Visualizing synapse formation in arborizing optic axons in vivo: dynamics and modulation by BDNF

In vivo imaging of synapse formation on a growing dendritic arbor

The form of a neuron's dendritic arbor determines the set of axons with which it may form synaptic contacts, thus establishing connectivity within neural circuits. However, the dynamic relationship between dendrite growth and synaptogenesis is not well understood. To observe both processes simultaneously, we performed long-term imaging of non-spiny dendritic arbors expressing a fluorescent postsynaptic marker protein as they arborized within the optic tectum of live zebrafish larvae. Our results indicate that almost all synapses form initially on newly extended dendritic filopodia. A fraction of these nascent synapses are maintained, which in turn stabilizes the subset of filopodia on which they form. Stabilized filopodia mature into dendritic branches, and successive iterations of this process result in growth and branching of the arbor. These findings support a 'synaptotropic model' in which synapse formation can direct dendrite arborization.

Structural dynamics of synapses in living animals

Recently, there has been increasing interest in the use of in vivo imaging approaches in the study of the way that synaptic circuits become established and the degree to which they stabilize in mature brains. We review progress since the first efforts, two decades ago, at in vivo synaptic imaging and highlight the more recent advances in molecular biology, optics and neurobiological imaging that have fueled a mini-renaissance in this line of inquiry. Many of the technical problems that limited early efforts still remain, but the rapid pace of molecular and optical innovation might soon transform this specialized field into one that is more 'mainstream'.

Transport of a synaptotagmin–YFP fusion protein in sympathetic neurons during early neurite outgrowth in vitro after transfection in vivo

Developing neurons are engaged in neurite outgrowth as well as the synthesis and transport of proteins involved in synaptic transmission. Very little is known about when transport is established in these rudimentary neurites. We used a novel technique to visualize protein transport during the early hours of neurite outgrowth in culture. Recombinant adenoviruses were used to express a synaptotagmin-YFP fusion protein in the superior cervical ganglia of neonatal rats in vivo and protein transport was examined in neuronal cultures established from the superior cervical ganglions (SCGs). We find that, as early as 4h in culture, synaptotagmin-YFP was present in the cytoplasm, lamellipodia, filopodia and growth cones. Protein expression appeared punctate in neurites at 8h in vitro and is consistent with a vesicular localization. These results indicate that the machinery to transport synapse-specific proteins is functional in rudimentary neurites at this time and indicates that this technique can be used to study early neuronal development.

Retinotopic map refinement requires spontaneous retinal waves during a brief critical period of development

During retinocollicular map development, spontaneous waves of action potentials spread across the retina, correlating activity among neighboring retinal ganglion cells (RGCs). To address the role of retinal waves in topographic map development, we examined wave dynamics and retinocollicular projections in mice lacking the beta2 subunit of the nicotinic acetylcholine receptor. beta2(-/-) mice lack waves during the first postnatal week, but RGCs have high levels of uncorrelated firing. By P8, the wild-type retinocollicular projection remodels into a refined map characterized by axons of neighboring RGCs forming focal termination zones (TZs) of overlapping arbors. In contrast, in P8 beta2(-/-) mice, neighboring RGC axons form large TZs characterized by broadly distributed arbors. At P8, glutamatergic retinal waves appear in beta2(-/-) mice, and later, visually patterned activity appears, but the diffuse TZs fail to remodel. Thus, spontaneous retinal waves that correlate RGC activity are required for retinotopic map remodeling during a brief early critical period.

Trk receptors: roles in neuronal signal transduction

Trk receptors are a family of three receptor tyrosine kinases, each of which can be activated by one or more of four neurotrophins-nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophins 3 and 4 (NT3 and NT4). Neurotrophin signaling through these receptors regulates cell survival, proliferation, the fate of neural precursors, axon and dendrite growth and patterning, and the expression and activity of functionally important proteins, such as ion channels and neurotransmitter receptors. In the adult nervous system, the Trk receptors regulate synaptic strength and plasticity. The cytoplasmic domains of Trk receptors contain several sites of tyrosine phosphorylation that recruit intermediates in intracellular signaling cascades. As a result, Trk receptor signaling activates several small G proteins, including Ras, Rap-1, and the Cdc-42-Rac-Rho family, as well as pathways regulated by MAP kinase, PI 3-kinase and phospholipase-C-gamma (PLC-gamma). Trk receptor activation has different consequences in different cells, and the specificity of downstream Trk receptor-mediated signaling is controlled through expression of intermediates in these signaling pathways and membrane trafficking that regulates localization of different signaling constituents. Perhaps the most fascinating aspect of Trk receptor-mediated signaling is its interplay with signaling promoted by the pan-neurotrophin receptor p75NTR. p75NTR activates a distinct set of signaling pathways within cells that are in some instances synergistic and in other instances antagonistic to those activated by Trk receptors. Several of these are proapoptotic but are suppressed by Trk receptor-initiated signaling. p75NTR also influences the conformations of Trk receptors; this modifies ligand-binding specificity and affinity with important developmental consequences.

Cell-cell signaling during synapse formation in the CNS

Synapses join individual nerve cells into a functional network. Specific cell-cell signaling events regulate synapse formation during development and thereby generate a highly reproducible connectivity pattern. The accuracy of this process is fundamental for normal brain function, and aberrant connectivity leads to nervous system disorders. However, despite the overall precision with which neuronal circuits are formed, individual synapses and synaptic networks are also plastic and can readily adapt to external stimuli or perturbations. In recent studies, several trans-synaptic signaling systems have been identified that can mediate various aspects of synaptic differentiation in the central nervous system. It appears that these individual pathways functionally cooperate, thereby generating robustness and flexibility, which ensure normal nervous system function.

Mechanisms of Synapse Assembly and Disassembly

The mechanisms that govern synapse formation and elimination are fundamental to our understanding of neural development and plasticity. The wiring of neural circuitry requires that vast numbers of synapses be formed in a relatively short time. The subsequent refinement of neural circuitry involves the formation of additional synapses coincident with the disassembly of previously functional synapses. There is increasing evidence that activity-dependent plasticity also involves the formation and disassembly of synapses. While we are gaining insight into the mechanisms of both synapse assembly and disassembly, we understand very little about how these phenomena are related to each other and how they might be coordinately controlled to achieve the precise patterns of synaptic connectivity in the nervous system. Here, we review our current understanding of both synapse assembly and disassembly in an effort to unravel the relationship between these fundamental developmental processes.

Synergistic effects of brain-derived neurotrophic factor and chondroitinase ABC on retinal fiber sprouting after denervation of the superior colliculus in adult rats

Damage to the adult CNS often causes devastating and permanent deficits because of the limited capacity of the brain for anatomical reorganization. The finding that collateral sprouting of uninjured fiber tracts mediates recovery of function prompts the search for experimental strategies that stimulate axonal plasticity after CNS trauma. Here we characterize treatments that promote the sprouting of undamaged retinal afferents into the denervated superior colliculus (SC) after a partial retinal lesion in the adult rat. Delivery of brain-derived neurotrophic factor (BDNF) was performed to enhance the intrinsic potential of retinal ganglion cells to reelongate their axons. Reduction of the neurite growth-inhibitory properties of the adult SC was accomplished via treatment with chondroitinase ABC (C-ABC), which degrades chondroitin sulfate proteoglycans. Retinal axons were labeled via intraocular injections of fluorescently tagged cholera toxin B subunit, and fiber sprouting within the denervated SC was measured by quantitative laser-scanning confocal microscopy 1 week after the retinal lesion. We found that both the administration of BDNF and the injection of C-ABC induce significant sprouting of retinal afferents into the collicular scotoma. Remarkably, the combined treatment with BDNF and C-ABC showed synergistic effects on axon growth. Colocalization analysis with anti-synapsin antibodies demonstrated synapse formation by the sprouting axons. These results suggest that the combined treatment with BDNF and C-ABC can be relevant in therapies for the repair of the damaged adult CNS.

N- and C-terminal domains of beta-catenin, respectively, are required to initiate and shape axon arbors of retinal ganglion cells in vivo

We used deletion mutants to study beta-catenin function in axon arborization of retinal ganglion cells (RGCs) in live Xenopus laevis tadpoles. A deletion mutant betacatDeltaARM consists of the N- and C-terminal domains of wild-type beta-catenin that contain, respectively, alpha-catenin and postsynaptic density-95 (PSD-95)/discs large (Dlg)/zona occludens-1 (ZO-1) (PDZ) binding sites but lacks the central armadillo repeat region that binds cadherins and other proteins. Expression of DeltaARM in RGCs of live tadpoles perturbed axon arborization in two distinct ways: some RGC axons did not form arbors, whereas the remaining RGC axons formed arbors with abnormally long and tangled branches. Expression of the N- and C-terminal domains of beta-catenin separately in RGCs resulted in segregation of these two phenotypes. The axons of RGCs overexpressing the N-terminal domain of beta-catenin developed no or very few branches, whereas axons of RGCs overexpressing the C-terminal domain of beta-catenin formed arbors with long, tangled branches. Additional analysis revealed that the axons of RGCs that did not form arbors after overexpression of DeltaARM or the N-terminal domain of beta-catenin were frequently mistargeted within the tectum. These results suggest that interactions of the N-terminal domain of beta-catenin with alpha-catenin and of the C-terminal domain with PDZ domain-containing proteins are required, respectively, to initiate and shape axon arbors of RGCs in vivo.

The amyloid precursor protein and its regulatory protein, FE65, in growth cones and synapses in vitro and in vivo

Although the Alzheimer amyloid protein precursor (APP) has been studied intensely for more than a decade, its function in neurons is unresolved. Much less is known about its binding partner FE65. We have shown recently that APP and FE65 synergistically regulate the movement of transfected cells. It remained to be shown whether endogenous APP and FE65 could play a similar role in vivo. Here, we show that FE65, like APP, is expressed at high levels in neurons. Using a combination of immunofluorescence, live imaging, and subcellular fractionation, we find that FE65 and APP localize in vitro and in vivo to the most motile regions of neurons, the growth cones. Within growth cones, APP and FE65 concentrate in actin-rich lamellipodia. Finally, APP and FE65 interact in nerve terminals, where they associate with Rab5-containing synaptic organelles but not with synaptic vesicles. Our data are consistent with a role for the APP/FE65 complex in regulation of actin-based membrane motility in neurons, which could be important for highly dynamic processes such as neurite growth and synapse modification.

Glial expression of Borna disease virus phosphoprotein induces behavioral and neurological abnormalities in transgenic mice

One hypothesis for the etiology of behavioral disorders is that infection by a virus induces neuronal cell dysfunctions resulting in a wide range of behavioral abnormalities. However, a direct linkage between viral infections and neurobehavioral disturbances associated with human psychiatric disorders has not been identified. Here, we show that transgenic mice expressing the phosphoprotein (P) of Borna disease virus (BDV) in glial cells develop behavioral abnormalities, such as enhanced intermale aggressiveness, hyperactivity, and spatial reference memory deficit. We demonstrate that the transgenic brains exhibit a significant reduction in brain-derived neurotrophic factor and serotonin receptor expression, as well as a marked decrease in synaptic density. These results demonstrate that glial expression of BDV P leads to behavioral and neurobiological disturbances resembling those in BDV-infected animals. Furthermore, the lack of reactive astrocytosis and neuronal degeneration in the brains indicates that P can directly induce glial cell dysfunction and also suggests that the transgenic mice may exhibit neuropathological and neurophysiological abnormalities resembling those of psychiatric patients. Our results provide a new insight to explore the relationship between viral infections and neurobehavioral disorders.

Biologic Transplantation and Neurotrophin-Induced Neuroplasticity After Traumatic Brain Injury

OBJECTIVE

In this review, we analyze progress in the treatment of traumatic brain injury with neurotrophins, growth factors and cell and tissue neurotransplantation. The primary objective of these therapies is to reduce neurologic deficits associated with the trauma by inducing neuroplasticity. These therapies are restorative and not necessarily neuroprotective.

MAIN OUTCOME MEASURES

An extensive literature on administration of neurotrophics factors and cell and tissue cerebral transplantation is reviewed. The effects of these therapeutic approaches on brain biochemical, molecular, cellular, and tissue responses are summarized.

CONCLUSION

The cumulative data indicate that cell therapy shows substantial promise in the treatment of neural injury.

Glutamate receptor delta2 subunit in activity-dependent heterologous synaptic competition

In the adult cerebellum, the glutamate receptor delta2 subunit (GluRdelta2) is selectively targeted to the spines of the distal Purkinje cell dendrites, the spiny branchlets, that are innervated by the parallel fibers. Although GluRdelta2 has no known channel function, it is presumed to be involved in the formation and stabilization of these synapses. After block of electrical activity by tetrodotoxin, GluRdelta2s appear in the postsynaptic densities of the proximal dendritic spines, which then lose their contact with climbing fibers and become ectopically innervated by parallel fibers. This phenomenon suggests that climbing fiber activity prevents GluRdelta2 targeting to proximal dendrites and that GluRdelta2s admitted to the postsynaptic density of the spine cause withdrawal of the silent climbing fiber. To test this hypothesis, we studied the distribution of GluRdelta2s in the rat cerebellum by immunoelectron microscopy during the recovery period that follows removal of the electrical block, and during the sprouting of climbing fibers that follows subtotal deletion of the parent inferior olivary neurons by administration of the drug 3-acetylpyridine. We found that after removal of the electrical block, the climbing fibers reinnervate proximal spines that bear GluRdelta2s and these subunits are successively repressed. Similarly, after subtotal lesion of the inferior olive, reinnervation of denervated Purkinje cells occurs on spines bearing GluRdelta2s. Thus, GluRdelta2s are not responsible for displacing silent climbing fibers. We propose instead that GluRdelta2s are associated with climbing fiber-to-Purkinje cell synapses, during development or at early stages of climbing fiber regeneration or sprouting, and are downregulated during the process of synapse maturation.

Neurotrophin secretion: current facts and future prospects

The proteins of the mammalian neurotrophin family (nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3) and neurotrophin-4/5 (NT-4/5)) were originally identified as neuronal survival factors. During the last decade, evidence has accumulated implicating them (especially BDNF) in addition in the regulation of synaptic transmission and synaptogenesis in the CNS. However, a detailed understanding of the secretion of neurotrophins from neurons is required to delineate their role in regulating synaptic function. Some crucial questions that need to be addressed include the sites of neurotrophin secretion (i.e. axonal versus dendritic; synaptic versus extrasynaptic) and the neuronal and synaptic activity patterns that trigger the release of neurotrophins. In this article, we review the current knowledge in the field of neurotrophin secretion, focussing on activity-dependent synaptic release of BDNF. The modality and the site of neurotrophin secretion are dependent on the processing and subsequent targeting of the neurotrophin precursor molecules. Therefore, the available data regarding formation and trafficking of neurotrophins in the secreting neurons are critically reviewed. In addition, we discuss existing evidence that the characteristics of neurotrophin secretion are similar (but not identical) to those of other neuropeptides. Finally, since BDNF has been proposed to play a critical role as an intercellular synaptic messenger in long-term potentiation (LTP) in the hippocampus, we try to reconcile this possible role of BDNF in LTP with the recently described features of synaptic BDNF secretion.

BDNF regulates spontaneous correlated activity at early developmental stages by increasing synaptogenesis and expression of the K+/Cl- co-transporter KCC2

Spontaneous neural activity is a basic property of the developing brain, which regulates key developmental processes, including migration, neural differentiation and formation and refinement of connections. The mechanisms regulating spontaneous activity are not known. By using transgenic embryos that overexpress BDNF under the control of the nestin promoter, we show here that BDNF controls the emergence and robustness of spontaneous activity in embryonic hippocampal slices. Further, BDNF dramatically increases spontaneous co-active network activity, which is believed to synchronize gene expression and synaptogenesis in vast numbers of neurons. In fact, BDNF raises the spontaneous activity of E18 hippocampal neurons to levels that are typical of postnatal slices. We also show that BDNF overexpression increases the number of synapses at much earlier stages (E18) than those reported previously. Most of these synapses were GABAergic, and GABAergic interneurons showed hypertrophy and a 3-fold increase in GAD expression. Interestingly, whereas BDNF does not alter the expression of GABA and glutamate ionotropic receptors, it does raise the expression of the recently cloned K(+)/Cl(-) KCC2 co-transporter, which is responsible for the conversion of GABA responses from depolarizing to inhibitory, through the control of the Cl(-) potential. Together, results indicate that both the presynaptic and postsynaptic machineries of GABAergic circuits may be essential targets of BDNF actions to control spontaneous activity. The data indicate that BDNF is a potent regulator of spontaneous activity and co-active networks, which is a new level of regulation of neurotrophins. Given that BDNF itself is regulated by neuronal activity, we suggest that BDNF acts as a homeostatic factor controlling the emergence, complexity and networking properties of spontaneous networks.

Presynaptic neurotrophin-3 increases the number of tectal synapses, vesicle density, and number of docked vesicles in chick embryos

To determine whether presynaptically derived neurotrophins may contribute to synaptic plasticity, we examined whether neurotrophin-3 (NT-3) changed the number, size, vesicle content, or vesicle distribution of synapses within the retinorecipient layers of the chick optic tectum. In this system, endogenous NT-3 derives presynaptically from retinal ganglion cell axons. Retinotectal synapses comprise the majority of synapses in superficial tectal layers, as demonstrated by destruction of retinotectal input by intraocular application of the drug monensin. To examine the effect of increased or decreased levels of NT-3, either exogenous NT-3 or monoclonal NT-3 blocking antibodies were injected into the optic tectum of 19-day-old chick embryos, spiked with radiolabeled protein to verify the success of injections and estimate effective concentrations. After 48 hours, the ultrastructure of superficial tectal layers was analyzed and compared with samples from control tecta injected with cytochrome C. NT-3 increased the number of synapses, synaptic vesicles/profile, synaptic vesicle densities, the number of docked vesicles, and the length of the synaptic profile. Deprivation of anterogradely transported endogenous NT-3 with NT-3 antibodies resulted in the opposite effect: decreased numbers of synapses, decreased vesicle densities, and decreased numbers of docked vesicles. Brain-derived neurotrophic factor (BDNF) had a largely different effect than NT-3. BDNF increased the density of vesicles and deprivation of endogenous TrkB ligands with TrkB fusion protein reduced the density of vesicles in the synapses, without effects on synapse number or docked vesicles. We conclude that anterogradely transported NT-3 affects synapse strength in a way that differs from that of presumably postsynaptic-derived BDNF.

The enigma of transmitter-selective receptor accumulation at developing inhibitory synapses

The control of synaptic inhibition is crucial for normal brain function. More than 20 years ago, glycine and gamma-aminobutyric acid (GABA) were shown to be the two major inhibitory neurotransmitters. They can be released independently from different terminals or co-released from the same terminal to activate postsynaptic glycine and GABA(A) receptors. The anchoring protein gephyrin is involved in the postsynaptic accumulation of both glycine and GABA(A) receptors. In lower brain regions, both receptors can be concentrated in synapses, whereas in higher brain regions, glycine receptors are mostly excluded from postsynaptic sites. The activation of glycine and/or GABA(A) receptors determines the strength and precise timing of inhibition. Therefore, tight regulation of postsynaptic glycine versus GABA(A) receptor localization is crucial for optimizing synaptic inhibition in neurons. This review focuses on recent findings and discusses questions concerning the specificity of postsynaptic inhibitory neurotransmitter receptor accumulation during inhibitory synapse formation and development.

Changes in brain-derived neurotrophic factor and trkB receptor in the adult Rana pipiens retina and optic tectum after optic nerve injury

In this study we used immunocytochemistry to investigate the distribution of brain-derived neurotrophic factor (BDNF) and its receptor tyrosine kinase (trkB) in retina and optic tectum of the frog Rana pipiens during regeneration after axotomy. We also measured changes in BDNF mRNA in retina and tectum. Retrograde labeling was used to identify retinal ganglion cells (RGCs) prior to quantification of the BDNF immunoreactivity. In control animals, BDNF was found in the majority of RGCs and displaced amacrine cells and in some cells in the inner nuclear layer (INL). After axotomy, BDNF immunoreactivity was reduced in RGCs but increased in the INL. BDNF mRNA levels in the retina remained high before and after axotomy. Three months after axotomy, after reconnection to the target, the staining intensity of many of the surviving RGCs had partially recovered. In the control tectum, BDNF staining was present in ependymoglial cells and in neurons throughout layers 4, 6, 8, and 9. After axotomy, BDNF staining in tectal neurons became more intense, even though mRNA synthesis was transiently down-regulated. In control retinas, trkB receptor immunostaining was present in most RGCs; no significant changes were observed after axotomy. In control tectum, trkB was detected only in ependymoglial cells. After axotomy, many neuronal cell bodies were transiently labeled. Our data are consistent with the hypothesis that a considerable fraction of the BDNF normally present in RGCs is acquired from their targets in the tectum. However, there are also intraretinal sources of BDNF that could contribute to the survival of RGCs.

Regulation by glycogen synthase kinase-3beta of the arborization field and maturation of retinotectal projection in zebrafish

The retinotectal projection is one of the best systems to study the molecular basis of synapse formation in the CNS because of the well characterized topographic connections and activity-dependent refinement. Here, we developed a presynaptic neuron-specific gene manipulation system in the zebrafish retinotectal projection in vivo using the nicotinic acetylcholine receptor beta3 (nAChRbeta3) gene promoter. Enhanced green fluorescent protein (EGFP) expression signals in living transgenic zebrafish lines carrying the nAChRbeta3 gene promoter-directed EGFP expression vector visualized the development of entire retinal ganglion cell (RGC) axon projection to the tectum. Microinjection of the nAChRbeta3 gene promoter-driven double-cassette vectors directing the expression of both dominant-negative glycogen synthase kinase-3beta (dnGSK-3beta) and EGFP enabled us to follow the development of individual RGCs and to examine the effect of the molecule on the axonal arborization and maturation of the same neurons in living zebrafish. We found that the expression of the dominant-negative form of zebrafish GSK-3beta suppressed the arborization field of RGC axon terminals in the tectum as estimated by the reduction of arbor branch length and arbor areas. Furthermore, the suppression of GSK-3beta activity increased the size of vesicle-associated membrane protein 2-EGFP puncta in RGC axon terminals at the early stage of innervation to the tectum. These results suggest that GSK-3beta regulates the arborization field and maturation of RGC axon terminals in vivo.

Cell type-selective expression of green fluorescent protein and the calcium indicating protein, yellow cameleon, in rat cortical primary cultures

A cell type-specific green fluorescent protein (GFP) expression system in rat cortical primary cultures has been developed for the fluorescence labeling of brain cells. Lipid-mediated transfection (lipofection) was employed, allowing the establishment of a convenient efficient system for the analysis of individual cells. To achieve cell type-specific labeling, GFP expression vectors containing the rat neuron-specific enolase (NSE) gene promoter, human glial fibril acidic protein (GFAP) gene promoter, human elongation factor (EF-1alpha) gene promoter, or human cytomegalovirus (CMV) immediate early promoter were constructed, and their specificities examined. Vectors containing the CMV or GFAP promoter resulted primarily in GFP expression in astrocytes, while those containing the EF1-alpha or NSE promoter resulted primarily in GFP expression in neurons. This labeling system was applied to the morphological analysis of living neurons and to cell type-selective calcium imaging. Confocal microscopy revealed that individual GFP-expressing neurons had processes, which were longer than 500 microm and bore spine-like protrusions. A calcium-indicating GFP variant, yellow cameleon (YC2.1), was expressed in the same system, and cell type-selective calcium imaging performed. On pharmacological stimulation, YC2.1-expressing neurons responded to depolarizing stimuli, but not to the metabotropic glutamate receptor agonist, trans-(1S,3R)-1-amino-1,3-cyclopentanedicarboxylic acid (tACPD), while astrocytes responded only to tACPD.

SCG10-related neuronal growth-associated proteins in neural development, plasticity, degeneration, and aging

Neuronal growth-associated proteins (nGAPs) are in general neuron-specific gene products whose expression correlates tightly with neuronal process outgrowth and/or regeneration, and are mostly good downstream targets of neurotrophin stimulation. Expression of genes encoding nGAPs such as GAP-43, SCG10, and stathmin is upregulated following lesioning of cortical and hippocampal regions of the adult rat brain. In the brains of aged animals, however, the magnitude of the response is reduced, whereas the time course of the response is mostly unchanged when compared with that for brains of young ones. Expression of GAP-43 and stathmin is reduced by aging, and is also changed in age-related neurodegenerative conditions such as Alzheimer's disease in humans. Certain nGAPs are induced during long-term potentiation (LTP) and also during critical periods of song-learning and ocular dominance column formation in birds and cats, respectively. Recent evidence further supports the idea that functional synaptic modulation is often associated with remodeling of synaptic structures. These results suggest that neurotrophin-responsive nGAPs serve as molecular markers of neuronal plasticity during development and aging, and that the neuronal plasticity decreases, at least in certain neuronal circuits, in the aged brain and neurodegenerative diseases. Recent findings on the roles of stathmin and SCG10-related proteins in microtubule destabilization and its functional block by phosphorylation further support the importance of the SCG10 family proteins in neuronal cytoskeletal regulation, particularly as to microtubule dynamics. We summarize here a decade of research on SCG10 and its related molecules with special interests to brain aging and disease.

The developing synapse: construction and modulation of synaptic structures and circuits

Synapse formation and stabilization in the vertebrate central nervous system is a dynamic process, requiring bi-directional communication between pre- and postsynaptic partners. Numerous mechanisms coordinate where and when synapses are made in the developing brain. This review discusses cellular and activity-dependent mechanisms that control the development of synaptic connectivity.

Techniques for gene transfer into neurons

To illuminate the function of the thousands of genes that make up the complexity of the nervous system, it is critical to be able to introduce and express DNA in neurons. Over the past two decades, many gene transfer methods have been developed, including viral vectors, liposomes and electroporation. Although the perfect gene transfer technique for every application has not yet been developed, recent technical advances have facilitated the ease of neuronal gene transfer and have increased the accessibility of these techniques to all laboratories. In order to select a transfection method for any particular experiment, the specific advantages and disadvantages of each technique must be considered.

Local and target-derived brain-derived neurotrophic factor exert opposing effects on the dendritic arborization of retinal ganglion cells in vivo

The dendritic and axonal arbors of developing retinal ganglion cells (RGCs) are exposed to two sources of BDNF: RGC dendrites are exposed to BDNF locally within the retina, and RGC axons are exposed to BDNF at the target, the optic tectum. Our previous studies demonstrated that increasing tectal BDNF levels promotes RGC axon terminal arborization, whereas increasing retinal BDNF levels inhibits RGC dendritic arborization. These results suggested that differential neurotrophic action at the axon versus dendrite might be responsible for the opposing effects of BDNF on RGC axonal versus dendritic arborization. To explore this possibility, we examined the effects of altering BDNF levels at the optic tectum on the elaboration of RGC dendritic arbors in the retina. Increasing tectal BDNF levels resulted in a significant increase in dendritic branching, whereas neutralizing endogenous tectal BDNF with function-blocking antibodies significantly decreased dendritic arbor complexity. Thus, RGC dendritic arbors react in opposing manners to retinal- versus tectal-derived BDNF. Alterations in retinal BDNF levels, however, did not affect axon terminal arborization. Thus, RGC dendritic arborization is controlled in a complementary manner by both local and target-derived sources of BDNF, whereas axon arborization is modulated solely by neurotrophic interactions at the target. Together, our results indicate that developing RGCs modulate dendritic arborization by integrating signals from discrete sources of BDNF in the eye and brain. Differential integration of spatially discrete neurotrophin signals within a single neuron may therefore finely tune afferent and efferent neuronal connectivity.

Interactions between TrkB signaling and serotonin excess in the developing murine somatosensory cortex: a role in tangential and radial organization of thalamocortical axons

Mice lacking monoamine oxidase A (MAOA) display high levels of brain serotonin during the first postnatal week, causing an exuberant outgrowth of thalamocortical axons (TCAs) in layer IV of the somatosensory cortex (S1). We asked whether this exuberance is attributable to abnormal TrkB signaling, because modulation of TrkB signaling during a critical period dramatically influences the segregation and the morphology of TCAs in layer IV of the visual cortex. Using in situ hybridization and ELISA immunoassays, we showed that the levels of trkB mRNA and BDNF and neurotrophin-4 (NT-4) proteins are normal in the thalamus and the cortex of mice lacking MAOA during barrel field formation. Because the release of BDNF and NT-4 could be abnormal in MAOA knock-out (KO) mice, we tested whether abnormal TrkB signaling is required for TCA exuberance in MAOA-KO mice by generating mice lacking both trkB and MAOA. Surprisingly, these mice exhibited more severe phenotypes than those found in MAOA-KO mice: a widespread tangential expansion of TCAs in layer IV of the cortex, resulting in a fusion of all sensory representations and a radial expansion of TCAs in layers II-III of the cortex. Careful examination of mice lacking trkB alone revealed subtle alterations of TCAs, with abnormal invasion of layer III. This study reveals the following: (1) expression of trkB, BDNF, and NT-4 are not modulated by an excess of serotonin during barrel formation, (2) TrkB signaling limits branching of TCAs in inappropriate supragranular cortical layers, and (3) serotonin and TrkB signaling act together to cluster thalamocortical axons in layer IV.

Molecular mechanisms of CNS synaptogenesis

Synapses of the mammalian CNS are asymmetric sites of cell-cell adhesion between nerve cells. They are designed to mediate the rapid and efficient transmission of signals from the presynaptic bouton of one neuron to the postsynaptic plasma membrane of a second neuron. Significant progress has been made in the characterization of the structural, functional and developmental assembly of CNS synapses. Recent progress has been made in understanding the molecular and cellular mechanisms that underlie synaptogenesis, in particular that of glutamatergic synapses of the CNS.

TrkB receptor signaling is required for establishment of GABAergic synapses in the cerebellum

Neurotrophins are essential to the normal development and maintenance of the nervous system. Neurotrophin signaling is mediated by Trk family tyrosine kinases such as TrkA, TrkB and TrkC, as well as by the pan-neurotrophin receptor p75NTR. Here we have deleted the trkB gene in cerebellar precursors by Wnt1-driven Cre--mediated recombination to study the function of the TrkB in the cerebellum. Despite the absence of TrkB, the mature cerebellum of mutant mice appears similar to that of wild type, with all types of cell present in normal numbers and positions. Granule and Purkinje cell dendrites appear normal and the former have typical numbers of excitatory synapses. By contrast, inhibitory interneurons are strongly affected: although present in normal numbers, they express reduced amounts of GABAergic markers and develop reduced numbers of GABAergic boutons and synaptic specializations. Thus, TrkB is essential to the development of GABAergic neurons and regulates synapse formation in addition to its role in the development of axon terminals.