Suppression of programmed neuronal death by sustained elevation of cytoplasmic calcium

An increase in intracellular levels of cyclic AMP produces trophic effects on striatal neurons developing in culture

Cyclic AMP-dependent kinases have been suggested to constitute signal transduction pathways involved in the regulation of neuronal development and survival. The present study examined whether elevated levels of cyclic AMP exhibit trophic activities on rat striatal neurons grown under serum-free culture conditions. Treatment with dibutyryl cyclic AMP, a permeable cyclic AMP, increased GABA uptake and immunocytochemically detectable levels of proteins such as c-Fos and calbindin-D28k. Neuronal survival was promoted by dibutyryl cyclic AMP only in lower density cultures. Chronic exposure of neurons to dibutyryl cyclic AMP enhanced the morphological development of calbindin-D28k-positive neurons. Furthermore, pretreatment with dibutyryl cyclic AMP afforded neuroprotection against N-methyl-D-aspartate-induced excitotoxicity. The dibutyryl cyclic AMP-induced trophic effects above were blocked by adenosine 3',5'-cyclic monophosphothioate, a specific inhibitor of cyclic AMP-dependent kinases. We also examined whether cyclic AMP is involved in trophic effects provided by membrane depolarization induced by high K+ and growth factors such as basic fibroblast growth factor and insulin-like growth factor-1. Depolarization, but not the growth factors, increased intracellular levels of cyclic AMP. Adenosine 3',5'-cyclic monophosphothioate diminished depolarization increases in GABA uptake, whereas it did not affect the trophic effect of the growth factors. Co-treatment with the growth factors and dibutyryl cyclic AMP produced additive effects on both increases in GABA uptake and neuroprotection against excitotoxicity. The present results indicate that cyclic AMP-dependent kinases play roles in mediating differentiation and survival of developing striatal neurons. Signalling pathways activated by either basic fibroblast growth factor or insulin-like growth factor-1 are independent of those involving cyclic AMP. In contrast, depolarization-induced trophic effects are mediated, at least in part, by cyclic AMP-dependent pathways. Protective actions of dibutyryl cyclic AMP against excitotoxic injury as well as the additive effects with the growth factors are of potential interest in the experimental therapy of acute or chronic neurodegenerative diseases.

A new twist in an old story: the role for crosstalk of neuronal and trophic activity

A number of recent findings suggest a reciprocal interaction between neurotransmitters and neurotrophins functioning at the level of the synapse, which may be relevant not only for plasticity changes in the mature nervous system, but also for the development of synaptic connectivity and for survival or maturation of neurons prior to target contact. Thus, neurotrophin-induced attenuation of frequency-dependent depletion of releasable synaptic vesicle pools of neurotransmitter at synapses may participate in Hebbian and non-Hebbian forms of LTP, as a characteristic of mature synaptic contacts. Subsequent to nerve/target contact, neurotrophins also appear to mediate contact-induced enhancement of neurotransmitter release; this may participate in a developmental improvement of synapse efficacy, stabilization of synaptic contacts, and maturation of "conductive" functional synapses. Coincident with a transmitter-induced elevation of cytosolic Ca2+ levels within growth cones, a local neurotrophin-mediated increase in released neurotransmitter occurring subsequent to stabilization of a distinct synaptic contact may then participate in the refinement of synapses with retention of those neurites affected by neurotrophins and withdrawal of those neurites not affected by neurotrophins. Finally, prior to nerve/target contact, Ca2+ channel-generated spontaneous neuronal activity as well as co-expression of neurotrophins and their receptors may play a role in maturational changes.

Mastoparan-induced apoptosis of cultured cerebellar granule neurons is initiated by calcium release from intracellular stores

We have recently reported that mastoparan, a peptide toxin isolated from wasp venom, induces apoptosis in cultured cerebellar granule neurons that can be blocked by cholera toxin, an activator of Gs. Measurements of intracellular free calcium concentration ([Ca2+]i) reveal that mastoparan induces a dramatic elevation of [Ca2+]i that is frequently followed by enhanced leakage of fura-2 out of the neurons, suggesting that this rise in [Ca2+]i may be due to a more generalized change in membrane permeability. However, the mastoparan-induced initial elevation of [Ca2+]i is maintained in the absence of extracellular Ca2+, suggesting that the rise of [Ca2+]i is from intracellular stores. This conclusion is supported by the observation that depletion of [Ca2+]i stores by pretreatment with either caffeine or thapsigargin attenuates both the rise in [Ca2+]i and cell death induced by mastoparan. Phospholipase C (PLC) inhibitors, neomycin and U73122 block mastoparan-induced increases of [Ca2+]i and protect against neuronal death. Pretreatment with cholera toxin, but not pertussis toxin, reduced the mastoparan-induced rise in [Ca2+]i. Taken together, our data suggest that mastoparan initiates cell death in cerebellar granule neurons by inducing Ca2+ release from intracellular stores, probably via activation of PLC and IP3. A secondary or parallel process results in disruption of plasma membrane integrity and may be ultimately responsible for the death of these neurons by mastoparan.

Mediation of neuronal apoptosis by enhancement of outward potassium current

Apoptosis of mouse neocortical neurons induced by serum deprivation or by staurosporine was associated with an early enhancement of delayed rectifier (IK) current and loss of total intracellular K+. This IK augmentation was not seen in neurons undergoing excitotoxic necrosis or in older neurons resistant to staurosporine-induced apoptosis. Attenuating outward K+ current with tetraethylammonium or elevated extracellular K+, but not blockers of Ca2+, Cl-, or other K+ channels, reduced apoptosis, even if associated increases in intracellular Ca2+ concentration were prevented. Furthermore, exposure to the K+ ionophore valinomycin or the K+-channel opener cromakalim induced apoptosis. Enhanced K+ efflux may mediate certain forms of neuronal apoptosis.

Neuronal protection from apoptosis by pituitary adenylate cyclase-activating polypeptide

Pituitary adenylate cyclase-activating polypeptide (PACAP) is known to have trophic effects on neurons. Apoptosis of PC12 cells was induced by depletion of serum and nerve growth factor (NGF) from culture medium. Not only high potassium-induced Ca2+ channel activation but PACAP-38 at physiological concentrations (10[-10] to 10[-8] M) protected PC12 cells from apoptosis. PACAP-38 increased Ca2+ uptake and intracellular Ca2+ concentrations in PC12 cells. The effects of PACAP-38 on cell survival and Ca2+ channels were eliminated by inhibitors for Ca2+ channels and protein kinase A, and mimicked by 8-bromo-cAMP. Mitogen-activated protein (MAP) kinase activity was stimulated by PACAP-38. These findings implicate that PACAP protects PC12 cells from apoptosis by activating Ca2+ channels via the cAMP-protein kinase A pathway to stimulate MAP kinase cascade.

Hypoxia-induced dysfunction in developing rat neocortex

Neocortical slices from young [postnatal day (P) 5-8], juvenile (P14-18), and adult (>P28) rats were exposed to long periods of hypoxia. Field potential (FP) responses to orthodromic synaptic stimulation, the extracellular DC potential, and the extracellular Ca2+ concentration ([Ca2+]o] were measured simultaneously in layers II/III of primary somatosensory cortex. Hypoxia caused a 42 and 55% decrease in the FP response in juvenile and adult cortex, respectively. FP responses recorded in slices from young animals were significantly more resistant to oxygen deprivation as compared with the juvenile (P < 0.01) and adult age group (P < 0.001) and declined by only 3% in amplitude. In adult cortex, hypoxia elicited, after 7 +/- 4.5 min (mean +/- SD), a sudden anoxic depolarization (AD) with an amplitude of 14 +/- 6 mV and a duration of 0.89 +/- 0.28 min at half-maximal amplitude. Although the AD onset latency was significantly longer in P5-8 (12.5 +/- 4.9 min, P < 0.001) and P14-18 (8.7 +/- 3.2 min, P < 0.002) cortex, the amplitude and duration of the AD was larger in young (45.7 +/- 7.6 mV, 2.19 +/- 0.71 min, both P < 0.001) and juvenile animals (29.9 +/- 9.1 mV, P < 0.001, 0.96 +/- 0.26 min, P > 0.05) when compared with the adults. The hypoxia-induced [Ca2+]o decrease was significantly (P < 0.002) larger in young cortex (1,115 +/- 50 microM) as compared with the adult (926 +/- 107 microM). Prolongation of hypoxia after AD onset for >5 min elicited in young and juvenile cortex a long-lasting AD with an amplitude of 40.5 mV associated with a decrease in [Ca2+]o by >1 mM. On reoxygenation, only slices from these age groups showed spontaneous repetitive spreading depression in 3 out of 26 cases. In adults, the same protocol caused a significantly (P < 0.05) smaller and shorter AD and never a spreading depression. However, recovery in synaptic transmission after this long-term hypoxia was better in young and juvenile cortex, indicating a prolonged or even irreversible deficiency in synaptic function in mature animals. Application of ketamine caused a 49% reduction in the initial amplitude of the AD in juvenile cortex but did not significantly affect the AD in slices from adult animals. These data indicate that the young and juvenile cortex tolerates much longer periods of oxygen deprivation as compared with the adult, but that a sufficiently long hypoxia causes severe pathophysiological activity in the immature cortex. This enhanced sensitivity of the immature cortex is at least partially mediated by activation of N-methyl-D-aspartate receptors.

Involvement of phosphatidylinositol-3 kinase in prevention of low K(+)-induced apoptosis of cerebellar granule neurons

Cerebellar granule neurons obtained from 9-day-old rats die in an apoptotic manner when cultured in serum-free medium containing a low concentration of potassium (5 mM). A high concentration of potassium (26 mM) in the culture medium and BDNF can effectively prevent this apoptosis. The survival effects of high potassium and BDNF were additive, and the effect of high potassium was not blocked by addition of anti-BDNF antibody. These observations indicated that these survival effects were independent. To examine which molecules are involved in the survival pathway induced by BDNF or high K+, we used wortmannin, a specific inhibitor of PI-3 kinase. Wortmannin blocked the survival effects of both BDNF and high K+ on cerebellar granule neurons. Furthermore, in vitro PI-3 kinase assay showed that treatment with BDNF or high K+ induced PI-3 kinase activity, which was diminished by addition of wortmannin. These results indicate that different survival-promoting agents, BDNF and high K+, can prevent apoptosis in cerebellar granule neurons via a common enzyme, PI-3 kinase.

Activity-dependent survival of rat cerebellar granule neurons is not associated with sustained elevation of intracellular Ca2+

Ca2+ plays a pivotal role for the activity-dependent survival of neurons. In primary culture of cerebellar granule neurons, we found that there is no significant difference in intracellular Ca2+ level in the survival-promoting condition (cultures in the presence of 25 mM KCl) and that in the apoptosis-inducing condition (cultures in the presence of 5 mM KCl). This was not due to the inactivation of voltage-dependent L-type Ca2+ channels in the survival-promoting condition, but due to the enhanced rate of the influx and the efflux of Ca2+ in the survival-promoting condition compared to that in the apoptosis-inducing condition. These results suggest that the activity-dependent survival of the granule neurons is not associated with sustained rise of intracellular Ca2+ but associated with the enhanced turnover rate of Ca2+.

Activity-dependent expression of parathyroid hormone-related protein (PTHrP) in rat cerebellar granule neurons. Requirement of PTHrP for the activity-dependent survival of granule neurons

To identify genes whose expression is neuronal activity-dependent, we used an mRNA differential display technique and discovered that parathyroid hormone-related protein (PTHrP) is expressed in an activity-dependent manner in primary cultures of rat cerebellar granule neurons. PTHrP mRNA was expressed as early as 1 h by the addition of KCl to a final concentration of 25 mM to the culture medium. This expression was induced by Ca2+ influx through voltage-dependent L-type Ca2+ channels and regulated at the transcriptional step. PTHrP mRNA was persistently expressed before and after the time of commitment of granule neurons to apoptosis when they are cultured in the presence of 25 mM KCl or both 150 microM N-methyl-D-aspartic acid and 15 mM KCl, both of which promote the survival of these neurons. PTHrP was rapidly secreted into the culture medium in a depolarization-dependent manner. Parathyroid hormone/PTHrP receptor mRNA was also expressed in the primary cultures, and its expression was up-regulated by KCl and/or N-methyl-D-aspartic acid. The addition of anti-PTHrP antiserum to the culture medium resulted in a reduction of the activity-dependent survival of the granule neurons. These results suggest that PTHrP is involved in an autocrine loop and required for the survival of granule neurons.

Synchronization of neuronal activity promotes survival of individual rat neocortical neurons in early development

Neural activity is thought to play a significant role during the development of the cerebral cortex. In this study, we examined the effects of global activity block or enhancement and the effects of patterned firing on the ability of cultured rat neocortical neurons to survive during the second week in vitro, beyond the beginning of synaptogenesis. Blockade of neuronal activity by adding tetrodotoxin (TTX) and increasing magnesium concentration in the medium strongly reduced the survival of cortical cells. Increasing neuronal activity by raising the external potassium concentration significantly improved the survival of cortical neurons. We postulated that in a developing neuronal network the survival of nerve cells is regulated by synaptically mediated events that involve changes in the intracellular calcium concentration. To examine this question further, we monitored the activity of the developing network by optically recording the intracellular calcium signals of many neurons simultaneously. These recordings show that in low magnesium neocortical neurons express synchronized oscillation of their intracellular calcium concentration. The ability of a network to synchronize the changes in intracellular calcium of multiple cells appeared gradually during the second week in culture, paralleled by both an increase in the synaptic density and a decline in the number of surviving neurons. By examining the fate of identified cells several days after a recording session, we found that those nerve cells that were co-activated with other neurons had a significantly higher chance to survive than cells that did not participate in synchronized events. These experiments demonstrate that during early cortical network development cortical neurons show synchronized firing activity and that the survival of neurons is at least partially dependent on this pattern of neuronal activity.

Inhibition of phosphatidylinositol 3-kinase activity blocks depolarization- and insulin-like growth factor I-mediated survival of cerebellar granule cells

Depolarizing concentrations of potassium promote the survival of many neuronal cell types including cerebellar granule cells. To begin to understand the intracellular mediators of neuronal survival, we have tested whether the survival-promoting effect of potassium depolarization on cerebellar granule cells is dependent on either mitogen-activated protein (MAP) kinase or phosphatidylinositol 3-kinase (PI-3-K) activity. In 7-day cerebellar granule cell cultures, potassium depolarization activated both MAP kinase and PI-3-K. Preventing the activation of MAP kinase with the MEK1 inhibitor PD98059 did not affect potassium saving. In contrast, the survival-promoting effect of 25 mM potassium was negated by the addition of 30 microM LY 294002 or 1 microM wortmannin, two distinct inhibitors of PI-3-K. The cell death induced by PI-3-K inhibition was indistinguishable from the cell death caused by potassium deprivation; LY 294002-induced death included nuclear condensation, was blocked by cycloheximide, and had the same time course as potassium deprivation-induced cell death. Cerebellar granule cells can also be maintained in serum-free medium containing either 100 ng/ml insulin-like growth factor I (IGF-I) or 800 microM cAMP. PI-3-K inhibition completely blocked the survival-promoting activity of IGF-I, but had no effect on cAMP-mediated survival. These data indicate that the survival-promoting effects of depolarization and IGF-I, but not cAMP, require PI-3-K activity.

Trophic support of cultured spiral ganglion neurons by depolarization exceeds and is additive with that by neurotrophins or cAMP and requires elevation of [Ca2+]i within a set range

Spiral ganglion neurons (SGNs) require both pre- and postsynaptic contacts to maintain viability. BDNF, NT-3, chlorphenylthio-cAMP, and depolarization (veratridine or elevated [K+]o) all promote survival of SGNs in vitro, depolarization being the most effective. Combining different trophic stimuli increases survival in an additive manner. Neurotrophins and depolarization maintain comparable soma size and neurite extension, but SGNs are shrunken in cAMP. Elevated [K+]o has a biphasic effect on SGN survival; survival improves as [K+]o is raised to 30 mM (30K) and falls as [K+]o is further increased; SGN survival in 80 mM [K+]o (80K) is poor relative to survival in 30K. These responses to elevated [K+]o are potentiated by an L-type channel agonist, whereas L-type Ca2+ channel blockers antagonize the trophic effect of depolarization. Four hours after depolarization, steady-state [Ca2+]i is elevated in SGNs in 30K and further elevated in SGNs in 80K. At 22 hr after depolarization, by which time death of neurons in 80K has begun, elevated [Ca2+]i levels in surviving neurons in 80K are not higher than those in neurons in 30K ( approximately 150-450 nM), suggesting that neurons with high [Ca2+]i are preferentially lost. Veratridine causes oscillatory increases in [Ca2+]i to 250-350 nM. Thus, [Ca2+]i is predictive of cell survival; [Ca2+]i elevated to 100-500 nM in a sustained or oscillatory manner permits SGN survival independent of exogenous neurotrophic factors. Higher [Ca2+]i is associated with cell death.

Transneuronal regulation of ribosomes after blockade of ionotropic excitatory amino acid receptors

Elimination of auditory nerve activity results in death and atrophy of neurons in the cochlear nucleus, nucleus magnocellularis (NM), of the chick. One early event believed to lead to cell death and atrophy is the disruption of ribosomes in the NM neuron. A useful assay for visualizing these ribosomal changes is immunolabeling with the antibody Y10B, which recognizes ribosomal RNA. Activity-dependent changes in Y10B labeling have been observed both in vivo, after unilateral cochlea removal and in vitro after unilateral auditory nerve stimulation. Although it is clear that activity is crucial for maintaining ribosomal integrity, the identity of the important transynaptic signal(s) is not known. It is possible that this trophic signal is glutamate, the neurotransmitter release from the auditory nerve. The present study investigates the role of ionotropic glutamate receptors in the activity-dependent regulation of ribosomes, as measured by the Y10B immunoreactivity. Brain slices containing the auditory nerve and NM on both sides were obtained from hatchling chicks. The auditory nerve on one side of the slice was stimulated for 1 h. The slice was then processed for Y10B immunoreactivity. As expected, greater Y10B immunolabeling was observed on the stimulated side of the slice. Unexpectedly, however, this immunolabeling difference was still observed after blocking NMDA receptors (50 microM DL-APV), non-NMDA receptors (20 microM CNQX), or blocking both ionotropic receptor subtypes (APV and CNQX). This was true even though CNQX eliminated driven postsynaptic potentials. These data suggest that ionotropic glutamate receptors are not necessary for the activity-dependent regulation of ribosomes in NM neurons.

GABAA receptor stimulation promotes survival of embryonic rat striatal neurons in culture

In order to clarify the functional role of gamma-aminobutyric acid (GABA) in developing brain, we investigated the effect of GABA on the survival of embryonic rat striatal neurons in dissociated cell culture. Chronic exposure of striatal cultures to GABA resulted in a significant increase in the number of surviving neurons. The effect of GABA was concentration-dependent (1-1000 microM) and was blocked by a GABAA receptor antagonist, bicuculline (100 microM), or a GABAA chloride channel blocker, picrotoxin (100 microM), but not by a GABAB receptor antagonist, 2-hydroxysaclofen (100 microM). In addition, the GABAA receptor agonist muscimol mimicked the effect of GABA, promoting cell survival in a concentration-dependent manner (0.01-100 microM), while the GABAB receptor agonist baclofen (up to 100 microM) had no significant effect. The GABA-induced enhancement of neuronal survival was suppressed by the L-type voltage-dependent Ca2+ channel blockers nifedipine (1-3 microM) and nicardipine (1-5 microM). Protein kinase inhibitors, H-7 (10-30 microM) or genistein (3 microM), also suppressed GABA-induced enhancement of neuronal survival. These results suggest that stimulation of GABAA receptors enhances survival of embryonic striatal neurons, and that the effect is mediated by Ca2+ influx through L-type voltage-dependent Ca2+ channels, initiating intracellular signaling cascades that involve activation of H-7- and genistein-sensitive protein kinases.

Brain-derived neurotrophic factor suppresses programmed death of cerebellar granule cells through a posttranslational mechanism

Cerebellar granule cells isolated from 7-d-old rats have been shown to die in vitro unless they are continuously exposed to elevated K+ (25 mM). Here we have characterized this neuronal death, and examined whether its major features are shared with those of sympathetic neurons following nerve growth factor (NGF) deprivation. Granule cells underwent active cell death accompanied by morphological features of apoptosis. Brain-derived neurotrophic factor (BDNF), but not NGF, was capable of preventing this neuronal death by acting posttranslationally. Moreover, semiquantitative RT-PCR, Northern blot, and immunoblot analyses showed that trkB, the signal-transducing receptor for BDNF, was upregulated during neuronal death of granule cells in vitro. These results extend recent findings for the role of BDNF in granule cell development, and suggest that BDNF plays a pivotal role on the regulation of the neuronal death/survival of granule cells.

Localization of voltage-sensitive Ca2+ fluxes and neuropeptide Y immunoreactivity to varicosities in SH-SY5Y human neuroblastoma cells differentiated by treatment with the protein kinase inhibitor staurosporine

The distribution of voltage-sensitive elevations of the level of Ca2+ in untreated SH-SY5Y cells and cells that had been induced to differentiate with staurosporine was investigated by monitoring fura-2 fluorescence in cell suspensions, and by using microfluorometry and quantitative fluorescence imaging on cell bodies and on cellular processes. Cell bodies of both types of cells displayed small Ca2+ elevations, which were composed of transient and sustained components. Elevations were partially sensitive to the L- and N-channel blockers nifedipine (1 microM) and omega-conotoxin GVIA (100 nM) respectively. Up to ten times Ca2+ elevations were observed in varicosities of treated cells than in cell bodies of treated and cells. These elevations were insensitive to compounds known to release Ca2+ from intracellular stores. Elevations of Ca2+ were sustained, and they were insensitive to 5 microM nifedipine, 100 nM omega-agatoxin IVA and 100 nM omega-conotoxin GVIA, and partially sensitive to 2 microM omega-conotoxin GVIA, indicating predominance of non-L-type, non-N-type, non-P-type channel activity. The intracellular localization of neuropeptide Y, a marker of differentiation in these cells, was also investigated by fluorescence immunocytochemistry. Varicosities of treated cells displayed marked fluorescence when viewed in a confocal microscope. These findings show that the varicosities of staurosporine-treated cells exhibit some of the functional properties of nerve terminals. The varicosities resemble boutons en passant nerve endings and they seem to express Ca2+ channels different from those in the cell body.

Resting potential of rat cerebellar granule cells during early maturation in vitro

The survival of rat cerebellar granule cells maintained in vitro is enhanced by a KCl-enriched medium. This effect is classically interpreted as resulting from a higher cytosolic calcium concentration. This implies the presence of voltage-dependent Ca2+ channels and a membrane potential that can respond to changes in external K+. Since previous studies cast a doubt on these two conditions, we reinvestigated the resting membrane potential and Ca2+ influxes in rat cerebellar granule neurones during the first week in vitro using a fluorescence imaging approach. Membrane potential was assessed with the fluorescent dye bis-oxonol, and intracellular free calcium with Fura-2. Resting potential was shown to progressively decrease from -40 mV at the first day in vitro to -60 mV at day 7. At all times in culture, as early as day 0, cells were depolarized when external KCl concentration was increased from 5 to 30 mM. This depolarization resulted in an increased cytosolic calcium concentration due to Ca2+ influx through L-type and N-type voltage-activated Ca2+ channels, functional at day 0. Gross estimations of the permeabilities of Na+ and Cl- were obtained at various times in culture by measuring the changes in resting potential brought about by a reduction of their external concentration. A progressive increase of the relative permeability to K+ ions seems to underlie the evolution of the resting potential with time.

Excitatory actions of GABA after neuronal trauma

GABA is the dominant inhibitory neurotransmitter in the CNS. By opening Cl- channels, GABA generally hyperpolarizes the membrane potential, decreases neuronal activity, and reduces intracellular Ca2+ of mature neurons. In the present experiment, we show that after neuronal trauma, GABA, both synaptically released and exogenously applied, exerted a novel and opposite effect, depolarizing neurons and increasing intracellular Ca2+. Different types of trauma that were effective included neurite transection, replating, osmotic imbalance, and excess heat. The depolarizing actions of GABA after trauma increased Ca2+ levels up to fourfold in some neurons, occurred in more than half of the severely injured neurons, and was long lasting (>1 week). The mechanism for the reversed action of GABA appears to be a depolarized Cl- reversal potential that results in outward rather than inward movement of Cl-, as revealed by gramicidin-perforated whole-cell patch-clamp recording. The consequent depolarization and resultant activation of the nimodipine sensitive L- and conotoxin-sensitive N-type voltage-activated Ca2+ channel allows extracellular Ca2+ to enter the neuron. The long-lasting capacity to raise Ca2+ may give GABA a greater role during recovery from trauma in modulating gene expression, and directing and enhancing outgrowth of regenerating neurites. On the negative side, by its depolarizing actions, GABA could increase neuronal damage by raising cytosolic Ca2+ levels in injured cells. Furthermore, the excitatory actions of GABA after neuronal injury may contribute to maladaptive signal transmission in affected GABAergic brain circuits.

TGF-beta-induced apoptosis of cerebellar granule neurons is prevented by depolarization

The regulation of programmed cell death in the developing nervous system involves target-derived survival factors, afferent synaptic activity, and hormone- and cytokine-dependent signaling. Cultured immature cerebellar granule neurons die by apoptosis within several days in vitro unless maintained in depolarizing (high) concentrations of potassium (25 mM K+). Here we report that transforming growth factors (TGF)-beta1, -beta2, and -beta3 accelerate apoptosis of these neurons when maintained in physiological (low) K+ medium (5mM K+) as assessed by measures of viability, quantitative DNA fragmentation, and nuclear morphology. TGF-beta-induced apoptosis of these neurons is not blocked by CNTF and LIF, cytokines that enhance neuronal survival when applied alone, or by IGF-I, which prevents apoptosis upon potassium withdrawal. In contrast, neurons that differentiate in high K+ medium for several days in vitro acquire resistance to TGF-beta-mediated cell death. Granule neurons maintained in either low or high K+ medium produce latent, but not bioactive, TGF-beta1 and -beta2. Because neutralizing TGF-beta antibodies fail to augment survival of low K+ neurons, the cerebellar neurons are apparently unable to activate latent TGF-beta. Thus, apoptosis of low K+ neurons is not attributable to endogenous production of TGF-beta. Taken together, our data suggest that TGF-beta may limit the expansion of postmitotic neuronal precursor populations by promoting their apoptosis but may support survival of those neurons that have maturated, differentiated, and established supportive synaptic connectivity.

Inhibition of protein phosphatases induces IGF-1-blocked neurotrophin-insensitive neuronal apoptosis

We have previously described the marine toxin okadaic acid (OKA) to be a potent neurotoxin for cultured rat cerebellar neurons. Here we show that OKA-induced neurodegeneration involves the DNA fragmentation characteristic of apoptosis and is protein synthesis-dependent. DNA fragmentation and neurotoxicity correlated with inhibition of protein phosphatase (PP) 2A rather than PP1 activity. Neurotrophins NT-3 and BDNF failed to protect from OKA-induced apoptotic neurotoxicity that was, however, totally prevented by insulin-like growth factor-1. Neuronal death by OKA was significantly reduced by protein kinase C inhibitors and by the L-type calcium channel agonist Bay K8644, while it was potentiated by the reduction of free extracellular calcium concentrations.

Expression of cyclin A decreases during neuronal apoptosis in cultured rat cerebellar granule neurons

Cultured cerebellar granule neurons died in an apoptotic manner when the K+ concentration in culture medium was lowered to the normal level (5 mM) after maturation of cells with a high concentration of K+ (26 mM). The changes in expression of 14 cell cycle-related genes in this CNS apoptosis model were analyzed by quantitative RT-PCR. Most of the genes analyzed were stable during apoptosis. The expression of cyclin A mRNA, however, transiently decreased 1 h after the induction of apoptosis, and recovered within 3 h to above the basal level. In this system, the level of cyclin D1, which has been reported to be up-regulated in apoptosis of NGF-deprived cultured sympathetic neurons, did not change. These results suggest that the molecular mechanisms in these two apoptosis models are different. To determine cyclin A protein level, we used an immunostaining method. The number of cyclin A-positive neurons decreased during apoptosis. Moreover, the numbers of MAP2- and cdk2-positive neurons also decreased in a similar manner. Taken together, these results suggest that there is a relationship between apoptosis and cell cycle, and that morphological changes during apoptosis result from cytoskeletal structure degradation.

Tachykinins protect cholinergic neurons from quinolinic acid excitotoxicity in striatal cultures

The neuroprotective effect of tachykinins against excitotoxic death of cholinergic neurons was studied in rat striatal cell cultures. Quinolinic acid (QUIN) and kainic acid (KA) produced a dose dependent decrease in choline acetyltransferase activity, but KA was more potent. Our results show that substance P (SP) totally reversed the toxicity induced by 125 microM QUIN but not by 40 microM KA. This effect was also observed using protease inhibitors or a SP-analog resistant to degradation, [Sar9]-Substance P. The survival of neuron specific enolase- and acetylcholinesterase (AChE)-positive cells after treatment with QUIN alone or in the presence of SP was also examined. We observed that, while a decrease in total cell number produced by QUIN was not prevented by SP treatment, AChE-positive cells were rescued from the toxic damage. To characterize the SP protective effect we used more selective agonists of the three classes of neurokinin (NK) receptors. [Sar9, Met(O2)11]-Substance P (NK1 receptor agonist), [Nle10]-Neurokinin A (NK2 receptor agonist) or [Me-Phe7]-Neurokinin B (NK3 receptor agonist) were all able to block the toxic effect of QUIN on cholinergic activity. These results show that tachykinins provide an important protective support for striatal neurons, suggesting a possible therapeutical benefit in neurodegenerative disorders affecting cholinergic neurons.

Spontaneous motoneuronal activity mediated by glycine and GABA in the spinal cord of rat fetuses in vitro

1. Spontaneous motoneuronal activity was monitored from the lumbar ventral roots in an isolated spinal cord preparation from rat fetuses at embryonic days (E) 13.5-18.5. 2. Spontaneous bursts that were synchronized in both left and right ventral roots were observed periodically (mean interval, 1.5-2.6 min) from E14.5 to 17.5. This activity was abolished in Ca(2+)-free saline or by application of tetrodotoxin (1 microM), indicating that it was synaptically mediated. 3. The glutamate receptor blocker kynurenate (4 mM) failed to block spontaneous bursts at E14.5-15.5, though it completely abolished them at E17.5. The glycine receptor antagonist strychnine (10 microM) completely blocked spontaneous bursts at E14.5-15.5. Bicuculline, a GABAA receptor antagonist, reduced the amplitude of the spontaneous bursts. 4. At E15.5, a brief application of glycine (250 microM to 2 mM) evoked excitatory responses resembling the spontaneous bursts in both time course and amplitude. Such glycine-induced responses were not observed under Ca(2+)-free conditions, suggesting that they were synaptically evoked. These synaptic responses were not blocked by kynurenate (4 mM), but they were abolished by strychnine (10 microM). 5. It is concluded that glycine and GABA generate the earliest spontaneous motor activity of the fetus and function transiently as excitatory transmitters in the embryonic spinal cord.

Non-erythroid alpha-spectrin breakdown by calpain and interleukin 1 beta-converting-enzyme-like protease(s) in apoptotic cells: contributory roles of both protease families in neuronal apoptosis

The cytoskeletal protein non-erythroid alpha-spectrin is well documented as an endogenous calpain substrate, especially under pathophysiological conditions. In cell necrosis (e.g. maitotoxin-treated neuroblastoma SH-SY5Y cells), alpha-spectrin breakdown products (SBDPs) of 150 kDa and 145 kDa were produced by cellular calpains. In contrast, in neuronal cells undergoing apoptosis (cerebellar granule neurons subjected to low potassium and SH-SY5Y cells treated with staurosporine), an additional SBDP of 120 kDa was also observed. The formation of the 120 kDa SBDP was insensitive to calpain inhibitors but was completely blocked by an interleukin 1 beta-converting-enzyme (ICE)-like protease inhibitor, Z-Asp-CH2OC(O)-2,6-dichlorobenzene. Autolytic activation of both calpain and the ICE homologue CPP32 was also observed in apoptotic cells. alpha-Spectrin can also be cleaved in vitro by purified calpains to produce the SBDP doublet of 150/145 kDa and by ICE and ICE homologues [ICH-1, ICH-2 and CPP32(beta)] to produce a 150 kDa SBDP. In addition, CPP32 and ICE also produced a 120 kDa SBDP. Furthermore inhibition of either ICE-like protease(s) or calpain protects both granule neurons and SH-SY5Y cells against apoptosis. Our results suggest that both protease families participate in the expression of neuronal apoptosis.

Oxygen-induced apoptosis in PC12 cells with special reference to the role of Bcl-2

We previously reported that PC12h cells are killed by a high oxygen atmosphere. In this study, we further characterized this oxygen-induced cell death and found apoptotic features, as follows. Firstly, chromatin condensation was observed in cells cultured in a 50% O2 atmosphere. Secondly, cycloheximide and cordycepin, protein and RNA synthesis inhibitors, respectively, prevented the oxygen-induced cell death in PC12h cells, suggesting that it is mediated by an intracellular death program. Thirdly, NGF, CPT-cAMP and depolarization by high potassium medium also effectively inhibited this apoptotic cell death in PC12h cells. The effect of high K+ is thought to be mediated by the influx of Ca2+ into cells through voltage-dependent Ca2+ channels, because nifedipine, an L-type Ca2+ channel blocker, inhibited the effect of high K+. In addition, since the oxygen-induced apoptosis was blocked by the antioxidant vitamin E, this oxygen toxicity is suggested to be mediated by reactive oxygen species. To further characterize this oxygen-induced apoptosis at the molecular level, we used PC12 cells overexpressing the proto-oncogene bcl-2. Although a large number of PC12 cells transfected with the control vector died in a 50% O2 atmosphere within 6 days, bcl-2-transfected PC12 cells survived and proliferated. These findings suggested that our system using PC12 cells will be a useful model with which to analyze the molecular mechanisms of apoptosis induced by oxidative stress in neuronal cells.

Involvement of p53 in DNA strand break-induced apoptosis in postmitotic CNS neurons

The tumour suppressor p53 gene serves as a critical regulator of the cell cycle and of apoptosis following the exposure of normal cells to DNA damage. To examine the role of p53 in postmitotic CNS neurons, we cultured cerebellar neurons from normal wild-type mice and mutant p53-null mice under various conditions inducing neuronal death. When cerebellar neurons from 15- to 16-day postnatal wild-type mice were treated with ionizing radiation or DNA-damaging agents, massive neuron death occurred after 24-72 h. In contrast, neurons from p53-/- mice evidently resisted gamma-irradiation and some DNA-damaging agents, such as etoposide and bleomycin. On the other hand, low-K+ medium-induced apoptosis of cerebellar neurons was not affected by p53 status. Neither cell cycle progression nor DNA synthesis occurred during cell death induced by gamma-irradiation and low-K+ medium, as well as in normal cultures of p53+/+ and p53-/- neurons. These results suggest that p53 is required for the apoptotic death of postmitotic cerebellar neurons induced by DNA strand breaks.

Growth conditions influence DNA methylation in cultured cerebellar granule cells

Growth conditions influenced DNA methylation in cultured cerebellar granule cells, as indicated by immunocytochemical analysis with monoclonal antibodies raised against 5-methylcytidine. In cultures grown under suboptimal conditions, i.e. in medium containing 10 instead of 25 mM K+, a substantial reduction in both the number of immunopositive cells and the intensity of immunostaining occurred at 4 days in vitro (DIV), a time which preceded the appearance of the morphological features of apoptosis. These results suggest that a reduction in DNA methylation is one of the biochemical events associated with the 'condemned phase' of apoptosis, in which granule cells grown under suboptimal conditions become committed to death.

Developmental changes in glutamate receptor-activated translocation of protein kinase C in cerebellar granule neurons

Developmental changes in glutamate receptor agonist-produced enhancement of 4-beta-[3H]phorbol-12,13-dibutyrate binding ([3H]-PDBu binding), indicative of an intracellular translocation of protein kinase C (PKC), were investigated in cerebellar granule cells. Our observations demonstrate that the magnitude of glutamate-, NMDA-, and kainate-produced enhancement of PKC translocation was dramatically decreased between 2 and 12 DIV, whereas there was only a minor reduction in the corresponding response caused by the non-NMDA receptor agonist, AMPA. The maximally enhanced stimulation of PKC translocation caused by glutamate and NMDA was significantly reduced already at 4 DIV, whereas a significant reduction of the kainate-induced enhancement of [3H]PDBu binding was not observed until 8 DIV. Glutamate- and NMDA-induced responses were effectively blocked by the specific NMDA receptor antagonists MK-801 (1 microM) and APV (100 microM) as well as by the addition of Mg2+ into assay media. In contrast, the non-NMDA receptor antagonist, CNQX (10 microM), effectively blocked the kainate-induced enhancement of [3H]PDBu binding, but had no effect on the NMDA- and glutamate-induced stimulation of PKC translocation. The metabotropic glutamate receptor agonist, ACPD (up to 250 microM), had no effect on the translocation of PKC. Taken together, our data support the working hypothesis that the rapidly occurring changes in the glutamate receptor agonist-produced translocation of PKC are most likely due to a differential maturation of glutamate ionotropic receptor subtypes and/or to development-dependent alterations in mechanisms responsible for the coupling between the glutamate receptor subtypes and the activation of PKC translocation in cerebellar granule neurons.

In vitro neuroprotective properties of the novel cholinergic channel activator (ChCA), ABT-418

Recent literature has shown that compounds interacting with neuronal nicotinic acetylcholine receptors (nAChRs) have the potential to be neuroprotective both in vitro and in vivo. ABT-418 is a novel ChCA that selectively stimulates discrete subtypes of the nAChRs and exhibits cognitive enhancing activity. In the present study, the neuroprotective effects of ABT-418 and (-)-nicotine, as measured by the release of lactate dehydrogenase (LDH) into the media, were investigated in a glutamate (Glu)-induced cytotoxicity assay using either primary rat cortical neurons or a human differentiated cell line, IMR 32. The neuroprotection elicited by ABT-418 and (-)-nicotine is both time and concentration dependent with an optimal concentration of 10 microM and an optimal pretreatment time of 2 h. ABT-418 remained neuroprotective and not cytotoxic to rat cortical cells following subacute exposure for 7 days. Protection appears to be mediated via an interaction with nAChRs, possibly the alpha 7 subtype, since the neuroprotection was prevented by alpha-bungaratoxin (alpha-Bgt) and methyllycaconitine (MLA), both selective alpha 7 antagonists. Removal of extracellular Ca2+ prevented the neuroprotective effects of ABT-418 and (-)-nicotine, consistent with the known ability of alpha 7 nAChRs to modulate calcium dynamics. These data support the idea that ABT-418 not only enhances cognition, but may possibly slow the progression of the neurodegenerative process.

Oxygen deprivation but not a combination of oxygen, glucose, and serum deprivation induces DNA degradation in mouse cortical neurons in vitro: attenuation by transgenic overexpression of CuZn-superoxide dismutase

The present work was designed to study the possible implication of apoptosis in ischemic neuronal death, a phenomenon that has been suggested to be involved in neurodegeneration following focal as well as global ischemia. In this study, mouse cortical neurons in primary culture were subjected to oxygen deprivation or oxygen, glucose, and serum deprivation to simulate hypoxia and "ischemia-like" conditions; also, cellular viability as well as DNA degradation were investigated. The results showed that DNA degradation occurred in neurons subjected to oxygen deprivation but not to oxygen and substrate deprivation together. This DNA degradation, resulting in a laddering by agarose gel electrophoresis, could be prevented by cycloheximide and actinomycin-D treatments, although these inhibitors were unable to reduce neuronal death. To investigate if DNA degradation could be elicited by an intracellular free radical generation during reoxygenation, transgenic neurons overexpressing copper-zinc superoxide dismutase were subjected to 9 h of oxygen deprivation and analyzed after 24 h of reoxygenation. The results showed a significant attenuation of DNA degradation in these cells and confirmed a possible relationship between reactive oxygen species and neuronal apoptosis. This study opens the way to further investigations regarding the involvement of an apoptotic process in necrotic neuronal death, and provides some new insights into the mechanisms underlying selective sensitivity of neuronal cells to oxygen and glucose deprivation.

High affinity block by nimodipine of the internal calcium elevation in chronically depolarized rat cerebellar granule neurons

Chronic depolarization enhances survival of cultured rat cerebellar granule neurons by elevating the internal calcium concentration ([Ca2+]i). In Fura2-loaded cells maintained in 25 mM KCl, [Ca2+]i was close to 150 nM and decreased to approximately 50 nM when KCl was lowered to 5.4 mM. The effect of nimodipine (IC50 = 0.45 nM) was similar to depolarization removal, while agatoxin IVA (up to 500 nM) was ineffective. In whole-cell-clamp experiments, the IC50 for current inhibition was 57 nM, while with transient, KCl-induced depolarizations, the dose dependence of nimodipine inhibition had a 'double-affinity' shape, with IC50(1) = 0.30 nM and IC50(2) = 71 nM. We concluded that L-type calcium channels are the main responsible of the elevated internal calcium level necessary for survival in these neurons. These channels do not inactivate and bind nimodipine with different affinity, depending on their state.

Selective up-regulation of alpha 1-chimaerin mRNA in SK-N-SH neuroblastoma cells by K+/-induced depolarisation

The expression of alpha 1-chimaerin, which encodes a neuron-specific GTPase-activating protein for p21rac, is spatially and temporally regulated in vivo. In vitro, expression of the mRNA of both alpha 1-chimaerin and its alternative spliced form, alpha 2-chimaerin, was up-regulated when human neuroblastoma SK-N-SH cells underwent neuronal-type differentiation in a serum-free medium. KCl-induced membrane depolarisation also specifically up-regulated alpha 1-chimaerin mRNA expression in SK-N-SH cells at the transcriptional level. The up-regulation of alpha 1-chimaerin expression by membrane depolarisation is not an immediate early event, and occurs 3 h after KCl treatment. It does not require de novo protein synthesis. The increase in calcium influx via the L-type voltage-sensitive calcium channel as the result of depolarisation is a key event leading to the up-regulation of alpha 1-chimaerin mRNA. alpha 1-Chimaerin expression was also found to respond positively to the hypertonic osmolarity changes. These results suggest that in vivo expression of alpha 1-chimaerin, a potential signal transduction molecule, may be regulated by neuronal/synaptic activity.

Induction of transmitter release at the neuromuscular junction prevents motoneuron death after axotomy in neonatal rats

Motoneurons to rat hindleg muscles die after neonatal nerve injury. Here we show that increasing transmitter release of motor nerve terminals by treatment with 4-aminopyridine, prior to nerve injury at three days, reduces the extent of motoneuron death. Retrograde labelling of soleus motoneurons was carried out in 10-week-old animals that had their sciatic nerve crushed on one side when they were three days old. Only 20% (+/- 4.2 S.E.M.) of the motoneurons survived the nerve injury. A group of animals similarly injured at three days had their calf muscles treated with 4-aminopyridine at birth, prior to nerve injury. In these animals a significantly higher percentage (51 +/- 6.6% S.E.M.) of soleus motoneurons survived. In order to assess the proportion of surviving alpha-motoneurons only, the number of motor units in both the soleus and extensor digitorum longus muscles was established by following the stepwise increments of twitch tension in response to increasing intensity of stimulation of the respective motor nerve. After nerve injury at three days only 18% (+/- 4.1% S.E.M.) of motor units to soleus and 28.5% (+/- 4.9% S.E.M.) to extensor digitorum longus survived and were able to reinnervate their respective muscles. If the nerve injury was preceded by local application of 4-aminopyridine, then the number of motor units present in the reinnervated muscles was significantly greater, so that in soleus 52.7% (+/- 5.4% S.E.M.) and in extensor digitorum longus 52.1% (+/- 2.4% S.E.M.) of motor units were present. This increase of motoneuron survival was reflected in a smaller weight loss and in a better restoration of force production by the pretreated as compared to untreated muscles on reinnervation after nerve injury. It is suggested that enhancing transmitter release from nerve endings in neonatal animals induces the motoneuron to become more resistant to nerve injury.

Apoptosis induced in neuronal cultures by either the phosphatase inhibitor okadaic acid or the kinase inhibitor staurosporine is attenuated by isoquinolinesulfonamides H-7, H-8, and H-9

Protein phosphorylation is kept in balance by an orchestrated action of kinases and phosphatases; when this balance is lost, neuronal apoptosis may occur. Okadaic acid (OKA), a marine toxin that inhibits specifically protein phosphatases 1 and 2A (EC 3.1.3.16), and staurosporine, an inhibitor of protein kinase C (PKC; EC 2.7.1.37), induced apoptosis in primary cultures of rat cerebellar granule neurons. We assayed apoptosis by the DNA gel electrophoresis, by the in situ TUNEL assay, and by morphological appearance following propidium iodide staining. Cell viability was assessed by the Trypan blue assay. Both OKA- and staurosporine-induced neuronal apoptosis were prevented by a macromolecular synthesis inhibitor actinomycin D and by a group of isoquinolinesulfonamide kinase inhibitors (H-7, 1-[5-isoquinolinesulfonyl]-2-methylpiperazine; H-8, N-¿2-[methylamino]ethyl¿-5-isoquinolinesulfonamide; H-9, N-(2-aminoethyl)-5-isoquinolinesulfonamide, but not by inhibitors of PKC, cyclic-GMP- and cyclic-AMP-dependent kinases, calcium/calmodulin-dependent kinases, tyrosine kinases, or by antioxidants. We postulate that a common mechanism, possibly an increased protein phosphorylation, is responsible for apoptosis triggered by an inhibition of phosphatases 1 and 2A and PKC. Elucidating the isoquinolinesulfonamide-sensitive mechanism may help us find new therapies for neurodegenerative diseases that involve apoptosis.

The survival of striatal cholinergic neurons cultured from postnatal 2-week-old rats is promoted by neurotrophins

Although the expression of nerve growth factor (NGF) in the rat striatum is the highest at 2 postnatal weeks (P2w), the action of NGF at that age has not been studied in detail. We examined the effects of several neurotrophic factors, including NGF, on striatal cholinergic neurons cultured from P2w rats. We also examined the effects of a cyclic AMP (cAMP) analog and high K(+)-evoked depolarization. NGF specifically promoted the survival of choline acetyltransferase (ChAT)-positive neurons, and consequently increased the ChAT activity per well, whereas it did not induce the ChAT activity per cholinergic neuron. NGF-responsiveness was the highest in striatal cultures from P2w rats, but it was almost lost in cultures from P4w rats. Brain-derived neurotrophic factor (BDNF), neurotrophin-4/5 (NT-4/5), and a cAMP analog had survival-promoting effects on striatal total neurons including cholinergic neurons. On the other hand, high K+ hardly promoted the survival of striatal cholinergic neurons in cultures from P2w rats, although it increased the viable number of total striatal neurons. High K+ did not increase the ChAT activity in any tested cultures from postnatal 3- to 28-day-old rats. These results demonstrated that NGF prevented the death of striatal cholinergic neurons in cultures from P2w rats, but not from P4w rats, and that high K+ could not rescue these deaths. We propose that cholinergic neurons in the striatum are programmed to die at P2w, and that this programmed cell death can be restored by neurotrophins, but not by depolarization.

The effects of insulin-like growth factor analogues on survival of cultured cerebral cortex and cerebellar granule neurones

Insulin and insulin-like growth factors (IGF-I, IGF-II) are closely related polypeptides which are found in the CNS and which promote neuronal survival and neurite outgrowth. They are each associated with specific cell surface receptors and several soluble binding proteins (IGFBPs) which are involved in regulating function and availability. Two analogues of IGF-I were produced by site directed mutagenesis: (Gln3, Ala4, Tyr15, (Leu16)IGF-1 (QAYL-IGF) and a B-chain mutant in which the first 16 amino acids of IGF-1 were replaced by the first 17 amino acids of insulin. These analogues have significantly reduced binding affinity for IGFBPs. Using glucose deprivation as a damaging stimulus and assaying lactate dehydrogenase released from cultures as a marker for cell death, we have investigated the effect of IGF analogues on cell death of cerebrocortical and cerebellar granule cell cultures. In the presence of IGF-I, QAYL-IGF or B-chain mutant, the amount of LDH released from cortical and cerebellar granule cell cultures was significantly reduced compared to control (no glucose), indicating that these molecules promote survival. Both QAYL and B-chain mutants, which have reduced affinity for IGFBPs, are as effective as IGF-I in promoting cell survival in conditions of glucose deprivation and their reduced affinity for IGFBPs has no apparent deleterious effect on their neuroprotective function. We also show that the neuroprotective effect of the IGF analogues is due to a direct effect on the neurones in these cultures and is independent of the presence of glia.

Stimulation of growth factor receptor signal transduction by activation of voltage-sensitive calcium channels

To understand the mechanisms by which electrical activity may generate long-term responses in the nervous system, we examined how activation of voltage-sensitive calcium channels (VSCCs) can stimulate the Ras/mitogen-activated protein kinase (MAPK) signaling pathway. Calcium influx through L-type VSCCs leads to tyrosine phosphorylation of the adaptor protein Shc and its association with the adaptor protein Grb2, which is bound to the guanine nucleotide exchange factor Sos1. In response to calcium influx, Shc, Grb2, and Sos1 inducibly associate with a 180-kDa tyrosine-phosphorylated protein, which was determined to be the epidermal growth factor receptor (EGFR). Calcium influx induces tyrosine phosphorylation of the EGFR to levels that can activate the MAPK signaling pathway. Thus, ion channel activation stimulates growth factor receptor signal transduction.

Nerve growth factor withdrawal induces the apoptotic death of developing septal cholinergic neurons in vitro: protection by cyclic AMP analogue and high potassium

Nerve growth factor regulates the developmental programmed cell death of certain neurons in the peripheral nervous system. The functions of nerve growth factor in the central nervous system are less well characterized. Nerve growth factor withdrawal results in the protein synthesis-dependent death of a large percentage of developing septal cholinergic neurons in sandwich tissue culture. In this study double labelling techniques were used to demonstrate that septal cholinergic neurons subjected to nerve growth factor withdrawal exhibit condensed chromatin and fragmented nuclei, and are labelled intensely for fragmented DNA. These degenerative changes are characteristic of apoptotic cell death. Half of the cholinergic neurons were committed to die and could no longer be rescued by nerve growth factor reapplication following approximately 16.5 h of nerve growth factor deprivation, whereas half of the cholinergic neurons could no longer be rescued by cycloheximide addition after only 9 h of nerve growth factor deprivation, suggesting that nerve growth factor and cycloheximide effect rescue by distinct mechanisms. Addition of a cyclic AMP analogue or depolarization with high K+, but not the general nuclease inhibitor aurintricarboxylic acid, prevented the death of cultured septal cholinergic neurons subjected to nerve growth factor withdrawal. Furthermore, these agents are capable of rescuing cholinergic neurons subjected to a period of nerve growth factor withdrawal after which addition of cycloheximide is no longer protective. Thus, nerve growth factor, cyclic AMP and high K+ can effect rescue after inhibition of translation ceases to be protective. These findings suggest that under defined conditions in vitro, withdrawal of nerve growth factor from septal cholinergic neurons during a critical period of development results in the apoptotic death of these CNS neurons, which can be prevented at the post-translational level by nerve growth factor, cyclic AMP and high K+.

Adrenocorticotropic hormone activation of adenylate cyclase in raphe neurons: multiple regulatory pathways control serotonergic neuronal differentiation

The RN46A cell line was derived from embryonic day 13 rat medullary raphe cells by infection with a retrovirus encoding the temperature-sensitive mutant of SV40 large T antigen (tsT-ag). The RN46A cell line is neuronally restricted and constitutively differentiates following a shift to nonpermissive temperature. Differentiated RN46A cells express low levels of tryptophan hydroxylase (TPH) but no detectable levels of serotonin (5-HT). Treatment of cultures with the adrenocorticotropic hormone peptide ACTH4-10 up-regulates the expression of TPH immunoreactivity in differentiated RN46A cells, but 5-HT synthesis requires initial treatment with ACTH4-10, followed by partial membrane depolarizing conditions. Up-regulation of TPH by ACTH4-10 is apparently due to activation of adenylate cyclase, whereas the increased 5-HT synthesis with membrane depolarization can be blocked with the voltage-sensitive Ca(2+)-channel blockers nifedipine and omega-conotoxin. ACTH4-10 treatment also markedly up-regulates the expression of the 5-HT reuptake transporter, as do dibutyryl cyclic AMP and forskolin; chronic membrane depolarization has no effect on 5-HT reuptake. The expression of the high-affinity 5-HT1A receptor is increased threefold by ACTH4-10 treatment during differentiation and fivefold by differentiation under partial membrane depolarizing conditions. Combining ACTH4-10 treatment and membrane depolarization does not increase expression of the 5-HT1A receptor further. 5-HT release is constitutive in ACTH-treated RN46A cells and linked to spontaneous synaptic vesicle fusion in RN46A cells. Considered with previous results, these data indicate that multiple effectors, ACTH, brain-derived neurotrophic factor, and membrane depolarization, have both distinct and overlapping effects that regulate specific elements of the serotonergic neuronal phenotype during differentiation and maturation.

Calcium influx induces neurite growth through a Src-Ras signaling cassette

We find that calcium influx through voltage-dependent calcium channels causes extensive neurite outgrowth in PC12 cells. The calcium signal transduction pathway promoting neurite outgrowth causes the rapid activation of protein tyrosine kinases, which include Src. Protein tyrosine phosphorylation results in the formation of an Shc/Grb2 complex, leading to Ras activation, MAP kinase activation, and the subsequent induction of the immediate early gene NGFI-A. Protein tyrosine phosphorylation, gene induction, and neurite outgrowth are inhibited by the expression of dominant negative forms of both Src and Ras, indicating a requirement for both proto-oncoproteins in calcium signaling. Our results suggest that a signaling cassette which includes Src and Ras is likely to underlie a broad range of calcium of actions in the nervous system.

Ionic regulation of endonuclease activity in PC12 cells

We have investigated the Ca2+ dependency of DNA degradation into nucleosome-sized fragments in intact chromaffin-like PC12 cells and PC12 nuclear fractions. In intact cells we were unable to trigger DNA fragmentation by inducing either transient or sustained elevations of cytoplasmic Ca2+ ([Ca2+]i) with the Ca2+ ionophore ionomycin. On the contrary, DNA fragmentation was induced in intact cells by the intracellular Zn2+ chelator NNN'N'-tetrakis-(2-pyridylmethyl)ethylenediamine (TPEN). To characterize further PC12 cell endonuclease activity, we then investigated digestion by purified PC12 cell fractions of exogenously added plasmids. In nuclear fractions two endonuclease activities were identified: an acidic (pH 5.0) endonuclease activity that was fully Ca2+- and Mg(2+)-independent; and a neutral (pH 7.6) endonuclease activity that was Ca(2+)-independent but Mg(2+)-dependent. Both endonuclease activities were inhibited by Zn2+. Nuclear membrane permeabilization greatly enhanced plasmid digestion at pH 7.6, but not at pH 5.0. This suggests that neutral endonuclease was located in a membrane-bound compartment, whereas acidic endonuclease was freely accessible to the substrate even in the presence of an intact nuclear membrane. In intact nuclei, digestion of genomic DNA could not be triggered by increasing the bivalent cation composition of the medium. On the contrary, in hypotonic medium we observed a large spontaneous nucleolytic DNA degradation that was increased by Zn2+ chelation. However, an acidic pH shift was a potent stimulus for DNA fragmentation in isotonic as well as hypotonic medium.

Glutamate as a hippocampal neuron survival factor: an inherited defect in the trisomy 16 mouse

The survival of cultured mouse hippocampal neurons was found to be greatly enhanced by micromolar concentrations of the excitatory neurotransmitter glutamate. Blockade of kainate/AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid) glutamate receptors increased the rate of neuron death, suggesting that endogenous glutamate in the cultures promotes survival. Addition of glutamate (0.5-1 microM) further increased neuron survival, whereas glutamate in excess of 20 microM resulted in increased death. Thus, the survival vs. glutamate dose-response relation is bell-shaped with an optimal glutamate concentration near 1 microM. We found that hippocampal neurons from mice with the genetic defect trisomy 16 (Ts16) died 2-3 times faster than normal (euploid) neurons. Moreover, glutamate, at all concentrations tested, failed to increase survival of Ts16 neurons. In contrast, the neurotrophic polypeptide basic fibroblast growth factor did increase the survival of Ts16 and euploid neurons. Ts16 is a naturally occurring mouse genetic abnormality, the human analog of which (Down syndrome) leads to altered brain development and Alzheimer disease. These results demonstrate that the Ts16 genotype confers a defect in the glutamate-mediated survival response of hippocampal neurons and that this defect can contribute to their accelerated death.

Brief potassium depolarization decreases levels of neurofilament proteins in CNS culture

Little is known about the effects of brief potassium depolarization that occurs concurrently with transient ischemia, epilepsy and head trauma. To investigate the effect of short-term depolarization on light (NF-L), middle (NF-M), and heavy (NF-H) neurofilament proteins and determine the role played by calcium in that effect, mixed septo-hippocampal cultures were exposed to 60 mM K+ for 6 min, in the presence of 0 to 11.8 mM Ca2+. Twenty-four hours later, neurofilament immunoreactivity in Western blots of depolarized cultures was decreased to 60% or less of control levels. Decreases were Ca2+-dependent, not due to cell loss, and affected both phosphorylated and nonphosphorylated proteins. The phosphorylation state of NF-M and NF-H influenced the degree of loss observed. Changes in the pattern of immunolabelling of neuritic processes were also associated with depolarization. Thus, brief potassium depolarization may contribute to cytoskeletal disruption following brain injury.

Characterization of the signaling interactions that promote the survival and growth of developing retinal ganglion cells in culture

The signaling mechanisms that control the survival of CNS neurons are poorly understood. Here we show that, in contrast to PNS neurons, the survival of purified postnatal rat retinal ganglion cells (RGCs) in vitro is not promoted by peptide trophic factors unless their intracellular cAMP is increased pharmacologically or they are depolarized by K+ or glutamate agonists. Long-term survival of most RGCs in culture can be promoted by a combination of trophic factors normally produced along the visual pathway, including BDNF, CNTF, IGF1, an oligodendrocyte-derived protein, and forskolin. These results suggest that neurotransmitter stimulation and electrical activity enhance the survival of developing RGCs and raise the question of whether the survival control mechanisms of PNS and CNS neurons are different.

Cytoskeletal derangements following central nervous system injury: modulation by neurotrophic gene transfection

This paper reviews important new evidence indicating that traumatic brain injury can produce more widespread derangements to the neuronal cytoskeleton than previously recognized. Although cytoskeletal derangements in axons have long been documented, recent data suggest that traumatic brain injury can produce structural derangements to dendrites and cell bodies as well. Many of these investigations have employed in vivo models to provide important insights into mechanisms possibly mediating the acute loss of cytoskeletal proteins, including disturbances in calcium homeostasis and activation of calcium-dependent proteolytic enzymes. However, we have little understanding of processes mediating the recovery of cytoskeletal proteins following injury. This paper provides recent evidence from in vitro models of central nervous system injury that neurotrophic proteins can enhance the recovery of the neuronal cytoskeleton. Neurotrophin-based therapy could employ either administration of exogenous neurotrophic proteins and/or transfection of cDNA for appropriate neurotrophins.

Moderate elevation of extracellular potassium transiently inhibits regeneration of sensory axons in cultured adult sciatic nerves

The adult frog dorsal root ganglia (DRG) together with the sciatic nerve (ScN) has previously been shown to survive in organ culture for several days. If a local test crush is made at the beginning of culturing, there is an initial delay of about 3 days before the sensory axons start to grow into the distal nerve stump at a rate of about 0.6-0.9 mm/day. The present results showed that axonal growth was unaffected in preparations maintained for 8 days in medium containing 10 mM K+ (5 mM is the physiological level). In contrast, the outgrowth was markedly reduced by 15 mM K+ and still more by 20 and 25 mM K+. The growth inhibition was partially counteracted by nifedipine, a Ca(2+)-channel antagonist. Other experiments clearly showed that high K+ exerted its effects during the early phase of the regeneration and lacked effects at later stages. The possibility that Ca(2+)-binding proteins, e.g. calbindin, which showed immunohistochemical reactivity in different structures, contribute to the growth adaptation to high K+ will be considered. The generality of the findings was supported by inhibition of axonal outgrowth of adult mouse sciatic sensory axons by high K+.

Time course of neurofilament protein loss following depolarization-induced injury in CNS culture

In septo-hippocampal cell cultures, brief potassium depolarization produces calcium-dependent decreases in neurofilament proteins and loss of fine neuritic processes within 24 h. It is not known whether neurons later exhibit delayed degeneration and die, live with enduring neurofilament loss, or restore neurofilament protein levels. Therefore, we exposed septohippocampal cultures to 6 min potassium depolarization (60 mM) with 2.8-11.8 mM extracellular Ca2+ and evaluated immunoreactivity for low, medium and heavy neurofilament proteins, neuronal number, and neuronal morphology for 10 days. Neuronal number remained unchanged; neurofilament protein levels recovered to between 31% and 99% of control levels, and fine neuritic processes reappeared.

Morphological and transmitter release properties are changed when sympathetic neurons are cultured in low Ca2+ culture medium

The stimulated elevation of [Ca2+]i can either promote neuronal survival or lead to Ca(2+)-mediated neurotoxicity. Similarly, growth cone mobility and neurite outgrowth may be promoted or arrested by elevated [Ca2+]i. We examined survival, development and transmitter release properties of chick sympathetic neurons maintained in culture medium containing varying concentrations of Ca2+. Neurons maintained in medium with no added Ca2+ or as low as 0.1 mM external Ca2+ show a dramatic change in growth and development compared to neurons kept in 1-2 mM Ca(2+)-containing medium. Furthermore, neurons in Ca(2+)-free medium (+ 100 microM EGTA) survived up to 24 h and, following change to 0.1 mM Ca2+, grew neurites and survived for several weeks. Neurons grown in high Ca2+ medium (0.6-2 mM) exhibited thick neurites, aggregated cell bodies, and neurites began to detach after six to eight days in culture. Neurons in low Ca2+ medium (no added Ca2+ to 0.3 mM) grew as single cells with extensive, thin branching neurites, remained firmly attached to the substrate and survived for several weeks. Neurons initially plated in 0.1 mM Ca2+ (or 2 mM Ca2+) medium and switched over to 2 mM (or 0.1 mM) Ca2+ medium after two days acquired the characteristic morphology of high (and low) Ca2+ medium over the next six days, demonstrating the plasticity of effects of external Ca2+. The above characteristic changes in growth of sympathetic neurons in low Ca2+ medium occurred when neurons were supported by 35 mM KCl or 30 nM phorbol 12,13-dibutyrate instead of nerve growth factor. The uptake and retention of tritiated norepinephrine in neurons grown in low or high Ca2+ medium were similar. However, basal release of [3H]norepinephrine in neurons maintained in low Ca2+ medium was one-third of that in neurons kept in high Ca2+ medium. Furthermore, electrically stimulated (10 pulses at 1 or 10 Hz) [3H]norepinephrine release from neurons grown in high Ca2+ had a high fractional release (> 1%) which did not change during six days in culture. On the other hand, fractional release in neurons grown in low Ca2+ medium for six to 10 days decreased about 50% and 75%, respectively, compared to release after two days in culture. The resulting low fractional release (< 0.5%) is characteristic of sympathetic neurons in neuroeffector organs.(ABSTRACT TRUNCATED AT 400 WORDS)