Ischemia-induced disturbances of polyamine synthesis

Effect of difluoromethylornithine on reperfusion injury after temporary middle cerebral artery occlusion

Polyamines have been shown to play an important role in the disturbance of the blood-brain barrier (BBB) in a number of pathological states including ischemia. BBB disturbances may be almost completely prevented by treating animals with the ornithine decarboxylase (ODC) inhibitor, alpha-difluoromethylornithine (DFMO). DFMO has been also shown to prevent N-Methyl-D-aspartate (NMDA) toxicity in tissue cultures. It has been suggested that the pathological disturbances in polyamine metabolism observed following cerebral ischemia, particularly the post-ischemic increase in putrescine, may contribute to the ischemic injury that is most evident in the CA1 subfield of the hippocampus. In this study, effects of DFMO in cerebral ischemia and reperfusion were examined. The results showed that inhibition of the polyamine system by DFMO decreased ischemic injury volume and brain tissue water content in a dose-dependent manner, without change in vital signs, including systemic arterial blood pressure, arterial partial oxygen pressure, regional cerebral blood flow and body temperature.

Ornithine decarboxylase activity in in vivo and in vitro models of cerebral ischemia

Ornithine decarboxylase (ODC) is considered the rate-limiting enzyme in polyamine biosynthesis, and an increase in putrescine after central nervous system (CNS) injury appears to be involved in neuronal death. Cerebral ischemia and reperfusion trigger an active series of metabolic events, which eventually lead to neuronal death. In the present study, ODC activity was evaluated following transient focal cerebral ischemia and reperfusion in rat. The middle cerebral artery (MCA) was occluded for 2 h in male rats with an intraluminal suture technique. Animals were sacrificed between 3 and 48 h of reperfusion following MCA occlusion, and ODC activity was assayed in cortex and striatum. ODC activity was also estimated in an in vitro ischemia model using primary rat cortical neuron cultures, at 6-24 h reoxygenation following 1 h oxygen-glucose deprivation (OGD). In cortex, following ischemia, ODC activity was increased at 3 h (P < .05), reached peak levels by 6-9 h (P < .001) and returned to sham levels by 48 h reperfusion. In striatum the ODC activity followed a similar time course, but returned to basal levels by 24 h. This suggests that ODC activity is upregulated in rat CNS following transient focal ischemia and its time course of activation is region specific. In vitro, ODC activity showed a significant rise only at 24 h reoxygenation following ischemic insult. The release of lactate dehydrogenase (LDH), an indicator for cell damage, was also significantly elevated after OGD. 0.25 mM alpha-difluoromethylornithine (DFMO) inhibited ischemia-induced ODC activity, whereas a 10-mM dose of DFMO appears to provide some neuroprotection by suppressing both ODC activity and LDH release in neuronal cultures, suggesting the involvement of polyamines in the development of neuronal cell death.

Role of polyamine metabolism in kainic acid excitotoxicity in organotypic hippocampal slice cultures

Polyamines are ubiquitous cations that are essential for cell growth, regeneration and differentiation. Increases in polyamine metabolism have been implicated in several neuropathological conditions, including excitotoxicity. However, the precise role of polyamines in neuronal degeneration is still unclear. To investigate mechanisms by which polyamines could contribute to excitotoxic neuronal death, the present study examined the role of the polyamine interconversion pathway in kainic acid (KA) neurotoxicity using organotypic hippocampal slice cultures. Treatment of cultures with N1,N(2)-bis(2,3-butadienyl)-1,4-butanediamine (MDL 72527), an irreversible inhibitor of polyamine oxidase, resulted in a partial but significant neuronal protection, especially in CA1 region. In addition, this pre-treatment also attenuated KA-induced increase in levels of lipid peroxidation, cytosolic cytochrome C release and glial cell activation. Furthermore, pre-treatment with a combination of cyclosporin A (an inhibitor of the mitochondrial permeability transition pore) and MDL 72527 resulted in an additive and almost total neuronal protection against KA toxicity, while the combination of MDL 72527 and EUK-134 (a synthetic catalase/superoxide dismutase mimetic) did not provide additive protection. These data strongly suggest that the polyamine interconversion pathway partially contributes to KA-induced neurodegeneration via the production of reactive oxygen species.

Spermidine/spermine N1-acetyl transferase activity in rat brain following transient focal cerebral ischemia and reperfusion

The polyamine system is very sensitive to different pathological states of brain and is perturbed after central nervous system (CNS) injury. Spermidine/Spermine N(1)-acetyl transferase (SSAT) is the key enzyme responsible for interconversion of spermine and spermidine to spermidine and putrescine respectively. In the present study, SSAT activity was evaluated in the rat CNS, following transient focal cerebral ischemia and reperfusion. The middle cerebral artery (MCA) was occluded for 2 h in male spontaneously hypertensive rats by an intraluminal suture technique. Animals were sacrificed at 3-24 h reperfusion following the MCA occlusion and SSAT activity was assayed in cortex and striatum. Results showed that SSAT activity was significantly increased at 12 h reperfusion in cortex and at 9, 12 and 18 h reperfusion in striatum following ischemia compared to sham or contralateral controls. These results demonstrate that polyamine catabolism in the rat CNS is altered following MCA occlusion. In the in vitro ischemia study, SSAT activity was evaluated in primary cortical neuronal cultures at 6-24 h re-oxygenation intervals following oxygen-glucose deprivation for 1 h, and the results from this group show that the enzyme activity increased by about 62% (P<0.05) at 24 h re-oxygenation. This study suggests that the increased SSAT activity may contribute to the increase in putrescine during the post-ischemic period.

Expression of ODC and its regulatory protein antizyme in the adult rat brain

Ornithine decarboxylase and its inhibitor protein, antizyme are key regulators of polyamine biosynthesis. We examined their expression in the adult rat brain using in situ hybridization and immunocytochemistry. Both genes were widely expressed and their expression patterns were mostly overlapping and relatively similar. The levels of antizyme mRNA were always higher than those of ornithine decarboxylase mRNA. The highest expression for both genes was detected in the cerebellar cortex, hippocampus, hypothalamic paraventricular and supraoptic nuclei, locus coeruleus, olfactory bulb, piriform cortex and pontine nuclei. Ornithine decarboxylase and antizyme mRNAs appeared to be localized in the nerve cells. ODC antibody displayed mainly cytoplasmic staining in all brain areas. Antizyme antibody staining was mainly cytoplasmic in the most brain areas, although predominantly nuclear staining was detected in some areas, most notably in the cerebellar cortex, anterior olfactory nucleus and frontal cortex. Our study is the first detailed and comparative analysis of ornithine decarboxylase and antizyme expression in the adult mammalian brain.

Protective effect of aminoguanidine on hypoxic-ischemic brain damage and temporal profile of brain nitric oxide in neonatal rat

Nitric oxide (NO) produced by inducible NO synthase contributes to ischemic brain damage. However, the role of inducible NO synthase-derived NO on neonatal hypoxic-ischemic encephalopathy has not been clarified. We demonstrate here that aminoguanidine, a relatively selective inhibitor of inducible NO synthase, ameliorated neonatal hypoxic-ischemic brain damage and that temporal profiles of NO correlated with the neuroprotective effect of aminoguanidine. Seven-day-old Wister rat pups were subjected to left carotid artery occlusion followed by 2.5 h of hypoxic exposure (8% oxygen). Infarct volumes (cortical and striatal) were assessed 72 h after the onset of hypoxia-ischemia by planimetric analysis of coronal brain slices stained with hematoxylin-eosin. Aminoguanidine (300 mg/kg i.p.), administered once before the onset of hypoxia-ischemia and then three times daily, significantly ameliorated infarct volume (89% reduction in the cerebral cortex and 90% in the striatum; p<0.001). NO metabolites were measured by means of chemiluminescence using an NO analyzer. In controls, there was a significant biphasic increase in NO metabolites in the ligated side at 1 h (during hypoxia) and at 72 h after the onset of hypoxia (p<0.05). Aminoguanidine did not suppress the first peak but significantly reduced the second one (p<0.05), and markedly reduced infarct size in a neonatal ischemic rat model. Suppression of NO production after reperfusion is a likely mechanism of this neuroprotection.

Effects of MDL 72527, a specific inhibitor of polyamine oxidase, on brain edema, ischemic injury volume, and tissue polyamine levels in rats after temporary middle cerebral artery occlusion

The possible effects of the polyamine interconversion pathway on tissue polyamine levels, brain edema formation, and ischemic injury volume were studied by using a selective irreversible inhibitor, MDL 72527, of the interconversion pathway enzyme, polyamine oxidase. In an intraluminal suture occlusion model of middle cerebral artery in spontaneously hypertensive rats, 100 mg/kg MDL 72527 changed the brain edema formation from 85.7 +/- 0.3 to 84.5 +/- 0.9% in cortex (p < 0.05) and from 79.9 +/- 1.7 to 78.4 +/- 2.0% in subcortex (difference not significant). Ischemic injury volume was reduced by 22% in the cortex (p < 0.05) and 17% in the subcortex (p < 0.05) after inhibition of polyamine oxidase by MDL 72527. There was an increase in tissue putrescine levels together with a decrease in spermine and spermidine levels at the ischemic site compared with the nonischemic site after ischemia-reperfusion injury. The increase in putrescine levels at the ischemic cortical and subcortical region was reduced by a mean of 45% with MDL 72527 treatment. These results suggest that the polyamine interconversion pathway has an important role in the postischemic increase in putrescine levels and that blocking of this pathway can be neuroprotective against neuronal cell damage after temporary focal cerebral ischemia.

Simultaneous assay of ornithine decarboxylase and polyamines after central nervous system injury in gerbil and rat

Ornithine decarboxylase (ODC) is considered the rate-limiting enzyme in polyamine biosynthesis. An increase in putrescine (a natural polyamine) synthesis after central nervous system (CNS) injury appears to be involved in blood-brain barrier dysfunction, development of vasogenic edema and neuronal death. An improved method is described to determine the ODC activity as well as polyamine levels from the same brain tissue. The polyamine results showed no significant differences from data obtained with the conventional assay. The advantages of this method are to: (1) minimize the number of animals needed for the study, and (2) eliminate any internal inconsistencies resulting from use of two independent groups of animals for ODC and polyamine measurements. Using this method, ODC activities and polyamine levels were measured in cortices and hippocampi from global transient ischemia of gerbils and traumatic brain injury (TBI) of rats.

Delayed treatment with aminoguanidine decreases focal cerebral ischemic damage and enhances neurologic recovery in rats

Delayed treatment with aminoguanidine (AG), a relatively selective inhibitor of inducible nitric oxide synthase, ameliorates brain damage produced by occlusion of the rat's middle cerebral artery (MCA). We investigated whether the protection exerted by AG is dose-dependent and whether it is associated with improved neurologic outcome. We also studied the effect of the timing of administration of AG relative to the induction of cerebral ischemia. Halothane-anesthetized spontaneously hypertensive rats underwent permanent MCA occlusion distal to the lenticulostriate branches. Neurologic deficits were assessed daily by the postural reflex test and beam balance test. Infarct volume was determined in thionin- stained sections 96 hours after ischemia and values corrected for swelling. Treatment with AG (intraperitoneally, twice daily), starting 24 hours after MCA occlusion, decreased neocortical infarct volume in comparison to vehicle-treated rats. After correction for swelling, the decrease was 8 +/- 12% at 50 mg/kg (n = 8; P > .05; analysis of variance), 25 +/- 13% at 100 mg/kg (n = 7; P < .05), 30 +/- 16% at 200 mg/kg (n = 7; P < .05) and 32 +/- 9% at 400 mg/kg (n = 5; P < .05). Twenty-four hours after induction of ischemia neurologic deficits scores did not differ between treated and untreated rats (P > .05). However, from 48 to 96 hours after ischemia, neurologic deficits improved significantly in rats treated with AG (100 to 400 mg/kg) compared to rats in which vehicle was administered (P < .05). The decrease in neocortical infarct volume was greatest when AG (100 mg/ kg; twice daily) was administered 12 (26 +/- 17%; n = 9) or 24 hours (25 +/- 13, n = 7) after MCA occlusion. The findings show that AG decreases ischemic brain damage dose-dependently and improves neurologic recovery. Delayed treatment with AG may be a therapeutic strategy to selectively target the evolution of ischemic damage that occurs in the post-ischemic period.

Temporal characteristics of the protective effect of aminoguanidine on cerebral ischemic damage

We investigated the temporal profile of the reduction in focal cerebral ischemic damage exerted by aminoguanidine (AG), an inhibitor of inducible nitric oxide synthase (iNOS). In anesthetized spontaneously hypertensive rats, the middle cerebral artery (MCA) was occluded distal to the origin of the lenticulostriate arteries. Rats were treated with vehicle (saline) or AG (100 mg kg-1, i.p.) immediately after MCA occlusion and, thereafter, two times per day. Rats were sacrificed 1(n = 7), 2(n = 8), 3 (n = 6) or 4 days (n = 5) after MCA occlusion. Injury volume (mm3) was determined in thionin-stained sections using an image analyzer. Volumes were corrected for ischemic swelling. Administration of AG up to 2 days after MCA occlusion did not reduce cerebral ischemic damage (p < 0.05 from vehicle; t-test). Treatment for a longer period decreased injury volume, the reduction averaging 21 +/- 5% at 3 days (p < 0.05) and 30 +/- 9% at 4 days (p < 0.05). Aminoguanidine did not affect ischemic brain swelling (p > 0.05). Administration of AG did not substantially modify arterial pressure, arterial blood gases, pH, hematocrit, plasma glucose and rectal temperature. We conclude that the protective effect of AG is time-dependent and occurs only when the drug is administered for longer than 2 days, starting after induction of ischemia. Because iNOS enzymatic activity develops more than 24 h after MCA occlusion [C. Iadecola, X. Xu, F. Zhang, E.E. El-Fakahany, M.E. Ross, Marked induction of calcium-independent nitric oxide synthase activity after focal cerebral ischemia, J. Cereb. Blood Flow, Metab. 14 (1995) 52-59; C. Iadecola, F. Zhang, X. Xu, R. Casey, M.E. Ross, Inducible nitric oxide synthase gene expression in brain following cerebral ischemia, J. Cereb. Blood Flow Metab. 15 (1995) 378-384.], the data support the hypothesis that the protective effect of AG is medicated by inhibition of iNOS in the post-ischemic brain.

Polyamines and cerebral ischemia

It has been well established that alterations in polyamine metabolism are associated with animal models of global ischemia. Recently, this has been extended to include models of focal ischemia and traumatic brain injury. There is much evidence to support the idea that polyamines may play a multifaceted detrimental role following ischemia reperfusion. Due to the deficit of knowledge about their physiology in the CNS, the link between ischemia-induced alterations in polyamine metabolism and neuronal injury remains to be substantiated. With the recent revelation that polyamines are major intracellular modulators of inward rectifier potassium channels and certain types of NMDA and AMPA receptors, the long wait for the physiologic relevance of these ubiquitous compounds may be in sight. Therefore, it is now conceivable that the alterations in polyamines could have major effects on ion homeostasis in the CNS, especially potassium, and thus account for the observed injury after cerebral ischemia.

Polyamines induce blood-brain barrier disruption and edema formation in the rat

Polyamines (PA) are derived from ornithine by the enzyme ornithine decarboxylase (ODC), which is activated very rapidly as acute and delayed responses to brain ischemia and trauma. Polyamines play a role in the disruption of the blood-brain barrier (BBB) in different pathological states. This study examined the effect of exogenous polyamines, administered intracerebrally (i.c.v.) or intracarotidly on BBB function. Putrescine, spermidine and spermine, given individually, were found to disrupt BBB integrity within 15 min of i.c.v. administration (p = 0.03; p = 0.0013; p = 0.042 vs saline treated rats, respectively). The effect was still evident after 1 h; however, since the saline treated rats also showed increased permeability of Evans blue at this time, there was no statistical difference between polyamines or saline treated rats 1 h post injection. When injected into the carotid artery, rapid increase in BBB permeability was found 1 min after putrescine and spermidine (p < 0.01 vs saline), with a slight decline at 15 min. A slower effect was noticed after spermine administration which reached significance only at 15 min. These results suggest a role for PA as mediators of vasogenic edema formation in the brain soon after brain injuries which induce increased production of these compounds.

Pharmacological implications of inward rectifier K+ channels regulation by cytoplasmic polyamines

The powerful combination of molecular biology and electrophysiology has allowed extraordinary progress in the field of ion channel structure-function. In fact, only 10 years have passed since the first amino acid sequence of a voltage-dependent ion channel, the Na+ channel, was deduced [1], and already the structural domains involved in ion channel permeation, block and gating have been identified in many channel types. Despite this progress, in most cases the correlation between specific domains and ion channel function is still speculative at present, due to the absence of direct structural information [2]. In this review we will describe recent progress in the field of structure-function of one class of K+ channels, the inward rectifiers (IRKs). In particular, we will review the sequences of structure-function experiments which have led to the discovery of a novel regulation of IRKs by cytoplasmic organic polycationic substances like polyamines (PAs). This discovery represents a paradigm for how structure-function information has preceded and made possible the identification of physiological mechanisms of ion channel regulation. Owing to the important role played by IRKs in the regulation of resting membrane potential, a major determinant of cellular transport and volume [3], and to the established link between PAs and cell growth and division, the direct regulation of IRKs by PAs assumes a critical importance for the pharmacological control of cell growth and neoplastic transformation.

Reexpression of developmentally regulated MAP2c mRNA after ischemia: colocalization with hsp72 mRNA in vulnerable neurons

Levels of mRNAs encoding the microtubule-associated proteins MAP2b and MAP2c as well as the 70-kDa stress protein [72-kDa heat shock protein (hsp72)] were evaluated in postischemic rat brain by in situ hybridization with oligonucleotide probes corresponding to the known rat sequences. Rats were subjected to 10-min cardiac arrest, produced by compression of major thoracic vessels, followed by resuscitation. The normally expressed MAP2b mRNA showed transient twofold elevations in all hippocampal neuron populations at 6-h recirculation, followed by a return to control levels by 24 h. MAP2b hybridization was progressively lost thereafter from the vulnerable CA1 and outer cortical layers, preceding both the fall in immunoreactive MAP2b and the eventual cell loss in these regions. The depletion of MAP2b mRNA coincided with an increase in the alternatively spliced MAP2c in vulnerable regions during 12-48 h of recirculation, precisely overlapping the late component of hsp72 expression that persisted in these cell populations. Previous studies have suggested that the initial induction of hsp72 provides an index of potential postischemic injury in neuron populations that may or may not be injured, while lasting hsp72 mRNA expression is associated with cell damage. In contrast, the present results demonstrate that MAP2c expression under these conditions occurs uniquely in neuron populations subject to injury. Available evidence suggests that MAP2c expression represents a plastic response in subpopulations of neurons that will survive in these regions, although it remains to be explicitly determined whether it may also be transiently expressed in dying cells. In any case, these observations demonstrate that reexpression of developmentally regulated MAP2c mRNA is a relatively late postischemic response in vulnerable cell populations, indicating that pathways regulating MAP2 splicing may be closely associated with mechanisms of neuron injury and/or recovery.

Increased immunostaining for L-ornithine decarboxylase occurs in neocortical neurons of Alzheimer's disease patients

We investigated the distribution of L-ornithine decarboxylase (ODC), an enzyme known to be involved in several developmental and restorative processes, in neocortical brain areas of Alzheimer's disease (AD) and normal patients by means of immunohistochemistry. While ODC immunoreactive material was only scarcely distributed in neocortical neurons of control brains, neocortical specimens from AD brains stood out by intense immunostaining for ODC. Dendrites and, to a lesser extent, axons of neurons from AD brains showed a strong immunoreaction to the enzyme, whereas neurons from non-affected brains displayed only a weak circumnuclear reaction pattern. Our results support the idea that neurorestorative processes take place in AD brains and that the ODC/polyamine system might be actively involved in this process.