Discrete roles for secreted and transmembrane semaphorins in neuronal growth cone guidance in vivo

Spatiotemporal changes in Netrin/Dscam1 signaling dictate axonal projection direction in Drosophila small ventral lateral clock neurons

Axon projection is a spatial- and temporal-specific process in which the growth cone receives environmental signals guiding axons to their final destination. However, the mechanisms underlying changes in axonal projection direction without well-defined landmarks remain elusive. Here, we present evidence showcasing the dynamic nature of axonal projections in Drosophila's small ventral lateral clock neurons (s-LNvs). Our findings reveal that these axons undergo an initial vertical projection in the early larval stage, followed by a subsequent transition to a horizontal projection in the early-to-mid third instar larvae. The vertical projection of s-LNv axons correlates with mushroom body calyx expansion, while the s-LNv-expressed Down syndrome cell adhesion molecule (Dscam1) interacts with Netrins to regulate the horizontal projection. During a specific temporal window, locally newborn dorsal clock neurons secrete Netrins, facilitating the transition of axonal projection direction in s-LNvs. Our study establishes a compelling in vivo model to probe the mechanisms of axonal projection direction switching in the absence of clear landmarks. These findings underscore the significance of dynamic local microenvironments in the complementary regulation of axonal projection direction transitions.

Looking at Developmental Neurotoxicity Testing from the Perspective of an Invertebrate Embryo

Developmental neurotoxicity (DNT) of chemical compounds disrupts the formation of a normal brain. There is impressive progress in the development of alternative testing methods for DNT potential in chemicals, some of which also incorporate invertebrate animals. This review briefly touches upon studies on the genetically tractable model organisms of Caenorhabditis elegans and Drosophila melanogaster about the action of specific developmental neurotoxicants. The formation of a functional nervous system requires precisely timed axonal pathfinding to the correct cellular targets. To address this complex key event, our lab developed an alternative assay using a serum-free culture of intact locust embryos. The first neural pathways in the leg of embryonic locusts are established by a pair of afferent pioneer neurons which use guidance cues from membrane-bound and diffusible semaphorin proteins. In a systematic approach according to recommendations for alternative testing, the embryo assay quantifies defects in pioneer navigation after exposure to a panel of recognized test compounds for DNT. The outcome indicates a high predictability for test-compound classification. Since the pyramidal neurons of the mammalian cortex also use a semaphorin gradient for neurite guidance, the assay is based on evolutionary conserved cellular mechanisms, supporting its relevance for cortical development.

A locust embryo as predictive developmental neurotoxicity testing system for pioneer axon pathway formation

Exposure to environmental chemicals during in utero and early postnatal development can cause a wide range of neurological defects. Since current guidelines for identifying developmental neurotoxic chemicals depend on the use of large numbers of rodents in animal experiments, it has been proposed to design rapid and cost-efficient in vitro screening test batteries that are mainly based on mixed neuronal/glial cultures. However, cell culture tests do not assay correct wiring of neuronal circuits. The establishment of precise anatomical connectivity is a key event in the development of a functional brain. Here, we expose intact embryos of the locust (Locusta migratoria) in serum-free culture to test chemicals and visualize correct navigation of identified pioneer axons by fluorescence microscopy. We define separate toxicological endpoints for axonal elongation and navigation along a stereotyped pathway. To distinguish developmental neurotoxicity (DNT) from general toxicity, we quantify defects in axonal elongation and navigation in concentration-response curves and compare it to the biochemically determined viability of the embryo. The investigation of a panel of recognized DNT-positive and -negative test compounds supports a rather high predictability of this invertebrate embryo assay. Similar to the semaphorin-mediated guidance of neurites in mammalian cortex, correct axonal navigation of the locust pioneer axons relies on steering cues from members of this family of cell recognition molecules. Due to the evolutionary conserved mechanisms of neurite guidance, we suggest that our pioneer axon paradigm might provide mechanistically relevant information on the DNT potential of chemical agents on the processes of axon elongation, navigation, and fasciculation.

Report on the First Symposium on Invertebrate Neuroscience held on 13-17th August 2019 at the Balaton Limnological Institute, MTA Centre for Ecological Research, Tihany, Hungary

This meeting report provides an overview of the oral and poster presentations at the first international symposium for invertebrate neuroscience. The contents reflect the contributions of invertebrate neuroscience in addressing fundamental and fascinating challenges in understanding the neural substrates of animal behaviour.

Characterization of plexinA and two distinct semaphorin1a transcripts in the developing and adult cricket Gryllus bimaculatus

Guidance cues act during development to guide growth cones to their proper targets in both the central and peripheral nervous systems. Experiments in many species indicate that guidance molecules also play important roles after development, though less is understood about their functions in the adult. The Semaphorin family of guidance cues, signaling through Plexin receptors, influences the development of both axons and dendrites in invertebrates. Semaphorin functions have been extensively explored in Drosophila melanogaster and some other Dipteran species, but little is known about their function in hemimetabolous insects. Here, we characterize sema1a and plexA in the cricket Gryllus bimaculatus. In fact, we found two distinct predicted Sema1a proteins in this species, Sema1a.1 and Sema1a.2, which shared only 48% identity at the amino acid level. We include a phylogenetic analysis that predicted that many other insect species, both holometabolous and hemimetabolous, express two Sema1a proteins as well. Finally, we used in situ hybridization to show that sema1a.1 and sema1a.2 expression patterns were spatially distinct in the embryo, and both roughly overlap with plexA. All three transcripts were also expressed in the adult brain, mainly in the mushroom bodies, though sema1a.2 was expressed most robustly. sema1a.2 was also expressed strongly in the adult thoracic ganglia while sema1a.1 was only weakly expressed and plexA was undetectable.

Genome-wide analysis of parent-of-origin effects in non-syndromic orofacial clefts

Parent-of-origin (PofO) effects, such as imprinting are a phenomenon where the effect of variants depends on parental origin. Conventional association studies assume that phenotypic effects are independent of parental origin, and are thus severely underpowered to detect such non-Mendelian effects. Risk of orofacial clefts is influenced by genetic and environmental effects, the latter including maternal-specific factors such as perinatal smoking and folate intake. To identify variants showing PofO effects in orofacial clefts we have used a modification of the family-based transmission disequilibrium test to screen for biased transmission from mothers and fathers to affected offspring, biased ratios of maternal versus paternal transmission, and biased frequencies of reciprocal classes of heterozygotes among offspring. We applied these methods to analyze published genome-wide single-nucleotide polymorphism (SNP) data from ∼2500 trios mainly of European and Asian ethnicity with non-syndromic orofacial clefts, followed by analysis of 64 candidate SNPs in a replication cohort of ∼1200 trios of European origin. In our combined analysis, we did not identify any SNPs achieving conventional genome-wide significance (P<5 × 10(-8)). However, we observed an overall excess of loci showing maternal versus paternal transmission bias (P=0.013), and identified two loci that showed nominally significant effects in the same direction in both the discovery and replication cohorts, raising the potential for PofO effects. These include a possible maternal-specific transmission bias associated with rs12543318 at 8q21.3, a locus identified in a recent meta-analysis of non-syndromic cleft (maternal-specific P=1.5 × 10(-7), paternal-specific P=0.17). Overall, we conclude from this analysis that there are subtle hints of PofO effects in orofacial clefting.

Developmental expression of cell recognition molecules in the mushroom body and antennal lobe of the locust Locusta migratoria

We examined the development of olfactory neuropils in the hemimetabolous insect Locusta migratoria with an emphasis on the mushroom bodies, protocerebral integration centers implicated in memory formation. Using a marker of the cyclic adenosine monophosphate (cAMP) signaling cascade and lipophilic dye labeling, we obtained new insights into mushroom body organization by resolving previously unrecognized accessory lobelets arising from Class III Kenyon cells. We utilized antibodies against axonal guidance cues, such as the cell surface glycoproteins Semaphorin 1a (Sema 1a) and Fasciclin I (Fas I), as embryonic markers to compile a comprehensive atlas of mushroom body development. During embryogenesis, all neuropils of the olfactory pathway transiently expressed Sema 1a. The immunoreactivity was particularly strong in developing mushroom bodies. During late embryonic stages, Sema 1a expression in the mushroom bodies became restricted to a subset of Kenyon cells in the core region of the peduncle. Sema 1a was differentially sorted to the Kenyon cell axons and absent in the dendrites. In contrast to Drosophila, locust mushroom bodies and antennal lobes expressed Fas I, but not Fas II. While Fas I immunoreactivity was widely distributed in the midbrain during embryogenesis, labeling persisted into adulthood only in the mushroom bodies and antennal lobes. Kenyon cells proliferated throughout the larval stages. Their neurites retained the embryonic expression pattern of Sema 1a and Fas I, suggesting a role for these molecules in developmental mushroom body plasticity. Our study serves as an initial step toward functional analyses of Sema 1a and Fas I expression during locust mushroom body formation.

Nitric oxide as a regulator of neuronal motility and regeneration in the locust embryo

Nitric oxide (NO) is known as a gaseous messenger in the nervous system. It plays a role in synaptic plasticity, but also in development and regeneration of nervous systems. We have studied the function of NO and its signaling cascade via cyclic GMP in the locust embryo. Its developing nervous system is well suited for pharmacological manipulations in tissue culture. The components of this signaling pathway are localized by histochemical and immunofluorescence techniques. We have analyzed cellular mechanisms of NO action in three examples: 1. in the peripheral nervous system during antennal pioneer axon outgrowth, 2. in the enteric nervous system during migration of neurons forming the midgut nerve plexus, and 3. in the central nervous system during axonal regeneration of serotonergic neurons after axotomy. In each case, internally released NO or NO-induced cGMP synthesis act as permissive signals for the developmental process. Carbon monoxide (CO), as a second gaseous messenger, modulates enteric neuron migration antagonistic to NO.

Localization of the phase-related 6-kDa peptide (PRP) in different tissues of the desert locust Schistocerca gregaria--immunocytochemical and mass spectrometric approach

A 6-kDa phase-related peptide (PRP) was recently identified from the hemolymph of the desert locust Schistocerca gregaria. Its presence in much higher concentrations in the crowd-reared (gregarious) phase than in the isolated-reared (solitarious) one suggests a role in phase polyphenism. However, when tested in a variety of classical bioassays, no activity could be found. We hoped that uncovering its site(s) of synthesis might yield hints as to possible functions. An antiserum was raised against the C-terminal 16 aa part of PRP for use in immunocytochemistry. No immunoreactivity was recorded in the fat body, midgut, or Malpighian tubules. The strongest positive immunostaining was observed in the follicle cells of the ovary and in the seminal vesicle tubes of the male accessory gland complex. Also, positive were a pair of large neurosecretory cells in the subesophageal ganglion, the storage part of the corpora cardiaca and some nerve fibers in the brain- and abdominal regions. An additional mass spectrometric analysis was successfully done in combination with a BLAST search to detect possible false positive staining. This confirmed the presence of genuine PRP in most of the immunopositive tissues. Additional experiments are needed to unravel the role of PRP.

Signaling of secreted semaphorins in growth cone steering

Despite a tremendous amount of progress in the identification and characterization of many new players as components of class 3 secreted semaphorin signaling in growth cone steering (Fig. 1), our understanding of the molecular mechanisms is far from complete. More questions remain to be answered: how are differential cytoskeletal changes within a growth cone achieved in response to semaphorins? What are the target(s) of cyclic nucleotide modulation? How does a growth cone make a reliable decision in response to a shallow gradient? And finally, how does a growth cone maintain its sensitivity to a decreasing concentration ofsemaphorins when it is growing away from the source? With a high degree of interest in the field with the development of novel technologies in analyzing growth cone steering, we expect to see a much more complete picture of semaphoring signaling in the near future.

The PLEXIN PLX-2 and the ephrin EFN-4 have distinct roles in MAB-20/Semaphorin 2A signaling in Caenorhabditis elegans morphogenesis

Semaphorins are extracellular proteins that regulate axon guidance and morphogenesis by interacting with a variety of cell surface receptors. Most semaphorins interact with plexin-containing receptor complexes, although some interact with non-plexin receptors. Class 2 semaphorins are secreted molecules that control axon guidance and epidermal morphogenesis in Drosophila and Caenorhabditis elegans. We show that the C. elegans class 2 semaphorin MAB-20 binds the plexin PLX-2. plx-2 mutations enhance the phenotypes of hypomorphic mab-20 alleles but not those of mab-20 null alleles, indicating that plx-2 and mab-20 act in a common pathway. Both mab-20 and plx-2 mutations affect epidermal morphogenesis during embryonic and in postembryonic development. In both contexts, plx-2 null mutant phenotypes are much less severe than mab-20 null phenotypes, indicating that PLX-2 is not essential for MAB-20 signaling. Mutations in the ephrin efn-4 do not synergize with mab-20, indicating that EFN-4 may act in MAB-20 signaling. EFN-4 and PLX-2 are coexpressed in the late embryonic epidermis where they play redundant roles in MAB-20-dependent cell sorting.

Pharmacological approaches to nitric oxide signalling during neural development of locusts and other model insects

A novel aspect of cellular signalling during the formation of the nervous system is the involvement of the messenger molecule nitric oxide (NO), which has been discovered in the mammalian vascular system as mediator of smooth muscle relaxation. NO is a membrane-permeant molecule, which activates soluble guanylyl cyclase (sGC) and leads to the formation of cyclic GMP (cGMP) in target cells. The analysis of specific cell types in model insects such as Locusta, Schistocerca, Acheta, Manduca, and Drosophila shows that the NO/cGMP pathway is required for the stabilization of photoreceptor growth cones at the start of synaptic assembly in the optic lobe, for regulation of cell proliferation, and for correct outgrowth of pioneer neurons. Inhibition of the NOS and sGC enzymes combined with rescue experiments show that NO, and potentially also another atypical messenger, carbon monoxide (CO), orchestrate cell migration of enteric neurons. Cultured insect embryos are accessible model systems in which the molecular pathways linking cytoskeletal rearrangement to directed cell movements can be analyzed in natural settings. Based on the results obtained from the insect models, I discuss current evidence for NO and cGMP as essential signalling molecules for the development of vertebrate brains.

Developmental and adult expression of semaphorin 2a in the cricketGryllus bimaculatus

Developmental guidance cues act to direct growth cones to their correct targets in the nervous system. Recent experiments also demonstrate that developmental cues are expressed in the adult mammalian nervous system, although their function in the brain is not yet clear. The semaphorin gene family has been implicated in the growth of dendrites and axons in a number of different species. While the expression of semaphorin and its influence on tibial pioneer neurons in the developing limb bud have been well characterized in the grasshopper, the expression of semaphorin 2a (sema2a) has not been explored in the adult insect. In this study we used polymerase chain reaction (PCR) with degenerate and gene-specific primers to clone part of the secreted form of sema2a from Gryllus bimaculatus. Using in situ hybridization and immunohistochemistry, we confirmed that sema2a mRNA and protein expression patterns in the embryonic cricket were similar to that seen in the grasshopper. We also showed that tibial neuron development in crickets was comparable to that described in grasshopper. An examination of both developing and adult cricket brains showed that sema2a mRNA and protein were expressed in the Kenyon cells in mushroom bodies, an area involved in learning and memory. Sema2a expression was most obvious near the apex of the mushroom body in a region surrounding the neurogenic tip, which produces neurons throughout the life of the cricket. We discuss the role of neurogenesis in learning and memory and the potential involvement of semaphorin in this process.

STOP and GO with NO: nitric oxide as a regulator of cell motility in simple brains

During the formation of the brain, neuronal cell migration and neurite extension are controlled by extracellular guidance cues. Here, I discuss experiments showing that the messenger nitric oxide (NO) is an additional regulator of cell motility. NO is a membrane permeant molecule, which activates soluble guanylyl cyclase (sGC) and leads to the formation of cyclic GMP (cGMP) in target cells. The analysis of specific cells types in invertebrate models such as molluscs, insects and the medicinal leech provides insight how NO and cyclic nucleotides affect the wiring of nervous systems by regulating cell and growth-cone motility. Inhibition of the NOS and sGC enzymes combined with rescue experiments show that NO signalling orchestrates neurite outgrowth and filopodial dynamics, cell migration of enteric neurons, glial migration and axonogenesis of pioneer fibers. Cultured insect embryos are accessible model systems in which cellular mechanisms of NO-induced cytoskeletal reorganizations can be analyzed in natural settings. Finally, I will outline some indications that NO may also regulate cell motility in the developing and regenerating vertebrate nervous system.

Growth cones turn and migrate up an immobilized gradient of the laminin IKVAV peptide

Growth cone navigation is guided by extrinsic environmental proteins, called guidance cues. Many in vitro studies have characterized growth cone turning up and down gradients of soluble guidance cues. Although previous studies have shown that axonal elongation rates can be regulated by gradients of surface-bound molecules, there are no convincing demonstrations of growth cones turning to migrate up a surface-bound gradient of an adhesive ligand or guidance cue. In order to test this mode of axonal guidance, we used a photo-immobilization technique to create grids and gradients of an adhesive laminin peptide on polystyrene culture dish surfaces. Chick embryo dorsal root ganglia (DRGs) were placed on peptide grid patterns containing surface-bound gradients of the IKVAV-containing peptide. DRG growth cones followed a path of surface-bound peptide to the middle of a perpendicularly oriented gradient with a 25% concentration difference across 30 microm. The majority of growth cones turned and migrated up the gradient, turning until they were oriented directly up the gradient. Growth cones slowed their migration when they encountered the gradient, but growth cone velocity returned to the previous rate after turning up or down the gradient. This resembles in vivo situations where growth cones slow at a choice point before changing the direction of axonal extension. Thus, these results support the hypothesis that mechanisms of axonal guidance include growth cone orientation by gradients of surface-bound adhesive molecules and guidance cues.

The tibial-1 pioneer pathway: an in vivo model for neuronal outgrowth and guidance

As neurons extend axons to their targets during development, growth cones must reorient their direction of migration in response to extracellular guidance cues. A variety of model systems have been employed in order to dissect the cellular and molecular mechanisms that underlie this complex process. One preparation, the developing grasshopper limb bud, has proved to offer a number of advantages in which to examine mechanisms of growth cone guidance and motility in vivo. First, the relatively large size of the embryonic nervous system allows for straightforward imaging of both fixed and live neurons in vivo. Second, the peripheral nerves generated in the limb bud are highly stereotyped. Third, intact embryos can be cultured for a period of days, allowing for fairly easy perturbations at precise developmental stages. Fourth, due to the ease of dissection, numerous cell biological and molecular techniques can be utilized in the limb bud. Finally, axon guidance molecules and mechanisms are conserved between grasshoppers and other organism, including vertebrates.

Temporal specific patterns of semaphorin gene expression in rat brain after kainic acid-induced status epilepticus

Mossy fiber sprouting and other forms of synaptic reorganization may form the basis for a recurrent excitatory network in epileptic foci. Four major classes of axon guidance molecules--the ephrins, netrins, slits, and semaphorins--provide targeting information to outgrowing axons along predetermined pathways during development. These molecules may also play a role in synaptic reorganization in the adult brain and thereby promote epileptogenesis. We studied semaphorin gene expression, as assessed by in situ hybridization, using riboprobes generated from rat cDNA in an adult model of synaptic reorganization, kainic acid (KA)-induced status epilepticus (SE). Within the first week after KA-induced SE, semaphorin 3C, a class III semaphorin, mRNA content is decreased in the CA1 area of the hippocampus and is increased in the upper layers of cerebral cortex. Another class III semaphorin, semaphorin 3F, is also decreased in CA1 and CA3 of hippocampus within the first week after KA-SE. These changes in gene expression are principally confined to neurons. By contrast, there was little change in the semaphorin 4C mRNA content of CA1 neurons at this time. No changes in expression of semaphorin 3A and 4C genes were detected 28 days after KA-induced SE. Regulation of semaphorin gene expression after KA-induced SE suggests that neurons may regulate the expression of axonal guidance molecules and thereby contribute to synaptic reorganization after injury of the mature brain. The anatomic locale of the altered semaphorin gene expression may serve as a marker for specific networks undergoing synaptic reorganization in the epileptic brain.

Gradients and growth cone guidance of grasshopper neurons

The generation of a functional nervous system is dependent on precise pathfinding of axons during development. This pathfinding is directed by the distribution of local and long-range guidance cues, the latter of which are believed to be distributed in gradients. Gradients of guidance cues have been associated with growth cone function for over a hundred years. However, little is known about the mechanisms used by growth cones to respond to these gradients, in part owing to the lack of identifiable gradients in vivo. In the developing grasshopper limb, two gradients of the semaphorin Sema-2a are necessary for correct neuronal pathfinding in vivo. The gradients are found in regions where growth cones make critical steering decisions. Observations of different growth cone behaviors associated with these gradients have provided some insights into how growth cones respond to them. Growth cones appear to respond more faithfully to changes in concentration, rather than absolute levels, of Sema-2a expression, whereas the absolute levels may regulate growth cone size.

Migrating mesoderm establish a uniform distribution of laminin in the developing grasshopper embryo

The basal lamina is composed of molecules which physically interact to form a network that serves as a migrational scaffold for many cell types. In the developing peripheral nervous system of the grasshopper, neuronal growth cones are intimately associated with the basal lamina as they migrate. Laminin is a major component of the basal lamina and is a potent promoter of neurite outgrowth in vitro. However, it is unclear what the source of laminin is or how the distribution of laminin within the basal lamina is established. To address this question, grasshopper laminin subunit genes were cloned. As expected, laminin was found within the basal lamina throughout the embryo, in particular in the limb bud, where its expression is coincident with the outgrowth and guidance of the Tibial (Til) pioneer neurons. Surprisingly, the synthesis of beta and gamma chains of laminin was restricted to migratory mesodermal cells, while in other nonmigratory tissues, such as epithelium and presumptive muscle, beta and gamma chains of laminin were not detected. In spite of this, laminin immunoreactivity in the basal lamina appears uniform and is available as a substrate for axonal outgrowth.

Nitric oxide: an unconventional messenger in the nervous system of an orthopteroid insect

Nitric oxide (NO) is a membrane-permeant messenger molecule generated from the amino acid L-arginine. NO can activate soluble guanylyl cyclase leading to the formation of cyclic GMP (cGMP) in target cells. In the nervous system, NO/cGMP signalling is thought to play essential roles in synaptic plasticity during development and also in the mature animal. This paper examines biochemical, cell biological, and physiological investigations of NO/cGMP signalling in the nervous system of the locust, a commonly used neurobiological preparation. Biochemical investigations suggest that an identical enzyme is responsible for both NO synthase (NOS) and NADPH-diaphorase activity after tissue fixation. Immunocytochemical staining of an olfactory center in the locust brain shows that NOS-immunoreactivity colocalizes with NADPH-diaphorase at the cellular level. The cytochemical staining of NO donor and target cells in adult animals suggests functions in olfaction, vision, and sensorimotor integration. During development, NO is implicated in axonal outgrowth and synaptogenesis. The cellular distribution of NO-responsive cells in neural circuits reflects potential functions of NO as a retrograde synaptic messenger, as an intracellular messenger, and as a lateral diffusible messenger independent of conventional synaptic connectivity.

Nitric oxide and cGMP influence axonogenesis of antennal pioneer neurons

The grasshopper embryo has been used as a convenient system with which to investigate mechanisms of axonal navigation and pathway formation at the level of individual nerve cells. Here, we focus on the developing antenna of the grasshopper embryo (Schistocerca gregaria) where two siblings of pioneer neurons establish the first two axonal pathways to the CNS. Using immunocytochemistry we detected nitric oxide (NO)-induced synthesis of cGMP in the pioneer neurons of the embryonic antenna. A potential source of NO are NADPH-diaphorase-stained epithelial cells close to the basal lamina. To investigate the role of the NO/cGMP signaling system during pathfinding, we examined the pattern of outgrowing pioneer neurons in embryo culture. Pharmacological inhibition of soluble guanylyl cyclase (sGC) and of NO synthase (NOS) resulted in an abnormal pattern of pathway formation in the antenna. Axonogenesis of both pairs of pioneers was inhibited when specific NOS or sGC inhibitors were added to the culture medium; the observed effects include the loss axon emergence as well as retardation of outgrowth, such that growth cones do not reach the CNS. The addition of membrane-permeant cGMP or a direct activator of the sGC enzyme to the culture medium completely rescued the phenotype resulting from the block of NO/cGMP signaling. These results indicate that NO/cGMP signaling is involved in axonal elongation of pioneer neurons in the antenna of the grasshopper.

Semaphorin function in the developing invertebrate peripheral nervous system

Different members of the semaphorin family of secreted and transmembrane guidance molecules play important and diverse roles during neuronal development. Within the developing grasshopper limb bud, two semaphorins are expressed in relatively non-overlapping and distinct expression patterns. The establishment of the tibial sensory projection within the limb bud relies on the combinatorial action of both semaphorins. In this review, we describe the function of the two semaphorins in axonal guidance and propose that a hierarchy of cues guide sensory neurons in the developing peripheral nervous system.Key words: semaphorin, axon guidance, grasshopper, peripheral nervous system, review.

A paradoxical gradient of a basal lamina-associated repellent is essential for pathfinding by the Ti1 pioneer axons in cockroach embryos

Perturbations of in situ axon growth with proteolytic enzymes and monoclonal antibodies were used to determine the role of gradient guidance cues in the formation of the Ti1 pioneer axon trajectory in cultured cockroach embryos. Treatment with enzymes that degrade the basal lamina indicated that this substrate contains both an elastase-sensitive proximal directing cue and a collagenase-sensitive distal directing cue. The latter is shown to be a repellent of axon growth and is identical to the PROD-2 antigen that is distributed in a gradient along the proximal-distal axis of the leg with high levels in proximal regions. This means that throughout the course of their growth the axons extend in the direction of increasing levels of repellent. At a critical decision point in the trajectory the axons change both the direction of growth and the substrate to which their growth cones adhere. PROD-2 plays an essential role in both of these processes.

Signalling by semaphorin receptors: cell guidance and beyond

Semaphorins are a large family of secreted or cell-bound signals, known to guide axons in developing nervous tissue. They are expressed in a variety of adult and embryonic tissues and are thought to have a broader spectrum of functions. Recent evidence suggests that semaphorins and their receptors play a key role in the control of cellular interactions, most likely in cell-cell repulsion. A subset of semaphorins interacts with neuropilins - cell-surface molecules lacking a signalling-competent cytoplasmic domain. Another large family of transmembrane molecules, namely plexins, bind specifically to semaphorins. Thus plexins, alone, or in association with neuropilins, behave as fully functional semaphorin receptors. The intracellular responses elicited by plexins are unknown, but their large cytoplasmic moiety, containing the strikingly conserved sex-plexin (SP) domain, is likely to trigger novel signal-transduction pathways.

Semaphorins and their receptors in vertebrates and invertebrates

The semaphorins are a family of intercellular signaling proteins that has grown to include 19 identified members in higher vertebrates. Several of its members act as axonal guidance molecules. One participates in signaling in the immune system. The majority, however, do not yet have known biological functions. Recent studies have shown that neuropilins and plexins act as receptors for semaphorins. The most important challenge for the future is to define the biological roles of semaphorins in vivo.

Ectopic semaphorin-1a functions as an attractive guidance cue for developing peripheral neurons

Transmembrane and secreted glycoproteins of the semaphorin family are typically classified as inhibitory neuronal guidance molecules. However, although chemorepulsive activity has been demonstrated for several semaphorin family members, little is known about the function of the numerous transmembrane semaphorins identified to date. Here we demonstrated that the extracellular semaphorin domain of a transmembrane semaphorin, semaphorin-1a, could actively perturb axon pathfinding in vivo when presented homogenously as a recombinant freely soluble factor. When ectopic overexpression was limited to defined epithelial regions, semaphorin-1a could directly steer axons by acting as an attractive guidance molecule.

Semaphorins: repulsive guidance molecules show their attractive side

Semaphorins are known to repel growth cones of developing axons, but a study of the grasshopper limb bud shows that they can also serve as attractive guidance cues.

Signal transduction underlying growth cone guidance by diffusible factors

Many diffusible axon guidance cues and their receptors have been identified recently. These cues are often found to be bifunctional, acting as attractants or repellents under different circumstances. Studies of cytoplasmic signaling mechanisms have led to the notion that the response of a growth cone to a particular guidance cue depends on the internal state of the neuron, which, in turn, is under the influence of other coincident signals received by the neuron. Furthermore, many diffusible guidance cues appear to share common cytoplasmic signaling pathways.