12 January 2011. Epileptic seizures are bad for neurons, but does it come down to zinc ions to finish them off? That is the conclusion of a report in the January 5 Journal of Neuroscience. Researchers at Baylor College of Medicine, led by first author Jing Qian and senior author Jeffrey Noebels, found that when they blocked zinc transport in mice, seizure-afflicted neurons were more likely to survive the attack. The researchers suggest that blockers of zinc transporters called Zips could be therapeutics for epilepsy. And with evidence mounting that epileptic seizures grip people with Alzheimer’s disease, the same medicine might warrant testing in them, too.
Although temporal lobe seizures are difficult to spot—a person may simply stop and stare or mumble for a bit—seizures are common in AD. According to one study, people with Alzheimer’s have an eightfold risk of seizure compared to the general population. Patients in their fifties—who usually have inherited early-onset AD—have a seizure risk 87-fold higher than normal (Amatniek et al., 2006; Bateman et al., 2011). Noebels and colleagues reported that mouse models for AD have non-convulsive temporal-lobe seizures (see ARF related news story on Palop et al., 2007). These could interfere with memory, Noebels suggested. In epilepsy and Alzheimer’s, he said, “we are looking at two sides of what could be the same coin” (also see ARF related news story).
A seizure is damaging while it happens—but in the hours following, additional damage can cause neurodegeneration. According to the current and other studies, zinc is somehow mixed up with both reducing the onset of seizures and exacerbating their aftermath (for a review, see Weiss et al., 2000).
First, zinc in the synapse guards against seizures, according to Ashley Bush of the University of Melbourne, Australia, who was not involved in the current study. Pre-synaptic cells pack a little zinc—via the Znt-3 transporter—into synaptic vesicles along with the neurotransmitters. Upon release, those neurotransmitters will bind to NMDA receptors and excite the post-synaptic cell. But if too many NMDA receptors are activated, the nerve potential goes into overdrive, causing excitotoxicity and seizure. Zinc ions dampen the neurotransmitters’ effect by also binding to NMDA receptors and blocking some of them. In this way, zinc ions prevent seizure. Among the evidence for this scenario: Chelating zinc ions worsens seizure activity (Dominguez et al., 2006).
But sometimes zinc fails to do this. The resulting overexcitation damages neurons to where they degenerate from stress after the seizure has passed. Noebels and colleagues addressed this form of damage in their study, and found that zinc import appears to enhance post-seizure damage.
Zinc has been implicated in AD many times before. For example, excess zinc boosts APP cleavage and amyloid-β plaque formation (Wang et al., 2010). The plaques act like a sponge for zinc, Bush said, sucking it up and preventing it from carrying out its normal role. Thus, zinc homeostasis could be a therapeutic target. Bush compared zinc’s activity to that of calcium, another ionic second messenger that can cause damage if its delicate balance is upset. “What this paper tells us is…that under certain pathological conditions, you can get an inappropriate influx of zinc into the cell, and that leads to neural death,” he said. And, he added, it corroborates evidence linking Alzheimer’s and epilepsy.
Noebels’s work helps explain how the intracellular zinc that participates in neurodegeneration gets into the neuron. Many possible entryways exist: Zinc gets in via glutamate receptors, GABA receptors, and ion channels. Another entry route, which Noebels said has received little attention, is through passive zinc importers called Zips in hippocampal pyramidal neurons. Qian and Noebels used mice deficient in Zip-1 and Zip-3 to examine their role in ion import as well as seizure response. These animals carry green fluorescent protein (GFP) in lieu of the Zip genes, so any cells that would normally express Zip glow green (Dufner-Beattie et al., 2006). When Qian looked in brain tissue, he found that the GFP was expressed slightly in the forebrain, brainstem, and cerebellum, but at high levels in the hippocampus.
To study zinc import via Zip, the scientists injected cells in brain slices with the zinc-sensitive fluorophore FluoZin-3. Neurons in hippocampal brain slices from the double knockout animals imported half as much zinc as wild-type cells, indicating that the Zips are key zinc importers in the hippocampus. Other Zips (there are at least 14 members in the family) or transmitter receptors may be responsible for the uptake.
The researchers next tested whether the Zip knockouts would respond differently to seizures. They injected kainic acid into month-old mice to induce a temporal lobe seizure, which they monitored by electroencephalography (EEG). A day later, the researchers sacrificed the animals and stained brain tissue with Fluoro-Jade, an indicator for dead or dying neurons. Qian saw that neurons in Zip1/3 double knockout mice were protected: Eleven of 43 mutant animals had damage in the most remote hippocampal subdomain, the CA-1, compared to 21 of 41 wild-type mice.
Where does the toxic zinc come from? The authors suspected that the synaptic vesicles released it, so they performed similar experiments with mice lacking the zinc transporter Znt-3, which packages zinc into synaptic vesicles for export. They found that Znt-3 knockout mice had less damage in the hippocampus than wild-type animals after seizures. Most of the 34 Znt-3-negative mice tested had no damage at all in the CA-1 area. Thus, the researchers concluded that in both cases—preventing zinc uptake or preventing its release into synapses—they protected neurons by blocking the traffic of zinc between the pre- and post-synaptic neuron.
These experiments suggest that pharmacologically sealing up Zip transporters could protect neurons after seizure, although it would be unlikely to prevent seizures from happening. Noebels intends to approach biotech partners in the hopes of developing selective Zip antagonists that would prevent import through those transporters. The first application would likely be a post-seizure treatment to protect neurons in people with severe epilepsy, he said.
Given the proposed links between Alzheimer’s and epilepsy, people with AD might benefit from the same strategy. “The last person you would want to lose cells in the hippocampus is someone with Alzheimer’s,” Noebels noted. Unfortunately, anti-seizure medicines often cause memory problems as a side effect, so while they could diminish one symptom, they might compound another. Targeting Zips instead might be a better approach, Noebels suggested.
But there are caveats. In Zip-less animals, induced seizures were more severe and sometimes lasted longer than in wild-type animals; 17 percent of the knockout animals died during those seizures, versus only 4 percent of wild-type mice. The authors cannot explain why that is; they suggest that it may be a result of abnormal development, since the animals lack Zip-1 and -3 from birth. It is not clear if blocking Zips pharmacologically in wild-type mice would also exacerbate seizures. “We are not saying to remove all zinc,” Noebels noted. “Zinc entering through these [Zip] transporters at high levels, such as during a seizure, is a toxic event…how to avoid that without impairing other zinc roles is key.” He doubts that reducing zinc import in this manner would induce seizures, because it would not interfere with synaptic zinc’s seizure-dampening activity.
There is reason to be very careful when interfering with zinc concentration, Bush suggested; after all, synaptic zinc prevents seizure. Altered zinc concentrations may cause AD-like symptoms: Bush has found that Znt-3 knockout mice have the same symptoms as AD model mice (see ARF related news story on Adlard et al., 2010).
“While trapping the zinc will cause epilepsy, allowing it to enter the post-synaptic cell would kill it,” Bush said. “What is needed is a gentle zinc-trapping molecule that facilitates its movement toward the NMDA receptor and does not allow it to enter the cell in an uncontrolled manner.” Bush and collaborators at Prana Biotechnology, Parkville, Australia, which Bush cofounded, are working on a zinc ligand, PBT2, that might fit the bill, he said (see ARF related news story on Adlard et al., 2008). Prana recently announced that it is moving forward with a large Phase 2b clinical trial, partly funded by local government.—Amber Dance.
Qian J, Xu K, Yoo J, Chen TT, Andrews G, Noebels JL. Knockout of Zn transporters Zip-1 and Zip-3 attenuates seizure-induced CA1 neurodegeneration. J Neurosci. 2010 5 Jan;31(1):97-104. Abstract