Brain-derived Neurotrophic Factor Expression In Astrocytes Research Articles

Ubiquitin ligase Triad1 promotes neurite outgrowth by inhibiting MDM2-mediated ubiquitination of the neuroprotective factor pleiotrophin

Spinal cord injury (SCI) is the most severe result of spine injury, but no effective therapy exists to treat SCI. We have previously shown that the E3 ubiquitin ligase Two RING fingers and DRIL 1 (Triad1) promotes neurite outgrowth after SCI. However, the mechanism by which Triad1 affects neuron growth and the potential involvement of its ubiquitination activity is unclear. Neuroprotective cytokine pleiotrophin (PTN) can promote microglia proliferation and neurotrophic factor secretion to achieve neuroprotection. We find using immunostaining and behavioral assays in rats that the expression of Triad1 and the PTN was peaked at 1 day after SCI and Triad1 improved motor function and histomorphological injury after SCI. We show using flow cytometry and astrocyte/neuronal coculture assays that Triad1 overexpression promoted PTN protein levels, neurotrophic growth factor (NGF) expression, brain-derived neurotrophic factor (BDNF) expression, astrocyte and neuronal viability, and neurite outgrowth but suppressed astrocyte apoptosis, while shRNA-mediated knockdown of Triad1 and PTN had the opposite effects. Ubiquitin ligase murine double mutant 2 (MDM2) has previously been demonstrated to participate in the process of neurite outgrowth and mediate ubiquitination of p53. Furthermore, we demonstrate overexpression of MDM2 downregulated PTN protein levels, NGF expression and BDNF expression in astrocytes, and inhibited neurite outgrowth of neurons. In addition, MDM2 facilitated PTN ubiquitination, which was reversed by Triad1. Finally, we show simultaneous sh-PTN and MDM2 overexpression attenuated the neurite outgrowth-promoting effect of Triad1 overexpression. In conclusion, we propose Triad1 promotes astrocyte-dependent neurite outgrowth to accelerate recovery after SCI by inhibiting MDM2-mediated PTN ubiquitination.

Downregulation of connexin 43 potentiates amitriptyline-induced brain-derived neurotrophic factor expression in primary astrocytes through lysophosphatidic acid receptor1/3, Src, and extracellular signal-regulated kinase

Connexin 43 (Cx43) expression is decreased in the prefrontal cortex of patients with depression, but its significance is still unknown. Neurotrophic factors, such as brain-derived neurotrophic factor (BDNF), are involved in the effects of antidepressant. However, the relationship between Cx43 expression and induction of brain-derived neurotrophic factor production by antidepressants is unknown. On the basis of our previous study, which showed that adrenergic receptors stimulation results in potentiation of BDNF expression in astrocytes with downregulated Cx43 expression, we investigated the induction of BDNF expression by amitriptyline, a tricyclic antidepressant, in Cx43-knockdown astrocytes. Amitriptyline treatment potentiated BDNF expression in Cx43-knockdown astrocytes compared with those treated with non-targeting small interfering RNA (siRNA). Using a pharmacological approach, we revealed that the potentiating effect of amitriptyline on BDNF expression was mediated by lysophosphatidic acid (LPA) receptor1/3 (LPA1/3) stimulation and subsequent activation of Src-extracellular signal-regulated kinase (ERK) signaling. These findings suggest that downregulation of Cx43 in patients with depression might contribute to the therapeutic efficacy of antidepressants rather than the pathogenesis of depression.

Emerging Roles of Astrocyte Kir4.1 Channels in the Pathogenesis and Treatment of Brain Diseases.

Inwardly rectifying Kir4.1 channels in astrocytes mediate spatial potassium (K+) buffering, a clearance mechanism for excessive extracellular K+, in tripartite synapses. In addition to K+ homeostasis, astrocytic Kir4.1 channels also play an essential role in regulating extracellular glutamate levels via coupling with glutamate transporters. Moreover, Kir4.1 channels act as novel modulators of the expression of brain-derived neurotrophic factor (BDNF) in astrocytes. Specifically, inhibition of astrocytic Kir4.1 channels elevates extracellular K+ and glutamate levels at synapses and facilitates BDNF expression in astrocytes. These changes elevate neural excitability, which may facilitate synaptic plasticity and connectivity. In this article, we summarize the functions and pharmacological features of Kir4.1 channels in astrocytes and highlight the importance of these channels in the treatment of brain diseases. Although further validation in animal models and human patients is required, astrocytic Kir4.1 channel could potentially serve as a novel therapeutic target for the treatment of depressive disorders and epilepsy.

A peroxisome deficiency–induced reductive cytosol state up-regulates the brain-derived neurotrophic factor pathway

The peroxisome is a subcellular organelle that functions in essential metabolic pathways, including biosynthesis of plasmalogens, fatty acid β-oxidation of very-long-chain fatty acids, and degradation of hydrogen peroxide. Peroxisome biogenesis disorders (PBDs) manifest as severe dysfunction in multiple organs, including the central nervous system (CNS), but the pathogenic mechanisms in PBDs are largely unknown. Because CNS integrity is coordinately established and maintained by neural cell interactions, we here investigated whether cell-cell communication is impaired and responsible for the neurological defects associated with PBDs. Results from a noncontact co-culture system consisting of primary hippocampal neurons with glial cells revealed that a peroxisome-deficient astrocytic cell line secretes increased levels of brain-derived neurotrophic factor (BDNF), resulting in axonal branching of the neurons. Of note, the BDNF expression in astrocytes was not affected by defects in plasmalogen biosynthesis and peroxisomal fatty acid β-oxidation in the astrocytes. Instead, we found that cytosolic reductive states caused by a mislocalized catalase in the peroxisome-deficient cells induce the elevation in BDNF secretion. Our results suggest that peroxisome deficiency dysregulates neuronal axogenesis by causing a cytosolic reductive state in astrocytes. We conclude that astrocytic peroxisomes regulate BDNF expression and thereby support neuronal integrity and function.

Citrus flavonoid 3,5,6,7,8,3',4'-heptamethoxyflavone induces BDNF via cAMP/ERK/CREB signaling and reduces phosphodiesterase activity in C6 cells.

Brain-derived neurotrophic factor (BDNF) is associated with onset of several central nervous system disorders, e.g., Parkinson's disease, Alzheimer's disease, depression, epilepsy, and chronic pain. In our previous in vivo studies using ischemic and depression mouse models, we revealed that citrus flavonoid 3,5,6,7,8,3',4'-heptamethoxyflavone (HMF) exerts neuroprotective effects by enhancing the expression of BDNF in astrocytes within the hippocampus. Therefore, in the present study, we examined the mechanism of BDNF induction by HMF in vitro using rat C6 glioma cells. C6 glioma cells were treated with HMF (10 μM) or HMF + U0126 (10 μM), HMF + H89 (1 μM), or HMF + K252a (200 nM) for 48 h. The protein level of mature BDNF (m-BDNF), phosphorylated-ERK (p-ERK) and phosphorylated-cAMP-response element binding protein (p-CREB) were measured using western blot analysis. To clarify the mechanism of HMF for increasing m-BDNF, the inhibitory effect of phosphodiesterase 4B (PDE4B) and PDE4D, and intracellular cAMP levels were examined by ELISA. Our findings revealed that the m-BDNF-inducing activity of HMF was abolished by U0126 but not by H89 or K252a. HMF was found to phosphorylate (activate) ERK and cAMP-response element binding protein (CREB), a BDNF transcription factor. HMF inhibited PDE4B and PDE4D activity. Moreover, 10 μM HMF elevated intracellular cAMP levels in C6 cells. These findings suggest that HMF might exert its neuroprotective effects by inducing m-BDNF expression in C6 cells, model cell line of astrocytes, via the activation of cAMP/ERK/CREB signaling and inhibiting PDE4B or PDE4D.

Inwardly Rectifying Potassium Channel Kir4.1 as a Novel Modulator of BDNF Expression in Astrocytes.

Brain-derived neurotrophic factor (BDNF) is a key molecule essential for neural plasticity and development, and is implicated in the pathophysiology of various central nervous system (CNS) disorders. It is now documented that BDNF is synthesized not only in neurons, but also in astrocytes which actively regulate neuronal activities by forming tripartite synapses. Inwardly rectifying potassium (Kir) channel subunit Kir4.1, which is specifically expressed in astrocytes, constructs Kir4.1 and Kir4.1/5.1 channels, and mediates the spatial potassium (K+) buffering action of astrocytes. Recent evidence illustrates that Kir4.1 channels play important roles in bringing about the actions of antidepressant drugs and modulating BDNF expression in astrocytes. Although the precise mechanisms remain to be clarified, it seems likely that inhibition (down-regulation or blockade) of astrocytic Kir4.1 channels attenuates K+ buffering, increases neuronal excitability by elevating extracellular K+ and glutamate, and facilitates BDNF expression. Conversely, activation (up-regulation or opening) of Kir4.1 channels reduces neuronal excitability by lowering extracellular K+ and glutamate, and attenuates BDNF expression. Particularly, the former pathophysiological alterations seem to be important in epileptogenesis and pain sensitization, and the latter in the pathogenesis of depressive disorders. In this article, we review the functions of Kir4.1 channels, with a focus on their regulation of spatial K+ buffering and BDNF expression in astrocytes, and discuss the role of the astrocytic Kir4.1-BDNF system in modulating CNS disorders.

Inhibition of Inwardly Rectifying Potassium (Kir) 4.1 Channels Facilitates Brain-Derived Neurotrophic Factor (BDNF) Expression in Astrocytes.

Inwardly rectifying potassium (Kir) 4.1 channels in astrocytes regulate neuronal excitability by mediating spatial potassium buffering. Although dysfunction of astrocytic Kir4.1 channels is implicated in the development of epileptic seizures, the functional mechanisms of Kir4.1 channels in modulating epileptogenesis remain unknown. We herein evaluated the effects of Kir4.1 inhibition (blockade and knockdown) on expression of brain-derived neurotrophic factor (BDNF), a key modulator of epileptogenesis, in the primary cultures of mouse astrocytes. For blockade of Kir4.1 channels, we tested several antidepressant agents which reportedly bound to and blocked Kir4.1 channels in a subunit-specific manner. Treatment of astrocytes with fluoxetine enhanced BDNF mRNA expression in a concentration-dependent manner and increased the BDNF protein level. Other antidepressants (e.g., sertraline and imipramine) also increased the expression of BDNF mRNA with relative potencies similar to those for inhibition of Kir4.1 channels. In addition, suppression of Kir4.1 expression by the transfection of small interfering RNA (siRNA) targeting Kir4.1 significantly increased the mRNA and protein levels of BDNF. The BDNF induction by Kir4.1 siRNA transfection was suppressed by the MEK1/2 inhibitor U0126, but not by the p38 MAPK inhibitor SB202190 or the JNK inhibitor SP600125. The present results demonstrated that inhibition of Kir4.1 channels facilitates BDNF expression in astrocytes primarily by activating the Ras/Raf/MEK/ERK pathway, which may be linked to the development of epilepsy and other neuropsychiatric disorders.

Glucose Can Epigenetically Alter the Gene Expression of Neurotrophic Factors in the Murine Brain Cells.

Glucose is believed to improve the memory in both human and mice, but the detailed insights were mostly elusive. In this study, we focused on two major neurotrophic factors, brain-derived neurotrophic factor (BDNF) and fibroblast growth factor 1 (FGF1), which are believed to be associated with the memory enhancement and assessed their expressional regulation among the murine neuronal and glial cells. Our findings showed that the glucose administration increased phosphorylated Akt, phosphorylated CREB, exon 1- and exon 4-specific BDNF transcripts, and FGF1 transcripts that are associated with the epigenetic changes expected to open the chromatin and a reduction in histone deacetylase 2 (HDAC2) in neurons and astrocytes of the murine hippocampus. The glucose administration enhanced the long-term potentiation and the number of dendritic spines in the CA1 and CA3 subfields of hippocampus. The intrahippocampal injection of short hairpin RNA against TrkB canceled the glucose-mediated memory enhancement. Like the glucose, we also report that the HDAC inhibitor can enhance the memory through the BDNF-TrkB pathway but it targeted different brain cell populations to enhance the BDNF and FGF1 transcripts. In addition, the soluble FGF1 treatments significantly increased the BDNF expression in astrocytes and neurons, suggesting that the glucose-mediated induction of the neurotrophic factors could contribute to the memory. Our study provides the valuable insights, explaining the distinctive neuronal and glial cell regulation of the neurotrophic factors by glucose and HDAC inhibitor, which could likely explain how our brain cells can control the release of neurotrophic factors.

Melanocortin 4 receptor activates ERK-cFos pathway to increase brain-derived neurotrophic factor expression in rat astrocytes and hypothalamus

Melanocortins are neuropeptides with well recognized anti-inflammatory and anti-apoptotic effects in the brain. Of the five melanocortin receptors (MCR), MC4R is abundantly expressed in the brain and is the only MCR present in astrocytes. We have previously shown that MC4R activation by the α-melanocyte stimulating hormone (α-MSH) analog, NDP-MSH, increased brain-derived neurotrophic factor (BDNF) expression through the classic cAMP–Protein kinase A–cAMP responsive element binding protein pathway in rat astrocytes. Now, we examined the participation of the mitogen activated protein kinases pathway in MC4R signaling. Rat cultured astrocytes treated with NDP-MSH 1 µM for 1 h showed increased BDNF expression. Inhibition of extracellular signal-regulated kinase (ERK) and ribosomal p90 S6 kinase (RSK), an ERK substrate, but not of p38 or JNK, prevented the increase in BDNF expression induced by NDP-MSH. Activation of MC4R increased cFos expression, a target of both ERK and RSK. ERK activation by MC4R involves cAMP, phosphoinositide-3 kinase (PI3K) and the non receptor tyrosine kinase, Src. Both PI3K and Src inhibition abolished NDP-MSH-induced BDNF expression. Moreover, we found that intraperitoneal injection of α-MSH induces BDNF and MC4R expression and activates ERK and cFos in male rat hypothalamus. Our results show for the first time that MC4R-induced BDNF expression in astrocytes involves ERK-RSK-cFos pathway which is dependent on PI3K and Src, and that melanocortins induce BDNF expression and ERK-cFos activation in rat hypothalamus.

Histone deacetylase inhibitors up-regulate astrocyte GDNF and BDNF gene transcription and protect dopaminergic neurons

Parkinson's disease (PD) is characterized by the selective and progressive loss of dopaminergic (DA) neurons in the midbrain substantia nigra. Currently, available treatment is unable to alter PD progression. Previously, we demonstrated that valproic acid (VPA), a mood stabilizer, anticonvulsant and histone deacetylase (HDAC) inhibitor, increases the expression of glial cell line-derived neurotrophic factor (GDNF) and brain-derived neurotrophic factor (BDNF) in astrocytes to protect DA neurons in midbrain neuron-glia cultures. The present study investigated whether these effects are due to HDAC inhibition and histone acetylation. Here, we show that two additional HDAC inhibitors, sodium butyrate (SB) and trichostatin A (TSA), mimic the survival-promoting and protective effects of VPA on DA neurons in neuron-glia cultures. Similar to VPA, both SB and TSA increased GDNF and BDNF transcripts in astrocytes in a time-dependent manner. Furthermore, marked increases in GDNF promoter activity and promoter-associated histone H3 acetylation were noted in astrocytes treated with all three compounds, where the time-course for acetylation was similar to that for gene transcription. Taken together, our results indicate that HDAC inhibitors up-regulate GDNF and BDNF expression in astrocytes and protect DA neurons, at least in part, through HDAC inhibition. This study indicates that astrocytes may be a critical neuroprotective mechanism of HDAC inhibitors, revealing a novel target for the treatment of psychiatric and neurodegenerative diseases.