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A major obstacle in Alzheimer’s disease (AD) research is the lack of predictive and translatable animal models that reflect disease progression and drug efficacy. Transgenic mice overexpressing amyloid precursor protein (App) gene manifest non-physiological and ectopic expression of APP and its fragments in the brain, which is not observed in AD patients. The App knock-in mice circumvented some of these problems, but they do not exhibit tau pathology and neuronal death. We have generated a rat model, with three familiar App mutations and humanized Aβ sequence knocked into the rat App gene. Without altering the levels of full-length APP and other APP fragments, this model exhibits pathologies and disease progression resembling those in human patients: deposit of Aβ plaques in relevant brain regions, microglia activation and gliosis, progressive synaptic degeneration and AD-relevant cognitive deficits. Interestingly, we have observed tau pathology, neuronal apoptosis and necroptosis and brain atrophy, phenotypes rarely seen in other APP models. This App knock-in rat model may serve as a useful tool for AD research, identifying new drug targets and biomarkers, and testing therapeutics.

Memory consolidation requires astrocytic microdomains for protein recycling; but whether this lays a mechanistic foundation for long-term information storage remains enigmatic. Here we demonstrate that persistent synaptic strengthening invited astrocytic microdomains to convert initially internalized (pro)-brain-derived neurotrophic factor (proBDNF) into active prodomain (BDNFpro) and mature BDNF (mBDNF) for synaptic re-use. While mBDNF activates TrkB, we uncovered a previously unsuspected function for the cleaved BDNFpro, which increases TrkB/SorCS2 receptor complex at post-synaptic sites. Astrocytic BDNFpro release reinforced TrkB phosphorylation to sustain long-term synaptic potentiation and to retain memory in the novel object recognition behavioral test. Thus, the switch from one inactive state to a multi-functional one of the proBDNF provides post-synaptic changes that survive the initial activation. This molecular asset confines local information storage in astrocytic microdomains to selectively support memory circuits., Beatrice Vignoli et al. examine potential molecular mechanisms of long-term storage information in mice. Their results suggest that astrocytes may help convert neuronal BDNF precursor into active prodomain and mature forms to enhance post-synaptic signaling and memory, providing further insight into the development of memory circuits.

Autism spectrum disorders (ASD) comprise a range of early age-onset neurodevelopment disorders with genetic heterogeneity. Most ASD related genes are involved in synaptic function, which is oppositely regulated by brain-derived neurotrophic factor (BDNF): the precursor proBDNF inhibits while mature BDNF (mBDNF) potentiates synapses. Here we generated a knock-in mouse line (BDNFmet/leu) in which the conversion of proBDNF to mBDNF is inhibited. Biochemical experiments revealed residual mBDNF but excessive proBDNF in the brain. Similar to other ASD mouse models, the BDNFmet/leu mice showed decreased brain volumes, reduced dendritic arborization, altered spines, and impaired synaptic transmission and plasticity. They also exhibited ASD-like phenotypes, including stereotypical behaviors, deficits in social interaction, hyperactivity, and elevated stress response. Interestingly, the plasma level of proBDNF, but not mBDNF, was significantly elevated in ASD patients. These results suggest that proBDNF level, but not Bdnf gene, is associated with autism-spectrum behaviors, and identify a potential blood marker and therapeutic target for ASD.

Repeated failures of "Aβ-lowering" therapies call for new targets and therapeutic approaches for Alzheimer's disease (AD). We propose to treat AD by halting neuronal death and repairing synapses using a BDNF-based therapy. To overcome the poor druggability of BDNF, we have developed an agonistic antibody AS86 to mimic the function of BDNF, and evaluate its therapeutic potential for AD. Method: Biochemical, electrophysiological and behavioral techniques were used to investigate the effects of AS86 in vitro and in vivo. Results: AS86 specifically activated the BDNF receptor TrkB and its downstream signaling, without affecting its other receptor p75NTR. It promoted neurite outgrowth, enhanced spine growth and prevented Aβ-induced cell death in cultured neurons, and facilitated Long-Term Potentiation (LTP) in hippocampal slices. A single-dose tail-vein injection of AS86 activated TrkB signaling in the brain, with a half-life of 6 days in the blood and brain. Bi-weekly peripheral administration of AS86 rescued the deficits in object-recognition memory in the APP/PS1 mouse model. AS86 also reversed spatial memory deficits in the 11-month, but not 14-month old AD mouse model. Conclusion: These results demonstrate the potential of AS86 in AD therapy, suggesting that neuronal and/or synaptic repair as an alternative therapeutic strategy for AD.

Brain-derived neurotrophic factor (BDNF) serves a pleiotropic role in the central nervous system, ranging from promoting neuronal survival and differentiation during development and synaptic modulation in the adult. An important, yet unanswered question is how BDNF could serve such diverse functions, sometimes in the same cell. At least two modes of BDNF actions have been elucidated so far based on BDNF signaling kinetics and/or the activity status of the responding neurons. Acute and gradual increases in extracellular BDNF concentrations elicit, respectively, transient and sustained activation of TrkB receptor and its downstream signaling, leading to differential molecular and cellular functions. In cultured neurons, sustained TrkB activation promotes neuronal dendritic arborization and spinogenesis, whereas transient TrkB activation facilitates dendritic growth and spine morphogenesis. In hippocampal slices, slow delivery of BDNF facilitates LTP, whereas fast application of BDNF enhances basal synaptic transmission in schaffer collateral synapses. High-frequency stimulation of neurons converts BDNF-induced TrkB signaling from a transient to a sustained mode. These initial insights lay the foundation for future investigation of the BDNF-TrkB pathway, and analogous signaling pathways to gain a comprehensive understanding to enable translational research. © 2018 Wiley Periodicals, Inc. Develop Neurobiol 78: 647-659, 2018.

BACKGROUND: Dentate gyrus (DG), a “gate” that controls information flow into the hippocampus, plays important roles in regulating both cognitive (e.g., spatial learning and memory) and mood behaviors. Deficits in DG neurons contribute to the pathogenesis of not only neurological, but also psychiatric, disorders, such as anxiety disorder. Whereas DG’s function in spatial learning and memory has been extensively investigated, its role in regulating anxiety remains elusive. METHODS: Using c-fos to mark DG neuron activation, we identified a group of embryonic born dorsal DG (dDG) neurons, which were activated by anxiogenic stimuli and specifically express osteocalcin (Ocn)-Cre. We further investigated their functions in regulating anxiety and the underlying mechanisms by using a combination of chemogenetic, electrophysiological, and RNA-seq methods. RESULTS: The Ocn-Cre(+) dDG neurons were highly active in response to anxiogenic environment, but had lower excitability and less presynaptic inputs than those of Ocn-Cre(−) or adult born dDG neurons. Activating Ocn-Cre(+) dDG neurons suppressed anxiety-like behaviors and increased adult DG neurogenesis, whereas ablating or chronic inhibiting Ocn-Cre(+) dDG neurons exacerbated anxiety-like behaviors, impaired adult DG neurogenesis, and abolished activity (e.g., voluntary wheel running) induced anxiolytic effect and adult DG neurogenesis. RNA-seq screening for factors induced by activation of Ocn-Cre(+) dDG neurons identified brain derived neurotrophic factor (BDNF), which was required for Ocn-Cre(+) dDG neurons mediated anti-anxiety-like behaviors and adult DG neurogenesis. CONCLUSIONS: These results demonstrate critical functions of Ocn-Cre(+) dDG neurons in suppressing anxiety-like behaviors, but promoting adult DG neurogenesis, and both functions are likely through activation of BDNF.

The distribution and (patho-)physiological role of neuropeptides in the adult and aging brain have been extensively studied. Galanin is an inhibitory neuropeptide that can coexist with ã-aminobutyric acid (GABA) in the adult forebrain. However, galanin's expression sites, mode of signaling, impact on neuronal morphology, and colocalization with amino acid neurotransmitters during brain development are less well understood. Here, we show that galaninergic innervation of cholinergic projection neurons, which preferentially express galanin receptor 2 (GalR2) in the neonatal mouse basal forebrain, develops by birth. Nerve growth factor (NGF), known to modulate cholinergic morphogenesis, increases GalR2 expression. GalR2 antagonism (M871) in neonates reduces the in vivo expression and axonal targeting of the vesicular acetylcholine transporter (VAChT), indispensable for cholinergic neurotransmission. During cholinergic neuritogenesis in vitro, GalR2 can recruit Rho-family GTPases to induce the extension of a VAChT-containing primary neurite, the prospective axon. In doing so, GalR2 signaling dose-dependently modulates directional filopodial growth and antagonizes NGF-induced growth cone differentiation. Galanin accumulates in GABA-containing nerve terminals in the neonatal basal forebrain, suggesting its contribution to activity-driven cholinergic development during the perinatal period. Overall, our data define the cellular specificity and molecular complexity of galanin action in the developing basal forebrain.

Brain-derived neurotrophic factor (BDNF) is a key regulator of neuronal plasticity and cognitive functions. BDNF val66met polymorphism, a human single-nucleotide polymorphism (SNP) in the pro-domain of BDNF gene, is associated with deficits in activity-dependent BDNF secretion and hippocampus-dependent memory. However, the underlying mechanism remains unclear. Here we show that in the BDNFMet/Met mouse line mimicking the human SNP, BDNF expression in the hippocampus was decreased. There was a reduction in the total number of cells in hippocampal CA1 region, while hippocampal expression of mRNAs for NR2a, 2b, GluR1, 2 and GABAARβ3 subunits were up-regulated. Although basal glutamatergic neurotransmission was unaltered, hippocampal long-term depression (LTD) induced by low-frequency stimulation was impaired, which was partially rescued by exogenous application of BDNF. Interestingly, 5-HT3a receptors were down-regulated in the hippocampus of BDNFMet/Met mice, whereas 5-HT2c receptors were up-regulated. Moreover, impaired LTD in BDNFMet/Met mice was reversed by 5-HT3aR agonist. Thus, these observations indicate that BDNF val66met polymorphism changes hippocampal synaptic plasticity via down-regulation of 5-HT3a receptors, which may underlie cognition dysfunction of Met allele carriers.

Brain-derived neurotrophic factor (BDNF) is a key regulator of neuronal plasticity and cognitive functions. BDNF val66met polymorphism, a human single-nucleotide polymorphism (SNP) in the pro-domain of BDNF gene, is associated with deficits in activity-dependent BDNF secretion and hippocampus-dependent memory. However, the underlying mechanism remains unclear. Here we show that in the BDNFMet/Met mouse line mimicking the human SNP, BDNF expression in the hippocampus was decreased. There was a reduction in the total number of cells in hippocampal CA1 region, while hippocampal expression of mRNAs for NR2a, 2b, GluR1, 2 and GABAARβ3 subunits were up-regulated. Although basal glutamatergic neurotransmission was unaltered, hippocampal long-term depression (LTD) induced by low-frequency stimulation was impaired, which was partially rescued by exogenous application of BDNF. Interestingly, 5-HT3a receptors were down-regulated in the hippocampus of BDNFMet/Met mice, whereas 5-HT2c receptors were up-regulated. Moreover, impaired LTD in BDNFMet/Met mice was reversed by 5-HT3aR agonist. Thus, these observations indicate that BDNF val66met polymorphism changes hippocampal synaptic plasticity via down-regulation of 5-HT3a receptors, which may underlie cognition dysfunction of Met allele carriers.

Magnetic manipulation of cell activity offers advantages over optical manipulation but an ideal tool remains elusive. The MagR protein was found through its interaction with cryptochrome (Cry) and the protein in solution appeared to respond to magnetic stimulation (MS). After we initiated an investigation on the specific role of MagR in cellular response to MS, a subsequent study claimed that MagR expression alone could achieve cellular activation by MS. Here we report that despite systematically testing different ways of measuring intracellular calcium and different magnetic stimulation protocols, it was not possible to detect any cellular or neuronal responses to magnetic stimulation in MagR-expressing HEK cells or primary neurons from the dorsal root ganglion and the hippocampus. By contrast, in neurons co-expressing MagR and channelrhodopin, optical but not magnetic stimulation increased calcium influx in hippocampal neurons. Our results indicate that MagR alone is not sufficient to confer cellular magnetic responses.

We have recently developed aged cortical neuron cultures from autopsied human brains with Alzheimer's disease (AD). During the culturing process, we found that glutamatergic cortical neurons from the AD brain lacked a response to glial cell line-derived neurotrophic factor (GDNF), including no axonal regrowth, and were starting to undergo apoptosis. Here we showed that, in cortical neurons from age- and gender-matched cognitively normal control (NC) subjects (NC neurons), GDNF enhanced the expression of GDNF family receptor subtype α1 (GFRα1), but not the other three subtypes (GFRα2, GFRα3, and GFRα4), whereas GDNF failed to induce GFRα1 expression in cortical neurons from the AD brain (AD neurons). The exogenous introduction of GFRα1, but not of its binding partner α1-neural cell adhesion molecule, or RET into AD neurons restored the effect of GDNF on neuronal survival. Moreover, between NC and AD neurons, the AMPA receptor blocker CNQX and the NMDA receptor blocker AP-5 had opposite effects on the GFRα1 expression induced by GDNF. In NC neurons, the presence of glutamate receptors was necessary for GDNF-linked GFRα1 expression, while in AD neurons the absence of glutamate receptors was required for GFRα1 expression by GDNF stimulation. These results suggest that, in AD neurons, specific impairments of GFRα1, which may be linked to glutamatergic neurotransmission, shed light on developing potential therapeutic strategies for AD by upregulation of GFRα1 expression.

During development, mammalian neuromuscular junctions (NMJs) transit from multiple-innervation to single-innervation through axonal competition via unknown molecular mechanisms. Previously, using an in vitro model system, we demonstrated that the postsynaptic secretion of pro-brain-derived neurotrophic factor (proBDNF) stabilizes or eliminates presynaptic axon terminals, depending on its proteolytic conversion at synapses. Here, using developing mouse NMJs, we obtained in vivo evidence that proBDNF and mature BDNF (mBDNF) play roles in synapse elimination. We observed that exogenous proBDNF promoted synapse elimination, whereas mBDNF infusion substantially delayed synapse elimination. In addition, pharmacological inhibition of the proteolytic conversion of proBDNF to mBDNF accelerated synapse elimination via activation of p75 neurotrophin receptor (p75(NTR)). Furthermore, the inhibition of both p75(NTR) and sortilin signaling attenuated synapse elimination. We propose a model in which proBDNF and mBDNF serve as potential "punishment" and "reward" signals for inactive and active terminals, respectively, in vivo.

Hippocampal development is coordinated by both extracellular factors like GABA neurotransmission and intracellular components like DISC1. We previously reported that SLC12A2-dependent GABA depolarization and DISC1 coregulate hippocampal neuronal development, and 2 SNPs in these genes linked to mRNA expression interactively increase schizophrenia risk. Using functional MRI, we now confirm this biological interaction in vivo by showing in 2 independent samples of healthy individuals (total N = 349) that subjects homozygous for both risk alleles evince dramatically decreased hippocampal area activation (Cohen’s d = 0.78) and connectivity (d = 0.57) during a recognition memory task. These data highlight the importance of epistatic models in understanding genetic association with complex brain phenotypes.

Increasing evidence suggests that synaptic dysfunction is a key pathophysiological hallmark in neurodegenerative disorders, including Alzheimer's disease. Understanding the role of brain-derived neurotrophic factor (BDNF) in synaptic plasticity and synaptogenesis, the impact of the BDNF Val66Met polymorphism in Alzheimer's disease-relevant endophenotypes - including episodic memory and hippocampal volume - and the technological progress in measuring synaptic changes in humans all pave the way for a 'synaptic repair' therapy for neurodegenerative diseases that targets pathophysiology rather than pathogenesis. This article reviews the key issues in translating BDNF biology into synaptic repair therapies.

Alzheimer's disease is a growing disease burden that affects millions of people worldwide and is expected to double by the year 2030 as the current standard of care only partially treats the symptom. A disease modifying treatment is in urgent need. A major challenge is to demonstrate disease-modification in the clinic, as the current treatment measures monitor mostly pathological biomarkers that are not linearly related to the clinical disease. Since synaptic dysfunction is directly related to cognitive deficits even in early stages of cognitive impairment, attempts are in progress to directly measure synaptic function non-invasively and some of the tools and pharmacological agents to measure synaptic function both in clinic and preclinical models are reviewed here. Finally, we propose measuring synaptic function could be used as an experimental medicine readout while testing disease-modifying agents in the clinic.

SummaryHow extrinsic stimuli and intrinsic factors interact to regulate continuous neurogenesis in the postnatal mammalian brain is unknown. Here we show that regulation of dendritic development of newborn neurons by Disrupted-in-Schizophrenia 1 (DISC1) during adult hippocampal neurogenesis requires neurotransmitter GABA-induced, NKCC1-dependent depolarization through a convergence onto the AKT-mTOR pathway. In contrast, DISC1 fails to modulate early-postnatal hippocampal neurogenesis when conversion of GABA-induced depolarization to hyperpolarization is accelerated. Extending the period of GABA-induced depolarization or maternal deprivation stress restores DISC1-dependent dendritic regulation through mTOR pathway during early-postnatal hippocampal neurogenesis. Furthermore, DISC1 and NKCC1 interact epistatically to affect risk for schizophrenia in two independent case control studies. Our study uncovers an interplay between intrinsic DISC1 and extrinsic GABA signaling, two schizophrenia susceptibility pathways, in controlling neurogenesis and suggests critical roles of developmental tempo and experience in manifesting the impact of susceptibility genes on neuronal development and risk for mental disorders.

Background Stress is causally associated with anxiety. Although the underlying cellular mechanisms are not well understood, the basal forebrain cholinergic neurons have been implicated in stress response. p75 NTR is a panneurotrophin receptor expressed almost exclusively in basal forebrain cholinergic neurons in adult brain. This study investigated whether and how p75 NTR , via regulation of the cholinergic system and hippocampal synaptic plasticity, influences stress-related behaviors. Methods We used a combination of slice electrophysiology, behavioral analyses, pharmacology, in vivo microdialysis, and neuronal activity mapping to assess the role of p75 NTR in mood and stress-related behaviors and its underlying cellular and molecular mechanisms. Results We show that acute stress enables hippocampal long-term depression (LTD) in adult wild-type mice but not in mice lacking p75 NTR . The p75 NTR mutant mice also exhibit two distinct behavioral impairments: baseline anxiety-like behavior and a deficit in coping with and recovering from stressful situations. Blockade of stress-enabled LTD with a GluA2-derived peptide impaired stress recovery without affecting baseline anxiety. Pharmacological manipulations of cholinergic transmission mimicked the p75 NTR perturbation in both baseline anxiety and responses to acute stress. Finally, we show evidence of misregulated cholinergic signaling in animals with p75 NTR deletion. Conclusions Our results suggest that loss of p75 NTR leads to changes in hippocampal cholinergic signaling, which may be involved in regulation of stress-enabled hippocampal LTD and in modulating behaviors related to stress and anxiety.

SummaryDisrupted-in Schizophrenia 1 (DISC1), a susceptibility gene for major mental disorders, encodes a scaffold protein that has a multifaceted impact on neuronal development. How DISC1 regulates different aspects of neuronal development is not well understood. Here, we show that Fasciculation and Elongation Protein Zeta-1 (FEZ1) interacts with DISC1 to synergistically regulate dendritic growth of newborn neurons in the adult mouse hippocampus, and that this pathway complements a parallel DISC1-NDEL1 interaction that regulates cell positioning and morphogenesis of newborn neurons. Furthermore, genetic association analysis of two independent cohorts of schizophrenia patients and healthy controls reveals an epistatic interaction between FEZ1 and DISC1, but not between FEZ1 and NDEL1, for risk of schizophrenia. Our findings support a model in which DISC1 regulates distinct aspects of neuronal development through its interaction with different intracellular partners and such epistasis may contribute to increased risk for schizophrenia.

Synaptic plasticity is the ability of synaptic connections between neurons to be strengthened or weakened; a process that is central to the information processing within the brain and which plays a particularly important role in enabling higher cognitive processes [1,2]. Its role in disease is becoming increasingly clear across a wide spectrum of CNS disorders. Thus, for example, dysfunctional synaptic plasticity has been reported in neurodegenerative disorders such as Alzheimer's Disease (AD) as well as in schizophrenia and in a range of disorders associated with learning disabilities [3]. Moreover, maladaptive plasticity processes in response to specific external challenges are believed to underlie disorders such as addiction and post-traumatic stress disorder (PTSD). The molecular basis of normal and disease plasticity is rapidly being unravelled such that synaptic plasticity now provides a unique platform from which to launch the hunt for highly innovative drugs to treat CNS disease by either, firstly, rectifying identifiable abnormalities in these processes, or secondly, utilizing these processes as a vehicle to rectify, or bypass, other mechanisms underlying disease. In this respect, recent advances have been made in studying synaptic plasticity in humans at the molecular through to clinical level and these approaches now provide a real opportunity to test synaptic plasticity as a treatment paradigm for a wide variety of CNS disorders.

Late-phase long-term potentiation (L-LTP), a cellular model for long-term memory (LTM), requiresde novoprotein synthesis. An attractive hypothesis for synapse specificity of long-term memory is “synaptic tagging”: synaptic activity generates a tag, which “captures” the PRPs (plasticity-related proteins) derived outside of synapses. Here we provide evidence that TrkB, the receptor of BDNF (brain-derived neurotrophic factor), may serve as a “synaptic tag.” TrkB is transiently activated by weak theta-burst stimulation (TBS) that induces only early-phase LTP (E-LTP). This TrkB activation is independent of protein synthesis, and confined to stimulated synapses. Induction of L-LTP by strong stimulation in one synaptic pathway converts weak TBS-induced E-LTP to L-LTP in a second, independent pathway. Transient inhibition of TrkB around the time of weak TBS to the second pathway diminished L-LTP in that pathway without affecting the first one. Behaviorally, weak training, which induces short-term memory (STM) but not LTM, can be consolidated into LTM by exposing animals to novel but not familiar environment 1 h before training. Inhibition of TrkB during STM training blocked such consolidation. These results suggest TrkB as a potential tag for synapse-specific expression of L-LTP and LTM.

Mechanisms underlying experience-dependent refinement of cortical connections, especially GABAergic inhibitory circuits, are unknown. By using a line of mutant mice that lack activity-dependent BDNF expression ( bdnf -KIV), we show that experience regulation of cortical GABAergic network is mediated by activity-driven BDNF expression. Levels of endogenous BDNF protein in the barrel cortex are strongly regulated by sensory inputs from whiskers. There is a severe alteration of excitation and inhibition balance in the barrel cortex of bdnf -KIV mice as a result of reduced inhibitory but not excitatory conductance. Within the inhibitory circuits, the mutant barrel cortex exhibits significantly reduced levels of GABA release only from the parvalbumin-expressing fast-spiking (FS) interneurons, but not other interneuron subtypes. Postnatal deprivation of sensory inputs markedly decreased perisomatic inhibition selectively from FS cells in wild-type but not bdnf -KIV mice. These results suggest that postnatal experience, through activity-driven BDNF expression, controls cortical development by regulating FS cell-mediated perisomatic inhibition in vivo.

Despite its increasing use in experimental and clinical settings, the cellular and molecular mechanisms underlying transcranial direct current stimulation (tDCS) remain unknown. Anodal tDCS applied to the human motor cortex (M1) improves motor skill learning. Here, we demonstrate in mouse M1 slices that DCS induces a long-lasting synaptic potentiation (DCS-LTP), which is polarity specific, NMDA receptor dependent, and requires coupling of DCS with repetitive low-frequency synaptic activation (LFS). Combined DCS and LFS enhance BDNF-secretion and TrkB activation, and DCS-LTP is absent in BDNF and TrkB mutant mice, suggesting that BDNF is a key mediator of this phenomenon. Moreover, the BDNF val66met polymorphism known to partially affect activity-dependent BDNF secretion impairs motor skill acquisition in humans and mice. Motor learning is enhanced by anodal tDCS, as long as activity-dependent BDNF secretion is in place. We propose that tDCS may improve motor skill learning through augmentation of synaptic plasticity that requires BDNF secretion and TrkB activation within M1.

BDNF has pleiotropic effects on neuronal development and synaptic plasticity that underlie circuit formation and cognitive function. Recent breakthroughs reveal that neuronal activity regulates BDNF cell biology, includingBdnftranscription, dendritic targeting and trafficking of BDNF mRNA and protein, and secretion and extracellular conversion of proBDNF to mature BDNF. Defects in these mechanisms contribute differentially to cognitive dysfunction and anxiety-like behaviors. Here we review recent studies, presented at a symposium at Neuroscience 2009, that describe regulatory mechanisms that permit rapid and dynamic refinement of BDNF actions in neurons.

Postsynaptic cells generate positive and negative signals that retrogradely modulate presynaptic function. At developing neuromuscular synapses, prolonged stimulation of muscle cells induces sustained synaptic depression. We provide evidence that pro–brain-derived neurotrophic factor (BDNF) is a negative retrograde signal that can be converted into a positive signal by metalloproteases at the synaptic junctions. Application of pro-BDNF induces a dramatic decrease in synaptic efficacy followed by a retraction of presynaptic terminals, and these effects are mediated by presynaptic pan-neurotrophin receptor p75 (p75NTR), the pro-BDNF receptor. A brief stimulation of myocytes expressing cleavable or uncleavable pro-BDNF elicits synaptic potentiation or depression, respectively. Extracellular application of metalloprotease inhibitors, which inhibits the cleavage of endogenous pro-BDNF, facilitates the muscle stimulation–induced synaptic depression. Inhibition of presynaptic p75NTR or postsynaptic BDNF expression also blocks the activity-dependent synaptic depression and retraction. These results support a model in which postsynaptic secretion of a single molecule, pro-BDNF, may stabilize or eliminate presynaptic terminals depending on its proteolytic conversion at the synapses.

The functions of the nervous system are mediated by neural circuits that are formed during development and modulated by experiences. Central to the assembly of neural circuits is the regulation of synaptic connectivity by synaptic molecules and neuronal activity. Extensive studies have focused on identifying molecules involved in synapse formation. Besides factors promoting synaptogenesis, several anti-synaptogenic factors have been discovered. These factors act in concert to ensure the timing and specificity of circuit formation. Moreover, progress has been made in revealing how neuronal activity governs the balance of excitatory and inhibitory synapses. Intriguingly, several transcription factors, as well as activity-dependent transcription of BDNF through promoter IV, have been shown to selectively regulate cortical inhibitory circuits by promoting GABAergic synapse formation.

Brain-derived neurotrophic factor (BDNF) and serotonin (5-hydroxytryptamine, 5-HT) are two seemingly distinct signaling systems that play regulatory roles in many neuronal functions including survival, neurogenesis, and synaptic plasticity. A common feature of the two systems is their ability to regulate the development and plasticity of neural circuits involved in mood disorders such as depression and anxiety. BDNF promotes the survival and differentiation of 5-HT neurons. Conversely, administration of antidepressant selective serotonin reuptake inhibitors (SSRIs) enhances BDNF gene expression. There is also evidence for synergism between the two systems in affective behaviors and genetic epitasis between BDNF and the serotonin transporter genes.

SummaryAdult neurogenesis occurs throughout life in discrete regions of the adult mammalian brain. Little is known about the mechanism governing the sequential developmental process that leads to integration of new neurons from adult neural stem cells into the existing circuitry. Here, we investigated roles of Disrupted-In-Schizophrenia 1 (DISC1), a schizophrenia susceptibility gene, in adult hippocampal neurogenesis. Unexpectedly, downregulation of DISC1 leads to accelerated neuronal integration, resulting in aberrant morphological development and mispositioning of new dentate granule cells in a cell-autonomous fashion. Functionally, newborn neurons with DISC1 knockdown exhibit enhanced excitability and accelerated dendritic development and synapse formation. Furthermore, DISC1 cooperates with its binding partner NDEL1 in regulating adult neurogenesis. Taken together, our study identifies DISC1 as a key regulator that orchestrates the tempo of functional neuronal integration in the adult brain and demonstrates essential roles of a susceptibility gene for major mental illness in neuronal development, including adult neurogenesis.

Brain-derived neurotrophic factor (BDNF) regulates diverse biological functions ranging from neuronal survival and differentiation during development to synaptic plasticity and cognitive behavior in the adult. BDNF disruption in both rodents and humans is associated with neurobehavioral alterations and psychiatric disorders. A unique feature of Bdnf transcription is regulation by nine individual promoters, which drive expression of variants that encode an identical protein. It is hypothesized that this unique genomic structure may provide flexibility that allows different factors to regulate BDNF signaling in distinct cell types and circuits. This has led to the suggestion that isoforms may regulate specific BDNF-dependent functions; however, little scientific support for this idea exists. We generated four novel mutant mouse lines in which BDNF production from one of the four major promoters (I, II, IV, or VI) is selectively disrupted (Bdnf-e1, -e2, -e4, and -e6 mice) and used a comprehensive comparator approach to determine whether different Bdnf transcripts are associated with specific BDNF-dependent molecular, cellular, and behavioral phenotypes. Bdnf-e1 and -e2 mutant males displayed heightened aggression accompanied by convergent expression changes in specific genes associated with serotonin signaling. In contrast, BDNF-e4 and -e6 mutants were not aggressive but displayed impairments associated with GABAergic gene expression. Moreover, quantifications of BDNF protein in the hypothalamus, prefrontal cortex, and hippocampus revealed that individual Bdnf transcripts make differential, region-specific contributions to total BDNF levels. The results highlight the biological significance of alternative Bdnf transcripts and provide evidence that individual isoforms serve distinct molecular and behavioral functions.

Expression of the Ca2+-dependent phospholipids binding protein annexin A2 (ANX2) in the brain is thought to be largely associated with brain pathological conditions such as tumor, inflammation, and neurodegeneration. The recent findings that ANX2 heterotetramer is involved in learning and neuronal activities necessitates a systematic investigation of the physiological expression of ANX2 in the brain. With combination of in situ hybridization and immunohistochemistry, ANX2 mRNA and protein were specifically detected in a group of GABAergic interneurons throughout the brain. Although ANX2 was absent from the interior of pyramidal neurons, it was found on the membrane and seemly the extracellular space of those neurons, where they closely co-localized with glutamate decarboxylase terminals. In cultured developing neurons, ANX2 was present at high concentrations in the growth cones co-distributing with several growth-associated proteins such as growth associated protein 43 (GAP43), turned on after division/Ulip/CRMP (TUC-4), tubulin, and tissue-plasminogen activator. It then became predominantly distributed on the membrane and mostly in axonal branches as neurons grew and extended synaptic networks. ANX2 was also secreted from cultured neurons, in a membrane-bound form that was Ca2+-dependent, which was significantly increased by neuronal depolarization. These results may have implications in the function and regulatory mechanism of ANX2 in the normal brain.

In the adult brain, the expression of NT-3 is largely confined to the hippocampal dentate gyrus (DG), an area exhibiting significant neurogenesis. Using a conditional mutant line in which the NT-3 gene is deleted in the brain, we investigated the role of NT-3 in adult neurogenesis, hippocampal plasticity, and memory. Bromodeoxyuridine (BrdU)-labeling experiments demonstrated that differentiation, rather than proliferation, of the neuronal precursor cells (NPCs) was significantly impaired in DG lacking NT-3. Triple labeling for BrdU, the neuronal marker NeuN, and the glial marker GFAP indicated that NT-3 affects the number of newly differentiated neurons, but not glia, in DG. Field recordings revealed a selective impairment in long-term potentiation (LTP) in the lateral, but not medial perforant path-granule neuron synapses. In parallel, the NT-3 mutant mice exhibited deficits in spatial memory tasks. In addition to identifying a novel role for NT-3 in adult NPC differentiation in vivo, our study provides a potential link between neurogenesis, dentate LTP, and spatial memory.

Parvalbumin-positive interneurons, which include basket and chandelier cells, represent a unique class of interneurons. By innervating the soma and the axonal initial segment of pyramidal cells, these interneurons can elicit powerful control on the output of pyramidal cells and consequently are important for a number of physiological processes in the mammalian brain. Recent evidence indicates that neurotrophins regulate the development and functions of parvalbumin-positive interneurons. Disruption of neurotrophin-mediated regulation of interneurons is thought to contribute to the pathological processes underlying CNS dysfunction. This review brings together recently described roles of neurotrophins in migration, differentiation, synaptogenesis during development, and acute effects of neurotrophins in transmission at inhibitory synapses, Cl-homeostasis, and network activity of cortical interneurons. The authors also discuss the importance of neurotrophin regulation of GABAergic neurons in schizophrenia and epilepsy.

Although neurotrophins elicit both acute and long-term effects, it is unclear whether the two modes of action are mediated by the same or different mechanisms. Using neuromuscular junction (NMJ) as a model system, we identified three characteristic features required for long-term, but not acute, forms of synaptic modulation by neurotrophin-3 (NT-3): endocytosis of NT-3-receptor complex, activation of the PI3 kinase substrate Akt, and new protein synthesis. Long-term effects were eliminated when NT-3 was conjugated to a bead that was too large to be endocytosed or when dominant-negative dynamin was expressed in presynaptic neurons. Presynaptic inhibition of Akt also selectively prevented NT-3-mediated long-term effects. Blockade of protein translation by the mammalian target of rapamycin inhibitor rapamycin prevented the long-term structural and functional changes at the NMJ, without affecting the acute potentiation of synaptic transmission by NT-3. These results reveal fundamental differences between acute and long-term modulation by neurotrophins.

Neurotrophins have diverse functions in the CNS. Initially synthesized as precursors (proneurotrophins), they are cleaved to produce mature proteins, which promote neuronal survival and enhance synaptic plasticity by activating Trk receptor tyrosine kinases. Recent studies indicate that proneurotrophins serve as signalling molecules by interacting with the p75 neurotrophin receptor (p75NTR). Interestingly, proneurotrophins often have biological effects that oppose those of mature neurotrophins. Therefore, the proteolytic cleavage of proneurotrophins represents a mechanism that controls the direction of action of neurotrophins. New insights into the 'yin and yang' of neurotrophin activity have profound implications for our understanding of the role of neurotrophins in a wide range of cellular processes.

Synaptic actions of brain-derived neurotrophic factor (BDNF) are 'gated' by cyclic AMP (cAMP), but the underlying molecular mechanisms remain unclear. Here we report that cAMP regulates BDNF function in mature hippocampal neurons by modulating the signaling and trafficking of its receptor TrkB. cAMP gated the TrkB tyrosine kinase with three characteristic features: BDNF-induced TrkB phosphorylation was attenuated by inhibitors of cAMP signaling, it was potentiated by cAMP analogs, and activation of the cAMP pathway alone had no effect. In addition, cAMP facilitated trafficking of TrkB to dendritic spines, possibly by promoting its interaction with synaptic scaffolding protein PSD-95. Norepinephrinergic and dopaminergic agonists, which elevate intracellular cAMP concentration, also enhanced TrkB phosphorylation and its translocation to spines. cAMP gated long-term modulation by BDNF of spine density, but not the number of primary dendrites. These results reveal a specific role of cAMP in controlling BDNF actions in the brain, and provide new insights into the molecular mechanism underlying cAMP gating.

Long-lasting forms of memory are generally believed to be mediated by protein synthesis-dependent, late-phase long-term potentiation (L-LTP). L-LTP exhibits at least two distinctive characteristics compared with early phase LTP (E-LTP): synaptic growth and requirement of gene transcription and new protein synthesis. In this review, we discuss the cellular and molecular mechanisms underlying the structural and functional changes of hippocampal synapses during L-LTP, in the context of long-term memory. We describe experiments that reveal the critical role of cAMP/protein kinase A and MAP kinase pathways, and the downstream transcription factor CREB. Because transcription-dependent long-term changes are input specific, we also discuss the role of “local protein synthesis” and “synaptic tagging” mechanisms that may confer synapse specificity. We then focus on brain-derived neurotrophic factor (BDNF) and tissue plasmigonen activator (tPA), two secreted proteins that have been repeatedly implicated in L-LTP. Biochemical and molecular biology experiments indicate that the expression and secretion of both factors are enhanced by strong tetanic stimulation that induces L-LTP as well as by training in hippocampal-dependent memory tasks. Inhibition of either tPA or BDNF by gene knockout and specific inhibitors results in a significant impairments in L-LTP and long-term memory. Further work will be required to address the relationship between BDNF and tPA in various forms of synaptic plasticity, and the mechanisms by which BDNF/tPA achieves synapse-specific modulation. Finally, we discuss how the aging process affects L-LTP and long-term memory.

Neurotrophins are synthesized first as precursors, followed by maturation through proteolytic removal of the "pro" region. Since pro- and mature neurotrophins elicit opposite functional effects by differential interactions with Trks and p75 receptors, extracellular cleavage represents a new way to control the synaptic functions of neurotrophins. A single nucleotide mutation in the pro-region appears to affect synaptic targeting and activity-dependent secretion of BDNF in hippocampal neurons. These results demonstrate new mechanisms by which neurotrophins regulate synaptic plasticity and memory function.

It is widely accepted that neuronal activity plays a pivotal role in synaptic plasticity. Neurotrophins have emerged recently as potent factors for synaptic modulation. The relationship between the activity and neurotrophic regulation of synapse development and plasticity, however, remains unclear. A prevailing hypothesis is that activity-dependent synaptic modulation is mediated by neurotrophins. An important but unresolved issue is how diffusible molecules such as neurotrophins achieve local and synapse-specific modulation. In this review, I discuss several potential mechanisms with which neuronal activity could control the synapse-specificity of neurotrophin regulation, with particular emphasis on BDNF. Data accumulated in recent years suggest that neuronal activity regulates the transcription of BDNF gene, the transport of BDNF mRNA and protein into dendrites, and the secretion of BDNF protein. There is also evidence for activity-dependent regulation of the trafficking of the BDNF receptor, TrkB, including its cell surface expression and ligand-induced endocytosis. Further study of these mechanisms will help us better understand how neurotrophins could mediate activity-dependent plasticity in a local and synapse-specific manner.

Glial cell line-derived neurotrophic factor (GDNF) is best known for its long-term survival effect on dopaminergic neurons in the ventral midbrain. A recent study showed that acute application of GDNF to these neurons suppresses A-type potassium channels and potentiates neuronal excitability. Here we have characterized the acute effects of GDNF on Ca2+ channels and synaptic transmission. GDNF rapidly and reversibly potentiated the high voltage-activated (HVA) Ca2+ channel currents in cultured dopaminergic neurons. Analyses of channel kinetics indicate that GDNF decreased the activation time constant, increased the inactivation and deactivation time constants of HVA Ca2+ channel currents. Ca2+ imaging experiments demonstrate that GDNF facilitated Ca2+ influx induced by membrane depolarization. To investigate the physiological consequences of the Ca2+ channel modulation, we examined the acute effects of GDNF on excitatory synaptic transmission at synapses made by these dopaminergic neurons, which co-release the transmitter glutamate. Within 3 min of application, GDNF increased the amplitude of spontaneous and evoked excitatory autaptic- or multiple-postsynaptic currents. The frequency as well as the amplitude of miniature excitatory postsynaptic currents was also increased. These results reveal, for the first time, an acute effect of GDNF on synaptic transmission and its potential mechanisms, and suggest that an important function of GDNF for midbrain dopaminergic neurons is the acute modulation of transmission and ion channels. Copyright (C) 2003 S. Karger AG, Basel.

An important aspect of synapse development is the clustering of neurotransmitter receptors in the postsynaptic membrane. Although MuSK is required for acetylcholine receptor (AChR) clustering at the neuromuscular junction (NMJ), the underlying molecular mechanisms remain unclear. We report here that in muscle cells, MuSK interacts with Dishevelled (Dvl), a signaling molecule important for planar cell polarity. Disruption of the MuSK-Dvl interaction inhibits Agrin- and neuron-induced AChR clustering. Expression of dominant-negative Dvl1 in postsynaptic muscle cells reduces the amplitude of spontaneous synaptic currents at the NMJ. Moreover, Dvl1 interacts with downstream kinase PAK1. Agrin activates PAK, and this activation requires Dvl. Inhibition of PAK1 activity attenuates AChR clustering. These results demonstrate important roles of Dvl and PAK in Agrin/MuSK-induced AChR clustering and reveal a novel function of Dvl in synapse development.

The brain-derived neurotropic factor (BDNF) Val66Met polymorphism has been associated with abnormalities of synaptic plasticity in animal models, and abnormalities in motor cortical plasticity have also been described in humans using transcranial direct current stimulation. No study has yet been done on plasticity in non-motor regions, and the effect of two Met alleles (i.e. ‘Met dose’) is not well understood. We studied the effect of the BDNF Val66Met polymorphism on the after-effects of transcranial direct current stimulation and tetanic auditory stimulation in 65 subjects (23; Val66Val, 22; Val66Met and 20; Met66Met genotypes). In the first session, motor evoked potentials (MEP) were recorded under stereotaxic guidance for 90 min after 9 min of anodal transcranial direct current stimulation (TDCS). In the second session, auditory-evoked potentials (AEP) were recorded before and after 2 min of auditory 13 Hz tetanic stimulation. There was a difference in MEP facilitation post-TDCS comparing Met carriers with non-Met carriers, with Met carriers having a modest late facilitation at 30–90 min. There was no difference in responses between Val66Met genotype and Met66Met genotype subjects. Tetanic auditory stimulation also produced late facilitation of N1-P2 AEP at 25 min, but there was no apparent effect of genetic status. This study indicates that Met66Met carriers behave like Val66Met carriers for TDCS-induced plasticity, and produce a late facilitation of MEPs. Auditory cortical plasticity was not affected by the BDNF Val66Met polymorphism. This study sheds light on the differences between auditory and motor cortical plasticity and the role of the BDNF Val66Met polymorphism.

Signaling mechanisms underlying neurotrophic regulation of synaptic transmission are not fully understood. Here we show that neurotrophin-3 (NT3)-induced potentiation of synaptic transmission at the neuromuscular synapses is blocked by inhibition of phosphoinositide-3 kinase, phospholipase C-gamma or the downstream IP3 receptors of phospholipase C-gamma, but not by inhibition of MAP kinase. However, neither stimulation of Ca2+ release from intracellular stores by photolysis of caged IP3, nor expression of a constitutively active phosphoinositide-3 kinase (PI3K(star)) in presynaptic motoneurons alone is sufficient to enhance transmission. Photo-uncaging of IP3 in neurons expressing PI3K(star) elicits a marked synaptic potentiation, mimicking the NT3 effect. These results reveal an involvement of PI3 kinase in transmitter release, and suggest that concomitant activation of PI3 kinase and IP3 receptors is both necessary and sufficient to mediate the NTS-induced synaptic potentiation.

Pro–brain-derived neurotrophic factor (proBDNF) and mature BDNF utilize distinct receptors to mediate divergent neuronal actions. Using new tools to quantitate endogenous BDNF isoforms, we found that mouse neurons secrete both proBDNF and mature BDNF. The highest levels of proBDNF and p75 were observed perinatally and declined, but were still detectable, in adulthood. Thus, BDNF actions are developmentally regulated by secretion of proBDNF or mature BDNF and by local expression of p75 and TrkB.

Neurotrophins are traditionally thought to be secretory proteins that regulate long-term survival and differentiation of neurons. Recent studies have revealed a previously unexpected role for neurotrophins in synaptic development and plasticity in diverse neuronal populations. In this review, we focus on the synaptic function of brain-derived neurotrophic factor (BDNF) in the hippocampus. Although a variety of in vitro experiments have shown the ability of BDNF to acutely modulate synaptic transmission, whether BDNF truly potentiates basal synaptic transmission in hippocampal neurons remains controversial. More consistent evidence has been obtained for the role of BDNF in long-term potentiation (LTP), a cellular model for learning and memory. BDNF also potentiates high frequency transmission by modulating the number of docked vesicles and the levels of the vesicle protein synaptobrevin and synaptophysin at the CA1 synapses. Both pre- and postsynaptic effects of BDNF have been demonstrated. Recent studies have begun to address the role of BDNF in late-phase LTP and in the development of hippocampal circuit. BDNF and other neurotrophins may represent a new class of neuromodulators that regulate neuronal connectivity and synaptic efficacy. J. Neurosci. Res. 58:76-87, 1999. Published 1999 Wiley-Liss, Inc.

Neurotrophic factors are endogenous soluble proteins that regulate long-term survival and differentiation of neurons of the peripheral and central nervous systems. These factors play an important role in the structural integrity of the nervous system, and therefore are good candidates as therapeutic agents for neurodegenerative diseases. However, recent studies have revealed some unexpected, novel roles of neurotrophic factors. Of particular significance is the discovery of the new functions of brain-derived neurotrophic factor (BDNF) and glia-derived neurotrophic factor (GDNF). Physiological experiments indicate that BDNF may serve as regulatory factors for synaptic transmission as well as for learning and memory. Gene targeting studies demonstrate that GDNF may be essential for development of the enteric nervous system (ENS) and kidney organogenesis. These results not only provide new insights into our understanding of the function of neurotrophic factors but may also have significant implications in the therapeutic usages of neurotrophic factors.

Although activity-dependent expression of neurotrophins has been studied extensively in the CNS, its physiological role during synapse development is not well established. At the developing neuromuscular junction in culture, exogenous application of the neurotrophin BDNF or NT-3 has been shown to acutely potentiate synaptic transmission and chronically promote synapse maturation. Using the same cell culture model, we have investigated activity-dependent neurotrophin expression in muscle cells and its role in developing neuromuscular synapses. Membrane depolarization, elicited by either depolarizing agents or repetitive electric stimulation, rapidly and specifically increased the levels of NT-3 mRNA in developingXenopus laevismuscle cells in culture. NT-3 gene expression also was enhanced by acetylcholine (ACh), the neurotransmitter that causes muscle membrane depolarization. The effects of depolarization were mediated by increasing intracellular calcium concentration. Moreover, factor(s) induced by membrane depolarization appeared to enhance synaptic transmission at the developing neuromuscular junction. The frequency of spontaneous synaptic currents (SSCs) recorded from neuromuscular synapses was increased significantly after treatment with conditioned medium from depolarized muscle cultures. The amplitude, rise time, and decay time of SSCs were not affected, indicating a presynaptic action of the conditioned medium. The effects of the conditioned medium were blocked, partially, by the NT-3 scavenger TrkC-IgG, suggesting that the potentiation of synaptic efficacy was attributable, at least in part, to elevated NT-3 as a consequence of muscle depolarization. Thus, activity-dependent expression of muscle NT-3 may contribute to the development of the neuromuscular synapse.

Neurotrophic factors are secreted proteins that regulate the long-term survival and differentiation of neurones in the peripheral and central nervous systems (PNS and CNS). Their profound effects on the structural integrity of the nervous system have led to intense interest in the potential use of the factors as therapeutic agents for neurodegenerative diseases. In the present review, we summarise the current data on the physiological role of these proteins based on in vitro investigations and animal models, and discuss their clinical potential as therapeutic agents in the treatment of neurological disorders. A rationale for neurotrophic factor therapy can be built for a specific neurological disease once a particular neurotrophic factor is identified to regulate vulnerable neurones in the affected areas. Progress has been made in the use of neurotrophic factors as an alternative therapy to conventional drug treatment, particularly for Alzheimer’s disease, Parkinson’s disease and peripheral neuropathy. Pr...