Molecular insights into the axon guidance molecules Sidestep and Beaten path

The transmembrane protein Sidestep (Side) functions as a substrate-bound attractant for motor axons in Drosophila. Outgrowing motor axons recognize Side via Beaten path Ia (Beat) and migrate along Side-expressing tissues. Here, we report a structure-function analysis of these guidance molecules using a variety of mutant lines and transgenic constructs. Investigation of Side mutants shows that the exchange of a single amino acid (L241H) in the second immunoglobulin domain disturbs Side function and subcellular localization. Overexpression of Side and Beat deletion constructs in S2 cells and muscles demonstrate that the first Ig domains of both proteins are necessary for their interaction. Furthermore, subcellular distributions of several Beat constructs identify functional domains and suggest a potential posttranslational processing step in ER compartments. In fact, fusing full-length Beat at both the N- and C-terminus with GFP and mCherry, respectively, shows that the N-terminal domain is transported to the plasma membrane and exposed on the cell surface, while the C-terminal domain accumulated in the nucleus. Taken together, these results give insights into the interaction of Side and Beat and imply that Beat might be subject to proteolytic cleavage during maturation.

Rearrangements in the musculature correlate with jumping behaviour in legless Mediterranean fruit fly larvae Ceratitis capitata (Tephritidae)

Larvae of holometabolic insects evolved different crawling strategies depending on the presence or absence of larval legs or life style. A rather unusual mode of locomotion has independently evolved in legless larvae of several dipteran species. Maggots of the Mediterranean fruit fly Ceratitis capitata developed an effective jumping mechanism to increase locomotion speed or to deter predators during the search for suitable pupation sites. Here, we use high-speed videography to visualize even the fastest movements during jump preparation and take-off. Quantification of kinetic and biometric parameters reveal that maggots jump up to 15-fold of their body length from a standing position and gain speed with 27 times the acceleration of gravity. Videos at high spatial resolution show the mechanism of latch formation and release in unprecedented detail. Mouth hooks insert in the caudal segment and raise a cuticular fold that serves as a handle to pressurize the body prior to launch. Since locomotion behaviour should be intrinsically linked to neuromuscular systems, we dissected third instar larvae and determined the precise pattern of abdominal muscles fibres. Compared to non-jumping dipteran larvae, such as Drosophila melanogaster, the overall arrangement is highly similar, but a few muscle fibres show characteristic re-arrangements in orientation and strength that are consistent with a role in bending and jumping. These results suggest that body wall muscles show adaptations to jumping behaviour in Ceratitis larvae, and possibly also in other species with different jumping techniques.

Dynamic monitoring of vital functions and tissue re-organization in Saturnia pavonia (Lepidoptera, Saturniidae) during final metamorphosis by non-invasive MRI

Magnetic resonance imaging (MRI) is the key whole-body imaging technology for observing processes within a living object providing excellent resolution and contrast between soft tissues. In the present work, we exploited the non-destructive properties of MRI to track longitudinally the dynamic changes that take place in developing pupae of the Emperor Moth (Saturnia pavonia) during the last days before eclosion. While in diapause pupae, body fluid was almost homogeneously distributed over the internal compartments, as soon as wings, legs, flight muscles and the head region were fully developed, a significant redistribution of water levels occurred between thoracic and abdominal regions. During the last two days before eclosion, the developing moths transferred substantial amounts of liquid into the gut and the labial gland, and in case of females, into developing eggs. Concomitantly, the volume of the air sacs increased drastically and their expansion/compression became clearly visible in time-resolved MR images. Furthermore, besides ventilation of the tracheal system, air sacs are likely to serve as volume reservoir for liquid transfer during development of the moths inside their pupal case. In parallel, we were able to monitor noninvasively lipid consumption, cardiac activity and haemolymph circulation during final metamorphosis.

Misregulation of Drosophila Sidestep Leads to Uncontrolled Wiring of the Adult Neuromuscular System and Severe Locomotion Defects

Holometabolic organisms undergo extensive remodelling of their neuromuscular system during metamorphosis. Relatively, little is known whether or not the embryonic guidance of molecules and axonal growth mechanisms are re-activated for the innervation of a very different set of adult muscles. Here, we show that the axonal attractant Sidestep (Side) is re-expressed during Drosophila metamorphosis and is indispensable for neuromuscular wiring. Mutations in side cause severe innervation defects in all legs. Neuromuscular junctions (NMJs) show a reduced density or are completely absent at multi-fibre muscles. Misinnervation strongly impedes, but does not completely abolish motor behaviours, including walking, flying, or grooming. Overexpression of Side in developing muscles induces similar innervation defects; for example, at indirect flight muscles, it causes flightlessness. Since muscle-specific overexpression of Side is unlikely to affect the central circuits, the resulting phenotypes seem to correlate with faulty muscle wiring. We further show that mutations in beaten path Ia (beat), a receptor for Side, results in similar weaker adult innervation and locomotion phenotypes, indicating that embryonic guidance pathways seem to be reactivated during metamorphosis.

Axon Guidance and Collective Cell Migration by Substrate-Derived Attractants

Neurons have evolved specialized growth structures to reach and innervate their target cells. These growth cones express specific receptor molecules that sense environmental cues and transform them into steering decisions. Historically, various concepts of axon guidance have been developed to better understand how axons reach and identify their targets. The essence of these efforts seems to be that growth cones require solid substrates and that major guidance decisions are initiated by extracellular cues. These sometimes highly conserved ligands and receptors have been extensively characterized and mediate four major guidance forces: chemoattraction, chemorepulsion, contact attraction and contact repulsion. However, during development, cells, too, do migrate in order to reach molecularly-defined niches at target locations. In fact, axonal growth could be regarded as a special case of cellular migration, where only a highly polarized portion of the cell is elongating. Here, I combine several examples from genetically tractable model organisms, such as Drosophila or zebrafish, in which cells and axons are guided by attractive cues. Regardless, if these cues are secreted into the extracellular space or exposed on cellular surfaces, migrating cells and axons seem to keep close contact with these attractants and seem to detect them right at their source. Migration towards and along such substrate-derived attractants seem to be particularly robust, as genetic deletion induces obvious searching behaviors and permanent guidance errors. In addition, forced expression of these factors in ectopic tissues is highly distractive too, regardless of the pattern of other endogenous cues. Thus, guidance and migration towards and along attractive tissues is a powerful steering mechanism that exploits affinity differences to the surroundings and, in some instances, determines growth trajectories from source to target region.

Sidestep-induced neuromuscular miswiring causes severe locomotion defects in Drosophila larvae

Mutations in motor axon guidance molecules cause aberrant projection patterns of motor nerves. As most studies in Drosophila have analysed these molecules in fixed embryos, the consequences for larval locomotion are entirely unexplored. Here, we took advantage of sidestep (side)-mutant larvae that display severe locomotion defects because of irreparable innervation errors. Mutations in side affected all motor nerve branches and all body wall regions. Innervation defects were non-stereotypical, showing unique innervation patterns in each hemisegment. Premature activation of Side in muscle precursors abrogated dorsal migration of motor nerves, resulting in larvae with a complete loss of neuromuscular junctions on dorsal-most muscles. High-speed videography showed that these larvae failed to maintain substrate contact and inappropriately raised both head and tail segments above the substrate, resulting in unique ‘arching’ and ‘lifting’ phenotypes. These results show that guidance errors in side mutants are maintained throughout larval life and are asymmetrical with respect to the bilateral body axis. Together with similar findings in mice, this study also suggests that miswiring could be an underlying cause of inherited movement disorders.

Impaired protein translation in Drosophila models for Charcot-Marie-Tooth neuropathy caused by mutant tRNA synthetases

Dominant mutations in five tRNA synthetases cause Charcot-Marie-Tooth (CMT) neuropathy, suggesting that altered aminoacylation function underlies the disease. However, previous studies showed that loss of aminoacylation activity is not required to cause CMT. Here we present a Drosophila model for CMT with mutations in glycyl-tRNA synthetase (GARS). Expression of three CMT-mutant GARS proteins induces defects in motor performance and motor and sensory neuron morphology, and shortens lifespan. Mutant GARS proteins display normal subcellular localization but markedly reduce global protein synthesis in motor and sensory neurons, or when ubiquitously expressed in adults, as revealed by FUNCAT and BONCAT. Translational slowdown is not attributable to altered tRNA(Gly) aminoacylation, and cannot be rescued by Drosophila Gars overexpression, indicating a gain-of-toxic-function mechanism. Expression of CMT-mutant tyrosyl-tRNA synthetase also impairs translation, suggesting a common pathogenic mechanism. Finally, genetic reduction of translation is sufficient to induce CMT-like phenotypes, indicating a causal contribution of translational slowdown to CMT.

The synaptic cytoskeleton in development and disease

The cytoskeleton forms the backbone of neuronal architecture, sustaining its form and size, subcellular compartments and cargo logistics. The synaptic cytoskeleton can be categorized in the microtubule-based core cytoskeleton and the cortical membrane skeleton. While central microtubules form the fundamental basis for the construction of elaborate neuronal processes, including axons and synapses, cortical actin filaments are generally considered to function as mediators of synapse dynamics and plasticity. More recently, the submembranous network of spectrin and ankyrin molecules has been involved in the regulation of synaptic stability and maintenance. Disruption of the synaptic cytoskeleton primarily affects the stability and maturation of synapses but also secondarily disturbs neuronal communication. Consequently, a variety of inherited diseases are accompanied by cytoskeletal malfunctions, including spastic paraplegias, spinocerebellar ataxias, and mental retardation. Since the primary reasons for many of these diseases are still unknown model organisms with a conserved repertoire of cytoskeletal elements help to understand the underlying biological mechanisms. The astonishing technical as well as genetic accessibility of synapses in Drosophila has shown that loss of the cytoskeletal architecture leads to axonal transport defects, synaptic maturation deficits, and retraction of synaptic boutons, before synaptic terminals finally detach from their target cells, suggesting that similar processes could be involved in human neuronal diseases.

Drosophila neuroligin 1 promotes growth and postsynaptic differentiation at glutamatergic neuromuscular junctions

Precise apposition of presynaptic and postsynaptic domains is a fundamental property of all neuronal circuits. Experiments in vitro suggest that Neuroligins and Neurexins function as key regulatory proteins in this process. In a genetic screen, we recovered several mutant alleles of Drosophila neuroligin 1 (dnlg1) that cause a severe reduction in bouton numbers at neuromuscular junctions (NMJs). In accord with reduced synapse numbers, these NMJs show reduced synaptic transmission. Moreover, lack of postsynaptic DNlg1 leads to deficits in the accumulation of postsynaptic glutamate receptors, scaffold proteins, and subsynaptic membranes, while increased DNlg1 triggers ectopic postsynaptic differentiation via its cytoplasmic domain. DNlg1 forms discrete clusters adjacent to postsynaptic densities. Formation of these clusters depends on presynaptic Drosophila Neurexin (DNrx). However, DNrx binding is not an absolute requirement for DNlg1 function. Instead, other signaling components are likely involved in DNlg1 transsynaptic functions, with essential interactions organized by the DNlg1 extracellular domain but also by the cytoplasmic domain.

Searching for guidance cues: follow the Sidestep trail

Neuronal growth cones migrate along stereotypic pathways to find and select their correct targets. Although it is widely believed that attractive and repulsive guidance molecules provide directional cues for migrating growth cones, it is still only poorly understood how these factors cooperate in a spatial and temporal manner. We have recently proposed that Drosophila motor axons recognize and follow a Sidestep-labeled substrate pathway from the ventral nerve cord to their peripheral target muscles. Sidestep (Side) is a transmembrane protein of the immunoglobulin superfamily and functions as a target-derived attractant. On motor axons, Beaten path Ia (Beat) is required to detect Side. In addition, Beat interacts with Side both genetically and biochemically, leading to the formation of heterophilic adhesion complexes in vitro. Since Side is expressed in sensory neurons, Beat-expressing motor axons fasciculate with sensory axons and use them as migratory substrates. In a similar process, motor axons contact a subset of Side-expressing glial cells, demonstrating that, during the period of axonal pathfinding, motor axons interact with all cell types that later will be the major constituents of peripheral nerves. Here, I discuss the idea that Sidestep-mediated attraction might represent the initial step for the controlled assembly of peripheral nerves.

No sidesteps on a beaten track: motor axons follow a labeled substrate pathway

The establishment of synaptic connections between motor neurons and muscle fibers is essential for controlled body movements in any higher organism. The wiring of the neuromuscular system in Drosophila serves as a model system for the identification of key regulatory proteins that control axon guidance and target recognition. Sidestep (Side) is a transmembrane protein of the immunoglobulin superfamily and plays a pivotal role in the coordination of motor axonal guidance decisions, as it functions as a target-derived attractant. Side, however, is expressed in a highly dynamic pattern during embryogenesis, making it difficult to deduce its precise function. We have recently shown that the expression of Side strongly correlates with the actual position of motor axonal growth cones. Motor axons seem to recognize and follow Side-positive surfaces until they reach their target fields. The motor neuronal protein Beaten path Ia (Beat) is required to detect Side. In beat mutant embryos, motor axons are no longer attracted to Side-expressing tissues. In addition, Beat and Side interact biochemically, forming heterophilic adhesion complexes in vitro. Here, I discuss the model that preferential adhesion of Beat-expressing growth cones to Side-labeled substrates could be a powerful mechanism to guide motor axons.

Drosophila motor axons recognize and follow a Sidestep-labeled substrate pathway to reach their target fields

During development of the Drosophila nervous system, migrating motor axons contact and interact with different cell types before reaching their peripheral muscle fields. The axonal attractant Sidestep (Side) is expressed in most of these intermediate targets. Here, we show that motor axons recognize and follow Side-expressing cell surfaces from the ventral nerve cord to their target region. Contact of motor axons with Side-expressing cells induces the down-regulation of Side. In the absence of Side, the interaction with intermediate targets is lost. Misexpression of Side in side mutants strongly attracts motor axons to ectopic sites. We provide evidence that, on motor axons, Beaten path Ia (Beat) functions as a receptor or part of a receptor complex for Side. In beat mutants, motor axons no longer recognize Side-expressing cell surfaces. Furthermore, Beat interacts with Side both genetically and biochemically. These results suggest that the tracing of Side-labeled cell surfaces by Beat-expressing growth cones is a major principle of motor axon guidance in Drosophila.

Drosophila ankyrin 2 is required for synaptic stability

Synaptic connections are stabilized through transsynaptic adhesion complexes that are anchored in the underlying cytoskeleton. The Drosophila neuromuscular junction (NMJs) serves as a model system to unravel genes required for the structural remodeling of synapses. In a mutagenesis screen for regulators of synaptic stability, we recovered mutations in Drosophila ankyrin 2 (ank2) affecting two giant Ank2 isoforms that are specifically expressed in the nervous system and associate with the presynaptic membrane cytoskeleton. ank2 mutant larvae show severe deficits in the stability of NMJs, resulting in a reduction in overall terminal size, withdrawal of synaptic boutons, and disassembly of presynaptic active zones. In addition, lack of Ank2 leads to disintegration of the synaptic microtubule cytoskeleton. Microtubules and microtubule-associated proteins fail to extend into distant boutons. Interestingly, Ank2 functions downstream of spectrin in the anchorage of synaptic microtubules, providing the cytoskeletal scaffold that is essential for synaptic stability.

Drosophila MICAL regulates myofilament organization and synaptic structure

The overall size and structure of a synaptic terminal is an important determinant of its function. In a large-scale mutagenesis screen, designed to identify Drosophila mutants with abnormally structured neuromuscular junctions (NMJs), we discovered mutations in Drosophila mical, a conserved gene encoding a multi-domain protein with a N-terminal monooxygenase domain. In mical mutants, synaptic boutons do not sprout normally over the muscle surface and tend to form clusters along synaptic branches and at nerve entry sites. Consistent with high expression of MICAL in somatic muscles, immunohistochemical stainings reveal that the subcellular localization and architecture of contractile muscle filaments are dramatically disturbed in mical mutants. Instead of being integrated into a regular sarcomeric pattern, actin and myosin filaments are disorganized and accumulate beneath the plasmamembrane. Whereas contractile elements are strongly deranged, the proposed organizer of sarcomeric structure, D-Titin, is much less affected. Transgenic expression of interfering RNA molecules demonstrates that MICAL is required in muscles for the higher order arrangement of myofilaments. Ultrastructural analysis confirms that myosin-rich thick filaments enter submembranous regions and interfere with synaptic development, indicating that the disorganized myofilaments may cause the synaptic growth phenotype. As a model, we suggest that the filamentous network around synaptic boutons restrains the spreading of synaptic branches.

At the next stop sign turn right: the metalloprotease Tolloid-related 1 controls defasciculation of motor axons in Drosophila

Navigation of motoneuronal growth cones toward the somatic musculature in Drosophila serves as a model system to unravel the molecular mechanisms of axon guidance and target selection. In a large-scale mutagenesis screen, we identified piranha, a motor axon guidance mutant that shows strong defects in the neuromuscular connectivity pattern. In piranha mutant embryos, permanent defasciculation errors occur at specific choice points in all motor pathways. Positional cloning of piranha revealed point mutations in tolloid-related 1 (tlr1), an evolutionarily conserved gene encoding a secreted metalloprotease. Ectopic expression of Tlr1 in several tissues of piranha mutants, including hemocytes, completely restores the wild-type innervation pattern, indicating that Tlr1 functions cell non-autonomously. We further show that loss-of-function mutants of related metalloproteases do not have motor axon guidance defects and that the respective proteins cannot functionally replace Tlr1. tlr1, however, interacts with sidestep, a muscle-derived attractant. Double mutant larvae of tlr1 and sidestep show an additive phenotype and lack almost all neuromuscular junctions on ventral muscles, suggesting that Tlr1 functions together with Sidestep in the defasciculation process.

The expression pattern of the Drosophila vesicular glutamate transporter: a marker protein for motoneurons and glutamatergic centers in the brain

To determine the functions of genes in distinct tissues during the development of Drosophila, it is often desirable to have genetic tools for targeted gene expression in restricted subsets of cells. Here, we report the identification of the enhancer trap line OK371-Gal4, which is expressed in a defined subset of neurons from embryonic stage 15 to adulthood. In the ventral nerve chord, it is expressed almost exclusively in motoneurons and in the brain in a limited number of neuronal clusters. The OK371 enhancer trap element is inserted in the proximity of the annotated gene CG9887, which encodes a Drosophila vesicular glutamate transporter (DVGLUT). In situ hybridization experiments using antisense probes against the mRNAs of DVGLUT and neighboring genes confirm that OK371-Gal4 detects an enhancer of DVGLUT. DVGLUT-specific antibodies detect its expression in identifiable motoneurons, which are known to be glutamatergic in Drosophila. DVGLUT initially appears in small cytoplasmic punctae in the somata of these motoneurons. As development proceeds, DVGLUT-positive particles are transported along motor axons and become concentrated at neuromuscular junctions (NMJs), where they colocalize with the synaptic vesicle marker synaptotagmin. We find that the DVGLUT-specific antibodies are valuable tools for the identification of motoneurons and other glutamatergic neurons. In addition, the OK371-Gal4 line can be used for the targeted expression of any gene in these cells. Given that vesicular glutamate transporters are essential for the uptake of the neurotransmitter glutamate into synaptic vesicles these tools provide a means to test gene function in these functionally important neurons.

Highwire Regulates Presynaptic BMP Signaling Essential for Synaptic Growth

Highwire (Hiw), a putative RING finger E3 ubiquitin ligase, negatively regulates synaptic growth at the neuromuscular junction (NMJ) in Drosophila. hiw mutants have dramatically larger synaptic size and increased numbers of synaptic boutons. Here we show that Hiw binds to the Smad protein Medea (Med). Med is part of a presynaptic bone morphogenetic protein (BMP) signaling cascade consisting of three receptor subunits, Wit, Tkv, and Sax, in addition to the Smad transcription factor Mad. When compared to wild-type, mutants of BMP signaling components have smaller NMJ size, reduced neurotransmitter release, and aberrant synaptic ultrastructure. BMP signaling mutants suppress the excessive synaptic growth in hiw mutants. Activation of BMP signaling, which in wild-type does not cause additional growth, in hiw mutants does lead to further synaptic expansion. These results reveal a balance between positive BMP signaling and negative regulation by Highwire, governing the growth of neuromuscular synapses.

wishful thinking encodes a BMP type II receptor that regulates synaptic growth in Drosophila

We conducted a large-scale screen for Drosophila mutants that have structural abnormalities of the larval neuromuscular junction (NMJ). We recovered mutations in wishful thinking (wit), a gene that positively regulates synaptic growth. wit encodes a BMP type II receptor. In wit mutant larvae, the size of the NMJs is greatly reduced relative to the size of the muscles. wit NMJs have reduced evoked excitatory junctional potentials, decreased levels of the synaptic cell adhesion molecule Fasciclin II, and synaptic membrane detachment at active zones. Wit is expressed by a subset of neurons, including motoneurons. The NMJ phenotype is specifically rescued by transgenic expression of Wit only in motoneurons. Thus, Wit appears to function as a presynaptic receptor that regulates synaptic size at the Drosophila NMJ.

E-cadherin binding prevents beta-catenin nuclear localization and beta-catenin/LEF-1-mediated transactivation

Beta-catenin is a multifunctional protein found in three cell compartments: the plasma membrane, the cytoplasm and the nucleus. The cell has developed elaborate ways of regulating the level and localization of beta-catenin to assure its specific function in each compartment. One aspect of this regulation is inherent in the structural organization of beta-catenin itself; most of its protein-interacting motifs overlap so that interaction with one partner can block binding of another at the same time. Using recombinant proteins, we found that E-cadherin and lymphocyte-enhancer factor-1 (LEF-1) form mutually exclusive complexes with beta-catenin; the association of beta-catenin with LEF-1 was competed out by the E-cadherin cytoplasmic domain. Similarly, LEF-1 and adenomatous polyposis coli (APC) formed separate, mutually exclusive complexes with beta-catenin. In Wnt-1-transfected C57MG cells, free beta-catenin accumulated and was able to associate with LEF-1. The absence of E-cadherin in E-cadherin-/- embryonic stem (ES) cells also led to an accumulation of free beta-catenin and its association with LEF-1, thereby mimicking Wnt signaling. beta-catenin/LEF-1-mediated transactivation in these cells was antagonized by transient expression of wild-type E-cadherin, but not of E-cadherin lacking the beta-catenin binding site. The potent ability of E-cadherin to recruit beta-catenin to the cell membrane and prevent its nuclear localization and transactivation was also demonstrated using SW480 colon carcinoma cells.

Signaling and adhesion activities of mammalian beta-catenin and plakoglobin in Drosophila

The armadillo protein of Drosophila and its vertebrate homologues, beta-catenin and plakoglobin, are implicated in cell adhesion and wnt signaling. Here, we examine the conservation of these two functions by assaying the activities of mammalian beta-catenin and plakoglobin in Drosophila. We show that, in the female germ line, both mammalian beta-catenin and plakoglobin complement an armadillo mutation. We also show that shotgun mutant germ cells (which lack Drosophila E-cadherin) have a phenotype identical to that of armadillo mutant germ cells. It therefore appears that armadillo's role in the germ line is solely in a complex with Drosophila E-cadherin (possibly an adhesion complex), and both beta-catenin and plakoglobin can function in Drosophila cadherin complexes. In embryonic signaling assays, we find that plakoglobin has no detectable activity whereas beta-catenin's activity is weak. Surprisingly, when overexpressed, either in embryos or in wing imaginal disks, both beta-catenin and plakoglobin have dominant negative activity on signaling, an effect also obtained with COOH-terminally truncated armadillo. We suggest that the signaling complex, which has been shown by others to comprise armadillo and a member of the lymphocyte enhancer binding factor-1/T cell factor-family, may contain an additional factor that normally binds to the COOH-terminal region of armadillo.

Expression of the Armadillo family member p120cas1B in Xenopus embryos affects head differentiation but not axis formation

The Armadillo family is formed by proteins which possess an Arm domain comprising multiple copies of a 42-amino-acid motif, the Arm repeat, initially described for the Drosophila segment polarity gene product Armadillo. The Arm domain serves in protein-protein interactions which are required for the family members Armadillo, beta-catenin and plakoglobin to mediate cell-cell adhesion and Wnt/Wingless signalling. Similarily, p120cas, the Arm domain containing src substrate, also binds to cadherins and becomes tyrosine phosphorylated in response to a variety of stimuli. However, a putative function of p120cas in adhesion or signalling has not yet been demonstrated. It has also not been shown until now that an Arm domain is a common signal transduction motif. Using Xenopus embryos we show by expression of murine p120cas1B (mp120cas1B) in ventral blastomeres that this catenin cannot replace beta-catenin function in dorsal axis formation. Thus, the presence of an Arm domain per se is not sufficient to activate the Wnt/Wg pathway. Indeed, injection of mp120cas1B into dorsal blastomeres led instead to delayed blastopore closure and posteriorized phenotypes with malformed head structures indicative of disturbed gastrulation movements. Because neither convergent extension behaviour nor adhesion to fibronectin was altered in the injected embryos we assume that mp120cas1B influences motility or orientation of migrating mesodermal cells.

beta-catenin is a target for the ubiquitin-proteasome pathway

beta-catenin is a central component of the cadherin cell adhesion complex and plays an essential role in the Wingless/Wnt signaling pathway. In the current model of this pathway, the amount of beta-catenin (or its invertebrate homolog Armadillo) is tightly regulated and its steady-state level outside the cadherin-catenin complex is low in the absence of Wingless/Wnt signal. Here we show that the ubiquitin-dependent proteolysis system is involved in the regulation of beta-catenin turnover. beta-catenin, but not E-cadherin, p120(cas) or alpha-catenin, becomes stabilized when proteasome-mediated proteolysis is inhibited and this leads to the accumulation of multi-ubiquitinated forms of beta-catenin. Mutagenesis experiments demonstrate that substitution of the serine residues in the glycogen synthase kinase 3beta (GSK3beta) phosphorylation consensus motif of beta-catenin inhibits ubiquitination and results in stabilization of the protein. This motif in beta-catenin resembles a motif in IkappaB (inhibitor of NFkappaB) which is required for the phosphorylation-dependent degradation of IkappaB via the ubiquitin-proteasome pathway. We show that ubiquitination of beta-catenin is greatly reduced in Wnt-expressing cells, providing the first evidence that the ubiquitin-proteasome degradation pathway may act downstream of GSK3beta in the regulation of beta-catenin.

Cadherin-catenin complex: protein interactions and their implications for cadherin function

Cadherins comprise a family of calcium-dependent glycoproteins that function in mediating cell-cell adhesion in virtually all solid tissues of multicellular organisms. In epithelial cells, E-cadherin represents a key molecule in the establishment and stabilization of cellular junctions. On the cellular level, E-cadherin is concentrated at the adherens junction and interacts homophilically with E-cadherin molecules of adjacent cells. Significant progress has been made in understanding the extra- and intracellular interactions of E-cadherin. Recent success in solving the three-dimensional structure of an extracellular cadherin domain provides a structural basis for understanding the homophilic interaction mechanism and the calcium requirement of cadherins. According to the crystal structure, individual cadherin molecules cooperate to form a linear cell adhesion zipper. The intracellular anchorage of cadherins is regulated by the dynamic association with cytoplasmic proteins, termed catenins. The cytoplasmic domain of E-cadherin is complexed with either beta-catenin or plakoglobin (gamma-catenin). Beta-catenin and plakoglobin bind directly to alpha-catenin, giving rise to two distinct cadherin-catenin complexes (CCC). Alpha-catenin is thought to link both CCC's to actin filaments. The anchorage of cadherins to the cytoskeleton appears to be regulated by tyrosine phosphorylation. Phosphorylation-induced junctional disassembly targets the catenins, indicating that catenins are components of signal transduction pathways. The unexpected association of catenins with the product of the tumor suppressor gene APC has led to the discovery of a second, cadherin-independent catenin complex. Two separate catenin complexes are therefore involved in the cross-talk between cell adhesion and signal transduction. In this review we focus on protein interactions regulating the molecular architecture and function of the CCC. In the light of a fundamental role of the CCC during mammalian development and tissue morphogenesis, we also discuss the phenotypes of embryos lacking E-cadherin or beta-catenin.

Cadherin-catenin complex: protein interactions and their implications for cadherin function

Cadherins comprise a family of calcium-dependent glycoproteins that function in mediating cell-cell adhesion in virtually all solid tissues of multicellular organisms. In epithelial cells, E-cadherin represents a key molecule in the establishment and stabilization of cellular junctions. On the cellular level, E-cadherin is concentrated at the adherens junction and interacts homophilically with E-cadherin molecules of adjacent cells. Significant progress has been made in understanding the extra- and intracellular interactions of E-cadherin. Recent success in solving the three-dimensional structure of an extracellular cadherin domain provides a structural basis for understanding the homophilic interaction mechanism and the calcium requirement of cadherins. According to the crystal structure, individual cadherin molecules cooperate to form a linear cell adhesion zipper. The intracellular anchorage of cadherins is regulated by the dynamic association with cytoplasmic proteins, termed catenins. The cytoplasmic domain of E-cadherin is complexed with either beta-catenin or plakoglobin (gamma-catenin). Beta-catenin and plakoglobin bind directly to alpha-catenin, giving rise to two distinct cadherin-catenin complexes (CCC). Alpha-catenin is thought to link both CCC's to actin filaments. The anchorage of cadherins to the cytoskeleton appears to be regulated by tyrosine phosphorylation. Phosphorylation-induced junctional disassembly targets the catenins, indicating that catenins are components of signal transduction pathways. The unexpected association of catenins with the product of the tumor suppressor gene APC has led to the discovery of a second, cadherin-independent catenin complex. Two separate catenin complexes are therefore involved in the cross-talk between cell adhesion and signal transduction. In this review we focus on protein interactions regulating the molecular architecture and function of the CCC. In the light of a fundamental role of the CCC during mammalian development and tissue morphogenesis, we also discuss the phenotypes of embryos lacking E-cadherin or beta-catenin.

Single amino acid substitutions in proteins of the armadillo gene family abolish their binding to alpha-catenin

Analysis of the calcium-dependent cell adhesion molecule E-cadherin has led to the identification of catenins, which are necessary for cadherin function. Growing evidence that cadherins and catenins are subjected to genetic alterations in carcinogenesis makes it especially important to understand protein-protein interactions within the cadherin-catenin complex. Here we report the identification and analysis of the alpha-catenin binding site in plakoglobin (gamma-catenin). Using N- and C-terminal truncations of plakoglobin, we identified a domain of 29 amino acids necessary and sufficient for binding alpha-catenin. The alpha-catenin binding site is fully encoded within exon 3 of plakoglobin but only partially represented in Armadillo repeat 1. This suggests that exons rather than individual Arm repeats encode functional domains of plakoglobin. Site-directed mutagenesis identified residues in the alpha-catenin binding site indispensable for binding in vitro. Analogous mutations in beta-catenin and Armadillo had identical effects. Our results indicate that single amino acid mutations in the alpha-catenin binding site of homologs of Armadillo could prevent a stable association with alpha-catenin, thus affecting cadherin-mediated adhesion.

The human plakoglobin gene localizes on chromosome 17q21 and is subjected to loss of heterozygosity in breast and ovarian cancers

The gene encoding human plakoglobin was mapped to chromosome 17q12-q22. An intragenic restriction fragment length polymorphism was used to localize the plakoglobin gene distal to locus KRT10 and proximal to the marker D17S858. The plakoglobin gene colocalizes with the polymorphic 17q21 marker UM8 on the same cosmid insert. This subregion of chromosome 17 is known to be particularly subjected to genetic alterations in sporadic breast and ovarian tumors. We show loss of heterozygosity of the plakoglobin gene in breast and ovarian tumors. We have identified a low-frequency polymorphism in the plakoglobin coding sequence which results in an arginine to histidine substitution at amino acid position 142 of the protein, as well as a silent mutation at nucleotide position 332 of the coding sequence. This polymorphism allowed us to demonstrate an allelic association of plakoglobin with predisposition to familial breast and ovarian cancers. Our results, together with the present knowledge about the biological function of plakoglobin, suggest that plakoglobin might represent a putative tumor suppressor gene for breast and ovarian cancers.

Beta-catenin mediates the interaction of the cadherin-catenin complex with epidermal growth factor receptor

Catenins mediate the linkage of classical cadherins with actin microfilaments and are part of a higher order protein structure by which cadherins are connected to other cytoplasmic and transmembrane proteins. The ratio of actin-bound to free cadherin-catenin complex, which varies depending on the type and growth rate of cells, is thought to be altered by cellular signals, such as those associated with mitosis, polarization of cells and growth factors during development. EGF induces an immediate tyrosine phosphorylation of beta-catenin and gamma-catenin (plakoglobin). We show here an association of the EGF-receptor with the cadherin-catenin complex. Using recombinant proteins we demonstrate the interaction of EGF-receptor and beta-catenin in in vitro kinase assays. This interaction is mediated by the evolutionarily conserved central "core" region of beta-catenin. These results suggest that catenins represent an important link between EGF-induced signal transduction and cadherin function.

Assembly of the cadherin-catenin complex in vitro with recombinant proteins

The cytoplasmic domain of classical cadherins is tightly associated with three proteins termed alpha-, beta- and gamma-catenin. These accessory proteins are of central importance for the adhesive properties of this class of cell adhesion molecules. In order to examine the molecular architecture of the cadherin-catenin complex in more detail we have expressed the catenins and the cytoplasmic domain of E-cadherin as fusion proteins in Escherichia coli, and analyzed the interaction of purified recombinant cadherin and catenins in combinatorial protein-protein interaction experiments. The cytoplasmic domain of E-cadherin cannot directly associate with alpha-catenin but interacts with high affinity with beta-catenin, whereas the binding of gamma-catenin (plakoglobin) to E-cadherin is less efficient. alpha- and beta-catenin assemble into a 1:1 heterodimeric complex. The analysis of various truncated beta-catenins revealed that an alpha-catenin binding site in beta-catenin is localized between amino acid positions 120 and 151. The central role of beta-catenin for the assembly of the heterotrimeric E-cadherin/alpha-catenin/beta-catenin complex in mixing experiments with all components was demonstrated. The reconstitution in vitro of the cadherin-catenin complex should allow the study of the interaction with signalling molecules and with the actin-based cytoskeleton.

[Carcinoma metastases in neck lymph nodes in occult primary tumor: survival rate depending from localization, stadium, histology and therapeutic method (48 patients)]

In patients with metastatic cancer in cervical nodes without evidence of a primary cancer, radiation therapy eventually supplemented by surgery is indicated. The treated volume should include the regions of the possible primary (pharynx, base of tongue, tonsil) besides the cervical node regions. Five-year cure rates are: about 40% in cases with cervical nodes only, more than 50% in cases with cervical nodes in one region only and not fixed, less than 10% in cases with supraclavicular nodes. In about 30% a primary tumor is found later. About 50% of these primaries is located in the supraclavicular volume and about 50% is located in the infraclavicular volume. In cases with supraclavicular nodes the majority of the primary tumors are located in the infraclavicular colume.