Ca(2+)-dependent inactivation of high-threshold Ca(2+) currents in hippocampal granule cells of patients with chronic temporal lobe epilepsy

Decrease of high voltage Ca2+ currents in the dentate gyrus granule cells by entorhinal amyloidopathy is reversed by calcium channel blockade

In the Alzheimer's disease (AD), entorhinal-hippocampal circuit is one of the earliest affected networks. There are some evidences indicating abnormal neuronal excitability and impaired synaptic plasticity in the dentate gyrus (DG) of AD animal model. However, the underlying mechanism leading to DG dysfunction particularly in the early phase of AD is not known. Since calcium dyshomeostasis has a critical role in the etiology of AD, it is possible that this phenomenon precedes electrophysiological alteration in the DG. Here, the effect of the amyloid pathogenesis in the entorhinal cortex (EC) on high activated Ca²⁺ currents in the DG granule cells was investigated. One week after bilaterally injection of amyloid beta (Aβ) 1-42 into the EC, Ca²⁺ currents in the DG granule cells were assessed by whole cell patch clamp. Voltage clamp recording showed the amplitude of high voltage calcium currents in the DG granule cells was decreased following EC amyloidopathy. However, the Ca²⁺ current decay was slower than control. Double-pulse recording revealed that Ca²⁺-dependent inactivation of calcium current (CDI) was more pronounced in the EC-Aβ group compared to the control group. However, chronic treatment by calcium channel blocker (CCBs), isradipine or nimodipine, reverse the Ca²⁺ currents toward the control level. On the other hand, there was no significant difference in the calbindin level in the DG of different groups. In conclusion, our results suggest that Aβ in the EC independent of calbindin level triggers a decreased Ca²⁺ currents along with increased CDI in the DG granule cells which may lead to further electrophysiological alterations in these cells, and treatment by CCBs could preserve normal calcium current and may ultimately normal function against the Aβ toxicity.

CD8(+) T Cell-Mediated Neuronal Dysfunction and Degeneration in Limbic Encephalitis

Autoimmune inflammation of the limbic gray matter structures of the human brain has recently been identified as major cause of mesial temporal lobe epilepsy with interictal temporal epileptiform activity and slowing of the electroencephalogram, progressive memory disturbances, as well as a variety of other behavioral, emotional, and cognitive changes. Magnetic resonance imaging exhibits volume and signal changes of the amygdala and hippocampus, and specific anti-neuronal antibodies binding to either intracellular or plasma membrane neuronal antigens can be detected in serum and cerebrospinal fluid. While effects of plasma cell-derived antibodies on neuronal function and integrity are increasingly becoming characterized, potentially contributing effects of T cell-mediated immune mechanisms remain poorly understood. CD8⁺ T cells are known to directly interact with major histocompatibility complex class I-expressing neurons in an antigen-specific manner. Here, we summarize current knowledge on how such direct CD8⁺ T cell–neuron interactions may impact neuronal excitability, plasticity, and integrity on a single cell and network level and provide an overview on methods to further corroborate the in vivo relevance of these mechanisms mainly obtained from in vitro studies.

Ca2+-dependent modulation of voltage-gated Ca2+ channels

BACKGROUND

Voltage-gated (Cav) Ca2+ channels are multi-subunit complexes that play diverse roles in a wide variety of tissues. A fundamental mechanism controlling Cav channel function involves the Ca2+ ions that permeate the channel pore. Ca2+ influx through Cav channels mediates feedback regulation to the channel that is both negative (Ca2+-dependent inactivation, CDI) and positive (Ca2+-dependent facilitation, CDF).

SCOPE OF REVIEW

This review highlights general mechanisms of CDI and CDF with an emphasis on how these processes have been studied electrophysiologically in native and heterologous expression systems.

MAJOR CONCLUSIONS

Electrophysiological analyses have led to detailed insights into the mechanisms and prevalence of CDI and CDF as Cav channel regulatory mechanisms. All Cav channel family members undergo some form of Ca2+-dependent feedback that relies on CaM or a related Ca2+ binding protein. Tremendous progress has been made in characterizing the role of CaM in CDI and CDF. Yet, what contributes to the heterogeneity of CDI/CDF in various cell-types and how Ca2+-dependent regulation of Cav channels controls Ca2+ signaling remain largely unexplored.

GENERAL SIGNIFICANCE

Ca2+ influx through Cav channels regulates diverse physiological events including excitation-contraction coupling in muscle, neurotransmitter and hormone release, and Ca2+-dependent gene transcription. Therefore, the mechanisms that regulate channels, such as CDI and CDF, can have a large impact on the signaling potential of excitable cells in various physiological contexts. This article is part of a Special Issue entitled Biochemical, biophysical and genetic approaches to intracellular calcium signaling.

Physiological studies of human dentate granule cells

The availability of human hippocampi obtained through surgery (usually for treatment of temporal lobe epilepsy) has allowed us to investigate the properties of the human dentate in a way that cannot be done with other brain regions. The dentate has been the primary focus of these studies because of its relative preservation in all patient specimens. Moreover, there is extensive synaptic reorganization of numerous neurotransmitter systems in this the fascia dentate (dentate gyrus and the hilus) in humans with specific forms of TLE. These changes are not evident in tissue from patients with seizure that begin outside the hippocampus, and, as a result, this tissue provides an invaluable resource for comparisons. Physiological data using both slices and acutely dissociated cells demonstrate that the granule cells have membrane properties similar to those of rodents although there are specific changes that appear to be associated with seizures. Similarly, in the non-sclerotic hippocampi, the synaptic properties are similar to those reported in rodents. There are also a number of parallels between the findings in humans and in status animal models of temporal lobe epilepsy. This review will cover analyses of membrane properties as well as of glutamatergic, GABAergic, and neuromodulatory systems. Thus, while there are a number of issues that invariably arise with studies of pathological human tissue, this tissue is ideally suited to verify and refine animal models of temporal lobe epilepsy. In addition, one can argue that human tissue provides the only resource to evaluate the ways that granule cells recorded from laboratory animals approximate human granule cell physiology.

Influence of Ca2+-binding proteins and the cytoskeleton on Ca2+-dependent inactivation of high-voltage activated Ca2+ currents in thalamocortical relay neurons

Ca2+-dependent inactivation (CDI) of high-voltage activated (HVA) Ca2+ channels was investigated in acutely isolated and identified thalamocortical relay neurons of the dorsal lateral geniculate nucleus (dLGN) by combining electrophysiological and immunological techniques. The influence of Ca2+-binding proteins, calmodulin and the cytoskeleton on CDI was monitored using double-pulse protocols (a constant post-pulse applied shortly after the end of conditioning pre-pulses of increasing magnitude). Under control conditions the degree of inactivation (34+/-9%) revealed a U-shaped and a sigmoid dependency of the post-pulse current amplitude on pre-pulse voltage and charge influx, respectively. In contrast to a high concentration (5.5 mM) of EGTA (31+/-3%), a low concentration (3 microM) of parvalbumin (20+/-2%) and calbindin(D28K) (24+/-4%) significantly reduced CDI. Subtype-specific Ca2+ channel blockers indicated that L-type, but not N-type Ca2+ channels are governed by CDI and modulated by Ca2+-binding proteins. These results point to the possibility that activity-dependent changes in the intracellular Ca2+-binding capacity can influence CDI substantially. Furthermore, calmodulin antagonists (phenoxybenzamine, 22+/-2%; calmodulin binding domain, 17+/-1%) and cytoskeleton stabilizers (taxol, 23+/-5%; phalloidin, 15+/-3%) reduced CDI. Taken together, these findings indicate the concurrent occurrence of different CDI mechanisms in a specific neuronal cell type, thereby supporting an integrated model of this feedback mechanism and adding further to the elucidation of the role of HVA Ca2+ channels in thalamic physiology.

Altered hippocampal expression of calbindin-D-28k and calretinin in GABAB(1)-deficient mice

Balb/c GABA(B(1))(-/-) mice develop complex epileptiform activity, including spontaneous and audiogenic generalized seizures, 6-8 weeks after birth. The neuronal systems involved in these epilepsies have not been identified yet. Because the hippocampus is critically involved in epileptiform activity, we now investigated whether this brain region exhibits seizure-related alterations. Using semi-quantitative immunohistochemistry, we studied the temporal and cellular hippocampal expression pattern of two seizure-sensitive calcium-binding proteins, calbindin-D-28k and calretinin, in GABA(B(1))(-/-) mice. One month after birth, before the onset of overt epileptiform activity, wild-type (WT) and GABA(B(1))(-/-) mice exhibit comparable expression profiles for the two calcium-binding proteins. Three months after birth, once the epileptic phenotype is established, we observe clear alterations in the expression of calcium-binding proteins in the dentate gyrus area. GABA(B(1))(-/-) mice exhibit a 50% decline in the staining intensity of calbindin-D-28k expressing neurons and a 70% increase in the number of calretinin-positive neurons when compared to WT littermates. Six months after birth, the down-regulation of calbindin-D-28k protein is even more pronounced, while the calretinin expression in GABA(B(1))(-/-) mice reverts to the pattern seen in WT littermates. Our data demonstrate that the absence of functional GABA(B) receptors causes epileptiform activity through a mechanism that crucially involves dentate gyrus granule cells, and that this pathological activity is accompanied by adaptive changes.

Differential and long-lasting alterations of high-voltage activated calcium currents in CA1 and dentate granule neurons after status epilepticus

The effects on high-voltage activated (HVA) calcium currents were examined in hippocampal CA1 cells and dentate gyrus (DG) granule neurons, 2 days (short-term; ST) and 2-3 months (long-term; LT) after electrically induced, limbic electrographic and behavioural seizures in rats. Whole-cell voltage-clamp recordings in dissociated CA1 neurons of LT rats showed a decrease in the sustained HVA calcium current amplitude and a faster inactivation of the current both in rats that had experienced a status epilepticus (post-SE rats) and those in which the stimulation did not lead to SE (non-SE rats). In CA1 neurons of LT-SE rats this resulted in a reduced Ca2+ entry through the HVA channels. Perforated-patch voltage-clamp recordings in dissociated DG granule neurons of LT-SE rats showed an increased sustained HVA current amplitude compared to controls and non-SE rats, leading to an increased Ca2+ entry via HVA calcium channels. Two days after SE, we observed an increased Ca2+ entry for a defined depolarization, although the change in HVA current amplitude and inactivation rate did not reach significance. We also observed a decrease in calbindin-D28k staining in DG post-SE neurons, but this change was not associated with a change in HVA current inactivation. The opposite changes in neuronal Ca2+ entry through HVA channels in CA1 vs. DG cells depended strongly on whether rats had experienced SE and later spontaneous seizure activity. These changes are likely to contribute to regionally different effects on local network excitability.

Altered expression of voltage-dependent calcium channel alpha(1) subunits in temporal lobe epilepsy with Ammon's horn sclerosis

Voltage-dependent calcium channels, the initial components in the calcium signalling cascade, are increasingly being recognised as relevant factors in the pathology of epilepsy. To further characterise their role in temporal lobe epilepsy associated with Ammon's horn sclerosis, we investigated the immunohistochemical distribution of five different voltage-dependent calcium channel alpha(1) subunits (alpha(1A), alpha(1B), alpha(1C), alpha(1D), alpha(1E)) in 14 hippocampal specimens of patients with Ammon's horn sclerosis in comparison with eight autopsy control cases. In epilepsy specimens an increased immunoreactivity was observed for alpha(1A), alpha(1B), alpha(1D) and alpha(1E) in the neuropil of the dentate gyrus molecular layer. Dentate gyrus granule cells and residual CA3 pyramidal neurones showed enhanced immunoreactivity for alpha(1A), while labelling of these neurones was decreased for alpha(1C). Astrocytes in Ammon's horn sclerosis specimens were strongly immunoreactive for the alpha(1C) subunit contrasting with an absent astrocytic alpha(1C) labelling in controls. Our results suggest that the expression of calcium channels in neurones and glial cells is dynamically regulated in temporal lobe epilepsy, supporting the relevance of calcium signalling pathways for this disease.

Ca2+ channels and epilepsy

The epilepsies encompass diverse seizure disorders afflicting as many as 50 million people worldwide. Many forms of epilepsy are intractable to current therapies and there is a pressing need to develop agents and strategies to not only suppress seizures, but also cure epilepsy. Recent insights from molecular genetics and pharmacology now point to an important role for voltage-dependent calcium channels in epilepsy. In this article, I first provide an introduction to the classification of the epilepsies and an overview of neuronal Ca(2+) channels. Next, I attempt to review the evidence for a role of Ca(2+) channels in epilepsy and the insights gained from genetics and pharmacology. Lastly, I describe new avenues for how such information might be exploited in the development of therapeutic reagents.

Upregulation of a T-type Ca2+ channel causes a long-lasting modification of neuronal firing mode after status epilepticus

A single episode of status epilepticus (SE) causes numerous structural and functional changes in the brain that can lead to the development of a chronic epileptic condition. Most studies of this plasticity have focused on changes in excitatory and inhibitory synaptic properties. However, the intrinsic firing properties that shape the output of the neuron to a given synaptic input may also be persistently affected by SE. Thus, 54% of CA1 pyramidal cells, which normally fire in a regular mode, are persistently converted to a bursting mode after an episode of SE induced by the convulsant pilocarpine. In this model, intrinsic bursts evoked by threshold-straddling depolarizations, and their underlying spike afterdepolarizations (ADPs), were resistant to antagonists of N-, P/Q-, or L-type Ca2+ channels but were readily suppressed by low (30-100 microm) concentrations of Ni2+ known to block T- and R-type Ca2+ channels. The density of T-type Ca2+ currents, but not of other pharmacologically isolated Ca2+ current types, was upregulated in CA1 pyramidal neurons after SE. The augmented T-type currents were sensitive to Ni2+ in the same concentration range that blocked the novel intrinsic bursting in these neurons (IC50 = 27 microm). These data suggest that SE may persistently convert regular firing cells to intrinsic bursters by selectively increasing the density of a Ni2+-sensitive T-type Ca2+ current. This nonsynaptic plasticity considerably amplifies the output of CA1 pyramidal neurons to synaptic inputs and most probably contributes to the development and expression of an epileptic condition after SE.

Modulation of Ca2+ currents in rat thalamocortical relay neurons by activity and phosphorylation

Rhythmic low and high frequency activity in thalamocortical networks depend critically on activation of low- and high-voltage-activated (LVA, HVA) Ca2+ currents. In order to test whether Ca2+ currents are modified during repetitive activation, acutely isolated thalamocortical relay neurons of rats, at postnatal days 12 (P12) to P20, were investigated using patch-clamp, Ca2+ imaging and Western blot techniques. High-voltage-activated, but not LVA Ca2+ currents were reduced significantly during 2 Hz stimulation. Ca2+ imaging experiments demonstrated a close correlation between the increase in intracellular Ca2+ levels and the decrease in HVA Ca2+ current amplitudes. Further examination of HVA Ca2+ currents revealed a 'U-shaped' inactivation curve and a time-dependent inactivation process that could be described by a two-exponential function. The 'U-shape' was significantly reduced, current amplitude was increased significantly and time-dependent inactivation revealed a one-exponential decline with Ba2+ as the charge carrier, following activation of the cAMP/PKA pathway, and following application of phosphatase inhibitors (ascomycin, calyculin A). Western blot analysis and the effect of ascomycin indicated an involvement of calcineurin in the inactivation process. Isolation of HVA Ca2+ current components by subtype-specific blockers revealed that changes in time-dependent inactivation, inactivation curve and current amplitude were carried mainly by L-type and N-type Ca2+ currents. Furthermore, Ca2+-dependent inactivation was operative during stimulation protocols mimicking tonic action potential firing. These data indicate a modulation of L- and N-type Ca2+ channels by phosphorylation, resulting jointly in an increased intracellular Ca2+ influx during activity of the ascending brainstem system, the latter occurring during states of wakefulness.

Calcium currents in rat dentate granule cells are altered after adrenalectomy

Adrenal corticosteroid hormones modulate voltage-gated calcium currents in rat CA1 hippocampal neurons. In the present whole-cell recording study we examined whether calcium currents in dentate granule cells are also under control of corticosteroids. In a first series of experiments, in which the calcium chelator BAPTA was added to the recording solution, the amplitude of calcium currents induced by a voltage step to -10 mV was found to be enhanced shortly (1 or 2 days) after adrenalectomy compared to sham operation. No enhancement was seen when adrenalectomized animals received a low dose of corticosterone in the drinking water. By contrast, 3 or 7 days after adrenalectomy calcium current amplitude was decreased. Starting 3 days after adrenalectomy, some of the granule cells underwent apoptosis. This caused a bias in the recorded cell population towards relatively apoptosis-resistant cells, suggesting that restricted calcium influx may be a key feature of cells withstanding the apoptotic route. In accordance, cells from a small percentage ( approximately 20%) of animals that resisted apoptosis after adrenalectomy also displayed small calcium currents. In a second series without BAPTA, thus focusing on the endogenous calcium-buffering capacity, we found that the time constant for the decay of the calcium current was decreased after adrenalectomy, probably due to enhanced calcium-dependent inactivation of the current. The data indicate that cellular calcium current characteristics of dentate granule cells are altered after adrenalectomy and that the alterations may in part determine the vulnerability to undergo apoptosis.

Surviving granule cells of the sclerotic human hippocampus have reduced Ca(2+) influx because of a loss of calbindin-D(28k) in temporal lobe epilepsy

In mesial temporal lobe epilepsy (mTLE), the predominant form of epilepsy in adults, and in animal models of the disease, there is a conspicuous loss of the intracellular Ca(2+)-binding protein calbindin-D(28k) (CB) from granule cells (GCs) of the dentate gyrus. The role of this protein in nerve cell function is controversial, but here we provide evidence for its role in controlling Ca(2+) influx into human neurons. In patients with Ammon's horn sclerosis (AHS), the loss of CB from GCs markedly increased the Ca(2+)-dependent inactivation of voltage-dependent Ca(2+) currents (I(Ca)), thereby diminishing Ca(2+) influx during repetitive neuronal firing. Introducing purified CB into GCs restored Ca(2+) current inactivation to levels observed in cells with normal CB content harvested from mTLE patients without AHS. Our data are consistent with the possibility of neuroprotection secondary to the CB loss. By limiting Ca(2+) influx through an enhanced Ca(2+)-dependent inactivation of voltage-dependent Ca(2+) channels during prolonged neuronal discharges, the loss of CB may contribute to the resistance of surviving human granule cells in AHS.