Cell polarity: fixing cell polarity with Pins

A protein complex is assembled in a step-wise manner at the apical pole of Drosophila neuroblasts. This complex organizes the apical-basal polarity of asymmetrically dividing neuroblasts, and may act via G-protein signalling.     

Cell polarity in Arabidopsis trichomes

Arabidopsis leaf trichomes are unicellular hairs that display a highly characteristic cell form that has a fixed orientation with respect to the basal-distal leaf axis. The genetic, molecular and cell biological analysis of trichome morphogenesis reveal that various cellular processes need to be coordinated including regulation of the cell cycle, the cell size and the actin and tubulin cytoskeleton. Here we will focus on what is known about the establishment and maintenance of positional information during trichome formation.     

Cell polarity from cell division

Apical-basal polarity of epithelial cells is critical for their symmetric versus asymmetric division and commonly thought to be established in interphase. In a novel type of cell division termed "mirror-symmetric", apical cell constituents accumulate during M-phase at the cleavage furrow, resulting in epithelial daughter cells with opposite apical-basal polarity.     

Cell polarity and epithelial oncogenesis

Pathologists have long recognized that tumour formation in epithelia leads to disruption of normal epithelial cell polarity. Despite this, few studies have taken advantage of new information on the biogenesis of cell polarity to analyse the process of epithelial oncogenesis. Recent studies of epithelial cell lines now indicate that the pattern of breakdown of polarity during oncogenesis may reflect the way in which normal epithelial cells achieve polarity. These results suggest not only a novel way to study the development of polarity in vitro, but also new ideas for the early detection of cancer.     

Cell polarity and Dictyostelium development

Cell polarity is essential for unicellular and multicellular stages of Dictyostelium development. Chemotaxis during early development requires each cell to rapidly reorganize its cytoskeleton to point towards a source of cAMP. This involves a balance between local induction of F-actin polymerization and suppression of pseudopods that point in other directions. Both the lipid phosphatidylinositol (3,4,5) trisphosphate and the soluble signal cGMP have been implicated in these processes, in addition to conserved and novel proteins. During later development cells adopt newly discovered, alternative modes of movement and interact through adhesion molecules. Finally, cells polarize secretion to particular regions of their surface.     

Cell polarity in development and cancer

The development of cancer is a multistep process in which the DNA of a single cell accumulates mutations in genes that control essential cellular processes. Loss of cell-cell adhesion and cell polarity is commonly observed in advanced tumours and correlates well with their invasion into adjacent tissues and the formation of metastases. Growing evidence indicates that loss of cell-cell adhesion and cell polarity may also be important in early stages of cancer. The strongest hints in this direction come from studies on tumour suppressor genes in the fruitfly Drosophila melanogaster, which have revealed their importance in the control of apical-basal cell polarity.     

Cell polarity and actin mRNA localization

In many species, intracellular mRNA localization is linked to cell polarity. In many cases however, mRNAs become localized as a result of a pre-existing cell-polarity, and they do not modify it. Remarkably, in the case beta actin mRNA in vertebrate, it has been shown that the transport and localization of this RNA is required for the establishment and maintenance of cell polarity. This occurs in fibroblasts, but, very interestingly, in immature neurons as well. This review will describe the functions and mechanisms of actin mRNA localization.     

Cell polarity: squaring the circle

Recent studies have found that Drosophila gene products required for zonula adherens formation in the ectoderm are also involved in the asymmetric cell division of the neuroblast. The results illustrate the reiterated use of groups of proteins to dictate cell polarity in epithelial and non-epithelial cells.     

Cell polarity: the PARty expands

Recent work has revealed an evolutionarily conserved trio of proteins that regulate cell polarity in epithelial cells, embryonic blastomeres and neural precursors. This common cell-polarity mechanism is used in cell-specific ways, as highlighted by the recent finding that at least two different types of asymmetric division are observed in Drosophila neural precursors.     

Cell flow and tissue polarity patterns

Planar tissue polarity is a fundamental feature of many epithelia. Large-scale cell polarity patterns govern the orientation of external structures such as hairs and cilia. Tissue polarity patterns arise from the collective organization of cells, which are polarized individually. Such cell and tissue polarities are reflected in anisotropic distributions of proteins of the planar cell polarity (PCP) pathway. Here we give an overview on recent progress in understanding how large-scale patterns of tissue polarity are controlled. We highlight the role of active mechanical events in the organization of polarity patterns during the development of the pupal fly wing. Patterns of cell flow are generated by mechanical stresses exerted on the tissue as well as by oriented cell divisions and neighbor exchanges. We discuss how the resulting tissue shear controls polarity orientation. We argue that the often-observed alignment of PCP either parallel or perpendicular to the long axis of developing tissues is a characteristic consequence of shear-induced polarity alignment. This principle allows for the versatile and robust generation of polarity patterns in tissues.