Ricin toxicity to microglial and monocytic cells

Microglial cells, like macrophages, are very sensitive to ricin, a galactose-specific toxic lectin belonging to the family of ribosome-inactivating proteins. This toxin can be taken up by most cells through the binding of its B chain to galactose-containing molecules on the cell membrane. In macrophagic cell types it can be internalised also by mannose receptors which are present on the surface of these cells. Endocytosis of the toxin by either pathway was evaluated by ricin toxicity to primary cultures of rat microglial cells and to a microglial N11 cell line in the presence or absence of lactose and mannan, which compete for the endocytosis via the ricin lectin chain or cellular mannose receptors, respectively. Results were compared with those obtained in cultures of mouse macrophages, human monocytes, and a monocytic JM cell line. All cultures were protected from ricin toxicity more by lactose than by mannan, indicating that ricin endocytosis via its lectin B chain is prevalent over that mediated by cellular mannose receptors. However, a partial protection by mannan was observed in all cases but not-stimulated N11 cells, either in the form of direct protection or of significant additional protection over that afforded by lactose. Mannose receptor expression by N11 cells was negative before, and positive after, treatment with endotoxin, as assessed by the specific binding of 125I-mannose-bovine serum albumin. Moreover, a partial protection from ricin toxicity by mannan was induced in the N11 microglial line after stimulation, consistently with an inducible expression of the mannose receptor by activated cells switched towards a microglial phenotype.

Neuronal nitric oxide synthase is permanently decreased in the cerebellum of rats subjected to chronic neonatal blockade of N-methyl-D-aspartate receptors

Pharmacological blockade of the (NMDA) receptor at critical stages of brain development may have long-lasting effects on brain chemistry and on animal behavior. We report here experiments in which the competitive NMDA receptor antagonist CGP 39551 was administered to rat pups from postnatal day 7 (P7) to P18. The stage of treatment was selected to primarily target the cerebellum, whose granule cells undergo post-mitotic migration and establishment of synaptic connections during this period. We focused our study on the long-term consequences of CGP 39551 treatment on the neuronal isoform of nitric oxide synthase (nNOS) since nNOS is highly expressed in the cerebellum and it is functionally linked to the NMDA receptor. Treated rats exhibited a long-lasting (up to P70) decrease in the intensity of nNOS immunocytochemical staining in the cerebellar cortex accompanied by a decrement of calcium-dependent NOS catalytic activity. A comparable decrease of enzyme activity was measured in the cerebral cortex, but not in the hippocampus, of adult rats. Other neurochemical markers (glutamatergic, gabaergic, purinergic) and glutamine synthetase were unchanged, while a cholinergic marker was slightly increased in the cerebellum of CGP 39551 treated animals. Taken together these data show that blockade of NMDA receptor during the critical period of formation and stabilization of neuronal circuits preferentially affects long-term nNOS expression and catalytic activity in the cerebellum.

Ornithine decarboxylase activity during development of cerebellar granule neurons

Ornithine decarboxylase (ODC), the key enzyme for polyamine biosynthesis, dramatically decreases in activity during normal cerebellar development, in parallel with the progressive differentiation of granule neurons. We have studied whether a similar pattern is displayed by cerebellar granule neurons during survival and differentiation in culture. We report that when granule cells were kept in vitro under trophic conditions (high K+ concentration), ODC activity progressively decreased in parallel with neuronal differentiation. Under nontrophic conditions (cultures kept in low K+ concentration), the enzymatic activity dropped quickly in parallel with an increased apoptotic elimination of cells. Cultures kept in high K+ but chronically exposed to 10 mM lithium showed both an increased rate of apoptotic cell death at 2 and 4 days in vitro and a quicker drop of ODC activity and immunocytochemical staining. A short chronic treatment of rat pups with lithium also resulted in transient decrease of cerebellar ODC activity and increased programmed cell death, as revealed by in situ detection of apoptotic granule neurons. The present data indicate that a sustained ODC activity is associated with the phase of survival and differentiation of granule neurons and that, conversely, conditions that favor their apoptotic elimination are accompanied by a down-regulation of the enzymatic activity.

Differential toxicity of protease inhibitors in cultures of cerebellar granule neurons

Involvement of proteases has been postulated in several neurodegenerative processes. Accordingly, protease inhibition has been proposed as a potential therapeutic tool to limit damage in some neuropathological states. The timed turn-over of proteins is, however, an essential biochemical process and its prolonged block may be dangerous to the cell. We report here data on toxicity consequent to 24-h exposure of cerebellar granule neurons in culture to inhibitors of different classes of proteases. Inhibition of calpains (calcium-activated cysteine proteases) resulted in dose-dependent neuronal death which largely occurred through apoptotic process. Leupeptin, an inhibitor acting on a broad spectrum of cellular serine proteases, was less toxic but resulted in definite morphological alteration of the cells. On the contrary, inhibitors of caspases, proteases belonging to the ICE (interleukin 1-beta converting enzyme) family, did not apparently damage granule neurons upon exposure for 24 h to high concentrations (up to 200 microM) of two inhibitors specific for ICE (Ac-YAVD-CHO) and CPP-32 (Ac-DEVD-CHO), respectively. These results suggest that inhibition of proteases that are activated by stressful stimuli but are not essential for the normal functioning of healthy cells, as it is likely the case for caspases, may not be harmful to neurons. Instead, the potential risks and side effects of prolonged inhibition of proteases such as calpains, that regulate the disposal and the turn-over of key cellular proteins, should be carefully tested in the assessment of possible neuroprotective roles.

Activation of the ornithine decarboxylase-polyamine system and induction of c-fos and p53 expression in relation to excitotoxic neuronal apoptosis in normal and microencephalic rats

Microencephalic rats obtained by gestational treatment with the DNA alkylating agent methylazoxymethanol, show a remarkable lack of sensitivity to excitotoxic neuropathology caused by systemic injections of the convulsant neurotoxin kainic acid. Taking advantage of this, we have studied in these rats, as well as in normal rats, the relationship between the induction of cellular signals supposedly related to cell death and the neuronal apoptosis consequent to kainic acid administration. While normal rats responded to the excitatory insult with a large and relatively long lasting increase of the activity of the enzyme ornithine decarboxylase and of the concentration of putrescine in some brain regions, these alterations were much smaller in microencephalic rats. Expression of c-fos in brain regions sensitive to kainic acid was quicker but lasted a noticeably shorter time in microencephalic rats as compared to normal animals. A profusion of apoptotic neurons, labeled by an in situ technique, were observed in the olfactory cortex, amygdala and hippocampus of normal rats injected with kainic acid, in particular 48 h and 72 h after drug administration. At corresponding time intervals and with similar topographic localization, neurons expressing p53 protein were observed. By contrast, microencephalic rats displayed only in a few cases and in a small number apoptotic neurons in restricted areas of the ventral hippocampus and entorhinal cortex. Noticeably, in these cases small populations of p53-expressing neurons were also present in the same areas. The present observations clearly show that oncogenes such as c-fos and p53, as well as ornithine decarboxylase which behaves as an immediate-early gene in the brain under certain circumstances, undergo noticeably lower and/or shorter induction in microencephalic rats exposed to excitotoxic stimuli. In these rats, therefore, the cellular signalling pathways studied here and related to excitotoxic sensitivity and commitment to cell death are downregulated as a probable consequence of altered brain wiring.

N-acetyl-serotonin (normelatonin) and melatonin protect neurons against oxidative challenges and suppress the activity of the transcription factor NF-kappaB

It is now well established that the formation of free radicals and oxidative stress-induced neuronal cell death can be involved in various neurodegenerative disorders, including Alzheimer's disease and Parkinson's disease. The pineal hormone melatonin has been suggested to be a neuroprotective antioxidant. To better understand the molecular mechanism of this activity, we compared the ability of melatonin and its precursor, N-acetyl-serotonin (normelatonin), to protect human neuroblastoma SK-N-MC cells and primary cerebellar granular neurons against oxidative stress. We found that normelatonin and melatonin have differential neuroprotective effects depending on the neuronal cell type. Normelatonin was more protective against hydrogen peroxide (H2O2) and glutamate-induced cell death in SK-N-MC cells compared to melatonin which was more effective to protect primary cerebellar granular neurons against the toxicity of H2O2, glutamate and N-methyl-D-aspartate when compared to normelatonin. At the molecular level, we tested the capacity of normelatonin and melatonin to inhibit the oxidative stress-induced NF-kappaB activation in both neuronal systems. Whereas normelatonin was more potent in the suppression of the activation of NF-kappaB by H2O2 in SK-N-MC cells compared to melatonin, no apparent differences in the extent of suppression could be detected in primary neurons. Normelatonin's and melatonin's neuroprotective activity in SK-N-MC neuroblastoma cells may be mediated by the suppression of NF-kappaB activation.

Neurotoxicity of polyamines and pharmacological neuroprotection in cultures of rat cerebellar granule cells

We have studied in a well-characterized in vitro neuronal system, cultures of cerebellar granule cells, the toxicity of polyamines endogenously present in the brain: spermine, spermidine, and putrescine. Twenty-four-hour exposure of mature (8 days in vitro) cultures to 1-500 microM spermine resulted in a dose-dependent death of granule cells, with the half-maximal effect being reached below 50 microM concentration. Putrescine was moderately toxic but only at 500 microM concentration. Spermidine was tested at 50 and 100 microM concentration and its toxicity was evaluated to be about 50% that of spermine. Neuronal death caused by spermine occurred, at least in part, by apoptosis. Spermine toxicity was completely prevented by competitive (CGP 39551) and noncompetitive (MK-801) antagonists of the NMDA receptor, but was unaffected by a non-NMDA antagonist (NBQX) or by antagonists of the polyamine site present on the NMDA receptor complex, such as ifenprodil. A partial protection from spermine toxicity was obtained through the simultaneous presence of free radical scavengers or through inhibition of the free radical-generating enzyme nitric oxide synthase, known to be partially effective against direct glutamate toxicity. The link between spermine toxicity and glutamate was further strengthened by the fact that, under culture conditions in which glutamate toxicity was ineffective or much reduced, spermine toxicity was absent or very much decreased. Exposure to spermine was accompanied by a progressive accumulation of glutamate in the medium of granule cell cultures. This was attributed to glutamate leaking out from dying or dead cells and was substantially prevented by the simultaneous presence of MK-801 or CGP 39551. The present results demonstrate that polyamines are toxic to granule cells in culture and that this toxicity is mediated through the NMDA receptor by interaction of exogenously added polyamines with endogenous glutamate released by neurons in the medium. The involvement of brain polyamines, in particular spermine and spermidine, in excitotoxic neuronal death is strongly supported by our present results.

Effects of chronic lithium treatment on ornithine decarboxylase induction and excitotoxic neuropathology in the rat

Young adult rats were chronically treated with lithium (2.5 mmol/kg/day) for 16 days. The day after the last lithium administration, rats were injected s.c. with the excitotoxic convulsant kainic acid (10 mg/kg). As compared to saline controls, lithium-treated rats had no apparent attenuation of convulsions. Furthermore, the induction of brain ornithine decarboxylase and the consequent increase of putrescine levels, an index related to the convulsant effects of kainic acid, were similar in saline- and lithium-treated rats. Other rats were unilaterally injected with ibotenic acid into the nucleus basalis magnocellularis: no differences were measured in cortical choline acetyltransferase (ChAT) decrease among saline- and lithium-treated rats. In both the above experiments, apoptotic cell death was monitored in relevant brain regions of saline- or lithium-treated rats through a specific in situ labeling method for fragmented DNA. Whilst morphological evidence for a reduced damage in the olfactory cortex and hippocampus of kainic acid-injected rats was not obtained, lithium-treated rats showed a lower decrease of specific neurochemical markers: [3H]D-aspartate uptake and glutamate decarboxylase. This result suggests that mechanisms of recovery, absent in saline-treated animals, are elicited by the excitotoxic insult in lithium-treated rats.

Toxicity of ricin and volkensin, two ribosome-inactivating proteins, to microglia, astrocyte, and neuron cultures

Ricin and volkensin, two potent toxins belonging to the family of ribosome-inactivating proteins (RIPs), have been largely exploited in recent years in in vivo experiments of neuronal degeneration consequent to suicide transport or immunolesioning. We have determined both the toxicity of, and the inhibition of, protein synthesis by ricin and volkensin in in vitro cultures enriched in microglial cells, astrocytes, or neurons. In microglial cultures, 50% of toxicity (estimated by LDH released from dead cells) after 24 h exposure to RIPs was obtained with volkensin at 2.2x10(-12) M concentration and 50% of protein synthesis inhibition at 2x10(-14) M concentration. Both values were higher by about one order of magnitude in astrocyte-enriched cultures. Toxicity of, and inhibition of, protein synthesis by, ricin were lower for both cell types by about 1 order of magnitude as compared to volkensin. Cerebellar granule neurons in culture survived remarkably well to 24 h exposure to ricin or volkensin, although their protein synthesis was effectively inhibited by the two toxins with a potency similar to that found for astrocytes. These results demonstrate that glial cells, in particular microglia, are very sensitive to RIPs toxicity and should, therefore, be a primary target of these toxins when injected in vivo. Thus, the damage observed after in vivo experiments could be partly related to diffusion of toxic substances from early-affected glial cells.

Effects of gestational or neonatal treatment with alpha-difluoromethylornithine on ornithine decarboxylase and polyamines in developing rat brain and on adult rat neurochemistry

Pregnant rats were treated for five consecutive days during gestation with s.c. injections of the ornithine decarboxylase (ODC) inhibitor alpha-difluoromethylornithine (DFMO). Treatment beginning at gestational days 13 or 14 was effective in inhibiting ODC and altering polyamine levels, and resulted in relatively small decreases in body and forebrain weight, but not in significant differences in adult neurochemistry. Neonatal rats were treated with DFMO from postnatal day 0 (PD 0) to PD 24. In addition to some somatic effects (decreased body weight, delayed eyelid opening and delayed fur growth) the postnatal treatment resulted in a permanent decrease in brain weight, which was mainly due to a dramatic decrease in cerebellar size. During treatment, and 3 days after the end of it, the levels of putrescine and spermidine, but not those of spermine, were consistently lower in the cerebellum and forebrain of DFMO-treated rats than in controls. On the other hand, ODC appeared strongly inhibited only during the first phase of the treatment and showed recovery, and also rebound of the activity, during the second part of the treatment. A screening of neurochemical markers related to cholinergic, GABAergic and glutamatergic neurons, as well to astrocytes and oligodendrocytes was performed in several brain regions (cerebellum, olfactory bulbs, cortex, striatum, hippocampus) of some of these rats once they became adults. Significant alterations for all the parameters tested, with the exception of the marker for the glutamatergic transmission, were measured in the undersized cerebellum of the neonatally DFMO-treated rats. A shorter neonatal treatment with DFMO (from PD 1 to 6) resulted, in the adult, in decreased cerebellar size and in neurochemical alterations, both very similar to those occurring after the prolonged treatment. In the other brain regions a few minor differences were noticed. The present results show that: (1) the brain polyamine system is differently regulated in foetuses with respect to newborns; (2) the effects of chronic ODC blockade are different on prenatally or postnatally proliferating neurons, due either to a lower sensitivity of gestationally proliferating neurons or to a subsequent recovery; and (3) chronic postnatal ODC inhibition has a strong effect on proliferating neurons, but little effect on further maturation of postmitotic neurons.

Chronic neonatal blockade of NMDA receptor does not affect developmental polyamine metabolism but results in altered response to the excitotoxic induction of ornithine decarboxylase

Neonatal rats were subjected to chronic blockade of the N-methyl-D-aspartate (NMDA) receptor through daily systemic administration of increasing doses of the competitive antagonist CGP 39551 from postnatal days 1-22. Treatment did not result in any significant alteration of the levels of putrescine, spermidine and spermine or in the constitutively expressed activity of the key enzyme for polyamine biosynthesis, ornithine decarboxylase (ODC), as evaluated at 10 and 20 days of age. However, in 30-day-old rats significant differences were observed in the process of excitotoxic ODC induction in the olfactory cortex and the hippocampus of chronically-treated rats: the increase of ODC activity caused by systemic administration of kainic acid took place more rapidly but it was shorter and apparently reached a smaller peak in treated animals as compared to controls. This result, in conjunction with previous data on neurochemistry and locomotor activity of similarly treated rats, strengthens the suggestion that functional alterations of some brain circuits may be the consequence of the blockade of NMDA receptor during the critical neonatal period of brain maturation.

Long-lasting effects of chronic neonatal blockade of N-methyl-D-aspartate receptor through the competitive antagonist CGP 39551 in rats

A competitive antagonist of the N-methyl-D-aspartate receptor, CGP 39551, was administered daily to neonatal rats with increasing doses from postnatal day 1 to 22. These animals displayed approximately 50% decrease of body weight at the end of treatment and, therefore, both normal and neonatally undernourished rats were used as controls. At a young adult stage (55-75 days of age) CGP 39551-treated rats showed a much higher spontaneous locomotor activity as compared to control groups. This hypermotility was counteracted by D1 and D2 dopamine antagonists while administration of methamphetamine increased, to the same extent, the differential basal locomotor activity of treated and control groups. The locomotor activity response to the N-methyl-D-aspartate channel blocker, dizocilpine maleate, was significantly shifted to the right for treated rats so that an equivalent increase of motility was obtained by doubling the dose effective for control animals. In in vivo microdialysis experiments, similar amounts of dopamine were collected from the striatum of treated and control rats after high K+ or methamphetamine stimulation, the only difference being a greater Ca2+ dependency of the depolarization-induced dopamine release in treated rats. Assays for different neurochemical parameters, carried out at 80-90 days of age, suggested some alteration of the balance between excitatory and inhibitory circuits in the basal ganglia of CGP 39551-treated rats. Tyrosine hydroxylase and calbindin immunostaining, as well as acetylcholinesterase histochemistry, revealed a similar picture in the striatum of treated and control rats. However, 5'-nucleotidase histochemistry showed a stronger and evenly distributed reactivity in the striatum of treated rats, opposite to the weaker and patchy localization of normal or undernourished controls. From the present results it is possible to conclude that chronic blockade of the N-methyl-D-aspartate receptor during neonatal brain maturation results in long-lasting alteration of locomotor activity which appears related to functional changes of the dopamine receptors as well as to an altered balance between various excitatory and inhibitory neurotransmitter and neuromodulatory systems.

Impaired neurogenesis by methylazoxymethanol in newborn rats results in transient reduction of ornithine decarboxylase and polyamines in the cerebellum, but not in the olfactory bulbs

Polyamines and the key enzyme for their biosynthesis, ornithine decarboxylase (ODC) play an important role in the control of neuronal proliferation and differentiation. Exposure to agents that interfere with normal cell maturation is expected to result in alteration of neuronal ODC developmental pattern. We have administered to newborn rats, about 6 and 30 hr after birth, 20 mg/kg of methylazoxymethanol acetate (MAM), an agent able to selectively kill dividing cells and we have evaluated ODC activity and polyamine levels in the cerebellum and ODC activity in the olfactory bulbs at various developmental stages starting from postnatal day 4 (PD 4) until PD 28. Cerebellar weight decreased by 22-50% at the different developmental stages in MAM-treated animals. A decline in ODC specific activity was observed at PD 4 and a decrease of putrescine levels at PD 4 and PD 6 in the cerebellum. At PD 10, however, both ODC activity and putrescine level were increased in MAM-treated animals. Spermidine levels were never affected by the treatment, while spermine was significantly decreased at PD 6 and PD 8. These results demonstrate that altered ontogenetic patterns of ODC activity and polyamine levels are the consequence of disturbance of the normal process of brain maturation. No significant differences in specific ODC activity were noticed in the olfactory bulbs of MAM-treated rats. This may be related to the more widespread time-span of neurogenesis in this region, a fact that is also revealed by the higher ODC activity constitutively expressed at times in which neurogenesis has ended in the rest of the brain.