Partial lithium-associated protection against apoptosis induced by C2-ceramide in cerebellar granule neurons

Patients receiving lithium therapy have a reduced prevalence of neurological and cardiovascular disorders

A variety of evidence from laboratory and animal studies suggests that lithium has neurotrophic and cytoprotective properties, and may ameliorate or prevent some disease states. We investigated whether such a protective effect can be observed in human psychiatric patients receiving lithium therapy. We carried out a retrospective chart review of 1028 adult psychiatric male and female outpatients attending four lithium clinics in metropolitan New York City. Patients were divided into two groups based on lithium usage, and the prevalence of neurological and cardiovascular disorders was compared. The main outcome measures were the occurrence in the two patient groups of a variety of neurological disorders and myocardial infarction. Odds ratios were calculated to assess the risk of having a disorder for patients receiving lithium compared to patients not receiving lithium: for seizures, the odds ratio was 0.097; for amyotrophic lateral sclerosis, the odds ratio was 0.112; for dementia not otherwise specified, the odds ratio was 0.112; and for myocardial infarction, the odds ratio was 0.30. Logistical regression analysis showed that lithium treatment is a significant negative predictive factor in the prevalence of each of these disease states, when age, duration of clinic attendance, and use of anti-psychotic medications are taken into account. Our results show that patients receiving regular lithium treatment have a reduced prevalence of some neurological disorders and myocardial infarctions. One possible explanation of these results is that a protective effect of lithium observed in laboratory and animal studies may also be present in human patients receiving regular lithium therapy.

Molecular actions and therapeutic potential of lithium in preclinical and clinical studies of CNS disorders

Lithium has been used clinically to treat bipolar disorder for over half a century, and remains a fundamental pharmacological therapy for patients with this illness. Although lithium's therapeutic mechanisms are not fully understood, substantial in vitro and in vivo evidence suggests that it has neuroprotective/neurotrophic properties against various insults, and considerable clinical potential for the treatment of several neurodegenerative conditions. Evidence from pharmacological and gene manipulation studies support the notion that glycogen synthase kinase-3 inhibition and induction of brain-derived neurotrophic factor-mediated signaling are lithium's main mechanisms of action, leading to enhanced cell survival pathways and alteration of a wide variety of downstream effectors. By inhibiting N-methyl-D-aspartate receptor-mediated calcium influx, lithium also contributes to calcium homeostasis and suppresses calcium-dependent activation of pro-apoptotic signaling pathways. In addition, lithium decreases inositol 1,4,5-trisphosphate by inhibiting phosphoinositol phosphatases, a process recently identified as a novel mechanism for inducing autophagy. Through these mechanisms, therapeutic doses of lithium have been demonstrated to defend neuronal cells against diverse forms of death insults and to improve behavioral as well as cognitive deficits in various animal models of neurodegenerative diseases, including stroke, amyotrophic lateral sclerosis, fragile X syndrome, as well as Huntington's, Alzheimer's, and Parkinson's diseases, among others. Several clinical trials are also underway to assess the therapeutic effects of lithium for treating these disorders. This article reviews the most recent findings regarding the potential targets involved in lithium's neuroprotective effects, and the implication of these findings for the treatment of a variety of diseases.

Effect of lithium salts on lactate dehydrogenase, adenylate kinase, and 1-phosphofructokinase activities

Inhibitions of 30 nM rabbit muscle 1-phosphofructokinase (PFK-1) by lithium, potassium, and sodium salts showed inhibition or not depending upon the anion present. Generally, potassium salts were more potent inhibitors than sodium salts; the extent of inhibition by lithium salts also varied with the anion. Li(2)CO(3) was a relatively potent inhibitor of PFK-1 but LiCl and lithium acetate were not. Our results suggest that extents of inhibition by monovalent salts were due to both cations and anions, and the latter needs to be considered before inhibition can be credited to the cation. An explanation for monovalent salt inhibitions is proffered involving interactions of both cations and anions at negative and positive sites of PFK-1 that affect enzyme activity. Our studies suggest that lithium cations per se are not inhibitors: the inhibitors are the lithium salts, and we suggest that in vitro studies involving the effects of monovalent salts on enzymes should involve more than one anion.

The role of lithium in the treatment of bipolar disorder: convergent evidence for neurotrophic effects as a unifying hypothesis

Lithium has been and continues to be the mainstay of bipolar disorder (BD) pharmacotherapy for acute mood episodes, switch prevention, prophylactic treatment, and suicide prevention. Lithium is also the definitive proof-of-concept agent in BD, although it has recently been studied in other psychoses as well as diverse neurodegenerative disorders. Its neurotrophic effects can be viewed as a unifying model to explain several integrated aspects of the pathophysiology of mood disorders and putative therapeutics for those disorders. Enhancing neuroprotection (which directly involves neurotrophic effects) is a therapeutic strategy intended to slow or halt the progression of neuronal loss, thus producing long-term benefits by favorably influencing outcome and preventing either the onset of disease or clinical decline. The present article: (i) reviews what has been learned regarding lithium's neurotrophic effects since Cade's original studies with this compound; (ii) presents human data supporting the presence of cellular atrophy and death in BD as well as neurotrophic effects associated with lithium in human studies; (iii) describes key direct targets of lithium involved in these neurotrophic effects, including neurotrophins, glycogen synthase kinase 3 (GSK-3), and mitochondrial/endoplasmic reticulum key proteins; and (iv) discusses lithium's neurotrophic effects in models of apoptosis and excitotoxicity as well as its potential neurotrophic effects in models of neurological disorders. Taken together, the evidence reviewed here suggests that lithium's neurotrophic effects in BD are an example of an old molecule acting as a new proof-of-concept agent. Continued work to decipher lithium's molecular actions will likely lead to the development of not only improved therapeutics for BD, but to neurotrophic enhancers that could prove useful in the treatment of many other illnesses.

Review of lithium effects on brain and blood

Clinicians have long used lithium to treat manic depression. They have also observed that lithium causes granulocytosis and lymphopenia while it enhances immunological activities of monocytes and lymphocytes. In fact, clinicians have long used lithium to treat granulocytopenia resulting from radiation and chemotherapy, to boost immunoglobulins after vaccination, and to enhance natural killer activity. Recent studies revealed a mechanism that ties together these disparate effects of lithium. Lithium acts through multiple pathways to inhibit glycogen synthetase kinase-3beta (GSK3 beta). This enzyme phosphorylates and inhibits nuclear factors that turn on cell growth and protection programs, including the nuclear factor of activated T cells (NFAT) and WNT/beta-catenin. In animals, lithium upregulates neurotrophins, including brain-derived neurotrophic factor (BDNF), nerve growth factor, neurotrophin-3 (NT3), as well as receptors to these growth factors in brain. Lithium also stimulates proliferation of stem cells, including bone marrow and neural stem cells in the subventricular zone, striatum, and forebrain. The stimulation of endogenous neural stem cells may explain why lithium increases brain cell density and volume in patients with bipolar disorders. Lithium also increases brain concentrations of the neuronal markers n-acetyl-aspartate and myoinositol. Lithium also remarkably protects neurons against glutamate, seizures, and apoptosis due to a wide variety of neurotoxins. The effective dose range for lithium is 0.6-1.0 mM in serum and >1.5 mM may be toxic. Serum lithium levels of 1.5-2.0 mM may have mild and reversible toxic effects on kidney, liver, heart, and glands. Serum levels of >2 mM may be associated with neurological symptoms, including cerebellar dysfunction. Prolonged lithium intoxication >2 mM can cause permanent brain damage. Lithium has low mutagenic and carcinogenic risk. Lithium is still the most effective therapy for depression. It "cures" a third of the patients with manic depression, improves the lives of about a third, and is ineffective in about a third. Recent studies suggest that some anticonvulsants (i.e., valproate, carbamapazine, and lamotrigene) may be useful in patients that do not respond to lithium. Lithium has been reported to be beneficial in animal models of brain injury, stroke, Alzheimer's, Huntington's, and Parkinson's diseases, amyotrophic lateral sclerosis (ALS), spinal cord injury, and other conditions. Clinical trials assessing the effects of lithium are under way. A recent clinical trial suggests that lithium stops the progression of ALS.

A systematic review of existing data on long-term lithium therapy: neuroprotective or neurotoxic?

Lithium is an efficacious agent for the treatment of bipolar disorder, but it is unclear to what extent its long-term use may result in neuroprotective or toxic consequences. Medline was searched with the combination of the word 'Lithium' plus key words that referred to every possible effect on the central nervous system. The papers were further classified into those supporting a neuroprotective effect, those in favour of a neurotoxic effect and those that were neutral. The papers were classified into research in humans, animal and in-vitro research, case reports, and review/opinion articles. Finally, the Natural Standard evidence-based validated grading rationale was used to validate the data. The Medline search returned 970 papers up to February 2006. Inspection of the abstracts supplied 214 papers for further reviewing. Eighty-nine papers supported the neuroprotective effect (6 human research, 58 animal/in vitro, 0 case reports, 25 review/opinion articles). A total of 116 papers supported the neurotoxic effect (17 human research, 23 animal/in vitro, 60 case reports, 16 review/opinion articles). Nine papers supported no hypothesis (5 human research, 3 animal/in vitro, 0 case reports, 1 review/opinion articles). Overall, the grading suggests that the data concerning the effect of lithium therapy is that of level C, that is 'unclear or conflicting scientific evidence' since there is conflicting evidence from uncontrolled non-randomized studies accompanied by conflicting evidence from animal and basic science studies. Although more papers are in favour of the toxic effect, the great difference in the type of papers that support either hypothesis, along with publication bias and methodological issues make conclusions difficult. Lithium remains the 'gold standard' for the prophylaxis of bipolar illness, however, our review suggests that there is a rare possibility of a neurotoxic effect in real-life clinical practice even in closely monitored patients with 'therapeutic' lithium plasma levels. It is desirable to keep lithium blood levels as low as feasible with prophylaxis.

Lithium blocks ethanol-induced modulation of protein kinases in the developing brain

Lithium has been shown to be neuroprotective against various insults including ethanol exposure. We previously reported that ethanol-induced apoptotic neurodegeneration in the postnatal day 7 (P7) mice is associated with decreases in phosphorylation levels of Akt, glycogen synthase kinase-3beta (GSK-3beta), and AMP-activated protein kinase (AMPK), and alteration in lipid profiles in the brain. Here, P7 mice were injected with ethanol and lithium, and the effects of lithium on ethanol-induced alterations in phosphorylation levels of protein kinases and lipid profiles in the brain were examined. Immunoblot and immunohistochemical analyses showed that lithium significantly blocked ethanol-induced caspase-3 activation and reduction in phosphorylation levels of Akt, GSK-3beta, and AMPK. Further, lithium inhibited accumulation of cholesterol ester (ChE) and N-acylphosphatidylethanolamine (NAPE) triggered by ethanol in the brain. These results suggest that Akt, GSK-3beta, and AMPK are involved in ethanol-induced neurodegeneration and the neuroprotective effects of lithium by modulating both apoptotic and survival pathways.

GSK-3beta acts downstream of PP2A and the PI 3-kinase-Akt pathway, and upstream of caspase-2 in ceramide-induced mitochondrial apoptosis

The signaling of glycogen synthase kinase-3beta (GSK-3beta) has been implicated in stress-induced apoptosis. However, the pro-apoptotic role of GSK-3beta is still unclear. Here, we show the involvement of GSK-3beta in ceramide-induced mitochondrial apoptosis. Ceramide induced GSK-3beta activation via protein dephosphorylation at serine 9. We previously reported that ceramide induced caspase-2 and caspase-8 activation, Bid cleavage, mitochondrial damage, and apoptosis. In this study, we found that caspase-2 activation and the subsequent apoptotic events were abolished by the GSK-3beta inhibitors lithium chloride and SB216763, and by GSK-3beta knockdown using short interfering RNA. We also found that ceramide-activated protein phosphatase 2A (PP2A) indirectly caused GSK-3beta activation, and that the PP2A-regulated PI 3-kinase-Akt pathway was involved in GSK-3beta activation. These results indicate a role for GSK-3beta in ceramide-induced apoptosis, in which GSK-3beta acts downstream of PP2A and the PI 3-kinase-Akt pathway, and upstream of caspase-2 and caspase-8.

Mood stabilizers target cellular plasticity and resilience cascades: implications for the development of novel therapeutics

Bipolar disorder is a devastating disease with a lifetime incidence of about 1% in the general population. Suicide is the cause of death in 10 to 15% of patients and in addition to suicide, mood disorders are associated with many other harmful health effects. Mood stabilizers are medications used to treat bipolar disorder. In addition to their therapeutic effects for the treatment of acute manic episodes, mood stabilizers are useful as prophylaxis against future episodes and as adjunctive antidepressant medications. The most established and investigated mood-stabilizing drugs are lithium and valproate but other anticonvulsants (such as carbamazepine and lamotrigine) and antipsychotics are also considered as mood stabilizers. Despite the efficacy of these diverse medications, their mechanisms of action remain, to a great extent, unknown. Lithium's inhibition of some enzymes, such as inositol monophosphatase and glycogen synthase kinase-3, probably results in its mood-stabilizing effects. Valproate may share its anticonvulsant target with its mood-stabilizing target or may act through other mechanisms. It has been shown that lithium, valproate, and/or carbamazepine regulate numerous factors involved in cell survival pathways, including cyclic adenine monophospate response element-binding protein, brain-derived neurotrophic factor, bcl-2, and mitogen-activated protein kinases. These drugs have been suggested to have neurotrophic and neuroprotective properties that ameliorate impairments of cellular plasticity and resilience underlying the pathophysiology of mood disorders. This article also discusses approaches to develop novel treatments specifically for bipolar disorder.

Lithium Treatment Prevents Neurocognitive Deficit Resulting from Cranial Irradiation

Curative cancer treatment regimens often require cranial irradiation, resulting in lifelong neurocognitive deficiency in cancer survivors. This deficiency is in part related to radiation-induced apoptosis and decreased neurogenesis in the subgranular zone of the hippocampus. We show that lithium treatment protects irradiated hippocampal neurons from apoptosis and improves cognitive performance of irradiated mice. The molecular mechanism of this effect is mediated through multiple pathways, including Akt/glycogen synthase kinase-3beta (GSK-3beta) and Bcl-2/Bax. Lithium treatment of the cultured mouse hippocampal neurons HT-22 induced activation of Akt (1.5-fold), inhibition of GSK-3beta (2.2-fold), and an increase in Bcl-2 protein expression (2-fold). These effects were sustained when cells were treated with lithium in combination with ionizing radiation. In addition, this combined treatment led to decreased expression (40%) of the apoptotic protein Bax. The additional genes regulated by lithium were identified by microarray, such as decorin and Birc1f. In summary, we propose lithium treatment as a novel therapy for prevention of deleterious neurocognitive consequences of cranial irradiation.

Sphingolipids in apoptosis, survival and regeneration in the nervous system

Simple sphingolipids such as ceramide, sphingosine and sphingosine 1-phosphate are key regulators of diverse cellular functions. Their roles in the nervous system are supported by extensive evidence derived primarily from studies in cultured cells. More recently animal studies and studies with human samples have revealed the importance of ceramide and its metabolites in the development and progression of neurodegenerative disorders. The roles of sphingolipids in neurons and glial cells are complex, cell dependent, and many times contradictory. In this review I will summarize the effects elicited by ceramide and ceramide metabolites in cells of the nervous system, in particular those effects related to cell survival and death, emphasizing the molecular mechanisms involved. I also discuss recent evidence for the implication of sphingolipids in the development and progression of certain dementias.

Dopamine-induced apoptosis in human melanocytes involves generation of reactive oxygen species

BACKGROUND

Previous studies have shown that significant increases in urinary and plasma levels of several monoamines and their metabolites characterize the onset of vitiligo and its progression. Recently, both epidermal keratinocytes and melanocytes were found to have the capacity for the biosynthesis of several catecholamines and serotonin. Some monoamines and their metabolites can induce apoptosis and cytotoxicity in neural cells. However, no previous report has investigated the potential role of these monoamines in inducing apoptosis or cytotoxicity in melanocytes.

OBJECTIVES

To study the effects of dopamine (DA), norepinephrine (NE), epinephrine (EP), and serotonin (5-HT) on melanocyte cytotoxicity and apoptosis.

METHODS

Primary cultures of normal human melanocytes established from the foreskins of normal individuals were treated with different concentrations of DA, NE, EP and 5-HT for 5 and 7 days. Cell viability was measured by the MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] assay. Melanocyte apoptosis was evaluated by morphological examination and flow cytometric analysis. We also measured the generation of reactive oxygen species (ROS) after DA treatment.

RESULTS

Among the four monoamines used in this study, only DA had an effect, dose-dependently decreasing the melanocyte viability at concentrations ranging from 0.01 to 100 micromol L(-1) (0.1 and 1 micromol L(-1), P < 0.05; 10 micromol L(-1), P < 0.01). In addition, DA-induced melanocyte apoptosis was evidenced by the increased percentage of sub-G1 cells from 7.71 +/- 0.28% (control) to 12.22 +/- 1.05% (0.1 micromol L(-1) DA) (P < 0.005), and treatment with the antioxidant N-acetylcysteine (NAC) reversed this apoptotic effect. DA treatment led to the generation of ROS, which could be prevented by pretreatment with NAC.

CONCLUSIONS

DA can induce melanocyte apoptosis, which might be related to the generation of ROS. This novel effect might play an important role in the development or progression of vitiligo, which is currently viewed as a disease process closely related to melanocyte apoptosis.

Implication of cyclin-dependent kinase 5 in the neuroprotective properties of lithium

Although numerous studies have demonstrated a neuroprotective and anti-apoptotic role of lithium in neuronal cell cultures, the precise mechanism by which this occurs, remains to be elucidated. In this study, we evaluated the lithium-mediated neuroprotection against colchicine-induced apoptosis in cultured cerebellar granule neurons. Previously, it has been demonstrated that colchicine mediates apoptosis in cerebellar granule neurons through cytoskeletal alteration and activation of an intrinsic pro-apoptotic pathway. Recently we also demonstrated a potential role of cyclin-dependent kinase 5 (cdk5) in this pathway. Here we report that colchicine induces dephosphorylation in Ser-9 and phosphorylation in Tyr-216, and thus activation, of glycogen synthase kinase-3beta in cerebellar granule neurons, and that this modification is inhibited by the presence of 5 mM lithium. However, the selective glycogen synthase kinase-3beta inhibitors SB-415286 and SB-216763 were unable to prevent colchicine-induced apoptosis in these cells, suggesting that the anti-apoptotic activity of lithium is not mediated by glycogen synthase kinase-3beta under these conditions. On the other hand, 5 mM lithium prevented the colchicine-induced increase in cdk5 expression and breakdown of cdk5/p35 to cdk5/p25. In addition, we show that up-regulation of cdk5/p25 is unrelated to inhibition of the activity of myocyte enhancer factor 2, a pro-survival transcription factor. These data suggest a previously undescribed neuroprotective mechanism of lithium associated with the modulation of cdk5/p35 or cdk5/p25 expression.

Lithium: Potential Therapeutics Against Acute Brain Injuries and Chronic Neurodegenerative Diseases

In addition to the well-documented mood-stabilizing effects of lithium in manic-depressive illness patients, recent in vitro and in vivo studies in rodents and humans have increasingly implicated that lithium can be used in the treatment of acute brain injuries (e.g., ischemia) and chronic neurodegenerative diseases (Alzheimer's disease, Parkinson's disease, tauopathies, and Huntington's disease). Consistent with this novel view, substantial evidences suggest that depressive illness is not a mere neurochemical disease, but is linked to gray matter atrophy due to the reduced number/size of neurons and glia in brain. Importantly, neurogenesis, that is, birth/maturation of functional new neurons, continues to occur throughout the lifetime in human adult brains (e.g., hippocampus); the neurogenesis is impaired by multiple not-fully defined factors (e.g., aging, chronic stress-induced increase of glucocorticoids, and excitotoxicity), accounting for brain atrophy in patients with depressive illness and neurodegenerative diseases. Chronic treatment of lithium, in agreement with the delayed-onset of mood-stabilizing effects of lithium, up-regulates cell survival molecules (e.g., Bcl-2, cyclic AMP-responsive element binding protein, brain-derived neurotrophic factor, Grp78, Hsp70, and beta-catenin), while down-regulating pro-apoptotic activities (e.g., excitotoxicity, p53, Bax, caspase, cytochrome c release, beta-amyloid peptide production, and tau hyperphosphorylation), thus preventing or even reversing neuronal cell death and neurogenesis retardation.

Lithium facilitates apoptotic signaling induced by activation of the Fas death domain-containing receptor

Background

Lithium, a mood stabilizer widely used to treat bipolar disorder, also is a neuroprotectant, providing neurons protection from apoptosis induced by a broad spectrum of toxic conditions. A portion of this neuroprotection is due to lithium's inhibition of glycogen synthase kinase-3. The present investigation examined if the neuroprotection provided by lithium included apoptosis induced by stimulation of the death domain-containing receptor Fas.

Results

Instead of providing protection, treatment with 20 mM lithium significantly increased apoptotic signaling induced by activation of Fas, and this occurred in both Jurkat cells and differentiated immortalized hippocampal neurons. Other inhibitors of glycogen synthase kinase-3, including 20 μM indirubin-3'-monoxime, 5 μM kenpaullone, and 5 μM rottlerin, also facilitated Fas-induced apoptotic signaling, indicating that the facilitation of apoptosis by lithium was due to inhibition of glycogen synthase kinase-3.

Conclusions

These results demonstrate that lithium is not always a neuroprotectant, and it has the opposite effect of facilitating apoptosis mediated by stimulation of death domain-containing receptors.

Hypertonic stress activates glycogen synthase kinase 3beta-mediated apoptosis of renal medullary interstitial cells, suppressing an NFkappaB-driven cyclooxygenase-2-dependent survival pathway

The survival of renal medullary interstitial cells (RMICs) requires their adaptation to rapid shifts in ambient tonicity normally occurring in the renal medulla. Previous studies determined that cyclooxygenase-2 (COX 2) activation is critical for this adaptation. The present studies find that these adaptive mechanisms are dampened by the simultaneous activation of an apoptotic pathway linked to a glycogen synthase kinase 3beta (GSK 3beta). Inhibition of GSK 3 by LiCl or specific small molecule GSK inhibitors increased RMIC survival following hypertonic stress, and transduction of RMICs with a constitutively active GSK 3beta (AdGSK 3betaA9) significantly increased apoptosis, consistent with a proapoptotic role of GSK 3beta. Following GSK 3beta inhibition, increased survival was accompanied by increased COX 2 expression and COX 2 reporter activity. In contrast, GSK 3beta overexpression reduced COX 2 reporter activity. Importantly, enhanced RMIC survival produced by GSK 3beta inhibition was completely dependent on COX 2 because it was abolished by a COX 2-specific inhibitor, SC58236. The signaling pathway by which GSK 3beta suppresses COX 2 expression was then explored. GSK 3beta inhibition increased both NFkappaB and beta-catenin activity associated with decreased IkappaB and increased beta-catenin levels. The increase in COX 2 following GSK 3beta inhibition was entirely blocked by NFkappaB inhibition using mutant IkappaB adenovirus. However, adenoviral overexpression of beta-catenin did not increase COX 2 levels. These findings suggest that GSK 3beta negatively regulates COX 2 expression and that GSK 3beta inhibitors protect RMICs from hypertonic stress via induction of NFkappaB-COX 2-dependent pathway.

Opposed effects of lithium on the MEK-ERK pathway in neural cells: inhibition in astrocytes and stimulation in neurons by GSK3 independent mechanisms

Lithium is widely used in the treatment of bipolar disorder, but despite its proven therapeutic efficacy, the molecular mechanisms of action are not fully understood. The present study was undertaken to explore lithium effects of the MEK/ERK cascade of protein kinases in astrocytes and neurons. In asynchronously proliferating rat cortical astrocytes, lithium decreased time- and dose-dependently the phosphorylation of MEK and ERK, with 1 mM concentrations achieving 60 and 50% inhibition of ERK and MEK, respectively, after a 7-day exposure. Lithium also inhibited [3H]thymidine incorporation into DNA and induced a G2/M cell cycle arrest. In serum-deprived, quiescent astrocytes, pre-exposure to lithium resulted in the inhibition of cell cycle re-entry as stimulated by the mitogen endothelin-1: under this experimental setting, lithium did not affect the rapid, peak phosphorylation of MEK taking place after 3-5 min, but was effective in inhibiting the long-term, sustained phosphorylation of MEK. Lithium inhibition of the astrocyte MEK/ERK pathway was independent of inositol depletion. Further, compound SB216763 inhibited Tau phosphorylation at Ser396 and stabilized cytosolic beta-catenin, consistent with the inhibition of glycogen synthase kinase-3 beta (GSK-3 beta), but failed to reproduce lithium effects on MEK and ERK phosphorylation and cell cycle arrest. In cerebellar granule neurons, millimolar concentrations of lithium enhanced MEK and ERK phosphorylation in a concentration-dependent manner, again through an inositol and GSK-3 beta independent mechanism. These opposing effects in astrocytes and neurons make lithium treatment a promising strategy to favour neural repair and reduce reactive gliosis after traumatic injury.

Prevalence of necrosis in C2-ceramide-induced cytotoxicity in NB16 neuroblastoma cells

The mechanism of cell death triggered by C2-ceramide was investigated using the NB16 neuroblastoma cell line. Treatment of NB16 cells with 20 microM C2-ceramide for 20 h resulted in approximately 75% loss of cell viability, but only 25% of cells were scored as apoptotic based on terminal deoxynucleotidyl transferase nick-end labeling. Ultrastructural analysis revealed early development of necrotic cytoplasmic vacuolization. After 20 h of treatment with C2-ceramide, the majority of cells possessed necrotic morphology with pronounced cytoplasmic vacuolization and without any nuclear changes, although a quarter of the cell population also exhibited clear perinuclear chromatin condensation characteristic of apoptosis. Flow cytometric analysis of cells labeled with both annexin V and propidium iodide showed the rapid accumulation of C2-ceramide-treated cells in the necrotic/late apoptotic fraction. In contrast, cells treated with tumor necrosis factor alpha plus cycloheximide (TNFalpha + CHX) first appeared in the early apoptotic fraction and then accumulated in the necrotic/late apoptotic fraction. Both C2-ceramide and TNFalpha + CHX increased caspase 8- and 3-like activities in cytosolic extracts; however, treatment of cells with the broad-spectrum caspase inhibitor N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone protected NB16 cells from TNFalpha + CHX-induced cell death but did not prevent C2-ceramide cytotoxicity. Although C2-ceramide triggered apoptosis in a fraction of the cells, cell death in the population was primarily caused by necrosis. Thus, C2-ceramide does not faithfully mimic the effects of apoptotic ligands such as TNFalpha, which are thought to be mediated by an accumulation of endogenous ceramide. The inhibition of phosphatidylcholine synthesis is a target for C2-ceramide-mediated cytotoxicity, and this work suggests that other agents that kill cells by inhibiting this pathway may also use a mixture of mechanisms, including necrosis as well as apoptosis.

Calcium-dependent prevention of neuronal apoptosis by lithium ion: essential role of phosphoinositide 3-kinase and phospholipase Cgamma

We examined the possibility that the neuroprotective effects of Li+ would depend upon the patterns of neuronal death, apoptosis versus necrosis, and whether Ca2+ as well as phosphoinositide 3-kinase (PI3-K) would mediate the neuroprotective effect of Li+. Cortical neurons treated with Li+ showed marked increase in [Ca2+]i within 2 min. Addition of BAPTA-acetoxymethyl ester, a selective Ca2+ chelator, abrogated the antiapoptotic effect of Li+. PI3-K was activated rapidly within 1 min after exposure to Li+, which mediated Ca2+-dependent neuroprotective effects of Li+. Activated PI3-K seemed to increase [Ca2+]i via the phospholipase Cgamma (PLCgamma) pathway. Antiapoptosis action of Li+ was prevented in the presence of U-73122, a selective phospholipase C inhibitor, and was not observed in PLCgamma1-null fibroblasts. In contrast to antiapoptosis action, administration of Li+ did not prevent neuronal cell necrosis by excitotoxicity or free radicals. Li+ selectively prevents apoptosis by increasing [Ca2+]i through activation of PI3-K and PLCgamma pathways.

A myocyte enhancer factor 2D (MEF2D) kinase activated during neuronal apoptosis is a novel target inhibited by lithium

Depolarization promotes the survival of cerebellar granule neurons via activation of the transcription factor myocyte enhancer factor 2D (MEF2D). Removal of depolarization induces hyperphosphorylation of MEF2D on serine/threonine residues, resulting in its decreased DNA binding and susceptibility to caspases. The subsequent loss of MEF2-dependent gene transcription contributes to the apoptosis of granule neurons. The kinase(s) that phosphorylates MEF2D during apoptosis is currently unknown. The serine/threonine kinase, glycogen synthase kinase-3 beta (GSK-3 beta), plays a pro-apoptotic role in granule neurons. To investigate a potential role for GSK-3 beta in MEF2D phosphorylation, we examined the effects of lithium, a non-competitive inhibitor of GSK-3 beta, on MEF2D activity in cultured cerebellar granule neurons. Lithium inhibited caspase-3 activation and chromatin condensation in granule neurons induced to undergo apoptosis by removal of depolarizing potassium and serum. Concurrently, lithium suppressed the hyperphosphorylation and caspase-mediated degradation of MEF2D. Moreover, lithium sustained MEF2 DNA binding and transcriptional activity in the absence of depolarization. Lithium also attenuated MEF2D hyperphosphorylation and apoptosis induced by calcineurin inhibition under depolarizing conditions, a GSK-3 beta-independent model of neuronal death. In contrast to lithium, MEF2D hyperphosphorylation was not inhibited by forskolin, insulin-like growth factor-I, or valproate, three mechanistically distinct inhibitors of GSK-3 beta. These results demonstrate that the kinase that phosphorylates and inhibits the pro-survival function of MEF2D in cerebellar granule neurons is a novel lithium target distinct from GSK-3 beta.

Suramin inhibits beta-bungarotoxin-induced activation of N-methyl-D-aspartate receptors and cytotoxicity in primary neurons

We demonstrated that beta-bungarotoxin (beta-BuTX), a snake presynaptic neurotoxin, exhibited a potent cytotoxic effect on cultured cerebellar granule neurons. The mechanism of action of beta-BuTX and the cytoprotective agents against beta-BuTX were studied. The neuronal death of cerebellar granule neurons induced by beta-BuTX was manifested with apoptosis and necrosis processes as revealed by neurite fragmentation, morphological alterations, and staining apoptotic bodies with the fluorescent dye Hoechst 33258. By means of microspectrofluorimetry and fura-2, we measured intracellular Ca2+ concentration, [Ca2+]i and found that [Ca2+]i was increased markedly prior to the morphological changes and cytotoxicity. The downstream pathway of the increased [Ca2+]i was investigated: there was increased production of free radicals, decreased mitochondrial membrane potential, and depleted cellular ATP content. MK801 and suramin effectively suppressed these detrimental effects of beta-BuTX. Furthermore, the [3H]MK801 binding was reduced by unlabeled MK801, beta-BuTX, and suramin. Thus, activation of N-methyl-D-aspartate (NMDA) receptors appeared to play a crucial role in the cytotoxic effects following betaBuTX exposure. In conclusion, the novel finding of this study was that a polypeptide beta-BuTX exerted a potent cytotoxic effect through sequential events, including activating NMDA receptors followed by increasing [Ca2+]i, ROS production, and impaired mitochondrial energy metabolism. Suramin, clinically used as a trypanocidal agent, was an effective antagonist against beta-BuTX. Data suggest that suramin might have value to detect the possible pathway of certain neuropathological disorders.

Activation of NMDA receptor partly involved in beta-bungarotoxin-induced neurotoxicity in cultured primary neurons

In this study, we demonstrated that a snake presynaptic toxin, beta-bungarotoxin (beta-BuTX), was capable of binding to NMDA receptors of the cultured primary neurons (cerebellar granule neurons, CGNs). We labeled beta-BuTX with fluorescent FITC (FITC-beta-BuTX) and showed that the binding of FITC-beta-BuTX was inhibited by unlabeled beta-BuTX and MK801 (an NMDA receptor antagonist). Meanwhile, the binding of [3H]-MK801 was also reduced by unlabeled MK801 and beta-BuTX. In addition, beta-BuTX produced a very potent neurotoxic effect on mature CGNs with the EC(50) of 3ng/ml (equivalent to 144pM), but was less effective in immature CGNs. We explored the signaling pathway of neuronal death and found that it was apparently due to the excessive production of reactive oxygen species (ROS) induced by beta-BuTX. MK801 and antioxidants (Vitamin C, N-acetylcysteine (NAC), melatonin, epigallocatechin gallate (EGCG), superoxide dismutase (SOD) and catalase) attenuated not only ROS production but also beta-BuTX-neurotoxicity. The downstream signaling of ROS was identified as the activation of caspase-3. Caspase inhibitor (z-DEVD-fmk) and antioxidants depressed both caspase-3 activation and neurotoxicity. Based on these findings and our previous reports, we conclude that the binding and activation of NMDA receptors by beta-BuTX was crucial step to produce the potent neurotoxic effect. The binding of NMDA receptors resulted in excessive Ca(2+) influx, followed by ROS production and activation of caspase-3. This snake toxin is considered not only to be a useful tool for exploring the death-signaling pathway of neurotoxicity, but also provides a model for searching neuroprotective agents.

Neuronal death signaling by beta-bungarotoxin through the activation of the N-methyl-D-aspartate (NMDA) receptor and L-type calcium channel

The aim of this study was to elucidate the mechanism of the neurotoxic effect of beta-bungarotoxin (beta-BuTX, a snake presynaptic neurotoxin isolated from the venom of Bungarus multicinctus) on cultured cerebellar granule neurons. beta-BuTX exerted a potent, time-dependent, neurotoxic effect on mature granule neurons. Mature neurons, with an abundance of neurite outgrowths, were obtained after 7-8 days in culture. By means of microspectrofluorimetry and fura-2, we measured the intracellular Ca(2+) concentration ([Ca(2+)](i)) and found it to be increased markedly. BAPTA-AM [1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tertrakis(acetoxymethyl ester)], EGTA, MK801 (dizocilpine maleate), and diltiazem prevented not only the elevation of [Ca(2+)](i), but also the beta-BuTX-induced neurotoxic effect. The signaling pathway involved in the elevation of [Ca(2+)](i) in beta-BuTX-induced neurotoxicity was studied. The results obtained indicated that beta-BuTX initially increased the production of reactive oxygen species and subsequently reduced mitochondrial membrane potential and depleted ATP. All of these events in the signaling pathway were blocked by MK801, diltiazem, EGTA, and BAPTA-AM. These findings suggest that the neurotoxic effect of beta-BuTX is mediated, at least in part, by a cascade of events that include the direct or indirect activation of N-methyl-D-aspartate (NMDA) receptors and L-type calcium channels that, in turn, lead to Ca(2+) influx, oxidative stress, mitochondrial dysfunction, and ATP depletion. Therefore, we suggest that this polypeptide neurotoxin, as a result of its high potency and irreversible properties, is a useful tool to elucidate the mechanisms of neurodegenerative diseases.

Lithium exposure enhances survival of NT2N cells (hNT neurons) in the hemiparkinsonian rat

Lithium (Li +) treatment of NTera2/D1 (or hNT Neurons) in culture increases tyrosine hydroxylase (TH) expression in this cell-line [Zigova et al., (1999) Exp. Neurol., 157, 251-258]. It is not known if these Li + treated cells maintain TH expression once transplanted into the striatum of the hemiparkinsonian rats. hNT neurons were either treated with 1 mm LiCl or left untreated and then transplanted into the striatum of Sprague-Dawley rats. Some cells were exposed to the lithium for 24 h in culture while others were exposed only briefly (2-3 h) just prior to transplantation. We also examined whether Li + treatment of the animal after transplantation (0.24% w/w lithium carbonate in chow) was effective in increasing neuronal survival. One week after transplantation, the animals were perfused with 4% paraformaldehyde and immunocytochemistry was performed on 30 micro m sections through the transplant. Human nuclear matrix antigen immunostaining demonstrated that there was significantly better survival of cells in the group treated briefly with lithium compared to all other groups. Brief exposure to lithium resulted in a greater expression of TH in situ as well. Neuron specific enolase immunohistochemistry showed that there was extensive fibre outgrowth in all groups. These results suggest that brief Li + exposure may enhance survival to over 60% and increase TH expression of hNT Neurons transplanted in the hemiparkinsonian rat nearly three-fold.

Mood stabilizers, glycogen synthase kinase-3beta and cell survival

Glycogen synthase kinase-3beta (GSK3beta) is a central figure in many intracellular signaling systems and is directly regulated by lithium. Substantial evidence now indicates that an important property of the mood stabilizer, lithium, is to influence GSK3beta-linked signaling pathways. This raises the possibility that other mood stabilizers act in a similar manner, which may include modulation of signaling systems leading to GSK3beta, direct regulation of GSK3beta or regulation of signaling intermediates downstream of GSK3beta. Downstream targets of GSK3beta, and thus potential targets of mood stabilizers, are several key transcription factors, including beta-catenin, AP-1, cyclic AMP-response element binding protein, NFkappaB, Myc, heat shock factor-1, nuclear factor of activated T-cells and CCAAT/enhancer-binding proteins. GSK3beta also is an important modulator of cell death, which may be a consequence of its regulatory effects on transcription factor activities. GSK3beta facilitates apoptosis, and lithium's inhibition of GSK3beta supports cell survival. Thus, signaling systems determining cell fate appear to be important targets of mood stabilizers, and these may include signaling pathways encompassing GSK3beta, including transcription factors regulated by GSK3beta.

Long-term lithium treatment prevents neurotoxic effects of beta-bungarotoxin in primary cultured neurons

Lithium is the most commonly used drug for the treatment of manic-depressive illness. The precise mechanisms underlying its clinical efficacy remain unknown. In this study, we found that long-term exposure to lithium chloride protected cultured cerebellar granule neurons (CGNs) against beta-bungarotoxin (beta-BuTX)-induced neurotoxicity. This neuroprotection was exhibited at the therapeutically relevant concentration of 1.2 mM lithium. Pretreatments for 3-5 days (long-term) were required for protection to occur; but a 3 hr treatment (short-term) was ineffective. In contrast, a longer treatment for 6-7 days or a higher concentration of 3 mM lithium led not only to loss of the neuroprotective effect but also to a neurotoxic effect. These findings suggest that lithium protection is limited to its narrow window of concentration and apparently relevant to its narrow therapeutic index in clinical application. Measurement of intracellular calcium [Ca(2+)](i) revealed that neurotoxic concentrations of beta-BuTX markedly increased [Ca(2+)](i), which could be attenuated by long-term, but not short-term, lithium treatment. Thus, the protection induced by lithium in CGNs was attributed to its inhibition of calcium overload. In addition, the Ca(2+) signaling pathway, including reactive oxygen species production and mitochondrial membrane potential reduction, along with the neurotoxic effect of beta-BuTX was blocked by long-term, but not short-term, lithium treatment. All of these results indicate that a crucial step for lithium protection is modulation of [Ca(2+)](i) homeostasis and that lithium neurotoxicity possibly, at least in part, is due to calcium overload. In conclusion, our results suggest that lithium, in addition to its use in treatment of bipolar depressive illness, may have an expanded use in intervention for neurotoxicity.

Native group-III metabotropic glutamate receptors are coupled to the mitogen-activated protein kinase/phosphatidylinositol-3-kinase pathways

We used cultured cerebellar granule cells to examine whether native group-III metabotropic glutamate (mGlu) receptors are coupled to the mitogen-activated protein kinase (MAPK) and phosphatidylinositol-3-kinase (PI-3-K) pathways. Cultured granule cells responded to the group-III mGlu receptor agonist, L-2-amino-4-phosphonobutanoate (l-AP4), with an increased phosphorylation and activity of MAPKs (ERK-1 and -2) and an increased phosphorylation of the PI-3-K target, protein kinase B (PKB/AKT). These effects were attenuated by the group-III antagonists, alpha-methyl-serine-O -phosphate (MSOP) and (R,S )-alpha-cyclopropyl-4-phosphonophenylglycine (CPPG), or by pretreatment of the cultures with pertussis toxin. l-AP4 also induced the nuclear translocation of beta-catenin, a downstream effector of the PI-3-K pathway. To assess the functional relevance of these mechanisms we examined the ability of l-AP4 to protect granule cells against apoptosis by trophic deprivation, induced by lowering extracellular K(+) from 25 to 10 mm. Neuroprotection by l-AP4 was attenuated by MSOP and abrogated by the compounds PD98059 and UO126, which inhibit the MAPK pathway, or by the compound LY294002, which inhibits the PI-3-K pathway. Taken together, these results show for the first time that native group-III mGlu receptors are coupled to MAPK and PI-3-K, and that activation of both pathways is necessary for neuroprotection mediated by this particular class of receptors.

An early increase in the disialoganglioside GD3 contributes to the development of neuronal apoptosis in culture

We induced apoptosis in primary cultures of cerebellar granule neurons by switching the growing medium into a medium containing lower concentrations of K(+) (5 or 10 mM instead of 25 mM) or, alternatively, by addition of staurosporine. The apoptotic phenotype was always preceded by an early increase in the intracellular levels of the disialoganglioside GD3, which peaked at 2-6 h and returned back to normal at 12 h. GD3 synthase, the enzyme that forms GD3 from the monosialoganglioside GM3, was also induced at early times after the induction of apoptosis in granule cells. Immunofluorescent staining showed that GD3 increased in neuronal cell bodies and neurites, but was never localized in cell nuclei. In cultures switched into a low K(+)-containing medium, exogenously applied GD3, but not the disialoganglioside GD1a, accelerated the development of neuronal apoptosis. In contrast, the antisense-induced knock-down of GD3 synthase was protective against granule cell death induced by lowering extracellular K(+) from 25 to 10 - but not 5 - mM. These results demonstrate that an early and transient increase in GD3 synthesis is one of the factors that contribute to the induction of neuronal apoptosis in culture.

Different dependence of lithium and valproate on PI3K/PKB pathway

OBJECTIVES

Acute treatment with valproate (VPA) or lithium (Li+) protects cerebellar granule cells (CGC) against apoptosis induced by low potassium (K+) (5 mM). As the protection induced by VPA is absolutely dependent on insulin, in contrast to the observed effects of Li+, we decided to study the different role of the PI3K/PKB pathway in the neuroprotective effects of both drugs.

METHODS

We have studied the neuroprotection elicited by Li+ or VPA in cultures of rat CGC. We induced the apoptosis by switching to a medium with a low concentration of K+ or by adding C2-ceramide to the cultures. We studied the effect of Li+ and VPA on viability and on the regulation of the PI3K/PKB pathway.

RESULTS AND CONCLUSIONS

Insulin also protects against low K(+)-induced apoptosis in CGC, probably through its interaction with an insulin-like growth factor receptor. Moreover, whereas Li+ protects against the apoptosis induced by C2-ceramide, VPA cannot, probably due to the inhibition of protein kinase B (PKB) caused in this apoptotic stimulus. These results suggest that VPA protects against low K(+)-induced apoptosis by acting on the PI3K/PKB pathway; however, VPA does not affect the increase of PKB activity caused by insulin in these cells. The protection by Li+ is independent of this transduction pathway. Moreover, Li+ blocks the caspase 3 activation induced by low K+, whereas neither VPA nor insulin affects this activation.

Lithium blocks the PKB and GSK3 dephosphorylation induced by ceramide through protein phosphatase-2A

The biochemical mechanism of apoptosis induced by ceramide remains still unclear, although it has been reported that dephosphorylation of PKB at Ser-473 may be a key event. In this article, we show that C(2)-ceramide (N-acetyl-sphingosine) induces the dephosphorylation of both protein kinase B (PKB) and glycogen synthase kinase-3 (GSK3) in cerebellar granule cells (CGC). We also show that lithium protects against the apoptosis induced by C(2)-ceramide by blocking the dephosphorylation of both kinases. Since lithium inhibits in vivo the observed protein phosphatase-2A (PP2A) activation induced by ceramide, we hypothesise that the neuroprotective action of lithium may be due to the inhibition of the PP2A activation by apoptotic stimuli.

Neuroprotective effects of lithium in cultured cells and animal models of diseases

Lithium, the major drug used to treat manic depressive illness, robustly protects cultured rat brain neurons from glutamate excitotoxicity mediated by N-methyl-D-aspartate (NMDA) receptors. The lithium neuroprotection against glutamate excitotoxiciy is long-lasting, requires long-term pretreatment and occurs at therapeutic concentrations of this drug. The neuroprotective mcchanisms involve inactivation of NMDA receptors, decreased expression of pro-apoptotic proteins, p53 and Bax, enhanced expression of the cytoprotective protein, Bcl-2, and activation of the cell survival kinase, Akt. In addition, lithium pretreatment suppresses glutamate-induced loss of the activities of Akt, cyclic AMP-response element binding protein (CREB), c-Jun - N-terminal kinase (JNK) and p38 kinase. Lithium also reduces brain damage in animal models of neurodegenerative diseases in which excitotoxicity has been implicated. In the rat model of stroke using middle cerebral artery occlusion, lithium markedly reduces neurologic deficits and decreases brain infarct volume even when administered after the onset of ischemia. In a rat Huntington's disease model, lithium significantly reduces brain lesions resulting from intrastriatal infusion of quinolinic acid, an excitotoxin. Our results suggest that lithium might have utility in the treatment of neurodegenerative disorders in addition to its common use for the treatment of bipolar depressive patients.

Glycogen synthase kinase-3beta, mood stabilizers, and neuroprotection

Glycogen synthase kinase-3beta (GSK-3beta) is a central component in many critical intracellular signaling mechanisms. These include the phosphatidylinositol 3-kinase/Akt cell survival pathway, which inhibits GSK-3beta activity. GSK-3beta itself inhibits the activation of several transcription factors, which are important cell survival factors, such as heat shock factor 1. These factors likely contribute to the recent revelation that GSK-3beta is a pro-apoptotic enzyme. Recently, lithium has been identified as a selective and direct inhibitor of GSK-3beta. Based on these findings, we have proposed that part of the neuroprotectant properties of lithium is due to its ability to inhibit GSK-3beta, and thus block the facilitation of apoptosis produced by GSK-3beta. Since several anticonvulsants recently have been shown to be effective mood stabilizers, we examined if these agents are capable of protecting cells from GSK-3beta-facilitated apoptosis. In addition to lithium, both valproic acid and lamotrigine, but not carbamazepine, provided protection from GSK-3beta-facilitated apoptosis in human neuroblastoma SH-SY5Y cells. These results demonstrate that several drugs therapeutic for bipolar disorder can provide neuroprotection by inhibiting the pro-apoptotic effects of GSK-3beta, providing new evidence that dysregulation of GSK-3beta may contribute to the pathophysiology of bipolar disorder.

Cerebellar granule cells as a model to study mechanisms of neuronal apoptosis or survival in vivo and in vitro

Granule cells of the cerebellum constitute the largest homogeneous neuronal population of mammalian brain. Due to their postnatal generation and the feasibility of well characterized primary in vitro cultures, cerebellar granule cells are a model of election for the study of cellular and molecular correlates of mechanisms of survival/apoptosis and neurodegeneration/neuroprotection. The present review mainly deals with recent data on mechanisms and factors promoting survival or apoptotic elimination of cerebellar granule neurons, with a particular focus on the molecular correlates at the level of gene expression and induction of cellular signal pathways. The in vivo development is first analysed with particular reference to the role played by several neurotrophic factors and by the NMDA subtype of glutamate receptor. Then, mechanisms of survival/apoptosis are examined in the model of primary in vitro cultures, where the role of neurotrophins acting on cerebellar granule cells is followed by the large deal of data coming from the paradigm of potassium/serum withdrawal. The role of some key genes of the Bcl family, of some kinase systems and of transcriptional factors is primarily highlighted. Furthermore, the involvement of mitochondria, free radicals and proteases of the caspase family is considered. Finally, the use of cerebellar granule neurons in primary culture to experimentally address the issue of neurodegeneration and pharmacological neuroprotection is considered, with some comments on models at the borderline between necrosis and apoptosis, such as the excitotoxic neuronal damage. The overlapping of cellular signal pathways activated in granule neurons by apparently unrelated stimuli, such as neurotrophins and neurotransmitters/neuromodulators is stressed to put into light the special 'trophic' role played by activity in neurons. Finally, the advantage of designing and performing conceptually equivalent experiments on cerebellar granule neurons during development in vivo and in vitro, is stressed. On the basis of the reviewed material, it is concluded that cerebellar granule neurons have acquired a special position in modern neuroscience as one of the most reliable models for the study of neural development, function and pathology.

The multifaceted roles of glycogen synthase kinase 3beta in cellular signaling

Glycogen synthase kinase-3beta (GSK3beta) is a fascinating enzyme with an astoundingly diverse number of actions in intracellular signaling systems. GSK3beta activity is regulated by serine (inhibitory) and tyrosine (stimulatory) phosphorylation, by protein complex formation, and by its intracellular localization. GSK3beta phosphorylates and thereby regulates the functions of many metabolic, signaling, and structural proteins. Notable among the signaling proteins regulated by GSK3beta are the many transcription factors, including activator protein-1, cyclic AMP response element binding protein, heat shock factor-1, nuclear factor of activated T cells, Myc, beta-catenin, CCAAT/enhancer binding protein, and NFkappaB. Lithium, the primary therapeutic agent for bipolar mood disorder, is a selective inhibitor of GSK3beta. This raises the possibility that dysregulation of GSK3beta and its inhibition by lithium may contribute to the disorder and its treatment, respectively. GSK3beta has been linked to all of the primary abnormalities associated with Alzheimer's disease. These include interactions between GSK3beta and components of the plaque-producing amyloid system, the participation of GSK3beta in phosphorylating the microtubule-binding protein tau that may contribute to the formation of neurofibrillary tangles, and interactions of GSK3beta with presenilin and other Alzheimer's disease-associated proteins. GSK3beta also regulates cell survival, as it facilitates a variety of apoptotic mechanisms, and lithium provides protection from many insults. Thus, GSK3beta has a central role regulating neuronal plasticity, gene expression, and cell survival, and may be a key component of certain psychiatric and neurodegenerative diseases.

Mechanisms of MPP(+) incorporation into cerebellar granule cells

Exposure of cerebellar granule cells to 1-methyl-4-phenylpiridinium (MPP(+)) results in cell death. We have studied the implication of various membrane transporter systems on MPP(+) neurotoxicity, including the dopamine transporter system (DAT) and cationic amino acid transporters (CAT). We have showed a partial protection against MPP(+) toxicity when the dopamine transporter is inhibited by 1-[2-[bis(4-fluorophenyl)methoxy]ethyl]4-(3-phenylpropyl)piperazinedihydrochloride (GBR-12909). However, almost full protection is only achieved by the simultaneous addition of GBR-12909 and cationic amino acids. These results suggest two ways system of MPP(+) entrance into cerebellar granule cells: the DAT with high activity and the CAT with low activity. We also demonstrated that 5,7-dichlorokynurenic acid (MK-801) failed to protect against MPP(+) exposure, evidencing that N-methyl-D-aspartate (NMDA) receptor is not involved in the MPP(+)-induced cell death.

Characterization of ceramide-induced apoptotic death in cerebellar granule cells in culture

Sphingomyelin signalling system has been involved in several examples of cell death through apoptosis. We have characterised the effect of exposure to the cell permeable ceramide analogue, C2-ceramide, on cultures of differentiated cerebellar granule cells. C(2)-ceramide was toxic to granule cells in a dose- and time-dependent way at concentrations higher than 10 microM. Ceramide exposure was accompanied by characteristic alterations of cell morphology, namely swollen cell bodies and punctuate appearance and arcuate direction of processes. The final outcome of ceramide exposure was a form of cell death largely apoptotic in nature. Hoechst stain, followed by counts of nuclei with normal appearance and size or with condensed chromatin and reduced size, revealed a large increase of the proportion of shrunken nuclei in treated cultures. In situ visualisation of fragmented DNA through the TUNEL technique, additionally marked cells undergoing apoptosis as a consequence of ceramide treatment. Accordingly, the DNA extracted from cultures exposed to C2-ceramide and subjected to agarose gel electrophoresis showed the peculiar ladder of fragmented low molecular weight DNA. Treatments with inhibitors of two caspases or of nitric oxide synthase were unable to rescue neurons exposed to ceramide, thus suggesting a neurotoxic action not primarily dependent on activation of death proteases or on nitric oxide production.

Lithium inhibits caspase 3 activation and dephosphorylation of PKB and GSK3 induced by K+ deprivation in cerebellar granule cells

Lithium protects cerebellar granule cells from apoptosis induced by low potassium, and also from other apoptotic stimuli. However, the precise mechanism by which this occurs is not understood. When cerebellar granule cells were switched to low potassium medium, the activation of caspase 3 was detected within 6 h, suggesting a role of caspase 3 in mediating apoptosis under conditions of low potassium. In the same conditions, lithium (5 mM) inhibited the activation of caspase 3 induced by low potassium. As lithium did not inhibit caspase 3 activity in vitro, these results suggest that this ion inhibits an upstream component that is required for caspase 3 activation. Lithium is known to inhibit a kinase termed glycogen sythase kinase 3 (GSK3), which is implicated in the survival pathway of phosphatidylinositol 3-kinase/protein kinase B (PI3K/PKB). Here we demonstrate that low potassium in the absence of lithium induces the dephosphorylation, and therefore the activation, of GSK3. However, when lithium was present, GSK3 remained phosphorylated at the same level as observed under conditions of high potassium. Low potassium induced the dephosphorylation and inactivation of PKB, whereas when lithium was present PKB was not dephosphorylated. Our results allow us to propose a new hypothesis about the action mechanism of lithium, this ion could inhibit a serine-threonine phosphatase induced by potassium deprivation.

C2-ceramide and reactive oxygen species inhibit pituitary adenylate cyclase activating polypeptide (PACAP)-induced cyclic-AMP-dependent signalling pathway

The pituitary adenylate cyclase activating polypeptide (PACAP) type I receptor, a seven-domain transmembrane receptor, is positively coupled to both adenylate cyclase and phospholipase C. PACAP exerts neurotrophic effects which are mainly mediated through the cAMP/protein kinase A pathway. Here we show that the cell-permeable C2-ceramide selectively blocks PACAP-activated cAMP production, without affecting phosphoinositide breakdown. Thus by blocking the neuroprotective cAMP signalling pathway, C2-ceramide will reinforce its direct death-inducing signalling. We found that a reactive oxygen species scavenger reversed the C2-ceramide effect and that H2O2 mimicked it. Together these data indicate that reactive oxygen species (ROS) mediates C2-ceramide-induced cAMP pathway uncoupling. This uncoupling did not involve ATP supply or Galphas protein function but rather adenylate cyclase function per se. Further, the tyrosine phosphatase inhibitors, but not the serine/threonine phosphatase inhibitors, prevent inhibition of cAMP production by ROS. This suggests that H2O2 requires a functional tyrosine phosphatase(s) to block PACAP-dependent cAMP production.

A novel evidence of different mechanisms of lithium and valproate neuroprotective action on human SY5Y neuroblastoma cells: caspase-3 dependency

Both lithium and valproate have been used in the treatment of manic-depressive illness with very limited understanding of their therapeutic mechanism of action. Recent literature suggests that blocking of potassium channels may be a common therapeutic mechanism of many antidepressant agents. To determine whether the commonly used antimanic agents could prevent potassium efflux-induced cell damage and apoptosis and the underlying mechanisms, we treated SH-SY5Y human neuroblastoma cells with the potassium ionophore, valinomycin (2-100 microM) and observed cell shrinkage, mitochondria damage, a significant increase in of lactate dehydrogenase (LDH) activity and caspase-3 protein expression. Cells treated with lithium (0.5-3 mM) or valproate (0.07-1.4 mM) alone produced no apoptotic morphological and biochemical changes while both mood stabilizers pretreatment reduced or prevented the apoptotic morphological changes. However, valinomycin-induced caspase-3 elevation was only prevented by lithium pretreatment while both lithium and valproate attenuated valinomycin-induced LDH release. Our results suggest that lithium and valproate share a common neuroprotective action against potassium efflux-induced cell apoptosis with different mechanisms.

Glycogen synthase kinase-3beta facilitates staurosporine- and heat shock-induced apoptosis. Protection by lithium

The potential role of glycogen synthase kinase-3beta in modulating apoptosis was examined in human SH-SY5Y neuroblastoma cells. Staurosporine treatment caused time- and concentration-dependent increases in the activities of caspase-3 and caspase-9 but not caspase-1, increased proteolysis of poly(ADP-ribose) polymerase, and induced morphological changes consistent with apoptosis. Overexpression of glycogen synthase kinase-3beta to levels 3.5 times that in control cells did not alter basal indices of apoptosis but potentiated staurosporine-induced activation of caspase-3, caspase-9, proteolysis of poly(ADP-ribose) polymerase, and morphological changes indicative of apoptosis. Inhibition of glycogen synthase kinase-3beta by lithium attenuated the enhanced staurosporine-induced activation of caspase-3 in cells overexpressing glycogen synthase kinase-3beta. In cells subjected to heat shock, caspase-3 activity was more than three times greater in glycogen synthase kinase-3beta-transfected than control cells, and this potentiated response was inhibited by lithium treatment. Thus, glycogen synthase kinase-3beta facilitated apoptosis induced by two experimental paradigms. These findings indicate that glycogen synthase kinase-3beta may contribute to pro-apoptotic-signaling activity, that inhibition of glycogen synthase kinase-3beta can contribute to anti-apoptotic-signaling mechanisms, and that the neuroprotective actions of lithium may be due in part to its inhibitory modulation of glycogen synthase kinase-3beta.

Different mechanisms of protection against apoptosis by valproate and Li+

Acute treatment with valproate and Li+ was found to protect cultured cerebellar granule cells against apoptosis induced by low K+ (5 mM). Because the protection was unaffected by MK801 (N-methyl-D-aspartate receptor inhibitor), an increase in glutamate release cannot be responsible for the observed neuroprotection. Insulin also protects against low-K+-induced apoptosis of cerebellar granule cells. This protection is totally dependent on LY294002 (a phosphatidylinositol 3-kinase inhibitor). These results suggest a role for phosphatidylinositol 3-kinase in the neuroprotection induced by insulin. Likewise, and in contrast with the results observed with Li+, the protection induced by valproate is also dependent on insulin and LY294002. Moreover, valproate (a branched-chain fatty acid) does not change the plasma membrane microviscosity under physiological conditions. These results suggest that valproate protects against low-K+-induced apoptosis by acting in the phosphatidylinositol 3-kinase/protein kinase B pathway. The protection by Li+ is independent of this transduction pathway.