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

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.

Kynurenine pathway in Parkinson's disease-An update

Parkinson's disease (PD) is a complex multi-factorial neurodegenerative disorder where various altered metabolic pathways contribute to the progression of the disease. Tryptophan (TRP) is a major precursor in kynurenine pathway (KP) and it has been discussed in various in vitro studies that the metabolites quinolinic acid (QUIN) causes neurotoxicity and kynurenic acid (KYNA) acts as neuroprotectant respectively. More studies are also focused on the effects of other KP metabolites and its enzymes as it has an association with ageing and PD pathogenesis. Until now, very few studies have targeted the role of genetic mutations in abnormal KP metabolism in adverse conditions of PD. Therefore, the present review gives an updated research studies on KP in connection with PD. Moreover, the review emphasizes on the urge for the development of biomarkers and also this would be an initiative in generating an alternative therapeutic approach for PD.

Minireview Exploring the Biological Cycle of Vitamin B3 and Its Influence on Oxidative Stress: Further Molecular and Clinical Aspects

Vitamin B3, or niacin, is one of the most important compounds of the B-vitamin complex. Recent reports have demonstrated the involvement of vitamin B3 in a number of pivotal functions which ensure that homeostasis is maintained. In addition, the intriguing nature of its synthesis and the underlying mechanism of action of vitamin B3 have encouraged further studies aimed at deepening our understanding of the close link between the exogenous supply of B3 and how it activates dependent enzymes. This crucial role can be attributed to the gut microflora and its ability to shape human behavior and development by mediating the bioavailability of metabolites. Recent studies have indicated a possible interconnection between the novel coronavirus and commensal bacteria. As such, we have attempted to explain how the gastrointestinal deficiencies displayed by SARS-CoV-2-infected patients arise. It seems that the stimulation of a proinflammatory cascade and the production of large amounts of reactive oxygen species culminates in the subsequent loss of host eubiosis. Studies of the relationhip between ROS, SARS-CoV-2, and gut flora are sparse in the current literature. As an integrated component, oxidative stress (OS) has been found to negatively influence host eubiosis, in vitro fertilization outcomes, and oocyte quality, but to act as a sentinel against infections. In conclusion, research suggests that in the future, a healthy diet may be considered a reliable tool for maintaining and optimizing our key internal parameters.

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.

NAD+ therapy in age-related degenerative disorders: A benefit/risk analysis

Nicotinamide adenine dinucleotide (NAD+) is an essential pyridine nucleotide that is present in all living cells. NAD+ acts as an important cofactor and substrate for a multitude of biological processes including energy production, DNA repair, gene expression, calcium-dependent secondary messenger signalling and immunoregulatory roles. The de novo synthesis of NAD+ is primarily dependent on the kynurenine pathway (KP), although NAD+ can also be recycled from nicotinic acid (NA), nicotinamide (NAM) and nicotinamide riboside (NR). NAD+ levels have been reported to decline during ageing and age-related diseases. Recent studies have shown that raising intracellular NAD+ levels represents a promising therapeutic strategy for age-associated degenerative diseases in general and to extend lifespan in small animal models. A systematic review of the literature available on Medline, Embase and Pubmed was undertaken to evaluate the potential health and/or longevity benefits due to increasing NAD+ levels. A total of 1545 articles were identified and 147 articles (113 preclinical and 34 clinical) met criteria for inclusion. Most studies indicated that the NAD+ precursors NAM, NR, nicotinamide mononucleotide (NMN), and to a lesser extent NAD+ and NADH had a favourable outcome on several age-related disorders associated with the accumulation of chronic oxidative stress, inflammation and impaired mitochondrial function. While these compounds presented with a limited acute toxicity profile, evidence is still quite limited and long-term human clinical trials are still nascent in the current literature. Potential risks in raising NAD+ levels in various clinical disorders using NAD+ precursors include the accumulation of putative toxic metabolites, tumorigenesis and promotion of cellular senescence. Therefore, NAD+ metabolism represents a promising target and further studies are needed to recapitulate the preclinical benefits in human clinical trials.

New Virulence Factors Identified in Pneumococcal Meningitis

Streptococcus pneumoniae causes bacterial meningitis with a high fatality rate globally. Patients who survive have a significant risk of lasting disabilities. Schmidt et al. have identified bacterial proteins that play a crucial role in pneumococcal meningitis: AliB, part of an oligopeptide transporter, and ComDE of the competence regulator.

Rubella Viruses Shift Cellular Bioenergetics to a More Oxidative and Glycolytic Phenotype with a Strain-Specific Requirement for Glutamine

The flexible regulation of cellular metabolic pathways enables cellular adaptation to changes in energy demand under conditions of stress such as posed by a virus infection. To analyze such an impact on cellular metabolism, rubella virus (RV) was used in this study. RV replication under selected substrate supplementation with glucose, pyruvate, and glutamine as essential nutrients for mammalian cells revealed its requirement for glutamine. The assessment of the mitochondrial respiratory (based on the oxygen consumption rate) and glycolytic (based on the extracellular acidification rate) rate and capacity by respective stress tests through Seahorse technology enabled determination of the bioenergetic phenotype of RV-infected cells. Irrespective of the cellular metabolic background, RV infection induced a shift of the bioenergetic state of epithelial cells (Vero and A549) and human umbilical vein endothelial cells to a higher oxidative and glycolytic level. Interestingly there was a RV strain-specific, but genotype-independent demand for glutamine to induce a significant increase in metabolic activity. While glutaminolysis appeared to be rather negligible for RV replication, glutamine could serve as donor of its amide nitrogen in biosynthesis pathways for important metabolites. This study suggests that the capacity of RVs to induce metabolic alterations could evolve differently during natural infection. Thus, changes in cellular bioenergetics represent an important component of virus-host interactions and could complement our understanding of the viral preference for a distinct host cell population.IMPORTANCE RV pathologies, especially during embryonal development, could be connected with its impact on mitochondrial metabolism. With bioenergetic phenotyping we pursued a rather novel approach in virology. For the first time it was shown that a virus infection could shift the bioenergetics of its infected host cell to a higher energetic state. Notably, the capacity to induce such alterations varied among different RV isolates. Thus, our data add viral adaptation of cellular metabolic activity to its specific needs as a novel aspect to virus-host evolution. In addition, this study emphasizes the implementation of different viral strains in the study of virus-host interactions and the use of bioenergetic phenotyping of infected cells as a biomarker for virus-induced pathological alterations.

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.

Streptococcus pneumoniae capsule determines disease severity in experimental pneumococcal meningitis

Streptococcus pneumoniae bacteria can be characterized into over 90 serotypes according to the composition of their polysaccharide capsules. Some serotypes are common in nasopharyngeal carriage whereas others are associated with invasive disease, but when carriage serotypes do invade disease is often particularly severe. It is unknown whether disease severity is due directly to the capsule type or to other virulence factors. Here, we used a clinical pneumococcal isolate and its capsule-switch mutants to determine the effect of capsule, in isolation from the genetic background, on severity of meningitis in an infant rat model. We found that possession of a capsule was essential for causing meningitis. Serotype 6B caused significantly more mortality than 7F and this correlated with increased capsule thickness in the cerebrospinal fluid (CSF), a stronger inflammatory cytokine response in the CSF and ultimately more cortical brain damage. We conclude that capsule type has a direct effect on meningitis severity. This is an important consideration in the current era of vaccination targeting a subset of capsule types that causes serotype replacement.

Increased levels of 3-hydroxykynurenine parallel disease severity in human acute pancreatitis

Inhibition of kynurenine 3-monooxygenase (KMO) protects against multiple organ dysfunction (MODS) in experimental acute pancreatitis (AP). We aimed to precisely define the kynurenine pathway activation in relation to AP and AP-MODS in humans, by carrying out a prospective observational study of all persons presenting with a potential diagnosis of AP for 90 days. We sampled peripheral venous blood at 0, 3, 6, 12, 24, 48, 72 and 168 hours post-recruitment. We measured tryptophan metabolite concentrations and analysed these in the context of clinical data and disease severity indices, cytokine profiles and C-reactive protein (CRP) concentrations. 79 individuals were recruited (median age: 59.6 years; 47 males, 59.5%). 57 met the revised Atlanta definition of AP: 25 had mild, 23 moderate, and 9 severe AP. Plasma 3-hydroxykynurenine concentrations correlated with contemporaneous APACHE II scores (R² = 0.273; Spearman rho = 0.581; P < 0.001) and CRP (R² = 0.132; Spearman rho = 0.455, P < 0.001). Temporal profiling showed early tryptophan depletion and contemporaneous 3-hydroxykynurenine elevation. Furthermore, plasma concentrations of 3-hydroxykynurenine paralleled systemic inflammation and AP severity. These findings support the rationale for investigating early intervention with a KMO inhibitor, with the aim of reducing the incidence and severity of AP-associated organ dysfunction.

Clinical significance of T cell metabolic reprogramming in cancer

Conversion of normal cells to cancer is accompanied with changes in their metabolism. During this conversion, cell metabolism undergoes a shift from oxidative phosphorylation to aerobic glycolysis, also known as Warburg effect, which is a hallmark for cancer cell metabolism. In cancer cells, glycolysis functions in parallel with the TCA cycle and other metabolic pathways to enhance biosynthetic processes and thus support proliferation and growth. Similar metabolic features are observed in T cells during activation but, in contrast to cancer, metabolic transitions in T cells are part of a physiological process. Currently, there is intense interest in understanding the cause and effect relationship between metabolic reprogramming and T cell differentiation. After the recent success of cancer immunotherapy, the crosstalk between immune system and cancer has come to the forefront of clinical and basic research. One of the key goals is to delineate how metabolic alterations of cancer influence metabolism-regulated function and differentiation of tumor resident T cells and how such effects might be altered by immunotherapy. Here, we review the unique metabolic features of cancer, the implications of cancer metabolism on T cell metabolic reprogramming during antigen encounters, and the translational prospective of harnessing metabolism in cancer and T cells for cancer therapy.

Genetic polymorphisms associated with the inflammatory response in bacterial meningitis

Background

Bacterial meningitis (BM) is an infectious disease that results in high mortality and morbidity. Despite efficacious antibiotic therapy, neurological sequelae are often observed in patients after disease. Currently, the main challenge in BM treatment is to develop adjuvant therapies that reduce the occurrence of sequelae. In recent papers published by our group, we described the associations between the single nucleotide polymorphisms (SNPs) AADAT +401C > T, APEX1 Asn148Glu, OGG1 Ser326Cys and PARP1 Val762Ala and BM. In this study, we analyzed the associations between the SNPs TNF -308G > A, TNF -857C > T, IL-8 -251A > T and BM and investigated gene-gene interactions, including the SNPs that we published previously.

Methods

The study was conducted with 54 BM patients and 110 healthy volunteers (as the control group). The genotypes were investigated via primer-introduced restriction analysis-polymerase chain reaction (PIRA-PCR) or polymerase chain reaction-based restriction fragment length polymorphism (PCR-RFLP) analysis. Allelic and genotypic frequencies were also associated with cytokine and chemokine levels, as measured with the x-MAP method, and cell counts. We analyzed gene-gene interactions among SNPs using the generalized multifactor dimensionality reduction (GMDR) method.

Results

We did not find significant association between the SNPs TNF -857C > T and IL-8 -251A > T and the disease. However, a higher frequency of the variant allele TNF -308A was observed in the control group, associated with changes in cytokine levels compared to individuals with wild type genotypes, suggesting a possible protective role. In addition, combined inter-gene interaction analysis indicated a significant association between certain genotypes and BM, mainly involving the alleles APEX1 148Glu, IL8 -251 T and AADAT +401 T. These genotypic combinations were shown to affect cyto/chemokine levels and cell counts in CSF samples from BM patients.

Conclusions

In conclusion, this study revealed a significant association between genetic variability and altered inflammatory responses, involving important pathways that are activated during BM. This knowledge may be useful for a better understanding of BM pathogenesis and the development of new therapeutic approaches.

Electronic supplementary material

The online version of this article (doi:10.1186/s12881-015-0218-6) contains supplementary material, which is available to authorized users.

Regulation of NAD biosynthetic enzymes modulates NAD-sensing processes to shape mammalian cell physiology under varying biological cues

In addition to its role as a redox coenzyme, NAD is a substrate of various enzymes that split the molecule to either catalyze covalent modifications of target proteins or convert NAD into biologically active metabolites. The coenzyme bioavailability may be significantly affected by these reactions, with ensuing major impact on energy metabolism, cell survival, and aging. Moreover, through the activity of the NAD-dependent deacetylating sirtuins, NAD behaves as a beacon molecule that reports the cell metabolic state, and accordingly modulates transcriptional responses and metabolic adaptations. In this view, NAD biosynthesis emerges as a highly regulated process: it enables cells to preserve NAD homeostasis in response to significant NAD-consuming events and it can be modulated by various stimuli to induce, via NAD level changes, suitable NAD-mediated metabolic responses. Here we review the current knowledge on the regulation of mammalian NAD biosynthesis, with focus on the relevant rate-limiting enzymes. This article is part of a Special Issue entitled: Cofactor-dependent proteins: evolution, chemical diversity and bio-applications.

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.

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.

Kynurenine Pathway Pathologies: do Nicotinamide and Other Pathway Co-Factors have a Therapeutic Role in Reduction of Symptom Severity, Including Chronic Fatigue Syndrome (CFS) and Fibromyalgia (FM)

The definition of dual tryptophan pathways has increased the understanding of the mind-body, body-mind dichotomy. The serotonergic pathway highlights the primary (endogenous) psychiatric disorders. The up-regulation of the kynurenine pathway by physical illnesses can cause neuropathic and immunological disorders1 associated with secondary neuropsychiatric symptoms.

Tryptophan and nicotinamide deficiencies fall within the protein energy malnutrition (PEM) spectrum. They can arise if the kynurenine pathway is stressed by primary or secondary inflammatory conditions and the consequent imbalance of available catabolic/anabolic substrates may adversely influence convalescent phase efficiency. The replacement of depleted or reduced NAD+ levels and other cofactors can perhaps improve the clinical management of these disorders.

Chronic fatigue syndrome (CFS) and fibromyalgia (FM) appear to meet the criteria of a tryptophan-kynurenine pathway disorder with potential neuroimmunological sequelae. Aspects of some of the putative precipitating factors have been previously outlined.2,3 An analysis of the areas of metabolic dysfunction will focus on future directions for research and management.

Kynurenines in the mammalian brain: when physiology meets pathology

The essential amino acid tryptophan is not only a precursor of serotonin but is also degraded to several other neuroactive compounds, including kynurenic acid, 3-hydroxykynurenine and quinolinic acid. The synthesis of these metabolites is regulated by an enzymatic cascade, known as the kynurenine pathway, that is tightly controlled by the immune system. Dysregulation of this pathway, resulting in hyper-or hypofunction of active metabolites, is associated with neurodegenerative and other neurological disorders, as well as with psychiatric diseases such as depression and schizophrenia. With recently developed pharmacological agents, it is now possible to restore metabolic equilibrium and envisage novel therapeutic interventions.