Analysis of transcriptional initiation and translatability of brain-derived neurotrophic factor mRNAs in the rat brain

An upstream enhancer and MEF2 transcription factors fine-tune the regulation of the Bdnf gene in cortical and hippocampal neurons

The myocyte enhancer factor (MEF2) family of transcription factors, originally discovered for its pivotal role in muscle development and function, has emerged as an essential regulator in various aspects of brain development and neuronal plasticity. The MEF2 transcription factors are known to regulate numerous important genes in the nervous system, including brain-derived neurotrophic factor (BDNF), a small secreted neurotrophin responsible for promoting the survival, growth, and differentiation of neurons. The expression of the Bdnf gene is spatiotemporally controlled by various transcription factors binding to both its proximal and distal regulatory regions. While previous studies have investigated the connection between MEF2 transcription factors and Bdnf, the endogenous function of MEF2 factors in the transcriptional regulation of Bdnf remains largely unknown. Here, we aimed to deepen the knowledge of MEF2 transcription factors and their role in the regulation of Bdnf comparatively in rat cortical and hippocampal neurons. As a result, we demonstrate that the MEF2 transcription factor-dependent enhancer located at -4.8 kb from the Bdnf gene regulates the endogenous expression of Bdnf in hippocampal neurons. In addition, we confirm neuronal activity-dependent activation of the -4.8 kb enhancer in vivo. Finally, we show that specific MEF2 family transcription factors have unique roles in the regulation of Bdnf, with the specific function varying based on the particular brain region and stimuli. Altogether, we present MEF2 family transcription factors as crucial regulators of Bdnf expression, fine-tuning Bdnf expression through both distal and proximal regulatory regions.

TrkB/BDNF signaling pathway and its small molecular agonists in CNS injury

As one of the most prevalent neurotrophic factors in the central nervous system (CNS), brain-derived neurotrophic factor (BDNF) plays a significant role in CNS injury by binding to its specific receptor Tropomyosin-related kinase receptor B (TrkB). The BDNF/TrkB signaling pathway is crucial for neuronal survival, structural changes, and plasticity. BDNF acts as an axonal growth and extension factor, a pro-survival factor, and a synaptic modulator in the CNS. BDNF also plays an important role in the maintenance and plasticity of neuronal circuits. Several studies have demonstrated the importance of BDNF in the treatment and recovery of neurodegenerative and neurotraumatic disorders. By undertaking in-depth study on the mechanism of BDNF/TrkB function, important novel therapeutic strategies for treating neuropsychiatric disorders have been discovered. In this review, we discuss the expression patterns and mechanisms of the TrkB/BDNF signaling pathway in CNS damage and introduce several intriguing small molecule TrkB receptor agonists produced over the previous several decades.

Revisiting the expression of BDNF and its receptors in mammalian development

Brain-derived neurotrophic factor (BDNF) promotes the survival and functioning of neurons in the central nervous system and contributes to proper functioning of many non-neural tissues. Although the regulation and role of BDNF have been extensively studied, a rigorous analysis of the expression dynamics of BDNF and its receptors TrkB and p75NTR is lacking. Here, we have analyzed more than 3,600 samples from 18 published RNA sequencing datasets, and used over 17,000 samples from GTEx, and ~ 180 samples from BrainSpan database, to describe the expression of BDNF in the developing mammalian neural and non-neural tissues. We show evolutionarily conserved dynamics and expression patterns of BDNF mRNA and non-conserved alternative 5' exon usage. Finally, we also show increasing BDNF protein levels during murine brain development and BDNF protein expression in several non-neural tissues. In parallel, we describe the spatiotemporal expression pattern of BDNF receptors TrkB and p75NTR in both murines and humans. Collectively, our in-depth analysis of the expression of BDNF and its receptors gives insight into the regulation and signaling of BDNF in the whole organism throughout life.

Untranslated regions of brain-derived neurotrophic factor mRNA control its translatability and subcellular localization

Brain-derived neurotrophic factor (BDNF) promotes neuronal survival and growth during development. In the adult nervous system, BDNF is important for synaptic function in several biological processes such as memory formation and food intake. In addition, BDNF has been implicated in development and maintenance of the cardiovascular system. The Bdnf gene comprises several alternative untranslated 5' exons and two variants of 3' UTRs. The effects of these entire alternative UTRs on translatability have not been established. Using reporter and translating ribosome affinity purification analyses, we show that prevalent Bdnf 5' UTRs, but not 3' UTRs, exert a repressive effect on translation. However, contrary to previous reports, we do not detect a significant effect of neuronal activity on BDNF translation. In vivo analysis via knock-in conditional replacement of Bdnf 3' UTR by bovine growth hormone 3' UTR reveals that Bdnf 3' UTR is required for efficient Bdnf mRNA and BDNF protein production in the brain, but acts in an inhibitory manner in lung and heart. Finally, we show that Bdnf mRNA is enriched in rat brain synaptoneurosomes, with higher enrichment detected for exon I-containing transcripts. In conclusion, these results uncover two novel aspects in understanding the function of Bdnf UTRs. First, the long Bdnf 3' UTR does not repress BDNF expression in the brain. Second, exon I-derived 5' UTR has a distinct role in subcellular targeting of Bdnf mRNA.

Effect of intravenous injection of antagomiR-1 on brain ischemia

Stroke is one of the leading causes of death in the world, but the underlying molecular mechanism of this disease remains elusive, thus it will be great challenges to finding appropriate protection. MicroRNAs are short, single-stranded, non-coding RNAs and recent studies have shown that they are aberrantly expressed in ischemic condition. Due to the fact that miR-1 has harmful effects on neural damages during brain ischemia, limited miR-1 has been proven to be protective in middle cerebral artery occlusion (MCAO). Here, the possible positive effect of intravenous injection of antagomiR-1 as a post-ischemic treatment on neurological deficits, infarct volume, brain edema and blood-brain barrier (BBB) permeability was evaluated. The rats were divided randomly into three experimental groups, each with 21 animals. MCAO surgery was performed on all groups and one hour later, 0.1 ml normal saline, 0.1 ml rapamycin and 300 pmol/g miR-1 antagomir (soluble in 0.1 ml normal saline), were injected intravenously into control, positive control and treatment group, respectively. After 24 h, neurologic deficits score, infarct volume, brain edema and BBB permeability were measured. The results indicated that post-treatment with miR-1 antagomir significantly improved neurological deficits and reduced infarction volume, brain edema, and BBB permeability. These data proved that there is a positive effects of antagomiR-1 on ischemic neuronal injury and neurological impairment. Due to the fact that microRNAs are able to protect the brain, it would be a promising therapeutic approach to stroke treatment.

Tristetraprolin is a novel regulator of BDNF

Brain-derived neurotrophic factor (BDNF) regulates multiple biological processes ranging from central nervous system development and function to neuroinflammation and myogenic differentiation and repair. While coordination of BDNF levels is central in determining the biological outcome, mechanisms involved in controlling BDNF levels are not fully understood. Here we find that both short (BDNF-S) and long (BDNF-L) BDNF 3’UTR isoforms contain conserved adenylate- and uridylate rich elements (AREs) that may serve as binding sites for RNA-binding proteins (ARE-BPs). We demonstrate that ARE-BPs tristetraprolin (TTP) and its family members butyrate response factor 1 (BRF1) and 2 (BRF2) negatively regulate expression from both BDNF-S and BDNF-L containing transcripts in several cell-lines and that interaction between TTP and AU-rich region in proximal 5’ end of BDNF 3’UTR is direct. In line with the above, endogenous BDNF mRNA co-immunoprecipitates with endogenous TTP in differentiated mouse myoblast C2C12 cells and TTP overexpression destabilizes BDNF-S containing transcript. Finally, RNAi-mediated knock-down of TTP increases the levels of endogenous BDNF protein in C2C12 cells. Our findings uncover TTP as a novel regulator of BDNF assisting future studies in different physiological and pathological contexts.

BAC-based cellular model for screening regulators of BDNF gene transcription

Background

Brain derived neurotrophic factor (BDNF) belongs to a family of structurally related proteins called neurotrophins that have been shown to regulate survival and growth of neurons in the developing central and peripheral nervous system and also to take part in synaptic plasticity related processes in adulthood. Since BDNF is associated with several nervous system disorders it would be beneficial to have cellular reporter system for studying its expression regulation.

Methods

Using modified bacterial artificial chromosome (BAC), we generated several transgenic cell lines expressing humanised Renilla luciferase (hRluc)-EGFP fusion reporter gene under the control of rat BDNF gene regulatory sequences (rBDNF-hRluc-EGFP) in HeLa background. To see if the hRluc-EGFP reporter was regulated in response to known regulators of BDNF expression we treated cell lines with substances known to regulate BDNF and also overexpressed transcription factors known to regulate BDNF gene in established cell lines.

Results

rBDNF-hRluc-EGFP cell lines had high transgene copy numbers when assayed with qPCR and FISH analysis showed that transgene was maintained episomally in all cell lines. Luciferase activity in transgenic cell lines was induced in response to ionomycin-mediated rise of intracellular calcium levels, treatment with HDAC inhibitors and by over-expression of transcription factors known to increase BDNF expression, indicating that transcription of the transgenic reporter is regulated similarly to the endogenous BDNF gene.

Conclusions

Generated rBDNF-hRluc-EGFP BAC cell lines respond to known modulators of BDNF expression and could be used for screening of compounds/small molecules or transcription factors altering BDNF expression.

miR-1, miR-10b, miR-155, and miR-191 are novel regulators of BDNF

Brain-derived neurotrophic factor (BDNF) is a secreted protein of the neurotrophin family that regulates brain development, synaptogenesis, memory and learning, as well as development of peripheral organs, such as angiogenesis in the heart and postnatal growth and repair of skeletal muscle. However, while precise regulation of BDNF levels is an important determinant in defining the biological outcome, the role of microRNAs (miRs) in modulating BDNF expression has not been extensively analyzed. Using in silico approaches, reporter systems, and analysis of endogenous BDNF, we show that miR-1, miR-10b, miR-155, and miR-191 directly repress BDNF through binding to their predicted sites in BDNF 3′UTR. We find that the overexpression of miR-1 and miR-10b suppresses endogenous BDNF protein levels and that silencing endogenous miR-10b increases BDNF mRNA and protein levels. Furthermore, we show that miR-1/206 binding sites within BDNF 3′UTR are used in differentiated myotubes but not in undifferentiated myoblasts. Finally, our data from two cell lines suggest that endogenous miR-1/206 and miR-10 family miRs act cooperatively in suppressing BDNF through their predicted sites in BDNF 3′UTR. In conclusion, our results highlight miR-1, miR-10b, miR-155, and miR-191 as novel regulators of BDNF long and short 3′UTR isoforms, supporting future research in different physiological and pathological contexts.

Neurotrophins: transcription and translation

Neurotrophins are powerful molecules. Small quantities of these secreted proteins exert robust effects on neuronal survival, synapse stabilization, and synaptic function. Key functions of the neurotrophins rely on these proteins being expressed at the right time and in the right place. This is especially true for BDNF, stimulus-inducible expression of which serves as an essential step in the transduction of a broad variety of extracellular stimuli into neuronal plasticity of physiologically relevant brain regions. Here we review the transcriptional and translational mechanisms that control neurotrophin expression with a particular focus on the activity-dependent regulation of BDNF.

Involvement of the 3'-untranslated region of the brain-derived neurotrophic factor gene in activity-dependent mRNA stabilization

Although gene transcription is controlled by neuronal activity, little is known about post-transcriptional regulation in neurons. Using cultured neurons, we found that the half-life of immediate-early gene transcripts is prolonged or shortened by membrane depolarization. Focusing on the activity-dependent stabilization of brain-derived neurotrophic factor (BDNF) mRNA, we constructed a series of plasmids, in which the short 3'-untranslated region (3'-UTR) of the BDNF gene was fused to the firefly luciferase gene, and found that the 3'-UTR prevented destabilization of luciferase mRNA through Ca(2+) signals evoked via depolarization. No such prevention was observed with the simian virus 40 late poly(A) site. The pre-mRNA covering the entire short 3'-UTR, where multiple poly(A) sites including novel ones are located, was stabilized. Deletion analyses of 3'-UTR revealed a core region (about 130 bases long) and a complementary region to be responsible for the prevention, well consistent with the formation of an extended stem-loop RNA structure and the production of poly(A) mRNAs. Thus, the mRNA stability is activity-dependently controlled in neurons and distinct regions of the 3'-UTR of BDNF mRNA are involved in stabilizing mRNA in response to Ca(2+) signals, suggesting a primary role of the RNA secondary structure affecting the availability of poly(A) sites in activity-dependent mRNA stabilization.

Differential expression and regulation of brain-derived neurotrophic factor (BDNF) mRNA isoforms in androgen-sensitive motoneurons of the rat lumbar spinal cord

Castration of adult male rats causes dendrites of the spinal nucleus of the bulbocavernosus (SNB) to retract. The neurotrophin brain-derived neurotrophic factor (BDNF) is implicated in mediating these androgenic effects on SNB dendrites. We previously found that castration decreases BDNF mRNA in SNB somata and BDNF protein in proximal SNB dendrites, effects not observed in nearby retrodorsolateral (RDLN) motoneurons. Given that different 5' non-coding exons of BDNF dictate specific subcellular targeting of BDNF mRNA, we set out to identify the specific BDNF transcripts regulated by androgens in SNB motoneurons. We used in situ hybridization to monitor the expression pattern of BDNF transcripts containing non-coding exons I, II, IV, and VI in SNB and RDLN motoneurons in gonadally intact and castrated male rats. While androgen-insensitive RDLN motoneurons expressed all four isoforms, SNB motoneurons contained low levels of BDNF exon IV and little, if any, BDNF exon I. Expression of BDNF isoforms containing exon II and VI was comparable in the two groups of motoneurons. Two weeks after castration, BDNF isoforms containing exon VI were significantly decreased in SNB motoneurons in an androgen-dependent manner, but unaffected in RDLN motoneurons. Because exon VI promotes dendritic localization of BDNF mRNA in other systems, androgens may regulate the dendrites of SNB motoneurons by altering expression of BDNF isoforms, thereby impairing targeting of BDNF protein to dendrites to regulate local synaptic signaling and dendritic structure.

Distinct 3'UTRs differentially regulate activity-dependent translation of brain-derived neurotrophic factor (BDNF)

Expression of the brain-derived neurotrophic factor (BDNF) is under tight regulation to accommodate its intricate roles in controlling brain function. Transcription of BDNF initiates from multiple promoters in response to distinct stimulation cues. However, regardless which promoter is used, all BDNF transcripts are processed at two alternative polyadenylation sites, generating two pools of mRNAs that carry either a long or a short 3'UTR, both encoding the same BDNF protein. Whether and how the two distinct 3'UTRs may differentially regulate BDNF translation in response to neuronal activity changes is an intriguing and challenging question. We report here that the long BDNF 3'UTR is a bona fide cis-acting translation suppressor at rest whereas the short 3'UTR mediates active translation to maintain basal levels of BDNF protein production. Upon neuronal activation, the long BDNF 3'UTR, but not the short 3'UTR, imparts rapid and robust activation of translation from a reporter. Importantly, the endogenous long 3'UTR BDNF mRNA specifically undergoes markedly enhanced polyribosome association in the hippocampus in response to pilocarpine induced-seizure before transcriptional up-regulation of BDNF. Furthermore, BDNF protein level is quickly increased in the hippocampus upon seizure-induced neuronal activation, accompanied by a robust activation of the tropomyosin-related receptor tyrosine kinase B. These observations reveal a mechanism for activity-dependent control of BDNF translation and tropomyosin-related receptor tyrosine kinase B signaling in brain neurons.

BAC transgenic mice reveal distal cis-regulatory elements governing BDNF gene expression

Brain-derived neurotrophic factor (BDNF), a member of the neurotrophin family of neurotrophic factors, has important functions in the peripheral and central nervous system of vertebrates. We have generated bacterial artificial chromosome (BAC) transgenic mice harboring 207 kb of the rat BDNF (rBDNF) locus containing the gene, 13 kb of genomic sequences upstream of BDNF exon I, and 144 kb downstream of protein encoding exon IX, in which protein coding region was replaced with the lacZ reporter gene. This BDNF-BAC drove transgene expression in the brain, heart, and lung, recapitulating endogenous BDNF expression to a larger extent than shorter rat BDNF transgenes employed previously. Moreover, kainic acid induced the expression of the transgenic BDNF mRNA in the cerebral cortex and hippocampus through preferential activation of promoters I and IV, thus recapitulating neuronal activity-dependent transcription of the endogenous BDNF gene. genesis 48:214–219, 2010. © 2010 Wiley-Liss, Inc.

Tissue-specific and neural activity-regulated expression of human BDNF gene in BAC transgenic mice

Background

Brain-derived neurotrophic factor (BDNF) is a small secreted protein that has important roles in the developing and adult nervous system. Altered expression or changes in the regulation of the BDNF gene have been implicated in a variety of human nervous system disorders. Although regulation of the rodent BDNF gene has been extensively investigated, in vivo studies regarding the human BDNF gene are largely limited to postmortem analysis. Bacterial artificial chromosome (BAC) transgenic mice harboring the human BDNF gene and its regulatory flanking sequences constitute a useful tool for studying human BDNF gene regulation and for identification of therapeutic compounds modulating BDNF expression.

Results

In this study we have generated and analyzed BAC transgenic mice carrying 168 kb of the human BDNF locus modified such that BDNF coding sequence was replaced with the sequence of a fusion protein consisting of N-terminal BDNF and the enhanced green fluorescent protein (EGFP). The human BDNF-BAC construct containing all BDNF 5' exons preceded by different promoters recapitulated the expression of endogenous BDNF mRNA in the brain and several non-neural tissues of transgenic mice. All different 5' exon-specific BDNF-EGFP alternative transcripts were expressed from the transgenic human BDNF-BAC construct, resembling the expression of endogenous BDNF. Furthermore, BDNF-EGFP mRNA was induced upon treatment with kainic acid in a promotor-specific manner, similarly to that of the endogenous mouse BDNF mRNA.

Conclusion

Genomic region covering 67 kb of human BDNF gene, 84 kb of upstream and 17 kb of downstream sequences is sufficient to drive tissue-specific and kainic acid-induced expression of the reporter gene in transgenic mice. The pattern of expression of the transgene is highly similar to BDNF gene expression in mouse and human. This is the first study to show that human BDNF gene is regulated by neural activity.

Changes in alternative brain-derived neurotrophic factor transcript expression in the developing human prefrontal cortex

In this study, we determined when and through which promoter brain-derived neurotrophic factor (BDNF) transcription is regulated during the protracted period of human frontal cortex development. Using quantitative real-time polymerase chain reaction, we examined the expression of the four most abundant alternative 5' exons of the BDNF gene (exons I, II, IV, and VI) in RNA extracted from the prefrontal cortex. We found that expression of transcripts I-IX and VI-IX was highest during infancy, whereas that of transcript II-IX was lowest just after birth, slowly increasing to reach a peak in toddlers. Transcript IV-IX was significantly upregulated within the first year of life, and was maintained at this level until school age. Quantification of BDNF protein revealed that levels followed a similar developmental pattern as transcript IV-IX. In situ hybridization of mRNA in cortical sections showed the highest expression in layers V and VI for all four BDNF transcripts, whereas moderate expression was observed in layers II and III. Interestingly, although low expression of BDNF was observed in cortical layer IV, this BDNF mRNA low-zone decreased in prominence with age and showed an increase in neuronal mRNA localization. In summary, our findings show that dynamic regulation of BDNF expression occurs through differential use of alternative promoters during the development of the human prefrontal cortex, particularly in the younger age groups, when the prefrontal cortex is more plastic.

BDNF splice variants from the second promoter cluster support cell survival of differentiated neuroblastoma upon cytotoxic stress

The neurotrophin brain-derived neurotrophic factor (BDNF) is a key survival factor for neural cells. In particular, in neuroblastoma tumour cells, expression of the BDNF/TrkB autocrine signalling system promotes a more malignant phenotype and resistance to chemotherapy. The human BDNF gene contains two clusters of upstream exons encoding the 5′UTR (exon 1 to exon 3 and exon 4 to exon 9a), these are alternatively spliced to a common exon 9, which contains the coding region and the 3′UTR. At least 34 different BDNF mRNA transcripts can be generated, although their physiological role is still unknown. The purpose of this study is to determine which BDNF transcript is involved in cell survival of the human neuroblastoma cell lines SH-SY-5Y (single-copy MYCN) and SK-N-BE (amplified MYCN). Expression of human BDNF mRNAs encoding all possible isoforms was characterised in the two neuroblastoma cell lines. We then investigated whether selective silencing of the different BDNF mRNAs using specific siRNAs could reduce cell survival in response to serum deprivation or the anticancer drugs cisplatin, doxorubicin and etoposide. We found that three isoforms located in the second exon cluster are essential for neuroblastoma cell survival under cytotoxic stress. Notably, promoters of the second exon cluster, but not the first, are controlled by Ca2+-sensitive elements.

BDNF up-regulates pre-pro-TRH mRNA expression in the fetal/neonatal paraventricular nucleus of the hypothalamus. Properties of the transduction pathway

Brain derived neurotrophic factor (BDNF) increases the levels of pre-pro-thyrotropin releasing hormone (TRH) mRNA in fetal rodent hypothalamic neurons that express TrkB receptors. The present studies aimed at better understanding the role of BDNF in establishing and maintaining the TRH phenotype in hypothalamic neurons during early development. To determine where BDNF regulates the expression of pre-pro-TRH mRNA in vivo, we compared the hypothalamic distribution of pre-pro-TRH mRNA to that of TrkB mRNA. Full-length TrkB (FL-TrkB) mRNA was detected earlier in development than pre-pro-TRH mRNA in the region that gives rise to the paraventricular nucleus of the hypothalamus (PVN). We also evaluated the effects of BDNF on the expression of pre-pro-TRH mRNA in vitro. BDNF up-regulated the levels of pre-pro-TRH mRNA in primary cell cultures obtained from the hypothalamus or the PVN of 17 days old fetuses or newborn rats. This effect was abolished by PD98059, an inhibitor of the mitogen-activated protein kinase kinase (MEK) 1/2 or 5. The effect of BDNF on pre-pro-TRH mRNA levels was reversible. The continuous application of BDNF led to a desensitization of the response at day 10 in vitro, an effect that correlated with a drop in the levels of FL-TrkB protein. In conclusion, BDNF enhances the expression of pre-pro-TRH mRNA in PVN neurons. This effect is reversible, decreases with time, and requires an active MEK. BDNF may contribute to the enhancement of pre-pro-TRH mRNA expression in the hypothalamic PVN during development.

Dissecting the human BDNF locus: bidirectional transcription, complex splicing, and multiple promoters

Brain-derived neurotrophic factor (BDNF), a member of the nerve growth factor family of neurotrophins, has central roles in the development, physiology, and pathology of the nervous system. We have elucidated the structure of the human BDNF gene, identified alternative transcripts, and studied their expression in adult human tissues and brain regions. In addition, the transcription initiation sites for human BDNF transcripts were determined and the activities of BDNF promoters were analyzed in transient overexpression assays. Our results show that the human BDNF gene has 11 exons and nine functional promoters that are used tissue and brain-region specifically. Furthermore, noncoding natural antisense RNAs that display complex splicing and expression patterns are transcribed in the BDNF gene locus from the antiBDNF gene (approved gene symbol BDNFOS). We show that BDNF and antiBDNF transcripts form dsRNA duplexes in the brain in vivo, suggesting an important role for antiBDNF in regulating BDNF expression in human.

Analysis of the stress response in rats trained in the water-maze: differential expression of corticotropin-releasing hormone, CRH-R1, glucocorticoid receptors and brain-derived neurotrophic factor in limbic regions

Glucocorticoids and corticotropin-releasing hormone (CRH) are key regulators of stress responses. Different types of stress activate the CRH system; in hypothalamus, CRH expression and release are increased by physical or psychological stressors while in amygdala, preferentially by psychological stress. Learning and memory processes are modulated by glucocorticoids and stress at different levels. To characterize the kind of stress provoked by a hippocampal-dependent task such as spatial learning, we compared the expression profile of glucocorticoid receptor (GR), pro-CRH and CRH-R1 mRNAs (analyzed by RT-PCR), in amygdala, hippocampus and hypothalamus and quantified serum corticosterone levels by radioimmunoassay at different stages of training. mRNA levels of brain-derived neurotrophic factor (BDNF) were also quantified due to its prominent role in learning and memory processes. Male Wistar rats trained for 1, 3 or 5 days in the Morris water-maze (10 trials/day) were sacrificed 5-60 min the after last trial. A strong stress response occurred at day one in both yoked and trained animals (increased corticosterone and hypothalamic pro-CRH and CRH-R1 mRNA levels); changes gradually diminished as the test progressed. In amygdala, pro-CRH mRNA levels decreased while those of BDNF augmented when stress was highest, in yoked and trained animals. Hippocampi, of both yoked and trained groups, had decreased levels of GR mRNA on days 1 and 3, normalizing by day 5, while those of pro-CRH and CRH-R1 increased after the 3rd day. Increased gene expression, specifically due to spatial learning, occurred only for hippocampal BDNF since day 3. These results show that the Morris water-maze paradigm induces a strong stress response that is gradually attenuated. Inhibition of CRH expression in amygdala suggests that the stress inflicted is of physical but not of psychological nature and could lead to reduced fear or anxiety.

The influence of specific noradrenergic and serotonergic lesions on the expression of hippocampal brain-derived neurotrophic factor transcripts following voluntary physical activity

Previous studies have shown that hippocampal brain-derived neurotrophic factor (BDNF) mRNA levels are significantly increased in rats allowed free access to exercise wheels and/or administered antidepressant medications. Enhancement of BDNF may be crucial for the clinical effect of antidepressant interventions. Since increased function of the noradrenergic and/or serotonergic systems is thought to be an important initial mechanism of antidepressant medications, we sought to test the hypothesis that noradrenergic or serotonergic function is essential for the increased BDNF transcription occurring with exercise. In addition, individual transcript variants of BDNF were examined, as evidence exists they are differentially regulated by discrete interventions, and are expressed in distinct sub-regions of the hippocampus. The neurotransmitter system-specific neurotoxins p-chloroamphetamine (serotonergic) and N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (noradrenergic) were administered to rats prior to commencing voluntary wheel-running activity. In situ hybridization experiments revealed an absence of exercise-induced full-length BDNF mRNA elevations in the hippocampi of noradrenergic-lesioned rats. In addition, the striking elevation of the exon I transcript in the dentate gyrus was removed with this noradrenergic lesion. In contrast, other transcript variants (exons II and III) were elevated in several hippocampal regions as a result of this lesion. In serotonin-lesioned rats, the significant increases in full-length BDNF, exon I and exon II mRNA levels were sustained without alteration (with the exception of exon IV in the cornus ammonis subregion 4, CA4). Overall, these results indicate that an intact noradrenergic system may be crucial for the observed ability of exercise to enhance full-length and exon I hippocampal BDNF mRNA expression. In addition, these results suggest that the promoter linked to exon I may provide a major regulatory point for BDNF mRNA expression in the dentate gyrus. Elevations of other exons, such as II and III, may require the activation of separate neurotransmitter systems and intracellular pathways.

BDNF and activity-dependent synaptic modulation

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.

Brain-derived neurotrophic factor gene organization and transcription in the zebrafish embryo

The gene encoding zebrafish brain-derived neurotrophic factor (BDNF) was cloned from a PAC genomic DNA library. The entire transcription unit was contained in two independently isolated clones that together encompass 120 kb of genomic DNA. The intron/exon organization of the zebrafish gene was found to be identical to that of the mammalian gene but only one promoter has so far been identified. The associated 5' exon is 67% identical to exon 1c of the rat BDNF gene. A search of the 5' flank of the cloned promoter for sequence similarities with known transcription factor binding sites revealed potential AP-1, CREB, and SP1 binding sites. Fusion constructs containing the cloned promoter and 1.7 kb of 5' flank and an enhanced green fluorescent protein reporter that becomes membrane-anchored were injected into 1-8 cell stage embryos. Expression was seen in notochord, muscle, epithelial and endothelial cells of the 1-day-old embryo in consonance with the endogenous gene. These results demonstrate that the cloned promoter mediates cell-specific expression.

Neurotrophins: which way did they go?

The classic view of the neurotrophin as a messenger produced by the postsynaptic cell that signals survival to the presynaptic cell has become expanded. Segal discusses the evidence that the neurotrophin brain-derived nerve growth factor (BDNF) can be also released from the presynaptic cell onto the dendritic spine of a postsynaptic cell to regulate the neuronal circuit in two directions: forward (from presynaptic cell to postsynaptic dendrite) and the classic backward direction (from postsynaptic cell to the presynaptic axon). These results remind us that in the brain, neurons exist in a complex circuit and one neuron's presynaptic cell is another neuron's postsynaptic cell. The evidence that a single growth factor may have different actions based on the localization of the receipt of the signal adds more complexity to the brain's neuronal circuitry.

Mismatch between BDNF mRNA and protein expression in the developing visual cortex: the role of visual experience

Brain-derived neurotrophic factor (BDNF) messenger RNA (mRNA) expression in the rat visual cortex of young and postnatal day 90 (P90) animals is developmentally regulated and influenced by visual experience. In the present paper we compared the expression of BDNF mRNA to the actual changes of BDNF protein occurring during postnatal development and verified whether BDNF protein distribution is controlled by visual activity. To achieve this aim we analysed BDNF mRNA and/or BDNF protein cellular distribution in the rat visual cortex at different postnatal ages by using immunohistochemistry and highly sensitive in situ hybridization. We found that before eye opening (P13), in all cortical layers a large number of visual cortical neurons contain BDNF mRNA with no detectable amount of BDNF protein. At later ages (P23 and P90), the number of BDNF-immunostained cells increases; most neurons are double labelled for BDNF mRNA and protein, and a small group of neurons is labelled only for BDNF protein. The cellular increase of BDNF immunolabelling is blocked in animals deprived of visual experience from birth (dark rearing), with a large population of neurons containing BDNF mRNA but not BDNF protein. This is similar to what is observed before eye opening. Exposure of dark-reared rats to a brief period (2 h) of light restores a good match between BDNF mRNA and BDNF protein cellular expression. We propose that visual experience controls the neuronal content of BDNF mRNA and BDNF protein in developing visual cortex.

Hippocampal BDNF mRNA shows a diurnal regulation, primarily in the exon III transcript

Endogenous expression levels of brain-derived neurotrophic factor (BDNF) mRNA were assessed using in situ hybridization to investigate whether there is a natural diurnal fluctuation in BDNF mRNA expression in the hippocampus of rats housed with a normal (12:12 h) light/dark cycle. BDNF expression was increased during lights out (dark-cycle) to 134%-158% of light-cycle levels in hippocampal regions CA1, CA3, and hilus. In addition, expression levels of the four BDNF transcript forms, exons I-IV, were assessed to evaluate whether expression of specific BDNF transcripts exhibited differential endogenous fluctuation. All exons had lowest levels of expression at either noon or 6 p.m. Significant correlations were found between exon expression level and time, with elevated expression occurring at dark-cycle timepoints. The exon III transcript showed the greatest diurnal change in expression in all hippocampal fields, with dark-cycle expression elevated to 219-419% of light-cycle expression level. In addition to exon III, dark-cycle exon II mRNA levels were elevated in all hippocampal subfields, to 140-180% of light-cycle levels, suggesting that the endogenous fluctuation in BDNF expression results predominantly from activation of the promoters linked to exons II and III. Previously we have shown that physical activity increases BDNF expression. The naturally occurring rise in BDNF expression during the dark-cycle, the time when rats are most physically active, may be due to increased activity and arousal levels. Because BDNF has a role in plasticity, the increase in BDNF expression during the time that a rat is maximally interacting with its surroundings may be part of an ongoing stimulus-encoding mechanism, or may be a mechanism to maximize information storage about the environment.

Regulation of brain-derived neurotrophic factor mRNA levels in hippocampus by neuronal activity

Neuronal activity increases synthesis of brain-derived neurotrophic factor (BDNF) mRNA in vivo and in vitro. We have investigated the pathways through which neuronal activity stimulated by kainic acid regulates BDNF mRNA levels in cultured hippocampal neurons and transgenic mice. Kainic acid induced the transcription of BDNF mRNA without influencing the mRNA stability. Interestingly, the half-life of the 4.2 kb BDNF transcript was much shorter than that of the 1.6 kb transcript (23 +/- 4 min. vs. 132 +/- 30 min). Increase in the BDNF mRNA levels by kainic acid was not blocked by the protein synthesis inhibitor cycloheximide demonstrating that BDNF is regulated as an immediate early gene in hippocampal neurons. Although calmodulin antagonists are known to abolish the effect of kainic acid on BDNF mRNA, this effect was very similar in Ca(+2)-calmodulin-dependent protein kinase II alpha knock-out mice and in wild-type mice. Surprisingly, even high doses of kainic acid failed to increase nerve growth factor (NGF) mRNA in mouse hippocampus although elevation in rat brain has been consistently observed.

Differential regulation of the expression of nerve growth factor, brain-derived neurotrophic factor, and neurotrophin-3 after excitotoxicity in a rat model of Huntington's disease

In the present study we have evaluated changes in nerve growth factor (NGF), brain-derived neurotrophic factor, and neurotrophin 3 (NT-3) mRNA expression induced by different glutamate receptor agonists injected into the neostriatum. Up-regulation of NGF expression was observed at 24 h after intrastriatal quinolinate injection, an N-methyl-D-aspartate receptor agonist, and this increase was maintained up to 7 days after lesion. NGF up-regulation was also apparent in alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) treatment from 6 to 16 h postinjection. Instead, BDNF was up-regulated only at 6 h after kainate or AMPA excitotoxicity. Interestingly, NT-3 mRNA was down-regulated from 10 to 16 h following AMPA lesion, while 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid injection enhanced NT-3 mRNA levels at 10 h. Our results show a specific neurotrophin response induced by stimulation of each glutamate receptor. These activity-dependent changes might be involved in neuronal plasticity processes and may underlie the differential vulnerability of striatal neurons observed in neurodegenerative disorders.

Dynamic changes of brain-derived neurotrophic factor protein levels in the rat forebrain after single and recurring kindling-induced seizures

Regional levels of brain-derived neurotrophic factor protein were measured in the rat brain using enzyme immunoassay following seizures evoked by hippocampal kindling stimulations. One stimulation, which induced a brief, single episode of epileptiform activity in hippocampus and piriform cortex but not in parietal cortex or striatum, gave rise to a transient increase of brain-derived neurotrophic factor levels in dentate gyrus and CA3 region and a decrease in piriform cortex. After 40 rapidly recurring seizures, with epileptiform activity also involving parietal cortex and striatum, increases were observed in dentate gyrus, CA3 and CA1 regions, piriform cortex and striatum. Maximum levels were reached at 2-24 h and brain-derived neurotrophic factor then returned to baseline except in dentate gyrus, where elevated protein content was sustained for four days. The differential regulation of brain-derived neurotrophic factor protein levels in various forebrain structures, which only partly correlates to messenger RNA changes, could indicate regional differences in protein release, antero- or retrograde transport, or brain-derived neurotrophic factor promotor activation. The dynamic changes of brain-derived neurotrophic factor levels in regions involved in the generation and spread of seizure activity may regulate excitability and trigger plastic responses in the post-seizure period.

Localization of putative calcium-responsive regions in the rat BDNF gene

Brain-derived neurotrophic factor (BDNF) has potent trophic and protective actions on CNS neurons, including mesencephalic dopaminergic neurons, ventral forebrain cholinergic neurons and spinal motor neurons. To evaluate the effects of calcium and other second messengers on BDNF gene transcription, C6 glioma cells were treated for 4 h with the calcium ionophore A23187, forskolin + isobutyl-methyl-xanthine (IBMX), or the phorbol ester, 12-O-tetradecanoyl-phorbol-13-acetate. Semi-quantitative RT-PCR analysis revealed that A23187 treatment increased BDNF transcripts containing the protein coding exon by 4.4-6.4-fold. Alternate BDNF transcripts were elevated to varying degrees after treatment with this ionophore and a subset of these transcripts was elevated following forskolin + IBMX treatment. When co-incubated with the RNA polymerase inhibitor, actinomycin D, A23187-induced increases were reduced or abolished, suggesting that calcium-mediated regulation of BDNF mRNA expression occurs at transcriptional as well as post-transcriptional levels. Transient transfection experiments employing reporter constructs containing serial 5' deletions of alternate BDNF promoters suggested that A23187-induced elevations in BDNF exon 1b, 1d and 1e containing transcripts are mediated by putative calcium-responsive regions flanking all three of these exons.

Comparative study of brain-derived neurotrophic factor messenger RNA and protein at the cellular level suggests multiple roles in hippocampus, striatum and cortex

Brain-derived neurotrophic factor (BDNF) is important for the development and trophic support of several neuronal groups in the rat. In the present study, the distribution of BDNF messenger RNA was studied by in situ hybridization, and the cellular localization of BDNF protein was investigated with anti-peptide antibodies. Anatomical investigations were also made in animals with prolonged epileptic seizures which show an enhanced expression of BDNF messenger RNA. Major forebrain areas studied were the hippocampus, striatum and cortex. The messenger RNA coding for the putative high-affinity receptor, tyrosine kinase B, was also visualized using in situ hybridization with a probe specific for the full-length form. In the hippocampus, granule cells and pyramidal neurons expressed BDNF messenger RNA and BDNF-like immunoreactivity. Interneurons in dendritic layers did not show labelling with either method. Tyrosine kinase B messenger RNA was found within neurons in all these regions. In the medial septum-diagonal band, nucleus basalis and lateral hypothalamus, neurons with punctate cytoplasmic immunofluorescence were found, and neurons in the lateral septum were diffusely positive for BDNF. In striatum, positive labelling of medium-sized neurons was found with the antibody, whereas BDNF messenger RNA was only detectable during seizures. A laminar pattern of neuronal labelling for BDNF messenger RNA and protein was found in the neocortex. The analysis of the anatomical distribution of BDNF-producing cells suggests a number of possible cellular interactions. In the hippocampus, BDNF might act in an autocrine or paracrine manner for granule cells and pyramidal neurons, and, in addition, may serve as a signal from these principal cells to interneurons. BDNF could be a target-derived and a locally produced trophic factor for cholinergic neurons in the medial septum. The expression of BDNF in the striatum suggests that this factor could be a target-derived factor for dopaminergic neurons of substantia nigra and/or work as an autocrine/ paracrine factor within the striatum itself.

Identification of brain-derived neurotrophic factor promoter regions mediating tissue-specific, axotomy-, and neuronal activity-induced expression in transgenic mice

The structure of rat brain-derived neurotrophic factor (BDNF) gene is complex; four 5' exons are linked to separate promoters and one 3' exon is encoding the BDNF protein. To analyze the relative importance of the regulatory regions in vivo, we have generated transgenic mice with six different promoter constructs of the BDNF gene fused to the chloramphenicol acetyl transferase reporter gene. High level and neuronal expression of the reporter gene, that in many respects recapitulated BDNF gene expression, was achieved by using 9 kb of genomic sequences covering the promoter regions that lie adjacent to each other in the genome (promoters I and II and promoters III and IV, respectively) and by including sequences of BDNF intron-exon splice junctions and 3' untranslated region in the constructs. The genomic regions responsible for the in vivo upregulation of BDNF expression in the axotomized sciatic nerve and in the brain after kainic acid-induced seizures and KCl-induced spreading depression were mapped. These data show that regulation of the different aspects of BDNF expression is controlled by different regions in vivo, and they suggest that these promoter constructs may be useful for targeted expression of heterologous genes to specific regions of the central and peripheral nervous systems in an inducible manner.