Special issue: The kynurenine pathway in aging

Association Between Tryptophan Metabolites, Physical Performance, and Frailty in Older Persons

Frailty is defined as a syndrome of physiological decline in late life, characterized by marked vulnerability to adverse health outcomes. A robust biomarker for frailty is still lacking. Tryptophan (TRP) metabolism through the kynurenine pathway (KP) plays essential roles in aging, the musculoskeletal system, and physical performance. In this study, we quantified 7 KP metabolites, including kynurenine (KYN), kynurenine acid (KYNA), quinolinic acid (QUIN), picolinic acid (PIC), 3-hydroxykynurenine (3-HK), 3-hydroxyanthranilic acid (3-HAA), and anthranilic acid (AA) using ultra-high-performance liquid chromatography and gas chromatography-mass spectrometry in the serum of 85 participants (median age 75; 65% female; 28 non-frail, 29 pre-frail, and 28 frail) at the Nepean Osteoporosis and Frailty (NOF) Study. We looked at the association between TRP metabolites and physical performance, sarcopenia, and frailty. After adjusting for age and sex, our results showed that KYN and KYN/TRP were associated with higher interleukin (IL)-6 levels (r = .324 and r = .390, respectively). KYNA and its ratios to other products (mainly KYNA/KYN, KYNA/QUIN, and KYNA/PIC) were associated with a lower likelihood of frailty by Fried’s criteria (OR 0.93 [0.88, 0.98], P = .009) and Rockwood index (r = −.241, P = .028) as well as a lower likelihood of sarcopenia (OR 0.88 [0.78, 1.00], P = .049). QUIN and QUIN/KYN showed an association with increased IL-6 (r = .293 and .204 respectively), higher likelihood of frailty (OR 1.02 [1.00, 1.04], P = .029 and OR 6.43 [2.23, 18.51], P = .001 respectively) and lower physical function (r = −.205 and r = −.292). In conclusion, different TRP metabolites have various associations with physical performance, frailty, and sarcopenia. Defining the underlying mechanisms may permit the development and validation of new biomarkers and therapeutics for frailty and musculoskeletal conditions targeting specific metabolites of the TRP catabolic pathway.

Exploring Early Detection of Frailty Syndrome in Older Adults: Evaluation of Oxi-Immune Markers, Clinical Parameters and Modifiable Risk Factors

Ageing is accompanied with a decline in several physiological systems. Frailty is an age-related syndrome correlated to the loss of homeostasis and increased vulnerability to stressors, which is associated with increase in the risk of disability, comorbidity, hospitalisation, and death in older adults. The aim of this study was to understand the relationship between frailty syndrome, immune activation, and oxidative stress. Serum concentrations of vitamins A and E were also evaluated, as well as inflammatory biomarkers (CRP and IL-6) and oxidative DNA levels. A group of Portuguese older adults (≥65 years old) was engaged in this study and classified according to Fried’s frailty phenotype. Significant increases in the inflammatory mediators (CRP and IL-6), neopterin levels, kynurenine to tryptophan ratio (Kyn/Trp), and phenylalanine to tyrosine ratio (Phe/Tyr), and significant decreases in Trp and Tyr concentrations were observed in the presence of frailty. IL-6, neopterin, and Kyn/Trp showed potential as predictable biomarkers of frailty syndrome. Several clinical parameters such as nutrition, dependency scales, and polypharmacy were related to frailty and, consequently, may influence the associations observed. Results obtained show a progressive immune activation and production of pro-inflammatory molecules in the presence of frailty, agreeing with the inflammageing model. Future research should include different dimensions of frailty, including psychological, social, biological, and environmental factors.

Mifepristone Increases Life Span of Virgin Female Drosophila on Regular and High-fat Diet Without Reducing Food Intake

Background: The synthetic steroid mifepristone is reported to have anti-obesity and anti-diabetic effects in mammals on normal and high-fat diets (HFD). We previously reported that mifepristone blocks the negative effect on life span caused by mating in female Drosophila melanogaster. Methods: Here we asked if mifepristone could protect virgin females from the life span-shortening effect of HFD. Mifepristone was assayed for effects on life span in virgin females, in repeated assays, on regular media and on media supplemented with coconut oil (HFD). The excrement quantification (EX-Q) assay was used to measure food intake of the flies after 12 days mifepristone treatment. In addition, experiments were conducted to compare the effects of mifepristone in virgin and mated females, and to identify candidate mifepristone targets and mechanisms. Results: Mifepristone increased life span of virgin females on regular media, as well as on media supplemented with either 2.5 or 5% coconut oil. Food intake was not reduced in any assay, and was significantly increased by mifepristone in half of the assays. To ask if mifepristone might rescue virgin females from all life span-shortening stresses, the oxidative stressor paraquat was tested, and mifepristone produced little to no rescue. Analysis of extant metabolomics and transcriptomics data suggested similarities between effects of mifepristone in virgin and mated females, including reduced tryptophan breakdown and similarities to dietary restriction. Bioinformatics analysis identified candidate mifepristone targets, including transcription factors Paired and Extra-extra. In addition to shortening life span, mating also causes midgut hypertrophy and activation of the lipid metabolism regulatory factor SREBP. Mifepristone blocked the increase in midgut size caused by mating, but did not detectably affect midgut size in virgins. Finally, mating increased activity of a SREBP reporter in abdominal tissues, as expected, but reporter activity was not detectably reduced by mifepristone in either mated or virgin females. Conclusion: Mifepristone increases life span of virgin females on regular and HFD without reducing food intake. Metabolomics and transcriptomics analyses suggest some similar effects of mifepristone between virgin and mated females, however reduced midgut size was observed only in mated females. The results are discussed regarding possible mifepristone mechanisms and targets.

The association of circulating kynurenine, a tryptophan metabolite, with frailty in older adults

Despite the accumulating evidence from in vitro and animal experiments supporting the role of kynurenine (a tryptophan metabolite) in a number of degenerative age-related changes, the relationship between kynurenine and frailty in older adults is not well understood. We collected blood samples from 73 participants who underwent a comprehensive geriatric assessment, measuring kynurenine levels using liquid chromatography-tandem mass spectrometry. We assessed the phenotypic frailty and the deficit accumulation frailty index using widely validated approaches proposed by Fried et al. and Rockwood et al., respectively. After adjusting for sex, age, and body mass index, the frail participants presented 52.9% and 34.3% higher serum kynurenine levels than those with robustness and prefrailty, respectively (P = 0.005 and 0.014, respectively). Serum kynurenine levels were positively associated with the frailty index, time to complete 5 chair stands, and patient health questionnaire-2 score and inversely associated with grip strength and gait speed (P = 0.042 to <0.001). Furthermore, the odds ratio per increase in serum kynurenine level for phenotypic frailty was approximately 2.62 (95% confidence interval = 1.22-5.65, P = 0.014). These data provide clinical evidence that circulating kynurenine might be a potential biomarker for assessing the risk of frailty in humans.