Caspase-3 expression by cerebellar granule neurons is regulated by calcium and cyclic AMP

Caspase-3 Activation Correlates With the Initial Mitochondrial Membrane Depolarization in Neonatal Cerebellar Granule Neurons

In this study we evaluated the effect of the reduction in the endoplasmic reticulum calcium concentration ([Ca²⁺]_ER), changes in the cytoplasmic calcium concentration ([Ca²⁺]_i), alteration of the mitochondrial membrane potential, and the ER stress in the activation of caspase-3 in neonatal cerebellar granule cells (CGN). The cells were loaded with Fura-2 to detect changes in the [Ca²⁺]_i and with Mag-fluo-4 to measure variations in the [Ca²⁺]_ER or with TMRE to follow modifications in the mitochondrial membrane potential in response to five different inducers of CGN cell death. These inducers were staurosporine, thapsigargin, tunicamycin, nifedipine and plasma membrane repolarization by switching culture medium from 25 mM KCl (K25) to 5 mM KCl (K5). Additionally, different markers of ER stress were determined and all these parameters were correlated with the activation of caspase-3. The different inducers of cell death in CGN resulted in three different levels of activation of caspase-3. The highest caspase-3 activity occurred in response to K5. At the same time, staurosporine, nifedipine, and tunicamycin elicited an intermediate activation of caspase-3. Importantly, thapsigargin did not activate caspase-3 at any time. Both K5 and nifedipine rapidly decreased the [Ca²⁺]_i, but only K5 immediately reduced the [Ca²⁺]_ER and the mitochondrial membrane potential. Staurosporine and tunicamycin increased the [Ca²⁺]_i and they decreased both the [Ca²⁺]_ER and mitochondrial membrane potential, but at a much lower rate than K5. Thapsigargin strongly increased the [Ca²⁺]_i, but it took 10 min to observe any decrease in the mitochondrial membrane potential. Three cell death inducers -K5, staurosporine, and thapsigargin- elicited ER stress, but they took 30 min to have any effect. Thapsigargin, as expected, displayed the highest efficacy activating PERK. Moreover, a specific PERK inhibitor did not have any impact on cell death triggered by these cell death inducers. Our data suggest that voltage-gated Ca²⁺ channels, that are not dihydropyridine-sensitive, load the ER with Ca²⁺ and this Ca²⁺ flux plays a critical role in keeping the mitochondrial membrane potential polarized. A rapid decrease in the [Ca²⁺]_ER resulted in rapid mitochondrial membrane depolarization and strong activation of caspase-3 without the intervention of the ER stress in CGN.

Apoptotic cell death regulation in neurons

Apoptosis plays a major role in shaping the developing nervous system during embryogenesis as neuronal precursors differentiate to become post-mitotic neurons. However, once neurons are incorporated into functional circuits and become mature, they greatly restrict their capacity to die via apoptosis, thus allowing the mature nervous system to persist in a healthy and functional state throughout life. This robust restriction of the apoptotic pathway during neuronal differentiation and maturation is defined by multiple unique mechanisms that function to more precisely control and restrict the intrinsic apoptotic pathway. However, while these mechanisms are necessary for neuronal survival, mature neurons are still capable of activating the apoptotic pathway in certain pathological contexts. In this review, we highlight key mechanisms governing the survival of post-mitotic neurons, while also detailing the physiological and pathological contexts in which neurons are capable of overcoming this high apoptotic threshold.

The steroid hormone 20-hydroxyecdysone upregulates calcium release-activated calcium channel modulator 1 expression to induce apoptosis in the midgut of Helicoverpa armigera

Animal steroid hormones stimulate extracellular Ca²⁺ influx into cells; however, the mechanism remains unclear. In this study, we determined that the Ca²⁺ influx induced by steroid hormone 20-hydroxyecdysone (20E) is mediated by the calcium release-activated calcium channel modulator 1 (CRACM1/Orai1). The Orai1 mRNA is highly expressed during midgut programmed cell death in the lepidopteran insect Helicoverpa armigera. 20E upregulated the expression of Orai1 in H. armigera larvae and in an epidermal cell line (HaEpi). Knockdown of Orai1 in HaEpi cells blocked 20E-induced Ca²⁺ influx, and the inhibitor of inositol 1, 4, 5-trisphosphate receptor (IP₃R) Xestospongin (XeC) blocked 20E-induced Ca²⁺ influx, suggesting that 20E, via Orai1, induces stored-operated Ca²⁺ influx. Orai1 interacts with stromal interaction molecule 1(Stim1) to exert its function in 20E-induced Ca²⁺ influx. 20E promotes Orai1 aggregation through G-protein-coupled receptors, phospholipase C gamma 1, and Stim1. Knockdown of Orai1 in the HaEpi cell line repressed apoptosis and maintained autophagy under 20E regulation. Knockdown of Orai1 in larvae delayed pupation, repressed midgut apoptosis, maintained the midgut in an autophagic state, and repressed 20E-pathway gene expression. These results revealed that steroid hormone 20E, via Orai1, induces Ca²⁺ influx to promote the transition of midgut from autophagy to apoptosis.

Reactive Oxygen Species Evoked by Potassium Deprivation and Staurosporine Inactivate Akt and Induce the Expression of TXNIP in Cerebellar Granule Neurons

The reactive oxygen species (ROS) play a critical role in neuronal apoptosis; however, the mechanisms are not well understood. It has been shown that thioredoxin-interacting protein (TXNIP) overexpression renders cells more susceptible to oxidative stress and promotes apoptosis and that the activation of PI3K/Akt pathway leads to a downregulation of TXNIP. Here, we evaluated the role of ROS in the regulation of Akt activity and the subsequent regulation of the TXNIP expression in a model of apoptotic death of cerebellar granule neurons (CGN). We observed that two apoptotic conditions that generate ROS at short times led to an increase in the expression of TXNIP in a time-dependent manner; antioxidants significantly reduced this expression. Also, H₂O₂ caused an increase in TXNIP expression. Moreover, apoptotic conditions induced inactivation of Akt in a time-dependent manner similar to TXNIP expression and H₂O₂ treatment led to Akt inactivation. Besides, the pharmacological inhibition of Akt increases TXNIP expression and induces CGN cell death. Together, these results suggest that ROS promote neuronal apoptosis through the Akt-TXNIP signaling pathway, supporting the idea that the PI3K/Akt pathway regulates the TXNIP expression. This study highlights the potential importance of this mechanism in neuronal death.

Peroxynitrite is Involved in the Apoptotic Death of Cultured Cerebellar Granule Neurons Induced by Staurosporine, but not by Potassium Deprivation

Nitric oxide (NO) regulates numerous physiological process and is the main source of reactive nitrogen species (RNS). NO promotes cell survival, but it also induces apoptotic death having been involved in the pathogenesis of several neurodegenerative diseases. NO and superoxide anion react to form peroxynitrite, which accounts for most of the deleterious effects of NO. The mechanisms by which these molecules regulate the apoptotic process are not well understood. In this study, we evaluated the role of NO and peroxynitrite in the apoptotic death of cultured cerebellar granule neurons (CGN), which are known to experience apoptosis by staurosporine (St) or potassium deprivation (K5). We found that CGN treated with the peroxynitrite catalyst, FeTTPs were completely rescued from St-induced death, but not from K5-induced death. On the other hand, the inhibition of the inducible nitric oxide synthase partially protected cell viability in CGN treated with K5, but not with St, while the inhibitor L-NAME further reduced the cell viability in St, but it did not affect K5. Finally, an inhibitor of the soluble guanylate cyclase (sGC) diminished the cell viability in K5, but not in St. Altogether, these results shows that NO promotes cell survival in K5 through sGC-cGMP and promotes cell death by other mechanisms, while in St NO promotes cell survival independently of cGMP and peroxynitrite results critical for St-induced death. Our results suggest that RNS are differentially handled by CGN during cell death depending on the death-inducing conditions.

Drug Development in Conformational Diseases: A Novel Family of Chemical Chaperones that Bind and Stabilise Several Polymorphic Amyloid Structures

The increasing prevalence of conformational diseases, including Alzheimer's disease, type 2 Diabetes Mellitus and Cancer, poses a global challenge at many different levels. It has devastating effects on the sufferers as well as a tremendous economic impact on families and the health system. In this work, we apply a cross-functional approach that combines ideas, concepts and technologies from several disciplines in order to study, in silico and in vitro, the role of a novel chemical chaperones family (NCHCHF) in processes of protein aggregation in conformational diseases. Given that Serum Albumin (SA) is the most abundant protein in the blood of mammals, and Bovine Serum Albumin (BSA) is an off-the-shelf protein available in most labs around the world, we compared the ligandability of BSA:NCHCHF with the interaction sites in the Human Islet Amyloid Polypeptide (hIAPP):NCHCHF, and in the amyloid pharmacophore fragments (Aβ17-42 and Aβ16-21):NCHCHF. We posit that the merging of this interaction sites is a meta-structure of pharmacophore which allows the development of chaperones that can prevent protein aggregation at various states from: stabilizing the native state to destabilizing oligomeric state and protofilament. Furthermore to stabilize fibrillar structures, thus decreasing the amount of toxic oligomers in solution, as is the case with the NCHCHF. The paper demonstrates how a set of NCHCHF can be used for studying and potentially treating the various physiopathological stages of a conformational disease. For instance, when dealing with an acute phase of cytotoxicity, what is needed is the recruitment of cytotoxic oligomers, thus chaperone F, which accelerates fiber formation, would be very useful; whereas in a chronic stage it is better to have chaperones A, B, C, and D, which stabilize the native and fibril structures halting self-catalysis and the creation of cytotoxic oligomers as a consequence of fiber formation. Furthermore, all the chaperones are able to protect and recondition the cerebellar granule cells (CGC) from the cytotoxicity produced by the hIAPP20-29 fragment or by a low potassium medium, regardless of their capacity for accelerating or inhibiting in vitro formation of fibers. In vivo animal experiments are required to study the impact of chemical chaperones in cognitive and metabolic syndromes.

Endogenous XIAP, but not other members of the inhibitory apoptosis protein family modulates cerebellar granule neurons survival

Programmed cell death plays a critical role during cerebellar development. In particular, it has been shown in vivo and in vitro that developing cerebellar granule neurons (CGN) die apoptotically. Apoptosis involves a series of morphological changes and the activation of caspases. Inhibitor of apoptosis proteins (IAPs) is implicated in negative regulation of caspase activation and apoptotic cell death. Although apoptotic death of CGN has been extensively studied, there is no information about the role of IAPs in the developing cerebellum. Here, we studied the participation of some members of IAPs in the survival of the developing rat CGN in culture and under physiological conditions. Under these conditions, we found a differential expression pattern of cIAP-1, cIAP-2, XIAP and survivin during cerebellar development in an age-dependent manner, highlighting the significant increase of XIAP levels. We also detected an interaction between XIAP and caspase 3 at postnatal day (P) 12 and 16. On the other hand, we found a significant decrease of XIAP levels in cultured CGN maintained in chronic potassium deprivation, an apoptotic condition, suggesting a possible relationship between XIAP levels and neuronal viability. Under these conditions, we also detected the interaction of XIAP with active caspase-3. The down-regulation of XIAP in CGN cultured under survival conditions (chronic potassium depolarization) induced a reduction of cell viability and an increment of apoptotic cells. These findings support the idea that IAPs could be involved in the survival of CGN and that XIAP might be critical for neuronal survival in cerebellar development and during chronic depolarization in cultured CGN through a mechanism involving caspase inhibition.

Substance P activates ADAM9 mRNA expression and induces α-secretase-mediated amyloid precursor protein cleavage

Altered levels of Substance P (SP), a neuropeptide endowed with neuroprotective and anti-apoptotic properties, were found in brain areas and spinal fluid of Alzheimer's disease (AD) patients. One of the hallmarks of AD is the abnormal extracellular deposition of neurotoxic beta amyloid (Aβ) peptides, derived from the proteolytic processing of amyloid precursor protein (APP). In the present study, we confirmed, the neurotrophic action of SP in cultured rat cerebellar granule cells (CGCs) and investigated its effects on APP metabolism. Incubation with low (5 mM) potassium induced apoptotic cell death of CGCs and amyloidogenic processing of APP, whereas treatment with SP (200 nM) reverted these effects via NK1 receptors. The non-amyloidogenic effect of SP consisted of reduction of Aβ(1-42), increase of sAPPα and enhanced α-secretase activity, without a significant change in steady-state levels of cellular APP. The intracellular mechanisms whereby SP alters APP metabolism were further investigated by measuring mRNA and/or steady-state protein levels of key enzymes involved with α-, β- and γ-secretase activity. Among them, Adam9, both at the mRNA and protein level, was the only enzyme to be significantly down-regulated following the induction of apoptosis (K5) and up-regulated after SP treatment. In addition to its neuroprotective properties, this study shows that SP is able to stimulate non-amyloidogenic APP processing, thereby reducing the possibility of generation of toxic Aβ peptides in brain.

Caspase-3 is a target gene of c-Jun:ATF2 heterodimers during apoptosis induced by activity deprivation in cerebellar granule neurons

Caspase-3, a key executor of neuronal apoptosis, is up-regulated and activated during apoptosis induced by activity deprivation in cerebellar granule neurons (CGNs). However, the transcriptional mechanism regulating caspase-3 during CGN apoptosis remains unknown. Here, we show that the caspase-3 gene is transactivated and its induction is preceded by c-Jun NH(2)-terminal kinase (JNK)/c-Jun:ATF2 pathway activation following activity deprivation in CGNs. We observed that caspase-3 induction is abolished by pharmacological inhibition of the JNK/c-Jun:ATF2 pathway. Destroying c-Jun:ATF2 heterodimers with dominant negative mutants of c-Jun and ATF2 or knockdown by small RNA interference reduced caspase-3 promoter activity and mRNA level. Furthermore, chromatin immunoprecipitation showed increased binding of c-Jun:ATF2 heterodimers to the caspase-3 promoter in response to activity deprivation in vivo. Site-directed mutagenesis of the caspase-3 promoter revealed that caspase-3 transcriptional activation depends primarily on an ATF site -233 to -225 nucleotides upstream of the start site. Taken together, these data demonstrate that caspase-3 is a target gene of c-Jun:ATF2 heterodimers during apoptosis induced by activity deprivation in CGNs.

Reactive oxygen species are related to ionic fluxes and volume decrease in apoptotic cerebellar granule neurons: role of NOX enzymes

Reactive oxygen species (ROS) are produced early during apoptosis of cerebellar granule neurons induced by low potassium (K5) and staurosporine (Sts). In addition, K5 and Sts activate NADPH oxidases (NOX). Recently, we described that K5 and Sts induce apoptotic volume decrease (AVD) at a time when ROS generation and NOX activity occur. In the present study, we evaluated the relationship between ROS generation and ionic fluxes during AVD. Here, we showed that K5- and Sts-induced AVD was inhibited by antioxidants and that direct ROS production induced AVD. Moreover, NOX inhibitors eliminated AVD induced by both K5 and Sts. Sts, but not K5, failed to induce AVD in cerebellar granule neurons from NOX2 knockout mice. These findings suggest that K5- and Sts-induced AVD is largely mediated by ROS produced by NOX. On the other hand, we also found that the blockage of ionic fluxes involved in AVD inhibited both ROS generation and NOX activity. These findings suggest that ROS generation and NOX activity are involved in ionic fluxes activation, which in turn could maintain ROS generation by activating NOX, leading to a self-amplifying cycle.

Role for apoptosis-inducing factor in the physiological death of cerebellar neurons

Apoptosis-inducing factor (AIF) is implicated in caspase-independent apoptotic-like death. AIF released from mitochondria translocates to the nucleus, where it mediates some apoptotic events such as chromatin condensation and DNA degradation. Here, the role of AIF in the neuronal death was studied under physiological conditions. When we analyzed the cellular localization of AIF during cerebellar development, we found a significant increase in the number of neurons with nuclear AIF localization in an age-dependent manner. On the other hand, cerebellar granule neurons (CGN) chronically cultured in low concentration of potassium (5 mM; K5) die with apoptotic-like characteristics after five days. In the present study we found that K5 induces a caspase-dependent apoptotic-like death of CGN as well as a late nuclear translocation of AIF. When CGN death induced by K5 was carried out in the presence of a general inhibitor of caspases, there was a slight decrement of cell death, but neurons eventually died by showing apoptotic-like features such as phosphatidylserine translocation and nuclear condensation. Besides, there was a significant increment of nuclear AIF translocation. These findings support the idea that AIF could be involved in apoptotic-like death of CGN and that it could be an alternative mechanism of neuronal death during cerebellar development.

Cooperative contributions of interferon regulatory factor 1 (IRF1) and IRF8 to interferon-γ-mediated cytotoxic effects on oligodendroglial progenitor cells

Background

Administration of exogenous interferon-γ (IFNγ) aggravates the symptoms of multiple sclerosis (MS), whereas interferon-β (IFNβ) is used for treatment of MS patients. We previously demonstrated that IFNγ induces apoptosis of oligodendroglial progenitor cells (OPCs), suggesting that IFNγ is more toxic to OPCs than IFNβ. Thus we hypothesized that a difference in expression profiles between IFNγ-inducible and IFNβ-inducible genes in OPCs would predict the genes responsible for IFNγ-mediated cytotoxic effects on OPCs. We have tested this hypothesis particularly focusing on the interferon regulatory factors (IRFs) well-known transcription factors up-regulated by IFNs.

Methods

Highly pure primary rat OPC cultures were treated with IFNγ and IFNβ. Cell death and proliferation were assessed by MTT reduction, caspse-3-like proteinase activity, Annexin-V binding, mitochondrial membrane potential, and BrdU-incorporation. Induction of all nine IRFs was comprehensively compared by quantitative PCR between IFNγ-treated and IFNβ-treated OPCs. IRFs more strongly induced by IFNγ than by IFNβ were selected, and tested for their ability to induce OPC apoptosis by overexpression and by inhibition by dominant-negative proteins or small interference RNA either in the presence or absence of IFNγ.

Results

Unlike IFNγ, IFNβ did not induce apoptosis of OPCs. Among nine IRFs, IRF1 and IRF8 were preferentially up-regulated by IFNγ. In contrast, IRF7 was more robustly induced by IFNβ than by IFNγ. Overexpressed IRF1 elicited apoptosis of OPCs, and a dominant negative IRF1 protein partially protected OPCs from IFNγ-induced apoptosis, indicating a substantial contribution of IRF1 to IFNγ-induced OPC apoptosis. On the other hand, overexpression of IRF8 itself had only marginal proapoptotic effects. However, overexpressed IRF8 enhanced the IFNγ-induced cytotoxicity and the proapoptotic effect of overexpressed IRF1, and down-regulation of IRF8 by siRNA partially but significantly reduced preapoptotic cells after treatment with IFNγ, suggesting that IRF8 cooperatively enhances IFNγ-induced OPC apoptosis.

Conclusions

This study has identified that IRF1 and IRF8 mediate IFNγ-signaling leading to OPC apoptosis. Therapies targeting at these transcription factors and their target genes could reduce IFNγ-induced OPC loss and thereby enhance remyelination in MS patients.

Differing in vitro survival dependency of mouse and rat NG2+ oligodendroglial progenitor cells

NG2 chondroitin sulfate proteoglycan is a surface marker of oligodendroglial progenitor cells (OPCs) in various species. In contrast to well-studied rat OPCs, however, we found that purified mouse NG2 surface positive cells (NG2(+) cells) require additional activation of cyclic AMP (cAMP) signaling for survival in a medium containing 30% B104 neuroblastoma conditioned medium supplemented with fibroblast growth factor-2 (B104CM+FGF2), whereas B104CM+FGF2 alone is sufficient for survival and selective proliferation of rat OPCs. After induction of in vitro differentiation, more than 90% of mouse NG2(+) cells became O4-positive, and a majority expressed myelin basic protein by 5 day of differentiation, which confirmed the identity of isolated mouse NG2(+) cells as OPCs. In comparison to rat OPCs, mouse OPCs in B104CM+FGF2 were less motile, and demonstrated lower basal phosphorylation levels of ERK1/2 and cAMP response element-binding protein (CREB) and a higher incidence of apoptosis mediated by the intrinsic pathway. Transient up-regulation of cAMP-CREB signaling partially inhibited apoptosis of mouse OPCs independently of the ERK pathway. This study demonstrates a difference in trophic requirements between mouse and rat OPCs, with an essential role for cAMP signaling to preserve viability of mouse OPCs.

Bone morphogenetic protein-6 promotes cerebellar granule neurons survival by activation of the MEK/ERK/CREB pathway

Bone morphogenetic proteins (BMPs) have been implicated in the generation and postnatal differentiation of cerebellar granule cells (CGCs). Here, we examined the eventual role of BMPs on the survival of these neurons. Lack of depolarization causes CGC death by apoptosis in vivo, a phenomenon that is mimicked in vitro by deprivation of high potassium in cultured CGCs. We have found that BMP-6, but not BMP-7, is able to block low potassium-mediated apoptosis in CGCs. The neuroprotective effect of BMP-6 is not accompanied by an increase of Smad translocation to the nucleus, suggesting that the canonical pathway is not involved. By contrast, activation of the MEK/ERK/CREB pathway by BMP-6 is necessary for its neuroprotective effect, which involves inhibition of caspase activity and an increase in Bcl-2 protein levels. Other pathways involved in the regulation of CGC survival, such as the c-Jun terminal kinase and the phosphatidylinositol 3-kinase (PI3K)-Akt/PKB, were not affected by BMP-6. Moreover, failure of BMP-7 to activate the MEK/ERK/CREB pathway could explain its inability to protect CGCs from low potassium-mediated apoptosis. Thus, this study demonstrates that BMP-6 acting through the noncanonical MEK/ERK/CREB pathway plays a crucial role on CGC survival.

Apoptosis and autophagy in rat cerebellar granule neuron death: Role of reactive oxygen species

Programmed cell death (PCD) has been defined as an active, controlled process in which cells participate in their own demise. Apoptosis, or type I PCD, has been widely characterized, both morphologically and biochemically. More recently, autophagy, the self-digesting mechanism involved in the removal of cytoplasmic long-lived proteins, has been involved in cell death, and type II PCD is defined as cell death occurring with autophagic features. Neurons can undergo more than one type of PCD as a backup mechanism when the traditional death pathway is inhibited or in response to a particular death-inducing stimulus. Reactive oxygen species (ROS) have been shown to be important signaling molecules in the execution of apoptosis and, more recently, in the autophagic pathway. In this work, we characterize apoptotic and autophagic cell death in rat cerebellar granule neuron (CGN) culture, a widespread model for the study of neuronal death. Potassium deprivation (K5) and staurosporine (STS) were used for death induction. We found apoptotic and autophagic features under both conditions. Caspase inhibition as well as autophagy inhibition by 3-methyl adenine decreased cell death. Moreover, CGN can undergo the alternative type of cell death when the other one is inhibited. An antioxidant or NADPH oxidase inhibitors delayed apoptosis and had no effect in autophagic features. Thus, we found that autophagy plays a role in cell death of CGN and that, when cells are treated with K5 or STS, both autophagy and ROS seem to promote apoptosis by independent mechanisms.

NOX2 mediates apoptotic death induced by staurosporine but not by potassium deprivation in cerebellar granule neurons

Neuronal apoptotic death involves the participation of reactive oxygen species (ROS), but their sources have not been completely elucidated. Previous studies have demonstrated that the ROS-producing enzyme NADPH oxidase is present in neuronal cells and that this enzyme could participate in the apoptotic neuronal death. Cerebellar granule neurons (CGN) undergo apoptosis when cells are transferred from a medium with 25 mM KCl (K25) to a 5 mM KCl (K5) medium or when they are treated with staurosporine (ST). Under these conditions, apoptotic death of CGN is dependent on ROS production. In this study, we evaluated the role of NOX2, an NADPH oxidase homolog, in the apoptotic death of CGN induced by two different conditions. In CGN from NOX2-deficient (ko) mice, a significantly lower rate of apoptotic death occurs compared with wild-type (wt) CGN. Also, caspase-3 activation, NADPH oxidase activity, and superoxide anion production induced by ST were markedly lower in ko neurons than in wt CGN. In contrast to the case with ST, when CGN were treated with K5, no differences were observed between ko and wt cells in any of the parameters measured. However, all NADPH oxidase inhibitors tested noticeably reduced cell death and apoptotic parameters induced by K5 in both wt and ko CGN. These results suggest that NOX2 could be necessary for apoptotic death induced by ST, but not by K5, which could require other member of the NOX family in the apoptotic process.

Calcium homeostasis and cone signaling are regulated by interactions between calcium stores and plasma membrane ion channels

Calcium is a messenger ion that controls all aspects of cone photoreceptor function, including synaptic release. The dynamic range of the cone output extends beyond the activation threshold for voltage-operated calcium entry, suggesting another calcium influx mechanism operates in cones hyperpolarized by light. We have used optical imaging and whole-cell voltage clamp to measure the contribution of store-operated Ca²⁺ entry (SOCE) to Ca²⁺ homeostasis and its role in regulation of neurotransmission at cone synapses. Mn²⁺ quenching of Fura-2 revealed sustained divalent cation entry in hyperpolarized cones. Ca²⁺ influx into cone inner segments was potentiated by hyperpolarization, facilitated by depletion of intracellular Ca²⁺ stores, unaffected by pharmacological manipulation of voltage-operated or cyclic nucleotide-gated Ca²⁺ channels and suppressed by lanthanides, 2-APB, MRS 1845 and SKF 96365. However, cation influx through store-operated channels crossed the threshold for activation of voltage-operated Ca²⁺ entry in a subset of cones, indicating that the operating range of inner segment signals is set by interactions between store- and voltage-operated Ca²⁺ channels. Exposure to MRS 1845 resulted in ∼40% reduction of light-evoked postsynaptic currents in photopic horizontal cells without affecting the light responses or voltage-operated Ca²⁺ currents in simultaneously recorded cones. The spatial pattern of store-operated calcium entry in cones matched immunolocalization of the store-operated sensor STIM1. These findings show that store-operated channels regulate spatial and temporal properties of Ca²⁺ homeostasis in vertebrate cones and demonstrate their role in generation of sustained excitatory signals across the first retinal synapse.

Decline in age-dependent, MK801-induced injury coincides with developmental switch in parvalbumin expression: somatosensory and motor cortex

MK801-induced activation of caspase-3 is developmentally regulated, peaking at postnatal day (P) 7 and decreasing with increasing postnatal age thereafter. Further, at P7, cells displaying activation of caspase-3 lack expression of calcium binding proteins (CaBPs). To further explore this relationship, we investigated postnatal expression of calbindin (CB), calretinin (CR) and parvalbumin (PV) in two brain regions susceptible to MK801-induced injury, the somatosensory cortex (S1) and layer II/III of motor cortex (M1/M2). Expression of CB and especially PV was low to absent prior to P7 but substantially increased from P7 through to P21 and adulthood. In contrast, CR expression was more variable at early developmental ages, stabilized to lower levels after P7 and showed a marked decline by P21. The results suggest that not only does calcium buffering capacity increase developmentally but also acquisition of enhanced buffering may be one mechanism by which neurons survive agent-induced alterations in calcium homeostasis.

A neurotoxic secretory phospholipase A2 induces apoptosis in motoneuron-like cells

Ammodytoxin A (AtxA) is a presynaptically neurotoxic secretory phospholipase A(2) from snake venom. The aim of this study was to investigate the mechanism of its cytotoxicity expressed against mouse motoneuronal NSC34 cells. AtxA displayed a potent dose- and time-dependent cytotoxicity that was associated with apoptosis and not necrosis, as revealed by a reduction of mitochondrial membrane potential, activation of caspase-3, and by the absence of propidium iodide staining. The cytotoxic- and apoptosis-inducing effects of AtxA were specific for the motoneuronal cells; human embryonic kidney (HEK293) and mouse myoblast (C2C12) cells were shown to be resistant to the toxin.

Role of NADPH oxidase in the apoptotic death of cultured cerebellar granule neurons

Cerebellar granule neurons (CGN) cultured in a medium containing 25 mM KCl and treated with staurosporine (ST) or transferred to a medium with 5 mM KCl (K5) die apoptotically. CGN death is mediated by an increase in reactive oxygen species (ROS) production. When CGN are treated with antioxidants all apoptotic parameters and cell death are markedly diminished, showing a central role for ROS in this process. Recently, it has been suggested that a possible ROS source involved in cell death is a NADPH oxidase. In that regard, we found expression in CGN of the components of NADPH proteins, p40phox, p47phox and p67phox, and p22phox, as well as three homologues of the catalytic subunit of this complex, NOX1, 2, and 4. The inhibition of NADPH oxidase with diphenylene iodonium or 4-(2-aminoethyl)benzenesulfonyl fluoride significantly reduced ROS production, NADPH oxidase activity, all the apoptotic events, and cell death induced by both K5 and ST. We conclude that ROS could be an early signal of apoptotic neuronal death and that NADPH oxidase, including NOX1, 2, and/or 4, could have a central role in apoptotic death induced by different conditions in these neurons.

The c-Jun-N-terminal kinase is involved in the neurotoxic effect of azaspiracid-1

AIMS

Azaspiracids (AZAs) are marine phycotoxins with an unknown mechanism of action, recently implicated in human intoxications. The predominant analog in nature, AZA-1 targets several organs in vivo, including the central nervous system and exhibits high neurotoxicity in vitro.

METHODS

We used pharmacological tools to inhibit the cytotoxic effect of the toxin in primary cultured neurons. Immunocytochemical techniques in combination with confocal microscopy were employed to examine the cellular mechanisms involved in the neurotoxic effect of AZA-1.

RESULTS

Several targets for azaspiracid-induced neurotoxicity, specifically the cAMP pathway, or protein kinase C and phosphatidylinositol 3-kinase activation were excluded. Interestingly, the specific c-Jun-N-terminal protein kinase (JNK) inhibitor SP 600125 protected cultured neurons against AZA-induced cytotoxicity. Immunocytochemistry experiments showed that AZA-1 increased the amount of phosphorylated JNK and caused nuclear translocation of the active protein that was prevented by SP 600125.

CONCLUSION

Our data constitute the relationship between azaspiracid-induced cytotoxicity and specific modifications in cellular transduction signals, specifically we found that JNK activation is associated with the cytotoxic effect of the toxin. These results should provide the basis to identify the mechanism of action of this group of toxins.

Novel action of epidermal growth factor on caspase 3 and its potential as a chemotherapeutic adjunct for neuroblastoma

PURPOSE

We previously reported that epidermal growth factor (EGF) induced cleaved caspase 3 expression and apoptosis in human neuroblastoma cells. We hypothesized that EGF upregulates total caspase 3 expression, thereby enhancing the cytotoxic potency of chemotherapeutic agents.

METHODS

Wild-type (WT) and doxorubicin-resistant (Dox-R) SK-N-SH neuroblastoma cells were incubated with EGF 0 to 250 ng/mL for 24 hours or with 5 ng/mL for 0 to 5 days, and total caspase 3 expression and transcription were determined by immunoblots and reverse transcription and polymerase chain reaction, respectively. Cell proliferation was determined after EGF (5 ng/mL) incubation for 1 to 5 days. Cells incubated with EGF (5 ng/mL) for 24 hours before doxorubicin treatment were used to determine the effect of EGF on cell replication and cleaved caspase 3 expression.

RESULTS

Wild-type and Dox-R cells had maximal total caspase 3 expression after incubation with EGF (5 ng/mL) for 3 and 5 days, respectively, and a corresponding increase in DNA transcription. Treating the cells with or without EGF (5 ng/mL) resulted in similar replication rate. However, cell death was increased by EGF pretreatment and doxorubicin when compared with that by doxorubicin alone (WT, 53% +/- 8%; Dox-R, 44% +/- 9%; P < .05). Cleaved caspase 3 expression was upregulated when cell death was increased.

CONCLUSION

Epidermal growth factor upregulates the expression and transcription of total caspase 3 in WT and Dox-R cells in a concentration- and time-dependent manner. Epidermal growth factor pretreatment augments the cytotoxic effect of doxorubicin.

Role of brain-derived neurotrophic factor in the protective action of N-methyl-D-aspartate in the apoptotic death of cerebellar granule neurons induced by low potassium

Several neurotrophic factors, including brain-derived neurotrophic factor (BDNF), and neurotransmitters, such as glutamate, may influence neuronal apoptotic death. Rat cerebellar granule neurons (CGN) cultured in low potassium (5 or 10 mM KCl) for more than 5 days in vitro (DIV) die apoptotically. These cells survive in the presence of high potassium (25 mM KCl, K25) or N-methyl-D-aspartate (NMDA), an agonist of glutamatergic receptors. CGN transferred from high to low potassium die apoptotically. Here, we characterized the effect of BDNF and NMDA on the apoptotic death induced by low potassium in CGN. Cell death of CGN by culturing in low potassium for 6 DIV was inhibited by BDNF and NMDA. When CGN were cultured in K25 and transferred to a low-potassium medium, 65% of neurons died after 48 hr. Under these conditions, BDNF, NMDA, or BDNF + NMDA increased CGN survival. Both BDNF and NMDA decreased caspase-9 activity and mRNA caspase-3 levels and activity induced by low potassium. CGN survival induced by BDNF is mediated by TrkB activation, whereas that induced by NMDA is mediated by NMDA receptor and TrkB activation. NMDA, but not BDNF, raised [Ca(2+)](i), which was reduced by low-potassium treatment. These results suggest that NMDA receptor stimulation induces CGN survival through the influx of extracellular Ca(2+) that may evoke the release of BDNF and the activation of TrkB. Complementary mechanisms induced by depolarization and changes in Ca(2+) levels would also contribute to the neuroprotection exerted by NMDA and potassium.

cAMP/protein kinase A signalling pathway protects against neuronal apoptosis and is associated with modulation of Kv2.1 in cerebellar granule cells

Previously, we have reported that apoptosis of cerebellar granular neurons induced by incubation in 5 mm K(+) and serum-free medium (LK-S) was associated with an increase in the delayed rectifier K(+) current (I(K)). Here, we show that I(K) associated with apoptotic neurons is mainly encoded by a Kv2.1 subunit. Silencing Kv2.1 expression by small interfering RNA reduces I(K) and increases neuron viability. Forskolin is able to decrease the I(K) amplitude recording from neurons of both the LK-S and control group, and prevents apoptosis of granule cells that are induced by LK-S. Dibutyryl cAMP mimicks the effect of forskolin on the modulation of I(K) and, accordingly, the inhibitor of protein kinase A, H-89, aborts the neuron-protective effect induced by forskolin. Whereas the expression of Kv2.1 was silenced by Kv2.1 small interfering RNA, the inhibition of forskolin on the current amplitude was significantly reduced. Quantitative RT-PCR and whole-cell recording revealed that the expression of Kv2.1 was elevated in the apoptotic neurons, and forskolin significantly depressed the expression of Kv2.1. We conclude that the protection against apoptosis via the protein kinase A pathway is associated with a double modulation on I(K) channel properties and its expression of alpha-subunit that is mainly encoded by the Kv2.1 gene.

Loss of calcium and increased apoptosis within the same neuron

Loss of neuronal calcium is associated with later apoptotic injury but observing reduced calcium and increased apoptosis in the same cell would provide more definitive proof of this apparent correlation. Thus, following exposure to vehicle or the calcium chelator, BAPTA (1-20 microM), primary cortical neurons were labeled with Calcium Green-1 which was then cross-linked with EDAC, prior to immuno-staining for various proteins. We found that BAPTA-induced changes in calcium were highly correlated with changes in expression of activated caspase-3 as well as the calcium binding proteins calbindin, calretinin, and parvalbumin. Additionally, in brain slices from P7 neonatal rats, BAPTA induced significant loss of calcium in a brain region we have previously shown to express only moderate levels of calcium binding proteins as well as display robust apoptosis following calcium entry blockade. In contrast, BAPTA had little influence on calcium levels in a brain region we have previously shown to express robust calcium binding proteins as well as display far less apoptosis following calcium entry blockade. These data suggest that the ability of developing neurons to buffer changes in calcium may be critical to their long-term survival.

Fragment 31–35 of β-amyloid peptide induces neurodegeneration in rat cerebellar granule cells via bax gene expression and caspase-3 activation

The amyloid beta-peptide (AbetaP) is the major protein component of brain senile plaques in Alzheimer's disease. The redox state of methionine-35 residue plays a critical role in peptide neurotoxic actions. We used the fragment 31-35 of AbetaP [AbetaP(31-35)], containing a single methionine-35 residue (Met-35), to investigate the relationship between the oxidative state of Met-35 and neurotoxic and pro-apoptotic actions induced by the peptide; in rat cerebellar granule cells (CGC), we compared the effects of AbetaP(31-35), in which the Met-35 is present in the reduced state, with those of a modified peptide with oxidized Met-35 [AbetaP(31-35)Met-35(OX)](,) as well as an AbetaP-derivative with Met-35 substituted by norleucine [AbetaP(31-35)Nle-35]. AbetaP(31-35) induced a time-dependent decrease in cell viability. AbetaP(31-35)Met-35(OX) was significantly less potent, but still induced a significant decrease in cell viability compared to control. No toxic effects were observed after treatment with AbetaP(31-35)Nle-35. AbetaP(31-35) induced a 2-fold increase in bax mRNA levels after 4h, whereas AbetaP(31-35)Met-35(OX) raised bax mRNA levels by 41% and AbetaP(31-35)Nle-35 had no effect. Finally, AbetaP(31-35) caused a 43% increase in caspase-3 activity after 24h; AbetaP(31-35)Met-35(OX) caused only a 18% increase, and AbetaP(31-35)Nle-35 had no effect. These findings suggest that AbetaP(31-35)-induced neurodegeneration in CGC is mediated by a selective early increase in bax mRNA levels followed by delayed caspase-3 activation; the redox state of the single Met-35 residue is crucial in the occurrence and extent of the above phenomena.

Developmental changes in Ca2+-regulated functions of early postnatal Purkinje neurons

Ca(2+) influx through L-type Ca(2+) channels regulates several different cellular processes in developing Purkinje neurons, including activation of transcription factors and expression of cellular proteins. In the current studies, we examined the age dependence of these actions of Ca(2+) during the early developmental period. Purkinje neurons acutely isolated from postnatal day 4-8 rat pups were studied. We also examined the sensitivity of the Ca(2+)-regulated processes to a toxic environmental factor (ethanol) known to show age-dependent actions on developing Purkinje neurons. Results show that Ca(2+) activation of the transcription factor cAMP-responsive element binding protein (CREB) and Ca(2+)-induced alterations in the level of the apoptotic enzyme caspase 3 show both dose and age dependence in the early-developing Purkinje neurons. Interestingly, the age dependence was opposite for the two proteins. Ca(2+) regulation of calbindin, a Ca(2+) binding protein, was dose dependent but showed little age dependence. Exposure to ethanol altered Ca(2+) activation of pCREB in an age-dependent manner but did not alter Ca(2+) regulation of caspase 3 or calbindin levels. Taken together, these results show that the downstream effects of Ca(2+) signaling have age-dependent components during early Purkinje neuron development. This age dependence may play an important role in the normal developmental program and could contribute to the critical window of sensitivity observed for certain toxic agents during early development.

Widespread Neonatal Brain Damage following Calcium Channel Blockade

An abundance of evidence exists that shows calcium channel blockade promotes injury in cultured neurons. However, few studies have addressed the in vivo toxicity of such agents. We now show that the L-type calcium channel antagonist nimodipine promotes widespread and robust injury throughout the neonatal rat brain, in an age-dependent manner. Using both isolated neuronal as well as brain slice approaches, we address mechanisms behind such injury. These expanded studies show a consistent pattern of injury using a variety of agents that lower intracellular calcium. Collectively, these observations indicate that postnatal brain development represents a transitional period for still developing neurons, from being highly sensitive to reductions in intracellular calcium to being less vulnerable to such changes. These observations directly relate to current therapeutic strategies targeting neonatal brain injury.

Effect of N-methyl-d-aspartate receptor blockade on caspase activation and neuronal death in the developing rat cerebellum

In vitro studies have demonstrated that N-methyl-D-aspartate (NMDA) receptor activation rescue cerebellar granule neurons (CGN) from apoptotic death. It has been suggested that this effect mimics the transient glutamate receptors activation by mossy fibers during cerebellar development. We reported previously that CGN from postnatal days 2-4 (P2-P4) rats increased cell survival in response to NMDA treatment. In this study, we evaluated the effect of dizocilpine (MK-801) administrated for three consecutive days on the apoptotic death of CGN during development. MK-801 treatment decreased the large number of CGN condensed nuclei found at P8, but this drug increased the proportion of condensed nuclei at P16. We also found a high activity of caspases during the first postnatal week that decreased during development. MK-801 treatment did not modify the activity of caspase-8 at any age, but decreased caspase-9 activity at P8 and increased the activity of caspase-1 (76%) at P8, caspase-3 (160%) at P16 and caspase-9 (50%) at P12. These results suggest that NMDA receptor stimulation regulates the activity of caspases in a differential way and plays an important role in the in vivo CGN death during postnatal development.

TRPC1 protects human SH-SY5Y cells against salsolinol-induced cytotoxicity by inhibiting apoptosis

Salsolinol, an endogenous neurotoxin, may be involved in the pathogenesis of Parkinson's disease. In this study, we sought to determine whether salsolinol-induced cytotoxicity in SH-SY5Y human neuroblastoma cells, a cloned cell line which expresses dopaminergic activity, could be prevented by overexpressing a Ca(2+) channel, transient receptor potential (TRPC1) protein. Exposure of SH-SY5Y cells to 500 microM salsolinol for 12 h resulted in a significant decrease in thapsigargin or carbachol-mediated Ca(2+) influx. Consistent with these results, SH-SY5Y cells treated with salsolinol showed approximately 60% reduction in TRPC1 protein levels. Confocal microscopy also showed that SH-SY5Y cells treated with salsolinol had a significant decrease in the plasma membrane staining of the TRPC1 protein. Interestingly, overexpression of TRPC1 increases TRPC1 protein levels and also protected SH-SY5Y neuroblastoma cells against salsolinol-mediated cytotoxicity as determined by 3,[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT) assay. The protective effect of TRPC1 was blocked by the addition of TRPC1 blockers lanthanum, or 2APB. Activation of TRPC1 protein by either thapsigargin or carbachol further protected SH-SY5Y cells from salsolinol treatments. Staining of SH-SY5Y cells with an apoptotic marker (YO-PRO-1) showed that TRPC1 protein protects against apoptosis. Furthermore, TRPC1 overexpression also inhibited cytochrome c release and decreased BAX protein levels required for apoptosis. Taken together, these findings suggest that the reduction in cell surface TRPC1 protein expression in response to salsolinol may be a contributory factor in cellular toxicity of the dopaminergic neurons. Furthermore, overexpression of TRPC1 could inhibit apoptotic complex thereby increasing neuronal cell survivability in Parkinson's disease.

Activation of the intrinsic cell death pathway, increased apoptosis and modulation of astrocytes in the cerebellum of diabetic rats

Poorly controlled diabetes mellitus results in structural and functional changes in many brain regions. We demonstrate that in streptozotocin-induced diabetic rats cell death is increased and proliferation decreased in the cerebellum, indicating overall cell loss. Levels of both the proform and cleaved forms of caspases 3, 6 and 9 are increased, with no change in caspases 7, 8 or 12. Colocalization of glial fibrillary acidic protein (GFAP) and cleaved caspase 3 and GFAP in TUNEL-positive cells increased in diabetic rats. Changes in GFAP levels paralleled modifications in proliferating cell nuclear antigen (PCNA), increasing at 1 week of diabetes and decreasing thereafter, and proliferating GFAP-positive cells were decreased in the cerebellum of diabetic rats. These results suggest that astrocytes are dramatically affected in the cerebellum, including an increase in cell death and a decrease in proliferation, and this could play a role in the structural and functional changes in this brain area in diabetes.

MEK-ERK signaling is involved in interferon-gamma-induced death of oligodendroglial progenitor cells

Oligodendrocytes are exposed to various cytokines in inflammatory lesions in the central nervous system. In this study, we focused on the direct effects of interferon-gamma (IFNG) on highly purified rat oligodendroglial cultures at different developmental stages. Among the three stages tested, IFNG had direct cytotoxic effects on actively proliferating oligodendrocyte progenitors but much less on immature oligodendrocytes and none on mature oligodendrocytes. This stage-specific susceptibility of progenitors to IFNG-induced cytotoxicity consisted of two components, delay in the G(1)/S transition of the cell cycle and increased cell death at least partly mediated by apoptosis, suggesting that progression of the cell cycle was tightly linked to this toxic mechanism. There was no functional difference in the signal transducers and activators of transcription (STAT) pathways between progenitors and mature oligodendrocytes as determined by induction of IRF1 mRNA in response to IFNG. We found that partial inhibition of the MEK-ERK pathway, one of the mitogen-activated protein kinase phosphorelay modules, by U0126 partially reversed the IFNG-induced cytotoxicity in progenitors. In addition, ERK activity was quickly down-regulated after in vitro differentiation of progenitors to immature oligodendrocytes. Therefore, we concluded that simultaneous activation of the STAT pathway by IFNG and of the ERK pathway by exogenous trophic factors played a role in the stage-specific IFNG-induced cytotoxicity in oligodendroglial progenitors. Our study has implications with respect to the mechanisms of periventricular leukomalacia in infants and of persistent demyelination in multiple sclerosis lesions in adults.

Effect of NMDA antagonists on the death of cerebellar granule neurons at different ages

Cerebellar granule neurons (CGN) are the most abundant neuronal type in the cerebellum. During development, these cells migrate from the external to the internal granule layer (IGL), where they receive excitatory glutamatergic and cholinergic contacts from mossy fibers. During this period of development a large proportion of CGN are eliminated via apoptosis. In vitro studies have demonstrated that when CGN are obtained from rats at postnatal day 8 (P8), the sustained activation of N-methyl-D-aspartate (NMDA) receptor at 2-4 days in vitro rescues neurons from cell death. The NMDA action on cultured CGN could mimic the in vivo actions of the transient activation of the glutamate receptors by the transmitter released by mossy fibers by P12. However, some results suggest that glutamate stimulation could be relevant for CGN at earlier stages of development. In this study we evaluated the effect of NMDA receptor stimulation or blockade on the cell death of both in vivo and cultured CGN obtained from P2 to P8 rats. Our results showed that the blockade of NMDA receptors with the antagonists D,L-2-amino-5-phosphonovaleric acid or dizocilpine (MK-801) reduces cell survival to 20-40%, whereas NMDA treatment increases neuronal survival by approximately 50-60%. In vivo, the treatment with MK-801 reduced the number of apoptotic CGN in the molecular layer (ML) from P5 to P8. These results suggest that NMDA receptor stimulation plays a critical role in the regulation of CGN death during the first week of rat cerebellar development.

Mk801-induced caspase-3 in the postnatal brain: Inverse relationship with calcium binding proteins

Age-dependent, neuronal apoptosis following N-methyl-D-aspartate receptor blockade has been linked to loss of calcium. To further explore this relationship, we examined expression of activated caspase-3, as well as the calcium binding proteins, calbindin-D 28K, calretinin and parvalbumin, following injection of vehicle or the N-methyl-D-aspartate receptor blocker, MK801, in postnatal day 7 or 21 rats. At postnatal day 7, MK801-induced activated caspase-3 expression was most frequently found in mutually exclusive cell populations to those expressing any of the three calcium binding proteins. For example, in the somatosensory cortex, most immunoreactivity for activated caspase-3 was found in layers IV/V, layered between areas of high calbindin or calretinin expression. Further, in the caudate putamen, activated caspase-3 rarely invaded zones of intense calbindin immunoreactivity. Suggesting expression patterns of these proteins were inversely related, these same brain regions no longer displayed MK801-induced activated caspase-3 at postnatal day 21, but instead robustly expressed calcium binding proteins. This later surge in expression was especially true for parvalbumin in regions such as the somatosensory and retrosplenial cortex, as well as the subicular complex. Calbindin-D 28K was also found to increase in the same regions though not as impressively as parvalbumin. Thus, developmental regulation of calcium binding protein expression may be a critical factor in age-dependent sensitivity to agents that disrupt calcium homeostasis in maturing neurons, providing a possible mechanistic explanation for age-dependent MK801 toxicity.

Brief exposure to NMDA produces long-term protection of cerebellar granule cells from apoptosis

Cerebellar granule cells (CGCs) require excitatory inputs to survive during their postnatal migration from the external to the internal granule cell layers. The lack of innervation of mossy fibres induces CGC death by apoptosis. In vitro, CGCs die by apoptosis in the presence of physiological concentrations of KCl (5 mm or K5) but they survive in the presence of depolarizing concentrations of KCl (25 mm or K25) or N-methyl-d-aspartate (NMDA) by a mechanism dependent on calcium influx. The addition of NMDA or K25, for only 24 h, to immature CGCs cultures [2 days in vitro (DIV)] was able to produce a remarkable and long-term protection until 8 DIV. Moreover, our data show that NMDA and K25-mediated long-lasting protection was related to an inhibition of caspase-3 activity. By using different protein kinase inhibitors, we have shown that the inhibition of caspase-3 activation by NMDA was dependent on the activation of tyrosine kinases and phosphatidylinositol 3-kinase (PI3-kinase). Moreover, an impairment in NMDA-mediated neuroprotection and caspase-3 inhibition was observed when the action of brain-derived neurotrophic factor (BDNF) was blocked. By contrast, K25-mediated neuroprotection was BDNF-independent and was mediated by a mitogen-activated protein kinase- and PI3-kinase-dependent inhibition of caspase-3.

Nuclear calpain regulates Ca2+-dependent signaling via proteolysis of nuclear Ca2+/calmodulin-dependent protein kinase type IV in cultured neurons

Accumulating evidence indicates that calpains can reside in or translocate to the cell nucleus, but their functions in this compartment remain poorly understood. Dissociated cultures of cerebellar granule cells (GCs) demonstrate improved long-term survival when their growth medium is supplemented with depolarizing agents that stimulate Ca(2+) influx and activate calmodulin-dependent signaling cascades, notably 20 mm KCl. We previously observed Ca(2+)-dependent down-regulation of Ca(2+)/calmodulin-dependent protein kinase (CaMK) type IV, which was attenuated by calpain inhibitors, in GCs supplemented with 20 mm KCl (Tremper-Wells, B., Mathur, A., Beaman-Hall, C. M., and Vallano, M. L. (2002) J. Neurochem. 81, 314-324). CaMKIV is highly enriched in the nucleus and thought to be critical for improved survival. Here, we demonstrate by immunolocalization/confocal microscopy and subcellular fractionation that the regulatory and catalytic subunits of m-calpain are enriched in GC nuclei, including GCs grown in medium containing 5 mm KCl. Calpain-mediated proteolysis of CaMKIV is selective, as several other nuclear and non-nuclear calpain substrates were not degraded under chronic depolarizing culture conditions. Depolarization and Ca(2+)-dependent down-regulation of CaMKIV were associated with significant alterations in other components of the Ca(2+)-CaMKIV signaling cascade: the ratio of phosphorylated to total cAMP response element-binding protein (a downstream CaMKIV substrate) was reduced by approximately 10-fold, and the amount of CaMK kinase (an upstream activator of CaMKIV) protein and mRNA was significantly reduced. We hypothesize that calpain-mediated CaMKIV proteolysis is an autoregulatory feedback response to sustained activation of a Ca(2+)-CaMKIV signaling pathway, resulting from growth of cultures in medium containing 25 mm KCl. This study establishes nuclear m-calpain as a regulator of CaMKIV and associated signaling molecules under conditions of sustained Ca(2+) influx.

Role of ionic fluxes in the apoptotic cell death of cultured cerebellar granule neurons

Cultured cerebellar granule neurons (CGC) increase survival in a medium containing 25 mM KCl (K25), and they die apoptotically when cultures are treated with staurosporine (St) or are transferred to a 5-mM KCl containing medium (K5). Apoptotic CGC show nuclear condensation and caspase-3 activation. Cell death induced by these conditions was partially prevented when cultures were maintained under alkaline conditions, which also induced a marked reduction of the caspase-3 activation. The acidification of the medium further increased cell death induced by both stimuli. Cultures transferred to K5 suffered an immediate intracellular alkalinization that remained constant during the time K5 was present. In contrast, St did not modify cytosolic pH at any of the evaluated times. On the other hand, DIDS, furosemide, and bumetanide prevented CGC death induced by K5 and St. Other drugs such as amiloride, EIPA, tamoxifen, NEM, or NPPB did not modify cell death induced by these conditions. Both DIDS and bumetanide markedly inhibited the processing and activation of caspase-3, and DIDS prevented the nuclear condensation induced by K5 and St. These findings suggest that pH is a condition that could contribute to the modulation of cell death induced by some stimuli and that other ions, such as potassium, could have a role in the initial phase of apoptotic death of CGC.

Survival of cultured cerebellar granule neurons can be maintained by Akt-dependent and Akt-independent signaling pathways

Cerebellar granule neurons can be maintained in culture by four factors: depolarizing levels of potassium (HK, 25 mM), cyclic AMP elevating agents and analogs (cyclic adenosine monophosphate, cAMP), insulin-like growth factor-1 (IGF-1), or lithium. We investigated the possibility that the signal transduction pathways utilized by these four survival factors might converge at a common molecular target and that the serine/threonine kinase Akt might be the convergent molecule. Previous research demonstrated that the four factors could phosphorylate and activate Akt; thus, using pharmacological inhibition of Akt and overexpression of an adenoviral delivered dominant negative Akt construct, we analyzed the role of Akt in the survival mediated by each factor. We found that although Akt is required for the survival mediated by IGF-1 and lithium, it is dispensable for the survival mediated by high potassium and cAMP.

Effect of Ginkgo biloba (EGb 761) on staurosporine-induced neuronal death and caspase activity in cortical cultured neurons

Previous studies suggest the protective potentiality of Ginkgo biloba (EGb 761) against apoptotic cell death induced by hydroxyl radicals, staurosporine, serum deprivation and beta-amyloid (betaA) peptide. We have extended these observations to cultured cortical neurons and studied the effect of EGb 761 on neuronal survival (evaluated as MTT reduction), the presence of condensed nuclei (monitored as Hoechst staining), the time-course of caspase-1, caspase-3 and caspase-9 activation (measured by cleavage of specific fluorescent substrates) and superoxide anion production (evaluated by hydroethidine staining) after the exposure to staurosporine. Results show that 200 microg/ml of EGb 761 increased cell survival and reduced the number of condensed nuclei after the exposure to 200 nM staurosporine. Vitamin E and the spin trapper alpha-phenyl-N-tert-butylnitrone (PBN) also significantly increased cell survival. In contrast, the broad-spectrum caspase inhibitors ZVAD and ZBIOT showed no protection. Similarly, selective inhibitors of caspase-1 (YVAD-CHO), caspase-2 (VDVAD-CHO), caspase-3 (DEVD-CHO) and caspase-8 (IETD-CHO) did not protect against cell damage induced by staurosporine. The protective effect of EGb 761 was not enhanced when coincubated with vitamin E or DEVD-CHO. Caspase-3 activity was maximally induced 5-8 h after staurosporine exposure. Both EGb 761 and vitamin E showed a tendency to decrease caspase-3 activity. In contrast, activation of caspase-1 and caspase-9 was not observed at any of the times studied after STS exposure. Exposure to staurosporine resulted in increased superoxide production that was maximal at 5 h. EGb 761 significantly inhibited superoxide production at short times after staurosporine exposure. Vitamin E and PBN also significantly reduced superoxide production. Results suggest that EGb 761 neuroprotective effect might be mediated by its well-known antioxidant activity, which might also influence caspase-3 activation. Inhibition of capase-3 induced by EGb 761 and vitamin E does not seem to contribute to their observed protective action.