Self-Assembly of Symmetric Copolymers in Slits with Inert and Attractive Walls

Although the behavior of the confined semi-dilute solutions of self-assembling copolymers represents an important topic of basic and applied research, it has eluded the interest of scientists. Extensive series of dissipative particle dynamics simulations have been performed on semi-dilute solutions of A5B5 chains in a selective solvent for A in slits using a DL-MESO simulation package. Simulations of corresponding bulk systems were performed for comparison. This study shows that the associates in the semi-dilute bulk solutions are partly structurally organized. Mild steric constraints in slits with non-attractive walls hardly affect the size of the associates, but they promote their structural arrangement in layers parallel to the slit walls. Attractive walls noticeably affect the association process. In slits with mildly attractive walls, the adsorption competes with the association process. At elevated concentrations, the associates start to form in wide slits when the walls are sparsely covered by separated associates, and the association process prevents the full coverage of the surface. In slits with strongly attractive walls, adsorption is the dominant behavior. The associates form in wide slits at elevated concentrations only after the walls are completely and continuously covered by the adsorbed chains.

Awakening the dormant: Role of axonal guidance cues in stress-induced reorganization of the adult prefrontal cortex leading to depression-like behavior

Major depressive disorder (MDD) is a chronic and disabling disorder affecting roughly 280 million people worldwide. While multiple brain areas have been implicated, dysfunction of prefrontal cortex (PFC) circuitry has been consistently documented in MDD, as well as in animal models for stress-induced depression-like behavioral states. During brain development, axonal guidance cues organize neuronal wiring by directing axonal pathfinding and arborization, dendritic growth, and synapse formation. Guidance cue systems continue to be expressed in the adult brain and are emerging as important mediators of synaptic plasticity and fine-tuning of mature neural networks. Dysregulation or interference of guidance cues has been linked to depression-like behavioral abnormalities in rodents and MDD in humans. In this review, we focus on the emerging role of guidance cues in stress-induced changes in adult prefrontal cortex circuitry and in precipitating depression-like behaviors. We discuss how modulating axonal guidance cue systems could be a novel approach for precision medicine and the treatment of depression.

The factors affecting neurogenesis after stroke and the role of acupuncture

Stroke induces a state of neuroplasticity in the central nervous system, which can lead to neurogenesis phenomena such as axonal growth and synapse formation, thus affecting stroke outcomes. The brain has a limited ability to repair ischemic damage and requires a favorable microenvironment. Acupuncture is considered a feasible and effective neural regulation strategy to improve functional recovery following stroke via the benign modulation of neuroplasticity. Therefore, we summarized the current research progress on the key factors and signaling pathways affecting neurogenesis, and we also briefly reviewed the research progress of acupuncture to improve functional recovery after stroke by promoting neurogenesis. This study aims to provide new therapeutic perspectives and strategies for the recovery of motor function after stroke based on neurogenesis.

[Central Slice Theorem-based Relationship between 1D-NPS Obtained by the Slit Method and 2D-NPS for CT Images]

PURPOSE

The method using a numerical slit (slit method) is used commonly to obtain the one-dimensional (1D) noise power spectrum (NPS) in computed tomography. However, the relationship between the 1D-NPS obtained by the slit method and the original two-dimensional (2D) NPS derived by the 2D Fourier transformation has not been elucidated clearly. The purpose of this study was to clarify their relationship based on the well-known central slice theorem (projection slice theorem) and validate it using computer simulation analysis.

METHODS

With the application of the central slice theorem, we described that the 1D-NPS obtained by the slit method was equal to the central slice (profile) in the 2D-NPS when we set the slit length to the maximum (i.e. the matrix size of the noise image). To verify this, we generated computer-simulated noise images with the known 2D-NPS (true 2D-NPS). From those images, we obtained the 1D-NPS that was obtained by the slit method and compared it with the central slice in the true 2D-NPS.

RESULTS

When we set the slit length to the maximum, the 1D-NPS obtained by the slit method showed good agreement with the central slice in the true 2D-NPS.

CONCLUSION

We clarified the relationship between the 1D-NPS obtained by the slit method and the 2D-NPS using a theoretical approach and the computer simulation. We had to maximize the slit length to achieve the accurate measurement of the 1D-NPS using the slit method.

Revealing the Genetic Instructions for Nervous System Wiring

Why did the axon cross the midline? A 1993 paper by Corey Goodman and colleagues described a genetic screen in fruit flies that pioneered the discovery of conserved families of axon guidance cues and receptors, highlighting fundamental processes underlying wiring specificity in the developing nervous system.

Ridge Minimization of Ablated Morphologies on ITO Thin Films Using Squared Quasi-Flat Top Beam

In this study, we explore the improvements in pattern quality that was obtained with a femtosecond laser with quasi-flat top beam profiles at the ablated edge of indium tin oxide (ITO) thin films for the patterning of optoelectronic devices. To ablate the ITO thin films, a femtosecond laser is used that has a wavelength and pulse duration of 1030 nm and 190 fs, respectively. The squared quasi-flat top beam is obtained from a circular Gaussian beam using slits with varying x-y axes. Then, the patterned ITO thin films are measured using both scanning electron and atomic force microscopes. In the case of the Gaussian beam, the ridge height and width are approximately 39 nm and 1.1 μm, respectively, whereas, when the quasi-flat top beam is used, the ridge height and width are approximately 7 nm and 0.25 μm, respectively.

Robo1 Forms a Compact Dimer-of-Dimers Assembly

Roundabout (Robo) receptors provide an essential repulsive cue in neuronal development following Slit ligand binding. This important signaling pathway can also be hijacked in numerous cancers, making Slit-Robo an attractive therapeutic target. However, little is known about how Slit binding mediates Robo activation. Here we present the crystal structure of Robo1 Ig1-4 and Robo1 Ig5, together with a negative stain electron microscopy reconstruction of the Robo1 ectodomain. These results show how the Robo1 ectodomain is arranged as compact dimers, mainly mediated by the central Ig domains, which can further interact in a "back-to-back" fashion to generate a tetrameric assembly. We also observed no change in Robo1 oligomerization upon interaction with the dimeric Slit2-N ligand using fluorescent imaging. Taken together with previous studies we propose that Slit2-N binding results in a conformational change of Robo1 to trigger cell signaling.

Regulation of Drosophila Brain Wiring by Neuropil Interactions via a Slit-Robo-RPTP Signaling Complex

The axonal wiring molecule Slit and its Round-About (Robo) receptors are conserved regulators of nerve cord patterning. Robo receptors also contribute to wiring brain circuits. Whether molecular mechanisms regulating these signals are modified to fit more complex brain wiring processes is unclear. We investigated the role of Slit and Robo receptors in wiring Drosophila higher-order brain circuits and identified differences in the cellular and molecular mechanisms of Robo/Slit function. First, we find that signaling by Robo receptors in the brain is regulated by the Receptor Protein Tyrosine Phosphatase RPTP69d. RPTP69d increases membrane availability of Robo3 without affecting its phosphorylation state. Second, we detect no midline localization of Slit during brain development. Instead, Slit is enriched in the mushroom body, a neuronal structure covering large areas of the brain. Thus, a divergent molecular mechanism regulates neuronal circuit wiring in the Drosophila brain, partly in response to signals from the mushroom body.

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In the Drosophila brain, mushroom bodies are a source of the Slit guidance cue

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Slit regulates axon growth in the vicinity of mushroom bodies via Robo receptors

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The phosphatase RPTP69D regulates Robo signaling in the brain

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RPTP69D regulates Robo3 membrane presentation independent of its enzymatic activity

The glycosylation pathway is required for the secretion of Slit and for the maintenance of the Slit receptor Robo on axons

Slit proteins act as repulsive axon guidance cues by activating receptors of the Roundabout (Robo) family. During early neurogenesis in Drosophila melanogaster, Slit prevents the growth cones of longitudinal tract neurons from inappropriately crossing the midline, thus restricting these cells to trajectories parallel to the midline. Slit is expressed in midline glial cells, and Robo is present in longitudinal axon tracts and growth cones. We showed that the enzyme Mummy (Mmy) controlled Slit-Robo signaling through mechanisms that affected both the ligand and the receptor. Mmy was required for the glycosylation of Slit, which was essential for Slit secretion. Mmy was also required for maintaining the abundance and spatial distribution of Robo through an indirect mechanism that was independent of Slit secretion. Moreover, secretion of Slit was required to maintain the fasciculation and position of longitudinal axon tracts, thus maintaining the hardwiring of the nervous system. Thus, Mmy is required for Slit secretion and for maintaining Robo abundance and distribution in the developing nervous system in Drosophila.

Structural Perspectives on Axon Guidance

Axon guidance relies on a combinatorial code of receptor and ligand interactions that direct adhesive/attractive and repulsive cellular responses. Recent structural data have revealed many of the molecular mechanisms that govern these interactions and enabled the design of sophisticated mutant tools to dissect their biological functions. Here, we discuss the structure/function relationships of four major classes of guidance cues (ephrins, semaphorins, slits, netrins) and examples of morphogens (Wnt, Shh) and of cell adhesion molecules (FLRT). These cell signaling systems rely on specific modes of receptor-ligand binding that are determined by selective binding sites; however, defined structure-encoded receptor promiscuity also enables cross talk between different receptor/ligand families and can also involve extracellular matrix components. A picture emerges in which a multitude of highly context-dependent structural assemblies determines the finely tuned cellular behavior required for nervous system development.

Slit Binding via the Ig1 Domain Is Essential for Midline Repulsion by Drosophila Robo1 but Dispensable for Receptor Expression, Localization, and Regulation in Vivo

The midline repellant ligand Slit and its Roundabout (Robo) family receptors constitute the major midline repulsive pathway in bilaterians. Slit proteins produced at the midline of the central nervous system (CNS) signal through Robo receptors expressed on axons to prevent them from crossing the midline, and thus regulate connectivity between the two sides of the nervous system. Biochemical structure and interaction studies support a model in which Slit binding to the first immunoglobulin-like (Ig1) domain of Robo receptors activates a repulsive signaling pathway in axonal growth cones. Here, we examine the in vivo functional importance of the Ig1 domain of the Drosophila Robo1 receptor, which controls midline crossing of axons in response to Slit during development of the embryonic CNS. We show that deleting Ig1 from Robo1 disrupts Slit binding in cultured Drosophila cells, and that a Robo1 variant lacking Ig1 (Robo1(∆Ig1)) is unable to promote ectopic midline repulsion in gain-of-function studies in the Drosophila embryonic CNS. We show that the Ig1 domain is not required for proper expression, axonal localization, or Commissureless (Comm)-dependent regulation of Robo1 in vivo, and we use a genetic rescue assay to show that Robo1(∆Ig1) is unable to substitute for full-length Robo1 to properly regulate midline crossing of axons. These results establish a direct link between in vitro biochemical studies of Slit-Robo interactions and in vivo genetic studies of Slit-Robo signaling during midline axon guidance, and distinguish Slit-dependent from Slit-independent aspects of Robo1 expression, regulation, and activity during embryonic development.

Robo2 acts in trans to inhibit Slit-Robo1 repulsion in pre-crossing commissural axons

During nervous system development, commissural axons cross the midline despite the presence of repellant ligands. In Drosophila, commissural axons avoid premature responsiveness to the midline repellant Slit by expressing the endosomal sorting receptor Commissureless, which reduces surface expression of the Slit receptor Roundabout1 (Robo1). In this study, we describe a distinct mechanism to inhibit Robo1 repulsion and promote midline crossing, in which Roundabout2 (Robo2) binds to and prevents Robo1 signaling. Unexpectedly, we find that Robo2 is expressed in midline cells during the early stages of commissural axon guidance, and that over-expression of Robo2 can rescue robo2-dependent midline crossing defects non-cell autonomously. We show that the extracellular domains required for binding to Robo1 are also required for Robo2's ability to promote midline crossing, in both gain-of-function and rescue assays. These findings indicate that at least two independent mechanisms to overcome Slit-Robo1 repulsion in pre-crossing commissural axons have evolved in Drosophila.

USP33, a new player in lung cancer, mediates Slit-Robo signaling

Ubiquitin specific protease 33 (USP33) is a multifunctional protein regulating diverse cellular processes. The expression and role of USP33 in lung cancer remain unexplored. In this study, we show that USP33 is down-regulated in multiple cohorts of lung cancer patients and that low expression of USP33 is associated with poor prognosis. USP33 mediates Slit-Robo signaling in lung cancer cell migration. Downregulation of USP33 reduces the protein stability of Robo1 in lung cancer cells, providing a previously unknown mechanism for USP33 function in mediating Slit activity in lung cancer cells. Taken together, USP33 is a new player in lung cancer that regulates Slit-Robo signaling. Our data suggest that USP33 may be a candidate tumor suppressor for lung cancer with potential as a prognostic marker.

A subset of chicken statoacoustic ganglion neurites are repelled by Slit1 and Slit2

Mechanosensory hair cells in the chicken inner ear are innervated by bipolar afferent neurons of the statoacoustic ganglion (SAG). During development, individual SAG neurons project their peripheral process to only one of eight distinct sensory organs. These neuronal subtypes may respond differently to guidance cues as they explore the periphery in search of their target. Previous gene expression data suggested that Slit repellants might channel SAG neurites into the sensory primordia, based on the presence of robo transcripts in the neurons and the confinement of slit transcripts to the flanks of the prosensory domains. This led to the prediction that excess Slit proteins would impede the outgrowth of SAG neurites. As predicted, axonal projections to the primordium of the anterior crista were reduced 2-3 days after electroporation of either slit1 or slit2 expression plasmids into the anterior pole of the otocyst on embryonic day 3 (E3). The posterior crista afferents, which normally grow through and adjacent to slit expression domains as they are navigating towards the posterior pole of the otocyst, did not show Slit responsiveness when similarly challenged by ectopic delivery of slit to their targets. The sensitivity to ectopic Slits shown by the anterior crista afferents was more the exception than the rule: responsiveness to Slits was not observed when the entire E4 SAG was challenged with Slits for 40 h in vitro. The corona of neurites emanating from SAG explants was unaffected by the presence of purified human Slit1 and Slit2 in the culture medium. Reduced axon outgrowth from E8 olfactory bulbs cultured under similar conditions for 24 h confirmed bioactivity of purified human Slits on chicken neurons. In summary, differential sensitivity to Slit repellents may influence the directional outgrowth of otic axons toward either the anterior or posterior otocyst.

RabGDI controls axonal midline crossing by regulating Robo1 surface expression

BACKGROUND

Axons navigate to their future synaptic targets with the help of choice points, intermediate targets that express axon guidance cues. Once they reach a choice point, axons need to switch their response from attraction to repulsion in order to move on with the next stage of their journey. The mechanisms underlying the change in axonal responsiveness are poorly understood. Commissural axons become sensitive to the repulsive activity of Slits when they cross the ventral midline of the CNS. Responsiveness to Slits depends on surface expression of Robo receptors. In Drosophila, Commissureless (Comm) plays a crucial regulatory role in midline crossing by keeping Robo levels low on precommissural axons. Interestingly, to date no vertebrate homolog of comm has been identified. Robo3/Rig1 has been shown to control Slit sensitivity before the midline, but without affecting Robo1 surface expression.

RESULTS

We had identified RabGDI, a gene linked to human mental retardation and an essential component of the vesicle fusion machinery, in a screen for differentially expressed floor-plate genes. Downregulation of RabGDI by in ovo RNAi caused commissural axons to stall in the floor plate, phenocopying the effect observed after downregulation of Robo1. Conversely, premature expression of RabGDI prevented commissural axons from entering the floor plate. Furthermore, RabGDI triggered Robo1 surface expression in cultured commissural neurons. Taken together, our results identify RabGDI as a component of the switching mechanism that is required for commissural axons to change their response from attraction to repulsion at the intermediate target.

CONCLUSION

RabGDI takes over the functional role of fly Comm by regulating the surface expression of Robo1 on commissural axons in vertebrates. This in turn allows commissural axons to switch from attraction to repulsion at the midline of the spinal cord.

Motor axon exit from the mammalian spinal cord is controlled by the homeodomain protein Nkx2.9 via Robo-Slit signaling

Mammalian motor circuits control voluntary movements by transmitting signals from the central nervous system (CNS) to muscle targets. To form these circuits, motor neurons (MNs) must extend their axons out of the CNS. Although exit from the CNS is an indispensable phase of motor axon pathfinding, the underlying molecular mechanisms remain obscure. Here, we present the first identification of a genetic pathway that regulates motor axon exit from the vertebrate spinal cord, utilizing spinal accessory motor neurons (SACMNs) as a model system. SACMNs are a homogeneous population of spinal MNs with axons that leave the CNS through a discrete lateral exit point (LEP) and can be visualized by the expression of the cell surface protein BEN. We show that the homeodomain transcription factor Nkx2.9 is selectively required for SACMN axon exit and identify the Robo2 guidance receptor as a likely downstream effector of Nkx2.9; loss of Nkx2.9 leads to a reduction in Robo2 mRNA and protein within SACMNs and SACMN axons fail to exit the spinal cord in Robo2-deficient mice. Consistent with short-range interactions between Robo2 and Slit ligands regulating SACMN axon exit, Robo2-expressing SACMN axons normally navigate through LEP-associated Slits as they emerge from the spinal cord, and fail to exit in Slit-deficient mice. Our studies support the view that Nkx2.9 controls SACMN axon exit from the mammalian spinal cord by regulating Robo-Slit signaling.

Nerve repulsion by the lens and cornea during cornea innervation is dependent on Robo-Slit signaling and diminishes with neuron age

The cornea, the most densely innervated tissue on the surface of the body, becomes innervated in a series of highly coordinated developmental events. During cornea development, chick trigeminal nerve growth cones reach the cornea margin at embryonic day (E)5, where they are initially repelled for days from E5 to E8, instead encircling the corneal periphery in a nerve ring prior to entering on E9. The molecular events coordinating growth cone guidance during cornea development are poorly understood. Here we evaluated a potential role for the Robo-Slit nerve guidance family. We found that Slits 1, 2 and 3 expression in the cornea and lens persisted during all stages of cornea innervation examined. Robo1 expression was developmentally regulated in trigeminal cell bodies, expressed robustly during nerve ring formation (E5-8), then later declining concurrent with projection of growth cones into the cornea. In this study we provide in vivo and in vitro evidence that Robo-Slit signaling guides trigeminal nerves during cornea innervation. Transient, localized inhibition of Robo-Slit signaling, by means of beads loaded with inhibitory Robo-Fc protein implanted into the developing eyefield in vivo, led to disorganized nerve ring formation and premature cornea innervation. Additionally, when trigeminal explants (source of neurons) were oriented adjacent to lens vesicles or corneas (source of repellant molecules) in organotypic tissue culture both lens and cornea tissues strongly repelled E7 trigeminal neurites, except in the presence of inhibitory Robo-Fc protein. In contrast, E10 trigeminal neurites were not as strongly repelled by cornea, and presence of Robo-Slit inhibitory protein had no effect. In full, these findings suggest that nerve repulsion from the lens and cornea during nerve ring formation is mediated by Robo-Slit signaling. Later, a shift in nerve guidance behavior occurs, in part due to molecular changes in trigeminal neurons, including Robo1 downregulation, thus allowing nerves to find the Slit-expressing cornea permissive for growth cones.

Slit/Robo-mediated axon guidance in Tribolium and Drosophila: divergent genetic programs build insect nervous systems

As the complexity of animal nervous systems has increased during evolution, developmental control of neuronal connectivity has become increasingly refined. How has functional diversification within related axon guidance molecules contributed to the evolution of nervous systems? To address this question, we explore the evolution of functional diversity within the Roundabout (Robo) family of axon guidance receptors. In Drosophila, Robo and Robo2 promote midline repulsion, while Robo2 and Robo3 specify the position of longitudinal axon pathways. The Robo family has expanded by gene duplication in insects; robo2 and robo3 exist as distinct genes only within dipterans, while other insects, like the flour beetle Tribolium castaneum, retain an ancestral robo2/3 gene. Both Robos from Tribolium can mediate midline repulsion in Drosophila, but unlike the fly Robos cannot be down-regulated by Commissureless. The overall architecture and arrangement of longitudinal pathways are remarkably conserved in Tribolium, despite it having only two Robos. Loss of TcSlit causes midline collapse of axons in the beetle, a phenotype recapitulated by simultaneous knockdown of both Robos. Single gene knockdowns reveal that beetle Robos have specialized axon guidance functions: TcRobo is dedicated to midline repulsion, while TcRobo2/3 also regulates longitudinal pathway formation. TcRobo2/3 knockdown reproduces aspects of both Drosophila robo2 and robo3 mutants, suggesting that TcRobo2/3 has two functions that in Drosophila are divided between Robo2 and Robo3. The ability of Tribolium to organize longitudinal axons into three discrete medial-lateral zones with only two Robo receptors demonstrates that beetle and fly achieve equivalent developmental outcomes using divergent genetic programs.

Integrins are required for cardioblast polarisation in Drosophila

BACKGROUND

The formation of a tubular organ, such as the heart, requires the communication of positional and polarity signals between migratory cells. Key to this process is the establishment of a new luminal domain on the cell surface, generally from the apical domain of a migratory cell. This domain will also acquire basal properties, as it will produce a luminal extracellular matrix. Integrin receptors are the primary means of cell adhesion and adhesion signaling with the extracellular matrix. Here we characterise the requirement of Integrins in a genetic model of vasculogenesis, the formation of the heart in Drosophila.

RESULTS

As with vertebrates, the Drosophila heart arises from lateral mesoderm that migrates medially to meet their contralateral partners, to then assemble a midline vessel. During migration, Integrins are among the first proteins restricted to the presumptive luminal domain of cardioblasts. Integrins are required for normal levels of leading edge membrane motility. Apical accumulation of Integrins is enhanced by Robo, and reciprocally, apicalisation of luminal factors like Slit and Robo requires Integrin function. Integrins may provide a template for the formation of a lumen by stabilising lumen factors like Robo. Subsequent to migration, Integrin is required for normal cardioblast alignment and lumen formation. This phenotype is most readily modified by other mutations that affect adhesion, such as Talin and extracellular matrix ligands.

CONCLUSION

Our findings reveal an instructive role for Integrins in communicating polarising information to cells during migration, and during transition to an epithelial tube structure.

A roundabout way to cancer

The Slit family of secreted proteins and their transmembrane receptor, Robo, were originally identified in the nervous system where they function as axon guidance cues and branching factors during development. Since their discovery, a great number of additional roles have been attributed to Slit/Robo signaling, including regulating the critical processes of cell proliferation and cell motility in a variety of cell and tissue types. These processes are often deregulated during cancer progression, allowing tumor cells to bypass safeguarding mechanisms in the cell and the environment in order to grow and escape to new tissues. In the past decade, it has been shown that the expression of Slit and Robo is altered in a wide variety of cancer types, identifying them as potential therapeutic targets. Further, studies have demonstrated dual roles for Slits and Robos in cancer, acting as both oncogenes and tumor suppressors. This bifunctionality is also observed in their roles as axon guidance cues in the developing nervous system, where they both attract and repel neuronal migration. The fact that this signaling axis can have opposite functions depending on the cellular circumstance make its actions challenging to define. Here, we summarize our current understanding of the dual roles that Slit/Robo signaling play in development, epithelial tumor progression, and tumor angiogenesis.

Robo1 and Robo2 have distinct roles in pioneer longitudinal axon guidance

Pioneer longitudinal axons grow long distances parallel to the floor plate and precisely maintain their positions using guidance molecules released from the floor plate. Two receptors, Robo1 and Robo2, are critical for longitudinal axon guidance by the Slit family of chemorepellents. Previous studies showed that Robo1(-/-);2(-/-) double mutant mouse embryos have disruptions in both ventral and dorsal longitudinal tracts. However, the role of each Robo isoform remained unclear, because Robo1 or 2 single mutants have mild or no errors. Here we utilized a more sensitive genetic strategy to reduce Robo levels for determining any separate functions of the Robo1 and 2 isoforms. We found that Robo1 is the predominant receptor for guiding axons in ventral tracts and prevents midline crossing. In contrast, Robo2 is the main receptor for directing axons within dorsal tracts. Robo2 also has a distinct function in repelling neuron cell bodies from the floor plate. Therefore, while Robo1 and 2 have some genetic overlap to cooperate in guiding longitudinal axons, each isoform has distinct functions in specific longitudinal axon populations.

Axon regeneration pathways identified by systematic genetic screening in C. elegans

The mechanisms underlying the ability of axons to regrow after injury remain poorly explored at the molecular genetic level. We used a laser injury model in Caenorhabditis elegans mechanosensory neurons to screen 654 conserved genes for regulators of axonal regrowth. We uncover several functional clusters of genes that promote or repress regrowth, including genes classically known to affect axon guidance, membrane excitability, neurotransmission, and synaptic vesicle endocytosis. The conserved Arf Guanine nucleotide Exchange Factor (GEF), EFA-6, acts as an intrinsic inhibitor of regrowth. By combining genetics and in vivo imaging, we show that EFA-6 inhibits regrowth via microtubule dynamics, independent of its Arf GEF activity. Among newly identified regrowth inhibitors, only loss of function in EFA-6 partially bypasses the requirement for DLK-1 kinase. Identification of these pathways significantly expands our understanding of the genetic basis of axonal injury responses and repair.

Navigating breast cancer: axon guidance molecules as breast cancer tumor suppressors and oncogenes

Slit, Netrin, Ephrin, and Semaphorin's roles in development have expanded greatly in the past decade from their original characterization as axon guidance molecules (AGMs) to include roles as regulators of tissue morphogenesis and development in diverse organs. In the mammary gland, AGMs are important for maintaining normal cell proliferation and adhesion during development. The frequent dysregulation of AGM expression during tumorigenesis and tumor progression suggests that AGMs also play a crucial role as tumor suppressors and oncogenes in breast cancer. Moreover, these findings suggest that AGMs may be excellent targets for new breast cancer prognostic tests and more effective therapeutic strategies.

Heparan sulfate proteoglycan specificity during axon pathway formation in the Drosophila embryo

Axon guidance is influenced by the presence of heparan sulfate (HS) proteoglycans (HSPGs) on the surface of axons and growth cones (Hu, [2001]: Nat Neurosci 4:695-701; Irie et al. [2002]: Development 129:61-70; Inatani et al. [2003]: Science 302:1044-1046; Johnson et al. [2004]: Curr Biol 14:499-504; Steigemann et al. [2004]: Curr Biol 14:225-230). Multiple HSPGs, including Syndecans, Glypicans and Perlecans, carry the same carbohydrate polymer backbones, raising the question of how these molecules display functional specificity during nervous system development. Here we use the Drosophila central nervous system (CNS) as a model to compare the impact of eliminating Syndecan (Sdc) and/or the Glypican Dally-like (Dlp). We show that Dlp and Sdc share a role in promoting accurate patterns of axon fasciculation in the lateral longitudinal neuropil; however, unlike mutations in sdc, which disrupt the ability of the secreted repellent Slit to prevent inappropriate passage of axons across the midline, mutations in dlp show neither midline defects nor genetic interactions with Slit and its Roundabout (Robo) receptors at the midline. Dlp mutants do show genetic interactions with Slit and Robo in lateral fascicle formation. In addition, simultaneous loss of Dlp and Sdc demonstrates an important role for Dlp in midline repulsion, reminiscent of the functional overlap between Robo receptors. A comparison of HSPG distribution reveals a pattern that leaves midline proximal axons with relatively little Dlp. Finally, the loss of Dlp alters Slit distribution distal but not proximal to the midline, suggesting that distinct yet overlapping pattern of HSPG expression provides a spatial system that regulates axon guidance decisions.

Expression and roles of Slit/Robo in human ovarian cancer

The Slit glycoproteins and their Roundabout (Robo) receptors regulate migration and growth of many types of cells including human cancer cells. However, little is known about the expression and roles of Slit/Robo in human ovarian cancer. Herein, we examined the expression of Slit/Robo in human normal and malignant ovarian tissues and its potential participation in regulating migration and proliferation of human ovarian cancer cells using two ovarian cancer cell lines, OVCAR-3 and SKOV-3. We demonstrated that Slit2/3 and Robo1 were immunolocalized primarily in stromal cells in human normal ovaries and in cancer cells in many histotypes of ovarian cancer tissues. Protein expression of Slit2/3 and Robo1/4 was also identified in OVCAR-3 and SKOV-3 cells. However, recombinant human Slit2 did not significantly affect SKOV-3 cell migration, and OVCAR-3 and SKOV-3 cell proliferation. Slit2 also did not induce ERK1/2 and AKT1 phosphorylation in OVCAR-3 and SKOV-3 cells. The current findings indicate that three major members (Slit2/3 and Robo1) of Slit/Robo family are widely expressed in the human normal and malignant ovarian tissues and in OVCAR-3 and SKOV-3 cells. However, Slit/Robo signaling may not play an important role in regulating human ovarian cancer cell proliferation and migration.

Understanding in vivo blood cell migration--Drosophila hemocytes lead the way

Drosophila embryonic hemocytes have emerged as a potent system to analyze the roles of key regulators of the actin and microtubule cytoskeletons live and in an in vivo context (see Table I and references therein). The relative ease with which live imaging can be used to visualize the invasive migrations of these highly motile macrophages and their responses to wound and chemoattractant signals make them a particularly appropriate and genetically tractable cell type to study in relation to pathological conditions such as cancer metastasis and inflammation. ( 1-3) In order to understand how signaling pathways are integrated for a coordinated response, a question with direct relevance to autoimmune dysfunction, we have sought to more fully characterize the inputs these cells receive in vivo over the course of their developmental dispersal. These studies have recently revealed that hemocyte migration is intimately associated with the development of the ventral nerve cord (VNC), a structure used by hemocytes to disperse over the embryo that itself requires this association for its correct morphogenesis. Crucially the VNC must separate from the epidermis to create a channel for hemocyte migration, revealing how constriction of extracellular space can be used to control cell migration in vivo. ( 4).

Slit-Robo signaling induces malignant transformation through Hakai-mediated E-cadherin degradation during colorectal epithelial cell carcinogenesis

The Slit family of guidance cues binds to Roundabout (Robo) receptors and modulates cell migration. We report here that ectopic expression of Slit2 and Robo1 or recombinant Slit2 treatment of Robo1-expressing colorectal epithelial carcinoma cells recruited an ubiquitin ligase Hakai for E-cadherin (E-cad) ubiquitination and lysosomal degradation, epithelial-mesenchymal transition (EMT), and tumor growth and liver metastasis, which were rescued by knockdown of Hakai. In contrast, knockdown of endogenous Robo1 or specific blockade of Slit2 binding to Robo1 prevented E-cad degradation and reversed EMT, resulting in diminished tumor growth and liver metastasis. Ectopic expression of Robo1 also triggered a malignant transformation in Slit2-positive human embryonic kidney 293 cells. Importantly, the expression of Slit2 and Robo1 was significantly associated with an increased metastatic risk and poorer overall survival in colorectal carcinoma patients. We conclude that engagement of Robo1 by Slit2 induces malignant transformation through Hakai-mediated E-cad ubiquitination and lysosomal degradation during colorectal epithelial cell carcinogenesis.

Presenilin-dependent receptor processing is required for axon guidance

The Alzheimer's disease-linked gene presenilin is required for intramembrane proteolysis of amyloid-β precursor protein, contributing to the pathogenesis of neurodegeneration that is characterized by loss of neuronal connections, but the role of Presenilin in establishing neuronal connections is less clear. Through a forward genetic screen in mice for recessive genes affecting motor neurons, we identified the Columbus allele, which disrupts motor axon projections from the spinal cord. We mapped this mutation to the Presenilin-1 gene. Motor neurons and commissural interneurons in Columbus mutants lacking Presenilin-1 acquire an inappropriate attraction to Netrin produced by the floor plate because of an accumulation of DCC receptor fragments within the membrane that are insensitive to Slit/Robo silencing. Our findings reveal that Presenilin-dependent DCC receptor processing coordinates the interplay between Netrin/DCC and Slit/Robo signaling. Thus, Presenilin is a key neural circuit builder that gates the spatiotemporal pattern of guidance signaling, thereby ensuring neural projections occur with high fidelity.

Midbrain dopaminergic axons are guided longitudinally through the diencephalon by Slit/Robo signals

Dopaminergic neurons from the ventral mesencephalon/diencephalon (mesodiencephalon) form vital pathways constituting the majority of the brain's dopamine systems. Mesodiencephalic dopaminergic (mdDA) neurons extend longitudinal projections anteriorly through the diencephalon, ascending toward forebrain targets. The mechanisms by which mdDA axons initially navigate through the diencephalon are poorly understood. Recently the Slit family of secreted axon guidance proteins, and their Robo receptors, have been identified as important guides for descending longitudinal axons. To test the potential roles of Slit/Robo guidance in ascending trajectories, we examined tyrosine hydroxylase-positive (TH+) projections from mdDA neurons in mutant mouse embryos. We found that mdDA axons grow out of and parallel to Slit-positive ventral regions within the diencephalon, and that subsets of the mdDA axons likely express Robo1 and possibly also Robo2. Slit2 was able to directly inhibit TH axon outgrowth in explant co-culture assays. The mdDA axons made significant pathfinding errors in Slit1/2 and Robo1/2 knockout mice, including spreading out in the diencephalon to form a wider tract. The wider tract resulted from a combination of invasion of the ventral midline, consistent with Slit repulsion, but also axons wandering dorsally, away from the ventral midline. Aberrant dorsal trajectories were prominent in Robo1 and Robo1/2 knockout mice, suggesting that an aspect of Robo receptor function is Slit-independent. These results indicate that Slit/Robo signaling is critical during the initial establishment of dopaminergic pathways, with roles in the dorsoventral positioning and precise pathfinding of these ascending longitudinal axons.

Perspectives of SLIT/ROBO signaling in placental angiogenesis

A novel family of evolutionally conserved neuronal guidance cues, including ligands (i.e., Slit, netrin, epherin, and semaphorin) and their corresponding receptors (i.e., Robo, DCC/Unc5, Eph and plexin/ neuropilin), has been identified to play a crucial role in axon pathfinding and branching as well as neuronal cell migration. The presence of commonalities in both neural and vascular developments has led to some exciting discoveries recently, which have extended the functions of these systems to vascular formation (vasculogenesis) and development (angiogenesis). Some of these ligands and receptors have been found to be expressed in the vasculature and surrounding tissues in physiological and pathological conditions. It is postulated that they regulate the formation and integrity of blood vessels. In particular, it has been shown that the Slit/Robo pair plays a novel role in angiogenesis during tumorigenesis and vascular formation during embryogenesis. Herein we summarize briefly the characteristics of this family of neuronal guidance molecules and discuss the extra-neural expression and function of the Slit/Robo pair in angiogenesis in physiological and pathological settings. We report expression of Robo1 protein in capillary endothelium and co-expression of Slit2 and Robo1 proteins in syncytiotrophoblast in healthy term human placental villi. These cellular expression patterns implicate that the Slit/Robo signaling plays an autocrine and/or paracrine role in angiogenesis and trophoblast functions. We also speculate a possible role of this system in pathophysiological placental angiogenesis.

Sublingual Immunotherapy: Clinical Indications in the WAO-SLIT Position Paper

Sublingual immunotherapy (SLIT) is a matter of only 20 years. Nonetheless, in this short period of time more than 60 randomized double blind placebo-controlled trials have been published, in addition to postmarketing surveillance studies and meta-analyses. The wide diffusion of SLIT in clinical practice and the large availability of experimental data prompted the WAO to publish a position paper on SLIT, to identify the indications, contraindications, and practical aspects of the treatment. On the basis of the available literature, SLIT is certainly indicated in allergic rhinitis in both adults and children. In this latter population, SLIT may exert a preventative effect on the development of asthma. The age seems not to represent a special problem. SLIT can be used also when asthma is associated to rhinitis, whereas it is not the first choice for the treatment of isolated asthma. The IgE-mediated mechanism and the clear identification of the causal role of the allergen are mandatory prerequisites for prescribing SLIT. The safety profile is excellent, but it is recommended that the first dose be given under medical supervision. Atopic dermatitis, latex allergy, and hymenoptera hypersensitivity are promising fields of use of SLIT, but they are still considered only experimental uses.

The Adam family metalloprotease Kuzbanian regulates the cleavage of the roundabout receptor to control axon repulsion at the midline

Slits and their Roundabout (Robo) receptors mediate repulsive axon guidance at the Drosophila ventral midline and in the vertebrate spinal cord. Slit is cleaved to produce fragments with distinct signaling properties. In a screen for genes involved in Slit-Robo repulsion, we have identified the Adam family metalloprotease Kuzbanian (Kuz). Kuz does not regulate midline repulsion through cleavage of Slit, nor is Slit cleavage essential for repulsion. Instead, Kuz acts in neurons to regulate repulsion and Kuz can cleave the Robo extracellular domain in Drosophila cells. Genetic rescue experiments using an uncleavable form of Robo show that this receptor does not maintain normal repellent activity. Finally, Kuz activity is required for Robo to recruit its downstream signaling partner, Son of sevenless (Sos). These observations support the model that Kuz-directed cleavage is important for Robo receptor activation.

Vascular Robo4 restricts proangiogenic VEGF signaling in breast

Formation of the vascular system within organs requires the balanced action of numerous positive and negative factors secreted by stromal and epithelial cells. Here, we used a genetic approach to determine the role of SLITs in regulating the growth and organization of blood vessels in the mammary gland. We demonstrate that vascularization of the gland is not affected by loss of Slit expression in the epithelial compartment. Instead, we identify a stromal source of SLIT, mural cells encircling blood vessels, and show that loss of Slit in the stroma leads to elevated blood vessel density and complexity. We examine candidate SLIT receptors, Robo1 and Robo4, and find that increased vessel angiogenesis is phenocopied by loss of endothelial-specific Robo4, as long as it is combined with the presence of an angiogenic stimulus such as preneoplasia or pregnancy. In contrast, loss of Robo1 does not affect blood vessel growth. The enhanced growth of blood vessels in Robo4(-/-) endothelium is due to activation of vascular endothelial growth factor (VEGF)-R2 signaling through the Src and FAK kinases. Thus, our studies present a genetic dissection of SLIT/ROBO signaling during organ development. We identify a stromal, rather than epithelial, source of SLITs that inhibits blood vessel growth by signaling through endothelial ROBO4 to down-regulate VEGF/VEGFR2 signaling.

Slit-Robo signaling mediates lymphangiogenesis and promotes tumor lymphatic metastasis

The Slit family of guidance cues binds to Roundabout (Robo) receptors to modulate neuronal, leukocytic, and endothelial migration. Slit-Robo signaling had been reported to function as chemoattractive signal for vascular endothelial cells during angiogenesis. In this study, we found that Robo1 was expressed in lymphatic endothelial cells to mediate the migration and tube formation of these cells upon Slit2 stimulation, which were specifically inhibited by the function-blocking antibody R5 to Slit2/Robo1 interaction. To further explore the lymphangiogenic effect and significance mediated by Slit-Robo signaling, we intercrossed Slit2 transgenic mice with a non-metastatic RIP1-Tag2 mouse tumor model, and found that transgenic overexpression of Slit2 significantly enhanced tumor lymphangiogenesis and subsequently promoted mesenteric lymph node metastasis of pancreatic islet tumors. Taken together, our findings reveal that through interacting with Robo1, Slit2 is a novel and potent lymphangiogenic factor and contributes to tumor lymphatic metastasis.

The SLIT-ROBO pathway: a regulator of cell function with implications for the reproductive system

The secreted SLIT glycoproteins and their Roundabout (ROBO) receptors were originally identified as important axon guidance molecules. They function as a repulsive cue with an evolutionarily conserved role in preventing axons from migrating to inappropriate locations during the assembly of the nervous system. In addition the SLIT-ROBO interaction is involved in the regulation of cell migration, cell death and angiogenesis and, as such, has a pivotal role during the development of other tissues such as the lung, kidney, liver and breast. The cellular functions that the SLIT/ROBO pathway controls during tissue morphogenesis are processes that are dysregulated during cancer development. Therefore inactivation of certain SLITs and ROBOs is associated with advanced tumour formation and progression in disparate tissues. Recent research has indicated that the SLIT/ROBO pathway could also have important functions in the reproductive system. The fetal ovary expresses most members of the SLIT and ROBO families. The SLITs and ROBOs also appear to be regulated by steroid hormones and regulate physiological cell functions in adult reproductive tissues such as the ovary and endometrium. Furthermore several SLITs and ROBOs are aberrantly expressed during the development of ovarian, endometrial, cervical and prostate cancer. This review will examine the roles this pathway could have in the development, physiology and pathology of the reproductive system and highlight areas for future research that could further dissect the influence of the SLIT/ROBO pathway in reproduction.

Functional diversity of Robo receptor immunoglobulin domains promotes distinct axon guidance decisions

Recognition molecules of the immunoglobulin (Ig) superfamily control axon guidance in the developing nervous system. Ig-like domains are among the most widely represented protein domains in the human genome, and the number of Ig superfamily proteins is strongly correlated with cellular complexity. In Drosophila, three Roundabout (Robo) Ig superfamily receptors respond to their common Slit ligand to regulate axon guidance at the midline: Robo and Robo2 mediate midline repulsion, Robo2 and Robo3 control longitudinal pathway selection, and Robo2 can promote midline crossing. How these closely related receptors mediate distinct guidance functions is not understood. We report that the differential functions of Robo2 and Robo3 are specified by their ectodomains and do not reflect differences in cytoplasmic signaling. Functional modularity of Robo2's ectodomain facilitates multiple guidance decisions: Ig1 and Ig3 of Robo2 confer lateral positioning activity, whereas Ig2 confers promidline crossing activity. Robo2's distinct functions are not dependent on greater Slit affinity but are instead due in part to differences in multimerization and receptor-ligand stoichiometry conferred by Robo2's Ig domains. Together, our findings suggest that diverse responses to the Slit guidance cue are imparted by intrinsic structural differences encoded in the extracellular Ig domains of the Robo receptors.

Involvement of the SLIT/ROBO pathway in follicle development in the fetal ovary

In humans and domestic mammals, pivotal processes in ovary development, including primordial follicle assembly, occur prenatally. These events are essential for determining fertility in adult life; however, they remain poorly understood at the mechanistic level. In mammals, the SLITs (SLIT1, SLIT2 and SLIT3) and their ROBO (ROBO1, ROBO2, ROBO3/RIG-1 and ROBO4/MAGIC ROBO) receptors regulate neural, leukocyte, vascular smooth muscle cell and endothelial cell migration. In addition, the SLIT/ROBO pathway has functional roles in embryonic development and in the adult ovary by inhibiting cell migration and promoting apoptosis. We therefore characterised follicle formation and investigated the expression and localisation of the ROBO/SLIT pathway in the ovine fetal ovary. Using RT-PCR, we identified SLIT2, SLIT3, ROBO1, ROBO2 and ROBO4 in sheep ovaries harvested across gestation. The real-time quantitative PCR results implied that ROBO2 expression and ROBO4 expression were elevated during the early stages of follicle formation and stayed abundant during primordial follicle maturation (P<0.05). Immunohistochemistry examination demonstrated that ROBO1 was localised to the pre-granulosa cells, while ROBO2, ROBO4 and SLIT2 were expressed in the oocytes of the developing primordial follicle. This indicates that in the fetal ovary, SLIT-ROBO signalling may require an autocrine and paracrine interaction. Furthermore, at the time of increased SLIT-ROBO expression, there was a significant reduction in the number of proliferating oocytes in the developing ovary (P<0.0001). Overall, these results suggest, for the first time, that the SLIT-ROBO pathway is expressed at the time of follicle formation during fetal ovary development.

Wiring Olfaction: The Cellular and Molecular Mechanisms that Guide the Development of Synaptic Connections from the Nose to the Cortex

Within the central nervous system, the olfactory system fascinates by its developmental and physiological particularities, and is one of the most studied models to understand the mechanisms underlying the guidance of growing axons to their appropriate targets. A constellation of contact-mediated (laminins, CAMs, ephrins, etc.) and secreted mechanisms (semaphorins, slits, growth factors, etc.) are known to play different roles in the establishment of synaptic interactions between the olfactory epithelium, olfactory bulb (OB) and olfactory cortex. Specific mechanisms of this system (including the amazing family of about 1000 different olfactory receptors) have been also proposed. In the last years, different reviews have focused in partial sights, specially in the mechanisms involved in the formation of the olfactory nerve, but a detailed review of the mechanisms implicated in the development of the connections among the different olfactory structures (olfactory epithelium, OB, olfactory cortex) remains to be written. In the present work, we afford this systematic review: the different cellular and molecular mechanisms which rule the formation of the olfactory nerve, the lateral olfactory tract and the intracortical connections, as well as the few data available regarding the accessory olfactory system. These mechanisms are compared, and the implications of the differences and similarities discussed in this fundamental scenario of ontogeny.

Midline governs axon pathfinding by coordinating expression of two major guidance systems

Formation of the neural network requires concerted action of multiple axon guidance systems. How neurons orchestrate expression of multiple guidance genes is poorly understood. Here, we show that Drosophila T-box protein Midline controls expression of genes encoding components of two major guidance systems: Frazzled, ROBO, and Slit. In midline mutant, expression of all these molecules are reduced, resulting in severe axon guidance defects, whereas misexpression of Midline induces their expression. Midline is present on the promoter regions of these genes, indicating that Midline controls transcription directly. We propose that Midline controls axon pathfinding through coordinating the two guidance systems.

Pioneer longitudinal axons navigate using floor plate and Slit/Robo signals

Longitudinal axons transmit all signals between the brain and spinal cord. Their axon tracts through the brain stem are established by a simple set of pioneer axons with precise trajectories parallel to the floor plate. To identify longitudinal guidance mechanisms in vivo, the overall role of floor plate tissue and the specific roles of Slit/Robo signals were tested. Ectopic induction or genetic deletion of the floor plate diverted longitudinal axons into abnormal trajectories. The expression patterns of the diffusible cues of the Slit family were altered in the floor plate experiments, suggesting their involvement in longitudinal guidance. Genetic tests of Slit1 and Slit2, and the Slit receptors Robo1 and Robo2 were carried out in mutant mice. Slit1;Slit2 double mutants had severe longitudinal errors, particularly for ventral axons, including midline crossing and wandering longitudinal trajectories. Robo1 and Robo2 were largely genetically redundant, and neither appeared to specify specific tract positions. However, combined Robo1 and Robo2 mutations strongly disrupted each pioneer tract. Thus, pioneer axons depend on long-range floor plate cues, with Slit/Robo signaling required for precise longitudinal trajectories.

Manipulating Robo expression in vivo perturbs commissural axon pathfinding in the chick spinal cord

In vertebrate embryos, most spinal commissural axons cross the ventral midline (VM) and project either alongside or significant distances away from the floor plate (FP). The upregulation of repulsive Robo1/2 receptors on postcrossing commissural axons, in mammals, presumably allows these axons to respond to the midline-associated repellents, Slit1-3, facilitating their expulsion from, and prohibiting their reentry into, the FP. Compelling data suggest that Robo3 represses Robo1/2 function on precrossing axons and that Robo1/2 inhibit attractive guidance receptors on postcrossing axons, thereby ensuring that decussated axons are selectively responsive to midline Slits. However, whether Robo1/2 expel decussated commissural axons from the VM and/or prevent their reentry into the FP has not been explicitly established in vivo. Furthermore, some commissural axons do not require Robo1/2 to elaborate appropriate contralateral projections in the mouse spinal cord. Here, we use unilateral in ovo electroporation together with Atoh1 and Neurog1 enhancer elements to visualize, and assess the consequences of manipulating Robo expression on, dl1 and dl2 chick commissural axons. In response to misexpressing a cytoplasmic truncation of Robo1 and/or Robo2, which should block all Robo-ligand interactions, postcrossing commissural axons extend alongside, but do not project away from or reenter the FP. In contrast, misexpression of full-length Robo2 prevents many commissural axons from crossing the VM. Together, these findings support key and selective in vivo roles for Robo receptors in presumably altering the responsiveness of decussated commissural axons and facilitating their expulsion from the VM within the chick spinal cord.

Molecular mechanisms controlling midline crossing by precerebellar neurons

Precerebellar neurons of the inferior olive (IO) and lateral reticular nucleus (LRN) migrate tangentially from the rhombic lip toward the floor plate following parallel pathways. This process is thought to involve netrin-1 attraction. However, whereas the cell bodies of LRN neurons cross the midline, IO neurons are unable to do so. In many systems and species, axon guidance and cell migration at the midline are controlled by Slits and their receptor Robos. We showed previously that precerebellar axons and neurons do not cross the midline in the absence of the Robo3 receptor. To determine whether this signaling by Slits and the two other Robo receptors, Robo1 and Robo2, also regulates precerebellar neuron behavior at the floor plate, we studied the phenotype of Slit1/2 and Robo1/2/3 compound mutants. Our results showed that many IO neurons can cross the midline in absence of Slit1/2 or Robo1/2, supporting a role for midline repellents in guiding precerebellar neurons. We also show that these molecules control the development of the lamellation of the inferior olivary complex. Last, the analysis of Robo1/2/3 triple mutants suggests that Robo3 inhibits Robo1/2 repulsion in precrossing LRN axons but not in IO axons in which it has a dominant and distinct function.

Robos and slits control the pathfinding and targeting of mouse olfactory sensory axons

Odorants are detected by olfactory receptor neurons (ORNs) located in the olfactory epithelium. In mice, ORNs expressing the same odorant receptor (OR) project to a single glomerulus out of 1800 in the olfactory bulb (OB). It has been proposed that OR-derived cAMP signals guide ORN axons to their glomeruli rather than OR themselves. Recently, it has also been shown that the axon guidance molecule Slit1 and its receptor Robo2 control the dorsoventral segregation of ORN axons as they are projecting to the OB. We have analyzed the development of olfactory projections in Slit1/Slit2 and Robo1/Robo2 single and double mutants. We show that in Robo1-/-;Robo2-/- mice, most ORN axons fail to enter the OB and instead project caudally into the diencephalon. Moreover, in these mice, ORN axons expressing the same OR project to several glomeruli at ectopic positions. Thus, Slit1, Slit2, Robo1, and Robo2 cooperate to control the convergence of ORN axons to the OB and the precise targeting of ORN axons to specific glomeruli.

Cellular components and signals required for the cardiac outflow tract assembly in Drosophila

Specification of cardiac primordia and formation of the Drosophila heart tube is highly reminiscent of the early steps of vertebrate heart development. We previously reported that the final morphogenesis of the Drosophila heart involves a group of nonmesodermal cells called heart-anchoring cells and a pair of derived from the pharyngeal mesoderm cardiac outflow muscles. Like the vertebrate cardiac neural crest cells, heart-anchoring cells migrate, interact with the tip of the heart, and participate in shaping the cardiac outflow tract. To better understand this process, we performed an in-depth analysis of how the Drosophila outflow tract is formed. We found that the most anterior cardioblasts that form a central outflow tract component, the funnel-shaped heart tip, do not originate from the cardiac primordium. They are initially associated with the pharyngeal cardiac outflow muscles and join the anterior aorta during outflow tract assembly. The particular morphology of the heart tip is disrupted in embryos in which heart-anchoring cells were ablated, revealing their critical role in outflow tract morphogenesis. We also demonstrate that Slit and Robo are required for directed movements of heart-anchoring cells toward the heart tip and that the cell-cell contact between the heart-anchoring cells and the ladybird-expressing cardioblasts is critically dependent on DE-cadherin Shotgun. Our observations suggest that the similarities between Drosophila and vertebrate cardiogenesis extend beyond the early developmental events.

Slits and Robos in the developing chicken inner ear

Mechanosensory hair cells in the chick inner ear synapse onto afferent neurons of the statoacoustic ganglion (SAG). During development, these neurons extend a central process to the brain and a peripheral process into one of eight sensory organs. A combination of cues, including chemoattractants and chemorepellents, direct otic axons to their peripheral targets. As a first step in evaluating the role of known axon guidance molecules, Slits and Robos, we examined expression of their transcripts in the chick inner ear from embryonic day 2-11 (Hamburger and Hamilton stages 14-37). Robo2 and slit2 are in migrating neuroblasts and the SAG, while both slits and robos are in the otic epithelium. We speculate that this family of signaling molecules may be involved in repulsion, first of otic neuroblasts and then of otic axons. Later our expression data revealed a potentially novel role for these molecules in maintaining sensory/nonsensory boundaries.

The Slit receptor Robo1 is predominantly expressed via the Dutt1 alternative promoter in pioneer neurons in the embryonic mouse brain and spinal cord

Robo1 is a member of the Roundabout (Robo) family of receptors for the Slit axon guidance cues. In mice (and humans), the Robo1 locus has alternative promoters producing two transcript isoforms, Robo1 and Dutt1. These isoforms have unique 5' termini, predicted to encode distinct N-terminal amino acids, but share the rest of their 3' exons. To determine the spatial expression of the Robo1 and Dutt1 isoforms, we generated isoform-specific RNA probes, and carried out in situ hybridization on E10.5 mouse embryos, the stage in early neuron differentiation when many major axon pathways are established. The two isoforms had distinct expression patterns that partially overlapped. Dutt1 was the predominant isoform, with widespread expression in regions of post-mitotic neurons and neuroepithelial cells. The Robo1 isoform had a distinct expression pattern restricted to subsets of neurons, many of which were Dutt1-negative. Dutt1 was the main isoform expressed in spinal cord commissural neurons. For both probes, the main hybridization signal was limited to two spots in the nuclei of individual cells. This study shows distinct expression patterns for the Dutt1 and Robo1 alternative promoters in the embryonic nervous system.

Cloning, expression, crystallization and preliminary X-ray analysis of the first two Ig domains from human roundabout 1 (Robo1)

Activation of Roundabout 1 (Robo1) by Slit proteins results in axon repulsion from the midline. Robo1 is a large transmembrane receptor expressed on the axon growth cone and the minimal Robo1-binding region required for Slit activation has been mapped to the N-terminal Ig1-2 domains. The cDNA encoding the first two Ig domains of Robo1 has been cloned and the protein has been expressed in HEK293 EBNA-1 mammalian cells. Here, the purification and crystallization conditions of this Robo1 construct are reported. The crystals are orthorhombic, space group P2(1)2(1)2, with unit-cell parameters a = 38.8, b = 69.4, c = 103.3 A and one molecule in the asymmetric unit. X-ray diffraction data have been collected to 2.8 A resolution on beamline ID29 at the ESRF.

Regeneration and maintenance of the planarian midline is regulated by a slit orthologue

Several families of evolutionarily conserved axon guidance cues orchestrate the precise wiring of the nervous system during embryonic development. The remarkable plasticity of freshwater planarians provides the opportunity to study these molecules in the context of neural regeneration and maintenance. Here we characterize a homologue of the Slit family of guidance cues from the planarian Schmidtea mediterranea. Smed-slit is expressed along the planarian midline, in both dorsal and ventral domains. RNA interference (RNAi) targeting Smed-slit results in the collapse of many newly regenerated tissues at the midline; these include the cephalic ganglia, ventral nerve cords, photoreceptors, and the posterior digestive system. Surprisingly, Smed-slit RNAi knockdown animals also develop morphologically distinguishable, ectopic neural structures near the midline in uninjured regions of intact and regenerating planarians. These results suggest that Smed-slit acts not only as a repulsive cue required for proper midline formation during regeneration but that it may also act to regulate the behavior of neural precursors at the midline in intact planarians.

Dual branch-promoting and branch-repelling actions of Slit/Robo signaling on peripheral and central branches of developing sensory axons

Secreted Slit proteins signal through Robo receptors and negatively regulate axon guidance and cell migration, but in vertebrates, Slit proteins can also stimulate branching and elongation of sensory axons and cortical dendrites in vitro. Here, we show that this branching activity is required for developing peripheral sensory arbors in vivo, because trigeminal sensory branching above the eye is reduced in Slit2;Slit3 double or Slit1,2,3 triple mutants. A similar phenotype is observed in Robo1;Robo2 double mutants, implicating Robo receptors in mediating this activity. Interestingly, by studying the central projection of sensory neurons in the spinal cord, we discovered that Slit ligands are also required for proper guidance of sensory branches during bifurcation but through a different cellular mechanism. In Slit1;Slit2 or Robo1;Robo2 double mutant mice, sensory axons enter the spinal cord prematurely because of the loss of an inhibitory guidance function on one of the daughter branches of each afferent during bifurcation. Together, our studies reveal that Slit/Robo signaling contributes to patterning both the peripheral and central branches of sensory neurons but via distinct positive branching and negative guidance actions, respectively.

Robo1 and Robo2 cooperate to control the guidance of major axonal tracts in the mammalian forebrain

The function of the nervous system depends on the precision of axon wiring during development. Previous studies have demonstrated that Slits, a family of secreted chemorepellent proteins, are crucial for the proper development of several major forebrain tracts. Mice deficient in Slit2 or, even more so, in both Slit1 and Slit2 have defects in multiple axonal pathways, including corticofugal, thalamocortical, and callosal connections. In the spinal cord, members of the Robo family of proteins help mediate the function of Slits, but the relative contribution of these receptors to the guidance of forebrain projections remains to be determined. In the present study, we addressed the function of Robo1 and Robo2 in the guidance of forebrain projections by analyzing Robo1-, Robo2-, and Robo1;Robo2-deficient mice. Mice deficient in Robo2 and, more dramatically, in both Robo1 and Robo2, display prominent axon guidance errors in the development of corticofugal, thalamocortical, and corticocortical callosal connections. Our results demonstrate that Robo1 and Robo2 mostly cooperate to mediate the function of Slit proteins in guiding the major forebrain projections.