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

Axons and synaptic boutons are highly dynamic in adult visual cortex

While recent studies of synaptic stability in adult cerebral cortex have focused on dendrites, how much axons change is unknown. We have used advances in axon labeling by viruses and in vivo two-photon microscopy to investigate axon branching and bouton dynamics in primary visual cortex (V1) of adult Macaque monkeys. A nonreplicative adeno-associated virus bearing the gene for enhanced green fluorescent protein (AAV.EGFP) provided persistent labeling of axons, and a custom-designed two-photon microscope enabled repeated imaging of the intact brain over several weeks. We found that large-scale branching patterns were stable but that a subset of small branches associated with terminaux boutons, as well as a subset of en passant boutons, appeared and disappeared every week. Bouton losses and gains were both approximately 7% of the total population per week, with no net change in the overall density. These results suggest ongoing processes of synaptogenesis and elimination in adult V1.

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).

Stabilization of axon branch dynamics by synaptic maturation

The developmental refinement of topographic projections in the brain is reflected in the dynamic sculpting of axonal arbors that takes place as connections between CNS structures form and mature. To examine the role of synaptogenesis and synaptic maturation in the structural development of axonal projections during the formation of the topographic retinotectal projection, we coexpressed cytosolic fluorescent protein (FP) and FP-tagged synaptophysin (SYP) in small numbers of retinal ganglion cells in living albino Xenopus laevis tadpoles to reveal the distribution and dynamics of presynaptic sites within labeled retinotectal axons. Two-photon time-lapse observations followed by quantitative analysis of tagged SYP levels at individual synapses demonstrated the time course of synaptogenesis: increases in presynaptic punctum intensity are detectable within minutes of punctum emergence and continue over many hours. Puncta lifetimes correlate with their intensities. Furthermore, we found that axon arbor dynamics are affected by synaptic contacts. Axon branches retract past faintly labeled puncta but are locally stabilized at intensely labeled SYP puncta. Visual stimulation for 4 h enhanced the stability of the arbor at intense presynaptic puncta while concurrently inducing the retraction of exploratory branches with only faintly labeled or no synaptic sites.

Evidence from in vivo imaging that synaptogenesis guides the growth and branching of axonal arbors by two distinct mechanisms

To explore the relationship between axon arbor growth and synaptogenesis, developing retinal ganglion cell (RGC) axon arbors in zebrafish optic tectum were imaged in vivo at high temporal and spatial resolution using two-photon microscopy. Individual RGC axons were dually labeled by expression of a cytosolic red fluorescent protein (DsRed Express) to mark arbor structure and a fusion of the synaptic vesicle protein synaptophysin with green fluorescent protein (Syp:GFP) to mark presynaptic vesicles. Analysis of time-lapse sequences acquired at 10 min intervals revealed unexpectedly rapid kinetics of both axon branch and vesicle cluster turnover. Nascent axonal branches exhibited short average lifetimes of 19 min, and only 17% of newly extended axonal processes persisted for periods exceeding 3 h. The majority (70%) of Syp:GFP puncta formed on newly extended axonal processes. Syp:GFP puncta also exhibited short average lifetimes of 30 min, and only 34% of puncta were stabilized for periods exceeding 3 h. Moreover, strongly correlated dynamics of Syp:GFP puncta and branch structure suggest that synaptogenesis exerts strong influences on both the extension and the selective stabilization of nascent branches. First, new branches form almost exclusively at newly formed Syp:GFP puncta. Second, stabilized nascent branches invariably bear Syp:GFP puncta, and the detailed dynamics of branch retraction suggest strongly that nascent synapses can act at branch tips to arrest retraction. These observations thus provide evidence that synaptogenesis guides axon arbor growth by first promoting initial branch extension and second by selective branch stabilization.

Oxytocin and estrogen promote rapid formation of functional GABA synapses in the adult supraoptic nucleus

We here investigated inhibitory synapse turnover in the adult brain using the hypothalamic supraoptic nucleus where new synapses form during different physiological conditions, in particular on oxytocin neurons largely controlled by GABAergic inputs and locally released oxytocin. Patch clamp recordings and ultrastructural analysis of the nucleus in acute slices from late gestating rats showed that oxytocin and estrogen promoted rapid formation of inhibitory synapses. Thus, after 2-h exposure to a combination of oxytocin and 17-beta estradiol, the frequency of miniature inhibitory postsynaptic currents was significantly enhanced. Since their amplitude and presynaptic GABA release probability were unmodified, this indicated an increased number of synapses. Electron microscopy confirmed increased densities of symmetric, putative GABAergic synapses within 2-h exposure to the peptide or steroid, effects which were reversible and oxytocin receptor mediated. Our observations thus offer direct evidence that hypothalamic GABAergic microcircuitries can undergo rapid and functional remodeling under changing neuroendocrine conditions.

TrkB has a cell-autonomous role in the establishment of hippocampal Schaffer collateral synapses

Neurotrophin signaling has been implicated in the processes of synapse formation and plasticity. To gain additional insight into the mechanism of BDNF and TrkB influence on synapse formation and synaptic plasticity, we generated a conditional knock-out for TrkB using the cre/loxp system. Using three different cre-expressing transgenic mice, three unique spatial and temporal configurations of TrkB deletion were obtained with regard to the hippocampal Schaffer collateral synapse. We compare synapse formation in mutants in which TrkB is ablated either in presynaptic or in both presynaptic and postsynaptic cells at early developmental or postdevelopmental time points. Our results indicate a requirement for TrkB at both the presynaptic and postsynaptic sites during development. In the absence of TrkB, synapse numbers were significantly reduced. In vivo ablation of TrkB after synapse formation did not affect synapse numbers. In primary hippocampal cultures, deletion of TrkB in only the postsynaptic cell, before synapse formation, also resulted in deficits of synapse formation. We conclude that TrkB signaling has a cell-autonomous role required for normal development of both presynaptic and postsynaptic components of the Schaffer collateral synapse.

Circulating insulin-like growth factor I and functional recovery from spinal cord injury under enriched housing conditions

Voluntary locomotor training as induced by enriched housing of rats stimulates recovery of locomotion after spinal cord injury (SCI). Generally it is thought that spinal neural networks of motor- and interneurons located in the ventral and intermediate laminae within the lumbar intumescence of the spinal cord, also referred to as central pattern generators (CPGs), are the 'producers of locomotion' and play a pivotal role in the amelioration of locomotor deficits after SCI. It has been suggested that locomotor training provides locomotor-specific sensory feedback into the CPGs, which stimulates remodeling of central nervous system pathways, including motor systems. Several molecules have been proposed to potentiate this process but the underlying mechanisms are not yet known. To understand these mechanisms, we studied the role of insulin-like growth factor (IGF) I in functional recovery from SCI under normal and enriched environment (EE) housing conditions. In a first experiment, we discovered that subcutaneous administration of IGF-I resulted in better locomotor recovery following SCI. In a second experiment, detailed analysis of the observed functional recovery induced by EE revealed full recovery of hindlimb coordination and stability of gait. This EE-dependent functional recovery was attenuated by alterations in the pre-synaptic bouton density within the ventral gray matter of the lumbar intumescence or CPG area. Neutralization of circulating IGF-I significantly blocked the effectiveness of EE housing on functional recovery and diminished the EE-induced alterations in pre-synaptic bouton density within the CPG area. These results support the use of IGF-I as a possible therapeutic aid in early rehabilitation after SCI.

Exercise differentially regulates synaptic proteins associated to the function of BDNF

We explored the capacity of exercise to impact select events comprising synaptic transmission under the direction of brain-derived neurotrophic factor (BDNF), which may be central to the events by which exercise potentiates synaptic function. We used a specific immunoadhesin chimera (TrkB-IgG) that mimics the BDNF receptor, TrkB, to selectively block BDNF in the hippocampus during 3 days of voluntary wheel running. We measured resultant synapsin I, synaptophysin, and syntaxin levels involved in vesicular pool formation, endocytosis, and exocytosis, respectively. Synapsin I is involved in vesicle pool formation and neurotransmitter release, synaptophysin, in the biogenesis of synaptic vesicles and budding, and syntaxin, in vesicle docking and fusion. Exercise preferentially increased synapsin I and synaptophysin levels, without affecting syntaxin. There was a positive correlation between synapsin I and synaptophysin in exercising rats and synapsin I with the amount of exercise. Blocking BDNF abrogated the exercise-induced increases in synapsin I and synatophysin, revealing that exercise regulates select properties of synaptic transmission under the direction of BDNF.

Retinotectal mapping: new insights from molecular genetics

The sensory and motor components of nervous systems are connected topographically and contain neural maps of the external world. The paradigm for such maps is the precisely ordered wiring of the output cells of the eye to their synaptic targets in the tectum of the midbrain. The retinotectal map is organized in development through the graded activity of Eph receptor tyrosine kinases and their ephrin ligands. These signaling proteins are arrayed in complementary expression gradients along the orthogonal axes of the retina and tectum, and provide both input and recipient cells with Cartesian coordinates that specify their location. Molecular genetic studies in the mouse indicate that these coordinates are interpreted in the context of neuronal competition for termination sites in the tectum. They further suggest that order in the retinotectal map is determined by ratiometric rather than absolute difference comparisons in Eph signaling along the temporal-nasal and dorsal-ventral axes of the eye.

Receptor tyrosine kinase B-mediated excitatory synaptogenesis

The receptor tyrosine kinase B, TrkB, is the high-affinity receptor for brain-derived neurotrophic factor (BDNF). Much evidence supports a role for TrkB signaling in excitatory synapse formation. There have been a number of recent advances in understanding the cell biology of TrkB-mediated excitatory synaptogenesis. The predominant mechanism by which TrkB supports excitatory synaptogenesis appears to be due to cell-autonomous signaling in both pre- and postsynaptic cells. This signaling appears to contribute to the growth and stabilization processes necessary for the net formation of synapses during development. Further, the molecular mechanisms by which TrkB contributes to these growth and stabilization processes are beginning to be elucidated.

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.

A Comparison of Experience-Dependent Plasticity in the Visual and Somatosensory Systems

In the visual and somatosensory systems, maturation of neuronal circuits continues for days to weeks after sensory stimulation occurs. Deprivation of sensory input at various stages of development can induce physiological, and often structural, changes that modify the circuitry of these sensory systems. Recent studies also reveal a surprising degree of plasticity in the mature visual and somatosensory pathways. Here, we compare and contrast the effects of sensory experience on the connectivity and function of these pathways and discuss what is known to date concerning the structural, physiological, and molecular mechanisms underlying their plasticity.

TrkB signaling regulates the developmental maturation of the somatosensory cortex

In the rodent central nervous system, the region of the cortex that responds to facial whisker stimulation is anatomically segregated into discrete regions called barrels. Each barrel is made up of layer IV cortical neurons that receive input from a separate whisker via innervation from the thalamus. It has been shown that neurotrophins play important roles in the development and plasticity of thalamic axon innervation into the visual and retrosplenial cortex. We now extend those findings to the investigation of the role of neurotrophin signaling in barrel cortex formation. We show that the neurotrophin receptor TrkB is expressed in the thalamus and cortex during the time of cortical innervation. The two TrkB ligands, brain derived neurotrophic factor (BDNF) and neurotrophin-4 (NT-4), are expressed in the cortex at this time. Mice lacking TrkB demonstrate a developmental delay in the segregation of thalamic axons within barrels. In TrkB mutants, thalamic axons are abnormally uniform within layer IV of the cortex at postnatal day 4 compared to their control littermates, but show clear segregation into barrels 2 days later. This phenotype is recapitulated in BDNF mutant mice, but not in NT-4 mutant mice. These results demonstrate that BDNF is the sole TrkB ligand responsible for this phenotype. Analysis of conditional knockout mice that lack TrkB within the cortex, and not the thalamus, does not show a delay in thalamic axon segregation. These results indicate that TrkB expression in thalamic axons is important for the appropriate timing of barrel cortex development.

ErbB1 receptor ligands attenuate the expression of synaptic scaffolding proteins, GRIP1 and SAP97, in developing neocortex

Scaffolding proteins containing postsynaptic density-95/discs large/zone occludens-1 (PDZ) domains interact with synaptic receptors and cytoskeletal components and are therefore implicated in synaptic development and plasticity. Little is known, however, about what regulates the expression of PDZ proteins and how the levels of these proteins influence synaptic development. Here, we show that ligands for epidermal growth factor receptors (ErbB1) decrease a particular set of PDZ proteins and negatively influence synaptic formation or maturation. In short-term neocortical cultures, concentrations of epidermal growth factor and amphiregulin (2-9 pM) decreased the expression of glutamate receptor interacting protein 1 (GRIP1) and synapse-associated protein 97 kDa (SAP97) without affecting postsynaptic density-95 (PSD-95) levels and glial proliferation. In long-term cultures, epidermal growth factor treatment resulted in a decrease in the frequency of pan-PDZ-immunoreactive aggregates on dendritic processes. A similar activity on the same PDZ proteins was observed in the developing neocortex following epidermal growth factor administration to rat neonates. Immunoblotting revealed that administered epidermal growth factor from the periphery activated brain ErbB1 receptors and decreased GRIP1 and SAP97 protein levels in the neocortex. Laser-confocal imaging indicated that epidermal growth factor administration suppressed the formation of pan-PDZ-immunoreactive puncta and dispersed those structures in vivo as well. These findings revealed a novel negative activity of ErbB1 receptor ligands that attenuates the expression of the PDZ proteins and inhibits postsynaptic maturation in developing neocortex.

Time-lapse analysis of retinal differentiation

The availability of new vital markers and the improvements in time-lapse video microscopy have increased our ability to observe developmental events in the retina while they are happening. Using this approach, advances have been made in the understanding of important issues such as how division patterns relate to cell fate determination, how post-mitotic progenitors reach their correct retinal layers, and finally, how retinal ganglion cell axons navigate out of the retina and to their final targets in the midbrain.

Molecular gradients and development of retinotopic maps

Gradients of axon guidance molecules have long been postulated to control the development of the organization of neural connections into topographic maps. We review progress in identifying molecules required for mapping and the mechanisms by which they act, focusing on the visual system, the predominant model for map development. The Eph family of receptor tyrosine kinases and their ligands, the ephrins, remain the only molecules that meet all criteria for graded topographic guidance molecules, although others fulfill some criteria. Recent reports further define their modes of action and new roles for them, including EphB/ephrin-B control of dorsal-ventral mapping, bidirectional signaling of EphAs/ephrin-As, bifunctional action of ephrins as attractants or repellents in a context-dependent manner, and complex interactions between multiple guidance molecules. In addition, spontaneous patterned neural activity has recently been shown to be required for map refinement during a brief critical period. We speculate on additional activities required for map development and suggest a synthesis of molecular and cellular mechanisms within the context of the complexities of map development.

Mechanisms of vertebrate synaptogenesis

The formation of synapses in the vertebrate central nervous system is a complex process that occurs over a protracted period of development. Recent work has begun to unravel the mysteries of synaptogenesis, demonstrating the existence of multiple molecules that influence not only when and where synapses form but also synaptic specificity and stability. Some of these molecules act at a distance, steering axons to their correct receptive fields and promoting neuronal differentiation and maturation, whereas others act at the time of contact, providing positional information about the appropriateness of targets and/or inductive signals that trigger the cascade of events leading to synapse formation. In addition, correlated synaptic activity provides critical information about the appropriateness of synaptic connections, thereby influencing synapse stability and elimination. Although synapse formation and elimination are hallmarks of early development, these processes are also fundamental to learning, memory, and cognition in the mature brain.

Nitric-oxide-directed synaptic remodeling in the adult mammal CNS

In adult mammals, learning, memory, and restoration of sensorimotor lost functions imply synaptic reorganization that requires diffusible messengers-mediated communication between presynaptic and postsynaptic structures. A candidate molecule to accomplish this function is the gaseous intercellular messenger nitric oxide (NO), which is involved in synaptogenesis and projection refinement during development; however, the role of NO in synaptic reorganization processes in adulthood remains to be established. In this work, we tested the hypothesis that this free radical is a mediator in the adult mammal CNS synaptic remodeling processes using a model of hypoglossal axonal injury recently developed by us. Axonal injury-induced disconnection of motoneurons from myocytes produces withdrawal of synaptic inputs to motoneurons and concomitant upregulation of the neuronal isoform of NO synthase (NOS-I). After recovery of the neuromuscular function, synaptic coverage is reestablished and NOS-I is downregulated. We also report, by using functional and morphological approaches, that chronic inhibition of the NO/cGMP pathway prevents synaptic withdrawal evoked by axon injury, despite the persistent muscle disconnection. After successful withdrawal of synaptic boutons, inhibition of NO synthesis, but not of cGMP, accelerated the recovery of synaptic coverage, although neuromuscular disconnection was maintained. Furthermore, protein S-nitrosylation was upregulated after nerve injury, and this effect was reversed by NOS-I inhibition. Our results suggest that during synaptic remodeling in the adult CNS, NO acts as a signal for synaptic detachment and inhibits synapse formation by cGMP-dependent and probably S-nitrosylation-mediated mechanisms, respectively. We also suggest a feasible role of NO in neurological disorders coursing with NOS-I upregulation.

Differential regulation of the growth-associated proteins GAP-43 and superior cervical ganglion 10 in response to lesions of the cortex and substantia nigra in the adult rat

Investigation of the elements underlying synapse replacement after brain injury is essential for predicting the neural compensation that can be achieved after various types of damage. The growth-associated proteins superior cervical ganglion-10 and growth-associated protein-43 have previously been linked with structural changes in the corticostriatal system in response to unilateral deafferentation. To examine the regulation of this response, unilateral cortical aspiration lesion was carried out in combination with ipsilateral 6-hydroxydopamine lesion of the substantia nigra, and the time course of the contralateral cortical molecular response was followed. Unilateral cortical aspiration lesion in rats corresponds with an upregulation of superior cervical ganglion-10 mRNA at 3 and 10 days post-lesion, and protein, sustained from three to at least 27 days following lesion. With the addition of substantia nigra lesion, the response shifts to an upregulation of growth-associated protein-43 mRNA at 3 and 10 days post-lesion, and protein after 10 days. Nigral lesion alone does not alter contralateral expression of either gene. Likewise, motor function assessment using the rotorod test revealed no significant long-term deficits in animals that sustained only nigrostriatal damage, but cortical lesion was associated with a temporary deficit which was sustained when nigrostriatal input was also removed. Growth-associated protein-43 and superior cervical ganglion-10, two presynaptic genes that are postulated to play roles in lesion-induced sprouting, are differentially upregulated in corticostriatal neurons after cortical versus combined cortical/nigral lesions. The shift in contralateral gene response from superior cervical ganglion-10 to growth-associated protein-43 upregulation and associated behavioral deficit following combined cortical and nigral denervation suggest that nigrostriatal afferents regulate cortical lesion-induced gene expression and ultimate functional outcome.

Automatic morphometry of synaptic boutons of cultured cells using granulometric analysis of digital images

Numbers, linear density, and surface area of synaptic boutons can be important parameters in studies on synaptic plasticity in cultured neurons. We present a method for automatic identification and morphometry of boutons based on filtering of digital images using granulometric analysis. Cultures of cortical neurons (DIV8 and DIV21) were fixed and marked with fluorescently labeled antibodies for synapsin I (a marker for synaptic boutons) and MAP-2 (a marker for dendrites). Images were acquired on a confocal microscope and automatically processed. Granulometry, a morphological operator sensitive to the geometry and size of objects, was used to construct a filter passing fuzzy fluorescent grains of a certain size. Next, the filter was overlaid with the original image (masking) and the positive pixels were identified by an integral intensity threshold (thresholding). Disjoint grains, representing individual boutons, were reconstructed from the connected pixels above the threshold, numbered and their area was measured. In total, 1498 boutons with a mean diameter of 1.63 +/- 0.49 microm (S.D.) were measured. Comparisons with manual counts showed that the proposed method was capable of identifying boutons in a systematic manner at the light microscopic level and was a viable alternative to manual bouton counting.

BDNF stabilizes synapses and maintains the structural complexity of optic axons in vivo

Brain-derived neurotrophic factor (BDNF) modulates synaptic connectivity by increasing synapse number and by promoting activity-dependent axon arbor growth. Patterned neuronal activity is also thought to influence the morphological maturation of axonal arbors by directly influencing the stability of developing synapses. Here, we used in vivo time-lapse imaging to examine the relationship between synapse stabilization and axon branch stabilization, and to better understand the participation of BDNF in synaptogenesis. Green fluorescent protein (GFP)-tagged synaptobrevin II was used to visualize presynaptic specializations in individual DsRed2-labeled Xenopus retinal axons arborizing in the optic tectum. Neutralizing endogenous tectal BDNF with function-blocking antibodies significantly enhanced GFP-synaptobrevin cluster elimination, a response that was paralleled by enhanced branch elimination. Thus, synapse dismantling was associated with axon branch pruning when endogenous BDNF levels were reduced. To obtain a second measure of the role of BDNF during synapse stabilization, we injected recombinant BDNF in tadpoles with altered glutamate receptor transmission in the optic tectum. Tectal injection of the NMDA receptor antagonists APV or MK801 transiently induced GFP-synaptobrevin cluster dismantling, but did not significantly influence axon branch addition or elimination. BDNF treatment rescued synapses affected by NMDA receptor blockade: BDNF maintained GFP-synaptobrevin cluster density by maintaining their addition rate and rapidly inducing their stabilization. Consequently, BDNF influences synaptic connectivity in multiple ways, promoting not only the morphological maturation of axonal arbors, but also their stabilization, by a mechanism that influences both synapses and axon branches.

MeCP2 in neurons: closing in on the causes of Rett syndrome

The discovery in 1999 that Rett syndrome (RTT) is caused by mutations in a gene encoding the methyl-CpG-binding repressor protein MECP2 provided a significant breakthrough in the understanding of this devastating disease. The subsequent production of Mecp2 knockout mice 2 years later provided an experimental resource to better understand how mutations in the MECP2 gene result in RTT. This paper reviews the recent progress in understanding when and where MeCP2 function becomes important in the developing brain, why MeCP2 protein levels are crucial, which genes are normally silenced by MeCP2, and how misexpression of these targets might lead to the clinical manifestations of RTT.

Activity dependence of cortical axon branch formation: a morphological and electrophysiological study using organotypic slice cultures

The influence of neuronal activity on cortical axon branching was studied by imaging axons of layer 2/3 neurons in organotypic slice cultures of rat visual cortex. Upper layer neurons labeled by electroporation of plasmid encoding yellow fluorescent protein were observed by confocal microscopy. Time-lapse observation of single-labeled axons showed that axons started to branch after 8-10 d in vitro. Over the succeeding 7-10 d, branch complexity gradually increased by both growth and retraction of branches, resulting in axon arbors that morphologically resembled those observed in 2- to 3-week-old animals. Electrophysiological recordings of neuronal activity in the upper layers, made using multielectrode dishes, showed that the frequency of spontaneous firing increased dramatically approximately 10 d in vitro and remained elevated at later stages. To examine the involvement of spontaneous firing and synaptic activity in branch formation, various blockers were applied to the culture medium. Cultures were silenced by TTX or by a combination of APV and DNQX but exhibited a homeostatic recovery of spontaneous activity over several days in the presence of blockers of either NMDA-type or non-NMDA-type glutamate receptors alone. Axonal branching was suppressed by TTX and AMPA receptor blockade but not by NMDA receptor blockade. We conclude that cortical axon branching is highly dynamic and that neural activity regulates the early developmental branching of upper layer cortical neurons through the activation of AMPA-type glutamate receptors.

Axon retraction and degeneration in development and disease

The selective elimination of axons, dendrites, axon and dendrite branches, and synapses, without loss of the parent neurons, occurs during normal development of the nervous system as well as in response to injury or disease in the adult. The widespread developmental phenomena of exuberant axonal projections and synaptic connections require both small-scale and large-scale axon pruning to generate precise adult connectivity, and they provide a mechanism for neural plasticity in the developing and adult nervous system, as well as a mechanism to evolve differences between species in a projection system. Such pruning is also required to remove axonal connections damaged in the adult, to stabilize the affected neural circuits, and to initiate their repair. Pruning occurs through either retraction or degeneration. Here we review examples of these phenomena and consider potential cellular and molecular mechanisms that underlie axon retraction and degeneration and how they might relate to each other in development and disease.

Brain-derived neurotrophic factor regulation of retinal growth cone filopodial dynamics is mediated through actin depolymerizing factor/cofilin

The molecular mechanisms by which neurotrophins regulate growth cone motility are not well understood. This study investigated the signaling involved in transducing BDNF-induced increases of filopodial dynamics. Our results indicate that BDNF regulates filopodial length and number through a Rho kinase-dependent mechanism. Additionally, actin depolymerizing factor (ADF)/cofilin activity is necessary and sufficient to transduce the effects of BDNF. Our data indicate that activation of ADF/cofilin mimics the effects of BDNF on filopodial dynamics, whereas ADF/cofilin inactivity blocks the effects of BDNF. Furthermore, BDNF promotes the activation of ADF/cofilin by reducing the phosphorylation of ADF/cofilin. Although inhibition of myosin II also enhances filopodial length, our results indicate that BDNF signaling is independent of myosin II activity and that the two pathways result in additive effects on filopodial length. Thus, filopodial extension is regulated by at least two independent mechanisms. The BDNF-dependent pathway works via regulation of ADF/cofilin, independently of myosin II activity.

Astrocytes in cerebral ischemic injury: morphological and general considerations

Asrocytic responses constitute one of the earliest and most prominent changes in the CNS following ischemic injury. Astrocytes are known to carry out critical functions such as maintenance of ionic homeostasis, prevention of excitotoxicity, scavenging free radicals, provision of nutrients and growth factors, promotion of neovascularization, and support of synaptogenesis and neurogenesis that potentially may influence the outcome of ischemic injury. This article reviews ischemia-associated alterations in astrocytes and their potential significance. Interactions with neurons, microglia, and endothelial cells are also considered. This article highlights the critical role of astrocytes in the CNS response to ischemic injury.

BDNF function in adult synaptic plasticity: the synaptic consolidation hypothesis

Interest in BDNF as an activity-dependent modulator of neuronal structure and function in the adult brain has intensified in recent years. Localization of BDNF-TrkB to glutamate synapses makes this system attractive as a dynamic, activity-dependent regulator of excitatory transmission and plasticity. Despite individual breakthroughs, an integrated understanding of BDNF function in synaptic plasticity is lacking. Here, we attempt to distill current knowledge of the molecular mechanisms and function of BDNF in LTP. BDNF activates distinct mechanisms to regulate the induction, early maintenance, and late maintenance phases of LTP. Evidence from genetic and pharmacological approaches is reviewed and tabulated. The specific contribution of BDNF depends on the stimulus pattern used to induce LTP, which impacts the duration and perhaps the subcellular site of BDNF release. Particular attention is given to the role of BDNF as a trigger for protein synthesis-dependent late phase LTP--a process referred to as synaptic consolidation. Recent experiments suggest that BDNF activates synaptic consolidation through transcription and rapid dendritic trafficking of mRNA encoded by the immediate early gene, Arc. A model is proposed in which BDNF signaling at glutamate synapses drives the translation of newly transported (Arc) and locally stored (i.e., alphaCaMKII) mRNA in dendrites. In this model BDNF tags synapses for mRNA capture, while Arc translation defines a critical window for synaptic consolidation. The biochemical mechanisms by which BDNF regulates local translation are also discussed. Elucidation of these mechanisms should shed light on a range of adaptive brain responses including memory and mood resilience.

Erythropoietin protects primary hippocampal neurons increasing the expression of brain-derived neurotrophic factor

Erythropoietin, the principal regulator of erythroids progenitor cells, also promotes neuronal survival. Using primary cultures of rat hippocampal neurons, we investigated whether erythropoietin could mediate neuroprotection favouring the transcription of brain-derived neurotrophic factor (BDNF). Erythropoietin 2.7 nm reduced by approximately 50% the neuronal death triggered by the prototypic neurotoxicant trimethyltin (TMT) and time-dependently induced BDNF mRNA. This effect resulted in an increased production of biologically active BDNF, which led to a sustained activation of the specific BDNF receptor tyrosine kinase B (TrkB). Reduction of TMT-induced neuronal death by erythropoietin was specifically prevented by a neutralizing anti-BDNF antibody (15 microg/mL), indicating the involvement of this neurotrophin in erythropoietin neuroprotective effect. Intracerebroventricular administration of erythropoietin in mice significantly increases BDNF mRNA expression in brain, supporting the idea of the involvement of this neurotrophin in erythropoietin action within the CNS. BDNF expression in neuronal cells is induced by activation of voltage Ca(2+)-channels and recruitment of Ca(2+)-sensitive transcription factors. Consistently, 2.7 nm erythropoietin increased intracellular Ca(2+) in 5 min and cAMP response element binding protein (CREB) phosphorylation at Ser 133 in 30 min. Both effects were abolished by 1 microm nitrendipine, a selective blocker of L-type voltage Ca(2+)-channels. These data demonstrate that erythropoietin activates the CREB transcription pathway and increases BDNF expression and production, which contributes to erythropoietin mediated neuroprotection.

Functional imaging reveals rapid development of visual response properties in the zebrafish tectum

The visual pathway from the retina to the optic tectum in fish and frogs has long been studied as a model for neural circuit formation. Although morphological aspects, such as axonal and dendritic arborization, have been well characterized, less is known about how this translates into functional properties of tectal neurons during development. We developed a system to provide controlled visual stimuli to larval zebrafish, while performing two-photon imaging of tectal neurons loaded with a fluorescent calcium indicator, allowing us to determine visual response properties in intact fish. In relatively mature larvae, we describe receptive field sizes, visual topography, and direction and size selectivity. We also characterize the onset and development of visual responses, beginning when retinal axons first arborize in the tectum. Surprisingly, most of these properties are established soon after dendrite growth and synaptogenesis begin and do not require patterned visual experience or a protracted period of refinement.

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.

A “deficient environment” in prenatal life may compromise systems important for cognitive function by affecting BDNF in the hippocampus

The intrauterine environment has the capacity to mold the prenatal nervous system. Particularly, recent findings show that an adverse prenatal environment produces structural defects of the hippocampus, a critical area sub-serving learning and memory functions. These structural changes are accompanied by a disruption in the normal expression pattern of brain-derived neurotrophic factor (BDNF) and its cognate tyrosine kinase B (TrkB) receptor. The important role that the BDNF system plays in neural modeling and learning and memory processes suggests that fetal exposure to unfavorable intrauterine conditions may compromise proper cognitive function in adult life. These findings have implications for disorders that involve a dysfunction in the BDNF system and are accompanied by cognitive deficits.

Phenotype of cerebellar glutamatergic neurons is altered in stargazer mutant mice lacking brain-derived neurotrophic factor mRNA expression

Brain-derived neurotrophic factor (BDNF) influences neuronal survival, differentiation, and maturation. More recently, its role in synapse formation and plasticity has also emerged. In the cerebellum of the spontaneous recessive mutant mouse stargazer (stg) there is a specific and pronounced deficit in BDNF mRNA expression. BDNF protein levels in the cerebellum as a whole are reduced by 70%, while in the granule cells (GCs) there is a selective and near total reduction in BDNF mRNA expression. Recently, we published data demonstrating that inhibitory neurons in the cerebella of stgs have significantly reduced levels (approximately 50%) of gamma-aminobutyric acid (GABA) and fewer, smaller inhibitory synapses compared to wildtype (WT) controls. Our current investigations indicate that the stargazer mutation has an even more pronounced effect on the phenotype of glutamatergic neurons in the cerebellum. There is a profound decrease in the levels of glutamate-immunoreactivity (up to 77%) in stg compared to WT controls. The distribution profile of presynaptic vesicles is also markedly different: stgs have proportionally fewer docked vesicles and fewer vesicles located adjacent to the active zone ready to dock than WTs. Furthermore, the thickness of the postsynaptic density (PSD) at mossy fiber-granule cell (MF-GC) and parallel fiber-Purkinje cell (PF-PC) synapses is severely reduced (up to 33% less than WT controls). The number and length of excitatory synapses, however, appear to be relatively unchanged. It is possible that at least some of theses changes in phenotype are directly attributable to the lack of BDNF in the cerebellum of the stg mutant.

Hippocampal BDNF mediates the efficacy of exercise on synaptic plasticity and cognition

We found that a short exercise period enhanced cognitive function on the Morris water maze (MWM), such that exercised animals were significantly better than sedentary controls at learning and recalling the location of the platform. The finding that exercise increased brain-derived neurotrophic factor (BDNF), a molecule important for synaptic plasticity and learning and memory, impelled us to examine whether a BDNF-mediated mechanism subserves the capacity of exercise to improve hippocampal-dependent learning. A specific immunoadhesin chimera (TrkB-IgG), that mimics the BDNF receptor, TrkB, to selectively bind BDNF molecules, was used to block BDNF in the hippocampus during a 1-week voluntary exercise period. After this, a 2-trial-per-day MWM was performed for 5 consecutive days, succeeded by a probe trial 2 days later. By inhibiting BDNF action we blocked the benefit of exercise on cognitive function, such that the learning and recall abilities of exercising animals receiving the BDNF blocker were reduced to sedentary control levels. Inhibiting BDNF action also blocked the effect of exercise on downstream systems regulated by BDNF and important for synaptic plasticity, cAMP response-element-binding protein (CREB) and synapsin I. Specific to exercise, we found an association between CREB and BDNF expression and cognitive function, such that animals who were the fastest learners and had the best recall showed the highest expression of BDNF and associated CREB mRNA levels. These findings suggest a functional role for CREB under the control of BDNF in mediating the exercise-induced enhancement in learning and memory. Our results indicate that synapsin I might also contribute to this BDNF-mediated mechanism.

In vivo trafficking and targeting of N-cadherin to nascent presynaptic terminals

N-cadherin is a prominent component of developing and mature synapses, yet very little is known about its trafficking within neurons. To investigate N-cadherin dynamics in developing axons, we used in vivo two-photon time-lapse microscopy of N-cadherin--green fluorescent protein (Ncad-GFP), which was expressed in Rohon-Beard neurons of the embryonic zebrafish spinal cord. Ncad-GFP was present as either stable accumulations or highly mobile transport packets. The mobile transport packets were of two types: tubulovesicular structures that moved preferentially in the anterograde direction and discrete-punctate structures that exhibited bidirectional movement. Stable puncta of Ncad-GFP accumulated in the wake of the growth cone with a time course. Colocalization of Ncad-GFP puncta with synaptic markers suggests that N-cadherin is a very early component of nascent synapses. Expression of deletion mutants revealed a potential role of the extracellular domain in appropriate N-cadherin trafficking and targeting. These results are the first to characterize the trafficking of a synaptic cell-adhesion molecule in developing axons in vivo. In addition, we have begun to investigate the cell biology of N-cadherin trafficking and targeting in the context of an intact vertebrate embryo.

Loss of brain-derived neurotrophic factor gene allele exacerbates brain monoamine deficiencies and increases stress abnormalities of serotonin transporter knockout mice

To study the neurochemical and behavioral effects of altered brain-derived neurotrophic factor (BDNF) expression on a brain serotonin system with diminished serotonin transport capability, a double-mutant mouse model was developed by interbreeding serotonin transporter (SERT) knockout mice with BDNF heterozygous knockout mice (BDNF +/-), producing SERT -/- x BDNF +/- (sb) mice. Prior evidence implicates serotonin and SERT in anxiety and stress responses. Some studies have shown that BDNF supports serotonergic neuronal development, leading to our hypothesis that reduced BDNF availability during development might exaggerate the consequences of absent SERT function. In the present study, brain serotonin and 5-hydroxyindol acetic acid concentrations in male sb mice were significantly reduced in the hippocampus and hypothalamus compared with wild-type control SB mice, BDNF-deficient Sb mice, and serotonin transporter knockout sB mice. The sb mice had significantly increased anxiety-like behaviors compared with SB, Sb, and sB mice as measured on the elevated plus maze test. These sb mice also had significantly greater increases in plasma adrenocorticotrophic hormone than mice with other genotypes after a stressful stimulus. Analysis of neuronal morphology showed that hypothalamic and hippocampal neurons exhibited 25-30% reductions in dendrites in sb mice compared with SB control mice. These findings support the hypothesis that genetic changes in BDNF expression interact with serotonin and other circuits that modulate anxiety and stress-related behaviors. Thus, this double-mutant mouse model should prove valuable in studying other gene x gene consequences for brain plasticity as well as in evaluating epistatic interactions of BDNF and serotonin transporter gene polymorphisms in neuropsychiatric disorders.

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.

Competition in neurite outgrowth and the development of nerve connections

During the development of the nervous system, neurons form their characteristic morphologies and become assembled into synaptically connected networks. In both neuronal morphogenesis and the development of nerve connections, competition plays an important role. Although the notion of competition is commonly used in neurobiology, there is little understanding of the nature of the competitive process and the underlying molecular and cellular mechanisms. In this chapter, we review a model of competition between outgrowing neurites, as well as various models of competition that have been proposed for the refinement of connections that takes place in the development of the neuromuscular and visual systems. We describe in detail a model that links competition in the development of nerve connections with the underlying actions and biochemistry of neurotrophic factors.

Brain-derived neurotrophic factor modulates GABAergic synaptic transmission by enhancing presynaptic glutamic acid decarboxylase 65 levels, promoting asynchronous release and reducing the number of activated postsynaptic receptors

Brain-derived neurotrophic factor is known to modulate the function of GABAergic synapses, but the site of brain-derived neurotrophic factor action is still a matter of controversy. This study was aimed at further dissecting the functional alterations produced by brain-derived neurotrophic factor treatment of GABAergic synaptic connections in cultures of the murine superior colliculus. The functional consequences of long-term brain-derived neurotrophic factor treatment were assessed by analysis of unitary evoked and delayed inhibitory postsynaptic currents in response to high frequency stimulation of single axons. It was found that brain-derived neurotrophic factor facilitated the asynchronous release, but had no effect on the probability of evoked release, the size of the readily releasable pool, and the paired-pulse behavior of evoked inhibitory postsynaptic currents. However, the amplitudes of evoked inhibitory postsynaptic currents, delayed inhibitory postsynaptic currents and miniature inhibitory postsynaptic currents were significantly reduced. Non-stationary fluctuation analysis revealed a decrease in the open channel number at the miniature/evoked inhibitory postsynaptic current peak, but no effect on the mean GABA(A) receptor single channel conductance. Quantitative immunocytochemistry uncovered a significant elevation of presynaptic levels of glutamic acid decarboxylase 65. Together, these findings indicate that brain-derived neurotrophic factor treatment induces pre- as well as postsynaptic changes. What effect predominates will depend on the presynaptic activity pattern: at low activation rates brain-derived neurotrophic factor-treated synapses display a pronounced postsynaptic depression, but at high frequencies this depression is fully compensated by an enhancement of asynchronous release.

Diversity in immune-cell interactions: states and functions of the immunological synapse

The contact-dependent exchange of signals between epithelial and neuronal cells results from close membrane-membrane appositions, which are stabilized for years by polarized adhesion, cytoskeletal assemblies and extracellular scaffold proteins. By contrast, owing to a lack of scaffold proteins, interactions between immune cells such as T lymphocytes and antigen-presenting cells (APCs) comprise a spectrum of structurally diverse and short-lived interaction modes that last from minutes to hours. Signals exchanged between T cells and APCs are generated in a specific contact region, termed the "immunological synapse", that coordinates cytoskeletal dynamics with the T-cell receptor (TCR), the engagement of accessory receptors and membrane-proximal signaling. Recent data shed light on the different physical and molecular interaction modes that occur between T cells and APCs, including their dynamics and transition stages, and their consequences for signaling, activation and T-cell effector function.

p75 neurotrophin receptor signaling regulates growth cone filopodial dynamics through modulating RhoA activity

The mechanisms by which neurotrophins regulate growth cone motility are unclear. We investigated the role of the p75 neurotrophin receptor (p75NTR) in mediating neurotrophin-induced increases in filopodial length. Our data demonstrate that neurotrophin binding to p75NTR is necessary and sufficient to regulate filopodial dynamics. Furthermore, retinal and dorsal root ganglion growth cones from p75 mutant mice are insensitive to neurotrophins but display enhanced filopodial lengths comparable with neurotrophin-treated wild-type growth cones. This suggests unoccupied p75NTR negatively regulates filopodia length. Furthermore, p75NTR regulates RhoA activity to mediate filopodial dynamics. Constitutively active RhoA blocks neurotrophin-induced increases in filopodial length, whereas inhibition of RhoA enhances filopodial lengths, similar to neurotrophin treatment. BDNF treatment of retinal neurons results in reduced RhoA activity. Furthermore, p75 mutant neurons display reduced levels of activated RhoA compared with wild-type counterparts, consistent with the enhanced filopodial lengths observed on mutant growth cones. These observations suggest that neurotrophins regulate filopodial dynamics by depressing the activation of RhoA that occurs through p75NTR signaling.

Functionally intact glutamate-mediated signaling in bipolar cells of the TRKB knockout mouse retina

In the juvenile trkB knockout (trkB−/−) mouse, retina synaptic communication from rods to bipolar cells is severely compromised as evidenced by a complete absence of electroretinogram (ERG)b-wave, even though the inner retina appears anatomically normal (Rohrer et al., 1999). Since it is well known that theb-wave reflects light-dependent synaptic activation of ON bipolar cellsviatheir metabotropic glutamate receptor, mGluR6, we sought to analyze the anatomical and functional integrity of the glutamatergic synapses at these and other bipolar cells in thetrkB−/−mouse. Although rod bipolar cells from wild-type juvenile mice were determined to be immunopositive for trkB, postsynaptic metabotropic and ionotropic glutamate receptor-mediated pathways in ON and OFF bipolar cells were found to be functionally intact, based on patch electrode recordings, using brief applications (“puffs”) of glutamate or its analog, 2-amino-4-phosphonobutyric acid (APB), a selective agonist for mGluR6 receptors. Ionotropic glutamate receptor function was assayed in OFF-cone bipolar and horizontal cells by applying exogenous glutamatergic agonists in the presence of the channel-permeant guanidinium analogue, 1-amino-4-guanidobutane (AGB). Electron-microscopic analysis revealed that the ribbon synapses between rods and postsynaptic rod bipolar and horizontal cells were formed at the appropriate age and appear to be structurally intact, and immunohistochemical analysis did not detect profound defects in the expression of excitatory amino acid transporters involved in glutamate clearance from the synaptic cleft. These data indicate that there does not appear to be evidence for postsynaptic deficits in glutamatergic signaling in the ON and OFF bipolar cells of mice lacking trkB.

Neuronal activation of Ras regulates synaptic connectivity

A synRas mouse model was used expressing constitutively activated Ha-Ras (Val12 mutation) in neurons to investigate the role of Ras-MAPkinase signalling for neuronal connectivity in adult brain. Expression of the transgene in the cortex of these mice starts after neuronal differentiation is completed and allows to directly investigate the effects of enhanced Ras activity in differentiated neurons. Activation of Ha-Ras induced an increase in soma size which was sensitive to MEK inhibitor in postnatal organotypic cultures. Adult cortical pyramidal neurons showed complex structural rearrangements associated with an increased size and ramification of dendritic arborization. Dendritic spine density was elevated and correlated with a twofold increase in number of synapses. In acute brain slices of the somatosensory and of the visual cortex, extracellular field potentials were recorded from layer II/III neurons. The input-output relation of synaptically evoked field potentials revealed a significantly higher basal excitability of the transgenic mice cortex compared to wild-type animals. In whole cell patch clamp preparations, the frequency of AMPA receptor-mediated spontaneous excitatory postsynaptic currents was increased while the ratio between NMDA and AMPA-receptor mediated signal amplitude was unchanged. A pronounced depression of paired pulse facilitation indicated that Ras contributes to changes at the presynaptic site. Furthermore, synRas mice showed an increased synaptic long-term potentiation, which was sensitive to blockers of NMDA-receptors and of MEK. We conclude that neuronal Ras is a common switch of plasticity in adult mammalian brain sculpturing neuronal architecture and synaptic connectivity in concert with tuning synaptic efficacy.

Neural activity and the dynamics of central nervous system development

Recent imaging studies show that the formation of neural connections in the central nervous system is a highly dynamic process. The iterative formation and elimination of synapses and neuronal branches result in the formation of a much larger number of trial connections than is maintained in the mature brain. Neural activity modulates development through biasing this process of formation and elimination, promoting the formation and stabilization of appropriate synaptic connections on the basis of functional activity patterns.

Induction of brain-derived neurotrophic factor in plaque-associated glial cells of aged APP23 transgenic mice

Brain-derived neurotrophic factor (BDNF) is a versatile neurotrophic factor that has been implicated in cell survival, cell differentiation, axonal growth, and activity-dependent synaptic plasticity. Changes in BDNF expression have also been reported during the course of several neurological disorders, including Alzheimer's disease (AD). The role of BDNF in AD, however, has remained elusive. To learn more about this neurotrophic factor, we investigated BDNF expression in brain of amyloid precursor protein overexpressing mice (APP23 transgenic mice). In situ hybridization revealed BDNF mRNA signals associated with amyloid plaques. Laser microdissection in combination with quantitative RT-PCR demonstrated a sixfold increase of BDNF mRNA in the immediate plaque vicinity, a threefold increase in a tissue ring surrounding the plaque, and control levels in interplaque areas comparable with those measured in age-matched nontransgenic mice. Double immunofluorescence localized BDNF to microglial cells and astrocytes surrounding the plaque. Cortical BDNF protein levels were quantified by ELISA demonstrating a >10-fold increase compared with age-matched controls. This upregulation of BDNF protein significantly correlated with the beta-amyloid load in the transgenic animals. Taken together, our data demonstrate a plaque-associated upregulation of BDNF in APP23 transgenic mice and implicate this neurotrophin in the regulation of inflammatory and axonal growth processes in the plaque vicinity.

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.

Live Optical Imaging of Nervous System Development

Although development of the nervous system is inherently a process of dynamic change, until recently it has generally been investigated by inference from static images. However, advances in live optical imaging are now allowing direct observation of growth, synapse formation, and even incipient function in the developing nervous system, at length scales from molecules to cortical regions, and over timescales from milliseconds to months. In this review, we provide technical background and present examples of how these new methods, including confocal and two-photon microscopy, GFP-based markers, and functional indicators, are being applied to provide fresh insight into long-standing questions of neural development.

EphrinB2a in the zebrafish retinotectal system

Members of the Eph-B family of receptors tyrosine kinase and their transmembrane ligands have been implicated in dorsoventral patterning of the vertebrate retinotectal projection. In the zebrafish retinotectal system, however, ephrinB2a is expressed strongly in the posterior tectum, in tectal neurons that form physical contacts with retinal ganglion cell (RGC) axons. In the gnarled mutant, where tectal neurons form ectopically in the pretectum, RGC axons stall before entering the tectum, or else are misrouted or branch aberrantly in the tectal neuropil. Ectopic expression of ephrinB2a in the anterior midbrain of wild-type embryos, with the aid of baculovirus, also inhibits RGC axon entry into the tectum. In vitro, zebrafish RGC axons are repelled by stripes of purified ephrinB2a. It is proposed that ephrinB2a may signal a subpopulation of RGC axons that they have reached their target neurons in the tectum.

Insights into activity-dependent map formation from the retinotectal system: A middle-of-the-brain perspective

The development of orderly topographic maps in the central nervous system (CNS) results from a collaboration of chemoaffinity cues that establish the coarse organization of the projection and activity-dependent mechanisms that fine-tune the map. Using the retinotectal projection as a model system, we describe evidence that biochemical tags and patterned neural activity work in parallel to produce topographically ordered axonal projections. Finally, we review recent experiments in other CNS projections that support the proposition that cooperation between molecular guidance cues and activity-dependent processes constitutes a general paradigm for CNS map formation.

The anterogradely transported BDNF promotes retinal axon remodeling during eye specific segregation within the LGN

Neurotrophins have been implicated in regulating many aspects of neuronal development and plasticity, including dendritic and axonal elaboration, by acting primarily as target derived trophic factors. Recently, we have shown that brain-derived neurotrophic factor (BDNF) is produced by retinal ganglion cells (RGCs) and travels in an anterograde direction along the optic nerve in neonatal rats. Here, we have assessed whether the anterogradely transported BDNF plays a role in shaping the retinogeniculate connectivity during development. We used intraocular injections of antisense oligonucleotides to suppress selectively retinal synthesis and anterograde transport of BDNF in rat pups. We found that in the absence of endogenous BDNF, RGC axons retract from their target in the dorsal lateral geniculate nucleus (dLGN). The blockade of BDNF action at the retinal level with the tyrosine kinase inhibitor, K252a, failed to produce this effect, suggesting an anterograde action of the endogenous BDNF. Moreover, the effects of BDNF removal on RGC fibers were evident only during a narrow temporal window coincident with the critical period for the retinothalamic refinement, indicating a role for BDNF on growth and elaboration of RGC axons rather than on their maintenance. Altogether these results propose a novel role for BDNF in the elaboration of retinogeniculate axons.