Roundabout 2 regulates migration of sensory neurons by signaling in trans

BACKGROUND

Roundabout (Robo) receptors and their ligand Slit are important regulators of axon guidance and cell migration. The development of Drosophila embryonic sense organs provides a neuronal migration paradigm where the in vivo roles of Slit and Robo can be assayed using genetics.

RESULTS

Here we show that Slit-Robo signaling controls migration of Drosophila larval sensory neurons that are part of the Chordotonal (Cho) stretch receptor organs. We used live imaging to show that abdominal Cho organs normally migrate ventrally during development, whereas thoracic Cho organs do not. Robo2 overexpression in cis (in the sensory neurons) or in trans (on neighboring visceral mesoderm) transforms abdominal organs to a thoracic morphology and position by blocking migration, while loss of Slit-Robo signaling produces a reverse transformation in which thoracic organs migrate ectopically. Rescue and tissue-specific knockout experiments indicate that trans signaling by Robo2 contributes to the normal positioning of the thoracic Cho organs. The differential positioning of Cho organs between the thorax and abdomen is known to be regulated by Hox genes, and we show that the essential Hox cofactor Homothorax, represses Robo2 expression in the abdominal visceral mesoderm.

CONCLUSIONS

Our results suggest that segment-specific neuronal migration patterns are directed through a novel signaling complex (the "Slit sandwich") in which Robo2 on the thoracic visceral mesoderm binds to Slit and presents it to Robo receptors on Cho neurons. The differential positioning of Cho organs between thorax and abdomen may be determined by Hox gene-mediated repression of robo2.

Combinatorial RNAi: a method for evaluating the functions of gene families in Drosophila

Individual members of gene families often have partially redundant functions during nervous system development, making conventional genetic analysis problematic. Here we review experiments showing that several genes can be silenced together by injection of double-stranded RNAs into wild-type Drosophila embryos. By dye-labeling single neuroblasts in injected embryos, the effects of multigene silencing on individual CNS axon pathways can now be examined.

Immunolocalization of synaptotagmin for the study of synapses in the developing antennal lobe of Manduca sexta

In the mature olfactory systems of most organisms that possess a sense of smell, synapses between olfactory receptor neurons and central neurons occur in specialized neuropil structures called glomeruli. The development of olfactory glomeruli has been studied particularly heavily in the antennal lobe of the moth Manduca sexta. In the current study, we address the development of synapses within the antennal lobe of M. sexta by reporting on the localization of synaptotagmin, a ubiquitous synaptic vesicle protein, throughout development. A cDNA clone coding for M. sexta synaptotagmin was characterized and found to encode a protein that shares 67% amino acid identity with Drosophila synaptotagmin and 56% amino acid identity with human synaptotagmin I. Conservation was especially high in the C2 domains near the C-terminus and very low near the N-terminus. A polyclonal antiserum (MSYT) was raised against the unique N-terminus of M. sexta synaptotagmin, and a monoclonal antibody (DSYT) was raised against the highly conserved C-terminus of D. melanogaster synaptotagmin. In Western blot analyses, both antibodies labeled a 60 kD protein, which very likely corresponds to synaptotagmin. On sections, both antibodies labeled known synaptic neuropils in M. sexta and yielded similar labeling patterns in the developing antennal lobe. In addition, DSYT detected synaptotagmin-like protein in three other insect species examined. Analysis of synaptotagmin labeling at the light microscopic level during development of the antennal lobe of M. sexta confirmed and extended previous electron microscopic studies. Additional synapses in the coarse neuropil and a refinement of synaptic densities in the glomeruli during the last one-third of metamorphic development were revealed.

Regulation of CNS and motor axon guidance in Drosophila by the receptor tyrosine phosphatase DPTP52F

Receptor-linked protein tyrosine phosphatases (RPTPs) regulate axon guidance and synaptogenesis in Drosophila embryos and larvae. We describe DPTP52F, the sixth RPTP to be discovered in Drosophila. Our genomic analysis indicates that there are likely to be no additional RPTPs encoded in the fly genome. Five of the six Drosophila RPTPs have C. elegans counterparts, and three of the six are also orthologous to human RPTP subfamilies. DPTP52F, however, has no clear orthologs in other organisms. The DPTP52F extracellular domain contains five fibronectin type III repeats and it has a single phosphatase domain. DPTP52F is selectively expressed in the CNS of late embryos, as are DPTP10D, DLAR, DPTP69D and DPTP99A. To define developmental roles of DPTP52F, we used RNA interference (RNAi)-induced phenotypes as a guide to identify Ptp52F alleles among a collection of EMS-induced lethal mutations. Ptp52F single mutant embryos have axon guidance phenotypes that affect CNS longitudinal tracts. This phenotype is suppressed in Dlar Ptp52F double mutants, indicating that DPTP52F and DLAR interact competitively in regulating CNS axon guidance decisions. Ptp52F single mutations also cause motor axon phenotypes that selectively affect the SNa nerve. DPTP52F, DPTP10D and DPTP69D have partially redundant roles in regulation of guidance decisions made by axons within the ISN and ISNb motor nerves.

Normal T-cell response and in vivo magnetic resonance imaging of T cells loaded with HIV transactivator-peptide-derived superparamagnetic nanoparticles

The present study analyzed the feasibility of using magnetic resonance imaging (MRI) to monitor T-cell homing in vivo after loading T cells with superparamagnetic iron oxide (CLIO) nanoparticles derivatized with a peptide sequence from the transactivator protein (Tat) of HIV-1. T cells were isolated from C57BL/6 (B6) mice and loaded with 0, 400, 800, 1600, or 8000 ng/ml of FITC conjugated CLIO-Tat (FITC-CLIO-Tat). There was a dose-dependent uptake of FITC-CLIO-Tat by T cells. Stimulation of FITC-CLIO-Tat loaded T cells with anti-CD3 (0.1 microg/ml) plus IL-2 (5 ng/ml) elicited normal activation and activation-induced cell death (AICD) responses, and normal upregulation of CD69, ICAM-1 (CD54), L-selectin (CD62L), and Fas. The FITC-CLIO-Tat loaded T cells (3 x 10(7)) were transferred intravenously (i.v.) into B6 mice and the in vivo MRI of mice was acquired using a spin-echo pulse sequence at 4.7 T with a Bruker Biospec system. Homing of T cells into the spleen was observed by a decrease in MRI signal intensity within 1 h after the transfer, which remained decreased for 2-24 h after transfer. These homing data were confirmed by FACS analysis and biodistribution analysis using 125I-CLIO-Tat. Thus, T cells can be efficiently loaded with FITC-CLIO-Tat without interfering with their normal activation and AICD, or homing to the spleen, and the biodistribution of FITC-CLIO-Tat loaded T cells can be monitored in vivo over time by MRI.

Radioimaging of implants in rats using Tc-99m-MDP

Radiopharmaceutical isotopes are widely used clinically to detect tumors of osteogenic origin. One example is Technetium-99m methylene diphosphonate (Tc-99m-MDP). When viewed with a gamma camera, the concentration of the isotope (increased gamma activity) indicates an area of increased bone activity. This technology provides an opportunity to measure bone growth around implants in vivo. The purpose of this study was to measure Tc-99m-MDP activity around titanium alloy implants placed in the tibiae of rats. Some implant sites were treated with a growth factor; other sites served as controls. The hypothesis tested was that implants placed with a growth factor would have greater associated Tc-99m-MDP activity. Twelve adult male Sprague-Dawley rats were anesthetized and surgical access to the medial proximal tibiae was obtained. Titanium alloy screw implants were placed in six animals along with 65 microgram of acidic fibroblast growth factor (FGF-1); the other six animals received implants only and served as controls. After five days, rats were injected with 1500 microCi of Tc-99m-MDP. After 3 hours, rats were imaged with a gamma camera. The Tc-99m-MDP intensity associated with each implant was quantified and the means for each group were compared using ANOVA. Implants treated with FGF-1 demonstrated significantly more Tc-99m-MDP activity than implants alone. This suggests that Tc-99m-MDP analysis may be a useful tool for determining bone growth around implants in laboratory animals in vivo.

Analysis of Ti-6Al-4V implants placed with fibroblast growth factor 1 in rat tibiae

Titanium-aluminum-vanadium (Ti-6Al-4V) implants were placed in the tibiae of 32 rats (male Sprague-Dawley, 350 g) to examine healing and bone response. Half of the implants were treated with fibroblast growth factor 1 (FGF-1) delivered in an activated fibrinogen matrix. Animals were injected with a radiopharmaceutical imaging agent, technetium-99m-methylene diphosphonate (Tc-99m-MDP), which concentrates in bone, especially in areas of higher osteoblastic activity. Binding of Tc-99m-MDP to the implant was detected in vivo by Anger gamma camera imaging. Fourteen days after implant surgery, specimens were recovered and prepared for histomorphometric analysis. Histologic examination revealed that samples treated with FGF-1 demonstrated significantly greater amounts of bone-to-implant contact (P < .05) compared to controls. Also, FGF-1-treated samples showed significantly greater amounts of bone (percent volume) adjacent to implants (P < .005). These findings were supported by analyses of the non-invasive Tc-99m-MDP images, which demonstrated significantly greater uptake of Tc-99m-MDP adjacent to FGF-1-treated implants (P < .05). Results of the experiments supported the hypothesis that FGF-1 could increase bone production around implants in a rat model.

A gain-of-function screen for genes controlling motor axon guidance and synaptogenesis in Drosophila

BACKGROUND

The neuromuscular system of the Drosophila larva contains a small number of identified motor neurons that make genetically defined synaptic connections with muscle fibers. We drove high-level expression of genes in these motor neurons by crossing 2293 GAL4-driven EP element lines with known insertion site sequences to lines containing a pan-neuronal GAL4 source and UAS-green fluorescent protein elements. This allowed visualization of every synapse in the neuromuscular system in live larvae.

RESULTS

We identified 114 EPs that generate axon guidance and/or synaptogenesis phenotypes in F1 EP x driver larvae. Analysis of genomic regions adjacent to these EPs defined 76 genes that exhibit neuromuscular gain-of-function phenotypes. Forty-one of these (known genes) have published mutant alleles; the other 35 (new genes) have not yet been characterized genetically. To assess the roles of the known genes, we surveyed published data on their phenotypes and expression patterns. We also examined loss-of-function mutants ourselves, identifying new guidance and synaptogenesis phenotypes for eight genes. At least three quarters of the known genes are important for nervous system development and/or function in wild-type flies.

CONCLUSIONS

Known genes, new genes, and a set of previously analyzed genes with phenotypes in the Adh region display similar patterns of homology to sequences in other species and have equivalent EST representations. We infer from these results that most new genes will also have nervous system loss-of-function phenotypes. The proteins encoded by the 76 identified genes include GTPase regulators, vesicle trafficking proteins, kinases, and RNA binding proteins.

Complex genetic interactions among four receptor tyrosine phosphatases regulate axon guidance in Drosophila

Four receptor-linked protein tyrosine phosphatases are selectively expressed on central nervous system axons in the Drosophila embryo. Published data show that three of these (DLAR, DPTP69D, DPTP99A) regulate motor axon guidance decisions during embryonic development. Here we examine the role of the fourth neural phosphatase, DPTP10D, by analyzing double-, triple-, and quadruple-mutant embryos lacking all possible combinations of the phosphatases. This analysis shows that all four phosphatases participate in guidance of interneuronal axons within the longitudinal tracts of the central nervous system. In the neuromuscular system, DPTP10D works together with the other three phosphatases to facilitate outgrowth and bifurcation of the SNa nerve, but acts in opposition to the others in regulating extension of ISN motor axons past intermediate targets. Our results provide evidence for three kinds of genetic interactions among the neural tyrosine phosphatases: partial redundancy, competition, and collaboration.

Receptor tyrosine phosphatases regulate axon guidance across the midline of the Drosophila embryo

Neural receptor-linked protein tyrosine phosphatases (RPTPs) are required for guidance of motoneuron and photoreceptor growth cones in Drosophila. These phosphatases have not been implicated in growth cone responses to specific guidance cues, however, so it is unknown which aspects of axonal pathfinding are controlled by their activities. Three RPTPs, known as DLAR, DPTP69D, and DPTP99A, have been genetically characterized thus far. Here we report the isolation of mutations in the fourth neural RPTP, DPTP10D. The analysis of double mutant phenotypes shows that DPTP10D and DPTP69D are necessary for repulsion of growth cones from the midline of the embryonic central nervous system. Repulsion is thought to be triggered by binding of the secreted protein Slit, which is expressed by midline glia, to Roundabout (Robo) receptors on growth cones. Robo repulsion is downregulated by the Commissureless (Comm) protein, allowing axons to cross the midline. Here we show that the Rptp mutations genetically interact with robo, slit and comm. The nature of these interactions suggests that DPTP10D and DPTP69D are positive regulators of Slit/Roundabout repulsive signaling. We also show that elimination of all four neural RPTPs converts most noncrossing longitudinal pathways into commissures that cross the midline, indicating that tyrosine phosphorylation controls the manner in which growth cones respond to midline signals.

The Drosophila SH2-SH3 adapter protein Dock is expressed in embryonic axons and facilitates synapse formation by the RP3 motoneuron

The Dock SH2-SH3 domain adapter protein, a homolog of the mammalian Nck oncoprotein, is required for axon guidance and target recognition by photoreceptor axons in Drosophila larvae. Here we show that Dock is widely expressed in neurons and at muscle attachment sites in the embryo, and that this expression pattern has both maternal and zygotic components. In motoneurons, Dock is concentrated in growth cones. Loss of zygotic dock function causes a selective delay in synapse formation by the RP3 motoneuron at the cleft between muscles 7 and 6. These muscles often completely lack innervation in late stage 16 dock mutant embryos. RP3 does form a synapse later in development, however, because muscles 7 and 6 are normally innervated in third-instar mutant larvae. The absence of zygotically expressed Dock also results in subtle defects in a longitudinal axon pathway in the embryonic central nervous system. Concomitant loss of both maternally and zygotically derived Dock dramatically enhances these central nervous system defects, but does not increase the delay in RP3 synaptogenesis. These results indicate that Dock facilitates synapse formation by the RP3 motoneuron and is also required for guidance of some interneuronal axons The involvement of Dock in the conversion of the RP3 growth cone into a presynaptic terminal may reflect a role for Dock-mediated signaling in remodeling of the growth cone's cytoskeleton.

Retinal axon target selection in Drosophila is regulated by a receptor protein tyrosine phosphatase

Different Drosophila photoreceptors (R cells) connect to neurons in different optic lobe layers. R1-R6 axons project to the lamina; R7 and R8 axons project to separate layers of the medulla. We show a receptor tyrosine phosphatase, PTP69D, is required for lamina target specificity. In Ptp69D mutants, R1-R6 project through the lamina, terminating in the medulla. Genetic mosaics, transgene rescue, and immunolocalization indicate PTP69D functions in R1-R6 growth cones. PTP69D overexpression in R7 and R8 does not respecify their connections, suggesting PTP69D acts in combination with other factors to determine target specificity. Structure-function analysis indicates the extracellular fibronectin type III domains and intracellular phosphatase activity are required for targeting. We propose PTP69D promotes R1-R6 targeting in response to extracellular signals by dephosphorylating substrate(s) in R1-R6 growth cones.

Profilin and the Abl tyrosine kinase are required for motor axon outgrowth in the Drosophila embryo

The ability of neuronal growth cones to be guided by extracellular cues requires intimate communication between signal transduction systems and the dynamic actin-based cytoskeleton at the leading edge. Profilin, a small, actin-binding protein, has been proposed to be a regulator of the cell motility machinery at leading edge membranes. However, its requirement in the developing nervous system has been unknown. Profilin associates with members of the Enabled family of proteins, suggesting that Profilin might link Abl function to the cytoskeleton. Here, genetic analysis in Drosophila is used to demonstrate that mutations in Profilin (chickadee) and Abl (abl) display an identical growth cone arrest phenotype for axons of intersegmental nerve b (ISNb). Moreover, the phenotype of a double mutant suggests that these components function together to control axonal outgrowth.

Tyrosine kinase inhibition produces specific alterations in axon guidance in the grasshopper embryo

Tyrosine kinase signaling pathways are essential for process outgrowth and guidance during nervous system development. We have examined the roles of tyrosine kinase activity in programming growth cone guidance decisions in an intact nervous system in which neurons can be individually identified. We applied the tyrosine kinase inhibitors herbimycin A and genistein to whole 40% grasshopper embryos placed in medium, or injected the inhibitors into intact grasshopper eggs. Both inhibitors caused interneuronal axons that normally would grow along the longitudinal connectives to instead leave the central nervous system (CNS) within the segmental nerve root and grow out toward the body wall muscles. In addition, herbimycin A produced pathfinding errors in which many longitudinal axons crossed the CNS midline. To study how this drug affected guidance decisions made by individual growth cones, we dye-filled the pCC interneuron, which normally extends an axon anteriorly along the ipsilateral longitudinal connective. In the presence of herbimycin A, the pCC growth cone was redirected across the anterior commissure. These phenotypes suggest that tyrosine kinase inhibition blocks a signaling mechanism that repels the growth cones of longitudinal connective neurons and prevents them from crossing the midline.

Receptor tyrosine phosphatases: the worm clears the picture

Recent work on the Caenorhabditis elegans clr-1 gene shows that the receptor tyrosine phosphatase that it encodes negatively regulates a receptor tyrosine kinase related to mammalian fibroblast growth factor receptors. This opens up a promising system for investigating receptor tyrosine phosphatase function.

Kinesin light chains are essential for axonal transport in Drosophila

Kinesin is a heterotetramer composed of two 115-kD heavy chains and two 58-kD light chains. The microtubule motor activity of kinesin is performed by the heavy chains, but the functions of the light chains are poorly understood. Mutations were generated in the Drosophila gene Kinesin light chain (Klc), and the phenotypic consequences of loss of Klc function were analyzed at the behavioral and cellular levels. Loss of Klc function results in progressive lethargy, crawling defects, and paralysis followed by death at the end of the second larval instar. Klc mutant axons contain large aggregates of membranous organelles in segmental nerve axons. These aggregates, or organelle jams (Hurd, D.D., and W.M. Saxton. 1996. Genetics. 144: 1075-1085), contain synaptic vesicle precursors as well as organelles that may be transported by kinesin, kinesin-like protein 68D, and cytoplasmic dynein, thus providing evidence that the loss of Klc function blocks multiple pathways of axonal transport. The similarity of the Klc and Khc (. Cell 64:1093-1102; Hurd, D.D., and W.M. Saxton. 1996. Genetics 144: 1075-1085) mutant phenotypes indicates that KLC is essential for kinesin function, perhaps by tethering KHC to intracellular cargos or by activating the kinesin motor.

Transmembrane glycoprotein gp150 is a substrate for receptor tyrosine phosphatase DPTP10D in Drosophila cells

We have begun to explore the downstream signaling pathways of receptor protein tyrosine phosphatases (RPTPs) that control axon guidance decisions in the Drosophila central nervous system. We have focused our studies on the adhesion molecule-like gp150 protein, which binds directly to and is an in vitro substrate for the RPTP DPTP10D. Here we show that gp150 and DPTP10D form stable complexes in Drosophila Schneider 2 (S2) cells and in wild-type larval tissue. We also demonstrate that the DPTP10D cytoplasmic domain is sufficient to confer binding to gp150. gp150 has a short cytoplasmic domain containing four tyrosines, all found within sequences similar to immunoreceptor family tyrosine-based activation motifs (ITAMs). We demonstrate that gp150 is tyrosine phosphorylated in wild-type larvae. In S2 cells, gp150 becomes tyrosine phosphorylated following incubation with PTP inhibitors or upon coexpression of the Dsrc tyrosine kinase. Phosphorylated Dsrc and an unknown 40-kDa phosphoprotein form stable complexes with gp150, thereby implicating them in a putative gp150 signaling pathway. When coexpressed with gp150, either full-length DPTP10D or its cytoplasmic domain mediates gp150 dephosphorylation whereas a catalytically inactive DPTP10D cytoplasmic domain does not. The neural RPTP DPTP99A can also induce gp150 dephosphorylation but does not coimmunoprecipitate with gp150. Taken together, the results suggest that gp150 transduces signals via phosphorylation of its ITAM-like elements. Phosphotyrosines on gp150 might function as binding sites for downstream signaling molecules, thereby initiating a signaling cascade that could be modulated in vivo by RPTPs such as DPTP10D.

The Caenorhabditis elegans seven-transmembrane protein ODR-10 functions as an odorant receptor in mammalian cells

The nematode Caenorhabditis elegans exhibits behavioral responses to many volatile odorants. Chemotaxis toward one such odorant, diacetyl (butanedione), requires the function of a seven-transmembrane receptor protein encoded by the odr-10 gene. To determine directly whether ODR-10 protein is an odorant receptor, it is necessary to express the protein in a heterologous system and show that it responds to diacetyl by activation of a G protein signaling pathway. Here we demonstrate that human cells expressing ODR-10 on their surfaces exhibit a transient elevation in intracellular Ca2+ levels after diacetyl application. Volatile compounds that differ from diacetyl only by the addition of a methyl group (2,3-pentanedione) or the absence of a keto group (butanone) are not ODR-10 agonists. Behavioral responses to these compounds are not dependent on odr-10 function, so ODR-10 specificity in human cells resembles in vivo specificity. The apparent affinity of ODR-10 for diacetyl observed in human cells is consistent with the diacetyl concentration ranges that allow efficient nematode chemotaxis. ODR-10 expressed in human cells also responds to two anionic compounds, pyruvate and citrate, which are metabolic precursors used for diacetyl production by certain bacterial species. Ca2+ elevation in response to ODR-10 activation is due to release from intracellular stores.

Regulated neurite tension as a mechanism for determination of neuronal arbor geometries in vivo

Transection and displacement experiments on isolated neurons in culture have shown that their neurites are under tension. Such tensile forces might be important in determining the structures of neuronal arbors in vivo. It has also been proposed that tension mechanisms generate the global folding patterns of the brain. It has been difficult to determine whether tension is important in vivo, however, because most neuronal arbors have complex three-dimensional structures that cannot be perturbed in a controlled manner. Here we describe a situation in which tension can be demonstrated and perturbed in an intact central nervous system (CNS). In the embryonic CNS neuropil of the grasshopper Schistocerca americana, the axon of a local serotonergic interneuron known as s1 forms a characteristic bifurcation. The geometry of this bifurcation node is highly conserved between embryos and held constant during development. Current models for the development of such geometries usually propose that they are created and maintained by neurite adhesion to localized substrates. Here we show that the structure of the s1 bifurcation node is likely to be determined by balanced tension between three fixed points. This was revealed by selectively transecting each of the branches that intersect at the node. Transections are followed by a rapid restructuring ('snapping') of the node geometry.

Competition and cooperation among receptor tyrosine phosphatases control motoneuron growth cone guidance in Drosophila

The neural receptor tyrosine phosphatases DPTP69D, DPTP99A and DLAR are involved in motor axon guidance in the Drosophila embryo. Here we analyze the requirements for these three phosphatases in growth cone guidance decisions along the ISN and SNb motor pathways. Any one of the three suffices for the progression of ISN pioneer growth cones beyond their first intermediate target in the dorsal muscle field. DLAR or DPTP69D can facilitate outgrowth beyond a second intermediate target, and DLAR is uniquely required for formation of a normal terminal arbor. A different pattern of partial redundancy among the three phosphatases is observed for the SNb pathway. Any one of the three suffices to allow SNb axons to leave the common ISN pathway at the exit junction. When DLAR is not expressed, however, SNb axons sometimes bypass their ventrolateral muscle targets after leaving the common pathway, instead growing out as a separate bundle adjacent to the ISN. This abnormal guidance decision can be completely suppressed by also removing DPTP99A, suggesting that DLAR turns off or counteracts a DPTP99A signal that favors the bypass axon trajectory. Our results show that the relationships among the tyrosine phosphatases are complex and dependent on cellular context. At growth cone choice points along one nerve, two phosphatases cooperate, while along another nerve these same phosphatases can act in opposition to one another.

Functional expression of the heteromeric "olfactory" cyclic nucleotide-gated channel in the hippocampus: a potential effector of synaptic plasticity in brain neurons

Cyclic nucleotide-gated (cng) channels are important components of signaling systems mediating sensory transduction. In vertebrate photoreceptors, light activates a signaling cascade that causes a decrease in intracellular cGMP concentrations, closing retinal cng channels. Signal transduction in olfactory receptor neurons is believed to proceed via G-protein-mediated elevation of intracellular cAMP in response to odorant binding by 7-helix receptors. cAMP opens the olfactory cng channel, which is highly permeable to Ca2+. Here we demonstrate by in situ hybridization and immunohistochemistry with subunit-specific antibodies that both subunits of the heteromeric rat olfactory cng channel are also widely expressed in the brain. Expression of the retinal rod cng channel, however, can be detected only in the eye. In the adult hippocampus, the olfactory cng channel is expressed on cell bodies and processes of CA1 and CA3 neurons. In cultured embryonic hippocampal neurons, the channel is localized to a subset of growth cones and processes. We recorded conductances with the electrophysiological characteristics of the heteromeric olfactory cng channel in excised inside-out patches from these cultured neurons. We also show that Ca2+ influx into hippocampal neurons in response to cyclic nucleotide elevation can be detected using fura-2 imaging. Cyclic nucleotide elevation has been implicated in several mechanisms of synaptic plasticity in the hippocampus, and these mechanisms also require elevation of intracellular Ca2+. Our results suggest that the "olfactory" cng channel could regulate synaptic efficacy in brain neurons by modulating Ca2+ levels in response to changes in cyclic nucleotide concentrations.

Tyrosine phosphorylation and axon guidance: of mice and flies

Recent genetic evidence suggests that tyrosine kinases and tyrosine phosphatases can control the guidance of specific growth cones. Within a family of related phosphatases or kinases, individual members can have partially redundant functions. Receptor phosphatases can work together at one guidance choice point, but in opposition at another. The specific combination of kinases and phosphatases active in a growth cone may be an important determinant of pathway choice. One mechanism by which these proteins could control guidance decisions is through regulation of adhesion between growth cones and axons.

Receptor tyrosine phosphatases are required for motor axon guidance in the Drosophila embryo

The receptor tyrosine phosphatases DPTP69D and DPTP99A are expressed on motor axons in Drosophila embryos. In mutant embryos lacking DPTP69D protein, motor neuron growth cones stop growing before reaching their muscle targets, or follow incorrect pathways that bypass these muscles. Mutant embryos lacking DPTP99A are indistinguishable from wild type. Motor axon defects in dptp69D dptp99A double mutant embryos, however, are much more severe than in embryos lacking only DPTP69D. Our results demonstrate that DPTP69D and DPTP99A are required for motor axon guidance and that they have partially redundant functions during development of the neuro-muscular system.

Tenascin-C mRNA is expressed in cranial neural crest cells, in some placodal derivatives, and in discrete domains of the embryonic zebrafish brain

A partial zebrafish tenascin-C cDNA clone was isolated from an embryonic zebrafish cDNA library on the basis of homology to mouse tenascin-C. The expression pattern in the head of embryonic zebrafish was analyzed by in situ hybridization. Tenascin-C mRNA was detected in neural crest cells during the period of their migration and differentiation. Expression also occurred in differentiating placodal tissues and in mesodermal cells. In the developing brain, tenascin-C mRNA was expressed in specific domains. In the hindbrain the pattern of the domains was dynamic. At 18 to 22 h postfertilization, expression was widespread in rhombomeres 3, 5, and 6, confined to periventricular cells in rhombomere 2, and not detectable in rhombomere 4. At 32 h postfertilization, tenascin-C was expressed at the rhombomere boundaries. In contrast to the hindbrain, the pattern in the forebrain and midbrain did not show any major changes between 22 and 32 h postfertilization. Domains expressing tenascin-C alternated with regions devoid of it. The most anterior domain of expression was observed at the telencephalic-diencephalic border, surrounding the optic recess. A second domain, at the border between the diencephalon and the midbrain, and a third domain, in the caudal midbrain tegmentum, appeared restricted to the basal plate. Additionally, expression of tenascin-C mRNA was detected in the hypothalamus and in the developing epiphysis. These expression patterns suggest that tenascin-C may play a role in neural crest cell migration and during the differentiation of neural crest, placodal, and mesodermal derivatives. In the developing brain, tenascin-C may be involved in the consolidation of different regional identities.

Use of 82Br- radiotracer to study transmembrane halide flux: the effect of a tranquilizing drug, chlordiazepoxide on channel opening of a GABAA receptor

We used the short-lived radionuclide, 82Br- to follow gamma-aminobutyrate (GABA) receptor-mediated halide exchange into membrane vesicles from rat cerebral cortex in millisecond and second time regions using quench-flow technique. The radioisotope was prepared by neutron capture [81Br-(n,gamma)82Br-] on irradiation of a natural isotope of bromine, 81Br- in a neutron flux. 82Br- decays by beta-emission with secondary gamma-emission. Possible advantages of 82Br- over 36Cl- in anion tracer measurements include, (a) a short lifetime (t1/2 = 35.3 hr), which alleviates contamination and disposal problems, (b) high counting efficiency (1.54) due to the secondary radiation, (c) measurement with a gamma-counter as well as a beta-counter, (d) a simple preparation not requiring subsequent purification steps giving a specific activity depending on the irradiation time. With 6 hr irradiation time the specific activity was sufficient to make measurements with < 1 mM Br-, which is less than the bromide concentration known to affect the properties of GABAA receptor. The radiotracers, 82Br- and 36Cl- could be compared with the same solution composition. In conditions where a direct effect of binding of halide to receptor does not contribute to a difference in measured ion-flux, 82Br- was translocated only marginally faster than 36Cl-. The effect of chlordiazepoxide (CDPX) (2-250 microM) on the progress of GABA (10 microM)-mediated 82Br- uptake was measured in a time range of 200 msec to 20 sec using quench-flow technique. The two phases of anion exchange previously reported in this experimental model with GABA alone were observed. The rate of 82Br- exchange was increased 2.3-fold at 30-60 microM CDPX and was not further increased with increasing [CDPX]. The rate of halide exchange is a measure of open channel concentration. The isotope exchange rate constant, J, in a membrane vesicle preparation, is a measure of the membrane permeability per internal volume/surface area, J = PmA/V. Receptor desensitization rate was also increased by CDPX, but unlike the isotope exchange rate, it continued to increase up to at least 250 microM CDPX.

Activation of cAMP-dependent protein kinase triggers a glial-to-neuronal cell-fate switch in an insect neuroblast lineage

BACKGROUND

The grasshopper median neuroblast (MNB) is a multipotent progenitor cell that produces neurons and midline glia in distinct temporal phases. The MNB generates pioneer neurons during its first few divisions, and then switches to production of midline glial precursors. After the glia have been produced, the MNB reverts to generating neurons. We have investigated the molecular mechanism underlying the transition from glia production back to neuron production in the MNB lineage.

RESULTS

We report evidence that this second transition in the MNB lineage is triggered by the activation of cAMP-dependent protein kinase (PKA). PKA is a heterodimer of a catalytic (PKA-C) and a cAMP-binding regulatory (R) subunit. The R subunit dissociates from PKA-C on binding cAMP, and free PKA-C than translocates into the nucleus. Nuclear localization of PKA-C can thus be used as an indicator of PKA activation within a cell. We have found that PKA-C is translocated into the nucleus at the time of the second switch in the MNB lineage. When PKA is prematurely activated in the MNB by microinjection of purified PKA-C, or by pharmacological agents that elevate intracellular cAMP levels, the glial-to-neuronal cell-fate switch takes place prematurely. Inhibition of PKA activity by microinjection of a peptide inhibitor, or by a non-hydrolyzable cAMP analog, blocks the glial-to-neuronal switch.

CONCLUSIONS

Our results imply that elevation of cAMP in the MNB, and the resultant activation of PKA, is likely to be a trigger for the glial-to-neuronal cell-fate transition within the MNB lineage.

Cell fate decisions in the grasshopper central nervous system

The central nervous system (CNS) of grasshopper embryos is similar in organization to the embryonic Drosophila CNS, but its neurons are much larger. The recent development of a culture system in which extensive CNS development occurs has allowed new types of experiments to be performed, including perturbation of gene expression within single neuroblast lineages.

A Drosophila receptor tyrosine phosphatase expressed in the embryonic CNS and larval optic lobes is a member of the set of proteins bearing the "HRP" carbohydrate epitope

Recent studies have defined several cell surface glycoproteins expressed in the developing nervous system of insect embryos that may be involved in axon outgrowth and guidance processes. These glycoproteins include the fasciclins and a group of receptor-linked protein tyrosine phosphatases (R-PTPs). In embryos, the fasciclins are localized to axonal subsets, while the R-PTPs appear to be expressed on most or all CNS axons. To identify other neuronal cell surface glycoproteins in the Drosophila embryo, we have taken a biochemical approach. This is based on the observation that antisera against horseradish peroxidase (HRP) recognize a carbohydrate epitope that is selectively expressed in the insect nervous system. A large number of neuronal glycoproteins (denoted "HRP proteins") apparently bear the HRP carbohydrate epitope. We have used polyclonal anti-HRP antibodies to purify these proteins from Drosophila embryos, and have obtained protein sequences from seven HRP protein bands. These data define three major HRP proteins as neurotactin, fasciclin I, and an R-PTP, DPTP69D. Western blotting data suggest that fasciclin II, neuroglian, DPTP10D, and DPTP99A are also HRP proteins. We show that DPTP69D, like the previously characterized R-PTPs, is localized to CNS axons in the embryo. In third instar larvae, DPTP69D expression is restricted to subsets of neuronal processes in the brain, ventral nerve cord, and eye disk. In the optic lobes, DPTP69D is localized to the neuropils of the lamina and medulla, and to an array of parallel thick bundles that may be the transmedullary fibers of the developing lobula complex.

An adhesion molecule-like protein that interacts with and is a substrate for a Drosophila receptor-linked protein tyrosine phosphatase

Receptor-linked protein tyrosine phosphatases (R-PTPs) are a large and diverse group of transmembrane signaling molecules. In Drosophila, four R-PTPs are localized to central nervous system axons in the embryo and may participate in assembly of the central nervous system axon array. In this paper, we identify and characterize a transmembrane glycoprotein, gp150, that selectively interacts with the catalytic domain of the axonal R-PTP DPTP10D. gp150 does not bind to a cysteine-to-serine active site mutant, and binding is inhibited by vanadate, suggesting that it interacts with the active site. It has an extracellular domain composed of 18 leucine-rich repeats, which are found in many adhesion molecules. Its short cytoplasmic domain contains 4 tyrosine residues in sequence contexts that suggest that they could interact with SH2 domain-containing effector molecules. The overall organization of the tyrosine motifs resembles that of antigen recognition activation motif signaling elements from receptors in the vertebrate immune system. The cytoplasmic domain of gp150 is a good substrate for v-Abl tyrosine kinase, and phosphorylated gp150 can be dephosphorylated efficiently in vitro by DPTP10D. We suggest that DPTP10D may function in vivo to regulate phosphorylation of gp150 and thereby control its interactions with downstream effectors.

The grasshopper median neuroblast is a multipotent progenitor cell that generates glia and neurons in distinct temporal phases

The midline of the insect CNS provides a good system for studying the control of neuronal and glial cell fates, because it contains a small number of distinct cell types that arise from a unique set of precursors. In this report we analyze the development of the CNS midline in the grasshopper embryo, focusing on the median neuroblast (MNB) cluster, which contains the majority of midline neurons. We have directly traced cell lineage in the MNB cluster by injecting the MNB with tracer dyes and following the development of its progeny in whole embryo cultures. As least three types of neuronal progeny are labeled with tracer dye in these experiments. In addition, we find that all of the midline glia are also derived from the MNB. Thus, this neuroblast is actually a multipotent progenitor cell. We also examined the temporal and spatial patterns of midline development by staining embryos with antibodies to neuronal and glial markers and to the protein product of the engrailed gene, which is transiently expressed by all MNB progeny. Our data show that neuronal and glial progeny are generated from the MNB in distinct temporal phases. A change in the orientation of the MNB's mitotic spindle correlates with the transition between two of the phases of progeny production.

Heteromeric olfactory cyclic nucleotide-gated channels: a subunit that confers increased sensitivity to cAMP

Olfactory receptor neurons respond to odorant stimulation with a rapid increase in intracellular cAMP that opens cyclic nucleotide-gated (cng) cation channels. cng channels in rat olfactory neurons are activated by cAMP in the low micromolar range and are outwardly rectifying. The cloned rat olfactory cng channel (rOCNC1), however, is much less sensitive to cAMP and exhibits very weak rectification. Here we describe the cloning and characterization of a second rat cng channel subunit, denoted rOCNC2. rOCNC2 does not form functional channels when expressed alone. When rOCNC1 and rOCNC2 are coexpressed, however, an outwardly rectifying cation conductance with cAMP sensitivity near that of the native channel is observed. In situ hybridization with probes specific for the two subunits shows that they are coexpressed in olfactory receptor neurons. These data indicate that the native olfactory cng channel is likely to be a heterooligomer of the rOCNC1 and rOCNC2 subunits.

Engrailed controls glial/neuronal cell fate decisions at the midline of the central nervous system

The molecular mechanisms involved in glial/neuronal fate decisions during embryonic development are largely unknown. Here we show that the segment-polarity gene engrailed, which encodes a homeodomain protein, controls these decisions within an insect CNS lineage. The grasshopper median neuroblast (MNB) generates both neurons and midline glia in distinct temporal phases. engrailed expression in MNB progeny can be inhibited by injection of antisense oligodeoxynucleotides into the MNB nucleus. This produces a phenotype in which the midline glia do not develop and extra midline neurons are generated. In the absence of engrailed function, midline glial precursors are apparently converted into neuronal precursors. Thus, engrailed is required for execution of the decision between the glial and neuronal fates.

Pair-rule expression patterns of even-skipped are found in both short- and long-germ beetles

Now that the genes controlling embryonic patterning have been identified in several model organisms, long-standing questions concerning the evolution of developmental systems are open to investigation. Examination of the expression of even-skipped in a variety of insects reveals that Insect germ-type designations apparently do not reflect the variations in the mechanisms of segmentation evident throughout insect phylogeny.

Drosophila protein tyrosine phosphatases

Seven protein tyrosine phosphatase (PTPase) genes have been identified in the fruit-fly Drosophila melanogaster. Four of these genes encode receptor-linked PTPases (R-PTPs) that are expressed on central nervous system axons in the embryo. Each axonal R-PTP has an extracellular domain that is homologous to vertebrate adhesion molecules and to identified mammalian R-PTPs. Two non-receptor PTPase genes have been isolated to date. One of these, corkscrew (csw), encodes an SH2 domain-containing PTPase that appears to be a homolog of mammalian PTP1D. Genetic evidence indicates that the csw PTPase is involved in the transduction of signals from receptor tyrosine kinases to their down-stream targets, which include Ras proteins.

Expression and structural studies of fasciclin I, an insect cell adhesion molecule

Fasciclin I is a lipid-linked cell-surface glycoprotein that can act as a homophilic adhesion molecule in tissue culture cells. It is thought to be involved in growth cone guidance in the embryonic insect nervous system. To facilitate structure-function studies, we have generated Chinese hamster ovary (CHO) cell lines expressing high levels of cell surface grasshopper and Drosophila fasciclin I. Grasshopper fasciclin I released by phospholipase C cleavage was purified on an immunoaffinity column and single crystals were obtained that diffracted to approximately 5-A resolution. We also generated CHO and Drosophila S2 cell lines that produce a secreted form of fasciclin I. Fasciclin I expressed in S2 cells contains significantly less carbohydrate than the protein expressed in CHO cells, and may therefore be more suitable for crystallization. Biochemical characterization of purified fasciclin I indicates that the extracellular portion exists as a monomer in solution. Circular dichroism studies suggest that fasciclin I is primarily alpha-helical. Its structure is therefore different from other known cell adhesion molecules, which are predicted to be elongated beta-sheet structures. This suggests that fasciclin I may define a new structural motif used to mediate adhesive interactions between cell surfaces.

Dynamic expression of the cell adhesion molecule fasciclin I during embryonic development in Drosophila

A number of different cell surface glycoproteins expressed in the central nervous system (CNS) have been identified in insects and shown to mediate cell adhesion in tissue culture systems. The fasciclin I protein is expressed on a subset of CNS axon pathways in both grasshopper and Drosophila. It consists of four homologous 150-amino acid domains which are unrelated to other sequences in the current databases, and is tethered to the cell surface by a glycosyl-phosphatidylinositol linkage. In this paper we examine in detail the expression of fasciclin I mRNA and protein during Drosophila embryonic development. We find that fasciclin I is expressed in several distinct patterns at different stages of development. In blastoderm embryos it is briefly localized in a graded pattern. During the germ band extended period its expression evolves through two distinct phases. Fasciclin I mRNA and protein are initially localized in a 14-stripe pattern which corresponds to segmentally repeated patches of neuroepithelial cells and neuroblasts. Expression then becomes confined to CNS and peripheral sensory (PNS) neurons. Fasciclin I is expressed on all PNS neurons, and this expression is stably maintained for several hours. In the CNS, fasciclin I is initially expressed on all commissural axons, but then becomes restricted to specific axon bundles. The early commissural expression pattern is not observed in grasshopper embryos, but the later bundle-specific pattern is very similar to that seen in grasshopper. The existence of an initial phase of expression on all commissural bundles helps to explain the loss-of-commissures phenotype of embryos lacking expression of both fasciclin I and of the D-abl tyrosine kinase. Fasciclin I is also expressed in several nonneural tissues in the embryo.

Alternative splicing of micro-exons creates multiple forms of the insect cell adhesion molecule fasciclin I

Fasciclin I is a homophilic cell adhesion molecule in insects that is dynamically expressed on a subset of axon pathways in the embryonic nervous system, and on a variety of other cells and tissues during development. The fasciclin I protein consists of four homologous 150 amino acid domains. In this article, we describe the complete sequence of the Drosophila fasciclin I (fasI) gene. The gene consists of 15 exons and is distributed over 14 kilobases of DNA. We examine the structure and temporal expression pattern of multiple fasciclin I mRNAs that differ in the lengths of their 3' untranslated regions. We also show that a highly conserved sequence at the end of the second domain can be altered by the addition of three or six amino acids that are encoded by two alternatively spliced 9 base pair (bp) micro-exons. In grasshopper fasciclin I mRNAs, there are 9 bp and 6 bp insertions at the same position. The first of these insertions is identical in sequence to the first fly micro-exon. The grasshopper insertions are not found together in the same mRNA, so grasshopper fasciclin I species differ by the addition of three or two extra amino acids to the second domain. The alternatively spliced mRNAs are differentially expressed during embryogenesis, and all three of them are present in nerve cord preparations. We suggest that the amino acids inserted by alternative micro-exon splicing may alter the binding specificity of fasciclin I.

Three receptor-linked protein-tyrosine phosphatases are selectively expressed on central nervous system axons in the Drosophila embryo

We describe the isolation of seven different protein-tyrosine phosphatase (PTPase) cDNAs from Drosophila embryos, three of which are primarily expressed in the central nervous system (CNS). The CNS-specific PTPases include the previously sequenced DLAR, as well as two novel PTPases (denoted DPTP10D and DPTP99A), which have extracellular domains consisting of multiple fibronectin type III repeats. Each of the Drosophila sequences is most closely related to a different human PTPase. The three PTPase mRNAs are expressed in different patterns of cells in the ventral nerve cord, and all three proteins are restricted to axons. DLAR and DPTP99A are apparently expressed on most or all axons, while DPTP10D is primarily localized to the anterior commissure and its junctions with the longitudinal tracts.

Characterization and cloning of fasciclin I and fasciclin II glycoproteins in the grasshopper

Monoclonal antibodies were previously used to identify two glycoproteins, called fasciclin I and II (70 and 95 kDa, respectively), which are expressed on different subsets of axon fascicles in the grasshopper (Schistocerca americana) embryo. Here the monoclonal antibodies were used to purify these two membrane-associated glycoproteins for further characterization. Fasciclin II appears to be an integral membrane protein, whereas fasciclin I is an extrinsic membrane protein. The amino acid sequences of the amino terminus and fragments of both proteins were determined. Using synthetic oligonucleotide probes and antibody screening, we isolated genomic and cDNA clones. Partial DNA sequences of these clones indicate that they encode fasciclins I and II.

Sequence analysis and neuronal expression of fasciclin I in grasshopper and Drosophila

The fasciclin I, II, and III glycoproteins are expressed on different subsets of axon bundles (fascicles) in insect embryos and are thus candidates for surface recognition molecules involved in growth cone guidance. Here we present the sequence of grasshopper fasciclin I and the identification and sequence of the Drosophila fasciclin I homolog. In both species, fasciclin I appears to be an extrinsic membrane protein with a signal sequence but no transmembrane region; the protein comprises four homologous domains of approximately 150 amino acids each. Antibodies against Drosophila fasciclin I reveal that it is expressed on the surface of a subset of commissural axon pathways in the embryonic central nervous system and on all sensory axon pathways in the peripheral nervous system. This pattern of expression is similar to that in grasshopper.

2-Aminopurine selectively inhibits the induction of beta-interferon, c-fos, and c-myc gene expression

The protein kinase inhibitor 2-aminopurine (2AP) blocks the induction of the human beta-interferon gene by virus or poly(I)-poly(C) at the level of transcription. This inhibition is specific, since 2AP does not inhibit induction of either the hsp70 heat-shock gene by high temperature or the metallothionein gene by cadmium or dexamethasone. However, 2AP does block the induction of the c-fos and c-myc proto-oncogenes by serum growth factors or virus, suggesting that a protein kinase may be involved in the regulation of these genes, as well as of the beta-interferon gene. However, different factors must be required for the induction of these three genes, since they are not coordinately regulated by the same inducers in most of the cell lines examined.