Yi-Hsuan Lee

Differential effects of natural polyphenols on neuronal survival in primary cultured central neurons against glutamate- and glucose deprivation-induced neuronal death

Neuronal injury in the central nervous system following ischemic insult is believed to result from glutamate toxicity and glucose deprivation. In this study, polyphenols isolated from Scutellaria baicalensis Georgi, including baicalin, baicalein, and wogonin, were investigated for their neuroprotective effects against glutamate/NMDA (Glu/NMDA) stimulation and glucose deprivation in primary cultured rat brain neurons. Cell death was accessed by lactate dehydrogenase (LDH) release assay for necrosis, and mitochondrial activity was accessed by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) reduction activity assay. It was found that both baicalin and baicalein decreased LDH release of the cultured neurons after 24 h treatment, whereas wogonin profoundly increased LDH release after 2 h treatment and resulted in neuronal death after 24 h. Glu/NMDA treatment profoundly increased LDH release and moderately decreased MTT reduction activity in an NMDA receptor-dependent manner. Both baicalin and baicalein significantly reduced Glu/NMDA-increased LDH release, in which baicalein is much more potent than baicalin. Glu/NMDA-increased intracellular calcium was also significantly attenuated by baicalin and baicalein. Baicalin and baicalein did not affect glutamate receptor binding activity, but baicalein did moderately decrease Glu/NMDA-induced nitric oxide (NO) production. In the glucose deprivation (GD) study, baicalein but not baicalin showed significant protective effects on the GD-increased LDH release, without affecting the GD-induced NO production, in cultured rat brain neurons. These results suggest that baicalein is the most effective compound among three polyphenols tested in preventing neurotoxicity induced by both glutamate and GD, whereas baicalin was only effective in preventing glutamate toxicity. Wogonin might have a neurotoxic effect on the brain.

Curcumin Provides Neuroprotection After Spinal Cord Injury

BACKGROUND Traumatic spinal cord injury (SCI) is a major cause of long-term disability. However, therapeutic agents targeting SCI are sorely lacking. The aim of this study was to investigate whether curcumin has neuroprotective effects after SCI in rats. MATERIALS AND METHODS Studies were performed in 39 male Sprague-Dawley rats after spinal cord hemisection. The animals were randomly divided into three groups: sham, vehicle, and curcumin. The Basso, Beattie, and Bresnahan (BBB) scale was used to evaluate functional outcome. Specimens were tested for histologic, terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick-end labeling (TUNEL), and immunohistochemical staining. Primary cultured astrocytes were used to test the inhibitory effect of curcumin on glial reactivation. RESULTS The BBB scores for the affected hindlimb after hemisection were significantly improved in the curcumin-treated group compared with the vehicle group (on d 3 and 7; P < 0.001). Immunohistochemistry of NeuN revealed remarkable neuronal loss in the vehicle group after hemisection. In comparison, curcumin significantly protected neurons after SCI (curcumin compared with vehicle; P < 0.001). Furthermore, curcumin significantly attenuated apoptosis after SCI (curcumin compared with vehicle; P < 0.001). RT-PCR demonstrated that the expression of glial fibrillary acidic protein (GFAP) was significantly inhibited by curcumin. CONCLUSIONS Curcumin inhibited apoptosis and neuron loss, quenched astrocyte activation, and significantly improved neurologic deficit 7 d after spinal cord hemisection. By down-regulating GFAP expression, curcumin seems to attenuate astrocyte reactivation, which may be beneficial for neuronal survival. This is the first report demonstrating the successful treatment of SCI by curcumin.

Neuronal activity enhances aryl hydrocarbon receptor-mediated gene expression and dioxin neurotoxicity in cortical neurons

The aryl hydrocarbon receptor (AhR) is a ligand‐activated transcription factor activated by dioxin and polyaromatic hydrocarbons. Recent studies have revealed that AhR activity in central neurons depends on the NMDA receptor. In this study, we investigated how the neuronal activity influence AhR‐mediated dioxin‐responsive gene expression and neurotoxicity. Our results show that activation of AhR by the selective agonist 2,3,7,8‐tetrachlorodibenzo‐p‐dioxin induced dioxin‐responsive gene expression and calcium entry, which were attenuated by AhR small interfering RNA, the NMDA receptor channel blocker MK801, and the action potential blocker tetrodotoxin (TTX). In addition, AhR‐mediated gene expression was enhanced in neurons during synaptogenesis (10 days in vitro) compared with younger neurons (4 days in vitro), as was sensitivity to TTX and MK801. Furthermore, TTX and MK801 differentially affected the association of AhR and its transcriptional co‐activator cAMP‐responsive‐element binding protein with the cytochrome P450 1A1 (cyp1A1) gene enhancer. Calcium/calmodulin‐dependent protein kinase IV, the cAMP‐responsive‐element binding protein activating enzyme, was also activated by 2,3,7,8‐tetrachlorodibenzo‐p‐dioxin in an activity‐dependent manner. Finally, we found that neuronal susceptibility to dioxin insult was also maturation and activity‐dependent. Together, the results suggest that neuronal activity may facilitate AhR‐mediated calcium signaling, which in turn enhances AhR‐mediated gene regulation and mediated maturation‐dependent dioxin neurotoxicity.

Mechanism of Neuroprotective Function of Taurine

Taurine has potent protective function against glutamate-induced neuronal injury presumably through its function in regulation of intracellular free calcium level, [Ca2+]i. In this communication, we report that taurine exerts its protective function through one or more of the following mechanisms: 1. Inhibition of glutamate-induced calcium influx through L-, N- and P/Q-type voltage-gated calcium channels and NMDA receptor calcium channel; 2. Attenuation of glutamate-induced membrane depolarization; 3. Prevention of glutamate-induced apoptosis via preventing glutamate-mediated down-regulation of Bcl-2; 4. Prevention of cleavage of Bcl-2 by calpain. This action of taurine is due to its inhibition on glutamate induced calpain activation. Based on these observations, we propose that taurine protects neurons against glutamate-induced neurotoxicity in part, by preventing glutamate-induced membrane depolarization, elevation of [Ca2+]i, activation of calpain, reduction of Bcl-2 and apoptosis.

Injury-induced Janus kinase/protein kinase C-dependent phosphorylation of growth-associated protein 43 and signal transducer and activator of transcription 3 for neurite growth in dorsal root ganglion

Elevation of corticosteroids and excessive glutamate release are the two major stress responses that occur sequentially during traumatic CNS injury. We have previously reported that sequential application of corticosterone and kainic acid (CORT + KA) mimicking the nerve injury condition results in synergistic enhancement of neurite outgrowth and expression of growth‐associated protein 43 (GAP‐43) in cultured dorsal root ganglion (DRG). GAP‐43 is known to promote neurite extension when phosphorylated by protein kinase C (PKC). In addition, PKC can phosphorylate the signal transducer and activator of transcription 3 (STAT3) at Ser727, which is phosphorylated primarily by Janus kinase (JAK) at Tyr705. In this study, we further examine the role of PKC in this stress‐induced growth‐promoting effect. In the cultured DRG neurons, the JAK inhibitor AG‐490 and the PKC inhibitor Ro‐318220 reduced the CORT + KA‐enhanced neurite growth effect when applied prior to CORT and KA treatment, respectively. Both AG‐490 and Ro‐318220 diminished the CORT + KA‐enhanced GAP‐43 expression, phosphorylation, and axonal localization. Furthermore, CORT + KA treatment synergistically phosphorylated STAT3 at Ser727 but not at Tyr705. Similar phenomena were observed in an animal model of acute spinal cord injury (SCI), in which phosphorylation of GAP‐43 and phospho‐Ser727‐STAT3 was elevated in the injured DRG 4 hr after the impact injury. Further treatment with the therapeutic glucocorticoid methylprednisolone enhanced the phosphorylation of GAP‐43 in both the DRG and the spinal cord of SCI rats. These results suggest that elevated glucocorticoids and overexcitation following CNS injury contribute to nerve regeneration via induction of JAK/PKC‐mediated GAP‐43 and STAT3 activities.

Methylprednisolone inhibits the expression of glial fibrillary acidic protein and chondroitin sulfate proteoglycans in reactivated astrocytes

Reactive gliosis caused by post‐traumatic injury often results in marked expression of chondroitin sulfate proteoglycan (CSPG), which inhibits neurite outgrowth and regeneration. Methylprednisolone (MP), a synthetic glucocorticoid, has been shown to have neuroprotective and anti‐inflammatory effects for the treatment of acute spinal cord injury (SCI). However, the effect of MP on CSPG expression in reactive glial cells remains unclear. In our study, we induced astrocyte reactivation using α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazole propionate (AMPA) and cyclothiazide to mimic the excitotoxic stimuli of SCI. The expression of glial fibrillary acidic protein (GFAP), a marker of astrocyte reactivation, and CSPG neurocan and phosphacan were significantly elevated by AMPA treatment. The conditioned media from AMPA‐treated astrocytes strongly inhibited neurite outgrowth of rat dorsal root ganglion neurons, and this effect was reversed by pretreatment with MP. Furthermore, MP downregulated GFAP and CSPG expression in adult rats with SCI. Additionally, both the glucocorticoid receptor (GR) antagonist RU486 and GR siRNA reversed the inhibitory effects of MP on GFAP and neurocan expression. Taken together, these results suggest that MP may improve neuronal repair and promote neurite outgrowth after excitotoxic insult via GR‐mediated downregulation of astrocyte reactivation and inhibition of CSPG expression.

Blocking Lymphocyte Trafficking with FTY720 Prevents Inflammation-Sensitized Hypoxic–Ischemic Brain Injury in Newborns

Intrauterine infection (chorioamnionitis) aggravates neonatal hypoxic–ischemic (HI) brain injury, but the mechanisms linking systemic inflammation to the CNS damage remain uncertain. Here we report evidence for brain influx of T-helper 17 (TH17)-like lymphocytes to coordinate neuroinflammatory responses in lipopolysaccharide (LPS)-sensitized HI injury in neonates. We found that both infants with histological chorioamnionitis and rat pups challenged by LPS/HI have elevated expression of the interleukin-23 (IL-23) receptor, a marker of early TH17 lymphocytes, in the peripheral blood mononuclear cells. Post-LPS/HI administration of FTY720 (fingolimod), a sphingosine-1-phosphate receptor agonist that blocks lymphocyte trafficking, mitigated the influx of leukocytes through the choroid plexus and acute induction of nuclear factor-κB signaling in the brain. Subsequently, the FTY720 treatment led to attenuated blood–brain barrier damage, fewer cluster of differentiation 4-positive, IL-17A-positive T-cells in the brain, less proinflammatory cytokine, and better preservation of growth and white matter functions. The FTY720 treatment also provided dose-dependent reduction of brain atrophy, rescuing >90% of LPS/HI-induced brain tissue loss. Interestingly, FTY720 neither opposed pure-HI brain injury nor directly inhibited microglia in both in vivo and in vitro models, highlighting its unique mechanism against inflammation-sensitized HI injury. Together, these results suggest that the dual hit of systemic inflammation and neonatal HI injury triggers early onset of the TH17/IL-17-mediated immunity, which causes severe brain destruction but responds remarkably to the therapeutic blockade of lymphocyte trafficking.

Astrocytic GAP43 Induced by the TLR4/NF-κB/STAT3 Axis Attenuates Astrogliosis-Mediated Microglial Activation and Neurotoxicity

Growth-associated protein 43 (GAP43), a protein kinase C (PKC)-activated phosphoprotein, is often implicated in axonal plasticity and regeneration. In this study, we found that GAP43 can be induced by the endotoxin lipopolysaccharide (LPS) in rat brain astrocytes both in vivo and in vitro. The LPS-induced astrocytic GAP43 expression was mediated by Toll-like receptor 4 and nuclear factor-κB (NF-κB)- and interleukin-6/signal transducer and activator of transcription 3 (STAT3)-dependent transcriptional activation. The overexpression of the PKC phosphorylation-mimicking GAP43S41D (constitutive active GAP43) in astrocytes mimicked LPS-induced process arborization and elongation, while application of a NF-κB inhibitory peptide TAT-NBD or GAP43S41A (dominant-negative GAP43) or knockdown of GAP43 all inhibited astrogliosis responses. Moreover, GAP43 knockdown aggravated astrogliosis-induced microglial activation and expression of proinflammatory cytokines. We also show that astrogliosis-conditioned medium from GAP43 knock-down astrocytes inhibited GAP43 phosphorylation and axonal growth, and increased neuronal damage in cultured rat cortical neurons. These proneurotoxic effects of astrocytic GAP43 knockdown were accompanied by attenuated glutamate uptake and expression of the glutamate transporter excitatory amino acid transporter 2 (EAAT2) in LPS-treated astrocytes. The regulation of EAAT2 expression involves actin polymerization-dependent activation of the transcriptional coactivator megakaryoblastic leukemia 1 (MKL1), which targets the serum response elements in the promoter of rat Slc1a2 gene encoding EAAT2. In sum, the present study suggests that astrocytic GAP43 mediates glial plasticity during astrogliosis, and provides beneficial effects for neuronal plasticity and survival and attenuation of microglial activation. SIGNIFICANCE STATEMENT Astrogliosis is a complex state in which injury-stimulated astrocytes exert both protective and harmful effects on neuronal survival and plasticity. In this study, we demonstrated for the first time that growth-associated protein 43 (GAP43), a well known growth cone protein that promotes axonal regeneration, can be induced in rat brain astrocytes by the proinflammatory endotoxin lipopolysaccharide via both nuclear factor-κB and signal transducer and activator of transcription 3-mediated transcriptional activation. Importantly, LPS-induced GAP43 mediates plastic changes of astrocytes while attenuating astrogliosis-induced microglial activation and neurotoxicity. Hence, astrocytic GAP43 upregulation may serve to indicate beneficial astrogliosis after CNS injury.

Curcumin Attenuates the Expression and Secretion of RANTES after Spinal Cord Injury In Vivo and Lipopolysaccharide-Induced Astrocyte Reactivation In Vitro

Curcumin has been proposed for treatment of various neuroinflammatory and neurodegenerative conditions, including post-traumatic inflammation during acute spinal cord injury (SCI). In this study, we examined whether curcumin anti-inflammation involves regulation of astrocyte reactivation, with special focus on the injury-induced RANTES (regulated on expression normal T-cell expressed and secreted) from astrocytes in acute SCI. Male Sprague-Dawley (SD) rats were subjected to impact injury of the spinal cord followed by treatment with curcumin (40 mg/kg i.p.). RANTES and inducible nitric oxide synthase expression as well as RANTES-positive astrocytes were all induced by injury accompanied by the elevation of lipid peroxidation, and attenuated by the curcumin treatment. In primary cultured rat astrocytes challenged with lipopolysaccharide (LPS) to mimic astrocyte reactivation following SCI, LPS induces robust increase of RANTES expression and the effect was also reduced by 1 μM curcumin treatment. Furthermore, cortical neurons cultured with astrocyte conditioned medium (ACM) conditioned with both LPS and curcumin (LPS-curcumin/ACM), which characteristically exhibited decreased RANTES expression when compared with ACM from astrocytes treated with LPS alone (LPS/ACM), showed higher level of cell viability and lower level of cell death as assessed by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) reduction activity assay and lactate dehydrogenase release assay, respectively. Knockdown of RANTES expression by siRNA (siRANTES) shows reduced RANTES expression and release from LPS-reactivated astrocytes, and ACM obtained from this condition (LPS-siRANTES/ACM) becomes less cytotoxic as compared with the LPS-ACM. Therefore, curcumin reduction of robust RANTES production in reactivated astrocytes both in vitro and in vivo may contribute to its neuroprotection and potential application in SCI.

Knockdown of the aryl hydrocarbon receptor attenuates excitotoxicity and enhances NMDA-induced BDNF expression in cortical neurons

NMDA receptors play dual and opposing roles in neuronal survival by mediating the activity‐dependent neurotrophic signaling and excitotoxic cell death via synaptic and extrasynaptic receptors, respectively. In this study, we demonstrate that the aryl hydrocarbon receptor (AhR), also known as the dioxin receptor, is involved in the expression and the opposing activities of NMDA receptors. In primary cultured cortical neurons, we found that NMDA excitotoxicity is significantly enhanced by an AhR agonist 2,3,7,8‐tetrachlorodibenzo‐p‐dioxin, and AhR knockdown with small interfering RNA significantly reduces NMDA excitotoxicity. AhR knockdown also significantly reduces NMDA‐increases intracellular calcium concentration, NMDA receptor expression and surface presentation, and moderately decreases the NMDA receptor‐mediated spontaneous as well as miniature excitatory post‐synaptic currents. However, AhR knockdown significantly enhances the bath NMDA application– but not synaptic NMDA receptor‐induced brain‐derived neurotrophic factor (BDNF) gene expression, and activating AhR reduces the bath NMDA‐induced BDNF expression. Furthermore, AhR knockdown reveals the calcium dependency of NMDA‐induced BDNF expression and the binding activity of cAMP‐responsive element binding protein (CREB) and its calcium‐dependent coactivator CREB binding protein (CBP) to the BDNF promoter upon NMDA treatment. Together, our results suggest that AhR opposingly regulates NMDA receptor‐mediated excitotoxicity and neurotrophism possibly by differentially regulating the expression of synaptic and extrasynaptic NMDA receptors.