In vivo cell sorting in complementary segmental domains mediated by Eph receptors and ephrins.

The restriction of intermingling between specific cell populations is crucial for the maintenance of organized patterns during development. A striking example is the restriction of cell mixing between segments in the insect epidermis and the vertebrate hindbrain that may enable each segment to maintain a distinct identity. In the hindbrain, this is a result of different adhesive properties of odd- and even-numbered segments (rhombomeres), but an adhesion molecule with alternating segmental expression has not been found. However, blocking experiments suggest that Eph-receptor tyrosine kinases may be required for the segmental restriction of cells. Eph receptors and their membrane-bound ligands, ephrins, are expressed in complementary rhombomeres and, by analogy with their roles in axon pathfinding, could mediate cell repulsion at boundaries. Remarkably, transmembrane ephrins can themselves transduce signals, raising the possibility that bi-directional signalling occurs between adjacent ephrin- and Eph-receptor-expressing cells. We report here that mosaic activation of Eph receptors leads to sorting of cells to boundaries in odd-numbered rhombomeres, whereas mosaic activation of ephrins results in sorting to boundaries in even-numbered rhombomeres. These data implicate Eph receptors and ephrins in the segmental restriction of cell intermingling.     

Crystal structure of an Eph receptor-ephrin complex.

The Eph family of receptor tyrosine kinases and their membrane-anchored ephrin ligands are important in regulating cell-cell interactions as they initiate a unique bidirectional signal transduction cascade whereby information is communicated into both the Eph-expressing and the ephrin-expressing cells. Initially identified as regulators of axon pathfinding and neuronal cell migration, Ephs and ephrins are now known to have roles in many other cell-cell interactions, including those of vascular endothelial cells and specialized epithelia. Here we report the crystal structure of the complex formed between EphB2 and ephrin-B2, determined at 2.7 A resolution. Each Eph receptor binds an ephrin ligand through an expansive dimerization interface dominated by the insertion of an extended ephrin loop into a channel at the surface of the receptor. Two Eph-Ephrin dimers then join to form a tetramer, in which each ligand interacts with two receptors and each receptor interacts with two ligands. The Eph and ephrin molecules are precisely positioned and orientated in these complexes, promoting higher-order clustering and the initiation of bidirectional signalling.     

The SH2/SH3 adaptor Grb4 transduces B-ephrin reverse signals

Bidirectional signals mediated by membrane-anchored ephrins and Eph receptor tyrosine kinases have important functions in cell-cell recognition events, including those that occur during axon pathfinding and hindbrain segmentation. The reverse signal that is transduced into B-ephrin-expressing cells is thought to involve tyrosine phosphorylation of the signal's short, conserved carboxy-terminal cytoplasmic domain. The Src-homology-2 (SH2) domain proteins that associate with activated tyrosine-phosphorylated B-subclass ephrins have not been identified, nor has a defined cellular response to reverse signals been described. Here we show that the SH2/SH3 domain adaptor protein Grb4 binds to the cytoplasmic domain of B ephrins in a phosphotyrosine-dependent manner. In response to B-ephrin reverse signalling, cells increase FAK catalytic activity, redistribute paxillin, lose focal adhesions, round up, and disassemble F-actin-containing stress fibres. These cellular responses can be blocked in a dominant-negative fashion by expression of the isolated Grb4 SH2 domain. The Grb4 SH3 domains bind a unique set of other proteins that are implicated in cytoskeletal regulation, including the Cbl-associated protein (CAP/ponsin), the Abl-interacting protein-1 (Abi-1), dynamin, PAK1, hnRNPK and axin. These data provide a biochemical pathway whereby cytoskeletal regulators are recruited to Eph-ephrin bidirectional signalling complexes.     

Association of the Shc and Grb2/Sem5 SH2-containing proteins is implicated in activation of the Ras pathway by tyrosine kinases.

The mammalian shc gene encodes two overlapping, widely expressed proteins of 46 and 52K, with a carboxy-terminal SH2 domain that binds activated growth factor receptors, and a more amino-terminal glycine/proline-rich region. These shc gene products (Shc) are transforming when overexpressed in fibroblasts. Shc proteins become phosphorylated on tyrosine in cells stimulated with a variety of growth factors, and in cells transformed by v-src (ref. 2), suggesting that they are tyrosine kinase targets that control a mitogenic signalling pathway. Here we report that tyrosine-phosphorylated Shc proteins form a specific complex with a non-phosphorylated 23K polypeptide encoded by the grb2/sem-5 gene. The grb2/sem-5 gene product itself contains an SH2 domain, which mediates binding to Shc, and is implicated in activation of the Ras guanine nucleotide-binding protein by tyrosine kinases in both Caenorhabditis elegans and mammalian cells. Consistent with a role in signalling through Ras, shc overexpression induced Ras-dependent neurite outgrowth in PC12 cells. These results suggest that Shc tyrosine phosphorylation can couple tyrosine kinases to Grb2/Sem-5, through formation of a Shc-Grb2/Sem-5 complex, and thereby regulate the mammalian Ras signalling pathway.     

Negative regulation of T-cell activation and autoimmunity by Mgat5 N-glycosylation

T-cell activation requires clustering of a threshold number of T-cell receptors (TCRs) at the site of antigen presentation, a number that is reduced by CD28 co-receptor recruitment of signalling proteins to TCRs. Here we demonstrate that a deficiency in beta1,6 N-acetylglucosaminyltransferase V (Mgat5), an enzyme in the N-glycosylation pathway, lowers T-cell activation thresholds by directly enhancing TCR clustering. Mgat5-deficient mice showed kidney autoimmune disease, enhanced delayed-type hypersensitivity, and increased susceptibility to experimental autoimmune encephalomyelitis. Recruitment of TCRs to agonist-coated beads, TCR signalling, actin microfilament re-organization, and agonist-induced proliferation were all enhanced in Mgat5-/- T cells. Mgat5 initiates GlcNAc beta1,6 branching on N-glycans, thereby increasing N-acetyllactosamine, the ligand for galectins, which are proteins known to modulate T-cell proliferation and apoptosis. Indeed, galectin-3 was associated with the TCR complex at the cell surface, an interaction dependent on Mgat5. Pre-treatment of wild-type T cells with lactose to compete for galectin binding produced a phenocopy of Mgat5-/- TCR clustering. These data indicate that a galectin-glycoprotein lattice strengthened by Mgat5-modified glycans restricts TCR recruitment to the site of antigen presentation. Dysregulation of Mgat5 in humans may increase susceptibility to autoimmune diseases, such as multiple sclerosis.     

The DIX domain targets dishevelled to actin stress fibres and vesicular membranes

Colorectal cancer results from mutations in components of the Wnt pathway that regulate beta-catenin levels. Dishevelled (Dvl or Dsh) signals downstream of Wnt receptors and stabilizes beta-catenin during cell proliferation and embryonic axis formation. Moreover, Dvl contributes to cytoskeletal reorganization during gastrulation and mitotic spindle orientation during asymmetric cell division. Dvl belongs to a family of eukaryotic signalling proteins that contain a conserved 85-residue module of unknown structure and biological function called the DIX domain. Here we show that the DIX domain mediates targeting to actin stress fibres and cytoplasmic vesicles in vivo. Neighbouring interaction sites for actin and phospholipid are identified between two helices by nuclear magnetic resonance spectroscopy (NMR). Mutation of the actin-binding motif abolishes the cytoskeletal localization of Dvl, but enhances Wnt/beta-catenin signalling and axis induction in Xenopus. By contrast, mutation of the phospholipid interaction site disrupts vesicular association of Dvl, Dvl phosphorylation, and Wnt/beta-catenin pathway activation. We propose that partitioning of Dvl into cytoskeletal and vesicular pools by the DIX domain represents a point of divergence in Wnt signalling.     

RIP3 mediates the embryonic lethality of caspase-8-deficient mice

Apoptosis and necroptosis are complementary pathways controlled by common signalling adaptors, kinases and proteases; among these, caspase-8 (Casp8) is critical for death receptor-induced apoptosis. This caspase has also been implicated in non-apoptotic pathways that regulate Fas-associated via death domain (FADD)-dependent signalling and other less defined biological processes as diverse as innate immune signalling and myeloid or lymphoid differentiation patterns. Casp8 suppresses RIP3-RIP1 (also known as RIPK3-RIPK1) kinase complex-dependent necroptosis that follows death receptor activation as well as a RIP3-dependent, RIP1-independent necrotic pathway that has emerged as a host defence mechanism against murine cytomegalovirus. Disruption of Casp8 expression leads to embryonic lethality in mice between embryonic days 10.5 and 11.5 (ref. 7). Thus, Casp8 may naturally hold alternative RIP3-dependent death pathways in check in addition to promoting apoptosis. We find that RIP3 is responsible for the mid-gestational death of Casp8-deficient embryos. Remarkably, Casp8(-/-)Rip3(-/-) double mutant mice are viable and mature into fertile adults with a full immune complement of myeloid and lymphoid cell types. These mice seem immunocompetent but develop lymphadenopathy by four months of age marked by accumulation of abnormal T cells in the periphery, a phenotype reminiscent of mice with Fas-deficiency (lpr/lpr; also known as Fas). Thus, Casp8 contributes to homeostatic control in the adult immune system; however, RIP3 and Casp8 are together completely dispensable for mammalian development.     

Hedgehog signalling activity of Smoothened requires phosphorylation by protein kinase A and casein kinase I

The Hedgehog (Hh) family of secreted proteins governs cell growth and patterning in animal development. The Hh signal is transduced by the seven-transmembrane protein Smoothened (Smo); however, the mechanism by which Smo is regulated remains largely unknown. Here we show that protein kinase A (PKA) and casein kinase I (CKI) regulate Smo cell-surface accumulation and activity in response to Hh. Blocking PKA or CKI activity in the Drosophila wing disc prevents Hh-induced Smo accumulation and attenuates pathway activity, whereas increasing PKA activity promotes Smo accumulation and pathway activation. We show that PKA and CKI phosphorylate Smo at several sites, and that phosphorylation-deficient forms of Smo fail to accumulate on the cell surface and are unable to transduce the Hh signal. Conversely, phosphorylation-mimicking Smo variants show constitutive cell-surface expression and signalling activity. Furthermore, we find that the levels of Smo cell-surface expression and activity correlate with its levels of phosphorylation. Our data indicate that Hh induces progressive Smo phosphorylation by PKA and CKI, leading to elevation of Smo cell-surface levels and signalling activity.     

Ephrin signalling controls brain size by regulating apoptosis of neural progenitors

Mechanisms controlling brain size include the regulation of neural progenitor cell proliferation, differentiation, survival and migration. Here we show that ephrin-A/EphA receptor signalling plays a key role in controlling the size of the mouse cerebral cortex by regulating cortical progenitor cell apoptosis. In vivo gain of EphA receptor function, achieved through ectopic expression of ephrin-A5 in early cortical progenitors expressing EphA7, caused a transient wave of neural progenitor cell apoptosis, resulting in premature depletion of progenitors and a subsequent dramatic decrease in cortical size. In vitro treatment with soluble ephrin-A ligands similarly induced the rapid death of cultured dissociated cortical progenitors in a caspase-3-dependent manner, thereby confirming a direct effect of ephrin/Eph signalling on apoptotic cascades. Conversely, in vivo loss of EphA function, achieved through EphA7 gene disruption, caused a reduction in apoptosis occurring normally in forebrain neural progenitors, resulting in an increase in cortical size and, in extreme cases, exencephalic forebrain overgrowth. Together, these results identify ephrin/Eph signalling as a physiological trigger for apoptosis that can alter brain size and shape by regulating the number of neural progenitors.     

The structural basis of Arfaptin-mediated cross-talk between Rac and Arf signalling pathways

Small G proteins are GTP-dependent molecular switches that regulate numerous cellular functions. They can be classified into homologous subfamilies that are broadly associated with specific biological processes. Cross-talk between small G-protein families has an important role in signalling, but the mechanism by which it occurs is poorly understood. The coordinated action of Arf and Rho family GTPases is required to regulate many cellular processes including lipid signalling, cell motility and Golgi function. Arfaptin is a ubiquitously expressed protein implicated in mediating cross-talk between Rac (a member of the Rho family) and Arf small GTPases. Here we show that Arfaptin binds specifically to GTP-bound Arf1 and Arf6, but binds to Rac.GTP and Rac.GDP with similar affinities. The X-ray structure of Arfaptin reveals an elongated, crescent-shaped dimer of three-helix coiled-coils. Structures of Arfaptin with Rac bound to either GDP or the slowly hydrolysable analogue GMPPNP show that the switch regions adopt similar conformations in both complexes. Our data highlight fundamental differences between the molecular mechanisms of Rho and Ras family signalling, and suggest a model of Arfaptin-mediated synergy between the Arf and Rho family signalling pathways.     

Casein kinase 1 gamma couples Wnt receptor activation to cytoplasmic signal transduction

Signalling by Wnt proteins (Wingless in Drosophila) has diverse roles during embryonic development and in adults, and is implicated in human diseases, including cancer. LDL-receptor-related proteins 5 and 6 (LRP5 and LRP6; Arrow in Drosophila) are key receptors required for transmission of Wnt/beta-catenin signalling in metazoa. Although the role of these receptors in Wnt signalling is well established, their coupling with the cytoplasmic signalling apparatus remains poorly defined. Using a protein modification screen for regulators of LRP6, we describe the identification of Xenopus Casein kinase 1 gamma (CK1gamma), a membrane-bound member of the CK1 family. Gain-of-function and loss-of-function experiments show that CK1gamma is both necessary and sufficient to transduce LRP6 signalling in vertebrates and Drosophila cells. In Xenopus embryos, CK1gamma is required during anterio-posterior patterning to promote posteriorizing Wnt/beta-catenin signalling. CK1gamma is associated with LRP6, which has multiple, modular CK1 phosphorylation sites. Wnt treatment induces the rapid CK1gamma-mediated phosphorylation of these sites within LRP6, which, in turn, promotes the recruitment of the scaffold protein Axin. Our results reveal an evolutionarily conserved mechanism that couples Wnt receptor activation to the cytoplasmic signal transduction apparatus.     

Homeobox gene Nkx2.2 and specification of neuronal identity by graded Sonic hedgehog signalling

During vertebrate development, the specification of distinct cell types is thought to be controlled by inductive signals acting at different concentration thresholds. The degree of receptor activation in response to these signals is a known determinant of cell fate, but the later steps at which graded signals are converted into all-or-none distinctions in cell identity remain poorly resolved. In the ventral neural tube, motor neuron and interneuron generation depends on the graded activity of the signalling protein Sonic hedgehog (Shh). These neuronal subtypes derive from distinct progenitor cell populations that express the homeodomain proteins Nkx2.2 or Pax6 in response to graded Shh signalling. In mice lacking Pax6, progenitor cells generate neurons characteristic of exposure to greater Shh activity. However, Nkx2.2 expression expands dosally in Pax6 mutants, raising the possibility that Pax6 controls neuronal pattern indirectly. Here we provide evidence that Nkx2.2 has a primary role in ventral neuronal patterning. In Nkx2.2 mutants, Pax6 expression is unchanged but cells undergo a ventral-to-dorsal transformation in fate and generate motor neurons rather than interneurons. Thus, Nkx2.2 has an essential role in interpreting graded Shh signals and selecting neuronal identity.     

A protein-tyrosine phosphatase with sequence similarity to the SH2 domain of the protein-tyrosine kinases.

The phosphorylation of proteins at tyrosine residues is critical in cellular signal transduction, neoplastic transformation and control of the mitotic cycle. These mechanisms are regulated by the activities of both protein-tyrosine kinases (PTKs) and protein-tyrosine phosphatases (PTPases). As in the PTKs, there are two classes of PTPases: membrane associated, receptor-like enzymes and soluble proteins. Here we report the isolation of a complementary DNA clone encoding a new form of soluble PTPase, PTP1C. The enzyme possesses a large noncatalytic region at the N terminus which unexpectedly contains two adjacent copies of the Src homology region 2 (the SH2 domain) found in various nonreceptor PTKs and other cytoplasmic signalling proteins. As with other SH2 sequences, the SH2 domains of PTP1C formed high-affinity complexes with the activated epidermal growth factor receptor and other phosphotyrosine-containing proteins. These results suggest that the SH2 regions in PTP1C may interact with other cellular components to modulate its own phosphatase activity against interacting substrates. PTPase activity may thus directly link growth factor receptors and other signalling proteins through protein-tyrosine phosphorylation.     

A role for Wnt signalling in self-renewal of haematopoietic stem cells

Haematopoietic stem cells (HSCs) have the ability to renew themselves and to give rise to all lineages of the blood; however, the signals that regulate HSC self-renewal remain unclear. Here we show that the Wnt signalling pathway has an important role in this process. Overexpression of activated beta-catenin expands the pool of HSCs in long-term cultures by both phenotype and function. Furthermore, HSCs in their normal microenvironment activate a LEF-1/TCF reporter, which indicates that HCSs respond to Wnt signalling in vivo. To demonstrate the physiological significance of this pathway for HSC proliferation we show that the ectopic expression of axin or a frizzled ligand-binding domain, inhibitors of the Wnt signalling pathway, leads to inhibition of HSC growth in vitro and reduced reconstitution in vivo. Furthermore, activation of Wnt signalling in HSCs induces increased expression of HoxB4 and Notch1, genes previously implicated in self-renewal of HSCs. We conclude that the Wnt signalling pathway is critical for normal HSC homeostasis in vitro and in vivo, and provide insight into a potential molecular hierarchy of regulation of HSC development.     

Attenuation of FGF signalling in mouse beta-cells leads to diabetes

Fibroblast growth factor (FGF) signalling has been implicated in patterning, proliferation and cell differentiation in many organs, including the developing pancreas. Here we show that the FGF receptors (FGFRs) 1 and 2, together with the ligands FGF1, FGF2, FGF4, FGF5, FGF7 and FGF10, are expressed in adult mouse beta-cells, indicating that FGF signalling may have a role in differentiated beta-cells. When we perturbed signalling by expressing dominant-negative forms of the receptors, FGFR1c and FGFR2b, in the pancreas, we found that that mice with attenuated FGFR1c signalling, but not those with reduced FGFR2b signalling, develop diabetes with age and exhibit a decreased number of beta-cells, impaired expression of glucose transporter 2 and increased proinsulin content in beta-cells owing to impaired expression of prohormone convertases 1/3 and 2. These defects are all characteristic of patients with type-2 diabetes. Mutations in the homeobox gene Ipf1/Pdx1 are linked to diabetes in both mouse and human. We also show that Ipf1/Pdx1 is required for the expression of FGFR1 signalling components in beta-cells, indicating that Ipf1/Pdx1 acts upstream of FGFR1 signalling in beta-cells to maintain proper glucose sensing, insulin processing and glucose homeostasis.