Neurotrophins are thought to be important for the survival and differentiation of vertebrate neurons. Roles have been suggested for target-derived neurotrophins, based both on their expression in target tissues at the time of neuron innervation, and on their effects on axonal sprouting. However, direct in vivo evidence of their involvement in axon arborization has remained elusive. We have used in vivo microscopy to follow individual optic axons over time, and have examined the role of the neurotrophin brain-derived neurotrophic factor (BDNF) in their development. Here we show that injection of BDNF into the optic tectum of live Xenopus laevis tadpoles increased the branching and complexity of optic axon terminal arbors. In contrast, injection of specific neutralizing antibodies to BDNF reduced axon arborization and complexity. The onset of these effects was rapid (within 2 hours) and persisted throughout the 24-hour observation period. Other neurotrophins had little or no significant effects. These results demonstrate the involvement of neurotrophins in the dynamic elaboration of axon terminals, and suggest a direct role for target-derived BDNF during synaptic patterning in the developing central nervous system.

Dynamic developmental changes in axon arbor morphology may directly reflect the formation, stabilization and elimination of synapses. We used dual-color imaging to study, in the live, developing animal, the relationship between axon arborization and synapse formation at the single cell level, and to examine the participation of brain-derived neurotrophic factor (BDNF) in synaptogenesis. Green fluorescent protein (GFP)-tagged synaptobrevin II served as a marker to visualize synaptic sites in individual fluorescently labeled Xenopus optic axons. Time-lapse confocal microscopy revealed that although most synapses remain stable, synapses are also formed and eliminated as axons branch and increase their complexity. Most new branches originated at GFP-labeled synaptic sites. Increasing BDNF levels significantly increased both axon arborization and synapse number, with BDNF increasing synapse number per axon terminal. The ability to visualize central synapses in real time provides insights about the dynamic mechanisms underlying synaptogenesis, and reveals BDNF as a modulator of synaptogenesis in vivo.

During development, neural networks are established in a highly organized manner, which persists throughout life. Neurotrophins play crucial roles in the developing nervous system. Among the neurotrophins, brain-derived neurotrophic factor (BDNF) is highly conserved in gene structure and function during vertebrate evolution, and serves an important role during brain development and in synaptic plasticity. BDNF participates in the formation of appropriate synaptic connections in the brain, and disruptions in this process contribute to disorders of cognitive function. In this review, we first briefly highlight current knowledge on the expression, regulation, and secretion of BDNF. Further, we provide an overview of the possible actions of BDNF in the development of neural circuits, with an emphasis on presynaptic actions of BDNF during the structural development of central neurons. '

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.The function of the nervous system critically relies on the establishment of precise synaptic connections between neurons and specific target cells (Fig. 1). During synaptogenesis, synapses form, mature, and stabilize and are also eliminated by a process that requires intimate communication between pre-and postsynaptic partners. Most of our understanding of synapse formation and stabilization comes from extensive studies performed at the neuromuscular junction (NMJ). However, recent advances in methodologies that include real-time imaging of living neurons have provided insight into the molecular, cellular, and activity-dependent processes that guide synaptogenesis in the developing central nervous system (CNS). This review highlights several aspects of vertebrate synaptogenesis and its relation to activity-dependent processes, from the cellular mechanisms by which neurons communicate with each other to establish synaptic contacts to the role of activity during the development of topographically ordered neuronal maps. Emphasis is placed on the development of central excitatory synapses, and some aspects of NMJ development are also discussed. Synaptogenesis: A Microscopic ViewIn the CNS, synapse assembly begins when axons approach their targets and establish contact with dendritic arbors or soma of their target neurons. Real-time imaging experiments demonstrate that both axonal and dendritic filopodia actively participate in synapse formation (Fig. 2). Highly dynamic interactions at contact sites of advancing axon growth cones and dendritic filopodia have been demonstrated in living zebrafish embryos in which pre-and postsynaptic partners were labeled with green fluorescent protein (GFP) (1). Highly motile dendritic filopodia in zebrafish embryos resemble those of mammalian developing central neurons undergoing synaptogenesis both in culture (2, 3) and in vivo (4). Dynamic filopodia are also present in developing axon arbors before synapse differentiation (5-8) and have been implicated in synapse formation (9). Real-time imaging of GFP-labeled synaptic components and functional imaging of presynaptic sites (labeled with FM 1-43, a vital dye that reveals activityevoked synaptic vesicle recycling) have revealed the time course and sequence of events in CNS synaptogenesis. Imaging GFP-tagged synaptobrevin II (also known as VAMP II, a synaptic vesicle protein) in cultured hippocampal neurons revealed that transport packets containing preassembled synaptic vesicle components begin to accumulate at presynaptic sites immediately after axons and dendritic filopodia establish initial contact (10). Presynaptic components are assembled very rapi...

Expression of the neurotrophin brain-derived neurotrophic factor (BDNF) and its receptor trkB in the ganglion cell layer of the Xenopus retina during retinal ganglion cell (RGC) dendritic arborization indicates that BDNF is spatially and temporally available to influence RGC morphological differentiation (; ). BDNF promotes RGC axon arborization in vivo by acting as a target-derived trophic factor (). To determine whether BDNF also acts locally to regulate RGC dendritic development in vivo, we altered retinal neurotrophin levels at the onset of dendritic arborization and assessed the resulting arbor morphologies of RGCs retrogradely labeled with fluorescent dextrans. Injecting neurotrophins or BDNF function-blocking antibodies coupled to microspheres provided local alterations of retinal neurotrophin levels. BDNF significantly decreased RGC dendritic arbor complexity, whereas neutralizing endogenous BDNF levels with function-blocking antibodies significantly increased dendritic arbor complexity. RGCs exposed to other neurotrophins, as well as RGCs in retinae treated with BDNF but in areas not directly exposed to the neurotrophin, developed dendritic arbors that were indistinguishable from controls, indicating that exogenous BDNF acts specifically and locally. In the tectum, where RGC axons arborize, BDNF had opposite effects. BDNF significantly increased RGC axon arbor complexity and anti-BDNF reduced RGC arborization. Thus, BDNF reduces RGC dendritic arborization within the retina and increases axon arborization in the tectum. These results indicate that BDNF can differentially modulate axonal and dendritic arborization within a single neuronal population in opposing manners and raise the possibility that differential modulation by a neurotrophic factor finely tunes the morphological differentiation program of a neuron.

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Netrin has been implicated in retinal ganglion cell (RGC) axon pathfinding in a number of species. In Xenopus laevis, RGC axons reaching their target in the optic tectum can be repelled by a netrin-1 gradient in vitro, suggesting that netrin may also function in wiring events that follow successful axon pathfinding. Here, we examined the contribution of netrin to RGC axon arborization and synapse formation at the target. Time-lapse confocal microscopy imaging of individual RGC axons coexpressing GFP-synaptobrevin and DsRed in the intact Xenopus brain demonstrated a role for deleted in colorectal cancer (DCC)-mediated netrin signaling. Microinjection of netrin-1 into the tectum induced a rapid and transient increase in presynaptic site addition that resulted in higher presynaptic site density over a 24 h observation period. Moreover, netrin induced dynamic axon branching, increasing branch addition and retraction; a behavior that ultimately increased total branch number. In contrast, microinjection of DCC function-blocking antibodies prevented the increase in presynaptic site number normally observed in control axons as well as the associated increase in branch number and axon arbor growth. Dynamic analysis of axon arbors demonstrated that the effects of anti-DCC on axon morphology and presynaptic connectivity were attributable to a specific decrease in new synapse and branch additions, without affecting the stability of existing synapses and branches. Together, these results indicate that, in the absence of DCC signaling, RGC axons fail to branch and differentiate, and support a novel role for netrin in later phases of retinotectal development.

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