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BACKGROUND: Adults with chronic kidney disease (CKD) exhibit alterations in tryptophan metabolism, mainly via the kynurenine pathway, due to higher enzymatic activity induced mainly by inflammation. Indoles produced by gut-microflora are another group of tryptophan metabolites related to inflammation and conditions accompanying CKD. Disruptions in tryptophan metabolism have been associated with various neurological and psychological disorders. A high proportion of CKD patients self-report symptoms of depression and/or anxiety and decline in cognitive functioning. This pilot study examines tryptophan metabolism in CKD and explores associations with psychological and cognitive functioning.

METHODS: Twenty-seven adults with CKD were part of 49 patients recruited to participate in a prospective pilot study, initially with an eGFR of 15-29 mL/min/1.73 m². Only participants with viable blood samples and complete psychological/cognitive data at a 2-year follow-up were included in the reported cross-sectional study. Serum samples were analysed by Liquid Chromatography coupled to Mass Spectrometry, for tryptophan, ten of its metabolites, the inflammation marker neopterin and the hypothalamic-pituitary-adrenal (HPA) axis marker cortisol.

RESULTS: The tryptophan breakdown index (kynurenine / tryptophan) correlated with neopterin (Pearson R = 0.51 P = 0.006) but not with cortisol. Neopterin levels also correlated with indoxyl sulfate (R = 0.68, P < 0.0001) and 5 metabolites of tryptophan (R range 0.5-0.7, all P ≤ 0.01), which were all negatively related to eGFR (P < 0.05). Higher levels of kynurenic acid were associated with lower cognitive functioning (Spearman R = -0.39, P < 0.05), while indole-3 acetic acid (IAA) was correlated with anxiety and depression (R = 0.52 and P = 0.005, R = 0.39 and P < 0.05, respectively).

CONCLUSIONS: The results of this preliminary study suggest the involvement of inflammation in tryptophan breakdown via the kynurenine pathway, yet without sparing tryptophan metabolism through the 5-HT (serotonin) pathway in CKD patients. The multiple moderate associations between indole-3 acetic acid and psychological measures were a novel finding. The presented pilot data necessitate further exploration of these associations within a large prospective cohort to assess the broader significance of these findings.

Item Type:	Refereed Article
Keywords:	Chronic kidney disease, Tryptophan, Kynurenine, Neopterin, Cortisol, Inflammation, Depression, Anxiety, Cognition
Research Division:	Medical and Health Sciences
Research Group:	Clinical Sciences
Research Field:	Nephrology and Urology
Objective Division:	Health
Objective Group:	Public Health (excl. Specific Population Health)
Objective Field:	Mental Health
UTAS Author:	Karu, N (Dr Naama Karu)
UTAS Author:	McKercher, C (Dr Charlotte McKercher)
UTAS Author:	Nichols, DS (Dr David Nichols)
UTAS Author:	Davies, N (Associate Professor Noel Davies)
UTAS Author:	Shellie, RA (Associate Professor Robert Shellie)
UTAS Author:	Hilder, EF (Professor Emily Hilder)
UTAS Author:	Jose, MD (Professor Matthew Jose)
ID Code:	112506
Year Published:	2016
Web of Science® Times Cited:	18
Deposited By:	Menzies Institute for Medical Research
Deposited On:	2016-11-15
Last Modified:	2017-11-01
Downloads:	135 View Download Statistics