Pneumococcal meningitis causes accumulation of neurotoxic kynurenine metabolites in brain regions prone to injury

PCSK9 Inhibition Reduces Depressive like Behavior in CUMS-Exposed Rats: Highlights on HMGB1/RAGE/TLR4 Pathway, NLRP3 Inflammasome Complex and IDO-1

Ample evidence has pointed to a close link between cardiovascular diseases (CVD) and depression. Inflammatory pathways including the high-mobility-group-box-1 protein, receptor-for-advanced-glycation-end-products and toll-like-receptor-4 (HMGB1/RAGE/TLR4) and nucleotide-binding domain (NOD)-like receptor protein 3 (NLRP3) inflammasome pathways are thought to be crucial players in this link. Activation of these pathways ends by releasing of different inflammatory mediators involved in CVD and depression pathophysiology. In the brain, this inflammatory process enhanced indoleamine2,3-dioxygenase-1 (IDO-1) activation with subsequent alteration in kynurenine/tryptophan levels causing depression. Based on the favorable anti-inflammatory effects of Alirocumab, the proprotein-convertase-subtilisin/kexin-type-9 (PCSK9) inhibitor, used in different CVD, this study was designed to investigate its potential antidepressant effect. The behavioral and neurochemical effects of concomitant treatment of Alirocumab at doses of (4, 8 and 16 mg/kg/week subcutaneously) in Wistar rats exposed to chronic unpredictable mild stress (CUMS) for 6 weeks were assayed. Alirocumab prevented CUMS-induced depressive-like-behaviors exhibited in open-field and forced-swimming tests, and hypothalamus-pituitary-adrenal axis hyperactivity (adrenal gland weight and serum corticosterone). Alirocumab prevented CUMS-induced alteration in hippocampal kynurenine/tryptophan levels and pro-inflammatory cytokines tumor-necrosis-factor-alpha, interleukin-1beta (IL-1β), IL-2 and IL-6. Western blot and PCR analysis showed that Alirocumab favorably modulated the HMGB1/RAGE/TLR4 axis, nuclear-factor-kappa-beta, NLRP3 inflammasome complex and IDO-1 in the hippocampus of CUMS rats. These effects were correlated to the level of PCSK9 expression. The behavioral and biochemical findings indicated the potential antidepressant effect of PCSK9 inhibition by Alirocumab.

The neurotoxic branch of the kynurenine pathway is highly activated in the central nervous system of patients with pneumococcal meningitis

BACKGROUND

Acute bacterial meningitis (ABM) causes excessive activation of N-methyl-D-aspartate receptors (NMDAr), leading to cortical and hippocampal neuron death. As opposite, enteroviral meningitis is more frequently benign. The kynurenine (KYN) pathway is the major catabolic route of tryptophan (TRP) and some of its metabolites are agonists or antagonists of NMDAr.

METHODS

In order to investigate the pathogen-specific patterns of KYN pathway modulation in the central nervous system of children with acute meningococcal (MM), pneumococcal (PM) or enteroviral (VM) meningitis, the cerebrospinal fluid (CSF) concentrations of TRP, KYN, kynurenic acid (KYNA) and quinolinic acid (QUINA) were evaluated by ultra-high performance liquid chromatography (uHPLC) coupled to mass spectrometry. In addition, CSF levels of IL-6, IL-10 and TNF-α were quantified by multi-analyte flow assay. The data was mined and integrated using statistical and machine learning methods.

RESULTS

The three forms of meningitis investigated herein up-regulated the neurotoxic branch of the KYN pathway within the intrathecal space. However, this response, represented by the concentration of QUINA, was six and nine times higher in PM patients compared to MM or VM, respectively. CSF levels of IL-6, TNF-α, and IL-10 were increased in MM and PM patients when compared to controls. In VM, CSF IL-6 and IL-10, but not TNF-α were increased compared to controls, although not reaching the high levels found in bacterial meningitis. No correlation was found between the concentrations or the ratios of any pair of KYN metabolites and any cytokine or standard cytochemical parameter tested.

CONCLUSIONS

CNS infection with meningococci, pneumococci, and enteroviruses intrathecally activate the KYN pathway, favoring its neurotoxic branch. However, in PM, higher CSF levels of QUINA, compared to MM and VM, may contribute to its poorer neurologic outcome.

Therapeutic potential of targeting kynurenine pathway in neurodegenerative diseases

Kynurenine pathway (KP), the primary pathway of L-tryptophan (Trp) metabolism in mammals, contains several neuroactive metabolites such as kynurenic acid (KA) and quinolinic acid (QA). Its imbalance involved in aging and neurodegenerative diseases (NDs) has attracted much interest in therapeutically targeting KP enzymes and KP metabolite-associated receptors, especially kynurenine monooxygenase (KMO). Currently, many agents have been discovered with significant improvement in animal models but only one aryl hydrocarbon receptor (AHR) agonist 30 (laquinimod) has entered clinical trials for treating Huntington's disease (HD). In this review, we describe neuroactive KP metabolites, discuss the dysregulation of KP in aging and NDs and summarize the development of KP regulators in preclinical and clinical studies, offering an outlook of targeting KP for NDs treatment in future.

A model of dopamine and serotonin-kynurenine metabolism in cortisolemia: Implications for depression

A major factor contributing to the etiology of depression is a neurochemical imbalance of the dopaminergic and serotonergic systems, which is caused by persistently high levels of circulating stress hormones. Here, a computational model is proposed to investigate the interplay between dopaminergic and serotonergic-kynurenine metabolism under cortisolemia and its consequences for the onset of depression. The model was formulated as a set of nonlinear ordinary differential equations represented with power-law functions. Parameter values were obtained from experimental data reported in the literature, biological databases, and other general information, and subsequently fine-tuned through optimization. Model simulations predict that changes in the kynurenine pathway, caused by elevated levels of cortisol, can increase the risk of neurotoxicity and lead to increased levels of 3,4-dihydroxyphenylaceltahyde (DOPAL) and 5-hydroxyindoleacetaldehyde (5-HIAL). These aldehydes contribute to alpha-synuclein aggregation and may cause mitochondrial fragmentation. Further model analysis demonstrated that the inhibition of both serotonin transport and kynurenine-3-monooxygenase decreased the levels of DOPAL and 5-HIAL and the neurotoxic risk often associated with depression. The mathematical model was also able to predict a novel role of the dopamine and serotonin metabolites DOPAL and 5-HIAL in the ethiology of depression, which is facilitated through increased cortisol levels. Finally, the model analysis suggests treatment with a combination of inhibitors of serotonin transport and kynurenine-3-monooxygenase as a potentially effective pharmacological strategy to revert the slow-down in monoamine neurotransmission that is often triggered by inflammation.

Kynurenic Acid Protects Against Ischemia/Reperfusion-Induced Retinal Ganglion Cell Death in Mice

Background: Glaucoma is an optic neuropathy and involves the progressive degeneration of retinal ganglion cells (RGCs), which leads to blindness in patients. We investigated the role of the neuroprotective kynurenic acid (KYNA) in RGC death against retinal ischemia/reperfusion (I/R) injury. Methods: We injected KYNA intravenously or intravitreally to mice. We generated a knockout mouse strain of kynurenine 3-monooxygenase (KMO), an enzyme in the kynurenine pathway that produces neurotoxic 3-hydroxykynurenine. To test the effect of mild hyperglycemia on RGC protection, we used streptozotocin (STZ) induced diabetic mice. Retinal I/R injury was induced by increasing intraocular pressure for 60 min followed by reperfusion and RGC numbers were counted in the retinal flat mounts. Results: Intravenous or intravitreal administration of KYNA protected RGCs against I/R injury. The I/R injury caused a greater loss of RGCs in wild type than in KMO knockout mice. KMO knockout mice had mildly higher levels of fasting blood glucose than wild type mice. Diabetic mice showed significantly lower loss of RGCs when compared with non-diabetic mice subjected to I/R injury. Conclusion: Together, our study suggests that the absence of KMO protects RGCs against I/R injury, through mechanisms that likely involve higher levels of KYNA and glucose.

Kynurenine Is a Cerebrospinal Fluid Biomarker for Bacterial and Viral Central Nervous System Infections

BACKGROUND

The tryptophan-kynurenine-nicotinamide adenine dinucleotide (oxidized; NAD+) pathway is closely associated with regulation of immune cells toward less inflammatory phenotypes and may exert neuroprotective effects. Investigating its regulation in central nervous system (CNS) infections would improve our understanding of pathophysiology and end-organ damage, and, furthermore, open doors to its evaluation as a source of diagnostic and/or prognostic biomarkers.

METHODS

We measured concentrations of kynurenine (Kyn) and tryptophan (Trp) in 221 cerebrospinal fluid samples from patients with bacterial and viral (due to herpes simplex, varicella zoster, and enteroviruses) meningitis/encephalitis, neuroborreliosis, autoimmune neuroinflammation (due to anti-N-methyl-D-aspartate receptor [NMDA] encephalitis and multiple sclerosis), and noninflamed controls (ie, individuals with Bell palsy, normal pressure hydrocephalus, or Tourette syndrome).

RESULTS

Kyn concentrations correlated strongly with CSF markers of neuroinflammation (ie, leukocyte count, lactate concentration, and blood-CSF-barrier dysfunction), were highly increased in bacterial and viral CNS infections, but were low or undetectable in NMDA encephalitis, multiple sclerosis, and controls. Trp concentrations were decreased mostly in viral CNS infections and neuroborreliosis. Multiple logistic regression analysis revealed that combinations of Kyn concentration, Trp concentration, and Kyn/Trp concentration ratio with leukocyte count or lactate concentration were accurate classifiers for the clinically important differentiation between neuroborreliosis, viral CNS infections, and autoimmune neuroinflammation.

CONCLUSIONS

The Trp-Kyn-NAD+ pathway is activated in CNS infections and provides highly accurate CSF biomarkers, particularly when combined with standard CSF indices of neuroinflammation.

The cerebral protective effect and mechanism of action of vitamin B6 adjuvant ceftriaxone in experimental pneumococcal meningitis

BACKGROUND

Pneumococcal meningitis is one of the most common infectious diseases with a high-mortality rate and long-term neurological sequelae, affecting up to 50% of survivors. Pneumococcal compounds are pro-inflammatory mediators that induce an innate immune response and tryptophan degradation through the kynurenine pathway. Vitamin B6 (vitB6) is an important vitamin which acts as a cofactor at the active sites of enzymes that catalyze a great number of reactions involved in the metabolism of tryptophan through the kynurenine pathway and may thus limit the accumulation of neurotoxic metabolites and preserve the cellular energy status. The aim of this study was to investigate the neuroprotective effect of adjuvant treatment with vitB6 in pneumococcal meningitis.

METHODS

The effects of vitB6 on the clinical symptoms, the expression of kynureninase (KYN), Kynurenic acid (KYNA), nicotinamide adenine dinucleotide (NAD) and cytokines in brain tissue and memory of infant Wistar rats subjected to pneumococcal meningitis were researched. At the same time, Kynurenine 3-monooxygenase (KMO) inhibitor Ro 61-8048 was applied in order to further investigate the brain protective effect of vitB6 in bacterial meningitis.

RESULTS

Adjuvant therapy of bacterial meningitis with vitB6 could improve the clinical symptoms, learning performance, lead to the maintenance of cellular NAD+ and ATP homeostasis and significantly down-regulate the levels of cytokines in the brain tissue by affecting the KYN pathway.

CONCLUSIONS

Adjuvant treatment with vitB6 in pneumococcal meningitis could exert neuroprotective effect via increasing the preservation of cellular energy through affecting the KYN pathway and reducing of the inflammatory response.

Alterations on Cellular Redox States upon Infection and Implications for Host Cell Homeostasis

The cofactors nicotinamide adenine dinucleotide (NAD⁺) and its phosphate form, NADP⁺, are crucial molecules present in all living cells. The delicate balance between the oxidized and reduced forms of these molecules is tightly regulated by intracellular metabolism assuring the maintenance of homeostatic conditions, which are essential for cell survival and proliferation. A recent cluster of data has highlighted the importance of the intracellular NAD⁺/NADH and NADP⁺/NADPH ratios during host-pathogen interactions, as fluctuations in the levels of these cofactors and in precursors' bioavailability may condition host response and, therefore, pathogen persistence or elimination. Furthermore, an increasing interest has been given towards how pathogens are capable of hijacking host cell proteins in their own advantage and, consequently, alter cellular redox states and immune function. Here, we review the basic principles behind biosynthesis and subcellular compartmentalization of NAD⁺ and NADP⁺, as well as the importance of these cofactors during infection, with a special emphasis on pathogen-driven modulation of host NAD⁺/NADP⁺ levels and contribution to the associated immune response.

Neuroprotective kynurenine metabolite indices are abnormally reduced and positively associated with hippocampal and amygdalar volume in bipolar disorder

Inflammation-related changes in the concentrations of kynurenine-pathway metabolites occur in depression secondary to medical conditions but have not been well characterized in primary bipolar disorder (BD), with contradictory results potentially attributable to the presence or absence of psychosis and/or medication effects. In contrast, reductions in hippocampal and amygdalar volume that theoretically reflect dendritic atrophy occurring in the context of a neurotoxic process are commonly reported in unmedicated BD patients. Here we tested whether the concentrations of putatively neuroprotective (kynurenic acid, KynA) and neurotoxic (3-hydroxy-kynurenine, 3HK and quinolinic acid, QA) kynurenine-pathway metabolites were altered in primary BD and whether these metabolites were associated with hippocampal and amygdalar volume. Twenty-five moderately-to-severely depressed unmedicated subjects and 38 moderately-to-severely depressed medicated subjects who met DSM-IV-TR criteria for BD, as well as 48 healthy controls (HCs) completed a structural MRI scan and provided a blood sample for kynurenine metabolite analysis, performed using high performance liquid chromatography with tandem mass spectrometry. Gray matter volumes were measured with the automated segmentation software, FreeSurfer. A putative neuroprotective index, KynA/QA, was significantly lower in the BD subjects relative to the HCs, a finding that was unrelated to current treatment with medication or a prior history of psychosis. Further, another putative neuroprotective index, KynA/3HK was positively associated with hippocampal volume in the BD group after controlling for age, sex, body mass index (BMI), and intracranial volume (ICV). Kyn/3HK was significantly associated with total amygdalar volume in the BD group, but after controlling for age, sex, BMI, but not ICV, this association was reduced to a trend. In addition, Kyn/3HK was positively associated with amygdalar volume in the HCs although the association was no longer significant after accounting for the effects of age, sex, and BMI. The results raise the possibility that BD-associated abnormalities in kynurenine metabolism may impact the structure of the hippocampus and amygdala, highlighting a pathway through which inflammation may exert neuropathological effects in the context of depression.

Putative neuroprotective and neurotoxic kynurenine pathway metabolites are associated with hippocampal and amygdalar volumes in subjects with major depressive disorder

Inflammation-related changes in the concentrations of kynurenine pathway metabolites occur in depression secondary to medical conditions but are not firmly established in primary mood disorders. Reductions in hippocampal and amygdalar volume that putatively reflect dendritic atrophy are widely reported in major depressive disorder (MDD). Here we tested whether the relative serum concentrations of putatively neuroprotective (kynurenic acid (KA)) and neurotoxic (3-hydroxykynurenine (3HK) and quinolinic acid (QA)) kynurenine pathway metabolites were altered in primary MDD and whether these metabolites were associated with hippocampal and amygdalar volume. A total of 29 moderately to severely depressed unmedicated subjects who met DSM-IV criteria for MDD and 20 healthy controls (HCs) completed a structural MRI scan and provided blood sample for kynurenine metabolite analysis, performed using high-performance liquid chromatography with tandem mass spectrometry. Cytokine concentrations were measured with ELISA and gray matter volumes were measured with the automated segmentation software, FreeSurfer. An a priori defined variable of interest, the KA/QA ratio, a putative neuroprotective index, trended lower in the MDD versus the HC group and correlated negatively with anhedonia but positively with the total hippocampal and amygdala volume in the MDD subjects. The post hoc data reduction methods yielded three principal components. Component 1 (interleukin-1 receptor antagonist, QA, and kynurenine) was significantly elevated in MDD participants versus the HCs, whereas component 2 (KA, tryptophan, and kynurenine) was positively correlated with hippocampal and amygdala volume within the MDD group. Our results raise the possibility that an immune-related imbalance in the relative metabolism of KA and QA predisposes to depression-associated dendritic atrophy and anhedonia.

The kynurenine pathway is involved in bacterial meningitis

BACKGROUND

Bacterial meningitis (BM) is characterized by an intense host inflammatory reaction, which contributes to the development of brain damage and neuronal sequelae. Activation of the kynurenine (KYN) pathway (KP) has been reported in various neurological diseases as a consequence of inflammation. Previously, the KP was shown to be activated in animal models of BM, and the association of the SNP AADAT + 401C/T (kynurenine aminotransferase II - KAT II) with the host immune response to BM has been described. The aim of this study was to investigate the involvement of the KP during BM in humans by assessing the concentrations of KYN metabolites in the cerebrospinal fluid (CSF) of BM patients and their relationship with the inflammatory response compared to aseptic meningitis (AM) and non-meningitis (NM) groups.

METHODS

The concentrations of tryptophan (TRP), KYN, kynurenic acid (KYNA) and anthranilic acid (AA) were assessed by HPLC from CSF samples of patients hospitalized in the Giselda Trigueiro Hospital in Natal (Rio Grande do Norte, Brazil). The KYN/TRP ratio was used as an index of indoleamine 2,3-dioxygenase (IDO) activity, and cytokines were measured using a multiplex cytokine assay. The KYNA level was also analyzed in relation to AADAT + 401C/T genotypes.

RESULTS

In CSF from patients with BM, elevated levels of KYN, KYNA, AA, IDO activity and cytokines were observed. The cytokines INF-γ and IL-1Ra showed a positive correlation with IDO activity, and TNF-α and IL-10 were positively correlated with KYN and KYNA, respectively. Furthermore, the highest levels of KYNA were associated with the AADAT + 401 C/T variant allele.

CONCLUSION

This study suggests a downward modulatory effect of the KP on CSF inflammation during BM.

Vitamin B6 prevents cognitive impairment in experimental pneumococcal meningitis

Streptococcus pneumoniae is the relevant cause of bacterial meningitis, with a high-mortality rate and long-term neurological sequelae, affecting up to 50% of survivors. Pneumococcal compounds are pro-inflammatory mediators that induce an innate immune response and tryptophan degradation through the kynurenine pathway. Vitamin B6 acts as a cofactor at the active sites of enzymes that catalyze a great number of reactions involved in the metabolism of tryptophan, preventing the accumulation of neurotoxic intermediates. In the present study, we evaluated the effects of vitamin B6 on memory and on brain-derived neurotrophic factor (BDNF) expression in the brain of adult Wistar rats subjected to pneumococcal meningitis. The animals received either 10 µL of artificial cerebral spinal fluid (CSF) or an equivalent volume of S. pneumoniae suspension. The animals were divided into four groups: control, control treated with vitamin B6, meningitis, and meningitis treated with vitamin B6. Ten days after induction, the animals were subjected to behavioral tests: open-field task and step-down inhibitory avoidance task. In the open-field task, there was a significant reduction in both crossing and rearing in the control group, control/B6 group, and meningitis/B6 group compared with the training session, demonstrating habituation memory. However, the meningitis group showed no difference in motor and exploratory activity between training and test sessions, demonstrating memory impairment. In the step-down inhibitory avoidance task, there was a difference between training and test sessions in the control group, control/B6 group, and meningitis/B6 group, demonstrating aversive memory. In the meningitis group, there was no difference between training and test sessions, demonstrating impairment of aversive memory. In the hippocampus, BDNF expression decreased in the meningitis group when compared to the control group; however, adjuvant treatment with vitamin B6 increased BDNF expression in the meningitis group. Thus, vitamin B6 attenuated the memory impairment in animals subjected to pneumococcal meningitis.

The kynurenine pathway contributes to long-term neuropsychological changes in experimental pneumococcal meningitis

Pneumococcal meningitis is a lethal form of bacterial infection in the central nervous system that often causes lifelong neurological sequelae, despite therapeutic advances. The contemporary view is that the inflammatory response to infection contributes to the functional disabilities among survivors of this disease. We previously have established a mouse model of neurobehavioural deficits, using an automated IntelliCage™ system that revealed long-term behavioural and cognitive deficits in C57BL/6J female mice cured of meningitis by ceftriaxone treatment. We now have investigated the roles of two kynurenine pathway enzymes, indoleamine dioxygenase-1 (IDO1) and tryptophan dioxygenase-2 (TDO2), in the pathomechanisms of pneumococcal meningitis. Since tryptophan metabolism has long been implicated in behavioural and cognitive modulation through the production of neuroactive compounds, we hypothesised that preventing the actions of these enzymes through gene knockout would be beneficial in mice subjected to pneumococcal infection. We found no significant effect of IDO1 or TDO2 on mortality. Post-meningitic wild-type mice showed long-term diurnal hypoactivity and nocturnal hyperactivity when they were exposed to an Intellicage adaptation test throughout both the light and dark phases. These changes were not apparent in IDO1(-/-) survivors, but were present in the TDO2(-/-) survivors. Both IDO1(-/-) and TDO2(-/-) survivors were not protected against developing long-term cognitive deficits as measured in IntelliCage-based patrolling or reversal tasks. Collectively, these observations suggest (i) involvement of the kynurenine pathway in causing some behavioural sequelae of pneumococcal meningitis and (ii) that this pathway might operate synergistically with, or independently of, other pathways to cause other aspects of neurological sequelae.

Activation of kynurenine pathway in ex vivo fibroblasts from patients with bipolar disorder or schizophrenia: cytokine challenge increases production of 3-hydroxykynurenine

Accumulating data suggest a causative link between immune stimulation, disturbed metabolism of tryptophan, and pathogenesis of bipolar disorder and schizophrenia. The goal of this study was to examine the production of kynurenic acid (KYNA), 3-hydroxykynurenine (3-HK) and the expression of kynurenine pathway enzymes involved in their synthesis and metabolism in cultured skin fibroblasts obtained from patients with bipolar disorder, schizophrenia or from healthy control individuals. The assessment was performed under basal conditions or following treatment with interferon (IFN)-γ, tumor necrosis factor (TNF)-α, interleukin (IL)-1β, IL-6, or their combinations, in cells exposed to exogenous kynurenine. In both groups of patients, the baseline production of KYNA and 3-HK was increased, as compared to control subjects. Case-treatment analyses revealed significant interactions between bipolar case status and IL-1β, IL-6, IFN-γ + TNF-α, or IFN-γ + IL-1β, as well as between schizophrenia case status and IL-1β, IFN-γ + TNF-α, or IFN-γ + IL-1β, in terms of higher 3-HK. Noteworthy, no case-treatment interactions in terms of KYNA production were found. Observed changes did not appear to correlate with the expression of genes encoding kynurenine aminotransferases (KATs), kynureninase (KYNU) or kynurenine-3-monooxygenase (KMO). The single nucleotide polymorphisms (SNPs), rs1053230 and rs2275163, in KMO influenced KYNA levels yet did not explain the case-treatment discrepancies. In conclusion, our present findings indicate the utility of skin-derived fibroblasts for kynurenines research and support the concept of kynurenine pathway alterations in bipolar disorder and schizophrenia. The increase in ratio between neurotoxic 3-HK and neuroinhibitory/neuroprotective KYNA following exposure to cytokines may account for altered neurogenesis and structural abnormalities characteristic for both diseases.

Vitamin B6 reduces hippocampal apoptosis in experimental pneumococcal meningitis

BACKGROUND

Bacterial meningitis caused by Streptococcus pneumoniae leads to death in up to 30% of patients and leaves up to half of the survivors with neurological sequelae. The inflammatory host reaction initiates the induction of the kynurenine pathway and contributes to hippocampal apoptosis, a form of brain damage that is associated with learning and memory deficits in experimental paradigms. Vitamin B6 is an enzymatic cofactor in the kynurenine pathway and may thus limit the accumulation of neurotoxic metabolites and preserve the cellular energy status. The aim of this study in a pneumococcal meningitis model was to investigate the effect of vitamin B6 on hippocampal apoptosis by histomorphology, by transcriptomics and by measurement of cellular nicotine amide adenine dinucleotide content.

METHODS AND RESULTS

Eleven day old Wistar rats were infected with 1x10(6) cfu/ml of S. pneumoniae and randomized for treatment with vitamin B6 or saline as controls. Vitamin B6 led to a significant (p > 0.02) reduction of hippocampal apoptosis. According to functional annotation based clustering, vitamin B6 led to down-regulation of genes involved in processes of inflammatory response, while genes encoding for processes related to circadian rhythm, neuronal signaling and apoptotic cell death were mostly up-regulated.

CONCLUSIONS

Our results provide evidence that attenuation of apoptosis by vitamin B6 is multi-factorial including down-modulation of inflammation, up-regulation of the neuroprotective brain-derived neurotrophic factor and prevention of the exhaustion of cellular energy stores. The neuroprotective effect identifies vitamin B6 as a potential target for the development of strategies to attenuate brain injury in bacterial meningitis.

Effects of pentoxifylline, 7-nitroindazole, and imipramine on tumor necrosis factor-α and indoleamine 2,3-dioxygenase enzyme activity in the hippocampus and frontal cortex of chronic mild-stress-exposed rats

OBJECTIVES

This study aimed to investigate the role of tumor necrosis factor (TNF)-α and the neuronal nitric oxide synthase enzyme in dysregulation of indoleamine 2,3-dioxygenase (IDO) enzyme, and hence serotonin availability in chronic mild stress (CMS), an animal model of depression.

METHODS

RATS WERE DIVIDED INTO FIVE GROUPS: two control and CMS-exposed for 6 weeks, and another three groups exposed to CMS and administered pentoxifylline 50 mg/kg/day intraperitoneally, 7-nitroindazole 40 mg/kg/day subcutaneously, or imipramine 20 mg/kg/day intraperitoneally for the previous 3 CMS weeks. Rats were assessed for neurochemical and immunohistochemical abnormalities.

RESULTS

Pentoxifylline-, 7-nitroindazole-, and imipramine-treated rats showed amelioration of CMS-induced behavioral deficits that was accompanied by significant reduction in kynurenine/serotonin molar ratio and nitrates/nitrites in frontal cortex and hippocampus. In the pentoxifylline and 7-nitroindazole groups, serum TNF-α was reduced relative to the CMS group (18.54 ± 0.85 and 19.16 ± 1.54 vs 26.20 ± 1.83 pg/mL, respectively; P < 0.05). Exposure to CMS increased TNF-α and IDO immunohistochemical staining scores in both hippocampus and midbrain raphe nuclei. 7-Nitroindazole and pentoxifylline significantly (P < 0.05) reduced TNF-α immunostaining in hippocampus and raphe nuclei, with significant (P < 0.01) reduction of IDO immunostaining in raphe nuclei. Likewise, imipramine reduced TNF-α immunostaining (P < 0.05) in hippocampus.

CONCLUSION

Neuronal nitric oxide synthase and TNF-α may play a concerted role in modulating IDO enzyme activity in CMS-exposed rats and provide additional evidence for possible alternative approaches to switch the neurobiological processes in depression.

3-Hydroxykynurenine: an intriguing molecule exerting dual actions in the central nervous system

Kynurenine pathway is gaining attention due to the many metabolic processes in which it has been involved. The tryptophan conversion into several other metabolites through this pathway provides neuronal and redox modulators useful for maintenance of major functions in the brain. However, when physiopathological conditions prevail - i.e. oxidative stress, excitotoxicity, and inflammation - preferential formation and accumulation of toxic metabolites could trigger factors for degeneration in neurological disorders. 3-Hydroxykynurenine has been largely described as one of these toxic metabolites capable of inducing oxidative damage and cell death; consequently, this metabolite has been hypothesized to play a pivotal role in different neurological and psychiatric disorders. Supporting evidence has shown altered 3-hydroxykynurenine levels in samples of patients from several disorders. In contrast, some experimental studies have provided evidence of antioxidant and scavenging properties inherent to this molecule. In this review, we explored most of literature favoring one or the other concept, in order to provide an accurate vision on the real participation of this tryptophan metabolite in both experimental paradigms and human brain pathologies. Through this collected evidence, we provide an integrative hypothesis on how 3-hydroxykynurenine is exerting its dual actions in the central nervous system and what will be the course of investigations in this field for the next years.

Acetylcholinesterase activity in the rat brain after pneumococcal meningitis

Pneumococcal meningitis is a life-threatening disease characterized by acute purulent infection of the meninges causing neuronal injury, cortical necrosis and hippocampal apoptosis. Cholinergic neurons and their projections are extensively distributed throughout the central nervous system. The aim of this study was to assess acetylcholinesterase activity in the rat brain after pneumococcal meningitis. In the hippocampus, frontal cortex and cerebrospinal fluid, acetylcholinesterase activity was found to be increased at 6, 12, 24, 48 and 96 hr without antibiotic treatment, and at 48 and 96 hr with antibiotic treatment. Our data suggest that acetylcholinesterase activity could be related to neuronal damage induced by pneumococcal meningitis.

Association of kynurenine aminotransferase II gene C401T polymorphism with immune response in patients with meningitis

Background

The kynurenine (KYN) pathway has been shown to be altered in several diseases which compromise the central nervous system (CNS) including infectious diseases such as bacterial meningitis (BM). The aim of this study was to assess single nucleotide polymorphisms (SNPs) in four genes of KYN pathway in patients with meningitis and their correlation with markers of immune response in BM.

Methods

One hundred and one individuals were enrolled in this study to investigate SNPs in the following genes: indoleamine-2,3-dioxygenase (IDO1 gene), kynureninase (KYNU gene), kynurenine aminotransferase I (CCBL1 gene), and kynurenine aminotransferase II (AADAT gene). SNP analyses were performed by primer-introduced restriction analysis-PCR (PIRA-PCR) followed by RFLP. Cytokines were measured using multiplex bead assay while immunoglobulins (IG) by immunodiffusion plates and NF-kappaB and c-Jun by dot blot assay.

Results

The variant allele of SNP AADAT+401C/T showed prevalent frequency in patients with BM. A significant decrease (p < 0.05) in TNF-α, IL-1β, IL-6, MIP-1αCCL3 and MIP-1β/CCL4 levels was observed in BM patients homozygous (TT) to the SNP AADAT+401C/T. Furthermore, a significant (p < 0.05) decrease in cell count was observed in cerebrospinal fluid (CSF) from patients with TT genotype. In addition, an increase in the IgG level in adults (p < 0.05) was observed. The variant allele for KYNU+715G/A was found with low frequency in the groups, and the SNPs in IDO1+434T/G, KYNU+693G/A, CCBL1+164T/C, and AADAT+650C/T had no frequency in this population.

Conclusions

This study is the first report of an association of SNP AADAT+401C/T with the host immune response to BM, suggesting that this SNP may affect the host ability in recruitment of leukocytes to the infection site. This finding may contribute to identifying potential targets for pharmacological intervention as adjuvant therapy for BM.

Inhibition of the kynurenine-NAD+ pathway leads to energy failure and exacerbates apoptosis in pneumococcal meningitis

Pneumococcal meningitis causes neurological sequelae, including learning and memory deficits in up to half of the survivors. In both humans and in animal models of the disease, there is apoptotic cell death in the hippocampus, a brain region involved in learning and memory function. We previously demonstrated that in an infant rat model of pneumococcal meningitis, there is activation of the kynurenine (KYN) pathway in the hippocampus, and that there was a positive correlation between the concentration of 3-hydroxykynurenine and the extent of hippocampal apoptosis. To clarify the role of the KYN pathway in the pathogenesis of hippocampal apoptosis in pneumococcal meningitis, we specifically inhibited 2 key enzymes of the KYN pathway and assessed hippocampal apoptosis, KYN pathway metabolites, and nicotinamide adenine dinucleotide (NAD) concentrations by high-performance liquid chromatography. Pharmacological inhibition of kynurenine 3-hydroxylase and kynureninase led to decreased cellular NAD levels and increased apoptosis in the hippocampus. The cerebrospinal fluid levels of tumor necrosis factor and interleukin-1α and -β were not affected. Our data suggest that activation of the KYN pathway in pneumococcal meningitis is neuroprotective by compensating for an increased NAD demand caused by infection and inflammation;this mechanism may prevent energy failure and apoptosis in the hippocampus.

Tracking the transcriptional host response from the acute to the regenerative phase of experimental pneumococcal meningitis

BACKGROUND

Despite the availability of effective antibiotic therapies, pneumococcal meningitis (PM) has a case fatality rate of up to 30% and causes neurological sequelae in up to half of the surviving patients. The underlying brain damage includes apoptosis of neurons in the hippocampus and necrosis in the cortex. Therapeutic options to reduce acute injury and to improve outcome from PM are severely limited.With the aim to develop new therapies a number of pharmacologic interventions have been evaluated. However, the often unpredictable outcome of interventional studies suggests that the current concept of the pathophysiologic events during bacterial meningitis is fragmentary. The aim of this work is to describe the transcriptomic changes underlying the complex mechanisms of the host response to pneumococcal meningitis in a temporal and spatial context using a well characterized infant rat model.

METHODS

Eleven days old nursing Wistar rats were infected by direct intracisternal injection of 2 x 106 cfu/ml of Streptococcus pneumoniae. Animals were sacrificed at 1, 3, 10 and 26 days after infection, the brain harvested and the cortex and hippocampus were sampled. The first two time points represent the acute and sub-acute phase of bacterial meningitis, whereas the latter represent the recovery phase of the disease.

RESULTS

The major events in the regulation of the host response on a transcriptional level occur within the first 3 days after infection. Beyond this time, no differences in global gene expression in infected and control animals were detectable by microarray analysis. Whereas in the acute phase of the disease immunoregulatory processes prevail in the hippocampus and the cortex, we observed a strong activation of neurogenic processes in the hippocampal dentate gyrus, both by gene expression and immunohistology starting as early as 3 days after infection.

CONCLUSIONS

Here we describe the cellular pathways involved in the host response to experimental pneumococcal meningitis in specified disease states and brain regions. With these results we hope to provide the scientific basis for the development of new treatment strategies which take the temporal aspects of the disease into account.

Oxidative stress in pneumococcal meningitis: A future target for adjunctive therapy?

Despite antibiotic therapy and supportive intensive care, the morbidity and mortality of pneumococcal meningitis remain unacceptably high. During the last years, reactive oxygen (ROS) and nitrogen species (RNS), and peroxynitrite, were found to be produced in large amounts during pneumococcal meningitis. Although most likely intended to fight the invasive pathogens, they seem to lead to substantial collateral damage instead. This is because ROS and RNS can exert a vast variety of toxic actions, e.g., through lipid peroxidation, DNA strand breakage followed by PARP activation and subsequent cellular energy depletion, production of inflammatory cytokines, and activation of matrix metalloproteinases. Animal models of pneumococcal meningitis have shown that these interactions contribute to massive meningeal inflammation, disruption of the blood-brain barrier, alterations of the cerebral autoregulation, neuronal cell death, and cochlear destruction. Thus, the production of ROS and RNS seems at least in part to be responsible for the poor outcome of patients with pneumococcal meningitis. In consequence, reactive oxygen and nitrogen species such as peroxynitrite have been investigated as potential targets for adjunctive therapy in pneumococcal meningitis. Among the multiple agents tested, one promising drug is N-acetyl-l-cysteine (NAC), which significantly reduced cerebral and cochlear complications in animal models of experimental pneumococcal meningitis.