Quinolinic acid levels in a murine retrovirus-induced immunodeficiency syndrome

A murine model of retroviral encephalopathy

This unit delineates the steps for production of a murine model of retroviral encephalopathy. The LP-BM5 infected mouse develops a chronic inflammation of the brain secondary to profound immune deficiency. The model is robust, develops rapidly and does not require the use of human pathogens. In addition, the behavioral and neurochemical characteristics of this model is reviewed.

Changes in neuronal protein expression in LP-BM5-infected mice

Murine acquired immunodeficiency syndrome (MAIDS) induced by LP-BM5 murine leukemia virus is used as a model of human immunodeficiency virus (HIV)-related neurologic dysfunction. Mice infected with LP-BM5 have mnemonic abnormalities (i.e., spontaneous alternation behavior in the Y-maze and performance in the Morris water maze) and biochemical alternations (i.e., cytokines, platelet-activating factor, quinolinate, glutamate and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor) that produce neurologic symptoms similar to those observed in HIV-related neurologic dysfunction. To identify proteins associated with dysmnesia in the MAIDS model, we examined the expression of neuronal proteins in LP-BM5-infected mice using two-dimensional polyacrylamide gel electrophoresis (2-DE). Neuronal protein expression in LP-BM5-infected mice was compared with that in non-infected mice using the Image Master 2D. We detected approximately 800 protein spots, of which 35 were distinguishable between non-infected and LP-BM5-infected mice. Most of these spots were downregulated in LP-BM5-infected mice. Three of the spots were identified as 14-3-3 protein zeta/delta, synapsin 2 and protein disulfide isomerase using a capillary nanoliquid chromatography tandem mass spectrometric system. We verified the expression levels of these proteins by Western blot. Analysis of these 35 spots could provide insight into mechanisms of dysmnesia in the MAIDS model of HIV-related neuronal dysfunction.

Crystal structure of Homo sapiens kynureninase

Kynureninase is a member of a large family of catalytically diverse but structurally homologous pyridoxal 5'-phosphate (PLP) dependent enzymes known as the aspartate aminotransferase superfamily or alpha-family. The Homo sapiens and other eukaryotic constitutive kynureninases preferentially catalyze the hydrolytic cleavage of 3-hydroxy-l-kynurenine to produce 3-hydroxyanthranilate and l-alanine, while l-kynurenine is the substrate of many prokaryotic inducible kynureninases. The human enzyme was cloned with an N-terminal hexahistidine tag, expressed, and purified from a bacterial expression system using Ni metal ion affinity chromatography. Kinetic characterization of the recombinant enzyme reveals classic Michaelis-Menten behavior, with a Km of 28.3 +/- 1.9 microM and a specific activity of 1.75 micromol min-1 mg-1 for 3-hydroxy-dl-kynurenine. Crystals of recombinant kynureninase that diffracted to 2.0 A were obtained, and the atomic structure of the PLP-bound holoenzyme was determined by molecular replacement using the Pseudomonas fluorescens kynureninase structure (PDB entry 1qz9) as the phasing model. A structural superposition with the P. fluorescens kynureninase revealed that these two structures resemble the "open" and "closed" conformations of aspartate aminotransferase. The comparison illustrates the dynamic nature of these proteins' small domains and reveals a role for Arg-434 similar to its role in other AAT alpha-family members. Docking of 3-hydroxy-l-kynurenine into the human kynureninase active site suggests that Asn-333 and His-102 are involved in substrate binding and molecular discrimination between inducible and constitutive kynureninase substrates.

Effects of LP-BM5 murine leukemia virus infection on errors and response time in a two-choice serial reaction time task in C57BL/6 mice

Human immunodeficiency virus type 1 (HIV-1) infection is often accompanied by cognitive, motor, and behavioral dysfunction. Cognitive function diminishes in indices of attention, psychomotor speed, and learning and memory. These are collectively termed acquired immunodeficiency syndrome dementia complex (ADC or neuroAIDS). Inoculation with the LP-BM5 murine leukemia virus (MuLV) causes profound immunosuppression (murine acquired immunodeficiency syndrome, or MAIDS) in C57BL/6 mice. Previous studies show that the LP-BM5 MuLV impairs learning and memory without gross motor impairment. Since learning and memory performance deficits can be related to attention deficits, we assessed the effect of LP-BM5 MuLV infection on sustained attention performance using a two-choice serial reaction time task. This task required the animals to detect a visual stimulus presented randomly on the right or the left unit and respond by a nose-poke in the illuminated hole within a 5 s period for water reward. The LP-BM5 MuLV infected group, like the control group, improved sustained attention performance until 7 weeks of virus infection in all measures including choice accuracy, response omission, and correct response time. However, during the late stage of infection, LP-BM5 MuLV infected mice showed selective sustained attention performance deficits. From 8 weeks after LP-BM5 MuLV infection, the virus infected mice started to lose their improved sustained attention performance in response omission and began to make correct responses more slowly than the control mice when the duration of stimulus light was 5 s. Moreover, at 13 and 14 weeks after LP-BM5 MuLV infection, the virus infected group made correct choices significantly less accurately than the control group when duration of stimulus light was shortest (1 s). These data show that LP-BM5 MuLV infection causes not only the previously reported learning and memory deficits but also produces sustained attention performance deficits in mice.

Kynurenines in the CNS: from endogenous obscurity to therapeutic importance

In just under 20 years the kynurenine family of compounds has developed from a group of obscure metabolites of the essential amino acid tryptophan into a source of intensive research, with postulated roles for quinolinic acid in neurodegenerative disorders, most especially the AIDS-dementia complex and Huntington's disease. One of the kynurenines, kynurenic acid, has become a standard tool for use in the identification of glutamate-releasing synapses, and has been used as the parent for several groups of compounds now being developed as drugs for the treatment of epilepsy and stroke. The kynurenines represent a major success in translating a basic discovery into a source of clinical understanding and therapeutic application, with around 3000 papers published on quinolinic acid or kynurenic acid since the discovery of their effects in 1981 and 1982. This review concentrates on some of the recent work most directly relevant to the understanding and applications of kynurenines in medicine.

Endogenous neurotoxins from tryptophan

In most tissues, including brain, a major proportion of the tryptophan which is not used for protein synthesis is metabolised along the kynurenine pathway. Long regarded as the route by which many mammals generate adequate amounts of the essential co-factor nicotinamide adenine dinucleotide, two components of the pathway are now known to have marked effects on neurones. Quinolinic acid is an agonist at the N-methyl-D-aspartate sensitive subtype of glutamate receptors in the brain, while kynurenic acid is an antagonist and, thus, a potential neuroprotectant. A third kynurenine, 3-hydroxykynurenine, is involved in the generation of free radicals which can also damage neurones. Quinolinic acid is increasingly implicated in neurodegenerative disorders, most especially the AIDS-dementia complex and Huntington's disease, while kynurenic acid has become a standard for the identification of glutamate-releasing synapses, and has been used as the parent for several groups of compounds now being developed as drugs for the treatment of epilepsy and stroke.

LP-BM5 infection impairs spatial working memory in C57BL/6 mice in the Morris water maze

Previous studies show that the LP-BM5 murine leukemia virus causes an acquired immunodeficiency syndrome in C57BL/6 mice (MAIDS) and impairs learning and memory without gross motor impairment. To assess spatial working memory impairment after LP-BM5 infection and the time course of this impairment, we tested mice in a modified working-memory version of the Morris water maze. Twenty mice were inoculated with LP-BM5; controls received medium (Minimum Essential Medium). In the test procedure, animals had two 1-min training sessions to learn the position of a randomly placed hidden platform. Thirty seconds after the second training session, animals were placed in the maze without the platform, and time and pathlength spent in each quadrant of the maze were measured. For 9 weeks after LP-BM5 infection, both groups showed preference for the target quadrant compared to the opposite quadrant. At 10 and 11 weeks after infection, the LP-BM5 virus infected mice lost this target quadrant preference. We conclude that LP-BM5 infection impaired spatial working memory in a modified working-memory version of the Morris water maze test in C57BL/6 mice at 10 and 11 weeks after virus infection.

Increased blood-brain barrier permeability in LP-BM5 infected mice is mediated by neuroexcitatory mechanisms

Serum protein levels in LP-BM5 infected mouse brains were investigated to gain insight into the contribution of blood-brain barrier (BBB) patency to the pathogenesis of retroviral encephalopathy. Evans blue uptake by the forebrain and cerebellum was significantly increased between 8-12 weeks post inoculation. Immunohistochemistry revealed foci of albumin, transferrin, alpha(2)-macroglobulin and IgG transudation around blood vessels particularly in the cerebral cortex and cerebellar vermis. These leaks were often associated with astrocytosis and apoptotic cells. Unlike the other serum proteins, IgG immunoreactivity extended from the circumventricular organs and disseminated throughout the brain parenchyma, accumulating on the plasma membranes of hippocampal and cortical neurons. Consistent with the chronic elevation of free glutamate levels in LP-BM5 infected mice, the increase in Evans blue uptake into the forebrain was completely reversed following dizocilpine administration. Thus, the chronic increase in free glutamate levels in LP-BM5 infected mouse brain contributes to BBB disruption. Furthermore, the CNS accumulation of serum proteins, particularly IgG, observed in these mice may increase osmotic load, impair neuronal function, and cause white matter pallor. Administration of NMDA receptor antagonists may prove useful in managing BBB permeability in those neuropathologies, such as HIV-associated dementia/cognitive/motor complex, having a glutamatergic component.

Stereospecificity of Pseudomonas fluorescens kynureninase for diastereomers of beta-methylkynurenine

The diastereomers of beta-methyl-L-kynurenine were prepared by preparative ozonolysis of the respective diastereomers of beta-methyl-L-tryptophan. A practical method for preparative enzymatic resolution of the diastereomers of beta-methyltryptophan was developed using carboxypeptidase A digestion of the N-trifluoroacetyl derivatives. The stereochemical assignment was confirmed by X-ray crystal structure determination of (2S, 3R)-threo-beta-methyl-L-tryptophan. (2S,3S)-erythro-beta-Methyl-L-kynurenine is a slow substrate for kynureninase from Pseudomonas fluorescens (k(cat)/K(m) = 0.1% that of L-kynurenine), producing anthranilic acid, while (2S,3R)-threo-L-kynurenine is about 390-fold less reactive than erythro. Rapid-scanning stopped-flow measurements show that beta-methyl substitution affects the rate of alpha-deprotonation of the L-kynurenine-pyridoxal-5'-phosphate Schiffs base. This is consistent with the stereoelectronic requirements of the reaction. These results are the first demonstration that beta-substituted kynurenines can be substrates for kynureninase, and may be useful in the design of mechanism-based inhibitors.

The anti-inflammatory effects of quinolinic acid in the rat

Quinolinic acid (QUIN) levels are elevated in patients and animals suffering from chronic infectious diseases. In the present study, male Sprague-Dawley rats were used to test the anti-inflammatory effects of QUIN using the carrageenan (CGN)-induced paw edema assay and the CGN sponge assay. Results of these studies indicate that QUIN (30, 100 or 300 mg/kg i.p.) caused a reduction of carrageenan-induced inflammation by as much as 80% at the highest dose. Moreover, QUIN reduced exudate volume and inhibited leukocyte migration in the sponge granuloma assay. In another experiment, the anti-inflammatory activity of QUIN was eliminated in adrenalectomized rats. QUIN did not reduce edema caused by arachidonic acid, bradykinin or compound 48/80. Neither morphine nor naloxone altered the anti-inflammatory activity of QUIN. These results may suggest that QUIN exerts its anti-inflammatory activity through a direct action on neutrophils or vascular permeability.

Characterization of learning and memory deficits in C57BL/6 mice infected with LP-BM5, a murine model of AIDS

Mice infected with an immunosuppressive murine leukemia virus mixture, LP-BM5 show a profound immunosuppression described as murine acquired immune deficiency syndrome (AIDS). In the present study, we characterized learning and memory deficits in C57BL/6 mice infected with LP-BM5. Spontaneous alternation behavior in a Y-maze and latent learning (spatial attention) in a water-finding test, as well as spatial reference and reversal learning in a water maze test, were significantly impaired in the mice infected with LP-BM5. These deficits appeared in the absence of any motoric and visual impairment as assessed by open-field, rotarod and visual water maze tests. These results suggest that cognitive functions are impaired in the mice infected with LP-BM5. Furthermore, LP-BM5-infected mice may be useful as a model for the AIDS dementia complex.

Increased discriminative stimulus potency of phencyclidine in C57B1/6 mice infected with the LP-BM5 retrovirus

Drug discrimination procedures in mice are used to study the neuropharmacology of a wide variety of drugs. In C57 B1/6 mice, infection with the LP-BM5 murine leukemia virus leads to a syndrome (murine acquired immunodeficiency syndrome-MAIDS) characterized by immunocompromise, neurochemical alterations, and learning and memory deficits. Because the neurochemical and behavioral changes suggest that altered glutamatergic neurotransmission follows LP-BM5 infection, we studied the effects of infection on discriminative stimulus properties of phencyclidine (PCP), a Ca2+ channel blocker at NMDA receptors. We also tested D-amphetamine and dizocilpine to assess the specificity of the discrimination. As expected, dizocilpine produced PCP-like responding. After animals were trained to discriminate PCP from saline, they were inoculated with LP-BM5 and the PCP dose-response functions repeatedly determined. The potency of PCP in this procedure was unchanged 3 weeks after infection, but was increased approximately fivefold 6 and 9 weeks after infection. Amphetamine 9 weeks after inoculation did not produce PCP-like responding, showing that the results were not caused by a loss of specificity of the discrimination. The time course for changes in PCP potency is similar to those of other behavioral and neurochemical changes reported after LP-BM5 infection. The results are consistent with an action of LP-BM5 infection at glutamatergic synapses.

Evidence of neuronal degeneration in C57B1/6 mice infected with the LP-BM5 leukemia retrovirus mixture

Mice infected with LP-BM5 develop a severe immunodeficiency accompanied by learning and memory deficits, gliosis, and neurotransmitter abnormalities. The neurochemical alterations are consistent with elevated excitotoxin levels, suggesting that infected mice may incur neuronal damage. Although the number of neocortical neurons was unchanged in mice 12 wk after LP-BM5 infection, the expression of cytoskeletal proteins declined, particularly in the frontal and parietal cortex as indicated by MAP2, NF-200, and synaptophysin immunoreactivity. In contrast, the number of striatal neurons decreased 19%. The remaining neurons were smaller, with fewer synaptic boutons, and showed decreased synaptophysin and NF-200, immunoreactivity. Immunoblots of cortex and striatum confirmed decreases in MAP2, NF-200 and synaptophysin expression. Finally, although NCAM expression decreased in the striatum, it increased in the cortex. These results indicate that LP-BM5-infected mice sustain significant neuronal damage, which may contribute to their behavioral deficits. Moreover, the increase in cortical NCAM expression suggests active synaptic remodeling to compensate for the persistent excitotoxic environment in these mice, whereas striatal neurons degenerate. These concurrent degenerative and compensatory processes may also occur in the brains of patients with AIDS dementia complex (ADC), who suffer extensive degeneration of the basal ganglia and cerebral cortex.

Sources of the neurotoxin quinolinic acid in the brain of HIV-1-infected patients and retrovirus-infected macaques

This study investigated the sources of quinolinic acid, a neurotoxic tryptophan-kynurenine pathway metabolite, in the brain and blood of HIV-infected patients and retrovirus-infected macaques. In brain, quinolinic acid concentrations in HIV-infected patients were elevated by > 300-fold to concentrations that exceeded cerebrospinal fluid (CSF) by 8.9-fold. There were no significant correlations between elevated serum quinolinic acid levels with those in CSF and brain parenchyma. Because nonretrovirus-induced encephalitis confounds the interpretation of human postmortem data, rhesus macaques infected with retrovirus were used to examine the mechanisms of increased quinolinic acid accumulations and determine the relationships of quinolinic acid to encephalitits and systemic responses. The largest kynurenine pathway responses in brain were associated with encephalitis and were independent of systemic responses. CSF quinolinic acid levels were also elevated in all infected macaques, but particularly those with retrovirus-induced encephalitis. In contrast to the brain changes, there was no difference in any systemic measure between macaques with encephalitis vs. those without. Direct measures of the amount of quinolinic acid in brain derived from blood in a macaque with encephalitis showed that almost all quinolinic acid (>98%) was synthesized locally within the brain. These results demonstrate a role for induction of indoleamine-2,3dioxygenase in accelerating the local formation of quinolinic acid within the brain tissue, particularly in areas of encephalitis, rather than entry of quinolinic acid into the brain from the meninges or blood. Strategies to reduce QUIN production, targeted at intracerebral sites, are potential approaches to therapy.

The pattern of neurotransmitter alterations in LP-BM5 infected mice is consistent with glutamatergic hyperactivation

To gain insight into the neurochemical pathologies contributing to AIDS dementia complex, neurotransmitter levels were measured in the brains of mice infected with the LP-BM5 leukemia retrovirus. These mice develop immunologic and cognitive deficits analogous to human HIV-1 infection. Met-enkephalin and substance-P levels declined approximately 50% in the striatum and hypothalamus beginning as early as 4 weeks after infection. Hippocampal met-enkephalin levels were reduced to 50% only at 12 weeks after inoculation. Significant decreases (60-70%) in acetylcholine concentrations were observed in the striatum, cerebral cortex and hippocampus by 12 weeks after virus inoculation, while striatal GABA concentrations decreased to 50-60% at 8-12 weeks after infection. Striatal somatostatin levels were unchanged. Administration of the NMDA receptor antagonists MK-801 or LY 274614 ameliorated the decline in striatal met-enkephalin levels observed in mice after 8 weeks of infection. This pattern of neurotransmitter depletion and the ability of NMDA receptor antagonists to attenuate the loss of striatal met-enkephalin are consistent with an excitotoxic lesion. Thus, the elevation of glutamate levels secondary to glial activation may contribute to the contemporaneous development of cognitive deficits observed in mice infected with the LP-BM5 virus.

The encephalopathy associated with murine acquired immunodeficiency syndrome

Mice infected with the LP-BM5 murine leukemia virus (MuLV) develop an immune deficiency syndrome together with an encephalopathy characterized by impairments in spatial learning and memory. These cognitive deficits are evident before the appearance of neuron loss and lymphoid cell invasion of the brain. Nonetheless, a prominent gliosis and a variety of neurochemical changes precede the development of cognitive deficits. The neurochemical abnormalities include significant decreases in striatal Met-enkephalin and substance P (but not somatostatin), increases in concentrations of quinolinic acid and platelet-activating factor, and alterations in brain fyn kinase. At this stage of the infection, some of these neurochemical changes can be reversed by glutamate receptor antagonists, cytokine inhibitors, and anti-retroviral agents. In later stages of the infection, however, the infected mice develop irreversible neuronal loss, invasion of hematopoietic cells, and increased viral burden in the CNS. In addition, motor-neuron dysfunction (hindlimb paralysis, weakness, and ataxia) and seizures are sometimes observed during the late stages of infection. Thus, the LP-BM5 MuLV-infected mouse is a useful model for studying the chronology of neurodegenerative changes, ranging from reversible neuron dysfunction to irreversible neuron loss, that are associated with retrovirus-induced immunodeficiency.

Altered tryptophan metabolism in mice with herpes simplex virus encephalitis: increases in spinal cord quinolinic acid

Mice infected with the herpes simplex virus, type-1, developed a paralysis which was associated with increased levels of the neurotoxin quinolinic acid (QUIN). The largest increases in QUIN were observed in the spinal cord with much smaller changes in the rostral forebrain or serum. The time course for the paralysis coincided with the increase in spinal cord QUIN, a maximal 40-fold elevation, at 7-10 days post infection. The time course suggested that the increases in QUIN were due to its local synthesis. Consistent with this possibility, herpes virus-infected mice had increased activities of indoleamine 2,3-dioxygenase and kynurenine hydroxylase (two key enzymes in QUIN formation), when compared to non-infected controls. Since QUIN is formed by activated macrophages, these new data are consistent with QUIN formation as part of the host response to a pathogen whose importance is discussed.

LP-BM5 infection impairs acquisition, but not performance, of active avoidance responding in C57B1/6 mice

LP-BM5 murine leukemia virus infection causes an AIDS-like syndrome--murine acquired immunodeficiency syndrome--in C57B1/6 mice and impairs spatial learning in the Morris water maze without gross motor impairment. We used a shuttle shock-avoidance procedure to examine the effects of LP-BM5 infection on learning and retention of avoidance behavior. Thirty mice were inoculated with LP-BM5; 30 received vehicle (DMEM) injections. Fifteen LP-BM5 and 15 DMEM mice were trained in avoidance 7 wk after inoculation; retention of the avoidance response was tested 4 wk later. The remaining mice were trained 11 wk after inoculation. In animals trained 7 wk after inoculation, the groups performed similarly, with a marginally significant tendency for LP-BM5-infected animals to make more avoidance responses. This group difference was significant when animals were retested at 11 wk. However, LP-BM5 animals trained 11 wk after inoculation made significantly fewer avoidance responses than controls trained at the same time. We conclude that in later stages of disease, LP-BM5 impairs response acquisition, but not performance, in the active avoidance procedure. Results extend the use of the LP-BM5-infected mouse as a model of AIDS dementia complex.

Cloning, sequence, and expression of kynureninase from Pseudomonas fluorescens

We have cloned the gene encoding kynureninase from Pseudomonas fluorescens using a restriction site polymerase chain reaction technique (RS-PCR) (G. Sarkar, R. T. Turner, and M. E. Bolander PCR Methods Appl. 2, 318-322, 1993) and expressed the enzyme in Escherichia coli DH5a F'. The kynureninase gene has an open reading frame (ORF) of 1251 base pairs that codes for a protein of 416 amino acids with a calculated molecular weight of 45,906. The protein purified from P. fluorescens has N-terminal threonine and an observed molecular weight of 45,787 by electrospray mass spectrometry, suggesting that the N-terminal methionine is removed by posttranslational processing. The complete gene was obtained by PCR and inserted into pTZ18U. The resultant plasmid was used to transform E. coli DH5alpha F', and these cells overexpressed kynureninase to about 37% of total soluble protein. The isolated recombinant protein has molecular weight and Km values identical to those of the native protein from P. fluorescens. The amino acid sequence exhibits 29% identity with those of rat and human kynureninases and 32% identity with the amino acid sequence translated from a Saccharomyces cerevisiae ORF. Alignment of the four sequences shows a highly conserved region which corresponds to the pyridoxal-5'-phosphate (PLP) binding site of rat kynureninase. Based on this alignment, we predict that Lys227 and Asp212 in P. fluorescens kynureninase are involved in pyridoxal-5'-phosphate binding. P. fluorescens kynureninase also exhibits significant homology to the nifS gene product, cysteine desulfurase, and to eucaryotic serine/pyruvate aminotransferases, suggesting that it is a member of subgroup IV of the aminotransferase family of PLP-dependent enzymes.

Pentoxifylline decreases brain levels of platelet activating factor in murine AIDS

Tumor necrosis factor-alpha (TNF-alpha) and platelet-activating factor (PAF) have been implicated in the pathogenesis of human immunodeficiency virus (HIV)-associated encephalopathy. The effects of pentoxifylline on brain PAF levels were examined in mice infected with the LP-BM5 murine leukemia virus (MuLV). Seven weeks after viral inoculation, significant increases in serum TNF-alpha and brain PAF levels were observed. One week of treatment with pentoxifylline initiated 6 weeks postinfection significantly reduced both serum TNF-alpha and brain PAF levels. A significant positive correlation was observed between the levels of these substances (r = 0.62; P < 0.01). This study demonstrates that pentoxifylline treatment was effective in decreasing the levels of TNF-alpha in the serum and PAF levels in the brain of mice infected with the LP-BM5 MuLV.

Regional changes in constitutive, but not inducible NOS expression in the brains of mice infected with the LP-BM5 leukemia virus

Potential neurotoxins such as nitric oxide have been implicated in the pathogenesis of acquired immunodeficiency syndrome (AIDS) dementia complex. The LP-BM5 murine leukemia-infected mice, which develop immunological and cognitive deficits reminiscent of human HIV-1 infection, were employed to investigate the changes in brain constitutive nitric oxide synthase (cNOS) and inducible nitric oxide synthase (iNOS) expression. Cerebellar and striatal cNOS enzymatic activity increased approximately 70% as early as 2 weeks after infection, declining to control levels by 12-16 weeks. In contrast, cNOS protein expression in the striatum and cerebellum was decreased 30% at 4 weeks, declining to 50% of control levels by 16 weeks post-infection. Staining intensity for cNOS, but not neuron number was reduced in the cerebral cortex, striatum, ventromedial hypothalamic nucleus and amygdala. Although iNOS protein expression was elevated in splenic monocytes, neither iNOS activity, mRNA nor protein was detected in the brains of mice 12 weeks after infection. These results indicate that neurons decrease cNOS protein expression to compensate for chronic cNOS activation, probably resulting from glutamatergic stimulation. The cNOS activation is contemporaneous with microglial activation in LP-BM5-infected mice, and precedes the development of cognitive deficits. Moreover, the lack of iNOS induction in either infected macrophages or glial elements suggests that iNOS is not necessary for the development of these cognitive deficits.

Gliosis in the LP-BM5 murine leukemia virus-infected mouse: an animal model of retrovirus-induced dementia

Mice infected with the LP-BM5 murine leukemia virus (MuLV) mixture develop severe immunosuppression, neurotransmitter abnormalities and cognitive impairments in the absence of significant viral or macrophage invasion of the CNS. The time-course of the changes in glial activation have been characterized in an effort to understand the cellular basis of the neurobehavioral abnormalities observed in these mice. Glial activation was determined by measuring the relative changes in F4/80 protein and GFAP immunoreactivity using immunoblots. Augmented F4/80 expression preceded that of GFAP, with global elevations of 4-6-fold at 3 weeks, sustained for up to 12 weeks after inoculation. GFAP immunoreactivity increased 2-fold only in the cerebral cortex and striatum 5 weeks postinfection, declining to control levels by 12 weeks. Immunohistochemistry revealed significant increases in microglial size and staining intensity in the cortex, corpus callosum and striatum, with the development of a unique population of highly ramified, intensely stained microglia and microglial nodules in the corpus callosum and striatum. No evidence of ameboid microglia was found. Astrocyte size and degree of ramification was increased in the hippocampus, cortex, striatum and corpus callosum. Thus, microgliosis is an early event in LP-BM5 infection, preceding astrocytosis, neurotransmitter loss, and development of cognitive deficits. Activated microglia may secrete neurotoxins leading to the neurochemical alterations and cognitive deficits observed in these mice. Because gliosis and microglial nodule formation are hallmarks of HIV-1 encephalopathy, LP-BM5 MuLV-infected C57/B16 mice may afford insights into the mechanisms contributing to the early stages of this syndrome.