Excitotoxic mechanisms contribute to the degeneration of hippocampal pyramidal neurons after recurrent seizures and brain ischemia. However, susceptibility differs, with CA1 neurons degenerating preferentially after global ischemia and CA3 neurons after limbic seizures. Whereas most studies address contributions of excitotoxic Ca2+ entry, it is apparent that Zn2+ also contributes, reflecting accumulation in neurons either after synaptic release and entry through postsynaptic channels or upon mobilization from intracellular Zn2+-binding proteins such as metallothionein-III (MT-III). Using mouse hippocampal slices to study acute oxygen glucose deprivation (OGD)-triggered neurodegeneration, we found evidence for early contributions of excitotoxic Ca2+ and Zn2+ accumulation in both CA1 and CA3, as indicated by the ability of Zn2+ chelators or Ca2+ entry blockers to delay pyramidal neuronal death in both regions. However, using knock-out animals (of MT-III and vesicular Zn2+ transporter, ZnT3) and channel blockers revealed substantial differences in relevant Zn2+ sources, with critical contributions of presynaptic release and its permeation through Ca2+- (and Zn2+)-permeable AMPA channels in CA3 and Zn2+ mobilization from MT-III predominating in CA1. To assess the consequences of the intracellular Zn2+ accumulation, we used OGD exposures slightly shorter than those causing acute neuronal death; under these conditions, cytosolic Zn2+ rises persisted for 10–30 min after OGD, followed by recovery over ∼40–60 min. Furthermore, the recovery appeared to be accompanied by mitochondrial Zn2+ accumulation (via the mitochondrial Ca2+ uniporter MCU) in CA1 but not in CA3 neurons and was markedly diminished in MT-III knock-outs, suggesting that it depended upon Zn2+ mobilization from this protein. SIGNIFICANCE STATEMENT The basis for the differential vulnerabilities of CA1 versus CA3 pyramidal neurons is unclear. The present study of events during and after acute oxygen glucose deprivation highlights a possible important difference, with rapid synaptic entry of Ca2+ and Zn2+ contributing more in CA3, but with delayed and long-lasting accumulation of Zn2+ within mitochondria occurring in CA1 but not CA3 pyramidal neurons. These data may be consistent with observations of prominent mitochondrial dysfunction as a critical early event in the delayed degeneration of CA1 neurons after ischemia and support a hypothesis that mitochondrial Zn2+ accumulation in the early reperfusion period may be a critical and targetable upstream event in the injury cascade.

https://www.jneurosci.org/content/jneuro/37/3/726.full.pdf

Differential Vulnerability of CA1 versus CA3 Pyramidal Neurons After Ischemia: Possible Relationship to Sources of Zn2+ Accumulation and Its Entry into and Prolonged Effects on Mitochondria.

Calcium signaling in pancreatic ductal epithelial cells: an old friend and a nasty enemy.

Intracellular zinc is a critical intermediate in the excitotoxic cascade

Zinc-induced neurotoxicity has been shown to play a role in neuronal damage and death associated with traumatic brain injury, stroke, seizures, and neurodegenerative diseases. During normal firing of "zinc-ergic" neurons, vesicular free zinc is released into the synaptic cleft where it modulates a number of postsynaptic neuronal receptors. However, excess zinc, released after injury or disease, leads to excitotoxic neuronal death. The mechanisms of zinc-mediated neurotoxicity appear to include not only neuronal signaling but also regulation of mitochondrial function and energy production, as well as other mechanisms such as aggregation of amyloid beta peptides in Alzheimer's disease. However, recent data have raised questions about some of our long-standing assumptions about the mechanisms of zinc in neurotoxicity. Thus, this review explores the most recent published findings and highlights the current mechanistic controversies.

Neurotoxicity of Zinc.

Ischemic stroke is a major cause of death and disabilities worldwide, and it has been long hoped that improved understanding of relevant injury mechanisms would yield targeted neuroprotective therapies. While Ca2+ overload during ischemia-induced glutamate excitotoxicity has been identified as a major contributor, failures of glutamate targeted therapies to achieve desired clinical efficacy have dampened early hopes for the development of new treatments. However, additional studies examining possible contributions of Zn2+, a highly prevalent cation in the brain, have provided new insights that may help to rekindle the enthusiasm. In this review, we discuss both old and new findings yielding clues as to sources of the Zn2+ that accumulates in many forebrain neurons after ischemia, and mechanisms through which it mediates injury. Specifically, we highlight the growing evidence of important Zn2+ effects on mitochondria in promoting neuronal injury. A key focus has been to examine Zn2+ contributions to the degeneration of highly susceptible hippocampal pyramidal neurons. Recent studies provide evidence of differences in sources of Zn2+ and its interactions with mitochondria in CA1 versus CA3 neurons that may pertain to their differential vulnerabilities in disease. We propose that Zn2+-induced mitochondrial dysfunction is a critical and potentially targetable early event in the ischemic neuronal injury cascade, providing opportunities for the development of novel neuroprotective strategies to be delivered after transient ischemia.

Mitochondrial Zn2+ Accumulation: A Potential Trigger of Hippocampal Ischemic Injury.

Zinc has been implicated in neurodegeneration following ischemia. In analogy with calcium, zinc has been proposed to induce toxicity via mitochondrial dysfunction, but the relative role of each cation in mitochondrial damage remains unclear. Here, we report that under conditions mimicking ischemia in hippocampal neurons - normal (2 mM) calcium plus elevated (> 100 μM) exogenous zinc - mitochondrial dysfunction evoked by glutamate, kainate or direct depolarization is, despite significant zinc uptake, primarily governed by calcium. Thus, robust mitochondrial ion accumulation, swelling, depolarization, and reactive oxygen species generation were only observed after toxic stimulation in calcium-containing media. This contrasts with the lack of any mitochondrial response in zinc-containing but calcium-free medium, even though zinc uptake and toxicity were strong under these conditions. Indeed, abnormally high, ionophore-induced zinc uptake was necessary to elicit any mitochondrial depolarization. In calcium- and zinc-containing media, depolarization-induced zinc uptake facilitated cell death and enhanced accumulation of mitochondrial calcium, which localized to characteristic matrix precipitates. Some of these contained detectable amounts of zinc. Together these data indicate that zinc uptake is generally insufficient to trigger mitochondrial dysfunction, so that mechanism(s) of zinc toxicity must be different from that of calcium.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/jnc.12489

Galina Kazanina

Papers

The interactive roles of zinc and calcium in mitochondrial dysfunction and neurodegeneration