Facilitation of granule cell epileptiform activity by mossy fiber-released zinc in the pilocarpine model of temporal lobe epilepsy

Zinc reduces antiseizure activity of neurosteroids by selective blockade of extrasynaptic GABA-A receptor-mediated tonic inhibition in the hippocampus

Zinc is an abundant trace metal in the hippocampus nerve terminals. Previous studies demonstrate the ability of zinc to selectively block neurosteroid-sensitive, extrasynaptic GABA-A receptors in the hippocampus (Carver et al, 2016). Here we report that zinc prevents the seizure protective effects of the synthetic neurosteroid ganaxolone (GX) in an experimental model of epilepsy. GABA-gated and tonic currents were recorded from dissociated dentate gyrus granule cells (DGGCs), CA1 pyramidal cells (CA1PCs), and hippocampal slices from adult mice. Antiseizure effects of GX and the reversal of these effects by zinc were evaluated in fully-kindled mice expressing generalized (stage 5) seizures. In electrophysiological studies, zinc blocked the GABA-evoked and GX-potentiated GABA-gated chloride currents in DGGCs and CA1PCs in a concentration-dependent fashion similar to the competitive GABA-A receptor antagonists bicuculline and gabazine. Zinc completely blocked GX potentiation of extrasynaptic tonic currents, but not synaptic phasic currents. In hippocampus kindling studies, systemic administration of GX produced a dose-dependent suppression of behavioral and electrographic seizures in fully-kindled mice with complete seizure protection at the 10 mg/kg dose. However, the antiseizure effects of GX were significantly prevented by intrahippocampal administration of zinc (ED₅₀, 150 μM). The zinc antagonistic response was reversible as animals responded normally to GX administration 24 h post-zinc blockade. These results demonstrate that zinc reduces the antiseizure effects of GX by selectively blocking extrasynaptic δGABA-A receptors in the hippocampus. These pharmacodynamic interactions have clinical implications in neurosteroid therapy for brain conditions associated with zinc fluctuations.

New insights into the pathogenesis and therapeutics of episodic ataxia type 1

Episodic ataxia type 1 (EA1) is a K⁺channelopathy characterized by a broad spectrum of symptoms. Generally, patients may experience constant myokymia and dramatic episodes of spastic contractions of the skeletal muscles of the head, arms, and legs with loss of both motor coordination and balance. During attacks additional symptoms may be reported such as vertigo, blurred vision, diplopia, nausea, headache, diaphoresis, clumsiness, stiffening of the body, dysarthric speech, and difficulty in breathing. These episodes may be precipitated by anxiety, emotional stress, fatigue, startle response or sudden postural changes. Epilepsy is overrepresented in EA1. The disease is inherited in an autosomal dominant manner, and genetic analysis of several families has led to the discovery of a number of point mutations in the voltage-dependent K⁺ channel gene KCNA1 (Kv1.1), on chromosome 12p13. To date KCNA1 is the only gene known to be associated with EA1. Functional studies have shown that these mutations impair Kv1.1 channel function with variable effects on channel assembly, trafficking and biophysics. Despite the solid evidence obtained on the molecular mechanisms underlying EA1, how these cause dysfunctions within the central and peripheral nervous systems circuitries remains elusive. This review summarizes the main breakthrough findings in EA1, discusses the neurophysiological mechanisms underlying the disease, current therapies, future challenges and opens a window onto the role of Kv1.1 channels in central nervous system (CNS) and peripheral nervous system (PNS) functions.

PIAS1-modulated Smad2/4 complex activation is involved in zinc-induced cancer cell apoptosis

Prostate cancer is one of the most frequently diagnosed cancers among men. Dietary intake of nutrients is considered crucial for preventing the initiation of events leading to the development of carcinoma. Many dietary compounds have been considered to contribute to cancer prevention including zinc, which has a pivotal role in modulating apoptosis. However, the mechanism for zinc-mediated prostate cancer chemoprevention remains enigmatic. In this study, we investigated the therapeutic effect of zinc in prostate cancer chemoprevention for the first time. Exposure to zinc induced apoptosis and resulted in transactivation of p21^WAF1/Cip1 in a Smad-dependent and p53-independent manner in prostate cancer cells. Smad2 and PIAS1 proteins were significantly upregulated resulting in dramatically increased interactions between Smad2/4 and PIAS1 in the presence of zinc in LNCaP cells. Furthermore, it was found that the zinc-induced Smad4/2/PIAS1 transcriptional complex is responsible for Smad4 binding to SBE1 and SBE3 regions within the p21^WAF1/Cip1 promoter. Exogenous expression of Smad2/4 and PIAS1 promotes zinc-induced apoptosis concomitant with Smad4 nuclear translocation, whereas endogenous Smad2/4 silencing inhibited zinc-induced apoptosis accompanying apparent p21^WAF1/Cip1 reduction. Moreover, the knockdown of PIAS1 expression attenuated the zinc-induced recruitment of Smad4 on the p21^WAF1/Cip1 promoter. The colony formation experiments demonstrate that PIAS1 and Smad2/4 silencing could attenuate zinc apoptotic effects, with a proliferation of promoting effects. We further demonstrate the correlation of apoptotic sensitivity to zinc and Smad4 and PIAS1 in multiple cancer cell lines, demonstrating that the important roles of PIAS1, Smad2, and Smad4 in zinc-induced cell death and p21^WAF1/Cip1 transactivation were common biological events in different cancer cell lines. Our results suggest a new avenue for regulation of zinc-induced apoptosis, and provide a model that demonstrates zinc endorses the Smad2/4/PIAS1 complex to activate the p21^WAF1/Cip1 gene that mediates apoptosis.

Modulation of neuronal signal transduction and memory formation by synaptic zinc

The physiological role of synaptic zinc has remained largely enigmatic since its initial detection in hippocampal mossy fibers over 50 years ago. The past few years have witnessed a number of studies highlighting the ability of zinc ions to regulate ion channels and intracellular signaling pathways implicated in neuroplasticity, and others that shed some light on the elusive role of synaptic zinc in learning and memory. Recent behavioral studies using knock-out mice for the synapse-specific zinc transporter ZnT-3 indicate that vesicular zinc is required for the formation of memories dependent on the hippocampus and the amygdala, two brain centers that are prominently innervated by zinc-rich fibers. A common theme emerging from this research is the activity-dependent regulation of the Erk1/2 mitogen-activated-protein kinase pathway by synaptic zinc through diverse mechanisms in neurons. Here we discuss current knowledge on how synaptic zinc may play a role in cognition through its impact on neuronal signaling.

Targeting BK (big potassium) channels in epilepsy

INTRODUCTION

Epilepsies are disorders of neuronal excitability characterized by spontaneous and recurrent seizures. Ion channels are critical for regulating neuronal excitability and, therefore, can contribute significantly to epilepsy pathophysiology. In particular, large conductance, Ca2+-activated K+ (BKCa) channels play an important role in seizure etiology. These channels are activated by both membrane depolarization and increased intracellular Ca2+. This unique coupling of Ca2+ signaling to membrane depolarization is important in controlling neuronal hyperexcitability, as outward K+ current through BKCa channels hyperpolarizes neurons.

AREAS COVERED

BKCa channel structure-function and the role of these channels in epilepsy pathophysiology.

EXPERT OPINION

Loss-of-function BKCa channel mutations contribute to neuronal hyperexcitability that can lead to temporal lobe epilepsy, tonic-clonic seizures and alcohol withdrawal seizures. Similarly, BKCa channel blockade can trigger seizures and status epilepticus. Paradoxically, some mutations in BKCa channel subunit can give rise to channel gain-of-function that leads to development of idiopathic epilepsy (primarily absence epilepsy). Seizures themselves also enhance BKCa channel currents associated with neuronal hyperexcitability, and blocking BKCa channels suppresses generalized tonic-clonic seizures. Thus, both loss-of-function and gain-of-function BKCa channels might serve as molecular targets for drugs to suppress certain seizure phenotypes including temporal lobe seizures and absence seizures, respectively.

Tissue plasminogen activator alters intracellular sequestration of zinc through interaction with the transporter ZIP4

Glutamatergic neurons contain free zinc packaged into neurotransmitter-loaded synaptic vesicles. Upon neuronal activation, the vesicular contents are released into the synaptic space, whereby the zinc modulates activity of postsynaptic neurons though interactions with receptors, transporters and exchangers. However, high extracellular concentrations of zinc trigger seizures and are neurotoxic if substantial amounts of zinc reenter the cells via ion channels and accumulate in the cytoplasm. Tissue plasminogen activator (tPA), a secreted serine protease, is also proepileptic and excitotoxic. However, tPA counters zinc toxicity by promoting zinc import back into the neurons in a sequestered form that is nontoxic. Here, we identify the zinc influx transporter, ZIP4, as the pathway through which tPA mediates the zinc uptake. We show that ZIP4 is upregulated after excitotoxin stimulation of the mouse, male and female, hippocampus. ZIP4 physically interacts with tPA, correlating with an increased intracellular zinc influx and lysosomal sequestration. Changes in prosurvival signals support the idea that this sequestration results in neuroprotection. These experiments identify a mechanism via which neurons use tPA to efficiently neutralize the toxic effects of excessive concentrations of free zinc.

Hippocampal zinc infusion delays the development of afterdischarges and seizures in a kindling model of epilepsy

PURPOSE

Zinc occurs in high concentration in synaptic vesicles of glutamatergic terminals including hippocampal mossy fibers. This vesicular zinc can be synaptically released during neuronal activity, including seizures. Zinc inhibits certain subtypes of N-methyl-D-aspartate (NMDA) and gamma-aminobutyric acid (GABA)(A) receptors. By blocking NMDA excitation or GABA inhibition, an excess of zinc may alter the excitability of hippocampal circuits, which contribute to the development of seizures.

METHODS

Twenty-one adult Wistar rats were implanted under anesthesia with Alzet pumps releasing vehicle, 10 microM ZnCl(2) or 1,000 microM ZnCl(2), at a rate of 0.25 microl/h continuously into the hippocampal hilus for 4 weeks. Kindling was performed by daily awake commissural stimulation at 60 Hz and afterdischarges were recorded from a dentate gyrus electrode. Development of behavioral Racine seizure stages was recorded by a blinded investigator.

RESULTS

The development of behavioral Racine seizure stages was delayed only in rats infused with 1,000 microM ZnCl(2) (p < 0.02). With completion of kindling at stimulation number 20, all groups had reached the same maximum level of behavioral seizures. The expected increased progression of afterdischarge duration was inhibited by both 10 microM ZnCl(2) and 1,000 microM ZnCl(2) infusion compared to control animals (p < 0.01). At stimulation number 18, all groups had reached the same maximum duration of afterdischarges.

DISCUSSION

We conclude that excess infused zinc delayed the development of behavioral seizures in a kindling model of epilepsy. These data support the hypothesis that zinc synaptically released during seizures may alter hippocampal excitability similar to zinc infused in our experiment.

Zinc-mediated transactivation of TrkB potentiates the hippocampal mossy fiber-CA3 pyramid synapse

The receptor tyrosine kinase, TrkB, is critical to diverse functions of the mammalian nervous system in health and disease. Evidence of TrkB activation during epileptogenesis in vivo despite genetic deletion of its prototypic neurotrophin ligands led us to hypothesize that a non-neurotrophin, the divalent cation zinc, can transactivate TrkB. We found that zinc activates TrkB through increasing Src family kinase activity by an activity-regulated mechanism independent of neurotrophins. One subcellular locale at which zinc activates TrkB is the postsynaptic density of excitatory synapses. Exogenous zinc potentiates the efficacy of the hippocampal mossy fiber (mf)-CA3 pyramid synapse by a TrkB-requiring mechanism. Long-term potentiation of this synapse is impaired by deletion of TrkB, inhibition of TrkB kinase activity, and by CaEDTA, a selective chelator of zinc. The activity-dependent activation of synaptic TrkB in a neurotrophin-independent manner provides a mechanism by which this receptor can regulate synaptic plasticity.

Chelatable zinc modulates excitability and seizure duration in the amygdala rapid kindling model

Zinc is present in high concentration in many structures of the limbic circuitry, however the role of zinc as a neuromodulator in such synapses is still uncertain. In this work, we verified the effects of zinc chelation in an animal model of epileptogenesis induced by amygdala rapid kindling. The basolateral amygdala was electrically stimulated ten times per day for 2 days. A single stimulus was applied on the third day. Stimulated animals received injections of PBS or the zinc chelator diethildythiocarbamate acid (DEDTC) before each stimulus series. Animals were monitored with video-EEG and were perfused 3h after the last stimulus for subsequent neo-Timm and Fluoro-Jade B analysis. Zinc chelation decreased the duration of both behavioral seizures and electrical after-discharges, and also decreased the EEG spikes frequency, without changing the progression of behavioral seizure severity. These results indicate that the zinc ion may have a facilitatory role during kindling progression.

Extracellular chelation of zinc does not affect hippocampal excitability and seizure-induced cell death in rats

In the nervous system, zinc can influence synaptic responses and at extreme concentrations contributes to epileptic and ischaemic neuronal injury. Zinc can originate from synaptic vesicles, the extracellular space and from intracellular stores. In this study, we aimed to determine which of these zinc pools is responsible for the increased hippocampal excitability observed in zinc-depleted animals or following zinc chelation. Also, we investigated the source of intracellularly accumulating zinc in vulnerable neurons. Our data show that membrane-permeable and membrane-impermeable zinc chelators had little or no effect on seizure activity in the CA3 region. Furthermore, extracellular zinc chelation could not prevent the accumulation of lethal concentrations of zinc in dying neurons following epileptic seizures. At the electron microscopic level, zinc staining significantly increased at the presynaptic membrane of mossy fibre terminals in kainic acid-treated animals. These data indicate that intracellular but not extracellular zinc chelators could influence neuronal excitability and seizure-induced zinc accumulation observed in the cytosol of vulnerable neurons.