Circulating asymmetric dimethylarginine and cognitive decline: A 4-year follow-up study of the 1936 Aberdeen Birth Cohort

Comparing Symmetric Dimethylarginine and Amyloid-β42 as Predictors of Alzheimer's Disease Development

Background

Physicians may soon be able to diagnose Alzheimer's disease (AD) in its early stages using fluid biomarkers like amyloid. However, it is acknowledged that additional biomarkers need to be characterized which would facilitate earlier monitoring of AD pathogenesis.

Objective

To determine if a potential novel inflammation biomarker for AD, symmetric dimethylarginine, has utility as a baseline serum biomarker for discriminating prodromal AD from cognitively unimpaired controls in comparison to cerebrospinal fluid amyloid-β42 (Aβ42).

Methods

Data including demographics, magnetic resonance imaging and fluorodeoxyglucose-positron emission tomography scans, Mini-Mental State Examination and Functional Activities Questionnaire scores, and biomarker concentrations were obtained from the Alzheimer's Disease Neuroimaging Initiative for a total of 146 prodromal AD participants and 108 cognitively unimpaired controls.

Results

Aβ42 (p = 0.65) and symmetric dimethylarginine (p = 0.45) were unable to predict age-matched cognitively unimpaired controls and prodromal AD participants. Aβ42 was negatively associated with regional brain atrophy and hypometabolism as well as cognitive and functional decline in cognitively unimpaired control participants (p < 0.05) that generally decreased in time. There were no significant associations between Aβ42 and symmetric dimethylarginine with imaging or neurocognitive biomarkers in prodromal AD patients.

Conclusions

Correlations were smaller between Aβ42 and neuropathological biomarkers over time and were absent in prodromal AD participants, suggesting a plateau effect dependent on age and disease stage. Evidence supporting symmetric dimethylarginine as a novel biomarker for AD as a single measurement was not found.

Circulating arginine metabolites in Alzheimer's disease and vascular dementia: A systematic review and meta-analysis

BACKGROUND

Alterations in nitric oxide (NO) synthesis have been reported in Alzheimer's disease and vascular dementia. However, as the measurement of NO in biological samples is analytically challenging, alternative, stable circulatory biomarkers of NO synthesis may be useful to unravel new pathophysiological mechanisms and treatment targets in dementia.

METHODS

We conducted a systematic review and meta-analysis of the circulating concentrations of arginine metabolites linked to NO synthesis, arginine, citrulline, asymmetric (ADMA) and symmetric (SDMA) dimethylarginine, and ornithine, in Alzheimer's disease and vascular dementia. We searched for relevant studies in PubMed, Scopus, and Web of Science from inception to the 31st of May 2023. The JBI checklist and GRADE were used to assess the risk of bias and the certainty of evidence, respectively.

RESULTS

In 14 selected studies, there were no significant between-group differences in arginine and ornithine concentrations. By contrast, compared to controls, patients with dementia had significantly higher ADMA (standard mean difference, SMD=0.62, 95% CI 0.06-1.19, p = 0.029), SDMA (SMD=0.70, 95% CI 0.34-1.35, p<0.001), and citrulline concentrations (SMD=0.50, 95% CI 0.08-0.91, p = 0.018). In subgroup analysis, the effect size was significantly associated with treatment with cholinesterase inhibitors and/or antipsychotics for ADMA, and underlying disorder (Alzheimer's disease), study continent, and analytical method for citrulline.

CONCLUSION

Alterations in ADMA, SDMA, and citrulline, biomarkers of NO synthesis, may be useful to investigate the pathophysiology of different forms of dementia and identify novel therapeutic strategies. (PROSPERO registration number: CRD42023439528).

The Effect of Fat Intake with Increased Omega-6-to-Omega-3 Polyunsaturated Fatty Acid Ratio in Animal Models of Early and Late Alzheimer's Disease-like Pathogenesis

This work aims to clarify the effect of dietary polyunsaturated fatty acid (PUFA) intake on the adult brain affected by amyloid pathology. McGill-R-Thy1-APP transgenic (Tg) rat and 5xFAD Tg mouse models that represent earlier or later disease stages were employed. The animals were exposed to a control diet (CD) or an HFD based on corn oil, from young (rats) or adult (mice) ages for 24 or 10 weeks, respectively. In rats and mice, the HFD impaired reference memory in wild-type (WT) animals but did not worsen it in Tg, did not cause obesity, and did not increase triglycerides or glucose levels. Conversely, the HFD promoted stronger microglial activation in Tg vs. WT rats but had no effect on cerebral amyloid deposition. IFN-γ, IL-1β, and IL-6 plasma levels were increased in Tg rats, regardless of diet, while CXCL1 chemokine levels were increased in HFD-fed mice, regardless of genotype. Hippocampal 3-nitrotyrosine levels tended to increase in HFD-fed Tg rats but not in mice. Overall, an HFD with an elevated omega-6-to-omega-3 ratio as compared to the CD (25:1 vs. 8.4:1) did not aggravate the outcome of AD regardless of the stage of amyloid pathology, suggesting that many neurobiological processes relevant to AD are not directly dependent on PUFA intake.

Arginine, Transsulfuration, and Folic Acid Pathway Metabolomics in Chronic Obstructive Pulmonary Disease: A Systematic Review and Meta-Analysis

There is an increasing interest in biomarkers of nitric oxide dysregulation and oxidative stress to guide management and identify new therapeutic targets in patients with chronic obstructive pulmonary disease (COPD). We conducted a systematic review and meta-analysis of the association between circulating metabolites within the arginine (arginine, citrulline, ornithine, asymmetric, ADMA, and symmetric, SDMA dimethylarginine), transsulfuration (methionine, homocysteine, and cysteine) and folic acid (folic acid, vitamin B6, and vitamin B12) metabolic pathways and COPD. We searched electronic databases from inception to 30 June 2023 and assessed the risk of bias and the certainty of evidence. In 21 eligible studies, compared to healthy controls, patients with stable COPD had significantly lower methionine (standardized mean difference, SMD = -0.50, 95% CI -0.95 to -0.05, p = 0.029) and folic acid (SMD = -0.37, 95% CI -0.65 to -0.09, p = 0.009), and higher homocysteine (SMD = 0.78, 95% CI 0.48 to 1.07, p < 0.001) and cysteine concentrations (SMD = 0.34, 95% CI 0.02 to 0.66, p = 0.038). Additionally, COPD was associated with significantly higher ADMA (SMD = 1.27, 95% CI 0.08 to 2.46, p = 0.037), SDMA (SMD = 3.94, 95% CI 0.79 to 7.08, p = 0.014), and ornithine concentrations (SMD = 0.67, 95% CI 0.13 to 1.22, p = 0.015). In subgroup analysis, the SMD of homocysteine was significantly associated with the biological matrix assessed and the forced expiratory volume in the first second to forced vital capacity ratio, but not with age, study location, or analytical method used. Our study suggests that the presence of significant alterations in metabolites within the arginine, transsulfuration, and folic acid pathways can be useful for assessing nitric oxide dysregulation and oxidative stress and identifying novel treatment targets in COPD. (PROSPERO registration number: CRD42023448036.).

Dietary nitrate, aging and brain health: the latest evidence

PURPOSE OF REVIEW

With an increasing population age, cognitive decline and age-associated neurodegenerative diseases are becoming increasingly prevalent and burdensome in society. Dietary supplementation with inorganic nitrate, which serves as a nitric oxide precursor, has been suggested as a potential nutritional strategy to improve brain health in older adults. In this review, we discuss recent findings in this area.

RECENT FINDINGS

A number of studies have emerged in the past 12-18 months exploring the effects of dietary nitrate supplementation on cognitive function, with typically (although not exclusively) null findings emerging. This research is characterized by small, acute/short-term studies, although observational studies and longer-duration randomised controlled trials are beginning to emerge. From the limited research reporting benefits of nitrate supplementation on cognitive function, one important discovery has been the identification of a potential pathway through which nitrate could impact cognitive health, involving modulation of the oral microbiome, which warrants further investigation.

SUMMARY

Despite some promising early findings, there is currently insufficient evidence to recommend increased dietary nitrate intake for the purpose of improving brain health. However, longer-term, larger-scale trials in potentially responsive groups are warranted to provide definitive evidence in this area.

The impact of uremic toxins on Alzheimer's disease

Alzheimer's disease (AD) is the most common type of dementia, pathologically characterized by accumulation of senile plaques and neurofibrillary tangles. Chronic kidney disease (CKD) is highly prevalent in elderly population closely associated with occurrence of dementia. Recent epidemiological and experimental studies suggest a potential association of CKD with AD. Both diseases share a panel of identical risk factors, such as type 2 diabetes; and hypertension. However, the relationship between CKD and AD is unclear. Lower clearance of a panel of uremic toxin including cystatin-C, guanidine, and adiponectin due to CKD is implied to contribute to AD pathogenesis. In this review we summarize the current evidence from epidemiological, experimental and clinical studies on the potential contribution of uremic toxins to AD pathogenesis. We describe outstanding questions and propose an outlook on the link between uremic toxins and AD.

The Emerging Role of Metabolism in Brain-Heart Axis: New Challenge for the Therapy and Prevention of Alzheimer Disease. May Thioredoxin Interacting Protein (TXNIP) Play a Role?

Alzheimer disease (AD) is the most frequent cause of dementia and up to now there is not an effective therapy to cure AD. In addition, AD onset occurs decades before the diagnosis, affecting the possibility to set up appropriate therapeutic strategies. For this reason, it is necessary to investigate the effects of risk factors, such as cardiovascular diseases, in promoting AD. AD shows not only brain dysfunction, but also alterations in peripheral tissues/organs. Indeed, it exists a reciprocal connection between brain and heart, where cardiovascular alterations participate to AD as well as AD seem to promote cardiovascular dysfunction. In addition, metabolic dysfunction promotes both cardiovascular diseases and AD. In this review, we summarize the pathways involved in the regulation of the brain-heart axis and the effect of metabolism on these pathways. We also present the studies showing the role of the gut microbiota on the brain-heart axis. Herein, we propose recent evidences of the function of Thioredoxin Interacting protein (TXNIP) in mediating the role of metabolism on the brain-heart axis. TXNIP is a key regulator of metabolism at both cellular and body level and it exerts also a pathological function in several cardiovascular diseases as well as in AD.