Transgenic animals as models in the study of the neurobiological role of polyamines

Polyamines and polyamine-metabolizing enzymes in schizophrenia: Current knowledge and concepts of therapy

Polyamines play preeminent roles in a variety of cellular functions in the central nervous system and other organs. A large body of evidence suggests that the polyamine pathway is prominently involved in the etiology and pathology of schizophrenia. Alterations in the expression and activity of polyamine metabolizing enzymes, as well as changes in the levels of the individual polyamines, their precursors and derivatives, have been measured in schizophrenia and animal models of the disease. Additionally, neuroleptic treatment has been shown to influence polyamine concentrations in brain and blood of individuals with schizophrenia. Thus, the polyamine system may appear to be a promising target for neuropharmacological treatment of schizophrenia. However, for a number of practical reasons there is currently only limited hope for a polyamine-based schizophrenia therapy.

GABAergic input through GABAB receptors is necessary during a perinatal window to shape gene expression of factors critical to reproduction such as Kiss1

Lack of GABAB receptors in GABAB1 knockout mice decreases neonatal ARC kisspeptin 1 (Kiss1) expression in the arcuate nucleus of the hypothalamus (ARC) in females, which show impaired reproduction as adults. Our aim was to selectively impair GABAB signaling during a short postnatal period to evaluate its impact on the reproductive system. Neonatal male and female mice were injected with the GABAB antagonist CGP 55845 (CGP, 1 mg/kg body wt sc) or saline from postnatal day 2 (PND2) to PND6, three times per day (8 AM, 1 PM, and 6 PM). One group was killed on PND6 for collection of blood samples (hormones by radioimmunoassay), brains for gene expression in the anteroventral periventricular nucleus-periventricular nucleus continuum (AVPV/PeN), and ARC micropunches [quantitative PCR (qPCR)] and gonads for qPCR, hormone contents, and histology. A second group of mice was injected with CGP (1 mg/kg body wt sc) or saline from PND2 to PND6, three times per day (8 AM, 1 PM, and 6 PM), and left to grow to adulthood. We measured body weight during development and parameters of sexual differentiation, puberty onset, and estrous cycles. Adult mice were killed, and trunk blood (hormones), brains for qPCR, and gonads for qPCR and hormone contents were obtained. Our most important findings on PND6 include the CGP-induced decrease in ARC Kiss1 and increase in neurokinin B (Tac2) in both sexes; the decrease in AVPV/PeN tyrosine hydroxylase (Th) only in females; the increase in gonad estradiol content in both sexes; and the increase in primordial follicles and decrease in primary and secondary follicles. Neonatally CGP-treated adults showed decreased ARC Kiss1 and ARC gonadotropin-releasing hormone (Gnrh1) and increased ARC glutamic acid decarboxylase 67 (Gad1) only in males; increased ARC GABAB receptor subunit 1 (Gabbr1) in both sexes; and decreased AVPV/PeN Th only in females. We demonstrate that ARC Kiss1 expression is chronically downregulated in males and that the normal sex difference in AVPV/PeN Th expression is abolished. In conclusion, neonatal GABAergic input through GABAB receptors shapes gene expression of factors critical to reproduction.

Epileptic seizures and oxidative stress in a mouse model over-expressing spermine oxidase

Several studies have demonstrated high polyamine levels in brain diseases such as epilepsy. Epilepsy is the fourth most common neurological disorder and affects people of all ages. Excitotoxic stress has been associated with epilepsy and it is considered one of the main causes of neuronal degeneration and death. The transgenic mouse line Dach-SMOX, with CD1 background, specifically overexpressing spermine oxidase in brain cortex, has been proven to be highly susceptible to epileptic seizures and excitotoxic stress induced by kainic acid. In this study, we analysed the effect of spermine oxidase over-expression in a different epileptic model, pentylenetetrazole. Behavioural evaluations of transgenic mice compared to controls showed a higher susceptibility towards pentylentetrazole. High-performance liquid chromatography analysis of transgenic brain from treated mice revealed altered polyamine content. Immunoistochemical analysis indicated a rise of 8-oxo-7,8-dihydro-2'-deoxyguanosine, demonstrating an increase in oxidative damage, and an augmentation of system xc- as a defence mechanism. This cascade of events can be initially linked to an increase in protein kinase C alpha, as shown by Western blot. This research points out the role of spermine oxidase, as a hydrogen peroxide producer, in the oxidative stress during epilepsy. Moreover, Dach-SMOX susceptibility demonstrated by two different epileptic models strongly indicates this transgenic mouse line as a potential animal model to study epilepsy.

Regulation of Excitatory Amino Acid Transmission in the Retina: Studies on Neuroprotection

Excitotoxicity occurs in neurons due to the accumulation of excitatory amino acids such as glutamate in the synaptic and extrasynaptic locations. In the retina, excessive glutamate concentrations trigger a neurotoxic cascade involving several mechanisms, including the elevation of intracellular calcium (Ca²⁺⁾ and the activation of α-amino-3-hydroxy 5-methyl-4-iso-xazole-propionic acid/kainate (AMPA/KA) and N-methyl-d-aspartate (NMDA) receptors leading to retinal degeneration. Both ionotropic glutamate receptors (iGluRs) and metabotropic glutamate receptors (mGluRs) are present in the mammalian retina. Indeed, due to the abundant expression of GluRs, the mammalian retina is highly susceptible to excitotoxic neurodegeneration. Excitotoxicity has been postulated to present a common downstream mechanism for several stimuli, including hypoglycemia, hypoxia, ischemia, and chronic neurodegenerative diseases. Experimental approaches to the study of neuroprotection in the retina have utilized insults that trigger hypoxia, hypoglycemia, or excitotoxicity. Using these experimental approaches, the neuroprotective potential of GluR agents, including the NMDA receptor modulators (MK801, ifenprodil, memantine); AMPA/KA receptor antagonist (CNQX); Group II and III mGluR agonists (LY354740, quisqualate); and Ca²⁺-channel blockers (diltiazem, lomerizine, verapamil, ω-conotoxin), and others (pituitary adenylate cyclase activating polypeptide, neuropeptide Y, acetylcholine receptor agonists) have been elucidated. In addition to corroborating the exocytotic role of excitatory amino acids in retinal degeneration, these studies affirm that multiple mechanism/s contribute to the prevention of damage caused by excitotoxicity in the retina. Therefore, it is feasible that several pathways are involved in protecting the retina from toxic insults in ocular neurodegenerative conditions such as glaucoma and retinal ischemia. Furthermore, these experimental models are viable tools for evaluating therapeutic candidates in ocular neuropathies.

Treatment with polyamine oxidase inhibitor reduces microglial activation and limits vascular injury in ischemic retinopathy

Retinal vascular injury is a major cause of vision impairment in ischemic retinopathies. Insults such as hyperoxia, oxidative stress and inflammation contribute to this pathology. Previously, we showed that hyperoxia-induced retinal neurodegeneration is associated with increased polyamine oxidation. Here, we are studying the involvement of polyamine oxidases in hyperoxia-induced injury and death of retinal vascular endothelial cells. New-born C57BL6/J mice were exposed to hyperoxia (70% O2) from postnatal day (P) 7 to 12 and were treated with the polyamine oxidase inhibitor MDL 72527 or vehicle starting at P6. Mice were sacrificed after different durations of hyperoxia and their retinas were analyzed to determine the effects on vascular injury, microglial cell activation, and inflammatory cytokine profiling. The results of this analysis showed that MDL 72527 treatment significantly reduced hyperoxia-induced retinal vascular injury and enhanced vascular sprouting as compared with the vehicle controls. These protective effects were correlated with significant decreases in microglial activation as well as levels of inflammatory cytokines and chemokines. In order to model the effects of polyamine oxidation in causing microglial activation in vitro, studies were performed using rat brain microvascular endothelial cells treated with conditioned-medium from rat retinal microglia stimulated with hydrogen peroxide. Conditioned-medium from activated microglial cultures induced cell stress signals and cell death in microvascular endothelial cells. These studies demonstrate the involvement of polyamine oxidases in hyperoxia-induced retinal vascular injury and retinal inflammation in ischemic retinopathy, through mechanisms involving cross-talk between endothelial cells and resident retinal microglia.

New Insights on Astrocyte Ion Channels: Critical for Homeostasis and Neuron-Glia Signaling

Initial biophysical studies on glial cells nearly 50 years ago identified these cells as being electrically silent. These first studies also demonstrated a large K(+) conductance, which led to the notion that glia may regulate extracellular K(+) levels homeostatically. This view has now gained critical support from the study of multiple disease models discussed herein. Dysfunction of a major astrocyte K(+) channel, Kir4.1, appears as an early pathological event underlying neuronal phenotypes in several neurodevelopmental and neurodegenerative diseases. An expanding list of other astrocyte ion channels, including the calcium-activated ion channel BEST-1, hemichannels, and two-pore domain K(+) channels, all contribute to astrocyte biology and CNS function and underpin new forms of crosstalk between neurons and glia. Once considered merely the glue that holds the brain together, it is now increasingly recognized that astrocytes contribute in several fundamental ways to neuronal function. Emerging new insights and future perspectives of this active research area are highlighted within.

SIGNIFICANCE STATEMENT

The critical role of astrocyte potassium channels in CNS homeostasis has been reemphasized by recent studies conducted in animal disease models. Emerging evidence also supports the signaling role mediated by astrocyte ion channels such as BEST1, hemichannels, and two-pore channels, which enable astrocytes to interact with neurons and regulate synaptic transmission and plasticity. This minisymposium highlights recent developments and future perspectives of these research areas.

A New Transgenic Mouse Model for Studying the Neurotoxicity of Spermine Oxidase Dosage in the Response to Excitotoxic Injury

Spermine oxidase is a FAD-containing enzyme involved in polyamines catabolism, selectively oxidizing spermine to produce H2O2, spermidine, and 3-aminopropanal. Spermine oxidase is highly expressed in the mouse brain and plays a key role in regulating the levels of spermine, which is involved in protein synthesis, cell division and cell growth. Spermine is normally released by neurons at synaptic sites where it exerts a neuromodulatory function, by specifically interacting with different types of ion channels, and with ionotropic glutamate receptors. In order to get an insight into the neurobiological roles of spermine oxidase and spermine, we have deregulated spermine oxidase gene expression producing and characterizing the transgenic mouse model JoSMOrec, conditionally overexpressing the enzyme in the neocortex. We have investigated the effects of spermine oxidase overexpression in the mouse neocortex by transcript accumulation, immunohistochemical analysis, enzymatic assays and polyamine content in young and aged animals. Transgenic JoSMOrec mice showed in the neocortex a higher H2O2 production in respect to Wild-Type controls, indicating an increase of oxidative stress due to SMO overexpression. Moreover, the response of transgenic mice to excitotoxic brain injury, induced by kainic acid injection, was evaluated by analysing the behavioural phenotype, the immunodistribution of neural cell populations, and the ultrastructural features of neocortical neurons. Spermine oxidase overexpression and the consequently altered polyamine levels in the neocortex affects the cytoarchitecture in the adult and aging brain, as well as after neurotoxic insult. It resulted that the transgenic JoSMOrec mouse line is more sensitive to KA than Wild-Type mice, indicating an important role of spermine oxidase during excitotoxicity. These results provide novel evidences of the complex and critical functions carried out by spermine oxidase and spermine in the mammalian brain.

Defining the role of polyamines in colon carcinogenesis using mouse models

Genetics and diet are both considered important risk determinants for colorectal cancer, a leading cause of death in the US and worldwide. Genetically engineered mouse (GEM) models have made a significant contribution to the characterization of colorectal cancer risk factors. Reliable, reproducible, and clinically relevant animal models help in the identification of the molecular events associated with disease progression and in the development of effictive treatment strategies. This review is focused on the use of mouse models for studying the role of polyamines in colon carcinogenesis. We describe how the available mouse models of colon cancer such as the multiple intestinal neoplasia (Min) mice and knockout genetic models facilitate understanding of the role of polyamines in colon carcinogenesis and help in the development of a rational strategy for colon cancer chemoprevention.

Effect of L-arginine on metabolism of polyamines in rat's brain with extrahepatic cholestasis

Cholestatic encephalopathy results from accumulation of unconjugated bilirubin and hydrophobic bile acids in the brain. The aim of this study was to determine disturbances of polyamine metabolism in the brains of rats with experimental extrahepatic cholestasis and the effects of L-arginine administration. Wister rats were divided into groups: I: sham-operated, II: rats treated with L-arginine, III: animals with bile-duct ligation (BDL), and IV: cholestatic-BDL rats treated with L-arginine. Increased plasma gamma-glutamyltransferase and alkaline phosphatase activity and increased bile-acids and bilirubin levels in BDL rats were reduced by administration of L-arginine (P < 0.001). Cholestasis increased the brain's putrescine (P < 0.001) and decreased spermidine and spermine concentration (P < 0.05). The activity of polyamine oxidase was increased (P < 0.001) and diamine oxidase was decreased (P < 0.001) in the brains of BDL rats. Cholestasis increased the activity of arginase (P < 0.05) and decreased the level of citrulline (P < 0.001). Administration of L-arginine in BDL rats prevents metabolic disorders of polyamines and establishes a neuroprotective role in the brain during cholestasis.

Altered subcellular localization of ornithine decarboxylase in Alzheimer's disease brain

The amyloid precursor protein can through ligand-mimicking induce expression of ornithine decarboxylase (ODC), the initial and rate-limiting enzyme in polyamine biosynthesis. We report here the regional distribution and cellular localization of ODC immunoreactivity in Alzheimer's disease (AD) brains. In frontal cortex and hippocampus of control cases, the most pronounced ODC immunoreactivity was found in the nucleus. In possible and definite AD the immunoreactivity had shifted to the cytoplasm. In cerebellum of control cases, ODC staining was found in a small portion of Purkinje cells, mostly in the nucleus. In AD, both possible and definite, the number of stained Purkinje cells increased significantly and immunoreactivity was shifted to the cytoplasm, even though it was still prominent in the nucleus. In conclusion, our study reveals an early shift of the ODC immunoreactivity in AD from the nuclear compartment towards the cytoplasm.

Animal disease models generated by genetic engineering of polyamine metabolism

The polyamines putrescine, spermidine and spermine are natural components of all living cells. Although their exact cellular functions are still largely unknown, a constant supply of these compounds is required for mammalian cell proliferation to occur. Studies with animals displaying genetically altered polyamine metabolism have shown that polyamines are intimately involved in the development of diverse tumors, putrescine apparently has specific role in skin physiology and neuroprotection and the higher polyamines spermidine and spermine are required for the maintenance of pancreatic integrity and liver regeneration. In the absence of ongoing polyamine biosynthesis, murine embryogenesis does not proceed beyond the blastocyst stage. The last years have also witnessed the appearance of the first reports linking genetically altered polyamine metabolism to human diseases.

Overproduction of cardiac S-adenosylmethionine decarboxylase in transgenic mice

The present study was designed to provide a better understanding of the role played by AdoMetDC (S-adenosylmethionine decarboxylase), the key rate-controlling enzyme in the synthesis of spermidine and spermine, in controlling polyamine levels and the importance of polyamines in cardiac physiology. The alphaMHC (alpha-myosin heavy chain) promoter was used to generate transgenic mice with cardiac-specific expression of AdoMetDC. A founder line (alphaMHC/AdoMetDC) was established with a >100-fold increase in AdoMetDC activity in the heart. Transgene expression was maximal by 1 week of age and remained constant into adulthood. However, the changes in polyamine levels were most pronounced during the first week of age, with a 2-fold decrease in putrescine and spermidine and a 2-fold increase in spermine. At later times, spermine returned to near control levels, whereas putrescine and spermidine levels remained lower, suggesting that compensatory mechanisms exist to limit spermine accumulation. The alphaMHC/AdoMetDC mice did not display an overt cardiac phenotype, but there was an increased cardiac hypertrophy after beta-adrenergic stimulation with isoprenaline ('isoproterenol'), as well as a small increase in spermine content. Crosses of the alphaMHC/AdoMetDC with alphaMHC/ornithine decarboxylase mice that have a >1000-fold increase in cardiac ornithine decarboxylase were lethal in utero, presumably due to increase in spermine to toxic levels. These findings suggest that cardiac spermine levels are highly regulated to avoid polyamine-induced toxicity and that homoeostatic mechanisms can maintain non-toxic levels even when one enzyme of the biosynthetic pathway is greatly elevated but are unable to do so when two biosynthetic enzymes are increased.

Genetic Manipulation of Polyamine Catabolism in Rodents

Activation of polyamine catabolism through the overexpression of spermidine/spermine N1-acetyltransferase (SSAT) in transgenic rodents does not only lead to distorted tissue polyamine homeostasis, manifested as striking accumulation of putrescine, appearance N1-acetylspermidine and reduction of tissue spermidine and/or spermine pools, but likewise creates striking phenotypic changes. The latter include loss of hair, lipoatrophy and female infertility. Forced expression of SSAT modulates skin, prostate and intestinal carcinogenesis, induces acute pancreatitis and blocks early liver regeneration. Although many of these features are directly attributable to altered tissue polyamine pools, some of them are more likely related to the greatly accelerated flux of the polyamines caused by activated catabolism and compensatorily enhanced biosynthesis.

Regional and temporal alterations of ODC/polyamine system during ALS-like neurodegenerative motor syndrome in G93A transgenic mice

Natural polyamines (putrescine, spermidine and spermine) are ubiquitous molecules known to regulate a number of physiological processes and suspected to play a role also in various pathological conditions. Changes in polyamine levels and in their biosynthetic enzymes have been described for some neurodegenerative diseases but the available data are incomplete and somewhat contradictory. We report here alterations of the key enzyme of the polyamine pathway, ornithine decarboxylase (ODC) catalytic activity and polyamine levels in different CNS areas from SOD1 G39A transgenic mice, an animal model for amyotrophic lateral sclerosis (ALS). ODC catalytic activity, was found significantly increased both in the cervical and lumbar spinal cord and, to a lesser extent in the brain stem of transgenic mice at a symptomatic stage of the disease (125-day-old mice), while no differences were present at a pre-symptomatic stage (55-day-old mice). In parallel with the increase of ODC activity putrescine levels were several times increased in both cervical and lumbar spinal cord and in the brain stem of 125-day-old SOD1 G39A mice. Higher order polyamines were not increased except for a significant increase of spermidine in the cervical spinal cord. The present data demonstrate considerable alterations of the ODC/polyamine system in a reliable animal model of ASL, consistent with their role in neurodegeneration and in particular in motor neuron diseases.

Toward the prediction of CNS drug-effect profiles in physiological and pathological conditions using microdialysis and mechanism-based pharmacokinetic-pharmacodynamic modeling

Our ultimate goal is to develop mechanism-based pharmacokinetic (PK)-pharmacodynamic (PD) models to characterize and to predict CNS drug responses in both physiologic and pathologic conditions. To this end, it is essential to have information on the biophase pharmacokinetics, because these may significantly differ from plasma pharmacokinetics. It is anticipated that biophase kinetics of CNS drugs are strongly influenced by transport across the blood-brain barrier (BBB). The special role of microdialysis in PK/PD modeling of CNS drugs lies in the fact that it enables the determination of free-drug concentrations as a function of time in plasma and in extracellular fluid of the brain, thereby providing important data to determine BBB transport characteristics of drugs. Also, the concentrations of (potential) extracellular biomarkers of drug effects or disease can be monitored with this technique. Here we describe our studies including microdialysis on the following: (1) the evaluation of the free drug hypothesis; (2) the role of BBB transport on the central effects of opioids; (3) changes in BBB transport and biophase equilibration of anti-epileptic drugs; and (4) the relation among neurodegeneration, BBB transport, and drug effects in Parkinson's disease progression.

Overexpression of ADC2 in Arabidopsis induces dwarfism and late-flowering through GA deficiency

We have obtained Arabidopsis thaliana transgenic plants constitutively overexpressing ADC2, one of the two genes encoding arginine decarboxylase (ADC) in Arabidopsis. These plants contained very high levels of putrescine (Put) but no changes were observed in spermidine and spermine contents. The results obtained from quantification of free and conjugated polyamines suggest that conjugation may be a limiting step for control of Put homeostasis within a non-toxic range for plant survival. Transgenic plants with increased levels of ADC2 transcript and elevated Put content showed dwarfism and late-flowering, and the phenotype was rescued by gibberellin A3 (GA3) application. The contents of bioactive GA4 and GA1, and of GA9 (a precursor of GA4), as well as the levels of AtGA20ox1, AtGA3ox1 and AtGA3ox3 transcripts (quantified by real-time PCR) were lower in the ADC2 overexpressor plants than in the wild type. No change in the expression of genes encoding earlier enzymes in the GA biosynthesis pathway was detected by microarray analysis. These results suggest that Put accumulation affects GA metabolism through the repression of biosynthetic steps catalyzed by GA 20-oxidase and GA 3-oxidase.

Characterization of transgenic mice with widespread overexpression of spermine synthase

A widespread increase in SpmS (spermine synthase) activity has been produced in transgenic mice using a construct in which the human SpmS cDNA was placed under the control of a composite CMV-IE (cytomegalovirus immediate early gene) enhancer-chicken beta-actin promoter. Four separate founder CAG/SpmS mice were studied. Transgenic expression of SpmS was found in all of the tissues examined, but the relative SpmS activities varied widely according to the founder animal and the tissue studied. Very large increases in SpmS activity were seen in many tissues. SpdS (spermidine synthase) activity was not affected. Although there was a statistically significant decline in spermidine content and increase in spermine, the alterations were small compared with the increase in SpmS activity. These results provide strong support for the concept that the levels of the higher polyamines spermidine and spermine are not determined only by the relative activities of the two aminopropyltransferases. Other factors such as availability of the aminopropyl donor substrate decarboxylated S-adenosylmethionine and possibly degradation or excretion must also influence the spermidine/spermine ratio. No deleterious effects of SpmS overexpression were seen. The mice had normal growth, fertility and behaviour up to the age of 12 months. However, breeding the CAG/SpmS mice with MHC (alpha-myosin heavy chain)/AdoMetDC (S-adenosylmethionine decarboxylase) mice, which have a large increase in S-adenosylmethionine decarboxylase expression in heart, was lethal. In contrast, breeding the CAG/SpmS mice with MHC/ODC (L-ornithine decarboxylase) mice, which have a large increase in cardiac ornithine decarboxylase expression, had a protective effect in preventing the small decrease in viability of the MHC/ODC mice.

X-linked spermine synthase gene (SMS) defect: the first polyamine deficiency syndrome

Polyamines (putrescine, spermidine, spermine) are ubiquitous, simple molecules that interact with a variety of other molecules in the cell, including nucleic acids, phospholipids and proteins. Various studies indicate that polyamines are essential for normal cell growth and differentiation. Furthermore, these molecules, especially spermine, have been shown to modulate ion channel activities of certain cells. Nonetheless, little is known about the specific cellular functions of these compounds, and extensive laboratory investigations have failed to identify a heritable condition in humans in which polyamine synthesis is perturbed. We report the first polyamine deficiency syndrome caused by a defect in spermine synthase (SMS). The defect results from a splice mutation, and is associated with the Snyder-Robinson syndrome (SRS, OMIM_309583), an X-linked mental retardation disorder. The affected males have mild-to-moderate mental retardation (MR), hypotonia, cerebellar circuitry dysfunction, facial asymmetry, thin habitus, osteoporosis, kyphoscoliosis, decreased activity of SMS, correspondingly low levels of intracellular spermine in lymphocytes and fibroblasts, and elevated spermidine/spermine ratios. The clinical features observed in SRS are consistent with cerebellar dysfunction and a defective functioning of red nucleus neurons, which, at least in rats, contain high levels of spermine. Additionally, the presence of MR reflects a role for spermine in cognitive function, possibly by spermine's ability to function as an 'intrinsic gateway' molecule for inward rectifier K(+) channels.

Genetic approaches to the cellular functions of polyamines in mammals

The polyamines putrescine, spermidine and spermine are organic cations shown to participate in a bewildering number of cellular reactions, yet their exact functions in intermediary metabolism and specific interactions with cellular components remain largely elusive. Pharmacological interventions have demonstrated convincingly that a steady supply of these compounds is a prerequisite for cell proliferation to occur. The last decade has witnessed the appearance of a substantial number of studies, in which genetic engineering of polyamine metabolism in transgenic rodents has been employed to unravel their cellular functions. Transgenic activation of polyamine biosynthesis through an overexpression of their biosynthetic enzymes has assigned specific roles for these compounds in spermatogenesis, skin physiology, promotion of tumorigenesis and organ hypertrophy as well as neuronal protection. Transgenic activation of polyamine catabolism not only profoundly disturbs polyamine homeostasis in most tissues, but also creates a complex phenotype affecting skin, female fertility, fat depots, pancreatic integrity and regenerative growth. Transgenic expression of ornithine decarboxylase antizyme has suggested that this unique protein may act as a general tumor suppressor. Homozygous deficiency of the key biosynthetic enzymes of the polyamines, ornithine and S-adenosylmethionine decarboxylase, as achieved through targeted disruption of their genes, is not compatible with murine embryogenesis. Finally, the first reports of human diseases apparently caused by mutations or rearrangements of the genes involved in polyamine metabolism have appeared.

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.

Polyamines and central nervous system injury: spermine and spermidine decrease following transient focal cerebral ischemia in spontaneously hypertensive rats

Polyamines (putrescine, spermidine and spermine) are ubiquitous cellular components, but their specific role in central nervous system (CNS) injury has yet to be characterized. CNS injury results in increased activities of ornithine decarboxylase and spermidine/spermine-N(1)-acetyltransferase, and accumulation of putrescine. The present study determined the polyamine profile in three models of CNS injury, in two different species (gerbil and rat) and two strains of rats (Sprague-Dawley and spontaneously hypertensive): (1) transient focal cerebral ischemia in spontaneously hypertensive rats (SHR); (2) traumatic brain injury in Sprague-Dawley rats; and (3) transient forebrain ischemia in gerbils. While there was a significant increase in putrescine in all three models, spermine and spermidine levels were unaltered in forebrain ischemia and traumatic brain injury. However, transient focal cerebral ischemia shows depletion of spermine and spermidine levels in injured hemisphere compared to contralateral region. Exogenous spermine significantly restored the spermine as well as spermidine levels in the ipsilateral hemisphere after transient focal cerebral ischemia, but did not alter putrescine levels or the ratio of spermidine to spermine. The loss of spermine in particular, may have several consequences that contribute to ischemic injury, including destabilization of chromatin, decreased mitochondrial Ca(2+) buffering capacity, and increased susceptibility to oxidative stress. Based on our and other studies, we propose a tentative antioxidant mechanism of spermine neuroprotection.

Gene expression profiling reveals alterations of specific metabolic pathways in schizophrenia

Dysfunction of the dorsal prefrontal cortex (PFC) in schizophrenia may be associated with alterations in the regulation of brain metabolism. To determine whether abnormal expression of genes encoding proteins involved in cellular metabolism contributes to this dysfunction, we used cDNA microarrays to perform gene expression profiling of all major metabolic pathways in postmortem samples of PFC area 9 from 10 subjects with schizophrenia and 10 matched control subjects. Genes comprising 71 metabolic pathways were assessed in each pair, and only five pathways showed consistent changes (decreases) in subjects with schizophrenia. Reductions in expression were identified for genes involved in the regulation of ornithine and polyamine metabolism, the mitochondrial malate shuttle system, the transcarboxylic acid cycle, aspartate and alanine metabolism, and ubiquitin metabolism. Interestingly, although most of the metabolic genes that were consistently decreased across subjects with schizophrenia were not similarly decreased in haloperidol-treated monkeys, the transcript encoding the cytosolic form of malate dehydrogenase displayed prominent drug-associated increases in expression compared with untreated animals. These molecular analyses implicate a highly specific pattern of metabolic alterations in the PFC of subjects with schizophrenia and raise the possibility that antipsychotic medications may exert a therapeutic effect, in part, by normalizing some of these changes.

Influence of SAMe on the modifications of brain polyamine levels in an animal model of depression

The mechanism(s) of the antidepressant activity of S-adenosyl-L-methionine (SAMe) have not yet been elucidated. SAMe is essential for the synthesis of polyamines, which have a key role in protein synthesis, cell proliferation, and neuronal plasticity. On the other hand, accumulating data indicate that depression is associated with a reduction in regional brain volume and that antidepressants increase neurogenesis in defined brain regions and also influence neuronal plasticity. Here we show that in a validated rat model of depression (chronic unpredictable mild stress-induced anhedonia) there is a significant reduction of putrescine, spermidine and spermine in the hippocampus, and of only putrescine in the nucleus accumbens septi. SAMe, at a fully antidepressant dose (300 mg/kg i.m., daily for 7 days), completely restores the levels of putrescine in the nucleus accumbens, and restores in part the levels of both spermidine and spermine in the hippocampus. These results may suggest (i) a role for brain polyamines in depression and in reward processes, and (ii) that the antidepressant effect of SAMe may be due, at least in part, to a normalization of putrescine levels in the nucleus accumbens septi.

Transgenic manipulation of the metabolism of polyamines in poplar cells

The metabolism of polyamines (putrescine, spermidine, and spermine) has become the target of genetic manipulation because of their significance in plant development and possibly stress tolerance. We studied the polyamine metabolism in non-transgenic (NT) and transgenic cells of poplar (Populus nigra x maximowiczii) expressing a mouse Orn decarboxylase (odc) cDNA. The transgenic cells showed elevated levels of mouse ODC enzyme activity, severalfold higher amounts of putrescine, a small increase in spermidine, and a small reduction in spermine as compared with NT cells. The conversion of labeled ornithine (Orn) into putrescine was significantly higher in the transgenic than the NT cells. Whereas exogenously supplied Orn caused an increase in cellular putrescine in both cell lines, arginine at high concentrations was inhibitory to putrescine accumulation. The addition of urea and glutamine had no effect on polyamines in either of the cell lines. Inhibition of glutamine synthetase by methionine sulfoximine led to a substantial reduction in putrescine and spermidine in both cell lines. The results show that: (a) Transgenic expression of a heterologous odc gene can be used to modulate putrescine metabolism in plant cells, (b) accumulation of putrescine in high amounts does not affect the native arginine decarboxylase activity, (c) Orn biosynthesis occurs primarily from glutamine/glutamate and not from catabolic breakdown of arginine, (d) Orn biosynthesis may become a limiting factor for putrescine production in the odc transgenic cells, and (e) assimilation of nitrogen into glutamine keeps pace with an increased demand for its use for putrescine production.

Human, bovine, and rabbit retinal glutamate-induced [3H]D-aspartate release: role in excitotoxicity

The pharmacological basis of glutamate-induced [3H]D-aspartate release was investigated in isolated human, bovine and rabbit retinas. Isolated mammalian retinas were preloaded with [3H]D-aspartate and then prepared for studies of neurotransmitter release using the superfusion method. Release of [3H]D-aspartate was elicited by K+ (50 mM) or by L-glutamate. In bovine retinas, L-glutamate, but not D-glutamate induced an overflow of [3H]D-aspartate that was partially inhibited by low external calcium, omega-conotoxin (10 nM) or nitrendipine (1 microM). Metabotropic glutamate receptor (GLUR) agonists also evoked [3H]D-aspartate release in both bovine and human retinas whereas polyamines only enhanced the excitatory effects of L-glutamate on [3H]D-aspartate release. Antagonists of GLURs and the polyamine site inhibited L-glutamate evoked [3H]D-aspartate overflow with the following rank order of potency: MCPG >ifenprodil > AP-5 > arcaine> MK-801. In conclusion, L-glutamate-induces a stereoselective, calcium-dependent release of [3H]D-aspartate from isolated mammalian retinas that can be mimicked by GLUR agonists (and blocked by both receptor and polyamine site antagonists).

On the mechanism of the spermine-exerted inhibition on alpha-thrombin-induced platelet activation

Previous reports have shown that various amines inhibited platelet activation, but no definitive conclusions on their action mechanism were drawn. We have further investigated the action of spermine on platelet responses evoked by alpha-thrombin and other agonists. Spermine inhibited in a concentration-dependent manner (1-10 mM), and more efficiently than spermidine and putrescine, the alpha-thrombin-induced (1.5 nM) platelet activation. Spermine added at a concentration that inhibited completely aggregation only partially affected the thrombin-induced increase in cytosolic Ca(2+) concentration, protein phosphorylation, and ATP secretion. The polyamine had little effect on the morphology of resting platelets, as measured by electron microscopy, thrombin hydrolytic activity, and fibrinogen clotting capacity but decreased the thrombin binding to platelets and isolated glycocalicin. Spermine partially inhibited the aggregation elicited by ADP, vasopressin, platelet-activating factor, thrombin receptor-activating peptide, fluoroaluminate, ionomycin, and dioctanoylglycerol but did not affect the cytosolic Ca(2+) increase induced by these agonists. The polyamine bound to both glycocalicin and platelets, and it inhibited the fibrinogen binding to stimulated platelets. The amount of 14C-spermine bound to resting cells decreased in the presence of the glycoprotein GPIb-antibody LJIB1, whereas the polyamine bound to activated platelets, which was higher than that tied to resting cells, was markedly reduced by LJCP8 or decorsin, a GPIIb/IIIa antibody and antagonist-peptide, respectively. These results indicate that spermine specifically inhibits the thrombin binding to GPIb of resting platelets and the fibrinogen binding to GPIIb/IIIa (integrin alpha(IIb)beta(3)) of activated platelets.

Overexpression of spermidine/spermine N-acetyltransferase in transgenic mice protects the animals from kainate-induced toxicity

We recently generated a transgenic mouse line with activated polyamine catabolism through overexpression of spermidine/spermine N1-acetyltransferase (SSAT). A detailed analysis of brain polyamine concentrations indicated that all brain regions of these animals showed distinct signs of activated polyamine catabolism, e.g. overaccumulation of putrescine (three- to 17-fold), appearance of N1-acetylspermidine and decreases in spermidine concentrations. In situ hybridization analyses revealed a marked overexpression of SSAT-specific mRNA all over the brain tissue of the transgenic animals. The transgenic animals appeared to tolerate subcutaneous injections of high-dose kainate substantially better as their overall mortality was less than 50% of that of their syngenic littermates. We used the expression of glial fibrillary acidic protein (GFAP) as a marker of brain injury in response to kainate. In situ hybridization analysis with GFAP oligonucleotide up to 7 days after the administration of sublethal kainate doses showed reduced GFAP expression in transgenic animals in comparison with their non-transgenic littermates. This difference was especially striking in the cerebral cortex of the transgenic mice where the exposure to kainate hardly induced GFAP expression. The treatment with kainate likewise resulted in loss of the hippocampal (CA3) neurons in non-transgenic but not transgenic animals. These results support our earlier findings indicating that elevated concentrations of brain putrescine, irrespective whether derived from an overexpression of ornithine decarboxylase, or as shown here, from an overexpression of SSAT, play in all likelihood a neuroprotective role in brain injury.

The cellular localization of the L-ornithine decarboxylase/polyamine system in normal and diseased central nervous systems

Natural polyamines, spermidine and spermine, and their precursor putrescine, are of considerable importance for the developing and mature nervous system. They exhibit a number of neurophysiological and metabolic effects in the nervous system, including control of nucleic acid and protein synthesis, modulation of ionic channels and calcium-dependent transmitter release. The polyamine system is also known to be involved in various brain pathologic events (seizures, stroke, Alzheimer's disease and others). While cerebral polyamine concentrations and the activities of polyamine-metabolizing enzymes have been studied in great detail, much less is known about the cells that are responsible for cerebral polyamine synthesis and interconversion. With the present review the attempt is made to show how exact knowledge about the regional distribution and cellular localization of polyamines and the polyamine-synthesizing enzymatic machinery (and especially of L-ornithine decarboxylase) may help to better understand the functional interplay between polyamines and other endogenous agents (transmitters, receptors, growth factors neuroactive drugs etc.). Polyamines have been localized both in neurones and glial cells. However, the main cellular locus of the ODC is the neuron--both in the immature and adult central nervous system. Each period of normal brain development and ageing seems to have its own, characteristic temporo-spatial pattern of neuronal ODC expression. During strong functional activation (kindling, epileptic seizures, neural transplantation) astrocytes and other non-neuronal cells do also express ODC and other polyamine-metabolizing enzymes. Astroglial expression of ODC is accompanied by an increase in glial fibrillary acidic protein in these cells. This shift in the cellular mechanisms of polyamine metabolism is currently far from being understood. In human brain diseases (Alzheimer's disease, schizophrenia) certain neurones show an increased expression of ODC, the first and rate-limiting enzyme of polyamine metabolism. Since polyamines are structurally related to psychoactive drugs (neuroleptics, antidepressants) the polyamine system might be of importance as a putative target for drug intervention in psychiatry.

Neuroprotective role of ornithine decarboxylase activation in transient focal cerebral ischaemia: a study using ornithine decarboxylase-overexpressing transgenic rats

Nuclear magnetic resonance imaging (MRI) was used to study dynamics of maturation and the size of ischaemic stroke lesions in rats with greatly increased activity of ornithine decarboxylase (ODC). Syngenic rats, either with or without chronic pre-ischaemic treatment with an ODC inhibitor, alpha-difluoromethylornithine (DFMO), as well as ODC-overexpressing transgenic rats were subjected either to transient middle cerebral artery (MCA) occlusion or permanent occlusion of the cortical branch of MCA. The two models were chosen to assess the role of ODC activity in damage caused by ischaemia and reperfusion, respectively. Diffusion of water was quantified by means of the trace of the diffusion tensor (D(av) = 1/3 Trace D) to assess the extent of energy failure and cytotoxic oedema, whereas the spin-spin relaxation time (T2) was used as a quantitative indicator of irreversible damage by MRI. Exposure to transient MCA occlusion resulted in significantly smaller stroke lesions in the ODC-overexpressing transgenic (246+/-14 mm3) than in syngenic (320+/-9 mm3) or DFMO-treated (442+/-63 mm3) rats as determined 48 h after the occlusion. The differences in sizes were due to smaller lesions in the cortical tissue (transgenic vs. syngenic) or both in cortical and striatal regions (transgenic vs. DFMO-treated animals). The degree of irreversible oedema was greater in DFMO-treated rats than in syngenic or transgenic animals indicating accelerated development of a permanent damage in the absence of ODC induction. Cortical infarct following permanent MCA occlusion developed faster in the DFMO-treated than in syngenic or transgenic rats as the lesion sizes at 10 h were 26.2+/-4.3 mm3, 14.2+/-2.3 mm3 and 12.3+/-1.9 mm3, respectively. However, the stroke volumes by 48 h were not statistically different in the three animal groups. The present data demonstrate that ODC activation is an endogenous neuroprotective measure in transient cerebral ischaemia.

Preserved induction of long-term potentiation in the stratum radiatum in the CA1 field of hippocampal slices from transgenic mice overexpressing ornithine decarboxylase and overproducing putrescine

The role of putrescine in synaptic neurotransmission and plasticity was studied using transgenic mice overexpressing ornithine decarboxylase (ODC), a polyamine-synthesizing enzyme. Transgenic mice were produced using the standard microinjection technique leading to elevated levels of putrescine in the periphery and in the brain. The experiments investigated whether or not ODC mice with elevated levels of putrescine show alterations in synaptic transmission and induction of long-term potentiation in the CA1 field of the hippocampus in vitro. Our results indicated that (1) putrescine levels in brain slices of the transgenic mice were more than ten times higher than those in fresh slices of control mice, although the absolute levels of putrescine and spermine decreased (by 15 and 40%, respectively) after 3-6 h incubation in vitro, while the levels of spermidine slightly increased (by 10%), (2) the excitatory synaptic response waveforms were wider (an increased half-width), and paired-pulse facilitation was somewhat reduced in ODC mice as compared to controls, and (3) potentiation of excitatory synaptic responses (measured 30-45 min after theta burst stimulation) did not differ between ODC and control mice. These results indicate that synaptic transmission is affected, but synaptic plasticity in the field CA1 assessed in vitro is not changed by elevated levels of intracellular putrescine.

A 1H/13C inverse 2D method for the analysis of the polyamines putrescine, spermidine and spermine in cell extracts and biofluids

The polyamines putrescine, spermidine and spermine are involved in the regulation of various metabolic processes. It is therefore desirable to detect and quantify the polyamines with NMR. We present the proton and carbon assignments for all polyamine signals obtained from PCA extracts of F98 glioma cells with high resolution using a semi-selective HSQC 2D-experiment. The biosynthesis of the polyamines in cell culture was examined using the labeled substrates [U-13C]glucose and [U-13C]glutamate. In such studies the high resolution of the semi-selective HSQC experiment at very high magnetic fields (14-19 T) allows the analysis of carbon-carbon couplings, and isotopomer patterns. The different effects of osmotic stress on the concentrations of polyamines and amino acids are also reported.

Long-term effects of status epilepticus induced by kainic acid on hippocampal polyamines

Putrescine has been suggested to have an inhibitory effect on the excitability of the central nervous system. In the present study we found that 2 and 3 weeks after status epilepticus induced by kainic acid, rats had increased concentrations of putrescine (3- and 1.7-fold, respectively) and spermidine (1.6- and 1.4-fold, respectively) in the hippocampus. These animals exhibited a higher susceptibility to pentylentetrazol than the saline group. In addition, several hours after the pentylentetrazol injection, the concentration of putrescine and spermidine increased again in the brain and also in the plasma. In conclusion, increased hippocampal putrescine and spermidine concentrations seem to be linked with a lower threshold of excitability.

Metabolism of polyamines in transgenic cells of carrot expressing a mouse ornithine decarboxylase cDNA

The metabolisms of arginine (Arg), ornithine (Orn), and putrescine were compared in a nontransgenic and a transgenic cell line of carrot (Daucus carota L.) expressing a mouse Orn decarboxylase cDNA. [14C]Arg, [14C]Orn, and [14C]putrescine were fed to cells and their rates of decarboxylation, uptake, metabolism into polyamines, and incorporation into acid-insoluble material were determined. Transgenic cells showed higher decarboxylation rates for labeled Orn than the nontransgenic cells. This was correlated positively with higher amounts of labeled putrescine production from labeled Orn. With labeled Arg, both the transgenic and the nontransgenic cells exhibited similar rates of decarboxylation and conversion into labeled putrescine. When [14C]putrescine was fed, higher rates of degradation were observed in transgenic cells as compared with the nontransgenic cells. It is concluded that (a) increased production of putrescine via the Orn decarboxylase pathway has no compensatory effects on the Arg decarboxylase pathway, and (b) higher rates of putrescine production in the transgenic cells are accompanied by higher rates of putrescine conversion into spermidine and spermine as well as the catabolism of putrescine.

Transgenic rats as models for studying the role of ornithine decarboxylase expression in permanent middle cerebral artery occlusion

BACKGROUND AND PURPOSE

Cerebral ischemia causes activation of ornithine decarboxylase (ODC) gene and subsequent accumulation of putrescine, which might either directly or indirectly affect the outcome of cerebral infarct. We developed a transgenic rat overexpressing human ODC, which was used to explore the effect of abnormally high putrescine concentration in the brain on the infarct volume after permanent middle cerebral artery (MCA) occlusion.

METHODS

The transgenic rats were produced by the pronuclear injection technique with the use of cloned human ODC gene. The right MCA was permanently occluded through craniotomy. ODC activity and polyamines were assayed in the infarcted and contralateral hemispheres. MRI was used to quantify T2 relaxation time, apparent diffusion constant (ADC), and infarct volume, which was also determined by 2,3,5-triphenyltetrazolium chloride.

RESULTS

Permanent MCA occlusion resulted in extensive activation of ODC, which was approximately sevenfold greater than in syngenic animals at 20 hours after occlusion. Consequently, putrescine increased from approximately 10 and 230 pmol/mg to 160 and 410 pmol/mg in the infarcted hemisphere of syngenic and transgenic animals, respectively, but all the other polyamines were unchanged. This high putrescine in the transgenic rats did not influence infarct size evolution, as determined by MRI, T2, ADC, or the infarct volume by 2,3,5-triphenyltetrazolium chloride at 48 hours.

CONCLUSIONS

Data from the ODC transgenic rat model show that the development of brain infarct after permanent MCA occlusion was not influenced by extensive levels of putrescine, indicating that this endogenous amine is not involved in maturation and spread of stroke lesion in vivo. Thus, it seems that ODC activation reflects an endogenous adaptation of neural cells to a noxious stimulus that does not directly influence lesion development.

Transgenic animals as new approaches in pharmacological studies

Transgenic animals are becoming useful tools for pharmacological studies. The use of transgenic technology raises two types of questions, "How are transgenic animals made?" and "What types of pharmacological questions can be answered using transgenic technologies?" Answers to these questions are discussed in this review. The production of animals with specific genetic alteration can be achieved by two strategies. The first involves the simple addition of DNA sequences to the chromosomes. The second strategy is to select particular genetic loci for site-specific changes. There are two well-established procedures for simple introduction of DNA into an animal genome, pronuclear DNA injection and transduction using a retrovirus. In contrast, methods for targeting specific DNA sequences to definite sites in the chromosomes are evolving rapidly. Some of these procedures can be used in combination to make a different variety of gene alterations in animals. Pharmacological studies where transgenic technology has been extensively used are discussed, including studies in the cardiovascular system, the nervous system, the endocrine system, cancer, and toxicology.