Target recognition and synaptogenesis by motor axons: responses to the sidestep protein

Axon guidance genes modulate neurotoxicity of ALS-associated UBQLN2

Mutations in the ubiquitin (Ub) chaperone Ubiquilin 2 (UBQLN2) cause X-linked forms of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) through unknown mechanisms. Here, we show that aggregation-prone, ALS-associated mutants of UBQLN2 (UBQLN2ALS) trigger heat stress-dependent neurodegeneration in Drosophila. A genetic modifier screen implicated endolysosomal and axon guidance genes, including the netrin receptor, Unc-5, as key modulators of UBQLN2 toxicity. Reduced gene dosage of Unc-5 or its coreceptor Dcc/frazzled diminished neurodegenerative phenotypes, including motor dysfunction, neuromuscular junction defects, and shortened lifespan, in flies expressing UBQLN2ALS alleles. Induced pluripotent stem cells (iPSCs) harboring UBQLN2ALS knockin mutations exhibited lysosomal defects while inducible motor neurons (iMNs) expressing UBQLN2ALS alleles exhibited cytosolic UBQLN2 inclusions, reduced neurite complexity, and growth cone defects that were partially reversed by silencing of UNC5B and DCC. The combined findings suggest that altered growth cone dynamics are a conserved pathomechanism in UBQLN2-associated ALS/FTD.

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.

The impact of Megf10/Drpr gain-of-function on muscle development in Drosophila

Recessive mutations in multiple epidermal growth factor-like domains 10 (MEGF10) underlie a rare congenital muscle disease known as MEGF10 myopathy. MEGF10 and its Drosophila homolog Draper (Drpr) are transmembrane receptors expressed in muscle and glia. Drpr deficiency is known to result in muscle abnormalities in flies. In the current study, flies that ubiquitously overexpress Drpr, or mouse Megf10, display developmental arrest. The phenotype is reproduced with overexpression in muscle, but not in other tissues, and with overexpression during intermediate stages of myogenesis, but not in myoblasts. We find that tubular muscle subtypes are particularly sensitive to Megf10/Drpr overexpression. Complementary genetic analyses show that Megf10/Drpr and Notch may interact to regulate myogenesis. Our findings provide a basis for investigating MEGF10 in muscle development using Drosophila.

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.

Deconstruction of the beaten Path-Sidestep interaction network provides insights into neuromuscular system development

An ‘interactome’ screen of all Drosophila cell-surface and secreted proteins containing immunoglobulin superfamily (IgSF) domains discovered a network formed by paralogs of Beaten Path (Beat) and Sidestep (Side), a ligand-receptor pair that is central to motor axon guidance. Here we describe a new method for interactome screening, the Bio-Plex Interactome Assay (BPIA), which allows identification of many interactions in a single sample. Using the BPIA, we ‘deorphanized’ four more members of the Beat-Side network. We confirmed interactions using surface plasmon resonance. The expression patterns of beat and side genes suggest that Beats are neuronal receptors for Sides expressed on peripheral tissues. side-VI is expressed in muscle fibers targeted by the ISNb nerve, as well as at growth cone choice points and synaptic targets for the ISN and TN nerves. beat-V genes, encoding Side-VI receptors, are expressed in ISNb and ISN motor neurons.

Within every organ of the body, cells must be able to recognise and communicate with one another in order to work together to perform a particular role. Each cell has a specific protein on its surface that acts like a molecular identity card, and which can form weak bonds with a complementary protein on another cell. There are thousands of different cell surface proteins, and the interactions between them – known collectively as the interactome – dictate the how cells interact with one another.

Many cell surface proteins are similar across species. Humans and fruit flies, for example, both possess a family of cell surface proteins that contain a region called the Immunoglobulin Superfamily domain. This family can be further divided into subfamilies, two of which are known as “Beats” and “Sides” for short. As the nervous system develops, nerve cells carrying a particular Beat protein interact with nerve or muscle cells carrying a corresponding Side protein. Yet while experiments have matched up many Beats and Sides, the partners of others remain unknown.

Li et al. have now developed a new technique called the Bio-Plex Interactome Assay to rapidly screen for interactions between multiple cell surface proteins in a single sample. Applying the technique to cells from fruit flies revealed new binding partners within the Beats and the Sides. After verifying several of these interactions, Li et al. explored the role of various Beats and Sides in the developing nervous system of fruit fly embryos by mapping the cells that display them on their surfaces.

This increased knowledge of the Beat-Side binding network should provide further insights into how connections form between nerve cells. The new screening technique could also eventually be used to map the cell surface protein interactome in humans. A number of key drugs, including the breast cancer drug Herceptin, target cell surface proteins. Identifying interactions among cell surface proteins could thus provide additional leads for developing new therapies.

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.

Unc-51 controls active zone density and protein composition by downregulating ERK signaling

Efficient synaptic transmission requires the apposition of neurotransmitter release sites opposite clusters of postsynaptic neurotransmitter receptors. Transmitter is released at active zones, which are composed of a large complex of proteins necessary for synaptic development and function. Many active zone proteins have been identified, but little is known of the mechanisms that ensure that each active zone receives the proper complement of proteins. Here we use a genetic analysis in Drosophila to demonstrate that the serine threonine kinase Unc-51 acts in the presynaptic motoneuron to regulate the localization of the active zone protein Bruchpilot opposite to glutamate receptors at each synapse. In the absence of Unc-51, many glutamate receptor clusters are unapposed to Bruchpilot, and ultrastructural analysis demonstrates that fewer active zones contain dense body T-bars. In addition to the presence of these aberrant synapses, there is also a decrease in the density of all synapses. This decrease in synaptic density and abnormal active zone composition is associated with impaired evoked transmitter release. Mechanistically, Unc-51 inhibits the activity of the MAP kinase ERK to promote synaptic development. In the unc-51 mutant, increased ERK activity leads to the decrease in synaptic density and the absence of Bruchpilot from many synapses. Hence, activated ERK negatively regulates synapse formation, resulting in either the absence of active zones or the formation of active zones without their proper complement of proteins. The Unc-51-dependent inhibition of ERK activity provides a potential mechanism for synapse-specific control of active zone protein composition and release probability.

Synaptic defects in a Drosophila model of congenital muscular dystrophy

The congenital muscular dystrophies present in infancy with muscle weakness and are often associated with mental retardation. Many of these inherited disorders share a common etiology: defective O-glycosylation of alpha-dystroglycan, a component of the dystrophin complex. Protein-O-mannosyl transferase 1 (POMT1) is the first enzyme required for the glycosylation of alpha-dystroglycan, and mutations in the POMT1 gene can lead to both Walker-Warburg syndrome (WWS) and limb girdle muscular dystrophy type 2K (LGMD2K). WWS is associated with severe mental retardation and major structural abnormalities in the brain; however, LGMD2K patients display a more mild retardation with no obvious structural defects in the brain. In a screen for synaptic mutants in Drosophila, we identified mutations in the Drosophila ortholog of POMT1, dPOMT1. Because synaptic defects are a plausible cause of mental retardation, we investigated the molecular and physiological defects associated with loss of dPOMT1 in Drosophila. In dPOMT1 mutants, there is a decrease in the efficacy of synaptic transmission and a change in the subunit composition of the postsynaptic glutamate receptors at the neuromuscular junction. We demonstrate that dPOMT1 is required to glycosylate the Drosophila dystroglycan ortholog Dg in vivo, and that this is the likely cause of these synaptic defects because (1) mutations in Dg lead to similar synaptic defects and (2) genetic interaction studies suggest that dPOMT1 and Dg function in the same pathway. These results are consistent with the model that dPOMT1-dependent glycosylation of Dg is necessary for proper synaptic function and raise the possibility that similar synaptic defects occur in the congenital muscular dystrophies.

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.

The divergent TGF-β ligand Dawdle utilizes an activin pathway to influence axon guidance inDrosophila

Axon guidance is regulated by intrinsic factors and extrinsic cues provided by other neurons, glia and target muscles. Dawdle (Daw), a divergent TGF-β superfamily ligand expressed in glia and mesoderm, is required for embryonic motoneuron pathfinding in Drosophila. In dawmutants, ISNb and SNa axons fail to extend completely and are unable to innervate their targets. We find that Daw initiates an activin signaling pathway via the receptors Punt and Baboon (Babo) and the signal-transducer Smad2. Furthermore, mutations in these signaling components display similar axon guidance defects. Cell-autonomous disruption of receptor signaling suggests that Babo is required in motoneurons rather than in muscles or glia. Ectopic ligand expression can rescue the daw phenotype, but has no deleterious effects. Our results indicate that Daw functions in a permissive manner to modulate or enable the growth cone response to other restricted guidance cues,and support a novel role for activin signaling in axon guidance.

The metalloprotease Tolloid-related and its TGF-β-like substrate Dawdle regulateDrosophilamotoneuron axon guidance

Proper axon pathfinding requires that growth cones execute appropriate turns and branching at particular choice points en route to their synaptic targets. Here we demonstrate that the Drosophila metalloprotease tolloid-related (tlr) is required for proper fasciculation/defasciculation of motor axons in the CNS and for normal guidance of many motor axons enroute to their muscle targets. Tlr belongs to a family of developmentally important proteases that process various extracellular matrix components, as well as several TGF-β inhibitory proteins and pro-peptides. We show that Tlr is a circulating enzyme that processes the pro-domains of three Drosophila TGF-β-type ligands, and, in the case of the Activin-like protein Dawdle (Daw), this processing enhances the signaling activity of the ligand in vitro and in vivo. Null mutants of daw, as well as mutations in its receptor babo and its downstream mediator Smad2, all exhibit axon guidance defects that are similar to but less severe than tlr. We suggest that by activating Daw and perhaps other TGF-β ligands, Tlr provides a permissive signal for axon guidance.

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.