Distinct functional domains of the Abelson tyrosine kinase control axon guidance responses to Netrin and Slit to regulate the assembly of neural circuits

Inter-plane feedback coordinates cell morphogenesis and maintains 3D tissue organization in the Drosophila pupal retina

How complex organs coordinate cellular morphogenetic events to achieve three-dimensional (3D) form is a central question in development. The question is uniquely tractable in the late Drosophila pupal retina, where cells maintain stereotyped contacts as they elaborate the specialized cytoskeletal structures that pattern the apical, basal and longitudinal planes of the epithelium. In this study, we combined cell type-specific genetic manipulation of the cytoskeletal regulator Abelson (Abl) with 3D imaging to explore how the distinct cellular morphogenetic programs of photoreceptors and interommatidial pigment cells (IOPCs) organize tissue pattern to support retinal integrity. Our experiments show that photoreceptor and IOPC terminal differentiation is unexpectedly interdependent, connected by an intercellular feedback mechanism that coordinates and promotes morphogenetic change across orthogonal tissue planes to ensure correct 3D retinal pattern. We propose that genetic regulation of specialized cellular differentiation programs combined with inter-plane mechanical feedback confers spatial coordination to achieve robust 3D tissue morphogenesis.

A tug of war between DCC and ROBO1 signaling during commissural axon guidance

Dynamic and coordinated axonal responses to changing environments are critical for establishing neural connections. As commissural axons migrate across the CNS midline, they are suggested to switch from being attracted to being repelled in order to approach and to subsequently leave the midline. A molecular mechanism that is hypothesized to underlie this switch in axonal responses is the silencing of Netrin1/Deleted in Colorectal Carcinoma (DCC)-mediated attraction by the repulsive SLIT/ROBO1 signaling. Using in vivo approaches including CRISPR-Cas9-engineered mouse models of distinct Dcc splice isoforms, we show here that commissural axons maintain responsiveness to both Netrin and SLIT during midline crossing, although likely at quantitatively different levels. In addition, full-length DCC in collaboration with ROBO3 can antagonize ROBO1 repulsion in vivo. We propose that commissural axons integrate and balance the opposing DCC and Roundabout (ROBO) signaling to ensure proper guidance decisions during midline entry and exit.

Orthogonal coupling of a 3D cytoskeletal scaffold coordinates cell morphogenesis and maintains tissue organization in the Drosophila pupal retina

How complex three-dimensional (3D) organs coordinate cellular morphogenetic events to achieve the correct final form is a central question in development. The question is uniquely tractable in the late Drosophila pupal retina where cells maintain stereotyped contacts as they elaborate the specialized cytoskeletal structures that pattern the apical, basal and longitudinal planes of the epithelium. In this study, we combined cell type-specific genetic manipulation of the cytoskeletal regulator Abelson (Abl) with 3D imaging to explore how the distinct cellular morphogenetic programs of photoreceptors and interommatidial pigment cells coordinately organize tissue pattern to support retinal integrity. Our experiments revealed an unanticipated intercellular feedback mechanism whereby correct cellular differentiation of either cell type can non-autonomously induce cytoskeletal remodeling in the other Abl mutant cell type, restoring retinal pattern and integrity. We propose that genetic regulation of specialized cellular differentiation programs combined with inter-plane mechanical feedback confers spatial coordination to achieve robust 3D tissue morphogenesis.

Abelson kinase's intrinsically disordered region plays essential roles in protein function and protein stability

Background

The non-receptor tyrosine kinase Abelson (Abl) is a key player in oncogenesis, with kinase inhibitors serving as paradigms of targeted therapy. Abl also is a critical regulator of normal development, playing conserved roles in regulating cell behavior, brain development and morphogenesis. Drosophila offers a superb model for studying Abl’s normal function, because, unlike mammals, there is only a single fly Abl family member. In exploring the mechanism of action of multi-domain scaffolding proteins like Abl, one route is to define the roles of their individual domains. Research into Abl’s diverse roles in embryonic morphogenesis revealed many surprises. For instance, kinase activity, while important, is not crucial for all Abl activities, and the C-terminal F-actin binding domain plays a very modest role. This turned our attention to one of Abl’s least understood features—the long intrinsically-disordered region (IDR) linking Abl’s kinase and F-actin binding domains. The past decade revealed unexpected, important roles for IDRs in diverse cell functions, as sites of posttranslational modifications, mediating multivalent interactions and enabling assembly of biomolecular condensates via phase separation. Previous work deleting conserved regions in Abl’s IDR revealed an important role for a PXXP motif, but did not identify any other essential regions.

Methods

Here we extend this analysis by deleting the entire IDR, and asking whether Abl∆IDR rescues the diverse roles of Abl in viability and embryonic morphogenesis in Drosophila.

Results

This revealed that the IDR is essential for embryonic and adult viability, and for cell shape changes and cytoskeletal regulation during embryonic morphogenesis, and, most surprisingly, revealed a role in modulating protein stability.

Conclusion

Our data provide new insights into the role of the IDR in an important signaling protein, the non-receptor kinase Abl, suggesting that it is essential for all aspects of protein function during embryogenesis, and revealing a role in protein stability. These data will stimulate new explorations of the mechanisms by which the IDR regulates Abl stability and function, both in Drosophila and also in mammals. They also will stimulate further interest in the broader roles IDRs play in diverse signaling proteins.

LIM and SH3 protein 1 localizes to the leading edge of protruding lamellipodia and regulates axon development

The actin cytoskeleton drives cell motility and is essential for neuronal development and function. LIM and SH3 protein 1 (LASP1) is a unique actin-binding protein that is expressed in a wide range of cells including neurons, but its roles in cellular motility and neuronal development are not well understood. We report that LASP1 is expressed in rat hippocampus early in development, and this expression is maintained through adulthood. High-resolution imaging reveals that LASP1 is selectively concentrated at the leading edge of lamellipodia in migrating cells and axonal growth cones. This local enrichment of LASP1 is dynamically associated with the protrusive activity of lamellipodia, depends on the barbed ends of actin filaments, and requires both the LIM domain and the nebulin repeats of LASP1. Knockdown of LASP1 in cultured rat hippocampal neurons results in a substantial reduction in axonal outgrowth and arborization. Finally, loss of the Drosophila homologue Lasp from a subset of commissural neurons in the developing ventral nerve cord produces defasciculated axon bundles that do not reach their targets. Together, our data support a novel role for LASP1 in actin-based lamellipodial protrusion and establish LASP1 as a positive regulator of both in vitro and in vivo axon development.

The first quarter of the C-terminal domain of Abelson regulates the WAVE regulatory complex and Enabled in axon guidance

Background

Abelson tyrosine kinase (Abl) plays a key role in axon guidance in linking guidance receptors to actin dynamics. The long C-terminal domain (CTD) of Drosophila Abl is important for this role, and previous work identified the ‘first quarter’ (1Q) of the CTD as essential. Here, we link the physical interactions of 1Q binding partners to Abl’s function in axon guidance.

Methods

Protein binding partners of 1Q were identified by GST pulldown and mass spectrometry, and validated using axon guidance assays in the embryonic nerve cord and motoneurons. The role of 1Q was assessed genetically, utilizing a battery of Abl transgenes in combination with mutation or overexpression of the genes of pulled down proteins, and their partners in actin dynamics. The set of Abl transgenes had the following regions deleted: all of 1Q, each half of 1Q (‘eighths’, 1E and 2E) or a PxxP motif in 2E, which may bind SH3 domains.

Results

GST pulldown identified Hem and Sra-1 as binding partners of 1Q, and our genetic analyses show that both proteins function with Abl in axon guidance, with Sra-1 likely interacting with 1Q. As Hem and Sra-1 are part of the actin-polymerizing WAVE regulatory complex (WRC), we extended our analyses to Abi and Trio, which interact with Abl and WRC members. Overall, the 1Q region (and especially 2E and its PxxP motif) are important for Abl’s ability to work with WRC in axon guidance. These areas are also important for Abl’s ability to function with the actin regulator Enabled. In comparison, 1E contributes to Abl function with the WRC at the midline, but less so with Enabled.

Conclusions

The 1Q region, and especially the 2E region with its PxxP motif, links Abl with the WRC, its regulators Trio and Abi, and the actin regulator Ena. Removing 1E has specific effects suggesting it may help modulate Abl’s interaction with the WRC or Ena. Thus, the 1Q region of Abl plays a key role in regulating actin dynamics during axon guidance.

Abl signaling directs growth of a pioneer axon in Drosophila by shaping the intrinsic fluctuations of actin

The fundamental problem in axon growth and guidance is understanding how cytoplasmic signaling modulates the cytoskeleton to produce directed growth cone motility. Live imaging of the TSM1 axon of the developing Drosophila wing has shown that the essential role of the core guidance signaling molecule, Abelson (Abl) tyrosine kinase, is to modulate the organization and spatial localization of actin in the advancing growth cone. Here, we dissect in detail the properties of that actin organization and its consequences for growth cone morphogenesis and motility. We show that advance of the actin mass in the distal axon drives the forward motion of the dynamic filopodial domain that defines the growth cone. We further show that Abl regulates both the width of the actin mass and its internal organization, spatially biasing the intrinsic fluctuations of actin to achieve net advance of the actin, and thus of the dynamic filopodial domain of the growth cone, while maintaining the essential coherence of the actin mass itself. These data suggest a model whereby guidance signaling systematically shapes the intrinsic, stochastic fluctuations of actin in the growth cone to produce axon growth and guidance.

Slit/Robo Signaling Pathway in Cancer; a New Stand Point for Cancer Treatment

Angiogenesis and metastasis are two critical steps for cancer cells survival and migration. The microenvironment of tumor sphere induces new blood vessels formation for enhancing tumor mass. Preexisting capillaries and postcapillary venules in tumors bring about new blood vessels. ROBO1-ROBO4 are transmembrane receptors family which act as guidance molecules of the nervous system. The SLITs family is secreted glycoproteins that bind to these receptors. SLIT-ROBO signaling pathway plays an important role in neurogenesis and immune response. Linkage between ROBOs and their ligands (SLITs) induce chemorepllent signal for regulation of axon guidance and leukocyte cell migration, recent finding shows that it is also involved in endothelial cell migration and angiogenesis in various type of cancers. In this article we review recent finding of SLIT-ROBO pathway in angiogenesis and metastasis.

Regulatory mechanisms of Robo4 and their effects on angiogenesis

Roundabout4 (Robo4) is a transmembrane receptor that belongs to the Roundabout (Robo) family of axon guidance molecules. Robo4 is an endothelial-specific receptor that participates in endothelial cell migration, proliferation, and angiogenesis and the maintenance of vasculature homeostasis. The purpose of this review is to summarize and analyze three main mechanisms related to the expression and function of Robo4 during developmental and pathological angiogenesis. In this review, static shear stress and the binding of transcription factors such as E26 transformation-specific variant 2 (ETV2) and Slit3 induce Robo4 expression and activate Robo4 during tissue and organ development. Robo4 interacts with Slit2 or UNC5B to maintain vascular integrity, while a disturbed flow and the expression of transcription factors in inflammatory or neoplastic environments alter Robo4 expression levels, although these changes have uncertain functions. Based on the mechanisms described above, we discuss the aberrant expression of Robo4 in angiogenesis-related diseases and propose antiangiogenic therapies targeting the Robo4 signaling pathway for the treatment of ocular neovascularization lesions and tumors. Finally, although many problems related to Robo4 signaling pathways remain to be resolved, Robo4 is a promising and potentially valuable therapeutic target for treating pathological angiogenesis and developmental defects in angiogenesis.

The Role of Apoptotic Signaling in Axon Guidance

Navigating growth cones are exposed to multiple signals simultaneously and have to integrate competing cues into a coherent navigational response. Integration of guidance cues is traditionally thought to occur at the level of cytoskeletal dynamics. Drosophila studies indicate that cells exhibit a low level of continuous caspase protease activation, and that axon guidance cues can activate or suppress caspase activity. We base a model for axon guidance on these observations. By analogy with other systems in which caspase signaling has non-apoptotic functions, we propose that caspase signaling can either reinforce repulsion or negate attraction in response to external guidance cues by cleaving cytoskeletal proteins. Over the course of an entire trajectory, incorrectly navigating axons may pass the threshold for apoptosis and be eliminated, whereas axons making correct decisions will survive. These observations would also explain why neurotrophic factors can act as axon guidance cues and why axon guidance systems such as Slit/Robo signaling may act as tumor suppressors in cancer.

Brain Tumor promotes axon growth across the midline through interactions with the microtubule stabilizing protein Apc2

Commissural axons must cross the midline to establish reciprocal connections between the two sides of the body. This process is highly conserved between invertebrates and vertebrates and depends on guidance cues and their receptors to instruct axon trajectories. The DCC family receptor Frazzled (Fra) signals chemoattraction and promotes midline crossing in response to its ligand Netrin. However, in Netrin or fra mutants, the loss of crossing is incomplete, suggesting the existence of additional pathways. Here, we identify Brain Tumor (Brat), a tripartite motif protein, as a new regulator of midline crossing in the Drosophila CNS. Genetic analysis indicates that Brat acts independently of the Netrin/Fra pathway. In addition, we show that through its B-Box domains, Brat acts cell autonomously to regulate the expression and localization of Adenomatous polyposis coli-2 (Apc2), a key component of the Wnt canonical signaling pathway, to promote axon growth across the midline. Genetic evidence indicates that the role of Brat and Apc2 to promote axon growth across the midline is independent of Wnt and Beta-catenin-mediated transcriptional regulation. Instead, we propose that Brat promotes midline crossing through directing the localization or stability of Apc2 at the plus ends of microtubules in navigating commissural axons. These findings define a new mechanism in the coordination of axon growth and guidance at the midline.

The establishment of neuronal connections that cross the midline of the animal is essential to generate neural circuits that coordinate the left and right sides of the body. Axons that cross the midline to form these connections are called commissural axons and the molecules and mechanisms that control midline axon crossing are remarkably conserved across animal evolution. In this study we have used a genetic screen in the fruit fly in an attempt to uncover additional players in this key developmental process, and have identified a novel role for the Brain Tumor (Brat) protein in promoting commissural axon growth across the midline. Unlike its previous described functions, in the context of midline axon guidance Brat cooperates with the microtubule stabilizing protein Apc2 to coordinate axon growth and guidance. Molecular and genetic analyses point to the conserved B box motifs of the Brat protein as key in promoting the association of Apc2 with the plus ends of microtubules. Brat is highly conserved and future studies will determine whether homologous genes play analogous roles in mammalian neural development.

Long disordered regions of the C-terminal domain of Abelson tyrosine kinase have specific and additive functions in regulation and axon localization

Abelson tyrosine kinase (Abl) is a key regulator of actin-related morphogenetic processes including axon guidance, where it functions downstream of several guidance receptors. While the long C-terminal domain (CTD) of Abl is required for function, its role is poorly understood. Here, a battery of mutants of Drosophila Abl was created that systematically deleted large segments of the CTD from Abl or added them back to the N-terminus alone. The functionality of these Abl transgenes was assessed through rescue of axon guidance defects and adult lethality in Abl loss-of-function, as well as through gain-of-function effects in sensitized slit or frazzled backgrounds that perturb midline guidance in the Drosophila embryonic nerve cord. Two regions of the CTD play important and distinct roles, but additive effects for other regions were also detected. The first quarter of the CTD, including a conserved PxxP motif and its surrounding sequence, regulates Abl function while the third quarter localizes Abl to axons. These regions feature long stretches of intrinsically disordered sequence typically found in hub proteins and are associated with diverse protein-protein interactions. Thus, the CTD of Abl appears to use these disordered regions to establish a variety of different signaling complexes required during formation of axon tracts.

Midline axon guidance in the Drosophila embryonic central nervous system

Studies in the fruit fly Drosophila melanogaster have provided many fundamental insights into the genetic regulation of neural development, including the identification and characterization of evolutionarily conserved axon guidance pathways and their roles in important guidance decisions. Due to its highly organized and fast-developing embryonic nervous system, relatively small number of neurons, and molecular and genetic tools for identifying, labeling, and manipulating individual neurons or small neuronal subsets, studies of axon guidance in the Drosophila embryonic CNS have allowed researchers to dissect these genetic mechanisms with a high degree of precision. In this review, we discuss the major axon guidance pathways that regulate midline crossing of axons and the formation and guidance of longitudinal axon tracts, two processes that contribute to the development of the precise three-dimensional structure of the insect nerve cord. We focus particularly on recent insights into the roles and regulation of canonical midline axon guidance pathways, and on additional factors and pathways that have recently been shown to contribute to axon guidance decisions at and near the midline.

Motor axon guidance in Drosophila

The Drosophila motor system starts to assemble during embryonic development. It is composed of 30 muscles per abdominal hemisegment and 36 motor neurons assembling into nerve branches to exit the CNS, navigate within the muscle field and finally establish specific connections with their target muscles. Several families of guidance molecules that play a role controlling this process as well as transcriptional regulators that program the behavior of specific motor neuron have been identified. In this review we summarize the role of both groups of molecules in the motor system as well as their relationship where known. It is apparent that partially redundant guidance protein families and membrane molecules with different functional output direct guidance decisions cooperatively. Some distinct transcriptional regulators seem to control guidance of specific nerve branches globally directing the expression of groups of pathfinding molecules in all motor neurons within the same motor branch.

Sema-1a Reverse Signaling Promotes Midline Crossing in Response to Secreted Semaphorins

Commissural axons must cross the midline to form functional midline circuits. In the invertebrate nerve cord and vertebrate spinal cord, midline crossing is mediated in part by Netrin-dependent chemoattraction. Loss of crossing, however, is incomplete in mutants for Netrin or its receptor Frazzled/DCC, suggesting the existence of additional pathways. We identified the transmembrane Semaphorin, Sema-1a, as an important regulator of midline crossing in the Drosophila CNS. We show that in response to the secreted Semaphorins Sema-2a and Sema-2b, Sema-1a functions as a receptor to promote crossing independently of Netrin. In contrast to other examples of reverse signaling where Sema1a triggers repulsion through activation of Rho in response to Plexin binding, in commissural neurons Sema-1a acts independently of Plexins to inhibit Rho to promote attraction to the midline. These findings suggest that Sema-1a reverse signaling can elicit distinct axonal responses depending on differential engagement of distinct ligands and signaling effectors.

Neurovascular patterning cues and implications for central and peripheral neurological disease

The highly branched nervous and vascular systems run along parallel trajectories throughout the human body. This stereotyped pattern of branching shared by the nervous and vascular systems stems from a common reliance on specific cues critical to both neurogenesis and angiogenesis. Continually emerging evidence supports the notion of later-evolving vascular networks co-opting neural molecular mechanisms to ensure close proximity and adequate delivery of oxygen and nutrients to nervous tissue. As our understanding of these biologic pathways and their phenotypic manifestations continues to advance, identification of where pathways go awry will provide critical insight into central and peripheral nervous system pathology.

Amplification of F-Actin Disassembly and Cellular Repulsion by Growth Factor Signaling

Extracellular cues that regulate cellular shape, motility, and navigation are generally classified as growth promoting (i.e., growth factors/chemoattractants and attractive guidance cues) or growth preventing (i.e., repellents and inhibitors). Yet, these designations are often based on complex assays and undefined signaling pathways and thus may misrepresent direct roles of specific cues. Here, we find that a recognized growth-promoting signaling pathway amplifies the F-actin disassembly and repulsive effects of a growth-preventing pathway. Focusing on Semaphorin/Plexin repulsion, we identified an interaction between the F-actin-disassembly enzyme Mical and the Abl tyrosine kinase. Biochemical assays revealed Abl phosphorylates Mical to directly amplify Mical Redox-mediated F-actin disassembly. Genetic assays revealed that Abl allows growth factors and Semaphorin/Plexin repellents to combinatorially increase Mical-mediated F-actin disassembly, cellular remodeling, and repulsive axon guidance. Similar roles for Mical in growth factor/Abl-related cancer cell behaviors further revealed contexts in which characterized positive effectors of growth/guidance stimulate such negative cellular effects as F-actin disassembly/repulsion.

A Sonic hedgehog coreceptor, BOC regulates neuronal differentiation and neurite outgrowth via interaction with ABL and JNK activation

Neurite outgrowth is a critical step for neurogenesis and remodeling synaptic circuitry during neuronal development and regeneration. An immunoglobulin superfamily member, BOC functions as Sonic hedgehog (Shh) coreceptor in canonical and noncanonical Shh signaling in neuronal development and axon outgrowth/guidance. However signaling mechanisms responsible for BOC action during these processes remain unknown. In our previous studies, a multiprotein complex containing BOC and a closely related protein CDO promotes myogenic differentiation through activation of multiple signaling pathways, including non-receptor tyrosine kinase ABL. Given that ABL and Jun. N-terminal kinase (JNK) are implicated in actin cytoskeletal dynamics required for neurogenesis, we investigated the relationship between BOC, ABL and JNK during neuronal differentiation. Here, we demonstrate that BOC and ABL are induced in P19 embryonal carcinoma (EC) cells and cortical neural progenitor cells (NPCs) during neuronal differentiation. BOC-depleted EC cells or Boc^-/- NPCs exhibit impaired neuronal differentiation with shorter neurite formation. BOC interacts with ABL through its putative SH2 binding domain and seems to be phosphorylated in an ABL activity-dependent manner. Unlike wildtype BOC, ABL-binding defective BOC mutants exhibit impaired JNK activation and neuronal differentiation. Finally, Shh treatment enhances JNK activation which is diminished by BOC depletion. These data suggest that BOC interacts with ABL and activates JNK thereby promoting neuronal differentiation and neurite outgrowth.

Slit-Robo signaling

Slits are secreted proteins that bind to Roundabout (Robo) receptors. Slit-Robo signaling is best known for mediating axon repulsion in the developing nervous system. However, in recent years the functional repertoire of Slits and Robo has expanded tremendously and Slit-Robo signaling has been linked to roles in neurogenesis, angiogenesis and cancer progression among other processes. Likewise, our mechanistic understanding of Slit-Robo signaling has progressed enormously. Here, we summarize new insights into Slit-Robo evolutionary and system-dependent diversity, receptor-ligand interactions, signaling crosstalk and receptor activation.

Abelson kinase acts as a robust, multifunctional scaffold in regulating embryonic morphogenesis

The importance of Abl kinase activity, the F-actin–binding site, and scaffolding ability in Abl’s many cell biological roles during Drosophila morphogenesis is examined. Abl is a robust multidomain scaffold with different protein motifs and activities contributing differentially to diverse cellular behaviors.

Abelson family kinases (Abls) are key regulators of cell behavior and the cytoskeleton during development and in leukemia. Abl’s SH3, SH2, and tyrosine kinase domains are joined via a linker to an F-actin–binding domain (FABD). Research on Abl’s roles in cell culture led to several hypotheses for its mechanism of action: 1) Abl phosphorylates other proteins, modulating their activity, 2) Abl directly regulates the cytoskeleton via its cytoskeletal interaction domains, and/or 3) Abl is a scaffold for a signaling complex. The importance of these roles during normal development remains untested. We tested these mechanistic hypotheses during Drosophila morphogenesis using a series of mutants to examine Abl’s many cell biological roles. Strikingly, Abl lacking the FABD fully rescued morphogenesis, cell shape change, actin regulation, and viability, whereas kinase-dead Abl, although reduced in function, retained substantial rescuing ability in some but not all Abl functions. We also tested the function of four conserved motifs in the linker region, revealing a key role for a conserved PXXP motif known to bind Crk and Abi. We propose that Abl acts as a robust multidomain scaffold with different protein motifs and activities contributing differentially to diverse cellular behaviors.

Dysregulated Dscam levels act through Abelson tyrosine kinase to enlarge presynaptic arbors

Increased expression of Down Syndrome Cell Adhesion Molecule (Dscam) is implicated in the pathogenesis of brain disorders such as Down syndrome (DS) and fragile X syndrome (FXS). Here, we show that the cellular defects caused by dysregulated Dscam levels can be ameliorated by genetic and pharmacological inhibition of Abelson kinase (Abl) both in Dscam-overexpressing neurons and in a Drosophila model of fragile X syndrome. This study offers Abl as a potential therapeutic target for treating brain disorders associated with dysregulated Dscam expression.

DOI:http://dx.doi.org/10.7554/eLife.05196.001

Information is transmitted through the brain by cells called neurons, which are connected into specific circuits and networks. As the brain develops, several different signaling molecules control how the connections between neurons develop. If these signals occur at the wrong time or wrong place, or in the wrong amount, the neurons may not connect in the right way; this is the cause of several brain disorders.

One of the signaling molecules that helps neural circuits to form in the developing brain is the Dscam protein. Having too much Dscam has been linked to neurons with enlarged presynaptic terminals. Presynaptic terminals are the parts of each neuron that send information on to the next cell, and when they are enlarged it results in the neuron not being able to communicate with other neurons in a targeted way. People with brain disorders including Down syndrome, epilepsy and possibly fragile X syndrome often have excessive amounts of Dscam.

It was not known precisely how Dscam signals within neurons. Sterne, Kim and Ye have now investigated this by exploring the effects of Dscam on a group of well-known neurons in the larvae of the fruit fly species Drosophila. The presynaptic terminals of single neurons in this group were labeled in the larvae using a genetic marker. This revealed that the neurons of larvae that had been engineered to produce too much Dscam had larger presynaptic terminals than normal.

Further investigation showed that for Dscam to influence how a presynaptic terminal grows, it must interact with another signaling protein called Abelson tyrosine kinase (or Abl for short). Therefore, the larger presynaptic terminals seen in larvae that produce too much Dscam are a result of the Dscam protein activating too much Abl.

There are several drugs that are approved for use in humans that suppress the activity of Abl. Sterne, Kim and Ye used two of these to treat fruit fly larvae, and found that they reversed the detrimental effects of extra Dscam on the larvae's neural circuit. Furthermore, the drugs fixed neural defects in a fruit fly model designed to reproduce the symptoms of fragile X syndrome.

Overall, the results presented by Sterne, Kim and Ye suggest that suppressing the abnormally high activity of the Abl protein could be a way of treating the brain disorders caused by having excessive amounts of the Dscam protein. The next step is to test whether Dscam and Abl interact in the same way in mammals and whether the proposed treatment is effective in treating mammalian models of disorders that involve dysregulated Dscam signaling.

DOI:http://dx.doi.org/10.7554/eLife.05196.002

Flamingo, a seven-pass transmembrane cadherin, cooperates with Netrin/Frazzled in Drosophila midline guidance

During central nervous system development, several guidance cues and receptors, as well as cell adhesion molecules, are required for guiding axons across the midline and along the anterior-posterior axis. In Drosophila, commissural axons sense the midline attractants Netrin A and B (Net) through Frazzled (Fra) receptors. Despite their importance, lack of Net or fra affects only some commissures, suggesting that additional molecules can fulfill this function. Recently, planar cell polarity (PCP) proteins have been implicated in midline axon guidance in both vertebrate and invertebrate systems. Here, we report that the atypical cadherin and PCP molecule Flamingo/Starry night (Fmi/Stan) acts jointly with Net/Fra signaling during midline development. Additional removal of fmi strongly increases the guidance defects in Net/fra mutants. Rescue and domain deletion experiments suggest that Fmi signaling facilitates commissural pathfinding potentially by mediating axonal fasciculation in a partly homophilic manner. Altogether, our results indicate that contact-mediated cell adhesion via Fmi acts in addition to the Net/Fra guidance system during axon pathfinding across the midline, underlining the importance of PCP molecules during vertebrates and invertebrates midline development.

Defying c-Abl signaling circuits through small allosteric compounds

Many extracellular and intracellular signals promote the c-Abl tyrosine kinase activity. c-Abl in turn triggers a multitude of changes either in protein phosphorylation or in gene expression in the cell. Yet, c-Abl takes part in diverse signaling routes because of several domains linked to its catalytic core. Complex conformational changes turn on and off its kinase activity. These changes affect surface features of the c-Abl kinase and likely its capability to bind actin and/or DNA. Two specific inhibitors (ATP-competitive or allosteric compounds) regulate the c-Abl kinase through different mechanisms. NMR studies show that a c-Abl fragment (SH3-SH2-linker-SH1) adopts different conformational states upon binding to each inhibitor. This supports an unconventional use for allosteric compounds to unraveling physiological c-Abl signaling circuits.