Brain aging and late-onset Alzheimer's disease: many open questions

From Young to Old: Mimicking Neuronal Aging in Directly Converted Neurons from Young Donors

A substantial challenge in human brain aging is to find a suitable model to mimic neuronal aging in vitro as accurately as possible. Using directly converted neurons (iNs) from human fibroblasts is considered a promising tool in human aging since it retains the aging-associated mitochondrial donor signature. Still, using iNs from aged donors can pose certain restrictions due to their lower reprogramming and conversion efficacy than those from younger individuals. To overcome these limitations, our study aimed to establish an in vitro neuronal aging model mirroring features of in vivo aging by acute exposure on young iNs to either human stress hormone cortisol or the mitochondrial stressor rotenone, considering stress as a trigger of in vivo aging. The impact of rotenone was evident in mitochondrial bioenergetic properties by showing aging-associated deficits in mitochondrial respiration, cellular ATP, and MMP and a rise in glycolysis, mitochondrial superoxide, and mitochondrial ROS; meanwhile, cortisol only partially induced an aging-associated mitochondrial dysfunction. To replicate the in vivo aging-associated mitochondrial dysfunctions, using rotenone, a mitochondrial complex I inhibitor, proved to be superior to the cortisol model. This work is the first to use stress on young iNs to recreate aging-related mitochondrial impairments.

N-Acetylcysteine Amide against Aβ-Induced Alzheimer's-like Pathology in Rats

Oxidative stress with a depletion of glutathione is a key factor in the initiation and progression of Alzheimer's disease (AD). N-Acetylcysteine (NAC), a glutathione precursor, provides neuroprotective effects in AD animal models. Its amide form, N-Acetylcysteine amide (NACA), has an extended bioavailability compared to NAC. This study evaluates the neuroprotective effects of NACA against Aβ1-42 peptide-induced AD-like pathology in rats. Male Wistar rats (2.5 months old) were divided into five groups: Normal Control (NC), Sham (SH), Aβ, Aβ + NACA and NACA + Aβ + NACA (n = 8 in all groups). AD-like pathology was induced by the intracerebroventricular infusion of Aβ1-42 peptide into the lateral ventricle. NACA (75 mg/kg) was administered either as a restorative (i.e., injection of NACA for 7 consecutive days after inducing AD-like pathology (Aβ + N group)), or as prophylactic (for 7 days before and 7 days after inducing the pathology (N + Aβ + N group)). Learning and memory, neurogenesis, expression of AD pathology markers, antioxidant parameters, neuroprotection, astrogliosis and microgliosis were studied in the hippocampus and the prefrontal cortex. All data were analyzed with a one-way ANOVA test followed by Bonferroni's multiple comparison test. NACA treatment reversed the cognitive deficits and reduced oxidative stress in the hippocampus and prefrontal cortex. Western blot analysis for Tau, Synaptophysin and Aβ, as well as a histopathological evaluation through immunostaining for neurogenesis, the expression of neurofibrillary tangles, β-amyloid peptide, synaptophysin, neuronal morphology and gliosis, showed a neuroprotective effect of NACA. In conclusion, this study demonstrates the neuroprotective effects of NACA against β-amyloid induced AD-like pathology.

Mitochondrial Permeability Transition: A Pore Intertwines Brain Aging and Alzheimer's Disease

Advanced age is the greatest risk factor for aging-related brain disorders including Alzheimer's disease (AD). However, the detailed mechanisms that mechanistically link aging and AD remain elusive. In recent years, a mitochondrial hypothesis of brain aging and AD has been accentuated. Mitochondrial permeability transition pore (mPTP) is a mitochondrial response to intramitochondrial and intracellular stresses. mPTP overactivation has been implicated in mitochondrial dysfunction in aging and AD brains. This review summarizes the up-to-date progress in the study of mPTP in aging and AD and attempts to establish a link between brain aging and AD from a perspective of mPTP-mediated mitochondrial dysfunction.

An agent-based model to investigate microbial initiation of Alzheimer's via the olfactory system

BACKGROUND

Alzheimer's disease (AD) is a degenerative brain disease. A novel agent-based modelling framework was developed in NetLogo 3D to provide fundamental insights into the potential mechanisms by which a microbe (eg. Chlamydia pneumoniae) may play a role in late-onset AD. The objective of our initial model is to simulate one possible spatial and temporal pathway of bacterial propagation via the olfactory system, which may then lead to AD symptoms. The model maps the bacteria infecting cells from the nasal cavity and the olfactory epithelium, through the olfactory bulb and into the olfactory cortex and hippocampus regions of the brain.

RESULTS

Based on the set of biological rules, simulated randomized infection by the microbe led to the formation of beta-amyloid (Aβ) plaque and neurofibrillary (NF) tangles as well as caused immune responses. Our initial simulations demonstrated that breathing in C. pneumoniae can result in infection propagation and significant buildup of Aβ plaque and NF tangles in the olfactory cortex and hippocampus. Our model also indicated how mucosal and neural immunity can play a significant role in the pathway considered. Lower immunities, correlated with elderly individuals, had quicker and more Aβ plaque and NF tangle formation counts. In contrast, higher immunities, correlated with younger individuals, demonstrated little to no such formation.

CONCLUSION

The modelling framework provides an organized visual representation of how AD progression may occur via the olfactory system to better understand disease pathogenesis. The model confirms current conclusions in available research but can be easily adjusted to match future evidence and be used by researchers for their own individual purposes. The goal of our initial model is to ultimately guide further hypothesis refinement and experimental testing to better understand the dynamic system interactions present in the etiology and pathogenesis of AD.

Involvement of the nitric oxide signaling in modulation of naringin against intranasal manganese and intracerbroventricular β-amyloid induced neurotoxicity in rats

Manganese -induced aggregation of the amyloid-β peptide (Aβ) is a hallmark molecular feature of Alzheimer's disease (AD). The current study was designed to investigate the effects of chronic administration of naringin against β-A_1-42 and manganese induced experimental model. Wistar rats received intracerebroventricular (ICV) β-A_1-42 once, intranasal manganese, naringin and nitric oxide modulators for 21 days and behavioral alterations were assessed. Mitochondrial enzymes, oxidative parameters, TNF-α, β-A_1-42 acetylcholinesterase (AChE) levels and manganese concentration were measured. ICV β-A_1-42 and intranasal manganese treated rats showed a memory deficit and significantly increased in β-A_1-42 level and manganese concentration, mitochondrial oxidative damage, AChE level and inflammatory mediator in the hippocampus and cortex. Chronic administration of naringin (40 and 80 mg/kg) significantly improved memory performance and attenuated the oxidative damage and mitochondrial dysfunction in Aβ with Mn treated rats. In addition, naringin also attenuates the pro-inflammatory cytokines like TNF-α, AChE, Amyloid deposition and Mn concentration. Further, pretreatment of N(G)-Nitro-L-arginine methyl ester (L-NAME) with (5 mg/kg) with lower dose of naringin significantly potentiated its protective effect. These results demonstrate that naringin offers protection against ICV β-A_1-42 and intranasal manganese induced memory dysfunction possibly due to its antioxidant, anti-inflammatory, anti-amyloidogenesis therefore, could have a therapeutic potential in Alzheimer's disease.

Ursolic and oleanolic acid derivatives with cholinesterase inhibiting potential

Triterpenoids are in the focus of scientific interest, and they were evaluated for many pharmacological applications among them their ability to act as inhibitors of cholinesterases. These inhibitors are still of interest as drugs that improve the life quality of patients suffering from age-related dementia illnesses especially of Alzheimer's disease. Herein, we prepared several derivatives of ursolic and oleanolic acid and screened them in Ellman's assays for their ability to inhibit acetylcholinesterase and/or butyrylcholinesterase, and for each of the active compounds the type of inhibition was determined. As a result, several compounds were shown as good inhibitors for acetylcholinesterase and butyrylcholinesterase even in a micromolar range. An ursolic acid derived hydroxyl-propinyl derivative 10 was a competitive inhibitor for butyrylcholinesterase with an inhibition constant of Ki = 4.29 μM, and therefore being twice as active as gold standard galantamine hydrobromide. The best inhibitor for acetylcholinesterase, however, was 2-methyl-3-oxo-methyl-ursoloate (18), acting as a mixed-type inhibitor showing Ki = 1.72 µM and Ki' = 1.28 μM, respectively.

Root-Securing and Brain-Fortifying Liquid Upregulates Caveolin-1 in Cell Model with Alzheimer's Disease through Inhibiting Tau Phosphorylation

In order to explore the effect of root-securing and brain-fortifying Liquid- (RSBFL-) mediated caveolin-1 (CAV-1) on phosphorylation of Tau protein and to uncover underlying mechanisms of RSBFL for the prevention and treatment of Alzheimer's disease (AD), hippocampal neurons isolated from neonatal SD rats and cultured in DMEM-F12 medium were induced by exogenous Aβ1–42 to establish a cell model with AD. Meanwhile, pEGFP-C1-CAV1 and CAV1-shRNA plasmids were transfected into hippocampal neurons for CAV-1 overexpression and silence, respectively. The serum containing RSBFL was prepared for the intervention of AD model cells. The expression of CAV-1, GSK-3β, and p-Tau in normal hippocampal neurons and AD model cells in the presence of serum containing RSBFL was evaluated. The model hippocampal neurons with AD induced by Aβ1–42 revealed an obvious CAV-1 inhibition, enhanced GSK-3β activity, and abnormal Tau phosphorylation. In contrast, the treatment with serum containing RSBFL could upregulate CAV-1 in AD hippocampal neurons (P < 0.05) with improved p-GSK-3β^Ser9 and reduced p-GSK-3β^Tyr216 (P < 0.01), as well as suppressed abnormal phosphorylation of Tau protein. Therefore, RSBFL has an excellent protective effect on hippocampal neurons through increasing CAV-1 expression, inhibiting GSK-3β activity, and reducing excessive abnormal phosphorylation of Tau protein.

Mechanisms of Alzheimer's Disease Pathogenesis and Prevention: The Brain, Neural Pathology, N-methyl-D-aspartate Receptors, Tau Protein and Other Risk Factors

The characteristic features of Alzheimer’s disease (AD) are the appearance of extracellular amyloid-beta (Aβ) plaques and neurofibrillary tangles in the intracellular environment, neuronal death and the loss of synapses, all of which contribute to cognitive decline in a progressive manner. A number of hypotheses have been advanced to explain AD. Abnormal tau phosphorylation may contribute to the formation of abnormal neurofibrillary structures. Many different structures are susceptible to AD, including the reticular formation, the nuclei in the brain stem (e.g., raphe nucleus), thalamus, hypothalamus, locus ceruleus, amygdala, substantia nigra, striatum, and claustrum. Excitotoxicity results from continuous, low-level activation of N-methyl-D-aspartate (NMDA) receptors. Premature synaptotoxicity, changes in neurotransmitter expression, neurophils loss, accumulation of amyloid β-protein deposits (amyloid/senile plaques), and neuronal loss and brain atrophy are all associated with stages of AD progression. Several recent studies have examined the relationship between Aβ and NMDA receptors. Aβ-induced spine loss is associated with a decrease in glutamate receptors and is dependent upon the calcium-dependent phosphatase calcineurin, which has also been linked to long-term depression.

The association of renin-angiotensin system blockade use with the risks of cognitive impairment of aging and Alzheimer's disease: A meta-analysis

A quantitative meta-analysis was performed to evaluate the association of renin-angiotensin system blockade (RASB) use with the incidence of cognitive impairment of aging and Alzheimer's disease (AD). Pubmed, Embase, and Cochrane Library databases were searched up to October 2015. Ten studies that assessed the relationship between RASB use and the incidence of cognitive impairment of aging or AD were included. When randomized trials and observational studies were combined, the use of RASB was significantly associated with a reduced risk of AD (risk ratio [RR], 0.80; 95% confidence interval [CI] 0.68-0.92) and cognitive impairment of aging (RR, 0.65; 95% CI 0.35-0.94) compared no use of RASB. Meanwhile, in an analysis of subgroups, both subjects with angiotensin converting enzyme inhibitor (ACEI) and angiotensin receptor blocker (ARB) use were lower incidence of AD (RR, 0.87; 95% CI 0.74-1.00; RR, 0.69; 95% CI 0.44-0.93, respectively) than those without, whereas, indirect comparison between ACEI and ARB revealed no significance in the risk of AD (RR, 1.27, 95% CI 0.85-1.89, p=0.245). In an analysis of cognitive impairment of aging, ARB use (RR, 0.40; 95% CI 0.02-0.78), rather than ACEI use (RR, 0.72; 95% CI 0.36-1.09), was shown to decrease the risk of cognitive impairment of aging. In conclusion, RASB treatments, regardless of the drug class, have benefits on prevention of AD, and the effects of ACEI may analogous to ARB. However, the benefit differs according to drug classes for cognitive impairment of aging, with ARB use, rather than ACEI use, being a potential treatment for reducing the incidence of cognitive impairment of aging.

Translation of Pre-Clinical Studies into Successful Clinical Trials for Alzheimer's Disease: What are the Roadblocks and How Can They Be Overcome?

Preclinical studies are essential for translation to disease treatments and effective use in clinical practice. An undue emphasis on single approaches to Alzheimer's disease (AD) appears to have retarded the pace of translation in the field, and there is much frustration in the public about the lack of an effective treatment. We critically reviewed past literature (1990-2014), analyzed numerous data, and discussed key issues at a consensus conference on Brain Ageing and Dementia to identify and overcome roadblocks in studies intended for translation. We highlight various factors that influence the translation of preclinical research and highlight specific preclinical strategies that have failed to demonstrate efficacy in clinical trials. The field has been hindered by the domination of the amyloid hypothesis in AD pathogenesis while the causative pathways in disease pathology are widely considered to be multifactorial. Understanding the causative events and mechanisms in the pathogenesis are equally important for translation. Greater efforts are necessary to fill in the gaps and overcome a variety of confounds in the generation, study design, testing, and evaluation of animal models and the application to future novel anti-dementia drug trials. A greater variety of potential disease mechanisms must be entertained to enhance progress.

The Janus face of the heme oxygenase/biliverdin reductase system in Alzheimer disease: it's time for reconciliation

Alzheimer disease (AD) is the most common form of dementia among the elderly and is characterized by progressive loss of memory and cognition. These clinical features are due in part to the increase of reactive oxygen and nitrogen species that mediate neurotoxic effects. The up-regulation of the heme oxygenase-1/biliverdin reductase-A (HO-1/BVR-A) system is one of the earlier events in the adaptive response to stress. HO-1/BVR-A reduces the intracellular levels of pro-oxidant heme and generates equimolar amounts of the free radical scavengers biliverdin-IX alpha (BV)/bilirubin-IX alpha (BR) as well as the pleiotropic gaseous neuromodulator carbon monoxide (CO) and ferrous iron. Two main and opposite hypotheses for a role of the HO-1/BVR-A system in AD propose that this system mediates neurotoxic and neuroprotective effects, respectively. This apparent controversy was mainly due to the fact that for over about 20years HO-1 was the only player on which all the analyses were focused, excluding the other important and essential component of the entire system, BVR. Following studies from the Butterfield laboratory that reported alterations in BVR activity along with decreased phosphorylation and increased oxidative/nitrosative post-translational modifications in the brain of subjects with AD and amnestic mild cognitive impairment (MCI) subjects, a debate was opened on the real pathophysiological and clinical significance of BVR-A. In this paper we provide a review of the main discoveries about the HO/BVR system in AD and MCI, and propose a mechanism that reconciles these two hypotheses noted above of neurotoxic and the neuroprotective aspects of this important stress responsive system.

Phosphorylation of tau protein as the link between oxidative stress, mitochondrial dysfunction, and connectivity failure: implications for Alzheimer's disease

Alzheimer's disease (AD) is defined by the concurrence of abnormal aggregates composed of phosphorylated tau protein and of abnormal cellular changes including neurite degeneration, loss of neurons, and loss of cognitive functions. While a number of mechanisms have been implicated in this complex disease, oxidative stress remains one of the earliest and strongest events related to disease progression. However, the mechanism that links oxidative stress and cognitive decline remains elusive. Here, we propose that phosphorylated tau protein could be playing the role of potential connector and, therefore, that a combined therapy involving antioxidants and check points for synaptic plasticity during early stages of the disease could become a viable therapeutic option for AD treatment.

Selenium homeostasis and antioxidant selenoproteins in brain: implications for disorders in the central nervous system

The essential trace element selenium, as selenocysteine, is incorporated into antioxidant selenoproteins such as glutathione peroxidases (GPx), thioredoxin reductases (TrxR) and selenoprotein P (Sepp1). Although comparatively low in selenium content, the brain exhibits high priority for selenium supply and retention under conditions of dietary selenium deficiency. Liver-derived Sepp1 is the major transport protein in plasma to supply the brain with selenium, serving as a "survival factor" for neurons in culture. Sepp1 expression has also been detected within the brain. Presumably, astrocytes secrete Sepp1, which is subsequently taken up by neurons via the apolipoprotein E receptor 2 (ApoER2). Knock-out of Sepp1 or ApoER2 as well as neuron-specific ablation of selenoprotein biosynthesis results in neurological dysfunction in mice. Astrocytes, generally less vulnerable to oxidative stress than neurons, are capable of up-regulating the expression of antioxidant selenoproteins upon brain injury. Occurrence of neurological disorders has been reported occasionally in patients with inadequate nutritional selenium supply or a mutation in the gene encoding selenocysteine synthase, one of the enzymes involved in selenoprotein biosynthesis. In three large trials carried out among elderly persons, a low selenium status was associated with faster decline in cognitive functions and poor performance in tests assessing coordination and motor speed. Future research is required to better understand the role of selenium and selenoproteins in brain diseases including hepatic encephalopathy.

Ginkgo biloba extract EGb 761® in the context of current developments in the diagnosis and treatment of age-related cognitive decline and Alzheimer's disease: a research perspective

In June 2011 a two-day expert meeting "The Ageing Brain" took place in Amsterdam, The Netherlands. The main aim was to discuss the available preclinical and clinical data on Ginkgo biloba special extract EGb 761® in the context of current developments in the diagnosis and treatment of age-related cognitive decline and Alzheimer's disease. 19 dementia experts covering the disciplines bio- and neurochemistry, gerontology, neurology, pharmacology, and psychiatry from Australia, Asia, Europe and North America reviewed available preclinical and clinical data for EGb 761® and identified core topics for future research. Based on a wide range of preclinical effects demonstrated for Ginkgo biloba, EGb 761® can be conceptualized as a multi-target compound with activity on distinct pathophysiological pathways in Alzheimer's disease (AD) and age-related cognitive decline. While symptomatic efficacy in dementia and mild cognitive impairment (MCI) has been demonstrated, interpretation of data from dementia prevention trials is complicated by important methodological issues. Bridging pre-clinical research and clinical research as well as deciding on suitable study designs for future trials with EGb 761® remain important questions. The participants of the "Ageing Brain" meeting on Ginkgo biloba special extract EGb 761® concluded that there is plenty of promising data, both pre-clinical and clinical, to consider future research with the compound targeting cognitive impairment in old age as a worthwhile activity.