Lilly Schwieler

Background: Inflammation alters the kynurenine pathway (KP), producing metabolites related to pain regulation. Previous microdialysis studies indicate increased levels of inflammatory markers in temporomandibular disorders myalgia (TMDM) that increase in response to tooth-clenching. This study aimed to elucidate the role of the KP in TMDM by investigating intramuscular KP metabolites in response to tooth-clenching. Methods: This study included 41 women; 15 TMDM myofascial pain with referral (MFP), 13 TMDM local myalgia (MYA), and 13 controls (CTR). Microdialysis was applied to the masseter muscle for 220 minutes to analyze tryptophan (TRP), l -kynurenine (KYN), 3-hydroxykynurenine (3-HK), quinolinic acid (QUIN) and nicotinamide (NAM). After baseline (140 minutes), participants performed a 20-minute tooth-clenching exercise followed by 1 hour of recovery. Pain variables were assessed throughout the experiment. Results: NAM levels increased significantly in response to tooth-clenching across all groups ( P < 0.001), while TRP, KYN, QUIN, and 3-HK increased only in MFP ( P < 0.001 to P = 0.041). Supervised multivariate analysis showed that changes in TRP, KYN, and QUIN levels in response to exercise, and NAM levels at baseline and recovery, significantly separated MFP from CTR and MYA ( P = 0.040). In addition, decreased concentration levels of mainly NAM and TRP were significantly associated with increased widespread pain ( P = 0.02) and pain during recovery ( P = 0.030). Conclusion: These findings indicate that intramuscular KP metabolites increase in response to tooth-clenching, particularly in MFP, distinguishing them from both CTR and MYA. This supports the link between intramuscular inflammation and exercise-induced alterations in the KP, providing further insights in the TMDM pathophysiology and differentiation of its subdiagnosis.

The kynurenine pathway (KP) is gaining attention in several clinical fields. Recent studies show that physical exercise offers a therapeutic way to improve ratios of neurotoxic to neuroprotective KP metabolites. Antioxidant supplementation can blunt beneficial responses to physical exercise. We here studied the effects of endurance training in the form of sprint interval training (SIT; three sessions of 4–6 × 30 s cycling sprints per week for three weeks) in elderly (~65 years) men exposed to either placebo (n = 9) or the antioxidants vitamin C (1 g/day) and E (235 mg/day) (n = 11). Blood samples and muscle biopsies were taken under resting conditions in association with the first (untrained state) and last (trained state) SIT sessions. In the placebo group, the blood plasma level of the neurotoxic quinolinic acid was lower (~30%) and the neuroprotective kynurenic acid to quinolinic acid ratio was higher (~50%) in the trained than in the untrained state. Moreover, muscle biopsies showed a training-induced increase in kynurenine aminotransferase (KAT) III in the placebo group. All these training effects were absent in the vitamin-treated group. In conclusion, KP metabolism was shifted towards neuroprotection after three weeks of SIT in elderly men and this shift was blocked by antioxidant treatment.

In children, tick-borne encephalitis and neuroborreliosis are common infections affecting the central nervous system. As inflammatory pathways including cytokine expression are activated in these children and appear to be of importance for outcome, we hypothesized that induction of the kynurenine pathway may be part of the pathophysiological mechanism. Inflammatory biomarkers were analyzed in cerebrospinal fluid from 22 children with tick-borne encephalitis (TBE), 34 children with neuroborreliosis (NB) and 6 children with no central nervous system infection. Cerebrospinal fluid levels of kynurenine and kynurenic acid were increased in children with neuroborreliosis compared to the comparison group. A correlation was seen between expression of several cerebrospinal fluid cytokines and levels of kynurenine and kynurenic acid in children with neuroborreliosis but not in children with tick-borne encephalitis. These findings demonstrate a strong induction of the kynurenine pathway in children with neuroborreliosis which differs from that seen in children with tick-borne encephalitis. The importance of brain kynurenic acid (KYNA) in both immune modulation and neurotransmission raises the possibility that abnormal levels of the compound in neuroborreliosis might be of importance for the pathophysiology of the disease. Drugs targeting the enzymes of this pathway may open the venue for novel therapeutic interventions.

One-fourth of children with cerebral malaria (CM) retain cognitive sequelae up to 2 years after acute disease. The kynurenine pathway of the brain, forming neuroactive metabolites, e.g. the NMDA-receptor antagonist kynurenic acid (KYNA), has been implicated in long-term cognitive dysfunction in other CNS infections. In the present study, the association between the kynurenine pathway and neurologic/cognitive complications in children with CM was investigated. Cerebrospinal fluid (CSF) concentrations of KYNA and its precursor kynurenine in 69 Ugandan children admitted for CM to Mulago Hospital, Kampala, Uganda, between 2008 and 2013 were assessed. CSF kynurenine and KYNA were compared to CSF cytokine levels, acute and long-term neurologic complications, and long-term cognitive impairments. CSF kynurenine and KYNA from eight Swedish children without neurological or infectious disease admitted to Astrid Lindgren’s Children’s Hospital were quantified and used for comparison. Children with CM had significantly higher CSF concentration of kynurenine and KYNA than Swedish children (P < 0.0001 for both), and CSF kynurenine and KYNA were positively correlated. In children with CM, CSF kynurenine and KYNA concentrations were associated with coma duration in children of all ages (P = 0.003 and 0.04, respectively), and CSF kynurenine concentrations were associated with worse overall cognition (P = 0.056) and attention (P = 0.003) at 12-month follow-up in children ≥5 years old. CSF KYNA and kynurenine are elevated in children with CM, indicating an inhibition of glutamatergic and cholinergic signaling. This inhibition may lead acutely to prolonged coma and long-term to impairment of attention and cognition.

Tryptophan degradation along the kynurenine pathway is of central importance for the immune function. Toll‐like receptors (TLRs), representing the first line of immune defence against pathogens, are expressed in various cell types. The most abundant expression is found on monocytes, macrophages and dendritic cells. The aim of this study was to investigate whether stimulation with different TLR ligands induces the kynurenine pathway in human peripheral monocytes. Cell supernatants were analysed using a liquid chromatography/mass spectrometry to measure kynurenine, kynurenic acid (KYNA), quinolinic acid (QUIN) and tryptophan. Stimulation of TLR‐2, TLR‐3, TLR‐4, TLR‐7/8 and TLR‐9 was found to induce the production of kynurenine, but only stimulation of TLR‐3 increased levels of further downstream metabolites, such as KYNA and QUIN. Stimulation of TLR‐1, TLR‐5 and TLR‐6 did not induce the kynurenine pathway. Taken together, this study provides novel evidence demonstrating that TLR activation induces a pattern of downstream tryptophan degradation along the kynurenine pathway in monocytes. The results of this study may implicate that TLRs can be used as new drug targets for the regulation of aberrant tryptophan metabolism along this pathway, a potential therapeutic strategy that may be of importance in several disorders.

Neuroinflammation is increasingly recognized as contributing to the pathogenesis of depression. Key inflammatory markers as well as kynurenic acid (KYNA) and quinolinic acid (QUIN), both tryptophan metabolites, have been associated with depressive symptoms and suicidality. The aim of the present study is to investigate the peripheral concentration of cytokines and tryptophan and kynurenine metabolites in patients with unipolar treatment-resistant depression before and after electroconvulsive therapy (ECT), the most effective treatment for depression. Cytokines in plasma from patients with major depressive disorder (MDD; n = 19) and healthy volunteers (n = 14) were analyzed with electrochemiluminescence detection. Tryptophan and kynurenine metabolites were detected with high-performance liquid chromatography (HPLC) and LC/MS. KYNA was analyzed in a second healthy control cohort (n = 22). Patients with MDD had increased plasma levels of interleukin (IL)-6 compared to healthy volunteers (P < 0.05). We also found an altered kynurenine metabolism in these patients displayed by decreased plasma levels of KYNA (P < 0.0001) as well as a significantly increased QUIN/KYNA ratio (P < 0.001). Plasma levels of tryptophan, kynurenine, and QUIN did not differ between patients and controls. Treatment with ECT was associated with a significant decrease in the plasma levels of tryptophan (P < 0.05), kynurenine (P < 0.01), and QUIN (P < 0.001), whereas plasma levels of KYNA did not change. The QUIN/KYNA ratio was found to significantly decrease in ECT-treated patients (P < 0.05). There was a significant inverse correlation between symptom severity and kynurenine levels at baseline (r = −0.67, P = 0.002). This study confirms an imbalanced kynurenine pathway in MDD supporting the hypothesis of a netstimulation of N-methyl-d-aspartic acid (NMDA) receptors in the disorder. Treatment with ECT profoundly decreased QUIN, an NMDA-receptor agonist previously suggested to be implicated in the pathogenesis of depression, an effect that might have bearing for the good clinical outcome of ECT.

Abstract. Holtze M, Mickiené A, Atlas A, Lindquist L, Schwieler L (Karolinska Institutet, Stockholm; Lithuanian University of Health Sciences, Kaunas, Lithuanian; Karolinska Institutet, Karolinska University Hospital, Huddinge; Karolinska Institutet, Karolinska University Hospital, Solna, Sweden). Elevated cerebrospinal fluid kynurenic acid levels in patients with tick‐borne encephalitis.J Intern Med2012; 272: 394–401.Background. Kynurenic acid (KYNA) is a neuroactive metabolite of tryptophan that is thought to regulate cognitive functions. Previous studies have shown that levels of KYNA increase during virus infection and that this metabolite interacts with the immune system.Objective. The aim of the study was to investigate whether patients with tick‐borne encephalitis (TBE), a viral infectious disease associated with long‐term cognitive impairment, have increased levels of KYNA in the cerebrospinal fluid (CSF).Methods. CSF KYNA was analysed using high‐performance liquid chromatography in 108 patients with TBE and 52 age‐matched controls. Patients were classified according to the severity of TBE: mild (47%), moderate (44%) or severe (9%).Results. Concentrations of CSF KYNA were considerably higher in patients with TBE (5.3 nmol L⁻¹) than in control subjects (0.99 nmol L⁻¹). KYNA concentration in the CSF varied greatly amongst individuals with TBE and increased (P < 0.05) with the severity of disease.Conclusions. This is the first study to demonstrate increased levels of CSF KYNA in patients with TBE. The importance of brain KYNA in both immune modulation and neurotransmission raises the possibility that abnormal levels of the compound in TBE might play a part in the pathophysiology of the disease. A detailed knowledge of endogenous brain KYNA during the course of CNS infection might yield further insights into the neuroimmunological role of the compound and may also provide new pharmacological approaches for the treatment of cognitive symptoms.