Extracellular Zn2+-Dependent Amyloid-β1-42 Neurotoxicity in Alzheimer's Disease Pathogenesis

Neuroprotective, Anti-Inflammatory and Antifibrillogenic Offerings by Emodin against Alzheimer's Dementia: A Systematic Review

Background: Alzheimer's disease (AD) is among the major causes of dementia in the elderly and exerts tremendous clinical, psychological and socio-economic constraints. Currently, there are no effective disease-modifying/retarding anti-AD agents. Emodin is a bioactive phytochemical with potent multimodal anti-inflammatory, antioxidant, and antifibrillogenic properties. In particular, emodin may result in significant repression of the pathogenic mechanisms underlying AD. The purpose of this review is to accumulate and summarize all the primary research data evaluating the therapeutic actions of emodin in AD pathogenesis. Methodology: The search, selection, and retrieval of pertinent primary research articles were systematically performed using a methodically designed approach. A variety of keyword combinations were employed on online scholarly web-databases. Strict preset inclusion and exclusion criteria were used to select the retrieved studies. Data from the individual studies were summarized and compiled into different sections, based upon their findings. Results: Cellular and animal research indicates that emodin exerts robust multimodal neuroprotection in AD. While emodin effectively prevents tau and amyloid-beta (Aβ) oligomerization, it also mitigates their neurotoxicity by attenuating neuroinflammatory, oxidative, and bioenergetic defects. Evidences for emodin-mediated enhancements in memory, learning, and cognition were also found in the literature. Conclusion: Emodin is a potential anti-AD dietary supplement; however, further studies are warrantied to thoroughly understand its target players and mechanisms. Moreover, human clinical data on emodin-mediated amelioration of AD phenotype is largely lacking, and must be addressed in the future. Lastly, the safety of exogenously supplemented emodin must be thoroughly evaluated.

Multifunctional Supramolecular Vesicles as Zn2+ -Triggered Microglial Modulator Alleviates Alzheimer's Disease

Zn2+ -induced β-amyloid protein (Aβ) aggregation and microglia activation are the predominant contributors in Alzheimer's disease (AD). Regulating intracephalic excessive Zn2+ is a promising therapeutic strategy for AD treatment. However, only inhibition of Zn2+ is hardly to repair continuous damages caused by activated microglia. Herein, an intelligent resveratrol-loaded supramolecular vesicles (RES-loaded vesicles) with zinc ion chelation function and responsive release capability are constructed to alleviate Aβ fibrillation, oxidative stress, and microglial dysfunction. The resveratrol encapsulation efficiency and drug loading efficiency are calculated to be 49.67% and 7.87%, respectively. In vitro studies demonstrate that the RES-loaded vesicles can modulate Zn2+ -dependent Aβ aggregation. More importantly, the cargoes will be released in zinc environment and further reprograms microglia from proinflammatory M1 phenotype toward anti-inflammatory M2 phenotype, which prevents spontaneous neuroinflammation and alleviates cytotoxicity of cultured cells from 29% to 12%. With the stereotactic or intranasal administration, RES-loaded vesicles can overcome the blood brain barrier, alleviate neuronal apoptosis, neuroinflammation, and ultimately ameliorate cognitive impairment in two AD mouse models. This work provides a new sight for taking advantage of Zn2+ to treat CNS disorders.

Role of micronutrients in Alzheimer's disease: Review of available evidence

Alzheimer's disease (AD) is one of the most common age-related neurodegenerative disorders that have been studied for more than 100 years. Although an increased level of amyloid precursor protein is considered a key contributor to the development of AD, the exact pathogenic mechanism remains known. Multiple factors are related to AD, such as genetic factors, aging, lifestyle, and nutrients. Both epidemiological and clinical evidence has shown that the levels of micronutrients, such as copper, zinc, and iron, are closely related to the development of AD. In this review, we summarize the roles of eight micronutrients, including copper, zinc, iron, selenium, silicon, manganese, arsenic, and vitamin D in AD based on recently published studies.

[Brain Function and Pathophysiology Focused on Zn2+ Dynamics]

The basal levels of intracellular Zn²⁺ and extracellular Zn²⁺ are in the range of ～100 pM and ～10 nM, respectively, in the brain. Extracellular Zn²⁺ dynamics is involved in both cognitive performance and neurodegeneration. The bidirectional actions are linked with extracellular glutamate and amyloid-β_1-42 (Aβ_1-42). Intracellular Zn²⁺ signaling via extracellular glutamate is required for learning and memory, while intracellular Zn²⁺ dysregulation induces cognitive decline. Furthermore, human Aβ_1-42, a causative peptide in Alzheimer's disease pathogenesis captures extracellular Zn²⁺ and readily taken up into hippocampal neurons followed by intracellular Zn²⁺ dysregulation. Aβ_1-42-mediated intracellular Zn²⁺ dysregulation is accelerated with aging, because extracellular Zn²⁺ is age-relatedly increased, resulting in Aβ_1-42-induced cognitive decline and neurodegeneration with aging. On the other hand, metallothioneins, zinc-binding proteins can capture Zn²⁺ released from intracellular Zn-Aβ_1-42 complexes and serve for intracellular Zn²⁺-buffering to maintain intracellular Zn²⁺ homeostasis. This review summarizes Zn²⁺ function and its neurotoxicity in the brain, and also the potential defense strategy via metallothioneins against Aβ_1-42-induced pathogenesis.

Mechanistic Impact of Zinc Deficiency in Human Development

Zinc (Zn) deficiency in humans is an emerging global health issue affecting approximately two billion people across the globe. The situation prevails due to the intake of Zn deficient grains and vegetables worldwide. Clinical identification of Zn deficiency in humans remains problematic because the symptoms do not appear until impair the vital organs, such as the gastrointestinal track, central nervous system, immune system, skeletal, and nervous system. Lower Zn body levels are also responsible for multiple physiological disorders, such as apoptosis, organs destruction, DNA injuries, and oxidative damage to the cellular components through reactive oxygen species (ROS). The oxidative damage causes chronic inflammation lead toward several chronic diseases, such as heart diseases, cancers, alcohol-related malady, muscular contraction, and neuro-pathogenesis. The present review focused on the physiological and growth-related changes in humans under Zn deficient conditions, mechanisms adopted by the human body under Zn deficiency for the proper functioning of the body systems, and the importance of nutritional and nutraceutical approaches to overcome Zn deficiency in humans and concluded that the biofortified food is the best source of Zn as compared to the chemical supplementation to avoid their negative impacts on human.

Structural Studies Providing Insights into Production and Conformational Behavior of Amyloid-β Peptide Associated with Alzheimer's Disease Development

Alzheimer's disease is the most common type of neurodegenerative disease in the world. Genetic evidence strongly suggests that aberrant generation, aggregation, and/or clearance of neurotoxic amyloid-β peptides (Aβ) triggers the disease. Aβ accumulates at the points of contact of neurons in ordered cords and fibrils, forming the so-called senile plaques. Aβ isoforms of different lengths are found in healthy human brains regardless of age and appear to play a role in signaling pathways in the brain and to have neuroprotective properties at low concentrations. In recent years, different substances have been developed targeting Aβ production, aggregation, interaction with other molecules, and clearance, including peptide-based drugs. Aβ is a product of sequential cleavage of the membrane glycoprotein APP (amyloid precursor protein) by β- and γ-secretases. A number of familial mutations causing an early onset of the disease have been identified in the APP, especially in its transmembrane domain. The mutations are reported to influence the production, oligomerization, and conformational behavior of Aβ peptides. This review highlights the results of structural studies of the main proteins involved in Alzheimer's disease pathogenesis and the molecular mechanisms by which perspective therapeutic substances can affect Aβ production and nucleation.

Dehydroeffusol Pprevents Amyloid β1-42-mediated Hippocampal Neurodegeneration via Reducing Intracellular Zn2+ Toxicity

Dehydroeffusol, a phenanthrene isolated from Juncus effusus, is a Chinese medicine. To explore an efficacy of dehydroeffusol administration for prevention and cure of Alzheimer's disease, here we examined the effect of dehydroeffusol on amyloid β_1-42 (Aβ_1-42)-mediated hippocampal neurodegeneration. Dehydroeffusol (15 mg/kg body weight) was orally administered to mice once a day for 6 days and then human Aβ_1-42 was injected intracerebroventricularly followed by oral administration for 12 days. Neurodegeneration in the dentate granule cell layer, which was determined 2 weeks after Aβ_1-42 injection, was rescued by dehydroeffusol administration. Aβ staining (uptake) was not reduced in the dentate granule cell layer by pre-administration of dehydroeffusol for 6 days, while increase in intracellular Zn²⁺ induced with Aβ_1-42 was reduced, suggesting that pre-administration of dehydroeffusol prior to Aβ_1-42 injection is effective for Aβ_1-42-mediated neurodegeneration that was linked with intracellular Zn²⁺ toxicity. As a matter of fact, pre-administration of dehydroeffusol rescued Aβ_1-42-mediated neurodegeneration. Interestingly, pre-administration of dehydroeffusol increased synthesis of metallothioneins, intracellular Zn²⁺-binding proteins, in the dentate granule cell layer, which can capture Zn²⁺ from Zn-Aβ_1-42 complexes. The present study indicates that pre-administration of dehydroeffusol protects Aβ_1-42-mediated neurodegeneration in the hippocampus by reducing intracellular Zn²⁺ toxicity, which is linked with induced synthesis of metallothioneins. Dehydroeffusol, a novel inducer of metallothioneins, may protect Aβ_1-42-induced pathogenesis in Alzheimer's disease.

[Alzheimer's disease pathogenesis focused on intracellular Zn2+ toxicity and its defense strategy]

The basal levels of intracellular Zn²⁺ and extracellular Zn²⁺ are in the range of ~100 pM and ~10 nM, respectively, in the hippocampus. Extracellular Zn²⁺ dynamics, which serves bidirectionally and involved in cognitive activity and cognitive decline, is modified by extracellular glutamate signaling and the presence of amyloid-β_1-42 (Aβ_1-42), a causative peptide in Alzheimer's disease (AD) pathogenesis. When human Aβ_1-42 reaches 100-500 pM in the extracellular compartment of the rat hippocampus, Zn-Aβ_1-42 complexes are produced and readily taken up into dentate granule cells in a synaptic activity-independent manner. Furthermore, intracellular Zn-Aβ_1-42 complexes release Zn²⁺ followed by intracellular Zn²⁺ dysregulation. Aβ_1-42-mediated intracellular Zn²⁺ toxicity is accelerated with aging, because extracellular Zn²⁺ is age-relatedly increased. We have reported that Aβ_1-42 released physiologically from neuron terminals disrupts intracellular Zn²⁺ homeostasis, resulting in age-related cognitive decline and neurodegeneration. Metallothioneins (MTs), zinc-binding proteins can capture Zn²⁺ released from intracellular Zn-Aβ_1-42 complexes and serve for intracellular Zn²⁺-buffering under acute intracellular Zn²⁺ dysregulation. Aβ_1-42-induced pathogenesis leads the AD development and its defense strategy may prevent the development. This review summarizes extracellular Zn²⁺-dependent Aβ_1-42 neurotoxicity, which is accelerated with aging, and the potential defense strategy against AD.