Brain-derived neurotrophic factor protection of cortical neurons from serum withdrawal-induced apoptosis is inhibited by cAMP

Beyond PDE4 inhibition: A comprehensive review on downstream cAMP signaling in the central nervous system

Cyclic adenosine monophosphate (cAMP) is a key second messenger that regulates signal transduction pathways pivotal for numerous biological functions. Intracellular cAMP levels are spatiotemporally regulated by their hydrolyzing enzymes called phosphodiesterases (PDEs). It has been shown that increased cAMP levels in the central nervous system (CNS) promote neuroplasticity, neurotransmission, neuronal survival, and myelination while suppressing neuroinflammation. Thus, elevating cAMP levels through PDE inhibition provides a therapeutic approach for multiple CNS disorders, including multiple sclerosis, stroke, spinal cord injury, amyotrophic lateral sclerosis, traumatic brain injury, and Alzheimer's disease. In particular, inhibition of the cAMP-specific PDE4 subfamily is widely studied because of its high expression in the CNS. So far, the clinical translation of full PDE4 inhibitors has been hampered because of dose-limiting side effects. Hence, focusing on signaling cascades downstream activated upon PDE4 inhibition presents a promising strategy, offering novel and pharmacologically safe targets for treating CNS disorders. Yet, the underlying downstream signaling pathways activated upon PDE(4) inhibition remain partially elusive. This review provides a comprehensive overview of the existing knowledge regarding downstream mediators of cAMP signaling induced by PDE4 inhibition or cAMP stimulators. Furthermore, we highlight existing gaps and future perspectives that may incentivize additional downstream research concerning PDE(4) inhibition, thereby providing novel therapeutic approaches for CNS disorders.

Evaluation of the effects of the T-type calcium channel enhancer SAK3 in a rat model of TAF1 deficiency

The TATA-box binding protein associated factor 1 (TAF1) is part of the TFIID complex that plays a key role during the initiation of transcription. Variants of TAF1 are associated with neurodevelopmental disorders. Previously, we found that CRISPR/Cas9 based editing of the TAF1 gene disrupts the morphology of the cerebral cortex and blunts the expression as well as the function of the CaV3.1 (T-type) voltage gated calcium channel. Here, we tested the efficacy of SAK3 (ethyl 8'-methyl-2', 4-dioxo-2-(piperidin-1-yl)-2'H-spiro [cyclopentane-1, 3'-imidazo [1, 2-a] pyridine]-2-ene-3-carboxylate), a T-type calcium channel enhancer, in an animal model of TAF1 intellectual disability (ID) syndrome. At post-natal day 3, rat pups were subjected to intracerebroventricular (ICV) injection of either gRNA-control or gRNA-TAF1 CRISPR/Cas9 viruses. At post-natal day 21, the rat pups were given SAK3 (0.25 mg/kg, p.o.) or vehicle for 14 days (i.e. till post-natal day 35) and then subjected to behavioral, morphological, and molecular studies. Oral administration of SAK3 (0.25 mg/kg, p.o.) significantly rescued locomotion abnormalities associated with TAF1 gene editing. SAK3 treatment prevented the loss of cortical neurons and GFAP-positive astrocytes observed after TAF1 gene editing. In addition, SAK3 protected cells from apoptosis. SAK3 also restored the Brain-derived neurotrophic factor/protein kinase B/Glycogen Synthase Kinase 3 Beta (BDNF/AKT/GSK3β) signaling axis in TAF1 edited animals. Finally, SAK3 normalized the levels of three GSK3β substrates - CaV3.1, FOXP2, and CRMP2. We conclude that the T-type calcium channel enhancer SAK3 is beneficial against the deleterious effects of TAF1 gene-editing, in part, by stimulating the BDNF/AKT/GSK3β signaling pathway.

Time and region-dependent manner of increased brain derived neurotrophic factor and TrkB in rat brain after binge-like methamphetamine exposure

Methamphetamine (MA), a synthetic derivate of amphetamine, has become a major drug of abuse worldwide. This study investigated the effect of binge-like MA dosing (4 x 4 mg/kg, s.c., 2 h (h) apart) at a range of different time points (from 2 h to 7 days after treatment) on brain-derived neurotrophic factor (BDNF) levels and its receptors, TrkB and p75^NTR. BDNF levels were significantly increased in the frontal cortex from 2 to 36 h after treatment, returning to normal within 48 h after treatment. In the striatum, BDNF expression was increased at 12 and 24 h after binge-like MA treatment and had returned to normal at 36 h. Increased expression of the TrkB receptor was observed in the frontal cortex at 2, 24 and 48 h after MA treatment and in the striatum at 24 and 48 h after the MA regimen. A significant increase in the p75^NTR receptor was also noted in the striatum but not the frontal cortex, and it was less pronounced than the effect on TrkB receptor expression. These findings show that the binge-like regimen of MA affects expression of BDNF and its receptors, particularly the TrkB receptor, in a time and region dependent manner, and highlights the importance of the frontal cortex and the striatum in the response following MA binge-like dosing.

Homeostatic interplay between electrical activity and neuronal apoptosis in the developing neocortex

An intriguing feature of nervous system development in most animal species is that the initial number of generated neurons is higher than the number of neurons incorporated into mature circuits. A substantial portion of neurons is indeed eliminated via apoptosis during a short time window - in rodents the first two postnatal weeks. While it is well established that neurotrophic factors play a central role in controlling neuronal survival and apoptosis in the peripheral nervous system (PNS), the situation is less clear in the central nervous system (CNS). In postnatal rodent neocortex, the peak of apoptosis coincides with the occurrence of spontaneous, synchronous activity patterns. In this article, we review recent results that demonstrate the important role of electrical activity for neuronal survival in the neocortex, describe the role of Ca²⁺ and neurotrophic factors in translating electrical activity into pro-survival signals, and finally discuss the clinical impact of the tight relation between electrical activity and neuronal survival versus apoptosis.

tPA promotes cortical neuron survival via mTOR-dependent mechanisms

Tissue plasminogen activator (tPA) is a thrombolytic agent commonly used in the treatment of ischemic stroke. While the thrombolytic effects of tPA have been well established, the impact of this blood-brain barrier (BBB) crossing drug on neurons is not known. Given the widespread use of tPA in the clinical setting and the strict therapeutic window established for effective use of the drug, we examined the molecular mechanisms mediating the impact of tPA on postnatal cortical neurons isolated from the mouse brain. Dissociated postnatal primary cortical neurons were treated with tPA and the effects on neuron survival were evaluated. Pharmacological inhibitors of several signaling pathways previously implicated in neuroprotection (mTOR, JAK/STAT, MAPK and PKA-dependent mechanisms) were used to pinpoint the mechanistic effectors of tPA on neuron survival in vitro. We report here that tPA treatment results in a time-dependent neuroprotective effect on postnatal cortical neurons that relies predominantly on Janus kinase (JAK) and mammalian target of rapamycin (mTOR) signaling mechanisms. Taken together, these data suggest that tPA promotes neuroprotection in a temporally-regulated manner and that both JAK and mTOR signaling effectors are critical mediators of this neuroprotective effect. The results suggest the possibility of targeting these defined mechanisms to potentially expand the therapeutic window for tPA.

Evaluation of cell damage in organotypic hippocampal slice culture from adult mouse: a potential model system to study neuroprotection

The use of organotypic hippocampal slice culture (OHSC) has become a powerful tool for studying cell damage in different neuropathological states, since it reproduces the basic morphological and functional properties of hippocampal neuronal network. However, the conventional OHSCs are established from postnatal animals rather than adult. Here we reevaluated the features of cell death in adult OHSC in detail and found potential utility for the study of neuroprotection. Organotypic culture of hippocampal slices from adult mice under conventional conditions led to a time-dependent and reproducible cell death. Around 6days in vitro (DIV), slices lost 50% of the cells, based on LDH release assessment. The cell death was greater than 90% after DIV 15. The cell loss was linearly correlated (r=0.944, P<0.01) with the time in culture. The electrophysiological responses to the stimulus in the cultured adult slices were accordingly reduced. The cell degeneration during adult OHSC might be utilized as a tool for studying neuroprotective effects in drug development. To illustrate this potential use, adult OHSCs were challenged with brain-derived neurotrophic factor (BDNF). We found that the continuous supplementation of 300ng/ml BDNF promoted cell survival of adult OHSC. Using immunohistochemistry and Western blot analyses of neuronal markers, we also demonstrated the pro-survival effects of BDNF on neurons in the adult OHSC system. It is suggested that OHSCs from adult mice might provide an alternative model system for neuronal degeneration, suitable for studying physiological factors and pharmacological compounds contributing to neuronal survival.

Intranigral transplantation of epigenetically induced BDNF-secreting human mesenchymal stem cells: implications for cell-based therapies in Parkinson's disease

It is thought that the ability of human mesenchymal stem cells (hMSC) to deliver neurotrophic factors might be potentially useful for the treatment of neurodegenerative disorders. The aim of the present study was to characterize signals and/or molecules that regulate brain-derived neurotrophic factor (BDNF) protein expression/delivery in hMSC cultures and evaluate the effect of epigenetically generated BDNF-secreting hMSC on the intact and lesioned substantia nigra (SN). We tested 4 different culture media and found that the presence of fetal bovine serum (FBS) decreased the expression of BDNF, whereas exogenous addition of epidermal growth factor (EGF) and basic fibroblast growth factor (bFGF) to serum-free medium was required to induce BDNF release (125 ± 12 pg/day/10⁶ cells). These cells were called hM(N)SC. Although the induction medium inhibited the expression of alpha smooth muscle actin (ASMA), an hMSC marker, and increased the nestin-positive subpopulation of hMSC cultures, the ability to express BDNF was restricted to the nestin-negative subpopulation. One week after transplantation into the SN, the human cells integrated into the surrounding tissue, and some showed a dopaminergic phenotype. We also observed the activation of Trk receptors for neurotrophic factors around the implant site, including the BDNF receptor TrkB. When we transplanted these cells into the unilateral lesioned SN induced by striatal injection of 6-hydroxydopamine (6-OHDA), a significant hypertrophy of nigral tyrosine hydroxylase (TH)(+) cells, an increase of striatal TH-staining and stabilization of amphetamine-induced motor symptoms were observed. Therefore, hMSC cultures exposed to the described induction medium might be highly useful as a vehicle for neurotrophic delivery to the brain and specifically are strong candidates for future therapeutic application in Parkinson's disease.

PTH inactivates the AKT survival pathway in the colonic cell line Caco-2

In previous works, we found that PTH promotes the apoptosis of human Caco-2 intestinal cells, through the mitochondrial pathway. This study was conducted to investigate the modulation of different players implicated in the AKT survival pathway in PTH-induced intestinal cell apoptosis. We demonstrate, for the first time, that PTH modulates AKT phosphorylation in response to apoptosis via the serine/threonine phosphatase PP2A. PTH treatment induces an association of AKT with the catalytic subunit of PP2A and increases its phosphatase activity. PTH also promotes the translocation of PP2Ac from the cytosol to the mitochondria. Furthermore, our results suggest that PP2A plays a role in hormone-dependent Caco-2 cells viability and in the cleavage of caspase-3 and its substrate PARP. The cAMP pathway also contributes to PTH-mediated AKT dephosphorylation while PKC and p38 MAPK do not participate in this event. Finally, we show that PTH induces the dissociation between 14-3-3 and AKT, but the significance of this response remains unknown. In correlation with PTH-induced Bad dephosphorylation, the hormone also decreases the basal association of 14-3-3 and Bad. Overall, our data suggest that in Caco-2 cells, PP2A and the cAMP pathway act in concert to inactivate the AKT survival pathway in PTH-induced intestinal cell apoptosis.

alpha-Tocopherol decreases the somatostatin receptor-effector system and increases the cyclic AMP/cyclic AMP response element binding protein pathway in the rat dentate gyrus

Neuronal survival has been shown to be enhanced by alpha-tocopherol and modulated by cyclic AMP (cAMP). Somatostatin (SST) receptors couple negatively to adenylyl cyclase (AC), thus leading to decreased cAMP levels. Whether alpha-tocopherol can stimulate neuronal survival via regulation of the somatostatinergic system, however, is unknown. The aim of this study was to investigate the effects of alpha-tocopherol on the SST signaling pathway in the rat dentate gyrus. To that end, 15-week-old male Sprague-Dawley rats were treated daily for 1 week with (+)-alpha-tocopherol or vehicle and sacrificed on the day following the last administration. No changes in either SST-like immunoreactivity (SST-LI) content or SST mRNA levels were detected in the dentate gyrus as a result of alpha-tocopherol treatment. A significant decrease in the density of the SST binding sites and an increase in the dissociation constant, however, were detected. The lower SST receptor density in the alpha-tocopherol-treated rats correlated with a significant decrease in the protein levels of the SST receptor subtypes SSTR1-SSTR4, whereas the corresponding mRNA levels were unaltered. G-protein-coupled-receptor kinase 2 expression was decreased by alpha-tocopherol treatment. This vitamin induced a significant increase in both basal and forskolin-stimulated AC activity, as well as a decrease in the inhibitory effect of SST on AC. Whereas the protein levels of AC type V/VI were not modified by alpha-tocopherol administration, ACVIII expression was significantly enhanced, suggesting it might account for the increase in AC activity. In addition, this treatment led to a reduction in Gialpha1-3 protein levels and in Gi functionality. alpha-Tocopherol did not affect the expression of the regulator of G-protein signaling 6/7 (RGS6/7). Finally, alpha-tocopherol induced an increase in the levels of phosphorylated cAMP response element binding protein (p-CREB) and total CREB in the dentate gyrus. Since CREB synthesis and phosphorylation promote the survival of many cells, including neurons, whereas SST inhibits the cAMP-PKA pathway, which is known to be involved in CREB phosphorylation, the alpha-tocopherol-induced reduction of SSTR observed here might possibly contribute, via increased cAMP levels and CREB activity, to the mechanism by which this vitamin promotes the survival of newborn neurons in the dentate gyrus.

Neuroprotective activities of enzymatically hydrolyzed peptides from porcine hide gelatin

Central nervous system disorders, including cerebrovascular disease, neurodegenerative diseases and head trauma are the most common cause of severe disability in adults and share a number of pathophysiological features. The therapeutic strategy of neuroprotection has been well accepted as one of the promising approaches in treating such brain disorders, and searching for the effective neuroprotective agents is still an open-ended task for neurologists and neuro-pharmacologists. In this study, we report for the first time that the enzymatic hydrolysates from type-B porcine hide gelatin has potent neuroprotective activity against H(2)O(2)- or serum deprivation-induced injuries of cultured SH-SY5Y cells. The peptides used in this study were prepared from type-B porcine hide gelatin digested with pepsin and papain. The neuroprotective activity of the porcine hide gelatin hydrolysate (PHH) was evaluated using MTT reduction assay. From the pre-screening of PHH, we found that the whole porcine hide gelatin hydrolysate obtained from papain digestion (PHH-I) showed significant neuroprotective activities (P<0.05). After further separation of PPH-I through SP-Sephadex C-50 and Sephadex G-25, only the fraction with smaller molecular weight from Sephadex G-25 (PHH-Ic) demonstrated potent neuroprotective activities (P<0.01). The active fraction showed a molecular mass between 1,000-3,000Da in SDS-polyacrylamide gel electrophoresis, and was rich in Glycine, Proline and Hydroxyproline in amino acid composition, indicating that peptides with a spectrum of molecular sizes and certain amino acids are critical for the neuroprotective activities of gelatin peptides. The viability of cultured cells treated with gelatin peptides was significantly improved in a dose-dependent manner. Further studies are necessary to establish the neuroprotective activity of hydrolyzed peptides for the neurons in vivo.

Opposing effects of retinoid signaling on astrogliogenesis in embryonic day 13 and 17 cortical progenitor cells

All-trans retinoic acid (RA) is a differentiation factor in many tissues. However, its role in astrogliogenesis has not been extensively studied. Here, we investigated the effect of RA on the regulation of astrogliogenesis at different cortical developmental stages. We prepared rat cortical progenitor cells from embryonic day (E) 13 and E17, which correspond to the beginning of neurogenic and astrogliogenic periods, respectively. Surprisingly, RA promoted astrogliogenesis at E17 but inhibited astrogliogenesis induced by ciliary neurotrophic factor (CNTF) at E13. The inhibitory effect of RA on astrogliogenesis at E13 was not due to premature commitment of progenitors to a neuronal or oligodendroglial lineage. Rather, RA retained more progenitors in a proliferative state. Furthermore, RA inhibition of astrogliogenesis at E13 was independent of STAT3 signaling and required the function of the alpha and beta isoforms of the RA receptors (RAR). Moreover, the differential response of E13 and E17 progenitors to RA was due to differences in the intrinsic properties of these cells that are preserved in vitro. The inhibitory effect of RA on cytokine-induced astrogliogenesis at E13 may contribute to silencing of any potential precocious astrogliogenesis during the neurogenic period.

The basal level of intracellular calcium gates the activation of phosphoinositide 3-kinase-Akt signaling by brain-derived neurotrophic factor in cortical neurons

Brain-derived neurotrophic factor (BDNF) mediates survival and neuroplasticity through the activation of phosphoinositide 3-kinase-Akt pathway. Although previous studies suggested the roles of mitogen-activated protein kinase, phospholipase C-gamma-mediated intracellular calcium ([Ca2+]i) increase, and extracellular calcium influx in regulating Akt activation, the cellular mechanisms are largely unknown. We demonstrated that sub-nanomolar BDNF significantly induced Akt activation in developing cortical neurons. The TrkB-dependent Akt phosphorylation at S473 and T308 required only phosphoinositide 3-kinase, but not phospholipase C and mitogen-activated protein kinase activity. Blocking NMDA receptors, L-type voltage-gated calcium channels, and chelating extracellular calcium by EGTA failed to block BDNF-induced Akt phosphorylation. In contrast, chelating [Ca2+]i by 1,2-bis(o-aminophenoxy)ethane-N,N,N ',N '-tetraacetic acid-acetoxymethyl ester (BAPTA-AM) abolished Akt phosphorylation. Interestingly, sub-nanomolar BDNF did not stimulate [Ca2+]i increase under our culture conditions. Together with that NMDA- and membrane depolarization-induced [Ca2+]i increase did not activate Akt, we conclude that the basal level of [Ca2+]i gates BDNF function. Furthermore, inhibiting calmodulin by W13 suppressed Akt phosphorylation. On the other hand, inhibition of protein phosphatase 1 by okadaic acid and tautomycin rescued Akt phosphorylation in BAPTA-AM and W13-treated neurons. We further demonstrated that the phosphorylation of phosphoinositide-dependent kinase-1 did not correlate with Akt phosphorylation at T308. Our results suggested novel roles of basal [Ca2+]i, rather than activity-induced calcium elevation, in BDNF-Akt signaling.

A novel Epac-Rap-PP2A signaling module controls cAMP-dependent Akt regulation

Rap1b has been implicated in the transduction of the cAMP mitogenic signal. It is phosphorylated and activated by cAMP, and its expression in models where cAMP is mitogenic leads to proliferation and tumorigenesis. Akt is a likely downstream effector of cAMP-Rap1 action. cAMP elevation induced a rapid and transient Akt inhibition that required activated and phosphorylated Rap1b. However, the mechanism(s) by which cAMP-Rap regulates Akt remains unclear. Here we show that (i) upstream regulators, PIK and PDK1, are not the target(s) of the cAMP inhibitory action; (ii) constitutively active Akt and calyculin A-stimulated Akt are resistant to cAMP inhibition, suggesting the action of a phosphatase; (iii) cAMP increases the rate of Akt dephosphorylation, directly implicating an Akt-phosphatase; (iv) Epac- and protein kinase A (PKA)-specific analogs synergistically inhibit Akt, indicating the involvement of both cAMP-dependent effector pathways; (v) H89 and dominant negative Epac 279E block cAMP-inhibitory action; (vi) Epac associates in a complex with Akt and PP2A, and the associated-phosphatase activity is positively modulated by cAMP in a PKA- and Rap1-dependent manner; (vii) like its action on Akt inhibition, PKA- and Epac-specific analogs synergistically activate Epac-associated PP2A; and (viii) dominant negative PP2A blocks cAMP-inhibitory action. Thus, we uncovered a novel cAMP-Epac/PKA-Rap1b-PP2A signaling module involved in Akt regulation that may represent a physiological event in the process of cAMP stimulation of thyroid mitogenesis.

Dephosphorylation of Akt in C6 cells grown in serum-free conditions corresponds with redistribution of p85/PI3K to the nucleus

Withdrawal of serum from cell cultures constitutes a useful model for the study of mechanisms involved in the regulation of Akt function in vitro. However, there have been several reports of changes in Akt activity that are not fully explained by the current model of phosphatidylinositol 3'-kinase (PI3K)/Akt signaling. We demonstrate the expected loss of Akt phosphorylation in C6 glioma cells cultured in serum-free conditions, yet we also observed a paradoxical increase in PI3K-lipid kinase activity in the same cultures. These events corresponded with relocalization of p85, the regulatory subunit of PI3K, to the perinuclear region and a local increase in PI3K-lipid kinase products. Treatment with platelet-derived growth factor (PDGF) maintained the association between p85 and the PDGF receptor during serum withdrawal and restored PI3K-lipid production at the plasma membrane. Although this protected Akt from dephosphorylation, it only slightly reversed cell-cycle arrest. These effects were not sensitive to treatment with epidermal growth factor, thus precluding a generalized role for growth factors. Our data suggest that loss of growth factor signaling, including PDGF signaling, may disrupt recruitment and/or anchoring of an active p85(PI3K) complex at the plasma membrane during serum withdrawal, which could account for the concurrent loss of Akt function.

Interleukin-1 beta impairs brain derived neurotrophic factor-induced signal transduction

The expression of IL-1 is elevated in the CNS in diverse neurodegenerative disorders, including Alzheimer's disease. The hypothesis was tested that IL-1 beta renders neurons vulnerable to degeneration by interfering with BDNF-induced neuroprotection. In trophic support-deprived neurons, IL-1 beta compromised the PI3-K/Akt pathway-mediated protection by BDNF and suppressed Akt activation. The effect was specific as in addition to Akt, the activation of MAPK/ERK, but not PLC gamma, was decreased. Activation of CREB, a target of these signaling pathways, was severely depressed by IL-1 beta. As the cytokine did not influence TrkB receptor and PLC gamma activation, IL-1 beta might have interfered with BDNF signaling at the docking step conveying activation to the PI3-K/Akt and Ras/MAPK pathways. Indeed, IL-1 beta suppressed the activation of the respective scaffolding proteins IRS-1 and Shc; this effect might involve ceramide generation. IL-1-induced interference with BDNF neuroprotection and signal transduction was corrected, in part, by ceramide production inhibitors and mimicked by the cell-permeable C2-ceramide. These results suggest that IL-1 beta places neurons at risk by interfering with BDNF signaling involving a ceramide-associated mechanism.

Brain-derived neurotrophic factor stimulates the transcriptional and neuroprotective activity of myocyte-enhancer factor 2C through an ERK1/2-RSK2 signaling cascade

Neurotrophin activation of myocyte-enhancer factor (MEF) 2C is one of the strongest pro-survival signaling pathways in developing neurons. To date, neurotrophin stimulation of MEF2C has been largely attributed to its direct phosphorylation by extracellular signal-regulated kinase (ERK) 5. Because MEF2C is not directly phosphorylated by ERK1/2 in vitro, it is generally assumed that the ERK1/2 signaling cascade does not regulate MEF2C. Surprisingly, we discovered that ERK1/2 are required for both the transcriptional and neuroprotective activity of MEF2C in cortical neurons stimulated by brain-derived neurotrophic factor. ERK1/2 stimulation of MEF2C is mediated by p90 ribosomal S6 kinase 2 (RSK2), a Ser/Thr protein kinase downstream of ERK1/2. RSK2 strongly phosphorylates purified recombinant MEF2C protein in vitro. Furthermore, RSK2 can directly phosphorylate MEF2C on S192, a consensus RSK2-phosphorylation site located in the transactivation domain of MEF2C. Substitution of S192 with a non-phosphorylatable alanine diminishes both the transcriptional and neuroprotective activity of MEF2C to an extent similar to mutation on S387, an established activating phosphorylation site. Together, our data identifies ERK1/2-RSK2 signaling as a novel mechanism by which neurotrophins activate MEF2C and promote neuronal survival.

Kainic acid triggers oligodendrocyte precursor cell proliferation and neuronal differentiation from striatal neural stem cells

Glutamate is an excitatory amino acid that serves important functions in mammalian brain development through alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)/ kainate receptor stimulation. Neural stem cells with self-renewal and multilineage potential are a useful tool to study the signals involved in the regulation of brain development. We have investigated the role played by AMPA/kainate receptors during the differentiation of neural stem cells derived from fetal rat striatum. The application of 1 and 10 microM kainic acid increased significantly the phosphorylation of the cyclic AMP response element binding protein (CREB), raised bromodeoxyuridine incorporation in O4-positive oligodendrocyte precursors, and increased the number of O1-positive cells in the cultures. Increased CREB phosphorylation and proliferation were prevented by the AMPA receptor antagonist 4-4(4-aminophenyl)-1,2-dihydro-1-methyl-2-propylcarbamoyl-6,7-methylenedioxyphthalazine (SYM 2206) and by protein kinase A and protein kinase C inhibitors. Cultures treated with 100 microM kainic acid showed decreased proliferation, a lower proportion of O1-positive cells, and apoptosis of O4-positive cells. None of these effects were prevented by SYM 2206, suggesting that kainate receptors take part in these events. We conclude that AMPA receptor stimulation by kainic acid promotes the proliferation of oligodendrocyte precursors derived from neural stem cells through a mechanism that requires the activation of CREB by protein kinase A and C. In the neurons derived from these cells, either AMPA or kainate receptor stimulation produces neuritic growth and larger cell bodies.

Genetic evidence for adenylyl cyclase 1 as a target for preventing neuronal excitotoxicity mediated by N-methyl-D-aspartate receptors

The excessive activation of N-methyl-D-aspartate (NMDA) receptors by glutamate results in neuronal excitotoxicity. cAMP is a key second messenger and contributes to NMDA receptor-dependent synaptic plasticity. Adenylyl cyclases 1 (AC1) and 8 (AC8) are the two major calcium-stimulated ACs in the central nervous system. Previous studies demonstrate AC1 and AC8 play important roles in synaptic plasticity, memory, and persistent pain. However, little is known about the possible roles of these two ACs in glutamate-induced neuronal excitotoxicity. Here, we report that genetic deletion of AC1 significantly attenuated neuronal death induced by glutamate in primary cultures of cortical neurons, whereas AC8 deletion did not produce a significant effect. AC1, but not AC8, contributes to intracellular cAMP production following NMDA receptor activation by glutamate in cultured cortical neurons. AC1 is involved in the dynamic modulation of cAMP-response element-binding protein activity in neuronal excitotoxicity. To explore the possible roles of AC1 in cell death in vivo, we studied neuronal excitotoxicity induced by an intracortical injection of NMDA. Cortical lesions induced by NMDA were significantly reduced in AC1 but not in AC8 knock-out mice. Our findings provide direct evidence that AC1 plays an important role in neuronal excitotoxicity and may serve as a therapeutic target for preventing excitotoxicity in stroke and neurodegenerative diseases.

Brain-derived Neurotrophic Factor Activates ERK5 in Cortical Neurons via a Rap1-MEKK2 Signaling Cascade

The extracellular signal-regulated kinase 5 (ERK5) is activated in neurons of the central nervous system by neurotrophins including brain-derived neurotrophic factor (BDNF). Although MEK5 is known to mediate BDNF stimulation of ERK5 in central nervous system neurons, other upstream signaling components have not been identified. Here, we report that BDNF induces a sustained activation of ERK5 in rat cortical neurons and activates Rap1, a small GTPase, as well as MEKK2, a MEK5 kinase. Our data indicate that activation of Rap1 or MEKK2 is sufficient to stimulate ERK5, whereas inhibition of either Rap1 or MEKK2 attenuates BDNF activation of ERK5. Furthermore, BDNF stimulation of MEKK2 is regulated by Rap1. Our evidence also indicates that Ras and MEKK3, a MEK5 kinase in non-neuronal cells, do not play a significant role in BDNF activation of ERK5. This study identifies Rap1 and MEKK2 as critical upstream signaling molecules mediating BDNF stimulation of ERK5 in central nervous system neurons.

Differential effects of Ca2+ and cAMP on transcription mediated by MEF2D and cAMP-response element-binding protein in hippocampal neurons

In neurons, the second messengers Ca(2+) and cAMP are mediators of transcriptional responses that are important for the development and function of the nervous system. The pro-survival neuronal transcription factors cAMP-response elementbinding protein (CREB) and myocyte enhancer factor-2 (MEF2) both stimulate gene expression in response to activity-dependent increases in the concentration of intracellular Ca(2+) ions. CREB is also activated by increases in intracellular cAMP. Here we have investigated whether the MEF2 family member MEF2D, similar to CREB, is also activated by cAMP in hippocampal neurons. We have shown that, unlike CREB, MEF2D is not activated by agents that increase intracellular cAMP. Moreover, increases in cAMP inhibit Ca(2+)-activated MEF2D-mediated gene expression. We have also shown that cAMP inhibits Ca(2+)-induced nuclear export of the MEF2 co-repressor HDAC5 and prevents Ca(2+)-stimulated nuclear import of the MEF2 co-activator NFAT3/c4. Our results suggest that cAMP interferes with MEF2D-mediated gene expression at multiple levels by antagonizing the derepression of MEF2D by HDAC5 and by inhibiting recruitment of the co-activator NFAT.

Inhibition of the spinal phosphoinositide 3-kinase exacerbates morphine withdrawal response

The present study investigates the roles of the spinal phosphoinositide 3-kinase (PI3K) signaling pathway in naloxone-precipitated withdrawal in acute and chronic morphine-dependent mice. There are two principal findings: (1) intrathecal pretreatment with wortmannin or LY294002, two structurally unrelated PI3K inhibitors, produced a dose-dependent increase of naloxone-precipitated withdrawal jumping, which was accompanied by an increased expression of spinal Fos protein in acute and chronic morphine-dependent mice; and (2) the expression of spinal p110gamma, the catalytic subunit PI3K, in the membrane fraction was significantly down-regulated by naloxone-precipitated withdrawal in acute and chronic morphine-dependent mice. This study provides new evidence showing that inactivation of the PI3K signaling pathway in the spinal cord may be involved in the expression of morphine withdrawal.

Upstream control of apoptosis by caspase-2 in serum-deprived primary neurons

During development as well as in pathological situations, neurons that fail to find appropriate targets or neurotrophic factors undergo cell death. Using primary cortical neurons subjected to acute serum-deprivation (SD), we have examined caspases activation, mitochondrial dysfunction and cell death parameters. Among a panel of metabolic, signaling and caspases inhibitors only those able to interfere with caspase-2 like activity protect primary neurons against SD-induced cell death. In situ detection and subcellular fractionation demonstrate a very early activation of cytosolic caspase-2, which controls Bax cleavage, relocalization and mitochondrial membrane permeabilization (MMP). Both z-VDVAD-fmk and a siRNA specific for caspase-2 abolish Bax changes, mitochondrial membranes permeabilization, as well as cytochrome c release-dependent activation of caspase-9/caspase-3, nuclear alterations, phosphatidylserine exposure, neurites dismantling and neuronal death. Hence, caspase-2 is an early checkpoint for apoptosis initiation in primary neurons subjected to serum deprivation.

Removal of G(ialpha1) constraints on adenylyl cyclase in the hippocampus enhances LTP and impairs memory formation

Stimulation of adenylyl cyclase in the hippocampus is critical for memory formation. However, generation of cAMP signals within an optimal range for memory may require a balance between stimulatory and inhibitory mechanisms. The role of adenylyl cyclase inhibitory mechanisms for memory has not been addressed. One of the mechanisms for inhibition of adenylyl cyclase is through activation of G(i)-coupled receptors, a mechanism that could serve as a constraint on memory formation. Here we report that ablation of G(ialpha1) by gene disruption increases hippocampal adenylyl cyclase activity and enhances LTP in area CA1. Furthermore, gene ablation of G(ialpha1) or antisense oligonucleotide-mediated depletion of G(ialpha1) disrupted hippocampus-dependent memory. We conclude that G(ialpha1) provides a critical mechanism for tonic inhibition of adenylyl cyclase activity in the hippocampus. We hypothesize that loss of G(ialpha1) amplifies the responsiveness of CA1 postsynaptic neurons to stimuli that strengthen synaptic efficacy, thereby diminishing synapse-specific plasticity required for new memory formation.

Role of the adenosine 3',5'-cyclic monophosphate (cAMP) in enhancing the efficacy of siRNA-mediated gene silencing in neuroblastoma cells

Gene-silencing activity mediated by siRNA has been demonstrated in mammalian cells; however, the mechanism of its regulation is not well understood. Since downregulation of a number of genes occurs during adenosine 3',5'-cyclic monophosphate (cAMP)-induced differentiation of neuroblastoma (NB) cells, it is possible that cAMP may play a role in regulating siRNA activity during differentiation. To study this, we utilized an NB cell line (NBP2-PN25) that expresses a short-lived green fluorescent protein (d2EGFP) under the CMV promoter. These cells were transfected with a retroviral plasmid that expresses U6 promoter-driven expression of siRNA targeted to d2EGFP and then were treated with cAMP-elevating agents (200 microg/ml RO20-1724, an inhibitor of cyclic nucleotide phosphodiesterase, and 1 microg/ml prostaglandin A1, a stimulator of adenylate cyclase) for 2 or 24 h. The siRNA activity was measured by determining the level of intensity of d2EGFP protein by flow cytometry, and the level of d2EGFP mRNA by real-time PCR. The results showed that cAMP-elevating agents enhanced U6-driven siRNA activity directed towards d2EGFP in NB cells 24 h after treatment. One of the mechanisms of action of cAMP is mediated via phosphatidylinositol 3-kinase (PI3K) inhibition; therefore, we have investigated the effect of a PI3K inhibitor on siRNA activity. This study showed that inhibition of PI3K also enhanced U6-driven siRNA activity towards d2EGFP. cAMP-stimulating agents increased U6 transcript levels, perhaps suggesting that increased siRNA activity may in part be due to an increase in transcriptional activity. When NB cells were transfected with a synthetic siRNA directed to d2EGFP, both cAMP elevation and PI3K inhibition similarly enhanced siRNA activity. Sodium butyrate, which inhibits the growth of NB cells similar to the effect produced by cAMP, did not affect U6-driven siRNA activity towards d2EGFP. Protein kinase C (PKC) activation or inhibition also failed to affect siRNA activity in NB cells. This study also showed that cAMP elevation and PI3K inhibition increases U6-driven siRNA activity directed towards an endogenous gene, p53. Our data suggest a role for the cAMP pathway in affecting the efficacy of siRNA system during differentiation of NB cells.

Regulation of neuroprotective activity of myocyte-enhancer factor 2 by cAMP-protein kinase A signaling pathway in neuronal survival

The transcription factor myocyte-enhancer factor 2 (MEF2) has been shown to be required for the survival of different types of neurons. However, the death- or survival-inducing second messenger pathways that regulate MEF2 activity remain to be fully elucidated. Membrane depolarization by KCl induces neuronal survival that is dependent upon MEF2-mediated gene transactivation. Here we report that membrane depolarizationinduced activation of MEF2 requires the cAMP-protein kinase A (PKA) pathway. Inhibition of the activity of cAMP-PKA pathway attenuates membrane depolarization-induced activation of MEF2 activity and neuronal survival, whereas enhancing the activity of this pathway prevents KCl withdrawal-induced inhibition of MEF2 and neuronal apoptosis. Moreover, PKA directly phosphorylates MEF2 at Thr-20 in vitro to increase MEF2 DNA binding activity. A mutation of Thr-20 to Ala renders MEF2 resistant to PKA phosphorylation in vitro and reduces its DNA binding activity. Transfection of this T20A mutant blocks survival and induces apoptosis in cultured cortical and cerebellar granule neurons. This study identifies the transcription factor MEF2 as a target of cAMP-PKA pathway and demonstrates that PKA phosphorylation of MEF2 is a key step in modulating its DNA binding activity and ability to promote neuronal survival.

Role of MAPK/ERK in Neurotrophin-4 Potentiation of Necrotic Neuronal Death

Neurotrophic factors have been proposed for the treatment of a variety of neurological diseases. However, to this point they have failed in clinical trials. One potential problem is that while neurotrophic factors attenuate apoptosis, they have the potential to enhance necrosis. In this study we show that neurotrophin-4 (NT-4) attenuated apoptotic neuronal death while potentiating necrotic neuronal death in cortical cultures. The protective effects of NT-4 were not blocked by the mitogen-activated protein kinase kinase (MEK) inhibitors PD098059 or U0126, while the injury potentiation by NT-4 was blocked by these inhibitors. NT-4 stimulated the phosphorylation of ERK1/2 and this phosphorylation was attenuated by U0126 and PD098059. The results indicate a disassociation between the pathway by which NT-4 potentiates necrosis, and that by which it attenuates apoptosis, and suggest that addition of a MEK inhibitor may enhance the beneficial effects of NT-4 in treating complex injuries such as occur in vivo.

Neural system-enriched gene expression: relationship to biological pathways and neurological diseases

To understand the commitment of the genome to nervous system differentiation and function, we sought to compare nervous system gene expression to that of a wide variety of other tissues by gene expression database construction and mining. Gene expression profiles of 10 different adult nervous tissues were compared with that of 72 other tissues. Using ANOVA, we identified 1,361 genes whose expression was higher in the nervous system than other organs and, separately, 600 genes whose expression was at least threefold higher in one or more regions of the nervous system compared with their median expression across all organs. Of the 600 genes, 381 overlapped with the 1,361-gene list. Limited in situ gene expression analysis confirmed that identified genes did represent nervous system-enriched gene expression, and we therefore sought to evaluate the validity and significance of these top-ranked nervous system genes using known gene literature and gene ontology categorization criteria. Diverse functional categories were present in the 381 genes, including genes involved in intracellular signaling, cytoskeleton structure and function, enzymes, RNA metabolism and transcription, membrane proteins, as well as cell differentiation, death, proliferation, and division. We searched existing public sites and identified 110 known genes related to mental retardation, neurological disease, and neurodegeneration. Twenty-one of the 381 genes were within the 110-gene list, compared with a random expectation of 5. This suggests that the 381 genes provide a candidate set for further analyses in neurological and psychiatric disease studies and that as a field, we are as yet, far from a large-scale understanding of the genes that are critical for nervous system structure and function. Together, our data indicate the power of profiling an individual biologic system in a multisystem context to gain insight into the genomic basis of its structure and function.

Neuroprotection of immortalized hippocampal neurones by brain-derived neurotrophic factor and Raf-1 protein kinase: role of extracellular signal-regulated protein kinase and phosphatidylinositol 3-kinase

We have investigated the molecular mechanisms of neurotrophin-mediated cell survival in HT22 cells, a murine cell line of hippocampal origin, expressing the brain-derived neurotrophic factor (BDNF) receptor TrkB as well as the TrkB.T1 splice variant. Stimulation with BDNF protected HT22-TrkB cells, but not HT22-TrkB.T1 cells, against programmed cell death induced by serum deprivation. BDNF did not, however, provide protection against oxidative glutamate toxicity, indicating that serum deprivation-induced cell death differs substantially from glutamate-induced cell death. Using a pharmacological strategy to block either the extracellular signal-regulated protein kinase (ERK) or the phosphatidylinositol 3-kinase (PI3) pathway, we show that activation of PI3 kinase is required for the neuroprotective activity of BDNF in HT22 cells. To further analyse the role of ERK in neuroprotection we expressed an inducible deltaRaf-1:ER fusion protein in HT22 cells. Activation of this conditionally active form of Raf-1 induced a sustained phosphorylation of ERK, and protected the cells from serum withdrawal-induced cell death. Inhibition of ERK activation at different time points revealed that a prolonged activation of ERK is essential to protect HT22 cells from cell death triggered by the withdrawal of serum, indicating that the duration of ERK activation is of major importance for its neuroprotective biological function.

Inhibition of protein kinase C promotes neuronal survival in low potassium through an Akt-dependent pathway

Cerebellar granule cell neurons undergo apoptotic cell death when subjected to serum-free conditions at physiological concentrations of potassium (5 mM). Protein kinase C (PKC) is known to play a role in preventing neuronal apoptosis under trophic factor deprivation, but its role in protecting cerebellar neurons from cell death under conditions of low potassium is unknown. This study sought to determine the involvement of PKC in neuronal survival and to determine if PKC regulated the phosphatidylinositol 3-kinase (PI 3-K)/Akt pathway in low physiologic concentrations of potassium. Incubation with a pan-PKC inhibitor, Ro-31-8220 (2 microm), or a specific PKCAlpha inhibitor, Gö6976, protected cerebellar granule cell neurons from low potassium-mediated cell death. In contrast, phorbol ester (TPA, 100 nm), a PKC activator, increased cell death. Incubation with, Ro-31-8220 rescued neurons from cell death induced by the PI 3-K inhibitor, LY294002, suggesting that Ro-31-8220 may affect Akt phosphorylation. Western blot analysis showed that serum-free, low potassium conditions decreased Akt phosphorylation, which was exacerbated by treatment with LY294002. In contrast, PKC inhibitors, Gö6976 or Ro-31-8220, increased Akt phosphorylation approximately two and four-fold, respectively in low potassium conditions. Because Akt activation appears to be critical in promoting neuronal survival under these culture conditions, increased Akt phosphorylation brought about by inhibiting PKC promotes neuronal survival.

Genetics of obsessive-compulsive disorders: new findings and challenges

A review of the current state of research in the genetics of obsessive-compulsive disorder (OCD) is presented. OCD is a neuropsychiatric condition that affects 1-2% of the population and often has an early age at onset of symptoms. OCD has been shown to be familial, and a major gene effect has been reported. However, phenotypic and genetic heterogeneity of OCD poses multiple challenges for locating susceptibility genes. Strategies such as the use of phenotypic subtyping (using tic disorders or other anxiety disorders) and endophenotyping based on brain mechanisms underlying OCD (functional brain imaging and neuropsychological measures) may open ways to understand the genetic components of OCD. Using child probands and extended families for linkage an association studies is another venue to obtain greater informative families for genetic studies. A better understanding of environmental triggers, OCD subtypes and OCD pathophysiology will lead to locating genes that confer risk to OCD.