Localizing the adhesive and signaling functions of plakoglobin

Plakoglobin (PKG) is a major component of cell-cell adhesive junctions. It is also closely related to the Drosophila segment polarity gene product armadillo and can induce a WNT-like neural axis duplication (NAD) phenotype in Xenopus [Karnovsky and Klymkowsky, 1995].* To define the regions of PKG involved in cell adhesion and inductive signaling, we examined the behavior of mutated forms of PKG in Xenopus. Deletion of amino acids 22 through 39 (in the Xenopus. PKG sequence) increased the apparent stability of the polypeptide within the embryo and increased its ability to induce a WNT-like, NAD phenotype when expressed in the vegetal hemisphere. The N-terminal “head” and first 6 “ARM” repeats of PKG, or the C-terminal “tail” and the last 3 “ARM” repeats, could be removed without destroying the remaining polypeptide's ability to induce a NAD phenotype. The nuclear localization of mutant PKGs, however, was not strictly correlated with the ability to induce a NAD phenotype, i.e., some inactive polypeptides still accumulate in nuclei. Removal of PKG's head and first ARM repeat, which includes its α-catenin binding site, resulted in a polypeptide that, when expressed in the embryo, generated a dramatic cell adhesion defect. Removal of the next three ARM repeats abolished this adhesion defect, suggesting that the polypeptide no longer competes effectively with endogenous catenins for binding to cadherins. Expression of a form of PKG truncated after the 5th ARM repeat produced a milder cell adhesion defect, whereas expression of a polypeptide truncated after the 8th ARM repeat had little apparent effect on cellular adhesion. Based on these observations, we conclude that functions related to stability and cellular adhesion reside in the N-terminal region of the polypeptide, whereas the ability to induce a NAD phenotype lies within repeats 6–10 of the central region. The function(s) of the C-terminal domain of PKG remain uncertain at this time. Dev. Genet. 20:91–102, 1997. © 1997 Wiley-Liss, Inc.     

Planar polarity specification through asymmetric subcellular localization of Fat and Dachsous

Two pathways regulate planar polarity: the core proteins and the Fat-Dachsous-Four-jointed (Ft-Ds-Fj) system. Morphogens specify complementary expression patterns of Ds and Fj that potentially act as polarizing cues. It has been suggested that Ft-Ds-Fj-mediated cues are weak and that the core proteins amplify them. Another view is that the two pathways act independently to generate and propagate polarity: if correct, this raises the question of how gradients of Ft and Ds expression or activity might be interpreted to provide strong cellular polarizing cues and how such cues are propagated from cell to cell. Here, we demonstrate that the complementary expression of Ds and Fj results in biased Ft and Ds protein distribution across cells, with Ft and Ds accumulating on opposite edges. Furthermore, boundaries of Ft and Ds expression result in subcellular asymmetries in protein distribution that are transmitted to neighboring cells, and asymmetric Ds localization results in a corresponding asymmetric distribution of the myosin Dachs. We show that the generation of subcellular asymmetries of Ft and Ds and the core proteins is largely independent in the wing disc and additionally that ommatidial polarity in the eye can be determined without input from the Ft-Ds-Fj system, consistent with the two pathways acting in parallel.     

Fat/Dachsous family cadherins in cell and tissue organisation

Precisely controlled organisation at the cellular and tissue level is crucial to establish and maintain complex organisms. The atypical cadherins Fat (Ft), Fat2 and Dachsous (Ds) contribute to this organisation by regulating growth and planar cell polarity. Here we describe the recent advances in understanding how these large cadherins coordinate these processes, and discuss additional progress extending their function in regulation of microtubules, migration and disease.     

Interactions between Fat and Dachsous and the regulation of planar cell polarity in the Drosophila wing

It was recently suggested that a proximal to distal gradient of the protocadherin Dachsous (Ds) acts as a cue for planar cell polarity (PCP) in the Drosophila wing, orienting cell-cell interactions by inhibiting the activity of the protocadherin Fat (Ft). This Ft-Ds signaling model is based on mutant loss-of-function phenotypes, leaving open the question of whether Ds is instructive or permissive for PCP. We developed tools for misexpressing ds and ft in vitro and in vivo, and have used these to test aspects of the model. First, this model predicts that Ds and Ft can bind. We show that Ft and Ds mediate preferentially heterophilic cell adhesion in vitro, and that each stabilizes the other on the cell surface. Second, the model predicts that artificial gradients of Ds are sufficient to reorient PCP in the wing; our data confirms this prediction. Finally, loss-of-function phenotypes suggest that the gradient of ds expression is necessary for correct PCP throughout the wing. Surprisingly, this is not the case. Uniform levels of ds drive normally oriented PCP and, in all but the most proximal regions of the wing, uniform ds rescues the ds mutant PCP phenotype. Nor are distal PCP defects increased by the loss of spatial information from the distally expressed four-jointed (fj) gene, which encodes putative modulator of Ft-Ds signaling. Thus, while our results support the existence of Ft-Ds binding and show that it is sufficient to alter PCP, ds expression is permissive or redundant with other PCP cues in much of the wing.     

Mammalian Fat and Dachsous cadherins regulate apical membrane organization in the embryonic cerebral cortex

Compartmentalization of the plasma membrane in a cell is fundamental for its proper functions. In this study, we present evidence that mammalian Fat4 and Dachsous1 cadherins regulate the apical plasma membrane organization in the embryonic cerebral cortex. In neural progenitor cells of the cortex, Fat4 and Dachsous1 were concentrated together in a cell–cell contact area positioned more apically than the adherens junction (AJ). These molecules interacted in a heterophilic fashion, affecting their respective protein levels. We further found that Fat4 associated and colocalized with the Pals1 complex. Ultrastructurally, the apical junctions of the progenitor cells comprised the AJ and a stretch of plasma membrane apposition extending apically from the AJ, which positionally corresponded to the Fat4–Dachsous1-positive zone. Depletion of Fat4 or Pals1 abolished this membrane apposition. These results highlight the importance of the Fat4–Dachsous1–Pals1 complex in organizing the apical membrane architecture of neural progenitor cells.     

Role of endoproteolytic processing in the adhesive and signaling functions of alphavbeta5 integrin

Some integrin alpha subunits undergo a post-translational cleavage in their extracellular domain. However, the role of this cleavage in integrin function is unclear. Enzymes involved in this maturation belong to the subtilisin-like endoprotease family (convertases). To understand the role of the alpha subunit cleavage in integrin function, we have designed stable transfectants (PDX39P cells) expressing alpha(1)-PDX, a convertase inhibitor. Immunoprecipitation of cell surface proteins from PDX39P showed that alpha(3), alpha(6) and alpha(v) integrins lack endoproteolytic cleavage. We have compared adhesion between PDX39P cells and mock-transfected cells on different extracellular matrix proteins. No difference in adhesion could be observed on laminin-1 and type I collagen, while attachment of PDX39P cells to vitronectin (ligand of the alpha(v)beta(5) integrin) was dramatically reduced. The reduced adhesion of PDX39P cells was not due to changes in integrin affinity as determined by solid-phase receptor assay in a cell-free environment. Intracellular signaling pathways activated by alpha(v) integrin ligation were also affected in PDX39P cells. It thus seems that the absence of endoproteolytic cleavage of alpha(v) integrins has important consequences on signal transduction pathways leading to alterations in integrin function such as cell adhesion.     

Drosophila Dachsous and Fat polarize actin-based protrusions over a restricted domain of the embryonic denticle field

Atypical cadherins Dachsous (Ds) and Fat coordinate the establishment of planar polarity, essential for the patterning of complex tissues and organs. The precise mechanisms by which this system acts, particularly in cases where Ds and Fat act independently of the ‘core’ frizzled system, are still the subject of investigation. Examining the deployment of the Ds–Fat system in different tissues of the model organism Drosophila, has provided insights into the general mechanisms by which polarity is established and propagated to coordinate outcomes across a field of cells. The Drosophila embryonic epidermis provides a simple model epithelia where the establishment of polarity can be observed from start to finish, and in the absence of proliferation, over a fixed number of cells. Using the asymmetric placement of f-actin during denticle assembly as a read-out of polarity, we examine the requirement for Ds and Fat in establishing polarity across the denticle field. Comparing detailed phenotypic analysis with steady state protein enrichment revealed a spatially restricted requirement for the Ds–Fat system within the posterior denticle field. Ectopic Ds signaling provides evidence for a model whereby Ds acts to asymmetrically enrich Fat in a neighboring cell, in turn polarizing the cell to specify the position of the actin-based protrusions at the cell cortex.     

Two frizzled planar cell polarity signals in the Drosophila wing are differentially organized by the Fat/Dachsous pathway

The regular array of distally pointing hairs on the mature Drosophila wing is evidence for the fine control of Planar Cell Polarity (PCP) during wing development. Normal wing PCP requires both the Frizzled (Fz) PCP pathway and the Fat/Dachsous (Ft/Ds) pathway, although the functional relationship between these pathways remains under debate. There is strong evidence that the Fz PCP pathway signals twice during wing development, and we have previously presented a Bidirectional-Biphasic Fz PCP signaling model which proposes that the Early and Late Fz PCP signals are in different directions and employ different isoforms of the Prickle protein. The goal of this study was to investigate the role of the Ft/Ds pathway in the context of our Fz PCP signaling model. Our results allow us to draw the following conclusions: (1) The Early Fz PCP signals are in opposing directions in the anterior and posterior wing and converge precisely at the site of the L3 wing vein. (2) Increased or decreased expression of Ft/Ds pathway genes can alter the direction of the Early Fz PCP signal without affecting the Late Fz PCP signal. (3) Lowfat, a Ft/Ds pathway regulator, is required for the normal orientation of the Early Fz PCP signal but not the Late Fz PCP signal. (4) At the time of the Early Fz PCP signal there are symmetric gradients of dachsous (ds) expression centered on the L3 wing vein, suggesting Ds activity gradients may orient the Fz signal. (5) Localized knockdown or over-expression of Ft/Ds pathway genes shows that boundaries/gradients of Ft/Ds pathway gene expression can redirect the Early Fz PCP signal specifically. (6) Altering the timing of ds knockdown during wing development can separate the role of the Ft/Ds pathway in wing morphogenesis from its role in Early Fz PCP signaling.     

Protein tyrosine phosphorylation and the adhesive functions of platelets

The intracellular signalling pathways that mediate changes in cell behavior induced by extracellular matrix and cell adhesion molecules are poorly understood. Studies on the regulation of tyrosine phosphorylation in platelets indicate that cell-to-cell aggregation mediated by fibrinogen binding to its integrin-family receptor, GP IIb-IIIa, and events regulated by the putative adhesion receptor, GP IV (CD36), involve tyrosine phosphorylation. Thus, tyrosine phosphorylation is implicated in cellular events crucial for hemostasis. It may also be involved in signaling mediated by integrin receptors in other cell types.     

Separating the spindle,checkpoint, and timer functions of BubR1

BubR1 performs several roles during mitosis, affecting the spindle assembly checkpoint (SAC), mitotic timing, and spindle function, but the interdependence of these functions is unclear. We have analyzed in Drosophila melanogaster the mitotic phenotypes of kinase-dead (KD) BubR1 and BubR1 lacking the N-terminal KEN box. bubR1-KD individuals have a robust SAC but abnormal spindles with thin kinetochore fibers, suggesting that the kinase activity modulates microtubule capture and/or dynamics but is relatively dispensable for SAC function. In contrast, bubR1-KEN flies have normal spindles but no SAC. Nevertheless, mitotic timing is normal as long as Mad2 is present. Thus, the SAC, timer, and spindle functions of BubR1 are substantially separable. Timing is shorter in bubR1-KEN mad2 double mutants, yet in these flies, lacking both critical SAC components, chromosomes still segregate accurately, reconfirming that in Drosophila, reliable mitosis does not need the SAC.     

Fat facets and Liquid facets promote Delta endocytosis and Delta signaling in the signaling cells

Endocytosis modulates the Notch signaling pathway in both the signaling and receiving cells. One recent hypothesis is that endocytosis of the ligand Delta by the signaling cells is essential for Notch activation in the receiving cells. Here, we present evidence in strong support of this model. We show that in the developing Drosophila eye Fat facets (Faf), a deubiquitinating enzyme, and its substrate Liquid facets (Lqf), an endocytic epsin, promote Delta internalization and Delta signaling in the signaling cells. We demonstrate that while Lqf is necessary for three different Notch/Delta signaling events at the morphogenetic furrow, Faf is essential only for one: Delta signaling by photoreceptor precluster cells, which prevents recruitment of ectopic neurons. In addition, we show that the ubiquitin-ligase Neuralized (Neur), which ubiquitinates Delta, functions in the signaling cells with Faf and Lqf. The results presented bolster one model for Neur function in which Neur enhances Delta signaling by stimulating Delta internalization in the signaling cells. We propose that Faf plays a role similar to that of Neur in the Delta signaling cells. By deubiquitinating Lqf, which enhances the efficiency of Delta internalization, Faf stimulates Delta signaling.     

Two separate molecular systems, Dachsous/Fat and Starry night/Frizzled, act independently to confer planar cell polarity

Planar polarity is a fundamental property of epithelia in animals and plants. In Drosophila it depends on at least two sets of genes: one set, the Ds system, encodes the cadherins Dachsous (Ds) and Fat (Ft), as well as the Golgi protein Four-jointed. The other set, the Stan system, encodes Starry night (Stan or Flamingo) and Frizzled. The prevailing view is that the Ds system acts via the Stan system to orient cells. However, using the Drosophila abdomen, we find instead that the two systems operate independently: each confers and propagates polarity, and can do so in the absence of the other. We ask how the Ds system acts; we find that either Ds or Ft is required in cells that send information and we show that both Ds and Ft are required in the responding cells. We consider how polarity may be propagated by Ds-Ft heterodimers acting as bridges between cells.     

Morphogen control of wing growth through the Fat signaling pathway

Organ growth is influenced by organ patterning, but the molecular mechanisms that link patterning to growth have remained unclear. We show that the Dpp morphogen gradient in the Drosophila wing influences growth by modulating the activity of the Fat signaling pathway. Dpp signaling regulates the expression and localization of Fat pathway components, and Fat signaling through Dachs is required for the effect of the Dpp gradient on cell proliferation. Juxtaposition of cells that express different levels of the Fat pathway regulators four-jointed and dachsous stimulates expression of Fat/Hippo pathway target genes and cell proliferation, consistent with the hypothesis that the graded expression of these genes contributes to wing growth. Moreover, uniform expression of four-jointed and dachsous in the wing inhibits cell proliferation. These observations identify Fat as a signaling pathway that links the morphogen-mediated establishment of gradients of positional values across developing organs to the regulation of organ growth.     

Late signaling in the activated platelets upregulates tyrosine phosphatase SHP1 and impairs platelet adhesive functions: Regulation by calcium and Src kinase

Sustained stimulation of platelets with protease-activated receptor agonists in presence of extracellular calcium was associated with tyrosine dephosphorylation of specific proteins of relative mobilities 35, 67, and 75 kDa. From phosphatase assays and inhibitor studies SHP1, a Src homology 2 (SH2) domain-containing tyrosine phosphatase expressed abundantly in hemopoietic cells, was found to be upregulated in platelets between 25 and 30 min following thrombin stimulation. Concomitantly, SHP1 was tyrosine phosphorylated by, and coprecipitated with, Src tyrosine kinase. SHP1 activation, association with Src and dephosphorylation of specific proteins were dependent on extracellular calcium and maintenance of a higher cytosolic calcium plateau. There was progressive impairment of platelet functions like aggregability and clot retraction, associated with downregulation of fibrinogen-binding affinity of integrin alpha(IIb)beta(3), in the platelets exposed to thrombin for 45 min. This could reflect the late physiological changes in platelets when the cells are consistently exposed to stimulatory signals under thrombogenic environment in vivo.     

mTOR complex 2 signaling and functions

The mechanistic target of rapamycin (mTOR) plays a central role in cellular growth and metabolism. mTOR forms two distinct protein complexes, mTORC1 and mTORC2. Much is known about the regulation and functions of mTORC1 due to availability of a natural compound, rapamycin, that inhibits this complex. Studies that define mTORC2 cellular functions and signaling have lagged behind. The development of pharmacological inhibitors that block mTOR kinase activity, and thereby inhibit both mTOR complexes, along with availability of mice with genetic knockouts in mTOR complex components have now provided new insights on mTORC2 function and regulation. Since prolonged effects of rapamycin can also disrupt mTORC2, it is worth re-evaluating the contribution of this less-studied mTOR complex in cancer, metabolic disorders and aging. In this review, we focus on recent developments on mammalian mTORC2 signaling mechanisms and its cellular and tissue-specific functions.