title: Activation of the kynurenine pathway and increased production of the excitotoxin quinolinic acid following traumatic brain injury in humans.
abstract: During inflammation, the kynurenine pathway (KP) metabolises the essential amino acid tryptophan (TRP) potentially contributing to excitotoxicity via the release of quinolinic acid (QUIN) and 3-hydroxykynurenine (3HK). Despite the importance of excitotoxicity in the development of secondary brain damage, investigations on the KP in TBI are scarce. In this study, we comprehensively characterised changes in KP activation by measuring numerous metabolites in cerebrospinal fluid (CSF) from TBI patients and assessing the expression of key KP enzymes in brain tissue from TBI victims. Acute QUIN levels were further correlated with outcome scores to explore its prognostic value in TBI recovery.
  
  
  METHODS
  Twenty-eight patients with severe TBI (GCS ≤ 8, three patients had initial GCS = 9-10, but rapidly deteriorated to ≤8) were recruited. CSF was collected from admission to day 5 post-injury. TRP, kynurenine (KYN), kynurenic acid (KYNA), QUIN, anthranilic acid (AA) and 3-hydroxyanthranilic acid (3HAA) were measured in CSF. The Glasgow Outcome Scale Extended (GOSE) score was assessed at 6 months post-TBI. Post-mortem brains were obtained from the Australian Neurotrauma Tissue and Fluid Bank and used in qPCR for quantitating expression of KP enzymes (indoleamine 2,3-dioxygenase-1 (IDO1), kynurenase (KYNase), kynurenine amino transferase-II (KAT-II), kynurenine 3-monooxygenase (KMO), 3-hydroxyanthranilic acid oxygenase (3HAO) and quinolinic acid phosphoribosyl transferase (QPRTase) and IDO1 immunohistochemistry.
  
  
  RESULTS
  In CSF, KYN, KYNA and QUIN were elevated whereas TRP, AA and 3HAA remained unchanged. The ratios of QUIN:KYN, QUIN:KYNA, KYNA:KYN and 3HAA:AA revealed that QUIN levels were significantly higher than KYN and KYNA, supporting increased neurotoxicity. Amplified IDO1 and KYNase mRNA expression was demonstrated on post-mortem brains, and enhanced IDO1 protein coincided with overt tissue damage. QUIN levels in CSF were significantly higher in patients with unfavourable outcome and inversely correlated with GOSE scores.
  
  
  CONCLUSIONS
  TBI induced a striking activation of the KP pathway with sustained increase of QUIN. The exceeding production of QUIN together with increased IDO1 activation and mRNA expression in brain-injured areas suggests that TBI selectively induces a robust stimulation of the neurotoxic branch of the KP pathway. QUIN's detrimental roles are supported by its association to adverse outcome potentially becoming an early prognostic factor post-TBI.
authors.affiliation: Department of Physiology, Monash University, Clayton, VIC, 3800, Australia. edwin.yan@monash.edu.
authors.forename: Edwin B
authors.initials: EB
authors.lastname: Yan
authors.affiliation: Department of Pharmacology and Therapeutics, The University of Melbourne, Melbourne, Australia.
authors.forename: Tony
authors.initials: T
authors.lastname: Frugier
authors.affiliation: Neuroinflammation group, Faculty of Medicine and Health Sciences, Macquarie University, Sydney, Australia.
authors.forename: Chai K
authors.initials: CK
authors.lastname: Lim
authors.affiliation: Neuroinflammation group, Faculty of Medicine and Health Sciences, Macquarie University, Sydney, Australia.
authors.forename: Benjamin
authors.initials: B
authors.lastname: Heng
authors.affiliation: Applied Neurosciences Program, Peter Duncan Neurosciences Research Unit, St Vincent's Centre for Applied Medical Research, Sydney, Australia.
authors.forename: Gayathri
authors.initials: G
authors.lastname: Sundaram
authors.affiliation: Hospital Queen Elizabeth, Karung Berkunci No. 2029, 88586, Kota Kinabalu, Sabah, Malaysia.
authors.forename: May
authors.initials: M
authors.lastname: Tan
authors.affiliation: Department of Neurosurgery, The Alfred Hospital, Melbourne, Australia.|Department of Surgery, Central Clinical School and Monash Institute of Medical Engineering, Monash University, Melbourne, Australia.
authors.forename: Jeffrey V
authors.initials: JV
authors.lastname: Rosenfeld
authors.affiliation: The Ritchie Centre, Hudson Institute of Medical Research, Monash Medical Centre, Melbourne, Australia.
authors.forename: David W
authors.initials: DW
authors.lastname: Walker
authors.affiliation: Neuroinflammation group, Faculty of Medicine and Health Sciences, Macquarie University, Sydney, Australia.
authors.forename: Gilles J
authors.initials: GJ
authors.lastname: Guillemin
authors.affiliation: Australian New Zealand Intensive Care Research Centre, Department of Epidemiology and Preventive Medicine, Monash University, Melbourne, Australia.|Department of Child Health, Barrow Neurological Institute, University of Arizona, Phoenix, AZ, USA.
authors.forename: Maria Cristina
authors.initials: MC
authors.lastname: Morganti-Kossmann
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D015415:Biomarkers / Q000378:metabolism; D001921:Brain / Q000378:metabolism; D001930:Brain Injuries / Q000175:diagnosis* / Q000378:metabolism* / Q000503:physiopathology; D016022:Case-Control Studies; D005260:Female; D023261:Glasgow Outcome Scale; D006801:Humans; D050503:Indoleamine-Pyrrole 2,3,-Dioxygenase / Q000378:metabolism; D007737:Kynurenine / Q000502:physiology*; D008297:Male; D008875:Middle Aged; D009498:Neurotoxins / Q000134:cerebrospinal fluid*; D011379:Prognosis; D017378:Quinolinic Acid / Q000134:cerebrospinal fluid*; D012333:RNA, Messenger / Q000378:metabolism; D015398:Signal Transduction / Q000502:physiology*; D014364:Tryptophan / Q000097:blood; D055815:Young Adult
chemical_list: D015415:Biomarkers; C576710:IDO1 protein, human; D050503:Indoleamine-Pyrrole 2,3,-Dioxygenase; D009498:Neurotoxins; D012333:RNA, Messenger; D007737:Kynurenine; D014364:Tryptophan; D017378:Quinolinic Acid