Stabilization of growing retinal axons by the combined signaling of nitric oxide and brain-derived neurotrophic factor

Intrinsic organization of the corpus callosum

The corpus callosum-the largest commissural fiber system connecting the two cerebral hemispheres-is considered essential for bilateral sensory integration and higher cognitive functions. Most studies exploring the corpus callosum have examined either the anatomical, physiological, and neurochemical organization of callosal projections or the functional and/or behavioral aspects of the callosal connections after complete/partial callosotomy or callosal lesion. There are no works that address the intrinsic organization of the corpus callosum. We review the existing information on the activities that take place in the commissure in three sections: I) the topographical and neurochemical organization of the intracallosal fibers, II) the role of glia in the corpus callosum, and III) the role of the intracallosal neurons.

Target-selective vertebrate motor axon regeneration depends on interaction with glial cells at a peripheral nerve plexus

A critical step for functional recovery from peripheral nerve injury is for regenerating axons to connect with their pre-injury targets. Reestablishing pre-injury target specificity is particularly challenging for limb-innervating axons as they encounter a plexus, a network where peripheral nerves converge, axons from different nerves intermingle, and then re-sort into target-specific bundles. Here, we examine this process at a plexus located at the base of the zebrafish pectoral fin, equivalent to tetrapod forelimbs. Using live cell imaging and sparse axon labeling, we find that regenerating motor axons from 3 nerves coalesce into the plexus. There, they intermingle and sort into distinct branches, and then navigate to their original muscle domains with high fidelity that restores functionality. We demonstrate that this regeneration process includes selective retraction of mistargeted axons, suggesting active correction mechanisms. Moreover, we find that Schwann cells are enriched and associate with axons at the plexus, and that Schwann cell ablation during regeneration causes profound axonal mistargeting. Our data provide the first real-time account of regenerating vertebrate motor axons navigating a nerve plexus and reveal a previously unappreciated role for Schwann cells to promote axon sorting at a plexus during regeneration.

nNOS-induced tyrosine nitration of TRKB impairs BDNF signaling and restrains neuronal plasticity

Nitric oxide (NO) has been long recognized as an important modulator of neural plasticity, but characterization of the molecular mechanisms involved - specially the guanylyl cyclase-independent ones - has been challenging. There is evidence that NO could modify BDNF-TRKB signaling, a key mediator of neuronal plasticity. However, the mechanism underlying the interplay of NO and TRKB remains unclear. Here we show that NO induces nitration of the tyrosine 816 in the TRKB receptor in vivo and in vitro, and that post-translational modification inhibits TRKB phosphorylation and binding of phospholipase Cγ1 (PLCγ1) to this same tyrosine residue. Additionally, nitration triggers clathrin-dependent endocytosis of TRKB through the adaptor protein AP-2 and ubiquitination, thereby increasing translocation of TRKB away from the neuronal surface and directing it towards lysosomal degradation. Accordingly, inhibition of nitric oxide increases TRKB phosphorylation and TRKB-dependent neurite branching in neuronal cultures. In vivo, chronic inhibition of neuronal nitric oxide synthase (nNOS) dramatically reduced TRKB nitration and facilitated TRKB signaling in the visual cortex, and promoted a shift in ocular dominance upon monocular deprivation - an indicator of increased plasticity. Altogether, our data describe and characterize a new molecular brake on plasticity, namely nitration of TRKB receptors.

A glia cell dependent mechanism at a peripheral nerve plexus critical for target-selective axon regeneration

A critical step for functional recovery from peripheral nerve injury is for regenerating axons to connect with their pre-injury targets. Reestablishing pre-injury target specificity is particularly challenging for limb-innervating axons as they encounter a plexus, a network where peripheral nerves converge, axons from different nerves intermingle, and then re-sort into target-specific bundles. Here, we examine this process at a plexus located at the base of the zebrafish pectoral fin, equivalent to tetrapod forelimbs. Using live cell imaging and sparse axon labeling, we find that regenerating motor axons from three nerves coalesce into the plexus. There, they intermingle and sort into distinct branches, and then navigate to their original muscle domains with high fidelity that restores functionality. We demonstrate that this regeneration process includes selective retraction of mistargeted axons, suggesting active correction mechanisms. Moreover, we find that Schwann cells are enriched and associate with axons at the plexus, and that Schwann cell ablation during regeneration causes profound axonal mistargeting. Our data provide the first real time account of regenerating vertebrate motor axons navigating a nerve plexus and reveal a previously unappreciated role for Schwann cells to promote axon sorting at a plexus during regeneration.

Transcriptome analysis in mice treated with vigabatrin identifies dysregulation of genes associated with retinal signaling circuitry

Vigabatrin (VGB; γ-vinyl-GABA) is an antiepileptic drug that elevates CNS GABA via irreversible inactivation of the GABA catabolic enzyme GABA-transaminase. VGB's clinical utility, however, can be curtailed by peripheral visual field constriction (pVFC) and thinning of the retinal nerve fiber layer (RNFL). Earlier studies from our laboratory revealed disruptions of autophagy by VGB. Here, we tested the hypothesis that VGB administration to animals would reveal alterations of gene expression in VGB-treated retina that associated with autophagy. VGB (140 mg/kg/d; subcutaneous minipump) was continuously administered to mice (n = 6 each VGB/vehicle) for 12 days, after which animals were euthanized. Retina was isolated for transcriptome (RNAseq) analysis and further validation using qRT-PCR and immunohistochemistry (IHC). For 112 differentially expressed retinal genes (RNAseq), two databases (Gene Ontology; Kyoto Encyclopedia of Genes and Genomes) were used to identify genes associated with visual function. Twenty four genes were subjected to qRT-PCR validation, and five (Gb5, Bdnf, Cplx9, Crh, Sox9) revealed significant dysregulation. IHC of fixed retinas verified significant down-regulation of Gb5 in photoreceptor cells. All of these genes have been previously shown to play a role in retinal function/circuitry signaling. Minimal impact of VGB on retinal autophagic gene expression was observed. This is the first transcriptome analysis of retinal gene expression associated with VGB intake, highlighting potential novel molecular targets potentially related to VGB's well known ocular toxicity.

Regulation of nerve growth and patterning by cell surface protein disulphide isomerase

Contact repulsion of growing axons is an essential mechanism for spinal nerve patterning. In birds and mammals the embryonic somites generate a linear series of impenetrable barriers, forcing axon growth cones to traverse one half of each somite as they extend towards their body targets. This study shows that protein disulphide isomerase provides a key component of these barriers, mediating contact repulsion at the cell surface in chick half-somites. Repulsion is reduced both in vivo and in vitro by a range of methods that inhibit enzyme activity. The activity is critical in initiating a nitric oxide/S-nitrosylation-dependent signal transduction pathway that regulates the growth cone cytoskeleton. Rat forebrain grey matter extracts contain a similar activity, and the enzyme is expressed at the surface of cultured human astrocytic cells and rat cortical astrocytes. We suggest this system is co-opted in the brain to counteract and regulate aberrant nerve terminal growth.

BDNF: A Key Factor with Multipotent Impact on Brain Signaling and Synaptic Plasticity

Brain-derived neurotrophic factor (BDNF) is one of the most widely distributed and extensively studied neurotrophins in the mammalian brain. Among its prominent functions, one can mention control of neuronal and glial development, neuroprotection, and modulation of both short- and long-lasting synaptic interactions, which are critical for cognition and memory. A wide spectrum of processes are controlled by BDNF, and the sometimes contradictory effects of its action can be explained based on its specific pattern of synthesis, comprising several intermediate biologically active isoforms that bind to different types of receptor, triggering several signaling pathways. The functions of BDNF must be discussed in close relation to the stage of brain development, the different cellular components of nervous tissue, as well as the molecular mechanisms of signal transduction activated under physiological and pathological conditions. In this review, we briefly summarize the current state of knowledge regarding the impact of BDNF on regulation of neurophysiological processes. The importance of BDNF for future studies aimed at disclosing mechanisms of activation of signaling pathways, neuro- and gliogenesis, as well as synaptic plasticity is highlighted.

Uncovering the regeneration strategies of zebrafish organs: a comprehensive systems biology study on heart, cerebellum, fin, and retina regeneration

Background

Regeneration is an important biological process for the restoration of organ mass, structure, and function after damage, and involves complex bio-physiological mechanisms including cell differentiation and immune responses. We constructed four regenerative protein-protein interaction (PPI) networks using dynamic models and AIC (Akaike’s Information Criterion), based on time-course microarray data from the regeneration of four zebrafish organs: heart, cerebellum, fin, and retina. We extracted core and organ-specific proteins, and proposed a recalled-blastema-like formation model to uncover regeneration strategies in zebrafish.

Results

It was observed that the core proteins were involved in TGF-β signaling for each step in the recalled-blastema-like formation model and TGF-β signaling may be vital for regeneration. Integrins, FGF, and PDGF accelerate hemostasis during heart injury, while Bdnf shields retinal neurons from secondary damage and augments survival during the injury response. Wnt signaling mediates the growth and differentiation of cerebellum and fin neural stem cells, potentially providing a signal to trigger differentiation.

Conclusion

Through our analysis of all four zebrafish regenerative PPI networks, we provide insights that uncover the underlying strategies of zebrafish organ regeneration.

Differential visual system organization and susceptibility to experimental models of optic neuropathies in three commonly used mouse strains

Mouse disease models have proven indispensable in glaucoma research, yet the complexity of the vast number of models and mouse strains has also led to confusing findings. In this study, we evaluated baseline intraocular pressure, retinal histology, and retinofugal projections in three mouse strains commonly used in glaucoma research, i.e. C57Bl/6, C57Bl/6-Tyr(c), and CD-1 mice. We found that the mouse strains under study do not only display moderate variations in their intraocular pressure, retinal architecture, and retinal ganglion cell density, also the retinofugal projections to the dorsal lateral geniculate nucleus and the superior colliculus revealed striking differences, potentially underlying diverging optokinetic tracking responses and visual acuity. Next, we reviewed the success rate of three models of (glaucomatous) optic neuropathies (intravitreal N-methyl-d-aspartic acid injection, optic nerve crush, and laser photocoagulation-induced ocular hypertension), looking for differences in disease susceptibility between these mouse strains. Different genetic backgrounds and albinism led to differential susceptibility to experimentally induced retinal ganglion cell death among these three mouse strains. Overall, CD-1 mice appeared to have the highest sensitivity to retinal ganglion cell damage, while the C57Bl/6 background was more resistant in the three models used.

BDNF-induced nitric oxide signals in cultured rat hippocampal neurons: time course, mechanism of generation, and effect on neurotrophin secretion

BDNF and nitric oxide signaling both contribute to plasticity at glutamatergic synapses. However, the role of combined signaling of both pathways at the same synapse is largely unknown. Using NO imaging with diaminofluoresceine in cultured hippocampal neurons we analyzed the time course of neurotrophin-induced NO signals. Application of exogenous BDNF, NT-4, and NT-3 (but not NGF) induced NO signals in the soma and in proximal dendrites of hippocampal neurons that were sensitive to NO synthase activity, TrkB signaling, and intracellular calcium elevation. The effect of NO signaling on neurotrophin secretion was analyzed in BDNF-GFP, and NT-3-GFP transfected hippocampal neurons. Exogenous application of the NO donor sodium-nitroprusside markedly inhibited neurotrophin secretion. However, endogenously generated NO in response to depolarization and neurotrophin stimulation, both did not result in a negative feedback on neurotrophin secretion. These results suggest that a negative feedback of NO signaling on synaptic secretion of neurotrophins operates only at high intracellular levels of nitric oxide that are under physiological conditions not reached by depolarization or BDNF signaling.

Cannabinoid-induced actomyosin contractility shapes neuronal morphology and growth

Endocannabinoids are recently recognized regulators of brain development, but molecular effectors downstream of type-1 cannabinoid receptor (CB1R)-activation remain incompletely understood. We report atypical coupling of neuronal CB1Rs, after activation by endo- or exocannabinoids such as the marijuana component ∆(9)-tetrahydrocannabinol, to heterotrimeric G12/G13 proteins that triggers rapid and reversible non-muscle myosin II (NM II) dependent contraction of the actomyosin cytoskeleton, through a Rho-GTPase and Rho-associated kinase (ROCK). This induces rapid neuronal remodeling, such as retraction of neurites and axonal growth cones, elevated neuronal rigidity, and reshaping of somatodendritic morphology. Chronic pharmacological inhibition of NM II prevents cannabinoid-induced reduction of dendritic development in vitro and leads, similarly to blockade of endocannabinoid action, to excessive growth of corticofugal axons into the sub-ventricular zone in vivo. Our results suggest that CB1R can rapidly transform the neuronal cytoskeleton through actomyosin contractility, resulting in cellular remodeling events ultimately able to affect the brain architecture and wiring.

Retrograde response in axotomized motoneurons: nitric oxide as a key player in triggering reversion toward a dedifferentiated phenotype

The adult brain retains a considerable capacity to functionally reorganize its circuits, which mainly relies on the prevalence of three basic processes that confer plastic potential: synaptic plasticity, plastic changes in intrinsic excitability and, in certain central nervous system (CNS) regions, also neurogenesis. Experimental models of peripheral nerve injury have provided a useful paradigm for studying injury-induced mechanisms of central plasticity. In particular, axotomy of somatic motoneurons triggers a robust retrograde reaction in the CNS, characterized by the expression of plastic changes affecting motoneurons, their synaptic inputs and surrounding glia. Axotomized motoneurons undergo a reprograming of their gene expression and biosynthetic machineries which produce cell components required for axonal regrowth and lead them to resume a functionally dedifferentiated phenotype characterized by the removal of afferent synaptic contacts, atrophy of dendritic arbors and an enhanced somato-dendritic excitability. Although experimental research has provided valuable clues to unravel many basic aspects of this central response, we are still lacking detailed information on the cellular/molecular mechanisms underlying its expression. It becomes clear, however, that the state-switch must be orchestrated by motoneuron-derived signals produced under the direction of the re-activated growth program. Our group has identified the highly reactive gas nitric oxide (NO) as one of these signals, by providing robust evidence for its key role to induce synapse elimination and increases in intrinsic excitability following motor axon damage. We have elucidated operational principles of the NO-triggered downstream transduction pathways mediating each of these changes. Our findings further demonstrate that de novo NO synthesis is not only "necessary" but also "sufficient" to promote the expression of at least some of the features that reflect reversion toward a dedifferentiated state in axotomized adult motoneurons.

Effects of nitric oxide on neuromuscular properties of developing zebrafish embryos

Nitric oxide is a bioactive signalling molecule that is known to affect a wide range of neurodevelopmental processes. However, its functional relevance to neuromuscular development is not fully understood. Here we have examined developmental roles of nitric oxide during formation and maturation of neuromuscular contacts in zebrafish. Using histochemical approaches we show that elevating nitric oxide levels reduces the number of neuromuscular synapses within the axial swimming muscles whilst inhibition of nitric oxide biosynthesis has the opposite effect. We further show that nitric oxide signalling does not change synapse density, suggesting that the observed effects are a consequence of previously reported changes in motor axon branch formation. Moreover, we have used in vivo patch clamp electrophysiology to examine the effects of nitric oxide on physiological maturation of zebrafish neuromuscular junctions. We show that developmental exposure to nitric oxide affects the kinetics of spontaneous miniature end plate currents and impacts the neuromuscular drive for locomotion. Taken together, our findings implicate nitrergic signalling in the regulation of zebrafish neuromuscular development and locomotor maturation.

Nitric oxide in the nervous system: biochemical, developmental, and neurobiological aspects

Nitric oxide (NO) is a very reactive molecule, and its short half-life would make it virtually invisible until its discovery. NO activates soluble guanylyl cyclase (sGC), increasing 3',5'-cyclic guanosine monophosphate levels to activate PKGs. Although NO triggers several phosphorylation cascades due to its ability to react with Fe II in heme-containing proteins such as sGC, it also promotes a selective posttranslational modification in cysteine residues by S-nitrosylation, impacting on protein function, stability, and allocation. In the central nervous system (CNS), NO synthesis usually requires a functional coupling of nitric oxide synthase I (NOS I) and proteins such as NMDA receptors or carboxyl-terminal PDZ ligand of NOS (CAPON), which is critical for specificity and triggering of selected pathways. NO also modulates CREB (cAMP-responsive element-binding protein), ERK, AKT, and Src, with important implications for nerve cell survival and differentiation. Differences in the regulation of neuronal death or survival by NO may be explained by several mechanisms involving localization of NOS isoforms, amount of NO being produced or protein sets being modulated. A number of studies show that NO regulates neurotransmitter release and different aspects of synaptic dynamics, such as differentiation of synaptic specializations, microtubule dynamics, architecture of synaptic protein organization, and modulation of synaptic efficacy. NO has also been associated with synaptogenesis or synapse elimination, and it is required for long-term synaptic modifications taking place in axons or dendrites. In spite of tremendous advances in the knowledge of NO biological effects, a full description of its role in the CNS is far from being completely elucidated.

Mitochondrial Dynamics in Retinal Ganglion Cell Axon Regeneration and Growth Cone Guidance

Failed axon regeneration and retinal ganglion cell (RGC) death after trauma or disease, including glaucomatous and mitochondrial optic neuropathies, are increasingly linked to mitochondrial dysfunction. Mitochondria are highly dynamic organelles whose size, organization, and function are regulated by a balance between mitochondrial fission and fusion. Mitochondria are ubiquitous in axonal growth cones both in vitro and in vivo and during development and regeneration. However, the roles that mitochondrial fission and fusion dynamics play in the growth cone during axon regeneration are largely unstudied. Here we discuss recent data suggesting mitochondria in the distal axon and growth cone play a central role in axon growth by integrating intrinsic axon growth states with signaling from extrinsic cues. Mitochondrial fission and fusion are intrinsically regulated in the distal axon in the growth cones of acutely purified embryonic and postnatal RGCs with differing intrinsic axon growth potentials. These differences in fission and fusion correlate with differences in mitochondrial bioenergetics; embryonic RGCs with high intrinsic axon growth potential rely more on glycolysis whereas RGCs with low intrinsic axon growth potential rely more on oxidative phosphorylation. Mitochondrial fission and fusion are also differentially modulated by KLFs that either promote or suppress intrinsic axon growth, and altering the balance between mitochondrial fission and fusion can differentially regulate axon growth rate and growth cone guidance responses to both inhibitory and permissive guidance cues.

Arachidonic acid and lipoxin A4 as possible endogenous anti-diabetic molecules

In both type 1 and type 2 diabetes mellitus, increased production of pro-inflammatory cytokines and reactive oxygen species (ROS) occurs that induce apoptosis of β cells and cause peripheral insulin resistance respectively though the degree of their increased production is higher in type 1 and less in type 2 diabetes mellitus. Despite this, the exact mechanism(s) that lead to increased production of pro-inflammatory cytokines: interleukin-6 (IL-6) and tumor necrosis factor-α (TNF-α) and ROS is not known. Studies showed that plasma concentrations of arachidonic acid (AA) and lipoxin A4 (LXA4) are low in alloxan-induced type 1 diabetes mellitus in experimental animals and patients with type 2 diabetes mellitus. Prior administration of AA, eicosapentaenoic and docosahexaenoic acids (EPA and DHA, respectively) and transgenic animals that produce increased amounts of EPA and DHA acids were protected from chemical-induced diabetes mellitus that was associated with enhanced formation of LXA4 and resolvins, while protectin D1 ameliorated peripheral insulin resistance. AA, LXA4, resolvins and protectins inhibit IL-6 and TNF-α production and suppress ROS generation. Thus, AA and lipoxins, resolvins and protectins may function as endogenous anti-diabetic molecules implying that their administration could be useful in the prevention and management of both types of diabetes mellitus.

The expression mechanism of the residual LTP in the CA1 region of BDNF k.o. mice is insensitive to NO synthase inhibition

BDNF and nitric oxide signaling both contribute to long-term potentiation (LTP) at glutamatergic synapses, but to date, few studies analyzed the interaction of both signaling cascades in the same synaptic pathway. Here we addressed the question whether the residual LTP in the CA1 region of hippocampal slices from heterozygous BDNF knockout mice (BDNF⁺/⁻) is dependent on nitric oxide (NO) signaling. Extracellular recording of synaptic field potentials elicited by presynaptic Schaffer collateral stimulation was performed in the CA1 region of hippocampal slices of 4- to 6-week-old mice, and LTP was induced by a theta burst stimulation protocol. Application of the nitric oxide inhibitor L-NAME (200 μM) strongly inhibited LTP by 70% in wildtype animals. This inhibition of LTP was not a consequence of altered basal synaptic properties. In CA1 of BDNF⁺/⁻ mice, stimulated with the same theta burst protocol, LTP was reduced by 50% as compared to wildtype animals. This impairment in the expression of LTP in BDNF⁺/⁻ mice did not result from an increased synaptic fatigue. The residual LTP in BDNF⁺/⁻ was not further reduced by preincubation of slices with L-NAME. These results suggest that BDNF and NO share overlapping intracellular signaling cascades to mediate LTP in CA1, and part of their signaling cascades are most likely arranged consecutively in the signaling pathway mediating LTP.

NO orchestrates the loss of synaptic boutons from adult "sick" motoneurons: modeling a molecular mechanism

Synapse elimination is the main factor responsible for the cognitive decline accompanying many of the neuropathological conditions affecting humans. Synaptic stripping of motoneurons is also a common hallmark of several motor pathologies. Therefore, knowledge of the molecular basis underlying this plastic process is of central interest for the development of new therapeutic tools. Recent advances from our group highlight the role of nitric oxide (NO) as a key molecule triggering synapse loss in two models of motor pathologies. De novo expression of the neuronal isoform of NO synthase (nNOS) in motoneurons commonly occurs in response to the physical injury of a motor nerve and in the course of amyotrophic lateral sclerosis. In both conditions, this event precedes synaptic withdrawal from motoneurons. Strikingly, nNOS-synthesized NO is "necessary" and "sufficient" to induce synaptic detachment from motoneurons. The mechanism involves a paracrine/retrograde action of NO on pre-synaptic structures, initiating a downstream signaling cascade that includes sequential activation of (1) soluble guanylyl cyclase, (2) cyclic guanosine monophosphate-dependent protein kinase, and (3) RhoA/Rho kinase (ROCK) signaling. Finally, ROCK activation promotes phosphorylation of regulatory myosin light chain, which leads to myosin activation and actomyosin contraction. This latter event presumably contributes to the contractile force to produce ending axon retraction. Several findings support that this mechanism may operate in the most prevalent neurodegenerative diseases.

Spinal motoneuron synaptic plasticity after axotomy in the absence of inducible nitric oxide synthase

Background

Astrocytes play a major role in preserving and restoring structural and physiological integrity following injury to the nervous system. After peripheral axotomy, reactive gliosis propagates within adjacent spinal segments, influenced by the local synthesis of nitric oxide (NO). The present work investigated the importance of inducible nitric oxide synthase (iNOS) activity in acute and late glial responses after injury and in major histocompatibility complex class I (MHC I) expression and synaptic plasticity of inputs to lesioned alpha motoneurons.

Methods

In vivo analyses were carried out using C57BL/6J-iNOS knockout (iNOS^-/-) and C57BL/6J mice. Glial response after axotomy, glial MHC I expression, and the effects of axotomy on synaptic contacts were measured using immunohistochemistry and transmission electron microscopy. For this purpose, 2-month-old animals were sacrificed and fixed one or two weeks after unilateral sciatic nerve transection, and spinal cord sections were incubated with antibodies against classical MHC I, GFAP (glial fibrillary acidic protein - an astroglial marker), Iba-1 (an ionized calcium binding adaptor protein and a microglial marker) or synaptophysin (a presynaptic terminal marker). Western blotting analysis of MHC I and nNOS expression one week after lesion were also performed. The data were analyzed using a two-tailed Student's t test for parametric data or a two-tailed Mann-Whitney U test for nonparametric data.

Results

A statistical difference was shown with respect to astrogliosis between strains at the different time points studied. Also, MHC I expression by iNOS^-/- microglial cells did not increase at one or two weeks after unilateral axotomy. There was a difference in synaptophysin expression reflecting synaptic elimination, in which iNOS^-/- mice displayed a decreased number of the inputs to alpha motoneurons, in comparison to that of C57BL/6J.

Conclusion

The findings herein indicate that iNOS isoform activity influences MHC I expression by microglial cells one and two weeks after axotomy. This finding was associated with differences in astrogliosis, number of presynaptic terminals and synaptic covering of alpha motoneurons after lesioning in the mutant mice.

Elevated expression of CAPON and neuronal nitric oxide synthase in the sciatic nerve of rats following constriction injury

Neuronal nitric oxide synthase (nNOS) has been implicated in peripheral nerve lesions and regeneration. The CAPON adaptor protein interacts with the PDZ domain of nNOS, helping to regulate nNOS activity at post-synaptic sites in neurones, but it is not known whether its expression is altered in sciatic nerves after chronic nerve constriction injury. In the present study, the spatiotemporal expression of CAPON was determined in chronically constricted rat sciatic nerves. Similar to the level of protein expression, CAPON mRNA was significantly up-regulated for almost 5weeks following sciatic nerve injury. Immunohistochemistry demonstrated that increased CAPON was found mainly in S-100-positive Schwann cells. In addition, co-immunoprecipitation demonstrated an interaction between CAPON and nNOS in Schwann cells and the interaction was enhanced in injured sciatic nerves. CAPON may be involved in peripheral nerve regeneration through regulation of nNOS activity.

Treadmill running improves long-term potentiation (LTP) defects in streptozotocin-induced diabetes at dentate gyrus in rats

OBJECTIVES

It has been demonstrated that exercise has neuroprotective effects in the central nervous system (CNS), especially in hippocampus. Previous studies have indicated that diabetes mellitus affects synaptic plasticity in the hippocampus leading to impairments in learning and memory. The aim of this study was to evaluate the effects of treadmill running on synaptic plasticity at dentate gyrus (DG) of streptozotocin-induced diabetic rats.

STUDY DESIGN

Experimental groups were the control, the diabetes and the diabetes-exercise groups. Long-term potentiation (LTP) in perforant path-DG synapses was assessed (by 400Hz tetanization) in order to investigate the effect of exercise on synaptic plasticity. Field excitatory post-synaptic potential (fEPSP) slope and population spike (PS) amplitude were measured.

RESULTS

With respect to the control group, fEPSP were significantly decreased in the diabetes group. However, there were no differences between responses of the diabetes-exercise group and the control.

CONCLUSION

The present results suggest that LTP induction in the dentate gyrus is affected under diabetic conditions and that treadmill running prevents these effects. The data suggest that treadmill running protect against diabetes-induced decrease of learning ability and memory function of the hippocampus.

Key roles of Ephs and ephrins in retinotectal topographic map formation

Cellular and molecular mechanisms involved in the development of topographic ordered connections in the central nervous system (CNS) constitute a key issue in neurobiology because neural connectivities are the base of the CNS normal function. We discuss the roles of the Eph/ephrin system in the establishment of retinotopic projections onto the tectum/colliculus, the most detailed studied model of topographic mapping. The expression patterns of Ephs and ephrins in opposing gradients both in the retina and the tectum/colliculus, label the local addresses on the target and give specific sensitivities to growth cones according to their topographic origin in the retina. We postulate that the highest levels of these gradients could signal both the entry as well as the limiting boundaries of the target. Since Ephs and ephrins are membrane-bound molecules, they may function as both receptors and ligands producing repulsive or attractant responses according to their microenvironment and play central roles in a variety of developmental events such as axon guidance, synapse formation and remodeling. Due to different experimental approaches and the inherent species-specific differences, some results appear contradictory and should be reanalyzed. Nevertheless, these studies about the roles of the Eph/ephrin system in retinotectal/collicular mapping support general principles in order to understand CNS development and could be useful to design regeneration therapies.

Effects of treadmill running on spatial learning and memory in streptozotocin-induced diabetic rats

Previous studies have shown an association between diabetes mellitus and impairments in learning and memory. These deficits were partially reversed by the use of insulin. Due to the fact that exercise has positive effects on many physiological systems, including the central nervous system, the present study, evaluated the effects of treadmill running on spatial learning and memory in streptozotocin (STZ)-induced diabetic rats. The exercise program was treadmill running at 17 meters per minute (m/min) at 0 degrees inclination for 40 minutes per day (min/day), 7 days/week, for 12 weeks. Experimental groups were: the control-rest, the control-exercise, the diabetes-rest and the diabetes-exercise. Spatial learning and memory was investigated by Morris water maze test in the rats after 12 weeks of diabetes induction and the exercise period. Our data showed that spatial learning and memory was significantly impaired in the diabetes-rest group with respect to the control-rest group. However, there were no differences between the other groups. The present results suggest that spatial learning and memory is affected under diabetic conditions and that treadmill running prevents these effects. The data correspond to the possibility that treadmill running is helpful in the prevention and alleviation of the cognitive decline in diabetes mellitus.

Changes in mRNA for CAPON and Dexras1 in adult rat following sciatic nerve transection

Peripheral nerve transection has been implicated to cause a production of neuronal nitric oxide synthase (nNOS), which may influence a range of post-axotomy processes necessary for neuronal survival and nerve regeneration. Carboxy-terminal post synaptic density protein/Drosophila disc large tumor suppressor/zonula occuldens-1 protein (PDZ) ligand of neuronal nitric oxide synthase (CAPON), as an adaptor, interacts with nNOS via the PDZ domain helping regulate nNOS activity at postsynaptic sites in neurons. And Dexras1, a small G protein mediating multiple signal transductions, has been reported to form a complex with CAPON and nNOS. A role for the physiologic linkage by CAPON of nNOS to Dexras1 has suggested that NO-mediated activation of Dexras1 is markedly enhanced by CAPON. We investigated the changes in mRNA for CAPON, Dexras1 and nNOS in the sciatic nerve, dorsal root ganglia and lumbar spinal cord of adult rat following sciatic axotomy by TaqMan quantitative real-time PCR and in situ hybridization combined with immunofluorescence. Signals of mRNA for CAPON and Dexras1 were initially expressed in these neural tissues mentioned, transiently increased at certain time periods after sciatic axotomy and finally recovered to the basal level. It was also found that nNOS mRNA underwent a similar change pattern during this process. These results suggest that CAPON as well as Dexras1 may be involved in the different pathological conditions including nerve regeneration, neuron loss or survival and even pain process, possibly via regulating the nNOS activity or through the downstream targets of Dexras1.

Kinesin-5 regulates the growth of the axon by acting as a brake on its microtubule array

Kinesin-5 is a homotetrameric motor protein that interacts with adjacent microtubules in the mitotic spindle. Kinesin-5 is also highly expressed in developing postmitotic neurons. Axons of cultured neurons experimentally depleted of kinesin-5 grow up to five times longer than controls and display more branches. The faster growth rates are accompanied by a doubling of the frequency of transport of short microtubules, suggesting a major role for kinesin-5 in the balance of motor-driven forces on the axonal microtubule array. Live-cell imaging reveals that the effects on axonal length of kinesin-5 depletion are caused partly by a lower propensity of the axon and newly forming branches to undergo bouts of retraction. Overexpression of wild-type kinesin-5, but not a rigor mutant of kinesin-5, has the inverse effect on axonal length. These results indicate that kinesin-5 imposes restrictions on the growth of the axon and does so at least in part by generating forces on the axonal microtubule array.

Effect of peripheral axotomy on gene expression of NIDD in rat neural tissues

Peripheral nerve lesion-induced production of neuronal nitric oxide synthase (nNOS) was implicated to influence a range of postaxotomy processes necessary for neuronal survival and nerve regeneration (Zochodne et al., Neuroscience, 91:1515-1527, 1999; Keilhoff et al., Journal of Chemical Neuroanatomy, 24:181-187, 2002, Nitric Oxide, 10:101-111, 2004). Protein-protein interactions represent an important mechanism in the control of NOS spatial distribution or activity (Alderton et al., Biochemical Journal, 357:593-615, 2001; Dedio et al., FASEB Journal, 15:79-89, 2001; Zimmermann et al., Proceedings of the National Academy of Sciences, 99:17167-17172, 2002). As one of the nNOS-binding proteins, nNOS-interacting DHHC domain-containing protein with dendritic mRNA (NIDD) has recently been identified to increase nNOS enzyme activity by targeting nNOS to the synaptic plasma membrane in a postsynaptic density protein 95/discs-large/zona occlusens-1 domain dependent manner (Saitoh et al., Journal of Biological Chemistry, 279:29461-29468, 2004). In this paper, we established a rat model with peripheral axotomy to investigate the gene expression patterns of NIDD in neural tissues using TaqMan quantitative real-time polymerase chain reaction and in situ hybridization combined with immunofluorescence. It revealed that NIDD mRNA was upregulated after sciatic nerve transection with the similar expressing styles as that of the nNOS in the injured nerves, corresponding dorsal root ganglia, and lumbar spinal cord. These findings imply that NIDD may be involved in the different pathological conditions including nerve regeneration, neuron loss or survival, and even pain process, possibly via regulating the enzyme nNOS activity.

Nitric oxide regulates growth cone filopodial dynamics via ryanodine receptor-mediated calcium release

Nitric oxide (NO) is a gaseous intercellular messenger involved in numerous processes during development, including wiring of the nervous system. Neuronal growth cones are responsible for establishing the correct connectivity in the nervous system, but how NO might affect neuronal pathfinding is not fully understood. We have demonstrated in a previous study that local application of a NO donor, NOC-7, via micropipette onto individual growth cones from Helisoma trivolvis B5 neurons results in an increase in filopodial length, a decrease in filopodial number and an increase in the intracellular calcium concentration ([Ca(2+)](i)). Moreover, these NO-induced effects were demonstrated to be mediated via an intracellular cascade involving soluble guanylyl cyclase, protein kinase G (PKG) and cyclic adenosine diphosphate ribose (cADPR). We now demonstrate that the increase in the [Ca(2+)](i) that results from local NO application is mediated via release from ryanodine receptor (RyR)-sensitive intracellular stores. We also show that PKG and RyRs are localized within growth cones and microinjection of cADPR mimics the effects of NO, providing further support that the NO-induced effects are mediated via cADPR. Lastly, we provide evidence that calcium influx across the plasma membrane is a necessary component of the NO-induced calcium increase; however, this calcium influx is secondary to the RyR-induced calcium release from intracellular stores. This study details a signalling pathway by which NO can cause changes in growth cone morphology and thus provides a mechanism by which NO could affect neuronal wiring by acting locally on individual growth cones during the pathfinding process.

S-nitrosylation of microtubule-associated protein 1B mediates nitric-oxide-induced axon retraction

Treatment of cultured vertebrate neurons with nitric oxide leads to growth-cone collapse, axon retraction and the reconfiguration of axonal microtubules. We show that the light chain of microtubule-associated protein (MAP) 1B is a substrate for S-nitrosylation in vivo, in cultured cells and in vitro. S-nitrosylation occurs at Cys 2457 in the COOH terminus. Nitrosylation of MAP1B leads to enhanced interaction with microtubules and correlates with the inhibition of neuroblastoma cell differentiation. We further show, in dorsal root ganglion neurons, that MAP1B is necessary for neuronal nitric oxide synthase control of growth-cone size, growth-cone collapse and axon retraction. These results reveal an S-nitrosylation-dependent signal-transduction pathway that is involved in regulation of the axonal cytoskeleton and identify MAP1B as a major component of this pathway. We propose that MAP1B acts by inhibiting a microtubule- and dynein-based mechanism that normally prevents axon retraction.

Embryonic motor neuron dendrite growth is stunted by inhibition of nitric oxide-dependent activation of soluble guanylyl cyclase and protein kinase G

We have examined the participation of a neuronal nitric oxide synthase (nNOS) signaling pathway in the elaboration of motor neuron dendrites during embryonic life. During chick embryogenesis, nNOS is expressed by interneurons that surround the motor neuron pools in the ventral horn. Pseudorabies virus tracing suggests that these cells, while juxtaposed to motor neurons are not synaptically connected to them. The downstream effectors, soluble guanylyl cyclase (sGC) and protein kinase G (PKG), are found in motor neurons as well as several other populations of spinal cord cells. To determine the functional significance of the nNOS/sGC/PKG signaling pathway, pharmacological inhibitors were applied to chick embryos and the effects on motor neuron dendrites monitored. Inhibition of nNOS activity led to a lasting reduction in the overall size and degree of branching of the dendritic tree. These alterations in dendritic architecture were also seen when the activity of sGC or PKG was blocked. Our results suggest that normal motor neuron dendrite elaboration depends, in part, on the activity-dependent generation of NO by ventral horn interneurons, which then activates sGC and PKG in motor neurons.

Nitric oxide synthase protein levels, not the mRNA, are downregulated in olfactory bulb interneurons of reeler mice

Homozygous mutations in the Reelin gene result in severe disruption of brain development. The histogenesis of layered regions, like the neocortex, hippocampus and the cerebellum, is most notably affected in mouse reeler mutants and similar traits are also present in mice lacking molecular components of the Reelin signalling pathway. Moreover, there is evidence for an additional role of Reelin in sustaining synaptic plasticity in adult networks. Nitric oxide is an important gaseous messenger that can modulate neuronal plasticity both in developing and mature synaptic networks and has been shown to facilitate synaptic changes in the hippocampus, cerebellum and olfactory bulb. We studied the distribution and content of neuronal nitric oxide synthase in the olfactory bulbs of reeler and wildtype mice. Immunocytochemistry reveals that Reelin and neuronal nitric oxide synthase containing interneurons are two distinct, non overlapping cell populations of the olfactory bulb. We show by in situ hybridization that both nitrergic and Reelin expressing cells represent only a subset of olfactory bulb GABAergic neurons. Immunoblots show that neuronal nitric oxide synthase protein content is decreased by two thirds in reeler mice causing a detectable loss of immunolabelled cells throughout the olfactory bulb of this strain. However, neuronal nitric oxide synthase mRNA levels, essayed by quantitative real-time RT-PCR, are unaffected in the reeler olfactory bulb. Thus, disruption of the Reelin signalling pathway may modify the turnover of neuronal nitric oxide synthase in the olfactory bulb and possibly affects nitric oxide functions in reeler mice.

Developmental regulation of sensory axon regeneration in the absence of growth cones

The actin filament (F-actin) cytoskeleton is thought to be required for normal axon extension during embryonic development. Whether this is true of axon regeneration in the mature nervous system is not known, but a progressive simplification of growth cones during development has been described and where specifically investigated, mature spinal cord axons appear to regenerate without growth cones. We have studied the cytoskeletal mechanisms of axon regeneration in developmentally early and late chicken sensory neurons, at embryonic day (E) 7 and 14 respectively. Depletion of F-actin blocked the regeneration of E7 but not E14 sensory axons in vitro. The differential sensitivity of axon regeneration to the loss of F-actin and growth cones correlated with endogenous levels of F-actin and growth cone morphology. The growth cones of E7 axons contained more F-actin and were more elaborate than those of E14 axons. The ability of E14 axons to regenerate in the absence of F-actin and growth cones was dependent on microtubule tip polymerization. Importantly, while the regeneration of E7 axons was strictly dependent on F-actin, regeneration of E14 axons was more dependent on microtubule tip polymerization. Furthermore, E14 axons exhibited altered microtubule polymerization relative to E7, as determined by imaging of microtubule tip polymerization in living neurons. These data indicate that the mechanism of axon regeneration undergoes a developmental switch between E7 and E14 from strict dependence on F-actin to a greater dependence on microtubule polymerization. Collectively, these experiments indicate that microtubule polymerization may be a therapeutic target for promoting regeneration of mature neurons.

Exuberance in the development of cortical networks

The cerebral cortex is the largest and most intricately connected part of the mammalian brain. Its size and complexity has increased during the course of evolution, allowing improvements in old functions and causing the emergence of new ones, such as language. This has expanded the behavioural and cognitive repertoire of different species and has determined their competitive success. To allow the relatively rapid emergence of large evolutionary changes in a structure of such importance and complexity, the mechanisms by which cortical circuitry develops must be flexible and yet robust against changes that could disrupt the normal functions of the networks.

Calcium signaling in neuronal motility

Neuronal motility is a fundamental feature that underlies the development, regeneration, and plasticity of the nervous system. Two major developmental events--directed migration of neuronal precursor cells to the proper positions and guided elongation of axons to their target cells--depend on large-scale neuronal motility. At a finer scale, motility is also manifested in many aspects of neuronal structures and functions, ranging from differentiation and refinement of axonal and dendritic morphology during development to synapse remodeling associated with learning and memory in the adult brain. As a primary second messenger that conveys the cytoplasmic actions of electrical activity and many neuroactive ligands, Ca(2+) plays a central role in the regulation of neuronal motility. Recent studies have revealed common Ca(2+)-dependent signaling pathways that are deployed for regulating cytoskeletal dynamics associated with neuronal migration, axon and dendrite development and regeneration, and synaptic plasticity.

RhoA-kinase and myosin II are required for the maintenance of growth cone polarity and guidance by nerve growth factor

Growth cones are highly polarized and dynamic structures confined to the tips of axons. The polarity of growth cones is in part maintained by suppression of protrusive activity from the distal axon shaft, a process termed axon consolidation. The mechanistic basis of axon consolidation that contributes to the maintenance of growth cone polarity is not clear. We report that inhibition of RhoA-kinase (ROCK) or myosin II resulted in unstable consolidation of the distal axon as evidenced by increased filopodial and lamellipodial extension. Furthermore, when ROCK or myosin II was inhibited lamellipodia formed at the growth cone migrated onto the axon shaft. Analysis of EYFP-actin dynamics in the distal axon revealed that ROCK negatively regulates actin polymerization and initiation of protrusive structures from spontaneously formed axonal F-actin patches, the latter being an effect attributable to ROCK-mediated regulation of myosin II. Inhibition of ROCK or myosin II blocked growth cone turning toward NGF by preventing suppression of protrusive activity away from the source of NGF, resulting in aborted turning responses. These data elucidate the mechanism of growth cone polarity, provide evidence that consolidation of the distal axon is a component of guidance, and identify ROCK as a negative regulator of F-actin polymerization underlying protrusive activity in the distal axon.

Changes in nNOS and NADPH diaphorase in frog retina and tectum after axotomy and FGF-2 application

We have shown previously that application of fibroblast growth factor-2 (FGF-2) to the cut optic nerve of the frog, Rana pipiens, augments the survival of retinal ganglion cells (RGCs). In this study, we examine the effects of axotomy and FGF-2 treatment upon the distribution of nitric oxide synthase (NOS) and NADPH diaphorase (NADPH-d) activity in the frog retina and tectum. We find that NOS and NADPH-d are largely absent from RGCs but present in amacrine neurons and in retinorecipient tectal layers. Axotomy alone has little effect on NOS expression or diaphorase activity, apart from slightly increasing the levels of expression in a subpopulation of amacrine cells that arborize in the On sublamina of the inner plexiform layer. FGF-2 application to the optic nerve down-regulates NOS expression and activity in the retina and up-regulates it in the tectum, particularly in retinorecipient layers. Electron microscopy of the optic nerve and neurofilament immunostaining of the tectum suggests that FGF-2 treatment increases the number of regenerating retinal axons arriving at the tectum. The effects in the retina and tectum are probably indirect, that in the retina being due to retrograde signaling from RGCs to amacrine neurons, and that in the tectum being due to re-induction of NOS expression in tectal neurons by the arrival of regenerating axons. At this stage, it appears unlikely that these changes in NOS play a role in the FGF-2's survival effect on RGCs.

Regulation of GDNF and its receptor components GFR-alpha1, -alpha2 and Ret during development and in the mature retino-collicular pathway

The development of the retino-tectal projection as part of the central visual pathway is accomplished around postnatal day (P) 10-14 in rodents, and trophic factors are important for topographic refinement of this projection. Emerging data indicate that GDNF may influence synaptic plasticity of this projection. To date, maturation-dependent kinetics of GDNF release and expression and biological function of single GDNF receptors along the retino-collicular pathway are ill-defined. Here, we examined mRNA and protein expression of GDNF and its multicomponent receptor complex in the retina and superior colliculus (SC) during postnatal development of the rat visual system, and after optic nerve (ON) injury by RT-PCR, immunoblotting and immunofluorescence. Stable mRNA transcription of GDNF and its receptors GFR-alpha1, -alpha2 and Ret was found in retina and SC throughout development into adulthood and after ON transection. Expression of GDNF protein increased during retinal development, declined in adulthood and was further reduced in injured retina. In the SC, GDNF peaked at P0, continuously declined with maturation, and was undetectable in the deafferentiated SC. GFR-alpha1 was abundant in retina and SC throughout, while GFR-alpha2 was not expressed. Since Ret was localized primarily to the vascular compartment, the receptor tyrosine kinase may play a minor role in neuronal GDNF signaling. In summary, we provide evidence for GDNF as survival and guidance factor during development of the retino-tectal projection with differential regulation in early and premature retina and SC. Postlesionally, midbrain targets do not induce GDNF, suggesting that retrograde GDNF is not essential for rescue of adult injured retinal ganglion cells (RGCs).

Cdc42 participates in the regulation of ADF/cofilin and retinal growth cone filopodia by brain derived neurotrophic factor

Rho family GTPases have important roles in mediating the effects of guidance cues and growth factors on the motility of neuronal growth cones. We previously showed that the neurotrophin BDNF regulates filopodial dynamics on growth cones of retinal ganglion cell axons through activation of the actin regulatory proteins ADF and cofilin by inhibiting a RhoA-dependent pathway that phosphorylates (inactivates) ADF/cofilin. The GTPase Cdc42 has also been implicated in mediating the effects of positive guidance cues. In this article we investigated whether Cdc42 is involved in the effects of BDNF on filopodial dynamics. BDNF treatment increases Cdc42 activity in retinal neurons, and neuronal incorporation of constitutively active Cdc42 mimics the increases in filopodial number and length. Furthermore, constitutively active and dominant negative Cdc42 decreased and increased, respectively, the activity of RhoA in retinal growth cones, indicating crosstalk between these GTPases in retinal growth cones. Constitutively active Cdc42 mimicked the activation of ADF/cofilin that resulted from BDNF treatment, while dominant negative Cdc42 blocked the effects of BDNF on filopodia and ADF/cofilin. The inability of dominant negative Cdc42 to block ADF/cofilin activation and stimulation of filopodial dynamics by the ROCK inhibitor Y-27632 indicate interaction between Cdc42 and RhoA occurs upstream of ROCK. Our results demonstrate crosstalk occurs between GTPases in mediating the effects of BDNF on growth cone motility, and Cdc42 activity can promote actin dynamics via activation of ADF/cofilin.

Local activation of the nitric oxide/cyclic guanosine monophosphate pathway in growth cones regulates filopodial length via protein kinase G, cyclic ADP ribose and intracellular Ca2+ release

Nitric oxide (NO) is a gaseous messenger that has been shown to affect growth cone motility and neurite outgrowth in several model systems, but how NO brings about its effects is not understood. We have previously demonstrated that global and long-term application of NO to Helisoma trivolvis B5 neurons results in a transient increase in filopodial length, decrease in filopodial number and decrease in neurite outgrowth, all of which are mediated via soluble guanylyl cyclase (sGC) and involve an increase in the intracellular Ca2+ concentration [S. Van Wagenen & V. Rehder (1999)Journal of Neurobiology, 39, 168-185; K.R. Trimm & V. Rehder (2004) European Journal of Neuroscience, 19, 809-818]. The goal of the current study was twofold: to investigate the effects of short-term NO exposure on individual growth cones and to further elucidate the downstream pathway through which NO exerts its effects. Local application of the NO donor NOC-7 for 10-20 ms via puffer micropipette resulted in a transient increase in filopodial length and a small decrease in filopodial number. We show evidence that these effects of NO are mediated via sGC, protein kinase G and cyclic ADP ribose, resulting in the release of Ca2+ from intracellular stores, probably of the ryanodine-sensitive type. These results suggest that growth cones expressing sGC are highly sensitive to local and short-term exposure to NO, which they may experience during pathfinding, and that the stereotyped response of transient filopodial elongation seen in B5 neurons in response to NO requires intracellular Ca2+ release.

Synergistic action of brain-derived neurotrophic factor and lens injury promotes retinal ganglion cell survival, but leads to optic nerve dystrophy in vivo

Trauma or disease in the CNS often leads to neuronal death and consequent loss of functional connections. The idea has been put forward that strategies aimed at repairing the injured CNS involve stimulation of both neuronal survival and axon regeneration. We tested this hypothesis in the adult rat retinocollicular system by combining two strategies: (i) exogenous administration of brain-derived neurotrophic factor (BDNF), a potent survival factor for damaged retinal ganglion cells (RGCs) and (ii) lens injury, which promotes robust growth of transected RGC axons. Our results demonstrate that BDNF and lens injury interact synergistically to promote neuronal survival: 71% of RGCs were alive at 2 weeks after optic nerve injury, a time when only approximately 10% of these neurons remain without treatment. Intravitreal injection of BDNF, however, led to regeneration failure following lens injury. The effect of BDNF could not be generalized to other growth factors, as ciliary neurotrophic factor did not cause a significant reduction of lens injury-induced regeneration. Growth arrest in optic nerves treated with BDNF and lens injury correlated with the formation of hypertrophic axonal swellings in the proximal optic nerve. These swellings were filled with numerous vesicular bodies, disorganized neurofilaments and degenerating organelles. Our results demonstrate that: (i) increased neuronal survival does not necessarily lead to enhanced axon regeneration and (ii) activation of survival and growth pathways may produce axonal dystrophy similar to that found in neurodegenerative disorders including glaucoma, Alzheimer's disease and multiple sclerosis. We propose that loss of axonal integrity may limit neuronal recovery in the injured, adult CNS.

Nitric Oxide and Synaptic Dynamics in the Adult Brain: Physiopathological Aspects

The adult brain retains the capacity to rewire mature neural circuits in response to environmental changes, brain damage or sensory and motor experiences. Two plastic processes, synaptic remodeling and neurogenesis, have been the subject of numerous studies due to their involvement in the maturation of the nervous system, their prevalence and re-activation in adulthood, and therapeutic relevance. However, most of the research looking for the mechanistic and molecular events underlying synaptogenic phenomena has been focused on the extensive synaptic reorganization occurring in the developing brain. In this stage, a vast number of synapses are initially established, which subsequently undergo a process of activity-dependent refinement guided by target-derived signals that act as synaptotoxins or synaptotrophins, promoting either loss or consolidation of pre-existing synaptic contacts, respectively. Nitric oxide (NO), an autocrine and/or paracrine-acting gaseous molecule synthesized in an activity-dependent manner, has ambivalent actions. It can act by mediating synapse formation, segregation of afferent inputs, or growth cone collapse and retraction in immature neural systems. Nevertheless, little information exists about the role of this ambiguous molecule in synaptic plasticity processes occurring in the adult brain. Suitable conditions for elucidating the role of NO in adult synaptic rearrangement include physiopathological conditions, such as peripheral nerve injury. We have recently developed a crush lesion model of the XIIth nerve that induces a pronounced stripping of excitatory synaptic boutons from the cell bodies of hypoglossal motoneurons. The decline in synaptic coverage was concomitant with de novo expression of the neuronal isoform of NO synthase in motoneurons. We have demonstrated a synaptotoxic action of NO mediating synaptic withdrawal and preventing synapse formation by cyclic GMP (cGMP)-dependent and, probably, S-nitrosylation-mediated mechanisms, respectively. This action possibly involves the participation of other signaling molecules working together with NO. Brain-derived neurotrophic factor (BDNF), a target-derived synaptotrophin synthesized and released postsynaptically in an activity-dependent form, is a potential candidate for effecting such a concerted action. Several items of evidence support an interrelationship between NO and BDNF in the regulation of synaptic remodeling processes in adulthood: i) BDNF and its receptor TrkB are expressed by motoneurons and upregulated by axonal injury; ii) they promote axon arborization and synaptic formation, and modulate the structural dynamics of excitatory synapses; iii) NO and BDNF each control the production and activity of the other at the level of individual synapses; iv) the NO/cGMP pathway inhibits BDNF secretion; and finally, v) BDNF protects F-actin from depolymerization by NO, thus preventing the collapsing and retracting effects of NO on growth cones. Therefore, we propose a mechanism of action in which the NO/BDNF ratio regulates synapse dynamics after peripheral nerve lesion. This hypothesis also raises the possibility that variations in this NO/BDNF balance constitute a common hallmark leading to synapse loss in the progression of diverse neurodegenerative diseases such as amyotrophic lateral sclerosis, Alzheimer's and Parkinson's diseases.

Nitric oxide synthesis is required for exercise-induced increases in hippocampal BDNF and phosphatidylinositol 3' kinase expression

Previous studies have shown that running exercise, either alone or in combination with antidepressant treatment, results in increased hippocampal BDNF levels. Nitric oxide (NO) is an important signaling molecule that has neuronal survival-promoting properties and has been shown to play an important role in plasticity associated with activating interventions. Herein, we administered the NO synthase (NOS) inhibitor, N-nitro-L-arginine methyl ester (L-NAME), in conjunction with the monoamine oxidase inhibitor (MAOI) antidepressant, tranylcypromine, and voluntary wheel-running exercise to determine whether the enhancement in full-length BDNF mRNA occurring with these interventions is dependent upon NO synthesis. Our results demonstrate that both chronic exercise and chronic exercise-plus-tranylcypromine lead to enhanced hippocampal BDNF mRNA and protein expression. NOS inhibition prevents this effect of chronic exercise, but only partly prevents the effects of the exercise/antidepressant combination. Thus, the robust enhancement in BDNF mRNA occurring with exercise appears to be NO synthesis-dependent, but the intervention including antidepressant may enhance BDNF expression through alternative intracellular mechanisms. In addition, because exercise and antidepressants have both been shown to activate survival-promoting genes, we evaluated the levels of hippocampal phosphatidylinositol 3' kinase (PI-3K), an important signaling molecule within a principal neuronal survival-promoting intracellular pathway. Like BDNF mRNA and protein, exercise increases the expression of PI-3K, whereas concomitant NOS inhibition prevents this increase in PI-3K immunoreactivity above control levels. Our results are discussed in light of possible overlapping, but distinct intracellular pathways activated by exercise and antidepressant treatment to bring about enhancements in BDNF expression and other survival-promoting effects. These findings further demonstrate the potential therapeutic potential of chronic exercise to supplement pharmacotherapeutic treatment of mood disorders.

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

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

Developmental pattern of NADPH-diaphorase positive neurons in chick optic tectum is sensitive to changes in visual stimulation

The chick retinotectal system is a suitable model to investigate the mechanisms involved in the establishment of synaptic connections in whose refinement nitric oxide was implicated. The purpose of this work was to describe the developmental pattern of the nitric oxide synthase (NOS)-positive neurons as well as to determine if it is sensitive to changes in visual stimulation. The NADPH-diaphorase histochemical method was used to describe and quantify NOS neurons in normally stimulated and subnormally stimulated chickens. Nine types of NOS neurons were identified; seven of them express NOS until adulthood, while two of them show only a transient expression. The developmental pattern of NOS neurons follows the process of laminar segregation. It can be divided into three phases. The first includes the onset of NOS expression in periventricular neurons and the formation of a deep network of NOS fibers during early development. These neurons do not show any significant change in subnormally stimulated animals. The second phase includes the appearance of two transient NOS populations of bipolar neurons that occupy the intermediate layers during the optic fibers ingrowth. One of them significantly changes in subnormally stimulated chicks. The third phase occurs when the transitory expression of bipolar neurons decreases. It includes NOS expression in six neuronal populations that innervate the superficial retinorecipient layers. Most of these cells suffer plastic changes in subnormally stimulated chicks. The diversity of neuronal types with regard to their morphology, location, and sensitivity to visual stimulation strongly suggests that they serve different functions.

Mechanisms of retinotopic map development: Ephs, ephrins, and spontaneous correlated retinal activity

This chapter summarizes mechanisms that control the development of retinotopic maps in the brain, focusing on work from our laboratory using as models the projection of retinal ganglion cells (RGCs) to the chick optic tectum (OT) or rodent superior colliculus (SC). The formation of a retinotopic map involves the establishment of an initial, very coarse map that subsequently undergoes large-scale remodeling to generate a refined map. All arbors are formed by interstitial branches that form in a topographically biased manner along RGC axons that overshoot their correct termination zone (TZ) along the anterior-posterior (A-P) axis of the OT/SC. The interstitial branches exhibit directed growth along the lateral-medial (L-M) axis of the OT/SC to position the branch at the topographically correct location, where it arborizes to form the TZ. EphA receptors and ephrin-A ligands control in part RGC axon mapping along the A-P axis by inhibiting branching and arborization posterior to the correct TZ. Ephrin-B1 acts bifunctionally through EphB forward signaling to direct branches along the L-M axis of the OT/SC to their topographically correct site. Computational modeling indicates that multiple graded activities are required along each axis to generate a retinotopic map, and makes several predictions, including: the progressive addition of ephrin-As within the OT/SC, due to its expression on RGC axon branches and arbors, is required to increase topographic specificity in branching and arborization as well as eliminate the initial axon overshoot, and that interactions amongst RGC axons that resemble correlated neural activity are required to drive retinotopic refinement. Analyses of mutant mice that lack early spontaneous retinal waves that correlate activity amongst neighboring RGCs, confirm this modeling prediction and show that correlated activity during an early brief critical period is required to drive the large-scale remodeling of the initially topographically coarse projection into a refined one. In summary, multiple graded guidance molecules, retinal waves and correlated spontaneous RGC activity cooperate to generate retinotopic maps.

Myosin II activity is required for severing-induced axon retraction in vitro

Understanding the mechanistic basis of the response of neurons to injury is directly relevant to the development of effective therapeutic approaches aimed at the amelioration of nervous system damage. Axons retract in response to severing. We investigated the mechanism of axon retraction in response to severing in vitro, testing the hypothesis that actomyosin contractility drives severing-induced axon retraction. Axon retraction commenced within 5 min following severing and correlated with actin filament accumulation at the site of severing. Depolymerization of actin filaments prevented retraction, demonstrating that actin filaments are required for severing-induced axon retraction. Direct inhibition of myosin II, using blebbistatin, minimized axon retraction in response to severing. Blocking RhoA-kinase (ROCK), a modulator of myosin II activity, inhibited axon retraction. Similarly, inhibiting myosin light chain kinase (MLCK) with a cell-permeable pseudo-substrate peptide also inhibited axon retraction. These data demonstrate that myosin II activity is required for severing-induced axon retraction in vitro, and suggest myosin II as a target for therapeutic interventions aimed at minimizing retraction following severing in vivo.

Activity-driven sharpening of the retinotectal projection: the search for retrograde synaptic signaling pathways

Patterned visual activity, acting via NMDA receptors, refines developing retinotectal maps by shaping individual retinal arbors. Because NMDA receptors are postsynaptic but the retinal arbors are presynaptic, there must be retrograde signals generated downstream of Ca(++) entry through NMDA receptors that direct the presynaptic retinal terminals to stabilize and grow or to withdraw. This review defines criteria for retrograde synaptic messengers, and then applies them to the leading candidates: nitric oxide (NO), brain-derived neurotrophic factor (BDNF), and arachidonic acid (AA). NO is not likely to be a general mechanism, as it operates only in selected projections of warm blooded vertebrates to speed up synaptic refinement, but is not essential. BDNF is a neurotrophin with strong growth promoting properties and complex interactions with activity both in its release and receptor signaling, but may modulate rather than mediate the retrograde signaling. AA promotes growth and stabilization of synaptic terminals by tapping into a pre-existing axonal growth-promoting pathway that is utilized by L1, NCAM, N-cadherin, and FGF and acts via PKC, GAP43, and F-actin stabilization, and it shares some overlap with BDNF pathways. The actions of both are consistent with recent demonstrations that activity-driven stabilization includes directed growth of new synaptic contacts. Certain nondiffusible factors (synapse-specific CAMs, ephrins, neurexin/neuroligin, and matrix molecules) may also play a role in activity-driven synapse stabilization. Interactions between these pathways are discussed.