Extracellular Zn2+ Is Essential for Amyloid β1-42-Induced Cognitive Decline in the Normal Brain and Its Rescue

Beneficial effect of Juncus effusus extract powder enriched with dehydroeffusol on the cognitive and dexterous performance of elderly people: A randomized, double-blind, placebo-controlled, parallel-group study

On the basis of the evidence that dehydroeffusol prevents human amyloid-β-induced memory deficit and neurodegeneration in mice, here we tested the effect of a Juncus effusus extract powder enriched with dehydroeffusol on the cognitive and dexterous performance of elderly people. A randomized, double-blind, placebo-controlled, parallel-group study was conducted in 41 participants (averaged age: 69 years) randomly divided into test and placebo groups who received a test tablet and a placebo tablet, respectively, once a day for 24 weeks. Changes in cognitive function were assessed using the Five-Cog test 24 weeks after the start of intake. The task scores of the test group were significantly higher in the clue recall and clock-drawing tasks than in the placebo group, suggesting that intake of J. effusus extract powder may improve the cognitive function of elderly people. Moreover, the task score of the test group was significantly higher for the assembly task in the dexterity test than the placebo group, suggesting that intake of J. effusus extract powder may improve the dexterous movement of elderly people. No adverse events were clinically observed during the study. The present study first suggests that intake of J. effusus extract powder enriched with dehydroeffusol is of benefit to the cognitive and dexterous performance of elderly people.

Insight into brain metallothioneins from bidirectional Zn2+ signaling in synaptic dynamics

The basal levels as the labile Zn2+ pools in the extracellular and intracellular compartments are in the range of ∼10 nM and ∼100 pM, respectively. The influx of extracellular Zn2+ is used for memory via cognitive activity and is regulated for synaptic plasticity, a cellular mechanism of memory. When Zn2+ influx into neurons excessively occurs, however, it becomes a critical trigger for cognitive decline and neurodegeneration, resulting in acute and chronic pathogenesis. Aging, a biological process, generally accelerates vulnerability to neurodegenerative disorders such as Alzheimer's disease (AD) and Parkinson's disease (PD). The basal level of extracellular Zn2+ is age relatedly increased in the rat hippocampus, and the influx of extracellular Zn2+ contributes to accelerating vulnerability to the AD and PD pathogenesis in experimental animals with aging. Metallothioneins (MTs) are Zn2+-binding proteins for cellular Zn2+ homeostasis and involved in not only supplying functional Zn2+ required for cognitive activity, but also capturing excess (toxic) Zn2+ involved in cognitive decline and neurodegeneration. Therefore, it is estimated that regulation of MT synthesis is involved in both neuronal activity and neuroprotection. The present report provides recent knowledge regarding the protective/preventive potential of MT synthesis against not only normal aging but also the AD and PD pathogenesis in experimental animals, focused on MT function in bidirectional Zn2+ signaling in synaptic dynamics.

Blockage of metallothionein synthesis via adrenaline β receptor activation invalidates dehydroeffusol-mediated prevention of amyloid β1-42 toxicity

Dehydroeffusol, a major phenanthrene in Juncus effusus, protects neurodegeneration induced by intracellular Zn2+ ferried by extracellular amyloid β1-42 (Aβ1-42). Here we focused on adrenaline β receptor activation and the induction of metallothioneins (MTs), intracellular Zn2+-binding proteins to test the protective mechanism of dehydroeffusol. Isoproterenol, an agonist of adrenergic β receptors elevated the level of MTs in the dentate granule cell layer 1 day after intracerebroventricular (ICV) injection. When Aβ1-42 was injected 1 day after isoproterenol injection, pre-injection of isoproterenol protected Aβ1-42 toxicity via reducing the increase in intracellular Zn2+ after ICV injection of Aβ1-42. On the basis of the effect of increased MTs by isoproterenol, dehydroeffusol (15 mg/kg body weight) was orally administered to mice once a day for 2 days. On day later, dehydroeffusol elevated the level of MTs and prevented Aβ1-42 toxicity via reducing Aβ1-42-mediated increase in intracellular Zn2+. In contrast, propranolol, an antagonist of adrenergic β receptors reduced the level of MTs increased by dehydroeffusol, resulting in invalidating the preventive effect of dehydroeffusol on Aβ1-42 toxicity. The present study indicates that blockage of MT synthesis via adrenaline β receptor activation invalidates dehydroeffusol-mediated prevention of Aβ1-42 toxicity. It is likely that MT synthesis via adrenaline β receptor activation is beneficial to neuroprotection and that oral intake of dehydroeffusol preventively serves against the Aβ1-42 toxicity.

Chronic memantine disrupts spatial memory and up-regulates Htr1a gene expression in the hippocampus of GPR39 (zinc-sensing receptor) KO male mice

GPR39 is a receptor involved in zincergic neurotransmission, and its role in regulating psychological functions is an active area of research. The purported roles of GPR39 at the cellular level include regulation of inflammatory and oxidative stress response, and modulation of GABAergic and endocannabinoid neurotransmission. GPR39 knock-out (KO) mice exhibit episodic-like and spatial memory (ELM and SM, respectively) deficits throughout their lifetime, and are similar in that respect to senescent wild-type (WT) conspecifics. Since a role for zinc has been postulated in neurodegenerative disorders, in this study we investigated the possibility of a pharmacological rescue of both types of declarative memory with memantine - a noncompetitive NMDAR antagonist used for slowing down dementia; or, a putative GPR39 agonist - TC-G 1008. First, we tested adult WT and GPR39KO male mice under acute 5 mg/kg memantine or vehicle treatment in an object recognition task designed to simultaneously probe the "what?", "where?" and "when?" components of ELM. Next, we investigated the impact of chronic memantine or TC-G 1008 on ELM and SM (Morris water maze, MWM) in both WT and GPR39KO mice. Following chronic experiments, we assessed with qRT-PCR hippocampal gene expression of targets previously associated with GPR39. We report: no effects of acute memantine on ELM; a tendency to improve the "where?" component of ELM in both WT and GPR39 KO mice following 12 days of memantine; and, a disruption of SM in GPR39KO mice after 24 days of memantine treatment. The latter result was associated with upregulation of Htr1a hippocampal expression.

Role of TPEN in Amyloid-β25-35-Induced Neuronal Damage Correlating with Recovery of Intracellular Zn2+ and Intracellular Ca2+ Overloading

The overproduction of neurotoxic amyloid-β (Aβ) peptides in the brain is a hallmark of Alzheimer's disease (AD). To determine the role of intracellular zinc ion (_iZn²⁺) dysregulation in mediating Aβ-related neurotoxicity, this study aimed to investigate whether N, N, N', N'‑tetrakis (2‑pyridylmethyl) ethylenediamine (TPEN), a Zn²⁺‑specific chelator, could attenuate Aβ_25-35‑induced neurotoxicity and the underlying mechanism. We used the 3-(4, 5-dimethyl-thiazol-2-yl)-2, 5-diphenyltetrazolium bromide assay to measure the viability of primary hippocampal neurons. We also determined intracellular Zn²⁺ and Ca²⁺ concentrations, mitochondrial and lysosomal functions, and intracellular reactive oxygen species (ROS) content in hippocampal neurons using live-cell confocal imaging. We detected L-type voltage-gated calcium channel currents (L-I_Ca) in hippocampal neurons using the whole‑cell patch‑clamp technique. Furthermore, we measured the mRNA expression levels of proteins related to the _iZn²⁺ buffer system (ZnT-3, MT-3) and voltage-gated calcium channels (Cav1.2, Cav1.3) in hippocampal neurons using RT-PCR. The results showed that TPEN attenuated Aβ_25-35‑induced neuronal death, relieved the Aβ_25-35‑induced increase in intracellular Zn²⁺ and Ca²⁺ concentrations; reversed the Aβ_25-35‑induced increase in ROS content, the Aβ_25-35‑induced increase in the L-I_Ca peak amplitude at different membrane potentials, the Aβ_25-35‑induced the dysfunction of the mitochondria and lysosomes, and the Aβ_25-35‑induced decrease in ZnT-3 and MT-3 mRNA expressions; and increased the Cav1.2 mRNA expression in the hippocampal neurons. These results suggest that TPEN, the Zn²⁺-specific chelator, attenuated Aβ_25-35‑induced neuronal damage, correlating with the recovery of intracellular Zn²⁺ and modulation of abnormal Ca²⁺-related signaling pathways.

The metal ion hypothesis of Alzheimer's disease and the anti-neuroinflammatory effect of metal chelators

Alzheimer's disease (AD), characterized by the β-amyloid protein (Aβ) deposition and tau hyperphosphorylation, is the most common dementia with uncertain etiology. The clinical trials of Aβ monoclonal antibody drugs have almost failed, giving rise to great attention on the other etiologic hypothesis regarding AD such as metal ions dysmetabolism and chronic neuroinflammation. Mounting evidence revealed that the metal ions (iron, copper, and zinc) were dysregulated in the susceptible brain regions of AD patients, which was highly associated with Aβ deposition, tau hyperphosphorylation, neuronal loss, as well as neuroinflammation. Further studies uncovered that iron, copper and zinc could not only enhance the production of Aβ but also directly bind to Aβ and tau to promote their aggregations. In addition, the accumulation of iron and copper could respectively promote ferroptosis and cuproptosis. Therefore, the metal ion chelators were recognized as promising agents for treating AD. This review comprehensively summarized the effects of metal ions on the Aβ dynamics and tau phosphorylation in the progression of AD. Furthermore, taking chronic neuroinflammation contributes to the progression of AD, we also provided a summary of the mechanisms concerning metal ions on neuroinflammation and highlighted the metal ion chelators may be potential agents to alleviate neuroinflammation under the condition of AD. Nevertheless, more investigations regarding metal ions on neuroinflammation should be taken into practice, and the effects of metal ion chelators on neuroinflammation should gain more attention. Running title: Metal chelators against neuroinflammation.

Metallothionein synthesis increased by Ninjin-yoei-to, a Kampo medicine protects neuronal death and memory loss after exposure to amyloid β1-42

BACKGROUND

It is possible that increased synthesis of metallothioneins (MTs), Zn²⁺-binding proteins is linked with the protective effect of Ninjin-yoei-to (NYT) on Zn²⁺ toxicity ferried by amyloid β_1-42 (Aβ_1-42).

METHODS

Judging from the biological half-life (18-20 h) of MTs, the effective period of newly synthesized MT on capturing Zn²⁺ is estimated to be approximately 2 days. In the present paper, a diet containing 3% NYT was administered to mice for 2 days and then Aβ_1-42 was injected into the lateral ventricle of mice.

RESULTS

MT level in the dentate granule cell layer was elevated 2 days after administration of NYT diet, while the administration reduced intracellular Zn²⁺ level increased 1 h after Aβ_1-42 injection, resulting in rescuing neuronal death in the dentate granule cell layer, which was observed 14 days after Aβ_1-42 injection. Furthermore, Pre-administration of NYT diet rescued object recognition memory loss via affected perforant pathway long-term potentiation after local injection of Aβ_1-42 into the dentate granule cell layer of rats.

CONCLUSION

The present study indicates that pre-administration of NYT diet for 2 days increases synthesis of MTs, which reduces intracellular Zn²⁺ toxicity ferried by extracellular Aβ_1-42, resulting in protecting neuronal death in the dentate gyrus and memory loss after exposure to Aβ_1-42.

The Zinc-sensing Receptor (GPR39) Modulates Declarative Memory and Age-related Hippocampal Gene Expression in Male Mice

As a neuromodulator, zinc regulates synaptic plasticity, learning and memory. Synaptic zinc is also a crucial factor in the development of toxic forms of amyloid beta protein and, subsequently, of Alzheimer's dementia (AD). Therefore, efforts to pinpoint mechanisms underlying zinc-dependent cognitive functions might aid AD research, by providing potential novel targets for drugs. One of the most understudied proteins in this regard is a zinc-sensing metabotropic receptor: GPR39. In this study we investigated the impact of GPR39 knock-out (KO) on age-related memory decline in mice of both sexes, by comparing them to age-matched wild-type (WT) littermates. We also tested the effects of a GPR39 agonist (TC-G 1008) on declarative memory of old animals, and its disruption in adult mice. We observed episodic-like memory (ELM) and spatial memory (SM) deficits in male GPR39 KO mice, as well as intact procedural memory in GPR39 KO mice regardless of age and sex. ELM was also absent in old WT male mice, and all female mice regardless of their genotype. Acute application of TC-G 1008 (10 mg/kg) reversed a deficit in two of three ELM components in old WT male mice, and had no promnesic effect on consolidation interference of ELM in adult WT mice. We discuss the possible neurobiological mechanisms and the translational value of these results for potential add-on pharmacotherapy of AD aimed at the zinc-sensing receptor.

Exposure of metal toxicity in Alzheimer's disease: An extensive review

Metals serve important roles in the human body, including the maintenance of cell structure and the regulation of gene expression, the antioxidant response, and neurotransmission. High metal uptake in the nervous system is harmful because it can cause oxidative stress, disrupt mitochondrial function, and impair the activity of various enzymes. Metal accumulation can cause lifelong deterioration, including severe neurological problems. There is a strong association between accidental metal exposure and various neurodegenerative disorders, including Alzheimer's disease (AD), the most common form of dementia that causes degeneration in the aged. Chronic exposure to various metals is a well-known environmental risk factor that has become more widespread due to the rapid pace at which human activities are releasing large amounts of metals into the environment. Consequently, humans are exposed to both biometals and heavy metals, affecting metal homeostasis at molecular and biological levels. This review highlights how these metals affect brain physiology and immunity and their roles in creating harmful proteins such as β-amyloid and tau in AD. In addition, we address findings that confirm the disruption of immune-related pathways as a significant toxicity mechanism through which metals may contribute to AD.

[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.

Isoproterenol injected into the basolateral amygdala rescues amyloid β1-42-induced conditioned fear memory deficit via reducing intracellular Zn2+ toxicity

On the basis of amyloid β (Aβ) peptides as triggers in atrophy of structures in the limbic system, here we postulated that Aβ_1-42-induced intracellular Zn²⁺ toxicity in the basolateral amygdala contributes to conditioned fear memory. Aβ_1-42 increased intracellular Zn²⁺ level in the amygdala after local injection of Aβ_1-42 into the basolateral amygdala, resulting in conditioned fear memory deficit via attenuated LTP at perforant pathway-basolateral amygdala synapses. Co-injection of isoproterenol, a beta-adrenergic receptor agonist, reduced Aβ_1-42-mediated increase in intracellular Zn²⁺, resulting in rescue of the memory deficit and attenuated LTP. The present study suggests that beta-adrenergic activity induced by isoproterenol in the basolateral amygdala rescues the impairment of conditioned fear memory by Aβ_1-42. The rescuing effect may be linked with reducing Aβ_1-42-induced intracellular Zn²⁺ toxicity. Furthermore, Aβ_1-42 injection into the basolateral amygdala also attenuated LTP at perforant pathway-dentate granule cell synapses, while co-injection of isoproterenol rescued it, suggesting that Aβ_1-42 toxicity in the basolateral amygdala also affects hippocampus-dependent memory. It is likely that beta-adrenergic receptor activation in the basolateral amygdala rescues the limbic system exposed to Aβ_1-42 toxicity.

Post-Ischemic Neurodegeneration of the Hippocampus Resembling Alzheimer's Disease Proteinopathy

In this review, we summarize, inter alia, the protein and gene changes associated with Alzheimer’s disease and their role in post-ischemic hippocampal neurodegeneration. In the hippocampus, studies have revealed dysregulation of the genes for the amyloid protein precursor metabolism and tau protein that is identical in nature to Alzheimer’s disease. Data indicate that amyloid and tau protein, derived from brain tissue and blood due to increased permeability of the blood–brain barrier after ischemia, play a key role in post-ischemic neurodegeneration of the hippocampus, with concomitant development of full-blown dementia. Thus, the knowledge of new neurodegenerative mechanisms that cause neurodegeneration of the hippocampus after ischemia, resembling Alzheimer’s disease proteinopathy, will provide the most important therapeutic development goals to date.

Role of zinc and copper ions in the pathogenetic mechanisms of traumatic brain injury and Alzheimer's disease

The disruption of homeostasis of zinc (Zn2+) and copper (Cu2+) ions in the central nervous system is involved in the pathogenesis of many neurodegenerative diseases, such as amyotrophic lateral sclerosis, Wilson's, Creutzfeldt-Jakob, Parkinson's, and Alzheimer's diseases (AD), and traumatic brain injury (TBI). The last two pathological conditions of the brain are the most common; moreover, it is possible that TBI is a risk factor for the development of AD. Disruptions of Zn2+ and Cu2+ homeostasis play an important role in the mechanisms of pathogenesis of both TBI and AD. This review attempts to summarize and systematize the currently available research data on this issue. The neurocytotoxicity of Cu2+ and Zn2+, the synergism of the toxic effect of calcium and Zn2+ ions on the mitochondria of neurons, and the interaction of Zn2+ and Cu2+ with β-amyloid (Abeta) and tau protein are considered.

TPEN attenuates amyloid-β25-35-induced neuronal damage with changes in the electrophysiological properties of voltage-gated sodium and potassium channels

To understand the role of intracellular zinc ion (Zn²⁺) dysregulation in mediating age-related neurodegenerative changes, particularly neurotoxicity resulting from the generation of excessive neurotoxic amyloid-β (Aβ) peptides, this study aimed to investigate whether N, N, N', N'-tetrakis (2-pyridylmethyl) ethylenediamine (TPEN), a Zn²⁺-specific chelator, could attenuate Aβ_25-35-induced neurotoxicity and the underlying electrophysiological mechanism. We used the 3-(4, 5-dimethyl-thiazol-2-yl)-2, 5-diphenyltetrazolium bromide assay to measure the viability of hippocampal neurons and performed single-cell confocal imaging to detect the concentration of Zn²⁺ in these neurons. Furthermore, we used the whole-cell patch-clamp technique to detect the evoked repetitive action potential (APs), the voltage-gated sodium and potassium (K⁺) channels of primary hippocampal neurons. The analysis showed that TPEN attenuated Aβ_25-35-induced neuronal death, reversed the Aβ_25-35-induced increase in intracellular Zn²⁺ concentration and the frequency of APs, inhibited the increase in the maximum current density of voltage-activated sodium channel currents induced by Aβ_25-35, relieved the Aβ_25-35-induced decrease in the peak amplitude of transient outward K⁺ currents (I_A) and outward-delayed rectifier K⁺ currents (I_DR) at different membrane potentials, and suppressed the steady-state activation and inactivation curves of I_A shifted toward the hyperpolarization direction caused by Aβ_25-35. These results suggest that Aβ_25-35-induced neuronal damage correlated with Zn²⁺ dysregulation mediated the electrophysiological changes in the voltage-gated sodium and K⁺ channels. Moreover, Zn²⁺-specific chelator-TPEN attenuated Aβ_25-35-induced neuronal damage by recovering the intracellular Zn²⁺ concentration.

Role of GPR39 in Neurovascular Homeostasis and Disease

GPR39, a member of the ghrelin family of G protein-coupled receptors, is zinc-responsive and contributes to the regulation of diverse neurovascular and neurologic functions. Accumulating evidence suggests a role as a homeostatic regulator of neuronal excitability, vascular tone, and the immune response. We review GPR39 structure, function, and signaling, including constitutive activity and biased signaling, and summarize its expression pattern in the central nervous system. We further discuss its recognized role in neurovascular, neurological, and neuropsychiatric disorders.

Preventive effect of Ninjin-yoei-to, a Kampo medicine, on amyloid β1-42-induced neurodegeneration via intracellular Zn2+ toxicity in the dentate gyrus

Ninjin-yoei-to (NYT), a Kampo medicine, has ameliorative effects on cognitive dysfunction via enhancing cholinergic neuron activity. To explore an efficacy of NYT administration for prevention and cure of Alzheimer’s disease, here we examined the effect of NYT on amyloid β_1-42 (Aβ_1-42)-induced neurodegeneration in the dentate gyrus. A diet containing 3% NYT was administered to mice for 2 weeks and human Aβ_1-42 was intracerebroventricularly injected. Neurodegeneration in the dentate granule cell layer of the hippocampus, which was determined 2 weeks after the injection, was rescued by administration of the diet for 4 weeks. Aβ staining (uptake) was not modified in the dentate granule cell layer by pre-administration of the diet for 2 weeks, while Aβ_1-42-induced increase in intracellular Zn²⁺ was reduced, suggesting that pre-administration of NYT prior to Aβ injection is effective for reducing Aβ_1-42-induced Zn²⁺ toxicity in the dentate gyrus. As a matter of fact, Aβ_1-42-induced neurodegeneration in the dentate gyrus was rescued by pre-administration of NYT. Interestingly, the level of metallothioneins, intracellular Zn²⁺-binding proteins, which can capture Zn²⁺ from Zn-Aβ_1-42 complexes, was elevated in the dentate granule cell layer by pre-administration of NYT. The present study suggests that pre-administration of NYT prevents Aβ_1-42-mediated neurodegeneration in the dentate gyurs by induced synthesis of metallothioneins, which reduces intracellular Zn²⁺ toxicity induced by Aβ_1-42.

Zinc

Since the discovery of manifest Zn deficiency in 1961, the increasing number of studies demonstrated the association between altered Zn status and multiple diseases. In this chapter, we provide a review of the most recent advances on the role of Zn in health and disease (2010-20), with a special focus on the role of Zn in neurodegenerative and neurodevelopmental disorders, diabetes and obesity, male and female reproduction, as well as COVID-19. In parallel with the revealed tight association between ASD risk and severity and Zn status, the particular mechanisms linking Zn2+ and ASD pathogenesis like modulation of synaptic plasticity through ProSAP/Shank scaffold, neurotransmitter metabolism, and gut microbiota, have been elucidated. The increasing body of data indicate the potential involvement of Zn2+ metabolism in neurodegeneration. Systemic Zn levels in Alzheimer's and Parkinson's disease were found to be reduced, whereas its sequestration in brain may result in modulation of amyloid β and α-synuclein processing with subsequent toxic effects. Zn2+ was shown to possess adipotropic effects through the role of zinc transporters, zinc finger proteins, and Zn-α2-glycoprotein in adipose tissue physiology, underlying its particular role in pathogenesis of obesity and diabetes mellitus type 2. Recent findings also contribute to further understanding of the role of Zn2+ in spermatogenesis and sperm functioning, as well as oocyte development and fertilization. Finally, Zn2+ was shown to be the potential adjuvant therapy in management of novel coronavirus infection (COVID-19), underlining the perspectives of zinc in management of old and new threats.

The Zinc-Sensing Receptor GPR39 in Physiology and as a Pharmacological Target

The G-protein coupled receptor GPR39 is abundantly expressed in various tissues and can be activated by changes in extracellular Zn²⁺ in physiological concentrations. Previously, genetically modified rodent models have been able to shed some light on the physiological functions of GPR39, and more recently the utilization of novel synthetic agonists has led to the unraveling of several new functions in the variety of tissues GPR39 is expressed. Indeed, GPR39 seems to be involved in many important metabolic and endocrine functions, but also to play a part in inflammation, cardiovascular diseases, saliva secretion, bone formation, male fertility, addictive and depression disorders and cancer. These new discoveries offer opportunities for the development of novel therapeutic approaches against many diseases where efficient therapeutics are still lacking. This review focuses on Zn²⁺ as an endogenous ligand as well as on the novel synthetic agonists of GPR39, placing special emphasis on the recently discovered physiological functions and discusses their pharmacological potential.

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.

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

The basal level of extracellular Zn²⁺ is in the range of low nanomolar (~ 10 nM) in the hippocampus. However, extracellular Zn²⁺ dynamics plays a key role for not only cognitive activity but also cognitive decline. Extracellular Zn²⁺ dynamics is modified by glutamatergic synapse excitation and the presence of amyloid-β_1-42 (Aβ_1-42), a causative peptide in Alzheimer's disease (AD). When human Aβ_1-42 reaches high picomolar (> 100 pM) in the extracellular compartment of the rat dentate gyrus, Zn-Aβ_1-42 complexes are readily formed and taken up into dentate granule cells, followed by Aβ_1-42-induced cognitive decline that is linked with Zn²⁺ released from intracellular Zn-Aβ_1-42 complexes. Aβ_1-42-induced intracellular Zn²⁺ toxicity is accelerated with aging because of age-related increase in extracellular Zn²⁺. The recent findings suggest that Aβ_1-42 secreted continuously from neuron terminals causes age-related cognitive decline and neurodegeneration via intracellular Zn²⁺ dysregulation. On the other hand, metallothioneins (MTs), zinc-binding proteins, quickly serve for intracellular Zn²⁺-buffering under acute intracellular Zn²⁺ dysregulation. On the basis of the idea that the defense strategy against Aβ_1-42-induced pathogenesis leads to preventing the AD development, this review deals with extracellular Zn²⁺-dependent Aβ_1-42 neurotoxicity, which is accelerated with aging.

Retention Period of Amyloid β1-42 in the Brain Extracellular Fluid as the Toxicological Determinant in Freely Moving Rats

The pathological significance of amyloid-β_1-42 (Aβ_1-42) dynamics is poorly understood in the brain extracellular compartment. Here we test which of the concentration or the retention is critical for Aβ_1-42 toxicity after injection of equal dose into dentate granule cell layer of freely moving rats. The toxicity of Aβ_1-42 (25 µM) was compared between injections at the rate of 0.25 µL/min for 4 min (fast injection) and 0.025 µL/min for 40 min (slow injection). Dentate gyrus long-term potentiation (LTP) was affected 1 and 2 h after the fast injection, but not 4 h. In contrast, LTP was affected even 72 h after the slow injection. Aβ_1-42 staining 5 min after finish of the slow injection was more intense in the dentate granule cell layer than of the fast injection. The present study indicates that the retention of Aβ_1-42 in the extracellular fluid is correlated with neuronal Aβ_1-42 uptake and plays a key role in Aβ_1-42 neurotoxicity. In the extracellular fluid of the dentate gyrus, the retention period of Aβ_1-42 is much more critical for Aβ_1-42 toxicity than Aβ_1-42 concentration. It is likely that Aβ_1-42 toxicity is accelerated by the disturbance of Aβ_1-42 metabolism in the dentate gyrus.

Zinc in the Brain: Friend or Foe?

Zinc is a trace metal ion in the central nervous system that plays important biological roles, such as in catalysis, structure, and regulation. It contributes to antioxidant function and the proper functioning of the immune system. In view of these characteristics of zinc, it plays an important role in neurophysiology, which leads to cell growth and cell proliferation. However, after brain disease, excessively released and accumulated zinc ions cause neurotoxic damage to postsynaptic neurons. On the other hand, zinc deficiency induces degeneration and cognitive decline disorders, such as increased neuronal death and decreased learning and memory. Given the importance of balance in this context, zinc is a biological component that plays an important physiological role in the central nervous system, but a pathophysiological role in major neurological disorders. In this review, we focus on the multiple roles of zinc in the brain.

Manganese Exposure Aggravates β-Amyloid Pathology by Microglial Activation

Human epidemiological evidence and animal experimental data suggest that chronic manganese (Mn) exposure increases the risk of Alzheimer’s disease (AD) and amyloid plaques, a hallmark of AD brain pathology, but the underlying mechanisms were not fully understood. Using the transgenic APP/PS1/Tau triple transgenic AD (3×Tg-AD) mouse model and mouse-derived microglia and neuroblastoma cell lines, we found that chronic 5-month Mn treatment increased beta amyloid peptide (Aβ) expression and Aβ plaques in the cerebral cortex and hippocampus in these 3×Tg-AD mice. Furthermore, we found that the β- and γ-secretase cleavage activities were markedly increased, while α-secretase cleavage activity was reduced in the brain of Mn-treated AD mice; these effects increase Aβ production and thus are amyloidogenic. Equally important, Mn treatment alone did not alter β-secretase 1 (BACE1) gene expression or Aβ production in amyloidogenic mutant amyloid precursor protein (APP) gene hAPPsw-transfected N2a cells (APPsw-N2a), but in APPsw-N2a cells either co-cultured with microglia or cultured with microglia-conditioned media, Mn exposure increased BACE1 expression and amyloidogenesis. We further determined that Mn exposure promoted the activation of microglia both in 3×Tg-AD mouse brains and in cultured microglia cells, and increased the secretion of the inflammatory cytokines interleukin-1β (IL-1β) and tumor necrosis factor-α (TNF-α). Taken together, these results suggest that Mn may increase the release of IL-1β and TNF-α from microglia that in turn stimulates the expression of BACE1 gene and protein and consequently Aβ production; this novel molecular mechanism not only advances our understanding about the amyloidogenic effect of chronic Mn exposure reported for special human populations but also indicates Mn dyshomeostasis as a potential contributor to AD pathogenesis.

Alzheimer risk factors age and female sex induce cortical Aβ aggregation by raising extracellular zinc

Aging and female sex are the major risk factors for Alzheimer's disease and its associated brain amyloid-β (Aβ) neuropathology, but the mechanisms mediating these risk factors remain uncertain. Evidence indicates that Aβ aggregation by Zn²⁺ released from glutamatergic neurons contributes to amyloid neuropathology, so we tested whether aging and sex adversely influences this neurophysiology. Using acute hippocampal slices, we found that extracellular Zn²⁺-elevation induced by high K⁺ stimulation was significantly greater with older (65 weeks vs 10 weeks old) rats, and was exaggerated in females. This was driven by slower reuptake of extracellular Zn²⁺, which could be recapitulated by mitochondrial intoxication. Zn²⁺:Aβ aggregates were toxic to the slices, but Aβ alone was not. Accordingly, high K⁺ caused synthetic human Aβ added to the slices to form soluble oligomers as detected by bis-ANS, attaching to neurons and inducing toxicity, with older slices being more vulnerable. Age-dependent energy failure impairing Zn²⁺ reuptake, and a higher maximal capacity for Zn²⁺ release by females, could contribute to age and sex being major risk factors for Alzheimer's disease.

Adrenergic β receptor activation reduces amyloid β1-42-mediated intracellular Zn2+ toxicity in dentate granule cells followed by rescuing impairment of dentate gyrus LTP

Adrenergic β receptor activation prevents human soluble amyloid β (Aβ)-induced impairment of long-term potentiation (LTP) in slices. On the basis of the evidence that human Aβ_1-42-induced impairment of LTP is due to Aβ_1-42-mediated Zn²⁺ toxicity, we postulated that adrenergic β receptor activation reduces Aβ_1-42-mediated intracellular Zn²⁺ toxicity followed by rescuing Aβ_1-42 toxicity. To test the effect of adrenergic β receptor activation, LTP was recorded at perforant pathway-dentate granule cell synapses of anesthetized rats 60 min after Aβ_1-42 injection into the dentate granule cell layer. Human Aβ_1-42-induced impairment of LTP was rescued by co-injection of isoproterenol, an adrenergic β receptor agonist, but not by co-injection of phenylephrine, an adrenergic α₁ receptor agonist. Isoproterenol did not reduce Aβ_1-42 uptake into dentate granule cells, but reduced increase in intracellular Zn²⁺ in dentate granule cells induced by Aβ_1-42. In contrast, phenylephrine did not reduce both Aβ_1-42 uptake and increase in intracellular Zn²⁺ by Aβ_1-42. In the case of human Aβ_1-40 and rat Aβ_1-42, which do not increase intracellular Zn²⁺, human Aβ_1-40- and rat Aβ_1-42-induced impairments of LTP were not rescued by co-injection of isoproterenol. The present study indicates that adrenergic β receptor activation reduces Aβ_1-42-mediated increase in intracellular Zn²⁺ in dentate granule cells, resulting in rescuing Aβ_1-42-induced impairment of LTP. It is likely that noradrenergic neuron activation by stimulating the locus coeruleus is effective for rescuing Aβ_1-42-induced cognitive decline that is caused by intracellular Zn²⁺ dysregulation in the hippocampus.

Zinc-mediated Neurotransmission in Alzheimer's Disease: A Potential Role of the GPR39 in Dementia

With more people reaching an advanced age in modern society, there is a growing need for strategies to slow down age-related neuropathology and loss of cognitive functions, which are a hallmark of Alzheimer's disease. Neuroprotective drugs and candidate drug compounds target one or more processes involved in the neurodegenerative cascade, such as excitotoxicity, oxidative stress, misfolded protein aggregation and/or ion dyshomeostasis. A growing body of research shows that a G-protein coupled zinc (Zn²⁺) receptor (GPR39) can modulate the abovementioned processes.

Zn²⁺ itself has a diverse activity profile at the synapse, and by binding to numerous receptors, it plays an important role in neurotransmission. However, Zn²⁺ is also necessary for the formation of toxic oligomeric forms of amyloid beta, which underlie the pathology of Alzheimer’s disease. Furthermore, the binding of Zn²⁺ by amyloid beta causes a disruption of zincergic signaling, and recent studies point to GPR39 and its intracellular targets being affected by amyloid pathology.

In this review, we present neurobiological findings related to Zn²⁺ and GPR39, focusing on its signaling pathways, neural plasticity, interactions with other neurotransmission systems, as well as on the effects of pathophysiological changes observed in Alzheimer's disease on GPR39 function.

Direct targeting of the GPR39 might be a promising strategy for the pharmacotherapy of zincergic dyshomeostasis observed in Alzheimer’s disease. The information presented in this article will hopefully fuel further research into the role of GPR39 in neurodegeneration and help in identifying novel therapeutic targets for dementia.

Preferential Neurodegeneration in the Dentate Gyrus by Amyloid β1-42-Induced Intracellular Zn2+Dysregulation and Its Defense Strategy

On the basis of the evidence that rapid intracellular Zn²⁺ dysregulation by amyloid β_1-42 (Aβ_1-42) in the normal hippocampus transiently induces cognitive decline, here we report preferential neurodegeneration in the dentate gyrus by Aβ_1-42-induced intracellular Zn²⁺ dysregulation and its defense strategy. Neurodegeneration was preferentially observed in the dentate granule cell layer in the hippocampus after a single Aβ_1-42 injection into the lateral ventricle but not in the CA1 and CA3 pyramidal cell layers, while intracellular Zn²⁺ dysregulation was extensively observed in the hippocampus in addition to the dentate gyrus. Neurodegeneration in the dentate granule cell layer was rescued after co-injection of extracellular and intracellular Zn²⁺ chelators, i.e., CaEDTA and ZnAF-2DA, respectively. Aβ_1-42-induced cognitive impairment was also rescued by co-injection of CaEDTA and ZnAF-2DA. Pretreatment with dexamethasone, an inducer of metalothioneins, Zn²⁺-binding proteins rescued neurodegeneration in the dentate granule cell layer and cognitive impairment via blocking the intracellular Zn²⁺ dysregulation induced by Aβ_1-42. The present study indicates that intracellular Zn²⁺ dysregulation induced by Aβ_1-42 preferentially causes neurodegeneration in the dentate gyrus, resulting in hippocampus-dependent cognitive decline. It is likely that controlling intracellular Zn²⁺ dysregulation, which is induced by the rapid uptake of Zn-Aβ_1-42 complexes, is a defense strategy for Alzheimer's disease pathogenesis.

Age-Dependent Modification of Intracellular Zn2+ Buffering in the Hippocampus and Its Impact

The basal concentrations of extracellular Zn²⁺ and intracellular Zn²⁺, which are approximately 10 nM and 100 pM, respectively, in the brain, are markedly lower than those of extracellular Ca²⁺ (1.3 mM) and intracellular Ca²⁺ (100 nM), respectively, resulting in much less attention paid to Zn²⁺ than to Ca²⁺. However, intracellular Zn²⁺ dysregulation, which is closely linked with glutamate- and amyloid β-mediated extracellular Zn²⁺ influx, is more critical for cognitive decline and neurodegeneration than intracellular Ca²⁺ dysregulation. It is estimated that the age-dependent increase in the basal concentration of extracellular Zn²⁺ in the hippocampus plays a key role in cognitive decline and neurodegeneration. The characteristics of extracellular Zn²⁺ influx in the hippocampus may be modified age-dependently, probably followed by modification of intracellular Zn²⁺ buffering that is closely linked with age-related cognitive decline and neurodegeneration. Reduction of intracellular Zn²⁺-buffering capacity may be linked with the pathophysiology of progressive neurodegeneration such as Alzheimer's disease. This paper deals with age-dependent modification of intracellular Zn²⁺ buffering in the hippocampus and its impact. On the basis of the estimated impact, we propose a potential defense strategy against Zn²⁺-mediated neurodegeneration, i.e., metallothionein induction in the hippocampus.

Acetylcholinesterase inhibitors in Alzheimer's disease influence Zinc and Copper homeostasis

BACKGROUND

Alzheimer's disease (AD) is the most common age-related neurodegenerative disease. An altered homeostasis of Zinc (Zn) and Copper (Cu), as well as a dysregulated expression of Zn-regulatory proteins have been previously described in AD. Acetylcholinesterase inhibitors (AChEI) are commonly used as AD treatment to improve cognitive function, but their effect on Zn homeostasis is still unexplored.

OBJECTIVES

The aims of this study were to define the metal dyshomeostasis in AD patients, to investigate AChEI influence on Zn homeostasis and inflammation, and to analyze the relationship between cognitive impairment at two-year follow-up and metal concentrations, considering AChEI use.

METHODS AND RESULTS

84 Healthy Elderly (HE) and 95 AD patients were enrolled (62 AchEI user and 33 AchEI naïve). HE showed similar plasma Zn and Cu concentrations and Cu/Zn ratio in comparison to AChEI users, but significantly higher Zn level, as well as lower Cu amount and Cu/Zn ratio than AChEI naïve patients. Moreover, AChEI users had increased Zn plasma level, reduced Cu amount, Cu/Zn ratio, and IL1β concentration and lower Zip2 lymphocytic expression vs. naïve patients. A multiple linear regression analysis showed that the MMSE score decline after two-year follow-up was reduced by AChEI therapy and was positively associated with plasma Zn decrease over time.

CONCLUSION

Our data revealed that AChEI use may affect peripheral Zn and Cu homeostasis in AD patients, decrease Cu/Zn ratio demonstrating a general reduction of inflammatory status in patients under AChEI treatment. Finally, AChEI influence on circulating Zn could be implicated in the drug-related slowdown of cognitive decline.

Zinc Exacerbates Tau Pathology in a Tau Mouse Model

Hyperphosphorylated tau protein is a key pathology in Alzheimer's disease (AD), frontotemporal dementia, chronic traumatic encephalopathy, and Parkinson's disease. The essential trace element zinc exacerbates tauopathy in vitro as well as in a Drosophila model of AD. However, the interaction has never been assessed behaviorally or biochemically in mammals. Zinc supplementation is prevalent in society, finding use as a treatment for macular degeneration and cataracts, and is also taken as an immune system booster with high levels appearing in multivitamins marketed toward the elderly. Using a transgenic mouse model that contains the human gene for tau protein (P301L), we assessed the effects of excess chronic zinc supplementation on tau pathology. Behavioral tests included nest building, circadian rhythm, Morris Water Maze, fear conditioning, and open field. Biochemically, total tau and Ser396 phosphorylation were assessed using western blot. Number of tangles were assessed by Thioflavin-S and free zinc levels were assessed by Zinpyr-1. Tau mice demonstrated behavioral deficits compared to control mice. Zinc supplementation exacerbated tauopathic deficits in circadian rhythm, nesting behavior, and Morris Water Maze. Biochemically, zinc-supplemented tau mice showed increased phosphorylation at pSer396. Zinc supplementation in tau mice also increased tangle numbers in the hippocampus while decreasing free-zinc levels, demonstrating that tangles were sequestering zinc. These results show that zinc intensified the deficits in behavior and biochemistry caused by tau.

Extracellular Zn2+-independently attenuated LTP by human amyloid β1-40 and rat amyloid β1-42

Human amyloid-β_1-40 (Aβ_1-40) and rat Aβ_1-42 have lower affinity for extracellular Zn²⁺ than human Aβ_1-42. Here we report extracellular Zn²⁺-independent attenuation of dentate gyrus long-term potentiation (LTP) by human Aβ_1-40 and rat Aβ_1-42. On the basis of the data that dentate gyrus LTP is extracellular Zn²⁺-dependently attenuated after local injection of human Aβ_1-42 (25 pmol, 1 μl) into the dentate gyrus, which increases intracellular Zn²⁺ in the dentate gyrus, the toxicity of human Aβ_1-40 and rat Aβ_1-42 was compared in the in vivo system with human Aβ_1-42. Dentate gyrus LTP was attenuated after injection of human Aβ_1-40 and rat Aβ_1-42 (25 pmol, 1 μl) into the dentate gyrus, which did not increase intracellular Zn²⁺ in the dentate gyrus. The attenuated LTP was not rescued by co-injection of CaEDTA, an extracellular Zn²⁺ chelator. The present study suggests that human Aβ_1-40 and rat Aβ_1-42 affect cognitive activity via extracellular Zn²⁺-independent mechanism at low micromolar concentration.

In vivo synaptic activity-independent co-uptakes of amyloid β1-42 and Zn2+ into dentate granule cells in the normal brain

Neuronal amyloid β_1–42 (Aβ_1–42) accumulation is considered an upstream event in Alzheimer’s disease pathogenesis. Here we report the mechanism on synaptic activity-independent Aβ_1–42 uptake in vivo. When Aβ_1–42 uptake was compared in hippocampal slices after incubating with Aβ_1–42, In vitro Aβ_1–42 uptake was preferentially high in the dentate granule cell layer in the hippocampus. Because the rapid uptake of Aβ_1–42 with extracellular Zn²⁺ is essential for Aβ_1–42-induced cognitive decline in vivo, the uptake mechanism was tested in dentate granule cells in association with synaptic activity. In vivo rapid uptake of Aβ_1–42 was not modified in the dentate granule cell layer after co-injection of Aβ_1–42 and tetrodotoxin, a Na⁺ channel blocker, into the dentate gyrus. Both the rapid uptake of Aβ_1–42 and Zn²⁺ into the dentate granule cell layer was not modified after co-injection of CNQX, an AMPA receptor antagonist, which blocks extracellular Zn²⁺ influx, Both the rapid uptake of Aβ_1–42 and Zn²⁺ into the dentate granule cell layer was not also modified after either co-injection of chlorpromazine or genistein, an endocytic repressor. The present study suggests that Aβ_1–42 and Zn²⁺ are synaptic activity-independently co-taken up into dentate granule cells in the normal brain and the co-uptake is preferential in dentate granule cells in the hippocampus. We propose a hypothesis that Zn-Aβ_1–42 oligomers formed in the extracellular compartment are directly incorporated into neuronal plasma membranes and form Zn²⁺-permeable ion channels.

Difference in ability for extracellular Zn2+ influx between human and rat amyloid β1-42 and its significance

The accumulation of amyloid-β_1-42 (Aβ_1-42), a constituively-generated peptide, in the brain is considered an upstream event in pathogenesis of Alzheimer's disease. Aβ_1-42-induced pathophysiology has been extensively studied in experimental mice and rats. However, neurotoxicity of murine Aβ_1-42 is much less understood than human Aβ_1-42. Here we report difference in ability for extracellular Zn²⁺ influx into dentate granule cells of rats between human and rat Aβ_1-42 and its significance. Human Aβ_1-42 rapidly increased intracellular Zn²⁺, which was determined with intracellular ZnAF-2, in dentate granule cells, 5 min after injection of Aβ_1-42 (25 μM, 1 μl) into the dentate gyrus, while rat Aβ_1-42 did not increase intracellular Zn²⁺. In vivo perforant pathway LTP was attenuated under pre-perfusion with 5 nM human Aβ_1-42 in artificial cerebrospinal fluid (ACSF) containing 10 nM Zn²⁺, recapitulating the concentration of extracellular Zn²⁺, but not with 5 nM rat Aβ_1-42 in ACSF containing 10 nM Zn²⁺. The present study suggests that rat Aβ_1-42 has lower affinity for extracellular Zn²⁺ than human Aβ_1-42 and does not capture Zn²⁺ in the extracellular compartment, resulting in no significant effect on cognitive activity of rat even in the range of very low nanomolar concentrations of endogenous Aβ_1-42.

Metal Toxicity Links to Alzheimer's Disease and Neuroinflammation

As the median age of the population increases, the number of individuals with Alzheimer's disease (AD) and the associated socio-economic burden are predicted to worsen. While aging and inherent genetic predisposition play major roles in the onset of AD, lifestyle, physical fitness, medical condition, and social environment have emerged as relevant disease modifiers. These environmental risk factors can play a key role in accelerating or decelerating disease onset and progression. Among known environmental risk factors, chronic exposure to various metals has become more common among the public as the aggressive pace of anthropogenic activities releases excess amount of metals into the environment. As a result, we are exposed not only to essential metals, such as iron, copper, zinc and manganese, but also to toxic metals including lead, aluminum, and cadmium, which perturb metal homeostasis at the cellular and organismal levels. Herein, we review how these metals affect brain physiology and immunity, as well as their roles in the accumulation of toxic AD proteinaceous species (i.e., β-amyloid and tau). We also discuss studies that validate the disruption of immune-related pathways as an important mechanism of toxicity by which metals can contribute to AD. Our goal is to increase the awareness of metals as players in the onset and progression of AD.

Protective effects of voltage-gated calcium channel antagonists against zinc toxicity in SN56 neuroblastoma cholinergic cells

One of the pathological site effects in excitotoxic activation is Zn²⁺ overload to postsynaptic neurons. Such an effect is considered to be equivalent to the glutamate component of excitotoxicity. Excessive uptake of Zn²⁺ by active voltage-dependent transport systems in these neurons may lead to significant neurotoxicity. The aim of this study was to investigate whether and which antagonists of the voltage gated calcium channels (VGCC) might modify this Zn²⁺-induced neurotoxicity in neuronal cells. Our data demonstrates that depolarized SN56 neuronal cells may take up large amounts of Zn²⁺ and store these in cytoplasmic and mitochondrial sub-fractions. The mitochondrial Zn²⁺ excess suppressed pyruvate uptake and oxidation. Such suppression was caused by inhibition of pyruvate dehydrogenase complex, aconitase and NADP-isocitrate dehydrogenase activities, resulting in the yielding of acetyl-CoA and ATP shortages. Moreover, incoming Zn²⁺ increased both oxidized glutathione and malondialdehyde levels, known parameters of oxidative stress. In depolarized SN56 cells, nifedipine treatment (L-type VGCC antagonist) reduced Zn²⁺ uptake and oxidative stress. The treatment applied prevented the activities of PDHC, aconitase and NADP-IDH enzymes, and also yielded the maintenance of acetyl-CoA and ATP levels. Apart from suppression of oxidative stress, N- and P/Q-type VGCCs presented a similar, but weaker protective influence. In conclusion, our data shows that in the course of excitotoxity, impairment to calcium homeostasis is tightly linked with an excessive neuronal Zn²⁺ uptake. Hence, the VGCCs types L, N and P/Q share responsibility for neuronal Zn²⁺ overload followed by significant energy-dependent neurotoxicity. Moreover, Zn²⁺ affects the target tricarboxylic acid cycle enzymes, yields acetyl-CoA and energy deficits as well.

Monitoring metal-amyloid-β complexation by a FRET-based probe: design, detection, and inhibitor screening

A FRET-based method was developed for monitoring metal–amyloid-β complexation and identifying inhibitors against such interaction.

Aggregation of amyloidogenic peptides could cause the onset and progression of neurodegenerative diseases, such as Alzheimer's disease and Parkinson's disease. These amyloidogenic peptides can coordinate to metal ions, including Zn(ii), which can subsequently affect the peptides' aggregation and toxicity, leading to neurodegeneration. Unfortunately, the detection of metal–amyloidogenic peptide complexation has been very challenging. Herein, we report the development and utilization of a probe (A-1) capable of monitoring metal–amyloid-β (Aβ) complexation based on Förster resonance energy transfer (FRET). Our probe, A-1, is composed of Aβ_1–21 grafted with a pair of FRET donor and acceptor capable of providing a FRET signal upon Zn(ii) binding even at nanomolar concentrations. The FRET intensity of A-1 increases upon Zn(ii) binding and decreases when Zn(ii)-bound A-1 aggregates. Moreover, as the FRET intensity of Zn(ii)-added A-1 is drastically changed when their interaction is disrupted, A-1 can be used for screening a chemical library to determine effective inhibitors against metal–Aβ interaction. Eight natural products (out of 145 compounds; >80% inhibition) were identified as such inhibitors in vitro, and six of them could reduce Zn(ii)–Aβ-induced toxicity in living cells, suggesting structural moieties useful for inhibitor design. Overall, we demonstrate the design of a FRET-based probe for investigating metal–amyloidogenic peptide complexation as well as the feasibility of screening inhibitors against metal-bound amyloidogenic peptides, providing effective and efficient methods for understanding their pathology and finding therapeutic candidates against neurodegenerative disorders.

Is Vulnerability of the Dentate Gyrus to Aging and Amyloid-β1-42 Neurotoxicity Linked with Modified Extracellular Zn2+ Dynamics?

The basal levels of extracellular Zn²⁺ are in the range of low nanomolar concentrations in the hippocampus and perhaps increase age-dependently. Extracellular Zn²⁺ dynamics is critical for cognitive activity and excess influx of extracellular Zn²⁺ into hippocampal neurons is a known cause of cognitive decline. The dentate gyrus is vulnerable to aging in the hippocampus and affected in the early stage of Alzheimer's disease (AD). The reasons remain unclear. Neurogenesis-related apoptosis may induce non-specific neuronal depolarization by efflux of intracellular K⁺ in the dentate gyrus and be markedly increased along with aging. Extracellular Zn²⁺ influx into dentate granule cells via high K⁺-induced perforant pathway excitation leads to cognitive decline. Modified extracellular Zn²⁺ dynamics in the dentate gyrus of aged rats is linked with vulnerability to cognitive decline. Amyloid-β_1-42 (Aβ_1-42) is a causative candidate for AD pathogenesis. When Aβ_1-42 concentration reaches picomolar in the extracellular compartment in the dentate gyrus, Zn-Aβ_1-42 is formed in the extracellular compartment and rapidly taken up into dentate granule cells, followed by Aβ_1-42-induced cognitive decline that is due to Zn²⁺ released from Aβ_1-42, suggesting that dentate granule cells are sensitive to extracellular Zn²⁺-dependent Aβ_1-42 toxicity. This paper deals with proposed vulnerability of the dentate gyrus to aging and Aβ_1-42 neurotoxicity.

How cellular Zn2+ signaling drives physiological functions

Zinc is an essential micronutrient affecting many aspects of human health. Cellular Zn²⁺ homeostasis is critical for cell function and survival. Zn²⁺, acting as a first or second messenger, triggers signaling pathways that mediate the physiological roles of Zn²⁺. Transient changes in Zn²⁺ concentrations within the cell or in the extracellular region occur following its release from Zn²⁺ binding metallothioneins, its transport across membranes by the ZnT or ZIP transporters, or release of vesicular Zn²⁺. These transients activate a distinct Zn²⁺ sensing receptor, ZnR/GPR39, or modulate numerous proteins and signaling pathways. Importantly, Zn²⁺ signaling regulates cellular physiological functions such as: proliferation, differentiation, ion transport and secretion. Indeed, novel therapeutic approaches aimed to maintain Zn²⁺ homeostasis and signaling are evolving. This review focuses on recent findings describing roles of Zn²⁺ and its transporters in regulating physiological or pathological processes.

Adrenergic β receptor activation in the basolateral amygdala, which is intracellular Zn2+-dependent, rescues amyloid β1-42-induced attenuation of dentate gyrus LTP

On the basis of the evidence that the basolateral amygdala (BLA) modulates hippocampal memory processes via synaptic plasticity, here we report that adrenergic β receptor activation in the BLA rescues amyloid β_1-42 (Aβ_1-42)-induced attenuation of long-term potentiation (LTP) at perforant pathway-dentate granule cell (DGC) synapses. When 500 μM isoproterenol (2 μl), an adrenergic β receptor agonist, was injected into the BLA 20 min before LTP induction, LTP was enhanced. Isoproterenol-mediated enhancement of LTP was blocked by co-injection with 100 μM ZnAF-2DA, an intracellular Zn²⁺ chelator, suggesting that intracellular Zn²⁺ is required for the intracellular signaling cascade after adrenergic β receptor activation in the BLA. Aβ_1-42-induced attenuation of LTP, which was induced by Aβ_1-42 injection into the dentate gyrus 60 min before LTP induction, was rescued by isoproterenol injection into the BLA 20 min before LTP induction, but not by 500 μM phenylephrine (2 μl), an adrenergic α₁ receptor agonist, injection into the BLA, which did not enhance LTP unlike the case of isoproterenol injection. Interestingly, Aβ_1-42-induced attenuation of LTP was also rescued by 100 μM isoproterenol injection into the BLA 20 min before LTP induction, which did not enhance LTP. The present study demonstrates that adrenergic β receptor activation in the BLA, which is linked with intracellular Zn²⁺ signaling, rescues Aβ_1-42-induced attenuation of dentate gyrus LTP. It is likely that adrenergic β receptor activation in the BLA is a strategy for rescuing Aβ_1-42-induced cognitive decline that is associated with hippocampal synaptic plasticity.

The Extracellular Zn2+ Concentration Surrounding Excited Neurons Is High Enough to Bind Amyloid-β Revealed by a Nanowire Transistor

The Zn²⁺ stored in the secretory vesicles of glutamatergic neurons is coreleased with glutamate upon stimulation, resulting in the elevation of extracellular Zn²⁺ concentration (CZn2+ex). This elevation of CZn2+ex regulates the neurotransmission and facilitates the fibrilization of amyloid-β (Aβ). However, the exact CZn2+ex surrounding neurons under (patho)physiological conditions is not clear and the connection between CZn2+ex and the Aβ fibrilization remains obscure. Here, a silicon nanowire field-effect transistor (SiNW-FET) with the Zn²⁺ -sensitive fluorophore, FluoZin-3 (FZ-3), to quantify the CZn2+ex in real time is modified. This FZ-3/SiNW-FET device has a dissociation constant of ≈12 × 10^-9 m against Zn²⁺ . By placing a coverslip seeded with cultured embryonic cortical neurons atop an FZ-3/SiNW-FET, the CZn2+ex elevated to ≈110 × 10^-9 m upon stimulation with α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA). Blockers against the AMPA receptor or exocytosis greatly suppress this elevation, indicating that the Zn²⁺ stored in the synaptic vesicles is the major source responsible for this elevation of CZn2+ex. In addition, a SiNW-FET modified with Aβ could bind Zn²⁺ with a dissociation constant of ≈633 × 10^-9 m and respond to the Zn²⁺ released from AMPA-stimulated neurons. Therefore, the CZn2+ex can reach a level high enough to bind Aβ and the Zn²⁺ homeostasis can be a therapeutic strategy to prevent neurodegeneration.

Zinc ion rapidly induces toxic, off-pathway amyloid-β oligomers distinct from amyloid-β derived diffusible ligands in Alzheimer's disease

Alzheimer’s disease (AD) is the most prevalent neurodegenerative disease in the elderly. Zinc (Zn) ion interacts with the pathogenic hallmark, amyloid-β (Aβ), and is enriched in senile plaques in brain of AD patients. To understand Zn-chelated Aβ (ZnAβ) species, here we systematically characterized ZnAβ aggregates by incubating equimolar Aβ with Zn. We found ZnAβ40 and ZnAβ42 both form spherical oligomers with a diameter of ~12–14 nm composed of reduced β-sheet content. Oligomer assembly examined by analytical ultracentrifugation, hydrophobic exposure by BisANS spectra, and immunoreactivity of ZnAβ and Aβ derived diffusible ligands (ADDLs) are distinct. The site-specific ¹³C labeled solid-state NMR spectra showed that ZnAβ40 adopts β-sheet structure as in Aβ40 fibrils. Interestingly, removal of Zn by EDTA rapidly shifted the equilibrium back to fibrillization pathway with a faster kinetics. Moreover, ZnAβ oligomers have stronger toxicity than ADDLs by cell viability and cytotoxicity assays. The ex vivo study showed that ZnAβ oligomers potently inhibited hippocampal LTP in the wild-type C57BL/6JNarl mice. Finally, we demonstrated that ZnAβ oligomers stimulate hippocampal microglia activation in an acute Aβ-injected model. Overall, our study demonstrates that ZnAβ rapidly form toxic and distinct off-pathway oligomers. The finding provides a potential target for AD therapeutic development.

Novel Defense by Metallothionein Induction Against Cognitive Decline: From Amyloid β1-42-Induced Excess Zn2+ to Functional Zn2+ Deficiency

The role of metallothioneins (MTs) in cognitive decline associated with intracellular Zn²⁺ dysregulation remains unclear. Here, we report that hippocampal MT induction defends cognitive decline, which was induced by amyloid β_1-42 (Aβ_1-42)-mediated excess Zn²⁺ and functional Zn²⁺ deficiency. Excess increase in intracellular Zn²⁺, which was induced by local injection of Aβ_1-42 into the dentate granule cell layer, attenuated in vivo perforant pathway LTP, while the attenuation was rescued by preinjection of MT inducers into the same region. Intraperitoneal injection of dexamethasone, which increased hippocampal MT proteins and blocked Aβ_1-42-mediated Zn²⁺ uptake, but not Aβ_1-42 uptake, into dentate granule cells, also rescued Aβ_1-42-induced impairment of memory via attenuated LTP. The present study indicates that hippocampal MT induction blocks rapid excess increase in intracellular Zn²⁺ in dentate granule cells, which originates in Zn²⁺ released from Aβ_1-42, followed by rescuing Aβ_1-42-induced cognitive decline. Furthermore, LTP was vulnerable to Aβ_1-42 in the aged dentate gyrus, consistent with enhanced Aβ_1-42-mediated Zn²⁺ uptake into aged dentate granule cells, suggesting that Aβ_1-42-induced cognitive decline, which is caused by excess intracellular Zn²⁺, can more frequently occur along with aging. On the other hand, attenuated LTP under functional Zn²⁺ deficiency in dentate granule cells was also rescued by MT induction. Hippocampal MT induction may rescue cognitive decline under lack of cellular transient changes in functional Zn²⁺ concentration, while its induction is an attractive defense strategy against Aβ_1-42-induced cognitive decline.

The Zinc Sensing Receptor, ZnR/GPR39, in Health and Disease

A distinct G-protein coupled receptor that senses changes in extracellular Zn²⁺, ZnR/GPR39, was found in cells from tissues in which Zn²⁺ plays a physiological role. Most prominently, ZnR/GPR39 activity was described in prostate cancer, skin keratinocytes, and colon epithelial cells, where zinc is essential for cell growth, wound closure, and barrier formation. ZnR/GPR39 activity was also described in neurons that are postsynaptic to vesicular Zn²⁺ release. Activation of ZnR/GPR39 triggers Gαq-dependent signaling and subsequent cellular pathways associated with cell growth and survival. Furthermore, ZnR/GPR39 was shown to regulate the activity of ion transport mechanisms that are essential for the physiological function of epithelial and neuronal cells. Thus, ZnR/GPR39 provides a unique target for therapeutically modifying the actions of zinc in a specific and selective manner.