Inhibition of medulloblastoma cell invasion by Slit

Invasion of brain tumor cells has made primary malignant brain neoplasms among the most recalcitrant to therapeutic strategies. We tested whether the secreted protein Slit2, which guides the projection of axons and developing neurons, could modulate brain tumor cell invasion. Slit2 inhibited the invasion of medulloblastoma cells in a variety of in vitro models. The effect of Slit2 was inhibited by the Robo ectodomain. Time-lapse videomicroscopy indicated that Slit2 reduced medulloblastoma invasion rate without affecting cell direction or proliferation. Both medulloblastoma and glioma tumors express Robo1 and Slit2, but only medulloblastoma invasion is inhibited by recombinant Slit2 protein. Downregulation of activated Cdc42 may contribute to this differential response. Our findings reinforce the concept that neurodevelopmental cues such as Slit2 may provide insights into brain tumor invasion.

Signal transduction in neuronal migration: roles of GTPase activating proteins and the small GTPase Cdc42 in the Slit-Robo pathway

The Slit protein guides neuronal and leukocyte migration through the transmembrane receptor Roundabout (Robo). We report here that the intracellular domain of Robo interacts with a novel family of Rho GTPase activating proteins (GAPs). Two of the Slit-Robo GAPs (srGAPs) are expressed in regions responsive to Slit. Slit increased srGAP1-Robo1 interaction and inactivated Cdc42. A dominant negative srGAP1 blocked Slit inactivation of Cdc42 and Slit repulsion of migratory cells from the anterior subventricular zone (SVZa) of the forebrain. A constitutively active Cdc42 blocked the repulsive effect of Slit. These results have demonstrated important roles for GAPs and Cdc42 in neuronal migration. We propose a signal transduction pathway from the extracellular guidance cue to intracellular actin polymerization.

Neuronal migration and the evolution of the human brain

A new study demonstrating a pathway for neuronal migration in humans, but not in monkeys, suggests that migration has a key role in the evolution of the brain, as well as its development.

Role of PI3-kinase in isoproterenol and IGF-1 induced ecNOS activity

Phosphatidylinositol 3-kinase (PI3-K) has been shown to mediate insulin and insulin-like growth factor-1 (IGF-1)-induced nitric oxide (NO) generation and, thus, vascular tone. A role for PI3-K in G-protein-coupled receptor signal transduction has been reported. As beta (beta2)-adrenergic vascular actions are partly dependent on NO, we have investigated the role of PI3-K in isoproterenol (Iso) and IGF-1 induced endothelial NO synthase (ecNOS) activity in rat aortic endothelial cells (RAEC). Cell lysates of RAEC, exposed to Iso (10 micromol/L) for 5 min and 6 h, and to IGF-1 (100 nM) for 10 min and 6 h, or pretreated with PI3-K inhibitor Wortmannin (WT), were used for measuring PI3-K activity, p85kDa regulatory protein, and citrulline production. Results show that Iso and IGF-1 increased a p85 subunit and citrulline production, and also enhanced 32P incorporation into PIP3. Pretreatment with WT inhibited Iso-stimulated ecNOS, as well as, PI3-K activity. Iso enhanced association of ecNOS with the triton X-100-insoluble fraction of RAEC. These data indicate that the endothelial cell PI3-K pathway mediates, in part, the release of NO and subsequent vasorelaxation in response to this beta-agonist, as well as, IGF-1.

The neuronal repellent Slit inhibits leukocyte chemotaxis induced by chemotactic factors

Migration is a basic feature of many cell types in a wide range of species. Since the 1800s, cell migration has been proposed to occur in the nervous and immune systems, and distinct molecular cues for mammalian neurons and leukocytes have been identified. Here we report that Slit, a secreted protein previously known for its role of repulsion in axon guidance and neuronal migration, can also inhibit leukocyte chemotaxis induced by chemotactic factors. Slit inhibition of the chemokine-induced chemotaxis can be reconstituted by the co-expression of a chemokine receptor containing seven transmembrane domains and Roundabout (Robo), a Slit receptor containing a single transmembrane domain. Thus, there is a functional interaction between single and seven transmembrane receptors. Our results reveal the activity of a neuronal guidance cue in regulating leukocyte migration and indicate that there may be a general conservation of guidance mechanisms underlying metazoan cell migration. In addition, we have uncovered an inhibitor of leukocyte chemotaxis, and propose a new therapeutic approach to treat diseases involving leukocyte migration and chemotactic factors.

Short-range guidance of olfactory bulb axons is independent of repulsive factor slit

During development, mitral cells, the major output neurons of the olfactory bulb, project their axons caudolaterally into the telencephalon and form the lateral olfactory tract (LOT). Two types of guidance cues have been suggested for this projection. First, a long-range factor Slit, which is secreted from the septum, repels mitral cell axons into a caudolateral direction. Second, the pathway of mitral cell axons contains a subset of neurons designated as lot cells, which guide the axons through short-range interactions. It is not clear how these two guidance cues relate to each other and how they share the physiological roles. Here we examined the behavior of mitral cell axons in organotypic culture on ectopic application of Slit and inhibition of endogenous Slit signaling. The results suggested that the short-range guidance cue in the LOT pathway functions independently from Slit. Furthermore, our results showed that removal of the septum and inhibition of Slit signaling did not affect the projection of mitral cell axons. Although the septum and exogenous Slit can repel olfactory bulb axons, our results cast doubts on the physiological relevance of the septum and endogenous Slit in guiding the projection of mitral cell axons.

The N-terminal leucine-rich regions in Slit are sufficient to repel olfactory bulb axons and subventricular zone neurons

The Slit proteins are a new family of secreted guidance cues involved in axon guidance and neuronal migration. Each mammalian Slit protein contains >1400 amino acid residues, with four leucine-rich regions (LRRs), nine epidermal growth factor repeats, a laminin G domain, and a C-terminal cysteine-rich domain. A receptor for Slit is the transmembrane protein Roundabout (Robo), whose extracellular part contains five Ig domains and three fibronectin type III repeats. We report here that the LRRs in Slit are sufficient for binding to the Ig domains of Robo. Mutant forms of Slit containing only the LRRs function as chemorepellents for axons projecting from the olfactory bulb both in vitro and in the telencephalon. The LRRs can repel neurons migrating from the anterior subventricular zone (SVZa) to the olfactory bulb in brain slices isolated from neonatal rodents. However, the LRRs do not show repulsive effects on the SVZa neurons migrating in collagen gels. Our results indicate that the same LRRs are sufficient for guiding both axon projection and neuronal migration and suggest that the other regions in the Slit proteins may be involved in regulating the diffusion and distribution of the Slit proteins. The fact that the same domains are involved in guiding axon projection and neuronal migration further strengthens the idea of a conserved guidance mechanism for these important processes.

Effects of intrahippocampal aniracetam treatment on Y-maze avoidance learning performance and behavioral long-term potentiation in dentate gyrus in rat

Effects of intrahippocampal treatment of aniracetam, a selective agonist for DL-alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproionic acid (AMPA) receptors, on Y-maze avoidance learning task and behavioral long-term potentiation (LTP) in perforant path-dentate gyrus were studied in freely moving rats by using in vivo electrophysiology combined with behavioral tests. The results were as follows: (1) intrahippocampal treatment of aniracetam reversibly enhanced basal synaptic transmission in perforant path to dentate gyrus in a dosage dependent manner; (2) aniracetam produced improvement in Y-maze learning performance when administration occurred 5 min prior to maze learning; (3) aniracetam administration significantly facilitated behavioral LTP in dentate gyrus, while the maximal amplitude of LTP has no significant difference when compared to saline group. The present results indicate that hippocampal AMPA receptors are involved in learning and memory.

Duplications and defects in the CYP2A6 gene: identification, genotyping, and in vivo effects on smoking

In humans, 80% of nicotine is metabolized to the inactive metabolite cotinine by the enzyme CYP2A6, which can also activate tobacco smoke procarcinogens (e.g., 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone). Previously, we demonstrated that individuals who are nicotine-dependent and have defective CYP2A6 alleles (*2, *3) smoked fewer cigarettes; however, we recognize that the genotyping method used for the CYP2A6*3 allele gave a high false-positive rate. In the current study we used improved genotyping methods to examine the effects of the defective CYP2A6*2 and CYP2A6*4 alleles on smoking behavior. We found that those with the defective alleles (N = 14) smoked fewer cigarettes per day than those homozygous (N = 277) for wild-type alleles (19 versus 28 cigarettes per day, P <.001). In addition, we identified a duplicated form of the CYP2A6 gene, corresponding to the gene deletion CYP2A6*4 allele, developed a genotyping assay, assessed the gene copy number, and examined its prevalence in Caucasian smokers (N = 296). We observed an ascending rank order for plasma cotinine and breath carbon monoxide levels (an index of smoke inhalation) in individuals with null (CYP2A6*2 and CYP2A6*4) alleles (N = 14), those homozygous for wild-type (CYP2A6*1/*1) alleles (N = 277), and those with our newly identified CYP2A6 gene duplication (N = 5). The phenotype, as determined by plasma nicotine/cotinine ratios, had a descending rank order for these three genotype groups that did not reach significance. Although further characterization is required for the duplication gene variant, these results extend our previous findings and suggest a substantial influence of CYP2A6 genotype and phenotype on smoking behavior.

[The molecular mechanism of neuronal migration]

A large amount of work since the late 1800 have shown that, during embryonic and postnatal development, the majority, if not all, neurons in mammalian nervous system have to migrate a certain distance to reach their final destination. An interesting question is how neurons are guided for their migration. We are interested in the molecular mechanisms underlying neuronal migration and our work published in 1999 indicates that there are diffusible molecules in the brain which can guide the direction of migrating neurons; specifically, a secreted protein called Slit is repulsive to neurons and its concentration gradient guides neuronal migration.

Regional and cellular distribution of CYP2D subfamily members in rat brain

1. Human CYP2D6 is present in brain, metabolizes many drugs and has been implicated in Parkinson's and Alzheimer's diseases and some cancers. It is still unclear which of the six known rat CYP2D subfamily members is/are homologous to human CYP2D6. 2. In this study, RT-PCR, Southern and Western blotting and immunohistochemical techniques were used to study the distribution of CYP2D subfamily member mRNA and proteins across 10 rat brain regions. CYP2D subfamily mRNA and protein levels were correlated with brain dextromethorphan O-demethylation (DOD), a measure of human CYP2D6 and rat CYP2D1 activities. 3. The data showed a strong relationship between CYP2D1 and CYP2D1-18 with brain DOD activity. In addition, it was shown that CYP2D proteins are present in brain mitochondrial as well as microsomal membranes. CYP2D subfamily member mRNA and proteins varied across brain regions and were highly concentrated in specific cell types. 4. These data strongly suggest that CYP2D1 and not CYP2D5 mediates DOD activity in rat brain, and may be the rat homologue of human CYP2D6. The highly localized nature of CYP2D indicates that in specific neurones enzyme levels may approach hepatic levels and, hence, contribute to local alterations in brain drug metabolism.

brakeless is required for photoreceptor growth-cone targeting in Drosophila

The R1-R6 subclass of photoreceptor neurons (R cells) in the Drosophila compound eye form specific connections with targets in the optic ganglia. In this paper, we report the identification of a gene, brakeless (bks), that is essential for R1-R6 growth cone targeting. In brakeless mutants, R1-R6 growth cones frequently fail to terminate migration in their normal target, the lamina, and instead project through it and terminate in the second optic ganglion, the medulla. Genetic mosaic analysis and transgene rescue experiments indicate that bks functions in R cells and not within the lamina target region. bks encodes a nuclear protein. We propose that it participates in a gene expression pathway regulating one or more growth cone components controlling R1-R6 targeting.

Embryonic expression and extracellular secretion of Xenopus slit

The slit genes have recently been found to encode proteins with a conserved chemorepulsive activity for axons in invertebrates and vertebrates. We have determined the expression pattern of a slit gene in Xenopus embryos. In the neural tube, slit is expressed at the ventral and dorsal midlines, and the motor neurons. slit is also expressed in a changing pattern in the retina. The full-length Xenopus Slit protein is secreted extracellularly, whereas its receptor Roundabout can not be secreted. Using a myc-tagged secreted Slit protein, we confirmed the binding of Slit to Roundabout expressed on the cell surface. These results confirm Slit-Roundabout interactions and the biochemical properties of Slit and Roundabout proteins, and further support the idea that Slit may guide axon projections in multiple regions of the embryo.

The SH2/SH3 adaptor protein dock interacts with the Ste20-like kinase misshapen in controlling growth cone motility

Recent studies suggest that the SH2/SH3 adaptor Dock/Nck transduces tyrosine phosphorylation signals to the actin cytoskeleton in regulating growth cone motility. The signaling cascade linking the action of Dock/Nck to the reorganization of cytoskeleton is poorly understood. We now demonstrate that Dock interacts with the Ste20-like kinase Misshapen (Msn) in the Drosophila photoreceptor (R cell) growth cones. Loss of msn causes a failure of growth cones to stop at the target, a phenotype similar to loss of dock, whereas overexpression of msn induces pretarget growth cone termination. Physical and genetic interactions between Msn and Dock indicate a role for Msn in the Dock signaling pathway. We propose that Msn functions as a key controller of growth cone cytoskeleton in response to Dock-mediated signals.

Fringe: defining borders by regulating the notch pathway

The Notch pathway mediates cell-cell interaction in many developmental processes. Multiple proteins regulate the Notch pathway, among these are the products of the fringe genes. The first fringe gene was identified in Drosophila, where it is involved in the formation of the dorsal/ventral border of the wing disc. It has now been found to be crucial for determining the dorsal/ventral border of the Drosophila eye. In vertebrates, fringe genes play roles in the formation of the apical ectodermal ridge, the dorsal/ventral border in the limb bud, and in the development of somitic borders. The roles of fringe in the neural tube or the eyes of vertebrate embryos are not clear, although it is unlikely that these roles are evolutionarily related to those in the same tissues in Drosophila. Genetic evidences suggest that Fringe protein functions by modulating the Notch signaling pathway, perhaps through differential regulation of Notch activation by different ligands; however, the mechanism underlying Fringe function remains to be investigated.

Transcription repression by Xenopus ET and its human ortholog TBX3, a gene involved in ulnar-mammary syndrome

T box (Tbx) genes are a family of developmental regulators with more than 20 members recently identified in invertebrates and vertebrates. Mutations in Tbx genes have been found to cause several human diseases. Our understanding of functional mechanisms of Tbx products has come mainly from the prototypical T/Brachyury, which is a transcription activator. We previously discovered ET, a Tbx gene expressed in Xenopus embryos. We report here that ET is an ortholog of the human Tbx3 and that ET is a repressor of basal and activated transcription. Functional dissection of the ET protein reveals a novel transcription-repression domain highly conserved among ET, human TBX3, and TBX2. These results reveal a new transcription repressor domain, show the existence of a subfamily of transcription repressors in the Tbx superfamily, and provide a basis for understanding etiology of diseases caused by Tbx3 mutations.

The mouse SLIT family: secreted ligands for ROBO expressed in patterns that suggest a role in morphogenesis and axon guidance

The Slit gene encodes a secreted molecule essential for neural development in Drosophila embryos. Here we report the identification of three Slit homologues in the mouse. We demonstrate that the mouse SLIT1 protein can bind ROBO1, a transmembrane receptor implicated in axon guidance. Both whole-mount and section in situ hybridization studies reveal unique and complementary patterns of expression of the three mouse Slit genes and of Robo1, both within the central nervous system and in other developing tissues. The complementary expression patterns of Slit and Robo1 and their in vitro interaction suggest a ligand-receptor relationship. The expression of all three Slit genes in the floor plate suggests that they are likely to share the same functional properties with their Drosophila homologue in midline neural development and axon guidance. The complementary expression of Slit and Robo1 in different subdivisions of the somites suggests their possible function in axon pathfinding and neural crest cell migration. The unique expression pattern in limb and other organs indicates additional potential functions of the Slit gene family.