Stimulation of kainate toxicity by zinc in cultured cerebellar granule neurons and the role of mitochondria in this process

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.

Streptozotocin causes neurotoxic effect in cultured cerebellar granule neurons

Streptozotocin (STZ) is a glucosamine-nitrosourea compound used for experimental simulation of sporadic Alzheimer's disease at intracerebroventricular administration in vivo. The studies of STZ influence on neurons of central nervous system performed on the primary cultures are practically absent. We have shown the application of STZ (1-5mM) in primary culture for 48h induced strong dose-dependent death in cultured cerebellar granule neurons. This toxic effect was decreased by pyruvate, insulin partially. Using the indicator Fluo-4 AM for measurements of intracellular calcium ions and tetramethylrhodamine ethyl ester (TMRE) for detection of changes of mitochondrial membrane potential in live cells we have shown that 5 h-exposure to STZ induced intensive increase of Fluo-4 and decrease TMRE fluorescence in neurons. STZ exposure caused considerable ultrastructural alterations in granule neurons: chromatin clumping, swelling of the endoplasmic reticulum and mitochondria, and disruption of the mitochondrial cristae. Probably, STZ significantly impaired glucose metabolism and mitochondrial function that, in turn, resulted in mitochondrial membrane potential damage, excessive calcium overload and neuronal death.

Zinc Modulates Nanosilver-Induced Toxicity in Primary Neuronal Cultures

Silver nanoparticles (NAg) have recently become one of the most commonly used nanomaterials. Since the ability of nanosilver to enter the brain has been confirmed, there has been a need to investigate mechanisms of its neurotoxicity. We previously showed that primary neuronal cultures treated with nanosilver undergo destabilization of calcium homeostasis via a mechanism involving glutamatergic NMDA receptors. Considering the fact that zinc interacts with these receptors, the aim of the present study was to examine the role of zinc in mechanisms of neuronal cell death in primary cultures. In cells treated with nanosilver, we noted an imbalance between extracellular and intracellular zinc levels. Thus, the influence of zinc deficiency and supplementation on nanosilver-evoked cytotoxicity was investigated by treatment with TPEN (a chelator of zinc ions), or ZnCl₂, respectively. Elimination of zinc leads to complete death of nanosilver-treated CGCs. In contrast, supplementation with ZnCl₂ increases viability of CGCs in a dose-dependent manner. Addition of zinc provided protection against the extra/intracellular calcium imbalance in a manner similar to MK-801, an antagonist of NMDA receptors. Zinc chelation by TPEN decreases the mitochondrial potential and dramatically increases the rate of production of reactive oxygen species. Our results indicate that zinc supplementation positively influences nanosilver-evoked changes in CGCs. This is presumed to be due to an inhibitory effect on NMDA-sensitive calcium channels.

Zinc: indications in brain disorders

Zinc is the authoritative metal which is present in our body, and reactive zinc metal is crucial for neuronal signaling and is largely distributed within presynaptic vesicles. Zinc also plays an important role in synaptic function. At cellular level, zinc is a modulator of synaptic activity and neuronal plasticity in both development and adulthood. Different importers and transporters are involved in zinc homeostasis. ZnT-3 is a main transporter involved in zinc homeostasis in the brain. It has been found that alterations in brain zinc status have been implicated in a wide range of neurological disorders including impaired brain development and many neurodegenerative disorders such as Alzheimer's disease, and mood disorders including depression, Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis, and prion disease. Furthermore, zinc has also been implicated in neuronal damage associated with traumatic brain injury, stroke, and seizure. Understanding the mechanisms that control brain zinc homeostasis is thus critical to the development of preventive and treatment strategies for these and other neurological disorders.

Dopamine- and zinc-induced autophagosome formation facilitates PC12 cell survival

Dopamine oxidation and divalent cations have been reported to induce neuronal cell death. Although autophagy is involved in neuronal cell death, it has also been suggested to facilitate cell survival. We sought to investigate the role of autophagy in PC12 cells and cultured neurons treated with dopamine and Zn2+. Cells expressing EGFP-LC3 were treated with high concentrations of dopamine and Zn2+, and the formation of EGFP-LC3 fluorescence aggregates was monitored. Our results showed a significant increase in the number of fluorescent puncta in the cytosol of PC12 cells treated with these chemicals. These treatments enhanced LC3 lipidation levels in PC12 cells. Decreasing the ATG7 protein level using specific small interference RNA (siRNA) and pretreating with phosphatidylinositol 3-phosphate kinase blockers, wortmannin and LY294002, inhibited puncta formation. Dopamine or Zn2+ treatment significantly elevated the intracellular Zn2+ concentration ([Zn2+] i ); however, inhibiting the [Zn2+] i elevation in dopamine-treated cells suppressed the puncta formation. LY294002 or siRNA-directed members of the autophagy pathway increased the fraction of phosphatidylserine present on the outer membrane leaflet in PC12 cells treated with dopamine or Zn2+, suggesting an increase in apoptosis. Primary embryonic midbrain neurons expressing EGFP-LC3 also displayed a significant increase in the number of fluorescent aggregates in cells upon treatment with dopamine or Zn2+. Dopamine or Zn2+ treatment significantly elevated the [Zn2+] i in neurons and caused neuronal death. Our results indicate that treating cells with dopamine and Zn2+ results in the activation of the autophagy pathway in an effort to enhance cell survival.

Intramitochondrial Zn2+ accumulation via the Ca2+ uniporter contributes to acute ischemic neurodegeneration

Ca(2+) and Zn(2+) have both been implicated in the induction of acute ischemic neurodegeneration. We recently examined changes in intracellular Zn(2+) and Ca(2+) in CA1 pyramidal neurons subjected to oxygen glucose deprivation (OGD), and found that Zn(2+) rises precede and contribute to the onset of terminal Ca(2+) rises ("Ca(2+) deregulation"), which are causatively linked to a lethal loss of membrane integrity. The present study seeks to examine the specific role of intramitochondrial Zn(2+) accumulation in ischemic injury, using blockers of the mitochondrial Ca(2+) uniporter (MCU), through which both Zn(2+) and Ca(2+) appear able to enter the mitochondrial matrix. In physiological extracellular Ca(2+), treatment with the MCU blocker, Ruthenium Red (RR), accelerated the Ca(2+) deregulation, most likely by disrupting mitochondrial Ca(2+) buffering and thus accelerating the lethal cytosolic Ca(2+) overload. However, when intracellular Ca(2+) overload was slowed, either by adding blockers of major Ca(2+) entry channels or by lowering the concentration of Ca(2+) in the extracellular buffer, Ca(2+) deregulation was delayed, and under these conditions either Zn(2+) chelation or MCU blockade resulted in similar further delays of the Ca(2+) deregulation. In parallel studies using the reactive oxygen species (ROS) indicator, hydroethidine, lowering Ca(2+) surprisingly accelerated OGD induced ROS generation, and in these low Ca(2+) conditions, either Zn(2+) chelation or MCU block slowed the ROS generation. These studies suggest that, during acute ischemia, Zn(2+) entry into mitochondria via the MCU induces mitochondrial dysfunction (including ROS generation) that occurs upstream of, and contributes to the terminal Ca(2+) deregulation.

Ion channels and zinc: mechanisms of neurotoxicity and neurodegeneration

Ionotropic glutamate receptors, such as NMDA, AMPA and kainate receptors, are ligand-gated ion channels that mediate much of the excitatory neurotransmission in the brain. Not only do these receptors bind glutamate, but they are also regulated by and facilitate the postsynaptic uptake of the trace metal zinc. This paper discusses the role of the excitotoxic influx and accumulation of zinc, the mechanisms responsible for its cytotoxicity, and a number of disorders of the central nervous system that have been linked to these neuronal ion channels and zinc toxicity including ischemic brain injury, traumatic brain injury, and epilepsy.