Planar cell polarity: one or two pathways?

In multicellular organisms, cells are polarized in the plane of the epithelial sheet, revealed in some cell types by oriented hairs or cilia. Many of the underlying genes have been identified in Drosophila melanogaster and are conserved in vertebrates. Here we dissect the logic of planar cell polarity (PCP). We review studies of genetic mosaics in adult flies - marked cells of different genotypes help us to understand how polarizing information is generated and how it passes from one cell to another. We argue that the prevailing opinion that planar polarity depends on a single genetic pathway is wrong and conclude that there are (at least) two independently acting processes. This conclusion has major consequences for the PCP field.     

Planar cell polarity: A bridge too far?

The mechanisms of planar cell polarity are being revealed by genetic analysis. Recent studies have provided new insights into interactions between three proteins involved in planar cell polarity: Flamingo, Frizzled and Van Gogh.     

Is cell polarity under mechanical control in plants?

Plant cells experience a tremendous amount of mechanical stress caused by turgor pressure. Because cells are glued to their neighbors by the middle lamella, supracellular patterns of physical forces are emerging during growth, usually leading to tension in the epidermis. Cortical microtubules have been shown to reorient in response to these mechanical stresses, and to resist them, indirectly via their impact on the anisotropic structure of the cell wall. In a recent study, we show that the polar localization of the auxin efflux carrier PIN1 can also be under the control of physical forces, thus linking cell growth rate and anisotropy by a common mechanical signal. Because of the known impact of auxin on the stiffness of the cell wall, this suggests that the mechanical properties of the extracellular matrix play a crucial signaling role in morphogenesis, notably controlling the polarity of the cell, as observed in animal systems.Key words: development, growth, auxin, microtubule, PIN1, stiffness, cell wall, biophysics, meristemThe current development of high throughput analyses of gene regulatory networks is feeding a very complex view of growth control and shape changes. To go beyond the accumulation of data, the identification of universal and parsimonious mechanisms explaining the robustness of morphogenesis becomes a central issue in today''s developmental biology.^1–3 Among them, the coupling between molecular and mechanical signals has the strong advantage of providing a simple way to coordinate cell behavior synchronously and over long distances. The role of such signals has been investigated in different systems and the contribution of mechanical forces to animal development is now widely accepted, as the expression of key genes (e.g., TWIST⁴) and key cellular events (e.g., mitotic spindle orientation⁵) have been shown to depend on the mechanical environment of the tissue. Several mechanosensors have also been identified.⁶In an earlier study, we showed that the orientation of the cortical microtubular cytoskeleton in plant shoot meristems depends on the principal direction of mechanical stress. Cortical microtubules are known to guide the deposition of the cellulose microfibrils in the cell wall and thus to control the main direction of growth, and consequently, shape. Evidence indicates that the epidermis is under tension, and therefore the shape of the tissue can influence the pattern of mechanical stress. In this framework, multicellular shape is transposed into a map of stress directions in the epidermis that can act as a supracellular instructional signal. By applying mechanical constraints on a meristem with GFP-marked microtubules, we were able to close the feedback loop: microtubule orientation became parallel to the externally applied stress, supporting a view in which mechanical stress controls cell behavior.⁷In a more recent study we showed that in addition to the cortical microtubules, the polar localization of the auxin efflux carrier PIN1 can also be controlled by its mechanical environment. In particular, we observed that, when viewed from the top, PIN1 is usually concentrated on the membrane that is parallel to the microtubule orientation. Furthermore, a single cell ablation, which induces both a circumferential pattern of stress around the wound and a circumferential orientation of microtubules, also induced a relocalization of PIN1 away from the wound on the circumferential membrane, consistent with the hypothesis that PIN1 would be preferentially recruited on the membrane undergoing the most tensile stress.⁸ Mechanistically, it is unclear how this could be achieved, but the PIN1 vesicle recycling machinery is likely to play a major role, since it is now well established that membrane tension inhibits endocytosis and favors exocytosis.⁹ In such a scenario, PIN1 would be trapped in a membrane as long as the tension of the membrane is higher than that of its neighbours.To further test the response of PIN1 to mechanical forces, we used a pharmacological approach. Figure 1 highlights the correlation between the predicted opposable impacts of isoxaben and oryzalin on stress and the response of PIN1. In the presence of isoxaben, a well known inhibitor of cellulose synthesis, the thickness of the cell wall is supposed to decrease. Knowing that mechanical stress is here defined as a force divided by the area of a section of the wall, stress is expected to increase after isoxaben treatment. When we treated PIN1-GFP meristems with isoxaben, we observed a “hyper” localization of PIN1, with in most cases a preferential localization of PIN1 along the supracellular stress patterns, and within the cell, a concentration of the signal at cell corners, predicted sites of stress maxima. In contrast, in the presence of oryzalin, which by depolymerising the microtubules leads to isotropic growth and thus isotropic stresses, PIN1 localization became more homogeneous.Open in a separate windowFigure 1Impact of isoxaben and oryzalin on the localization of PIN1 in meristematic cells. The PIN1-GFP signal (in black) is very heterogenous in the control meristematic cells, consistent with the preferential localization of PIN1 to one side of the cells. Sometimes the signal is even restricted to one cell corner. After microtubule depolymerization with oryzalin, cell growth becomes more isotropic, and while PIN1 localization remains heterogenous, the signal becomes more widespread on each plasma membranes and thus tends to homogeneity. In contrast, after isoxaben treatment (which inhibits cellulose synthesis and thus is predicted to increase stress levels), the PIN1-GFP protein concentrates at the corners of the cells.⁸It seems therefore plausible that mechanical stress acts as a common instructional signal for both microtubule-dependent cell anisotropy and PIN1/auxin-dependent growth rate. Mathematical modeling further supported this proposal. Several successful models for the generation of organ patterns in the meristem assume an ability of individual cells to sense auxin concentration in their neighbours.^10–16 However to date no mechanism had been proposed to explain how one cell could measure the concentration of auxin in its vicinity. One of the main implications of our study is that, if PIN1 can respond to the mechanical status of the wall, then it also integrates auxin concentration of the neighboring cells, indirectly, as auxin loosens the cell wall, allowing cell expansion. Using such a hypothesis, computer simulations were able to reproduce the stereotyped pattern of organogenesis in the shoot further confirming the plausibility of the model.It must be noted however that our work does not exclude other hypotheses. In particular, it has recently been proposed that the ROP2 and ROP6 proteins, well known effectors of cell polarity, could respond differently to ABP1-dependent auxin signaling, thus providing a model in which cell-cell communication via ROP could “measure” local differences in auxin between neighbors.¹⁷ These different scenarios could actually be reconciled some day, especially knowing that Rho proteins in animals have been involved in the responses to mechanical forces.¹⁸ Last, the control of PIN1 polar localization by its mechanical environment could actually reveal a more universal response of cells to the stiffness and tension of the extracellular matrix. Similarly, animal motile (and polar) cells can sense the rigidity of their substrate^19–21 and respond by reinforcing the cytoskeleton at the cell cortex.^22–25     

Barrett's esophagus: treatment or observation of a major precursor factor of esophageal cancer?

Barrett's esophagus (BE) is a major precursor factor of esophageal cancer (EC). The appropriate management of patients with BE depends on the presence or not of dysplasia and the type of dysplasia that occurs. Due to the small proportion of BE patients that progress to cancer, the value of surveillance programs are a matter of debate. On the contrary, in high risk group of patients surveillance programs have significant impact. Large prospective trials are needed to define the optimal management strategy. Elucidation of carcinogenesis' steps and signal transduction pathways could reveal potential biomarkers in the order of early prediction for a highly malignant neoplasm with dismal prognosis. An efficacious tailored-made manner focusing to the safety profile and associated costs should be practised for less severe disease. In this review a thorough investigation of all available methods dealing with the clinical management of BE is provided.     

Regulatory T cell subsets in human cancer: are they regulating for or against tumor progression?

Regulatory T cells (Treg) play a key role in maintaining the balance of immune responses in human health and in disease. Treg come in many flavors and can utilize a variety of mechanisms to modulate immune responses. In cancer, inducible (i) or adaptive Treg expand, accumulate in tissues and the peripheral blood of patients, and represent a functionally prominent component of CD4+ T lymphocytes. Phenotypically and functionally, iTreg are distinct from natural (n) Treg. A subset of iTreg expressing ectonucleotidases, CD39 and CD73, is able to hydrolyze ATP to 5′-AMP and adenosine (ADO) and thus mediate suppression of those immune cells which express ADO receptors. iTeg can also produce prostaglandin E₂ (PGE₂). These iTreg, expanding in response to tumor antigens and cytokines such as TGF-β or IL-10, are presumably responsible for the suppression of anti-tumor immune responses and for successful tumor escape. On the other hand, in cancers associated with prominent inflammatory infiltrates, e.g., colorectal carcinoma or certain types of breast cancer, iTreg down-regulate excessive inflammation by producing ADO and/or PGE₂ and protect the host from tissue injury and tumor development. Thus, iTreg utilizing the same adenosine pathway play a key but dual role in cancer, and their plasticity is controlled and driven by the microenvironment. Thus, monitoring for the frequency and functions of iTreg rather than nTreg is important in cancer. In addition, elimination of iTreg by various available strategies prior to immunotherapies may not be beneficial in all cases and needs to be undertaken with caution.     

Coordinating cell polarity and cell cycle progression: what can we learn from flies and worms?

Spatio-temporal coordination of events during cell division is crucial for animal development. In recent years, emerging data have strengthened the notion that tight coupling of cell cycle progression and cell polarity in dividing cells is crucial for asymmetric cell division and ultimately for metazoan development. Although it is acknowledged that such coupling exists, the molecular mechanisms linking the cell cycle and cell polarity machineries are still under investigation. Key cell cycle regulators control cell polarity, and thus influence cell fate determination and/or differentiation, whereas some factors involved in cell polarity regulate cell cycle timing and proliferation potential. The scope of this review is to discuss the data linking cell polarity and cell cycle progression, and the importance of such coupling for asymmetric cell division. Because studies in model organisms such as Caenorhabditis elegans and Drosophila melanogaster have started to reveal the molecular mechanisms of this coordination, we will concentrate on these two systems. We review examples of molecular mechanisms suggesting a coupling between cell polarity and cell cycle progression.     

Cardiac cell: a biological laser?

We present a new concept of cardiac cells based on an analogy with lasers, practical implementations of quantum resonators. In this concept, each cardiac cell comprises a network of independent nodes, characterised by a set of discrete energy levels and certain transition probabilities between them. Interaction between the nodes is given by threshold-limited energy transfer, leading to quantum-like behaviour of the whole network. We propose that in cardiomyocytes, during each excitation-contraction coupling cycle, stochastic calcium release and the unitary properties of ionic channels constitute an analogue to laser active medium prone to "population inversion" and "spontaneous emission" phenomena. This medium, when powered by an incoming threshold-reaching voltage discharge in the form of an action potential, responds to the calcium influx through L-type calcium channels by stimulated emission of Ca2+ ions in a coherent, synchronised and amplified release process known as calcium-induced calcium release. In parallel, phosphorylation-stimulated molecular amplification in protein cascades adds tuneable features to the cells. In this framework, the heart can be viewed as a coherent network of synchronously firing cardiomyocytes behaving as pulsed laser-like amplifiers, coupled to pulse-generating pacemaker master-oscillators. The concept brings a new viewpoint on cardiac diseases as possible alterations of "cell lasing" properties.     

T cell–tumor cell: a fatal interaction?

Fas (Apo-1/CD95) is a cell-surface protein that is responsible for initiating a cascade of proteases (caspases) culminating in apoptotic cell death in a variety of cell types. The function of the Fas/FasL system in the dampening of immune responses to infectious agents through the autocrine deletion of activated T cells has been well documented. More recently, it has been proposed that tumor cells express FasL, presumably to avoid immune detection. In this review, we focus on the role of the interaction of Fas and FasL in the modulation of antitumor responses. We critically examine the evidence that FasL is expressed by tumor cells and explore alternative explanations for the observed phenomena in vitro and in vivo. By reviewing data that we have generated in our laboratory as well as reports from the literature, we will argue that the Fas/FasL system is a generalized mechanism used in an autocrine fashion to regulate cell survival and expansion in response to environmental and cellular cues. We propose that FasL expression by tumor cells, when present, is indicative of a perturbed balance in the control of proliferation while “immune privilege” is established by “suicide” of activated antitumor T cells, a form of activation-induced cell death. Received: 5 May 1998 / Accepted: 20 May 1998     

One renegade cancer stem cell?

The CSC compartment represents the subpopulation of tumor cells with clonogenic potential and the ability to initiate new tumors. Besides self renewal, one of their main features is their ability to differentiate into variety of cells within the tumor. The question remains whether this potential resides within the single CSC or whether many different CSCs are necessary to generate a heterogeneous population of tumor cells. There is an increasing amount of evidence showing that single CSC indeed has the potential to reconstitute complete tumor phenotype. This is likely to be a general phenomenon and it has been demonstrated in many tumors so far. Here we show that single GBM CSCs have multilineage potential, although not exclusively. Furthermore, our results show that CSCs originating from same tumor are not necessarily uniform in respect to their differentiation potential.     

Student report--the fight against cancer and angiogenesis inhibitors: are we entering a new era?

Chemoresistance is currently the main cause of failure in the treatment of cancer which, despite extensive research, remains unsolved. In this report the theoretical assumptions underlying antiangiogenic therapy are described and future perspectives and limits are discussed.     

The cancer stem cell theory: is it correct?

The cancer stem cell hypothesis posits that tumor growth is driven by a rare subpopulation of cells, designated cancer stem cells (CSC). Studies supporting this theory are based in large part on xenotransplantation experiments wherein human cancer cells are grown in immunocompromised mice and only CSC, often constituting less than 1% of the malignancy, generate tumors. Herein, we show that all colonies derived from randomly chosen single cells in mouse lung and breast cancer cell lines form tumors following allografting histocompatible mice. Our study suggests that the majority of malignant cells rather than CSC can sustain tumors and that the cancer stem cell theory must be reevaluated.     

Iron: Key player in cancer and cell cycle?

BackgroundIron is an essential element for growth and metabolic activities of all living organisms but remains in its oxyhydroxide ferric ion form in the surrounding. Unavailability of iron in soluble ferrous form led to development of specific pathways and machinery in different organisms to make it available for use and maintain its homeostasis. Iron homeostasis is essential as under different circumstances iron in excess as well as deprivation leads to different pathological conditions in human.ObjectiveThis review highlights the current findings related to iron excess as well as deprivation with regards to cellular proliferation.ConclusionsIron excess is extensively associated with different types of cancers viz. colorectal cancer, breast cancer etc. by producing an oxidative stressed condition and alteration of immune system. Ironically its deprivation also results in anaemic conditions and leads to cell cycle arrest at different phases with mechanism yet to be explored. Iron deprivation arrests cell cycle at G1/S and in some cases at G2/M checkpoints resulting in growth arrest. However, in some cases iron overload arrests cell cycle at G1 phase by blocking certain signalling pathways. Certain natural and synthetic iron chelators are being explored from few decades to combat diseases caused by alteration in iron homeostasis.     

Is cytoskeletal tension a major determinant of cell deformability in adherent endothelial cells?

We tested the hypothesis that mechanical tension in thecytoskeleton (CSK) is a major determinant of cell deformability. To confirm that tension was present in adherent endothelial cells, weeither cut or detached them from their basal surface by a microneedle. After cutting or detachment, the cells rapidly retracted. This retraction was prevented, however, if the CSK actin lattice was disrupted by cytochalasin D (Cyto D). These results confirmed thatthere was preexisting CSK tension in these cells and that the actinlattice was a primary stress-bearing component of the CSK. Second, todetermine the extent to which that preexisting CSK tension could altercell deformability, we developed a stretchable cell culture membranesystem to impose a rapid mechanical distension (and presumably a rapidincrease in CSK tension) on adherent endothelial cells. Altered celldeformability was quantitated as the shear stiffness measured bymagnetic twisting cytometry. When membrane strain increased 2.5 or 5%,the cell stiffness increased 15 and 30%, respectively. Disruption ofactin lattice with Cyto D abolished this stretch-induced increase instiffness, demonstrating that the increased stiffness depended on theintegrity of the actin CSK. Permeabilizing the cells with saponin andwashing away ATP and Ca²⁺ did notinhibit the stretch-induced stiffening of the cell. These resultssuggest that the stretch-induced stiffening was primarily due to thedirect mechanical changes in the forces distending the CSK but not toATP- or Ca²⁺-dependent processes.Taken together, these results suggest preexisting CSK tension is amajor determinant of cell deformability in adherent endothelial cells.

     

A role for Wnt/planar cell polarity signaling during lens fiber cell differentiation?

Wnt signaling through frizzled (Fz) receptors plays key roles in just about every developmental system that has been studied. Several Wnt-Fz signaling pathways have been identified including the Wnt/planar cell polarity (PCP) pathway. PCP signaling is crucial for many developmental processes that require major cytoskeletal rearrangements. Downstream of Fz, PCP signaling is thought to involve the GTPases, Rho, Rac and Cdc42 and regulation of the JNK cascade. Here we report on the localization of these GTPases and JNK in the lens and assess their involvement in the cytoskeletal reorganisation that is a key element of FGF-induced lens fiber cell differentiation.     

Follicle cell processes: a shark thing?

Follicle cell processes (FCP) are identified in two species of carcharhinid shark (Selachii) but are absent in the little skate Leucoraja erinacea (Batoidea). This suggests that FCPs are either a unique structure that evolved in selachians or were lost by the batoids after their divergence, some 280 mya . The presence of FCPs in the selachians would be consistent with the evolution of large oocytes in this group of animals.     

Planar polarity: out of joint?

Epithelial structures, such as the wing hairs and ommatidia in Drosophila, are aligned in the plane of the epithelium. This planar polarity requires the transmembrane receptor Frizzled. Recent studies have shed new light on mechanisms that could be involved in generating or transducing the polarity signal.     

Cytotoxicity of δ-tocotrienols from Kielmeyera coriacea against cancer cell lines

In the search for new anti-cancer compounds, Brazilian Cerrado plant species have been investigated. The hexane root bark extract of Kielmeyera coriacea lead to a mixture of ??-tocotrienol (1) and its dimer (2). The structures of both compounds 1 and 2 were established based on detailed 1D and 2D NMR and EI-MS analyses. The cytotoxicity of the mixture was tested against four human tumor cell lines in the following cultures: MDA-MB-435 (melanoma), HCT-8 (colon), HL-60 (leukemia), and SF-295 (glioblastoma), and displayed IC₅₀ values ranging from 8.08 to 23.58 ??g/mL. Additional assays were performed in order to investigate the mechanism of action of the mixture (1 + 2) against the human leukemia cell line HL-60. The results suggested that the mixture suppressed leukemia growth and reduced cell survival, triggering both apoptosis and necrosis, depending on the concentration.