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Background: The kynurenic acid (KYNA) hypothesis for schizophrenia is partly based on studies showing increased brain levels of KYNA in patients. KYNA is an endogenous metabolite of tryptophan (TRP) produced in astrocytes and antagonizes N-methyl-D-aspartate and α7* nicotinic receptors., Methods: The formation of KYNA is determined by the availability of substrate, and hence, we analyzed KYNA and its precursors, kynurenine (KYN) and TRP, in the cerebrospinal fluid (CSF) of patients with schizophrenia. CSF from male patients with schizophrenia on olanzapine treatment (n = 16) was compared with healthy male volunteers (n = 29)., Results: KYN and KYNA concentrations were higher in patients with schizophrenia (60.7 ± 4.37 nM and 2.03 ± 0.23 nM, respectively) compared with healthy volunteers (28.6 ± 1.44 nM and 1.36 ± 0.08 nM, respectively), whereas TRP did not differ between the groups. In all subjects, KYN positively correlated to KYNA., Conclusion: Our results demonstrate increased levels of CSF KYN and KYNA in patients with schizophrenia and further support the hypothesis that KYNA is involved in the pathophysiology of schizophrenia.

Mesenchymal stem cells (MSC) display unique suppressive properties on T-cell immunity, thus representing an attractive vehicle for the treatment of conditions associated with harmful T-cell responses such as organ-specific autoimmunity and graft-versus-host disease. Toll-like receptors (TLR) are primarily expressed on antigen-presenting cells and recognize conserved pathogen-derived components. Ligation of TLR activates multiple innate and adaptive immune response pathways to eliminate and protect against invading pathogens. In this work, we show that TLR expressed on human bone marrow-derived MSC enhanced the immunosuppressive phenotype of MSC. Immunosuppression mediated by TLR was dependent on the production of immunosuppressive kynurenines by the tryptophan-degrading enzyme indoleamine-2,3-dioxygenase-1 (IDO1). Induction of IDO1 by TLR involved an autocrine interferon (IFN)-beta signaling loop, which was dependent on protein kinase R (PKR), but independent of IFN-gamma. These data define a new role for TLR in MSC immunobiology, which is to augment the immunosuppressive properties of MSC in the absence of IFN-gamma rather than inducing proinflammatory immune response pathways. PKR and IFN-beta play a central, previously unidentified role in orchestrating the production of immunosuppressive kynurenines by MSC.

We evaluated the synthesis of nitric oxide (NO) and of the neurotoxic kynurenine metabolites 3OH-kynurenine and quinolinic acid (QUIN) in interferon-gamma (IFN-gamma)-activated macrophages of the murine BACl.2F5 cell line with the aim of investigating the roles of mononuclear phagocytes in inflammatory neurological disorders. IFN-gamma induced indoleamine 2,3-dioxygenase (IDO) and NO synthase (NOS) and increased the synthesis of 3OH-kynurenine, QUIN, and NO that accumulated in the incubation medium where they reached neurotoxic levels. Macrophage exposure to norharmane, an IDO inhibitor, resulted in a decreased formation of not only the kynurenine metabolites but also NO. The inhibition of NO synthesis could not be ascribed to reduced NADPH availability or decreased NOS induction. Norharmane inhibited NOS activity also in coronary vascular endothelial cells and in isolated aortic rings. Our findings suggest that activated macrophages release large amounts of neurotoxic molecules and that norharmane may represent a prototype compound to study macrophage involvement in inflammatory brain damage.

The tryptophan residue of talopeptin, which is a specific inhibitor for thermolysin, was converted into kynurenine by ozonization followed by acid-catalyzed hydrolysis, and (Trp leads to Kyn) talopeptin (Kyn-talopeptin) thus obtained was purified with gel-chromatography. The inhibitor constant of Kyn-talopeptin, K1, and the dissociation constant of thermolysin-Kyn-talopeptin complex, Kd, directly obtained by fluorometric titration were in good agreement with each other. These values were found to be about 10 times larger than those of intact talopeptin, but both inhibitors showed a similar pH dependence. Upon the binding of Kyn-talopeptin with thermolysin, the protein fluorescence of thermolysin decreases by about 60%, while the kynurenine fluorescence (measured at 450 nm when excited at 360 nm) of the inhibitor increases about 14 times. The measurements of the excitation and fluorescence spectra of EI complex strongly indicated the energy transfer from tryptophan residue(s) (the donor) of the enzyme to kynurenine residue (the acceptor) of the inhibitor. The distance between the donor and the acceptor was roughly estimated to be 18 A. This value is in good agreement with the one expected from the crystallography of phosphoramidon-thermolysin complex. The binding process was studied kinetically with the stopped-flow method over the pH range between 4.5 and 8.5, by monitoring the decrease in the fluorescence intensity of the enzyme tryptophan caused by the complex formation. Comparison of the data with those previously obtained for talopeptin-thermolysin system revealed that the replacement of the tryptophan residue by kynurenine of the inhibitor does not affect the apparent second-order association rate constant, kon, seriously.