Deprivation of nerve growth factor rapidly increases purine efflux from cultured sympathetic neurons

N-alpha-p-tosyl-L-lysine chloromethyl ketone (TLCK) suppresses neuritic degeneration caused by different experimental paradigms including in vitro Wallerian degeneration

Accumulating evidence indicates that neurite degeneration occurs via a distinct mechanism from somal death programs. We have previously shown that neuritic ATP level in sympathetic neurons decreases, whereas somal ATP level remains unaltered during degeneration caused by the microtubule-disrupting agent, vinblastine. Moreover, caspase activation occurs only in cell soma, supporting the view of somal apoptosis and neuritic necrosis. Therefore, the ATP level of neurites is crucial for their degeneration; it appears to correlate with membrane blebbing or beading which precedes late whole fragmentation of neurites under these conditions. Based on these metabolic and morphological criteria, we have tested the effects of various protease inhibitors on vinblastine-induced neurite degeneration in superior cervical ganglia from neonatal mice. Among agents tested, N-alpha-p-tosyl-L-lysine chloromethyl ketone (TLCK), the trypsin-like serine protease inhibitor, but not N-p-tosyl-L-phenylalanine chloromethyl ketone (TPCK), the chymotrypsin-like serine protease inhibitor, protected sympathetic neurites from beading formation, neuritic fragmentation and a decrease in their ATP level. The commitment time for the saving effect of TLCK occurred around 7 h following treatment with vinblastine, at a time point after microtubule degradation (2 h) and before massive beading formation (later than 12 h). Moreover, TLCK was also capable of suppressing Wallerian degeneration in culture and neuritic degeneration following withdrawal of NGF in a dose-dependent manner. These results strongly suggest that TLCK intervenes in a common step in the cascade of neuritic degeneration caused by these different experimental paradigms and provides a helpful clue for identifying such a molecular step.

Nerve Growth Factor is Required for Induction of c-Fos Immunoreactivity by Serum, Depolarization, Cyclic AMP or Trauma in Cultured Rat Sympathetic Neurons

Nerve growth factor (NGF) induces transient Fos-immunoreactivity (Fos-IR) independently of any other factor, both in newly isolated rat sympathetic neurons and in established cultures after NGF deprivation. The same proportion of neurons that express Fos-IR in response to NGF also survive. In addition to direct stimulation of Fos-IR expression, the presence or recent exposure to NGF is required to obtain Fos-IR expression by other stimuli. In newly isolated neurons no Fos-IR is detected in response to stimulation by serum alone and a response to depolarization or cyclic AMP is obtained only if neurons are stimulated within a short period after ganglion excision. In established cultures none of these stimuli, nor the trauma of cutting neurites or spiking cell bodies with a microinjection needle induce Fos-IR unless NGF is present or had been removed for <8 - 16 h. The lack of response is not due to a general decrease in the rate of protein or RNA synthesis. These findings show that in regenerating sympathetic neurons NGF induces c-Fos and suggest that NGF may activate a master trigger that is required for c-Fos expression to be induced by other stimuli.

Programmed cell death in neurons: focus on the pathway of nerve growth factor deprivation-induced death of sympathetic neurons

Extensive programmed cell death (PCD) occurs in the developing nervous system. Neuronal death occurs, at least in part, because neurons are produced in excess during development and compete with each other for the limited amounts of the survival-promoting trophic factors secreted by target tissues. Neuronal death is apoptotic and utilizes components that are conserved in other PCD pathways. In this review, we discuss the mechanism of trophic factor-dependent neuronal cell death by focusing on the pathway of nerve growth factor (NGF) deprivation-induced sympathetic neuronal death. We describe the biochemical and genetic events that occur in NGF-deprived sympathetic neurons undergoing PCD. Participation of the Bcl-2 family of proteins and the interleukin-1beta-converting enzyme family of proteases (caspases) in this and other models of neuronal death is also examined. The order and importance of these components during NGF deprivation-induced sympathetic neuronal death are discussed.

Novobiocin modulates colchicine sensitivity in parental and multidrug-resistant B16 melanoma cells

The effect of the antibiotic agent novobiocin on the sensitivity of melanoma cells to colchicine and vinblastine was examined in drug-sensitive and drug-resistant B16 melanoma cells. A cell line COL/R was selected for colchicine resistance. The COL/R cell line (resistant to 80 ng/ml colchicine) was found to possess the multidrug-resistant (MDR) phenotype. The cells were shown to be cross-resistant to vinblastine and Adriamycin and to overexpress P glycoprotein. P glycoprotein activity was assessed by using the rhodamine 123 accumulation test. Rhodamine accumulation was markedly decreased in COL/R cells as compared to the parental B16 cells. Verapamil reversed drug resistance and increased rhodamine accumulation in COL/R cells. Novobiocin in combination with colchicine or vinblastine synergistically inhibited the proliferation of parental B16 cells. In COL/R cells, novobiocin markedly decreased colchicine resistance and increased rhodamine accumulation. These data show that novobiocin increases the sensitivity of both parental and MDR melanoma cells to microtubule-disrupting cytotoxic drugs.

Expression of the cyclic AMP-dependent protein kinase (PKA) catalytic subunit from a herpes simplex virus vector extends the survival of rat sympathetic neurons in the absence of NGF

Superior cervical ganglion neurons from neonatal rats are dependent on nerve growth factor for their survival both in vivo and in vitro. In culture this requirement can be largely replaced by cAMP or its analogues. Since activation of protein kinase A by cAMP is likely to be the pathway by which it exerts its survival-promoting effect, we have tested the feasibility of using herpes simplex virus (HSV) as a vector for expressing survival-promoting genes in neurons by cloning the catalytic subunit of the cAMP-dependent protein kinase (PKAcat) with a metallothionein gene promoter into the HSV thymidine kinase gene by homologous recombination. About 95% of the neurons became infected using 2.5 p.f.u. per cell. When this construct was used to express PKAcat in superior cervical ganglion neurons, in the presence of nerve growth factor (NGF) increases of 1.9- to 2.4-fold in PKA activity were found 8-10 h after infection; levels remained elevated (1.4- to 2.1-fold) up to 18 h, returning to basal by 24 h. After infection in the absence of NGF, cumulative activity over 24 h was approximately 3.5-fold lower in the first 24 h. Although the level of the inhibitory regulatory subunit type I was raised by 18 h, this is unlikely to completely explain the transient activity of PKAcat. When neurons were induced to express maximum PKAcat levels in the presence of NGF and then deprived of NGF, survival was extended by up to 2 days, demonstrating a direct role for PKA in promoting survival. By this time, some neurite degeneration was beginning which appeared to be partly due to toxic effects of the virus. However, replenishment with NGF supported further survival, showing that at this time the neurons were still viable. Similar rates of survival were obtained using a tsK-based PKAcat vector, but no significant survival was obtained with parental HSV or tsK virus strains. These data demonstrate the feasibility, and highlight some of the problems, of using HSV-based vectors as tools for expressing functional survival proteins in sympathetic neurons.

Temporal analysis of events associated with programmed cell death (apoptosis) of sympathetic neurons deprived of nerve growth factor

The time course of molecular events that accompany degeneration and death after nerve growth factor (NGF) deprivation and neuroprotection by NGF and other agents was examined in cultures of NGF-dependent neonatal rat sympathetic neurons and compared to death by apoptosis. Within 12 h after onset of NGF deprivation, glucose uptake, protein synthesis, and RNA synthesis fell precipitously followed by a moderate decrease of mitochondrial function. The molecular mechanisms underlying the NGF deprivation-induced decrease of protein synthesis and neuronal death were compared and found to be different, demonstrating that this decrease of protein synthesis is insufficient to cause death subsequently. After these early changes and during the onset of neuronal atrophy, inhibition of protein synthesis ceased to halt neuronal degeneration while readdition of NGF or a cAMP analogue remained neuroprotective for 6 h. This suggests a model in which a putative killer protein reaches lethal levels several hours before the neurons cease to respond to readdition of NGF with survival and become committed to die. Preceding loss of viability by 5 h and concurrent with commitment to die, the neuronal DNA fragmented into oligonucleosomes. The temporal and pharmacological characteristics of DNA fragmentation is consistent with DNA fragmentation being part of the mechanism that commits the neuron to die. The antimitotic and neurotoxin cytosine arabinoside induced DNA fragmentation in the presence of NGF, supporting previous evidence that it mimicked NGF deprivation-induced death closely. Thus trophic factor deprivation-induced death occurs by apoptosis and is an example of programmed cell death.

Simultaneous analysis of adenosine 3',5'-cyclic monophosphate accumulation and adenosine 5'-triphosphate metabolism in cultured cells preincubated with [2-3H]adenine

A simple and sensitive method based on metabolic labeling was developed for the simultaneous analysis of cyclic AMP accumulation and ATP metabolism in small numbers of cultured cells. Cells are preincubated overnight with [2-3H]adenine to label the ATP pool to a high specific activity. After cell stimulation the metabolites are extracted in a small volume of aqueous acetic acid and chloroform and separated without further manipulation by one-dimensional thin-layer chromatography and the radioactivity incorporated is determined by liquid scintillation counting. With ATP labeled to about 6 Ci/mmol, the lower limit of cyclic AMP detection is 2 fmol, a sensitivity that is comparable to the radioimmunoassay of acetylated cyclic AMP. In primary neurons and a neural cell line, for example, levels of ATP and its metabolites change when large amounts of cyclic AMP are generated, each with its unique pattern. ATP is also depleted when metabolic energy is consumed concomitantly with stimulation of cyclic AMP production by agonists, probably as a result of an increase in ion pump activity following cation influx. As ATP is utilized for cyclic AMP production and simultaneously for many other processes, an assessment of its metabolism in parallel with that of cyclic AMP is critical. We suggest that the method described here is particularly advantageous over other methods for this purpose.

The death programme in cultured sympathetic neurones can be suppressed at the posttranslational level by nerve growth factor, cyclic AMP, and depolarization

We have examined whether sympathetic neurones that have lost the potential to be rescued by protein and RNA synthesis inhibitors after a period of nerve growth factor (NGF) deprivation are irreversibly committed to die. We found that 15 h after withdrawal of NGF from 7-day cultures of neonatal rat superior cervical ganglion neurones, 50% of the neurones lost the potential to be rescued by cycloheximide but that NGF rescued most of the neurones. By 22 h after NGF withdrawal, only 10% of the neurones were rescued by inhibition of macromolecular synthesis with cycloheximide, puromycin, or actinomycin D, but as many as 60-80% of the neurones were rescued by NGF. This is after the time at which a DNA "ladder," consistent with cell death by apoptosis, was first detected (18 h). As long as 27 h of NGF withdrawal was required before 50% of the neurones lost the potential to be rescued by NGF. The survival-promoting agent 8-(4-chlorophenylthio)cyclic AMP (CPTcAMP) or depolarization with 50 mM KCl (HK) rescued neurones with kinetics similar to those of NGF, and rescue by all three agents did not require protein synthesis. Thus, NGF, CPTcAMP, and HK can rescue neurones deprived of NGF at much later times than either protein or RNA synthesis inhibitors by acting at the posttranslational level, a finding suggesting that initiation of the cell death programme in sympathetic neurones is not an irreversible step.