Early Stages of Late Onset Alzheimer's Disease

Thyroid hormone levels in Alzheimer disease: a systematic review and meta-analysis

BACKGROUND AND OBJECTIVE

Thyroid hormone (TH) disturbances are perceived to contribute to cognitive impairment and dementia. However, there is no consensus on the association between TH levels and Alzheimer Disease (AD). In this study, we aimed to compare serum and cerebrospinal fluid (CSF) TH levels in AD patients to controls by performing a meta-analysis.

METHODS

We systematically searched online databases for papers comparing CSF or serum TH levels in AD patients to controls, and performed a meta-analysis on the extracted data.

RESULTS

Out of 1604 records identified, 32 studies were included. No significant difference in serum TSH (standardized mean difference (SMD): -0.03; 95% confidence interval (CI): -0.22-0.16), total T4 (SMD: 0.10; 95% CI: -0.29-0.49), and free T4 (SMD: 0.25; 95% CI: -0.16-0.69) levels were observed. However, there was significantly lower serum total T3 (SMD: -0.56; 95%CI: -0.97 to -0.15) and free T3 (SMD: -0.47; 95%CI: -0.89 to -0.05) levels in AD group compared to controls. Subgroup analyses on studies including only euthyroid patients did not show any significant difference in either of the thyroid hormone levels. Also, no significant difference in CSF total T4 and total T3/total T4 ratios but significantly lower CSF total T3 (SMD: -2.45; 95% CI: -4.89 to -0.02) in AD group were detected.

CONCLUSION

Serum total and free T3 and CSF total T3 levels are significantly lower in AD patients compared to controls. The temporality of changes in thyroid hormone levels and AD development should be illustrated by further longitudinal studies.

Examining the Role of the Noradrenergic Locus Coeruleus for Predicting Attention and Brain Maintenance in Healthy Old Age and Disease: An MRI Structural Study for the Alzheimer's Disease Neuroimaging Initiative

The noradrenergic theory of Cognitive Reserve (Robertson, 2013-2014) postulates that the upregulation of the locus coeruleus-noradrenergic system (LC-NA) originating in the brainstem might facilitate cortical networks involved in attention, and protracted activation of this system throughout the lifespan may enhance cognitive stimulation contributing to reserve. To test the above-mentioned theory, a study was conducted on a sample of 686 participants (395 controls, 156 mild cognitive impairment, 135 Alzheimer's disease) investigating the relationship between LC volume, attentional performance and a biological index of brain maintenance (BrainPAD-an objective measure, which compares an individual's structural brain health, reflected by their voxel-wise grey matter density, to the state typically expected at that individual's age). Further analyses were carried out on reserve indices including education and occupational attainment. Volumetric variation across groups was also explored along with gender differences. Control analyses on the serotoninergic (5-HT), dopaminergic (DA) and cholinergic (Ach) systems were contrasted with the noradrenergic (NA) hypothesis. The antithetic relationships were also tested across the neuromodulatory subcortical systems. Results supported by Bayesian modelling showed that LC volume disproportionately predicted higher attentional performance as well as biological brain maintenance across the three groups. These findings lend support to the role of the noradrenergic system as a key mediator underpinning the neuropsychology of reserve, and they suggest that early prevention strategies focused on the noradrenergic system (e.g., cognitive-attentive training, physical exercise, pharmacological and dietary interventions) may yield important clinical benefits to mitigate cognitive impairment with age and disease.

New frontiers in Alzheimer's disease diagnostic: Monoamines and their derivatives in biological fluids

Current diagnosis of Alzheimer's disease (AD) relies on a combination of neuropsychological evaluations, biomarker measurements and brain imaging. Nevertheless, these approaches are either expensive, invasive or lack sensitivity to early AD stages. The main challenge of ongoing research is therefore to identify early non-invasive biomarkers to diagnose AD at preclinical stage. Accumulating evidence support the hypothesis that initial degeneration of profound monoaminergic nuclei may trigger a transneuronal spread of AD pathology towards hippocampus and cortex. These studies aroused great interest on monoamines, i.e. noradrenaline (NA), dopamine (D) ad serotonin (5-HT), as early hallmarks of AD pathology. The present work reviews current literature on the potential role of monoamines and related metabolites as biomarkers of AD. First, morphological changes in the monoaminergic systems during AD are briefly described. Second, we focus on concentration changes of these molecules and their derivatives in biological fluids, including cerebrospinal fluid, obtained by lumbar puncture, and blood or urine, sampled via less invasive procedures. Starting from initial observations, we then discuss recent insights on metabolomics-based analysis, highlighting the promising clinical utility of monoamines for the identification of a molecular AD signature, aimed at improving early diagnosis and discrimination from other dementia.

Indoleamine concentrations in cerebrospinal fluid from patients with Alzheimer type and Binswanger type dementias before and after administration of citalopram, a synthetic serotonin uptake inhibitor

We studied changes in the concentrations of serotonin (5-HT), kynurenine, and other indoleamines in the cerebrospinal fluid of patients with Alzheimer type dementia (ATD) and those with Binswanger type vascular dementia (VDBT), and changes in these indoleamine concentrations 2 weeks after administration of citalopram, a 5-HT uptake inhibitor. The concentrations of total 5-HT (p < 0.005) and kynurenine (p < 0.005) were significantly decreased in ATD patients in comparison to those of the controls. After citalopram administration, there was a remarkable increase in 5-HT concentration (249%, p < 0.0001) and a significant decrease in 5-HIAA concentration (22%, p < 0.02). In the VDBT patients, only 5-HT showed a significant decrease (p < 0.005) in comparison to the control values. It also increased significantly (214%) after citalopram administration. The 5-HT/tryptophan and kynurenine/tryptophan ratios were not correlated for the controls, but did significantly for the ATD and VDBT patients; after citalopram treatment, the increase in 5-HT/tryptophan was correlated significantly with that of kynurenine/tryptophan for ATD, but not for VDBT. These results suggest that both the serotonin and kynurenine pathways are impaired in ATD; whereas, the serotonin pathway alone is in VDBT, and that these impairments are ameliorated by the administration of citalopram.

Concentrations of serotonin and its related substances in the cerebrospinal fluid in patients with Alzheimer type dementia

We studied concentrations of free and total serotonin (5-hydroxytryptamine, 5-HT) and its related substances in the cerebrospinal fluid from patients with Alzheimer type dementia (ATD) compared with controls. In ATD patients, concentrations of total 5-HT, tryptophan, 5-hydroxytryptophan (5-HTP), melatonin, kynurenine, and 3-hydroxykynurenine decreased significantly. The rate of concentration of tryptophan metabolites to that of tryptophan was significantly reduced for total 5-HT and 3-hydroxykynurenine only. The 5-hydroxyindoleacetic acid (5-HIAA)/5-HT ratio was significantly larger in ATD patients than in controls. The greater reduction in the 3-hydroxykynurenine concentration (81% vs. controls) than in the total 5-HT concentration (51% vs. controls) suggests that the metabolism of tryptophan to 5-HT and 3-hydroxykynurenine is in favor of 5-HT.

Slowed synthesis of DNA and methionine is a pathogenetic mechanism common to dementia in Down's syndrome, AIDS and Alzheimer's disease?

This is a presentation of the hypothesis of a pathogenetic mechanism common to the dementia seen in Alzheimer's disease (AD), Down's Syndrome (DS) and the acquired immunodeficiency syndrome (AIDS). As there is experimental evidence of defective DNA repair capacity in AD and DS, unrepaired damage to DNA occurs in these diseases and may lead to complete breakdown of cellular function and ultimate cell death. Cobalamin and folate are coordinated in a vulnerable key position in the synthesis of DNA and S-adenosylmethionine (SAM). Cobalamin/folate deficiency, a significant feature in senile dementia of Alzheimer type and in AIDS-related dementia complex, will result in concomitant slowed synthesis of DNA and SAM. The enzyme cystathionine-beta-synthetase (CBS) has been localized to the chromosome band 21q22.3. Owing to gene dosage, CBS activity is increased in trisomy 21. As a consequence, cobalamin/folate metabolism is inhibited, which leads to slowing of DNA and SAM synthesis in DS patients. Amyloidosis is a hallmark of AD and DS brain neuropathology and recent experimental findings support the view that amyloid or amyloid precursors stimulate DNA synthesis, which is in agreement with the hypothesis presented in this paper. In summary, demented patients with cobalamin/folate deficiency, trisomy 21 and human immunodeficiency virus (HIV) infection display a simultaneous downregulation of DNA and SAM synthesis, which may indicate a pathway common to the dementia seen in AD, DS and AIDS.

Disturbance of the 5-hydroxytryptamine metabolism in brains from patients with Alzheimer's dementia

The 5-hydroxytryptamine (5-HT) system in the human brain is sensitive to aging. In dementia of the Alzheimer type (AD/SDAT), there are significantly reduced concentrations of 5-HT and 5-hydroxyindoleacetic acid (5-HIAA). 5-HT-sensitive imipramine binding is reduced by almost 50%, indicating a loss of presynaptic 5-HT terminals. There also seems to be reduced tryptophan hydroxylase activity in some brain areas. In cerebrospinal fluid (CSF) from AD/SDAT patients, the concentration of 5-HIAA is reduced, and the accumulation of 5-HIAA after probenecid loading is diminished. Biochemical findings together with structural findings in the raphe nuclei indicate that the disturbance of the 5-HT system is of the same magnitude as the disturbance of the cholinergic system. Reduced activity in the 5-HT system may be of importance for activity in the hypothalamus. There is an increased concentration of arginine vasopressin, which may explain the increased activity in the hypothalamic-pituitary-adrenal axis seen in patients with AD/SDAT. This activity is reduced when a selective 5-HT reuptake blocker is given. Pharmacological treatment with 5-HT reuptake blockers improves emotional disturbances, confusion, anxiety and depressed mood in patients with AD/SDAT.