Mechanisms, locations, and kinetics of synaptic BDNF secretion: an update. Neurosci Res. 2009;65:11-22. [PMID: 19523993 DOI: 10.1016/j.neures.2009.06.004]

Multiple approaches to investigate the transport and activity-dependent release of BDNF and their application in neurogenetic disorders

Studies utilizing genetic and pharmacological manipulations in rodent models and neuronal cultures have revealed myriad roles of brain-derived neurotrophic factor (BDNF). Currently, this knowledge of BDNF function is being translated into improvement strategies for several debilitating neurological disorders in which BDNF abnormalities play a prominent role. Common among the BDNF-related disorders are irregular trafficking and release of mature BDNF (mBDNF) and/or its prodomain predecessor, proBDNF. Thus, investigating the conditions required for proper trafficking and release of BDNF is an essential step toward understanding and potentially improving these neurological disorders. This paper will provide examples of disorders related to BDNF release and serve as a review of the techniques being used to study the trafficking and release of BDNF.

Specific sets of intrinsic and extrinsic factors drive excitatory and inhibitory circuit formation

How are excitatory (glutamatergic) and inhibitory (GABAergic) synapses established? Do distinct molecular mechanisms direct differentiation of glutamatergic and GABAergic synapses? In the brain, glutamatergic and GABAergic synaptic connections are formed with specific patterns. To establish such precise synaptic patterns, neurons pass through multiple checkpoints during development, such as cell fate determination, cell migration and localization, axonal guidance and target recognition, and synapse formation. Each stage offers key molecules for neurons/synapses to obtain glutamatergic or GABAergic specificity. Some mechanisms are based on intrinsic systems to induce gene expression, whereas others are based on extrinsic systems mediated by cell-cell or axon-target interactions. Recent studies indicate that specific formation of glutamatergic and GABAergic synapses is controlled by the expression or activation of different sets of molecules during development. In this review, the authors outline stages critical to the determination of glutamatergic or GABAergic specificity and describe molecules that act as determinants of specificities in each stage, with a particular focus on the synapse formation stage. They also discuss possible mechanisms underlying glutamatergic and GABAergic synapse formation via synapse-type specific synaptic organizers.

NMDA-dependent switch of proBDNF actions on developing GABAergic synapses

The brain-derived neurotrophic factor (BDNF) has emerged as an important messenger for activity-dependent development of neuronal network. Recent findings have suggested that a significant proportion of BDNF can be secreted as a precursor (proBDNF) and cleaved by extracellular proteases to yield the mature form. While the actions of proBDNF on maturation and plasticity of excitatory synapses have been studied, the effect of the precursor on developing GABAergic synapses remains largely unknown. Here, we show that regulated secretion of proBDNF exerts a bidirectional control of GABAergic synaptic activity with NMDA receptors driving the polarity of the plasticity. When NMDA receptors are activated during ongoing synaptic activity, regulated Ca(2+)-dependent secretion of proBDNF signals via p75(NTR) to depress GABAergic synaptic activity, while in the absence of NMDA receptors activation, secreted proBDNF induces a p75(NTR)-dependent potentiation of GABAergic synaptic activity. These results revealed a new function for proBDNF-p75(NTR) signaling in synaptic plasticity and a novel mechanism by which synaptic activity can modulate the development of GABAergic synaptic connections.

Increased basal plasma brain-derived neurotrophic factor levels in sprint runners

OBJECTIVE

Exercise is known to enhance circulating brain-derived neurotrophic factor (BDNF) levels in healthy humans. BDNF changes have been measured in endurance but not in strength exercise. The present study aimed to investigate whether anaerobic activity such as sprinting differentially alters basal plasma BDNF concentration.

METHODS

Brazilian sprinters (100 m) at either the international (Olympics and Outdoor World Championships) (n = 14) or the domestic level (n = 8), and sedentary subjects (n = 15), were recruited. Plasma BDNF concentrations were analyzed by enzyme-linked immunosorbent assay.

RESULTS

The basal plasma BDNF concentrations were significantly higher in the international and the domestic sprinters than in the sedentary subjects. In addition, sprinters at the international level had higher plasma BDNF concentrations than those at the domestic level.

CONCLUSION

Our findings suggest that increased basal plasma BDNF level is related to enhanced exercise performance.

UNC-73/trio RhoGEF-2 activity modulates Caenorhabditis elegans motility through changes in neurotransmitter signaling upstream of the GSA-1/Galphas pathway

Rho-family GTPases play regulatory roles in many fundamental cellular processes. Caenorhabditis elegans UNC-73 RhoGEF isoforms function in axon guidance, cell migration, muscle arm extension, phagocytosis, and neurotransmission by activating either Rac or Rho GTPase subfamilies. Multiple differentially expressed UNC-73 isoforms contain a Rac-specific RhoGEF-1 domain, a Rho-specific RhoGEF-2 domain, or both domains. The UNC-73E RhoGEF-2 isoform is activated by the G-protein subunit Gαq and is required for normal rates of locomotion; however, mechanisms of UNC-73 and Rho pathway regulation of locomotion are not clear. To better define UNC-73 function in the regulation of motility we used cell-specific and inducible promoters to examine the temporal and spatial requirements of UNC-73 RhoGEF-2 isoform function in mutant rescue experiments. We found that UNC-73E acts within peptidergic neurons of mature animals to regulate locomotion rate. Although unc-73 RhoGEF-2 mutants have grossly normal synaptic morphology and weak resistance to the acetylcholinesterase inhibitor aldicarb, they are significantly hypersensitive to the acetylcholine receptor agonist levamisole, indicating alterations in acetylcholine neurotransmitter signaling. Consistent with peptidergic neuron function, unc-73 RhoGEF-2 mutants exhibit a decreased level of neuropeptide release from motor neuron dense core vesicles (DCVs). The unc-73 locomotory phenotype is similar to those of rab-2 and unc-31, genes with distinct roles in the DCV-mediated secretory pathway. We observed that constitutively active Gαs pathway mutations, which compensate for DCV-mediated signaling defects, rescue unc-73 RhoGEF-2 and rab-2 lethargic movement phenotypes. Together, these data suggest UNC-73 RhoGEF-2 isoforms are required for proper neurotransmitter signaling and may function in the DCV-mediated neuromodulatory regulation of locomotion rate.

Activity-dependent transcription of BDNF enhances visual acuity during development

In the developing Xenopus tadpole, conditioning with 20 min of visual stimulation leads to increased proBDNF protein levels in the tectum measured 4 hr later. Following conditioning, the ability to induce direction selectivity in tectal neurons, as well as both retinotectal long-term potentiation and depression, thought to underlie this phenomenon, was strongly facilitated. This facilitation was blocked by knockdown of BDNF expression in tectal neurons. Animals that had been exposed to visual conditioning and subsequently received normal visual input for 7-11 hr exhibited higher spatial frequency thresholds of tectal cell responses to counterphasing gratings than nonconditioned control animals. An improvement in visual acuity was confirmed by enhanced sensitivity to counterphasing gratings in a behavioral test. These results indicate that brief sensory stimulation, by initiating nuclear transcription and de novo protein synthesis of BDNF, can facilitate the refinement of response properties in the developing visual system.

MicroRNA function and neurotrophin BDNF

MicroRNAs (miRs), endogenous small RNAs, regulate gene expression through repression of translational activity after binding to target mRNAs. miRs are involved in various cellular processes including differentiation, metabolism, and apoptosis. Furthermore, possible involvement of miRs in neuronal function have been proposed. For example, miR-132 is closely related to neuronal outgrowth while miR-134 plays a role in postsynaptic regulation, suggesting that brain-specific miRs are critical for synaptic plasticity. On the other hand, numerous studies indicate that BDNF (brain-derived neurotrophic factor), one of the neurotrophins, is essential for a variety of neuronal aspects such as cell differentiation, survival, and synaptic plasticity in the central nervous system (CNS). Interestingly, recent studies, including ours, suggest that BDNF exerts its beneficial effects on CNS neurons via up-regulation of miR-132. Here, we present a broad overview of the current knowledge concerning the association between neurotrophins and various miRs.

Contribution of metabotropic GABA(B) receptors to neuronal network construction

In the 1980s, Bowery and colleagues discovered the presence of a novel, bicuculline-resistant and baclofen-sensitive type of GABA receptor on peripheral nerve terminals, the GABA(B) receptor. Since this pioneering work, GABA(B) receptors have been identified in the Central Nervous System (CNS), where they provide an important inhibitory control of postsynaptic excitability and presynaptic transmitter release. GABA(B) receptors have been implicated in a number of important processes in the adult brain such as the regulation of synaptic plasticity and modulation of rhythmic activity. As a result of these studies, several potential therapeutic applications of GABA(B) receptor ligands have been identified. Recent advances have further shown that GABA(B) receptors play more than a classical inhibitory role in adult neurotransmission, and can in fact function as an important developmental signal early in life. Here we summarize current knowledge on the contribution of GABA(B) receptors to the construction and function of developing neuronal networks.

SNARE protein expression in synaptic terminals and astrocytes in the adult hippocampus: a comparative analysis

Several evidences suggest that astrocytes release small transmitter molecules, peptides, and protein factors via regulated exocytosis, implying that they function as specialized neurosecretory cells. However, very little is known about the molecular and functional properties of regulated secretion in astrocytes in the adult brain. Establishing these properties is central to the understanding of the communication mode(s) of these cells and their role(s) in the control of synaptic functions and of cerebral blood flow. In this study, we have set-up a high-resolution confocal microscopy approach to distinguish protein expression in astrocytic structures and neighboring synaptic terminals in adult brain tissue. This approach was applied to investigate the expression pattern of core SNARE proteins for vesicle fusion in the dentate gyrus and CA1 regions of the mouse hippocampus. Our comparative analysis shows that astrocytes abundantly express, in their cell body and main processes, all three protein partners necessary to form an operational SNARE complex but not in the same isoforms expressed in neighbouring synaptic terminals. Thus, SNAP25 and VAMP2 are absent from astrocytic processes and typically concentrated in terminals, while SNAP23 and VAMP3 have the opposite expression pattern. Syntaxin 1 is present in both synaptic terminals and astrocytes. These data support the view that astrocytes in the adult hippocampus can communicate via regulated exocytosis and also indicates that astrocytic exocytosis may differ in its properties from action potential-dependent exocytosis at neuronal synapses, as it relies on a distinctive set of SNARE proteins.

Brain-derived neurotrophic factor plasma variation during the different phases of the menstrual cycle in women with premenstrual syndrome

Premenstrual syndrome (PMS) is characterized by a cluster of psychological and somatic symptoms that begin during the late luteal phase of the menstrual cycle and disappear after the onset of menses. Since PMS might be caused by an alteration in the cyclical hormonal modifications and ovarian steroids are directly involved in the regulation of mood, affective and cognitive functions and influence neurotrophins expression, in particular the brain-derived neurotrophic factor (BDNF), we aimed to evaluate whether plasma BDNF levels in women with PMS differ from those of normally menstruating women without PMS. Sixty-two women were divided into two groups: one group of women (n=35) with PMS and one group (n=27) composed by normally menstruating women. Plasma samples were collected at day 7 (follicular phase) and day 21 (luteal phase) of the menstrual cycle. Plasma BDNF of the control group significantly increased (p<0.001) from the follicular phase (402.90±74.41pg/ml) to the luteal phase (1098.79±146.49pg/ml). On the other hand, in the PMS group plasma BDNF levels significantly decreased (p<0.001) from the follicular phase (412.45±78.35pg/ml) to the luteal phase (233.03±75.46pg/ml) Luteal BDNF levels of the PMS women were significantly lower than those of the control group (p<0.001). In women with PMS, plasma BDNF followed a decreasing trend during the ovarian cycle, in opposition to the increasing trend observed in women without PMS. The lower luteal BDNF levels of the PMS women might be a consequence of an altered hormonal response and might play a role in the onset of the symptoms PMS related.

Prenatal stress differentially alters brain-derived neurotrophic factor expression and signaling across rat strains

BACKGROUND

Psychiatric illness and anxiety disorders have strong neurodevelopmental components. Environmental insults such as prenatal exposure to stress and genetic differences in stress responses may affect brain development.

METHODS

A rat model of random variable prenatal stress was used to study the expression and processing of hippocampal brain-derived neurotrophic factor (BDNF) in the offspring of the stressed rat dams. To account for unknown genetic influences that may play a role in the outcome of this prenatal stress paradigm, three different rat strains with known differences in stress responsivity were studied: Fischer, Sprague-Dawley, and Lewis rats (n=132).

RESULTS

Multiple disparities in mRNA expression levels of BDNF, and transcripts related to its processing and signaling were found in the three strains. Of the numerous splice variants transcribed from the BDNF gene, the transcript containing BDNF exon VI was most aberrant in the prenatally stressed animals. Protein levels of both uncleaved proBDNF and mature BDNF were also altered, as was intra-cellular signaling by phosphorylation of the neurotrophic tyrosine kinase receptor type 2 (NTRK2, TrkB) and mitogen-activated protein kinase (Erk 1/2). Changes were not only dependent on prenatal stress, but were also strain dependent, demonstrating the importance of genetic background.

CONCLUSION

BDNF signaling provides both positive neurotrophic support for neurons and negative apoptotic effects, both of which may contribute to behavioral or neurochemical outcomes after prenatal exposure to stress. Differential processing of BDNF after prenatal stress in the three rat strains has implications for human subjects where genetic differences may protect or exacerbate the effects of an environmental stressor during fetal development.

Strain dependent effects of prenatal stress on gene expression in the rat hippocampus

Multiple animal models have been developed to recapitulate phenotypes of the human disease, schizophrenia. A model that simulates many of the cognitive and sensory deficits of the disorder is the use of random variable prenatal stress (PS) in the rat. These deficits suggest a molecular origin in the hippocampus, a brain region that plays a role in the regulation of stress. To study both hippocampal gene expression changes in offspring of prenatally stressed dams and to address genetic variability, we used a random array of prenatal stressors in three different rat strains with diverse responses to stress: Fischer, Sprague-Dawley, and Lewis rats. Candidate genes involved in stress, schizophrenia, cognition, neurotrophic effects, and immunity were selected for assessment by real-time quantitative PCR under resting conditions and following a brief exposure to restraint stress. PS resulted in significant differences in gene expression in the offspring that were strain dependent. mRNA expression for the N-methyl-D-aspartate receptor subtype 2B (Grin2b) was increased, and tumor necrosis factor-alpha (Tnfα) transcript was decreased in PS Sprague-Dawley and Lewis rats, but not in the Fischer rats. Expression of brain-derived neurotrophic factor (Bdnf) mRNA in the hippocampus was increased after an acute stress in all controls of each strain, yet a decrease was seen after acute stress in the PS Sprague-Dawley and Lewis rats. Expression of the glucocorticoid receptor (Nr3c1) was decreased in the Fischer strain when compared to Lewis or Sprague-Dawley rats, though the Fischer rats had markedly higher α7 nicotinic receptor (Chrna7) expression. The expression differences seen in these animals may be important elements of the phenotypic differences seen due to PS and genetic background.

Mechanism of GABAB receptor-induced BDNF secretion and promotion of GABAA receptor membrane expression

Recent studies have shown that GABA(B) receptors play more than a classical inhibitory role and can function as an important synaptic maturation signal early in life. In a previous study, we reported that GABA(B) receptor activation triggers secretion of brain-derived neurotrophic factor (BDNF) and promotes the functional maturation of GABAergic synapses in the developing rat hippocampus. To identify the signalling pathway linking GABA(B) receptor activation to BDNF secretion in these cells, we have now used the phosphorylated form of the cAMP response element-binding protein as a biological sensor for endogenous BDNF release. In the present study, we show that GABA(B) receptor-induced secretion of BDNF relies on the activation of phospholipase C, followed by the formation of diacylglycerol, activation of protein kinase C, and the opening of L-type voltage-dependent Ca(2+) channels. We further show that once released by GABA(B) receptor activation, BDNF increases the membrane expression of β(2/3) -containing GABA(A) receptors in neuronal cultures. These results reveal a novel function of GABA(B) receptors in regulating the expression of GABA(A) receptor through BDNF-tropomyosin-related kinase B receptor dependent signalling pathway.

Presynaptic BDNF promotes postsynaptic long-term potentiation in the dorsal striatum

Brain-derived neurotrophic factor (BDNF) facilitates the formation of long-term potentiation (LTP) in hippocampus, but whether this involves release from presynaptic versus postsynaptic pools is unclear. We therefore tested whether BDNF is essential for LTP in dorsal striatum, a structure in which the neurotrophin is present only in afferent terminals. Whole-cell recordings were collected from medium spiny neurons in striatal slices prepared from adult mice. High-frequency stimulation (HFS) of neocortical afferents produced a rapid and stable NMDA receptor-dependent potentiation. The ratio of AMPA to NMDA receptor-mediated components of the EPSPs was substantially increased after inducing potentiation, suggesting that the response enhancement involved postsynaptic changes. In accord with this, paired-pulse response ratios, a measure of transmitter release kinetics, were reduced by elevated calcium but not by LTP. Infusion of the BDNF scavenger TrkB-Fc blocked the formation of potentiation, beginning with the second minute after HFS, without reducing responses to HFS. These results suggest that presynaptic pools of BDNF can act within 2 min of HFS to support the formation of a postsynaptic form of LTP in striatum.

Neurotrophins in lung health and disease

Neurotrophins (NTs) are a family of growth factors that are well-known in the nervous system. There is increasing recognition that NTs (nerve growth factor, brain-derived neurotrophic factor and NT3) and their receptors (high-affinity TrkA, TrkB and TrkC, and low-affinity p75NTR) are expressed in lung components including the nasal and bronchial epithelium, smooth muscle, nerves and immune cells. NT signaling may be important in normal lung development, developmental lung disease, allergy and inflammation (e.g., rhinitis, asthma), lung fibrosis and even lung cancer. In this review, we describe the current status of our understanding of NT signaling in the lung, with hopes of using aspects of the NT signaling pathway in the diagnosis and therapy of lung diseases.

Brain-derived neurotrophic factor and the development of structural neuronal connectivity

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

Understanding proneurotrophin actions: Recent advances and challenges

Neurotrophins are initially synthesized as larger precursors (proneurotrophins), which undergo proteolytic cleavage to yield mature forms. Although the functions of the mature neurotrophins have been well established during neural development and in the adult nervous system, roles for the proneurotrophins in developmental and injury-induced cell death, as well as in synaptic plasticity, have only recently been appreciated. Interestingly, both mature neurotrophins and proneurotrophins utilize dual-receptor complexes to mediate their actions. The mature neurotrophin coreceptors consist of the Trk receptor tyrosine kinases and p75(NTR), wherein Trk transduces survival and differentiative signaling, and p75(NTR) modulates the affinity and selectivity of Trk activation. On the other hand, proneurotrophins engage p75(NTR) and the structurally distinct coreceptor sortilin, to initiate p75(NTR)-dependent signal transduction cascade. Although the specificity of mature neurotrophins vs. proneurotrophins actions is due in part to the formation of distinct coreceptor complexes, a number of recent studies highlight how different p75(NTR)-mediated cellular actions are modulated. Here, we review emerging evidence for a novel transmembrane mechanism for ligand-specific p75(NTR) activation and several mechanisms by which p75(NTR)-dependent apoptotic and nonapoptotic responses can be selective activated.

BDNF signaling in the formation, maturation and plasticity of glutamatergic and GABAergic synapses

In the past 15 years numerous reports provided strong evidence that brain-derived neurotrophic factor (BDNF) is one of the most important modulators of glutamatergic and GABAergic synapses. Remarkable progress regarding localization, kinetics, and molecular mechanisms of BDNF secretion has been achieved, and a large number of studies provided evidence that continuous extracellular supply of BDNF is important for the proper formation and functional maturation of glutamatergic and GABAergic synapses. BDNF can play a permissive role in shaping synaptic networks, making them more susceptible for the occurrence of plastic changes. In addition, BDNF appears to be also an instructive factor for activity-dependent long-term synaptic plasticity. BDNF release just in response to synaptic stimulation might be a molecular trigger to convert high-frequency synaptic activity into long-term synaptic memories. This review attempts to summarize the current knowledge in synaptic secretion and synaptic action of BDNF, including both permissive and instructive effects of BDNF in synaptic plasticity.

Marcadores Biológicos da Depressão:

Introdução. A depressão é um dos mais prevalentes transtornos psi­quiátricos e tem sido associada a alterações nas vias de sinalização que regulam a neuroplasticidade e a sobrevivência celular. Objetivo. O objetivo deste estudo foi revisar a relação entre neurotrofinas e mo­delos experimentais de depressão, bem como a forma em que os me­dicamentos antidepressivos podem estar atuando na estimulação da produção dos fatores neurotróficos. Método. Uma revisão de litera­tura através de pesquisa nas bases de dados MEDLINE e SCIELO, utilizando a combinação dos seguintes descritores: modelo animal de depressão, fatores neurotróficos e BDNF, e suas respectivas traduções no idioma inglês. Resultados. Foram apresentados dados que revela­ram que o fator neurotrófico derivado do cérebro apresenta um papel crucial na regulação da atividade sináptica e plasticidade, relacionando a depressão maior com danos nas vias de sinalização celular. Entre­tanto, para a transcrição desta neurotrofina é necessário a ativação de mecanismos de sinalização, incluindo cálcio, CREB, MEK, MeCP2, CaMKII e hormônios. Conclusão. Após esses argumentos foi possível rever o conceito sobre a fisiopatologia e os mecanismos envolvidos para o desenvolvimento da doença e com isso sugerir possíveis estra­tégias a fim de desenvolver novos fármacos para melhorar a eficácia do tratamento.