Functional interactions between Fat family cadherins in tissue morphogenesis and planar polarity.

The atypical cadherin fat (ft) was originally discovered as a tumor suppressor in Drosophila and later shown to regulate a form of tissue patterning known as planar polarity. In mammals, four ft homologs have been identified (Fat1-4). Recently, we demonstrated that Fat4 plays a role in vertebrate planar polarity. Fat4 has the highest homology to ft, whereas other Fat family members are homologous to the second ft-like gene, ft2. Genetic studies in flies and mice imply significant functional differences between the two groups of Fat cadherins. Here, we demonstrate that Fat family proteins act both synergistically and antagonistically to influence multiple aspects of tissue morphogenesis. We find that Fat1 and Fat4 cooperate during mouse development to control renal tubular elongation, cochlear extension, cranial neural tube formation and patterning of outer hair cells in the cochlea. Similarly, Fat3 and Fat4 synergize to drive vertebral arch fusion at the dorsal midline during caudal vertebra morphogenesis. We provide evidence that these effects depend on conserved interactions with planar polarity signaling components. In flies, the transcriptional co-repressor Atrophin (Atro) physically interacts with Ft and acts as a component of Fat signaling for planar polarity. We find that the mammalian orthologs of atro, Atn1 and Atn2l, modulate Fat4 activity during vertebral arch fusion and renal tubular elongation, respectively. Moreover, Fat4 morphogenetic defects are enhanced by mutations in Vangl2, a 'core' planar cell polarity gene. These studies highlight the wide range and complexity of Fat activities and suggest that a Fat-Atrophin interaction is a conserved element of planar polarity signaling.

Organising cells into tissues: new roles for cell adhesion molecules in planar cell polarity.

Planar cell polarity (PCP) is the coordinated organization of cells within the plane of the epithelium, first described in Drosophila. A Frizzled signalling pathway dedicated to PCP (the non-canonical Frizzled pathway) acts through Dishevelled and small G proteins, as does the classical Wnt pathway, but then diverges downstream of Dishevelled. Recent studies have demonstrated a crucial role for several atypical cadherin molecules (Fat, Dachsous and Flamingo) in controlling PCP signalling. Recent work has also indicated that the first sign of PCP during development is the polarized localization of PCP proteins (Frizzled, Flamingo, Dishevelled, etc). Exciting new data reveal that this PCP pathway is conserved to man.

Nuclear localization of magphinins, alternative splicing products of the human trophinin gene.

Human magphinin proteins are translation products of differentially spliced transcripts from the 5' region of the human trophinin gene (TRO), whose 3' region encodes trophinin, a unique cell adhesion molecule involved in human embryo implantation. Magphinins belong to the MAGE (melanoma-associated antigen) family, and a previous study of mouse magphinins showed their expression in male and female germ cells, suggesting a role in germ cell development. Here, we characterized the structure and subcellular localization of human magphinins. Confocal microscopy analysis of ectopically expressed magphinins revealed that magphinin-alpha and -beta localize in the cytoplasm, whereas magphinin-gamma lacking the peptide encoded by exon-3 is nuclear. Following Triton X-100 extraction, DNA digestion, and high salt extraction magphinin-gamma remained nuclear, suggesting strong association with the nuclear matrix. A series of magphinin-gamma deletion mutants were generated and assayed for localization, which showed that the N-terminal region of the MAGE homology domain is necessary for nuclear localization. When magphinin-gamma was expressed in NIH3T3 cells, cells underwent G1 arrest. These results suggest that human magphinin-gamma inhibits cell cycle progression through nuclear activity.

Apical cell adhesion molecule, trophinin, localizes to the nuclear envelope.

Trophinin mediates homophilic and apical cell adhesion between trophoblastic cells and endometrial epithelial cells, which is potentially the initial attachment step in human embryo implantation. Since trophinin is an atypical membrane protein without the signal sequence, it is possible that trophinin localizes to the cytoplasm. By treating trophinin-expressing trophoblastic cells with a series of detergents, we found significant levels of endogenous trophinin in the cytoplasm, particularly at the nuclear envelope (NE). Fluorescence photobleaching of GFP-trophinin expressed in COS-1 cells showed the stable association of trophinin with the NE, suggesting an additional role of trophinin besides apical cell adhesion.

Localization of cAMP-dependent protein kinase in the actin and microtubule cytoskeletons in mouse hippocampal neurons.

Cyclic adenosine monophosphate-dependent protein kinase (PKA) is involved in various biological functions in neurons. To investigate the subcellular localization of PKA, we stained cultured hippocampal neurons with anti-PKA catalytic subunit antisera. PKA catalytic subunit colocalized with microtubules (MTs) in dendrites as well as with the actin filaments (F-actin) in growth cones. After treatment with cytochalasin B, the colocalization of PKA catalytic subunits with MTs was enhanced, whereas the colocalization with F-actin was suppressed. This result indicates that PKA is anchored to the actin and MT cytoskeletons, and disruption of F-actin releases PKA to the cytoplasm, which then leads to an increase in the amount of PKA in MT domains in the neuron.

Phosphorylation of the tumor suppressor fat is regulated by its ligand Dachsous and the kinase discs overgrown.

The Drosophila tumor suppressor gene fat encodes a large cadherin that regulates growth and a form of tissue organization known as planar cell polarity (PCP). Fat regulates growth via the Hippo kinase pathway, which controls expression of genes promoting cell proliferation and inhibiting apoptosis (reviewed in). The Hippo pathway is highly conserved and is implicated in the regulation of mammalian growth and cancer development. Genetic studies suggest that Fat activity is regulated by binding to another large cadherin, Dachsous (Ds). The tumor suppressor discs overgrown (dco)/Casein Kinase I delta/epsilon also regulates Hippo activity and PCP. The biochemical nature of how Fat, Ds, and Dco interact to regulate these pathways is poorly understood. Here we demonstrate that Fat is cleaved to generate 450 kDa and 110 kDa fragments (Fat(450) and Fat(110)). Fat(110) contains the cytoplasmic and transmembrane domain. The cytoplasmic domain of Fat binds Dco and is phosphorylated by Dco at multiple sites. Importantly, we show Fat forms cis-dimers and that Fat phosphorylation is regulated by Dachsous and Dco in vivo. We propose that Ds regulates Dco-dependent phosphorylation of Fat and Fat-associated proteins to control Fat signaling in growth and PCP.

Loss of Fat4 disrupts PCP signaling and oriented cell division and leads to cystic kidney disease.

Tissue organization in Drosophila is regulated by the core planar cell polarity (PCP) proteins Frizzled, Dishevelled, Prickle, Van Gogh and Flamingo. Core PCP proteins are conserved in mammals and function in mammalian tissue organization. Recent studies have identified another group of Drosophila PCP proteins, consisting of the protocadherins Fat and Dachsous (Ds) and the transmembrane protein Four-jointed (Fj). In Drosophila, Fat represses fj transcription, and Ds represses Fat activity in PCP. Here we show that Fat4 is an essential gene that has a key role in vertebrate PCP. Loss of Fat4 disrupts oriented cell divisions and tubule elongation during kidney development, leading to cystic kidney disease. Fat4 genetically interacts with the PCP genes Vangl2 and Fjx1 in cyst formation. In addition, Fat4 represses Fjx1 expression, indicating that Fat signaling is conserved. Together, these data suggest that Fat4 regulates vertebrate PCP and that loss of PCP signaling may underlie some cystic diseases in humans.

Significant differences between mouse and human trophinins are revealed by their expression patterns and targeted disruption of mouse trophinin gene.

Trophinin has been identified as a membrane protein mediating apical cell adhesion between two human cell lines: trophoblastic HT-H cells, and endometrial epithelial SNG-M cells. Expression patterns of trophinin in humans suggested its involvement in embryo implantation and early placental development. The mouse trophinin gene maps to the distal part of the X chromosome and corresponds to human chromosome Xp11.21-22, the locus where the human trophinin gene maps. Western blot analysis indicates that the molecular weight of mouse trophinin is 110 kDa, which is consistent with the calculated value of 107 kDa. Positive signals for trophinin proteins were detected in preimplantation mouse embryos at the morula and blastocyst stages. Implanting blastocysts do not show detectable levels of trophinin protein, demonstrating that trophinin is not involved in blastocyst adhesion to the uterus in the mouse. Mouse embryo strongly expressed trophinin in the epiblast 1 day after implantation. Trophinin protein was not found in the mouse uteri and placenta after 5.5 days postcoitus (dpc). Targeted disruption of the trophinin gene in the mouse showed a partial embryonic lethality in a 129/SvJ background, but the cause of this lethality remains undetermined. The present study indicates significant differences between mouse and human trophinins in their expression patterns, and it suggests that trophinin is not involved in embryo implantation and placental development in the mouse.