Functional Role of BDNF Production from Unique Promoters in Aggression and Serotonin Signaling

Unveiling the role of astrogliosis in Alzheimer's disease Pathology: Insights into mechanisms and therapeutic approaches

Alzheimer's disease (AD) is one of the most debilitating age-related disorders that affect people globally. It impacts social and cognitive behavior of the individual and is characterized by phosphorylated tau and Aβ accumulation. Astrocytesmaintain a quiescent, anti-inflammatory state on anatomical level, expressing few cytokines and exhibit phagocytic activity to remove misfolded proteins. But in AD, in response to specific stimuli, astrocytes overstimulate their phagocytic character with overexpressing cytokine gene modules. Upon interaction with generated Aβ and neurofibrillary tangle, astrocytes that are continuously activated release a large number of inflammatory cytokines. This cytokine storm leads to neuroinflammation which is also one of the recognizable features of AD. Astrogliosis eventually promotes cholinergic dysfunction, calcium imbalance, oxidative stress and excitotoxicity. Furthermore, C5aR1, Lcn2/, BDNF/TrkB and PPARα/TFEB signaling dysregulation has a major impact on the disease progression. This review clarifies numerous ways that lead to astrogliosis, which is stimulated by a variety of processes that exacerbate AD pathology and make it a suitable target for AD treatment. Drugs under clinical and preclinical investigations that target several pathways managing astrogliosis and are efficacious in ameliorating the pathology of the disease are also included in this study. D-ALA2GIP, TRAM-34, Genistein, L-serine, MW150 and XPro1595 are examples of few drugs targeting astrogliosis. Therefore, this study may aid in the development of a potent therapeutic agent for ameliorating astrogliosis mediated AD progression.

The role of the prefrontal cortex in modulating aggression in humans and rodents

Accumulating evidence suggests that the prefrontal cortex (PFC) plays an important role in aggression. However, the findings regarding the key neural mechanisms and molecular pathways underlying the modulation of aggression by the PFC are relatively scattered, with many inconsistencies and areas that would benefit from exploration. Here, we highlight the relationship between the PFC and aggression in humans and rodents and describe the anatomy and function of the human PFC, along with homologous regions in rodents. At the molecular level, we detail how the major neuromodulators of the PFC impact aggression. At the circuit level, this review provides an overview of known and potential subcortical projections that regulate aggression in rodents. Finally, at the disease level, we review the correlation between PFC alterations and heightened aggression in specific human psychiatric disorders. Our review provides a framework for PFC modulation of aggression, resolves several intriguing paradoxes from previous studies, and illuminates new avenues for further study.

Neurotrophins and Their Receptors: BDNF's Role in GABAergic Neurodevelopment and Disease

Neurotrophins and their receptors are distinctly expressed during brain development and play crucial roles in the formation, survival, and function of neurons in the nervous system. Among these molecules, brain-derived neurotrophic factor (BDNF) has garnered significant attention due to its involvement in regulating GABAergic system development and function. In this review, we summarize and compare the expression patterns and roles of neurotrophins and their receptors in both the developing and adult brains of rodents, macaques, and humans. Then, we focus on the implications of BDNF in the development and function of GABAergic neurons from the cortex and the striatum, as both the presence of BDNF single nucleotide polymorphisms and disruptions in BDNF levels alter the excitatory/inhibitory balance in the brain. This imbalance has different implications in the pathogenesis of neurodevelopmental diseases like autism spectrum disorder (ASD), Rett syndrome (RTT), and schizophrenia (SCZ). Altogether, evidence shows that neurotrophins, especially BDNF, are essential for the development, maintenance, and function of the brain, and disruptions in their expression or signaling are common mechanisms in the pathophysiology of brain diseases.

Sustained Depolarization Induces Gene Expression Pattern Changes Related to Synaptic Plasticity in a Human Cholinergic Cellular Model

Neuronal gene expression in the brain dynamically responds to synaptic activity. The interplay among synaptic activity, gene expression, and synaptic plasticity has crucial implications for understanding the pathophysiology of diseases such as Alzheimer's disease and epilepsy. These diseases are marked by synaptic dysfunction that affects the expression patterns of neuroprotective genes that are incompletely understood. In our study, we developed a cellular model of synaptic activity using human cholinergic neurons derived from SH-SY5Y cell differentiation. Depolarization induction modulates the expression of neurotrophic genes and synaptic markers, indicating a potential role in synaptic plasticity regulation. This hypothesis is further supported by the induction kinetics of various long non-coding RNAs, including primate-specific ones. Our experimental model showcases the utility of SH-SY5Y cells in elucidating the molecular mechanisms underlying synaptic plasticity in human cellular systems.

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.

Distinct roles of Bdnf I and Bdnf IV transcript variant expression in hippocampal neurons

Brain-derived neurotrophic factor (Bdnf) plays a critical role in brain development, dendritic growth, synaptic plasticity, as well as learning and memory. The rodent Bdnf gene contains nine 5' non-coding exons (I-IXa), which are spliced to a common 3' coding exon (IX). Transcription of individual Bdnf variants, which all encode the same BDNF protein, is initiated at unique promoters upstream of each non-coding exon, enabling precise spatiotemporal and activity-dependent regulation of Bdnf expression. Although prior evidence suggests that Bdnf transcripts containing exon I (Bdnf I) or exon IV (Bdnf IV) are uniquely regulated by neuronal activity, the functional significance of different Bdnf transcript variants remains unclear. To investigate functional roles of activity-dependent Bdnf I and IV transcripts, we used a CRISPR activation system in which catalytically dead Cas9 fused to a transcriptional activator (VPR) is targeted to individual Bdnf promoters with single guide RNAs, resulting in transcript-specific Bdnf upregulation. Bdnf I upregulation is associated with gene expression changes linked to dendritic growth, while Bdnf IV upregulation is associated with genes that regulate protein catabolism. Upregulation of Bdnf I, but not Bdnf IV, increased mushroom spine density, volume, length, and head diameter, and also produced more complex dendritic arbors in cultured rat hippocampal neurons. In contrast, upregulation of Bdnf IV, but not Bdnf I, in the rat hippocampus attenuated contextual fear expression. Our data suggest that while Bdnf I and IV are both activity-dependent, BDNF produced from these promoters may serve unique cellular, synaptic, and behavioral functions.

An ancient polymorphic regulatory region within the BDNF gene associated with obesity modulates anxiety-like behaviour in mice and humans

Obesity and anxiety are morbidities notable for their increased impact on society during the recent COVID-19 pandemic. Understanding the mechanisms governing susceptibility to these conditions will increase our quality of life and resilience to future pandemics. In the current study, we explored the function of a highly conserved regulatory region (BE5.1) within the BDNF gene that harbours a polymorphism strongly associated with obesity (rs10767664; p = 4.69 × 10-26). Analysis in primary cells suggested that the major T-allele of BE5.1 was an enhancer, whereas the obesity-associated A-allele was not. However, CRISPR/CAS9 deletion of BE5.1 from the mouse genome (BE5.1KO) produced no significant effect on the expression of BDNF transcripts in the hypothalamus, no change in weight gain after 28 days and only a marginally significant increase in food intake. Nevertheless, transcripts were significantly increased in the amygdala of female mice and elevated zero maze and marble-burying tests demonstrated a significant increase in anxiety-like behaviour that could be reversed by diazepam. Consistent with these observations, human GWAS cohort analysis demonstrated a significant association between rs10767664 and anxiousness in human populations. Intriguingly, interrogation of the human GTEx eQTL database demonstrated no effect on BDNF mRNA levels associated with rs10767664 but a highly significant effect on BDNF-antisense (BDNF-AS) gene expression and splicing. The subsequent observation that deletion of BE5.1 also significantly reduced BDNF-AS expression in mice suggests a novel mechanism in the regulation of BDNF expression common to mice and humans, which contributes to the modulation of mood and anxiety in both species.

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.

Brain-derived neurotrophic factor: Its role in energy balance and cancer cachexia

Brain-derived neurotrophic factor (BDNF) plays an important role in the development of the central and peripheral nervous system during embryogenesis. In the mature central nervous system, BDNF is required for the maintenance and enhancement of synaptic transmissions and the survival of neurons. Particularly, it is involved in the modulation of neurocircuits that control energy balance through food intake, energy expenditure, and locomotion. Regulation of BDNF in the central nervous system is complex and environmental factors affect its expression in murine models which may reflect to phenotype dramatically. Furthermore, BDNF and its high-affinity receptor tropomyosin receptor kinase B (TrkB), as well as pan-neurotrophin receptor (p75NTR) is expressed in peripheral tissues in adulthood and their signaling is associated with regulation of energy balance. BDNF/TrkB signaling is exploited by cancer cells as well and BDNF expression is increased in tumors. Intriguingly, previously demonstrated roles of BDNF in regulation of food intake, adipose tissue and muscle overlap with derangements observed in cancer cachexia. However, data about the involvement of BDNF in cachectic cancer patients and murine models are scarce and inconclusive. In the future, knock-in and/or knock-out experiments with murine cancer models could be helpful to explore potential new roles for BDNF in the development of cancer cachexia.

Regulation of Satiety by Bdnf-e2-Expressing Neurons through TrkB Activation in Ventromedial Hypothalamus

The transcripts for Bdnf (brain-derived neurotrophic factor), driven by different promoters, are expressed in different brain regions to control different body functions. Specific promoter(s) that regulates energy balance remain unclear. We show that disruption of Bdnf promoters I and II but not IV and VI in mice (Bdnf-e1^-/-, Bdnf-e2^-/-) results in obesity. Whereas Bdnf-e1^-/- exhibited impaired thermogenesis, Bdnf-e2^-/- showed hyperphagia and reduced satiety before the onset of obesity. The Bdnf-e2 transcripts were primarily expressed in ventromedial hypothalamus (VMH), a nucleus known to regulate satiety. Re-expressing Bdnf-e2 transcript in VMH or chemogenetic activation of VMH neurons rescued the hyperphagia and obesity of Bdnf-e2^-/- mice. Deletion of BDNF receptor TrkB in VMH neurons in wildtype mice resulted in hyperphagia and obesity, and infusion of TrkB agonistic antibody into VMH of Bdnf-e2^-/- mice alleviated these phenotypes. Thus, Bdnf-e2-transcripts in VMH neurons play a key role in regulating energy intake and satiety through TrkB pathway.

Diverse Functions of Multiple Bdnf Transcripts Driven by Distinct Bdnf Promoters

The gene encoding brain-derived neurotrophic factor (Bdnf) consists of nine non-coding exons driven by unique promoters, leading to the expression of nine Bdnf transcripts that play different roles in various brain regions and physiological stages. In this manuscript, we present a comprehensive overview of the molecular regulation and structural characteristics of the multiple Bdnf promoters, along with a summary of the current knowledge on the cellular and physiological functions of the distinct Bdnf transcripts produced by these promoters. Specifically, we summarized the role of Bdnf transcripts in psychiatric disorders, including schizophrenia and anxiety, as well as the cognitive functions associated with specific Bdnf promoters. Moreover, we examine the involvement of different Bdnf promoters in various aspects of metabolism. Finally, we propose future research directions that will enhance our understanding of the complex functions of Bdnf and its diverse promoters.

Distinct roles of Bdnf I and Bdnf IV transcript variant expression in hippocampal neurons

Brain-derived neurotrophic factor (Bdnf) plays a critical role in brain development, dendritic growth, synaptic plasticity, as well as learning and memory. The rodent Bdnf gene contains nine 5' non-coding exons (I-IXa), which are spliced to a common 3' coding exon (IX). Transcription of individual Bdnf variants, which all encode the same BDNF protein, is initiated at unique promoters upstream of each non-coding exon, enabling precise spatiotemporal and activity-dependent regulation of Bdnf expression. Although prior evidence suggests that Bdnf transcripts containing exon I (Bdnf I) or exon IV (Bdnf IV) are uniquely regulated by neuronal activity, the functional significance of different Bdnf transcript variants remains unclear. To investigate functional roles of activity-dependent Bdnf I and IV transcripts, we used a CRISPR activation (CRISPRa) system in which catalytically-dead Cas9 (dCas9) fused to a transcriptional activator (VPR) is targeted to individual Bdnf promoters with single guide RNAs (sgRNAs), resulting in transcript-specific Bdnf upregulation. Bdnf I upregulation is associated with gene expression changes linked to dendritic growth, while Bdnf IV upregulation is associated with genes that regulate protein catabolism. Upregulation of Bdnf I, but not Bdnf IV, increased mushroom spine density, volume, length, and head diameter, and also produced more complex dendritic arbors in cultured rat hippocampal neurons. In contrast, upregulation of Bdnf IV, but not Bdnf I, in the rat hippocampus attenuated contextual fear expression. Our data suggest that while Bdnf I and IV are both activity-dependent, BDNF produced from these promoters may serve unique cellular, synaptic, and behavioral functions.

Bidirectional effects of voluntary exercise on the expression of Bdnf isoforms in the hippocampus of Hatano rat strains displaying different activity levels

Brain-derived neurotrophic factor has functional mRNA isoforms, whose expression is assumed to mediate the beneficial effects of exercise in neuropsychiatric disorders. This study aims to reveal the mechanism of intensity-dependent effects of voluntary exercise, focusing on the expression of Bdnf mRNA isoforms in Hatano rats. Animals with different voluntary activity were housed in cages with a locked or unlocked wheel for 5 weeks. The expression levels of Bdnf isoforms and the corresponding coding sequences (CDS) were measured in the hippocampus using real-time polymerase chain reaction (PCR). We found that exercise increased the expression of Bdnf isoform containing exon 1 in the high-intensity-running strain and decreased the expressions of Bdnf exon 1, 3, 6, 7, 8, and 9a in mild-intensity-running animal. The expression of Bdnf CDS was increased by exercise in both strains. These results suggest that expressions of Bdnf isoforms depend on the intensities of voluntary exercise, but the involvement of subjects' genetic background could not be excluded. Our finding also implies that the bidirectional effects of exercise may not be mediated via the final product of Bdnf.

A thalamic-primary auditory cortex circuit mediates resilience to stress

Resilience enables mental elasticity in individuals when rebounding from adversity. In this study, we identified a microcircuit and relevant molecular adaptations that play a role in natural resilience. We found that activation of parvalbumin (PV) interneurons in the primary auditory cortex (A1) by thalamic inputs from the ipsilateral medial geniculate body (MG) is essential for resilience in mice exposed to chronic social defeat stress. Early attacks during chronic social defeat stress induced short-term hyperpolarizations of MG neurons projecting to the A1 (MG^A1 neurons) in resilient mice. In addition, this temporal neural plasticity of MG^A1 neurons initiated synaptogenesis onto thalamic PV neurons via presynaptic BDNF-TrkB signaling in subsequent stress responses. Moreover, optogenetic mimicking of the short-term hyperpolarization of MG^A1 neurons, rather than merely activating MG^A1 neurons, elicited innate resilience mechanisms in response to stress and achieved sustained antidepressant-like effects in multiple animal models, representing a new strategy for targeted neuromodulation.

The updated development of blood-based biomarkers for Huntington's disease

Huntington's disease is a progressive neurodegenerative disease caused by mutation of the huntingtin (HTT) gene. The identification of mutation carriers before symptom onset provides an opportunity to intervene in the early stage of the disease course. Optimal biomarkers are of great value to reflect neuropathological and clinical progression and are sensitive to potential disease-modifying treatments. Blood-based biomarkers have the merits of minimal invasiveness, low cost, easy accessibility and safety. In this review, we summarized the updated development of blood-based biomarkers for HD from six aspects, including neuronal injuries, oxidative stress, endocrine functions, immune reactions, metabolism and differentially expressed miRNAs. The blood-based biomarkers presented and discussed in this review were close to clinical applicability and might facilitate clinical design as surrogate endpoints. Exploration and validation of robust blood-based biomarkers require further standard and systemic study design in the future.

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.

On Associations between Fear-Induced Aggression, Bdnf Transcripts, and Serotonin Receptors in the Brains of Norway Rats: An Influence of Antiaggressive Drug TC-2153

The Bdnf (brain-derived neurotrophic factor) gene contains eight regulatory exons (I-VIII) alternatively spliced to the protein-coding exon IX. Only exons I, II, IV, and VI are relatively well studied. The BDNF system and brain serotonergic system are tightly interconnected and associated with aggression. The benzopentathiepine TC-2153 affects both systems and exerts antiaggressive action. Our aim was to evaluate the effects of TC-2153 on the Bdnf exons I-IX's expressions and serotonin receptors' mRNA levels in the brain of rats featuring high aggression toward humans (aggressive) or its absence (tame). Aggressive and tame adult male rats were treated once with vehicle or 10 or 20 mg/kg of TC-2153. mRNA was quantified in the cortex, hippocampus, hypothalamus, and midbrain with real-time PCR. Selective breeding for high aggression or its absence affected the serotonin receptors' and Bdnf exons' transcripts differentially, depending on the genotype (strain) and brain region. TC-2153 had comprehensive effects on the Bdnf exons' expressions. The main trend was downregulation in the hypothalamus and midbrain. TC-2153 increased 5-HT_1B receptor hypothalamusc mRNA expression. For the first time, an influence of TC-2153 on the expressions of Bdnf regulatory exons and the 5-HT_1B receptor was shown, as was an association between Bdnf regulatory exons and fear-induced aggression involving genetic predisposition.

A novel intergenic enhancer that regulates Bdnf expression in developing cortical neurons

Brain-derived neurotrophic factor (BDNF) promotes neuronal differentiation and survival and is implicated in the pathogenesis of many neurological disorders. Here, we identified a novel intergenic enhancer located 170 kb from the Bdnf gene, which promotes the expression of Bdnf transcript variants during mouse neuronal differentiation and activity. Following Bdnf activation, enhancer-promoter contacts increase, and the region moves away from the repressive nuclear periphery. Bdnf enhancer activity is necessary for neuronal clustering and dendritogenesis in vitro, and for cortical development in vivo. Our findings provide the first evidence of a regulatory mechanism whereby the activation of a distal enhancer promotes Bdnf expression during brain development.

Function of brain-derived neurotrophic factor in the hypothalamus: Implications for depression pathology

Depression is a prevalent mental health disorder and is the number one cause of disability worldwide. Risk factors for depression include genetic predisposition and stressful life events, and depression is twice as prevalent in women compared to men. Both clinical and preclinical research have implicated a critical role for brain-derived neurotrophic factor (BDNF) signaling in depression pathology as well as therapeutics. A preponderance of this research has focused on the role of BDNF and its primary receptor tropomyosin-related kinase B (TrkB) in the cortex and hippocampus. However, much of the symptomatology for depression is consistent with disruptions in functions of the hypothalamus including changes in weight, activity levels, responses to stress, and sociability. Here, we review evidence for the role of BDNF and TrkB signaling in the regions of the hypothalamus and their role in these autonomic and behavioral functions associated with depression. In addition, we identify areas for further research. Understanding the role of BDNF signaling in the hypothalamus will lead to valuable insights for sex- and stress-dependent neurobiological underpinnings of depression pathology.

Buspirone, a 5-HT1A agonist attenuates social isolation-induced behavior deficits in rats: a comparative study with fluoxetine

Social isolation is a potent stressor in both humans and animals that results in increased anger-like emotion, (anger in humans), aggression and suicidal ideation in humans [suicidal trait-related behavior in rats (STRB)]. The study's purpose was to compare the effects of buspirone (BUS) and fluoxetine (Flx) on social isolation-induced behavior deficits in rats. The male Wistar rats were randomized into six groups and caged individually for 14 days except for the non stress control (nSC) group. They were then divided into the following groups, stress control (SC), Flx (30), BUS (10), BUS (20) and BUS (40) and treated from day 14 to day 28. On the last day of treatment behavior parameters were recorded. Serum cortisol, blood pressure (BP) measurement, magnetic resonance imaging (MRI) of the rat's brain and brain-derived neurotrophic factor (BDNF) expression were performed. SC group showed a significant increase in anger-like emotion, aggression, irritability score, learned helplessness, increased cortisol level and reduced BDNF. These behavioral deficits were attenuated by BUS and Flx, Both were found to be equally beneficial in preventing anger-like emotions and aggression. Flx, which has been found to promote suicidal thoughts in people, did not reduce irritability in rats, showing that it did not affect it. BUS significantly improved all behavioral traits also reduced cortisol levels, significantly increased BDNF and normalized BP. Neuroimaging studies in SC brains showed a reduction in amygdala size compared to nSC, BUS treatment mitigated this reduction. Buspirone is effective in preventing social isolation induced behavioural-deficits.

Brain-derived neurotrophic factor in Alzheimer's disease and its pharmaceutical potential

Synaptic abnormalities are a cardinal feature of Alzheimer's disease (AD) that are known to arise as the disease progresses. A growing body of evidence suggests that pathological alterations to neuronal circuits and synapses may provide a mechanistic link between amyloid β (Aβ) and tau pathology and thus may serve as an obligatory relay of the cognitive impairment in AD. Brain-derived neurotrophic factors (BDNFs) play an important role in maintaining synaptic plasticity in learning and memory. Considering AD as a synaptic disorder, BDNF has attracted increasing attention as a potential diagnostic biomarker and a therapeutical molecule for AD. Although depletion of BDNF has been linked with Aβ accumulation, tau phosphorylation, neuroinflammation and neuronal apoptosis, the exact mechanisms underlying the effect of impaired BDNF signaling on AD are still unknown. Here, we present an overview of how BDNF genomic structure is connected to factors that regulate BDNF signaling. We then discuss the role of BDNF in AD and the potential of BDNF-targeting therapeutics for AD.

Downregulation of Bdnf Expression in Adult Mice Causes Body Weight Gain

Gain or loss of appetite and resulting body weight changes are commonly observed in major depressive disorders (MDDs). Brain-derived neurotrophic factor (BDNF) is broadly expressed in the brain and is thought to play a role in the pathophysiology of MDDs and obesity. Congenital loss of function of BDNF causes weight gain in both humans and rodents; however, it is not clear whether acquired loss of function of BDNF also affects body weight. Thus, we exploited mutant mice in which the Bdnf expression level is regulated by the tetracycline-dependent transcriptional silencer (tTS)-tetracycline operator sequence (tetO) system. Time-controlled Bdnf expression using this system allowed us to establish congenital and acquired loss of function of Bdnf in mice. We demonstrated that changes in Bdnf expression influenced body weight during not only the developmental stage but also the adult stage of mice. Although it is still unclear whether acquired Bdnf loss of function in rodents mimics the pathology of MDD, our findings may bridge the mechanistic gap between MDDs and body weight gain in line with BDNF dysfunction.

Induction of Bdnf from promoter I following electroconvulsive seizures contributes to structural plasticity in neurons of the piriform cortex

BACKGROUND

Electroconvulsive therapy (ECT) efficacy is hypothesized to depend on induction of molecular and cellular events that trigger neuronal plasticity. Investigating how electroconvulsive seizures (ECS) impact plasticity in animal models can help inform our understanding of basic mechanisms by which ECT relieves symptoms of depression. ECS-induced plasticity is associated with differential expression of unique isoforms encoding the neurotrophin, brain-derived neurotrophic factor (BDNF).

HYPOTHESIS

We hypothesized that cells expressing the Bdnf exon 1-containing isoform are important for ECS-induced structural plasticity in the piriform cortex, a highly epileptogenic region that is responsive to ECS.

METHODS

We selectively labeled Bdnf exon 1-expressing neurons in mouse piriform cortex using Cre recombinase dependent on GFP technology (CRE-DOG). We then quantified changes in dendrite morphology and density of Bdnf exon 1-expressing neurons.

RESULTS

Loss of promoter I-derived BDNF caused changes in spine density and morphology in Bdnf exon 1-expressing neurons following ECS.

CONCLUSIONS

Promoter I-derived Bdnf is required for ECS-induced dendritic structural plasticity in Bdnf exon 1-expressing neurons.

Leptin regulates exon-specific transcription of the Bdnf gene via epigenetic modifications mediated by an AKT/p300 HAT cascade

Leptin is an adipocyte-derived hormone with pleiotropic functions affecting appetite and mood. While leptin's role in the regulation of appetite has been extensively studied in hypothalamic neurons, its function in the hippocampus, where it regulates mood-related behaviors, is poorly understood. Here, we show that the leptin receptor (LepRb) colocalizes with brain-derived neurotrophic factor (BDNF), a key player in the pathophysiology of major depression and the action of antidepressants, in the dentate gyrus of the hippocampus. Leptin treatment increases, whereas deficiency of leptin or leptin receptors decreases, total Bdnf mRNA levels, with distinct expression profiles of specific exons, in the hippocampus. Epigenetic analyses reveal that histone modifications, but not DNA methylation, underlie exon-specific transcription of the Bdnf gene induced by leptin. This is mediated by stimulation of AKT signaling, which in turn activates histone acetyltransferase p300 (p300 HAT), leading to changes in histone H3 acetylation and methylation at specific Bdnf promoters. Furthermore, deletion of Bdnf in the dentate gyrus, or specifically in LepRb-expressing neurons, abolishes the antidepressant-like effects of leptin. These findings indicate that leptin, acting via an AKT-p300 HAT epigenetic cascade, induces exon-specific Bdnf expression, which in turn is indispensable for leptin-induced antidepressant-like effects.

BDNF Overexpression in the Ventral Hippocampus Promotes Antidepressant- and Anxiolytic-Like Activity in Serotonin Transporter Knockout Rats

BDNF plays a pivotal role in neuroplasticity events, vulnerability and resilience to stress-related disorders, being decreased in depressive patients and increased after antidepressant treatment. BDNF was found to be reduced in patients carrying the human polymorphism in the serotonin transporter promoter region (5-HTTLPR). The serotonin knockout rat (SERT^-/-) is one of the animal models used to investigate the underlying molecular mechanisms of depression in humans. They present decreased BDNF levels, and anxiety- and depression-like behavior. To investigate whether upregulating BDNF would ameliorate the phenotype of SERT^-/- rats, we overexpressed BDNF locally into the ventral hippocampus and submitted the animals to behavioral testing. The results showed that BDNF overexpression in the vHIP of SERT^-/- rats promoted higher sucrose preference and sucrose intake; on the first day of the sucrose consumption test it decreased immobility time in the forced swim test and increased the time spent in the center of a novel environment. Furthermore, BDNF overexpression altered social behavior in SERT^-/- rats, which presented increased passive contact with test partner and decreased solitary behavior. Finally, it promoted decrease in plasma corticosterone levels 60 min after restraint stress. In conclusion, modulation of BDNF IV levels in the vHIP of SERT^-/- rats led to a positive behavioral outcome placing BDNF upregulation in the vHIP as a potential target to new therapeutic approaches to improve depressive symptoms.

Brain-Derived Neurotrophic Factor in Neonatal Seizures

Brain-derived neurotrophic factor (BDNF), a member of the neurotrophin family, has an extensively studied classical role in neuronal growth, differentiation, survival, and plasticity. Neurotrophic, from the Greek neuro and trophos, roughly translates as "vital nutrition for the brain." During development, BDNF and its associated receptor tyrosine receptor kinase B are tightly regulated as they influence the formation and maturation of neuronal synapses. Preclinical research investigating the role of BDNF in neurological disorders has focused on the effects of decreased BDNF expression on the development and maintenance of neuronal synapses. In contrast, heightened BDNF-tyrosine receptor kinase B activity has received less scrutiny for its role in neurological disorders. Recent studies suggest that excessive BDNF-tyrosine receptor kinase B signaling in the developing brain may promote the hyperexcitability that underlies refractory neonatal seizures. This review will critically examine BDNF-tyrosine receptor kinase B signaling in the immature brain, its role in the emergence of refractory neonatal seizures, and the potential of targeting BDNF-TrkB signaling as a novel antiseizure strategy.

Brain-derived neurotrophic factor in cerebrospinal fluid and plasma is not a biomarker for Huntington's disease

Brain-derived neurotrophic factor (BDNF) is implicated in the survival of striatal neurons. BDNF function is reduced in Huntington's disease (HD), possibly because mutant huntingtin impairs its cortico-striatal transport, contributing to striatal neurodegeneration. The BDNF trophic pathway is a therapeutic target, and blood BDNF has been suggested as a potential biomarker for HD, but BDNF has not been quantified in cerebrospinal fluid (CSF) in HD. We quantified BDNF in CSF and plasma in the HD-CSF cohort (20 pre-manifest and 40 manifest HD mutation carriers and 20 age and gender-matched controls) using conventional ELISAs and an ultra-sensitive immunoassay. BDNF concentration was below the limit of detection of the conventional ELISAs, raising doubt about previous CSF reports in neurodegeneration. Using the ultra-sensitive method, BDNF concentration was quantifiable in all samples but did not differ between controls and HD mutation carriers in CSF or plasma, was not associated with clinical scores or MRI brain volumetric measures, and had poor ability to discriminate controls from HD mutation carriers, and premanifest from manifest HD. We conclude that BDNF in CSF and plasma is unlikely to be a biomarker of HD progression and urge caution in interpreting studies where conventional ELISA was used to quantify CSF BDNF.

Intronic enhancer region governs transcript-specific Bdnf expression in rodent neurons

Brain-derived neurotrophic factor (BDNF) controls the survival, growth, and function of neurons both during the development and in the adult nervous system. Bdnf is transcribed from several distinct promoters generating transcripts with alternative 5' exons. Bdnf transcripts initiated at the first cluster of exons have been associated with the regulation of body weight and various aspects of social behavior, but the mechanisms driving the expression of these transcripts have remained poorly understood. Here, we identify an evolutionarily conserved intronic enhancer region inside the Bdnf gene that regulates both basal and stimulus-dependent expression of the Bdnf transcripts starting from the first cluster of 5' exons in mouse and rat neurons. We further uncover a functional E-box element in the enhancer region, linking the expression of Bdnf and various pro-neural basic helix–loop–helix transcription factors. Collectively, our results shed new light on the cell-type- and stimulus-specific regulation of the important neurotrophic factor BDNF.

Klotho, BDNF, NGF, GDNF Levels and Related Factors in Withdrawal Period in Chronic Cannabinoid Users

Klotho and neurotropic factors have recently been shown to be related to some psychiatric disorders and neurocognitive disorders, but there is no study on this issue within substance users. In this study, brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF), glial derived neurotrophic factor (GDNF) and klotho serum levels of a patient group consisting of 27 chronic cannabis users according to the DSM-V and 27 healthy volunteers were compared, and their relationships with other the clinical features of other patients were evaluated. Clinical scales, the Buss-Perry Aggression Scale, and the Substance Craving Scale were repeated on the first day of hospitalisation and on the seventh day of withdrawal. BDNF, GDNF, NGF and klotho levels were analysed using the ELISA method. There was no differences between the cannabinoid use disorder group and the control group regarding their klotho and other neurotrophic levels, but initiation age of cannabis use was negatively correlated with these levels. In addition, there was a relationship between verbal aggression scores and BDNF and NGF levels. There was a positive correlation between klotho and neurotrophic factors in all groups (patient group Day 1, patient group Day 7, control group) (p < 0.01). When comparing the difference between the correlations using the cocor (a comprehensive solution for the statistical comparison of correlations), the klotho-GDNF and klotho-NGF correlations for the first day of the patient group and the control group were different. In this study, rather than a difference in klotho levels and neurotropic factors, a significant relationship between these markers and each other and clinical parameters was demonstrated; further studies are needed to understand the exact mechanism.

Cryptotanshinone ameliorates CUS-induced depressive-like behaviors in mice

Objectives

Cryptotanshinone (CPT), a natural quinoid diterpene, isolated from Salvia miltiorrhiza, has shown various pharmacological properties. However, its effect on chronic unpredictable stress (CUS)-induced depression phenotypes and the underlying mechanism remain unclear. Therefore, the aim of this study was to investigate whether CPT could exert an antidepressant effect.

Methods

We investigated the effects of CPT in a CUS-induced depression model and explored whether these effects were related to the anti-inflammatory and neurogenesis promoting properties by investigating the expression levels of various signaling molecules at the mRNA and protein levels.

Results

Administration of CPT improved depression-like behaviors in CUS-induced mice. CPT administration increased the levels of doublecortin-positive cells and reversed the decrease in the expression levels of brain-derived neurotrophic factor (BDNF)/tyrosine kinase receptor B (TrkB) signaling transduction, as well as the downstream functional proteins, phosphorylated extracellular regulated protein kinases (p-ERK), and cyclic adenosine monophosphate (cAMP)-response element-binding protein levels (p-CREB) in hippocampus. CPT treatment also inhibited the activation of microglia and suppressed M1 microglial polarization, while promoting M2 microglial polarization by monitoring the expression levels of arginase 1 (Arg-1) and inducible nitric oxide synthase (iNOS), and further inhibited the expression of proinflammatory cytokines, including interleukin (IL)-1, IL-6, and tumor necrosis factor-α (TNF-α), and increased the expression of the anti-inflammatory cytokine IL-10 by regulating nuclear factor-κB (NF-κB) activation.

Conclusions

CPT relieves the depressive-like state in CUS-induced mice by enhancing neurogenesis and inhibiting inflammation through the BDNF/TrkB and NF-κB pathways and could therefore serve as a promising candidate for the treatment of depression.

Synaptic control of DNA methylation involves activity-dependent degradation of DNMT3A1 in the nucleus

DNA methylation is a crucial epigenetic mark for activity-dependent gene expression in neurons. Very little is known about how synaptic signals impact promoter methylation in neuronal nuclei. In this study we show that protein levels of the principal de novo DNA-methyltransferase in neurons, DNMT3A1, are tightly controlled by activation of N-methyl-D-aspartate receptors (NMDAR) containing the GluN2A subunit. Interestingly, synaptic NMDARs drive degradation of the methyltransferase in a neddylation-dependent manner. Inhibition of neddylation, the conjugation of the small ubiquitin-like protein NEDD8 to lysine residues, interrupts degradation of DNMT3A1. This results in deficits in promoter methylation of activity-dependent genes, as well as synaptic plasticity and memory formation. In turn, the underlying molecular pathway is triggered by the induction of synaptic plasticity and in response to object location learning. Collectively, the data show that plasticity-relevant signals from GluN2A-containing NMDARs control activity-dependent DNA-methylation involved in memory formation.

Molecularly Defined Hippocampal Inputs Regulate Population Dynamics in the Prelimbic Cortex to Suppress Context Fear Memory Retrieval

BACKGROUND

Context fear memory dysregulation is a hallmark symptom of several neuropsychiatric disorders, including generalized anxiety disorder and posttraumatic stress disorder. The hippocampus (HC) and prelimbic (PrL) subregion of the medial prefrontal cortex have been linked with context fear memory retrieval in rodents, but the mechanisms by which HC-PrL circuitry regulates this process remain poorly understood.

METHODS

Spatial and genetic targeting of HC-PrL circuitry was used for RNA sequencing (n = 31), chemogenetic stimulation (n = 44), in vivo calcium imaging (n = 20), ex vivo electrophysiology (n = 8), and molecular regulation of plasticity cascades during fear behavior (context fear retrieval) (n = 16).

RESULTS

We showed that ventral HC (vHC) neurons with projections to the PrL cortex (vHC-PrL projectors) are a transcriptomically distinct subpopulation compared with adjacent nonprojecting neurons, and we showed complementary enrichment for diverse neuronal processes and central nervous system-related clinical gene sets. We further showed that stimulation of this population of vHC-PrL projectors suppresses context fear memory retrieval and impairs the ability of PrL neurons to dynamically distinguish between distinct phases of fear learning. Using transgenic and circuit-specific molecular targeting approaches, we demonstrated that unique patterns of activity-dependent gene transcription associated with brain-derived neurotrophic factor signaling within vHC-PrL projectors causally regulated activity in excitatory and inhibitory PrL neurons during context fear memory retrieval.

CONCLUSIONS

Together, our data show that activity-dependent brain-derived neurotrophic factor release from molecularly distinct vHC-PrL projection neurons modulates postsynaptic signaling in both inhibitory and excitatory PrL neurons, modifying activity in discrete populations of PrL neurons to suppress freezing during context fear memory retrieval.

dotdotdot: an automated approach to quantify multiplex single molecule fluorescent in situ hybridization (smFISH) images in complex tissues

Multiplex single-molecule fluorescent in situ hybridization (smFISH) is a powerful method for validating RNA sequencing and emerging spatial transcriptomic data, but quantification remains a computational challenge. We present a framework for generating and analyzing smFISH data in complex tissues while overcoming autofluorescence and increasing multiplexing capacity. We developed dotdotdot (https://github.com/LieberInstitute/dotdotdot) as a corresponding software package to quantify RNA transcripts in single nuclei and perform differential expression analysis. We first demonstrate robustness of our platform in single mouse neurons by quantifying differential expression of activity-regulated genes. We then quantify spatial gene expression in human dorsolateral prefrontal cortex (DLPFC) using spectral imaging and dotdotdot to mask lipofuscin autofluorescence. We lastly apply machine learning to predict cell types and perform downstream cell type-specific expression analysis. In summary, we provide experimental workflows, imaging acquisition and analytic strategies for quantification and biological interpretation of smFISH data in complex tissues.

TrkB agonists prevent postischemic emergence of refractory neonatal seizures in mice

Refractory neonatal seizures do not respond to first-line antiseizure medications like phenobarbital (PB), a positive allosteric modulator for GABAA receptors. GABAA receptor-mediated inhibition is dependent upon electroneutral cation-chloride transporter KCC2, which mediates neuronal chloride extrusion and its age-dependent increase and postnatally shifts GABAergic signaling from depolarizing to hyperpolarizing. Brain-derived neurotropic factor-tyrosine receptor kinase B activation (BDNF-TrkB activation) after excitotoxic injury recruits downstream targets like PLCγ1, leading to KCC2 hypofunction. Here, the antiseizure efficacy of TrkB agonists LM22A-4, HIOC, and deoxygedunin (DG) on PB-refractory seizures and postischemic TrkB pathway activation was investigated in a mouse model (CD-1, P7) of refractory neonatal seizures. LM, a BDNF loop II mimetic, rescued PB-refractory seizures in a sexually dimorphic manner. Efficacy was associated with a substantial reduction in the postischemic phosphorylation of TrkB at Y816, a site known to mediate postischemic KCC2 hypofunction via PLCγ1 activation. LM rescued ischemia-induced phospho-KCC2-S940 dephosphorylation, preserving its membrane stability. Full TrkB agonists HIOC and DG similarly rescued PB refractoriness. Chemogenetic inactivation of TrkB substantially reduced postischemic neonatal seizure burdens at P7. Sex differences identified in developmental expression profiles of TrkB and KCC2 may underlie the sexually dimorphic efficacy of LM. These results support a potentially novel role for the TrkB receptor in the emergence of age-dependent refractory neonatal seizures.

Rbfox1 Is Expressed in the Mouse Brain in the Form of Multiple Transcript Variants and Contains Functional E Boxes in Its Alternative Promoters

The RNA-binding protein RBFOX1 is an important regulator of neuron development and neuronal excitability. Rbfox1 is a dosage-sensitive gene and in both mice and humans, decreased expression of Rbfox1 has been linked to neurodevelopmental disorders. Alternative promoters drive expression of Rbfox1 transcript isoforms that encode an identical protein. The tissue- and developmental stage-specific expression of these isoforms, as well as the underlying regulatory mechanisms, are, however, unclear. Here, we set out to capture all of the Rbfox1 transcript isoforms and identify transcriptional mechanisms that regulate brain-specific Rbfox1 expression. Isoform sequencing identified multiple alternative Rbfox1 transcript variants in the mouse cerebral cortex, including transcripts with novel first exons, alternatively spliced exons and 3′-truncations. Quantitative RT-PCR determined the expression of the alternative first exons in the developing cerebral cortex and different subregions of the juvenile brain. Alternative first exons were found to be highly stage- and subregion specific in their expression patterns suggesting that they fulfill specific functions during cortex development and in different brain regions. Using reporter assays we found that the promoter regions of the two first exons E1B and E1C/E1C.1 contain several functional E-boxes. Together, we provide an extensive picture of Rbfox1 isoform expression. We further identified important regulatory mechanisms that drive neuron-specific Rbfox1 expression. Thus, our study forms the basis for further research into the mechanisms that ensure physiological Rbfox1 expression in the brain. It also helps to understand why, in patients with neurodevelopmental disorders deletion of individual RBFOX1 transcript isoforms could affect brain function.

Brain-derived neurotrophic factor and schizophrenia

The brain-derived neurotrophic factor (BDNF) is a secretory growth factor that promotes neuronal proliferation and survival, synaptic plasticity and long-term potentiation in the central nervous system. Brain-derived neurotrophic factor biosynthesis and secretion are chrono-topically regulated processes at the cellular level, accounting for specific localizations and functions. Given its role in regulating brain development and activity, BDNF represents a potentially relevant gene for schizophrenia, and indeed BDNF and its non-synonymous functional variant, rs6265 (C → T, Val → Met) have been widely studied in psychiatric genetics. Human and animal studies have indicated that brain-derived neurotrophic factor is relevant for schizophrenia-related phenotypes, and that: (1) fine-tuned regulation of brain-derived neurotrophic factor secretion and activity is necessary to guarantee brain optimal development and functioning; (2) the Val → Met substitution is associated with impaired activity-dependent secretion of brain-derived neurotrophic factor; (3) disruption of brain-derived neurotrophic factor signaling is associated with altered synaptic plasticity and neurodevelopment. However, genome-wide association studies failed to associate the BDNF locus with schizophrenia, even though a sub-threshold association exists. Here, we will review studies focused on the relationship between the genetic variation of BDNF and schizophrenia, trying to fill the gap between genetic risk per se and insights from molecular biology. A deeper understanding of brain-derived neurotrophic factor biology and of the epigenetic regulation of brain-derived neurotrophic factor and its interactome during development may help clarifying the potential role of this gene in schizophrenia, thus informing development of brain-derived neurotrophic factor-based strategies of prevention and treatment of this disorder.

CREB Family Transcription Factors Are Major Mediators of BDNF Transcriptional Autoregulation in Cortical Neurons

BDNF signaling via its transmembrane receptor TrkB has an important role in neuronal survival, differentiation, and synaptic plasticity. Remarkably, BDNF is capable of modulating its own expression levels in neurons, forming a transcriptional positive feedback loop. In the current study, we have investigated this phenomenon in primary cultures of rat cortical neurons using overexpression of dominant-negative forms of several transcription factors, including CREB, ATF2, C/EBP, USF, and NFAT. We show that CREB family transcription factors, together with the coactivator CBP/p300, but not the CRTC family, are the main regulators of rat BDNF gene expression after TrkB signaling. CREB family transcription factors are required for the early induction of all the major BDNF transcripts, whereas CREB itself directly binds only to BDNF promoter IV, is phosphorylated in response to BDNF-TrkB signaling, and activates transcription from BDNF promoter IV by recruiting CBP. Our complementary reporter assays with BDNF promoter constructs indicate that the regulation of BDNF by CREB family after BDNF-TrkB signaling is generally conserved between rat and human. However, we demonstrate that a nonconserved functional cAMP-responsive element in BDNF promoter IXa in humans renders the human promoter responsive to BDNF-TrkB-CREB signaling, whereas the rat ortholog is unresponsive. Finally, we show that extensive BDNF transcriptional autoregulation, encompassing all major BDNF transcripts, occurs also in vivo in the adult rat hippocampus during BDNF-induced LTP. Collectively, these results improve the understanding of the intricate mechanism of BDNF transcriptional autoregulation.SIGNIFICANCE STATEMENT Deeper understanding of stimulus-specific regulation of BDNF gene expression is essential to precisely adjust BDNF levels that are dysregulated in various neurological disorders. Here, we have elucidated the molecular mechanisms behind TrkB signaling-dependent BDNF mRNA induction and show that CREB family transcription factors are the main regulators of BDNF gene expression after TrkB signaling. Our results suggest that BDNF-TrkB signaling may induce BDNF gene expression in a distinct manner compared with neuronal activity. Moreover, our data suggest the existence of a stimulus-specific distal enhancer modulating BDNF gene expression.

Manipulation of a genetically and spatially defined sub-population of BDNF-expressing neurons potentiates learned fear and decreases hippocampal-prefrontal synchrony in mice

Brain-derived neurotrophic factor (BDNF) signaling regulates synaptic plasticity in the hippocampus (HC) and prefrontal cortex (PFC), and has been extensively linked with fear memory expression in rodents. Notably, disrupting BDNF production from promoter IV-derived transcripts enhances fear expression in mice, and decreases fear-associated HC-PFC synchrony, suggesting that Bdnf transcription from promoter IV plays a key role in HC-PFC function during fear memory retrieval. To better understand how promoter IV-derived BDNF controls HC-PFC connectivity and fear expression, we generated a viral construct that selectively targets cells expressing promoter IV-derived Bdnf transcripts ("p4-cells") for tamoxifen-inducible Cre-mediated recombination (AAV8-p4Bdnf-ER^T2CreER^T2-PEST). Using this construct, we found that ventral hippocampal (vHC) p4-cells are recruited during fear expression, and that activation of these cells causes exaggerated fear expression that co-occurs with disrupted vHC-PFC synchrony in mice. Our data highlight how this novel construct can be used to interrogate genetically defined cell types that selectively contribute to BDNF-dependent behaviors.

A subpopulation of Bdnf-e1-expressing glutamatergic neurons in the lateral hypothalamus critical for thermogenesis control

Objective

Brown adipose tissue (BAT)–mediated thermogenesis plays a key role in energy homeostasis and the maintenance of body temperature. Previous work suggests that brain-derived neurotrophic factor (BDNF) is involved in BAT thermogenesis, but the underlying neural circuits and molecular mechanism remain largely unknown. This is in part due to the difficulties in manipulating BDNF expression in different brain regions through different promoters and the lack of tools to identify neurons in the brain specifically involved in BAT thermogenesis.

Methods

We have created several lines of mutant mice in which BDNF transcription from a specific promoter was selectively disrupted by replacing Bdnf with green fluorescent protein (GFP; Bdnf-e1, -e4, and -e6^−/− mice). As such, cells expressing Bdnf-e1, -e4, or -e6 were labeled with GFP. To identify BAT-connected thermogenesis neurons in brain, we applied the retrograde pseudorabies virus labeling method from BAT. We also used chemogenetic tools to manipulate specific neurons coupled with BAT temperature recording. Moreover, we developed a new TrkB agonist antibody to rescue the BAT thermogenesis deficits.

Results

We show that selective disruption of Bdnf expression from promoter 1 (Bdnf-e1) resulted in severe obesity and deficits of BAT-mediated thermogenesis. Body temperature response to cold was impaired in Bdnf-e1^−/− mice. BAT expression of Ucp1 and Pcg1a, genes known to regulate thermogenesis, was also reduced, accompanying a decrease in the sympathetic activity of BAT. Staining of cells expressing Bdnf-e1 transcript, combined with transsynaptic, retrograde-tracing labeling of BAT-connected neurons, identified a group of excitatory neurons in lateral hypothalamus (LH) critical for thermogenesis regulation. Moreover, an adaptive thermogenesis defect in Bdnf-e1^−/− mice was rescued by injecting an agonistic antibody for TrkB, the BDNF receptor, into LH. Remarkably, activation of the excitatory neurons (VGLUT2+) in LH through chemogenetic tools resulted in a rise of BAT temperature.

Conclusions

These results reveal a specific role of BDNF promoter I in thermogenesis regulation and define a small subset of neurons in LH that contribute to such regulation.

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Only Bdnf-e1^−/−, but not Bdnf-e4^−/− or Bdnf-e6^−/−, mutant mice exhibited deficiencies of BAT thermogenesis.

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Neurons that are both Bdnf-e1 expressing and BAT-connected were found only in LH.

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BAT-connected neurons in LH are glutamatergic.

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Activation of the LH glutamatergic neurons resulted in an increase in BAT temperature.

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Administration of TrkB agonist antibody in LH rescued thermogenesis deficits.

Obese mice exposed to psychosocial stress display cardiac and hippocampal dysfunction associated with local brain-derived neurotrophic factor depletion

INTRODUCTION

Obesity and psychosocial stress (PS) co-exist in individuals of Western society. Nevertheless, how PS impacts cardiac and hippocampal phenotype in obese subjects is still unknown. Nor is it clear whether changes in local brain-derived neurotrophic factor (BDNF) account, at least in part, for myocardial and behavioral abnormalities in obese experiencing PS.

METHODS

In adult male WT mice, obesity was induced via a high-fat diet (HFD). The resident-intruder paradigm was superimposed to trigger PS. In vivo left ventricular (LV) performance was evaluated by echocardiography and pressure-volume loops. Behaviour was indagated by elevated plus maze (EPM) and Y-maze. LV myocardium was assayed for apoptosis, fibrosis, vessel density and oxidative stress. Hippocampus was analyzed for volume, neurogenesis, GABAergic markers and astrogliosis. Cardiac and hippocampal BDNF and TrkB levels were measured by ELISA and WB. We investigated the pathogenetic role played by BDNF signaling in additional cardiac-selective TrkB (cTrkB) KO mice.

FINDINGS

When combined, obesity and PS jeopardized LV performance, causing prominent apoptosis, fibrosis, oxidative stress and remodeling of the larger coronary branches, along with lower BDNF and TrkB levels. HFD/PS weakened LV function similarly in WT and cTrkB KO mice. The latter exhibited elevated LV ROS emission already at baseline. Obesity/PS augmented anxiety-like behaviour and impaired spatial memory. These changes were coupled to reduced hippocampal volume, neurogenesis, local BDNF and TrkB content and augmented astrogliosis.

INTERPRETATION

PS and obesity synergistically deteriorate myocardial structure and function by depleting cardiac BDNF/TrkB content, leading to augmented oxidative stress. This comorbidity triggers behavioral deficits and induces hippocampal remodeling, potentially via lower BDNF and TrkB levels. FUND: J.A. was in part supported by Rotary Foundation Global Study Scholarship. G.K. was supported by T32 National Institute of Health (NIH) training grant under award number 1T32AG058527. S.C. was funded by American Heart Association Career Development Award (19CDA34760185). G.A.R.C. was funded by NIH (K01HL133368-01). APB was funded by a Grant from the Friuli Venezia Giulia Region entitled: "Heart failure as the Alzheimer disease of the heart; therapeutic and diagnostic opportunities". M.C. was supported by PRONAT project (CNR). N.P. was funded by NIH (R01 HL136918) and by the Magic-That-Matters fund (JHU). V.L. was in part supported by institutional funds from Scuola Superiore Sant'Anna (Pisa, Italy), by the TIM-Telecom Italia (WHITE Lab, Pisa, Italy), by a research grant from Pastificio Attilio Mastromauro Granoro s.r.l. (Corato, Italy) and in part by ETHERNA project (Prog. n. 161/16, Fondazione Pisa, Italy). Funding source had no such involvement in study design, in the collection, analysis, interpretation of data, in the writing of the report; and in the decision to submit the paper for publication.

[11C](R)-Rolipram positron emission tomography detects DISC1 inhibition of phosphodiesterase type 4 in live Disc1 locus-impaired mice

Although still a matter of controversy, disrupted in schizophrenia protein 1 (DISC1) was suggested as a potential inhibitor of phosphodiesterase 4 (PDE4). We used Disc1 locus impairment (LI) mice to investigate the interaction between PDE4 and DISC 1 in vivo and in vitro. [¹¹C](R)-Rolipram binding was measured by PET in LI (n = 11) and C57BL/6 wild-type (WT, n = 9) mice. [¹¹C](R)-Rolipram total distribution volumes (V_T) were calculated and corrected for plasma-free fraction (f_P) measured in a separate group of LI (n = 6) and WT (n = 7) mice. PDE4 enzyme activity was measured using in vitro samples of cerebral cortices from groups of LI (n = 4), heterozygote (n = 4), and WT (n = 4) mice. Disc1 LI mice showed a 41% increase in V_T (18 ± 6 vs. 13±4 mL/cm³, P = 0.04) compared to WT mice. V_T/f_P showed a 73% significant increase (90 ± 31 vs. 52 ± 15 mL/cm³, P = 0.004) in Disc1 LI compared to WT mice. PDE4 enzymatic activity assay confirmed in vivo findings showing significant group differences (p < 0.0001). In conclusion, PDE4 activity was increased in the absence of critical DISC1 protein isoforms both in vivo and in vitro. Additionally, [¹¹C](R)-Rolipram PET was sensitive enough to assess altered PDE4 activity caused by PDE4-DISC1 interaction.

Enriched environment alleviates stress-induced dry-eye through the BDNF axis

The number of patients with dry eye disease (DED) is increasing, and DED has become an urgent public health problem. A comorbidity of mental disorders has been reported in DED patients. We hypothesized that physical and psychological stressors impair tear secretion. To examine the relationship between stress loading and decreased tear secretion, we established a stress-induced DED mouse model, which permitted us to address the underlying mechanism of pathogenesis and resilience. Enriched environment (EE) was an effective intervention to prevent and alleviate stress-induced decreased tear secretion. Because stress loading resulted in decreased brain-derived neurotrophic factor (BDNF) expression while EE resulted in increased expression, we focused on the role of BDNF in tear secretion. Using two distinct Bdnf gene knockdown mice, we evaluated whether BDNF was a deterministic factor in regulating tear secretion in healthy and stressed conditions. Bdnf knockdown mice showed decreased basal tear secretion and loss of stress tolerance by EE for tear secretion. These results suggest that BDNF expression is related to tear secretion and to the pathology of DED.

Disruption of brain-derived neurotrophic factor production from individual promoters generates distinct body composition phenotypes in mice

Brain-derived neurotrophic factor (BDNF) is a key neuropeptide in the central regulation of energy balance. The Bdnf gene contains nine promoters, each producing specific mRNA transcripts that encode a common protein. We sought to assess the phenotypic outcomes of disrupting BDNF production from individual Bdnf promoters. Mice with an intact coding region but selective disruption of BDNF production from Bdnf promoters I, II, IV, or VI (Bdnf-e1^-/-, -e2^-/-, -e4^-/-, and -e6^-/-) were created by inserting an enhanced green fluorescent protein-STOP cassette upstream of the targeted promoter splice donor site. Body composition was measured by MRI weekly from age 4 to 22 wk. Energy expenditure was measured by indirect calorimetry at 18 wk. Food intake was measured in Bdnf-e1^-/- and Bdnf-e2^-/- mice, and pair feeding was conducted. Weight gain, lean mass, fat mass, and percent fat of Bdnf-e1^-/- and Bdnf-e2^-/- mice (both sexes) were significantly increased compared with wild-type littermates. For Bdnf-e4^-/- and Bdnf-e6^-/- mice, obesity was not observed with either chow or high-fat diet. Food intake was increased in Bdnf-e1^-/- and Bdnf-e2^-/- mice, and pair feeding prevented obesity. Mutant and wild-type littermates for each strain (both sexes) had similar total energy expenditure after adjustment for body composition. These findings suggest that the obesity phenotype observed in Bdnf-e1^-/- and Bdnf-e2^-/- mice is attributable to hyperphagia and not altered energy expenditure. Our findings show that disruption of BDNF from specific promoters leads to distinct body composition effects, with disruption from promoters I or II, but not IV or VI, inducing obesity.

Cortistatin-expressing interneurons require TrkB signaling to suppress neural hyper-excitability

Signaling of brain-derived neurotrophic factor (BDNF) via tropomyosin receptor kinase B (TrkB) plays a critical role in the maturation of cortical inhibition and controls expression of inhibitory interneuron markers, including the neuropeptide cortistatin (CST). CST is expressed exclusively in a subset of cortical and hippocampal GABAergic interneurons, where it has anticonvulsant effects and controls sleep slow-wave activity (SWA). We hypothesized that CST-expressing interneurons play a critical role in regulating excitatory/inhibitory balance, and that BDNF, signaling through TrkB receptors on CST-expressing interneurons, is required for this function. Ablation of CST-expressing cells caused generalized seizures and premature death during early postnatal development, demonstrating a critical role for these cells in providing inhibition. Mice in which TrkB was selectively deleted from CST-expressing interneurons were hyperactive, slept less and developed spontaneous seizures. Frequencies of spontaneous excitatory post-synaptic currents (sEPSCs) on CST-expressing interneurons were attenuated in these mice. These data suggest that BDNF, signaling through TrkB receptors on CST-expressing cells, promotes excitatory drive onto these cells. Loss of excitatory drive onto CST-expressing cells that lack TrkB receptors may contribute to observed hyperexcitability and epileptogenesis.