Transcriptome profiling analysis of the mechanisms underlying the BDNF Val66Met polymorphism induced dysfunctions of the central nervous system

FKN/CX3CR1 axis facilitates migraine-Like behaviour by activating thalamic-cortical network microglia in status epilepticus model rats

Background

The incidence of migraines is higher among individuals with epilepsy than in healthy individuals, and these two diseases are thought to shared pathophysiological mechanisms. Excitation/inhibition imbalance plays an essential role in the comorbidity of epilepsy and migraine. Microglial activation is crucial for abnormal neuronal signal transmission. However, it remains unclear whether and how microglia are activated and their role in comorbidities after being activated. This study aimed to explore the characteristics and mechanism of microglial activation after seizures and their effect on migraine.

Methods

Model rats of status epilepticus (SE) induced by intraperitoneal injection of lithium chloride (LiCl)-pilocarpine and migraine induced by repeated dural injections of inflammatory soup (IS) were generated, and molecular and histopathologic evidence of the microglial activation targets of fractalkine (FKN) signalling were examined. HT22-BV2 transwell coculture assays were used to explore the interaction between neurons and microglia. LPS (a microglial agonist) and FKN stimulation of BV2 microglial cells were used to evaluate changes in BDNF levels after microglial activation.

Results

Microglia were specifically hyperplastic and activated in the temporal lobe cortex, thalamus, and spinal trigeminal nucleus caudalis (sp5c), accompanied by the upregulation of FKN and CX3CR1 four days after seizures. Moreover, SE-induced increases in nociceptive behaviour and FKN/CX3CR1 axis expression in migraine model rats. AZD8797 (a CX3CR1 inhibitor) prevented the worsening of hyperalgesia and microglial activation in migraine model rats after seizures, while FKN infusion in migraine model rats exacerbated hyperalgesia and microglial activation associated with BDNF-Trkb signalling. Furthermore, in neuron-microglia cocultures, microglial activation and FKN/CX3CR1/BDNF/iba1 expression were increased compared with those in microglial cultures alone. Activating microglia with LPS and FKN increased BDNF synthesis in BV2 microglia.

Conclusions

Our results indicated that epilepsy facilitated migraine through FKN/CX3CR1 axis-mediated microglial activation in the cortex/thalamus/sp5c, which was accompanied by BDNF release. Blocking the FKN/CX3CR1 axis and microglial activation are potential therapeutic strategies for preventing and treating migraine in patients with epilepsy.

Fractalkine signaling regulates oligodendroglial cell genesis from SVZ precursor cells

Neural and oligodendrocyte precursor cells (NPCs and OPCs) in the subventricular zone (SVZ) of the brain contribute to oligodendrogenesis throughout life, in part due to direct regulation by chemokines. The role of the chemokine fractalkine is well established in microglia; however, the effect of fractalkine on SVZ precursor cells is unknown. We show that murine SVZ NPCs and OPCs express the fractalkine receptor (CX3CR1) and bind fractalkine. Exogenous fractalkine directly enhances OPC and oligodendrocyte genesis from SVZ NPCs in vitro. Infusion of fractalkine into the lateral ventricle of adult NPC lineage-tracing mice leads to increased newborn OPC and oligodendrocyte formation in vivo. We also show that OPCs secrete fractalkine and that inhibition of endogenous fractalkine signaling reduces oligodendrocyte formation in vitro. Finally, we show that fractalkine signaling regulates oligodendrogenesis in cerebellar slices ex vivo. In summary, we demonstrate a novel role for fractalkine signaling in regulating oligodendrocyte genesis from postnatal CNS precursor cells.

Impaired brain fractalkine-CX3CR1 signaling is implicated in cognitive dysfunction in diet-induced obese mice

INTRODUCTION

A diet high in saturated fat is well known to affect neuronal function and contribute to cognitive decline in experimental animals and humans. Fractalkine released from neurons acts on its receptor, CX3C chemokine receptor 1 (CX3CR1), in the microglia to regulate several brain functions. The present study addressed whether fractalkine-CX3CR1 signaling in the brain, especially the hippocampus, contributes to the cognitive deficits observed in diet-induced obese (DIO) mice.

RESEARCH DESIGN AND METHODS

Mice were given 60% high-fat diet for 16 weeks. The expression of fractalkine and CX3CR1 in the hippocampus, amygdala and prefrontal cortex of DIO mice was analyzed. Cognitive ability in the Y-maze test and hippocampal glutamate receptors and synaptic markers were observed in DIO and CX3CR1 antagonist-treated mice. Regulation of fractalkine and CX3CR1 expression in the hippocampus was examined following administration of a selective insulin-like growth factor-1 (IGF-1) receptor inhibitor and a tyrosine receptor kinase B (TrkB) antagonist in normal mice.

RESULTS

DIO mice exhibited significant cognitive deficits in the Y-maze test and decrease in fractalkine and CX3CR1 in the hippocampus and amygdala compared with mice fed a control diet (CD mice). Administration of the CX3CR1 antagonist 18a in normal mice induced significant cognitive deficits in the Y-maze test. DIO mice and CX3CR1 antagonist-treated mice exhibited significant decreases in protein levels of NMDA (N-methyl-D-aspartate) receptor subunit (NR2A), AMPA (α-amino-5-methyl-3-hydroxy-4-isoxazole propionate) receptor subunit (GluR1) and postsynaptic density protein 95 in the hippocampus compared with their respective controls. Furthermore, plasma IGF-1 and hippocampal brain-derived neurotrophic factor were significantly decreased in DIO mice compared with CD mice. Administration of a selective IGF-1 receptor inhibitor and a TrkB antagonist in normal mice significantly decreased fractalkine and CX3CR1 in the hippocampus.

CONCLUSIONS

These findings indicate that the cognitive decline observed in DIO mice is due, in part, to reduced fractalkine-CX3CR1 signaling in the corticolimbic system.

Association between BDNF rs6265 polymorphisms and postoperative cognitive dysfunction in Chinese Han Population

INTRODUCTION

Brain-derived neurotrophic factor (BDNF) plays a critical role in the pathogenesis of postoperative cognitive dysfunction (POCD). In present study, we aimed to assess the possible association between POCD and BDNF rs6265 polymorphisms.

METHODS

124 patients aged 60 years or older scheduled for elective surgery under general anesthesia and 25 age- and gender-matched healthy volunteers were recruited. POCD was identified using a neuropsychological test battery administered preoperatively, 7 days, and 3 months after surgery. Genotyping of rs6265 was performed using polymerase chain reaction amplification and restriction fragment length polymorphism analysis.

RESULTS

99 patients and 25 healthy controls were finally enrolled in the analysis. 29(29.3%) and 18(18.2%) of 99 patients had POCD at 7 days and 3 months after surgery, respectively. The patients carrying a G allele at the rs6265 locus showed a lower risk for POCD than an A allele carriers on postoperative 7 days, but not 3 months after surgery (OR = 0.67; 95% CI: 0.47-0.96; p = .017; OR = 0.69; 95% CI: 0.42-1.13; p = .14, respectively). The risk of POCD at 7 days following surgery was significantly lower in additive model (OR = 0.41; 95% CI: 0.2-0.84; p = .015) and dominant model (OR = 0.35; 95% CI: 0.13-0.96; p = .042).

CONCLUSION

We tentatively demonstrate that BDNF rs6265 polymorphisms might be associated with occurrence of POCD at 7 days after surgery and the A > G mutant at the rs6265 locus be likely a protective factor for early POCD in Chinese Han population.

Kynurenine pathway is altered in BDNF Val66Met knock-in mice: Effect of physical exercise

The Brain-Derived Neurotrophic Factor (BDNF) Val66Met polymorphism has been correlated with increased predisposition to develop cognitive and psychiatric disorders, and with a reduced response to some therapeutic treatments. However, the mechanisms underlying these impairments are currently not completely understood. Remarkably, kynurenine pathway alterations have also been implicated in cognitive and psychiatric disorders. Moreover, recent evidence suggests that physical exercise may promote beneficial effects by controlling kynurenine metabolism in the muscle. The aim of the present study was to assess whether the kynurenine pathway was differentially regulated in sedentary and exercising wild-type (BDNF^Val/Val) and homozygous knock-in BDNF Val66Met (BDNF^Met/Met) mice. We found that plasma and hippocampal levels of kynurenic acid and the hippocampal mRNA levels of IDO1 and KAT2 protein levels were increased in BDNF^Met/Met mice and were not modulated by physical exercise. On the contrary, KAT1 protein levels in the gastrocnemius muscle were reduced, whereas MCP1 mRNA in the gastrocnemius muscle and GFAP protein in the hippocampus were increased in BDNF^Met/Met mice compared to BDNF^Val/Val mice, and reduced by physical exercise. Physical exercise increased plasmatic kynurenine levels only in BDNF^Met/Met mice, and protein levels of KAT1 and KAT4 in the gastrocnemius muscle and hippocampus respectively, regardless of the genotype. Finally, we found that physical exercise was able to enhance the hippocampal-dependent memory only in the BDNF^Val/Val mice. Overall our results showing an overactivation of the kynurenine pathway in the BDNF^Met/Met mice may suggest a possible mechanism underlying the cognitive deficits reported in the BDNF Val66Met carriers.

CX3CL1 rs170364 gene polymorphism has a protective effect against major depression by enhancing its transcriptional activity

Studies have shown that adult hippocampal neurogenesis may be a cause of depression. CX3CL1 is a chemokine that plays an important role in adult neurogenesis. This study aimed to investigate the relationship between CX3CL1 polymorphisms (rs170364) and the risk of depression. A case-control study of 502 patients with major depression and 504 gender-matched and age-matched healthy controls was performed. All subjects were recruited from the Chinese Han population. Next-generation sequencing was used to genotype the CX3CL1 rs170364 locus. In addition, the effect of the rs170364 polymorphism on transcription of CX3CL1 was investigated through the use of luciferase reporter constructs and in vitro analysis in SH-SY5Y cells. Our results demonstrated that the T allele and GT + TT genotype of the CX3CL1 rs170364 locus were associated with a reduced risk of major depression. Subgroup analysis found that this significant association was consistently found in females but not in males. In vitro experiments found that the rs170364 mutation enhanced the transcriptional activity of CX3CL1. These results suggest that T allele and GT + TT genotypes of the CX3CL1 rs170364 locus may be a protective factor against the onset of depression in the Chinese Han population, especially in females. SNP rs170364 enhances the transcriptional activity of CX3CL1.

BDNF Val66Met polymorphism alters food intake and hypothalamic BDNF expression in mice

Obesity, a rising public health burden, is a multifactorial disease with an increased risk for patients to develop several pathological conditions including type 2 diabetes mellitus, hypertension, and cardiovascular disease. Increasing evidence suggests a relationship between the human brain-derived neurotrophic factor (BDNF) Val66Met single-nucleotide polymorphism (SNP) and obesity, although the underlying mechanisms of this connection are still not completely understood. In the present study, we found that homozygous knock-in BDNF^Met/Met mice were overweight and hyperphagic compared to wildtype BDNF^Val/Val mice. Increased food intake was associated with reduction of total BDNF and BDNF1, BDNF4 and BDNF6 transcripts in the hypothalamus of BDNF^Met/Met mice. In contrast, in the white adipose tissue total BDNF and Glut4 expression levels were augmented, while sirtuin 1 and leptin receptor (Ob-R) expression levels were reduced in BDNF^Met/Met mice. Moreover, plasmatic leptin levels were decreased in BDNF^Met/Met mice. However, BDNF^Val/Val and BDNF^Met/Met mice showed a similar response to the insulin tolerance test and glucose tolerance test. Altogether, these results suggest that BDNF Val66Met SNP strongly contributes to adipose tissue pathophysiology, resulting in reduced circulating leptin levels and hypothalamic expression of BDNF, which, in turn, promote increased food intake and overweight in BDNF^Met/Met mice.

In vitro affinity optimization of an anti-BDNF monoclonal antibody translates to improved potency in targeting chronic pain states in vivo

The role of brain-derived neurotrophic factor (BDNF) signaling in chronic pain has been well documented. Given the important central role of BDNF in long term plasticity and memory, we sought to engineer a high affinity, peripherally-restricted monoclonal antibody against BDNF to modulate pain. BDNF shares 100% sequence homology across human and rodents; thus, we selected chickens as an alternative immune host for initial antibody generation. Here, we describe the affinity optimization of complementarity-determining region-grafted, chicken-derived R3bH01, an anti-BDNF antibody specifically blocking the TrkB receptor interaction. Antibody optimization led to the identification of B30, which has a > 300-fold improvement in affinity based on BIAcore, an 800-fold improvement in potency in a cell-based pERK assay and demonstrates exquisite selectivity over related neurotrophins. Affinity improvements measured in vitro translated to in vivo pharmacological activity, with B30 demonstrating a 30-fold improvement in potency over parental R3bH01 in a peripheral nerve injury model. We further demonstrate that peripheral BDNF plays a role in maintaining the plasticity of sensory neurons following nerve damage, with B30 reversing neuron hyperexcitability associated with heat and mechanical stimuli in a dose-dependent fashion. In summary, our data demonstrate that effective sequestration of BDNF via a high affinity neutralizing antibody has potential utility in modulating the pathophysiological mechanisms that drive chronic pain states.

BDNF Val66Met Polymorphism, the Allele-Specific Analysis by qRT-PCR - a Novel Protocol

Background: Alteration in brain-derived neurotrophic factor (BDNF) production is a marker of neuropathological conditions, which has led to the investigation of Val66Met polymorphism occurring in the human BDNF gene (BDNF). Presently, there are no reported methods available for the analysis of Val66Met impact on human BDNF functioning. Purpose: To develop a qRT-PCR protocol for the allele-specific expression evaluation of the Val66Met polymorphism in BDNF. Methods: Using RNA extracted from muscle samples of 9 healthy volunteers (32.9 ± 10.3 y) at rest and following a maximal effort aerobic capacity exercise test, a protocol was developed for the detection of Val66/Met66 allele-specific BDNF expression in Real-Time Quantitative Reverse Transcription PCR (qRT-PCR) - relative to housekeeping genes - and validated by absolute quantification in Droplet Digital Polymerase Chain Reaction (ddPCR). Results: Differences in the relative values of BDNF mRNA were confirmed by ddPCR analysis. HPRT1 and B2M were the most stable genes expressed in muscle tissue among different metabolic conditions, while GAPDH revealed to be metabolic responsive. Conclusion: Our qRT-PCR protocol successfully determines the allele-specific detection and changes in BDNF expression regarding the Val66Met polymorphism.

Surgical Trauma Exacerbates Cognitive Deficits and Neuroinflammation in Aged Rats: The Role of CX3CL1-CX3CR1 Signaling

Age is the most prominent risk factor for the development of postoperative cognitive dysfunction. The present study investigated the role of CX3CL1-CX3CR1 signaling in age-related differences in surgery-induced cognitive deficits and neuroinflammation. Adult and aged male Sprague-Dawley rats were subjected to partial hepatectomy or partial hepatectomy with intracerebroventricular infusion of CX3CL1. On postoperative days 3, 7, and 14, the rats were subjected to an open field test and the Morris water maze test. Hippocampal interleukin-1β, CX3CL1, CX3CR1, brain derived neurotrophic factor (BDNF), ionized calcium-binding adapter molecule 1 (Iba-1), and Arginase-1 (Arg1) levels were measured. Age exacerbated cognitive impairment and increased neuroinflammation following surgery. Surgery-induced decreases in CX3CL1 and CX3CR1 proteins were accompanied by increased microglial activation, as indicated by increased Iba-1 expression. Corresponding decline in Arg1 and BDNF levels were observed. Treatment with CX3CL1 decreased proinflammatory cytokines expression, increased BDNF and Arg1 levels in the brain, and enhanced behavioral recovery. The surgery-induced decreases in CX3CL1 and CX3CR1 expression exacerbated postoperative cognitive deficits and exaggerated neuroinflammatory responses in this rodent model. Treatment with CX3CL1 attenuated these effects, at least partly by inhibiting microglial activation, decreasing the associated production of proinflammatory cytokines, and enhancing BDNF expression.

Global epigenetic analysis of BDNF Val66Met mice hippocampus reveals changes in dendrite and spine remodeling genes

Brain-derived neurotrophic factor (BDNF), a neurotrophin highly expressed in the hippocampus, plays crucial roles in cognition, neuroplasticity, synaptic function, and dendritic remodeling. The common human Val66Met polymorphism of BDNF has been implicated in the pathophysiology of neuropsychiatric and neurodegenerative disorders, and in the outcome of pro-adaptive and therapeutic treatments. Altered gene-expression profile has been previously shown in BDNF Val66Met knock-in mice, which recapitulate the phenotypic hallmarks of individuals carrying the BDNF Met allele. The aim of this study was to investigate the impact of the BDNF Val66Met polymorphism in the knock-in mouse model on two hippocampal epigenetic marks for transcriptional repression and activation, respectively: trimethylation of lysine 27 on histone H3 (H3K27me3) and acetylation of histone H3 (AcH3), using a genome-wide approach. Chromatin immunoprecipitation followed by deep sequencing of immunoprecipitated DNA (ChIP-Seq) was carried out with specific antibodies for H3K27me3 and AcH3. Our results revealed broad alteration of H3K27me3 and AcH3 marks association profiles in BDNF^Met/Met , compared to BDNF^Val/Val mice. Bioinformatics analysis showed changes in several biological functions and related pathways, affected by the presence of the polymorphism. In particular, a number of networks of functional interaction contained BDNF as central node. Quantitative PCR analysis confirmed epigenetically related significant changes in the expression of five genes: Dvl1, Nos3, Reln, Lypd6, and Sh3gl2. The first three are involved in dendrite and spine remodeling, morphological features altered in BDNF^Met/Met mice. This work in homozygous knock-in mice shows that the human BDNF Val66Met polymorphism induces an array of histone H3 epigenetic modifications, in turn altering the expression of select genes crucial for structural and functional neuronal features.

Lgl1 deficiency disrupts hippocampal development and impairs cognitive performance in mice

Cellular polarity is crucial for brain development and morphogenesis. Lethal giant larvae 1 (Lgl1) plays a crucial role in the establishment of cell polarity from Drosophila to mammalian cells. Previous studies have found the importance of Lgl1 in the development of cerebellar, olfactory bulb, and cerebral cortex. However, the role of Lgl1 in hippocampal development during the embryonic stage and function in adult mice is still unknown. In our study, we created Lgl1-deficient hippocampus mice by using Emx1-Cre mice. Histological analysis showed that the Emx1-Lgl1^-/- mice exhibited reduced size of the hippocampus with severe malformations of hippocampal cytoarchitecture. These defects mainly originated from the disrupted hippocampal neuroepithelium, including increased cell proliferation, abnormal interkinetic nuclear migration, reduced differentiation, increased apoptosis, gradual disruption of adherens junctions, and abnormal neuronal migration. The radial glial scaffold was disorganized in the Lgl1-deficient hippocampus. Thus, Lgl1 plays a distinct role in hippocampal neurogenesis. In addition, the Emx1-Lgl1^-/- mice displayed impaired behavioral performance in the Morris water maze and fear conditioning test.

Fractalkine/CX3CR1 is involved in the cross-talk between neuron and glia in neurological diseases

Fractalkine (CX3C chemokine ligand 1, CX3CL1) is an essential chemokine, for regulating adhesion and chemotaxis through binding to CX3CR1, which plays a critical role in the crosstalk between glial cells and neurons by direct or indirect ways in the central nervous system (CNS). Fractalkine/CX3CR1 axis regulates microglial activation and function, neuronal survival and synaptic function by controlling the release of inflammatory cytokines and synaptic plasticity in the course of the neurological disease. The multiple functions of fractalkine/CX3CR1 make it exert neuroprotective or neurotoxic effects, which determines the pathogenesis. However, the role of fractalkine/CX3CR1 in the CNS remains controversial. Whether it can be used as a therapeutic target for neurological diseases needs to be further investigated. In this review, we summarize the studies highlighting fractalkine/CX3CR1-mediated effects and discuss the potential neurotoxic and neuroprotective actions of fractalkine/CX3CR1 in brain injury for providing useful insights into the potential applications of fractalkine/CX3CR1 in neurological diseases.

Brain-Derived Neurotrophic Factor (BDNF): Novel Insights into Regulation and Genetic Variation

Since its discovery, brain-derived neurotrophic factor (BDNF) has spawned a literature that now spans 35 years of research. While all neurotrophins share considerable overlap in sequence homology and their processing, BDNF has become the most widely studied neurotrophin because of its broad roles in brain homeostasis, health, and disease. Although research on BDNF has produced thousands of articles, there remain numerous long-standing questions on aspects of BDNF molecular biology and signaling. Here we provide a comprehensive review, including both a historical narrative and a forward-looking perspective on advances in the actions of BDNF within the brain. We specifically review BDNF’s gene structure, peptide composition (including domains, posttranslational modifications and putative motif sites), mechanisms of transport, signaling pathway recruitment, and other recent developments including the functional effects of genetic variation and the discovery of a new BDNF prodomain ligand. This body of knowledge illustrates a highly conserved and complex role for BDNF within the brain, that promotes the idea that the neurotrophin biology of BDNF is diverse and that any disease involvement is likely to be equally multifarious.

Translational profiling of stress-induced neuroplasticity in the CA3 pyramidal neurons of BDNF Val66Met mice

Genetic susceptibility and environmental factors (such as stress) can interact to affect the likelihood of developing a mood disorder. Stress-induced changes in the hippocampus have been implicated in mood disorders, and mutations in several genes have now been associated with increased risk, such as brain-derived neurotrophic factor (BDNF). The hippocampus has important anatomical subdivisions, and pyramidal neurons of the vulnerable CA3 region show significant remodeling after chronic stress, but the mechanisms underlying their unique plasticity remain unknown. This study characterizes stress-induced changes in the in vivo translating mRNA of this cell population using a CA3-specific enhanced green fluorescent protein (EGFP) reporter fused to the L10a large ribosomal subunit (EGFPL10a). RNA-sequencing after isolation of polysome-bound mRNAs allows for cell-type-specific, genome-wide characterization of translational changes after stress. The data demonstrate that acute and chronic stress produce unique translational profiles and that the stress history of the animal can alter future reactivity of CA3 neurons. CA3-specific EGFPL10a mice were then crossed to the stress-susceptible BDNF Val66Met mouse line to characterize how a known genetic susceptibility alters both baseline translational profiles and the reactivity of CA3 neurons to stress. Not only do Met allele carriers exhibit distinct levels of baseline translation in genes implicated in ion channel function and cytoskeletal regulation, but they also activate a stress response profile that is highly dissimilar from wild-type mice. Closer examination of genes implicated in the mechanisms of neuroplasticity, such as the NMDA and AMPA subunits and the BDNF pathway, reveal how wild-type mice upregulate many of these genes in response to stress, but Met allele carriers fail to do so. These profiles provide a roadmap of stress-induced changes in a genetically homogenous population of hippocampal neurons and illustrate the profound effects of gene-environment interactions on the translational profile of these cells.

Role of microglia-neuron interactions in diabetic encephalopathy

In the central nervous system, the primary immune cells, the microglia, prevent pathogenic invasion as the first line of defense. Microglial energy consumption is dependent on their degree of activity. Microglia express transporters for the three primary energy substrates (glucose, fatty acids, glutamine) and regulate diabetic encephalopathy via microglia-neuron interactions. Microglia may play a sentry role for rapid protection or even ablation of impaired neurons. Neurons exhibit hyperactivity in response to hyperglycemia, hyperlipidemia, and neurotoxic factors and release potential microglial activators. Microglial activation is also regulated by proinflammatory factors, caspase-3 activity, P2X₇ receptor, interferon regulatory factor-8, and glucocorticoids. Modulation of microglia in diabetic encephalopathy may involve CX3CL1, p38 MAPK, purinergic, and CD200/CD200R signaling pathways, and pattern recognition receptors. The microglia-neuron interactions play an important role in diabetic encephalopathy, and modulation of microglial activation may be a therapeutic target for diabetic encephalopathy.

The antidepressant roles of Wnt2 and Wnt3 in stress-induced depression-like behaviors

Wnts-related signaling pathways have been reported to play roles in the pathogenesis of stress-induced depression-like behaviors. However, there is relatively few direct evidence to indicate the effect of Wnt ligands on this process. Here, we investigated the role of Wnts in mediating chronic restraint stress (CRS)-induced depression-like behaviors. We found that CRS induced a significant decrease in the expression of Wnt2 and Wnt3 in the ventral hippocampus (VH) but not in the dorsal hippocampus. Knocking down Wnt2 or Wnt3 in the VH led to impaired Wnt/β-catenin signaling, neurogenesis deficits and depression-like behaviors. In contrast, overexpression of Wnt2 or Wnt3 reversed CRS-induced depression-like behaviors. Moreover, Wnt2 and Wnt3 activated cAMP response element-binding protein (CREB) and there was CREB-dependent positive feedback between Wnt2 and Wnt3. Finally, fluoxetine treatment increased Wnt2 and Wnt3 levels in the VH and knocking down Wnt2 or Wnt3 abolished the antidepressant effect of fluoxetine. Taken together, our study indicates essential roles for Wnt2 and Wnt3 in CRS-induced depression-like behaviors and antidepressant.

CX3CL1/CX3CR1 in Alzheimer's Disease: A Target for Neuroprotection

CX3C chemokine ligand 1 (CX3CL1) is an intriguing chemokine belonging to the CX3C family. CX3CL1 is secreted by neurons and plays an important role in modulating glial activation in the central nervous system after binding to its sole receptor CX3CR1 which mainly is expressed on microglia. Emerging data highlights the beneficial potential of CX3CL1-CX3CR1 in the pathogenesis of Alzheimer's disease (AD), a common progressive neurodegenerative disease, and in the progression of which neuroinflammation plays a vital role. Even so, the importance of CX3CL1/CX3CR1 in AD is still controversial and needs further clarification. In this review, we make an attempt to present a concise map of CX3CL1-CX3CR1 associated with AD to find biomarkers for early diagnosis or therapeutic interventions.

Effects of the BDNF Val66Met Polymorphism on Anxiety-Like Behavior Following Nicotine Withdrawal in Mice

INTRODUCTION

Nicotine withdrawal is characterized by both affective and cognitive symptoms. Identifying genetic polymorphisms that could affect the symptoms associated with nicotine withdrawal are important in predicting withdrawal sensitivity and identifying personalized cessation therapies. In the current study we used a mouse model of a non-synonymous single nucleotide polymorphism in the translated region of the brain-derived neurotrophic factor (BDNF) gene that substitutes a valine (Val) for a methionine (Met) amino acid (Val66Met) to examine the relationship between the Val66Met single nucleotide polymorphism and nicotine dependence.

METHODS

This study measured proBDNF and the BDNF prodomain levels following nicotine and nicotine withdrawal and examined a mouse model of a common polymorphism in this protein (BDNF(Met/Met)) in three behavioral paradigms: novelty-induced hypophagia, marble burying, and the open-field test.

RESULTS

Using the BDNF knock-in mouse containing the BDNF Val66Met polymorphism we found: (1) blunted anxiety-like behavior in BDNF(Met/Met) mice following withdrawal in three behavioral paradigms: novelty-induced hypophagia, marble burying, and the open-field test; (2) the anxiolytic effects of chronic nicotine are absent in BDNF(Met/Met) mice; and (3) an increase in BDNF prodomain in BDNF(Met/Met) mice following nicotine withdrawal.

CONCLUSIONS

Our study is the first to examine the effect of the BDNF Val66Met polymorphism on the affective symptoms of withdrawal from nicotine in mice. In these mice, a single-nucleotide polymorphism in the translated region of the BDNF gene can result in a blunted withdrawal, as measured by decreased anxiety-like behavior. The significant increase in the BDNF prodomain in BDNF(Met/Met) mice following nicotine cessation suggests a possible role of this ligand in the circuitry remodeling after withdrawal.

BDNF Val66met and 5-HTTLPR polymorphisms predict a human in vivo marker for brain serotonin levels

Brain-derived neurotrophic factor (BDNF) has been implicated in multiple aspects of brain function including regulation of serotonin signaling. The BDNF val66met polymorphism (rs6265) has been linked to aspects of serotonin signaling in humans but its effects are not well understood. To address this, we evaluated whether BDNF val66met was predictive of a putative marker of brain serotonin levels, serotonin 4 receptor (5-HT4 ) binding assessed with [11C]SB207145 positron emission tomography, which has also been associated with the serotonin-transporter-linked polymorphic region (5-HTTLPR) polymorphism. We applied a linear latent variable model (LVM) using regional 5-HT4 binding values (neocortex, amygdala, caudate, hippocampus, and putamen) from 68 healthy humans, allowing us to explicitly model brain-wide and region-specific genotype effects on 5-HT4 binding. Our data supported an LVM wherein BDNF val66met significantly predicted a LV reflecting [11C]SB207145 binding across regions (P = 0.005). BDNF val66met met-carriers showed 2-9% higher binding relative to val/val homozygotes. In contrast, 5-HTTLPR did not predict the LV but S-carriers showed 7% lower neocortical binding relative to LL homozygotes (P = 7.3 × 10(-6)). We observed no evidence for genetic interaction. Our findings indicate that BDNF val66met significantly predicts a common regulator of brain [11C]SB207145 binding, which we hypothesize reflects brain serotonin levels. In contrast, our data indicate that 5-HTTLPR specifically affects 5-HT4 binding in the neocortex. These findings implicate serotonin signaling as an important molecular mediator underlying the effects of BDNF val66met and 5-HTTLPR on behavior and related risk for neuropsychiatric illness in humans.

Modulating neurotoxicity through CX3CL1/CX3CR1 signaling

Since the initial cloning of fractalkine/CX3CL1, it was proposed that the only known member of the CX3C or δ subfamily of chemotactic cytokines could play some significant role in the nervous system, due to its high expression on neurons. The pivotal description of the localization of the unique CX3CL1 receptor, CX3CR1, on microglial cells, firmed up by the generation of cx3cr1^GFP/GFP mice, opened the road to the hypothesis of some specific key interactions between microglia and neurons mediated by this pair. This expectation has been indeed supported by recent exciting evidence indicating that CX3CL1-mediated microglia-neuron interaction modulates basic physiological activities during development, adulthood and aging, including: synaptic pruning; promoting survival of neurons and neural precursors; modulating synaptic transmission and plasticity; enhancing synapse and network maturation; and facilitating the establishment of neuropathic pain circuits. Beyond playing such fascinating roles in physiological conditions, CX3CL1 signaling has been implicated in different neuropathologies. Early papers demonstrated that the levels of CX3CL1 may be modulated by various toxic stimuli in vitro and that CX3CL1 signaling is positively or negatively regulated in EAE and MS, in HIV infection and LPS challenge, in epilepsy, in brain tumors, and in other neuropathologies. In this review we focus on the experimental evidence of CX3CL1 involvement in neuroprotection and survey the common molecular and cellular mechanisms described in different brain diseases.