Zinc, Copper, and Calcium: A Triangle in the Synapse for the Pathogenesis of Vascular-Type Senile Dementia

Abstract

Zinc (Zn) and copper (Cu) are essential for normal brain functions. In particular, Zn and Cu are released to synaptic clefts during neuronal excitation. Synaptic Zn and Cu regulate neuronal excitability, maintain calcium (Ca) homeostasis, and play central roles in memory formation. However, in pathological conditions such as transient global ischemia, excess Zn is secreted to synaptic clefts, which causes neuronal death and can eventually trigger the pathogenesis of a vascular type of senile dementia. We have previously investigated the characteristics of Zn-induced neurotoxicity and have demonstrated that low concentrations of Cu can exacerbate Zn neurotoxicity. Furthermore, during our pharmacological approaches to clarify the molecular pathways of Cu-enhanced Zn-induced neurotoxicity, we have revealed the involvement of Ca homeostasis disruption. In the present review, we discuss the roles of Zn and Cu in the synapse, as well as the crosstalk between Zn, Cu, and Ca, which our study along with other recent studies suggest may underlie the pathogenesis of vascular-type senile dementia.

Keywords: calcium homeostasis; endoplasmic reticulum stress; ischemia; reactive oxygen species (ROS).

Figure 1

Roles of Zn and Cu in the synapse under physiological and pathophysiological conditions: (A) Roles of Zn and Cu under normal physiological conditions. Zn and Cu are secreted from presynaptic vesicles to the synaptic clefts with glutamate during neuronal firing. Secreted Zn binds to N-methyl-d-aspartic acid-type glutamate receptors (NMDA-R) and amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainite type glutamate receptors (A/K-R) and inhibits their functions. Zn enhances the expression of the zinc transporter ZnT-1 or the translocation of ZnT-1 to synaptic membranes. Thereafter, ZnT-1 binds to NMDA-R and L-type voltage-gated Ca²⁺ channels (L-VGCC) to regulate overall brain excitability and maintain Ca homeostasis. Secreted Cu regulates NMDA-R and A/K-R by binding with normal cellular prion protein (PrP^C). PrP^C also regulates Zn²⁺ influx by binding with A/K-R. Secreted Zn and Cu spill over to neighboring synapses and inhibit or potentiate neuronal excitability in a biphasic dose-dependent manner. Finally, Zn and Cu contribute to information processing and memory formation. ZIP-4 can reuptake secreted Zn, and CTR1 can reuptake Cu once it has been reduced to Cu⁺ by amyloid precursor protein (APP). The subtle balance of Zn, Cu, and Ca maintains synaptic functions. (B) Roles of Zn and Cu under pathophysiological conditions. In transient global ischemia conditions, widespread, prolonged excitation occurs in the brain, and excess Zn and Cu can be secreted to synaptic clefts with glutamate. Secreted excess Zn causes the acute reduction of the GluR2 subunit of A/K-R, and Ca²⁺-permeable A/K-R (Ca-A/K-R) appears. Both Ca²⁺ and Zn²⁺ are intracellularly translocated via Ca-A/K-R, NMDA-R, and L-VGCC. Increased intracellular Ca²⁺ levels ([Ca²⁺]i) and intracellular Zn²⁺ levels ([Zn²⁺]i) then cause various neurodegenerative pathways, ultimately triggering the pathogenesis of vascular-type senile dementia. Colored circles represent Zn, Cu, Ca, and glutamate.

Figure 1

Roles of Zn and Cu in the synapse under physiological and pathophysiological conditions: (A) Roles of Zn and Cu under normal physiological conditions. Zn and Cu are secreted from presynaptic vesicles to the synaptic clefts with glutamate during neuronal firing. Secreted Zn binds to N-methyl-d-aspartic acid-type glutamate receptors (NMDA-R) and amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainite type glutamate receptors (A/K-R) and inhibits their functions. Zn enhances the expression of the zinc transporter ZnT-1 or the translocation of ZnT-1 to synaptic membranes. Thereafter, ZnT-1 binds to NMDA-R and L-type voltage-gated Ca²⁺ channels (L-VGCC) to regulate overall brain excitability and maintain Ca homeostasis. Secreted Cu regulates NMDA-R and A/K-R by binding with normal cellular prion protein (PrP^C). PrP^C also regulates Zn²⁺ influx by binding with A/K-R. Secreted Zn and Cu spill over to neighboring synapses and inhibit or potentiate neuronal excitability in a biphasic dose-dependent manner. Finally, Zn and Cu contribute to information processing and memory formation. ZIP-4 can reuptake secreted Zn, and CTR1 can reuptake Cu once it has been reduced to Cu⁺ by amyloid precursor protein (APP). The subtle balance of Zn, Cu, and Ca maintains synaptic functions. (B) Roles of Zn and Cu under pathophysiological conditions. In transient global ischemia conditions, widespread, prolonged excitation occurs in the brain, and excess Zn and Cu can be secreted to synaptic clefts with glutamate. Secreted excess Zn causes the acute reduction of the GluR2 subunit of A/K-R, and Ca²⁺-permeable A/K-R (Ca-A/K-R) appears. Both Ca²⁺ and Zn²⁺ are intracellularly translocated via Ca-A/K-R, NMDA-R, and L-VGCC. Increased intracellular Ca²⁺ levels ([Ca²⁺]i) and intracellular Zn²⁺ levels ([Zn²⁺]i) then cause various neurodegenerative pathways, ultimately triggering the pathogenesis of vascular-type senile dementia. Colored circles represent Zn, Cu, Ca, and glutamate.

Figure 2

The coexistence of Cu exacerbates Zn-induced neurotoxicity: (A) Various concentrations of ZnCl₂ were applied to GT1-7 cells with or without 20 µM CuCl₂. After 24 h of exposure, cell viability was analyzed using a Cell-Titer Glo2 assay. Data are expressed as the mean ± standard error of the mean (SEM), n = 6. (B) Morphological changes in GT1-7 cells were observed by fluorescent microscopy stained by carboxyfluorescein after 24 h of exposure to [a] control, [b] 20 µM CuCl₂, [c] 30 µM ZnCl₂, and [d] 20 µM CuCl₂ with 30 µM ZnCl₂. (C) Effects of sodium pyruvate and dantrolene on Zn neurotoxicity and Cu/Zn neurotoxicity. [i]: GT1-7 cells were exposed to 50 µM ZnCl₂ with or without sodium pyruvate (Pyr; 1~2 mM) and dantrolene (2.5~10 µM). After 24 h, cell viability was analyzed using a Cell-Titer Glo2 assay. Data are expressed as the mean ± SEM, n = 6. * p < 0.05, ** p < 0.01 (vs. Zn alone). [ii]: GT1-7 cells were exposed to 20 µM CuCl₂ and 30 µM ZnCl₂ with or without sodium pyruvate (Pyr; 1~2 mM) and dantrolene (2.5~10 µM). After 24 h, cell viability was analyzed using a Cell-Titer Glo2 assay. Data are expressed as the mean ± SEM, n = 6. * p < 0.05, ** p < 0.01 (vs. Cu/Zn alone).

Figure 3

Effects of substances related to Ca influx on Zn neurotoxicity and Cu/Zn neurotoxicity: (A) GT1-7 cells were exposed to various concentrations of ZnCl₂ with or without 150 mM KCl. (B) GT1-7 cells were exposed to 20 µM CuCl₂ with various concentrations of ZnCl₂, with or without 150 mM KCl. After 24 h, cell viability was analyzed using a Cell-Titer Glo2 assay. Data are expressed as the mean ± SEM, n = 6. (C) GT1-7 cells were exposed to 50 µM ZnCl₂ with or without nimodipine (nimo), AlCl₃, and GdCl₃. After 24 h, cell viability was analyzed using a Cell-Titer Glo2 assay. Data are expressed as the mean ± SEM, n = 6. * p < 0.05, ** p < 0.01 (vs. Zn). (D) GT1-7 cells were exposed to 20 µM CuCl₂ and 30 µM ZnCl₂ (Cu/Zn) with or without nimodipine (Nimo), AlCl₃, and GdCl₃. After 24 h, cell viability was analyzed using a Cell-Titer Glo2 assay. Data are expressed as the mean ± SEM, n = 6. ** p < 0.01 (vs. Cu/Zn).

Figure 4

Hypothetical schema of the molecular pathways of Cu-enhanced Zn-induced neurotoxicity. Under pathological conditions, such as transient global ischemia and excess Zn and Cu, are secreted into the synaptic cleft. Zn and Ca are then intracellularly translocated via Ca-A/K-R, NMDA-R, and L-VGCC. Elevated [Zn²⁺]_i then triggers the ER stress pathway and stress-activated protein kinase/c-Jun amino-terminal kinase (SAPK/JNK) pathway, resulting in apoptotic neuronal death. Dantrolene, an inhibitor of the ER stress pathway, and SP600125, an inhibitor of the SAPK/JNK signaling pathway, attenuate Cu/Zn neurotoxicity. Zn also inhibits NAD⁺ and causes energy depletion in the mitochondria. Furthermore, Zn causes elevated [Ca²⁺]_i, which then triggers various neurodegenerative processes. Substances that induce [Ca²⁺]_i elevation (e.g., KCl) enhance Cu/Zn neurotoxicity, whereas substances that inhibit [Ca²⁺]_i elevation (e.g., nimodipine, Al³⁺, and Gd³⁺) attenuate the neurotoxicity. It is also possible that Cu influences Zn-induced neurotoxicity because Cu produces reactive oxygen species (ROS), which upregulate the ER stress and SAPK/JNK pathways. Thus, the presence of Cu exacerbates Zn-induced neuronal death. Additionally, seleno-L-methionine (Se-Met) and a conjugated protein consisting of thioredoxin and human serum albumin (HSA-Trx) suppress ROS production and attenuate Cu/Zn neurotoxicity. It is also possible that Cu causes elevated [Ca²⁺]_i and enhances Zn-induced neurotoxicity. Ultimately, the coexistence of Cu and Zn in the same synapse triggers neurodegenerative pathways and eventually induces the pathogenesis of vascular-type senile dementia. Colored circles represent Zn, Cu, and Ca.