Systemic perturbations of the kynurenine pathway precede progression to dementia independently of amyloid-β

Increasing evidence suggests that kynurenine pathway (KP) dyshomeostasis may promote disease progression in dementia. Studies in Alzheimer's disease (AD) patients confirm KP dyshomeostasis in plasma and cerebrospinal fluid (CSF) which correlates with amyloid-β and tau pathology. Herein, we performed the first comprehensive study assessing baseline levels of KP metabolites in participants enrolling in the Australian Imaging Biomarkers Flagship Study of Aging. Our purpose was to test the hypothesis that changes in KP metabolites may be biomarkers of dementia processes that are largely silent. We used a cross-sectional analytical approach to assess non-progressors (N = 73); cognitively normal (CN) or mild cognitive impairment (MCI) participants at baseline and throughout the study, and progressors (N = 166); CN or MCI at baseline but progressing to either MCI or AD during the study. Significant KP changes in progressors included increased 3-hydroxyanthranilic acid (3-HAA) and 3-hydroxyanthranilic acid/anthranilic acid (3-HAA/AA) ratio, the latter having the largest effect on the odds of an individual being a progressor (OR 35.3; 95% CI between 14 and 104). 3-HAA levels were hence surprisingly bi-phasic, high in progressors but low in non-progressors or participants who had already transitioned to MCI or dementia. This is a new, unexpected and interesting result, as most studies of the KP in neurodegenerative disease show reduced 3-HAA/AA ratio after diagnosis. The neuroprotective metabolite picolinic acid was also significantly decreased while the neurotoxic metabolite 3-hydroxykynurenine increased in progressors. These results were significant even after adjustment for confounders. Considering the magnitude of the OR to predict change in cognition, it is important that these findings are replicated in other populations. Independent validation of our findings may confirm the utility of 3-HAA/AA ratio to predict change in cognition leading to dementia in clinical settings

The Science of Sungrazers, Sunskirters, and Other Near-Sun Comets

This review addresses our current understanding of comets that venture close to the Sun, and are hence exposed to much more extreme conditions than comets that are typically studied from Earth. The extreme solar heating and plasma environments that these objects encounter change many aspects of their behaviour, thus yielding valuable information on both the comets themselves that complements other data we have on primitive solar system bodies, as well as on the near-solar environment which they traverse. We propose clear definitions for these comets: We use the term near-Sun comets to encompass all objects that pass sunward of the perihelion distance of planet Mercury (0.307 AU). Sunskirters are defined as objects that pass within 33 solar radii of the Sun’s centre, equal to half of Mercury’s perihelion distance, and the commonly-used phrase sungrazers to be objects that reach perihelion within 3.45 solar radii, i.e. the fluid Roche limit. Finally, comets with orbits that intersect the solar photosphere are termed sundivers. We summarize past studies of these objects, as well as the instruments and facilities used to study them, including space-based platforms that have led to a recent revolution in the quantity and quality of relevant observations. Relevant comet populations are described, including the Kreutz, Marsden, Kracht, and Meyer groups, near-Sun asteroids, and a brief discussion of their origins. The importance of light curves and the clues they provide on cometary composition are emphasized, together with what information has been gleaned about nucleus parameters, including the sizes and masses of objects and their families, and their tensile strengths. The physical processes occurring at these objects are considered in some detail, including the disruption of nuclei, sublimation, and ionisation, and we consider the mass, momentum, and energy loss of comets in the corona and those that venture to lower altitudes. The different components of comae and tails are described, including dust, neutral and ionised gases, their chemical reactions, and their contributions to the near-Sun environment. Comet-solar wind interactions are discussed, including the use of comets as probes of solar wind and coronal conditions in their vicinities. We address the relevance of work on comets near the Sun to similar objects orbiting other stars, and conclude with a discussion of future directions for the field and the planned ground- and space-based facilities that will allow us to address those science topics

Plasma neurofilament light chain and amyloid-β are associated with the kynurenine pathway metabolites in preclinical Alzheimer’s disease

Background Blood markers indicative of neurodegeneration (neurofilament light chain; NFL), Alzheimer’s disease amyloid pathology (amyloid-β; Aβ), and neuroinflammation (kynurenine pathway; KP metabolites) have been investigated independently in neurodegenerative diseases. However, the association of these markers of neurodegeneration and AD pathology with neuroinflammation has not been investigated previously. Therefore, the current study examined whether NFL and Aβ correlate with KP metabolites in elderly individuals to provide insight on the association between blood indicators of neurodegeneration and neuroinflammation. Methods Correlations between KP metabolites, measured using liquid chromatography and gas chromatography coupled with mass spectrometry, and plasma NFL and Aβ concentrations, measured using single molecule array (Simoa) assays, were investigated in elderly individuals aged 65–90 years, with normal global cognition (Mini-Mental State Examination Score ≥ 26) from the Kerr Anglican Retirement Village Initiative in Ageing Health cohort. Results A positive correlation between NFL and the kynurenine to tryptophan ratio (K/T) reflecting indoleamine 2,3-dioxygenase activity was observed (r = .451, p < .0001). Positive correlations were also observed between NFL and kynurenine (r = .364, p < .0005), kynurenic acid (r = .384, p < .0001), 3-hydroxykynurenine (r = .246, p = .014), anthranilic acid (r = .311, p = .002), and quinolinic acid (r = .296, p = .003). Further, significant associations were observed between plasma Aβ40 and the K/T (r = .375, p < .0005), kynurenine (r = .374, p < .0005), kynurenic acid (r = .352, p < .0005), anthranilic acid (r = .381, p < .0005), and quinolinic acid (r = .352, p < .0005). Significant associations were also observed between plasma Aβ42 and the K/T ratio (r = .215, p = .034), kynurenic acid (r = .214, p = .035), anthranilic acid (r = .278, p = .006), and quinolinic acid (r = .224, p = .027) in the cohort. On stratifying participants based on their neocortical Aβ load (NAL) status, NFL correlated with KP metabolites irrespective of NAL status; however, associations between plasma Aβ and KP metabolites were only pronounced in individuals with high NAL while associations in individuals with low NAL were nearly absent. Conclusions The current study shows that KP metabolite changes are associated with biomarker evidence of neurodegeneration. Additionally, the association between KP metabolites and plasma Aβ seems to be NAL status dependent. Finally, the current study suggests that an association between neurodegeneration and neuroinflammation manifests in the periphery, suggesting that preventing cytoskeleton cytotoxicity by KP metabolites may have therapeutic potential

Alterations in serum kynurenine pathway metabolites in individuals with high neocortical amyloid-β load: A pilot study

The kynurenine pathway (KP) is dysregulated in neuroinflammatory diseases including Alzheimer’s disease (AD), however has not been investigated in preclinical AD characterized by high neocortical amyloid-β load (NAL), prior to cognitive impairment. Serum KP metabolites were measured in the cognitively normal KARVIAH cohort. Participants, aged 65–90 y, were categorised into NAL+ (n = 35) and NAL− (n = 65) using a standard uptake value ratio cut-off = 1.35. Employing linear models adjusting for age and APOEε4, higher kynurenine and anthranilic acid (AA) in NAL+ versus NAL− participants were observed in females (kynurenine, p = 0.004; AA, p = 0.001) but not males (NALxGender, p = 0.001, 0.038, respectively). To evaluate the predictive potential of kynurenine or/and AA for NAL+ in females, logistic regressions with NAL+/− as outcome were carried out. After age and APOEε4 adjustment, kynurenine and AA were individually and jointly significant predictors (p = 0.007, 0.005, 0.0004, respectively). Areas under the receiver operating characteristic curves were 0.794 using age and APOEε4 as predictors, and 0.844, 0.866 and 0.871 when kynurenine, AA and both were added. Findings from the current study exhibit increased KP activation in NAL+ females and highlight the predictive potential of KP metabolites, AA and kynurenine, for NAL+. Additionally, the current study also provides insight into he influence of gender in AD pathogenesis