional adhesion molecules (JAMs): more molecules with dual functions?

Junctional adhesion molecules (JAMs) are members of an immunoglobulin subfamily expressed by leukocytes and platelets as well as by epithelial and endothelial cells, in which they localize to cell-cell contacts and are specifically enriched at tight junctions. The recent identification of extracellular ligands and intracellular binding proteins for JAMs suggests two functions for JAMs. JAMs associate through their extracellular domains with the leukocyte beta2 integrins LFA-1 and Mac-1 as well as with the beta1 integrin alpha4beta1. All three integrins are involved in the regulation of leukocyte-endothelial cell interactions. Through their cytoplasmic domains, JAMs directly associate with various tight junction-associated proteins including ZO-1, AF-6, MUPP1 and the cell polarity protein PAR-3. PAR-3 is part of a ternary protein complex that contains PAR-3, atypical protein kinase C and PAR-6. This complex is highly conserved through evolution and is involved in the regulation of cell polarity in organisms from Caenorhabditis elegans and Drosophila to vertebrates. These findings point to dual functions for JAMs: they appear to regulate both leukocyte/platelet/endothelial cell interactions in the immune system and tight junction formation in epithelial and endothelial cells during the acquisition of cell polarity.

Humpty Dumpty and All That ...

“…the primitive way of breaking eggs before we eat them, was upon the larger end: but his present Majesty's grandfather, while he was a boy, going to eat an egg, and breaking it according to the ancient practice, happened to cut one of his fingers. Whereupon the Emperor his Father, published an edict, commanding all his subjects, upon great penalties, to break the smaller end of the eggs.” (Jonathan Swift, Gulliver's Travels ) The study of polarity in biological systems has a long and illustrious past. The bilateral symmetry of most organisms contrasts with the major differences between the upper and lower halves of the organisms, be they animals or plants. Even the apparent symmetry across the body can be deceptive: The internal organs have a well-defined, predictable, left-right asymmetry in their organization, which again is widely preserved within vertebrates. It should then come as no surprise that even single-celled organisms such as bacteria possess and propagate intrinsically polar characteristics. This issue describes a variety of topics that are connected by the issue of polarity, loosely conforming to the prescription “From Cell to Organism.” Shapiro and colleagues (p. 1942) examine the issue of polarity in bacteria, with respect to extracellular structures such as flagellae and the intrinsic linear polarity of bacteria due to their rodlike shape. The localization of membrane proteins, signaling apparatus, and cytoskeletal components are all orchestrated to allow these organisms to respond to external cues such as those involved in chemotaxis toward food sources or mating partners. Pellettieri and Seydoux (p. [1946][1]) describe how anterior-posterior polarity is established and maintained within the egg and look at the variety of molecular mechanisms used in different organisms, concentrating mainly on the worm Caenorhabditis elegans and the fruit fly Drosophila . Keller (p. 1950) explains how these early polarity markers go on to be translated into organs and organisms through polarized cell movements during embryonic development. Knust and Bossinger (p. [1955][2]) examine epithelial cell polarity in organisms ranging from worms to flies to vertebrates, and compare and contrast the mechanisms involved in establishing and maintaining polarity. Intriguingly, many of the same molecular players involved in epithelial polarity are also important in the cell movements outlined by Keller. Finally, Dickson (p. 1959) describes how one of the most structurally and functionally polarized cells—the neuron—exploits its characteristics to establish specific connections within the developing nervous system. Establishing and maintaining polarity and the recognition and exploitation of gradients are key features of living organisms. When polarity is disrupted early in development, dire consequences for the body plan result. Throughout development and into the adult organism, polarized cells and organismal polarity remain important in promoting and maintaining health. Knowing top from bottom and left from right are key characteristics in nearly all forms of life, and refining our understanding of the cellular and molecular mechanisms involved in these processes remains an important aspect of ongoing research in cell and developmental biology and neuroscience. [1]: /lookup/doi/10.1126/science.1072162 [2]: /lookup/doi/10.1126/science.1072161

Meiotic maturation induces animal-vegetal asymmetric distribution of aPKC and ASIP/PAR-3 in Xenopus oocytes.

The asymmetric distribution of cellular components is an important clue for understanding cell fate decision during embryonic patterning and cell functioning after differentiation. In C. elegans embryos, PAR-3 and aPKC form a complex that colocalizes to the anterior periphery of the one-cell embryo, and are indispensable for anterior-posterior polarity that is formed prior to asymmetric cell division. In mammals, ASIP (PAR-3 homologue) and aPKCgamma form a complex and colocalize to the epithelial tight junctions, which play critical roles in epithelial cell polarity. Although the mechanism by which PAR-3/ASIP and aPKC regulate cell polarization remains to be clarified, evolutionary conservation of the PAR-3/ASIP-aPKC complex suggests their general role in cell polarity organization. Here, we show the presence of the protein complex in Xenopus laevis. In epithelial cells, XASIP and XaPKC colocalize to the cell-cell contact region. To our surprise, they also colocalize to the animal hemisphere of mature oocytes, whereas they localize uniformly in immature oocytes. Moreover, hormonal stimulation of immature oocytes results in a change in the distribution of XaPKC 2-3 hours after the completion of germinal vesicle breakdown, which requires the kinase activity of aPKC. These results suggest that meiotic maturation induces the animal-vegetal asymmetry of aPKC.