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Author(s): Yihui Cui [1, 2]; Yan Yang [1, 2]; Zheyi Ni [1]; Yiyan Dong [1]; Guohong Cai [3]; Alexandre Foncelle [4, 5]; Shuangshuang Ma [1]; Kangning Sang [1]; Siyang Tang [...]

In mammalian neurons, the precise accumulation of sodium channels at the axonal initial segment (AIS) ensures action potential initiation. This accumulation precedes the immobilization of membrane proteins and lipids by a diffusion barrier at the AIS. Using single-particle tracking, we measured the mobility of a chimeric ion channel bearing the ankyrin-binding motif of the Nav1.2 sodium channel. We found that ankyrin G (ankG) limits membrane diffusion of ion channels when coexpressed in neuroblastoma cells. Site-directed mutants with decreased affinity for ankG exhibit increased diffusion speeds. In immature hippocampal neurons, we demonstrated that ion channel immobilization by ankG is regulated by protein kinase CK2 and occurs as soon as ankG accumulates at the AIS of elongating axons. Once the diffusion barrier is formed, ankG is still required to stabilize ion channels. In conclusion, our findings indicate that specific binding to ankG constitutes the initial step for Nav channel immobilization at the AIS membrane and precedes the establishment of the diffusion barrier. doi/ 10.1083/jcb.201003042

Regenerative responses to axonal injury involve changes in gene expression; however, little is known about how such changes can be induced from a distant site of injury. In this study, we describe a nerve crush assay in Drosophila melanogaster to study injury signaling and regeneration mechanisms. We find that Wallenda (Wnd), a conserved mitogen-activated protein kinase (MAPK) kinase kinase homologous to dual leucine zipper kinase, functions as an upstream mediator of a cell-autonomous injury signaling cascade that involves the c-Jun [NH.sub.2]-terminal kinase MAPK and Fos transcription factor. Wnd is physically transported in axons, and axonal transport is required for the injury signaling mechanism. Wnd is regulated by a conserved E3 ubiquitin ligase, named Highwire (Hiw)in Drosophila. Injury induces a rapid increase in Wnd protein concomitantly with a decrease in Hiw protein. In hiw mutants, injury signaling is constitutively active, and neurons initiate a faster regenerative response. Our data suggest that the regulation of Wnd protein turnover by Hiw can function as a damage surveillance mechanism for responding to axonal injury. doi/ 10.1083/jcb.201006039

Atypical Rho-guanine nucleotide exchange factors (Rho-GEFs) that contain Dock homology regions (DHR-1 and DHR-2) are expressed in a variety of tissues; however, their functions and mechanisms of action remain unclear. We identify key conserved amino acids in the DHR-2 domain that are criticai for the catalytic activity of Dock-GEFs (Dock1-4). We further demonstrate that Dock-GEFs directly associate with WASP family verprolin-homologous (WAVE) proteins through the DHR-1 domain. Brain-derived neurotrophic factor (BDNF)-TrkB signaling recruits the Dock3/WAVE1 complex to the plasma membrane, whereupon Dock3 activates Rac and dissociates from the WAVE complex in a phosphorylation-dependent manner. BDNF induces axonal sprouting through Dock-dependent Rac activation, and adult transgenic mice overexpressing Dock3 exhibit enhanced optic nerve regeneration after injury without affecting WAVE expression levels. Our results highlight a unique mechanism through which Dock-GEFs achieve spatial and temporal restriction of WAVE signaling, and identify Dock-GEF activity as a potential therapeutic target for axonal regeneration., Dock family proteins | brain-derived neurotrophic factor | Fyn | axonal regeneration | optic nerve doi/ 10.1073/pnas.0914514107

Repair of damage to the central nervous system (CNS) is inhibited by the presence of myelin proteins that prevent axonal regrowth. Consequently, growth inhibitors and their common receptor have been identified as targets in the treatment of injury to the CNS. Here we describe the structure of the extracellular domain of the neurite outgrowth inhibitor (Nogo) in a membrane-like environment. Isoforms of Nogo are expressed with a common C terminus containing two transmembrane (TM) helices. The ectodomain between the two TM helices, Nogo-66, is active in preventing axonal growth [GrandPre T, Nakamura F, Vartanian T, Strittmatter SM (2000) Nature 403:439-444]. We studied the structure of Nogo66 alone and in the presence of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) vesicles and dodecylphosphocholine (DPC) micelles as membrane mimetics. We find that Nogo-66 is largely disordered when free in solution. However, when bound to a phosphocholine surface Nogo-66 adopts a unique, stable fold, even in the absence of TM anchors. Using paramagnetic probes and protein-DPC nuclear Overhauser effects (NOEs), we define portions of the growth inhibitor likely to be accessible on the cell surface. With these data we predict that residues (28-58) are available to bind the Nogo receptor, which is entirely consistent with functional assays. Moreover, the conformations and relative positions of side chains recognized by the receptor are now defined and provide a foundation for antagonist design. peripheral | lipid surface | CNS | membrane protein | nmr doi/ 10.1073/pnas.0911817107

Guidance of axons by molecular gradients is crucial for wiring up the developing nervous system. It often is assumed that the unique signature of such guidance is immediate and biased turning of the axon tip toward or away from the gradient. However, here we show that such turning is not required for guidance. Rather, by a combination of experimental and computational analyses, we demonstrate that growth-rate modulation is an alternative mechanism for guidance. Furthermore we show that, although both mechanisms may operate simultaneously, biased turning dominates in steep gradients, whereas growth-rate modulation may dominate in shallow gradients. These results suggest that biased axon turning is not the only method by which guidance can occur. chemotaxis | growth cone | nerve growth factor | neural development | computational neuroscience doi: 10.1073/pnas.0909254107

Cytosolic cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) often mediate antagonistic cellular actions of extracellular factors, from the regulation of ion channels to cell volume control and axon guidance. We found that localized cAMP and cGMP activities in undifferentiated neurites of cultured hippocampal neurons promote and suppress axon formation, respectively, and exert opposite effects on dendrite formation. Fluorescence resonance energy transfer imaging showed that alterations of the amount of cAMP resulted in opposite changes in the amount of cGMP, and vice versa, through the activation of specific phosphodiesterases and protein kinases. Local elevation of cAMP in one neurite resulted in cAMP reduction in all other neurites of the same neuron. Thus, local and Long-range reciprocal regulation of cAMP and cGMP together ensures coordinated development of one axon and multiple dendrites. 10.1126/science.1179735

Phosphoinositide (PI) lipids are intracellular membrane signaling intermediates and effectors produced by localized PI kinase and phosphatase activities. Although many signaling roles of PI kinases have been identified in cultured cell lines, transgenic animal studies have produced unexpected insight into the in vivo functions of specific PI 3- and 5-kinases, but no mammalian PI 4-kinase (PI4K) knockout has previously been reported. Prior studies using cultured cells implicated the PI4K2[alpha] isozyme in diverse functions, including receptor signaling, ion channel regulation, endosomal trafficking, and regulated secretion. We now show that despite these important functions, mice lacking PI4K2[alpha] kinase activity initially appear normal. However, adult [Pi4k2a.sup.GT/GT] animals develop a progressive neurological disease characterized by tremor, limb weakness, urinary incontinence, and premature mortality. Histological analysis of aged [Pi4k2a.sup.GT/GT] animals revealed lipofuscin-like deposition and gliosis in the cerebellum, and loss of Purkinje cells. Peripheral nerves are essentially normal, but massive axonal degeneration was found in the spinal cord in both ascending and descending tracts. These results reveal a previously undescribed role for aberrant PI signaling in neurological disease that resembles autosomal recessive hereditary spastic paraplegia. genetrap | hereditary spastic paraplegia | phosphoinositide | lipofuscin | neurodegeneration

Axon guidance by molecular gradients plays a crucial role in wiring up the nervous system. However, the mechanisms axons use to detect gradients are largely unknown. We first develop a Bayesian 'ideal observer' analysis of gradient detection by axons, based on the hypothesis that a principal constraint on gradient detection is intrinsic receptor binding noise. Second, from this model, we derive an equation predicting how the degree of response of an axon to a gradient should vary with gradient steepness and absolute concentration. Third, we confirm this prediction quantitatively by performing the first systematic experimental analysis of how axonal response varies with both these quantities. These experiments demonstrate a degree of sensitivity much higher than previously reported for any chemotacting system. Together, these results reveal both the quantitative constraints that must be satisfied for effective axonal guidance and the computational principles that may be used by the underlying signal transduction pathways, and allow predictions for the degree of response of axons to gradients in a wide variety of in vivo and in vitro settings. axon guidance | chemotaxis | growth cone | nerve growth factor | nerve regeneration

All but the smallest-diameter axons in the central nervous system are myelinated, but the signals that initiate myelination are unknown. Our prior work has shown that integrin signaling forms part of the cell--cell interactions that ensure only those oligodendrocytes contacting axons survive. Here, therefore, we have asked whether integrins regulate the interactions that lead to myelination. Using homologous recombination to insert a single-copy transgene into the hypoxanthine phosphoribosyl transferase (hprt) locus, we find that mice expressing a dominant-negative [beta]1 integrin in myelinating oligodendrocytes require a larger axon diameter to initiate timely myelination. Mice with a conditional deletion of focal adhesion kinase (a signaling molecule activated by integrins) exhibit a similar phenotype. Conversely, transgenic mice expressing dominant-negative 133 integrin in oligodendrocytes display no myelination abnormalities. We conclude that 131 integrin plays a key role in the axoglial interactions that sense axon size and initiate myelination, such that loss of integrin signaling leads to a delay in myelination of small-diameter axons.

During development, Schwann cells (SCs)interpret different extracellular cues to regulate their migration, proliferation, and the remarkable morphological changes associated with the sorting, ensheathment, and myelination of axons. Although interactions between extracellular matrix proteins and integrins are critical to some of these processes, the downstream signaling pathways they control are still poorly understood. Integrin-linked kinase (ILK) is a focal adhesion protein that associates with multiple binding partners to link integrins to the actin cytoskeleton and is thought to participate in integrin and growth factor-mediated signaling. Using SC-specific gene ablation, we report essential functions for ILK in radial sorting of axon bundles and in remyelination in the peripheral nervous system. Our in vivo and in vitro experiments show that ILK negatively regulates Rho/Rho kinase signaling to promote SC process extension and to initiate radial sorting. ILK also facilitates axon remyelination, likely by promoting the activation of downstream molecules such as AKT/protein kinase B.

Slow Wallerian degeneration ([Wld.sup.S]) encodes a chimeric Ube4b/nicotinamide mononucleotide adenylyl transferase 1 (Nmnat1) fusion protein that potently suppresses Wallerian degeneration, but the mechanistic action of [Wld.sup.S] remains controversial. In this study, we characterize [Wld.sup.S]-mediated axon protection in vivo using Drosophila melanogaster. We show that Nmnat1 can protect severed axons from autodestruction but at levels significantly lower than [Wld.sup.S], and enzyme-dead versions of Nmnat1 and [Wld.sup.S] exhibit severely reduced axon-protective function. Interestingly, a 16-amino acid N-terminal domain of [Wld.sup.S] (termed N16) accounts for the differences in axon-sparing activity between [Wld.sup.S] and Nmnat1, and N16-dependent enhancement of Nmnat1-protective activity in [Wld.sup.S] requires the N16-binding protein valosin-containing protein (VCP)/TER94. Thus, [Wld.sup.S]-mediated suppression of Wallerian degeneration results from VCP-N16 interactions and Nmnatl activity converging in vivo. Surprisingly, mouse Nmnat3, a mitochondrial Nmnat enzyme that localizes to the cytoplasm in Drosophila cells, protects severed axons at levels indistinguishable from [Wld.sup.S]. Thus, nuclear Nmnat activity does not appear to be essential for [Wld.sup.S]-like axon protection.

The slow Wallerian degeneration ([Wld.sup.S]) protein protects injured axons from degeneration. This unusual chimeric protein fuses a 70-amino acid N-terminal sequence from the Ube4b multiubiquitination factor with the nicotinamide adenine dinucleotide-synthesizing enzyme nicotinamide mononucleotide adenylyl transferase 1. The requirement for these components and the mechanism of [Wld.sup.S]-mediated neuroprotection remain highly controversial. The Ube4b domain is necessary for the protective phenotype in mice, but precisely which sequence is essential and why are unclear. Binding to the AAA adenosine triphosphatase valosin-containing protein (VCP)/p97 is the only known biochemical property of the Ube4b domain. Using an in viva approach, we show that removing the VCP-binding sequence abolishes axon protection. Replacing the [Wld.sup.S] VCP-binding domain with an alternative ataxin-3-derived VCP-binding sequence restores its protective function. Enzyme-dead [Wld.sup.S] is unable to delay Wallerian degeneration in mice. Thus, neither domain is effective without the function of the other. [Wld.sup.S] requires both of its components to protect axons from degeneration.

MS results from destruction of the protective myelin sheath surrounding axons, which prevents the transmission of nerve impulses. Precursors of oligodendrocytes, the cells capable of myelinating axons, are preserved in demyelinating lesions; however, why these precursors do not differentiate into mature oligodendrocytes and remyelinate axons is unknown. Contactin is a noncanonical Notch receptor ligand that mediates oligodendrocyte differentiation. In this issue of the JCI, Nakahara et al. show that Contactin is abundantly expressed on demyelinated axons in human chronic MS lesions and that Notch1 is activated in oligodendrocyte precursor cells (see the related article beginning on page 169). However, Notch1 intracellular domain coassociates with the nuclear transporter Importin [beta] but fails to show evidence of nuclear translocation. These cytoplasmic aggregates also contain TAT-interacting protein 30 kDa (TIP30), a proapoptotic factor, which inhibits nuclear transport and, consequently, Notch1-mediated oligodendrocyte differentiation and remyelination. These data target TIP30 as a new pathogenic factor in MS., MS is an inflammatory, demyelinating disease of the CNS that is thought to be mediated by an immune attack directed against oligodendrocytes and myelin. Clinically, patients usually present in early [...]

Author(s): William M. Howe; Paul J. Kenny Opposing forces shape our everyday lives -- for instance, stimuli can encourage us to move or stop, and events can make us happy [...]

Byline: Yasufumi Hayano (Neuroscience Laboratories, Graduate School of Frontier Biosciences, Osaka University, Osaka, Japan); Nobuhiko Yamamoto (Neuroscience Laboratories, Graduate School of Frontier Biosciences, Osaka University, Osaka, Japan, nobuhiko@fbs.osaka-u.ac.jp) Keywords: Axon [...]

Loss of function after neurological injury frequently occurs through the interruption of axonal connectivity, rather than through cell loss. Functional deficits persist because a multitude of inhibitory factors in degenerating myelin and astroglial scar prevent axonal growth in the adult brain and spinal cord. Given the high clinical significance of achieving functional recovery through axonal growth, substantial research effort has been, and will be, devoted toward this desirable goal. Unfortunately, the labels commonly used in the literature to categorize post-injury axonal anatomy might hinder advancement. In this article, we present an argument for the importance of developing precise terms that describe axonal growth in terms of the inciting event, the distance of axonal extension and the timing of axonal growth. The phenotypes produced by molecular interventions that overcome astroglial scar or myelin-associated inhibitors are refrained and discussed in this context.

Vascular sympathetic innervation is an important determinant of blood pressure and blood flow. The mechanisms that determine vascular sympathetic innervation are not well understood. The present study tests the hypothesis that vascular-derived artemin promotes the development of sympathetic innervation to blood vessels by promoting sympathetic axon growth. RT-PCR and Western analyses indicate that artemin is expressed by cultured vascular smooth muscle and arteries, and artemin coreceptors, glial cell-derived neurotrophic factor family receptor [alpha]3 and ret, are expressed by postganglionic sympathetic neurons. The effects of artemin on axon growth were assessed on explants of neonatal rat sympathetic ganglia. In the presence, but not in the absence, of nerve growth factor, exogenous artemin stimulated neurite growth. Femoral arteries (FA) from adult rats contain artemin, and these arteries stimulated sympathetic neurite growth. Growth in the presence of FA was 92.2 [+ or -] 11.9 mm, and that in the absence of FA was 26.3 [+ or -] 5.4 mm (P < 0.05). FA stimulation of axon growth was reduced by an antibody that neutralized the activity of artemin (P < 0.05). These data indicate that artemin is expressed in arteries, and its receptors are expressed and functional in the postganglionic sympathetic neurons that innervate them. This suggests that artemin may be a determinant of vascular sympathetic innervation. vascular smooth muscle; sympathetic nervous system; axon growth doi:10.1152/ajpheart.00859.2006

The molecular mechanism and significance of endocytic processes involved in directional axon elongation are not well understood. The Unc-51 family of serine/threonine kinases was shown to be important for axon growth and was also linked to endocytosis, providing an entry point to study this problem. We found that mouse Unc-51-like kinase 1/2 (Ulk1/2) proteins are localized to vesicular structures in growth cones of mouse spinal sensory neurons. RNAi-mediated knockdown of Ulk1 and/or Ulk2 resulted in impaired endocytosis of nerve growth factor (NGF), excessive axon arborization, and severely stunted axon elongation. The evidence also indicates that Ulk1/2 mediates a non-clathrin-coated endocytosis in sensory growth cones. Interestingly, NGF can induce the interaction of Ulk1 with TrkA receptor complexes through promoting K63-polyubiquitination of Ulk1 and binding of Ulk1 to the scaffolding protein p62. These results and additional studies suggest that Ulk1/2 proteins regulate filopodia extension and neurite branching during sensory axon outgrowth, probably through regulating TrkA receptor trafficking and signaling. axon growth | dorsal root ganglion neurons | endocytosis | p62

The mammalian odorant receptors (ORs) comprise a large family of G protein-coupled receptors that are critical determinants of both the odorant response profile and the axonal identity of the olfactory sensory neurons in which they are expressed. Although the pathway by which ORs activate odor transduction is well established, the mechanism by which they direct axons into proper glomerular relationships remains unknown. We have developed a gain-of-function approach by using injection of retroviral vectors into the embryonic olfactory epithelium to study the ORs' contribution to axon guidance. By ectopically expressing ORs, we demonstrate that functional OR proteins induce axonal coalescence. Furthermore, ectopic expression of G[alpha] mutants reveals that activation of the signal transduction cascade is sufficient to cause axonal convergence into glomeruli. Analysis of G[alpha] subunit expression indicates that development and odorant transduction use separate transduction pathways. Last, we establish that the generation of cAMP through adenylyl cyclase 3 is necessary to establish proper axonal identity. Our data point to a model in which axonal sorting is accomplished by OR stimulation of cAMP production by coupling to Gas. axon guidance | development | olfaction

The navigation of axons toward their targets is a highly dynamic and precisely regulated process during nervous system development. The molecular basis of this navigation process is only partly understood. In Caenorhabditis elegans, we isolated the RNAi-hypersensitive strain nre-1(hd20) lin-15b(hd126), which allows us to phenocopy axon guidance defects of known genes by feeding RNAi. We used this mutant strain to systematically screen 4,577 genes on chromosomes I and III for axon guidance phenotypes. We identified 93 genes whose down-regulation led to penetrant ventral cord fasciculation defects or motoneuron commissure out-growth defects. These genes encode various classes of proteins, ranging from secreted or putative cell surface proteins to transcription factors controlling gene expression. A majority of the genes is evolutionary conserved and previously uncharacterized. In addition, we found axon guidance functions for known genes like pry-1, a component of the Wnt-signaling pathway, and ced-1, a receptor required for the engulfment of neurons undergoing apoptosis during development. Our screen provides insights into molecular pathways operating during the generation of neuronal circuits and provides a basis for a more detailed analysis of gene networks regulating axon navigation. Wnt | neuron | development | axon navigation

Genes whose expression profiles are correlated with the expressed odorant receptors (ORs) were searched to study how OR-instructed axonal fasciculation is controlled. The findings reveal that a specific set of adhesive/repulsive molecules, whose expression levels are determined by OR molecules, regulate the axonal fasciculation of OSNs during the process of glomerular map formation.

New studies related to discrete identities of olfactory sensory neurons, which converted into a spatial map of axonal connections, are reported. It was found that levels of cAMP signals derived from olfactory receptors (ORs) could direct targeting of axons along an axis and that ORs and neural activity regulate expression of adhesion/guidance molecules in mosaic patterns that could sort axons into discrete locations.

The mechanisms of nerve conduction block induced by high-frequency biphasic electrical currents were investigated using a lumped circuit model of the myelinated axon based on Frankenhaeuser-Huxley (FH) model or Chiu-Ritchie-Rogart-Stagg-Sweeney (CRRSS) model. The FH model revealed that the constant activation of potassium channels at the node under the block electrode, rather than inactivation of sodium channels, is the likely mechanism underlying conduction block of myelinated axons induced by high-frequency biphasic stimulation. However, the CRRSS model revealed a different blocking mechanism where the complete inactivation of sodium channels at the nodes next to the block electrode caused the nerve conduction block. The stimulation frequencies to observe conduction block in FH model agree with the observations from animal experiments (greater than 6 kHz), but much higher frequencies are required in CRRSS model (greater than 15 kHz). This frequency difference indicated that the constant activation of potassium channels might be the underlying mechanism of conduction block observed in animal experiments. Using the FH model, this study also showed that the axons could recover from conduction block within 1 ms after termination of the blocking stimulation, which also agrees very well with the animal experiments where nerve block could be reversed immediately once the blocking stimulation was removed. This simulation study, which revealed two possible mechanisms of nerve conduction block in myelinated axons induced by high-frequency biphasic stimulation, can guide future animal experiments as well as optimize stimulation waveforms for electrical nerve block in clinical applications. Index Terms--Axon, electrical stimulation, high-frequency, model, nerve block.

The growth of axons is fundamental to the development and repair of brain circuitry. We show here that Mst3b, a neuron-specific homolog of the yeast kinase Ste20, is critical for axon outgrowth. Mst3b is activated in response to trophic factors, and suppressing its expression (via siRNAs) or its function (by a dominant-negative mutant) blocks axon outgrowth. Inosine, a purine nucleoside that stimulates axon outgrowth, activates Mst3b kinase activity, whereas 6-thioguanine, a purine analog that blocks outgrowth, inhibits the activity of this kinase. These findings place Mst3b as a key regulator of axon outgrowth and help explain the purine sensitivity of this process. inosine | neural development | regeneration | signal transduction

The effects of SoxNeuro gene expression on development of axons, using chromatin immunoprecipitation studies, are presented.

The laboratory mouse has emerged as a primary model organism for studying axon regeneration after experimental spinal cord injury, owing to its genetic amenability. Mutant mouse models are contributing significantly to our understanding of the molecular mechanisms of axon regeneration failure in the adult mammalian central nervous system (CNS), in particular regarding the role of axon-growth inhibitors. Here, we discuss recent advances in understanding axon regeneration failure that have been made using genetically modified mice, focusing on the inhibitory influences in the CNS, and we illustrate the advantages of using the mouse as a surrogate organism to study axon regeneration and spinal cord repair.

Study is presented to report that Spineless (Ss), the Drosophila homolog of the mammalian aryl hydrocarbon (dioxin) receptor (Ahr), regulates dendrite diversity in the dendritic arborization (da) sensory neurons. It is suggested that one evolutionarily conserved role for Ahr in neuronal development concerns the diversification of dendrite morphology.

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.ydbio.2006.06.021 Byline: Hans-Martin Pogoda (a), Nitzan Sternheim (a), David A. Lyons (a), Brianne Diamond (a), Thomas A. Hawkins (a), Ian G. Woods (a), Dimple H. Bhatt (b), Clara Franzini-Armstrong (c), Claudia Dominguez (a), Naomi Arana (a), Jennifer Jacobs (a), Rebecca Nix (a), Joseph R. Fetcho (d), William S. Talbot (a) Keywords: Myelin; Zebrafish; Schwann cell; Oligodendrocyte; Axon Abstract: The myelin sheath insulates axons in the vertebrate nervous system, allowing rapid propagation of action potentials via saltatory conduction. Specialized glial cells, termed Schwann cells in the PNS and oligodendrocytes in the CNS, wrap axons to form myelin, a compacted, multilayered sheath comprising specific proteins and lipids. Disruption of myelinated axons causes human diseases, including multiple sclerosis and Charcot-Marie-Tooth peripheral neuropathies. Despite the progress in identifying human disease genes and other mutations disrupting glial development and myelination, many important unanswered questions remain about the mechanisms that coordinate the development of myelinated axons. To address these questions, we began a genetic dissection of myelination in zebrafish. Here we report a genetic screen that identified 13 mutations, which define 10 genes, disrupting the development of myelinated axons. We present the initial characterization of seven of these mutations, defining six different genes, along with additional characterization of mutations that we have described previously. The different mutations affect the PNS, the CNS, or both, and phenotypic analyses indicate that the genes affect a wide range of steps in glial development, from fate specification through terminal differentiation. The analysis of these mutations will advance our understanding of myelination, and the mutants will serve as models of human diseases of myelin. Author Affiliation: (a) Department of Developmental Biology, Stanford University School of Medicine, Stanford, CA 94305, USA (b) Department of Neurobiology and Behavior, State University of New York at Stony Brook, Stony Brook, NY 11794, USA (c) Department of Cell and Developmental Biology, University of Pennsylvania School of Medicine, Philadelphia, PA 19104, USA (d) Department of Neurobiology and Behavior, Cornell University, Ithaca, NY 14853, USA Article History: Received 28 April 2006; Revised 13 June 2006; Accepted 13 June 2006

During the development of the nervous system, neurons must first migrate to their appropriate locations and then send out axons to make connections. Various environmental cues guide these migrating neurons [...]

Using a mutagenesis screen based on the alpha1-tubulin driven green fluorescent protein reporter expression obtained from transgenic zebrafish, the breaks in genetic loci crucial for for neuronal and axonal development are analyzed.

A study examines the impact of ephrin receptor B2 class in the development of axons in the auditory system.

We used diffusion tensor imaging (DTI) to study 2 patients with traumatic brain injury. The first patient recovered reliable expressive language after 19 years in a minimally conscious state (MCS); [...]

Vascular sympathetic innervation is an important determinant of blood pressure and blood flow. The mechanisms that determine vascular sympathetic innervation are not well understood. Recent studies indicate that vascular endothelial cells (EC) express semaphorin 3A, a repulsive axon guidance cue. This suggests that EC would inhibit the growth of axons to blood vessels. The present study tests this hypothesis. RT-PCR and Western analyses confirmed that rat aortic vascular ECs expressed semaphorin 3A as well as other class 3 semaphorins (sema 3s). To determine the effects of EC-derived sema 3 on sympathetic axons, axon outgrowth was assessed in cultures of neonatal sympathetic ganglia grown for 72 h in the absence and presence of vascular EC. Nerve growth factor-induced axon growth in the presence of ECs was 50 [+ or -] 4% (P < 0.05) of growth in the absence of ECs. ECs did not inhibit axon growth in the presence of an antibody that neutralized the activity of sema 3 (P > 0.05). RT-PCR and Western analyses also indicated that sema 3s were expressed in ECs of intact arteries. To assess the function of sema 3s in arteries, sympathetic ganglia were grown in the presence of arteries for 72 h, and the percentage of axons that grew toward the artery was determined: 44 [+ or -] 4% of axons grew toward neonatal carotid arteries. Neutralization of sema 3s or removal of EC increased the percentage of axons that grew toward the artery (71 [+ or -] 8% and 72 [+ or -] 8%, respectively). These data indicate that vascular EC-derived sema 3s inhibit sympathetic axon growth and may thus be a determinant of vascular sympathetic innervation. sympathetic nervous system; endothelium

The role of Caenorhabditis elegans Eph receptor tyrosine kinase and variable abnormal protein 1 in guiding axons towards precise positions is presented.

Studies indicate that entire leucine-rich repeat (LPR) region of nogo-66 receptor (NgR1) is required for ligand binding and that the adjacent C-terminal region (CT stalk) for the NgR1 contributes to interaction with the coreceptors. The disulfide structure of full length NgR1 is analyzed to provide structure based information for these interactions.

During development, neurons are guided to their targets by short-and long-range attractive and repulsive cues. MICAL, a large multidomain protein, is required for the combined action of semaphorins and plexins in axon guidance. Here, we present the structure of the N-terminal region of MICAL ([MICAL.sub.fd]) determined by x-ray diffraction to 2.0 [Angstrom] resolution. The structure shows that [MICAL.sub.fd] is an FAD-containing module structurally similar to aromatic hydroxylases and amine oxidases. In addition, we present biochemical data that show that [MICAL.sub.fd] is a flavoenzyme that in the presence of NADPH reduces molecular oxygen to [H.sub.2][O.sub.2] ([K.sub.m,NAPDH] = 222 [micro]M; [k.sub.cat] = 77 [sec.sup.-1]), a molecule with known signaling properties. We propose that the [H.sub.2][O.sub.2] produced by this reaction may be one of the signaling molecules involved in axon guidance by MICAL. hydrogen peroxide | hydroxylase | monooxygenase | x-ray diffraction

Herein, the well-known cable equation for nonmyelinated axon model is extended analytically for myelinated axon formulation. The myelinated membrane conductivity is represented via the Fourier series expansion. The classical cable equation is thereby modified into a linear second order ordinary differential equation with periodic coefficients, known as Hill's equation. The general internal source response, expressed via repeated convolutions, uniformly converges provided that the entire periodic membrane is passive. The solution can be interpreted as an extended source response in an equivalent nonmyelinated axon (i.e., the response is governed by the classical cable equation). The extended source consists of the original source and a novel activation function, replacing the periodic membrane in the myelinated axon model. Hill's equation is explicitly integrated for the specific choice of piecewise constant membrane conductivity profile, thereby resulting in an explicit closed form expression for the transmembrane potential in terms of trigonometric functions. The Floquet's modes are recognized as the nerve fiber activation modes, which are conventionally associated with the nonlinear Hodgkin-Huxley formulation. They can also be incorporated in our linear model, provided that the periodic membrane point-wise passivity constraint is properly modified. Indeed, the modified condition, enforcing the periodic membrane passivity constraint on the average conductivity only leads, for the first time, to the inclusion of the nerve fiber activation modes in our novel model. The validity of the generalized transmission-line and cable equation models for a myelinated nerve fiber, is verified herein through a rigorous Green's function formulation and numerical simulations for transmembrane potential induced in three-dimensional myelinated cylindrical cell. It is shown that the dominant pole contribution of the exact modal expansion is the transmembrane potential solution of our generalized model. Index Terms--Cable equation, Floquet's theory, Hill's equation, myelinated axon, nerve activation, transmission-line model.

The current model of basal ganglia rests on the idea that the striatofugal system is composed of two separate (direct and indirect) pathways originating from distinct cell populations in the striatum. The striatum itself is divided into two major compartments, the striosomes and the matrix, which differ by their neurochemical makeup and input/output connections. Here, neurons located in either striosomes or the extrastriosomal matrix in squirrel monkeys were injected with biotin dextran amine, and their labeled axons were entirely reconstructed with a camera lucida. Twenty-four of 27 reconstructed axons arborized into the three main striatal targets (external pallidum, globus pallidus, and substantia nigra pars reticulata), a finding that is at odds with the concept of a dual striatofugal system. Axons of striosomal neurons formed several columnar terminal fields in the substantia nigra pars reticulata. These data indicate that the substantia nigra pars compacta is neither the only nor the main target of striosomal neurons, a finding that calls for a reevaluation of the organization of the striatonigral projection system. anatomy | basal ganglia | matrix | striatum | striosomes

Eliminating assembled neurofilaments (NFs) from axons or misaccumulating NFs in motor neuron cell bodies strongly slows disease in mouse models of mutant superoxide dismutase 1 (SOD1)-induced amyotrophic lateral sclerosis. One proposal for how reducing axonal NFs can increase survival is that the multiphosphorylated tail domains of the two larger NF subunits act in motor neuron cell bodies as phosphorylation sinks where they mitigate cyclin-dependent kinase 5 dysregulation induced by mutant SOD1. Elimination by gene targeting in mice of the NF medium and NF heavy tail domains and their 58 known phosphorylation sites accelerates aberrant phosphorylation of other neuronal substrates while leaving overall NF content unaltered. However, disease onset is significantly delayed and survival is extended, inconsistent with the ameliorative property of altered NF content protecting by serving as substrates for dysregulation of any NF kinase. Moreover, at comparable disease stages significantly more surviving motor neurons and axons were found in SOD1 mutant mice deleted in the NF tails than in similar mice with wild-type NFs. This finding supports noncell autonomous toxicity in SOD1 mutant-mediated amyotrophic lateral sclerosis: removal of the NF tails slows damage developed directly within motor neurons, but SOD1 mutant damage within nonneuronal supporting cells reduces motor neuron functionality. axonal transport | axonal cytoskeleton | cyclin-dependent kinase 5

The concept of postembryonic development of the lateral line system is examined and have revealed an unexpected triangular relationship when migrating sensory precursors guide axons, axons guide glia and glia control the formation of sensory organs. The remarkable accessibility of the lateral line system may allow a detailed analysis of cell migration and polarization, and may clarify the complex interactions between sensory precursor cells, neurons and glia during development.

The development of the embryonic blood vascular and lymphatic systems requires the coordinated action of several transcription factors and growth factors that target endothelial and periendothelial cells. The new advances in the understanding of the roles of these axonal pathfinding molecules in vascular remodeling and vessel guidance, indicating that neuronal axons and vessel sprouts use common molecular mechanisms for navigation in the body is summarized.

Adult urodele amphibians possess the unique ability to regenerate amputated limbs and to re-innervate these regenerating structures; however, the factors involved in mediating this re-innervation are largely unknown. Here, we investigated the role of retinoic acid (RA) and one of its receptors, RAR[beta], in the reciprocal neurotropic interactions between regenerating limb blastemas and spinal cord explants from the adult newt Notophthalmus viridescens. First, we showed that retinoic acid induced directed axonal outgrowth from cultured spinal cord tissue. This RA-induced outgrowth was significantly reduced when spinal cord explants were pre-treated with either the synthetic RAR pan antagonist, LE540, or the specific RAR[beta] antagonist, LE135. The role of RAR[beta] was also investigated using co-cultured regenerating limb blastemas and spinal cord explants. Blastemas induced significantly more axonal outgrowth from the near side of co-cultured explants, than from the far side (when cultured less than 1 mm apart). This blastema-induced directed outgrowth from co-cultured spinal cord explants was also abolished in the presence of the RAR[beta] antagonist, LE135. These data strongly suggest that endogenous retinoic acid is one of the tropic factors produced by the blastema and that it may be capable of guiding re-innervating axons to their targets. Moreover, this interaction is likely mediated by the retinoic acid [beta] nuclear receptor. Keywords: Regeneration; Neurotrophic factors; Newt; Retinoic acid; Blastema; Axonal outgrowth; Regeneration; RAR[beta]

A study was conducted to show that the growth factor Midkine-a, expressed in the paraxial mesoderm, is required in the formation of the medial floor plate in zebrafish. It is concluded that in zebrafish, trunk-derived signals are required for complete medial floor plate (MFP) formation from a common pool of organizer-derived midline precursor cells.

The patterning of the nervous system is achieved through coordinated action of a variety of repulsive or attractive neuronal guidance factors that direct the growth of growing axons to specific pathways. It is suggested that similar sets of factors also guide migration of endothelial cells.