Current understanding of metal ions in the pathogenesis of Alzheimer’s disease

Erroneous deposition of iron, copper, zinc, or elevation of calcium in different brain regions can promote Aβ overproduction, tau hyperphosphorylation and their aggregation/accumulation. The abnormality of the related metal transporters is a key factor inducing the incorrect distribution of metal ions in the brain.

Iron is an essential nutrient but high levels of iron are toxic mainly due to the catalytic generation of destructive hydroxyl radicals. Iron is transferred into the neurons via transferrin (Tf), divalent metal transporter 1 (DMT1), and lactoferrin (Lf), while it is transferred out of neurons by ferroportin (FPN). As a multifunctional iron-binding globular glycoprotein, Lf has high affinity for Fe³⁺. Iron can be transported to tissues and organs with blood circulation by bounding to Tf. Therefore, the dysregulation of DMT1, Lf, Tf and FPN not only affects the distribution but also accumulation of iron in the brain.

An increased iron level is correlated with the amount of Aβ plaques and tau pathologies, and iron increases Aβ toxicity by impeding the ordered aggregation of Aβ [31]. The binding of iron to Aβ or to tau induces Aβ aggregation and tau hyperphosphorylation, leading to formation of senile plaques and NFTs, and as well as the increased neurotoxicity of Aβ and tau [32, 33]. Iron increases APP translation and Aβ42 production/accumulation in the retina with no observed change in secretase levels or cleavage activities [34]. Fe²⁺ ions bound to the N-terminal region of Aβ, which can modify Aβ and generate oxygen radicals to induce damages on the membrane surface [35]. In a human neuroblastoma cell line (SHSY5Y), elevated Fe³⁺ can bind to APP mRNA and promote APP translation, and the Fe³⁺ can activate β-secretase and thus induces Aβ production [36]. It was also shown that huperzine A could attenuate Aβ and tau pathologies in APP/PS1 mice, while feeding the animals with a high iron diet abolished the protective effect of the huperzine A [37]. The increased iron inhibits α-secretase through down-regulating furin [38]. These studies together suggest a detrimental role of iron in AD. Therefore, maintaining iron homeostasis can inhibit APP translation and Aβ overproduction by activating α-secretase [39]. On the other hand, it was reported that iron could promote APP translation to attenuate the toxicities of lead (Pb) [40], suggesting a protective role of iron in APP metabolism. In addition to the effects of iron on APP as mentioned above, APP could reversely affect iron, i.e., to stabilize the iron exporter FPN1, and the heavy subunit of the iron-storage protein, ferritin; by which the elevated APP can regulate metabolisms of brain and peripheral iron to attenuate brain iron elevation during aging [41].

Fe²⁺ can promote tau phosphorylation by activating cyclin-dependent kinases-5 (CDK5) and glycogen synthase kinase-3β (GSK-3β), while the iron chelator deferoxamine can reduce the degree of iron-induced tau hyperphosphorylation in the mouse brain [42]. Iron-mediated tau hyperphosphorylation may be caused by the activation of extracellular regulated protein kinase1/2 (ERK1/2) and mitogen-activated protein kinase (MAPK) pathway [43, 44]. On the other hand, it was also observed that treatment of the cultured hippocampal neuron with Fe³⁺ decreases tau phosphorylation, which correlates with a decreased activity of CDK5 [45]. The differences of the experimental systems used in the studies may at least partially contribute to the observed discrepancy. A recent stduy has also shown that tau protein is reqiured to mediate iron export, which can prevent ferroptotic damage after ischemic stroke [46].

In AD patients, an increased copper depositing has been detected and it can interact with Aβ and tau proteins to promote the pathological aggregation and deposition of Aβ and tau proteins.

Copper ions can bind to Aβ and induce oligomer formation, and eventually lead to Aβ aggregation. Cu²⁺-stabilized Aβ1–42 interacts with the lipid bilayer and thus increases the permeability of membrane [47]. However, nitration of Aβ1–42 can dramatically inhibit the Cu²⁺-induced Aβ1–42 oligomerization and neurotoxicity in vitro [48]. Furthermore, Cu²⁺ significantly affects the amyloid cascade, by which it interacts with APP through a Cu²⁺-binding domain [49]. Copper chelator can inhibit the activity of β-secretase [50]. In a preclinical AD model, small amounts of copper in drinking water can induce Aβ accumulation and significantly retard the learning ability [51]. Interestingly, it was also reported that the complex formed by Aβ, monoamine neurotransmitters and Cu²⁺ can mediate Cu²⁺ translocation, by which it reduces Aβ toxicity [52].

Excess copper can promote tau hyperphosphorylation and the copper chelating agents attenuate tau phosphorylation in human neuroblastoma cells. Suppressed levels of copper in plasma and brain resulted in a marked attenuation of tau phosphorylation in transgenic mouse expressing human tau [53]. The copper-responsive transcription factor, Sp1, can bind to the promoter of tau gene, which also links copper to the transcription of tau [54].

Regarding the alterations or role of zinc in AD, large numbers of controversial results have been reported. Some demonstrated that zinc concentration in senile plaques and neuropils of AD increased 2~3-fold compared with the controls, while others showed a decreased zinc concentration in hippocampus or amygdala of AD patients. The reasons for the discrepancy are not very clear, different experimental systems used for the study could be at least one of them. These evidences strongly suggest that zinc is required for normal neural functions while excessive or misdirected zinc is harmful to the neural cells.

By analyzed 48 independent plaques in the hippocampus, James et al. demonstrated that the areal concentrations (ng cm^− 2) of iron, copper and zinc were significantly higher in plaques than in the surrounding neutrophils in APP/PS1 mice. Interestingly, only the level of zinc in the plaques remained elevated after adjustment of tissue density [55]. In addition to increasing protein deposition, direct binding of zinc to Aβ also reduces the solubility of Aβ and thus exacerbates damage to neurons by increasing Aβ aggregation [56]. Immediately after addition of Aβ1–40, zinc ions can block the cation channel on the surface of the membrane, but subsequent Aβ fiber formation fragments the membrane that cannot be stopped by zinc [57]. Zinc ions are associated with APP processing, and interference in the hydrolysis of the APP by zinc results in abnormal cleavage of APP and deposition of Aβ, possibly owing to the decreased α-secretase cleavage of the zinc-bound APP or the increased Aβ production from APP in the presence of zinc. Additionally, a disintegrin and metalloprotease 10 (ADAM10), whose function can be activated by zinc, can also catalyzes the non-amyloidogenic α-secretase cleavage of the APP [58, 59].

Excessive of zinc also induces tau aggregation and formation of the NFTs. Zinc can directly bind to tau monomers and stimulate the phosphorylation of tau proteins by activate GSK-3β, ERK1/2, and c-Jun N-terminal kinase (JNK) [60, 61]. Zinc also induces protein phosphatase 2A (PP2A) inactivation and tau hyperphosphorylation through Src-dependent pathway, which ultimately leads to a net increase in phosphorylated tau that may exacerbate AD-like tau pathologies. In genetically modified mice with high expression of human tau protein, zinc chelating agent inhibited the activation of Src, thereby reduced the inhibitory phosphorylation of PP2A at Tyr307 with attenuation of tau phosphorylation and aggregation [62, 63].

Intracellular Ca²⁺ dysregulation is an early manifestation of AD. Studies have shown that Ca²⁺ concentrations near Aβ deposits are significantly increased, and the increase of Ca²⁺ likely plays a key role in the cognitive deficits associated with AD [64, 65]. Ca²⁺ elevation can promote Aβ production and the cellular toxicity of Aβ, while the elevation of Aβ in turn contributes to the increased intracellular Ca²⁺. Therefore, a positive feedback loop may exist between Ca²⁺ and Aβ, which may exacerbate the neurodegeneration and cognitive deficits in AD patients [66].

Aβ can promote the opening of voltage-dependent calcium channels, which in turn increases the intracellular concentration of Ca²⁺ [67]. An increased intracellular Ca²⁺ concentration can promote the overexpression of L-type calcium channel subtype (Cav1.2) in the hippocampal cell membranes in models of AD, which further promote the influx of Ca²⁺ [68]. Majority reports suggest that the intracellular calcium overload can promote Aβ production and aggregation. For instance, the elevated cytoplasmic Ca²⁺ by inhibiting of sarcoplasmic/endoplasmic reticulum (S/ER) calcium ATPase (SERCA) or liberating Ca²⁺ release via the ryanodine receptor (RyR) can robustly activate β-secretase and thus increase Aβ production [69]. Therefore, inhibition of Ca²⁺ influx can reduce the neurotoxicity of Aβ oligomers, levels of insoluble Aβ1–40 and Aβ1–42 in the hippocampus of AD transgenic mice [70]. Studies also demonstrated that Ca²⁺ ions actively participate in Aβ-promoted membrane damage, i.e., Ca²⁺ ions inhibit Aβ-mediated membrane poration but enhance membrane fragmentation by lipid loss due to fiber growth on the membrane surface [71].

Given that many kinases can be activated by Ca²⁺, dysregulation of Ca²⁺ homeostasis, such as seen during ER stress, can increase tau phosphorylation [14, 72, 73]. Conversely, intracellular tau accumulating can also induce ER stress and cause Ca²⁺ overload, the latter induces dephosphorylation of Ca²⁺-calmodulin-dependent protein kinase IV (CaMKIV) and cAMP-responsive element binding protein (CREB) by activating calcineurin [5]; the Ca²⁺ overload can also activate JAK2-STAT1 signaling, and the upregulated STAT1 directly binds to the specific GAS elements of N-methyl-D-aspartate receptors (NMDARs) and thus inhibits the transcription of NMDARs [14]; the cleaved tau-induced STAT1 elevation also activates BACE1 to promote Aβ production [74]; all of which reveal new mechanisms underlying tau-induced synapse impairments and cognitive deficits.

Oxidative stress is a state of cellular damage caused by free radicals because of the insufficient functioning of the antioxidant system [75]. Under physiological conditions, the peroxidation-antioxidant system is in equilibrium. When this balance is disturbed, the body responds to oxidative stress by producing free radicals, including reactive oxygen species (ROS), such as •O₂, H₂O₂, and •OH, and reactive nitrogen species (RNS), such as •ONOO and •NO.

Neurons in the brain are extremely sensitive to free radicals. DNA damage, protein oxidation, lipid peroxidation, and production of advanced glycosylation end products (AGEs) in the AD brains are usually related to free radical attacks and metal imbalances [76, 77]. Oxidative stress has been wildly detected in AD patients and the animal models, and the imbalance of metal ions, such as iron, copper, zinc, and calcium, can cause oxidative stress, tau hyperphosphorylation, Aβ deposition, cross-linking of nerve fibers and nerve cell damages, which are closely related to the pathogenesis of AD.

The reversible transition of iron from Fe²⁺ to Fe³⁺ catalyzes the electron transfer reaction. However, excessive redox reactions can disrupt the transition. The iron content in the brain is tightly regulated. When the concentration of iron in the brain is too high, oxidative stress occurs through the Haber-Weiss and Fenton reaction that directly generates ROS. The overproduction of ROS subsequently causes lipid peroxidation of the neuronal membrane, DNA damage, and neuron impairment in the brain [78, 79].

Oxidative stress has been observed in mild cognitive impairment with a correlated elevation of iron in glial cells and cerebellum. The glia act as immune cells in the brain and participate in maintaining homeostasis and multiple brain functions. Though the role of cerebellum in AD has not been extensively studied, the increased iron and free radical generation had been detected in both glial cells and cerebellum in the early stage of AD [80], and in cerebellum, loss of Purkinje cells and synaptic alterations in the mossy fibers, granule cell dendrites, parallel fibers and Purkinje cell dendrites with substantial loss of dendritic spines had been shown. High levels of iron were precipitated with Aβ plaques, and the iron can promote toxic Aβ oligomer formation with production of ROS, leading to mitochondrial dysfunction and cell death [81].

Copper is a cofactor for Cu/Zn-superoxide dismutase and plays a key role in scavenging ROS. An imbalanced homeostasis of copper can induce oxidative stress and cause ROS overproduction by Fenton and Haber-Weiss reactions [82]. Interferes with the bidirectional and copper-dependent communication between neurons and astrocytes could eventually lead to various brain diseases.

APP and Aβ have copper binding sites, and their interaction with copper can produce ROS, including •OH, which can cause oxidative damage to Aβ itself and the surrounding proteins and lipids. Moreover, elevated copper reduces the level of GSH, a substrate for enzymes that remove ROS, and enhances the production and cytotoxic effects of ROS [83]. The expression of copper-dependent enzymes, such as superoxide dismutase 1 (SOD1) and antioxidant protein 1 (ATOX1), was markedly reduced in multiple microarray studies of AD patients, reinforcing the lack of copper [84]. In addition to the antioxidant role, SOD1 also has anti-inflammatory functions. Therefore, reduced expression of SOD1 can exacerbate ROS accumulation and the chronic neuroinflammation [85].

Although zinc is recognized to be a redox-inert metal ion, there are still evidences showing an active involvement of zinc in redox metabolism. In the brain, metallothionein 3 (MT3) is one of the major players in maintain the homeostasis of zinc, which is chelated as metal-thiolate clusters. MT3 regulates zinc in a copper-related manner, and the zinc in MT3 can exchange with copper in the Aβ-Cu complex and inhibit the oxidative damage caused by the Aβ-Cu complex, Zn²⁺ can also protect sulfhydryl groups from oxidation in the cells [86]. Therefore, downregulation of zinc or MT3 as observed in the AD neurons can contribute to the oxidative damages.

Under pathological conditions, excessive zinc is released from presynaptic neurons and astrocytes, which causes NADPH-oxidase activation and ROS production in neurons, and microglial activation, and eventually exacerbates neuronal death [87]. Furthermore, Zn²⁺ is an inhibitor of many enzymes, therefore, excessive zinc can cause multiple metabolic abnormalities by deregulating related enzyme activities. The mitochondrial respiratory chain is particularly sensitive to Zn²⁺, and elevated Zn²⁺ in mitochondria promotes ROS production. Excessive zinc, which can be caused by an increased release from metalloproteins, can induce Aβ production and deposition, and thereby triggering a cascade reaction. Therefore, the oxidative and nitrosative stresses can cause an uncontrolled zinc elevation and Aβ deposition, while zinc and Aβ accumulation can conversely lead to oxidative stress and cytotoxicity, forming a vicious cycle.

Calcium-mediated oxidative stress is largely inseparable from iron. Recent studies show that iron-mediated calcium signaling leads to the downstream activation of a kinase cascade involved in synaptic plasticity. Under physiological conditions, iron-mediated production of ROS promotes a normal calcium-dependent signaling pathway, while excessive iron promotes oxidative stress, leading to an unrestricted increase in calcium signaling that impairs mitochondrial function and other downstream targets [88, 89]. Activation of microglia and astrocytes in the brain of AD patients can induce ROS and RNS production with dose-dependent increases of glutamate release and calcium entry, leading to neuron death.

Autophagy, a highly conservative proteolysis system driven by the lysosome, can remove dysfunctional organelles and misfolded proteins and thus plays an important role in coping with various adverse environmental stresses and maintain cell vitality [90, 91]. An insufficient autophagy can cause intracellular protein accumulation and thus impair the cellular functions, while excessive autophagy can also destroy the cell microenvironment and cause cell damages. Though both insufficient autophagy and excessive autophagy have been detected in the AD brains, an insufficient autophagy may play a more important role in the accumulation of the misfolded proteins during AD [92, 93], such as tau and Aβ accumulation. Interestingly, a recent study showed that intracellular accumulation of tau proteins could in turn aggravate autophagy deficits by disrupting IST1-regulated ESCRT-III complex formation and autophagosome-autolysosome fusion, which formed a vicious cycle between tau accumulation and autophagy deficit in AD and the related tauopathie [13]. Therefore, autophagy should be a promising therapeutic target for AD, and metal ions change the autophagy processes.

The activity of ubiquitin proteasome system (UPS) is decreased in the AD brains, and the UPS activity is closely intertwined with autophagy functions. Moreover, the copper, Fe and zinc ions can bind to UPS components, e.g., the proteasome and ubiquitin can and thus modify their activity [94].

The increased iron accumulation is commonly seen in the brains of AD patients. Uptake of iron components and lysosomal accumulation of iron-related lipofuscin over time induces autophagy deficits. The iron-rich autolysosomes can produce more ROS that can cause damage to autolysosomal membranes [95]. Ferritin is an intracellular protein that stores iron; therefore, autophagy deficit-related ferritin elevation plays an important role in accumulating iron and mediating the iron toxicities in the AD [96, 97]. Additionally, an increasing iron in the cytoplasm will saturate ferritin and thus prevent ferritin from entering the autolysosome which exacerbates iron accumulation. On the other hand, a rapidly increased iron can activate AMP-activated protein kinase (AMPK)/mammalian target of rapamycin (mTOR) and promote lysosomal degradation of Aβ, which suggests a protective role of a rapidly increased iron in Aβ removal. It is reported that iron exposure in the neonatal period has long-lasting harmful effects on the UPS which can induce memory impairment [98]. The proteasome inhibitor lactacystin causes a marked increase in labile iron and the aggregation of ubiquitin-conjugated proteins prior to cell injury and death in vitro [99].

In vitro experiments have shown that excessive addition of copper to the cell culture medium can induce autophagy, and the autophagy here is recognized as a cellular defense mechanism that can reduce cell death [100]. The elevation of copper has been observed in lysosomes, the organelles involved in autophagy [101]. In the redox cycle of metal ions, elevation of copper promotes autophagy and apoptosis of glioma cells by reactive oxygen species and JNK activation [102, 103]. The activity of proteasome can be inhibited by copper at mM level and copper-chelator complexes can inhibit it [104]. And it is reported Cu²⁺ ions inhibited interaction of nerve growth factor (NGF)1–14 and ubiquitin [105] .

Zinc plays a role in the regulation of both basal and stress-induced autophagy. When the concentration of Zn²⁺ is high, the level of autophagy will be increased [106]. Zinc promotes autophagy with the mechanisms involving ERK1/2 activation, which can phosphorylate beclin-1 and thus facilitate the beclin1-PI3K complex formation during autophagic process; zinc can also promote degradation of mTOR, a negative regulator of autophagy, and thus lead to cell autophagy [107]. It was also reported that zinc oxide nanoparticles-induced autophagy may be associated with oxidative stress and the inflammatory process in primary astrocyte cultures [108]. Addition of increasing Zn²⁺ may interact with specific regions of ubiquitin and promote protein-protein contacts [109]. And zinc caused UPS impairment resulting in α-synuclein aggregation subsequently leading to dopaminergic neurodegeneration [110]. In PC12 cell, zinc-induced autophagosome formation facilitates cell survival [111]. These data suggest that the increased zinc in the AD brain may be potentially protective in removal of stress-induced abnormal proteins/organelles by autophagy, although the pathological role of the increased zinc in AD-like tau and Aβ pathologies and protective role of zinc chelator were also reported [112].

In physiological conditions, the extracellular calcium concentration is thousands-fold higher than that of the intracellular calcium. In the face of metabolic pressures, such as hypoxia and nutrient deprivation, transient receptor potential mucolipin 1 (TRPML1) mediates outflow and increases the concentration of Ca²⁺ in the cytoplasm [113]. With increase of intracellular Ca²⁺, autophagic vesicles is accumulated and autophagy is stimulated, the mechanisms involve Ca²⁺/calmodulin-dependent protein kinase kinase β (CaMKKβ)/AMPK activation and mTORC1 inhibition [114, 115]. Application of BAPTA-AM, a chelator of cytoplasmic Ca²⁺, can inhibit the accumulation of autophagic vesicles, which confirms the role of Ca²⁺ in stimulating the autophagy [116].

Synapse impairment is an early pathological feature of AD, which is positively correlated with cognitive dysfunction in AD [117]. The homeostasis of metal ions is critical for maintaining synaptic function.

Iron is closely related to synaptic functions. It has been reported that the dietary iron is required for synapse formation and iron deficiency reduces synapse formation in the drosophila clock circuit. Iron mediates NMDAR-dependent stimulation of calcium-induced pathways and hippocampal synaptic plasticity. Non-transferrin-bound iron (NTBI) contributes to the “oxidative tone” which is important for both basal synaptic transmission and long-term potentiation [118]. In the presence of elevated iron, increased synaptic activity can cause iron overload which concurs to the cytotoxic effects, as seen in early stage of AD patients [119]. On the other hand, long-term excessive iron exposure induces learning and memory deficits in mice with widespread molecular alterations including synapse- and memory-associated proteins [120].

Copper is an essential component of neuronal transmission. Upon synaptic depolarization, copper release at micromolar concentrations from copper-containing vesicles into the synaptic cleft modulates synaptic functions [121, 122]. Once released onto the synaptic cleft, copper can act post-synaptically as a high-affinity blocker of NMDAR and glutamatergic ɑ-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR), thereby producing a marked inhibition of glutamate-mediated neurotransmission [123]. Copper can also control, directly or indirectly, the activity of γ-aminobutyric acid (GABA) and P2X receptors and thus affect neurotransmission and neuronal excitability. Copper can also modulate synaptic vesicles trafficking and protein interactions, while the neurotransmission can in turn affect copper trafficking and delivery in neuronal cells.

As mentioned above, copper can promote Aβ production and tau hyperphosphorylation. In turn, the aggregating vulnerable proteins, such as APP, prion protein, α-synuclein, show copper-binding domains. Therefore, these proteins may act as copper buffers at synapses and participate in the interplay between copper and the neurotransmitters receptors. Mutations of copper pump ATP7A are responsible for disorders with a prominent neurodegenerative component, suggesting that ATP7A may play a pivotal role in the release of copper at synapses. The involvement of copper in synaptic transmission may open new insights for therapeutic interventions of AD.

Among different types of metal ions, zinc is the most extensively studied and recognized one to be involved in regulating synaptic functions. Zinc ion is a potent allosteric inhibitor of NMDA receptors. By its mobile form accounting for 10~20% of total zinc, the mZn presents in the excitatory presynaptic vesicles with the concentration > 100 μM, while the inhibitory neurons have been recognized as no-zinc cells. Excessive intracellular accumulation of Zn²⁺ induces toxicities through multiple mechanisms, such as increasing ROS production through damaging mitochondrial functions, as often seen after the acute phase of transient ischemia or epilepsy.

Among the three major routes of divalent cation entry, i.e., the NMDA channels, the voltage-sensitive Ca²⁺ channels (VSCCs), and the Ca²⁺-permeable AMPA/kainate (Ca-A/K) channels, the Ca-A/K channels show the highest permeability to exogenously applied Zn²⁺. In a model of trans-synaptic Zn²⁺ movement occurring under conditions of oxygen and glucose deprivation (OGD), it was observed that blockage of NMDAR or VSCC by using MK-801 or Gd³⁺ increased Zn²⁺ accumulation, whereas the blocking Ca-A/K channels by using 1-naphthyl acetyl spermine (NAS) or application of the extracellular Zn²⁺ chelator (Ca²⁺ EDTA) decreased Zn²⁺ accumulation in the pyramidal neurons of hippocampal CA1 and CA3 subsets. Simultaneously, significant neuron death was shown in the presence of MK-801 and Gd³⁺, whereas the injury was attenuated by NAS or Ca²⁺ EDTA. These data suggest a pivotal role of Ca-A/K channel, a route for rapid and direct Zn²⁺ entry, in zinc accumulation and the neuronal toxicities. Similar to that seen in OGD, downregulation of GluR2 in the dendrites of pyramidal neurons progressively increases the number of Ca-A/K channels with an increased Zn²⁺ inflow and an increased neuron loss during aging and AD [124].

Intriguingly, the Ca-A/K channel is highly expressed in hippocampal pyramidal neurons, basal forebrain acetyl-cholinergic neurons, and the forebrain somatostatin neurons, while these neurons are prominently injured in AD patients. This evidence imply that proper zinc entry must be protective during AD. The synaptic activity could improve neuronal resistance to apoptosis via synaptically released zinc, whereas lowering Zn²⁺ concentration by using intracellular chelators can cause apoptotic neuronal death [125]. In the central nervous system, the brain-derived neurotrophic factor (BDNF) can modulate critically the neuronal activities, covering from neuronal differentiation and survival to synaptogenesis and activity-dependent synaptic plasticity [126, 127]. The production and maturation of BDNF in the brain is regulated by Cu²⁺-dependent metalloproteinases. In SHSY5Y cells, Cu²⁺ treatment decreased pro-BDNF level in cells and increased pro- and mature BDNF levels in the medium [128] with a strong decrease of the proliferative activity of both cleaved BDNF (1–12) and the full length protein [129]. Application of zinc chelator induced reduction of BDNF level, synaptic plasticity-related proteins and dendritic spine density in vivo, which further confirm that appropriate amount of Zn²⁺ is essential in brain development and the synaptic functions [130].

At the cellular level, mZn is loaded onto the presynaptic vesicles by zinc transport protein ZnT3, by which zinc plays a role in modulating neurotransmission and plasticity of glutamatergic neurons. The mice with ZnT3 knock out exhibit learning and memory deficits only at 6-months but not at 3-months of age. These age-dependent cognitive deficits are associated with significantly decreased levels of key hippocampal proteins involved in learning and memory. As aggregation of extracellular Aβ may trap this synaptic pool of Zn²⁺, ZnT3 knock out may represent a phenocopy for the synaptic and memory deficits of AD. Interestingly, this ZnT3 deletion mice also showed a reduced synaptic localization of Aβ oligomerization compared to that in wild-type mice [131], which implies a functional role of Aβ oligomer in the synapses.

In AD, increased levels of Ca²⁺ in dendrites and dendritic spines have important effects on synaptic dysfunction. Calcineurin is an important enzyme that mediates the signaling pathway of Ca²⁺ in the central nervous system. Calcineurin can work cooperatively with Aβ or tau and contribute to the loss of dendritic spines and synapses, leading to cognitive deficit in mouse models of AD, thus use of calcineurin inhibitors can reverse or improve these impairments [5, 132].

Mitochondria are especially abundant in synapses, where they provide energy for calcium homeostasis and synaptic functions. The synaptic mitochondria are more sensitive to calcium overload damage than non-synaptic mitochondria [133]. Cytoplasmic Ca²⁺ can flow into the mitochondria and activate the calcium-dependent mitochondrial matrix dehydrogenase to produce ATP. However, the cytoplasmic Ca²⁺ overload causes excessive Ca²⁺ influx into the mitochondria. The mitochondrial Ca²⁺ overload induces ROS overproduction, and thus results in mitochondrial outer membrane permeabilization (MOMP) and the subsequent release of pro-apoptotic factors into the cytoplasm. The pro-apoptotic factors, including cytochrome C and apoptosis inducing factor, activate caspases that induce apoptosis [134]. During long-term potentiation (LTP), microtubule associated protein 1B (MAP1B) phosphorylation and local concentrations of Ca²⁺-calmodulin-dependent kinase II (CaMKII) were increased [135]. CaMKII is responsible for phosphorylating MAP2, which enhances synaptic response [136]. The human tau accumulation impairs synapse and memory by activating Ca²⁺-dependent calcineurin and suppressing nuclear CaMKIV/CREB signaling, which reveals a new mechanism underlying the human tau-induced synaptic toxicity [5].

To help the readers quickly understand the key points, we summarized how metal ions (Fe, Cu, Zn and Ca) are involved in AD-like neurodegeneration and memory deficits, mainly from their effects on oxidative stress, Aβ and tau pathologies, autophagic imbalance, and synaptic impairment (Fig. 1).