Distribution of synaptic vesicle proteins within single retinotectal axons ofXenopus tadpoles

Beyond the cytoskeleton: The emerging role of organelles and membrane remodeling in the regulation of axon collateral branches

The generation of axon collateral branches is a fundamental aspect of the development of the nervous system and the response of axons to injury. Although much has been discovered about the signaling pathways and cytoskeletal dynamics underlying branching, additional aspects of the cell biology of axon branching have received less attention. This review summarizes recent advances in our understanding of key factors involved in axon branching. This article focuses on how cytoskeletal mechanisms, intracellular organelles, such as mitochondria and the endoplasmic reticulum, and membrane remodeling (exocytosis and endocytosis) contribute to branch initiation and formation. Together this growing literature provides valuable insight as well as a platform for continued investigation into how multiple aspects of axonal cell biology are spatially and temporally orchestrated to give rise to axon branches. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 76: 1293-1307, 2016.

Mechanisms underlying the initiation and dynamics of neuronal filopodia: from neurite formation to synaptogenesis

Filopodia are finger-like cellular protrusions found throughout the metazoan kingdom and perform fundamental cellular functions during development and cell migration. Neurons exhibit a wide variety of extremely complex morphologies. In the nervous system, filopodia underlie many major morphogenetic events. Filopodia have roles spanning the initiation and guidance of neuronal processes, axons and dendrites to the formation of synaptic connections. This chapter addresses the mechanisms of the formation and dynamics of neuronal filopodia. Some of the major lessons learned from the study of neuronal filopodia are (1) there are multiple mechanisms that can regulate filopodia in a context-dependent manner, (2) that filopodia are specialized subcellular domains, (3) that filopodia exhibit dynamic membrane recycling which also controls aspects of filopodial dynamics, (4) that neuronal filopodia contain machinery for the orchestration of the actin and microtubule cytoskeleton, and (5) localized protein synthesis contributes to neuronal filopodial dynamics.

Synaptic maturation of the Xenopus retinotectal system: effects of brain-derived neurotrophic factor on synapse ultrastructure

Synaptogenesis is a dynamic process that involves structural changes in developing axons and dendrites as synapses form and mature. The visual system of Xenopus laevis has been used as a model to study dynamic changes in axons and dendrites as synapses form in the living brain and the molecular mechanisms that control these processes. Brain-derived neurotrophic factor (BDNF) contributes to the establishment and refinement of visual connectivity by modulating retinal ganglion cell (RGC) axon arborization and presynaptic differentiation. Here, we have analyzed the ultrastructural organization of the Xenopus retinotectal system to understand better the maturation of this synaptic circuit and the relation between synapse ultrastructure and the structural changes in connectivity that take place in response to BDNF. Expression of yellow fluorescent protein (YFP) followed by preembedding immunoelectron microscopy was used to identify RGC axons specifically in living tadpoles. Injection of recombinant BDNF was used to alter endogenous BDNF levels acutely in the optic tectum. Our studies reveal a rapid transition from a relatively immature synaptic circuit in which retinotectal synapses are formed on developing filopodial-like processes to a circuit in which RGC axon terminals establish synapses with dendritic shafts and spines. Moreover, our studies reveal that BDNF treatment increases the number of spine synapses and docked vesicle number at YFP-identified synaptic sites within 24 hours of treatment. These fine structural changes at retinotectal synapses are consistent with the role that BDNF plays in the functional maturation of synaptic circuits and with dynamic, rapid changes in synaptic connectivity during development.

Synapses are regulated by the cytoplasmic tyrosine kinase Fer in a pathway mediated by p120catenin, Fer, SHP-2, and beta-catenin

Localization of presynaptic components to synaptic sites is critical for hippocampal synapse formation. Cell adhesion–regulated signaling is important for synaptic development and function, but little is known about differentiation of the presynaptic compartment. In this study, we describe a pathway that promotes presynaptic development involving p120catenin (p120ctn), the cytoplasmic tyrosine kinase Fer, the protein phosphatase SHP-2, and β-catenin. Presynaptic Fer depletion prevents localization of active zone constituents and synaptic vesicles and inhibits excitatory synapse formation and synaptic transmission. Depletion of p120ctn or SHP-2 similarly disrupts synaptic vesicle localization with active SHP-2, restoring synapse formation in the absence of Fer. Fer or SHP-2 depletion results in elevated tyrosine phosphorylation of β-catenin. β-Catenin overexpression restores normal synaptic vesicle localization in the absence of Fer or SHP-2. Our results indicate that a presynaptic signaling pathway through p120ctn, Fer, SHP-2, and β-catenin promotes excitatory synapse development and function.

Developmental period for N-methyl-D-aspartate (NMDA) receptor-dependent synapse elimination correlated with visuotopic map refinement

During a short perinatal interval, N-methyl-D-aspartate receptor (NMDAR) function is essential to a process in which spontaneous retinal waves focus retinal axon arbors in the superficial layers of the rodent superior colliculus (sSC). Here we provide evidence that this NMDAR-dependent axonal refinement occurs through elimination of uncorrelated retinal synapses arising from disparate loci, rather than stabilization of topographically appropriate inputs. The density of synaptic release sites within fluorescently labeled retinal terminals was counted in double-labeling experiments using confocal microscopy and antibodies against synaptophysin or synapsin-1. Chronic NMDAR blockade from birth increased retinal axon synapse density at postnatal days (P) 6, 8, and 10, suggesting that NMDAR currents reduce synapse density during the refinement period. With assay at P14, after focal arborization has been established, the effect disappeared. Conversely, chronic NMDA treatment, known to induce functional synaptic depression in the sSC, decreased retinocollicular synapse density at P14, but not earlier, during the refinement period (P8). Thus during the development of retinocollicular topographic order, there is a period when NMDAR activity predominantly eliminates retinal axon synapses. We were able to extend this period by using retinal lesions to reduce synaptic density in a defined zone. Synapse density on intact retinocollicular axons sprouting into this zone was increased by NMDAR blockade, even when examined at P14. Thus, the period of NMDAR-dependent synaptic destabilization is terminated by a factor related to the density and refinement of retinal arbors.

Coordinated motor neuron axon growth and neuromuscular synaptogenesis are promoted by CPG15 in vivo

We have used in vivo time-lapse two-photon imaging of single motor neuron axons labeled with GFP combined with labeling of presynaptic vesicle clusters and postsynaptic acetylcholine receptors in Xenopus laevis tadpoles to determine the dynamic rearrangement of individual axon branches and synaptogenesis during motor axon arbor development. Control GFP-labeled axons are highly dynamic during the period when axon arbors are elaborating. Axon branches emerge from sites of synaptic vesicle clusters. These data indicate that motor neuron axon elaboration and synaptogenesis are concurrent and iterative. We tested the role of Candidate Plasticity Gene 15 (CPG15, also known as Neuritin), an activity-regulated gene that is expressed in the developing motor neurons in this process. CPG15 expression enhances the development of motor neuron axon arbors by promoting neuromuscular synaptogenesis and by increasing the addition of new axon branches.

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.

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

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.

Postnatal development of corticospinal axon terminal morphology in the cat

The corticospinal system undergoes important postnatal development, leading to the mature topography and specificity of connections. The purpose of this study was to determine the time-course of development of corticospinal axonal branching and varicosity density within the cervical gray matter. Corticospinal neurons were labeled after small injections of the anterograde tracer biotinylated dextran amine into the primary motor cortex of cats. Tracer injection and transport times were adjusted to examine labeling at 25, 35, 55, and 75 days and in adults. We measured the numbers and lengths of nonreconstructed terminal and preterminal branches and the numbers and locations of axon varicosities. We found significant age-dependent increases in all morphologic measures. At 25 days, corticospinal axon branching was sparse, with only a few scattered varicosities. By day 35, the mean number of branches, varicosities per branch, and varicosity density increased. Several morphologic measures did not increase between day 35 and 55, but further changes occurred between 55 days and maturity. Beginning around day 55, there was extensive development of small terminal axon branches with high densities of varicosities. We also found, by using spatial point analysis, that there was an age-dependent increase in varicosity clustering. Our results show for the first time that terminal and preterminal corticospinal axon branches increase in complexity during a protracted early postnatal period. This developmental period extended beyond the early postnatal period of activity-dependent refinement of the topography of terminations. Comparison with the time-course of maturation of the cortical motor representation revealed development of substantial, albeit incomplete, branching and varicosity density of CS axons before cortical motor circuits effectively drive their spinal targets.

A method for measuring colocalization of presynaptic markers with anatomically labeled axons using double label immunofluorescence and confocal microscopy

Information concerning the location and distribution of presynaptic neurotransmitter release sites within anatomically labeled axons would be of value for a large number of studies in functional anatomy, development, and plasticity. Here we report a method for localizing presynaptic sites within identified arbors of interest using anterograde anatomical tracer injections to label axonal projections and synaptic vesicle protein (SVP) antibodies to label presumptive presynaptic terminals. The axons and presynaptic sites are independently visualized with double label immunofluorescence and confocal microscopy. Stacks of images representing adjacent focal planes are collected, and image processing techniques are applied to identify the location of each axonal branch segment and each cluster of SVP label in three-dimensional space. Segmentation of the SVP label into distinct pixel clusters in three-dimensional space, followed by colocalization of these clusters with the labeled axons (object-based analysis), yields much more reliable and sensitive measures of colocalization than a simple determination of the number (or summed intensities) of colocalized pixels in a single optical section (pixel-based analysis). The method has been extended to measure the colocalization of antigens that are not located at the presynaptic terminal with a labeled population of axons.

NMDA receptor activity stabilizes presynaptic retinotectal axons and postsynaptic optic tectal cell dendrites in vivo

To investigate the role of N-methyl-D-aspartate (NMDA) receptor activity in the stability of the presynaptic axon arbor and postsynaptic dendritic arbors in vivo, we took time-lapse confocal images of single DiI-labeled Xenopus retinotectal axons and optic tectal neurons in the presence and absence of the NMDA receptor antagonist, APV. Retinotectal axons or tectal neurons were imaged at 30-min intervals over 2 h, or twice over a 24-h period. Retinal axons in animals exposed to DL-APV (100 microM) showed an increase in rates of branch additions and a decrease in branch lifetimes over 2 h compared to untreated axons. Under the same experimental conditions, tectal neurons showed a decreased rate of branch tip additions and retractions. APV treatment over 24 h had no apparent effect on axon arbor morphology, but did decrease tectal cell dendritic arbor elaboration. These observations demonstrate that NMDA receptor activity in postsynaptic neurons stabilizes pre- and postsynaptic neuronal morphology in vivo.. However, when NMDA receptor activity is blocked, presynaptic retinal axons respond with increased arbor dynamics while postsynaptic tectal cell dendrites decrease arbor dynamics. Such differential responses of pre- and postsynaptic partners might increase the probability of coactive afferents converging onto a common target under conditions of lower NMDA receptor activity.

Light-induced calcium influx into retinal axons is regulated by presynaptic nicotinic acetylcholine receptor activity in vivo

Visual activity is thought to be a critical factor in controlling the development of central retinal projections. Neuronal activity increases cytosolic calcium, which was hypothesized to regulate process outgrowth in neurons. We performed an in vivo imaging study in the retinotectal system of albino Xenopus laevis tadpoles with the fluorescent calcium indicator calcium green 1 dextran (CaGD) to test the role of calcium in regulating axon arbor development. We find that visual stimulus to the retina increased CaGD fluorescence intensity in retinal ganglion cell (RGC) axon arbors within the optic tectum and that branch additions to retinotectal axon arbors correlated with a local rise in calcium in the parent branch. We find three types of responses to visual stimulus, which roughly correlate with the ON, OFF, and SUSTAINED response types of RGC reported by physiological criteria. Imaging in bandscan mode indicated that patterns of calcium transients were nonuniform throughout the axons. We tested whether the increase in calcium in the retinotectal axons required synaptic activity in the retina; intraocular application of tetrodotoxin (10 microM) or nifedipine (1 and 10 microM) blocked the stimulus-induced increase in RGC axonal fluorescence. A second series of pharmacological investigations was designed to determine the mechanism of the calcium elevation in the axon terminals within the optic tectum. Injection of bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid-AM (BAPTA-AM) (20 mM) into the tectal ventricle reduced axonal calcium levels, supporting the idea that visual stimulation increases axonal calcium. Injection of BAPTA (20 mM) into the tectal ventricle to chelate extracellular calcium also attenuated the calcium response to visual stimulation, indicating that calcium enters the axon from the extracellular medium. Caffeine (10 mM) caused a large increase in axonal calcium, indicating that intracellular stores contribute to the calcium signal. Presynaptic nicotinic acetylcholine receptors (nAChRs) may play a role in axon arbor development and the formation of the topographic retinotectal projection. Injection of nicotine (10 microM) into the tectal ventricle significantly elevated RGC axonal calcium levels, whereas application of the nAChR antagonist alphaBTX (100 nM) reduced the stimulus-evoked rise in RGC calcium fluorescence. These data suggest that light stimulus to the retina increases calcium in the axon terminal arbors through a mechanism that includes influx through nAChRs and amplification by calcium-induced calcium release from intracellular calcium stores. Such a mechanism may contribute to developmental plasticity of the retinotectal system by influencing both axon arbor elaboration and the strength of synaptic transmission.